POLITECNICO DI MILANO
Tesi di Dottorato di
FRANCISCO FERNANDEZ PALACIO
Matricola 802404
DESIGN AND APPLICATIONS OF HALOGEN-BONDED RESPONSIVE
MATERIALS
DIPARTIMENTO DI CHIMICA,
MATERIALI
E
INGEGNERIA CHIMICA
“Giulio Natta”
Dottorato di Ricerca in
Chimica Industriale e
Ingegneria Chimica (CII)
XXVIII cicle
2012 - 2015
Coordinatore: Prof. Tiziano Faravelli
Tutor: Prof. Luca Lietti
Supervisor: Prof. Pierangelo Metrangolo
2
To my parents, Esther and Carlos.
To my brother Pablo.
A mis padres Esther y Carlos.
A mi hermano Pablo.
3
Acknowledgements.
Thanks to everyone who has supported me throughout my doctoral studies
over these three years.
A very special thanks to the most important person, Priscilla.
¿Qué te voy a decir?
4
List of Publications
Manuscripts in preparation
“Zinc(II) coordination networks decorated with azobenzene halogen bonding
donors”
Francisco Fernandez-Palacio, Marco Saccone, Luca Catalano, Tullio Pilati,
Giancarlo Terraneo, Pierangelo Metrangolo, Giuseppe Resnati
Ready for publication, intention to submit to CrystEngComm
“Photoinduced phase transitions in halogen-bonded liquid crystals”
Francisco Fernandez-Palacio, Mikko Poutanen, Marco Saccone, Antti Siiskonen,
Giancarlo Terraneo, Giuseppe Resnati, Olli Ikkala, Pierangelo Metrangolo and Arri
Priimagi.
Ready for publication, intention to submit to Angew. Chem. Int. Ed.
“Halogen bond-directed nanostructuring of block copolymers”
Francisco Fernandez-Palacio, Roberto Milani, Marco Saccone, Alessandro Luzio,
Gabriella Cavallo, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi, and
Olli Ikkala
Ready for publication, intention to submit to J. Am. Chem. Soc.
“Halogen bond in ionic liquid crystals”
Francisco Fernandez-Palacio, Gabriella Cavallo, Giancarlo Terraneo, Marco
Saccone, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
“Halogen bonding in Molecular Clips Co-Crystal with I2”
Luca Catalano, Francisco Fernandez-Palacio, Giancarlo Terraneo, Lyle Isaacs,
Giuseppe Resnati, Pierangelo Metrangolo
Ready for publication, intention to submit to CrystEngComm.
5
Conferences
“Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals”
(Presenting Author)
F. Fernandez-Palacio, M. Poutanen, M. Saccone, G. Terraneo, A. Siiskonen, G.
Resnati, P. Metrangolo, A. Priimagi.
Abstract and Poster
RSC Macrocylic and Supramolecular Chemistry Group. Durham (United
Kingdom)
December, 2015
“Zn(II) Coordination Networks based on an Azobenzene-containing Halogen
Bond-Donor Ligand” (Presenting Author)
F. Fernandez-Palacio, M. Saccone, L. Catalano, T. Pilati, G. Terraneo, G. Resnati,
P. Metrangolo.
Abstract and Poster
RSC Macrocylic and Supramolecular Chemistry Group. Durham (United
Kingdom)
December, 2015
“Fast and Efficient Photoinduced Phase Transitions in Halogen-Bonded Liquid
Crystals” (Presenting Author)
Francisco Fernandez-Palacio, Mikko Poutanen, Marco Saccone, Giancarlo
Terraneo, Pierangelo Metrangolo, Giuseppe Resnati, Arri Priimagi.
Abstract and Poster. POSTER PRIZE, by Crystal Growth and Design
2nd ICSU/IUPAC Workshop on Crystal Engineering. Como (Italy)
August-September 2015
“Zinc(II) Coordination Networks based on an Azobenzene-containing Halogen-
Bond Donor Ligand” (Presenting Author)
Francisco Fernandez-Palacio, Marco Saccone, Tullio Pilati, Pierangelo Metrangolo,
Giuseppe Resnati
Abstract and Poster
2nd ICSU/IUPAC Workshop on Crystal Engineering. Como (Italy)
August-September 2015
6
“Halogen-Bond Driven Self-Assembly of Borromean Systems” (Co-Author)
Gabriella Cavallo, Francisco Fernandez-Palacio, Vijith Kumar, Frank Meyer,
Tullio Pilati, Pierangelo Metrangolo, Giuseppe Resnati, Giancarlo Terraneo
Abstract and Poster
2nd ICSU/IUPAC Workshop on Crystal Engineering. Como (Italy)
August-September 2015
“Self-Assembly of Photo-Switchable Fluorinated Liquid Crystals through Halogen
Bonding” (Presenting Author)
Francisco Fernandez-Palacio, Marco Saccone, Mikko Poutanen, Tullio Pilati,
Gabriella Cavallo, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Oral Presentation
21st International Symposium on Fluorine Chemistry and 6th International
Symposium In Fluorous Technologies. Como (Italy)
August 2015
“Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals” (Presenting
Author)
Francisco Fernandez-Palacio, Mikko Poutanen, Marco Saccone, Tullio Pilati,
Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Abstract and Poster
10th The International Symposium on Macrocyclic and Supramolecular Chemistry.
Strasbourg (France)
June, 2015
“Tunable halogen-bonded responsive systems: A closer look at solid state” (Co-
Author)
Giancarlo Terraneo, Francisco Fernandez-Palacio, Gabriella Cavallo, Valentina
Dichiarante, Marco Saccone, Giuseppe Resnati, Pierangelo Metrangolo, Arri
Priimagi.
Poster
Gordon Research Conference. Artificial Molecular Switches & Motors. Easton,
Massachusetts, (United States)
June, 2015
7
“Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals” (Co-
Author)
Mikko Poutanen, Francisco Fernandez-Palacio, Marco Saccone, Tullio Pilati,
Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Poster
Gordon Research Conference. Artificial Molecular Switches & Motors. Easton,
Massachusetts, (United States)
June, 2015
“Halogen-Bonded Photoresponsive Liquid Crystals” (Co-Author)
Marco Saccone, Francisco Fernandez-Palacio, Mikko Poutanen, Tullio Pilati,
Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Abstract and Poster
Gordon Research Conference. Self-Assembly and Supramolecular Chemistry.
Lucca (Italy)
May, 2015
“Borromean Systems via Anion Driven Self-assembly: Topology Invariance and
Metric Tuning on Anion Change” (Presenting Author)
F. Fernandez-Palacio, F. Meyer, T. Pilati, P. Metrangolo, G. Resnati
Abstract and Poster
First International symposium on Halogen Bonding. Porto Cesareo (Italy)
June, 2014
“Organic frameworks formed via hydrogen and halogen bonding orthogonal self-
assembly.” (Presenting Author)
F. Fernandez-Palacio, L. Colombo, J. Martí-Rujas, G. Terraneo, T. Pilati, P.
Metrangolo, G. Resnati.
Abstract and Poster
Past, Present, and Future of Crystallography@Politecnico di Milano. Milan (Italy)
July, 2013
8
Index
Index of figures 11
Index of tables 16
State of the art 17
1. Introduction 18
1.1. What is Halogen Bonding? 18
1.1.1. Halogen Bonding: History 21
1.1.2. Halogen Bonding in Supramolecular Chemistry 22
1.2. Responsive materials 24
1.2.1. Photoresponsive materials 24
1.2.2. Azobenzene system 25
1.2.3. Responsive does not only mean Photo- 26
1.3. Liquid Crystals 27
1.3.1. Halogen-Bonded Liquid Crystals 31
1.3.2. Photoresponsive Halogen-Bonded Liquid Crystals 32
1.3.3. Ionic Liquid Crystals 33
1.4. Metal Organic Frameworks 35
1.4.1. Photoswitchable Metal Organic Frameworks 36
1.5. Block Co-Polymers 39
9
2. Neutral and Ionic Halogen-Bonded Liquid Crystals 45
2.1. Objectives 45
2.2. Materials 48
2.3. Results and discussion 50
2.3.1. Structural analysis 50
2.3.2. Mesophase characterization 54
2.3.3. Photochemical studies 57
2.4. Experimental part 63
2.4.1. General procedure for the synthesis of AZO molecules 63
2.4.2. Crystal structure determination 64
2.5. Conclusions 68
2.6. Halogen-bonded ionic liquid crystals 69
2.6.1. Objectives 69
2.6.2. Materials, results and discussion 69
2.6.3. Conductivity studies 73
2.6.4. Conclusions 76
3. Metal Organic Frameworks 77
3.1. Objectives 77
3.2. Materials 80
3.3. Results and discussion 81
3.3.1. {[Zn(1)(Py)2](2-propanol)}n (3) 81
3.3.2. {[Zn(1)2(4,4’-bipyridyl)2](DMF)2}n (4) 83
3.3.3. {[Zn(2)(1,2-di(4-pyridyl)ethylene](DMF)1.81}n (5) 85
3.3.4. Photochemical studies 86
3.4. Experimental part 88
3.4.1. Synthesis of AZO molecules 88
3.4.2. Synthesis of MOFs 89
3.5. Conclusions 91
10
4. Block Co-Polymers 92
4.1. Objectives 92
4.2. Materials 94
4.3. Results and discussion 96
4.4. Experimental part 106
4.4.1. Preparation of samples for AFM 106
4.4.2. Removal of DIPFO in the complex 106
4.4.3. Preparation of samples for IR 106
4.4.4. Preparation of samples for TGA 107
4.4.5. Metalation 107
4.5. Conclusions 108
General conclusions and future perspectives 109
References and notes 111
11
Index of figures
Figure 1. General scheme for the formation of halogen bonds. Y is a carbon,
nitrogen or halogen atom, X is the electrophilic halogen atom (XB donor,
Lewis acid) and D is a donor of electron density (XB acceptor, Lewis base). ........... 18
Figure 2. Molecular electrostatic potential, in Hartrees, mapped on the 0.001
electrons Bohr-3
isodensity surface. Color code: positive electrostatic potential
in red and negative electrostatic potential in blue11
.................................................... 20
Figure 3. Photoisomerization of trans azobenzene to cis azobenzene
and viceversa. .............................................................................................................. 25
Figure 4. Examples of calamitic mesogens.12
............................................................ 28
Figure 5. Smectic phases, showing layered structure: (left) Smectic A, and
(right) Smectic C (tilted). ............................................................................................ 29
Figure 6. On the left Smectic phase. On the right Nematic phase. On the top
schematic views of the different phases. On the bottom, POM images for the two
phases........................................................................................................................... 29
Figure 7. Examples of discotic liquid crystals.12
........................................................ 30
Figure 8. The three kinds of bent-core liquid crystals................................................ 31
Figure 9. First example of Liquid Crystal formed through Halogen Bonding.71
....... 32
Figure 10. Simple scheme of Metal Organic Framework Synthesis. ......................... 35
Figure 11. The azobenzene forming the MOF structure can be switched from the
trans to the cis state by UV light (ν1) and vice versa by visible light (ν2).98
.............. 37
Figure 12. Structure of the azobenzene-containing linkers and MOFs.104
................. 38
Figure 13. Schematic representation of supramolecular assembly formation
through hydrogen bonding between the P4VP block of PS-b-P4VP copolymer
and any halogen-bond donor molecule. ...................................................................... 40
Figure 14. Mean-field prediction of the thermodynamic equilibrium phase
structures for conformationally symmetric diblock melts. fA is the volume
fraction. ........................................................................................................................ 40
12
Figure 15. A theoretical phase diagram (depending on temperature) for a
conformationally symmetric block co-polymer melt. ................................................. 41
Figure 16. On the left perpendicular and on the left parallel orientation between
cylindrical order and the film. ..................................................................................... 43
Figure 17. Schematic sketch of the fabrication process. ............................................ 43
Figure 18. Photoinduced phase transition of LC-azobenzene mixtures. Tcc refers
to the LC-isotropic phase transition temperature of the mixture with the
cis form, while Tct refers to that of the mixture with the trans form.134
...................... 46
Figure 19. The complexes prepared in this study are assembled by halogen
bonding between promesogenic stilbazole molecules (left) and photoresponsive
halogen-bond donors (right). ....................................................................................... 47
Figure 20. On the left, crystal packing of cis-AZO12. On the right, crystal
packing of trans-AZO12.144
....................................................................................... 51
Figure 21. X-ray spacefill model of the AZO10-ST1 complex showing both the
N···I halogen bonding and arene-perfluoroarene quadrupolar interactions.144
.......... 52
Figure 22. The crystal structure of the complex AZO12-ST1 reveals the XB
interaction and the segregation of the aliphatic chains from the aromatic cores. ....... 52
Figure 23. Crystal structure of the complex AZO8-ST2.144
...................................... 53
Figure 24. X-ray Powder diffraction for sample AZO12-ST1. In red calculated,
in black simulated. ....................................................................................................... 53
Figure 25. X-ray Powder diffraction for sample AZO10-ST1. In blue calculated,
in black simulated. ....................................................................................................... 54
Figure 26. POM images for a typical Nematic phase (left) for the complex AZO8-
ST8 and Smectic A phase (right) in the complex AZO12-ST8. ................................ 56
Figure 27. Chart of the thermal behavior of the studied complexes. Crystal phase
is in blue, Smectic phase in red and Nematic phase in green. All the transitions are
reported on heating, with the exception of the AZO12-ST12 complex. .................... 57
Figure 28. The normalized absorbance spectra of the AZOm and STn molecules,
represented by AZO10 and ST2, respectively. The figure shows also the
13
photostationary spectra under illumination wavelengths of 365 nm, 395 nm
and 457 nm, and the calculated spectrum of the cis-isomer for the AZO10
molecule. ..................................................................................................................... 58
Figure 29. The photoinduced nematic-to-isotropic transition and the reverse
transition of AZO10-ST8 observed under POM. ....................................................... 60
Figure 30. On the left the birefringence and absorbance measurements of the
photoinduced nematic-to-isotropic transition. On the right the absorbance
behavior under different illumination conditions. ....................................................... 61
Figure 31. Transition crystal-to-isotropic liquid of AZO10-ST8 at 95 °C (10 °C
below melting point) with 395 nm light. The initial crystal (1) melts into isotropic
liquid (2) after ca. 30 s of illumination. The recrystallization (3) occurs through a
partial nematic phase formation approximately 3 minutes after the UV light
illumination. The recrystallization proceeds as a front due to uneven illumination
conditions under the microscope. The end state is fully crystalline material (4)
without phase separation. ............................................................................................ 62
Figure 32. Crystal structure of the compounds (left to right, top to bottom):
trans-AZO8, trans-AZO10, trans-AZO12 and cis-AZO12.144
............................... 64
Figure 33. From the top to bottom: Crystal structure of the ST1-AZO12; ST1-
AZO10 and ST2-AZO8.144
......................................................................................... 65
Figure 34. The complexes prepared in this study are assembled by halogen
bonding between imidazolium iodides “IMIn” (left) and photoresponsive
halogen-bond donors AZOm (right). .......................................................................... 69
Figure 35. POM image of the 2AZO12-IMI2 at 70°C (cooling). ............................. 70
Figure 36. DSC of IMI2, AZO12 and 2AZO-IMI2 ................................................. 71
Figure 37. Crystal structure of 2AZO12-IMI12.144
................................................... 72
Figure 38. Crystal structure of 2-stilbeneC8-IMI10.144
............................................ 72
Figure 39. Crystal structure of N,N-azo halogen-bond donor and IMI8 iodide.144
... 73
Figure 40. On the left IMI12 conductivity. On the right 2AZO12-IMI2
conductivity. ................................................................................................................ 74
14
Figure 41. Comparison between conductivity of 2AZO12-IMI2 and IMI12 .......... 74
Figure 42. On the left compound 1, on the right compound 2 ................................... 78
Figure 43. Organic commercial linkers used in this work. ........................................ 81
Figure 44. On the left the coordination polymer 3, projected along its main
axis. On the right projected approximately orthogonal to main axis.144
..................... 82
Figure 45. A single complete net projected along the a axis, showing the cage
that contains two DMF molecules (larger balls).144
.................................................... 83
Figure 46. A single complete net projected along the b axis, where halogen
bond (I•••O) is evident.144
............................................................................................ 84
Figure 47. Interdigitating of three 2D layers projected along a axis using
different colors. ............................................................................................................ 85
Figure 48. Left: packing view along a showing the bidimensional pipe network
containing DMF in Mercury style. Right: the same viewed along c. In all the
plots, the DMF is omitted and only one of the disordered conformers is reported, for
clarity.144
...................................................................................................................... 86
Figure 49. Left, UV-vis spectra of 1 before (black curve) and after (coloured
curves) irradiation using 457 nm light. Right, time development of the absorbance
of 1. .............................................................................................................................. 87
Figure 50. Left, UV-vis spectra of 2 before (black curve) and after (coloured
curves) irradiation using 457 nm light. Right, time development of the absorbance
of 2. .............................................................................................................................. 87
Figure 51. On the top, a dual height (left) and phase (right) AFM images for the
polymer (blank). On the bottom, a dual height (left) and phase (right) AFM images
for the polymer plus DIPFO (complex) ...................................................................... 96
Figure 52. A dual height (left) and phase (right) AFM images for the polymer
with DIPFO showing tetragonal order. ....................................................................... 97
Figure 53. Distribution parameters of diameter bulk (left) and distance center-to-
center (right) ................................................................................................................ 97
15
Figure 54. A dual height (left) and phase (right) AFM images for the
complex at different speeds (from the top to the bottom, 500, 1000 and
2000 rpm respectively) ................................................................................................ 98
Figure 55. TEM images .............................................................................................. 99
Figure 56. SAXS patterns of PS-b-P4VP (trace 1) and PS-b-P4VP(DIPFO)
(trace 2) samples prepared by drop casting. .............................................................. 100
Figure 57. Comparison of IR spectrum of DIPFO, PS-b-P4VP and complex ......... 101
Figure 58. Comparison of TGA for PS-b-P4VP, complex and DIPFO. .................. 102
Figure 59. Comparison of Raman spectrum for PS-b-P4VP, complex
and DIPFO. ................................................................................................................ 103
Figure 60. A dual height (left) and phase (right) AFM images of a thin complex
film after washing with ethanol. ................................................................................ 104
Figure 61. ATR-FTIR spectra of PS-b-P4VP, DIPFO and of a thin spin-coated
complex film, both as prepared and after ethanol washing. ...................................... 104
Figure 62. AFM topographic micrograph on the left and section profile on the
right of gold nanostructures prepared by metalation of the hollow template left
after washing the thin complex film in ethanol, and subsequent removal of the
polymer template by acetone. .................................................................................... 105
Figure 63. Chart of the size distribution of gold nanodots. ...................................... 105
16
Index of tables
Table 1. Thermal data for the LC complexes ............................................................. 55
Table 2. Crystallographic data for the compounds trans-AZO12,
trans-AZO10, trans-AZO8 and cis-AZO12 ............................................................. 66
Table 3. Crystallographic data for the complexes AZO12-ST1,
AZO10-ST1 and AZO8-ST2. .................................................................................... 67
Table 4. Thermal data for the LC complexes ............................................................. 70
Table 5. Crystallographic data for the complexes 2AZO12-IMI12,
2stilbeneC8-IMI10 and 2N,N-azo-IMI8. .................................................................. 75
Table 6. Crystallographic data for the compounds 3, 4, 5. ......................................... 90
17
State of the art
From the prehistory, humans have been creating materials with the goal of
helping their daily chores. Therefore, knowing the problem, one can design
apparatus to overcome common drawbacks and it is particularly attractive to
layout materials from smaller things. Applying this concept to chemistry, the design
of materials from small molecules and the interaction between them,
give us the definition of the “Supramolecular Chemistry”.
Recently, Supramolecular Chemistry has been deeply studied for many groups on
every side of the world, in which molecules respond to stimuli applied from diverse
non-internal sources by undergoing reversible transformations
between two different states. The importance
of these phenomena rises from the molecules undergoing changes in their
noted characteristics (e.g. electronic and topological) and then acting as switching
different elements in functional materials. Thus, designing a functional system
responding to external stimuli, one can create a material which provides us with
predictable response.
In the recent years, several studies related to halogen bonding have been
developed by numerous groups. The remarkable interest in this non-covalent
interaction may find explanation in its properties (specificity, robustness and high
directionality), which present more advantages than others non-covalent interactions
frequently used in Supramolecular Chemistry.
Therefore, highly motivated from the discussion above mentioned, I considered
three well known topics in supramolecular chemistry to review the studies developed
until now, adding halogen bonding properties in order to improve their characteristics.
These topics are responsive Liquid Crystals (LC), Metal Organic Frameworks (MOFs)
and block co-polymers.
18
CHAPTER 1
Introduction
1.1. What is Halogen Bonding?
Recently, IUPAC has defined Halogen Bonding (XB)1 as “an attractive
interaction between an electrophilic region associated with a halogen atom in a
molecular entity and a nucleophilic region in another or the same, molecules entity”.
To clarify the general scheme Y-X…
D, illustrated in figure 1has been introduced2 to
define a halogen bond.
Figure 1. General scheme for the formation of halogen bonds. Y is a carbon, nitrogen or halogen
atom, X is the electrophilic halogen atom (XB donor, Lewis acid) and D is a donor of electron density (XB acceptor, Lewis base).3
19
In this scheme, the Lewis acid X (halogen atom) is covalently bound to Y
(carbon, nitrogen or halogen atom) which non-covalently interacts with the Lewis
Base D (N, O, S, Se, Cl, Br, I, Halides…). Halogens participating in halogen bonding
are iodine (I), bromine (Br), chlorine (Cl) and, rarely,4 fluorine (F). All four halogens
act as halogen-bond donors (as proven through theoretical and experimental data)
following the general trend: F < Cl < Br < I, with iodine normally forming the
strongest interactions since it is most likely polarizable.5 Y can be any atom (e.g., C,
N or halogen). Of particular interest is the case where Y is a halogen because it is a
dihalogen. Dihalogens (I2, Br2, etc.) tend to form strong halogen bonds.6-7
In the same
way, the presence of electron-withdrawing groups (for example fluorine atoms) near
the atom covalently bound to the halogen, increases the strength of halogen bond
considerably. 8-9-10
In organic halides, the electron density is anisotropically distributed around
halogen atoms (as you can see in figure 2) where the molecular electrostatic potential
for every halogen atom is represented.11
According to this scheme, it is possible to
note that the positive potential along the C-X axis increases on moving from F to I, in
the same way as the polarizability of the halogen atom. Therefore, an important area
of positive charge is shown for I, while for the F atom, the electrostatic potential
remains negative.12
Thus, a negatively charged belt surrounds the smaller halogens
element and, indeed, this anisotropic distribution of the electron density makes
possible for halogen atoms to work at the same time as XB donors and acceptors.
Nevertheless, the magnitude of the positive area increases as the electron-
withdrawing effect of the neighboring groups increases and, therefore, theoretical
calculations prove it is possible to carefully tune the strength of the halogen bond in a
given halocarbon by modifying the substituents on the carbon skeleton.8-10
20
Figure 2. Molecular electrostatic potential, in Hartrees, mapped on the 0.001 electrons Bohr-3 isodensity surface. Color code: positive electrostatic potential in red and negative electrostatic
potential in blue11
The well-localized electron positive region described above, permits to dispose
the electron pair of other, or the same, molecule towards the halogen atom, and it
plays a crucial role for the orientation of this interaction. For this reason, halogen
bonding is markedly directional, and the angle between the atom covalently bound to
the halogen, the halogen and the electron rich atom usually approximates to 180°.2-13
Another important point is that the presence of halogen atoms in a molecule
decreases its hydrophilic character. It has been reported that polar solvents have low
interaction in the energies and geometries of halogen bond in solution.14
However,
since it does not happen with hydrogen bond, halogen bond is considered as a
hydrophobic equivalent of the hydrogen bonding.
All the benefits of halogen bonding described above, particularly the high
directionality, linearity and robustness, have been recognized and much used in
crystal engineering,15
medicinal chemistry16
and, more recently, in the design of
functional materials.17
Thus, halogen bond is presented as an attracting interaction for
supramolecular chemistry. Furthermore, halogen bonding has been used for other
applications in different topics as anion recognition,18
polymers,19
nanoparticles,20
catalysis21
among many others.
21
1.1.1. Halogen Bonding: History.
In the mid-19th
century the first systems involving halogen atoms as electrophilic
“sticky” sites in self-organization processes due to NH3.I2 and pyridine-alkyl iodides
adducts being isolated, was described.22-23
About sixty years later, Benesi and
Hildrebrand24
published a paper where they described the UV-Vis changes that
accompany the spontaneous complexation in non-polar solvents of various aromatic
hydrocarbons with I2. In the 1950s, Robert S. Mulliken developed a detailed theory of
electron donor-acceptor complexes, classifying them as being outer or inner
complexes.25-26-27
The Mulliken theory has been used to describe the mechanism
through which halogen bond formation occurs. Outer complexes were those in which
the intermolecular interaction between the electron donor and acceptor were weak
and had very little charge transfer. Inner complexes have extensive charge
redistribution.
In 1969, Odd Hassel won the Nobel Prize in Chemistry (shared with Derek H. R.
Barton) for “his contributions to the development of the concept of conformation and
its applications in chemistry”. These contributions were based on the studies about X-
ray crystallographic measurements of Br2 complexes with benzene and dioxane in
which it was provided evidence that these intermolecular complexes involve close
contacts between electron-donor and acceptor molecules (with interatomic separation
significantly shorter than the sum of their Van der Waals radii).28-29
Therefore, in his
Nobel lecture,30
he highlighted the importance of intermolecular interactions
involving halogen atoms to direct supramolecular self-assembly.31
Nowadays, there are many groups around the world working on Halogen
Bonding from many different topics. In graphic 1, it is possible to note how the
number of papers, which have “halogen bond” as topic, has been raising to reach
more than 200 papers in 2014. Therefore, recently, the IUPAC introduced the
definition of Halogen Bonding.1 In June 2014, the first “International Symposium On
Halogen Bonding” was celebrated. It was held on Porto Cesareo (Italy)32
with almost
22
200 participants. The Second Symposium will be held on Gotherburg (Sweden) in
June 2016.33
For all these reasons, I can quote Metrangolo and co-workers2 saying
“the halogen bonding concept is still in its infancy”
Graphic 1. Number of papers including “halogen bond” in the title. Source: Web of Science
1.1.2. Halogen Bonding in Supramolecular Chemistry.
In his Nobel Price Lecture, Professor Jean-Marie Lehn defined Supramolecular
Chemistry as “the chemistry of the intermolecular bond, covering the structures and
functions of the entities formed by association of two or more chemical species”.34
The forces, which permit to assemble these domains, may vary from strong covalent
bonds into every single molecules up to weak interactions, (including electrostatic
interactions, ion-dipole interaction, dipole-dipole interaction, hydrogen bonding,
halogen bonding…). This allows individual molecules to held together with non-
covalent intermolecular forces to form a bigger unit called supramolecule,35
where
individuals have its own organization, their stability and tendency to associate or
isolate.
23
Therefore, the main advantage of supramolecular chemistry is to provide
structures in which the properties are given by the cooperation between the small
constituents. Thus, supramolecular chemistry is playing an important role in concepts
as molecular self-assembly, host-guest chemistry, folding, molecular recognition,
dynamic covalent chemistry.36
In addition, supramolecular chemistry is crucial to the
understanding of many biological processes from cell structure to vision which relies
on these forces for structure and function.
As it was discussed above, the weak interactions play a critical role in the final
structure of the supramolecules. In organic molecules, the halogen atoms are usually
located at the periphery of them and prone to be involved in intermolecular
interactions. For example, halogen bonding is also presented as a functional, effective
and reliable interaction to direct intermolecular recognition processes in gas, liquid
and solid phases. In fact, in the last decade many examples have been reported using
halogen bonding to direct the self-assembly of non-mesomorphic components into
supramolecular liquid crystals,37
to afford and tune second-order nonlinear optical
responses,38
to control the structural and physical properties of conducting and
magnetic molecular materials,39
to separate mixtures of enantiomers and other
isomers,40
to exert supramolecular control on the reactivity in the solid state,41
to
optimize the binding of ligands in a receptor, molecular folding, and other
biopharmacological properties,42
as well as to bind anions in solution and in the solid
state.43
One of the most important advantages working with supramolecular structures is
to design and predetermine these macro-domains in order to obtain specific
properties. This has a great potential for developing useful materials and, in this
context, halogen bonding has been deeply studied for self-assembled systems.19-44
24
1.2. Responsive materials
1.2.1. Photoresponsive materials.
Several studies on material sciences have been developed, where molecules (or
materials) respond to the stimuli of an applied external source by undergoing
reversible transformations between two different conformations or isomers.45
The
importance of this fact is that molecules undergo changes in their characteristics (e.g.
electronic and topological) and can act as switching elements in functional
materials.46
Therefore, this relationship among material-answer opens to a plethora of
combinations.
Particularly, modern lasers are used in order to achieve fast response times and
focus on fine-tuned light stimulus of a specific wavelength on localized areas.
Photochromism46
is the process by which a large number of compounds, for instance
azobenzenes, interconvert between two different isomers when they are stimulated in
the absorption spectra and, for this reason, azobenezenes are the most studied
molecule that gives photoresponsive properties to the materials. The variation in the
electronic structure entails that this materials are useful because of their particular
properties in many areas, such as refractive index,47
luminescence,48
electrical
conductance49
and optical rotation.50
In this Doctoral Thesis photoresponsive materials designed through halogen
bonding will be discussed. This combination is particularly attractive and commonly
used in the recent years,37
due to the well-known and above highlighted properties of
this interaction as, for example, the high directionality,51
which can enhance the
optical performance52
and the ability to tune the interactive strength between the
building blocks via single halogen atom mutation at the binding site.
25
1.2.2. Azobenzenes system.
Azobenzenes are molecules containing two aromatic rings held together by a
nitrogen-nitrogen double bond. Due to the N=N link, they exist in two stereoisomeric
forms, trans and cis (Fig. 3). Actually, 'azobenzene' is the term used to refer to the
generic molecule, but it is extended to the molecules with different chemical groups
which replaced the aromatic hydrogen.
Figure 3. Photoisomerization of trans azobenzene to cis azobenzene and viceversa.
At room temperature, the trans form is predominant because it is more stable
than the cis form due to thermodynamically reasons, mainly because of close
proximity of the rings in the latter that leads to steric repulsion. Through the
absorption of UV-Vis radiation, trans form can be switched to the cis and the process
depends on the wavelength of the absorbed photon and on the transition involved (π
→ π* for the trans azobenzene). The effect produced on the absorption spectrum on
azobenzenes molecules attaching functional groups to the rings has been deeply
studied.
The wavelengths at which azobenzene isomerization occurs depend on the
particular structure of each azomolecule and, for this reason, azobenzenes have been
divided into three different classes,53
according to the absorption spectra. The first
ones (yellow) are azobenzene type materials, which are electronically and chemically
similar to the azobenzene. These compounds show a higher intensity π-π* absorption
in the ultraviolet region but low-intensity n-π* absorption in the visible.
Aminobenzenes are the second type (orange) and they are characterized by a weak
26
pull-push character because they are ortho- or para- substituted with electron-donor
groups and tend to closely spaced n-π* and π-π* bands in the visible. The last ones
are the pseudostilbenes molecules (red), which present a particularly hard push-pull
character because they are substituted on the 4 and 4’ position by functional groups
which are strong electron donors or acceptors and, for this reason, the strong π-π*
band is red shifted and overlaps with the n-π* one.54
1.2.3. Responsive does not only mean Photo-
In addition to the photoresponsive materials explained above, many materials
can be sensitive to a different number of factors, such as washing solvent,
temperature, humidity, pH or the intensity of light and can undergo changes in many
different ways, like altering color or transparency, becoming permeable to water or
changing shape among others. Therefore, in the last years, many studies also focused
on these “changes” in order to design systems which provide an answer required
when they are stimulated not only with light. Particularly, as in medical applications,
the polymers are often used so that the answer cannot be induced by light and, for this
reason, other stimuli, as the change of temperature, have been studied.55
27
1.3. Liquid Crystals
The liquid crystalline state is often quoted as the fourth state of matter56-57
because of its ability to form the so-called mesophases. Their properties are
intermediate between those of the crystalline solid state and those of the liquid state.
Therefore, liquid crystals flow like liquids, but they are anisotropic compounds.58-59
In the late 1880s, liquid crystals were discovered by Friedrich Richard Reinitzer,60
when he was experimenting with cholesteryl benzoate. However, it was one year later
when Otto Lehmann defined what liquid crystals are for the first time.61
For almost a
hundred years, the interest on liquid crystals remained entirely academic up to the
1970s, when they were used and exploited in flat-panel displays.62
After that moment,
several new applications for liquid crystals have been found, such as optical
imaging,63
medical applications64
and erasable optical disks65
among many others.
Depending on the order of molecules within the liquid crystal phase, we can
distinguish several different kinds of them. Therefore, the first classification depends
on how the liquid crystal phase is reached. If liquid crystallinity is induced by
temperature or solvent they are called thermotropic or lyotropic liquid crystals
respectively.
For a better understanding, we need to introduce the concept of anisotropy,
because it helps us distinguish liquid-crystalline molecules (mesogens) from those
that are not liquid crystalline. Anisotropy is the property of being directionally
dependent. Isotropy is the opposite because it implies identical properties in all
directions. Usually, liquid crystals systems have one axis, which is very different
from the other two. When there are one short and two long axes we have disc-like
molecules while when there are one long and short axes we have rod-sharped
molecules.
Attending to their molar mass, liquid crystals can be classified in low molar
mass (e.g. non-polymeric) and high molar mass (e.g. polymeric). Only low molecular
28
mass liquid crystals have been studied in this Doctoral Thesis, while the high
molecular mass ones will not be consider in the following discussion.
Low molar mass liquid crystals are divided into three major categories
depending on the kind of molecule which forms the liquid crystal phase (calamitic,
discotic and bent-core). Calamitic mesogens is characterized by a rod-shaped rigid
core formed by two or more rings and one flexible chain at least (Fig. 4).
Figure 4. Examples of calamitic mesogens.12
The liquid crystal phases of calamitic mesogens may be classified into two
types: Smectic (Sm) and Nematic (N). In the Smetic phase, molecules are arranged in
layers, with the long molecular axis approximately perpendicular to the laminar
planes. The only long-range order extends along this axis, with the result that
individual layers can slip over each other (in a manner similar to that observed in
graphite). Within one layer there is a certain amount of short-range order. Although
there are many categories of smectic phases depending on the angle between the layer
and the direction of the molecules, in this Doctoral Thesis I will only consider the
Smectic A (molecules orthogonal to the layers) and Smectic C (the molecules are
tilted) (Fig 5).
29
Figure 5. Smectic phases, showing layered structure: (left) Smectic A, and (right) Smectic C (tilted).66
The Nematic phase is the most disordered of the liquid crystal phases and
molecules are aligned in the same direction but are free to randomly drift around, as it
similarly happens in an ordinary liquid (Fig. 6).
Figure 6. On the left Smectic phase. On the right Nematic phase. On the top schematic views of the different phases.67 On the bottom, POM images for the two phases.
30
On the other hand, discotic liquid crystals are disposed around a fairly flat core
structure and are usually surrounded by six or eight peripheral alkyl(oxy) chains (Fig.
7). Calamitics molecules tend to form smectic mesophases whereas discotics
molecules self-organize into columnar mesophases.
Figure 7. Examples of discotic liquid crystals.12
The third major category is the bent-core (also called banana liquid crystals)
and it is characterized by the angle between the two parts of the molecule. Each of
these two parts is formed by at least two aromatic rings and, in addition, by one long
chain. In some case, one part contains the aromatic rings and another contains the
long chain. Despite the fact that constituent molecules of these mesophases are not
chiral, they show polar order and chiral superstructures in their LC mesophases. As it
is possible to see in figure 8, three types of bent-core liquid crystals are noted.
31
Figure 8. The three kinds of bent-core liquid crystals.68
1.3.1. Halogen-Bonded Liquid Crystals
The use of non-covalent interactions for the induction of liquid-crystalline
order is particularly attractive, because it is possible to reach liquid crystal phases by
mixing molecules which do not show liquid crystal phase by themselves. Hydrogen
bonding has been deeply used in this area.69
For instance, in one of the first cases
studied,70
several liquid crystals were formed through hydrogen bonding between
alkoxystilbazoles and various substituted phenols where neither component was
liquid crystalline. Thus, hydrogen bonding represents an example of a non-covalent
intermolecular interaction capable of inducing mesomorphism from non-
mesomorphic species.
The first example of non-mesomorphic tectons forming liquid-crystal phases
induced by halogen bonding was reported in 2004.71
It was demonstrated that liquid
crystal phase was reached by mixing 4-alkoxystilbazole and iodopentafluorobenzene,
32
and halogen-bonded interaction was confirmed by X-ray single-crystal. Thermotropic
smectic A and nematic phases were detected by microscope (Fig. 9).
Figure 9. First example of Liquid Crystal formed through Halogen Bonding.71
In order to compare different halogen-bonded liquid crystals, the non-
mesogenic halogen bonding acceptor was also mixed to bromopentafluorobenzene
but no evidence was found that the thermal behavior of the free stilbazole changed.
As mentioned above, this fact can be explained because the strength of the halogen
bonding interaction depends on the polarizability of the halogen atom.
Right after these studies, the versatility of halogen bonding in liquid crystals in
trimers formed (2:1) between stilbazoles with diiodoperfluoroalkanes72
and
diiodotetrafluorobenzene, was also confirmed.73
These were the first studies on a
topic which is still in its first years.
1.3.2. Photoresponsive Halogen-Bonded Liquid Crystals
As I anticipated in the introduction, halogen bonding is a high directional non-
covalent interaction which shares many features with the much better-known
hydrogen bonding.74
In 2002, Ikeda et al.75
have published a seminal work for doped
covalent liquid crystals. A photoinduced phase transition in liquid crystals doped with
azobenzene derivatives was studied earlier under the polarized microscope. In order
to induce the nematic-to-isotropic transition phase, a uniform UV light of wavelength
33
(365 nm) was used. It was assumed that the isotropic phase appears as a microscopic
domain, formed around the cis isomer of azobenzene derivative molecules. Their
model is based on randomly positioned formation and growth of circularly shaped
isotropic domains, characterized by the constant growth rate of their radius during the
irradiation.
In a paper published in 2012 by Priimagi, Metrangolo, Resnati et al.37
it was
demonstrated that liquid crystals assembled by halogen bonding had an unique light-
responsive properties. In particular, they studied the photoaligneament of halogen
bonding liquid crystals and the efficiency of surface-relief-grating formation of a
complex between a non-mesogenic halogen-bond donor with an azo-group and a non-
mesogenic alkoxystilbazole that acts as a halogen boning acceptor. Regarding the
photoaligneament, after a film done by spin-coating was irradiated, the absorbance
changes demonstrating that the system was alienated perpendicularly to the sample
with time. The second thing verified in this paper was the efficient surface-relief-
grating formation of the halogen-bonded supramolecular liquid crystals. Starting with
the initial film thickness of 250 nm after irradiation, the depth of the sample was 600
nm increasing the modulation 2.4 times.
Recently, Yinjie Chen et al.76
published a study on photoresponsive liquid
crystals based on the formation of halogen bonding interaction between azopyridines
and molecular iodine and bromine. Although photochemical phase transition was
induced by UV irradiation with iodine molecule, no changes were still observed in
brominated compounds due to the different polarizability of the halogen atoms.
1.3.3. Ionic Liquid Crystals
Ionic liquid crystals (ILC) are a class of liquid-crystalline compounds which
contain anions and cations. The ionic character means that some of the properties of
the ionic liquid crystals significantly differ from those of conventional liquid crystals.
One of the most important features of ionic liquid crystals is the ion conductivity.
Another significant characteristic is the ionic interaction that stabilizes lamellar
34
mesophases. In addition, ionic liquid crystals show uncommon mesophases (nematic
columnar phase).77
From an application point of view, due to the fact that ionic liquid
crystals combine the properties of ionic liquids and liquid crystals, the interest on this
topic has been growing in the last years.78-79-80
35
1.4. Metal Organic Frameworks
In recent years, the use of metals in crystal engineering has attracted considerable
attention and lead to the development of concepts defined by IUPAC, such as
coordination polymers81
(CP) and Metal Organic Frameworks (MOF),81-82-83
crystalline materials composed of self-assembled organic ligands and metal cations.84
The understanding of molecular recognition and intermolecular interactions that
drives the crystal packing in solids is a hot topic in present-day research in view of
the design and synthesis of new materials.85
In this kind of materials, single metal ions or metal clusters are used as rigid
nodes, while multidentate organic molecules possessing diverging coordination sites
are used as linkers in order to build one, two or three-dimensional architectures (Fig.
10).
Figure 10. Simple scheme of Metal Organic Framework Synthesis.86
36
Since a large variety of organic ligands and metal centers can be used for the
construction of non-porous and porous networks, the possible combination between
them, and in consequence, the number of MOF that are possible to obtain are infinite.
Therefore, the features of the metal and organic ligand play a key role in the
properties of the final network. Moreover, the periodical arrangement of the atoms in
the crystalline structure might introduce additional advantages for specific
applications (e.g. an ordered arrangement of identical catalytic sites).87-88-89
For this
reason, an important field in the studies of MOFs and CPs is the introduction of
different functional groups in order to obtain specific properties. Due to these
characteristics, MOFs and CPs are interesting because of their structural and
functional tunability which allows plethora of applications in many topics, such as
biology and medicine,90
sensor techniques,91
luminescent and magnetic materials,92-93
gas storage and separation94
or even catalysis.95
The synthesis of MOFs is an important field. There are mainly two different
techniques used to obtain the crystalline structures. The first one is the hydrothermal
reaction96
where the synthesis of single crystal is performed in an apparatus
consisting of a steel pressure vessel (autoclave) where a nutrient is supplied along
with water. Keeping the volume constant and supplying temperature of ramp, the
reaction within the autoclave is developed. The main disadvantage of the method is
the impossibility of observing the crystals as they grow. On the other hand,
isothermal techniques are based on growing slowly (even for weeks) single crystals
from a hot solution.
1.4.1. Photoswitchable Metal Organic Frameworks
As explained above, Metal Organic Frameworks are usually pre-designed in
order to obtain specific properties that permit us to use them for many applications. In
recent years, MOFs have been pre-designed introducing photoswitchable molecules
(Fig. 11) which provide the system with photoresponsive properties.97
Some of these
37
studies are oriented to demonstrate the remote-controlled release of guest molecules
thanks to the photoswitching of the azobenzene in the MOF structure.98-99
On the
other hand, the goal of any other studies is to demonstrate how the adsorption
capacity of gas molecules, for example carbon dioxide, can be changed.100-101-102-103
Figure 11. The azobenzene forming the MOF structure can be switched from the trans to the cis state by UV light (ν1) and vice versa by visible light (ν2).98
Recently an interesting work has been published104
where two similar MOFs
have been compared as both “a priori” photoswitchables. As it is possible to see in
figure 12, both MOFs are similar in their structure. The first one is formed by copper
which is the metal center, to which both 4,4’-bipyridyl and 3-azobenzene-4,4-
biphenyldicarboxylic as photoswitchable molecule, are linked. Instead, the
photoswitchable molecule on the second MOF is the 3-azobenzene-4,4-bipyridine
which is also linked to the copper atoms. As shown in figure 12, both azobenzenes
can be switched to the bent cis state by UV light. The azobenezenes present a
reversible behavior. They go back to trans state when they are irradiated with visible
light. However, when these azobenzenes are placed on the MOF, the behavior is
different. While in MOF a) the photoisomerizations is enabled, the
photoisomerization in MOF b) is sterically hindered.
38
Figure 12. Structure of the azobenzene-containing linkers and MOFs.104
39
1.5. Block Co-polymers
A polymer is a large molecule composed of many repeated subunits
(monomers). In 1963 Karl Ziegler and Giulio Natta, Professor from the Politecnico di
Milano, were awarded with the Nobel Prize for "for their discoveries in the field of
the chemistry and technology of high polymers".105
After this moment, a whole new
topic on polymers has been opened and therefore, due to their broad range of
properties, both synthetic and natural polymers play an essential role in everyday life.
When two or more different of these monomers are united together to
polymerize, their result is called co-polymer. Of particular interest is a kind of co-
polymer called block co-polymer, which are made up of blocks of different
polymerized monomers. Since the term polymer represents an entered world, only
block co-polymers will be discussed in this Doctoral Thesis, along with their
applications.
I focused my studies on the ability of block co-polymers which remain one of
the most extensively studied and utilized classes of macromolecules,106
due to their
high capacity to induce microphase separation, which has generated significant
interest in their application.107
Therefore, the formation of ultrathin films of
predetermined morphology with well-defined order and well-known dimensions has
been a hot-topic in recent years. Particularly interesting is the combination of these
organic self-assembled molecules with inorganic components having electronic
properties because it permits the fabrication of electronic materials.108
Additionally,
they may be used for microfiltration, but they are also widely used as templates to
obtain functional materials.109
For instance, phase separation is generated by binding (usually one) polymer,
which forms the co-polymer block, with small molecule surfactants. Generally, the
most widely studied polymer is poly(4-vinylpyridine-co-styrene)110-111
because of its
facility to form hydrogen bonding through its pyridine (Fig. 13). In fact, there are
40
many examples in literature using hydrogen bonding to achieve phase separation.112-
113-114 Phase separation leads to multiscale structured assemblies with features at
length scales of the order of ten to hundred nanometres.115-116
Figure 13. Schematic representation of supramolecular assembly formation through hydrogen bonding between the P4VP block of PS-b-P4VP copolymer and any halogen-bond donor molecule.
These co-polymers tend to self-assembly in many different possible ways.
Depending on the organization and the volume fraction of the space occupied (or the
free space) we can mainly distinguish four categories: spheres (s), cylinders (c),
bicontinous cubic (g) and lamellae (l) as is reported in figure 14.117
Figure 14. Mean-field prediction of the thermodynamic equilibrium phase structures for conformationally symmetric diblock melts. fA is the volume fraction.118
41
Nevertheless, to exploit these morphologies, it is essential to know and
understand the control factors and the transitions between them. For instance,
depending on the Flory–Huggins interaction parameter between the monomer units,
the length of the block copolymers (N) and the composition (f), different structures
are formed due to the balancing of the enthalpic interfacial energy between the blocks
and the entropic chain stretching energy of the individual blocks.
Figure 15. A theoretical phase diagram (depending on temperature) for a conformationally symmetric block co-polymer melt.119
The deposition of the sample in the substrate plays a critical role in the
ordering of block co-polymers thin films.120
Therefore, there are mainly two
methodologies to deposit the sample: spin-coating and drop-cast. The first one, a
polymer solution with the linker molecule, is deposited on a substrate undergoing
rapid rotation. The centrifugal forces play a critical role and the solution flows off the
rotating substrate forming a uniform film while simultaneously evaporating and,
consequently, when the solvent is completely evaporated the thin film is formed. The
second one consists of dropping a low volume of the solution containing both the
polymer and the linker and, after the evaporation of the solvent, thin film is obtained.
In this case, the process is far less violent in comparison to the previous one. In
addition, less volatile solvents are conventionally more utilized than chloroform. In
42
this methodology the formation of the polymer film allows the solvent to evaporate
more slowly.121
After the deposition, in the case that order is not detected (something very
common), it is possible to create (or improve) order through the annealing. Annealing
of the sample is obtained through high temperature or by using vapors of another (or
the same) solvent. In the first case, the temperature has a remarkable influence on the
final arrangement of the block co-polymer. Indeed, as shown in figure 15 (in the
diagram of the different phases) it was emphasized that every temperature has a
different influence in the final disposition. This change in the order has a particular
importance when you are closed to the glass transition temperature of the polymer.
On the second hand, solvent annealing is used to improve the order in the films but,
unfortunately, the thin film behavior becomes even more complicated in comparison
to the temperature annealing. However, the main advantages of using solvent
annealing are that there is no danger for polymer degradation and the time required to
achieve the order is remarkably reduced.122
In some cases, the long range order can
even be greatly improved.123
Unfortunately, there are many more important aspects to
consider such as, for instance, the solvent evaporation rate, the selectivity of the
solvent and the vapor pressure. In fact, depending on the speed at which the solvent
evaporates, the final morphology can change dramatically. For example, by using a
fast solvent evaporation, kinetically (non-equilibrium) non-ordered structures are
favored while, with a slow solvent evaporation, the thermodynamic equilibrium is
reached and the structures are more ordered.124
Therefore, the election of the solvent
used highly contributes to the final result and, for example, it is possible to reach non-
thermodynamic equilibrium structures but well ordered.
Since we are studying polymers formed by more than one component, it is
obvious that solvents do not equally interact with both constituents. Therefore, by
changing the composition of the solvents, the interaction with the block co-polymer
will be more (or less) efficient and, consequently, it is possible, for instance, to
change the system morphology from lamellar to cylindrical to spherical....125
43
Certainly, the interaction between the surface where the sample is placed and
the thickness of the sample is highly important in the final structuration of the
polymer. Usually, these films are parallel alienated to the surface,126
but actually,
looking from an applicative point of view (fabrication of nanomaterials), the most
attractive orientation is the perpendicular one (Fig. 16). Indeed, many forms on how
to redirect the preferred microdomain orientation have been reported,127
such as, for
example, electric fields, solvent interactions and by using rough substrates.128
Figure 16. On the left perpendicular and on the left parallel orientation between cylindrical order and the film.
The perpendicular orientation is the most attractive one since it is possible to
pre-design a space made of cylinders, which can work as templates for the creation of
films of ordered nanoparticles. Therefore, the controlled incorporation of
nanoparticles into self-assembled block copolymers has attracted a great deal of
interest in recent years.129-130
Additionally, this methodology is a well-known way to
improve the properties of materials at the nanometer scale.
Figure 17. Schematic sketch of the fabrication process.131
44
For a better understanding of the process, in figure 17 the fabrication process is
schematically shown. As widely highlighted in this discussion, the first important
point is to obtain perpendicular cylinders. Thus, in the second step it is possible to
deposit pre-synthesized inorganic nanoparticles selectivity into the P4VP domains.
The red part in the representation of the polymer concerns the pyridine domains,
which obviously have interaction with the deposited nanoparticles. When these
inorganic materials have reached the organization, the last step of the process is to
remove the polymer used as template. There are mainly three methodologies to
remove the polymer. The first one is by heating the sample in air furnace at high
temperature (pyrolysis)132
the second one is by oxygen plasma etching,133
and the last
one is by washing the sample with an organic solvent. These methodologies leave
behind arrays of metallic nanodots on the surface.
45
CHAPTER 2
Neutral and Ionic Halogen-Bonded
Liquid Crystals
2.1. Objectives
Based on the works already mentioned in the introduction,37-72-75-76
we considered
that the recently developed supramolecular Low-Molecular Weight Liquid
Crystalline Actuators (LMWLCA) have not been investigated in depth yet. However,
it has been demonstrated134
that the main option to assemble these LMWLCA
consists of doping a fraction of azobenzene molecules in nematic liquid crystals as 4-
pentyl-4’-cyanobiphenyl. Therefore, in order to destroy the liquid crystal alignment
by inducing a transition liquid crystal-to-isotropic phase, the isomerization of small
quantity of azobenzene molecules from the trans to cis is enough. This change on
even small amount of it, permits the propagation of isotropic phase on the whole
sample thought an efficient cooperative molecular motions (Fig. 18).
46
Figure 18. Photoinduced phase transition of LC-azobenzene mixtures. Tcc refers to the LC-isotropic phase transition temperature of the mixture with the cis form, while Tct refers to that of the
mixture with the trans form.134
Unfortunately, this approach mainly presents two problems. On the first hand,
sometimes the correspondent cis-azobeneze studied presents problems in the
solubility on the liquid crystal phase causing phase separation on the sample. To
overcome this problem, the azobenzene quantity should be reduced.135
On the other
hand, the nematic-to-isotropic phase transition cannot be induced at an arbitrary
temperature in the whole nematic phase range of the mixture, as it is shown in figure
18. The phase transition can be induced only at temperatures higher than the phase
transition temperature of the mixture with the azobenzene-derivative fully as cis-
isomer.
It was considered that the versatility of the supramolecular approach to
LMWLCAs should provide the possibility to overcome the problems described
above. In order to assemble the molecules which form our tectons, halogen bonding
was chosen because, as I anticipated many times in this Doctoral Thesis, it has been
proven particularly reliable and robust in self-assembly.
The key of this work136
was to assemble dimeric liquid crystals where the
photoactive alkoxyazobenzene molecule containing an iodoperfluoroarene ring acting
as the halogen-bond donor moiety with the halogen bond acceptor is a 4,4’-
alkoxystilbazole derivative. Therefore, three photoresponsive halogen bond137
donors,
47
synthetized for the first time in this research project, and five promesogenic stilbazole
molecules were used. Thus, in this work, fifteen new supramolecular liquid crystals
are presented. Their general scheme is depicted in figure 19.
Figure 19. The complexes prepared in this study are assembled by halogen bonding between promesogenic stilbazole molecules (left) and photoresponsive halogen-bond donors (right).
These compounds were labeled as STn and AZOm, where n and m are the
numbers of carbon atoms in the alkyl chain. All the liquid crystals described in this
work feature enantiotropic mesophases, except for the complex with the longest alkyl
chain, which exhibited a monotropic LC phase.
48
2.2. Materials
The starting materials were purchased from Sigma-Aldrich. Commercial HPLC-
grade solvents were used without further purification, except for acetonitrile, used as
solvent for the synthesis of azobenzenes, which was dried over CaH2 and distilled
prior use. 1H,
13C and
19F NMR spectra were recorded at room temperature on a
Bruker AV 400 or AV500 spectrometer, using CDCl3 as solvent. 1H NMR and
13C
NMR spectroscopy chemical shifts were referenced to tetramethylsilane (TMS) using
residual proton or carbon impurities of the deuterated solvents as standard reference,
while 19
F NMR spectroscopy chemical shifts were referenced to an internal CFCl3
standard.
The LC textures were studied with a Leica DM2700M polarized light optical
microscope equipped with a Linkam Scientific LTS 350 heating stage and a Canon
EOS 6D camera. The melting points were also determined on a Reichert instrument
by observing the melting process through an optical microscope. The attenuated total
reflectance Fourier transform infrared (ATR-FTIR) spectra were obtained using a
Nicolet Nexus FTIR spectrometer. The values, given in wave numbers, were rounded
to 1 cm-1
using automatic peak assignment. Mass spectra were recorded on a Bruker
Esquire 3000 PLUS. X-ray powder diffraction experiments were carried out on a
Bruker D8 Advance diffractometer operating in reflection mode with Ge-
monochromated Cu Kα radiation (λ=1.5406 Å) and a linear position-sensitive
detector. Powder X-ray diffraction data were recorded at ambient temperature, with a
2θ range of 5−40°, a step size 0.016°, and exposure time of 1.5 s per step.
The photoresponsivity in the liquid crystal phase was studied in planar liquid
crystal cells with a gap of 2 μm. The temperature control was done with Linkam
Scientific LTS 350 heating stage, and the spectra were measured with USB 2000+
spectrometer (Ocean Optics, Inc.) with a deuterium-halogen light source. The
birefringence data was measured using 820 nm wavelength laser and crossed
49
polarizers. The sample was placed in between the polarizers at a 45° angle, and the
laser intensity was measured with a photodiode. The illumination of the samples was
done with 365 nm and 395 nm high power LEDs (Thorlabs), equipped with 10 nm,
OD 4.0, band pass filters (Edmund Optics), or then with a 457 nm laser.
50
2.3. Results and discussion
2.3.1. Structural analysis
In order to demonstrate the halogen bonding interaction in our complexes, single
crystals of some combinations were grown. In addition, the single crystals of the
three new trans-azobenzenes (trans-AZO8, trans-AZO10 and trans-AZO12) and
the isomer cis-AZO12 were also obtained (Fig. 20). For this reason, I will take the
opportunity to describe all these single crystals structures in detail in order to
determinate, besides the halogen bonding, which interactions are involved in the
crystal packing.
It is remarkably interesting to compare both isomers cis- and trans- of the
AZO12. Therefore, studying the crystal packing of cis-AZO12, it is possible to
conclude that the nitrogen lone pairs plays a crucial role in the packing, since it
makes more accessible to interact with Lewis acids, transforming the azo unit in a
good halogen-bond acceptor, as shown in figure 20. The distance between the iodine
and the nitrogen of the neighbor molecule is 2.995 Å and C-I···N angle 170.2°. The
overall crystal packing nicely illustrates strong segregation between aromatic and
aliphatic parts of the molecule. The aromatic moieties of cis-AZO12 interact to form
a columnar arrangement, running through the crystallographic c-axis, thanks to the
tilted-off set stacking interactions occurring between the fluorinated surfaces. On the
other hand, the crystal structure of trans-AZO12 allows to determinate that halogen
bonding is not detected and, in addition, there is not a very clear segregation between
the aliphatic and aromatic parts. However, a weak interaction between
perfluoroarene-perfluoroarene systems is shown, as you can also see in figure 20
(distance between centroids 4.693 Å). Additionally, weak F···H contacts occur
between neighboring molecules.
51
Figure 20. On the left, crystal packing of cis-AZO12. On the right, crystal packing of trans-
AZO12.144
Even though fifteen combinations were studied on this work, only three of them
leave us single crystals which were able to be analyzed by X-ray diffraction. These
combinations are AZO10-ST1, AZO12-ST1 and AZO8-ST2. The X-ray diffraction
for the complex AZO10-ST1 mainly shows two different kinds of interactions, being
the first one halogen bonding N···I-C with a distance N···I of 2.792 Å and C-I···N
angle of approximately 174.3°. These numbers are well in line with those previously
reported for azobenzene-methoxystilbazole complexes138
and reflect the high linearity
of the halo structures.139
The second force of the crystal packing is the arene-
perfluoroarene quadrupolar interaction between neighboring AZO10 molecules in
different planes, as you can see in figure 21. The observed distance between the
centroids of the fluorinated and non-fluorinated rings is 3.698 Å, very close to that of
the benzene-hexafluorobenzene dimer.140
These interactions are strong (6.1 kcal mol-1
for the benzene hexafluorobenzene adduct)141
and very useful in crystal
engineering,142
and in liquid-crystal self-assembly.143
52
Figure 21. X-ray spacefill model of the AZO10-ST1 complex showing both the N···I halogen bonding and arene-perfluoroarene quadrupolar interactions.144
The crystal structure of the complex AZO12-ST1 shows an halogen bonding
between iodine atom of the perfluorinated azo molecule and the pyridine nitrogen of
the stilbazole, with a distance N···I of 2.817 Å and C-I···N angle of approximately
174.5°, very close to the distance already reported for AZO10-ST1. In addition, since
the alkyl chains are two alkyl units longer than the previous ones, segregation is
detected between them forming layers with a length of 31 Å (Fig. 22). Although the
skeleton is not so different, no aromatic interactions between arenes have been
detected.
Figure 22. The crystal structure of the complex AZO12-ST1 reveals the XB interaction and the segregation of the aliphatic chains from the aromatic cores.144
The third case studied, (AZO8-ST2) shows the smaller distance N···I (2.771
Å) with an angle approximately 178.6°. Also, in this case, no aromatic interactions
between arene and perfluoroarene rings have been shown. However, several weak C-
53
H aliphatic and C-H aromatic contacts also contribute to the crystal packing of our
complexes.
Figure 23. Crystal structure of the complex AZO8-ST2.144
Powder X-ray diffraction analysis of the samples was performed in order to
demonstrate that the stoichiometry of the complex determined by single crystal is
also representative of the entire bulk sample (Fig. 24 and Fig. 25).
Figure 24. X-ray Powder diffraction for sample AZO12-ST1. In red calculated, in black simulated.
54
Figure 25. X-ray Powder diffraction for sample AZO10-ST1. In blue calculated, in black simulated.
2.3.2. Mesophase Characterization
I want to underline that the starting compounds used in this work do not show
liquid crystal phase at any temperature and, that all the temperature transitions
presented here, have been checked by both Differential Scanning Calorimetry (DSC)
and Polarized Optical Microscopy (POM). In order to properly determinate the
mesophase by POM, planar cells with a thickness of 10 µm were used. In the cases
where it was possible, single crystals of the complexes grown from solution were
used for the mesophase analysis. In the other cases, the complexes were prepared by
melting together equimolar amounts of the halogen-bond donor and acceptor. In table
1 the temperature transitions are shown of every sample checked by POM and
confirmed by DSC, which give us the energy implicated in. Metrangolo et al.145
have
already reported a similar table.
55
Complex Transition T [oC] ∆H[kJ/mol]
AZO12-ST12 I-N (110.5) 14
N-SmA (101) 1
SmA-Cr (90.5) 89.1
AZO12-ST8 Cr-SmA 97.1 49.8
SmA-N 108 -
N-I 114 5.3
AZO12-ST4 Cr-N 98.2 46.4
N-I 117.8 7.8
AZO12-ST2 Cr-N 90.4 61.2
N-I 122.2 8.7
AZO12-ST1 Cr-N 80.7 40.2
N-I 112.6 3.8
AZO10-ST12 Cr-SmA 100.2 51.5
SmA-N 107 -
N-I 114.6 5.5
AZO10-ST8 Cr-N 95.5 59.1
N-I 115.5 9
AZO10-ST4 Cr-N 98.5 53.4
N-I 120.4 8
AZO10-ST2 Cr-N 101.5 58.4
N-I 125.3 10.1
AZO10-ST1 Cr-N 94 69
N-I 124.3 6.7
AZO8-ST12 Cr-SmA 92.5 29.9
SmA-N 108.6 0.3
N-I 112.3 4.9
AZO8-ST8 Cr-N 85.5 41.8
N-I 115 7.9
AZO8-ST4 Cr-N 90.1 48.1
N-I 125 7.3
AZO8-ST2 Cr-N 86.2 40.9
N-I 125 7
AZO8-ST1 Cr-N 87.2 32.9
N-I 121 5.8
Table 1. Thermal data for the LC complexes
56
Only with the exception of the complex formed by the longest chains, which it
was observed as monotropic liquid crystal phase, all the complexes feature
enantiotropic liquid crystal phases. As it is possible to see in table 1, all the
complexes present nematic phase, identified by the presence of schlieren texture (Fig.
26). Smectic A phase was detected in four cases (AZO12-ST12, AZO12-ST8,
AZO10-ST12, AZO8-ST12). These combinations correspond to the complexes, of
which the sum of the number of carbons of the both chains is equal, or even more
than twenty. This is well in line with the observation that long alkyl chains tend to
present smectic phase.146-147
Figure 26. POM images for a typical Nematic phase (left) for the complex AZO8-ST8 and Smectic A phase (right) in the complex AZO12-ST8.
In figure 27 a graphic with the thermal behavior of the complexes studied is
reported. If we examine the series by fixing the alkyl chain of the AZO molecule, we
immediately see that the nematic phase range increases by decreasing the alkyl chain
length of the STn molecule, reaches a maximum at n(ST)=2 and remains almost
constant for n=1. As it was anticipated in the previous paragraph, this proves that
long alkyl chains in the stilbazole molecules tend to destabilize the nematic phase
which is favored with short alkyl chains.
57
Figure 27. Chart of the thermal behavior of the studied complexes. Crystal phase is in blue, Smectic phase in red and Nematic phase in green. All the transitions are reported on heating, with
the exception of the AZO12-ST12 complex.
2.3.3. Photochemical studies
Firstly, the cis-to-trans isomerization of every azobenzene by means of
polarized optical spectroscopy in solution at room temperature and then in complexes
in the liquid crystal phases was studied. Regarding the photochemical properties, the
discussion is restricted to AZO10 because the three azobenzene studied shown
similar spectra and isomerization behavior. In figure 28, the absorption spectra of the
AZOm molecules measured from dilute (10-5
M) DMF solution is reported. The
molecules are more stable in trans-conformation in thermal equilibrium but it is
possible to obtain cis-conformation upon irradiation with UV light. In figure 28 we
58
can also see the different states upon illumination with 365 nm, 395 nm and 457 nm,
showing that isomerization highly depends on the illumination wavelength. Based on
the Fischer’s method148
that permits us to calculate the spectrum of the cis-isomer, the
cis-fractions were estimated to be respectively 88%, 84% and 24%. Moreover, the
absorbance spectra for the stilbazole, was studied (fig. 28). It is important to note that
the stilbazole spectrum partially overlaps with the AZOm absorption but, upon
illumination with light wavelengths above 395 nm, the STn molecules do not show
isomerization. Therefore, for the following discussion of this chapter, illumination
with light wavelengths of 395 nm will be use.
Figure 28. The normalized absorbance spectra of the AZOm and STn molecules, represented by AZO10 and ST2, respectively. The figure shows also the photostationary spectra under illumination wavelengths of 365 nm, 395 nm and 457 nm, and the calculated spectrum of the cis-isomer for the
AZO10 molecule.
Particularly interesting is the study of the cis-state of the azobenzenes because
of the unusually long life-time, equal to 13 days at 20°C. It is so stabile that cis-
conformation of AZO12 was crystallized and single crystal determined through X-
ray diffraction, as it was reported in the structural discussion. This fact is well in line
59
with the results published by Bleger et al.,149
where it was demonstrated that the o-
fluorination of azobenzene remarkably stabilizes the half-life of the cis-isomer.
After the studies on the photochemical properties of our molecules, it is now
time to study the behavior of these dyes in the liquid crystal phase. Therefore, one of
the main points of this work is the study of the isothermal nematic-to-isotropic
transition in the supramolecular liquid crystalline phase through the
photoresponsivity of the halogen-bond donor molecules. In order to ensure a
complete and very detailed study of the mesophases, oriented planar (2 µm) liquid
crystal cells were used. Due to the difference in the chemical environment,
cooperative effects and ordered nature of the liquid crystal phases, also the
photochemical behavior in the liquid crystal phases may differ from the solution
behavior. Therefore, studying the photochemistry in situ during the transitions is
important. I want to underline that it was demonstrated that the transition can be
achieved at any temperature over the whole LC interval and no differences were
found between transitions from nematic and smectic phase.
As it was proven and previously highlighted that all our complexes exhibit
similar and reproducible behavior, the isothermal transition for the AZO10-ST8
complex at 110ºC is shown in figure 29. Therefore, the sample was placed on the
liquid crystal cell detecting the homogenous alignment of the liquid crystal
complexes, due to the fact that the sample exhibited a colored bright image when
viewed through crossed polarizers with the director axis set to ±45º in respect to the
polarizer, but an opaque image was obtained when the axes coincided (Fig. 29a).
When irradiating the sample (395 nm), the liquid crystal phase disappeared within 5
seconds, as you can see in figure 29b, indicating the transition liquid crystal-to-
isotropic phase. Sample was irradiated for 35 seconds, but not changes were detected
in the POM. After the irradiation was switched off, although it was not instantaneous
(significant delay), the liquid crystal phase reappeared again figure 29c. The last
image figure 29 shows small black spots, which should be isotropic droplets that
disappear after longer recovery time.
60
Figure 29. The photoinduced nematic-to-isotropic transition and the reverse transition of AZO10-ST8 observed under POM.
To better understand the different transitions occurred in the previous
experiment, absorbance at 400 (coinciding with the absorption band of the
azobenzene), birefringence, and absorbance 700 (optical scattering monitored at
700 nm, where neither of the compounds absorb) were simultaneously measured at
different stages of the illumination cycle. The results are shown in figure 30. Before
the irradiation, the birefringence revealed high values, probably related to the good
orientation of the liquid crystal phase. The domains were oriented in the same
direction due to the planar liquid crystal cell. As described above, illumination of the
sample led to nematic-to-isotropic transition within 3 seconds and, consequently, to a
sharp drop in the birefringence showing the disappearance of the orientational order.
Regarding the absorbance at 400 nm and 700 nm, the photoinduced phase transition
is accompanied with a rapid increase in both. The pick at 400 nm arises from a
combination of increased scattering due to phase separation. The change in the UV-
Vis absorption spectrum due to a red-shift of the π-π* absorbance band of the
azobenzenes, is attributed to the disruption of molecular packing under UV
illumination. On the other hand, another peak is shown at 700 nm, indicating that the
photoinduced phase separation also takes place upon liquid crystal-to-isotropic phase
transition.
61
The sample is illuminated for 35 seconds and, during this time, the only
measurable change is the absorbance at 400 nm since it continues to decrease because
of trans-cis isomerization. After stopping the irradiation, the azobenzene starts to
thermally relax to the trans-form. It is necessary to wait for 130 seconds until the
birefringence starts to recover. At the same time, the absorbance at 400 nm reaches
the maximum, meaning that the isomerization cis-trans has started. In the absorbance
at 700 nm another pick is detected which means that a new phase transition is
happing in our sample. Indeed, as it was seen figure 29 from the POM images, it is
remarkable that the nematic phase reappears as small domains with a phase
separation process, which is now significantly slower than before starting the
illumination.
Figure 30. On the left the birefringence and absorbance measurements of the photoinduced nematic-to-isotropic transition. On the right the absorbance behavior under different illumination
conditions.
In addition to these features, our systems can undergo a crystalline-to-liquid
phase transition under irradiation with the same wavelength. Actually, this transition
has been recently reported happening in azobenzene-containing molecules, but not in
azo-containing supramolecular or liquid crystal systems.150
In order to show this, the
same complex (AZO10-ST8) was used at 85°C, where, obviously, the sample is in
the crystal phase. Therefore, a clear and reversible crystal-to-isotropic transition was
verified, as shown in figure 31. The crystalline phase melts within ca. 30 s irradiation,
after which isotropic liquid is observed. Four minutes after ceasing the irradiation, the
62
recrystallization occurs through a partial nematic liquid crystal phase. The result is a
homogeneous crystalline phase. It confirms that the halogen bond drives the
supramolecular complexation for the photoresponsive liquid crystals and, in addition,
the significant delay and the partial nematic phase confirm again that the transition is
due to the isomerization.
Figure 31. Transition crystal-to-isotropic liquid of AZO10-ST8 at 85 °C (10 °C below melting point) with 395 nm light. The initial crystal (1) melts into isotropic liquid (2) after ca. 30 s of illumination.
The recrystallization (3) occurs through a partial nematic phase formation approximately 3 minutes after the UV light illumination. The recrystallization proceeds as a front due to uneven
illumination conditions under the microscope. The end state is fully crystalline material (4) without phase separation.
63
2.4. Experimental part
2.4.1. General Procedure for the Synthesis of AZO molecules
Reactions were carried out in oven-dried glassware under a nitrogen atmosphere.
A solution of 4-iodo-2,3,5,6-tetrafluoroaniline (3.715 mmol) in dry acetonitrile (5
mL) was added dropwise to a solution of nitrosonium tetrafluoroborate (3.715 mmol)
in acetonitrile (5 mL) at –30 °C. After 1 hour of additional stirring at –30 °C the
appropriated alkoxybenzene (7.43 mmol) was added dropwise. The resulting solution
was stirred overnight at room temperature and then water (15 mL) was added. The
mixture was extracted three times with CH2Cl2. The organic layers were collected,
dried over Na2SO4 and the solvent was removed under reduced pressure. The residue
was purified by column chromatography using hexane as eluent to yield the AZO
molecules (20-25%).
AZO12: Yield 25%, m.p. 74 °C;
1H NMR (400 MHz, CDCl3): δ= 7.94 (d, J = 8
Hz, 2H), 7.01 (d, J = 8 Hz, 2H), 4.06 (t, J = 6 Hz, 2H), 1.83 (q, J = 8 Hz, 2H), 1.48
(q, J = 8 Hz, 2H), 1.28 (m, 16H), 0.89 ppm (t, J = 4, 3H); 13
C NMR (126 MHz,
CDCl3): δ=163.68, 147.66, 147.61 (ddt, 1JCF = 246 Hz,
2JCF = 13 Hz,
3JCF = 5 Hz),
139.90 (dd, 1JCF = 265 Hz,
2JCF = 15 Hz), 133.32 (t, J = 14 Hz), 125.85, 115.08, 71.60
(t, J = 28 Hz), 68.76, 32.08, 29.80, 29.74, 29.51, 29.27, 26.13, 22.84, 14.25 ppm; 19
F
NMR (471 MHz, CDCl3): δ= -121.74 (m, 2F), -150.60 ppm (m, 2F). IR: 2919, 2850,
1599, 1578, 1472, 1407, 1252, 1143, 980, 836, 587, 525 cm-1
. MS/ESI m/z 564.3
found 587.0 (M+Na+).
AZO10: Yield 25%, m.p. 73 °C;
1H NMR (400 MHz, CDCl3): δ= 7.97 (d, J = 12
Hz, 2H), 7.01 (d, J = 8 Hz, 2H), 4.07 (t, J = 6 Hz, 2H), 1.83 (q, J = 8 Hz, 2H), 1.48
(q, J = 8 Hz, 2H), 1.28 (m, 12H), 0.89 ppm (t, J = 8, 3H); 13
C NMR (126 MHz,
CDCl3): δ= 163.69, 147.67 (ddt, 1JCF = 246 Hz,
2JCF = 13 Hz,
3JCF = 5 Hz), 139.90
(dd, 1JCF = 265 Hz,
2JCF = 15 Hz), 133.32 (t, J = 14 Hz), 125.86, 115.10, 68.78, 32.05,
29.71, 29.51, 29.47, 29.27, 26.14, 22.83, 14.24 ppm; 19
F NMR (471 MHz, CDCl3):
64
δ= -121.77 (m, 2F), -150.65 ppm (m, 2F). IR: 2919, 2850, 1599, 1575, 1472, 1408,
1252, 1144, 980, 858, 587, 525 cm-1
. MS/ESI m/z 536.2 found 559.0 (M+Na+).
AZO8: Yield 20%, m.p. 72 °C;
1H NMR (400 MHz, CDCl3): δ= 7.94 (d, J = 8
Hz, 2H), 7.01 (d, J = 8 Hz, 2H), 4.06 (t, J = 6 Hz, 2H), 1.83 (q, J = 8 Hz, 2H), 1.48
(q, J = 8 Hz, 2H), 1.30 (m, 8H), 0.90 ppm (t, J = 4, 3H); 13
C NMR (126 MHz,
CDCl3): δ=163.68, 147.66, 147.61 (ddt, 1JCF = 246 Hz,
2JCF = 14 Hz,
3JCF = 5 Hz),
139.90 (dd, 1JCF = 265 Hz,
2JCF = 15 Hz), 133.32 (t, J = 13 Hz), 125.81, 114.99, 71.67
(t, J = 28 Hz), 68.69, 31.95, 29.48, 29.74, 29.38, 29.24, 26.12, 22.81, 14.25 ppm; 19
F
NMR (471 MHz, CDCl3): δ= -121.75 (m, 2F), -150.66 ppm (m, 2F). IR: 2924, 2855,
1601, 1578, 1474, 1408, 1245, 1146, 979, 849, 567, 531 cm-1
. MS/ESI m/z 508.2
found 530.9 (M+Na+).
2.4.2. Crystal structure determination
As advanced in the structural analysis and as shown in figure 32, the structure
of the newly synthetized trans-AZOm molecules was proven by single crystal X-ray
diffraction analysis. The long half-life of these azobencenes allowed us to grow
single crystal of cis-AZO12.
Figure 32. Crystal structure of the compounds (left to right, top to bottom): trans-AZO8, trans-AZO10, trans-AZO12 and cis-AZO12.144
65
In addition, we performed single crystal X-ray diffraction of our complexes
(Fig. 33) and high quality data was obtained for three of them (AZO12-ST1;
AZO10-ST1 and AZO8-ST2). The azobenzene derivatives and the stilbazoles were
separately dissolved in CHCl3 at room temperature in 1:1 ratio, under saturated
conditions. The saturated solutions containing the halogen-bond donor and the
halogen bond acceptor were then mixed in a clear borosilicate glass vial, which was
left open. The solvent was allowed to slowly evaporate at room temperature for three
days until the formation of good-quality single crystals occurred.
Figure 33. From the top to bottom: Crystal structure of the ST1-AZO12; ST1-AZO10 and ST2-AZO8.144
The crystals were measured using Mo-Kα radiation on a Bruker KAPPA
APEX II diffractometer with a Bruker KRIOFLEX low temperature device. Crystal
structures were solved by direct hydrogen atoms were refined anisotropically and
hydrogen atoms were refined using difference Fourier map or positioned
geometrically. method and refined against F2 using SHELXL97.
151 Packing diagrams
66
were generated using the CSD software Mercury 3.3.152
The non- hydrogen atoms
were refined anisotropically and hydrogen atoms were refined using difference
Fouries map or positioned geometrically.
trans-AZO12 trans-AZO10 trans-AZO8 cis-AZO12
Chemical Formula C24H29F4IN2O C22H25F4IN2O C20H21F4IN2O C24H29F4IN2O
Formula weight 564.39 536.34 508.29 564.39
Temperature (K) 103(2) 103(2) 103(2) 103(2)
Crystal system Triclinic Triclinic Triclinic Monoclinic
Space group P-1 P-1 P-1 P 21/c
a (Å) 7.4374(9) 7.3742(6) 8.680(3) 8.5802(16)
b (Å) 10.2935(12) 10.3488(8) 9.209(3) 33.663(5)
c (Å) 16.536(2) 15.1393(12) 14.060(5) 8.752(2)
(°) 105.237(10) 72.642(4) 107.04(3) 90.00
β (°) 101.641(10) 82.274(4) 99.89(3) 109.255(16)
(°) 95.607(12) 83.512(4) 99.970(10) 90.00
Volume (Å3) 1181.0(2) 1089.47(15) 1028.3(6) 2386.5(8)
Z 2 2 2 4
Density (gcm-3
) 1.587 1.635 1.642 1.571
μ (mm-1
) 1.406 1.519 1.605 1.391
F (000) 568 536 504 1136
ABS Tmin, Tmax 0.5528, 0.7061 0.5035, 0.5926 0.5337, 0.6935 0.7911, 0.8621
θmin, max (°) 2.08, 35.74 2.07, 39.81 2.38, 23.25 2.42, 27.52
No. of reflections. 39236 41970 35868 5073
No. of independent
reflections.
10045 10422 2490 3840
No of parameter 290 272 237 290
No of restraints - - 174 -
R_all, R_obs 0.0379, 0.0301 0.0321, 0.0234 0.1272, 0.0815 0.0633, 0.0633
wR2_all, wR
2_obs 0.0795, 0.0749 0.0548, 0.0514 0.2375, 0.2176 0.0587, 0.0534
∆ρmin, max (eÅ-3
) -1.717, 1.390 -0.627, 0.612 -1.198, 1.334 -0.986, 0.574
G.o.F 1.071 1.068 1.114 1.047
CCDC
Table 2. Crystallographic data for the compounds trans-AZO12, trans-AZO10, trans-AZO8 and cis-AZO12.
67
AZO12-ST1 AZO10-ST1 AZO8-ST2
Chemical Formula C24H29F4IN2O, C14H13NO
C22H25F4IN2O, C14H13NO
C20H21F4IN2O, C15H15NO
Formula weight 775.65 747.59 733.57
Temperature (K) 103(2) 103(2) 150(2)
Crystal system Triclinic Triclinic Monoclinic
Space group P-1 P-1 P 21/c
a (Å) 7.7366(6) 7.602(2) 8.9690(3)
b (Å) 10.9857(8) 20.914(6) 8.4501(4)
c (Å) 21.3039(19) 21.785(5) 42.8603(19)
(°) 100.364(4) 105.819(12) 90.00
β (°) 94.900(4) 99.102(12) 94.291(2)
(°) 99.552(4) 92.312(14) 90.00
Volume (Å3) 1744.0(2) 3277.7(15) 3239.2(2)
Z 2 4 4
Density (gcm-3
) 1.477 1.515 1.504
μ (mm-1
) 0.977 1.037 1.048
F (000) 792 1520 1488
ABS Tmin, Tmax 0.7245, 0.8374 0.6644, 0.7686 0.6684, 0.7363
θmin, max (°) 1.92, 30.00 1.95, 36.15 2.28, 34.51
No. of reflections. 34300 72347 38414
No. of independent
reflections.
10000 28821 12504
No of parameter 435 833 406
No of restraints - - -
R_all, R_obs 0.0622, 0.0388 0.0526, 0.0330 0.0515, 0.0426
wR2_all, wR
2_obs
0.0728, 0.0662 0.0729, 0.0664 0.0945, 0.0912
∆ρmin, max (eÅ-3
) -0.764, 0.638 -0.598, 0.715 -0.717, 0.844
G.o.F 1.017 1.017 1.163
CCDC
Table 3. Crystallographic data for the complexes AZO12-ST1, AZO10-ST1 and AZO8-ST2.
68
2.5. Conclusions
Fifteen new halogen-bonded mesogens were prepared and studied in great detail
in this work. In particular, the photoinduced phase transitions was investigated
thoroughly. Although, these transitions have been investigated in the seminal work of
Ikeda,75
for the dye doped liquid crystals, the recently developed supramolecular
LMWLCA have not been investigated so far. Given the versatility of the
supramolecular halogen bonding approach, and the possibility to overcome the
problems associated to common dye-doped liquid crystals, we expect this
methodology to be more and more explored in the next future.
69
2.6. Halogen-bonded ionic liquid crystals
2.6.1. Objectives
The goal of this project was to design a system of ionic liquid crystals
assembled through halogen bond, based on the molecules synthetized in this Doctoral
Thesis in order to study the ion conductivity of the samples.153
Liquid crystal phase is
formed by preferential channels where an ion could go across them increasing the
conductivity in respect to the isotropic phase, where the anarchical disposition of the
molecules obstructs these movements.154-155
Moreover, by using photoresponsive
molecules, we obtain a system that, a priori, is possible to control through the
light.156
2.6.2. Materials, results and discussion
We combined all our photoresponsive dyes with many imidazolium iodide with
different long alkyl chains (Fig. 34). None of the compounds considered (exception
1-dodecyl-3-methylimidazolium iodide IMI12)157
in this work show liquid crystal
phase by themselves. All the temperature transitions shown here have been checked
by both, Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy
(POM).
Figure 34. The complexes prepared in this study are assembled by halogen bonding between imidazolium iodides “IMIn” (left) and photoresponsive halogen-bond donors AZOm
(right).
70
We studied that some of these combinations present a large range of liquid
crystal phase and, in particular, the best results for the combinations are highlighted
in table 4. AZO12-IMI2 was the combination more deeply studied since it presents a
very lager range of liquid crystallinity by cooling, and it would permit us to develop
our experiments at a determined temperature without problems regarding the stability
of the sample (Fig. 35).
Complex Transition T [oC]
2AZO12-IMI2 I-SmA (82)
SmA-Cr (38)
AZO12-IMI2 Cr-SmA 70
SmA-I 125
AZO10-IMI2 Cr-SmA 110
SmA-I 128
Table 4. Thermal data for the LC complexes
Nevertheless, the main drawback that I found was to determinate the proper
ratio that both compounds are combined to obtain the liquid crystal phase. To
demonstrate the combination between them, single crystal is required and the
structure should be solved through the X-ray diffraction. Although more than two
hundred tests were done to obtain the single crystal (including solvent evaporation,
solvent diffusion,158
heat and fast cooling with dry ice, heat and slow cooling 0.01
°C/min, high different concentrations…among many others), no single crystal of the
combination AZO12-IMI2 was obtained.
Figure 35. POM image of the 2AZO12-IMI2 at 70°C (cooling).
71
In order to demonstrate the stoichiometric combination between both
compounds, the DSC of the complex and the starting materials by themselves was
studied to see if any change is detected. Therefore, it was demonstrated that in the
DSC of 2AZO12-IMI2 the temperature transitions change in comparison to the
starting materials and, in addition, no peaks corresponding to IMI2 and AZO12 were
detect (Fig. 36). For all of these reasons, it was confirmed that the sample is
completely homogeneous and this is an unequivocal signal that the stoichiometry is
2AZO12-IMI2.
Figure 36. DSC of IMI2, AZO12 and 2AZO-IMI2
Although it was not possible to obtain single crystal for the desired
combination, a crystal of the combination AZO12-IMI12 was isolated, as you can
see in figure 37. As it could be predictable for this kind of systems8-159
involving
iodide ion within the general scheme C-I···I-···I-C, the stoichiometry was 2:1. The
distance I···I- are 3.574 and 3.475 and C-I···I
- and I···I
-···I angles are 163.58°,
170.84° and 149.72° respectively, which gives an almost linear arrangement to both
fluorinated rings and the iodide anion.
72
Figure 37. Crystal structure of 2AZO12-IMI12.144
My research group provided me with crystal structures160
of similar systems in
order to demonstrate that these systems have usually stoichiometry 2:1. Therefore, in
figure 38, where the crystal structure between two molecules of stilbene C8 (STC8)
and one molecule of 1-decyl-3-methylimidazolium iodide (IMI10) is shown, single
crystal was also isolated between two molecules of the same halogen-bond donor and
one with one of 1-ethyl-3-methylimidazolium iodide (IMI2). In addition, the
structures formed between 2STC8-IMI2, 2STC10-IMI8 and 2STC12-IMI10 were
studied. Regarding the crystallographic data of the combination shown in figure 38,
this system also shows parameters consisting of halogen-bonded system. The I…
I-
distances are longer in respect to the previous one (3.487 Å and 3.575 Å) and the C-
I…
I- and I
…I
-…I angles are 160.32°, 172.59° and 154.38° respectively, which denote a
slight difference with the previous case. However, these data are very similar to all
the combinations quoted in this paragraph.
Figure 38. Crystal structure of 2-stilbeneC8-IMI10.144
73
For the sake of comparison, the complex between an azo-dye with a smaller
chain and some imidazolium iodide was checked. In particular, the single crystal
structure of the well-known52
photoresponsive azo-dye with N-N-dimethylamino
group (N,N-azo) and 1-octyl-3-methylimidazolium iodide (IMI8) was obtained as
you can see in figure 39. In this case, the distance I···I- are 3.490 and 3.404 and C-
I···I- and I···I
-···I angles are 164.97°, 179.06° and 149.85° respectively, and very
much in line with the crystallographic structures previously reported. Unfortunately,
this system does not present liquid crystal phase at any temperature.
Figure 39. Crystal structure of N,N-azo halogen-bond donor and IMI8 iodide.144
2.6.3. Conductivity studies
As already mentioned, the main idea of this work was to study the ion
conductivity in a liquid crystal phase. With this proposal, considering the results
reported in this chapter, 2AZO12-IMI2 was the best candidate to develop further
studies. Therefore, we started testing the sample using a comb-shaped gold electrode,
as it was many times reported for this kind of systems.153-156
In order to properly contradistinguish the conductivity regarding our designed
system, we compared our measurements to the conductivity of the 1-dodecyl-3-
methylimidazolium iodide (IMI12) by itself, as you can see in figure 40.
74
Figure 40. On the left IMI12 conductivity. On the right 2AZO12-IMI2 conductivity.
Unexpectedly, conductivity decreases for both IMI12 and 2AZO12-IMI2,
going from the liquid to the liquid crystal phase. By studying the graphs, it is not
possible to detect changes in the phase transition temperatures. It could be an effect
of the noise of the measurement because the high value of the resistance comes into
play. In addition, comparing both sample studies, IMI12 shows higher conductivity
than 2AZO12-IMI2 (Fig. 41). Indeed, conductivity values of both materials are very
low, even comparable to of the high quality deionized water (0,055 µS cm-1
) while
the conductivity reported in the Kato’s studies are around 1 µS cm-1
.156
Figure 41. Comparison between conductivity of 2AZO12-IMI2 and IMI12
75
2AZO12-IMI12 2stilbeneC8-IMI10 2N,N-azo-IMI8
Chemical Formula 2C24H29F4IN2O, C16H31N2I
2C22H23F4IO, C14H27N2I
2C14H10F4IN3, C12H23N2I
Formula weight 1507.11 1362.88 1168.52
Temperature (K) 103(2) 123(2) 103(2)
Crystal system Triclinic Triclinic Monoclinic
Space group P-1 P-1 P 2 1
a (Å) 14.9747(15) 9.5648(12) 10.406(2)
b (Å) 18.898(2) 14.7815(18) 7.8092(14)
c (Å) 25.034(3) 21.592(3) 26.717(5)
(°) 87.618(12) 102.28(2) 90.00
β (°) 76.317(10) 93.35(2) 92.90(2)
(°) 76.576(10) 100.96(2) 90.00
Volume (Å3) 6694.6(13) 2912.9(7) 2168.3(7)
Z 4 2 2
Density (gcm-3
) 1.495 1.554 1.790
μ (mm-1
) 1.468 1.677 2.236
F (000) 3048.0 1364 1140
ABS Tmin, Tmax 0.731, 0.863 0.6247, 0.7462 0.6319, 0.7471
θmin, max (°) 0.954, 31.57 2.48, 31.13 0.922, 32.00
No. of reflections. 42910 61119 18631
No. of independent
reflections.
27374 7583 17551
No. of parameter 1503 683 536
No. of restraints - - 1
R_all, R_obs 0.0836, 0.0374 0.0294, 0.0219 0.0236, 0.0204
wR2_all, wR
2_obs 0.0903, 0.0729 0.0557, 0.0520 0.0449, 0.0439
∆ρmin, max (eÅ-3
) -0.882, 3.564 -0.627, 0.612 -0.435, 1.254
G.o.F 1.017 1.016 1.017
CCDC
Table 5. Crystallographic data for the complexes 2AZO12-IMI12, 2stilbeneC8-IMI10 and 2N,N-azo-IMI8.
76
2.6.4. Conclusions
Unfortunately, the results achieved did not satisfy our expectative. On the one
hand, although we are pretty sure about the key combination of the molecules, we
were not able to empirically demonstrate (single crystal) what the real stoichiometry
is when these two non-mesophase compounds present liquid crystal phase put
together. On the second hand, the ion conductivity did not work as we wanted. We
individuated two main possible causes. The first one is that the long chains of our
molecules stopple the channels where the ions had to pass. The second possibility
could be that it is necessary to add a third compound on the sample in order to
improve the movement of the anions. For instance, an iodine molecule can
contribute153
to the equilibrium I- + I2 I3
-. Also a second option is to dope the
sample with a Lithium salt for the same considerations explained above.
When this Doctoral Thesis was written, my efforts were directed to design a
better system which could help us achieve the expected results. With this goal in
mind, we started a collaboration with Professor Takashi Kato161
in order to improve
the system. Therefore, this is work in progress in this direction.
77
CHAPTER 3
Metal Organic Frameworks
3.1. Objectives
Recently, as it was advanced in the introduction, MOFs have been studied in
the context of photoresponsive materials. Although almost ten papers have shown
photoresponsive MOFs, no one of them reported Halogen Bonding interaction within.
Considering all the properties of Halogen Bonding previously highlighted, it is now
time to prepare systems where both photochromic moieties and halogen-bond donor
sites are present in the metal organic network.162
In order to carry this work out,163
a photoresponsive molecule was designed
and synthetized. This molecule presents four remarkable parts which is worth to
underline because they give different properties to the azobenzene when incorporated
into the network (Fig. 42). Therefore, the molecule was designed as an azobenezene
which possess an azo-group between two benzene-rings, as advanced in figure 3.
Secondly, the molecule was nicely decorated with a N,N-dimethylamino group
because it promotes efficient cis-trans-cis cycling upon irradiation.37-164
In the second
78
aromatic ring, two carboxylates groups were incorporated, both of them in meta-
position regarding the azo-group, in order to extend the coordination into the
network.165
Finally, the molecule owns three iodine atoms in orto- and para-
positions, with the goal of acting as halogen-bond donor within the supramolecular
structure.
Figure 42. On the left compound 1, on the right compound 2
For sake of comparison, a very similar ligand was synthetized, which does not
have halogen-bond donor groups. The goal of this study was to demonstrate how the
presence of these halogen-bond donor influences the final result of the network
providing the supramolecular structure some more coordination points in comparison
to the network done with the molecule 2. Additionally, it is worth noting that, in
literature, the presence of azobenzenes containing halogen-bond donor groups is
scarce.37-166-167
A very important point in the design of metal organic frameworks is the choice
of the metal. In this work, Zn(II) was chosen to prepare the coordination networks
because of some favorable features it shows when it is the center of the
supramolecular structure. These characteristics are, for instance, that this metal tends
to form bonds with a greater degree of covalence and it forms much more stable
79
complexes with N- and S- donors.168
In fact, zinc carboxylates have been shown to
crystallize with a range of two- and three- dimensionally connected lattices.169-170
Complexes of zinc are mostly tetra- or hexa- coordinated, even though penta-
coordinated complexes are known.171-172
In addition, Zn(II) is air stable and easy to
handle and its complexes are among the most stable in the Irving-Williams series.173
Moreover, zinc represents an important metal to study because it is an essential trace
element for humans and other animals and, recently, several papers have been
published showing networks with zinc as metallic center.174
80
3.2. Materials
All reagents and solvents were purchased from Aldrich and used without
further purification. Distilled water was used for synthetic manipulations. 1H and
13C
NMR spectra were recorded at room temperature on a Bruker AV400 spectrometer,
using CDCl3 as solvent. 1H NMR and
13C NMR chemical shifts were referenced to
tetramethylsilane (TMS) using the residual proton impurities of the deuterated
solvents as standard reference. Melting points were determined on a Reichert
instrument by observing the transition process though an optical microscope. ATR-
FTIR spectra were obtained with a Nicolet Nexus FTIR spectrometer. The values,
given in wave numbers, were rounded to 1 cm-1
using automatic peak assignment.
Mass spectra were recorded on a BRUKER Esquire 3000 PLUS. The single crystal
X-ray structure were determined on a Bruker Kappa Apex II diffractometer at 103 K
using a fine-focus MoKα tube, λ=0,71073 Å. Data collection and reduction were
performed by SMART and SAINT and absorption correction, based on multi-scan
procedure, by SADABS. The structure were solved by SIR92 and refined on all
independent reflections by full-matrix least-squares based on Fo2 by using SHELX-
97. All the non-hydrogen atoms were refined anisotropically. The UV-Vis spectra
were collected from both films and diluted (10-5
M) DMF solutions with an Oceans
Opticals USB2000+ fiber-optic spectrometer and a DH-2000-BAL light source, both
in dark and under irradiation (457 nm, 50 mW/cm2). Thermal cis-trans isomerization
was studied by exciting the chromophores to the cis-state with a circularly polarized
pump beam (457 nm, 50 mW/cm2) and monitoring the transmittance changes after
blocking the pump. The probe was a fiber-coupled xenon lamp equipped with proper
bandpass filters. The signal was detected with a photodiode and a lock-in amplifier.
81
3.3. Results and discussion
In addition to the dyes explained above, three different organic molecules were
used in order to collaborate with the assembly of the MOF. These linkers are the 4,4'-
Bipyridine, 1,2-Di(4-pyridyl)ethylene and even pyridine (Fig. 43). The choice of
these molecules was encouraged because they are well-known spacers highly used in
coordination polymers.175
Obviously at least two active-coordination points are
required to obtain extended structures. However, pyridine only has one nitrogen
capable to act as atom-linker. This fact is particularly attractive, because the structure
has to grow in two-dimensions instead of tri-dimensions or, in other words, pyridine
is useful in order to block positions in the metallic center.176
Figure 43. Organic commercial linkers used in this work.
3.3.1. {[Zn(1)(Py)2](2-propanol)}n (3)
The hydrothermal reaction among Zn(NO3)2·6H2O, 2-propanol, pyridine and 1
led to the formation of the derivative 3 (Fig. 44) that was isolated as red crystals. This
3 crystallizes in the monoclinic P21/n space group and presents 1D chains, probably
due to the explanation of the “block positions” pyridine above developed. The
formula is [Zn(1)](Py)2]n, acting 1 as bridge which connects the metal cations. These
chains are connected to the nearest one thanks to halogen bonding. In 1 two of the
three iodine atoms from each molecule are involved in a halogen bond interaction.
Iodine in position 2 forms halogen bond with the iodine in position 4 belonging to the
parallel chain. Thus, iodine in position 4 forms halogen bond with iodine in position
2 of other chain. The solvent molecule is only involved in one interaction: a hydrogen
82
bonding is observed between the -OH group of the isopropanol molecule and the
carboxylate group of 1 (O-H···O=C).
The zinc center is bonded to two pyridine molecules, blocking two of the
coordination positions, and to two 1 ligands forming a tetrahedral coordination
center. The presence of pyridine as a ligand could be limiting the formation of a
structure of greater dimension. Each azobenzene molecule 1 is linked to the metal
atom through the carboxylate group in a monodentate fashion with a distance O-Zn of
1.939 Å. The other molecule 1 coordinated to the Zn(II) center is also linked to the
carboxylate group in a monodentate fashion, but with a O-Zn distance of 1.946 Å.
Considering the zinc atoms on the same plane allows us to divide the network it in
two parts (Fig. 44). In the upper part, one pyridine molecule (one for each zinc ions)
forms an angle between itself and the plane of 93.92°. The other pyridine has three
atoms (two carbons and the nitrogen) crossed by the plane. The remaining three
atoms are below the plane. The molecule forms an angle of 30.21° with the plane. On
the bottom of the plane, two azobenzene molecules are bound.
Figure 44. On the left the coordination polymer 3, projected along its main axis. On the right projected approximately orthogonal to main axis.144
83
3.3.2. {[Zn(1)2(4,4’-bipyridyl)2](DMF)2}n (4)
After the studies of 3, and since pyridine cannot work as a multi-dentate ligand,
it was wondering how the system could act if a bidentate ligand was placed in the
supramolecular structure. Thus, after several days at room temperature a solution of
Zn(NO3)2∙6H2O with 4,4’-bipyridyl and 1 in a mixture of DMF, EtOH, and water,
afforded dark red crystals of the framework 4 and single crystal X-ray analyses
revealed its structure, showing a tetranuclear derivative that crystallizes in the
monoclinic C2/c space group.
Figure 45. A single complete net projected along the a axis, showing the cage that contains two DMF molecules (larger balls).144
Four zinc atoms show a distorted hexagonal geometry and the axial positions
are occupied by two monodentate carboxylate groups coming from the azobenzene
ligands 1. The equatorial plane is defined by four 4-4’dipyridil linked each of them to
two zinc ions, forming a square. Iodine atoms 1A point to the center of the square,
84
which interact with the oxygen atom of the DMF through a halogen bond (O···I
distance 2.930 Å). Each iodine comes from two different azobenzene molecules, one
of them from the top and the other one from the bottom of the square. As shown in
figure 46, two DMF molecules are hosted in the center of the cavity, and is the
halogen bond which supports the storage of this solvent molecule. Overall the
structure presents a sandwich geometry. Above and below this level there are the
azobenzenes. There is a halogen bond between the planes that results particularly
interesting, due to the fact that it involves the iodine I3A of the azo molecule and the
nitrogen N2A of the -N=N- group. I···N distance is 2.979 Å, and C-I···N angle is
166.45°. This is shorter than the only other case of halogen bonding involving the
nitrogen atom of an azo group found in CCDC database (TEGFUI, 3.240 Å,
152.5°).52
Figure 46. A single complete net projected along the b axis, where halogen bond (I•••O) is evident.144
85
3.3.3. {[Zn(2)(1,2-di(4-pyridyl)ethylene)](DMF)1.81}n (5)
As stated previously, we synthetized 2 which has the same structure as 1 but
without iodine atoms. This new azobenzene was used to obtain 5, under isothermal
conditions and using the same solvent in the synthesis of 4. In compound 5, as in 3,
the Zn atoms are tetrahedrally coordinated with 2 acting as bridge between two metal
atoms and therefore, Zn is bonded to two 1,2-di(4-pyridyl)ethylene, that, being
chemically symmetric also link two Zn atoms. These bridges form a zig-zag 2D
network from where, to both sides, the 2 molecule lean out, this 2D layers
interdigitate (Fig. 47).
Figure 47. Interdigitating of three 2D layers projected along a axis using different colors.
The 3D structure shows a planar rectangular network of channels occupied by
two independent molecules of DMF, one of these, linked two dipyridylethylene is
ordered, while the second is less bonded, and positioned in the larger channel of the
network. Last DMF molecule is so mobile that the refinement of it population
converge at 0.81 rather then 1.00, probably because the time needed for the
manipulation of the crystal before its freezing was sufficient for a partial loss of DMF
(Fig. 48).
86
Figure 48. Left: packing view along a showing the bidimensional pipe network containing DMF in Mercury style. Right: the same viewed along c. In all the plots, the DMF is omitted and only one of
the disordered conformers is reported, for clarity.144
3.3.4. Photochemical studies
To assess the reliability of our dyes as photoswitching units for our coordination
structures, we studied their photochemistry in diluted solution in DMF (10-5
M). The
UV-Vis spectra of both 1 (Fig. 49) and 2 (Fig. 50) are quite similar, displaying an
absorption maximum at 400 and 435 nm respectively. We also measured the cis
thermal half-lifetimes upon UV-VIS irradiation at 457 nm of the corresponding trans
azobenzene using reported procedures, and we obtained the values τ = 3000 s and τ =
850 s for 1 and 2 respectively.
87
Figure 49. Left, UV-vis spectra of 1 before (black curve) and after (coloured curves) irradiation using 457 nm light. Right, time development of the absorbance of 1.
Figure 50. Left, UV-vis spectra of 2 before (black curve) and after (coloured curves) irradiation using 457 nm light. Right, time development of the absorbance of 2.
88
3.4. Experimental part
3.4.1. Synthesis of AZO molecules
Compound 1 (E)-4-((2,4,6-triiodoisophtalic)phenyl)diazenyl)-N,N-
dimethylbenzenamine: In a round bottom flask 5-amino-2,4,6triiodosophtalic acid
(0.560 g, 0.001 mmol) was dissolved in 3 ml of water. The solution was cooled to -
3oC. After 10 minutes, 0.3 mL concentrated HCl was added. In an Erlenmeyer,
sodium nitrite (1eq) was dissolved in 0.8 of water previously cooled at the same
temperature. When each solution was solved, the mixtures were put together.
Sodium acetate and N,N–dimethylaniline were solved in a mixture of ethanol: water
(2 mL : 0.8 mL). This solution was also cooled down to -3 oC and added, after 20
minutes, to the amine solution. The reaction was stirred for 16 hours at room
temperature. Then, water was added and the mixture was extracted tree times with
ethyl acetate. The mixture was dried (Na2SO4) and evaporated under vacuum. After
that, a solution of NaOH (0.01M) was added up to solid was solved. The solution was
washed several times with ethyl acetate. Then aqueous solution was acidified with
HCl (pH=1). Finally, the mixture was extracted with ethyl acetate (for two times) and
the solvent was removed under vacuum. Yield 55%. M.p. 237 °C (dec). 1H NMR
(DMSO, 400 MHz, 293 K, δ ppm): 7.81 (d, 3JHH = 8 Hz, 2H), 6.88 (d,
3JHH = 8 Hz,
2H), 3.10 (s, 6H). 13
C NMR (DMSO, 100 MHz, 293 K, δ ppm): 169.21, 148.96,
147.19, 146.55, 141.26, 140.18, 125.31, 111.45, 92.59, 88.90, 85.67, 30.54. FTIR:
νmax = 3323, 2911, 2637, 2504, 1729, 1697, 1598, 1545, 1516, 1317, 1248, 1179,
1126, 931,917, 902, 750 cm-1
. Anal. Calcd for C16H12N3O4I3: C, 27.81; H, 1.75; N,
6.08%. Found: C, 27.62; H, 1.72; N, 5.99%. MS/ESI m/z 691.0, found 692.1 (M +
H+).
Compound 2 (E)-4-(isophtalicphenyl)diazenyl)-N,N-
dimethylbenzenamine: This compound was synthetized using the same procedure as
per compound 1. The only one change regarded the use of 5-aminoisophtalic acid
89
(0.001 mmol) instead of 5-amino-2,4,6triiodosophtalic acid. Yield 62%. M.p. 251 °C
(dec) 1H NMR (DMSO, 400 MHz, 293 K, δ ppm): 8.47 (s, 1H). 8.45 (s, 2H), 7.85 (d,
3JHH = 8 Hz, 2H), 6.85 (d,
3JHH = 8 Hz, 2H), 3.08 (s, 6H).
13C NMR (DMSO, 100
MHz, 293 K, δ ppm): 166.70, 153.60, 153.25, 142.94, 132.97, 130.44, 126.43,
125.80, 112.10. FTIR: νmax = 2819, 2663, 2549, 1692, 1602, 1562, 1522, 1454,
1397, 1366, 1277, 1148, 916, 813, 858, 726, 692 cm-1
. Anal. Calcd for C16H15N3O4:
C, 61.34; H, 4.83; N, 13.41%. Found: C, 61.11; H, 4.63; N, 13.67%. MS/ESI m/z
313.3, found 314.1 (M + H+).
3.4.2. Synthesis of MOFs
Compound 3 {[Zn(1)(Py)2](2-propanol)}n. A mixture of 0.032 g (0.107
mmol) of Zn(NO3)2·6H2O, 0.036 g (0.052mmol) of compound 1, 0.009 g (0.106
mmol) of pyridine, 0.053 mL of a solution 2M (NaOH in H2O) and 0.132 mL of 2-
propanol was sealed in a 1.5 mL Teflon-lined autoclave and heated at 80 °C for 32
hours. Then the autoclave was slowly cooled to room temperature. The mixture was
filtered and compound 1 was isolated as red crystals.
Compound 4 {[Zn(1)2(4,4’-bipyridyl)2](DMF)2}n. A mixture of 0.008 g
(0,027mmol) of Zn(NO3)2·6H2O, 0.012g (0.0179 mmol) of compound 1, 0.010g
(0.064mmol) of 4,4’-bipyridine was solved in 5 mL of DMF, 2 mL of ethanol and 1
mL of H2O. The solution was stirred for 10 min. The mixture was filtered and kept at
room temperature. After some days, a red single crystal was obtained.
Compound 5 {[Zn(2)(1,2-di(4-pyridyl)ethylene)](DMF)1.81}n. 0.008 g (0.027
mmol) of Zn(NO3)2·6H2O, 0.006g (0.0179 mmol) of compound 2, 0.012g
(0.064mmol) of 1,2-di(4-pyridyl)ethylene were dissolved in 5 mL of DMF, 2 mL of
ethanol and 1 mL of H2O. The mixture was stirred for 10 min. After that, it was
filtered and kept at room temperature. After some days compound 7 was isolated as
red crystals.
90
Compound 3 Compound 4 Compound 5
Chemical Formula
2C26H20I3N5O4Zn. C3H8O
C52H38I6N10O8Zn. 2C3H7NO
C28H23N5O4Zn. 1.81C3H7NO
Formula weight 1885.17 1903.89 690.97
Temperature (K) 103(2) 103(2) 103(2)
Crystal system monoclinic monoclinic monoclinic
Space group P21/n C2/c P21/c
a (Å) 8.813(2) 24.0554(18) 10.1389(5)
b (Å) 23.763(5) 11.3346(9) 14.9139(8)
c (Å) 14.620(3) 22.9563(18) 23.1236(12)
(°) 90.00 90.00 90.00
β (°) 91.91(2) 98.893(8) 95.194(2)
(°) 90.00 90.00 90.00
Volume (Å3) 3060.1(11) 6184.0(8) 3482.2(3)
Z 2 4 4
Density (gcm-3
) 2.046 2.045 1.316
μ (mm-1
) 3.873 3.461 0.757
Dimensions (mm-3) 0.04, 0.04, 0.21 0.06, 0.07, 0.33 0.07, 0.04, 0.22
Colour, form Red, needle Red, needle Orange, prism
ABS Tmin, Tmax 0.5437, 0.6752 0.4380, 0.5769 0.6977, 0.7456
θmax 25.68 31.51 27.91
No. of reflections. 35345 58923 77266
No. of independent
reflections.
5805 10264 7964
No of parameter 409 483 431
No of restraints 142 229 406
R_all, R_obs 0.077, 0.049 0.063, 0.048 0.115, 0.079
wR2_all, wR
2_obs 0.115, 0.104 0.130, 0.122 0.257, 0.227
∆ρmin, max (eÅ-3
) -2.00, 4.29 -3.65, 2.72 -0.64, 0.95
G.o.F 1.040 1.037 1.057
CCDC
Table 6. Crystallographic data for the compounds 3, 4, 5.
91
3.5. Conclusions
This work has permitted us to synthetized two new azobenzenes and using
them, obtained three new metal organic frameworks. Therefore, the structures were
obtained in different conditions, hydrothermal or isothermal and several halogen
bonding interactions were found in the frameworks containing iodine. As far as I
know, these are the first coordination networks involving both azobenzene molecules
and halogen bonding interactions. In this sense, I want to underline the importance of
halogen bonding in the construction of coordination polymers. Furthermore,
preliminary photochemical studies of the azobenzenes 1 and 2 in solution confirm
that their cis isomer lifetimes177-178
matches the bistability requested for the controlled
release of a guest. Such outcomes are important in view of the design and synthesis
of future halogen-bonded azobenzene-containing photoresponsive MOF.
92
CHAPTER 4
Block Co-polymers
4.1. Objectives
As advanced in the introduction, block co-polymers have been widely studied
in recent years especially for the construction of patterns based on microdomains
formed by many different forms. Roukolainen et al.179
first demonstrated the
formation of these patterns through the hydrogen bonding by pentadecylphenol and
PS-b-P4VP. Until now, the construction of these patterns was mainly studied by self-
assembly through hydrogen bonding among other interactions.
As it was many times highlighted in the discussion of this Doctoral Thesis,
halogen bonding presents many more advantages than the better known hydrogen
bonding. Despite this, halogen bonding has only been scarcely used in polymeric
systems up to now. Very recently, it has been shown that halogen bonding can be
used to induce long-range orientation in polymer self-assemblies44
and, in fact, there
are some examples using halogen bonding in polymer-topic such as, for instance,
93
layer-by-layer assemblies,180
long-range polymer alignment liquid crystalline
supramolecular polymers,181
supramolecular gels,182
solution assembly of
complementary polymer blocks,19
light responsive polymers52
and molecularly
imprinted polymers.183
My effort was to investigate how incorporation of halogen
bonding into block co-polymers studies can supply and even improve the results
obtained using hydrogen bond since many years.
The first important point was the choice of the best halogenated molecule
candidate and, obviously, in this choice, the strength of the halogen-bond donor
played a very important role. Considering the studies shown before (Fig. 3), iodine is
the best halogen-bond donor because of its remarkable positive behavior, which is
even stronger if fluorine atoms are placed in the closest positions. Since its length is
so comparable to the previous studies developed with hydrogen bonding, 1,8-
diiodoperfluorooctane (DIPFO) was chosen as the molecule. In the same way, the
choice of the polymer was also an important decision. Considering the previous
studies already published, we used the Polystyrene-block-4-vinylpyridine (PS-b-
P4VP) (Fig. 13). In fact, it has been reported that, when mixed, 1Py:1Iodine the
P4VP and α,ω-iodoperfluoroalkanes co-assemble to form com-like structure.184
94
4.2. Materials
The polymer Polystyrene-block-4-vinylpyridine (PS-b-P4VP) Mn=41300 (PS)
and 8200 (P4VP), (PDI =1.13) was purchased from Polymer Source. 1,8-
Diiodoperfluorooctane was purchased from ChemSpider. Chloroform and ethanol
were purchased from Aldrich Sigma Chemicals and used as are. Infrared spectra were
recorded in transmission mode, in the far-IR region (600-100 cm-1
, 4 cm-1
resolution)
and mid-IR region (500-4000 cm-1
, 4 cm-1
resolution) with a Nicolet iS50 FT-IR
spectrometer equipped with a DTGS detector. The AFM images were taken on a
Agilent 5500 Atomic Force Microscope operated in the Acoustic Mode using silicon
tapping mode tips (nominal radius 7 nm). The average size and distance of the
cylindrical P4VP(DIPFO) domains were evaluated by ImageG software. The full
image shown was used in order to obtain the values. Solid sample was placed on the
TGA after solvent was removed. Analyses were performed on a TGA Q500 (TA
Instruments) at a heating rate of 5°C/min, from 25°C to 250°C under nitrogen
atmosphere ( flow rate 45 ml/min). In order to prepare the sample for TEM, PS-b-
P4VP and DIPFO were dissolved in CHCl3 separately at 0.2 g.mL-1
concentration.
The PS-b-P4VP solution was added dropwise to the DIPFO solution in order to reach
a final 1:2 DIPFO:vinylpyridine molar ratio and the final solution was stirred for one
hour to allow complexation, than it was deposited on a Teflon slide and left under a
fumehood overnight to allow the solvent to evaporate. For the actual TEM
investigations, the specimens were cooled down to -187 oC after sectioning and
cryotransferred to the TEM device in order to keep sections relatively intact. SAXS
measurements were performed with a setup consisting of a Bruker Microstar
microfocus X-ray source with a rotating anode (λ = 1.54 Å) and Montel optics. The
beam from the X-ray source was further adjusted by four sets of four-blade slits,
which resulted about 1 x 1 mm beam at the sample position. The scattered beam was
detected with Hi-Star 2D area detector (Bruker). For measurements, the sample to
detector distance was set to 0.59 m to see the desired length scale in the
measurements. The measured 2D scattering data is azimuthally averaged to obtain
95
one-dimensional SAXS data. Raman spectra were acquired at RT by using a Horiba
Xplora MicroRaman instrument equipped with an Olympus BX-41 Microscope. An
excitation wavelength of 785 nm was used. Laser power was attenuated by neutral
density filters with a final power density of B0.07 mW mm-2
. The low wavenumber
detection limit is 140 cm-1
. Each spectrum was acquired with an exposure time of 5 s
over 35 cycles. The raw data were first corrected from the baseline, using the JASCO
Nicolet FTIR software, Omnic 9.0, between 200 and 560 cm-1
. Obtained data were
subsequently normalized, for the sake of comparison, and plotted using Origin Pro 8.
Thin complex films were coated through Physical Vacuum Deposition (PVD) using a
MB-ProVap-3 glove-box workstation (Tungsten source).
96
4.3. Results and discussion
We developed our study based on morphological investigations by AFM. The
first step was to demonstrate that halogen bonding has an influence on the
supramolecular organization of the polymer while the polymer by itself does not.
Therefore, in figure 51 the differences between the polymer and the polymer with
DIPFO (done by spin-coating at 500 rpm), are shown. The differences between both
are well evident and, therefore, it can be deduced that halogen bonding critically
influences the final organization of the thin films block co-polymers. Moreover, it is
possible to conclude that, in this case, microphase separation does not occur because
of selectivity of the solvent, as it is highly known and reported in the papers. Thus,
thin film spin-coated from the complex solution displayed clear microphase
separation. These nanostructures were mostly arranged in a hexagonal pattern, as
demonstrated in figure 51. However, domains with tetragonal order were also found,
as shown in figure 52.
Figure 51. On the top, a dual height (left) and phase (right) AFM images for the polymer (blank). On the bottom, a dual height (left) and phase (right) AFM images for the polymer plus DIPFO
(complex)
97
Figure 52. A dual height (left) and phase (right) AFM images for the polymer with DIPFO showing tetragonal order.
The parameters of distribution of diameter bulk and distance center-to-center
were studied and shown in figure 53. Cylindrical P4VP/DIPFO domains of roughly
average 27.2 ± 5.7 nm diameter were observed, embedded in a polystyrene matrix
with an approximate center-to-center of about 45.3 ± 5.8 nm.
Figure 53. Distribution parameters of diameter bulk (left) and distance center-to-center (right)
As previously stated, the samples were done by spin-coating at 500 rpm. In
order to demonstrate the versatility of the method, the AFMs done at different speeds
(1000 rpm and 2000 rpm) are shown in figure 54. No notable differences were
detected between them. Thus, for the whole development of this work, 500 rpm was
used and results can be applied to the rest of the speeds.
98
Figure 54. A dual height (left) and phase (right) AFM images for the complex at different speeds (from the top to the bottom, 500, 1000 and 2000 rpm respectively)
In figure 55 the TEM images are shown for our sample. As it is possible to see
in these figures, cylindrical domains have been observed rather than spherical
microphases, which are mostly arranged in a hexagonal pattern. These cylindrical
domains consist of a core-tube formed by P4VP/DIPFO halogen–bonded complexes
surrounded by a PS corona. In addition to halogen-bonding between the nitrogen
99
atom of P4VP and the both iodine atoms (every iodine is linked to one different
pyridine)184
of DIPFO, interactions between the fluorinated tails should also play a
role in the segregation process, as it has been reported in preliminary works.185
Figure 55. TEM images
All the studies already presented in this chapter, give us the intuition that
halogen bond remarkably influences the final organization of the structure. However,
I considered crucial to “empirically” confirm the presence of halogen bond in this
orientated domains. Therefore, SAXS, TGA, FTIR and RAMAN were used to
unequivocally demonstrate the halogen bond presence.
100
First the packing and structures were determined by SAXS,186
where all peaks
were fitted to Lorentzian profile for clarity. Two main reflection maxima at q2=0.016
Å-1
and q3= 0.019 Å-1
along with a well-pronounced higher order composite reflection
between 0.04 and 0.05 Å-1
were detected (figure 56), indicating that ordering took
place in the PS-b-P4VP(DIPFO) system. This higher order reflection has a maximum
at ca. 0.043 Å-1
which closely correlates with the higher order peak ratios of √7q2 and
√5q3, designating both hexagonal and tetragonal packing, respectively. Despite the
absence of expected √3q2 and √2q3 secondary reflections (note the weak scattering
between 0.02 Å-1
and 0.03 Å-1
, where the reflections become most likely screened by
the intense main peaks), we suggest a coexistence of hexagonal and tetragonal
packings with spacing between the adjacent P4VP (DIPFO) domains of ca. 45 nm
(q2=0.016 Å-1) and 38 nm (0.019 Å-1
), respectively. In fact, the coexistence of these
two phases has been previously reported.187
Compared to the PS-b-P4VP (DIPFO)
complex, the pure copolymer shows a main broad reflection at q1 = 0.021 Å-1
and a
secondary weak reflection at ca. 0.056 with a ratio of 1:√7, indicating only a poor
hexagonal local order, with tentative spacing of ca. 34 nm.
Figure 56. SAXS patterns of PS-b-P4VP (trace 1) and PS-b-P4VP(DIPFO) (trace 2) samples prepared
by drop casting.
101
Considering the above SAXS results, it is possible to conclude that by mixing
PS-b-P4VP with DIPFO, a new halogen-bonded P4VP(DIPFO) domain was formed,
where interdomain spacing increased from 34 nm to 38-45 nm. Also, due to complex
formation, the local polymer order was dramatically improved including well-defined
hexagonal and tetragonal morphologies.
In addition to the SAXS, in order to demonstrate the presence of halogen
bonding interaction in our system, FTIR, TGA and RAMAN have been also studied.
Therefore, in figure 57, IR spectrum of DIPFO (green), PS-b-P4VP (red) and
complex (blue) are shown. It is possible to note the typical188-189
red-shifts (2-4 cm-1
)
of the C-F stretching bands, which are located at 1112 cm-1
and 1090 cm-1
in pure
DIPFO. Moreover, not significant changes are detected in the peaks relative to the
polymer before and after complexation.
Figure 57. Comparison of IR spectrum of DIPFO, PS-b-P4VP and complex
102
In figure 58, TGA of the solid complex, DIPFO and PS-b-P4VP are shown.
There is a 33.95% of weight loss, which is well in line with the stoichiometry
quantity of DIPFO when the sample is 1 mol of DIPFO: 2 mol of 4-vinylpyridine
monomer unit, so that it confirms the stoichiometry of our complex. Particularly
interesting is to note that it is necessary to reach almost 190 °C to remove all the
DIPFO of the sample, while, when the DIPFO is not linked, the sample evaporates at
105 °C. This change in the boiling point is attributed to the interaction between
DIPFO and the PS-b-P4VP.
Figure 58. Comparison of TGA for PS-b-P4VP, complex and DIPFO.
In figure 59, the RAMAN spectrum is shown where it is possible to note the
red shift of the Raman C-I band from 291.5 to 279.0 cm-1
. These shifts are well in
line with some studies previously reported.188-189
103
Figure 59. Comparison of Raman spectrum for PS-b-P4VP, complex and DIPFO.
After the new fast and efficient nanostructuring of block copolymers directed
by halogen bonding was proven, my efforts were focused on removing the small-
molecule additive which acts as directed, keeping the same pattern obtained. In other
words, the next step is to remove DIPFO contained within our cylinders in order to
create void tubes. The non-covalent nature of halogen bonding allowed us to remove
the DIPFO by washing it with ethanol, according to the analogous concept already
illustrated.112
Ethanol was chosen because it is a good solvent for DIPFO. In addition,
as shown in figure 60, no changes in the structural organization of the thin films after
treatment are detected since ethanol does not swell the PS polymer matrix.
The hexagonal arrangement of the washed film was maintained, as it is
possible to see in figure 60, although the roughness of the exposed PS matrix surface
seems to have slightly increased. On the other hand, notable differences were found
in the phase contrast of the thin film surfaces before and after washing. These
changes are in line with the fact that one compound was removed, since it seems that
just one kind of material is on the film.
104
Figure 60. A dual height (left) and phase (right) AFM images of a thin complex film after washing with ethanol.
The complete removal of the fluorinated molecule is demonstrated in figure 61
where IR spectra of the washed sample are shown, where no absorption bands of
DIPFO are detected.
Figure 61. ATR-FTIR spectra of PS-b-P4VP, DIPFO and of a thin spin-coated complex film, both as prepared and after ethanol washing.
105
In order to prove the presence of hollow spaces in the cylindrical structures,
metalation experiments were studied, where gold evaporated and deposited on the top
of the ethanol washed thin films. Therefore, after removing the polymer template
with a washing in acetone, film with gold nanodots (Fig. 62) with a diameter average
of 46.9 ± 10.3 nm figure 63 was created and studied by AFM. In addition, this film
confirms that DIPFO can be removed from the polymer through the treatment
previously explained.
Figure 62. AFM topographic micrograph on the left and section profile on the right of gold nanostructures prepared by metalation of the hollow template left after washing the thin complex
film in ethanol, and subsequent removal of the polymer template by acetone.
Figure 63. Chart of the size distribution of gold nanodots.
106
4.4. Experimental part
4.4.1. Samples for AFM
Samples for AFM were prepared by mixing two solutions previously prepared
separately. Both compounds PS-b-P4VP and DIPFO were separately solved in
chloroform (1 mol of DIPFO: 2 mol of 4-vinylpyridine monomer unit) and placed in
an ultrasonic bath till no solid was observed. After that, solution A was then added
drop-by-drop to DIPFO solution, while the solution was placed again in an ultrasonic
bath. The resulting solution was used as it is. Complex thin films were prepared by
spin-coating onto glass film (cleaned previously with oxygen and plasma) at different
speeds.
4.4.2. Removal of DIPFO in the complex
The same film done for AFM was placed in a simply ethanol solution
overnight. After that, sample was dried with an air flow.
4.4.3. Preparation of samples for IR
Samples for IR were prepared by mixing two solutions previously prepared.
Both compounds PS-b-P4VP and DIPFO were separately solved in chloroform (1
mol of DIPFO: 2 mol of 4-vinylpyridine monomer unit) and placed in an ultrasonic
bath till no solid was observed. After that, solution A was then added drop-by-drop to
DIPFO solution, while the solution was placed again in an ultrasonic bath. The
resulting solution was used as it is. Complex thin films were prepared by spin-coating
onto glass film a 500 rpm. After that, the solvent was removed with air flow, the film
was placed in transmission IR.
107
4.4.4. Preparation of samples for TGA
Samples for TGA were prepared by mixing two solutions previously
prepared. Both compounds PS-b-P4VP and DIPFO were separately solved in
chloroform (1 mol of DIPFO: 2 mol of 4-vinylpyridine monomer unit) and placed in
an ultrasonic bath till no solid was observed. After that, solution A was then added
drop-by-drop to DIPFO solution, while the solution was placed again in an ultrasonic
bath. Finally, the solvent was removed by air flow and the solid left was placed on
the TGA instrument.
4.4.5. Metalation
Thin complex films were coated with a 5 nm thick gold layer through Physical
Vacuum Deposition (PVD). Evaporation was carried out at a pressure of 5×10-6
mbar
with a constant rate of 0.2-0.3 Å/sec.
108
4.5. Conclusions
In this work, it was demonstrated that halogen bonding is also an effective tool
to enhance microphase separation in solid block copolymer systems. One of the most
important result achieved is that through halogen bond between a polymer block and
an iodoperfluoroalkane it is possible to obtain nanopatterned solid films, even upon
simple spin-coating from a non-block-selective solvent, and in the absence of
annealing treatments. Finally, it was shown that the small molecule additive DIPFO
can be easily removed from the polymer matrix to result in hollow nanostructures,
according to a concept which was previously demonstrated only for hydrogen bonded
systems.
109
General conclusions and future perspectives
In this doctoral thesis, the exceptional features of the halogen bonding1
were
applied and exploited in three well-known topics in Supramolecular Chemistry
(Liquid Crystals, Metal Organic Frameworks and Block Co-Polymers).
Although Ikeda et al75
has published a work for dye doped covalent liquid
crystals, the recently developed supramolecular low molecular weight
photocontrollable liquid crystals have not been investigated in depth yet. The
versatility of the supramolecular approach to LMWLCAs may provide the possibility
to overcome common problems and encountered with dye doped liquid crystals, e.g.,
phase separation of the dye. Our approach using halogen bonding has been
demonstrated particularly reliable and robust. In fact, we have obtained low
molecular weight photocontrollable liquid crystals with unprecedented features such
as a fast and efficient photoinduced phase transition from the liquid crystalline to the
isotropic state and from the crystalline to the isotropic state.
The MOFs work has allowed me to synthetize two new azo-dyes and obtain
three new coordination networks that display diverse dimensionality. These structures
were obtained in different conditions, by hydrothermal synthesis or isothermal
evaporation, and by using different organic molecules as additional linkers
(bipyridine, dipyridylethylene, and also pyridine). As far as I know, they are the first
coordination networks involving both azobenzenes and halogen bonding. In fact,
single crystal X-ray diffraction studies revealed that iodine atoms function as good
halogen bond-donor sites coordinating the solvent included in the framework.162
Our
preliminary photochemical studies confirm the formation of the cis isomer of the
ligand in solution. Such outcomes are important in view of the design and synthesis
of halogen-bonded azobenzene-containing photoresponsive MOF.
110
The results achieved in the block co-polymer work, point out that halogen
bonding is an effective tool to enhance microphase separation in these systems. The
formation of halogen bonding domains selectively involving one polymer block
allowed an immediate assembly into nanopatterned solid films, even upon simple
spin-coating from a non-block-selective solvent, and in the absence of annealing
treatments. Finally, I have shown that the small molecule additive DIPFO can be
easily removed from the polymer matrix to result in hollow nanostructures, according
to a concept which was previously demonstrated only for hydrogen bonded systems.
Based on the complementarity of halogen and hydrogen bonds, my findings open up
new possibilities in the fields of surface patterning and nanotemplating to tune e.g.
the optical, magnetic, conducting, transport or sensing properties of materials.
111
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