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Accepted Manuscript
Tuning of light colour emission from new polyperyleneimides containing oxadiazoleand siloxane moieties
Mariana-Dana Damaceanu, Catalin-Paul Constantin, Maria Bruma, Mariana Pinteala
PII: S0143-7208(13)00162-9
DOI: 10.1016/j.dyepig.2013.04.035
Reference: DYPI 3934
To appear in: Dyes and Pigments
Received Date: 22 February 2013
Revised Date: 29 April 2013
Accepted Date: 30 April 2013
Please cite this article as: Damaceanu M-D, Constantin C-P, Bruma M, Pinteala M, Tuning of light colouremission from new polyperyleneimides containing oxadiazole and siloxane moieties, Dyes and Pigments(2013), doi: 10.1016/j.dyepig.2013.04.035.
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TUNING OF LIGHT COLOUR EMISSION FROM NEW POLYPERYLENEIMIDES
CONTAINING OXADIAZOLE AND SILOXANE MOIETIES
Mariana-Dana Damaceanu*, Catalin-Paul Constantin, Maria Bruma and Mariana Pinteala
"Petru Poni" Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A,
Iasi-700487, ROMANIA
Abstract
Four new aromatic poly(peryleneimides) containing electron-withdrawing oxadiazole
rings and flexible tetramethyldisiloxane units in the backbone were prepared by a one step
polycondensation reaction at high temperature of four aromatic diamines containing
preformed oxadiazole units with a mixture, in different ratios, of perylenetetracarboxylic
dianhydride and tetramethyldisiloxane-1,3-bis(4-phthalic anhydride). The structure of these
polymers was confirmed by FT-IR and 1H-NMR spectroscopy. The solubility, thermal
stability and glass transition temperature of the poly(peryleneimides) were measured and
discussed in correlation with their chemical structure. The solid polymers were studied by X-
ray diffraction which revealed a semicrystalline state consisting in face-to-face arranged
columns of perylenediimide units for two of the polymers. The film-forming ability and the
morphology of the resulting thin films were investigated using scanning electron microscopy
which showed that two of the polymer films were organized into self-assembled rod-like
structures. An extensive study of the photo-optical properties of these polymers highlighted
the ability of the color of the emitted light to be modulated as a function of the excitation
wavelengths, as was surveyed in the chromaticity diagrams. The Förster excitation energy
transfer phenomenon from oxadiazole to perylenediimide chromophores was observed to
occur for some of the polymers, for which the oxygen bridge appears to be responsible. In
solution, high fluorescence quantum yields were obtained, up to 24 %, while, in the solid
state, low fluorescence emission was attained due to aggregation. These poly(peryleneimides)
could be considered as promising candidates for high-performance materials to be used in
novel optoelectronic applications, e.g. chemiluminescent sensors.
Keywords: poly(peryleneimides); oxadiazole; high temperature materials; thin films;
chromaticity coordinates; excitation energy transfer.
* Corresponding author. Address: "Petru Poni" Institute of Macromolecular Chemistry, Aleea
Grigore Ghica Voda 41A, Iasi-700487, ROMANIA, email: [email protected]
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1. INTRODUCTION
Research and development in the field of materials science and technology are
directed toward the preparation of new materials capable of overcoming specific problems,
and improving, or reducing the cost of present-day industrial processes. In this regard, if
science may be considered essential for the welfare of humankind, then materials science
clearly contributes to ongoing social and economic progress. In contrast to conventional
polymers, high performance polymers are characterized by specific criteria, such as higher
thermal resistance, higher mechanical strength, lower specific densities, higher conductivity,
higher thermal, electrical or sound insulation properties, superior flame resistance etc [1].
Polyimides containing six-membered imide rings, such naphthylimide or peryleneimide
represent one of the most important classes of high performance polymers. Peryleneimides
and their derivatives have been a subject of research for the last 10 decades. With their
promising electroactive and photoactive properties, peryleneimides and their derivatives have
attracted much attention for potential applications in organic molecular electronics [2,3]. Of
particular interest is their use as light emitting materials for the fabrication of
electroluminescent display devices [4,5]. The syntheses of a variety of peryleneimides have
provided fundamental understanding of their electronic properties at the molecular level. The
correlation of the electrochemical and photophysical properties with their molecular structures
is crucial for rational exploration of new materials for optoelectronic applications [6].
Perylenediimides have a deep-lying highest occupied molecular orbital (HOMO) energy level
and a rigid, planar core that was used as viable building blocks to n-type organic
semiconductors. Their molecular architecture represented by fused aromatic rings provides
high electron affinity/mobility, and excellent photochemical and thermal stability [7,8]. In
fact, the electron acceptor character is a common property of aromatic diimides and arises
from the strong electron-withdrawing character of the imide groups [9]. Polymers containing
perylenediimides are also excellent candidates for creating self-organized molecular
electronic materials, they are one of the few n-type organic semiconductor systems that have
demonstrated high charge carrier mobility in thin film devices and structures, due to the
strong π-π interactions between perylenediimide units, which can facilitate the self-
organization [10,11].
A critical challenge, however, is the poor solubility of these fused arylene systems in
organic solvents, rendering it difficult to employ cost-effective solution-processing techniques
[12,13]. The solubility issue of these arylene molecules has been addressed by employing a
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diimide structure whose imide nitrogens are modified with alkyl tails, or by attaching
substituents to the main core at the bay positions. Unfortunately, many of these tail structures
and side groups create steric hindrance that inhibits the extended π-stacking required for
superior optoelectronic properties [14]. One alternative method of blending low molecular
weight perylene imides with polymers to prepare smooth films has the inherent problem of
phase separation [15].
One approach to soluble polymers containing perylenedimide units is to synthesize
copolymers containing them. Oligomers or polymers with end-capped perylene monoimides
as polysiloxanes [16], polyfluorenes and polyindenofluorenes [17,18], copolymers having
perylenediimides incorporated into the polymer chains [19,20] or graft copolymers as
perylene monoimide side chains on a polyfluorene [17], as well as more sophisticated
architectures such as dendrimers [21] were synthesized over time.
Perylene-containing polyimides showed poor solubility; they dissolve only in highly
polar solvents, such as m-cresol or concentrated H2SO4 because of the strong π-π stacking
between perylene moieties. Therefore, improving the polyperyleneimide solubility is an
important issue for the chemistry of high performance polymers. The main possibilities to
achieve enhanced solubility are either introduction of bulky substituents on the carbocyclic
scaffold [22] or copolymerization with other monomers containing flexible units. The
introduction of tetramethyldisiloxane units into already thermally stable polyimides has been
shown to yield several attractive properties while retaining many of the excellent properties.
The presence of the siloxane components in the structure of polyimides allows for increased
impact resistance, excellent adhesion, gas permeability and reduced water absorption, while
maintaining the thermal and mechanical stability that is adequate for most microelectronic,
printed circuits, and aerospace applications [23-26].
On the other hand, oxadiazole-based polymers present a combination of properties,
such as mechanical strength, stiffness, thermal and chemical stability, that makes them good
candidates for application as high temperature fibers, reinforcement materials, graphitized
fibers and membranes for gas separation [27]. Particularly, the outstanding thermal stability of
poly(1,3,4-oxadiazoles) was ascribed to the fact that the oxadiazole ring is spectrally and
electronically equivalent to the p-phenylene ring structure. Among organic chromophores,
oxadiazoles show high thermal and hydrolytic stability. Their good electron affinity makes
these compounds usable as electron transport materials in light emitting diodes (LEDs). The
prominent example is PBD, 2-(4-tert-butylbiphenyl)-5-phenyl-1,3,4-oxadiazole, which is
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used in molecular [28] and in polymeric devices [29]. Also two-photon absorption [30] and
NLO properties of oxadiazole compounds were reported [31].
A combination of several structural modifications, that is, the incorporation of 1,3,4-
oxadiazole rings, perylenediimide units and flexible groups in the polymer backbones,
minimizes the trade-off between the solubility and desired properties of wholly aromatic
polyimides. This work describes the synthesis and photo-physical properties of new
polyperyleneimides containing flexible siloxane bridges and oxadiazole moieties in the main
chain. The basic properties of these copolymers such as solubility, inherent viscosity, thermal
stability and glass transition temperature were thoroughly investigated. The photo-optical
behaviour of these polymers was investigated in detail in order to highlight the ability of
emission wavelength tuning. The crystallinity and morphology of thin films made therefrom
were investigated as well.
2. EXPERIMENTAL
2.1. Starting materials
4-Fluorobenzoic acid, hydrazine hydrate, trimellitic anhydride chloride, 1,2-
dichlorotetramethyldisilane, glacial acetic acid, anhydrous diethyl ether, 1-methyl-2-
pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO) and
anhydrous N,N-dimethylformamide (DMF), were purchased from Sigma-Aldrich. Ortho-
phosphoric acid, 4-aminobenzoic acid, 3-aminophenol, 4-aminophenol, 5-amino-naphthol,
benzoic acid and acetic anhydride were purchased from Merck. Chloroform was purchased
from Chimopar Bucharest. Solvents were purified by standard procedures and handled in a
moisture-free atmosphere. All other chemicals were of reagent grade and used as received
without further purification.
2.2. Synthesis of the monomers
2,5-Bis(p-aminophenyl)-1,3,4-oxadiazole, M1, 2,5-bis[4-(p-aminophenoxy)
phenylene]-1,3,4-oxadiazole, M2, 2,5-bis[4-(m-aminophenoxy)phenylene]-1,3,4-oxadiazole,
M3 and 2,5-bis-[4-(5-amino-naphthalene-1-yloxy)-phenyl]-1,3,4-oxadiazole, M4, whose
structures are shown in scheme 1, were synthesized by known procedures [32,33].
3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA, M5) was purchased from
Aldrich and purified following a procedure described in the literature [34].
Tetramethyldisiloxane-1,3-bis(4-phthalic anhydride), M5, was prepared by the
reaction of trimellitic anhydride acid chloride with 1,2-dichlorotetramethyldisilane, in the
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presence of a catalyst made from a mixture of bis(benzonitrile)palladium chloride and
triphenylphosphine, following a published procedure [23].
2.3. Synthesis of the polymers
Copolyimides P1-P4 have been prepared by a one-step polycondensation reaction of
an oxadiazole-containing diamine, M1-M4, with a mixture of dianhydrides M5 and M6,
taken in different molar ratios. The reactions were carried out in NMP with 3.5% LiCl, in the
presence of benzoic acid as catalyst, at a concentration of 7% total solids, under a nitrogen
stream and at high temperatures (200-210°C).
In the case of the two dianhydrides with different reactivity, the less active
dianhydride monomer PTCDA (M5) was added in the first stage of reaction to the solution of
diamine in NMP, and then the more active dianhydride monomer, M6, was added to achieve
an even more random incorporation of the two monomer units in the polymer chain. Owing to
the low solubility of M5 in NMP with LiCl at room temperature, higher reaction temperature
was necessary to achieve better solubility. The reaction mixture was stirred and heated to 200-
210°C and allowed to react for 10 h, then gradually cooled to room temperature. The
generated water was removed through a gentle nitrogen flow, and hence the reaction balance
was moved to the formation of polyimide.
The following example illustrates the general procedure. In a 100 mL three-necked,
round-bottomed flask, equipped with a mechanical stirrer and nitrogen inlet and outlet,
diamine M4 (2.144 g, 4 mmol) and NMP with 3.5% LiCl (40 mL) as solvent were introduced.
After complete solubilization of the diamine, dianhydride M5 (0,392 mg, 1 mmol) was
charged. The reaction mixture was stirred and purged with a gentle nitrogen flow for about
half an hour and then M6 (1.25 g, 3 mmol) and NMP with 3.5% LiCl (10 mL) were added.
The reaction mixture was stirred and heated to 200-210°C and allowed to react for 10 h, then
gradually cooled to room temperature. Part of the resulting dark red copolyimide solution, P4,
was poured onto glass plates in order to prepare thin films, while the rest was poured into
water to precipitate the solid polymer. The dark red polymer was washed with plenty of water
and finally treated with ethanol in a Soxhlet apparatus for 1 day in order to remove the
unreacted monomers and the high boiling solvent. Finally, copolyimide P4 was obtained as a
red powder after drying in an oven, under vacuum, at 100°C for 6 h.
2.4. Preparation of polymer films
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Films of copolymers P1-P4 were prepared by casting a polymer solution of 7%
concentration in NMP with LiCl onto glass plates, followed by gradual heating from room
temperature up to 210°C, and kept at 210°C for 1 h. Dark-red, opaque coatings resulted
having strong adhesion to the glass support. The resulting polymer films could not be stripped
off the plates, being brittle.
Very diluted polymer solutions in NMP with concentration of 0.5-1% were used to
obtain very thin films having the thickness in the range 100-150 nanometers onto glass or
quartz plates by drop-casting technique. These films, as deposited, were gradually heated up
to 210°C in the same way as described earlier to remove the solvent and were used for
scanning electron microscopy (SEM) and photo-optical properties investigations.
2.5. Measurements
The 1H NMR spectra of the polymers have been recorded on a Bruker Advance III 400
MHz instrument, using DMSO-d6 and tetramethylsiloxane as standard.
The infrared spectra of the polymers were recorded on FT-IR Bruker Vertex 70
Spectrophotometer in transmission mode, by using KBr pellets.
The inherent viscosities of the polymers were determined at 20°C, by using NMP-
polymer solutions of 0.5 g/dL concentration, with an Ubbelohde viscometer.
The thermal stability of the polymers was investigated by thermogravimetric analysis
(TGA) using a STA 449 F1 Jupiter device (Netzsch, Germany) operating at a heating rate of
10°K/min, in nitrogen, from 30°C to 900°C. The onset on the TG curve was considered to be
the beginning of decomposition or the initial decomposition temperature (IDT). The
temperature of maximum rate of decomposition which is the maximum signal in differential
thermogravimetry (DTG) curves was also recorded.
The glass transition temperature (Tg) of the precipitated polymers was determined by
using a DSC 200 F3 Maia (Netzsch, Germany) equipment. Approximately 4 to 6 mg of each
polymer were crimped in aluminium pans and run in nitrogen atmosphere with a heat-cool-
heat profile from 30°C to 200-250°C at 10°K/min. The mid-point temperature of the change
in slope of the DSC signal of the second heating cycle was used to determine the glass
transition temperature values of the polymers.
Model molecules for a polymer fragment were obtained by molecular mechanics
(MM+) by means of the HyperChem program, Version 7.5. The same program was used to
visualize the structures obtained after energy minimization. The calculations were carried out
with full geometry optimization (bond lengths, bond angles and dihedral angles).
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Wide Angle X Ray Diffraction (WAXD) was performed on a Bruker D8 ADVANCE
Difractometer, using the Ni-filtered Cu-Kα radiation (λ=0.1541 nm). A MRI-WRTC–
temperature chamber (with nitrogen inert atmosphere) and a MRI-TCPU1-Temperature
Control and Power Unit were used. The working conditions were 36 kV and 30 mA. All the
diffractograms were investigated in the range 5÷40° (2 theta degrees), at room temperature.
Initial samples for X-Ray measurements were powders obtained as described in the synthesis
procedures.
The morphology of the free-standing films was investigated by scanning electron
microscopy (SEM) using a Quanta 200 ESEM.
The UV-Vis absorption and photoluminescence spectra of copolyimides were
registered with Specord M42 apparatus and Perkin Elmer LS 55 apparatus, respectively, by
using very diluted polymer solutions (aprox. 10-5 M) and very thin films having the thickness
in the nanometer range deposited on quartz plates.
The absolute values of fluorescence quantum yield (Φfl) of the samples was measured
on a FluoroMax-4 spectrofluorometer equipped with a Quanta-phi integrating sphere
accessory Horiba Jobin Yvon, by exciting the corresponding compound solutions at different
wavelengths corresponding to absorption maxima, at room temperature. The solution
concentration was optimized to obtain an absorbance around 0.055. The slit widths and
detector parameters were optimized to maximize but not saturate the excitation Rayleigh
peak, in order to obtain a good optical luminescence signal-to-noise ratio.
3. RESULTS AND DISCUSSION
3.1. Synthesis of polymers and basic characterization
The copolyimides P1-P4 reported here are based on two dianhydrides, one containing
perylene unit, M5, and another one containing tetramethyldisiloxane group, M6, and various
aromatic diamines containing a preformed oxadiazole ring, M1-M4. The structures of the
monomers are shown in Scheme 1.
..................................... Scheme 1
.....................................
A one-step polycondensation reaction of a diamine M1-M4 with a mixture of perylene
tetracarboxylic dianhydride, M5, and tetramethyldisiloxane-1,3-bis(4-phthalic anhydride),
M6, in NMP with 3.5% LiCl solutions, in the presence of benzoic acid as catalyst and by
heating at high temperatures, yielded copolyimides P1-P4 as shown in Scheme 2.
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............................... Scheme 2
...............................
The 1H-NMR spectrum of copolyimide P4 (figure 1) was recorded in order to confirm
the structure of the polymer. Thus, in the aromatic region of P4 exhibited multiplets due to the
aromatic protons of phenylene, perylenediimide and phthalimide rings, as follows: 8.34-7.88
(48H, m), 7.84–7.36 (32H, m), 7.35–7.02 (26H, m). In the aliphatic region, the polymers
exhibited multiple singlets readily distinguished by a 0.04–0.17 ppm shift that can be
attributed to the CH3 protons of the dimethylsiloxane units that are present in syn–anti
conformations: 0.53, 0.47, 0.38, 0.21, 0.17 and 0.06 ppm. Moreover, the integrals values
ratios of the aromatic and aliphatic signals evidenced the total incorporation of the diamine
and dianhydride segments in the polymer chain P4 in the ratio: M4 : M5 : M6 = 4 : 3 : 1.
Unfortunately, due to the extremely low solubility of the other polymers (P1-P3) in DMSO,
their 1H-NMR spectra could not be obtained with sufficiently good resolution for detailed
analysis.
..................................... Figure 1
.....................................
The FT-IR spectra of the synthesized copolyimides provided evidence that all of the
perylene, tetramethyldisiloxane and oxadiazole units were successfully incorporated into the
copolyimide chain (figure 2).
..................................... Figure 2
..................................... The polymers showed characteristic five-imide ring absorptions coming from
siloxane-conaining dianhydride in the range of 1774-1776 cm-1 (asymmetrical C=O imide
stretching), 1721-1724 cm-1 (symmetrical C=O imide stretching) and 722-734 cm-1 (imide
ring deformation) in their FT-IR spectra. All the polymers exhibited strong absorption bands
in the range of 1664-1672 cm-1 typical for the vibrations of six-member imide rings of
PTCDA. Characteristic absorption peaks of the 1,3,4-oxadiazole ring were evident in the
range of 961-967 cm-1 and 1012-1019 cm-1 (=C–O–C= stretching). C-H and C=C linkage in
the aromatic rings showed absorption peaks at 3055-3119 cm-1 and, respectively, 1497-1486
cm-1 and 1593-1597 cm–1. C-H and C=C linkages in the phenylene, naphthalene or perylene
cores showed absorption peaks at 3120-3080 cm-1 and, respectively, 1490-1480 cm-1 and
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1590-1600 cm-1. The incorporation of the tetramethyldisiloxane units in the polymer chain
was evidenced by the clear absorption peaks in the range of 2958-2852 cm-1 (aliphatic C–H
stretching, CH3 groups), 1243-1256 cm-1 (Si–CH3 deformation), 1120-1179 cm-1 (silicon–
phenyl bond), 1064-1066 cm-1 (Si–O–Si asymmetric stretching) and 785-796 cm-1 (Si–CH3
stretching). The strong absorption bands attributed to the presence of aromatic ether stretching
in FTIR spectra of polymers P2, P3 and P4, in the range of 1243-1248 cm-1 is overlapped
with aliphatic C–Si deformation bands.
It is known that the majority of polyheteroarylenes based on PTCDA are difficult to
process, being infusible and insoluble in common organic solvents and soluble only in m-
cresol. This behaviour is due to the rigid nature of perylenediimide unit which dictates the
overall shape of the corresponding macromolecules and thus facilitates the strong interchain
interactions, and due to the compact aggregation of the polymer chains which occurs during
imidization carried out at high temperatures [35]. Our poly(peryleneimides) containing
oxadiazole and siloxane units are soluble in one convenient aprotic amidic solvent, which is
NMP, at a concentration of 0.5-1% due to the presence of flexible tetramethyldisiloxane units
and oxadiazole rings that introduce ‘‘kinks” in the macromolecular structure of these
copolyimides, thus resulting in a higher solubility compared with their analogues without
these units. Oxadiazole rings and the flexible groups, such as ether or tetramethyldisiloxane,
introduce a deviation from the linearity of the chain and disturb the tight packing of the
polymer chains and make the shape of the respective macromolecules far from a linear rigid
rod which is characteristic to aromatic polyimides based only on PTCDA.
The inherent viscosity values of these polymers are in the range of 0.1-0.14 dL/g.
These low values of viscosity can be explained in terms of lower electrophilic reactivity of
six-membered anhydride rings compared to their five-membered analogs, which may be
largely attributed to the fact that a six-membered anhydride has no strain [36]. Comparable
values of viscosity have been obtained by us for similar copolyimides made from the same
PTCDA and oxadiazole-containing diamines, but with another co-dianhydride containing
flexible units, namely hexafluoroisopropylidene bis(phthalic anhydride) [37]; thus it is clearly
proved that the viscosity of the resulting polyperyleneimides depends strongly on the structure
of the monomers and on the reaction conditions, and that the introduction of rigid perylene
moieties into polyimides hampers the formation of products with high viscosity values. Here,
we intended to use as much as possible eco-friendly reaction conditions, avoiding the
utilization of m-cresol, which is the most frequently used solvent in the synthesis of such
polymers; m-cresol has an offensive odor and is more dangerous to health when compared
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with NMP. Due to the insufficient solubility of these polymers in proper solvents as
dimethylformamide, the molecular weights measurement by gel permeation chromatography
was not possible.
3.2. Thermal properties
The thermal properties of the polyperyleneimides P1-P4 were evaluated by means of
DSC and TGA. The thermal behavior data of the polymers are summarized in Table 1 and
representative TGA curves are shown in Figure 3.
..................................... Table 1. Figure 3.
...................................... The polymers showed excellent thermal stability, as expected in the case of
polyperyleneimides. The initial decomposition temperatures (IDT) of the siloxane-containing
polyimides were about 457–490°C, the temperatures of 10% gravimetric loss (T10%), that are
important criterion for evaluation of thermal stability, were in the range of 459-499°C, while
the temperature corresponding to the maximum rate of decomposition was above 501°C,
indicating a high thermal stability. For each sample, the degradation process is not complete,
the char yields of those polyimides at 800°C being higher than 49%. The polymer containing
ether bridges and only para-catenation, P2, shows the highest decomposition temperatures,
being of 490°C, while the copolyimide containing meta-catenation, P3, shows a slightly lower
decomposition temperature, of 475°C, as evaluated by the values of the temperature when the
decomposition starts. Unexpectedly, polyperyleneimide P4 showed the lowest initial
decomposition temperature and the highest char yield at 800°C, up to 57%. Probably, the
segment formed by the rigid, but voluminous unit of naphthalene connected through an ether
linkage to the rest of the macromolecule reduces the chain interactions.
The glass transition temperature of the polymers measured by differential scanning
calorimetry (DSC) is in the range of 187-227°C. The thermograms of those polymers are
shown in figure 4, in which broad endothermic inflections can by observed.
...................................... Figure 4
.........................................
Generally, the glass transition temperatures of polymers depend upon the chain
flexibility, free volume and interchain stacking, being an indication of molecular mobility. In
our case, the values of glass transition temperatures are in close relation with the structure of
the copolymers, being dependent on the structure of the diamine component. For example,
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polyperyleneimide P2 has the lowest glass transitions temperature (186°C) because the
macromolecules are much more linear, the para-attachment of ether linkages to phenyl rings
in this aromatic and heterocyclic structure leading to highly rigid rod-like macromolecules.
Thus, the macromolecules can slide past each other, being connected with a smaller quantity
of energy necessary for the relaxation of the macromolecules. The difference between
copolymers P1 and P2 consists of the absence of ether groups in P1, which leads to less
flexible macromolecules, and this is correlated with the higher glass transition temperature
(200°C). Although the P3 copolymer contains ether groups in the structure, coming from the
diamine component, the value of the glass transition temperature is high (195°C). The
presence of some meta-linkages in this polymer backbone results in less mobile
macromolecules, due to the steric effect of the meta-catenated phenylene rings that dictate the
packing of the macromolecules in a more coiled form and thus, a higher energy is necessary
to increase the mobility of the macromolecules. Polyperyleneimide P4 has the highest value
of glass transition temperature (227°C), for which a collection of effects could be responsible
including steric hindrance, rigidity and free volume induced by naphthalene moiety that
behaves like a meta-catenated penylene unit. All these data are sustained by the molecular
models (4 repeating units) of these polymers that visualize the structures obtained after energy
minimization and clearly evidenced more linear, rod -like but still flexible shape of polymer
chains P1 and P2, and more coiled conformations of polymer chains P3 and P4. Figure 5
presents representative molecular models of these polymers.
.................................. Figure 5
........................................
3.3. X-ray diffraction studies
The synthesized copolymers P1-P4 contain a perylene unit which has the tendency to
form columnar phases because of the strong π-π interactions. The ability of self-assembly in
supramolecular architectures was checked by wide angle X-ray diffraction (XRD)
measurements. XRD investigation revealed that the siloxane-containing polyperyleneimides
P3 and P4 showed only one broad halo that indicates an amorphous state due to molecular
disorder (figure 6). Most probably, the molecular disorder was induced by the meta-catenated
phenylene units in P3, and by naphthyl units that behave like meta-catenated penylene
moieties in P4 polymer main chains, disrupting their planarity and hence the possibility of
self-assembling.
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................................... Figure 6
................................... The XRD patterns of polyperyleneimides P1 and P2 are quite different. P1 shows the
X-ray pattern consisting in two amorphous halos in the range of 10–20o and 20–30o (2 theta
degrees), and seven peaks located around 9.28o, 10.51o, 12.27o, 23.2o, 24.76°, 26.06° and
27.62o (2 theta degrees) (figure 6). This XRD pattern indicates the presence of what we define
as a semicrystalline state, which was formed during solution polycondensation reaction of
polyperyleneimide P1. In the diffraction pattern of P2, only a broad intense peak appeared at
15.22° (2 theta degrees), indicating a lower degree of crystallinity of P2 compared to P1. The
position and peak shape are clearly dependent on structure of the diamine monomer and on
the monomers ratio used in the polymers syntheses.
Usually, as confirmed by X-ray diffraction of several single crystals, the perylene
units exhibit flat π-systems, which are arranged in stacks showing a parallel orientation of the
neighboring, cofacially stacked perylene cores [38]. In the case of X-ray diffraction pattern of
P1, the π-π stacking of the perylene cores into columns gives two peaks at 24.76o and 27.62o
(2 theta degrees) being located above a diffuse halo, which means that the columns are
slightly disordered and can slide past each other. The explanation for the close stacking into
perylene columns could be the rigid imide rings which restrict the rotation and enhance the
interactions of perylene discs to stabilize the columns by facilitating face-to-face packing
motifs. Taking into account that the supramolecular architecture is dictated by π-stacking of
the perylene cores, the reflections in the middle angle region (9.28o, 10.51o, 12.27o, 2 theta
degrees for P1 and 15.22°, 2 theta degrees for P2) may be attributed to the tilted perylene
discs [37]. This fact indicates that the columnar stacks have various orientations. In
conclusion, only P1 and P2 are able to form supramolecular architectures consisting in
columns of perylene units that are statistically distributed into amorphous part of the polymer
sample. This conclusion is consistent with SEM images which will be discussed later.
3.4. Morphological studies of the polymer films
Very thin films having the thickness in the 100-150 nanometer range were deposited
by drop-casting technique onto glass plates, by using very diluted solutions of polymers P1-
P4 (concentration of 10-3 M). The quality and the morphology of these films were studied by
scanning electron microscopy (SEM). The solid-state packing behavior of these copolymers
was hypothesized based on their aggregation characteristics in solid state [37], and the
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morphology of the obtained supramolecular assembly was intuitively reflected in SEM
images (Figure 7). SEM images of the deposited copolyimide films show that the polymers
P1 and P2 are organized into self-assembled well-ordered building blocks that form
supramolecular rod-like structures; these aggregates mainly result from π-π stacking
interactions of perylenediimide units. The crystalline films of the other two polymers, P3 and
P4 are not organized in a rod-like structure, implying that the π-π stacking interactions of
perylenediimide units in these polymers are hampered by the presence of kinking units, meta-
phenylene and naphthalene; these data are in close correlation with TGA data and molecular
modelling obtained for these polymers.
..................................... Figure 7
.......................................
3.5. Photo-optical behavior of polymers
Since both 1,3,4-oxadiazole and perylenediimide are known fluorophores that emit
light in the blue and, respectively, red spectral range, the synthesized copolymers P1-P4
containing these chromophores were investigated in detail with respect to their photo-optical
properties. The photo-optical behavior of P1-P4 was assessed on the basis of the UV-Vis
absorption and photoluminescence spectra which were recorded for polymer solutions in
NMP and for thin films made from NMP solutions, after excitation with UV or visible light.
The chromaticity diagrams were also recorded in order to survey the color of the emitted
light.
Figures 8 and 9 show the absorption spectra of NMP solutions and of cast films from
NMP solutions of siloxane-containing copolyimides P1-P4.
................................... Figure 8 Figure 9
................................... All the oxadiazole-containing copoly(peryleneimide)s show one strong UV absorption
maximum at 301-310 nm, and two peaks at 471-486 nm and 513-517 nm, in NMP solutions,
the spectra being quite identical (Table 2).
............................ Table 2
............................ The absorption maxima at 301-310 nm of these polyimides are mainly determined by
the diphenyl-1,3,4-oxadiazole chromophore, while the absorption peaks at 471-486 nm and
513-517 nm are due to the chain segments containing perylenediimide units and are much
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lower in intensity as compared with the absorption bands characteristic of diphenyl-1,3,4-
oxadiazole. In films cast from NMP all of the copolyimides presented the main absorption
maximum between 300 and 307 nm due to the π-π* transitions involving the oxadiazole
chromophore, while very weak peaks appeared in the range of 502-543 nm, being attributed
to the absorption of perylenediimide units. No significant shift was noticed in the films as
compared with the absorption of the solutions in the case of polymers P2, P3 and P4, while
the absorption maximum of P1 is centered at 302 nm, being blue-shifted with respect to the
absorption of the isolated molecules (table 2). Interestingly, while most conjugated polymers
exhibit bathochromic shifts from solution to solid state, the absorption spectra of this new
polymer exhibits hypsochromic shifts in thin films, which is indicative of a solid-state
organization of the oxadiazole chromophore through aggregation in a parallel way, plane to
plane stacking, to form a sandwich-type arrangement. Similar behaviour was observed by us
in the case of other polymers containing the phenyloxadiazole chromophore [39]. The absence
of any aggregation with respect to oxadiazole units in the case of polymers P2, P3 and P4
could be explained in terms of presence of two ether aromatic linkages in the diamine
segment, which partially disturb the electronic conjugation and prevent the close packing of
the macromolecules through physical interactions. The noticeable red-shift of the absorption
maxima characteristic to perylenediimide units in the solid state with respect to solution could
be explained by the fact that the surrounding chromophore polarity became weaker, and the
intermolecular interaction of the polymer backbones became stronger. In fact, the tendency of
perylenediimide units to form columnar phases through strong π-π interactions is well known
[40], these aggregates preventing the self-absorbance of visible light.
The photoluminescence (PL) ability of these polymers was investigated by exciting
with light of different wavelengths, and the color of the PL emission was surveyed on the
basis of chromaticity coordinates, according to the available CIE 1931 (Comission
Internationale de I'Éclairage) standard [41]. Maximum emission wavelength values of
copoly(peryleneimide)s P1-P4 in solution and the CIE coordinates are collected in Table 3.
............................. Table 3
........................... By exciting at the maximum absorption wavelength characteristic to diphenyl-1,3,4-
oxadiazole chromophore (308-310 nm) and at 360 nm, polyperyleneimide P1 displayed
similar spectra with one strong and large emission band centered at 417 nm and,
respectiveley, 429 nm, in the blue domain, due to the emission of oxadiazole, imide and
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phenylene rings, and one maximum in the green spectral range, at 522 nm due to the emission
of perylenediimide moieties (Figure 10).
............................... Figure 10
.................................
When excited with light of 300 nm, polyimides P2 and P3 behave similarly, showing
one weak emission band in the UV domain, centered at about 371 nm, and one strong band
within 526-529 nm range. Polyperyleneimide P3 presented one additional shoulder at 581 nm
characteristic for perylenediimide fluorophore. The bands from the UV domain of these
polymers are red-shifted to 454 nm and 447 nm (blue domain), when excitation was
performed with light of 360 nm wavelength (Table 3, Figure 11).
.............................. Figure 11
.................................. In a previous study regarding the photo-optical behavior of related polymers containing
haxafluoroisopropylidene units instead of tetramethyldisiloxane we have demonstrated that
the PL emission that occured in the UV-domain is mainly determined by segments containing
five-imide rings and not by the diphenyloxadiazole chromophore, and it is shifted towards the
blue region by lowering of the energy of excitation [37]. The PL spectra of the present
polymers P1, P2 and P3 also show a very strong emission of perylenediimide moieties, while
the emission of oxadiazole moieties is completely quenched in case of polyimides P2 and P3,
when the excitation was done with the wavelength characteristic to diphenyloxadiazole (300-
310 nm). Because the perylenediimide moieties did not directly absorb these wavelengths of
light, an excitation energy transfer, known as the FRET phenomenon, from the oxadiazole to
the perylenediimide moieties must have taken place. In the case of P2 and P3 the entire
emitted light by oxadiazole was absorbed by perylenediimide, whereas in the case of P1 only
part of excitation energy was transferred to perylenediimide. A similar effect was reported by
us and by other authors on related polymers [42]. It appears that in the excited state, the
oxygen acts as a better bridge for the delocalization of the charge. When P2 and P3 solutions
were excited with 360 nm, the emission at 520-526 nm characteristic to perylenediimide was
weak, being the same intensity as that from the blue domain (figure 11). In this case, the
phenyloxadiazole units do not absorb light of 360 mn, and the emission from perylenediimide
may come from an excitation energy transfer from conjugated segments containing the five-
membered imide ring.
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Different behavior must be noted in the case of P1 which does not contain ether
bridges. By excitation with the UV light of 360 nm, the PL spectrum consist of two strong
bands of similar intensity, at 429 and 522 nm, while by excitation at 310 nm a similar
spectrum was obtained, with a less intense band in the blue domain (Figure 10). This means,
that the emitted blue light is much stronger by excitation at 360 nm and the FRET
phenomenon is less intensive as compared with excitation at 310 nm. This behavior is clearly
reflected in the fluorescence quantum yield (Φfl) values being 2.2 times higher at 360 nm (Φfl
= 10.91 %) as compared with 310 nm (Φfl = 4.92 %). Very low quantum yield of fluorescence
was obtained for P2 and P3, below 0.5 %, by excitation at about 300 nm, and around 1 % by
excitation at 360 nm, probably due to the limitation of the apparatus that allowed only the
light emitted in the blue spectral range to be measured.
Poly(peryleneimide) P4 showed both UV (386 nm) and blue light emission at 418 and
437 nm, coming from five-imide and naphthalene moieties and a weak and broad emission at
514 nm, due to the perylenediimide fluorophore upon excitation at 308 nm (figure 10). It
should be noted here that the emission band from the blue domain is very narrow and intense,
being much higher in intensity as compared with the absorption bands characteristic of
perylenediimide. This band decreases in intensity when the excitation is made with 360 nm
irradiation, while the PL intensity band at 514 nm increases. Taking into account that the PL
peak in UV domain is due to the five-imide ring emission and this polymer contains ether
bridges, we may assume that the emission in the blue domain is due to the naphthalene
fluorophore and an excitation energy transfer took place from the oxadiazole, and perhaps the
naphthalene, to the perylenediimide, for which ether bridges appear to be responsible, as in
the case of polyperyleneimides P2 and P3. This statement is reinforced by the photo-physical
behavior of this polymer upon excitation at 360 nm, a UV light that is not absorbed by the
oxadiazole ring, but only the naphthalene unit, the PL spectrum in the blue and green regions
being similar to that obtained by excitation at 308 nm (figure 10). Even in this case, the FRET
pehenomenon from naphthlene to perylenediimide units took placed to some extent. The
values of the quantum yield of fluorescence are also low, as in the case of P2 and P3, being
0.65 %, by excitation at 308 nm, and 0.84 % by excitation at 360 nm, proving the existence of
the excitation energy transfer at both excitation wavelengths.
In the study of color perception, one of the first mathematically defined color spaces is
the CIE-1931 XYZ color space [41]. The chromatic sensation of human eyes to a specific
optical spectrum is usually characterized by a chromaticity diagram. According to the
available CIE standard, the chromaticity coordinates of our polymers depend on the excitation
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wavelength. As shown in figure 12, they are located in the blue and green region of the
chromaticity diagram, except for polymer P3, at 303 nm excitation, which are out of the
diagram. The chromaticity coordinates of P2 and P4, obtained by excitation at the maximum
absorption wavelength characteristic to diphenyloxadiazole unit are located on the spectral
locus and correspond to pure hue blue light of a single wavelength. The CIE coordinates
corresponding to P2, P3 and P4, obtained by exciting at 360 nm, are shifted inside of the
chromaticity diagram, indicating a less saturated light, coming from the mixing of blue and
green. Polyperyleneimide P1 has very close CIE coordinates by excitation at both 310 and
360 nm, being located in the blue region, which means that this polymer emits similar, pure
blue light by excitation at these two wavelengths.
....................................... Figure 12
......................................... The emission spectra of copoly(peryleneimide)s P1-P4 in NMP solution excited by a
473-524 nm light, showed one broad peak at 532-552 nm, except for P3 that exhibited two
peaks at 537 and 581 nm, which corresponded to the emission of perylenediimide moieties
(Table 3). The values of fluorescence quantum yield depend on the excitation wavelength and
on the structure of the polymer. Generally, the highest PL intensities and fluorescence
quantum yields were obtained at the highest energy of excitation (480-485 nm) corresponding
to the perylenediimide. The absolute values of fluorescence quantum yield (Φfl) of the
copolymers P1-P4 in solution, measured using an integrating sphere, reach 23.87 % and
17.99 % for P1, 12.95% and 9.50 % for P4, 9.69 % and 4.42 % for P3, by excitation at 480
nm and at 525 nm, respectively. P2 showed a very low value of fluorescence quantum yield
(Φfl = 0.75), when excited at 502 nm. The chromaticity coordinates of all siloxane-containing
poly(peryleneimides), obtained by excitation at the maximum absorption wavelength
characteristic to perylenediimide are located on the spectral locus and correspond to pure
saturated orange monochromatic light (figure 12).
The photoluminescent behavior of copolyperyleneimides P1-P4 changed significantly
from solution to solid state. The light-emitting ability of these polymers in films was observed
only by exciting at the maximum absorption wavelength characteristic to the diphenyl-1,3,4-
oxadiazole chromophore and 360 nm (table 4). Representative spectra of polymer P4 are
shown in figure 13. The PL spectra of these polymer films obtained by excitation with about
300 nm, exhibit one emission maxima in the UV domain within 353-366 nm, several
shoulders in the blue domain, in the range of 418-422 nm, 439-444 nm and 481-485 nm, due
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to the emission of diphenyl-oxadiazole, phthalimide, phenylene segments and, additionally, in
the case of P4, due to the naphthalene units. Very weak emission peaks characteristic to
perylenediimide moieties centered at 522 and 537 nm were registered only in the case of P3,
the more flexible and less packed polymer.
............................... Table 4
Figure 13 ...................................
When the excitation was performed with UV light of 360 nm, the PL spectra of all
polymers were similar and displayed two sharp strong PL peaks in the blue domain at 412-
416 nm and 434-438 nm, and a shoulder at 471-474 nm, with no emission peaks characteristic
to perylenediimide moiety. No emission attributed to this chromophore was observed when
the excitation was performed at even higher wavelengths. This behavior can be ascribed to the
high content of the perylenediimide cromophore which leads to fluorescence quenching
through aggregation. The CIE coordinates corresponding to polymers P1-P4, obtained by
exciting at about 300 and 360 nm are inside of the chromaticity diagram, in the blue region,
indicating the ability of these polymers to emit blue light of different shades: hue pure blue
light or a less saturated light, coming from the mixing of blue and green or blue and violet
colours (figure 12). Very low fluorescence quantum yields, less than 1%, were obtained in
thin films at both excitation wavelengths, proving that the aggregation played an important
role in fluorescence behavior of these polyperyleneimides in the solid state.
4. CONCLUSIONS
Study of the photo-optical behaviour with an emphasis on the ability of light colour
modulation of new poly(peryleneimides) containing two chromophores, oxadiazole and
perylenediimide, and flexible siloxane bridges in the main chain was the main objective of
this work. The copolymers were successfully synthesized by one step solution
polycondensation reaction at high temperatures and their structures were confirmed by FT-IR
and 1H-NMR analyses. These copoly(oxadiazole-peryleneimide)s were soluble in 1-methyl-2-
pyrrolidinone and their solutions were cast into transparent coatings having strong adhesion to
the various supports. These copolymers are highly thermostable, up to 457°C, and display
glass transition in the range of 187-227°C. X-ray diffraction studies revealed a semicrystalline
state only for two polymers, coming from the π-stacking of the perylene cores, and an
amorphous state for the other two polymers. The self-assembling of the semicrystalline
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copolyimides in thin films into π stacking rods was evidenced by scanning electron
microscopy. The photo-optical properties of these copolymers were assessed on the basis of
UV-Vis absorption and photoluminescence spectra recorded for polymer solutions and for
thin films, after excitation with UV or visible light. The fluorescence quantum yields and
chromaticity diagrams were recorded, as well, in order to survey the colour of the emitted
light. The light emission of these polymers depends on the structure of the diamine
component and on the excitation wavelengths. Thus, by excitation with UV light of different
wavelengths all polymers showed blue and green light emission in solution, as was surveyed
in the chromaticity diagrams. The FRET phenomenon from oxadiazole to perylenediimide
chromophores was observed to occur for some polymers, for which the oxygen bridge appears
to be essential. By excitation with visible light of different wavelengths, pure hue orange light
of various shades and high fluorescence quantum yields, up to 24 %, were obtained. In solid
state, the light-emitting ability of these polymers was observed only by exciting at the
maximum absorption wavelength characteristic to the diphenyl-1,3,4-oxadiazole chromophore
and 360 nm. The chromaticity coordinates are located in the blue region, indicating the ability
of these polymers to emit blue light of various shades: hue pure blue light or a less saturated
light, coming from the mixing of blue and green or blue and violet colours. The nice balance
of high-performance properties encountered in these copolymers containing perylenediimide,
oxadiazole and siloxane units provides an important guideline for the design of new materials
that can be used to fabricate optical devices with high stability.
ACKNOWLEDGEMENTS:
The financial support provided by CNCSIS-UEFISCDI through the Project PN II-RU,
code TE_221, no. 31/2010 is acknowledged with great pleasure.
The authors are grateful to Dr. Ioana Moleavin from ''Petru Poni'' Institute of
Macromolecular Chemistry, Iasi-Romania, for the fluorescence quantum yields
measurements.
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List of figure and table captions
Scheme 1. Structures of the monomers (diamines and dianhydrides).
Scheme 2. Synthesis of polyperyleneimides containing oxadiazole and siloxane units.
Figure 1. NMR spectrum of P4 (top: aromatic region, down: aliphatic region)
Figure 2. FTIR spectrum of siloxane-containing polyperyleneimide P3
Figure 3. TGA curves of polyperyleneimide P1.
Figure 4. DSC curves of siloxane-containing polyperyleneimides P1-P4
Figure 5. Models of polyperyleneimides containing oxadiazole and tetramethyldisiloxane
units, P1 and P4.
Figure 6. XRD patterns of siloxane- and oxadiazole-containing polyperyleneimides P1-P4.
Figure 7. SEM image of a thin film made from P1
Figure 8. UV-vis absorption spectra in solution of siloxane-containing polyperyleneimides
P1-P4 (the inset represents a zoom of the region were perylenediimide units absorb)
Figure 9. UV-vis absorption spectra in solid state of siloxane-containing polyperyleneimides
P1-P4 (the inset represents a zoom of the region were perylenediimide units absorb)
Figure 10. PL spectra of P1 and P4 in NMP solution
Figure 11. PL spectra of P2 in NMP solution when excited with light of various wavelengths
Figure 12. Chromaticity diagrams of polyperyleneimides P1-P4.
Figure 13. Photoluminescence spectra of thin films of copolyimide P4.
Table 1. Thermal properties of siloxane-containing polyperyleneimides P1-P4
Table 2. UV-vis absorption properties of polymers P1-P4
Table 3. Photoluminescence (PL) and chromaticity coordinates of polymers P1-P4 in solution
Table 4. Photoluminescence (PL) and chromaticity coordinates of polymer films P1-P4
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Highlights
• New polyperyleneimides containing oxadiazole and siloxane units were prepared
• Some of the thin films prepared were organized into self-assembled rod-like structures
• FRET phenomenon from oxadiazole to perylenediimide units was observed in solution
• High fluorescence quantum yields, up to 24 %, were obtained in solution
• The color of the emitted light was modulated as a function of the excitation wavelength
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Table 1. Thermal properties of siloxane-containing polyperyleneimides P1-P4
Polymer Tg (oC) IDT (
°C) T10% (°C) Tmax (
°C) W800 (%)
P1 200 470 485 505 49.85
P2 187 490 499 519 49.56
P3 195 475 481 510 49.86
P4 227 457 459 501 56.9
Tg = glass transition temperature;
IDT = onset on the TG curve;
T10% = temperature of 10% weight loss on the TG curve;
Tmax= temperature of maximum rate of decomposition
W800 = residue at 800°C
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Table 2. UV-vis absorption properties of polymers P1-P4
Polymer P1 P2 P3 P4
UV, sol
λmax, nm
310, 332s, 486,
524 300, 473, 513 303, 487, 513
308, 459s, 486,
517
UV, film
λmax, nm
302, 503, 519,
541 302, 503, 518
300, 502, 519,
543 307, 502, 538
λmax - wavelength of the maximum absorption peak
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Table 3. Photoluminescence (PL) and chromaticity coordinates of polymers P1-P4 in solution
PL (303-310 nm) PL (360 nm) PL (480-490 nm) PL (504-525 nm)
Polymer
λλλλem, nm CIE
X Y
λλλλem, nm CIE
X Y
λλλλem, nm
CIE X Y
λλλλem, nm
CIE X Y
P1 417, 522 0.175 0.108
429, 522 0.186 0.122
540, 579s 0.471 0.517
581 0.556 0.443
P2 371, 526 0.176 0.006
454, 520 0.180 0.427
532 - 576 0.524 0.474
P3 371, 529,
581 0.463 1.571
447, 526, 580
0.049 0.602
534, 580 0.493 0.495
537, 581 0.557 0.442
P4 386, 418, 437s, 514
0.166 0.009
418, 444, 514
0.110 0.349
552 0.486 0.507
560 0.542 0.457
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Table 4. Photoluminescence (PL) and chromaticity coordinates of polymer films P1-P4
PL (300-307 nm) λλλλem, nm
PL (360 nm) λλλλem, nm
Polymer Film
CIE X
Y Film
CIE X
Y
P1 365, 422, 444, 485 0.242 0.322
414, 438, 470s 0.163 0.194
P2 353, 422, 443, 485 0.239 0.318
416, 434, 470s, 522, 537
0.155 0,185
P3 360, 418, 442, 482 0.216 0.306
412, 436, 471s 0.273 0.237
P4 366, 419, 439, 481 0.257 0.309
416, 436, 474s 0.166 0.205
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