Structure and Properties of Functionalized BithiophenesilaneMonodendrons
Ray Gunawidjaja,† Yuriy N. Luponosov,‡ Feifei Huang,† Sergei A. Ponomarenko,‡
Aziz M. Muzafarov,‡ and Vladimir V. Tsukruk*,†
†School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245,and ‡Enikolopov Institute of Synthetic Polymer Materials of the RAS, Profsoyuznaya 70, Moscow, Russia
Received March 4, 2009. Revised Manuscript Received April 9, 2009
This study reports a focal group modification of bithiophenesilane monodendrons and its effect on their molecularordering in solution, bulk, and surface. We investigated hydrophobic MDnmonodendrons and COOH-functionalizedMDn-COOHmonodendrons with generations, n=0, 1, 2, and 3.We observed that increasing the number of branchesled to the progressive blue shift, indicating distorted packing of branched thiophene fragments of MDn. In contrast,MDn-COOHmonodendrons showed a progressive red shift with the increasing generation number, indicating gradualdomination of σ-π interactions. Moreover, the introduction of a focal carboxylic group resulted in the formation of ahighly crystalline state for the linear MD0-COOH compound with separated alkyl tail-thiophene packing, whichlimits π-π interactions. Increasing branching in the COOH-containing monodendrons resulted in a hydrophobic-hybrophilic balance sufficient to form stable and uniform Langmuir monolayers at the air-water at a modest surfacepressure (<10 mN/m), easily transferrable to a solid substrate. However, a further increase in the thickness of thesurface layers from tens to hundreds of nanometers via Langmuir-Blodgett (LB) deposition or spin casting is limitedby the formation of globular surface aggregates because of strong intermolecular interactions. A modest red shiftobserved for condensed Langmuir monolayers indicates densification of thiophene branches and limited intramono-layer crystallization, which preserves photoluminescence. In contrast, thicker surface films showed a significant redshift, confirming a dense molecular packing with strong π-π interactions, which results in photoluminescencequenching.
Introduction
Thin-film organic semiconducting materials are considered forprospective applications in microelectronic devices, includingorganic light-emitting diodes (OLEDs) and field-effect transis-tors.1 This is because of their low cost, ease of fabrication, wide-viewing angle, and low power consumption, in contrast toinorganic light-emiting diode. The band gap of an organicsemiconductor is determined by the lowest unoccupiedmolecularorbit (LUMO) and highest unoccupiedmolecular orbit (HOMO)energy bands that is tunable with molecular architecture.2 Inaddition to the molecular architecture, it is well-established thatcontrol over long-range molecular orientations and local mole-cular packing is a prerequisite for high-performance organic-based systems, and therefore, the understanding of the structure-property relationship, especially in the solid state, is critical in thedesign of thin-film systems.3 Small molecules that are comprised
of rod and coil segments can assemble into unique and well-defined nanoscale assemblies that are rarely seen in regular linearmolecules,4 e.g., barrel-like, left-helical tubular, nanoribbons,spherical, cylindrical, toroidal, and microporous structures.5
These unique assemblies with, frequently, intriguing photolu-minscent properties are the result of conformational and shapemismatch between the rigid rod and flexible segments covalentlybonded within a single molecule, and the driving force forassembly are weak interactions that include hydrogen bonding,electrostatic, van der Waals interactions, π-π interactions, andhydrophobic-hydrophilic balance.6
In addition to their ability to control the assembly by means ofweak interactions and molecular architectures, the rod-like seg-ments often possess useful functionalities with intriguing photo-physical and electrochemical properties. Thiophene is oneexample of a rigid molecular fragment that is both chemicallystable and has a unique combination of properties.7 Thiophenemolecules are easy to functionalize, and depending upon thefunctional groups and chain architecture, these molecules canemit light across the entire visible range.8 These compounds aremainly investigated as prospective conducting, luminescent, andsensing materials. Organic semiconductors, such as polythio-phenes, are commercially appealing because of their low costand facile processing, both ofwhich are enabled by solution-basedassembly. While unsubstituted conjugated organic oligomershave the tendency to crystallize into herringbone structures,charge mobility across a thin-film organic semiconductor mightbe altered when the π orbitals overlap as a result of electronic
Mucci, A.; Iarossi, D. Sens. Actuators, B 2000, 68, 203.(2) McNeill, R.; Siudak, R.; Wardlaw, J. H.; Weiss, D. E. Aust. J. Chem. 1963,
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T. C. T.; Mutsaers, K. A. H. A.; Cantatore, E.; Wondergem, H. J.; Werzer, O.;Resel, R.; Kemerink,M.; Kirchmeyer, S.;Muzafarov, A.M.; Ponomarenko, S. A.;de Boer, B.; Blom, P. W.M.; de Leeuw, D. M.Nature 2008, 455, 956. (b) Tsukruk,V. V. Prog. Polym. Sci. 1997, 22, 247.(4) (a) Stupp, S. I. Curr. Opin. Colloid Interface Sci. 1998, 3, 20. (b) Cho, B.-K.;
Kim, H.-J.; Chung, Y.-W.; Lee, B.-I.; Lee, M. Adv. Polym. Sci. 2008, 12, 138. (c)May, S. J.; Zheng, J.-G.; Wessels, B. W.; Lauhon, L. J. Adv. Mater. 2005, 17, 598.(5) (a) Genson, K. L.; Holzmueller, J.; Ornatska, M.; Yoo, Y.-S.; Park, M.-H.;
Lee, M. S.; Tsukruk, V. V.Nano Lett. 2006, 6, 435. (b) Tsukruk, V. V.; Genson, K.L.; Peleshanko, S.; Markutsya, S.; Greco, A.; Lee, M.; Yoo, Y. Langmuir 2003, 19,495. (c) Prokhorova, S. A.; Sheiko, S. S.; M
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coupling between adjacent molecules, resulting in higher mobilityfor face-face arrangement in contrast to edge-face arrange-ment.9
Usually, nonfunctionalized polythiophene molecules are inso-luble in conventional organic solvents, which makes their proces-sing very difficult. Thus, to enhance their solubility and ability toprocess with conventional wet chemistry, various thiophene-based architectures have been prepared, e.g., linear, branched,oligomer, star-shaped, and all-thiophene dendrimers.9 To thisend, Reitzel et al. demonstrated that amphiphilic regioregularpolythiophene derivatives at the air-water interface could adoptedge-on orientation.10 Similarly, to obtain vertically orientedthiophenes, Senkovskyy et al. proposed to directly grow poly-thiopehene brush from a solid surface.11 Conversely, in the case ofOLED applications, π stacking is not desirable to avoid loweredluminescence quantum efficiency as a result of quenching12 orundesired secondary emission.13Reducedπ stacking is achievablein nonlinear thiophene molecules, e.g., dendrimers where the πstacking is impeded by shape persistency and multiple bran-ching.12c Alternatively, bulky substituents can be introduced toinhibit a close face-face packing.14 Longer oligothiophene frag-ments were found to have increased luminescent efficiency andsmaller energy gaps, as a result of increased conjugation length.15
Additionally, intermolecular conjugation is most pronouncedwhen the molecule adapts a planar conformation.
Highly branched structures are particularly interesting becausethey can be tailored with multiple functionalities at their periph-erals, which hence increases their solubility. Additionally, sym-metrical and asymmetrical dendritic architectures can effectively
provide site isolation and inhibit crystallization.16 The number ofperipheral groups or dendrimer generation can be varied tocontrol their properties.17 Dendrimers can also be categorizedinto flexible and rigid, according to their linking groups.18 Rigiddendrimers are shape-persistent, while flexible dendrimers allowfor branch folding. Their optical properties are governed by inter-and intramolecular interactions and thus depend upon theirmolecular ordering. It is worth noting that, although dendriticarchitectures have been associated with shielding of thiophenefragments, Malenfant et al. demonstrated that a functionalizedpolythiophene dendrimer can be conductive.19
Our previous studies have shown that in a dilute solution theσ-π conjugation between silicon atoms and aromatic π systemsof oligothiophenes in bithiophenesilane dendrimers can lead up to30% photoluminescence quantum yield without significantchange in the energy gap.20 This value is significant comparedto the parent bithiophenes that possess only 2-6% photolumi-nescence quantum yield. Therefore, it is of interest to elucidatetheir film-forming ability for a possible application in OLEDdevices with spin-cast and Langmuir-Blodgett (LB) techniquesto be considered as a firstmodel step in the formation of thin filmson solid support.
The current study involves representatives of a recently intro-duced class of flexible bithiophenesilanemonodendrons as well astheir new chemically modified amphiphilic counterparts.20 Amonodendron is unique fromdendrimer, in that, its asymmetricalshape allows for the incorporation of a focal terminal group.Here, we discuss bithiophenesilane monodendrons and functio-nalized COOH-containing monodendrons with generation num-ber, n = 0, 1, 2, and 3 (Scheme 1). From here onward, thefollowing notations will be used:MDn-COOH andMDn for nthgeneration carboxyl-terminated monodendrons and fully hydro-phobic monodendrons, respectively (Figure 1).
The focus of this work is on understanding the role of bran-ched architecture and focal groups on inter- and intramolecular
Scheme 1. Schematic for nth Generation Monodendron, where n= 0-3 and x = 0 for Hydrophobic MDnMonodendrons and x = COOH forAmphiphilic MDn-COOH Monodendrons
(9) Murphy, A. R.; Frechet, M. J. Chem. Rev. 2007, 107, 1066.(10) Reitzel, N.; Greve, D. R.; Kjaer, K.; Howes, P. B.; Jayaraman, M.; Savoy,
S.; McCullough, R. D.; McDevitt, J. T.; Bjornholm, T. J. Am. Chem. Soc. 2000,122, 5788.(11) Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.;
Kuckling, D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626.(12) (a) Perepichka, I. F.; Perepichka, D. F.; Meng, H.; Wudl, F. Adv. Mater.
2005, 17, 2281. (b) Taraneker, P.; Qiao, Q.; Jiang, H.; Schanze, K. S.; Reynolds, J.R. J. Am. Chem. Soc. 2007, 129, 8958. (c) Xia, C.; Fan, X.; Licklin, J.; Advincula,R. C.; Gies, A.; Nonidez, W. J. Am. Chem. Soc. 2004, 126, 8735.(13) Takagi, K.; Momiyama, M.; Ohta, J.; Yuki, Y.; Matsuoka, S.; Suzuki, M.
Macromolecules 2007, 40, 8807.(14) Bolognesi, A.; Schieroni, A. G.; Botta, C.; Marinelli, M.; Mendichi, R.;
(15) Bair, J. S.; Harrison, R. G. J. Org. Chem. 2007, 72, 6653.
(16) Burn, P. L.; Lo, S.-C.; Samuel, I. D. W. Adv. Mater. 2007, 19, 1675.(17) Furuta, P.; Frechet, J. M. J. Am. Chem. Soc. 2003, 125, 13173.(18) Zhang, Y.; Zhao, C.; Yang, J.; Kapiamba, M.; Haze, O.; Rothberg, L. J.;
Ng, M.-K. J. Org. Chem. 2006, 71, 9475.(19) Malenfant, P. R. L.; Frechet, J. M. J. Macromolecules 2000, 33, 3634.(20) Borshchev, O. V.; Ponomarenko, S. A.; Surin, N. M.; Kaptyug, M. M.;
Buzin,M. I.; Pleshkova, A. P.; Demchenko, N. V.;Myakushev, V. D.;Muzafarov,A. M. Organometallics 2007, 26, 5165.
interactions in the solid state, which in turn, govern theirmolecular ordering. In the solid state, the asymmetrical mono-dendrons are expected to root on a single end, from which nthgeneration branched molecules emanate, hence providing high-grafting density compared to their linear counterparts, while atthe same time, affecting crystallization behavior and π-π inter-actions.21 Initially, the film-forming ability was investigated bymeans of spin coating. Then, the LB technique was mainlyemployed to systematically study the aggregation behavior. Inaddition, optical absorption and emission spectra were obtainedto provide additional insight on their inter- and intramolecularinteractions.
Experimental Section
Materials. Chemicals. n-Butyllithium (1.6 and 2.5M solu-tions in hexane) (Acros Organics) and 5-hexyl-2,20-bithiophene(Stark) were used as received. Carbon dioxide was dried bypassing through aDrierite gas-drying unit (Sigma-Aldrich) beforeuse. Tetrahydrofuran (THF) was dried over CaH2 and distilledfromLiAlH4. The residual solvent was removed in vacuumbelow1Torr at 40 �C.All reactions, unless statedotherwise,were carriedout under an inert atmosphere using anhydrous solvents.
Synthesis of MDn-COOH, for n = 0-3. The synthesis ofinitial hydrophobic monodendrons was described in detail else-where.22 Functionalizedmonodendrons have been synthesized asfollows. To obtain MD0-COOH, a solution of 8.8 mmol ofbutyllithium in 3.51 mL of hexane (2.5 M solution) was addeddropwise to a solution of MD0 (2.2 g, 8.8 mmol) in 70 mL THF,while the temperature was kept between -75 and -70 �C, andthen the solution was stirred at -78 �C for 1 h. Next, 300 mL
(13.2 mmol) of carbon dioxide was introduced for 1 min, keepingthe temperature below-60 �C.The solutionwas stirred at-70 �Cfor 30min. Itwas thenpoured into100mLofwater, containinganequimolar amount of hydrochloric acid (13.2 mL of 1 M HCl).After vigorous shaking, three extractions with 100 mL of diethylether were carried out. The organic solution was dried oversodium sulfate. The solvent was evaporated in vacuum, and theresidue was dried at 1 Torr to give 2.56 g of crude product in areaction yield of 95% (according to 1HNMR).The crude productwas purified by recrystallization from a hexane/ethylacetatemixture (2:1) to give pure compound MD0-COOH (2.3 g,89%), mp 120 �C (literature value of 112 �C).23 IR ν (KBr):1678 cm-1 (CdO). 1H NMR (250MHz, DMSO-CCl4) δ: 0.88 (t,3H,J=6.7Hz), 1.24-1.40 (overlappedpeaks, 6H), 1.63 (m, 2H,M= 5, J = 6.7 Hz), 2.78 (t, 2 H, J= 7.3 Hz), 6.74 (d, 1 H, J=3.6Hz), 7.11 (d, 1 H, J=4.28Hz), 7.14 (d, 1 H, J=3.7Hz), 7.56(d, 1 H, J = 4.28 Hz), 12.80 (broadened signal, 1 H). 13C NMR(125 MHz, CDCl3) δ: 14.07, 22.56, 28.73, 30.21, 31.48, 31.52,123.30, 125.24, 125.39, 129.55, 133.47, 135.91, 146.65, 147.90,167.82. MS-EI (70 eV): m/z 294.0748 (M+, requires 294.0744).
MD1-COOH was obtained as described for compoundMD0-COOH using 1.00 g (1.40 mmol) of compound MD1,0.93 mL of 1.6 M butyllithium solution in hexane (1.4 mmol),48 mL (2.10 mmol) of carbon dioxide, and 30 mL of THF to givethe crude product in a reaction yield of 98% (according to 1HNMR). The crude product was purified by gradient columnchromatography on silica gel (eluents: 1:3 toluene/hexane andethyl acetate) to give pure compound MD1-COOH (0.79 g,75%), mp 116 �C. IR ν (film): 1671 cm-1 (CdO). 1H NMR(250MHz,DMSO-CCl4)δ: 0.88 (t, 6H, J=6.7Hz), 0.93 (s, 3H),1.23-1.39 (overlapped peaks, 12H), 1.64 (m, 4H,M=5, J= 6.7Hz), 2.76 (t, 4 H, J=7.3 Hz), 6.69 (d, 2 H, J=3.7 Hz), 7.03 (d, 2H,J=3.7Hz), 7.21 (d, 2H,J=3.7Hz), 7.26 (d, 1H,J=3.7Hz),7.29 (d, 2 H, J= 3.7 Hz), 7.35 (d, 1 H, J= 3.7 Hz), 7.45 (d, 1 H,J = 3.7 Hz), 7.59 (d, 1 H, J = 3.7 Hz), 12.88 (broadened signal,1 H). 13C NMR (125MHz, CDCl3) δ:-0.28, 14.06, 22.55, 28.72,
Figure 1. Synthesis and structural formulas of nth generation carboxyl-terminatedMDn-COOH and fully hydrophobic bithiophenesilanemonodendrons, MDn, with n= 0, 1, 2, and 3 (I = BuLi, II = CO2, and III = HCl/H2O).
(21) (a) Dendrimers and Other Dendritic Polymers; Fr�echet, J. M. J., Tomalia,D. A., Eds.; Wiley: New York, 2001; p 647.(b) Dendritic Molecules: Concepts,Syntheses, Perspectives; Newkome, G. R., Moorefield, C. N., Vogtle, F., Eds.;VCH: Weinheim, Germany, 1996; p 261.(c) Peleshanko, S.; Tsukruk, V. V. Prog.Polym. Sci. 2008, 33, 523.(22) Luponosov, Y. N.; Ponomarenko, S. A.; Surin, N. M.; Muzafarov, A. M.
Org. Lett. 2008, 10, 2753. (23) Kobmehl, G.; Labahn, B. Z. Naturforsch., B: Chem. Sci. 1996, 58, 286.
MD2-COOH was obtained as described for compoundMD0-COOH using 1.11 g (0.7 mmol) of compound MD2,0.41 mL of 1.6 M butyllithium solution in hexane (0.7 mmol),23mL (10mmol) of carbondioxide, and 35mLofTHF togive thecrude product in a reaction yield of 99% (according to 1HNMR).The crude product was purified by gradient column chromato-graphy on silica gel (eluents: 1:3 toluene/hexane and ethyl acetate)to give pure compoundMD2-COOH (0.95 g, 83%). IR ν (film):1671 cm-1 (CdO). 1H NMR (250 MHz, DMSO-CCl4) δ: 0.87(t, 12 H, J = 6.7 Hz), 0.91 (s, 6 H), 0.95 (s, 3 H), 1.23-1.36(overlapped peaks, 24H), 1.62 (m, 8 H,M=5, J= 7.3 Hz), 2.75(t, 8 H, J= 7.3 Hz), 6.68 (d, 4 H, J= 3.7 Hz), 7.02 (d, 4 H, J=3.1 Hz), 7.20 (d, 4 H, J=3.7 Hz), 7.26 (d, 1 H, J=4.3 Hz), 7.28(d, 4 H, J = 3.7 Hz), 7.31-7.41 (overlapped peaks, 9H), 7.47(d, 1 H, J=3.7 Hz), 7.58 (d, 1 H, J=4.3 Hz), 12.86 (broadenedsignal, 1H). 13CNMR (125MHz,CDCl3) δ:-0.32,-0.18, 14.10,22.58, 28.73, 30.16, 31.54, 31.56, 123.99, 124.36, 124.77, 124.84,125.71, 125.78, 125.91, 132.82, 133.68, 134.27, 134.89, 134.78,137.85, 138.00, 138.06, 143.79, 144.33, 145.20, 145.97. 29Si NMR(100 MHz, CDCl3) δ: -25.24, -24.92. Anal. Calcd forC84H90O2S14Si3 (%): C, 60.60; H, 5.45; S, 26.96; Si, 5.06. Found:C, 60.63; H 5.49; S, 26.78; Si, 5.10. MALDI-MS: m/z 1665.2359(M+, requires 1665.2350).
MD3-COOH was obtained as described for compoundMD0-COOH using 1.3 g (0.4 mmol) of compound MD3, 0.24mL of 1.6 M butyllithium solution in hexane (0.4 mmol), 13 mL(0.6mmol) of carbondioxide, and 35mLofTHF to give the crudeproduct in a reaction yield of 40% (according to 1H NMR). Thecrude product was purified by gradient column chromatographyon silica gel (eluents: 1:1 toluene/hexane and ethyl acetate) to givepure compound MD3-COOH (0.26 g, 20%). IR ν (film): 1671cm-1 (CdO). 1HNMR (250MHz,DMSO-CCl4) δ: 0.85 (t, 24H,J = 6.7 Hz), 0.90 (s, 12 H), 0.93 (s, 9 H), 1.21-1.36 (overlappedpeaks, 48 H), 1.61 (m, 16 H, M = 5, J = 7.3 Hz), 2.73 (t, 16 H,J=7.3 Hz), 6.65 (d, 8 H, J=3.7 Hz), 6.99 (d, 8 H, J=3.1 Hz),7.17 (d, 8 H, J=3.1 Hz), 7.23 (d, 1 H, J= 3.7 Hz), 7.26 (d, 8 H,J = 3.7 Hz), 7.28-7.39 (overlapped peaks, 25 H), 7.44 (d, 1 H,J = 3.1 Hz), 7.57 (d, 1 H, J = 4.3 Hz). 13C NMR (125, MHz,CDCl3) δ: 0.26, 0.24, 0.19, 14.08, 22.55, 28.71, 30.14, 31.51, 31.53,123.96, 124.33, 124.73, 124.81, 125.66, 125.70, 125.75, 125.79,126.88, 132.82, 134.09, 134.26, 134.43, 134.72, 135.71, 137.83,137.94, 138.05, 143.87, 144.14, 144.24, 145.15, 145.92. 29Si NMR(100 MHz, CDCl3) δ: -25.25, -25.09, -25.05. Anal. Calcd forC176H186O2S30Si7 (%):C, 60.54;H, 5.37; S, 27.55; Si, 5.63. Found:C, 60.67; H, 5.57; S, 27.20; Si, 5.60.MALDI-MS:m/z 3599.3499(M+ + Ag, requires 3599.3490).
Substrates Preparation. Silicon substrates (1 � 2 cm) [100](Semiconductor Processing) with a native silicon oxide layer with1.6 nm thickness were cleaned with Piranha solution (3:1 concen-trated sulfuric acid and hydrogen peroxide mixture, caution),abundantly rinsed with Nanopure water (18.2MΩ cm), and driedwith drynitrogen stream inaccordancewith the usual procedure.24
They served as hydrophilic substrates for film depositions, whichwere prepared either by spin coating from 1 mg/mL (1500 rpm,30 s) chloroform solution or LB deposition. Quartz slides wereprepared similarly: cut into 1� 2 cm, rinsedwithNanopure water,sonicated at room temperature, and finally treated with Piranhasolution. Similar to silicon substrates, quartz slideswere last rinsedwith Nanopure water and dried with dry nitrogen stream.
Methods. 1H NMR spectra were recorded on a Bruker WP-250 SY spectrometer at 250 MHz using NMR-grade CDCl3 or
CCl4-DMSO mixture as a solvent. 13C NMR spectra wererecorded on a Bruker DRX500 spectrometer at 125.76 MHz,and 29Si NMR spectra were recorded on a Bruker DRX500spectrometer at 99.36 MHz, using NMR-grade CDCl3 as asolvent. Mass spectra (MALDI-MS) were recorded on a BrukerDaltonics Reflex-III mass spectroscope in the positive-ion regimeusing the reflection mode with a target voltage of 20 kV. 2,4,9-Antracenetriolwas usedas amatrix. The sampleswere preparedby dissolution of the compounds under investigation in chloro-form (C=10-4-10-6M) andmixedwith a solution of thematrix(20 mg/mL) in chloroform in the ratio of 1:1. Mass spectra ofMD0-COOH were recorded on Micromass M@ldi MALDI-TOF MS working in reflectron mode using dihydroxybenzoicacid (10 mg/mL in THF) as a matrix and CF3COOAg (1 mg/mLTHF) as an additive. Samples were prepared by dissolution ofMD0-COOH in THF (1mg/mL) andmixedwith the matrix andadditive in a 1:1:1 volume ratio. Mass spectra of MD0-COOHwere recorded on Micromass M@ldi MALDI-TOF MS work-ing in reflectronmode using dihydroxybenzoic acid (10mg/mL inTHF) as a matrix and CF3COOAg (1 mg/mL THF) as anadditive. Samples were prepared by dissolution ofMD0-COOHin THF (1 mg/mL) and mixed with the matrix and additive in a1:1:1 volume ratio.
TheLBstudieswere conductedusing aKSV2000minitroughatroom temperature, according to the usual procedure adapted inour lab.25A40-120μLsolution (0.26-1.4mg/mL) in chloroform(HPLC grade) was evenly dispersed in several droplets onto thesurface of theNanopurewater. It was then left for 30min to allowfor the evaporation of the chloroform. Compression of themonolayers was conducted at a speed of 5 mm/min. The limitingcross-sectional areaA0was determinedby the steepest tangent risein the surface pressure, which provided evidence of the formationof a condensed monolayer.26
Effective film thicknesses were obtained with a M-2000Uspectroscopic ellipsometer (Woolam), equipped with WVASE32analysis software. Images were obtained from a Dimension-3000atomic force microscope (AFM) and a Leica DM4000 opticalmicroscope. AFM topographical images were generated in the“light” tapping mode with low-amplitude set point (ratio within0.90-1.0) to avoid monolayer damage.27 The AFM cantilevershave spring constants in the range of 40-60 N/m. Scanning rateswere chosen between 1.0 and 2.0 Hz, depending upon the scanarea that ranges from40� 40 to 1� 1 μm2, in accordancewith theprocedures adapted in our lab.28
Fluorescence spectra were obtained with a RF-5301PC spec-trofluorophotometer (Shimadzu). Quartz slides were used forUV/vis measurement with a UV-2450 spectrophotometer (Shi-madzu). Emission spectra were registered in the wavelength rangeof 250-600 nm for excitation wavelengths in the range of 250-400 nm. In all cases, the optical density of the solutions used forthe measurements of quantum yield did not exceed 0.05 in a10 mm thick quartz cuvette. Density was measured according tothe Archimedes method using a glycerine-water mixture as thefluid medium.
X-ray diffraction data were collected with an Alpha1-Philipsanalytical diffractometer. Analysis of X-ray diffraction data andthe unit cell determinationwas performedwithMDI Jade 7X-raydiffraction software. TREOR90 algorithm was used for spacegroup and peak assignments.29 Molecular models were built with
(24) (a) Szunerits, S.; Boukherroub, R. Langmuir 2006, 22, 1660. (b) Sheller, N.B.; Petrash, S.; Foster, M. D.; Tsukruk, V. V. Langmuir 1998, 14, 4535.
(25) (a) Larson, K.; Vaknin, D.; Villavicencio, O.; McGrath, D. V.; Tsukruk, V.V. J. Phys. Chem. B 2002, 106, 7246. (b) Peleshanko, S.; Sidorenko, A.; Larson, K.;Villavicencio, O.; Ornatska, M.; McGrath, D. V.; Tsukruk, V. V. Thin Solid Films2002, 406, 233.
(26) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991.
(27) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385.(28) (a) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791. (b) Tsukruk, V.
V. Rubber Chem. Technol. 1997, 70, 430.(29) Pecharsky, V. K.; Zavalij, P. Y. In Fundamentals of Powder Diffraction
and Structural Characterization of Materials; Kluwer Academic Publishers:Norwell, MA, 2003.
Materials Studio 3.1 software. Force fields for energy minimiza-tions were assigned using the PCFF method available in theMaterials Studio Discover 3.1 package.
Results and Discussion
Photoluminescent Properties of BithiophenesilaneMono-dendrons. The branched bithiophenesilane monodendrons stu-died here have molecular weights below 4000 Da (Table 1). Theyare composed of bithiophene fragments (from 1 to 15), which arelinked together by silicon atoms. Both the branched dendriticarchitectures and the hexyl chains that are attached to thebithiophene peripheries make them well-soluble in commonorganic solvents. Themolar extinction coefficients, ε, asmeasuredfrom Beer-Lambert’s law for a series of dilute concentrations of
MDn in THF (C<5� 10-5M) show the increase from 13000 to350 000M-1 cm-1 (Table 1). ForMDn-COOH, the values rangefrom 23500 and 295 000 M-1 cm-1. Except for the highestgeneration, n = 3, these values are generally smaller for MDnthan that for MDn-COOH. The greatest and lowest values areachieved for MD3 and MD0, respectively. The linear plot ofabsorbance versus concentration confirms that, at the givenconcentrations (<5 � 10-5 M), these molecules are non-inter-acting (not shown).30
The measured molar extinction coefficients, ε, for monoden-drons studied here are much larger than the ε for thiophene
Figure 2. UV/vis spectra of a dilute solution of (a) MDn and (b) MDn-COOH in THF. Photo-emission spectra of a dilute solution of(c) MDn and (d) MDn-COOH.
Table 1. Properties of MDn-COOH and MDn Dendrimers
dendrimers having similar generations that were reported by Xiaet al.12c For dendrimers having 3, 6, 7, 14, and 15 thiophenemolecules directly linked at the R positions, reported values of εare between 9700 and 56400 M-1 cm-1. These dendrimers havesolubilizing hexyl chains at the periphery. Thus, the main differ-ence between our systems and compounds from Xia et al. is thatour dendrons are composed of bithiophene fragments that areisolated by silicon branching points that break conjugation.Another comparison can be made for phenyl-cored thiophenedendrimers, which also, because of conjugation, show a smallervalue of ε for dendrimers that contain 12-32 thiophene mole-cules. The values of ε for these compounds ranged from 76000 to112 000 M-1 cm-1.34
The presence of a focal COOH group in the monodendronscauses the absorption and fluorescence maxima to red shift withrespect to MDn monodendrons, and the shift is most significantfor the linear MDn-COOH compound (Figure 2). This is due tothe electron-withdrawing nature of the carboxylic group,31 as wellas increased conjugation length. Dilute THF solution absorptionmaxima ofMDnmonodendrons, which corresponds to the π-π*electronic transition,32 lie between 308 and 333 nm,whereas diluteTHF solution absorption maxima of MDn-COOH monoden-drons are between 335 and 342 nm. These values are smaller thanthose for a similar thiophene molecule, R-hexadecylterthiophene(C16-3T), which absorbs at 366 nm. The red-shifted absorption
maximum in C16-3T is due to the increased conjugation length inthe terthiophenes as compared to bithiophenes.33 The alkylsubstituent is electron-donating and, depending upon the positionof the thiophene ring, can cause either a red or blue shift.34 Whenplaced at the R,ω position, the alkyl substituent leads to bettercoplanarity of the molecule and therefore causes a red shift. Onthe contrary, the alkyl substituent at the β position will induce ablue shift because of some torsion of the molecule along the longaxis, as a result of steric interactions induced by the pendant alkylgroup.
It is worth noting that the UV/vis maximum absorption forMDn monodendrons red shifts for n = 1-3 as compared ton = 0, but in the case of MDn-COOH, a blue shift is observed(panels a and b of Figure 2 and Figure 3a). The contrasting trendsin the shift in peak position appear to be symmetrical. Red shift isusually observed for higher generation dendrimers because ofincreased intramolecular interactions, which is consistent withthat observed for the MDn monodendrons. However, when thelarger molecule experiences steric limitations that, in turn, causesthe dendron fragments to twist out of plane, it leads to a slightdecrease in conjugation. When this distortion occurs, a blue shiftin the UV/vis maximumabsorption spectrum is observed instead,which is seen for the amphiphilic MDn-COOH monoden-drons.35
Photo-emission spectra in dilute THF solution were obtainedfor all samples using excitation wavelength that corresponds totheir λabs-max (panels c anddofFigure 2 andFigure 3b). Inparallelwith the trend in the UV/vis peak positions, the presence of aCOOH-withdrawing group causes the λfl-max to red shift inMDn-COOH with respect to MDn monodendrons. Thephoto-emission peak maximum occurs between 372 and 387 nm(violet) for MDn monodendrons and between 377 and 406 nm(violet-indigo) for MDn-COOHmonodendrons (Table 1). Theemission spectrum is typically a mirror image of the absorptionspectrum. This is especially true when the electronic excitationdoes not greatly influence the nuclear geometry.36
In addition, multiple peaks were observed with increasingdendron generations. This is indicative of additional intramole-cular energy transfer from higher band gap, which, in this case, isfor isolated bithiophene molecules, to the lowest band gap forinteracting bithiophene fragmentswithin the singlemonodendronmolecules having multiple fragments.12c
Bulk Structure and Properties. MD0-COOH is a pale-yellow solid powder, whereas its hydrophobic MD0 counterpartis a viscous liquid at room temperature. The rest of the mono-dendron samples exhibit waxy or oily appearance at roomtemperature that suggests amorphous structures. The pale-yellowpowder exhibits fluorescence when observed under UV light,λexc = 380 nm (Figure 4a).
Differential scanning calorimetry (DSC) measurements on allmonodendron samples revealed that only MD0-COOH andMD1-COOH show sharp endothermic transitions, which im-plies the presence of the crystalline state (see Figures S1 and S2 inthe Supporting Information). Moreover, MD0-COOH showstwo enantiotropic phase transitions during first and second heat-ing curves at 121 �Cwith the enthalpy of 99 J/g and at 169 �Cwiththe enthalpy of 5.6 J/g (see Figure S2b in the SupportingInformation). However, upon cooling, the lower peak splits into
Figure 3. Plots of the (a) number of bithiophene fragments versusUV/vis peak position (nm) and (b) number of bithiophene frag-ments versus photo-emission peak position (nm) in dilute THFsolution.
(31) Bourgeaux, M.; Skene, W. G. J. Org. Chem. 2007, 72, 8882.(32) Qiu, Y. Q.; Xu, Y.; Zhu, D. B. Synth. Met. 1997, 84, 197.(33) Murphy, A. R.; Chang, P. C.; VanDyke, P.; Liu, J.; Frechet, J. M. J.;
Subramian, V.; DeLongChamp, D. M.; Sambasivan, S.; Fischer, D. A.; Lin, E. K.Chem. Mater. 2005, 17, 6033.
(34) Yassar, A.; Horowitz, G.; Valat, P.; Wintgens, V.; Hmyene, M.; Deloffre,F.; Srivastava, P.; Lang, P.; Garnier, F. J. Phys. Chem. 1995, 99, 9155.
(35) Mitchell, W. J.; Kopidakis, N.; Rumbles, G.; Ginley, D. S.; Shaheen, S. E.J. Mater. Chem. 2005, 15, 4518.
(36) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; SpringerScience: Singapore, 2006.
two sharp transitions, indicating recrystallization or the presenceof a mesophase. The first peak at 102 �C possesses the enthalpy of45 J/g, and a second peak at 89 �C has the enthalpy of 52 J/g.Optical polarizing microscopy upon cooling in the temperatureinterval of 170-103 �C showed schlieren texture, characteristic ofa nematic mesophase, and in the interval of 102-60 �C showed a
less familiar bright texture, which probably, corresponds to anordered smectic mesophase (see Figure S3 in the SupportingInformation). Further cooling led to a dark crystal texture. ForMD1-COOH, a phase transition occurs at 118 �C with theenthalpy of 52 J/g during the first heating scan only. The secondscan for this substance shows only the glass transition in theregion of -6 �C, indicating its amorphous state.
Indeed, sharp peaks with no discernible broad halo peaks areobtained from X-ray diffraction measurements, confirming thehigh crystallinity of MD0-COOH sample, as concluded fromDSC (Figure 4b). Despite evidence of crystallinity from DSCmeasurements, X-ray diffraction of MD1-COOH did not yieldsufficient crystalline peaks for a meaningful interpretation (notshown). Exhaustive indexing of MD0-COOH yields a primitivemonoclinic crystal system with P21 (4) being the most likelyspace group, with the c dimension closely corresponding thelength of a single MD0-COOH molecule, 1.7 nm (Figure 5).The corresponding unit cell parameters are a = 1.4 nm, b =0.7 nm, c= 1.7 nm, R= 90�, β= 114.5�, and γ= 90�, and unitcell volume is 1596 nm3. Considering the measured density of1.12( 0.01 g/cm3, we can conclude the presence of fourmoleculesper unit cell, which yields the packing coefficient of 0.75.37Head-head packing configuration suggested here forMD0-COOHcanbe stabilized by hydrogen bonding between COOH groups andhydrophobic interactions between alkyl chains (Figure 5).
The packing for linear biothiphene molecules suggested hereclosely resembles that reported for 4-[[4-(alkoxy)phenyl]azo]ben-zoic acid.38 Both compounds contain a aromatic rigid backbonethat is capable of forming either a herringbone or a π-stack
structure with a hydrophobic alkyl chain on one end and ahydrophilic carboxylic group on the other end. A single-crystalX-ray diffraction study of 4-[[4-(alkoxy)phenyl]azo]benzoic acidsrevealed a low symmetry space group, P-1, belonging to thetriclinic crystal system.Surface Films fromMonodendrons. The solution of mono-
dendrons spin cast onto hydrophilic silicon substrates formedaggregates with facets forMD0-COOH,whereasMD0 formed auniform film (compare panels c and d of Figure 4). All othercompounds, i.e., the amphiphilic MDn-COOH and fully hydro-phobic MDn monodendrons, showed significant dewetting(Figure 6).
Closer examination of these surface films suggests that themultilayered structures is composed of globular aggregates resid-ing on an underlying monolayer (Figure 7). The thickness of theunderlying monolayer, also verified from AFM scratch test, waswithin 0.6-5.5 nm (Table 2). These values are within the height ofa single monodendron molecule estimated from the molecular
model (Figure 8). The effective thickness of these films rangedbetween 5 and 11 nm as measured with ellipsometry (Table 2).AFMheight images enabled direct measurement of the heights ofthe globular aggregates, which ranged from 4.5 to 325 nm,indicating the formation of bulk aggregates (Table 2).
The dewetting of the upper layer into globular aggregates can beassociated with aggregation, which minimizes the total surfaceenergy that implies weak intermolecular interactions between thesuccessive layers. It iswell-known that filmswith thickness less than100 nm might undergo three different stages of dewetting instabil-ities on solid substrates: (1) cylindrical holes are formed sporadi-cally throughout a smooth film; (2) the holes grow into rims, whicheventually turn into “cellular” structures; (3) and finally they turninto droplets.39 Apparently, the final stage of dewetting is observedfor thin surface films from our monodendrons. It is worth noting
Figure 6. Representative dark-field optical images of spin-cast nth generation carboxyl-terminated, MD1 andMD2, (a and c, respectively)and fully hydrophobic bithiophenesilane monodendrons, MD1-COOH and MD2-COOH (b and d, respectively). All images are of thesame magnification.
Table 2. Characteristics of Spin-Coated nth Generation Carboxyl-Terminated, MDn-COOH, and Fully Hydrophobic BithiophenesilaneMonodendrons, MDn, with n = 0-, 1, 2, and 3
that similar surface behavior was previously reported for otherdendritic thiophene derivatives.12c
The spin-coated film of 0th generation MD0-COOH mono-dendron showed a blue shift (Figure 9). In contrast, the UV/visabsorption spectra of the surface films of other MDn-COOH(n=1-3) monodendrons showed red-shifted maxima comparedto the dilute solution, indicating significant molecular aggrega-tion, as suggested fromAFMstudies (Table 1). Themost extremered shift is observed forMD1, indicating enhanced intermolecularinteractions and improved conjugation.40 Moreover, intermole-cular interactions caused the peak widths of the absorptionspectra towiden, and additional shoulder peaks are also observed(Figure 9). As a result of significant intermolecular interactionsand quenching, a relatively weak photoluminescence was ob-tained for these films with photoluminescence for MDn-COOHseries, following the same trend as MDn series.
Generally, this surface behavior corresponds to that discussedin a theoretical study on amphiphilic R,ω-substituted oligothio-phenes on hydrophilic and hydrophobic substrates, which con-cluded that the overall surface morphology is determined by thebalance of molecule-substrate and molecule-molecule interac-tions.41 Three possible scenarios were considered: (1) comparable
molecule-molecule and molecule-substrate interactions; (2) themolecule-molecule interaction is much stronger than the mole-cule-substrate interaction; and (3) the molecule-substrate inter-action is much stronger than the molecule-molecule interaction.In the first case, a uniform layered growth should be observed. Inthe second case, 3D layered island growth is observed. In the thirdcase, the first layer of molecules will lie parallel to the substrate,but the subsequent deposition should result in aggregation. Ourresults clearly show that the MD0-COOH monodendrons stu-died here fall under the second category. The much strongermolecule-molecule interactions led to their crystallization,resulting in a 3D aggregation. The higher generation monoden-drons fall under the third category; i.e., the first layer ofmoleculesforms a uniformmonolayer spread evenlyover the solid substrate,but there is no sufficient intermolecular energy to build the nextuniform molecular layer. As a result, the excess molecules ratherform globular aggregates, thus preventing the formation of thin(from tens to hundreds of nanometers in thickness) uniformsurface layers. Apparently, a similar situation is realized if surfacefilms of comparable thickness are fabricated with sequentiallayered deposition, as will be discussed in the next section.Langmuir Monolayers from COOH-Containing Mono-
dendrons.The isothermof the fully hydrophobicMDnmoleculesdid not show an increase in surface pressure when compressed(not shown). Only the carboxyl-terminated monodendronsformed a stable Langmuir monolayer at the air-water interfacebecause of the presence of the highly polar focal group(Figure 10a). The MDn-COOH series for n = 1-3 show theconventional Langmuir behavior that is characteristic for ba-lanced amphiphilic compounds.42 The Langmuir isotherms ofMDn-COOH monodendrons for n = 1-3 show an extendedplateau and collapsed at an intermediate surface pressure,between 10 and 20 mN/m (Figure 10a). The cusp on the isothermrepresents the transition from a strong elastic response to aviscous response.43 The overall shape of the Langmuir isothermswas similar to that reported for rigid third generation poly(benzylether) monodendrons with oligo(ethylene glycol) tails.44 Thederived areas per molecule from their Langmuir isotherms forMDn-COOHmonodendrons for n=1-3 are in the same rangewith photochromic amphiphilic azobenzenemonodendrons, withcarboxylic terminal group having the same number of alkyl tails(1, 2, 4, and 8).45 The surface area per molecule ranged from 0.24to 1.50 nm2 for these molecules.
The area permolecule derived from the tangent line of the slopethat lead to the collapsed point corresponds to the limitingmolecular area in the condensed state.26 It increases from 0.02to 0.94 nm2 from 0th to 3rd generation monodendrons(Figure 10a and Table 3). The difference in the surface area permolecule is significant between the first and secondgenerationbutis smaller between the second and third generation, i.e., 0.28versus 0.06 nm2. The area per molecule for MD0-COOH isinconsistent with even the most extreme scenario, where thethiophene rings adopt an edge-on orientation on the water sur-face, which should occupy a minimum area of about 0.2 nm2.32
Thus, we concluded that, for this compound, the unrealisticlimiting area per molecule indicates strong aggregation behaviorwith the formation of bulky aggregates (Figure 10a).
Figure 7. Representative AFM topography of spin-cast (a and b)MD1 andMD1-COOH, (c and d)MD2 andMD2-COOH, and(e and f) MD3 and MD3-COOH. The Z scale is 300 nm.
(40) Dingemans, T. J.; Bacher, A.; Thelakkat, M.; Pedersen, L. G.; Samulski, E.T.; Schmidt, H.-W. Synth. Met. 1999, 105, 171.(41) Surin, M.; Leclere, P.; De Feyter, S.; Abdel-Mottaleb, M. M. S.; De
Schryver, F. C.; Henze, O.; Feast, W. J.; Lazzaroni, R. J. Phys. Chem. B 2006,110, 7898.
(42) Dos Santos, C. G.; de Melo, C. P.; Souto Maior, R. Synth. Met. 1995, 71,2083.
(43) Deng, J.; Hottle, J. R.; Polidan, J. T.; Kim, H.-J.; Farmer-Creely, C. E.;Viers, B. D.; Esker, A. R. Langmuir 2004, 20, 109.
(44) Kampf, J. P.; Frank, C. W.; Malmstr::om, E. E.; Hawker, C. J. Langmuir
1999, 15, 227.(45) Genson, K. L.; Holzmuller, J.; Villancencio, O. F.; McGrath, D. V.;
Vaknin, D.; Tsukruk, V. V. J. Phys. Chem. B 2005, 109, 20393.
Molecular modeling of monodendrons in possible conforma-tions with branches predominantly oriented vertically and mini-mized energy predicts a slightly larger area per molecule, from0.39 to 2.3 nm2 than that observed experimentally (Figure 8 andTable 3). This suggests that themolecules are in amore condensedstate within a dense Langmuir monolayer than that presented inFigure 8. The increasing limiting surface area per molecule withincreasing monodendron generation contradicts that estimatedfrom the corresponding molecular models (Table 3). The devia-tion from linearity in the area per molecule may be explained bythe increasing role of intramolecular interaction with increasing
monodendron generation, because of crowding of peripheralgroups. This can be seen from the plot of surface pressure versusarea per bithiophene fragments for the compression-decompres-sion cycle, which reveals that the value of per bithiophenefragment area per molecule indeed decreases with increasingmonodendron generation (Figure 10b). The graph also showsthat within a constant surface pressure P= 5 mN/m for MDn-COOH, n = 1-3, the isotherms are nearly reversible, indicatingthat the monolayers are compressible and capable of recoveryduring decompression because of the presence of the siliconspacer atoms.
Figure 8. Molecular models of MDn-COOH molecules, for n= 0-3: (top row) top view and (bottom row) side view.
Figure 9. UV/vis absorption spectra of spin-cast films: (a) MDn and (b) MDn-COOH, with n=0-3. Graphs have been offset for clarity.
Figure 10. (a) Langmuir isotherms of carboxyl-terminated MDn-COOH monodendrons with n = 0-3. (b) Reversibility of Langmuirisotherms for MDn-COOH with n= 1-3.
Surface Morphologies and Optical Properties of Lang-muirMonolayers. The transfer ratio onto a silicon substrate forMD0-COOH monolayers was low (0.2-0.3), indicating theabsence of uniformmonolayers.46 Optical microscopy confirmedthat MD0-COOH formed 3D aggregates even at very lowsurface pressure, P = 0.5 mN/m (Figure 11a). The height ofthese aggregates determined fromAFM images was about 60 nm,indicating their multilayer nature (Figure 11c). The aggregatesformed sporadically, wherever nucleation sites happen, but theytypically initiate along the edges of the LB trough. Uponcompression, the number of nucleation sites increased; hence,denser aggregates are formed. At high surface pressure, highaspect ratio needle-like aggregates have been seen in optical andAFM images (Figure 11b). For high-concentration MD0-COOH solution (2.3 mg/mL, instead of 0.5 mg/mL), theseaggregates formed as soon as the chloroform evaporated. Incomparison to the crystals seen in spin-casted film, themajority ofthese crystals possess one-dimensional shape (compare Figure 4cto Figure 11b).
The X-ray diffraction peaks for these MD0-COOH aggre-gates occurred at 2θ= 5.8�, 10.7�, 17.4�, and 24.1� (Figure 11d).They correspond to the primary spacing of 15.2 A and higherorders at 8.2, 5.01, and 3.7 A, respectively. These spacings followclosely to the (00l) indexing from a layered packing with 1.5 nm
thickness, which is very close to but slightly lower than the lengthof the extended molecules, indicating either tilting or partialinterdigitation of molecules. Indeed, it has been demonstratedon many occasions that the rod-like molecules with alkyl tailstend to form layered structures with the alkyl tails, which caneither form interdigitated packing or skewed packing in thec direction.25,47,48
The blue shift in the maximum UV/vis absorption spectrumwas observed for the needle crystals from 342 nm inTHF solutionto 335 nm (Figure 12a). This shift suggests that the thiophenemolecules in the (a, b) plane are arranged into a herringbonestructure rather than a direct π-π-stack arrangement of back-bones (inset in Figure 11d).49,50 The intensity of the main peakincreases and the intensity of the secondary peak at 270 nmdecreases, indicating the dominance of bithiophene absorption ina solid monolayer state. A theoretical study on thiophene dimerinteractions by Tsuzuki et al. concluded that, for unsubstitutedandR-substituted thiophenes, herringbone arrangement is always
Table 3. Langmuir Isotherms Data for nth Generation Carboxyl-Terminated, MDn-COOH, with n = 1, 2, and 3
monolayer thickness (nm)
sample namesarea per
molecule (nm2)estimated area
per molecule (nm2) P = 0.5 mN/m P = 5 mN/m P = 20 mN/m
Figure 11. Fluorescence images of LB-depositedMD0-COOHmonodendron at (a)P=0.5mN/mand (b)P=20mN/m (λexc=360 nm).(c) AFM topographical image of b, z=200 nm. (d) X-ray diffraction ofMD0-COOHLB film at P=20mN/m. The inset is the proposedlamella arrangement of MD0-COOHmolecules.
(46) Li, B.; Marand, H.; Esker, A. R. J. Polym. Sci., Part B: Polym. Phys. 2007,45, 3300.
(47) (a) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126, 4318.(b) Collard, D. M.; Stoakes, M. S Chem. Mater. 1994, 6, 850.
(48) (a) Sidorenko, A.;Houphouet-Boigney, C.; Villavicencio, O.; Hashemzadeh,M.McGrath, D. V.; Tsukruk, V. V. Langmuir 2000, 16, 10569. (b) Tsukruk, V. V.;Luzinov, I.; Larson, K. J. Mater. Sci. Lett. 2001, 20, 873.
(49) Zhang, X.; Johnson, J. P.; Kampf, J.W.;Matzger, A. J.Chem.Mater. 2006,18, 3470.
(50) Yamamoto, T.; Komarudin, D.; Arai, M.; Lee, B.-L.; Saganuma, H.;Asakawa, N.; Inoue, Y.; Kubota, K.; Sasaki, S.; Fukuda, T.; Matsuda, H. J. Am.Chem. Soc. 1998, 120, 2047.
preferred over π-π stacking because of much larger interactionenergy between perpendicular thiophene dimers than parallelthiophene dimers.51 Moreover, π-π stacking is preferred for
the β substituent because it destabilizes perpendicular dimers.Therefore, the proposed molecular arrangement for the R-sub-stituted MD0-COOH molecules derived from X-ray, UV/vis,and photo-emission data is depicted in the inset of Figure 11d. Itshows a lamellar arrangement, where the thiophenemolecules are
Figure 12. Spectra of the LB film of MD0-COOH at P= 20 mN/m and in dilute THF solution: (a) UV/vis and (b) fluorescence.
Figure 13. (a-i) LBmonolayer of (a, d, and g)MD1-COOH (b, e, and h)MD2-COOH, and (c, f, and i)MD3-COOHbithiophenesilanesat different surface pressures, P= 0.5 mN/m (a-c), 5 mN/m (d-f), and 20 mN/m (g-i). The Z scale is 5 nm for a-f and 100 nm for g-i.
(51) Tsuzuki, S.; Honda, K.; Azumi, R. J. Am. Chem. Soc. 2002, 124, 12200.
non-coplanar. Similar to its bulk structure, both the presence of acrystallizable alkyl chain and hydrogen bonding between thecarboxylic groups on the other end of the bithiophene moleculescould promote alternating hydrophilic/hydrophobic packing.
Fluorescence measurement for these crystals shows that theemission peak red shifts in the solid state with respect to the diluteTHF solution case, i.e., 421 nm in the solid state versus 406 nm ina dilute THF solution (Figure 12b). Additionally, the emissionpeak is narrower in the solid state and is accompanied with theappearance of a shoulder at 443 nm. The red shift of the emissionmaximum may indicate that the molecules are distorted in thesolid state to accommodate planar lamellar structure. A secondabsorption peak is obviously red-shifted in the solid state at283 nm. The more intense absorption peak corresponds to theabsorption peak because of bithiophene units, and the less intenseabsorption peak corresponds to a single thiophene unit. For thesingle thiophene unit, the absorption peak does not depend uponthe solvent used and occurs at 238 nm.22 The red shift corrobo-rates with the increased planarization of the bithiophene unit thatfacilitates energy transfer.
Unlike linear molecule (0th generation), all branched COOH-containing molecules formed uniform Langmuir monolayers onsolid substrates (Figure 13). Representative surface morphologiesof Langmuir monolayers forMDn-COOH (n=1-3) depositedatP=0.5, 5, and 20mN/m showed similar trends over the rangeof surface pressures. The highest surface pressure,P=20mN/m,represents the surface morphologies following the cusp transitionfor MD1-MD3-COOH molecules. At a low surface pressure,P = 0.5 mN/m, very smooth and uniform monolayers wereobtainedwith root-mean-square (rms) surface roughness of about0.15 nm2 within a 1 � 1 μm2 area. The thickness of thesemonolayers is close to the height of a single MDn-COOHmolecule (between 1.5 and 2.9 nm) (Table 3 and Figure 8). Uponfurther compression to P = 5 mN/m, the monolayer thicknessincreased slightly, up to 3.4 nm, which can be attributed to theincreasing molecular density and further vertical alignment ofthiophene branches. Finally, at P = 20 mN/m (above thetransition on the isotherm), it became clear that we observe theonset of the formation of the globular aggregates as a result of theprecollapse of the monolayers with a significant increase in rmssurface roughness to approximately 10 nm over a 1 � 1 μm2 area(Figures 10a and 13).
UV/vis spectroscopy allowed us to follow and confirm changesin molecular packing discussed above (photoluminescence inten-sitywas tooweak to be unambiguously recorded). Indeed,UV/visspectra of MDn-COOH molecules deposited at several repre-sentative surface pressures, P = 0.5, 5, and 20 mN/m, showed asystematic increase in the absorbance intensity (up to 7-fold forthe highest pressure), indicating the densification of the mono-layer packing. As a result of the densification, the UV/visabsorption peak of the monolayer LB film is more red-shift thanthe spin-coated film (compare Table 1 to Table 4). The densifica-tion of the monolayer packing is confirmed from the increasingmonolayer thickness and decreasing surface area available permolecules (Figure 14 and Table 4), as well as the shift in the UV/vis absorbance peak, as observed with an increasing surface
pressure (Figure 14). In all cases, the UV/vis absorbance peakfor all generations at the lowest and medium surface pressure issignificantly red-shifted as compared to isolated molecules insolution from 335 to 339-352 nm (Table 4). This shift confirmsthat these MDn-COOH (n = 1-3) molecules show enhancedintermolecular interaction of thiophene branches in the solid
Table 4. UV/Vis for LB-Deposited MDn-COOH Monolayers, for n = 1-3, at Various Surface Pressures
state. At P = 20 mN/m, following monolayer collapse, theposition of UV absorption maximum for MD1-COOH signifi-cantly red shifts even more, but for MD2-COOH and MD3-COOH, it is maintained (Table 4). At this point, the globularaggregates observed for collapsed monolayers are composed ofweakly interacting MDn-COOH molecules in random confor-mation (Figure 14). A significant shift in the UV/vis absorptionpeak is usually observed for π-conjugated polymer systems thatexhibit strong intermolecular interactions as a result of planariza-tion of the molecular backbones organized in close π-π stacks inthe condensed, highly crystalline state.52 However, a modest redshift observed in the case of condensed Langmuir monolayer(within 4-17 nm) indicates only modest densification of thio-phene branches with the unlikely formation of π-π stacks and,thus, very limited or no intramonolayer crystallization. In con-trast to the 0th generation monodendron, the photo-emissionmeasurement unfortunately did not show discernible spectra forthe high generation monodendrons in the solid state, MDn-COOH (n=1-3). This is consistentwith the spin-coated films, asdiscussed earlier.
Finally, LB deposition of COOH-containing monodendronsresulted in consistent linear thickness growth only for MD2-COOH and MD3-COOH generations, and the subsequentlayers were not transferrable for MD1-COOH when multi-layered deposition was attempted (Figure 15 and Table 5). Theincrement per layer is close to the molecular dimension (∼3 nm),indicating a consistent amount of materials added at eachdeposition cycle. However, a dramatic increase in surface micro-
roughness indicates significant dewetting because of the poorintermolecular interactions between successive layers.
Conclusions
In conclusion, we observed that the fluorescence peak ofMDnmonodendrons in dilute solution progressively blue shift withincreasing generation, indicating a somewhat distorted packing ofbranched thiophene fragments with an increasing number ofjunctions. In contrast, functionalized MDn-COOH monoden-drons showed a progressive red shift with the increasing genera-tion number, indicating gradual domination of σ-π interactionsand increasing the conjugation length. Moreover, the introduc-tion of a carboxylic group resulted in the formation of a highlycrystalline state of linearmolecule (0th generation) with separatedalkyl tail-thiophene packing, resulting in limited π-π interac-tions, and thus,MD0-COOHmonodendrons exhibit discerniblephotoluminescence in the solid state. On the other hand, thephotoluminescence measurement of all other monodendron deri-vatives showed nondiscernible spectra in the solid state.
Figure 15. AFM topographical images ofMDn-COOHLB films (n=1-3) at P=5mN/m, for (a) 1 layer, (b) 5 layers, and (c) 11 layers.The Z scale is 200 nm.
Table 5. Measured Film Thickness forMultilayer MDn-COOHLBFilms
We demonstrated that adding branched thiophene fragmentsin the COOH-containing compounds resulted in achievinghydrophobic-hybrophilic balance sufficient to form stable,uniform, and elastic Langmuir monolayers with the thick-ness of 2-3 nm at the air-water at a modest surface pressure(<10 mN/m) easily transferrable to a solid substrate. However,further increasing the thickness of the surface layers and thefabrication of ultrathin films (from tens to hundreds of nan-ometers) via multilayer LB deposition or spin casting is limitedby the dewetting and the formation of globular surface aggre-gates on top of the first monolayer because of strong intermo-lecular interactions. A modest red shift observed for condensedLangmuir monolayers (below 18 nm) indicates minor densi-fication of thiophene branches and limited intramonolayercrystallization and, thus, potential for the preservation of highphotoluminescence. In contrast, thicker surface films showed a
very significant red shift, confirming a dense molecular packingwith strong π-π interactions, which results in strong photo-luminescence quenching.
Acknowledgment. This work is supported by NSF-DMRGrant 0756273 and RFBR Grant 07-03-01037. Authors thankDr.N.M. Surin (ISPMRAS) forUV/vis investigations, Dr.M. I.Buzin (INEOS RAS) for DSC measurements, and Drs. A. P.Pleshkova (INEOS RAS) and I. Leschiner (Chemistry Depart-ment of MSU) for MALDI-TOF measurements.
Supporting Information Available: DSC curves for MDnand MDn-COOH (Figure S1), DSC curves for MD0 andMD0-COOH (Figure S2), and TOPM images of MD0-COOH (Figure S3). This material is available free of chargevia the Internet at http://pubs.acs.org.
