The Role of Intrachain and Interchain Interactions of Regioregular Poly

Aug 26, 2008 - Department of Chemistry, UniVersity of Patras, Patras 26500, Greece, and Foundation for Research and. Technology Hellas, Institute of ...
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Langmuir 2008, 24, 11103-11110

11103

The Role of Intrachain and Interchain Interactions of Regioregular Poly(3-octylthiophene) Chains on the Optical Properties of a New Amphiphilic Conjugated Random Copolymer in Solution Andreas A. Stefopoulos,†,‡ Christos L. Chochos,*,†,§ Georgios Bokias,† and Joannis K. Kallitsis†,‡ Department of Chemistry, UniVersity of Patras, Patras 26500, Greece, and Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Processes (FORTH-ICEHT), P.O. Box 1414, Patras 26500, Greece ReceiVed April 15, 2008. ReVised Manuscript ReceiVed July 7, 2008 The synthesis of a random copolymer through free radical copolymerization of a properly vinyl monofunctionalized regioregular poly(3-octylthiophene) (rrP3OT) macromonomer and N,N′-dimethylacrylamide (DMAM) is presented. The optical properties of the copolymer in water and in several organic solvents of varying polarity, as well as in THF/water and THF/methanol mixtures, were explored using UV-vis and photoluminescence spectroscopy. It is demonstrated that the rrP3OT chains adopt a coil conformation in solvents such as tetrahydrofuran (THF) and chloroform with the appearance of the absorption and emission maxima at 439 and 565 nm, respectively. On the contrary, the rrP3OT chains are organized on a single chain packing form (intrachain interactions) in polar solvents such as ethanol and methanol, as it is verified with the observation of the characteristic three vibronic features of the absorption spectra of the copolymer with maxima at 513, 550, and 603 nm and emission maxima at 560 nm. However, when water is used as solvent, the rrP3OT chains self-assemble into a stacklike structure due to the increased interchain interactions, as confirmed by the different aggregation process of the rrP3OT chains in the THF/water mixture, the broader absorption spectrum in water compared to those recorded in ethanol and methanol, and the 80 nm red-shifted emission maximum, centered at 640 nm.

1. Introduction With the introduction of π-conjugated systems in electronic devices, the detailed understanding of the supramolecular interactions between the individual π-conjugated molecules has become one of the most challenging scientific research areas. A crucial prerequisite for high-performance devices is an efficient control over the ordering of the semiconducting polymers in the solid state.1 Thus, molecular self-assembling processes of the conjugated polymers preferably from solution leading to ordered structures are highly desired. Such organizational phenomena are often supported by preorganization in solution. Hence, the understanding and control of aggregation processes of conjugated polymers in solution are important steps toward the design of molecular electronics by self-organization. Regioregular poly(3-alkylthiophene)s (rrP3ATs) are among the most widely studied conducting polymers, having potential applications for fabrication of light-emitting diodes,2 thin film transistors,3 chemical sensors,4 and plastic solar cells.5 It is known that the optical and electronic properties of the rrP3ATs are * To whom correspondence should be addressed. E-mail: chochos@ chem.demokritos.gr. Telephone: (+30)210-6503667. † University of Patras. ‡ Institute of Chemical Engineering and High Temperature Processes (FORTH-ICEHT). § Present Address: National Center for Scientific Research “Demokritos”, Institute of Physical Chemistry, Neapoleos Aghia Paraskevi, 15310 Athens, Greece. (1) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491. (2) (a) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123. (b) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J. B. P. H.; Kuijk, K. E.; Hart, C. M.; Cantatore, E.; Herwig, P. T.; van Breemen, A. J. M. M.; de Leeuw, D. M. Nature 2001, 414, 599. (3) (a) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 174. (b) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685.

influenced by their self-organizing properties6 which in turn are affected by the processing conditions and molecular weight.7 rrP3ATs generally adopt a coil (planar) conformation in solution, while their self-organization in the solid state involves a high degree of intrachain order (main chains align parallel to each other; single chain polymer packing) and subsequently a formation of a two-dimensional π-stacked lamellar structure due to increased interchain interactions.3b,6e However, the exact structures of the supramolecular assemblies of substituted regioregular polythiophenes or oligothiophenes in a particular environment is a topic of extensive research efforts.8 Additionally, another issue concerning the rrP3ATs is the influence of the intrachain and interchain interactions on the optical properties of the materials, which still remains a subject of debate.9 Despite that, it is well recognized that the absorption and emission maxima of the (4) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (b) Barbarella, G.; Zambianchi, M.; Pudova, O.; Paladini, V.; Ventola, A.; Cipriani, F.; Gigli, G.; Cingolani, R.; Citro, G. J. Am. Chem. Soc. 2001, 123, 11600. (c) Bernier, S.; Garreau, S.; Bera-Aberem, M.; Gravel, C.; Leclerc, M. J. Am. Chem. Soc. 2002, 124, 12463. (5) (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (b) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197. (c) Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D. L. Appl. Phys. Lett. 2007, 90, 163511. (6) (a) Faid, K.; Frechette, M.; Ranger, M.; Mazerolle, L.; Levesque, I.; Leclerc, M. Chem. Mater. 1995, 7, 1390. (b) Yue, S.; Berry, G. C.; McCullough, R. D. Macromolecules 1996, 29, 933. (c) Yang, C.; Orfino, F. P.; Holdcroft, S. Macromolecules 1996, 29, 6510. (d) Yamamoto, T.; Komarudin, D.; Arai, M.; Lee, B.-L.; Suganuma, H.; Asakawa, N.; Inoue, Y.; Kubota, K.; Sasaki, S.; Fukuda, T.; Matsuda, H. J. Am. Chem. Soc. 1998, 120, 2047. (e) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2000, 39, 2679. (f) Garreau, S.; Leclerc, M.; Errien, N.; Louarn, G. Macromolecules 2003, 36, 692. (g) Kiriy, N.; Jahne, E.; Alder, H.-J.; Schneider, M.; Kiriy, A.; Gorodyska, G.; Minko, S.; Jehnichen, D.; Simon, P.; Fokin, A. A.; Stamm, M. Nano Lett. 2003, 3, 707. (7) (a) Zen, A.; Saphiannikova, M.; Neher, D.; Grenzer, J.; Grigorian, S.; Pietsch, U.; Asawapirom, U.; Janietz, S.; Scherf, U.; Lieberwirth, I.; Wegner, G. Macromolecules 2006, 39, 2162. (b) Kline, R. J.; McGehee, M. D. Polym. ReV. 2006, 46, 27.

10.1021/la801178m CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

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rrP3ATs in solution are centered between 440 and 450 nm and at 565-570 nm, respectively,10 while a vibronic feature with the appearance of the characteristic three absorption peaks (electronic transitions) at higher wavelengths and an emission maximum at ∼660 nm are presented in the solid state.5b,11 Until now, side-chain functionalization, with the use of various organic groups12 or end modified alkyl side chain groups13 or functionalization of the 4-position of the 3-alkylthiophene ring,14 has been demonstrated to be the most common method to alter the physical and electronic properties of the rrP3ATs or even to prepare water-soluble polythiophene derivatives.8e,12f,g On the other hand, another interesting approach is the end group modification of the rrP3ATs and the further modulation of their properties upon reacting with different parts or monomers in order to achieve materials with the desired properties. Limited research has been done in this area though, because of the limitations of the procedures published to date for the end functionalization of the conjugated polymers. However, the intensive research effort of McCullough et al. in the area of the rrP3ATs opened new ways toward the easy preparation of end functionalized rrP3ATs15 that can be further integrated into more complex polymeric architectures.16 In this work, we propose a simple synthetic approach toward the preparation of copolymers based on end functionalized rrP3ATs. The preparation of a vinyl monofunctionalized poly(8) (a) Apperloo, J. J.; Janssen, R. A. J.; Malenfant, P. R. L.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001, 123, 6916. (b) Schenning, A. P. A. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavallini, M.; Cooper, H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Lecle`re, P.; McDonell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002, 124, 1269. (c) Westenhoff, S.; Abrusci, A.; Feast, W. J.; Henze, O.; Kilbinger, A. F. M.; Schenning, A. P. H. J.; Silva, C. AdV. Mater. 2006, 18, 1281. (d) Henze, O.; Feast, W. J.; Gardebien, F.; Jonkheijm, P.; Lazzaroni, R.; Lecle`re, P.; Meijer, E. W.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2006, 128, 5923. (e) Tu, G.; Li, H.; Forster, M.; Heiderhoff, R.; Balk, L. J.; Sigel, R.; Scherf, U. Small 2007, 3, 1001. (f) Ellinger, S.; Kreyes, A.; Ziener, U.; Hoffmann-Richter, C.; Landfester, K.; Moller, M. Eur. J. Org. Chem. 2007, 5686. (g) Pan, H.; Liu, P.; Li, Y.; Wu, Y.; Ong, B. S.; Zhu, S.; Xu, G. AdV. Mater. 2007, 19, 3240. (h) Curtis, M. D.; Nanos, J. I.; Moon, H.; Jahng, W. S. J. Am. Chem. Soc. 2007, 129, 15072. (i) Mena-Osteritz, E. AdV. Mater. 2002, 14, 609. (9) (a) Sakurai, K.; Tachibana, H.; Shiga, N.; Terakura, C.; Matsumoto, M.; Tokura, Y. Phys. ReV. B 1997, 56, 9552. (b) Brown, P. J.; Sirringhaus, H.; Harrison, M.; Shkunov, M.; Friend, R. H. Phys. ReV. B 2001, 63, 125204. (c) Jiang, X. M.; ¨ sterbacka, R.; Korovyanko, O.; An, C. P.; Horovitz, B.; Janssen, R. A. J.; O Vardeny, Z. V. AdV. Funct. Mater. 2002, 12, 587. (d) Brown, P. J.; Thomas, D. S.; Ko¨hler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Phys. ReV. B 2003, 67, 064203. (e) Kobayashi, T.; Hamazaki, J.-I.; Kunugita, H.; Ema, K.; Endo, T.; Rikukawa, M.; Sanui, K. Phys. ReV. B 2003, 67, 205214. (f) Spano, F. C. J. Chem. Phys. 2005, 122, 234701. (g) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Phys. ReV. Lett. 2007, 98, 206406. (10) (a) Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233. (b) McCullough, R. D. AdV. Mater. 1998, 10, 93. (11) Kim, Y.; Cook, S.; Kirkpatrick, J.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Heeney, M.; Hamilton, R.; McCulloch, I. J. Phys. Chem. C 2007, 111, 8137. (12) (a) van Hutten, P. F.; Gill, R. E.; Herrera, J. K.; Hadziioannou, G. J. Phys. Chem. 1995, 99, 3218. (b) Levesque, I.; Leclerc, M. Chem. Mater. 1996, 8, 2843. (c) Levesque, I.; Bazinet, P.; Roovers, J. Macromolecules 2000, 33, 2952. (d) Hong, X. M.; Collard, D. M. Macromolecules 2000, 33, 6916. (e) Sakurai, S.-I.; Goto, H.; Yashima, E. Org. Lett. 2001, 3, 2379. (f) de Boer, B.; van Hutten, P. F.; Ouali, L.; Grayer, V.; Hadziioannou, G. Macromolecules 2002, 35, 6883. (g) Brustolin, F.; Goldoni, F.; Meijer, E. W.; Sommerdijk, N. A. J. M. Macromolecules 2002, 35, 1054. (13) (a) Bjornholm, T.; Greve, D. R.; Reitzel, N.; Hassenkam, T.; Kjaer, K.; Howes, P. B.; Larsen, N. B.; Bogelund, J.; Jayaraman, M.; Ewbank, P. C.; McCullough, R. D. J. Am. Chem. Soc. 1998, 120, 7643. (b) Yu, J.; Holdcroft, S. Macromolecules 2000, 33, 5073. (c) Stokes, K. K.; Heuze, K.; McCullough, R. D. Macromolecules 2003, 36, 7114. (d) Mattu, J.; Johansson, T.; Holdcroft, S.; Leach, G. W. J. Phys. Chem. B 2006, 110, 15328. (14) (a) Li, Y.; Vamvounis, G.; Holdcroft, S. Macromolecules 2001, 34, 141. (b) Li, Y.; Vamvounis, G.; Yu, J.; Holdcroft, S. Macromolecules 2001, 34, 3130. (c) Li, Y.; Vamvounis, G.; Holdcroft, S. Macromolecules 2002, 35, 6900. (d) Chochos, C. L.; Economopoulos, S. P.; Deimede, V.; Gregoriou, V. G.; Lloyd, M. T.; Malliaras, G. G.; Kallitsis, J. K. J. Phys. Chem. C 2007, 111, 10732. (e) Economopoulos, S. P.; Chochos, C. L.; Gregoriou, V. G.; Kallitsis, J. K.; Barrau, S.; Hadziioannou, G. Macromolecules 2007, 40, 921. (15) (a) Jeffries-EL, M.; Sauve, G.; McCullough, R. D. Macromolecules 2005, 38, 10346. (b) Jeffries-EL, M.; Sauve, G.; McCullough, R. D. AdV. Mater. 2004, 16, 1017.

Stefopoulos et al. Scheme 1. Synthetic Route Toward the Preparation of the Vinyl-Functionalized Regioregular Poly(3-octylthiophene) Macromonomer (P1) and the Amphiphilic Conjugated Copolymer (P2)

(3-octylthiophene) macromonomer (rrP3OT; P1) was accomplished, that can subsequently be copolymerized with vinyl monomers, such as N,N′-dimethylacrylamide (DMAM), through conventional free radical copolymerization in common organic solvents, providing a new amphiphilic conjugated random copolymer (P2) (Scheme 1). The versatility of this method relies on the variety of many commercially available functional vinyl monomers that can be used as comonomers for the radical copolymerization, providing copolymers based on rrP3ATs, with different functions and properties. As our first target was the preparation of a water-soluble product, we have chosen to incorporate a low fraction of a rather low molecular weight rrP3OT chain in the copolymer synthesized. To our knowledge, no reports exist on the preparation of comblike copolymers based on a hydrophilic backbone, such as PDMAM, and pendent regioregular poly(3-alkylthiophene) chains, although the synthesis of products based on a hydrophobic backbone containing styrene or chloromethyl styrene units has been recently reported.16d,g The conformational and optical behavior of the hydrophobic conjugated rrP3OT chain coupled with the highly polar nonionic DMAM unit in solvents of varying polarity has attracted our interest in the present work, as the final copolymer P2 was found to present excellent solubility both in water and in a large variety of common organic solvents (tetrahydrofuran, chloroform, dichloroethane, ethyl acetate, acetone, ethanol, methanol). The optical properties of P2 in solution were investigated by means of UV-vis and photoluminescence spectroscopy, and they mainly result from a different preassembly of the rrP3OT chains. The solvent-selective optical response (both UV-vis and photoluminescence) indicates the existence of different kinds of preorganization of the rrP3OT chains. Finally, the influence of the intrachain and interchain interactions of the rrP3OT chains on the optical properties of the copolymer is analyzed.

2. Experimental Section Instrumentation and Measurements. The structures of the synthesized materials were clarified by 1H NMR spectroscopy with a Bruker Avance DPX 400 MHz spectrometer. Gel permeation chromatography (GPC) measurements were carried out using a polymer laboratory chromatographer with two Ultra Styragel linear columns (104, 500 A), UV detector polystyrene standards, and CHCl3 as eluent, at 25 °C with a flow rate of 1 mL/min. UV-vis spectra were recorded using a Hewlett-Packard 8452A Diode array. Continuous wave photoluminescence was measured on a PerkinElmer LS45 spectrofluorometer. The concentration of P2 for the solvatochromic study was fixed at 0.5 mg/mL (0.1 µM in rrP3OT chains, based on the 1H NMR results), while for the rest studies (solvent mixtures, thermochromic properties) the concentration of

Copolymers Based on rrP3ATs P2 was fixed at 0.3 mg/mL (0.06 µM in rrP3OT chains). The excitation wavelength was set at the absorption maximum (440 or 513 nm) in the different solvents, while for the solvent mixtures it was kept fixed at 440 nm. For the determination of the fluorescence quantum yield of P2 in THF and methanol, a solution of quinine sulfate in 0.1 N H2SO4 with a quantum yield of Φs ) 0.54 ( 0.02 was used as a reference.17 Monomer and Polymer Synthesis. All the solvents and reagents were purchased from Aldrich and used without further purification unless otherwise stated. All reactions were run under inert atmosphere (argon). Tetrahydrofuran (THF) was distilled with benzophenone and metallic sodium. 2,5-Dibromo-3-octylthiophene14d and 4-vinylphenylboronic acid M118 were synthesized according to published procedures. The H/Br rrP3OT was synthesized based on synthetic procedures reported in the literature,19 and the synthesis involves the Ni-catalyzed chain-growth polymerization of 2-bromo-3-octylthiophene-5-magnesium bromide. The crude polymer was purified by Soxhlet extraction with solvents of an increased solubility for P3OT (methanol, acetone, hexane, and chloroform). The polymer used in this work was isolated from the hexane extraction (Yield: 34% for the hexane fraction). 1H NMR (400 MHz, CDCl ): δ 0.89 (t, 3H), 1.29 (m, 8H), 1.40 3 (m, 2H), 1.69 (t, 2H), 2.80 (t, 2H), 6.98 (s, 1H). GPC: Mn, 4350; PDI, 1.6. 1H NMR: DPn, 13; Mn, 2600. Synthesis of Vinylphenyl Terminated rrP3OT (P1). rrP3OT (100 mg), 4-vinylphenylboronic acid (M1) (80.7 mg, 0.55 mmol), and 12.6 mg of tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] were placed together in a reaction flask. The flask was degassed and filled with argon several times. THF (10 mL) and 2 M Na2CO3 (0.6 mL) were added, and the mixture was heated at reflux for 48 h under argon atmosphere. The mixture was poured into methanol to precipitate the crude polymer. The polymer was filtered, collected, and washed by Soxhlet extraction with methanol, acetone (in order to remove the excess of M1), and chloroform. The macromonomer P1 (91 mg) was collected from the chloroform extraction in quantitative yields. 1H NMR (400 MHz, CDCl ): δ 0.89 (t, 3H), 1.29 (m, 8H), 1.40 3 (m, 2H), 1.69 (t, 2H), 2.80 (t, 2H), 6.98 (s, 1H). GPC: Mn. 5200; PDI, 1.7. 1H NMR: DPn, 13; Mn, 2626. Synthesis of the Amphiphilic Copolymer (P2). A solution of P1 (20 mg, 7.6 µmol, based on the Mn value found from the 1H NMR characterization for the rrP3OT chains), DMAM (0.4 mL, 3.9 mmol), and AIBN (0.5 mg in 0.5 mL THF) in dry THF (3 mL) was degassed and flushed with argon three times. The mixture was heated at 80 °C under stirring in a sealed tube for 3 days. After cooling to room temperature, the reaction mixture was precipitated in hexane (60 mL) and the precipitate was filtered and collected. The copolymer was dissolved in water, in order to remove any excess of P1, and filtered, and the filtrate was evaporated under vacuum. The red solid formed was dried under high vacuum for 48 h, affording 350 mg of the desired compound.

3. Results and Discussion 3.1. Synthesis. Our primary target was to synthesize the vinyl monofuctionalized rrP3OT macromonomer. We used a two-step process slightly different from the one reported by McCullough et al.15 for the end functionalization of rrP3ATs, similar to a (16) (a) Liu, J.; Sheina, E.; Kowalewski, T.; McCullough, R. D. Angew. Chem. 2002, 114, 2002. (b) Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2004, 126, 6550. (c) Radano, C. P.; Scherman, O. A.; Stingelin-Stutzmann, N.; Muller, C.; Breiby, D. W.; Smith, P.; Janssen, R. A. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 12502. (d) Sivula, K.; Ball, Z. T.; Watanabe, N.; Fre´chet, J. M. J. AdV. Mater. 2006, 18, 206. (e) Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.; Kuckling, D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626. (f) Dai, C.-A.; Yen, W.-C.; Lee, Y.-H.; Ho, C.-C.; Su, W.-F. J. Am. Chem. Soc. 2007, 129, 11036. (g) Chen, X.; Gholamkhass, B.; Han, X.; Vamvounis, G.; Holdcroft, S. Macromol. Rapid Commun. 2007, 28, 1792. (17) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (18) (a) Dondoni, A.; Ghiglione, C.; Marra, A.; Scoponi, M. J. Org. Chem. 1998, 63, 9535. (b) Economopoulos, S. P.; Andreopoulou, A. K.; Gregoriou, V. G.; Kallitsis, J. K. Chem. Mater. 2005, 17, 1063.

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work presented recently.20 First, we synthesized and isolated the H/Br terminated rrP3OT (Scheme 1) as described in the Experimental Section. The 1H NMR spectrum of rrP3OT is presented in Figure 1a, which allows the accurate determination of the molecular weight of the rrP3OT based on the integration of the peak at 2.8 ppm, assigned to the a-methylene protons of the octyl groups and the broad peak at 2.6-2.5 ppm, assigned to the methylene protons on the first carbon substituent (b and b′) on the end units. For example, the number of the repeat units n for the rrP3OT is equal to 13, corresponding to Mn ) 2600. This molecular weight is lower than that obtained from the GPC measurements (Mn ∼ 4350), in agreement with previous works.15 Afterward, a palladium-mediated Suzuki coupling reaction21 was performed between rrP3OT and M1 for the preparation of the vinylphenyl terminated rrP3OT (P1) macromonomer (Scheme 1). Despite our efforts, we were not able to detect the characteristic peaks of the vinyl protons in the 1H NMR spectra of P1. However, in a parallel work of our group,22 when we have made the Suzuki coupling reaction between the same rrP3OT precursor and the 2-(4-boronic acid phenoxy)tetrahydro-2H-pyran, the 1H NMR spectrum was in agreement with the proposed structure, showing the characteristic peaks both of the rrP3OT and the phenyltetrahydropyranyl-protecting group. Therefore, we decided to proceed to the polymerization of P1 with N,N′-dimethylacrylamide (DMAM) and characterize the resulting material with different spectroscopic techniques, in order to verify the insertion of the rrP3OT macromonomer. Among the various techniques to polymerize vinyl monomers, the most widely used still remains free radical polymerization. This technique requires mild reaction conditions and is suitable for a wide range of functional monomers. Thus, free radical polymerization was applied for the copolymerization of P1 with a typical water-soluble comonomer, for example, N,N′-dimethylacrylamide (DMAM) using AIBN as the initiator in THF. The resulting copolymer P2 (Scheme 1) was found to be soluble in a variety of solvents (from highly polar protic to weakly polar aprotic solvents) such as water, methanol, ethanol, acetone, THF, chloroform, dichloroethane, and ethyl acetate. The 1H NMR spectrum of P2 is presented in Figure 1b and shows the characteristic peaks of typical poly(N,N′-dimethylacrylamide) (PDMAM), along with a small peak at δ ) 0.8 ppm assigned to the -CH3 protons of the alkyl side chain of P1. The calculation of the molecular weight of P2 was assessed by GPC and recorded at 440 nm, that is, at the long-wavelength absorption maximum of the rrP3OT precursor of P1 and copolymer P2 in dilute solution, based on calibration with polystyrene standards, revealing a Mn value of 22 300 and a polydispersity index (Mw/Mn) of 1.9. Furthermore, we can also calculate the composition ratio of the rrP3OT chains in the copolymer, from the 1H NMR spectra of P2, based on the aliphatic -CH3 protons at δ ) 0.8 ppm of P1 and the signal at δ ) 1.3 ppm corresponding to the -CH protons in the vinyl position of the P1 macromonomer and DMAM, taking into account that the number of the repeat units n of P1 is equal to 13. Thus, the composition ratio of P1 macromonomer in the copolymer P2 is 0.05% (mol/mol), that is, about the onefourth of the feed composition, 0.19% (mol/mol). 3.2. Optical Properties. 3.2.1. SolVatochromism. The optical behavior of P2 in solvents of different polarity was studied using (19) (a) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Macromolecules 2005, 38, 8649. (b) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542. (20) Zhang, Q.; Russell, T. P.; Emrick, T. Chem. Mater. 2007, 19, 3712. (21) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (22) Stefopoulos, A. A.; Chochos, C. L.; Prato, M.; Pistolis, G.; Papagelis, K.; Petraki, F.; Kennou, S.; Kallitsis, J. K. Chem.sEur. J., published online Aug 4, http://dx.doi.org/10.1002/chem.200800683.

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Figure 1. 1H NMR spectra (400 MHz) of (a) rrP3OT and (b) the amphiphilic conjugated copolymer P2. The asterisk denotes the solvent used (CDCl3). (c) GPC graph of P1 and P2 in chloroform.

Figure 2. (a) Absorption and (b) photoluminescence spectra of P2 in the different solvents. (c) Photoluminescence intensity and the emission maxima of P2 versus the different solvents. In (a), the absorption spectra of P2 in ethanol, methanol, and water are presented independently.

UV-vis and photoluminescence spectroscopy (Figure 2), monitoring the changes in the π-π* transitions of the rrP3OT

chains. It should be mentioned that all solvents used here are good solvents for PDMAM, while the solubility of the rrP3OT

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Figure 3. Absorption spectra of P2 collected in (a) THF/methanol mixture and (b) THF/water mixture.

chains depends on the polarity of the solvent used. As seen, both the absorption (Figure 2a) and emission (Figure 2b) spectra of P2 depend strongly on the polarity of the solvent used. Thus, in the less polar aprotic solvents, THF and chloroform, the absorption maximum of P2 is centered at 439 and 443 nm, respectively (Figure 2a), while the photoluminescence intensity is comparable for both solvents (Figure 2b). These observations suggest that the rrP3OT chains of P2 adopt a coil conformation in these two solvents. In the case of dichloroethane, the absorption maximum is centered at 447 nm and a significant broadening of the absorption spectrum is observed in the region 500-650 nm (Figure 2a). This observation, together with the important decrease of the photoluminescence intensity of P2 (Figure 2b), suggests that a small fraction of the rrP3OT chains are possibly involved in aggregates in dichloroethane, while the major fraction is still well soluble, adopting the coil conformation. Aggregation is further enhanced in ethyl acetate, where both a main absorption peak at 513 nm (and the two vibronic peaks at 550 and 603 nm) and a shoulder at 443 nm are observed (Figure 2a), while photoluminescence quenching is more pronounced here than in dichloroethane. Therefore, it is evident that the rrP3OT chains of P2 coexist between two conformational structures (a coil structure and an aggregated one) in ethyl acetate. Finally, in highly polar solvents, aprotic (acetone) or protic (ethanol, methanol, and water), P2 exhibits similar absorption spectra with a main absorption peak at 513 nm and two vibronic peaks at 550 and 603 nm (Figure 2a), while the emission intensity of P2 is quenched as the polarity of the solvent increases (Figure 2b), revealing that the aggregated structures of rrP3OT chains are predominant in these solvents. To better quantify the significant quenching process observed in the highly polar solvents, we have determined the fluorescence quantum yield (Φ) in two representative solvents, namely, THF and methanol. In fact, Φ was found to be 0.21 in THF, whereas in methanol (highly polar solvent) Φ was less than 10-3. So far, we examined the emission spectra of P2 dissolved in solvents of varying polarity with respect to emission intensity changes and not to emission maxima alteration. Both phenomena, change of the photoluminescence intensity and shift of the emission maxima of P2 in these solvents, are summarized in Figure 2c. It is clear that in all other solvents the progressive decrease of the emission intensity of P2 is not accompanied by any significant shift of the emission maximum, with the exception of water. Thus, the emission maximum of P2 in water is centered at 640 nm, while the emission maxima in all other solvents are observed in the region of 560-565 nm. Since the absorption

spectra of P2 in the polar protic solvents (water, ethanol, and methanol) are similar in shape to those of the rrP3ATs in the solid state,5b,11,14d one would expect that the emission maxima should observed around 660 nm. However, only in water the emission maximum at 640 nm is close to that value. Consequently, the question rising is why does P2 exhibit the emission maxima at 560-565 nm in all the other solvents, especially in ethanol and methanol, whereas in water at 640 nm. In an attempt to explain this unusual effect, we proceeded to a detailed investigation of the optical properties of P2 dissolved in mixtures of a good solvent (THF) with either methanol or water. 3.2.2. SolVent Mixtures. For a deeper understanding of the different emission maximum of P2 observed in water, as compared to the other solvents and especially the polar ones, we investigated the optical properties of P2 in THF/methanol and THF/water mixtures. The absorption and photoluminescence spectra of P2 in such solvent mixtures are presented in Figures 3 and 4, respectively. As shown in Figure 3a, the π-π* absorption band of the rrP3OT chains of P2 is located at 439 nm in pure THF, indicative of the coil conformation, while a bathochromic shift with the gradual appearance of the characteristic peaks at 513, 550, and 603 nm is observed upon addition of methanol, indicative of a conformational change toward the formation of aggregates in pure methanol. The distinct isosbestic point observed at 480 nm with increasing methanol content confirms the presence of only two conformational forms, coil and aggregate form, also documented by a characteristic color change from light yellow to purple, as shown in Figure 3a. In contrast, the absorption characteristics of P2 are different in THF/water mixtures compared to those recorded in THF/ methanol mixtures (Figure 3b). Even though the bathochromic shift of the π-π* absorption band from 439 nm in pure THF to the band with the characteristic vibronic features at 513, 550, and 603 nm in pure water is presented, the absorption spectra of the intermediate THF/water mixture ratios are qualitatively different from those of the corresponding THF/methanol mixtures. Similar to the known cononsolvency behavior of the homopolymer PDMAM in mixtures of water with organic solvents such as dioxane and acetone,23 it was found that PDMAM is not soluble in THF/water mixtures with low water contents. Thus, it was not possible to follow the optical properties of P2 in THF/ water mixtures with a water content less than 50%. However, it is evident from Figure 3b that an obvious isosbestic point is (23) (a) Pagonis, K.; Bokias, G. Polymer 2004, 45, 2149. (b) Orakdogen, N.; Okay, O. Polymer 2006, 47, 561.

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Figure 4. Photoluminescence spectra of P2 collected in different solvent mixtures of (a) THF/methanol and (b) THF/water. (c) Variation of the photoluminescence intensity of P2 upon addition of methanol or water.

not observed this time. Moreover, it is seen that the absorption spectra at intermediate water contents are broader, while the absorption maxima are 10 nm red-shifted and absorption peaks at lower energies are observed compared with the absorption spectrum of P2 in pure THF. Furthermore, it is worthy to mention that the characteristic vibronic features at 513, 550, and 603 nm in THF/methanol mixtures are already observed at a 40% methanol content, while a water content higher than 70% is needed in the case of THF/water mixtures to detect a similar behavior, although water is a more polar solvent than methanol. Finally, the possible concentration-dependence of the absorption spectra of P2 was checked in some THF/water mixtures. Within the concentration range investigated, we did not notice any significant changes of the characteristics of the absorption band of rrP3OT with the variation of the polymer concentration. The aforementioned observations indicate that the conformational change of the rrP3OT chains of P2 probably follows two different self-assembly procedures in the two solvent mixtures. For this reason, parallel with the absorption study, the photoluminescence behavior of P2 in THF/methanol and THF/ water mixtures was also investigated (Figure 4). Progressive quenching of the emission intensity of P2 is observed, by the subsequent addition of methanol (Figure 4a) or water (Figure 4b). The change in the emission intensity as a function of methanol or water added is depicted in Figure 4c, showing no noteworthy differentiation between the two solvents.

However, the normalized emission spectra of P2 in the THF/ methanol and THF/water mixtures (Figure 5) present a very interesting trend. While no significant variation is observed in the emission maxima of P2 by the addition of methanol (Figure 5a), a significant red-shift of the emission maximum is revealed by the addition of water (Figure 5b). The variation of the emission maxima of P2 upon addition of methanol or water is summarized in Figure 5c. It is interesting to note that the emission maxima of P2 in the THF/water mixtures with a water content of 50, 60, and 70% are about 27 nm red-shifted compared to those recorded in pure THF. This result, in combination with the 10 nm redshifted absorption spectra of P2 recorded in these contents (Figure 3b), indicates the formation of an alternative type of aggregate, probably J-type aggregates,9d,24 for the rrP3OT chains of P2 upon addition of water in the THF solution, up to a 70% water content. Further water addition leads to the appearance of the characteristic vibronic features at 513, 550, and 603 nm on the absorption spectra of P2 (Figure 3b) and the shift of the emission maxima of P2 to higher wavelengths (Figure 5b,c). In the case of pure water, two peaks are clearly observed in the emission spectrum of P2 (Figure 5b), a minor peak at 592 nm due to the contribution from the J-type aggregates and the major peak with a maximum emission at 640 nm. On the basis of the above experimental findings, one should consider the role of the intrachain and interchain interactions of (24) Kim, J. Pure Appl. Chem. 2002, 74, 2031.

Copolymers Based on rrP3ATs

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Figure 5. Normalized photoluminescence spectra of P2 collected in different solvent mixtures of (a) THF/methanol and (b) THF/water. (c) Variation of the emission maxima of P2 upon addition of methanol or water.

the rrP3OT chains in order to explain the observed optical properties of P2. From the study of the optical properties (absorption and photoluminescence) of P2 in THF/methanol mixtures, it can be concluded that, despite the higher emission intensity of the coil form, compared to the lower emission intensity of the aggregate species of rrP3OT chains of P2 (Figure 4), all the other emission characteristics (shape and emission maxima) are similar and only a blue-shift of 5 nm is detected in pure methanol (Figure 5a,c). Therefore, a reasonable starting point is to consider that the emission characteristics of P2 in methanol arise from the intrachain states of the aggregate species of the rrP3OT macromonomer, since they have similar emission characteristics with the coil form of the rrP3OT chains of P2. The optical properties of P2 in methanol and consequently in ethanol can then be well explained from a single chain packing form of the rrP3OT chains, which involves only intrachain interactions. The low molecular weight (∼2600) and the very low composition ratio (0.05% molar ratio) of the rrP3OT chains of the copolymer P2 also support the argument that the emission properties of P2 in methanol and ethanol occur from the packing form of isolated rrP3OT chains. Besides, the observation of the one-dimensional aggregation of the rrP3ATs in solution has been presented.6g On the other hand, the emission characteristics of P2 in water (more polar solvent than methanol) may be attributed to increased interchain interactions, apparently based on a different self-assembly procedure of rrP3OT chains upon addition of water to THF. This is supported by the observation of the emission

maximum at 640 nm, close to the emission maxima of the rrP3ATs observed in the solid state, and the broader absorption spectrum of P2 recorded in water compared to methanol and ethanol (plot of ethanol, methanol, and water in Figure 2a) (similar absorption maxima though). Thus, the rrP3OT chains of P2 in water seem to be organized in a stacklike structure, in contrast to the single chain packing conformation formed in methanol and ethanol. 3.2.3. Thermochromism. It is well-known that polythiophene derivatives, especially regioregular, exhibit thermochromic properties, both in solution and in the solid state.6a,c,f,12b In order to check whether this happens in P2, we investigated the absorption behavior upon heating the solution of P2 in ethyl acetate. This solvent was chosen, because, as mentioned previously, the coil form of rrP3OT chains of P2 coexists with aggregate species in ethyl acetate at room temperature and it was expected that the equilibrium between these two states would be sensitive to temperature variations. Thus, the absorption spectra of P2 were recorded between room temperature and 75 °C (a temperature close to the boiling point of the solvent). It is clearly observed that, upon heating, the intensity of the bands at 513, 550, and 603 nm decreases, while at the same time the intensity of the band at 440 nm increases (Figure 6). Besides, an obvious isosbestic point at 470 nm is observed as the temperature rises, verifying that this change is in fact an equilibrium process. Thus, while the rrP3OT chains of P2 are present in two conformational states at room temperature, the conformation of the rrP3OT chains changes upon heating,

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Figure 6. UV-vis spectra of P2 collected at different temperatures in ethyl acetate solution.

adopting the coil form at high temperature. In fact, this change is witnessed visually from the color change of the solution from purple to yellow upon heating (Figure 6). The observed color change is reversible, since the yellow solution at 75 °C returns to purple after cooling at room temperature.

4. Conclusions An easily applicable synthetic route toward the preparation of copolymers based on regioregular poly(3-alkylthiophene)s (rrP3ATs) is presented in this work, through the radical copolymerization of a vinyl end monofunctionalized regioregular poly(3-alkylthiophene) macromonomer with vinyl functional monomers. As an example, we have effectively synthesized and extensively characterized by means of UV-vis and photoluminescence spectroscopy techniques a new amphiphilic conjugated random copolymer through the copolymerization of a regioregular poly(3-octylthiophene) (rrP3OT) macromonomer with a large excess of N,N′-dimethylacrylamide (DMAM). This copolymer was found to be readily soluble both in water and in a large variety of organic solvents, permitting the correlation

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between the conformational states and the optical properties of the rrP3OT chains in solution with the polarity of the solvent. It is shown that the copolymer exhibits interesting solvatochromic phenomena, as a result of the controllable conformational structures adopted by the rrP3OT chains in solvents and solvent mixtures of different polarity. This behavior could be applied to form preassembled structures prior to film casting and, thus, modulating the properties of the final product. Nevertheless, the most significant conclusion from the systematic study of the optical properties (absorption and photoluminescence) of the copolymer in various solvents and solvent mixtures is that the rrP3OT chains may be found in three different conformational states in solution, depending on the polarity of the solvent. Thus, in low polarity solvents such as THF or chloroform, the rrP3OT chains adopt a coil conformation, while in polar protic solvents (ethanol, methanol, and water) aggregation of rrP3OT chains is favored. Furthermore, the emission characteristics of the copolymer in ethanol and methanol indicate the self-organization of the rrP3OT chains on a single chain packing form (only intrachain interactions are allowed), whereas the formation of a stacklike structure, due to the increased interchain interactions, is favored in water. Moreover, when ethyl acetate is used as solvent, it is evident that at room temperature the rrP3OT chains coexist in the coil and the single chain packing form while upon heating the coil form predominates, leading to the observation of thermochromic phenomena in solution. Finally, as a future extension of this work, all the factors of the copolymer, especially the molecular weight and the composition ratio of the rrP3OT macromonomer, affecting its optical properties in solution along with the screening of different vinyl monomers should be extensively examined. Acknowledgment. Financial support from the European Commission through the NMP3-CT-2006-033228 program is acknowledged. We thank Prof. Georges Hadziioannou and Dr. Nicola Leclerc (Laboratoire d’Inge´nierie des Polyme`res pour les Hautes Technologies (LIPHT), Ecole Europe´enne de Chimie, Polyme`res et Mate´riaux (ECPM), UMR 7165, CNRS - Universite´ Louis Pasteur, Strasbourg, France) for helpful discussions. LA801178M