Chemical and Electrochemical Polymerization of 3-Alkylthiophenes on

Langmuir , 1999, 15 (11), pp 3752–3758. DOI: 10.1021/la981330o ... Cite this:Langmuir 15, 11, 3752-3758 ... Anton Kiriy. Journal of the American Che...
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Langmuir 1999, 15, 3752-3758

Chemical and Electrochemical Polymerization of 3-Alkylthiophenes on Self-assembled Monolayers of Oligothiophene-Substituted Alkylsilanes Seiji Inaoka and David M. Collard* School of Chemistry and Biochemistry, and the Polymer Education and Research Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received September 25, 1998. In Final Form: February 8, 1999 Monolayers of oligothiophene-substituted alkylsilanes, chlorodimethyl(11-(3-(2,2′bithienyl))undecyl)silane and chlorodimethyl(11-(3′-(2,2′:5′,2′′-terthienyl))undecyl)silane, are subject to electrochemical oxidation within the monolayer to afford more highly conjugated oligomers with lower redox potentials. The electrochemical polymerization of 3-methylthiophene is promoted by monolayers of the oligothiophene-substituted silanes on the electrode surface to form smooth, highly adherent films of poly(3methylthiophene). The effect of monolayers of an electroactive monomer on the rate of deposition of poly(3-methylthiophene) is compared to the effect of low concentrations of oligomers in solution. Chemical polymerization of 3-octylthiophene on substrates modified with the redox-active silanes gives films of poly(3-octylthiophene) which display reversible solvatochromism without dissolving the polymer. These thin films are sensitive to low concentrations of chloroform in the vapor phase or in aqueous solution.

The formation of self-assembled monolayers (SAMs) on solid substrates provides a convenient method for the modification of surface properties.1 SAMs of functionalized adsorbates have been used to modify the chemical and electrochemical properties of substrates for the deposition of films of a number of materials with interesting electronic and optical properties. Further elaboration of these thin films to provide components of optical or electronic devices might be achieved by selective deposition of materials on micron-scale patterns of functional SAMs.2 Such patterns have been used as templates for the selective electroless deposition of metals,3 deposition of minerals,4 chemical vapor-phase deposition,5 electrostatic adsorption of polymer multilayers,6 initiation of chain growth polymerization,7 and electrooxidative formation of conjugated polymers.8-11 Oligothiophenes and polythiophenes have recently attracted a great deal of attention for use in optical and electronic devices.12 Since the conjugated π-orbitals con-

stitute a one-dimensional array of electron density, the organization of these chains by self-assembly provides approaches to “molecular electronic” devices.13 For example, vacuum sublimation of R,ω-substituted and unsubstituted oligothiophenes affords single crystals, with the long molecular axis tilted to the substrate, which have been used in transistors.14 Self-assembly of isolated conjugated molecules in an insulating matrix of adsorbates affords molecular wires perpendicular to the surface for the study of single-molecule conductivity.15 Alternatively, assemblies in which the arylene backbone lies parallel to the substrate surface have been prepared by adsorption of β-substituted oligomers from solution.16 Self-assembled monolayers may also be used to prepare ordered assemblies of conjugated polymers. Adsorbates bearing monomeric units can be polymerized within the monolayer.17,18 Alternatively, SAM-modified electrodes can be used as substrates for deposition of films of conjugated polymers by electrooxidative polymerization of suitable monomers. SAMs of thiols and silanes bearing pyrrole,19-23

(1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology 1996, 7, 452. (3) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calvert, J. M. Chem. Mater. 1993, 5, 148. (4) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehnig, M. C. Langmuir 1994, 10, 619. (5) Potochnik, S. J.; Pehrsson, P. E.; Hsu, D. S. Y.; Calvert, J. M. Langmuir 1995, 11, 1841. (6) Clark, S. L.; Montague, M.; Hammond, P. T. Supramol. Sci. 1997, 4, 141. (7) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 592. Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 602. (8) Rozsnyai, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 5993. Rozsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913. Rozsnyai, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309. (9) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526. (10) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. Huang, Z.; Wang, P.-C.; Feng, J.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. M. Synth. Met. 1997, 85, 1375. (11) Sayre, C. N.; Collard, D. M. J. Mater. Chem. 1997, 7, 909.

(12) Roncalli, J. Chem. Rev. 1992, 92, 711. (13) Rubner, M. F. In Molecular Electronics; Ashwell, G. J., Ed.; Wiley: New York, 1992; p 65. (14) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.-L.; Garnier, F. Chem. Mater. 1995, 7, 1337. Horowitz, G.; Delannoy, P.; Bouchriha, H.; Deloffre, F.; Fave, J.-L.; Garnier, F.; Hajlaoui, R.; Heyman, M.; Kouki, F.; Valat, P.; Wintgens, V.; Yassar, A. Adv. Mater. 1994, 6, 752. (15) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (16) Ba¨uerle, P. Adv. Mater. 1992, 4, 102. Rabe, J. P.; Bauerle, P.; Fischer, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 303. (17) Willicut, R. J.; McCarley, R. M. J. Am. Chem. Soc. 1994, 116, 10824. (18) Zotti, G.; Schiavon, G.; Zacchin, S.; Berlin, A.; Pagani, G.; Canavesi, A. Langmuir 1997, 13, 2694. (19) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (20) Willicut, R. J.; McCarley, R. M. Langmuir 1995, 11, 296. (21) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (22) Wurm, D. B.; Brittain, S. T.; Kim, Y. T. Langmuir 1996, 12, 3756. (23) Smela, E. Langmuir 1998, 14, 2996.

Introduction

10.1021/la981330o CCC: $18.00 © 1999 American Chemical Society Published on Web 04/28/1999

Polymerization of 3-Alkylthiophenes on SAMs

bithiophene,24 dithienylpyrrole,25,26 and aniline,27-29 have been used to modify electrodes for electrochemical deposition of conjugated polymers by oxidative polymerization of monomers in contacting solution. The modification of the electrode surface facilitates the formation of dense, adherent films of a polymer. The monolayers might therefore be used to integrate conjugated polymers with conventional electronic materials, or to provide a template for epitaxial growth of ordered arrays. Here, we report the effect of monolayers of oligothienylsubstituted alkylsilanes, 1-3, on the electrochemical and chemical oxidative deposition of polythiophenes on conductors and insulators. Substitution of the tether on the oligomer was made perpendicular to the long axis of the conjugated chain in order to organize the oligothiophenes parallel to the substrate in an orientation amenable to R-coupling to afford monolayers of longer oligomers and polymers. Detailed studies of the structure of monolayers formed by a ω-(2,2′:5′,2′′-terthien-5-yl)alk-1-ylthiol, endsubstituted thiol analogue of silane-terthiophene 3 appeared recently.30,31 We chose to study alkylsilane adsorbates in order to modify both insulating oxide surfaces (glass) and hydrated metal surfaces (gold, platinum). The use of thienyl-, bithienyl-, and terthienyl-substituted adsorbates allows for the study of adsorbed monomers with a range of oxidation potentials. In addition to preliminary evidence for electrochemical polymerization within the monolayer, we report the effect of these adsorbates on the chemical oxidation of monomers in solution and the properties of the deposited polymer films.

Results and Discussion Synthesis. Synthetic routes to adsorbates 1-3 are shown in Schemes 1 and 2. 3-(6-Bromohexyl)thiophene, 6, was prepared by NiCl2(dppp)-catalyzed32 coupling of 3-bromothiophene and the Grignard reagent prepared from 1-bromo-6-(4-methoxyphenoxy)hexane, 4, followed by treatment with HBr in acetic anhydride.33 The Grignard reagent derived from 6 was treated with allyl bromide in the presence of Li2CuCl434 to afford 9-(3-thienyl)-1-nonene, 7, which was hydrosilated35 with chlorodimethylsilane in the presence of chloroplatinic acid to afford 1. (24) Ng, S. C.; Miao, P.; Chen, Z. K.; Chan, H. S. O. Adv. Mater. 1998, 10, 782. (25) Kowalik, J.; Tolbert, L. M.; Ding, Y.; Bottomley, L.; Vogt, K.; Kohl, P. Synth. Met. 1995, 55, 1171. (26) Mekhalif, Z.; Lazarescu, A.; Hevesi, L.; Pireaux, J. J.; Delhalle, J. J. Mater. Chem. 1998, 8, 545. (27) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (28) Sato, N.; Nonaka, T. Chem. Lett. 1995, 805. (29) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (30) Liedberg, B.; Yang, Z.; Engquist, I.; Wirde, M.; Gelius, U.; Go¨tz, G.; Ba¨uerle, P.; Rummel, R.-M.; Ziegler, C.; Go¨pel, W. J. Phys. Chem. B 1997, 101, 5951. (31) Michalitsch, R.; El Kassmi A.; Lang, P.; Yassar, A.; Garnier, F. J. Chim. Phys. 1998, 95, 1339. (32) (a) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.; Fujioka, A.; Kodama, S.; Nakajima, I.; Minato, A.; Kumada, M. Bull. Chem. Soc. Jpn. 1976, 49, 1958. (b) Tamao, K.; Kodama, S.; Nakajima, I.; Kumada, K.; Minato, A.; Suzuki, K. Tetrahedron 1982, 38, 3347. (33) Ba¨uerle, P.; Wurthner, F.; Heid, S. Angew. Chem., Int. Ed. Engl. 1990, 29, 419. (34) Tamura, M.; Kochi, J. Synthesis 1971, 303. (35) (a) Speier, J. L.; Zimmerman, R.; Webster, J. J. Am. Chem. Soc. 1956, 78, 2278. (b) Speier, J. L.; Webster, J.; Barnes, G. H. J. Am. Chem. Soc. 1957, 79, 974. (c) Tamao, K.; Yoshida, J.; Yamamoto, H.; Kakui, T.; Matsumoto, H.; Takahashi, M.; Kurita, A.; Murata, M.; Kumada, K. Organometallics 1982, 1, 355.

Langmuir, Vol. 15, No. 11, 1999 3753 Scheme 1

Scheme 2

3-Bromo-2,2′-bithiophene and 3′-bromo-2,2′:5′:2′′-terthiophene were prepared by reaction of 2-thienylmagnesium bromide with 2,3-dibromothiophene and 2,3,5tribromothiophene,36 respectively, according to the method by Carpita et al.37 Reaction of bromobithiophene (8) or bromoterthiophene (11) with the Grignard reagent derived from 10-bromo-1-undecene (prepared by entrainment with ethylene bromide) in the presence of NiCl2(dppp) gave the corresponding 10-undecenyl-substituted oligothiophenes 9 and 12. Although this method is a convenient method for the synthesis of 3-alkylthiophenes from alkylmagnesium halides, we could not avoid the formation of ≈5% of the isomerized internal alkene. In control experiments we showed that ω-alkenyl-1-magnesium bromides isomerize in the presence of NiCl2(dppp) to give internal alkenes. Chloroplatinic acid-catalyzed hydrosilation of 9 and 12 with chlorodimethylsilane gave 2 and 3, respectively. While the hydrosilation of internal alkenes to give terminal silanes via isomerization has been reported,38 we did not optimize the conditions for complete hydrosilation of the internal alkene present in samples of 9 and 12. Thus, samples of chlorosilanes 2 and 3 contained a small amount of unreacted internal alkenes which are not absorbed on the substrates used for the formation of the monolayers. Formation of Monolayers. Initial attempts to prepare monolayers of 11-(oligothienyl)-1-trichlorosilylundecane led to multilayers through formation of Si-O-Si bridges. The area beneath the oxidative curve of the cyclic voltammogram of surfaces modified with these trichlorosilanes integrated to more charge than would be expected for a single layer of redox-active adsorbates. Use of the monochlorides avoided this problem. Monolayers were formed by the immersion of substrates into a solution of appropriate chlorosilane followed by chlorotrimethylsilane to fill pinholes in the monolayer.39 Cyclic Voltammetry of SAMs. Cyclic voltammograms of electrodes modified with monolayers of 2 and 3 (36) Janda, M.; Srogl, J.; Stibor, I.; Nemec, M.; Vopatrna, P. Synthesis 1972, 545. (37) Carpita, A.; Rossi, R. Gazz. Chim. Ital. 1985, 115, 575. (38) Saam, J. C.; Speier, J. L. J. Am. Chem. Soc. 1958, 80, 4104. (39) Davis, C. A.; Graves, P. R.; Healy, P. C.; Myhra, S. Appl. Surf. Sci. 1993, 72, 419.

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Inaoka and Collard

Figure 1. Cyclic voltammogram of a gold electrode modified with silane monolayers: A, 2; B, 3.

immersed in a monomer-free electrolyte are shown in Figure 1. Thiophene-substituted alkylsilane 1 did not show a significant oxidation peak up to a potential limit of +1500 mV, consistent with the high oxidation potential of 3-alkylthiophenes. Bithiophene-substituted alkylsilane 2 shows an onset for oxidation at ≈+1050 mV and a peak at +1200 mV on the first excursion to positive potential (Figure 1A). The oxidative charge obtained by the integration of the peak in the cyclic voltammogram (Qoxid ) 110 µC/cm2) corresponds to a molecular density of 23 Å2/ molecule (assuming a one-electron oxidation). This is similar to the molecular density of the chlorodimethyloctadecylsilane monolayer determined by Ru¨he et al. (224 ng/cm2, corresponding to 23 Å2/molecule).40 A small peak corresponding to bithienyl oxidation appears in the second sweep to high potential and the peak disappears upon continued cycling. This suggests that all of the bithiophene is not oxidized upon the first excursion to high potential, or that the radical cation reacts only slowly (on the time scale of the experiment) and that some of it is reduced upon reversing the potential sweep (i.e., the oxidation is partly reversible). A small inflection at +1200 mV in the first reductive segment might correspond to the reduction of a small portion of the radical cation. Although bithiophene radical cations are very reactive, the reversibility of this couple could arise from the restricted geometry of the tethered unit and a resulting retardation of coupling and side reactions. Repeated potential cycling results in the gradual disappearance of the peak corresponding to oxidation of the bithiophene and the appearance of a broad redox wave at ≈+810 mV (oxidation). The appearance of this wave is consistent with the formation of more conjugated redox-active oligomeric (or polymeric) chains. Monolayers of the terthiophene-substituted alkylsilane 3 shows an onset for oxidation at ≈+650 mV and a peak at +900 mV on the first excursion to positive potential (Figure 1B). The oxidative charge obtained by the integration of the peak in the cyclic voltammogram (Qoxid ) 77 µC/cm2) corresponds to a molecular density of 21 Å2/ molecule. A new peak appears at ≈+750 mV (oxidation), which decreases slightly upon repeated cycling. This behavior is consistent with coupling of tethered-terthiophene radical cations to form more conjugated species. Similar evidence for oxidative coupling of tethered electroactive monomers, and the oxidative instability of the resulting species, has been presented for pyrrole-substituted alkanethiol monolayers.20 (40) Ru¨he, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 7, 2236.

Figure 2. Cyclic voltammogram of 3-methylthiophene on gold electrodes modified with silane adsorbates: A, no monolayer; B, 1; C, 2; D, 3; E, 13.

Electrochemical Polymerization of Monomers on SAM-Modified Electrodes. Cyclic voltammograms of unmodified and silane-modified gold electrodes in a solution of a 3-methylthiophene monomer are shown in Figure 2. The onset for oxidation of the monomer is approximately +1350 mV at an unmodified electrode (Figure 2A). Excursion to potentials only slightly positive of the onset of monomer oxidation (+1375 mV) does not give rise to the deposition of a polymer owing to the formation of only low concentrations of the radical cation and lack of nucleation required for film growth. The monomer is polymerized by cycling the potential up to +1400 mV (versus SCE) to afford a thin film of a redoxactive polymer with a broad reversible voltammetric wave at approximately +800 mV. Repeated cycling of electrodes modified with chlorodimethyldecylsilane, 13, to +1375 mV indicates a smaller double-layer charging current and no evidence of polymer deposition (Figure 2E). This is consistent with the effects of densely packed monolayers which block electron transfer between the electrode and redox-active probes in contacting solutions.41 Although previous studies suggest that hydrophobic monolayers (alkanethiols on gold) eventually accelerate the electrooxidative deposition of redox-active polymers (polyaniline),42 the data shown in Figure 2E indicates that monolayers reported here do not promote polymer deposition under these conditions. Similarly, electrodes modified with a SAM of the thiophene-substituted silane 1 do not show significant polymer deposition (Figure 2B). The 3-thienyl substituent is not oxidized under these conditions (see above), and the monolayer effectively blocks oxidation of 3-methylthiophene in contacting solution. In contrast, electrodes modified with a SAM of the bithienyl-substituted silane 2 show continued growth of a broad redox wave on cycling the potential to +1375 mV in 3-methylthiophene, corresponding to the deposition of (41) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 6107. Miller, C.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (42) Sayre, C. N.; Collard, D. M. Langmuir 1997, 13, 714.

Polymerization of 3-Alkylthiophenes on SAMs

poly(3-methylthiophene) (Figure 2C). The 3-alkylbithienyl group of 2 (Eonset ) +1050 mV) is subject to oxidation at lower potentials than those of the monomer in solution (Eonset ) +1350 mV). Films of poly(3-methylthiophene) are strongly bound to 2-modified electrodes compared to films deposited on unmodified electrodes. We attribute this to incorporation of the surface-bound monomer into the polymer chain. However, the acceleration of the polymerization over that for the reaction on unmodified electrodes requires further explanation. The mechanisms for polymerization of electroactive monomers have been discussed for many years. Experimental observations are consistent with chain growth in the early stages of the electrochemical polymerization of pyrrole in aqueous media,43 and coupling of two radical cations to give a dication44 for the polymerization of N-substituted pyrroles in nonaqueous media,45 and various hetrocyclic pentamers.46 Several examples of the acceleration of the polymerization of thiophenes47 and anilines48 by the addition of oligomers to the reaction mixture were presented by Wei et al. For example, oligothiophenes (2,2′-bithiophene and 2,2′:5′:2′′-terthiophene) significantly lower the induction period for electrochemical polymerization of thiophene and increases the rate of polymerization during later potential cycles. Since 3-methylthiophene in solution is subject to oxidation, but not polymerization, at +1375 mV on unmodified electrodes, we suggest that the acceleration of the early stages of polymerization of 3-methylthiophene on bithienyl-bearing SAMs is due to the presence of the radical cation of both the surface-immobilized bithienyl groups and 3-methylthiophene in solution, which couple to provide an immobilized oligomer. An alternative explanation which takes Wei’s observations into account is for the reaction between oxidized surface-immobilized bithienyl groups and neutral 3-methylthiophene. The strategy of our electrochemistry experiments is not to add bithiophene to the monomer solution but to place the bithienyl groups on the electrode surface by modification of the electrodes with bithienyl-substituted silane. Though the total amount of bithienyl groups in the system is much smaller than that in the experiments with bithiophene in solution, there is a much larger effective concentration of bithienyl groups at the electrode surface. Thus, a bithienyl group on the surface is oxidized to the radical cation at a lower potential than that at which the soluble monomer is oxidized. Coupling between the immobilized radical cation and a neutral monomer in solution provides a terthiophene which is subject to oxidation and chain extension by the same mechanism. This is, of course, consistent with the observation that initiation of thiophene polymerization requires a large overpotential; once initiated, polymerization proceeds by reaction between polymeric radical cations and monomer in solution without excursion to high potential. (43) Qiu, Y.-J.; Reynolds, J. R. J. Polym. Sci. A 1992, 30, 1315. (44) (a) Heinze, J. Topics in Current Chemistry Vol. 152; SpringerVerlag: Berlin, 1990; p 1. (b) Eigen, M.; Kruse, W.; Maas, G.; De Maeyer, L. Progress in Reaction Kinetics Vol. 2; Pergamon Press: Oxford, 1990; p 284. (c) Debye, P. Trans. Electrochem. Soc. 1942, 82, 265. (45) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Saveant, J.-M. J. Phys. Chem. 1991, 95, 10158. (46) Audebert, P.; Catel, J.-M.; Le Coustumer, G.; Duchenet, V.; Hapiot, P. J. Phys. Chem. 1995, 99, 11923. (47) (a) Wei, Y.; Chan, C.-C.; Tian, J.; Jang, G.-W.; Hsueh, F. Chem. Mater. 1991, 3, 888. (b) Wei, Y.; Jang, G.-W.; Chan, C.-C. J. Polym. Sci. C 1990, 28, 219. (c) Wei, Y.; Tian, J. Macromolecules 1993, 26, 457. (48) Wei, Y.; Tang, X.; Sun, Y. J. Polym. Sci. A 1989, 27, 2385.

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Figure 3. Absorption spectra of poly(3-octylthiophene) deposited chemically on glass substrates modified with silane monolyers: A, no monolayer; B, 13; C, 1; D, 2; E, 3.

Electrodes modified with a SAM of the terthienylsubstituted silane 3 shows slow continued growth of a broad redox wave corresponding to the deposition of poly(3-methylthiophene) (Figure 2D). Wei’s experiments with oligomers in solution showed that the addition of 2,2′:5′: 2′′-terthiophene to a solution of monomer increases the rate of polymerization more than the addition of 2,2′bithiophene.46 In contrast, our results show that the growth of the polymer is much slower on electrodes modified with the terthienyl-substituted adsorbate than the bithienyl-bearing monolayers. This result can be explained with the difference in reactivity of bithienyl and terthienyl groups (Figure 1). Although polymerizations proceed by reaction of a surface-immobilized oligothiophene radical cation with monomer in solution, coupling of two immobilized oligomers or reaction with nucleophiles (e.g., water) would lead to the consumption of active chain ends without polymerization. When a low concentration of terthiophene is in solution,46 reaction between two terthiophenes is rare, and chain extension by reaction with thiophene predominates. However, in our experiments, a terthienyl group on the modified electrode is surrounded by neighboring terthienyl groups. Considering the oxidation potential of the terthienyl group compared to that of 3-methylthiophene and the time scale (sweep rate) of our experiments, the terthienyl radical cation might have time to react with a neighboring terthienyl to form a sexithienyl group before coupling to 3-methylthiophene. Chemical Polymerization of 3-Alkylthiophenes on SAM-Modified Glass. Glass substrates (Fisher or VWR glass slides) were modified with thienyl- or oligothienylsubstituted alkylsilanes, 1-3, or chlorodimethyldecylsilane, 13. Chemical polymerization of 3-octylthiophene was carried out on these modified substrates with FeCl3 as an oxidizing agent. Ultraviolet-visible spectra of chemically deposited poly(3-octylthiophene) on glass substrates are shown in Figure 3. Poly(3-octylthiophene) deposited on unmodified glass and glass modified with alkylsilane 13 is washed away after thorough rinsing with methylene chloride, (Figure 3A,B). However, polymerization on glass substrates modified with thienyl-substituted silane 1, bithienylsubstituted silane 2, and terthienyl-substituted silane 3 results in absorptions with λmax around 520 nm (Figure 3C-E). The polymer films are smooth, uniform, and

3756 Langmuir, Vol. 15, No. 11, 1999

Figure 4. Absorption spectra of poly(3-octylthiophene)on glass modified with 2 deposited by oxidative polymerization of 3-octylthiophene (1 mM) with FeCl3 in CH2Cl2: A, 2.5 equiv; B, 5 equiv; C, 10 equiv.

strongly adhered and passed a peel test with Scotch tape, suggesting that the poly(3-octylthiophene) is covalently connected to the substrates modified with monolayers substituted with monomeric units. The amount of polymer deposited on substrates modified with 2 by chemical oxidation of monomer in solution (25 mM 3-octylthiophene in CH2Cl2) varies inversely with the concentration of FeCl3 (2.5 to 20 equiv). The use of less oxidant in solution results in more polymer deposition on the modified surface Figure 4. Olinga and Francois49 noted that the rate of polymerization of thiophene is proportional to the surface area of FeCl3 in chemical oxidations, with suspended oxidant acting as sites for nucleation of polymer growth. The tethered thienyl or oligothienyl groups on the substrate in our polymerizations must therefore compete as nucleation sites with solid FeCl3 particles. At low concentrations of oxidant, polymerization on the modified substrate is favored and the reaction is effectively activated by the surface. Solvatochromism and Doping of Immobilized Polymer Films. Films of poly(3-octylthiophene) deposited on substrates modified with dimer-substituted adsorbates (2) are strongly adherent. In addition, the polymer does not dissolve in solvents in which poly(3-ocylthiophene) is usually soluble (e.g., CHCl3, CH2Cl2, THF). The thin films display solvatochromism similar to that displayed by poly(3-alkylthiophenes) in solution:50,51 λmax shifts from 520 nm in air to 445 nm in CH2Cl2, and the absorption is sensitive to solvent composition (e.g., THF/methanol, Figure 5). This observation is similar to that observed for a solution of poly(3-alkylthiophene) in the mixture of CHCl3 and MeOH, for a solution of poly(alkylsulfonatethiophene) in mixtures of THF and MeOH,52 and for a solution of poly(3-(10-hydroxydecyl))thiophene in mixtures of 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidinone (DMPU) and MeOH.53 The solvatochromism is entirely reversible upon changing the solvent. Spectra of a film in air, water, chloroform (liquid), chloroform vapor, (49) Olinga, T.; Francois, B. Synth. Met. 1995, 69, 297. (50) Yoshino, K.; Morita, S.; Yin, X.-H.; Kawai, T. Jpn. J. Appl. Phys. 1993, 32, L547. Inganas, O.; Salaneck, W. R.; Osterholm, J.-E.; Laasko, J. Synth. Met. 1988, 22, 395. (51) Ingana¨s, O.; Salaneck, W. R.; O ¨ sterholm, J.-E.; Laakso, J. Synth. Met. 1988, 22, 395. (52) Patil, A. O. Synth. Met. 1989, 28, C495. (53) Bertinelli, F.; Della Casa, C. Polymer 1996, 37, 5469.

Inaoka and Collard

Figure 5. Solvatochromism: absorption spectra of tethered poly(3-octylthiophene) on glass modified with 2 in THF/ methanol: A, 0/100; B, 20/80; C, 40/60; D, 60/40; E, 80/20; F, 100/0.

Figure 6. Solvatochromism: absorption spectra of tethered poly(3-octylthiophene) on glass modified with 2 in A, air; B, water; C, water saturated with chloroform; D, chloroform vapor; E, chloroform.

and chloroform-saturated water are shown in Figure 6. The spectra of films in air and water are similar, corresponding to an unswollen film which possesses highly conjugated segments. In chloroform (liquid or vapor, Figure 6D,E) the lower value of λmax arises from swelling of the film to afford a sample with a lower average conjugation length. Interestingly, the spectra of the film in chloroform-saturated water shows a broader peak with intermediate absorption maxima (480 nm). An isobestic point at 470 nm supports the direct transformation between more- and less-conjugated states. One unique feature of this polymer on modified glass is that the solvatochromic material is covalently bound to the substrate and is not dissolved in “good” solvents. These observations suggest that the conformational behavior (i.e., solvation and solvent-induced structural changes) of tethered polymer chains at the interface are similar to those of the polymer in solution. However, since the polymer is not dissolved, the study of solvatochromism is

Polymerization of 3-Alkylthiophenes on SAMs

not limited to solvents for the polymer. This reversible solvatochromism of thin immobilized films of poly(3octylthiophene) in response to low concentrations of organic solvent in water or air might provide for a rapid, robust sensor for organic analytes in the atmosphere or aqueous solution. Conclusions. Alkylsilanes with thienyl, bithienyl, or terthienyl tail groups were synthesized. Monolayers of these molecules were formed on a gold electrode surface and glass slides. These monolayers have different oxidation potentials, depending on the identity of the electroactive (oligo)thiophene. Irreversible changes in the oxidation peak in the cyclic voltammetry of modified electrodes in the absence of additional monomer imply that the tethered groups couple together to form longer thienyl oligomers. In the presence of 3-methylthiophene the bithiophene-substituted alkylsilane-modified electrode most significantly increases the rate of polymerization, resulting in the deposition of a smooth film of poly(3methylthiophene) on the electrode. Chemical polymerization of 3-octylthiophene on substrates modified with thienyl- or oligothienyl-substituted silanes gives smooth, well-adhered thin films of a highly conjugated polymer. Poly(3-octylthiophene) on modified substrates shows reversible solvatochromism without dissolving the polymer in good solvents such as CH2Cl2, CHCl3, and THF. Experimental Section Reagents. Thiophene, 3-bromothiophene, 3-methylthiophene, 1,6-dibromohexane, 4-methoxyphenol, 8-bromo-1-octene, 10undecylenyl alcohol, iron(III) chloride, allyl bromide, ethylene bromide, and [1,3-bis(diphenylphosphino)propane]nickel(II) chloride (NiCl2(dppp)) were obtained from the Aldrich Chemical Co. (Milwaukee, WI) and used as received. Tetrahydrofuran (THF) and diethyl ether (Fisher) were purified by distillation from sodium or potassium benzophenone ketyl. Copper(II) chloride (Fisher) was recrystallized from methanol, dried in an oven at 150 °C, and used immediately to prepare a 0.1 M solution of lithium copper chloride (0.1 M), which was prepared by dissolving 1 equiv of copper(II) chloride and 2 equiv of lithium chloride (Aldrich) in anhydrous THF. A solution of chloroplatinic acid was prepared in freshly distilled propanol. 11-Chloro-1undecene was prepared from 10-undecenyl alcohol by treatment with 4-toluenesulfonyl chloride in pyridine and then converted to the corresponding bromide (using NaBr in acetone). Spectroscopy. 1H and 13C NMR analyses were carried out on a Varian Gemini 300 spectrometer at 300 and 75 MHz, respectively. IR analysis was carried out on a Nicolet 520 FT-IR spectrometer. Melting points were measured on a Mel-temp II melting point analyzer. Electrochemistry. Cyclic voltammograms were recorded on a Bioanalytical Systems BAS-100 electrochemical analyzer with a saturated calomel electrode (SCE) and platinum wire or gold counter electrode in anhydrous propylene carbonate (Aldrich) containing 0.1 M LiClO4. Cyclic voltammograms were determined at 100 mV s-1 and were integrated graphically. 3-Bromothiophene was treated with the Grignard reagent derived from 1-bromo-6-(4-methoxyphenoxy)hexane, 4, (bp 100105 °C/1.0 mmHg; mp 46-47 °C) to afford 3-(6-(4-methoxyphenoxy)hexyl)thiophene, 5, which was converted to 3-(6-bromohexyl)thiophene, 6, according to the method of Ba¨uerle et al.33 bp 113 °C/1.5 mmHg (lit.33 90 °C/1 × 10-3 mmHg). 1H NMR (CDCl3) δ: 1.2-1.55 (m, 4H), 1.65 (p, J ) 7 Hz, 2H, C2 CH2), 1.89 (p, J ) 7 Hz, 2H, C5 CH2), 2.63 (t, J ) 7 Hz, 2H, C1 CH2), 3.41 (t, J ) 7 Hz, 2H, C6 CH2), 6.92 (dd, J ) 1.5 Hz, 3 Hz, 1H, thienyl-C2 CH), 6.94 (dd, J ) 1.5 Hz, 4.8 Hz, 1H, thienyl-C4 CH), 7.23 (dd, J ) 3 Hz, 4.8 Hz, 2H, thienyl-C5 CH). IR (neat): 3100 (thiophene C-H stretch), 2930, 2850 (methylene C-H stretch), 1450 (thiophene out-of-plane), 850, 820, 770 cm-1 (thiophene rings). 3-(8-Nonen-1-yl)thiophene (7). 6-(3-Thienyl)hexyl-1-magnesium bromide was prepared by heating a mixture of 3-(6bromohexyl)thiophene (2.0 g, 8.0 mmol), magnesium (1.2 g, 48 mmol) and ethylene dibromide (3.8 g, 20 mmol) in anhydrous

Langmuir, Vol. 15, No. 11, 1999 3757 ether (20 mL) at reflux for 1 h. The Grignard reagent was added dropwise to the stirred solution of allyl bromide (0.97 g, 8.0 mmol) and Li2CuCl4 (0.3 mL of 0.1 M solution in THF) in ether (20 mL) at -10 °C. The mixture was stirred overnight at room temperature, washed with 5% HCl (3 × 150 mL), and dried over MgSO4, and the solvent was removed under reduced pressure. Purification by flash column chromatography (silica gel, petroleum ether) gave 7 (0.60 g, 36%) as a colorless liquid. 1H NMR (CDCl3) δ: 1.2-1.45 (m, 8H), 1.62 (p, J ) 7 Hz, 2H, C2 CH2), 2.04 (q, J ) 7 Hz, 2H, C7 CH2), 2.62 (t, J ) 7 Hz, 2H, C1 CH2), 4.9-5.1 (m, 2H, C9 CH2), 5.7-5.9 (m, 1H, C8 CH), 6.92 (dd, J ) 3 Hz, 1 Hz, 1H, thienyl-C2 CH), 6.93 (dd, J ) 1 Hz, 4.8 Hz, 1H, thienyl-C4 CH), 7.24 (dd, J ) 4.8 Hz, 3 Hz, 1H, thienyl-C5 CH). IR (neat): 3078 (thiophene C-H stretch), 2927, 2862 (methylene C-H stretch), 1466 (thiophene out-of-plane), 999, 913 cm-1 (out-ofplane CdC bend). Chlorodimethyl(9-(3-thienyl)nonyl)silane (1). A mixture of 6 (0.63 g, 3.0 mmol), chlorodimethylsilane (1.5 g, 15 mmol), and H2PtCl6 (0.03 mL of a 0.1 M solution in 2-propanol) was heated at reflux for 20 h. Purification by microdistillation gave 1 (0.65 g, 71%) as a colorless liquid. 1H NMR (CDCl3) δ: 0.36 (s, 6H, Si(CH3)2), 0.78 (t, J ) 7 Hz, 2H, C1 CH2), 1.2-1.4 (m, 12H), 1.59 (p, J ) 7 Hz, 2H, C8 CH2), 2.58 (t, J ) 7 Hz, 2H, C9 CH2), 6.89 (dd, J ) 1 Hz, 3 Hz, 1H, thienyl-C2 CH), 6.93 (dd, J ) 1 Hz, 4.8 Hz, 1H, thienyl-C4 CH), 7.22 (dd, J ) 3 Hz, 4.8 Hz, 1H, thienyl-C5 CH2). 3-Bromo-2,2′-bithiophene (8). The title compound was prepared according to the method of Carpita et al.37 1H NMR (CDCl3) δ: 7.03 (d, J ) 5.2 Hz, 1H, thienyl-C4 CH), 7.09 (dd, d ) 1.2 Hz, 3.1 Hz, 1H, thienyl-C4′ CH), 7.21 (d, J ) 5.2 Hz, 1H, thienyl-C5 CH), 7.36 (dd, J ) 1.2 Hz, 5.3 Hz, 1H, thienyl-C5′ CH), 7.43 (dd, J ) 1.2 Hz, 3.1 Hz, 1H, thienyl-C3′ CH). IR (neat): 3106, 3085 (aromatic C-H stretch), 1492, 1342-1082 (CdC ring stretch), 863, 839 cm-1 (ring CdC bend). 3-(10-Undecen-1-yl)-2,2′-bithiophene (9). 10-Undecylenyl1-magnesium bromide was prepared by heating a mixture of 11-bromo-1-undecene (3.8 g, 16 mmol) and magnesium (0.40 g, 16 mmol) in anhydrous ether (20 mL) at reflux for 1 h. The Grignard reagent was treated with 8 (2.0 g, 8.2 mmol) and NiCl2(dppp) (40 mg, 74 µmol) in ether (10 mL) and worked up according to the procedure described above for the synthesis of 5. Purification by flash column chromatography (silica gel, petroleum ether) gave 9 (0.84 g, 32%) as a colorless liquid containing ≈5% of the internal alkene. 1H NMR (CDCl3) δ: 1.2-1.4 (m, 14H), 1.59 (p, J ) 7 Hz, 2H, C2 CH2), 2.03 (q, J ) 7 Hz, 2H, C9 CH2), 2.73 (t, J ) 7 Hz, 2H, C1 CH2), 4.95 (m, 2H, C11 CH2), 5.80 (m, 1H, C10 CH), 6.92 (d, J ) 4.8 Hz, 1H, thienyl-C4 CH), 7.05 (dd, J ) 3.6 Hz, 5.4 Hz, 1H, thienyl-C4′ CH), 7.10 (dd, J ) 1 Hz, 3.6 Hz, 1H, thienyl-C3′ CH), 7.15 (d, J ) 4.8 Hz, 1H, thienyl-C5 CH), 7.28 (dd, J ) 1 Hz, 5.4 Hz, 1H, thienyl-C5′ CH). 13C NMR (CDCl3) δ: 28.86, 29.04, 29.34, 29.42, 30.65, 33.75, 114.18, 123.81, 125.36, 126.07, 127.40, 129.98, 139.36, 139.79. IR (neat): 3112, 3073 (thiophene C-H stretch), 2925, 2853 (methylene C-H stretch), 1639, 1466, 1415 (thiophene out-of-plane), 1300-700 cm-1 (thiophene ring). Chlorodimethyl(11-(3-(2,2′-bithienyl))undecyl)silane (2). 9 was treated with chlorodimethylsilane and H2PtCl6 according to the procedure given above for the synthesis of 7 to give 10 (77%) as a colorless liquid containing ≈5% of unreacted internal alkene. 1H NMR (CDCl3) δ: 0.38 (s, 6H, Si(CH3)2), 0.79 (t, J ) 7 Hz, 2H, C1 CH2), 1.2-1.4 (m, 16H), 1.60 (p, J ) 7 Hz, 2H, C10 CH2), 2.72 (t, J ) 7 Hz, 2H, C11 CH2), 6.92 (d, J ) 4.8 Hz, 1H, thienyl-C4 CH), 7.05 (dd, J ) 3.6 Hz, 5.4 Hz, 1H, thienyl-C4′ CH), 7.10 (dd, J ) 1 Hz, 3.6 Hz, 1H, thienyl-C3′ CH), 7.15 (d, J ) 4.8 Hz, 1H, thienyl-C5 CH), 7.28 (dd, J ) 1 Hz, 5.4 Hz, 1H, thienyl-C5′ CH). 13C NMR (CDCl3) δ: 1.53, 18.89, 22.86, 29.03, 29.15, 29.42, 29.53, 30.63, 32.88, 123.8, 125.3, 126.1, 127.4, 130.0, 139.8. 2,3,5-Tribromothiophene(10). The title compound was prepared according to the method of Janda et al.36 and purified by flash column chromatography (silica gel, petroleum ether) to give 10 (89%) as a yellow liquid. 1H NMR (CDCl3) δ: 6.91 (s, 1H, thienyl-C5 CH). IR (neat): 3105 (aromatic C-H stretch), 1508, 1418, 1305 (CdC ring stretch), 1136, 1004 (in-plane CdH bend), 815 cm-1 (ring CdC bend).

3758 Langmuir, Vol. 15, No. 11, 1999 3′-Bromo-2,2′:5′,2′′-terthiophene (11). The title compound was prepared according to the method of Carpita.37 Purification by flash column chromatography (silica gel, petroleum ether) gave 11 (76%) as a colorless liquid. 1H NMR (CDCl3) δ: 7.04 (dd, J ) 3.3 Hz, 4.8 Hz, 1H, thienyl-C3′′ CH), 7.09 (s, 1H, thienyl-C4′ CH), 7.09 (dd, J ) 3.3 Hz, 4.8 Hz, 1H, thienyl-C4 CH), 7.19 (dd, J ) 1.5 Hz, 3.3 Hz, 1H, thienyl-C3′′ CH), 7.27 (dd, J ) 1.5 Hz, 4.8 Hz, 1H, thienyl-C5′′ CH), 7.36 (dd, J ) 1.5 Hz, 4.8 Hz, 1H, thienyl-C5 CH), 7.43 (dd, J ) 1.5 Hz, 3.3 Hz, 1H, thienyl-C3 CH). 13C NMR (CDCl ) δ: 107.85, 124.31, 125.35, 126.12, 126.60, 3 127.30, 127.64, 127.98, 130.77, 134.21, 135.70. IR (neat): 3105, 3079 (aromatic C-H stretch), 1644, 1493, 1420, 1223 (CdC ring stretch), 1078, 1045 (in-plane CdH bend), 848, 815, 709 cm-1 (ring CdC bend). 3′-(10-Undecen-1-yl)-2,2′:5′,2′′-terthiophene (12). A solution of 11 was treated with the Grignard reagent of 11-bromo1-undecene according to the method described above for the synthesis of 9. Column chromatography (petroleum ether, silica gel) gave 12 (62%) as a colorless liquid, containing ≈5% of the internal alkene.1H NMR (CDCl3) δ: 1.2-1.4 (m, 12H), 1.63 (p, J ) 7 Hz, 2H, C2 CH2), 2.02 (q, J ) 7 Hz, 2H, C9 CH2), 2.70 (t, J ) 7 Hz, 2H, C1 CH2), 4.9-5.0 (m, 2H, C11 CH2), 5.7-5.9 (m, 1H, C10 CH), 6.99 (dd, J ) 3.3 Hz, 5.1 Hz, 1H, thienyl-C4′′ CH), 7.00 (s, 1H, thienyl-C4′ CH), 7.05 (dd, J ) 3.3 Hz, 5.1 Hz, 1H, thienyl-C4 CH), 7.11 (dd, J ) 1 Hz, 3.3 Hz, 1H, thienyl-C3′′ CH), 7.14 (dd, J ) 1 Hz, 3.3 Hz, 1H, thienyl-C5′′ CH), 7.19 (dd, J ) 1 Hz, 5.1 Hz, 1H, thienyl-C5 CH), 7.29 (dd, J ) 1 Hz, 5.1 Hz, 1H, thienyl-C3 CH). 13C NMR (CDCl3) δ: 28.83, 29.02, 29.20, 29.31, 29.35, 29.40, 30.45, 33.71, 114.16, 123.61, 124.35, 125.37, 125.92, 126.57, 127.47, 127.89, 129.62, 135.21, 136.00, 137.33, 139.32, 140.37. Chlorodimethyl(11-(3′-(2,2′:5′,2′′-terthienyl))undecyl)silane (3). 12 was hydrosilated according to the method described above for the synthesis of 2 to give 3 (91%) as a light-brown liquid containing ≈5% of unreacted internal alkene. 1H NMR (CDCl3) δ: 0.38 (s, 6H, Si(CH3)2), 0.79 (t, J ) 7 Hz, 2H, C1 CH2), 1.2-1.4 (m, 16H), 1.60 (p, J ) 7 Hz, 2H, C10 CH2), 2.72 (t, J ) 7 Hz, 2H, C11 CH2), 6.99 (dd, J ) 3.3 Hz, 5.1 Hz, 1H, thienyl-C4′′ CH), 7.00 (s, 1H, thienyl-C4′ CH), 7.05 (dd, J ) 3.3 Hz, 5.1 Hz,

Inaoka and Collard 1H, thienyl-C4 CH), 7.11 (dd, J ) 1 Hz, 3.3 Hz, 1H, thienyl-C3′′ CH), 7.14 (dd, J ) 1 Hz, 3.3 Hz, 1H, thienyl-C5′′ CH), 7.19 (dd, J ) 1 Hz, 5.1 Hz, 1H, thienyl-C5 CH), 7.29 (dd, J ) 1 Hz, 5.1 Hz, 1H, thienyl-C3 CH). Chlorodimethyldecylsilane (13). A mixture of 1-decene (2.0 g, 14 mmol), chlorodimethylsilane (2.8 g, 30 mmol), and H2PtCl6 (0.03 mL of a 0.1 M solution in 2-propanol) was heated at reflux for 20 h. Purification by microdistillation gave 13 (2.9 g, 87%) as a colorless liquid. 1H NMR (CDCl3) δ: 0.37 (s, 6H, Si(CH3)2), 0.78 (t, J ) 7 Hz, 2H, C1 CH2), 0.86 (t, J ) 7 Hz, 3H, C10 CH3), 1.2-1.3 (m, 16H). 13C NMR (CDCl3) δ: 1.51, 14.00, 18.89, 22.59, 22.88, 29.18, 29.23, 29.45, 29.56, 31.85, 32.91. Preparation of SAMs. Substrates (glass microscope slides and gold evaporated on glass microscope slides coated with a chromium layer) were cleaned with H2SO4/30% H2O2 (7:3) (“piranha solution”, CAUTION: extreme care must be taken to avoid contact with organic materials), rinsed with water, and then rinsed with acetone in a Soxhlet extractor overnight. Surface modification was carried out by immersion of the substrates in a solution of appropriate chlorosilane (ca. 10 mM) in heptane for 16 h, followed by the immersion of the substrates in the solution of chlorotrimethylsilane (ca. 10 mM) in heptane for 2 h. The modified substrates were rinsed with either hexane and dried under the flow of nitrogen. Chemical Polymerization of Thiophene Derivatives on Monolayer-Modified Substrates. Modified substrates were immersed in solution of thiophene, 3-methylthiophene, or 3-octylthiophene (1 mmol) in CH2Cl2 (20 mL). Anhydrous FeCl3 was added to the solution, and the mixture was stirred for 16 h. The substrates were rinsed with a suspension of FeCl3 in CH2Cl2 (20 mL) to remove soluble polymer, then dedoped with acetone (3 × 20 mL) in an ultrasonic cleaner, and stored under N2.

Acknowledgment. This research was supported through awards from NSF (CAREER Award) and Research Corporation (Cottrell Scholarship). LA981330O