Self-Assembly of C60 π-Extended Tetrathiafulvalene (exTTF) Dyads

Langmuir 2006, 22, 10619-10624

10619

Self-Assembly of C60 π-Extended Tetrathiafulvalene (exTTF) Dyads on Gold Surfaces† Marı´a Sierra,‡ M. A Ä ngeles Herranz,‡ Sheng Zhang,§ Luis Sa´nchez,‡ Nazario Martı´n,*,‡ and Luis Echegoyen*,§ Departamento de Quı´mica Orga´ nica, Facultad de Ciencias Quı´micas, UniVersidad Complutense de Madrid, E-28040 Madrid, Spain, and Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634 ReceiVed April 27, 2006. In Final Form: July 25, 2006 The first self-assembly of a C60 π-extended tetrathiafulvalene (exTTF) dyad on a gold surface is reported. Four fullerene derivatives, two of them containing p-quinonoid π-extended tetrathiafulvalenes (exTTFs), have been synthesized, and their solution electrochemistry has been investigated by means of cyclic voltammetry. Fullerene-containing SAMs of thioctic acid derivatives 3 and 6 have also been investigated by cyclic voltammetry. The cyclic voltammograms of both compounds exhibit three reversible reduction waves, and for compound 6, one irreversible oxidation process corresponding to the oxidation of the exTTF subunit is observed. Stable self-assembled monolayers (SAMs) of fullerene derivative 3 were formed on gold surfaces, whereas dyad 6 does not present a very clear electrochemical response, most probably as a result of structural rearrangements on the monolayer or charge transfer between the C60 and exTTF moieties.

Introduction A variety of biological processes and artificial devices, such as photosynthesis and optoelectronic devices, respectively, are based on donor-acceptor interactions. Therefore, understanding electron-transfer (ET) reactions between the donor and the acceptor moieties is essential for a better knowledge of the primary processes involved in photosynthesis as well as to develop systems mimicking this natural process.1,2 A large number of photo- and electroactive systems based on donor-acceptor (D-A) assemblies have been synthesized to determine the structureproperty tradeoff resulting from the interactions between the donor and acceptor units.3 However, most of the literature reports on D-A systems are studies carried out in the solution phase, and only a few examples in which these systems are immobilized †

Part of the Electrochemistry special issue. * To whom correspondence should be addressed. E-mail: nazmar@ quim.ucm.es. Tel: (+34)-91-394-4227. Fax: (+34)-91-394-4103. E-mail: [email protected]. Tel: 864-656-0778. Fax: 864-656-6613. ‡ Universidad Complutense de Madrid. § Clemson University. (1) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 849. (c) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (d) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vols. 1-5. (e) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57. (f) Imahori, H.; Mori, Y.; Matano, Y. J. Photochem. Photobiol. 2003, C4, 51. (2) (a) Atienza, C. M.; Ferna´ndez, G.; Sa´nchez, L.; Martı´n, N.; Wienk, M. M.; Sa´ Dantas, I.; Janssen, R. A. J.; Rahman, G. M. A.; Guldi, D. M. Chem. Commun. 2006, 514. (b) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (c) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (d) Riedel, I.; von Hauff, E.; Parisi, J.; Martı´n, N.; Giacalone, F.; Dyakonov, V. AdV. Funct. Mater. 2005, 15, 1979. (e) Winder C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077. (f) Wienk, M. W.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371. (g) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (3) For some recent examples, see (a) Chernick, E. T.; Mi, Q.; Kelley, R. F.; Weiss, E. A.; Jones, B. A.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2006, 128, 4356. (b) Kim, O.-K.; Je, J.; Melinger, J. S. J. Am. Chem. Soc. 2006, 128, 4532. (c) Thompson, A. L.; Ahn, T.-S.; Thomas, K. R. J.; Thayumanavan, S.; Martı´nez, T. J.; Bardeen, C. J. J. Am. Chem. Soc. 2005, 127, 16348. (d) Perepichka, D. F.; Bryce, M. R. Angew. Chem., Int. Ed. 2005, 44, 5370 (e) Okamoto, K.; Imahori, H.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 7014. (f) Fukuzumi, S.; Okamoto, K.; Yoshida, Y.; Imahori, H.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2003, 125, 1007.

on a solid support have been reported.4 Among the many techniques explored, self-assembling electroactive organic molecules on solid substrates have been found to be a sensible choice for making thin films avalaible for the investigation of their physical properties. Thus, self-assembled monolayers (SAMs) formed by chemisorption yield well-defined and, on occasion, robust and stable structures that can operate in unison.5 The materials obtained by the application of SAM techniques offer efficient routes for the engineering of nanometer-scale devices for use in organic electronics.6 Fullerene-based materials exhibit interesting characteristics such as photovoltaic responses,2 superconductivity upon doping with alkali metals,7 nonlinear optical properties,8 and biological activity.9 Furthermore, fullerenes have frequently been used as (4) See, for example, (a) Umeyama, T.; Imahori, H. Photosynth. Res. 2006, 87, 63. (b) Kim, K.-S.; Kang, M.-S.; Ma, H.; Jen, A. K.-Y. Chem. Mater. 2004, 16, 5058. (c) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124, 532. (5) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (b) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (c) Cooke, G. Angew. Chem., Int. Ed. 2003, 42, 4860. (6) (a) Whitesides, G. M. Small 2005, 1, 172. (b) Jiang, P.; Morales, G. M.; You, W.; Yu, L. Angew. Chem., Int. Ed. 2004, 43, 4471. (c) Fan, F.-R. F.; Yao, Y.; Cai, L.; Cheng, L.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2004, 126, 4035. (d) Special Issue on Organic Electronics, Chem. Mater. 2004, 16, 4381. (e) Wassel, R. A.; Gorman, C. B. Angew. Chem., Int. Ed. 2004, 43, 5120. (f) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (g) Fox, M. A. Acc. Chem. Res. 1999, 32, 201. (7) (a) Boutorine, A. S.; Tokuyama, H.; Takasugi, M.; Isobe, H.; Nakamura, E.; Helene, C. Angew. Chem., Int. Ed. Engl. 1994, 33, 2462. (b) Kajzar, F.; Taliani, C.; Danieli, R.; Rossini, S.; Zamboni, R. Chem. Phys. Lett. 1994, 217, 418. (c) Tutt, L. W.; Kos, A. Nature 1992, 356, 225. (d) Hebard, A. F.; Rosseinski, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. M. T.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (e) Holczer, K.; Klein, O.; Huang, S.-M.; Kaner, R. B.; Fu, K.-J.; Whetten, R. L.; Diederich, F. Science 1991, 252, 1154. (8) (a) Herance, J. R.; Peris, E.; Vidal, J.; Bourdelande, J. L.; Marquet, J.; Garcı´a, H. Chem. Mater. 2005, 17, 4097. (b) Janner, A. M.; Jonkman, H. T.; Sawatzky, G. A. Phys. ReV. B 2001, 63, 085111/1. (c) Hoshi, H.; Yamada, T.; Ishikawa, K.; Takezoe, H.; Fukuda, A. Phys. ReV. B 1995, 52, 12355. (9) (a) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807. (b) Da Ros, T.; Prato, M. Chem. Commun. 1999, 663. (c) Dugan, L. L.; Turetsky, D. M.; Du, C.; Lobner, D.; Wheeler, M.; Almli, C. R.; Shen, C. K. F.; Luh, T. Y.; Choi, D. W.; Lin, T. S. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 9434. (d) Jensen, A.; Wilson, S. R.; Schuster, D. I. Bioorg. Med. Chem. 1996, 4, 767. (e) Friedman, S. H.; DeCamp, D. L.; Sijbesma, R. P.; Srdanov, G.; Wuld, F.; Kenyon, G. L. J. Am. Chem. Soc. 1993, 115, 6506.

10.1021/la061142v CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006

10620 Langmuir, Vol. 22, No. 25, 2006

Sierra et al.

Figure 1. Cyclic voltammograms of SAMs of an exTTF derivative (molecular structure on the left) in THF solutions containing 0.1 M TBAPF6 at different scan rates (50 mV s-1 to 2 V s-1).

an efficient electron acceptor component in D-A linked molecules.10 Since the pioneering work of Mirkin et al.11 to form well-defined SAMs of a C60 thiol derivative on gold surfaces, several reports have been published concerning C60 derivative SAMs by using donor-fullerene dyads and triads, and these have been systematically used to prepare optoelectronic devices.12 Echegoyen et al.13 reported SAMs of a fullerene derivative containing a 1,10-phenanthroline adduct on gold surfaces. They also prepared stable SAMs of oligothiophene-fulleropyrrolidine dyads by spontaneous adsorption on Au (111).14 More recently, an ordered SAM of a C60-anthrylphenylacetylene hybrid on a gold surface has demonstrated interesting electronic properties.15 The combination of different acceptor moieties with tetrathiafulvalene (TTF) and its derivatives,16 which are well known for their good electron donor ability, renders D-A conjugates very useful as models for fundamental investigations on charge-transfer interactions and molecular rectifiers.17,18 Furthermore, TTF derivatives can be reversibly oxidized to the thermodynamically stable radical cation and dication states,16 which have been favorably used to prepare molecular-level devices such as molecular switches, molecular shuttles, and even molecular logic circuits.19 p-Quinonoid π-extended tetrathiafulvalenes (exTTFs) have been investigated for the construction of different C60based nanoconjugates, and they are particularly interesting because, upon oxidation, exTTF shows a remarkable gain of (10) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Prato, M. J. Mater. Chem. 1997, 7, 1097. (c) Martı´n, N.; Sa´nchez, L.; Illescas, B.; Pe´rez, I. Chem. ReV. 1998, 98, 2527. (d) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445. (e) Diederich, F.; Go´mez-Lo´pez, M. Chem. Soc. ReV. 1999, 28, 263. (f) Guldi, D. M. Chem. Commun. 2000, 321. (g) Reed, C. A.; Bolskar, R. D. Chem. ReV. 2000, 100, 1075. (h) Gust, D.; Moore, T. A.; Moore, A. L. J. Photochem. Photobiol., B 2000, 58, 63. (i) Guldi, D. M.; Martı´n, N. J. Mater. Chem. 2002, 12, 1978. (j) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22. (k) Nierengarten, J. F. Top. Curr. Chem. 2003, 228, 87. (l) Guldi, D. M.; Prato, M. Chem. Commun. 2004, 2517. (m) Sa´nchez, L.; Martı´n, N.; Guldi, D. M. Angew. Chem., Int. Ed. 2005, 44, 5374. (n) Segura, J. L.; Martı´n, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34, 31. (o) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen, L.; Tome´, J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.sEur. J. 2005, 11, 3375. (p) Guldi, D. M. J. Phys. Chem. B, 2005, 109, 11432. (q) Guldi, D. M.; Zerbetto, F.; Georgakilas, V.; Prato, M. Acc. Chem. Res. 2005, 38, 38. (11) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598. (12) (a) Chukharev, V.; Vuorinen, T.; Efimov, A.; Tkachenko, N. V.; Kimura, M.; Fukuzumi, S.; Imahori, H.; Lemmetyinen, H. Langmuir 2005, 21, 6385. (b) Imahori, H.; Fukuzumi, S. AdV. Funct. Mater. 2004, 14, 525. (c) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (d) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099. (13) Domı´nguez, O.; Echegoyen, L.; Cunha, F.; Tao, N. J. Langmuir 1998, 14, 821. (14) Liu, S. G.; Marineau, C.; Raimundo, J.-M.; Roncali, J.; Echegoyen, L. Chem. Commun. 2001, 913. (15) Kang, S. H.; Ma, H.; Kang, M.-S.; Kim, K.-S.; Jen, A. K.-Y.; Zareie, M. H.; Sarikaya, M. Angew. Chem., Int. Ed. 2004, 43, 1512.

aromaticity and planarity.20 Only recently, we investigated the preparation of SAMs from exTTF derivatives and demonstrated that they form highly stable SAMs with surface-confined electrochemistry (Figure 1).21 Our previous work on structurally well defined donor-acceptor arrays that incorporate an exTTF as an electron donor and C60 as an electron acceptor has documented the numerous benefits of testing exTTF/C60 couples for ET reactions.22 In the current work, we take this example one step further, and our objective is to adsorb and address these two redox-active organic molecules on surfaces and to self-assemble systems composed of these molecules. For these purposes, we designed a new C60-exTTF dyad endowed with a 1,2-dithiolane unit to ensure strong adherence of the structure to the gold surface. To the best of our knowledge, no previous examples have been reported in which C60-exTTF dyads are self-assembled on a gold surface. Herein we report the synthesis of a new C60-exTTF dyad and the investigation of its electrochemical properties in solution as well as after SAM formation. Experimental Section Chemicals and General Methods. All reagents were of commercial quality and were used as supplied unless otherwise stated; solvents were distilled before use. N-(4-Hydroxyphenyl)-fullerene derivative 2 and 2-formyl-9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (4) were synthesized according to the protocols reported in the literature.15,23 Flash chromatography was performed using silica gel (Merck, kiesegel 60, 230-240 mesh). Analytical thin-layer chromatography (TLC) was performed using aluminum-coated Merck Kieselgel 60 F254 plates. NMR spectra were recorded on a Bruker AC-300 or (16) (a) Herranz, M. A.; Sa´nchez, L.; Martı´n, N. Phosphorus, Sulfur Silicon Relat. Elem. 2005, 180, 1133. (b) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104, 4891. (c) Segura, J. L.; Martı´n, N. Angew. Chem., Int. Ed. 2001, 40, 1372. (d) Bryce, M. R. J. Mater. Chem. 1999, 11, 11. (17) For charge-transfer interactions, see (a) Zhang, G.; Zhang, D.; Zhao, X.; Ai, X.; Zhang, J.; Zhu, D. Chem.sEur. J. 2006, 12, 1067. (b) Dı´az, M. C.; Illescas, B. M.; Martı´n, N.; Viruela, R.; Viruela, P. M.; Ortı´, E.; Brede, O.; Zilbermann, I.; Guldi, D. M. Chem.sEur. J. 2004, 10, 2067. (c) Dumur, F.; Gautier, N.; Gallego-Planas, N.; Sahin, Y.; Levillain, E.; Mercier, N.; Hudhomme, P.; Masimo, M.; Girlando, A.; Lloveras, V.; Vidal-Gancedo, J.; Veciana, J.; Rovira, C. J. Org. Chem. 2004, 69, 2164. (d) Gautier, N.; Dumur, F.; Lloveras, V.; Gancedo, J. V.; Veciana, J.; Rovira, C.; Hudhomme, P. Angew. Chem., Int. Ed. 2003, 42, 2765. (e) Perepichka, D. F.; Bryce, M. R.; McInnes, E. J. L.; Zhao, J. Org. Lett. 2001, 3, 1431. (18) For TTF-based molecular rectifiers, see (a) Ho, G.; Heath, J. R.; Kondratenko, M.; Perepichka, D. F.; Arseneault, K.; Pe´zolet, M.; Bryce, M. R. Chem.sEur. J. 2005, 11, 2914. (b) Perepichka, D. F.; Bryce, M. R.; Pearson, C.; Petty, M. C.; McInnes, E. J. L.; Zhao, J. P. Angew. Chem., Int. Ed. 2003, 42, 4636. (c) Perepichka, D. F.; Bryce, M. R.; Perepichka, I. F.; Lyubchik, S. B.; Godbert, N.; Christensen, C. A.; Batsanov, A. S.; Levillain, E.; McInnes, E. J. L.; Zhao, J. P. J. Am. Chem. Soc. 2002, 124, 14227.

Self-Assembly of TetrathiafulValene Dyads a Bruker Avance AV-500 spectrometer at 298 K using partially deuterated solvents as internal standards. Coupling constants (J) are denoted in Hz, and chemical shifts (δ), in ppm. Multiplicities and abbreviations are denoted as follows: s ) singlet, d ) doublet, qv ) quintet, m ) multiplet, and br ) broad. Melting points were measured on a Thermolab apparatus. FTIR spectra were recorded as KBr pellets on a Perkin-Elmer 257 spectrometer. UV-vis spectra were recorded with a Varian Cary 50 spectrophotometer using CHCl3 as the solvent. Mass spectra with electrospray ionization (ESI) were recorded on an HP1100MSD spectrometer. Synthesis of N-(4-Hydroxyphenyl)-fullerene Derivative 5. To a solution of C60 (0.100 g, 0.14 mmol) in chlorobenzene was added N-(4-hydroxyphenyl)-glycine (1) (0.41 mmol) and aldehyde 4 (61 mg, 0.15 mmol). The mixture was heated to reflux under argon for 16 h. After cooling to room temperature, the solvent was evaporated, and the crude compound was purified by column chromatography (silica gel, CS2/CHCl3). Further purification was accomplished by subsequent precipitation and centrifugation cycles with cyclohexane, ether, and methanol to give 5 as a brown solid (isomeric mixture)24 in 30% yield with a mp >300 °C. 1H NMR (CDCl3/CS2, 300 MHz): δ 7.97 (br s, 1.2H), 7.95 (br s, 0.8H), 7.71-7.58 (m, 4H), 7.33-7.18 (m, 8H,), 7.12 (d, J ) 7.3 Hz, 4H), 6.81-6.74 (m, 4H), 6.28-6.04 (m, 8H), 6.04 (s, 1.2 H), 5.97 (s, 0.8 H), 5.55 (d, J ) 8.6 Hz, 1.2H), 5.50 (d, J ) 8.6 Hz, 0.8H), 4.91 (d, J ) 8.6 Hz, 1.2H), 4.87 (d, J ) 8.6 Hz, 0.8H), 4.49 (br s, 1.2 H) 4.43 (br s, 0.8H). 13C NMR (CDCl3/CS2, 75 MHz): δ 155.65, 153.50, 148.11, 147.40, 146.78, 146.70, 146.63, 146.40, 146.14, 146.01, 145.70, 145.50, 145.27, 145.08, 144.93, 143.20, 143.06, 142.61, 142.56, 142.42, 142.05, 140.79, 140.14, 138.61, 137.70, 137.22, 137.04, 136.68, 136.12, 135.74, 135.55, 135.41, 134.99, 126.64, 126.27, 126.14, 125.73, 125.60, 125.05, 124.98, 123.13, 122.70, 122.49, 122.02, 119.06, 118.01, 117.75, 116.98, 82.23, 70.21, 69.33, 68.71. FTIR (KBr): 1624, 1508, 1458, 1253, 1172, 640, 526 cm-1. UV-vis (CHCl3) λmax, (log ): 433 (3.09), 328 (5.22), 257 (5.84) nm. MS m/z (ESI): 1233 (M+, 100). General Procedure for the Syntheses of Fullerene Disulfides 3 and 6. To a solution of the corresponding alcohol-functionalized 2 or 5 (0.30 mmol) in chlorobenzene was added 1,3-dicyclohexylcarbodiimide (DCC) (0.37 mmol) and 4-(dimethylamino)-pyridine (DMAP) (0.15 mmol), and the mixture was stirred at room temperature for 20 min. Then thioctic acid (0.32 mmol) was added, (19) See, for example, (a) Balzani, V.; Clemente-Leon, M.; Credi, A.; Ferrer, B.; Venturi, M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1178. (b) Jang, S. S.; Jang, Y. H.; Kim, Y.-H.; Goddard, W. A.; Flood, A. H.; Laursen, B. W.; Tseng, H.-R.; Stoddart, J. F.; Jeppesen, J. O.; Choi, J. W.; Steuerman, D. W.; DeIonno, E.; Heath, J. R. J. Am. Chem. Soc. 2005, 127, 1563. 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J. 2005, 11, 7199. (b) Handa, S.; Giacalone, F.; Haque, S. A.; Palomares, E.; Martı´n, N.; Durrant, J. R. Chem.sEur. J. 2005, 11, 7440. (c) Sa´nchez, L.; Sierra, M.; Martı´n, N.; Guldi, D. M.; Wienk, M. W.; Janssen, R. A. J. Org. Lett. 2005, 7, 1691. (d) Giacalone, F.; Segura, J. L.; Martı´n, N.; Guldi, D. M. J. Am. Chem. Soc. 2004, 126, 5340. (e) Sa´nchez, L.; Pe´rez, I.; Martı´n, N.; Guldi, D. M. Chem.sEur. J. 2003, 9, 2457. (f) Gonza´lez, S.; Martı´n, N.; Guldi, D. M. J. Org. Chem. 2003, 68, 779. (g) Kodis, G.; Liddell, P. A.; de la Garza, L.; Moore, A. L.; Moore, T. A.; Gust, D. J. Mater. Chem. 2002, 12, 2100. (h) Herranz, M. A.; Martı´n, N.; Ramey, J.; Guldi D. M. Chem. Commun. 2002, 2968. (i) Martı´n, N.; Sa´nchez, L.; Guldi, D. M. Chem. Commun. 2000, 113. (23) Martı´n, N.; Pe´rez, I.; Sa´nchez, L.; Seoane, C. J. Org. Chem. 1997, 62, 5690. (24) Compounds 5 and 6 showed resonance splitting in their 1H NMR spectra, which has previously been accounted for in comparable systems by the equilibrium existing between the two different conformations of the exTTF units. See Herranz, M. A.; Illescas, B.; Martı´n, N.; Luo, Ch.; Guldi, D. M. J. Org. Chem. 2001, 65, 5728.

Langmuir, Vol. 22, No. 25, 2006 10621 and the solution was stirred for 16 h. After this period of time, the solvent was evaporated, and the product was purified by column chromatography (silica gel, CS2/toluene). Subsequently, the solvent was evaporated to dryness, and the solid residue was further purified by precipitation and centrifugation cycles with cyclohexane, ether, and methanol to afford 3 or 6 as a brown solid. 3. 83% yield, mp > 300 °C. 1H NMR (CDCl3/CS2, 300 MHz): δ 7.53 (2H, d, J ) 9.00 Hz), 7.41 (2H, d, J ) 9.00 Hz), 5.45 (4H, s), 3.85 (1H, q, J ) 6.9 Hz), 3.41 (2H, m), 2.7-2.8 (3H, m), 2.3-1.8 (7H, m). FTIR (KBr): 526, 1128, 1170, 1188, 1215, 1458, 1508, 1541, 1635, 1749 cm-1. 13C NMR (CDCl3/CS2, 75 MHz): δ 171.01, 154.90, 148.10, 147.06, 146.85, 146.56, 146.38, 146.06, 145.54, 145.32, 143.87, 143.45, 142.97, 142.87, 142.70, 141.09, 136.99, 123.42, 117.82, 78.42, 70.48, 64.28, 60.05, 57.32, 41.31, 39.83, 35.09, 34.99, 31.03, 30.02, 25.92. UV-vis (CHCl3) λmax, (log ): 329 (5.32), 258 (5.87) nm. MS m/z (ESI): 1029 (M+, 100). 6 (Isomeric Mixture).24 96% yield, mp > 300 °C. 1H NMR (CDCl3/ CS2, 300 MHz): δ 8.01 (br s, 1.2H), 7.98 (br s, 0.8H), 7.72-7.58 (m, 4H), 7.41-7.22 (m, 8H,), 7.09 (d, J ) 7.3 Hz, 4H), 6.81-6.74 (m, 4H), 6.29-6.17 (m, 8H), 6.14 (s, 1.2 H), 6.07 (s, 0.8 H), 5.72 (d, J ) 8.6 Hz, 1.2H), 5.60 (d, J ) 8.6 Hz, 0.8H), 5.05 (d, J ) 8.6 Hz, 1.2H), 4.98 (d, J ) 8.6 Hz, 0.8H), 3.56 (q, J ) 8.50 Hz, 2H), 3.20-3.14 (m, 4H), 2.59-2.46 (m, 6H), 1.92 (m, 2H), 1.60 (m, 8H), 1.48 (m, 4H). 13C NMR (CDCl3/CS2, 75 MHz): δ 171.91, 155.98, 153.96, 147.81, 146.84, 146.70, 146.28, 146.01, 145.70, 145.22, 145.13, 144.87, 143.15, 143.08, 142.60, 142.42, 141.95, 140.77, 140.13, 138.60, 137.21, 136.94, 136.72, 135.41, 134.99, 127.30, 126.59, 126.38, 126.24, 125.68, 125.54, 125.14, 124.95, 123.87, 122.68, 122.52, 121.96, 118.86, 117.88, 117.65, 116.83, 69.41, 68.61, 56.89, 40.86, 39.27, 34.68, 34.39, 30.19, 29.46, 27.47; 23.61; FTIR (KBr): 526, 1012, 1122, 1170, 1201, 1458, 1506, 1541, 1558, 1652, 1749 cm-1. UV-vis (CHCl3) λmax, (log ): 430.16 (3.48), 327 (5.18), 257 (5.74) nm. MS m/z (ESI): 1421 (M+, 100). Electrochemistry. All of the electrochemical measurements were performed with a CHI 660 electrochemical workstation (CHI Instruments Inc., Austin, TX). Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) or tetrabutylammonium perchlorate (TBAClO4) in dry CH2Cl2 or anhydrous 1,2-dichlorobenzene was used as the supporting electrolyte (degassed with argon). A platinum wire was employed as the counter electrode. A Ag/Ag+ electrode and a Ag wire were used as the reference electrodes for the monolayer and solution electrochemistry, respectively. In the case of solution electrochemistry, ferrocene (Fc) was added as an internal reference, and the potentials were referenced relative to the Fc/Fc+ couple. The potentials for the monolayer electrochemistry were referenced relative to the Ag/Ag+ couple. A glassy carbon electrode, polished with aluminum paste and ultrasonicated in deionized water and in a CH2Cl2 bath, was used as the working electrode for the solution electrochemistry. SAM-modified gold bead electrodes were used as working electrodes for monolayer electrochemistry. Monolayer Preparation. Gold bead electrodes were prepared by heating a gold wire in a natural gas/O2 flame followed by cooling in deionized water. A glass capillary was inserted from the opposite side of the gold beads and melted onto the wire, insulating the gold electrodes. The gold beads were electrochemically cleaned as reported previously.25 Monolayers on gold were prepared by the immersion of freshly prepared gold beads in 1 mM solutions of compounds 3 and 6 (Scheme 1) in toluene. After removal, the gold beads were rinsed with copious amounts of toluene and dried in a stream of argon. The CVs of the SAMs were measured in a solution of acetonitrile containing 0.1 M TBAPF6, which was thoroughly purged with argon.

Results and Discussion The synthesis procedure used for the preparation of fullerene derivatives 2, 3, 5, and 6 is shown in Scheme 1. Compounds 2 (25) (a) Zhang, S.; Echegoyen, L. Tetrahedron 2006, 62, 1947. (b) Zhang, S.; Palkar, A.; Fragoso, A.; Prados, P.; de Mendoza, J.; Echegoyen, L. Chem. Mater. 2005, 17, 2063. (c) Zhang, S.; Echegoyen, L. J. Org. Chem. 2005, 70, 9874. (d) Zhang, S.; Echegoyen, L. J. Am. Chem. Soc. 2005, 127, 2006. (e) Zhang, S.; Echegoyen, L. Org. Lett. 2004, 6, 791 and references therein.

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Sierra et al.

Scheme 1. Synthesis of C60-exTTF Dyad 6 and Reference Compound 3

and 5 were obtained by the 1,3-dipolar cycloaddition reaction26 of paraformaldehyde or 2-formyl-9,10-bis(1,3-dithiol-2-ylidene)9,10-dihydroanthracene (4)23 with N-(4-hydroxyphenyl)-glycine (1) and C60 in chlorobenzene in 39 and 30% yields, respectively. The preparation of thioctic acid derivatives 3 and 6 was subsequently carried out starting from corresponding alcoholfunctionalized fullerene derivatives 2 and 5 following the previously described esterification procedure with thioctic acid in the presence of DCC/DMAP.27 As previously noticed, this direct coupling reaction to prepare disulfides is straightforward and leads to the expected compounds in good yields, 83 (3) and 96% (6). The solution electrochemistry of compounds 3 and 6 was investigated by cyclic voltammetry in 1,2-dichlorobenzene and CH2Cl2, respectively. Figure 2 shows the CV of compound 3. Three reduction processes were observed at -1.16, -1.54, and -2.11 V versus Fc/Fc+, respectively, which are due to the first three reduction processes of the C60 cage. The first and second reduction waves are reversible, and two additional broad reoxidation peaks appeared at -0.81 and -1.27 V versus Fc/ Fc+, respectively, after scanning past the third reduction peak. This observation suggests that scanning to more negative potentials leads to chemical reactions of the reference compound after multiple reductions. The CV of compound 6 is shown in Figure 3. The first three reduction peaks observed at -0.99, -1.37, and -1.92 V versus Fc/Fc+ correspond to the fulleropyrrolidine unit. The first two reduction processes of compound 6 are reversible, as are those for compound 3. Figure 3 also shows an electrochemically irreversible oxidation process that corresponds to the oxidation of the exTTF subunit.21,28 exTTFs, unlike TTF, upon oxidation exhibit a single two-electron chemically reversible oxidation (26) For examples of 1,3-dipolar cycloadditions on fullerenes, see (a) Tagmatarchis, N.; Prato, M. Synlett 2003, 768. (b) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519. (27) (a) Herranz, M. A.; Colonna, B.; Echegoyen, L. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5040. (b) Cooke, G.; Duclair, F. M. A.; Rotello, V. M.; Stoddart, J. F. Tetrahedron Lett. 2000, 41, 8163. (28) (a) Liu, S.-G.; Pe´rez, I.; Martı´n, N.; Echegoyen, L. J. Org. Chem. 2000, 65, 9092. (b) Pe´rez, I.; Liu, S.-G.; Martı´n, N.; Echegoyen, L. J. Org. Chem. 2000, 65, 3796.

process to form a dicationic state, and this oxidation is accompanied by a drastic geometric change from highly distorted butterfly-like geometry in the neutral state to a planar aromatic hydrocarbon skeleton in the dicationic state.20 The CV of the SAMs obtained from compound 3 shows the expected three one-electron cathodic waves with E01 ) -0.93, E02 ) -1.31, and E03 -1.80 V versus Ag/Ag+ corresponding to the first three reductions of the fullerene fragment (Figure 4a). The potentials were not referenced to internal Fc/Fc+ because the SAMs blocked this compound from approaching the electrode surface. Within the range of scan rates investigated, peak currents increased linearly with the scan rate, but the peak potentials and peak-to-peak separations were independent of the scan rate. These characteristics together with the Gaussian shape of the CV waves are typical of surface-confined electroactive species.25 Figure 4b shows the reductive electrochemistry of SAMs of 6. The cathodic electrochemical response of monolayers of 6 is also consistent with a redox system confined to the gold electrode surface. However, the waves observed are, unlike those of SAMs of 3, not as well-defined or reversible. Compared to the SAMs of 3, those of 6 exhibit much broader reduction peaks. Additionally, the CV changed dramatically upon successive scans,

Figure 2. Cyclic voltammogram of 3 recorded in 1,2-dichlorobenzene containing 0.1 M Bu4ClO4. Scan rate, 100 mV s-1.

Self-Assembly of TetrathiafulValene Dyads

Figure 3. Cyclic voltammogram of 6 recorded in CH2Cl2 containing 0.1 M Bu4PF6. Scan rate, 100 mV s-1.

Langmuir, Vol. 22, No. 25, 2006 10623

Figure 5. Cyclic voltammograms recorded in CH3CN containing 0.1 M Bu4NPF6 of SAMs grown from 6 at variable scan rates (100600 mV s-1).

Figure 6. Cathodic scan of the CV recorded in CH3CN containing 0.1 M Bu4NPF6 of SAMs grown from 6 and tert-butylcalix[8]arene.

Figure 4. Reductive part of the CVs recorded in CH3CN of SAMs grown from (a) 3 and (b) 6.

indicating some structural rearrangement of the compound and/ or charge transfer between C60 and the exTTF subunits. Scanning anodically, the SAMs of 6 show an irreversible broad oxidation peak at 0.52 V versus Ag/Ag+. This peak corresponds to the oxidation of the exTTF fragment (Figure 5). Increasing the scan rate leads to improved reversibility, and an additional oxidation is observed at 0.12 V versus Ag/Ag+, for which we do not yet have a clear explanation. In an attempt to prepare SAMs with better preorganization and to observe improved electrochemical responses for the SAMs of 6, mixed monolayers were grown from toluene solution containing octanethiol and compound 6. However, the electrochemical response observed did not change appreciably. Because C60 is known to bind calix[8]arene,29 it was added to see if it

would inhibit the aggregation of C60 and result in a better monolayer response. Thus, SAMs were grown from toluene solutions of compound 6 in the presence of excess tert-butylcalix[8]arene. As shown in Figure 6, three reasonably defined reduction waves were observed at -0.92, -1.31, and -1.83 V versus Ag/Ag+, respectively, corresponding to the redox processes of C60. However, the first two reduction peaks are still very broad. Compared with the redox responses of the SAMs of the fullerene derivative used as reference (3), the reduction peaks of SAMs grown with dyad 6 are much broader, and the electrochemical response keeps changing upon successive scans under all of the conditions used. Experiments are underway to grow monolayers with similar dyads possessing different molecular designs to further probe charge-transfer interactions in SAMs of C60-exTTF derivatives.

Conclusions A new C60-exTTF dyad with gold surface anchoring groups has been prepared. SAMs of this compound and a reference fullerene derivative were formed on gold surfaces and characterized by cyclic voltammetry. SAMs of 6, however, do not exhibit very reproducible electrochemical responses, presumably because of structural rearrangements of the monolayer or charge-transfer events between the C60 and the exTTF subunits. Further investiga(29) (a) Pan, G.-B.; Liu, J.-M.; Zhang, H.-M.; Wan, L.-J.; Zheng, Q.-Y.; Bai, C.-L. Angew. Chem., Int. Ed. 2003, 42, 2747. (b) Atwood, J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994, 368, 229. (c) Suzuki, T.; Nakashima, K.; Shinkai, S. Chem. Lett. 1994, 699.

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tions will include the preparation of new dyads with different molecular designs for SAM formation and an investigation of the photoelectrochemical properties of these C60-based monolayers. Acknowledgment. This work was supported by the MEC of Spain (project CTQ2005-02609/BQU), the CAM (project P-PPQ-

Sierra et al.

000225-0505), and the U.S. National Science Foundation (grant CHE-0509989). M.S. and M.A.H. thank the MEC and CICYT of Spain for a studentship and a Ramo´n y Cajal contract, respectively. LA061142V