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

Marıa Sierra,‡ M. AÄ ngeles Herranz,‡ Sheng Zhang,§ Luis Sanchez,‡ Nazario Martı´n,*,‡ andLuis Echegoyen*,§

Departamento de Quı´mica Organica, Facultad de Ciencias Quı´micas, UniVersidad Complutense deMadrid, 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. Fourfullerene derivatives, two of them containingp-quinonoidπ-extended tetrathiafulvalenes (exTTFs), have been synthesized,and their solution electrochemistry has been investigated by means of cyclic voltammetry. Fullerene-containing SAMsof thioctic acid derivatives3 and6 have also been investigated by cyclic voltammetry. The cyclic voltammogramsof both compounds exhibit three reversible reduction waves, and for compound6, one irreversible oxidation processcorresponding to the oxidation of the exTTF subunit is observed. Stable self-assembled monolayers (SAMs) offullerene derivative3 were formed on gold surfaces, whereas dyad6 does not present a very clear electrochemicalresponse, 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, suchas photosynthesis and optoelectronic devices, respectively, arebased on donor-acceptor interactions. Therefore, understandingelectron-transfer (ET) reactions between the donor and theacceptor moieties is essential for a better knowledge of the primaryprocesses involved in photosynthesis as well as to develop systemsmimicking this natural process.1,2 A large number of photo- andelectroactive systems based on donor-acceptor (D-A) as-semblies have been synthesized to determine the structure-property tradeoff resulting from the interactions between thedonor and acceptor units.3 However, most of the literature reportson D-A systems are studies carried out in the solution phase,and only a few examples in which these systems are immobilized

on a solid support have been reported.4 Among the manytechniques explored, self-assembling electroactive organic mol-ecules on solid substrates have been found to be a sensible choicefor making thin films avalaible for the investigation of theirphysical 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 Thematerials obtained by the application of SAM techniques offerefficient routes for the engineering of nanometer-scale devicesfor use in organic electronics.6

Fullerene-based materials exhibit interesting characteristicssuch as photovoltaic responses,2 superconductivity upon dopingwith alkali metals,7 nonlinear optical properties,8 and biologicalactivity.9 Furthermore, fullerenes have frequently been used as

† 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:luis@clemson.edu. 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.;SaDantas, 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.

(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.Small2005, 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.Nature1992, 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.Nature1991, 350, 600. (e) Holczer, K.; Klein, O.; Huang,S.-M.; Kaner, R. B.; Fu, K.-J.; Whetten, R. L.; Diederich, F.Science1991, 252,1154.

(8) (a) Herance, J. R.; Peris, E.; Vidal, J.; Bourdelande, J. L.; Marquet, J.;Garcıa, 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.

10619Langmuir2006,22, 10619-10624

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

an efficient electron acceptor component in D-A linkedmolecules.10Since the pioneering work of Mirkin et al.11 to formwell-defined SAMs of a C60 thiol derivative on gold surfaces,several reports have been published concerning C60 derivativeSAMs by using donor-fullerene dyads and triads, and these havebeen systematically used to prepare optoelectronic devices.12

Echegoyen et al.13 reported SAMs of a fullerene derivativecontaining a 1,10-phenanthroline adduct on gold surfaces. Theyalso prepared stable SAMs of oligothiophene-fulleropyrrolidinedyads by spontaneous adsorption on Au (111).14 More recently,an ordered SAM of a C60-anthrylphenylacetylene hybrid on agold surface has demonstrated interesting electronic properties.15

The combination of different acceptor moieties with tetrathi-afulvalene (TTF) and its derivatives,16which are well known fortheir good electron donor ability, renders D-A conjugates veryuseful as models for fundamental investigations on charge-transferinteractions and molecular rectifiers.17,18 Furthermore, TTFderivatives can be reversibly oxidized to the thermodynamicallystable radical cation and dication states,16 which have beenfavorably used to prepare molecular-level devices such asmolecular switches, molecular shuttles, and even molecular logiccircuits.19p-Quinonoidπ-extended tetrathiafulvalenes (exTTFs)have been investigated for the construction of different C60-based nanoconjugates, and they are particularly interestingbecause, upon oxidation, exTTF shows a remarkable gain of

aromaticity and planarity.20 Only recently, we investigated thepreparation of SAMs from exTTF derivatives and demonstratedthat they form highly stable SAMs with surface-confinedelectrochemistry (Figure 1).21

Our previous work on structurally well defined donor-acceptorarrays that incorporate an exTTF as an electron donor and C60

as an electron acceptor has documented the numerous benefitsof testing exTTF/C60 couples for ET reactions.22 In the currentwork, we take this example one step further, and our objectiveis to adsorb and address these two redox-active organic moleculeson surfaces and to self-assemble systems composed of thesemolecules. For these purposes, we designed a new C60-exTTFdyad endowed with a 1,2-dithiolane unit to ensure strongadherence of the structure to the gold surface. To the best of ourknowledge, no previous examples have been reported in whichC60-exTTF dyads are self-assembled on a gold surface. Hereinwe report the synthesis of a new C60-exTTF dyad and theinvestigation of its electrochemical properties in solution as wellas after SAM formation.

Experimental Section

Chemicals and General Methods.All reagents were of com-mercial quality and were used as supplied unless otherwise stated;solvents were distilled before use.N-(4-Hydroxyphenyl)-fullerenederivative2 and 2-formyl-9,10-bis(1,3-dithiol-2-ylidene)-9,10-di-hydroanthracene (4) were synthesized according to the protocolsreported 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 60F254 plates. NMR spectra were recorded on a Bruker AC-300 or

(10) (a) Imahori, H.; Sakata, Y.AdV. Mater. 1997, 9, 537. (b) Prato, M.J.Mater. Chem.1997, 7, 1097. (c) Martı´n, N.; Sanchez, 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-Lopez, 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., B2000, 58, 63. (i) Guldi, D. M.; Martın, 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.Langmuir2005, 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. B2000, 104, 2099.

(13) Domınguez, O.; Echegoyen, L.; Cunha, F.; Tao, N. J.Langmuir1998,14, 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.

(16) (a) Herranz, M. A.; Sa´nchez, L.; Martı´n, N. Phosphorus, Sulfur SiliconRelat. 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.

Figure 1. Cyclic voltammograms of SAMs of an exTTF derivative (molecular structure on the left) in THF solutions containing 0.1 MTBAPF6 at different scan rates (50 mV s-1 to 2 V s-1).

10620 Langmuir, Vol. 22, No. 25, 2006 Sierra et al.

a Bruker Avance AV-500 spectrometer at 298 K using partiallydeuterated solvents as internal standards. Coupling constants (J) aredenoted in Hz, and chemical shifts (δ), in ppm. Multiplicities andabbreviations are denoted as follows: s) singlet, d) doublet, qv) quintet, m) multiplet, and br) broad. Melting points weremeasured on a Thermolab apparatus. FTIR spectra were recordedas KBr pellets on a Perkin-Elmer 257 spectrometer. UV-vis spectrawere recorded with a Varian Cary 50 spectrophotometer using CHCl3

as the solvent. Mass spectra with electrospray ionization (ESI) wererecorded on an HP1100MSD spectrometer.

Synthesis ofN-(4-Hydroxyphenyl)-fullerene Derivative 5.Toa solution of C60 (0.100 g, 0.14 mmol) in chlorobenzene was addedN-(4-hydroxyphenyl)-glycine (1) (0.41 mmol) and aldehyde4 (61mg, 0.15 mmol). The mixture was heated to reflux under argon for16 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 bysubsequent precipitation and centrifugation cycles with cyclohexane,ether, and methanol to give5 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 ε): 433 (3.09), 328 (5.22), 257 (5.84) nm. MSm/z (ESI):1233 (M+, 100).

General Procedure for the Syntheses of Fullerene Disulfides3 and 6.To a solution of the corresponding alcohol-functionalized2 or 5 (0.30 mmol) in chlorobenzene was added 1,3-dicyclohexyl-carbodiimide (DCC) (0.37 mmol) and 4-(dimethylamino)-pyridine(DMAP) (0.15 mmol), and the mixture was stirred at roomtemperature for 20 min. Then thioctic acid (0.32 mmol) was added,

and the solution was stirred for 16 h. After this period of time, thesolvent was evaporated, and the product was purified by columnchromatography (silica gel, CS2/toluene). Subsequently, the solventwas evaporated to dryness, and the solid residue was further purifiedby precipitation and centrifugation cycles with cyclohexane, ether,and methanol to afford3 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 ε):329 (5.32), 258 (5.87) nm. MSm/z (ESI): 1029 (M+, 100).

6(Isomeric Mixture).2496% 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 ) 8.6Hz, 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, (logε): 430.16 (3.48), 327 (5.18),257 (5.74) nm. MSm/z (ESI): 1421 (M+, 100).

Electrochemistry.All of the electrochemical measurements wereperformed with a CHI 660 electrochemical workstation (CHIInstruments Inc., Austin, TX). Tetrabutylammonium hexafluoro-phosphate (TBAPF6, 0.1 M) or tetrabutylammonium perchlorate(TBAClO4) in dry CH2Cl2 or anhydrous 1,2-dichlorobenzene wasused as the supporting electrolyte (degassed with argon). A platinumwire was employed as the counter electrode. A Ag/Ag+ electrodeand a Ag wire were used as the reference electrodes for the monolayerand solution electrochemistry, respectively. In the case of solutionelectrochemistry, ferrocene (Fc) was added as an internal reference,and the potentials were referenced relative to the Fc/Fc+ couple. Thepotentials for the monolayer electrochemistry were referenced relativeto the Ag/Ag+ couple. A glassy carbon electrode, polished withaluminum paste and ultrasonicated in deionized water and in a CH2-Cl2 bath, was used as the working electrode for the solutionelectrochemistry. SAM-modified gold bead electrodes were used asworking electrodes for monolayer electrochemistry.

Monolayer Preparation. Gold bead electrodes were preparedby heating a gold wire in a natural gas/O2 flame followed by coolingin deionized water. A glass capillary was inserted from the oppositeside of the gold beads and melted onto the wire, insulating the goldelectrodes. The gold beads were electrochemically cleaned as reportedpreviously.25 Monolayers on gold were prepared by the immersionof freshly prepared gold beads in 1 mM solutions of compounds3and6 (Scheme 1) in toluene. After removal, the gold beads wererinsed with copious amounts of toluene and dried in a stream ofargon. The CVs of the SAMs were measured in a solution ofacetonitrile containing 0.1 M TBAPF6, which was thoroughly purgedwith argon.

Results and DiscussionThe synthesis procedure used for the preparation of fullerene

derivatives2, 3, 5, and6 is shown in Scheme 1. Compounds2

(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.(c) Steuerman, D.; Tseng, H.; Peters, A.; Flood, A.; Jeppesen, J.; Nielsen, K.;Stoddart, J. F.; Heath, J.Angew. Chem., Int. Ed.2004, 43, 6486. (d) Tseng, H.;Vignon, S.; Stoddart, J. F.Angew. Chem., Int. Ed.2003, 42, 1491. (e) Jeppesen,J.; Vignon, S.; Stoddart, J. F.Chem.sEur. J. 2003, 9, 4611. (f) Pease, A. R.;Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R.Acc. Chem.Res.2001, 34, 433.

(20) (a) Yamashita, Y.; Kobayashi, Y.; Miyashi, T.Angew. Chem., Int. Ed.Engl. 1989, 28, 1052. (b) Bryce, M. R.; Moore, A. J.; Hasan, M.; Ashwell, G.J.; Fraser, A. T.; Clegg, W.; Hursthouse, M. B.; Karaulov, A. I.Angew. Chem.,Int. Ed. Engl.1990, 29, 1450. (c) Martı´n, N.; Sanchez, L.; Seoane, C.; Ortı´, E.;Viruela, P. M.; Viruela, R. M.J. Org. Chem.1998, 63, 1268.

(21) Herranz, M. A.; Yu, L.; Martı´n, N.; Echegoyen, L.J. Org. Chem.2003,68, 8379.

(22) See, for example, (a) Guldi, D. M.; Giacalone, F.; de la Torre, G.; Segura,J. L.; Martın, N. Chem.sEur. 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.;Martın, 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.; dela 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.; Sanchez, L.; Guldi, D. M.Chem. Commun.2000, 113.

(23) Martın, N.; Perez, I.; Sanchez, L.; Seoane, C.J. Org. Chem.1997, 62,5690.

(24) Compounds5 and6 showed resonance splitting in their1H NMR spectra,which has previously been accounted for in comparable systems by the equilibriumexisting 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.

(25) (a) Zhang, S.; Echegoyen, L.Tetrahedron2006, 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.

Self-Assembly of TetrathiafulValene Dyads Langmuir, Vol. 22, No. 25, 200610621

and5 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)23with N-(4-hydroxyphenyl)-glycine(1) and C60 in chlorobenzene in 39 and 30% yields, respectively.The preparation of thioctic acid derivatives3 and 6 wassubsequently carried out starting from corresponding alcohol-functionalized fullerene derivatives2 and 5 following thepreviously described esterification procedure with thioctic acidin the presence of DCC/DMAP.27 As previously noticed, thisdirect coupling reaction to prepare disulfides is straightforwardand leads to the expected compounds in good yields, 83 (3) and96% (6).

The solution electrochemistry of compounds3 and 6 wasinvestigated by cyclic voltammetry in 1,2-dichlorobenzene andCH2Cl2, respectively. Figure 2 shows the CV of compound3.Three reduction processes were observed at-1.16,-1.54, and-2.11 V versus Fc/Fc+, respectively, which are due to the firstthree reduction processes of the C60 cage. The first and secondreduction waves are reversible, and two additional broadreoxidation 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 negativepotentials leads to chemical reactions of the reference compoundafter multiple reductions.

The CV of compound6 is shown in Figure 3. The first threereduction peaks observed at-0.99,-1.37, and-1.92 V versusFc/Fc+ correspond to the fulleropyrrolidine unit. The first tworeduction processes of compound6 are reversible, as are thosefor compound3. Figure 3 also shows an electrochemicallyirreversible oxidation process that corresponds to the oxidationof the exTTF subunit.21,28exTTFs, unlike TTF, upon oxidationexhibit a single two-electron chemically reversible oxidation

process to form a dicationic state, and this oxidation isaccompanied by a drastic geometric change from highly distortedbutterfly-like geometry in the neutral state to a planar aromatichydrocarbon skeleton in the dicationic state.20

The CV of the SAMs obtained from compound3 shows theexpected three one-electron cathodic waves withE0

1 ) -0.93,E0

2 ) -1.31, andE03 -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+ becausethe SAMs blocked this compound from approaching the electrodesurface. Within the range of scan rates investigated, peak currentsincreased linearly with the scan rate, but the peak potentials andpeak-to-peak separations were independent of the scan rate. Thesecharacteristics together with the Gaussian shape of the CV wavesare typical of surface-confined electroactive species.25

Figure 4b shows the reductive electrochemistry of SAMs of6. The cathodic electrochemical response of monolayers of6 isalso consistent with a redox system confined to the gold electrodesurface. However, the waves observed are, unlike those of SAMsof 3, not as well-defined or reversible. Compared to the SAMsof 3, those of 6 exhibit much broader reduction peaks.Additionally, the CV changed dramatically upon successive scans,

(26) For examples of 1,3-dipolar cycloadditions on fullerenes, see (a)Tagmatarchis, N.; Prato, M.Synlett2003, 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.; Martın, 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.

Figure 2. Cyclic voltammogram of3 recorded in 1,2-dichloroben-zene containing 0.1 M Bu4ClO4. Scan rate, 100 mV s-1.

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

10622 Langmuir, Vol. 22, No. 25, 2006 Sierra et al.

indicating some structural rearrangement of the compound and/or charge transfer between C60and the exTTF subunits. Scanninganodically, the SAMs of6 show an irreversible broad oxidationpeak at 0.52 V versus Ag/Ag+. This peak corresponds to theoxidation of the exTTF fragment (Figure 5). Increasing the scanrate leads to improved reversibility, and an additional oxidationis observed at 0.12 V versus Ag/Ag+, for which we do not yethave a clear explanation.

In an attempt to prepare SAMs with better preorganizationand to observe improved electrochemical responses for the SAMsof 6, mixed monolayers were grown from toluene solutioncontaining octanethiol and compound6. However, the electro-chemical response observed did not change appreciably. BecauseC60 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 bettermonolayer response. Thus, SAMs were grown from toluenesolutions of compound6 in the presence of excesstert-butyl-calix[8]arene. As shown in Figure 6, three reasonably definedreduction waves were observed at-0.92,-1.31, and-1.83 Vversus Ag/Ag+, respectively, corresponding to the redox processesof C60. However, the first two reduction peaks are still verybroad.

Compared with the redox responses of the SAMs of thefullerene derivative used as reference (3), the reduction peaksof SAMs grown with dyad6 are much broader, and theelectrochemical response keeps changing upon successive scansunder all of the conditions used. Experiments are underway togrow monolayers with similar dyads possessing differentmolecular designs to further probe charge-transfer interactionsin SAMs of C60-exTTF derivatives.

Conclusions

A new C60-exTTF dyad with gold surface anchoring groupshas been prepared. SAMs of this compound and a referencefullerene derivative were formed on gold surfaces and character-ized by cyclic voltammetry. SAMs of6, however, do not exhibitvery reproducible electrochemical responses, presumably becauseof structural rearrangements of the monolayer or charge-transferevents between the C60and 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.

Figure 3. Cyclic voltammogram of6 recorded in CH2Cl2 containing0.1 M Bu4PF6. Scan rate, 100 mV s-1.

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

Figure 5. Cyclic voltammograms recorded in CH3CN containing0.1 M Bu4NPF6 of SAMs grown from6at variable scan rates (100-600 mV s-1).

Figure 6. Cathodic scan of the CV recorded in CH3CN containing0.1 M Bu4NPF6 of SAMs grown from6andtert-butylcalix[8]arene.

Self-Assembly of TetrathiafulValene Dyads Langmuir, Vol. 22, No. 25, 200610623

tions will include the preparation of new dyads with differentmolecular designs for SAM formation and an investigation of thephotoelectrochemical properties of these C60-based monolayers.

Acknowledgment. This work was supported by the MEC ofSpain (project CTQ2005-02609/BQU), the CAM (project P-PPQ-

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

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