Long-distance electron transfer across molecule-nanocrystalline semiconductor interfaces.

Tremendous insights into the mechanisms for electron transfer have been gained over the last 15 years by studying donorspacer-acceptor model systems of varying complexity.1 Important and classic experiments involving redox active molecules separated by hydrocarbon,2,3 peptidic,4 protein,5 and DNA6 spacers, have thoroughly revealed the distance dependence for electrontransfer processes in fluid solutions. Related studies of surfacemediated electron transfer have utilized molecules positioned at variable distances from planar surfaces and electrodes by means of self-assembled monolayers,7 inert gas spacers,8 and LangmuirBlodgett films.9 To date, however, no systematic studies have been performed of fixed-distance electron transfer across semiconductor nanoparticles. Electronic interactions across molecule-nanoparticle interfaces are finding applications in several emerging fields of chemistry and provide the basis for new classes of molecular devices.10 Control over the distance between molecules and nanoparticles will ultimately lead to a deeper understanding of interfacial electron transfer. Previous researches have attempted to fix the distance with limited success.11,12 Here we report a new strategy for studying fixed-distance electron transfer at nanoparticle interfaces and the first experiments with metal oxide nanocrystallites that have resulted in rapid (ket > 108 s-1) interfacial electron transfer over an 18 Å distance.13 The general strategy is to utilize a tripod-shaped organic molecule as a rigid, three-point anchor that can position a redox active molecule at a variable, yet fixed distance with respect to the surface of a semiconductor nanoparticle, Figure 1.13,14 By incorporating three surface-binding groups into one molecule, the orientation shown in Figure 1 is thermodynamically favored.15 A chromophoric electron donor was employed so that the kinetic rate constants for interfacial electron transfer to, kcs, and from, kcr, a semiconductor nanocrystallite can be quantified spectroscopically after selective light excitation. The first “molecular tripod” prepared, Ru(Ad-Ph-E-phen)(bpy)2(PF6)2, 1, is an adamantane derivative having three phenyl arms each terminating with an ester group and a fourth phenylethynyl arm bearing the sensitizer, Ru(phen)(bpy)2(PF6)2. The Ru complex 1 was prepared from the ligand Ad-Ph-E-phen, which was recently synthesized in our laboratories.17 Infrared measurements of 1 revealed a single asymmetric CO stretch at 1708 cm-1. An acetonitrile solution of 1 displayed the expected metal-to-ligand charge transfer, MLCT, band in the visible region (λmax ) 450 nm, ) 16 200 M-1 cm-1) and roomtemperature photoluminescence (λmax ) 624 nm) with a long excited-state lifetime, τ ) 1.44 μs. Tripod 1 was bound to mesoporous thin films of anatase TiO2 or, for some experiments, insulating ZrO2 particles. For brevity, surface-bound 1 is abbreviated as 1/TiO2 or 1/ZrO2. The TiO2 nanocrystallites were approximately 20 nm in diameter and were deposited as ∼10 μm thick, mesoporous films on tin-oxide coated glass, glass, or sapphire substrates. Spectroscopic, electrochemical, and photo(1) (a) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143-150 and references therein. (b) Meyer, T. J. Acc. Chem. Res. 1989, 22, 364. (c) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (2) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673. (3) (a) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Green, N. J. J. Phys. Chem. 1989, 93, 1173; (b) Penfield, K. W.; Miller, J. R.; Paddon-Row, M. N.; Cotsaris, E.; Oliver, A. M.; Hush, N. S. J. Am. Chem. Soc. 1987, 108, 5061. (4) Isied, S. S.; Vassilian, A.; Wishart, J. F.; Creutz, C.; Schwartz, H. A.; Sutin, N. J. Am. Chem. Soc. 1988, 109, 635. (5) (a) Gray, H. B.; Winkler, J. R. Annu. ReV. Biochem. 1996, 65, 537. (b) Netzel, T. L. In Organic and Inorganic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 1998; Vol. 2, pp 1-54. (6) (a) Murphy, C. J.; Arkin, M. R.; Jenkins, Y.; Ghatlia, N. D.; Bossmann, S. H.; Turro, N. J.; Barton, J. K. Science 1993, 262, 1025. (b) Arkin, M. R.; Stemp, E. D. A.; Holmlin, R. E.; Barton, J. K.; Hormann, A.; Olson, E. J. C.; Barbara, P. F. Science 1996, 273, 475. (c) Lewis, F. D.; Wu, T.; Zhang, Y.; Letsinger, R. L.; Greenfield, S. R.; Wasielewski, M. R. Science 1997, 277, 673. (7) (a) Li, T. T.-T.; Liu, H. Y.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 1233. (b) Chidsey, C. E. D. Science 1991, 251, 919. (c) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (d) Kasmi, A. E.; Wallace, J. M.; Bowden, E. F.; Binet, S. M.; Linderman, R. J. J. Am. Chem. Soc. 1998, 120, 225. (e) Cotton, T. M.; Heald, R. L. J. Phys. Chem. 1987, 91, 3891. (8) (a) Rossetti, R.; Brus, L. E. J. Chem. Phys. 1982, 76, 1146. (b) Whitmore, P. M.; Robota, H. 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(13) This is the distance of the Ru center from the plane defined by the three surface-bound O (the footprint). This distance is therefore most appropriate for interfacial charge recombination since interfacial charge separation occurs from an electron localized on a ligand in the MLCT excited state. (14) The footprint of 1 is ∼70 Å2, which covers about five Ti atoms on the (001) face of anatase TiO2. (15) As surface-binding groups were chosen esters, that are known to form strong bonds to TiO2 with high adduct formation constants, Kad ) (2-10) × 104 M-1: (a) Nazeeruddin, M. K.; Liska, P.; Moser, J.; Vlachopoulos, N.; Grätzel, M. HelV. Chim. Acta 1990, 73, 1788. (b) Péchy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Grätzel, M. Chem. Commun. 1995, 65. (16) Abbreviations used in this paper: bpy ) 2,2′-bipyridine; phen ) 1,10-phenanthroline; Ad-Ph-E-phen ) 1-[4-(5-(1,10-phenanthrolinyl)ethynyl)phenyl]-3,5,7-(4-carboethoxyphenyl)adamantane; dcb ) 4,4′-(COOH)22,2′-bipyridine; deeb ) 4,4′-(COOEt)2-2,2′-bipyridine. (17) Guo, W.; Galoppini, E.; Rydja, G.; Pardi, G. Tetrahedron Lett. 2000, 7419. Figure 1. Schematic representation of a surface-bound molecular tripod (left) and structure of Ru(Ad-Ph-E-phen)(bpy)2(PF6)2 1 (right). 4342 J. Am. Chem. Soc. 2001, 123, 4342-4343