Structural Characterization and Photoelectrochemical Properties of the Self-Assembled Monolayers of Tris(2,2‘-bipyridine)ruthenium(II)−Viologen Linked Compounds Formed on the Gold Surface

Steady-state spectroscopic studies of tris(2,2‘-bipyridine)ruthenium(II)−viologen (Ru 2+ −V 2+ ) linked thiol derivatives with different spacer-chain lengths (denoted as RuC n VC m S, where ( n, m ) = (11, 2), (7, 6), (7, 2), and (3, 6)) and different number ratios of Ru 2+:V 2+ = 1:2, Ru(C 7 VC 6 S), have been carried out, together with the corresponding reference compounds without Ru 2+ or V 2+ . Absorption and cyclic voltammetric (CV) measurements indicated no appreciable electronic interactions between the two chromophores (Ru 2+ and V 2+ ) in the ground state. While luminescence spectral measurements revealed the occurrence of efficient photoinduced electron transfer from the photoexcited Ru 2+ (*Ru 2+ ) to V 2+ . The electron transfer efficiency was also better for the 1:2 complex as compared with the corresponding 1:1 complex. Self-assembled monolayers (SAMs) of these compounds were fabricated on a gold electrode. XPS and CV measurements revealed immobilization of the compounds on the gold surface via gold−sulfur bonding. In the presence of triethanolamine, all modified electrodes with the Ru 2+ −V 2+ linked compounds showed clear anodic photocurrents. The presence of the V 2+ moiety was crucial for obtaining larger photocurrents A shorter spacer-chain length between the V 2+ moiety and the terminal thiol group ( m = 2) afforded a larger photocurrent, while a moderate spacer-chain length between the Ru 2+ and the V 2+ moieties was also a key for obtaining a larger photocurrent. A possible mechanism for photocurrent generation is discussed.