Effect of Surface Immobilization on Intramolecular and Intermolecular Electron Transfer in a Chromophore−Donor−Acceptor Assembly

A chromophore−donor−acceptor assembly [Ru(bpyCOOH)(bpyCH2MV2+) (bpyCH2PTZ)]4+ (1) (where bpyCOOH = 4-carboxylic acid-4‘-methyl-2,2‘-bipyridine, bpyCH2MV2+ = 1-[(4‘-methyl-2,2‘-bipyridin-4-yl)methyl]-1‘-methyl-4,4‘-bipyridinediium, and bpyCH2PTZ = 10-[(4‘-methyl-2,2‘-bipyridin-4-yl)methyl]phenothiazine) has been adsorbed on the surface of nanocrystalline ZrO2 and its excited state properties studied by emission and transient absorption spectroscopy. In deaerated acetonitrile solution, the complex emits weakly with an emission quantum yield of φem ≈ 0.01 with an excited-state lifetime of τ ≈ 20 ps. Emission from the surface-adsorbed complex is intense, with φem ≈ 0.4 and τ ≈ 40 ns. The increase in emission on the surface is likely due to a significant inhibition to the electron-transfer quenching of the metal-to-ligand charge transfer (MLCT) excited state caused by surface adsorption-induced changes in the redox potentials. Transient (nanosecond time scale) absorption monitoring, following laser flash photolysis, reveals the presence of a transient or transients that are formed during the flash. Transient spectral changes that occur during and after the flash are consistent with the formation and decay of the intermediate ZrO2−[Ru(bpyCOOH)(bpyCH2MV+•)(bpyCH2PTZ+•)]4+. It returns to the ground state by both intramolecular and intermolecular processes. Intramolecular electron transfer occurs with k BET = 6.3 × 106 s-1 (τ = 160 ns), which is comparable to the rate constant for back-electron transfer in solution. The back-electron transfer is a second-order process and is much slower, with k BET = 390 M-1 s-1 (τ = 2.6 ms).