Evidence that ΔS Controls Interfacial Electron Transfer Dynamics from Anatase TiO to Molecular Acceptors.

Recombination of electrons injected into TiO with molecular acceptors present at the interface represents an important loss mechanism in dye-sensitized water oxidation and electrical power generation. Herein, the kinetics for this interfacial electron transfer reaction to oxidized triphenylamine (TPA) acceptors was quantified over a 70° temperature range for para-methyl-TPA (Me-TPA) dissolved in acetonitrile solution, 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid (a-TPA) anchored to the TiO, and a TPA covalently bound to a ruthenium sensitizer, [Ru(tpy-CH-POH)(tpy-TPA)] "RuTPA", where tpy is 2,2':6',2''-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol for Me-TPA and 22 ± 1 kJ mol for a-TPA, values that were insensitive to the identity of different sensitizers. Recombination to RuTPA proceeded with E = 27 ± 1 kJ mol that decreased to 19 ± 1 kJ mol when recombination occurred to an oxidized para-methoxy TPA (MeO-TPA) dissolved in CHCN. Eyring analysis revealed a smaller entropy of activation |ΔS| when the a-TPA was anchored to the surface or covalently linked to the sensitizer, compared to that when Me-TPA was dissolved in CHCN. In all cases, Eyring analysis provided large and negative ΔS values that point toward unfavorable entropic factors as the key contributor to the barrier that underlies the slow recombination kinetics that are generally observed at dye-sensitized TiO interfaces.