Evidence that ΔS^‡ Controls Interfacial Electron Transfer Dynamics from Anatase TiO₂ to Molecular Acceptors

Recombination of electrons injected into TiO₂ 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 TiO₂, and a TPA covalently bound to a ruthenium sensitizer, [Ru(tpy-C₆H₄-PO₃H₂)(tpy-TPA)]²⁺ "RuTPA", where tpy is 2,2':6',2''-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol^-1 for Me-TPA and 22 ± 1 kJ mol^-1 for a-TPA, values that were insensitive to the identity of different sensitizers. Recombination to RuTPA⁺ proceeded with E_a = 27 ± 1 kJ mol^-1 that decreased to 19 ± 1 kJ mol^-1 when recombination occurred to an oxidized para-methoxy TPA (MeO-TPA) dissolved in CH₃CN. 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 CH₃CN. 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 TiO₂ interfaces.