Evidence that △S Controls Interfacial Electron Transfer Dynamics from Anatase TiO_2 to Molecular Acceptors

摘要

Recombination of electrons injected into TiO_(2) 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_(2), and a TPA covalently bound to a ruthenium sensitizer, [Ru(tpy-C_(6)H_(4)-PO_(3)H_(2))(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 CH_(3)CN. Eyring analysis revealed a smaller entropy of activation |Δ S ~(‡)| when the a -TPA was anchored to the surface or covalently linked to the sensitizer, compared to that when Me -TPA was dissolved in CH_(3)CN. In all cases, Eyring analysis provided large and negative Δ S ~(‡) 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_(2) interfaces.