ABSTRACT
Photogalvanic cells are photoelectrochemical systems wherein the semiconductor electrode is not a participant in primary photoinduced charge formation. The discovery of photoelectrochemical systems that successfully exploit secondary (thermal) electron injection at dye-semiconductor interfaces may enable studies of electron transfer at minimal driving force for electron injection into the semiconductor. In this study, we have examined thermal electron transfer from molecular sensitizers to nanostructured semiconductor electrodes composed of titanium dioxide nanorods by means of transient spectroscopy and the assembly and testing of photoelectrochemical cells. Electron-accepting molecular dyes have been studied alongside an arylamine electron donor. Thermal injection is estimated for a naphthacenequinone radical anion as a multiexponential decay process with initial decay lifetimes of 6 and 27 ps. The ambient electric field present during charge separation at a surface-adsorbed dye monolayer causes Stark shifts of the radical ion pair absorbance peaks that confounded kinetic estimation of thermal injection for a fullerene sensitizer. Electron-accepting dyes that operate by thermal injection into titanium dioxide function better in solid-state photoelectrochemical cells than in liquid-junction cells due to the kinetic advantage of solid-state cells with respect to photoinduced acceptor-quenching to form the necessary radical anion sensitizers.
