Photoelectrochemistry in semiconductor particulate systems. 17. Photosensitization of large-bandgap semiconductors: charge injection from triplet excited thionine into zinc oxide colloids

Abstract

The kinetics and mechanism of a diffusion-controlled interfacial charge transfer between excited triplet thionine and ZnO colloids in ethanol have been elucidated with laser flash photolysis. The singlet excited state of thionine, which has a lifetime of 450 ps in ethanol, did not participate in the charge injection process. However, the triplet excited state of thionine was quenched by ZnO colloids. The triplet lifetime ({tau}{sub T}) decreased from 88 to 15 {mu}s with increasing ZnO colloid concentration. The maximum charge injection efficiency ({theta}{sub TH{center_dot}}2+) which was measured from the yield of dye cation radical was 0.1. The increase in the rate constant for reverse electron transfer (k{sub r}) at high ZnO concentration resulted in a small decrease in charge-transfer efficiency. The dependence of {tau}{sub T}, {theta}{sub TH{center_dot}}2+, and k{sub r} on the concentration of ZnO is described. 49 refs., 9 figs.