RATE CONSTANTS FOR CHARGE INJECTION FROM EXCITED SENSITIZER INTO SNO2, ZNO, AND TIO2 SEMICONDUCTOR NANOCRYSTALLITES

Independent microwave absorption and luminescence measurements have be en carried out to monitor the charge injection from excited (2,2-bipyr idine)(2,2'-bipyridine-4,4'-dicarboxylic acid)ruthemium(II) cation, Ru (bpy)(2)(dcbpy)(2+), into SnO2, ZnO, and TiO2 nanocrystallites. The lu minescence decay showed at least two components, and the faster of the two decay processes gave rate constants of (1-3) x 10(8) s(-1) The gr owth of microwave absorption was delayed from the laser pulse by a pro cess showing a rate constant similar to that of the fast decay portion of the luminescence. With ZnO, a second, slower growth was seen in th e microwave absorption, and its rate corresponded with that of the slo wer luminescence decay. The appearance of microwave conductivity at ra tes corresponding with that of the luminescence decay directly confirm s heterogeneous electron transfer from excited dye to the semiconducto r particle. The existence of two rates suggests differing adsorption a nd/or injection sites. In contrast to the behavior of excited Ru(bpy)( 2)(dcbpy)(2+), charge injection from the excited singlet states of chl orophyll a and b on SnO2 film was very fast and could not be resolved (k > 5 x 10(8) s(-1)) in the microwave experiment.