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

Independent microwave absorption and luminescence measurements have been carried out to monitor the charge injection from excited bis(2,2-bipyridine)(2,2'-bipyridine-4,4'-dicarboxylic acid)ruthemium(II) cation, Ru(bpy)(2)(dcbpy)(2+), into SnO2, ZnO, and TiO2 nanocrystallites. The luminescence 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 growth of microwave absorption was delayed from the laser pulse by a process showing a rate constant similar to that of the fast decay portion of the luminescence. With ZnO, a second, slower growth was seen in the microwave absorption, and its rate corresponded with that of the slower luminescence decay. The appearance of microwave conductivity at rates corresponding with that of the luminescence decay directly confirms heterogeneous electron transfer from excited dye to the semiconductor particle. The existence of two rates suggests differing adsorption and/or injection sites. In contrast to the behavior of excited Ru(bpy)(2)(dcbpy)(2+), charge injection from the excited singlet states of chlorophyll 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.