Ultrafast interfacial charge separation processes from the singlet and triplet MLCT states of Ru(bpy)(2)(dcbpy) adsorbed on nanocrystalline SnO2 under negative applied bias

We have observed the stimulated emission from the (MLCT)-M-1 state of Ru(bp y)(2)(dcbpy) adsorbed on SnO2 nanocrystal by femtosecond pump-probe spectro scopy under applied bias (V-ex) for the first time. The luminescence from t he (MLCT)-M-3 state has been also observed by picosecond time-resolved meas urement. Observed lifetimes of the stimulated emission and the luminescence are 70 fs and 130 ps, respectively, at zero applied voltage. Both these li fetimes continuously increase with increasing negative bias and reach 320 f s (stimulated emission) and 6 ns (luminescence) at V-ex = -0.7 V. The chang e in the lifetime of luminescence and of stimulated emission under negative applied bias is considered to be due to the change in the interfacial CS r ate, although the possibility of the bias dependent desorption of the dye c annot be completely excluded. We analyzed the result by assuming that the c hanges in the lifetimes of the stimulated emission and the luminescence are due to the change in the interfacial CS rate constant. This analysis shows that interfacial CS occurs from both the (MLCT)-M-1 and (MLCT)-M-3 states. The competition between CS and intersystem crossing (ISC) from (MLCT)-M-1 occurs in similar to 100 fs time region. Assuming there is no nonradiative relaxation pathways from (MLCT)-M-1 except for CS and for ISC, the rate con stant of ISC was estimated to be 1/(320 fs). The ratio of the numbers of el ectrons injected from the (MLCT)-M-1 and the (MLCT)-M-3 states was 0.76:0.2 2. This means that unnegligible number of electrons are injected to SnO2 fr om the (MLCT)-M-1 state. (C) 2000 American Institute of Physics. [S0021-960 6(00)70232-7].