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 ¹MLCT state of $Ru(bpy)2(dcbpy)$ adsorbed on $SnO2$ nanocrystal by femtosecond pump–probe spectroscopy under applied bias $(Vex)$ for the first time. The luminescence from the ³MLCT state has been also observed by picosecond time-resolved measurement. Observed lifetimes of the stimulated emission and the luminescence are 70 fs and 130 ps, respectively, at zero applied voltage. Both these lifetimes continuously increase with increasing negative bias and reach 320 fs (stimulated emission) and 6 ns (luminescence) at $Vex=−0.7$ V. The change 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 rate, although the possibility of the bias dependent desorption of the dye cannot be completely excluded. We analyzed the result by assuming that the changes 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 ¹MLCT and ³MLCT states. The competition between CS and intersystem crossing (ISC) from ¹MLCT occurs in ∼100 fs time region. Assuming there is no nonradiative relaxation pathways from ¹MLCT except for CS and for ISC, the rate constant of ISC was estimated to be 1/(320 fs). The ratio of the numbers of electrons injected from the ¹MLCT and the ³MLCT states was 0.76:0.22. This means that unnegligible number of electrons are injected to $SnO2$ from the ¹MLCT state.