Photosensitization aspects of pinacyanol H-aggregates.: Charge injection from singlet and triplet excited states into SnO2 nanocrystallites

The singlet and triplet excited-state behavior of a symmetric carbocyanine dye, 1,1'-diethyl-2,2'-carbocyanine (commonly referred as pinacyanol), adsorbed on SiO2 and SnO2 nanocrystallites has been investigated using transient absorption spectroscopy. The adsorption of the dye molecules on the negatively charged SiO2 or SnO2 colloids results in H-type aggregation. When excited with a 532 nm laser pulse we observe a short-lived (tau < 30 ps) singlet excited state of the H-aggregate on the SiO2 surface. In contrast to this observation, a long-lived cation radical formation is seen on the SnO2 surface. The dependence of the cation radical yield on the concentration of SnO2 colloids and the intensity of laser pulse excitation confirms direct electron transfer between the excited aggregate and SnO2 colloids. Both singlet and triplet excited states of the pinacyanol aggregate participate in the charge injection process on the SnO2 surface with heterogeneous electron-transfer rate constants of >5 x 10(10) and 7 x 10(9) s(-1), respectively. Such a charge injection process is also confirmed from the photocurrent generation at a dye-modified SnO2 electrode. The fast reverse electron transfer between the photoinjected electron and the cation radical of the dye aggregate is considered to be a major limiting factor in achieving high photoconversion efficiencies.