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), adso rbed on SiO2 and SnO2 nanocrystallites has been investigated using transien t absorption spectroscopy. The adsorption of the dye molecules on the negat ively charged SiO2 or SnO2 colloids results in H-type aggregation. When exc ited with a 532 nm laser pulse we observe a short-lived (tau < 30 ps) singl et excited state of the H-aggregate on the SiO2 surface. In contrast to thi s observation, a long-lived cation radical formation is seen on the SnO2 su rface. The dependence of the cation radical yield on the concentration of S nO2 colloids and the intensity of laser pulse excitation confirms direct el ectron transfer between the excited aggregate and SnO2 colloids. Both singl et and triplet excited states of the pinacyanol aggregate participate in th e 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. S uch a charge injection process is also confirmed from the photocurrent gene ration at a dye-modified SnO2 electrode. The fast reverse electron transfer between the photoinjected electron and the cation radical of the dye aggre gate is considered to be a major limiting factor in achieving high photocon version efficiencies.