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 SiO 2 and SnO 2 nanocrystallites has been investigated using transient absorption spectroscopy. The adsorption of the dye molecules on the negatively charged SiO 2 or SnO 2 colloids results in H-type aggregation. When excited with a 532 nm laser pulse we observe a short-lived (τ < 30 ps) singlet excited state of the H-aggregate on the SiO 2 surface. In contrast to this observation, a long-lived cation radical formation is seen on the SnO 2 surface. The dependence of the cation radical yield on the concentration of SnO 2 colloids and the intensity of laser pulse excitation confirms direct electron transfer between the excited aggregate and SnO 2 colloids. Both singlet and triplet excited states of the pinacyanol aggregate participate in the charge injection process on the SnO 2 surface with heterogeneous electron-transfer rate constants of >5 × 10 10 and 7 × 10 9 s -1, respectively. Such a charge injection process is also confirmed from the photocurrent generation at a dye-modified SnO 2 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.