Photophysical and photochemical events during the photosensitization of Hypocrellin A on a colloidal CdS semiconductor

UV¯visible spectroscopy, fluorescence quenching measurements, and single-photon counting have been employed to study electron transfer from excited Hypocrellin A (HA) into the conduction band of a colloidal CdS semiconductor. It has been determined that HA dye adsorbed on colloidal CdS with an apparent association constant of 940 (mol/l)-1, and fluorescence lifetime measurements gave a rate constant of 5.16×109 s-1 for the electron transfer process. The transferred electrons could be used to reduce the substrate 2,2,6,6tetramethyl-1-piperdinyloxy (TEMPO). In this regard electron paramagnetic resonance spectroscopy was used to study the photoreduction of TEMPO by electrons in the CdS semiconductor. The results suggested that CdS photoreduction efficiency was enhanced by photosensitization of HA in visible region.