Abstract
The apparent association constant ( k app) between Hypocrellin A (HA) and CdS sol was determined by the fluorescence quenching method to be 1400 M −1. TEMPO (2,2,6,6-tetramethyl-1-piperdinyloxy), a stable free radical, was used in the kinetic study of the reduction reaction taking place on the surface of a CdS colloidal semiconductor because after TEMPO accepts an electron and a proton, its signal would be lost. Kinetics of the reaction was determined by the electron paramagnetic resonance (EPR) method. The reaction order of TEMPO is different from that of the spin elimination reaction in a hematoporphyrin (HP) and HA system and the difference may be due to the adsorption effect of colloidal particles. When HA was added, the rate of EPR increased greatly. By comparing rate constants, the HA-CdS system was revealed to be 350 times more efficient than CdS sol in the photoreduction of TEMPO under visible light. It suggests that HA can be used as an efficient sensitizer of a colloidal semiconductor in the application of solar energy.
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