Photo-induced electron transfer between hypocrellins and nano-sized semiconductor CdS - EPR study on the kinetics of photosensitized reduction in HA-CdS and HB-CdS systems

Both HA-CdS and HB-CdS (Hys-CdS, Hys represents HA, HE) complex systems wer e established according to the dynamics of heterogeneous electron-transfer process mu = E-S*/S - E-CB <0. In these systems, the electron transferring from (1)Hys* to conduction band of CdS is feasible. Determined from the flu orescence quenching, the apparent association constants (K-app) between Hyp ocrellin A (HA), Hypocrellin B (HB) and CdS sol. were about 940 (mol/L)(-1) , 934 (mol/L)(-1), respectively. Fluorescence lifetime measurements gave th e rate constant for the electron transfer process from (1)HA*, (HB)-H-1* in to conduction band of CdS semiconductor as 5.16x10(9) s(-1) 5.10 X 10(9) s( -1), respectively. TEM PO (2,2,6,6-tetramethy-1-piperdinyloxy), a stable ni troxide radical, was used in the kinetic study of the reduction reaction ta king place on the surface of a CdS colloidal semiconductor, kinetics equati on of the reaction was determined with the electron paramagnetic resonance (EPR) method, and the reaction order of TEMPO is zero. When Hys were added, the rate of EPR increased greatly. By comparing rate constants, the Hys-Cd S systems were revealed to be about 350 times more efficient than CdS sol. alone in the photoreduction of TEMPO under visible light. It suggests that Hys can be used as efficient sensitizers of a colloidal semiconductor in th e application of solar energy.