Photo-induced electron transfer between hypocrellins and nano-sized semiconductor CdS

Abstract

Both HA-CdS and HB-CdS (Hys-CdS, Hys represents HA, HB) complex systems were established according to the dynamics of heterogeneous electron-transfer process$\mu = E_{S^* /S^ + } - E< 0$. In these systems, the electron transferring from¹Hys* to conduction band of CdS is feasible. Determined from the fluorescence quenching, the apparent association constants (K_app) between Hypocrellin A (HA), Hypocrellin B. (HB) and CdS sol. were about 940 (mol/L)⁻¹, 934 (mol/L)⁻¹, respectively. Fluorescence lifetime measurements gave the rate constant for the electron transfer process from¹HA*,¹HB* into conduction band of CdS semiconductor as 5.16 × 10⁹ s⁻¹, 5.10 × 10⁹ s⁻¹, respectively. TEMPO (2,2,6,6-tetramethy-1-piperdinyloxy), a stable nitroxide radical, was used in the kinetic study of the reduction reaction taking place on the surface of a CdS colloidal semiconductor, kinetics equation 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-CdS 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 the application of solar energy.