Excited States and Intermediates in the Photochemistry of Chromium(III) Complexes Studied by Flash Method

Abstract

Abstract Photoreactions of K3[CrIII(NCS)6] and NH4[CrIII(NCS)4(NH3)2] were investigated by means of a flash technique. At low temperature (−87°C), the lowest excited doublet states (2Eg) of the complexes were easily observed on d-d excitation in acetone, the yields decreasing rapidly with rising temperature. In methanol-ethyl–eneglycol-water, 2Eg states of both complexes were easily observed at dry-ice temperature, but not at −46°C. Thus, the lowest excited quartet state (4T2g), the precursor of 2Eg state, should rapidly undergo photolysis at room temperature before being converted into the 2Eg state. The rate of thermal degradation of 4T2g state including chemical reactions was calculated to be ca. 1010 s−1 at room temperature. The decay rate of 2Eg state was estimated to be ca. 105 s−1 at room temperature. In the presence of organic compounds having carbonyl group, a strong and sharp absorption band near the UV region was observed at room temperature. Irradiation of UV light produced (NGS)2− radical for both complexes.