Mechanism and relaxation kinetics of photo-induced valence tautomerism of [Co(phen)(3,5-DBSQ)2]·C6H5Cl

Photo-illumination effects have been measured for [CoIII-LS(phen)(3,5-DBSQ)(3,5-DBCat)]·C6H5Cl, where LS, phen, 3,5-DBSQ and 3,5-DBCat denote low-spin, 1,10-phenanthroline, 3,5-di-tert-butyl-1,2-semiquinonate and 3,5-di-tert-butyl-1,2-catecholate, respectively. When the Co compound was illuminated at 5 K, the magnetization value, μeff increased from 1.7 to 2.7μB. The UV-Vis absorption spectra after illumination show that the absorption band at around 800 nm, characteristic of the [CoII-HS(phen)(3,5-DBSQ)2] state (HS denotes high-spin), increased and the charge transfer band from 3,5-DBCat to 3,5-DBSQ at 2500 nm was reduced. IR spectra show that the CO stretching peak at 1280 cm−1 is significantly decreased. These results confirm that a photo-induced intra-molecular electron transfer, [CoIII-LS(phen)(3,5-DBSQ)(3,5-DBCat)]·C6H5Cl↔[CoII-HS(phen)(3,5-DBSQ)2]·C6H5Cl, was induced by 532 or 830 nm light. The relaxation kinetics of valence tautomerism was studied. For T≥25 K, kVT0=0.205 s−1 and Ea=110.38 cm−1. Below 25 K, the rate constant for the valence tautomeric relaxation as T approaches the 0 K, which suggests that tunneling is the predominant mechanism for the relaxation.