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, mu(eff) increased from 1.7 to 2.7 mu(B). 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 greater than or equal to 25 K, k(VT)(0) = 0.205 s(-1) and E-a = 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. (C) 2004 Elsevier B.V. All rights reserved.