Investigation of photochromic cluster systems based on molybdenum oxides by ESR spectroscopy

The electron-spin resonance (ESR) spectra of cluster polyoxometalate system s-a finely dispersed powder of the (NH4)(6)[Mo7O24]-citric acid complex, mo lybdic acid, and molybdenum(VI) oxide-are investigated. The initial samples are colored under exposure to ultraviolet (UV) irradiation (photochromic e ffect) and thermal annealing. The ESR signal (g(perpendicular to) = 1.94, g (parallel to) 1.92) which is observed for the (NH4)(6)[Mo7O24]-citric acid photocolored samples corresponds to an electron of the molybdenum atom. Thi s is in agreement with the data derived from the electronic spectrum. In ad dition, the (NH4)(6)[Mo7O24]-citric acid colored system exhibits an ESR sig nal (g = 2.02) which corresponds to a hole at the organic ligand. This conf irms the previously advanced model of intramolecular electron transfer unde r UV irradiation. The thermally colored molybdic acid has a similar ESR spe ctrum (g = 1.88, 1.92, 1.93, and 1.98). For the ether samples, the ESR sign al is trot observed. It is demonstrated that an unpaired electron of molybd enum atoms is substantially delocalized over all metal atoms in the cluster . (C) 2001 MAlK "Nauka/Interperiodica".