Oxygen Quenching of Electronically Excited Hexanuclear Molybdenum and Tungsten Halide Clusters

Article abstract of DOI:10.1021/j100374a029

Quenching of the electronically excited Y6(2-) (M = Mo(II), W(II), X,Y = Cl,Br,I) ions by molecular oxygen has been investigated.Stern- Volmer analysis of emission intensity and lifetime data reveals that the rate constants for oxygen quenching of the Y6(2-) ions are similar (k_q^obs = 8.1(37) * 1E7 M^-1 s^-1) with the exception of the Y6(2-) clusters, which exhibit significantly greater quenching rates (k_q^obs = 2.1(5) * 1E9 M^-1 s^-1).Photosensitized oxidation of 1-methylcyclohexene and 1,2-dimethylcyclohexene by all Y6(2-) clusters yields products expected for the reaction of the olefins with singlet oxygen.No evidence of radical autooxidation products were detected.However, the measured quantum yields for the photooxidation of 2,3-diphenyl-p-dioxene by only the Y6(2-) (M = no W; X = no I) clusters are in agreement with the values calculated from a kinetic scheme involving the exclusive production of singlet oxygen by direct energy transfer; observed quantum yields of Y6(2-) -photosensitized reactions are not consistent with this scheme.One explanation for the enhanced oxygen quenching rates of the Y6(2-) excited states (Y6(2-)(excit)) and anomalous observed quantum yields is the contribution of an electron-transfer pathway to the quenching reaction.Transient absorption spectra for the reaction between W6I14(2-)(excit.) and oxygen, however, do not display transients attributable to electron-transfer products.Accordingly, we ascribe the enhanced quenching rate of Y6(2-)(excit.) by oxygen to greater adiabaticity of the energy-transfer reactions of these ions as compared to their homologous cluster counterparts.The absence of an electron-transfer contribution to the Y6(2-)-cluster photosensitized production of ¹O2 (Y6(2-)(excit.) + O2 -> Y6(1-) + O2(1-) -> Y6(2-) + ¹O2) parallels the results observed for the photosensitized production of ¹O2 by RuL3(2+) (L = plypyridyl) systems, which also produce singlet oxygen exclusively by energy transfer despite the existence of potential electron-transfer pathway.