OXYGEN QUENCHING OF ELECTRONICALLY EXCITED HEXANUCLEAR MOLYBDENUM AND TUNGSTEN HALIDE CLUSTERS

Quenching of the electronically excited [M6X8]Y62- (M = MoII, WII; 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 [M6X8]Y62- ions are similar (kqobs = 8.1 (37) × 107 M-1 s-1) with the exception of the [W6I8]Y62- clusters, which exhibit significantly greater quenching rates (kqobs = 2.1 (5) × 109 M-1 s-1). Photosensitized oxidation of 1-methylcyclohexene and 1,2-dimethylcyclohexene by all [M6X8]Y62- 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 [M6X8]Y62- (M ≠ W; X ≠ 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 [W6I8]Y62--photosensitized reactions are not consistent with this scheme. One explanation for the enhanced oxygen quenching rates of the [W6I8]Y62- excited states ([W6I8]Y62-*) and anomalous observed quantum yields is the contribution of an electron-transfer pathway to the quenching reaction. Transient absorption spectra for the reaction between W6I142-* and oxygen, however, do not display transients attributable to electron-transfer products. Accordingly, we ascribe the enhanced quenching rate of [W6I8]Y62-* 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 [M6X8]Y62--cluster photosensitized production of 1O2 ([M6X8]Y62-* + O2 → [M6X8]Y6- + O2- → [M6X8]Y62- + 1O2) parallels the results observed for the photosensitized production of 1O2 by RuL32+ (L = polypyridyl) systems, which also produce singlet oxygen exclusively by energy transfer despite the existence of potential electron-transfer pathways. © 1990 American Chemical Society.