Early events in decatungstate photocatalyzed oxidations: A nanosecond laser transient absorbance reinvestigation

Transient absorbance changes following nanosecond flash excitation of M-4[W10O32]( )(M = Na+ or (n-C4H9)(4)N+ = Q(+)) in acetonitrile reveal a long-lived ligand-to-metal charge-transfer (LMCT) intermediate [W10O32](4-t) (phi similar to 0.50, tau > 74 ns) as the reactive species in photocatalyzed organic oxidations. From its long lifetime and the lack of detectable emission between 400 and 900 nm, this LMCT intermediate is assigned to a "hot" ground-state isomer which is quenched reductively to [W10O32](5-) Or its protonated derivative [HW10O32](4-)dagger Up to 1 mu s after flash excitation of Ni-4[W10O32], the kinetics and quantum yields are measured for the reductive quenching of [W10O32](4-t) by thiocyanate and four classes of organic substrates, Aromatic amines, thianthrene, and thiocyanate quench [W10O32](4-t) by electron transfer at or near diffusion-controlled rates, and the alkanes and 2-butanol quench [W10O32](4-t) by hydrogen atom transfer with rate constants in the range 10(7)-10(8) M-1 s(-1). yields of the geminate pairs do not require photoexcitation of ground-state adducts, substrate-[W10O32](4-), but do require protonation of the reduction product [W10O32](5-).