ELECTRON-TRANSFER REACTIONS OF TRIPLET 9-ARYLXANTHENIUM AND 9-ARYLTHIOXANTHENIUM CATIONS

Triplet excited states of 9-arylxanthenium and 9-arylthioxanthenium cations have been characterized using laser flash photolysis. The triplet cations absorb at approximately 300 nm and are long-lived (tau > 5-10 mus) in the absence of electron donors. They undergo efficient electron transfer reaction with a variety of aromatic donors, including biphenyl, naphthalene, substituted benzenes, and the precursor alcohols, to give the corresponding xanthenyl or thioxanthenyl radicals and the radical cation of the aromatic donor. Both the radical and radical cations are readily detectable by transient techniques. The rate constants for electron transfer quenching of the triplet cations decrease with increasing oxidation potential of the donor, as expected from the Rehm-Weller equation. The triplet cations are somewhat poorer oxidizing agents than the singlet excited states of the same cations due to their lower excitation energy. However, the triplet cations are much more useful sensitizers in that they give substantially higher yields of cage escape from the initial geminate pair than the corresponding singlets, based on a lower limit of 25% for biphenyl radical cation formation from the triplet 9-phenylxanthenium cation.