Reactivity and photophysical behavior of chromone triplet. A laser flash photolysis study

Abstract

The chromone triplet ([Formula: see text] in acetonitrile) is produced in quantitative yields upon 308- or 337.1-nm laser pulse excitation and is characterized by submicrosecond lifetimes in solutions at room temperature. The short-lived nature of the triplet is attributable to intrinsically fast T1 [Formula: see text] S0 intersystem crossing, nearly diffusion-limited self-quenching, and facile interactions with solvents in the form of charge and hydrogen-atom transfer. The unusually high self-quenching rate constants, (0.9–4.0) × 109 M−1 s−1, are related in a major part to the presence of the ene double bond at which the photocycloaddition of the triplet may occur; this is supported by large bimolecular rate constants [Formula: see text] observed for chromone triplet quenching by various alkene derivatives. Although, based on low-temperature photophysical behaviors, the lowest triplet state of chromone in polar solvents is expected to be of reduced n,π* character, the reactivity of the triplet toward hydrogen donors is very pronounced in acetonitrile (for example, [Formula: see text] for tri-n-butylstannane and 2-propanol, respectively). Carbon tetrachloride and benzene prove to be facile quenchers of chromone triplet; the quenching interactions probably involve charge transfer, the carbonyl triplet acting as a donor and an acceptor, respectively. The electrophilic role of chromone triplet in the quenching by benzene derivatives is supported by a good correlation between [Formula: see text] and quencher oxidation potential.