Effect of Solvents on the Photoenolization ofo-Methylanthrone at Low Temperatures. Evidence for H-Atom Tunneling from Nonequilibrating Triplets

Abstract

The reactivity of 1,4-dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d8 (1,4-DMAT) toward hydrogen transfer was investigated in methylcyclohexane (MCH), ethanol (EtOH), and 2,2,2-trifluoroethanol (TFE) by comparing their triplet-state emission spectra and lifetimes with those of compounds without the o-methyl substituent. A predominantly n,π* lowest triplet state in nonpolar solvents such as MCH changes toward a less reactive π,π* configuration in polar solvents such as EtOH and TFE. In EtOH, 1,4-DMAT exhibits a temperature-dependent dual emission while the corresponding protio compound (1,4-MAT) decays by reaction at all temperatures analyzed (ca. 15−90 K). Although compounds without the o-methyl group also presented dual emission, their relative intensities and lifetimes were shown to be temperature-independent, indicating the lack of thermal equilibration between the two triplets below 90 K. In the case of 1,4-DMAT in EtOH, an Arrhenius plot based on the data from the short-lived component (mainly n,π*) and detection of the enol between 19 and 80 K revealed that deuterium transfer occurs by quantum-mechanical tunneling (QMT). In agreement with results in MCH where a short-lived n,π* state predominates, the D-tunneling rate constant was estimated to be ca. (1.5−7.5) × 103 s-1. The phosphorescence of anthrone derivatives in TFE was long-lived, with a strong π,π* character, and highly heterogeneous, but not dual. A large isotope effect on phosphorescence is assigned to the isotope-dependent tunneling reaction from a predominant π,π* state.