Photochemistry of methylated p -benzoquinones

Abstract

The transient absorption at 490 nm, observed on flash photolysis of duroquinone in a variety of solvents, has been confirmed as the triplet state rather than the isomer which has recently been proposed. In ethanolic solution, the triplet state decays to give the semiquinone radical, while in liquid paraffin the evidence is not conclusive owing to an overlap of the radical absorption by the triplet at 410 nm. The short-lived triplet absorption is observed at 490 nm on photolysis of a number of methylated p-benzoquinones, previous failures to observe this absorption being attributed to inadequate time resolution. The lifetime of this species is increased by methyl substitution into the quinone ring and/or the addition of water to an ethanolic solution. This phenomenon is attributed to a stabilization of the (π, π*) triplet state relative to the ( n , π*) triplet state, and a lowest triplet state of (π, π*) character in duroquinone. Deprotonation of this state is proposed as a primary photochemical process in polar solvents.