Pulse radiolysis of 9,10-anthraquinones. Part 2.—Triplet excited states

Abstract

The triplet excited states of anthraquinone, 1- and 2-piperidinoanthraquinone and 1- and 2-aminoanthraquinone have been studied in benzene using pulse radiolysis. The triplet-triplet absorption spectra and extinction coefficients were characterized via energy-transfer techniques for all but the 1-piperidino derivative. The lowest triplet energy levels of the piperidino- and amino-anthraqqinones were similarly located. The lifetime of anthraquinone triplet in benzene is short (τ= 180 ns), and it reacts with oxygen and propan-2-ol with rate constants of 1.5 × 109 and 2.1 × 107 M–1 s–1, respectively. No reaction of the triplet with its own ground state could be detected. The triplet states of 2-piperidino-, 1-amino- and 2-amino-anthraquinone are much longer-lived in benzene than anthraquinone. 2-Piperidinoanthraquinone triplet reacts with oxygen with a rate constant of 1.9 × 109 M–1 s–1. No reaction of 2-piperidinoanthraquinone triplet with its own ground state or with propan-2-ol could be detected. The different properties of the piperidino- and amino-anthraquinone triplets, in comparison with anthraquinone triplet itself, are considered to arise because their lowest triplet levels are charge-transfer in character, rather than nπ*.