Triplet excited states of 1,4-disubstituted anthraquinones: possible evidence for association of quinones in solution

Abstract

The triplet excited states of 1,4-disubstituted anthraquinones containing NH₂, NHMe, NHAr and OH groups have been studied in benzene using pulse radiolysis and flash photolysis. Triplet-triplet absorption spectra, extinction coefficients and triplet energy levels were determined by energy transfer techniques. The E_T values lie in the range 100–176 kJ mol^–1. Triplet lifetimes (t_½∼ 3 µs) are typical of substituted anthraquinone compounds, the rapid decay being attributed to reversible hydrogen abstraction across the intramolecular hydrogen bond.

Comparison of the concentrations of biphenyl triplet acting as donor and the loss of singlet dye as acceptor in the energy transfer experiments suggests that these quinones are in an associated state in benzene solutions at concentrations as low as 10^–5–10^–7 mol dm^–3.