Triplet-state photophysics of retinal analogues. Interaction of polyene triplets with the di-t-butylnitroxy radical

Abstract

Using 337.1 nm laser flash photolysis, a number of all-trans retinal analogues with varying chain lengths have been investigated for triplet quenching by interaction with the stable free radical di-t-butylnitroxy (DTBN) and by energy transfer to a series of acceptors with triplet energies in the range 88–171 kJ mol^–1. The triplet energies of C₁₇ aldehyde (immediate lower homologue of retinal), retinol and retinal are estimated to be 150–160, 140–150 and ⩽ 140 kJ mol^–1, respectively. The rate constants for polyene triplet quenching by DTBN are higher than those for aromatic hydrocarbons of comparable triplet energies by 1–2 order(s) of magnitude. As with aromatic hydrocarbon triplets, the rate constant for polyenal triplet quenching by DTBN first decreases and then increases as the triplet energy of polyenals is gradually decreased; the minimum is at a triplet energy of ca. 140 kJ mol^–1 corresponding to retinal. A temperature-dependence study shows low or negative activation enthalpies at relatively high temperatures (295–353 K) which are explainable in terms of a pre-equilibrium for the formation of a complex between DTBN and a polyenal triplet in the course of the quenching interaction.