The relaxation of electronically excited molecules: Effects of alkyl substitution on the S2 and T1 states of benzopyranthione in solution

Abstract

The effects of n-alkyl group substitution on the photophysics, photochemistry and spectroscopy of benzopyranthione (BPT), a rigid model aromatic thione, have been investigated. The absorption, emission and excitation spectra of BPT and its 2-substituted n-C4H9 (I) and n-C10H21 (II) derivatives, their S2 → S0 fluorescence and T1 → S0 phosphorescence quantum yields and their S2 and T1 lifetimes have been measured in several solvents, in the presence and absence of oxygen. Perfluorinated solvents have been used to minimize the effects of solute–solvent interactions on the excited state dynamics. The rate constants of radiative and non-radiative decay of the S2 and T1 states and of their bimolecular quenching by molecular oxygen and by ground state thione have been obtained. The spectra of I and II are nearly identical in every respect, except that the S2←S0 absorption spectrum of II exhibits a greater absorbance on its red edge, attributable to its larger density of low-frequency torsional states. The expected effect of intramolecular interaction of the longer alkyl chain with the strongly H-abstracting CS group was not observed in the S2 states of II compared with I, and this lack of effect was confirmed by showing that the excited state lifetimes were unaffected by a 40-fold change in the viscosity of the perfluoroalkane solvents. By contrast, a significantly shorter lifetime of the triplet state of II compared with I suggests that intramolecular interaction beween the hydrocarbon tail of II and the excited CS group may induce non-radiative relaxation in the longer-lived triplets.