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

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 S-2 --> S-0 fluorescence and T-1 --> S-0 phosphorescence quantum yields and their S-2 and T-1 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 E solvent interactions on the excited state dynamics. The rate constants of radiative and non-radiative decay of the S-2 and T-1 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 S-2 <-- S-0 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 C=S group was not observed in the S-2 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 C=S group may induce non-radiative relaxation in the longer-lived triplets.