Triplet states of aromatic thioketones supported on cellulose

Abstract

Diffuse reflectance, emission and transient diffuse reflectance measurements of the spectra and decay kinetics of several aromatic thioketones supported on cellulose have provided information concerning the second excited singlet state, S₂, the lowest triplet state, T₁, and transient chemical intermediates formed when these materials are excited in the near UV. Thermally activated delayed fluorescence from the lowest excited singlet state, S₁, is a minor component of the emission at room temperature, but not at 77 K because back-intersystem crossing is eliminated, resulting in a substantial lengthening of the triplet lifetime. At room temperature, the triplet states have lifetimes of the order of microseconds and the chemical intermediates have lifetimes of hundreds of microseconds. Comparisons of the triplet spectra in polar and non-polar media with those on cellulose show that T₁ is of π,π* configuration on the solid support, as expected if the thione is adsorbed in a polar microenvironment. The triplet decay times, but not the spectra, indicate that the thiones are in different microenvironments when they are adsorbed from acetonitrile compared with methanol. The latter have decay times characteristic of single molecules; the former may indicate the thiones are adsorbed as aggregates. Singlet molecular oxygen, O₂(¹Δ_g), observed directly by transient emission at 1270 nm, is formed with near unit efficiency from the thione triplets in fluid acetonitrile solution, but oxygen has no measurable effect on the triplet spectra and decay times when the thione is supported on cellulose.

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