Triplet states of aromatic thioketones supported on cellulose

Diffuse reflectance, emission and transient diffuse reflectance measurement s of the spectra and decay kinetics of several aromatic thioketones support ed on cellulose have provided information concerning the second excited sin glet state, S-2, the lowest triplet state, T-1, and transient chemical inte rmediates formed when these materials are excited in the near UV. Thermally activated delayed fluorescence from the lowest excited singlet state, S-1, 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 stat es have lifetimes of the order of microseconds and the chemical intermediat es have lifetimes of hundreds of microseconds. Comparisons of the triplet s pectra in polar and non-polar media with those on cellulose show that T-1 i s of pi,pi* configuration on the solid support, as expected if the thione i s adsorbed in a polar microenvironment. The triplet decay times, but not th e spectra, indicate that the thiones are in different microenvironments whe n they are adsorbed from acetonitrile compared with methanol. The latter ha ve decay times characteristic of single molecules; the former may indicate the thiones are adsorbed as aggregates. Singlet molecular oxygen, O-2((1)De lta(g)), observed directly by transient emission at 1270 nm, is formed with near unit efficiency from the thione triplets in fluid acetonitrile soluti on, but oxygen has no measurable effect on the triplet spectra and decay ti mes when the thione is supported on cellulose.