Photochromism in p-methylbenzoylthioacetone and related β-thioxoketones

Irradiation of p-methylbenzoylthioacetone with UV or visible light brings about spectral changes characteristic for the photochromic behavior of β-thioxoketones. The nature of the initial species and of the photochromic product can be assigned based on spectral studies of the electronic and IR spectra in rare gas matrices combined with quantum chemical calculations. Additional arguments for the vibrational assignments are provided by using polarized light to induce the phototransformation, and by subsequent measurements of linear dichroism on partially aligned samples. Comparison with the results previously obtained for three related molecules: thioacetylacetone, p-methyl(thiobenzoyl)acetone, and monothiodibenzoylmethane reveals a common pattern of the photochromic reaction. In all four molecules, the initial species corresponds to an intramolecularly hydrogen-bonded enolic molecular structure, and the main photochromic product to an “open”, nonchelated enethiolic counterpart. On the basis of a computational TD-DFT study of ground and excited electronic states, possible mechanisms of the photochromic transformation are discussed.