Temperature influence on deactivation paths and tautomeric equilibrium of some photochromic Schiff bases studied by time-resolved and stationary spectroscopy

The effect of temperature on spectroscopic properties (stationary and time-resolved) of selected compounds belonging to the salicylideneaniline and hydroquinone families of photochromic Schiff bases is studied. The nonradiative decay of the excited keto tautomer consists of two components: the temperature-independent one (internal conversion, dominant below 200 K) and the temperature-dependent one (assigned to structural changes). The parameters of these components are remarkably different in the two families of the photochromic compounds. The influence of the hydrogen bonding ability of the solvent on the tautomeric equilibrium between the enol and keto forms indicates changes in the order of the ground state energy levels when going from methanol to trifluoroethanol.