Kinetics and mechanism of the photochromic transformations of N-salicylidene-4-hydroxy-3,5-dimethylaniline and its complex with uranium(VI) dioxide

The title compounds were prepared and some of their photophysical and photochemical properties studied. The anil (LH) and its complex with uranium (C) were found to possess very similar spectral and kinetic characteristics, namely absorption and luminescence spectra, photoisomer spectra and isomerization kinetics. From these measurements, it is concluded that the photoexcitation of the compounds in polar solvents (methanol and acetonitrile) leads to proton transfer in the excited sinlet state with the formation of a trans zwitterionic structure. The trans zwitterion emits light and undergoes isomerization to form the cis zwitterion. The latter compound is a photochromic intermediate, which participates in cis-trans isomerization (rate constant kiso = (3–4) × 104 s−1) with the formation of the initial anil or the uranyl complex. An energy-level scheme of the reactions is presented.