Photochromic cycle of two N-salicylidene-p-X-anilines revealed by the dynamics over 10 orders of magnitude

The photoexcited processes in two compounds belonging to the salicylidene-p-X-anilines family were investigated by time resolved absorption and emission spectroscopy in the femto-to-millisecond time domain. The studies were conducted in two solutions of different polarity and were supplemented by investigations in solid state (microcrystalline powder). In the polar solvent, both compounds share similar fluorescence properties. In the non-polar solvent, one of the compounds exhibits only the tautomeric emission, while for the other compound, the fluorescence is almost quenched, but still visible as a co-existence of the normal and tautomeric emissions. These different emission characteristics in a non-polar solvent were correlated with the aggregate formation. From femto- to miliseconds, various deactivation channels and conformers of the known tautomeric forms have been evidenced altogether with an additional fluorescence relaxation appearing only in molecular aggregates. A complete and detailed image of the photochromic cycles emerges by correlating structural dynamics with the experimentally observed kinetic and spectroscopic parameters over 10 orders of magnitude time-scale. (C) 2019 Elsevier B.V. All rights reserved.