On–Off Mechano-responsive Switching of ESIPT Luminescence in Polymorphic N-Salicylidene-4-amino-2-methylbenzotriazole

We report the synthesis of a luminescent N-salicylidene aniline derivative, N-salicylidene-4-amino-2-methylbenzotriazole (1), and the study of its polymorphism and photophysical properties. Three phases showing yellow (1-Y), orange (1-O), and red (1-R) fluorescence have been isolated and characterized by thermal and single crystal X-ray analysis. The photoluminescence results from excited-state intramolecular proton transfer process and the quantum yield is strongly dependent on polymorphism (Φ1-Y = 0.87, Φ1-O = 0.11, Φ1-R = 0.028). The poorly emitting 1-R can be easily prepared, converted to the bright 1-Y by grinding, and reverted to 1-R through melting and annealing, giving rise to a luminescence on–off mechano-responsive cycle. The different photophysical properties are explained with the variable π-overlap and molecular conformation changes in the three polymorphs, characterized by a very similar crystal packing. By DFT calculations, the absorption properties were explained as dependent on the torsion angle between the two planar portions of the molecule, which affects the equilibrium between enol and keto forms in the ground state.