Fine-Tuning Molecular Structure of Salicylaldehyde Schiff Base for Solid-State Photochromic Mechanism

Salicylaldehyde Schiff base moiety is the classic photochromic core. The study of the absorption and fluorescence color-changing mechanism of such molecules in the solid state remains a challenge. Herein, two similar organic small molecules were designed and synthesized by replacing the hydroxyl group in the salicylaldehyde Schiff base moiety with an ethoxy group. Their optical properties were compared to elucidate critical structure–photoresponse correlations. The hydroxyl-containing molecule can undergo the excited-state intramolecular proton transfer (ESIPT) process under continuous ultraviolet light stimulation in the solid state, transforming from enol forms to keto forms, causing bathochromic shifts in absorption and fluorescence spectra. It has obvious reversible photochromism and aggregation-induced emission (AIE) characteristics. Contrastingly, the ethoxy-substituted derivative shows no change in its absorption spectra and emits almost no fluorescence under the same conditions. The hydroxyl group plays a crucial role in the photochromic and solid-state fluorescence properties of this type of molecule. These findings offer useful guidance for designing stimuli-responsive salicylaldehyde Schiff base molecules.