Harnessing the Magic Methyl Effect: Manipulating Photoluminescence and Photochromism of Salicylaldehyde Benzoylhydrazones in the Solid State

Photoluminescence and photochromism are pivotal in developing solid-state photoresponsive materials, yet achieving precise tuning through subtle molecular structure modifications remains challenging. Herein, we demonstrate a universal "magic methyl" strategy to effectively manipulate the photoluminescence and photochromism of salicylaldehyde benzoylhydrazones in the solid state. Interestingly, the In (n = 1-4) series exhibits weak photochromism but excitation-dependent (Ex-De) emission with green fluorescence (cis-keto) and cyan fluorescence (excimer), an exceptional phenomenon to Kasha's rule. More importantly, by replacing the hydrogen atom in the ─NHCO─ group with a methyl group, the obtained IIn (n = 1-4) series displays quenched fluorescence but pronounced photochromism. This subtle structural modification allows IIn series to reversibly switch the powder colors between white and orange under 365 and 520 nm light irradiation within seconds. The photochromic behavior persists from ambient temperature to -193 °C across different substituents. Notably, compound II3, featuring a fluorine substituent, exhibits an exceptionally long half-life of ∼240 min for its out-of-equilibrium trans-keto tautomer, attributed to intramolecular hydrogen bonding from ─F···HN─ within this form. These exceptional photophysical properties suggest promising applications in anti-counterfeiting, smart coatings, and encrypted information storage, providing insightful strategies for developing organic small molecule based solid-state photoresponsive materials.