An ESIPT Active Salicylaldehyde Schiff Base With Crystallinity‐Dependent Solid‐State Photochromism and Applications in Latent Fingerprint Visualization

The salicylaldehyde Schiff base moiety is a classic photochromic core, yet elucidating its solid-state photochromic mechanism and realizing its practical applications still pose substantial challenges. Herein, a novel ESIPT-active salicylaldimine derivative TSP was facilely synthesized through a condensation reaction between salicylaldehyde derivative and 3-phenylpropan-1-amine under mild conditions, and fully characterized. X-ray crystallography indicates that intramolecular O-H⋯N hydrogen bonds endow TSP with favorable ESIPT behavior. TSP exhibits remarkable crystallinity-dependent photochromism: highly crystalline β-TSP undergoes rapid green-to-yellow fluorescence conversion under UV irradiation within 30 s, while α-TSP with low crystallinity shows no obvious response under the same conditions. Theoretical calculations and molecular packing analyses reveal that the loose molecular packing facilitates efficient photoisomerization in β-TSP. Furthermore, TSP-based magnetic powder was developed for high-resolution latent fingerprint imaging, enabling clear visualization of level 1-3 details of latent fingerprints on various substrates with high contrast and good transferability. This work demonstrates that TSP is a promising crystallinity-dependent photochromic material with great potential in forensic fingerprint detection.