High-Resolution Latent Fingerprinting by AIE-Active Light-Harvesting BODIHYs and pH-Induced Configurational Molecular Switching in Hydrazones

Latent fingerprint (LFP) detection is essential in criminal investigations, especially for identifying suspects. This study delves into the advancements in LFP imaging using artificial light-harvesting systems (ALHSs). These systems demonstrate high energy transfer efficiency and the antenna effect of BODIHYs (B1–B3) when combined with Rhodamine 6G (R6G), which led to the generation of high-resolution fluorescence images of LFPs, capturing details at levels 1–3 with great success. In these systems, BODIHYs (B1–B3) serve as a donor, while R6G serves as an acceptor. The BODIHYs were created from specific compounds: (E)-N′-(4-benzoylphenyl)picolinohydrazonoyl cyanide (L1), (Z)-N′-(4-benzoylphenyl)benzo[d]-thiazole-2-carbohydrazonoyl cyanide (L2), and (Z)-ethyl 2-(benzo[d]thiazol-2-yl)-2-(2-(4-benz-oylphenyl)hydrazono)acetate (L3). The ligands and BODIHYs exhibit strong emission properties in solution, aggregate, and solid states. The vital roles of various interactions in aggregation-induced emission (AIE; L2 and B1–B3) and aggregation-caused quenching (ACQ; L1) have been rationalized by X-ray single-crystal analyses. Notably, the hydrazone L3 contains a secondary hydrogen bond acceptor (−COOC2H5) and displays intramolecular hydrogen bonding, functioning as an effective molecular switch. Its ability to rapidly and reversibly transition between E and Z configurations under pH variations has been clearly demonstrated, which has practical application in inkless writing.