Structure–Driven Intramolecular Charge Transfer Modulation in Imidazole Fluorophores for Selective Hydrazine Detection and Forensic Applications

Abstract

Hydrazine is a highly toxic, potentially carcinogenic compound widely used in industry, necessitating sensitive detection. Meanwhile, latent fingerprint identification requires high-contrast, high-resolution imaging. Both demand rapid and visually discernible signals, motivating the development of integrated materials for simultaneous chemical sensing and forensic imaging. Herein, we report a rationally designed imidazole-based fluorophore, DPI–DCN, that integrates reaction-based hydrazine sensing with high-resolution latent fingerprint (LFP) visualization. By incorporating a strongly electron-withdrawing dicyanovinyl unit, DPI–DCN establishes an efficient donor–acceptor framework, leading to enhanced intramolecular charge transfer (ICT) and pronounced bathochromic shifts relative to its ester analogue, DPI–DET. These photophysical characteristics are supported by density functional theory calculations, which reveal a reduced HOMO–LUMO gap and favorable orbital separation. DPI–DCN exhibits excellent selectivity toward hydrazine, displaying a distinct fluorescence “turn-off” response with a low detection limit of 1.94 μM. Mechanistic investigations using 1H NMR and mass spectrometry confirm that nucleophilic addition of hydrazine to the dicyanovinyl moiety disrupts π-conjugation and suppresses ICT, accounting for the observed optical changes. Beyond solution-phase sensing, DPI–DCN functions as an effective solid-state fluorescent powder for LFP development, enabling high-contrast imaging across diverse substrates, including aged fingerprints. Notably, multilevel fingerprint features up to level III are clearly resolved, demonstrating its capability for detailed forensic analysis. This work establishes a direct structure–property–function relationship and presents a versatile molecular platform that bridges chemical sensing and forensic imaging, offering new opportunities for integrated analytical and security applications.