Alkyl-chain engineering of Schiff base AIEgens: tuning solid-state emission for versatile latent fingerprint imaging

Overcoming solid-state quenching and optimizing substrate affinity remain critical challenges for fluorescent probes in latent fingerprint (LFP) imaging. Herein, we report the rational alkyl-chain engineering of four Schiff base AIEgens (DSB-R; R = ethyl, butyl, hexyl, octyl) to systematically modulate their solid-state packing and lipophilicity. Driven by the restriction of intramolecular motion (RIM) mechanism, the n-hexyl-substituted derivative (DSB-Hex) achieved the optimal balance, exhibiting a high solid-state quantum yield of 47.0% and specific affinity for sebaceous fingerprint residues. To establish practical forensic boundary conditions, DSB-Hex was evaluated across three LFP visualization strategies. The powder-dusting method (2 wt% DSB-Hex/talc) demonstrated universal substrate applicability with exceptional environmental stability over 30 days. The immersing method (80 vol% EtOH/H₂O) yielded the highest signal-to-noise ratio (G_R/G₀ ∼ 20–25), successfully visualizing level 1–3 details, including minute sweat pores (<500 µm). Conversely, the spraying method provides insufficient contrast due to rapid solvent dispersion, highlighting a limitation for field deployment. Ultimately, this work demonstrates strategic alkyl-chain engineering as a powerful approach to tailor ESIPT fluorophores for versatile, high-resolution forensic imaging.