Substituent-regulated charge transfer, aggregation-induced emission, and lipid recognition in naphthalimide fluorophores for latent fingerprint imaging.

Abstract

Sensitive visualization of latent fingerprints (LFPs) through the detection of specific biomolecular constituents is essential for forensic analysis. Herein, we report a series of naphthalimide-based fluorescent probes bearing systematically varied substituents for LFP development. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations show that, among the investigated compounds, the triphenylamine-functionalized derivative NA-TPA has the smallest HOMO-LUMO gap (2.94 eV), the longest charge-transfer distance (D index = 6.68 Å), and the lowest exciton binding energy (2.49 eV). These electronic characteristics are consistent with the pronounced intramolecular charge-transfer (ICT) character and aggregation-induced emission (AIE) behavior of NA-TPA. NA-TPA also exhibits pronounced solvatochromism, stable emission in the aggregated state, favorable in vitro biocompatibility, substantial lipophilicity (experimentally determined logP = 1.42), and high selectivity toward triglycerides. As a "turn-on" fluorescent probe, NA-TPA displays a 3.1-fold increase in fluorescence intensity together with a 56 nm blue shift upon interaction with triglycerides. Binding-energy calculations (-66.12 kcal·mol-1) and atom-resolved contribution analyses further support the observed selectivity and elucidate the preferential interaction of NA-TPA with triglycerides. Moreover, NA-TPA enables high-contrast visualization of LFPs on diverse substrates and resolves well-defined ridge characteristics, including Level 3 features such as sweat pores. The structure-property-performance relationships established in this study provide a rational framework for designing fluorescent probes for forensic applications.

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