Benzothiophene-hydroxy naphthalene hybrid: An investigation of fluorescence features for advanced security and forensic applications

• Synthesis and characterization of Benzothiophene–Hydroxy Naphthalene hybrid (BTNH) that exhibits AIE and ICT. • BTNH shows tunable fluorescence emission under various stimuli including solvents, pH, mechanical force and excitation wavelength. • Durable offset prints on paper with excellent lightfastness, rub resistance, and advanced covert security features. • High-contrast level 1–3 fingerprint details detected on non-/semi-porous substrates under daylight and on complex surfaces under 380 nm UV light. Advanced luminescent materials with distinct fluorescence properties play a crucial role as security ink pigments and latent fingerprint developers. A new structural hybrid BTNH that incorporates benzothiophene and hydroxy naphthalene units, linked through a C=N-NH-CO spacer is synthesized as a dual-emissive pigment with superior optical performance for versatile use in security printing and latent fingerprinting. The small molecule shows aggregation-induced emission, intramolecular charge transfer, and tunable fluorescence emission under different stimuli including solvents, pH, mechanical force and excitation source. Offset prints are obtained using a water-based ink formulation with BTNH as pigment on magnetic ink character recognition (MICR) paper which is specially designed for banking and financial documents to allow automated reading of characters using magnetic properties. The prints exhibited a switch in fluorescence color triggered by mechanical rubbing and exposure to dichloromethane, along with different excitation wavelength-dependent emission patterns from both rubbed and unrubbed areas. Additionally, the emission color demonstrated sensitivity to various solvents and pH levels, thereby providing multi-level covert security features that enhance the potential of BTNH ink for anti-counterfeiting applications. The print proofs exhibited good lightfastness and rub resistance, validating the durable nature of the ink film in security printing. Beyond security applications, BTNH proved to be highly effective in latent fingerprint detection under daylight on nonporous substrates, enabling high-contrast visualization of ridge patterns, minutiae, and sweat pores, eliminating the need for traditional UV excitation. Moreover, levels 1 and 2 features of latent fingermarks on semi-porous substrates were also visualized under daylight. Further, under 380 nm UV illumination, the fluorescence emission of BTNH enabled the clear visualization of fingerprints on complex compact disc surface, revealing level 1 to 3 details. Thus, with its unique fluorescence properties, structural/chemical stability, biocompatibility and scalable synthesis, BTNH stands out as a next-generation material for security and forensic advancements.