AIE-Integrated D–A–D-Structured Dual-State Emissive Coumarin Derivative: Applications in pH Sensing, Vapor Luminescence, and Latent Fingerprint Detection

Abstract A donor−π–acceptor−π–donor (D–A–D) fluorophore exhibiting aggregation-induced emission (AIE) and dual-state emission (DSE) characteristics was developed and found to display excellent photophysical properties in both solution and aggregated states. The thermodynamically favored E-configuration of the fluorophore was established by DFT calculations and single-crystal X-ray diffraction analysis. The fluorophore showed pronounced solvatochromism, with the emission maximum shifting from 552 to 618 nm depending on solvent polarity, together with large Stokes shifts (3880–5230 cm–1), reflecting efficient intramolecular charge transfer (ICT). High molar absorptivity (up to 4.8 × 104 M–1 cm–1) and solvent-dependent fluorescence quantum yields further highlight its favorable light-harvesting and emissive characteristics. Notably, the system shows characteristic AIE activity in DMSO:water mixtures, where fluorescence intensity is significantly enhanced because of restriction of intramolecular motion (RIM) in the aggregated state. Furthermore, the probe exhibits pH-responsive fluorescence arising from protonation-induced modulation of the ICT pathway, along with acid–base reversibility that enables repeatable ON/OFF/ON fluorescence switching. In addition, the protonation site upon trifluoroacetic acid (TFA) treatment was identified using 1H NMR titration studies supported by DFT calculations. The fluorophore further demonstrates intense solid-state emission suitable for confocal fluorescence imaging, enabling high-contrast visualization of latent fingerprints with clearly distinguishable ridge patterns and minutiae characteristics. Beyond its sensing capabilities, the molecule exhibits multiple stimuli-responsive features including tunable luminescence, mechanochromism, acidochromism, and vapor-phase sensing performance in both strip- and film-based platforms.