An Organic Fluorescent Probe for Selective Picric Acid Detection: Experimental and Theoretical Investigations With Smartphone and Real‐Time Applications

High‐precision optical sensing of picric acid (PA) is crucial due to its pronounced toxicity and explosive properties. PA possesses several distinctive physicochemical attributes, including strong proton donor‐acceptor functionality, an exceptionally low pK a , high electron deficiency and excellent aqueous solubility, which make its selective detection particularly challenging. In this study, a novel fluorescent probe, NPI‐Phol, was rationally designed, synthesized and thoroughly characterized for the selective recognition of PA. Comprehensive photophysical investigations, supported by theoretical calculations, revealed that NPI‐Phol exhibits pronounced fluorescence quenching in the presence of PA compared to other nitroaromatic analytes, demonstrating superior selectivity and a low detection limit of 18.2 nM, with a quenching constant 4.331 X 10 4 M −1 . Mechanistic studies indicate that the sensing response arises from a synergistic contribution of excited‐state intramolecular proton transfer and photoinduced electron transfer processes. Experimental validation using UV–visible spectroscopy, steady‐state fluorescence measurements, nuclear magnetic resonance (NMR) analysis and density functional theory calculations confirms the sensing mechanism. Notably, computational results reveal efficient intermolecular charge transfer from NPI‐Phol to PA in the lowest unoccupied molecular orbital, accounting for the observed fluorescence quenching. Furthermore, the practical applicability of the probe was demonstrated through successful PA detection in diverse water matrices, on thin‐layer chromatography plates and on filter paper, along with real‐time analysis using a smartphone‐assisted platform. Overall, this work provides valuable insights into the rational development of efficient fluorescent chemosensors for selective and real‐time detection of PA.