Excited-State Intramolecular Proton Transfer (ESIPT) Driven Emission Switching in a Coordination Polymer for Selective Detection of Nitroaniline Isomers.

Abstract

Nitroaniline (NA) isomers are important, yet hazardous dye industry intermediates, require rapid and selective detection. Although conventional coordination polymers can detect NA through fluorescence quenching or wavelength shifting, however their limited signal discrimination often complicates the detection process. In this context, excited-state intramolecular proton transfer (ESIPT)-regulated emission switching offers distinct advantages by enabling dual-to-single emission transitions, visual color change, and improved analyte discrimination through the modulation of proton transfer pathways. Herein, an ESIPT-active Cd(II)-based coordination polymer has been synthesized using 2,5-dihydroxyterephthalate (dht) as an ESIPT-active ligand. The framework exhibits ESIPT-mediated dual emission in polar solvents, with the most pronounced response in water, producing intense yellow luminescence. Notably, exposure to aqueous solutions of NA isomers selectively perturbed the water-assisted ESIPT process, resulting in a distinct dual-to-single emission transition accompanied by fluorescence quenching and a readily noticeable visual color change from yellow to colorless. In contrast, other aromatic amines produced negligible disruption of the dual-emission response, highlighting the excellent selectivity of the sensing platform. Generalized-gradient-approximation-based DFT calculations by guest-induced modulation of competing proton-transfer pathways explain the observed ESIPT response. This study demonstrates that ESIPT-regulated dual-to-single emission switching can provide an advanced luminescent CP-based sensor for the selective detection and discrimination of nitroaniline isomers.