Seven-membered Heterocycle Containing Coumarin-Based Probe for Selective Detection of Chemical Warfare Agent Phosgene in Solution and Gas Phase

Abstract

Herein we report two seven-membered heterocycle containing coumarin-based probes (2 and 3), functionalized with ethylenediamine and o-phenylenediamine, for the selective detection of the highly toxic chemical warfare agent phosgene. The structures were comprehensively characterized using 1H and 13C NMR, ESI-HRMS and FT-IR. Solid state structure of probe 3 was determined using single-crystal X-ray diffraction. Both the probes were employed for luminescent turn-on detection of phosgene. However, in case of 2, poor selectivity and emission at ~365 nm limit its practical applicability as it is not visible to the naked eye. In contrast, probe 3 displayed ~330-fold enhancement in emission intensity upon phosgene exposure, accompanied by an increase in fluorescence lifetime (1.65 ns to 3.05 ns). 1H NMR spectra revealed that a nucleophilic addition of phosgene induces intramolecular cyclization, leading to the formation of five and six-membered rings, which finally restricts intramolecular twisting and enhances luminescence. Probe 3 demonstrates a rapid response time (~94 s) and a very low detection limit (17.8 nM). TD-DFT calculations support the proposed reaction mechanism and elucidate the electronic transitions involved. Finally, probe 3 coated paper strip was utilized to detect the different concentrations of phosgene vapor without any interference from other competitive analytes.