A seven-membered heterocycle containing – coumarin-based probe for selective detection of chemical warfare agent phosgene in solution and the 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. The 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 the 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 a ∼330-fold enhancement in emission intensity upon phosgene exposure, accompanied by an increase in fluorescence lifetime (1.65 ns to 3.05 ns). The 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, a probe 3-coated paper strip was utilized to detect different concentrations of phosgene vapor without any interference from other competitive analytes.