Ratiometric detection of lethal oxalyl and thionyl chloride by a thiophene-appended phenanthridine sensor

Abstract

As fatal and potentially accessible reagents, oxalyl and thionyl chloride have become severe concerns for public safety. Therefore, it is essential to develop effective methods to discriminate them. Herein, a new fluorescent sensor, 2-(7,8,13,14-tetrahydrodibenzo[a, i]phenanthridin-5-yl)-4,6-di(thiophen-2-yl)aniline (TPTA) has been designed with the incorporation of classical phenanthridine fluorophore and o-aniline functionality to explore the versatile sensing capabilities. Due to the electrophilic character of chlorocarbonyls, the detection process occurs between TPTA and SOCl2, (COCl)2 via nucleophilic addition and subsequent cyclization, which produces new, green emissive cyclic adducts. An inhibited excited-state intramolecular proton transfer (ESIPT) followed by the ICT process endows sensitive ratiometric detection of oxalyl and thionyl chloride with a low limit of detection (4.15 and 2.93 nM). The limit of quantification (LOQ) was calculated to be (9.81 and 13.8 nM for SOCl2 and (COCl)2, which is far lower than the LD50 values. Because of its reaction-based sensing mechanism, robust selectivity with rapid response time has been achieved towards SOCl2 and (COCl)2 (<5 sec) with good stability. The anti-interference properties reveal that the sensor TPTA can efficiently detect SOCl2 and (COCl)2 even in the presence of other analytes. Further, the detection mechanism of the TPTA induced by oxychlorides was studied and well supported by NMR, HRMS, and DFT theoretical calculations. Remarkably, smartphone-assisted concentration-dependent test kit experiments enhance the practical utility of TPTA and facilitate the real-time detection of oxychlorides. Hence, TPTA has proven to be an inevitable and promising tool for the sensitive recognition oxychloride species.