Small Electron-Rich Isomeric Solid-State Emitters with Variation in Coplanarity and Molecular Packings: Rapid and Ultralow Recognition of TNT

Fast, on-the-spot detection of TNT (2,4,6-trinitrotoluene) in ultralow concentration is in high demand to prevent illegal transport and find explosive formulations before and after the explosion. This work demonstrates an optoelectronic strategy to identify TNT selectively in solution, solid, and vapor phases by rapid fluorescence (FL) quenching as low as 15.24 parts per quadrillion (ppq) level using a couple of easily accessible, structurally diverse positional isomeric electron-rich solid-state emitters as promising small molecules. These solid-state emitters display superior sensitivity in comparison to many reported polymeric materials. The aggregates (almost aqueous solution) of these probes could detect TNT with a detection limit of 39-65 nM. Our mechanistic investigations using steady-state and lifetime FL studies reveal that the quenching effect in the ground state is practically negligible. Still, the probes' associated frontier molecular orbitals energy levels favor the photoinduced electron transfer process selectively with TNT. We establish the role of relatively planar molecules for the inclination to the quenching process than the twisted ones. The probes were drop-cast or dispersed on a simple filter paper and employed successfully for onsite TNT detection. The impregnation of these probes on a well-known polymeric platform could also realize a safer detection even during the rainy season. A rapid quenching efficiency of yellowish-green emission is beneficial while treating TNT-mixed soil (0.2 mg of TNT in 2 g of soil) with the probe's aggregated 90% aqueous solution. Ample crystal void space, available interlayer distances, and suitable orbitals energy gap cooperatively result in a rapid ppq level detection of TNT even in the solid state.