Highly conjugated carbazole and pyrrolo[1,2-a]quinoxaline based small molecules for fluorescent detection of nitroexplosives

Abstract

Explosive detection has been a need of the current times to ensure security of people & fluorescent small molecules have become popular for their rapid and selective detection. Herein, we report two highly conjugated small organic fluorophores i.e.1,3,6,8-tetraethynyl-9-hexyl-9H-carbazole (Tetra) based on carbazole & 5-(4′-(pyrrolo[1,2-a]quinoxalin-4-yl)-[1,1′-biphenyl]-4-yl)pyrrolo[1,2-c]quinazoline (PQ-1) based on pyrroloquinoxaline scaffolds, both of which showed excellent quenching of fluorescence in the presence of nitroexplosive molecules. Linear Stern Volmer plots were obtained with different quencher molecules giving low detection limits even under visual inspection. A spectral overlap between the absorption spectra of picric acid (PA) and emission spectra of Tetra and a decrease in fluorescence lifetime of fluorophore with increasing concentration of quencher molecules confirm dynamic quenching with Forster Resonance Energy Transfer (FRET) mechanism. PQ-1 on the other hand showed a static mode of quenching indicated by no change in fluorescence lifetimes. DFT studies were done to identify the HOMO-LUMO bandgap, which are in good agreement with the experimental photophysical properties of the molecules. A low detection limit of 6.68 × 10−7 M for picric acid & 2.53 × 10−6 M for 2,4,6-TNT was observed using Tetra. Similarly, detection limit of 4.45 × 10−6 M & 1.84 × 10−6 M was found for PQ-1 towards picric acid & 2,4,6-TNT respectively. Interestingly, both Tetra & PQ-1 sensed aliphatic nitrocompounds like nitromethane with low detection limits of 4.57 × 10−7 M & 9.37 × 10−7 M respectively.