Aggregation induced emission enhancement (AIEE) of tripodal pyrazole derivatives for sensing of nitroaromatics and vapor phase detection of picric acid

Five pyrazole-based tripodal ligands, compounds 1-5 , based on a 1,3,5-triethylbenzene scaffold, show aggregation-induced emission enhancement (AIEE) and their application as fluorescent probes for detection of nitroaromatics is investigated. All five compounds are weakly fluorescent in THF, but their fluorescence intensity increases with the addition of a poor solvent (water), causing nanoaggregation, as confirmed by the changes in the UV-vis and emission spectra, and light scattering techniques. The nanoaggregates exhibit time-dependent emission characteristics and can serve as sensors for the detection of nitroaromatic compounds. The selective detection of picric acid (PA) over other nitroaromatic compounds by 1-3 is attributed to the photoinduced electron transfer from the trispyrazole to the quencher, as confirmed by TD-DFT calculations. Furthermore, compounds 1 and 2 were tested for sensing of PA in the vapor and solid phases by means of changes in the emission spectra observable by the naked eye. The supramolecular assemblies of 1 and 3 with PA were structurally characterized by X-ray crystallography. Organosoluble tris-pyrazole compounds aggregate in organic/aqueous solvent mixtures, showing aggregation-induced emission enhancement (AIEE), the latter being quenched by picric acid.