2-(Benzothiazol-2-yl)pyren-1-ol, a new excited state intramolecular proton transfer-based fluorescent sensor for nitroaromatic compounds

2-(Benzothiazol-2-yl)pyren-1-ol (P3-NS) was developed as a excited-state intramolecular proton transfer (ESIPT)- based sensor for nitroaromatic compounds (NACs). Results of studies of its photophysical and optical properties show that P3-NS exists in various, solvent dependent isomeric forms arising from rotation about the C–C bond connecting the benzothiazole and pyren-1-ol rings and the location of the pyrene-OH proton. The results of density functional theory (DFT) calculations enabled identification of the major structures of P3-NS dissolved in different solvents. Upon electronic excitation in nonpolar or weakly polar solvents, P3-NS undergoes an ESIPT reaction to form a product that only weakly fluoresces. In contrast, P3-NS exists in a strongly fluorescent deprotonated form in highly polar solvents. The results of time-dependent DFT calculations indicate that fluorescence quenching of the product generated by ESIPT reaction of P3-NS is caused by intersystem crossing at a conical intersection between S₁ and T₂ states. The anionic form of P3-NS, present in highly polar solvents, was shown to be a fluorescence sensor for nitroaromatic compounds (NACs). Fluorescence quenching by NACs occurs by protonation of the anionic form of P3-NS (static quenching) and by photoinduced electron transfer from the anionic form to NACs (dynamic quenching).