Novel thiazoline-phenothiazine based push-pull molecules as fluorescent probes for volatile acids detection

A series of novel acceptor-donor-acceptor type of push-pull molecules 3a-3e containing acid-sensitive component based on phenothiazine-5-oxide and thiazoline as donor and acceptor units, respectively were synthesized in excellent yields of 84-95 %. The photophysical processes of 3a-3e were explored with the aim to switch absorption and emission property by an external analyze, such as acid/base. Compounds 3a-3e displayed strong fluorescence in solution as well as in solid state. The presence of thiazoline unit on the pendant phenyl rings makes them suitable for volatile acid (TFA and HCl) sensing. The protonation of thiazoline moeities leads to the red-shifted band in both absorption and emission studies due to the strong intramolecular charge transfer (ICT) interactions. These spectral changes were found to be reversible on the addition of base triethylamine (TEA). Visual color change was also observed from colorless to yellow on the addition of an acid which was reversed back to colorless on the addition of TEA. Compound 3c showed minimum detection limit of 0.98 ppm in case of TFA. Compound 3a showed minimum detection limit of 13.1 ppb in case of HC1. The HOMO-LUMO energy gap of 3a-3e and their protonated forms were calculated by density functional theory (DFT) studies.