Structural isomerism induces pH dependent AIE-coupled ESIPT: an in-depth spectroscopic exploration with application in amine vapor sensing

A novel excited state intramolecular proton transfer probe 3-(benzo[d]thiazol-2-yl)-4-hydroxy-5-methoxybenzaldehyde (3BTHMB) was synthesized and characterized using NMR, ESIMS and single crystal diffraction studies. The steady state and time resolved studies using the time correlated single photon counting (TCSPC) technique revealed that 3BTHMB shows ESIPT reaction with a time constant of 0.10-0.15 ns and longer local emission and anion emissions of time constants 0.5-2.0 ns and 3.0-4.0 ns, respectively, that co-exist in polar solution. In a polar protic solution like methanol, the ESIPT process is damped. Interestingly, 3BTHMB shows aggregation induced emission (AIE) solely in an acidic environment, evidenced by the enhanced quantum yield of emission (∼35%) in acidic solution as well as a long lifetime component of 7.4 ns that corresponds to the lifetime in the solid state. The AIE phenomenon was explored using the DLS technique where enhancement of the hydrodynamic radius in an acidic medium was observed. The judicious positioning of the formyl group in 3BTHMB gives the mentioned probe to show AIE in an acidic medium, as opposed to its previously reported structural isomer BTHMB, which showed no AIE phenomenon as well as its parent molecule 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP), which showed AIE in neutral pH. The AIE property of 3BTHMB was exploited by successful and rapid detection of amine vapor in the solid state, qualitative ratiometric detection of aqueous pH, by observing the colour change under UV light in acidic (yellow-green) and basic to neutral (blue) medium. The above results pave the way for multiutility fluorescent probes for environmental purposes along with a deep understanding of the underlying photophysical processes which bestow the probes with such outstanding utilities.