Benzimidazole and benzothiazole fluorophores with large Stokes shift and intense sky-blue emission in aggregation as Al3+ and Pb2+ sensors

New fluorophores based on 2-(2'-hydroxyphenyl)benzimidazole (HBZ) and 2-(2'-hydroxyphenyl)benzothiazole (HBT) for metal ion sensing were designed and synthesized using a simple method. The photophysical behaviour of these fluorophores were investigated in various solvents using UV-visible and fluorescence spectra. All the heterocycles showed strong excited state intramolecular proton transfer (ESIPT) characteristics with remarkably large Stokes shift (190-252 nm). Spatial charge distribution in the frontier molecular orbitals also demonstrated the ESIPT mechanism through intramolecular charge transfer. Time resolved fluorescence measurements for these heterocycles showed two long-life decay mechanisms which may be attributed to excited state enol and keto emission. These intense sky-blue emitters also exhibited aggregation induced blue shifted emissions due to restriction of intramolecular rotation processes. Fluorescence sensing studies for metal ions revealed the good selectivity of these fluorophores towards Al3+ and Pb2+ . Theoretical computations performed using density functional theory methods showed two possible geometric configurations for Al3+ binding.