Fluorescent properties based on ESIPT and TICT of novel acylhydrazone-based probe and its sensing mechanism for Al3+: A TD-DFT investigation

By using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the fluorescent process and sensing mechanism of N′-(2-hydroxybenzylidene)isoquinoline-3-carbohydrazide (NHIC) probe for Al3+ detection were explored in detail. For NHIC, twisted intramolecular charge transfer (TICT) is induced by existed state intramolecular proton transfer (ESIPT). Probe NHIC can undergo ESIPT process in the stepwise pattern, and then a stable double proton-transfer (DPT) structure is formed. There are two non-emissive TICT states (TICT1 and TICT2) existing in NHIC. TICT2 is responsible for the weak fluorescence observed experimentally. After the addition of Al3+, the coordination bond is formed between NHIC and Al3+, the TICT state is prohibited, and the strong fluorescence is switched on. It is evident that the twisted C-N single bond belonging to acylhydrazone part of NHIC results in its TICT state. We hope that our study can throw some light on the development of switch-on probe.