Recent progress in ESIPT-based fluorescent chemosensors for detection of Zn2+, Mg2+and Cu2+

Sensing cations is an up-and-coming area of scientific research within chemical sensors, given its extensive applicability across clinical, biological, and environmental domains. Among various photophysical mechanisms, excited-state intramolecular proton transfer (ESIPT)-based probes offer distinct advantages due to their photophysical characteristics, including a narrowed emission band, sensitivity to solvent polarity, red-shifted fluorescence emission, high quantum yield, and other favorable properties. The ESIPT mechanism involves two sets of normal (N) and tautomer (T) energy levels which contribute to establishing four stable electronic energy levels in single proton transfer systems. Fluorescent probes utilizing the ESIPT phenomenon in the selective detection of cations have become versatile tools in scientific exploration. The progress of ESIPT-based fluorescent probes typically involves a design strategy aimed at inhibiting the ESIPT process, and, in turn, quenching fluorescence emission. In the "turn-on" fluorescent mechanism, when the chemosensor binds with metal ions, it disrupts fast non-radiative processes, enhancing structural stability and leading to a CHEF (chelation-enhanced fluorescence) effect. The cations in the complexes are commonly coordinated by tridentate ligands, leading to distinctive spectral changes that enable the detection of cations. This review article aims to present a comprehensive overview of recent progress and potential applications related to ESIPT-based dyes in sensing three main cations Zn2+, Mg2+ and Cu2+. The unique properties and mechanisms of ESIPT-based probes for cation sensing are elucidated through various validation approaches, offering insights into their design, performance, and future prospects in the field of chemical sensing.