Excited-state proton transfer (ESPT) is a fundamental photophysical process that underpins the fluorescence behavior of many organic and organometallic molecules. In chemosensing, ESPT provides a highly selective and sensitive pathway for detecting target analytes through distinct optical responses. This project investigates the ESPT mechanism in newly designed thiosemicarbazone- and pincer-ligand-based receptors, focusing on their interactions with environmentally and biologically relevant ions such as fluoride, cyanide, and hydroxide. The study integrates experimental spectroscopy (UV–Vis, fluorescence, NMR titrations) with advanced computational methods (DFT, TD-DFT, PES, NBO, and transition state analyses) to unravel the electronic and structural factors governing ESPT. The outcomes of this work are expected to guide the rational design of next-generation chemosensors with enhanced selectivity, sensitivity, and real-world applicability in environmental monitoring and biomedical diagnostics.