Substitution‐Dependent Excited‐State Behavior and Acid‐Responsive Fluorescence of Methoxylated Coumarin–Thiosemicarbazones: Experimental and TD‐DFT Insights

ABSTRACT

Methoxylated coumarin–thiosemicarbazone derivatives CT6 , CT7 , and CT8 were synthesized and systematically investigated to elucidate their excited‐state properties, solid‐state fluorescence, and acid‐sensing behavior. In solution, all three compounds exhibited broad emission bands, large Stokes shifts, and solvent‐dependent fluorescence, consistent with excited‐state relaxation and partial intramolecular charge redistribution within the coumarin–thiosemicarbazone framework. At low temperature (77 K), restricted molecular motion reduced the efficiency of non‐radiative decay pathways, leading to enhanced emission intensity. In the solid state, CT6 and CT7 displayed weak fluorescence, whereas CT8 showed efficient blue emission. Protonation experiments revealed markedly different responses: CT6 showed only modest fluorescence enhancement, CT7 displayed a pronounced turn‐on response, whereas CT8 underwent strong fluorescence quenching. DFT calculations on the possible protonated/tautomeric forms indicate that the CT6/7/8‐2 form, associated with protonation at the imine/azomethine nitrogen, is thermodynamically favored for all three derivatives. Therefore, the contrasting acid‐sensing outputs arise not from fundamentally different preferred protonation sites, but from substitution‐dependent modulation of the electronic structure and excited‐state deactivation pathways after protonation. Furthermore, nonlinear optical calculations revealed substitution‐dependent polarization responses, with CT8 showing the largest first hyperpolarizability and CT7 displaying the strongest second hyperpolarizability within the series. These findings demonstrate how subtle substitution differences at the 6‐, 7‐, and 8‐positions of the coumarin core govern fluorescence efficiency, protonation response, and solid‐state behavior, providing design principles for acid‐sensitive fluorophores and efficient solid‐state emitters.