Identifying and harnessing ESIPT properties of a strategically designed bichromophoric coumarin-benzothiazole...

Molecules capable of excited-state intramolecular proton transfer (ESIPT) have garnered considerable attention in recent decades due to their distinct photophysical properties and wide-ranging applications. However, the integration of ESIPT processes into molecular-scale logic circuits remains relatively underexplored. In this work, we present the design, synthesis, and photophysical characterization of a bichromophoric coumarin-benzothiazole conjugate, 8-[1-(6-Chloro-benzothiazol-2-ylimino)-ethyl]-7-hydroxy-chromen-2-one (CBHC) investigated in both non-polar solvent (N-Hexane) and a protic polar solvent (ethanol) using steady-state and time-resolved fluorescence spectroscopy. Experimental findings demonstrate that CBHC undergoes ESIPT to form keto-amine tautomer in nonpolar N-Hexane, while in polar protic ethanol, the enol-imine form predominates. Quantum chemical calculations based on DFT and TD-DFT further support the feasibility of excited-state proton transfer, with potential energy curve (PEC) analysis confirming that photoinduced charge redistribution increases the basicity of the azomethine nitrogen, thereby facilitating the proton transfer process. Additionally, the photophysical behavior of CBHC was examined under varying pH and thermal conditions. Leveraging its dual emission profiles in different solvents, a series of basic and complex molecular logic gates were constructed, with light acting as the central control input. This optical switching capability enables logic gate interconversion without altering chemical inputs. Furthermore, the fluorescence on/off response facilitates its localization in microenvironments of differing polarity and proticity. Overall, the unique opto-chemical characteristics of this newly developed coumarin-benzothiazole Schiff base highlight its potential as a versatile platform for next-generation smart materials, with promising applications in molecular electronics, sensing, and programmable systems.