Synthesis, Characterization, and BSA Binding Properties of Carboxylated Merocyanine-Based Fluorophores

This study describes the synthesis of new carboxylated merocyanine dyes by Knoevenagel condensation between 4-carboxybenzaldehyde and indolium/benzoindolium- and benzothiazolium-based coupling compounds. The condensations were performed in the presence of ammonium acetate, and the products were obtained in good yields after simple purification. These merocyanines exhibit UV-A-to-blue absorption and blue-to-green fluorescence emission, characterized by relatively large Stokes shift values (∼5000 cm^-1). In addition, quantum chemical calculations were conducted to better explore the electronic and photophysical properties of the merocyanines under study. Thermal analysis via thermogravimetric analysis (TGA) revealed distinct decomposition stages for the merocyanines, with stability up to 200 °C. Cyclic voltammetry revealed irreversible waves for donor oxidation and acceptor reduction. On the basis of the onset potentials, the highest occupied molecular orbital (HOMO) energies were estimated to be between -5.38 and -5.47 eV, and the lowest unoccupied molecular orbital (LUMO) energies were calculated to range from -3.20 to -3.24 eV. These values suggest a narrow electrochemical band gap of 2.07 to 2.13 eV. Finally, fluorescence quenching experiments using the intrinsic fluorescence of the Trp residues in BSA were successfully applied to these compounds, indicating strong interactions with this protein via a static mechanism. The docking simulations corroborated the interaction between the merocyanines and BSA.