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

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