Photophysical Properties of Sinapic Acid and Ferulic Acid and Their Binding Mechanism with Caffeine

Sinapic acid (SA) and ferulic acid (FA) are bioactive compounds used in the food, pharmaceutical, and cosmetic industries due to their antioxidant properties. In this work, we studied the photophysical properties of SA and FA in different solvents and concentrations and their interactions with caffeine (CF), using ultraviolet-visible (UV-Vis), fluorescence spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The findings show that the quantum yield, fluorescence lifetime, radiative decay rates, and non-radiative decay rates of SA and FA are influenced by the concentrations and solvent polarity. The interaction between SA and FA with CF was also studied using UV-Vis and fluorescence spectroscopy. The results indicate that the CF quenched the fluorescence intensity of SA and FA by static quenching due to the formation of a non-fluorescent complex. The van't Hoff equation suggests that the van der Waals forces and hydrogen bonds force were responsible for the interaction between SA and CF, as indicated by a negative change in enthalpy ( $$  < 0) and a negative change in entropy ( $$  < 0). On the other hand, the interaction between FA and CF was primarily controlled by electrostatic force, as indicated by a negative change in enthalpy ( $$ < 0) and a positive change in entropy ( $$ > 0). The negative change in Gibbs free energy ( $$ ) indicates that both compounds underwent a spontaneous binding process.