Spectroscopic and theoretical evaluation of feruloyl derivatives: Insights into their electronic properties

The spectroscopic characterization of ferulic acid (FA), ethyl ferulate (EF), 1-feruloyl-glycerol (FG), and 1,3-diferuloyl-glycerol (DFG) was done using absorbance spectrophotometry and fluorescence excitation-emission matrix (EEM) analysis. Absorbance measurements demonstrated that each feruloyl derivative (FA, EF, FG, and DFG) existed in two forms in water at pH 4.6, a fully neutral form (X, X = FA, EF, FG, or DFG) and a mono-deprotonated form (X⁻). Absorbance spectral analysis demonstrated that the neutral forms of FA, EF, FG, and DFG each were excited from the ground state (S₀) to the first (S₁), second (S₂), fourth (S₄), and sixth (S₆) excited states; the mono-deprotonated states were excited to S₁, S₂, and S₄ excited states. EEM analysis was able to indicate excitation was from the neutral ground state, [X]-S₀ to the first [X]-S₁, second [X]-S₂, and fourth [X]-S₄ neutral ground states for EF; [X]-S₁ and [X]-S₄ for FG and DFG. EEM indicated that the mono-deprotonated form ground state, [X⁻]-S₀, was excited to [X⁻]-S₂ and [X⁻]-S₅ for FA⁻, [X⁻]-S₂ for EF⁻, FG⁻, and DFG⁻. Fluorescence emission analysis as a function of pH demonstrated that there was one detectable path of emission for all feruloyl derivative (FA, EF, FG, and DFG), from the neutral first excited state, [X]-S₁, to the deprotonated ground state, [X⁻]-S₀. Molecular electrostatic potential modelling suggests that the oxygen of each feruloyl derivative is associated with the electrostatic interactions, but glycerol hydroxyl group of FG and DFG have insignificant impact. Implications are that EEM of FA, EF, FG, and DFG has the potential to be a simple method of detection during synthesis.