Experimental and theoretical study of the fluorescence emission of ferulic acid: Possible insights into the fluorescence properties of humic substances

Ferulic acid ((E)-3-(4-hydroxy-3-methoxy-phenyl)prop-2-enoic acid, hereinafter FA) is a building block of plant cell walls that is commonly found in lignocellulose. As such, it is a potential component of humic substances produced by microbial degradation of plant spoils. The fluorescence excitation-emission matrix spectra of FA have an interesting humic-like shape, with bands that can be assimilated to the A and C regions of humic substances. Therefore, the study of FA photoluminescence might provide interesting insight into the still unknown processes that lay behind the fluorescence properties of humic compounds. FA is a weak diprotic acid that occurs in three different forms in aqueous solution (neutral H2FA, singly deprotonated HFA− and doubly deprotonated FA2−), which have slightly different absorption and emission properties. The “A-like” fluorescence emission of the FA species is accounted for by excitation from the ground singlet state S0 to singlet excited states higher than the first (S4 for H2FA, S5 for HFA−, and a state higher than S2 for FA2−), followed by radiationless deactivation to the first excited singlet state (S1), and by fluorescence emission according to the S1 → S0 transition. In contrast, the “C-like” emission is mainly caused by S0 → S1 excitation combined with S1 → S0 emission, but there is also a minor contribution from the S0 → S2 excitation that becomes significant for HFA−. The uneven variations with pH of the wavelengths of the maximum FA radiation absorption and fluorescence emission can be rationalised in the framework of the energy levels of the frontier (HOMO and LUMO) molecular orbitals of the different FA species. These levels are affected by charge interaction between the relevant electrons and the neutral (protonated) or negative (deprotonated) groups of each species.