Department of Chemistry, University of Torino, Via Pietro Giuria 5,7, 10125, Torino, Italy.
Department of Chemistry, University of Torino, Via Pietro Giuria 5,7, 10125, Torino, Italy.
Spectrochim Acta A Mol Biomol Spectrosc. 2020 Mar 5;228:117587. doi: 10.1016/j.saa.2019.117587. Epub 2019 Oct 12.
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 HFA, singly deprotonated HFA and doubly deprotonated FA), 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 S to singlet excited states higher than the first (S for HFA, S for HFA, and a state higher than S for FA), followed by radiationless deactivation to the first excited singlet state (S), and by fluorescence emission according to the S → S transition. In contrast, the "C-like" emission is mainly caused by S → S excitation combined with S → S emission, but there is also a minor contribution from the S → S 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.
阿魏酸（（E）-3-（4-羟基-3-甲氧基-苯基）-2-烯酸，以下简称 FA）是植物细胞壁的组成部分，通常存在于木质纤维素中。因此，它是微生物降解植物残渣产生的腐殖质的潜在成分。FA 的荧光激发-发射矩阵光谱具有有趣的腐殖质样形状，其带可以与腐殖质的 A 和 C 区域同化。因此，FA 光致发光的研究可能为了解腐殖质化合物荧光性质背后未知的过程提供有趣的见解。FA 是一种弱酸，在水溶液中以三种不同形式存在（中性 HFA、单质子化 HFA 和双质子化 FA），它们具有略有不同的吸收和发射特性。FA 物种的“类似 A 的”荧光发射归因于从基态 S 到高于第一激发态（HFA 的 S，HFA 的 S 和 FA 的高于 S 的状态）的单重激发态的激发，然后无辐射失活到第一激发单重态（S），并根据 S → S 跃迁进行荧光发射。相比之下，“类似 C 的”发射主要是由 S → S 激发与 S → S 发射结合引起的，但对于 HFA 也有来自 S → S 激发的较小贡献。FA 辐射吸收和荧光发射的最大波长随 pH 的不均匀变化可以用不同 FA 物种的前线（HOMO 和 LUMO）分子轨道能级的框架来合理化。这些能级受到相关电子与每个物种的中性（质子化）或负（去质子化）基团之间的电荷相互作用的影响。
