Photoreactivity of humic-like polyphenol material under irradiation with different wavelengths explored by FTICR MS and deuteromethylation

The goal of this study was to establish reactivity of lignin-derived synthetic polyphenolic material under irradiation by ultraviolet (254 nm) and visible (460 and 525 nm) light in order to deeper examine relationships between the optical properties of this complex mixture and its individual constituents. In all photoirradiation experiments, blue shift of the fluorescence spectrum was observed. We aimed at understanding whether these changes could be explained on the basis of the chromophore interactions hypothesis, which implies destruction of electron-acceptor pairs via free radical transformations to be responsible for the alteration of optical properties. For this, changes in molecular composition were explored by Fourier transform ion cyclotron resonance mass spectrometry. Irradiation with UV resulted in a pronounced oxidation of polyphenols, which was manifested in the van Krevelen diagram by the formation of components with higher O/C ratio. At the same time, irradiation by visible light had led to the appearance of more condensed molecules depleted of oxygen. Consideration of changes in relative contribution of 500 most abundant components in polyphenol materials revealed higher transformation yields under UV light as compared to the visible light. Further studies using deuteromethylation followed by Fourier transform ion cyclotron resonance mass spectrometry enabled to enumerate the number of carboxylic groups in individual components of the parent polyphenol material. It was shown that at all wavelengths irradiation mainly impacted carboxylic-rich unsaturated and aromatic compounds, which can be considered as strong electron-acceptors. We suggest that their transformation is responsible for the blue shift of fluorescence spectrum, thus emphasizing the role of chromophore interaction mechanism of the optical properties formation.