Acidity Constants of Lignin Model Compounds in the Electronically Excited State in Water–N,N-Dimethylformamide Mixtures

The acidity constants of a number of guaiacyl phenols simulating structural fragments of natural lignins in the electronically excited state in water–N,N-dimethylformamide mixtures are determined by fluorescence spectroscopy using the Forster cycle. The differentiating effect of an aprotic solvent on the acidity of phenols is shown. It is found that the pKa values in the excited state are associated with the properties of para-substituents with respect to the phenolic hydroxyl group. The highest capacity for acid ionization during photoexcitation is exhibited by compounds containing an electron-withdrawing carbonyl group.