Reliable experimental method for determination of photoacidity revealed by quantum chemical calculations

Abstract

Photoacids are aromatic acids that exhibit significantly different acidities when they are electronically excited. Three experimental methods have been extensively used to determine the photoacidity, pK(a)*: fluorescence titration, the Forster cycle, and time-resolved experiments. However, the photoacidities determined by these experimental methods are not consistent. In this work, we used a theoretical method to evaluate the reliability of experimentally determined pK(a)* values. In particular, density functional theory (DFT) and time-dependent DFT calculations were used to obtain the changes in Gibbs free energy for acid dissociation reactions which are directly related to pK(a)* values. The Forster cycle, which is frequently used to experimentally determine the photoacidity due to its simplicity, yielded inconsistent results depending on how the transition energy was defined. We evaluated six empirical parameters extracted from the absorption and emission spectra of acidic and basic species of photoacids to adequately define the transition energy in the Forster cycle. And we found that the pK(a)* values obtained using the optical bandgap as the transition energy in the Forster cycle were in the best agreement with the results of quantum chemical calculations.