Revisiting photoacidity using R*NH2 photoacids

Time-resolved and steady-state measurements of proton transfer reactions for the two main types of neutral photoacids, the R*NH2 and R*OH photoacids, are compared and analyzed through free-energy correlations. We show that significant similarities in the proton transfer mechanisms exist in the proton dissociation of the two types of photoacids and that the observed differences in the free-energy correlations are likely to be caused by differences in the local reorganization energy emerging from the solvent. While both the R*NH2 and R*OH photoacids may release the proton to the solvent, R*NH2 photoacids are too weak to do so noticeably within the short excited-state lifetime of the photo-excited photoacids. For this reason, we constructed for R*NH2 photoacids a free-energy correlation between the proton transfer rate of the NH2 group and a series of strong Brønsted-base proton-acceptors R*NH2 + B ⇄ R*NH- + BH+ using steady-state and time-resolved fluorescence spectroscopies. We discuss how this correlation compares with the free-energy correlation of a series of R*OH photoacids of various strengths, which proton-dissociate in water according to R*OH ⇄ R*O- + H+ or proton-dissociate to a strong base like the proton transfer reactions we study with the R*NH2 photoacids.