Experimental and Theoretical Characterization of Ultrafast Water-Soluble Photochromic Photoacids.

UV-visible transient absorption spectroscopy and quantum mechanical simulations are combined to elucidate the photochemical mechanism of two metastable merocyanine/spiropyran photoacids, 2-[()-2-(2-hydroxyphenyl)ethenyl]-3,3-dimethyl-1-(3-sulfopropyl)-3-indol-1-ium (phenylhydroxy-MCH) and 2-[()-2-(1-indazol-7-yl)ethenyl]-3-(3-sulfopropyl)-1,3-benzothiazol-3-ium (indazole-MCH). Transient absorption spectra demonstrate that -acid isomerization to the form results in deprotonation on a picosecond time scale. Ring closure to form spiropyran follows promptly from the appropriate conformation or follows at longer time delays (≫3.5 ns) following a barrier crossing for single-bond isomerization to the appropriate conformation. Consistent with the results of Berton et al. [ 2020, 11, 8457-8468] , we find that -phenylhydroxy-MCH is a stronger acid than -phenylhydroxy-MCH. The decrease in p upon isomerization is further investigated to benchmark quantum chemical methods for their accuracy. Calculations were performed with nine levels of theory including continuum solvent models and explicit water. The calculations are not sufficient to describe the Δp following isomerization of these photoacids, and more work is necessary to properly evaluate the physical basis for the acidity of the photoacids.