Monitoring ultrafast intramolecular proton transfer processes in an unsymmetric β-diketone.

We report the experimental determination of the intramolecular enol-enol tautomerization rate of an unsymmetric β-diketone, benzoylacetone, with femtosecond transient absorption in the ultraviolet. Initially, there is an equilibrium of two possible enolic structures in solution, which is disturbed upon UV excitation by exciting a disproportionate fraction of one enolic form. Comparison to symmetric β-diketones, acetylacetone and dibenzoylmethane, suggests that ground-state proton transfer gives rise to additional dynamics in benzoylacetone due to the dissimilarity of the two enolic forms. In the excited state of the molecules, the intramolecular H-bond is initially broken, followed by photochemical processes towards rotamer structures. Our studies therefore disclose intramolecular proton transfer among electronic ground as well as excited states of benzoylacetone. Considering the importance of β-diketones as a common model of enol-enol tautomerization and their resemblance to enzymatic enolates, the present study provides valuable information on the ultrafast mechanism of intramolecular proton transfer processes.