Control and activation of chemical processes by light promotes mechanistic studies of chemical and biochemical reactions. Photoacids, i.e., molecules that significantly decrease pKaupon radiation, open exciting opportunities for photo-control of the local environment, by changing the acid-base equilibrium on a nanosecond timescale, with applications in photolithography, protein folding and drug delivery. However, competing channels of excitation energy relaxation through conical intersections and intersystem crossings often result in non-adiabatic dynamics and short-lived excited states. Thus, it is hard to tell a priory whether a particular compound will exhibit a long-lived deprotonated excited state and can be utilized as a photoacid.In this work we apply ab initio excited state methods to understand ultrafast dynamics in a prototypic photoacid 3-cyano-6-hydroxycoumarin (CHCM). In particular, we characterize electronic excited states in CHCM along the proton transfer coordinate. In the gas phase, a conical intersection between bright p?p* and dark p?s* excited states is observed, which is a common motif of deprotonation dynamics in aromatic chromophores. However, our results suggest that inclusion of water solvent, which strongly destabilizes p?s* state, eliminates the conical intersection as a plausible route for nonradiative deexcitation. These results call for a careful reevaluation of our understanding of excited state proton transfer dynamics in solvated chromophores.

Photoacids under investigation: Theoretical modeling of excited state proton transfer to solvent in solvent

248th National Meeting (2014)

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