Abstract
The rapid growth of time-resolved spectroscopies and the theoretical advances in ab initio molecular dynamics (AIMD) pave the way to look at the real-time molecular motion following the electronic excitation. Here, we exploited the capabilities of AIMD combined with a hybrid implicit/explicit model of solvation to investigate the ultrafast excited-state proton transfer (ESPT) reaction of a super photoacid, known as QCy9, in water solution. QCy9 transfers a proton to a water solvent molecule within 100 fs upon the electronic excitation in aqueous solution, and it is the strongest photoacid reported in the literature so far. Because of the ultrafast kinetics, it has been experimentally hypothesized that the ESPT escapes the solvent dynamics control (Huppert et al., J. Photochem. Photobiol. A2014,277, 90). The sampling of the solvent configuration space on the ground electronic state is the first key step toward the simulation of the ESPT event. Therefore, several configurations in the Franck–Condon region, describing an average solvation, were chosen as starting points for the excited-state dynamics. In all cases, the excited-state evolution spontaneously leads to the proton transfer event, whose rate is strongly dependent on the hydrogen bond network around the proton acceptor solvent molecule. Our study revealed that the explicit representation at least of three solvation shells around the proton acceptor molecule is necessary to stabilize the excess proton. Furthermore, the analysis of the solvent molecule motions in proximity of the reaction site suggested that even in the case of the strongest photoacid, the ESPT is actually assisted by the solvation dynamics of the first and second solvation shells of the water accepting molecule.
Highlights
Light irradiation adds new dimensions to the conventional ground-state chemistry
Unraveling the complex aspects of excited-state proton transfer (ESPT) reactions at the molecular level, with solvent molecules acting as the proton acceptor, is extremely difficult.[4−9] a wide range of time and space scales are in play.[10,11]
We explored the effects of the quantum mechanical/molecular mechanical (QM/MM) partition size on the excited-state reaction dynamics, collecting S1 ab initio molecular dynamics (AIMD) trajectories with an increasing number of water molecules considered at the QM level
Summary
Light irradiation adds new dimensions to the conventional ground-state chemistry. The strongly perturbed electronic structure, reached when molecules get excited, leads to a reactive behavior that ground-state chemistry cannot perform. Ensemble averages extracted from the NPBC/AIMD simulations provide essential insights on the equilibrium solvation of the actors in play (proton donor and acceptor) while excited-state non-equilibrium dynamics give access to the mechanistic details of the ESPT to the solvent In this perspective, the crucial issue is the correct setup of the quantum mechanical/molecular mechanical (QM/MM) layout, namely how large the QM region should be. Huppert and co-workers interpreted these decay data individuating in the oxygen donor−oxygen acceptor intermolecular vibrational mode the rate-limiting step for the ultrafast kinetics.[4,33,34] According to this interpretation, the O−O stretching mode controls and assists in reducing the oxygen−oxygen distance of the proton-transferring complex, leading to the PT This would represent a unique case because the solvation dynamics is usually supposed to be the ratedetermining step for ultrafast ESPT to the solvent.[5].
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