Modeling Excited-State Proton Transfer to Solvent: A Dynamics Study of a Super Photoacid with a Hybrid Implicit/Explicit Solvent Model

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. A 2014, 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.

Water-Mediated Excited State Proton Transfer of Pyranine–Acetate in Aqueous Solution: Vibrational Fingerprints from Ab Initio Molecular Dynamics

“…In a recent work, we explored the ESPT reaction of a superphotoacid to the solvent by means of the same QM/MM hybrid scheme. 86 Despite the ultrafast reactivity (100 fs), we found that the ESPT is assisted by the solvation dynamics of the water molecules belonging to the first and second solvation shell of the first accepting water molecule. Additionally, we found that this event can be reproduced only when solvation shells around the proton are taken into account at the full QM level.…”

“… 30 , 35 The ESPT kinetics is however strongly dependent on the nuclear structure of the starting configuration, i.e., the intermolecular distances of the proton donor–acceptor pair and their relative orientation, in addition to the solvation around the reactive core. 3 , 4 , 86 If the starting nuclear configuration is structurally prepared for the ESPT reaction, it reasonable to expect an ESPT in the subpicosecond time scale.…”

Quantum and semiclassical dynamical studies of nonadiabatic processes in solution: achievements and perspectives

“…the bulk solvent, when molecularity is less important, will probably represent a significant advance in this field. Since QM/MM methods with localised basis sets are more straightforwardly applied without periodic boundary conditions, non-periodic approaches, like the generalized liquid optimized boundary (GLOB) model, 44,221,222 will probably play an important role. Recent attempts to combine GLOB ideas with the use of polarizable FFs look very promising.…”

“…117 For short time-scales a wise increase of the QM subset to include some solvent molecules is an option, at least in the case of well-defined specific solute-solvent interactions. 44 On the other hand, the number of discrete QM solvent molecules necessary to simulate bulk properties is a currently open question, even for static properties. 216,224 In terms of the dynamics methods to be combined with explicit solvent approaches, at-the-state of the art the most popular and widely used are those that propagate all nuclear degrees of freedom with classical dynamics.…”

AutoSolvate: A Toolkit for Automating Quantum Chemistry Design and Discovery of Solvated Molecules

“…Although implicit solvent models, such as the polarizable continuum models, [18][19][20][21][22][23][24] are now widely available in QC packages and the corresponding molecular property data set curation can be easily automated with the aforementioned QC workflows, the accuracy of these calculations cannot always meet our needs in design and discovery. Accurate prediction of many molecular properties requires QC calculation in explicit solvents due to the existence of proton transfer, 25,26 hydrogen bonds, [27][28][29] or other strong solute-solvent interactions. Setting up QC calculations of explicitly solvated molecules usually involves multiple steps, including building the structure of the solute molecule in a solvent box, generating a customized force field for solute/solvent, running molecular dynamics (MD) simulations at molecular mechanical (MM) and hybrid quantum mechanical and molecular mechanical (QM/MM) [30][31][32][33][34] level of theories to obtain equilibrated solvation configurations, and the final QC calculation of molecular properties for the microsolvated molecule extracted from the solvent box.…”