Intrinsic and Dynamical Reaction Pathways of an Excited State Proton Transfer

Abstract

The detailed knowledge of excited state proton transfer mechanisms in complex environments is of paramount importance in chemistry. However, the definition of an effective reaction coordinate and the understanding of the driving force of the reaction can be difficult from both the experimental and the theoretical points of view. Here we analyzed by theoretical approaches the mechanism and the driving forces of the excited state proton transfer reaction occurring between the 7-hydroxy-4-(trifluoromethyl)coumarin photoacid and the 1-methylimidazole base molecules in toluene solution. In particular, we compared the intrinsic and the dynamical reaction pathways, obtained by integrating the reaction coordinate, and by performing ab initio simulations of molecular dynamics, respectively. Time-dependent density functional theory and polarizable solvation continuum models were adopted to define the excited state potential energy surface. Results were analyzed by means of the D(CT) electronic density based index. Our findings suggest that the reaction coordinate is mainly composed of several intra- and intermolecular modes of the reactants. An analysis of both the intrinsic coordinate and the dynamical results shows that the charge transfer induced by electronic excitation of the coumarin molecule is the main proton transfer driving force. With regards to the methodological validation, the combination of ab initio molecular dynamics with time-dependent density functional theory appears to be feasible and reliable to study excited state proton transfer reactions in the condensed phase.

Excited‐State Proton Transfer and Intramolecular Charge Transfer in 1,3‐Diketone Molecules

“…From the electronic point of view, the reorganization of the electronic density upon excitation and thus the degree of charge transfer associated to a given electronic excitation can be measured using simple descriptors such as the D CT index previously introduced and applied also to ESPT systems by some of us …”

“…Current computational approaches can be helpful in the characterization of the ES phenomena, due to the possibility of clearly pointing out and analyzing the different factors affecting a particular path on the ES potential energy surfaces (ES‐PESs) . In this respect, the use of electronic‐density‐based indexes has demonstrated a valid contribution to the analysis of the charge transfer character of a given transition and in the characterization of ESPT reactions …”

Comparing the performance of TD‐DFT and SAC‐CI methods in the description of excited states potential energy surfaces: An excited state proton transfer reaction as case study

“…The activation energy for the proton transfer computed in solution (Δ E # ) is computed to be 1.9 kcal/mol, as expected larger than what previously reported at the TD‐B3LYP/6‐31 + G(d,p) level of theory (0.25 kcal/mol). Nonetheless, in SAC‐CI results, a plateau in the R OH = 1.10–1.30 Å range (corresponding to the transition state region) is found.…”

“…To decouple structural from electronic effects, the same geometries will be firstly considered for all ES calculations performed using DFs. In particular, the structures obtained using a global hybrid (GH) have been considered as it has been previously shown that this level of theory enables to correctly describe the reaction profile in solution when compared to available experimental data …”

Computational Insights into Excited‐State Proton‐Transfer Reactions in Azo and Azomethine Dyes

“…Not surprisingly, these methodologies have been extensively applied to study ESPT reactions . Moreover, the same approach can be applied, with due care, to the analysis of nonradiative events …”