A simple model of solvent-induced symmetry-breaking charge transfer in excited quadrupolar molecules

Abstract

A simple model has been developed to describe the symmetry-breaking of the electronic distribution of A-D-A type molecules in the excited state, where D is an electron donor and A and A are identical acceptors. The origin of this process is usually associated with the interaction between the molecule and the solvent polarization that stabilizes an asymmetric and dipolar state, with a larger charge transfer on one side than on the other. An additional symmetry-breaking mechanism involving the direct Coulomb interaction of the charges on the acceptors is proposed. At the same time, the electronic coupling between the two degenerate states, which correspond to the transferred charge being localised either on A or A, favours a quadrupolar excited state with equal amount of charge-transfer on both sides. Because of these counteracting effects, symmetry breaking is only feasible when the electronic coupling remains below a threshold value, which depends on the solvation energy and the Coulomb repulsion energy between the charges located on A and A. This model allows reproducing the solvent polarity dependence of the symmetry-breaking reported recently using time-resolved infrared spectroscopy.

Solvent tuning of photochemistry upon excited-state symmetry breaking

“…3b NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-15681-3 ARTICLE decreases of the fluorescence intensity are those expected for a transition from a state with the excitation delocalized over 3 or 2 branches to a state with the excitation confined on a single branch 36 . ES-SB occurs with O and Q in all polar solvents (Supplementary Discussion), including the weakly polar ethers or the quadrupolar benzene, as predicted by the Ivanov model [53][54][55] . Its dynamics are the same for both O and Q and depend only on the solvent.…”

“…with the decoherence of the exciton 36 . The Ivanov symmetry-breaking model [53][54][55] can be applied to evaluate the dipolar character, D, of the relaxed S 1 state. It is based on two experimentally accessible parameters: the electronic interbranch coupling, V, and the fluorosolvatochromic shift in a given medium with respect to a non-polar solvent.…”

Modeling Infrared Spectral Dynamics upon Symmetry Breaking of a Photo-Excited Quadrupolar Dye

“…It A-π-D-π-A molecule with a pyrrolopyrrole D core and two cyanophenyl A sub-units. 45,52 For larger D values, the frequency splitting reaches a minimum, and suggests a rise for D > 1.…”

“…The theoretical model of ES-SB based on a two-essential-states approach was presented in ref. 52 and its extension that includes the effect of electron-vibration coupling can be found in ref. 50.…”

“…includes the coupling between the two zwitterionic states, V , which determines the splitting of the one-and two-photon absorption bands, H s describes the dipolar solute-solvent interactions, H c accounts for the Coulombic interactions, H v0 and U int are intramolecular vibration and electron-vibration interactions, respectively. Within the self-consistent field approximation, the Hamiltonian of the interaction of a polar molecule with a polar medium is 39,52 H s = −λDD (4) where the dissymmetry parameter, D,…”