The effect of acceptor–donor–acceptor molecule bending on the symmetry breaking charge transfer and transition dipole moment

This study investigates the influence of molecular bending on excited-state symmetry-breaking charge transfer (SBCT) and transition dipole moments (TDMs) in donor-acceptor (A-D-A/D-A-D) systems with identical acceptors (or donors). The main conclusion is that SBCT, or exciton localization, is hindered in bent molecules. The proposed model introduces a key novelty: two reaction coordinates are required to describe SBCT in bent molecules, whereas linear systems can be adequately described by using a single coordinate. The model predicts that (i) bending reduces the degree of dissymmetry and induces nonmonotonic changes in the dipole moment, and (ii) the TDM of fluorescence exhibits monotonic increase or decrease with bending, contingent on the parity of the lower excited state. Crucially, the two-dimensional framework accounts for both the magnitude changes and rotational effects of the dipole moment and the TDM. The predictions of the model are aligned with available experimental data, validating its conclusions. Additionally, the article provides simple formulas for quantifying symmetry-breaking extent and TDM magnitudes (for absorption and fluorescence), offering practical utility for experimentalists in designing studies and interpreting results. These insights advance the design of optoelectronic materials, where controlled symmetry breaking is critical.