Beyond the Stokes shift: Quantifying excited-state symmetry breaking effects in steady-state spectra of quadrupolar molecules

We present an approach for modeling and interpreting the optical spectra of donor–acceptor aggregates, demonstrated on A–π–D–π–A/D–π–A–π–D systems with identical electron-donor (D) and electron-acceptor (A) units. The key advance is a detailed description of electronic–vibrational transitions incorporating symmetry-breaking charge transfer. Analysis of the results reveals the excited-state structural changes in the molecule caused by symmetry breaking, which manifest themselves in increasing the Huang–Rhys factors by up to a factor of 2.9. This unexpected outcome challenges the generally accepted view that normal vibrations are not significantly affected by charge transfer. Comparison of the simulated and experimental steady-state spectra demonstrates the high accuracy of the method. The model parameters are estimated using an algorithm that imitates swarm intelligence. The approach is based on a theory that addresses the interaction of the electronic subsystem of the molecule with both the intramolecular degrees of freedom and solvent polarization, which leads to an adiabatic excited-state free energy curve. The adiabatic effects on the steady-state spectra are discussed.