Wavefunction-based simulations of 2D electronic spectroscopy of conjugated polymers: Signatures of exciton transport and coherent vibronic dynamics at finite temperature

Abstract

Spectroscopic signatures of exciton transport and vibronic coupling in two-dimensional electronic spectroscopy (2DES) are studied for an oligothiophene chain as a minimal model for intra-chain exciton migration in poly-(3-hexylthiophene). Generalizing our previous approach [Brey et al., Faraday Discuss. 237, 148 (2022)], a first-principles parameterized Frenkel Hamiltonian is combined with a collective high-frequency lattice mode and a set of ring-torsional modes whose thermal fluctuations drive exciton migration. A wavefunction-based quantum–classical treatment is employed, where quantum Langevin friction via the Kostin equation acts on the collective lattice mode, while the torsional modes evolve under a classical Langevin equation at finite temperature. Single wavefunction realizations exhibit largely adiabatic, diffusive exciton motion across the 20-site lattice under periodic boundary conditions. 2DES spectra are computed using the equation-of-motion phase matching approach (EOM-PMA) within a wavefunction setting. In line with experimental observations, a pronounced vibronic fine structure is observed, which is modulated by spectral diffusion due to fluctuation-induced changes in the exciton extension and localization.