Vibronic effects and destruction of exciton coherence in optical spectra of J-aggregates: A variational polaron transformation approach

Bloemsma, E. A., Silvis, M. H., Stradomska, A., Knoester, J.

Chemical Physics 481, 250–261 (2016). DOI: 10.1016/j.chemphys.2016.06.018.

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Using a symmetry adapted polaron transformation of the Holstein Hamiltonian, we study the interplay of electronic excitation-vibration couplings, resonance excitation transfer interactions, and temperature in the linear absorption spectra of molecular J-aggregates. Semi-analytical expressions for the spectra are derived and compared with results obtained from direct numerical diagonalization of the Hamiltonian in the two-particle basis set representation. At zero temperature, we show that our polaron transformation reproduces both the collective (exciton) and single-molecule (vibrational) optical response associated with the appropriate standard perturbation limits. Specifically, for the molecular dimer excellent agreement with the spectra from the two-particle approach for the entire range of model parameters is obtained. This is in marked contrast to commonly used polaron transformations. Upon increasing the temperature, the spectra show a transition from the collective to the individual molecular features, which results from the thermal destruction of the exciton coherence.