Optical spectroscopy and system-bath interactions in molecular aggregates with full configuration interaction Frenkel exciton model

- Standard Frenkel exciton model is extended to include inter-band coupling.
- It is formally linked with configuration interaction method of quantum chemistry.
- Spectral shifts due to inter-band coupling are found in molecular aggregates.
- Effects of peak amplitude redistribution in two-dimensional spectra are found.

Standard application of the Frenkel exciton model neglects resonance coupling between collective molecular aggregate states with different number of excitations. These inter-band coupling terms are, however, of the same magnitude as the intra-band coupling between singly excited states. We systematically derive the Frenkel exciton model from quantum chemical considerations, and identify it as a variant of the configuration interaction method. We discuss all non-negligible couplings between collective aggregate states, and provide compact formulae for their calculation. We calculate absorption spectra of molecular aggregate of carotenoids and identify significant band shifts as a result of inter-band coupling. The presence of inter-band coupling terms requires renormalization of the system-bath coupling with respect to standard formulation, but renormalization effects are found to be weak. We present detailed discussion of molecular dimer and calculate its time-resolved two-dimensional Fourier transformed spectra to find weak but noticeable effects of peak amplitude redistribution due to inter-band coupling.

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