Theory of Anisotropic Circular Dichroism of Excitonically Coupled Systems
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We present a simple exciton theory for the description of anisotropic circular dichroism (ACD) spectra. The latter is the difference in absorption between left- and right circularly polarized light of an oriented pigment-protein complex. The theory is applied to the baseplate of green sulfur bacteria, which consist of arrays of bacteriochlorophyll (BChl) a dimers with strong intra dimer and weak inter dimer couplings. It is demonstrated that only the combined analysis of calculated ACD and CD spectra for the present baseplate BChl dimer allows for an unambiguous determination of the parameters of the exciton Hamiltonian from experimental data. The analysis of experimental absorption (OD) and linear dichroism (LD) spectra further suggests that either the baseplate structure which is obtained from NMR has to be refined, or in addition to the dimers seen in the NMR structure and in the CD and ACD spectra, BChl a monomers are present in the baseplate sample. A refined dimer structure is presented which explains all four (ACD, CD, OD, LD) optical spectra.