Theory of linear absorption spectra of biological and non-biological chromophore complexes

Abstract

Linear frequency-domain absorption spectra of chromophore complexes are studied within a Frenkel-exciton model including static and dynamic disorder. While static disorder is accounted for by explicite numerical ensemble averaging, dynamic disorder is described via the coupling of excitons to low-frequency vibrational modes (solvent or protein modes) and to high-frequency (intra-molecular) modes. Using a time-dependent formulation of the absorption density matrix theory can be applied. It is shown that the non-Markovian version of the Quantum Master Equation offers a certain approximation for the absorption based on a partial summation with respect to the exciton–vibrational coupling. In a first application the approach is used to describe absorption spectra of phenylacetylene dendrimers where static disorder is introduced to account for solvent induced deviations from the ideal structure. A detailed analysis of the long-wavelength tail underlines the dominance of structure fluctuations in the outer part of the dendrimer. While a concrete classification of the low-frequency modes remains impossible (according to the presence of static disorder) a qualitative estimate of the high-frequency modes becomes feasible. In a second application linear absorption of the photosynthetic antenna complex LHC-II of higher plants is analyzed and different structural parameters (Huang Rhys factor, inhomogeneous width of site energies, exciton state life times) are deduced. In particular the measured temperature dependence of the absorption spectrum can be well reproduced. A simultaneous fit of the circular dichroism spectrum at 77 K is used to discriminate between two different structural models.