Localized and Delocalized Electronic Excitations in Biological and Artificial Antenna Complexes

Abstract

The Collective Electronic Oscillators (CEO) approach based on the Time-Dependent Hartree-Fock approximation is applied to analyze localization and delocalization of electronic excitations, charge and energy transfer in LH2 aggregates (biological antenna complexes) and in phenylacetylene dendrimers (artificial antenna macromolecules). Two-dimensional analysis of transition density matrices indicates that electron-hole pairs created upon optical excitation in LH2 aggregate are localized within separate chromophores. In addition LH2 has a band of low-lying charge-transfer states unaccessible with linear absorption spectroscopy. In phenylacetylene dendrimers, electronic excitation are shown to be localized within linear segments connected by benzene rings substituted at the meta-position. These results are used to construct effective Frenkel exciton hamiltonians in these biological and artificial light-harvesting antenna complexes.

Density Matrix Analysis and Simulation of Electronic Excitations in Conjugated and Aggregated Molecules

“…Optical properties of chlorophylls and porphyrins have drawn considerable attention − because of their fundamental and biological relevance. We applied the CEO/INDO/S approach to study the electronic excitations of bacteriochlorophyll- a (Bchl- a ) ,, from LH2 complex of photosynthetic bacteria Rhodospirillum molischianum (βB850). , The calculated properties of individual Bchl- a , both isolated and in a dielectric medium, are summarized in Table . This table reveals the significant impact of solvation on the linear absorption spectrum, reflecting the need for properly incorporating the dielectric environment in accurate computations …”

“…The CEO/INDO/S approach was applied toward the study of the electronic excitations of Bchl- a and carotenoid aggregates of LH2 complex of Rs. molischianum . ,, The electronic spectra of carotenoids and Bchl were analyzed in Sections IIIB and IIIE, respectively. Below we investigate the relevant electronic modes of the Bchl- a dimers, study the effects of aggregation in the LH2 complex, and compute interchromophore couplings.…”

“…Two techniques have been employed for calculating the electronic couplings: a spectroscopic procedure that is based on computing the Davydov's splitting in the dimer spectrum, , and a Coulombic method that computes the electrostatic interactions between transition densities of individual pigments. , Since the former takes both Coulomb and exchange interactions into account whereas the latter only incorporates the Coulomb interactions, comparison of the results allows the separation electrostatic (Förster) and electron exchange (Dexter) contributions to interchromophore couplings. Examination of the computed couplings 93,94,327 shows that it is essential to take into account dielectric medium effects in order to reproduce experimental transition dipoles of thechromophores (see Table ) and intermolecular couplings.…”

“…Optical properties of chlorophylls and porphyrins have drawn considerable attention [315][316][317][318][319][320][321][322][323][324][325][326] because of their fundamental and biological relevance. We applied the CEO/INDO/S approach to study the electronic excitations of bacteriochlorophyll-a (Bchla) 93,94,327 from LH2 complex of photosynthetic bacteria Rhodospirillum molischianum (βB850). 328,318 The calculated properties of individual Bchl-a, both isolated and in a dielectric medium, are summarized in Table 1.…”

“…molischianum. 93,94,327 The electronic spectra of carotenoids and Bchl were analyzed in Sections IIIB and IIIE, respectively. Below we investigate the relevant electronic modes of the Bchl-a dimers, study the effects of aggregation in the LH2 complex, and compute interchromophore couplings.…”