Optical Spectra and Localization of Excitons in Inhomogeneous Helical Cylindrical Aggregates

TL;DR

This paper investigates the optical spectra and exciton localization in inhomogeneous helical cylindrical aggregates, demonstrating improved spectral agreement with experiments by including disorder effects and analyzing anisotropic exciton localization.

Contribution

It introduces a combined numerical and analytical approach to account for static disorder in optical spectra and exciton localization in helical cylindrical aggregates.

Findings

Disorder improves agreement with experimental spectra.

Exciton localization is strongly anisotropic and follows chiral wave function properties.

Circular dichroism depends on cylinder length despite exciton localization.

Abstract

We study the linear optical properties of helical cylindrical molecular aggregates accounting for the effects of static diagonal disorder. Absorption, linear dichroism, and circular dichroism spectra are presented, calculated using brute force numerical simulations and a modified version of the coherent potential approximation that accounts for finite size effects. Excellent agreement between both approaches is found. It is also shown that the inclusion of disorder results in a better agreement between calculated and measured spectra for the chlorosomes of green bacteria as compared to our previous report, where we restricted ourselves to homogeneous cylinders [J. Phys. Chem. B 106, 11474 (2002)]. We also investigate the localization properties of the excitons responsible for the optical response. By analyzing an autocorrelation function of the exciton wave function, we find a strongly anisotropic localization behavior, closely following the properties of chiral wave functions which previously have been found for homogenoeus helical cylinders [J. Chem. Phys. 121, 946 (2004)]. It is shown that the circular dichroism spectrum may still show a strong dependence on the cylinder length, even when the exciton wave function is localized in a region small compared to the cylinder's size.