Single molecule spectroscopy of disordered circular aggregates: A perturbation analysis

Abstract

Analytical expressions for various spectroscopic observables are derived for statically disordered circular aggregates using first order perturbation theory. Quantities relevant to single molecule spectroscopy experiments on circular aggregates such as the distribution of energy splittings between levels that are degenerate in the absence of disorder, and the distribution of intensity of levels that are forbidden in the absence of disorder are derived. For simplicity only diagonal disorder is considered here. The approach, however, can be extended to include other types of disorder. Expressions are written in terms of standard deviations of Gaussian disorder, the symmetry number of the aggregate, and in some cases intermolecular couplings. The approach is compared to numerical simulations and shows excellent agreement over a large range of parameters. The expressions, however, provide a more direct framework to examine and understand the effects of disorder on spectroscopy of aggregates than do the simulations. The potential utility of the expressions for quantifying disorder and intermolecular couplings from single molecule experiments is also demonstrated.

Spectral Dynamics of Individual Bacterial Light-Harvesting Complexes: Alternative Disorder Model

“…The model of the B850 antenna ring needed to include the excitonic interaction between pigments, the static disorder of pigment site energies, and the coupling of the pigment electronic excitations to intra-and intermolecular vibrations/phonons, which would give rise to the so-called dynamic disorder. The FL spectral profile of the B850 was determined by the interplay of these two types of disorder (25)(26)(27). Typically, exciton models assume that structural fluctuations change the realization of the static disorder (a set of deviations of electronic transition energies of the pigments from the value in the ideally homogeneous ring), whereas parameters of the dynamic disorder remain unchanged.…”

Single complex line shapes of the B850 band of LH2

“…The light harvesting complex 2 ͑LH2͒ of purple bacteria [1][2][3][4] has been the subject of numerous experimental 2-24 and theoretical [2][3][4][25][26][27][28][29][30][31][32][33][34][35][36][37][38] studies since its first crystal structure determination. 1 LH2 consists of two bands, B800 and B850.…”

“…These theoretical understandings are important in view of recent experiments 14 -19 and for future progresses in the SCS of the B850 band. Important theoretical contributions 2-4, [25][26][27][28][29][30][31][32][33][34][35][36][37][38] have been made to various spectroscopic aspects of the B850 band, but no theoretical work exists addressing the details of the line shape in the context of SCS.…”

THEORY OF SINGLE-MOLECULE SPECTROSCOPY: Beyond the Ensemble Average

“…In a disordered condensed phase [e.g., low-temperature glasses, bacterial antena system LH2 (45,46), and defected crystal], each SM interacts with a unique nanoenvironment. Hence, the spectra of an individual SM have unique features [see e.g., (44,47,48)]. Therefore, in a disordered host, one must consider many SM whose features are investigated individually.…”