Excitons and Disorder in Molecular Nanotubes: A 2D Electronic Spectroscopy Study and First Comparison to a Microscopic Model

Abstract

The efficiency of natural light-harvesting complexes relies on delocalization and directed transfer of excitation energy on spatially well-defined arrangements of molecular absorbers. Coherent excitation delocalization and long-range molecular order are also central prerequisites for engineering energy flows in bioinspired devices. Double-wall cylindrical aggregates have emerged as excellent candidates that meet these criteria. So far, the experimental signatures of exciton relaxation in these tubular supramolecules could not be linked to models encompassing their entire spatial structure. On the basis of the power of two-dimensional electronic spectroscopy, we characterize the motion of excitons in the three-fold band structure of the bitubular aggregate C8S3 through temporal, energetic, and spatial attributes. Accounting for intra-as well as interwall electronic interactions in the framework of a Frenkel exciton basis, we employ numerical computations using inhomogeneous and homogeneous microscopic models. The calculations on large but finite structures identify disorder-induced effects, which become increasingly relevant for higher energy states and give insight into the topology of the excited state manifold. Calculations in the infinite homogeneous limit capture the phenomena evidenced in the experimental two-dimensional patterns. Our results provide a basis for understanding recently reported correlated fluctuations of excitonic absorption bands and interband coherences in tubular aggregates.

Mixtures of achiral amphiphilic cyanine dyes form helical tubular J-aggregates

“…It is a challenging task to use this structural information to refine the crude and simple models used so far for a theoretical calculation of the excitonic band structure [27,61] The essential problem to be solved is to find a molecular packing model that is consistent with the boundary conditions imposed by the presented reconstructed volume and to use this model to explain the excitonic band structure. One can assume, that the chromophors are stacked within each layer plane-to-plane, whereas the octyl chains are intercalating within the $3.6 nm thick bilayer due to the hydrophobic effect.…”

“…These boundary conditions may help to resolve more detailed features of the optical spectra by comparison with theoretical calculations, but the situation is complicated by additional aspects. Structural investigations on the previously reported tubular C8S3 J-aggregates (in the absence of methanol) were the starting point in attempting a quantitative molecular interpretation of the nonlinear optical spectra [61]. For the modelling of the optical exciton dynamics the chromophores were arranged in a doublelayered cylindrical geometry, where the interlayer distance corresponds to the experimental value of $3.6 nm.…”

Vibronic effects and destruction of exciton coherence in optical spectra of J-aggregates: A variational polaron transformation approach

“…A well-known recent example concerns the coherence of excitons in lightharvesting antenna complexes, such as FMO, and its contribution to the excitation transport efficiency in these systems [42][43][44][45][46][47][48][49][50][51]. Another recent example of interest relates to the coherence between electronic excitations on inner and outer walls of double-walled cylindrical molecular aggregates [52][53][54][55][56][57].…”

Quantum Diffusion on Molecular Tubes: Universal Scaling of the 1D to 2D Transition

“…The parameters for J, σ, Γ, and R from top to bottom are deduced from Refs. [36], [39], [14], and [40], respectively. The other quantities are calculated using Eqs.…”