Robust excitons inhabit soft supramolecular nanotubes

Abstract

Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions-prerequisites for efficient energy transport-are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient lightharvesting devices even from soft, supramolecular materials. supramolecular assembly | self-assembled excitonic nanoscale systems | photosynthesis | exciton theory | light-harvesting antennae systems T he most remarkable materials that demonstrate the ability to capture solar energy are natural photosynthetic systems such as those found in primitive marine algae and bacteria (1-10). Their light-harvesting (LH) antennae are crucial components, because they absorb the light and direct the resulting excitation energy efficiently to a reaction center, which then converts these excitations (excitons) into charge-separated states (1,4,11,12). Although the noncovalent interactions that link the individual molecules within the LH antennae are weak, the excitation transfer interactions between the molecules are relatively strong; new excited states, so-called Frenkel excitons (13), are generated that are delocalized over a number of molecules (1). These delocalized excitons are key to nature's efficiency and are therefore of high interest (14)(15)(16)(17)(18)(19)(20).To create such efficient LH systems, nature assembles molecular subunits into individual supramolecular structures, which are then further assembled into close-packed superstructures (1, 4, 7-10, 12, 21). This hierarchical assembly is a generic motif of nature's photosynthetic systems. As with natural systems, assembling artificial LH devices from supramolecular structures will require close packing into hierarchical assemblies to maximize the amount of absorbed light (19). Therefore, key to our ability to tune materials properties for efficient LH applications is a basic understanding of the role of each level of the hierarchy: from the individual molecule, to the individual supramolecular building block, to the close-packed assembly. Whereas the role of the individual molecules in the excitonic properties of the building blocks is well-studied (1, 7-10, 20, 22-37), the effect of structural hierar...

Nanohybrids from nanotubular J-aggregates and transparent silica nanoshells

“…The diameters of the J-aggregate tube are similar as observed for the bare J-aggregates (13 nm outer diameter and 6.5 nm inner diameter). 2 Obviously, the sol-gel process does not cause significant changes to the size and morphology of the J-aggregate nanotubes as is further supported by optical spectroscopy. In Fig.…”

Macroscopic coherence as an emergent property in molecular nanotubes

“…States with a large dipole strength can also couple between themselves efficiently, inducing a super-transfer coupling between distant molecular aggregates [31] or different parts of the same aggregate as we show here. Delocalized single excitonic states over a large number of molecules are called macroscopic coherent states and they are studied both for applications and basic science [44][45][46][47][48][49][50].…”

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].…”