Supramolecular assemblies that interact with light have recently garnered much interest as well-defined nanoscale materials for electronic excitation energy collection and transport. However, to control such complex systems it is essential to understand how their various parts interact and whether these interactions result in coherently shared excited states (excitons) or in diffusive energy transport between them. Here, we address this by studying a model system consisting of two concentric cylindrical dye aggregates in a light-harvesting nanotube. Through selective chemistry we are able to unambiguously determine the supramolecular origin of the observed excitonic transitions. These results required the development of a new theoretical model of the supramolecular structure of the assembly. Our results demonstrate that the two cylinders of the nanotube have distinct spectral responses and are best described as two separate, weakly coupled excitonic systems. Understanding such interactions is critical to the control of energy transfer on a molecular scale, a goal in various applications ranging from artificial photosynthesis to molecular electronics.

Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid substrates Eisele, Doerthe M.; Knoester, Jasper; Kirstein, Stefan; Rabe, Juergen P.; Vanden Bout, David A.; Rabe, Jürgen P. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Since the optical properties of the tubular J-aggregates strongly depend on their specific supramolecular structure, the absorption and emission spectra from the sample can be used to determine if the molecular structure of the aggregates has changed upon deposition onto the substrate. Because the tubules on the substrate are highly dilute, emission spectra rather than the very weak absorption spectra were used. Emission spectra were collected rather than excitation spectra because of the extremely small Stokes shift for the emission. Emission spectra of tubular J-aggregates in solution (red) and after preparation on a quartz surface (black)via spin-coating and slowly drying in air (c) Idem, but now the black line is the spectrum of aggregates prepared on quartz via the drop flow technique and drying by blowing with nitrogen.As shown in the manuscript, the sample prepared by the drop-flow technique and carefully dried in air in a black box had spectra that were nearly identical in both position and width to the solution, indicating no significant morphological and structural changes upon deposition.3

ABSTRACT:We report 1.6 ± 1 μm exciton transport in self-assembled supramolecular light-harvesting nanotubes (LHNs) assembled from amphiphillic cyanine dyes. We stabilize LHNs in a sucrose glass matrix, greatly reducing light and oxidative damage and allowing the observation of exciton− exciton annihilation signatures under weak excitation flux. Fitting to a onedimensional diffusion model, we find an average exciton diffusion constant of 55 ± 20 cm 2 /s, among the highest measured for an organic system. We develop a simple model that uses cryogenic measurements of static and dynamic energetic disorder to estimate a diffusion constant of 32 cm 2 /s, in agreement with experiment. We ascribe large exciton diffusion lengths to low static and dynamic energetic disorder in LHNs. We argue that matrix-stabilized LHNS represent an excellent model system to study coherent excitonic transport. KEYWORDS: J-aggregate, molecular aggregate, exciton, exciton diffusion, coherent exciton, exciton delocalization E xcitons are bound electron−hole pairs generated upon absorption of a photon or through charge carrier injection. Photosynthetic organisms and organic electronics make use of ordered molecular aggregates as excitonic antennas, with energy transport out-competing radiative and nonradiative decay channels leading to near-unity internal quantum efficiencies. 1,2 Like electronic conduction, molecular exciton conduction falls largely in two regimes: hopping and delocalization. In the hopping regime, interaction with the environment (the reorganization energy) exceeds the dipole−dipole coupling (λ reorg > J), leading to Forster resonance dominated transport. In the delocalized regime, dipole−dipole coupling exceeds the reorganization energy leading to Redfield transport. 3,4 Efficient conduction of spin-singlet excitons requires a balance of these two regimes, with both coherent quantum delocalization and incoherent resonance energy transfer playing a role in natural and artificial light-harvesting systems. 3,5−7 However, extracting principles of design from disordered complex biological and polymer systems is a significant challenge. 8 This study probes singlet exciton transport in self-assembled light harvesting nanotubes (LHNs). LHNs are quasi one-dimensional Jaggregates consisting of ordered amphiphillic cyanine dyes that form extended transition dipoles with concentrated oscillator strength in a lower-energy, highly emissive state. 9 LHNs show remarkably high overall coupling, negligible reorganization energies, and high structural uniformity resulting in large delocalization lengths. LHNs are an excellent model material for exploring the relationship between quantum delocalization and energy transport in a system where λ reorg ≪ J (coherent regime). 10−12 However, spectroscopic studies of LHNs have been hampered by difficulties in sample preparation 13 and photoinstability. 14 As a result, studies of exciton transport in LHNs have yielded highly variable results, 15−17 with estimates of transport ranging from 30 to 300 nm ...