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 ...

Photosynthetic antennae and organic electronic materials use topological, structural, and molecular control of delocalized excitons to enhance and direct energy transfer. Interactions between the transition dipoles of individual chromophore units allow for coherent delocalization across multiple molecular sites. This delocalization, for specific geometries, greatly enhances the transition dipole moment of the lowest energy excitonic state relative to the chromophore and increases its radiative rate, a phenomenon known as superradiance. In this study, we show that ordered, self-assembled light-harvesting nanotubes (LHNs) display excitation-induced photobrightening and photodarkening. These changes in quantum yield arise due to changes in energetic disorder, which in turn increases/decreases excitonic superradiance. Through a combination of experiment and modeling, we show that intense illumination induces different types of chemical change in LHNs that reproducibly alter absorption and fluorescence properties, indicating control over excitonic delocalization. We also show that changes in spectral width and shift can be sensitive measures of system dimensionality, illustrating the mixed 1-2D nature of LHN excitons. Our results demonstrate a path forward for mastery of energetic disorder in an excitonic antenna, with implications for fundamental studies of coherent energy transport.

Enhancing photoluminescent emission (PL) in the near-infrared-infrared (NIR-IR) spectral region has broad applications from solar energy conversion to biological imaging. We show that self-assembled molecular dye J-aggregates (light-harvesting nanotubes, LHNs) can increase the PL emission of NIR PbS quantum dots (QDs) in both liquid and solid media more than 8-fold, promoted primarily by a long-range antenna effect and efficient Förster resonance energy transfer (FRET) from donor to acceptor. To create this composite material and preserve the optical properties of the nanocrystals, we performed an in situ ligand substitution followed by a functionalization reaction using click-chemistry. This resulted in PbS QDs soluble in an aqueous environment compatible with the molecular J-aggregates (LHNs). Theoretical and experimental results demonstrate that long-range diffusive exciton transport in LHNs enables efficient energy transfer to low concentrations of QDs despite there being no direct binding between molecular donors and QD acceptors. This suggests a broad application space for mixed light harvesting and photophysically active nanocomposite materials based on self-assembling molecular aggregates.