Long-Range Exciton Transport in Anthracene-Based Supramolecular Mesostructures and Its Control by Surface Plasmons

Abstract

Efficient singlet exciton transport is essential for optoelectronic applications, but in organic solids exciton diffusion is limited to tens of nanometers. We study exciton transport in supramolecular nanofibers self-assembled from 9,10-bis­(phenylethynyl)­anthracene (BPEA) derivatives end-capped with hydrophilic dendritic structures. Hydrogen bonds determine the nanofiber structure and induce J-aggregate character of the BPEA chromophores. Position-dependent fluorescence lifetime reveals exciton diffusion lengths up to 350 nm and diffusion coefficients up to 0.7 cm²/s, among the highest reported for organic solids. Quantum-chemical calculations combined with exciton diffusion simulations qualitatively reproduce the spectral properties and diffusion behavior. Structural rigidity, exciton delocalization over 2–3 monomers, and mixing of the locally excited and charge-transfer states are proposed as factors enabling the long-range transport. Additionally, plasmonic nanohole gold substrates enhance exciton transport by more than 2-fold, with a nanofiber orientation revealing the role of local electric field in the plasmonic enhancement of the Förster-type exciton transport.

Exciton transport in organic solids is a crucial process governing the performance of organic optoelectronic materials, such as charge generation in organic photovoltaics, emission in light-emitting devices, and energy migration in light-harvesting systems. Frenkel excitons in organic semiconductors are tightly bound due to low dielectric screening and, generally, diffusion lengths of singlet excitons in organic solids are limited to 5–20 nm over a wide range of materials because of energetic disorder, presence of defects or weak intermolecular electronic coupling. Low exciton diffusion lengths limit the performance, e.g., in organic photovoltaics as the excitons need to migrate toward the electron/hole extracting layers to generate photocurrent. , Even though effective mixing of donor and acceptor parts in a heterojunction can overcome the low exciton diffusion length, there are challenges to consistently extract carriers in a stable device structure. − Therefore, understanding, manipulating and improving exciton diffusion in organic solids are essential steps for enhancing their performance in optoelectronic devices.

Experimental evidence emerging over the past 10–15 years shows that in well-ordered supramolecular assemblies excitons can migrate over distances of hundreds of nanometers or more, in contrast to the existing assumptions about exciton localization in molecular materials. Highly ordered chromophore systems, such as crystalline frameworks, nanotubes, conjugated polymers, and supramolecular nanofibers have shown that precise molecular packing can result in strong electronic coupling, anisotropic transport, and exciton diffusion lengths far exceeding those in amorphous materials. − In the crystalline chromophore lattices of diphenylanthracene, long-range (∼100 nm) directional transfer of excimer-like excitations results from the high anisotropy of the crystalline structure. Supramolecular polymers of phenanthrene doped with pyrene acceptors show energy transfer along the polymer over distances >100 nm, suggesting a contribution of a quantum coherent process. Long-range transport over hundreds of nanometers has also been observed in polyfluorene-based conjugated polymer nanofibers with highly crystalline π–π-stacked donor core and acceptor terminal blocks, and explained due to high structural order and reduced dynamic disorder. , Bundles of J-aggregated cylindrical nanotubes of the C8S3 carbocyanine dye have shown exciton diffusion on the order of few hundred nanometers. Later, stabilization of the same nanotubes in a glass matrix lead to 1.6 μm exciton transport due to low static and dynamic energetic disorder. On the other hand, the bundling strategy is not universal as shown by an example of bundles of nanofibers with disordered energy landscape leading to localized excitons compared to single nanofibers. Conjugated polymer poly­(indacenodithiophene-co-benzothiadiazole) (IDTBT) films with both H- and J-type interactions show efficient interchain exciton diffusion with reduced exciton–exciton annihilation, but with diffusion length (∼75 nm) shorter compared to single polymer chains. Other than J-aggregate structures, long-range 1D exciton transport on the order of several microns has been observed in supramolecular H-aggregated nanofibers self-assembled of carbonyl-bridged triarylamines.

The variety of materials and molecular interactions supporting the long-range exciton transport make it challenging to provide unified theoretical picture of the phenomena. While Förster-type incoherent hopping dominates energy transport in disordered systems, transport in supramolecular materials can involve a regime where excitons transiently delocalize over multiple chromophores, ,, accelerating migration by enhanced effective coupling over lengths an order of magnitude larger than predicted by classical hopping models. , Further, spatial correlations in energy transitions among the monomeric units in supramolecular fibers can preserve extended exciton wave functions and facilitate long-range migration. , Additionally, hybridization of singlet exciton with charge-transfer (CT) states has been identified as a potential mechanism for enhancing exciton diffusion in donor–acceptor systems. , Thus, we are far from having a universal design principle that would lead to long-range exciton transport, and it is important to understand the intrinsic molecular factors governing exciton migration and explore strategies for its enhancement in organic systems.

Here, we use position-dependent microscopic fluorescence lifetime measurements to visualize and study exciton transport along bis­(phenylethynyl)­anthracene (BPEA) cores of supramolecular nanofibers. We show that the excitons can migrate in the J-aggregated structures to lengths of up to 350 nm with a diffusion coefficient of up to 0.7 cm²/s, and that the exciton diffusion can be significantly enhanced by a plasmonic substrate composed of regular array of nanoholes in a gold film.

To realize precise arrangement of the BPEA chromophores in the supramolecular nanofibers, two amide groups were introduced at both ends of the chromophore. 1D hydrogen bonding formed by the amides locks the chromophores into a well-defined stacking geometry and suppresses the formation of trap sites. Dendritic hydrophilic side chains were introduced to ensure sufficient solubility while preserving the amide-directed arrangement of the chromophores. The molecular structure is shown in Figure a. The nanofibers are fabricated via self-assembly as described in the Materials and Methods section, resulting in well-ordered fiber-like structures. The atomic force microscopy (AFM) image (Figure c) shows a typical network morphology comprising interconnected thin nanofibers and fiber bundles, with diameters varying between 100 and 400 nm. Adjustment of the concentration enables the preparation of sufficiently isolated nanofibers for the optical experiments (Figure d). The absorption and photoluminescence (PL) spectra of these nanofibers (Figure e) exhibit a moderate Stokes shift with both bands red-shifted relative to the corresponding monomer spectra. The emission spectrum of the nanofibers displays two major peaks and a weaker shoulder, corresponding to the 0–0, 0–1, and 0–2 vibrational transitions at 502, 535, and 565 nm, respectively. The monomer 0–0, 0–1, and 0–2 peaks are located at 476, 505, and 530 nm, respectively. The observed 26 nm red shift is consistent with the predominantly J-aggregated nature of the fluorophores within the supramolecular structure.

1.

Characterization of the supramolecular nanofibers. (a) Chemical structure of the monomer and (b) schematic of the self-assembled nanofiber formed by hydrogen bonds between the amide groups of individual monomers. (c) AFM image of the supramolecular nanofibers cast on a coverslip. (d) Simultaneous AFM and PL images of an area with isolated nanofibers suitable for PL measurements. (e) Absorption (green) and emission (red) spectra of the nanofibers measured under a microscope. The absorption spectrum is taken from a single nanofiber. The blue and black lines are the spectra of monomer solutions in a 1:1 (v/v) mixture of dichloromethane and methanol.

To evaluate the exciton diffusion, the supramolecular assemblies formed in a solution were drop-cast on a microscope coverslip. The exciton diffusion was measured using two complementary microscopic techniques: (1) static measurement of the PL intensity profile under focused laser excitation and (2) position-dependent PL lifetime measurement. The techniques yield exciton transport length L _T and exciton diffusion length L _D, both defined below. Mainly L _T will be used to discuss the results. The static measurement was performed by focusing a 457 nm continuous-wave laser into a near diffraction-limited spot on individual nanofibers and imaging the resulting PL intensity profile using an array detector (EM-CCD camera). The excitation spot appears nearly circular, whereas the emission profiles are slightly elongated along the nanofiber axis (Figure a), indicating the presence of exciton diffusion. The extent of diffusion was quantified by fitting the excitation and emission cross-sectional intensity profiles along the nanofiber long axis with Gaussian functions. The exciton transport length (L _T) was calculated using the relationship $L_{\mathrm{T}}=\sqrt{{{\sigma }_{\mathrm{em}}}^2-{{\sigma }_{\mathrm{exc}}}^2}$ , where σ_em and σ_exc represent the standard deviations of the PL and excitation laser profiles, respectively.

2.

Exciton diffusion in the nanofibers. (a) Microscopic images of a confocal laser spot (top) and of PL from a nanofiber upon the confocal excitation (bottom). The scale bar corresponds to 1 μm. (b) Corresponding excitation and emission cross sections of the laser (red) and the nanofiber PL (black), fitted with Gaussian functions to extract the transport length of 294 nm. (c) Distribution (violin plot) of the exciton transport lengths. (d) PL images upon wide-field (top) and confocal (bottom) illumination of a supramolecular nanofiber. (e) PL decay curve at the confocal excitation location (red), together with its multiexponential fit (blue) and instrument response function (gray). (f) 2D map of normalized PL intensity decay plotted as a function of the position on the nanofiber. (g) MSD constructed by fitting cross sections of the 2D map at different times. (h) Distributions of the diffusion coefficient D (left) and transport lengths L _T (right) obtained from the MSD analysis.

An example of the excitation and PL cross sections (Figure b) shows reasonably good Gaussian fits, yielding a transport length of 294 nm. The L _T values obtained from 20 different nanofiber locations are distributed (Figure c) between 50 and 300 nm, with an average value of 175 nm.

Because the static PL method is prone to artifacts and its results can be affected by the phenomenon of waveguiding, we further investigated the exciton dynamics using the position-dependent PL lifetime measurements. The nanofibers were confocally excited with a focused picosecond pulsed laser (485 nm) with the energy of 0.9 nJ/pulse. The emission magnification was optimized to achieve effective coverage of the avalanche photodiode (APD) detection area below the diffraction limit. The APD mounted on a piezostage was scanned across the emission profile along the nanofiber long axis to record the PL lifetime at increasing distances from the excitation position. To enable the position-dependent PL measurements, nanofibers aligned horizontally in the image plane were selected, as shown in the example in Figure d. The PL decay within the first 30 ns at the excitation position (Figure e) exhibited multiexponential behavior with an average lifetime of ∼1.1 ns. The PL lifetime was measured along the emission profile with a step size of ∼70 nm (corresponding to a 30 μm APD step) and plotted as a function of the position in the 2D map shown in Figure f. The normalized PL intensity shows progressive spatial broadening with increasing time after the laser excitation (Figure f), consistent with excitons migrating away from the excitation site prior to radiative recombination.

By fitting the PL cross sections at different times with Gaussian functions, the mean-square displacement (MSD) is determined using MSD = σ­(t)² – σ(0)² = 2Dt ^α, where σ­(t) represents the standard deviation of the cross section at time t, D is the exciton diffusion coefficient, and α is a nonlinearity parameter describing deviations from free diffusion (α = 1 corresponding to free diffusion). A linear fit to the initial region of the MSD curve (Figure g) yielded a diffusion coefficient of 0.22 cm²/s. The exciton transport length L _T, visually estimated from saturation of the MSD curve, was 260 nm. To exclude the effect of exciton–exciton annihilation on the observed data, we carried out reference experiments with an order of magnitude lower laser energy of 0.08 nJ/pulse (Figure S1). There is no significant difference or trend in the transport length L _T or diffusion coefficient D between the two fluences, indicating negligible contribution of exciton–exciton annihilation to the exciton transport.

Further, the diffusion coefficient is related to exciton diffusion length L _D as $L_{\mathrm{D}}=\sqrt{{\tau }_{\mathrm{av}}D}$ , where τ_av represents an average PL lifetime. Using this relationship and the lifetime of 1.1 ns gives a L _D of 156 nm. Measurements on a statistical ensemble of 35 locations on different nanofibers show that there is a large distribution of D and L _T values (Figure h) and that D reaches up to 0.7 cm²/s (with average D ∼ 0.17 cm²/s) and L _T up to 350 nm (with average L _T ∼ 205 nm). The corresponding distribution of L _D is presented in Figure S2a. The broad distributions of the exciton diffusion parameters are likely related to local variations in the nanofiber structure, interaction with the substrate and presence of local defects. The local defect is likely a structural disorder in intermolecular distance or orientation (off-diagonal disorder) that causes change in the electronic coupling. Such defects are occasionally demonstrated in asymmetric PL broadening (Figure S3), where the presence of a defect causes the excitons to migrate to different distances on the two sides from the excitation spot.

The change of the slope or saturation of the MSD at longer times in the Figures g and S3c is not predicted by the MSD equation and is most likely caused by slower diffusion of triplet states. Triplets transfer their energy by less efficient Dexter mechanism; however, their longer lifetime allows them to continue migrating even after singlet excitons decay within a few nanoseconds. Such a crossover from singlets to triplets, enabled by reverse intersystem crossing (see the Supporting Information for a detailed explanation), manifests itself as the apparent change of the MSD. The different singlet and triplet diffusion coefficients result in the different slopes in the MSD plots. This interpretation is supported by the observation of delayed fluorescence at longer times in the PL decay (Figure S4 and Table S1) with long components of 49 and 190 ns. A similar crossover from singlet to triplet exciton diffusion has recently been reported in a donor–acceptor organic film. An alternative explanation of the saturation of the MSD would be a more abundant presence of defects that would cause the diffusion to slow down at longer times due to increased probability of trapping (subdiffusion). We are not excluding this possibility but consider it less likely because the 2D plots are symmetricif the saturation was due to the presence of defects, there would have to be defects on either side from the excitation spot, at the same distances. If the defects are random, this is unlikely. Also, random distribution of defects would preclude the observed minimum transport length of ∼100 nm.

The above values of exciton transport length L _T and diffusion coefficient D are among the highest reported for singlet excitons in organic solids (Table S2). One of the factors contributing to the efficient transport is likely the structurally well-ordered and robust nature of the nanofibers. Their structure is determined by the hydrogen bonds between the amide groups which fix the BPEA unites at relatively large intermolecular distances of 4.9 Å [determined by molecular dynamics (MD) calculations], while the surrounding dendrons protect the chromophores from the environment. The structure is not determined by π–π stacking as in conventional molecular aggregates but by the hydrogen bonds and the steric effect of the dendrons. The BPEA units are not fully cofacially stacked but adopt a slipped, offset arrangement, resulting in a ladder-type geometry and the J-type coupling. In contrast, covalent linking of the same BPEA molecules to the walls of nanopores in anodic aluminum oxide lead to intermolecular spacing of ∼2.8 Å and to a pronounced excimer character of the transitions. Such decreased spacing and random structural arrangement resulted in exciton diffusion lengths of only a few tens of nanometers. In analogy with other 1D exciton diffusion systems, we assume that the diffusion predominantly occurs via dipole–dipole Förster resonant energy transfer (FRET), where after the initial downhill energy relaxation of excitons, the population reaches a quasi-equilibrium state and subsequently can hop to a neighboring site with a similar density of states.

Another recently proposed factor of long-range exciton migration is the hybridization of singlet excitons with CT states. To examine this possibility and to assess the degree of CT mixing, we carried out excited-state calculations based on the fragment molecular orbital (FMO) method. In this approach, the excited state wave functions of the whole supramolecular aggregate are approximated as a superposition of local exciton (LE) states and CT states. The nanofiber was modeled by a 30 monomer (30-mer) section of the supramolecular aggregate, the structure of which was obtained from MD simulation in solution. The MD optimization calculation involved the whole monomer structure including the dendritic units and solvent molecules, with hydrogen bonding between the amide groups of the dendritic units being the primary factor determining the structure. For the FMO calculations, only the core BPEA units from the MD-optimized structure were used (Figure a). The calculated absorption spectrum of the 30-mer (Figure b) reproduces the experimental spectrum (overlaid in Figure b) well and exhibits a peak at 484 nm (2.559 eV), corresponding to a 27 nm red shift relative to the calculated monomer spectrum (Table S3, with a peak at 457 nm). A comparable 26 nm red shift is observed experimentally between the absorption of the nanofiber and the monomer solution (Figure ). Visualization of the excited states was performed on shorter 11-mer sections of the aggregates, and natural transition orbital pairs of the brightest state (Figure c) reveal delocalization of electron density over 2–3 monomer units. The calculated transition dipole moments (TDMs) of the total excited states decomposed into components perpendicular and parallel to the long axis of the 30-mer structure (Figure d,e) confirm the strong orbital coupling. The color code representing the contributions of LE and CT states to individual transitions shows varying CT character and an overall significant LE–CT mixing. These results were further used to theoretically reproduce the exciton diffusion along the nanofibers. This was achieved by propagating the time-dependent exciton wave function on the 30-mer and calculating the MSD of the exciton as

$$\mathrm{MSD}(t)=\sum _{\mathit{I}}{(R_{\mathit{I}}-R_{\mathrm{init}})}^2{p}_{\mathit{I}}(t)$$

where p _I (t) is the exciton population on the Ith molecule, R _I is the center of mass of the Ith molecule, and R _init is the center of mass of the molecule where the exciton was created at t = 0.

3.

Quantum-chemical calculations. (a) Structure of a 30 monomer (30-mer) of the BPEA core determined by molecular dynamics and used for the FMO calculations. (b) Calculated absorption spectrum of the 30-mer (blue) and experimental spectrum measured on a single nanofiber (red). (c) Dominant natural transition orbital pairs of three brightest states, calculated on a 11-mer subunit. TDM components perpendicular (d) and parallel (e) with the 30-mer long axis. The color code represents the degree of mixing of LE and CT states, with green corresponding to 100% CT and red to 100% LE character. (f) Time-dependent MSD calculated by propagating the time-dependent exciton wave function on the 30-mer.

The time-dependent MSD shown in Figure f shows initially a linear increase, as expected for normal incoherent exciton diffusion, but saturates quickly due to the finite size of the 30-mer section, confirming fast exciton transport. Using the initial linear region of the MSD curve, we calculated the diffusion coefficient D of 0.38 cm²/s. This value is in qualitative agreement with the experimental results, lying within the measured range of D and close to the mean value of 0.17 cm²/s. We realize that the limited size and the resulting limited time scale (1 ps) of the simulation makes a straightforward extrapolation of the diffusion coefficient to the nanosecond range difficult and that the obtained value is mostly indicative. Generally, when excitons encounter defects in the real system the diffusion slows down at later times. Still, the calculated diffusion coefficient (in the first 1 ps) and the experimental one on nanoseconds are on the same order, indicating that the exciton transport is mostly defect-free.

The spatially extended transition molecular orbitals indicate the involvement of CT states and suggest a partially coherent character of the exciton transport. The similar energy levels of LE and CT states (Figure S5) in FMO analysis further support a defect free arrangement of monomers. We observed extended exciton diffusion in nanofibers of various thickness, indicating formation of bundles of nanofibers. Previously, nanofiber bundles were shown to suppress long-range exciton transport due to disordered excited-state energy landscape. Our observation of long-range exciton migration even in apparently thick bundles of nanofibers suggest effective protection provided by dendritic groups to the chromophoric environment.

Surface plasmons generated by the interaction of light with noble metal nanostructures are known to enhance the efficiency of FRET process between donor and acceptor molecules or nanoparticles. − Since FRET is one of the primary mechanisms governing the exciton diffusion in supramolecular nanostructures, it can be expected that such diffusion would be enhanced through coupling with surface plasmons. To examine this possibility, we employed plasmonic substrates composed of arrays of nanoholes (with 250 nm diameter and 40 nm depth) fabricated in a 100 nm gold film and arranged in a square lattice with a periodicity of 500 nm. The periodic architecture of the nanohole array ensures a spatially uniform plasmonic effect on the supramolecular nanofibers deposited on the substrates. Details regarding the substrate preparation and characterization have been reported elsewhere.

The plasmonic effect on the exciton diffusion was examined by depositing the nanofibers on the gold nanohole array substrate (Figure a). The morphology of the nanofibers on the plasmonic substrate is similar to that on the glass (Figure S6). Prior to the nanofiber deposition, the substrate was coated with a 12 nm layer of poly­(vinyl alcohol) (PVA) to prevent PL quenching by the metallic surface. We have not observed any difference in photostability between nanofibers on glass and on the plasmonic substrate, indicating negligible effect of local plasmonic heating. An example of the static PL intensity profile measurement (Figure b,c) does demonstrate more pronounced elongation of the emission pattern, corresponding to an exciton transport length L _T of 734 nm. The maximum enhancement of exciton diffusion is achieved when the nanofibers are aligned parallel to the nanohole array. The calculated transport lengths for 67 locations on different nanofibers, oriented at various angles relative to the nanohole lattice direction (as defined in Figure a), reveal a distinct angular dependence of the diffusion enhancement.

4.

Plasmonic enhancement of exciton diffusion. (a) AFM image of the plasmonic substrate with gold nanohole array (top) and definition of the angle θ that the nanofiber takes with respect to the array horizontal direction (bottom). (b) Microscopic images of a confocal laser spot (left) and of PL from a nanofiber upon the confocal excitation (right). The scale bar corresponds to 1 μm. (c) Corresponding excitation and emission cross sections of the laser (red) and the nanofiber PL (black), fitted with Gaussian functions to extract the transport length of 734 nm. (d) Angle-dependent exciton transport length L _T of nanofibers deposited on gold nanohole arrays.

Although a certain degree of heterogeneity in the diffusion behavior (spread of the L _T values) is observed across different orientations, the overall angular trend is clearly evident in Figure a. The heterogeneity likely arises from variations in local nanofiber quality and uncertainty in the estimation of the excitation laser cross sections. The exciton transport length reaches a maximum of approximately 1100 nm when θ is 0°, 90°, or 180°, corresponding to perfect alignment of nanofibers with the nanohole lattice direction. In contrast, the transport length decreases to a minimum of approximately 400 nm when θ is 45° or 135°. These transport lengths are significantly larger than those for nanofibers deposited on the glass substrates (Figure ), demonstrating clear plasmonic enhancement of the exciton diffusion.

The broader emission profiles for nanofibers aligned with the gold nanohole array can result either from excitation by plasmons propagating in the substrate, or from enhancement of the exciton diffusion process itself. To discern the two possibilities, we employed the position-dependent, spatially resolved PL lifetime technique. The plasmon propagation is a very fast process, and therefore it would not be resolved with the sub-ns resolution of the setup. Representative PL images (Figure a) and the corresponding 2D map of the PL intensity decay versus position (Figure b) reveal a significant broadening of the PL spatial profile in time. The linear region of the MSD plot (Figure c) elucidates this effect, yielding a diffusion coefficient D of 0.50 cm²/s. The observed broadening occurs on a nanosecond time scale, indicating the intrinsic exciton diffusion process enhanced by the plasmonic contribution. The exciton transport length L _T for this example was estimated at ∼410 nm, substantially longer than the values measured for nanofibers on glass substrates. Statistical analysis of D and L _T values obtained from 32 locations across multiple nanofibers (Figure d) yield D reaching up to 1.3 cm²/s (with average D ∼ 0.43 cm²/s) and L _T up to 550 nm (with average L _T ∼ 335 nm). Distribution of L _D is presented in Figure S2b. These results reveal more than a 2-fold enhancement in exciton diffusivity on the gold nanohole substrate compared to the glass. The distributions of D and L _T values are broad, both on glass (Figure ) and on the nanohole array. While on glass the variations in the exciton diffusion parameters are related to local nanofiber structure, the additional broadening of both the D and L _T distributions on the plasmonic substrate originates from variations in the position of the nanofiber relative to the nanoholes, with the lower D and L _T values likely corresponding to nanofibers positioned and excited between the nanoholes.

5.

Time-resolved monitoring of the plasmonic enhancement. (a) PL images upon wide-field (top) and confocal (bottom) illumination of a supramolecular nanofiber deposited on the plasmonic substrate. (b) 2D map of normalized PL intensity decay plotted as a function of the position on the nanofiber. (c) MSD constructed by fitting cross sections of the 2D map at different times. (d) Distributions of the diffusion coefficient D (left) and transport lengths L _T (right) obtained from the MSD analysis. (e) Calculated electric field enhancement around one nanohole upon 502 nm light incident normal to the nanohole substrate, for horizontal polarization along the nanohole array (top) and for polarization tilted by 45° (bottom).

The enhancement of exciton diffusion can be attributed to plasmonic enhancement of the FRET process. Such enhancement arises from the interaction of both the absorption and emission TDMs of the acceptor and donor, respectively, with the local density of states (LDOS) of the plasmonic structure. The plasmonic enhancement factor of FRET is proportional to the product of the LDOS at the donor and acceptor positions, and as such is determined by local field distribution. To visualize this distribution, we performed finite difference time-domain simulations of the nanohole array at 502 nm. Figure e shows the electric field distribution around a single nanohole and its surrounding region for light polarized along the nanohole array axis and at 45° relative to it. Along the nanohole array direction, the field is enhanced continuously toward neighboring holes, whereas at 45° this continuity is disrupted. This difference accounts for the angular dependence of the exciton diffusion enhancement observed in Figure d. Nanofibers aligned parallel to the nanoholes experience, through the parallel components of TDMs (Figure e), the effect of continuous field (LDOS) that causes the long-distance exciton diffusion enhancement beyond the size of a single nanohole (Figure S7a). In contrast, at 45°, the enhancement is constrained to the vicinity of a nanohole (Figure S7b), and the resulting transport length is comparable, or only slightly larger than, that measured on glass substrates.

In summary, we demonstrated efficient long-range exciton transport in supramolecular BPEA-based nanofibers and revealed a pronounced plasmonic enhancement of exciton transport on gold nanohole array substrates. Static and time-resolved PL measurements consistently show exciton transport lengths reaching hundreds of nanometers on a glass substrate. Excited-state calculations based on the FMO method helped to identify strong mixing between LE and CT states, as well as delocalization of the electron density over several monomer units as the factors contributing to the long-range exciton transport. 2-fold enhancement of the exciton diffusivity under optimal plasmonic coupling conditions is explained by plasmon enhancement of the FRET process. These results highlight the combined importance of molecular-scale electronic structure and photonic environment in governing exciton transport and demonstrate that plasmonic engineering provides a viable strategy for substantially enhancing exciton diffusion in organic supramolecular systems. Further, the observation of asymmetric diffusion points to the possibility of actively controlling the exciton transport by external stimuli, such as localized mechanical or optical modification, electric field, etc., paving the way for the development of excitonic electronic components.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.6c02411.

• Materials preparation and data measurement methods, reverse intersystem crossing, FMO calculations for the model nanofiber structures, simulation of exciton diffusion for 30-mer, supporting Figures S1–S7, and supporting Tables S1–S3 (PDF)

The authors declare no competing financial interest.