Temperature-Dependent Exciton Properties of Two Cylindrical J‑Aggregates

The temperature dependence of the steady-state excitonic absorption and emission spectral features of the J-aggregates of the amphiphilic cyanine dye 3,3′-bis­(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) was examined over the temperature range from 77 to 298 K. Two C8S3 J-aggregate structures were investigated: well-separated, double-walled nanotubes and bundles of agglomerated nanotubes that spontaneously assemble over long periods of storage. Absorption and emission spectral line broadening and Stokes shift are presented, and the responses of both aggregates are evaluated as a function of temperature. Both J-aggregates exhibit two fluorescence bands. We found that, across the measured temperature range, the ratio of the nanotube’s emission bands is well described with Boltzmann statistics, while that of the bundles is not. Additionally, the relative quantum yield of the nanotubes increased dramatically upon cooling, while the bundles’ quantum yield exhibited a significantly smaller increase over the same temperature rangean observation we attribute to the bundles’ greater absolute quantum yield

Quantifying the Polarization of Exciton Transitions in Double-Walled Nanotubular J‑Aggregates

A fully consistent model for the exciton band structure of double-walled 3,3′-bis­(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) J-aggregates was developed using reduced linear dichroism (LD^r) spectroscopy on flow aligned samples. Chemical oxidation was utilized to “turn off”outer wall optical absorption and produce stable aggregate samples with a simplified absorption profile associated only with the nanotube inner wall. The oxidized aggregates were aligned in a flow cell to collect LD^r spectra; these spectra reveal a series of both polarized and isotropic transitions. Four spectral transitions, assigned to be purely parallel or perpendicular to the aggregate long axis, that fit both the experimental LD^r and isotropic spectra were used create a model for oxidized J-aggregate excitonic absorption. The LD^r spectral study was repeated using pristine J-aggregates, and the spectrum for the full double-walled J-aggregates could be fit using six total transitions: four from the oxidized fit and two additional transitions distinct to the outer wall. A quantitative model that agrees with experimental absorption and emission spectral results and aligns with current theory was constructed wherein the energies and polarizations of excitonic transitions remained consistent for both the unperturbed and chemically oxidized C8S3 J-aggregates. The polarization studies also reveal, in contrast to the strongly polarized transitions that comprise the low-energy region of the excitonic aggregate spectrum, that the high-energy absorption is unpolarized and attributed to highly localized exciton transitions that arise due to disorder

Direct Measurement of Energy Migration in Supramolecular Carbocyanine Dye Nanotubes

Exciton transport lengths in double-walled and bundled cylindrical 3,3′-bis- (2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimida-carbocyanine (C8S3) J-aggregates were measured using direct imaging of fluorescence from individual aggregates deposited on solid substrates. Regions identified in confocal images were excited with a focused laser spot, and the resulting fluorescence emission was imaged onto an electron multiplying charged coupled device camera. A two-dimensional Gaussian fitting scheme was used to quantitatively compare the excitation beam profile to the broadened aggregate emission profiles. The double-walled tubes exhibit average exciton transport lengths of 140 nm, while exciton transport in the bundled nanotubes was found to be remarkably long, with distances reaching many hundreds of nanometers. A steady-state one-dimensional diffusion model for the broadening of the emission profiles yields diffusion coefficients of 120 nm² ps^–1 for the nanotubes and 7000 nm² ps^–1 for the aggregate bundles. The level of structural hierarchy dramatically affects the exciton transport capabilities in these artificial light-harvesting systems, and energy migration is not limited to a single dimension in J-aggregate bundles

Carbon Optically Transparent Electrodes for Electrogenerated Chemiluminescence

This study investigates pyrolyzed photoresist film (PPF)-based carbon optically transparent electrodes (C-OTEs) for use in electrogenerated chemiluminescence (ECL) studies. Oxidative–reductive ECL is obtained with a well-characterized ECL system, C8S3 J-aggregates with 2-(dibutylamino)­ethanol (DBAE) as coreactant. Simultaneous cyclic voltammograms (CVs) and ECL transients are obtained for three thicknesses of PPFs and compared to nontransparent glassy carbon (GC) and the conventional transparent electrode indium tin oxide (ITO) in both front face and transmission electrode cell geometries. Despite positive potential shifts in oxidation and ECL peaks, attributed to the internal resistance of the PPFs that result from their nanoscale thickness, the PPFs display similar ECL activity to GC, including the low oxidation potential (LOP) observed for amine coreactants on hydrophobic electrodes. Reductive–oxidative ECL was obtained using the well-studied ECL luminophore Ru­(bpy)₃²⁺, where the C-OTEs outperformed ITO because of electrochemical instability of ITO at very negative potentials. The C-OTEs are promising electrodes for ECL applications because they yield higher ECL than ITO in both oxidative–reductive and reductive–oxidative ECL modes, are more stable in alkaline solutions, display a wide potential window of stability, and have tunable transparency for more efficient detection of ECL

Spectroelectrochemical Investigation of an Electrogenerated Graphitic Oxide Solid–Electrolyte Interphase

This study investigates electrogenerated graphitic oxides (EGO) on the surface of carbon optically transparent electrodes (C-OTEs) using a combined UV–vis spectroelectrochemical approach. By monitoring the π–π* aromatic carbon transition for reduced GO (270 nm) and GO (230 nm), we observe the growth of GO in KCl upon applying oxidizing potentials. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) are used to confirm sample composition and location of salt ions within the electrode. Formation of EGO stable enough to be observed by UV–vis is found to be unique to alkali chloride supporting electrolytes due to formation of a solid–electrolyte interphase (SEI) which incorporates the alkali cation to stabilize the negatively charged oxygen functional groups while the presence of chloride anion acts as a passivation agent that protects the electrode surface from dissolution. The spectroelectrochemical approach highlights the detection and study of EGO that cannot be detected by electrochemical measurements. Specifically, the amount of EGO observed by UV–vis scales with increasing cation size (Li⁺, Na⁺, K⁺) despite all the cations showing identical electrochemical response

Revealing the Chemistry and Morphology of Buried Donor/Acceptor Interfaces in Organic Photovoltaics

With power conversion efficiencies (PCEs) of <13% and plagued by stability issues, organic photovoltaics (OPVs) still lack wide adoption, despite significant recent advances. Currently, the most progress in OPV device performance is achieved by “trial-and-error” preparation procedures that lead to complex and largely unknowndespite tremendous analytical effortsmorphologies. Here, we demonstrate a proof-of-principle, chemical imaging methodology that combines experimental high spatial sensitivity and chemical selectivity with theoretical modeling, capable of analyzing the three-dimensional composition and morphology of virtually any device. Allowing the precise measurement of composition and direct visualization of film morphology with depth, our approach reveals the intricate buried donor/acceptor (D/A) interface of a model polymer/fullerene system, poly­(3-hexylthiphene-2,5-diyl)/[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT/PCBM). In particular, our technique is able to identify and quantify the D/A interface length, that is, the extent of molecular mixing at the D/A interface, a parameter crucial for device performance, yet never measured. Extracting this parameter allows demonstrating that, contrary to the general understanding, when starting with a fully mixed D/A phase in our model system, thermal annealing, which is known to substantially (however limited) increase the device performance by phase segregation, does not create but small amounts of pure phases, leaving the device mostly mixed, which limits the performance improvement

Mimicking Conjugated Polymer Thin-Film Photophysics with a Well-Defined Triblock Copolymer in Solution

Conjugated polymers (CPs) are promising materials for use in electronic applications, such as low-cost, easily processed organic photovoltaic (OPV) devices. Improving OPV efficiencies is hindered by a lack of a fundamental understanding of the photophysics in CP-based thin films that is complicated by their heterogeneous nanoscale morphologies. Here, we report on a poly­(3-hexylthiophene)-<i>block</i>-poly­(<i>tert</i>-butyl acrylate)-<i>block</i>-poly­(3-hexylthiophene) rod–coil–rod triblock copolymer. In good solvents, this polymer resembles solutions of P3HT; however, upon the addition of a poor solvent, the two P3HT chains within the triblock copolymer collapse, affording a material with electronic spectra identical to those of a thin film of P3HT. Using this new system as a model for thin films of P3HT, we can attribute the low fluorescence quantum yield of films to the presence of a charge-transfer state, providing fundamental insights into the condensed phase photophysics that will help to guide the development of the next generation of materials for OPVs

The Effects of Aggregation on Electronic and Optical Properties of Oligothiophene Particles

Solution processing of oligothiophene molecules is shown to produce a range of particles with distinct morphologies. Once isolated on a substrate, the optical and electronic properties of individual particles were studied. From polarized scanning confocal microscopy experiments, distinct particles that are identifiable by shape were shown to have similar emission spectra except in regard to the 0–0 vibronic band intensity. This suppression of the 0–0 vibronic band correlates to the amount of energetic disorder present in a weakly coupled H-aggregate. The studied particles ranged from moderate to almost complete suppression of the 0–0 vibronic band when compared to the emission spectrum of the isolated molecule in solution. All particles were found to have a high degree of geometric order (molecular alignment) as observed from the fluorescence dichroism (FD) values of around 0.7–0.8 for all the studied morphologies. The structural and electronic properties of the particles were investigated with Kelvin probe force microscopy (KPFM) to measure the local contact potential (LCP) difference, a quantity that is closely related to the differences in intermolecular charge distribution between the oligothiophene particles. The LCP was found to vary by as much as 70 mV between different oligothiophene particles and a trend was observed that correlated the LCP changes with the amount of energetic disorder present, as signified by the suppression of the 0–0 vibronic peak in the emission spectra. Combined polarized scanning confocal microscopy studies, along with KPFM measurements, help to provide fundamental insights into the role of morphology, molecular packing, and intermolecular charge distributions in oligiothiophene particles

Conformational Effect on Energy Transfer in Single Polythiophene Chains

Herein we describe the use of regioregular (<i>rr-</i>) and regiorandom (<i>rra-</i>) P3HT as models to study energy transfer in ordered and disordered single conjugated polymer chains. Single molecule fluorescence spectra and excitation/emission polarization measurements were compared with a Förster resonance energy transfer (FRET) model simulation. An increase in the mean single chain polarization anisotropy from excitation to emission was observed for both <i>rr-</i> and <i>rra-</i>P3HT. The peak emission wavelengths of <i>rr-</i>P3HT were at substantially lower energies than those of <i>rra-</i>P3HT. A simulation based on FRET in single polymer chain conformations successfully reproduced the experimental observations. These studies showed that ordered conformations facilitated efficient energy transfer to a small number of low-energy sites compared to disordered conformations. As a result, the histograms of spectral peak wavelengths for ordered conformations were centered at much lower energies than those obtained for disordered conformations. Collectively, these experimental and simulated results provide the basis for quantitatively describing energy transfer in an important class of conjugated polymers commonly used in a variety of organic electronics applications

Excitonic Energy Migration in Conjugated Polymers: The Critical Role of Interchain Morphology

Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in <i>regioregular</i> poly­(3-hexylthiophene) (<i>rr</i>-P3HT) enables long-range interchain energy migration, while disordered packing in <i>regiorandom</i> poly­(3-hexylthiophene) (<i>rra</i>-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to <i>rr</i>-P3HT, interchain energy migration in poly­(3-(2′-methoxy-5′-octylphenyl)­thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes