The synthesis and morphological investigation of a series of polychromophore polymers composed of oligomeric bis(2-ethylhexyl)-p-phenylenevinylene (BEH-PPV) (trimer, pentamer, and heptamer) monomers copolymerized with rigid morphological directing groups (morphons) are described. The polymerization was carried out using a Sonogashira cross-coupling polymerization between monomers composed of an iodo-terminated BEH-PPV oligomer and a bis(phenylacetylene)-containing morphon. The rigid morphons are prepared from adamantane and diamantane frameworks that are composed solely of sp3 carbons, inhibit conjugation between BEH-PPV oligomers, and direct the local polymer morphology in either a bent or linear vector, respectively. The morphological properties of the polychromophore polymers were interrogated via single molecule fluorescence spectroscopy, thin-film absorption and fluorescence spectroscopy, and atomic force microscopy. Significant morphological variation was found upon substituting the morphon, as well as the chromophore size, with the most ordered structures being accessed with diamantane morphons.

We have synthesized and characterized a polymer comprising a polynorbornene backbone with perylene diimide (PDI) pendant groups. The polymer exhibits intramolecular self-assembly into molecular aggregates that can be controlled via solvent quality. In a medium that solvates the polymer backbone well the PDI chromophores self-assemble into characteristic H-type aggregates aligned along the polymer backbone. In contrast, in a solvent medium that does not solvate the polymer well the polynorbornene backbone collapses into a globular conformation preventing the aggregation of the PDI chromophores, which display monomer-like photophysics. The polymer photophysics are recorded under a variety of conditions, including lifetimes and relative quantum yields. Furthermore, we utilize the weakly coupled H-aggregate model to quantify the exciton bandwidth of the polymer structure and demonstrate that polymer thin films can retain the same chromophore aggregated morphology as the fully solvated polymer.

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)-block-poly(tert-butyl acrylate)-block-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.

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.

Scanning confocal photocurrent microscopy has been used to characterize carrier collection efficiency in lateral bulk heterojunction devices. By analyzing the photocurrent mappings within these devices, the lateral extents of the space charge regions has been measured and reported. Modulation via white light bias or increased voltage bias is also shown to increase the size of the space charge regions.

Herein we describe the use of regioregular (rr-) and regiorandom (rra-) 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 rr- and rra-P3HT. The peak emission wavelengths of rr-P3HT were at substantially lower energies than those of rra-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.

A series of alternating copolymers containing oligomeric bis(2-ethylhexyl)-p-phenylenevinylene (BEH-PPV) chromophores and conformational-flexible n-decyl or tetraethylene glycol chains were prepared. The polymerization was carried out using Sonogashira coupling conditions between monomers composed of an iodo-terminated PPV oligomer (trimer, pentamer, or septamer) and a bis(phenylacetylene)-containing flexible chain. Polymers containing the n-decyl chain attained higher molecular weights compared to the tetraethylene glycol-containing polymers. 4-Ethynylanisole-capped oligomers (trimer, pentamer, or septamer) were prepared, and their solution photophysical properties were compared to the analogous polymeric materials. The solution optical properties of the polymers were primarily determined by chromophore length of the constituent oligomers. In contrast, the thin film fluorescence spectra of the polymers showed substantial differences between n-decyl and tetraethylene glycol containing materials, suggesting significant changes in the degree of interchain coupling in the solid state. The control of effective conjugation length afforded by these materials makes them a promising system for understanding electronic trap states in conjugated polymers.

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.

For conjugated polymer materials, there is currently a major gap in understanding between the fundamental properties observed in single molecule measurements and the bulk electronic properties extracted from measurements of highly heterogeneous thin films. New materials and methodologies are needed to follow the evolution from single chain to bulk film properties as multiple chains begin to interact. In this work, we used a controlled solvent vapor annealing process to assemble single chains of phenylene–vinylene conjugated polymers into aggregates that can be individually spectroscopically interrogated. This approach allowed us to probe the effects of interchain coupling in isolated conjugated polymer nanodomains of controlled size. By assembling these aggregates from building blocks of both pristine MEH–PPV and MEH–PPV derivatives containing structure-directing ortho- or para-terphenyl inclusions, we were able to control the ordering of these nanodomains as measured by single aggregate polarization anisotropy measurments. Depending on the individual chain constituents, these aggregates varied from highly anisotropic to nearly isotropic, respectively facilitating or inhibiting interchain coupling. From the single chain fluorescence lifetimes, we demonstrated that these structure directing inclusions effectively break the phenylene–vinylene conjugation, allowing us to differentiate interchain electronic effects from those due to hyper-extended conjugation. We observed well-defined bathochromic shifts in the fluorescence spectra of the aggregates containing extensive interchain interactions, indicating that low-energy exciton traps in MEH–PPV are the result of coupling interactions between neighboring chain segments. These results demonstrate the power of the synthetic inclusion approach to control properties at not just the single chain level, but as a comprehensive approach toward ground-up design of bulk electronic properties.Keywords: conjugated polymers; organic electronics; photoluminescence; self-assembly; structure−property relationships

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.Keywords: aggregate; Kelvin probe force microscopy; morphology; oligothiophene; spectroscopy

This study investigates superradiant organic dye J-aggregates as a potential new class of aqueous luminophores for electrogenerated chemiluminescence (ECL). Simultaneous cyclic voltammograms (CVs) and ECL transients are obtained from the self-assembled double-walled tubular J-aggregates formed from the amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) immobilized on glassy carbon electrodes in the presence of the oxidative−reductive coreactant 2-(dibutylamino)ethanol (DBAE). ECL is produced by both the direct oxidation of DBAE at the electrode and the catalytic oxidation of DBAE by the C8S3 J-aggregates. Optimization studies of the DBAE concentration and pH of the electrolyte show the most intense ECL signal was obtained with ∼17 mM DBAE as coreactant (saturated solution in 1 M KNO3) at pH 12.85, an effect of DBAE solubility and pKa. The overlaid ECL spectrum and the fluorescence spectrum were in good agreement, confirming that the ECL emission is associated with the singlet exciton delocalized on the tubular C8S3 J-aggregates. Amphiphilic J-aggregates are promising new systems for ECL applications because of their unique characteristics such as accessible redox chemistry in the aqueous potential window, increased fluorescence emission, and narrow emission lines.

The effect of molecular weight on the formation of β phase poly(9,9′-dioctylfluorene) (PF8) was studied in dilute solutions. Temperature-dependent fluorescence experiments of unique synthetic batches as well as size-excluded single batches of polyflourene were studied. Each batch had unique molecular weight, tetrahedral defect concentration, and polydispersity index (PDI). Polyflourene was found to exhibit a temperature-dependent transition between two phases with distinct electronic transition signatures: the α (primary) phase and the β (secondary) phase. In dilute solutions, the temperature at which the polymer exhibited a conversion between these phases showed a clear dependence on molecular weight. We model this transition temperature for β phase formation using the mean field theory for the coil–globule transition developed by Isaac Sanchez. Results show that temperature affects the average end-to-end distance corresponding to increases in secondary electronic absorption and that the dependence on temperature related to the coil–globule transition.

The redox chemistry of supramolecular nanotubes self-assembled from amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) in aqueous solution was investigated by spectroelectrochemistry. The absorption spectra during the redox-reaction were analyzed by singular value decomposition (SVD) and semiempirical quantum mechanical calculations. Previously unavailable absorption spectra were modeled for each species of the reaction at every point throughout the oxidation and reduction process. The expected peak absorption wavelengths of these species present throughout the electrochemical cycles were calculated using the semiempirical method ZINDO/S (Zerner method of intermediate neglect of differential overlap for spectroscopy). The spectral assignments from the proposed species derived from the SVD analysis were confirmed with the ZINDO/S calculations, supporting the assignment of the key species as well as the intermediates in the proposed redox-reaction scheme. The oxidized species have spectral signatures that agree with the calculated spectral maxima of isolated species, indicating that the resulting products are not electronically perturbed by aggregation, in strong contrast to the unoxidized dye.

The regioregularity of a conjugated polymer can greatly affect bulk film morphologies and properties. However, it remains unclear how regioregularity affects the conformation of isolated individual chains where interchain interactions are absent. Here, the effect of the regioregularity on the conformations adopted by regioregular (rr-) and regiorandom (rra-) poly(3-hexylthiophene) (P3HT) chains was studied using single molecule fluorescence excitation polarization spectroscopy. While every rr-P3HT chain within an ensemble was found to fold into a highly ordered conformation, single rra-P3HT chains adopted a wide variety of conformations, ranging from highly ordered to isotropic. This distribution is likely due to variations in the positions of nonhead-to-tail linkages of the side-chains along the backbone of the different polymer chains. Molecular dynamics simulation on atomistic models of rr- and rra-P3HT chains supports the effect of regioregularity on the collapsed conformations. These results demonstrate that side-chains govern the morphology of P3HT, even at the single chain level.Keywords: conjugated polymers; molecular dynamics simulation; morphology; P3HT; single molecule spectroscopy;

The spectroscopic properties of 2-[4′-(dimethylamino)phenyl]-benzothiazole (BTA-2) in solution and in the presence of amyloid fibrils were investigated using absorption and fluorescence spectroscopy. Solution studies show that BTA-2 forms micelles in aqueous solutions, but that the dye can be solvated upon the addition of acetonitrile (CH3CN). BTA-2 binds to amyloid fibrils in solution leading to a characteristic blue-shift in the emission spectrum and an increase in fluorescence intensity. However, in solutions with increasing CH3CN concentration, there was a marked decrease in binding of the BTA-2 to fibrils. Studies demonstrating the effect of BTA-2 concentration on binding were performed. A comparison with the standard amyloid fluorescent marker, thioflavin T (ThT), showed that BTA-2 is more fluorescent, making it an excellent dye to label amyloid samples.

Rotational motions of probe molecules in poly(cyclohexyl acrylate) (PCA) were investigated by both ensemble fluorescence recovery after photobleaching (FRAP) and single-molecule spectroscopy at temperatures near the glass transition temperature (Tg) of the polymer host. FRAP measurements of the ensemble anisotropy decay show a nonexponential decay with β values of 0.5−0.6 when fit by a stretched exponential function. The relationship between the average relaxation time and temperature follows the Williams−Landel−Ferry equation, whereas β shows no temperature dependence over this range. The same system was also studied by single-molecule spectroscopy at 2 °C above the Tg of PCA. The rotational dynamics of the probe molecule can be measured by the autocorrelation function of the linear dichroism signals. Each single-molecule correlation function was fit to the stretched exponential function. The results from all single-molecule data yield broad distributions of both the correlation times (τ) and β values. The average of the single-molecule correlation times agrees with the ensemble relaxation time, and the sum of all single correlation functions has a nonexponential decay that is almost identical to the ensemble anisotropy decay. The ensemble β values are smaller than the average β values in the single-molecule experiments, demonstrating that the system exhibits heterogeneous dynamics. However, the dynamics are not described by an ensemble of molecules that all have single-exponential correlation functions with different time constants.

The binding of two amyloid fibril stain dyes, thioflavin T (ThT) and its neutral analog 2-[4′-(dimethylamino)phenyl]-benzothiazole (BTA-2), are measured using near-field scanning optical microscopy (NSOM), which is able to image individual amyloid fibrils. Polarized NSOM images reveal that both dyes bind to the fibrils with the long axis of the molecule aligned parallel to the long axis of the fibrils. This indicates that the dyes bind along the surface of the β-sheet within the grooves of the fibril that run parallel to the fibril axis. The similarity in the binding motifs of the two dyes shows that electrostatic interaction of the charged amine group on the ThT dye plays a minimal role in the affinity of the dyes for the amyloids. The polarized NSOM images confirm that the enhanced fluorescence of the ThT and BTA-2 result from binding of the monomeric dye rather than micelles or excimer species.

Scanning confocal photocurrent microscopy has been used to characterize carrier collection efficiency in lateral bulk heterojunction devices. By analyzing the photocurrent mappings within these devices, the lateral extents of the space charge regions has been measured and reported. Modulation via white light bias or increased voltage bias is also shown to increase the size of the space charge regions.