Near-Field Scanning Optical Microscopy Studies of Nanoscale Order in Thermally Annealed Films of Poly(9,9-diakylfluorene)

Near-field scanning optical microscopy (NSOM) is used to characterize nanoscale topographic and fluorescence features in thermally annealed films of the conjugated polymer polyfluorene. Thin films of polyfluorenes with either two hexyl (1), octyl (2), or dodecyl (3) alkyl groups at the 9 position were studied. Annealed films were made by holding the films above their respective liquid crystalline phase transition temperature and then rapidly cooling the films. Upon annealing, the films show large spectroscopic and morphological changes. The emission spectra films of 1 show a large increase in emission at wavelengths greater than 500, while films of 2 and 3 show small or no change in the long wavelength emission. Polarized NSOM images of all three films show that the films organize into highly ordered nanoscale domains. The order in the films is found to be largest in the polymer with the shortest alkyl chains growing progressively less ordered with increasing chain length. Films of 3 have domains on the order of 15 nm, while films of 1 and 2 have domains 25−30 nm in size. The domains in films of 1 have additional translational order as they align into larger ribbonlike polymer structures. NSOM imaging at two wavelengths reveals that intra- and interpolymer emitting species are found nearly uniformly throughout all three films. Small insoluble clusters that remain in the annealed films show no contrast in the polarization or wavelength images. The spectroscopy and NSOM together show that close packing of polymer chains in films of 1 can provide highly ordered films but only at the expense of increased excimer emission. The dioctyl polymer (2) has an ideal alkyl chain length to be able to achieve high molecular order while maintaining a minimum of interpolymer interactions. Films of 3 with the longest alkyl substituent show poor polymer order while maintaining a substantial component of interpolymer emission

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

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

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

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

Aqueous Electrogenerated Chemiluminescence of Self-Assembled Double-Walled Tubular J-Aggregates of Amphiphilic Cyanine Dyes

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

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

Effect of Film Morphology on the Energy Transfer to Emissive Green Defects in Dialkyl Polyfluorenes

The formation of a ketone defect at the 9-site along the backbone of dialkyl polyfluorenes has been shown to be directly involved in the degradation of the polymer's emission from blue to an undesirable green. Films of poly(9,9‘-dihexylfluorene) (PFH) with and without ketone defects were annealed above their liquid crystalline phase transition in an inert argon atmosphere, and their emission spectra were collected in order to study the effect of morphology on the energy transfer to ketone defects. The annealing was performed in situ in the fluorometer, allowing for a direct comparison of the absolute changes in the emission spectra. Annealing of the films resulted in regions of highly aligned polymer chains as confirmed by atomic force microscopy. After annealing, the fluorescence spectra of pristine films (without ketone defects) exhibited no green emission, indicating the lack of thermal oxidation in the inert atmosphere. However, these films did show an increase in fluorescence quantum yield, revealing that high polymer order does not lead to interchain electronic species that quench the excited states. Annealing of partially photobleached PFH films revealed that an increase in the polymer chain order of a film containing a few defects resulted in an increase in green emission and decrease in blue without the creation of further defects. The increase in green emission combined with the decreased blue can only be the result of increased energy transfer from pristine chromophores to ketone sites, as the aligned polymer chains increase exciton diffusion. PFH films containing defects that were annealed beneath the LC temperature of the polymer did not result in any spectral changes, indicating that alignment of polymer chains was necessary for the increased energy transfer to the defect sites

Well-Defined Alternating Copolymers of Oligo(phenylenevinylene)s and Flexible Chains

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

Single- and Double-Layer Graphenes as Ultrabarriers for Fluorescent Polymer Films

Graphene offers great potential for electrodes in flexible organic optoelectronic devices. Moreover, it may function as a permeation barrier to protect a device from chemical degradation under ambient conditions. Here, we report on the chemical and structural stability of graphene in situ on a conjugated polymer film. Fluorescence and scanning force microscopies were used to probe the degradation kinetics of the fluorescent polymer protected from ambient by graphene. We demonstrate that defect-free single-layer graphene efficiently protects the polymer from oxygen and water in the ambient, reaching the technological requirements on ultrabarriers, but we also observe a growing number of individual permeable defects in the single-layer graphene resulting from photoinduced structural degradation of the graphene. In contrast, double-layer graphene remains free of permeable defects, which we attribute to the structural independence of the two single layers. This suggests that graphenes can function as both a transparent electrode and a barrier layer in future optoelectronic devices