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

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

Evaluation of Lithium Ion Insertion Reactivity via Electrochromic Diffraction-Based Imaging

A microscope−CCD setup has been developed as an analytical tool for the detection of diffraction from one-dimensional redox-active transition-metal oxide gratings prepared with a combination of microtransfer molding (μTM) and cathodic electrodeposition. The diffraction efficiencies (DE) of tungsten trioxide, WO3, and binary molybdenum−tungsten trioxide, Mo0.6W0.4O3, gratings were measured during Li+ insertion/deinsertion experiments performed with both cyclic voltammetry and chronoamperometry in 1 M LiClO4/propylene carbonate. The DE was evaluated in terms of the optical constants of the grating materials determined by spectroscopic ellipsometry (SE) measurements of Li+ insertion/deinsertion into unpatterned thin films. The effect of grating thickness and the amount of inserted charge on DE has been analyzed. The diffraction method is used to quantitatively estimate lithium ion diffusion coefficients of electrochemically active metal oxide gratings

Singular Value Decomposition Analysis of Spectroelectrochemical Redox Chemistry in Supramolecular Dye Nanotubes

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