L.E. Horsburgh

An efficient electron-transporting polymer for light-emitting diodes

The efficient operation of polymer light-emitting diodes (LEDs) requires balanced injection and transport of electrons and holes. This has stimulated much research into suitable electron-injecting and transporting materials. We report the use of polypyridine as an efficient electron-transporting polymer. We have achieved much-improved LED performance by incorporating polypyridine as an electron-transporting layer in a poly(p-phenylene vinylene) (PPV) LED and optimizing layer thicknesses to balance transport of electrons and holes. The external quantum efficiency of these LEDs is 0.25%, 60 times greater than similar devices without the electron-transporting layer.

Polarized luminescence from self-assembled, aligned, and cleaved supramolecules of highly ordered rodlike polymers

A hierarchical self-assembly in comb-shaped supramolecules of conjugated rodlike polymers is reported. The supramolecules consist of poly(2,5-pyridinediyl), acid dopants, and hydrogen bonded alkyl side chains. A thermotropic smectic state with an exceptionally large coherence length is formed without additional solvent. This allows facile overall alignment resulting in high dichroism and polarized photoluminescence. Solid films are formed by cleaving side groups from the supramolecules which retain the optical anisotropy together with the high photoluminescence quantum yield of pristine polymer.

Protonation effects on the photophysical properties of poly(2,5-pyridine diyl)

We have recently shown that poly(2,5-pyridine diyl) (PPY) can be synthesized to yield a polymer with high photoluminescence quantum yield (PLQY) in the solid state, and that it is an excellent electron transport material. To explore the photophysical properties of PPY further, we have used a range of acidic “dopants” to protonate the nitrogen sites on each ring and made observations on how this affects the optical properties of the resultant protonated PPY films. In general, we find that sulphonic acids have the greatest effect, causing perturbations to both the ground-state and excited-state properties of the PPY. These changes occur with only moderate reduction of the PLQY, whereas nonsulphonic acids cause a larger reduction in PLQY without significantly affecting the ground- or excited-state energy levels. These aspects of the photophysics of PPY can be described using a simple ring torsion argument. This model can also account for the observed shifts between solution state and solid-state emission wav...

Effect of Chain Rigidity and Effective Conjugation Length on the Structural and Photophysical Properties of Pyridine-Based Luminescent Polymers

Effect of Chain Rigidity and Effective Conjugation Length on the Structural and Photophysical Properties of Pyridine-Based Luminescent Polymers

Self-Organized Liquid Phase and Co-Crystallization of Rod-Like Polymers Hydrogen-Bonded to Amphiphilic Molecules**

Application to rod-likepolymers (such as p-conjugated electroactive polymers)could lead to possible new applications, but is not straight-forward because such polymers tend to be infusible andpoorly soluble. For polymers containing rod-like moieties,self-organization has been achieved based on di-block co-polymers with flexible blocks,

Observation of in-plane optical anisotropy of spin-cast rigid-rod electroluminescent polymer films

Transmittance difference spectroscopy has been applied to study the optical properties of spin-cast rigid-rod polymer films. Significant in-plane optical anisotropy is observed for both the real and the imaginary parts of the transmittance of the poly(2,5-pyridine diyl) films. This in-plane optical anisotropy is a result of the partial alignment of the polymer chains, oriented radially outward from the center of the film, during the spin casting process. This observation has important implications for emissive polymer devices, particularly for devices sensitive to polarization direction and those using waveguide confinement where relatively long propagation distance within the films are required.

Synthesis and Characterisation of Polypyridines

Abstract We report the synthesis of poly(2,5-pyridinediyl) (PPY), and synthesis of the novel polymerpoly(2,6-pyridinediyl). Details of the characterisation of the polymers are reported. The effects upon the photophysical properties of restricting conjugation within the polymer chain arc discussed.

THE ELECTRONIC STRUCTURE OF POLY(PYRIDINE-2,5-DIYL) INVESTIGATED BY SOFT X-RAY ABSORPTION AND EMISSION SPECTROSCOPIES

The electronic structure of the poly-pyridine conjugated polymer has been investigated by resonant and nonresonant inelastic X-ray scattering and X-ray absorption spectroscopies using synchrotron radiation. The measurements were made for both the carbon and nitrogen contents of the polymer. The analysis of the spectra has been carried out in comparison with molecular orbital calculations taking the repeat-unit cell as a model molecule of the polymer chain. The simulations indicate no significant differences in the absorption and in the non-resonant X-ray scattering spectra for the different isomeric geometries, while some isomeric dependence of the resonant spectra is predicted. The resonant emission spectra show depletion of the {\pi} electron bands in line with symmetry selection and momentum conservation rules. The effect is most vizual for the carbon spectra; the nitrogen spectra are dominated by lone pair n orbital emission of {\sigma} symmetry and are less frequency dependent.

Self-organized supramolecules of poly(2,5-pyridinediyl)

Polypyridines are remarkably stable and good charge transport materials having high photoluminescence quantum yield. They form supramolecules that form lamellar self-organized structures due to bonding between pyridine and sulphonic acid unit and polar nonpolar effects combined. Some examples are demonstrated here using synchrotron radiation and small angle X-ray scattering (SAXS) method.

Electronic structure of pristine and sodium doped poly(p-pyridine)

The electronic structure of pristine and sodium-doped poly(p-pyridine) has been studied using both ultraviolet and x-ray photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum-chemical calculations. Electronic band-structure calculations are performed for isolated chains with different connectivity patterns (head-to-tail and head-to-head), using the valence effective Hamiltonian (VEH) method, with geometries derived from optimizations using the Austin Model 1 Hamiltonian. The density-of-valence-states are derived directly from the VEH band structure. Excellent agreement is obtained between the theoretical simulations and the experimental data, which allows for a detailed assignment of the different peaks in the spectra. The C(1s) and N(1s) shake-up spectra of poly(p-pyridine) are analyzed on the basis of corresponding data for pyridine in the gas phase. Upon sodium doping of poly(p-pyridine), new states are observed within the otherwise forbidden energy gap. These ...