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A small/wide-angle X-ray scattering and grazing incidence diffraction study of comb-shaped supramolecules of conjugated poly(2,5-pyridinediyl), acid dopant and hydrogen bonded amphiphilic side chains is reported. In solution, polymers are dissolved rodlike particles. When the side-chains are introduced, polymers self-assemble in hierarchic liquid crystals (LC). Diffraction patterns of aligned LC show h00, 020, and 004 reflections, and additional small-angle reflections along the polymer axis. A triangular correlation function indicating a very large correlation length is seen along the smectic axis. An aligned solid structure can be formed by cleaving side chains from the aligned LC.

We studied the photophysical properties of poly(2,5-pyridine diyl). The blue emission in solution originates from singlet state, while the green emission in thin film consists of phosphorescence as well as delayed fluorescence. Oxygen complexation increases triplet population in thin film. At low excitation energies, emission in thin film originates mostly from triplet state, while at higher excitation energies, delayed fluorescence becomes major part in the emission.

The structure formation and phase behavior of supramolecular hairy-rod polymers consisting of rodlike polymer chains with physically bonded side chains are investigated in the melt state using small-angle X-ray scattering. The supramolecules consist of poly(2,5-pyridinediyl), complexed by methanesulfonic acid to form poly(2,5-pyridinium methane sulfonates), to which octyl gallates are hydrogen bonded. These comblike supramolecules self-organize in rodlike assemblies in a square or oblique lattice or form lamellar structures. Moreover, nematic (solid) and macrophase separated structures are observed. These results are collected in a phase diagram in the high polymer fraction limit and they are in a clear qualitative resemblance with recent theoretical modeling. The differences and similarities between the experiments and theory are discussed.

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.

The photophysical properties of poly(2,5-pyridine diyl) (PPY) are characterized in detail using steady state measurements, time correlated single photon counting techniques, and excitation pulse energy dependence of the photoluminescence intensity. Our results show that the blue emission from dilute solution is prompt fluorescence (lifetime ∼10 ps). On the other hand, the green emission from thin solid film consists of phosphorescence as well as delayed fluorescence with lifetimes of ∼4.8 ns and 1.1 ns, respectively. Oxygen complexation plays an important role and increases triplet population in thin film. At low excitation pulse energies, emission in thin film originates mostly from the triplet state, while at higher excitation pulse energies, delayed fluorescence becomes the major part in the emission. Also, at low temperature (20 K), an excimerlike emission is observed to the red side of the PPY emission spectrum in thin film, which is only induced by pressure.

Using pulse radiolysis and triplet energy transfer has enabled us to measure the triplet energies in a broad range of different pi-conjugated polymers. In all cases we find that the 1 (3)B(u) is of order 0.6 to 1 eV below the 1 (1)B(u), indicative of localized triplet states with strong electron-electron correlation. We also observe that the 1 (1)A(g)-1 (3)B(u) gap decreases linearly as the 1 (1)A(g)-1 (1)B(u) gap decreases even though polymers with very different structure have been studied. This surprising result suggests that polymers with singlet gap <1.3 eV will have a triplet ground state.

Pulse radiolysis was used to determine the triplet state energy (2.3eV) of Poly(2,5-pyridine diyl) (PPY) film which was found to be coincident with the emission energy. Detailed time-resolved photoluminescence (PL) and pump-probe measurements have been applied to study the photoexcited state relaxation dynamics. In films, a very large spectral red-shift (∼0.35eV) for the PL occurred within the first 100ps whereas no spectral red-shift was observed for the PPY in solution. This result shows clear evidence for the evolution of short-lived singlet emission (S1→S0) at ∼450nm to long-lived triplet emission (T1→S0) at ∼520nm for the PPY thin film. Streak camera measurement indicates the long live component has a decay time constant of several ns. The picosecond photo-induced triplet state absorption (T1→T2 transition) peaks at ∼600nm as measured by pump-probe which is consistent with both the radiolysis and cw photo-induced absorption measurements. A triplet lifetime of ∼6ns is measured which is again consistent with the streak camera measurement. These results lead us to believe that the long live component of the emission from PPY film is in fact phosphorescence. Furthermore, there is evidence that oxygen plays a very important role in the fast triplet radiative lifetime in PPY films.

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 new states can be assigned to the formation of bipolarons.

X-ray diffraction on samples in the form of a deposited thin layer, a crushed powder and a flake were used to deduce models for the crystalline structure of polypyridine (-(C5H3N)(n)-), a conjugated polymer of great interest for applications. A monoclinic unit cell with lattice parameters a = 7.47 Angstrom, b = 5.83 Angstrom, c = 4.25 Angstrom, gamma = 108.7 degrees containing two chains running along the c-axis and with the molecular plane normal to the a-axis gives a good fit with the observations. The interchain packing distance is about 3.6 Angstrom, i.e. in the same range as intermolecular distances found in conductive charge transfer salts. The deposited layer is highly anisotropic, with the a-c-plane parallel to the substrate. (C) 2001 Elsevier Science B.V. All rights reserved.

The charged states of the conjugated polymers poly(2-methoxy,5-(2′-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) and poly(2,5-pyridinediyl) (PPY) have been studied by pulse radiolysis. Following pulse radiolysis of argon-saturated solutions of MEH-PPV in chloroform, a new absorption is seen to grow in over a few hundred microseconds. This has a principal absorption at 1.43 eV and a weaker, low energy band (⩽0.80 eV), and is assigned to the positive one-electron charge state (positive polaron) of MEH-PPV. The slow absorption decay is unaffected by oxygen. Negative charge states (negative polarons), with absorptions around 1.4 eV, are produced upon pulse radiolysis of MEH-PPV in argon-saturated solutions in tetrahydrofuran (THF) or benzonitrile. A small solvatochromic shift is observed. In contrast to the behavior of the positive polaron, the MEH-PPV negative charge carriers decay fairly rapidly, and are readily quenched by molecular oxygen. Previous results on chemically produced positive and negative charge states of conjugated polymers and oligomers are discussed on the basis of these assignments, and comparison is made with theoretical calculations. Using benzophenone as a charge scavenger, pulse radiolysis of formic acid is shown to generate one-electron reducing species. Various derivatives of PPY, including a regioregular polymer (rPPY) and a hexyl substituted compound (HPPY) have been studied by pulse radiolysis in formic acid solution. With rPPY, new absorptions are observed at 2.59 and 1.40 eV, and are assigned to the one-electron reduced species. These are strongly quenched by molecular oxygen. With HPPY, the lower energy transition is broadened to give a maximum below 1.21 eV and a shoulder at 1.65 eV. The differences between rPPY and HPPY are interpreted on the basis of differences in the rigidity of the polymer. The relevance of these assignments to the identification of charged species in photoinduced absorption measurements is indicated.

We report on the use of insulating Langmuir-Blodgett (LB) films to improve the quantum efficiency of light emitting devices based on polymeric LB films. The insertion of arachidic acid layers between a poly-(2-methoxy,5-(2'ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) film and an aluminium cathode was found to decrease the turn-on voltage for electroluminescence and to double the quantum efficiency. In contrast, depositing the fatty acid layer between the polymer and the indium tin oxide anode produced little change in the device efficiency. Dual-layer structures using MEH-PPV as the light emissive layer and poly-(p-(3-hexyl pyridylene)) as an electron transport layer were also tested. Again, an increase in device efficiency could be obtained by placing arachidic acid LB layers immediately beneath the metallic cathode.

Poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) and poly(6-hexylpyridine-2,5-diyl) (PHPY) Langmuir-Blodgett films have been used as emitting layer and electron transporting layer, respectively, in light emitting diodes. The electrical and opto-electrical characteristics of the dual-layer devices have been compared to single layer devices based on MEH-PPV. AC impedance measurements over the frequency range 5 Hz-13 MHz have been used to provide an insight into the electrical equivalent circuit of the devices. The external quantum efficiency of the dual-layer structure was found to be approximately ten times higher than that of the single layer device.

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.