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We report here structural and photophysical studies on polyacetylene prepared by various precursor routes. We find that control of stereoregularity, molecular weight and molecular weight distribution of the precursor allows improvements in ordering parameters, such as molecular orientation and crystallographic order of the final polymer. The photophysical properties of polyacetylene also depend on the particular precursor preparation route. The peak in the π–π∗ absorption lies between 2.15 and 2.65 eV, depending on the precursor; also, photoinduced metastable band-gap states vary in position, shape and strength.

NH4Ni(mnt)2. H2O shows localized moments with antiferromagnetic coupling at room temperature. The antiferromagnetism persists down to 100K at which point there is an abrupt change in the slope of the susceptibility corresponding to an anomaly observed in the resistivity. Below 100K there is evidence for ferromagnetic coupling between the spins. The crystal has an orthorhombic space group Pnam, with a=12.096A, b=3. 918A, c=29. 216A, U=1384. 8A3 and Z = 4, and the crystal structure is highly anisotropic, in common with other charge transfer salts of this type. The anions form uniform stacks along the b-axis in which the molecules are tilted by 26° to the stacking axis. Results are presented here on the electrical and magnetic behaviour of this salt.

Polymerisation of two mono-substituted triphenylamines gave 4a and 4b. Their use as a hole-transporting layer in poly(p-phenylenevinylene) electroluminescent devices resulted in a decrease of device efficiency compared to devices without the additional layer.

We report on the magnetic and transport properties of the organic salt NH 4 Ni(mnt) 2 H 2 O where (mnt) is the maleonitrile dithiolate ligand. This material is antiferromagnetic down to 100K, below which there is a transition to a paramagnetic state with ferromagnetic exchange. This is accompanied by an increase in resistivity by a factor of two which we consider indicative of a structural phase transition. Long range ferromagnetic order is seen below 4.9K.

Unoriented and stretch-oriented polyacetylene samples prepared via an improved Durham precursor polymer have been investigated. The standard catalysts for the Durham precursor polymerization have been replaced by a Schrock-type molybdenum-based initiator, which gives a narrower distribution in molecular weight and better stereoregularity in the precursor polymer. We have looked at the process of orientation achieved by stretching free-standing films during thermal conversion. Mixtures of high and lower molecular weight precursor polymers, giving a bimodal distribution, are more readily stretched than polymers with a monomodal molecular weight distribution, and we find that these oriented films show significantly higher levels of orientation and larger crystallite sizes than films prepared from the standard Durham-route precursor. Unoriented films of polyacetylene show optical absorption similar to those of the standard Durham polyacetylene, though there are significant differences in the photoinduced absorption spectra.

Poly(p-phenylene vinylene) [PPV] is fast becoming one of the most widely studied of the conjugated polymers. Its preparation via a precursor polymer synthesis allows ready processing and presents many opportunities for subtle control of the physical and electronic properties of the final polymer. A number of precursor routes have now been developed and we report an X-ray and electron diffraction study of the effects of different synthesis routes on the structure of the PPV polymer films thus prepared.

Results of electron diffraction experiments on unoriented and stretct-oriented poly( p -phenylene vinylene) [PPV], poly(2,5-dimethyl- p -phenylene vinylene) [PDMePV], poly(2,5-dimethoxy- p -phenylene vinylene) [PDMeOPV] and poly(2,5-diethoxy- p -phenylene vinylene) [PDEOPV] are presented. We make a comparison of these closely related materials and discuss the effect that different 2,5-substituent groups have on the state of order.

Conjugated polymers such as poly(p-phenylenevinylene), (PPV, see Figure), show real promise in optoelectronic applications. A new synthetic route to improved PPV is presented, involving the thermal conversion of a precursor polymer containing rigid rod conjugated segments joined by flexible spacer groups. A high degree of interchain ordering results which influences the optical response of the material.

The electronic structure of highly-oriented poly(p-phenylenevinylene) (PPV) and unoriented poly(2,5-thienylenevinylene) (PTV) were investigated by high-resolution electron energy-loss spectroscopy (EELS). Performing a Kramers-Kronig analysis, the momentum-dependent dielectric functions could be derived. For the oriented PPV samples, the dielectric functions parallel and perpendicular to the chain axis were determined. In order to study the influence of the vinylene linkages on the electronic properties of these materials, the results were compared with previous measurements on polyparaphenylene (PPP) and polythiophene (PT).

We report measurements of electron and X-ray diffraction on highly oriented films of poly(p-phenylene vinylene), PPV and poly(2,5-dimethoxy phenylene vinylene), DMPPV. We find that DMPPV shows very much better translational ordering than PPV, and we consider that this is due to the locking of the methoxy side-groups on adjacent chains.

We report here the properties of polyacetylene prepared via a precursor polymer, modified from the standard Durham precursor by photoisomerization of the monomer to 3,6-bis(trifluoromethyl)pentacyclo-[6.2.0.02,4,O3,6,O5,7]dec-9-ene. This modification gives a precursor with improved thermal stability, in that conversion to polyacetylene only occurs above 60°C. The conversion is highly exothermic, but the method is suitable for the formation of thin films. There is evidence from infra-red and X-ray photoelectron spectroscopy measurements for retention of fluorine in the thermally treated polymer, and we consider that this is probably due to isomerization of some of the side groups on the precursor polymer, rather than their elimination. Polyacetylene produced in the form of thin unoriented films is highly disordered, with a coherence length interchain order determined from X-ray scattering of only 1.8 nm. The electronic and vibrational structure of these films is much affected by the disorder, with the peak in the π-π∗ optical absorption band at 2.65 eV, and correspondingly high Raman-active vibrational modes. We compare also the properties of polymers prepared with different degrees of polymerization, achieved by addition of a chain transfer agent during polymerization of the monomer to form the precursor polymer. There is some evidence from the Raman spectra that the lower molecular weight precursors produce better ordered polyacetylene.

Characterisation by X-ray and electron diffraction of conjugated polymers prepared by different routes is presented. Modification of the precursor routes to polyacetylene and poly( p -phenylenevinylene) has a significant effect on intra- and interchain ordering. Also, attachment of side-groups to poly( p -phenylenevinylene) allows control of the state of order, both interchain and intrachain.

We present work which has been directed towards the understanding of the factors which affect the degree of order and orientation in the family of conjugated polymers based on poly(phenylene vinylene). We have found that by increasing the bulk of the leaving group and/or the rigidity of precursor polymers to poly(phenylene vinylene) the intra-chain order increases whereas the inter-chain order decreases. This has allowed us to prepare poly(phenylene vinylene)s which have more one-dimensional character.

Studies on the physical properties of conjugated polymers have been hampered in the past by difficulties in purification and sample fabrication, arising from the intractibility associated with the presence of a rigid chain backbone. The use of a precursor-route synthesis, in which the processing is performed for a non-conjugated precursor polymer (that can subsequently be converted to the conjugated material through a thermal treatment) has greatly improved the possibilities for the preparation of samples in a variety of forms, including the homogeneous thin films required for optical and electro-optical processing applications.