J.H.F. Martens

PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE IN CONJUGATED POLYMERIC SYSTEMS

The basic working principles and construction of electroluminescent polymer devices are described. The opportunities for combining creative synthetic chemistry and imaginative device physics to address issues of colour, efficiency, and control of processing of semiconducting polymeric materials are reviewed. New advances in the construction of multilayer devices and controlled syntheses of poly(p-phenylenevinylene) are reported.

Hole-transporting compounds for multi-layer polymer light-emitting diodes

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.

Nh4Ni(mnt)2H2O— a low-dimensional organic ferromagnet

Abstract 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.

Structural order in poly(p-phenylene vinylene)

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.

The effect of side groups on the structure and ordering of poly(p-phenylene vinylene) derivatives

Abstract 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.

Control of order in poly(arylene vinylene) conjugated polymers

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.

Electronic properties of polyacetylene prepared by the Durham photoisomer route

Abstract 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.

NH4Ni(mnt)2.H2O). A New 1D Organic Ferromagnet

Abstract 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.

Structure and optical properties of polyacetylene prepared via an improved Durham-route precursor polymer

Abstract 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.