Gary J. Richards

Heterocyclic polycatenar liquid crystals

We report the synthesis of polycatenar liquid crystals incorporating 2,5‐disubstituted 1 3 4‐oxadiazole and 1 3 4‐thiadiazole rings joined by a combination of carbon–carbon single and double bonds (–CH = CH–). The ratio of the aromatic core to the aliphatic chains was varied systematically by changing the number of the aliphatic chains, from two to six, and their length, from short to very long, i.e. from methoxy to hexadecyloxy. The shape anisotropy of the core was varied by exchanging the oxygen atom in the 1 3 4‐oxadiazole for a sulfur atom to form the corresponding 1 3 4‐thiadiazole ring with a smaller deviation from coaxiality of the bonds in the 2,5‐positions. The shape anisotropy of the core was increased by the presence of an additional phenylenevinylene unit in a series of tetracatenar oxadiazoles. We report the synthesis, physical properties and polymerization of a polycatenar reactive mesogen in a columnar phase to form a polycatenar polymer network.

Synthesis and mesomorphic behaviour of novel light‐emitting liquid crystals

We report the results of a systematic study of the structure–mesomorphic behaviour relationships of a diverse range of light‐emitting liquid crystals, but especially nematic 2,7‐disubstituted‐9,9‐dialkylfluorenes. The dependence of the mesomorphic behaviour and transition temperatures on the nature and length of the terminal chains, the nature, position and number of lateral substituents and the number and nature of aromatic rings with and without heteroatoms in the central core is studied. The results of these studies are used to design polymerizable, light‐emitting crystals (reactive mesogens) with a nematic phase having a high clearing point and a melting point below room temperature for facile OLED fabrication.

Photoluminescence study of crosslinked reactive mesogens for organic light emitting devices

We study the spectroscopic properties of luminescent liquid crystals which show a glassy nematic phase at room temperature and then form polymer networks by polymerization using ultraviolet light. The reactive mesogens possess fluorene-based aromatic cores with either diene or acrylate photoreactive groups at the end of aliphatic spacers. The photoluminescence quantum efficiency is enhanced when a rigid polymer backbone is formed by crosslinking of the diene endgroups. Spectral shifts of the vibronic transitions confirm an increase in the viscosity of the matrix following photopolymerization. Continuous and time-resolved photoluminescence measurements show that the quantum efficiency is limited by exciton diffusion to traps. Either the diffusion constant or the density of traps is reduced by photopolymerization.

Electroluminescent Liquid Crystals

Abstract A new class of liquid crystals for use as transport layers and/or emission layers in organic light-emitting diodes has been prepared. Depending on the nature and number of the terminal substituents enatiotropic nematic, smectic and columnar phases are observed. Chain branching gives rise to materials with a low melting point. Several of these liquid crystals electroluminesce in the visible region. The charge transport properties of some of these materials have been evaluated.

Stokes-parameter analysis of the polarization of light transmitted through a chiral nematic liquid-crystal cell

A spectroscopic Stokes polarimeter is used to directly measure the linearly, circularly, and randomly polarized components of light obtained on transmission of unpolarized light through thick chiral nematic liquid-crystal cells in the stop band. The Stokes parameters are simulated to fit the experimental data by use of the Berreman 4 x 4 transfer matrix by means of the Jones and Stokes vectors and taking into account multiple reflections at the interfaces of the cell. Excellent agreement is obtained. The transmitted light through a commercial cell is mainly circularly polarized at normal incidence, but a significant linearly polarized component is also observed. The model shows that this results from refractive-index mismatching at the liquid-crystal-alignment-layer interface, but a small linearly polarized component remains even with optimized index matching. An improved device configuration incorporating random defects at the exit boundary of the liquid crystal gives a highly circularly polarized output with virtually no linear or unpolarized components.

Pulsed laser deposition of small molecules for organic electroluminescence

A XeCl excimer laser at 308 nm is used for the pulsed laser deposition (PLD) of a range of novel electroluminescent molecules. An optimum fluence dependent on the absorption length is found as a compromise between deposition rate and photodecomposition. There is minimal degradation of the aromatic luminescent core of the molecule when the fluence is optimised. Electroluminescent devices are prepared and show similar properties to those incorporating spin-cast films. Photodecomposition is observed for films deposited using higher fluences. The decomposition mechanism is ablative and some photoproducts are oxidised at high fluences. Thin films of reactive mesogens, containing end-groups which can undergo free radical polymerisation, are also prepared by PLD. These are not polymerised on the substrate by the deposition process. Subsequent irradiation with ultraviolet light results in an insoluble cross-linked network.

Electroluminescent Nematic Polymer Networks with Polarised Emission

Abstract Electroluminescence with a polarisation ratio of 12:1 from a uniformly aligned nematic network is reported. Diene photo-active end-groups were used, which polymerise by a selective cyclisation reaction. Surface alignment was achieved for the first time using a doped polymer photoalignment layer, oriented by exposure to polarised UV light. Threshold voltages between 2 V and 8 V were found and a brightness of 90 cd m−2 was obtained. Both the PL and EL intensities were higher after photochemical crosslinking.