Stuart P. Kitney

Heterocyclic reactive mesogens: synthesis, characterisation and mesomorphic behaviour

Novel heterocyclic and photopolymerizable liquid crystalline materials (reactive mesogens) with smectic phases have been synthesized and characterized. A selection of heterocyclic rings, such as benzothiazole, benzothiadiazole and pyrimidine, has been incorporated into the aromatic core to control the electrochemical/luminescence properties and the structural geometry. Particular emphasis is focused on structure–property relationships, in which the variation of molecular structure and its subsequent effect on the liquid crystalline transition temperatures have been investigated.

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.

Electroluminescent segmented liquid crystalline trimers

The synthesis and liquid crystal behaviour of light‐emitting trimeric liquid crystals consisting of three mesomorphic moieties connected by aliphatic spacers are reported. The combination of an aromatic light‐emitting central core and two cholestanyl (dihydrocholesteryl) groups induces a helical liquid crystalline phase with circularly polarised photoluminescence and electroluminescence. These segmented trimers are designed to possess a high glass transition temperature below which the structure of liquid crystalline phases can be fixed. The effects of odd and even spacers, spacer length and the presence of large lateral substituents on the liquid crystal behaviour and the glass transition temperature of these trimers were studied. Electroluminescence from a segmented liquid crystalline trimer is reported for the first time.

Organic electroluminescence using polymer networks from smectic liquid crystals

We report the synthesis of a red light‐emitting and photopolymerizable smectic liquid crystal (reactive mesogen). We investigate the suitability of polymer networks formed from smectic reactive mesogens for use in organic light‐emitting diodes (OLEDs). The use of mixtures of smectic reactive mesogens is shown to lower the processing temperature for the fabrication of OLEDs to room temperature. We also report efficient energy transfer from a nematic polymer network host to a smectic light‐emitting dopant and polarized emission from a polymer network formed from an aligned smectic reactive mesogen.

Grazing Incidence X-ray Diffraction of a Photoaligned Nematic Semiconductor

Grazing incidence X-ray diffraction is used to find the thin film morphology of an extended molecule with an irregular alternating fluorene-thiophene structure, which is used to obtain linearly polarized electroluminescence and the photovoltaic effect. The material has a room temperature nematic glassy phase and is uniaxially aligned in the plane of the film using photoalignment techniques. Two distinct intermolecular separations of 0.45 and 1.5 nm are identified showing that the molecules are lamellar. The lamellae stack with only local order and the two short axes of the lamellae have no preferred orientation at the surface or bulk of the film. Neighboring molecules show a wide range of longitudinal displacements along the axis of the director, as expected for a nematic liquid crystal with no positional order. There is, however, a dominant feature corresponding to a longitudinal offset of 0.51 nm. Unlike some other fluorene-containing semiconductors where microphase separation of the side chains inhibits close packing of neighboring molecules, the lamellar structure and 0.45 intermolecular spacing found here allows pi-pi intermolecular interactions for efficient carrier transport. We obtain a room temperature hole mobility up to 3.4 x 10-3 cm2 V-1 s-1 using a time-of-flight technique.

Optical Properties of Light-Emitting Nematic Liquid Crystals: A Joint Experimental and Theoretical Study

Semiempirical quantum-chemical calculations are used to simulate the optical properties of a series of green light-emitting nematic liquid crystals containing fluorene, thiophene, or thienothiophene groups with solid-state photoluminescence quantum efficiencies up to 0.36. We use a simple model of two parallel and closely spaced molecules in an anticofacial configuration to study intermolecular interactions in the solid state and slide one past the other to mimic the high orientational and low positional order of the nematic phase. We find that switching between H and J aggregates can be triggered by longitudinal displacements of the molecules with respect to one another by an extent that closely follows the chemical structure of the interacting chromophores. We discuss the implications of aggregate formation for efficient light emission in conjugated oligomers and polymers that show nematic or smectic order.

One-step photoembossing for submicrometer surface relief structures in liquid crystal semiconductors.

We report a new single-step method to directly imprint nanometer-scale structures on photoreactive organic semiconductors. A surface relief grating is spontaneously formed when a light-emitting, liquid crystalline, and semiconducting thin film is irradiated by patterned light generated using a phase mask. Grating formation requires no postannealing nor wet etching so there is potential for high-throughput fabrication. The structured film is cross-linked for robustness. Gratings deeper than the original film thickness are made with periods as small as 265 nm. Grating formation is attributed to mass transfer, enhanced by self-assembly, from dark to illuminated regions. A photovoltaic device incorporating the grating is discussed.

Carbazole nematic liquid crystals

A number of calamitic 2,7-diary-N-alkyl-substituted carbazoles with an enantiotropic nematic phase have been prepared. Branching of the aliphatic chain attached to the nitrogen atom in the carbazole ring leads to significantly lower liquid crystal transition temperatures. These new materials show a lower ionisation potential than fluorene analogues and blue photoluminescence in solution and as thin solid films.

A new approach to photoalignment: photo-addition to a self-assembled monolayer

Photoalignment used to control the orientation of nematic liquid crystals in LCDs usually involves the photochemical modification of the chemical structure of an organic thin film so that an optical and/or steric anisotropy is developed. We now demonstrate a novel photoadditive alternative whereby small photoreactive molecules are bound to a photoreactive SAM surface by irradiation with polarised UV so that the surface properties are changed and a surface anisotropy is developed.

Calamitic liquid crystal blends for organic photovoltaics

Four different perylene-based electron-acceptors having similar electron affinities, but different thermotropic phases are blended with nematic liquid crystalline electron-donors with a fluorene-thiophene structure to form single layer photovoltaic devices. Best results are obtained when the nematic donor is mixed with an amorphous acceptor to give a supercooled nematic glass at room temperature. Atomic force microscopy operating in the phase contrast mode reveals phase separation on a nanometer scale with a broad distribution of domain sizes peaking at 26 nm. We correlate the morphology of the different blends with the performance of the photovoltaic devices. Power conversion efficiencies up to 0.9 % are obtained with excitation at 470 nm.