We present a critical review of semiconducting/light emitting, liquid crystalline materials and their use in electronic and photonic devices such as transistors, photovoltaics, OLEDs and lasers. We report that annealing from the mesophase improves the order and packing of organic semiconductors to produce state-of-the-art transistors. We discuss theoretical models which predict how charge transport and light emission is affected by the liquid crystalline phase. Organic photovoltaics and OLEDs require optimization of both charge transport and optical properties and we identify the various trade-offs involved for ordered materials. We report the crosslinking of reactive mesogens to give pixellated full-colour OLEDs and distributed bi-layer photovoltaics. We show how the molecular organization inherent to the mesophase can control the polarization of light-emitting devices and the gain in organic, thin-film lasers and can also provide distributed feedback in chiral nematic mirrorless lasers. We update progress on the surface alignment of liquid crystalline semiconductors to obtain monodomain devices without defects or devices with spatially varying properties. Finally the significance of all of these developments is assessed.

This review examines surface alignment of liquid crystals by exposure to light. Two distinct effects are discussed: illumination with polarized light induces a surface anisotropy to an alignment layer and hence a preferred in-plane orientation of the overlying liquid crystal director. Alternatively, a photochemical reaction of the alignment surface changes the liquid crystal anchoring conditions from homogeneous to homeotropic. We discuss how cis/trans isomerization, crosslinking and photodegradation are used to produce photoalignment layers and we show how the performance of the materials can be optimized by molecular design. Pretilted alignment is discussed and theoretical models are introduced to explain the photoalignment process. A range of display and non-display applications for photoalignment is presented.

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A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syndimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.

Blue, green, and red polymerizable light‐emitting liquid crystals have been patterned photolithographically in a full‐color liquid‐crystal electroluminescent display (see Figure). A new hole‐transporting photoalignment copolymer is also reported and the spatial patterning of the polarization direction of emission is demonstrated.