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This thesis presents a study of the role of surface effects in the reorientation mechanism in a photorefractive-like liquid crystal-polymer hybrid structure. The photorefractive properties of liquid crystals were first discovered in 1994. Liquid crystals are very attractive as photorefractive materials due to their high nonlinear optical response caused by collective molecular reorientation, which can be induced by light and applied electric field. They are also low cost compared with photorefractive crystals. Liquid crystals are also excellent adaptive materials, ideal for writing dynamic re-writable gratings. . Photorefractive effects have been observed in both pure liquid crystals and polymers such as PVK, but also in certain hybrid structures of both liquid crystal and polymer layers. Recent experiments have shown that these effects are only apparent for certain combinations of liquid crystals and polymer surface layer, suggesting the cause to be surface-mediated at the interface between the two, and not due to bulk effects in the liquid crystal or the polymer. In this thesis I present characterisation and analysis of such hybrid structures and discuss a hypothesis for the underlying mechanism, supported by experimental and theoretical investigation. When investigating surface effects, I also developed a system for characterisation of the molecular alignment in samples. This has been shown to be valid for molecular angles 0-16.30 and 48.8-900 from the surfaces. I also present an analysis of 'pretilt' measurement methods in which I have found the Crystal Rotation Method (CRM) to be most suited to our needs. I used this experimental method and the characterisation program to inspect cells that had been 'damaged' and exhibited long-term memory effects. I found that for some polymer-liquid crystal combinations, such as LC1294PVK, the surface 'damage' is due to applying electric field and light illumination. A mathematical model for liquid crystal reorientation under a spatially varying field is presented. This model is based on the Landau De-Gennes theory and is used to simulate photorefractive gratings in liquid crystal cells with varying thickness and grating size. It has been found that the grating spacing to cell thickness ratio governs the penetration of electric field into the liquid crystal bulk. These simulations are useful for predicting the conditions for highest diffraction efficiency. Finally, the birefringence measurements I carried out were used to validate the theoretical simulations and to obtain simulation parameters for the experimental cells investigated

Due to the general interest on the alignment effect of the surface-bound ultrathin layers, this thesis presents research of the investigation of anchoring properties for liquid crystal (LC) molecules adsorbed on a solid substrate, using dual polarization interferometry (DPI). A new theoretical framework is designed based on the dual polarization interferometer for the detection of anisotropic information and the average anchoring angle of the adsorbed molecules, provided the liquid crystal bulk optogeometric parameters are well known. In the experiment, the nematic compound 4'-n-pentyl-4- cyanobiphenyl (5CB) is applied onto the silicon oxynitride surface, and a complete wetting monolayer with the average 56 ° polar angle and 16.6 Å thickness is observed in the stabilized stage. The results match quite well with the theoretical predictions in terms of the DPI phase change ratio. The thesis also extends the research into the functionalized substrate, in particular the Langmuir-Blodgett (LB) films covered substrate, which can give alignment effect to the deposited LC molecules. The LB forming molecule quinolinium tricyanoquinodimethanide (Q3CNQ) with long hydrocarbon chain (typically C18H37-Q3CNQ) is chosen. In order to justify the functionality of this material as an alignment layer, the electronic structure and the optical absorptive properties of this molecule in the LB phase are explored both in experiment and theory. Based on the very controversial history of this Q3CNQ compound, a robust computational LB model is built to examine the ground state, optimal geometry and the optical absorption features. A 530 nm absorption band is obtained to conclude the properties of the Q3CNQ LB layers, and also used to compare with our own experimental results.

This thesis describes contributions made as part of the EPSRC-funded project Age Concern: Crystallographic Software for the Future. Work has been done in various areas of small molecule crystallographic soft- ware development, both within the smtbx (Small Molecule Toolbox) and the Olex2 software. Chapter 2 details the work that was done towards the smtbx-based re- finement that was developed as part of the “Age Concern” project. A framework was created enabling the inclusion of observations of restraint in the refinement, and new restraints on geometry and anisotropic displacement parameters were added. Refinement of (pseudo-)merohedrally twinned structures was implemented. In Chapter 3 a description of the determination of absolute structure by various methods is given. The methods of Hooft et al. [2008] and Flack [1983] have been implemented, and a quantitative comparison made between the two methods. Chapter 4 discusses the method of van der Sluis and Spek [1990] for the refinement of structures containing severely disordered regions. This method has been implemented and a modification designed to give improved results when one or more low angle reflections are missing is proposed and tested, and shown to be beneficial. Chapter 5 introduces a new module, iotbx.cif, which has been added to the cctbx (Computational Crystallography Toolbox), providing a comprehensive set of tools for the manipulation of Crystallographic Information Files (CIFs).