Novel polycatenar bent-core Schiff-base metallomesogens from derivatives of 1,1 O-phenanthroline ([MCb(LPhen-n)]), 2,2-bipyridine ([MCb(L Bipy-n)]) and 5,5'-dimethyldipyrromethane ((tc)_[M(LDipy-n)] and ex-[M(L Dipy-n)]) have been investigated in this body of work. The mesomorphic properties of these first- and second-row transition metal complexes have been studied. Further to the examination of the compounds in the liquid crystalline state, single crystal X-ray studies of short chain analogues were performed to determine the coordination geometry and the degree of selfassembly of the molecules in the solid state. Chapter 1 introduces the field of liquid crystals and metallomesogens, with a focus on thermotropic liquid crystals and their nomenclature, physical properties and applications. The historical background of the field is briefly explored and previous research on bent-core metallomesogens from the Schroder group in Nottingham has been reviewed. The characterisation of liquid crystalline mesophases, namely by polarised optical microscopy, differential scanning calorimetry and X-ray diffraction, are described. Further discussion is dedicated to the X-ray diffraction patterns generated by columnar mesophases. The chapter finishes with a description of the aims of the project. Chapter 2 commences with an introduction to liquid crystals derived from 1,1 O-phenanthroline. Following this is a description of the synthesis and characterisation of mesomorphic metal-free ligands, LPhen-n (n = 10, 12, 14, 16), four novel series of metallomesogens and two non-mesomorphic series of complexes derived from 1,10-phenanthroline, [MCb(LPhen-n)] (M = Mn2+, Fe2~, C02+, Ni2+, Cu2+. Zn2+~ LPhen = 2.9-bis-[3' A ',5' -tri(alkoxy)phenyliminon1ethyl]- 1,1 O-phenanthroline: n = 8, 10, 12, 14, 16). Structural determination by single crystal X-ray diffraction of the analogous methoxy complexes [MCb(LPhen-l)] (M = Mn2+, C02+, Ni2+, Zn2+), and the complex without any lateral aliphatic groups [CuCb(LPhen-O)], revealed the metal (II) complexes to have either distorted trigonal bipyramidal, square pyramidal or octahedral coordination geometry. The mesomorphic behaviour of the complexes [MCb(LPhen-n)] (M = Mn2+, C02+, Ni2+, Zn2+; n = 8, 10, 12, 14, 16) and the metal-free ligands LPhen-n (n = 10, 12, 14, 16) is columnar (with the exception of the non-mesomorphic [CoCb(LPhen-8)]), and the 2D symmetries of these mesophases vary between hexagonal, rectangular and oblique. Chapter 3 is introduced with a discussion of liquid crystalline compounds derived from 2,2' -bipyridine. Subsequently, the synthesis and characterisation of four new series of metallomesogens and two nonmesomorphic compounds derived from 2,2' -bipyridine, [MCb(L Bipy-n)] (M = Mn2+, Fe2+, C02+ Cu2+ and n = 16; M = Ni2+, Zn2+ and n = 10, 12, 14, 16; L Bipy = 6,6' -bis-[3' ,4',5' -tri(alkoxy)phenyliminomethyl]-2,2' -bipyridine) are detailed. Single crystal X-ray diffractometry revealed the coordination geometry• of [MnCb(LBipy-l)], [CoCb(LBipY-l)] and [NiCb(LBipy-l)] to be octahedral, whereas [ZnCb(LBipY-l)] is distorted trigonal bipyramidal. The complexes [MCb(L Bipy-n)] (M = Mn2+, C02+ and n = 16; M = Ni2+, Zn2+ and n = 10, 12, 14, 16) exhibit mesomorphic character and again generate columnar mesophases. Finally, Chapter 4 begins with a discussion on pyrrole-derived liquid crystals. Consequently, the synthesis and characterisation of hexacatenar d compoun s [M(LDipH/)] \: (M -- 2H ,Zn 2+, Pd2+.' n = 10 ,_l', 1'1"t o 16', x. = 1, ,-), tetracatenar compounds tc_[M(LDip~-16)1\ (M = 2H, Zn2+, Pd2T ; x = 1, 2) and extended dicatenar compounds ex_[M(LDipy-n)]x (M = 2H, Zn2+, Pd2+; x = 1. 2) are described. Characterisation by X-ray diffraction of single crystals of [Zn(LDipy-l)h, ex-[Zn(LDipy-l)h show they exhibit a distorted tetrahedral geometry, forming double stranded helical structures, while ex-[Pd(L Dipyl)] has a distorted square planar geometry. The metal-free ligands H2LDip~-1/ (n = 10, 12, 14, 16) and complexes [Zn(LDipy-16)h and [Pd(LDipy-n)] (n = 12, 14, 16) all exhibit narrow mesomorphic temperature ranges and unidentified mesophases. The tetracatenar compound tc_[Zn(LDipy-16)h generates a columnar hexagonal mesophase and the complex tc_[Pd(LDipy-16)] generates an unidentified liquid crystalline phase, whereas the metal-free ligand tc-H2L Dipy-16 has no mesomorphic character. Finally, two of the extended dicatenar compounds ex-H 2L DipY-16 and ex-[Zn(L Dipy-16)h are non-mesomorphic, while ex-[Pd(L Dipy-16)] was found to have a smectic A phase.

In this thesis, a hybrid photorefractive liquid crystal cell structure with the addition of a thin 40nm Gold layer is proposed that demonstrates significant photorefractive control of Surface Plasmon Polaritons (SPP). The photorefractive effects are generated through optically controlling the conductivity of a ~100nm photoconducting poly-N-vinyl-carboxyl (PVK) layer. Therefore, when a potential is applied across the cell, the liquid crystal alignment and the SPP wavevector is able to be controlled with light. The aim for developing this device is for the eventual demonstration of SPP gain to offset the high optical losses and increase the characteristically short propagation length of SPP. The mechanism we intend to use to demonstrate gain is analogous to the asymmetric energy transfer in a wave mixing system for two laser beams used to typically characterise photorefractive materials. We first characterise the electrical and optical behaviour of the novel photorefractive plasmonic structure proposed with uniform illumination. Our system demonstrates a good photorefractive wavevector shift of 0.207μm-1 for a 1.24eV SPP; this shift is in excess of the FWHM of the SPP resonance in the attenuated total reflection spectrum (0.154μm-1). However, the electric behaviour of the system is found to be highly complex and cannot be fully characterised by an equivalent electrical circuit. In addition, due to electronic stability issues, we require a slow AC potential to demonstrate consistent photorefractive effects. In a step towards realising SPP gain, we then consider the SPP interaction with a refractive index grating written into the liquid crystal layer with the interference pattern of crossed laser beams. We find that a SPP is diffracted into additional SPP modes. Our investigation then determines the ideal parameters that maximise the energy transfer by examining the diffraction efficiency dependence of each variable of the system. The maximum energy transfer observed is 25.3±2.3% for a 1.05eV SPP from a 4μm grating. With the assistance of a numerical simulation of our system we present a series of qualitative and semi-analytical descriptions to describe the mechanisms behind the observed trends. We discover that the diffraction efficiency is dependent of three important effects; the orientation of the grating, the penetration depth of the SPP into the liquid crystal and the magnitude of the periodic electric field in the liquid crystal. In addition, to fully describe the quantitative values observed we must also consider the presence of a thin 100nm region of the liquid crystal near the photoconductor interface that does not strongly respond to the applied electric field due to anchoring forces

- / -

This thesis consists of five studies on the applications of the molecular field theory to model systems of biaxial molecules which form biaxial and uniaxial nematic and smectic A phases. The first study extends the original theory for biaxial nematic phases of D2h symmetry to allow the phase symmetry to be C2h. In the second study, a dipolar interaction is introduced to the original model of biaxial nematic phases formed from V-shaped molecules to explain a disagreement between theory and experiment. This leads to the stabilisation of the novel polar biaxial nematic phase. In the third study, we introduce molecular flexibility at a simplified level into an existing model of V-shaped molecules to investigate its effects on the stability of the biaxial nematic phases. The fourth study aims to explain and predict various effects of magnetic field on the uniaxial nematic to isotropic phase transition for a system of rigid V-shaped molecules. In the fifth study, we develop a model for biaxial smectic A phases. The theory is simplified by using several approximations which facilitates the calculations.

The behavior of colloidal liquid crystals in confinement is addressed on the single particle level using laser scanning confocal microscopy. We seek to disentangle how equilibrium director fields are controlled by the complex interplay between confinement, elasticity and surface anchoring. First, we study the nematic phase confined to wedge structured channels. Varying the wedge opening angle leads to a splay to bend transition mediated by a defect in the bulk of the wedge. Our results are in quantitative agreement with lattice Boltzmann simulations, and we show that comparison between experiments and simulation yields a new method to obtain the splay-to-bend elasticity ratios of colloidal and biological liquid crystals. Next, we extend our study of the wedge structured channels to the cholesteric phase, and measure a splay to twist transition with increasing wedge angle. We directly visualise the 3D nature of the twisted state, and explain how the transition is intricately determined by the anchoring strength and the splay, bend, and twist elasticities. Next, we investigate the effect of rectangular confinement on the nematic phase. The rectangle aspect ratio is systematically varied and we observe five distinct director fields. Comparison with computations of the Frank-Oseen energies yields the extrapolation length, which we find to be of the order of the rod length. Next, we confine the nematic phase to annular geometries of varying dimensions, and observe the novel director fields that are adopted. We approach a level of confinement which is of the order of the particle size. Interpreting our observations with Monte Carlo simulations, which take into account the finite size of the particles, illuminates the applicability of continuum theories down to microscopic lengthscales. We finish with a study of the isotropic-nematic interface in bulk and confinement. We show that parallel anchoring occurs at the interface, and measure the width of the interface to be of the order of the rod length.

Flexoelectricity in liquid crystals is thought to be due to a coupling between dielectric properties and shape anisotropy of the molecules and described by the fiexoelectric coefficients el and e3. Two experiments are needed to measure el and e3 and it is usual to measure the difference (el - e3) and the sum (el + e3) and then calculate el and e3· The first experiment to measure the difference (el - e3) uses a TN structure with an in-plane applied electric field. Due to the dielectric coupling, the director aligns with the electric field and due to the fiexoelectric effect, the director tilts out of plane. This tilt is measured optically using two laser beams at oblique incidence, e.g. 45°. Using a theoretical model the experimental data is fitted and the difference (el - e3) extracted. The second experiment to measure the sum (el + e3) uses a Pi cell. Applying an ac voltage the transmission through the device is a repeating oscillating signal which contains 1st and 2nd harmonics. The 1st harmonic corresponds to the fiexoelectric effect and the 2nd harmonic to the dielectric effect. Using a lock-in amplifier, the harmonics were measured and the sum (el + e3) extracted using a theoretical model to fit the experimental data. Unfortunately, the data proved the experiment to be unreliable and another method was developed, which uses a BAN cell. The third experiment uses simple pulses in a BAN cell and also measures the sum (el + e3). The big disadvantage of the BAN cell is an internal voltage, which is created by the homeotropic alignment layer and the fiexoelectric polarisation. The internal voltage has the same effect on the director profile as the fiexoelectric effect, which is a big problem in measuring fiexoelectricity. Using a material, which is non ionic and has no fiexoelectricity, the internal bias could be measured and taken into account. Applying short de pulses of opposite sign, the fiexoelectric effect can be observed by the optical response and can be measured. Using these experiments, a number of investigation are being carried out such as the correlation between fiexoelectricity and the molecular structure, ions, elastic properties, molecular orientation, dielectric anisotropy 6E, and order parameter S. The results showed that fiexoelectricity only depends on ions and dielectric properties which was very interesting and surprising at the same time.

Les cristaux liquides ferroélectriques (CLF), photosensibles, sont susceptibles de générer des charges de surfaces s’ils sont soumis à un champ électrique ; cette propriété est mise à profit dans les dispositifs d’affichages à cadence rapide. Les CLF utilisés dans ce travail présentent une phase smectique C* ferroélectrique ; l’un est un mélange commercial utilisé comme matériau hôte, l’autre, un dérivé azobenzénique photosensible utilisé comme dopant (1, 3, 5%). Les propriétés optiques et ferroélectriques ont été étudiées en fonction de la concentration, de la température et du champ électrique, sans et sous illumination UV (à 365 nm) en phase SmC* ; le dopage et l’illumination engendrent la réduction de l’ordre smectique ; l’illumination UV induit la photo-isomérisation Trans-Cis, la polarisation spontanée diminue avec l’ajout de dopant et l’illumination UV. Des mesures du temps de vol des charges (trous/électrons) photogénérées par impulsion laser- technique développée au laboratoire par ce travail – mettent en évidence un photocourant de type dispersif et ambipolaire, dont l’intensité varie avec le champ électrique et la température. L’influence du champ interne propre à la phase SmC se traduit sur le photocourant par le passage d’intensités négatives vers des intensités positives pour les trous et pour les électrons. Les trous présentent des valeurs de mobilités plus élevées que les électrons. Le mécanisme de conduction, déduit des modèles d’Arrhénius et de Bassler, est un déplacement de charge par sauts successifs entre molécules voisines. Pour les mélanges, le dopant réduit le temps de transit des charges et conduit à des valeurs de mobilités élevées et constantes avec la température ; le dopant agit en renforçant la cohésion des couches smectiques, et réduit la viscosité du mélange. En configuration Cis le signal conserve son caractère ambipolaire et dispersif mais tend à s’atténuer avec le dopage et à disparaître sous UV. / Ferroelectrical materials possess the remarkable property to orienting themselves and generating surfaces charges under electric field. Dipole movements can be observed by measuring internal electric field or spontaneous polarization under alternative electric field. These organic materials are widely used in fast display devices. The aim of this work was to propose a direct current measurement of photoconduction phenomenon in ferroelectric non-photosensitive and photosensitive liquid crystals by charge carrier transport in SSFLC devices using time of flight methodology. The proposed method is based on charge carrier transport determination which providing access to characteristics parameters like mobility. Differentprocedures have been presented for mobility estimation from adding increasing dopant concentrations, electrical field and temperature, as well as charge behaviour pattern for exploitation and description. This materials show real influence of several parameters on the photocurrent shape signal and different mobility values under large scales of temperature. The validation of the method on photosensitive ferroelectric materials and obtained results by data treatment show that this method can be used to characterize another liquid crystal whatever its phase including SmC*.

- / -

- / -

Pulsed laser deposition (PLD) is a quick, versatile technique for crystal film growth. Multi-beam PLD extends the basic PLD setup to include multiple lasers and targets, and has shown immense promise for the engineering of custom crystal films and structures. The full potential of the technique has not, however, yet been fully explored. The experiments in this thesis have been designed to investigate, extend and improve multibeam PLD, to provide new avenues for fabrication of sophisticated designer films and to find applications that truly exploit the technique's potential. The effect of relative delay between plasma plumes on crystal properties was investigated and was found to influence lattice constant for delay values of ≤400 μs due to higher energy ion bombardment of the growing film. The shutter technique for multibeam crystal engineering was developed and demonstrated via the automated growth of garnet mixed films, superlattices and chirped structures. The method was used to grow crystalline garnet Bragg reflectors with a range of designs, including π-phase shifted and quarter-wave stacks with up to 145 layers and 99% peak reflectivity. A Gaussian profile, grating-strength apodised Bragg stack was grown, with the resulting reduction in side band reflections observed as expected. This represents the first known example of such sophisticated crystal engineering by PLD. Routes to using single- and multi-beam PLD for rapid prototyping of laser crystals were also explored. Double-clad crystalline channel waveguides were fabricated via physical micromachining of PLD-grown garnet multilayer films and subsequent overgrowth. Millimetre-sized crystalline features were grown via single-beam PLD through shadow masks and funnels, culminating in fabrication of a hybrid garnet crystal by a combination of multi-beam PLD and plume funnelling. The results in this thesis represent steps towards the true exploitation of PLD, but much is still to be done. Many routes for future improvement have been suggested, building on the novel techniques developed for this thesis, including the growth of sesquioxides as component layers of functional Bragg stacks capable of withstanding high powers and temperatures.