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We report on observations of optical switching in bent-core liquid crystals possessing azocinnamoyl groups in both elongated side arms. UV-activated switching was observed in two of the synthesised materials, both of which were studied optically and with dielectric spectroscopy. Polarising microscopy was used to measure the changes in optical transmittance, and while no illumination effect was seen in the nematic phase, visible changes were observed in the smectic phase, most pronounced in the close vicinity of the smectic to nematic-phase transition. Switching dynamics was characterised, and we found that the average switching time varies strongly with temperature. The relaxation process is faster (below (Formula presented.)) and exhibits no significant temperature dependence. Dielectric measurements have shown that the observed smectic C phases were not polar with the molecules possessing a rather small electric dipole moment. Upon illumination, small differences in the dielectric permittivity were observed only in the smectic phase, suggesting that the illumination effect can be attributed to changes in the molecular conformational and orientational order.

Experimental and theoretical investigations of the twist-bend nematic (NTB) phase in liquid crystals have recently received significant attention for reasons of unusual and interesting characteristics of the NTB. Though NTB was predicted to exist for rigid bent-core LCs by Ivan Dozov in 2001, nevertheless it has unambiguously been proven to exist only in a few bent-core systems so far. Here the characteristics of both the nematic (N) and NTB phases are investigated for a hockey shaped achiral rigid bent-core LC, called BCI, in planar and homeotropic aligned cells, using polarizing optical microscopy, electro-optics, pyroelectricity and wide band dielectric spectroscopy. Spontaneous polarization measured for a bias field of 2.2 V/μm of a chiral domain using the pyroelectric effect is found to be only ∼5 nC/cm2. A measurement of the spontaneous polarization using pyroelectricity in general is unaffected by the sample’s dc conductivity except when its conductance dominates the admittance. As helix of NTB is partially unwound by the field, results lead to the identification of NTB as polar and chiral. The first and the second harmonics of the applied field are observed of higher amplitudes in NTB and N phases, respectively. The hockey-shaped bent-core system exhibits a large negative dielectric anisotropy over a wide range of temperatures than observed for any other compound so far. The large negative dielectric anisotropy is the highly desirable characteristic parameter of the NTB for exploring the field induced phases at relatively lower field strengths. The orientational order parameter characteristically jumps at the N-NTB transition temperature. The complex dielectric permittivity is measured as a function of frequency in the range 1 Hz to 10 MHz, the analysis of results reveales two collective modes in the dielectric spectra. Amplitude of the lower frequency mode is much higher than of higher frequency, former is assigned to the hydrodynamic mode q=qzz^ with z-dependent rotation of the heloconical director n^(r), this in turn involves compression and dilation of the pseudo-layers. The higher frequency mode corresponds to fluctuations of the tilt director and closer to the transition temperature, it exhibits a typical soft mode characteristic feature.

A series of bent-core liquid crystals possessing the azocinnamoyl unit in both elongating side arms was synthesized. The chain length was kept fixed for each molecule (C12H25), whereas the substituents at the central and outer rings were varied. The LC phases were assigned by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. The investigated compounds are suitably diverse to reveal some aspects of the relationship between molecular structure and the mesomorphic properties. Namely, non-substituted parent compound is crystalline only and the methyl group in position 2 or the chlorine atom in position 4 of the central ring suppresses LC phase formation. Introduction of the strong electron-withdrawing nitro group between the side arms on the central ring leads to a B7 phase. Compounds possessing a bromine atom or two chlorine atoms in the neighbourhood of the ester groups form LC phases typical for rod-like molecules, namely a nematic – smectic phase sequence. The results are compared with those reported for the azobenzoyl analogues.

Photosensitive liquid crystals represent an important class of functional materials that experience rapid development. Hereby, we present novel bent-core liquid crystals bearing a lateral substitution on the central core and in the vicinity of the photosensitive unit—an azo group. The azo group enables fast (E)-to-(Z)-isomerization upon irradiation with UV-light and visible light, while the substitution facilitates the high stability of the photochemically formed (Z)-isomer. The effectiveness of the irradiation and the composition of photostationary states was determined by UV/Vis and 1H NMR spectroscopy. A nematic phase formed by the materials was characterized by differential scanning calorimetry and optical polarizing microscopy. We show that the materials easily change their relative configuration of the N=N double bond not only in solution, but also in the mesophase, which leads to fast isothermal phase transition from the nematic phase to isotropic liquid.

We present small-angle X-ray scattering, polarized optical microscopy and electric current measurements of a sulfur-containing bent-core liquid crystal material for characterization of the layer and director structures, thermally and electrically driven transitions between antiferroelectric and ferroelectric structures and switching properties. It was found that the material has polarization-modulated homochiral synclinic ferroelectric (SmCsPFmod), homochiral anticlinic antiferroelectric (SmCaPA) and racemic synclininc antiferroelectric (SmCsPA) structures that can be reversibly switched between each other either thermally and/or electrically. High switching polarization combined with softness of the liquid crystalline structure makes this compound a good candidate for applications in high-power capacitors and electrocaloric devices.

A series of asymmetric bent-core liquid crystals, bearing the stilbene moiety in the side wing, was synthesised and characterised. Structural modifications of the non-stilbene wing resulted in formation of various mesophases. Compounds 1 and 3 with an ester linking group between the phenyl rings in this wing have an enantiotropic non-polar smectic-type mesophase, while compound 2, incorporating the biphenyl moiety, has a ferroelectric SmCaPF phase. The molecules of 2 undergo photoisomerisation in solution under UV irradiation, while the mesophases are unstable and UV irradiation leads to irreversible changes. The mesophases of 1 and 3 do not show any electro-optical switching and polarisation current. Applying low frequency AC voltage on a planarly aligned film of compound 2, smectic domains grow, while the birefringence and an image flickering increase. The observed features of these compounds identify stilbene as an attractive moiety in the design of advanced materials based on bent-core molecules.

Two homologue achiral bent-core liquid crystals (LCs), BCN66 and BCN84, in their nematic phases are studied by dielectric spectroscopy in the frequency range 10 Hz–10 MHz. In each of these compounds, two relaxation processes are identified and assigned to (i) collective dynamics of molecules in nanometer-sized cybotactic clusters and (ii) individual molecular relaxations, in the ascending order of frequency of the probe field. The temperature and the bias electric field dependence of the dielectric strength and relaxation frequency for these processes are shown to give rise to sharpness in cluster boundaries, increased size and volume fraction in the LC nematic phase. The effect of the bias field on the LC cell is similar to reducing its temperature; both variables increase the cluster size and volume fraction and give rise to sharp cluster boundaries. The findings confirm that dielectric spectroscopy is a powerful and an extremely useful technique to provide a deeper understanding of the mechanism of cybotactic cluster formation in the isotropic liquid and the nematic phase of LCs as a function of temperature and the bias field.

We report on the synthesis and characterization of bent‐core liquid crystal (LC) compounds and the preparation of mixtures that provide an optically isotropic antiferroelectric (OI‐AFLC) liquid crystal display mode over a very wide temperature interval and well below room temperature. From the collection of compounds synthesized during this study, we recognized that several ternary mixtures displayed a modulated SmCaPA phase down to below −40 °C and up to about 100 °C on both heating and cooling, as well as optical tilt angles in the transformed state of approximately 45° (optically isotropic state). The materials were fully characterized and their liquid crystal as well as electro‐optical properties analyzed by polarized optical microscopy, differential scanning calorimetry, synchrotron X‐ray diffraction, dielectric spectroscopy, and electro‐optical tests.

Two isomeric cyanostilbene photoswitchable bent-core mesogens with polar liquid crystal phases in which macroscopic polarization and luminescence can be light-modulated are introduced. Z/E isomerization or [2+2] cycloaddition photochemical processes occur depending on the chemical structure, which make the compounds very innovative multifunctional advanced materials., This work was financially supported by MICINN-FEDER and MINECO-FEDER of Spain-UE (Project Nos. MAT2011-27978-C02-02, MAT2012-38538-C03-01, MAT2012-38538-C03-02, and MAT2015-66208-C3-1-P) and the Aragón Government and FSE (Project E04), and the JAE Predoc-CSIC program (fellowship to M. Martínez-Abadia) and the Gobierno Vasco (GI/IT-449-10).

Azo-functionalized materials are one of the appealing groups of the functionalized materials owing to their photoswitching behaviour and have been explored for various potential applications viz., optical data storage, sensor, display devices, nonlinear materials and molecular switches. Recently, azo-functionalized bent-core liquid crystals (BCLCs) have gained significant attention because they have dual properties of BCLCs and azobenzene, which enables to generate new multifaceted functional and smart materials. In this report, the recently synthesized azobenzene containing bent-core mesogens and its subclass, the so-called hockey stick and V-shaped molecules are summarized. The mesomorphic behaviour of reported BCLCs affected by the type of central core unit, the nature, number and position of the lateral substituents and the type and length of the terminal chain are discussed. The photoisomerization process of these photoresponsive BCLCs in solid, solution and mesophase, as well as the impact of light on the chemical and electrical properties of them, are discussed.

The splay, twist, and bend elastic constants of an oxadiazole based bent-core liquid crystal have been measured as functions of temperature throughout the nematic phase. The splay elastic constant (K11) behaves similarly to other bent-core systems, though it exhibits rather high values, varying between 8–14 pN. Further, anomalously low, temperature-independent values of both the twist and bend constants (K22 and K33) are found. The combination of such low values of both constants (∼0.15 pN and ∼1.2 pN) and temperature independence has not been observed in any type of nematic material previously. This unusual behaviour has been further investigated using two independent theoretical approaches; a molecular theory based on atomistic simulations of the real molecular structure and an analytical molecular-field theory. Computational calculations show good agreement with the experimental results for both the twist and bend constants. The statistical theory introduces a term related to chiral fluctuations that affects the twist constant, while the inclusion of polarity is known to explain low values of the bend constant. The additional terms enable one to understand the origin of the low values of both elastic constants and to describe their weak temperature dependence. It also offers an explanation for the spontaneous chiral segregation observed in some nematic bent-core systems. Mixtures of this unusual oxadiazole material are made with another similar material (C5-Ph-ODBP-Ph-OC12) that has been extensively studied that exhibits somewhat higher twist and bend elastic constants to explore the behaviour as a function of concentration.

A presente tese descreve a síntese e caracterização de 10 novas séries de moléculas na forma de banana contendo os anéis isoxazolina e isoxazol como reais candidatos na preparação de cristais líquidos polares. Foram avaliados parâmetros estruturais tais como o tipo de função conectora do centro curvado com os braços mesogênicos, a natureza do heterociclo e a sua posição relativa ao núcleo. A síntese dos compostos contou com a metodologia de preparação do anel isoxazolina, a cicloadição [3+2] 1,3-dipolar entre alcenos e óxidos de nitrila, os quais foram gerados pelas oximas preparadas a partir de aldeídos alifáticos e aromáticos. Todas as isoxazolinas foram oxidadas aos seus respectivos isoxazóis utilizando dióxido de manganês. Foram utilizadas ainda reações de alquilação, redução, desproteção, hidrogenólise, olefinação, adição de aminas à aldeídos e esterificação. Todas as moléculas sintetizadas foram caracterizadas por RMN de 1H e 13C, além de serem observadas por MOLP para determinação dos seus pontos de fusão. As moléculas que apresentaram comportamento mesomórfico foram ainda caracterizadas por DSC, XRD e voltagem triangular. No capítulo 3 é descrita a síntese dos isoftalatos 19a-b, 20a-b, 26a-b e 27a-b. No subgrupo dos Isoftalatos derivados de isoxazóis e isoxazolinas 3,5-diarilsubstituídos foi possível a síntese apenas do composto 11d que não apresentou comportamento líquido cristalino No subgrupo dos isoftalatos derivados de isoxazóis e isoxazolinas 3-alquil-5-arilsubstituídos as isoxazolinas 19a-b não apresentaram comportamento líquido-cristalino. Foram encontradas mesofases para os compostos 20a-b que ainda não foram determinadas com as técnicas disponíveis. Os isoxazóis 20a-b apresentaram uma provável Blue Phase logo após o resfriamento do isotrópico, que rapidamente se converte em uma provável mesofase monotrópica ferroelétrica com texturas e padrão de XRD peculiares até então não observada na literatura. No subgrupo dos isoftalatos derivados de isoxazóis e isoxazolinas 3-aril-5-alquilsubstituídos 26a-b e 27a-b não foi observado comportamento líquido-cristalino. O capítulo 4 descreve a síntese das isoftaliminas 35, 36, 44, 45, 50a-b, 51a-b, 56a-d, 57a-d, 63a-b e 64a-b. No subgrupo dos isoxazóis e isoxazolinas 3,5-diarilsubstituídos 35, 36, 44 e 45 foram observadas as mesofases B7 e B1 apenas com a inversão da orientação do anel isoxazol como braçomesogênicos dos compostos finais, enquanto que as isoxazolinas não apresentaram mesofases. A mesofase B7 do composto 36 apresentou comportamento antiferroelétrico enquanto que a mesofase B1 do composto 45 não mostrou resposta frente ao campo elétrico aplicado, além disso, o XRD mostrou que tal mesofase colunar B1 pode ser uma fase 3D modulada. No subgrupo das isoxazolinas e isoxazóis 3-alquil-5-arilsubstituídos 50a-b e 51a-b foram observadas fases do tipo DC para as isoxazolinas 50a-b, a qual deve ser confirmada por FFTEM. No subgrupo dos materiais 3-aril-5-alquilsubstituídos 56a-d, 57a-d, 63a-b e 64a-b foram observadas mesofases SmXPF para os compostos 56b-d. Os isoxazóis 57a-d apresentaram texturas similares, porém não apresentaram mesofase, mas os mesmos seguem o mesmo padrão de difração das isoxazolinas do capítulo 3. Apenas a isoxazolina 63b dos compostos perfluorados apresentou mesofase No capítulo 5 é descrita a síntese de ésteres não-simétricos contendo os heterociclos isoxazol e isoxazolinas como núcleo central 71a-f, 82, 83, 84 e 85. No primeiro subgrupo todos os ésteres cinâmicos 71a-f apresentaram comportamento mesomórfico com grandes faixas de temperaturas nas mesofases. No segundo subgrupo as isoxazolinas 82 e 83 apresentaram comportamento completamente distinto, onde só foi observada a formação de uma mesofase SmB para o composto que tem a posição éster localizada na direção do nitrogênio do heterociclo. Já os isoxazóis 84 e 85 deste subgrupo apresentaram as mesofases N e SmC em temperaturas bastante similares, porém, a observação desses materiais em uma cela alinhada revelou o crescimento de filamentos na transição N-SmC apenas para o composto 85, o qual também possui a porção éster na direção do nitrogênio do anel isoxazol. O capítulo 6 traz a síntese dos ésteres e iminas simétricos 88, 89, 94 e 95 utilizando os heterociclos como núcleo central, os quais apresentaram mesofases SmC e N. Além disso, o diéster 89 derivado de isoxazol apresentou a mesma característica que o composto 85 do capítulo 5, apresentando uma transição N-SmC com crescimento de filamentos perpendiculares à direção de alinhamento da amostra dentro da cela, podendo estar relacionada à uma mesofase NTB. This thesis describe the synthesis and characterization of 10 new series of banana shaped molecules containing the isoxazoline and isoxazole rings as real players for preparation of polar liquid crystals. It was evaluate structural parameters such as the type of connecting function of the bent core with the mesogenic arms, the heterocycle nature and its position relative to the central core. The synthesis of the compounds included the methodology of preparation of the isoxazoline ring, the [3 + 2] 1,3-dipolar cycloaddition between alkenes and nitrile oxides, which were generated by the oximes prepared from aliphatic and aromatic aldehydes. Furthermore, all isoxazolines were oxidized to its respective isoxazoles using manganese dioxide. Besides the described methodologies, alkylation, reduction, deprotection, hydrogenolysis, olefination, addition of amines to the aldehydes and esterification reactions were used. All the synthesized molecules were characterized by 1H NMR and 13C NMR, and also observed by POM for determination of its melting points. The molecules previously analyzed by POM that showed mesomorphic behavior were characterized by DSC, XRD and triangular voltage still. Chapter 3 describes the synthesis of isophthalates 19a-b, 20a-b, 26a-b e 27a-b. In the subgroup of isophthalates derived from isoxazoles and isoxazolines 3,5-diarylsubstituted it was possible to synthesize only compound 11d which did not show liquid crystalline behavior In the subgroup of the isophthalates derived from isoxazoles and isoxazolines 3-alkyl-5-arylsubstituted the isoxazolines 19a-b did not show liquid crystalline behavior. It was found mesophases for compounds 20a-b that still could not be determined with the available techniques. The isoxazoles 20a-b presented a probable Blue Phase soon after cooling from the isotropic which quickly converts into a probable ferroelectric monotropic mesophase with peculiar textures and pattern of XRD until then not observed in the literature. In the subgroup of the isophthalates of isoxazoles and isoxazolines 3-aryl-5-alkylsubstituted 26a-b and 27a-b no liquid crystalline behavior was observed. Chapter 4 describes the synthesis of isophthalimines 35, 36, 44, 45, 50a-b, 51a-b, 56a-d, 57a-d, 63a-b and 64a-b. In the subgroup of the isoxazoles 3,5-diarylsubstituted 35, 36, 44 and 45 the B7 and B1 mesophases were observed only with the inversion of the isoxazole ring orientation as mesogenic arm of thefinal compounds whereas the isoxazolines did not present mesophases. The B7 mesophase of the compound 36 showed antiferroelectric switching while the B1 mesophase of the compound 45 showed no response to the applied electric field, in addition, the XRD showed that such B1 columnar mesophase could be a 3D modulated phase one. In the subgroup of 3-alkyl-5-arylsubstituted isoxazolines and isoxazoles 50a-b and 51a-b DC phases were observed for isoxazolines 50a-b which should be confirmed by FFTEM. The SmXPF mesophase were observed for componds 56b-d in the subgroup of 3-aryl-5-alkylsubstituted materials 56a-d, 57a-d, 63a-b e 64a-b. The isoxazoles 57a-d presented similar textures although did not showed mesophase, but they follow the same diffraction pattern of the chapter 3 isoxazolines. Only the isoxazoline 63b of the perfluorinated compounds showed mesophase In chapter 5 is described the synthesis of non-symmetric esters containing isoxazol and isoxazolines heterocycles as central core 71a-f, 82, 83, 84 e 85. In the first subgroup all the cinnamic esters 71a-f showed mesomorphic behavior with large mesophases temperature ranges. In the second subgroup the isoxazolines 82 and 83 showed completely different behavior which only was observed the SmB mesophase formation for compound having the ester position towards the heterocycle nitrogen. Furthermore, the isoxazoles 84 and 85 of this subgroup showed the N and SmC mesophases at very similar temperatures, however, the observation of these materials in an aligned cell revealed the filamentary growth in the N-SmC transition only for compound 85, which also has the ester moiety in the direction of the isoxazole ring nitrogen. Chapter 6 brings forward the synthesis of symmetrical esters and imines 88, 89, 94 and 95 using the heterocycles as the central cores, which showed SmC and N mesophases. Moreover, the isoxazole derived diester 89 showed the same feature as compound 85 of Chapter 5, exhibiting a N-SmC transition with filament growth perpendicular to the rubbing direction of the cell which may be related to a NTB mesophase.

We have previously reported the first realization of an orthogonal ferroelectric bent-core SmAPF phase by directed design in mesogens with a single tricarbosilane-terminated alkoxy tail. Given the potentially useful electrooptic properties of this phase, including analog phase-only electrooptic index modulation with optical latching, we have been exploring its “structure space”, searching for novel SmAPF mesogens. Here, we report two classes of these—the first designed to optimize the dynamic range of the index modulation in parallel-aligned cells by lowering the bend angle of the rigid core, and the second expanding the structure space of the phase by replacing the tricarbosilane-terminated alkyl tail with a polyfluorinated polyethylene glycol oligomer.

Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (NTB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the NTB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the NTB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1″,7″-bis-4-(4′-cyanobiphenyl-4′-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase.

In the thesis we study the structure and response of bent-core liquid crystals in the orthogonal ferroelectric Smectic-A (SmAP$_F$) phase in thin planar cells. We construct a phenomenological continuum model to study the structure in thin planar cells. A set of molecules within a small volume is presented by the director ($vec{n}$), which defines the average direction of the long molecular axes within this volume, and by the polar director ($vec{p}$), which points in the direction of local polarization. We choose a geometry in which the director in the smectic layer is constant and the polar director varies across the cell. The polar director structure inside the cell is determined by a competition among the torques due to the bulk elasticity, electrostatic effects and surface anchoring. The equilibrium profile structure of the polar director is obtained by minimization of the free energy. We find the polar director profile in a cell as a function of the type and strength of the surface anchoring, bulk elastic constants and cell thickness. The effect of the external electric bias field on the structure in the cell is studied, as well. Bent-core molecules have a permanent electric dipole moment. In the external electric field the electric torque tends to rotate the dipoles in the direction of the external field. The polar director profile in the cell in external bias field thus depends on the competition among three effects: anchoring at the surfaces, the elastic properties of the bulk and the influence of the external electric field. By the rotation of molecules in the external electric field, optical properties of the cell are changed, therefore the SmAP$_F$ phase is a promising phase for use in displays with high response time, high contrast, continuous gray level and wide viewing angle. The response of the SmAP$_F$ phase to an alternating external electric field (the dielectric response) is also considered. The dielectric response of the SmAP$_F$ phase consists of two modes: the phase and amplitude mode. The phase mode is due to fluctuations in the orientation of the local direction of the spontaneous polarization and the amplitude mode is due to the change in the magnitude of spontaneous polarization. The frequency of the phase and amplitude mode and the dielectric permittivity are calculated numerically as a function of the bias external DC electric field, cell thickness, the type and strength of surface anchoring and the ratio between the bend and splay elastic constants. Analytical solution for a very specific case of chosen parameters is also obtained. Theoretically obtained dependencies are in agreement with the reported experimental measurements. In the thesis we study the effect of different types of surface anchoring and for this purpose three different types of cells are defined. The polar director profile and the dielectric response is calculated in all three types of cells. The type I cell has polar surface anchoring of equal strengths at both surfaces. The type II cell has, in addition to the polar surface anchoring of equal strengths at both surfaces, a nonpolar anchoring at the bottom surface. The type III cell has polar surface anchoring at both surfaces but not of equal strengths. We predict that by comparison of the dielectric response of the SmAP$_F$ phase in all three types of cells the type of the polarization splay in bent-core liquid crystals can be determined. The thesis is divided into three parts. In the first part the basic physical properties of ferroelectric liquid crystals, focusing on the bent-core liquid crystals, are discussed. In the second part phenomenological theoretical model is developed. In the third part of the thesis a dielectric response in external bias field is studied. V doktorski disertaciji obravnavamo strukturo in dielektrični odziv tekočih kristalov z ukrivljeno sredico v feroelektrični smetični-A (SmAP$_F$) fazi v tankih planarnih celicah. Za preučevanje struktur v tankih planarnih celicah zgradimo minimalni fenomenološki kontinuumski model. Skupek molekul v majhni prostornini opišemo z direktorjem ($vec{n}$), ki določa povprečno usmerjenost vseh dolgih osi molekul v skupku, in s polarnim direktorjem ($vec{p}$), ki kaže v smeri lokalne polarizacije. V izbrani geometriji je direktor v smektičnih plasteh konstanten, spreminja se le smer polarnega direktorja. Ravnovesno strukturo polarnega direktorja v celici določa tekmovanje med navori zaradi elastičnosti, elektrostatičnega učinkom in površinskega sidranja. Dobimo jo z minimizacijo proste energije. Strukturo polarnega direktorja v celici poiščemo v odvisnosti od vrste in jakosti površinskega sidranja, velikosti elastičnih konstant ter od debeline celice. Obravnavamo tudi strukturo krajevnega ureditvenega parametra v celici v odvisnosti od jakosti konstantnega zunanjega električnega polja. Ukrivljene molekule tekočih kristalov imajo permanentni električni dipol. V zunanjem električnem polju na dipol deluje navor, ki želi zavrteti dipol v smeri polja. V tem primeru je krajevna odvisnost polarnega direktorja odvisna od površinskega sidranja, elastičnih lastnosti materiala in jakosti polja. Z vrtenjem dipolov v zunanjem električnem polju se spremenijo optične lastnosti tekočekristalne celice, zato je SmAP$_F$ faza obetavna za uporabo v zaslonih s hitrim odzivnim časom, visoko ločljivostjo, dobrim kontrastom, zvezno sivo skalo ter širokim vidnim kotom. Še posebej nas zanima dielektrični odziv SmAP$_F$ faze. Z izmeničnim zunanjim poljem na tekoči kristal delujemo na dva načina: zanihamo smer lokalne polarizacije, torej smer polarnega direktorja (fazni način nihanja), in lokalno večamo in manjšamo stopnjo urojenosti električnih dipolov ter s tem velikost lokalne polarizacije (amplitudni način nihanja). Z numeričnim pristopom raziščemo, kako jakost električnega polja, debelina celice, jakost ter vrsta površinskega sidranja in razmerje velikosti med elastičnima konstantama za upogibno in pahljačasto deformacijo, vplivajo na frekvenco faznega in amplitudnega načina nihanja. Za specifično izbrane parametre izračunamo tudi analitično rešitev. Napovedi teoretičnega modela se ujemajo z rezultati obstoječih eksperimentalnih meritev. V disertaciji preučimo tudi učinek različnih vrst sidranj na površini in zato definiramo tri različne vrste celic. V vseh treh vrstah poiščemo ravnovesno strukturo polarnega direktorja ter dielektrični odziv. V prvi vrsti celice je na obeh površinah celice enako močno polarno sidranje. V drugi vrsti dodamo poleg enako močnega polarnega sidranja na obeh površinah še nepolarno sidranje na spodnji površini. V tretji vrsti celice je na obeh površinah polarno sidranje, ki pa ni enako močno. Napovemo, da s primerjavo dielektričnega odziva SmAP$_F$ faze v vseh treh vrstah celic lahko določimo vrsto pahljačaste deformacije spontane polarizacije v tekočih kristalih iz ukrivljenih molekul. Doktorsko delo je sestavljeno iz treh sklopov. V prvem predstavimo osnovne fizikalne lastnosti feroelektričnih tekočih kristalov s poudarkom na lastnostih tekočih kristalov iz ukrivljenih molekul. V drugem sklopu razvijemo fenomenološki kontinuumski model, v tretjem delu pa obravnavamo dielektrični odziv v zunanjem električnem polju.

Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4ʹ-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to ~ 3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to ~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds.

We have previously reported the first realization of an orthogonal ferroelectric bent-core SmAPF phase by directed design in mesogens with a single tricarbosilane-terminated alkoxy tail. Given the potentially useful electrooptic properties of this phase, including analog phase-only electrooptic index modulation with optical latching, we have been exploring its “structure space”, searching for novel SmAPF mesogens. Here, we report two classes of these—the first designed to optimize the dynamic range of the index modulation in parallel-aligned cells by lowering the bend angle of the rigid core, and the second expanding the structure space of the phase by replacing the tricarbosilane-terminated alkyl tail with a polyfluorinated polyethylene glycol oligomer.

The Inside Cover picture highlights how an optically isotropic antiferroelectric liquid‐crystal display mode can be accomplished by using ternary bent‐core liquid crystal mixtures operating from well below room temperature up to 100 °C. More details can be found in the Communication by T. Hegmann and co‐workers on page 196 in Issue 2, 2017 (DOI: 10.1002/open.201600138).

We have determined experimentally the magnitude of the difference in the splay and bend flexoelectric coefficients, |e 1 − e 3|, of an oxadiazole bent-core liquid crystal by measuring the critical voltage for the formation of flexodomains together with their wave number. The coefficient |e 1 − e 3| is found to be a factor of 2–3 times higher than in most conventional calamitic nematic liquid crystals, varying from 8 pCm−1 to 20 pCm−1 across the ∼60 K—wide nematic regime. We have also calculated the individual flexoelectric coefficients e 1 and e 3, with the dipolar and quadrupolar contributions of the bent-core liquid crystal by combining density functional theory calculations with a molecular field approach and atomistic modelling. Interestingly, the magnitude of the bend flexoelectric coefficient is found to be rather small, in contrast to common expectations for bent-core molecules. The calculations are in excellent agreement with the experimental values, offering an insight into how molecular parameters contribute to the flexoelectric coefficients and illustrating a huge potential for the prediction of flexoelectric behaviour in bent-core liquid crystals.

The splay and bend elastic constants of the bent-core oxadiazole material [C5-Ph-ODBP-Ph-OC12] have been investigated as a function of temperature across the nematic phase. The bend constant K-33 is found to take values of similar to 3.0 pN and to be almost temperature independent, whereas, the splay constant K-11 increases monotonically from similar to 3.5 pN close to the isotropic phase transition to values of similar to 9 pN deep in the nematic phase. No pretransitional divergence is observed in either K-11 or K-33 at temperatures approaching the underlying phase. This behavior of the elastic constants is distinct from that observed in rodlike liquid crystal systems but appears to share characteristics with the few other bent-core nematic systems studied to date. We discuss the interdependence of the elastic constants, the birefringence, and the order parameter to allow a comparison of the observed behavior with theory. We show that calculations of the elastic constants via molecular-field theory and atomistic modeling are in excellent qualitative as well as good quantitative (within 2 pN) agreement with the measurements across the temperature range, offering a deeper understanding of the elasticity in bent-core nematic materials than has been, hitherto, available.

25 pages; International audience; We extend the Landau theory of bent-core mesophases and d-wave high-Tc superconductors by considering additional secondary pseudo-proper order parameters. These systems exhibit a remarkable analogy relating their symmetry groups, list of phases and an infinite set of physical tensors. This analogy lies upon an internal dual structure shared by the two theories. We study the dual operator transforming rotations into translations in liquid crystals, and gauge symmetries into rottions in superconductors. It is used to classify the bent-core line defects, and to analyze the electronic gap structure of lamellar d-wave superfluids.