María Blanca Ros

Cyanostilbene bent-core molecules: a route to functional materials

The synthesis and characterization of novel compounds, that incorporate the high current interest functional cyanostilbene unit, are reported. Four examples, derived from 3,4′-biphenylene, have a 2D molecular-shape that promotes a variety of bent-core liquid crystalline phases (columnar, polar smectic C, and dark-conglomerate phases). The mesomorphism has been characterized by optical microscopy, differential scanning calorimetry, X-ray diffraction, and by detailed electrooptic and dielectric studies. The liquid crystalline properties can be modulated depending on the position of the cyano group on the CC bond (α- or β-isomers) and the number of cyanostilbene units in the molecule (one or two). The polar smectic C mesophases have an antiferroelectric ground state. Furthermore, the multiresponsive nature of the cyanostilbene structure has led to very attractive and diverse photoactivity, including luminescence and second harmonic generation. The results obtained broaden the possibilities for the use of bent-core liquid crystals to design novel multiresponsive soft materials.

Multiresponsive luminescent dicyanodistyrylbenzenes and their photochemistry in solution and in bulk

Multiresponsive luminescent materials with dicyanodistyrylbenzene chromophores incorporated into bent-core and rod-like molecules have been prepared and characterized. Rod-like compounds exhibit nematic or smectic liquid crystal phases, whereas bent-core compounds melt directly to the isotropic liquid at high temperatures. Fluorescence properties are very sensitive to external stimuli such as heat, light, pressure and solvent vapors. All compounds undergo Z/E photoisomerization in solution under UV irradiation. In addition, photochemical activity in bulk has been investigated. Whereas the crystal phases are stable under UV irradiation, the liquid crystal phases are unstable giving rise to Z/E photoisomerization and [2+2]-photocycloaddition products. As a consequence, photoinduced phase changes are observed. This study shows the versatilty of dicyanodistyrylbenzene-based liquid crystals as novel types of photoreactive materials.

On the supramolecular packing of bent-shaped molecules: the influence of the central bent-core on the liquid crystalline behaviour

Two series of bent-shaped molecules have been synthesized and their supramolecular self-organization at variable temperature has been studied. The majority of these bent-compounds form liquid crystalline phases and the mesomorphic behaviour of them has been fully characterized by polarised optical microscopy, calorimetry, X-ray diffraction and dielectric techniques. A series of bis-4-n-tetradecyloxybenzoyloxybenzoates derived from naphthalene, resulting in a change of the bending angle, have proved that when these cores bend around 120°, a polar lamellar packing (SmCaPA) can be induced, whereas when the angle was around 60°, an orthogonal lamellar arrangement (SmA) that responded to an electric field, are found to occur. Liquid-crystal order can also be achieved over a significant range of temperatures by using appropriate bis-4,4′-disubstituted phenylene derivatives (–Ar–X–Ar–). Groups such as –CO–, SO– and –CH2– connecting the aromatic rings induce the SmCaPA phase while a sulfide group (–S–) leads to columnar order. Further, the formation of isotropic textures as chiral conglomerates or a field-induced phase transition have also been observed as unusual properties of the self-organizations of these compounds.

The SmA phase of a bent-core V-shaped compound: structure and electric-field response

The smectic A (SmA) phase of a V-shaped mesogen with a molecular bending angle of 60° was studied. According to X-ray measurements, the molecules are arranged with the ‘bow-string’ direction parallel to the smectic layer. This point was confirmed by birefringence analysis and the result is compatible with a non-polar structure. A detailed study of the (001) smectic layer reflection versus temperature was also carried out to determine the smectic translational order parameter. The results indicate that the material presents a high degree of translational order. In addition, the electric field response of the material was studied by means of second harmonic generation. This technique turned out to be highly sensitive to small structural distortions due to the V-shape of the constituent molecules.

Effect of Molecular Flexibility on the Nematic-to-Isotropic Phase Transition for Highly Biaxial Molecular Non-Symmetric Liquid Crystal Dimers

In this work, a study of the nematic (N)–isotropic (I) phase transition has been made in a series of odd non-symmetric liquid crystal dimers, the α-(4-cyanobiphenyl-4’-yloxy)-ω-(1-pyrenimine-benzylidene-4’-oxy) alkanes, by means of accurate calorimetric and dielectric measurements. These materials are potential candidates to present the elusive biaxial nematic (NB) phase, as they exhibit both molecular biaxiality and flexibility. According to the theory, the uniaxial nematic (NU)–isotropic (I) phase transition is first-order in nature, whereas the NB–I phase transition is second-order. Thus, a fine analysis of the critical behavior of the N–I phase transition would allow us to determine the presence or not of the biaxial nematic phase and understand how the molecular biaxiality and flexibility of these compounds influences the critical behavior of the N–I phase transition.

Insight into the supramolecular organization of columnar assemblies with phototunable chirality

New supramolecular associations with photoaddressable azobenzene units have been synthesized in order to better understand the structure of these complexes and their mesophases. The complexes are formed by a melamine core and three V-shaped acids and they exhibit columnar mesomorphism. The acids consist of two different arms, one derived from azobenzene and the other derived from phenyl benzoate. This design has allowed decreasing Tg values and also avoiding the tendency to crystallize, favouring regular and reproducible structures, together with the possibility of photoresponse. The mesomorphic properties of the complexes have been studied by POM, DSC and X-ray diffraction, showing Colh or Colr mesophases. CD activity of these materials in the mesophase (that reveals chiral structures) has been controlled by irradiation with CPL. XRD and AFM studies have been carried out to check the molecular organization in the films before and after CPL exposure, which confirmed that the mesophase retains its structure after switching the corresponding supramolecular chirality.

Self‐Assembled α‐Cyanostilbenes for Advanced Functional Materials

In the specific context of condensed media, the significant and increasing recent interest in the α-cyanostilbene (CS) motif [uf8ffAruf8ffCHuf8feC(CN)uf8ffAruf8ff] is relevant. These compounds have shown remarkable optical features in addition to interesting electrical properties, and hence they are recognized as very suitable and versatile options for the development of functional materials. This progress report is focused on current and future use of CS structures and molecular assemblies with the aim of exploring and developing for the next generations of functional materials. A critical selection of illustrative materials that contain the CS motif, including relevant subfamilies such as the dicyanodistyrylbenzene and 2,3,3-triphenylacrylonitrile shows how, driven by the self-assembly of CS blocks, a variety of properties, effects, and possibilities for practical applications can be offered to the scientific community, through different rational routes for the elaboration of advanced materials. A survey is provided on the research efforts directed toward promoting the self-assembly of the solid state (polycrystalline solids, thin films, and single crystals), liquid crystals, nanostructures, and gels with multistimuli responsiveness, and applications for sensors, organic light-emitting diodes, organic field effect transistors, organic lasers, solar cells, or bioimaging purposes.

Photoresponsive cyanostilbene bent-core liquid crystals as new materials with light-driven modulated polarization

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.

Influence of the chain length on the nematic-to-isotropic phase transition for the odd members of a highly non-symmetric pyrene-based series of liquid crystal dimers

ABSTRACT This paper describes a detailed study of the nematic (N)-isotropic (I) phase transition in the homologous series of liquid crystal dimers, the α-(4-cyanobiphenyl-4ʹ-oxy)-ω-(1-pyreniminebenzylidene-4ʹ-oxy)alkanes (CBOnO.Py) by means of calorimetric and dielectric measurements as a function of temperature. It is concluded that for this transition, the latent heat or the entropy change decreases as the chain length of the odd dimers decreases, and this decrease is consistent with the observed tricritical behaviour. Graphical Abstract

Influence of internal flexibility on the double glass transition in a series of odd non-symmetric liquid crystal dimers characterised by dielectric measurements

ABSTRACT Dielectric measurements (thermally stimulated depolarisation currents and broadband dielectric spectroscopy) have been performed near the glass transition to study the glass transition on the odd non-symmetric liquid crystal (LC) dimers of the series α-(4-cyanobiphenyl-4-oxy)-ω-(1-pyreniminebenzylidene-4-oxy) alkanes (CBOnO.Py) with n ranging from 3 to 9. A previous study [S. Diez-Berart et al., Materials 8 (2015) 3334] carried out in CBO11O.Py showed the presence of two glass transition temperatures, attributed to different molecular motions of the terminal groups. The study performed allows us to analyse the molecular dynamics in the rest of the series and determine the role played by the flexible spacer. Parallel and perpendicular orientations of the molecular director with regards to the probe electric field have been investigated. The low and intermediate observed relaxations are explained in the framework of Stocchero’s theoretical model [M. Stocchero, J. Chem. Phys. 121 (2004) 8079] for the dielectric behaviour of non-symmetric LCs dimers, as independent end-over-end rotations of each terminal semi-rigid unit. As the length of the spacer chain in the series of compounds decreases, the different relaxations become progressively more coupled at the glass transition. Numerical simulations of the calorimetric response from the obtained kinetic parameters show good agreement with experimental behaviour. Graphical Abstract