Influence of fluorine substituent on the mesomorphic properties of five-ring ester banana-shaped molecules

The synthesis and characterization of more than a hundred achiral compounds composed of banana-shaped molecules and belonging to eleven different homologous series are reported. All these symmetrical five-ring compounds are esters and are derived from resorcinol. The compounds have been systematically substituted by fluorine on the phenyl rings of the arms of these banana-shaped molecules. The mesophases exhibited by these compounds have been characterized using a combination of polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction studies. Electro-optical investigations have been carried out on several compounds and both ferroelectric and antiferroelectric properties are observed. The influence of the position of the fluorine substituent on the nature of the mesophase obtained has been examined.     

Monofluorinated unsymmetrical bent-core mesogens

The synthesis and mesomorphic properties of 30 bent-core compounds with a fluorine substituent in one of the outer rings are reported. The banana-shaped compounds are all derived from resorcinol and contain esters as linking groups between the five aromatic rings. The different mesophases have been characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical investigations. The compounds with the longer terminal chains exhibit an antiferroelectric SmCP phase. Upon introduction of a fluorine substituent the layer spacing increases, as compared with the corresponding unsubstituted compound. The introduction of one terminal vinyl group in the mono-substituted bent-core mesogens has no significant influence on the liquid crystalline properties and the switching remains antiferroelectric. Due to the introduction of the terminal double bond, these banana-shaped compounds are suitable for the preparation of siloxane polymers or attachment to a hydrogen-terminated silicon surface.     

SmCP-networked films obtained by in situ photopolymerization of neat reactive banana-shaped liquid crystals

The synthesis and characterization of reactive banana-shaped compounds have been carried out, and their ability to be photopolymerized in their SmCP mesophase has been assessed. The presence of a SmCP liquid crystalline phase in these compounds has been confirmed by X-ray studies. The polymerization of these molecules has been demonstrated by calorimetric techniques as well as by the preparation and characterization of SmCP-ordered free films that are mechanically stable at room temperature. Furthermore, polymerized films exhibit second harmonic generation activity at room temperature in the absence of an electric field.     

Liquid crystalline dimeric compounds with an alkylene spacer

Two series of compounds with dimeric molecules have been synthesized and characterized. These molecules consist of two non-mesogenic units linked by an alkylene spacer through an ester linkage. The effects of varying the terminal as well as the spacer chain length on the mesomorphic properties have been studied. For compounds with an even number of carbon atoms in the spacer chains, the observed textures in the mesophases are reminiscent of those seen for the smectic phases of banana-shaped molecules.     

Liquid crystalline dimeric compounds with an alkylene spacer

Two series of compounds with dimeric molecules have been synthesized and characterized. These molecules consist of two non-mesogenic units linked by an alkylene spacer through an ester linkage. The effects of varying the terminal as well as the spacer chain length on the mesomorphic properties have been studied. For compounds with an even number of carbon atoms in the spacer chains, the observed textures in the mesophases are reminiscent of those seen for the smectic phases of banana-shaped molecules.     

Banana-shaped mesogens: effect of lateral substituents on seven-ring esters containing a biphenyl moiety

The synthesis and characterization of five homologous series of symmetrical compounds composed of banana-shaped molecules containing a biphenyl moiety are reported. All these compounds are non-Schiff's bases and are esters. The effects of lateral substituents such as fluoro, methyl and ethyl in the side arms of these molecules are examined. These substituents have a strong influence in reducing the clearing temperatures. Banana phases such as B 1 , B 2 and B 6 were observed in the above series of compounds. The mesophases were characterized by a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic studies.     

Effect of lateral substituents on the mesophases formed by some achiral banana-shaped molecules

Several compounds composed of banana-shaped molecules and exhibiting mesophases are reported. The effect of different lateral substituents on the type of mesophase formed is examined and a comparison made with unsubstituted compounds. The mesophases have been characterized using techniques such as polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The B₂ phase shows antiferroelectric switching behaviour.     

Banana-shaped mesogens: effect of lateral substituents on seven-ring esters containing a biphenyl moiety

The synthesis and characterization of five homologous series of symmetrical compounds composed of banana-shaped molecules containing a biphenyl moiety are reported. All these compounds are non-Schiff's bases and are esters. The effects of lateral substituents such as fluoro, methyl and ethyl in the side arms of these molecules are examined. These substituents have a strong influence in reducing the clearing temperatures. Banana phases such as B1, B2 and B6 were observed in the above series of compounds. The mesophases were characterized by a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic studies.     

Synthesis and mesomorphic properties of banana-shaped compounds derived from 2,7-dihydroxynaphthalene

Thirty eight compounds composed of achiral banana-shaped molecules belonging to three new homologous series have been synthesized and their mesomorphic properties investigated. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry, miscibility methods and X-ray diffraction measurements. All the compounds reported exhibit mesomorphic properties. Most of the homologues in the three series show B₁ and B₂ phases and the transition temperatures of these are lower in the fluoro substituted compounds. Some of the lower homologues of the fluoro substituted derivatives also show the nematic phase.     

Effect of lateral substituents on the mesophases formed by some achiral banana-shaped molecules

Several compounds composed of banana-shaped molecules and exhibiting mesophases are reported. The effect of different lateral substituents on the type of mesophase formed is examined and a comparison made with unsubstituted compounds. The mesophases have been characterized using techniques such as polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The B₂ phase shows antiferroelectric switching behaviour.     

Synthesis and mesomorphic properties of banana-shaped compounds derived from 2,7-dihydroxynaphthalene

Thirty eight compounds composed of achiral banana-shaped molecules belonging to three new homologous series have been synthesized and their mesomorphic properties investigated. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry, miscibility methods and X-ray diffraction measurements. All the compounds reported exhibit mesomorphic properties. Most of the homologues in the three series show B₁ and B₂ phases and the transition temperatures of these are lower in the fluoro substituted compounds. Some of the lower homologues of the fluoro substituted derivatives also show the nematic phase.     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Quantum chemical studies on the conformational behaviour of substituted banana-shaped mesogens with a central 1,3-phenylene unit

Ab initio and DFT calculations on the HF/STO-3G and B3LYP/6-31G(d) level were performed on the conformational behaviour of isolated banana-shaped molecules of 1,3-phenylene bis[4-(4-n-hexyloxyphenyliminomethyl)benzoate] systems (P-6-O-PIMB). The influence of small substituents in both the central phenyl ring and the external phenyl rings on the shape, polarity and flexibility of these molecules was investigated by one- and two-fold relaxed potential energy scans in a systematic way. The effect of substituents on the global polarity of banana-shaped mesogens was analysed by the magnitude and direction of the dipole moment and its components in relation to the long axis of the molecules. Moreover, a simple model for the calculation of the bending angle was tested for banana-shaped molecules with a central 1,3-phenylene unit. The findings for the isolated banana-shaped molecules are correlated with solid state X-ray and liquid crystalline state NMR results. Banana-shaped molecules with both hexyloxy (P-6-O-PIMB) and hexyl (P-6-PIMB) terminal chains are included to study the effect of substituents in the external phenyl rings on the flexibility of these chains. An attempt will be made to correlate the results with experimental findings on banana-shaped mesogens.     

Liquid crystal phases of achiral banana-shaped molecules: a computer simulation study

The phase behaviour of achiral banana-shaped molecules was studied by computer simulation. The banana-shaped molecules were described by model intermolecular interactions based on the Gay-Berne potential. The characteristic molecular structure was considered by joining two calamitic Gay-Berne particles through a bond to form a biaxial molecule of point symmetry group C 2v with a suitable bending angle. The dependence on temperature of systems of N =1024 rigid banana-shaped molecules with bending angle ϕ=140° has been studied by means of Monte Carlo simulations in the isobaric-isothermal ensemble ( NpT ). On cooling an isotropic system, two phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. For the first time by computer simulation of a many-particle system of banana-shaped molecules, at low temperature an untilted smectic phase showing a global phase biaxiality and a spontaneous local polarization in the layers, i.e. a local polar arrangement of the steric dipoles, with an antiferroelectric-like superstructure could be proven, a phase structure which recently has been discovered experimentally. Additionally, at intermediate temperature a nematic-like phase has been proved, whereas close to the transition to the smectic phase hints of a spontaneous achiral symmetry breaking have been determined. Here, in the absence of a layered structure a helical superstructure has been formed. All phases have been characterized by visual representations of selected configurations, scalar and pseudoscalar correlation functions, and order parameters.     

Helical superstructures in the mesophase of compounds derived from 2-cyanoresorcinol

The mesophase exhibited by four members of a new homologous series of compounds composed of banana-shaped molecules derived from 2-cyanoresorcinol is reported. The characteristic optical textures showing helical filamentary growth patterns, coupled with the results from X-ray diffraction studies of these seven-ring compounds containing a biphenyl moiety indicate that the mesophase is indeed B7.     

Helical superstructures in the mesophase of compounds derived from 2-cyanoresorcinol

The mesophase exhibited by four members of a new homologous series of compounds composed of banana-shaped molecules derived from 2-cyanoresorcinol is reported. The characteristic optical textures showing helical filamentary growth patterns, coupled with the results from X-ray diffraction studies of these seven-ring compounds containing a biphenyl moiety indicate that the mesophase is indeed B 7 .     

Efficient Syntheses of Novel Fluoro‐Substituted Pentacenes and Azapentacenes: Molecular and Solid‐State Properties

Non‐symmetrical 6,13‐disubstituted pentacenes bearing trifluoromethyl and aryl substituents have been synthesized starting from pentacenequinone. Diazapentacenes with a variety of fluorine substituents were prepared either via a Hartwig–Buchwald aryl amination route or by a S_NAr strategy. As a result of a non‐symmetric substitution pattern containing electron‐donating substituents in combination with electron‐accepting fluorine substituents, the synthesized compounds feature distinct molecular dipoles. All compounds are analyzed regarding their optoelectronic properties in solution with special focus on the frontier orbital energies as well as their molecular packing in the crystal structures. The analyses of isolated molecules are complemented by thin‐film studies to examine their solid‐state properties. A precise comparison between these and the molecular properties gave detailed insights into the exciton binding energies of these compounds, which are explained by means of a simple model considering the molecular packing and polarizabilities.     

Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.     

Synthesis and mesomorphic properties of three-benzene-ring-containing banana-shaped liquid crystals

A homologous series of three-benzene-ring-containing banana-shaped compounds, N,N-bis(4-alkylcarbonyloxybenzylidene)benzene-1,3-diamine, were synthesised by esterifying aliphatic acids with the three-benzene-ring bent core 1,3-phenylene-bis(4-hydroxybenzylideneamine). As the number of carbon atoms in the aliphatic acids increased from 1 to 12, 12 novel banana-shaped compounds resulted with increasing lengths in their terminal tails. The mesomorphic properties of this homologous series of three-benzene-ring-containing banana-shaped compounds were characterised by means of polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies. Our results have demonstrated that this series of banana-shaped compounds can form mesophases, although each of their bent cores contains only three benzene rings.