Cyanobiphenyl Units Research Articles

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase: on the role played by the length and parity of the spacer

ABSTRACT Six members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkanes are reported and referred to as CBnCB in which n = 1, 15, 16, 17, 19 and 20 and indicates the number of methylene units in the spacer separating the two cyanobiphenyl units. The behaviour of CB3CB is revisited. The temperature dependence of the refractive indices, optical birefringence and dielectric permittivities measured in the nematic, N, phase for selected homologues are reported. The dimers with n ≥ 15 showed an enantiotropic N phase, and for the odd members the twist-bend nematic, NTB, phase was observed. CB3CB shows a direct NTB-isotropic, I, transition, whereas for CB1CB a virtual NTB-I transition is found. The temperature dependence of the bend elastic constant, K 33 , measured in the oblique helicoidal cholesteric state formed by mixtures of CBnCB with a chiral additive S811, shows strong non-monotonous behaviour with a deep minimum near the transition point to the NTB phase. The minimum value of K 33 decreases as n increases. The long even members of the CBnCB series show similar values of TNI to their odd-membered counterparts, but their estimated values of TNTB N are considerably lower. This is attributed to molecular shape and its effect on K 33 .

Influence of the hemisphere on Janus structures containing cyanobiphenyl units

ABSTRACT New Janus-type compounds were synthesised with two different hemispheres structures and properties. The structures contain cyanobiphenyl units linked with 3,4,5-tribenzyloxybenzene or 3,4,5-tridecyloxybenzene units. Also, the influence of the 1,3,4-thiadiazole heterocycle was studied. The mesomorphic properties were studied by differential scanning calorimetry, polarised light optical microscopy and thermal gravimetric analysis. The mesophases obtained depend on the structure of the hemispheres, even obtaining mesophases different from the initial ones. The new materials synthesised presented photoluminescent properties, which were studied in solution.

Liquid-crystalline pillar[5]arene-based [2]rotaxanes

ABSTRACT Liquid-crystalline mechanically interlocked molecules have been prepared from a clickable pillar[5]arene-containing [2]rotaxane building block incorporating ethynylated stoppers and dendritic mesogenic subunits of generation 0, 1 and 2 under copper-catalysed alkyne-azide cycloaddition conditions. The liquid-crystalline properties of the resulting pillar[5]arene-based rotaxanes bearing 4, 8 and 16 peripheral cyanobiphenyl units have been investigated and compared to those of corresponding model compounds lacking the macrocyclic component.

DNMR measurements of an asymmetric odd liquid crystal dimer: determination of the intramolecular angle and the degree of order of the two rigid cores.

In this work, we present a Deuteron Nuclear Magnetic Resonance (DNMR) study of the non-symmetric odd liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py), formed by a pro-mesogenic cyanobiphenyl unit and a bulky pyrene-containing unit, linked via alkoxy flexible chain. We have synthesized two partially deuterated samples: one with the deuterium atoms in the cyanobiphenyl moiety (dCBO7O.Py) and the other one with the deuterium atoms in the pyrenimine-benzylidene unit (CBO7O.dPy). We have performed angular distribution analysis in the SmA glassy state, obtaining the degree of order of both rigid cores and an estimation of the internal molecular angle between both structures. With the results from the angular study, we have been able to determine the degree of order of both rigid units in either the N phase and the SmA phase, far enough from the glass transition. Both rigid cores have the same degree of order close to the nematic-isotropic phase transition, but as the compound is cooled down, the degree of order of the cyanobiphenyl moiety is clearly higher than that of the pyrene-containing unit. The critical behaviour of the order parameter of the pyrene-containing moiety is consistent with the fact that, for CBO7O.Py, the N-I phase transition is tricritical, which seems to indicate that the uniaxial order parameter of the dimer is dominated by the degree of order of the pyrene-containing core.

Environmentally Stable Chiral-Nematic Liquid-Crystal Elastomers with Mechano-Optical Properties

Chiral-nematic liquid crystal (N* LC) elastomers exhibit mechano-optical responsive behavior. However, practical sensor applications have been limited by the intrinsic sensitivity of N* LC elastomers to environmental conditions, such as temperature. Although densely cross-linked LC network polymers exhibit high thermal stability, they are not proper for the mechanical sensor due to high glass transition temperatures and low flexibility. To overcome these issues, we focused on enhancing thermal stability by introducing noncovalent cross-linking sites via intermolecular interactions between LC molecules bonded to the polymer network. N* LC elastomers with a cyanobiphenyl derivative as a side-chain mesogen exhibited mechano-optical responsive behavior, with a hypsochromic shift of the reflection peak wavelength under an applied tensile strain and quick shape and color recovery owing to high elasticity. Notably, the N* LC elastomers showed high resistance to harsh environments, including high temperatures and various solvents. Interactions, such as π–π stacking and dipole–dipole interactions, between the cyanobiphenyl units can act as weak cross-links, thus improving the thermal stability of the LC phase without affecting the mechano-optical response. Thus, these N* LC elastomers have great potential for the realization of practical mechano-optical sensors.

Synthesis and properties of supramolecular gels based on tetrathiafulvalene and cyanobiphenyl units

ABSTRACT For the development of self-assembly donor-σ-acceptor (D-σ-A) molecules, a series of D-σ-A low-molecular mass organogelators, consisting of tetrathiafulvalene (D) and cyanobiphenyl (A) connected by a thioalkanoyloxy spacer of varying lengths modified with hydrophobic chains incorporating amide groups on the side, was designed and readily synthesized. Their gelation properties were studied. The results showed that the gelation capability of these compounds was highly dependent on the alkyl chain length in the spacer group and the TTF and cyanobiphenyl units. Derivative 1a with short alkyl chain (n = 4) efficiently gelated in some aromatic solvents, forming opaque organogels. These gelators reacted with F4TCNQ to form stable charge-transfer complex gels in ethanol. Clear evidence of self-assembled micro/nanoflower, fibrous, or tubular morphologies in gelation state was obtained by scanning electron microscopy. It was found that the molecules adopted lamellar and rectangular columnar molecular packing models in the gel phase by X-ray diffraction. The native gels underwent a reversible gel-sol phase transition upon exposure to external stimuli. The gel-sol transition of the organogel was stimulated by the addition of fluoride anions. Interestingly, all gels showed an irreversible gel-sol transition triggered by TFA and TEA. The formation of flower-like morphologies of TTF-based derivatives in the gel state was observed.

Synthesis and mesocrystalline properties of liquid crystal compounds with tetrathiafulvalene and two cyanobiphenyl units

ABSTRACTA series of ethylenedithiotetrathiafulvalene (EDT-TTF) compounds with cyanobiphenyl moieties attached with different length alkyl chains has been designed and synthesized. The phase behavior of these derivatives was investigated by a combination of polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. Among the derivatives, the derivatives with spacer group lengths of n = 6–12 (3b–3d) show a distinct glassy transition during the heating and cooling processes, and 3c (n = 8) and 3d (n = 12) show liquid crystal (LC) phases below the isotropic temperature. An electrochemical study shows that the longer the spacer group, the more readily the target compound is oxidized. Density functional theory calculations show that the LC molecules are typical donor–σ-bridge–acceptor molecules.

Selenium-linked liquid crystal dimers for twist-bend nematogens

The recently discovered twist-bend nematic (NTB) phase has become an intensively studied topic in supramolecular chemistry and condensed matter science. Herein, a new molecular design for twist-bend nematogens employing the selenoether (C–Se–C) linkage, instead of the conventional methylene or ether linkages, is introduced. We synthesized CBSenSeCB dimers, consisting of two cyanobiphenyl (CB) units connected by two selenoether linkers sandwiching an odd-numbered alkylene spacer (n = 3, 5, 7, and 9) and revealed their phase transitions and mesomorphism. CBSenSeCB (n = 5, 7, and 9) can potentially form mesophases; notably, CBSe7SeCB formed the NTB phase from the conventional uniaxial nematic phase, which was cooled to below room temperature. The structure of the all-trans model of CBSe7SeCB, optimized by the density functional theory, suggests that the molecular bend angle between the para axes of the cyanobiphenyl moieties is ∼90°, which is more bent than those of previously reported twist-bend nematogens. Such a keen curvature is ascribed to the small C–Se–C angle of ∼101°. In addition, the molecular bend angles of the twist-bend dimers, including the present and previously reported ones, correlate linearly with their phase-transition temperatures. The present study paves the way for the molecular design of new twist-bend nematic liquid crystals based on chalcogen linkages.

Rhodium catalyzed template-assisted distal para-C-H olefination.

Rhodium catalysis has been extensively used for ortho-C-H functionalization reactions, and successfully extended to meta-C-H functionalization. Its application to para-C-H activation remains an unmet challenge. Herein we disclose the first example of such a reaction, with the Rh-catalyzed para-C-H olefination of arenes. The use of a Si-linked cyanobiphenyl unit as a traceless directing group leads to highly para-selective arene-olefin couplings.

Azobenzene-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACTThe synthesis and characterisation of two new sets of non-symmetric liquid crystal dimers is reported, the 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexanes (CB6OABX) and 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yloxy)pentanes (CBO5OABX). The terminal substituents are methyl, methoxy, butyl, butyloxy, nitrile and nitro. All the CB6OABX dimers exhibit twist-bend nematic (NTB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the NTB phase. The transitional behaviour of the non-symmetric dimers is compared to that of the corresponding symmetric dimers, the 1,5-bis(4-substitutedazobenzene-4′-yloxy)pentanes (XABO5OABX) and either 1,7-bis(4-cyanobiphenyl-4′-yl)heptane or 1,5-bis(4-cyanobiphenyl-4′-yloxy)pentane. The XABO5OABX dimers all show a nematic phase and in addition, the butyl homologue exhibits a smectic A phase. The difference in transitional behaviour between the CB6OABX and CBO5OABX dimers is attributed to the difference in their molecular shapes arising from different bond angles between the para axis of the cyanobiphenyl unit and the first bond in the spacer. Specifically, the all-trans conformation of a CBO5OABX dimer is more linear than that of the corresponding CB6OABX dimer. Differences within each set of dimers are attributed to changes in the average molecular shape and the strength of the mixed mesogen interaction on varying the terminal group. Crystal structures are reported for CB6OABOMe, CBO5OABNO2 and MeOABO5OABOMe.

Preparation of colloidal solution of silica encapsulating cyanobiphenyl unit-capped ZnO QD emitting in the blue region.

The encapsulation of ZnO nanoparticles (5 nm) coated by cyanobiphenyl units by the sol-gel technique leads to spherical ZnO@SiO2 nanoparticles displaying blue emission under UV excitation.

Two Glass Transitions Associated to Different Dynamic Disorders in the Nematic Glassy State of a Non-Symmetric Liquid Crystal Dimer Dopped with g-Alumina Nanoparticles

In the present work, the nematic glassy state of the non-symmetric LC dimer α-(4-cyanobiphenyl-4′-yloxy)-ω-(1-pyrenimine-benzylidene-4′-oxy) undecane is studied by means of calorimetric and dielectric measurements. The most striking result of the work is the presence of two different glass transition temperatures: one due to the freezing of the flip-flop motions of the bulkier unit of the dimer and the other, at a lower temperature, related to the freezing of the flip-flop and precessional motions of the cyanobiphenyl unit. This result shows the fact that glass transition is the consequence of the freezing of one or more coupled dynamic disorders and not of the disordered phase itself. In order to avoid crystallization when the bulk sample is cooled down, the LC dimer has been confined via the dispersion of γ-alumina nanoparticles, in several concentrations.

Synthesis and characterization of novel azobenzene-based mesogens and their organization at the air–water and air–solid interfaces

Eight new oligomeric mesogens are reported, consisting of an azobenzene-based core attached to which are four 4-cyanobiphenyl units via flexible alkyl spacers (n = 5–12). Their chemical structures were determined by 1H NMR, 13C NMR, IR, UV-Vis and Raman spectroscopy, and elemental analysis. The thermotropic liquid crystalline properties of these materials were investigated by POM, DSC and XRD. The oligomers containing n = 8 and n = 10 were found to exhibit a monotropic nematic (N) phase while others were non-mesomorphic. Langmuir monolayers and Langmuir–Blodgett films of the nematic compounds (n = 10) were studied at the air–water interface (Langmuir film) and the air–solid interface. Surface manometry studies on the Langmuir monolayer showed that the film had nearly zero surface pressure at a large area per molecule (Am ≥ 0.55 nm2). The film on compression showed a gradual increase in surface pressure and finally collapsed at an Am of about 0.15 nm2 with a collapse pressure of about 60 mN m−1. Brewster angle microscopy (BAM) images (during compression) showed that dark regions coexisting with grey spots in large areas transformed to a uniform grey region with an increase in surface density and finally collapsed exhibiting bright regions. Atomic force microscopy studies (AFM) on the LB films, transferred onto freshly cleaved hydrophilic mica substrates, exhibited a network of thin fibers with the height of fibers varying between ∼4 nm to 80 nm which could be due to π–π stacking and dipolar interactions associated with the cyanobiphenyl units. On a hydrophobic silicon substrate, the LB transfer yielded a multilayer film which dewetted to form nanodroplets. We carried out temperature dependent AFM studies of the nematic compounds which showed the reversible formation of aligned fibers (∼20 to 40 μm) in the mesophase. In brief, our study provides new approaches for the realization of controlling the anisotropic properties of an ordered phase.

Thermotropic luminescent clustomesogen showing a nematic phase: a combination of experimental and molecular simulation studies.

Octahedral Mo6 nanoclusters are functionalized with two organic ligands containing cyanobiphenyl (CB) units, giving luminescent hybrid liquid crystals (LC). Although the mesogenic density around the bulky inorganic core is constant, the two hybrids show different LC properties. Interestingly, one of them shows a nematic phase, which is particularly rare for this kind of supermolecular system. This surprising result is explained by using large-scale molecular dynamic simulations.

Liquid crystal dimers and the twist-bend nematic phase. The preparation and characterisation of the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates

Eleven members of the homologous series of liquid crystal dimers, the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates, have been synthesised and their transitional properties characterised. These dimers consist of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages. All eleven members exhibit exclusively nematic behaviour. The nematic–isotropic transition temperatures, TNI, and associated entropy changes, ∆SNI/R, exhibit pronounced alternations as the length and parity of the spacer is varied; this is characteristic behaviour of liquid crystal dimers. The transitional properties of the ester-linked dimers are compared with the corresponding materials having either ether, methylene or carbonate linkages between the spacer and mesogenic units. For short spacer lengths and both odd- and even-membered dimers, the ester-linked materials show the highest values of TNI and the methylene-linked the lowest. For longer spacer lengths, TNI of the carbonate-linked dimers fall between those of the corresponding ester- and ether-linked dimers. The ether-linked materials show the largest alternation in ∆SNI/R on varying spacer length and the carbonate-linked dimers the lowest. This behaviour is interpreted in terms of the molecular geometry and it is suggested that the ether- and ester-linked odd-membered dimers have rather similar shapes. A phase diagram constructed using binary mixtures of the pentyl member of this ester-linked series and the known twist-bend nematogen, 1,7-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB), is presented. The twist-bend nematic–nematic transition temperature of the mixtures shows a striking convex curvature as the concentration of CB7CB is decreased, and so it is not possible to estimate a virtual twist-bend nematic–nematic transition temperature for the ester-linked material.

The design and investigation of porphyrins with liquid crystal properties at room temperature

The design, synthesis and investigation of a series of porphyrins displaying smectic liquid crystal phase behaviour at room temperature are described. The molecules are constructed in a modular approach, consisting of porphyrin units linked via hydrocarbon and siloxane spacers to calamitic cyanobiphenyl units. The investigation of the phase behaviour, based on optical polarizing microscopy, differential scanning calorimetry and XRD studies, is discussed in detail.

Diketopyrrolopyrrole‐based liquid crystalline conjugated donor–acceptor copolymers with reduced band gap for polymer solar cells

AbstractA new liquid crystalline (LC) acceptor monomer 2,5‐bis[4‐(4′‐cyanobiphenyloxy)dodecyl]‐3,6‐dithiophen‐2‐yl‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione (TDPPcbp) was synthesized by incorporating cyanobiphenyl mesogens into diketopyrrolopyrrole (DPP). The monomer was copolymerized with bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′] dithiophene (BDT) and N‐9′‐heptadecanylcarbazole (CB) donors to obtain donor–acceptor alternating copolymers poly[4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(thiophen‐5‐yl)‐2,5‐bis[4‐(4′‐cyanobiphenyloxy)dodecyl]‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione] (PBDTDPPcbp) and poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐3,6‐bis(thiophen‐5‐yl)‐2,5‐bis[4‐(4′‐cyano‐biphenyloxy)dodecyl]‐2,5‐dihydropyrrolo[3, 4‐c]pyrrole‐1,4‐dione] (PCBTDPPcpb) with reduced band gap, respectively. The LC properties of the copolymers, the effects of main chain variation on molecular packing, optical properties, and energy levels were analyzed. Incorporating the mesogen cyanobiphenyl units not only help polymer donors to pack well through mesogen self‐organization but also push the fullerene acceptor to form optimized phase separation. The bulk heterojunction photovoltaicdevicesshow enhanced performance of 1.3% for PBDTDPPcbp and 1.2% for PCBTDPPcbp after thermal annealing. The results indicate that mesogen‐controlled self‐organization is an efficient approach to develop well‐defined morphology and to improve the device performance. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Supermolecular Chiral Mesogenic Tripedes

A novel series of chiral liquid crystalline tripedes Glucoside and Mannoside derivatives G(n) and M(n) (n=1-3) have been synthesised. The inner cores consist of methyl α-D-Glucoside G or methyl α-D-Mannoside M, regioselectively functionalised at the less hindered position C6, with tert-butyldimethylsilyl (TBDMS), hydroxyl or carboxylic acid moieties. The cores, which can acquire several flexible conformations, are attached to rod-like smectogenic-preferring cyanobiphenyl units, by means of a flexible hexanoyl spacer. These Glyco-Supermolecules exhibit chiral nematic (N*) and smectic A (SmA) phases. The combined effects of core chirality and functional groups on thermal and mesomorphic characteristics are discussed.

Building liquid crystals from the 5-fold symmetrical pillar[5]arene core

Comparison of the liquid-crystalline properties of a pillar[5]arene core functionalized with 10 mesogenic cyanobiphenyl units with those of a corresponding model compound revealed the strong influence of the macrocyclic pillar[5]arene core on the mesomorphic properties.

Mesogens Mediated Self-Assembly in Applications of Bulk Heterojunction Solar Cells Based on a Conjugated Polymer with Narrow Band Gap

The ability to control the molecular organization of electronically active liquid-crystalline polymer provides opportunities to develop easy-to-process yet highly ordered supramolecular systems in the field of photovoltaics. Here, we report a novel donor−acceptor type liquid-crystalline copolymer, poly{9,9-bis[6-(4′-cyanobiphenyloxy)hexyl]fluorene-alt-5,5-(4′,7′-di-2-thienyl-2′,1,3′-benzothiadiazole)}, PFcbpDTBT, which contains both electron-donating fluorene and electron-accepting benzothiadiazole units. Incorporating the electron withdrawing cyanobiphenyl units not only narrowed the band gap of the copolymer but also help main chain form spontaneously through self-organization. The films with structural anisotropy can endow the PFcbpDTBT with special features, including absorption band red-shift; fluorescence enhancement; lower lying LUMO level, and crystallinity improvement. When blended with PCBM, the PFcbpDTBT enables the acceptors to adopt the preferential well-oriented arrangement in both surface a...