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Smectic liquid crystals with a low layer contraction of less than 1% upon an SmA to SmC transition are referred to as 'de Vries-like'. These materials have attracted considerable attention as possible alternative to problems with surface-stabilized ferroelectric liquid crystals in liquid crystal displays. The 'de Vries-like' properties of liquid crystals with nanosegregating carbosilane segments are enhanced by lengthening the carbosilane end-groups from mono- to tricarbosilane. The observed enhancement is thought to arise from an increase in the cross-section of the free volume in the hydrocarbon sub-layer. The hypothesis is tested by assuming that dimers with a tricarbosilane linking group have smaller time-averaged cross-sections. This paper reports the synthesis and characterization of two homologous series of liquid crystals and an investigation of their mesogenic properties as model systems employing liquid crystalline monomers (QL39-n) and dimers (QL40-n) comprising 2-phenylpyrimidine cores and tricarbosilane end-groups and spacers, respectively. We find that the QL39-n monomers form only a tilted SmC phase, whereas the QL40-n dimer forms an orthogonal SmA phase due to the tricarbosilane spacer will not have a spherically shape on the time average like that predicted for the tricarbosilane end-group. Results are discussed in the context of the timeaveraged cross-sections of the materials. [ABSTRACT FROM AUTHOR]

We investigate the origins of 'de Vries-like' liquid crystalline behaviour by introducing an ethynyl spacer in the core of the tricarbosilane-terminated 5-phenylpyrimidine mesogens QL16-6 and QL17-6. The rationale for this structural change is based on the assumption that an ethynyl spacer would create more free volume in the core sub-layer and therefore decrease the orientational order parameter S2 in the SmA phase. The tricarbosilane-terminated mesogens WL41-5 and WL42-6 with a 5-(phenylethynyl)pyrimidine core in either a normal or inverted orientation were synthesised, and their mesomorphic and 'de Vries-like' properties characterised using polarised optical microscopy, differential scanning calorimetry, birefringence measurements, small–angle (SAXS) and 2D X-ray scattering. Reduction factors R derived from SAXS and optical tilt angle measurements suggest that neither WL41-5 (R = 0.49) nor WL42-6 (R = 0.80) exhibit 'de Vries-like' properties. The S2(T) profiles show an increase in orientational order with decreasing temperature and a sharp discontinuity at the SmA-SmC transition, which is consistent with 'de Vries-like' behaviour. However, the Leff(T) profiles suggest an increase in interdigitation that reduces the positive effect of increasing S2 in compensating for the molecular tilt. [ABSTRACT FROM AUTHOR]

We report the results of a study in which we introduce a tricarbosilane end-group in a mesogenic scaffold derived from an axially chiral biphenyl with a large transverse dipole moment. This structural modification is intended to promote the formation of a chiral smectic C (SmC*) phase, with the aim of discovering a mesogen with a virtual transition temperature to a proper ferroelectric phase (To) that approaches the crystal-SmC* melting point. We show herein that the combination of a 4-pentylbicyclo[2.2.2]octane-1-carboxylate chain and a tricarbosilane-terminated 4-alkoxybenzoate chain gives a mesogen (R,S)-WL43 forming an enantiotropic orthogonal smectic A (SmA) phase and a higher order tilted smectic phase. Substitution of the 4-pentylbicyclo[2.2.2]octane-1-carboxylate group with a trans-4-pentylcyclohexane-1-carboxylate group gives a mesogen (R,S)-WL45 forming monotropic SmA and SmC phases. The SmC* phase formed by the enantiomerically pure (R)-WL45 exhibits Goldstone-mode switching characteristic of a surface-stabilized FLC, but its spontaneous polarization PS is too low to be measured by conventional methods. [ABSTRACT FROM AUTHOR]

The recent discovery of a new lyotropic liquid crystal phase, the structure and properties of which are analogous to the chiral ferroelectric smectic C-phase (SmC*) in thermotropics, was based on a tailored amphiphile structure in which a tilt-promoting mesogenic core was linked to a chiral diol-headgroup via a hydrophilic ethylene glycol spacer [J. R. Bruckner et al., Angew. Chem., Int. Ed., 2013, 52, 8934–8937]. However, so far there is only one example of this general amphiphile structure known to form the new lyotropic SmC* phase in mixtures with water and with formamide. In an attempt to systematically elucidate the underlying structure–property relations we now report two new amphiphiles leading to lyotropic SmC* phases. These amphiphiles are derived from the parent amphiphile structure by (i) an elongation of the hydrophilic ethylene glycol spacer and (ii) an exchange of the original 2-phenylpyrimidine core by an even more tilt-promoting fluorenone core. Our investigations also reveal that the formation of the lyotropic SmC* phase is highly sensitive to exchanging or enlarging a building block of the primary material. [ABSTRACT FROM AUTHOR]

A homologous series of tri-, di- and monocarbosilane mesogens with a 5-phenylpyrimidine core and a chiral (S,S)-2,3-difluorohexyloxy chain were synthesized and characterized. We show that shortening the carbosilane end-group has a detrimental effect on ‘de Vries-like’ properties, causing an increase in reduction factor R from 0.17 for the tricarbosilane QL32-6 to 0.38 for the monocarbosilane QL34-6, both at the point of maximum layer contraction upon transition from the orthogonal SmA* to the tilted SmC* phase. This increase in R correlates with an increase in birefringence Δn, a broadening in the SmA* temperature range and a decrease in optical tilt angle ϑopt in the SmC* phase, which are all consistent with an increase in orientational order and a decrease in lamellar order. The electroclinic effect (ECE) in the SmA* phase decreases with decreasing length of the carbosilane end-group, which is consistent with previous observations of high ECE in ‘de Vries-like’ liquid crystals. On the other hand, spontaneous polarization PS measurements suggest that polar ordering in the SmC* phase is not strongly dependent on R. [ABSTRACT FROM AUTHOR]

Chiral periodic mesoporous organosilica (PMO) materials have been shown to deracemise a configurationally achiral, but conformationally racemic liquid crystal in which the PMO is embedded. In particular, application of an electric fieldEin the liquid crystal’s smectic-Aphase results in a rotation of the liquid-crystal director by an angle proportional toE, which is detected optically – this is the so-called ‘electroclinic’ effect. Here we present results from electroclinic measurements as a function of frequency and temperature, which allow us to distinguish the component of optical signal that arises from liquid-crystal chirality induced within the PMO’s chiral pores from that induced just outside the silica colloids. Our central result is that the overwhelming source of our electrooptic signal emanates from outside the PMO, and that the contribution from the liquid crystal embedded in the chiral pores is much smaller and below the noise level. [ABSTRACT FROM PUBLISHER]

The phase diagrams of binary mixtures of smectic C (SmC) mesogens – the carbosilane-terminated mesogen 2-(6-chlorohexyloxy)-5-(4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl)-1,3,4-thiadiazole (QL13-6) and one of nine conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole mesogens (QL14-m/n)–with a molecular length ratioQL13-6/QL14-m/nranging from 1.24 to 1.46 show the induction of a smectic A (SmA) phase in the mole fraction range 0.1 ≤xQL13-6 ≤ 0.6. The results show that the temperature range of the induced SmA phase is remarkably sensitive to the molecular length ratio, and that this approach to tuning the mesomorphic properties of SmA–SmC liquid crystal mixtures may provide a powerful new tool in the formulation of mixtures for SSFLC and electroclinic applications. [ABSTRACT FROM AUTHOR]

The chiral carbosilane-terminated liquid crystal 2-[(2S,3S)-2,3-difluorohexyloxy]-5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]pyrimidine (QL32-6) undergoes a smectic A*-smectic C* phase transition with a maximum layer contraction of only 0.2%. It exhibits an electroclinic effect (ECE) comparable to that reported for the ‘de Vries-like’ liquid crystal 8422[2F3] and shows no appreciable optical stripe defects due to horizontal chevron formation. [ABSTRACT FROM AUTHOR]

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. A defining structural characteristic of most ‘de Vries-like’ materials is a nanosegregating element that strongly promotes the formation of smectic layers. We show herein the effect of varying the length of a nanosegregating carbosilane end-group on ‘de Vries-like’ properties in three series of 5-phenyl-1,3,4-thiadiazole mesogens with chloro-terminated alkoxy chains (QL13-n, QL18-n and QL19-n). Shortening the end-group from a tricarbosilane to a monocarbosilane causes a decrease in ‘de Vries-like’ character, as measured by the reduction factor R, which is consistent with a decrease in nanosegregation and quality of the lamellar ordering. Measurements of the orientational order parameter S2 by 2D X-ray scattering of smectic monodomains show a concomitant increase in S2 that is consistent with the hypothesis that ‘de Vries-like’ behavior in these materials arises from a combination of high lamellar order and low orientational order. The requirement of a nanosegregating structural element to achieve ‘de Vries-like’ behavior is confirmed by the characterization of the chloro-terminated mesogen 2-(6-chlorohexyloxy)-5-(4-hexyloxyphenyl)-1,3,4-thiadiazole (QL28-6/6), which undergoes a SmA–SmC transition with a maximum layer contraction of 4.1% and behaves like a conventional smectogen. [ABSTRACT FROM AUTHOR]

Two sterically equivalent series of phenoxy-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals were synthesised, and their mesogenic properties were characterised by polarised optical microscopy and differential scanning calorimetry (DSC). The phenoxy end-group causes a significant increase in melting point and inhibits – at least partially – the mesomorphism of these materials relative to the parent isomers; in most cases, the broad enantiotropic SmC phase formed by the parent isomers is suppressed by the addition of the phenoxy end-group. However, detailed analyses by small-angle X-ray scattering and monodomain 2D X-ray scattering suggest that these compounds form a SmA phase with a partially intercalated bilayer structure in which the phenoxy end-groups are nanosegregated. Such an intercalated bilayer structure might enable the tuning of smectogenic properties by appropriate substitution of the phenoxy end-groups. [ABSTRACT FROM AUTHOR]

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. In an effort to expand the library of ‘de Vries-like’ liquid crystals required for the formulation of chevron-free ferroelectric liquid crystal mixtures, we report the synthesis of a homologous series of tricarbosilane 5-phenylpyrimidine liquid crystals QL16-n using an improved synthetic route, and the characterization of their liquid crystalline and ‘de Vries-like’ properties. Measurements of orientational order parameters S2 and effective molecular lengths Leff by monodomain 2D X-ray scattering suggest that ‘de Vries-like’ behavior in series QL16-n is due to the combined effect of an increase in S2 and a decrease in bilayer interdigitation, thus causing a smectic layer expansion that compensates for the molecular tilt in the SmC phase. We also show how the optical tilt angle in the SmC phase may be optimized for SSFLC displays―without compromising ‘de Vries-like’ properties―by shortening the tricarbosilane end-group to a dicarbosilane. Two of the new materials reported herein, 5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]-2-hexyloxypyrimidine (QL16-6) and 2-hexyloxy-5-[4-(12,12,14,14-tetramethyl-12,14-disilapentadecyloxy)phenyl]pyrimidine (QL24-6) rank among the best ‘de Vries-like’ materials reported heretofore, with broad SmC phases and reduction factors R of 0.17 and 0.18, respectively, at a reduced temperature T−TAC = −10 K. We also show that inverting the orientation of the 5-phenylpyrimidine core in the homologous series QL17-n causes a suppression of ‘de Vries-like’ properties. These results suggest that non-covalent core–core interactions in the intercalated smectic bilayers formed by these mesogens may influence ‘de Vries-like’ behavior. [ABSTRACT FROM AUTHOR]

The axially chiral mesogens 5-alkoxy-5′-[(4-alkoxybenzoyl)oxy]-2,2′-spirobiindan-1,1′-dione (QL7-n) were synthesised as racemic mixtures and resolved as single enantiomers by preparative chiral phase HPLC. The shorter homologues (R)-QL7-8andQL7-9form enantiotropic N* and SmC* phases, whereas the longer homologues (R)-QL7-10andQL7-12form an enantiotropic SmC* phase only. Texture analysis showed that the helical pitch of the N* phase is below the wavelength range of visible light; a helical pitch of 187 nm was measured atT – TNI= –5 K by selective reflection using enantiomerically enriched mixtures of (R)- and (S)-QL7-8(0.35 ≤ ee ≤ 0.80). Spontaneous polarisations were measured as a function of temperature in the SmC* phase by the triangular wave method.PSvalues at saturation range from 102 nC cm–2for (R)-QL7-8to 120 nC cm−2for (R)-QL7-9, which are up to three times greater than the highest polarisation previously reported for an axially chiral SmC* mesogen. Optical tilt angles θ were measured as a function of temperature and showed a sharp rise consistent with first-order N*–SmC* or I–SmC* transitions, with values at saturation ranging from 42 to 44°. [ABSTRACT FROM PUBLISHER]

Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having either a thioester or oxoester linkage group in the core. The presence of the carbon nanotubes resulted in chiral signatures being observed in the liquid crystals, including an electroclinic effect (a rotation of the liquid crystal director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic A phases, and a macroscopic helical twist of the liquid crystal director in the nematic phase. For both experiments the chiral signatures for the thioester liquid crystals were found to be an order of magnitude larger than those of the oxoesters. We speculate that the much larger strength of the thioester's chiral properties is a result of stronger non-covalent interactions between the liquid crystal molecule and carbon nanotube. [ABSTRACT FROM AUTHOR]

A series of axially chiral 5,5'- and 6,6'-dialkanoyloxy-2,2'-spirobiindan-1,1'-dione dopants, (R)-2 and (R)-4a-4c were synthesized in optically pure form and their ferroelectric polarization powers, δp, measured in four liquid crystal hosts with isotropic (I)-nematic (N)-smectic A (SmA)-smectic C (SmC) phase sequences. The results show that the sign of polarization P S induced by (R)-2 and (R)-4a-4c follows the same trend as that previously reported for the 5,5'- and 6,6'-diheptyloxy-2,2'-spirobiindan-1,1'-dione dopants, (R)-1 and (R)-3. The polarization induced by (R)-2 in the host DFT is below detection limits, and the sign of P S was found to invert as a function of temperature at mole fractions as low as 0.01. On the other hand, the polarization power of the 6,6'-diheptanoyloxy dopant (R)-4b in the host NCB76 is -1449 nC cm-2, the fourth highest value reported so far, and more than three times the δp value of the 5,5'-diheptanoyloxy analogue (R)-2 in that host (+474 nC cm-2). Results of 2H NMR experiments suggest that (R)-4b exerts stronger local perturbations in NCB76 than (R)-2, and that these perturbations may be chiral in nature. [ABSTRACT FROM AUTHOR]

The polarization powers δp of four chiral dopants with (R,R)-2,3-difluorooctyloxy side-chains were measured in four liquid crystal hosts with isotropic (I)-nematic (N)-smectic A (SmA)-smectic C (SmC) phase sequences. The four chiral dopants differ in terms of their core structures: 2-phenylpyridine (MDW950), biphenyl (5), 2-phenylpyrimidine (6) and 2-(3-nitrophenyl)pyrimidine (7). In each case, δp varies with the structure of the liquid crystal host, which is consistent with the behaviour of so-called Type II dopants that normally feature a chiral core structure. The δp(host) profile was found to depend on the degree of biaxiality of the dopant core structure, and on the degree of steric coupling between the chiral 2,3-difluorooctyloxy side-chain and the core. Conformational analyses at the B3LYP/6-31G* level suggest that the 2,3-difluorooctyloxy side-chain is conformationally more rigid than conventional chiral side-chains due to the added electrostatic repulsion of the two adjacent fluoro groups combined with the hyperconjugative 'gauche effect', and may therefore have a higher degree of biaxiality on the time average. This biaxial character should make the chiral side-chain more sensitive to variations in quadrupolar ordering imposed by the SmC phase of the liquid crystal host, and may therefore explain the dependence of δp on the host structure reported herein. [ABSTRACT FROM AUTHOR]

The chiral fluorenone mesogens ( R )‐1‐fluoro‐2‐(2‐octyloxy)‐7‐(4‐undecyloxybenzoyloxy)fluoren‐9‐one (2) and ( R )‐2‐(2‐octyloxy)‐7‐(4‐undecyloxybenzoyloxy)‐3‐azafluoren‐9‐one (3) were synthesized using a combined ortho ‐directed and remote metalation strategy, which also incorporates a Suzuki–Miyaura crosscoupling step. These compounds form chiral SmC* liquid crystal phases with reduced polarizations (P o ) of +475 and +332 nC cm -2 at 10 K below the Curie point, respectively. These values are considerably larger than that previously reported for the unsubstituted fluorenone ( R )‐2‐(2‐octyloxy)‐7‐(4‐undecyloxybenzoyloxy)fluoren‐9‐one (1), which is +111 nC cm -2 at 10 K below the Curie point. Molecular modelling based on the Boulder model suggests that the larger polarizations result from a conformational bias of the fluorenone core dipole moment along the polar axis of the SmC* phase caused by stereo‐polar coupling with the chiral 2‐octyloxy side chain. [ABSTRACT FROM AUTHOR]

The addition of the achiral biphenyl dopant 2,2′,6,6′-tetramethyl-4,4′-bis(4- n -nonyloxybenzoyloxy)biphenyl ( 3 ) or its dithionoester or dithioester analogue ( 4 , 5 ) to a 4 mol % mixture of the atropisomeric biphenyl dopant ( R )-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis(4- n -nonyloxybenzoyloxy)biphenyl, ( R )- 1 , in the phenylpyrimidine SmC host PhP1 produces a significant amplification of the spontaneous polarization induced by ( R )- 1 . This amplification may be due to a chiral perturbation by ( R )- 1 which causes a shift in the equilibrium between enantiomeric conformations of the achiral dopant. The degree of polarization amplification afforded by the achiral dopant, as expressed by the polarization amplification factor PAF , varies with the nature of the linking group. This may be ascribed to different rotational distributions of the core transverse dipole moments relative to the polar axis of the SmC* phase and/or to differences in lateral bulk of the polar linking groups. The latter may affect the degree of chiral molecular recognition achieved by 3 - 5 in the binding site of the SmC* phase. [ABSTRACT FROM AUTHOR]

Two new ferroelectric liquid crystals, 4-[(S)-1-(ethoxycarbonyl)]ethyl 4-[4-(9-decenyloxy)biphenyl]azobenzoate (10) and its ester analogue 4-[(S)-1-(ethoxycarbonyl)]ethyl {[4-(9-decenyloxy)]biphenyl-4'-yl}carbonyloxybenzoate (17), and side chain polysiloxane copolymers and terpolymers containing various amounts of 10 and 17 as mesogenic side groups have been synthesized. Their mesomorphic and physical properties have been characterized. Both 10 and 17 exhibit isotropic, SmA*, SmC*, and Sm(I or F)* phases. The copolymer with 17 as the mesogenic group and terpolymers having low content of the azo compound 10 show a SmC* phase over a large temperature range down to room temperature. The copolymer with the azo compound 10 as the mesogenic group is amorphous. [ABSTRACT FROM AUTHOR]

Two series of mesogens derived from the non-activated spiropyran dyes 6-hydroxy-1 ,3 ,3- trimethylspiro\[2H -1-benzopyran-2,2-indoline] (series trimethylspiro\[2 H -1-benzopyran-2,2-indoline] (series 2 ) have been synthesized. Analysis by differential scanning calorimetry, polarizing microscopy and X-ray diffraction showed compounds in series 1 form monotropic nematic and SmA phases, while compounds in series 2 form only a monotropic nematic phase. [ABSTRACT FROM AUTHOR]

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. We show that binary mixtures of ‘de Vries-like’ and conventional SmC mesogens with a molecular length ratio of 1.34 undergo a SmA–SmC phase transition with a maximum layer contraction ranging from 1.0 to 1.9% depending on the mixture composition. [ABSTRACT FROM AUTHOR]

A nematic twist cell having a thickness gradient was filled with a mixture containing a configurationally achiral liquid crystal (LC) and chiral dopant. A chiral-based linear electrooptic effect was observed on application of an ac electric field. This 'electroclinic effect' varied monotonically with d, changing sign at d=d0 where the chiral dopant exactly compensated the imposed twist. The results indicate that a significant chiral electrooptic effect always exists near the surfaces of a twist cell containing molecules that can be conformationally deracemized. Additionally, this approach can be used to measure the helical twisting power (HTP) of a chiral dopant in a liquid crystal. [ABSTRACT FROM AUTHOR]

The solubility of two partially deuterated thioindigo dopants in a smectic liquid crystal host was evaluated by variable temperature [sup 2] H NMR spectroscopy and polarized microscopy. 2H NMR spectra showed that the dopant (±)-6,6'-bis(2-octyloxy)-5,5-dinitrothioindigo-d[sub 6] forms a homogeneous solution with the smectic phases of the liquid crystal host (±)-4-(4-methylhexyloxy)phenyl 4-decyloxybenzoate (PhB) up to its saturation point of 3 mol %. These results are consistent with polarized microscopy observations of the dopant crystallizing out of solution upon reaching a concentration of 3 mol %. On the other hand, [sup 2] H NMR spectra of (±)-5,5'-dichloro-6,6'-bis(2-octyloxy)thioindigo-d[sub 6] dissolved in PhBshowed evidence of a partitioning of the solution between smectic and isotropic microdomains, which increases with increasing dopant concentration-from 1.2 to 9.1 mol %. To a large extent, this smectic/isotropic microphase separation could not be detected by polarized microscopy. These results suggest that [sup 2] H NMR spectroscopy can provide a more accurate determination of the occurrence and extent of microphase separation in doped liquid crystal samples. [ABSTRACT FROM AUTHOR]