Ferroelectric lyo-mesophase of banana-shaped molecules

Preliminary results are reported on the first ferroelectric 'lyo-mesophases' of banana-shaped molecules. We studied the effect of the non-polar solvents p-xylene and m-xylene on the macroscopic properties of banana-shaped molecules having a B₇ phase. It was observed that by adding only 15% of solvent the isotropic-B₇ transition temperature was lowered from 130°C to 70°C and the ferroelectric electro-optical switching range was extended from 1-2°C to more than 50°C, thus maintaining the switching even at room temperature. The xylene molecules act as spacers, maintaining larger separations between the banana-molecules, thus reducing the polar packing order. These observations provide unique opportunities for tuning the intermolecular interactions of banana-shaped molecules and modifying their macroscopic properties.     

Two ferroelectric phases of a columnar dibenzopyrene

The ferroelectric switching of columnar 1,2,5,6,8,9,12,13-octakis-((S)-2-heptyloxy-propanoyloxy)dibenzo[e,l]pyr-2-heptyloxy-propanoyloxy)dibenzo[e,1]pyrene was studied in detail between 60°C and the clearing point 115°C. The switching angle (optical tilt angle) is ±24.5° up to 10V μm^-1 and ±37° at higher field strengths. The electric polarization is 60nC cm^-2 in the low field phase and 180nC cm^-2 in the high field phase. The switching rate has apparent activation energies that increase with voltage from 3 × 10^-19 to 10^-18 J. It varies roughly with a power of the voltage, the exponent increasing with decreasing temperature from 2 to 5. At equal voltages, switching is faster in the low field than in the high field phase. We tentatively infer the structures of the two columnar lattices from the ratio of polarizations and other data. From the switching angles, we calculate a tilt angle of 44° for the aromatic cores of the disc-like molecules stacked in the columns. Finally, we point out possible advantages of ferroelectric columnar liquid crystals over their smectic counterparts in electro-optical displays.     

Synthesis and characterization of achiral banana-shaped liquid crystalline molecules containing bisnaphthyl moieties

Three analogous series of symmetric banana-shaped liquid crystalline molecules containing bisnaphthyl units were synthesized and characterized. The effects of linking groups (on the side wings) and lateral meta-fluoro substitutions (on the middle outer rings) on the mesogenic properties were examined. The type of mesophase depends on the lengths of terminal alkoxy chains. Thus, achiral molecules with shorter flexible chains (n = 8) exhibit a rectangular columnar (B₁) phase, while analogous derivatives with longer flexible chains (n = 12) display the B₂ phase. All lateral meta-fluoro substituted analogues (series II) possess the lowest isotropization temperatures and the narrowest mesophasic ranges of the B₁ and B₂ phases. The B₁ and B₂ phases were confirmed by X-ray diffraction, polarizing optical microscopy (POM) and electro-optical (EO) switching experiments. An electric field-induced transition from an antiferroelectric (tristable) state to a ferroelectric (bistable) state was observed in the EO measurements. Spontaneous polarization (by switching current response), tilt angle of chiral domains (by POM), and transmittance-voltage measurements of the B₂ phase in related compounds have been surveyed in this study.     

Dielectric studies of different phases of a thin-layered ferroelectric liquid crystal with a small pitch and high spontaneous polarization

Dielectric spectroscopy of a ferroelectric liquid crystal with planar texture in the frequency range 100 Hz to 1 MHz has been carried out as a function of temperature from 25°C to 60°C at different DC fields varying from 0 to 12 kVcm^-1. From the measured dielectric strengths and relaxation frequencies, it is observed that the material has a S_c*-S_A* transition at 56·1°C and a S_A*-I transition at 58°C. The data of the S_c* phase have been assigned to the Goldstone mode. The rotational viscosity and elastic constant have been calculated from the observed data. The capacitance shows an abrupt drop at the critical DC field where unwinding of the helix occurs. The measured value of the critical field decreases with temperature and agrees to literature data from electrooptical experiments. On applying a DC field greater than 6·5 kV cm^-1 at room temperature, the liquid crystal shows the unwound S_c* and S_A* phases.     

Dielectric characterization of samples showing the SmCPA/B7 transition

Dielectric measurements on samples consisting of banana-shaped molecules with a polymorphism I/SmCP_A/B₇ have been performed for the first time. The dielectric increment for the reorientation of the molecules about the long axis, and the corresponding relaxation time, surprisingly decrease at the SmCP_A/B₇ transition. This effect is discussed in terms of the undulated structure which partially destroys the ferroelectric short range order. Decreasing temperature gives an even stronger hindrance for this reorientation and could explain controversial results obtained in the B₇ phases of different samples. At lower temperatures a further mesophase was detected. This was additionally observed by calorimetric and atomic force microscopy methods.     

Ferroelectric switching in a novel bent-shaped mesogen having two non-mesogenic units linked by an alkylene spacer

We report the synthesis, X-ray diffraction results and electro-optical switching measurements for a bent-shaped mesogen having two non-mesogenic units linked by an alkylene spacer. The novelty of the molecular structure lies in the carbonyl group of the ester linkage being directly attached to the spacer unit, unlike for banana-shaped molecules reported so far, in which it is one oxygen atom away from the spacer or the central aromatic unit. The compound shows two mesophases: the high temperature mesophase is a tilted smectic phase showing ferroelectric switching characteristics; the low temperature phase is more highly ordered with textural features similar to that of the B₃ banana phase.     

New low molar mass organosiloxanes with unusual ferroelectric properties

A new low molar mass chiral organosiloxane mesogen and its racemic analogue have been synthesized and their mesomorphic and ferroelectric properties investigated. The chiral derivative, denoted A*B, exhibits one tilted enantiotropic ferroelectric smectic C mesophase over a broad temperature range, with very high tilt angles and moderate spontaneous polarization (36° and 19 nC cm^-2 at 20°C). The achiral siloxane derivative, denoted A*B, exhibits one broad enantiotropic smectic C phase. Preliminary electro-optic measurements indicate that the spontaneous polarization is weakly dependent on temperature between 10°C and 50°C, the latter being the S*_c to isotropic phase transition. The tilt angle and layer spacing are temperature independent, and current response times of less than 200 μs were measured at 25°C for fields of 10 V μ^-1. These results are discussed in comparison with those for side chain polymer liquid crystal structures and other low molar mass ferroelectric materials.     

Non-symmetric bent-core mesogens with one terminal vinyl group

Two series of non-symmetric banana-shaped compounds, both with one alkyl and one alkenyl terminal tail, have been synthesized and studied. Both series were compared with the corresponding series with two saturated terminal alkyl tails. All the compounds have a bent central 1,3-phenylene bis(4-benzoyloxy)benzoate core; their mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and switching current response experiments. In all four series one of the terminal tails is varied from OC₈H₁₇ to OC₁₆H₃₃. The other terminal tails are OC₁₁H₂₃, O(CH₂)₉CH = CH₂, OC₁₀H₂₁ and O(CH₂)₈CH = CH₂. The short-tailed compounds show monotropic or enantiotropic B₁ phases and the long-tailed compounds the B₂ phase. The introduction of one terminal vinyl group slightly lowers the transition temperatures. The introduction of a second terminal vinyl group further suppresses the liquid crystalline properties. All compounds with B₂ phases have layer spacings that suggest a tilt of ∼45° of the bent molecules in the layers, and their switching behaviour is antiferroelectric.     

Thermal decomposition of potassium cobalt hexacyanoferrate(II)

Potassium cobalt hexacyanoferrate(II), K₂CoFe(CN)₆ · 1.4H₂O, loses its water when heated up to 170°C, and the anhydrous compound begins to decompose above 230°C. The cyanide groups are evaporated off in the temperature range 230–350°C, and the solid products thus formed are K₂CO₃, Fe₂O₃, Co₃O₄ and CoFe₂O₄. In the range 550–900°C, the cobalt-containing compounds become CoO, and K₂CO₃ probably partly decomposes to K₂O, so that the product mixture at 900°C is K₂CO₃/K₂O, Fe₂O₃ and CoO. Above this temperature, K₂CO₃ decomposes to K₂O.     

A coloured ferroelectric side chain polymer

Coloured ferroelectric liquid-crystalline side chain copolymers containing 5 per cent and 15 per cent of an azo dye were synthesized and characterized by DSC, GPC and optical microscopy. Order parameters, S, of the azo compound exceeding 0·8 were measured in the frozen smectic phase for the 5 per cent copolymer. The copolymers exhibit fast electrooptic switching in the range of a few hundred microseconds to milliseconds in the S_c* phase. Both linear (i.e. electroclinic-like) and ferroelectric switching have been observed. Tilt angles of ∼19° and spontaneous polarization of ∼300nC cm^-2 have been recorded in the S_c* phase.     

AFM investigations of a sample showing the B2 phase

Atomic force microscopy (AFM) images reveal that the liquid crystalline B₂ phase has no helical superstructure, in contrast to the B₇ phase. This is demonstrated on comparing the AFM images of focal-conic domains in the smectic A, cholesteric, B₂ and B₇ phases. The B₂ and B₇ phases are phases formed by banana-shaped molecules. The results point to the existence of ferroelectric clusters with a random size distribution which also cause the low frequency dielectric relaxation.     

Dehydration of magnesium sulphate heptahydrate investigated by quasi isothermal—quasi isobaric TG

With the help of the quasi isothermal-quasi isobaric technique, completed with DTA and thermomicroscopic examinations, several new observations have been made regarding the dehydration process of MgSO₄ · 7 H₂O. It was found that under given conditions the material, first at 50°C and then at 95°C, melts in an incongruous way. In the course of the latter transformation, a ternary system consisting of solid MgSO₄ · 3 H₂O, a solution phase saturated with respect to the trihydrate, and a water vapour phase, is formed. The saturated solution reaches its boiling point at 105°C. Without any temperature change, the system loses four moles of water and solid MgSO₄ · 3 H₂O remains. This decomposes at 115°C and a mixture consisting of MgSO₄ · H₂O and MgSO₄ · 2 H₂O forms, the proportion of which depends on the experimental conditions. At 150°C, the latter compound loses one mole of water. The MgSO₄ · H₂O maintains constant weight up to 310°C, above which temperature the remaining water of crystallization is removed.     

Reactions of tetrakis(trifluoromethyl) diphosphine and bis(trifluoromethyl) phosphine with ruthenium carbonyl clusters; X-ray structure of tetraruthenium carbonyl cluster derivatives

Reaction of [Ru₃(CO)₁₂ with (CF₃)₂P---P(CF₃)₂ in p-xylene at 140°C yielded the compounds [Ru₄(CO)₁₃{μ-P(CF₃)₂}₂] (1), [Ru₄(CO)₁₄{μ-P(CF₃)₂}₂] (2) and [Ru₄(CO)₁₁{μ-P(CF₃)₂}₄] (3). Reaction with [(μ-H)₄Ru₄(CO)₁₂] under similar conditions yielded [(μ-H)₃Ru₄(CO)₁₂{μ-P(CF₃)₂}] (4). All four compounds have been characterised by X-ray crystallography. The fluxional behaviour of the hydrides in 4 has also been studied by variable-temperature NMR spectroscopy. Compounds 1, 2 and 4 were also obtained from the reactions of Ru₃(CO)₁₂ with (CF₃)₂PH in dichloromethane at 80°C.     

Temperature dependence of the rate constant for reactions of hydrated electrons with H, OH and H2O2

The temperature dependence of the rate constants, for the reactions of hydrated electrons with H atoms, OH radicals and H₂O₂ has been determined. The reaction with H atoms, studied in the temperature range 20–250°C gives k(20°C) = 2.4 × 10¹⁰M^-1s¹ and the activation energy E_A = 14.0 kJ mol^-1 (3.3 kcal mol^-1). For reaction with OH radicals the corresponding values are, k(20°C) = 3.1 × 10¹⁰M^-1s^-1 and E_A = 14.7 kJ mol^-1 (3.5 kcal mol^-1) determined in the temperature range 5–175°C. For reaction with H₂O₂ the values are, k(20°C) = 1.2 × 10¹⁰M^-1s^-1 and E_A = 15.6 kJ mol^-1 (3.7 kcal mol^-1) measured from 5–150°C. Thus, the activation energy for all three fast reactions is close to that expected for diffusion controlled reactions. As phosphates were used as buffer system, the rate constant and activation energy for the reaction of hydrated electron with H₂PO₄^- was determined to k(20°C) = 1.5 × 10⁷M^-1s^-1 and E_A = 7.4 kJ mol^-1 (1.8 kcal mol^-1) in the temperature range 20–200°C.     

Fluorescent ferroelectric liquid crystalline copolyacrylates

Fluorescent liquid crystalline side chain polymers were synthesized by copolymerization of a ferroelectric monomer and 5 per cent of various blue fluorescent naphthalic imide dye comonomers. Those copolymers were characterized by DSC, X-ray, GPC and optical microscopy. In favourable cases, fast switching fluorescent ferroelectric polymers resulted, exhibiting high tilt angles (up to ∼ 34°) and spontaneous polarization values (up to ∼ 115 nC cm^-2) in the S*_c phase. One fluorescent copolymer shows orthogonal smectic phases exclusively due to the structure of the incorporated fluorescent comonomer. In this case a strong electroclinic effect and high induced tilt angles (12° 10 V μm^-1) have been observed in the S_a phase. Order parameters, S, of the dye moieties up to 0.64 were measured in the room temperature S_b phase for the copolymers     

Reactions of tetrafluorophthalic and difluoropyromellitic acids with sulphur tetrafluoride

Fluorination of tetrafluorophthalic acid (1) with an SF₄/HF mixture at 190–300°C afforded,beside the expected perfluoro-o-xylene (2) and perfluoro-o-toluyl fluoride (4), considerableamounts of octafluoro-1,3-dihydroisobenzofuran (3). The reaction with difluoropyromelliticacid (5) at 190°C gave a mixture of perfluoro-2,4,5-trimethylbenzoyl fluoride (9),perfluoro-2,5-dimethylterephthaloyl difluoride (10a), perfluoro-2,4-dimethylisophthaloyl difluoride(10b) and perfluoro-5-methyl-6-fluoroformyl-1,3-dihydroisobenzofuran (11), but at 300°C perfluorodurene (6), perfluoro-5,6-dimethyl-1,3-dihydroisobenzofuran (7) and perfluoro-2,4,5-trimethylbenzoyl fluoride (9) were obtained in a 3:1:0.8 ratio.     

The X-ray analysis of dihydroheliangine monochloroacetate

In connection with heliangine, extracted from leaves of Helianthus tuberosus L., the crystal structure of dihydroheliangine monochloroacetate, C₂₂H₂₉C₇Cl, has been determined by X-ray methods. The tetragonal unit cell with dimensions, A = 13·77, C = 11·95 Å, contains four molecules, the space group being P4₁ - C₄² or P4₃ - C₄⁴. Using a three-dimensional Patterson function and minimum functions, a majority of atomic positions were determined. Further elucidation of the structure was continued by alternative application of successive least squares treatment and Fourier syntheses. The structure thus determined is fully consistent with the chemical results obtained by Morimoto et al. The final R-value is 0·134 (using 1680 data with sin² θ/λ² < 0·20) or 0·152 (using 2419 data with sin²θ/λ² < 0·27).     

The effects of transition metal complexes on the permeation of small gas molecules through cellulose acetate membranes

Cellulose acetate (CA) membranes containing RuCl₃·3H₂O and RhCl [P(C₆H₅)₃]₃ were prepared reproducibly. Such membranes, on treatment with CO, formed metal-carbonyl species at relatively low temperature. The Ru-carbonyls formed in CA were quite stable at 40°C in comparison with the Rh-carbonyl species and, interestingly, there was no permeation of CO gas through the ruthenium-containing CA membrane at 40°C. However, the permeation of other gas molecules, such as H₂, N₂ and O₂, through the same membrane was reduced only slightly, probably due to the cross-linking effect of the transition metal complexes in CA. It was found that essentially pure H₂ gas could be recovered from a 1: 1 mixture of H₂ and CO gases using ruthenium-containing cellulose acetate membranes.     

Ferroelectric liquid crystals with a thioester linking group positioned at the chiral tail

A homologous series of chiral thioesters, hexyl (2S)-2-[6-(4-alkanoyloxyphenyl)benzoyloxy-2'-naphthyl]thiopropionates, HnPBNTP (n=7-11) was synthesized, and the mesophases and electro-optical properties were studied. All the materials possess a ferroelectric SmC* phase, which was confirmed by the switching current behaviour. It was found that compounds with a shorter alkyl chain length (n=7-9) displayed SmA* and SmC* phases, while, compounds with a longer alkyl chain length (n=10 and 11), displayed the SmC* phase exclusively. Spontaneous polarization and apparent tilt angle were also measured. The maximum P_s values are in the range 15-26 nC cm^-2, and the maximum θ values are in the range 23°-28°.     

An investigation of the structure of butadiene by high resolution infra-red and raman spectroscopy

The rotational Raman spectra of butadiene and butadiene-d₆ are found to consist of discrete lines having small ≈0·4 cm⁻¹) yet almost constant spacings, as would be expected for symmetric or nearly symmetric top molecules. An infra-red absorption band (Type C) of butadiene at 908 cm⁻¹ is observed to have a spacing of about 2·5 cm⁻¹. Both the Raman and infra-red spectra provide evidence for the trans structure of the butadiene molecule. From the rotational constants A″ and ″ the following structural parameters were obtained: C=C---C) = 122·9 ± 0·5° rC---C) = 1·47₆ ± 0·010 Å dy]somewhat shorter than recently determined from electron-diffraction experiments).