K. Goubitz

Solid-State Structure and Spectroscopy of Chromoionophoric Acridinium Derivatives

Abstract A comparative study is presented of the chromoionophoric and fluoroion-ophoric behaviour of 1 and 2, which both consist of an aza-crownether-ionophore integrated with a twisted 9-aryl-acridinium chromophore. In 2 the degree of ground state twisting in the chromophore is enhanced relative to that in 1. While the cation induced spectral shifts (i.e. chromoionophoric effect) in 2 are not significantly different from those in 1, the cation binding properties of 2 are enhanced significantly. This is explained in terms of better preorganisation of 2 and higher charge density of the anilino nitrogen in 2 as supported by X-ray data. In addition to their chromoionophoric properties, the non-fluorescing 1 and 2 show an acridiniumtype of fluorescence upon binding of H+ and Ag+ ions. This fluoroionophoric effect is also significantly enhanced in 2 where restricted internal rotational freedom allows for a higher fluorescence quantum yield of the complex.

Crystal and Molecular Structure of the Nematogenic Compound 4-Cyanophenyl-4′-n-Heptylbenzoate (CPHB)

Abstract It is now well established that for a proper understanding and interpretation of several physical properties of liquid crystalline phases, a knowledge of the molecular arrangement in the crystalline state is very useful. A preliminary survey of the present knowledge of the solid-mesophase relationships has been given by Bryan1. The classical view is that in typical nematogen crystals the long narrow molecules lie more or less parallel and are interleaved giving what was described by Bernal and Crowfoot2 as an imbricated packing and that the transformation from the solid to the nematic phase is characterised by the breakdown of the positional order of the molecules but not of the orientational order. Leadbetter3 has however pointed out that although this is true of at least the majority of the small number of cases so far known, it is a generalisation which must at this stage be treated with caution. Hence the determination of the crystal and molecular structure of the title nematogenic compound w...

An X-Ray Study of Cyanophenylpyrimidines Part III. Crystal Structure of 5-(trans-4- Heptylcyclohexyl)-2-(4-Cyanophenyl) Pyrimidine

Abstract The compound C24H31N3 at higher temperatures shows a smectic liquid crystal phase followed by a nematic phase. The crystals belong to space group P21/c, with a = 13.0422(6), b = 17.7464(11) and c = 9.5950(5) A, β = 107.170(1)°, Z = 4. The structure has been solved by the direct method program system SIMPEL83 and refined to an R-value of 0.041. Molecules are packed in associated pairs running parallel to the a axis. An X-ray study at higher temperatures shows that the smectic phase is also built ap from similar pairs of molecules.

The molecular and crystal structure of a polar mesogen 4-cyanobiphenyl-4′-hexylbiphenyl carboxylate

The crystal structure of the compound 4-cyanobiphenyl-4′-hexylbiphenyl carboxylate (6CBB), which exhibits both monolayer smectic A and nematic phases, has been determined by direct methods using single crystal X-ray diffraction data. The structure is triclinic with the space group P-1 and Z=2. The unit cell parameters are a=9.3511(7)u2009Å, b=11.2456(7)u2009Å, c=13.1417(6)u2009Å, α=85.872(4)°, β=76.258(5)° and γ=70.697(5)°. The molecule is found to be slightly bow-shaped although the alkyl chain is in all-trans conformation. The phenyl rings in 6CBB are non-coplanar. The packing of the molecules in the crystalline state is found to be a precursor to the smectic A phase structure. Comparison of crystal structures and packing of the four-ring 6CBB with those of the two-ring nCB or nOCB compounds has been made to explain the observed phase behaviour.

Crystal and Molecular Structure of a Cybotactic Nematic Compound bis(4′-n-Butoxybenzal—Chloro-1,4-Phenylinediamine

Abstract The crystal and molecular structure of a cybotactic nematic compound bis-(4′-n-butoxybenzal—chloro-1,4-phenylinediamine (BOCP in short) has been reported. The crystals belong to monoclinic system with space group P21/c, a = 36.693, b = 14.940, c = 6.1058 A, β = 93.89° and Z = 4. The structure was determined by direct methods, with a final R-value of 0.049 (Rw = 0.070) for 1979 observed reflections. The molecules are almost in their extended conformation. The molecules are inclined to the ab-plane, the planar aromatic core region being almost parallel to the ac-plane. Molecular packing in relation to the liquid crystal phase is discussed.

An X-ray Study of Cyanophenyl Pyrimidines. Part I-Crystal Structure of PCCPP

Abstract The crystal and molecular structures of the nematogen 5-(trans-4-ethylcyclohexyl)-2-(4-cyanophenyl)-pyrimidine have been determined by direct methods. The crystal belongs to the monoclinic system with space group P2/a, a = 18.437, b = 7.1027, c = 15.342 ° β = 106.07° and Z = 4. Least squares refinement leads to R = 0.049 (Rw = 0.048) for 2246 observed reflections. The molecules are in an almost fully extended conformation. Phenyl and pyrimidine rings are highly planar and lie almost in the ac plane. The cyclohexane ring is in chair form. Pairs of molecules related by a centre of symmetry are packed into sheets in the ac plane and these sheets are stacked along the b-axis. The structure is of a common type for a nematogen and the transition to the mesophase is a simple displacive type. The pairing of molecules resulting from the interaction between opposed dipoles of cyano-groups exists in both the crystalline and the nematic state.

X-Ray Structural Analysis in the Crystalline Phase of a Nematogenic Fluoro-Phenyl Compound

Crystal and molecular structures of a nematogenic compound 4′-(3,4-difluoro phenyl)-4-propyl-bicyclohexyl (3ccp-ff) has been determined by direct methods using single crystal X-ray diffraction data. The compound (C21 H30 F2) crystallizes in the monoclinic system with the space group P21/n and Z = 4. The unit cell parameters are a = 16.293(15) Å, b = 6.278(7) Å, c = 18.3064) Å and β – 96.846)°. The two cyclohexyl rings are almost coplanar, so also are the phenyl ring and the propyl chain. The cyclohexyl groups are in chair conformation. Imbricated mode of packing of the molecules is found in the crystalline state which is precursor to the nematic phase structure. Several close van der Waals interactions are found to exist between the neighbouring molecules. Short-range antiparallel type of association is found to exist between the neighbouring molecules in crystalline as well as in nematic phase.

X-ray structural analysis of a mesogenic compound of N,N'-bis-(p-butoxybenzylidene)-alpha,alpha'-bi-p-toluidine

Abstract Crystal and molecular structure of a mesogenic compound N,N′-Bis-p-(Butoxybenzylidene)-α,α′-bi-p-Toluidine has been determined by direct methods. The compound crystallizes in triclinic system with space group P1. Cell parameters are a = 6.116(3), b = 7.916(3), c = 31.421 (8)A, α = 92.39(2), β = 92.35(3), y = 96.59(2)° and z = 2. The two symmetrical fractions of the molecule are almost planar but the angle between these planes is 63.5°. The molecules are arranged in tilted layers which is precursor to the tilted smectic phase. Crystal-mesophase transition is discussed.

Structures and Properties of an Alkenyl Liquid Crystalline Compound

Abstract The crystal and molecular structure of the mesomorphic compound 4(3″-Pentenyl)4′(cyano)1,1′-bicyclohexane having the molecular formula C5H9−C6H10−C6H10−CN has been determined by direct methods. The crystal belongs to the monoclinic system with space group P21/n, a = 5.609, b >= 31.390, c >=9.562 A β >= 98.89° and Z >= 4. Least squares refinement leads to R >= 0.065 (Rw >= 0.081) for 2326 observed reflections. The packing in the solid crystalline state of the compound is discussed and is compared with that derived from X-ray diffraction data in the liquid crystalline state.

Effect of molecular conformation on the mesogenic properties of a partially fluorinated nematogenic compound investigated by X-ray diffraction and dielectric measurements

A bicyclohexyl trifluorophenyl compound has been studied in both the crystalline and the nematic phases by X-ray diffraction and dielectric methods. Difference in molecular geometry, conformation, packing, density and orientation of molecular dipole moments explains substantial increase of melting point and decrease of nematic range in the compound compared to structurally very closely related systems. The facts that the apparent length of molecules is about 1.35 times more than the most extended molecular length and the average value of the dielectric constant is about 6% less than the extrapolated isotropic phase values give conclusive evidence of existence of short-range antiparallel order in fluorinated compounds like cyanobiphenyls and isothiocyanates supporting our previous conclusion (Haldar S, Mandal PK, Prathap SJ, Guru Row TN, Haase W. X-ray studies of the crystalline and nematic phases of 4′-(3,4,5-trifluorophenyl)-4-propylbicyclohexyl. Liq Cryst. 2008;35:1307–1312). About 5 V dc bias is sufficient for switching the orientation of the molecules from homogeneous to homeotropic configuration. Dielectric anisotropy and splay elastic constant exhibit similar decreasing trend with temperature. Only one strong Debye-type dielectric absorption process, connected with reorientations of the molecules around the short axis, is observed. Dielectric spectra fitted to modified Cole-Cole function revealed that the relaxation frequency increases from 2.6 MHz to 4 MHz exhibiting Arrhenius-type behaviour. Activation energy of the thermally activated relaxation process is about 32.4 kJ/mole.