Smectic E Research Articles

Rich polymorphism in 4‐propyl‐4′‐thiocyanato‐1,1′‐biphenyl (3TCB) revealed by adiabatic calorimetry

The thermal behaviour of 4‐propyl‐4′‐thiocyanatobiphenyl (3TCB) was studied by adiabatic calorimetry. In addition to the isotropic liquid and smectic E (SmE) phases, five crystalline phases were detected. Such a rich polymorphism was not observed for the other low‐n members of the nTCB homologous series (n = 2–5). The observed anomalies are analysed and the possible origin for their existence is suggested. The temperature quench of the SmE phase leads to a metastable crystalline state with a multiple‐stage relaxation process.

Charge-carrier transport in smectic mesophases of biphenyls

The charge-transport properties of ω,ω′-alkylalkoxylbiphenyls, one of the simplest calamitic (rodlike) liquid crystals exhibiting smectic mesophases, were investigated using the time-of-flight experiments. A mobility of ∼10−3 cm2∕V s was determined for holes in both smectic B (SmB) and smectic E (SmE) phases. This high electronic mobility, comparable to the highest mobilities in amorphous organic semiconductors, indicates that the self-organization of such simple molecules including small aromatic compounds is quite effective in enhancing the charge transport in organic materials. The mobilities of both smectic mesophases, however, depend on temperatures, which is not the case for other typical smectic liquid crystals reported previously. Interestingly, the mobility in the SmE phase exhibits a Poole–Frenkel type of field dependence. Detailed experimental studies have shown that this field dependence is not due to the trapping effect of carriers caused by chemical contaminants or structural defects such as domain boundaries and dislocation, but arises from the widely distributed density of states with a distribution width of 0.11–0.13 eV for the SmE phase, as estimated based on a two-dimensional Gaussian disorder model. The phase-specific carrier transport in biphenyls may provide us with unique insights leading to a more complete understanding of the charge carrier transport in the mesophase materials.

Molecular Dynamics and Residual Entropy in the Soft Crystal, SmE Phase, of 4-Butyl-4‘-isothiocyano-1,1‘-biphenyl

Molecular dynamics and resulting disorder in the soft crystal, smectic E (SmE) phase, were studied in detail for the title compound, 4-butyl-4'-isothiocyano-1,1'-biphenyl (4TCB), by (1)H NMR spectroscopy and adiabatic calorimetry. The ordered crystal phase of 4TCB was realized for the first time under ambient pressure after long two-step annealing and used as the reference state in the analysis of the experimental results. Four motional modes were identified in the SmE phase through the analysis of the (1)H NMR T(1). The residual entropy was determined as ca. 6 J K(-1) mol(-1). This magnitude implies that most of the disorder in the SmE phase at high temperatures is removed on cooling except for the head-to-tail disorder of the rod-shaped 4TCB molecule. Standard thermodynamic functions are tabulated below 375 K.

Synthesis and Properties of Hexakis(6-octyl-2-azulenyl)benzene as a Multielectron Redox System with Liquid Crystalline Behavior

[structure: see text] This paper describes the cyclotrimerization reaction of di(2-azulenyl)acetylenes (2a,b) catalyzed by Co2(CO)8 to produce hexa(2-azulenyl)benzene derivatives (1a,b). The cyclooligomerization of 2a and 2b utilizing CpCo(CO)2 as a catalyst produced (eta5-cyclopentadienyl)[tetra(2-azulenyl)cyclobutadiene]cobalt complexes (3a,b). The redox behavior of hexakis(6-octyl-2-azulenyl)benzene (1b), bis(6-octyl-2-azulenyl)acetylene (2b), and the cobalt complexes 3a and 3b along with 6-octyl-2-phenylazulene (19) was examined by cyclic voltammetry (CV). The reduction of compound 1b exhibited multiple-electron transfers in one step upon CV with a reduction potential similar to that of compound 19. However, the CVs of compounds 2b, 3a, and 3b were characterized by stepwise waves because of the reduction of each azulene ring. The mesomorphic behaviors of 1b, 2b, and 19 were also studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques. A new series of azulene derivatives, 1b, 2b, and 19, substituted by a long alkyl chain at the 6-position shows mesomorphism with crystalline polymorphs. Compound 1b showed a large temperature range of hexagonal columnar mesophases (Col(ho)) from 115.5 to 199.9 degrees C. Compound 2b has rectangular columnar (Col(ro)), smectic E (S(E)), and nematic (N) mesophases. Compound 19 exhibited an S(E) mesophase.

A novel pentaerythritol‐based carbosilane liquid crystalline dendrimer containing 12 nitroazobenzene groups on the periphery

AbstractA novel liquid crystalline dendrimer with peripheral mesogenic units was successfully prepared. Azo‐reaction and Williamson synthesis were employed in the preparation of the mesogenic unit 4–[4–(6‐hydroxyhexyloxy)‐phenylazo]nitrobenzene (M‐NO2). A terminal SiCl functional carbosilane dendrimer based on pentaerythritol was used as dendritic scaffold and subsequently functionalized with the aforementioned groups. Investigation of the liquid crystalline properties of the mesogen‐functionalized dendrimer PCSi‐1G‐NO2 by polarizing optical microscopy, DSC. and X‐ray diffraction showed that it exhibits smectic E (SE) phase, different from the corresponding mesogenic unit, which shows nematic phase. Furthermore. the temperatures of both the melting point and the clearing point of the mesogen‐functionalized dendrimer decrease, and the temperature region of the SE phase is wider than that of the nematic phase.

Carrier Transport in Calamitic Mesophases of Liquid Crystalline Photoconductor 2-Phenylnaphthalene Derivatives

We have characterized carrier transport in mesophases of 2-phenyl-naphthalene derivatives by transient photocurrent measurements. In contrast to ionic conduction of isotropic and nematic (N) phase, electronic and ambipolar carrier transport was observed in smectic A (SmA), smectic B (SmB), and smectic E (SmE) phases. In particular, anomalous large mobilities on the order of 10−2cm2/Vs were obtained in SmE phase. Carrier mobilities in mesophases of 2-phenynaphthalene derivatives were increased stepwise corresponding to sophistication of molecular ordering from N to SmE. This indicated a dominant effect of molecular ordering on the carrier transport of calamitic mesophases.

Carrier Transport Properties of Calamitic Liquid Crystalline Photoconductors

Carrier transport properties of a calamitic liquid crystalline photoconductor (Calamitic LCPC), 2-phenynaphthalene derivatives were investigated by steady state and transient photocurrent measurements. These materials exhibited calamitic mesophases such as nematic (N), smectic A (SmA), smectic B (SmB), and smectic E (SmE) phases, which depended on hydrocarbon chains attached to a core moiety of 2-phenylnaphthalene. A large photoconductive anisotropic ratio over 50 was observed under light illumination due to molecular alignment. The transient photocurrents from time-of-flight measurements exhibited non-dispersive and ambipolar carrier transits in all the mesophases, whereas only small current decays were observed in polycrystalline phase.The carrier transport in the smectic phases was electronic, while the ionic conduction was dominant in both of isotropic and nematic phases. The mobility was increased stepwise when the phase transition took place with a decrease in temperature. The mobility for smectic phases was independent of temperature and applied electric field, while those in isotropic and N phases depended on temperature positively. The highest mobility of 10−2 cm2/Vs was obtained for both electrons and holes in SmE, which was four orders of magnitude faster than those of the conventional disordered photoconductors.These results indicate how effective the molecular alignment is in order to upgrade the carrier transport in spite of a small π-conjugate core moiety of 2-phenylnaphthalene.

Structures of wholly aromatic copolyesters composed of 4-hydroxybenzoic acid, biphenol and terephthalic acid: 1. Effects of copolymer composition on thermal properties

The structures and phase transitions of thermotropic liquid-crystalline copolymers composed of 4- hydroxybenzoic acid (HBA), terephthalic acid (TPA) and biphenol (BP) (HBA/TPA/BP copolymer) with five different copolymer compositions were investigated using X-ray diffraction and thermal analyses. The HBA/TPA/BP copolymers exhibited nematic liquid-crystalline characteristics at elevated temperatures, and complicated fine structures formed in the solid state when cooled from the anisotropic nematic melt. The crystal to smectic E (SE) type and SE type to hexagonal phase transitions inevitably occurred at approximately 100°C and 300°C, respectively, in the samples of varied copolymer compositions. Particularly, the molecular chain of almost the same monomer fraction (HBA/TPA/BP = 35/32.5/32.5) showed sharp endothermic peaks on the differential scanning calorimetry curve; thereby, the crystallite size was considered to be comparatively uniform. The SE-type structure was frozen in almost all the quenched samples. Highly ordered crystallites were formed in the samples when they were annealed above 300°C where the hexagonal interchain packing order was formed.

Conformational Change of Liquid Crystalline Ethyl-4'-n-Octyloxy Biphenyl Carboxylate at the Liquid Crystal Phase Transitions

The temperature dependence of Raman spectra of ethyl-4'-n-octyloxy biphenyl carboxylate (EOBC) has been observed in the spectral range of 10–2000 cm-1. Raman peaks at 523, 415, 355 and 242 cm-1 have shown a drastic change in their intensity at the phase transition from the crystalline solid to the liquid crystal. Particularly, the 415 cm-1 peak emerging during the phase transition from crystalline solid to smectic E (SmE) discontinuously increases at the subsequent higher temperature phases. This can be assigned to the vibrational mode of the aromatic frame. The appearance of this peak is explained by the change in the dihedral angle between two benzene rings of biphenyl in EOBC at the phase transition from the crystalline solid to SmE. This change occurs discontinuously upon going to higher temperature phases. In EOBC, smectic polymorphism seems to arise from the change of the molecular conformation of the aromatic frame.