Two types of binary mixtures were examined to optimize the siloxanyl fraction by filling the gap between the two Cub-phaseforming molecules with di- and tri-siloxanyl terminals. Adding siloxanyl amount to the disiloxanyl system largely inhibited crystallization, increasing the stability at room temperature of the meta-stable Ia3d phase obtained by cooling from the high-temperature phase. The effect was prominent for the mixtures containing both di- and tri-siloxanyl compounds. The most prominent result was obtained for the 50:50 mixture; the Ia3d phase was quite stable and survived at room temperature after more than 1 year, as if it were like a thermodynamically stable phase.
(Phys. Chem. Chem. Phys., 25, 19891-19898 (2023).)
Interface geometry and skeletal graphs are two complementary characterizations of micro-phase separated organizations in soft matter. This paper explores the possibility of analyzing the latter instead of the former to get insights into aggregation structures. Analyses of the ideality of vertices geometry (closeness to the equiangular three-coordination) and the spatial homogeneity of the vertex distribution strongly suggest that the pair of noh nets, recently proposed for thermotropic liquid crystals of rodlike molecules without the interface between counter-chiral domains, is beneficial besides the srs twins if the cubic symmetry is presumed as an experimental input.
(Bull. Chem. Soc. Jpn., 96, 607-613 (2023))
Chiral aggregation structure spontaneously formed by achiral rodlike molecules, a long-time unsolved problem in liquid crystal science, has been clarified by applying a new crystallographic algorithm recently developed while utilizing aggregation characteristics of this type. Bicontinuously interwoven networks characterize it similarly to the neighboring Gyroid phase in a phase diagram against the alkyl chain length and temperature. However, the network connectivity is significantly different from the bicontinuous networks that have been either known for related compounds or assumed for this phase. The network is compatible with the homochiral arrangement of rodlike molecules with successive twists by a proper angle between adjacent junctions.
(Soft Matter, 19, 1194-1201 (2023))
Spontaneous formation of structures with "cubic symmetry" is observed by molecular dynamics simulation.We show that many phases with cubic symmetry with planes of edge dislocations appear when the Sm layers are allowed to break up at relatively low pressures. The phases obtained can be interpreted as bend-grain-boundary phases consisting of edge dislocations, the counterpart of twist-grain-boundary phase consisting of screw dislocations.
(Liq. Cryst., 50, 1383-1391 (2023))
A recent intriguing finding that a helical network arrangement forms the bicontinuous cubic phase is attracting great attention for its possibility for new routes to asymmetric synthesis by achiral molecules. However, the design of the molecular structure for the cubic phase is still unrevealed. Herein, we show that a nonsymmetric core molecule with larger naphthalene and smaller benzene moieties at each side of the central linkage and the same disiloxanyldecyloxy terminal at both terminals is the first example for forming both single-layered and double-layered core assembly modes in the Ia3d phase as a single molecule system. The molecule forms the former mode at high temperatures as a thermodynamically stable phase, similarly to the symmetric naphthalene core system, whereas, on cooling below a temperature (ca. 350 K), a metastable Ia3d phase forms a double-layered core state down to room temperature, which is common to the benzene core system. As another effect of the nonsymmetric core, the cubic phase is maintained at room temperature for more than 100 days with slight distortion. Infrared spectral studies and quantum chemical calculations suggested the easy transformation between the two core assembly modes. The core nonsymmetry can be a versatile fine-tuning of the core assembly mode and phase stability for the cubic phase molecules.
(Chem. Eur. J., 27, 10293-10302 (2021))
1,2-Bis(4'-n-alkoxybenzoyl)hydrazine [BABH(n), n is the number of carbon atoms in the alkyl chain] exhibits two different Ia3d cubic phases depending on the chain length (5 =< n =< 13 and 15 =< n =< 22). The molecular packing modes, not only of molecular cores but also of alkyl chains, are investigated through a maximum entropy method (MEM) and an analysis of the chain-length dependence of the body diagonal of the unit cell. The analyses revealed the difference in molecular packing. The short-chain Ia3d structure of the BABH(n) (5 =< n =< 13) is constructed by single-layered core motifs and bilayers of alkyl chains, of which the orientation is tilted from the normal to the layer. The long-chain Ia3d structure (15 =< n =< 22) is formed by double-layered core motifs and monolayers of alkyl chains, of which the orientation is normal to the layer. Based on the molecular packing modes, the reentrant behavior between the two Ia3d phases was clarified. It was revealed that the alkyl chains of BABH(n) molecules play an essential role in the formation and stability of the two Ia3d structures.
(Phys. Chem. Chem. Phys., 21, 23705-23712 (2019))
Effect of the preference for non-parallel alignment without a preferred twist sense of neighboring headless spins is examined through computer simulations on extended Maier-Saupe models on the simple cubic lattice with/without next-nearest neighbor interactions. Phase diagrams containing four phases are constructed for a varying degree of the preference for the local twist and for a strength of the next-nearest-neighbor interaction. It is shown that the preference for the local twist brings about not only the instability for the nematic (uniaxial) order but also that for a spatial order to reduce the frustration arising from the local twist. The next-nearest-neighbor interaction can produce even a weak spatial order of local chirality. The results are discussed in relation to the formation of a liquid crystalline gyroid phase and chiral phases in ensembles consisting of anti-spindle molecules that are axially symmetric and achiral.
(J. Phys. Soc. Jpn., 86, 084602 (2017))
Mirror symmetry breaking was observed by mixing of equimolar amounts of two gyroid phase-forming achiral molecules, one having a short and the other with a long tail. This fact indicates the importance of a critical chain length for the appearance of the chiral bicontinuous cubic (Cubbi) phase. It also demonstrates that the molecular-level mobility is closely connected to the formation and stabilisation of those Cubbi phases.
(Phys. Chem. Chem. Phys., 18, 17341-17344 (2016))
Introduction of oligodimethyl siloxane segments at the termini of the alkyl tails has been employed to stabilize bicontinuous cubic (Cubbi) phase of a chain-core-chain type molecule having 1,2-bis(benzoyl)hydrazine central core with two chains attached at the 4' position of each benzoyl moiety. In this study, three silylated molecules, bis-C10Si2, bis-C10Si3, and C10Si2-C8C=C were synthesized, where "CnSim" represents the number of carbon and silicon atoms in the chain and "bis" means the two chains being the same whereas the last one is asymmetric with respect to the core. The phase behaviors were examined by using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction techniques. All three compounds forms Cubbi phases and their formations were compared including their parent compound bis-C18. It was clearly revealed that the introduction does effectively suppress crystallization at low temperatures and as a result, stabilizes the Cubbi phases, in an extreme case down to room temperature. The semi-quantitative analyses in terms of lattice constant and three-dimensional electron density map help us to better understand the selfassembly
process in the Cubbi phases. The study also revealed the introduction of oligodimethyl siloxane segments is not only an alternative of hydrocarbon segment but is able to provide versatile design strategy for obtaining stable formation of the Cubbi phases.
(Phys. Chem. Chem. Phys., 18, 9013-9020 (2016))
Assuming the twisted arrangement of rodlike molecules as the origin of the chirality as in the existing model, a new model of the molecular arrangement in the cubic "Im3m" phase is proposed. The adoption of a basic structure different from that assumed in the existing model resolves most difficulties of the model including the random placement of defects concerning the sense of twist.
(Phys. Chem. Chem. Phys., 18, 3280-3284 (2016))
Two cubic (Cub)-phase forming compounds, 4'-n-hexadecyloxy-3'-cyanobiphenyl-4-
carboxylic acid (ACBC-16) and its 3'-nitro analogue (ANBC-16) were studied by infrared (IR) spectroscopy and quantum chemical calculation at DFT/B3LYP level. In contrast with the identical stabilization energies for dimerization and frequencies of aromatic ring C=C stretching (nu_(C=C)ring) band predicted for their respective model compounds, in the temperature region of the Cub phase, the molar fraction of hydrogen-bonded COOH groups estimated for ACBC-16 was different from, and by ca. 5 % at maximum larger than, that for ANBC-16, and the nu_(C=C)ring band frequency of ACBC-16 was by 3 cm-1 lower than that of ANBC-16. These results can be ascribed to the different steric effects of the side groups; the CN group would make possible the close contact of neighboring phenyl rings while the bulky NO2 group would not, giving slightly more stabilized dimerization of ACBC-16 in the Cub phase than in ANBC-16.
(Liq. Cryst., 42, 143-157 (2015))
Phase transitional behavior of 1,2-bis(4'-n-hexyloxybenzoyl)hydrazine (BABH(6)) was studied by differential scanning calorimetry (DSC) and polarizing microscopy. While the DSC trace above melting temperature is consistent with the existing report, it is not for crystalline states. A number of tiny and spiky anomalies was observed in the first cooling run whereas the number of anomalies decreases, and finally disappeared in repeated runs. Microscopic observation revealed that the tiny and spiky anomalies arise from collapsing of crystals. When the sample was cooled from the cubic phase (430 K), the tiny and spiky anomalies revived on the cooling process, which can be ascribed to the liquid-crystallinity of the cubic phase.
(Thermochim. Acta, 590, 160 (2014))
Phase behavior of a thermotropic cubic mesogen, 1,2-bis(4'-n-hexyloxybenzoyl)hydrazine [BABH(6)] was investigated under pressure up to about 55 MPa using a polarizing optical microscope equipped with a high-pressure optical cell. BABH(6) shows the crystal (Cr) - cubic (Cub) - isotropic liquid (I) phase transition at ambient pressure on heating. The smectic C (SmC) phase was induced above 32 MPa, showing the unusual phase sequence of Cr - Cub - SmC - I similar to those in BABH(n) (n = 8 - 10). The boundary between Cub and SmC phases exhibited a negative slope dT/dP of about -1.0 K MPa-1.
(Liq. Cryst., 41, 731 (2014))
Maximum entropy analyses of small-angle X-ray diffraction patterns of a series of title compounds [1,2-bis(4'-n-alkyloxybenzoyl)hydrazine, n: number of carbon atoms in an alkyl group] yield a new description of the so-called gyroid phase. The structure is described as two sets of connected triangles, instead of jungle gyms consisting of rods, embedded in two spaces separated by a mathematical gyroid. The reconstructed electron density provides new evidence of molecular packing: While molecules having short alkyl chains laterally aggregate to form single layers of triangular shape with nearly vertical alignments, those with long chains split into two groups on both sides of the triangular planes. The formation of double layers of the molecular cores is tolerable with the possible formation of hydrogen bonds between shifted molecules, and adjusts the volume fraction of the core part to attain the stability of the reentrant gyroid phase upon chain elongation.
(J. Phys. Soc. Jpn., 81, 094601 (2012): Paper of Editors' Choice)
[Commentary by T. Dotera at JPSJ site]
The transformation between two thermotropic bicontinuous cubic (Cub) phases has kinetically been determined by time-resolved X-ray diffraction. The system investigated was 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (denoted as BABH-n, where n is the number of carbon atoms in the alkyl chain). Transitions from the Ia3d to Im3m phases of BABH-13 and from the Im3m to Ia3d phases of BABH-16 were induced by a temperature jump. Although the two Cub phases have quite different aggregation structures, the transitions between them appeared to occur without any diffractive intermediate states. A clear increase in the rate of transformation was observed upon increasing the extent of superheating, and the rate was slower than that reported previously for lyotropic systems. In addition, a comparison between the two transformations revealed that the rate of the Im3m to Ia3d transformation (of BABH-16) is ca. 5 times faster than that of Ia3d to Im3m (of BABH-13).
(Langmuir, 26, 11605 (2010))
Extensive application of chemical thermodynamics to exotic aggregation formed in thermotropic liquid crystals is briefly described. Through thermodynamic analyses and considerations of experimental results on liquid crystals, the unexpected sharing of common properties by thermo- and lyotropic liquid crystals is demonstrated. In some thermotropic liquid crystals, the terminal alkyl chain attached to the molecular core is highly disordered, as indicated by the magnitude of configurational entropy. The molten chain serves as intramolecular solvent (self-solvent), as evidenced by the close similarity between phase diagrams against chain length and composition in the binary system with n-alkane. These facts lead to the quasi-binary picture of thermotropic liquid crystals. Consideration of the thermodynamic potential expanded in terms of density fluctuation gives a new insight into the multicontinuous phases formed in simple systems consisting of anisotropic, rodlike particles.
(Pure & Appl. Chem., 81, 1783 (2009), review)
Process of structure determination of liquid-crystalline superstructures formed in a mesogenic series, bis(n-alkoxybenzoyl)hydrazine [BABH(n); n, the number of carbon atoms in the alkoxy group], is described. The chain-length (n) dependence of relative diffraction intensities from the Ia3d phase resolves the phase problem, leading to the structural description that the molecular centers are on the rods forming two interpenetrating jungle gyms. Theoretical consideration on the stability of superstructures and systematic MEM analysis reveal the coexistence of two aggregation modes (rods forming an extending jungle gym and closed sheets forming spherical shells) for the Im3m phase.
(J. Crystallogr. Soc. Jpn., 51, 169 (2009), review in Japanese)
The heat capacities of two members of a representative cubic mesogen, 4'-alkoxy-3'-nitrobiphenyl-4-carboxylic acid, were measured precisely. In accordance with the tendency reported previously for other members, the heat capacity in the cubic phase is smaller than that in the neighboring smectic C phase. The chain-length dependence of the reduction in the heat capacity upon the phase transition showed opposing trend depending on symmetry of the cubic phase (Ia3d or Im3m). This finding is rationalized by assuming the existence of two aggregation modes (rod and shell) in the Im3m phase in contrast to only the former mode in the Ia3d phase.
(Chem. Phys. Lett., 469, 157 (2009))
Structure of a complex superstructure self-organized by thermotropic mesogen, 1,2-bis(4'-n-alkoxybenzoyl)-hydrazine [BABH(n), where n is the number of carbon atoms in an alkoxy chain] was studied while paying special attention to the structure at the molecular level. Maximum entropy (MEM) analysis revealed that the molecular cores form two kinds of aggregates: Jungle gym with 3-fold junctions roughly on P minimal surface and spherical shells.
(J. Phys. Chem. B, 112, 12179 (2008))
It is shown by extending the Landau theory of freezing that a tendency to form multicontinuous complex structures is possessed by simple systems consisting of rigid rodlike particles. The excellent stability of the Gyroid phase, which is most widely observed in nature, is naturally explained. An exotic structure found in thermotropic liquid crystals can be further regarded as an example.
(J. Phys. Soc. Jpn., 77, 093601 (2008))
The phase transitions of a homologous series of 1,2-bis(4'-n-alkoxybenzoyl) hydrazine (BABH-n, where n is the number of carbon atoms in the alkyl chain and in this investigation varies from 4 to 22) were investigated by differential scanning calorimetric (DSC), polarizing optical microscopic (POM), X-ray diffraction (XRD), and infrared (FT-IR) measurements. The formation of bicontinuous type cubic (Cub) mesophases was observed enantiotropically for n > 6 and only on cooling for n = 5. The structures were examined by XRD, which revealed the presence of two types with symmetries Ia3d and Im3m, depending on n, and for n = 13, 15, and 16, phase transitions between the two Cub phases were observed. The FT-IR studies elucidated the formation of intermolecular hydrogen bonding between the CdO bond of one molecule and the NH group of another molecule, whose binding strength was temperature-dependent. The temperature dependence of the Cub lattice parameter (da/dT) varied from a large positive to a large negative value with increasing alkyl chain length n, which can be well-explained in terms of the temperature-responsive shape change of the constituent molecules. The packing structures of two Ia3d-Cub phases formed by shorter and longer alkyl chain members are compared and discussed. It is revealed that two competitive mechanisms are mainly operated on the self-organization, i.e., the preferential orientation of the long axes of the aromatic core parts parallel to each other and microsegregation between the aromatic core and alkyl chain parts of the molecules; the former mechanism is effective in the shorter chain members, whereas the latter is predominant in the longer chain members.
(Chem. Mater., 20, 3675 (2008))
The cubic (Cub) phase structure of a thermotropic mesogen 1,2-bis(4'-n-alkoxybenzoyl) hydrazine (BABH-n, where n is the number of C atoms in the aliphatic tail) was examined by small-angle X-ray scattering. The BABH-n system exhibits two types of Cub phases, Ia3d type and Im3m type. In the phase diagram as a function of the tail length (n) the Im3m-Cub phase region (n = 13-16) was sandwiched by two Ia3d-Cub phase regions with n < 13 and n > 16. On the basis of the triply periodic minimal surface (TPMS) picture, the Ia3d and Im3m structures are described by the gyroid (G) and doubled-P (PP) surfaces, respectively. In this paper we focused attention on the internal structure of the Ia3d-Cub phase at the molecular level. By examining how the relative intensity of the 220 reflection with respect to the 211 reference peak varies with n, we have successfully determined the position of the aliphatic tails, which are located on the G-TPMSs. As far as we know, this is the first approach to clarifying the internal structure of the thermotropic Ia3d-Cub phase. The usefulness of the systematic studies with respect to the tail length n has also been demonstrated.
(J. Appl. Crystallogr., 40, s279 (2007))
The structure type of bicontinuous cubic phase formed by 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (BABH) was strongly dependent on the alkoxy chain length, showing the phase sequence of Ia3d - Im3m - Ia3d with lengthening the chain.
(Chem. Lett., 35, 362 (2006))
The cubic phase structure of 4'-n-hexadecyloxy-3'-cyanobiphenyl-4-carboxylic acid (ACBC-16) was examined by X-ray diffraction. Unlike the octadecyloxy homologue showing an Im3m-type cubic phase, the cubic phase of ACBC-16 was of Ia3d type, both on heating and on cooling, similarly to the corresponding nitro-substituted analogue (ANBC-16). The lattice dimension a at 453 K was a =11.0 nm, 2.5% larger than the value for ANBC-16 and rather close to the value of ANBC-17 or -18. It is expected that the appearance of the cubic phase type, as a function of the number of carbon atoms n in the alkoxy chain in the ACBC-n series, is essentially the same as in the ANBC-n series, but shifted towards shorter n by 1 or 2. In the latter ANBC-n series, the cubic phase type is Ia3d for 15 <= n <= 18, while an Im3m type is formed for 19 <= n <= 21, both on heating and on cooling.
(Liq. Cryst., 33, 75 (2006))
Heat capacity of cubic mesogen ACBC(16) was measured by adiabatic calorimetry. Besides the known condensed phases, a new crystalline phase was found to undergo a glass transition around 165 K. The first-order nature of the SmC-cubic phase transition at 431.15 K was confirmed by observation of supercooling of the cubic phase for the first time. The entropy of this transition (DeltatrsS) lies on an extrapolating line of the chain-length vs DeltatrsS plot for the SmC - Im3m transition in ANBC, suggesting nearly the same core size for ANBC and ACBC. A broad hump in the Cp curve above the transition to isotropic liquid at 474.30 K was demonstrated to continue to a low-temperature side if the isotropic liquid was supercooled. The experimental results are discussed in the light of the structural model of the cubic phases in the classical cubic mesogens by the authors.
(Liq. Cryst., 30, 861 (2003))
Heat capacity of a thermotropic mesogen ANBC(22) showing two cubic mesophases was measured by adiabatic calorimetry between 13 and 480 K. Excess enthalpies and entropies due to phase transitions were determined. A small thermal anomaly due to the cubic Im3m -> cubic Ia3d phase transition was successfully
detected. Through an analysis of chain-length dependence of the entropy of
transition, the sequence of two cubic mesophases (with space groups Ia3d
and Im3m) is deduced for thermotropic mesogens ANBC( n).
It is shown that the disorder of the core arrangement decreases in the
order of SmC -> cubic (Im3m) -> cubic (Ia3d) while that
of the chain in the reverse order cubic (Ia3d) -> cubic ( Im3m)
-> SmC.
(Phys. Rev. E, 65, 031719 (2002))
Characteristics of cubic liquid crystals are demonstrated through heat capacity measurements, comparison of phase diagram, and analysis of chain-length dependence of the entropy of SmC -> cubic transition. Molecular structures of cubic mesogens similarly consists of a central rigid core and long terminal alkyl chains. Alkyl chains in cubic mesogens (ANBC and BABH) attain large entropy on phase transitions in the solid state, resulting in highly disordered state in liquid crystalline phases. The chains behave like a solvent in the cubic liquid crystalline phase. The phase diagrams of some cubic mesogens show that the diagram is primarily determined by attractive interaction in molecular lateral direction and by geometrical factor, but is insensitive to the details of intermolecular interaction. The chain-length dependence of DeltatrsS(SmC ->cubic)
shows that the molecular core in the cubic phase is more ordered in terms
of entropy than in the SmC phase whereas the chain is more disordered by
0.5 J K-1 (mol of CH 2)-1 in the cubic
phase.
(EKISHO (Liq. Cryst.), 5, 20 (2001), review in Japanese)
Phase diagrams of some compounds showing optically isotropic mesophases are compared as a function of the number of paraffinic carbon atoms per molecular core. In spite of a wide range of the transition temperatures, the number of paraffinic carbon atoms required for the appearance of the isotropic mesophases is limited within a narrow range, irrespective of molecular structure and/or intermolecular interaction. Combining this finding with the previous results obtained from thermodynamic studies, a possible framework toward the molecular-statistical modeling is proposed.
(Liq. Cryst., 27, 1555 (2000))
Heat capacity of ANBC(18) has been measured at the temperatures 8 K to 490 K by adiabatic calorimetry. All known phases were detected. The temperatures, enthalpies and entropies of transitions were determined for the phase transitions observed. On the basis of the entropy of transition to SmC phase from the D or the cubic phases, it is pointed out that the D phase of ANBC and the cubic phase of BABH might be identical in nature. It is shown that the arrangement of "molecular" core has higher degree of order in the isotropic (D and cubic) phases than in SmC phase, whereas the terminal alkoxy chains are more disordered in the isotropic phases than in SmC phase. The degrees of disorder in the D and cubic phases relative to the SmC phase are very similar in terms of the entropy of transition per methylene group. The inverted phase sequence in ANBC (SmC -> D on heating) and BABH (cubic -> SmC) can be accounted for in terms of the competing roles in the entropy between the molecular core and the chains.
(Liq. Cryst., 26, 1185 (1999))
Heat capacity of ANBC(16) has been measured between 15 K and 500 K by adiabatic calorimetry. Three (one known and two newly-found) crystal-crystal phase transitions and all known liquid crystalline phases (SC , cubic
D and SA) were detected. The temperatures, enthalpies and
entropies of transitions were determined for all the phase transitions
observed. The entropy of transition is very small for the transition from/to
the cubic D mesophase. The results are compared with the thermal properties
of another cubic mesogen, BABH(8). The logical possibility is pointed
out that the cubic mesophases of ANBC(16) and BABH(8) are of the identical
higher-order structure, while discussing why they are immiscible.
(Liq. Cryst., 26, 341 (1999))
The molar heat capacity of the thermotropic cubic mesogen 1,2-bis(4- n-octyloxybenzoyl)hydrazine, BABH(8) for
short, with a purity of 99.43 mole per cent has been precisely measured
with an aduiabatic calorimeter at temperatures between 14 K and 480 K.
The enthalpy and entropy gained at each phase transition across the
phase sequence of {crystal(2) -> crystal(1) -> cubic mesophase ->
SC -> isotropic liquid} have been determined. The existence
of a solid-solid phase transition with a fairly large entropy change seems
to be necessary for the alkyl moieties attached to both sides of the molecule
to play the role of "solvent" in the cubic mesophase. On the basis of curvature
elasticity consideration, the small energy difference between the cubic
and SC phases is favorably accounted for in terms of the jointed-rod
micelles model. The reason for the immiscibility of BABH(8) with the cubic
D mesogen, 4'- n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acids
is discussed in terms of the large difference in their molecluar size
and of the "structure breaking" arising from the admixture of heterogeneous
hydrogen bonds.
(Liq. Cryst., 26, 219 (1999))
Phase behavior of the binary systems, ANBC(nC) - n-tetradecane
for nC = 8, 16 and 18, was examined by using a DSC,
paying a special attention to the role of the alkoxy chain of the ANBC
molecule in the D phase. The dependence of the SC - D and
D - isotropic liquid transition temperatures upon apparent average number
of paraffinic carbon atoms closely resembles the nC
dependence in a series of neat ANBC's, demonstrating that the alkoxy chain
behaves, at least in part, like a solvent in lyotropic liquid crystals.
The D phase was not detected in ANBC(8) - n-tetradecane.
(Liq. Cryst., 24, 525 (1998))
See also Quasi-Binary Picture of Thermotropics and Lyotropic Liquid Crystal