Eutectic mixtures with plastic columnar discotics: molecular structure,phase morphology and kinetics of phase separation

We report the properties of eutectic mixtures of triphenylenes displaying a highly ordered columnar phase with a low molar mass non‐discotic compound. Such highly ordered triphenylenes display large charge carrier mobilities which are strongly controlled by the state of order in the discotic phase. The motivation was to establish how the state of order—molecular order, phase morphology, temperature ranges of phase stabilities and macroscopic orientational order—can be influenced by mixing. The studies reveal that the molecular order, in particular the mutual arrangement of the columns and the intracolumnar order, are unaffected by dilution of the discotic compound, whereas the phase morphology and the kinetics of phase separation change significantly with dilution. Rod‐shaped discotic domains with a hexagonal cross‐sectional area are formed via a nucleation process and the rods grow linearly as a function of time. Both the pure discotic phase as well as the discotic domains forming during phase separation can be macroscopically ordered by orientation layers.     

Theoretical analysis of texture dependent extensional viscosity of discotic mesophases

Rheological functions for uniaxial extensional flows predicted by a previously selected and validated constitutive equation (CE) for discotic mesophases are presented. The predicted relations between extensional viscosities, flow-induced microstructure, processing conditions, and material parameters of discotic mesophases are characterized and discussed. It is found that, in contrast to rod-like nematics, two distinct uniaxial extensional viscosities need to be defined to characterize the extensional rheological functions of discotic mesophases completely. The model predicts non-Troutonian extensional viscosities of discotic nematics, such as strain thinning and strain thickening, depending on the process temperature, and the ratio of viscous to elastic stress contributions. The uniaxial extensional viscosities are also found to depend strongly on the flow-induced microstructure. The rheological analysis is then used to characterize the relations between extensional flow viscosities and the classical microstructures that arise during the industrial fiber spinning of discotic mesophase pitches.     

盘状液晶材料（1）聚集态结构及分子工程

介绍了盘状液晶材料的相分类和不同聚集态结构的特点，并对盘状液晶体系的分子工程和其它扩展性研究进展了综述。     

Experimental studies on the phase diagram and physical properties of mixture of calamitic and discotic nematic liquid crystals

Physical studies on mixture of calamitic and discotic nematic liquid crystals are meagre although they are potential for optimising physical properties. Here, we report experimental studies on the phase diagram and physical properties of mixtures of ambient temperature discotic and calamitic nematic liquid crystals. A substantial decrease in several physical properties such as birefringence, dielectric anisotropy and elastic constants are observed with increasing wt% of discotic compound. On the other hand a large increase in the rotational viscosity is observed. Based on the experimental results a simple model of mutual orientation of the rod-like and disc-like molecules is proposed.     

On the effects of doping calamitic and discotic nematics with materials of the opposite aspect ratio

In this paper the inter-mixing of calamitic and discotic materials is discussed. The systems are found to be immiscible near the middle of the phase diagram, as was predicted theoretically. However, a degree of miscibility is seen at each end of the phase diagram, allowing some interesting effects to be explored. A negative permittivity anisotropy is induced in a discotic material by dissolution of a polar calamitic dopant. The relaxation behaviour of the dopant in the material is studied and found to be similar to that in the pure calamitic, as far as can be determined in this experiment. The elastic constants of a calamitic mixture are enhanced by addition of a discotic dopant. This is believed to be due to the packing of the discotic molecules in such a way as to hinder director distortions, and may be useful for changing the values of calamitic elastic constants.     

Liquid crystal parameters through image analysis

A technique which combines image analysis and polarising optical microscope (POM) is a useful tool for the physical investigation of discotic liquid crystals, such as hexabutoxytriphenylene and hexahexyloxytriphenylene. This investigation includes the phase transitions, optical properties and order parameter as a function of temperature. Textures of discotic liquid crystals are captured as a function of temperature using POM. These microscopic textures are analysed using MATLAB software to compute statistical parameters, Legendre moments, optical parameters and order parameters of discotic liquid crystal samples. Compared with other techniques in the literature, the proposed methodology is a reliable and very simple technique for the physical investigation of liquid crystals.     

SYNTHESIS AND CHARACTERIZATION OF A NEW THREE-ARMS-NINE-CHAINS DISCOTIC LIQUID CRYSTAL

A new three-arms-nine-chains discotic molecule has been synthesized,and theaggregation structure of the compound was measured by x-ray diffraction,infraredspectra,polarizing microscope,etc.From these evidences,the new discotic liquid crystalis found to have the discotic lammelar phase.     

Eutectic mixtures with plastic columnar discotics: molecular structure, phase morphology and kinetics of phase separation

We report the properties of eutectic mixtures of triphenylenes displaying a highly ordered columnar phase with a low molar mass non-discotic compound. Such highly ordered triphenylenes display large charge carrier mobilities which are strongly controlled by the state of order in the discotic phase. The motivation was to establish how the state of order—molecular order, phase morphology, temperature ranges of phase stabilities and macroscopic orientational order—can be influenced by mixing. The studies reveal that the molecular order, in particular the mutual arrangement of the columns and the intracolumnar order, are unaffected by dilution of the discotic compound, whereas the phase morphology and the kinetics of phase separation change significantly with dilution. Rod-shaped discotic domains with a hexagonal cross-sectional area are formed via a nucleation process and the rods grow linearly as a function of time. Both the pure discotic phase as well as the discotic domains forming during phase separation can be macroscopically ordered by orientation layers.     

Design of low-molecular-mass and polymeric discotic mesogenes

Key directions in the molecular design of thermotropic low-molecular-mass and polymeric discotic mesogens are considered. The data on relationships between the molecular structure of disklike compounds, oligomers, and polymers formed on their basis and the manifestation of mesomorphism are generalized. Some application areas of low-molecular-mass and polymeric discotic mesogens are discussed.     

Fractal analysis of a discotic texture

The dendritic-type texture displayed by low molar mass discotic charge transfer systems possessing a columnar hexagonal ordered phase has been subjected to a fractal analysis. The diffusion limited aggregation approach was used to simulate the growth of the texture. The results are that the texture displays a fractal geometry and that the prominent features of the discotic texture are correctly reflected by the simulated clusters, including the fractal dimension. The conclusion is that the texture is conditioned by the lattice structure of the discotic phase and the kinetic processes, the sticking rules in the terms of diffusion limited aggregation model, responsible for the growth of the texture.     

Triphenylene-based discotic liquid crystal dimers, oligomers and polymers

Discotic liquid crystals are unique nanostructures with remarkable electronic and optoelectronic properties. Triphenylene derivatives play a major role in the research on discotic liquid crystals. Following recent reviews of the chemistry of triphenylene-based monomeric liquid crystals, this article now reviews the chemistry and physical properties of triphenylene-based discotic dimeric, oligomeric and polymeric liquid crystals.     

Star-like discotic liquid crystals

The game of molecular engineering of liquid crystalline systems via a combination of molecular architecture and functionalization of mesogens has opened interesting perspectives in the field of liquid crystals. Here we report on the synthesis and characterization of star-like heptameric triphenylenes. The discotic subunits in these molecules have been arranged in a hexagonal fashion around a central discotic core, thus leading to a new type of molecular arrangement for discotic liquid crystals. These oligomers do not crystallize and combine a well-defined structure with a discotic polymer-like processing.     

苯并菲盘状侧链液晶聚合物的大分子工程

盘状液晶聚合物兼具盘状液晶的光电性能和聚合物的柔韧性以及优异的成膜加工性能,有望发展成为新一代先进有机聚合物柔性光电功能材料.本文介绍苯并菲盘状侧链液晶聚合物的研究进展,主要结合我们研究组的工作,重点评述采用受控/活性自由基聚合方法可控合成这类侧链液晶聚合物以及对分子量效应和间隔基长度影响等基本问题的阐明.我们采用可逆加成-断裂链转移(RAFT)聚合第一次成功实现了分子量窄分布的苯并菲聚丙烯酸酯侧链盘状液晶聚合物的可控制备.首先提出盘单元局部簇集的分立短柱堆积(DCS)模型,合理解释了聚合度20左右出现的显著分子量效应,尤其揭示并提出了盘状侧链液晶聚合物的正耦合效应(PCE)理论,即短间隔基的较强耦合作用更有利于其有序柱状相的形成,与棒状侧链液晶聚合物经典的柔性长间隔基去耦合理论形成鲜明对照,补充了缺失的理论短板.基于这些原则,我们设计合成的丁氧基苯并菲聚丙烯酸酯侧链盘状液晶聚合物,经飞行时间谱(TOF)测试,表现出比文献报道值高1~2个数量级的载流子迁移率.进一步在手性客体分子掺杂诱导组装形成单手性螺旋结构聚合物复合物,以及拓扑受限环状聚合物和嵌段共聚物的受控制备等方面开展了比较系统的大分子工程实践.盘状侧链液晶聚合物的可控制备及其显著不同于棒状液晶聚合物体系的一些基本特征的阐明,为这类重要有机聚合物半导体材料的理性设计与可控合成提供理论指导,也为加速推进其光电器件化应用奠定基础.     

Self-assembled discotic nematic liquid crystals formed by simple hydrogen bonding between phenol and pyridine moieties

New discotic nematic liquid crystals have been prepared through intermolecular hydrogen bonding between the core of 1,3,5-trihydroxybenzene (phloroglucinol, PG) or 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) and the peripheral molecules of stilbazole derivatives. The various nematic phases formed by new hydrogen bonding building blocks were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The first discotic complexes of PG and trans-4-alkoxy-4′-stilbazoles exhibited nematic columnar (N_C) and hexagonal columnar phases depending on the length of alkyl chains, which were considered as the basic discotic structure. Several structural variations on the building blocks were attempted to examine their effects on the liquid crystalline properties of discotic complexes. The nematic lateral phase (N_L) with enhanced intercolumnar order was observed for the complexes of PG and trans-4-cyanoalkoxy-4′ stilbazoles due probably to the strong dipole interactions between cyano groups at the end of alkoxy chains. By introducing the nonlinear structure in three arms of supramolecular discotic mesogen, a discotic nematic phase (N_D) was observed for the complex of THPB and trans-4-octyloxy-4 -stilbazole. The single hydrogen bonding between phenol and pyridine moieties in this study provides a simple and effective method for preparing the rarely found discotic nematic liquid crystals.     

Orientation control and fixation of discotic liquid crystal

We established hybrid, horizontal, vertical, and vertical-twisted alignment of discotic liquid crystals on web-coating thin film. Newly developed materials are the photopolymerizable discotic liquid crystals, in order to fix desired novel alignment, the alignment promoters that deposit toward air surface during web coating, followed by drying of solvent, and then accomplish desired alignment of liquid crystals, the alignment layers that promote desired alignment of liquid crystals from the substrate side and the chiral agents, which have strong chiral powers in small amounts. Hybrid aligned discotic liquid crystal thin film has proven quite effective in expanding the viewing angle of thin film transistor liquid crystal displays.     

氮杂苯并菲类n-型有机半导体材料研究进展

苯并菲盘状液晶是一类新型的有机电子学材料.该类材料多数以空穴传输功能为主,能传输电子的n-型材料较少.氮杂苯并菲是与苯并菲衍生物非常相似的一种杂环化合物,材料结构中引入了氮原子,吸电子能力得到增强,是潜在的n-型有机半导体材料,具有重要的应用价值.本文系统回顾了氮杂苯并菲类盘状液晶材料的研究进展,分类讨论了材料的分子结构,其中包括二、四、六氮杂苯并菲,以及它们的合成方法和物理化学性能,论述了材料在光电子领域的最新使用进展,并在此基础上,对该类液晶材料作为n-型有机半导体在光电子器件领域的应用前景进行了展望.     

First examples of functionalized triphenylene discotic dimers: molecular engineering of advanced materials

A series of novel functionalized triphenylene discotic dimers was synthesized starting from 2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylene. Nitration of monohydroxypentaalkoxytriphenylene gave the mononitromonohydroxypentaalkoxytriphenylene which was alkylated with 2-bromoethanol. The resulting alcohol was coupled with various diacids. These compounds are unique in that they possess an electron withdrawing group (and consequently a large dipole moment) connected directly to the aromatic core. It is well known that connecting two discotic molecules together via a spacer (discotic dimers) stabilizes the columnar mesophase significantly and often leads to the formation of glassy materials. The introduction of functionality into LCs allows the variation of their properties on a wide scale and opens the route to new synthetic supramolecular systems for various device applications.     

Substrate-induced phases: transition from a liquid-crystalline to a plastic crystalline phase via nucleation initiated by the substrate

The presence of substrate-induced phases in thin films is an intriguing phenomenon with the physical and chemical factors responsible for their formation are not yet clearly understood. In this article, we present the structure and morphological changes associated with a substrate-induced phase in a discotic liquid crystal. The thin films of the discotic molecule are characterised by a combination of various X-ray diffraction methods to investigate the structural properties. Atomic force microscopy and polarised optical microscopy are used to determine the thin film morphologies. This is the first experimental proof of the presence of a substrate-induced phase in discotic liquid crystal, although they are known in thin films of molecular crystals. Moreover, we sought to decipher how the substrate-induced phase behaves with the evolution of time, temperature and changes at the interfaces. This work gives a unique example where the two-dimensional liquid-crystalline phase converts to a three-dimensional crystal plastic phase because of nucleation caused by the solid substrate over a time scale of a month or longer.     

A plastic columnar discotic phase Dp

A plastic columnar discotic phase is reported for an asymmetrically substituted triphenylene. It is characterized by a three-dimensional crystal-like registry of ordered columns in a hexagonal lattice while the disc molecules within the columns are able to rotate. At the phase transition from the normal discotic hexagonal phase to the new phase only very minute changes in structure and dynamics occur.     

Self-organization of disc-like molecules: chemical aspects

The hierarchical self-assembly of disc-shaped molecules leads to the formation of discotic liquid crystals. These materials are of fundamental importance not only as models for the study of energy and charge migration in self-organized systems but also as functional materials for device applications such as, one-dimensional conductors, photoconductors, light emitting diodes, photovoltaic solar cells, field-effect transistors and gas sensors. The negative birefringence films formed by polymerized nematic discotic liquid crystals have been commercialized as compensation foils to enlarge the viewing angle of commonly used twisted nematic liquid crystal displays. To date the number of discotic liquid crystals derived from more than 50 different cores comes to about 3000. This critical review describes, after an in-depth introduction, recent advances in basic design principles and synthetic approaches towards the preparation of most frequently encountered discotic liquid crystals.