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.     

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.     

2H NMR studies of phase behaviour and molecular motions of doped discotic liquid-crystalline systems

A discotic triphenylene monomer as well as a dimer and a main chain polymer, all substituted with heptyloxy side groups, were doped with an electron acceptor (2,4,7-trinitrofluorenone (TNF)) to give charge transfer complexes. These doped systems were aligned in a magnetic field, thus proving their liquid crystallinity. ²H NMR measurements show that the electron acceptor molecules are incorporated into the columns built of triphenylene cores. In the charge transfer complex with the triphenylene monomer almost all the electron acceptor molecules stack in the columns even close to the clearing temperature T₁, while for the dimer and especially for the polymer a significant fraction of the TNF molecules exhibits isotropic motion, which is attributed to their location in the region between the columns, already way below T₁. This isotropically distributed part increases on approaching T₁. Fast rotation of the discs around their column axes takes place in the monomer and is quenched in the dimer and the polymer, due to the interlinking of the columns by the spacer. The electron acceptor molecules, on the other hand, exhibit free rotation in all samples, even in the charge transfer complexes with the triphenylene dimer and polymer. In the side group labelled triphenylenes increased ²H NMR spectral narrowing is detected in the charge transfer complex samples compared with the corresponding pure triphenylenes. This indicates higher side group mobility in the doped systems, since the incorporation of TNF molecules increases the spacing between the discotic units.     

Structure formation in doped discotic polymers and low molar mass model systems

Doping of low molar mass materials or polymers, possessing disc-like units, with electron acceptors leads to the stabilization of columnar discotic phases or even to the induction of such phases in compounds which either display a nematic discotic phase or only an amorphous phase in the absence of the electron acceptor. The induced columnar phase corresponds frequently to a hexagonally ordered one. We have observed, however, in addition the induction of new columnar phases such as the rectangularly ordered (D^ro) and the columnar nematic phase (N^c). The enhancement of the tendency towards the formation of columnar phases is a consequence of electron acceptor—electron donor complex formation. Using a model proposed by de Jeu to describe the induction of smectic phases by complex formation we are able to account qualitatively for the experimental findings.     

A triphenylene-containing side chain liquid crystalline ladder-like polysiloxane and its highly ordered superstructure

A liquid crystalline discotic triphenylene-containing side chain ladder-like polysiloxane (LPS) derivative was prepared by hydrosilylation. The superstructure was investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and isotherm surface pressure ( ∪)-area ( A ) diagrams based on LB-film experiments. The XRD results suggest that the discotic triphenylene units in the side chains stack to form a columnar structure and the columns further align with each other to form a board-like superstructure because of the semi-rigidity of the ladder-like backbone. This suggestion has been confirmed by parallel nanowire-like stripes observed in the AFM image. Mixing small amounts of the corresponding low molar mass molecules with the polymer leads to a more ordered and denser columnar stacking, as shown by sharpened XRD patterns and a ∪-A curve with a steeper slope, higher collapse pressure and lower collapse area than those obtained either for the low molar mass molecules or the polymer individually.     

Mesomorphic donor-acceptor twin molecules with covalently linked sheet-like pentaalkyne and nitrofluorenone sub-units

Charge-transfer (CT) twin mesogens are presented which consist of sheet-like pentayne donor moieties with different peripheral substituents chemically linked with nitro substituted fluorenone sub-units via flexible alkyl spacers. A novel plastic rectangular columnar (Colrp) phase is reported for the laterally unsubstituted member of the pentayne based CT twins. The phase is characterized by a three-dimensional crystal-like correlation of ordered columns in a rectangular lattice, while the molecular sub-units are still able to perform relaxation motions. The attachment of peripheral substituents to the pentayne moieties results in a distortion of the three-dimensional positional order and the five-fold alkyl substituted homologues exhibit a rectangular columnar ordered (Colro) mesophase. The rectangular lattice symmetries originate from the chemical linkage of the flat donor and acceptor parts of the molecules placed in an alternating manner within neighbouring columns, and the regular intracolumnar periodicity is enhanced by charge-transfer interactions within the columns. A further structural modification consists of the incorporation of an asymmetric carbon into the spacer sequence resulting in pentayne based CT-twin mesogens displaying a nematic columnar mesophase with a helical twisting of the columns (N*col). Dielectric investigations reveal an unusual dynamic behaviour of the donor-acceptor pentaynes. The glass relaxation process is characterized by the occurrence of two relaxation modes, both following a WLF behaviour, a feature which has not previously been reported to our knowledge for columnar phase-forming disc-like liquid crystal materials.     

Modular assembly of elliptical mesogens

Discotic mesogens featuring a pyridine ring were synthesized, and were found either to form ordered hexagonal columnar liquid crystalline phases or melt directly from a crystal to an isotropic liquid, depending on the position of the pyridyl nitrogen atom. Binary mixtures of the mesogenic pyridine derivatives with a similar discotic mesogen having a carboxylic acid group resulted in the formation of modular elliptical complexes through hydrogen bonding. The binary mixtures were found to exhibit ordered hexagonal columnar or ordered rectangular columnar and nematic mesophases, depending on the length of the alkyl chains, and displayed dramatically different properties from their constituent components. Binary mixtures of the non-mesogenic pyridine derivatives with carboxylic acid-functionalized discotic mesogens did not result in the formation of hydrogen-bonded complexes.     

Discotic charge transfer twins Structure and mesophase behaviour of covalently linked triphenylenes and trinitrofluorenones

Discotic charge transfer twins, a novel class of discotic liquid-crystalline compounds were studied. These compounds consist of triphenylene units (as donors) which are chemically linked via flexible spacers of various lengths to trinitrofluorenone units (acting as acceptor). They display a liquid-crystalline phase over a wide temperature range extending up to 240-260°C. Based on X-ray analysis a structural model is proposed for the liquid-crystalline phase: the molecules are arranged in columns in such a way that mixed stacks occur, the intercolumnar packing possesses an orthorhombic symmetry. The neighbouring columns are connected along specific directions via flexible spacers which give rise to highly anisotropic structural properties of this columnar liquid-crystalline phase.     

Photophysical properties of novel fluorescent poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes

In the present study, photophysical properties of fluorescent poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes have been studied by means of stationary and time-resolved fluorescence spectroscopy (in ethanol at room temperature). The luminescent quantum yields and efficiency for the energy transfer from β-diketonate ligands to Eu(III) ion have been determined for the studied complexes by using diffusion-enhanced fluorescence resonance energy transfer. Obtained results show effect of the polymer ligands upon photophysical properties of the complexes and a relation has been established with length of the oxyethylene spacer between two phosphonate groups. The Förster radiuses of the synthesized compounds with SulfoRhodamine 101 as acceptor have been calculated. Measured distances between molecules of the donors and acceptor at identical acceptor/donor molar ratios have been illustrated the difference in structure of the ternary and polymer complexes in solution even at low concentration.     

A room temperature discotic mesogenic dyad based-on triphenylene and pentaalkynylbenzene

A facile synthesis of a novel room temperature discotic mesogenic dyad based on triphenylene and pentaalkynylbenzene linked via flexible alkyl spacer is reported. The thermotropic liquid crystalline (LC) property of the compound was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffractometry (XRD). The LC property of the dyad was further manipulated by preparing charge transfer (CT) complexes with trinitrofluorenone (TNF). The compound self-assembles into a columnar hexagonal mesophase and exhibits an excellent fluorescent emission property which has possible potential for various opto-electronic applications.     

Self-assembly of semifluorinated dendrons attached to electron-donor groups mediates their pi-stacking via a helical pyramidal column

Semifluorinated first-generation self-assembling dendrons attached via a flexible spacer to electron-donor molecules induce pi-stacking of the donors in the center of a supramolecular helical pyramidal column. These helical pyramidal columns self-organize in various columnar liquid crystal phases that mediate self-processing of large single crystal liquid crystal domains of columns and self-repair their intracolumnar structural defects. In addition, all supramolecular columns exhibit a columnar phase at lower temperatures that maintains the helical pyramidal columnar supramolecular structure and displays higher intracolumnar order than that in the liquid crystals phases. The results described here demonstrate the universality of this concept, the power of the fluorous phase or the fluorophobic effect in self-assembly and the unexpected generality of pyramidal liquid crystals.     

Electronic delocalization in discotic liquid crystals: a joint experimental and theoretical study

Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2',3'-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper pi-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the pi-electrons along the discotic stacks.     

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.     

A computer simulation of model discotic dimers

We set up a model for discotic liquid crystal dimers and study, by means of Monte Carlo simulations, their phase behaviour and self–assembling properties, in comparison with the simpler monomeric case. Each discotic dimer is described by two oblate Gay–Berne ellipsoids connected by a flexible spacer, modelled by a harmonic “spring” of three different lengths. We find that dimerization in general yields produces a significant change on the phase behaviour, with an increase of the columnar–nematic transition temperature, a widening of the nematic region and the apparent suppression of the crystalline phase in favour of the columnar phase up to very low temperatures. Longer spacers prove to ease the formation of columns and to increase the orientational order. Contribution to the Fernando Bernardi memorial issue.     

Induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system

The induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system is achieved by mixing hexakispentyloxytriphenylene 1 with a long chain derivative of trinitrofluorenone 3. The difference in chain length has a strong influence on the packing behaviour due to steric effects. The long hydrocarbon chains of the acceptor introduce a strong asymmetry into the electron donor acceptor complex. It could be shown by differential scanning calorimetry, optical microscopy and X-ray measurements that a nematic columnar phase is formed. In this mesophase the triphenylenes form columns but no hexagonal or orthorhombic lattice is built up. Each column behaves like a rod-like nematic mesogen. To prove that the long hexadecane alkyl chains of the acceptor are responsible for this induction, the acceptor 3 was mixed with the non-liquid-crystalline triphenylene derivative 2 containing six hexadecyloxy side groups. The long alkyl chains of the acceptor dissolve perfectly in the side chain region of the discs. No asymmetry is induced and the columns formed can be arranged on a hexagonal lattice resulting in a D_ho phase.     

Novel donor-acceptor triple mesogens incorporating disc-like and rod-like molecular sub-units

Novel non-symmetric charge-transfer trimers are presented which consist of an electron-rich penta-alkynylbenzene group with different peripheral substituents and a flat nitrofluorenone acceptor moiety both of which are chemically linked to the terminal positions of a rod-like azobenzene unit. The laterally unsubstituted member of the pentayne based CT-triple compounds and the five-fold methyl modified homologue exhibit an enantiotropic mesophase which can be obtained as a frozen-in glassy state at room temperature. The X-ray investigations reveal a nematic columnar (NCol) like molecular arrangement for the mesophases of both compounds. The columns are formed through an intercalated stacking of the flat donor and acceptor sub-units of different molecules. The rigid rod-shaped moieties are aligned with their long axes orthogonal rather than parallel relative to the columns axis. A further elongation of the five lateral alkyl substituents of the donor group enhances the steric frustration between the disc-like and the rod-like molecular groups and the five-fold pentyl substituted pentayne donor-acceptor trimer no longer exhibits mesomorphic properties. Dielectric investigations confirm the occurrence of a glass transition well below the clearing temperature and reveal the presence of thermally activated secondary relaxation processes in the low temperature range. For these trimers containing an azo group, an additional secondary relaxation (beta2-process) is observed, which is probably caused by a local motion of the azo group within the spacer segment.     

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.     

Dilatometric investigation of thermotropic linear chain copper (II) alkanoates

The molar volumes of three binuclear copper (II) alkanoates (cupric octadecanoate, docosanoate, and tetracosanoate) were measured over the temperature range 30°–200 °C, which encompasses their phase transition to a columnar mesophase. A sharp volume increase in a narrow temperature interval about the transition was observed, consistent with a first-order phase transition. Values obtained for the partial molar volume of a methylene group in the columnar mesophase (16.9±0.3 cm³/mol) are between those for crystalline paraffins and for other smectic or discotic mesogens, respectively, indicating a disordered state, but not a complete fusion, of the aliphatic chains of the copper soaps in their columnar mesophase. The stacking period of the binuclear copper complexes in a column was derived from a combination of dilatometric and X-ray data. Its value, 4.64±0.08 Å, is independent of the chain length of the complex and of the temperature.