Theme issue: liquid crystals beyond display applications

Liquid crystals (LCs) represent a unique state of matter combining order and mobility. This combination is of importance for the application of LC materials in different areas, where LC displays are the most prominent. These light and low energy consuming displays have significantly changed our daily life in recent decades as they contributed to the rapid development of mobile communication and information processing systems. Hence, LCs represent the first widely used organic electronic materials. However, there are many more applications of LC materials which are already in use or which are presently under development. LC polymers, for example, can form high strength fibers (e.g. Kevlar®) and chiral LCs are applied for temperature sensors, polarized light reflecting and photonic band-gap materials. Ferroelectric and pyroelectric LCs are useful for light modulators, for sensor applications, etc. Optical data storage, especially volumetric and holographic storage, may provide solutions for the needs of the 21st century. Major efforts are presently being made in utilizing the LC state to organize π-conjugated semiconducting organic molecules in such a way that high charge carrier mobilities can be achieved. This is of importance for the development of devices based on self assembled semiconducting organic materials for photovoltaic cells, organic transistors and OLEDs. The optical anisotropy of LC systems also allows the generation of polarized light emitting OLEDs and polarized fluorescence is of interest for security labels. As well as electron conducting materials, direction dependent ion conducting materials are also of significant importance (T. Kato et al., Angew. Chem., Int. Ed., 2006, 45, 38). LC polymer networks lead to novel stimulus-response materials that provide numerous potential applications including soft actuators and components of artificial muscles (K. Urayama, Macromolecules, 2007, 40, 2277). Liquid crystals are also of growing interest for numerous biomedical applications (S. J. Woltman et al., Nat. Mater., 2007, 6, 929). Moreover, in a more general sense, the combination of order and mobility is a basic requirement for self organization and structure formation in living systems. Hence, LC self assembly in aqueous systems (amphiphilic LC, lyotropic systems) and the use of peptide sequences and DNA structures (M. Nakata et al., Science, 2007, 318, 1276) as building blocks for mesomorphic materials become of growing interest.

The development of new applications is based on the development of new LC materials and for this purpose the fundamental understanding of the basic relations between molecular structure, liquid crystalline phase structures and other materials properties is required. Therefore, the focus of this theme issue is on materials design. Some aspects of this research, such as LC dendrimers, LC π-conjugated semiconductors, LC glycolipids and LC polygonal cylinder phases, have been covered in a recent special issue of Chemical Society Reviews (Chem. Soc. Rev. 2007, 36(12), 1845–2128). In the present theme issue, devoted to LC beyond display applications, focus is more on those areas which were not covered by these recent reviews. The 21 papers in this theme issue highlight some of the latest research on new LC materials, covering six major themes which are:

—Bent-core mesogens

—Nanoparticles, nanotubes in LCs, LC nanoparticles and nanotubes

—All organic magnetic LCs

—Unconventional molecular structures and new phase structures

—Bioinspired thermotropic and lyotropic systems

—Chiral LCs and chiral induction

Bent-core mesogens

In 1996, bent-core molecules were at first reported by Niori et al. (J. Mater. Chem., 1996, 6, 1231). Since this pioneering work the occurrence of new and intriguing polar mesophases, the formation of supramolecular chirality, noticeable optical, ferroelectric and antiferroelectric responses were found, which made these compounds one of the most exciting new areas of LC research (R. A. Reddy et al., J. Mater. Chem., 2006, 16, 907). In the Feature Article by Etxebarria and Ros attention is focused on new applications of these materials, such as for nonlinear optics, flexoelectricity, photoconductivity, molecular electronics and for the design of biaxial nematics. The giant flexoelectric effects of some bent-core LCs could lead to a technological breakthrough in conversion of mechanical into electric energy at the molecular level. Pelzl et al. investigated the influence of triple bonds in the terminal chains on the phase structures, first examples of bent-core mesogens based on m-carboranes or adamantanes as bent units are described by Pociecha et al. and complex types of modulated lamellar and columnar phases are reported by Gorecka et al.

Fig. 1 Schematic representation of a rod-like (a) and bent-core mesogen (b) incorporating conjugated electron donor–acceptor systems for use in nonlinear optics (see Etxebarria and Ros).

Nanoparticles, nanotubes in LCs, LC nanoparticles and nanotubes

The current state of research of carbon nanotubes (CNTs) dispersed in thermotropic and lyotropic LC phases and LC formation by functionalized CNTs themselves is reviewed by Lagerwall and Scalia. In particular, the formation of fibers with well aligned CNTs is a challenging task with great importance. Carbon nanotubes in discotic LCs were investigated by Bisoyi and Kumar. Mourad et al. report about hexagonal columnar phases formed by silica coated gibbsite particles which were used as templates for the directed growth of ordered macroporous silica. TiO₂, SnO₂, ZnO and CdTe nanoparticles were functionalized by Zorn et al. with diblock copolymers to form LC phases in organic solvents as well as in oligomeric and polymeric matrices. Thiol, thioacetate and xanthate end-substituted bent-core molecules are described by Marx et al. These compounds were used for the preparation of gold nanoparticles decorated with bent-core mesogens.

Fig. 2 Hexagonal columnar phase (right) of colloidal gibbsite platelets (left, see Mourad et al.).

All-organic magnetic LCs

Magnetic all-organic LCs and in particular multifunctional materials based on the combination of liquid crystalline, magnetic and ferroelectric properties are highlighted by Tamura et al. Uchida et al. report about magnetic LC droplets which were moved on a water surface by means of a permanent magnet.

Fig. 3 Attraction of a LC droplet by a permanent magnet (see Uchida et al.).

Unconventional molecules and new phase structures

An overview of cholesterol based liquid crystalline dimers is given by Yelamaggad et al., and LC dimers and oligomers with unusual molecular structures, forming hierarchical structures due to molecular shape, molecular topology and micro-scale segregation are reported in the Feature Article of Yoshizawa. Lehmann et al. report “non-symmetric” star shaped molecules incorporating naphthalene units which show a rich polymorphism ranging from lamellar via cubic to columnar phases and which were studied as potential semiconducting LC materials. The structure of the body-centered cubic Im3̄m phase of polycatenar compounds was investigated by Zeng et al. by sophisticated X-ray methods, revealing the highly complex triple network structure of this mesophase.

Fig. 4 The complex Im3̄m cubic phase of polycatenar mesogens (see Zeng et al. and cover image).

Bioinspired thermotropic and lyotropic systems

In the Feature Article by Lim et al. the aqueous self assembly of rod–coil molecules is reviewed with special focus on peptide based helical rods. Nano-pharmacophores for drug delivery, gene delivery and membrane activity and other nano-biomaterials are expected for these systems. Ionic self assembly was used by Faul et al. to induce LC properties in a peptide based receptor molecule and the induced LC phase is influenced by binding of peptidic guests.

Fig. 5 Self-assembled structures of rod–coil molecules in aqueous solution (see Lim et al.).

Chiral LC and chiral induction

The present state of art in chiral photochemical induction in calamitic, columnar and bent-core LC is reviewed in a Feature Article by Tejedor et al. Tunable lasers, using cholesteric LC as photonic band gap materials, are reported by Manabe et al. Chirality effects in dimesogens are discussed in the Feature Article of Yelamaggad et al. and spontaneous formation of different types of chiral superstructures is reported by Pelzl et al. for achiral bent-core mesogens.

Fig. 6 Chiral superstructures formed by achiral bent-core LCs (see Pelzl et al.).

In addition to these major themes the report of Matharu et al. describes the use of side-chain polymers with thiophene substituted azobenzene units for optical data storage, and Pisula et al. report about the self-organization of hydrogen-bonded oligo(p-phenylenevinylene)s.

I would like to thank all the authors for their excellent contributions to this theme issue effort in submitting a rich variety of high quality peer-reviewed manuscripts. I also wish to thank the editorial and production staff at the RSC for their assistance. Finally, I think that this theme issue is a good indicator of the rapid development in this field. The interfaces with other disciplines are very strong and I hope therefore that this issue will provide a valuable reference for the research community working in this steadily growing field and also for those from other fields which are interested in using LC self assembly to organize different kinds of materials into soft, but well defined, functional superstructures.

Carsten Tschierske, Guest Editor

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