The challenging task, to further improve device efficiencies in organic electronics, is accompanied with the development of novel self‐organizing semiconducting organic compounds.
Active materials for efficient photovoltaics, in particular, need to satisfy a number of criteria, including a high optical density over the visible and near infrared spectral regime, high charge carrier mobilities and a large exciton diffusion length. The conventional approach focuses on
π‐conjugated polymers or oligomers, which possess the inherent ability to self‐organize via
crystallization and to conduct electric charges along their backbone as well as via interchain transport.38,39 These highly processable materials exhibit charge carrier mobilities in the order of 0.1 cm2∙V‐1∙s‐1, such as determined for regioregular poly(3‐hexylthiophene).40 Another attractive strategy exploits the hierarchical self‐assembly41,42 of small π‐conjugated molecules into supramolecular assemblies by non‐covalent interactions as alternative classes of functional materials with innovative design.43‐45 The programmed, supramolecular self‐organization of small molecular building blocks into well‐defined nanostructural architectures via multiple intermolecular driving forces is recognized as one of the key techniques for the “bottom‐up”
approach in nanotechnology.46 Amongst the various new materials for organic electronic applications, conjugated liquid crystals (CLCs) are currently contemplated as an auspicious novel class of organic semiconductors because they combine order and dynamics.47
Figure 7. (a) Typical length‐scales encountered in organic electronics and control of order achievable with conjugated liquid crystalline (CLC) semiconductors. (b) Schematic representation of calamitic and discotic semiconductors. (reproduced from Geerts et al.47).
CLCs offer the decisive advantage of controlling order in the bulk as well as at interfaces and at all length‐scales from the molecular to the macroscopic scale (Fig. 7). Additionally, liquid crystalline materials possess the unique propensity to form highly organized films, which can be obtained by cheap processing techniques from solution. For conjugated liquid crystals it can be differentiated between calamitic (rod‐like) and discotic (disc‐like) mesogens. Despite their different molecular shape, they differ basically in the dimensionality of charge transport or exciton migration and in the extent of their orbital overlap. For calamitic mesogens, smectic mesophases with a two‐dimensional charge transport and for discotic mesogens columnar mesophases featuring a one‐dimensional charge transport are the usually observed phase organizations for contributing towards charge migration.
But also the spatial organization on the mesoscopic length scale (5 ‐ 100nm) of π‐conjugated dye‐entities as molecular building blocks under a programmed manner by making use of intermolecular interactions such as hydrogen‐bonding, dipole‐dipole, van der Waals or π‐π interactions is a topic of particular interest for scientists in this field. It was proposed to call this field of research “supramolecular electronics”.45 In these objects, smallest dimensions are combined with a high degree of order.48 The hierarchical formation of fibrous nanostructures building up a 3D network under thermodynamic control is controllable and may thus be implemented in various organic device applications. In this context, especially hydrogen bonding motifs or solvophobic effects, e.g. in organogelators49,50 offer an excellent structure‐directing tool to position well‐defined objects at predefined positions in order to construct nanotubes,51 nanowires52 or xerogels53 of electronic components in the nanometer range (Fig. 8).
Figure 8. (a) Proposed scheme of supramolecular gel formations (reproduced from Hiromitsu et al.54).
Columnar Liquid Crystals Formed by π‐Conjugated Systems
Liquid crystals (LCs) are unique functional soft materials which possess both, mobility and order, ranging from the macroscopic to the molecular level. In addition to the classical states of matter – solid, liquid, gaseous ‐ liquid crystals are accepted as a fourth state of matter.55 The liquid crystalline phenomenon was described firstly for cholesterol derivatives by F. Reinitzer in 188856 and recognized as novel state of matter by O. Lehmann.57 Immense efforts in research and development have pushed this topic into a mature field of modern science and many mesophase forming classes of compounds are known nowadays. The special properties of liquid crystals are used in various modern materials, e.g. thermotropic calamitic mesogens in active matrix liquid crystal displays (AM‐LCD)58 or high strength synthetic polyaramid fibers such as Kevlar® which are spun from a lyotropic melt.59 The liquid crystalline phase of matter can be characterized by attributes in‐between those of a conventional, fluid isotropic liquid and those of a solid crystal with orientational and/or positional long‐range order co‐instantaneously.60,61 As in liquid crystalline phases, the molecules are able to diffuse like the molecules of a liquid, but still maintain some degree of ordering to a greater or lesser extent, they are also denominated as mesophases (greek: μεσοσ = middle). Generally, liquid crystals are divided into two categories, thermotropic and lyotropic LCs. Thermotropic LCs exhibit a phase transition into the LC‐phase upon temperature change, whereas lyotropic LCs62 exhibit phase transitions as a function of temperature and concentration of the mesogens in a solvent (a typically example are amphiphilic molecules in water). Besides liquid crystals, also condis‐phases63, plastic crystals64 and their corresponding glasses, as well as micro‐phase separated copolymers65 belong to the class of mesomorphic materials. Any compound that is able to form a mesophase is called a mesogene.
Mesogenes can consist of only one molecule (molecular mesogene), or they can be build up of several – even different – individual molecules (supramolecular mesomorphism). Liquid crystalline phases can be classified according to the degree of order (1D – 3D) and orientation of the mesogens in the liquid crystalline state into different types of phases as nematic, smectic or columnar. The molecular origin for the formation of a mesophase can mainly be attributed to three different principles: anisotropy, aggregation and segregation. When melting to the liquid crystalline phase, the mesogen must exhibit structural principles in order to maintain parts of its organization or orientation. As molecular shape is an important factor in determining whether certain molecules will self‐assemble into liquid crystalline phases, the molecules may usefully be classified according to their anisometric geometry as calamitic‐ (rod‐like) discotic‐ (disc‐like), banana‐ (bent‐like), sanidic‐ (board‐like) or pyramidic‐ (conical or cone‐shaped) mesogens.66,67
A prominent class of molecular mesogens, which holds potential for future semiconducting appliances comprises thermotropic liquid crystals with a disc‐shaped geometry of the mesogens (DLCs).68‐70 After speculations on the existence of mesomorphism of coin‐like mesogens by D. Vorländer,71 the liquid crystalline state of so‐called discotic mesogens was discovered by S. Chandrasekhar in 1977.72 Generally, these molecules consist of a disc‐shaped rigid π‐conjugated core and bear flexible substituents at the periphery, generally hydrocarbon chains which are attached by connecting groups, such as ether, thioether, ester or amide groups.
Scheme 1 presents several prominent examples of discotic molecules, which form columnar superstructures due to π‐π interactions.