Templating Crystal Growth at the Nanometer‐Scale with a Monotropic Columnar Mesophase

Abstract

Since their discovery, columnar mesophases have become increasingly important in fundamental research and in practical applications [1,2] due to their peculiar supramolecular architectures that allow one-dimensional charge transport. Although the most studied columnar phases are formed by disc-shaped molecules, it is now recognized that such phases can also be formed by dendrimers, [3,4] main-or side-chain polymers with [5,6] or without [7±9] mesogenic moieties, phasmids, [10,11] and board-shaped molecules. [12] For most applications, the organization of the columnar phase at different spatial scales, as well as its temperature and time stabilities are of primary importance. Indeed, at the local scale the mutual mesogen orientation and intracolumnar ordering directly affects charge migration [13,14] and could possibly be tailored to maximize electron orbital overlap. The electronic and optical properties of the liquid-crystalline (LC) phases also strongly depend on the large-scale structure. In particular, the organization at the mesoscopic scale, ranging from several nanometers to some hundreds of nanometers, must be tailored to control features such as the size and orientation of the LC monodomains. This spatial scale is, however, not easily assessable by conventional techniques [15] as it falls in the gap between the resolution of conventional optical observations and X-ray diffraction. As far as temperature and time stability of the phases are concerned, it is worth noting that the phase diagrams of systems forming LC columnar mesophases are generally complex. This is often not only due to a rich polymorphism of these systems, but also to the heating/cooling irreversibility of phases, i.e., the appearance of monotropic phases. The latter are typically observed on cooling when the phase that is energetically more stable is bypassed because of the slow (re)crystallization kinetics. It is important to note that crystallization of the mesophases was previously studied in some detail for highly ordered smectic phases (G and H) formed by calamitic molecules. [16] Compared to the monotropic nematic and smectic phases formed by polymers, [17,18] the columnar LC monotropic phases have been less extensively studied. [19±22] Two of the first LC molecules displaying monotropic columnar phases were observed by Giroud-Godquin et al. [23] (in metallomesogens) and by Billard et al. [24] (in rufigallol hexa-n-octanoate). In the first case, the room temperature (RT) crystal was formed on cooling via a rectangular columnar phase, which was absent during subsequent heating. [19] In the second case, the columnar monotropic phase was converted into another stable, columnar mesophase before reaching the RT crystalline phase. [20] Another interesting type of monotropic phase formed by HOBT-C8 molecules was studied by Cheng and co-workers, [21,22] who found an intermediate monotropic crystalline phase between a mesophase and a stable crystal. Some other examples of columnar monotropic phases were reported for non-conventional star-...

Homeotropic Alignment of Columnar Liquid Crystals in Open Films by Means of Surface Nanopatterning

“…2a), which displays various types of common structural defects such as disclinations. [19] Figure 2b shows an AFM phase image of an annealed sample of 2 measured at a smaller scale. Two zones delimited by a vertical darker boundary are visible in the image.…”

“…[19] A typical X-ray fiber diffraction pattern of the Cr phase ( Fig. 1, middle right panel) displays a series of sharp meridional reflections, which is the typical signature of a helical superstructure.…”

“…To this end, rather thick films (about 100 nm thick) were spun-cast on a Si wafer. Large-scale film morphologies of 1 and 2 visualized shortly after their annealing in the isotropic phase show a layer-like structure [17,19] with characteristic layer thicknesses of 3.1 and 4.5 nm, respectively. These values are in good agreement with the intercolumnar distances determined from the XRD measurements.…”

“…In the Col h phase the molecules are not tilted. Molecule 2 exhibits a more complex phase diagram [18,19] . The molecular structure and phase behavior of molecules 1 and 2 is given in the left and right panels, respectively.…”

Columnar Mesophases with 3D Order from New Functional Nonconventional Star‐Shaped Mesogens

“…[4] The smallest star of the series transforms at room temperature into a soft columnar crystal, templated by a columnar hexagonal mesophase. [5] The perfect alignment in the range of several micrometers and the high order along the columns render such materials highly attractive for plastic electronics, if chromophores can be incorporated without changing the mesomorphic behavior. We decided to introduce naphthalene derivatives to oligobenzoate scaffolds, due to their relatively small size and excellent semiconductor properties, found in crystals, [6] smectic LCs, [7] columnar mesophases, [8] and polymers.…”