ABSTRACT
Background During the past several years, the interest in the design and study of shape-persistent macrocycles with an interior in the nanometer regime has considerably increased. From the structural point of view, most compounds are based on the phenylene, phenylene acetylene or phenylene butadiynylene backbone, or they contain a mixture of these structural elements. However, apart from meeting the synthetic challenge, the supramolecular chemistry of rigid rings is currently investigated with considerable eort. For example, shape-persistent macrocycles can act as host structures for appropriate guest molecules; they can form 1D aggregates (in solution or gas phase) or regular 2D lattices after deposition at dened surfaces. [1-8] Furthermore, shape-persistent macrocycles are interesting mesogenes for discotic liquid crystalline materials. [9-12]
e common design principle of conventional discotic liquid crystals (LCs) is a more or less rigid core (disk-like or macrocyclic) with peripheral exible side groups that point outward. [13-16] ey can be used for a variety of dierent optic and electronic applications, for example as materials for photovoltaics (in the columnar phase) [17-19] or as compensation layers in display technology (in the nematic phase). [20-22]
Recently, we could show that shape-persistent macrocycles with xed intraannular side chains (e.g. 1) can also exhibit liquid crystalline behavior. [23,24] Compound 1 is a shape-persistent macrocycle which is based on the phenyleneethynylene-butadiynylene backbone. As it generally holds for rigid compounds, no matter if they are rod-like or cyclic, exible side groups need to be added to keep these compounds tractable, i.e. soluble or/and meltable. For this purpose, 1 contains four long octadecyloxy side groups pointing inside the ring and eight propyloxy groups at the adaptable positions of the compound. e term adaptable means that the orientation of substituents at that position can be inuenced by an external parameter. [25,26]
Compound 1 melts at 134°C to form a nematic mesophase that becomes isotropic at 159°C. However, compared to all previously reported discotic LCs, this compound is composed of a rigid periphery and the exible side chains point inward. Hence, 1 can be described as a discotic LC with an inverted structure (gure 1). [23,24]
By comparing the structure of 1, and other shape-persistent macrocycles, with their thermal behavior we could identify some preliminary guidelines for the observation of liquid crystallinity in those compounds. Among these is the necessity of the rings to ll their interior more or less with their own alkyl chains and the absence of bulky peripheral side groups, both in order to prevent an interlocking of the rings. However, the number of compounds that follow this new design principle is still rather limited. is was a motivation for us to synthesize additional macrocycles and to explore their thermal behavior.
