Wide-Range 2D Lattice Correlation Unveiled for Columnarly Assembled Triphenylene Hexacarboxylic Esters

摘要

Realization of monodomain liquid crystals (LCs) is a grand challenge in materials science. Herein we propose that even rather small mesogens such as triphenylene, when function-alized with polar ester groups, can develop an exceptionally wide-range 2D lattice correlation in discotic columnar LCs. We recognized a particular role of ester substituents in the course of a study on the LC assembly of triphenylene hexacarboxylic esters (1_n, n = 2, 4, 6, 8, and 10; Scheme 1a). Our initial motivation of this study was to investigate if π polarization of triphenylene by electron-withdrawing substituents may enhance its π stacking. In relation to this possibility, Bock et al. reported, though without particular notions, that thermally stable columnar LCs are formed with triphenylene tri- and tetracarboxylic esters as mesogens. As soon as we started to explore the LC behaviors of 1_n, we noticed that these molecules are unique in a sense that their LC columns align homeotropically on a wide variety of substrates. Although the homeotropic orientation is well-known for certain discotic LC molecules, it is highly dependent on the type of substrate. Furthermore, we found with a big surprise that columnarly assembled 1_n commonly display an extremely sharp X-ray diffraction (XRD) peak owing to the (100) plane. Inspired by these observations, we revealed that the columnar LCs of 1_n feature an exceptionally wide-range 2D lattice correlation. As highlighted herein, the successful crystal structure analysis of 1_1 allowed us to recognize that the ester carbonyl groups of 1_n can exert an intercolumnar dipole-dipole interaction. This interaction possibly forces the LC columns to be correlated tightly over a wide range.