Synthesis, Crystal Structure, and Photophysical Properties of Triphenylene 2,3,6,7-Tetracarboxylic Ester-Based Discotic Liquid Crystals

Abstract

Herein, we report the synthesis and mesomorphic properties of a new series of hexasubstituted triphenylene tetraesters with a mix of four β-alkoxycarbonyl and two adjacent β-alkoxy chains. The key synthetic step involves the efficient fourfold oxidation of methyl groups (CH3) into carboxylic acid (CO2H) functions in the precursor TPCm-(CH3)4 (m = 4, 6, 8, 10) bearing four methyl groups. The crystal structure of the short-chain tetraester TPC4-(COOC4H9)4 showed that two molecules form a parallel slip-stacked dimer with an interplanar spacing of 3.38 Å. Furthermore, two antiparallelly arranged dimers stack up on top of each other to form a one-dimensional columnar assembly with a short interplanar distance of 3.32 Å. Most of the tetrafunctional tetraesters with good solubility exhibit a columnar mesophase that is broad enough between ambient temperature and a moderate clearing temperature of approximately 120 °C, making them attractive for facile processability in future device fabrication. A comparison with structurally related hexaether and hexaester derivatives of triphenylene revealed that these tetrafunctional mesogens had higher clearing temperatures and broader columnar phases. However, similar phase behavior and nearly identical optical properties were observed between the mixed-substituted tetraester and diester. Computational studies at the density functional theory level suggest that increasing the number of peripheral alkoxycarbonyl functions from two to four leads to a reduction in both the HOMO and LUMO levels without affecting the HOMO–LUMO gap.