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

Herein, we report the synthesis and mesomorphic propertiesof anew series of hexasubstituted triphenylene tetraesters with a mixof four beta-alkoxycarbonyl and two adjacent beta-alkoxy chains.The key synthetic step involves the efficient fourfold oxidation ofmethyl groups (CH3) into carboxylic acid (CO2H) functions in the precursor TPC ( m ) -(CH ( 3 ) ) ( 4 ) (m = 4,6, 8, 10) bearing four methyl groups. The crystal structure of theshort-chain tetraester TPC ( 4 ) -(COOC ( 4 ) H ( 9 ) ) ( 4 ) showed that two moleculesform a parallel slip-stacked dimer with an interplanar spacing of3.38 angstrom. Furthermore, two antiparallelly arranged dimers stackup on top of each other to form a one-dimensional columnar assemblywith a short interplanar distance of 3.32 angstrom. Most of the tetrafunctionaltetraesters with good solubility exhibit a columnar mesophase thatis broad enough between ambient temperature and a moderate clearingtemperature of approximately 120 degrees C, making them attractive forfacile processability in future device fabrication. A comparison withstructurally related hexaether and hexaester derivatives of triphenylenerevealed that these tetrafunctional mesogens had higher clearing temperaturesand broader columnar phases. However, similar phase behavior and nearlyidentical optical properties were observed between the mixed-substitutedtetraester and diester. Computational studies at the density functionaltheory level suggest that increasing the number of peripheral alkoxycarbonylfunctions from two to four leads to a reduction in both the HOMO andLUMO levels without affecting the HOMO-LUMO gap. Synthesis and mesomorphic propertiesof a new series ofhexasubstituted triphenylene tetraesters with a mix of four beta-alkoxycarbonyland two adjacent beta-alkoxy chains.