Heptasubstituted triphenylene 2,3-dicarboxylic esters discotic liquid crystals with three different types of peripheral substitution

ABSTRACT

Hexasubstituted triphenylene (1a) containing four alkoxy and two adjacent methoxycarbonyl tails can be smoothly brominated or nitrated at its α-position(s) of two electron-rich rings to give a series of bromides and nitro compounds through electrophilic aromatic substitutions. Positions of bromo or nitro groups in target compounds were unambiguously established by combined analysis of 2D  nuclear magnetic resonance experiments including nuclear Overhauser effect and heteronuclear multiple-bond correlation which indicated that monobromination or mononitration occurred preferentially at position 8 or 5 of 1a, respectively. Calculations of molecular geometry on 2a–e reveal that the α-Br or α-NO₂ group(s) would induce different degrees of helical deformation in the triphenylene core and the degree of non-planarity decrease in the following order: 2c > 2b > 2e > 2a > 2d. In contrast to highly twisted 2b–c and 2e, which did not form a mesophase, the formation of a hexagonal columnar phase at room temperature even down to −50°C was observed and confirmed by polarising optical microscopy and X-ray diffraction for sterically less hindered, monosubstituted 2a and 2d.

Graphical Abstract

KEYWORDS:

• Heptasubstituted triphenylene
• discotic liquid crystal
• helical deformation
• triphenylene 2, 3-dicarboxylic ester

Acknowledgements

This work was supported by the National Natural Science Foundation of China [numbers 21072140, 51273133 and 21402187]; the Key Research and Development Projects in Sichuan Province [number 2017SZ0075]; and the Special Funds for Sharing Large Precision Equipment [numbers DJ2015-33 and DJ2015-51] at Sichuan Normal University.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the National Natural Science Foundation of China [numbers 21072140, 51273133 and 21402187]; the Key Research and Development Projects in Sichuan Province [number 2017SZ0075]; and the Special Funds for Sharing Large Precision Equipment at Sichuan Normal University [numbers DJ2015-33 and DJ2015-51].