Structural isomerism of functionalised triphenylene discotic liquid crystals

Abstract

Exclusive synthesis and characterisation of new dinitrohexa(alkoxy)triphenylene compounds were carried out. Nitration of 2,3,6,7,10,11-hexa(alkoxy)triphenylene with fuming nitric acid yielded 2,3,6,7,10,11-hexa(alkoxy)-1,5-dinitrotriphenylene with regioisomer 2,3,6,7,10,11-hexa(alkoxy)-1,8-dinitrotriphenylene and that of 1-chloro-2,3,6,7,10,11-hexa(alkoxy)triphenylene yielded 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,12-dinitrotriphenylene with regioisomer 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,9-dinitrotriphenylene. These isomers were separated by column chromatography and the molecular structures were confirmed by spectroscopic and spectrometric analyses. The liquid crystalline properties were investigated by optical polarising microscopy, differential scanning calorimetry (DSC) and X-ray analysis. The compound 2,3,6,7,10,11-hexa(butyloxy)-1,5-dinitrotriphenylene is a crystalline solid, whereas 2,3,6,7,10,11-hexa(butyloxy)-1,8-dinitrotriphenylene exhibits columnar phases. The higher homologues exhibit hexagonal columnar mesophase with a wide thermal range. Interestingly, 2,3,6,7,10,11-hexa(alkoxy)-1,8-dinitrotriphenylene has the higher clearing temperature compared to its 2,3,6,7,10,11-hexa(alkoxy)-1,5-dinitrotriphenylene isomer. Similarly, 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,9-dinitrotriphenylene is liquid and 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,12-dinitrotriphenylene exhibits hexagonal columnar mesophase at room temperature.

Keywords:

• triphenylene
• columnar mesophase
• discotic liquid crystals
• isomerism
• nitro compounds

Acknowledgements

The author is grateful to Professor Hideo Takezoe, Professor K. A. Suresh and Dr Praveer Asthana for their helpful discussions and encouragement.