Improved mesophase stability of benzoxazole derivatives via dipole moment modification

Abstract

By modifying the molecular dipole moments with lateral monofluorine substituent, improved mesophase stabilities were obtained for novel benzoxazole derivatives, 2-(4ʹ-alkoxy-3-fluorobiphenyl-4-yl)-benzoxazole liquid crystals (coded as nPPF(3)Bx). The series of nPPF(3)Bx with lateral monofluorine substituent ortho to benzoxazole group have larger calculated dipole moments by about 2 D than the corresponding fluorine-substituted analogs (compounds I) with lateral monofluorine ortho to alkoxy group; it is interesting to note that they show lower melting and clearing points but better mesophase stability with wider mesophase ranges for the molecules with both polar (NO₂, Cl) and nonpolar (CH₃, H) terminal groups. Meanwhile, compounds nPPF(3)Bx show greater red-shifted photoluminescence emissions than compounds I, which suggest that π–π interaction between molecules is reinforced by the enhanced dipole–dipole interaction caused by increased dipole moments. These results suggest that modification of the molecular dipole moment is an effective method to improve the mesophase stability of the classical mesogenic compounds.

Keywords:

• dipole moment
• benzoxazole derivatives
• lateral monofluorine
• mesophase stability

Acknowledgements

We are grateful to Professor Wenliang Wang at Shaanxi Normal University for theoretical calculations at Shaanxi Normal University.

Additional information

Funding

The authors acknowledge the financial support by Shaanxi Natural Science Foundation [2014JM7270], Key Technologies R&D Program of Xian [CXY1430(2)], the Key Technologies R&D Program of Shaanxi Province [2014K10-06], NSF of China [51373092], Program for Key Science & Technology Innovation Team of Shaanxi Province [2012KCT-21], Program for Changjiang Scholars and Innovative Research Team in University [IRT_14R33], and the Fundamental Research Funds for the Central Universities [GK201503038, GK201501007, GK201501002].