Structural assignment and molecular order of three-ring mesogen by ¹³C NMR spectroscopy in mesophase

ABSTRACT

A homologous series of rod-like molecules with three phenyl rings in the core and terminal alkoxy chains are synthesised from mesogenic two-ring aldehyde by coupling with non-mesogenic 4-alkoxy anilines. The mesophase properties are evaluated with hot-stage optical polarising microscopy and differential scanning calorimetry, and accordingly, all the molecules exhibited enantiotropic nematic and smectic C phases along with monotropic low-temperature smectic phases. For a representative homologue, the existence of smectic C phase is further confirmed by noticing a sharp reflection at small angle region in powder X-ray diffraction which varies with change in temperature. The main focus of the investigation, however, is the clear demonstration of chemical shift assignment of static ¹³C NMR of a representative three-phenyl ring mesogen in smectic C phase. In this novel approach, the static ¹³C NMR spectral data of synthetic mesogenic intermediate namely two-ring aldehyde are utilised for the chemical shift assignment of three-ring mesogens. Further, the orientational order parameter of two-ring aldehyde in smectic A and three-ring mesogen in smectic C phase is carried out by measuring the ¹³C-¹H dipolar couplings by 2D separated local field spectroscopy.

GRAPHICAL ABSTRACT

KEYWORDS:

• Calamitic mesogens
• smectic C phase
• XRD
• polymesomorphism
• 2D SLF

Acknowledgements

The authors thank Prof. K. V. Ramanathan, NMR Research Centre, Indian Institute of Science, Bangalore, India for his support and keen interest in this work. The authors are thankful to Mr. E. Varathan, Dr. V. Subramanian and Dr. A. B. Mandal, CLRI, for their help and support. We are also indebted to Prof. V. A. Raghunathan and Ms. K. N. Vasudha, Raman Research Institute, Bangalore, India for the powder X-ray measurements. MGR thanks Council of Scientific and Industrial Research (CSIR), New Delhi for Senior Research Fellowship.

Disclosure statement

No potential conflict of interest was reported by the authors.

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