¹³C NMR investigations and molecular order of nematogens with biphenyl and bithiophene at terminus

ABSTRACT

Four rod-like mesogens in which biphenyl and bithiophene units are located at one terminus are synthesised and the occurrence of enantiotropic nematic phase is established by hot-stage optical polarising microscopy and differential scanning calorimetry. For biphenyl based mesogens (BP-1 and BP-2), the mesophase is observed irrespective of number of phenyl rings in the amine moiety. For bithiophene (BT-1), on the other hand, the mesophase is only noted when the amine component has two rings. The mesophase range for BP-1 is found to be 163.7^◦C and for BT-1 and BP-2, the values are 84.7^◦C and 11.1^◦C, respectively. BP-1 and BT-1 are subjected to ¹³C NMR investigation in nematic phase to determine the local order and accordingly, oriented 1D ¹³C and 2D SAMPI-4 NMR experiments are accomplished. The remarkable feature of the SAMPI-4 spectrum of BP-1 measured at 145^◦C in nematic phase is observation of the ¹³C-¹H dipolar coupling value of 16.67 kHz for terminal carbon (C18). For BT-1, however, the dipolar coupling value for C18 is found to be 8.60 kHz in nematic phase (138^◦C). Such large values of ¹³C-¹H dipolar couplings for the terminal ring carbons of the nematogens offer orientation of the rings of the core unit directly.

Graphical Abstract

KEYWORDS:

• Nematic
• biphenyl
• bithiophene
• ¹³C NMR
• ¹³C-¹H Dipolar couplings
• order parameter

Acknowledgements

The authors would like to acknowledge the support of Dr. K. J. Sreeram, Director, CSIR-CLRI and Prof. K. V. Ramanathan, NMR Research Centre, Indian Institute of Science, Bangalore for detailed discussions related to order parameters. Authors also acknowledge the help of Dr. V. Subramanian and Ch.Venkata Surya Kumar, CSIR-CLRI for DFT calculations. A.A.B. acknowledges the financial support from Council of Scientific and Industrial Research (CSIR), New Delhi, in the form of Senior Research Fellowship. CSIR-CLRI Communication Number: A/2020/CTS/CSIR-SRF/1477.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

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