Probing molecular ordering in the nematic phases of para-linked bimesogen dimers through NMR studies of flexible prochiral solutes

ABSTRACT

The quadrupolar splittings of perdeuteriated n-decane dissolved in nematic phases formed by mesogenic dimers of the CBnCB series, for n = 7,9,10,11, are measured throughout the entire temperature range of these phases. These results are reported together with related measurements using the common nematic phase of 5CB as a solvent for n-decane. The data obtained from the ¹³C spectra of the cyanobiphenyl mesogenic units of the monomeric and dimeric solvent molecules yield the order parameter of those units. The information obtained from this set of experiments is used to elucidate the structure of the low temperature (N_X) and the high temperature (N) nematic phases of CBnCB dimers with n = 7,9,11. The polar twisted nematic (N_PT) model is found to provide a consistent description not only of our experimental results but also of NMR measurements previously reported in the literature for these phases. These findings suggest that the high temperature nematic (N) is not a common, locally uniaxial and apolar nematic, but rather a nematic phase consisting of N_PT clusters. The twist-bend (N_TB) model, often identified with the N_X phase, is shown to be inadequate to account even qualitatively for crucial features of the experimental findings.

NMR studies of perduteriated n-decane dissolved in mesogenic dimers of the CBnCB series elucidate the structure of the low and high temperature nematic phases of the odd-n dimers.

Graphical abstract

KEYWORDS:

• Nenatic-nematic phase transitions
• NMR
• potential of mean torque
• symmetric dimers
• prochiral solutes

Acknowledgments

Z.A. and C. W. acknowledge funding through the EPSRC projects EP/J004480 and EP/M015726.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Additional information

Funding

This work was supported by the EPSRC [EP/J004480,EP/M015726].