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

Leah M Heist,Edward T Samulski,Chris Welch,Ziauddin Ahmed,Georg H Mehl,Alexandros G Vanakaras,Demetri J Photinos

doi:10.1080/02678292.2019.1711214

Leah M Heist, Edward T Samulski + Show 5 more

Open Access

https://doi.org/10.1080/02678292.2019.1711214

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Abstract

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 (NX) and the high temperature (N) nematic phases of CBnCB dimers with n = 7,9,11. The polar twisted nematic (NPT) 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 NPT clusters. The twist-bend (NTB) model, often identified with the NX 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.

Highlights

The definitive characterization[1,2,3,4] of a second, lower temperature, nematic in the phase sequence of the symmetric, odd-spacer length, cyano-biphenyl dimers CBnCB, terminated a two decade uncertainty as to the nature of this phase
The NX phase in the odd-spacer length CBnCB dimers shows a distinct difference from its higher temperature nematic phase, a macroscopically uniaxial and apolar phase hereafter denoted by N for brevity
The NX phase exhibits a ~10 nm spatial modulation of the orientational ordering that persists over domain sizes exceeding the μm range

Summary

Introduction

The definitive characterization[1,2,3,4] of a second, lower temperature, nematic in the phase sequence of the symmetric, odd-spacer length, cyano-biphenyl dimers CBnCB, terminated a two decade uncertainty as to the nature of this phase. [29,30,31,32] The ordering of small rigid solutes in both nematic phases exhibited by dimers of the CBnCB series has been studied experimentally [2,33] by NMR and the results were analysed consistently within the NPT model.[28] It was found that three distinct mechanisms could in principle underlie the appearance of enantiotopic discrimination in the NX phase. For a flexible solute molecule that (i) undergoes rapid reorientations, (ii) visits rapidly all the accessible conformations and (iii) diffuses rapidly, traveling several pitch lengths (h) within a domain of specific handedness in the NPT phase, the averaging indicated by the angular brackets in eq(1) for any molecular quantity Q is expressed as 1 h. In this case the PMT reduces to that of the chord model [34,42] in its simplest (additive) form

CB mesogenic units

Findings

Discussion and Conclusions