Molecular flexibility in nematics: from alkanes to dimer mesogens

Abstract

The ‘chord model’ of flexible alkane solute ordering in the nematic phase (D. J. Photinos, E. T. Samulski and H. Toriumi, J. Phys. Chem., 1990, 94, 4688; 4694) is extended to dimer molecules. Using the same parametrization of the modular potential of mean torque that successfully describes alkane solutes in the nematic field and monomer mesogens in neat phases, we are able to generate correctly the segmental orientational ordering profiles of a variety of mesogenic and non-mesogenic dimer solutes in a nematic solvent. The effects of the linkage between the mesogenic core and flexible spacer on the ordering properties of the dimers are explored by studying the homologous series of cyanobiphenyl (CB) ethers, CBO—(CH2)N—OCB, (N= 2–10) with different primary structural topologies, i.e. with para, meta and ortho linkages. The observed 2H-NMR spectra of para-linked dimers with N= 9, 10 and of ortho-linked dimer with N= 10 are readily reconstructed using the simplest parametrization of the chord model potential. This parametrization is then used to predict the 2H-NMR spectra of the respective meta-linked dimers and also to make inferences about how the propensities of the topologically different series of dimers for conforming to the nematic field depend on spacer length and spacer parity. Finally, the effect of the core size on the overall ordering of dimers is studied by considering para-cyanophenyl (CP) ether linked dimers, CPO—(CH2)N—OCP. It is found that, although the NMR spectra of the spacer chain in the CPO dimer series are not profoundly different from those of the respective CBO dimers, the average value of their orientational energy is indicative of substantially reduced (self) orientability of these dimers. Generally, these findings corroborate the predictive power of the chord model and its utility for describing orientational ordering of flexible solutes, even when molecular topology is complex.