Journal of Physical Chemistry B 2015-09-03

Structure-Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens.

Rashmi Prabhu, Channabasaveshwar V Yelamaggad

Index: J. Phys. Chem. B 119 , 11935-52, (2015)

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Abstract

In this contribution, we present the results of our extensive investigations on the synthesis and phase behavior of five series of dimer-like compounds formed by covalently linking a promesogenic, cyanobiphenyl core, with a nonmesogenic N-(n-alkyl)-salicylaldimine segment via an oxy(oligomethylene)oxy spacer of varying length and parity. This work is a continuation of our previous short study where the occurrence of re-entrant nematic (Nre) phase seemed to be vitally dependent on the parity of the spacer and the length of the terminal chain. Thus, working along these lines the effects on the thermal (Nre phase) behavior of the various spacers and the terminal tails have been investigated comprehensively. In particular, with the aim of exploring the fundamental relationships between molecular structural features and thermal properties, five spacers, namely, oxy(hexyl)oxy, oxy(heptyl)oxy, oxy(octyl)oxy, oxy(nonyl)oxy, and oxy(decyl)oxy spacers, have been used. Each series consists of six mesogens, as the terminal tail attached to the salicylaldimine core has been varied from n-pentyl to n-decyl. For the sake of comparison and completeness, previously reported materials have also been studied in detail, and data are included here. The characterization results derived from the optical and calorimetric studies clearly illustrate a pronounced odd-even effect; however, the effect attenuates as the length of the spacer is elongated. The even-members show higher clearing temperatures and enthalpies of transition than those of adjoining odd-members. Three series of mesogens possessing an even-parity spacer show thermodynamically stable nematic (N) and/or smectic A (SmA) phase(s); the lower homologues display only the N phase, and the middle ones exhibit both N and SmA phases while the higher members show the SmA phase solely. In contrast, the vast majority of the odd-spaced compounds of two series show a monotropic N phase. X-ray diffraction study confirms the partial bilayer order of the SmA (SmAd) phase with L < d < 2L where d is the layer spacing and L is the molecular length. Contrary to what was previously noted, none of the new compounds synthesized favor the stabilization of the Nre phase. At the present time, it is extremely difficult to point out the exact cause for the origin of re-entrant behavior in some compounds exclusively, as they hardly differ in their structure. It was also found that the phase transitional properties of even-members are comparable to that observed for 4'-n-alkoxy-4-cyanobiphenyls (nOCBs) and 4-n-alkyl-4-cyanobiphenyls (nCBs). In essence, our study validates the point that dimer-like compounds resemble liquid crystal (LC) dimers, as well as monomers to some degree, in the context of their thermal behavior; thus, they can be regarded as the bridging structures between LC monomers and dimers.