What makes a liquid crystal? The effect of free volume on soft matter

Abstract

In this article, we probe the formation of liquid crystal and soft crystal phases as a consequence of minimising the free volume of the system either through design engineering of molecular shape or through deformation of molecular architecture. Following this concept, a number of realisations were made, for example, smectic A phases with variable layer spacing, smectic C phases without layer shrinkage, lattices of free space and fibres in N TB phases.Prologue: At the start of my thesis research in 1974, the knowledge of the structures of smectic liquid crystals was somewhat limited. Even though his thought predated the discovery of ferreoelectric behaviour in liquid crystals, George Gray had the idea that smectic C (SmC) liquid crystals might underpin potential new display applications. As I launched into synthesising new SmC variants, I started to think about why molecules should tilt over in their layers, and then onwards to question why do certain materials exhibit different forms of smectic B (SmB) phases, which was before their classification as hexatic and crystal B phases, etc. In those days I became very much interested in how the molecules pack together in condensed phases. With protractors, compasses and graph paper, the Cartesian coordinates of the atoms of a variety of molecular structures in their all trans forms were located, and their mass axes were determined for their structures by computer methods using ticker-tape which used to shoot out of the machine in volumes to yield just one result -the minimum moment of inertia. Flipping the structures about their inertia axes by 180°yielded the surface contours and the rotational volumes of the molecules. Packing them together in ways to minimise the free volume gave snapshot pictures of the local structures of various mesophases.[1] Over the passing years, I discarded this methodology for the following reasons: (1) the molecules in liquid crystal phases are rotationally disordered and are in dynamic fluctuation as demonstrated by neutron scattering studies [2,3]; (2) X-ray data showed that the molecules pack close enough together that they interpenetrate each other's independent rotational volumes, meaning they share each other's space [4,5]; (3) there were no ways to simulate the interpenetration of space, and what was free space and what was not; (4) there were only a few families of materials that could provide full data sets on structure versus phase formation; to this day there are still relatively few full homologous series of materials being reported; and lastly (5) there was no discussion of how to determine the energy cost of not fully packing space with molecules in order to minimise the remaining free volume. However, with the advent of research exploring liquid crystal materials of unconventional structure and new modelling methods such as density functional theory (DFT) simulations becoming available, our work again began to explore the steric packing arrangements of the molecules in condensed phases. [6][7][8][9][10] Then recently, one o...

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

“…The result is that compound 3 exhibits the twist-bend phase, but compound 10 does not. Thus, we feel it is likely that it is the gross shape of the molecule (and possibly the free volume [50]) that dictates the incidence of the twist-bend phase. It is presently an open question as to how changes in the average bend angle impact on the properties and structure of the twist bend phase, e.g.…”

Synthesis and Study of Chemical, Thermal, Mesomorphic, and Optical Properties of Terphenyls Modified with Nitrile Groups

“…The absorption maxima for terphenyls with and without one CN group are located at 295 nm; the sample with one CN group shows a hyperchromic effect. The absorption maximum of T12-2CN is shifted to 286 nm (hypsochromic effect as discussed elsewhere [14,28]). From this characterization, it is possible to confirm that only one CN group is necessary for luminescent applications as demonstrated by PL characterization.…”

“…The oligo-p-terphenylenes are of particular interest because the smallest (terphenyl, quaterphenyl, and quinquephenyl) are able to not only radiate a high energy light in the blue region of the spectrum but also display a rich thermotropic polymorphism. These oligomers are commonly modified with flexible chains [12,13] and other groups [14,15] to disrupt the -interactions that hold them close together. Such disruption increases their entropy and improves the number of conformations that can take place to effectively decrease their melting point and improve their solubility.…”

The 2015 Luckhurst-Samulski Prize

“…The effect of free volume on soft matter, Goodby and co-workers provide a fascinating insight into the formation of liquid crystal and soft crystal phases in terms of free volume [10]. This involves the minimization of the free volume of the system either by molecular design, or by deformation of the molecular archictecture.…”