Dr Isabel M Saez

01904 322607
E-mail: isabel.saez@york.ac.uk

Liquid crystals and functional materials

Our research focuses on liquid crystals with new molecular architectures and functional liquid crystals. The approach we follow in our work is creating liquid crystal supermolecules by attaching functional moieties, through covalent bonds, to a liquid crystalline molecule. In this way we use the liquid crystalline state as the organising medium to induce the assembly of functional moieties. This is a very powerful tool since a great variety of mesophase morphologies can be achieved to control the structuring in one or more dimensions.

Some of the aspects we investigate are described below:

Liquid crystal dendrimers

Dendrimers owe their unique physical properties to their well-defined, highly branched architecture, their globular shape and their multiple peripheral functionalities, offering an unprecedented and exquisite level of control on the molecular architecture.

Our work on liquid crystal dendrimers is based on the use of a central core unit (organic or inorganic) to which dendritic branches functionalised on the periphery by mesogenic units are covalently attached. Complex molecular architectures combined with the effects of microsegregation provide a wide variety of supermolecules with fascinating self-assembling properties. We use a variety of liquid crystal dendritic materials based on soft and hard cores, and show that the mesomorphism can be tuned to give lamellar or columnar mesophases depending on the generation and the structure of the mesogen attached to the dendritic core.

We have studied extensively dendrimers using polyhedral octasilsesquioxane core, a rigid inorganic cage of cubic symmetry displaying eight radial binding points.

When this core is functionalised by cyanobiphenyl mesogens, smectic A and C phases were obtained. Side-on attachment of the mesogens to the core produced a hexadecamer liquid crystal dendrimer that exhibits chiral nematic, columnar hexagonal and columnar rectangular phases (Figure 1). This dendrimer is the first one in which the change from lamellar to columnar arrangement takes place at the first generation. It is proposed that the increase of molecular surface curvature, imposed by the combination of the core and short spacer, induces the transition from lamellar to columnar phase, a situation that mirrors the behaviour of lyotropic systems.

structure

Texture of the hexagonal columnar phase of the first generation silsesquioxane dendrimer.
Figure 1. Texture of the hexagonal columnar phase of the first generation silsesquioxane dendrimer.

Current work on octasilsesquioxane-based liquid crystal dendrimers is focused towards determining the effect of dendritic generation on the mesomorphic properties.

When soft cores are used, a different picture emerges. Use of pentaerythritol as core and branches of the dendrimer has produced liquid crystal dendrimers with a variety of shapes and ensuing diversity on phases exhibited (unpublished).

  1. A Liquid Crystalline Silsesquioxane Dendrimer Exhibiting Chiral Nematic and Columnar Mesophases.
    I. M. Saez, J. W. Goodby and R. M. Richardson, Chem. Eur. J., 2001, 7, 2758-2764.
  2. Chiral Nematic Octasilsesquioxanes.
    I. M. Saez and J. W. Goodby, J Mater Chem, 2001, 11, 2845-2851.

Janus dendritic liquid crystals

We have developed a new concept “Janus” liquid crystals.
Materials with diversely functionalized sides or faces may help us realize supramolecular objects that might recognize left from right or top from bottom. In this work we have developed a new design concept, termed “Janus liquid crystals”, as materials that contain two differentiated dendritic hemispheres carrying two different types of mesogenic units in a segregated fashion. The materials show different mesophases depending on which hemisphere carries which mesogen, clearly showing that the recognition at molecular level translates into different macroscopic properties. The ultimate aim is to produce self-assembling materials that will register selective features in a way similar to the recognition processes used by proteins.


Reproduced by permssion of Wiley-VCH Verlag GmbH

Design of a Janus liquid crystal
Figure 2. Design of a “Janus “ liquid crystal

(a) g -2.8 SmC* 33.8 N* 60.8 Iso °C

(b) g -7.9 N* 38.2 Iso °C

Figure 3. Structure of “Janus” liquid crystals (a) and reversed core (b)

chiral nematic phase structure chiral smectic C phase texture
Figure 7. The chiral nematic (left) and chiral smectic C (right) phase textures of (a)

Multipedes/ Functional Dendritic Liquid Crystals

The molecular design of multipedes is based on a central focal point to which mesogenic units are covalently attached. They represent a class of materials intermediate between monomeric and dendritic.

Polyhedral silsesquioxanes are regarded as the smallest silica particle. Chiral nematic liquid crystalline materials result when the cubic octasilsesquioxane core is functionalized with eight side-on mesogenic moieties. These giant supermolecular materials display remarkably wide temperature range chiral nematic phases and glass transitions near room temperature.

Figure 4
chiral smectic C phase texture
g 23.7 N* 116.9 Iso Liq

In this group we have explored the extraordinary versatility of pentaerythritol and TRIS (2-amino-2-(hydroxymethyl)propane-1,3-diol) to produce chiral nematic multipedes containing two different types of mesogenic units.

A group of chiral nematic multipedal materials based on pentaerythritol containing two types of mesogenic sub-units were synthesised. The presence of different types of mesogens, the topology of the attachment to the core (end-on and side-on) and the chemical nature of the linking connection to the PE scaffold (ether, ester and tetramethyldisiloxane moieties) were used to tailor the mesomorphic properties. The crystalline state was found to be suppressed for most of the materials once they had experienced a first isotropisation process; all exhibited glass transitions near room temperature and accompanying wide temperature range mesophases.

Figure 5
K 36.2 N* 58.8 Iso °C
K 36.2 N* 58.8 Iso °C

Figure 6
g -20 N* 18.6 Iso °C
g -20 N* 18.6 Iso °C

When adequately functionalized by active molecular units, they provide an entry into the fabrication of self-assembled structures with the specific physical properties of such functional units.

Current work in this area is focused on the synthesis and assembling properties of porphyrin functional materials. The photophysical characterisation of these materials is carried out in collaboration with Dr Bosca, Universidad Politécnica de Valencia (Spain).