isabel m. saez and john w. goodby- supermolecular liquid crystals

Supermolecular liquid crystals

Isabel M. Saez and John W. Goodby

Received 31st August 2004, Accepted 12th November 2004

First published as an Advance Article on the web 29th November 2004

DOI: 10.1039/b413416h

In this article we review recent work on the development and study of the properties of self-

organising supermolecular and supramolecular materials, that are in size comparable to small

proteins. We take the concept of the creation of dendritic liquid crystals, and apply it to the

creation of new materials with single identifiable entities, so that they are monodisperse or are

single compounds. We show how functionality can be in-built into such materials so that self-

organising functional systems can be created.

Introduction

The engines of living systems are based on supermolecular and

supramolecular self-organising and self-assembling systems of

discrete structure and topology, where supermolecular

describes a giant molecule made up of covalently bound

smaller identifiable components, and supramolecular means

a system made up of multiple components that are not

covalently bound together. For example, proteins, although

peptide polymers, have defined and reproducible primary

compositions of amino acids, specified a-helical and b-pleated

secondary constructions, and gross topological tertiary

structures. Moreover, highly specific functionality, and thereby

the ability to perform selective chemical processing, is in-

built into such molecular machines. Concomitantly, the

study of materials that self-assemble into supramolecular

structures with desirable functionality and physical

properties at nano- and mesoscopic length scales is

currently an exciting area of intense research, and provides a

‘‘bottom-up’’ approach to the design and synthesis of

functional materials.1

Until recently, most liquid crystals have been designed to be

either low molar weight for displays or high molecular weight

for prototypical high yield strength polymers.2 Supermolecular

liquid crystals, on the other hand, combine the unique traits of

the self-organisation of discrete low molecular weight materi-

als with those of polymers in their ability to form secondary

and tertiary structures. Furthermore, super- and supra-

molecular liquid crystals3 exhibit a variety of physical proper-

ties which make them attractive for applications in the fields of

nanoscience, materials and biology. They offer a very elegant

and effective way of adding functionalisation together with an

exquisite and unprecedented level of control of the precise

nature and location of specific functionalities and overall

molecular architectures, as in proteins. As a consequence, the

initial interest in super- and supra-molecular liquid crystal

synthesis has shifted towards functional dendrimers because of

the latter’s rich supramolecular chemistry and self-assembling

properties.4 The very properties of precise control of function-

ality and molecular architecture are also essential ingredients

in the molecular engineering of liquid crystals for controlling

and fine-tuning the physical properties that ultimately define

Dr Isabel M. Saez obtainedher PhD in from theUniversity of Alcala deHenares, Madrid, and afterworking with Professor P.M. Maitlis in Sheffield andDr W. A. Graham inEdmonton, Canada, onmechanistic organometallicchemistry, she joined theLiquid Crystal Group at theUniversity of Hull in 1992,a n d w a s L e v e r h u l m eResearch Fellow for foury e a r s . S h e n o w h a sa permanent appointmenti n H u l l a s a S e n i o r

University Research Fellow. Recently her work on liquidc r y s t a l s w i t h J a n u s - l i k e m i c r o p h a s e s e g r e g a t e dmolecular structures was highlighted in Chemical andEngineering News and the Annual Review of the AmericanChemical Society.

Professor John W. Goodby iscurrently Chairman of theB r i t i s h L i qu id Cry s t a lSociety, and until recentlywas the President of theInternational Liquid CrystalSociety. He studied for hisPhD under the guidance ofProfessor G. W. Gray FRSCBE before moving to AT&TBell Laboratories in the USAwhere he spent nine years asSupervisor of the LiquidCrystal Materials Group. In1996 he was the first recipientof the G. W. Gray Medal of

the British Liquid Crystal Society, in 1994 he was the AmershamSenior Research Fellow of the Royal Society, in 2002/3 he wasthe Tilden Lecturer of the Royal Society of Chemistry, and in2002 he was awarded an Honorary Doctorate from TrinityCollege, Dublin. Currently, he is Professor of MaterialsChemistry in the University of York.

Isabel M. Saez John W. Goodby

FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

26 | J. Mater. Chem., 2005, 15, 26–40 This journal is � The Royal Society of Chemistry 2005

the self-organising process that leads to mesophase formation.2

Thus, super- and supra-molecular liquid crystals provide

unique materials for the study of the structures and properties

of self-organising and self-assembling processes.3,4

Defining the structures of supermolecules

In the fledgling field of super- and supra-molecular liquid

crystals, many different terms have been introduced which

describe some of the same structures. Fig. 1 shows some

classical molecular architectures which can be used to describe

the gross structures of super- and supramolecular liquid

crystals. For example, two mesogenic units, ie molecular

entities that favour mesophase formation, can be tied end to

end to give linear supermolecular materials. If the mesogenic

groups are the same they are called dimers, but if they are

different they are referred to as bimesogens. The mesogenic

units may also be tied together laterally, rather than end to

end, or they may have terminal units tied to lateral units to

give T-shaped dimers or bimesogens. With trimers the

situation becomes more complicated because not only are

there linearly and laterally linked possible structures, there are

structures where the mesogenic units could be linked to a

central point, creating a ‘‘molecular knot’’. In a similar way,

tetra-, penta-, etc substituted supermolecules can be created.

Increased numbers of mesogenic units attached to a central

point can be created by introducing a central scaffold upon

which to build the supermolecular structures. Thus cyclic,

caged or hyperbranched scaffolds can be utilised.

Such supermolecular materials can be thought of as

dendritic structures when the mesogenic units are all of the

same type. Fig. 2 depicts the general structure of a dendrimer

where repeating Y-shaped units are linked together one shell

on top of another to create various generations of the

dendrimer. Typically identical mesogenic units are located at

the surface to the dendritic scaffold, thereby creating a

dendritic liquid crystal, or a polypedal (an object having many

feet which are the same) supermolecule. Alternatively, if the

mesogenic units are mostly different with respect to one

another, then the supermolecule will have many different feet,

and thus could be termed a multipedal supermolecule. There is

a clear distinction between these two types of supermolecular

system; the polypedal supermolecule, like a polymer, can be

subject to polydispersity, whereas the multipedal supermole-

cular material is not; it can be similar in constitution to a

protein and have a primary structure. In fact Newkome et al.

make the observation that ‘‘on the preparation of dendritic

macromolecules, the continued inability to ascertain the absolute

homogeneity of the resultant structures has led many to claim

monodisperse character of their products’’.5

There is of course a further distinction between a super-

molecule and a supramolecular system. A supermolecule is a

giant molecular entity that is made up of covalently bonded

identifiable molecular units, thus it is similar in constitution to

that of a tertiary structure of a protein. A supramolecular

system, on the other hand, is a self-assembled, non-covalently

bonded entity where complete molecular units are brought

together through non-covalent forces to create a complex

structure similar in constitution to that of a quaternary

structure of a protein. Fig. 3 shows both of the supermolecular

and supramolecular constructions. Thus, a supermolecule has

a clearly defined chemical constitution, whereas for the

supramolecular system it is possible to have variations in the

constitution depending on how many molecular units are

required to create the self-assembling, and hence self-organis-

ing, complex system.

Fig. 1 Templates for the design of liquid crystalline supermolecular

materials. The mesogenic units are shown as ellipsoids of differing

colours, however, the materials may be constructed with mesogenic

units of the same type.

Fig. 2 A dendritic scaffold, polypedal (many feet the same) super-

molecule which can be subject to polydispersity, and a multipedal

(many different feet) supermolecule which is not necessarily subject to

polydispersity.

Fig. 3 (a) The architecture of a supermolecule, and (b) the complex

self-organised structure of a supramolecular entity.

This journal is � The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 26–40 | 27

Through the incorporation of chemically or physically

active moieties into the structures of super- and supra-

molecular systems, ‘‘functional’’ or ‘‘smart’’ self-organising

materials can be created. Such material systems are related

to proteins in that they can be designed to have specific

structures and properties. For example, Fig. 4 shows a

template for a supermolecular system where two functional

units A and B are incorporated into its structure. A and B may

interact with each other or with other chemical entities

introduced to the supermolecular system. The design of such

‘‘smart’’ functional supermolecular materials is expected to

have a strong impact on the future development of materials

science, as this area of research offers a powerful alternative

for managing the awkward gap in the length scale which

exists between top-down miniaturisation and bottom-up

nanofabrication.

The self-organisation processes

The way in which supermolecules can self-organise is

dependent to a large degree on simple structural features,

such as the density of the mesogenic units attached to the

periphery of the central scaffold, their orientation of attach-

ment, and the degree to which they are decoupled from the

scaffold. For example, the density of mesogenic groups

attached to the periphery can effectively change the overall

gross shape of the structure of the supermolecule from being

rod-like, to disc-like to spherulitic. Thus, the structure of the

systems at a molecular level can be considered as being

deformable, where each type of molecular shape will support

different types of self-organised mesophase structure. Thus, for

supermolecular materials, rod-like systems will support the

formation of calamitic mesophases (including the various

possibilities of smectic polymorphism), disc-like systems tend

to support columnar mesophases, and spherulitic systems form

cubic phases, as shown in Fig. 5.

Similar structure-self-organising properties are also found

for supramolecular systems, and indeed in some cases far more

complex polymorphism and a richer variety of mesophase

types are found.6

Although there have not been extensive research studies into

the effects that the type (rod-like, disc-like or spherulitic) of

mesogenic group attached to the central scaffold has on

mesophase formation, it is clear for rod-like shaped mesogenic

groups that the orientation of the attachment can markedly

influence the type of mesophase formed and polymorphism of

any smectic phases formed. As with side chain liquid crystal

polymers (SCLCPs), lateral attachment (side-on) of the

mesogenic units often leads to supermolecular systems

exhibiting nematic phases, whereas for terminal attachment

(end-on) smectic phases appear to predominate, as shown in

Fig. 6.

In addition to number density and orientation of attach-

ment, the degree to which the mesogenic units are

decoupled from the central structure is important. The shorter

the linking unit, the more likely the material will act as

single supermolecular entity, whereas the longer the spacer the

more likely it is that the properties of the individual

mesogenic groups will dominate the overall properties of the

material.

Apart from these coarse property–structure activity

relationships, a secondary level of structure needs to be

considered in defining the finer points of such relationships.

For example, the central scaffolds might be considered to be

soft (flexible) or hard (rigid), there might be blocks of

mesogenic groups of one type or another, there could be

microphase segregating units incorporated into the periphery

or the scaffold of the material, mesogenic units might be

incorporated into the scaffold as well as the periphery, it is also

possible to incorporate metallomesogenic units as well as

conventional organic mesogenic moieties, and chirality might

be introduced into any part of the structure. Fig. 7 shows some

examples of templates for supermolecular materials bearing

blocks of mesogenic units, some of which are functionalised.

The variety of templates available is thus very large, indicating

the potential for the development of novel materials with

designer properties.

Through self-assembly and self-organisation processes,

liquid crystalline phases have opened up new perspectives in

materials science towards the design and engineering of

supramolecular materials.1a,d,e The self-organisation in two-

and three-dimensional space offered by the liquid crystalline

medium is an ideal vehicle to explore and control the

organisation of matter on the nanometre to the micrometre

scale which are key to the emerging development of

nanotechnology.Fig. 4 Template structure of a smart or functional supermolecular

material.

Fig. 5 Effect of the number density of mesogens on the surface of the

supermolecular structure on the formation of various mesophases.


In the following sections we describe the structures of

novel polypedal, multipedal and functional supermolecular

materials.

Liquid crystalline polypedes (dendrimers)

As noted above, various morphologies have been documented

for dendritic and hyperbranched liquid crystal polymers, and

this topic has recently received intense interest.4 Although the

exhaustive revision of this area is outside the scope of this

article, several types can be easily identified: a) hyperbranched

polymers and liquid crystal dendrimers in which the mesogenic

groups form part of each branching unit; b) dendrimers

without mesogenic groups that form liquid crystal phases,

where the formation of the mesophases is due to self-assembly

process of the constituents dendrons, aided by microphase

segregation, and c) liquid crystal dendrimers formed by

attachment of liquid crystal moieties to the periphery of a

suitable core dendrimer, where the mesogenic units are located

on the surface of the dendritic core.

The first thermotropic liquid crystalline dendrimer, I, was

reported by Percec and Kawasumi for a hyperbranched

polyether structure that consisted of an AB2 monomer,

with a mesogenic unit based on conformational isomerism.

Different end-groups were used to terminate the polymer,

and the gauche conformation was found to favour the

dendritic geometry in the solid state; however the anti

conformation was obtained at higher temperatures and

this was found to stabilise the formation of the nematic

phase.7

Fig. 6 Orientation of attachment of rod-like mesogenic groups to a central scaffold and mesophases formed for supermolecular systems, shown in

comparison to the structures of side chain liquid crystal polymers.

Fig. 7 Templates for supermolecular materials bearing A–B blocks of

mesogenic units, some of which are functionalised.


Lattermann et al reported the first metallomesogenic

dendrimer when they described results on trigonal bipyramidal

metal complexes of ethyleneimine dendrimers of the first and

second generation, based on derivatives of tris(2-aminoethyl)-

amine. Complexes of cobalt, nickel, copper and zinc were

prepared and found to exhibit relatively low temperature

mesophases, which generally possessed hexagonal columnar

structures. These materials therefore provided the first

examples of metallomesogenic dendrimers.8

The groups of Shivaev and Frey reported simultaneously the

first examples of materials that are classed as conventional

liquid crystal dendrimers. In these reports they described the

synthesis of liquid crystalline carbosilane dendrimers, which

differ from the above dendritic liquid crystals in that the

mesogenic groups are located only at the surface.9 Thus the

materials combine a flexible, dendritic carbosilane inner core

which is functionalised on the surface with cyanobiphenyl

groups or cholesteryl groups, attached by flexible spacers via

either classical hydrosilylation reactions or through esterifica-

tions; see Fig. 8 for example. This material has 36 mesogenic

moieties attached to its periphery, however, the flexibility of

the central scaffold allows the supermolecule to deform its

shape to give a rod-like overall form, thereby supporting the

formation of calamitic mesophases. Remarkably, although

the individual mesogenic groups would normally support the

formation of nematic phases at temperatures above room

temperature, the supermolecule exhibits a wider temperature

range for the mesomorphic state (SmA), with a glassy phase

being formed well below room temperature. In this series of

materials up to 124 mesogens have been attached to the

periphery, and at higher number densities of mesogens, i.e. for

higher generations of the dendritic scaffold, columnar phases

start to appear. However, the introduction of columnar phases

at higher generations is indicative of the flexibility of the

scaffold which allows for a wide variety of conformers to be

present in the lower generations that support the formation of

rod-like gross shapes of the supermolecule.9

Within the now conventional group of liquid crystal

dendrimers, we have reported a group of liquid crystalline

dendrimers based on the octasilsesquioxane core, which

provides eight primary (radial) branches for derivatisation,

allowing an increased packing of the mesogens around the

dendritic core at earlier generations; moreover, the rigid

framework of the octasilsesquioxane core offers the possibility

of a contrasting comparison with dendrimers derived from

entirely flexible scaffolds, among others PPI, PAMAM,

carbosiloxanes and esters.10

The first example we discuss, supermolecule 1, is an octamer

based the octasilsesquioxane core unit.11 This material was

prepared and purified in such a way that by HPLC and 29Si

NMR spectroscopy it was shown to be a single compound

without dispersity. Unlike the materials described above, this

material exhibits smectic polymorphism with SmC and SmA

phases being formed. The incorporation of cyanobiphenyl

mesogenic moieties means that the material has interesting

dielectric properties, and the presence of the smectic C phase

indicates that with the introduction of chirality the material

would also be ferroelectric and pyroelectric. The mesophase

sequence of g 212.8 Cryst1 4.7 Cryst2 39.0 SmC 74.2 SmA

102.9 uC Iso Liq, demonstrates that the dendritic structure of

the supermolecule is constrained to being rod-like, and that the

rod-like conformers tilt over at the smectic A to smectic C

Fig. 8 Example of a typical liquid crystal dendrimer with its mesogenic units covalently attached to the outer surface of a flexible scaffold.9


phase transition, as shown in Fig. 9. Thus, the structures of

both the smectic A and the smectic C phases have alternating

organic and inorganic and layers. As the inorganic and organic

layers have differing refractive indices, the mesophase struc-

tures are essentially nano-structured birefringent slabs.

The number of mesogenic groups can be increased in the

periphery by the hydrosilylation of the first generation

hexadecavinyl octasilsesquioxane dendimer 2 with cyanobi-

phenyl mesogens containing terminal Si–H groups, thereby

affording the liquid crystal multipede 3, which contains sixteen

cyanobiphenyl groups attached to the dendritic core.11 For this

material the 1H NMR spectrum showed well resolved

resonances for all the protons of the cyanobiphenyl mesogenic

groups, indicating complete conformational freedom of the

mesogenic units in solution. Complementary 29Si NMR

spectroscopy shows that the reaction of the cyanobiphenyl

mesogens with the vinyl units of 3 was incomplete, and

although the distribution was much sharper than normally

found for polymers, nevertheless the material shows a degree

of dispersity and is not a unique compound. The molecular

weight of 3 was under-estimated by SEC, an effect frequently

encountered in dendrimers, which has been interpreted as

evidence of a globular conformation in solution.

Multipede 3 shows a phase sequence of g 217.5 SmC

63.1 SmA 91.7 uC Iso Liq. Thus the effective doubling of the

number of mesogenic units has the effect of lowering the

clearing point, the SmC to SmA transition and the glass

transition relative to 1. This thermal behaviour is in contrast

with that normally found for liquid crystal dendrimers based

on flexible dendritic cores, where the clearing points increase

and the glass transitions remain almost constant with

generation number.10

The mesophase behaviour of 3 implies that the dendrimer

must, like supermolecule 1, have a rod-like shape in order that

it can pack in layers, where the molecules within the layers are

disordered and the layer structure is diffuse, revealing that the

mesogenic state deforms the globular environment of the

dendrimer. This is remarkable given that the number of

mesogenic units is doubled relative to the number of

octasilsesquioxane scaffolds, see Fig. 10, and where as a

consequence one would expect curvature in the packing of the

mesogenic units to occur due to their bunching about the

scaffolds.

As noted earlier, in general, as for traditional side-chain

liquid crystal polymers, the end-on attachment of the

mesogenic moieties affords smectic phases, whereas side-on


mesogenic sub-units lead to nematic phases. By appending

laterally attached chiral mesogenic units to the octasilsesquiox-

ane scaffold we targeted the induction of the chiral nematic

phase into this class of materials. Supermolecule 4, the size of a

small globular protein (i.e. MW = 6560), was prepared as a

single material with a dispersity of 1, and as predicted it

exhibited a chiral nematic phase with a helical macrostructure.

The material exhibits an extraordinarily long temperature

range for the chiral nematic phase. A transition from a glassy

state to the chiral nematic phase occurs near to room

temperature and then the phase extends over ninety degrees

before transforming to the isotropic liquid at 116.9 uC.12

The local structure of the chiral nematic phase is shown in

Fig. 11 where the supermolecules are shown as having rod-like/

tubular structures where the mesogenic units are expected to

intermingle between the supermolecules. The chiral nematic

phase will have a helical macrostructure superimposed

upon the local nematic ordering as shown in the figure.

Remarkably the material has a helical pitch of approximately

2 mm, which is a relatively short pitch considering the size of

the polypede, and approximately the same as the pitch that

which would be produced by the individual mesogenic units

without spacers being attached. However, unlike for the

mesogenic units, the pitch is relatively temperature insensitive.

Thus the surface of the polypede acts as a molecular

recognition surface, in a similar way to recognition surfaces

are created in surfactant systems and Janus grains as described

by de Gennes.13

By changing the chiral moiety associated with the liquid

crystal units, the physical properties of the supermolecular

system can be changed and/or fine-tuned. For example,

changing the chiral group from 2-methylbutyl to 2-octyl, as

for polypede 5 (MW = 6889) in comparison to 4, the glass to

chiral nematic phase transition is lowered to a value below

room temperature, and similarly the clearing point is

substantially reduced. However, the chiral nematic phase still

exists over a temperature range of approximately 60 uC, which

is quite remarkable considering that the low molar mass

equivalents melt above 70 uC, thus supermolecular systems

provide access to liquid crystal phases over much wider

temperature ranges than the low molar mass equivalents.12

The pitch of the helix for compound 5 was found to be

approximately 0.2–0.3 mm, thus the material selectively reflects

Fig. 9 The structures and phase transition for the smectic A and

smectic C phases of multipede 1, where the spheres represent the

terminal cyano moieties.

Fig. 10 Schematic drawing of the bilayer structure of the smectic A

phase of multipede 3. The 16 mesogenic units (red cylinders and

spheres which represent the cyano groups) per molecule are

accommodated in the layers without the introduction of curvature in

the packing of the mesogenic units together.


visible light over a wide temperature range. Moreover the pitch

is relatively temperature insensitive thus the material can be

used in large area non-absorbing polarizers, or in optical notch

filters or reflectors. In addition, in the glassy state the helical

macrostructure of the chiral nematic phases is retained, thus

similar applications are possible.

As with the terminally appended systems, it was possible

to bifurcate the scaffold so that twice the number of

mesogenic units could be bound to the central scaffold.

Thus, the first generation octasilsesquioxane dendrimer 6,

which is the size of a small protein (MW = 14653) possessing

sixteen side-on attached mesogenic units, was prepared.

After purification, the material was found to be monodisperse

by MALDI-TOF spectrometry, size exclusion chromatogra-

phy and 29Si NMR spectroscopy; see Fig. 12 for the NMR

spectrum.

Compound 6 was found to exhibit chiral nematic phase (see

the texture in Fig. 13), hexagonal disordered columnar (see

Fig. 11 Schematic representation of the local nematic structure and

the helical structure of the chiral nematic phase of dendrimer 4.


Fig. 14) and rectangular disordered columnar phases, in the

sequence g 5.4 Col*rd 30 Col*hd 102.3 N* 107.7 uC Iso Liq.14

Thus the increase in the number density of the mesogens

bound to the central scaffold transforms the situation from the

octamers 4 and 5, which exhibit calamitic phases, to one where

the hexadecamer 6 exhibits columnar phases. However, the

formation of columnar phases when the dendrimer

possesses rod-like mesogenic groups is not easy to visualise

unless the mesogenic groups surround the octasilsesquioxane

core to give a cotton-reel-like structure. From the point of

view of the mesophase structure, the best fit to the X-ray data

was obtained using such a ‘‘cotton-reel’’ model, which suggests

that in the hexagonal columnar phase the dendrimer assumes

a cylindrical shape that has approximately the same height

as its diameter. The long axes of the mesogenic units are

roughly parallel to or slightly tilted with respect to the

rotational axis that is normal to the cylinder, ie they are

nematic-like, and they are packed together side by side on the

outer surface of the cylinder, in a disordered fashion, as shown

in Fig. 15.

The mesogenic behaviour of 6 emphasizes the role played by

the octasilsesquioxane core when compared with the related

side-chain liquid crystalline polysiloxanes.15 It is remarkable

that the thermal stability of the chiral nematic phase is similar

in both the polymer and the dendrimer, suggesting that the

cubic core does not perturb significantly the associations

between the mesogens, which are necessary to support the

chiral nematic phase.

Similarly, the formation of the columnar phases in the

dendrimer 6, but not in the side-chain polysiloxanes, indicates

that the silsesquioxane core assists in the interaction of

neighbouring units resulting in the formation of the hexagonal

and rectangular disordered structures, presumably through

segregation of the siloxane cores from the mesogenic units

in distinct columns. These remarkable new mesophase

structures, which are essentially variations of ‘‘tubular

nematic–columnar phases’’, are favoured by the spacer length

used to attach the mesogen to the dendrimer core, since it

Fig. 14 The homeotropic and rectilinear defect textures of the

hexagonal columnar phase of supermolecule 6 (6100).

Fig. 15 Schematic structures of the hexagonal tubular nematic (left)

and rectangular tubular nematic (right) phases. The mesogenic groups

are roughly parallel to the columns but their positions are disordered.

Thus the columns effectively act as the directors of the nematic phase.

Fig. 12 The 29Si NMR spectrum of supermolecule 6 showing the

resonances for four identifiable silicon atoms.

Fig. 13 The Grandjean plane texture of the chiral nematic phase of

supermolecule 6. The blue iridescent colour is due to the selective

reflection of light from the helical macrostructure (6100).


has been kept relatively short (five methylene units) in order

to prevent full decoupling of the mesogenic motions from the

silsesquioxane core and by forcing the mesogens to pack

closely together around the dendritic core. The structures of

the ‘‘hexagonal tubular nematic–columnar’’ and ‘‘rectangular

tubular nematic–columnar’’ phases are shown schematically

together in Fig. 15.

The hybrid structures of such ‘‘tubular nematic–columnar

liquid crystal phases’’ lend themselves as potential model

systems for the development of photonic band-gap materials,

where large difference in refractive indices between the

inorganic and organic sections can be engineered into the

system through design and synthesis. In addition, the chiral

nematic phase of the hexadecamer shows the selective

reflection of blue light indicating that the pitch of the chiral

nematic phase is sub-micron, approximately 0.2 to 0.3 mm.

By swapping the position of chiral group of the mesogen

with the linking group to the central scaffold, polypede 7

is created where the mesogens are attached end-on. As

predicted, the material does not exhibit columnar phases,

but instead reverts to calamitic phases, with chiral nematic

and chiral smectic C* phases being found. In terms of material

properties the nematic phase of the supermolecule possesses

a helical macrostructure and therefore reflects light. The

chiral smectic C* phase is ferroelectric as was shown by

electrical field studies, however the polarization was not

determined because of the slowness of the response of the

material.

Liquid crystalline multipedes and giant molecules

Multipedes are essentially a step away from polymer and

dendritic systems towards the complex super- and supra-

molecular materials found in living organisms, where materials

with selective functional properties are incorporated into self-

assembling and/or self-organising states of matter. If we

consider firstly the two polypedes 8 and 9, tetramer 8 has

four laterally appended chiral mesogenic groups and therefore

exhibits a chiral nematic phase as expected. Similarly, tetramer

9, which has terminally appended mesogenic groups, exhibits a

smectic phase as expected.

Replacement of one of the mesogenic groups in either of the

tetramers by a mesogenic group from the other tetramer

results in two new supermolecules 10 and 11.16 These materials

are no longer dendrimers but are molecules in their own right

with discrete structures. The introduction of a terminally

appended mesogen into a supermolecule which has predomi-

nantly laterally appended mesogens in the periphery, as for 10,

results in a lowering of the relative melting point, but the

mesomorphic properties remain nematic, which is expected as

the terminally appended mesogen serves to increase the

disordering in the system.

However, when a laterally appended mesogen is

introduced into a system where all of the other mesogens are

terminally appended, as for 11, the laterally appended

mesogen acts as a disrupter to the orderly packing of the

molecules together. This results in the suppression of the

formation of smectic modifications and support for

nematic phases. Thus compound 9, which is smectic A, is

converted into a chiral nematic phase as in 11. A

comparison of the nematic structures of 10 and 11 is shown

in Fig. 16.


Interestingly, material 11 has a nematic phase at room

temperature, with a glassification point at 220 uC. In

comparison 4-pentyl-49-cyanobiphenyl has a melting point of

24 uC and a clearing point of 34 uC. Furthermore, mechanical

shearing of specimens of the nematogenic materials 10 and 11

show that the samples have low viscosity, indicating that they

have rheological properties similar to those of low molar mass

mesogens.

Fine tuning of material properties therefore can be achieved

by using mixtures of mesogens attached to the periphery of the

scaffold. In addition, the disordering induced can substantially

lower melting points and widen temperature ranges of

desirable liquid crystal phases. Furthermore, such materials

are miscible with low molar mass materials, and can be used to

modify their physical properties.

Janus liquid crystals

One of the more intriguing and challenging aspects in materials

science is understanding the molecular recognition and self-

assembling processes in materials with diversely functionalised

faces or sides, which can yield supramolecular objects that may

recognise and select left from right, or top from bottom, as

described by de Gennes.13 For example, Janus grains,17a block

co-polymers in the form of Janus micelles,17b segregated

amphiphilic dendrimers,17c–e shape-persistent macromolecu-

les17f and polar colloidal particles17g are abiotic examples of

such materials that self-organise, like proteins, in a pre-

programmed fashion.

We have described a new concept for the design of self-

assembling functional liquid crystals18 as ‘‘Janus’’ liquid

crystalline molecular materials in the form of segmented

structures that contain two different types of mesogenic units,

which can favour different types of mesophase structure,

grafted onto the same scaffold, to create giant molecules that

contain different hemispheres (‘‘Janus’’ refers to materials with

two faces, such as fluorocarbon/hydrocarbon or hydrophilic/

hydrophobic, etc, for example a Janus supermolecule with

hydroxyl and hydrocarbon faces has been recently reported19).

Thus, we studied the complementary materials 12 and 13,

Fig. 16 Schematic representation of the nematic phases formed by

multipedes 10 and 11 which have mixed lateral and terminal mesogenic

groups.


based on a central scaffold made up of pentaerythritol and

tris(hydroxymethyl)aminomethane units linked together,

where one unit carries three cyanobiphenyl (CB) (smectic

preferring) and the other three chiral phenyl benzoate (PB)

(chiral nematic preferring) mesogenic moieties or vice versa,20

see Fig. 17.

Both Janus compounds, 12 and 13, were isolated as single

compounds in a glassy state at room temperature. The

MALDI-TOF mass spectra of 13 are shown in Fig. 18.

The predicted isotopic distribution is shown together with

the experimental isotopic distribution for the mass ion. The

two distributions overlay each other identically confirming

the purity of the material and demonstrating the utility of

MALDI-TOF in the determination of structure in super-

molecular systems.

On heating 12, a broad melting endotherm with onset at

33.8 uC (DH = 6.26 kJ mol21) was followed by a transition

from the liquid crystal state to the liquid at 60.8 uC (DH =

2.73 kJ mol21). The cooling cycle from the isotropic liquid

showed a broad, weak exotherm, onset at 60.3 uC, marking the

transition to the chiral nematic phase. A second exotherm

occurred on cooling, onset 36.1 uC, marking a second order

transition to a chiral smectic C* phase. Further cooling

induced a glass transition at approximately 22.8 uC. This

sequence of events was reproducible in subsequent heating and

cooling cycles. It is noteworthy that the transitions have

extremely low DH/DS values, suggesting that the system is

relatively disordered and highly flexible. In addition, it is

important to note that the formation of chiral mesophases by

compound 12 means that the nematic phase is thermochromic

and the smectic C* phase is ferroelectric and pyroelectric and

will exhibit electrostrictive properties.

In contrast, compound 13 exhibits only one enantiotropic

transition by DSC which occurs between a chiral nematic

phase and the isotropic liquid, in the form of a weak and broad

peak at 38.2 uC (DH = 0.87 kJ mol21). The only other thermal

event present was a glass transition below room temperature at

27.9 uC. Only when the sample was left standing at room

temperature for three weeks did a broad, strong endotherm

with onset at 31.6 uC with a shoulder at 42.6 uC occur.

However, these thermal events were not present in successive

heating and cooling cycles, suggesting that crystallisation only

occurs on standing after a long period of time.

Comparison of the phase behaviour of compounds 12 and

13 shows clearly that the overall topology of the molecule in

respect to the inner core (i.e. which hemisphere carries what

mesogen) plays a significant role in determining the type of

Fig. 17 Conceptual design of Janus liquid crystalline supermolecular

systems, where the Janus head is composed of a nematic liquid crystal

(right) and a smectic liquid crystal (left).

Fig. 18 MALDI-TOF spectra for compound 13; (a) the full spectral

details, (b) the predicted isotopic distribution for the mass ion, and (c)

measured isotopic distribution.


mesophase formed, since in both cases the number of

mesogens of each type and the core are the same, and simply

by placing them in different hemispheres the mesophase

exhibited is changed.

The manipulation of the structural fragments (mesogenic

units, central scaffold, and linking units) in the molecular

design of such supermolecular systems potentially allows us to

vary mesophase type and therefore the physical properties and

potential applications of materials. Thus the molecular design

of these systems is flexible and potentially capable of

incorporating functional units, thereby allowing us to take

some steps towards the molecular and functional complexity

found in living systems.

Functional liquid crystalline supermolecules

The concept of creating functional materials by incorporating

a certain functionality within a liquid crystalline molecule,

through covalent attachment, is a bottom-up approach to

self-organising functional materials. In collaboration with

R. Deschenaux (University of Neuchatel, Switzerland), in the

following examples we incorporated fullerene as the functional

unit into self-organising supermolecular systems. Fullerene

was selected because of its interesting physical properties and

in order to see if such a large non-mesogenic unit would affect

mesomorphic properties. Thus, the synthesis of fullerene-

containing liquid crystals was expected to open new avenues

in the design of self-organised structures containing the

fullerene unit.21 This approach to self-organisation appears

to be particularly interesting for other functional groups which

are not well adapted to being organised in nanoscale

architectures.

The three-dimensional structural analogy between fullerene

and the octasilsesquioxane was exploited designing chiral

nematic fullerene dendrimers.22 These were synthesized by the

Bingel reaction of fullerene and dendritic malonates of several

generations, carrying chiral nematic mesogens attached in a

side-on fashion. Fullerene dendrimer 14 shows an enantio-

tropic chiral nematic phase, with a phase sequence of g 26 N*

69 uC Iso Liq. The values of the pitch of the helix of the

chiral nematic phase obtained indicate that C60 fits within

the helical structure formed by the mesogens themselves

without causing any significant perturbation of the pitch.

This, in turn, implies that although the large C60 unit

disturbs the mesogenic interactions, therefore lowering the

clearing point, it can be effectively camouflaged within the

self-organising chiral nematic medium, provided that

enough mesogenic sub-units are available in the dendritic

addend, without markedly suppressing the liquid crystalline

state.


Indeed when the number density of the mesogenic is

increased, thereby reducing the weight fraction of

fullerene units in the supermolecular structure, see 15, the

liquid crystal phases become increasingly more stable and

the spherical fullerene unit becomes increasingly hidden

within the liquid crystal matrix.22b This result indicates that

the liquid crystal units have the ability to incorporate and

organise non-mesogenic functional units into the self-

organising state, without too much detriment to the liquid

crystal itself.

Thus, we can apparently control the periodic ordering of

non-mesogenic functional units through the process of self-

organisation. Similar approaches recently have been taken by

Sakai and Matile23 and Zhuravel et al,24 but in both of these

cases, amino acids were used to induce self-assembly, and

ultimately self-organisation.

Conclusions

In this brief review, we have demonstrated the potential for

creating a wide variety of novel supermolecular and supramo-

lecular materials with designer structural and functional

properties. In our development of new materials we have

borne in mind the philosophy and approach used by living

systems in the creation of monodisperse and discrete super-

molecular materials with specific functionality. In particular,

the design features employed in the creation of proteins have

been used in the creation of materials which do not bear any

common atomistic relationship with proteins themselves. In

this way, this new field lies open for the development of

materials for which their design is only limited by human

imagination.

Acknowledgements

The authors are very grateful to The Leverhulme Trust for

financial support.

Isabel M. Saez and John W. GoodbyLiquid Crystals and Organic Materials Research Group, University ofYork, Heslington, York, UK YO10 5DD.

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isabel m. saez and john w. goodby- supermolecular liquid crystals

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