Core chirality based tailoring of the liquid crystalline properties of supermolecular tetrapedes

Article abstract of DOI:10.1039/b927008f

A series of novel supermolecular star-shaped tetrapede chiral liquid crystals (LCs), incorporating carbohydrates, methyl α-d-glucopyranoside or methyl α-d-mannopyranoside, as chiral cores surrounded by four cyanobiphenyl mesogenic groups linked by either C6 or C11 alkyl chain spacers, have been synthesised. These non-conventional thermotropic carbohydrate-based chiral liquid crystals exhibit chiral nematic and smectic A mesophases over broad temperature ranges. The mesomorphic properties were characterized by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The effect of the flexible spacer length in addition to the stereogenic and conformational properties of the chiral cores is discussed.

Full text of DOI:10.1039/b927008f

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www.rsc.org/softmatter | Soft Matter
Core chirality based tailoring of the liquid crystalline properties of
supermolecular tetrapedes†
*
Abdelhak Belaissaoui, Stephen J. Cowling, Isabel M. Saez and John W. Goodby
Received 22nd December 2009, Accepted 24th February 2010
First published as an Advance Article on the web 25th March 2010
DOI: 10.1039/b927008f
A series of novel supermolecular star-shaped tetrapede chiral liquid crystals (LCs), incorporating
carbohydrates, methyl a-D-glucopyranoside or methyl a-D-mannopyranoside, as chiral cores
surrounded by four cyanobiphenyl mesogenic groups linked by either C6 or C11 alkyl chain spacers,
have been synthesised. These non-conventional thermotropic carbohydrate-based chiral liquid crystals
exhibit chiral nematic and smectic A mesophases over broad temperature ranges. The mesomorphic
properties were characterized by polarized optical microscopy (POM) and differential scanning
calorimetry (DSC). The effect of the flexible spacer length in addition to the stereogenic and
conformational properties of the chiral cores is discussed.
Thereby, the liquid crystalline properties are related to the
1. Introduction
overall molecular topology in relation to the inner core (Fig. 2),
aspect ratios, conformational dynamics,3 dipolar properties and
specific inter- and intra-molecular interactions. Moreover, at the
macroscopic level, nanosegregation of building blocks and their
space filling efficiency in condensed phase are two competing
driving forces for the molecular self-assembly in liquid-crystal-
line phases.4
One of the most fascinating and challenging fields in material
science is the design and development of novel materials,
endowing predictable or desirable functionalities and physical
properties.
As a consequence of the self-assembling properties of liquid
crystals and their self-organized dynamic structures, the control
and fine-tuning of their mesomorphic and physical properties can
be achieved by structural engineering at the molecular level.
In recent years, the design and functionalisation of super-
molecular liquid crystals (Fig. 1) have been the subject of
growing and intense interest as a result of their molecular
architectures,1 which combine the self-organizing properties of
low molecular weight LCs and the ability to form secondary or
tertiary conformational structures of polymeric-like liquid crys-
tals.2 Furthermore, they provide the possibility of incorporating
specific functionalities with precise nature and location, allowing
the elaboration of highly functional materials with unique
tailored properties. Hence, the molecular engineering approach
to highly structured materials leads to further understanding of
self-assembling and self-organizing processes and subsequently
fine-tuning control of mesophase formations.
Fig. 1 Primary structures of polypedal supermolecules with mesogens as
(a) end-on (b) side-on (c) tetramer supermolecule with end-on mesogenic
groups.
There are various factors influencing the mesomorphic
behaviour of supermolecular liquid crystals and are particularly
based on: (i) the degree of flexibility/rigidity, dimensional prop-
erties (linear, planar or three dimensional) and structural/chem-
ical nature (stereogenic properties, hydrophobic, hydrophilic,
aromatic,.) of the inner scaffold; (ii) the length of the spacer,
which controls the degree of decoupling of the mesogens from the
core; (iii) the nature and density of the mesogenic units at the
periphery, as well as the topology of the attachments to the core
(end-on and side-on).
Department of Chemistry, University of York, York, YO10 5DD, UK.
E-mail: jwg500@york.ac.uk
Fig. 2 Schematic representations of primary structures of molecular
tetrapedes displaying the orientational effect of the tetravalent chiral core
on the overall shape anisometry.
† Electronic supplementary information (ESI) available: Experimental
procedures, POM micrographs and MALDI-TOF. See DOI:
10.1039/b927008f
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Our aim, in an approach to chiral dendritic supermolecular
LCs,1 is to modify their functional behaviour and investigate the
manifestation of the stereogenic structural properties on their
mesomorphic behaviour. This can be achieved by incorporating
chirality within the dendritic structure, as multivalent core and
branching points, which can affect the microenvironment of the
dendrimer and ultimately their mesomorphic properties.
Carbohydrates represent a large class of liquid crystals that
form thermotropic as well as lyotropic mesophases.5 The driving
force for the mesomorphic behaviour in amphiphilic carbohy-
drate derivatives is the microphase segregation. Hence, chiral
thermotropic mesophases are not observed in amphiphilic
carbohydrate liquid crystals, despite their chiral nature and
optical activity. Surprisingly, there have been very few reports on
the synthesis and mesomorphic properties of supermolecular
liquid crystals with carbohydrates as chiral cores and mesogenic
units attached at the periphery.6
The profound importance of carbohydrates in molecular
recognition processes,7 combined with their structurally and
stereochemically diverse commercial availability, makes them
particularly attractive as a promising source for novel liquid
crystalline materials. In this context, we incorporated enantio-
merically pure carbohydrate derived units, as cores for the
synthesis of chiral dendritic supermolecules.
Fig. 3 Conceptual synthetic strategies applied for the synthesis of
tetramer supermolecules Gn and Mn (n ¼ 1 and 2).
approach, corresponding to the synthetic strategy A, consists of
assembling stepwise the branched tetramer from the core to the
periphery. In the first step, the four spacers are attached to the
core, followed by tetrafold coupling of the end-on pro-mesogenic
groups. In contrast, in the convergent synthetic approach B, the
arms consisting of the mesogen and alkyl spacer are prepared
initially before their attachment to the core.
In this paper, we report the synthesis of supermolecular chiral
liquid crystals incorporating either methyl a-D-glucopyranoside
ormethyl a-D-mannopyranoside as the chiral coressurrounded by
four cyanobiphenyl mesogenic groups linked by a spacer either
penta- or deca-methylene carboxylate units (Scheme 1). The two
carbohydrate cores differ only at the C2 position with the
R configuration for the methyl a-D-glucoside and S for the methyl
a-D-mannoside. We demonstrate the effect of the stereogenic and
associated conformational properties of the core and the length of
the flexible spacer on the liquid crystalline properties.
The synthesis of tetramers Gn and Mn (n ¼ 1 and 2) is outlined
in Scheme 2. 11-(4-Cyanobiphenyl-40-yloxy)undecanoic acid 3a
and 6-(4-cyanobiphenyl-40-yloxy)hexanoic acid 3b were obtained
in excellent yield by the oxidation of the corresponding alcohols
11-(4-cyanobiphenyl-40-yloxy)undecanol 2a and 6-(4-cyanobi-
phenyl-40-yloxy)hexanol 2b with Jones reagent in acetone.
The esterification reactions of methyl a-D-glucopyranoside
and methyl a-D-mannopyranoside with the carboxylic acids 3a
and 3b in CH2Cl2 using DCC as coupling reagent in the presence
of a catalytic amount of DMAP, afforded the corresponding
tetramers Gn and Mn (n ¼ 1 and 2) in quantitative yields and as
monodisperse structures. The reactions were monitored by GPC
to completion for two weeks. The slow kinetic rate is most likely
owing to the low solubility of the carboxylic acids and their
corresponding intermediates in CH2Cl2.
2. Results and discussions
2.1. Synthesis
Two conceptually different synthetic strategies, summarised in
Fig. 3, can be applied for the preparation of the tetramer
supermolecules Gn and Mn (n ¼ 1 and 2). The divergent
Alternatively, the carboxylic acids 3a and 3b were synthesised
following synthetic route 2 (Scheme 2), by etherification reaction
of ethyl-11-bromo-undecanoate and ethyl-6-bromo-hexanoate
with 4-cyano-40-hydroxybiphenyl 1, followed by hydrolysis
reaction in KOH ethanolic solution.8 However, the coupling of
the resulting carboxylic acids 3a and 3b with methyl a-D-gluco-
pyranoside and methyl a-D-mannopyranoside yielded the tetra-
mers Gn and Mn (n ¼ 1 and 2) as polydisperse materials.
1
Evidence from H NMR showed a residual downfield proton
signal at 8.05 ppm, assigned to two aromatic protons adjacent to
an amide group. In addition, two highfield residual resonances,
a triplet at 1.4 ppm and a quadruplet at 4.4 ppm, were recorded
and assigned to ethyl ester group.
As suggested by 1H NMR investigation, the observed
Scheme 1 Molecular structures of the chiral supermolecular carbohy-
drate-based liquid crystal tetramers.
distribution can be rationalised by the partial coupling of the
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Scheme 2 Reagents and conditions: (i) K2CO3/KI, butanone, (iia) Jones reagent, acetone, (iib) KOH/EtOH, (iii) DCC/CH2Cl2.
glucoside and mannoside cores with ethyl-11-(4-carbamoylbi-
phenyl-40-yloxy)undecanoate and ethyl-6-(4-carbamoylbiphenyl-
40-yloxy)hexanoate, present in slight amount in the reaction
mixture. The amide derivatives are byproducts obtained in very
low yields, resulting from the partial hydrolysis of cyano groups
under strong basic conditions to amide moieties, during the
hydrolysis reactions of the esters 2c and 2d (Scheme 2).
These results suggest that the optimal synthetic approach to
obtain Gn and Mn, as supermolecular monodisperse structures,
can be achieved by using the convergent synthetic strategy B
(Fig. 3) with the arms prepared via synthetic route 1 (Scheme 2),
involving the etherification reaction using mild deprotonating
agent K2CO3 and the consecutive oxidation with chromic acid.
Moreover, the divergent synthetic strategy A (Fig. 3) has been
attempted (Scheme 2). Esterification reactions of 11-bromo-
undecanoic acid and 6-bromo-hexanoic acid with methyl
a-D-glucopyranoside and methyl a-D-mannopyranoside using
DCC as coupling reagent in CH2Cl2 yielded the tetrabromo-
tetramers Cn and Dn (n ¼ 1 and 2), followed by etherification
reactions with the 4-cyano-40-hydroxybiphenyl 1 to obtain the
tetramers Gn and Mn (n ¼ 1 and 2). However, the esterification
reactions of the glucoside and mannoside cores with 11-bromo-
undecanoic acid and 6-bromo-hexanoic acid did not progress to
completion and as a consequence the purification of the branched
tetramers proved to be difficult. Furthermore, the monodispersity
of Gn and Mn might be hardly controllable from the etherification
reactions of Cn and Dn with 4-cyano-40-hydroxybiphenyl.
2.2. Mesomorphic properties
The mesomorphic properties of the synthesised compounds were
investigated by polarized optical microscopy (POM) and differ-
ential scanning calorimetry (DSC).
The optical defect textures of the tetramers G1 and M1, con-
sisting of long alkyl spacer C11 and either methyl a-D-glucoside
or methyl a-D-mannoside cores respectively, exhibit enantio-
tropic SmA mesophases with typical fan-shaped textures.
The DSC traces of G1 and M1 were recorded, during the third
heating and cooling run, at a scanning rate of 10 C minꢁ1. On
ꢀ
the heating run of G1 (Fig. 4), the first endothermic transition at
lower temperature region corresponds to the glassy state
obtained at midpoint 21.0 ꢀC and a second endothermic
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Fig. 4 DSC thermograms of star-shaped liquid crystals G1 and M1.
Table 1 Phase transition temperatures (ꢀC) and corresponding enthalpy
values (J gꢁ1
corresponding to R and S respectively. The increased ordering
for the glucoside tetramer is seemingly owing to specific spatial
conformation of the arms in relation to the chiral core (Fig. 6),
revealing an expression of the chirality9 at macromolecular level
in thermotropic mesophases. This is remarkable, as the spacer
length C11 is sufficiently long that one would expect that the
mesogens would be highly decoupled from the core.10 Macro-
scopically, the SmA mesophase formation for G1 and M1 can be
rationalized by the high flexibility of the cyanobiphenyl meso-
genic arms, as a consequence of the conformationally dynamic
cores (Fig. 5 and 6), but mainly owing to the long alkyl chain
spacer, allowing the most stable arrangements of the mesogen
units in their dipole-driven anti-parallel correlations.11
)
Gluco-tetrapedes
Manno-tetrapedes
G1 Iso 100.9
M1 Iso 87.2
(DH ¼ ꢁ7.69) SmA 25.4 Tg
G2 Iso 112.0
(DH ¼ ꢁ1.15) N* 41.9 Tg
(DH ¼ ꢁ6.18) SmA 15.6 Tg
M2 Iso 84.4
(DH ¼ ꢁ1.44) N* 35.2 Tg
transition with peak signal at 104.1 ꢀC (DH ¼ 7.22 J gꢁ1). Upon
cooling, two exothermic transitions were observed, the first peak
at 100 C (DH ¼ ꢁ7.69 J gꢁ1) and a broad glassy transition at
ꢀ
midpoint 25.4 ꢀC. The measured thermal properties are sum-
Similarly, the defect textures of the compounds G2 and M2
were investigated by thermal polarized optical microscopy. After
slow cooling from isotropic state and annealing, G2 exhibits
a fine-grain texture while M2 displays a poorly defined granular
texture. Evidences from POM suggest right handed helical
ordering for G2 and M2, thereby indicating chiral nematic
mesophases.
marised in Table 1.
The ability of pyranose monosaccharides to access more than
one low energy conformation, as a consequence of their con-
formationally dynamic nature (Fig. 5), clearly has an effect on
their phase behaviour in the condensed phase.
This is the case when comparing compounds G1 and M1,
which differ only in configuration at the stereogenic centre C2,
Fig. 5 Conformational diversity associated with glucoside and manno-
side confers high versatility and flexibility to the chiral cores (arms are
hidden for clarity). Glucoside chair conformations (1) with four arms in
a, all in axial positions, and b, all in equatorial positions. Mannoside
chair conformations (2) with four arms in c, three axial and one equa-
torial, and d, three equatorial and one axial.
Fig. 6 Topological orientations of the four arms in the two chair
conformations associated with Gn (a and b) and Mn (c and d) (n ¼ 1
and 2) incorporating glucoside and mannoside inner cores, respectively.
Groups are hidden for clarity.
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comparison to M1/M2, relatively wider mesophase temperature
ranges, respectively.
Comparison of the transition temperatures reveals a decrease
in the isotropisation temperature from the glucosides Gn (n ¼ 1
and 2) to mannosides Mn (n ¼ 1 and 2), thereby indicating lower
thermal stability of the mannoside mesophases.
The clearing-point temperature depression upon changing the
chiral core structure from glucoside to mannoside can be
attributed to the orientational and conformational properties
associated with each inner chiral core (Fig. 5 and 6), influencing
the thermal stability and dictating the phase behaviour.
When attaching long spacer C11, glycoside and mannoside
tetramers G1 and M1 display wide temperature range SmA
mesophases. In contrast, incorporating a shorter spacer C6
induces right-handed chiral nematic mesophases associated with
an increase in the glassy temperature transition, regardless of the
chiral core structure.
The molecular modelling geometries of the optimized Gn and
Mn (n ¼ 1 and 2) molecules in gas phase using the molecular
mechanics 2 (MM2) force fields, shown in Fig. 8, reveal large free
volume between the four individual arms.
For all the models, the MeO groups attached to the anomeric
carbons C1 are in axial positions. The core conformation with
the lowest steric energy in Gn (n ¼ 1 and 2), corresponds to the
model with all four arm substituents in equatorial positions
(Fig. 6b). On the other hand, for the mannoside derivatives Mn
(n ¼ 1 and 2), it is attributed to 3,4,5-triequatorial with an axial
arm substituent at C2 position (Fig. 6d).
Fig. 7 DSC thermograms of star-shaped liquid crystals G2 and M2.
Phase transition behaviours of G2 and M2 were analysed by
DSC. The thermograms are shown in Fig. 7 and the transition
temperatures are listed in Table 1. Both materials, which are in
the glassy state at room temperature, undergo enantiotropic
transitions during the DSC analysis.
In the condensed state, any voids have to be compensated for.4
This can be achieved by internal folding and/or hosting. Since
star-shaped structures Gn and Mn present large internal molec-
ular free volume in single molecule state, modelling their supra-
molecular organisation into mesophases would be challenging,
due the structural complexity and conformational diversity.
There are various elements of structural flexibility within the
tetramers: conformationally dynamic cores, the alkyl chain
spacers and the four connecting ester groups.
On the heating cycle of G2, the ꢀendothermic glassy broad
transition occurred at midpoint 43.5 C, which was followed by
a second transition with a peak signal at 111.2 ꢀC (DH ¼ 1.39 J gꢁ1)
marking the transition from chiral nematic to isotropic phase.
The cooling cycle showed a first exotherm with its peak at
108.9 ꢀC (DH ¼ ꢁ1.15 J gꢁ1), associated with the isotropic–chiral
nematic transition. Further cooling induced a broad glassy
transition at midpoint 41.9 ꢀC.
Due to the higher flexibility of the alkyl chain spacers C11, the
tendency of G1 and M1 to efficiently fill space in liquid crystal
Interestingly, tetrapedes Gn and Mn (n ¼ 1 and 2) displayed
relatively broad DSC signals corresponding to anisotropic–
isotropic transitions. This can be attributed to the gradual
conformational changes affecting the packing properties of the
glucoside and mannoside bulks during their corresponding
anisotropic–isotropic transitions.
It is noteworthy that, in comparison with G1 and M1, both G2
and M2 presented broader DSC signals and extremely low
enthalpy values for the anisotropic–isotropic transitions on
cooling and heating cycles, indicating relatively disordered
mesophases. In addition, on cooling G2 rapidly developed
a grainy texture associated with a right-handed helicity suggest-
ing a chiral nematic mesophase, while M2 showed similar phase
ordering and handness with a relatively poorly defined texture.
On the other hand, G1 and M1 displayed higher enthalpy
values for the anisotropic–isotropic phase transitions on cooling
and heating, reflecting an enhanced mesogenic ordering (SmA) in
comparison to G2 and M2. In addition, G1/G2 gave rise to, in
Fig. 8 Side-on view 3D space-filling models of Gn and Mn (n ¼ 1 and 2)
tetramer conformers after energy minimisation at 0 K in gas phase using
the molecular mechanics 2 (MM2).
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state would be higher in comparison with G2 and M2 bearing
shorter spacers C6.
Notes and references
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3. Conclusions
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A novel series of tetrapodal star-shaped carbohydrate-based
chiral liquid crystals have been synthesised. The inner cores
consist of glucoside or mannoside derivatives, which can acquire
several flexible conformations. The effect of the flexible spacer
length on the liquid crystalline property was investigated.
G1 and M1, incorporating long spacer C11, display wide
temperature range SmA mesophases. In the case of G2 and M2,
bearing shorter arms C6, the tetramers exhibit chiral nematic
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Acknowledgements
This work has been financed by EPSRC Grant EP/E064299/1
within the EUROCORES Programme SONS II of the Euro-
pean Science Foundation. We also thank the EPSRC
Mass Spectrometry Service at Swansea for MALDI-TOF MS
analyses.
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