Chiral nematic organo-siloxane oligopodes based on an axially chiral binaphthalene core

Article abstract of DOI:10.1039/c2cc33050d

The synthesis of a novel class of organosiloxane oligopodes, based on an axially chiral binaphthalene core is described and their mesogenic properties are fully characterised. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc33050d

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Chiral nematic organo-siloxane oligopodes based on an axially chiral
binaphthalene core
Christopher P. J. Schubert, M. Gabriela Tamba and Georg H. Mehl*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The synthesis of a novel class of organosiloxane oligopodes,
based on an axially chiral binaphthalene core, is described
and their mesogenic properties are fully characterised.
The development of axially chiral systems has received a
10 considerable amount of attention in recent years. Such structures
are widely employed in chiral catalysis,1 natural product
synthesis,2 non-linear optics3, 4 and as chiral templates.5, 6 Axially
chiral systems are particularly useful due to their ability to
effectively transfer chirality, and the relative ease of obtaining
15 materials of high optical purity, when compared to centrally
chiral molecular structures. This feature has made them of
significant interest for liquid crystal research.7-10
Work on axially chiral systems, which show liquid crystalline
phase behaviour (vs chiral dopants), has focused on atropisomers
20 of oligophenyls,10-12 allenes,8 and chiral binaphthalenes.13-17
Chiral binaphthalene and its derivatives, such as 1,1’-bi(2-
naphthol) 3 (Scheme 1), are widely employed to induce a
substantial helical superstructure upon solvation in a non-chiral
liquid crystal host (chiral dopant), inducing the chiral nematic
25 (N*) phase.18-22 Whilst the excellent structural compatibility of
chiral binaphthalene derivatives towards liquid crystal host
solvents has led to the successful development of powerful chiral
dopants, these materials generally show little, if any, mesophase
behaviour.13, 14 This makes the development of powerfully chiral
30 liquid crystal mesogens highly attractive and desirable.
Surprisingly, efforts to induce liquid crystalline behaviour in
binaphthalene derivatives have proven limited. Notable
exceptions are concerned with the mesogenic functionalisation of
the binaphthalene core.14-17 The problems associated with
35 inducing mesogenic behaviour in binaphthalene derivatives can
be easily rationalised. The three-dimensional non-planar
orientation of the naphthyl planes hinders molecular ordering.
Mesogenic functionalisation is required to compensate for this
disordering effect. Functionalisation at the 2,2’ and 6,6’-positions
40 is commonly employed to introduce mesogenic groups via a
flexible spacer. Smectic14 and monotropic blue and chiral nematic
phases15-17 have been observed in materials possessing terminally
linked mesogenic units in the 2,2’,6,6’ and 2,2’-positions
respectively. However, materials exhibiting an enantiotropic
45 chiral nematic phase have yet to be realised. Previous work
focused on the development of liquid crystalline organosiloxane
tetrapode systems, bearing laterally connected mesogenic units,
provided a strategy towards realising nematic phase behaviour.23-
50 Fig. 1 Schematic illustration of binaphthalene oligopodes bearing laterally
linked organo-siloxane mesogens
25
A conceptually related organosiloxane-binaphthalene system
was expected to induce mesophase properties.
In this article we report the synthesis and full characterisation
55 of analogous organosiloxane oligopode materials, based on a
chiral binaphthalene molecular core.
Control of the mesogenic properties was an important factor in
the structural design. Essentially, the attachment of laterally
linked mesogenic units was predicted to suppress higher ordered
60 LC phases. Indeed, the mesogenic precursor 8 exhibits only the
nematic phase. The inclusion of organo-siloxane moieties within
the flexible spacer would provide convenient access to the target
compounds, whilst additionally contributing to reduce the
viscosity and melting point of the target systems.
65
The binaphthalene intermediates 4 and 7 were prepared from
optically pure (S)-1,1’-bi(2-naphthol) 3 as shown in Scheme 1.
Alkylation of the hydroxyl functionalities with 5-bromopentene,
under Williamson ether conditions, afforded precursor 4. Initial
bromination of 3 and subsequent alkylation of the hydroxyl
70 groups provided dibromide 6. Suzuki-Miyaura cross-coupling
with allylboronic acid pinacol ester afforded precursor 7.
Hydrosilylation of the olefinic intermediates, using Karstedt’s
catalyst, and siloxane mesogen 8,26,27 with a long lateral spacer,
designed to promote nematic phase behaviour produced the target
75 oligopode systems 1 and 2 in 63% and 57% yields respectively
(Scheme 2).
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20
Fig. 3 Optical microscopy of dimer 1 (a) contact experiment on cooling at
96 °C with nematic mesogen 8 (left); (b) contact experiment on cooling at
85 °C; (c) planar aligned cell on cooling; (d) HTP measurement using
Grandjean-Cano wedge cell in nematic host 5CB at ambient temperature.
i) 5-Bromopentene / K2CO3 / MeCN; ii) Br2 / DCM / -78 °C; iii)
Allylboronic acid pinacol ester / PdCl2(dppf) / CsF / DME / 60 °C
25 binaphthyl unit, was found to display only monotropic liquid
crystalline phase behaviour, as observed on cooling from the
isotropic at 78.2 °C (0.24 J g-1), with the onset of slow
crystallisation observed below 48.8 °C.
Scheme 1 Synthesis of olefinic oligopode precursors.
In contrast, the tetrameric material, constructed with four
30 mesogenic units linked to the binaphthyl central core, revealed an
enantiotropic (thermodynamically stable) chiral nematic phase, as
confirmed by OPM and DSC studies (Fig. 2). On heating, a chiral
nematic phase was observed at 90.7 °C, preceded by crystal-
crystal transitions at lower temperatures, turning to an isotropic
35 liquid at 98.0 °C (0.12 J g-1). Formation of the chiral nematic
phase at 98.2 °C (0.24 J g-1) was observed on cooling from the
isotropic state, with slow crystallisation below occurring 68.2 °C.
The observed OPM textures were somewhat unspecific, with
characteristics of oily streak textures, with similar appearance to
40 planar textures previously reported for organosiloxane elastomers
containing chiral binaphthalene units.28 Thus, electro-optical
cells, of a 2µm thickness, with a polyimide coating favouring
planar alignment were prepared for compounds 1 and 2, and the
characteristic Grandjean texture for a chiral nematic phase was
45 observed (Fig. 3c).‡ However, as the greyish colour texture did
not change much over the stability range of the chiral nematic
phase, the question remained as to whether short or long pitch
helical structures were formed.
5
i) Karstedt’s catalyst / Toluene / rt
Scheme 2 Binaphthalene oligopode synthesis
Table 1 Thermal properties of oligopode materials and organosiloxane
mesogen.
Compound Transition Ttrans(°C) Transition Ttrans(°C)(ΔHtrans/J g-1)
1
2
8
C →N*
C →N*
C →N
(68.2)a
90.7
97.7
N*→I
N*→I
N→I
(98.2)a (0.24)
98.0 (0.12)
115.7 (0.59)
a On cooling
10
A contact experiment, between dimer 1 and the nematic
50 precursor mesogen 8, was therefore prepared to further classify
the nature of the phase (Fig. 3a-b). The materials were placed
between glass substrates and allowed to freely mix in the
isotropic state, with no discernable phase boundary observed on
cooling from the isotopic liquid. The solvation of dimer 1 (as an
55 isotropic liquid) into the nematic material formed an oily streak
texture. Further cooling led to the co-existence of the chiral
nematic texture and the oily streak texture in the area of the pure
mesogen. Crystallisation of 1 and the onset of phase separation
occurred at lower temperatures. The observed texture and
60 appearance of colour, with the decreasing concentration of dimer
1 in the mixed region, suggested that the pure material had a short
pitch length beyond the visible region of the spectrum.
Fig. 2 OPM texture and DSC thermograph of tetramer 2 (10° min-1)
Oligopode purity was confirmed by gel permeatation
chromatography (GPC).
15
Phase assignments and thermal transitions were initially
determined by optical polarizing microscopy (OPM) and
differential scanning calorimetry (DSC). Mesogenic properties
were observed in both dimer 1 and tetramer 2 (Table 1). Dimer 1,
comprising two liquid crystal mesogens linked to the central
To investigate the degree of helical twist further the helical
twisting powers of compounds 1 and 2 were recorded. The
65 relationship between the molecular structure of chiral
binaphthalenes and their ability to induce the chiral nematic
2
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50 ‡ Electro-optical polyimide test cell –KSRO-02/A111N1NSS05, cell gap
2 m; Grandjean-Cano wedge KCRK-03, 0.3 mm, tanθ 0.0083. E.H.C.
Co. Ltd.
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high. The HTP of compounds 1 and 2 were determined using a
Grandjean-Cano wedge cell with 4-pentyl-4’-cyanobiphenyl
10 (5CB) as the nematic host (Figure 3d).‡ The HTPs were found to
be 27.9 μm-1 and 40.2 μm-1 for compounds 1 and 2 respectively,
and higher than the majority of the conformationally flexible
dopants previously reported.19, 28 Significantly, the pitch lengths
were found to in the order of 6.4-6.5 μm for a 5% w/w
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ray diffraction (XRD). Fig. 4 shows the 2θ scan of the recorded
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pattern. It is noted that the experimental set-up, using capillaries
in a micro furnace with a magnetic field perpendicular to the
direction of the X-ray beam, did not allow for a distinction
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25 the alignment in the external field was very limited, with only
two intensities at 2θ = 3.17 (2.77 nm) and 2θ = 18.73 (0.47 nm)
were detected. The position of the reflection did not change to
any extent over the stability range of the liquid crystal phase. The
intensities can be associated with the approximate length of the
30 aromatic groups and the lateral distances of the molecules.
In summary, we have prepared a novel class of organosiloxane
oligopodes, based on an axially chiral binaphthalene core, and
fully characterised their liquid crystalline properties. The
integration of laterally substituted mesogenic units results in the
35 complete suppression of higher ordered liquid crystal phases. The
tetrapodal system 2 represents the first binaphthalene derivative
to display enantiotropic chiral nematic phase behaviour. The
systems also display high helical twist in their respective chiral
nematic phase.
5
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40
We thank the EPSRC National Mass Spectrometry Service
Centre for their support. This work was supported through the EU
7th Framework Program NANOGOLD.
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Notes and references
a Department of Chemistry, University of Hull, Hull, UK, HU6 7RX. Fax:
100 27 Yu, C. H.; Schubert, C. P. J.; Welch, C.; Tang, B. J. Tamba, M. G.;
Mehl, G. H.; J. Am. Chem. Soc. 2012, 134, 5076.
45 +44 (0) 1482466410; Tel: +44 (0) 1482465590; E-mail:
g.h.mehl@hull.ac.uk
† Electronic Supplementary Information (ESI) available: Full
experimental procedures, data and liquid crystal properties. See
DOI: 10.1039/b000000x/
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