Enhancement of the cholesteric induction power by macrocyclization in liquid crystal dimers with a chiral spacer

Article abstract of DOI:10.1039/c0jm03161e

A liquid crystal dimer with a chiral spacer increases the helical twisting power in a nematic liquid crystal solvent by a factor of four because the two tails are connected. In the macrocyclized dimer, the two mesogens are fixed in a skewed overlapping al

Full text of DOI:10.1039/c0jm03161e

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Enhancement of the cholesteric induction power by macrocyclization in liquid
crystal dimers with a chiral spacer†
*
Manabu Itoh, Masatoshi Tokita, Hiromitsu Hegi, Teruaki Hayakawa, Sungmin Kang and Junji Watanabe
Received 21st September 2010, Accepted 26th November 2010
DOI: 10.1039/c0jm03161e
chiral molecules, such as binaphthyls,11,12 biphenyls13 and helicenes,14
A liquid crystal dimer with a chiral spacer increases the helical
are widely applied as chiral dopants.
twisting power in a nematic liquid crystal solvent by a factor of four
because the two tails are connected. In the macrocyclized dimer, the
two mesogens are fixed in a skewed overlapping alignment with the
handedness determined by the spacer’s chirality.
In this paper, we demonstrate that the b value of chiral LC dimers
can be enhanced by macrocyclization. The chiral dimer has two
mesogenic units connected by an alkyl spacer with a chiral carbon:
A proven method of obtaining a chiral nematic liquid crystal (LC) is
the addition of a chiral dopant to a nematic LC.1,2 The chiral nematic
(or cholesteric) phase is formed in such a way that the molecular
chirality of the dopant is transferred to the nematic phase organiza-
tion, and the phase formation causes the nematic director to rotate
perpendicular to an axis in a helical manner with pitch P inversely
proportional to the concentration (molar fraction) of chiral dopant
c. The handedness of the helix and the magnitude of P are intrinsic to
every chiral compound and are different for every host–guest
combination.1
This linear (L)-dimer(R) with allyl tails is macrocyclized by ring-
closing metathesis under high dilution in the presence of the first
Grubbs catalyst to yield the cyclic (C)-dimer(R):15
The efficiency of the dopant in inducing helical organization is
expressed by the parameter helical twisting power (HTP), defined as
b ¼ (Pcr)ꢀ1, where r is the enantiometric excess of the dopant. The
sign of b is considered positive if the induced cholesteric helix is
right-handed. This parameter reflects the amount of dopant needed
to reach a cholesteric phase with a certain pitch.
Two of these molecules with biphenyl moieties form enantiotropic
smectic A LCs, which can be easily dissolved in nematic dopants. We
measured the values of b by doping these dimers into a nematic LC of
40-cyanobiphenyl-4-yl 4-butylbenzoate (CBPBB):16
Two approaches have been used to prepare high-HTP chiral
dopants. The first approach is mesogenic functionalization of the
chiral dopant, where a chiral molecule is functionalized with
a mesogenic group resembling the host nematic LC to enhance the
solubility and interactions with the LC host.3–6 For (R)-octane-2-ol,
b is 0.8 mmꢀ1 in an LC host of MBBA (p-methoxybenzylidene-
p-butylaniline), but increases to 19.4 mmꢀ1 by functionalization with
a resembling MBBA mesogen.3 The second approach is reduction in
the conformation flexibility by the formation of a chiral coordination
complex or use of a reasonably rigid backbone.7–10 For the tris
(pyridyl)amine-based ligand, b is negligibly small, but complexation
with Cu(I) or Cu(II) greatly increases it up to 98 mmꢀ1.7 Inherently
For the (C)-dimer(R), b is ꢀ38.4 mmꢀ1, the absolute value of which
is more than four times that of the (L)-dimer(R) (ꢀ9.2 mmꢀ1). The
mechanism of HTP enhancement by macrocyclization is discussed
below.
Fig. 1 shows the phase diagrams of the binary mixtures of
(C)-dimer(R)/CBPBB and (L)-dimer(R)/CBPBB. As the temperature
decreases, CBPBB forms the following phases in the order listed:
isotropic liquid (Iso), nematic (N), smectic A (SmA) and finally,
crystal. The N phase is formed in a wide temperature range 74–247
ꢁC, which decreases with the addition of the chiral dimer. However,
this wide temperature range does not impede the measurement of b,
because chiral nematic phases with helical pitches, measured by
circular dichroism (CD) spectroscopy, are formed over a sufficiently
Department of Organic and Polymeric Materials, Tokyo Institute of
Technology, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan. E-mail:
mtokita@polymer.titech.ac.jp; Fax: +81-3-5734-2888; Tel: +81-3-5734-
2834
† Electronic Supplementary Information (ESI) available: Synthesis
procedure for the dimers, optical micrograph of the BP phase, CD
1
spectra of the mixtures and H-NMR spectra of dimers in chloroform.
See DOI: 10.1039/c0jm03161e/
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Fig. 1 The phase transition temperatures of (a) (C)-dimer(R)/CBPBB
and (b) (L)-dimer(R)/CBPBB mixtures determined by optical microscopy
during cooling of the isotropic liquid at a rate of 10 ꢁC minꢀ1
.
wide temperature range for (C)-dimer(R)/CBPBB and (L)-dimer(R)/
CBPBB mixtures at dimer molar fractions of <10 and <50 mol%,
respectively. The (C)-dimer(R)/CBPBB mixture, which is cooled from
the Iso phase and observed by polarized optical microscopy, forms
a mosaic texture, characteristic of a blue phase over a narrow
temperature range (ꢂ1 ꢁC, for example, 235–236 ꢁC at a dimer molar
fraction of 8.1%), and then forms the chiral N phase (ESI, Fig. S2†).
The value of HTP for (C)-dimer(R) is more than four times larger
than that for the (L)-dimer(R). Accordingly, blue-coloured chole-
steric LC film can be prepared by doping a dimer in CBPBB. The
required doping concentration of the (C)-dimer(R) is just 8 mol%,
whereas for the (L)-dimer(R), it is 35 mol%. We doped samples of
CBPBB with various molar fractions of (C)- and (L)-dimers and
determined the cholesteric pitch using CD spectroscopy of the planar
aligned film samples (thickness ca. 10 mm). All the spectra exhibit
a positive peak due to the selective light reflection characteristic of
chiral nematic LCs (Fig. 2a), indicating that the induced helical
structure is left-handed. The maximum wavelength lm of the reflec-
tion band is related to the optical pitch nP by lm ¼ nP, where the
refractive index n is assumed to be a common value of 1.5. The
pitches are weakly dependent on temperature (ꢀdP/dT < 1.7 nm
ꢁCꢀ1 in the temperature range 30–5 ꢁC below the isotropization
temperature Ti (ESI, Fig. S3†)); hence, P measured at T ¼ Ti ꢀ 10 is
regarded as the standard pitch. Fig. 2b shows the plot of reciprocal
pitch Pꢀ1 against the molar fraction of the chiral dimer. The data fall
on a straight line in both mixture systems. From the slope of the line
and the handedness of the induced helix, we estimate b of the
(C)-dimer(R) as ꢀ38.4 mmꢀ1, the absolute value of which is more
than four times that of the (L)-dimer(R) (ꢀ9.2 mmꢀ1).
Fig. 3 CD (top) and UV (bottom) spectra of (C)-dimer(R) (blue line)
and (L)-dimer(R) (red line) at concentrations of 2 ꢃ 10ꢀ5 Mꢀ1 in chlo-
roform. Dashed lines show the spectra of the enantiomers.
Thus, macrocyclization increases the absolute value of b of the
chiral dimer. A possible reason is the skewed arrangement of the two
mesogens that face each other in the (C)-dimer. Fig. 3 shows CD and
UV spectra of the chiral dimers in chloroform measured at 25 ꢁC. In
the UV spectra, the (L)- and (C)-dimers(R) are absorbed with similar
values of 3 at a peak wavelength of 295 nm due to p–p* electronic
transitions in the biphenyl moiety. In the CD spectra, peaks of the
first positive and second negative Cotton effects are evident in the
region 250–350 nm, which indicates that the two mesogenic moieties
of the (L)- and (C)-dimers(R) overlap counter-clockwise. This is
illustrated in Fig. 4.17 Mirror-image CD spectra are observed for the
enantiomers, i.e. the (L)- and (C)-dimers(S), indicated that the
handedness of such skewed overlapping of the mesogens is deter-
mined by the handedness of the chiral spacer.
Interestingly, the CD peaks are much larger for the (C)-dimers
than for the (L)-dimers. This phenomenon can be simply explained as
Fig. 2 (a) Typical reflection CD spectra observed at T ¼ Ti ꢀ 10 in a (C)-
dimer(R)/CBPBB mixture system; (b) reciprocal pitch Pꢀ1 of a cholesteric
helix in the chiral nematic LC of (C)-dimer(R)/CBPBB (square) and (L)-
dimer(R)/CBPBB (triangle) mixtures as a function of the molar content
of the chiral dimer.
Fig. 4 The proposed configurations of (a) (C)-dimer(R) and (b) (L)-
dimer(R) in a nematic solvent.
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In summary, the HTP of an LC dimer having a chiral spacer is
efficiently enhanced by macrocyclization. In the macrocyclized dimer,
the two mesogens facing each other are fixed in a skewed arrange-
ment and the handedness is determined by the handedness of the
spacer. Such an intermolecular skewed configuration is effective in
enhancing the HTP. Macrocyclization of chiral LC dimers can thus
be utilized as a new strategy for obtaining high-HTP chiral dopants.
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This research was supported by a Grant-in-Aid for Creative Research
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