A photoswitchable and thermally stable axially chiral dithienylperfluorocyclopentene dopant with high helical twisting power

Article abstract of DOI:10.1039/c3tc30611a

A chiral dithienylperfluorocyclopentene molecule bearing two bridged binaphthyl units was designed and synthesized by a Suzuki-Miyaura cross-coupling reaction between chiral binaphthyl iodide and dithienylperfluorocyclopentene- derived bis(boronic ester). Its photoresponsive properties were investigated in both organic solvent and liquid crystal media. The UV-vis spectra exhibited typical photochromic changes of diarylethenes upon UV irradiation. The CD spectral changes upon light irradiation indicated that the conformation of binaphthyl units and the chiroptical properties of this molecule could be modulated by light. More importantly, when using as a chiral dopant in nematic liquid crystals, this molecule could induce cholesteric liquid crystals with very high helical twisting powers. At very low doping concentrations, this dopant was able to induce a reversible isothermal phase transition between nematic and cholesteric phases upon light irradiation. The photochemical control of the pitch length of cholesteric phases at higher doping concentrations enabled the reversible reflection wavelength control in the visible region. Superior thermal stability and excellent fatigue resistance were also observed during the photoswitching process, which are important properties for applications.

Full text of DOI:10.1039/c3tc30611a

Journalof
Materials Chemistry C
Materials for optical and electronic devices
www.rsc.org/MaterialsC
Volume 1 | Number 25 | 7 July 2013 | Pages 3889–4044
ISSN 2050-7526
PAPER
Quan Li et al.
A photoswitchable and thermally stable axially chiral
dithienylperfluorocyclopentene dopant with high helical twisting power
2050-7526(2013)1:25;1-E

Journal of
Materials Chemistry C
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PAPER
A photoswitchable and thermally stable axially chiral
dithienylperfluorocyclopentene dopant with high
helical twisting power†
Cite this: J. Mater. Chem. C, 2013, 1,
3917
a
a
b
a
*
Yannian Li, Mengfei Wang, Augustine Urbas and Quan Li
A chiral dithienylperfluorocyclopentene molecule bearing two bridged binaphthyl units was designed and
synthesized by Suzuki–Miyaura cross-coupling reaction between chiral binaphthyl iodide and
a
dithienylperfluorocyclopentene-derived bis(boronic ester). Its photoresponsive properties were
investigated in both organic solvent and liquid crystal media. The UV-vis spectra exhibited typical
photochromic changes of diarylethenes upon UV irradiation. The CD spectral changes upon light
irradiation indicated that the conformation of binaphthyl units and the chiroptical properties of this
molecule could be modulated by light. More importantly, when using as a chiral dopant in nematic
liquid crystals, this molecule could induce cholesteric liquid crystals with very high helical twisting
powers. At very low doping concentrations, this dopant was able to induce a reversible isothermal
phase transition between nematic and cholesteric phases upon light irradiation. The photochemical
control of the pitch length of cholesteric phases at higher doping concentrations enabled the reversible
reflection wavelength control in the visible region. Superior thermal stability and excellent fatigue
resistance were also observed during the photoswitching process, which are important properties
for applications.
Received 3rd April 2013
Accepted 10th May 2013
DOI: 10.1039/c3tc30611a
www.rsc.org/MaterialsC
wavelength l of the selective reection is dened by l ¼ nP,
where P is the pitch length of the helical structure across which
Introduction
the LC directors rotate a full 360ꢀ and n is the average refractive
index of the LC material. The ability of a chiral molecule to
induce a twist in the director orientation of an achiral NLC is
quantied as the helical twisting power (HTP, bM) and
expressed in the equation bM ¼ (PC)ꢁ1, where C is the molar
concentration of the chiral dopant. The reversible photo-
isomerization of a chiral dopant oen results in a change in the
HTP and therefore the reection wavelength of the cholesteric
phase can be reversibly tuned by light (Fig. 1). This provides
opportunities as well as challenges in fundamental science that
may pave the way for applications of CLC materials in color
lters and reectors,6–8 tunable lasers,9,10 and reection
displays11,12 which require no driving electronics and can be
exible.
Stimuli responsive self-organized architectures with functional
properties hold great promise in design and fabrication of
smart so materials. Cholesteric liquid crystals (CLCs) repre-
sent such a striking example due to their unique self-organized
helical arrangement of mesogen molecules and their property
of selective reection of light.1–5 Moreover, the weak intermo-
lecular interaction in liquid crystals (LCs) provides the possi-
bilities of being responsive to a number of stimuli, such as light,
heat, electric eld, and mechanical stress. Among all stimuli,
light is particularly attractive due to the advantages such as easy
addressability, fast response time and potential for remote
control in a wide range of ambient environments. Photo-
responsive CLCs can be made by doping chiral photochromic
molecules into an achiral nematic LC (NLC) host. The resulting
mixture can self-organize into controllable, optically tunable
helical superstructures, i.e. the cholesteric mesophase, which
can selectively reect light according to Bragg's law. The central
High HTPs are of key importance for the performance of
photoresponsive CLC materials since low HTPs give rise to the
need for higher dopant concentrations for a specic pitch
length. This oen leads to phase separation, coloration and can
alter the desired physical properties of the NLC host. To date,
various kinds of photochromic molecules have been reported as
chiral dopants for LCs, such as azobenzenes,13–19 overcrowded
alkenes,20–23 diarylethenes,24–32 spirooxazines33,34 and ful-
gides.35,36 Among them, diarylethenes are of particular interest
owing to their thermal stability and fatigue resistance. Upon
irradiation with UV light, they can transform from a colorless
aLiquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA. E-mail: qli1@
kent.edu; Fax: +1-330-672-2796; Tel: +1-330-672-1537
bMaterials and Manufacturing Directorate, Air Force Research Laboratory WPAFB,
Ohio 45433, USA
† Electronic supplementary information (ESI) available: Measurement of pitch
and HTP, thermal stability, fatigue resistance, CD spectra and copies of 1H
NMR and 13C NMR. See DOI: 10.1039/c3tc30611a
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open-ring form to a colored closed-ring form. The closed form is
thermally stable and the reverse photoisomerization to open
form can occur only by visible light irradiation. The photo-
chemically reversible switching of diarylethenes with thermal
stability in both states constitutes the basis for their widespread
applications in optical devices.37 In comparison with the large
body of literature describing the reection color tuning of CLCs
using chiral azobenzene molecules, the development of chiral
diarylethene dopants with satisfactory functionalities for device
performance, e.g. high HTPs, remains a challenging task.
The most frequently investigated diarylethenes are dithie-
nylethenes with
a cyclopentene or peruorocyclopentene
bridge. Although non-uorinated dithienylcyclopentenes
possess the advantage of low synthetic cost with acceptable
performance, dithienylperuorocyclopentenes are more attrac-
tive due to their superior thermal stability and excellent fatigue
resistance, which are crucial traits for switching and application
purposes.38 In continuation of our research on diary-
lethenes,31,32 here we report the synthesis and characterization
of a new dithienylperuorocyclopentene (R,R)-1 containing two
bridged binaphthyl units (Scheme 1). This compound was
found not only to exhibit higher HTPs than previously reported
diarylethene dopants but also to show excellent thermal
stability and fatigue resistance. The intent behind the design of
the molecular structure is derived from (1) axially chiral
binaphthyl derivatives, particularly bridged ones are powerful
CLC inducers with very high HTPs,39,40 (2) dithienylper-
uorocyclopentene is selected as the core due to its excellent
photochromic properties and fatigue resistance, and (3) the
binaphthyl units can be directly attached to the photochromic
core by the Suzuki–Miyaura protocol. These relatively more rigid
structures are expected to increase HTPs of the resulting
molecules.41
Fig. 1 Schematic illustration of the mechanism for the pitch length and reflec-
tion wavelength photo-tuning of the cholesteric phase using a chiral photo-
chromic dopant.
Results and discussion
Synthesis of compound (R,R)-1
Compound (R,R)-1 was synthesized by a Suzuki–Miyaura cross-
coupling reaction between binaphthyl-derived iodide (R)-3 and
bis(boronic ester) 7 (Scheme 1).42 The intermediate (R)-3 was
readily prepared in two steps starting from (R)-binol. The
etherication of (R)-binol with diiodomethane gave the bridged
product (R)-2, which underwent direct orthometalation in the
presence of tert-butyllithium followed by quenching with
iodine. By controlling the ratio of tert-butyllithium to (R)-2,
mono-iodide (R)-3 was achieved in acceptable yield with a small
portion of 3,30-diiodide byproduct. Another key intermediate
dichloride 6 was prepared according to a known method43 from
2-chloro-5-methyl thiophene 4, which was brominated to
provide 3-bromo-5-chloro-2-methyl thiophene 5. Treatment of 5
with n-butyllithium and the following double substitution
reaction with peruorocyclopentene gave dichloride 6 in a
moderate yield, which was converted to bis(boronic ester) 7
followed by the Suzuki–Miyaura cross-coupling reaction with
iodide (R)-3 to afford the product (R,R)-1 in good yield. All the
target compound and intermediates were fully characterized by
1H NMR, 13C NMR, and HRMS or elemental analysis.
Scheme 1 Synthesis of intermediates and compound (R,R)-1. Reaction condi-
tions: (a) CH2I2, K2CO3, acetone; (b) (1) t-BuLi, THF, ꢁ78 ꢀC, (2) I2; (c) Br2, CHCl3; (d)
(1) n-BuLi, Et2O, (2) perfluorocyclopentene; (e) (1) n-BuLi, (2) B(OBu)3; (f)
Pd(PPh3)4, aq. Na2CO3.
3918 | J. Mater. Chem. C, 2013, 1, 3917–3923
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Photoisomerization in organic solvent
Compound (R,R)-1 is chemically and thermally stable, and its
photoresponsive properties in solution were examined by UV-
vis spectroscopy (Fig. 2). The solution of (R,R)-1 in hexane was
colorless and the UV spectrum showed maximum absorption at
279 nm, and there was no absorption observed in the visible
region. Upon UV irradiation at 310 nm, the solution color
changed immediately from colorless to fresh blue. Accordingly,
there was a gradual decrease seen in the absorption at 279 nm
accompanied by the appearance of a new absorption band in
the visible region with a maximum at 612 nm (Fig. 2A), indi-
cating the transformation of (R,R)-1 from its open form to the
closed form. An isosbestic point remained at 331 nm during
this process. The photostationary state (PSS310nm) was reached
within 45 s and this irradiated state was thermally stable and
could be switched back to the initial state upon visible light
irradiation at 630 nm in about 10 minutes as evidenced by the
restoration of the absorption spectra (Fig. 2B). The thermal
stability of the closed form of (R,R)-1 at PSS310nm was monitored
by the absorbance at 612 nm (A) compared with the initial
absorbance (A0) of PSS310nm. The plot of ln(A/A0) versus time
shows the rst order kinetics of the thermal decay and the half-
life time is calculated to be 115 days (Fig. S1 and S2†).
Furthermore, the photoisomerization in organic solvent also
exhibited excellent fatigue resistance as revealed by the
repeated irradiation of the sample with UV and visible light over
many cycles (Fig. S3†).
Fig. 3 CD spectral changes of (R,R)-1 (10 mM in hexane) upon UV irradiation at
310 nm for different times. Note: D3 was calculated based on the molar
concentration of (R,R)-1. Taking into account that there are two binaphthyl units,
the conformation change of binaphthyl was estimated from the value of D3/2.
strong bisignated exciton couplets between 200 and 250 nm in
the initial state, which are assigned to the coupling of the two 1Bb
transitions located on distinct naphthalene rings.44 The intense
negative exciton couplet at 229 nm associated with a positive
couplet at 216 nm reect the absolute R conguration of the
binaphthyl moiety in the molecule. The couplets at around 280
1
1
and 320 nm are related to the La and Lb transitions of naph-
thalene. Dramatic changes in shape and intensity were found in
CD spectra when irradiated the solution with UV light at 310 nm.
The exciton couplet at 229 nm weakened signicantly with a
1
small bathochromic shi in maximum to 231 nm. For the La
and 1Lb transitions, the intensity of the positive band at around
280 nm decreased with a negative shoulder developing at
264 nm, while the negative band at 320 nm became nearly at.
Binaphthyl derivatives can rotate along the carbon–carbon
bond between the 1 and 10 positions in a certain range. The
intensity change of 1Bb transitions is actually a sign of the
conformation change in the binaphthyl moiety with variation in
the dihedral angle (q) between two naphthalene rings.45
Although the binaphthyls in (R,R)-1 are xed with a methyl-
enedioxy tether and thus have less exibility, a slight variation
of the dihedral angle is still possible, which may cause a
dramatic difference in chiroptical properties.39,40,44 According to
Rosini's correlation between the D3 of the low energy compo-
The chiroptical property of (R,R)-1 in organic solvent was
investigated by CD spectra at room temperature. D3 is dened as
the difference value between the molar excitation coefficients of
le (3L) and right (3R) circularly polarized light, respectively. As
shown in Fig. 3, the CD spectra of (R,R)-1 in hexane exhibited two
1
nent of Bb exciton couplet (around 230 nm) and the dihedral
angle of binaphthyl derivatives,45 the q for the open form
isomers was estimated to be around 60ꢀ, which is consistent
with the fact that methylenedioxy bridged binaphthyl has a
rigid structure with a quite narrow dihedral angle.46,47 The
decrease in D3 indicates that the dihedral angle in (R,R)-1 may
be slightly increased during photoisomerization.45 It is note-
worthy that the photocyclization of (R,R)-1 generates two new
chiral centers with the conguration of (S,S) or (R,R), and thus
forms two diastereomers in closed form (Fig. S4†). However, no
Cotton effect in the CD spectra was observed in the visible
region around 612 nm corresponding to the absorption band of
the closed form (Fig. S5†), indicating that no diastereoselective
photoisomerization occurred in this photoisomerization
process. Therefore the chiroptical properties of (R,R)-1 and their
changes upon UV irradiation are mainly attributed to the
binaphthyl units and the variation in dihedral angle.30
Fig. 2 Absorption spectral changes of (R,R)-1 (10 mM in hexane) upon UV irra-
diation at 310 nm for different times (A) and the reverse process upon visible light
irradiation at 630 nm (B). Inset: color change before and after UV irradiation.
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Table 1 HTPs of (R,R)-1 at different states in NLC hosts
HTP (bM)
Photoisomerization in LC media
It is established that the dihedral angle in the binaphthyl
derivative plays a key role in their cholesteric induction
behaviors, such as HTP and handedness of the induced chole-
steric phase.46,47 Both the transoid conformation (q > 90ꢀ) and
the cisoid conformation (q < 90ꢀ) of binaphthyl derivatives can
efficiently induce CLCs with high HTPs, while the quasi-
orthogonal conformations (q ꢂ 90ꢀ) only have very weak induc-
tion ability. For R conguration, the transoid conformation
induces a right-handed CLC and the cisoid conformation results
in a le-handed CLC. A change in q can cause a dramatic
difference in the cholesteric induction abilities, i.e., HTPs.
Inspired by the chiroptical property information obtained from
CD spectra, the photoresponsive behaviors of (R,R)-1 in liquid
crystals were then investigated.
NLC 4-pentyl-40-cyanobiphenyl (5CB) was used as the host
material. A cholesteric phase was observed under the polarized
microscope with the characteristic ngerprint texture at a low
doping concentration of 0.26 mol% (Fig. 4A), indicating that
(R,R)-1 exhibited very high HTP at the initial state. The pitch
length of the cholesteric helix was measured by Cano's
method.48 When the mixture was lled into a wedge cell, the
photoinduced pitch change was directly evidenced by short-
ening the distance between the Cano's lines upon UV irradia-
tion (Fig. 4B and C). According to the relationship between HTP
and pitch, it was concluded that the HTP of (R,R)-1 was
increased during photoisomerization.
By doping different concentrations of (R,R)-1, the inverse of
the pitch (1/P) proportionally increases with increase in the
doping concentration and the absolute values of HTP are then
determined as the slopes by plotting 1/P against dopant
concentrations (Fig. S7†). The HTPs of (R,R)-1 in different NLCs
at both states were measured and the results are summarized in
Table 1. The handedness of these CLCs was determined by the
contact method.49 As seen in Table 1, (R,R)-1 induced le-
handed CLCs in all three hosts due to the cisoid form of the
binaphthyl moiety, and the high HTPs at both the initial state
and PSS310nm are ascribed to the narrow dihedral angle of (R,R)-
1.39 In addition to the high HTPs, the photoisomerization led to
a dramatic increase in HTPs, which might be the result of the
dihedral angle change of the binaphthyl units in this mole-
cule.46,47 Using different LC hosts resulted in different inter-
molecular associations between the dopant and the host as
evidenced by the variation in HTP values and changes in
magnitude upon UV irradiation. It was found that NLC 5CB not
only gave the highest HTPs but also showed the largest HTP
HTP changeb
(DbM/bM)
NLCa
Initial state
PSS310nm
PSS630nm
E7
5CB
Zli-1132
ꢁ114
ꢁ147
ꢁ124
ꢁ145
ꢁ188
ꢁ141
ꢁ116
ꢁ150
ꢁ118
+27%
+28%
+14%
a
E7 is a eutectic mixture of LC components commercially designed
for display applications, and 5CB is 40-pentyl-4-biphenylcarbonitrile.
Zli-1132 is a mixture of 4-(4-alkylcyclohexyl)benzonitrile and 4-(4-
b
alkylcyclohexyl)-40-cyanobiphenyl derivatives. Percent change in bM
observed between the initial state and PSSUV
.
change upon UV irradiation. The HTP of (R,R)-1 at the initial
state in 5CB was 147 mmꢁ1 while the highest HTP of the known
chiral diarylethenes recently reported is 109 mmꢁ1 at the initial
state in 5CB,31 which was further increased to 188 mmꢁ1. To the
best of our knowledge, this is the highest value among the
known diarylethene dopants so far.24–32
Light-induced isothermal phase transition
The light-induced isothermal phase transition between nematic
and cholesteric is interesting and has been demonstrated with a
few chiral dopants utilizing their large variations in HTPs
between two different states. Compound (R,R)-1 was found to be
able to induce the cholesteric mesophase even with the
concentration as low as 0.13 mol% due to its high HTP at the
initial state. When the doping concentration was further
reduced to 0.1 mol%, the LC mixture exhibited an apparent
nematic phase which was conrmed by the conoscopic obser-
vation (Fig. 5A), i.e. the pitch was too long to be detected at this
low doping concentration. Upon UV irradiation at 310 nm, a
characteristic ngerprint texture of the cholesteric phase grad-
ually appeared due to the increase of HTP and the consequent
formation of a helical superstructure of the cholesteric phase
(Fig. 5B and C). The cholesteric phase at this irradiated state
was thermally stable and the reverse process to the nematic
phase occurred with visible light irradiation at 630 nm. All the
experiments were carried out at room temperature and heating
from light irradiation was not detected and thought to be
minimal at the exposure intensity of 30 mW cmꢁ2
.
Fig. 5 Cross-polarized optical textures of 0.1 mol% of (R,R)-1 in 5CB in a 5 mm
thick homeotropic cell upon UV irradiation for 0 s (A, conoscopic observation), 5 s
(B) and 30 s (C).
Fig. 4 Cross-polarized optical texture of 0.26 mol% of (R,R)-1 in 5CB (A) and the
pitch change before (B) and after (C) UV irradiation observed in a wedge cell.
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Journal of Materials Chemistry C
wavelength was shied to ca.550 nm and the cell showed a
green reection color, which was further tuned to blue with a
reection wavelength of ca. 470 nm at PSS310nm. The reverse
process was achieved with 630 nm visible light irradiation and it
took about 10 minutes for the LC mixture to restore to the initial
state (Fig. S8†). It is worth noting here that the LC mixture
exhibited superior thermal stability in any irradiated state
which was revealed by no obvious reection wavelength shiing
being detected when the cell was stored in the dark (Fig. S9†).
The excellent fatigue resistance was examined by the repeated
switching between the initial state and PSS310nm over many
cycles with no degradation in cell properties detected
(Fig. S10†). Combined with the superior thermal stability and
excellent fatigue resistance, the reection color tuning enabled
by (R,R)-1 is extremely promising for future applications.
Reection color control
Compound (R,R)-1 showed very good solubility in all the three
NLC hosts. For the reection wavelength tuning, we used 5CB as
the NLC host due to the higher HTPs and larger HTP variation.
At rst, a LC mixture was formulated by doping 1.38 mol% of
(R,R)-1 into 5CB, which was capillary-lled into a 5 mm thick
glass cell with anti-parallel rubbing directions. The cell was
painted black on one side and the reection spectra were
recorded. At the initial state, the cell showed a reection band
in the near IR region with a central wavelength of ca. 800 nm,
and no reection color was observed visually (Fig. 6). Aer UV
irradiation for 60 s to reach the photostationary state, the
reection band shied to the visible region with a central
wavelength of ca. 640 nm, and a red reection color was visually
observed in the cell. This pronounced hypochromic shi in
reection wavelength was a result of shortening in pitch, which
was also in accordance with the increase in HTPs of (R,R)-1
during photoisomerization.
Conclusion
A new axially chiral dithienylperuorocyclopentene dopant
containing two bridged binaphthyl units was designed and
synthesized. Its photoresponsive properties were investigated in
both organic solvent and LC media. The photochromic behav-
iors in organic solvent were characterized by UV-vis and CD
spectra. When used as a chiral dopant, it was found that this
molecule exhibited very high HTPs in different NLC hosts. The
light-induced isothermal phase transition between nematic and
cholesteric was achieved with this dopant at a very low doping
concentration. The reection wavelength of the CLCs with
higher doping concentrations was reversibly tuned in the visible
region. Moreover, the photochemical switching also showed
superior thermal stability at both states and excellent fatigue
resistance. The high HTPs combined with excellent thermal
stability and fatigue resistance are believed to be helpful for the
development of more powerful molecular switches and repre-
sent progress towards practical applications.
A higher doping concentration of 1.80 mol% enabled the
phototuning of reection wavelength in the visible region as
demonstrated in Fig. 7. The reection color of the cell at the
initial state was orange-red with a central reection wavelength
of ca. 600 nm. Upon UV irradiation for 15 s, the reection
Experimental section
Materials and methods
Fig. 6 Real cell images and reflection spectra of 1.38 mol% of (R,R)-1 in 5CB
All chemicals and solvents were purchased from commercial
suppliers and used without further purication. Tetrakis-
(triphenylphosphine)palladium (Pd(PPh3)4) and (R)-1,10-bi(2-
naphthol) were purchased from Sigma-Aldrich. 1H and 13C NMR
before (black) and after UV irradiation for 60 s (red).
spectra were recorded in CDCl3 or THF-d8. 1H (400 MHz) and 19
F
(376 MHz) NMR spectra were recorded on a Bruker 400 spec-
trometer and 13C (50 MHz) was recorded on a Varian 200
spectrometer. Chemical shis are in d units (ppm) with the
residual solvent peak or TMS as the internal standard. The
coupling constant ( J) is reported in hertz (Hz). NMR splitting
patterns are designated as follows: s, singlet; d, doublet; t,
triplet; and m, multiplet. Column chromatography was carried
out on silica gel (230–400 mesh). Analytical thin layer chroma-
tography (TLC) was performed on commercially coated 60 mesh
F254 glass plates. Spots were rendered visible by exposing the
plate to UV light. Melting points are uncorrected. Elemental
analysis was performed by Robertson Microlit Inc. The mass
spectrum was taken by Mass Spectrometry & Proteomics Facility
Fig. 7 Real cell images and reflection spectra of 1.80 mol% of (R,R)-1 in 5CB
upon UV irradiation at 310 nm for 0 s (orange), 15 s (green) and 60 s (blue).
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of Ohio State University. Textures and disclination line distance (382 mg, 45%). Mp: 208–210 ꢀC. 1H NMR (CDCl3, 400 MHz) d ¼
changes were observed by optical microscopy using a Leitz or 8.50 (s, 1H), 8.00 (d, J ¼ 9.2 Hz, 1H), 7.94 (dd, J ¼ 8.8, 1.2 Hz,
Nikon polarizing microscope. UV-vis spectra were measured by 1H), 7.83 (d, J ¼ 8.4 Hz, 1H), 7.50–7.43 (m, 5H), 7.33–7.28 (m,
a Perkin Elmer Lambda 25 Spectrometer. Circular dichroism 2H), 5.71 (d, J ¼ 3.6 Hz, 1H), 5.67 (d, J ¼ 3.6 Hz, 1H). 13C NMR
(CD) spectra were recorded on a J-715 spectropolarimeter (Jasco (CDCl3, 50 MHz) d ¼ 151.5, 149.3, 139.2, 133.1, 132.0, 131.7,
Inc.). The UV and visible light irradiation was carried out using 130.7, 128.4, 127.3, 127.0, 126.9, 126.7, 126.5, 126.2, 125.8,
a xenon light source 100 W through a lter at 310 nm or 630 nm. 125.1, 120.8, 102.6, 90.2. Anal. calcd for C21H13IO2: C, 59.45, H,
Reection spectra were examined with an Ocean Optics spec- 3.09; found: C, 59.22, H, 2.94%. HRMS (ESI) calcd for
trometer collecting spectra in the dark. The achiral NLCs E7,
5CB and Zli-1132 were used in the study. E7 is a eutectic mixture
C
21H13IO2Na+: 446.9858, found: 446.9840.
of LC components commercially designed for display applica- 3-Bromo-5-chloro-2-methylthiophene (5) (ref. 43)
tions and 5CB is 40-pentyl-4-biphenylcarbonitrile. Zli-1132 is a
A solution of bromine (4.8 g, 30 mmol) in chloroform (10 mL)
mixture of 4-(4-alkylcyclohexyl)benzonitrile and 4-(4-alkylcyclo-
was added dropwise into an ice-cooled solution of 2-chloro-5-
hexyl)-40-cyanobiphenyl derivatives.
methyl thiophene 4 (3.99 g, 30 mmol) in chloroform (50 mL).
Aer addition of the bromine, the reaction mixture was stirred
for 2 h at room temperature, and subsequently poured into
Measurement of pitch and HTP
The CLCs were prepared by weighing appropriate amounts of water (100 mL). The mixture was extracted with dichloro-
host liquid crystal and the dopant into a vial followed by mixing methane (3 ꢃ 50 mL). The combined organic layers were
them with the addition of a few drops of dichloromethane. Aer washed with 1 M Na2S2O3, dried over anhydrous Na2SO4,
evaporation of the solvent under reduced pressure, the mixture ltered and concentrated. The crude product was puried by
was loaded into the wedge cell by capillary action at room ash column chromatography to give the product as a yellow oil
1
temperature. The pitch was then determined by measuring the (5.5 g, 87%). H NMR (CDCl3, 200 MHz) d ¼ 6.73 (s, 1H), 2.32
intervals of Cano's lines appearing on the surfaces of wedge- (s, 3H). 13C NMR (CDCl3, 50 MHz) d ¼ 133.1, 128.4, 126.8, 107.5,
type liquid crystalline cells. Three different concentrations were 14.7. The NMR data are in accordance with the literature.
used by this method for each sample, and the HTP was deter-
mined by plotting 1/P (mmꢁ1) against the concentration of the 1,2-Bis(5-chloro-2-methyl-3-thienyl)peruorocyclopentene (6)
dopant C (mol%) according to the equation bM ¼ 1/(PC).
(ref. 43)
To a stirred solution of 3-bromo-5-chloro-2methylthiophene
(3.0 g, 14.2 mmol) in anhydrous diethyl ether (50 mL), n-butyl-
(R)-2,20-Methylenedioxy-1,10-binaphthyl (R)-2 (ref. 31)
A mixture of (R)-1,10-bi(2-naphthol) (1.43 g, 5 mmol), diiodo- lithium (9.75 mL, 1.6 M in hexane, 15.6 mmol) was added
methane (4.01 g, 15 mmol) and anhydrous K2CO3 (4.14 g, dropwise at ꢁ78 ꢀC and the resulting solution was stirred for 15
30 mmol) in acetone (50 mL) was stirred magnetically and min. Pre-cooled peruorocyclopentene (1.38 g, 6.5 mmol) was
reuxed until the reaction was complete as monitored by TLC. added immediately and the mixture was kept at this tempera-
Aer cooling to room temperature, the reaction mixture was ture and stirred for another 4 h. Then the mixture was diluted
poured into water and extracted with ether (3 ꢃ 50 mL). The with diethyl ether (50 mL) and the reaction was quenched with
combined organic layer was washed with water, dried over an aqueous solution of hydrochloric acid. The aqueous layer
Na2SO4 and concentrated. The crude product was puried by was extracted with diethyl ether (3 ꢃ 50 mL) and the combined
ash column chromatography over silica gel to give the product organic layers were washed with an aqueous solution of
as a white solid (1.37 g, 92%). Mp: 186–188 ꢀC. 1H NMR (CDCl3, NaHCO3 (50 mL), water (2 ꢃ 50 mL) and brine (50 mL), dried
200 MHz) d ¼ 8.01–7.92 (m, 4H), 7.54–7.41 (m, 6H), 7.35–7.30 over anhydrous Na2SO4, ltered and concentrated. The crude
(m, 2H), 5.70 (s, 2H). 13C NMR (CDCl3, 50 MHz) d ¼ 151.2, 132.2, product was puried by ash column chromatography to give
131.8, 130.3, 128.4, 126.9, 126.08, 126.05, 125.0, 120.9, 103.1. the product as an off-white solid (1.08 g, 38%). 1H NMR (CDCl3,
HRMS (ESI) calcd for C21H14O2Na+: 321.0891, found: 321.0898. 400 MHz) d ¼ 6.89 (s, 2H), 1.89 (s, 6H). The NMR data are in
accordance with the literature. Anal. calcd for C15H8Cl2F6S2: C,
(R)-3-Iodo-2,20-methylenedioxy-1,10-binaphthyl (R)-3 (ref. 31)
41.20, H, 1.84; found: C, 41.35, H, 1.73%.
To a THF solution (40 mL) of (R)-2 (596 mg, 2 mmol) at ꢁ78 ꢀC
under nitrogen was added t-BuLi (1.5 mL, 1.7 M in pentane,
2.5 mmol). The solution was stirred at this temperature for 1 h.
(R,R)-1,2-Bis[2-methyl-5-(2,20-methylenedioxy-1,10-binaphthyl-
3-yl)-3-thienyl]peruorocyclopentene ((R,R)-1)
To the resulting solution was added a solution of I2 (660 mg, To a solution of 6 (218 mg, 0.5 mmol) in anhydrous THF (10 mL)
2.6 mmol) in THF (10 mL). The reaction mixture was allowed to was added n-BuLi (0.65 mL, 1.6 M in hexane, 1 mmol) dropwise,
warm to room temperature and stirred for an additional 10 h. and the mixture was stirred at room temperature for 1 h. Then
The excess iodine was reduced with 1 M Na2S2O3. The reaction B(OBu)3 (345 mg, 1.5 mmol) was added and stirred at room
mixture was poured into water, and extracted with ethyl acetate temperature for another 1 h followed by addition of (R)-3
(2 ꢃ 20 mL). The combined organic layers were dried over (425 mg, 1 mmol), 20% Na2CO3 (8 mL) aqueous solution and
Na2SO4 and concentrated. The residue was puried by ash Pd(PPh3)4 (35 mg, 0.03 mmol). The reaction mixture was heated
column chromatography to give the product as a white solid and stirred for 10 h. Aer the mixture was cooled to room
3922 | J. Mater. Chem. C, 2013, 1, 3917–3923
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temperature, the organic layer was separated and the water layer 19 R. Thomas, Y. Yoshida, T. Akasaka and N. Tamaoki, Chem.–
was extracted with ethyl acetate (2 ꢃ 10 mL). The combined Eur. J., 2012, 18, 12337–12348.
organic layers were washed, dried over Na2SO4 and concen- 20 B. L. Feringa, N. P. M. Huck and H. A. van Doren, J. Am.
trated. The crude product was puried by ash column chro- Chem. Soc., 1995, 117, 9929–9930.
matography over silica gel to give (R,R)-1 as an off-white solid 21 R. A. van Delden, N. Koumura, N. Harada and B. L. Feringa,
(202 mg, 42%). Mp: 250–252 ꢀC. 1H NMR (CDCl3, 400 MHz) d ¼
Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4945–4949.
8.23 (s, 2H), 7.99 (d, 8.8 Hz, 2H), 7.96–7.92 (m, 4H), 7.67 (s, 2H), 22 R. Eelkema, M. M. Pollard, J. Vicario, N. Katsonis,
7.51–7.42 (m, 10H), 7.34–7.28 (m, 4H), 5.67 (d, J ¼ 3.6 Hz, 2H),
B. S. Ramon, C. W. M. Bastiaansen, D. J. Broer and
B. L. Feringa, Nature, 2006, 440, 163.
5.61 (d, J ¼ 3.2 Hz, 2H), 2.09 (s, 6H). 13C NMR (CDCl3, 100 MHz)
d ¼ 151.2, 147.3, 142.6, 136.7, 132.1, 131.8, 131.6, 131.5, 130.6, 23 A. Bosco, M. G. M. Jongejan, R. Eelkema, N. Katsonis,
128.4, 127.6, 127.4, 126.8, 126.3, 126.2, 126.0, 125.9, 125.8,
125.6, 125.4, 125.1, 120.7, 102.4, 14.4. 19F NMR (THF-d8, 376
E. Lacaze, A. Ferrarini and B. L. Feringa, J. Am. Chem. Soc.,
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Anal. calcd for C57H34F6O4S2: C, 71.24, H, 3.57; found: C, 71.47,
1082.
H, 3.32%. MS (ESI): m/z 983 (M + Na+).
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Lett., 2000, 654–655.
26 T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie and M. Irie,
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Acknowledgements
This work is supported by the Air Force Office of Scientic 27 T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie and M. Irie,
Research (AFOSR FA9550-09-1-0254 and FA9550-09-1-0193).
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