Rationally designed axially chiral diarylethene switches with high helical twisting power

Article abstract of DOI:10.1002/chem.201403705

Three rationally designed axially chiral diaryle-thene switches were synthesized and their application as chiral dopants for phototunable cholesteric liquid crystal devices was investigated. Design of these molecules was based on the combination of photoc

Full text of DOI:10.1002/chem.201403705

DOI: 10.1002/chem.201403705
Full Paper
&
Molecular Switches
Rationally Designed Axially Chiral Diarylethene Switches with
High Helical Twisting Power
Yannian Li,[a] Mengfei Wang,[a] Hao Wang,[a] Augustine Urbas,[b] and Quan Li*[a]
Abstract: Three rationally designed axially chiral diaryle-
thene switches were synthesized and their application as
chiral dopants for phototunable cholesteric liquid crystal de-
vices was investigated. Design of these molecules was based
on the combination of photochromic dithienylcyclopentene
core with bridged binaphthyl units as chiral precursors. Aro-
matic groups were introduced to the molecules at 6,6’-posi-
tions of binaphthyls through a Suzuki–Miyaura coupling re-
action. Their helical twisting powers (HTPs) are significantly
higher than those of the known chiral diarylethenes report-
ed as chiral dopants so far. Photocyclization of these mole-
cules upon light irradiation brought out dramatic variation
in HTPs between different states. The primary colors, red,
green, and blue, were obtained in reflection on light irradia-
tion and with thermal stability. Moreover, a multi-switchable
photodisplay was demonstrated using one of these chiral
molecular switches.
Introduction
cholesteric phase, which can selectively reflect light according
to Bragg’s law.[4] The ability of a chiral dopant to twist a nemat-
ic phase is quantified as helical twisting power (HTP, b) and ex-
pressed as b=1/(PC), where P is the pitch length of the helical
structure and C is the molar fraction of the chiral dopant. The
reflection wavelength of CLCs is defined as l=nP, where n is
the mean refractive index of the LC medium. When chiral dia-
rylethenes are employed as chiral dopants, the molecular con-
formation change during photoisomerization will bring out
variation in HTPs accompanied by tuning in pitch length and
reflection wavelength (Figure 1), which would open the door
for CLCs to applications such as in color filters and reflectors,[5]
tunable lasers,[6] and optically addressed displays[7] that require
no driving electronics and can be made flexible.
Photochromic molecules that can reversibly transform between
different forms upon light irradiation have proven extremely
fascinating for the design and creation of intelligent functional
materials and devices.[1] Among various kinds of photochromic
molecules, diarylethenes are of particular interest owing to
their superior thermal stability and excellent fatigue resistance,
which is crucial for their applications.[2] Upon irradiation with
UV light, diarylethenes can transform from the colorless ring-
open form to the colored ring-closed form, whereas the re-
verse isomerization occurs only with visible light irradiation
and the ring-closed forms are thermally stable. Because of the
different physical and chemical properties at different states,
the reversible switching constitutes the basis for the applica-
tions of diarylethenes in various areas, such as memories and
switches.
Undoubtedly, high HTP is the most desirable attribute of
a chiral dopant for device performance. Low HTPs will give rise
to the requirement of high doping concentrations, which often
In recent years, chiral diarylethenes have found important
application in phototunable liquid crystal (LC) materials, espe-
cially as chiral dopant for the fabrication of photoresponsive
cholesteric LCs (CLCs).[3] CLCs can be formulated by doping
a chiral molecule into an achiral nematic LC and the resulting
mixture can self-organize into a helical superstructure, that is,
[a] Dr. Y. Li,+ M. Wang,+ H. Wang, Prof. Q. Li
Liquid Crystal Institute and Chemical Physics Interdisciplinary Program
Kent State University, Kent, OH, 44240 (USA)
Fax: (+1)330-672-2796
E-mail: qli1@kent.edu
[b] Dr. A. Urbas
Materials and Manufacturing Directorate
Air Force Research Laboratory WPAFB, OH, 45433 (USA)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403705.
Figure 1. Illustration of the reversible tuning of the reflection wavelength in
cholesteric liquid crystals doped with chiral photochromic molecules.
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leads to phase separation and/or coloration, and alters the de-
sired physical properties of LC host. As an example of pursuing
high HTPs, an azobenzene dopant has been reported with HTP
up to 304 mmÀ1.[8] For the diarylethenes dopants, it turns out
that the HTPs have been dramatically increased in our recent
reports;[3j–m] however, there are still considerable room to im-
prove. In continuation with our previous work on light-driven
chiral molecular switches, here we report three new chiral dia-
rylethene molecules with very high HTPs (Figure 2). The intent
Scheme 1. Synthesis of 1a–c.
coupling with 6a–c, compounds 1a–c were successfully syn-
thesized in good yields.
All three of the compounds are thermally and chemically
stable, and their photoresponsive properties were initially char-
acterized in organic solvent. For example, the UV/Vis spectra of
1a in hexane exhibited absorption in UV region, and there was
no absorption observed in the visible region (Figure 3). Upon
UV irradiation, the solution color changed immediately from
Figure 2. Design and chemical structures of axially chiral dithienylcyclopen-
tene molecules 1a–c.
behind the design of these molecules is based on following
criteria: 1) a widely used dithienylcyclopentene core was se-
lected for photoswitching purpose; 2) binaphthyl derivatives,
especially bridged ones, have been proven to be powerful hel-
icity inducers and therefore they were linked to photochromic
core by Suzuki–Miyaura coupling;[9] and 3) rigid structures nor-
mally enhance the HTPs and additional aromatic rings were in-
troduced through the 6 and 6’ positions of binaphthyl moiety
with another Suzuki–Miyaura coupling reaction. This type of
structure may resemble the rod-like LC molecules, and their
dopant-host compatibility may also result in high HTPs.[10]
Results and Discussion
The synthesis of the three compounds is depicted in
Scheme 1. Intermediates 6a–c were prepared starting from (S)-
1,1’-bi(2-naphthol), which was brominated followed by reac-
tion with diiodomethane to give the bromo-substituted
bridged binapthyl derivative 3 in excellent yield. The Suzuki–
Miyaura reaction of 3 with corresponding boronic acid or ester
in the presence of palladium catalyst and sodium carbonate af-
forded coupling product 5a–c, which was treated with tert-bu-
tyllithium at À788C followed by quenching with iodine. By
controlling the ratio of tert-butyllithium to 5a–c, mono-iodide
product 6a–c was achieved in acceptable yield. Dichloride 7 is
a well-known intermediate for dithienylcyclopentene synthesis
and was prepared according to a known method.[11] By conver-
sion of 7 to bis(boronic ester) and another Suzuki–Miyaura
Figure 3. Absorption spectra changes of 1a in hexane (10 mm) upon UV irra-
diation at 310 nm (30 mWcmÀ2, top), followed by visible irradiation at
550 nm (30 mWcmÀ2, bottom). Inset: solution color change from colorless to
purple upon UV irradiation.
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colorless to purple (Figure 3, inset). Accordingly, there was
a gradual decrease observed in the absorption band in UV
region accompanied by the appearance of a new absorption
band in the visible region with a maximum at 558 nm. This is
the characteristics of photochromic behavior, indicating that
the transformation of 1a from its open form to the closed
form.[2] The photostationary state (PSSUV) was achieved within
60 s and this irradiated state was thermally stable as monitored
by absorption spectra (Supporting Information, Figure S3). The
reverse isomerization to open forms was realized by visible
light irradiation at 550 nm in 15 min and the absorption spec-
tra restored to initial state. The fatigue resistance of the photo-
isomerization process was examined by repeated and alterna-
tive irradiation with UV and visible light over many cycles (Sup-
porting Information, Figure S3). No obvious degradation was
observed.
and thus have less flexibility, a slight variation of the dihedral
angle is still possible, which may cause dramatic changes in
chiroptical properties.[3j–m]
It is known that the photocyclization of diarylethene mole-
cules generates a pair of enantiomers with S,S or R,R configura-
tion of two newly generated chiral centers derived from anti-
parallel style of open form. In the presence of axial chirality of
the binaphthyl units, the closed form existed as two diastereo-
mers with chiralities of (S,S,S,S) and (S,R,R,S) (Supporting Infor-
mation, Figure S5). However, no Cotton effects in CD spectra
were observed in the visible region corresponding to the ab-
sorption band of the closed forms of all three compounds, in-
dicating that no diastereoselective photoisomerization oc-
curred in photoisomerization process. Therefore the chiroptical
properties of 1a–c and the related changes upon UV irradia-
tion are mainly attributed to the conformation change of bi-
naphthyl units and the variation in dihedral angle.
The chiroptical properties of these compounds were investi-
gated by CD spectroscopy at room temperature. As shown in
Figure 4, The CD spectra of 1a exhibited strong bisignated ex-
Inspired by the photoisomeization in organic solvent and
chiroptical properties change, we examined their photorespon-
sive behavior in liquid crystals medium as chiral dopants. At
first, compound 1a was doped into commercially available
nematic LC 4-pentyl-4’-cyanobiphenyl (5CB). Surprisingly, it was
found cholesteric phases were induced even at very low
doping concentration of 0.1 mol% as confirmed by the obser-
vation of characteristic fingerprint and oily streak textures
(Figure 5).[14] This indicated that 1a exhibited very high HTP at
the initial state. When a mixture of 0.26 mol% of 1a in 5CB
Figure 4. CD spectra changes of 1a in hexane (10 mm) upon UV irradiation
at 310 nm.
citon couplet between 230 and 280 nm (Figure 4), which is re-
1
Figure 5. Fingerprint texture (A) and oily streak texture (B) of 0.1 mol% 1a
in LC 5CB observed in a homeotropic cell and homogeneous cell, respective-
ly.
lated to the coupling of the two Bb transitions located on dis-
tinct naphthalene rings.[12] The intensive positive exciton cou-
plet at 267 nm associated with negative couplet at 248 nm re-
flects the absolute S configuration of the binaphthyl moiety in
the molecule. The couplets at around 300 and 340 nm are at-
tributed to the 1La and 1Lb transitions of naphthalene. It is note-
worthy here that these couplets appeared at longer wave-
was filled into a wedge cell, the photoinduced pitch change
was directly evidenced by the shortening in the distance be-
tween two adjacent Cano’s lines upon UV irradiation (Figure 6).
The pitch length and the HTPs were measured using the well-
known Cano’s method;[15] that is, the pitch length P=2Rtanq,
where the R is the distance between two adjacent Cano’s line
and q is the wedge angle of the cell. By doping different con-
centrations of 1a, 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 concentration. The HTPs of 1a at
the initial state was calculated to be 205 mmÀ1, which was in-
creased to 266 mmÀ1 upon UV irradiation.
1
length than where they typically locate. For example, the Bb
couplets for binaphthyls were normally found between 200–
250 nm for binapthyl derivatives compared with 230–280 nm
observed in these compounds. This might result from the ex-
tended conjugation systems with the incorporation of aromat-
ic groups into binapthyl units. Upon irradiation at 310 nm, the
1Bb exciton couplets showed a slight bathochromic shift associ-
ated with variance in intensity. Distinct changes were also ob-
1
1
served for La and Lb couplets. These changes of CD spectra
are actually a sign of the conformation change in binaphthyl
moiety with variation in dihedral angle.[13] Although the bi-
naphthyls in these three compounds are fixed with a tether
By using similar method, the HTPs of all of these compounds
at different states were determined and are summarized in
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formations (q~908) only have very weak induction ability. For S
configuration, the transoid conformation induces a right-
handed CLC and the cisoid conformation results in a left-
handed CLC. The minor variation in q may cause a dramatic
difference in the cholesteric induction abilities, that is, HTPs.
All three compounds induced right-handed cholesteric phase
which resulted from the cisoid form of bridged binaphthyl
units. The very high HTPs of these three compounds may origi-
nate from our molecule design as aforementioned. The meth-
ylene bridged binaphthyls are stabilized as cisoid conforma-
tions and have very powerful helicity induction. The incorpora-
tion of aromatic groups at the 6 and 6’ position and the result-
ing elongated structures are more compatible with rod-like
nematic mesogens with high HTPs. The variation in HTPs upon
light irradiation might be a result of a molecular conformation
change and the subtle dopant–host interactions.
Figure 6. The Cano’s lines and their distance change of 0.26 mol% of 1a in
LC 5CB before (A) and after (B) UV irradiation with 310 nm.
Table 1. HTPs of 1a–c at different states in nematic LC hosts.
[b]
Dopant
LC host[a]
HTP bM [mmÀ1
]
DbM/bM
Initial
PSS310nm
PSS550nm
1a
1a
1a
1b
1b
1b
1c
1c
1c
5CB
E7
Zli-1132
5CB
E7
Zli-1132
5CB
E7
+205
+149
+188
+248
+173
+224
+278
+224
+256
+266
+172
+244
+239
+156
+219
+223
+190
+234
+208
+150
+191
+246
+172
+222
+275
+220
+254
+30
+15
+29
À4
On the basis of the high HTPs of 1a and the dramatic in-
crease in LC 5CB, we determined to tune the reflection wave-
length in the visible region. A mixture of 1.3 mol% of 1a in
5CB was capillary-filled into a 5 mm thick glass cell with anti-
parallel rubbing directions. The cell was painted black on one
side and the reflection spectra were recorded. As seen in
Figure 7, the reflection color of the cell at the initial state was
À10
À2
À20
À15
À9
Zli-1132
[a] E7 is a eutectic mixture of LC components commercially designed for
display applications, and 5CB is 4’-pentyl-4-biphenylcarbonitrile. Zli-1132
is a mixture of 4-(4-alkylcyclohexyl)benzonitrile and 4-(4-alkylcyclohexyl)-
4’-cyanobiphenyl derivatives. [b] Percent change in bM observed be-
tween initial state and PSS310nm.
Table 1. Their handedness were also determined by contact
method and the sign “+” represents a right-handed cholester-
ic phase.[16] All three of the compounds induced right-handed
CLCs in different LC hosts. It was found that they exhibit very
high HTPs at both states. These values are much higher than
those in our previous reports on high HTP diarylethene dop-
ants with a value of 156 mmÀ1 [3j]
The HTP of 1c in 5CB is as
.
high as 278 mmÀ1, which is the highest value among all report-
ed chiral diarylethene dopants to date. It is interesting that the
three compounds exhibit different photoresponsive behavior
as chiral dopants. All the HTPs of 1a in three LC hosts in-
creased dramatically upon UV irradiation. 1b had a slightly
higher HTP than 1a at the initial state, which experienced
a minor decrease during photoisomerization. 1c showed the
highest HTPs among three compounds at open forms; howev-
er, a relatively larger decrease was observed during the trans-
formation into closed forms. Using different LC hosts for the
same dopant resulted in different intermolecular associations
between the dopant and the host, as revealed by the different
HTP values and the change magnitude of HTP. 5CB gave the
highest HTPs for all three compounds with largest change for
1a and 1c, while E7 resulted in relatively lower HTPs than
other two hosts.
Figure 7. Reflection color changes observed in cell (2 cmꢁ2 cm) and corre-
sponding reflection spectra of 1.3 mol% of 1a in 5CB upon UV irradiation at
310 nm for 0 s (red), 20 s (green), and 60 s (blue).
red with a central reflection wavelength of ca. 610 nm. Upon
UV irradiation for 20 s, the reflection wavelength shifted to ca.
550 nm and the cell showed a green reflection color, which
was further tuned to blue with a reflection wavelength of ca.
480 nm at PSS310nm. The reverse process was achieved with
550 nm visible-light irradiation and it took about 10 min for
the LC mixture to restore the initial state. It is worth noting
that the LC mixture exhibited superior thermal stability at any
It is established that the dihedral angle in binaphthyl deriva-
tive plays key roles in their cholesteric induction behaviors,
such as HTP and handedness of induced cholesteric phase.[9b]
Both the transoid conformation (q>908) and the cisoid confor-
mation (q<908) of binaphthyl derivatives can efficiently
induce CLCs with high HTPs, while the quasi-orthogonal con-
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irradiated state, which was revealed by no obvious reflection
wavelength shifting when the cell was stored in the dark. 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. Com-
bined with the superior thermal stability and excellent fatigue
resistance, the reflection color tuning enabled by 1a is ex-
tremely promising for future applications.
est value reported for chiral diarylethene dopant to date. Their
photoresponsive behaviors were investigated and the reflec-
tion color tuning was demonstrated by using dopant 1a. Fur-
thermore, a thermally stable multi-switchable photodisplay
was demonstrated. This work would provide new insight into
designing and developing novel diarylethene molecules as
photoresponsive liquid crystal materials for practical applica-
tions and further stimulate rational design of light-driven chiral
switches or motors.
The photo-addressed display utilizing 1a as chiral dopant
was also demonstrated in Figure 8. The same cell in Figure 7
was used and the central square area was coated with ITO
layer. The reflection color of the cell was driven to blue with
UV irradiation. After covering with a photomask, the symbol
Experimental Section
Materials and methods
All of the chemicals and solvents were purchased from commercial
supplies and used without further purification. 1H and 13C NMR
spectra were recorded in CDCl3. 1H NMR (400 MHz) was recorded
on a Bruker 400 spectrometer and 13C NMR (50 MHz) was recorded
on a Varian 200 spectrometer. Chemical shifts are in d (ppm) with
the residual solvent peak or TMS as the internal standard. The cou-
pling constant J is reported in Hz. NMR splitting patterns are desig-
nated as follows: s singlet, d doublet, t triplet, m multiplet. Column
chromatography was carried out on silica gel (230–400 mesh). Ana-
lytical thin-layer chromatography (TLC) was performed on commer-
cially 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. Mass
spectrum was taken by Mass Spectrometry and Proteomics Facility
of Ohio State University. Textures and disclination line distance
changes were observed by optical microscopy using a Leitz or
Nikon polarizing microscope. UV/Vis spectra were measured by
a PerkinElmer Lambda 25 Spectrometer. Circular dichroism (CD)
spectra were recorded on a J-715 spectropolarimeter (Jasco Inc.)
The UV and visible light irradiation was carried out with a xenon
light source 100 W through a filter at 310 nm or 550 nm. Reflection
spectra were examined with an Ocean Optics spectrometer collect-
ing spectra in the dark. The achiral NLCs E7, 5CB, and Zli-1132 were
used in the study. E7 is a eutectic mixture of LC components com-
mercially designed for display applications, and 5CB is 4’-pentyl-4-
biphenylcarbonitrile. Zli-1132 is a mixture of 4-(4-alkylcyclohexyl)-
benzonitrile and 4-(4-alkylcyclohexyl)-4’-cyanobiphenyl derivatives.
Figure 8. A photo-addressed and multi-switchable CLC display in a planar
state through a photomask (A). The image was hidden by applying a voltage
pulse in a focal conic state (B) and was made to reappear by a mechanical
force (C). Note: the central square of the cell was coated with conductive
ITO electrode layer. Top: representation of cholesteric textures; bottom:
demonstration of an image.
“b” with green reflection was recorded by visible light irradia-
tion at 550 nm (Figure 8A). The image recorded was thermally
stable and can be erased by UV irradiation. The cell was rewrit-
able many times, which is ascribed to the excellent fatigue re-
sistance of the chiral dopant 1a. More interestingly, when
a voltage of 20 V was applied across the cell, the LC mixture in
ITO coated area was switched from planar state to focal conic
Measurement of pitch and HTP
state, which is
a scattering, multidomain structure with
The CLCs were prepared by weighing appropriate amount of host
liquid crystal and the dopant into a vial followed by mixing them
with the addition of a few drops of dichloromethane. After evapo-
ration of the solvent under reduced pressure, the mixture was
loaded into the wedge cell by capillary action at room tempera-
ture. The pitch was then determined by measuring the intervals of
Cano’s lines appearing on the surfaces of wedge-type liquid crys-
talline cells. Three different concentrations were used by this
method for each sample, and the HTP were determined by plot-
ting 1/P (mmÀ1) against concentration of the dopant C (mol%) ac-
cording to the equation b=1/(PC).
random alignment of the cholesteric helices. The scattered
light buried the image, as shown in Figure 8B. The image can
be stored at this state and reappear by simply applying a me-
chanical force onto the cell (Figure 8C). The LC mixture re-
turned to planar state owing to the shear-flow-induced align-
ment effect.[8h]
Conclusions
We have rationally designed and synthesized three novel di-
thienylcyclopentene chiral dopants with high HTPs. The design
of these compounds is based on the combination of photo-
chromic core with binapthyl groups. Aromatic groups were in-
troduced through the 6,6’-position of binaphtyl moiety. All of
these compounds were found to exhibit exceptionally high
HTPs and a HTP of 278 mmÀ1 was achieved, which is the high-
Synthesis
Compound 3 was synthesized according to a previous report.[17]
M.p.: 204–2058C. H NMR (CDCl3, 400 MHz): d=8.10 (d, J=2.4 Hz,
2H), 7.89 (d, J=8.8 Hz, 2H), 7.50 (d, J=8.8 Hz, 2H), 7.38 (dd, J=
9.2, 2.4 Hz, 2H), 7.31 (d, J=9.2 Hz, 2H), 5.69 ppm (s, 2H). 13C NMR
(CDCl3, 50 MHz): d=151.6, 132.9, 130.5, 130.4, 129.6, 128.3, 125.9,
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122.2, 119.2, 103.2 ppm. Elemental analysis calcd (%) for
C21H12Br2O2: C 55.30, H 2.65; found: C 55.28, H 2.54. MS (MALDI): m/
z 454 [M+H+].
6b: M.p.: 106–1088C. 1H NMR (CDCl3, 400 MHz): d=8.56 (s, 1H),
8.14 (s, 1H), 8.05 (d, J=8.8 Hz, 1H), 8.02 (s, 1H), 7.66–7.58 (m, 8H),
7.52 (d, J=8.8 Hz, 1H), 7.31 (d, J=8.4 Hz, 4H), 5.75 (d, J=3.6 Hz,
1H), 5.70 (d, J=3.6 Hz, 1H), 2.69 (t, J=8.0 Hz, 4H), 1.71–1.63 (m,
4H), 1.46–1.37 (m, 4H), 0.97 ppm (t, J=7.2 Hz, 6H). 13C NMR
(CDCl3, 50 MHz): d=151.4, 149.3, 142.6, 142.3, 139.4, 138.4, 137.84,
137.75, 137.6, 133.5, 132.1, 131.0, 130.9, 129.02, 128.98, 127.5,
127.2, 127.1, 126.7, 126.2, 126.0, 125.8, 125.7, 124.5, 121.2, 102.6,
90.6, 35.3, 33.6, 22.4, 14.0 ppm. Elemental analysis calcd (%) for
C41H37IO2: C 71.51, H 5.42; found: C 71.48, H 5.31. MS (MALDI): m/z
688 [M+H+].
6c: M.p.: 128–1308C. 1H NMR (CDCl3, 400 MHz): d=8.58 (s, 1H),
8.21 (d, J=1.2 Hz, 1H), 8.09–8.07 (m, 2H), 7.81–7.78 (m, 4H), 7.71
(d, J=7.6 Hz, 4H), 7.69–7.58 (m, 8H), 7.54 (d, J=8.8 Hz, 1H), 7.29
(d, J=8.0 Hz, 4H), 5.76 (d, J=3.6 Hz, 1H), 5.71 (d, J=3.2 Hz, 1H),
2.67 (t, J=7.6 Hz, 4H), 1.70–1.66 (m, 4H), 1.39–1.36 (m, 8H),
0.93 ppm (t, J=7.2 Hz, 6H). 13C NMR (CDCl3, 50 MHz): d=151.6,
149.5, 142.4, 142.3, 140.5, 140.3, 139.5, 139.1, 138.8, 138.0, 137.9,
137.8, 137.4, 133.5, 132.1, 131.2, 128.9, 127.6, 127.5, 126.9, 126.7,
126.1, 125.8, 125.7, 124.7, 121.3, 102.7, 90.7, 35.6, 31.6, 31.2, 22.6,
14.0 ppm. Elemental analysis calcd (%) for C55H49IO2: C 76.03,
H 5.68; found: C 76.31, H 5.57. MS (MALDI): m/z 868 [M+H+].
General procedure for the synthesis of 5a–c
The mixture of 3 (1 mmol), 4 (2.2 mmol), [Pd(PPh3)4] (0.1 mmol),
aqueous Na2CO3 (20%, 5 mL), and THF (10 mL) was refluxed under
nitrogen for 4 h. The reaction mixture was extracted with ethyl
acetate (3ꢁ10 mL) and the combined organic layer was dried over
Na2SO4. The solvent was removed under reduced pressure and the
residue was purified by flash column chromatography to give the
corresponding product 5a–c in 85, 93, and 84% yields, respective-
ly.
5a: M.p.: 265–2668C. 1H NMR (CDCl3, 400 MHz): d=8.16 (d, J=
2.0 Hz, 2H), 8.06 (d, J=8.8 Hz, 2H), 7.75–7.72 (m, 4H), 7.65 (d, J=
8.8 Hz, 2H), 7.61 (dd, J=8.8, 1.6 Hz, 2H), 7.54–7.47 (m, 6H), 7.40–
7.38 (m, 2H), 5.73 ppm (s, 2H). 13C NMR (CDCl3, 50 MHz): d=151.4,
140.7, 137.7, 132.1, 131.3, 130.6, 128.9, 127.4, 127.3, 126.1, 126.0,
125.8, 121.4, 103.2 ppm. Elemental analysis calcd (%) for C33H22O2:
C 87.98, H 4.92; found: C 87.68, H 4.82. MS (ESI): m/z 473 [M+Na+].
5b: M.p.: 175–1768C. 1H NMR (CDCl3, 400 MHz): d=8.14 (d, J=
1.6 Hz, 2H), 8.04 (d, J=9.2 Hz, 2H), 7.66–7.59 (m, 8H), 7.51 (d, J=
8.4 Hz, 2H), 7.30 (d, J=8.4 Hz, 4H), 5.73 (s, 2H), 2.68 (t, J=8.0 Hz,
4H), 1.70–1.63 (m, 4H), 1.46–1.37 (m, 4H), 0.96 ppm (t, J=7.2 Hz,
6H). 13C NMR (CDCl3, 50 MHz): d=151.2, 142.3, 138.0, 137.6, 132.1,
131.2, 130.5, 129.0, 127.4, 127.1, 126.0, 125.8, 121.3, 103.2, 35.3,
33.6, 22.4, 14.0 ppm. Elemental analysis calcd (%) for C41H38O2:
C 87.51, H 6.81; found: C 87.61, H 6.89. MS (ESI): m/z 585 [M+Na+].
5c: M.p.: 276–2788C. 1H NMR (CDCl3, 400 MHz): d=8.21 (s, 2H),
8.07 (d, J=8.8 Hz, 2H), 7.81 (d, J=8.4 Hz, 4H), 7.72 (d, J=8.4 Hz,
4H), 7.68–7.67 (m, 4H), 7.59 (d, J=8.4 Hz, 4H), 7.54 (d, J=8.8 Hz,
2H), 7.29 (d, J=8.4 Hz, 4H), 5.75 (s, 2H), 2.67 (t, J=8.0 Hz, 4H),
1.72–1.65 (m, 4H), 1.39–1.36 (m, 8H), 0.93 ppm (t, J=7.2 Hz, 6H).
13C NMR (CDCl3, 50 MHz): d=151.4, 142.3, 140.2, 139.2, 137.9,
137.2, 132.2, 131.4, 130.6, 128.9, 127.6, 127.4, 126.9, 126.0, 125.7,
121.5, 103.2, 35.6, 31.6, 31.2, 22.6, 14.0 ppm. Elemental analysis
calcd (%) for C55H50O2: C 88.91, H 6.78; found: C 89.12, H 6.60. MS
(ESI): m/z 742 [M+H+].
General procedure for the synthesis of 1a–c
A solution of nBuLi (1 mmol) was added dropwise to a solution of
7 (0.5 mmol) in anhydrous THF (10 mL), and the mixture was
stirred at room temperature for 1 h. Then B(OBu)3 (345 mg,
1.5 mmol) was added and stirred at room temperature for another
1 h followed by addition of 6 (1 mmol), 20% Na2CO3 (8 mL) aque-
ous solution, and [Pd(PPh3)4] (0.03 mmol). The reaction mixture was
heated to reflux and stirred for 10 h. After the mixture was cooled
to room temperature, the organic layer was separated and the
water layer was extracted with ethyl acetate (2ꢁ10 mL). The com-
bined organic layer was dried over Na2SO4 and the solvent was re-
moved under reduced pressure. The residue was purified by flash
column chromatography to give corresponding product 1a–c in
67, 73, and 75% yields, respectively.
1a: M.p.: 235–2378C. 1H NMR (CDCl3, 400 MHz): d=8.26 (s, 2H),
8.13 (d, J=10.8 Hz, 4H), 8.00 (d, J=8.8 Hz, 2H), 7.75–7.69 (m, 8H),
7.66–7.60 (m, 4H), 7.54 (s, 4H), 7.51–7.33 (m, 16H), 5.63–5.60 (m,
4H), 2.95–2.91 (m, 4H), 2.18 (s, 6H), 2.18–2.12 ppm (m, 2H).
13C NMR (CDCl3, 50 MHz): d=151.3, 147.8, 140.7, 140.6, 138.1,
137.6, 136.4, 136.1, 134.9, 134.1, 132.04, 131.96, 131.4, 130.6, 130.3,
128.8, 128.0, 127.6, 127.4, 127.3, 127.2, 127.13, 127.10, 126.1, 126.0,
125.8, 125.6, 121.3, 102.5, 38.4, 23.1, 14.3 ppm. Elemental analysis
calcd (%) for C81H56O4S2: C 84.05, H 4.88; found: C 83.78, H 4.76. MS
(MALDI): m/z 1156 [M+H+].
1b: M.p.: 208–2108C. 1H NMR (CDCl3, 400 MHz): d=8.24 (s, 2H),
8.12 (d, J=10.4 Hz, 4H), 7.98 (d, J=8.8 Hz, 2H), 7.67–7.61 (m, 12H),
7.54 (s, 4H), 7.44 (s, 2H), 7.41 (d, J=8.8 Hz, 2H), 7.30 (d, J=7.6 Hz,
4H), 7.27–7.25 (m, 4H), 5.62 (d, J=3.6 Hz, 2H), 5.59 (d, J=3.2 Hz,
2H), 2.95–2.92 (m, 4H), 2.71- 2.64 (m, 8H), 2.17 (s, 6H), 2.18–2.15
(m, 2H), 1.69–1.63 (m, 8H), 1.44–1.37 (m, 8H), 0.99–0.94 ppm (m,
12H). 13C NMR (CDCl3, 50 MHz): d=151.2, 147.6, 142.3, 138.1, 138.0,
137.9, 137.6, 136.4, 136.0, 134.9, 134.2, 132.1, 132.0, 131.3, 130.5,
130.2, 128.9, 127.9, 127.5, 127.1, 127.0, 126.1, 125.7, 125.6, 121.2,
102.5, 38.4, 35.3, 33.6, 23.1, 22.4, 14.3, 14.0 ppm. Elemental analysis
calcd (%) for C97H88O4S2: C 84.31, H 6.42, S 4.64; found: C 84.05,
H 6.31, S 4.40. MS (MALDI): m/z 1380 [M+H+].
General procedure for the synthesis of 6a–c
A tBuLi solution (1.2 mmol) was added to a solution of 5 (1 mmol)
in anhydrous THF (10 mL) at À788C under nitrogen. The solution
was stirred at this temperature for 1 h. A THF solution (5 mL) of
iodine (1.3 mmol) was added to the reaction mixture. The reaction
mixture was then allowed to warm to room temperature and
stirred for an additional 10 h. The excess iodine was reduced with
1m Na2S2O3. The reaction mixture was poured into water and ex-
tracted with ethyl acetate (3ꢁ20 mL). The combined organic layer
was dried over Na2SO4 and the solvent was removed under re-
duced pressure. The residue was purified by flash column chroma-
tography to give the corresponding product 6a–c in 52, 53, and
48% yields, respectively.
6a: M.p.: 228–2308C. 1H NMR (CDCl3, 400 MHz): d=8.57 (s, 1H),
0.97 (d, J=0.8 Hz, 1H), 8.07 (d, J=8.4 Hz, 1H), 8.03 (d, J=1.2 Hz,
1H), 7.74–7.69 (m, 4H), 7.62–7.59 (m, 4H), 7.54–7.47 (m, 5H), 7.41–
7.39 (m, 2H), 5.74 (d, J=3.2 Hz, 1H), 5.70 ppm (d, J=3.6 Hz, 1H).
13C NMR (CDCl3, 50 MHz): d=151.6, 149.4, 140.5, 140.3, 139.5,
138.4, 137.8, 133.4, 132.1, 131.1, 131.0, 128.9, 127.6, 127.53, 127.46,
127.3, 126.7, 126.2, 126.1, 126.0, 125.7, 124.9, 121.3, 102.6,
90.7 ppm. Elemental analysis calcd (%) for C33H21IO2: C 68.76,
H 3.67; found: C 68.87, H 3.68. MS (ESI): m/z 599 [M+Na+].
1c: M.p.: 200–2028C. 1H NMR (CDCl3, 400 MHz): d=8.26 (s, 2H),
8.18 (s, 2H), 8.14 (d, J=1.2 Hz, 2H), 8.00 (d, J=8.8 Hz, 2H), 7.79
(dd, J=6.8, 1.6 Hz, 4H), 7.75–7.69 (m, 8H), 7.64 (d, J=8.4 Hz, 8H),
&
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d) A. Hochbaum, Y. Jiang, L. Li, S. Vartak, S. Faris, Dig. Tech. Pap. Soc. Inf.
Disp. Int. Symp. 1999, 30, 1063–1065.
7.60–7.54 (m, 12H), 7.45 (s, 2H), 7.42 (d, J=8.8 Hz, 2H), 7.27 (t, J=
8.0 Hz, 8H), 5.64 (d, J=3.6 Hz, 2H), 5.62 (d, J=3.6 Hz, 2H), 3.00–
2.89 (m, 4H), 2.69–2.63 (m, 8H), 2.21 (s, 6H), 2.21–2.15 (m, 2H),
1.70–1.65 (m, 8H), 1.40–1.35 (m, 16H), 0.94–0.90 ppm (m, 12H).
13C NMR (CDCl3, 50 MHz): d=151.3, 147.7, 142.2, 140.1, 139.2,
139.0, 137.89, 137.85, 137.6, 137.1, 136.4, 136.2, 135.0, 134.1, 132.1,
132.0, 131.4, 130.7, 130.3, 128.9, 128.1, 127.5, 127.44, 127.37, 127.3,
127.15, 127.12, 126.8, 126.1, 125.9, 125.7, 125.5, 121.3, 102.5, 38.2,
35.6, 31.6, 31.2, 23.2, 22.6, 14.3, 14.0 ppm. Elemental analysis calcd
(%) for C125H112O4S2: C 86.17, H 6.48, S 3.68; found: C 86.16, H 6.22,
S 3.49. MS (MALDI): m/z 1741 [M+H+].
[6] a) V. I. Kopp, B. Fan, H. K. M. Vithana, A. Z. Genack, Opt. Lett. 1998, 23,
1707–1709; b) S. Furumi, N. Tamaoki, Adv. Mater. 2010, 22, 886–891.
[7] a) N. Tamaoki, S. Song, M. Moriyama, H. Matsuda, Adv. Mater. 2000, 12,
94–97; b) Q. Li, Y. Li, J. Ma, D.-K. Yang, T. J. White, T. J. Bunning, Adv.
Mater. 2011, 23, 5069–5073; c) E. Montbach, N. Venkataraman, A. Khan,
I. Shiyanovskaya, T. Schneider, J. W. Doane, L. Green, Q. Li, SID Int. Symp.
Dig. Tech. Pap. 2008, 39, 919–922.
[8] For selected papers on azobenzene dopants: a) S. Pieraccini, G. Gottar-
elli, R. Labruto, S. Masiero, O. Pandoli, G. P. Spada, Chem. Eur. J. 2004,
10, 5632–5639; b) Q. Li, L. Green, N. Venkataraman, I. Shiyanovskaya, A.
Khan, A. Urbas, J. W. Doane, J. Am. Chem. Soc. 2007, 129, 12908–12909;
c) M. Mathews, N. Tamaoki, J. Am. Chem. Soc. 2008, 130, 11409–11416;
d) T. J. White, R. L. Bricker, L. V. Natarajan, N. V. Tabiryan, L. Green, Q. Li,
T. J. Bunning, Adv. Funct. Mater. 2009, 19, 3484–3488; e) L. Green, Y. Li,
T. White, A. Urbas, T. Bunning, Q. Li, Org. Biomol. Chem. 2009, 7, 3930–
3933; f) M. Mathews, R. S. Zola, S. Hurley, D.-K. Yang, T. J. White, T. J.
Bunning, Q. Li, J. Am. Chem. Soc. 2010, 132, 18361–18366; g) J. Ma, Y.
Li, T. White, A. Urbas, Q. Li, Chem. Commun. 2010, 46, 3463–3465; h) L.
Chen, Y. Li, J. Fan, H. K. Bisoyi, D. A. Weitz, Q. Li, Adv. Optical Mater.
2014, 2, 845–848; i) R. Thomas, Y. Yoshida, T. Akasaka, N. Tamaoki,
Chem. Eur. J. 2012, 18, 12337–12348; j) Y. Li, M. Wang, T. J. White, T. J.
Bunning, Q. Li, Angew. Chem. 2013, 125, 9093–9097; Angew. Chem. Int.
Ed. 2013, 52, 8925–8929; k) Y. Xie, D. Fu, O. Jin, H. Zhang, J. Wei, J.
Guo, J. Mater. Chem. C 2013, 1, 7346–7356; l) L. Wang, H. Dong, Y. Li, C.
Xue, L. Sun, C. Yan, Q. Li, J. Am. Chem. Soc. 2014, 136, 4480–4483.
[9] a) C. Rosini, L. Franzini, P. Salvadori, G. P. Spada, J. Org. Chem. 1992, 57,
6820–6824; b) G. Proni, G. P. Spada, J. Org. Chem. 2000, 65, 5522–5527.
[10] M. Goh, K. Akagi, Liq. Cryst. 2008, 35, 953–965.
[11] a) L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa,
Eur. J. Org. Chem. 2003, 155–166; b) M. Akazawa, K. Uchida, J. J. D.
de Jong, J. Areephong, M. Stuart, G. Caroli, W. R. Browne, B. L. Feringa,
Org. Biomol. Chem. 2008, 6, 1544–1547.
[12] L. Di Bari, G. Pescitelli, P. Salvadori, J. Am. Chem. Soc. 1999, 121, 7998–
8804.
[13] a) G. Gottarelli, M. Hibert, B. Samori, G. Solladiꢂ, G. P. Spada, R. Zimmer-
mann, J. Am. Chem. Soc. 1983, 105, 7318–7321; b) G. Gottarelli, G. P.
Spada, R. Bartsch, G. Solladiꢂ, R. Zimmermann, J. Org. Chem. 1986, 51,
589–592; c) C. Rosini, I. Rosati, G. P. Spada, Chirality 1995, 7, 353–358;
d) C. Rosini, S. Superchi, H. W. I. Peerlings, E. W. Meijer, Eur. J. Org. Chem.
2000, 61–71.
[14] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim, 2003.
[15] a) F. Grandjean, C. R. Hebd. Seances Acad. Sci. 1921, 172, 71–74; b) R.
Cano, Bull. Soc. Fr. Mineral. 1968, 91, 20–27.
Acknowledgements
This work is supported by the Air Force Office of Scientific Re-
search (AFOSR FA9559-09-0254 and FA9559-09-1–0193).
Keywords: axial chirality · liquid crystals · molecular switches ·
photochromism · reflection color
[1] a) B. L. Feringa, Molecular Switches, Wiley-VCH, Weinheim, 2001; b) In-
telligent Stimuli-Responsive Materials: From Well-Defined Nanostructures
to Applications (Ed.: Q. Li), Wiley, Hoboken, New Jersey, 2013.
[2] M. Irie, Chem. Rev. 2000, 100, 1685–1716.
[3] a) C. Denekamp, B. L. Feringa, Adv. Mater. 1998, 10, 1080–1082; b) K.
Uchida, Y. Kawai, Y. Shimizu, V. Vill, M. Irie, Chem. Lett. 2000, 654–655;
c) T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie, M. Irie, Mol. Cryst. Liq.
Cryst. 2000, 345, 287–292; d) T. Yamaguchi, T. Inagawa, H. Nakazumi, S.
Irie, M. Irie, Chem. Mater. 2000, 12, 869–871; e) T. Yamaguchi, T. Inaga-
wa, H. Nakazumi, S. Irie, M. Irie, Mol. Cryst. Liq. Cryst. 2001, 365, 861–
866; f) T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie, M. Irie, J. Mater.
Chem. 2001, 11, 2453–2458; g) T. van Leeuwen, T. C. Pijper, J. Aree-
phong, B. L. Feringa, W. R. Browne, N. Katsonis, J. Mater. Chem. 2011, 21,
3142–3246; h) Y. Li, A. Urbas, Q. Li, J. Org. Chem. 2011, 76, 7148–7156;
i) H. Hayasaka, T. Miyashita, M. Nakayama, K. Kuwada, K. Akagi, J. Am.
Chem. Soc. 2012, 134, 3758–3765; j) Y. Li, A. Urbas, Q. Li, J. Am. Chem.
Soc. 2012, 134, 9573–9576; k) Y. Li, Q. Li, Org. Lett. 2012, 14, 4362–
4365; l) Y. Li, M. Wang, A. Urbas, Q. Li, J. Mater. Chem. C 2013, 1, 3917–
3923; m) Y. Li, C. Xue, M. Wang, A. Urbas, Q. Li, Angew. Chem. 2013, 125,
13948–13952; Angew. Chem. Int. Ed. 2013, 52, 13703–13707.
[4] a) N. Tamaoki, T. Kamei, J. Photochem. Photobiol. C 2010, 11, 47–61; b) S.
Pieraccini, S. Masiero, A. Ferrarini, G. P. Spada, Chem. Soc. Rev. 2011, 40,
258–271; c) V. A. Mallia, N. Tamaoki, Chem. Soc. Rev. 2004, 33, 76–84;
d) R. Eelkema, Liq. Cryst. 2011, 38, 1641–1652; e) Y. Wang, Q. Li, Adv.
Mater. 2012, 24, 1926–1945.
[16] G. W. Gray, D. G. McDonnell, Mol. Cryst. Liq. Cryst. 1976, 34, 211–217.
[17] J.-W. Park, M. D. Ediger, M. M. Green, J. Am. Chem. Soc. 2001, 123, 49–
56.
[5] a) N. A. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi,
K. Ishikawa, H. Takezoe, Nat. Mater. 2008, 7, 43–47; b) M. Mitov, N. Des-
saud, Nat. Mater. 2006, 5, 361–364; c) J. Lub, P. van de Witte, C. Door-
nkamp, J. P. A. Vogels, R. T. Wegh, Adv. Mater. 2003, 15, 1420–1425;
Received: May 27, 2014
Revised: July 10, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 8
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FULL PAPER
Three axially chiral diarylethene
switches were synthesized and their
photoresponsive properties were char-
acterized in both organic solvent and
liquid crystal media. These rigid dithie-
nylcyclopentene structures exhibit very
high helical twisting power. The primary
colors, red, green, and blue, were ob-
tained in reflection on light irradiation
and with thermal stability, and a multi-
switchable photodisplay was demon-
strated.
&
Molecular Switches
Y. Li, M. Wang, H. Wang, A. Urbas, Q. Li*
&& – &&
Rationally Designed Axially Chiral
Diarylethene Switches with High
Helical Twisting Power
&
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Chem. Eur. J. 2014, 20, 1 – 8
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!