Reversible photocyclization of achiral dithienylperfluorocyclopentene dopants in a ferroelectric liquid crystal: Bistable SSFLC photoswitching

Article abstract of DOI:10.1039/b403188a

The spontaneous polarization (P_S) of a ferroelectric liquid crystal is modulated reversibly by photocyclization (λ = 313 nm) of dithienyl- and dibenzothienylperfluorocyclopentene dopants with relatively high aspect ratios. Photocyclization of these dopants causes a decrease of P _S due to a photomechanical effect which may originate from a loss of conformational flexibility of the dopant upon photocyclization. The degree of P_S photomodulation increases with dopant concentration up to the solubility limit of 3 mol%. The maximum polarization photomodulation, ΔP_S = 16.2 nC cm^-2, is achieved at 6 K below the Curie point with the dopant 1,2-bis(5-(4-heptyloxyphenyl)-2-methylthien-3-yl) perfluorocyclopentene (3c, 3 mol%). The resulting P_S photoswitch is fatigue resistant and photochemically bistable. Polarized spectroscopy experiments suggest that the dopants form homogeneous solutions with the FLC host and are integrated within the layer structure of the SmC* phase, with their molecular long axes approximately coincident with the director. To the best of our knowledge, this is the first example of a bistable ferroelectric liquid crystal photoswitch to be reported in the literature.

Full text of DOI:10.1039/b403188a

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A R T I C L E
Reversible photocyclization of achiral dithienylperfluoro-
cyclopentene dopants in a ferroelectric liquid crystal: bistable
SSFLC photoswitching
Kenneth E. Maly,a Peng Zhang,a Michael D. Wand,b Erwin Buncela and
Robert P. Lemieux*a
aDepartment of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
bDisplaytech Inc., 2602 Clover Basin Drive, Longmont, Colorado, 80503, USA.
E-mail: lemieux@chem.queensu.ca
Received 1st March 2004, Accepted 10th June 2004
First published as an Advance Article on the web 2nd August 2004
The spontaneous polarization (PS) of a ferroelectric liquid crystal is modulated reversibly by photocyclization
(l ~ 313 nm) of dithienyl- and dibenzothienylperfluorocyclopentene dopants with relatively high aspect ratios.
Photocyclization of these dopants causes a decrease of PS due to a photomechanical effect which may originate
from a loss of conformational flexibility of the dopant upon photocyclization. The degree of PS
photomodulation increases with dopant concentration up to the solubility limit of 3 mol%. The maximum
polarization photomodulation, DPS ~ 16.2 nC cm22, is achieved at 6 K below the Curie point with the dopant
1,2-bis(5-(4-heptyloxyphenyl)-2-methylthien-3-yl)perfluorocyclopentene (3c, 3 mol%). The resulting PS
photoswitch is fatigue resistant and photochemically bistable. Polarized spectroscopy experiments suggest that
the dopants form homogeneous solutions with the FLC host and are integrated within the layer structure of
the SmC* phase, with their molecular long axes approximately coincident with the director. To the best of our
knowledge, this is the first example of a bistable ferroelectric liquid crystal photoswitch to be reported in the
literature.
constant temperature, the net result is a decrease of PS or, in
Introduction
some cases, a complete loss of polarization due to a phase
transition. Another approach is to combine a photochromic
chiral dopant with an achiral SmC liquid crystal host and
modulate PS via a photoinduced change in the polarization
power of the chiral dopant. For example, we have shown that
PS can be increased via the trans–cis photoisomerization of
chiral thioindigo dopants that maintain a rod-like shape in
both isomeric forms.11 In this case, PS photomodulation is
attributed to the increase in transverse dipole moment of the
thioindigo core, which is sterically coupled to chiral side-
chains, and is achieved without concomitant destabilization of
the SmC* phase.
Ikeda et al. showed that the photomechanical effect causes a
lowering of the coercive force Ec required to switch the SSFLC,
and that photoswitching of an azobenzene-doped SSFLC can
be achieved via PS photomodulation by setting the applied
electric field E between Ec thresholds corresponding to the
trans-state and photostationary cis-state (Fig. 1a).5 Komitov
et al. reported that SSFLC photoswitching can also be achieved
via trans–cis photoisomerization of an achiral azobenzene
dopant in a SmC* liquid crystal host which undergoes an
inversion in the sign of PS as a function of temperature.10 In
this case, SSFLC photoswitching results from a shift in the PS
vs. T curve characteristic of the photomechanical effect. By
setting the temperature of the doped SSFLC film between the
inversion temperatures Tinv corresponding to the trans- and cis-
states, the photomechanical effect produces an isothermal
inversion of the sign of PS and switching of the SSFLC
(Fig. 1b). A photoinduced inversion of the sign of PS has also
been achieved via transverse dipole modulation using a
combination of chiral dopants which induce polarizations of
opposite signs: a chiral thioindigo dopant with a positive
polarization power (1dp) and a photoinert chiral dopant with a
negative polarization power (2dp).12 More recently, we
achieved PS sign reversal using a single ‘‘ambidextrous’’
The vast majority of liquid crystal display (LCD) devices
currently on the market use nematic liquid crystals as electro-
optical material.1 However, smectic C liquid crystals formed by
chiral molecules (SmC*) have received considerable attention
as light shutters for the next generation of LCD devices because
they can be switched ON and OFF about 103 times faster than
nematic liquid crystals.2 The chiral SmC* phase is a liquid
crystal phase with diffuse layer ordering and a uniform tilt
orientation within each layer. In a surface-stabilized planar
alignment, the SmC* phase is ferroelectric, i.e., it is possesses a
spontaneous electric polarization (PS) that can be coupled to an
electric field E to produce a light shutter with ON and OFF
states corresponding to opposite tilt orientations.3 Surface-
stabilized ferroelectric liquid crystal (SSFLC) light shutters are
characterized by electro-optical switching times on the order of
microseconds, which makes them attractive for some photonic
and telecommunication applications.4 In the past ten years, a
growing interest in photonic liquid crystal materials has led a
number of research groups to investigate optical switching
mechanisms for SSFLC light shutters (photoswitch) based on
the photomodulation of PS.5–13 Devices using photoswitchable
SSFLC materials would enable the reversible inscription of
images, diffraction gratings and waveguides, and could be
useful in dynamic holography and optical data storage
applications.
One approach to photomodulate PS is based on the change
in molecular shape of chiral and achiral photochromic dopants
such as 4, 4’-disubstituted azobenzenes.5–10 In a ferroelectric
SmC* host, the trans–cis photoisomerization of an azobenzene
dopant destabilizes the SmC* phase due to the change in shape
of the azobenzene chromophore from rod-like (trans) to bent
(cis). This so-called photomechanical effect causes a shift of the
PS vs. temperature profile by lowering the transition tempera-
ture to a non-ferroelectric SmA* or N* phase (Curie point). At
2 8 0 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 8 0 6 – 2 8 1 2
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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conventional smectic liquid crystals. Recently, Frigoli and
Mehl reported that the dithienylethene 1a can be integrated in a
mesogenic structure by linking it to cyanobiphenyl mesogens
via flexible spacers, and that mesogenic properties are
modulated by photocyclization. For example, 1b forms a
broad SmC phase and a narrow nematic phase, whereas the
photocyclized isomer 2b forms an unidentified highly ordered
SmX phase and a broad nematic phase.18
In this paper, we extend the use of dithienylethene dopants to
ferroelectric SmC* liquid crystals. Our approach to the design
of dithienylethene dopants for SmC* photoswitches is to
extend the conjugation of two well-known dithienylperfluor-
ocyclopentene chromophores with 4-alkoxyphenyl side-chains.
This modification afforded dopants with higher aspect ratios
and shifted the absorption maxima of the uncyclized forms to
longer wavelengths to avoid spectral overlap with the SmC*
host. We report on the effect of photocyclization of the
dithienyl dopants 3a–d and the dibenzothienyl dopant 4 on the
spontaneous polarization of a ferroelectric SmC* host and
describe the first examples of a photochemically bistable FLC
photoswitch based on these materials.
Fig. 1 Switching of a SSFLC film by (a) photomodulation of PS below
a coercive force threshold EC (top) and (b) photoinduced inversion of
PS (bottom). In both cases, a dc electric field is maintained across the
film and the PS vector is perpendicular to the plane of the page.
thioindigo dopant containing a chiral side-chain with 1dp that
is sterically coupled to the thioindigo core, and a chiral side-
chain with 2dp that is decoupled from the core.13 In both cases,
the photoinduced increase in transverse dipole moment of the
thioindigo core raises the polarization induced by the 1dp unit
above that of the 2dp unit, thereby inverting the sign of PS.
Interestingly, none of the FLC photoswitches reported thus
far is photochemically bistable (i.e., a system in which each
photostate is stable indefinitely in the dark) due to the thermal
reversion of cis-isomers to the thermodynamically more stable
trans-isomers, which limits their usefulness in optical data
storage applications. To address this problem, we have recently
undertaken a study of photochromic dithienylethene dopants
in SmC* liquid crystal hosts.14 The discovery of dithieny-
lethenes as photochromic compounds is relatively new,15 and
they are considered to be among the most promising
photochromic materials for optical data storage and other
photoswitching applications because of their fatigue resistance
and the thermal stability of both photoisomers.16 These
compounds undergo a reversible conrotatory photocyclization
reaction, as shown in Scheme 1. Dithienylethenes bearing
chiral substituents have been doped in nematic liquid crystals
to induce chiral nematic (N*) phases and shown to cause a
reversible modulation of the N* helical pitch upon photo-
cyclization.17 Despite their proven usefulness in modulating the
bulk properties of nematic phases, simple dithienylethenes such
as 1a have inherent limitations as photochromic dopants in
more ordered smectic phases, including a low aspect ratio and
overlap of the UV spectrum of the uncyclized form with that of
Results
Synthesis
The known dithienylperfluorocyclopentene 3a was prepared
according to Lehn et al., as shown in Scheme 2.19 Demethyla-
tion of 3a with BBr3 gave the diol 5, which was alkylated to give
the derivatives 3b–d. The known dibenzothienylperfluorocy-
clopentene 6 was prepared according to Matsuda and Irie and
iodinated to give 7.20 Extension of the conjugated backbone
was achieved by Suzuki–Miyaura cross-couplings to give 8,
which was demethylated with BBr3 and alkylated with
bromoheptane to give 4. Dithienylethenes are in dynamic
equilibria between parallel and antiparallel conformations in
solution, and the conrotatory photocyclization can proceed
only from the latter. These two conformations can be
distinguished by H NMR spectroscopy in the case of 4 (vide
infra) because of steric crowding about the two thienylethene
bonds, which raises the activation barrier to ca. 70 kJ mol21 21
The 35:65 parallel:antiparallel ratio measured by NMR
spectroscopy is consistent with that reported in the literature
for the parent compound.22
1
.
Photochromism
Extending the conjugation of the dithienylperfluorocyclopen-
tene and dibenzothienylperfluorocyclopentene chromophores
to give 3 and 4 resulted in bathochromic shifts of lmax of 65 and
34 nm, respectively.20,23 As shown in Fig. 2, irradiation of 2 6
1025 M solutions of the dopants 3c and 4 in benzene at l ~
313 nm resulted in photocyclization, and the appearance of
colors characteristic of the cyclized products: blue and purple,
Scheme 1
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 8 0 6 – 2 8 1 2
2 8 0 7

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Scheme 2 Reagents and conditions: (a) 2 eq Br2, CHCl3, 90–100%. (b) Pd(Ph3P)4, Na2CO3, H2O, THF, reflux, 45–52%. (c) (i) s-BuLi, THF, 278 uC;
(ii) perfluorocyclopentene, THF, 278 uC, 75–80 %. (d) BBr3, CH2Cl2,278 uC, 82–84%. (e) CnH2n11Br, K2CO3, 2-butanone, reflux, 40–65%.
(f) 1-heptanol, DIAD, Ph3P, THF, 25 uC, 46%. (g) n-BuLi, THF, 278 uC; (ii) CH3I, 74–84%. (h) I2, H5IO6, H2SO4, AcOH, 72%.
respectively. The degree of photocyclization of 3c and 4 in
dilute benzene solutions was determined by 1H NMR spectro-
scopy in conjunction with UV–vis spectroscopy. Irradiation of
more concentrated solutions of 3c and 4 in deuterated benzene
at 313 nm resulted in partial cyclization, as shown by the
appearance of new signals corresponding to the methyl protons
of the cyclized products (Fig. 3). Absorbances of the NMR
solutions at lmax of the cyclized products were measured by
UV–vis spectroscopy, and the extinction coefficients of the
cyclized products were then calculated by factoring in the
cyclized:uncyclized ratios obtained by integration of the
corresponding NMR signals. Using these extinction coeffi-
cients and the spectral data from Fig. 2, photoconversions of
90% (3c) and 100% (4) were derived, which are consistent with
literature data for similar compounds in dilute solutions. For
instance, Irie reported that compounds like 3 and 4 without alkoxy
groups undergo photocyclization at 313 nm with photoconver-
sions of 80% and 74%, respectively,20,24 and showed that the
addition of electron-donating substituents at the para-position of
the phenyl groups can increase the photoconversion by 10–20%.23
Fig. 3 Methyl 1H NMR signals of compounds 3c (7.2 6 1023 M) and
4 (7.2 6 1023 M) in deuterated benzene before (bottom) and after
irradiation (top) at l ~ 313 nm. The labels indicate the signals
corresponding to the methyl protons of the uncyclized (u) and cyclized
(c) forms, including the parallel (pu) and antiparallel (au) conformers of
uncyclized 4.
Fig. 2 UV–vis absorption spectra of 3c (2.1 6 1025 M) and 4 (2.3 6
1025 M) in benzene before (—) and after irradiation (—) at l ~ 313 nm.
2 8 0 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 8 0 6 – 2 8 1 2

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Polarization photomodulation
The compounds 3a–d and 4 were doped in a ferroelectric liquid
crystal host consisting of a 10 mol% mixture of the Displaytech
compound MDW950 in the achiral liquid crystal PhP.25 In all
cases, mixtures with dopant mole fractions greater than 3 mol%
showed signs of phase separation by polarized microscopy and
were discarded. Each doped FLC mixture was introduced in a
rubbed polyimide-coated ITO glass cell with a 4 mm spacing
and aligned by slow cooling from the isotropic liquid phase to
the SmC* phase in a hot stage. The spontaneous polarization
PS was measured as a function of temperature T before and
after irradiation of the SSFLC films at 313 nm for 3 min using a
450 W Xe arc lamp fitted with an interference filter. In a first
experiment, PS vs. T plots for 1 and 3 mol% mixtures of 3c in
the SmC* host were obtained (Fig. 4a). Extrapolation of the
plots to PS ~ 0 prior to photocyclization reveals that 3c causes
a substantial decrease of the SmC*–SmA* phase transition
temperature (Curie point, TC), from 84 uC without dopant to
65 uC for the 3 mol% mixture. Photocyclization of 3c causes the
PS vs. T plots to shift to lower temperatures, the effect being
more pronounced at the higher dopant mole fraction, which is
consistent with a photomechanical destabilization of the SmC*
phase. As shown in Fig. 4b, the photomechanical effect
achieved with 3 increases with the length of the alkoxy side-
chains, but appears to level off at C7. Photocyclization of the
dibenzothienyl dopant 4 also causes a shift in the PS vs T plot,
but to a lesser extent than the dithienyl analogue 3c (Fig. 4c).
Fig. 5 Modulation of PS for a FLC mixture doped with 4 at 3 mol%
during alternating irradiation with UV (l ~ 313 nm) and visible (l w
455 nm) light at 64 uC (n ~ number of cycles).
SSFLC film containing 3 mol% of 3c was kept in the dark at
55 uC for 4 days after irradiation at 313 nm. No increase of PS
indicative of a thermal reversion of the photocyclization was
observed over that period of time.
Polarized spectroscopy
The cyclized photoproducts from 3 and 4 are characterized by a
broad visible absorption band around 560–600 nm (Fig. 2) with
a transition dipole moment oriented parallel to the long axis of
the chromophores.26 These compounds should therefore
exhibit linear dichroism in the SmC* host if they are uniformly
integrated within the lamellar structure of the SmC* phase. To
assess the orientational order of the cyclized photoproducts
from 3 and 4 in the SmC* host, polarized UV–vis spectroscopy
measurements were performed on SSFLC films containing
3 mol% of 3a–d and 4. The doped FLC mixtures were carefully
aligned in 4 mm ITO glass cells by slow cooling from the
isotropic liquid phase to the SmC* phase and then irradiated at
313 nm. Absorption spectra were acquired as a function of the
angle formed by the polarizer and the smectic layer normal
while applying a positive or negative 10 V dc field across the
SSFLC films. As shown in Fig. 6, the absorbance of the
cyclized photoproducts from 3c and 4 in the SmC* phase at
T 2 TC ~ 210 K is characterized by a relatively strong linear
dichroism corresponding to order parameters of 0.70 and 0.82,
respectively.27 The order parameter increases with the alkoxy
chain length in 3a–d, from 0.58 to 0.78. The dichroic tilt angle,
i.e., the angle formed by the polarizer and the layer normal at
maximum absorbance, is approximately the same (¡2u) as the
SmC* tilt angle measured by polarized microscopy, and
decreases with increasing temperature, which is consistent
with the temperature dependence of molecular tilt in the SmC*
At any given temperature in the SmC* phase, the result of
this photomechanical effect is either a decrease in absolute
value of PS or a complete loss of polarization due to an
isothermal transition to the non-ferroelectric SmA* phase. The
maximum polarization photomodulation DPS achieved with 3c
(3 mol%) is 16.2 nC cm22 at 6 K below the Curie point (T 2 TC ~
26 K), which is double the photomodulation achieved with
4 (3 mol%), 7.7 nC cm22 at T 2 TC ~ 24 K. In all cases, the
high polarization state was fully restored by reversing the
photocyclization using visible light at l w 455 nm. The PS
photomodulation cycle could be repeated a number of times
without any sign of photochemical degradation, as shown in
Fig. 5, for example. To test the bistability of this photoswitch, a
Fig. 4 Spontaneous polarization PS as a function of temperature T. (a) FLC mixtures with no dopant (triangles), doped with 3c at 1 mol% (circles)
and 3 mol% (squares) before and after irradiation at l ~ 313 nm (open and filled symbols, respectively). (b) FLC mixtures doped with 3a (circles), 3b
(squares), 3c (diamonds) and 3d (triangles) at 3 mol% before and after irradiation at l ~ 313 nm. (c) FLC mixtures with no dopant (triangles) and
doped with 4 at 3 mol% (circles) before and after irradiation at l ~ 313 nm.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 8 0 6 – 2 8 1 2
2 8 0 9

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Discussion
The polarization photomodulation achieved with dopants 3c
and 4 is comparable to that generally achieved with azobenzene
dopants.5–10 The decrease in TC caused by doping these
compounds in the SmC* host is consistent with a destabiliza-
tion of the SmC* phase, which is further enhanced by the
photocyclization of the dopants. However, molecular modeling
at the semiempirical AM1 level suggests that, unlike azoben-
zene dopants, the photocyclization of dithienylperfluorocyclo-
pentenes results in relatively modest changes in molecular
shape. As shown in Fig. 7, the AM1 model of 3c in its most
extended form has a bent shape that should indeed disrupt the
lamellar ordering of the SmC* phase, and which remains more
or less the same upon photocyclization. It is possible that the
photomechanical effect achieved with these dopants is due to a
difference in the conformational degrees of freedom of the
uncyclized and cyclized chromophores. The uncyclized chro-
mophores are conformationally more flexible by virtue of
rotation about the thienylethene single bonds, and should
therefore be more adaptable to the lamellar ordering imposed
by the SmC* phase. However, the dopant loses its conforma-
tional flexibility upon photocyclization, effectively locking in
the unfavorable bent shape of the chromophore, which should
cause a greater destabilization of the SmC* phase. The AM1
model of 4 has approximately the same bent shape as 3c, but
the photocyclized form has a higher, more favorable aspect
ratio, which is consistent with the higher order parameter
measured by polarized spectroscopy and may explain why the
photomechanical effect achieved with 4 is significantly less than
with 3c. The increase in photomechanical destabilization with
increasing length of the alkoxy side-chains in the series 3a–d is
also consistent with this mechanism.
Summary
The spontaneous polarization of a ferroelectric liquid crystal
can be modulated by the reversible photocyclization of
dithienyl- and dibenzothienylperfluorocyclopentene dopants.
The photomodulation of PS is due to a photomechanical effect
which may originate from a loss of conformational flexibility of
the dopant upon photocyclization. The degree of photomodu-
lation increases with dopant concentration up to 3 mol%, and
the resulting PS photoswitch is fatigue resistant and photo-
chemically bistable. To the best of our knowledge, this is the
first example of a bistable SSFLC photoswitch to be reported
in the literature.
Fig. 6 Polar plots of linearly polarized light absorbance vs. angle
formed by the polarizer and the layer normal for 4 mm SSFLC films of
3 mol% mixtures of 3c (top) and 4 (bottom) in the SmC* host after
irradiation at l ~ 313 nm. The measurements were taken at T 2 TC
210 K while applying a dc field of 110 V (open circles) and 210 V
~
(filled circles) across the films.
Experimental
General
phase. These results strongly suggest that 3 and 4 form
homogeneous solutions with the SmC* host and are integrated
within the layer structure of the SmC* phase, with their
molecular long axes approximately coincident with the
director.
1H and 13C NMR spectra were recorded on Bruker Avance 300
and 400 spectrometers in deuterated chloroform and deuter-
ated benzene. The chemical shifts are reported in d (ppm)
relative to tetramethylsilane. Low resolution electron impact
Fig. 7 Space-filling models of 3c (top, left) and 4 (bottom, left) and their photoproducts.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 8 0 6 – 2 8 1 2
2 8 1 0

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(EI) mass spectra were recorded on a Fisons VG Quattro triple
quadrupole mass spectrometer; peaks are reported as m/z (%
intensity relative to the base peak). High resolution EI mass
spectra were performed by the University of Ottawa Regional
Mass Spectrometry Center or the University of Toronto Mass
Spectrometry Center. Elemental analyses were performed by
MHW Laboratory (Phoenix, AZ). Melting points were
determined on a Mel-Temp II melting point apparatus and
are uncorrected. Polarized microscopy analyses were per-
formed using a Nikon Eclipse E600 POL polarized light
microscope fitted with a Linkam LTS 350 hot stage. Solution
visible spectra were recorded on a Varian Cary-3 UV–visible
spectrometer in spectrophotometric grade benzene or deuter-
ated benzene. Flash chromatography was performed using 40–
63 mm (230–400 mesh) silica gel (Silicycle) and thin layer
chromatography was performed using silica gel 60 F254 plates
(EM Science). Molecular modeling was performed at the AM1
level using MOPAC as implemented on Chem3D Pro, version
4.0.
4H), 6.90 (d, J ~ 9.0 Hz, 4H), 7.14 (s, 2H), 7.44 (d, J ~ 9.0 Hz,
4H); 13C NMR (100 MHz, CDCl3) d 14.4, 14.8, 22.9, 26.3, 29.4,
29.5, 30.1, 68.5, 115.2, 121.5, 126.1, 126.3, 127.2, 140.5, 142.5,
159.4; MS (EI) m/e 748 (M1, 89), 733 (7), 537 (15), 522 (16),
277 (19), 167 (27), 149 (100); UV (C6H6, open) lmax 298 (log e
4.70); HRMS (EI) calcd for C41H46F6O2S2 748.2850, found
748.2843.
1,2-Bis(5-(4-nonyloxyphenyl)-2-methylthien-3-
yl)perfluorocyclopentene (3d)
The procedure used for the synthesis of 3b was repeated with 5
(246 mg, 0.44 mmol), 1-bromononane (201 mg, 0.97 mmol) and
K2CO3 (250 mg, 1.81 mmol) in 2-butanone (15 mL) to give
1
163 mg (46%) of 3d as white needles: mp 82–83 uC; H NMR
(300 MHz) d 0.89 (t, 3H, J ~ 7 Hz), 1.29–1.50 (m, 12H),
1.79 (m, 2H), 1.94 (s, 3H), 3.97 (t, 2H, J ~ 7 Hz), 6.90 (d, 2H,
J ~ 9 Hz), 7.15 (s, 2H), 7.45 (d, 2H, J ~ 9 Hz); 13C NMR
(75 MHz) 14.5, 14.9, 23.1, 26.4, 29.7, 29.9, 32.2, 68.6, 115.3,
121.6, 126.1, 126.4, 127.3, 140.6, 142.6, 159.5; MS (EI) m/z: 807
(M 1 3, 3), 806 (M 1 2, 9), 805 (M 1 1, 22), 804 (M1, 42), 537
(24), 522 (22), 401 (13), 207 (15), 149 (100), 137 (55), 129 (14), 126
(25), 121 (20). Anal. calcd for C45H54F6O2S2: C, 67.14; H, 6.76; F,
14.16; S, 7.97. Found: C, 67.36; H, 6.87; F, 14.43; S, 7.84.
Materials
All reagents, solvents and chemicals were obtained from
commercial sources and used without further purification,
unless otherwise noted. Methylene chloride was freshly distilled
from P2O5 under nitrogen. Tetrahydrofuran (THF) was freshly
distilled from Na/benzophenone under nitrogen. (R,R)-5-(2,3-
1,2-Bis(6-(4-methoxyphenyl)-2-methylbenzothiophen-3-
yl)perfluorocyclopentene (8)
difluorooctyl)-2-(4-octylphenyl)pyridine
(MDW950)
was
Under an Ar atmosphere, Pd(PPh3)4 (83 mg, 0.072 mmol),
Na2CO3 (1.3 g), water (5 mL) and 4-methoxyphenylboronic
acid (0.45 g, 2.9 mmol) were added to a solution of 7 (0.52 g,
0.72 mmol) in THF (5 mL). The mixture was stirred under
reflux for 24 h, then poured into water and extracted twice with
ether. The combined extracts were washed with water, dried
(MgSO4) and concentrated. The residue was purified by flash
chromatography on silica gel (9:1 hexanes/ethyl acetate) to give
8 (0.22 g, 45%) as a pale yellow solid: mp 128–130 uC; 1H NMR
(300 MHz, CDCl3) d 2.28 (s, 6H, antiparallel conformer), 2.54
(s, 6H, parallel conformer), 3.83 (s, 6H, parallel), 3.86 (s, 6H,
antiparallel), 6.94–7.02 (m, 4H), 7.46–7.98 (m, 10H), parallel/
antiparallel ~ 35:65; MS (EI) m/e 680 (M1, 100), 650 (14), 340
(27); HRMS (EI) calcd for C37H26F6O2S2 680.1278, found
680.1274.
supplied by Displaytech, Inc. 1,2-Bis(5-(4-methoxyphenyl)-
2-methylthien-3-yl)perfluorocyclopentene (3a),19 1,2-bis(5-
(4-hydroxyphenyl)-2-methylthien-3-yl)perfluorocyclopentene
(5),19 and 1,2-bis(6-iodo-2-methylbenzothiophen-3-yl)perfluor-
ocyclopentene (7)20 were prepared according to published
procedures and shown to have the expected physical and
spectral properties.
1,2-Bis(5-(4-butyloxyphenyl)-2-methylthien-3-
yl)perfluorocyclopentene (3b)
Under an Ar atmosphere, a mixture of 5 (71 mg, 0.13 mmol),
1-bromobutane (59 mg, 0.43 mmol), and K2CO3 (117 mg,
0.85 mmol) in 2-butanone (10 mL) was refluxed for 24 h. After
cooling, the mixture was poured into water and extracted twice
with ether. The combined extracts were dried (MgSO4),
concentrated and the residue purified by flash chromatography
on silica gel (9:1 hexanes/ethyl acetate). Recrystallization from
ethanol gave 25 mg (30%) of 3b as a light blue solid: mp 131–
133 uC; 1H NMR (400 MHz) d 0.98 (t, 3H, J ~ 8 Hz), 1.50 (m,
2H), 1.77 (m, 2H), 1.94 (s, 3H), 3.98 (t, 2H, J ~ 8 Hz), 6.89 (d,
2H, J ~ 8 Hz), 7.15 (s, 1H), 7.44 (d, 2H, J ~ 8 Hz); 13C NMR
(100 MHz) d 13.8, 14.5, 19.2, 31.2, 67.8, 114.9, 121.2, 125.7,
126.0, 126.9, 140.2, 142.2, 159.0; MS (EI) m/z: 666 (M 1 2, 4),
665 (M 1 1, 9), 664 (M1, 23), 271 (16), 226 (12), 215 (13), 212
(11), 198 (21), 197 (19), 169 (73), 167 (30), 156 (21), 152 (13),
149 (100), 137 (42), 136 (24), 129 (21), 107 (34), 105 (30);
HRMS (EI) calcd for C35H34F6O2S2: 664.1904, Found:
664.1905.
1,2-Bis(6-(4-hydroxyphenyl)-2-methylbenzothiophen-3-
yl)perfluorocyclopentene (9)
Under an Ar atmosphere, a 1.0 M solution of BBr3 in heptanes
(1.2 mL) was added dropwise to a solution of 8 (120 mg,
0.17 mmol) in CH2Cl2 (3.5 mL) cooled to 278 uC. The mixture
was stirred at 278 uC for 6 h, then allowed to warm to room
temperature. Water was slowly added and the mixture was
stirred for 1.5 h., then extracted twice with ether. The combined
extracts were dried (MgSO4) and concentrated to give 9 (97 mg,
84%) which was used in the next step without further
purification. 1H NMR (300 MHz, CDCl3) d 2.28 (s, 6H,
antiparallel conformer), 2.52 (s, 6H, parallel conformer), 6.90–
6.98 (m, 4H), 7.44–7.90 (m, 10H), parallel/antiparallel ~ 35:65.
1,2-Bis(5-(4-heptyloxyphenyl)-2-methylthien-3-
yl)perfluorocyclopentene (3c)
1,2-Bis(6-(4-heptyloxyphenyl)-2-methylbenzothiophen-3-
yl)perfluorocyclopentene (4)
Under an Ar atmosphere, diisopropylazodicarboxylate (38 mg,
0.19 mmol) was added dropwise to a stirred solution of 5
(50 mg, 0.09 mmol), 1-heptanol (37 mg, 0.32 mmol), and
triphenylphosphine (52 mg, 0.2 mmol) in dry THF (5 mL).
After stirring overnight at room temperature, the mixture was
concentrated and the residue purified by flash chromatography
on silica gel (9:1 hexane/ethyl acetate). Recrystallization from
EtOH gave 31 mg (46%) of 3c as a light blue solid: mp 90–92 uC;
1H NMR (400 MHz, CDCl3) d 0.89 (t, J ~ 7.0 Hz, 6H), 1.32–
1.47 (m, 16H), 1.79 (m, 4H), 1.94 (s, 6H), 3.98 (t, J ~ 6.5 Hz,
A solution of 9 (80 mg, 0.12 mmol), 1-bromoheptane (72 mg,
0.40 mmol) and K2CO3 (110 mg, 0.80 mmol) in 2-butanone
(8 mL) was refluxed for 48 h. After cooling to room
temperature, the mixture was poured into water and extracted
twice with ether. The combined extracts were washed with
water, dried (MgSO4) and concentrated. The residue was
purified by flash chromatography on silica gel (15:1 hexanes/
ethyl acetate) and recrystallization from ether-hexanes to give 4
(67 mg, 65%) as a white solid: mp 57–58 uC; 1H NMR
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 8 0 6 – 2 8 1 2
2 8 1 1

View Article Online
2
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(300 MHz, CDCl3) d 0.91 (t, J ~ 6.0 Hz, 6H), 1.34–1.49 (m,
16H), 1.83 (m, 4H), 2.26 (s, 6H, antiparallel conformer), 2.54 (s,
6H, parallel conformer), 3.99 (t, 4H, parallel), 4.03 (t, 4H,
antiparallel), 6.96–7.02 (m, 4H), 7.46–7.87 (m, 10H), parallel/
antiparallel ~ 35:65; UV (C6H6, open) lmax 292 (log e 4.64);
MS (EI) m/e 848 (M1, 8), 750 (3), 652 (6), 326 (6), 149 (10).
Anal. calcd for C49H50F6O2S2: C, 69.32; H, 5.94. Found: C,
69.25; H, 5.99.
3
4
PS photomodulation experiments
Surface-stabilized FLC films were prepared using rubbed
polyimide-coated ITO glass cells (4 mm spacing, 0.25 cm2
addressed area) supplied by Displaytech Inc. The filled cells
were fitted in a Linkam LTS 350 hot stage with the addressed
area fully exposed through the hot stage aperture and slowly
cooled (0.2 K min21) from the isotropic phase to the SmC*
phase while applying a 10 Hz, 6 V mm21 triangular ac field
across the film. The SSFLC/hot stage assembly was positioned
vertically in front of a PTI 450 W high-pressure Xe arc lamp
fitted with an IR water filter and either an interference filter
(l ~ 313 nm) or a UV cutoff filter (l w 455 nm). The
spontaneous polarization (PS) was measured as a function of
temperature by the triangular wave method (6 V mm21, 80–
100 Hz)28 using a Displaytech APT III polarization testbed
before and after irradiation of the SSFLC films for 3 min at l ~
313 nm. Irradiation of the SSFLC films at l w 455 nm restored
the original polarization state. Tilt angles were measured by
polarized microscopy as half the rotation between two
extinction positions corresponding to opposite polarization
orientations.
5
6
7
8
9
10 L. Komitov, O. Tsutsumi, C. Ruslim, T. Ikeda, K. Ichimura and
K. Yoshino, J. Appl. Phys., 2001, 89, 7745.
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Chem., 1999, 9, 1679; (c) J. Z. Vlahakis and R. P. Lemieux,
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Phys. Lett., 1998, 73, 279; (b) B. Saad, T. Galstyan, L. Dinescu and
R. P. Lemieux, Chem. Phys., 1999, 245, 395; (c) L. Dinescu and
R. P. Lemieux, Adv. Mater., 1999, 11, 42.
13 J. Z. Vlahakis, M. D. Wand and R. P. Lemieux, J. Am. Chem.
Soc., 2003, 125, 6862.
14 For a preliminary communication, see: K. E. Maly, M. D. Wand
and R. P. Lemieux, J. Am. Chem. Soc., 2002, 124, 7898.
15 M. Irie and M. Mohri, J. Org. Chem., 1988, 53, 803.
16 (a) M. Irie, Chem. Rev., 2000, 100, 1685; (b) M. Irie, in Molecular
Switches, ed. B. L. Feringa, Wiley-VCH, Weinheim, 2001;
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167.
17 (a) C. Denekamp and B. L. Feringa, Adv. Mater., 1998, 10, 1080;
(b) K. Uchida, Y. Kawai, Y. Shimizu, V. Vill and M. Irie, Chem.
Lett., 2000, 654; (c) T. Yamagushi, T. Inagawa, H. Nakazumi,
S. Irie and M. Irie, Chem. Mater., 2000, 12, 869; (d) T. Yamaguchi,
T. Inagawa, H. Nakazumi, S. Irie and M. Irie, J. Mater. Chem.,
2001, 11, 2453.
18 M. Frigoli and G. H. Mehl, ChemPhysChem, 2003, 4, 101.
19 S. H. Kawai, S. L. Gilat, R. Ponsinet and J.-M. Lehn, Chem. Eur.
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20 K. Matsuda and M. Irie, Chem. Eur. J., 2001, 7, 3466.
21 K. Uchida, Y. Nakayama and M. Irie, Bull. Chem. Soc. Jpn., 1990,
63, 1311.
Linear dichroism experiments
Surface-stabilized FLC films were prepared and mounted in a hot
stage as described above and irradiated at l ~ 313 nm prior to
spectral acquisition. The SSFLC/hot stage assembly was
positioned vertically between an Ocean Optics UV–vis light
source and photodetector coupled to an Ocean Optics USB 2000
Miniature Fiber Optic Spectrometer interfaced to a PC. A linear
polarizer on a radial mount was positioned between the light
source and the sample and oriented so that the plane of
polarization was coincident with the rubbing direction of the
SSFLC cell at the start of each experiment. A baseline spectra of a
dopant-free sample of the SmC* host was first acquired and
subtracted from spectra of the doped FLC mixtures. Absorption
spectra were acquired as a function of the angle formed by the
polarizer and the smectic layer normal while applying a positive
or negative 10 V dc field across the SSFLC films.
22 M. Takeshita, N. Kato, S. Kawauchi, T. Imase, J. Watanabe and
M. Irie, J. Org. Chem., 1998, 63, 9306.
23 M. Irie, K. Sakemura, M. Okinaka and K. Uchida, J. Org. Chem.,
1995, 60, 8305.
Acknowledgements
24 M. Irie, T. Lifka, K. Uchida, S. Kobatake and Y. Shindo, Chem.
Commun., 1999, 747.
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (Discovery and Strategic Project
grants), the Canada Foundation for Innovation and the
Ontario Challenge Fund (Premier’s Research Excellence
Award to R.P.L.) for support of this work.
25 The dopant MDW950 induces a negative spontaneous polariza-
tion. All PS values reported herein are absolute values.
W. N. Thurmes, M. D. Wand, R. T. Vohra, K. M. More and
D. M. Walba, Liq. Cryst., 1993, 14, 1061.
26 T. Yamada, K. Muto, S. Kobatake and M. Irie, J. Org. Chem.,
2001, 66, 6164.
27 B. Bahadur, in Handbook of Liquid Crystals, ed. D. Demus,
J. W. Goodby, G. W. Gray, H.-W. Spiess, and V. Vill, Wiley-
VCH, Weinheim, 1998.
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