Reversible photochemical control of cholesteric liquid crystals with a diamine-based diarylethene chiroptical switch

Article abstract of DOI:10.1039/c0jm03626a

Upon addition of a chiral dopant to a nematic liquid crystal, amplification of molecular chirality can occur and consequently a cholesteric liquid crystal is formed. A major challenge in materials science consists in designing efficient chiral dopants tha

Full text of DOI:10.1039/c0jm03626a

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PAPER
Reversible photochemical control of cholesteric liquid crystals with a
diamine-based diarylethene chiroptical switch†
Thomas van Leeuwen, Thomas C. Pijper, Jetsuda Areephong, Ben L. Feringa,* Wesley R. Browne*
and Nathalie Katsonis*
Received 24th October 2010, Accepted 24th December 2010
DOI: 10.1039/c0jm03626a
Upon addition of a chiral dopant to a nematic liquid crystal, amplification of molecular chirality can
occur and consequently a cholesteric liquid crystal is formed. A major challenge in materials science
consists in designing efficient chiral dopants that allow for control over chiral amplification by use of an
external trigger, for example by irradiation with light, and thereby achieving the control of the dynamic
and responsive structure of cholesteric liquid crystals. Here, a chiral photochromic switch bearing two
chiral imine units connected via phenyl spacers was synthesized and characterized in solution, where it
can be photo-chemically converted from a colourless ring-opened form 1o to a coloured ring-closed
form 1c, reversibly. We show that a small amount of 1o used as a dopant induces the formation of
a stable cholesteric liquid crystal. The retention of the photochromic properties of 1, when used as
a chiral dopant, allows for reversible photocontrol over the period of the cholesteric helix, and shows
the highest values of helical twisting power achieved so far with diarylethene-based photoswitchable
dopants.
Introduction
irradiation with UV light and returned to the open form by
16
irradiation with visible light. The thermal stability of both the
open and the closed form is an important property of diary-
lethenes, which contrasts with, for example, the thermal revers-
ibility observed for azobenzenes and spiropyrans. This thermal
stability allows for the state of diarylethenes to be controlled
Phototunable chiral liquid crystals (LCs) are dynamic materials
that have demonstrated extensive applicability in several fields of
materials science including optical rewritable information
1
2
storage and dynamic colour control. A very promising
approach consists of using these tunable materials to design
3
smart and broadband reflective devices. Phototunbale chole-
selectively by UV and visible irradiation. The synthesis of dia-
17
rylethene-based mesogens has been reported.
Although
steric liquid crystals can be formed by using switchable molecules
as chiral dopants for nematic liquid crystals. A major challenge
to create these materials is to design and synthesize switchable
chiral dopants that are not only excellent helicity inducers but
also photoswitches with a high thermal stability. A wide range of
chiral photoswitchable dopants providing control over the
period of cholesteric liquid crystals has been developed in recent
attractive, this approach is hindered both by the synthetic cost of
the mesogens and by their high transition temperatures. Alter-
natively, the use of chiral photoswitches as dopants allows
envisioning of precise and reversible photocontrol over the hel-
icity and the pitch of cholesteric liquid crystals (the pitch of
a cholesteric helix is defined as the unit length for a complete
rotation). The propensity of a chiral dopant to induce cholesteric
order in a nematic environment is characterised by its helical
twisting power (HTP). So far, diarylethene derivatives have
4
years. While azobenzene-based chiral dopants have received
5,6
most attention, the properties of other classes of photo-
7
responsive molecules as dopants have been investigated also,
ꢀ1
shown moderate HTPs, up to 28.6 mm as reported by Irie and
15d
coworkers.
8
including fulgides, olefins, chiroptical overcrowded alkenes
9
10
11,12
(
as well as structurally analogous molecular motors),
and
This value remains significantly lower than HTPs reported for
ꢀ1 18
13–15
diarylethenes.
some azobenzenes, which can reach 158 mm . However,
Diarylethenes are a class of compounds that can be switched
from a colourless open form to a coloured closed form upon
despite their high efficiency in inducing chiral order, the use of
azobenzenes as chiral photoswitchable dopants is hampered by
their lack of thermal stability—one way switching from the trans
to the cis form can be controlled by irradiation with UV light but
the kinetics of the reverse reaction are entirely determined by the
structure of the azobenzene-based molecule and thus the reverse
reaction is often not controllable by an external trigger. While
Stratingh Institute for Chemistry, University of Groningen, 9747 AG
Groningen, The Netherlands. E-mail: b.l.feringa@rug.nl; w.r.browne@
rug.nl; n.h.katsonis@rug.nl; Fax: +31 50 363 42 96; Tel: +31 50 363 42 35
† Electronic Supplementary Information (ESI) available: IR and Raman
spectra of compounds 1 and 5. See DOI: 10.1039/c0jm03626a/
3
142 | J. Mater. Chem., 2011, 21, 3142–3146
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Fig. 1 Diarylethene-based liquid crystal dopant 1 in its open (1o) and closed (1c) form.
1
3
diarylethenes provide a prospectful alternative to azobenzenes
because of the thermal stability of both open and closed forms,
their HTPs have to be improved in view of future applications
where visible light is employed because high dopant ratios have
to be used when chiral dopants have a low HTP, which gives rise
to several related problems, e.g. phase separation.
designs earlier reported by our group. This red shift in the
absorption spectrum of the closed form results in an increase in
the photostationary state. The choice of the central moiety was
motivated by the fact that perfluorinated cyclopentene-based
systems provide better photochemical stability for diarylethenes,
1
9
compared to perhydrogenated moieties. The design of 1 also
took into account that enhancing the mesogenic character of the
molecule enhances its solubility in nematic hosts, thereby
allowing higher doping ratios to be used without the problem of
phase separation.
In this contribution, we report on the synthesis of diarylethene
1
and its photochromic properties both in solution and as a chiral
dopant (Fig. 1). To the best of our knowledge, the HTP of 1 used
as a chiral switch is the highest reported for a diarylethene-based
dopant. Importantly, the HTP value of diarylethene 1 is suffi-
ciently high to induce the formation of a cholesteric mesophase in
both its open (1o) and its closed form (1c), in contrast to the
majority of previous designs where only the open form showed
Diimine diarylethene 1o was prepared using a two-step
synthesis from the dichloro-substituted diarylethene 2 via deriv-
20
ative 5. Conversion of 5o to 1o was achieved by condensation
with a chiral amine (Scheme 1). This approach can potentially
allow for facile variation through the use of other chiral
secondary amines.
13,14,15b
a non-negligible HTP.
Finally, we demonstrate that the
photochromic properties of 1 in solution are retained in the
liquid crystalline matrix and that photocontrol of the whole
mesophase can be achieved reversibly by irradiation with UV
and visible light. With optimization of their HTPs, diarylethene-
based dopants are prospectful alternatives to azobenzene-based
dopants for reversible photocontrol of the structure of chole-
steric liquid crystals.
The photoswitching behaviour of 1o in solution was studied
1
1
using H NMR, UV/Vis, and CD spectroscopy. The H NMR
spectrum of 1o shows the absorption of two methyl groups
attached to the thiophene moieties at 1.99 ppm and the aromatic
protons between 7.20 and 8.40 ppm. Irradiation of the sample at
365 nm results in a decrease in the absorption at 1.99 ppm and
the appearance of a new band at 2.20 ppm in addition to the
shifting of one of the aromatic absorptions to 6.73 ppm, which is
Results and discussion
21
indicative of the formation of the closed-ring isomer 1c. Based
The design of diarylethene 1 is based on two chiral imine units
connected to a diarylethene photochromic unit through phenyl
spacers. The chiral units are introduced using phenyl-aldehyde
moieties, which react readily with chiral amines (Scheme 1).
Incorporation of phenyl groups directly attached to the diary-
lethene unit allows for extension of the conjugated core of the
molecule, which induces a red shift in the absorption of the
closed form, compared to 1-phenylethylimine-based diarylethene
on the integration of the absorption bands assigned to the methyl
groups, the ratio 1o:1c at the photostationary state (PSS365nm
)
was determined to be 2 : 98. This excellent value is an important
feature of diarylethene 1 in view of future applications.
The UV/Vis spectrum of a sample of 1o in hexane (2.0 ꢁ
ꢀ5
10 M) shows an absorption maximum at 322 nm (Fig. 2a).
Irradiation at 365 nm results in the appearance of a broad
absorption with a maximum at 595 nm, indicative of the
Scheme 1
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Fig. 2 UV-vis absorption (a) and CD spectra (b) of 1o (solid lines) and at PSS365 nm in hexane (2.0 ꢁ 10ꢀ M) at 20 C (dashed lines).
5
ꢂ
Fig. 3 Textures observed with 5CB doped with 1o (2,9 wt%) using
polarized optical microscopy under homeotropic anchoring conditions
(on a glass slide functionalised with octadecyltrichlorosilane). Size of
both images: 156 mm ꢁ 121 mm a) before irradiation at 365 nm and b)
after 10 min irradiation at 365 nm.
Fig. 4 CD spectra of 1 in 5CB: spectrum of 5CB doped with 1o (solid
line) and spectrum of the cholesteric mixture at PSS365nm (dashed line).
The spectra were recorded at normal incidence and the cell was rotated
through several angles around the direction of the beam to ensure the
absence of artefacts arising from linear dichroism. The inset shows the
result of monitoring of two cycles of irradiation with UV and visible light,
by using CD spectroscopy. This monitoring demonstrates reversible
switching of 1 in a liquid crystalline environment.
22
formation of 1c. An isosbestic point was maintained at 343 nm.
In contrast, the CD spectrum of 1o in hexane did not change
significantly upon irradiation at 365 nm, indicating that switch-
ing of the diarylethene unit does not induce a CD effect in
addition to the CD signal attributed to the chirality of the imine
groups (Fig. 2b).
0
Doping of nematic liquid crystals 4 -pentyl-4-biphenylcarbo-
nitrile (5CB) or E7 with 2.9 and 2.1 wt% of 1o respectively
cholesteric liquid crystals. Nematic liquid crystals based on non-
chiral compounds, e.g. 5CB and E7, do not give rise to CD
signals. However, the presence of a chiral molecular dopant can
induce CD signals through the formation of chiral molecular
architectures, such as the helical ordering of mesogens in chole-
steric liquid crystals. Planar orientation of the sample was
ensured by using an adapted cell (see experimental section) and
checked by observing the characteristic oily streak texture by
optical microscopy, under cross polarizers. The CD spectrum of
both 1o and 1c shows an intense negative band at 340 nm, which
corresponds to the absorption region for 5CB (Fig. 4). With the
values of cholesteric pitch we are using (see Table 1), the
23
resulted in the observation of a fingerprint-like texture that is
characteristic of the formation a stable cholesteric mesophase
(
Fig. 3a). When the cholesteric mixture of 1o in 5CB was irra-
diated at 365 nm, the characteristic fingerprint texture was
retained (Fig. 3b); however, the periodicity of the fingerprint
texture increased, indicating that the pitch of the cholesteric helix
had lengthened, and consequently that 1c has a lower HTP than
1
o. That the mesophase remained cholesteric indicates that the
HTP of 1c is sufficient to induce the formation of a stable
cholesteric liquid crystal. This observation contrasts with the
behaviour of previously described diamine-based diarylethene
1
3
14,15b
dopants and other diarylethene-based dopants,
for which
Table 1 Pitch values, HTP and handedness of the cholesteric liquid
crystals formed by doping nematic hosts 5CB and E7 with diarylethene 1
the HTP of the closed form is too low to induce the formation of
a cholesteric mesophase. Subsequent irradiation of the sample
with visible light (l > 420 nm) resulted in the formation of
a cholesteric texture with the same periodicity as observed prior
to irradiation at 365 nm.
HTP of
1o (mm
HTP at
PSS (mm )
ꢀ
1
ꢀ1
ꢀ1
Nematic host
wt-% of 1
Pitch (mm
)
)
5CB
E7
2.9
2.1
2.5
2.7
ꢀ44
ꢀ50
ꢀ30
ꢀ35
Circular dichroism (CD) was employed to study the photo-
chemically induced modification in the helical structure of the
3
144 | J. Mater. Chem., 2011, 21, 3142–3146
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reflection band of the cholesteric LC is to the red of the
absorption band. Moreover, the dielectric anisotropy and the
linear dichroism of 5CB are both positive, therefore it is possible
to determine the handedness of the cholesteric helix induced by
AS400. Melting points (uncorrected) were determined using
a B u€ chi B-545 melting point device. Mass spectra (HRMS) were
obtained using a Thermo Scientific LTQ XL and LTQ Orbitrap
XL spectrometer.
24
1
o and 1c based on induced CD spectra. The negative bands in
the absorption region of 5CB indicate that a left-handed chole-
steric helix is formed, and therefore that the HTP of 1o and 1c are
negative. The sign of HTPs determined by CD spectroscopy is in
agreement with earlier reports using a related dopant in the same
Spectroscopy
UV-vis spectra were recorded at room temperature using
a JASCO V-630 spectrophotometer in 1 cm pathlength quartz
cuvettes (conc. of 1o 2.0 ꢁ 10ꢀ M). CD spectra were recorded on
JASCO J-815 spectropolarimeter using a custom built sample
holder for liquid crystal cells, at normal incidence. The cells
purchased from EHC (Japan) had the following specifications:
thickness 8 mm, planar alignment, antiparallel, with capillary fill.
UV irradiation experiments were performed with a Spectroline
ENB-280C/FE UV lamp (range: 320–380 nm with a maximum at
365 nm). Irradiation experiments with visible light were per-
formed with a Thorlabs OSL1-EC fiber illuminator together with
a long pass filter (range: 450–750 nm).
5
13
nematic host. Moreover, the relative intensities of the CD
spectra indicate that the HTP of 1 at the PSS365 nm is lower
(
in magnitude), than the HTP of 1o.
The switching of 1 in a cholesteric liquid crystalline environ-
ment was reproduced over a few cycles without any significant
alteration of the CD spectra shown in Fig. 4.
The absolute values of HTP of 1o and 1PSS in 5CB and E7 were
25
determined with the Grandjean–Cano method (Table 1). The
HTPs measured did not differ significantly between 5CB and E7,
which can be rationalised on the basis of the similarity of the
molecular composition of the two nematic hosts.
Microscopy
Conclusions
Liquid crystalline textures were observed by polarized optical
microscopy. Pitches were determined by the Grandjean-Cano
technique using a plan-convex converging lens (radius ¼
In conclusion, we have synthesized a chiral diarylethene dopant
with higher helical twisting power than other diarylethene-based
dopants reported so far. Its light-switchable properties were
demonstrated, both in solution and in a liquid crystalline matrix.
Diarylethene 1 was used as a chiral dopant in nematic hosts 5CB
and in E7. To the best of our knowledge, in comparison to
2
5.119 mm).
Synthesis
0
3
,3 -(Perfluorocyclopent-1-ene-1,2-diyl)bis(5-(5-aldehyde)-phe-
0
13–15
previously reported chiral diarylethene-based dopants,
dia-
nylchloro-2-methylthiophene) (5). 3,3 -(Perfluorocyclopent-1-ene-
,2-diyl)bis(5-chloro-2-methylthiophene) (2, 3.30 g, 7.55 mmol)
rylethene 1 shows the best doping performances reported for
a chiral diarylethene so far. Because the aromatic core of 1 was
extended in comparison to previous designs of chiral diary-
1
was dissolved in a flame dried flask in anhydrous Et O (150 mL).
2
n-BuLi (11.7 mL, 1.6 M in hexane, 18.7 mmol) was added slowly
by syringe at rt and the mixture was stirred for 30 min. Tribu-
tylborate (5.22 mL, 19.3 mmol) was added dropwise and the
mixture was stirred for 40 min at rt. In a second flask, a mixture
of p-bromobenzaldehyde (4, 2.82 g, 15.2 mmol), tetrakis-
13
lethene dopants, this molecule, used as a dopant in nematic
liquid crystals, was shown to induce the formation of a chole-
steric mesophase in both its open and closed forms. Importantly
and in contrast with azobenzene-based chiral dopants, both 1c
and 1o are thermally stable and thus they both induce the
formation of thermally stable cholesteric liquid crystals.
(
(
(
triphenylphosphine)palladium(0) (0.33 g, 0.29 mmol), THF
2 3
60 mL), 2 M aqueous Na CO (40 mL), and ethylene glycol
We found that the open form 1o, which has a flexible chemical
structure, has a higher HTP than the closed form 1c, whose
structure is more rigid. This result is in agreement with recent
investigations by Akagi et al. who reported powerful helicity
ꢂ
3 mL) was heated to 50 C. The reaction mixture containing the
crude boronic ester 3 was added slowly by syringe after which the
resulting mixture was heated at reflux for 16 h. The mixture was
diluted with Et O (150 mL) and washed with brine (100 mL). The
2
26
inducers with a flexible structure.
brine solution was extracted with diethyl ether (100 mL) and the
combined organic layers were dried over Na SO and concen-
Finally, through irradiation with UV and visible light,
reversible photocontrol of the period of the cholesteric helix was
achieved. These results provide new opportunities for the
development of light-driven chiral molecular switches as building
blocks or components for smart functional materials.
2
4
trated in vacuo. Purification by column chromatography (silica
gel, pentane–ethyl acetate 5 : 1, R 0.3) yielded dialdehyde 5 as
a brown solid (2.4 g, 55%). mp 199–200 C; H NMR (400.1
MHz, CDCl
f
ꢂ
1
3
) d 10.01 (s, 2H), 7.90 (d, J ¼ 8.2 Hz, 4H), 7.70 (d,
13
J ¼ 8.3 Hz, 4H), 7.44 (s, 2H), 2.01 (s, 6H); C NMR (75 MHz,
Experimental
CDCl ) d 191.4 (d), 143.5 (s), 140.9 (s), 138.9 (s), 135.6 (s), 130.7
3
(
d), 126.4 (s), 125.9 (d), 124.4 (d), 14.9 (q); HRMS (APCI): calcd
ꢀ
Materials
for C H F O S ([M ꢀ H] ): 575.0569, found: 575.0537.
2
9
17
6
2 2
All reagents are of commercial grade and used as received unless
stated otherwise. Solvents for spectroscopic measurements were
0
0
N,N -(((4,4 -(Perfluorocyclopent-1-ene-1,2-diyl)bis(5-methyl-
thiophene-4,2-diyl))bis(4,1-phenylene))bis(methanylylidene))bis-
(1-phenylethanamine) (1). L(-)-a-Methylbenzylamine (0.12 g, 1.0
UVASOL (Merck) grade. The liquid crystalline materials E7 and
0
4
-pentyl-4-biphenylcarbonitrile (5CB) were purchased from
0
0
Merck Japan and from Aldrich, respectively. NMR spectra of
samples dissolved in CDCl were recorded using a Varian Unity
mmol) was added to a mixture of 4,4 -(4,4 -(perfluorocyclopent-
1-ene-1,2-diyl)bis(5-methylthiophene-4,2-diyl))dibenzaldehyde
3
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(
0.20 g, 0.35 mmol), MgSO (0.42 g 3.5 mmol), and anhydrous
4
9 (a) S. N. Yarmolenko, L. A. Kutulya, V. V. Vashchenko and
L. V. Chepeleva, Liq. Cryst., 1994, 16, 877; (b) M. Brehmer, J. Lub
and P. van de Witte, Adv. Mater., 1998, 10, 1438.
CH Cl (100 mL) in a flame dried flask. The resulting mixture
2
2
was stirred for 16 h at rt. The MgSO was removed by filtration
4
1
0 (a) B. L. Feringa, N. P. M. Huck and H. A. van Doren, J. Am. Chem.
Soc., 1995, 117, 9929; (b) R. A. van Delden, M. B. van Gelder,
N. P. M. Huck and B. L. Feringa, Adv. Funct. Mater., 2003, 13, 319.
1 (a) R. A. van Delden, N. Koumura, N. Harada and B. L. Feringa,
Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4945; (b) R. Eelkema and
B. L. Feringa, Chem.–Asian J., 2006, 1, 367.
and the filtrate was concentrated in vacuo, yielding a dark
brown oil which was crystallized from EtOH affording diimine
1
1
7
8
7
2
as a brown solid (140 mg, 50%). Analytical data: mp 68.4–
ꢂ
1
0.0 C; H NMR (400 MHz, CDCl
3
) d 8.37 (s, 2H), 7.79 (d, J ¼
.3 Hz, 4H), 7.57 (d, J ¼ 8.3 Hz, 4H), 7.44 (d, J ¼ 7.1 Hz, 4H),
.39–7.31 (m, 6H), 7.25 (t, J ¼ 7.3 Hz, 2H), 4.56 (q, J ¼ 6.6 Hz,
12 (a) R. Eelkema, M. M. Pollard, J. Vicario, N. Katsonis, B. S. Ramon,
C. W. M. Bastiaansen, D. J. Broer and B. L. Feringa, Nature, 2006,
4
40, 163; (b) R. Eelkema, M. M. Pollard, N. Katsonis, J. Vicario,
1
3
H), 1.99 (s, 6H), 1.61 (d, J ¼ 6.6 Hz, 6H); C NMR (126 MHz,
D. J. Broer and B. L. Feringa, J. Am. Chem. Soc., 2006, 128, 14397;
(c) A. Bosco, M. G. M. Jongejan, R. Eelkema, N. Katsonis,
E. Lacaze, A. Ferrarini and B. L. Feringa, J. Am. Chem. Soc., 2008,
130, 14615.
CDCl ) d 158.8 (d), 145.2 (s), 142.1 (s), 141.8 (s), 136.0 (s), 135.3
3
(
s), 129.1 (d), 128.6 (d), 127.0 (d), 126.8 (d), 126.1 (s), 125.7 (d),
1
23.1 (d), 80.0 (d), 25.0 (q), 14.8 (q); F NMR (376 MHz,
9
1
1
1
3 C. Denekamp and B. L. Feringa, Adv. Mater., 1998, 10, 1080.
4 K. Uchida, Y. Kawai, Y. Shimizu, V. Vill and M. Irie, Chem. Lett.,
2000, 29, 654.
CDCl ) d ꢀ110.0 (t, J ¼ 5.1 Hz, 4F), 131.8–131.9 (m, 2F);
3
+
([M + H] ): 783.2297,
HRMS (ESI): calcd for C45
found: 783.2287.
H
37
F
6
N
2
S
2
1
5 (a) T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie and M. Irie, Mol.
Cryst. Liq. Cryst., 2000, 345, 287; (b) T. Yamaguchi, T. Inagawa,
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Acknowledgements
This work was partially supported by a VENI (NK) and a VIDI
grant (WRB) from the Netherlands Organisation for Scientific
Research (NWO-CW) and by FOM (TCP).
1
1
1
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