Photochemically reversible and thermally stable axially chiral diarylethene switches

Article abstract of DOI:10.1021/ol3018165

A series of dithienylcyclopentenes containing axially chiral 1,1′-binaphthyl units were successfully synthesized by a Suzuki-Miyaura protocol. All these compounds exhibited photochemically reversible isomerization with thermal stability in both organic solvent and a liquid crystal (LC) host. When doping into an achiral LC host, some of them exhibited very high helical twisting powers. Reversible reflection wavelength tuning in the visible region and LC phase switching between nematic and cholesteric upon light irradiation were demonstrated.

Full text of DOI:10.1021/ol3018165

ORGANIC
LETTERS
2
012
Vol. 14, No. 17
362–4365
Photochemically Reversible and
Thermally Stable Axially Chiral
Diarylethene Switches
4
Yannian Li and Quan Li*
Liquid Crystal Institute, Kent State University, Kent, Ohio 44240, United States
qli1@kent.edu
Received July 2, 2012
ABSTRACT
0
A series of dithienylcyclopentenes containing axially chiral 1,1 -binaphthyl units were successfully synthesized by a SuzukiꢀMiyaura protocol.
All these compounds exhibited photochemically reversible isomerization with thermal stability in both organic solvent and a liquid crystal (LC)
host. When doping into an achiral LC host, some of them exhibited very high helical twisting powers. Reversible reflection wavelength tuning in
the visible region and LC phase switching between nematic and cholesteric upon light irradiation were demonstrated.
Photochromic molecules thatcan reversibly changetheir
absorption spectra upon light irradiation have attracted a
great deal of attention because of their promising applica-
tions as smart light-driven molecular switches and artificial
It is also known that a chiral molecule can introduce its
chirality into an achiral nematic liquid crystal (LC) host to
form a self-organized, optically tunable helical superstruc-
3
ture, i.e. cholesteric LC (CLC), which can selectively reflect
1
light according to Bragg’s law. The reflection wavelength is
defined by λ = np, where p is the pitch length of the helical
structure and n is the average refractive index of the LC
material. The ability of a chiral molecule to twist the nematic
mesogens is expressed as helical twisting power (HTP),
nanomachines. Among all the photochromic compounds,
diarylethenes are particularly fascinating owing to their
superior thermal stability and excellent fatigue resistance.
Upon irradiation with UV light, they can transform from a
colorless open-ring form to a colored closed-ring form.
The reverse process is thermally stable and occurs only by
visible light irradiation. The photochemically reversible
switching of diarylethenes with good thermal stability
constitutes the basis for their widespread applications,
(4) (a) Ikeda, T. J. Mater. Chem. 2003, 13, 2037–2057. (b) Mallia,
V. A.; Tamaoki, N. Chem. Soc. Rev. 2004, 33, 76–84. (c) White, T. J.;
McConney, M. E.; Bunning, T. J. J. Mater. Chem. 2010, 20, 9832–9847.
(d) Tamaoki, N.; Kamei, T. J. Photochem. Photobiol. C: Photochem.
Rev. 2010, 11, 47–61. (g) Eelkema, R. Liq. Cryst. 2011, 38, 1641–1652.
2
such as photomemories and switches.
(
5) For selected papers on azobenzene dopants: (a) Pieraccini, S.;
Gottarelli, G.; Labruto, R.; Masiero, S.; Pandoli, O.; Spada, G. P.
Chem.;Eur. J 2004, 10, 5632–5639. (b) Li, Q.; Green, L.; Venkataraman,
N.; Shiyanovskaya, I.; Khan, A.; Urbas, A.; Doane, J. W. J. Am. Chem.
Soc. 2007, 129, 12908–12909. (c) Mathews, M.; Tamaoki, N. J. Am.
Chem. Soc. 2008, 130, 11409–11416. (d) White, T. J.; Bricker, R. L.;
Natarajan, L. V.; Tabiryan, N. V.; Green, L.; Li, Q.; Bunning, T. J. Adv.
Funct. Mater. 2009, 19, 3484–3488. (e) Green, L.; Li, Y.; White, T.;
Urbas, A.; Bunning, T.; Li, Q. Org. Biomol. Chem. 2009, 7, 3930–3933.
(f) Mathews, M.; Zola, R. S.; Hurley, S.; Yang, D.-K.; White, T. J.;
Bunning, T. J.; Li, Q. J. Am. Chem. Soc. 2010, 132, 18361–18366. (g) Ma,
J.; Li, Y.; White, T.; Urbas, A.; Li, Q. Chem. Commun. 2010, 46, 3463–
3465. (h) Li, Q.; Li, Y.; Ma, J.; Yang, D.-K.; White, T. J.; Bunning, T. J.
Adv. Mater. 2011, 23, 5069–5073.
(
1) (a) Irie, M. Photo-reactive Materials for Ultrahigh Density Optical
Memory; Elsevier: Amsterdam, 1994. (b) Raymo, F. M.; Tomasulo, M.
Chem. Soc. Rev. 2005, 34, 327–336. (c) Gust, D.; Moore, T. A.; Moore,
A. L. Chem. Commun. 2006, 1169–1178.
(2) (a) Irie, M. Chem. Rev. 2000, 100, 1685–1716. (b) Tian, H.; Yang,
S. Chem. Soc. Rev. 2004, 33, 85–97. (c) Tian, H.; Wang, S. Chem.
Commun. 2007, 781–792.
(
3) (a) Eelkema, R.; Feringa, B. L. Org. Biomol. Chem. 2006, 4, 3729–
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926–1945.
3
1
1
0.1021/ol3018165 r 2012 American Chemical Society
Published on Web 08/21/2012

ꢀ
1
which can be quantified as β = (pc) , where c is the chiral
dopant concentration. When a chiral photochromic com-
pound is employed as a dopant, it is possible to control the
pitch length and the reflection wavelength of the helical
superstructure by light, which paves the way to applications
in optically addressable displays, optical data storage, and
these molecules utilizes the combination of the photo-
chromic dithienylcyclopentene core with two axially chiral
binaphthyl units, which has proven to be powerful helicity
8
inducers for CLCs.
4
photonics. However, compared with other intensively
5
studied photoresponsive dopants such as azobenzenes, less
attention was paid to diarylethenes. To date only a few ex-
amples with chiral diarylethene dopants have been reported
Table 1. SuzukiꢀMiyaura Reaction between Boronate Ester 2
and Binaphthyl-Derived Halides 3aꢀh
6
mostly because of their low HTPs or very minor tunability.
Scheme 1
Monohalogenated binaphthyl derivatives (S)-3a,
(
the coupling reactions (see Supporting Information). The
boronate ester 2 was generated in situ from dichloride 1
S)-3cꢀh, and (R)-3b were designed and synthesized for
To tailor chiral diarylethenes with satisfactory function-
alities for device performance, e.g., high HTPs or remark-
able tunabilities, we present here a series of novel photo-
chemically reversible and thermally stable axially chiral
dithienylcyclopentene switches (S,S)-4a, (S,S)-4cꢀf, and
9
and directly used for the coupling reaction with halides 3.
A standard Suzuki reaction condition was employed to
examine the reactivity of the substrates, in which 5 mol %
ofPd(PPh ) was usedasthe catalystand aqueousNa CO
3
4
2
3
(
R,R)-4b, which were successfully constructed by a Suzukiꢀ
9b
was used as the base. A good yield (78%) was obtained
for the reaction of (S)-3a and 2 under this condition
7
Miyaura coupling protocol (Scheme 1). The design of
(Table1, entry 1). Other attempts such as screening bases
or solvents did not improve the yield notably. Aryl bro-
mide with electron-donating groups shows only low to
moderate reactivities in Suzuki reactions, evidencing the
good yield might result from the high nucleophilicity of
perhydrocyclopentene-based boronate ester 2. The reactions
(
082. (b) Uchida, K.; Kawai, Y.; Shimizu, Y.; Vill, V.; Irie, M. Chem.
Lett. 2000, 654–655. (c) Yamaguchi, T.; Inagawa, T.; Nakazumi, H.;
Irie, S.; Irie, M. Mol. Cryst. Liq. Cryst. 2000, 345, 287–292.
d) Yamaguchi, T.; Inagawa, T.; Nakazumi, H.; Irie, S.; Irie, M. Chem.
Mater. 2000, 12, 869–871. (e) Yamaguchi, T.; Inagawa, T.; Nakazumi,
H.; Irie, S.; Irie, M. Mol. Cryst. Liq. Cryst. 2001, 365, 861–866.
6) (a) Denekamp, C.; Feringa, B. L. Adv. Mater. 1998, 10, 1080–
1
(
(f) Yamaguchi, T.; Inagawa, T.; Nakazumi, H.; Irie, S.; Irie, M.
J. Mater. Chem. 2001, 11, 2453–2458. (g) van Leeuwen, T.; Pijper,
T. C.; Areephong, J.; Feringa, B. L.; Browne, W. R.; Katsonis, N.
J. Mater. Chem. 2011, 21, 3142–3246. (h) Li, Y.; Urbas, A.; Li, Q. J. Org.
Chem. 2011, 76, 7148–7156. (i) Hayasaka, H.; Miyashita, T.; Nakayama,
M.; Kuwada, K.; Akagi, K. J. Am. Chem. Soc. 2012, 134, 3758–3765.
(8) (a) Rosini, C.; Franzini, L.; Salvadori, P.; Spada, G. P. J. Org.
Chem. 1992, 57, 6820–6824. (b) Proni, G.; Spada, G. P. J. Org. Chem.
2000, 65, 5522–5527.
(9) (a) Lucas, L. N.; de Jong, J. J. D.; van Esch, J. H.; Kellogg, R. M.;
Feringa, B. L. Eur. J. Org. Chem. 2003, 155–166. (b) Akazawa, M.;
Uchida, K.; de Jong, J. J. D.; Areephong, J.; Stuart, M.; Caroli, G.;
Browne, W. R.; Feringa, B. L. Org. Biomol. Chem. 2008, 6, 1544–1547.
(j) Li, Y.; Urbas, A.; Li, Q. J. Am. Chem. Soc. 2012, 134, 9573–9576.
(7) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
Org. Lett., Vol. 14, No. 17, 2012
4363

of (R)-3b, (S)-3c, and (S)-3d with 2 under the same reaction
conditions also gave the corresponding products with good
yields (Table 1, entries 2ꢀ4). When the bromides (S)-3e and
Table 2. HTPs of Diarylethene Dopants at Different States in
Nematic LC Host
a
(
S)-3g were used as the substrates, only low yields of
products (S,S)-4d and (S,S)-4e were obtained. This might
be due to the strong electron-donating effect and the steric
hindrance of methoxymethoxy groups at the 2,2 -position.
By using more active iodide substrates (S)-3f and (S)-3h, the
yields can be successfully improved to 77% and 68%
respectively (Table 1, entries 5, 6).
HTP (β)
b
c
dopant
LC host
initial state
PSSUV
Δβ/β (%)
0
4
4
4
4
4
4
4
4
4
4
4
4
a
a
b
c
c
c
d
d
e
e
f
E7
þ154
þ156
ꢀ154
ꢀ115
ꢀ87
ꢀ118
ꢀ77
ꢀ86
þ14
þ4
þ160
þ173
ꢀ160
ꢀ99
ꢀ65
ꢀ114
ꢀ70
ꢀ82
þ19
þ6
þ4%
þ11%
þ4%
Zli-1132
E7
E7
ꢀ14%
ꢀ25%
ꢀ3%
Zli-1132
5CB
E7
ꢀ9%
5CB
ꢀ4%
E7
þ40%
þ50%
þ100%
þ44%
Zli-1132
E7
þ11
þ9
þ22
þ13
f
Zli-1132
a
β = 1/(pc), where p is the pitch of the cholesteric phase, c is the
molar fraction of the dopant in LC host. E7 is a eutectic mixture of LC
b
components commercially designed for display applications, and 5CB is
0
4
alkylcyclohexyl)benzonitrile and 4-(4-alkylcyclohexyl)-4 -cyanobiphe-
-pentyl-4-biphenylcarbonitrile. Zli-1132 is
a
mixture of 4-(4-
0
c
nyl derivatives. Percent change in β observed from initial state to
PSSUV
.
3
Figure 1. CD spectra of (S,S)-4a (20 μM in CH CN) upon UV
irradiation. It takes about 60 s to reach the photostationary
state.
Their HTPs in different nematic LC hosts were measured in
wedge cells by the GrandjeanꢀCano method, and the results
are summarized in Table 2. Using different LC hosts resulted
in different intermolecular associations between the dopant
and the host as evidenced by the variation in HTP values and
change upon UV irradiation. Interestingly, it was found that
the HTPs and the changes were dependent on the incorpora-
tion style of binaphthyl units. Compounds (S,S)-4a,c,d and
(R,R)-4b in which the binaphthyl was introduced through the
6-position exhibited very high HTPs, while incorporation of
binaphthyl through the 3-position gave the products (S,S)-4e
and (S,S)-4f with low HTPs and a large change upon UV
irradiation. For example, the HTP of (S,S)-4a in E7 at the
The photoswitching properties of (S,S)-4a, (S,S)-4cꢀf,
and (R,R)-4b were first investigated in organic solvents.
The solution of (S,S)-4a in THF was colorless at the initial
state. Upon UV irradiation at 310 nm, the solution color
changed immediately to fresh purple, indicating the trans-
5
a
formation of (S,S)-4a from its open form to closed form.
The irradiated state was thermally stable and was able to
switch back to its initial state by visible light irradiation at
ꢀ
2
50 nm (30 mW cm ). The absorption spectra are in-
5
cluded in the Supporting Information (Figure S2). The
excellent fatigue resistance was also confirmed by repeated
irradiation of the solution with UV and visible light
ꢀ
1
initial state was 154 μm while compound (S,S)-4e exhibited
ꢀ
1
a very low HTP of 11 μm with a remarkable increase to 22
ꢀ
1
(Figure S4).The chiroptical properties of (S,S)-1a were
investigated by CD spectra. As shown in Figure 1, the
strongest bisignated exciton couplet between 210 and 250
μm upon UV irradiation. It is established that the dihedral
angle (θ) of two naphthalenes in binaphthyl derivatives plays
8
b
a key role in their cholesteric induction abilities. Both the
transoid conformation (θ>90°) andthecisoid conformation
(θ > 90°) of binaphthyl derivatives can efficiently induce the
helicity with high HTPs, while the quasi-orthogonal confor-
mations (θ ≈ 90°) only have very weak induction ability. For
the S configuration, the transoid conformation induces a left-
handed CLC and the cisoid conformation induces a right-
handed CLC. To interpret the propertyꢀstructure relation-
ship, the handedness of the induced CLCs was examined by
the contact method and the sign (ꢀ) and (þ) represent left-
handed and right-handed screw sense, respectively. As ex-
pected, (S,S)-4a induced right-handed CLCs, which can be
attributed to the fact that the binaphthyl unit is fixed with a
methylene tether and thus has a small dihedral angle of
1
nm is due to the coupling of the two B transitions located
b
on distinct naphthalene rings, while the couplets at around
1
1
2
80 and 320 nm are related to the L and L transitions of
a b
1
0
naphthalene. Upon UV irradiation, the exciton couplet
at 230 nm weakened significantly while a shoulder exciton
developed at 270 nm. The distinct changes gave clear
evidence that the conformation of binaphthyl moieties
was affected by photoisomerization; therefore, the chirop-
tical properties of this compound can be modulated by
light.
Doping these compounds into nematic hosts can efficiently
induce the formation of cholesteric LCs with a characteristic
fingerprint texture (see Supporting Information Figure S6).
8
a
∼
55°. Compound (R,R)-4b as the enantiomer of (S,S)-4a
has the same HTP values with the opposite handedness. The
unbridged binaphthyls in compounds (S,S)-4c and (S,S)-4d
(10) Di Bari, L.; Pescitelli, G.; Salvadori, P. J. Am. Chem. Soc. 1999,
21, 7998–8804.
1
4
364
Org. Lett., Vol. 14, No. 17, 2012

are conformationally more flexible, and the left-handedness
indicated that the binaphthyls might stabilize in a transoid
conformation. Based on the very low HTPs, the θ in (S,S)-4e
and (S,S)-4f were estimated to be ∼90° which might be due to
the rigid structures and the steric hindrance between substitu-
cholesteric phase appeared due to the dramatic increase
of HTP and formation of a helical superstructure. The
cholesteric state is thermally stable and can be driven back
to the nematic state upon visible light irradiation.
0
tion groups at 2,2 positions and the dithienylcyclopentene
core. This kind of structure was also found to be very sensitive
to photoisomerization as revealed by the large variation in
HTPs. These results also confirmed the subtle dependence of
cholesteric induction behavior on the molecular structure of
both the dopant and the nematic LC host.
Figure 3. Crossed polarized optical texture and conoscopic
observations (inset) of 4.4 wt % of (S,S)-4f in E7 in 5 μm thick
homeotropic aligned cell before (left) and after (right) UV
irradiation at 310 nm.
In conclusion, a series of novel photochemically rever-
sible and thermally stable dithienylcyclopentene switches
containing two axially chiral binaphthyl units were de-
signed and synthesized. The SuzukiꢀMiyaura coupling
between binaphthyl-derived halides with diboronate ester
gave the products with good yields. Their photoswitching
properties were characterized in both organic solvents and
LC media. Furthermore, their cholesteric induction beha-
vior was found to be dependent on the incorporation style
of binaphthyl units. Attachment of binaphthyl through the
Figure 2. Reflection spectra changes of 5.0 wt % of (S,S)-4a in
Zli-1132 in 5 μm thick planar cell. The reflection colors (inset)
were taken from a polarized reflective mode microscope.
6-position generated very high HTP compounds, among
which (S,S)-4a was successfully employed to tune the
reflection color in the visible range. When the binaphthyl
units were introduced through the 3-position, the products
show significant changes in HTP during photoisomeriza-
tion. Compound (S,S)-4f was found to be capable of
reversibly switching the LC phase between nematic and
cholesteric. The novel structures, high HTPs, and the
interplay between structure and chiroptical properties
would provide impetus toward developing more powerful
diarylethene switches for practical applications.
Utilization of the high HTP of (S,S)-4a enabled a visible
light reflective cholesteric LC to be formulated by doping
only 5.0 wt % of (S,S)-4a into the achiral nematic LC Zli-
1
132. This mixture was capillary-filled into a 5 μm thick
planar aligned cell with antiparallel rubbing directions. As
seen in Figure 2, the reflection wavelength of this cell at the
initial state is ∼620 nm, which was blue-shifted to 560 nm
upon UV irradiation due to the increase of HTP. The
photoswitching process was also confirmed by the direct
observation of the reflection color changing from red to
green under a reflective mode microscope (Figure 2, inset).
The reverse process can be achieved by visible light irradia-
tion at 550 nm.
Acknowledgment. This work is supported by the Air
Force Office of Scientific Research (AFOSR FA9550-09-
1-0193).
Due to the significant HTP changes upon light irradia-
tion, (S,S)-4f was successfully employed as an LC switch,
i.e. reversibly switching the LC phase between nematic
and cholesteric. When doping 4.4 wt % of (S,S)-4f into E7,
the low HTP at the initial state is not sufficient for helicity
induction and the microscopic texture with conoscopic
observation confirmed the nematic state (Figure 3). Upon
UV irradiation, a characteristic fingerprint texture of the
Supporting Information Available. Details of the synthe-
1
13
sis, characterization data, copies of H and C NMR
spectra, and measurement of HTP. This material is
available free of charge via the Internet at http://pubs.
acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 17, 2012
4365