The photochromic and self-assembling properties of diarylethenes having chiral amphiphilic chains at the reactive carbon atoms

Article abstract of DOI:10.1039/b822880a

A photochromic diarylethene derivative having chiral polyethylene glycol side chains at the reactive carbon atoms was synthesized. The diarylethene showed a photochromic reaction upon irradiation with UV and visible light in both ethyl acetate and water.

Full text of DOI:10.1039/b822880a

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LETTER
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The photochromic and self-assembling properties of diarylethenes having
chiral amphiphilic chains at the reactive carbon atomsw
Takashi Hirose,ab Masahiro Iriec and Kenji Matsuda*ab
Received (in Montpellier, France) 19th December 2008, Accepted 5th February 2009
First published as an Advance Article on the web 24th February 2009
DOI: 10.1039/b822880a
A
photochromic diarylethene derivative having chiral
cyclization was observed.5 On the contrary, PEG chains at
the periphery seem to be advantageous for intermolecular chiral
recognition between the stacked molecules. In order to see the
effect of the substitution position of the amphiphilic chain,
diarylethene derivative 2a, which has chiral PEG chains at
the reactive carbon atoms, was designed (Scheme 1). The chiral
PEG chains at the reactive carbon atoms seem to be advanta-
geous for stereoselective cyclization due to the controlled
conformation of the open-ring isomer but disadvantageous for
intermolecular chiral recognition.
polyethylene glycol side chains at the reactive carbon atoms
was synthesized. The diarylethene showed a photochromic
reaction upon irradiation with UV and visible light in both ethyl
acetate and water. The open- and the closed-ring isomers showed
a non-exciton-coupled-type of CD spectrum, and the compound
underwent a diastereoselective cyclization reaction.
Photochromic diarylethene derivatives are promising candidates
for future optoelectronic devices.1 Among them, compounds that
switch optical rotation are nominees for non-destructive readout.2
Photochemical conrotatory cyclization produces two enantiomeric
closed-ring isomers (R,R and S,S), in which the chirality originates
from the asymmetric carbon atoms. By controlling the molecular
conformation of the open-ring isomer, a stereoselective cyclization
is possible. The P-helix conformer of the open-ring isomer
produces an (R,R) isomer and the M-helix conformer produces
an (S,S) isomer. Such stereoselective cyclizations in various chiral
environments have been accomplished in solution and in the
single-crystalline phase.3
The synthesis of diarylethene 2a was performed according
to the general synthetic method for diarylethenes (see ESIw).
The expression of chirality in compound 1 in our previous
report was found to be opposite to that described, and
the chirality has been corrected in Scheme 1.6 It was found
that the SN2 reaction of tosylated PEG and phenol derivatives
converts the stereocenter from (S)- to (R)-chirality. The
structure of diarylethene 2a was confirmed by NMR and mass
spectroscopy. The absorption and CD spectral changes
of diarylethenes 2 are summarized in Fig. 1. Diarylethenes 2
showed excellent photochromic performance in both ethyl
acetate and water, in the same manner as previously reported
for compound 1. In contrast with 1, both the open-ring 2a
and closed-ring 2b isomers showed a CD signal, both in ethyl
acetate and water. The strong exciton-coupled-type signal
was not observed in all cases. The exciton-coupled-type CD
signal, which was observed for the closed-ring isomer 1b in
water, significantly decreased. This result indicates that in
the cases of both the open- and closed-ring isomers of 2,
the origin of the CD signal is not mainly the transition dipole
moments in a spatially-twisted position, but the chiral
structure of the molecule itself.
It is known that compounds that have both a hydrophobic
core and hydrophilic side chains self-assemble into nano-
structures in polar solvents.4 We have reported that amphi-
philic diarylethene derivative 1 self-assembles by hydrophobic
interactions (Scheme 1).5 It is of interest that the methyl group
introduced near to the hydrophobic core as a chiral source
induces a chiral supramolecular environment to exhibit an
exciton-coupled-type of CD spectrum only for closed-ring
isomer 1b.
Herein, we report the synthesis and photochromic proper-
ties of a diarylethene derivative having chiral polyethylene
glycol (PEG) chains at the reactive carbon atoms.
Because previously synthesized compound 1a has chiral
PEG chains at the periphery of the diarylethene molecule,
conformational control was not effective and no diastereoselective
a Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, JST, CREST,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: kmatsuda@sbchem.kyoto-u.ac.jp; Fax: +81 75-383-2739
b Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka
819-0395, Japan
c Department of Chemistry and Research Center for Smart Molecules,
Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo
171-8501, Japan
w Electronic supplementary information (ESI) available: Full experi-
mental procedure, the results of HPLC analyses and the calculated CD
spectrum. See DOI: 10.1039/b822880a
Scheme 1 Photochromism of diarylethene derivatives having hexa-
ethylene glycol side chains with a chiral methyl group.
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Fig. 2 CD spectra of the two separated diastereomers in ethyl
acetate. Black and gray solid lines denote the two diastereomers of
closed-ring isomer 2b. The gray dashed line is corresponding open-ring
isomer 2a, measured after irradiating the diastereomer of the gray
solid line with visible light. The gray dotted line is the sample in the
photostationary state measured after progressive irradiation with
313 nm light.
An aqueous solution of open-ring isomer 2a was turbid at
room temperature (25 1C), but the clouding cleared at 15 1C.
This phenomenon is typical lower critical solution temperature
(LCST) behavior, which is often observed in PEG derivatives.7
The measurement of de in aqueous solution gave 3.0%
and 2.6% for the sample irradiated at 25 1C and 15 1C,
respectively. The de did not vary by changing the temperature
below or above the LCST. The observed de value was similar
to the value observed in ethyl acetate solution, suggesting that
the conformation of the open-ring isomer is similar in both
solvents. This result also indicates that the LSCT transition
does not influence the diastereoselection significantly.
It is known that the (R,R) isomer of the closed-ring isomer of
1,2-bis(2-methyl-5-phenyl-3-thienyl)hexafluorocyclopentene shows
a negative CD signal.8 The origin of the observed CD signal
should be excess (R,R) isomer, meaning that the P-helix confor-
mer should be energetically favorable. The detailed mechanism is
under consideration.
Fig. 1 Absorption spectral changes of compound 2 (a) in water and
(b) in ethyl acetate. Corresponding CD spectral changes (c) in water
and (d) in ethyl acetate. CD spectra of compound 1 (e) in water and (f)
in ethyl acetate. The black line denotes the open-ring isomer and the
gray line denotes the sample in the photostationary state (PSS) under
irradiation with 313 nm light.
Closed form 1b has PEG chains at both ends of the molecule.
Therefore, its shape is rather rod-like, and it may form a stable
aggregate. On the contrary, 2b has two long PEG chains per-
pendicular to the molecular plane. Therefore, it is expected that
molecular aggregates of 2b will not be stable because of its central
fat core in the PEG thread. This may be the origin of the weak
aggregation and weak exciton-coupled CD signal seen for 2b.
Since open-ring isomer 2a took the conformation biased to
P- or M-helical conformation, diastereoselective cyclization
was expected. The diastereomer excess (de) of the cyclization
reaction in ethyl acetate was measured by HPLC analysis
using a chiral column (DAICEL, CHIRALPAC AD-H).
The observed de was 3.6%. Although the de is not significant,
it is large enough to give a CD signal. The CD spectra of
the separated diastereomers of closed-ring isomer 2b were
measured (Fig. 2), the diastereomers showing mirror-image
spectra. Upon irradiation with visible light, both the (R,R) and
(S,S) isomers exhibited a decrease in their CD signal to give a
CD spectrum identical to that of the open-ring isomer shown
in Fig. 1(d). Progressive irradiation with UV light gave a CD
spectrum identical to that of 2b, as shown in Fig. 1(d). By
calculating the CD intensity with 3.6% de, the observed
spectrum was clearly reproduced (see ESIw).
In conclusion, in contrast to the 1a/1b system, in the 2a/2b
system, the exciton-coupled-type CD signal was not strong,
indicating that self-assembly into a chiral nanostructure was
not significant. On the contrary, both the open-ring 2a and
closed-ring 2b isomers show a non-exciton-coupled-type CD
signal, and a slight diastereoselective cyclization was observed.
The effect of the chiral amphiphilic chain varies, depending on
the substitution position in the photochromic diarylethene
molecule.
This work was supported by CREST, JST, by a Grant-
in-Aid for Young Scientists (A) (no. 19685013) and by a
Grant-in-Aid for Science Research in the Priority Area
‘‘New Frontiers in Photochromism’’ (471) (no. 19050009)
from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan. T. H. acknowledges JSPS
for a young scientist fellowship.
Although the CD spectrum of open-ring isomer 2a was
not of the exciton-coupled-type, the CD signal of the visible
band of closed-ring isomer 2b was slightly influenced by
the exciton-coupled signal. However, the strength of the
exciton-coupled signal was very small compared to previously
reported 1b.
Experimental
Synthesis of 2a
To a solution of (S)-3-bromo-2-(2-{2-[2-(2-{2-[2-(2-methoxy-
ethoxy)-ethoxy]-ethoxy}-ethoxy)-ethoxy]-ethoxy}-1-methyl-
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ethoxymethyl)-5-phenyl-thiophene9 (90 mg, 0.149 mmol) in
dry THF (0.5 mL) was slowly added an n-butyllithium–hexane
solution (1.6 M, 0.098 mL, 0.156 mmol) at À78 1C. After the
addition of a solution of perfluorocyclopentene (10 mL,
0.074 mmol) in dry THF (1 mL), the reaction mixture was
further stirred for 1 h at that temperature. The reaction
was quenched by the addition of water. The reaction product
was extracted with ether (Â3), and the organic layer was
washed with brine (Â2), dried over MgSO4, filtered
and evaporated. The crude product was purified by silica gel
column chromatography (ethyl acetate : acetone = 1 : 0 4 1 : 1)
to yield 2a (22 mg, 0.018 mmol, 24%) as a blue oil.
1H NMR (CDCl3, TMS, 400 MHz): d 1.04 (d, J = 6 Hz,
6 H), 3.39 (s, 6 H), 3.35–3.69 (m, 54 H), 4.02–4.15 (m, 4 H),
7.31–7.37 (m, 4 H), 7.38–7.45 (m, 4 H) and 7.57–7.63 (m, 4 H).
MALDI-TOF MS (m/z): calc. for C59H82F6O16S2Na 1247.48
[M + Na]+, found 1247.29 [M + Na]+.
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Corresponding closed-ring isomer UV-vis: lmax = 585 nm.
Photochemical measurements
Absorption spectra were measured on a Hitachi U-3500
spectrophotometer. CD spectra were recorded on a JASCO
J-720S spectrophotometer. Photoirradiation was carried out
using a USHIO 500 W super high-pressure mercury lamp or a
USHIO 500 W xenon lamp. Mercury lines of 313 and 578 nm
were isolated by passing the light through a combination of
band-pass filter (UV-D33S) or sharp-cut filter (Y-48) and
monochromator (Ritsu MC-20L). Each diastereomer was
separated by chiral HPLC (DAICEL, CHIRALPAC AD-H
[4.6 Â 250 mm, 5 mm], hexane : 2-propanol : ethanol = 10 : 65 : 25,
flow rate = 0.25 mL minÀ1) for compounds 2.
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9 For the full synthetic procedure, see the ESIw.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
1334 | New J. Chem., 2009, 33, 1332–1334