Regulation of folding and photochromic reactivity of terarylenes through a host-guest interaction

Article abstract of DOI:10.1002/chem.201101495

The photochromic reactivity of terarylenes is integrated with molecular folding that is controlled through a host-guest interaction. A thieno[3, 2,b]pyridine unit is introduced into a photochromic terarylene structure as an aryl unit to form a guest-inter

Full text of DOI:10.1002/chem.201101495

FULL PAPER
DOI: 10.1002/chem.201101495
Regulation of Folding and Photochromic Reactivity of Terarylenes through a
Host–Guest Interaction
Takuya Nakashima,* Ryosuke Fujii, and Tsuyoshi Kawai*[a]
Abstract: The photochromic reactivity
of terarylenes is integrated with molec-
ular folding that is controlled through a
interconversion between photochro-
mic-reactive and unreactive conforma-
tions. In methanol, the intermolecular
interaction between terarylene species
and the solvent molecule slows the rate
of interconversion and increases the
population of the photochromic-active
form, whereas the unreactive confor-
mation is dominant in hexane. Crystal-
structural studies demonstrated the
perfect regulation of molecular folding
between a photochromic-active form
and an unreactive conformation by
changing the solvents for recrystalliza-
tion. Single crystals prepared from sol-
utions in methanol showed reversible
photochromic reactivity, whereas re-
crystallization from solutions in hexane
did not show this reactivity. X-ray crys-
tallographic studies of single crystals
from solutions in methanol demonstrat-
ed that the photochromic molecules
bind a solvent methanol molecule at
the guest-interacting site to regulate
the molecular conformation into a pho-
tochromic-active form in collaboration
with specific intramolecular interac-
tions, whereas crystals from solutions
in hexane possess the photochromic-
unreactive conformation.
host–guest interaction.
A
thieno-
ACHTUNGTRENNUNG[3,2,b]pyridine unit is introduced into a
photochromic terarylene structure as
an aryl unit to form a guest-interacting
site. Thienopyridine-containing terary-
lenes showed solvent-dependent photo-
chromic reactivity in solution. A terar-
ylene moiety that contains two thieno-
pyridyl units showed significantly high
photocoloration reactivity as high as
88% of photocyclization quantum
yield in methanol, whereas that value
was only 24% in hexane. A tempera-
Keywords: crystal growth · foldam-
ers · heterocycles · photochromism ·
supramolecular chemistry
1
ture-dependent H NMR spectroscopic
study in different solvents indicated an
Introduction
grate photochromic reactivity with molecular folding con-
trolled through a host–guest interaction.
Over the past decade, foldamers have emerged as artificial
oligomers that fold into well-defined secondary structures in
solutions, thus mimicking biopolymers.[1] The functions of
foldamers have been demonstrated at different levels. One
is the dynamic switching of conformations in response to ex-
ternal stimuli, such as temperature, solvent,[2] proton con-
centration,[3] metal coordination,[4] presence of small
guests,[5] and light irradiation,[6] thus demonstrating the sens-
ing capability by changing their optical properties. Another
functionality exerted by folding, which is more attractive
with respect to a biomimetic approach, includes guest recog-
nition[7] and catalytic activity[8] based on their secondary
structures. Our use of terarylenes,[9] which could be regarded
as a building unit of oligoarylene foldamers, aims to inte-
Photoresponsive molecular materials have attracted much
interest because their physicochemical properties are modu-
lated with precise spatiotemporal control in a remote and
noninvasive manner, which is required for a wide range of
applications.[10] Photoswitching systems based on diaryle-
thenes[11] and terarylens[9] have been extensively studied be-
cause they undergo thermally irreversible photoisomeriza-
tion with high efficiency. Recent advances highlight their
use for the regulation of biochemical activities.[12] Because
enzymes possess a binding pocket favorable for incorporat-
ing a substrate with a specific geometry, the regulation of
ground-state folding of photochromic inhibitors is of partic-
ular importance in the control of enzymatic activity.[12b] The
ground-state molecular geometry also plays a crucial role in
the photochromic reactivity as exemplified by an opsin/reti-
nal complex.[13] In this context, the control of molecular
folding has been a major subject for developing highly effi-
cient artificial photochromic materials.
[a] Dr. T. Nakashima, R. Fujii, Prof. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
8916-5 Takayama, Ikoma, Nara 630-0192 (Japan)
Fax : (+81)743-72-6179
The photoinduced pericyclization reaction of diaryle-
thenes[11] and terarylenes[9] are known to proceed only from
the C2-symmetrical conformation of the reactive hexatriene
moiety. Diarylethenes possess two conformations with the
two rings in mirror symmetry (parallel conformation) and in
C2 symmetry (antiparallel conformation) in solution, thus re-
sulting in a maximum quantum yield of about 50%.[11] Vari-
E-mail: ntaku@ms.naist.jp
tkawai@ms.naist.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101495.
Chem. Eur. J. 2011, 17, 10951 – 10957
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10951

ous attempts have been made to regulate the ground-state
geometry of diarylethenes in solution so that they have C2
symmetry.[14] In contrast, terarylenes, which are composed of
three heteroaromatic rings, offer a wide accessibility to the
control of molecular folding in terms of the design of intra-
and intermolecular interactions through the combination of
various heteroaromatic compounds. The appropriate design
of intramolecular interactions between arylene units in ter-
arylenes sometimes offers a high photochromic performance
with photocyclization quantum yields of over 70% in solu-
tion.[15] Very recently, we proposed a concept for the design
of highly reactive photochromic terarylenes with a confor-
mation that is directed into the photoreactive conformation
through multiple intramolecular interactions to produce a
photon-quantitative reaction.[15c] In the present study, the in-
tegration of the photochromic reactivity of terarylenes with
controlled folding through intermolecular interactions has
been demonstrated for the first time. The solvent-dependent
molecular folding of terarylenes was studied in terms of
photochromic reactivity, 1H NMR spectroscopic analysis,
and X-ray crystal structures. A host–guest interaction with a
solvent molecule perfectly regulates the molecular folding
and thereby the photochromic reactivity of terarylenes in a
single-crystalline state.
Results and Discussion
Solvent-dependent photochromic properties: ThienoACHTUNGTRENNUNG[3,2-
b]pyridine-containing terarylenes 1a and 2a were synthe-
sized by conventional cross-coupling reactions of the corre-
sponding arylene units. Their chemical structures were con-
firmed with high-resolution mass-spectrometric, 1H and
13C NMR spectroscopic, and X-ray single-crystal analysis. Ir-
radiation of solutions of 1a and 2b with UV light at l=
313 nm resulted in the immediate changes in the UV/Vis ab-
sorption spectra that are typical for diarylethenes and terar-
ylenes. Compound 1a had no absorption band in the visible
range in hexane and a new absorption band appeared at
around l=560 nm, which corresponds to the formation of
1b (Figure 1a). The chemical structure of 1b was confirmed
1
by H NMR spectroscopic analysis after separation from the
colored solution by means of reverse-phase HPLC with
methanol as the eluent. The change in the absorption spec-
trum upon photoirradiation was accompanied by an isosbes-
tic point at l=270 nm, which clearly indicates the two-com-
ponent photochromic reaction between the open form 1a
Photochromic terarylenes capable of binding a guest are
composed of a central 2-phenylthiazole unit and one (1a) or
two (2a) thienopyridyl rings as side-chain aryl units
(Scheme 1). A CH···S hydrogen-bonding[9b,16] or a S···N het-
eroatom-contact[15c,17] interaction for 1a and 2a, respectively,
would harness the rotation of one side-chain aromatic ring
(Scheme 1, left). In low-polar media, the electrostatic repul-
sion between the lone pairs of electrons on the pyridyl and
thiazolyl nitrogen atoms flips the thienopyridyl ring to form
a photochromic inactive conformation (I form). The I form
would be also stabilized by the intramolecular CH···N inter-
action between the methyl protons at a reactive carbon
center and the central thiazolyl nitrogen atom. Meanwhile,
in a protic media such as methanol, the methanol molecule
is expected to interact with both the aromatic nitrogen
atoms through conventional OH···N hydrogen bonding.[18]
Binding of a methanol molecule would effectively suppress
the rotation about the thiazolyl–thienopyridyl bond, there-
fore directing the adoption of a photochromic active confor-
mation (A form).
Figure 1. Absorption spectral changes of 1 in a) hexane (3.5ꢁ10À5 m) and
b) methanol (1.2ꢁ10À5 m) and 2 in c) hexane (1.6ꢁ10À5 m) and d) metha-
nol (8.7ꢁ10À6 m). Open form (solid lines), closed form (bold lines), and
photostationary state under irradiation with light at l=313 nm (dashed
lines).
and the closed form 1b. The
conversion ratio between 1a
and 1b at the photostationary
state achieved by irradiation
with UV light (l=313 nm) was
estimated to be 52% in hexane.
Irradiation of the colored solu-
tion with visible light led to the
recovery of an absorption spec-
trum identical to that of the ini-
tial solution of 1a. Similar pho-
Scheme 1. Folding and photochromism of terarylenes 1a and 2a.
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Photoresponsive Molecular Systems Based on Supramolecular Interactions
FULL PAPER
tochromic behavior was also observed for 1a in methanol
and for 2a in hexane/methanol (Figure 1b–d). The photo-
chromic properties of 1 and 2 in hexane and methanol are
summarized in Table 1. The quantum yields of the photo-
chromic reaction were evaluated by using a standard proce-
dure with 1,2-bis[2-methylbenzo[b]thiophen-3-yl]perfluoro-
cyclopentene[19] as a reference compound.
Table 1. Absorption maxima and coefficients of the open- and closed-
ring isomers of 1 and 2 with the quantum yields F in solution.
Isomer
lmax [nm]
Fo–c
Fc–o
(e [104 mÀ1 cmÀ1]
)
1a
1b
2a
2b
301 (1.8),[a] 299 (1.9)[b]
560 (1.1),[a] 561 (1.3)[b]
301 (2.0),[a] 299 (1.9)[b]
561 (1.3),[a] 571 (1.3)[b]
0.30,[a] 0.60[b]
–
–
0.22,[a] 0.16[b]
–
0.24,[a] 0.88[b]
–
0.26,[a] 0.16[b]
[a] In hexane. [b] In methanol.
The photocyclization quantum yield of 1a was Fo–c =0.30
in hexane, which dramatically increased to Fo–c =0.60 in
methanol. Likewise, 2a showed much larger quantum yield,
which was as high as Fo–c =0.88, in methanol than that in
hexane of Fo–c =0.24. This result is a good contrast to the
photochromic reactivity of the previously reported terary-
lene with a molecular conformation that was stabilized by
intramolecular CH···N hydrogen bonding, thus showing a
lower photocyclization quantum yield of 49% in methanol
than that in hexane ((98Æ2)%).[15c] The quantum yield of
the solvent-dependent photocyclization can be explained as
discussed above in terms of solvent-dependent molecular
folding (Scheme 1). The lower Fo–c value of 1a relative to
2a in methanol reflects the nature of intramolecular interac-
tions; that is, the weak CH···S hydrogen bonding, which is
expected to tether one side-chain aryl (benzothienyl) unit
for 1a, should weaken in a protic media and result in loose
anchoring of the photoreactive conformation. In contrast,
the S···N interaction in 2a might be rather electrostatic in
nature and be valid to sustain the photoreactive A form,
even in methanol. The solvent-dependence of the quantum
yield of the photocyloreversion FcÀo was not as prominent
as that observed for Fo–c. Note that the sum of the quantum
yields of the photocyclization and -cycloreversion reactions
is slightly larger than 1 for 2a in methanol, whereas the
both values might have substantial wavelength dependence.
The solvent-dependent molecular folding was further stud-
Figure 2. Temperature-dependent 1H NMR spectra of 1a in a) CD3OD
and b) [D14]hexane with c) peak assignment for (A).
servation indicates that the speed of interconversion be-
1
tween conformers is faster than the H NMR timescale at
room temperature, but is almost comparable at 233 K and
slower below 213 K. Therefore, the peak positions appear as
the average of chemical shifts that originate from quickly in-
terconverting conformers at 293 K. The increased interac-
tion between 1a and the solvent molecule might slow the ro-
À
tation about the thiazolyl thienopyridyl bond on decreasing
the temperature. Given the interconversion between two
distinct conformers, the rotation indeed involves a number
of conformers, each peak could be assigned to methyl
groups (depicted in Figure 2c). The pair of peaks aꢂ and bꢂ
originate from the photochromic reactive A form because
the chemical shifts at such an upfield region of below d=
2 ppm could be attributed to the ring-current effect of the
side-chain aryl rings at the opposite side.[15c] Meanwhile, the
a protons are expected to participate in the intramolecular
CH···N hydrogen-bonding interaction with the nitrogen
atom of the central thiazolyl ring in the unreactive I form,
thus leading to the most downfield shift (d=2.68 ppm). A
proton-decoupling study at 183 K confirmed that the signals
aꢂ and bꢂ have the same chemical structural origin as a and
b, respectively.[20] The proportion of photoreactive A is
roughly estimated to be 55% at room temperature from the
relative peak positions based on the evaluation of the intrin-
sic chemical shifts of these conformers from the spectrum at
183 K and the assumption that they are not temperature de-
pendent. The proportion is also estimated to be 68% at
183 K from the peak integral ratio.[20] By using the Boltz-
mann distribution relationship, the proportion of the photo-
reactive conformer (i.e., the A form) is evaluated to be
62% at 293 K. These rough approximations seem to be in
agreement with the value of Fo—c =60%. Likewise, com-
pound 2a, which gave much a higher quantum yield in
1
ied by H NMR spectroscopic analysis in solution and X-ray
crystallography.
1
Temperature-dependent H NMR spectroscopic study: Fig-
ure 2a shows the change in the chemical shifts of the methyl
protons on the reactive carbon centers of 1a in CD3OD
with temperature as the most obvious example. Two methyl
peaks appeared at d=2.15 and 2.20 ppm at room tempera-
ture, which merged into a broad peak at 233 K. The broad
peak again split into four distinct peaks at 213 K and those
four peaks sharpened with decreasing temperature. This ob-
Chem. Eur. J. 2011, 17, 10951 – 10957
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10953

T. Kawai et al.
methanol (Fo–c =88%), showed two methyl peaks at d=
2.00 and 2.10 ppm. The more downfield shifts of these peaks
relative to those of 1a (d=2.15 and 2.20 ppm) indicate the
larger contribution of photoreactive A form than 1a at
room temperature, and the proportion is supposed to be
almost 100% at 183 K from the upfield chemical shifts.[20]
The methyl peaks of 1a in [D14]hexane are substantially
separated from each other at room temperature (Figure 2b),
and the profile, which includes the chemical-shift values d=
2.60 and 2.25 ppm, is quite similar to that of the I form (Fig-
ure 2a, peaks a and b) observed in CD3OD at low tempera-
ture. The slight upfield shifts relative to those of the I form
(d=2.70 and 2.25 ppm) indicates the contribution of the A
form. Although the peaks showed a slight upfield shift with
a certain degree of broadening by decreasing the tempera-
ture, neither fusion nor resplitting of the signals was ob-
served, thus indicating that a faster interconversion occurred
1
between the conformers than on the H NMR timescale and
that the A form was not the majority species, even at 183 K
1
in [D14]hexane. Thus, the temperature-dependent H NMR
spectroscopic study explains well the relationship between
the solvent-dependent photocyclization reactivity and the
proportion of the photoreactive conformations.
Figure 4. ORTEP drawings (showing 50% probability displacement ellip-
soids) of the crystals a) 1a-MeOH, b) 1a, c) 2a-MeOH, and d) 2a. The
hydrogen atoms are omitted for clarity.
Solvent-dependent crystal structures: Finally, the solvent-de-
pendent crystal structures were studied. Both 1a and 2a
could be recrystallized from solutions in hexane and metha-
nol. The solvent-dependence was prominent in the photo-
chromic reactivity in the single-crystalline state. The single
crystals 1a-MeOH and 2a-MeOH that were obtained from
solutions in methanol turned blue after UV-light irradiation
and were bleached with visible-light irradiation (Figure 3),
whereas crystals 1a and 2a obtained from solutions in
hexane showed no photochromic performance. The wave-
lengths of the peak absorption of the colored state were
both at about l=620 nm, which are substantially longer
than those observed in solutions, thus indicating intermolec-
ular interactions within the crystalline packing structure.[9b]
Figure 4 shows X-ray crystallographic structures of the ob-
tained crystals,[21–25] and clear solvent-dependence in the
crystal structures was observed. Interestingly, crystals 1a-
MeOH and 2a-MeOH from solutions in methanol bind a
methanol molecule at the N1···N2 binding site to form inclu-
sion complexes and adopt the photoreactive A forms (Fig-
ure 4a and c). Table 2 summarizes the atomic distances of
C7···C10 (reactive carbon centers) and N1···N2 and the pho-
tochromic reactivity in a single-crystalline state. Both the
distances of C7···C10 and N1···N2 observed for crystals 1a-
MeOH and 2a-MeOH are apparently shorter than those in
crystals 1a and 2a from hexane. The binding of a methanol
molecule effectively shortens the N1···N2 distance. In addi-
tion, intramolecular interactions at the opposite side (left)
were observed. The estimated distance between (C17)···H
and S2 for crystal 1a-MeOH was 2.96 ꢃ, which is shorter
than the sum of the van der Waals radii of the S and H
atoms (1.85 and 1.2 ꢃ, respectively). The S2···N3 distance
for crystal 2a-MeOH (i.e., 3.21 ꢃ) is also shorter than the
sum of the van der Waals radii of the S and N atoms (i.e.,
1.55 ꢃ). Combined with the intermolecular interaction with
a methanol molecule, these intramolecular interactions are
considered to bring reactive C10 closer to C7, thus produc-
ing the photoreactive A forms. The C7···C10 distances ob-
served in crystals 1a-MeOH and 2a-MeOH are both below
Table 2. Structural and photochromic properties of crystals of 1a and 2a.
Crystal
C7···C10 [ꢃ]
N1···N2 [ꢃ]
Photochromism
1a-MeOH[a]
1a[b]
3.72
4.24
3.65
4.31
3.19
4.20
3.19
3.53
yes
no
yes
no
2a-MeOH[a]
2a[b]
Figure 3. Single-crystalline photochromism of a) 1a-MeOH and b) 2a-
MeOH. Absorption spectra before (black lines) and after (blue lines) the
UV irradiation.
[a] Recrystallized from methanol. [b] Recrystallized from hexane.
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Photoresponsive Molecular Systems Based on Supramolecular Interactions
FULL PAPER
4 ꢃ, which is short enough for single-crystalline photo-
chromism.[19]
On the other hand, the geometry of 1a in the crystal from
hexane is apparently unsuitable for the photocyclization re-
action (Figure 4b). The weak hydrogen bonding between
À
(C8) H and N2 (about 2.4 ꢃ) stabilizes the nonreactive I
form (Scheme 1), which was also suggested by the H NMR
1
spectroscopic study at low temperature. Although the crystal
structure of 2a from hexane appears to possess the photo-
reactive A form (Figure 4d), the C7···C10 distance (4.31 ꢃ)
is still longer than those in crystals 1a-MeOH and 2a-
MeOH. This distance in the reaction center (i.e., 4.31 ꢃ) is
classified as that of a nonreactive conformation on the basis
that single-crystalline photochromism requires the distance
in the reaction center to be shorter than 4.2 ꢃ.[26] In fact,
crystal 2a showed no single-crystalline photochromism. The
electrostatic repulsion that operates between the lone pairs
of electrons on the N1 and N2 atoms would make the dis-
tances of N1···N2, and thereby C7···C10, long. Thus, the
host–guest interaction with a solvent molecule perfectly reg-
ulates the molecular folding and consequent photochromic
reactivity of 1a and 2a in the single-crystalline state. The in-
clusion of hexane in the crystal of 2a was also observed,
whereas no specific intermolecular interaction was found be-
tween hexane and 2a. The inter- and intramolecular interac-
tions of the sort mentioned above would be effective to a
1
certain degree in solutions as discussed in the H NMR spec-
troscopic study and responsible for the solvent-dependent
photochromic reactivity.
Scheme 2. Synthesis of 1a and 2a. Reagents and conditions: a) nBuLi,
1,2-dimethyldisulfane/diethyl ether; b) trimethyl(prop-1-ynyl)silane,
[PdCl2A(PPh3)2], CuI/THF, triethylamine, tetrabutylammonium fluoride;
c) iodine/dichloromethane; d) 1) nBuLi/diethyl ether, 2) 2-isopropoxy-
CTHUNGTRENNUNG
4,4,5,5-tetramethyl-1,3,2-dioxaborolane; e) 7, [PdACHTNUTRGNEUNG(PPh3)4], PPh3, K3PO4/
H2O, 1,4-dioxane.
Conclusion
In summary, we have designed photochromic teraylenes
with a guest-binding site. Terarylenes 1a and 2a showed
higher photocyclization reactivity in methanol than in
hexane. A temperature-dependent 1H NMR spectroscopic
and X-ray crystallographic studies demonstrated that the in-
termolecular interaction with a methanol molecule regulates
the molecular folding into the photoreactive conformation
in collaboration with specific intramolecular interactions in
solution and in crystals. The molecular design based on
supramolecular interactions should provide a photorespon-
sive molecular system, with a responsivity and supramolec-
ular arrangement that can be controlled in a dynamic
manner through not only host–guest interactions but also
light irradiation.[6]
(400 MHz). Semipreparative HPLC was performed on a JASCO LC-
2000 Plus Serie. Mass spectra were measured with a JEOL JMS-700 mass
spectrometer. Absorption spectra in solution were studied with a JASCO
V-570 spectrophotometer. Photoirradiation was carried out with
a
USHIO 500 W ultrahigh-pressure mercury lamp or a 500 W Xenon
short-arc lamp as the exciting light sources. Monochromic light was ob-
tained by passing the light through a monochromator (Shimadzu SPG-
120S, l=120 mm, f=3.5). Absorption spectra in the single-crystalline
phase were measured by using an Olympus BX-51 polarizing microscope
connected with a Hamamatsu PMA-11 photodetector with an optical
fiber. The polarizer and analyzer were set in parallel to each other. X-ray
crystallographic analyses were carried out with
a Rigaku R-AXIS
RAPID/s imaging plate diffractometer with MoKa radiation.
2-Bromo-3-(methylthio)pyridine (4): n-Butyllithium (1.6m in hexane,
13 mL, 21 mmol) was added slowly to a solution of 2,3-dibromopyridine
(5.1 g, 22 mmol) in dry THF (90 mL) at À788C under argon. The reaction
mixture was stirred for 1 h at that temperature. Dimethyldisulfide
(4.5 mL, 43 mmol) was added dropwise, and the mixture was stirred for
another 1 h. The reaction mixture was allowed to warm to room tempera-
ture. Water was added to the reaction mixture, which was extracted with
ethyl acetate. The combined organic fractions were washed with water,
dried with anhydrous magnesium sulfate, filtered, and concentrated.
Column chromatography on silica gel (hexane/ethyl acetate 4:1) afforded
4 as a brown oil in 96% yield (4.2 g, 20 mmol). 1H NMR (300 MHz,
CDCl3, TMS): d=2.50 (s, 3H), 7.40 (m, 2H), 8.24 ppm (d, J=2 Hz, 1H).
Experimental Section
General: Compounds 1a and 2a were prepared according to the reaction
pathway depicted in Scheme 2. The chemical structures of 1a and 2a
were confirmed by high-resolution mass-spectrometric, 1H and 13C NMR
1
spectroscopic, and elemental analysis. H NMR spectra were recorded on
3-(Methylthio)-2-(prop-1-ynyl)pyridine (5): A 500 mL four-necked flask
a JEOL AL-300 spectrometer (300 MHz). The chemical shifts were re-
corded relative to trimethylsilane (TMS). Temperature-dependent
1H NMR spectra were measured using a JEOL ECP 400 spectrometer
was charged with
4 (4.2 g, 21 mmol), trimethyl(prop-1-ynyl)silane
(3.1 mL, 21 mmol), CuI (0.19 g, 1.0 mmol), and tetra-n-butylammonium
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T. Kawai et al.
fluoride in THF (1 m, 60 mL) in triethylamine (150 mL). After the solu-
tion was degassed by bubbling with N2 gas for 30 min, [PdCl2A(PPh3)2]
2.14 (s, 3H), 7.27–7.37 (m, 2H), 7.52 (m, 3H), 8.09 (m, 2H), 8.25 (d, J=
8.1, 2H), 8.44 (d, J=8.0, 1H), 8.59 ppm (d, J=8.0, 1H); 13C NMR
(75 MHz, CD2Cl2, TMS) d=15.57, 15.70, 118.58, 119.04, 124.90, 126.87,
128.74, 128.94, 129.35, 129.89, 130.05, 130.41, 132.27, 132.35, 134.31,
144.84, 144.90, 147.22, 147.40, 148.24, 155.39, 155.43, 168.22 ppm; EI-
HRMS: m/z calcd for C25H17N3S3 [M]+: 455.060, found: 455.058; elemen-
tal analysis (%) calcd for C25H17N3S3: C 65.90, H 3.76, N 9.22, found: C
66.06, H 3.57, N 9.23.
CTHUNGTRENNUNG
(0.70 g, 5.0 mol%) was added to the reaction flask. The reaction mixture
was stirred at 408C under argon for 15 h. The crude product was extract-
ed with ethyl acetate. The combined organic fractions were washed with
water, dried with anhydrous magnesium sulfate, filtered, and concentrat-
ed. Column chromatography on silica gel (hexane/ethyl acetate 4:1) af-
forded
5 as a
yellow oil in 84% yield (2.8 g 17 mmol). 1H NMR
(300 MHz, CDCl3, TMS): d=2.18 (s, 3H), 2.47 (s, 3H), 7.16 (m, 1H),
7.42 (d, J=5.1 Hz, 1H), 8.28 ppm (d, J=4.5 Hz, 1H).
3-Iodo-2-methylthienoACHTUNGTRENNUNG[3,2-b]pyridine (6): A 500 mL four-necked flask
was charged with 5 (2.4 g, 15.0 mmol) in dichloromethane (150 mL).
Iodine in dichloromethane (150 mL) was added dropwise to the solution.
The reaction mixture was stirred at room temperature for 2 h. The crude
product was extracted with ethyl acetate. The combined organic fractions
were washed with water, dried with anhydrous magnesium sulfate, fil-
tered, and concentrated. Column chromatography on silica gel (hexane/
ethyl acetate 4:1) afforded 6 as a yellow crystal in 75% yield (3.1 g,
11 mmol). 1H NMR (300 MHz, CD2Cl2, TMS): d=2.66 (s, 3H), 7.26 (m,
1H), 8.06 (m, 1H), 8.67 ppm (m, 1H).
Acknowledgements
We thank Prof. A. Ikeda for fruitful discussions and F. Asanoma and S.
1
Katao for the low-temperature H NMR spectroscopic and X-ray crystal-
lographic measurements, respectively. This work was partly supported by
the Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of Japan with Grants-in-Aid for Scientific Research on Priority
Area “New Frontiers in Photochromism” (No. 21021017).
2-Methyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thienoACTHUNTGRNEUNG[3,2-b]pyri-
dine (7): nBuLi (1.6m in hexane, 1.3 mL, 2.1 mmol) was added slowly to
a solution of 6 (0.55 g, 2.0 mmol) in dry diethyl ether (10 mL) under
argon at À788C. The reaction mixture was stirred for 2 h at À788C. 2-Iso-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.44 mL, 2.1 mmol) was
added dropwise, and the mixture was further stirred overnight. The reac-
tion mixture was allowed to warm to room temperature. Water was
added to the reaction mixture, which was extracted with ethyl acetate.
The combined organic fraction was washed with water, dried with anhy-
drous magnesium sulfate, filtered, and concentrated to give 7 as a yellow
oil in 93% yield (0.51 g, 1.8 mmol). The obtained oil was used without
further purification.
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ACHTUNGTRENNUNG(PPh3)4] (60 mg, 0.05 mmol) was added to the reaction mixture, which
was stirred at 1008C under N2 for 3 days. The reaction mixture was ex-
tracted with diethyl ether and the organic layer was dried with anhydrous
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1
(0.10 g, 0.22 mmol). H NMR (300 MHz, [D4]methanol, TMS): d=2.18 (s,
3H), 2.21 (s, 3H), 7.18–7.24 (m, 3H), 7.52 (m, 3H), 7.61 (d, J=7.5 Hz,
1H), 7.73 (d, J=8.1 Hz, 1H), 8.08 (m, 2H), 8.24 (d, J=8.7, 1H),
8.49 ppm (br, 1H). 1b: NMR (300 MHz, [D4]methanol, TMS): d=1.56 (s,
3H), 2.12 (s, 3H), 7.03–7.20 (m, 3H), 7.45–7.53 (m, 6H), 8.14 (m, 2H),
8.50 ppm (d, J=7.5 Hz, 1H); 13C NMR (75 MHz, CD2Cl2, TMS): d=
5.077, 15.611, 118.666, 122.143, 122.702, 123.360, 124.330, 124.609,
126.837, 128.662, 129.385, 129.435, 129.895, 130.577, 132.345, 134.129,
138.116, 140.155, 141.215, 144.807, 147.298, 148.383, 155.618,
+
167.702 ppm; FAB-HRMS: m/z calcd for C26H19N2S3 [M+H]+: 455.07;
found: 455.061; elemental analysis (%)calcd for C26H18N2S3: C 68.69, H
3.99, N 6.16, found: C 68.57, H 3.88, N 6.18.
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bromo-2-phenylthiazole (9, 64 mg, 0.20 mmol), PPh3 (26 mg, 0.1 mmol),
and K3PO4 (2m) in water/1,4-dioxane (40 mL, 10:30 v/v). After the solu-
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a colorless solid in 30% yield (28 mg,
0.060 mmol). 1H NMR (300 MHz, [D4]methanol, TMS) d=2.03 (s, 3H),
10956
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 10951 – 10957

Photoresponsive Molecular Systems Based on Supramolecular Interactions
FULL PAPER
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7.9368(4), c=15.7682(8) ꢃ, b=92.2797(13)8, monoclinic, space
3
group P21/c, Z=4, V=2205.70(20) ꢃ , 1calcd =1.369 gcmÀ3 21209 re-
¯
flections measured (2q=55.08), 5045 unique reflections (Rint
=
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b=10.9253(4), c=11.3647(4) ꢃ, a=78.8332(11), b=65.3106(9), g=
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75.9211(10)8, triclinic, space group P1, Z=2, V=1179.97(7) ꢃ3,
¯
1calcd =1.370 gcmÀ3; 11846 reflections measured (2q=54.98), 5373 in-
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[23] Crystallographic data for 2a: C28H24N3S3, a=10.664(1), b=
11.091(1), c=11.262(1) ꢃ, a=96.615(3), b=99.404(3), g=
3
¯
108.466(3)8, triclinic, space group P1, Z=2, V=1226.5(2) ꢃ , 1calcd
=
1.350 gcmÀ3; 12288 reflections measured (2q=54.98), 5604 inde-
pendent reflections (Rint =0.0446); structure solved by direct meth-
ods and refined with a full matrix against all F2 data; hydrogen
atoms were calculated in riding positions; wR=0.1489, R=0.0493.
[24] Crystallographic data for 2a-MeOH: C26H21N3S3O, a=10.9411(5),
b=10.9862(5), c=11.2090(7) ꢃ, a=77.136(2), b=65.596(2), g=
3
¯
75.726(2)8, triclinic, space group P1, Z=2, V=1177.9(1) ꢃ , 1calcd
=
1.375 gcmÀ3; 11837 reflections measured (2q=54.9), 5369 independ-
ent reflections (Rint =0.0253); structure solved by direct methods
and refined with a full matrix against all F2 data; hydrogen atoms
were calculated in riding positions; wR=0.1181, R=0.0349.
[25] CCDC-819722 (1a), CCDC-819723 (1a-MeOH), CCDC-819724
(2a), and CCDC-819725 (2a-MeOH) contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Received: May 16, 2011
[20] See the Supporting Information.
Published online: August 16, 2011
Chem. Eur. J. 2011, 17, 10951 – 10957
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10957