Dynamic molecular tweezers composed of dibenzocyclooctatetraene units: Synthesis, properties, and thermochromism in host-guest complexes

Article abstract of DOI:10.1002/chem.200900623

Novel dynamic molecular tweezers (DMTs) 3a, 3b, 4a, 4b, and 5 b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units, were designed and synthesized. The cyclooctatetraene (COT) rings of these DMTs readily invert in solution, and the molecula

Full text of DOI:10.1002/chem.200900623

DOI: 10.1002/chem.200900623
Dynamic Molecular Tweezers Composed of DibenzocyclooctaACTHNUTRGNEtUNG etraene Units:
Synthesis, Properties, and Thermochromism in Host–Guest Complexes
Tomohiko Nishiuchi,[a] Yoshiyuki Kuwatani,[b] Tohru Nishinaga,[a] and Masahiko Iyoda*[a]
Abstract: Novel dynamic molecular
tweezers (DMTs) 3a, 3b, 4a, 4b, and
5b, composed of two tub-shaped diben-
zocyclooctatetraene (DBCOT) units,
were designed and synthesized. The cy-
clooctatetraene (COT) rings of these
DMTs readily invert in solution, and
the molecular structure shows rigid syn
and anti forms in an equilibrium mix-
ture in solution. The syn and anti con-
formers can be observed by NMR. The
isomerization barriers of 3a, 3b, 4a,
4b, and 5b are in the range of 16.5–
21.3 kcalmolꢀ1, depending on steric re-
pulsion between substituents of the
COT rings and protons of the central
benzene ring. These DMTs form com-
plexes with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ) and 1,2,4,5-
tetracyano-benzene (TCNB) in solu-
tion and in the solid state. The binding
abilities of these DMTs increase with
electron-donating substituents on COT,
which increase the electron densities of
the cavity of the syn form, as supported
by theoretical calculations. In addition,
elongation of the terminal alkoxy
chains of the DMTs was found to
cause the enhancement of van der
Waals contact with guest molecules.
Therefore, 5b, which has CH2OMe
groups on the COT rings and longer
ethoxy groups on the terminal benzene
rings, showed the highest electron den-
sity of the cavity and hence the highest
binding ability with the electron-defi-
cient guest molecules. Interestingly, sol-
utions of 3b, 4b, and 5b show thermo-
chromism in the presence of DDQ. A
solution of 3b or 4b with DDQ in
CHCl3 is green due to charge-transfer
interaction at room temperature and
the color changes from green to yellow
upon heating to 608C and from green
to blue upon cooling to ꢀ408C, where-
as the high complexation ability of 5b
with DDQ only shows a change in the
shade of blue.
Keywords: cyclooctatetraene · den-
sity functional calculations · host–
guest systems · molecular tweezers ·
thermochemistry
and defined space inside the molecules, they function as a
host molecule and form molecular complexes with a number
of different guests.[6] Although cyclacenes, the basic belt-
shaped units of (n,0) carbon nanotubes,[7] have also been
theoretically investigated and proposed as an interesting
synthetic target,[8] no cyclacenes have been prepared to date,
presumably due to their internal strain.[9]
Introduction
Belt-shaped conjugated systems have been attracting the at-
tention of not only synthetic[1–3] but also theoretical chem-
ists.[4] One of the most interesting properties of belt-shaped
molecules is a host–guest interaction in supramolecular
chemistry.[5] Because molecular belts have a preorganized
As analogous systems, Gleiter and co-workers recently re-
ported metal complexes of
a
belt-like molecule,
[a] T. Nishiuchi, Prof. Dr. T. Nishinaga, Prof. Dr. M. Iyoda
Department of Chemistry
[4,8]3cyclacene (beltene)[10] and [6.8]3cyclacene 1.[11] In these
molecules, tub-shaped cyclooctatetraene (COT) rings are
fused with planar four- or six-membered rings. Thus, the in-
ternal strain of the beltene and 1 is small and hence beltene
and 1 are stable in light and air.
Graduate School of Science and Engineering
Tokyo Metropolitan University
Hachioji, Tokyo 192-0397 (Japan)
Fax : (+81) 042-677-2525
E-mail: iyoda@tmu.ac.jp
Although the beltene and 1 have unique structures, the
cavity of these molecules is too small to include any guest
from a viewpoint of host–guest chemistry. In this regard, the
cleft-shaped molecule 2, which is a partial structure of 1
with two dibenzocyclooctatetraene (DBCOT) units, is inter-
esting. Molecule 2 is considered to exist as an equilibrium
mixture of syn and anti forms owing to ring inversion of
[b] Dr. Y. Kuwatani
VSN Inc.
Nishimiyahara 2-1-3,
Yodogawa-Ku,
Osaka 532-0004 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900623.
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FULL PAPER
sired positions. First, 7a and 7b were prepared from 6 and
two 2-bromo-4,5-dialkoxybenzaldehydes by the Sonogashira
reaction. The two acetylene units of 7a and 7b were re-
duced to cis olefins by partial hydrogenation using Pd on
BaSO4 to afford 8a and 8b, respectively. Next, 10a and 10b
were prepared by deprotection of the tert-butyldimethylsilyl
groups in 8a and 8b, followed by mesylation of the hydroxyl
groups in 9a and 9b. When 10a and 10b were reacted with
NaCN, the formation of cyclized eight-membered ring com-
1
DBCOT (DG° =12 kcalmolꢀ1).[12] Furthermore, because the
face-to-face distance of the two terminal benzene rings in
the syn form is calculated to be 6.6 ꢁ in a preliminary
model structure at the PM3 level, 2 can function as molecu-
lar tweezers to incorporate a planar guest between the two
benzene rings.[13] With regard to molecular tweezers, Klꢂrn-
erꢃs and Harmataꢃs groups extensively investigated various
systems with a rigid cleft structure,[14,15] whereas Fukazawaꢃs
group investigated flexible systems.[16] Unlike the presence
of various stable conformations for Fukazawaꢃs molecules,
the most remarkable characteristic of the 2 molecular tweez-
ers is that 2 takes only two conformers, i.e., the syn and anti
forms. Among the two conformers, only the syn form func-
tions as a molecular tweezers that interconvert with the anti
form. Thus, 2 can act as a new host molecule for dynamic
molecular recognition,[17] and it thus has potential for use as
a molecular switch.[18]
pounds was observed by H NMR.[21] Therefore, a mixture
of linear and cyclized compounds was used for the next con-
densation reaction by DBU without purification to produce
molecular tweezers 3a and 3b in 63 and 58% yields from
10a and 10b, respectively. The dialdehydes 4a and 4b were
synthesized by reduction of 3a and 3b using DIBAL-H to
afford 4a and 4b in 40 and 81% yields, respectively. The
low yield of 4a is probably due to the low solubility of 3a.
The methoxymethyl derivative 5b was obtained from 4b in
68% yield.
Structural properties of 3a, 3b, 4a, 4b, and 5b: To estimate
the relative energies (Erel) of the syn and anti forms, optimi-
zation of the molecular structures of 3b, 4b and 5b were
performed at the B3LYP/6-31G(d) level[22] (Table 1). From
the calculations, these Erel values are very small, and almost
no difference in energy was observed between the syn and
1
Herein, we report the results of our investigations on the
synthesis, structure, and unique binding behavior of dynamic
molecular tweezers (DMTs) 3a, 3b, 4a, 4b, and 5b. The
anti forms. In the H NMR spectra of DMTs at room tem-
perature, both syn and anti forms were observed indepen-
AHCTUNGTREGdNNNU ently. As shown in Table 1, the observed differences in free
energy (DG8) between syn and anti isomers are small
(<1 kcalmolꢀ1), but slightly larger than the calculated re-
sults. On the NMR timescale, exchange rates of syn and anti
isomers of these DMTs are slow at room temperature owing
to steric repulsion between COT substituents and protons of
the central benzene ring. Therefore, we performed VT
1H NMR and determined the isomerization barriers of these
DMTs. Although both syn and anti forms of 3b were ob-
served at 308C (Figure 1), the signals of the syn and anti
forms gradually coalesced to give signals assigned as the
molecule with an apparent C2 symmetry at higher tempera-
tures. As shown in Table 2, the isomerization barriers (DG°)
of 3a and 3b are 16.5 kcalmolꢀ1, whereas the barriers of 4a,
4b, and 5b are approximately 21.0 kcalmolꢀ1. These results
indicate that the steric repulsion of the CN group is smaller
than that of the CHO and CH2OMe groups due to the
linear structure of the CN group. In addition, the theoretical
calculations for the inversion barriers (DE) of 3’, 4’, and 5’,
which were estimated by the energy difference between the
tub and planar structures, gave values (3’: 14.1, 4’: 20.1, 5’:
20.6 kcalmolꢀ1) similar to the observed inversion barriers of
3a, 3b, 4a, 4b, and 5b.
binding abilities of these DMTs are strongly dependent on
both substituents of the terminal benzene rings and two
COT units. Furthermore, fine-tuning of the charge-transfer
interaction of the DMTs and DDQ in CHCl3 solutions has
successfully led unique thermochromism.[19]
Results and Discussion
Synthesis of molecular tweezers 3a, 3b, 4a, 4b, and 5b: The
key step in the synthesis of these compounds is the forma-
tion of COT rings. Although a variety of simple methods for
the preparation of COT and its derivatives have been re-
ported,[20] we employed a stepwise pathway to construct
COT rings of DMTs as shown in Scheme 1. The stepwise re-
actions enabled us to introduce substituent groups at the de-
Among these DMTs, single crystals of 4a were obtained
from DMF, and the crystal structure of 4a·2DMF was deter-
mined by X-ray analysis (Figure 2). Although the syn form
of 4a is slightly more stable than the anti form of 4a in
DMF solution, 4a takes the anti form in the solid state. The
dihedral angle between the central and terminal benzene
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6839

M. Iyoda et al.
Scheme 1. Synthesis of dynamic molecular tweezers 3a, 3b, 4a, 4b, and 5b.
[a]
Table 1. Calculated relative energies (Erel
)
and observed free energies
Table 2. Isomerization barriers (DG°) and coalescence temperatures
(TC) of 3a, 3b, 4a, 4b, and 5b, and calculated inversion barriers (DE°) of
DBCOT derivatives of 3’, 4’, and 5’.
(DG8)[b] of anti and syn isomers of 3b, 4b, and 5b.
DG° [kcalmolꢀ1
]
TC [8C]
DE° [kcalmolꢀ1
]
3a
3b
3’
4a
4b
4’
16.5
16.5
–
21.2
20.8
–
50
50
–
135
130
–
–
–
14.1
–
–
Erel [kcalmolꢀ1
]
DG8 [kcalmolꢀ1
]
anti
0.00
0.06
0.00
syn
anti
0.00
0.89
0.00
syn
3b: R1 =CN
0.08
0.00
0.02
0.59
0.00
0.39
4b: R1 =CHO
5b: R1 =CH2OMe
20.1
[a] Calculated at the B3LYP/6-31G(d) level. [b] Determined by 1H NMR
measurements in CDCl3.
Figure 2. Crystal structures of 4a: a) ellipsoid drawing, b) side view,
c) packing structures of 4a (wireframe) with DMF (CPK model).
1
Figure 1. VT H NMR spectra of 3b (500 MHz in CDCl2CDCl2).
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Dynamic Molecular Tweezers
FULL PAPER
rings is 95.98, which is slightly narrower than in the case of
the parent DBCOT (998).[23] As shown in Figure 2c, DMF
was incorporated in the cavity formed by two adjacent 4a
molecules.
Table 3 summarizes the binding constants KC of DMTs
with TCNB, DDQ, or 1,3,5-trinitrobenzene (TNB)[24] togeth-
er with thermodynamic parameters DHDDQ and DSDDQ of
the complexation with DDQ. Irrespective of slight structural
differences among DMTs 3a, 3b, 4a, 4b, and 5b, the bind-
ing constants were increased by elongation of terminal
alkoxy chains from OMe to OEt groups and by increasing
the electron-donating ability of COT substituents from CN,
CHO to CH2OMe groups. Thus 5b has the highest binding
constants.
The DHDDQ of 3b and 4b , which have OEt groups, shows
more favorable binding with DDQ by 1.2–1.4 kcalmolꢀ1
compared to the binding of 3a and 4a, which have OMe
groups. On the other hand, the DSDDQ terms for 3b and 4b
are less favorable than those of 3a and 4a by 0.7 and 1.6 eu,
respectively. These results indicate that the van der Waals
interaction between terminal alkoxy chains of DMTs and
DDQ was increased by the elongation of the chains from
OMe to OEt, whereas increased flexibilities of these alkoxy
chains resulted in the negatively larger DSDDQ terms. More-
over, the DHDDQ values of 4a and 4b, which have CHO
groups, show more favorable binding with DDQ than those
of 3a and 3b, which have CN groups, by about
1.0 kcalmolꢀ1, and also 5b , which has CH2OMe groups,
shows more favorable binding with DDQ than 4b by about
1.7 kcalmolꢀ1. From these results, we thought COT substitu-
ents of CN, CHO, and CH2OMe were likely to affect the
electron densities of the binding sites in DMTs.
Binding properties of DMTs: The binding abilities and be-
haviors of DMTs 3–5 with various p acceptors were investi-
1
gated by analyzing UV/Vis and H NMR spectra. Among p
acceptors, these DMTs formed host–guest complexes in so-
lution with 1,2,4,5-tetracyanobenzene (TCNB), 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ), and 1,3,5-trinitroben-
zene (TNB), whereas no complexation was observed with
other small acceptors such as 7,7,8,8-tetracyano-p-quinodi-
methane (TCNQ), TCNQ-F4, pyridinium salts, and viologen.
In particular, the CT interaction between these DMTs and
DDQ caused a color change from green to blue. The UV/
Vis titration of 3b with DDQ is shown in Figure 3. The CT
In order to estimate the electrostatic effect on the binding
of DMTs with guest molecules, we optimized the structures
of the syn form (B3LYP/6-31G* level) and calculated the
electrostatic potential surfaces (ESPs) of 3b, 4b, and 5b
(Figure 5). From these ESPs, the electron densities of the
cavities of 3b, 4b, and 5b show marked differences, and the
negative character of the central and terminal benzene rings
increases in the order of 3b<4b<5b. Therefore, it is clear
that these differences in ESP affect the binding ability (DH)
with electron-deficient guest molecules. This electrostatic
effect of substituents on COT units efficiently extended into
whole molecules, indicating that the COT units are effective-
ly conjugated with the central and terminal benzene rings, in
spite of the bending p systems.
Figure 3. UV/Vis titration of 3b with DDQ (3b: 0.5 mm). The concentra-
tions of DDQ used are (from bottom to top) 0.0, 0.25, 0.5, 1.0, 1.5, 2.0,
2.5, 4.0, 5.0 mm. The insert is a Jobꢃs plot ([3b] + [DDQ]=1.0 mm) at
640 nm (solvent: CHCl3).
absorption of 3b with DDQ appeared at 640 nm (Fig-
ure 3a), and Jobꢃs plot ([3b] + [DDQ]=1.0 mm) at 640 nm
indicates the formation of a 1:1 complex (Figure 3b).
On the other hand, the DSDDQ values of 3b, 4b, and 5b
decrease in the order of 3b>4b>5b. If syn forms of the
DMTs have the preorganization effect[25] derived from the
cavity sizes, DSDDQ values have to increase in the order of
3b<4b<5b because the cavity sizes of DMTs (the distance
between both terminal oxygen atoms from theoretical calcu-
lation) decrease in the order of 3b (9.293 ꢁ)>4b
(9.147 ꢁ)>5b (8.631 ꢁ). Therefore, in this case, the preor-
ganization effect caused by the cavity sizes seems not to be
important and flexibilities of the COT substituents are con-
sidered to be a more effective factor in the DSDDQ terms.
This can be rationalized by decreasing the flexibilities of
COT substituents upon complexation with guest molecules.
The structures of the complexes of TCNB@4b and
TCNB@5b were determined by X-ray crystallography and
1
The H NMR titration experiments are also useful for in-
vestigating the binding behavior and binding constants of
DMTs. The experiment of 3b with DDQ is shown in
Figure 4 as a representative result. Although no shift of the
anti protons was observed as the concentration of DDQ in-
creased, the syn protons gradually shifted to either the
upper or lower field. The complexation-induced shift (CIS)
of the terminal benzene protons He and Hf is 0.28 ppm, and
the relative intensities of the syn peaks gradually increased.
These results indicate that the equilibrium is shifted toward
the syn isomer by the addition of DDQ and that only the
syn isomer forms the 1:1 complex with DDQ.
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M. Iyoda et al.
Figure 4. The NMR titration of 3b with DDQ (3b: 1 mm, solvent: CDCl3).
Table 3. Binding constants of KC [mꢀ1] of DMTs with TCNB, DDQ, or
the results of TCNB@4b are shown in Figure 6. As shown in
Figure 6b, TCNB and terminal benzene rings stack through
p–p interactions at a distance of 3.33–3.40 ꢁ, and the dihe-
TNB together with thermodynamic parameters of DHDDQ [kcalmolꢀ1
and DSDDQ [calmolꢀ1 Kꢀ1] of DDQ complex.[24]
]
Substrate
3a
3b
4a
4b
5b
TCNB
DDQ
TNB
<300
530
240
–
730
70
–
1100
360
28
10000[a]
1570
35
40
[b]
[b]
[b]
–
DHDDQ
DSDDQ
ꢀ2.4
ꢀ3.6
ꢀ3.2
ꢀ4.6
ꢀ6.3
ꢀ0.8
ꢀ1.5
ꢀ2.4
ꢀ4.0
ꢀ7.0
[a] Determined by UV/Vis titration (solvent: CHCl3). [b] The KC value
could not be determine because the interaction was very weak.
Figure 6. Crystal structures of TCNB@4b: a) ellipsoid drawing, b) side
view, c) top view, d) packing structures.
dral angles between terminal and central benzene rings in
4b are about 908. Moreover, the distance between the
proton of TCNB and the central benzene ring of 4b is
2.54 ꢁ, indicating the presence of a CH–p interaction (Fig-
ure 6c). The CH–p interaction would result in improved
binding of TCNB than DDQ, in spite of it being the weaker
acceptor. In the packing structure, two complexes stack
through p–p interactions, and the chloroform molecules
from the recrystallizing solvent are located on the opposite
side of the TCNB (Figure 6d). The terminal ethoxy groups
Figure 5. Electrostatic potential surfaces and the values of molecular
electrostatic potentials (MEPs, calculated position of MEP is 2.0 ꢁ above
the central benzene ring) of 3b, 4b, and 5b (B3LYP/6-31G*//B3LYP/6-
31G*).
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Dynamic Molecular Tweezers
FULL PAPER
of 4b were found to cover the guest molecule of TCNB, sug-
gesting that the ethoxy chains play an important role in the
binding of the guest molecule as observed in the binding
study.
Although many molecular tweezers have been reported,
most studies focus on the relationship between the p plane
area of recognition sites and the binding ability because
compounds with a rigid and wide p plane readily interact
with many electron-deficient molecules by p–p and/or CT
interactions. From this point of view, our synthesized DMTs
show unique molecular recognition by means of not only p–
p, CH–p, and CT interactions, but also by van der Waals in-
teractions between flexible alkoxy chains.
from green to blue upon cooling to ꢀ408C. On the other
hand, 5b with DDQ only shows a change in the shade of
blue. This thermochromism has a good response to changes
in temperature. A CHCl3 solution of DDQ is yellow and
that of the DDQ@DMT complex is blue. Therefore, the
principle of this thermochromism is related to the mixing
ratio of yellow and blue; the color is decided by the molar
Table 4. The ratio of [DDQ@DMTs]/
ACHTUNGTREN[NUNG DDQ] (DDQ: 1 mm, DMTs:
1 mm) in CHCl3 solutions calculated by means of a vanꢃt Hoff plot.[26]
T [8C] [DDQ@3b]/[DDQ] [DDQ@4b]/[DDQ] [DDQ@5b]/[DDQ]
A
E
E
G
N
ACHTUNGTRENNUNG
60 0.10
40 0.14
20 0.20
0.12
0.18
0.28
0.45
0.74
1.24
0.31
0.50
0.85
1.39
2.34
4.61
0
0.30
Thermochromic properties of DDQ@DMT complexes: Al-
though 3b and 4b have a lower binding ability than 5b
owing to electron-withdrawing substituents on the COT
units, it is interesting that these solutions of DDQ com-
plexes exhibit thermochromic properties. Figure 7 shows sol-
utions of 3b, 4b, and 5b with DDQ in CHCl3 at various
temperatures.
ꢀ20 0.46
ꢀ40 0.70
ratio of the DDQ@DMT complex to free DDQ. Table 4
shows the molar ratio of DDQ@3b, DDQ@4b, and
DDQ@5b complexes to free DDQ ([DDQ@DMTs]/ACHTUNG-
TERNNUG
at a ratio below 0.14, green at a ratio range of 0.18 to 0.28,
and blue at a ratio over 0.30. From these results, the COT
substituents adequately tuned not only the binding ability
but also the thermochromism.
The CT bands of the complexes of 3b (640 nm), 4b
(655 nm), and 5b (680 nm) with DDQ at 208C showed red-
shifts according to the donating ability of the substituents on
the COT rings. These results are consistent with Mulliken
2
charge–transfer theory,[27] namely, hnCT =IPꢀEAꢀe0 /er,
2
where IP, EA, and e0 /er denote the first ionization energy
of the donor, the electron affinity of the acceptor, and the
electrostatic work term, respectively, where e is the dielec-
tric constant of the solvent, and r is the plane-to-plane dis-
tance between donor and acceptor.[28] On the other hand, as
shown in Figure 8, the absorption maxima of the CT band
Figure 7. Solutions of 3b, 4b, and 5b with DDQ in CHCl3 (DDQ: 1 mm,
DMTs: 1 mm) at variable temperatures (60 to ꢀ408C).
The solutions of 3b and 4b with DDQ in CHCl3 are
green at room temperature. However, the color gradually
changes from green to yellow upon heating to 608C and
Figure 8. VT UV/Vis spectra of 5b with DDQ (5b: 0.5 mm, DDQ:
0.5 mm, CHCl3) at 608C to ꢀ408C.
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M. Iyoda et al.
2-bromo-4,5-dimethoxybenzaldehyde (7.72 g, 31.5 mmol), [PdACHTUNGTRENNUNG(PPh3)4]
of 5b in DDQ solution shifted to a longer wavelength from
665 to 715 nm with decreasing temperature. Similar redshifts
were also observed in the case of 3b (630!680 nm) and 4b
(635!690 nm). The reason for this redshift is not clear. One
possible explanation could be a reduction in the distance be-
tween DDQ and the terminal aromatic rings of DMTs due
to suppression of the thermal vibrations of the complex,
(345 mg, 0.3 mmol), and CuI (114 mg, 0.6 mmol) in Et3N (150 mL) was
stirred for 1 h under an argon atmosphere at 758C. A mixture of CHCl3
and aq. NH4Cl solution was added into the reaction mixture, which was
then separated. The organic layer was washed with 2m HCl, water, and
brine and then dried over magnesium sulfate. After evaporation of the
solvent, the residue was purified by column chromatography (silica gel,
benzene). Recrystallization from ether/hexane gave 7a. Yield: 9.70 g,
13.05 mmol (87.0%); yellow crystals; m.p. 208–2098C; MS (LDI-TOF):
2
which may affect the electrostatic work term (e0 /er). Irre-
1
m/z: 742 [M]+; H NMR (CDCl3): d=10.50 (s, 2H), 7.70 (s, 2H), 7.45 (s,
spective of the presence of redshifts in the CT bands, these
CT absorptions cause a similar blue color.
2H),7.05 (s, 2H), 4.96 (s, 4H), 4.01 (s, 6H), 3.98 (s, 6H), 0.98 (s, 18H),
0.16 ppm (s, 12H); 13C NMR (CDCl3): d=190.0, 153.6, 150.0, 141.8,
130.2, 129.7, 121.2, 120.1, 114.3, 108.3, 92.4, 91.3, 63.2, 56.3, 56.2, 26.0,
18.5, ꢀ5.2 ppm; elemental analysis calcd (%) for C42H54O8Si2: C 67.89, H
7.33; found: C 67.70, H 7.19.
Bis-1,4-(tert-butyldimethylsiloxymethyl)-bis-2,5-(2’-formyl-4’,5’-diethoxy-
phenyl-ethynyl)benzene (7b): Compound 7b was prepared by a proce-
dure similar to that used for 7a. Yield: 11.7 g, 14.7 mmol, 98.0%; yellow
crystals; m.p. 180–1828C; MS (LDI-TOF): m/z: 798 [M]+; 1H NMR
(CDCl3): d=10.47 (s, 2H), 7.69 (s, 2H), 7.43 (s, 2H),7.03 (s, 2H), 4.95 (s,
4H), 4.22 (q, J=7.0 Hz, 4H), 4.20 (q, J=7.0 Hz, 4H), 1.53 ( t, J=7.0 Hz,
3H), 1.50 ( t, J=7.0 Hz, 3H), 0.97 (s, 18H), 0.15 ppm (s, 12H); 13C NMR
(CDCl3): d=190.0, 153.3, 149.5, 141.8, 129.9, 129.6, 120.9, 120.1, 115.4,
109.5, 92.2, 91.4, 64.8, 64.6, 63.1, 26.0, 18.5, 14.5, ꢀ5.2 ppm; elemental
analysis calcd (%) for C46H62O8Si2: C 69.13, H 7.82; found: C 69.09; H
7.82.
Conclusions
In summary, we synthesized novel dynamic molecular tweez-
ers 3a, 3b, 4a, 4b, and 5b composed of two DBCOT units.
These DMTs form an equilibrium mixture of anti and syn
isomers in solution and these isomers were observed inde-
pendently in NMR spectra at room temperature. The iso-
merization barriers of the anti and syn forms depend on the
bulkiness of the COT substituents. From the investigations
of binding behavior of DMTs, only the syn form could inter-
act with guest molecules such as DDQ and TCNB, and the
electron-donating substituents on the COT rings and elonga-
tion of terminal alkoxy groups were found to enhance the
binding abilities. In particular, in spite of 3b and 4b having
a lower binding ability than 5b, both DDQ@3b and
DDQ@4b solutions show clear thermochromism from
yellow to green and green to blue, depending on the molar
Bis-1,4-(tert-butyldimethylsiloxymethyl)-bis-2,5-(2’-formyl-4’,5’-dimeth-
AHCTUNGERTGoNNUN xyphenyl-ethenyl)benzene (8a): A mixture of 7a (0.74 g, 1.0 mmol), Pd
on BaSO4 (0.5 g), and quinoline (1 mL) in THF (25 mL) was stirred for
3 h at room temperature under an H2 atmosphere. After Pd on BaSO4
was removed by filtration, the solution was concentrated. The residue
was dissolved in CH2Cl2, washed with 2m HCl and water, and dried over
magnesium sulfate. After evaporation of solvent, the residue was purified
by column chromatography (silica gel, CHCl3). Recrystallization from
CH2Cl2/hexane gave 8a. Yield: 0.67 g, 0.89 mmol (89.7%); yellow crys-
tals; m.p. 172–1738C; MS (EI): m/z: 746 [M]+; 1H NMR (CDCl3): d=
10.15 (s, 2H), 7.31 (s, 2H), 7.08 (d, J=12.2 Hz, 2H),7.06 (s, 1H), 6.84 (d,
J=11.9 Hz, 2H), 6.55 (s, 2H), 4.48 (s, 4H), 3.90 (s, 6H), 3.57 (s, 6H),
0.80 (s, 18H), ꢀ0.07 ppm (s, 12H); 13C NMR (CDCl3): d=189.9, 153.3,
148.5, 137.8, 135.2, 133.4, 130.8, 128.2, 126.63, 126.60, 112.4, 109.7 , 62.8,
55.9, 55.9, 25.8, 18.2, ꢀ5.6 ppm; elemental analysis calcd (%) for
C42H58O8Si2: C 67.52, H 7.83; found: C 67.46, H 7.81.
ratio of [DDQ@DMTs]/
ACHTUNGTNER[NUNG DDQ]. The unique thermochro-
ACHTUNGTRENNUNG
recognition, and, to our knowledge, this type of thermo-
chromism has not been previously reported.
Bis-1,4-(tert-butyldimethylsiloxymethyl)-bis-2,5-(2’-formyl-4’,5’-diethoxy-
phenyl-ethenyl)benzene (8b): Compound 8b was prepared by a proce-
dure similar to that used for 8a. Yield: 1.35 g, 1.68 mmol, 84.0%; yellow
crystals; m.p. 130–1318C; MS (EI): m/z: 802 [M]+; 1H NMR (CDCl3):
d=10.15 (s, 2H), 7.30 (s, 2H), 7.09 (d, J=12.2 Hz, 2H),7.04 (s, 2H), 6.80
(d, J=12.2 Hz, 2H), 6.50 (s, 2H), 4.47 (s, 4H), 4.11 (q, J=7.0 Hz, 4H),
3.74 (q, J=7.0 Hz, 4H), 1.45 (t, J=7.0 Hz, 6H), 1.22 (t, J=7.0 Hz, 6H),
0.80 (s, 18H), ꢀ0.07 ppm (s, 12H); 13C NMR (CDCl3): d=190.0, 152.8,
148.0, 137.7, 134.8, 133.5, 130.5, 128.1, 127.0, 126.4, 113.6 111.2 , 64.23,
64.16, 62.8, 25.8, 18.2, 14.6, 14.2, ꢀ5.6 ppm; elemental analysis calcd (%)
for C46H66O8Si2: C 68.79, H 8.28; found: C 68.75, H 8.30.
Experimental Section
General: 1H NMR and 13C NMR spectra were recorded on a JEOL LA-
500 instrument (1H: 500 MHz; 13C{1H}: 125 MHz) at 238C unless stated
otherwise. Chemical shift values are given in d (ppm) relative to internal
SiMe4 or residual solvent. Mass spectra were recorded on SHIMADZU
GCMS-QP2010 and KRATOS AXIMA-CFR instruments. Only the
more intense or structurally diagnostic mass spectral fragment ion peaks
are reported. Electronic spectra were recorded on a SHIMADZU UV-
VIS-NIR scanning spectrophotometer (Model UV-3101-PC). Melting
points were determined with a Yanaco MP-500D melting point appara-
tus. Elemental analyses were performed in the microanalysis laboratory
of Tokyo Metropolitan University. Column chromatography was carried
out on Merck silica gel60, 70–230 mesh ASTM, and Daiso silica gel
1001W. All solvents were dried by conventional procedures and distilled
before use. DDQ was purified by column chromatography on silica gel
with CHCl3/EtOAc (v/v=10:1) and recrystallized from CHCl3. Commer-
cially available TNB was dissolved in CH2Cl2 and dried with MgSO4. All
other commercially available materials were of reagent grade, unless
stated otherwise.
Bis-1,4-(2’-formyl-4’,5’-dimethoxyphenylethenyl)-bis-2,5-(hydroxymethyl)-
benzene (9a): A solution of 8a (3.74 g, 5.0 mmol) and 2m HCl (20 mL)
in THF (100 mL) was stirred for 1 h at room temperature, and then
hexane (200 mL) was added. The resulting white precipitate (9a) was fil-
tered and rinsed with EtOH and ether to give 9a. Yield: 2.20 g,
4.24 mmol (84.8%); white powder; m.p. 221–2238C; MS (EI): m/z: 518
[M]+; 1H NMR ([D6]DMSO): d=10.05 (s, 2H), 7.28 (s, 2H), 7.16 (d, J=
12.2 Hz, 2H),7.00 (s, 1H), 6.88 (d, J=11.9 Hz, 2H), 6.67 (s, 2H), 4.97 (t,
J=5.2 Hz, 2H), 4.26 (d, J=5.2 Hz, 4H), 3.79 (s, 6H), 3.46 ppm (s, 6H);
13C NMR ([D6]DMSO): d=190.3, 152.9, 148.1, 138.4, 134.6, 133.4, 130.6,
127.9, 126.9, 126.2, 112.5, 110.2, 60.5, 55.45, 55.41 ppm; elemental analysis
calcd (%) for C30H30O8: C 69.49, H 5.83; found: C 69.48, H 5.94.
Computational methods: All calculations were conducted with Gaussi-
an03 programs. The geometries were optimized with the restricted Becke
hybrid (B3LYP) at the 6-31G(d) level.
Bis-1,4-(2’-formyl-4’,5’-diethoxyphenylethenyl)-bis-2,5-(hydroxymethyl)-
benzene (9b): Compound 9b was prepared by a procedure similar to that
used for 9a. Yield: 2.04 g, 3.56 mmol, 88.9%; yellow powder; m.p. 183–
Bis-1,4-(tert-butyldimethylsiloxymethyl)-bis-2,5-(2’-formyl-4’,5’-dimeth-
ACHTUNGTRENNUNG
oxyphenyl-ethynyl)benzene (7a): A mixture of 6[29] (6.22 g, 15.0 mmol),
6844
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6838 – 6847

Dynamic Molecular Tweezers
FULL PAPER
1848C; MS (EI): m/z: 574 [M]+; 1H NMR ([D6]DMSO): d=10.03 (s,
2H), 7.25 (s, 2H), 7.14 (d, J=11.9 Hz, 2H), 6.99 (s, 2H), 6.84 (d, J=
11.9 Hz, 2H), 6.64 (s, 2H), 4.96 (t, J=5.2 Hz, 2H), 4.24 (d, J=5.2 Hz,
4H), 4.03 (q, J=7.0 Hz, 4H), 3.71 (q, J=7.0 Hz, 4H), 1.31 (t, J=7.0 Hz,
6H), 1.11 ppm (t, J=7.0 Hz, 6H); 13C NMR ([D6]DMSO) d=190.4,
152.2, 147.3, 138.3, 134.3, 133.4, 130.3, 127.8, 127.0, 126.0, 113.4, 111.5,
63.7, 63.7, 60.5, 14.6, 14.1 ppm; elemental analysis calcd (%) for
C30H30O8: C 71.06, H 6.67; found: C 71.03, H 6.80.
CHCl3/EtOAc v/v=10:1). Recrystallization from DMF gave 4a. Yield:
100 mg, 0.19 mmol (39.5%); yellow crystals; m.p. 277–2788C (decomp);
MS (LDI-TOF): m/z: 506 [M]+; 1H NMR (CDCl3): d=9.74–9.73 (sꢄ2,
2H), 7.57–7.54 (sꢄ2, 2H), 6.77–6.76 (sꢄ2, 2H), 6.74–6.53 (m, 8H), 3.87–
3.80 ppm (m, 12H); 13C NMR (CDCl3): d=193.0, 192.8, 154.0, 153.6,
149.7, 149.5, 148.4, 148.3, 143.7, 143.6, 136.5, 136.4, 132.6, 132.4, 132.4,
131.9, 131.8, 131.8, 131.1, 130.7, 129.7, 129.6, 127.1, 126.8, 112.0, 111.9,
110.9, 110.8, 56.0, 55.9, 55.8, 55.7 ppm; HR-MS (EI): m/z: calcd for
C32H26O6S2 [M]+: 506.1729; found: 506.1707.
Bis-1,4-(2’-formyl-4’,5’-dimethoxyphenylethenyl)-bis-2,5-(methanesulfon-
Molecular tweezers 4b: Compound 4b was prepared by a procedure sim-
ilar to that used for 4a. Yield: 360 mg, 0.640 mmol, 81.0%; yellow crys-
tals from CH2Cl2/hexane; m.p. 250–2528C (decomp); MS (LDI-TOF): m/z:
562 [M]+; 1H NMR (CDCl3): d=9.72–9.71 (sꢄ2, 2H), 7.55–7.52 (sꢄ2,
2H), 6.75–6.74 (sꢄ2, 2H), 6.72–6.51 (m, 8H), 4.09–3.99 (m, 8H), 1.45–
1.38 ppm (m, 12H); 13C NMR (CDCl3): d=193.1, 192.9, 154.2, 153.9,
149.5, 149.3, 148.0, 147.9, 143.6, 143.5, 136.5, 136.4, 132.7, 132.4, 132.2,
131.8, 131.7, 131.7, 131.1, 130.7, 129.8, 129.6, 126.9, 126.8, 113.8, 113.5,
113.0, 112.8, 64.7, 64.5, 64.4, 64.3, 14.8, 14.7ꢄ2, 14.6 ppm; elemental anal-
ysis calcd (%) for C32H24N2O4: C 76.85, H 6.09; found: C76.81, H 6.12.
ACHTUNGTRENNUNGyl-methyl)benzene (10a): To a solution of 9a (1.04 g, 2.0 mmol) and Et3N
(1.66 mL, 12.0 mmol) in DMF (100 mL) was added methanesulfonyl
chloride (0.46 mL, 6.0 mmol) dropwise at ꢀ558C under an N2 atmos-
phere. The solution was stirred for 15 min at the same temperature. The
reaction was quenched by addition of aqueous saturated NaHCO3, and
the mixture was extracted with CHCl3. The organic layer was washed
with 2m HCl, water, and brine, and then dried over magnesium sulfate.
1
The solvent volume was reduced to
=
by evaporation. The resulting
5
yellow precipitate (10a) was filtered and rinsed with hexane to give 10a.
Yield: 1.35 g, 2.0 mmol, (100%); yellow crystals; m.p. 171–1728C
(decomp); MS (EI): m/z: 674 [M]+; 1H NMR (CDCl3): d=10.06 (s, 2H),
7.31 (s, 2H), 7.22 (d, J=12.25 Hz, 2H),7.01 (s, 2H), 6.87 (d, J=11.9 Hz,
2H), 6.53 (s, 2H), 4.98 (s, 4H), 3.92 (s, 6H), 3.66 (s, 6H), 2.88 ppm (s,
6H); 13C NMR (CDCl3): d=190.1, 153.7, 149.0, 135.9, 134.2, 132.8, 132.0,
130.3, 128.8, 126.9, 112.4, 110.8, 68.1, 56.3, 56.2, 37.9 ppm; HR-MS (EI):
m/z: calcd for C32H34O12S2 [M]+: 674.1491; found: 674.1570.
Molecular tweezers 5b: To a solution of 4b (107 mg, 0.19 mmol) in THF
(50 mL) was added NaBH4 (8 mg, 0.20 mmol) at 08C under an N2 atmos-
phere. After the mixture was stirred for 3 h, the reaction was quenched
by addition of aqueous saturated NH4Cl solution, and the mixture was
extracted with CHCl3. The organic layer was washed with H2O and brine
and then dried over magnesium sulfate. After evaporation of the solvent,
the residue was dissolved in THF (20 mL) and the solution was added to
a suspension of NaH (60%, in oil; 85 mg, 2 mmol) in THF (20 mL)
under an N2 atmosphere. After the mixture was stirred for 10 min, CH3I
(0.6 mL, 9.7 mmol) was added dropwise and stirred overnight at 608C.
The reaction was quenched by addition of H2O, and the mixture was ex-
tracted with CH2Cl2. The organic layer was washed with H2O and brine,
and then dried over magnesium sulfate. After evaporation of solvent, the
residue was purified by column chromatography (silica gel, CHCl3/
EtOAc v/v=10:1). Recrystallization from CH2Cl2/hexane gave 5b. Yield:
76.6 mg, 0.13 mmol (68.4%, 2 steps); colorless powder; m.p. 236.5–
237.58C (decomp); MS (LDI-TOF) m/z: 594 [M]+; 1H NMR (CDCl3):
d=6.81–6.79 (sꢄ2, 2H), 6.69–6.67 (sꢄ2, 2H), 6.67–6.57 (6.65 (d, J=
10.0 Hz), 6.62(s), 6.59 (d, J=10.0 Hz), 4H), 6.55–6.54 (sꢄ2, 2H), 6.50–
6.47 (sꢄ2, 2H), 4.26–4.07 (m, 4H), 4.05–3.96 (m, 8H), 3.40–3.38 (sꢄ2,
6H), 1.42–1.36 ppm (m, 12H); 13C NMR (CDCl3): d=147.8ꢄ2, 147.6,
147.4, 139.5, 139.4, 137.2, 137.0, 136.7, 136.6, 133.4, 133.3, 131.7, 131.6,
130.3, 129.8, 129.6, 129.4, 129.4, 129.3, 128.6, 128.0, 113.6, 113.5, 113.4,
113.1, 64.5., 64.4, 64.3, 64.2, 58.1ꢄ2, 57.9ꢄ2, 14.8ꢄ4 ppm; HR-MS (EI):
m/z: calcd for C38H42O6 [M]+: 594.2941; found: 594.2981.
Bis-1,4-(2’-formyl-4’,5’-diethoxyphenylethenyl)-bis-2,5-(methanesulfonyl-
ACHTUNGTRENNUNGmethyl)benzene (10b): 10b was prepared by a procedure similar to that
used for 10a. Yield: 2.53 g, 3.46 mmol 100%; yellow crystals; m.p. 156–
1588C (decomp); MS (EI): m/z: 730 [M]+; 1H NMR (CDCl3): d=10.05
(s, 2H), 7.30 (s, 2H), 7.22 (d, J=11.9 Hz, 2H), 6.99 (s, 2H), 6.83 (d, J=
11.9 Hz, 2H), 6.48 (s, 2H), 4.96 (s, 4H), 4.13 (q, J=7.0 Hz, 4H), 3.83 (q,
J=7.0 Hz, 4H), 2.88 (s, 6H), 1.46 (t, J=7.0 Hz, 6H), 1.30 ppm (t, J=
7.0 Hz, 6H); 13C NMR (CDCl3): d=190.0, 153.1, 148.3, 135.8, 133.7,
132.6, 131.8, 130.4, 128.3, 126.5, 113.3, 112.2, 68.1, 64.5, 64.5, 37.7, 14.6,
14.3 ppm; elemental analysis calcd (%) for C36H42O12S2 :C 59.16, H 5.79;
found: C 58.90, H 5.79.
Molecular tweezers 3a: To a solution of 10a (628 mg, 0.93 mmol) in
MeCN (140 mL) was added NaCN (147 mg, 3.0 mmol) in DMF (60 mL)
dropwise at room temperature under an N2 atmosphere. After stirring for
2 h, the reaction was quenched by addition of 2m aqueous HCl and ex-
tracted with CHCl3. The organic layer was washed with water and brine,
and then dried over magnesium sulfate. After evaporation of the solvent,
the residue was dissolved in MeCN (100 mL), and DBU (1 mL) was
added at 1108C under an N2 atmosphere. The mixture was stirred for 1 h.
The resulting white precipitate (3a) was filtered and rinsed with EtOH
and ether to give 3a. Yield: 291 mg, 0.58 mmol (62.5%); white powder;
m.p. 308–3108C (decomp); MS (LDI-TOF): m/z: 500 [M]+; 1H NMR
(C2D2Cl4 908C): d=7.50 (s, 2H), 7.06 (s, 2H), 6.68 (s, 4H), 6.57 (s, 2H),
6.55 (s, 2H), 3.83 (s, 6H), 3.81 ppm (s, 6H); 13C NMR (C2D2Cl4 908C):
d=153.4, 152.1, 151.7, 140.5, 136.9, 136.5, 133.9, 133.3, 132.8, 128.9, 122.7,
118.9, 116.0, 115.3, 59.3, 59.1 ppm; elemental analysis calcd (%) for
C32H24N2O4: C 76.78, H 4.83; N, 5.60; found: C76.77, H 4.70; N, 5.68.
X-ray structural analysis: Intensity data were collected on a Bruker
SMART APEX diffractometer with graphite-monochromated MoKa radi-
ation (l=0.71073 ꢁ), a Rigaku AFC7R diffractometer with graphite-
monochromated MoKa radiation (l=0.71069 ꢁ), and Rigaku Mercury
CCD diffractometer with graphite-monochromated MoKa radiation (l=
0.71073 ꢁ). The structure was solved by direct methods (SHELXTL) and
refined by the full-matrix least-squares method on F2 (SHELXL-97). All
non-hydrogen atoms were refined anisotropically, and all hydrogen atoms
were placed using AFIX instructions.
Molecular tweezers 3b: Compound 3b was prepared by a procedure sim-
ilar to that used for 3a. Yield: 343 mg, 0.580 mmol, 58.3%; white
powder; m.p. 260–2628C (decomp); MS (LDI-TOF): m/z: 556 [M]+;
1H NMR (C2D2Cl4, 908C): d=7.48 (s, 2H), 7.04 (s, 2H), 6.66 (s, 4H),
6.57 (s, 2H), 6.55 (s, 2H), 4.03 (m, 8H), 1.39 ppm (m, 12H); 13C NMR
(C2D2Cl4, 908C): d=153.3, 151.81, 151.77, 140.5, 136.9, 136.4, 133.8,
133.2, 133.0, 129.0, 122.7, 118.7, 118.1, 117.7, 68.3, 68.0, 17.0, 17.8 ppm; el-
emental analysis calcd (%) for C32H24N2O4: C 77.68, H 5.79; N, 5.03;
found: C77.39, H 5.80; N, 5.27.
4a: Single crystals were obtained by recrystallization from DMF.
C32H26N6·2C3H7NO; FW=652.73, triclinic, P1, a=9.402(2), b=11.334(3),
c=9.2127(11) ꢁ; a=105.388(14), b=95.383(14), g=112.766(19)8; V=
851.3(3) ꢁ3, Z=1; 1cald =1.273 gcmꢀ3. The refinement converged to R1 =
0.0483, wR2 =0.1460 [I>2s(I)], GOF=0.999.
¯
TCNB@4b: Single crystals were obtained by recrystallization from
¯
CHCl3. C36H34O6·C10H2N4·CHCl3; FW=860.16, triclinic, P1, a=10.761(3),
b=13.910(4), c=15.552(4) ꢁ; a=84.952(5), b=73.093(4), g=76.461(4)8;
V=2164.8(10) ꢁ3; Z=2; 1cald =1.320 gcmꢀ3. The refinement converged
to R1 =0.0680, wR2 =0.1953 [I>2s(I)], GOF=1.025.
Molecular tweezers 4a: To a solution of 3a (250 mg, 0.5 mmol) in CH2Cl2
(60 mL) was added DIBAL-H (1m solution, 4 mL, 4.0 mmol) at ꢀ788C
under an N2 atmosphere. After stirring for 2 h, the reaction was
quenched by addition of aqueous saturated NH4Cl solution. The mixture
was extracted with CH2Cl2. The organic layer was washed with water and
brine and then dried over magnesium sulfate. After evaporation of the
solvent, the residue was purified by column chromatography (silica gel,
TCNB@5b: Single crystals were obtained by recrystallization from
¯
CHCl3. C38H42O6·C10H2N4; FW=772.87, triclinic, P1, a=11.2476(16), b=
11.5599(16), c=16.233(2) ꢁ; a=92.630(8), b=98.272(7), g=95.209(7)8;
V=2076.3(5) ꢁ3; Z=2; 1cald =1.236 gcmꢀ3. The refinement converged to
R1 =0.0842, wR2 =0.2031 (I>2s(I)), GOF=1.292.
Chem. Eur. J. 2009, 15, 6838 – 6847
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6845

M. Iyoda et al.
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Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science and Technology,
Japan. We would like to thank Prof. W. Fujita (Tokyo Metropolitan Uni-
versity) for the low-temperature X-ray analyses and Prof. Y. Mazaki (Ki-
tasato University) for VT UV/Vis measurements. We also thank Dr. H.
Enozawa and Dr. M. Takase (Tokyo Metropolitan University) and Dr.
M. Hasegawa (Kitasato University) for helpful discussions.
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1
served by H NMR spectroscopy.
Received: March 9, 2009
Published online: June 5, 2009
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6847