A light-controlled molecular brake with complete ON-OFF rotation

Article abstract of DOI:10.1002/chem.200902123

A light-controlled molecular machine based on cyclic azobenzenophanes consisting of a dioxynaphthalene rotating unit and a photoisomerizable dioxyazobenzene unit bridged by methylene spacers is reported. In compounds 1 and 2, 1,5- and 2,6-dioxynaphthalene moieties, respectively, are linked to p-dioxyazobenzene by different methylene spacers (n = 2 in 1a and 2; n = 3 in 1b), whereas a 1,5-dioxynaphthalene moiety is bonded to m-dioxyazobenzene by bismethylene spacers in 3. In 1b and 2, the naphthalene ring can rotate freely in both the trans and cis states at room temperature. The rotation speed can be controlled either by photoinduced reversible trans-cis (E-Z) isomerization of the azobenzene or by keeping the system at low temperature, as is evident from its NMR spectra. Furthermore, for the first time, we demonstrate a light-controlled molecular brake, wherein the rotation of the naphthalene moiety through the cyclophane is completely OFF in the trans isomer of compound 3 due to its smaller cavity size. Such restricted rotation imparts planar chirality to the molecule, and the corresponding enantiomers could be resolved by chiral HPLC. However, the rotation of the naphthalene moiety is rendered ON in the cis isomer due to its increased cavity size, and it is manifested experimentally by the racemization of the separated enantiomers by photoinduced E-Z isomerization.

Full text of DOI:10.1002/chem.200902123

FULL PAPER
DOI: 10.1002/chem.200902123
A Light-Controlled Molecular Brake with Complete ON–OFF Rotation
Meethale C. Basheer,[a] Yoshimi Oka,[b] Manoj Mathews,[b] and Nobuyuki Tamaoki*[a, b]
Abstract: A light-controlled molecular
machine based on cyclic azobenzeno-
phanes consisting of a dioxynaphtha-
lene rotating unit and a photoisomeriz-
able dioxyazobenzene unit bridged by
methylene spacers is reported. In com-
pounds 1 and 2, 1,5- and 2,6-dioxy-
naphthalene moieties, respectively, are
linked to p-dioxyazobenzene by differ-
ent methylene spacers (n=2 in 1a and
2; n=3 in 1b), whereas a 1,5-dioxy-
naphthalene moiety is bonded to m-di-
trans and cis states at room tempera-
ture. The rotation speed can be con-
trolled either by photoinduced reversi-
ble trans–cis (E–Z) isomerization of
the azobenzene or by keeping the
system at low temperature, as is evi-
dent from its NMR spectra. Further-
more, for the first time, we demon-
brake, wherein the rotation of the
naphthalene moiety through the cyclo-
phane is completely OFF in the trans
isomer of compound 3 due to its small-
er cavity size. Such restricted rotation
imparts planar chirality to the mole-
cule, and the corresponding enantio-
mers could be resolved by chiral
HPLC. However, the rotation of the
naphthalene moiety is rendered ON in
the cis isomer due to its increased
cavity size, and it is manifested experi-
mentally by the racemization of the
separated enantiomers by photoin-
duced E–Z isomerization.
strate
a
light-controlled molecular
Keywords: azo compounds · molec-
ular brake · molecular machines ·
oxyazobenzene
by
bismethylene
photochemistry
isomerization
·
photo-
spacers in 3. In 1b and 2, the naphtha-
lene ring can rotate freely in both the
Introduction
reversibly halted the rotation of the rigid triptycene wheel.
However, light-controlled molecular systems have many ad-
vantages compared to chemically-controlled systems, be-
cause the formation of waste products during a chemical
process will compromise the operation of the machine
unless they are removed from the system as in the case of
natural systems.[2b,4] One of the first attempts to design such
a light-controlled system was described by Feringa and co-
workers, wherein control of the rate of rotation of a xylyl
rotor by cis–trans isomerization of a thioxanthene-based
sterically overcrowded alkene was demonstrated.[5] The
presence of o-methyl groups on the rotor was expected to
cause restricted rotation about the arene–arene single bond
due to steric interactions with the naphthalene moiety in the
upper part of the molecule. Consequently, the rate of rota-
tion was presumed to be different in the cis and trans iso-
mers. J.-S. Yang and co-workers recently reported a pentip-
tycene-derived light-controlled molecular brake that could
function at room temperature.[6] In their design, the rigid
pentiptycene group served as a four-bladed wheel, and an
E–Z photoisomerizable dinitrostyryl group functioned as a
photoresponsive brake unit. In the resulting molecular
brake, the pentiptycene rotor showed distinct rates of rota-
tion in the trans and cis states, which differed by about nine
orders of magnitude. In this context, it is noteworthy that a
The ability to control motion at a molecular level is one of
the prerequisites for the design of molecular machines.[1]
Many efforts have been directed towards the design of
chemically- or light-controlled linear and rotary motions.[1,2]
Among the most elegant examples have been the “molecu-
lar brakes” designed by Kelly and co-workers,[3] which con-
sisted of a rigid triptycene wheel with a bipyridyl molecular
recognition unit as a brake. This chemically-controlled
brake was operated by coordination of a metal ion at a rec-
ognition site, which induced a conformational change that
[a] Dr. M. C. Basheer, Prof. Dr. N. Tamaoki
Research Institute for Electronic Science, Hokkaido University
N20, W10, Sapporo, Hokkaido 001-0020 (Japan)
Fax : (+81)11-706-9357
E-mail: tamaoki@es.hokudai.ac.jp
[b] Dr. Y. Oka, Dr. M. Mathews, Prof. Dr. N. Tamaoki
National Institute of Advanced Industrial Science
and Technology (AISI)
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902123.
Chem. Eur. J. 2010, 16, 3489 – 3496
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3489

molecular system with complete ON–OFF rotation has not
yet been achieved.
ing unit linked to photoisomerizable dioxyazobenzene by
methylene spacers were synthesized. In compounds 1 and 2,
1,5- and 2,6-dioxynaphthalene units, respectively, are con-
nected to p-dioxyazobenzene by different methylene spacers
(n=2 in 1a and 2; n=3 in 1b), while in 3 a 1,5-dioxynaph-
thalene unit is bonded to m-dioxyazobenzene by bismethy-
lene spacers. An overview of the synthetic strategy used to
obtain the target compounds is presented in Scheme 1.
Recently, azobenzene-containing systems have attracted
much attention for the construction of light-controlled mo-
lecular-level machines due to their E–Z photoisomerization
properties, which induce drastic changes in molecular length
and occupied volume.[7] In our previous reports, we de-
scribed the synthesis of some cyclic azobenzene dimers and
it was observed that the isomer-
ization of the azobenzene unit
in the frame of these cycles had
an enormous effect on the
structure or motion of other
parts of the molecules.[8] More
recently, we have reported the
synthesis of a cyclic azobenze-
nophane consisting of a 1,5-di-
oxynaphthalene moiety bonded
at the meta positions of azoben-
zene by bismethylene spacers.
This system showed conforma-
tional restriction on the free ro-
tation of the naphthalene
moiety, thereby imparting the
molecule with planar chirality,
and the corresponding enantio-
mers could be resolved and
preparatively
separated
by
chiral HPLC.[9] Photoinduced
E–Z isomerization of this com-
pound and its separated enan-
tiomers proceeds without race-
mization, indicating that the
size of the cyclophane cavity is
not large enough in either the
Scheme 1. Synthetic route to the target compounds.
trans or cis state to allow free
rotation of the naphthalene ring
through it. This compound was successfully employed as a
chiroptical switch in commercially available nematic liquid
crystals, allowing the attainment of phototunable reflection
colors. In the work described herein, a variety of cyclic azo-
benzenophanes with 1,5- or 2,6-dioxynaphthalene rotating
units bonded at the meta or para positions of photoisomeriz-
able dioxyazobenzene by different methylene spacers have
been investigated. Furthermore, we demonstrate for the first
time a “molecular brake” exhibiting a complete ON–OFF
rotation, wherein rotation of the naphthalene moiety is com-
pletely OFF (brake on) in the trans isomer of the cyclic azo-
benzenophane, but is ON (brake off) in the cis isomer as a
result of photoinduced E–Z isomerization of the azobenzene
moiety.
Compounds 1–3 were successfully synthesized by reduction
of their corresponding dinitro compounds. The crude prod-
ucts obtained were purified by column chromatography on
silica gel followed by recrystallization from CH2Cl2/hexane
by the vapor diffusion method to obtain the required com-
pounds in their pure trans forms. The structures of these
compounds were determined by 1H NMR, 13C NMR, and
mass spectrometry (ESI or MALDI-TOF). Moreover,
single-crystal X-ray analyses corroborated the suggested
structures of the respective compounds.
In order to obtain a preliminary insight into the rotation
behavior of the naphthalene units in these azobenzeno-
1
phanes, their H NMR spectra were carefully examined. It
was found that at room temperature trans-1b (600 MHz,
CD2Cl2) displays three resonances at d=2.20, 3.86, and
4.62 ppm
due
to
the
methylene
protons
Results and Discussion
(-OCHa CHb CHc2O-), two resonances at d=7.13 and
2
2
7.18 ppm due to the azobenzene protons, and three resonan-
ces at d=6.34, 7.06, and 7.27 ppm due to the naphthalene
protons (Figure 1). Correspondingly, three resonances due
Synthesis and structural characterization: A variety of cyclic
azobenzenophanes (1–3) consisting of a naphthalene rotat-
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Chem. Eur. J. 2010, 16, 3489 – 3496

Light-Controlled Molecular Brake
FULL PAPER
cis-1 was obtained by first dissolving trans-1 in CH2Cl2, irra-
diating the solution with UV light, and then separating the
corresponding cis product by column chromatography on
silica gel. The product was subsequently recrystallized from
chloroform/hexane. However, suitable crystals of cis-2/3
could not be obtained in this way. The crystal structures of
trans-1, cis-1, trans-2, and trans-3 are shown in Figure 2. The
Figure 1. 1H NMR spectra of 1b and 3 in CD2Cl2 and CDCl3, respectively,
at room temperature.
to the methylene protons at d=2.26, 3.99, and 4.45 ppm,
two resonances due to the azobenzene protons at d=6.51
and 6.58 ppm, and three resonances due to the naphthalene
protons at d=6.88, 7.18, and 7.75 ppm were obtained for
cis-1b. Similar observations were made in the case of com-
pound 2.[10] These results suggest that rotation of the naph-
thalene moiety occurs rapidly on the NMR time scale. In
contrast, four resonances at d=3.90, 4.38, 4.43, and
4.69 ppm due to the methylene protons (-OCHaHbCHcHdO-),
three resonances at d=6.91, 7.20, and 7.25 ppm due to the
azobenzene protons, and three resonances at d=6.23, 6.92,
and 7.11 ppm due to the naphthalene protons were observed
in the case of trans-1a in C2D2Cl4 at room temperature.[10]
The room temperature 1H NMR spectrum of trans-3 fea-
tures three resonances at d=4.38, 4.61, and 4.76 ppm due to
Figure 2. X-ray crystal structures of trans-1–3 and cis-1.
most stable structure of cis-3 was predicted by DFT calcula-
tion.[10] The presence of ring strain in both trans-1a and
trans-1b was verified by the 12–148 deviations in the torsion
À
À
angles of their C N=N C bonds from that (1808) of unsub-
stituted azobenzene.[12] Moreover, analysis of the structures
obtained from single-crystal X-ray diffraction data indicated
that the long axes of the azobenzene and naphthalene rotat-
ing units were close and positioned parallel to each other in
the trans isomers of compounds 1, 2, and 3. However, in the
cis isomers of 1a, 1b, and 3, the azobenzene was found to
be positioned away from the naphthalene rotating unit, leav-
ing a large cavity size that may permit fast rotation of the
naphthalene unit through it.
c
the methylene protons (-OCHaHbCH2 O-), four resonances
at d=6.42, 6.82, 7.06, and 7.64 ppm due to the azobenzene
protons, and three resonances at d=7.23, 7.29, and 7.33 ppm
due to the naphthalene protons (Figure 1). In these spectra,
the splitting of the aliphatic (-OCHaHb-) proton signals is
due to the diastereotopic nature of the molecule when the
exchange of the enantiomers is rendered slow on the NMR
time scale. This feature of splitting of the aliphatic proton
signals is unchanged up to 373 K in [D6]DMSO (Figure S10
in the Supporting Information). Thus, these results suggest
that restricted rotation of the naphthalene unit through the
cyclophane cavity on the NMR time scale imparts planar
chirality to the molecule.[9,11]
Absorption spectra and photochemical isomerization:
Figure 3 shows the changes in the absorption spectrum of 3
that occurred upon irradiation with light of wavelengths 366
and 436 nm in tetrahydrofuran (THF) at room temperature.
Initially, compound
3 exhibited absorption maxima at
227 nm (e/mÀ1 cmÀ1 31366) and in the region 260–400 nm
(lmax =297 nm, e/mÀ1 cmÀ1 10573). The former band is due to
the 1Bb transition of the naphthalene chromophore, while
1
1
Single-crystal X-ray analysis and computational prediction
of the molecular structures: Recrystallization from CH2Cl2/
hexane by the vapor diffusion method yielded orange crys-
tals of 1–3 suitable for X-ray crystal analysis. A crystal of
the latter is due to the La and Lb transitions of the naph-
thyl group and the p–p* transition of the azobenzene chro-
mophore. Moreover, a very weak absorption band due to
the n–p* transition of azobenzene at 440 nm can also be
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N. Tamaoki et al.
seen. Upon irradiation with light of wavelength 366 nm to
its photostationary state (PSS), due to E–Z isomerization,
the intensity of the p–p* band decreased with no concomi-
tant increase in the intensity of the n–p* band. The reverse
spectral changes were obtained upon irradiation with light
of wavelength 436 nm. We confirmed that the photochemi-
cal E–Z and Z–E isomerization processes could be repeated
by alternating between irradiation at 366 and at 436 nm for
more than ten cycles without any fatigue (inset in Figure 3).
mine the rates of rotation of the naphthalene unit through
the cyclophane cavity. NMR spectra of trans/cis-1b at room
temperature indicate that the naphthalene moiety rotates
rapidly on the NMR time scale, showing three signals due to
the aliphatic protons. However, upon decreasing the temper-
ature from 203 to 183 K the dynamic processes are slowed,
resulting in broadening of the line width and eventually
leading to separation of the signals due to the methylene
protons of trans-1b into six resonances at d=2.16, 2.32,
3.85, 3.90, 4.55, and 4.87 ppm, which we assign to the
CHaHa’CHbHb’CHcHc’ protons (Figure 4). The signals of the
aromatic protons of the azobenzene unit are also broad-
ened, eventually separating into three resonances at d=
6.85, 7.22, and 7.37 ppm (two of the four expected resonan-
ces are coincident). In contrast, the signals of the naphtha-
lene protons remain essentially unchanged, even at 183 K,
as they are unaffected by the rotation process. Similar phe-
nomena occur in other 1,5-naphthalenophane systems.[14] It
is also noteworthy that the number of signals in the NMR
spectrum of cis-1b remained the same, even at 183 K (Fig-
ure S8 in the Supporting Information), which indicates that
the naphthalene unit rotates more rapidly in the cis form
than in the trans form due to the larger cavity of the cyclo-
phane. NMR spectral analysis of the trans and cis isomers of
compound 2 indicated that the rotation of the naphthalene
unit is rapid at room temperature compared to that at lower
temperatures (Figures S3 and S9 in the Supporting Informa-
tion).
Figure 3. Evolution of the absorption spectrum of 3 in THF upon irradia-
tion at room temperature: a) initial state before irradiation, b) PSS after
irradiation at 366 nm, and c) PSS after irradiation at 436 nm (reverse re-
action). The inset shows the absorption spectral changes observed at
340 nm after alternating irradiations at 366 and 436 nm over ten complete
cycles.
The PSS composition of trans-3 upon UV/Vis irradiation, as
determined by HPLC, is discussed in the following section.
Compound 1b exists as a stable trans isomer in solution at
room temperature when maintained in the dark, whereas
compound 1a is transformed from the trans to the cis
isomer in the dark to reach an equilibrium state with an
isomer ratio of 42:58. Similar phenomena have been ob-
served for cyclic azobenzenes connected through their 4-
and 4’-positions.[8g,13] In acetonitrile at room temperature,
the molar extinction coefficients (e/mÀ1 cmÀ1) of trans-1a at
298, 355, and 442 nm were found to be 29855, 21600, and
1680, respectively, while those for trans-1b at 298, 358, and
452 nm were measured as 34232, 20200, and 2720, respec-
tively. Photoirradiation of trans-1/2 with UV light (lmax
=
366 nm) caused the absorption spectra to change,[10] with the
presence of isosbestic points indicating E–Z isomerization
of azobenzene. Irradiation of the resulting solutions with
visible light (lmax =436 nm) brought about the reverse spec-
tral changes, with the same isosbestic points as those ob-
served for the E–Z reactions. By NMR analysis, the isomer
compositions at the photostationary states were estimated to
be 6:94 and 44:56 for 1a, 6:94 and 50:50 for 1b, and 12:88
and 64:36 for 2 after irradiation at 366 and 436 nm, respec-
tively.
Figure 4. 1H NMR spectra of trans-1b at various temperatures.
The rate of rotation of the naphthalene ring through the
cyclophane cavity was determined by simulating the reso-
nance signal of the methylene protons nearest to the naph-
thalene ring.[15] Simulated spectra of trans-1b with the corre-
Variable-temperature NMR studies (VT-NMR): VT-NMR
studies of compounds 1b and 2 were conducted to deter-
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Light-Controlled Molecular Brake
FULL PAPER
sponding rate constants, along with matching experimental
spectra at different temperatures, are shown in Figure 5. The
activation parameters for its rotation obtained from the Ar-
rhenius plot were found to be DH° =24.4 kJmolÀ1, DS° =
À67.6 JmolÀ1 KÀ1, and DG° =37.5 kJmolÀ1 at 193 K. The ex-
perimental spectra of cis-1b and the well-fitted simulated
spectra are shown in the Supporting Information. The vari-
ous activation parameters calculated for the rotation of the
naphthalene moiety in cis-1b were found to be DH° =
um seen between the trans and cis isomers of this compound
at room temperature may preclude optical resolution into
one of the isomers. On the other hand, chiral HPLC of
trans-3 could be performed and the respective resolved
enantiomers were separated at retention times of 28.4 min
(trans-3A) and 35.1 min (trans-3B) using THF/hexane (3:7) as
eluent at a flow rate of 1 mLminÀ1 on a Chiralpak IB
column (Figure 6a). Furthermore, the resolved enantiomers
were isolated in pure form, as shown in Figure 6b. The pho-
toinduced isomerization properties, as well as the isomer
compositions at the photostationary states after irradiation
with light of wavelengths 366 and 436 nm, were also investi-
gated. On exposure of trans-3 to light of wavelength 366 nm,
E–Z photoisomerization was induced and a peak due to cis-
3 at a retention time of 44.7 min was obtained. Irradiation
of the resulting solution with light of wavelength 436 nm in-
duced the reverse reaction and hence the intensity of the
cis-3 peak decreased with the formation of trans-3 (Fig-
ure 6a). More interestingly, photoisomerization of the first-
eluted enantiomer, trans-3A, resulted in peaks consistent
with completely racemized trans-3 and cis-3 in the photosta-
tionary states after irradiations at 366 and 436 nm, as shown
in Figure 6b. Thus, the photoinduced E–Z isomerization of
the separated enantiomers proceeded with racemization, re-
vealing that the cavity of the cyclophane is sufficiently large
in the cis isomer to permit rotation of the naphthalene ring
through it. Thus, we succeeded in demonstrating a “molecu-
lar brake” with ON–OFF rotation function in a simple
system, wherein rotation of the naphthalene unit was com-
pletely OFF (brake on) in the trans isomer of the cyclic azo-
27.9 kJmolÀ1,
DS° =À14.1 JmolÀ1 KÀ1,
and
DG° =
30.7 kJmolÀ1 at 193 K.[10] The different values of the rate
constant
k
obtained for trans-1b (210 sÀ1) and cis-1b
(7000 sÀ1) after simulation at 188 K supported the photoin-
duced switching of the rotational speed of the naphthalene
unit through the change in size of the cyclophane cavity
upon isomerization of the azobenzene unit.
Figure 5. Temperature-dependent NMR signals of the methylene protons
adjacent to the naphthalene ring (600 MHz in CH2Cl2) of compound
trans-1b (left) and the corresponding simulated spectra with the rate con-
stants (right).
Molecular brake with ON–OFF rotation: In cyclophanes 1b
and 2, rotation of the naphthalene unit occurs in both the
trans and cis isomers, and the speed of rotation can be con-
trolled either by photoinduced reversible E–Z isomerization
of the azobenzene moiety or by keeping the system at low
temperature, as is evident from the NMR spectra. Neverthe-
less, photochemically-controlled switching on and off of the
rotation is more interesting and challenging, with potential
1
in the realm of molecular machines. The H NMR spectra of
compounds 1a and 3 at room temperature provide informa-
tion on the restricted rotation of the naphthalene ring
through the cyclophane cavity, which can impart planar chir-
ality to the molecule. However, optical resolution of com-
pound 1a was not possible by any of the chiral columns
available in our laboratory. Moreover, the thermal equilibri-
Figure 6. Chromatograms of a) a racemic mixture of trans-3 before irradi-
ation, the PSS after irradiation at 366 nm, and the PSS after irradiation
at 436 nm, and b) trans-3A before irradiation, the PSS after irradiation at
366 nm, and the PSS after irradiation at 436 nm, as obtained by chiral
HPLC (Chiralpak IB column; eluent: THF/hexane, 30:70, v/v; flow rate:
1 mLminÀ1).
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N. Tamaoki et al.
benzenophane but ON (brake off) in the cis isomer as a
result of photoinduced E–Z isomerization of the azobenzene
moiety (Scheme 2). By chiral HPLC analysis, the PSS ratios
for 3 were calculated as 16:84 (trans/cis) and 67:33 (cis/
trans) after irradiation at 366 and 436 nm, respectively.
The thermodynamic parameters obtained for racemization
in the cis form were DH° =32.5 kJmolÀ1, DS° =
À180 JmolÀ1 KÀ1, and DG° =81.2 kJmolÀ1 at 193 K.[10]
1
Hence, based on H NMR, chiral HPLC, and CD spectral
studies, we have unambiguously demonstrated the unique
behavior of 3 as a molecular brake.
Conclusion
We have described the design and synthesis of a light-con-
trolled molecular machine based on an azobenzenophane
consisting of a dioxynaphthalene rotating unit and a dioxya-
zobenzene photoisomerizable moiety connected by methyl-
ene spacers of different lengths. In the course of our studies,
three kinds of azobenzenophanes have been assembled and
their molecular-mechanical properties have been investigat-
ed. Our previous study showed that a cyclophane in which a
1,5-dioxynaphthalene moiety was bonded at the meta posi-
tions of azobenzene by bismethylene spacers had sufficiently
large conformational restriction on free rotation of the
naphthalene in both the trans and cis isomers to form re-
solvable planar-chiral enantiomers.[9] In the current design,
1,5-dioxynaphthalene and 2,6-dioxynaphthalene units
bonded at the para positions of dioxyazobenzene by tri-
methylene (1b) and bismethylene (2) spacers, respectively,
have been found to function as molecular rotors with con-
trollable rates of rotation of the naphthalene moiety through
the cyclophane cavity. More interestingly, a cyclophane
having a 1,5-dioxynaphthalene moiety attached to the meta
positions of dioxyazobenzene by bismethylene spacers (3)
has been demonstrated to function as a light-controlled mo-
lecular brake, wherein free rotation of the naphthalene unit
is completely stopped in the trans isomer (brake on) but is
switched on (brake off) in the cis isomer after photoinduced
E–Z isomerization of the azobenzene. Thus, our design
strategy has illustrated that control of the rotation of the
naphthalene moiety can be achieved by selection of an ap-
propriate spacer length between the rotating and isomeriz-
ing units as well as by the choice of position for attachment
of the spacer. We anticipate that such control over molecu-
lar motion may be exploited in the future development of
artificial molecular machines to achieve molecular-level
transportation.
Scheme 2. Schematic illustration of the photochemically controlled mo-
lecular brake system.
To further probe the chiral properties of compound 3, CD
spectra of the pure separated enantiomers obtained after
chiral HPLC were measured in THF, and the results are
shown in Figure 7. It can be seen that trans-3A shows weak
negative CD bands at 294 and 444 nm, a positive CD band
at 327 nm, and a strong positive exciton couplet centered at
218 nm. As expected, the second-eluted enantiomer trans-3B
showed the complete mirror image spectrum to that of
trans-3A. Exposure of trans-3A to light of wavelength 366 nm
caused E–Z isomerization and led to a gradual decrease in
the CD signal and finally a silent CD signal was obtained at
the PSS. Irradiation of the resulting solution with light of
wavelength 436 nm reversed the isomerization, with no sig-
nature of any CD signal.[10] Similar observations were made
in the case of trans-3B.[10] Thus, CD studies were also indica-
tive of photoinduced racemization of the pure enantiomers
due to E–Z isomerization upon irradiation with light of
wavelengths 366 or 436 nm. At temperatures below 273 K, it
was possible to measure the racemization rate in the cis
form by monitoring the change in intensity of the character-
istic transient CD band at around 426 nm, with a band of
opposite sign due to the trans isomer emerging after irradia-
tion at 366 nm (Figure S16 in the Supporting Information).
Experimental Section
General: All solvents and chemicals were obtained from commercial
sources and used without further purification, unless otherwise stated.
1H NMR (300, 400, or 600 MHz) and 13C NMR (75 or 400 MHz) spectra
were recorded on Varian Gemini 300, JEOL ECX 400, and Varian
Gemini 600 NMR spectrometers using tetramethylsilane as an internal
standard. Matrix-assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF MS) was performed on an Applied Biosys-
tems Voyager-DE pro instrument. X-ray crystallographic data were ac-
quired using a Bruker Smart Apex diffractometer. Absorption spectra
were recorded on a JASCO J-830 spectrophotometer. CD spectra were
Figure 7. CD spectra of the separated enantiomers in THF at room tem-
perature: a) trans-3A (first-eluted enantiomer) and b) trans-3B (second-
eluted enantiomer).
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recorded on
a
JASCO J-720 spectropolarimeter. Photoisomerization
argon atmosphere. The reaction mixture was then poured into excess
water (100 mL) and the precipitate formed was collected by filtration
and washed with ethyl acetate to obtain the pure compound 8 (1.1 g,
71%) as a brown solid. 1H NMR (300 MHz, CDCl3, 258C, TMS): d=
6.89–7.88 (m, 14H), 4.51–4.56 ppm (m, 8H); MS (MALDI-TOF): m/z:
calcd for C26H23N2O8 [M+H]+: 491.14; found: 491.24.
studies were conducted using radiation from a super-high-pressure mer-
cury lamp (500 W, USHIO Inc.) after passage through appropriate filters
(366 or 436 nm). High-performance liquid chromatography (HPLC) was
conducted on a Hitachi Elite La Chrome HPLC system using a CHIR-
ALPAK IB column (DAICEL Chemical Industries Ltd.). Photostation-
ary-state compositions were determined by NMR analysis in the case of
compounds 1 and 2 and by HPLC analysis in the case of compound 3.
All line-shape simulations were performed using the program gNMR
V4.1.0, provided by Cherwell Scientific, Oxford, Great Britain.
trans-3:
A solution of compound 8 (1.0 g, 2.04 mmol) in dry THF
(300 mL) was added dropwise over 4 h to a solution of LiAlH4 (776 mg,
20.4 mmol) in dry THF (100 mL) under an argon atmosphere. The reac-
tion mixture was refluxed during the addition of the dinitro compound
and was then left to stir at room temperature for 12 h. After cooling the
mixture in an ice bath, the reaction was carefully quenched by the addi-
tion of water. The precipitate formed was filtered off and the solvent was
evaporated under reduced pressure. The residual orange solid obtained
was redissolved in ethyl acetate, and this solution was washed with water
and dried over anhydrous MgSO4. The solvent was removed under re-
duced pressure and the product was purified by column chromatography
on silica gel to afford 1 (70 mg, 8%) as an orange solid. 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=7.64 (d, J=8.6 Hz, 2H), 7.33 (d, J=
7.7 Hz, 2H), 7.29 (d, J=6.0 Hz, 2H), 7.23 (t, J=8.04 Hz, 2H), 7.06 (tt,
J=2.3 Hz, 2.4 Hz, 2H), 6.82 (d, J=7.68 Hz, 2H), 6.42 (s, 2H), 4.76 (dt,
J=5.3, 4.76 Hz, 2H), 4.61 (t, J=3.28 Hz, 4H), 4.38 ppm (tt, J=3.4 Hz,
2H); 13C NMR (400 MHz, CDCl3, 258C, TMS): d=147.4, 142.8, 142.3,
123.6, 122.1, 120.1, 116.7, 114.3, 112.2, 106.7, 105.2, 73.9, 73.5 ppm; MS
(MALDI-TOF): m/z: calcd for C26H23N2O4 [M+H]+: 427.16.
General procedure for the synthesis of compounds 1 and 2: A mixture of
an appropriate amount of 1,5- or 2,6-dihydroxynaphthalene (20 mmol)
and potassium carbonate (K2CO3) (20 mmol) in dry DMF (100 mL) was
stirred at room temperature for 1 h. A solution of 4 (46 mmol) in DMF
(50 mL) was then added, and the mixture was heated at 808C for 48 h.
After cooling to room temperature, the solvent was distilled off under re-
duced pressure. The insoluble residue obtained was washed with di-
chloromethane (CH2Cl2) and aqueous K2CO3 solution, and dried under
vacuum overnight. The highly insoluble powders containing compound 5
or 6 were used for the next step reaction without further purification.
Thus, this compound (20 mmol) was added to a suspension of LiAlH4
(200 mmol) in dry 1,4-dioxane (300 mL), and the mixture was heated
under reflux for 48 h under a nitrogen atmosphere. After cooling the re-
action mixture in an ice bath, water (20 mL) was carefully added. The
precipitate formed was filtered off, and the solvent was removed from
the filtrate by evaporation under vacuum. The residual orange solid was
then redissolved in CH2Cl2, and the solution was washed with water,
dried, and concentrated under reduced pressure. The residue was purified
by column chromatography on silica gel followed by recrystallization
from CH2Cl2/hexane to afford the desired product. trans-1a: Orange
solid, 1.2%. 1H NMR (600 MHz, C2D2Cl4, 258C, TMS): d=7.25 (dd, J1 =
15.6, J2 =7.8 Hz, 4H), 7.20 (d, J=7.8 Hz, 2H), 7.11 (d, J=8.4 Hz, 2H),
6.92 (t, J=8.4 Hz, 2H), 6.91 (d, J=7.8 Hz, 2H), 6.23 (d, J=8.4 Hz, 2H),
4.69 (dd, J1 =12.6 Hz, J2 =7.8 Hz, 2H), 4.43 (dd, J1 =12.6, J2 =4.2 Hz,
2H), 4.38 (dd, J1 =12.6, J2 =7.8 Hz, 2H), 3.90 ppm (dd, J1 =12.6, J2 =
4.2 Hz, 2H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=159.3, 152.1,
149.9, 123.0, 121.7, 121.2, 115.3, 113.8, 103.2, 72.8, 66.6 ppm; MS (ESI):
m/z: calcd for C26H23N2O4 [M+H]+: 427.16; found 427.20. trans-1b:
Orange solid, 2.5%. 1H NMR (600 MHz, CD2Cl2, 258C, TMS): d=7.27
(d, J=8.4 Hz, 2H), 7.18 (d, J=8.9 Hz, 4H), 7.13 (d, J=9.1 Hz, 4H), 7.06
(t, J=7.7 Hz, 2H), 6.34 (d, J=7.7 Hz, 2H), 4.62 (t, J=5.4 Hz, 4H), 3.86
(t, J=5.6 Hz, 4H), 2.20 ppm (q, J=5.5 Hz, 4H); 13C NMR (75 MHz,
CDCl3, 258C, TMS): d=162.2, 153.5, 147.7, 125.7, 124.5, 123.8, 119.1,
114.1, 104.1, 69.2, 63.4, 31.5 ppm; MS (ESI): m/z: calcd for C28H27N2O4
[M+H]+: 455.19; found 455.24. trans-2: Orange solid, 1.0%. 1H NMR
(600 MHz, CD2Cl2, 258C, TMS): d=7.35 (d, J=9.0 Hz, 4H), 7.12 (d, J=
8.7 Hz, 2H), 7.08 (d, J=9.0 Hz, 4H), 6.63 (dd, J1 =8.7 Hz, J2 =2.7 Hz,
4H), 6.37 (d, J=2.7 Hz, 2H), 4.62 (t, J=3.6 Hz, 4H), 4.25 ppm (t, J=
3.6 Hz, 4H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=158.1, 153.5,
148.3, 128.7, 126.9, 123.8, 121.4, 116.5, 107.1, 71.1, 65.8 ppm; MS (ESI):
m/z: calcd for C26H23N2O4 [M+H]+: 427.19; found 427.21.
Acknowledgements
This work was supported by a grant-in-aid for science research in a prior-
ity area “New Frontiers in Photochromism (No. 471)” from the Ministry
of Education, Culture, Sports, Science, and Technology (MEXT), Japan.
We thank Dr. Midori Goto and Ms. Hiroko Kobashi from AIST Tsukuba
for helping with the X-ray crystallographic and variable-temperature
1H NMR spectroscopic analyses, respectively.
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[10] See the Supporting Information.
Received: July 30, 2009
Revised: December 3, 2009
Published online: February 5, 2010
[11] It is well known that the azo moiety N=N is rotating very rapidly,
even in the macrocyclic structure.[8] Hence, a 3,3-disubstituted azo-
3496
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