Induction of molecular chirality by circularly polarized light in cyclic azobenzene with a photoswitchable benzene rotor

Article abstract of DOI:10.1002/chem.201003526

New phototriggered molecular machines based on cyclic azobenzene were synthesized in which a 2,5-dimethoxy, 2,5-dimethyl, 2,5-difluorine or unsubstituted-1,4-dioxybenzene rotating unit and a photoisomerizable 3,3′-dioxyazobenzene moiety are bridged together by fixed bismethylene spacers. Depending upon substitution on the benzene moiety and on the E/Z conformation of the azobenzene unit, these molecules suffer various degrees of restriction on the free rotation of the benzene rotor. The rotation of the substituted benzene rotor within the cyclic azobenzene cavity imparts planar chirality to the molecules. Cyclic azobenzene 1, with methoxy groups at both the 2- and 5-positions of the benzene rotor, was so conformationally restricted that free rotation of the rotor was prevented in both the E and Z isomers and the respective planar chiral enantiomers were resolved. In contrast, compound 2, with 2,5-dimethylbenzene as the rotor, demonstrated the property of a light-controlled molecular brake, whereby rotation of the 2,5-dimethylbenzene moiety is completely stopped in the E isomer (brake ON, rotation OFF), while the rotation is allowed in the Z isomer (brake OFF, rotation ON). The cyclic azobenzene 3, with fluorine substitution on the benzene rotor, was in the brake OFF state regardless of E/Z photoisomerization of the azobenzene moiety. More interestingly, for the first time, we demonstrated the induction of molecular chirality in a simple monocyclic azobenzene by circular-polarized light. The key characteristics of cyclic azobenzene 2, that is, stability of the chiral structure in the E isomer, fast racemization in the Z isomer, and the circular dichroism of enantiomers of both E and Z isomers, resulted in a simple reversible enantio-differentiating photoisomerization directly between the E enantiomers. Upon exposure to r- or l-circularly polarized light at 488 nm, partial enrichment of the (S)- or (R)-enantiomers of 2 was observed. Copyright

Full text of DOI:10.1002/chem.201003526

DOI: 10.1002/chem.201003526
Induction of Molecular Chirality by Circularly Polarized Light in Cyclic
Azobenzene with a Photoswitchable Benzene Rotor
P. K. Hashim, Reji Thomas, and Nobuyuki Tamaoki*[a]
Abstract: New phototriggered molecu-
lar machines based on cyclic azoben-
zene were synthesized in which a 2,5-
dimethoxy, 2,5-dimethyl, 2,5-difluorine
or unsubstituted-1,4-dioxybenzene ro-
tating unit and a photoisomerizable
stricted that free rotation of the rotor
was prevented in both the E and Z iso-
mers and the respective planar chiral
enantiomers were resolved. In contrast,
compound 2, with 2,5-dimethylbenzene
as the rotor, demonstrated the property
of a light-controlled molecular brake,
whereby rotation of the 2,5-dimethyl-
benzene moiety is completely stopped
in the E isomer (brake ON, rotation
OFF), while the rotation is allowed in
the Z isomer (brake OFF, rotation
ON). The cyclic azobenzene 3, with flu-
orine substitution on the benzene
rotor, was in the brake OFF state re-
gardless of E/Z photoisomerization of
the azobenzene moiety. More interest-
ingly, for the first time, we demonstrat-
ed the induction of molecular chirality
in a simple monocyclic azobenzene by
circular-polarized light. The key char-
acteristics of cyclic azobenzene 2, that
is, stability of the chiral structure in the
E isomer, fast racemization in the Z
isomer, and the circular dichroism of
enantiomers of both E and Z isomers,
resulted in a simple reversible enantio-
differentiating photoisomerization di-
3,3’-dioxyazobenzene
moiety
are
bridged together by fixed bismethylene
spacers. Depending upon substitution
on the benzene moiety and on the E/Z
conformation of the azobenzene unit,
these molecules suffer various degrees
of restriction on the free rotation of
the benzene rotor. The rotation of the
substituted benzene rotor within the
cyclic azobenzene cavity imparts planar
chirality to the molecules. Cyclic azo-
benzene 1, with methoxy groups at
both the 2- and 5-positions of the ben-
zene rotor, was so conformationally re-
rectly between the
E enantiomers.
Upon exposure to r- or l-circularly po-
larized light at 488 nm, partial enrich-
ment of the (S)- or (R)-enantiomers of
2 was observed.
Keywords: azo compounds · chiral-
ity · circular dichroism · molecular
devices · photochromism
Introduction
science behind photoinduced chirality is also attracting the
attention of many researchers owing to its the possible ap-
plications in chiral sensors[13] and switches.[14]
Induction of chirality in molecules or molecular assemblies
has attracted the interest of chemists for a long time, partic-
ularly in connection with homochiral structures in nature.[1–5]
In addition to the academic interest, the induction of chirali-
ty is also important in modern chemistry due to the in-
creased number of applications in material science[6] and
technology.[7,8] Several physical factors were found to con-
tribute to the enantiomeric imbalance in a racemic system.
The various physical factors so far used to achieve such en-
richment include mechanical stirring[9] and the introduction
of a chiral environment in the form of chiral solvents[10] or
chiral additives.[11] In this context, the chiral nature of light
has also been used for chiral induction; indeed, the chirality
of light is believed to be one of the origins of homochiral
structures observed in nature.[1,12] Moreover, the intriguing
Following studies by le Bel[15] and Van’t Hoff[16] that dem-
onstrated the potential usefulness of l- or r-circular polarized
light (l- or r-CPL), several attempts have been made to ach-
ieve enantiomeric enrichment using CPL. Among the vari-
ous photocontrolled chiral induction processes, reversible
enantio-differentiating photoisomerization is known to take
place in a number of compounds for which photoresolution
and photoracemization occur with CPL and normal light, re-
spectively.[17–19] In 1996, Feringa and co-workers repor-
ted[8b,18] dynamic photoresolution mediated by CPL irradia-
tion on a sterically overcrowded alkene system, but the pro-
cess required relatively long irradiation time and the enan-
tiomeric excess was less than anticipated. Azobenzene is an-
other notable class of photochromic molecule with
remarkable E/Z isomerization that has been widely used for
the sophisticated fabrication and development of molecular
machines,[20] molecular conductors,[21] optical memory devi-
ces,[22] and optical switches.[23,14e–g] In addition, there are sev-
eral reports in the literature on azobenzene-based polymers
and liquid crystals for which the chiral molecular order is in-
troduced by CPL irradiation.[24] In our previous report we
described the partial photoresolution of a bicyclic azoben-
[a] P. K. Hashim, Dr. R. Thomas, Prof. 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
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003526.
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FULL PAPER
zene dimer with a relatively large difference in the molar
extinction coefficient between enantiomers.[25] However, this
bicyclic system, with two photoresponsive components, re-
sulted in a complicated enantio-differentiating photoisome-
rization path with three isomers (EE, EZ and ZZ), which
made the precise estimation of actual enantiomeric enrich-
ment upon CPL irradiation difficult. To the best of our
knowledge, no further examples of efficient systems exhibit-
ing sufficient photoresolution within short irradiation times
have been reported in the literature.
Our group is particularly interested in the design and syn-
thesis of azobenzene molecular systems to explore various
applications such as chiral dopants in liquid crystals and
photodriven brake systems.[14,26] Recently, we reported the
synthesis of a planar chiral azobenzene consisting of a di-
Scheme 1. Schematic illustration of chirality induction by CPL. This
study did not address which of the enantiomers R or S was enriched by l-
or r-CPL irradiation.
ACHTUNGTRENNUNGoxynaphthalene moiety that was cyclically bonded at the
meta positions of azobenzene by bismethylene spacers,
which was successfully employed as a chiroptical switch in
commercially available liquid crystals, giving rise to photo-
tunable reflection colors.[14a] More recently, in a similar
design, by tuning the spacer lengths we demonstrated for
the first time a complete ON/OFF switching of a naphtha-
lene rotor by E/Z photoisomerization of an azobenzene
braking element.[26]
methyl or fluorine) can affect the rotation, we considered
this to be negligible, and expected steric factors to deter-
mine the rotational speed of the substituted benzene rotors.
That is, the size of the substituent, as well as control of the
volume of the cavity by photoisomerization, can be the de-
ciding factors for rotor movement.
In the present work we demonstrate a new class of cyclic
azobenzene with a substituted benzene rotor showing a pho-
toswitchable braking property. We studied the effect of
steric bulk of the substituents at the 2- and 5-positions of
the benzene ring on the rotational behavior of the rotor in a
fixed cyclic azobenzene cavity. Similar to our previous naph-
thalene-based cyclic azobenzene, the compound with a 2,5-
dimethyl benzene rotor showed enantiomer stability in the
E isomer and fast racemization in the Z isomer. In addition,
this molecule exhibited a reasonably high De value, which
allowed it to be used in a chiral induction study with CPL.
The compound with a 2,5-dimethyl benzene rotor exhibits a
simple reversible enantio-differentiating photoisomerization
path between the E enantiomers, along with a considerable
induced circular dichroism (ICD) within a short period of
CPL irradiation (Scheme 1). To best of our knowledge, this
is the first monocyclic azobenzene that shows an enantio-
meric excess that is induced by CPL irradiation.
The synthetic strategies used to obtain the target mole-
cules are shown in Scheme 2. Compounds 1, 2, and 4 were
synthesized in three steps through reduction of the corre-
sponding dinitro compounds under dilute conditions. How-
ever, in the case of compound 3, with difluoro benzene as
rotor, an extremely low yield of the corresponding dinitro
compound was observed. Therefore, we adopted another
route in which the cyclization was carried out by conducting
a Mitsunobu reaction under dilute conditions (see Experi-
mental Section).
1
Figure 1 shows the H NMR spectra of (E)-1, (E)-2, and
(E)-3 recorded at room temperature. The spectrum of (E)-1
displayed a doublet of triplets at d=4.63–4.58 ppm (2H)
and multiplet resonances including a hidden doublet of trip-
lets at d=4.52–4.37 ppm (6H) for the bismethylene protons
[-OCHaHbCH2O-]. 1H NMR spectra of (E)-2, with methyl
substituents on the benzene ring, clearly displayed two sets
of separated doublet of triplets at d=4.64 and 4.31 ppm
(2H each, -OCHaHbCH2O-) and another resonance at d=
4.42 ppm due to aliphatic methylene protons. The split reso-
nances for Ha and Hb can be explained by a halted rotation
of the benzene unit, which results in a different environment
for these diastereotopic protons. This result provided a pre-
liminary insight into the degree of rotation of the benzene
moiety through the cyclic azobenzene cavity for (E)-1 and
(E)-2, which imparts planar chirality to the molecule.
NOESY experiments (see Figure S3 in the Supporting Infor-
mation) indicated that the protons at d=4.52–4.37 ppm for
(E)-1 and at d=4.64 ppm for (E)-2 are close to these aro-
matic protons (i.e., far from the methyl or methoxy substitu-
ents) of the 2,5-disubstituted benzene rotor. In contrast, the
1H NMR spectra for (E)-3, with fluorine substituents at the
Results and Discussion
Synthesis and conformational analysis: In the present study,
we designed and synthesized a new series of cyclic azoben-
zenes (1–4) consisting of either an unsubstituted or a 2,5-dis-
ubstituted (methoxy, methyl, or fluorine) 1,4-dioxybenzene
rotating unit, and a 3,3’-dioxyazobenzene photoisomerizable
moiety, bridged together by fixed bismethylene spacers. The
incorporation of a 2,5-disubstituted 1,4-dioxybenzene ring
system can impart an element of planar chirality to the mol-
ecule, provided there is no free rotation of the benzene unit
through the cavity of macrocycle.[25] Although the difference
in the electronic properties of the substituents (methoxy,
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N. Tamaoki et al.
the NMR study, owing to the
absence of diastereotopic hy-
drogen atoms. Presumably, this
molecule should also exhibit
free rotation of the benzene
rotor in the E isomer because
the hydrogen atom is smaller
than the fluorine atom.
Single-crystal X-ray analyses
of (E)-1 and (E)-2 were carried
out to confirm the molecular
structures (Figure 2). Analysis
of crystal structure indicated
that, in the E isomers, the
planes of the azobenzene and
benzene rotating units were
positioned reasonably close
and were in a perpendicular
orientation to each other,
keeping the methoxy or methyl
substituents away from the
cavity.
In summary, rotation of the
benzene rotor is prevented in
compounds 1 and 2 in their E
form, whereas it is allowed in
compounds 3 and 4 on the
NMR timescale. This restricted
rotation of the disubstituted
rotor through the cyclic azo-
benzene cavity imparts planar
chirality to molecules 1 and 2,
and would be expected to
make the compounds optically
resolvable by chiral HPLC.
However, optical resolution
would not be possible for com-
pound 3 due to the fast rota-
tion of the difluorobenzene
rotor.
Photoisomerization of cyclic
azobenzenes: To investigate
the photochemical isomeriza-
tion of the compounds, UV/Vis
spectroscopy and HPLC stud-
ies were carried out at room
temperature. Figure 3 shows
the change in the UV/Vis ab-
Scheme 2. Synthetic route to the molecules 1–4.
sorption of 2 in ethyl acetate,
2,5-positions of the benzene rotor, displayed two triplets
without splitting at d=4.51 and 4.42 ppm for aliphatic pro-
tons (Figure 1), clearly indicating the allowed rotation of the
2,5-difluorobenzene rotor through the macrocyclic cavity in
the E isomer. In the case of compound 4, with an unsubsti-
tuted benzene rotor, the extent of rotation of the benzene
ring through macrocyclic cavity could not be evaluated from
on irradiation with light of wavelengths 366 and 436 nm.
Upon irradiation at 366 nm, a gradual decrease was seen in
the intensity of the p–p* transition band at 327 nm, with a
notable increase in the n–p* transition band at 410–500 nm
due to photochemical E/Z isomerization. The reverse spec-
tral changes were observed upon irradiation at 436 nm. Sim-
ilar spectral changes were observed in the case of com-
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Induction of Molecular Chirality
FULL PAPER
Figure 3. Absorption spectral change of 2 in ethyl acetate upon irradia-
tion at room temperature; a) initial state before irradiation (solid line);
b) photostationary state, PSS366 nm (dashed line); c) PSS436 nm (dotted line).
An enlarged view of n–p* band is shown in the inset.
Effect of the bulkiness of the substituent and photoisomeri-
zation on rotor motion: We studied the effect of the bulki-
ness of the substituent on the rotatory behavior and wanted
to introduce a photoswitchable element into the molecular
system that would allow complete ON/OFF brake control.
A correlation between the bulkiness of the substituent on
the benzene rotor and its rotatory behavior was observed in
the 1H NMR study, and was further confirmed by chiral
HPLC analyses. The HPLC trace showed two peaks for (E)-
1 and (E)-2, whereas a single peak was seen for (E)-3. We
were even able to separate enantiomers of (E)-1 and (E)-2
on a preparative scale. This observation was in good agree-
ment with the planar chiral nature of (E)-1 and (E)-2, which
originates from the completely stopped rotation of the sub-
stituted benzene moiety. To check the possibility of racemi-
zation in the Z state, HPLC analysis was carried out using a
chiral column.
Figure 1. 1H NMR spectra of cyclic azobenzene (E)-1, (E)-2, and (E)-3 in
CDCl3 at room temperature.
On irradiating racemic (E)-1, which exhibited two peaks
corresponding to (E)-1A (Rt =22.57 min) and (E)-1B (Rt =
26.29 min), at 366 nm, two new peaks assignable to (Z)-1A
(Rt =50.54 min) and (Z)-1B (Rt =52.84 min) were observed;
subsequent irradiation at 436 nm resulted in increased inten-
sity of the parent peaks. Starting with pure enantiomer (E)-
1A, exposure at 366 nm induced the formation of a single
new peak (Rt =50.54 min) assignable to (Z)-1A and subse-
quent irradiation at 436 nm resulted in increased intensity of
the parent peak without the formation of a peak corre-
sponding to (E)-1B. These results confirmed that, even in the
Z form, racemization does not occur due to rotation of the
2,5-dimethoxybenzene rotor through the macrocyclic cavity
(for chromatograms see Figure S8 in the Supporting Infor-
mation). In contrast, interesting results were observed in the
case of 2, with less bulky methyl groups at the 2-, and 5-po-
sitions of the benzene rotor. Racemic (E)-2, which exhibited
two peaks ((E)-2A at Rt =18.84 min and (E)-2B at Rt =
22.04 min) by chiral HPLC, formed a new, single peak (Rt =
59.03 min) assignable to (Z)-2 upon irradiation at 366 nm;
Figure 2. X-ray crystal structures of (E)-1 (left) and (E)-2 (right).
pounds 1, 3, and 4 (see the Supporting Information Fig-
ACHTUNGTRENNUNGures S4 to S6). The photostationary states (PSS366 nm or
PSS436 nm) for compounds 1–4 were attained by irradiating at
366 or 436 nm. The E to Z isomer compositions at the PSS
were estimated by HPLC analysis to be 15:85 and 79:21 for
1, 11:89 and 75:25 for 2, and 7:93 and 85:15 for 3, respec-
tively, at 366 and 436 nm. From the PSS366 nm, slow thermal
back reaction from Z to E was seen to occur (rate constant
k=1.94ꢁ10ꢀ6 sꢀ1 at 25.08C) as observed by the change in
the UV/Vis absorption spectra of 2 (see the Supporting In-
formation).
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N. Tamaoki et al.
this is in sharp contrast to the result obtained with com-
pound 1, which formed two Z isomers (Figure 4a). These re-
sults gave a preliminary insight into the fast racemization in
Figure 5. CD spectra of enantiomers of 1 in ethyl acetate; a) (E)-1A,
b) (E)-1B, c) PSS366 nm from (E)-1A and d) PSS366 nm from (E)-1B.
Figure 4. Chromatograms of a) racemic mixture of (E)-2 before irradia-
tion, the PSS after irradiation at 366 nm (PSS366 nm), and the PSS after ir-
radiation at 436 nm (PSS436 nm) (top to bottom), and b) (E)-2A before irra-
diation, the PSS366 nm and the PSS436 nm (top to bottom), obtained by chiral
clearly shows the absence of racemization in both the E and
Z isomers of 1 (see the Supporting Information). Figure 6
shows the CD spectra recorded for the freshly separated
HPLC with a flow rate at 1.5 mLminꢀ1
.
the Z form of 2. Subsequent irradiation at 436 nm resulted
in increased intensity of the parent peaks. To further con-
firm the racemization behavior of the Z isomer, a separated
sample of pure (E)-2A was exposed to UV light. The HPLC
chromatogram of the irradiated enantiomer (E)-2A showed
a peak corresponding to (Z)-2, as observed in the case of
racemic 2. Irradiation of resulting solution at 436 nm in-
duced a reverse isomerization, giving rise to two peaks with
the same peak area (Rt =18.84 and 22.04 min) that matched
those of the racemized (E)-2A and (E)-2B (Figure 4b). Thus,
the photoinduced E/Z isomerization of the separated enan-
tiomers proceeded with racemization, revealing that the
macrocyclic cavity for the Z isomer is sufficiently large to
permit rotation of the 2,5-dimethylbenzene rotor. In con-
trast, with compounds (E)-1 and (E)-2, the HPLC chroma-
togram for (E)-3 showed a single peak (Rt =15.24 min).
Compound (E)-3, upon irradiation at 366 nm, formed a new
peak (Rt =25.43 min) assignable to (Z)-3; visible irradiation
(436 nm) resulted in reversal to the E form with the same
nature of the parent peak. These results clearly indicate the
free rotation of 2,5-difluorobenzene in the macrocyclic
cavity in both the E and Z forms.
To understand the chiroptical properties and racemization
behavior of compounds 1 and 2, CD spectroscopic studies of
pure, separated enantiomers were carried out. The CD spec-
trum of (E)-1A shows broad positive and negative bands at
307 and 433 nm, respectively, and the mirror image spec-
trum was observed in the case of (E)-1B (Figure 5). After
UV irradiation, (E)-1A gave rise to a new CD spectrum with
weaker positive bands at 300 and 324 nm, along with a more
intense negative band at 416 nm; the mirror image spectra
was observed for (E)-1B. The reversal of the CD spectra to
nearly the original state upon irradiation with visible light
Figure 6. CD spectra of enantiomers of (E)-2 in ethyl acetate; a) (E)-2A,
b) (E)-2B and c) PSS366 nm from (E)-2A.
enantiomers of compound 2. The CD spectrum of (E)-2A
(line a) shows broad positive and negative bands at 301 and
450 nm, respectively, and the complete mirror image spec-
trum (line b) was observed in the case of (E)-2B. Interesting-
ly, irradiation of (E)-2A with light at 366 nm led to a gradual
decrease in the CD signal, resulting in annihilation of the
CD bands at the PSS (Figure 6). Irradiation of the resulting
solution at 436 nm reversed the isomerization, keeping the
CD signal silent. A similar observation was also made in the
case of (E)-2B. Thus, CD studies were also indicative of pho-
toinduced racemization of the pure enantiomers due to E/Z
isomerization upon irradiation with light at 366 nm. Never-
theless, at ꢀ108C the CD band did not disappear at the PSS
upon UV irradiation; instead, a new CD band assignable to
the Z isomer was formed. The intensity of this new CD
band gradually decreased over 100 min in the dark at
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Induction of Molecular Chirality
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ꢀ108C (see Figure S11 in the Supporting Information). This
indicates that rate of rotation of the 2,5-dimethylbenzene
moiety is lowered at ꢀ108C. From the results of the HPLC
and CD experiments we are able to conclude that the rotor
motion is controlled both by the bulkiness of the substituent
on the rotor and by E/Z photoisomerization of the azoben-
zene moiety. Compound 1, with methoxy substituents on the
benzene rotor, represents the brake ON (rotation OFF)
state in both the E and Z isomers, while 2, with less bulky
methyl substituents, is in the brake ON state in the E
isomer and the brake OFF (rotation ON) state in the Z
isomer (Scheme 3). Compound 3, with fluorine substituents,
through a chiral discrimination path from the electronic
ground states to a common excited state[18,27] (Scheme 4a).
However, in the case of compound 2, we propose a new
Scheme 4. Photochemical reaction with chiral induction by CPL irradia-
tion. a) Reference [18,28] with an excited state for racemization; b) Com-
pound 2 with a ground state Z isomer for racemization.
chiral discrimination path involving enantio-differentiating
photoisomerization between the ground states of R and S
enantiomers of E (ER and ES) and a common (or fast race-
mizing) ground state of the Z form (Scheme 4b). Thus,
when a racemic mixture is irradiated with nonpolarized
light, both R-to-R (solid line) and S-to-S (dotted line) E/Z
isomerization pathways have the same efficiency, whereas
upon CPL irradiation, one of the enantiomers may interact
preferentially with r- or l-CPL, causing an enantiomeric im-
balance to develop during repeated isomerization with the
reverse Z/E isomerization path (thick line) (Scheme 4b).
Compound 2 possessed properties that were suitable for a
chiral induction study by CPL; there was no racemization in
E isomer and fast racemization in Z isomer in the electronic
ground state at room temperature, and photoisomerization
from E to Z and vice versa took place to reach the E-en-
riched PSS at 488 nm with a large De at the same wave-
length. The racemate (R,S)-(E)-2, upon CPL irradiation at
488 nm for 15 min resulted in enrichment of one of the
enantiomers as evidenced by the change in the CD spectrum
from inert to active. Further irradiation with CPL at the
same wavelength with l- and r-CPL, induced an active CD
that switched between positive and negative signs at the n–
p* transition band (Figure 6).
Because the peak shape and position of the induced CD
spectrum at the monitored wavelength for the solution irra-
diated with CPL matched those of one of the pure enantio-
mers of (E)-2, we were satisfied that one of the enantiomers
was partially enriched. Further irradiation with nonpolarized
light resulted in a silent CD spectrum, that is, the compound
returned to a racemic state. This modulation was entirely re-
producible in independent experiments and no deterioration
of the modulated CD signal was seen during eight cycles
(Figure 7, inset)
Scheme 3. Schematic representation of a photoinduced molecular brake.
is in the brake OFF state regardless of changes in cavity size
as a result of photoisomerization. Compound 4 can also be
expected to be in the brake OFF state in both the E and Z
isomers, because the fluorine atom in compound 3 is re-
placed with smaller hydrogen atoms in 4.
A study of the transition state of the rotational motion
using space-filling molecular models gives further evidence
for the brake action of compound 2. Rotation of the ben-
zene moiety in (E)-2 is blocked by the aromatic part of the
azobenzene, so the construction of an acceptable model for
the transition state is impossible. In (Z)-2, the rotor part is
accommodated within the macrocycle cavity in the transi-
tion state. Due to the small size of the fluorine atom in com-
pound 3, the rotor part is accommodated within the macro-
cycle cavity in the transition state even for the E isomer,
which allows clear rotation in both the E and Z isomers (see
Figure S14, in the Supporting Information). In comparison
with our previous design, for which we demonstrated a com-
plete ON/OFF switching by tuning the macrocyclic cavity
size with different methylene spacers, here we achieved a
similar ON/OFF switching of the benzene rotor by tuning
the size of the rotor within the macrocyclic cavity with
methylene spacers of fixed length.
Photoresolution by circular polarized light: The main objec-
tive of our study was to examine the possibility of chiral in-
duction of racemic 2 by exposure to CPL. Among the vari-
ous monocyclic azobenzenes synthesized, compound 2 was
found to be most suitable for a photoresolution process in
which irradiation of a racemate with r- or l-CPL will cause a
partial enrichment of one of the enantiomers. Several com-
pounds have been reported to undergo CPL photoresolution
The maximum enantiomeric excess (ee) under l-CPL was
obtained by using Equation (1); it is assumed that the molar
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N. Tamaoki et al.
Conclusion
We have introduced a new class of light-driven molecular
machines, based on macrocycles consisting of a 2,5-disubsti-
tuted-1,4-dioxybenzene rotating unit and a 3,3’-dioxyazoben-
zene photoisomerizable moiety bridged together by fixed
bismethylene spacers. Among the different kinds of cyclic
azobenzenes synthesized, compound 2, with a 2,5-dimethyl-
benzene as rotor, was demonstrated to be a light-controlled
molecular brake in which free rotation of rotor was com-
pletely stopped in the E isomer (brake ON, rotation OFF),
whereas the rotation was allowed in the Z isomer (brake
OFF, rotation ON). Compound 1, with bulkier methoxy sub-
stituents at the 2,5-positions of the benzene rotor showed
high conformational restriction of the free rotation of the
rotor in both the E and Z isomers. Compound 3, with a fluo-
rine-substituted rotor, was observed to be in the brake OFF
state, irrespective of changes to the cavity size as a result of
photoisomerization. Thus, our design strategy showed that
control of rotation of the benzene moiety can be achieved
by appropriate functionalization on the benzene rotor as
well as by photoisomerization of the azobenzene unit. More
interestingly, cyclic azobenezene 2 has been successfully em-
ployed as a chiral sensor that can be used to show enantio-
meric imbalance by detecting a chiral environment, such as
CPL. Upon irradiation of racemate 2 with r- or l-CPL at
488 nm, we were able to repeatedly perform partial enrich-
ment of the S or R enantiomers, respectively. We consider
that this type of simple cyclic azobenzene constitutes a
model that may be use to explain the asymmetric imbalance
of molecules in nature through a new photoresolution path,
that is, enantio-differentiation through photoisomerization
between the ground states of stable R and S enantiomers of
the E form and a rapidly racemizing ground state of the Z
form.
Figure 7. CD spectrum of 2 in ethyl acetate upon irradiation with r- and
l-CPL at 488 nm. The difference in the CD absorption at 450 nm for a so-
lution of 2 upon alternating irradiation with r- and l-CPL and nonpolar-
ized light (n=0–16) with error bars estimated by a standard error of
mean values of independent experiments (inset).
extinction coefficient of the S-E isomer was larger than that
of the R-E and the S-Z isomer was larger than the R-Z
isomer for l-CPL (see the Supporting Information for de-
tailed calculations).
½R ꢀ Eꢁ ꢀ ½S ꢀ Eꢁ 2eZDeE ꢀ 2eEDeZ
ð1Þ
¼
4eEeZ ꢀ DeEDeZ
½R ꢀ Eꢁ þ ½S ꢀ Eꢁ
Experimentally obtained values of 125, 207, 1.72 and
1.44 Lmolꢀ1 cmꢀ1 for eE, eZ, DeE, and DeZ at 488 nm, respec-
tively, were substituted into the equation to calculate the
maximum enantiomeric excess under l-CPL irradiation
[Eq. (2)].[29]
½R ꢀ Eꢁ ꢀ ½S ꢀ Eꢁ
ð2Þ
ꢂ 100 ¼ 0:34%
½R ꢀ Eꢁ þ ½S ꢀ Eꢁ
By using the experimentally obtained value of DeE at
450 nm for the pure enantiomer and the induced CD value
of 0.57 mdeg at 450 nm for the 2.2 mm solution of racemic 2
at PSS488 nm, we calculated the photoinduced ee of the E
isomer to be 0.30%, which is in reasonably good agreement
with the theoretically estimated value.
In our previous bicyclic system, in which photoisomeriza-
tion among the three isomers EE, EZ and ZZ produced a
complex induced CD spectrum with contributions from not
only the EE isomer but also from the EZ isomer, only an in-
complete evaluation of ee was possible. In contrast, com-
pound 2, with its simple E/Z photoisomerization path, made
it possible to clearly show the induction of molecular chirali-
ty in the E isomer, which was observed as an induced CD
band with the corresponding spectral features of pure E
enantiomers.
Experimental Section
All solvents and chemicals were obtained from commercial sources and
used without further purification, unless otherwise stated. NMR (1H and
13C) spectra were recorded with a JEOL ECX 400 spectrometer using
tetramethylsilane as an internal standard. Matrix-assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-TOF MS) was per-
formed with an Applied Biosystems Voyager-DE pro instrument. X-ray
crystallographic data were acquired with a Bruker Smart Apex diffrac-
tometer. Absorption spectra were recorded with an Agilent 8453 spectro-
photometer. CD spectra were recorded with a JASCO J-S720 spectro-
photometer. Photoisomerization studies were conducted using radiation
from a super-high-pressure mercury lamp (500W, USHIO Inc.) after pas-
sage through appropriate filters (366 or 436 nm). High-pressure liquid
chromatography (HPLC) was conducted with a Hitachi Elite La Chrome
HPLC system using a CHIRALPAK IB (DAICEL Chemical Industries
Ltd.) column to separate the enantiomers and analyze the E/Z and enan-
tiomer ratio. Mixtures of ethyl acetate and hexane with ratios of 30:70
(for compound 1 and 3) and 15:85 (for compound 2) were used as eluent
for the HPLC experiments. The E/Z isomer ratios at photostationary
states were determined by NMR analysis or by HPLC monitored at a
common isosbestic point, 273 nm.
7310
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Chem. Eur. J. 2011, 17, 7304 – 7312

Induction of Molecular Chirality
FULL PAPER
General procedure for the synthesis of compounds 1, 2, and 4: A solution
of dinitro compounds 5, 6, or 8 (2 mmol) in anhydrous THF (300 mL)
was added dropwise over 6 h to a solution of LiAlH4 (20 mmol) in anhy-
drous THF (100 mL) under an argon atmosphere. The reaction mixture
was heated to reflux during the addition of the dinitro compound and
was then stirred at RT for 12 h. After cooling the mixture in an ice bath,
the reaction mixture was carefully quenched by the addition of water.
The precipitate formed was filtered off and the solvent was evaporated
under reduced pressure. The residual orange solid was redissolved in
ethyl acetate or dichloromethane, and this solution was washed with
water, brine, and dried over anhydrous MgSO4. The solvent was removed
under reduced pressure and the product was purified by column chroma-
tography on silica gel to afford the desired product.
Compound (E)-1: Yield: 4.2% yield; orange solid; 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.48 (d, J=6.8 Hz, 2H), 7.37 (dd, J=8.0, 8.0 Hz,
2H), 7.13 (d, J=4.2 Hz, 2H), 7.05 (d, J=8.1 Hz, 2H), 6.7 (s, 2H), 4.62
(m, 2H), 4.37–4.52 (m, 6H), 3.69 ppm (s, 6H); 13C NMR (400 MHz,
CDCl3, 258C, TMS): d=159.0, 153.3, 144.7, 142.6, 130.1, 121.1, 115.1,
107.8, 104.5, 68.1, 65.3, 57.0 ppm; MS (MALDI-TOF): m/z calcd for
C24H24N2O6: 437.16 [M+H]+; found: 437.89.
Compound (E)-2: Yield: 3.3%; orange solid; 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.47 (d, J=7.9 Hz, 2H), 7.36 (dd, J=7.9, 7.9 Hz,
2H), 7.03–7.05 (d, J=8.6 Hz, 4H), 6.73 (s, 2H), 4.65 (m, 4H), 4.39–4.44
(m, 4H), 4.32 (m, 2H), 2.01 ppm (s, 6H); 13C NMR (400 MHz, CDCl3,
258C, TMS): d=159.1, 153.2, 150.2, 130.0, 126.4, 120.9, 116.2, 115.5,
107.8, 67.0, 65.1, 16.4 ppm. MS (MALDI-TOF): m/z calcd for
C24H24N2O4: 405.18 [M+H]+; found: 405.13.
Compound (E)-4: Yield: 3.5%; orange solid; 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.49 (d, J=7.8 Hz, 2H), 7.36 (dd, J=7.9, 7.9 Hz,
2H), 7.10–7.03 (m, 4H), 6.87 (s, 4H), 4.52 (t, J=5.6 Hz, 4H), 4.49 ppm
(t, J=5.4 Hz, 4H); 13C NMR (400 MHz, CDCl3, 258C, TMS): d=159.2,
153.7, 152.5, 130.0, 120.9, 117.0, 115.6, 108.5, 66.9, 65.2 ppm.
Compound 7: A mixture of 2-(3-nitrophenoxy)ethanol (1.83 g, 10 mmol),
Zn powder (3.92 g, 60 mmol), and sodium hydroxide (2.4 g, 60 mmol) in
ethanol/water (5:2, 70 mL) was heated to reflux for 20 min (disappear-
ance of red color was observed). The reaction mixture was filtered and
the filtrate was purged with air for 15 min. The filtrate was concentrated,
extracted with ethyl acetate, washed with water, and dried over MgSO4.
The ethyl acetate was evaporated and the resulting orange solid was dis-
solved in hot methanol. Orange plate-like crystals of 7 were obtained
(66% yield) by slow cooling of the solution. 1H NMR (400 MHz,
CD3CN, 258C, TMS): d=7.34–7.30 (m, 6H), 7.25–6.90 (m, 2H), 3.92 (t,
J=4.8 Hz, 4H), 3.63 (t, J=7.7 Hz, 4H), 2.81 ppm (t, J=5.9 Hz, 2H); MS
(MALDI-TOF): m/z calcd for C8H9NO4: 303.13 [M+H]+; found: 303.19.
Compound 9: Diisopropyl azodicarboxylate (40% in toluene, 4.36 g,
21.57 mmol) was added dropwise to a mixture of 3-nitrophenol (2 g,
14.38 mmol), 2-bromoethanol (2.2 g, 17.27 mmol), and PPh3 (5.66 g,
21.57 mmol) in THF (10 mL) at 08C under an argon atmosphere. After
completion of the addition, the reaction mixture was allowed to slowly
warm to RT and stirred overnight. Thereafter, the solvent was evaporat-
ed, the residue was extracted with CH2Cl2, and the combined extracts
were dried over anhydrous MgSO4. The product was purified by column
chromatography on silica gel to obtain 9 (2.5 g, 71%) as a pale-yellow
liquid. 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.24–7.87 (m, 4H),
4.38 (t, J=5.7 Hz, 2H), 3.68 ppm (t, J=5.7 Hz, 2H); 13C NMR
(400 MHz, CDCl3, 258C, TMS): d=158.7, 149.2, 130.3, 121.8, 116.4,
109.2, 68.5, 28.8 ppm; MS (MALDI-TOF): m/z calcd for C8H9BrNO3:
245.97 [M+H]+; found: 245.28.
Induced circular dichroism by CPL irradiation: Prior to every measure-
ment, the CD instrument was purged with nitrogen for at least 20 min
and the temperature was set to 25.08C. The spectra were collected be-
tween 400–550 nm with a standard sensitivity of 100 mdeg, a data pitch of
0.5 nm, a bandwidth of 5 nm, a scanning speed of 20 nmsꢀ1, and a re-
sponse of 2 s using a quartz cuvette (1 cm path length). Baseline correc-
tion and a binomial smoothing was applied to the induced spectral data.
Samples were prepared in ethyl acetate solvent at a concentration of
2.2 mm and reference CD data were collected in the same solvent. The
molecules obeyed the Beer–Lambert law at the monitored wavelength,
suggesting that no molecular aggregation occurred at the given concen-
tration (see the Supporting Information). Direct irradiation of the pure
racemate with nonpolarized light using an Ar+ laser (488 nm) followed
by irradiation through film filters (TCPR or TCPL of MeCan Imaging
Inc. Japan) to produce r- or l-CPL at the same wavelength for about
15 min induced the CD signal. The filters were alternatively changed to
produce r- or l-CPL to assess the reproducibility of the enrichment of the
enantiomers.
Synthesis of 3: Diisopropyl azodicarboxylate (DIAD; 1.62, 8 mmol) was
added dropwise over 7 h to a mixture of 2,5-difluoro-1,4-hydroquinone
(292 mg, 2 mmol), 7 (604 mg, 2 mmol) and triphenylphospine (1.57 g,
6 mmol) in THF (300 mL) maintained at 08C under an argon atmos-
phere. The reaction mixture was slowly brought to RT and stirring was
continued for 3 d. The solvent was evaporated and the residue was direct-
ly purified by chromatography over silica gel (ethyl acetate/hexane, 3:7)
to obtain 3 as an orange solid (8 mg, 0.7% yield). 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.51 (d, J=4.4 Hz, 2H), 7.39 (dd, J=2.5, 7.9 Hz,
2H), 7.05 (m, 4H), 6.86 (dd, J=9.8, 9.8 Hz, 2H), 4.51 (t, J=5.9 Hz, 4H),
4.42 ppm (t, J=5.7 Hz, 4H); 13C NMR (400 MHz, CDCl3, 258C, TMS):
d=158.7, 153.1, 144.1, 141.8, 137.1, 130.1, 121.1, 115.9, 107.9, 68.4,
65.1 ppm; MS (MALDI-TOF): m/z calcd for C22H18N2O4F2: 413.12
[M+H]+; found: 413.10.
Compounds 5 and 6: A mixture of 2,5-dimethoxy or 2,5-dimethyl-1,4-di-
hydroxybenzene (10 mmol), and K2CO3 or NaH (22 mmol) in anhydrous
DMF (20 mL) was stirred at RT for 1 h under an argon atmosphere. A
solution of 9 (22 mmol) in anhydrous DMF (20 mL) was then added
slowly and the mixture was heated to 608C for 12 h. After cooling to RT,
the reaction mixture was added to an excess of water and the formed
precipitate was washed with ethyl acetate and dried under vacuum over-
night. The highly insoluble powder containing 5 or 6 was used for the
next reaction without further purification.
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. Shin-ichiro Noro and Dr. Tsuyoshi Fukaminato, RIES,
Hokkaido University for helping with the X-ray crystallographic analysis.
Compound 5: Yield: 78.9%; pinkish powder; 1H NMR (400 MHz,
[D6]DMSO, 258C, TMS): d=7.76 (d, J=8.0 Hz, 2H), 7.72–7.70 (dd, J=
2.4, 2.4 Hz, 2H), 7.55 (dd, J=8.2, 8.2 Hz, 2H), 7.44 (d, J=8.1 Hz, 2H),
6.72 (s, 2H), 4.38 (t, J=6.0 Hz, 4H), 4.26 (t, J=5.3 Hz, 4H), 3.65 ppm (s,
6H); MS (MALDI-TOF): m/z calcd for C24H24N2O10: 501.15 [M+H]+;
found: 501.79.
Compound 6: Yield: 30%; off-white powder; 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.86 (dd, J=8.1 Hz, 2H), 7.83–7.79 (dd, J=2.3,
2.3 Hz, 2H), 7.44 (dd, J=8.2, 8.2 Hz, 2H), 7.30 (d, J=8.3 Hz, 2H), 6.70
(s, 2H), 4.39 (t, J=4.7 Hz, 4H), 4.30 (t, J=4.7 Hz, 4H), 2.18 ppm (s,
6H); MS (MALDI-TOF): m/z calcd for C24H24N2O8: 469.16 [M+H]+;
found: 469.97.
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Received: December 7, 2010
Published online: May 12, 2011
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