A self-locking molecule operative with a photoresponsive key

Article abstract of DOI:10.1021/ja0632308

Rotary host 1, composed of a ferrocene unit as a rotary module, is conformationally locked internally in apolar solvents such as benzene by a double intramolecular Zn-N coordination between the zinc porphyrin and aniline units, attached to each cyclopenta

Full text of DOI:10.1021/ja0632308

Published on Web 08/16/2006
A Self-Locking Molecule Operative with a
Photoresponsive Key
Takahiro Muraoka,† Kazushi Kinbara,*,‡ and Takuzo Aida*
Contribution from Department of Chemistry and Biotechnology, School of Engineering, and
Center for NanoBio Integration, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan, and PRESTO, Japan Science Technology Agency (JST), 4-1-8 Honcho,
Kawaguchi, Saitama 332-0012, Japan
Received May 9, 2006; E-mail: kinbara@macro.t.u-tokyo.ac.jp; aida@macro.t.u-tokyo.ac.jp
Abstract: Rotary host 1, composed of a ferrocene unit as a rotary module, is conformationally locked
internally in apolar solvents such as benzene by a double intramolecular Zn-N coordination between the
zinc porphyrin and aniline units, attached to each cyclopentadienyl (Cp) ring. Upon addition of the cis form
of 1,2-bispyridylethylene (cis-2) to (+)-1 ([cis-2]/[(+)-1] ) 5.0), an enantiomer of 1, the intramolecular Zn-N
coordination bonds in (+)-1 are readily cleaved to form an externally locked, cyclodimeric one-to-one complex
(+)-1⊃cis-2, accompanying a rotation of the ferrocene module, as visualized by CD spectroscopy. In contrast,
use of trans-2, in place of cis-2, under otherwise identical conditions to the above, did not result in releasing
the internal double lock of (+)-1. Such a large difference between the isomers of 2 in the affinity toward
host 1, along with their capabilities of photochemical interconversion, allowed for the demonstration of a
reversible self-locking operation of 1. Namely, the externally locked state of 1, as in the form of 1⊃cis-2,
spontaneously retrieves the internally locked state, after the release of 2 from 1 upon cis-to-trans
photochemical isomerization of ligating 2, while the backward photochemical isomerization of 2 in the
presence of 1 results in switching of 1 to its externally locked state.
Introduction
undergoes a conformational change and wraps around the
autoinhibitory segment to allow its dissociation from the active
Self-locking systems are useful for fail-safe operations and
utilized for a variety of equipments in our daily life. An essential
feature of self-locking systems is that they are normally locked
and kept dormant, while they can be unlocked with proper keys
whenever necessary. Furthermore, when the keys are released,
self-locking systems automatically retrieve the locked states.
Such self-locking mechanisms are also seen in biological
systems, especially in signal transduction pathways.1 For
example, Ca2+/calmodulin-dependent protein kinase II (CaM-
kinase II) is the representative of self-locking enzymes.2 In its
resting state, the active site is hybridized with an autoinhibitory
segment and sterically protected from the access of substrates.
On the other hand, once cofactor calmodulin binds Ca2+, it
site. Hence, the enzyme is activated to start phosphorylation.
However, when Ca2+ is dissociated from calmodulin, the
autoinhibitory segment is liberated from calmodulin and allowed
to reassemble with the active site of the enzyme. Consequently,
CaM-kinase II automatically returns to its resting state. Thus,
in this sequential event, calmodulin is the key to switch on and
off the enzymatic activity of self-locking CaM-kinase II, while
Ca2+ serves to activate the “calmodulin” key to change the
priority of the two competing interactions involving the auto-
inhibitory segment.
In the present paper, we report a novel artificial self-locking
system, designed by combining rotary host 1 (Figure 1) carrying
internal locking units with photochromic 1,2-bispyridylethylene
2 as an external key.3 The self-locking operation of 1 was
demonstrated by a photochemical priority change of competing
intra- and intermolecular interactions (Scheme 1).4
† The University of Tokyo.
‡ PRESTO, JST.
(1) (a) Introduction to Cellular Signal Transduction; Ari, S., Ed.; Birkhauser:
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Cell 2000, 103, 211-225. (d) Pawson, T.; Nash, P. Genes DeV. 2000, 14,
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J. AM. CHEM. SOC. 2006, 128, 11600-11605
10.1021/ja0632308 CCC: $33.50 © 2006 American Chemical Society

Self-Locking Molecule Operative with Photoresponsive Key
A R T I C L E S
cis-2 in 1⊃cis-2 is isomerized into its trans form, resultant
trans-2 detaches from rotary host 1, thereby disabling the
external locking. Consequently, 1 spontaneously retrieves, via
a rotary motion, the internally double-locked state. Thus,
compound 2 is regarded as a photoresponsive key for executing
the self-locking operation of 1. Rotary host 1 has a planar
chirality at the ferrocene module, and use of the enantiomers
of 1 could enable chiroptical visualization of its conformational
motions in response to the photochemical isomerization of 2.9
Synthesis and Conformational Aspects of Host (+)-1.
Dibromoferrocene derivative 3 having two aniline units,9 in a
racemic form, was subjected to preparative chiral HPLC on a
column Chiralpak IA. One of the enantiomers of 3, eluted
slower, was allowed to react with a zinc porphyrin boronate
(see Experimental Section for details), and resulting compound
1 was unambiguously characterized by means of 1H NMR
spectroscopy and MALDI-TOF-MS spectrometry, along with
electronic absorption, circular dichroism (CD), and infrared (IR)
spectroscopies. Host 1 displayed absorption bands at 280-390
nm in benzene due to the ferrocene module, along with intense
visible absorption bands at 433.5 (Soret band), 563.0, and 605.0
nm (Q-bands) due to the zinc porphyrin units. Host 1 is
enantiomerically denoted as (+)-1, as it showed intense positive
CD bands at 353.4 and 436.8 nm, along with a weak negative
CD band at 303.4 nm (see Supporting Information, Figure S1).
Figure 1. Molecular model of internally double-locked 1. Red and yellow
parts denote aromatic and amino groups of the aniline units, respectively.
Pink parts represent zinc atoms coordinating with the aniline amino groups.
The iron atom of the ferrocene module is represented in purple. Hydrogen
atoms are omitted for clarity. The structure was optimized by a molecular
mechanics calculation with an MMFF force field using Spartan’02.
Results and Discussion
Host 1 is composed of a ferrocene unit as a rotary module,5
which bears zinc porphyrin and aniline units at each cyclopen-
tadienyl (Cp) ring. The rotary motion of 1 allows the aniline
groups to come closer to the zinc porphyrin units, forming two
Zn-N coordination bonds simultaneously.6 Although the in-
teraction between zinc porphyrins and aniline is rather weak,
such a double intramolecular Zn-N coordination is strong
enough to lock the rotary motion of 1 (internal double lock)
(Figure 1). Thus, host 1 is self-locked in appropriate apolar
solvents such as benzene. On the other hand, compound 2 bears
two pyridine units, capable of coordinating to the zinc porphyrin
units in 1, in competition with the internal aniline groups. This
compound is photochromic, where UV irradiation allows for
its trans-to-cis isomerization,7 while visible light irradiation in
the presence of triplet sensitizers such as zinc porphyrins allows
for the backward isomerization.8 We found that rotary host 1 is
kept self-locked conformationally when mixed with, e.g., 5 equiv
of compound 2 adopting a trans configuration (trans-2), since
1 hardly interacts with trans-2 under the conditions employed
(Scheme 1, top right). However, when 2 is photochemically
isomerized into its cis form, 1 turns to accommodate resultant
cis-2 at its binding site, to form stable cyclodimeric 1⊃cis-2
having two zinc-pyridyl coordination bonds (Scheme 1, left).
Namely, the internal double lock is released, and 1 is trans-
formed into an externally locked state. On the other hand, when
Rotary host (+)-1 in benzene displayed absorptions in the
Soret and Q-band regions at 433.5, 563.0, and 605.0 nm, which
are obviously red shifted from those of reference compound 4
without aniline units (424.5, 522.0, and 591.0 nm; see Sup-
porting Information, Figure S2). This spectral profile is attribut-
able to an intramolecular coordination between the zinc
porphyrin and aniline units.6 Upon incremental increase of the
concentration from 10-7 to 10-4 M, neither the electronic
absorption nor the CD bands of (+)-1 exhibited any shifts, but
they were monotonically enhanced, following the Lambert-
1
Beer’s law (Figure 2). In the H NMR spectrum of (+)-1 in
benzene-d6, a characteristic signal due to aniline NH2 appeared
at δ -2.89 ppm,10 indicating that the aniline groups coordinate
to the zinc porphyrin units and are magnetically shielded (Figure
3).6 Considering the molecular structure, these spectral profiles
clearly indicate that 1 is in a self-locked state by adopting a
head-to-tail geometry with the two intramolecular Zn-N
coordination bonds (internal double lock; Figure 1).
(5) Gardner, A. B.; Howard, J.; Waddington, T. C.; Richardson, R. M.;
Tomkinson, J. Chem. Phys. 1981, 57, 453-460.
(6) (a) Borovkov, V. V.; Hembury, G. A.; Inoue, Y. Acc. Chem. Res. 2004,
37, 449-459. (b) Huang, X.; Fujioka, N.; Pescitelli, G.; Koehn, F. E.;
Williamson, R. T.; Nakanishi, K.; Berova, N. J. Am. Chem. Soc. 2002,
124, 10320-10335.
(7) Itokazu, M. K.; Polo, A. S.; Arau´jo de Faria, D. L.; Bignozzi, C. A.;
Murakami Iha, N. Y. Inorg. Chim. Acta 2001, 313, 149-155.
(8) (a) Iseki, Y.; Watanabe, E.; Mori, A.; Inoue, S. J. Am. Chem. Soc. 1993,
115, 7313-7317. (b) Whitten, D. G.; Wildes, P. D.; DeRosier, C. A. J.
Am. Chem. Soc. 1972, 94, 7811-7823. (c) Whitten, D. G.; McCall, M. T.
J. Am. Chem. Soc. 1969, 91, 5097-5103.
Complexation Behaviors of (+)-1 with trans- and cis-2.
According to CPK model studies, trans-2 adopts an extended
(9) (a) Muraoka, T.; Kinbara, K.; Aida, T. Nature 2006, 440, 512-515. (b)
Muraoka. T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003,
125, 5612-5613.
(10) This signal was not observed when the solvent contains D2O.
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A R T I C L E S
Muraoka et al.
Scheme 1. Molecular Structures of Internally Double-Locked 1 and Externally Locked 1⊃cis-2, and Schematic Representation of the
Self-locking Operation in Response to Photochemical Isomerization of 2a
a Ar groups are omitted for clarity.
geometry, while cis-2 adopts an angular geometry. Interestingly,
we found that these two isomeric forms show quite different
affinities toward host 1, where cis-2 is much favored over trans-
2. Upon titration with cis-2 at 20 °C in benzene, electronic
absorption spectroscopy of (+)-1 (2.4 × 10-6 M) displayed a
blue shift in the Soret absorption band from 433.5 to 428.0 nm
with a clear isosbestic point at 430.0 nm. This spectral change
appeared to take place monotonically and then saturated at a
[cis-2]/[(+)-1] mole ratio of 7.0 (see Supporting Information,
Figure S3). The Job’s plot, upon mixing (+)-1 with cis-2 in
benzene, showed an absorption maximum at a [cis-2]/[(+)-1]
mole ratio of unity (Figure 4a). Considering the monotonic
spectral change in the titration experiment, we conclude that
(+)-1 and cis-2 form a cyclodimeric one-to-one complex (+)-
1⊃cis-2 with only a negligibly small spectral contribution, if
any, of its acyclic intermediate. From these results, the associa-
tion constant Kassoc of (+)-1 with cis-2 was evaluated as 7.5 ×
105 M-1 (see Supporting Information, Figure S4a), which is
roughly 2 orders of magnitude greater than that of (+)-1 with
pyridine (see Supporting Information, Figures S4b and S5). Such
a large Kassoc value is obviously due to the two-point complex-
ation between (+)-1 and cis-2 (Figure 4, top). Namely, the
Figure 3. 1H NMR spectrum of (+)-1 in C6D6 at 20 °C. Red and blue
arrows represent the signals due to the amino and aromatic protons of the
aniline groups, respectively. Inset: An enlarged spectrum at the amino
proton region.
Figure 2. Concentration dependencies of the circular dichroism (CD)
intensity (b, solid line, Y1) and absorbance (9, broken line, Y2) of (+)-1
at 352.0 nm in benzene at 20 °C.
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Figure 5. Circular dichroism (CD) spectral changes of (+)-1 in benzene
(2.5 × 10-6 M) at 20 °C upon titration with trans-2 ([trans-2]/[(+)-1] )
(a) 0, 5, 10, 40, 100, (b) 200, 400, 600, 1000, 1400, and 3000). Arrows
indicate the directions of spectral changes.
Figure 4. (a) Job’s plot upon mixing (+)-1 with cis-2 in benzene at 20
°C. (b) Circular dichoism (CD) spectral change of (+)-1 in benzene (2.4 ×
10-6 M) at 20 °C upon titration with cis-2 ([cis-2]/[(+)-1] ) 0.0, 0.12,
0.23, 0.46, 0.70, 0.93, 1.4, 1.9, 2.8, 3.7, and 7.0). Arrows indicate the
directions of spectral changes.
Supporting Information, Figure S6). Although this absorption
spectral change appeared to be monotonic, the accompanying
CD spectral change (Figure 5), though moderate, displayed a
rather complicated dependence on the guest/host ratio and
essentially different from that observed with cis-2 as the guest
(Figure 4b). With [trans-2]/[(+)-1] up to 100, the CD spectrum
changed most explicitly at the absorption bands of the zinc
porphyrin and ferrocene units, exhibiting isosbestic points at
402.6 and 438.8 nm (Figure 5a). While such a CD spectral
change once appeared to subside with [trans-2]/[(+)-1] in a
range from 100 to 200, further incremental increase of the guest/
host ratio again resulted in a clear spectral change (isosbestic
point ) 433.6 nm) but only at the zinc porphyrin absorption
(Figure 5b). When the guest/host ratio was greater than 3000,
the CD spectrum of (+)-1 no longer changed. Such a compli-
cated CD spectral change likely originates from the extended
geometry of trans-2 capable of forming, upon interaction with
1, macrocyclic and linear polymeric structures, along with 1:1
and 2:1 acyclic guest/host complexes. For the monotonic change
in Figure S6, these species are likely indistinguishable from one
another by absorption spectroscopy. As expected, titration of
internally double-locked state of 1 can be unlocked by the
coordination with cis-2. We also found that the absorption
spectral change in the titration experiment (Figure S3) is
accompanied by a large, monotonic CD spectral change at the
absorption bands of the zinc porphyrin and ferrocene units,
exhibiting isosbestic points at 318.6 and 431.6 nm (Figure 4b).
The CD spectral change at the absorption band of the ferrocene
unit is indicative of its rotary motion accompanied by the
coordination with cis-2 (Figure 4, top). On the other hand, the
appearance of the marked split Cotton effect at the Soret
absorption band of (+)-1⊃cis-2 (415-455 nm) indicates that
the two zinc porphyrin units in (+)-1 are forced, by ligation
with cis-2, to come closer to one another and undergo exciton
coupling.6
In sharp contrast, when trans-2 was used in place of cis-2,
(+)-1 displayed only a subtle absorption spectral change upon
addition of even 200 equiv of trans-2 with respect to (+)-1.
Nevertheless, at a closer look, the spectral change took place
little by little until the guest/host ratio reached 3000 (see
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Muraoka et al.
(+)-1 with pyridine gave rather simple absorption and CD
spectral changes, where the final spectra obtained were identical
to those of (+)-1 in the presence of a large excess (e.g., 3000
equiv) of trans-2 (see Supporting Information, Figure S5).
Nevertheless, for the objective of the present study, one of the
most valuable results is the much higher affinity of cis-2 than
trans-2 toward 1, suggesting that the self-locking and unlocking
of (+)-1 are reversibly operative by photochemical isomerization
of 2 (Scheme 1).
Motion of (+)-1 upon Photoisomerization of 2. Considering
a rather poor ability of trans-2 in photochemical isomerization,11
along with the relative binding affinity of the isomeric forms
of 2 toward 1, we investigated a possibility of photoresponsive
conformational change of (+)-1 (4.2 × 10-6 M) in the presence
of 5.0 equiv of 2 with respect to (+)-1. In benzene at 20 °C,
the mixture showed, in addition to an absorption band due to
trans-2 at 300 nm, characteristic visible absorption bands due
to the zinc porphyrin units of (+)-1, which are virtually identical
to those in the absence of trans-2 (see Supporting Information,
Figure S7). Upon irradiation at 323 ( 10 nm to allow a trans-
to-cis isomerization of 2, the benzene solution displayed a
monotonic decrease in intensity of the 300-nm absorption band.7
At the same time, the Soret absorption band of (+)-1 showed
a blue shift from 433.0 to 428.0 nm with an isosbestic point at
430.0 nm (Figure 6a). This spectral change subsided in 90 min.
As expected, the absorption spectral change thus observed was
accompanied by a clear CD spectral change (Figure 6b), where
the CD band due to (+)-1 in the Soret absorption region was
significantly enhanced and turned to show a split Cotton effect.
Even more importantly, the CD band centered at 350 nm due
to the ferrocene module of (+)-1 exhibited an inversion of its
sign from positive to negative, indicating a rotary motion of
the ferrocene module. These spectral changes are quite analo-
gous to those observed for (+)-1 upon titration with cis-2
(Figures 4b and S3). Therefore, the cis form of 2 is generated
upon photoirradiation of trans-2 and incorporated into (+)-1
to form one-to-one complex (+)-1⊃cis-2. As observed by
HPLC, 34 mol % of trans-2 (1.7 equiv with respect to (+)-1)
isomerized after the 90-min photoirradiation. Taking into
account the association constant of cis-2 toward (+)-1, the yield
of (+)-1⊃cis-2 was estimated as 93 mol %.
Upon exposure of the isomerized mixture to visible light (λ
> 420 nm) at 20 °C, an absorption spectral change opposite to
the above took place to reach a plateau in 20 min (see
Supporting Information, Figure S8). In particular, the appearance
of the absorption band at 300 nm due to trans-2 clearly indicates
the occurrence of a backward cis-to-trans photoisomerization
of 2. Accordingly, the split Cotton effect at the Soret absorption
band of the zinc porphyrin units disappeared with a reversal of
the CD sense at 350 nm due to the ferrocene module (Figure
6c). HPLC analysis of the reaction mixture confirmed the
presence of trans-2 with only a trace amount of cis-2 (<1 mol-
%). As already described, this photochemical isomerization upon
irradiation at λ > 420 nm is most likely sensitized by the zinc
porphyrin units in host 1 as a triplet photosensitizer.8 In fact,
cis-2 itself did not isomerize upon direct photoexcitation but
isomerized into its trans form in the presence of zinc tetraphe-
nylporphyrin upon visible light irradiation. Hence, in our system,
Figure 6. (a) Electronic absorption and (b) circular dichroism (CD) spectral
changes of a mixture of (+)-1 and trans-2 ([(+)-1] ) 4.2 × 10-6 M; [trans-
2]/[(+)-1] ) 5.0) in benzene at 20 °C upon irradiation with UV light (λ )
323 ( 10 nm) for 5.0, 10, 15, 20, 30, 40, 50, 60, 70, and 90 min. (c) CD
spectral change of a mixture of (+)-1 and 2 ([(+)-1] ) 4.2 × 10-6 M;
[2]/[(+)-1] ) 5.0) in benzene at 20 °C upon irradiation with visible light
(λ > 420 nm) for 0.2, 0.5, 1.0, 2.0, 5.0, 10, and 20 min after 90-min UV
irradiation in (b). Arrows indicate the directions of spectral changes.
a tight, two-point complexation between (+)-1 and cis-2 in the
form of (+)-1⊃cis-2 is considered to play an important role in
the efficient photoisomerization of cis-2.
From all the above observations, it is obvious that, at [2]/[1]
) 5.0, the internal lock of 1 cannot be unlocked by trans-2
(11) In the absence of 1 in benzene at 20 °C, irradiation of trans-2 (3.4 × 10-5
M) with UV light (λ ) 323 ( 10 nm) resulted in a photostationary state
in 7 h to furnish the ratio [cis-2]/[trans-2] of 87/13.
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residues as internal standards. Matrix-assisted laser desorption/ionization
time-of-flight mass (MALDI-TOF-MS) spectrometry was performed
with a dithranol as a matrix on an Applied Biosystems BioSpectrometry
Workstation model Voyager-DE STR spectrometer. Electronic absorp-
tion and infrared spectra were recorded on JASCO type U-best 560
and FT/IR-610 spectrometers, respectively. Circular dichroism (CD)
spectra were recorded on a JASCO type J-720 spectropolarimeter.
Methods. Photoirradiation was carried out at 20 °C on degassed
benzene solutions of samples in a 10-mm thick quartz cell under Ar,
using a 150-W xenon arc lamp with a band-pass filter (Kenko) of λ )
323 ( 10 nm for UV irradiation or a cut filter (Kenko) of λ > 420 nm
for visible light irradiation. Titration experiments were conducted at
20 °C on benzene solutions of samples in a 10-mm thick quartz cell.
Concentration dependencies of electronic absorption and CD spectra
of benzene solutions of (+)-1 were evaluated using 1, 10, and 50-mm
thick quartz cells.
Synthesis of (+)-1.15 Water (5 mL) was added to a toluene solution
(20 mL) of a mixture of the zinc complex of 5-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-10,15,20-tri(4-methyl-phenyl)porphyrin16 (157
mg, 2.0 × 10-4 mol), an enantiomer of 37 (41 mg, 4.6 × 10-5 mol),
Pd(PPh3)4 (16 mg, 1.4 × 10-5 mol), and Cs2CO3 (60 mg, 1.8 × 10-4
mol), and the resulting mixture was degassed by freeze-pump-thaw
cycles and then refluxed under Ar for 10 h in the dark. Then, water
(50 mL) was added to the reaction mixture, and the aqueous phase
that separated was extracted with CH2Cl2 (3 × 100 mL). The combined
organic extract was dried over anhydrous Na2SO4 and evaporated to
dryness under reduced pressure. The powdery residue was chromato-
graphed on Bio-Beads with toluene as an eluent, followed by silica
gel with CH2Cl2 as an eluent, to allow isolation of 1 as purple solid in
22% yield (20 mg, 9.98 × 10-6 mol). IR (KBr; cm-1): 3421, 2958,
2925, 2858, 1726, 1599, 1460, 1379, 1338, 1274, 1122, 1072, 997,
796, 719. 1H NMR (270.05 MHz; CDCl3; 20 °C; ppm): δ -2.36 (br,
4H), 1.24 (br, 2H), 2.48 (br, 2H), 2.66 (s, 12H), 2.71 (s, 6H), 5.01 (br,
2H), 5.12 (br, 2H), 5.53 (br, 2H), 5.87 (br, 2H), 6.14 (br, 2H), 7.40-
7.50 (m, 16H), 7.54 (d, 8H, J ) 7.7 Hz), 7.92 (br, 8H), 8.03 (d, 4H, J
) 7.7 Hz), 8.11 (d, 4H, J ) 7.3 Hz), 8.77 (br, 4H), 8.81 (d, 4H, J )
4.5 Hz), 8.88 (d, 4H, J ) 4.5 Hz), 8.98 (br, 4H). MALDI-TOF-MS
(dithranol): m/z 2005 ([M + H+] calcd: 2005). UV-vis (benzene):
λmax 433.5, 563.0, 605.0 nm.
incapable of two-point complexation with 1, but it is readily
unlocked and fixed into an externally locked state (1⊃cis-2)
via a rotary motion, when trans-2 is photoisomerized into cis-
2. On the other hand, when cis-2 in 1⊃cis-2 is isomerized back
into the trans form, the release of 2 from 1 takes place, thereby
allowing 1 to spontaneously retrieve its internally locked state.
Therefore, host 1 may be called a self-locking molecule, whose
operation is switched on and off reversibly by using guest 2 as
a photoresponsive key.
Conclusions
By the combination with bispyridylethylene 2 as a photore-
sponsive key, we have demonstrated chiroptical visualization
of the self-locking operation of a ferrocene-based chiral rotary
host 1. In this system, the intramolecular interaction (internal
lock), operative between the zinc porphyrin and aniline units,
is designed to compete with intermolecular interactions with
photochromic 2. Since the affinity of cis-2 toward 1 is much
higher than that of trans-2, the priority of these two competing
interactions can be switched in response to the photochemical
isomerization of 2. Namely, the self-locking operation of 1 can
be executed by the isomerization of cis-2 in 1⊃cis-2 into trans-
2. When compared with the self-locking operation of a Ca2+
/
calmodulin-dependent kinase II system, where calmodulin serves
as the key in response to Ca2+ as the switching stimuli, our
artificial system utilizes ultraviolet and visible light for enabling
and disabling the unlocking function of 2 for self-locked 1,
respectively. While such a self-locking operation has not been
focused yet in the design of artificial molecular machin-
ery,3,4,9,12,13 we believe that this conception may open a new
door to stimuli-responsive molecular devices.
Experimental Section
Materials. Toluene, Pd(PPh3)4, and Cs2CO3 were purchased from
Kanto Kagaku, Tokyo Chemical Industry, and Wako Chemicals,
respectively, and used as received. trans-2 was purchased from Aldrich
and recrystallized from hexane. cis-2 was synthesized according to the
literature method.14 For column chromatography, Wakogel C-300HG
(particle size 40-60 µm, silica) and Bio-Beads S-X3 (BIO RAD) were
used.
Measurements. 1H NMR spectra were recorded on JEOL type GSX-
270 and GSX-500 spectrometers, where chemical shifts were deter-
mined with respect to nondeuterated or partially deuterated solvent
Acknowledgment. T.M. thanks the JSPS Young Scientist
Fellowship. This research was partially supported by the
Ministry of Education, Science, Sports and Culture, Grant-in-
Aid for Scientific Research on Priority Areas “Life Surveyors”
(to K.K.).
Supporting Information Available: Synthesis of 5-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-10,15,20-tri(4-methyl-phe-
nyl)porphyrin and 4. Absorption spectrum of 4. Absorption
spectra, titration data, and curve fitting profiles of (+)-1 upon
titration with cis-2 and pyridine. This material is available free
of charge via the Internet at http://pubs.acs.org.
(12) Selected reviews of molecular machinery: (a) Kinbara, K.; Aida, T. Chem.
ReV. 2005, 105, 1377-1400. (b) Kelly, T. R. Acc. Chem. Res. 2001, 34,
514-522. (c) Collin, J.-P.; Dietrich-Buchecker, C.; Gavin˜a, P.; Jimenez-
Molero, M. C.; Sauvage, J.-P. Acc. Chem. Res. 2001, 34, 477-487. (d)
Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int.
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J. V.; Kay, E. R.; Leigh, D. A. Science 2004, 306, 1532-1537. (c) Badjic,
J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F. Science 2004, 304,
1308-1312. (d) Thordarson, P.; Bijsterveld, E. J. A.; Rowan, A. E.; Nolte,
R. J. M. Nature 2003, 424, 915-918. (e) Ishii, D.; Kinbara, K.; Ishida, Y.;
Ishii, N.; Okochi, M.; Yohda, M.; Aida, T. Nature 2003, 423, 628-632.
(14) Whitten, D. G.; McCall, M. T. Tetrahedron Lett. 1968, 9, 2755-2758.
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(15) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
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