Stimuli-responsive folding and unfolding of a polymer bearing multiple cerium(IV) bis(porphyrinate) joints: Mechano-imitation of the action of a folding ruler

Article abstract of DOI:10.1002/anie.201205584

A pivotal guest role: A new porphyrin polymer, poly(PorZn·DD) (see picture, pink/purple), composed of a porphyrinatozinc and a porphyrin double-decker complex as a repeating unit was synthesized. In poly(PorZn·DD), porphyrinatozinc complexes recognize a divalent amine (tan/red) to induce an intramolecular pivoting motion through the rotation of porphyrin double-decker complexes and the polymer undergoes shortening and compaction.

Full text of DOI:10.1002/anie.201205584

Angewandte
Chemie
DOI: 10.1002/anie.201205584
Supramolecular Device
Stimuli-Responsive Folding and Unfolding of a Polymer Bearing
Multiple Cerium(IV) Bis(porphyrinate) Joints: Mechano-imitation of
the Action of a Folding Ruler**
Masayuki Shibata, Satoshi Tanaka, Tomohiro Ikeda, Seiji Shinkai,* Kenji Kaneko, Soichiro Ogi,
and Masayuki Takeuchi*
The design of molecular machines, in which the properties
and functions are controlled by photo-, redox-, and host–
guest-type interactions, has attracted growing attention.[1,2]
Interest has been focused on synthetic actuating molecules
that exhibit contraction/expansion motion, because a typical
biological molecular machine, myosin, can directly convert
chemical energy derived from ATP hydrolysis into mechan-
ical motion with extremely high efficiency. In a synthetic
system, it has been recently demonstrated that a sliding
motion,[3,4] a spring-like motion,[5] and a conformational
change[6] of synthetic molecules induce the contraction/
expansion of their structures triggered by external stimuli.
Challenges still remain in the design and synthesis of a new
molecule toward organic mechano-responsive materials that
deliberately undergo structural changes in length. It occurred
to us that the rational combination of a pivoting motion,[2e,7]
like a “folding ruler”, and a stimuli responsive moiety enables
us to construct a new class of actuating molecules.
lent amine as a modulator to induce an intramolecular
pivoting motion through the rotation of cerium(IV) bis(por-
phyrinate) complexes[8] that act as a joint and deliberately
undergo structural changes in length (Scheme 1). Further-
more, we successfully monitored and visualized the reversible
morphological changes that show folding and unfolding by
transmission electron microscopy (TEM).
Poly(PorZn·DD) was synthesized by the reaction of 1 with
tris(acetylacetonato) cerium(III) trihydrate in 1,2,4-trichloro-
benzene under reflux conditions, as shown in Scheme 2; in
this reaction, peripheral porphyrinatozinc moieties react
intermolecularly to give poly(PorZn·DD). During the reac-
tion, besides oligo- and poly(PorZn·DD), the intramolecu-
larly cyclized compound C1 was also formed; we hypothe-
sized that C1 would be an intermediate of this polymerization
We report herein the synthesis and dynamic guest
response of a porphyrin polymer, poly(PorZn·DD), com-
prised of a porphyrinatozinc and a cerium(IV) bis(porphyri-
nate) double-decker complex as a repeating unit. In poly-
(PorZn·DD), porphyrinatozinc complexes recognize a diva-
[*] Dr. T. Ikeda, Dr. S. Ogi, Prof. Dr. M. Takeuchi
Organic Materials Group,
National Institute for Materials Science (NIMS)
1-2-1 Sengen, Tsukuba 305-0047 (Japan)
E-mail: takeuchi.masayuki@nims.go.jp
Homepage: http://www.nims.go.jp/macromol
M. Shibata, S. Tanaka
Department of Chemistry and Biochemistry,
Kyushu University (Japan)
Prof. S. Shinkai
Institute for Advanced Study, Kyushu University
Fukuoka 819-0395 (Japan)
E-mail: shinkai_center@mail.cstm.kyushu-u.ac.jp
Prof. S. Shinkai
Institute of Systems, Information Technologies and Nanotechnol-
ogies (ISIT)
Fukuoka 819-0385 (Japan)
Prof. K. Kaneko
HVEM Laboratory, Kyushu University (Japan)
Scheme 1. Schematic illustration of the folding action of a polymer
with rotatable joints and recognition sites for a modulator; cerium(IV)
bis(porphyrinate) is the joint and porphyrinatozinc is the modulator
recognition site. Each porphyrin ligand in the cerium(IV) bis(porphyri-
nate)-based rotor undergoes a stepping rotation. Pivoting motion
induced by the modulator binding gives rise to changes in the length
of the polymer.
[**] This study was supported partially by a Grant-in-Aid for Scientifc
Research for Priority Area “Coodination Programming” (area 2107)
and “Innovative Nanobio” (area 521) to M.T. from MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205584.
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similarity between the UV/Vis spectrum of
poly(PorZn·DD) and the additive sum of por-
phyrinatozinc and the double-decker complex
indicates that intra- and interpolymer stacking
of the porphyrin complexes does not occur in
this system. To appraise the recognition-driven
conformational change, we evaluated the for-
mation of the poly(PorZn·DD) with divalent
amine guest in chloroform using UV/Vis spec-
tral changes that occurred upon the successive
addition of a guest molecule. We chose 1,4-
diazabicyclo[2.2.2]octane (DABCO) as a guest
because the distance between two zinc metals in
the sandwiched DABCO·(porphyrinatozinc)2
was reported to be 7.0 ꢀ,[9] which is slightly
longer than the distance (5.1 ꢀ) of meso-aryls in
a cerium(IV) bis(porphyrinate) (Supporting
Information, Figure S1); a slight difference
would be negligible owing to the flexible
propylene linkage inserted between porphyri-
Scheme 2. Synthesis of poly(PorZn·DD).
natozinc and cerium(IV) bis(porphyrinate).
Given such a distance, we deduced that the
successive DABCO binding induces the folded
reaction, however purified C1 did not undergo further
reaction to yield oligo- and poly(PorZn·DD). We thus
purified the reaction mixture using gel permeation chroma-
tography (GPC) to remove the low molecular weight fraction,
and we obtained poly(PorZn·DD) with number-averaged
molecular weight (Mn) of 27000, calculated using a poly-
(styrene) standard (found a 10-mer on average for the
poly(PorZn·DD)).
The Soret band of porphrinatozinc in poly(PorZn·DD) in
chloroform appeared at 425.5 nm, which is identical to that of
1. No band attributable to p–p stacking among porphyrin
complexes in poly(PorZn·DD) was observed. In addition, the
geometry of poly(PorZn·DD). Upon addition of DABCO, the
lmax values of the Soret and Q bands of the porphyrinatozinc
moiety shifted to longer wavelengths with tight isosbestic
points at 424.0 nm and 557.0 nm, whereas no spectral changes
in the Soret band (397.0 nm) of cerium(IV) bis(porphyrinate)
was observed (Figure 1 and Figure S2); these changes are
consistent with those observed from studies of other porphyr-
inatozinc–amine coordination systems.[10] We estimated the
stoichiometry of the complex formed between DABCO and
PorZn·DD (PorZn·DD = 20 mm) from molar ratio plots,
which clearly indicated the formation of a 1:2 DABCO/
PorZn·DD complex and did not convert to a 1:1 complex, at
Figure 1. a) UV/Vis spectral changes of poly(PorZn·DD) upon addition of DABCO (0–28 mm) in chloroform at 258C ([PorZn·DD] =20 mm, path
length is 1 mm). Inset: A Plot of the absorbance at 561.5 nm of poly(PorZn·DD) versus [DABCO]/[PorZn·DD]. b) Schematic illustration of the
poly(PorZn·DD)·DABCO complex. c) Hydrodynamic diameters of poly(PorZn·DD) with (red) and without (blue) DABCO analyzed by dynamic light
scattering. The contracted form (calculated to be 7ꢀ6ꢀ1.6 nm in size, see Figure S7) retains some rotational freedom; we infer this would be one
of the reasons why the average size did not change greatly.
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least up to [DABCO] = 30 mm (even at a lower concentration
of [PorZn·DD] = 1.0 mm corresponding to [poly-
(PorZn·DD)·DABCO composites, which had a height of
1.6 nm (Figure 2b); this unimolecular height strongly sup-
ports the view that the folding processes by the DABCO
binding take place in solution to afford the dot morphologies.
Electron microscopic images of poly(PorZn·DD) present
information regarding how poly(PorZn·DD) is folded and
contracted by DABCO. As for the UV/Vis spectroscopy and
DLS profile, we prepared a solution of poly(PorZn·DD)
([PorZn·DD] = 1.0 mm) and a solution with the addition of
DABCO ([DABCO] = 0.5 mm); we then obtained solution-
dip films on a TEM grid without staining. The samples were
subjected to TEM. First, we confirmed that poly(PorZn·DD)
forms fibrous morphologies that are approximately 10–20 nm
in width and several hundred nanometers in length (Figure 3a
and Figure S7); this is probably because bundling of poly-
(PorZn·DD) occurs during the solution-dip process. Thin
fibers can be seen faintly with a length of 50–100 nm on the
TEM grid, however the contrast is not strong enough to
confirm the width. Interestingly, when DABCO was added to
the solution of poly(PorZn·DD) to form poly-
(PorZn·DD)·DABCO, the fibrous morphology assigned to
poly(PorZn·DD) disappeared and we only observed dot
structures having dimensions of 10–20 nm (Figure 3b). Fur-
thermore, an energy dispersive X-ray spectroscopic (EDX)
study of poly(PorZn·DD)·DABCO revealed that the dot
structure indeed contains cerium (Figure S6). Because the
size of the dot morphologies is slightly larger than the
calculated structure (Figure S7), we deduce that DABCO
binding mainly takes place intramolecularly, however, some
cross-linking and partial ladder formation could also occur, as
shown in Figure 3c. Addition of acid to poly-
(PorZn·DD)·DABCO should induce dissociation of the
DABCO because of the formation of DABCO·2H+; upon
protonation with trifluoroacetic acid (TFA), the lmax value of
(PorZn·DD)] = ca. 0.1 mm). A plot of the absorbance at
425.5 nm versus [DABCO]/[PorZn·DD] shows a linear rela-
tionship and is saturated at the ratio of 0.5 (logKass is assumed
to be greater than seven), suggesting a 1:2 stoichiometry of
DABCO and PorZn·DD (Figure S3). Neither piperidine nor
1,3-di(4-piperidyl)propane, which do not fit the folding
geometry, showed such high affinities toward poly-
(PorZn·DD). The high association of DABCO suggests that
the binding of DABCO to poly(PorZn·DD) most likely
occurs intramolecularly, although interpolymer cross-linking
or ladder formation[11] of polymers cannot be fully ruled out.
We deduce, however, that full ladder formation rarely
happens because it is difficult to construct a structural
model for the ladder between DABCO and two poly-
(PorZn·DD)s owing to the steric hindrance between the
DD moieties within a polymer chain.
The folding structures of poly(PorZn·DD) induced by
DABCO binding in solution were further investigated using
dynamic light scattering (DLS), atomic force microscopy
(AFM), and TEM. The average size of poly(PorZn·DD) in
chloroform at 258C was measured to be approximately 12 nm
in chloroform ([PorZn·DD] = 20 mm) with a relatively large
distribution, probably owing to its polymer structure having
intrinsically rotatable joints (Figure 1c). Upon DABCO
addition ([DABCO] = 28 mm), we still observed an average
size of ca. 12 nm, however, the size dispersity decreased, and
this is consistent with the contraction of poly(PorZn·DD).
Judging from the average size of 12 nm in the presence of
DABCO, we would assume there is cross-linking or partial
ladder formation of the polymers. Figure 2 shows an AFM
image of poly(PorZn·DD) spin-coated from a 1.0 mm solution
in chloroform, in which the linear- or granular-shaped poly-
(PorZn·DD) assembly is well dispersed on highly ordered
pyrolytic graphite (HOPG), reflecting the larger distribution
of the DLS profile. From the height profile we calculate the
average height to be 1.4 nm; poly(PorZn·DD) probably lies
on HOPG in a monolayer having an edge-on orientation. By
mixing with DABCO ([DABCO] = 0.5 mm), instead, we only
observed the dot or granular shape for the poly-
the
Soret
band
for
porphrinatozinc
in
poly-
Figure 3. Electron micrographs (no staining) of a) poly(PorZn·DD),
b) poly(PorZn·DD)·DABCO, and d) poly(PorZn·DD)·DABCO after addi-
tion of TFA. c) Schematic illustration of the folding and unfolding
action of poly(PorZn·DD) upon addition of DABCO and then TFA.
Figure 2. AFM images of poly(PorZn·DD) on HOPG spin-coated from
a) a chloroform solution of poly(PorZn·DD) and b) a chloroform
solution of a 1:2 mixture of DABCO and PorZn·DD in 2ꢀ2 mm.
Angew. Chem. Int. Ed. 2013, 52, 397 –400
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(PorZn·DD)·DABCO shifts to a shorter wavelength of
425.5 nm with the same isosbestic points as those observed
for the complexation process with DABCO (Figure S5).
Eventually, this TFA treatment results in a reversal to
uncomplexed poly(PorZn·DD), without demetallation of
the porphyrinatozinc or decomposition of the cerium(IV)
bis(porphyrinate). Figure 3d shows a TEM image of the TFA-
treated sample (10 equiv. to DABCO) in which once again the
fibrous morphologies were the dominant feature. This
confirms that DABCO association and dissociation induce
the folding and unfolding actions of poly(PorZn·DD) and the
resultant morphological changes.
In conclusion, we have successfully synthesized a new
porphyrin polymer bearing porphyrinatozinc as the DABCO
recognition site and cerium(IV) bis(porphyrinate) as the
rotatable joints; successive DABCO binding to poly-
(PorZn·DD) modulates the distance between the two por-
phyrinatozinc moities and leads to the folded and contracted
conformation. Furthermore, we visualized the morphological
changes reflecting folded and unfolded structures by means of
AFM and TEM. Such action resembles the pivoting motion of
a folding ruler. Further efforts towards installing photo-
responsive or redox moieties in addition to this molecular-
recognition-based folding process could lead to the gener-
ation of a new class of actuating materials. Work along these
lines is in progress.
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Experimental Section
All starting materials and solvents were purchased from Tokyo Kasei
1
Chemicals or Wako Chemicals and used as received. The H NMR
spectra were recorded on a Brucker DRX 600 (600 MHz) spectrom-
eter. Chemical shifts are reported in ppm downfield from tetrame-
thylsilane as the internal standard. Mass spectral data were obtained
using a Perceptive Voyager RP MALDI-TOF mass spectrometer and/
or a JEOL JMS HX110A high-resolution magnetic sector FAB mass
spectrometer. UV/Vis spectra were recorded using Shimadzu UV-
2500 PC spectrophotometer. Compound 1 and poly(PorZn·DD) were
synthesized according to Scheme S1 and S2 in the Supporting
Information. TEM images were acquired using a TECNAI-20 FEI
(accelerating voltage: 200 kV). A sample solution was placed on
a copper TEM grid upon a carbon support film. The TEM grid was
dried under reduced pressure for 6 h prior to TEM observation.
Received: July 14, 2012
Revised: September 10, 2012
Published online: November 14, 2012
Keywords: folding · molecular machinery ·
.
molecular recognition · porphyrinoids
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