Thermally Induced Intra-Carboxyl Proton Shuttle in a Molecular Rack-and-Pinion Cascade Achieving Macroscopic Crystal Deformation

Article abstract of DOI:10.1002/anie.201607886

Proton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra-carboxyl proton shuttle can be generated in a molecular assembly akin to a rack-and-pinion cascade via a thermally induced single-crystal-to-single-crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4′-azopyridine (azpy) molecule connects to two biphenyl-3,3′,5,5′-tetracarboxylic acid (H₄BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack-and-pinion cascade, which mainly involves 1) an intra-carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H₄BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.

Full text of DOI:10.1002/anie.201607886

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International Edition: DOI: 10.1002/anie.201607886
German Edition: DOI: 10.1002/ange.201607886
Molecular Machines
Thermally Induced Intra-Carboxyl Proton Shuttle in a Molecular Rack-
and-Pinion Cascade Achieving Macroscopic Crystal Deformation
You-Gui Huang, Yoshihito Shiota, Sheng-Qun Su, Shu-Qi Wu, Zi-Shuo Yao, Guo-Ling Li,
Shinji Kanegawa, Soonchul Kang, Takashi Kamachi, Kazunari Yoshizawa, Katsuhiko Ariga,
and Osamu Sato*
Abstract: Proton transport via dynamic molecules is ubiqui-
tous in chemistry and biology. However, its use as a switching
mechanism for properties in functional molecular assemblies is
far less common. In this study, we demonstrate how an intra-
carboxyl proton shuttle can be generated in a molecular
assembly akin to a rack-and-pinion cascade via a thermally
induced single-crystal-to-single-crystal phase transition. In
a triply interpenetrated supramolecular organic framework
(SOF), a 4,4’-azopyridine (azpy) molecule connects to two
biphenyl-3,3’,5,5’-tetracarboxylic acid (H4BPTC) molecules to
form a functional molecular system with switchable mechan-
ical properties. A temperature change reversibly triggers
a molecular movement akin to a rack-and-pinion cascade,
which mainly involves 1) an intra-carboxyl proton shuttle
coupled with tilting of the azo molecules and azo pedal motion
and 2) H4BPTC translation. Moreover, both the molecular
motions are collective, and being propagated across the entire
framework, leading to a macroscopic crystal expansion and
contraction.
tems.[13–17] For example, the coordination-coupled proton
relay process that is important in histidine residues[18,19] has
been replicated in some types of hydrazine-based rotary
switches.[14,15] Moreover, carboxyl groups can ionize under the
right conditions, releasing the proton from the hydroxy group
with the negative charge “flip-flopping” back and forth
between the two oxygen atoms. Reversely, the ionized
carboxyl groups can be protonated with the assistance of
a stronger acid.[20] This characteristic makes it possible to
initiate an intra-carboxyl proton shuttle using a dynamic
molecule.
Therefore, we synthesized a supramolecular organic
framework (SOF) comprising of 4,4’-azopyridine (azpy) and
biphenyl-3,3’,5,5’-tetracarboxylic acid (H4BPTC) to generate
a molecular assembly akin to a rack-and-pinion cascade.
H4BPTC can form hydrogen bonds with azpy molecules.
Moreover, ethylene and azo groups are both well-known to
undergo pedal motion,[21–24] which may result in fine modu-
lation of the electronic structures of ethylene or azo-based
molecules.[25] The ethylene group has been used to build
a molecular machine akin to rack-and-pinion gears.[21] There-
fore, we envisaged that the pyridine moiety in azpy may serve
as a proton acceptor and facilitate proton transfer in the
carboxyl group.
L
ong-distance proton transport is ubiquitous in chemistry
and biology[1–4] and plays a crucial role in a variety of chemical
and biological activities including viral replication,[5] proton
pumping through membrane protein channels,[6] and the
removal of acetyl moieties from histone tails.[7] For example,
proton conduction in influenza M2 proton channels is
regulated by a proton-shuttling process that is dynamically
assisted by an imidazolium ring flip; this process represents
a key step in virus replication.[8] Proton transport in these
systems is facilitated by hydrogen-bonded networks that serve
as proton conduits through which the proton relay via the
Grotthuss and/or vehicle mechanisms.[9–12] Although replicat-
ing the functions of these biological systems in synthetic
materials is extremely challenging, great achievements have
been accomplished by building prototypes that can mimic the
complicated ion-transport pathways of biological sys-
Herein, we show that an SOF comprising azpy and
H4BPTC molecules undergoes a thermally induced intra-
carboxyl proton shuttle in a single-crystal to single-crystal
manner. All molecular motions in the phase transition are
collective and are propagated throughout the single crystal,
leading to macroscopic crystal expansion and contraction. To
the best of our knowledge, this is the first example of
breathing motion in a wine-rack-type framework resulting
from
a single-crystal to single-crystal phase transition,
wherein proton shuttling via a dynamic molecule plays
a key role.
The supramolecular framework [(H4BPTC)(azpy)2]n (1)
was obtained via cocrystallization of H4BPTC and azpy in
methanol. Single-crystal X-ray crystallography at 333 K
revealed that compound 1 crystallizes in the centrosymmetric
space group C2/c (Tables S1 and S2 in the Supporting
Information) with a triply-interpenetrated supramolecular
framework structure (Figure S1b). The asymmetric unit
contains an azpy molecule and half a H4BPTC molecule
(Figure 1b). The azo group of the azpy molecule is disordered
over two sites with occupancies of 0.80 and 0.20, and the
populations of the two conformers change with temperature,
implying dynamic disorder (site occupancies found at 363 K:
0.75 and 0.25)[26] (Table S3). Therefore, the azpy molecules
[*] Dr. Y.-G. Huang, Y. Shiota, S.-Q. Su, S.-Q. Wu, Z.-S. Yao, G.-L. Li,
S. Kanegawa, S. Kang, T. Kamachi, K. Yoshizawa, Prof. O. Sato
Institute for Materials Chemistry and Engineering, Kyushu University
744 Motooka Nishi-ku, Fukuoka, 819-0395 (Japan)
E-mail: sato@cm.kyushu-u.ac.jp
Dr. Y.-G. Huang, Prof. K. Ariga
World Premier International (WPI) Center for Materials Nanoarch-
itectonics (MANA), Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki, 305-0044 (Japan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607886.
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Figure 1. The asymmetric unit of a) the LT phase and b) the HT phase
of compound 1. ORTEP drawing at 30% probability. The green sphere
Figure 3. a,c) Structural presentation of the deformation of the rhom-
bic grid in compound 1 in the phase transition. b) Crystal photographs
showing significant crystal expansion or contraction in the phase
transition.
represents the transported proton in the phase transition.
undergo a conformation interconversion via azo-pedal-
motion in this phase of the crystal. The dihedral angles
between the two phenyl rings in the H4BPTC molecule (f)
and the two pyridyl rings in the azpy molecule (a) are 32.28
and 13.28 (Table S4), respectively. Each H4BPTC connects to
its four neighbors by azpy by two kinds of hydrogen bonds (O-
H···N: 2.713–2.792 ꢀ) forming a (4,4) two-dimensional (2D)
structure (Tables S5 and S6). A rhombic grid with a hinged
angle q of 79.78, which is formed by four H4BPTC and four
azpy molecules, is the basic building unit in the 2D structure
(Figures 2b and 3c). This grid is akin to a framework with
Thermogravimetric analysis (TGA) showed that com-
pound 1 decomposes at ca. 440 K (Figure S3). Differential
scanning calorimetry (DSC) curves showed a multistep
reversible phase transition at a temperature far lower than
the decomposition point (Figure 4b). The reversible phase
Figure 4. a) PXRD pattern evolution of compound 1 showing the
reversible phase transition, b) DSC curves of compound 1 showing the
reversible multistep phase transition.
transition is supported by variable-temperature powder X-ray
diffraction (PXRD) and IR spectra (Figures 4a and S4). In
the PXRD patterns, the peaks indexed to [112], [51À1], and
[51À2] exhibited significant and reversible shifts near the
phase transition temperature (Figure 4a). To further eluci-
date the structural dynamics of compound 1 during the
reversible phase transition, variable-temperature crystal
structure determinations were performed in the range 123–
363 K. The unit cell parameters undergo an abrupt change at
the phase transition temperature (Figures S2a, S2b). During
the phase transition, thermal expansion along the a and b axes
was negative, an exceptionally large positive thermal expan-
sion occurred along the c axis. This exceptionally large
anisotropic thermal expansion can be rationalized by system-
atically comparing the crystal structures at 333 K and 273 K.
The structure of compound 1 was found to transition to
a low-temperature (LT) phase below 273 K; this phase
belongs to the noncentrosymmetric space group Cc
(Tables S1 and S2), implying that a symmetry breaking
process had occurred (Figure S5). In comparison with the
high-temperature (HT) phase, the asymmetric unit of the LT
phase is doubled, containing one H4BPTC and two azpy
molecules denoted as azpyi and azpyii, respectively (Fig-
ure 1a). The structure of the LT phase is also a triply
interpenetrated supramolecular framework based on rhombic
Figure 2. a,b) The structural reorganization of the rhombic grid in
compound 1 during the phase transition. c) Schematic of the rack-and-
pinion cascade movement achieving the intra-carboxyl proton shuttle
in compound 1. The green spheres represent the transported protons,
and the minor conformers of azo groups in the HT phase are
highlighted in yellow.
a “wine-rack” topology.[27] In the rhombic grid, the azpy
molecules act as the sides and bridge the H4BPTC molecules
as vertices with an angle of 109.88 to the shorter diagonal of
the rhombic grid (d) (Table S4 and Figures 3c, S2d). Because
of the large cavities present in each single framework, three
identical frameworks triply interpenetrate parallely to form
a dense framework, in which each rhombic grid crosses its
eight neighbors from the other two single frameworks twice
(Figure S1b).
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grids (Figures 2a, S1a, S6), but the geometries of the rhombic
Furthermore, it is interesting that intra-carboxyl proton
grids in the two phases are quite different. In response to the
temperature change, the structural reorganizations of the
rhombic grid primarily proceeded through two motions:
1) intra-carboxyl proton shuttle coupled with tilting of azo
molecules and azo-pedal-motion, and 2) H4BPTC translation
(Figure 2). Accompanying these motions, the configurations
of both H4BPTC and azpy changed slightly (Figures S2c and
S7). These molecular motions lead to a symmetry breaking
(i.e., the total symmetry decreases from four symmetry
operations (E, i, C2, sh) to two (E, sh)). The 21-fold axis and
the inversion center i disappear in the LT phase (Figure S5).
The phase transition occurs in a manner of reversible single-
crystal to single-crystal with an Aizu notation of 2/mFm.[28]
Structural determinations were performed on a single crystal
of compound 1 heated from 123 K to 363 K and then cooled
back to 123 K. The crystallographic data quality of the LT
phase was not affected by the thermal cycle (Tables S1 and
S2).
shuttle is achieved by the phase transition. As shown in
Figure 2, azpyi is hydrogen bonded to O1 and O6 in the HT
phase, while it is hydrogen bonded to O2 and O5 in the LT
phase. This implies that intra-carboxyl electron-coupled
proton transfer occurred during the phase transition, pro-
vided that the carboxylic groups are protonated in both
phases. Although a difference Fourier analysis yielded
nominal H-atom positions, both near O atoms (Figures S8–
S13), the electron density of H atoms is usually too small to
allow an accurate determination of their positions. Never-
theless, an alternative means of determining whether a car-
boxylic group is protonated or not is to scrutinize its local p
molecular geometry.[31] It is well known that deprotonation of
a carboxylic group leads to delocalization in the p-electron
=
À
system such that the C O ( ꢀ 1.21 ꢀ) and C O ( ꢀ 1.31 ꢀ)
[32,33]
=
bonds change to two identical C O bonds ( ꢀ 1.25 ꢀ).
À
The C O bond lengths can be obtained with sufficient
accuracy to discriminate between the protonated and depro-
tonated carboxylic groups in case that no structural disorder
exists in the carboxylic groups. As shown in Table 1, both
carboxylic groups (C7-O1-O2 and C16-O5-O6) that hydrogen
bond to azpyi show characteristics of protonation in both
phases, and each single and double bond “pair” interchanged
before and after the phase transition. These characterizations
indicate that the hydrogen bonds between azpyi and H4BPTC
are neutral O-H···N hydrogen bonds in both phases. Protons
reside on the O1 and O6 atoms in the HT phase and on the O2
and O5 atoms in the LT phase.
As shown in Figures 2 and 3, in the rhombic grid of the HT
phase, all four of the azpy molecules are symmetry related
and their azo groups are undergoing pedal motion. After the
phase transition, the disorder disappeared, implying that the
=
azo pedal motion was switched off in the LT phase. The N N
bonds of the major conformers of azpyi and azpyii are
approximately parallel with center-to-center distances of
=
18.19 ꢀ in the HT phase, while in the LT phase, the N N
bonds are in a criss-crossed arrangement with a twist angle of
59.68 and a center-to-center distance of 14.83 ꢀ. Because of
the phase transition, the C2 symmetry related azpyi and azpyii
molecules present in the HT phase became crystallographi-
cally independent in the LT phase. To accommodate this
movement, the configurations of both azpyi and azpyii slightly
changed with the dihedral angles (a and g) between the
pyridyl rings changing from a common value of 13.28 to 16.88
and 7.58, respectively (Figure S2c and Table S4).
Table 1: Bond lengths [ꢀ] of carboxyl groups in H4BPTC for compound
1 LT phase at 273 K and compound 1 HT phase at 333 K.
Bond (LT phase)
À
À
C7 O1
À
C7 O2
1.199(6)
1.327(6)
1.306(6)
1.214(6)
C15 O7
1.303(6)
1.214(6)
1.301(6)
1.216(6)
À
C15 O8
À
À
C8 O3
C16 O5
In addition to the on-off switching of azo-pedal-motion,
the azpyi molecules were also significantly reoriented in the
plane of the rhombic grid, with the angle (d) changing from
109.88 at 333 K to 116.68 at 273 K (Figures 3, S2d, and
Table S4). The H4BPTC molecules underwent a translation
along the diagonals of the rhombic grid (Figures 2 and 3). As
a result, the rhombic grid “breathed” like a wine-rack in
response to the temperature change. From 333 K to 273 K, the
longer diagonal expanded by 1.52% resulting in the ca.
1.53% expansion of the b axis and the shorter diagonal
contracted by 3.48% resulting in the ca. 3.21% contraction of
the c axis (Figures 3 and S2a). The large anisotropic thermal
expansion is comparable to those of some other molecular
crystals that show significant crystal deformation in their
phase transitions.[29,30] Accompanying the translation, the
phenyl rings in the H4BPTC molecules rotated slightly, with
the angle f increasing from 32.28 at 333 K to 35.08 at 273 K
(Figure S2c and Table S4). The dihedral angles between the
carboxylic acid groups of H4BPTC and their parent phenyl
rings also changed from 9.78, 9.78, 14.78, and 14.78 at 333 K to
3.18, 13.78, 15.38, and 17.68 at 273 K, respectively. The LT
phase shows a large anisotropic thermal expansion in the
range 123–313 K (Table S7).
À
À
C8 O4
C16 O6
Bond (HT phase)[a]
À
À
C7 O1
À
C7 O2
1.301(5)
C15 O7
1.301(5)
1.201(5)
1.221(5)
1.295(5)
À
1.201(5)
1.295(5)
1.221(5)
C15 O8
À
À
C8 O3
C16 O5
À
À
C8 O4
C16 O6
[a] The atoms are labeled in accordance with those in the LTphase. Intra-
carboxyl proton shuttles occurred in the C7-O1-O2 and C16-O5-O6 two
carboxylic acid groups during the phase transition.
In order to get insights into the phase transition, density
functional theory (DFT) calculations were performed at the
B3LYP/6-31G** level of theory. The energy barriers for the
molecular motions which may occur in the phase transition
were calculated. Two possible pathways for intra-carboxyl
proton transfer, i.e., 1) direct proton hopping between two
carboxyl O atoms and 2) proton relay via the pyridyl N atom
of azpy, were compared (Figure S14). The direct proton-
hopping pathway has a very high activation barrier (33 kcal
molÀ1) (Tables S14 and S15). In contrast, the activation
barrier for the proton relay pathway is much smaller at
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18 kcalmolÀ1 (Tables S16 and S17). Therefore, for the phase
transition in compound 1, the proton relay pathway is
energetically preferred. The presence of the azpy with pyridyl
N atom facilitates the intra-carboxyl proton transfer. Based
on the results of the DFT calculations, we may propose
a reasonable pathway as shown in Figure S15 (Tables S18–
S22). Proton migration coupled with tilting of azo molecules
and azo-pedal-motion occurred in the phase transition with
transition states of 18 kcalmolÀ1, and on–off switching of azo-
pedal motion was achieved. Notably, the pedal motion of
monitored by a microscope equipped with a heating-and-
cooling stage.
single crystal ( ꢀ 0.05 mm ꢁ 0.30 mm ꢁ
A
1.65 mm) contracted or expanded about 4% along its long
axis (Figure 3b) during cooling or heating in the range 273–
333 K. The magnitude of the expansion is consistent with the
change observed crystallographically in the c axis. Further-
more, the crystal exhibited no evidence of appreciable fatigue
after five cycles (Figure S22). The reversible expansion and
contraction processes of a single crystal up to 1.82 mm in
length in the range 123–333 K were recorded on two videos
(Supporting Information).
neutral azpy involves an energy barrier of 9.9 kcalmolÀ1
,
while that of diprotonated azpy (H2azpy2+) is only 6.7 kcal
molÀ1 (Figure S16 and Tables S10–S13). This result suggests
that ionization of the carboxyl groups can kinetically assist the
azo pedal motion.
In conclusion, an intra-carboxyl proton shuttle, which is
facilitated by the presence of azo molecules with pyridine
ring, has been observed for the first time in the thermally
induced, multistep single-crystal-to-single-crystal phase tran-
sition of a supramolecular framework. The phase transition
involves proton migration, which is coupled with azo-pedal
motion, a change in orientation of azo molecules, and
displacement of H4BPTC molecules. All the motions are
collective and are propagated throughout the framework,
leading to the macroscopic expansion and contraction of the
crystal. These observations may provide some valuable
insights into the design of materials that can replicate the
functions of biological systems. The macroscopic change in
the crystal shape harnessed by molecular movements prom-
ises a wide range of potential applications in the production of
thermal actuators and artificial muscle.
We successfully synthesized compound 1-d4 [(D4BPTC)-
(azpy)2]n which is isostructural to the HT phase of compound
1 (Table S9 and Figures S16, S17). For compound 1-d4, single-
crystal X-ray crystallography and DSC showed no phase
transition in the range 123–393 K (Table S8 and Figures S19,
S20). This substantial isotope effect implies that the protons
of the carboxyl groups do play an important role in the phase
transition of compound 1, hence supporting the proposed
proton relay mechanism,[34–36] The populations of the two azpy
conformers in compound 1-d4 also vary with temperature
(Table S3), which further indicates the dynamic disorder of
azpy observed in the HT phase of compound 1. The pedal
motion of neutral azpy molecules in compound 1-d4 was
switched off at 123 K much lower than that in compound
1 (Figure S21). Therefore, the phase transition in compound
1 enormously raised the azo-pedal-motion off temperature
from 123 K to 313 K. Unlike the LT phase of compound 1, all
of the three crystallographic axes of compound 1-d4 show
positive thermal expansion in the range 123–373 K (Table S9
and Figure S19).
Acknowledgements
Y.-G.H. thanks the Japan Society for the Promotion of
Science for a postdoctoral fellowship (No. P13033). This work
was supported by a JSPS KAKENHI Grant, number
16H00843.
Although azo-pedal-motion, molecular tilting, and the
breathing of the wine-rack-type grids are not rare in the
crystal structure transformations,[27,29,37–39] in this case, all of
the three movements have been observed in the same crystal.
In addition, intra-carboxyl proton shuttle was realized by the
molecular movements. Moreover, the molecular aggregate of
azpyi with its two adjacent H4BPTC molecules can be
considered as an analog to a rack-and-pinion cascade at the
molecular level. In this molecular assembly, an azpyi molecule
with two transported protons acts as the pinion cascade that
experiences rotation movement. Because of the hydrogen
bonds between azpyi and H4BPTC, the rotation of the cascade
pinion gear supports a correlated translation of two H4BPTC
rack gears, resulting in the conversion of rotary motion to
linear motion (Figure 2c).
Keywords: crystal deformation · molecular machine ·
phase transitions · proton shuttle
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Received: August 12, 2016
Revised: September 19, 2016
Published online: && &&, &&&&
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Molecular Machines
Y.-G. Huang, Y. Shiota, S.-Q. Su, S.-Q. Wu,
Z.-S. Yao, G.-L. Li, S. Kanegawa, S. Kang,
T. Kamachi, K. Yoshizawa, K. Ariga,
O. Sato*
&&&& — &&&&
Thermally Induced Intra-Carboxyl Proton
Shuttle in a Molecular Rack-and-Pinion
Cascade Achieving Macroscopic Crystal
Deformation
Mechanics in miniature: An intra-carbox-
yl proton shuttle coupled with the collec-
tive motion of constituent molecules has
been observed in a single-crystal-to-
tling process upon heating and cooling
leads to a reversible macroscopic crystal
deformation (see picture; azpy=4,4’-
azopyridine, H4BPTC=biphenyl-3,3’,5,5’-
tetracarboxylic acid).
single-crystal phase transition. The shut-
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!