Materials Chemistry
RESEARCH HIGHLIGHTS
One-Dimensional Redox CopperOrganophosphonate by XRD and XANES
Protein motors such as myosin (from skeletal muscle) or kinesin (from brain) are composed of
mats of bipolar 1-dimensional filaments. These filaments will be set into motion either by an
electrochemical reaction or using a chemical signal, corresponding to a reversible contraction/stretched process. To mimic the motions taking place in biological motors is, of course,
very challenging. In principle, an important aspect is that the mimetic nano-filaments should
possess mixed copper(II/I) and phosphate ligands and be self-assembled by much weaker π-π
interactions. It is therefore our interests to explore the kind of supramolecular filaments that
may be reversibly tuned simply by inserting and extracting ion species through an applied voltage. Such biomimetic concepts could lead to a new class of the battery-like actuations.
To perform the functionality of supramolecular machines, we propose that the following
requirements shall be satisfied: (i) construction of a
one-dimensional supramolecular chain structure
with redox metal centres, especially mixeded
copper (II, I) centres, (ii) introduction of metal-ions
intercalation into the crystal at an arbitrary position,
and (iii) introduction of crystal volume change
through an applied voltage. Here we present a new
supramolecular actuator, 1DOP-Cu, synthesized
under hydrothermal condition. The structure of
1DOP-Cu is determined by single crystal X-ray
diffraction. Figure 1 shows the struc-tural packing
of 1DOP-Cu which is similar to natural musclefibers; the monoclinic structure reveals 1D inorganic infinite zigzag chains, [– Cu(II) – O3P–
(CH2)2 – PO3 – Cu(II) –], at which metal centers are
incorporated the phen ligands. The macroscopic
sheet actuators are composed of mats of individual
nanofiber bundles joined by mechani-cal entanglement and π-π interaction acting along the
(a)
crystallographic a-axis.
The X-ray diffraction (XRD) patterns of
1DOP-Cu, both with and without Li insertion, are
presented in Figure 2. It is worth noting that the
distance between layers (d002) remains constant
whereas the dimension of the unit cell in the
corrugated sheets changes, which is caused by the
uneven stress along the a and b axes. The variation
of the strain ∆a/ao and ∆b/bo is 3.8 % and 6.2 %,
respectively. The larger strain in the b-axis direction
during actuation could be due to the contributions
from the weakness of π-π repulsions between
chains. It clearly demonstrates that the whole
thickness of the sheet increase due to the strong
deformation of the redox chain structure when it is
charged negatively.
The deformation mechanism of supramolecular chain is investigated by solid-state nuclear
magnetic resonance (SSNMR) and X-ray
absorption spectroscopy to elucidate the Liinserted state of 1DOP-Cu. SSNMR appears that
(b)
b
Fig. 1: (a) An ORTEP plot of a section of an infinite chain in 1DOP-Cu. (b) Space-filling view of the ab plane.
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Materials Chemistry
(2 0 0)
(0 2 1)
(0 0 2)
RESEARCH HIGHLIGHTS
1DOP-Cu
(2 0 0)
(0 4 0)
I
Li@1DOP-Cu
Fig. 4: XANES spectra of 1DOP-Cu and the Li@1DOP-Cu.
10
20
30
40
2-Theta
Fig. 2: The XRD patterns (synchrotron radiation, λ =
1.32633 Å) of 1DOP-Cu and the Li@1DOP-Cu. ( 1DOP-Cu :
monoclinic, space group P2(1)/n, a0 = 9.1579(18) Å, b0 =
30.916(6) Å, c0 = 12.882(3) Å, β0 = 110.52(3)；V0 =
3415.7(12) Å3, Z = 4, F(000) = 1864, σ = 1.796 Mg/m-3,
2θmax = 55o；R1 = 0.0778, wR2(F2) = 0.2586, and GOF =
1.086; Li@1DOP-Cu still has a monoclinic structure with
the lattice constants a = 9.51 , b = 32.84, c = 12.88 Å, β
= 110.5；V = 3767 Å3)
the narrow line-width (full width at half height, ~
400 H) offers a conclusive proof for lithium ion
motion in a crystalline 1DOP-Cu. It is appropriate
to look into the molecular structure of the Liinserted state of 1DOP-Cu. Considering that the
participation of Li-ion coordination environments
in reversible charge-discharge reaction takes place
at the bonding orbitals of Cu – O – P – O;
evidence for the partial electronic reduction of
Cu(II) was provided by 31P-{1H} MAS (magic
angle spinning) NMR data (Figure 3). Owing to
the paramagnetic characteristic of the bonding
orbital of Cu(II, d9) – O – P – O, 31P NMR of
1DOP-Cu reveals a significant down-field chemical shift with associated spinning sidebands in
the range of 200 – 1000 ppm, whereas a new
chemical shift of 31 ppm is significant in the Liinserted state of Cu(I, d10) – O – P – O … Li+.
Fig. 3: Monitoring the redox process in solid actuator by
31
P-{1H} MAS NMR.
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Finally, it is necessary to examine whether
the coordination geometry of Cu(II) or Cu(I) is
changed as well in the redox process, a key concept
of artificial machine postulated by Salvage. X-ray
absorption near edge spectra (XANES) is a powerful probe for characterizing the ligand field geometry, electronic structure, and oxidation state of the
metal centers. The synchrotron radiation XANES
spectra for Li (de)insertion states are shown in
Figure 4. A comparison of the XANES spectra
reveals that the Li-inserted state has a shoulder or
partially-resolved peak on the rising edge at
8985.4 eV in addition to 1s → 3d pre-edge
transition at 8991.8 eV, suggesting different environments in the geometry around the metal ions. It
is observed that only a pre-edge bump due to 1s →
3d transition exists in XANES spectra of squarepyramidal copper-based 1DOP-Cu. The pronounced
shoulder on the low energy side of the edge was
assigned to 1s → 4pz transition in accordance with
the destabilization of 4pz metal orbital, where z is
the elongation axis. This suggests distortion from a
square-pyramidal to a square planar environment.
It is reasonable to assume that 1DOP-Cu could
afford electrochemically muscle-like machines,
within basal planes allowing the conversion of the
5-coordinate situation (Cu(II)) to 4-coordinate
binding mode (Cu(I)) and vice versa.
In summary, the reduction of Cu(II) provides
an approach for studying the actuating mechanism
of natural molecular motor. We have demonstrated
that XRD and XANES can be applied to determine
the strong deformation of chain structure with redox
metal Cu (I, II).
Materials Chemistry
RESEARCH HIGHLIGHTS
BEAMLINES
17A1 Wiggler beamline
17C1 Wiggler beamline
EXPERIMENTAL STATIONS
X-ray Powder Diffraction end station
X-ray Absorption Spectroscopy end station
AUTHOR
K. J. Lin
Department of Chemistry, National Chung Hsing
University, Taichung, Taiwan
PUBLICATIONS
‧ C. Y. Cheng, S. J. Fu, C. J. Yang, W. H. Chen,
K. J. Lin, G. H. Lee, and Y. Wang, Angew. Chem.
Int. Edit., 42, 1937 (2003).
‧ L. L. Li, C. J. Yang, W. H. Chen, and K. J. Lin,
Angew. Chem. Int. Edit., 43, 1825 (2003).
‧ S. J. Fu, C. Y. Cheng, W. H. Chen, K. J. Lin, and
H. M. Kao, Angew. Chem. Int. Edit. 43, 4186
(2004).
‧ L. J. Chen, M. R. Prasad, C. Y. Cheng, and
K. J. Lin, Adv. Mater. (2004), in press.
CONTACT E-MAIL
[email protected]
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