Towards Rational Modulation of In-Plane Molecular Arrangements in Metal-Organic Framework Nanosheets

Article abstract of DOI:10.1002/cplu.201402150

A strategy to modulate the in-plane structural arrangement in preferentially oriented crystalline metal-organic framework (MOF) nanosheets assembled by a two-dimensional interfacial reaction between porphyrin units and metal ion linkers is reported. Start

Full text of DOI:10.1002/cplu.201402150

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DOI: 10.1002/cplu.201402150
Towards Rational Modulation of In-Plane Molecular
Arrangements in Metal–Organic Framework Nanosheets
Rie Makiura,*[a, b] Ryo Usui,[a] Yuta Sakai,[c] Akihiro Nomoto,[c] Akiya Ogawa,[c] Osami Sakata,[d]
and Akihiko Fujiwara[e]
A strategy to modulate the in-plane structural arrangement in
preferentially oriented crystalline metal–organic framework
(MOF) nanosheets assembled by a two-dimensional interfacial
reaction between porphyrin units and metal ion linkers is re-
ported. Starting with a tetratopic porphyrin MOF nanofilm,
NAFS-2, the framework size and shape are modified by em-
ploying specially designed building units, a trans-ditopic and
an expanded tetratopic porphyrin, and Cu2+ linkers. Reducing
the number of binding parts affords a MOF nanosheet, NAFS-
31, with a distorted in-plane structure. Extension of the periph-
eral substituents, while maintaining the tetratopic porphyrin
geometry, results in marked unit cell size enlargement in an
undistorted square grid in the MOF nanofilm, NAFS-41. The ex-
quisite geometric control that these structural modifications
entail is valuable to allow switching of chemical/physical prop-
erties of the nanosheets and lead to realization of their use in
nanotechnological applications.
Introduction
Much effort has been spent in creating nanomaterials with 2D
regularity[1–7] because of their anticipated unique chemical/
physical properties, as well as the ever-increasing need to scale
down the components of nanodevices. The 2D nanosheets of
various materials, including graphite,[2] diamond-like carbon,[3]
metal oxides,[4] transition-metal chalcogenides,[5] coordination
polymers,[6,7] organic polymers,[8] and their hybrids,[9] have
been prepared and shown to exhibit special properties such as
high charge mobility,[2] ultrafast permeation of organic sol-
vents,[3] superior insulating capacity,[4] and high H2 production
by photocatalytic activity.[5] Most of them were obtained by ex-
foliation of their bulk states. Another rational way to create or-
dered 2D structures with nanometer-scale precision is a direct
bottom-up approach that utilizes self-assembly processes of
building components. Applying the appropriate choice of
building units together with a suitable connection manner,
such
as
covalent/coordination/hydrogen
bonds
and
p–p/van der Waals interactions, can provide a variety of 2D
nanoarchitectures.
[a] Dr. R. Makiura, R. Usui
Coordination materials composed of organic building units
and metal ion linkers enable a rich variety of structural designs
because of the unlimited number of possible combinations of
their components. In addition, coordination compounds with
porosity, called porous coordination polymers (PCPs) or metal–
organic frameworks (MOFs), exhibit versatile functionalities at-
tributed to their highly regulated frameworks and to their
available inner porosity.[10,11] We previously reported the crea-
tion of a number of highly ordered MOF nanofilms assembled
at the air/liquid interface.[7] The 2D interfacial reactions be-
tween molecular building units and metal ion connectors re-
sulted in the assembly of 2D MOF nanosheets, which could be
transferred to a variety of solid substrates, while retaining their
highly crystalline nature.
Nanoscience and Nanotechnology Research Center
Osaka Prefecture University, 1-2 Gakuen-cho
Naka-ku, Osaka, 599-8570 (Japan)
E-mail: r-makiura@21c.osakafu-u.ac.jp
[b] Dr. R. Makiura
PRESTO, Japan Science and Technology Agency
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
[c] Y. Sakai, Dr. A. Nomoto, Prof. A. Ogawa
Department of Applied Chemistry
Graduate School of Engineering, Osaka Prefecture University (Japan)
[d] Prof. O. Sakata
Synchrotron X-ray Station at SPring-8
National Institute for Materials Science (NIMS) (Japan)
[e] Dr. A. Fujiwara
Research & Utilization Division, Japan Synchrotron
Radiation Research Institute (JASRI/SPring-8) (Japan)
An important issue in exploring the potential of this sheet
fabrication technique is to demonstrate its versatility to pro-
vide diverse types of well-ordered nanofilms of controllable
structure. We have reported—by varying the building compo-
nents—the successful modulation of the stacking motif along
the growth direction perpendicular to the substrate surface in
MOF nanofilms, while maintaining the same molecular ar-
rangement in the direction parallel to the surface.[7c,f] Our moti-
vation in this study was to achieve tuning of the in-plane mo-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402150.
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This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
This article is part of the “Early Career Series”. To view the complete
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lecular arrangement of MOF
nanofilms together with the si-
multaneous regulation of their
porosity.
The power of the methodolo-
gy of utilizing 2D interfacial reac-
tions at the air/liquid interface
was demonstrated by employing
porphyrin building units. Por-
phyrins, including metallopor-
phyrins, are excellent candidates
as molecular building blocks for
assembling 2D architectures be-
cause they possess 2D square-
planar geometry. Furthermore,
they are very stable molecules
and exhibit a rich variety of
properties attributed to their p-
conjugated
electrons
and
center-coordinated metal ions.
Facile substitution of the periph-
eral parts of the molecule with
various functional groups and of
the center-coordinated metals
provides a rich variety of por-
phyrin derivatives, while retain-
ing their planarity.[12] Modulation
of the layer stacking motif in
MOF nanofilms was demonstrat-
ed previously by simply chang-
Scheme 1. Molecular structures of the porphyrin building units, 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin
(H2TCPP), trans-5,15-diphenyl-10,20-di(4-carboxyphenyl)porphine (trans-H2DCPP) and 5,10,15,20-tetrakis[4-(4-car-
boxyphenylethynyl)phenyl]porphine (H2TCPEPP).
ing the porphyrin-based molecular building unit from a free-
base porphyrin to a metalloporphyrin.[7c,f] The metalloporphyrin
allowed the introduction of pillaring molecules at the axial
sites of the center-coordinated metal ions, resulting in the en-
largement of the interlayer spacing. Herein, we present anoth-
er general protocol for how the strategic geometric design of
porphyrin-based molecular building units combined with the
use of 2D interfacial reactions at the air/liquid interface under
ambient conditions can allow exquisite tuning of the in-plane
molecular arrangement of MOF nanosheets.
the other hand, H2TCPEPP, which was specifically designed and
synthesized for this study, is a tetratopic ligand, but its size is
significantly enlarged—the molecular size is now approximate-
ly 24 ꢁ (the distance between the two oxygen atoms of neigh-
boring carboxylic acids), which represents a substantial in-
crease relative to the corresponding size of MTCPP of approxi-
mately 15 ꢁ.
Assembly of the nanosheets was initiated by spreading a so-
lution of molecular building blocks onto the surface of an
aqueous solution containing metal ion joints filled in a Lang-
muir trough at room temperature (Figure 1). An important
point in the successful application of the nanosheet growth
method, which utilizes the air/liquid interface, is the selection
of the molecular building units; these should not be easily dis-
solved in water. Porphyrins are excellent candidates when con-
sidering this point. In addition, the appropriate selection of sol-
vent for the spread solution is of paramount importance be-
cause of the requirement that it does not mix with the sub-
phase solution and is highly volatile. To satisfy these condi-
tions, we employed a mixed solvent of chloroform and
methanol to dissolve the porphyrin molecules. Attempts to
fabricate the nanosheets by using a spread solution in single
solvents (methanol or THF) miscible with water resulted in the
formation of poorly crystalline films.
Results and Discussion
In our previous study, we successfully assembled highly crystal-
line MOF nanosheets by combining tetratopic porphyrin mole-
cules (MTCPP: M=H2, Co, Pd; TCPP=5,10,15,20-tetrakis(4-car-
boxyphenyl)porphyrin) with copper(II) ion connectors.[7] As
proof-of-concept components, with the aim of rationally mod-
ulating the in-plane structural arrangement of 2D arrays, we
selected two related porphyrin molecules as building units
(Scheme 1), trans-5,15-diphenyl-10,20-di(4-carboxyphenyl)por-
phine (trans-H2DCPP) and 5,10,15,20-tetrakis[4-(4-carboxyphe-
nylethynyl)phenyl]porphine (H2TCPEPP), while retaining the
copper(II) ions as linkers. The trans-H2DCPP molecule is the
same size as the MTCPP porphyrins, but differs in its denticity:
it incorporates only two carboxylic substituents in the trans pe-
ripheral phenyl parts and can act as a linear ditopic ligand. On
After spreading the molecular building unit solution, the sur-
face was compressed by barriers and the surface pressure, p,
was monitored during compression. The same spread solutions
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Figure 1. Schematic image of the assembly process of trans-DCPP–Cu (NAFS-31) and TCPEPP–Cu (NAFS-41) nanosheets at the air/liquid interface. The solution
of trans-H2DCPP or H2TCPEPP molecular building units was spread onto the aqueous solution of Cu(NO3)2·3H2O in a Langmuir trough. The surface pressure,
p, was controlled by the surface compression of double-sided barriers and was kept constant during deposition onto the solid substrates.
of trans-H2DCPP and H2TCPEPP were also applied on a pure
water subphase for comparison. Surface pressure–mean molec-
ular area (p–A) isotherms are shown in Figure 2. A larger mean
molecular area was observed in the case of a subphase of an
aqueous solution of copper(II) ions for both trans-H2DCPP and
H2TCPEPP; this implies that the molecular arrangement at the
air/liquid interface is changed by the reaction between the
molecular building units and copper(II) ions, and copper-incor-
porating nanofilms are formed (NAFS-31: trans-DCPP–Cu;
NAFS-41: TCPEPP–Cu). The smaller molecular areas observed
for NAFS-31 and NAFS-41 (88 and 56 ꢁ2 per molecule, respec-
tively, at p=5 mNmꢀ1) than that for NAFS-2 (130 ꢁ2 per mole-
cule) may be attributed to the formation of multilayered nano-
sheets. The 2D nanosheets on the liquid surface at a surface
pressure of 5 mNmꢀ1 were then transferred to a silicon sub-
strate by the horizontal dipping method (the substrate surface
is parallel to the subphase surface during dipping) at room
temperature and AFM measurements were undertaken. AFM
topological images for thin films of NAFS-31 and NAFS-41 (one
deposition cycle) show the existence of a number of sheet do-
mains of uniform height (Figure 3). Sheet domains of approxi-
mately 50–100 nm gather together to form secondary domains
with sizes of a few hundred nanometers in both films. The
average height of the small domains in NAFS-31 and NAFS-41
is about 1.7 and 2.1 nm (Figure 3), respectively; this implies
that the sheets are not monomolecular, for which a thickness
of about 3 ꢁ is expected,[7a] but are composed of several
layers.
The 2D nanosheets were also transferred onto a quartz sub-
strate in the same way and UV/Vis absorption spectroscopic
measurements were undertaken (Figure 4). The Soret band
shift for NAFS films formed on the aqueous solution of cop-
per(II) ions indicated that a change in the electronic state of
both trans-H2DCPP and H2TCPEPP was induced by coordination
between CuII ions and the carboxylate units, leading to the for-
mation of copper-mediated 2D nanosheets.[7b] A change in the
Q band at lꢁ500–700 nm also implies the introduction of
copper(II) ions into the central parts of the porphyrins, leading
to the formation of metalloporphyrin units (Figure 4a and b).
Evolution of the UV/Vis absorption spectra of NAFS-31 and
NAFS-41 on quartz substrates with successive cycles of sheet
deposition, rinsing, solvent immersion, and drying are shown
Figure 2. Surface pressure–mean molecular area (p–A) isotherms (red lines) for NAFS-31 (a) and NAFS-41 (b). The p–A isotherms for the same building units
collected on pure water as a subphase are also shown for comparison (black lines).
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(30 deposition cycles) fabricated
on Si(100) substrates are shown
in Figure 6. The observation of
a number of sharp peaks in the
profiles indicates that both
NAFS-31 and NAFS-41 are highly
crystalline in the substrate plane
direction. From the peak posi-
tions, we attempted to deter-
mine a crystal system and the
unit cell metrics. NAFS films
composed of the tetratopic
TCPP units adopt metrically tet-
ragonal structures with in-plane
lattice parameters, a=b=16.4–
16.6 ꢁ,[7] which coincide with
those of comparable bulk crys-
tals composed of TCPP units.[12f,g]
For NAFS-31, the observed peak
positions of the Bragg reflections
cannot be described with a tet-
ragonal unit cell, but can be in-
dexed as 110, 200, 210, 220, 330,
Figure 3. AFM height images (top) and cross-sectional analysis (bottom) of NAFS-31 (a) and NAFS-41 (b) nano-
sheets (one deposition cycle). The films were formed at a surface pressure of 5 mNmꢀ1 and transferred onto sili-
con substrates. The cross-sectional analysis part is marked by a white line in the height images.
and 400 up to
a scattering
angle, 2q, of 188 on a metrically
rhombic unit cell with basal
in Figure 4c and d. A linear increase in absorbance with an in-
creasing number of cycles implies that each growth cycle leads
to deposition of the same amount of material (Figure 4e and
f).
plane dimensions, a=b=23.00(5) ꢁ and g=96.2(3)8. The lat-
tice size is approximately equal to the value obtained by multi-
plying the lattice dimension of the NAFS-1 sheets (16.46 ꢁ)[7a]
p
by 2, which corresponds to the basal plane diagonal of the
To examine the bonding manner of the NAFS films, IR ab-
sorption measurements were undertaken. The IR spectra of
NAFS-31 and NAFS-41 deposited on silicon substrates (30 dep-
osition cycles) are shown in Figure 5 together with those of
the trans-H2DCPP and H2TCPEPP films fabricated on pure
water. A strong absorption assigned to the C=O stretching
mode of the carboxylic acid groups appears at n˜ ꢁ1700 cmꢀ1
in the reference films fabricated on pure water. On the other
hand, it is commonly observed that four carboxylic acid
groups can form a binuclear paddle-wheel unit together with
two copper(II) ions. When such Cu2(COO)4 paddle-wheel secon-
dary building units (SBUs) are formed, the COO stretching
mode of the carboxylate ion (COOꢀ) becomes IR active (n˜
ꢁ1400 cmꢀ1). A distinct absorption at n˜ ꢁ1400 cmꢀ1 observed
in NAFS-31 and NAFS-41 provides strong evidence for the for-
mation of binuclear paddle-wheel SBUs in both films. The evo-
lution of the IR absorption spectra of NAFS-31 and NAFS-41
with the number of deposition cycles is shown in Figure S1 in
the Supporting Information. The continuous increase in ab-
sorbance with increasing number of cycles also confirms suc-
cessful stacking of individual nanosheets up to 30 deposition
cycles.
NAFS-1 unit cell. The rhombic unit cell can be accounted for
by the molecular arrangement shown in the right panel of Fig-
ure 6a. Four trans-H2DCPP molecules are connected through
binuclear copper paddle-wheel units to form a slightly distort-
ed checkerboard motif. The distortion from square to rhombic
geometry may be attributed to the reduced number of bind-
ing parts in trans-H2DCPP. The tetratopic TCPP leads to a sym-
metric square grid assembly, which reflects the shape of the
molecule and square disposition of its connecting ligands. On
the other hand, ditopic trans-H2DCPP, in which the connecting
carboxylate units are arranged linearly, cannot form an undis-
torted square grid and leads to the development of a small in-
plane rhombic distortion. As shown in the left panel of Fig-
ure 6a, the simulation of the in-plane XRD pattern for NAFS-31
is in good agreement with experimental results for this struc-
tural model.
All six observed peaks up to a scattering angle, 2q, of 128 in
the GIXRD profile of NAFS-41 composed of the hyperexpanded
tetratopic H2TCPEPP indexed as hk0, namely, 100, 110, 200,
220, 300, and 310, on a metrically tetragonal unit cell with
basal plane dimensions, a=b=25.63(2) ꢁ (Figure 6b). This is
significantly enlarged when compared with the lattice size of
NAFS films with MTCPP building units (a=b=16.4–16.6 ꢁ).[7]
The symmetry and size of the unit cell can be accounted for
by a structural model (Figure 6b, right panel) that incorporates
an in-plane structural motif comprised of H2TCPEPP units
linked by binuclear Cu2(COO)4 paddle-wheel SBUs and resulting
To obtain detailed information on the molecular arrange-
ment in the sheets (parallel to the substrate surface), synchro-
tron XRD measurements were performed on beamline BL13XU
(l=1.553 ꢁ) at SPring-8. Grazing-incidence in-plane X-ray dif-
fraction (GIXRD) patterns for NAFS-31 and NAFS-41 nanosheets
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Figure 4. UV/Vis absorption spectra of NAFS-31 (a) and NAFS-41 (b) nanosheets assembled on an aqueous solution of Cu(NO3)2·3H2O (red lines) or pure water
(black lines). The films were formed at a surface pressure of 5 mNmꢀ1 and transferred onto quartz substrates. Film growth of NAFS-31 (c, e) and NAFS-41 (d,
f) was followed by UV/Vis absorption spectroscopy. The spectra were collected after successive cycles of sheet transfer, rinsing/water immersion, and drying.
Each spectrum in c) and d) is labeled with the corresponding cycle number. Plots of maximum absorbance of the porphyrin Soret band versus the number of
film growth cycles are shown in e) and f).
in a similar 2D checkerboard pattern to that encountered in
NAFS films composed of MTCPP. The simulated in-plane XRD
profile (Figure 6b, left panel) obtained by using the construct-
ed structural model for NAFS-41 is in good agreement with ex-
perimental results. The IR spectroscopic measurements are in
agreement with structural models of both NAFS-31 and NAFS-
41, in which the binuclear paddle-wheel SBUs present provide
the necessary linker points to develop the 2D order of the film.
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tained with
a 1.0 ꢁ Connolly
radius, are also included. The
size of the in-plane unit cell of
NAFS-31 is approximately twice
that of NAFS-2—there are two
porphyrin molecular units in the
NAFS-31 unit cell. The checker-
board framework of NAFS-2 ac-
commodates a single type of
distorted hexagonal pore with
an aperture of approximately
10.5 ꢁ. On the other hand, the
absence of half of the binuclear
Cu2(COO)4 paddle-wheel connec-
tors in NAFS-31 partially opens
the framework walls and impos-
es a different shape on the void
space. In addition to the distort-
ed hexagonal pores found in
NAFS-2, there are also elongated
narrow-shaped pores, which
result from merging of the small-
er pores following framework
wall removal. On the other hand,
NAFS-41 and NAFS-2 are isoretic-
Figure 5. IR absorption spectra of NAFS-31 (a) and NAFS-41 (b) nanosheets (30 deposition cycles) assembled on
an aqueous solution of Cu(NO3)2·3H2O (red lines) or pure water (black lines). The films were formed at a surface
pressure of 5 mNmꢀ1 and transferred onto silicon substrates.
The estimated mean molecular areas for NAFS-31 and NAFS-
41, taking into account structural information derived from
XRD measurements, are 526 and 657 ꢁ2 per molecule, respec-
tively. These values are larger than the observed mean molecu-
lar areas (88 and 56 ꢁ2 per molecule at p=5 mNmꢀ1, respec-
tively) consistent with the formation of multilayers, as revealed
by AFM height analysis. In addition, the average crystalline
sheet domain size is estimated at about 30 nm for both NAFS-
31 and NAFS-41 nanosheets from the full-width at half-maxi-
mum (FWHM) of the most intense reflections (100 for NAFS-
31; 110 for NAFS-41) by using Scherrer’s equation (Scherrer
constant=1.84). This is comparable to the domain size ob-
served in the AFM images.
ular, with the unit cell of NAFS-41 retaining the same square-
planar topology as that in NAFS-2. However, linear extension
of the four organic struts in NAFS-41 is accompanied by a dis-
tinct expansion of the unit cell by approximately 9 ꢁ (25.63 ꢁ
in NAFS-41; 16.48 ꢁ in NAFS-2) together with a simultaneous
increase in the pore apertures. The dimension of the long diag-
onal of the pore apertures (the distance between the facing
Connolly surfaces) estimated from the in-plane structural
model of NAFS-41 is about 19.6 ꢁ, which is double the size of
that in NAFS-2 (ꢁ10.5 ꢁ), with a void area (ꢁ196 ꢁ2) four
times larger than that of NAFS-2 (ꢁ43 ꢁ2).
Conclusion
We also performed XRD measurements for the same NAFS-
31 and NAFS-41 films in an out-of-plane scattering geometry,
which was sensitive to the lattice parameter in the film growth
direction. However, no diffraction peaks were observed up to
a scattering angle of 308 (Figure S2 in the Supporting Informa-
tion), which implied that neither the NAFS-31 nor the NAFS-41
nanofilms displayed crystalline order along the film stacking di-
rection. Although the in-plane GIXRD measurements demon-
strate that both NAFS-31 and NAFS-41 nanosheets are endow-
ed with highly crystalline order parallel to the substrate surface
and are preferentially oriented with the sheet plane parallel to
the substrate surface, the substrate deposition procedure of
the island-like domains, as seen in the AFM images, does not
lead to a highly ordered stacking manner in the vertical direc-
tion to the substrate.
We demonstrated the successful modulation of the in-plane
molecular arrangement in highly crystalline preferentially ori-
ented MOF nanosheets, NAFS-31 (trans-DCPP–Cu) and NAFS-41
(TCPEPP–Cu), by utilizing two-dimensional interfacial reactions
at the air/liquid interface under operationally simple ambient
conditions. By employing a trans-ditopic molecular building
unit, trans-H2DCPP resulted in the formation of nanosheets
(NAFS-31) with a rhombic unit cell. This represents a distortion
of the square-grid topology encountered in analogous NAFS
films (NAFS-1, NAFS-2, and NAFS-13),[7] which are assembled by
using tetratopic MTCPP molecules, and is driven by the ab-
sence of half of the linking struts. At the same time, NAFS-31
possesses two types of pores, in addition to hexagonal pores
found in the NAFS family networks; partial removal of the
framework wall also results in long, narrow-shaped apertures
by joining pairs of neighboring pores. Finally, the combination
of a hyper-expanded tetratopic porphyrin, H2TCPEPP, and cop-
per(II) ion connectors allows the assembly of the NAFS-41
Figure 7 provides a summary of the in-plane molecular ar-
rangements of NAFS-31 and NAFS-41 together with that of
NAFS-2 for comparison.[7c] Calculated Connolly surfaces, which
are graphical representations of the available empty space ob-
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along the direction vertical to
the substrate by successive
layering of individual nanosheets
with desired lateral molecular ar-
rangements. This offers the pos-
sibility of their use in nanotech-
nological applications, such as
molecular filters and traps, that
require the presence of a gradi-
ent in pore size variation along
the direction parallel to the mo-
lecular path, that is, vertical to
the substrate.
Experimental Section
Synthesis
4-{(Trimethylsilyl)ethynyl}benzal-
dehyde:[13] Ethynyltrimethylsilane
(0.848 mL, 6.00 mmol) was added
to a solution of 4-bromobenzalde-
hyde (0.925 g, 5.00 mmol), bis(tri-
phenylphosphine)palladium(II) di-
chloride (35.1 mg, 0.05 mmol), and
CuI (9.52 mg, 0.05 mmol) in trie-
thylamine (10.5 mL). The reaction
mixture was then heated at reflux
for 38 h under a nitrogen atmos-
phere. After the resulting mixture
was filtered through a pad of silica
gel by using AcOEt as the eluent,
the eluate was concentrated under
reduced pressure. Purification was
conducted by column chromatog-
raphy on silica gel (hexanes/AcOEt)
to give 4-{(trimethylsilyl)ethynyl}-
benzaldehyde
(0.989 g,
98%).
1H NMR (400 MHz, CDCl3): d=
10.00 (s, 1H), 7.82 (d, J=8.2 Hz,
2H), 7.60 (d, J=8.2 Hz, 2H),
0.27 ppm
(s,
9H);
13C NMR
(100 MHz, CDCl3): d=190.9, 135.4,
132.2, 129.2, 129.0, 103.7, 98.7,
ꢀ0.39 ppm.
4-Ethynylbenzaldehyde:[14] K2CO3
(0.0676 g, 0.489 mmol) was added
to a solution of 4-{(trimethylsilyl)e-
Figure 6. GIXRD patterns (green dots) for NAFS-31 (a) and NAFS-41 (b) nanosheets (30 deposition cycles; left
panels). In-plane lattice parameters and indexing of the Bragg peaks are shown in the plots. Derived structural
models for NAFS-31 (a) and NAFS-41 (b) are shown in the right panels. Calculated GIXRD patterns (black lines)
and reflection positions (black bars) obtained by using the structural models are also shown.
thynyl}benzaldehyde (0.989 g, 4.89 mmol) in MeOH (16 mL) and
the resulting mixture was stirred for 3 h at room temperature. The
mixture was concentrated in vacuo and extracted with diethyl
ether. The organic layer was dried over anhydrous MgSO4 and the
solvent was evaporated in vacuo to give 4-ethynylbenzaldehyde
nanosheet with an isoreticular structure, similar to that of
NAFS-1, NAFS-2, and NAFS-13, but with a pronounced inflation
of the lattice size from 16.4–16.6 to 25.63 ꢁ. This is accompa-
nied by a drastic enlargement of the pore aperture and void
area to coincide with the increased size of the organic linker.
The versatility of the interfacial bottom-up nanosheet growth
technique demonstrated herein can be employed to address
a major challenge in the field, namely, to pave the way to-
wards obtaining preferentially oriented, ordered, hierarchical,
porous structures with a predesigned variation in the size and
shape of the pore apertures.[10] We will investigate the creation
of such hetero-nanostructures with regulated pore modulation
1
(0.449 g, 71%). H NMR (400 MHz, CDCl3): d=10.02 (s, 1H), 7.84 (d,
J=8.2 Hz, 2H), 7.64 (d, J=8.2 Hz, 2H), 3.29 ppm (s, 1H); 13C NMR
(100 MHz, CDCl3):
81.1 ppm.
d 191.3, 135.9, 132.7, 129.5, 128.3, 82.6,
5,10,15,20-Tetrakis{4-(ethyl-4-ethynylbenzoate)}porphyrin:[13,14]
4-Ethynylbenzaldehyde (0.449 g, 3.45 mmol) was added to a solu-
tion of ethyl 4-iodobenzoate (0.828 g, 3.00 mmol), bis(triphenyl-
phosphine)palladium(II) dichloride (21.1 mg, 0.03 mmol), and CuI
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Figure 7. Summary of the in-plane structural arrangements in porphyrin-based 2D MOF nanosheets assembled at the air/liquid interface. Molecular building
units (top) and corresponding in-plane crystalline structures (bottom) are shown together with the unit cells obtained from synchrotron GIXRD measurements
and calculated Connolly surfaces.
(5.70 mg, 0.03 mmol) in triethylamine (6.3 mL). The reaction mix-
ture was then stirred under reflux for 40 h under a nitrogen atmos-
phere. After the resulting mixture was filtered through a pad of
silica gel by using AcOEt as the eluent, the eluate was concentrat-
ed under reduced pressure. Purification was conducted by column
chromatography on silica gel (hexanes/AcOEt) to give 4-[2-(4-for-
mylphenyl)ethynyl]benzoic acid ethyl ester (0.886 g, 92%). BF3OEt2
(0.0142 g, 1.00 mmol) was added to a solution of 4-[2-(4-formyl-
phenyl)ethynyl]benzoic acid ethyl ester (0.2783 g, 1.00 mmol) and
pyrrole (0.0671 g, 1.00 mmol) in CH2Cl2 (100 mL). The reaction mix-
ture was then stirred at room temperature under a nitrogen at-
mosphere. After 1 h, 2,3-dichloro-5,6-dicyano-p-benzoquinone
(0.227 g, 1.00 mmol) was added and the reaction mixture was
stirred at room temperature overnight under a nitrogen atmos-
phere. After an aqueous solution of NaOH (0.5 mmol) was added
to the resulting mixture, the reactant was extracted with CH2Cl2.
The organic layer was then dried over anhydrous MgSO4 and the
solvent was evaporated. The residue was purified by column chro-
matography on silica gel (eluent/CHCl3/acetone=19:1) to give
5,10,15,20-tetrakis{4-(ethyl-4-ethynylbenzoate)}porphyrin (0.0818 g,
nyl)phenyl]porphyrin (0.0846 g (including water)). MS (FAB): m/z:
1190 [M+H]+.
Solutions
trans-DCPP–Cu (NAFS-31): A 0.2 mm solution of trans-H2DCPP
(mixed chloroform/methanol solvent (5:2, v/v)) was used as
a spread solution and a 1 mm aqueous solution of Cu(NO3)2·3H2O
was used as a subphase.
TCPEPP–Cu (NAFS-41): A 0.5 mm solution of H2TCPEPP (mixed
chloroform/methanol solvent (9:1, v/v)) was used as a spread solu-
tion and a 1 mm aqueous solution of Cu(NO3)2·3H2O was used as
a subphase.
Nanosheet preparation
A
polytetrafluoroethylene (PTFE) Langmuir trough (375ꢂ75ꢂ
5 mm3, 0.16 L) was filled with the subphase solution. The surface
of the subphase was carefully cleaned by mild surface-touch vac-
uuming. The solution containing trans-H2DCPP or H2TCPEPP was
spread onto the subphase with a microsyringe. Surface pressure–
area (p–a) isotherm measurements were performed with a KSV
minitrough system by using a continuous pressing speed for two
barriers of 10 mmminꢀ1. The 2D nanosheets of NAFS-31 or NAFS-
41 at a surface pressure of 5 mNmꢀ1 were deposited onto the sub-
strate by the horizontal dipping method at room temperature
(process 1). The substrate with the sheets was then rinsed with
a flow of distilled water, immersed into distilled water for 3 min
under sonication, and finally dried under a flow of nitrogen (pro-
cess 2). To stack additional layers, the 2D nanosheets on the sub-
1
6%). H NMR (400 MHz, CDCl3): d=8.89 (s, 8H), 8.24 (d, J=8.2 Hz,
8H), 8.12 (d, J=8.2 Hz, 8H), 7.97 (d, J=8.2 Hz, 8H), 7.75 (d, J=
8.2 Hz, 8H), 4.43 (q, J=6.9 Hz, 8H), 1.45 ppm (t, J=7.4 Hz, 12H).
5,10,15,20-Tetrakis{4-(4-carboxyphenylethynyl)phenyl}porphyr-
in:[15] A 40% aqueous solution of KOH (3.2 mL) was added to a solu-
tion of 5,10,15,20-tetrakis{4-(ethyl-4-ethynylbenzoate)}porphyrin in
a 2:1 mixture of THF/CH3OH (53 mL). The reaction mixture was
then stirred at 408C for 2 h, acidified with concentrated HCl (pH 5),
and extracted with THF/CH2Cl2 (1:1; 2ꢂ100 mL). The organic layer
was dried over anhydrous MgSO4 and the solvent was evaporated
in vacuo to give crude 5,10,15,20-tetrakis[4-(4-carboxyphenylethy-
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of the layers deposited was controlled by the number of cycles of
sheet deposition (process 1) and rinsing/solvent immersion/drying
(process 2). The same spread solution was spread onto a pure
water subphase for comparison. The two barriers were compressed
at the same speed of 10 mmminꢀ1 and the 2D array was again
transferred onto the substrate by the horizontal dipping method.
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a multiaxis diffractometer at beamline BL13XU (l=1.553 ꢁ) in
SPring-8. Helium gas was supplied through the cell during the
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below the critical angle for total external reflection of the sub-
strate. The GIXRD measurements were performed at a=0.178. Dif-
fraction from the sample surface was detected in the in-plane di-
rection with a detection angle to the surface of 0.178. A Soller slit
(0.48) was placed between the sample and the detector to reduce
the background contribution.
Acknowledgements
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We thank the Japan Society for the Promotion of Science (JSPS)
“Grants-in-Aid for Scientific Research on Innovative Areas for Co-
ordination Programming (22108524, 24108735)”, the Japan Sci-
ence and Technology Agency (JST) Core Research for Evolutional
Science and Technology (CREST) and Precursory Research for Em-
bryonic Science and Technology (PRESTO) for a project of “Molec-
ular technology and creation of new functions”, the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) “Spe-
cial Coordination Funds for Promoting Science and Technology
(SCF)”, the Kao Foundation for Arts and Sciences, the INAMORI
foundation, the Japan Prize Foundation, and the Royal Society
(UK) “International Exchange Scheme” for financial support. The
synchrotron radiation experiments were performed at the
BL13XU beamline of SPring-8 with the approval of the Japan
Synchrotron Radiation Research Institute (JASRI; proposal nos.
2011B1904, 2012A1668, 2013A1668) and the Nanotechnology
Support Project of the Ministry of Education, Culture, Sports, Sci-
ence and Technology (proposal no. 2011A1656). We also thank
Hiroaki Yamanaka for help with the XRD measurements.
Keywords: interfaces
·
metal–organic
frameworks
·
nanosheets · polymers · porphyrinoids
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Received: May 21, 2014
Published online on July 18, 2014
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ChemPlusChem 2014, 79, 1352 – 1360 1360