Coordinatively immobilized monolayers on porous coordination polymer crystals

Article abstract of DOI:10.1002/anie.201001063

Surface-specific: Coordinatively immobilized monolayers (CIMs) of fluorescent dyes were fabricated on specific single-crystal surfaces of porous coordination polymers (PCPs) (see picture). This approach enables the fabrication of functional PCP crystal su

Full text of DOI:10.1002/anie.201001063

Angewandte
Chemie
DOI: 10.1002/anie.201001063
Functional Monolayers
Coordinatively Immobilized Monolayers on Porous Coordination
Polymer Crystals**
Mio Kondo, Shuhei Furukawa,* Kenji Hirai, and Susumu Kitagawa*
The formation of self-assembled monolayers (SAMs),
whereby chemisorbed organic molecules spontaneously
organize on metal or metal oxide substrates,[1] is the most
developed method used to modify the interfacial properties of
surfaces, such as work function and wettability, and to impart
new functions, such as switching properties and catalytic
reactivity. Porous coordination polymers (PCPs) or metal–
organic frameworks (MOFs),[2] comprising an alternate
arrangement of metal ions and bridging ligands, can be
utilized as molecular-based crystalline substrates for the
assembly of functional molecules on their surfaces. That use
allows better tuning of their porous properties, which is
advantageous for applications such as selective sorption,[3]
heterogeneous catalysis,[4] and chemical sensing,[5] as the
incorporation of guest molecules is influenced by the
interfacial structures of PCP crystals. Herein we focus on
the development of the methodology to form SAM-like
monolayers on PCP surfaces and show the fabrication of a
fluorescent organic monolayer on targeted surfaces of PCP
crystals by taking advantage of the equilibrium state of
coordination bonds.
nucleation of PCP crystals on the surface.[7] The lability of
coordination bonds is such that within a PCP material, in
which the ligands participate in the construction of the
crystalline framework, bond cleavage or ligand exchange
reactions triggered by chemical or physical stimuli can occur
and have been characterized, despite the diffusion and
confinement restrictions of the porous structure.[8] The
surfaces of such materials once in solution will be extremely
sensitive to the coordination equilibrium, whereby the
organic ligand and solvent molecules compete to terminate
the surfaces by coordination bonds.[9] When appropriate
postsynthesis reaction media have been found, that is, the
crystals do not degrade and the reactivity of the crystal
surfaces is preserved, the use of the coordination equilibrium
then allows specific and selective functionalization of the
surfaces by desirable organic molecules provided that the
latter have the same coordination ability (Figure 1a).
The relatively weak interactions of coordination bonds
that dominate the construction of PCPs are also useful to
hybridize PCPs with other materials. There have been reports
that demonstrate the direct formation of PCPs on the SAM-
modified substrates.[6] In this system, the coordination moiety
introduced into the SAM molecules plays a key role in
determining the orientation of the crystal growth. Moreover,
the coordination equilibrium between the metal precursor
and the SAM has a significant impact on the initiation of
[*] Dr. M. Kondo, Dr. S. Furukawa, Prof. S. Kitagawa
Institute for Integrated Cell-Material Sciences (iCeMS)
Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501 (Japan)
Figure 1. a) Surface modification process, and b) ligand-exchange reac-
tion with BODIPY dyes (SL1–3).
Dr. S. Furukawa, Prof. S. Kitagawa
ERATO Kitagawa Integrated Pores Project
Japan Science and Technology Agency (JST)
Kyoto Research Park Bldg #3, Shimogyo-ku, Kyoto 600-8815 (Japan)
E-mail: shuhei.furukawa@kip.jst.go.jp
Unlike the “PCP-on-PCP” (or “MOF-on-MOF”) concept
that we recently demonstrated, in which the epitaxial growth
of the shell crystal requires the careful choice of bridging
ligands to laterally match the lattice distances at the interfaces
between two crystals,[10] the formation of an organic mono-
layer allows the free choice of functional molecules by a
simple ligand-exchange reaction involving coordination
bonds on the crystal surface alone. Our strategy to decorate
the crystal surface differs from a few pioneering studies on
surface modifications of zeolites[11] and PCPs,[12] as we take
advantage of the formation of coordinatively immobilized
monolayers (CIMs) instead of the immobilization of func-
tional molecules through covalent bonds. Unlike SAMs on
metal substrates, the assembled structure of CIMs is most
K. Hirai, Prof. S. Kitagawa
Department of Synthetic Chemistry & Biological Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: kitagawa@sbchem.kyoto-u.ac.jp
[**] M.K. is grateful to JSPS Research Fellowships for Young Scientists.
This work was supported by Grants-in-Aid for Young Scientists (B)
from MEXT (Japan) (20750106). We acknowledge Y. Kinoshita, Dr.
R. Matsuda, Dr. M. Higuchi, and Dr. R. Kitaura for their preliminary
studies, and Dr. C. Bonneau for fruitful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001063.
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likely determined by the lattice structure of PCPs, directly
assisted by coordination bonds with metal ions at the lattice
points.
A series of tetragonal frameworks, [{Zn2(bdc)2(dabco)}n]
(F1) and [{Zn2(ndc)2(dabco)}n] (F2), (bdc = 1,4-benzenedi-
carboxylate, ndc = 1,4-naphthalenedicarboxylate, dabco =
1,4-diazabicyclo[2.2.2]octane), in which two different coordi-
nation bonds—namely zinc–carboxylate and zinc–amine—
participate to construct the framework, was selected as
substrate PCP crystals.[13] Both F1 and F2 exhibit rectangular
prism morphology. Four surfaces are terminated by zinc–
carboxylate bonds, denoted as the {100} surfaces, and the
other two surfaces are terminated by zinc–amine bonds,
denoted as the {001} surfaces. Because the metal–carboxylate
coordination bonds participate in constructing most PCPs, the
{100} surfaces were targeted to emphasize the applicability of
our concept. The well-known fluorescent dye, boron dipyrro-
methene (BODIPY),[14] as shown in Figure 1b, was chosen as
the functional organic moiety for detecting the formation of
CIMs on PCP crystals by using confocal laser scanning
microscopy (CLSM).
Figure 2. Representations of surface-modified crystals (left), CLSM
images (middle) and transmission images (right) of SL1/F2 at z=A
and z=B in a) the c axis orientation and b) the a axis orientation.
Scale bars: A, 180 mm; B, 190 mm; C, 130 mm; D, 130 mm.
The reaction conditions (temperature, concentration of
BODIPY dyes, and soaking time) to modify the crystal
surfaces were optimized to avoid the degradation of the
crystals. More moderate conditions than that for the synthesis
of F2 (1208C, 48 h) were therefore applied, and it is known
that the ligand-exchange reaction of the zinc paddlewheel
units could proceed even in such moderate conditions, as
previously described in the synthesis of PCPs composed of the
zinc paddlewheel units by liquid-phase epitaxy[6f]
.
On the basis of this strategy, the ligand-exchange reactions
with the BODIPY dyes with carboxylic acid moieties
(denoted surface ligands SL1–3; Figure 1b) on the surfaces
of F1 and F2 were performed according to the following
procedure. Single crystals of F1 or F2 were immersed in
0.05 mm solutions of BODIPY dyes (SL1–3) in dehydrated
DMF for 3 h at 458C, and then washed with dehydrated DMF
10 times. The PCP crystals were also treated with BODIPY
dyes without a coordination moiety as references (denoted
reference ligands RL1–3 as shown in Schemes S1 and S2 in
the Supporting Information).
The results of the CLSM studies shown in Figure 2
demonstrate that the surface-modified PCP crystals (denoted
surface ligand/framework SL1/F2) exhibited emission only at
the crystal surfaces by excitation at 635 nm, whereas the
crystals treated with RL1 exhibited no fluorescence (Fig-
ure S1 in the Supporting Information). The fluorescence
spectra of SL1/F2 under the CLSM conditions (obtained at
the red area in Figure 2) correspond to that of SL1 in DMF as
shown in Figure 3a, which indicates that the fluorescence
obtained under the microscope originated from the BODIPY
dye SL1.
Figure 3. Fluorescence spectra of a) SL1 in DMF (solid line) and SL1/
F2 under the CLSM conditions (dashed line) and b) SL2 in DMF (solid
line) and SL2/F3 under the CLSM conditions (dashed line).
fluorescence at four sides of the crystal. The other orientation
shown in Figure 2b exhibited a different trend—fluorescence
at the whole face of the crystal at the focal point z = A, and
fluorescence at only two sides at the focal point z = B. In both
orientations, four surfaces of the crystals had intense fluores-
cence due to the BODIPY dye (SL1).
Note that two different crystal orientations of SL1/F2
were observed under CLSM conditions (Figure 2). Horizon-
tally sliced CLSM images of one of the crystal orientations at
two different focal points are shown in Figure 2a. Whereas no
fluorescence was detected in the sliced image at the focal
point z = A (the bottom of the crystal), slicing at the focal
point z = B (the middle of the crystal) gave an image with
Because of the tetragonal crystal system, only four {100}
surfaces of the crystal have zinc–carboxylate coordination
bonds in the [100] direction, in which the ligand-exchange
reaction with SL1 can take place. To elucidate the modified
surfaces of SL1/F2, face index analysis as the same crystal
investigated by CLSM was carried out by using a single-
crystal X-ray diffractometer. Indeed, the four surfaces
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modified with BODIPY dyes were identified as the {100}
surfaces and the remaining two surfaces without modification
as the {001} surfaces. Hence, the BODIPY dye with a
monocarboxylic acid moiety (SL1) recognizes the {100}
surfaces of the tetragonal PCP crystal (F2) and is immobilized
by a ligand-exchange reaction only on these surfaces. A
similar tendency, namely that the BODIPY dyes (SL1–3)
recognize the {100} surfaces of F1, leading to the formation of
the surface modified PCP crystals (SL1–3/F1), was observed
(Figures S3–11 in the Supporting Information).
surface modification of F3 was performed according to the
following procedure. Single crystals of F3 were immersed in
0.5 mm solutions of BODIPY dyes (SL2) in methanol for 20 h
at 608C, and then washed with dehydrated DMF 10 times. A
higher concentration of BODIPY dye, higher reaction
temperature, and longer reaction time than were used for
the Zn-based tetragonal frameworks were required to pro-
mote the ligand-exchange reaction. These harsher reaction
conditions are required because copper–carboxylate coordi-
nation bonds are stronger than zinc–carboxylate bonds. The
CLSM sliced images (Figure 5) at different heights, and the
fluorescence spectrum of SL2/F3 under the microscope
The change of surface morphology upon surface modifi-
cation was investigated by atomic force microscopy (AFM) as
shown in Figure 4. Figure 4a shows a topographic image of
Figure 4. AFM images of the (100) surfaces of a) F2 and c) SL1/F2.
Averaged topographic profiles of the selected parts (indicated as white
boxes) of the (100) surfaces of b) F2 and d) SL1/F2.
the (100) surface of the bare crystal (F2). The step-edge
structure with the height (about 1 nm) corresponding to the
length (1.09 nm) of the a axis[13] clearly reveals the atomically
flat nature of the (100) surface. Surprisingly, the surface of the
modified crystal SL1/F2 still maintained a similar step-edge
structure with the corresponding step height, even though the
morphology of the step-edge structure changed into an
obscure feature, and the terraces became rather rough. The
step structure observed in the modified crystal indicates the
formation of a highly uniform monolayer of SL1 without
aggregation because the chemical structure of SL1 does not
induce the stable formation of uniform multilayers. There-
fore, it is suggested that the monolayer of BODIPY dyes is
most likely fabricated on the {100} surface of the PCP crystal.
To confirm the general applicability of our method, crystal
surface modification of [{Cu3(btc)2}n],[15] (HKUST-1, F3),
(btc = 1,3,5-benzenetricarboxylate) was also performed. The
Figure 5. Representations of surface-modified crystals (left), CLSM
images (middle), and transmission images (right) of SL2/F3 at z=A–
E. The sequential slicing images from the bottom of crystal (at z=A)
to the top of crystal (at z=E) reveal the octahedral crystal morphology
of F3.
(Figure 3b and Figure S12 in the Supporting Information)
indicated that all the crystal surfaces of F3 were eventually
covered and that the dye molecules were not incorporated
into the pores. These results clearly reveal that the coordina-
tively immobilized monolayer can be constructed on the
various surfaces of PCP crystals terminated by reactive
carboxylate ligands.
¯
framework F3 has a cubic crystal system Fm3m, in which the
paddlewheel-type Cu2 dimer units are connected by btc
ligands. The crystal structure of F3 exhibits regular octahedral
morphology, and all the surfaces are denoted as the {111}
faces terminated by btc ligands,[6e] which should have a similar
surface structure to the {100} surfaces of F1 and F2. The
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In conclusion, we provided the first clear evidence for the
formation of monolayers assisted by coordination bonds with
the immobilization of dyes on PCP single-crystal surfaces. The
key to ensuring that the reaction only takes place on crystal
surfaces is the dynamic nature of PCP crystal surfaces,
namely, coordination equilibria. The versatility of this
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can be integrated.
Received: February 21, 2010
Published online: June 22, 2010
Keywords: coordination polymers · interfaces · monolayers ·
metal–organic frameworks · surface modification
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