Towards covalent organic frameworks with predesignable and aligned open docking sites

Article abstract of DOI:10.1039/c4cc01825g

A strategy for the synthesis of covalent organic frameworks with open docking sites is developed. The docking sites are ordered on the channel walls and structurally predesignable for meeting various types of noncovalent interactions, thus opening a way towards designing supramolecular materials based on crystalline porous organic frameworks. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01825g

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Towards covalent organic frameworks with
predesignable and aligned open docking sites†
Cite this: Chem. Commun., 2014,
50, 6161
Xiong Chen, Ning Huang, Jia Gao, Hong Xu, Fei Xu and Donglin Jiang*
Received 11th March 2014,
Accepted 12th April 2014
DOI: 10.1039/c4cc01825g
www.rsc.org/chemcomm
A strategy for the synthesis of covalent organic frameworks with open anchoring open docking sites, whereas the vertices are usually
docking sites is developed. The docking sites are ordered on the channel highly substituted for the development of the skeleton of COFs and
walls and structurally predesignable for meeting various types of non- positions available for further appending docking sites are thus
covalent interactions, thus opening a way towards designing supra- limited. In contrast, the edge units have more free positions for
molecular materials based on crystalline porous organic frameworks.
functionalization. Based on this structural feature, we explored the
edge units of COFs for loading open docking sites. We demon-
Covalent organic frameworks (COFs) are a class of crystalline porous strated the concept by using imine-linked COFs as scaffolds, into
polymers with an atomically precise and predictable network of which four kinds of edge units with different functionalities were
building blocks in two-dimensional (2D) or three-dimensional (3D) introduced. We synthesized 4,40,400,40 00-(pyrene-1,3,6,8-tetrayl) tetra-
topology.1 Compared with other crystalline porous materials, COFs aniline (PyTTA) bearing four amino groups as the vertices and
are unique in that they are made from lightweight elements linked by developed four different aldehydes as the edge units, including
covalent linkages. A distinct feature is that COFs allow total control 2,5-dihydroxyterephthalaldehyde (DHTA), 2,3-dihydroxy terephthal-
over structures, including skeleton and pore size and shape. In aldehyde (2,3-DHTA), [2,20-bipyridine]-5,50-dicarbaldehyde (2,20-
particular, 2D COFs integrate organic building blocks into covalent BPyDCA) and [3,30-bipyridine]-6,60-dicarbaldehyde (3,30-BPyDCA),
2D sheets and layered frameworks, which constitute periodic p whereas –OH, –(OH)2 and 2,20- and 3,30-bipyridine docking sites
columnar arrays and directional open nanochannels. 2D COFs have were embedded in the channel walls. The –OH and pyridine
emerged as a powerful platform for designing functional polymers groups are well-established for various supramolecular construc-
with luminescence,2 photoconduction,2,3 sensing,2b catalysis,4 charge tions. Condensation of PyTTA and aldehydes in mixture solvents
transport3a,b,5 and separation6 and photovoltaic7 properties.
in the presence of an acetic acid catalyst at 120 1C afforded the
From a synthetic perspective, COFs are attractive motifs for Py-DHPh COF, Py-2,3-DHPh COF, Py-2,20-BPyPh COF and Py-3,30-
incorporation of open docking sites into ordered alignment, which BPyPh COF in good isolated yields (Scheme 1 and ESI†).
constitute a new class of COFs for supramolecular architecture.
We optimized the solvothermal conditions, including solvents
However, construction of COFs with open docking sites remains a and reaction times, for the synthesis of highly crystalline COFs (ESI†).
synthetic challenge. Here we report a strategy for the construction of Fig. S1 (ESI†) summarizes the X-ray diffraction (XRD) results. The
2D COFs with open docking sites in the skeletons. We highlight that COFs exhibited characteristic CQN vibration stretches for imines at
the docking sites are ordered on the channel walls and are structu- 1608–1622 cmꢀ1 (Fig. S2 and Table S1, ESI†). Elemental analysis
rally designable for meeting various types of noncovalent interactions. confirmed that the C, H and N contents of the COFs were close to the
Our strategy for embedding open docking sites is targeted on calculated values expected for an infinite 2D sheet (Table S2, ESI†).
the walls of one-dimensional (1D) channels of 2D COFs; this Field-emission scanning electron microscopy images revealed belt or
configuration allows an easy access of guest molecules through flake-like morphologies (Fig. S3, ESI†). Thermal gravimetric analysis
the 1D channels to the docking sites for supramolecular construc- suggested that the COFs are stable up to 400 1C (Fig. S4, ESI†).
tion. Both the vertices and edges of 2D COFs can be explored for
These COFs are highly crystalline materials with strong XRD
signals (Fig. 1). The Py-DHPh COF exhibited diffraction peaks at
3.681, 5.281, 7.281, 8.561, 11.281, 15.381 and 23.761, which are assign-
able to the (110), (020), (220), (030), (040), (060) and (001) facets,
respectively (Fig. 1A). The Py-2,3-DHPh COF demonstrated XRD peaks
at 3.721, 5.501, 7.541, 8.721, 11.401, 12.221, 15.321 and 23.981, which
are attributed to the (110), (020), (220), (030), (040), (240), (440) and
Department of Materials Molecular Science, Institute for Molecular Science,
National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji,
Okazaki 444-8787, Japan. E-mail: jiang@ims.ac.jp
† Electronic supplementary information (ESI) available: Methods, XRD data,
structural simulation, FT-IR and electronic absorption spectra, TGA, SEM and
gas adsorption analysis. See DOI: 10.1039/c4cc01825g
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Scheme 1 Schematic representation of the imine-linked pyrene COFs with predesigned and aligned open docking sites (phenol and pyridine units in red).
Pawley refinements (Fig. S5A, D, G and J, green curves, ESI†)
confirmed the above peak assignments as evidenced by their
negligible differences (black curves).
XRD pattern simulation by using AA-stacking mode repro-
duced the XRD signal position and peak intensity (Fig. S5A, D,
G and J, blue curves, ESI†). This layer structure results in open
1D channels (Fig. S5B, E, H and K, ESI†). In sharp contrast,
staggered AB stacking mode with offset by a/2 and b/2 gave XRD
patterns (purple curves) that could not reproduce the experi-
mental XRD patterns. In this case, the pores are overlapped by
neighboring sheets (Fig. S5C, F, I and L, ESI†). In the case of
AA-stacking mode, the a, b, c values for Py-DHPh COF are a =
35.82 Å, b = 31.90 Å, c = 4.69 Å; for Py-2,3-DHPh COF are a =
36.15 Å, b = 31.30 Å, c = 3.60 Å; for Py-2,20-BPyPh COF are
a = 42.14 Å, b = 36.90 Å, c = 4.33 Å and for Py-3,30-BPyPh COF are
a = 40.55 Å, b = 39.08 Å, c = 3.60 Å, respectively (Table S3, ESI†).
To investigate whether the 1D channels of the COFs are
accessible and their porosity, nitrogen sorption isotherm measure-
ments at 77 K were conducted. These COFs exhibited reversible type
IV sorption curves (Fig. 2A–D),9 which are characteristics of meso-
porous materials. The Brunauer–Emmett–Teller (BET) surface areas
were calculated to be as high as 1895, 1932, 2349 and 2200 m2 gꢀ1
for the Py-DHPh COF, Py-2,3-DHPh COF, Py-2,20-BPyPh COF and
Fig. 1 XRD patterns of (A) Py-DHPh COF, (B) Py-2,3-DHPh COF, (C) Py-2,20-
Py-3,30-BPyPh COF, respectively. These surface areas are among the
highest for the imine-linked COFs reported based on N2 sorption; in
particular, the value for the Py-2,20-BPyPh COF exceeds those of all
other imine-linked COFs.4,5a,8 The total pore volumes were estimated
BPyPh COF and (D) Py-3,30-BPyPh COF. Insets show the graphical view of a
2 ꢁ 2 porous framework in the AA-stacking mode.
(001) facets, respectively (Fig. 1B). The Py-2,20-BPyPh COF displayed to be 1.45, 1.51, 1.85 and 1.49 cm3 gꢀ1 for the Py-DHPh COF, Py-2,3-
main diffractions at 3.161, 4.581, 6.381, 9.741, 12.981 and 23.861, which DHPh COF, Py-2,20-BPyPh COF and Py-3,30-BPyPh COF, respectively
can be assigned to the (110), (020), (220), (330), (440), and (001) facets, (Table S4, ESI†). We evaluated the pore size distributions by using
respectively (Fig. 1C). The Py-3,30-BPyPh COF showed main peaks at the nonlocal density function theory (NLDFT) method. The COFs
3.161, 4.801, 6.381, 7.441, 9.641, 12.921 and 23.541, which can be have one main peak centered at 1.99, 1.95, 2.45, and 2.52 nm, which
attributed to (110), (020), (220), (030), (330), (240) and (001) facets, are close to their theoretical values (Fig. S6, ESI†). These results
respectively (Fig. 1D). The presence of (001) facets indicates that indicate that the docking sites on the channel walls are accessible to
these 2D COFs have periodic orders in all three dimensions. guest molecules for supramolecular constructions.
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confirmed that the crystallinity was maintained in the VO@Py-
2,3-DHPh COF. The N2 sorption curves (Fig. 2E) revealed micro-
porous characteristics, whereas the BET surface area was evaluated
to be 1048 m2 gꢀ1 (Fig. 2F). The pore volume was evaluated to be
1.16 cm3 gꢀ1 (Fig. S8B, ESI†). The VO@Py-2,3-DHPh COF consists
of only one kind of micropore with a size of 1.82 nm (Fig. S8B,
ESI†). Interestingly, the absorption band was broadened to longer
wavelength with the peak maximum red-shifted from 458 to
468 nm (Fig. S8C and D, ESI†). This is caused by the coordination
of VQO to the edge units of the COFs. The VO@ Py-2,3-DHPh COF
samples are stable in various solvents and are thermally stable up
to 300 1C (Fig. S9, ESI†). Because VQO sites are well-established
catalytic centers,10 the VO@Py-2,3-DHPh COF with dense VQO
units confined within the open channels constitutes an intriguing
heterogeneous catalytic system worthy of further investigation.
In summary, we have developed a general strategy for the
construction of a new class of COFs that offer open docking sites
on the channel walls. These COFs feature ordered alignment of
binding sites and predesignable skeletons. Metallation converts
the open frameworks into supramolecular COFs with dense and
aligned catalytic VQO sites confined within the nanochannels.
These remarkable results open a way to supramolecular architecture
based on crystalline open structures of COFs.
Fig. 2 Nitrogen sorption isotherms of (A) Py-DHPh COF, (B) Py-2,3-DHPh
COF, (C) Py-2,20-BPyPh COF, (D) Py-3,30-BPyPh COF and (E) VO@Py-2,3-
DHPh COF measured at 77 K. (F) BET surface areas of the COFs (from left
to right, the bars correspond to the Py-DHPh COF, Py-2,3-DHPh COF, Py-2,20-
BPyPh COF, Py-3,30-BPyPh COF and VO@Py-2,3-DHPh COF).
Imine linkage constitutes one type of conjugation bond, how
the docking sites affect the p conjugation of COFs and thus their p
electronic properties is an interesting point to be elucidated. We
conducted solid-state absorption spectroscopy to evaluate the
absorption bands of COFs. The Py-DHPh COF, Py-2,3-DHPh
COF, Py-2,20-BPyPh COF, and Py-3,30-BPyPh COF exhibited absorp-
tion bands at 469, 458, 442 and 445 nm, respectively (Fig. S7, ESI†),
which were 53, 42, 26 and 29 nm redshifted in comparison with
the PyTTA monomer (416 nm). These results indicate an extended
p conjugation over the 2D sheet of the COFs. The difference in the
redshift suggests that the edge units with different docking sites
can perturb the conjugation status. The docking sites with phenol
groups prefer a planar sheet conformation that promotes p conju-
gation, whereas the docking sites with bipyridine units cause a
twisted conformation of the 2D sheets and trigger pyridine nitrogen
induced p–p overlap that localizes the p clouds.
To demonstrate the potential of these crystalline COFs as scaffolds
for the supramolecular construction, we conducted additional experi-
ments. We chose the Py-2,3-DHPh COF as a scaffold and metallated
the catechol groups with vanadium(IV)-oxy acetylacetonate [VO(acac)2]
(Scheme 2 and ESI†). Treatment of the Py-2,3-DHPh COF sample
having a BET surface area of 1432 m2 gꢀ1 with VO(acac)2 in THF at
50 1C for 24 h caused a clear color change from red to black and led
to quantitative isolation of VO@Py-2,3-DHPh COF. The resulting
VO@Py-2,3-DHPh COF consists of VQO groups on the channel walls
with an almost quantitative conversion (0.96 VQO/catechol in molar
ratio), as determined by inductively coupled plasma atomic emission
spectroscopy analysis (ESI†). The XRD patterns (Fig. S8A, ESI†)
This work was supported by a Grant-in-Aid for Scientific
Research (A) (24245030) from MEXT, Japan.
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Scheme 2 Synthesis of the VO@Py-2,3-DHPh COF.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 6161--6163 | 6163