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condensation reaction of tetraformyl-bicarbazole and aromatic experimental PXRD pattern of Cz-COF1 displayed strong signals
DOI: 10.1039/C7CC07024A
diamines. The resulting Cz-COFs were insoluble in common at 4.90° and 9.88° (Fig. 1a, red curve), which are assignable to
organic solvents and were chemically stable in various aqueous the (110) and (220) facets, respectively. In addition, signals at
conditions including HCl (1 M) and NaOH (1 M) for 3 days. The 6.94 , 14.90 , and 22.32 corresponding to respectively the
°
°
°
COFs were characterized by various analytic methods (Figs. S1- (200), (420), and (001) facets were also observed. Pawley
S18, ESI†). Elemental analysis revealed that the C, H, and N refinement (Table S2, ESI†) using a C222 space group with lattice
contents were close to the theoretical values of infinite 2D parameters of a = 27.067
Å, b = 27.735 Å, and c = 3.993 Å
sheets (Table S1, Electronic Supporting Information, ESI†). yielded a PXRD pattern (Fig. 1a, green curve) that agrees with
Thermogravimetric analysis revealed that both Cz-COFs are the experimentally observed pattern, as indicated by their
stable up to 450 °C in nitrogen (Fig. S1, ESI†). The Fourier negligible difference (Fig. 1a, black curve). Similarly, Cz-COF2
transform infrared spectra showed strong C=N stretching exhibited strong signals at 3.99
vibration bands at 1618 cm–1 (Fig. S2, ESI†), indicating the with peaks at 5.71
(200), 12.10
1b, red curve). Pawley refinement (Table S3, ESI†) using a C222
space group with lattice parameters of a = 33.274 , b = 32.220
, and c = 3.976 yielded a PXRD pattern (Fig. 1b, green curve)
°
(110) and 8.02
°
(220) together
°
°
(420), and 21.78
°
(001) (Fig.
formation of imine linkages.
Å
110
Å
Å
110
(b)
(a)
that can reproduce the experimental curve with small
difference (Fig. 1b, black curve). These small differences were
evidenced by their small Rwp (4.87%) and Rp (3.71%) values for
Cz-COF1 and Rwp (3.61%) and Rp (2.83%) values for Cz-COF2, and
suggest the correctness of the above PXRD assignments.
220
200
220
001
200
420
420
001
A density-functional tight-binding method, which included a
Lennard-Jones dispersion, was used to simulate the optimum
structures of Cz-COFs. From the calculations, there is only one
possible orthorhombic crystal system for the COFs from the
bicarbazole and diamines. Using the optimal monolayer
structure, the AA and AB stacking models were generated and
optimized. The bicarbazole molecule consists of two carbazole
subunits connected with a N
owing to the steric hindrance of the protons on the proximate
phenyl units. Indeed, their twisted angle is as high as 73.5 for
the bicarbazole monomer. In the monolayer, the twisted angles
decreased to 48.9 and 57.9 for Cz-COF1 and Cz-COF2,
–N bond in which they are twisted
°
°
°
respectively, leading to more planar structures for the
bicarbazole vertices. Notably, the twisted angles become much
smaller in the stacked frameworks; the twisted angle is only
Fig. 1 PXRD profiles of (a) Cz-COF1 and (b) Cz-COF2 for experimentally observed (red),
Pawley refined (green), difference between experimental and calculated data (black),
calculated for the AA (blue) and AB (yellow) stacking models. Insets are structures for Cz-
COF1 and Cz-COF2 with the AA (upper) and AB (bottom) stacking models.
37.2° for Cz-COF1 and 42.1° for Cz-COF2. These decrements in
the twist angle along with the formations of 2D polygon
monolayer and frameworks are profound and indicate that the
bicarbazole units although with an extremely large dihedral
angle, can trigger a significant conformational change upon
polycondensation and fit the two carbazole subunits into the 2D
frameworks.
The atomic-level construction of Cz-COFs was further
investigated by solid-state 13C cross-polarization magic-angle
spinning (CP/MAS) NMR spectroscopy (Fig. S3, ESI†). The peaks
at 157 ppm corresponding to the C atoms of the C=N linkages
were observed for both COFs. Notably, the peaks at 148 ppm
assigned to the carbon atoms bonding to the N atoms of the
carbazole rings were also observed. The signals at 191 ppm for
the end group of C=O almost disappeared, indicating that most
aldehyde groups have been consumed in the condensation.
Besides, another peak at 127 ppm was observed for Cz-COF2,
which was assigned to the carbon atoms of the biphenyl edge
units. Field emission scanning electron microscopy revealed
that Cz-COFs have plate-like morphology (Fig. S4a and b, ESI†).
High-resolution transmission electron microscopic images
showed the presence of porous textures in Cz-COFs (Fig. S4c and
d, ESI†).
In the stacked structure, Cz-COF1 adopts an AA stacking
model of a space group of C222 with a = 27.3616
Å, b = 27.4776
Å
, and c = 3.9778
Å
(Fig. 1a, upper inset, and Table S4, ESI†).
The simulated PXRD pattern of the AA stacking model (Fig. 1a,
blue curve) matched the experimental peak positions and
intensities, whereas the staggered AB stacking model (Fig. 1a,
yellow curve) did not reproduce the data. The staggered AB
model (Table S5, ESI†) resulted in covered pores by the
neighboring sheets (Fig. 1a, bottom inset). Similarly, Cz-COF2
also assumes an AA stacking model of a space group of C222
with a = 33.1865
Å, b = 32.485 Å, and c = 3.9868 Å (Fig. 1b,
upper inset, and Table S6, ESI†). The simulated PXRD pattern of
the AA stacking model (Fig. 1b, blue curve) also matched well
the experimental peak positions and intensities, whereas the
staggered AB stacking model (Fig. 1b, orange curve) did not
Both Cz-COFs are highly crystalline polymers, as revealed by
powder X-ray diffraction (PXRD) measurements. The
2 | J. Name., 2012, 00, 1-3
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