Journal of the American Chemical Society
Article
adsorption, which degassed at least five times before isotherm
collection.
Scheme 1. Schematic Representation of the Synthesis of
FCOF-5
Synthesis of 1,2,4,5-Tetrakis[(4-formylphenoxy)methyl]-
benzene (TFMB). A suspension of 1,2,4,5-tetrakis(bromomethyl)-
benzene (4.50 g, 10 mmol), p-hydroxybenzaldehyde (5.25 g, 43
mmol), and potassium carbonate (2.76 g, 20 mmol) in CH3CN (450
mL) was refluxed for 18 h. After cooling to room temperature, the
reaction mixture was poured into water. The precipitate was filtered
and washed with water and methanol. The crude product was purified
by column chromatography [SiO2:CH2Cl2/ethyl acetate = 15/1] to
1
yield TFMB as a white solid (4.1 g, 67% yield). H NMR (400 M,
DMSO-d6, ppm): δ = 9.85 (s, 4H), 7.83 (d, J = 8.8 Hz, 8H), 7.74 (s,
2H), 7.19 (d, J = 8.8 Hz, 8H), 5.40 (s, 8H). 13C NMR (100 MHz,
DMSO-d6, ppm): δ = 191.2, 162.9, 134.7, 131.6, 129.8, 129.0, 115.2,
67.1. HR-MS (MALDI-TOF): m/z calcd for C38H30O8: 637.1838 [M
+ Na]+; found: 637.1833 [M + Na]+.
Synthesis of FCOF-5. TFMB (49.2 mg, 0.08 mmol) and TAPM
(30.4 mg, 0.08 mmol) were placed in a Pyrex tube with mesitylene
(2.8 mL), chloroform (1.2 mL), and 12 M aqueous acetic acid (0.4
mL). The tube was flash frozen in a liquid nitrogen bath, degassed
through three freeze−pump−thaw cycles, sealed under vacuum, and
then heated at 110 °C for 7 days. The mixture was cooled to room
temperature, and the resulting precipitate was filtered, exhaustively
washed by Soxhlet extractions with tetrahydrofuran and dichloro-
methane, and dried at 90 °C under vacuum for 24 h. The activated
FCOF-5 was isolated as a light-yellow powder (60 mg, 75% yield).
Elemental analysis: Calculated for (C63H46N4O4)n: C, 81.97%; H,
5.02%; N, 6.07%. Found: C, 80.26%; H, 5.01%; N, 6.00%.
Vapor-Triggered Transformation of FCOF-5. A 5 mL beaker
charged with 20 mg of FCOF-5 was put in a weighing bottle, which
was filled with 2 mL of THF. After being kept at room temperature
for a certain time, the samples was taken out and then investigated by
TGA and PXRD immediately.
Fabrication of FCOF-5@PVDF Film. FCOF-5 powder (40 mg)
was mixed with poly(vinylidene fluoride) (PVDF) (40 mg) in DMF
(0.5 mL). After stirring for 10 min, the mixture was drop-casted onto
a glass plate by scraper and placed in a preheated oven at 120 °C for 4
h.15 After cooling to room temperature, the light-yellow film was
detached from the substrate after being immersed into ethanol and
dried at 80 °C for 30 min. Owing to the contraction of the unit cell of
FCOF-5, the composite film was curled in the initial state.
Vapor-Triggered Actuations of the FCOF-5@PVDF Film. In a
typical experiment, a glass bottle with diameter of 2.5 cm and height
of 9 cm was charged with about 3 mL of organic solvents at room
temperature. A piece of film (20 mm × 3 mm) was placed into the
bottle and then taken out to study the vapor-triggered actuation
behaviors. To further visualize the breathing behavior of FCOF-5, we
prepared the petal of a flower from the FCOF-5@PVDF film. The
flower was inserted into silica sand in a glass box (27 cm × 22 cm × 7
cm), and then the organic solvent vapors were purged into the box.
Interestingly, the closed flower petals gradually bloomed and then
changed back to the initial shape after exposure to air.
starting from a molecular building block with C−O single
bonds in the backbone. By using the advanced continuous
rotation electron diffraction (cRED) technique12 and after
merging 17 cRED data sets, we successfully determined the
crystal structure of FCOF-5, which adopts a 6-fold inter-
penetrated pts topology. Interestingly, FCOF-5 did not adsorb
N2 but exhibited the ability in adsorption of organic solvent
vapors, e.g., tetrahydrofuran (THF). Moreover, the adsorption
or desorption of THF molecules within FCOF-5 can
remarkably induce expansion or contraction of the framework,
indicating a breathing motion. Finally, by utilizing this
molecular-level stimuli-responsive behavior, a smart soft
polymer composite film with FCOF-5 was fabricated, which
can show a reversible vapor-triggered shape transformation.
EXPERIMENTAL SECTION
■
Materials. All reagents and solvents, unless otherwise noted, were
purchased from commercial sources and used without further
purification. 1,2,4,5-Tetrakis(bromomethyl)benzene and 4-hydroxy-
benzaldehyde were purchased from Innochem. 1,2,4,5-Tetrakis[(4-
formylphenoxy)methyl]benzene (TFMB)13 and tetra(p-amino-
phenyl)methane (TAPM)14 were synthesized according to the
reported procedures.
1
Characterization. H and 13C NMR spectra were measured on a
Bruker Fourier 400 M spectrometer. High-resolution mass spectra
were collected on Bruker Solarix. Elemental analysis was conducted
on a Flash EA 1112. Fourier transform infrared (FT-IR) spectra were
recorded on a Nicolet iN10 micro FTIR spectrometer. Powder X-ray
diffraction (PXRD) patterns were obtained on a Rigaku SmartLab X-
ray diffractometer with Cu Kα line focused radiation at 45 kV and 200
mA or a Rigaku MiniFlex 600 X-ray diffractometer. Thermogravi-
metric analysis (TGA) from 30 to 800 °C was performed on a TA-
Q500 in a nitrogen atmosphere by using a 10 °C/min ramp without
equilibration delay. Field-emission scanning electron microscope (FE-
SEM) was performed on a ZEISS SIGMA operating at an accelerating
voltage ranging from 0.1 to 20 kV. Solid-state NMR spectra were
conducted at ambient pressure on a Bruker AVANCE III 400 M
spectrometer by using a standard CP-TOSS pulse sequence (cross-
polarization with total suppression of sidebands) probe with 4 mm
(outside diameter) zirconia rotors. Cross-polarization with TOSS was
used to acquire 13C data at 100.37 MHz. The 13C 90° pulse widths
were 4 μs. The decoupling frequency corresponded to 72 kHz. The
TOSS sample-spinning rate was 12 kHz. The recycle delay was 3 s.
The 13C chemical shifts are given relative to glycine as 176.03 ppm.
Nitrogen isotherms at 77 K were collected on a Quantachrome
Autosorb-iQ2 automated gas sorption analyzer. Before measurement,
the samples were degassed in a vacuum at 90 °C for 24 h. The
tetrahydrofuran vapor adsorption isotherms were collected by using a
MicrotracBELSopr-Aqua3 adsorption apparatus with a water
circulator bath. Anhydrous tetrahydrofuran was used for vapor
RESULTS AND DISCUSSION
■
In consideration of the challenges mentioned above, we believe
it is relatively easy to construct such 3D COFs through
condensation of one flexible and one rigid building blocks.
Therefore, we designed and synthesized a flexible building
block, 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]benzene
(TFMB, Scheme 1), which has flexible C−O single bonds in
the backbone. Following our topology design strategy to build
3D COF,3d,e,4a we then chose the reported tetra(p-
aminophenyl)methane (TAPM) as the rigid precursor, which
can react with TFMB to form desirable FCOF-5 via [4 + 4]
imine condensation reactions. We should emphasize here, due
to the free rotation of the C−O single bonds in TFMB, it is a
big challenge to synthesize FCOF-5, especially toward high
crystalline samples for further structural determination (Figure
2124
J. Am. Chem. Soc. 2021, 143, 2123−2129