Crystalline C - C and C=C Bond-Linked Chiral Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.0c11050

While crystalline covalent organic frameworks (COFs) linked by C-C bonds are highly desired in synthetic chemistry, it remains a formidable challenge to synthesize. Efforts to generate C-C single bonds in COFs via de novo synthesis usually afford amorphous structures rather than crystalline phases. We demonstrate here that C-C single bond-based COFs can be prepared by direct reduction of C=C bond-linked frameworks via crystal-to-crystal transformation. By Knoevenagel polycondensation of chiral tetrabenzaldehyde of dibinaphthyl-22-crown-6 with 1,4-phenylenediacetonitrile or 4,4′-biphenyldiacetonitrile, two olefin-linked chiral COFs with 2D layered tetragonal structure are prepared. Reduction of olefin linkages of the as-prepared CCOFs produces two C-C single bond linked frameworks, which retains high crystallinity and porosity as well as high chemical stability in both strong acids and bases. The quantitative reduction is confirmed by Fourier transform infrared and cross-polarization magic angle spinning 13C NMR spectroscopy. Compared to the pristine structures, the reduced CCOFs display blue-shifted emission with enhanced quantum yields and fluorescence lifetimes, while the parent CCOFs exhibit higher enantioselectivity than the reduced analogs when be used as fluorescent sensors to detect chiral amino alcohols via supramolecular interactions with the built-in crown ether moieties. This work provides an attractive strategy for making chemically stable functionalized COFs with new linkages that are otherwise hard to produce.

Full text of DOI:10.1021/jacs.0c11050

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Crystalline CC and CC Bond-Linked Chiral Covalent Organic
Frameworks
Chen Yuan, Shiguo Fu, Kuiwei Yang, Bang Hou, Yan Liu,* Jianwen Jiang, and Yong Cui*
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ABSTRACT: While crystalline covalent organic frameworks (COFs) linked by C−C bonds are highly desired in synthetic
chemistry, it remains a formidable challenge to synthesize. Efforts to generate C−C single bonds in COFs via de novo synthesis
usually afford amorphous structures rather than crystalline phases. We demonstrate here that C−C single bond-based COFs can be
prepared by direct reduction of CC bond-linked frameworks via crystal-to-crystal transformation. By Knoevenagel
polycondensation of chiral tetrabenzaldehyde of dibinaphthyl-22-crown-6 with 1,4-phenylenediacetonitrile or 4,4′-biphenyldiaceto-
nitrile, two olefin-linked chiral COFs with 2D layered tetragonal structure are prepared. Reduction of olefin linkages of the as-
prepared CCOFs produces two C−C single bond linked frameworks, which retains high crystallinity and porosity as well as high
chemical stability in both strong acids and bases. The quantitative reduction is confirmed by Fourier transform infrared and cross-
polarization magic angle spinning 13C NMR spectroscopy. Compared to the pristine structures, the reduced CCOFs display blue-
shifted emission with enhanced quantum yields and fluorescence lifetimes, while the parent CCOFs exhibit higher enantioselectivity
than the reduced analogs when be used as fluorescent sensors to detect chiral amino alcohols via supramolecular interactions with
the built-in crown ether moieties. This work provides an attractive strategy for making chemically stable functionalized COFs with
new linkages that are otherwise hard to produce.
synthesis of chiral COFs (CCOFs) linked by less reversible
bonds has not yet be explored.8 In this work, we illustrated that
C−C single bond linked CCOFs can be synthesized by
reduction of olefin-linked frameworks via crystal-to-crystal
transformation. Interestingly, postsynthetic methods have
recently been explored to convert imine linkages in COFs to
other linkages such as amide and amine.9
Macrocyclic compounds such as crown ethers have been the
focus of intense research interest for decades because of their
superior selectivity in complexation with metal ions and
INTRODUCTION
■
Covalent organic frameworks (COFs), an attractive class of
porous crystalline polymers,1 have been widely explored for
applications in diverse areas such as storage,2 separation,3 and
catalysis4 because of their tunable structures and chemical
properties. The crystallinity, stability, and functions of COFs
are dictated by the type of covalent linkages that join the
molecular building blocks. By far, covalent linkages varying
from reversible boroxine, imine, and hydrazone; less reversible
triazine, phenazine, and oxazole; to almost nonreversible dioxin
and olefin have been reported to construct COFs.5 It is
believed that making C−C single bond-linked COFs would
address the chemical stability issues of the network linkages
while also producing materials with new properties. However,
efforts to produce C−C single bonds in COFs via de novo
synthesis typically afford amorphous structures rather than
crystalline phases.6 On the other hand, despite their potential
use in enantioselective catalysis, separation, and sensing,7 the
Received: October 20, 2020
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Scheme 1. Synthesis of the COFs
organic cations.10 Chiral crown ethers are capable of binding
protonated chiral primary amines with high stereoselectivity
and affinity and have been extensively employed to resolve or
distinguish the enantiomers of chiral amines. However, chiral
crown ethers have not yet been used in construction of CCOFs
for enantioselective processes, in spite of several related
metal−organic frameworks (MOFs) having been reported.11
Herein, an enantiopure tetrabenzaldehyde derived from
dibinaphthyl-22-crown-6 (BINOL2-C, Scheme 1), which has
as the dialdehyde primary functionality and 22-crown-6
secondary functionality, was designed and synthesize to
crystallize CCOFs. It is worth noting that, owing to its axial
chirality characteristic, optically active 1,1′-bi-2-naphthol
(BINOL) has become a key source of chirality for organic
synthesis and materials science and provides an attractive
platform for asymmetric catalysis and enantioselective
recognitions.12 By Knoevenagel polycondensation of
BINOL2-C with 2,2′-(1,4-phenylene)diacetonitrile (PDAN)
or 2,2′-(biphenyl-4,4′-diyl) diacetonitrile (BPDAN), the first
two examples of olefin-linked CCOFs that have a 2D layered
tetragonal structure were prepared. Reduction of olefin
linkages of the as-prepared frameworks produces the first
two examples of C−C single bond-linked CCOFs, which
retains crystallinity and porosity as well as chemical stability
(Scheme 1). Compared to the parent CCOFs, the reduced
frameworks display blue-shifted emission with enhanced
quantum yields and fluorescence lifetimes; whereas the parent
CCOFs exhibit better enantioselectivity than the reduced
materials when used as fluorescent sensors to detect amino
alcohols via supramolecular interactions with the immobilized
crown ether moieties.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. CCOFs 17 and 18. The
enantiopure monomer BINOL2-C was prepared by Suzuki
coupling reaction between bromobenzene derivate of dibi-
naphthyl-22-crown-6 and 4-formylphenylboronic acid. CCOFs
17 and 18 were then prepared by solvothermal reactions of
BINOL2-C (0.064 mmol), 2,2′-(1,4-phenylene) diacetonitrile
(PDAN; 0.128 mmol) or 2,2′-(biphenyl-4,4′-diyl)-
diacetonitrile (BPDAN; 0.128 mmol) in dioxane (0.5 mL),
methanol (0.5 mL) and acetonitrile (0.2 mL) or mesitylene
(0.1 mL) in the presence of KOH solution as a catalyst at 120
°C. After 4 days, the reactions afforded bright yellow
microcrystalline solids in about 85% yields. Notably, although
K+ and crown ethers tend to form stable host−guest adducts,
inductively coupled plasma optical emission spectrometer
(ICP-OES) of the digested crystals showed the contents of
potassium are only 0.009% and 0.021% for CCOFs 17 and 18,
respectively.
B
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Figure 1. (a−d) PXRD patterns of the four CCOFs with the experimental profiles in red, Pawley refined in black, calculated in dark green, and the
difference between the experimental and refined PXRD patterns in blue.
The as-prepared COFs were characterized by a variety of
spectroscopic techniques. The Fourier transform infrared (FT-
IR) spectra of the two sp2-carbon-linked conjugated CCOFs
(sp2c-CCOFs) show a great decrease of the characteristic C
O vibration band (1690 cm−1) and the newly appearance of
the CN vibration band (1620 cm−1), indicating the
successful polymerization (Figure S1). The 13C cross-polar-
ization magic angle spinning (CP-MAS) NMR spectra show
that the aldehyde carbon peaks were barely observed and the
CC linkages were observed at ∼110 and ∼190 ppm (Figure
3b,c). The chemical shifts of other fragments are in good
agreement with those of the monomers. Thermal gravimetric
analysis (TGA) reveals that both sp2c-CCOFs are stable up to
350 °C (Figure S2). Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images show that
both of them possess uniform layered morphology (Figures 3d
and S3). Circular dichroism (CD) spectra of these sp2c-
CCOFs made from (R,R)- and (S,S)-enantiomers of the
dibinaphthyl-22-crown-6 monomers are mirror images of each
other, which is indicative of their enantiomeric nature (Figure
S4).
cell parameters were obtained (a = 53.5 Å, b = 40.7 Å, c = 8.3
Å, α = 135.6°, β = 91.2°, and γ = 88.9°). The experimental
PXRD pattern for CCOF 17 with P1 space group exhibited
main diffraction peaks at 3.55°, 6.02°, 7.13°, 9.21°, 10.46°, and
11.85°, which were assigned to the (110), (310), (220), (420),
(330), and (1−51) facets, respectively. This PXRD pattern was
in good agreement with the simulated pattern based on AA
stacking structure of CCOF 17, suggesting that the material
holds a single-pore structure. The lattice modeling and Pawley
refinement (Materials Studio, version 7.0) provide good
agreement factors (Rp = 0.59% and Rwp = 1.14%).
The experimental PXRD pattern for CCOF 18 with a P1
space group exhibited main peaks at 3.36°, 6.61°, 7.03°, 7.97°,
10.44°, and 11.92°, which were assigned to the (110), (020),
(220), (320), (251), and (620) facets, respectively. This
PXRD pattern agreed well with the simulated pattern based on
the AA stacking structure. The refinement results yield unit cell
parameters are nearly equivalent to the predictions that a =
51.5 Å, b = 50.6 Å, c = 9.3 Å, α = 145.2°, β = 87.7°, and γ =
91.9° (Rp = 0.21% and Rwp = 0.45%). PXRD patterns were also
calculated for the two CCOFs on the other 2D and 3D
structures, but all of the calculated PXRD patterns did not
match the experimental patterns well (Figures S5−S12).
CCOFs 17 and 18 are therefore proposed to have a slipped
AA stacking structure (Figure 2).
Crystal Structure. The crystalline structures of the CCOFs
were determined by powder X-ray diffraction (PXRD) analysis
with Cu Kα radiation (Figure 1). Considering the geometry of
the precursors and the connection patterns, a few 2D and 3D
topologies are reasonable. However, considering their layered
morphology observed by SEM and TEM, 2D topologies are
more rational. After a geometrical energy minimization based
on the 2D net with AA stacking mode for CCOF 17, the unit
We failed to calculate the pore size distributions of these two
sp2c-CCOFs by measuring nitrogen sorption isotherms
because of the low adsorption of N2. This is probably due to
the distortion of frameworks containing flexible crown ether
C
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Figure 2. Top views of the corresponding refined structure of (a) CCOF 17 (left) and CCOF 17-R (right) and (b) CCOF 18 (left) and CCOF
18-R (right). Side views of the slipped AA stacking structure of (c) CCOF 17 and (d) CCOF 18. Carbon, gray; nitrogen, blue; oxygen, red;
hydrogen, light gray.
moieties upon solvent removal.13a So, we used a dye uptake
assay to evaluate the pore size distribution of the CCOFs.9b,13
We carried out dye-uptake studies by soaking the COFs in a
solution of dyes with different sizes for 24 h. The dye solution
was decanted, and the COFs were washed several times to
remove dye molecules adsorbed on the external surfaces of the
solids. Dye molecules and COFs after spectral separation can
be assigned to red and green fluorescence by confocal
fluorescence microscopy (CFM), respectively (Figures 4b
and S13). The CFM results showed the uniform distributions
of dyes A−C with molecular sizes ranging from 1.40 × 1.60
nm2 to 2.20 × 2.40 nm2. However, the sterically bulky dye D
(3.20 × 3.60 nm2) was only attached to the surfaces of the
COFs and cannot enter the pores from the open channels,
probably due to its larger size. The dye absorption amount was
determined by measuring the UV−vis spectra in THF (Figures
S14 and S15). As shown in Figure 4c, remarkable size
selectivity was observed for the dye uptake: the COFs had very
significant uptake of dyes A−C (1.4−7.9 wt%) but had only
negligible uptake of dye D (<0.3 wt%). The dye uptake
experiment indicated that both the maximum pore diameter
and opening of the two COFs were in the range of 2.4 to 3.2
nm, consistent with the crystal structures.
bonds in CCOFs, we selected a commercially available
compound, 2,3-diphenylacrylonitrile (DPAN), as a model
molecule. Among various reductive conditions tested, we
identified that the best method involving the use of NaBH4, by
which quantitative conversion of DPAN to 2,3-diphenyl-
propionohnitrile (DPPN) was realized (Figure 3a). This was
confirmed by the shift of CN vibration peaks from 2218 to
2241 cm−1 in the FT-IR spectrum of the product. 13C NMR
spectra of the model compounds are another evidence for the
reduction of CC bond. The peaks belong to sp2-carbon at
126 and 143 ppm disappeared completely and new peaks at 40
and 42 ppm emerged, indicating the formation of C−C bond.
The mixture of CCOF 17 or 18 (10 mg), NaBH4 (100 mg),
and 2 mL of i-PrOH stood at r.t. for 3 days, then another part
of NaBH4 (100 mg) was added, and the mixture stood for
another 4 days to achieve the successfully conversion of olefin
CCOFs to C−C bond linked 17-R and 18-R (see the
Supporting Information for details). The quantitative reduc-
tion was confirmed by the appearance of a new peak at 2241
cm−1 from the CN vibration band in the FT-IR spectrum of
both CCOFs 17-R and 18-R (Figure 4b,c). Besides, the 13C
CP-MAS NMR spectra showed peaks shifts due to the
formation of a C−C bond. Peaks that belong to sp2 carbon at
103 and 189 ppm disappeared completely, and new peaks at 35
and 60 ppm emerged for CCOF 17-R. For CCOF 18-R, peaks
that belong to sp2 carbon at 111 and 189 ppm disappeared
CCOFs 17-R and 18-R. Both of them were prepared by
direct reduction of the olefin-linked CCOFs via crystal-to-
crystal transformation. In order to identify a suitable condition
to realize solid-state conversion of CC linkages to C−C
D
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Figure 3. Scheme for reduction of (a) the model molecule, (b) CCOF 17, and (c) CCOF 18. Corresponding FTIR spectra and 13C NMR spectra
are in the right. *, sidebands. (d) SEM images of the parent and reduced CCOFs.
when new peaks at 36 and 61 ppm emerged, indicating the
ratio of the peaks were observed, verifying that the crystalline
structures of COFs were retained after reduction (Figure 1c,d).
The additional flexibility of C−C bond linkages compared to
CC bond linkages prevents tight stacking between layers
successful reduction of CCOF 18.
PXRD patterns of the reduced CCOFs are almost the same
with the pristine CCOFs, although decreases in signal-to-noise
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Figure 4. (a) Structure and molecular sizes of dyes A−D. (b) CFM images obtained from CCOF 17 after incubation with dyes A−D, respectively.
(c) Different dye uptake of CCOFs by UV−vis spectroscopy.
and leads to slightly decreased crystalline. When finishing the
geometrical energy minimization based on the 2D net mode,
the unit cell parameters of CCOF 17-R were obtained (a =
51.1 Å, b = 40.8 Å, c = 8.6 Å, α = 137.6°, β = 91.1°, and γ =
89.0°). The experimental PXRD pattern for CCOF 17 with P1
space group exhibited main diffraction peaks at 3.55°, 6.08°,
7.15°, 9.32°, and 11.96°, which were assigned to the (110),
(310), (220), (420), and (2−51) facets, respectively. This
PXRD pattern was in good agreement with the simulated
pattern based on AA stacking structure of CCOF 17-R. The
lattice modeling and Pawley refinement provide good agree-
ment factors (Rp = 0.33% and Rwp = 0.68%). For CCOF 18-R,
the experimental PXRD pattern with P1 space group exhibited
peaks at 3.36°, 6.23°, 7.07°, 7.97°, 10.48°, and 11.94°, which
were assigned to the (110), (310), (220), (320), (330), and
(040) facets, respectively. This PXRD pattern also agreed well
with the simulated pattern based on the AA stacking structure
of CCOF 18-R. The refinement results yield unit cell
parameters that are nearly equivalent to the predictions that
a = 50.7 Å, b = 50.0 Å, c = 9.7 Å, α = 144.1°, β = 88.3°, and γ =
90.8° (Rp = 0.27% and Rwp = 0.69%). So, CCOFs 17-R and 18-
R are proposed to have the architectures as illustrated in Figure
2.
the range of 2.4 to 3.2 nm, indicating retention of porous
structures of the parent frameworks (Figures 4c and S13−
S15). SEM and TEM images showed that both reduced
frameworks had a uniform layered morphology (Figures 3d
and S3), indicating that the crystal morphology remained
unchanged after the solid-state conversion. In TGA curves of
CCOF 17-R and 18-R, the weight loss was also observed in the
temperature above 350 °C (Figure S2), which is almost the
same as the parent CCOFs.
Chemical Stability. To evaluate the chemical stability of
CCOFs, we immersed them for 1 week in various solvents,
including THF, DMF, MeOH, water, concentrated HCl (6
M), and aqueous KOH (14 M) solutions. From PXRD data
(Figures 5a,b and S16), CCOFs 17 and 18 are stable and
retained their crystallinity. Notably, 17 and 18 exhibited nearly
no weight loss (<1 wt%) in common organic solvents, even
under strong acidic (6 M HCl) and basic (14 M KOH)
conditions; the residual weight percentages were as high as 93
and 88 wt% for 17 and 95 and 93 wt% for 18 (Figure S17).
Also, we did the dye A uptake experiment to check the
preservation of pore structure. It was found that the dye
absorption amount of 17 and 18 after treating with various
solvents showed almost no decrease, indicating retention of
their porous structures (Figure 5c,d).
Dye uptake experiments and the CFM results revealed that
the maximum pore diameter of CCOFs 17-R and 18-R were in
F
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Figure 5. PXRD patterns of (a) CCOFs 17 and 17-R and (b) CCOF 18 and 18-R after treatment in different solvents for 1 week. Dye A uptake of
(c) CCOFs 17 and 17-R and (d) CCOFs 18 and 18-R after treatment in different solvents for 1 week by UV−vis spectroscopy.
As expected, CCOFs 17-R and 18-R keep crystallinity and
porosity after soaking in concentrated HCl (6 M) and aqueous
KOH (14 M) solutions for 1 week, as indicated by PXRD
(Figure 5a,b). From the residual weight percentages, they
exhibit nearly no weight loss (<1 wt%) in organic solvents and
little weight loss (<10 wt%) under strong acid (6 M HCl) and
base (14 M KOH; Figure S17). After these treatments, CCOF
17-R and 18-R can uptake 3−6 and 5−8 wt% of dye A,
respectively, which are close to those of the as-synthesized
samples (Figure 5c,d). The present frameworks represent the
most stable CCOFs that have been reported thus far (Figure
S18).8 Obviously, the good chemical stability of these CCOFs
are attributed to the stable CC bond and C−C bond
linkages.
Enantioselective Fluorescence Recognition. The
photophysical properties of CCOFs were studied by UV−vis,
fluorescent and circular dichroism (CD) spectra, and
compared with the homogeneous control P-BINOL2-C,
which was prepared by the Knoevenagel condensation of
BINOL2-C and phenylacetonitrile (for details see section 2.3
in the Supporting Information).
yields (ϕPL) of 5.8% and 6.5%, respectively (Figure 6 and
Table S1), different from P-BINOL2-C, which emitted around
495 nm with ϕPL of 17.5%. Time-resolved fluorescence
spectroscopy revealed that 17 and 18 have a lifetime (τ) of
0.83 and 0.88 ns, which is smaller than 1.65 ns of P-BINOL2-C
(Table S1). The decreased ϕPL and τ values suggest that the
π−π interactions between layers of sp2c-CCOFs facilitate the
nonradiation energy transfer. Interestingly, after reduction, the
emission maxima of 17-R and 18-R were blue-shifted to 507
and 493 nm with ϕPL of 19.0% and 20.5% and τ of 2.10 and
2.04 ns, respectively. Compared to the parent CCOFs, the
reduced CCOFs show increased ϕPL and τ values. It is likely
that the newly formed C−C bond linkages destroyed the π-
conjugated structure of sp2c-CCOFs, leading to the decrease of
π−π interactions between COF layers and limiting the
nonradiation energy transfer.14 In addition, CD spectra of
CCOFs made from the opposite enantiomers were mirror
images of each other, suggesting their enantiomeric nature
(Figure S4), consistent with their UV−vis spectra (Figure
6a,b).
Chiral discrimination of compounds with amino and
hydroxyl functionalities is of critical importance in many
areas of analytical chemistry and biotechnology.15 The
presence of chiral functional crown ether groups in these
fluorescent CCOFs makes them good candidates for
enantioselective recognition of chiral molecules. Crown ethers
have a unique architectural feature that allows them to form
various complexes with organic ammonium ions.16 Primary
As shown in Figure 6, CCOFs 17, 17-R, 18, and 18-R
absorb light in the UV and visible regions, with absorption
maxima at 422, 396, 422, and 397 nm, respectively, which are
red-shifted by 5−40 nm in comparison to that of the BINOL2-
C monomer, due to the conjugation in the crystalline
structures. Upon illumination with UV light, CCOFs 17 and
18 gave emission maxima at 527 and 511 nm with quantum
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Figure 6. (a,b) Solid-state UV−vis spectra of CCOFs. Corresponding photographs of the parent and reduced CCOFs are in the right. (c,d) The
solid-state fluorescence spectra of CCOFs. Corresponding photographs of the parent and reduced CCOFs under 365 nm UV light are in the right.
ammonium guests can bind to crown ethers via hydrogen
bonds between their N+−H bonds and the free electron pair
atoms in crown ethers (Figure 7a). As a result, CCOFs were
tested for fluorescence enhancement or quenching by amino
alcohols.
Microcrystalline particles of the (R)-CCOFs with an average
size of 1.4−2.9 μm were prepared by vigorous stirring with a
magnetic stir bar and then suspended in MeOH to prepare a
stock suspension with the CCOF concentration of 1 × 10−5
mol/L. The suspension was acidified by an aqueous HClO4
solution (pH 1). Aliquots containing different amounts of D-
and L-amino alcohol were added to the acidic MeOH
suspensions of (R)-CCOFs (3 mL). The fluorescence emission
signals of the CCOF suspension in the presence of different
amounts of amino alcohols were measured. As shown in Figure
7c, when CCOF 17 was treated with phenylglycinol (PGL),
D-tryptophanol was faster than that by the L-enantiomer, and
the fluorescence intensity was decreased to 43% and 66% of
the original level, respectively. Based on the linear Stern−
̈
Volmer equation, the measured absorbance [I0/I] varied as a
function of [M] in a linear relationship (Figure S20), where I0
is the maximum fluorescence intensity before exposure to the
analyte and I is the maximum intensity after exposure to the
analyte, indicating 1:1 stoichiometry of the interaction between
the crown ether of 17 and tryptophanol. The Ksv constants
were calculated to be 820.5 M−1 and 340.1 M−1 with the D-
and L-enantiomers, respectively, affording the quenching ratio
[QR = Ksv(D‑tryptophanol)/Ksv(L‑tryptophanol)] of 2.41.
Figure 7e depicts the effect of the pH value of HClO4
aqueous solution on fluorescence intensity. HClO4 aqueous
solutions in different pH value were added to the suspension of
CCOF 17, and the fluorescence intensity enhancement after
treated with 0.083 μM D-phenylglycinol was measured.
Increased hydrogen ion concentration produced an increase
in the observed fluorescence intensity change. This tendency of
change is very similar to the retention factor change when
using chiral crown ether as liquid chromatographic stationary
phases,17 indicating that the large enantioselectivity factors of
amino alcohols are presumably because of the ability of the
preferred enantiomers to “fill” the chiral cavity available to
them on complexation with the crown ether entities in CCOF
17. The formation of amino alcohol-crown ether adducts may
affect proton-transfer-assisted charge-transfer excited state,
leading to the fluorescence intensity changes.
the emission at 529 nm was enhanced obviously by
D
enantiomers, while the intensity was almost no change with
L-enantiomer, indicative of enantioselectivity. The fluorescence
intensity was increased to 1.36 and 1.02 times that of the
original value by D- and L-phenylglycinol, respectively. The
corresponding Benesi−Hildebrand plots were shown in Figure
7d. The association constants KBH of 17 were calculated to be
2338.60
149.38 and 158.88
10.41 M−1 with L- and D-
phenylglycinol, respectively, affording an enantioselectivity
factor [EF = KBH(L‑phenylglycinol)/KBH(D‑phenylglycinol)] of 14.72.
Besides, phenylalaninol (PAL) can also selectively enhance the
fluorescence of CCOF 17 (Figures 7f and S20), with the EF of
12.85.
In contrast, when treated with both D- and L-tryptophanol
(TPL), the fluorescence of 17 was quenched and followed
To microscopically elucidate the interactions between (R)-
CCOF 17 and D-/L-protonated PGL and reveal the chiral
sensing mechanism, density functional theory (DFT) calcu-
lations were performed. First, potential energy surface scan was
̈
Stern−Volmer (S−V) behavior. The quenching rate caused by
H
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Figure 7. (a) Operation principle of the CCOF sensor. (b) Structures of the examined amino alcohols. (c) Fluorescence emission spectra of (R)-17
with increasing concentration of D-phenylglycinol in solution. (d) Benesi−Hildebrand plots of the fluorescence emissions of (R)-17 enhanced by D-
and L-phenylglycinol. (e) The effect of pH values of HClO4 aqueous solutions on fluorescence intensity. (f) The comparative EFs/QRs of the (R)-
P-BINOL2-C and (R)-CCOFs for three amino alcohols.
conducted to identify the most stable conformations of isolated
D-/L-protonated PGL (Figures S22 and S23), which were then
used to explore the host−guest interactions. Various initial
binding structures were examined for both enantiomers
(Figure S24) over a two-layer ONIOM cluster model
representing (R)-CCOF 17 (Figure S25). Figure 8 depicts
the simplified structures of the most stable conformers of D-/L-
protonated PGL with (R)-CCOF 17 (denoted as D-complex
and L-complex, respectively). The corresponding complete
structures with adjacent 2D COF layers can be found in
Figures S26 and S27. In the D-complex, D-protonated PGL
interacts with the crown ether group in (R)-CCOF 17 through
three hydrogen bonds with H···O distances of 1.93, 2.46, and
2.72 Å and NH···O angles of 152°, 149°, and 136°,
respectively. In the L-complex, L-protonated PGL coordinates
with the crown ether group through three hydrogen bonds
Figure 8. Side (upper) and top (lower) views of the lowest-energy
structures of D-/L-protonated PGL with (R)-CCOF 17 (denoted as D-
complex and L-complex, respectively). The relative Gibbs energies in
methanol with respect to D-complex at 298.15 K are indicated in
with H···O distances of 1.79, 2.49, and 2.27 Å and NH···O
angles of 167°, 114°, and 144°, respectively. The phenyl group
in D-/L-protonated PGL points away/toward the confined
space constituted by adjacent 2D COF layers (Figures S26 and
S27). At room temperature (298.15 K) and with methanol as a
solvent, the relative Gibbs energy for L-complex is estimated to
be 4.4 kcal/mol with respect to D-complex, indicating a
preferential binding and sensing of D-enantiomer by (R)-
CCOF 17, which mainly results from more favorable host−
guest coordination through hydrogen bonds and less steric
hindrance imposed by the surrounding framework in D-
parentheses. The lengths (in Å) of hydrogen bonds involved in the
host−guest interactions are labeled. The adjacent layers surrounding
the host−guest complexes are omitted for clarity. C: gray, H: white,
O: red, and N: blue.
complex as compared to that for the L-complex. These
theoretical results well support the experimental observations
(Figure 7).
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The homogeneous control P-BINOL2-C showed obvious
fluorescence quenching toward all three amino alcohols, quite
different from the observed for CCOF 17. The QR values
ranging from 1.28 to 1.30 were much smaller than the QR/EF
values (4.72, 12.85, and 2.41) of CCOF 17. This CCOF
material is thus much sensitive than its molecular analog. Such
a difference indicates that the porous CCOF is capable of
providing a well-defined chiral environment for high
enantiodiscrimination of amino alcohol enantiomer in
comparison with P-BINOL2-C. It should be noted that
CCOF 17 show higher or comparable stereoselectivity to
those reported BINOL-based sensors including organic
molecules, organic oligomers, polymers, and coordination
assemblies (Table S2).18
All four CCOFs adopt a 2D layered tetragonal structure with
slipped AA stacking and possess high crystallinity and porosity
as well as high chemical stability in strong acids and bases. The
newly formed C−C bond linkages leads to blue-shifted
emissions of the parent structures with enhanced quantum
yields and fluorescence lifetimes, and the olefin linked CCOFs
show higher enantioseletivity than their reduced structures
when they were utilized as fluorescence sensors for chiral
amino alcohols. To the best of our knowledge, the present
reduced frameworks are the first two examples of C−C bond-
linked COFs and the unreduced frameworks are the first two
examples of CC bond-linked CCOFs. This work will
promote the design and synthesis of more chemical stable
COFs with strong linkages and new functions based on the
solid-state conversions.
The change of fluorescence intensity of the CCOF is
probably caused by static enhancement via the formation of a
crown ether-amino alcohol adduct, which can affect the
proton-transfer-assisted charge-transfer excited state.19 For
phenylglycinol and phenylalaninol, the formation of host−
guest adducts between protonated amines and the immobilized
crown ethers may lead to an increase in the interlayer space of
COFs and weaken the π−π interactions between CCOF layers,
thereby causing the fluorescence intensity enhancement.14 In
contrast, besides binding the crown ethers, tryptophanol can
establish π−π interactions with the COF layers via the indole
rings, as well as hydrogen interactions with crown ethers via
the indole NH groups. Such newly formed π−π stacking
interactions may play a dominant role in influencing the COF
fluorescence, causing decrease in the fluorescence intensity.20
Further study is needed to understand the mechanism of the
selective fluorescence changes. The static nature of the
complexation is suggested by the consistent fluorescence
lifetimes of the CCOF 17 suspensions before and after titration
with D-phenylglycinol and D-tryptophanol (lifetime, τ0, 1.15 vs
1.09 ns and 1.15 vs 1.53 ns, respectively). As the noncovalent
interactions of the CCOFs with the enantiomers of amino
alcohol afford different diastereomeric complexes, distinct
fluorescence intensity change is thus detected.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c11050.
■
sı
Detailed synthetic procedures, FT-IR spectra, TGA
traces, TEM images, CD spectra, modeling details, dye
uptake experiment, fluorescence data, DFT calculations,
and gas adsorption (PDF)
Crystal structure data of CCOF 17 (CIF)
Crystal structure data of CCOF 17-R (CIF)
Crystal structure data of CCOF 18 (CIF)
Crystal structure data of CCOF 18-R (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
Yan Liu − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, P. R. China;
orcid.org/0000-0002-7560-519X; Email: liuy@
sjtu.edu.cn
As shown in Figure 7f, under otherwise identical conditions,
CCOF 18 can also detect the three amino alcohols, with lower
EF or QR to phenylglycinol, ohenylalaninol, and tryptophanol
(3.26, 8.39, and 1.72, respectively) than CCOF 17. In contrast,
the reduced CCOFs showed low fluorescence change and
selectivity toward the amino alcohols, with the EF/QR of 1.30,
1.53, and 1.84 for CCOF 17-R and 2.41, 4.50, and 1.69 for
CCOF 18-R, respectively. It is clear that, for a given analyte,
the enantioselectivity efficiency of C−C bond linked CCOF
was lower than the CC bond linked CCOF. This is probably
caused by the destruction of the π-conjugate system after
reduction, which may reduce the transmission and expression
of chiral information in the 2D CCOF systems.21 In addition,
PXRD indicated that all CCOFs remained crystalline after
treatment with D-phenylglycinol, indicative of the good
stability of the frameworks (Figure S21).
Yong Cui − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, P. R. China;
orcid.org/0000-0003-1977-0470; Email: yongcui@
sjtu.edu.cn
Authors
Chen Yuan − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, P. R. China;
orcid.org/0000-0003-0334-2594
Shiguo Fu − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, P. R. China
Kuiwei Yang − Department of Chemical and Biomolecular
Engineering, National University of Singapore, Singapore
117585, Singapore
Bang Hou − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, P. R. China;
orcid.org/0000-0002-3803-9520
CONCLUSION
■
We have demonstrated direct synthesis of the first two olefin-
based CCOFs by Knoevenagel condensation of linear
diacetonitriles and chiral tetraaldehyde derived from dibi-
naphthyl-22-crown-6. After postsynthetic reduction of the
olefin linkages, the CCOFs were transformed into two
isostructural C−C bond-linked frameworks. The complete
reduction of CC bonds was proved by FT-IR and 13C NMR.
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pubs.acs.org/JACS
Article
Nanoparticles Confined in a Thioether-Containing Covalent Organic
Framework and Their Catalytic Applications. J. Am. Chem. Soc. 2017,
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Langenhahn, T.; Schwarze, M.; Schomaecker, R.; Thomas, A.;
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Jianwen Jiang − Department of Chemical and Biomolecular
Engineering, National University of Singapore, Singapore
117585, Singapore; orcid.org/0000-0003-1310-9024
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c11050
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Foundation of China (Grant Nos. 21620102001, 91856204,
91956124, and 21875136), the National Key Basic Research
Program of China (2016YFA0203400), Key Project of Basic
Research of Shanghai (18JC1413200 and 19JC1412600),
Shanghai Rising-Star Program (19QA1404300), and the
Ministry of Education of Singapore and the National
University of Singapore (C-261-000-207-532/C-261-000-777-
532 and R-279-000-574-114).
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