Donor-acceptor type [4+3] covalent organic frameworks: sub-stoichiometric synthesis and photocatalytic application

Article abstract of DOI:10.1007/s11426-019-9696-3

Three unprecedented 2D [4+3] covalent organic frameworks (TTCOF-1, TTCOF-2, and TTCOF-3) have been prepared by substoichiometric condensation of tetratopic and tritopic monomers, overcoming the limitations of the design rules of conventional topologies. By reticulating the tetraphenylethylene (TPE)-based and triazine-based moieties into COF frameworks, novel electron donor-acceptor (D-A) type structures were obtained. These TTCOFs have good photocatalytic activity in aerobic C(sp³)-H bond functionalization and arylboronic acid oxidation driven by visible light, with yields up to 94%. This can expedite possibilities of COFs with new structural and topological complexities and can also expand the application of COF-based photocatalysis in synthetic chemistry.

Full text of DOI:10.1007/s11426-019-9696-3

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-019-9696-3
SPECIAL ISSUE: Celebrating the 100th Anniversary of Chemical Sciences in Nanjing University
Donor-acceptor type [4+3] covalent organic frameworks:
sub-stoichiometric synthesis and photocatalytic application
Qiaobo Liao†, Wentao Xu†, Xin Huang, Can Ke, Qi Zhang, Kai Xi* & Jin Xie*
State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210023, China
Received December 13, 2019; accepted February 3, 2020; published online April 2, 2020
Three unprecedented 2D [4+3] covalent organic frameworks (TTCOF-1, TTCOF-2, and TTCOF-3) have been prepared by sub-
stoichiometric condensation of tetratopic and tritopic monomers, overcoming the limitations of the design rules of conventional
topologies. By reticulating the tetraphenylethylene (TPE)-based and triazine-based moieties into COF frameworks, novel
electron donor-acceptor (D-A) type structures were obtained. These TTCOFs have good photocatalytic activity in aerobic
C(sp3)–H bond functionalization and arylboronic acid oxidation driven by visible light, with yields up to 94%. This can expedite
possibilities of COFs with new structural and topological complexities and can also expand the application of COF-based
photocatalysis in synthetic chemistry.
COF, sub-stoichiometric synthesis, crystal engineering, photocatalysis, aerobic oxidation
Citation:
Liao Q, Xu W, Huang X, Ke C, Zhang Q, Xi K, Xie J. Donor-acceptor type [4+3] covalent organic frameworks: sub-stoichiometric synthesis and
photocatalytic application. Sci China Chem, 2020, 63, https://doi.org/10.1007/s11426-019-9696-3
1 Introduction
which can be used for the synthesis of D-A COFs
[3,4,9,10]. For instance, tetraphenylethylene (TPE)-based
units have previously been used as an electron donor [23–
27] and the electron-deficient triazine ring has been es-
teemed as a good electron-acceptor [28–30]. Although they
have been widely used to synthesize COFs [9,30–36], 2D
COFs constructed with TPE-based and triazine-based
monomers have not been reported previously. The key
challenge stems from the undeveloped topology from tet-
ratopic monomers with C2 symmetry and tritopic monomers
with C3 symmetry. Therefore, it is still desired to broaden
the topology diagram of 2D COFs to increase their diversity
[37].
In the prevalent topology diagrams of COFs [32,37–40],
the monomers are reacted stoichiometrically and the func-
tional groups of the building blocks are completely con-
sumed [33]. Accordingly, we rationalized that sub-
stoichiometric synthesis would be one feasible manner to
diversify COF topologies. It is worth noting that very re-
Covalent organic frameworks (COFs), an intriguing class of
crystalline porous organic materials, can be constructed by
incorporating organic building blocks into extended peri-
odic networks through strong covalent bonds [1–7]. Two-
dimensional COFs have emerged in the past decade to be
auspicious in photocatalysis due to their extended con-
jugation and periodic π-π arrays [8–15], in which the donor-
acceptor (D-A) structures are important to enhance their
performances in optoelectronic devices and photocatalysis
[11,16–21]. Generally, sophisticate monomers bearing
specific moieties, such as triphenylene [16,18,20], pyrene
[19,11,22], benzothiadiazole [18] are necessary for the
synthesis of D-A COFs. However, the currently prevalent
topologies usually lead to a very limited array of monomers
†These authors contributed equally to this work.
*Corresponding authors (email: xikai@nju.edu.cn; xie@nju.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

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Liao et al. Sci China Chem
cently Banerjee and Lotsch et al. [41] and Yaghi et al. [42]
have reported several 2D COFs based on sub-stoichio-
metric condensation, opening a new door towards functio-
nalized COFs. Since the symmetries of TPE-based
monomers and triazine-based monomers are C2 and C3,
respectively, we envisioned that sub-stoichiometric [4+3]
condensation of these monomers would be a good choice to
fabricate D-A COFs. In this work, by linking tetratopic
TPE-based monomers and the tritopic triazine-based
monomers with π-conjugated spacers of the imine bonds,
three novel D-A COFs (termed TTCOFs, tetratopic-tritopic
covalent organic frameworks) with non-conventional
topologies are constructed successfully in terms of [4+3]
condensation, with potential application in photocatalysis
(Figure 1). Importantly, TTCOF-2 is featured by its high
crystallinity, and large porosity as well as the D-A structure,
and shows outstanding performance for aerobic oxidation
reactions under visible-light irradiation with high product
yields up to 94%.
Figure 1 Schematic representation of a donor-acceptor structure con-
taining the electron donor tetraphenylethylene (TPE) and an acceptor,
triazine (color online).
(TFPT), 0.2 mL of o-DCB/n-butanol (v/v=7:3) and 0.05 mL
of 12 M aqueous acetic acid solution were charged into a
2 mL glass tube. The mixture was degassed by three freeze-
pump-thaw cycles. The glass tube was flame-sealed under
vacuum and then heated at 120 °C for 9 d. After cooling, the
mixture was filtered through a 0.22 μm PTFE membrane and
washed with THF several times to remove unreacted mono-
mers, the catalyst and solvent. The filter cake was further
purified by Soxhlet extraction using THF for 24 h, followed
by drying under supercritical CO2 flow for 3 h. Yield: 95.6%.
Synthesis of HCOF-1. 40.2 mg (0.30 mmol) of TA,
70.8 mg (0.20 mmol) of TAPT, 1.0 mL of o-DCB/n-butanol
(v/v=7:3) and 0.1 mL of 6 M aqueous acetic acid solution
were charged into a 5 mL glass tube. The mixture was de-
gassed by three freeze-pump-thaw cycles. The glass tube was
flame-sealed under vacuum and then heated at 120 °C for
3 d. After cooling, the mixture was filtered through a
0.22 μm PTFE membrane and washed with THF several
times to remove unreacted monomers, the catalyst and sol-
vent. The filter cake was further purified by Soxhlet ex-
traction using anhydrous THF for 24 h, followed by drying
under supercritical CO2 flow. Yield: 41.8%.
2 Experimental
2.1 Synthesis of TTCOFs
Synthesis of TTCOF-1. 20.0 mg (0.045 mmol) of (4,4′,4′′,4′′′-
(ethene-1,1,2,2-tetrayl) tetrabenzaldehyde (ETTB), 21.3 mg
(0.06 mmol) of (2,4,6-tris(4-aminophenyl)-1,3,5-triazine
(TAPT), 0.2 mL of 1,4-dioxane and 0.05 mL of 12 M aqueous
acetic acid solution were charged into a 2 mL glass tube. The
mixture was degassed by three freeze-pump-thaw cycles. The
glass tube was flame-sealed under vacuum and then heated at
120 °C for 6 d. After cooling, the mixture was filtered through
a 0.22 μm PTFE membrane and washed with tetrahydrofuran
(THF) several times to remove unreacted monomers, the cat-
alyst and solvent. The filter cake was further purified by
Soxhlet extraction using THF for 24 h, followed by drying
under supercritical CO2 flow. Yield: 86.3%.
Synthesis of TTCOF-2. 33.9 mg (0.045 mmol) of 4′,4′′′,4′′′′′,
4′′′′′′′-(1,2-ethene-diylidene) tetrakis[1,1′-biphenyl]-4-car-
boxaldehyde (ETBC), 21.3 mg (0.06 mmol) of TAPT,
0.2 mL of o-DCB/n-butanol (v/v=7:3) and 0.05 mL of 12 M
aqueous acetic acid solution were charged into a 2 mL glass
tube. The mixture was degassed by three freeze-pump-thaw
cycles. The glass tube was flame-sealed under vacuum and
then heated at 120 °C for 5 d. After cooling, the mixture was
filtered through a 0.22 μm PTFE membrane and washed with
THF several times to remove unreacted monomers, the cat-
alyst and solvent. The filter cake was further purified by
Soxhlet extraction using THF for 24 h, followed by drying
under supercritical CO2 flow. Yield: 73.3%.
2.2 Photochemistry part
1,2,3,4-tetrahydroisoquinoline derivatives 1 (0.2 mmol) and
TTCOF-2 (6.0 mg, 0.0014 mmol) were placed in a trans-
parent Schlenk tube equipped with a stirring bar. The sol-
vents of MeNO2 (2.0 mL) were added under air condition.
The reaction mixture was stirred under the irradiation of two
45 W blue LEDs (distance app. 4.0 cm from the bulb) at
room temperature for 18 h. When the reaction finished, the
mixture was quenched with water and extracted with ethyl
acetate (3×10 mL). The organic layers were combined and
concentrated under vacuo. The product was purified by flash
column chromatography on silica gel (petroleum ether: ethyl
acetate).
Synthesis of TTCOF-3. 23.6 mg (0.06 mmol) of 4,4′,4′′,
4′′′-(ethene-1,1,2,2-tetrayl)-tetraaniline (ETTA)), 31.5 mg
(0.09 mmol) of 2,4,6-tris(4-formylphenyl)-1,3,5-triazine

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Liao et al. Sci China Chem
3 Results and discussion
3.1 Characterizations of TTCOFs
In this work, three D-A type 2D COFs were prepared by
[4+3] condensation of tetratopic TPE-based monomers with
C2 symmetry ETTB, ETBC and ETTA with tritopic triazine-
based monomers with C3 symmetry TAPT and TFPT (Figure
2). The resulting products are denoted as TTCOF-1 (from
ETTB and TAPT), TTCOF-2 (from ETBC and TAPT) and
TTCOF-3 (from ETTA and TFPT). By co-condensation of
monomers in a flame-sealed glass tube at 120 °C, TTCOF-1
and TTCOF-2 were obtained as yellow powders and
TTCOF-3 as a red powder, and all of them are insoluble in
common organic solvents, such as acetone, dichloromethane,
methanol, tetrahydrofuran, and N,N-dimethylformamide.
The scanning electron microscopy (SEM) images (Figures
S3–S5, Supporting Information online) reveal that these
TTCOFs adopt irregular morphology with different dimen-
sions which may be caused by polycrystalline nature of these
COFs [43]. Thermogravimetric analysis (TGA) demon-
strates that all three TTCOFs exhibit high thermal stability
with the major derivative thermogravimetric (DTG) peaks
over 500 °C under a N2 atmosphere (Figures S6–S8). A de-
crease in the apparent mass of TTCOF-2 can be ascribed to
water molecules absorbed into the channels. Elemental
analysis of TTCOFs shows that C contents are below the
theoretical values, while the H contents are larger than ex-
pected (Table S1, Supporting Information online). This result
may stem from the absorbed water molecules, which is also
consistent with TGA analysis.
Figure 3 (a) PXRD patterns of TTCOFs. Insets: the corresponding
stacking reflections. (b) The PXRD patterns of the as-prepared TTCOF-2
(blue curve), and TTCOF-2 after heating at 120 °C for 6 d (red curve). (c–
e) HRTEM images of TTCOF-2 (scale bars: 5 and 10 nm). Insets: Fourier-
filtered images of the selected regions in the red squares in the TTCOF-2
images (scale bars: 5 and 10 nm) (color online).
3.2 Structural elucidation of TTCOFs
The powder X-ray diffraction (PXRD) patterns of all three
TTCOFs show several strong diffraction peaks (Figure 3(a)).
The main peaks of these TTCOFs possess small full-width at
half-maximum (FWHM) values of 0.239, 0.214 and 0.229,
respectively, confirming the crystalline nature of the
TTCOFs. Besides, the crystallite sizes estimated by Scher-
rer’s equation using the Jade software (v. 6.5) [44] are 31.3,
65.5 and 46.6 nm, respectively. These results reflect highest
crystallinity of TTCOF-2, which is further demonstrated by
high-resolution transmission electron microscopy (HRTEM)
(Figure 3(c–e)). Lattice fringes corresponding to main re-
flections (100), (120), (200) and (220) can be observed
clearly from the images, agreeing well with PXRD results.
Moreover, many smaller fringes with d-spacing distances
less than 0.3 nm can also be found in Figure 3(e), which may
attribute to the cutting effect [45]. Although a drop-off at
around 100 °C is found in the TGA curve of TTCOF-2, its
crystallinity remains almost intact when heated at 120 °C in
air for 6 d (Figure 3(b)), revealing the excellent thermal
stability of the crystalline structure. Considering that the
planar triazine-cord monomers and the propeller-shaped
TPE-based monomers afford highly crystalline 2D COFs
[34,36,37], we envisioned that TTCOFs tend to adopt largely
planar 2D structures. The insets of Figure 3(a) show stacking
Figure 2 Schematic representation of the synthesis of TTCOFs by [4+3]
condensation reactions (color online).

4
Liao et al. Sci China Chem
reflections of TTCOFs centered at 22.4°, 22.8° and 19.5°,
respectively, suggesting the π-π stacking distances of
0.396 nm for TTCOF-1, 0.390 nm for TTCOF-2 and
0.455 nm for TTCOF-3. The stacking reflections provide
proof toward our hypothesis for 2D structures of these
TTCOFs.
Possible crystal models were therefore constructed based
on the reasonable 2D tth and bex nets in the light of the
reticular chemistry structure resource (RCSR) [46] using the
Materials Studio software package. For both topologies, four
possible stacking configurations, i.e., eclipsed (AA), stag-
gered (AB) and two serrated stacking modes (a-AB and b-
AB) were examined (Figure 4, Figures S9, S10). The cal-
culated PXRD pattern of the unit cell with the b-AB stacking
mode of bex net was found to match well with the experi-
mental results from TTCOF-1 and TTCOF-2 (Figure 4(a),
Figure S9(a), khaki curves), except for a weak reflection at
2.8° of TTCOF-1 which was not found in the simulated
pattern. We posit that some of the ETTB linkers were absent,
giving rise to the defective unit cell shown in Figure 4(f).
One can clearly observe this peak accompanied with main
reflections in the calculated pattern of the defective unit cell
(Figure 4(a), light blue curve). In view of the relative in-
tensities of peaks, the defective unit cell is speculated to be a
fraction of the crystalline TTCOF-1. Conversely, the simu-
lation results suggest that TTCOF-3, an isomer of TTCOF-1,
adopts a tth net topology rather than a bex topology (Figure
S10(a), red curve). Although Lan et al. [47] have reported
two 3D COFs with ffc topology using a similar symmetrical
combination of monomers, the simulated PXRD patterns of
unit cells with this topology for TTCOFs fail to match the
experimental patterns, ruling out this possibility in our case
(Figure 4(a), Figures S9(a), S10(a)).
Nitrogen sorption analysis was carried out to determine the
permanent porosity and pore size distributions of TTCOFs.
For all samples, the N2 isotherms measured at 77 K have a
typical type I shape (Figure 5(a)) [9], suggesting that mi-
cropores exist in the TTCOFs. The Brunauer-Emmett-Teller
(BET) surface area of TTCOF-2 is up to 1996 m2 g−1, cal-
culated from the low-pressure region of the adsorption iso-
therm (Figure S12), and the total pore volume is 1.05 cm3 g−1
(P/P0=0.99). The BET surface areas of TTCOF-1 and
TTCOF-3 are calculated to be 978 and 740 m2 g−1 (Figures
S11, S13), and their total pore volumes are 0.51 and
0.45 cm3 g−1, respectively. The porosity of TTCOF-2 is
much higher than that of the other two TTCOFs, which may
mainly originate from the higher crystallinity of TTCOF-2
[48]. To analyze the pore size distributions in TTCOFs, the
adsorption isotherms were fitted using a nonlocal density
functional theory (NLDFT) model (Figure 5(b)). The results
reveal that TTCOF-1 has a microporous channel with aver-
age pore widths of 13.5 Å, which is in good agreement with
the simulated structure with a b-AB stacking mode of bex
Figure 4 (a) Experimental and simulated PXRD patterns of TTCOF-1.
The proposed unit cells of TTCOF-1 with bex topology: (b) the eclipsed
(AA) mode, (c) the staggered (AB) mode, (d, e) the serrated stacking
modes with adjacent sheets slipped by 1/2 of the unit cell distances along a
axis (a-AB) and b axis (b-AB), respectively, (f) the defective unit cell based
on bex-b-AB stacking mode. The proposed unit cells of TTCOF-1 with tth
topology: (g) the eclipsed (AA) mode, (h) the staggered (AB) mode, (i, j)
the serrated stacking mode with adjacent sheets slipped by 1/2 of the unit
cell distances along a axis (a-AB) and b axis (b-AB). (k) The proposed 3D
unit cell of TTCOF-1 with ffc topology. C: grey; H: white; O: red; N: blue
(color online).
net. In terms of the preliminary analysis of TTCOFs,
TTCOF-2 has a topologic structure with only one kind of
micropore, which is similar to that of TTCOF-1. However,
TTCOF-2 possesses two kinds of micropores with average
pore widths of 13.5 and 17.2 Å. In addition, TTCOF-3 also
has two main distributions at 5.0 and 14.4 Å, smaller than
those of the predicted unit cell with the eclipsed mode of tth
net (6.6 and 17.7 Å). It is noteworthy that the first pore is
probably smaller than 5.0 Å, with the measuring range of 5–
50 Å. These mismatches suggest that TTCOF-2 and TTCOF-
3 possess the more complicated stacking modes than the
eclipsed or staggered structures [49].
In the light of the dual-pore structures of TTCOF-2 and
TTCOF-3, these TTCOFs were proposed to adopt slightly
serrated stacking modes. According to the PSD results, either
layers in the unit cells of TTCOF-2 and TTCOF-3 were
displaced by 4.5 and 4 Å, respectively, to form serrated
stacking modes (Figure 6(d, f)). All reasonable structures
were geometry-optimized by the Forcite molecular dynamics
module, giving rise to slightly inclined unit cells. The cal-
culated PXRD patterns of the unit cells reproduce the ex-
perimental patterns well (Figure 6(a, c, e), black and green
curves). Pawley refinement was carried out for the simulated
structures of TTCOFs (Figure 6(a, c, e), red curves), and
produced good agreement factors (TTCOF-1: Rwp=7.71%,
Rp=4.28%, a=22.31 Å, b=44.77 Å, c=8.14 Å, α=94.16°,
β=93.72° and γ=90.74°; TTCOF-2: Rwp=9.59%, Rp=7.03%,
a=28.45 Å, b=61.98 Å, c=8.06 Å, α=79.34°, β=88.44° and
γ=87.39°; TTCOF-3: Rwp=3.71%, Rp=2.95%, a=49.20 Å,
b=47.36 Å, c=9.25 Å, α=91.81°, β=87.43° and γ=119.89°).

5
Liao et al. Sci China Chem
According to the Pawley-refined unit cell of TTCOF-3 with
tth topology, the distances between the free aldehyde groups
and the nearest atoms on the framework are ~3.0 Å away
(Figure 6(g)). Due to the propeller shape of TPE-based
moiety, the phenyl rings on the framework rotate to max-
imize the distances and so decrease the van der Waals in-
teractions.
Abundant remnant aldehyde groups rather than amine
groups are anchored on the backbones according to the
predicted crystal results of TTCOFs. The FTIR spectra
(Figure 7(a)) of all TTCOFs reveal the characteristic imine
stretching mode at ~1620 cm−1, indicating the occurrence of
imine condensation. Moreover, peaks at around 1700 cm−1
arising from the stretching vibration band of C=O are much
more intense than those of stoichiometric crystalline COFs
such as HCOF-1 (synthesized using terephthalaldehyde and
2,4,6-tris(4-aminophenyl)-1,3,5-triazine, Figures S17, S18),
indicating that there are free aldehyde groups remaining in
the synthesized TTCOFs (Figure 7(a)). In contrast, the weak
peaks at ~3300–3500 cm−1 imply that the amine functional-
ities have been nearly exhausted during the condensation
reaction (Figures S14–S16). Solid-state 13C cross-polariza-
tion magic angle spinning (CP-MAS) NMR spectroscopy
further confirmed the presence of the unreacted aldehyde
functionalities (Figure 7(b)). The characteristic signals at 159
and 190 ppm are assigned to the imine and the aldehyde
groups, respectively. These intense signals of the aldehyde
groups are comparable to those of the reported TPE-COF-II
which has many remaining aldehyde groups [32], suggesting
the signals arise not only from the edge groups [8] but also
from those inside the lattice of the TTCOFs. Those results
indicate that aldehyde groups are abundant in the frame-
works, and further verify the unusual crystal structures of the
TTCOFs. The foregoing results strongly suggest that
Figure 5 (a) Nitrogen sorption isotherms of TTCOFs at 77 K. (b) Pore
size distribution (PSD) profile of TTCOFs, calculated from the nonlocal
density theory model (color online).
Figure 6 Experimental PXRD patterns (black curve), Pawley-refined
patterns (red curve) and simulated patterns (green curve) for (a) TTCOF-1,
(c) TTCOF-2 and (e) TTCOF-3. (b) Top view and side view of the Pawley-
refined unit cell of TTCOF-1 with b-AB stacking mode of bex net. (d) Top
view and side view of the Pawley-refined unit cell of TTCOF-2 with 4.5 Å-
serrated b-AB stacking mode of bex net. (f) Top view and side view of the
Pawley-refined unit cell of TTCOF-3 with 4 Å-serrated AB mode for tth
net. (g) The distances between uncondensed aldehyde groups and the
nearest atoms on the frameworks (color online).
Figure 7 (a) FTIR spectra, (b) solid-state 13C CP-MAS NMR spectra, (c)
UV-Vis spectra and (d) the estimated band gaps of TTCOFs (color online).

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Liao et al. Sci China Chem
Table 1 Optimization of the reaction conditionsa)
TTCOFs with sophisticated bex and tth topology were
constructed successfully via sub-stoichiometric [4+3] con-
densation. By means of reversible coupling reactions and π-π
stacking interactions [4], crystalline lattices stitched by TPE
and triazine motifs are constructed, forming electron D-A
structures in TTCOFs.
Photocatalyst
Light source
Conversion (%)b)
Entry
1
2
TTCOF-1c)
TTCOF-2d)
TTCOF-3e)
–
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
23 W CFL
23 W CFL
Blue LEDs
Blue LEDs
87
99 (89)
65
3.3 Photocatalysis
3
Recently, D-A type photocatalysts, such as organic types
(carbazolyl dicyanobenzenes, acridiniums and pyryliums)
[50–54] and MOFs [55,56], have been used widely in pho-
toredox catalysis for organic synthesis. In view of the peri-
odic D-A structures, the synthesized TTCOFs may have
photochemical characteristics. Solid state diffuse reflectance
UV/Vis spectra show that TTCOF-1 and TTCOF-2 possess
similar absorptions with the strongest absorption peak at
~420 nm while TTCOF-3 has a red-shifted absorption peak
at ~500 nm (Figure 7(c)), which is also shown by optical
images (Figure S19). The optical band gaps of TTCOF-1,
TTCOF-2 and TTCOF-3 calculated by the Kubelka-Munk
function, are 2.04, 2.13 and 1.73 eV, respectively (Figure 7
(d)). The lower bandgap of TTCOF-3 may origin from the
tth net which has a higher degree of conjugation. In com-
parison with the well-known C3N4 with a band gap of
2.78 eV [57], these TTCOFs show a narrower band gap and
enhance the intensity of visible-light absorption, implying
their potential as photocatalysts [58–61].
4
83
5
TTCOF-2
–
70
6
12
7f)
8g)
TTCOF-2
TTCOF-2
64
10
Pd/Pt removed-
TTCOF-2h)
9
Blue LEDs
95
a) Reaction conditions: 1a (0.2 mmol), TTCOF (6 mg), MeNO2 (2 mL),
air, blue LEDs, rt, 18 h. b) Determined by GC. The number in parentheses
is the isolated yield. c) TTCOF-1 (6 mg, 0.002 mmol). d) TTCOF-2 (6 mg,
0.0014 mmol). e) TTCOF-3 (6 mg, 0.0008 mmol). f) argon atmosphere. g)
Under dark conditions. h) Synthesized by monomers treated by a com-
mercial scavenger agent to remove trace Pd and Pt.
photocatalyst can give a much better reuslt (70% versus
12%). In addition, we found that without the TTCOF-2, no
reaction could occur for the reaction of oxidation of ar-
ylboronic acids to corresponding phenols (5c). Moreover, to
exclude the possibility that cross coupling was catalyzed by
residue Pd or Pt, Pd/Pt removed-TTCOF-2 was synthesized
using monomer treated by a commercial scavenger agent to
remove trace Pd and Pt (Table S2). The catalytic efficiency
of this COF has almost not been changed, indicating that the
residue of Pd and Pt catalytic role is less likely in these
organic transformations (entries 9, Table 1). These control
experiments may indicate the photocatalytic reactivity of
prepared TTCOF-2.
The C(sp3)–H bonds are ubiquitous in organic molecules.
Visible-light-driven selective α-functionalization of tertiary
amines affords a powerful platform with which to construct
diverse amine derivatives under mild reaction conditions and
thus has attracted significant attention in recent years [62–
67]. In our study, 2-phenyl-1,2,3,4-tetrahydroisoquinoline
was selected as a substrate due to its appearance in a wide
range of natural compounds [68] and its use in the aza-Henry
reaction [69–73]. Based on the results from our UV/Vis
absorption spectra, blue LEDs were used as visible light
source. Screening of TTCOFs showed that TTCOF-2 was the
most effective (entries 1–3, Table 1). Under irradiation by
blue LEDs, the desired aerobic cross-dehydrogenative cou-
pling between 2-phenyl-1,2,3,4-tetrahydroisoquinoline and
nitromethane occurs smoothly with TTCOF-2 as a photo-
catalyst, furnishing a product (2a) in 89% yield (entry 2,
Table 1). Since the use of blue LEDs, the model reaction can
give the desired coupled product with and without TTCOF-2.
In the presence of TTCOF-2, a slightly better result was
obtained (entries 2 and 4, Table 1). The control experiments
suggest that visible light and O2 can promote aza-Henry
reaction, which is consistent with König and Gschwind’s
work [71]. Accordingly, it is difficult to determine its cata-
lytic reactivity. Much to our delight, under the irradiation
with 23 W CFL, the use of TTCOF-2 as the heterogeneous
As shown in Figure 8(A), direct aerobic cross-dehy-
drogenative coupling between 2-phenyl-1,2,3,4-tetra-
hydroisoquinoline and nitromethane or ketones is successful.
Generally, TTCOF-2 shows a better photocatalytic activity
for the electron-drawing substituents. A variety of N-arylte-
trahydroisoquinolines can be employed to give the desired
products (2a–2f and 3a–3f) in moderate to good yields. α-
aminonitriles are important building blocks for further sub-
sequent transformations to α-amino acids [74]. With
TTCOF-2 as a photocatalyst, selective α-cyanation can
readily occur and the corresponding products (4a–4g) are
produced in good to excellent yields. Recently, Wang et al.
[75] acquired access to a series of ultrastable benzoxazole-
based COFs, and showed that they can be successfully used
in the photocatalytic transformation of arylboronic acids to
phenols. In the light of Wang’s work, we hypothesized that
TTCOF-2 may catalyze the reaction for the synthesis of

7
Liao et al. Sci China Chem
Figure 9 The proposed mechanism (color online).
sorption, structural simulation and Pawley refinement. More
interestingly, the TTCOFs adopt D-A structures with TPE
moieties as donors and triazine moieties as acceptors, which
enable the superior photoactivity of these COFs in the
aerobic oxidation reaction. The TTCOFs bearing frustrated
networks possess abundant unreacted aldehyde groups in the
regular pores, allowing for downstream post-modification.
Such TTCOFs have potential applications in gas separation,
proton conduction and photo-catalyzed synthesis.
Acknowledgements This work was supported by the Ministry of Science
and Technology of China (2017YFA0700503), the Fundamental Research
Funds for the Central Universities (020514380149), 1000-Youth Talent Plan
and Nanjing University Graduate Interdisciplinary Research and Innovation
Project (2018CL05).
Figure 8 The visible-light-mediated aerobic organic transformations
catalyzed by TTCOF-2. (A) Selective C(sp3)–H bond functionalization: [a]
1 (0.2 mmol), TTCOF-2 (6.0 mg, 0.0014 mmol), MeNO2(2 mL), air, blue
LEDs, 18 h; [b] 1 (0.2 mmol), TTCOF-2 (6.0 mg, 0.0014 mmol), L-proline
(20 mol%, 0.04 mmol), acetone (2 mL), air, blue LEDs, 24 h; [c] 1
(0.2 mmol), TTCOF-2 (6.0 mg, 0.0014 mmol), TMSCN (1.5 equiv.,
0.3 mmol), MeCN (2 mL), air, blue LEDs, 24 h; (B) arylboronic acid
(0.2 mmol), TTCOF-2 (4.0 mg, 0.00094 mmol), iPr2NEt (1.0 mmol),
MeCN/H2O (1.6/0.4 mL), air, blue LEDs, 24 h. [d] Without TTCOF-2
(color online).
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
phenols. When the reaction was carried out with 0.2 mmol
scales, 4 mg TTCOF-2 showed an excellent catalytic activity
and the corresponding phenol products were obtained in
good yields (5a–5d). TTCOF-2 is easily recovered from the
reaction mixture and excellent catalytic activity with 95%
conversion was observed after 2 cycles (see Supporting In-
formation online for details). Based on König’s work [71], a
possible reaction mechanism was proposed (Figure 9). Under
irradiation, an electron in the unit of TTCOF-2 transfers to its
electron acceptors. Thus this species absorbs an electron
from tetrahydroisoquinolines (1) to generate radical cation,
just like the excited [Ru(bpy)3]2+*. The following chemical
bond formation steps are similar to the previous works
[70,71].
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