Construction of Fully Conjugated Covalent Organic Frameworks via Facile Linkage Conversion for Efficient Photoenzymatic Catalysis

Article abstract of DOI:10.1021/jacs.0c00923

Covalent organic frameworks (COFs) with improved stability and extended ?-conjugation structure are highly desirable. Here, two imine-linked COFs were converted into ultrastable and ?-conjugated fused-aromatic thieno[3,2-c]pyridine-linked COFs (B-COF-2 and T-COF-2). The successful conversion was confirmed by infrared and solid-state ¹³C NMR spectroscopies. Furthermore, the structures of thieno[3,2-c]pyridine-linked COFs were evaluated by TEM and PXRD. It is noted that a slight difference in the structure leads to totally different photoactivity. The fully ?-conjugated T-COF-2 containing triazine as the core exhibited an excellent photocatalytic NADH regeneration yield of 74% in 10 min.

Full text of DOI:10.1021/jacs.0c00923

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Communication
Construction of Fully Conjugated Covalent Organic Frameworks via
Facile Linkage Conversion for Efficient Photoenzymatic Catalysis
Yuancheng Wang, Hui Liu, Qingyan Pan, Chenyu Wu, Wenbo
Hao, Jie Xu, Renzeng Chen, Jian Liu, Zhibo Li, and Yingjie Zhao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00923 • Publication Date (Web): 12 Mar 2020
Downloaded from pubs.acs.org on March 12, 2020
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Construction of Fully Conjugated Covalent Organic
Frameworks via Facile Linkage Conversion for Efficient
Photoenzymatic Catalysis
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Yuancheng Wang,†,+ Hui Liu,†,+ Qingyan Pan,† Chenyu Wu,† Wenbo Hao,† Jie Xu,† Renzeng
Chen,† Jian Liu,‡ Zhibo Li,*,† and Yingjie Zhao*,†
†Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education Department; College of Polymer
Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
‡College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042,
China
Supporting Information Placeholder
ferromagnetic materials and so on.35, 38-39, 42, 45 Although the
ABSTRACT: Covalent organic frameworks (COFs) with
fully -conjugated COFs is fascinating, to obtain crystallinity
is still extremely difficult. The reversibility of the conjugated
bond is usually bad which is unfavorable for the self-
adjusting. Yaghi,31 Liu,49 Lotsch33 and Baek32 reported a two-
steps post-synthetic strategy to convert the reversible imine
bond into more stable and -electron delocalized linkages
such as thiazole, oxazole and quinolone. This is undoubtedly
an easier and much more efficient way to achieve the aims of
high crystallinity, improved electron delocalization and
stability simultaneously as the highly ordered imine skeleton
is already set at the first step. Herein, we report the synthesis
of two ultrastable fully -conjugated 2D COFs (B-COF-2 and
T-COF-2) linked by thieno[3,2-c]pyridine units achieved by a
post-oxidative cyclization (Scheme 1). The basic chemistry
for constructing the ordered skeleton is still Schiff base
reaction. More interestingly, the dynamic imine bond and
the neighborly thiophene can be converted into thieno[3,2-
c]pyridine through an oxidative cyclization (Scheme 1). The
thieno[3,2-c]pyridine as a new linkage was first reported in
constructing COFs. The obtained COFs show ultrastability
and good electron delocalization. Besides the conventional
powder X-ray diffraction (PXRD) characterization analysis,
high resolution TEM (HRTEM) clearly revealed the
honeycomb-like internal structure of the COFs. Considering
the advantages of stability and conjugation together with
unique 2D structure, the photo-catalysis performance of the
obtained COFs was further studied. Given the unique 2D
molecular structure and the suitable band gap, the obtained
improved stability and extended -conjugation structure are
highly desirable. Here, two imine-linked COFs were
converted into ultrastable and -conjugated fused-aromatic
thieno[3,2-c]pyridine-linked COFs (B-COF-2 and T-COF-2).
The successful conversion was confirmed by infrared and
solid-state 13C NMR spectroscopies. Furthermore, the
structures of thieno[3,2-c]pyridine-linked COFs were
evaluated by TEM and PXRD. It is noted that a slight
difference in the structure leads to totally different photo-
activity. The fully -conjugated T-COF-2 containing triazine
as the core exhibited excellent photo-catalytic NADH
regeneration yield of 74% in 10 min.
Covalent Organic Frameworks (COFs) are crystalline porous
materials with precise structures. COFs have attracted
considerable attention due to their unique structures. These
features make COFs useful for application in gas storage and
separation,1-5 heterogeneous catalysis,6-9 optoelectronics,10-15
sensing,16-19 energy storage,20-21 etc. The basic principles to
design and synthesize highly ordered structures lie in the
reversible formation of covalent bonds. Until now, the
mostly used linkages for constructing COFs are limited in B-
O,22-24 B-N,25 Si-O26-27 and C=N bond28-29. The self-adjusting
property of these bonds can facilitate the formation of well-
ordered structures. However, the chemical stability and
conjugation are sacrificed due to the poor stability and weak
electron delocalization of these bonds. Many efforts have
been made to explore new chemistry to construct highly
COFs show great potential in photo-catalysis as
a
semiconductor material. Interestingly, slightly variation in
the structure led to totally different photo-activity. T-COF-2
containing triazine as the core exhibited the best NADH
regeneration yield of 74% within 10 min which presents
higher efficiency than most of the reported materials.
30-32
stable and electron delocalized COFs. Oxazole,8,
thiazole,31, 33 vinylene,34-38 sp2 carbon-carbon linkages (cyano-
group-included),39-42 β-hydroxyimine or hydroxylamine,43
aryl-ether,44 pyrazine,45-46 dioxin,47 imidazole48 as novel
linkages have been used in making ultrastable COFs. It is
worth mentioning that some of the linkages mentioned
above such as sp2 carbon-carbon linkages could make fully -
conjugated COFs with ultrastability which broadens the
application fields to organic electronics, supercapacitor,
The building block A containing both thiophene and
amine groups was synthesized via Suzuki coupling reaction.
Two aldehyde building blocks B and C adopt benzene and
triazine as cores, respectively. First, two imine-linked COFs
were synthesized through the imine condensation reaction of
photo-catalysis,
cathode
for
lithium-ion
batteries,
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A+B and A+C in a mixture of 1, 2-dichlorobenzene/n-
Spengler reaction between the β-carbon of the thiophene
and the carbon of the imine gives the corresponding
thieno[3,2-c]pyridine (Ref-2) in a yield of 89%. B-COF-2 and
T-COF-2 were also prepared by the same reaction (For
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butanol/acetic acid under 120 °C for 3 days, giving crystalline
yellow solids B-COF-1 and T-COF-1, respectively. A reference
compound Ref-1 was also prepared by the same reaction
between A and benzaldehyde. Further cyclization via Pictet-
details
see
supporting
information).
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Scheme 1. Synthesis and structures of B-COF-1, 2; T-COF-1, 2; and Ref-1, 2.
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NH2
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B-COF-2
H2
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OHC
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T-COF-1
T-COF-2
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imine-linked precursor (B-COF-1 and T-COF-1) with a
decomposition temperature up to 460 °C (Figure S2).
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The crystallinities of the materials were then evaluated by
PXRD (Figure 2a-b). The four COFs all exhibit good
crystallinities. Especially B-COF-2 and T-COF-2 are still
highly crystalline after the cyclization reaction. No obvious
shift of the prominent diffraction peaks was observed
compared to the as-prepared imine-linked COFs, indicating
the preservation of the highly ordered framework. The
detailed structure information was given by comparison of
the experimental PXRD profiles and the predicted profiles.
As shown in Figure S3, the PXRD profiles calculated for AA
stacking mode match excellently with the experimental
profiles. B-COF-1, 2 and T-COF-1, 2 were concluded to be AA
stacking. Furthermore, we performed nitrogen adsorption
experiments to examine their porosities. The Brunauer-
Emmett-Teller (BET) surface areas were observed to be 810
m2 g-1, 620 m2 g-1, 952 m2 g-1, 829 m2 g-1 for B-COF-1, 2 and T-
COF-1, 2 respectively. The loss of the BET surface area after
linkage conversion was attributed to the decrease of pore
size and crystallinity. According to the pore size distribution
data calculated by using non-local density functional theory
(NLDFT) model, the pore size displayed an obvious
reduction (Figure S4).
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Figure 1. C CP-MAS NMR spectra of B-COF-1, 2 (a) and T-
COF-1, 2 (b).
To verify the chemical structures, the 13C CP/MAS and FT-
IR spectra of B-COF-1, 2 and T-COF-1, 2 were measured and
compared, respectively. As depicted in Figure 1a-1b, the
carbon peak of the C=N bond at about 157 ppm proved the
existence of imine bonds in B-COF-1 and T-COF-1. After acid
catalyzed cyclization reaction, C=N bonds were supposed to
become
a part of the newly formed pyridine ring.
Figure 2. (a-b) The PXRD patterns of B-COF-1, 2 (a) and T-
COF-1, 2 (b); (c-d) Nitrogen adsorption-desorption isotherms
(77 K) of B-COF-1, 2 (c) and T-COF-1, 2 (d).
Corresponding to the structural change, the carbon peak of
the C=N bonds was found to move to 154 ppm in B-COF-2
and T-COF-2. A similar change was also observed for Ref-2
and Ref-1. Besides, the most obvious characteristic of the
structural difference for Ref-2 was found to be the carbon
peak shift from 117 ppm to 123 ppm after cyclization. As
expected, B-COF-2 and T-COF-2 entirely inherit the
representative feature. The carbon peak at 115 ppm of B-
COF-1 and T-COF-1 disappeared and a new peak at 123 ppm
appeared in B-COF-2 and T-COF-2. These significant
changes indicated that the linkage was successfully
converted into thieno[3,2-c]pyridine. FT-IR spectra also
confirmed the conversion (Figure S1a-b). The C=N stretching
modes characteristic of the imine were observed at about
1618 cm-1 for B-COF-1 and T-COF-1. After cyclization, the
signal displayed an obvious attenuation. Meanwhile, a new
stretch signal appeared at about 1557 cm-1. All the results
above proved the formation of the imine bonds and the
followed conversion of the linkages. Thermogravimetric
analysis showed that the thieno[3,2-c]pyridine-linked COFs
(B-COF-2 and T-COF-2) possess better stability than their
The chemical stability of converted COFs B-COF-2 and T-
COF-2 were then examined. After submergence in HCl (12
M) or NaOH (12 M) solution at 50 °C for 24 hours, the
crystallinities remain. No obvious differences were observed
from the PXRD after the acid or base treatment (Figure S5),
which demonstrates that the novel thieno[3,2-c]pyridine
linkages in B-COF-2 and T-COF-2 bring excellent stability
even under harsh conditions.
The periodic structural characteristics of B-COF-2 and T-
COF-2 were also visualized by HRTEM. As shown in Figure 3,
B-COF-2 and T-COF-2 displayed clear honeycomb-like
porous structures. The HRTEM images combining with the
corresponding Fast Fourier transformation (FFT) further
confirmed the preservation of crystallinity and the structure.
The bright spots indicate a center-to-center distance of the
pore of ~2 nm in both B-COF-2 and T-COF-2, which is in
excellent agreement with the simulated structure model (2.3
nm, Figure S3). In sharp contrast, B-COF-1 and T-COF-1 are
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extremely unstable under the electron beam. HRTEM could
towards NAD+. In the end, the enzymatically active 1,4-
NADH is exclusively generated (Figure S6). Subsequently,
the solar energy was stored as the NADH which could
activate various enzyme-mediated synthetic processes.52 As
shown in Figure 4, the peak at 340 nm is the characteristic
absorbance peak of NADH which could indicate the
regeneration efficiency of the NADH. The imine-linked B-
COF-1 showed no photoactivity at all, while B-COF-2
displayed a regeneration yield of 5.5%. Interestingly, the
NADH regeneration yield of T-COF-2 displayed a dramatic
increase compared with T-COF-1, reaching up to 74% in 10
min. This is a relatively high efficiency compared with most
of other reported materials. Such obvious improvement of
NADH regeneration efficiency arising from structure
variation further demonstrated the successful conversion of
linkages. The significant improvement of photoactivity could
be ascribed to the extended conjugated structure which
facilitates the electron delocalization. The photocurrent
response was performed to indicate the separation efficiency
of the photo-induced electron-hole pairs. As shown in Figure
S12, Large enhancement of Iph
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not be obtained. These results further prove the excellent
stability of converted COFs bring from the thieno[3,2-
c]pyridine linkages.
As we know, nicotinamide adenine dinucleotide (NADH)
as a typical coenzyme, whose regeneration process is the key
step for solar energy utilization and biomass production
since it plays an important role in various biochemical
transformations.50 Many materials have been applied for
NADH regeneration. 80% of NADH regeneration yield can
be obtained within 1hour by CdS-coated SiO2 beads as the
photocatalyst51 TiO2-CdS nanotubes can reach to 75.2%
within 120 min.50 The carbon nitride materials showed
photo-regeneration yield of ~80 % in 45 min.52 We recently
reported a sp2-carbon linked
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Figure 3. (a) TEM image of B-COF-2; (b) TEM image of B-
COF-2 enlarged view of a selected area in panel (a); (c)
Fourier-filtered image of selected areas of B-COF-2, Inset:
Fast Fourier Transform (FFT) from the selected areas; for T-
COF-2 see (d), (e), (f), respectively.
2D COF with triazine as central units which give a 90%
photo-regeneration yield in 12 mins.42 Considering the fully
conjugated structures, the COFs may act as good candidates
for photocatalytic NADH regeneration. The photophysical
properties of the COFs were measured first. According to the
solid-state absorption spectra, obvious blue-shift occurred
after the linkage conversion (Figure S7). Solid-state UV-vis
measurements and cyclic voltammetry were performed to
evaluate the band gap and LUMO energy level of B-COF-2
and T-COF-2, respectively (Figure S7, S13). The band gaps of
T-COF-2 and B-COF-2 are calculated to be 2.33 and 2.38 eV,
respectively. Figure S13a shows the onset of reduction
potential, from which the corresponding LUMO of B-COF-2
was calculated to be -3.53 eV. A HOMO level of -5.91 eV was
evaluated based on the optical band gap value. The LUMO
and HOMO levels of T-COF-2 were measured to be -3.50 and
-5.83 eV. This electronic configuration suggests photo-
activated COFs possess the thermodynamic driving force to
reduce mediator to further transfer the hydride to the
NAD+.54
Figure 4. Experimental results of the NADH regeneration
rates for B-COF-1, 2 and T-COF-1, 2, respectively: (a-d)
absorbance changes of NADH (e-f) photocatalytic NADH
regeneration kinetics.
for T-COF-2 was observed, indicating better separation of
charged carriers than B-COF-2. In addition, according to the
NADH regeneration yield of the different COFs, we could
speculate that the nitrogen-rich triazine units brings a
positive effect on photocatalysis. The free electron pair and
the electron poor character may act as the active site for
interface redox reaction.53 In addition, T-COF-2 can strongly
bind with NAD+ due to the - stacking interactions between
the adenine subunit of NAD+ and the -conjugated 2D
surface of T-COF-2. The charge transfer complex as a pre-
state could facilite the photoinduced electron transfer
between T-COF-2 and NAD+.42 Similar phenomenon has also
been found in the g-C3N4 as a photocatalast.52, 55-56 To prove
The photocatalysis performance for the regeneration of
NADH was then evaluated. Here, the COFs materials play
the role of collecting solar energy in the form of light-excited
electrons. The electrons are then transferred to
[Cp*Rh(bpy)(H2O)]2+, followed by coupling with one proton
to form the hydridehodium [Cp*Rh(bpy)(H)]+ (M, in
abbreviation), which acts as a hydride transfer reagent
this
proposal,
a
control
experiment
without
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[Cp*Rh(bpy)(H)]+ was performed. The photoinduced
Frameworks (COFs) for Efficient CO2 Separation. Chem. Mater. 2016,
28, 1277-1285.
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electron transfer should in principle work without a further
mediator ([Cp*Rh(bpy)(H)]+). As illustrated in Figure S9,
without [Cp*Rh(bpy)(H)]+, NADH can still be moderately
regenerated, the efficiency reach to 43.8% in 10 min. The
photocatalytic stability of T-COF-2 was then tested. As
shown in Figure S10, no obvious decrease was observed after
three cycles of the NADH regeneration experiment which
indicated the excellent photo stability of T-COF-2.
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Lu, S.; Hu, Y.; Wan, S.; McCaffrey, R.; Jin, Y.; Gu, H.;
In summary, we demonstrated the synthesis of two
ultrastable fully -conjugated thieno[3,2-c]pyridine-linked
2D COFs via a new two-steps synthetic strategy. This method
allows the preservation of crystallinity and periodic
frameworks. The semiconductor properties were also
successfully improved after the linkage conversion. B-COF-2
and T-COF-2 exhibited NADH regeneration yield of 5.5%
and 74.0% over 10 min, respectively, more efficient than the
imine-linked precursor. The two-steps strategy provides a
convenient method for the preparation of conjugated COFs
and tuning of the semiconductor properties for
photocatalysis.
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ASSOCIATED CONTENT
Supporting Information
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Haldar, S.; Chakraborty, D.; Roy, B.; Banappanavar, G.;
Rinku, K.; Mullangi, D.; Hazra, P.; Kabra, D.; Vaidhyanathan, R.,
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The Supporting Information is available free of charge on the
ACS Publications website. Synthetic procedures, structural
simulation, FT-IR, TGA, UV-Vis, NMR, including Figure S1-
S16 (PDF).
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Highly Emissive Covalent Organic Frameworks. J. Am. Chem. Soc.
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AUTHOR INFORMATION
Corresponding Author
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Based,
Lin, G.; Ding, H.; Yuan, D.; Wang, B.; Wang, C., A Pyrene-
Fluorescent Three-Dimensional Covalent Organic
Framework. J. Am. Chem. Soc. 2016, 138, 3302-3305.
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Xue, M.; Yan, Y.; Valtchev, V.; Qiu, S.; Fang, Q., Three-Dimensional
Tetrathiafulvalene-Based Covalent Organic Frameworks for Tunable
Electrical Conductivity. J. Am. Chem. Soc. 2019, 141, 13324-13329.
*yz@qust.edu.cn
*zbli@qust.edu.cn
Author Contributions
+Y.W. and H.L. contributed equally.
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Li, X.; Gao, Q.; Wang, J.; Chen, Y.; Chen, Z. H.; Xu, H. S.;
Tang, W.; Leng, K.; Ning, G. H.; Wu, J.; Xu, Q. H.; Quek, S. Y.; Lu, Y.;
Loh, K. P., Tuneable Near White-emissive Two-dimensional
Covalent Organic Frameworks. Nat. Commun. 2018, 9, 2335.
Notes
The authors declare no competing financial interests.
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Jiang, J.; Cui, Y., Chiral BINOL-Based Covalent Organic Frameworks
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ACKNOWLEDGMENT
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Liu, X.; Huang, D.; Lai, C.; Zeng, G.; Qin, L.; Wang, H.; Yi,
The work was supported by the National Natural Science
Foundation of China (21905150), the National Young
Thousand Talents Program, China Postdoctoral Science
Foundation (2019M652338).
H.; Li, B.; Liu, S.; Zhang, M.; Deng, R.; Fu, Y.; Li, L.; Xue, W.; Chen,
S., Recent Advances in Covalent Organic Frameworks (COFs) as A
Smart Sensing Material. Chem. Soc. Rev. 2019, 48, 5266-5302.
(18)
Ascherl, L.; Evans, E. W.; Gorman, J.; Orsborne, S.;
Bessinger, D.; Bein, T.; Friend, R. H.; Auras, F., Perylene-Based
Covalent Organic Frameworks for Acid Vapor Sensing. J. Am. Chem.
Soc. 2019, 141, 15693-15699.
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