Diacetylene Functionalized Covalent Organic Framework (COF) for Photocatalytic Hydrogen Generation

Article abstract of DOI:10.1021/jacs.7b11255

Covalent organic frameworks (COFs) are crystalline, highly porous, two- or three-dimensional polymers with tunable topology and functionalities. Because of their higher chemical stabilities in comparison to their boron-linked counterparts, imine or β-ketoenamine linked COFs have been utilized for a broad range of applications, including gas storage, heterogeneous catalysis, energy storage devices, or proton-conductive membranes. Herein, we report the synthesis of highly porous and chemically stable acetylene (-C≡C-) and diacetylene (-C≡C-C≡C-) functionalized β-ketoenamine COFs, which have been applied as photocatalyst for hydrogen generation from water. It is shown that the diacetylene moieties have a profound effect as the diacetylene-based COF largely outperforms the acetylene-based COF in terms of photocatalytic activity. As a combined effect of high porosity, easily accessible diacetylene (-C≡C-C≡C-) functionalities and considerable chemical stability, an efficient and recyclable heterogeneous photocatalytic hydrogen generation is achieved.

Full text of DOI:10.1021/jacs.7b11255

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Article
Diacetylene Functionalized Covalent Organic Framework
(COF) for Photocatalytic Hydrogen Generation
Pradip Pachfule, Amitava Acharjya, Jerome Roeser, Thomas Langenhahn,
Michael Schwarze, Reinhard Schomaecker, Arne Thomas, and Johannes Schmidt
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Dec 2017
Downloaded from http://pubs.acs.org on December 29, 2017
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Diacetylene Functionalized Covalent Organic Framework (COF) for
Photocatalytic Hydrogen Generation
Pradip Pachfule,†,§ Amitava Acharjya,†,§ Jérôme Roeser,† Thomas Langenhahn,† Michael Schwarze,‡ Reinhard Schomäcker,‡ Arne
Thomas†* and Johannes Schmidt†
† Department of Chemistry/Functional Materials, Technische Universität Berlin, Hardenbergstraße 40, 10623 Berlin, Germany.
‡ Technische Universität Berlin, Institut für Chemie, TC 08, Straße des 17. Juni 124, 10623 Berlin, Germany.
Supporting Information
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cell, which is decreasing the interlayer attraction.30–32 Recent-
ABSTRACT: Covalent organic frameworks (COFs) are crys-
ly. Dichtel et. al. showed that the rate of COF growth in-
creases with enhanced van der Waals surface and planarity of
the linkers, as the stacking of the two-dimensional (2D)
polymers influences the rate of COF formation.30 Because of
the appreciably lower activation energy, the van der Waals
interactions of larger aromatic systems stabilizes the polymer
stacking intermediate, which further enhances the 2D COF
formation as compared to small aromatic cores. In this re-
gard, the synthesis of highly porous 2D COFs originating
from small organic cores (e.g. phenyl) separated by –ethyne
(acetylene, −C≡C−), and especially by –buta-1,3-diyne (di-
acetylene, −C≡C−C≡C−) moieties, seems to be an immense
challenge, as these extended organic linkers should on the
one hand yield large pore sizes but on the other have very
weak van der Waals interactions, and both factors does not
assist for interlayer stacking of the 2D polymers, which is the
rate-determining step in COF synthesis.30,33
Despite that the synthesis of diacetylene-bridged COFs
seem quite challenging, such structures could be of high
interest especially due to their highly conjugated structures,
high charge career mobility and ability to provide active sites
for facile migration of photogenerated excitons to the sur-
face.34 Indeed materials having diacetylene moieties have
received much attention in photocatalysis, optoelectronics
and photo/thermochromism,34–36 and therefore polymers
and small molecules having diacetylene moieties in their
backbone have been explored largely.36–38 However, COFs
comprised of these moieties have not been explored so far,
except few examples of diacetylene-based boroxines or boro-
nate ester-based COFs. These COFs have however not been
explored for any applications, most probably due to the syn-
thetic difficulties described above.33
Herein, we present the synthesis of two novel
β−ketoenamine COFs, TP-EDDA and TP-BDDA, bearing
acetylene and diacetylene moieties, via an acid-catalyzed
solvothermal reaction. TP-EDDA and TP-BDDA have been
synthesized by the reaction of 1,3,5-triformylphloroglucinol
(TP, 63 mg, 0.3 mmol) with 4,4'-(ethyne-1,2-diyl)dianiline
(EDDA, 94 mg, 0.45 mmol) or 4,4'-(buta-1,3-diyne-1,4-
diyl)dianiline (BDDA, 105 mg, 0.45 mmol), respectively, in
the presence of 3 M acetic acid (0.5 mL) using 1:1 mesity-
lene/dioxane (3 mL) as the solvents [Sections S2, in the Sup-
porting Information (SI)]. Keto-enol tautomerization provid-
ed by ketoenamine formation of amines with TP has been
utilized to achieve COFs with essential chemicals stability.39
In order to synthesize the diacetylene-based crystalline TP-
talline, highly porous, two- or three-dimensional polymers
with tunable topology and functionalities. Because of their
higher chemical stabilities in comparison to their boron-
linked counterparts, imine or
β−ketoenamine linked COFs
have been utilized for a broad range of applications, includ-
ing gas storage, heterogeneous catalysis, energy storage de-
vices or proton-conductive membranes. Herein, we report
the synthesis of highly porous and chemically stable acety-
lene (−C≡C−) and diacetylene (−C≡C−C≡C−) functionalized
β
−ketoenamine COFs, which have been applied as photo-
catalyst for hydrogen generation from water. It is shown that
the diacetlyene moieties have a profound effect as the di-
acetylene based COF largely outperforms the acetylene based
COF in terms of photocatalytic activity. As a combined effect
of high porosity, easily accessible diacetylene (−C≡C−C≡C−)
functionalities and considerable chemical stability, an effi-
cient and recyclable heterogeneous photocatalytic hydrogen
generation is achieved.
INTRODUCTION
Porous materials such as activated carbons, zeolites, metal-
organic frameworks (MOFs) or covalent organic frameworks
(COFs) have been tested and utilized for a variety of applica-
tions.1–6 Ordered crystalline materials have attracted much
attention, because of the possibility to precisely predict and
position atoms or functional groups within the large and
accessible pore surfaces.7–9 Due to their unique ordered
channel structures, relatively large pore apertures, high
chemical and thermal stabilities, and defined chemical na-
ture of their pore walls, COFs have received increasing inter-
est for gas storage and separation, heterogeneous catalysis,
proton conductivity, optoelectronics and energy applica-
tions.10–15 Using various organic building units and covalent
bond forming reactions, COFs with variable porosities, func-
tionalities and morphologies have been synthesized.16–25 As
constructed entirely from covalent bonds, COFs often show
good chemical stabilities,14,26,27 which can be further en-
hanced by introducing structures which enable keto to enol
taoutomerism28 and/or hydrogen bonding, especially in
imine-linked and other nitrogen-containing COFs.19,29
The synthesis of COFs with various building units such as
phenyl, biphenyl, naphthalene, anthracene, pyrene, triazine,
etc. have been explored in the last few years.15,19 Such studies
have shown that, generally, the rate of COF formation de-
creases when larger pore sizes are obtained, probably due to
the corresponding higher fraction of empty space per unit
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BDDA COF, we have carried out a series of experiments to
optimize the reaction time and
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Figure 1. Synthesis and characterization of TP-EDDA and TP-BDDA COFs. (a) Scheme of the synthesis of TP-EDDA and TP-BDDA
COFs. (b, e) Top and side views of TP-EDDA and TP-BDDA showing the ideal eclipsed (AA) structures. (c, d) Experimental, Pawley-
refined and simulated powder X-ray diffraction patterns (AA stacking) and a difference plot for TP-EDDA and TP-BDDA COFs.
temperature. It was found that prolonging the reaction to 4
days, as compared to the typical 2 or 3 days reaction time
reported for the synthesis of β−ketoenamine COFs, was nec-
essary to yield a material with satisfactory crystallinity and
long range order, confirming the above mentioned assump-
tions by Dichtel et. al. on the influence of the linker size and
aromaticity on the reaction rate of COF formation (Figure
S3).30 Notably, when the reaction was further maintained for
8 days, again a material with low crystallinity was observed,
suggesting the necessity of the precise reaction time and
temperature for the synthesis of crystalline TP-BDDA (Figure
S3).
mentally measured patterns. Good agreement was found
with the eclipsed stacking modes (Figure 1c and 1d, violet
curves), whereas the diffraction patterns of the staggered
stacking models did not reproduce the data (Figure S9, red
curves), thus apparently confirming the crystallization of
both the COFs in eclipsed (AA) stacking arrangement. A full
profile Pawley fitting was then carried out to refine the final
unit cell parameters, which has led to satisfactorily low re-
sidual values and acceptable profile differences (Figure 1c and
1d).
The permanent porosity of TP-EDDA and TP-BDDA
COFs was assessed by N2 sorption measurements at 77 K
(Figure 2a). The calculated Brunauer–Emmett–Teller (BET)
and Langmuir surface area values for TP-EDDA were 523 and
625 m2g–1, respectively. The TP-BDDA COF showed slightly
increased, but still moderate BET (758 m2g–1) and Langmuir
(905 m2g–1) surface areas. Most probably, the low degree of
π−π stacking among the adjacent COF layers, which causes
an offset stacking in the framework, is responsible for the
moderate surface area in TP-EDDA and TP-BDDA. A similar
phenomenon has been observed previously in a diacetylene-
based boronate ester COF, which was further verified using
molecular dynamics and density functional theory calcula-
tions, suggesting the horizontal offset of adjacent layers by
1.7-1.8 Å.33,40 The completion of the reaction and the struc-
tural integrity of the framework through the formation of β-
ketoenamine functionalities have been confirmed by Fourier
Transform Infrared Spectroscopy (FTIR) analyses.39 TP-
EDDA and TP-BDDA COFs exhibit the characteristic signals
corresponding to C‒N and C=C bonds at ∼1251 and ∼1451
cm−1, respectively (Figure S3). These results have been further
corroborated by the 13C solid-state cross-polarization magic-
RESULT AND DISCUSSION
Powder X-ray diffraction (PXRD) analyses were performed to
elucidate the structural features of the as-synthesized COFs.
Both patterns are dominated by an intense reflection in the
low-angle region, at 3.0 and 2.7 two-theta degrees for TP-
EDDA and TP-BDDA, respectively, that can be assigned to
the (100) facet of a primitive hexagonal lattice. Additionally,
the presence of further weak reflections and a broad reflec-
tion at ∼25 two-theta degrees for TP-EDDA and TP-BDDA
COFs that can be assigned to the (001) facet, confirm the
synthesis of two-dimensional COFs in a crystalline and π−π
stacked form (Figure 1c and 1d). The structural models for
TP-EDDA and TP-BDDA were constructed by generating the
expected two-dimensional hexagonal layers with hcb topolo-
gy and modeling stacking sequences with eclipsed (AA) and
staggered (AB) arrangement in the P6 (No. 168) and P63 (No.
173) space groups, respectively (Section S4). After geomet-
rical optimization of the models, their simulated PXRD pat-
tern were calculated and compared to the respective experi-
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angle-spinning (CP-MAS) nuclear magnetic resonance
of the COFs in water is of course an important prerequisite
for their application in photocatalytic hydrogen generation
from water. Moreover, even in HCl (3 M) for 1 day, crystallin-
ity of the COFs was essentially retained (Figure S6).
Photocatalytic hydrogen generation from water using or-
ganic semiconductors is a rapidly growing field, initiated by
the discovery of visible light hydrogen evolution using
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(NMR) analyses, which show the characteristic signal of the
carbonyl carbon (C=O) at ∼180−184 ppm and C−N bond at
∼144 ppm, respectively (Figure 2b). The appearance of the
representative signals for acetylene (−C≡C−) and diacetylene
(−C≡C−C≡C−) functionalities for TP-EDDA and TP-BDDA
COFs at ∼87 ppm (TP-EDDA)
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Figure 3. Characterizations of TP-BDDA. (a) SEM image of TP-
BDDA showing flower-like morphologies. (b) PXRD patterns of
TP-BDDA after 3 days treatment in water showing COF stability.
polymeric carbon nitride.41 Since then, several porous organic
materials such as heptazine/triazine based polymers42–44 and
conjugated microporous polymers45–47 are reported to exhibit
hydrogen evolution activity under visible light.48,49 Notably,
for COFs, just triazine containing frameworks have so far
been shown to exhibit considerable photocatalytic activity
for hydrogen evolution.15,50,51 Considering the significant
porosity, high stability in water and presence of acetylene
(−C≡C−) and diacetylene (−C≡C−C≡C−) functionalities in the
COF backbone, the photocatalytic H2 evolution performance
of TP-EDDA and TP-BDDA was evaluated. To distinctly
analyze the effect of acetylene and diacetylene moieties on
optical and catalytic properties, an isoreticular COF with
similar pore apertures constituted from terphenylene edges
(TP-DTP COF, DTP: 4,4′′-diamino-p-terphenyl) was synthe-
sized (Section S5).24,52 Diffuse reflectance ultraviolet–
visible spectroscopy (UV-Vis) spectra show distinct change
between the COFs (Figure 4a). TP-DTP exhibits an absorb-
ance edge around 500 nm, whereas the absorbance edge is
red shifted to 520 nm and 525 nm, for TP-EDDA for TP-
BDDA COFs, respectively. The absorbance tail also extended
up to 675 nm for TP-BDDA, showing the distinct red shift in
absorbance. Optical band gaps calculated from Tauc plots
are 2.42, 2.34 and 2.31 eV, respectively, for TP-DTP, TP-EDDA
and TP-BDDA (Figure 4a, inset), which are ideal for photo-
catalytic water splitting, that requires a minimum band gap
of ~ 1.8 eV.41
Continuous photocatalytic hydrogen evolution experi-
ments were conducted in a Teflon reactor fitted with quartz
glass window (Figure 4d, Figure S1). An optimized ratio of 9:1
for water:triethanolamine (TEOA) was used, where TEOA
acts as the sacrificial electron donor to capture the photo-
generated holes of the photocatalyst (Figure S21). Photo-
deposited Pt (from H2PtCl6) acted as the co-catalyst for pro-
ton reduction. All COFs were tested for long term (10 h)
hydrogen evolution under visible light (≥395 nm). Pressure
increment due to hydrogen evolution vs time profile was
recorded and subsequently converted to volume vs time
profiles. TP-BDDA showed continuous hydrogen evolution
over 10 h with an average rate of 324±10 µmol h-1 g-1, whereas
no pressure increase was observed for both TP-EDDA and
TP-DTP COF (Figure 4b). For the latter two COFs, GC results
from the head space gas injection implied the production of
Figure 2. Characterization of TP-EDDA and TP-BDDA COFs. (a)
N2 sorption isotherms for TP-BDDA and TP-EDDA. (b) 13C CP–
MAS solid-state NMR spectroscopy analyses of TP-BDDA and
TP-EDDA.
and two signals at ∼71 and ∼79 ppm (TP-BDDA) validates
their presence in backbones, respectively.
Scanning and transmission electron microscopy
(SEM/TEM) analyses show that TP-EDDA and TP-BDDA
crystallize in a flower like morphology (Figure 3a, Figure S4),
where each individual crystallites have the dimensions in the
hundred nanometer range. The structural stability of the
solvent exchanged and as-synthesized TP-EDDA and TP-
BDDA was evaluated by thermogravimetric analyses, show-
ing a major weight loss starting at 450 °C together with some
weight loss below 200 °C, probably due to the removal of
trapped solvent molecules (Figure S5).
To investigate the chemical stabilities of TP-EDDA and
TP-BDDA, 50 mg of each COF was submerged in various
solvents. The samples were kept immersed in the respective
liquids for 3 days and after washing with water analyzed for
their structural stabilities. Notably, TP-BDDA and TP-EDDA
retained their crystalline structure after dispersion in water,
Methanol, THF, MeOH, Cyclohexanone, DMSO and DMF for
3 days (Figure 3b; Figure S6). Furthermore, the sample could
be completely recovered after this treatment, thus showing
no sign of decomposition and dissolution. The high stability
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negligible amount of hydrogen for TP-DTP (20±5 µmol h-1g-1)
duction, which was further confirmed by photocurrent re-
sponse measurement and electrochemical impedance spec-
troscopy analyses (Figure S22, S23). Beside lowering of the
band-gap, it can be assumed
and low hydrogen evolution for TP-EDDA (30±5 µmol h-1g-1)
(Figure 4b and 4c). These results imply that the conjugated
diacetlylene moiety (−C≡C−C≡C−) is playing an important
role for enhancing the activity for photochemical water re-
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Figure 4. (a) UV-Vis diffuse reflectance spectra of TP-BDDA, TP-EDDA and TP-DTP. The inset shows Tauc plots and optical
images of the COF powders. (b) Photocatalytic hydrogen evolution performance of TP-BDDA, TP-EDDA and TP-DTP catalysts
under visible light irradiation (≥395 nm) using TEOA as sacrificial agent. (c) Comparison of photocatalytic hydrogen evolution
rates. (d) Reactor setup for hydrogen evolution experiments.
from studies on molecular counterparts that the diacetylene-
moieties are responsible for high charge career mobility, i.e.
for the facile migration of photogenerated excitons to the
surface of the photocatalyst.34,36
In conclusion, we report the first successful preparation of
porous 2D COFs including –ethyne (acetylene, −C≡C−) or –
buta-1,3-diyne (diacetylene, −C≡C−C≡C−) moieties. The
comparison of the photocatalytic hydrogen generation per-
formance of TP-BDDA with acetylene (TP-EDDA) and phe-
nyl (TP-DTP) moiety containing COFs showed the signifi-
cant influence of diacetylene moieties on photocatalytic
hydrogen evolution activity. To the best of our knowledge,
this is a first successful demonstration of photocatalytic
hydrogen generation using a COF catalyst without presence
of any heteronuclear molecular functionalities (triazine- or
heptazine-based moieties) inside the structure, further em-
phasizing the importance of the diacetylene diad in photoca-
talysis.
To quantify the photocatalytic activity of the TP-BDDA
over the spectral distribution, apparent quantum efficiency
(AQE) for hydrogen evolution was measured as a function of
incident light wavelength using bandpass filters with central
wavelength of 420 and 520 nm. A maximum apparent quan-
tum efficiency of 1.8 was obtained with 520 nm light, which is
very close to the absorbance maxima of the TP-BDDA (Fig-
ure S14 and S15). Further, a long term hydrogen evolution
experiment for 60 h was conducted to elucidate the stability
of the TP-BDDA catalyst (3 cycles). When TEOA was used as
sacrificial agent, hydrogen evolution slightly decreases after
20 hours, however addition of fresh TEOA reactivates the
system (Figure S16). After the photocatalytic experiments,
recovered TP-BDDA was washed thoroughly and fully char-
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.xxxxxxx. Mate-
rials and methods, synthesis and characterization of COFs,
simulation for theoretical COF structures and photocatalysis
experiment details (PDF).
13
acterized by FTIR and C CP-MAS solid state NMR spectros-
copy analyses, showing complete preservation of the COF
structural integrities and intact conjugated moieties within
the COF backbone (Figure S17). The PXRD analyses of recov-
ered TP-BDDA, however, showed a significant loss of long-
range ordering, probably due exfoliation of the COF
nanosheets in presence of water during the photocatalysis
experiments, which was reported previously also for other
COFs in photocatalytic applications.50,53 Nonetheless, we
could regenerate the crystalline material by subjecting the
material after photocatalysis into the initial synthetic condi-
tions, with characteristic peaks 40.6 and 47.1° corresponding
to the Pt(0) nanoparticles (Figure S17 and S18). After photo-
catalysis experiments, highly dispersed Pt nanoparticles were
found to be decorated (Figure S19 and S20) in TP-BDDA
matrix, as confirmed the SEM and TEM analyses of the re-
covered sample.15,51,54 To confirm that the source of liberated
hydrogen is indeed water, a labeling experiment was carried
out with D2O, producing D2 in the photocatalytic experi-
ment, as confirmed by the MS analyses of the evolved gas
during photocatalysis (Figure S25).
AUTHOR INFORMATION
Corresponding Author
*arne.thomas@tu-berlin.de
ORCID
Pradip Pachfule: 0000-0002-0804-541X
Arne Thomas: 0000-0002-2130-4930
Author Contributions
§ P.P. and A.A. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Support from the German Science Foundation for project
TH1463/15-1 and within the Cluster of Excellence UniCat is
acknowledged. P.P. thanks the Alexander von Humboldt
foundation for postdoctorate fellowship. We thank Prof. M.
CONCLUSIONS
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