Vinylene-Linked Covalent Organic Frameworks by Base-Catalyzed Aldol Condensation

Article abstract of DOI:10.1002/anie.201905886

Two 2D covalent organic frameworks (COFs) linked by vinylene (?CH=CH?) groups (V-COF-1 and V-COF-2) are synthesized by exploiting the electron deficient nature of the aromatic s-triazine unit of C₃-symmetric 2,4,6-trimethyl-s-triazine (TMT). Th

Full text of DOI:10.1002/anie.201905886

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Accepted Article
Title: Vinylene-Linked Covalent Organic Frameworks by Base-
Catalyzed Aldol Condensation
Authors: Amitava Acharjya, Pradip Pachfule, Jerome Roeser, Franz-
Josef Schmitt, and Arne Thomas
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905886
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Vinylene-Linked Covalent Organic Frameworks by Base-
Catalyzed Aldol Condensation
Amitava Acharjya,a Pradip Pachfule,a Jérôme Roeser,a Franz-Josef Schmitt,a and Arne Thomasa*
Abstract: The development of new linkages and synthetic routes for
the formation of 2D covalent organic frameworks (COFs) is a
challenging task. Herein, we report the synthesis of two vinylene (-
CH=CH-) linked COFs (V-COF-1 and V-COF-2), by exploiting the
electron deficient nature of the aromatic s-triazine unit of C3-
symmetric 2,4,6-trimethyl s-triazine (TMT). The acidic terminal
methyl hydrogens of TMT can easily be abstracted by a base,
resulting in a stabilized carbanion, which further undergoes aldol
condensation with multitopic aryl aldehydes to be reticulated into
extended crystalline frameworks (V-COFs). Both V-COF-1 (with
terepthalaldehyde (TA)) and V-COF-2 (with 1,3,5-tris(p-
formylphenyl)benzene (TFPB)) are polycrystalline and exhibit
permanent porosity and BET surface areas of 1341 m2.g-1 and
627 m2.g-1 respectively. Due to the close proximity (3.52 Å) of the
pre-organized vinylene linkages within adjacent 2D layers stacked in
eclipsed fashion, [2+2] photo-cycloadditon in V-COF-1 was observed
to form covalent crosslinks between the COF layers.
cyanovinylene [-CH=C(CN)-] linkages hold the promise for the
generation of chemically stable COFs as the cyanovinylene
linkage is significantly less prone to hydrolysis compared to
boroxine, boronate ester or imine linkages, mostly used for the
synthesis of COFs.[10,43] The challenge to crystallize a COF by
reversible [-CH=C(CN)-] bond formation was overcome by
implementing reversible Knoevenagel condensation of 1,4
phenylenediacetonitrile (PDAN) with multitopic aldehyde-
functionalized building blocks. Such COFs featuring fully
conjugated backbones are also promising materials in terms of
optoelectronic or photovoltaic applications.[44]
Herein, we report the synthesis of two purely vinylene (-
CH=CH-) linked 2D COFs (V-COF-1 and V-COF-2) achieved by
base-catalyzed reversible aldol condensation, exploiting the
highly electron deficient s-triazine core of 2,4,6-trimethyl s-
triazine (TMT). The methyl protons of TMT are acidic and
undergo base catalyzed aldol condensation with benzaldehyde
to yield 2,4,6 tri-styryl triazine quantitatively (Scheme 1a).[45,46] V-
COF-1 and V-COF-2 were obtained by extending this model
reaction to terepthalaldehyde (TA) and 1,3,5-tris(4-formyl)phenyl
benzene (TFPB) (Scheme 1b). During our investigation on the
base-catalyzed pathway, Yaghi et. al. reported the acid-
catalyzed synthesis of a biphenyl analogue of V-COF-1, which
however could not be synthesized after extensive trials
employing basic conditions.[47] On the other hand, we were so
far also unable to synthesize crystalline V-COF-1 and -2 using
the reported acid-catalyzed pathway, showing that the choice of
catalytic pathway and solvent can be crucial to obtain V-COFs
with varying backbone structure. V-COF-1 and V-COF-2,
possess surface areas (SABET) of 1341 m2·g-1 and 627 m2·g-1,
respectively. Although chemically and thermally stable, we
identified that V-COF-1 is photo-responsive, as the pre-
organized vinylene groups of adjacent 2D layers offer a platform
for [2+2] cycloaddition upon UV-Vis irradiation fulfilling Schmidt’s
criteria for photo-cyclization.[48]
The discovery of Covalent Organic Frameworks (COFs)
demonstrated that organic building blocks could be reticulated
via strong covalent bonds into well-ordered 2D and 3D extended
crystalline frameworks.[1–6] These ordered crystalline materials
feature defined porosities and functionalities and have
consequently garnered increasing attention in various
applications such as gas storage and separation,[7,8] energy
storage,[9,10] photovoltaics,[9,11] opto-electronics,[12,13] proton and
ion conduction[14–19] and heterogeneous catalysis.[20–26] The
formation of such extended crystalline frameworks is generally
achieved by implementing reversible condensation reactions in
order to polymerize rigid building blocks. Careful control over the
rate of reversible bond formation allow the construction of the
desired crystalline framework by applying the principles of
reticular chemistry.[27–29] Until recently, linkages (i.e. bonds
formed to reticulate the building blocks) reported for COF
synthesis were mostly based on reversible B-O,[1,2,16,30,31] B-N,[32]
Si-O[33,34] and C-N bond formation[35–39]. The reversible nature of
those linkages affected by the insertion-deletion of water
molecules, allow the formation of the desired extended
crystalline frameworks, but was also identified to be detrimental
in terms of chemical stability, especially in aqueous media.[28,40]
Indeed, although thermally robust, many COFs are prone to
hydrolysis and structural collapse in aqueous/acidic media or
even in contact with moisture. Several strategies, including keto-
enol tautomerization,[35] interlayer stacking optimization[36] or
post-functionalization[41,42] have been applied to circumvent
these limitations, yielding COFs featuring high chemical and
thermal stability.
In this context, recently reported 2D COFs based on
Scheme 1. V-COF synthesis. (a) Illustration of the base catalyzed aldol
condensation of 2,4,6 trimethyl s-triazine (TMT) and Terepthalaldehyde (TD)
to yield 2,4,6 tristylryl s-triazine (TST). (b) Schematic representation of
reticulation of the crystalline V-COFs by the condensation of 2,4,6 trimethyl s-
triazine with ditopic or tritopic aldehydes respectively.
[a]
Mr. A. Acharjya, Dr. P. Pachfule, Dr. J. Roeser, Dr. F-J. Schmitt,
Prof. Dr. A. Thomas.
Department of Chemistry-Functional Meterials
Technische Universität Berlin
Hardenbergstr. 40, BA2, 10623 Berlin (Germany)
E-mail: arne.thomas@tu-berlin.de
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Figure 1. Synthesis and structural characterization of V-COFs (a) Synthetic conditions for the reticulation of V-COF-1 and V-COF-2, (b) PXRD pattern of the V-
COF-1 and V-COF-2: experimental pattern (black), Pawley refined profile curve (red), Bragg diffractions (brown), difference (orange), simulated pattern from
eclipsed (green) and (c) Structural models of both V-COF-1 and V-COF-2 with their respective stacking patterns.
Syntheses of V-COF-1 and V-COF-2 were performed by
solvothermal condensation of 2,4,6 trimethyl s-triazine (TMT)
with terepthalaldehyde (TA) and 1,3,5-tris(4-formyl)phenyl
benzene (TFB), in 7:1 and 1:1 (methanol: mesitylene) solvent
mixtures respectively, for 4 days at 180 °C in presence of NaOH
as base (Figure 1a). Both the COFs were collected as light-
yellow polycrystalline solids by filtration and further washed with
methanol, water and acetone before being dried under vacuum
at 100 °C overnight.
Crystallinity of V-COF-1 and V-COF-2 was assessed by
powder X-ray diffraction (PXRD) analyses. The experimental
PXRD pattern of V-COF-1 and V-COF-2 confirmed the formation
of a crystalline framework with no evidence of remaining starting
material (Figure S2). Given the connectivity of the building
blocks, several models with different stacking modes were
generated in hexagonal 2D nets with hcb topology (Section S4).
A good match was found between the experimentally obtained
PXRD patterns and the calculated patterns of a fully eclipsed
model (Figure 1, Figure S3, S5). The final lattice parameters
were extracted after Pawley refinement and V-COF-1 was found
to crystallize in a hexagonal unit cell (P6/m, a = b = 21.6934 Å,
c = 3.5232 Å, RP = 2.66 %; RWP = 3.40 %) with similar unit cell
parameters as the isoreticular LZU-1 constructed from benzene
nodes and imine linkages.[49] V-COF-2 was poorly crystalline as
evidenced by the broad diffraction peaks observed
stretching frequency of starting aldehyde monomers at 1690
cm-1 and appearance of a new band at 1631 cm-1 attributed to -
C=C- stretching indicated complete condensation of the starting
building blocks and the successful formation of the vinylene
linkage in V-COF-1 and V-COF-2.
13C cross-polarization magic angle spinning nuclear
magnetic resonance (CP-MAS NMR) spectroscopy confirmed
the full conversion of starting monomers as no residual carbonyl
resonance located at δ~190 ppm could be detected. The two
expected signals for the vinylene (-C=C-) carbons at ~138.1
ppm and ~132.1 ppm in COFs could be unambiguously
assigned within the aromatic carbon signals (figure 2b, Figure
S7). V-COF-1 and V-COF-2 surprisingly exhibited two distinct
signals for the aromatic triazine carbon atoms at ~172.9 and
~166.4 ppm in the 13C CP-MAS NMR spectra, respectively,
while just one peak, as expected, was observed for the
molecular model compounds. Splitting of the triazine signals has
been also observed in the solid state 13C NMR spectrum of
Melamine and was attributed to different H-bonding environment
leading to two chemically inequivalent carbon atoms.[50,51]
However, this explanation could be ruled out here, since there is
no electron deficient hydrogen donor within the building blocks.
Instead, the second triazine environment is caused by a [2+2]
cycloaddition between vinylene groups.[52–54] This is corroborated
by the presence of a broad signal in the aliphatic region (δ~38.1
ppm) indicating the formation of cyclobutane moieties in the
COFs (see below).
experimentally, but was found to crystallize in
a similar
̅
hexagonal unit cell (space group: P6, a = b = 17.5632 Å, c=
3.4544 Å, Rp = 2.82% ; Rwp = 3.58%) (Figure 1, Figure S4, S6).
Fourier transform infrared (FT-IR) spectroscopy analyses
were applied to assess the structural integrity of the crystalline
framework (Figure 2a). Complete disappearance of -C=O-
The permanent porosity of the V-COF-1 and V-COF-2
frameworks was evaluated by low-pressure nitrogen (N2) and
argon (Ar) sorption studies on evacuated samples, at 77 K and
87 K, respectively. A steep gas uptake in the low relative
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Figure 2. Structural characterization of V-COFs (a) FT-IR analyses of V-COFs with the starting materials (TMT and TD) and model compound (TST). (b) 13C CP-
MAS NMR spectra of V-COF-1 and V-COF-2 and model compound (TST). (C) N2 sorption measurements of V-COF-1 and V-COF-2 powders.
pressure range (p/p0 < 0.05) of the N2 adsorption branch shows
the microporosity of the samples (Figure 2c). The Brunauer-
Emmett-Teller surface areas (SABET) calculated from the N2
adsorption were found to be 1341 m2.g-1 and 627 m2.g-1,
respectively. The SABET of V-COF-1 is almost twice as high as
the SABET reported for the isoreticular LZU-1 COF (729 m2·g-
1).[55,56] The corresponding pore size distribution of 1.69 nm was
derived by fitting the Ar adsorption branch data at 87 K with a
cylindrical quenched-solid density functional theory (QSDFT)
model, and matched well with the theoretically calculated pore
size of 1.6 nm of the proposed eclipsed structural model (Figure
S8). The pore size distribution of V-COF-2 was found to be
centered at 0.82 nm, derived by fitting the N2 adsorption branch
data at 77K with the QSDFT model and again matched well with
theoretically calculated pore size of 0.8 nm (Figure S9).
Architectural stability of the V-COF-1 and V-COF-2 was
investigated by thermogravimetric analysis (TGA) under N2
atmosphere (Figure S10, S11). After an initial weight loss of ~2
wt% due to desorption of adsorbed solvents below 100 °C, the
frameworks remained stable until 400 °C. Chemical stability of
crystalline V-COF-1 and V-COF-2 was further investigated in
harsh acidic and basic conditions (Figure S12, S13). Both V-
COF-1 and V-COF-2 were found to be stable in concentrated
acidic and basic medium for at least 4 days. V-COF-1 was also
found to be stable in common organic solvents such as THF,
acetone and DMF, for 4 days, with no signs of decomposition
(Figure S14).
Considering the high crystallinity and conjugated structure,
we investigated the optical properties of V-COF-1. The diffuse
reflectance UV-Vis (UV-DRS) spectrum of V-COF-1 shows a
distinct red shift in absorbance with respect to the monomers
(TD and TMT), indicating a higher degree of conjugation (Figure
S15). V-COF-1 shows an absorbance edge at ~410 nm,
whereas the absorbance tail is extended up to ~535 nm.
Interestingly, V-COF-1 was found to be photosensitive, as the
light yellow colour of pristine COF powders slowly faded with
time upon sunlight irradiation, indicating a structural change
within V-COF-1 (Figure 3a). To further elucidate the effect of
light irradiation, V-COF-1 powder was irradiated directly with UV-
Vis light (λ~320-500 nm) considering that the maximum
absorbance of the COF lies within this wavelength range
(Section S10). 13C CP-MAS NMR spectra of the irradiated (48h)
sample revealed the formation of cyclobutane moieties between
the COF layers. At the same time, the vinylene carbon signal
(δ~138.1, 132.4 ppm) intensity significantly decreased and the
ratio of the triazine carbon signals (at δ~172.9 and ~166.4)
reversed compared to the pristine COF (Figure 3b). A broad
signal at (δ~ 38.1 ppm) at the aliphatic region of the 13C CP-
MAS NMR spectra proved the formation of cyclobutane moieties
unambiguously.
Solid-state supramolecular photochemistry offers an
efficient way to engineer new functional polymers by taking
advantage of the preorganization of reactive moieties in the
crystalline matrix.[57–59] In general, different strategies such as
hydrogen bonding interaction, π···π stacking and metal ligand
interactions have been explored to control the topochemical
arrangement of the reactive modules. Efficient π···π stacking of
2D layers within 2D COFs offer a suitable platform for the
preorganization of photo-reactive moieties in a particular fashion
to undergo photochemical reaction.[60] Topochemical [2+2]
photocycloaddition in solid state requires parallel orientation of
two double bonds within a distance of 3.5-4.2Å, entailed by
Schmidt’s criteria.[48] Thus, preorganization of the vinylene based
templates are necessary for controlled synthesis of new 2D or
3D polymers. Schlüter et. al. reported the synthesis of a 2D
polymer by topochemical [2+2] photocycloaddition of
a
trivinyleneic monomer, whereas numerous vinyleneic building
units inside metal organic frameworks (MOFs) or coordination
polymers are also reported to undergo [2+2] cycloaddition.[61,62]
Considering the high crystallinity and eclipsed stacking of 2D
layers, V-COF-1 offers a suitable platform for [2+2] photo-
cycloaddition within the vinylene linkages of adjacent layers. Still,
this is the first report where parallel oriented vinylene moieties of
adjacent layers undergo [2+2] cycloaddition within the 2D layers
of a covalent organic framework.
The effect of formation of cyclobutane from the vinylene
linkages was also observed in the fluorescence emission
spectrum of the V-COF-1 powders measured over time with light
irradiation (Figure 3d). The pristine COF powders dispersed in
ethanol was found to be highly fluorescent upon excitation of
λ~365 nm. With light irradiation over time, a fast quenching a of
fluorescence emission together with the a blue-shift of the
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In summary, we report the successful crystallization of two
new vinylene linked 2D COFs (V-COF-1 and V-COF-2),
achieved by base catalyzed aldol condensation of TMT with
ditopic terepthalaldehyde (TD) and tritopic 1,3,5-tris(p-
formylphenyl)benzene (TFPB). This work demonstrates
a
successful strategy to exploit highly electron deficient s-triazine
core for the crystallization of 2D vinylene linked COFs by base
catalyzed condition. Both the COFs were found to be chemically
stable in concentrated acidic as well as in basic condition. [2+2]
photo-cycloaddition within the columnar π···π stacked 2D-
layers of a COF was observed for the first time upon UV-vis light
irradiation on V-COF-1 powders. Loss of crystallinity of V-COF-1
was observed after light irradiation due to formation of highly
strained cyclobutane rings within the 2D layers, as confirmed by
13C CP-MAS NMR spectrum. However, the framework remains
porous with a SABET of 1093 m2.g-1 even after exposure to UV-
vis irradiation for long time (48h). Therefore, our focus is now
dedicated towards the cycloreversion of the cyclobutanes, which
could lead to a photo-switchable crystalline framework and could
Figure 3. Effect of light irradiation on V-COF-1. (a) Photographs of powder V-
COF-1 before and after light irradiation in dispersion (b) Changes in 13C CP-
MAS NMR spectrum before and after light irradiation. (c). Change in N2
sorption isotherm of V-COF-1 before (V-COF-1) and after light irradiation (V-
COF-1-Light). (d) Fluorescence emission quenching upon irradiation of light
with time, monitored by fluorescence emission spectroscopy.
be implemented as
a rewritable memory storage device.
However, such a task is extremely difficult to achieve in solid
polycrystalline state, as the [2+2] cycloreversion from
cyclobutane to parent vinylenes can follow different pathways.
Thus, suitable conditions to photoswitch the framework is
currently under investigation.
emission maximum was observed and fluorescence emission
completely quenched after 10 minutes. This is expected, as the
degree of conjugation decreases upon formation of cyclobutane
from conjugated vinylenes.[63]
Acknowledgements
This work is funded by the “Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) under Germany´s
Excellence Strategy – EXC 2008/1 (UniSysCat) – 390540038".
A.A. thanks the BIG-NSE and Unicat for doctoral fellowship. We
thank Mrs. Christina Eichenauer and Mrs. Maria Unterweger for
their assistance. Dr. Matthias Trunk, Mr. Nicolas Chaoui and Dr.
Johannes Schmidt are acknowledged for the fruitful discussions.
Crystallinity of V-COF-1 was affected upon the [2+2]
photo-cycloaddition, as sp2-hybridized vinylene carbons convert
to sp3-cyclobutane species causing severe strain within the
crystalline lattice (Figure 4). To compensate the strain within the
2D layers, V-COF-1 framework renders amorphous (Figure S17).
Such a change in crystallinity upon [2+2] photocycloaddition
within crystalline lattices is expected and has been widely
investigated in vinylene-based monomer single crystals,
sometimes with photosalient effect.[64] Interestingly, the
framework remained porous even after exposure to UV-vis
irradiation for long time (48h) with a SABET of 1093 m2.g-1 (Figure
3c).
Keywords: Covalent Organic Frameworks (COFs)• Vinylene
linked• Porous polymer • π···π stacking • [2+2] cycloaddition
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This article is protected by copyright. All rights reserved.

10.1002/anie.201905886
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Amitava Acharjya,a Pradip Pachfule,a
Jérôme Roeser,a Franz-Josef Schmitt,a
and Arne Thomasa*
Page No. – Page No.
Vinylene Linked Covalent Organic
Frameworks by Base-Catalyzed Aldol
Condensation
The synthesis of two vinylene-linked (-CH=CH-) Covalent Organic Frameworks
(COFs) by base-catalyzed aldol condensation has been reported. Structural
properties of highly crystalline COF has been investigated.
This article is protected by copyright. All rights reserved.