Triazine Functionalized Porous Covalent Organic Framework for Photo-organocatalytic E- Z Isomerization of Olefins

Article abstract of DOI:10.1021/jacs.9b01891

Visible light-mediated photocatalytic organic transformation has drawn significant attention as an alternative process for replacing thermal reactions. Although precious metal/organic dyes based homogeneous photocatalysts have been developed, their toxic and nonreusable nature makes them inappropriate for large-scale production. Therefore, we have synthesized a triazine and a keto functionalized nonmetal based covalent organic framework (TpTt) for heterogeneous photocatalysis. As the catalyst shows significant absorption of visible light, it has been applied for the photocatalytic uphill conversion of trans-stilbene to cis-stilbene in the presence of blue light-emitting diodes with broad substrate scope via an energy transfer process.

Full text of DOI:10.1021/jacs.9b01891

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Communication
A Triazine Functionalized Porous Covalent Organic Framework
for Photo-organocatalytic E-Z Isomerization of Olefins
Mohitosh Bhadra, Sharath Kandambeth, Manoj K. Sahoo, Matthew
Addicoat, Dr. Ekambaram Balaraman, and Rahul Banerjee
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 04 Apr 2019
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a,b
b
a,b
d
Mohitosh Bhadra, Sharath Kandambeth, Manoj K. Sahoo, Matthew Addicoat, Ekambaram Bala-
*a,b
*c
raman and Rahul Banerjee
a
Academy of Scientific and Innovative Research (AcSIR), CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune-411008, India
b
Physical/Materials and Organic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-
4
c
11008, India
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus,
Mohanpur, 741246, India
d
School of Science and Technology, Nottingham Trent University, Clifton Lane, NG11 8NS Nottingham, United Kingdom
Supporting Information Placeholder
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compounds have been used for the E to Z isomerization of al-
kenes. However, the recyclability of these expensive photocata-
lysts becomes a key issue due to their homogenous nature and low
photochemical stability. Hence, there is a high demand for devel-
oping a novel heterogeneous, chemically stable photocatalyst for
the economic and energy efficient E to Z conversion of alkenes.
ABSTRACT: Visible light-mediated photocatalytic organic
transformation has drawn significant attention as an alternative
process for replacing thermal reactions. Although precious met-
al/organic dyes based homogeneous photocatalysts have been
developed, their toxic and non-reusable nature makes them inap-
propriate for large-scale production. Therefore, we have synthe-
sized a triazine and a keto functionalized non-metal based cova-
lent organic framework (TpTt) for heterogeneous photocatalysis.
As the catalyst shows significant absorption of visible light, it has
been applied for the photocatalytic uphill conversion of trans-
stilbene to cis-stilbene in the presence of blue LEDs with broad
substrates scope via an energy transfer process.
Covalent Organic Frameworks (COFs), a novel class of porous
crystalline polymers, have recently emerged as heterogeneous
1
3
catalysts for various organic transformations. COFs, because of
their highly ordered, predesignable and functionalizable porous
structures, allow precise integration of catalytic centers in the
framework matrix in a well-defined manner. Keeping all these
features in perspective, we have successfully synthesized a novel
heterogeneous COF based porous photocatalyst for the E to Z
isomerization of alkenes. The photocatalytic COF is constructed
from two distinct photoactive building blocks (triazine and β-
ketoenamine) having unique photo-sensitizing properties. The
triazine core should catalyze E to Z photo-isomerization of an
Alkene/olefin functional groups are essential organic building
units in many synthetic polymers and drugs.
synthesis of olefins is important due to their large scope of
applications in synthesizing anticancer drugs,
chromatic lasers and industrial dyes. However, most of the
methods to synthesize alkenes lead to the formation of the
thermodynamically more stable trans (E) form while the direct
synthesis of the energetically less stable cis (Z) form is
remarkably challenging and has been restricted to a limited
number of available synthetic methods. Most of these synthetic
methods lack high stereoselectivity and often require high energy
and expensive homogeneous metal catalysts. The probable
solution for an economic and stereoselective synthesis of the Z
isomer should be to follow the conventional photo-assisted E to Z
isomerization strategy. However, most of the common olefin
compounds lack absorption in the visible range. Thus, the E to Z
transformation can happen only by a high energy UV radiation
1
a-c
Stereoselective
1
d,e
2
14
scintillators,
alkene by facilitating strong π-π interaction with the E alkenes.
Additionally, keto functionalities present in the β-ketoenamine
3
4,5
core could help to enhance the lifetime of the excited triplet
12,15
state.
Moreover, due to the irreversible nature of the β-
ketoenamine formation reaction, the novel triazine functionalized
hybrid COF can offer high chemical stability even upon irradia-
tion of light.
6
13f,g
The COF (TpTt) was constructed by performing reaction
between melamine/1,3,5-Triazine-2,4,6-triamine (Tt) and 2,4,6-
Triformylphloroglucinol (Tp) aldehyde (Figure 1a, Section S2).
Previously, several polymers with triazine moiety were made in
7
16
the presence of a metal catalyst and higher temperatures.
Therefore, finding a suitable method for synthesizing a crystalline
porous TpTt COF from a melamine building block without using
a metal catalyst was very challenging due to its poor reactivity.
After several trials with different solvent combinations, we have
8
approach, which is neither a green, nor a safe process. The
alternative solution for this problem would be to use more
abundant visible light to perform this E to Z isomerization of
olefins with the help of a photocatalyst. However, designing such
catalyst is a challenging task as the ideal photocatalyst needs to
feature high photochemical stability, optimum band gap,
absorption maxima in the visible range, and a long-lived excited
produced
a moderately crystalline TpTt COF using the
DMAc:DMSO (2:1) solvent combination. The PXRD patterns of
TpTt display two main characteristic peaks at 2θ = 9.7° and 27.4°,
which correspond to the reflections from the 100 and 002 planes
respectively (Figure 1b). From the d spacing of the 002 peaks, we
have calculated the interlayer π-π stacking distance between the
individual COF layers to be 3.5 Å (Figure 1c). To find out a
9
state. In the literature, photocatalysts such as metal polypyridyl
complexes, organic dyes such as riboflavin, and aromatic keto
1
0
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Figure 1. (a) Schematic representation of the TpTt synthesis. (b) Comparison of the experimental PXRD patterns of TpTt (black) with the
simulated (blue) simulated after Pawley refinement (red), and difference plot (green). (c) Space-filling AA eclipsed stacking models of the
13
TpTt along c and a-axis direction. (d) C CP-MAS solid-state NMR and (e) SEM and (f) TEM images of TpTt.
reasonably fitting modeled structure of TpTt, several stacking
possibilities such as AA eclipsed, AB staggered and AA inclined
models were constructed. From the Pawley refinement studies, we
have observed that the simulated PXRD patterns of the AA ec-
lipsed and AA inclined structures have a decent agreement with
the experimental PXRD pattern (Section S3, Table S1). The suc-
cessful incorporation of β-ketoenamine links in TpTt is indicated
In comparison to the starting materials Tp (254 and 340 nm)
and Tt (250 nm), the absorption maxima of TpTt appears at a
higher wavelength in the visible range due to extended conjuga-
tion (Figure 2a). The optical band gap of TpTt is calculated as
2.74 eV from the UV-Vis spectra by using the Tauc plot (Figure
S8). Also, we have calculated (using DFTB) the energy difference
between the HOMO and LUMO orbitals of TpTt COF in several
stacking modes. Among them, the theoretical energy difference
between the HOMO and LUMO of TpTt in the AA slip mode is in
good agreement with the calculated energy difference from expe-
riment (Figure 3b, S17, Table S6, S7). The visible light absorption
capacity, together with the ideal band gap, makes TpTt an ideal
catalyst for the E to Z photo-isomerization reaction.
-
1
-
by the appearance of intense peaks at 1620 cm (-C=O), 1523 cm
1
-1
(
-C=C) and 1236 cm (-C-N) in the FTIR spectra (Figure S3).
1
3
The C CP-MAS spectra of TpTt displays characteristic signals
of the carbonyl carbon (-C=O) of the β-ketoenamine core at 184.9
ppm and the doublet peaks at 166.8 ppm and 163.2 ppm for aro-
matic carbons of the triazine core. The rest of the peaks corres-
ponding to the sp carbons appear in the range of 147.7 ppm to
1
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2
After observing the significant visible light absorbing ability of
the photocatalyst, the activated TpTt catalyst is directly utilized
for the trans to cis isomerization reaction by choosing trans-
stilbene as the benchmark substrate for the optimization process.
When the trans-stilbene (0.5 mmol) in 5 mL DMF solvent in the
presence of a catalytic amount of TpTt COF (4mg) was irradiated
using blue LEDs at room temperature, we have observed the
formation of cis-stilbene (GC analysis). A 20% yield of cis-
stilbene is observed after the initial 2 hours of reaction time. The
yield of the cis-stilbene increased steadily with reaction time and
obtained the maximum yield (90%) at an exposure time of 18
hours (Figure 2b). Later, we have screened the effect of various
solvents and observed that DMF is the optimal solvent for this
transformation. It gave 90% yield of the cis-stilbene under
standard reaction condition (Table 1, entry 1, Table S2). As the
amount of catalyst and solvent increase, the formation of the
product also increases and 4 mg TpTt catalyst and 5 mL DMF
solvent are optimal for this transformation (Table S3, S4). Under
similar photocatalytic conditions, melamine (Tt) yielded cis-
stilbene with a trace amount (Table 1, entry 3). To understand the
necessity of the COF catalyst in this isomerization reaction, we
have conducted a blank photocatalytic experiment without using
TpTt in the presence of visible light (Table 1, entry 2). No peak is
observed in the GC corresponding to cis-stilbene, which signifies
1
09.5 ppm (Figure 1d, S4). SEM (Figure 1e, S9) and TEM (Fig-
ure 1f, S10) images reveal that TpTt crystallites possess fibrillar
morphology. Thermogravimetric analysis (TGA) under N atmos-
phere shows the thermal stability of TpTt up to 200 °C (Figure
S7).
2
The N adsorption isotherm of TpTt COF displays a type-I re-
2
2
-1
versible isotherm with 277 m g accessible surface area calcu-
lated by using the Brunauer–Emmett–Teller (BET) method. Fur-
thermore, nonlocal density functional theory (NLDFT) calcula-
tions reveal that TpTt shows a narrow pore size distribution, with
a peak maxima at 1.3 nm (Section S6, S7). The pore size
distribution of TpTt and the length of the trans-stilbene indicate
that the maximum reaction happens on the surface of the
crystallites. It is experimentally observed that the porosity and the
crystallinity in the framework structure (TpTt) play an important
role during the photo-organocatalytic E-Z isomerization process
and exhibit better catalytic performance compared to similar
amorphous polymer (TpTt-Poly) (Section S19). The solid-state
UV-Vis diffuse reflectance spectrum of the TpTt powder shows
broad absorption spectra in the visible region, having a two peak
maximum at 350 nm and 525 nm.
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(Table 1, entry 5) and rt (Table 1, entry 6) in absence of blue LED
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light. No product (cis-stilbene) is formed under these conditions.
TpTt has shown good yield towards the trans to cis
photoisomerization of different stilbene substrates (Table 2). The
significance of visible light source is essential for this
transformation was investigated by performing the light on-off
experiment over time (Figure 2c). The reaction proceeds when the
light is on, and the conversion stops when the light is turned off,
which indicates that the reaction happens through a photocatalytic
pathway. To check the recyclability of the TpTt catalyst, we have
carried out the photocatalytic activity test up to four cycles. Even
after four cycles, the TpTt catalyst showed similar photocatalytic
activity (Section S15). The unchanged FTIR, PXRD, TEM and
gas adsorption isotherm of TpTt catalyst after 20 hours of photo
exposure in DMF suggests the high photostability of the catalyst
(Section S17).
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Table 2. Porous COF Catalyzed Photoisomerization of
a,b
Alkene Substrates Scope.
Figure 2. (a) Solid state UV-Vis diffuse reflectance spectra of
TpTt (pink), Tp (red) and Tt (blue). (b) The photocatalytic yield
of cis-stilbene at different time intervals. (c) Light on off
experiment study over time. (d) The photocatalytic yield of cis-
stilbene in the absence and presence of a free radical scavenger
(R.S.) TEMPO.
that in the absence of the catalyst, only higher energy UV
radiation is necessary for the trans to cis isomerization process.
To prove that the reaction is neither thermodynamically nor
kinetically controlled, we have performed the reactions at 80 °C
a
All the reaction were conducted with 0.5 mmol of trans-stilbene
(1 equiv.), 4 mg TpTt catalyst, in 5 mL N,N-dimethylformamide
a
at rt under blue LEDs irradiation (total 36 W; each LED has 1 W
Table 1. Optimization of the Reaction Conditions.
b
power) for 18h. Isolated yields.
To explore the catalytic reaction mechanism, we have per-
formed a controlled photocatalytic reaction using a radical sca-
venger (R.S.) TEMPO. In the presence of 4 eq TEMPO R.S. in
the reaction medium, the yield of the cis product decreases signif-
icantly [~3%] under the optimized conditions, which confirms
that the reaction proceeds through a biradical intermediate state
b
entry
catalyst
TpTt
temp
rt
solvent
DMF
DMF
DMF
DMF
DMF
DMF
yield[%]
1
0b-c,11c,18
(Figure 2d, Table 1, entry 4).
To get further insight about
1
2
3
90
trace
trace
trace
-
the reaction mechanism, we have performed theoretical calcula-
tions of energy states of different possible reaction intermediates
during the isomerization reaction (Section S18). The results indi-
cate that the first TpTt absorbs visible light and gets excited from
the ground state to the first singlet excited state (S to S ). After
-
rt
Melamine (Tt)
TpTt + TEMPO
TpTt
rt
0
1
[
c]
the intersystem crossing (ISC) TpTt reaches the energetically
4
rt
more stable triplet excited state (T ) and subsequently interacts
1
1
9
[
d]
o
with trans-stilbene and transfers its energy to the trans-stilbene.
This energy transfer helps trans-stilbene to convert itself into its
5
6
80 C
[d]
TpTt
rt
-
biradical triplet intermediate state (T ). This triplet intermediate
1
state then gets converted to the product cis-stilbene (Figure 3a).
a
All reactions were conducted with 0.5 mmol of trans-stilbene (1
In conclusion, for the first time, we could successfully develop
a heterogeneous TpTt photocatalyst for the visible light-induced
isomerization reaction of trans to cis-stilbene. The hybrid TpTt
photocatalyst is synthesized from two types of distinct
photoactive building blocks. Due to the β-ketoenamine linked
equiv.), 4 mg TpTt catalyst, in 5 mL solvents at rt under blue
LEDs irradiation (total 36 W; each LED has 1 W power) for 18h.
b
c
Based on GC analysis using n-decane as an internal standard. 4
d
equiv. TEMPO was added into the reaction mixture. In absence
of blue LEDs source.
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Figure 3. (a) Mechanistic representation of trans to cis photoisomerization of stilbene using the TpTt COF catalyst. (b) Pictorial
representation of HOMO and LUMO orbitals and their energy levels for the TpTt COF catalyst. The energy levels of the orbitals are not
exactly to scale.
structure, the novel TpTt catalyst displays high chemical stability
even upon irradiation of light and broad substrates scope and
retains its photocatalytic activity even after four consecutive
reaction cycles. We believe that the novel COF photocatalyst will
be a promising candidate for the scalable and cost-effective
synthesis of industrially important cis olefins.
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**]Although the pictorial representation of both the COFs (COF-TpMA
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