2D Poly(arylene vinylene) Covalent Organic Frameworks via Aldol Condensation of Trimethyltriazine

Article abstract of DOI:10.1002/anie.201906976

Designing structural order in electronically active organic solids remains a great challenge in the field of materials chemistry. Now, 2D poly(arylene vinylene)s prepared as highly crystalline covalent organic frameworks (COFs) by base-catalyzed aldol con

Full text of DOI:10.1002/anie.201906976

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Accepted Article
Title: 2D Poly(arylene vinylene) Covalent Organic Frameworks via
Aldol Condensation of Trimethyltriazine
Authors: Thaksen Jadhav, Yuan Fang, William Patterson, Chenghao
Liu, Ehsan Hamzehpoor, and Dmitrii F Perepichka
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906976
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Angewandte Chemie International Edition
10.1002/anie.201906976
2
D Poly(arylene vinylene) Covalent Organic Frameworks via Aldol Condensation of
Trimethyltriazine
Thaksen Jadhav, Yuan Fang, William Patterson, Cheng-Hao Liu, Ehsan Hamzehpoor, Dmitrii F.
Perepichka*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec
H3A 0B8, Canada
*
dmitrii.perepichka@mcgill.ca
Designing structural order in electronically active organic solids remains a great challenge in
the field of materials chemistry. Here, we report 2D poly(arylene vinylene)s prepared as highly
crystalline covalent organic frameworks (COFs) by base-catalyzed aldol condensation of
trimethyltriazine with aromatic dialdehydes. The synthesized polymers are highly emissive
(
quantum yield of up to 50%), as commonly observed in their 1D analogues poly(phenylene
2
vinylene)s. The inherent well-defined porosity (surface area ~1000 m /g, pore diameter ~11 Å
for the terephthaldehyde derived COF-1) and 2D structure of these COFs also present a new
set of properties and are likely responsible for the environmentally-sensitive emission color.
The COF-1 is highly hydrophilic and reveals a dramatic macroscopic structural reorganization
that has not been previously observed in framework materials.
Since the discovery of electrically conductive polyacetylene ~40 years ago,[1] π-conjugated
polymers have attracted wide attention from academic and industrial researchers, owed to their
special optoelectronic properties and numerous potential applications in semiconducting devices,
[
2]
explosion sensors, bioimaging, etc. The 2004 demonstration of a very high charge mobility in a
single layer graphene has revolutionized the solid state physics, giving birth to the rapidly growing
[
3]
field of two-dimensional (2D) materials. Numerous opportunities could result from ‘marrying’
the two fields if efficient methods for synthesis and processing of 2D conjugated polymers were
[
4]
available. To date, the two general approaches to synthesis of 2D conjugated polymers are (i)
on-surface polymerization which delivers small ‘patches’ of 2D polymers on metallic substrates[5]
and (ii) dynamic solution polymerization which can produces 2D polymers in bulk quantities,[6]
as stacked sheets.
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The latter approach has been pioneered in 2005 for non-conjugated boroxine polymers with
[
7]
open porosity, dubbed Covalent Organic Frameworks (COFs). Due to their well-defined porosity
and large active surface area, COFs have gained significant attention for applications in gas and
[
8]
energy storage, and catalysis. π-Conjugated COFs can be prepared by dynamic polycondensation
[
9]
[10]
of aromatic amines with aldehydes and ketoenols. While promising optoelectronic properties
have been reported in some of these,[11] the highly polarized C-N links might limit the charge
carrier delocalization (which marginalizes semiconducting applications of 1D conjugated
polyazomethines).[12] New efficient methods for preparing C-C linked 2D polymers are thus highly
desirable. Very recently, several groups have reported cyanovinylene linked COFs prepared by
Knoevenagel condensation of multidentate benzylcyanides (acting as C-H acids) with
dialdehydes.[13] These materials are 2D analogues of poly(phenylene cyanovinylene)s CN-PPV[14]
that have been used as semiconducting polymers in organic light emitting diodes[15] although not
as commonly as the parent, non-cyanated PPVs.[2b] Recognizing the remarkable advantages of
2
[13]
these first sp -carbon linked COFs, we also point out some limitations such as out-of-plane
distortions caused by steric clash of the CN group with the adjacent aromatic rings and increased
reactivity of the polarized cyanovinylene groups (hydrolysis, Michael addition, intramolecular
cyclization by electrophilic attack of CN on the aromatic rings.[14]
With this in mind, we considered the use of methyl groups activated by an electron deficient
heterocycle to synthesize –CH=CH– linked COFs as sterically unencumbered planar sheets with
maximized π-electron overlap. Placing methyl groups on a triazine unit increases their acidity so
that deprotonation in basic conditions generates nucleophilic carbanions which can react with
aldehydes forming a C=C double bond. We note that related acid-catalyzed aldol condensation of
trimethyltriazine with aromatic aldehydes has been known since 1960s and that an amorphous
polymer resulting from the reaction of trimethyltriazine with terephthalic aldehyde has been
reported.[16] During the preparation of this manuscript, a crystalline COF synthesized by acid
catalyzed aldol condensation of trimethyltriazine with biphenyldicarbaldehyde has been
communicated by Yaghi and co-workers.[17]
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Scheme 1. Synthesis of COFs via Aldol condensation of trimethyltriazine (dioxane / ethanol / 1
M aq. NaOH, 100 °C/3 days) (thicker bonds are used to indicates newly formed vinyl bonds).
The reported 2D poly(arylene vinylene)s were synthesized by the aldol-like condensation between
trimethyltriazine and terephthalaldehyde (COF-1), naphthalene-2,6-dicarbaldehyde (COF-2) and
2
,5-dimethoxyterephthaldehyde (COF-3) catalyzed by aqueous alkali hydroxides (Scheme 1). To
aid the characterization of the COFs, we also synthesized the corresponding reference compounds
and 2 using modified literature procedure.[18] The formation of vinylene links was confirmed by
1
infrared (IR), Raman and 13C cross polarization magic angle spinning (CP-MAS) NMR
spectroscopy in the solid-state. The IR spectra of COF-1 and COF-2 closely resemble those of the
–1
–1
reference 1 and 2. The vinylene stretch (1631 cm for COF-1; 1626 cm for COF-2) is blue
–
1
shifted by around 6 cm with respect to reference 1 and 2, possibly signifying the decreasing bond
order due to extended -conjugation (Figures 1a and S3a). The Raman spectra further supports the
–
1
–1
formation of vinylene links (1635 cm ) and disappearance of the methyl (2927 cm ) and carbonyl
–
1
13
(
1685 cm ) stretches of the monomers (Figures 1b). The C NMR peaks of the COFs were
assigned by comparing with corresponding reference compounds 1 and 2. The most downfield
shifted peak ~170 ppm corresponds to the triazine ring and the broad hump at ~120-140 ppm is
due to phenylenevinylene (naphthalenevinylene) links (Figures 1c and S3b).
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Angewandte Chemie International Edition
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a)
b)
c)
COF-1
1
631
1635
C=C
C=C
c
COF-1
Reference-1
1637
C=C
a
a
1
637
b
b
C=C
Reference 1
1685
3070
Aromatic CH
C=O
Terephthaldehyde
Trimethyl triazine
c
1685
Terephtaldehyde
C=O
2927
-H C-H
2
1
535
Trimethyl trazine
C=N
1
000
1500
2000
12500
3000
500
1000
1500
2000
2500
3000
200
150
100
50
0
-
-1
Wavelength (cm )
Raman Shift / cm
Chemical Shift (ppm)
Figure 1. (a) Comparison of the IR and Raman spectra of COF-1, reference 1, terephthaldehyde
1
3
and trimethyltriazine; and (b) C CP-MAS NMR spectra of COF-1 (solid state), reference 1
CDCl solution).
(
3
While an amorphous polymer is readily formed from trimethyltriazine and terephthalic
aldehyde at room temperature (precipitates in 60 min after addition of aq. NaOH), screening of
reaction conditions (solvent, temperature) was performed to improve the crystallinity of the desired
COFs (Table S1-S3). A characteristic (100) diffraction peak of COF-1 has been observed at 2θ =
5
.10.2° at elevated reaction temperature in multiple solvent compositions, but the best
crystallinity was observed in dioxane/ethanol/water solvent mixture, manifested as sharpening of
this peak and appearance of the additional peaks at 2θ = 8.4 (110), 9.7 (200), 12.8 (210) (Figures
2
and S2). The small shift of the (100) peak from 5.3° (d-spacing 16.7 Å) to 4.9° (d-spacing 18.0
Å) is likely a result of decreased slippage in the stacks of covalent polymer sheets. The full width
half maximum (FWHM) of (100) peaks in the most crystalline COF-1 is ~0.3 corresponds to an
average correlation length (crystallite size) of at least ~30 nm. Refinement of the PXRD diffraction
pattern predicts a primitive hexagonal lattice with unit cell parameters of a = b = 21.3 Å, c = 6.9
Å;  =  = 90,  = 120.
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Figure 2. (a, c) PXRD patterns and Pawley refinement of COF-1 (Rwp = 1.95%, Rp = 1.55%),
COF-2 (Rwp = 2.00%, Rp = 1.48%) and COF-3 (Rwp = 2.29%, Rp = 1.78%). b) DFT optimized
(B3LYP/6-31G(d)) model of COF-1 with superimposed HOMO and LUMO topologies. Bottom
left and top right displaying HOMO and LUMO structures, respectively. d) Photograph showing
swelling effect on dry COF-1 pallete; before adding water drops (left) and after adding water drops
(right). e) Comparison of PXRD patterns of dry and water immersed COF-1.
A similar optimization of the solvent was also carried out in order to obtain the crystalline
materials with naphthalenedialdehyde and dimethoxyterephthaldehyde (Tables S2 and S3). The
PXRD of the optimized COF-2 shows a (100) reflection at 2θ = 4.2°, consistent with hexagonal
unit cell parameters a = b = 26.1 Å, c = 6.9 Å;  =  = 90,  = 120 (Figure 2c). The observed 4.8
Å elongation of the unit cell of COF-2 compare to COF-1 is in agreement with the substituting
naphthalene for phenyl moieties. Somewhat lower crystallinity was observed with COF-2
compared to COF-1, manifested in broadened diffraction peaks (FWHM ~0.45).
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The permanent porosity of activated COF samples was revealed by Brunauer−Emmett−Teller
(
BET) method using N
2
adsorption at 77 K. Reversible type−I isotherms were observed with BET
2
surface area of up to 880 m /g (COF-1, Figure S8). The pore size distribution shows essentially
monodisperse pores of ~11 Å (Figure S8) which is somewhat smaller than the van der Waals
diameter of the pore (16.4 Å based on the DFT, Figure 2b), most likely due to a slipped stack
arrangement of the covalent sheets.
Scanning electron microscopy (SEM) of the most crystalline COF samples showed globular
polycrystalline particles of ~2-6 m size (Figures 3a and S9). High resolution transmission
electron microscopy (HR-TEM) revealed a periodic (~3.4 Å) pattern which corresponds to π-
stacked 2D covalent sheets of COF-1 (Figure 3b). This distance is equal to half of the unit cell
vector c predicted from the PXRD. Individual 2D polymer sheets of ~0.5 m size can be exfoliated
from the bulk COF powders by sonication in N-methylpyrrolidone (Figure 3c).
Figure 3. Electron microscopy of COF-1: (a) SEM image; (b) HR-TEM image showing a stacked
layers arrangement; (c) TEM image of exfoliated sheets.
The COFs are highly stable thermally (up to 400 °C, Figure S5) and chemically (1-week immersion
in 12 M NaOH and 11 M HCl, Figure S6). Furthemore, acid treatment of COFs leads to a
remarkable ‘annealing’ effect improving the crystalline order as revealed in PXRD pattern (Figure
2
S7) and increased BET surface area, up to 1008 m /g for COF-1 (Figure S8).
Interestingly, addition of protic solvents (water, ethanol) to a crystalline sample of COF-1 results
in immediate (a fraction of a second) multi-fold expansion of its volume (Figure 2d and SI video).
No swelling was observed for the amorphous samples of this material (prepared by room
temperature polymerization). The PXRD of the water swelled COF-1 shows a significant increase
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Angewandte Chemie International Edition
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in the intensities of (h10) relative to (h00) (Figure 2e, S10). Pawley refinement predicts a slight
change of the unit cell parameters (a = b = 21.6 Å, c = 4.0 Å;  = 60;  =  = 90) upon the
addition of water (Figures S10b). The apparent small expansion of both in-plane polymer
periodicity and interlayer stacking distance is likely attributable to intercalation of water molecules
which form hydrogen bonds with triazine nitrogen atoms. We note that such behavior has been
never reported for any triazine-based polymers. Many triazine-based COFs and microporous
polymers have been prepared (arylene-linked, via polycondensation of dinitriles;[19] imine-linked,
via aldehyde-amine polycondensation,[20][21] but most of these reveal hydrophobic (and even
superhydrophobic) behavior.[20]
The diffuse reflectance spectra of COF-1 and COF-2 powders shows absorption on-sets at
~
470 nm (2.6 eV) and ~540 nm (2.3 eV), respectively (Figure 4). These values are lower than the
band-gaps predicted at B3LYP/6-31G(d) level of theory for corresponding 2D polymer layers
3.30 eV and 3.23 eV, respectively) which can be attributed to electrostatic stabilization of the
(
hole/electron pairs and inter-layer electronic coupling. The observed red shift of absorption with
respect to the reference compounds 1 (362 nm, 3.42 eV) and 2 (395 nm, 3.14 eV) are a signature
of an extended π-conjugation. The cross-conjugation through ‘meta-linked” triazine moiety, as
revealed by the nodes of HOMO on the triazine carbons, certainly limits the electronic coupling,
as the HOMO band dispersion is only 8 meV according to DFT (Table S4). On the other hand,
significantly better dispersion (249 meV) and lack of nodes are predicted for the LUMO, pointing
to potential electron conductivity in these systems upon n-doping (Figure 2b and S12).
Strong greenish-blue (max = 511 nm, photoluminiscence quantum yield (PLQY) 25%) and
yellow (max = 532 nm, PLQY 15%) emission was observed for COF-1 and COF-2 powders in
air (at ambient humidity), respectively. The radiative life-times (~4.3 ns for COF-1, ~2.4 ns for
COF-2) divulge the fluorescence nature of the emission process. The color of the emission as well
as the fluorescence efficiency are sensitive to the environment. For example, dispersing COF-1 in
low polarity dichloromethane leads to stronger (PLQY = 52%) blue-shifted (max = 508 nm)
emission, while attenuated (PLQY = 15%) red-shifted (max = 555 nm) emission was observed in
water (Figure 4 and Table S6). This high efficiency (among the highest values reported for π-
conjugated COFs[13d, 22]) and tunable emission of synthesized arylene vinylene COFs make them
of interest for optoelectronic applications.
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Air
DCM
H
2
O
AcOH aq.HCl
Air
b)
Calculated
band gap
Calculated
band gap
a)
1.0
0.5
0.0
1.0
0.5
Absorption
Air
DCM
Water
Acetic acid
aq. HCl
300
400
500
600
700300
400
500
600
700
Wavelength /nm
exc
Figure 4. Absorption (dotted lines) and photoluminescence (solid lines,  = 430 nm) spectra of
a) COF-1 and b) COF-2 in solid state and dispersed in solvents with different polarities. The inset
shows photograph of the COFs in different environment under 365 nm illumination.
In conclusion we have established an efficient synthetic strategy for crystalline poly(arylene
vinylene)s COFs using base-catalyzed aldol condensation of trimethyltriazines with a series of
aromatic dialdehydes. The synthesized COFs exhibit excellent hydrolytic stability, retaining
their crystallinity in strong acidic and basic conditions. The synthesized COFs are highly
fluorescent; the quantum yield (up to 50%) and the color of the emission depend on the
environment: a red shift and attenuation was observed in polar solvents. The crystalline COF-
1
is highly hydrophilic and shows a multifold expansion of bulk volume upon contact with
water (and methanol). Such behavior is not observed in the amorphous sample of the same COF,
and is strikingly different from hydrophobic behavior of most COFs (including other triazine-
based microporous polymers). This report will opens a new pathway to design and synthesize
highly fluorescent, porous and sensitive COFs which are useful in semiconducting and sensors
application.
Acknowledgements. This work was supported by US Army Office for Scientific Research
(
W911NF-17-1-0126) and NSERC (Discovery Grant). TJ acknowledges FQRNT PBEEE
international post-doctoral fellowship. YF acknowledges NSERC and FQRNT post-doctoral
fellowships. We thank Dr. R. Stein (McGill University) for help with solid state NMR.
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Table of Contents Graphics:
Crystalline 2D poly(arylene vinylene)s COFs have been synthesized via base-catalyzed aldol
condensation of trimethyltriazine with a series of aromatic dialdehydes. The resulting COFs are
hydrolytically stable and highly fluorescent (quantum yield 50%). Addition of protic solvents
results in a dramatic and reversible expansion of COF’s bulk volume and red shifted emission.
Keywords:
Covalent organic frameworks; 2D materials; -Conjugation; Polymers; Aldol reaction
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