Conjugated Covalent Organic Frameworks via Michael Addition-Elimination

Article abstract of DOI:10.1021/jacs.6b12005

Dynamic covalent chemistry enables self-assembly of reactive building blocks into structurally complex yet robust materials, such as covalent organic frameworks (COFs). However, the synthetic toolbox used to prepare such materials, and thus the spectrum o

Full text of DOI:10.1021/jacs.6b12005

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Article
Conjugated Covalent Organic Frameworks via Michael Addition#Elimination
Malakalapalli Rajeswara Rao, Yuan Fang, Steven De Feyter, and Dmitrii F Perepichka
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12005 • Publication Date (Web): 14 Jan 2017
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Conjugated Covalent Organic Frameworks via Michael Addi-
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tion‒Elimination
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†
†‡
‡
†
M. Rajeswara Rao, Yuan Fang, Steven De Feyter and Dmitrii F. Perepichka*
†
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal H3A 0B8, Quebec, Canada
‡
Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven – University of Leuven,
Celestijnenlaan, 200 F, Bꢀ3001 Leuven, Belgium
Supporting Information Placeholder
ABSTRACT: Dynamic covalent chemistry enables selfꢀassembly of reactive building blocks into structurally complex yet robust
materials, such as Covalent Organic Frameworks (COFs). However, the synthetic toolbox used to prepare such materials, and thus
the spectrum of attainable properties, is very limited. For πꢀconjugated COFs, the Schiff base condensation of aldehydes and
amines is the only general dynamic reaction, but the resulting imineꢀlinked COFs display only a moderate electron delocalization
and are susceptible to hydrolysis, particularly in acidic conditions. Here we report a new dynamic polymerization based on Michael
addition‒elimination reaction of structurally diverse βꢀketoenols with amines, and use it to prepare novel two dimensional (2D) πꢀ
conjugated COFs, as crystalline powders and exfoliated micronꢀsize sheets. πꢀConjugation is manifested in these COFs in signifiꢀ
cantly reduced band gap (1.8ꢀ2.2 eV), solid state luminescence and reversible electrochemical doping creating midꢀgap (NIR abꢀ
sorbing) polaronic states. The βꢀketoenamine moiety enables protonation control of electron delocalization through the 2D COF
sheets. It also gives rise to direct sensing of triacetone triperoxide (TATP) explosive through fluorescence quenching.
INTRODUCTION
a modest (0.2ꢀ0.3 eV) reduction in the bandgap in comparison
to molecular reference compounds (Table S1).
Covalent Organic Frameworks (COFs) are porous crystalline
solids obtained by 2D or 3D polymerization of organic buildꢀ
Herein, we present a novel and general synthetic approach
towards crystalline COFs with improved πꢀdelocalization,
using Michael additionꢀelimination reaction of βꢀketoenols
and aromatic amines (Scheme 1). The resulting βꢀketoenamine
linked COFs display an enhanced hydrolytic stability endowed
by the intramolecular C=O…HN hydrogen bonding, and can
be prepared from a broad variety of electrophilic and nucleoꢀ
philic building blocks. The electron delocalization in these
COFs leads to a pronounced bandꢀgap reduction (0.3–0.6 eV),
reversible electrochemical doping, and lightꢀemitting properꢀ
ties. The combination of the πꢀconjugation
1
2
ing blocks. They emerge as new promising materials for gasꢀ
3
4
and energyꢀstorage, catalysis, and semiconducting device
5
applications. Achieving crystalline order in COFs relies on
the reversibility of the used chemical reaction; carrying out the
polymerization under dynamic equilibrium conditions allows
the system to selfꢀassemble towards thermodynamic miniꢀ
mum, via errorꢀcorrection mechanism (i.e. unlinking and reꢀ
1
linking improperly connected building blocks). However,
1
e
only a handful of reactions are sufficiently dynamic to enable
such mechanism: boronic acid selfꢀcondensation and polyconꢀ
1
,6
densation with aromatic diols, aldehyde/amine condensation
Scheme 1: Michael additionꢀelimination reaction. Mechanistic
pathway (a); synthesis and metathesis of enamines 3P (b) and
1TPA (c).
7
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(
Schiff base reaction), nitroso dimerization and imidation
reactions. Among these, Schiff base reaction forming imine
bonds is currently the only dynamic covalent chemistry suitaꢀ
ble for making πꢀconjugated COFs.
Electron delocalization in conventional, 1D πꢀconjugated
polymers gives rise to their special optoelectronic properties,
that are employed in organic lightꢀemitting diodes, photovoltaꢀ
a)
H
H
1
0
OH
O+ OH
OH+OH
R'
O
R'NH2/H+
-H+
-H O
2
O
HN
R
R
R
NH
+H O
2
R
NH2
R'
R'
b)
HO
11
O
OH
Ethanol/H+
OHN
N
O HN
ics, photodetectors, fieldꢀeffect transistors, sensors, etc. Even
N
H
O
O
H
Ethanol/H+
NH2
Excess
O
more intriguing opportunities could be created if equally effiꢀ
cient electron delocalization in πꢀfunctional materials is exꢀ
NH2
O
O
O
of
(T)
NH
OH
3
nd
12
NH
tended in the 2 dimension. These opportunities, however,
are barely explored in COFs, although a significant progress
has been made in 2D conjugated networks synthesized via
surfaceꢀcatalyzed polymerization and other nonꢀequilibrium
3P
3T
O
c)
O
OH
H
Ethanol/H+
N
NH O
13
N
H2N
NH2
reactions. Indeed, due to its high polarization, the imine linkꢀ
age of Schiff base COFs is not very efficient in supporting πꢀ
delocalization between the connected units, and most COFs
prepared via the dynamic covalent chemistry approach display
1
N
HN
O
1
TPA
NH2
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Scheme 2. Synthesis of COFs via Michael additionꢀelimination under solvothermal conditions. i) 5/5/1 dioxane:mesitylene:6M aceꢀ
tic acid; 130 °C/3 days; ii) 5/100/1 dioxane:mesitylene:6 M aq. acetic acid; 130 °C/3 days.
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with specific hostꢀguest interactions within the pores of the
COF, gives rise to a remarkable sensing behavior. The fluoꢀ
rescence of 3BD and 3'PD is effectively quenched by the di-
rect interaction with triacetone triperoxide (TATP). This is a
notoriously dangerous explosive, but its detection is more
challenging than that of common nitro explosives due to its
relative chemical inertness (although a fluorescent detector for
both, Fido Paxpoint, has already been commercialized by ICx
3P and T reveals a significant exchange over a course of few
hours at room temperature in CDCl (Figure S1). Furthermore,
3
heating 3P with an excess of T (80 °C, ethanol/catalytic
amount of acid) leads to quantitative formation of 3T (Figures
1, S2). These experiments show that the reaction is highly
reversible, and potentially suitable for formation of COFs.
The COFs were synthesized by reacting diꢀ and tritopꢀ
ic βꢀketoenols (
2
,
3
,
3') with aromatic amines (PD, BD,
TPA) at 130 C in the presence of aqueous acetic acid
(Scheme 2). The resulting orange (3BD 3'PD) to deepꢀ
14
o
Technologies ) We note that the βꢀketoenamine linker resemꢀ
15
bles that reported by Banerjee and coꢀworkers for a COF
,
obtained via tautomerisation of βꢀhydroxyimine (prepared by
Schiff base condensation). This transformation relied on the
instability on 1,3,5ꢀtrihydroxysubstituted benzene ring and is
thus limited to one particular building block (trihydroxytriꢀ
mesic aldehyde). In contrast, our approach has a broad scope
of electrophilic substrates, and we demonstrate its feasibility
with four readily available βꢀketoenol precursors.
red (3PD and 2TPA) powders were filtered, purified by
Soxhlet extraction (THF over 24 hrs) and dried under
o
vacuum at 120 C.
The infrared (IR) spectra of all synthesized COFs
showed a complete disappearance of characteristic NꢀH
–
1
stretching of the primary amines (3470 and 3420 cm );
–
1
the CꢀO stretching at ~1206 cm is replaced by a new
–
1
band at 1200 cm attributed to the formed C–N bond
Figure 1a and Figures S3ꢀ5). The overall IR spectra of
(
RESULTS and DISCUSSION
the COFs closely resemble that of molecular reference
compounds (3P, Figure 3a; 2P and 3’P, Figure S3ꢀ5). The
chemical structures of the COFs were further confirmed
To demonstrate the diversity of our approach, we have syntheꢀ
sized four βꢀketoenolꢀfunctionalized building blocks 1–3 and
1
3
by C solid state NMR spectroscopy, and the signals
3
' (Schemes 1, 2) in a oneꢀstep reaction of corresponding aceꢀ
13
were identified with the help of the C non quaternary
tophenone precursors with ethyl formate (yield = 74ꢀ82%, SI).
To verify the chemical selectivity and dynamic nature of the
proposed Michael additionꢀelimination polymerization, we
have studied an exchange reaction of the reference compound
suppression (NQS) measurements (Figures 1b and S6,7).
All COFs showed a characteristic signal at ~190 ppm,
corresponding to
3
P with toluidine (T). In situ NMR monitoring of a mixture of
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trast, staggered (AB) orientations show significant deviations
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from the observed PXRD patterns (Figures S8ꢀ11).
The permanent porosities were determined by
BrunauerꢀEmmettꢀTeller (BET) method using N adsorpꢀ
2
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tion at 77k, for supercritical CO activated COF samꢀ
2
ples. All COFs displayed reversible type−I isotherms with
a very small hysteresis (Figures S12, 13). The smallest
2
unit cell COF 3PD revealed BET surface area of 505 m /g
3
−1
and pores volume of 0.36 cm g which is comparable to
4a,15a
imineꢀlinked COFs with similar unit cell.
However,
enlarging the unit cell by 0.75 nm in 3BD unexpectedly
led to a slightly lower BET surface area of 478 m /g, and
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even much lower porosity was revealed by the COFs
3'PD and 2TPA (Table S2). We also note that when the
activation was carried out by simple vacuum drying (120
ºC), ca. twice lower surface areas were measured for all
COFs.
Figure 1. IR and NMR characterization of the COFs. a) Compariꢀ
son of the IR spectra of COF 3PD, reference compound (3P) and
13
building blocks (3 and PD); b) comparison of solid state
C
crossꢀpolarization magic angle spinning (CPꢀMAS) and NQS
1
3
NMR spectra of 3PD COF and solution C NMR spectrum of 3P
reference. Asterisks indicate spinning side bands.
the α,βꢀunsaturated carbonyl (C=O) carbon. The αꢀcarbon
signal (b) of ketoenamine in the COFs is shifted upꢀfield to 93
ppm in comparison to ketoenol 3 (99 ppm), in line with the Cꢀ
N linkage. All the signals are fully consistent with those of
reference compounds (3P, Figure 1b; 3'P and 2P, SI).
Powder Xꢀray diffraction (PXRD) of the prepared COFs
(
3
3
Figure 2 and Figures S8ꢀ11) reveals strong peaks at 2θ =
.52° (24.5 Å), 2.90° (30.8 Å) and 2.29° (38.6 Å) for 3PD,
16
BD and 3'PD, respectively. These correspond to (100) reꢀ
flections defined by the periodicity of the covalent network.
The broad peak at 2θ ~26° for 3PD corresponds to the (001)
reflection, indicating an interlayer distance of ~3.4 Å. Higher
order reflection peaks were also observed for 3PD (Figure 2a)
but not the other COFs, suggesting partial loss of crystallinity
with increasing length of the linkers. The Scherrer analysis of
(100) peak width for 3PD suggests an average crystalline coꢀ
herence length on the order of 10ꢀ15 nm. Density functional
1
2b
theory (DFT) calculations of 2D polymers corresponding to
individual COF sheets were carried out under periodic boundꢀ
ary conditions (PBC), using the B3LYP functional. 3D models
of COFs were built from the corresponding DFTꢀoptimized
Figure 2. PXRD patterns and simulated crystal structures of
COFs. Experimental PXRD of asꢀsynthesized (solid lines) and
simulated (broken lines) XRD pattern using the eclipsed AA
stacking of 3PD (a) and 3BD (b). Inset shows the top and side
views of the COFs structures. The simulated 2D unit cell parameꢀ
ters are: a = b = 29.7 Å, c = 3.4 Å for 3PD; a = b = 37.4 Å, c =
3.4 Å for 3BD.
2
D polymer sheets, assuming 3.4 Å van der Waals spacing
between the covalent sheets, and using various stacking shifts.
The simulated PXRD patterns of the resulting model in an
eclipsed orientation (AA stacking with a small stacking shift,
see SI) matched well the experimental data (Figure 2). In conꢀ
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This strongly suggests that partial collapse of the frameꢀ IR spectrum) which could be attributed to structural rearꢀ
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work takes place during the activation, and further optiꢀ
mization of the activation procedure might lead to higher
open porosity.
rangement induced by partial hydrolysis. A control experiment
with Schiff base COF lacking Hꢀbonding stabilization
4a
(TFBPD,
prepared
by
condensation
of
1,3,5ꢀ
Scanning electron microscopy (SEM) of asꢀprepared
COF powders showed several hundred nm size globular
particles with no clear signs of crystallinity (Figures 3a
and S14ꢀ17). A very different picture, however, emerges
from the microscopy of exfoliated samples. Both transꢀ
mission electron microscopy (TEM) and atomic force
microscopy (AFM) measurement on 3PD samples preꢀ
pared by sonication of ground COF powders in toluene
reveals ~halfꢀmicron large sheets with a uniform height in
the range of 0.7ꢀ4.5 nm (Figures 3bꢀd and S18). This corꢀ
responds to a stack of ~2ꢀ15 COF sheets. The apparent
structural integrity of the covalent sheets on the order of
hundreds of nm is not revealed in the SEM images, possiꢀ
bly due to their crumpling in the asꢀprepared COF powꢀ
ders. We note that single layers of 2D conjugated polyꢀ
mers are of interest for the
triformylbenzene and PD, see SI) hydrolyzes with complete
dissolution in 9 M HCl within hours, and also rapidly loses its
crystallinity in hot water (Figures S20–22).
Diffuse reflectance spectra of the synthesized COFs reꢀ
vealed a red absorption edge at ~565ꢀ685 nm, corresponding
to the optical band gap E of ~2.2ꢀ1.8 eV (Figure 4a). 3PD
g
COF with the smallest unit cell has E = 2.03 eV; this is inꢀ
g
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creased to E = 2.19 eV (3BD) and 2.17 eV (3'PD) upon inꢀ
g
troduction of additional phenylene links in the unit cell. Deꢀ
planarization of the polymer network due to Ph…Ph twist
might be partially responsible for the increased bandꢀgap, altꢀ
1
2b
hough diminished conjugation is also theoretically predicted
for fully planar 2D polymers networks upon extending the
links between the nodes. On the other hand, introducing an
electronꢀrich (triphenyl amine) unit in the covalent network of
2TPA gives rise to contraction of E (1.79 eV), which is
g
commonly observed in linear (1D) conjugated polymers with
11
alternating DꢀA structure.
Relative to molecular reference compounds 1TPA, 3P, 3'P
(representing 2D unit cells of these COFs), the bandꢀgap of
COFs is contracted by 0.34ꢀ0.62 eV (Figure 4a, Table S2) due
to πꢀdelocalization through the polymer sheet. However, this is
limited by crossꢀconjugation through the metaꢀconnected benꢀ
zene core of 3 and 3’ or through nitrogen lone pair of TPA.
Nevertheless, the βꢀketoenamineꢀbased COFs show a lower
bandꢀgap vs imineꢀbased COFs, as revealed by comparing
3
=
PD (E = 2.03 eV) to corresponding imine linked TFBPD (E
g g
4a
2.45 eV) (Figure S24). This is could be attributed to the Dꢀ
A interactions (carbonylꢀamine) in the former.
The DFT calculations for individual 2D polymer sheets
predict the same trend of band gap evolution: 3.16 eV for
3PD; 3.27 eV for 3BD and 3.18 eV for 3'PD; and 2.67 eV for
2TPA. In all four COFs, the HOMO is mostly centered over
the arylamine moieties and appears increasingly localized with
enlarging the unit cell of the COF (Figure S25ꢀ28). However,
the LUMO is delocalized through the entire network, with
exception for 2TPA where the electronꢀrich TPA units reveal
very small LUMO density. This is not favorable for inꢀplane
conductivity, a common problem for all reported COFs.
COFs 3BD and 3'PD exhibit orange luminescence in the
Figure 3. Morphology of 3PD. a) SEM image of COF powꢀ
der. b) TEM image of a COF nanosheet exfoliated by soniꢀ
cation in toluene c) AFM image of COF nanosheets on mica in
water. d) Crossꢀsection analysis of (c).
solid state with a broad band at λmax 547 and 560 nm (λ 450
ex
nm), respectively. These are bathochromically shifted vs refꢀ
potential grapheneꢀlike electron confinement effects and
erence compounds 3P (λmax 510 nm) and 3'P (λmax 480 nm)
1
2a
expected efficient charge transport properties.
Onꢀ
(
Figure S29). The lifeꢀtime of the emission is very short (<0.5
surface growth of such materials is an explosively growꢀ
nsec) pointing to fluorescent nature of the process. Interestingꢀ
ly, the luminescence of dried COFs is visibly lower than that
of dispersions in solvents (CH Cl , THF, diethyl ether, toluene
1
3aꢀd
ing research field,
but is presently limited to metallic
surfaces, which hampers their applications. In this respect,
2
2
18,15b
exfoliation of COFs
to access such materials.
provides an alternative approach
etc.).
Fluorescence quenching of linear (1D) conjugated polyꢀ
mers in the presence of strong electron acceptors is one of the
most attractive methods for detection of nitroꢀbased exploꢀ
Thermal gravimetric analysis (TGA) of the activated COFs
showed an onset of thermal decomposition temperatures at ca.
19
3
00 °C (Figure S19). As earlier demonstrated for other COFs,
sives. Chargeꢀtransfer interactions between a nitro acceptor
and electron rich polymer, and efficient exciton diffusion
through the polymer chain, lead to very high selectivity and
sensitivity of this simple detection method. On the other hand,
conventional conjugated polymers have no specific interacꢀ
tions with triacetone triperoxide (TATP), an extremely danꢀ
gerous and readily accessible explosive, implicated in many
terrorist attacks. While a number of TATP detection methods
intramolecular Hꢀbonding can dramatically suppress hydrolyꢀ
1
0f,15a
sis of these materials.
Likewise, our COFs showed a reꢀ
markable hydrolytic stability in a strong acid (9 M HCl) as
o
well as in hot water (50 C) displaying no signs of decomposiꢀ
tion in IR and PXRD analysis after over one week exposure. A
partial loss in the PXRD signal intensity was noticed upon
exposure of 3PD to 9 M NaOH (but no discernible change in
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tion of TATP into suspension of 3BD or 3'PD (ca. 0.02
have been reported and an already commercialized Fido Paxꢀ
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point is capable of detecting both, most of these are based on
detection of hydroperoxides, either present as impurities or
generated inꢀsitu through chemical or photochemical treatment
mg/mL) leads to a gradual decrease of the emission signal
with a concomitant blue shift, and with detection onꢀset of ~1
ꢀM of TATP (Figures 4c,d and S31ꢀ33). We note that while
sensing of picric acid and other nitroaromatics by fluorescent
14,20
of TATP.
21
Our COFs 3BD and 3'PD exhibit fluorescence quenching
in the presence of both nitroꢀ (picric acids, Figure S30) and
peroxideꢀbased (TATP) explosives. Titration of CH Cl soluꢀ
COFs was reported,
TATP detection by COFs or
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Figure 4. a) Diffuse reflectance spectra of all COFs and their corresponding reference compounds. b) Spectroelectrochemical
measurements of 2TPA (insets show the cyclic voltammogram and the photograph of COFꢀcovered ITO electrode). Photoluminesꢀ
cence spectra of 3’PD (c) and 3BD (d) (λex = 450 nm) dispersions in CH Cl and fluorescence quenching in the presence of TATP
2
2
(inset shows the photograph of 3’PD dispersion, before and after adding TATP).
other πꢀconjugated polymers is unprecedented. No acid treatꢀ
ment or photolysis, used to generate H O in other sensing
applications. However, the possibility of reversible doping in
conjugated COFs is not yet well established. The only pertiꢀ
nent work we know of describes chemical pꢀdoping of
2
2
20
systems, was applied. Furthermore, in these conditions (up to
23
1mM concentration), neither H O itself nor tꢀBuOOH quench
tetrathiafulvaleneꢀcontaining COF using iodine. Here, we
present the first case of electrochemical doping of the 2TPA
COF. Films of 2TPA were solvothermally grown on conducꢀ
tive ITO glass (Figure 4b, inset). Cyclic voltammetry of this
sample reveals a quasiꢀreversible oxidation wave at Epa ~ 0.7
V that can be associated with electron loss from the TPA unit.
Recording VisꢀNIR absorption of this film upon progressive
stepwise increase of the oxidation potential leads to attenuaꢀ
tion of the 460 nm band corresponding to the neutral COF and
the appearance of two new bands at λmax 395 nm and 1020 nm
(Figure 4b). Based on the topology of the HOMO (Figure S28)
it is expected that the formed radical cations are primarily
located on the TPA moieties, although the red shift of absorpꢀ
2
2
the fluorescence of our COFs (Figures S35,36). The direct
energy or electron transfer from excited COFs to TATP is
unlikely due to large optical gap (>3 eV) and very high
LUMO of the latter. The observed fluorescence quenching can
most probably be attributed to an oxidation of the enamine
moiety by TATP, which is supported by the change of the
color in these COFs (redꢀshift) in TATP solution (Figures S31,
S32). Nevertheless, the environment of the COF pores controlꢀ
ling the interaction with TATP, while not yet fully understood,
appears critically important. Indeed, control experiments with
the oligomeric enamines 3P and 3'P showed no response to
TATP (tested up to 1 mM).
2
2
.+
D
oping of conjugated polymers is required to create free
tion of 2TPA (0.85 eV) relative to the reference compound
.
+
charge carriers and is an essential part in most of their device
1TPA (1.03 eV Figure S38) suggests their appreciable interꢀ
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action/delocalization through the COF. The doping is comꢀ
METHODS
PD and 3BD. Triketoenol 3 (35.0 mg, 0.121 mmol) and diaꢀ
mine PD (19.7 mg, 0.182 mmol) or BD (33.4 mg, 0.181
mmol) were suspended in a degassed mixture of 1,4ꢀdioxane
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pletely reversible as evident by restoration of the original
spectrum of the neutral COF upon applying a reductive potenꢀ
tial (–0.5 V, Figure S39). 2TPA can also be doped chemically
3
(I vapor), producing a very similar NIR absorption to the
2
(
1
3 ml), mesitylene (3 ml) and 6 M aq. acetic acid (0.5 ml) in a
electrochemically doped films (Figure S40).
The ketoenamine moiety also allows to tune the πꢀ
conjugation via protonation (acid doping). Exposing COFs
2 ml high pressure glass vessel. The mixture was degassed
o
for 10 min and tube was sealed and heated to 130 C for three
days. The precipitate was filtered and washed with 1,4ꢀ
dioxane, water and acetone (10 ml each) and subjected to
Soxhlet extraction with THF for 24 hrs to remove the trapped
guest molecules. The powder was dried at 120 °C under vacuꢀ
um overnight to yield 3PD (35 mg, 83%) and 3BD (40 mg,
78%) as red and orange solids, respectively.
3
PD, 3BD, 3'PD and 2TPA to trifluoroacetic acid (TFA)
leads to a red shift of their absorption bands by 150ꢀ200 nm
Eg = 1.45ꢀ1.81 eV, Figures S41, 42). This is attributed to enꢀ
(
0
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0
hanced electronic delocalization through the protonated keꢀ
toenamine bridge. Indeed, the efficient electron delocalization
through the formally crossꢀconjugated βꢀ ketoenamine moiety
could be explained by the resonance contribution of the βꢀ
iminomethyleneꢀenol tautomer that leads to partial bond
length equalization across the link (Scheme 3).
Scheme 3: Tautomerism and resonance electron delocalꢀ
ization in βꢀketoenamine links. Intramolecular proton
transfer converts crossꢀconjugation of βꢀketoenamine (left)
into direct conjugation of βꢀiminomethyleneꢀenol (right).
The latter resonance form is enforced by protonation (botꢀ
tom).
2TPA was prepared using the above synthetic protocol, from
diketoenol 2 (22.5 mg, 0.103 mmol) and triamine TPA (19.9
mg, 0.068 mmol). The COF was isolated as a red powder (27
mg, 84%).
3
’PD was prepared using the above synthetic protocol except
the solvent combination was changed to 1,4ꢀdioxane (0.3 ml),
mesitylene (5.7 ml) and 6M acetic acid (0.5 ml), from trikeꢀ
toenol (3’) (30.0 mg, 0.058 mmol) and diamine PD (9.4 mg,
.086 mmol). The COF was isolated as an orange powder (27
mg, 80%).
0
TATP and picric acid sensing. Stock solutions (10 mM) of
recrystallized TATP and commercial grade (SigmaꢀAldrich)
picric acid were prepared in dichloromethane. A homogeneous
dispersion of 3BD or 3’PD (0.02 mg/ml) in dichloromethane
was made by sonication (10 mins). The fluorescence titrations
were carried out via gradual addition of aliquots of stock soluꢀ
tion of the analyte (TATP/picric acid) to the 10 ml of COF
dispersed solution and the spectra were recorded 2 min after
Protonation of this link increases the contribution of the latter
resonance form, enabling direct πꢀconjugation. In comparison,
molecular references 2P, 3P and 3'P show no noticeable color
change and only a minor red shift of absorption upon exposure
to TFA (by 15ꢀ20 nm, Figure S43), once again highlighting
each addition. The excitation wavelength (λ ) was at 450 nm
ex
and the corresponding emission wavelength was tested from
80 to 800 nm. Each test was repeated at least three times
4
showing reproducible fluorescence intensities. Fluorescence
extinction by TATP, with similar efficiency, was also obꢀ
served for 3BD and 3’PD dispersions in diethyl ether; howevꢀ
er, no appreciable change of their fluorescence was observed
upon addition of TATP to acetonitrile and ethanol dispersion.
Also, control experiments with H O and tertꢀbutyl peroxide
24
the optoelectronic effect of πꢀconjugation in our COFs.
CONCLUSIONS
2
2
In summary, we have developed a new dynamic covalent
polymerization based on Michael additionꢀelimination reacꢀ
tion of a series of aromatic βꢀketoenols with arylamines. Apꢀ
plicable to a variety of easily accessible monomers, this reacꢀ
tion constitutes the third general methodology for preparation
of crystalline porous Covalent Organic Frameworks. Comparꢀ
ing to the other two reported families of COF (prepared by
boronic acid and Schiff base condensation), the βꢀketoenamine
COFs exhibit much higher hydrolytic stability and provide a
more efficient πꢀelectron delocalization. The latter property is
manifested in significant band gap reduction and reversible
electrochemical doping creating NIRꢀabsorbing midꢀgap
states. 3BD and 3'PD COFs are fluorescent and their emission
is efficiently quenched in the presence of both nitroaromatic
(up to 1 mM, in both dichloromethane and diethyl ether)
showed no quenching of 3BD and 3’PD emission.
ASSOCIATED CONTENT
Synthetic procedures and characterization data of the synthesized
13
compounds. Computational protocols, PXRD data, C solidꢀstate
NMR spectra, BET sorption isotherms, additional SEM and AFM
images, chemical stability tests, additional fluorescence sensing
and spectroelectrochemical data.
AUTHOR INFORMATION
Corresponding Author
*
E mail: dmitrii.perepichka@mcgill.ca
(picric acid) and peroxide (TATP) explosives. This fluorescent
detection of TATP is direct (that is not mediated by hydroperꢀ
oxides), although its precise mechanism is yet to be elucidatꢀ
ed. Our synthetic strategy opens new opportunities to develop
electronically functional COFs, and could enable new applicaꢀ
tions in semiconducting and sensing devices.
ACKNOWLEDGMENT
This work was supported by NSERC of Canada through Discovꢀ
ery Grant and MESRST of Québec through International Collaboꢀ
ration Grant. MRR thanks FQRNT for PBEEE fellowship. SDF
thanks generous support by the Fund of Scientific Research –
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Journal of the American Chemical Society
Flanders (FWO), in the framework of a joint project between
Flanders and Quebec.
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