A Dual-Function Highly Crystalline Covalent Organic Framework for HCl Sensing and Visible-Light Heterogeneous Photocatalysis

Article abstract of DOI:10.1021/acs.macromol.1c00574

Covalent organic frameworks (COFs) offer great potential for various advanced applications such as photocatalysis, sensing, and so on because of their fully conjugated, porous, and chemically stable unique structural architecture. In this work, we have designed and developed a truxene-based ultrastable COF (Tx-COF-2) by Schiff-base condensation between 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 5,5,10,10,15,15-hexamethyl-10,15-dihydro-5H-diindeno(1,2-a:1′,2′-c)fluorene-2,7,12-tricarbaldehyde (Tx-CHO) for the first time. The resulting COF possesses excellent crystallinity, permanent porosity, and high Brunauer-Emmett-Teller (BET) surface areas (up to 1137 m2 g-1). The COF was found to be a heterogeneous, recyclable photocatalyst for efficient conversion of arylboronic acids to phenols under visible-light irradiation, an environmentally friendly alternative approach to conventional metal-based photocatalysis. Besides, Tx-COF-2 provides an immediate naked-eye color change (<1 s) and fluorescence "turn-on"phenomena upon exposure to HCl. The response is highly sensitive, with an ultralow detection limit of up to 4.5 nmol L-1.

Full text of DOI:10.1021/acs.macromol.1c00574

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Article
A Dual-Function Highly Crystalline Covalent Organic Framework for
HCl Sensing and Visible-Light Heterogeneous Photocatalysis
Yogendra Nailwal, A. D. Dinga Wonanke, Matthew A. Addicoat, and Santanu Kumar Pal*
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ABSTRACT: Covalent organic frameworks (COFs) offer great potential for
various advanced applications such as photocatalysis, sensing, and so on because
of their fully conjugated, porous, and chemically stable unique structural
architecture. In this work, we have designed and developed a truxene-based
ultrastable COF (Tx-COF-2) by Schiff-base condensation between 1,3,5-tris(4-
aminophenyl)benzene (TAPB) and 5,5,10,10,15,15-hexamethyl-10,15-dihydro-
5H-diindeno(1,2-a:1′,2′-c)fluorene-2,7,12-tricarbaldehyde (Tx-CHO) for the
first time. The resulting COF possesses excellent crystallinity, permanent
porosity, and high Brunauer−Emmett−Teller (BET) surface areas (up to 1137
m2 g−1). The COF was found to be a heterogeneous, recyclable photocatalyst for
efficient conversion of arylboronic acids to phenols under visible-light
irradiation, an environmentally friendly alternative approach to conventional metal-based photocatalysis. Besides, Tx-COF-2
provides an immediate naked-eye color change (<1 s) and fluorescence “turn-on” phenomena upon exposure to HCl. The response
is highly sensitive, with an ultralow detection limit of up to 4.5 nmol L−1.
interest as a monomer in the construction of functional
conjugated microporous polymers (CMPs) and COFs for
various applications.47−54 Herein, for the first time, we have
designed and synthesized a truxene-based [3 + 3] COF using
Schiff-base chemistry and demonstrated its dual function both
as a heterogeneous photocatalyst and a naked-eye acid sensor.
Recently, photocatalysis has become a vital category in
organic synthesis due to its mild reaction conditions such as
room temperature and atmospheric pressure, simple setup,
and, most importantly, an environmentally friendly alternative
to the metal-based catalysts.55,56 However, inorganic transition
metal complexes57−63 and organic dyes64−67 have already
shown various chemical transformations as homogeneous
photocatalysts. These metal-based photocatalysts and organic
dyes are commercially available, and they possess excellent
stability. However, their inherent disadvantages, separation
cost of homogeneous catalysts after completion of the reaction,
and so on restrict their extensive work in large-scale synthesis
used for commercial applications. Therefore, the demand for
metal-free, heterogeneous catalysts with high stability, easy
recovery, and reusability from the reaction mixture could
reduce the aforementioned concerns.
INTRODUCTION
■
Covalent organic frameworks (COFs) are the emerging class
of porous organic materials, constructed from various building
blocks, connected via strong covalent organic bonds.1−6 In
recent years, COFs have gained immense interest because of
their extraordinary properties such as high porosity, low
density, extreme crystallinity, and their capability of introduc-
ing a new organic moiety inside the stable framework.7−10
Because of these properties, COFs have expanded their
dimensions into various applications, including gas storage
and separation,11−13 sensing,14−17 energy storage and con-
version,18−26 proton conduction,27,28 optics,29−33 cataly-
sis,34−43 and so on. In terms of photocatalysis, they have
acquired enormous attention because of the combination of
their π-conjugated skeleton, uniform pore structure, tunable
pore channels, high surface area, and, most importantly, their
insolubility in all organic solvents, which is the key feature for
their heterogeneous character. These features demonstrate that
COFs could be considered as a potential candidate for
photocatalytic applications.
Among several π-conjugated molecules used as a monomer
for the construction of COFs, our particular interest is in the
heptacyclic truxene unit. Truxene can be considered as 1,3,5-
triphenylbenzene clipped through three bridging methylene
groups in a plane. These three methylene groups aid
planarization of the system and extend the π-conjugation by
forming three additional fused five-membered rings.44 This
unique structural motif provides truxene to be an attractive
photoactive core and enhances electron-donating capabil-
ities.45,46 The flat trigonal geometry of the truxene has raised
Received: March 14, 2021
Revised: June 3, 2021
Published: June 22, 2021
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Figure 1. (a) Synthetic scheme of Tx-COF-2 (inset: the physical appearance of Tx-COF-2). (b) PXRD patterns of Tx-COF-2: experimental (red)
and Pawley refined (blue, circled), difference plot (dark gray) between the refined and experimental PXRD patterns, and Bragg positions (green).
Simulated PXRD pattern of (c) slipped AA stacking mode (wine) and (d) staggered AB stacking mode (blue) of Tx-COF-2.
Figure 2. (a) Solid-state 13C NMR spectrum of Tx-COF-2. (b) Nitrogen adsorption and desorption isotherms of Tx-COF-2. (c) The pore size
distribution (PSD) was calculated from the QSDFT method of Tx-COF-2. (d) HRTEM image of Tx-COF-2. (e) Zoomed-in HRTEM image of
the marked yellow area in (d), showing a periodicity of 0.48 nm. (f) FFT filter applied on (e). (g) Corresponding height profiles of the marked line
(cyan line) in (f). (h) SAED patterns Tx-COF-2 showing the presence of various crystalline phases. (i) PXRD profiles of Tx-COF-2 as-synthesized
(black) and after treating with 12 M NaOH (red), 12 M HCl (blue), TFA (green), water (purple), and visible-light (orange) for 5 days.
In this article, we demonstrate the synthesis of Tx-COF-2
(Figure 1a) by performing a reaction between 1,3,5-tris(4-
aminophenyl)benzene (TAPB) and 5,5,10,10,15,15-hexameth-
yl-10,15-dihydro-5H-diindeno(1,2-a:1′,2′-c)fluorene-2,7,12 tri-
carbaldehyde (Tx-CHO). The reaction was conducted under
solvothermal conditions and heated at 120 °C for 3 days by
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employing a solvent combination of mesitylene/dioxane (1:1,
v/v) and 6 M acetic acid as a catalyst. Interestingly, Tx-COF-2
exhibited high BET surface area (SBET) up to 1137 m2 g−1,
extraordinary stability in harsh chemical conditions such as 12
M NaOH or 12 M HCl, and irradiation of visible light for a
minimum of 5 days. Tx-COF-2 showed excellent photo-
catalytic activity for the aerobic oxidative hydroxylation of
arylboronic acids. It preserved its crystallinity and activity even
after the 10th photocatalytic cycle. Moreover, Tx-COF-2 was
shown to be a selective “switch-on” sensor for HCl. Upon
exposure to HCl, it became fluorescent and reverted to its
negligible fluorescent intensity (pristine) after exposure to
NH3 vapor. Most importantly, a naked-eye reversible change in
color from light yellow to orange was also observed when
treated alternatively with HCl and NH3 vapor both in solid and
in the dispersed medium of Tx-COF-2.
Table 2. Substrate Scope of Tx-COF-2 in Oxidative
Hydroxylation of Arylboronic Acids to Phenols
a
RESULTS AND DISCUSSION
In a typical synthesis, Schiff-base condensation reaction was
used to construct Tx-COF-2 between 1,3,5-tris(4-amino-
■
a
Table 1. Control Experiments
a
Conditions: Tx-COF-2 (10 mg, 0.012 mmol), arylboronic acid (0.12
mmol), CH3CN:H2O (1.6 mL:0.4 mL), DIPEA (5.0 equiv),
b
irradiation of 20 W white LEDs. Ar = aryl group. Isolated yield.
b
entry visible light photocatalyst atmosphere additives yield (%)
mmol), and 0.5 mL of 6 M aqueous acetic acid, heated in a
Schlenk tube for 72 h under solvothermal conditions (Figure
1a, Scheme S2). The solvothermal conditions were screened
with various solvent combinations, and the best crystallinity
was observed in mesitylene/dioxane (1:1, v/v) solvent mixture
(Figure S4). To characterize the successful formation of Tx-
COF-2, we first employed Fourier transform infrared (FT-IR)
spectroscopy. As shown in the FT-IR spectra of Tx-COF-2
(Figure S3), the stretching band of CO at 1695 cm−1 in Tx-
CHO and that of amine (N−H, ∼3437, 3357 cm−1) in TAPB
disappeared. The appearance of a new vibrational band at
∼1624 cm−1 indicated the formation of an imine bond (C
N) in Tx-COF-2, which is consistent with prior studies.68
Furthermore, solid-state 13C cross-polarization−magic angle
spinning nuclear magnetic resonance (CP-MAS NMR)
spectroscopy of Tx-COF-2 revealed peaks at 22 and 46
1
2
3
4
5
on
on
on
off
on
Tx-COF-2
Tx-COF-2
Tx-COF-2
Tx-COF-2
no catalyst
air
O2
N2
air
air
−
−
−
−
−
99
99
N.D.
N.D.
N.D.
c
c
c
a
Conditions: Tx-COF-2 (10 mg, 0.012 mmol), 4-carboxyphenylbor-
onic acid (0.12 mmol), CH3CN:H2O (1.6 mL:0.4 mL), DIPEA (5.0
equiv), irradiation of 20 W white LEDs, 48 h. Isolated yield. N.D. =
b
c
not detected.
phenyl)benzene (TAPB) (27.5 mg, 0.078 mmol),
5,5,10,10,15,15-hexamethyl-10,15-dihydro-5H-diindeno(1,2-
a:1′,2′-c)fluorene-2,7,12-tricarbaldehyde (Tx-CHO, detailed
synthesis and characterization are given in Supporting
Information, Scheme S1, Figures S1 and S2) (40 mg, 0.078
Figure 3. (a) Reusability of Tx-COF-2 for oxidative hydroxylation of 4-carboxyphenylboronic acid. (b) Nitrogen adsorption and desorption
isotherms of Tx-COF-2 before (olive) and after (blue) the 10th catalytic cycle.
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Figure 4. (a) UV−vis spectrum of the Tx-COF-2 (inset: Kubelka−Munk function plot to evaluate the bandgap of Tx-COF-2). (b) Calculated
position of the conduction band and the valence band of Tx-COF-2, the reduction potential of O2, and the oxidation potential of DIPEA.
Figure 5. Probable reaction mechanism for the photocatalytic conversion of arylboronic acids to phenols by Tx-COF-2 as a photocatalyst.
ppm, originating from the methyl groups and tertiary carbon of
the truxene unit, respectively. The other signals in the range
120−150 ppm can be assigned to the aromatic carbons present
in the framework. In addition, a signal at 157.9 ppm is
indicative of the existence of the imine linkage in Tx-COF-2
(Figure 2a), which supports the formation of the reticular
framework.
kJ/mol) than that of AB stacking (−256.7 kJ/mol) further
supported that Tx-COF-2 adopted slipped AA stacking. The
obtained powder pattern was refined by Pawley refinement
which gave a unit cell with parameters a = 25.6 Å, b = 26.1 Å, c
= 9.4 Å, α = 94.4°, β = 90.5°, and γ = 62.6°. The final Rwp and
Rp values converged to 5.29% and 2.26%, respectively.
The porous properties of Tx-COF-2 were evaluated by the
nitrogen adsorption−desorption experiment, measured at 77
K. As shown in Figure 2b, Tx-COF-2 displayed a typical type I
reversible isotherm, indicating a microporous nature. The BET
surface area was calculated to be 1137 m2 g−1 (Figure S6)
compared to the theoretical surface area of 1357 m2 g−1.
Furthermore, the quenched solid density functional theory
(QSDFT) method was used to calculate the pore size
distribution (PSD). The method showed a narrow pore size
distribution (PSD) with a prominent peak at 2.3 nm, which is
in good agreement with the d-spacing value (2.4 nm) at 2θ =
3.62° obtained from the PXRD analysis (Figure 2c). The
morphologies of the Tx-COF-2 were studied by electron
microscopy. A particulate morphology was revealed by field
emission scanning electron microscopy (FE-SEM) of Tx-COF-
2 (Figure S7). In addition, the finer details of the internal
structure of Tx-COF-2 were revealed by HRTEM analysis.
HRTEM analysis showed an ordered, hexagonal porous
structure that was in line with the structural model. The
observed well-defined lattice fringe with a periodicity of 0.48
nm (Figure 2d,e) corresponds to the (002) plane of the refined
The crystalline structure of Tx-COF-2 was confirmed by
powder X-ray diffraction (PXRD) analysis. As shown in Figure
1b, the PXRD pattern of Tx-COF-2 showed high crystallinity,
with the first intense peak at a low 2θ value of 3.62°, which
corresponds to (100) reflection plane, along with minor peaks
at 6.31°, 7.30°, 9.68°, 13.22°, and 18.99° corresponding to the
(210), (200), (101), (231), and (002) facets, respectively. To
elucidate the structure of Tx-COF-2 and to calculate the unit
cell parameters, a monolayer of Tx-COF-2 (hcb net) was built,
and several stacking configurations were generated, including
eclipsed (AA), slipped stacking (slipped AA), and staggered
stacking (AB) models (Figure 1c,d and Figure S5). These
structural models were optimized by density-functional tight-
binding (DFTB) calculations as implemented in DFTB+ 19.1.
The comparison between the simulated PXRD pattern and
experimental PXRD data indicated that the experimental
PXRD profile matched well to the simulated pattern with
slipped AA stacking in the triclinic system, P1 space group
(Figure 1c and Figure S5). Additionally, the lower value of the
stacking energy of the model with slipped AA stacking (−355.1
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Figure 6. (a) Photographs of Tx-COF-2 suspension in 1,4-dioxane after exposure to HCl and NH3 vapor: naked eye (up) and in the UV light
(down). (b) Fluorescence spectra of Tx-COF-2 suspension (0.5 mg mL−1) in 1,4-dioxane at λex = 478 nm upon addition of HCl from 0 to 65.3
μmol L−1. (c) Solid-state UV−vis spectra of Tx-COF-2: as-synthesized (black), after treatment with HCl (blue) and NH3 (red) vapor. (d) Solid-
state fluorescence spectra of Tx-COF-2: as-synthesized (black), after exposure to HCl (blue) and NH3 (red) vapor (λex = 478 nm). (e) Calibration
curve of fluorescence intensity versus HCl concentration. (f) Schematic representation of the protonation and deprotonation mechanism of the Tx-
COF-2 after alternate exposure to HCl and NH3 vapor.
PXRD pattern (2θ = 18.9°, d = 0.46 nm) and well-matched
with the layer-to-layer distance (0.43 nm) of the slipped AA
model. Furthermore, the selected area electron diffraction
(SAED) pattern revealed various highly crystalline phases
present in the Tx-COF-2 (Figure 2h).
of arylboronic acids to produce corresponding phenols could
be done by using homogeneous photoredox catalysts, such as
Ir or Ru complexes75 and methylene blue (MB).76 Ruthenium
and iridium complexes such as Ru(bpy)3Cl2 and Ir(PPy)3 are
most commonly used for visible light photocatalysts.77
Ru(bpy)3Cl2 and similar complexes also have been used for
the water splitting78,79 and the reduction of CO2 to methane,80
OLEDs,81,82 and initiator for polymerization reactions83−85 or
in photodynamic therapy.86 Ru(bpy)3Cl2 has also been used as
visible light photoredox catalyst for α-alkylation of aldehydes,87
[2 + 2] cycloaddition,88 and reductive dehalogenation of
activated alkyl halides.89 Compared to these earlier works, our
designed COF is the first example of highly stable truxene-
based functional material constructed by Schiff-base chemistry.
Tx-COF-2 could provide a heterogeneous catalytic environ-
ment in mild and harsh reaction conditions, which helps to
easily achieve the reusability and recyclability of the photo-
catalyst.
In the recent past, a couple of COFs containing benzoxazole
core69,70 and vinylene-bridged COFs90 have already shown
photocatalytic activity for the transformation of boronic acids
to corresponding phenols. We believe that in our work the
photocatalytic activity of Tx-COF-2 could be a result of the
highly electron-rich truxene (Tx-CHO) unit.91−93 Thus, we
selected the conversion of 4-carboxyphenylboronic acid to 4-
hydroxybenzoic acid as a standard reaction for the precise
evaluation of the reaction. The elements that influence this
transformation, such as photocatalyst, light source, oxygen
source, and sacrificial agent, were evaluated systematically. As
shown in Table 1, the yield of the reaction can be obtained up
to 99% with N,N-diisopropylethylamine (DIPEA) as a
sacrificial agent and Tx-COF-2 as a photocatalyst under an
oxygen atmosphere or air in the presence of visible light. In the
absence of Tx-COF-2, no conversion was observed (Table 1,
Tx-COF-2 showed excellent thermal as well as chemical
stability. The thermogravimetric analysis indicated that Tx-
COF-2 was stable up to 375 °C under a nitrogen atmosphere
(Figure S8). PXRD assessed the chemical stability of the Tx-
COF-2 after 5 days of treatment in water, TFA, 12 M HCl, and
12 M NaOH. In all these harsh conditions, Tx-COF-2 retained
its crystallinity. Furthermore, most of the reported imine-
linked COFs degraded upon irradiation of light for a long
time;69,70 Tx-COF-2 showed no degradation in the crystal-
linity after irradiation with 20 W white LED for 5 days (Figure
2i). Furthermore, the high hydrophobic nature (Figure S9) of
the synthesized COF protects the imine bonds of Tx-COF-2
from hydrolysis and therefore retains its high crystallinity. This
possibly contributes to the high stability of Tx-COF-2 toward
harsh acid and base conditions, consistent with the prior
reports.71 The high stability toward harsh chemicals and
photostability of Tx-COF-2 render it an excellent candidate for
heterogeneous photocatalysis.
The extensively conjugated skeleton, high photostability,
and chemical stability of Tx-COF-2 led us to think whether it
could act as a heterogeneous photocatalyst without using
metal. We selected visible-light-mediated oxidative hydrox-
ylation of arylboronic acids to phenols to check its photo-
activity since metal-free heterogeneous photocatalysis provides
an environmentally friendly substitute to the traditional-metal-
based redox chemical processes. Additionally, phenols are well-
known, adaptable intermediates and constituents in natural
products, polymers, and pharmaceutical industries.72−74 In
previous reports, it was illustrated that oxidative hydroxylation
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entry 5), which suggested that Tx-COF-2 has a significant part
as a photocatalyst. Similarly, the transformation of the reaction
was hard to observe when conducted in the absence of visible
light and the nitrogen atmosphere (Table 1, entries 3 and 4). It
could be concluded from all these control experiments that
light source, oxygen, and photocatalyst are necessary for this
reaction.
The superiority of Tx-COF-2 as a photocatalyst was further
examined by recycling experiments. Because of the insoluble
nature of Tx-COF-2 in all organic solvents, it was simply
removed from the reaction mixture by centrifugation after the
completion of the reaction. The experiment was performed on
4-carboxyphenylboronic acid. As shown in Figure 3a, without
any reactivation procedure or specific treatment, Tx-COF-2
gave excellent results for the conversion of 4-carboxy-
phenylboronic acid to 4-hydroxybenzoic acid even after 10
runs as a photocatalyst. The crystallinity of recycled Tx-COF-2
catalyst was fully comparable with that of the as-synthesized
Tx-COF-2, as observed from PXRD, with a small decrease in
the intensity69,70 (Figure S12). Furthermore, after the 10th
catalytic run, Tx-COF-2 retained its microporous nature with a
slight drop in the BET surface area, as shown by the nitrogen
adsorption and desorption isotherms (Figure 3b, Figures S13
and S14).
conversion is shown in Figure 5. Initially, an excited
intermediate Tx-COF-2* was formed, upon irradiation of
visible light, followed by an electron extraction from DIPEA by
single-electron transfer (SET) pathway to generate a radical
anion Tx-COF-2•− which left a radical cation DIPEA•+. Then
the Tx-COF-2•− was oxidized by molecular oxygen to restore
•−
Tx-COF-2, used in upcoming cycles. In addition, O2 was
filled in the vacant p-orbital of boron atom, leading to the
generation of A. A hydrogen atom was pulled out from
DIPEA•+ to produce intermediate B. Next, loss of a −OH− ion
and rearrangement formed intermediate C. Finally, intermedi-
ate C hydrolyzed to afford the desired phenolic product D.
As mentioned earlier, the Tx-COF-2 was very stable in
contact with 12 M HCl, which also caused a visual change of
the color from yellow to orange. This phenomenon motivated
us to examine the sensing properties of Tx-COF-2 toward
HCl. Upon exposure to HCl vapor, the suspension (0.5 mg
mL−1 in 1,4-dioxane) of Tx-COF-2 went through a naked-eye
color change from light yellow to orange. When the HCl
treated suspension was subjected to light at 365 nm of
wavelength, an orange fluorescence emission color was also
observed (Figure 6a). This color change was completely
reversible when HCl treated Tx-COF-2 exposed to the NH3
vapor. The color changes occurred in fractions of a second
faster than those previously reported COF-based HCl
sensors.95,96 Notably, as-synthesized Tx-COF-2 was very
weakly fluorescent, but the HCl-treated suspension of Tx-
COF-2 showed remarkable enhancement in the fluorescence
intensity. Thus, Tx-COF-2 could be used as a “turn-on” sensor
because it offered a high increment in the fluorescence
intensity after treating with HCl vapor and reverted to its
pristine weakly fluorescent nature after exposure to NH3 vapor.
Besides, no degradation in the fluorescence intensity as well as
in naked-eye color changes up to 10 alternating cycles of
exposure to HCl and NH3 vapor confirmed the outstanding
reversibility and structural integrity of Tx-COF-2 as HCl
sensor. We then investigated the relationship between the
fluorescent intensity of 1,4-dioxane suspended Tx-COF-2 (0.5
mg mL−1) and the HCl concentration (diluted in 1,4-dioxane).
We observed that upon the addition of a very little amount of
HCl concentration (8 μmol L−1) to Tx-COF-2 suspension the
weak-intensity peaks at 495 and 515 nm started disappearing,
and a new peak appeared at 558 nm (λex = 478 nm) (Figure
6b). The fluorescence intensity enhanced gradually upon
increasing the HCl concentration from 8 to 65.3 μmol L−1 and
remained unchanged after that. The lowest limit of detection
was found to be 4.5 nmol L−1 from the calibration curve in the
range of HCl concentration from 1 to 55 μmol L−1, which
showed linear correlation (R2 = 0.9864) with detection at 558
nm (Figure 6e). The fluorescence lifetime decay of Tx-COF-2
was found to be 0.49 ns at 65.3 μmol L−1 HCl concentration
(Figure S18). A possible mechanism of sensing toward HCl is
shown in Figure 6f. The Tx-COF-2 presumably undergoes
protonation at the imine nitrogen atom, which takes part in the
conjugation of the COF and results in an apparent change in
fluorescence emission as well as in color.97 Previously reported
COFs support the fact of protonation at the nitrogen atom
present in the COFs.98 Next, the reusability and recyclability of
Tx-COF-2 as HCl sensor were investigated and shown in
Figure S16. Tx-COF-2 was recovered by the addition of a
solution of triethylamine (1.6 mmol L−1) from the HCl-treated
Tx-COF-2 at a concentration (1.6 mmol L−1) until the pH is
equal to 7. Moreover, the selectivity of the Tx-COF-2 toward
We then tested the versatility of the Tx-COF-2 as a
photocatalyst for the conversion of different derivatives of
arylboronic acid to the corresponding phenols. As shown in
Table 2, Tx-COF-2, as a photocatalyst, can easily convert all
the substituted arylboronic acids to the corresponding phenols.
Usually, the reaction rate is faster with the substrates bearing
electron-deficient groups (Table 2, entries 1−5) than that of
the substrates bearing electron-rich groups (Table 2, entries 6
and 7). In reported literature, this selectivity has been
explained according to the fact that the empty p-orbital of
boron atoms present in electron-deficient arylboronic acid
substrates are more available for the O2 radical anion.94
•−
To get a deeper understanding of the photocatalytic
procedure, various electronic properties of Tx-COF-2 were
studied. Initially, the UV−vis absorption spectrum of Tx-COF-
2 showed a broad light absorption edge in the range 300−700
nm, indicating that Tx-COF-2 can absorb light in the visible
region (Figure 4a). The optical band gap (Eg) of Tx-COF-2
was determined by the Kubelka−Munk plot and estimated to
be 2.65 eV, indicating its semiconducting nature (Figure 4a,
inset). Cyclic voltammetry (CV) was further recorded to
determine the oxidative potential of Tx-COF-2. The CV
experiment displayed an oxidation peak with an onset value of
+0.93 V versus the saturated calomel electrode (SCE), which
corresponds to the valence band potential (EVB) (Figure
S10).70 The approximate conduction band potential (ECB) was
evaluated by using the formula ECB = EVB − Eg. Thus, the ECB
value of Tx-COF-2 was estimated to be −1.72 V. The
reduction potential (Ered) of O2 and the oxidation potential
(Eox) of DIPEA versus SCE are −0.86 V and +0.90 V,
respectively. For oxidation of DIPEA and reduction of O2, a
photocatalyst is expected to possess a EVB value greater than
Eox of DIPEA and a ECB value less than Ered of O2.94 The ECB
and EVB of Tx-COF-2 entirely meet the above-mentioned
criteria (Figure 4b). In addition, electron paramagnetic
resonance (EPR) spectroscopy was performed to confirm the
•−
generation of O2 upon addition of the superoxide radical
scavenger 5,5-dimethyl-1-pyrroline N-oxide (DMPO), shown
in Figure S11. A probable reaction mechanism for the
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HCl was investigated by treating Tx-COF-2 suspension to
several acids with a concentration of 2.0 mmol L−1. As shown
in Figure S17, in comparison with the other acids, only HCl
produced a notable enhancement in the fluorescence intensity
of Tx-COF-2 when excited at 478 nm.
cyclic voltammetry (CV), fluorescence experiments and
computational details (Figures S20−S22, Tables S1−S3)
of Tx-COF-2 (PDF)
Video S1 (MP4)
Next, we investigated the naked-eye detection of HCl vapor
by Tx-COF-2 in a solid state. A color change from yellow to
orange was observed when Tx-COF-2 powder was exposed to
HCl vapor and returned to its natural color after the exposure
of NH3 vapor (Figure S15 and Video S1). In the UV−vis
spectra, a red-shift in the absorbance maxima was observed
after exposure to the HCl vapor and regenerated with NH3
vapor (Figure 6c). In addition, when we recorded fluorescence
spectra of Tx-COF-2 powder, fluorescence emission was
observed for the HCl-treated Tx-COF-2 powder and returned
to as-synthesized nonfluorescent powder when exposed to
NH3 vapor (Figure 6d). Moreover, the crystallinity of Tx-
COF-2 remained unaltered after alternate exposure to HCl and
NH3 vapors (Figure S19). In conclusion, these phenomena
were observed presumably because of the protonation at the
nitrogen center in the Schiff-base backbone of the Tx-COF-
2.99 As the crystallinity of Tx-COF-2 remained unchanged
after exposure to HCl, we believe that the structural integrity
due to the presence of truxene core in Tx-COF-2 might also be
contributing toward HCl sensing. Compared to the previous
reports, this is the first report in COFs where we
simultaneously observed a “turn-on” fluorescence phenomena
and a naked-eye color change when Tx-COF-2 was exposed to
HCl.
AUTHOR INFORMATION
Corresponding Author
■
Santanu Kumar Pal − Department of Chemical Sciences,
Indian Institute of Science Education and Research (IISER)
Mohali, Manauli 140306, India; orcid.org/0000-0003-
4101-4970; Email: skpal@iisermohali.ac.in,
santanupal.20@gmail.com
Authors
Yogendra Nailwal − Department of Chemical Sciences, Indian
Institute of Science Education and Research (IISER) Mohali,
Manauli 140306, India; orcid.org/0000-0003-3770-
6464
A. D. Dinga Wonanke − School of Science and Technology,
Nottingham Trent University, NG11 8NS Nottingham,
United Kingdom
Matthew A. Addicoat − School of Science and Technology,
Nottingham Trent University, NG11 8NS Nottingham,
United Kingdom; orcid.org/0000-0002-5406-7927
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.1c00574
Notes
The authors declare no competing financial interest.
CONCLUSIONS
■
ACKNOWLEDGMENTS
In summary, we have synthesized a novel dual-function
truxene-based COF using Schiff-base chemistry. Based on
our results, Tx-COF-2 was found to be highly crystalline with
permanent porosity and high chemical stability even in harsh
chemical conditions, for example, 12 M HCl, 12 M NaOH, and
so on. Tx-COF-2 has been used for visible-light heterogeneous
photocatalytic transformation of arylboronic acids to phenols
with high efficiency. Most importantly, Tx-COF-2 has many
advantages as a photocatalyst such as a reusability and
insolubility in various organic solvents. Hence, we believe
that Tx-COF-2 could be a promising photocatalyst for
chemical transformations. Besides, we tested Tx-COF-2 for
HCl sensing and found very high sensitivity and selectivity
toward HCl. We observed the “turn-on” phenomenon in the
fluorescence spectroscopy accompanied by naked eye color
change in solid as well as in the dispersed state in rapid
response time (<1 s). Tx-COF-2 can sense HCl up to 4.5
nmol L−1, which is the first example of very low detection for
HCl sensing in COFs. This COF was found to be an excellent
photocatalyst and sensor for HCl vapor. We believe that our
work will open up several new strategies to develop simple and
smart materials for practical applications, from naked eye
readable sensors to photocatalysts.
■
M.A.A. acknowledges HPC resources through Materials
Chemistry Consortium EP/P020194. S.K.P. acknowledges
Project File No. CRG/2019/000901/OC from DST-SERB.
Y. N. acknowledges IISER Mohali for fellowship. We thank
NMR, SAXS/WAXS, FE-SEM, TEM, Liquid nitrogen facility
and all other departmental facilities at IISER Mohali. We
sincerely thank Dr. Adrene Freeda D’cruz and Supreet Kaur for
critical reading of the manuscript. We thank Dr. Sanchita
Sengupta for help in CV measurement. We also thank Dr.
Manisha Devi and Dr. Joydip De for many helpful discussions.
We sincerely thank Dr. Kaushik Ghosh at INST, Mohali for
help in contact angle measurement.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.1c00574.
Detailed synthetic procedure of monomers and Tx-
COF-2, 1H and 13C NMR spectra of substrates (Figures
S23−S40), FT-IR spectra, FE-SEM images, TGA curve,
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