A highly fluorescent covalent organic framework as a hydrogen chloride sensor: roles of Schiff base bonding and π-stacking

Article abstract of DOI:10.1039/d0tc01872d

In this paper we report the extremely crystalline structures, high thermal stabilities, and strong fluorescence emissions of covalent organic frameworks (COFs) based on linked carbazole units. We have synthesized three stable luminescent carbazole-linked COFs, namely, BCTB-PD, BCTA-TP, and BCTB-BCTA, through Schiff base condensations of 4,4′,4′′,4′′′-([9,9′-bicarbazole]-3,3′,6,6′-tetrayl)tetrabenzaldehyde (BCTB-4CHO) with p-phenylenediamine (PD), of 4,4′,4′′,4′′′-([9,9′-bicarbazole]-3,3′,6,6′-tetrayl)tetraaniline (BCTA-4NH2) with terephthalaldehyde (TP), and of BCTB-4CHO with BCTA-4NH2, respectively. These COFs had large Brunauer-Emmett-Teller surface areas (up to 2212 m2 g-1) and outstanding thermal stabilities (decomposition temperatures of up to 566 °C). Interestingly, the intramolecular charge transfer (ICT) and fluorescence properties of these COFs were strongly influenced by their types of Schiff base bonding (BCTB-4CHN or BCTA-4NCH) and the degrees of π-stacking between their COF layers. For example, ICT from the electron-donating carbazole group to the acceptor through the Schiff base units of the type BCTB-4CHN and increasing the π-stacking distance enhanced the fluorescence emission from the COF. Moreover, BCTB-BCTA, the most fluorescent of our three COFs, functioned as a fluorescent chemosensor for HCl in solution, with outstanding sensitivity and a rapid response. This journal is

Full text of DOI:10.1039/d0tc01872d

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ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
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Highly fluorescent covalent organic framework as hydrogen chloride sensor: Roles
of Schiff base bonding and π-stacking
Ahmed F. M. EL-Mahdy,a,* Ming-Yi Lai,a and Shiao-Wei Kuoa,b,*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
In this paper we report the extremely crystalline architectures, high thermal stabilities, and strong fluorescence emissions of covalent
organic frameworks (COFs) based on linked carbazole units. We synthesized three stable luminescent carbazole-linked COFs—BCTB-PD,
BCTA-TP, and BCTB-BCTA—through Schiff base condensations of 4,4´,4´´,4´´´-([9,9´-bicarbazole]-3,3´,6,6´-tetrayl)tetrabenzaldehyde
10 (BCTB-4CHO) with p-phenylenediamine (PD), of 4,4´,4´´,4´´´-([9,9´-bicarbazole]-3,3´,6,6´-tetrayl)tetraaniline (BCTA-4NH2) with
terephthalaldehyde (TP), and of BCTB-4CHO with BCTA-4NH2, respectively. These COFs had large Brunauer–Emmett–Teller surface
areas (up to 2212 m2 g–1) and outstanding thermal stabilities (decomposition temperatures up to 566 °C). Interestingly, the
intramolecular charge transfer (ICT) and fluorescence properties of these COFs were strongly influenced by their types of Schiff base
bonding (BCTB-4CH=N or BCTA-4N=CH) and the degrees of π-stacking between their COF layers. For example, ICT from the electron-
15 donating carbazole group to the acceptor through Schiff base units of the type BCTB-4CH=N and increasing the π-stacking distance
enhanced the fluorescence emission from the COF. Moreover, BCTB-BCTA, the most fluorescent of our three COFs, functioned as a
fluorescent chemosensor for HCl in a solution, with outstanding sensitivity and a rapid response.
moieties (as core donor units) have been linked with electron-
Introduction
withdrawing groups (as acceptors) to obtain OSCs having power
conversion efficiencies (PCEs) of up to 2.56%.19 In addition, the
use of carbazole as a core donor and malononitrile and
Carbazole is an electron-rich tricyclic aromatic heterocyclic
compound comprising two benzene rings fused to the sides of a
pyrrole ring. Carbazole-based materials are generally highly
stable under harsh chemical and environmental conditions.
Furthermore, because of their attractive electron-donating and
charge-transporting properties, carbazole-based materials have
been used in a variety of applications, including energy storage,
photoluminescent probes, and organic solar cells (OSCs).1–10 As
an example of energy storage applications, we have recently
prepared the carbazole-based covalent organic frameworks
(COFs) Car-TPA, Car-TPP, and Car-TPT, containing carbazole units
in their hexagonal backbones and exhibiting good specific
capacitances (up to 17.4 F g–1 at a current density of 0.2 A g–1),
through [3+3] and one-pot polycondensations.5 Moreover, TFP-
diketopyrrolopyrrole as terminal units has provided OSCs with
21
PCEs of up to 5.3 and 2.3%, respectively.20,
All of these
previously reported carbazole-based luminescent materials have
been non-crystalline; because crystalline carbazole-based
luminescent materials would presumably be high surface area,
more luminescent and extremely high thermal stability, their
preparation should lead to the development of systems
displaying new and attractive properties.
COFs are porous crystalline polymers having two-
dimensional (2D) or three-dimensional long-range periodic
structures.22–24 COFs are assembled on the basis of the principle
of reticular chemistry, specifically through the cross-linking of
organic building blocks through covalent bonds.25–27 Because of
their excellent crystallinity, permanent porosity, high surface
areas, and high thermal stability,28–31 hundreds of COFs have
been synthesized over the past decade with impressive
applications in the fields of catalysis, gas adsorption, separation,
Car,
polycondensation of 1,3,5-triformylphloroglucinol with 9-(4-
aminophenyl)carbazole-3,6-diamine, has exhibited high
a
β-ketoenamine–tethered COF, obtained through
electrochemical efficiency (up to 149.1 F g–1 at a current density
of 2 A g–1).4 In terms of photoluminescence applications,
carbazole derivatives have been applied as fluorescent probes
for the detection of reactive sulfur, ionic, and oxygen species;
the detection of biomacromolecules (e.g., RNA, DNA); and the
identification of microenvironments (e.g., pH, viscosity,
polarity).11–18 In OSCs, carbazole-based materials have been used
as non-fullerene acceptors (NFAs). For example, carbazole
energy storage, medicine, optoelectronics, proton conduction,
25, 27, 32–45
and chemical sensing.4,
A diverse array of covalent
linkages, typically based on B–O, C–C, N–N, C–N, and B–N bonds,
has been used to construct the frameworks of COFs with various
pore sizes, structures, and functions.46–49 Although the majority
of previously prepared imine-linked COFs have been non-
fluorescent or only weakly fluorescent, some highly fluorescent
COFs have been constructed recently by using highly emissive
building linkers (e.g., pyrene, tetraphenylethylene) or non-
emissive building linkers (e.g., hydrazone, hydroquinoline).50–55
When used as chemosensors, fluorescent COFs have distinct
a. Department of Materials and Optoelectronic Science, Center of Crystal
Research, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan. E-mail:
ahmed1932005@gmail.com and E-mail: kuosw@faculty.nsysu.edu.tw
b. cDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical
University, Kaohsiung, 807, Taiwan
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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H2
N
N
NH2
NH2
OHC
CHO
CHO
NH2
NH2
CHO
CHO
N
N
N
N
H2
OHC
Results and discussion
BCTB-4CHO
PD
TP
BCTA-4NH2
(b)
(a)
We used Suzuki–Miyaura reactions to synthesize the new
precursor tetraaldehyde BCTB-4CHO and tetraamine BCTA-
4NH2. Briefly, the couplings of 3,3´,6,6´-tetrabromo-9,9´-
bicarbazole (BC-4Br) with 4-formylphenylboronic acid and
4-aminophenylboronic acid pinacol ester, performed in 1,4-
dioxane/water (1:0.2, v/v) in the presence of
tetrakis(triphenylphosphine)palladium(0) as catalyst and
K2CO3 as base, resulted in the formation of BCTB-4CHO and
BCTA-4NH2, respectively, in excellent yields. Fourier
transform infrared (FTIR) and nuclear magnetic resonance
(NMR) spectroscopy confirmed the chemical structures of
these precursors (Fig. S1–12). The FTIR spectrum of BCTB-
4CHO featured adsorption bands at 2803–2717, 1685, and
1589 cm–1 representing the vibrations of aldehydic C–H,
C=O, and aromatic C=C bonds, respectively (Fig. S3); the
spectrum of BCTA-4NH2 featured three sharp adsorption
bands at 3437, 3366, and 3209 cm–1 representing
asymmetric and symmetric vibrations of N–H bonds, as well
as a band at 1618 cm–1 representing aromatic C=C
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
BCTA-TP COF
BCTB-PD COF
(c)
(d)
N
N
N
N
N
N
N
N
N

N
N
N
BCTB-BCTA COF
Scheme 1. (a–c) Schematic representation of strategies for the
construction of the (a) BCTB-PD, (b) BCTA-TP, and (c) BCTB-BCTA
COFs. (d) Top view of the strongest fluorescent BCTB-BCTA COF,
revealing the A–A stacking model
1
vibrations (Fig. S4). The H NMR spectrum of BCTB-4CHO
advantages over non-fluorescent ones, because of their more
rapid response times, greater sensitivity to changes in their
chemical environment, superior physiochemical stability, and
monolithic pore sizes.56 Various fluorescent COFs have been
developed recently as chemosensors for mercury ions, pH, 2,4,6-
trinitrophenol, electron-rich arenes, iron (III), and anions.55, 57–63
We are, however, aware of only one COF-based sensor for HCl:
ETBA-DAB, a COF devolved by Cui et al. that functioned as a
chemosensor for the sensitive detection of HCl gas.64 Because
the porous crystalline structures of COFs can accommodate gas
molecules within their inner channels, there appears to be much
room for the development of new COF-based chemosensors.
Although the above examples confirm that it is possible to
prepare highly fluorescent COFs, there remains a lack of guiding
principles on how to build photoluminescent COFs with
enhanced emissive properties. Therefore, in this present study,
we investigated some of the underlying characteristics affecting
the photoluminescence of COFs. We have synthesized three
luminescent carbazole-linked COFs—BCTB-PD, BCTA-TP, and
BCTB-BCTA—through the reactions of 4,4´,4´´,4´´´-([9,9´-
featured a singlet at 10.06 ppm representing the four
aldehydic CHO groups, in addition to signals for the
aromatic CH groups in the range from 9.06 to 6.92 ppm (Fig.
S9); the spectrum of BCTA-4NH2 featured a singlet at 5.18
ppm for the four amino NH2 groups, in addition to signals
for the aromatic CH groups in the range 8.63–6.70 ppm (Fig.
S11). The 13C NMR spectrum of BCTB-4CHO featured a
signal at 193.80 ppm for the aldehydic CHO carbon nuclei
(Fig. S10), while that of BCTA-4NH2 featured a signal at
149.14 ppm for the carbon nuclei of the CNH2 groups (Fig.
S12).
Scheme 1 displays our syntheses of the three COFs from
the new precursors. We obtained the target BCTB-PD and
BCTB-BCTA COFs through Schiff base condensations of
BCTB-4CHO with PD and BCTA-4NH2, respectively, and the
target BCTA-TP COF through condensation of BCTA-4NH2
with TP; all of these reactions were performed under
solvothermal conditions, using a co-solvent of n-butanol/o-
dichlorobenzene (1:1, v/v) containing 6 M acetic acid (10
v%) at 120 °C for 72 h. The chemical compositions of the as-
prepared COFs were confirmed using FTIR and solid state
NMR spectroscopy. The characteristic vibration bands of
the aldehydic CHO and amino NH units of the starting
materials BCTB-4CHO, BCTA-4NH2, PD, and TP were
completely absent from the FTIR spectra of the BCTB-PD,
BCTA-TP, and BCTB-BCTA COFs, implying that Schiff base
condensations had occurred between the various building
blocks. New adsorption bands appeared at 1620, 1621, and
1622 cm–1 in the FTIR spectra of the BCTB-PD, BCTA-TP, and
bicarbazole]-3,3´,6,6´-tetrayl)tetrabenzaldehyde
(BCTB-4CHO)
with p-phenylenediamine (PD), of 4,4´,4´´,4´´´-([9,9´-
bicarbazole]-3,3´,6,6´-tetrayl)tetraaniline (BCTA-4NH2) with
terephthalaldehyde (TP), and of BCTB-4CHO with BCTA-4NH2,
respectively (Scheme 1). We have found that the type of Schiff
base bonding units and the degree of π-stacking between the
COF layers influenced the fluorescence of these COFs. In
addition, the BCTB-BCTA COF functioned as a chemosensor for
HCl.
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Figure 1. (a) FTIR and (b) solid state 13C CP/MAS NMR spectra of the BCTB-PD (olive), BCTA-TP (red), and BCTB-BCTA (blue) COFs. (c–e)
TEM and (f–h) FE-SEM images of the (c, f) BCTB-PD, (d, g) BCTA-TP and (e, h) BCTB-BCTA COFs.
BCTB-BCTA COFs, respectively, representing their imino C=N
vibrations (Fig. 1a and S13–S15). The solid state 13C NMR
spectra of the BCTB-PD, BCTA-TP, and BCTB-BCTA COFs
featured signals at 157.38, 158.67, and 159.13 ppm,
respectively, for their C=N groups (Fig. 1b and S16–S18).
Each of these synthesized COFs exhibited excellent thermal
stability, as determined through thermogravimetric analysis
(TGA) under a N2 atmosphere. Fig. S19 and Table S1 reveal
that BCTB-PD and BCTB-BCTA were the most stable of our
three COFs, with high decomposition temperatures (Td10) of
566 and 522 °C, respectively, and char yields of 72 and 71
wt.%, respectively; in contrast, the value of Td10 of BCTA-TP
COF was 402 °C and its char yield was 48 wt.%. Thus, the
use of BCTB-4CHO as the aldehydic building linker greatly
enhanced the COFs, when compared with the effect of the
other aldehydic TP building linker. Transmission electron
microscopy (TEM) and field-emission scanning electron
microscopy (FESEM) revealed the morphologies of the as-
prepared COFs. The structures of the BCTB-PD, BCTA-TP,
and BCTB-BCTA COFs featured porous networks (Fig. 1c–e
and S20–S22). The FESEM images of the BCTB-PD and BCTB-
BCTA COFs revealed regularly distributed and loose
aggregates, respectively, while BCTA-TP COF displayed small
stripe-like crystallites having lengths of approximately
hundreds of nanometers and widths of tens of nanometers
(Fig. 1f–h and S23).
We used powder X-ray diffraction (PXRD) to examine the
crystallinities of these new COFs. The BCTB-PD, BCTA-TP,
and BCTB-BCTA COFs possessed excellent crystallinities, as
evidenced by the appearance of strong sharp peaks at
values of 2θ of 3.23, 3.42, and 4.47°, respectively, in their
PXRD patterns (Fig. 2a–c and S24–S26), representing their
reflection (110) facets. Furthermore, the pattern of the
BCTB-PD COF featured a set of other peaks at values of 2θ
of 6.91 and 10.35°, attributed to the reflection (210) and
(310) facets; these signals appeared at 6.92 and 10.44° for
BCTA-TP COF and at 9.14 and 13.84° for BCTB-BCTA COF.
The last minor signals for the BCTB-PD, BCTA-TP, and BCTB-
BCTA COFs appeared at values of 2θ of 21.89, 21.92, and
21.25°, respectively, corresponding to their reflection (001)
facets. From these PXRD patterns, we used the Bragg
equation to calculate the d-spacings between the 110
planes of the COFs (d110) and the π-stacking distances
between their stacked interlayers. The values of d110 values
for the BCTB-PD, BCTA-TP, and BCTB-BCTA COFs were 2.73,
2.58, and 1.97 nm, respectively, with π-stacking distances of
3.99, 4.05, and 4.18 Å, respectively (Table S2). To assess the
crystalline framework structures of the BCTB-PD, BCTA-TP,
and BCTB-BCTA COFs, we used Material Studio software to
model credible 2D layered structures featuring A–A eclipsed
and A–B staggered stacking.65 As revealed in Fig. 2a–c and
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Figure 2. (a–c) PXRD patterns of the (a) BCTB-PD, (b) BCTA-TP and (c) BCTB-BCTA COFs: experimental patterns (black), simulated Pawley
refined patterns (red), their difference (blue), and simulated patterns obtained from the A–A (purple) and A–B (olive) stacking models.
(d) N2 sorption isotherms recorded at 77 K and (e) pore size distributions of the BCTB-PD (olive), BCTA-TP (red), and BCTB-BCTA (blue)
COFs.
S24–S26, the experimental PXRD patterns (Fig. 2a–c, black
curves) of the BCTB-PD, BCTA-TP, and BCTB-BCTA COFs
were more consistent with the simulated PXRD patterns for
the A–A models (Fig. 2a–c, magenta curves), while they
deviated from those of the A–B models (Fig. 2a–c, olive
curves), suggesting π-eclipsed stacking of the layers in our
new COFs. Moreover, we subjected these A–A eclipsed
packing models to subsequent Pawley refinement to
produce theoretical PXRD patterns (Fig. 2a–c, red curves)
that were consistent with the experimental ones (Fig. 2a–c,
black curves), with only slight differences (Fig. 2a–c, blue
curves). The refinement of the A–A eclipsed models
provided the following unit cell parameters: for the BCTB-
PD COF, a = 36.10 Å, b = 42.11 Å, c = 4.07 Å, and α = β = γ =
90°; for the BCTA-TP COF, a = b = 16.22 Å, c = 3.57 Å, and α
= β = γ = 90°; for the BCTB-BCTA COF, a = 28.80 Å, b = 31.63
Å, c = 4.04 Å, and α = β = γ = 90° (Fig. S27–S32, Tables S3–
S8).
Nitrogen adsorption/desorption analyses revealed the
permanent porosity properties of the as-prepared COFs
(Fig. 2d). Initially, the COFs were activated by degassing
under vacuum at 100 °C for 20 h. All of the activated COFs
provided classical type I isotherms at 77 K, suggesting that
they were microporous materials. According to the
Brunauer–Emmett–Teller (BET) model, the BCTB-PD COF
featured the highest BET surface area of 2212 m2 g–1, with
the BCTA-TP and BCTB-BCTA COFs having surface areas of
645, and 1098 m2 g–1, respectively (Table S2). We used
nonlocal density functional theory (NLDFT) to estimate the
pore size distributions of the BCTB-PD, BCTA-TP, and BCTB-
BCTA COFs, providing micropore sizes of 1.62, 1.55, and
1.10 nm, respectively, and pore volumes of 0.28, 0.31, and
0.35 cm3 g–1, respectively (Fig. 2e, Table S2). The
significantly higher BET surface area of BCTB-PD COF over
BCTA-TP COF could be attributed to the morphology
difference of BCTB-PD COF compared to BCTA-TP COF
(Figures 1c-h and S23). The PXRD patterns our COFs showed
nonalteration of the positions and intensities of the peaks
after nitrogen adsorption/desorption analyses (Figures S24-
S26, dark yellow curves).
Because several carbazole-based materials have
prominent photoluminescence properties,1–3 we examined
the photophysical properties of the precursors BCTB-4CHO
and BCTA-4NH2 by measuring their UV–Vis absorption and
fluorescence emission spectra in various solvents. Both
BCTB-4CHO and BCTA-4NH2 are highly soluble in various
high- and low-polarity solvents, including tetrahydrofuran
(THF), MeOH, dichloromethane (DCM), acetone, and
dimethylformamide (DMF). As illustrated in Fig. S33 and
S34, the UV–Vis absorption maxima of BCTB-4CHO and
BCTA-4NH2 at a concentration of 10–4 M appeared in the
regions 280–360 and 260–360 nm, respectively,
representing the π–π* transitions of the conjugated
biscarbazolyl and phenyl moieties. Interestingly, these
absorption maxima were affected strongly by the solvent;
for BCTB-4CHO, they appeared at 310 nm in MeOH, 312 nm
in THF, 314 nm in
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Figure 3. (a–c) Photographic images of the (a) BCTB-PD, (b) BCTA-TP, and (c) BCTB-BCTA COFs dispersed in DMF (1 mg mL–1) under
excitation at 365 nm. (d–f) Fluorescence spectra of the (d) BCTB-PD, (e) BCTA-TP and (f) BCTB-BCTA COFs dispersed in various solvents
(1 mg mL–1) under excitation at 365 nm. (g–i) Fluorescence emission maxima of the (g) BCTB-PD, (h) BCTA-TP, and (i) BCTB-BCTA COFs.
DCM, 316 nm in DMF, and 318 nm in acetone, while for
BCTA-4NH2 they appeared at 294 nm in MeOH, 296 nm in
THF, 298 nm in DCM, 302 in DMF, and 314 nm in acetone.
The blue-shifting of the absorption maxima in MeOH
presumably arose from hydrogen bonding between the
CHO groups in BCTB-4CHO and the NH2 groups in BCTA-
4NH2 with the solvent, stabilizing the BCTB-4CHO and BCTA-
4NH2 precursors in their ground states.66 On the other
hand, the red-shifting of the absorption maxima in acetone
presumably arose from π-stacking of BCTB-4CHO or BCTA-
4NH2.67
We investigated the fluorescence properties of BCTB-
4CHO and BCTA-4NH2 at a concentration of 10–4 M in
solvents of various polarities (Fig. S35a–f and S36a–f). The
fluorescence emission maxima of BCTB-4CHO in THF,
MeOH, DCM, acetone, and DMF appeared at 449, 464, 448,
451, and 466 nm, respectively; for BCTA-4NH2, they
appeared at 429, 484, 423, 446, and 486 nm, respectively.
Thus, the fluorescence emission wavelengths of BCTB-4CHO
and BCTA-4NH2 were strongly affected by the polarity of the
solvent, with an increase in solvent polarity causing an
increase in the fluorescence emission wavelength
increased; that is, a solvatochromism effect.68 We attribute
this solvatochromism (red-shifting) of the fluorescence
emission in polar solvents to hydrogen bonding of MeOH
(OH) and acetone and DMF (C=O) with the CHO groups of
BCTB-4CHO and the NH2 groups of BCTA-4NH2, thereby
stabilizing the polar excited states of BCTB-4CHO and BCTA-
4NH2 and accelerating their intramolecular charge-transfer
processes (solvent relaxation phenomenon).69 The
fluorescence images of BCTB-4CHO and BCTA-4NH2 in
various solvents revealed that both precursors exhibited
blue emissions (Fig. S35g and S36g).
Fluorescent materials having highly conjugated aromatic
systems typically exhibit strong fluorescence emissions only
in diluted solutions; conversely, their fluorescence
emissions are generally quenched in the solid state or in
solution at high concentrations—a phenomenon known as
aggregation-caused quenching (ACQ),70 which arises from
strong face-to-face noncovalent interactions (π-stacking).
Consequently, we investigated the effect of the
concentration on the fluorescence emissions of BCTB-4CHO
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Figure 4. (a) Fluorescence spectra of the BCTB-BCTA COF dispersed in DMF (1 mg mL–1) under excitation at 365 nm in the presence of
various amounts of HCl. (b) Photographs of the BCTB-BCTA COF dispersed in DMF (1 mg mL–1) under excitation at 365 nm after the
exposure to HCl and then NH3 vapor. (c) Calibration curve of the fluorescence intensity of the BCTB-BCTA COF plotted with respect to
the concentration of HCl. (d) Mechanism of the protonation and deprotonation of the BCTB-BCTA COF.
and BCTA-4NH2 in THF, MeOH, DCM, acetone, and DMF. As
revealed in Fig. S35a–f and S36a–f, the emission intensities
decreased upon increasing the concentration of BCTB-4CHO
and BCTA-4NH2 in each of these solvents, confirming their
ACQ behavior. According to the excellent fluorescence
properties of the precursors BCTB-4CHO and BCTA-4NH2,
we studied the photoluminescence behavior of our BCTB-
PD, BCTA-TP, and BCTB-BCTA COFs by measuring their
fluorescence emissions when dispersed as crystallites in
various solvents. Typically, we prepared dispersions of the
COF crystallites in the various solvents at a concentration of
1 mg mL–1. Interestingly, excitation of the suspensions of
the BCTB-PD and BCTB-BCTA COF crystallites in high polarity
solvents; pyridine, acetone, N-methyl-2-pyrrolidone (NMP),
synthesized COFs showed very weak fluorescence emissions
in THF. Moreover, we recorded the fluorescence emission
spectra of these COF suspension solutions. Under 365-nm
light, the suspensions of the BCTB-PD COF in pyridine,
acetone, DMF, and NMP provided strong fluorescence
emission maxima near 462, 469, 471, and 474 nm,
respectively, while in dioxane and EA provided moderate-
to-weak fluorescence at 423 and 440 nm, respectively (Fig.
3d) (Table S9). The suspensions of the BCTA-TP COF in
dioxane, EA, pyridine, acetone, DMF, and NMP exhibited
the weakest fluorescence emissions, with maxima near 399,
402, 420, 422, 430, and 443 nm, respectively (Fig. 3e) (Table
S9). Among our three COFs, BCTB-BCTA COF exhibited the
strongest fluorescence emissions in pyridine, acetone, DMF,
and NMP, with maxima near 461, 465, 472, and 476 nm,
respectively, while in dioxane and EA exhibited moderate-
to-weak fluorescence at 411 and 437 nm, respectively (Fig.
3f) (Table S9). This red-shifting of the fluorescence emission
upon increasing the polarity of the solvent indicated that
noncovalent interactions (e.g., hydrogen bonding, dipole
effects) between the COFs and the polar solvent molecules
enhanced the stability of the excited states of the COFs and
and DMF with light from an ultraviolet lamp at
a
wavelength of 365 nm resulted in strong fluorescence,
either blue-green or light-blue, that was visible to the naked
eye; while, the suspensions of the BCTB-PD and BCTB-BCTA
COFs in dioxane and ethyl acetate (EA) showed moderate-
to-weak pale violet fluorescence. In contrast, the
suspensions of BCTA-TP COF in the same solvents exhibited
weak, almost dark, fluorescence colors (Fig. 3a–c). All the
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thus enhanced the excited-state intramolecular charge
transfer (ICT).71,72 All of the COF crystallites dispersed in THF
displayed very weak fluorescence emissions, resulting from
the lower polarity of THF. Thus, our as-prepared COFs
emitted strong or very strong fluorescence only in solvents
of moderate to high polarity. To support these
observations, we recorded the absolute photoluminescence
quantum yield (PLQY) of our BCTB-PD, BCTA-TP, and BCTB-BCTA
COFs as crystallites in various solvents. Table S9 summarized the
PLQY values of our COFs which indicated that BCTB-PD COF
provided strong fluorescence with PLQY values of 12.8, 13.5,
Taking advantage of its strong fluorescence emission, we
employed the BCTB-BCTA COF as a sensitive and efficient
fluorescence sensor for the detection of HCl. First, we
investigated the influence of the concentration of HCl on
the sensitivity of the fluorescence of the BCTB-BCTA COF, by
measuring its fluorescence spectra in DMF (1 mg mL–1) in
the presence of various amounts of HCl. Fig. 4a reveals that
the addition of a trace amount of HCl (1 mmol L–1) induced
a considerable change in the fluorescence spectrum of the
BCTB-BCTA COF; the fluorescence emission peak having its
maximum at 472 nm disappeared completely, while and a
new peak appeared with its maximum at 495 nm (PLQY =
23.8%). This new fluorescence peak decreased in intensity
upon increasing the concentration of HCl in the range from
1 to 25 mmol L–1, but it remained approximately unchanged
when increasing the concentration of HCl thereafter to 50
and 100 mmol L–1. The relationship between the
fluorescence emission intensity of the peak at 495 nm and
the HCl concentration was linear in the range 1–25 mmol L–
1; it was described by the equation: I = 1648.56 – 48.89C,
where I is the fluorescence emission intensity and C is the
15.9, and 16.3% in pyridine, acetone, DMF, and NMP
,
respectively, and moderate fluorescence with PLQY values of 9.9
and 9.6% in dioxane and EA; for BCTA-TP COF, these values
were the lowest values of 1.1, 0.9, 3.1, 3.8, 5.1, and 5.6% in
pyridine, acetone, DMF, and NMP, respectively. BCTB-BCTA
COF showed the highest PLQY values of 17.6, 19.3, 20.8, and
21.2 % in dioxane, EA, pyridine, acetone, DMF, and NMP,
respectively, in addition to moderate values of 13.8 and
12.5% in dioxane and EA, respectively. The PLQY values of three
COFs in THF were very weak (1%). These findings suggest
that the use of BCTB-4CHO as the linker for Schiff base
condensation produced BCTB-4CH=N units that were
significant electron- donating groups and provided a COF
(BCTB-PD) capable of strong fluorescence emission; in
contrast, BCTA-4NH2 produced BCTA-4N=CH units that were
weak electron-donating groups and provided a COF (BCTA-
TP) displaying weak fluorescence emission. In addition, the
use of BCTB-4CHO, instead of TP, as the aldehydic linker in
concentration of HCl (mmol L–1), with
a correlation
coefficient (R2) of 0.989. The limit of detection (LOD) was
calculated to be 10 nmol L–1 (Fig. 4b), according to LOD =
3S/b, where b is the slop of the calibration curve and S is
the standard deviation of the fluorescence response.
Next, we investigated the naked-eye detection of HCl
gas by exposing a suspension of the BCTB-BCTA COF in DMF
(1 mg L–1) to HCl vapor under light at a wavelength of 365
nm. As revealed in Fig. 4c, upon exposure to HCl vapor, the
fluorescence emission color of the COF suspension was
changed from blue-green to green within a response time
of approximately 1 s. Upon subsequent exposed to NH3
vapor, the green color emitted by the BCTB-BCTA COF
returned to its original blue-green. No degradation in
fluorescence occurred upon alternating the exposure of the
BCTB-BCTA COF to HCl and NH3 vapors for up to 10 cycles,
confirming its excellent reproducibility when used as an HCl
sensor. In addition, the crystallinity of BCTB-BCTA COF after
10 times recycling was examined as shown in Figure S43
which revealed the nonalteration of peaks in the PXRD
pattern, indicating the promoting utilization of BCTB-BCTA
the Schiff base condensation produced
a strongly
fluorescent COF (BCTB-BCTA), confirming the importance of
BCTB-4CHO as a linker. The highest fluorescence emission
intensities among our three COFs were those of the BCTB-
PD and BCTB-BCTA COFs, presumably because of significant
ICT from the electron-donating carbazole BCTB-4CH=N
units, which acted as donors to the phenyl or BCTA moieties
acting as acceptors; within the BCTA-TP COF, however, ICT
from the BCTA-4N=CH units to the phenyl groups was weak
(Fig. 3g–i). In addition, the fluorescence emission intensity
of the BCTB-BCTA COF was stronger than that of the BCTB-
PD COF because of the greater π-stacking and crystallinity in
the latter structure (π-stacking distances: 4.05 Å in the
BCTB-PD COF, 4.18 Å in the BCTB-BCTA COF) (Table S2).
Thus, the type of Schiff base (BCTB-4CH=N or BCTA-4N=CH)
and the strength of π-stacking between the COF layers both
had major effects on the fluorescence behavior of the COFs.
Furthermore, the chemical stability and crystallinity of
BCTB-PD, BCTA-TP, and BCTB-BCTA COFs into the solvents
were investigated by immersing 50 mg of each COF in THF,
dioxane, EA, pyridine, acetone, DMF, and NMP for 3days
and then isolated using vacuum filtration and dried at 120
°C overnight under vacuum. As shown in Figures S37-S42,
Retention of FTIR and PXRD peaks with non-significantly
change after the immersing into the solvents, revealed the
outstanding chemical stability and crystallinity of the COFs
in these solvents.
COF as HCl sensor. Fig. 4d provides
a schematic
representation of the mechanism of HCl sensing when using
our BCTB-BCTA COF. At a concentration of HCl of 1 mmol L–
1, protonation of the imino nitrogen atoms in the COF
architecture may have occurred, increased the planarity of
the units in the BCTB-BCTA COF and, hence, expanding its
conjugated system, resulting in
a
red-shift in the
74
fluorescence emission.73,
At higher HCl concentrations,
protonation of the other nitrogen atoms in the COF
architecture would also occur, interfering with the ICT of
the BCTB-BCTA COF.
The protonation of imine bonds in the skeleton of BCTB-
BCTA COF was confirmed by recording the FTIR spectrum of
BCTB-BCTA COF after treatment with 1 mmol L-1 HCl
concentration (known as BCTB-BCTA COF-HCl). As shown in
Figure S44, BCTB-BCTA COF-HCl spectrum observed the
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appearance of a new peak at 1595 cm-1 attributed to the
C=NH+ stretching vibration, accompanied by an alleviation
of the stretching vibration of the imine (C=N) peak, which
compatible with the previous study.75,76 As a result, the
imine nitrogen atoms in the skeleton of BCTB-BCTA COF
underwent a rapid protonation in the presence of HCl (1
mmol L-1) which affects the conjugate structure of the
skeleton of BCTB-BCTA COF and thus caused this red-
shifting in the fluorescence emission and color of the
COF.73,74 In addition, it has been reported that the
protonated imine groups (C=NH+) are stronger electron-
acceptors than their free imine (C=N) groups, thus
increased the charge transfer process and decreased the
transitions energies.76 Similarly, The C=NH+ groups in the
skeleton of BCTB-BCTA COF-HCl would be increased the
charge transfer process from the electron-donating
carbazole units to the BCTA moieties and decreased their
transition energies, leading to this red-shifting.
Furthermore, these concepts were supported by measuring
the fluorescence life-time decays of BCTB-BCTA COF and
BCTB-BCTA COF-HCl. The fluorescence life-time of BCTB-
BCTA COF was 4.13 ns, while after the addition of 1 mmol L-
Conflicts of interest
“There are no conflicts to declare”.
Acknowledgements
This study was supported financially by Ministry of Science
and Technology, Taiwan, under contracts MOST 106-2221-
E-110-067-MY3, 108-2638-E-002-003-MY2, 108-2218-E-110-
013-MY3, and 108-2221-E-110-014-MY3. We also thank Mr.
Hsien-Tsan Lin of the Regional Instruments Center at National
Sun Yat-Sen University for help with the TEM measurement.
Notes and references
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We have synthesized three stable luminescent COFs (BCTB-
PD, BCTA-TP, BCTB-BCTA) through [4+2] and [4+4]
polycondensations of BCTB-4CHO with PD, of BCTA-4NH2
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g–1. More interestingly, the type of Schiff bases (BCTB-
4CH=N or BCTA-4N=CH) and the strength of π-stacking
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suitable for HCl sensing. This COF displayed a highly
sensitive (up to 10 nmol L–1) and rapid response toward the
spectroscopic detection of HCl in solution. Consequently,
this paper describes a new strategy for the construction of
potentially luminescent COFs for use as responsive sensing
platforms.
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The extremely crystalline architectures, high thermal stabilities,
and strong fluorescence emissions of covalent organic
frameworks based on linked carbazole units.
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