A photoluminescent covalent triazine framework: CO2 adsorption, light-driven hydrogen evolution and sensing of nitroaromatics

Article abstract of DOI:10.1039/c6ta04623a

A highly photoluminescent (PL) porous covalent triazine-based framework (PCTF-8) is synthesized from tetra(4-cyanophenyl)ethylene by using trifluoromethanesulfonic acid as the catalyst at room temperature. Due to triazine units in the framework, the PCTF-

Full text of DOI:10.1039/c6ta04623a

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A photoluminescent covalent triazine framework: CO2 adsorption,
light-driven hydrogen evolution and sensing of nitroaromatics
Asamanjoy Bhunia,*a,b Dolores Esquivel,a,c Subarna Dey,b Ricardo Fernández-Terán,d Yasutomo
Goto,e Shinji Inagaki,e Pascal Van Der Voort,a and Christoph Janiak*b
.Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A highly photoluminescent (PL) porous covalent triazine-based framework (PCTF-8) is synthesized from tetra(4-cyano-
phenyl)ethylene by using trifluoromethanesulfonic acid as catalyst at room temperature. Due to triazine units in the
framework, the PCTF-8 exhibits excellent thermal stabilities (>400 °C). The Brunauer–Emmett–Teller (BET) specific surface
area of PCTF-8 is 625 m2/g which is lower than the one obtained from synthesis under Lewis acid conditions (ZnCl2). At 1
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bar and 273 K, the PCTF-8 adsorbs significant amount of CO2 (56 cm3 g− ) and CH4 (17 cm3 g− ) which is highly comparable
to nanoporous 1,3,5-triazine frameworks (NOP-1-6, 29-56 cm3/g). This nitrogen rich framework exhibits good ideal
selectivity (61:1 (85% N2: 15% CO2) at 273 K, 1 bar). Thus, it can be used as a promising candidate for potential applications
in post-combustion CO2 capture and sequestration technologies. In addition, photoluminescence properties as well as the
sensing behaviour towards nitroaromatics have been demonstrated. The fluorescence emission intensity of PCTF-8 is
quenched by ca. 71 % in presence of 2,4,6-trinitrophenol (TNP). From time-resolved studies, a static quenching behaviour
was found. This high photoluminescence property is used for hydrogen evolving organic photocatalysis from water in
presence of a sacrificial electron donor and a cocatalyst.
1
1
π
–
conjugated materials are also a promising platform for CO2
capture because of the interaction between the quadrupole
moments of CO2 and the π clouds in the porous organic
Introduction
–
Porous organic materials have attracted considerable interest
because of their unique physicochemical properties in various
technological and energy-related applications such as gas
materials.6 The incorporation of high amounts of nitrogen in
the porous materials can facilitate CO2 capture. Such
multifunctionality has been a long term challenge for the
development of porous photofunctional materials which have
great interest as chemical sensors, e.g. to detect explosive
nitroaromatic compounds.7 The sensitivity and selectivity in
the detection of these compounds by porous materials suffer
from analyte–host interactions, chemical stability and so on. π-
Conjugated triazine based porous materials may be attractive
chemical sensors because of their high stability and high
porosity. Due to high electron mobilities and electron
withdrawing character of triazine ring, these materials have
been widely used in synthetic chemistry and optoelectronics.8
Currently, this type of materials has attracted great interest as
organic photocatalysts for hydrogen production via
photocatalytic water splitting under visible light.9 Their
chemical structures, analogous to g-C3N4, in combination with
their tunable porosity convert them in attractive candidates
for light-induced hydrogen evolution.10 Lostch et al. reported
the first example of a covalent organic framework (COF) which
was active in visible-light-induced hydrogen production from
water using platinum as a cocatalyst.9a More recently, a yellow
colour phenyl-triazine oligomer with high crystallinity
exhibited the highest activity obtained for metal-free organic
pthotocatalysts in the absence of noble metal catalyst.9b
storage, separation, catalysis etc.1,2 Specially, π
–conjugated
porous organic polymers are an integral part of research for
light emitting, light harvesting and chemo-sensing.3 Due to
their rigidity on the molecular level, they have a high affinity to
aggregate in solution and the solid state, resulting typically in
aggregation caused quenching (ACQ) effects.4 The aggregation
of conjugated polymers can be prevented by molecular
approaches based on site isolation with bulky polymeric
matrices.5 This approach can improve the luminescent
properties of the polymeric materials. At the same time
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Herein we report a new strategy for the construction of Infrared (IR) spectra were obtained on a Bruker FT-IR Tensor
photoluminescent materials based on π
conjugated building 37 Spectrometer in the 4000-550 cm–1 region with 2 cm–1
–
blocks. We employed tetra(4-cyanophenyl)ethylene as a single resolution as KBr disks. UV−vis absorption and fluorescence
component for the synthesis of a porous covalent triazine- emission spectra were obtained using Jasco V-670 and FP-6500
based framework (PCTF) using trifluoromethanesulfonic acid spectrometers, respectively. Fluorescence quantum yields
(TFMS) as the catalyst at room temperature, in which the were evaluated using an absolute photoluminescence
tetraphenylethylene (TPE) units are directly connected to quantum yield measurement system equipped with
a
strong electron accepting triazine rings. Recently, some of us calibrated integrating sphere and a multi-channel
reported PCTFs with the TPE unit by using the ionothermal spectrometer (C9920–02, Hamamatsu Photonics). The
ZnCl2 route.11 The structure of CTFs from ionothermal excitation wavelength was 395 nm. 1H and 13C spectra were
reactions with ZnCl2 are in-between well-defined COFs and recorded on a Bruker Avance DRX-500 instruments. 1H and 13
C
porous carbon materials.12 TFMS-catalyzed condensation NMR chemical shifts are given in ppm relative to SiMe4 (δ = 0.0
frameworks13,14 offer significant advantages: (1) lower ppm) with calibration against the residual protonated solvent
temperature and shorter reaction time (2) avoiding undesired signal (CDCl3: 7.26 (1H) and 77.0 (13C)). 13C cross-polarization
decomposition and condensation reactions such as C-H bond (CP) MAS NMR measurement was performed at 100.6 MHz at
cleavage and carbonization which otherwise lead to defect in a sample spinning frequency of 5 or 6 kHz using a Bruker
the framework (3) absence of ZnCl2 contamination in the Avance 400 spectrometer with a 7 mm zirconia rotor. For the
porous materials. These advantages are attractive to obtain 13C CP-MAS NMR measurements, the repetition delay was 5 s,
reasonably well-defined CTFs which can exhibit a broad verity the contact time was 1.75 ms, and the pulse width was 4.5 μs
of physical properties including chemical sensing, gas (1H 90° pulse). Elemental (CNH) analyses were carried out with
storage/separation and organic photocatalysis. So far, CTFs a PerkinElmer 2400 series 2 elemental analyser. Powder X-ray
have been used mainly in gas storage and separation.11,13 diffraction (PXRD) data was collected on a Bruker D2 Phaser
There are also few reports on the use of CTFs for diffractometer using a flat sample holder (also a flat silicon,
heterogeneous catalysis including light-induced hydrogen low background sample holder) and Cu Kα1/α2 radiation with
evolution,9,15 as catalytic support in liquid phase reactions,16 as λ = 1.5418 Å at 30 kV covering 2theta angles 5-80° over a time
adsorbents in liquids,17 due to their chemical and thermal of 2 h, that is. 0.01°/sec. Diffractograms were obtained on flat
stabilities. In spite of their many applications, their layer sample holders where at low angle the beam spot is
photoluminescence was only briefly noted14 to the best of our strongly broadened so that only a fraction of the reflected
knowledge. Thus, we show in this paper that PCTF-8 can be radiation reaches the detector which leads to low relative
used not only for gas adsorption/separation but also exhibits intensities measured at 2θ< 7°. For hygroscopic or air-sensitive
highly selective for the detection of 2,4,6-trinitrophenol (TNP), samples, the sample holder can be sealed with a dome.
over other nitroaromatic analytes such as 4-Nitrotoluene (NT), Scanning electron microscopy (SEM) images were obtained
Nitrobenzene (NB), 2,6-Dinitrotoluene (2, 6-DNT), 2,4- using an ESEM Quanta 400 FEG SEM equipped with a
Dinitrotoluene (2,4-DNT), 2,4-Dinitrophenol (DNP) together secondary electron detector. Thermogravimetric analyses
with light-induced hydrogen evolution.
(TGA) were carried out at a ramp rate of 5 °C/min under air
with a Netzsch Thermo-Microbalance Aparatus TG 209 F3
Tarsus. A coulometric Karl-Fischer titration (KFT) for the
determination of the water content was carried out with a
Karl-Fischer titration apparatus AQUA 40.00 with headspace
module. The solid CTF sample was heated to 170 °C in the
head space module and the liberated water transferred to the
measurement cell.
Sorption isotherms were measured using a Micromeritics ASAP
2020 automatic gas sorption analyser equipped with oil-free
vacuum pumps (ultimate vacuum <10–8 mbar) and valves,
which guaranteed contamination free measurements. The
sample was connected to the preparation port of the sorption
analyser and degassed under vacuum until the outgassing rate,
i.e. the rate of pressure rise in the temporarily closed manifold
with the connected sample tube, was less than 2 µTorr/min at
the specified temperature 200 °C. After weighing, the sample
tube was then transferred to the analysis port of the sorption
analyser. All used gases (H2, He, N2, CO2, CH4) were of ultra-
high purity (UHP, grade 5.0, 99.999%) and the STP volumes are
given according to the NIST standards (293.15 K, 101.325 kPa).
Helium gas was used for the determination of the cold and
warm free space of the sample tubes. H2 and N2 sorption
Scheme 1 Synthesis of the nitrile linker and PCTF-8.
Experimental section
Materials
All chemicals were purchased from commercial suppliers
(Sigma-Aldrich, Acros Organics, and Alfa Aesar chemical
company) and used without further purification, unless stated
otherwise. Tetra(4-cyanophenyl)ethylene was synthesized
according to the reported procedures.11 Chloroform was
distilled from P2O5 under a nitrogen atmosphere.
Instrumentation
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isotherms were measured at 77 K (liquid nitrogen bath), extraction for 24 h with acetone. The final product was dried in
whereas CO2 and CH4 sorption isotherms were measured at vacuum. Yield: 180 mg, 83 % for (C30H16N4)n.
293±1 K (passive thermostating) and 273.15 K (ice/deionized
water bath). The heat of adsorption values and the DFT pore
Results and discussion
size distributions (‘N2 DFT slit pore’ model) were calculated out
using the ASAP 2020 v3.05 software.
Synthesis and spectroscopic characterization
Photocatalytic H2 production experiments were performed
PCTF-8, was synthesized by the TFMS catalyzed reaction from
tetra(4-cyanophenyl)ethylene ( ) at room temperature as a
with Solar Box 3000 equipped with a Xe lamp (300 W). For
each experiment, the polymer powder (10 mg) was suspended
in a phosphate buffer solution (pH-7, 8 mL) containing
triethanolamine (TEoA) or MeOH as a sacrificial donor (2 mL).
1
yellow powder in 83% yield (Scheme 1). In FT-IR the
carbonitrile band in the precursors around 2226 cm–1 has
significantly decreased in intensity which indicates only few
free nitrile groups in PCTF-8 (Fig. 1, top). The intense bands
around 1503, 1362 and 800 cm–1 correspond to the formed
triazine rings.12 In the solid-state 13C CP/MAS NMR spectrum
(Fig. 1, bottom) the major resonances at 174, 141 and 134
ppm can be assigned to the triazine ring (-CAr = N-), ethylene
carbon (-C=C-) and aromatic carbon (Ar) atoms, respectively.12
Peaks around 124 and 114 ppm were associated to residual
cyano end groups.14 Powder X-ray diffraction of PCTF-8
confirmed its expected amorphous nature (Fig. S1, ESI†).
Dihydrogen hexachloroplatinate (5
ꢀL, 8% wt aqueous
solution) was added as precursor for the in-situ formation of
platinum cocatalyst. The solution was ultrasonicated for 15
min before being degassed by nitrogen bubbling for 30 min.
The flask was illuminated in a 90
with a 300 W Xe lamp for the time specified. Gas samples were
taken with gas-tight syringe and quantified by gas
° angle at a distance of 22 cm
a
chromatography (Shimadzu GC-2010 Plus) equipped with a
barrier discharge ionization detector (BID).
Time-correlated
single
photon
counting
(TCSPC)
measurements were performed using a pulsed diode laser
source (Edinburgh Instruments EPL405) operating at 404.6 nm
with a pulse FWHM of ca. 77.1 ps. A neutral density filter was
used to attenuate the beam to obtain photon counts of
approx. 1% or less of the incoming light intensity. The detector
used was a Hamamatsu MCP-photomultiplier tube R3809U-51
PCTF-8
Monomer
(cooled to ca. -40
℃), the signal was passed to a discriminator
(Ortec 9307) and then into a TAC (Ortec 566, 50 ns time range
used). The electrical trigger signal from the laser was also
passed through a discriminator (Tennelec TC454) and on to the
TAC (Ortec 566). The TAC output was read by a DAQ-1 MCA
computer card using 4096 channels and collected with Horiba
Jobin Yvon DataStation 2.3 software. All measurements were
done in reverse mode at 10 MHz and under magic angle
polarisation. A cut-off filter (GG3) was used to block scattered
excitation light. The instrument response function (IRF) were
collected by using a dilute solution of Ludox as a standard
scattering sample. Fluorescence lifetimes were obtained by
iterative reconvolution of the IRF and the collected decay
curves, by fitting to a multiexponential decay model using in-
house scripts.
−
CN
2500
2000
1500
1000
500
−
1
Wavenumber (cm
)
Synthesis
A
50
ml
Schlenk
flask
was
charged
with
trifluoromethanesulfonic acid (0.45 g, 3.0 mmol) in 8 mL dry
CHCl3 under N2 atmosphere. The reaction mixture was stirred
and cooled to 0 °C. Tetra(4-cyanophenyl)ethylene (217 mg, 0.5
mmol) in 30 mL dry CHCl3 was added dropwise into the acid
solution with stirring over 30 min and the temperature of 0 °C
was maintained for another 2 h. Then, the resulting solution
was stirred for 12 h at room temperature. An NH3/H2O
solution (0.5 mol/L) was added until the reaction mixture
became neutral and the mixture was stirred for another 2 h.
The yellow product was isolated by filtration and washed with
water (3 × 50 mL), THF (3 × 30 mL), acetone (3 × 30 mL),
chloroform (3 × 30 mL) and further purified by Soxhlet
Fig. 1 IR spectra of monomer and PCTF-8 (top). Solid-state 13C CP/MAS NMR spectrum
of PCTF-8 (bottom). Signals e, f are associated to residual cyano end groups and *
belongs to sidebands.
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Thermogravimetric analysis (TGA) showed that PCTF-8 triazine nitrogen due to its large polarizability and quadrupole
exhibited good thermal stability up to 420 °C under air (Fig. S2, moment.
ESI†). Elemental analysis (Table S1, ESI†) of PCTF-8 nearly The isosteric heat of adsorption from the CO2 adsorption
matches the calculated values corrected with the result of KFT isotherms at 273 and 293 K (Fig. S4, ESI†) at zero loading is 37
analysis which determined the water content of the sample kJ/mol and rapidly drops at adsorbate loadings of only a few
(Fig. S3, ESI†). It became apparent that the PCTF-8 is cm3/g. The heat of adsorption stays around 29 kJ/mol for
hygroscopic and adsorbs about 10wt% of moisture from loadings between 3 to ~30 cm3/g. When the loading is
ambient air upon storage under ambient conditions after increased beyond 30 cm3/g the heat of adsorption value
vacuum drying at 120 °C. A similar hygroscopic behaviour of appears to increase slightly which should not be over
related CTFs was reported recently by Janiak et al.18 and may interpreted however in view of the error margin of readily ±3
be a common phenomenon to CTFs which may also explain the kJ/mol.13,22 This heat of CO2 adsorption of ~29 kJ/mol for PCTF-
frequently observed mismatch in theoretical and experimental 8 stays well above the heat of liquefaction of bulk CO2 with 17
elemental (CHN) analysis. Under SEM the PCTF-8 particles are kJ/ mol23 or around the isosteric enthalpy of adsorption for
small and largely irregular as well as angular shaped with a CO2 on activated charcoals (e.g. BPL: 25.7 kJ mol–1, A10: 21.6 kJ
wide range of sizes (greater than 1 μm) (Fig. 2).
mol–1, and Norit R1 Extra: 22.0 kJ mol–1.24 The high Qst value
can be attributed to the high polar framework and the pore
size effect. The high adsorption enthalpy at zero coverage is
explained by the initial filling of the small ultramicropores with
4
Å diameter (Fig. S5, ESI‡) with adsorbate−surface
interactions to both sides or ends of the CO2 molecules.13
If the heat of adsorption would indeed exhibit an increase with
CO2 uptake, this means that a simultaneous, exothermic
process must take place, such as possibly the rearrangement
of already adsorbed CO2 molecules towards
a
closer,
energetically more favorable configuration.
CH4 sorption of PCTF-8 were 17 cm3g− at 273 K and 10 cm3g−
at 293 K (Fig. 4) which are comparable with the values for CTFs
from our previous work.11,13 From the available single-gas
adsorption isotherms, the CO2 selectivity over N2 of PCTF-8
was calculated at 273 by using the Henry equation and the
ideal adsorbed solution theory (IAST) in order to understand
greater affinity toward CO2. The calculated adsorption
selectivity of CO2/N2 is 24 and 61 at 273 K using the Henry
equation and the ideal adsorbed solution theory (IAST),
respectively (Fig. S6 and secꢀon 8 in ESI†), which are
comparable with the values for PCTF-1 to -7 from our previous
work as well as other CTFs.11,13,14,25,26 This comparably large
value of PCTF-8 for CO2 may be attributed to the presence of
more basic nitrogen moieties.
1
1
Fig. 2 SEM images of PCTF-8
Gas sorption
The N2 sorption isotherm shows a steep slope at low P/P0
values and a type I character typical for microporous materials
(Fig. 3 and Table S2, ESI†).19 The calculated BET surface area of
1
625 m2g−
is higher than for similar tetrakis(4-
carboxyphenyl)ethylene linker-based fluorescent MOFs ( 244-
1
317 m2g− ).20 The surface area of PCTF-8 is in the range of
other TFMS-catalysed CTFs with 2 to 1152 m2g−1.14 The ratio of
micropore volume to total pore volume (V0.1/Vtot) of 0.78
suggests that the material contains a very high fraction of
micropores. A narrow distribution of micropores centered at 5,
5.9, 6.8, 11.8 and 14.8 Å was observed (insert in Fig. 3). PCTF-8
1
showed H2 sorption of 112 cm3g− at 77 K (with no hysteresis)
that is very close to other CTFs.13 The CO2 adsorption
1
capacities in activated PCTF-8 are 56 cm3g− at 273 K and 35
cm3g−1 at 293 K, respectively (Fig. 4). The CO2 uptake of PCTF-8
was comparable to CTF-type polymers synthesized by TFMS,
1
which showed CO2 uptakes of 20–93 cm3g− at 273 K and 1
bar.14 Such uptake capacity is also comparable to nanoporous
1,3,5-triazine frameworks (NOP-1-6, 29-56 cm3/g).21 The CO2
uptake of PCTF-8 is comparable to PCTF-1 which has about
three times the BET surface area of PCTF-8.11 This is due to the
higher nitrogen content and intact triazine units of PCTF-8
from the milder synthesis conditions as CO2 interacts with the
Fig. 3 N2 sorption isotherms at 77 K and NL-DFT pore size distribution profile (insert) of
PCTF-8.
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To compare the emission behaviour of PCTF-8 synthesized
under different conditions (Brønsted acid and ionothermal
conditions), we have measured the emission spectrum of black
coloured PCTF-1 that was synthesized by using ionothermal
condiꢀons (Fig. S5, ESI†). PCTF-1 does not show appreciable
emission because of partial carbonization in the framework.
Therefore, highly emissive properties of CTFs seem only
possible when avoiding carbonization by preparation at room
temperature. In the solid-state PCTF-8 shows
a strong
greenish-yellow fluorescence with an emission maximum at
562 nm when excited at 395 nm and a fluorescence quantum
yield (Φꢀ) of 31% at room temperature (Fig. 5b). This high
value, compared to the extremely low quantum yield observed
for the monomer in solution, is primarily attributed to the
immobilization and restricted rotation of the aromatic rings in
the tetraphenylethene units of the framework.30,31
From its ultraviolet-visible spectrum, the optical band gap of
PCTF-8 was estimated to be 2.25 eV. In principle, this band gap
is large enough to overcome the endothermic character of the
water-splitting reaction. To investigate this property,
photocatalytic hydrogen evolution experiments were
performed with PTCF-8 and Pt-loaded PCTF-8 in the presence
of methanol or triethanolamine sacrificial electron donor (10
mol%) in buffered aqueous solutions. Without any platinum
co-catalyst, no hydrogen evolution was observed. After loading
with 2.3 wt% Pt, PCTF-8 showed H2 production from buffered
(pH 7) aqueous solutions containing sacrificial electron donors
under simulated sunlight (Fig. S8, ESI†). The total amount of H2
evolved of Pt-loaded PCTF-8 (10 mg) in a buffered aqueous
methanol or TEoA solutions when irradiated for 20 h was 1780
Fig.4 H2, CO2, and CH4 isotherms at 1 bar of PCTF-8.
Photophysical properties
Absorption and photoluminescence spectra of PCTF-8 and its
1) were measured at room temperature (Fig. 5).
monomer (
The monomer
1
dispersed in CH2Cl2 shows several absorption
bands below 400 nm attributed to the π-π* transitions of the
TPE chromophore.27,28 Upon excitation at 350 nm, the
monomer emits a weak blue emission with λmax = 389 nm (Fig.
5a). This type of subunit is known to show an aggregation
induced emission in the solid state or concentrated solutions.
The weak emission is due to the non-restricted rotation of
phenyl rings in dilute solutions.29,30 On the other hand, a solid-
state absorption spectrum of PCTF-8 exhibits
a broad
and 2370
ꢀ
mol g-1, respectively. These moderate values of
absorption band centred at around 400 nm ranging from 300
to 600 nm, which can be attributed to the TPE units (Fig. 5b).
These different absorption profiles are responsible for the
different colour; the monomer shows a light yellowish colour
owing to absorbing UV light but reflecting all visible light, while
PCTF-8 is a bright yellow powder due to absorbing blue light
(Fig. 5).
hydrogen production could be due to the poor crystallinity of
PCTF-8 which hinders the diffusion of light-induced charge
carriers to the surface.9
To examine the sensing behaviour of PCTF-8, luminescence
spectrometric quenching titration experiments were
performed by gradual addition of nitroaromatic analytes (TNP,
NT or NB, 2,4-DNP, 2,6-DNT, 2,4-DNT, cf. Fig. 6) to PCTF-8
dispersed in acetonitrile at room temperature (Fig. 6, 7a, and
secꢀon 11 in ESI†).
100
80
60
40
20
0
Fig. 5 Absorption (red line) and fluorescence spectrum (blue line) for (a) monomer 1 (in
CH2Cl2, λexc 350 nm) and (b) the corresponding PCTF-8 solid (λexc 395 nm). Photos of
PCTF-8 are shown under ambient light (c) and UV- light (d).
Fig. 6 Fluorescence quenching (%) of PCTF-8 with nitroaromatic analytes in acetonitrile
dispersion.
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6
(a)
(b)
(c)
IRF
0 µL
20 µL
10000
0 mol/L
1× 10-6 mol/L
700
40 µL
60 µL
80 µL
100 µL
120 µL
140 µL
160 µL
200 µL
250 µL (~30 ꢀM)
1000
100
10
5
4
3
5
K
= 1.32 x 10
SV
1
10
20
30
400
500
600
Wavelength (nm)
800
0
10
20
30
40
[TNP] (
ꢀM)
Time (ns)
Fig. 7 (a) Fluorescence quenching of PCTF-8 by TNP (0–1 × 10–6 M) at room temp. (1 mg in 3 mL acetonitrile, λꢁꢂꢃ ꢄ 395 nm); (b) Stern–Volmer plots for the quenching of PCTF-8
by TNP in acetonitrile dispersion (λꢁꢂꢃ ꢄ 395 nm); (c) Fluorescence decay profile of PCTF-8 (λꢁꢂꢃ ꢄ 405 nm) with different aliquots of TNP added, for a maximum quencher
concentration of ca. 30 µM.
Upon successive additions of nitroaromatic analytes, the lifetime of PCTF-8 was fitted to three exponential decay
fluorescence emission intensity was increasingly quenched, components (ꢅꢆ ꢄ 1.324, Fig. S22 in ESI†): 0.5 ns (49%), 2.1 ns
with TNP being the most effective quencher, for a maximum (42%) and 5 ns (1%). This decay behaviour can be rationalised
quenching of ca. 71% of the fluorescence emission intensity. in terms of the size distribution of the polymer, and due to the
The other analytes show similar quenching behaviour with the fact that emission comes from a heterogeneous suspension,
following order: TNP >> NT > NB > DNP > 2,6 DNT > 2,4 DNT where different aggregation behaviours can be observed.
(Fig. 6). Following the quenching experiment over time up to
sixty minutes, it is evident that the quenching occurs very
Conclusions
rapidly and does not increase over time (Fig. S16, ESI†).
To quantify the sensitivity towards the nitroaromatic analytes,
the Stern−Volmer (SV) quenching constants (KSV
In conclusion, we have used TPE units in the preparation of a
) were
porous and photoluminescent covalent triazine-based
framework (PCTF) by using TFMS as catalyst. This PCTF-8
showed photoluminescence behaviour under UV light,
whereas the material PCTF-1 prepared by the high-
temperature ZnCl2 route was completely non-emissive. PCTF-8
shows sensitive detection towards nitroaromatic analytes with
trinitrophenol (TNP) quenching ca. 71 % of the PCTF-8
fluorescence emission intensity. The quenching mechanism
was found to be static in nature, because the emission
lifetimes did not change with increasing addition of the
analytes. From optical transmission measurement, we
estimated the band gap of about 2.25 eV which determines
the efficiency as photocatalysts for hydrogen evolution. The
total amount of H2 evolved of Pt-loaded PCTF-8 in aqueous
methanol or TEoA was 1780 and 2370 μmol g-1, when
irradiated for 20 h. Hence, CTFs are not only interesting for gas
adsorption/separation but also hold a great potential for
hydrogen production and for the development of new
luminescent chemosensors.
calculated by using the standard linear curve-fitting of the
fluorescence intensity (I0/I) in the presence of different
concentrations of the analytes (1 × 10–5 – 5 × 10–5 mol/L) (Fig.
7b, Fig. S17-S21 in ESI†). The obtained quenching constants
(KSV) are in the order TNP >> DNP > NB > 2,6-DNT > 2,4-DNT >
NT. KSV of TNP was 1.32×105 L mol–1 (Fig. 7b), which is
comparable with previously reported frameworks.32
To further understand the nature of the quenching process
involved in “turn-off” fluorescence sensing by PCTF-8, we
measured the emission decays by time-correlated single
photon counting at different concentrations of TNP (added as
aliquots of a stock solution in acetonitrile), as it has the highest
quenching efficiency of the studied nitroaromatic compounds.
From the decays shown in Fig. 7c, it can be observed that the
lifetime is independent of the concentration of the quencher
in the studied range, which strongly suggests that a static
quenching mechanism is taking place. The static quenching
behaviour of PCTF-8 with TNP can be explained in terms of the
formation of
a ground-state complex due to a strong
interaction with the analyte (See ESI† for discussion).33 The
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Acknowledgements
11 A. Bhunia, V. Vasylyeva and C. Janiak, Chem. Commun., 2013, 49,
A.B. thanks Ghent Univ. (GOA nr. 01G00710). D.E. thanks
Scientific Research-Foundation Flanders (FWO), Spanish
Ministry of Economy and Competitiveness (Project MAT2013-
44463-R), Junta de Andalucia (Project P10-FQM-6181 and
Andalucia Talent Hub) and FEDER Funds. This work was partly
supported by ACT-C, Japan Science and Technology Agency
(JST) and the HHU SFF fund. We thank Prof. Romero-Salguero
at the University of Córdoba for his suggestions and support.
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA04623A
Table of contents
A highly photoluminescent (PL) porous covalent triazine-based framework (PCTF-8) is
synthesized from tetra(4-cyanophenyl)ethylene with a high fluoresencence quantum yield
(31%) at room temperature. The fluorescence emission intensity of PCTF-8 is quenched ~71 %
by TNP and it follows static pathway. Pt-loaded PCTF-8 showed H2 production under visible-
light illumination.