Bimodal Functionality in a Porous Covalent Triazine Framework by Rational Integration of an Electron-Rich and -Deficient Pore Surface

Article abstract of DOI:10.1002/chem.201600109

A porous covalent triazine framework (CTF) consisting of both an electron-deficient central triazine core and electron-rich aromatic building blocks is reported. Taking advantage of the dual nature of the pore surface, bimodal functionality has been achieved. The electron deficiency in the central core has been utilized to address one of the pertinent problems in chemical industries, namely separation of benzene from its cyclic saturated congener, that is, cyclohexane. Also, by virtue of the electron-rich aromatic rings with Lewis basic sites, aqueous-phase chemical sensing of a nitroaromatic compound of highly explosive nature (2,4,6-trinitrophenol; TNP) has been achieved. The present compound supersedes the performance of previously reported COFs in both the aspects. Notably, this reports the first example of pore-surface engineering leading to bimodal functionality in CTFs.

Full text of DOI:10.1002/chem.201600109

DOI: 10.1002/chem.201600109
Full Paper
&
Covalent Organic Frameworks
Bimodal Functionality in a Porous Covalent Triazine Framework
by Rational Integration of an Electron-Rich and -Deficient Pore
Surface
Avishek Karmakar,[a] Amrit Kumar,[a] Abhijeet K. Chaudhari,[a] Partha Samanta,[a]
Aamod V. Desai,[a] Rajamani Krishna,[b] and Sujit K. Ghosh*[a]
Abstract: A porous covalent triazine framework (CTF) con-
sisting of both an electron-deficient central triazine core and
electron-rich aromatic building blocks is reported. Taking ad-
vantage of the dual nature of the pore surface, bimodal
functionality has been achieved. The electron deficiency in
the central core has been utilized to address one of the per-
tinent problems in chemical industries, namely separation of
benzene from its cyclic saturated congener, that is, cyclohex-
ane. Also, by virtue of the electron-rich aromatic rings with
Lewis basic sites, aqueous-phase chemical sensing of a nitro-
aromatic compound of highly explosive nature (2,4,6-trinitro-
phenol; TNP) has been achieved. The present compound su-
persedes the performance of previously reported COFs in
both the aspects. Notably, this reports the first example of
pore-surface engineering leading to bimodal functionality in
CTFs.
considerable porosity.[4] Covalent triazine frameworks (CTFs)
are an important subclass of COFs that have been known for
their chemical and thermal stability.[5] The ease of syntheses by
simple acid-catalysed room-temperature reaction allows an
added advantage in the synthetic procedures. Pre-designing
CTF in view of specific functionalities is desired because of
growing demand of incorporating multifarious functionalities
in the same material. For example, simultaneous incorporation
of both electron-rich and electron-deficient functionalities in
the CTF material would pave way for fabrication of dual-func-
tionalized pore surface. The central triazine ring in CTF formed
by cyclotrimerisation reaction of the CꢀN bonds renders an in-
herent electron deficiency to framework. Likewise, if electron
rich aromatic building blocks with Lewis basic sites can utilized
to constitute a CTF, then applications related to both electron
deficient and/or rich pore surfaces can be achieved strategical-
ly (Scheme 1).
Introduction
The development of multifunctional materials has been
a theme of cutting-edge research in this changing world. Since
the dawn of the age of industries, development of cheap ma-
terials with multiple functionalities which can be produced in
a bulk scale and therefore be utilized in intensive industries, is
an imperative issue. Various materials have been have been
tried and tested by the people in both industries and academ-
ia to integrate multifunctional character in the same material.[1]
Metal–organic frameworks (MOFs), which belong to the porous
materials domain, are one such class of multifunctional materi-
als which have been well-known for their plethoric applications
in almost all spheres ranging from gas storage, separations, to
more recent applications in clean-energy and fuel-cell applica-
tions.[2] Recently a new class of similar porous materials, known
as covalent-organic frameworks (COFs), have been introduced,
which are a unique class of crystalline organic polymers with
extended porous ordered structures built from pre-designed
simple organic motifs.[3] Owing to the lightweight composition
of these materials, COFs have low mass densities and possess
[a] A. Karmakar, A. Kumar, A. K. Chaudhari, P. Samanta, A. V. Desai,
Dr. S. K. Ghosh
Department of Chemistry, Indian Institute of Science Education and
Research (IISER) Pune
Dr. Homi Bhabha Road, Pashan, Pune 411008 (India)
E-mail: sghosh@iiserpune.ac.in
Homepage: http://www.iiserpune.ac.in/~sghosh/
[b] R. Krishna
University of Amsterdam (the Netherlands)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
chem.201600109.
Scheme 1. Representation of the bimodal functionality in CTF-IP10 by virtue
of the electron-deficient and electron-rich pore surface.
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Owing to the paucity in the electronic cloud centrally, CTFs
can serve as a selective adsorbent for electron-rich aromatic
molecules.[6] The differential electronic attraction of CTFs to-
wards two molecules of similar sizes but different electron
densities such as benzene and cyclohexane may result in sepa-
ration of these two C6 cyclic congeners. Separation of ben-
zene/cyclohexane is of paramount importance because ben-
zene in its pure form is one of the vital building blocks in
chemical industries, with a production of about 43.7 metric
tonnes annually.[7] Cyclohexane, which is a by-product of the
hydrogenation of benzene, impedes in separation-based indus-
tries because of the closeness in the physical properties with
benzene (Table S1 in the Supporting Information). CTFs, by the
virtue of its large porous nature and electron-deficient pore
surface, can act as a selective adsorbent for benzene molecules
and separate it from cyclohexane by a much less energy/cost-
demanding adsorption process. On the other hand, it is well
known that molecules comprised of electron-rich substituents
or functional organic sites can sense electron-deficient nitroar-
omatic compounds (NACs).[8] NACs are the key constituents of
explosives that are used worldwide. Among them, TNP is a re-
nowned explosive that not only poses a threat to security, but
is also known to be an environmental pollutant.[9] Considering
its uses in intensive chemical industries, TNP causes unavoida-
ble contamination in ground waters and soil. Therefore detec-
tion of even trace amount of TNP is crucial in view of the cata-
strophic effects it causes to the environment. Fluorometric
methods of detection of TNP are known for the ease of han-
dling procedures and the very low detection level measure-
ments.[10] Considering the electron-deficient nature of most of
these NACs and TNP in particular, if a luminescent CTF with in-
corporated electron-rich substituent can be designed then an
efficient sensor for trace amount NACs detection can be fabri-
cated. Considering the high chemical and thermal stability of
CTF-based materials, aqueous-phase detection by fluorometric
methods would serve as an added advantage in prompt detec-
tion of TNP or other NACs.
rich unsaturated moiety (benzene) over its saturated congener
(cyclohexane). Moreover, the CTF has ethereal linkage that con-
nects the aromatic benzene rings that not only makes the CTF
electron-rich but also indulges in specific hydrogen-bonding
interaction resulting in chemical sensing of a NAC, namely TNP.
The very high degree of quenching specifically for TNP even in
concurrent presence of other nitro analytes. Although multi-
functionality in porous frameworks like MOFs and zeolites are
well-known, pore surface engineering resulting in bimodal
functionality in the CTFs is not reported till date.
Results and Discussion
CTF-IP10 was prepared in good yield by acid-catalysed room
temperature reaction of a tricyanomonomer (PCN-M1) in CHCl3
as a solvent (Figure 1; see the Supporting Information for full
details). The formation of the polymeric trimerised compound
was monitored by IR spectra of the product. The IR spectra of
the PCN-M1 and that of CTF-IP10 showed considerable differ-
ence, as the CꢀN stretching frequency of the monomer at
about 2300 cmÀ1 was absent in the resulting CTF, as shown in
Figure 2a. Moreover the peaks characteristic of the triazine
ring at about 1350 cmÀ1 and 1600 cmÀ1 appeared, indicative of
the formation of the new polymeric compound. Thermogravi-
metric analysis of CTF-IP10 performed under N2 atmosphere re-
vealed very high thermal stability of more than 5008C (Fig-
ure 2d). The thermal stability of the precursor on the other
hand was much less, which is typical for a discrete organic
compound. PXRD pattern of CTF-IP10 shows a peak around
2q=4.88 indicative of the crystalline nature of the polymeric
framework (Figure 2c). However exact structure elucidation
could not be achieved owing to absence of any long-range or-
dering in the bulk sample. Such type of weak crystalline nature
is often observed in CTFs prepared by a room-temperature
method.
The 13C solid-state NMR spectra shows peak for the triazine
ring carbon atoms around 170 ppm and other peaks between
d=90–200 ppm, which corresponds to other aromatic carbon
atoms (Figure 2b). The solid-state field emission scanning elec-
tron microscopy (FE-SEM) images show assemblage of particles
of CTF-IP10 resulting forming a sheet-like structure (Figure S4
Herein, we present a new porous covalently linked triazine-
based framework (CTF-IP10), built from a tripodal cyano-based
monomer. The CTF owing to the electron deficiency of the tri-
azine rings in the core shows a selective uptake of p-electron-
Figure 1. a) Representation of the formation of CTF-IP10 and b) space-filling diagram of a single net of CTF-IP10 showing electron-rich and electron-deficient
units.
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Figure 2. a) IR spectra of PCN-M1 and that of CTF-IP10 showing disappearance of the CN stretch and appearance of peaks for triazine rings in CTF-IP10.
b) 13C-SS-NMR for CTF-IP10. c) PXRD pattern of the CTF-IP10 compound and d) TGA profile of the desolvated phase of CTF-IP10.
in the Supporting Information). The UV/Vis solid-state reflec-
tance spectra for PCN-M1 and the CTF compound are distinctly
different owing to the increase in the conjugation, resulting in
a prominent shift of around 80 nm in the CTF (Figure S3 in the
Supporting Information). N2 and CO2 adsorption at low tem-
peratures (77 K and 195 K, respectively) and CO2 adsorption at
298 K (Figure 4a; Figure S5 and Figure S7 in the Supporting In-
formation) showed sufficient porosity, which is typical for such
microporous polymeric frameworks. The pore-size distribution
curve shows an aperture size of 0.87 nm (Figure S8 in the Sup-
porting Information). The Brunauer–Emmett–Teller (BET) sur-
face area of CTF-IP10 was calculated to be 358 m2 gÀ1 and pore
Figure 3. a) Structure of the repeating unit of CTF-IP10 and b) electronic po-
tential map of the repeating unit showing the electron-deficient and -rich
surface.
volume of 0.2207 cm3 gÀ1. In view of the intrinsic electron defi-
ciency and the highly porous nature of CTF-IP10, as also evi-
dent from the DFT calculations, by mapping the electron po-
tential of the repeating unit (Figure 3), as calculated with the
Gaussian 09 Rev D program suite using DFT (structure opti-
mized by density functional theory at the B3LYP/6-31G* level),
benzene vapour adsorption was carried out at 298 K using the
desolvated compound to check its response towards p-elec-
tron-rich guest molecules.
zene molecules, thereby acting as an efficient adsorbent for
benzene molecules. TGA analysis of CTF-IP10 exposed to va-
pours of 1:1 benzene/cyclohexane shows a steady loss corre-
sponding to the adsorbed benzene vapours inside the porous
framework. A similar exposure to pure cyclohexane solvent
showed no such loss validating the fact that only benzene
could be adsorbed selectively (Figure S11, S12 in the Support-
ing Information). UV/Vis reflectance spectra of the sample ex-
posed to benzene vapour showed a prominent shift with re-
spect to CTF-IP10; however, samples exposed to cyclohexane
vapour showed no such striking shift in the wavelength (Fig-
ure S13 in the Supporting Information). All of these results
prove that that benzene is selectively adsorbed by CTF-IP10
As anticipated, benzene adsorption showed a very high
uptake of about 12.43 molkgÀ1, whereas the saturated cyclic
congener (cyclohexane) measurement showed negligible
uptake (Figure 4). The reason is attributed to the very strong
interaction between the electron-rich benzene molecules and
the p-electron-deficient triazine ring of CTF-IP10. It is hypothe-
sized that the porous framework containing electron-deficient
motifs undergo a stacking interaction with the incoming ben-
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sponge for benzene molecule. To further study in detail, we
evaluated Bz/Cy separation by utilizing the ideal adsorbed so-
lution theory (IAST) calculations. Figure 5a shows the experi-
mental data for pure component isotherms for benzene and
cyclohexane in CTF-IP10; the continuous solid lines are Lang-
muir–Freundlich fits; the parameters are specified in Table S2
in the Supporting Information. For fitting purposes, only the
adsorption branches of the isotherms were considered. IAST
calculations for benzene uptake capacity for equimolar ben-
zene/cyclohexane mixtures in CTF-IP10 showed that for pres-
sures exceeding about 1 kPa, the adsorbed phase contain pre-
dominantly benzene; this indicates good separation character-
istics, as shown in Figure 5b. Figure 5c shows IAST calculations
for adsorption selectivity, Sads, for equimolar benzene/cyclohex-
ane mixtures. We note that the value of Sads for CTF-IP10 is in
excess of about 50. In view of the significantly higher uptake
capacity of benzene in CTF-IP10, we should expect that the
separation performance of benzene/cyclohexane mixtures with
CTF-IP10 would be significantly higher than that of DAT-MOF-
1, the separations of which were reported in our earlier
work.[11] Breakthrough simulations, carried out with the
method described in our earlier work,[12] show that clear sepa-
rations of equimolar benzene/cyclohexane mixtures are achiev-
able with CTF-IP10; see Figure 5d. A video animation (see the
Supporting Information) demonstrates the superior selective
adsorption ability of CTF-IP10 for benzene over cyclohexane.
In our endeavour to exploit effect of electron-rich building
units on electron-deficient NACs, a fluorometric detection
Figure 4. Top: CO2 adsorption isotherm of CTF-IP10 at 195 K. Bottom: ben-
&
&
*
*
) and cyclohexane adsorption ( , ) at 298 K.
zene adsorption (
,
both from the perspective of the single component adsorption
isotherm and also from binary mixtures of both benzene and
cyclohexane. Such a targeted syntheses of CTF for selective ad-
sorption of benzene makes it act as selective molecular
Figure 5. a) Experimental data for pure component isotherms for benzene and cyclohexane in CTFIP-10; the continuous solid lines are Langmuir–Freundlich
fits. b) IAST calculations for benzene uptake capacity for equimolar benzene/cyclohexane mixtures in CTFIP-10. c) IAST calculations for adsorption selectivity,
for equimolar benzene/cyclohexane mixtures by CTF-IP10. d) Breakthrough simulations for Bz/Cy in a fixed bed of CTF-IP10 at 298 K.
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study was carried out in disperse phase with the aquatically
stable fluorescent CTF-IP10. In a typical experiment, 2 mg of
the solid sample was dispersed in 1:0.25 mixture of H2O/EtOH
and incremental addition from a stock solution of 1 mmol ni-
trobenzene (NB), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitroto-
luene (2,6-DNT), 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol
(TNP), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) in 5 mL of
water was carried out. When excited at around 340 nm (Fig-
ure S14 in the Supporting Information), other nitroanalytes ex-
hibited negligible to moderate quenching behaviour, whereas
for TNP highest quenching effect on CTF-IP10 was observed
(Figure 7a; Figures S15–S22 in the Supporting Information).
About 98% fluorescence quenching was observed in the case
of TNP upon addition of only 80 mL of the analyte (Figure 6).
Figure 6. Fluorescence spectra of CTF-IP10 upon incremental addition of
TNP in 1:0.25 mixture of H2O/EtOH showing changes in photoluminescence
under UV light.
Figure 7. a) Fluorescence response of CTF-IP10 towards various nitroaromat-
ic compounds showing maximum response towards TNP and b) SV plot of
TNP titration at low concentration.
The electron-deficient TNP and other NACs can diffuse through
the porous channels of the framework as a result of suitable
interactions with the electron-rich substituents of CTF-IP10. Of
them, TNP causes the maximum quenching effect. The
quenching constant (Ksv) as calculated by Stern–Volmer equa-
tion showed a very high value of 8”105 LmolÀ1 at low concen-
tration (Figure 7b), which is one of the highest known Ksv
value for TNP in the COF regime. Encouraged by the above
result, a competitive assay was performed to check whether
the efficiency of quenching was consistent in presence of
other competing nitro analytes. In a systematically designed
experimental procedure, other competing analytes were added
to CTF-IP10 and fluorescence spectra were recorded. A little
change in fluorescence intensity was observed as compared to
the parent compound. However, when TNP was added to the
same solution, a sharp decrease in fluorescence intensity was
observed. This cycle was repeated for three consecutive times
and consistent results were observed (FigureS25 in the Sup-
porting Information). Thus the efficacy of CTF-IP10 towards se-
lective sensing of TNP was unaffected even in multicomponent
mixtures of other analytes. This showed that even in concur-
rent presence of other nitroanalytes, the CTF-IP10 showed a far
superior response towards TNP. The stability of CTF-IP10 in
TNP was checked by dipping the desolvated compound in
TNP solution for about 10 days, after which PXRD was record-
ed. The CTF showed retention in crystallinity confirming the
high chemical stability of the present compound. To investi-
gate the mechanism of quenching, we turned to calculate the
HOMO–LUMO energies for the nitro analytes by DFT by using
the B3LYP/6-31G* method with Gaussian 09 package. The
LUMO orbitals of TNP are the lowest-lying (Figure 8), which is
indicative of the fact that very fast and efficient photoinduced
electron transfer may occur from the p orbitals of the electron-
rich components of CTF-IP10, that is, the phenyl rings with
ethereal linkage. TNP, being the most electron-deficient spe-
cies, readily accepts the electron from the HOMO orbital of the
donor, resulting in significant fluorescence quenching.
To validate the mechanism of the electron donation from
the conduction band of the CTF to the LUMO of the analyte,
a control experiment in which other phenol derivatives, such
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pound in both the aspects supersedes both the sensing effi-
ciency of TNP and selectivity towards benzene reported by
previously reported CTFs. Pre-designing CTFs for specific func-
tionalities in a targeted fashion can thus open up new avenues
to develop multifunctional materials, to which significant at-
tention is currently being given.
Experimental Section
PCN-M1 was synthesized according to the previously reported pro-
cedure with slight modification. All other reagents and solvents
were commercially available and used without further purification.
The X-ray powder pattern was recorded on a Bruker D8 Advanced
X-ray diffractometer at room temperature using Cu Ka radiation
(l=1.5406 Š). FTIR spectra were measured on NICOLET 6700 FT-IR
Spectrophotometer using KBr Pellets. Thermogravimetric analyses
was obtained in the temperature range of 30–8008C on PerkinElm-
er STA 6000 analyser under a N2 atmosphere at a heating rate of
108C min. All fluorescence measurements were done on JobinYvon
Fluoroax-4 spectro fluorometer. 1H and 13C NMR was recorded in
400 MHz Jeol ECS-400 Instrument. The UV/Vis measurements were
performed using Chemito SPECTRASCAN UV-2600.
Figure 8. HOMO and LUMO energies of electron-deficient nitro analytes cal-
culated by DFT.
as nitrophenol and 2,4-dinitrophenol, were titrated to observe
the quenching behaviour. As expected the percentage quench-
ing values were significantly lower compared to TNP (32% for
NP and 56% for 2,4-DNP; Figure S24 in the Supporting Infor-
mation). The trend follows the exact order of electron deficien-
cy in such nitrophenols. TNP, being the most electron-deficient
among the three owing to the presence of three electron-with-
drawing ÀNO2 groups, interacts significantly with the electron-
rich p-orbitals of CTF-IP10, resulting in a prompt quenching re-
sponse. Also the absorption spectra of TNP showed a consider-
able overlap with the emission spectra of the CTF-IP10 as com-
pared to other non-emissive analytes, correlating with the fact
that an energy-transfer process is also responsible for the
quenching mechanism (Figure S26 in the Supporting Informa-
tion). The possibility of a hydrogen-bonding interaction of the
ether oxygen and/or basic nitrogen atoms of the triazine ring
of the compound with the acidic phenolic hydrogen of TNP
may also result in significant host–guest complexation, result-
ing in such effective and interesting fluorescence response. To
rule out the effect of self-absorption of TNP, we performed
fluorescence measurement at different excitation wavelengths
(lexc =360 nm, 390 nm and 400 nm) upon addition of TNP solu-
tion to CTF-IP10. As evident from the fluorescence profile (Fig-
ure S27 in the Supporting Information), negligible quenching
was observed. The above observation also corroborated the
high Ksv value for TNP at low concentrations. However, a non-
linearity in the Stern–Volmer curve was observed indicative of
both static and dynamic energy transfer mechanism occurring
herein.
Synthesis of PCN-M1: In a 50 mL flask, phloroglucinol (1 mmol)
and K2CO3 (4.5 mmol) were suspended in DMF (10 mL). The sus-
pension was heated at reflux temperature for 4 h. Then the reac-
tion mixture was cooled to about 608C and p-fluorobenzonitrile
(3.2 mmol) was added and heating continued at 1508C for 24 h.
On cooling, the reaction mixture was poured into 500 mL of water
and the precipitated solid was collected and washed with water
followed by methanol. The yield of the product was 86%. The
crude product was purified by recrystallization from ethanol to
afford pure compound PCN-M1.
Synthesis of CTF-IP10: Trifluoromethanesulfonic acid (0.6 g,
4 mmol) and CHCl3 (10 mL) were charged into a pre-dried two-
neck round-bottom flask under an N2 atmosphere. The mixture
was cooled to 08C and PCN-M1 (0.430 g, 1 mmol) in CHCl3 (80 mL)
was added into the solution dropwise over 30 min. The mixture
was stirred at 08C for another 2 h before left overnight at room
temperature. The solution turned red and solid precipitates were
formed. Then, the mixture was poured into water (200 mL) con-
taining ammonia solution (10 mL, 0.6 mmol) and stirred for 2 h.
The precipitates were filtered and washed with water, ethanol, ace-
tone, and chloroform successively. CTF-IP10 (1.1 g, 86% yield) was
obtained as a light-yellow solid. Elemental analysis (%) of CTF-IP10:
found: C 67.53, H 3.41, N 10.66.
Acknowledgements
P.S. is thankful to UGC for research fellowship. Av.K. and A.V.D.
are thankful to IISERPune for research fellowship. We are grate-
ful to IISER Pune for research facilities. DST (Project No. GAP/
DST/CHE-12-0083) is acknowledged for the financial support.
Conclusion
In conclusion, we have successfully employed a strategy of
pore-surface engineering to achieve bimodal functionality in
a porous covalent triazine framework. The combination of
both electron-rich and electron-deficient building blocks in the
present compound have been strategically exploited in ad-
dressing two issues of great concern, namely aqueous-phase
detection of a compound of highly explosive nature (TNP) and
separation of benzene molecules from its saturated congener
(cyclohexane). Notably, the performance of the present com-
Keywords: bimodal
functionality
·
covalent
organic
frameworks · sensing · separation · triazine
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Received: January 11, 2016
Revised: January 18, 2016
Published online on February 18, 2016
Chem. Eur. J. 2016, 22, 4931 – 4937
www.chemeurj.org
4937
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