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Phosphoric Acid Loaded Azo (−NN−) Based Covalent Organic

Framework for Proton Conduction
Suman Chandra,†,† Tanay Kundu,†,† Sharath Kandambeth,† Ravichandar BabaRao,‡ Yogesh Marathe,†
Shrikant M. Kunjir,† and Rahul Banerjee*,†
†
Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
‡
Materials Science and Engineering Division, Commonwealth Scientific and Industrial Research Organization (CSIRO), Clayton,
Victoria 3168, Australia
*
S Supporting Information
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atures are major limitations for the future development of proton

ABSTRACT: Two new chemically stable functional conducting materials. COFs are relatively lightweight, exhibit a
crystalline covalent organic frameworkds (COFs) (Tp- wide variety of functionality, and possess high thermal stability
Azo and Tp-Stb) were synthesized using the Schiff base and membrane processability like polymers. These features
reaction between triformylphloroglucinol (Tp) and 4,4′- ensure their sustainability in harsh fuel cell operating conditions
azodianiline (Azo) or 4,4′-diaminostilbene (Stb), respec- and a high degree of internal ordering as in MOFs that facilitates
tively. Both COFs show the expected keto-enamine form, loading and the transport of proton conducting substrates.
and high stability toward boiling water, strong acidic, and Despite these promising features, COFs have never been tested
basic media. H 3 PO 4 doping in Tp-Azo leads to for proton conduction due to their instability6 in ambient
immobilization of the acid within the porous framework, humidity conditions.
which facilitates proton conduction in both the hydrous (σ We recently overcame this stability problem and developed a
= 9.9 × 10−4 S cm−1) and anhydrous state (σ = 6.7 × 10−5 general strategy for synthesizing COFs that showcase the
S cm−1). This report constitutes the first emergence of stability toward a strong acid (9 N HCl) and moderately strong
COFs as proton conducting materials. base (6 N NaOH), even upon isoreticulation and functionaliza-
tion.7 Also, the COF backbone can be suitably functionalized,
which can further enhance the interaction with carrier molecules
C ovalent Organic Frameworks (COFs) are 2D or 3D
crystalline materials made by linking lighter elements (e.g.,
B, C, N, O) via covalent bonds in a periodic manner.1 COFs are
to aid proton conduction pathways. H3PO4 loaded PBI based
membranes are well documented in literature for high proton
conductivity (ca. 10−2 S cm−1) at fuel cell operating temper-
synthesized and subsequently crystallized by means of reversible atures.8 Other than the benzimidazole group, the azo group can
covalent bond formation reactions such as boronic acid also play a vital role in stabilizing H3PO4.9 The crystal structure of
trimerization, boronate ester formation, and the Schiff base the protonated azobenzene unit [ref code: WEVXAX]9a with the
reaction. Structurally, COFs are closely related to metal−organic H-bonded H2PO4− anion hints at the potential of azo
frameworks (MOFs),2 where coordination bonds link metal ions functionalized COFs to act as proton conducting materials.
and organic struts. Although COFs have shown excellent The extension of the azobenzene monomeric unit in higher
promise as semiconductive devices,3d sensors,3e,f and in gas dimensions to form a porous COF with a 1D channel may
storage and separation,3a proton conductivity in COFs is still
facilitate unidirectional proton transport, while crystallinity and
unexplored. In recent years, proton conducting materials have
chemical stability may be beneficial in providing mechanistic
spurred tremendous interest among researchers due to their
insight. In this regard, we present an azo functionalized COF
application in fuel cells, sensors, and electronic devices.4 Nafion
(Tp-Azo) made by the Schiff base reaction between triformyl-
based proton conducting membranes are considered as the
phloroglucinol (Tp) and 4,4′-azodianiline (Azo). This COF
benchmark in this field. Such membranes exhibit high proton
conductivity (ca. 10−1 S cm−1) at moderate temperature (60−80 exhibits excellent structural stability, porosity, and crystallinity
°C) under high relative humidity (98% RH).4c Yet, the high cost even after acid treatment. As mentioned before, we conceived
of perfluorinated membranes with less efficiency at fuel cell this idea of a H3PO4 loaded azo functionalized COF (PA@Tp-
operating temperatures (ca. 120 °C) encouraged researchers to Azo) being a good proton conductor after noticing the crystal
search for alternative materials. In this context, MOFs with structure of H3PO4 treated 4-aminoazobenzene, where H3PO4
loaded carrier molecules (e.g., imidazole, triazole, mineral acids) finds an anchoring site at the azo center by protonating it, while
have been envisaged for high temperature proton conduction the H2PO4− anions are stabilized via H-bonds.9a PA@Tp-Azo
applications.5 However, MOFs suffer poor hydrolytic stability shows proton conductivity in humid and anhydrous conditions
with very low pH tolerance of the occluded guests. As a result, (9.9 × 10−4 and 6.7 × 10−5 S cm−1, respectively). To elucidate the
rupture of the coordination bonds and the framework backbone structure/property relationship, we also synthesized the nonazo
limits its applicability in fuel cell operating conditions. In
addition, the high gravimetric weight of the MOF, difficulty in Received: March 8, 2014
forming compact membranes, and instability at higher temper- Published: April 23, 2014

© 2014 American Chemical Society 6570 dx.doi.org/10.1021/ja502212v | J. Am. Chem. Soc. 2014, 136, 6570−6573
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counterpart, i.e. stilbene functionalized COF (Tp-Stb) which peaks at 5.5°, 6.4°, 8.4°, and 27° (001 plane). The π−π stacking
shows less stability, crystallinity, porosity, and much less proton distance between COF layers was calculated to be 3.3 Å from the
conductivity than Tp-Azo because of the nonavailability of an d spacing between 001 planes. On the other hand, Tp-Stb
anchoring site. exhibits an intense peak at 2θ = 3.2° followed by a broad peak at
The syntheses of Tp-Azo and Tp-Stb were done by reacting 5.5°. A possible 2D model was built with an eclipsed structure in
Tp (63 mg, 0.3 mmol) with Azo (96 mg, 0.45 mmol) or Stb (128 the hexagonal space group (P6/m) and a staggered structure in
mg, 0.45 mmol) using (1:1) dimethylacetamide and o-dichloro- the P1 space group, by using the software Crystal 09.10 The
benzene as solvent (3 mL) (Figure 1). The reactants were first experimental PXRD pattern matches well with the simulated
pattern of the eclipsed stacking model (Figures S1 and S3,
Supporting Information (SI)). In order to find out the unit cell
parameters, Pawley refinements were done for both of the COFs.
The unit cell value was found to be (a = b = 31.5 Å, c = 3.3 Å) for
Tp-Azo and (a = b = 30.500 Å, c = 3.49 Å) for Tp-Stb. The
intensity ratio of the first and second peak in the eclipsed form
matches with the PXRD of Tp-Stb. The energy calculation of
both eclipsed and staggered forms of Tp-Stb also suggests an
eclipsed stacking model with lower energy (Section S-3, SI).
The FT-IR spectra of Tp-Azo and Tp-Stb indicate the total
consumption of starting materials due to the disappearance of the
N−H stretching bands (3100−3300 cm−1) of Azo or Stb and the
carbonyl stretching bands (1639 cm−1) of Tp (Figure 2b). The
strong peak at 1578 cm−1 arises due to the CC stretching in the
keto-form similar to the FT-IR spectrum of the reference
compound 2,4,6-tris((phenyldiazenyl)phenylamino-
methylene)cyclohexane-1,3,5-trione. Most of the FT-IR peaks
of Tp-Azo and Tp-Stb match well with those of the reference
compound (Figure 2b). The CO peaks (1619 cm−1) of Tp-
Azo and Tp-Stb get merged with the CC stretching band
(1582 cm−1). The isolation of Tp-Azo and Tp-Stb as the keto-
form was confirmed by 13C CP-MAS solid state NMR. Both
COFs show carbonyl (CO) carbon signals at δ = 181 and 186
ppm for Tp-Azo and Tp-Stb respectively. In the starting
material, the trialdehyde carbonyl (CO) carbon resonates at a
downfield position around δ = 192 ppm. The absence of a peak at
δ = 192 ppm in the 13C CP-NMR spectrum also indicates the
total consumption of the starting materials (Figures 2c and S-8,
Figure 1. (a) Crystal structure of 4-[(E)-phenyl-diazenyl]anilinium
dihydrogen phosphate.9a (b) Schematic of Tp-Azo and Tp-Stb SI). To investigate the protonation of the azo bond by
synthesis. phosphoric acid in PA@Tp-Azo, we have done 31P CP-NMR,
which shows two distinct peaks at δ −1.31 ppm and δ −14.3
ppm. The 31P resonance peak at δ −1.31 ppm is attributed to the
dispersed by ultrasonication for 10 min and then degassed undissociated H3PO4 and the shoulder at δ −14.3 ppm
through three freeze−pump−thaw cycles. The tubes were then corresponds to the H2PO4− anion, which indicates the
vacuum sealed and placed in an isotherm oven for 3 days at 120 protonation of the azo bond. However, in PA@Tp-Stb, only a
°C. Finally, the material was filtered out and washed with dry single intense peak at δ −0.88 ppm, corresponding to
acetone and dried under vacuum at 150 °C for 12 h. The PXRD undissociated H3PO4, has been observed and the peak at δ
patterns of Tp-Azo indicate an intense peak at 2θ = 3.2°, which −14.3 ppm was absent. This explains the absence of the H2PO4−
corresponds to 100 plane reflections (Figure 2a), with minor anion due to the lack of protonation sites in Tp-Stb (Figure 3b).

Figure 2. (a) Observed PXRD patterns of Tp-Azo (blue) and Tp-Stb (green) compared with simulated eclipsed patterns. (b) FT-IR spectra of Tp-Azo
and Tp-Stb compared with starting material Tp, Azo, and Stb. (c) 13C NMR comparison of Tp-Azo (blue), Tp-Stb (green) against the reference
compound Tp-Azo monomer = 2,4,6-tris(((4-((E)-phenyldiazenyl)phenyl)amino)methylene)cyclohexane-1,3,5-trione (red).

6571 dx.doi.org/10.1021/ja502212v | J. Am. Chem. Soc. 2014, 136, 6570−6573

Journal of the American Chemical Society Communication

cases (Figure S9, SI), which is indicative of H3PO4 loading. The

protonation of the −NN− functionality in 4-aminoazoben-
zene9a by H3PO4 and the well established phenomena of color
change of methyl orange in acidic medium9b suggested to us that
azo-groups are susceptible to protonation and can stabilize
counteranions such as phosphate or dihydrogen phosphate.
Moreover, H3PO4 exhibits high proton conductivity (10−1 S
cm−1) due to low volatility (>158 °C) and high proton mobility
resulted from the extended H-bonding utilizing three ionizable
O−H bonds. So we decided to apply this concept in COFs for
the synthesis of azo based proton conducting COFs. H3PO4
loading in Tp-Azo and Tp-Stb was achieved by simply
immersing the evacuated COF materials in 1.5 M H3PO4 for 2
h. Further washing with a copious amount of water (×2)
followed by activating overnight at 353 K under dynamic vacuum
lead to H3PO4 loaded PA@Tp-Azo and PA@Tp-Stb (Figure
4a). It is noteworthy that the H3PO4 loaded COFs exhibit

Figure 3. (a) Changes in the UV−vis spectra of monomer of Tp-Azo

with increasing concentration of H3PO4. (b) 31P CP-NMR of H3PO4
treated Tp-Azo and Tp-Stb. (c) PXRD patterns of HCl (red), boiling
water (orange) treated and as-synthesized Tp-Azo (blue). (d) N2
adsorption isotherms of Tp-Azo (blue), boiling water (orange) and
acid treated Tp-Azo (red). (e) PXRD patterns of HCl treated (blue)
and as-synthesized Tp-Stb (green). (f) N2 adsorption isotherms of as-
synthesized Tp-Stb (green) and 9 N HCl treated Tp-Stb (blue).

SEM and TEM images showed that Tp-Azo and Tp-Stb

crystallize with a flower-like morphology with the aggregation of
a large number of petals with an average length of 40−50 nm
(Sections S-9 and S-10, SI).
Thermogravimetric analysis (TGA) of the activated Tp-Azo
and Tp-Stb show thermal stability up to 350 °C, with a gradual
weight loss of 50% after 360 °C due to the decomposition of the
framework (Section S-8, SI). The permanent porosity of Tp-Azo
and Tp-Stb were evaluated by the N2 adsorption isotherm at 77 Figure 4. (a) Schematic of H3PO4 doping in COFs. Proton conductivity
K, which showed a reversible type IV adsorption isotherm. The of PA@Tp-Azo in (b) anhydrous and (c) hydrous conditions. (d)
surface areas of the activated COFs, calculated using the BET Proton conductivity of PA@Tp-Stb in hydrous conditions. (e)
Arrhenius plot for PA@Tp-Azo in hydrous conditions.
model, were 1328 and 422 m2/g for Tp-Azo and Tp-Stb,
respectively (Figure 3d,f). The lower surface area of Tp-Stb may
be due to the poor crystallinity and not so uniform channels identical IR spectra and 13C NMR spectra along with moderate
resulting from the lower solubility of Stb precursors in organic crystallinity and porosity compared to the parent COFs (Figures
solvents. The pore size distribution of Tp-Azo and Tp-Stb shows S7 and S21). Tp-Azo possesses a higher acid loading (5.4 wt %)
a narrow distribution of 1.6−2.5 nm (Section S-7, SI). as compared to Tp-Stb (2.8 wt %), which is evident from the
The stability of Tp-Azo and Tp-Stb were assayed by TGA analysis. The proton conductivities of Tp-Azo, Tp-Stb,
immersing 50 mg of COFs in 20 mL of boiling water or by PA@Tp-Azo, and PA@Tp-Stb were measured in both hydrous
standing in 20 mL strong mineral acids (9 N HCl/1.5 M H3PO4) and anhydrous conditions. The conductivities were determined
and bases (3−6 N NaOH). Interestingly, both Tp-Azo and Tp- from the semicircles in the Nyquist plots (Figure 4b−d).
Stb remain stable, crystalline, and porous while directly Interestingly, both Tp-Azo and Tp-Stb exhibit almost zero
submerged in boiling water for several days (7 days), as verified conductivity, which signifies that the COF backbones are acting
by PXRD, FT-IR spectra, and N2 adsorption isotherm studies. as a support. The proton conductivity of PA@Tp-Azo and PA@
They also exhibit strong acid (9 N HCl) stability with near Tp-Stb were measured from 295 to 415 K. Conductivity values
retention of molecular crystallinity. However, base treated (3−6 gradually increase upon heating, which reaches a maximum at a
N NaOH) COFs show moderate base stability with partial temperature of ca. 332−340 K and then decreases gradually upon
retention of crystallinity (Figures S20 and S23). The N2 increasing the temperature. The proton conductivity of PA@Tp-
adsorption isotherm of the 9 N HCl treated Tp-Azo indicates Azo was measured as 6.7 × 10−5 S cm−1 at 340 K under
the retention of its intrinsic porosity, while Tp-Stb shows a anhydrous conditions. This value was highly humidity-depend-
decrease in porosity after the acid treatment. However, 1.5 M ent and increased upon humidification. Finally, PA@Tp-Azo
H3PO4 treatment shows a significant loss of porosity in both exhibited a proton conductivity of 9.9 × 10−4 S cm−1 at 332 K
6572 dx.doi.org/10.1021/ja502212v | J. Am. Chem. Soc. 2014, 136, 6570−6573
Journal of the American Chemical Society

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Communication

under 98% relative humidity (RH) (Section S-13, SI). The AUTHOR INFORMATION
proton conductivity of PA@Tp-Azo is lower than that of its Corresponding Author
MOF counterparts, i.e. ferrous oxalate dihydrate5e (1.3 × 10−3 S r.banerjee@ncl.res.in
cm −1 at 298 K, 98% RH) and 1,2,4-triazole loaded β-PCMOF2 Author Contributions
(5 × 10−4 S cm−1 at 423 K, 20% RH) (full comparison in Table †
S.C. and T.K. contributed equally.
S3 in SI). Surprisingly, PA@Tp-Stb shows almost zero proton
conductivity in anhydrous conditions, while exhibiting a poor Notes
proton conductivity value of 2.3 × 10−5 S cm−1 at 332 K under The authors declare no competing financial interest.
98% RH. Notably, PA@Tp-Azo exhibited an activation energy
value of 0.11 eV (Figure 4e), which is much lower than that of
Nafion (0.22 eV) and its MOF counterparts (Table S3, SI)
■ ACKNOWLEDGMENTS
S.C. and T.K. acknowledge UGC and CSIR (New Delhi, India)
operating under humid conditions. Clearly, PA@Tp-Azo and for JRF and SRF. R.B. acknowledges [CSC0122 and CSC0102]
PA@Tp-Stb show distinct proton conductivity behavior under and DST (SB/SI/IC-32/2013) for funding. We acknowledge Dr.
similar conditions, although they are treated with the same T. G. Ajithkumar, Dr. K. Sreekumar, and Dr. C. Ramesh for
NMR, Proton Conduction, and PXRD facilities.

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amount of H3PO4. Nevertheless, Tp-Azo exhibits a distinct color
change (red to black) upon H3PO4 treatment, while the color of
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*
S Supporting Information
Synthetic procedures, PXRD, 13C solid state NMR, TGA, UV,
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
6573 dx.doi.org/10.1021/ja502212v | J. Am. Chem. Soc. 2014, 136, 6570−6573