Sulfonated graphenes catalyzed synthesis of expanded porphyrins and their supramolecular interactions with pristine graphene

Article abstract of DOI:10.1007/s12039-014-0731-8

A newer synthesis of sulfonic acid functionalized graphenes have been developed, which have been characterized, examined as heterogeneous solid acid carbocatalyst in the synthesis of selected expanded porphyrins in different reaction conditions. This envi

Full text of DOI:10.1007/s12039-014-0731-8

c
J. Chem. Sci. Vol. 126, No. 6, November 2014, pp. 1729–1736. ꢀ Indian Academy of Sciences.
Sulfonated graphenes catalyzed synthesis of expanded porphyrins
and their supramolecular interactions with pristine graphene
SWETA MISHRA, SMRITI ARORA, RITIKA NAGPAL
and SHIVE MURAT SINGH CHAUHAN∗
Bioorganic Laboratory, Department of Chemistry, University of Delhi, Delhi 110 007, India
e-mail: smschauhan@chemistry.du.ac.in
MS received 15 March 2014; revised 29 May 2014; accepted 04 June 2014
Abstract. A newer synthesis of sulfonic acid functionalized graphenes have been developed, which have
been characterized, examined as heterogeneous solid acid carbocatalyst in the synthesis of selected expanded
porphyrins in different reaction conditions. This environment-friendly catalyst avoids the use of toxic catalysts
and enhances the yields of porphyrinoids. The non-covalent interaction of porphyrinoids has also been studied
with exfoliated graphene solution in organic solvents by UV-Visible and fluorescence spectroscopy.
Keywords. Sulfonated graphene; expanded porphyrins; heterogeneous catalysis; solid acid; non-covalent
interaction.
1. Introduction
and calix[4]pyrroles using graphite oxide as solid
acid catalyst in different reaction conditions.19 Next
focused on the synthesis of sulfonic acid function-
alized graphene via bulk functionalization by direct
exfoliation of graphite. Here, we report the synthe-
sis of porphyrins and expanded porphyrins catalyzed
by these sulfonated graphene based solid acids, to
the best of our knowledge for the first time. These
solid acids are eco-friendly and recyclable up to five
times.
Graphene, an important allotropic member of carbon,
has received much attention owing to its interest-
ing properties in material science with wide range of
applications in energy devices, electronics, catalysis,
sensors, reinforced composites and biomedicines.1–6
Several effective techniques developed for preparing
graphene sheets limit their biological applications due
to the insolubility of graphite or graphene.7 Several
authors have tried chemical functionalization to pre-
vent the agglomeration of single layer graphene and
maintain the inherent properties of graphene.8
2. Experimental
Porphyrins are ubiquitous pigments in nature
and play key roles in biological processes.9 They
have also emerged as functional dyes, non-linear
optical materials, ion receptors and stable organic
radicals.10–12 Porphyrins consist of four pyrrole rings
connected by meso-methine bridges, while the name
‘expanded porphyrin’ stems from their larger num-
ber of pyrrole rings.13–17 These expanded porphyrins
explain the difference between aromatic, stable antiaro-
matic species and conformationally twisted Möbius
aromatic species.18 Earlier, expanded porphyrins have
been synthesized by simple Rothemund–Lindsey pro-
tocol that involves BF3.OEt2 catalyzed condensation
of pyrrole and 2,3,4,5,6-pentafluorobenzaldehyde in
dichloromethane and subsequent oxidation with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).12 Re-
cently, we have reported the synthesis of dipyrromethanes
2.1 Synthesis of sulfonated graphene
Graphite (100 mg) was exfoliated using ultrasonication
for 2 h. (1% wt) SDS was added to the reaction mixture.
Diazonium salt of sulfanilic acid was synthesized using
HBF4 and sulfanilic acid. Diazonium salt (35 mg, 0.13
mmol) was added to graphite suspension and stirred for
16 h at 35◦C. After centrifuging and rinsing with water
several times and the product was dried in vacuum at
50◦C. FTIR (KBr, cm−1) 3437 (br), 1194, 1126, 1036,
1005, 820; Raman shift (cm−1): 1354 (D band), 1581
(G band) ID/IG = 0.32.
2.2 Reaction of pyrrole with aryl aldehydes in
presence of different carbocatalyst
2.2a Method A: Reaction of pyrrole with aryl alde-
hyde: A mixture of pyrrole (276 µL, 4 mmol) and
∗For correspondence
1729

1730
Sweta Mishra et al.
aldehyde (4 mmol) and sulfonated graphene 4 (170 mg) 3. Results and Discussion
in o-DCB (60 mL) was stirred at room temperature
3.1 Synthesis of catalysts
under inert atmosphere for 2 h followed by addition of
DDQ with further stirring for 2 h. Progress of reaction
was monitored by TLC. After completion of reaction,
catalyst was filtered and rinsed with chloroform fol-
lowed by ethanol. DDQ (10 g) was then added in the
reaction mixture with further stirring for 2 h. Reaction
mixture was passed through a short alumina plug and
then chromatographed on silica gel 60–120 mesh (9:1
hexane: ethyl acetate) to obtain mixture of desired prod-
ucts leaving behind the polymeric materials. Repeated
chromatography of the mixture was done on silica gel
using hexane and chloroform (4:6) as eluent to obtain
purple (Rf = 0.8), blue (Rf = 0.6), violet (Rf = 0.5),
red (Rf = 0.3) and dark green fraction (Rf = 0.1)
solid. Mixture of hexaphyrins was further separated by
preparative TLC.
Diazonium salt was used in the chemical modifica-
tions of graphite and graphene oxide.20,21 Sulfonated
graphene 3 has been synthesized by diazotisation of r-
GO (scheme 1). Bulk functionalization of graphite was
done by exfoliating graphite in 1 wt% aqueous solution
of SDS under ultrasonication followed by the reaction
with tetrafluoroborate diazonium salt of sulfanilic acid
at 35◦C for 16–18 h to give 4 (scheme 2). SDS was
added to bring the water-soluble, cationic diazonium
reagents in close proximity of the highly hydrophobic
graphene sheets. The prepared catalysts were character-
ized completely by Raman, FTIR, TEM, SEM, XRD
and TGA techniques.
3.2 Characterization of catalysts
Raman spectroscopy is a powerful tool for charac-
terizing materials possessing conjugated carbon sp2-
hybridized bonds, such as graphene, due to their intense
Raman signals. The corresponding spectrum for sul-
fonated graphene 4 shows a red shifted D band with
increased intensity at 1351 cm−1 indicating disruption
of the sp2 carbon network as a result of function-
alization (ID/IG = 0.42 for 4 vs 0.19 for exfoliated
graphene),22 G band at 1585 cm−1 and a blue shifted
2D band at 2720 cm−1 with respect to pristine graphite
(figure S4).23 Functionalization was further confirmed
by IR spectroscopy and elemental analysis. IR spectrum
show peaks at 1194 cm−1, 1126 cm−1, and 1036 cm−1
(two νS−O and one νS−phenyl), 1007 cm−1 (νC−H in-plane
bending) and 830 cm−1 (out-of-plane hydrogen wag-
ging) are characteristic vibrations of a phenylsulfonic
acid group (figure S5).24 In elemental analysis, the sul-
phur content was estimated to be 1.5% in 4 and 0.8 %
for 3 (table S1). The amount of sulfonic acid groups
covalently anchored onto the graphene lattice was
calculated by conductometric titration. The pH and
sulfonic acid content in catalyst 4 was found to be 2.9
and 55.71 µmol/g respectively (which was five times
higher as compared to 3) (Eq. 1) showing high acidic
2.2b Method B: Reaction of dipyrromethane with aryl
aldehyde: A mixture of dipyrromethane (2 mmol),
aryl aldehyde (2 mmol) and sulfonated graphene 4 (170
mg) in o-DCB (30 mL) was taken in a round bot-
tom flask under inert atmosphere for 2 h. Progress
of reaction was monitored by TLC. After comple-
tion of reaction, catalyst was filtered and rinsed with
chloroform followed by ethanol. DDQ (10 g) was
then added in the reaction mixture with further stir-
ring for 2 h. Reaction mixture was passed through
a short alumina plug and then chromatographed on
silica gel 60–120 mesh (9:1 hexane:ethyl acetate) to
obtain mixture of desired products leaving behind
the polymeric materials. Repeated chromatography
of the mixture was done on silica gel using hex-
ane and chloroform (4: 6) as eluent to obtain
purple, blue, violet and green coloured fractions.
Mixture of hexaphyrins was further separated by
preparative TLC.
2.3 Non-covalent interaction of porphyrinoids
with graphene in organic solvents
Graphite (50 mg) was ultrasonicated in o-dichloro- content and proving it to be a better catalyst as shown
benzene (50 mL) for 5–6 h. The graphene dispersion by results.
was centrifuged at 10,000 rpm for 30 min and the super-
Pristine graphite is thermally stable up to 800◦C
under nitrogen. The TGA graph (figure S8) of 4
natant was collected. Stock solutions of 1.0 × 10−3
M
were prepared by dissolving different porphyrinoids in revealed a weight loss, in the temperature range of 200–
o-DCB. To study the effect, exfoliated graphene (0.25 500◦C, which is attributed to the thermal decomposition
mg/mL) was added to 20 µl stock solutions of por- of sulfonic acid groups attached to graphene sheets.3,25
phyrinoids and the changes in absorption spectra were The weight loss that occurred above 500◦C is due to the
recorded after every addition.
thermal decomposition of defects created at the time of

Synthesis of porphyrinoids by sulfonated graphene
1731
Scheme 1. Synthesis of sulfonated graphene using reduced graphene oxide.
functionalization. The XRD analysis exhibits that the concentration (67 mM) gave a mixture of products
(002) diffraction peak of graphite appears at 26.6◦, and which were separated by repeated column chromatog-
the interlayer spacing is 0.34 nm which was increased raphy and preparative TLC to give tetraarylporphyrin
to 0.38 nm in 4 after functionalization (figure S7). SEM (8a), N-fused pentaphyrin (9a), hexaphyrin (10a)
and TEM images of 4 showed that material exhibited and octaphyrin (11a) in 20, 8, 14 and 1% yield
crumpled sheet like appearances despite of the presence (scheme 3). The synthesized products were charac-
1
of functional groups (figure S2, S3).26
terized by UV-Vis, H NMR and mass spectrometry
(figure S9–S18).
Among the prepared catalysts, catalyst 4 showed
higher yields of porphyrins and expanded porphyrins
as compared to 3 and 2 (table 1, entry 1–3). Other
solid acids such as Amberlyst-15, humic acid and
graphene oxide gave only corresponding porphyrin
or dipyrromethane and no higher homologues were
observed (table 1, entry 7, 8, 9). Reaction conditions of
this cyclo-oligomerization were then optimized using
different solvents and temperature. Among the solvents
listed in table 2, o-dichlorobenzene gave very high
3.3 One-pot synthesis of porphyrins and expanded
porphyrins
Firstly, we tried to mimic the Rothemund–Lindsey
protocol involving reaction of pyrrole and aldehyde
6a (reactant concentration 10 mM) in dichloromethane
which gave corresponding porphyrinogen in presence
of sulfonated graphenes and on oxidation with DDQ
gave the corresponding tetraarylporphyrin (8a) while
the same reaction when carried out with higher reactant
Scheme 2. Sulfonation of graphite using diazonium salt based ionic liquid.

1732
Sweta Mishra et al.
Reactant Conc.
67mM
8
9
Ar-CHO
catalyst/DDQ
5
6
Ar = a) C6F5
b) 2,6-Cl2C6H3
c) 2,4,6-Me2C6H2
d) C6H5
10
11
Scheme 3. Synthesis of porphyrinoids in presence of sulfonated graphene 4 in different reaction conditions.
Table 1. Reaction of pyrrole and aldehyde (6a) in dichloromethane using various acids [a]
Entry
Catalyst
Product Distribution (% Yield) [b]
1
2
3
4
5
6
7
8
9
4
3
2
8a (20, 12 [c]), 9a (8, 1 [c]), 10a (14, 9 [c]), 11a (1)
8a (10), 9a (3), 10a (12), 11a (traces)
8a (10), 9a (1), 10a (12), 11a (traces)
8a (12), 9a (15), 10a (18), 11a (5) + higher homologue
8a (14), 9a (10), 10a (20), 11a (4) + higher homologue
8a (10), 9a (8),10a (15),11a (1)
8a (20), 9a (1), 10a (5)
BF3.OEt2
Methane sulfonic acid
p−toluene sulfonic acid
Amberlyst-15
Humic acid
GO
7a
7a
[a] Reaction was conducted under inert atmosphere, reaction time: 2 h, rt. [b] 8: porphyrin, 9:
N-fused pentaphyrin, 10: hexaphyrin, 11: octaphyrin, Isolated yields. [c] Yield after five runs.
Table 2. Reaction of pyrrole and aldehyde (6a) in different solvents using sulfonated graphene
4 [a]
Entry
Solvent
Product Distribution (% Yield) [b],[c]
1
2
CH2Cl2
CHCl3
8a (20), 9a (8), 10a (14), 11a (1)
8a (18), 9a (5), 10a (15), 11a (1)
3
4
5
6
o-Dichlorobenzene
1,2,4-Trichlorobenzene
DMF
NMP
THF
MeOH
No Solvent
8a (10), 9a (15), 10a (25), 11a (5)
8a (5), 9a (8), 10a (10), 11a (traces)
−
−
7
−
8
−
9 [d]
8a (8), 10a (5), 12a (6) and other products [e]
[a] Reaction was conducted under inert atmosphere, reaction time: 2 h, rt. [b] 8: porphyrin, 9: N-
fused pentaphyrin, 10: hexaphyrin, 11: octaphyrin, 12: corrole. [c] Isolated yields. [d] Reaction time:
1 h, 80 oC. [e] open chain compounds and polymer.
yields of expanded porphyrins, N-fused pentaphyrin providing high surface area while expanded porphyrins
(9a), hexaphyrin (10a) and octaphyrin (11a) in 15, 25, were not observed in basic and polar solvents (DMF,
5% yield, due to effective dispersion of catalyst and thus NMP, THF).

Synthesis of porphyrinoids by sulfonated graphene
1733
Table 3. Reaction of pyrrole and different aryl aldehyde
in dichloromethane using catalyst 4 [a]
3.4 Formation of symmetrical meso-substituted
porphyrins, expanded porphyrins and mixed
porphyrins
Entry
Aldehyde
Product Distribution (% Yield) [b],[c]
Reaction of dipyrromethane 7a (figure 1) with 6a
afforded porphyrin and expanded porphyrins selectively
with only even number of pyrrole units in higher
yields at room temperature itself (scheme 4) while
with other acids such as methane sulfonic acid and p-
toluenesulfonic acid such selectivity is observed only at
0◦C (table 4).27
1
2
3
4
6a
6b
6c
6d
8a (20), 9a (8), 10a (14), 11a (1)
8b (15), 9b (traces), 10b (5)
8c (11)
8d (8)
[a] Reaction was conducted under inert atmosphere, reaction
time: 2 h, rt. [b] 8: porphyrin, 9: N-fused pentaphyrin, 10:
hexaphyrin, 11: octaphyrin. [c] Isolated yields.
A high yield of meso-pentafluorophenyl substituted
hexaphyrin was obtained when catalyst loading (4) was
7.5 wt% (figure 2). The catalyst 4 was reusable up to
five runs after which yields were decreased to almost
half (figure 3). Raman spectrum of recovered catalyst 4
showed red shifting in the D and G bands with increased
ID/IG ratio, which was attributed to the presence of car-
bonaceous byproducts such as polymeric materials on
the surface of the catalyst (figures S19, S20), hence
resulting in decreased reactivity.26 A higher catalytic
turnover number (TON = 133) and non-toxicity, proves
4 to be a better catalyst.
Reaction of pyrrole with different aryl aldehydes
were examined under similar conditions. The best
result was obtained with pentafluorobenzaldehyde, due
to the stabilities of both intermediate expanded por-
phyrinogens on acidolysis and expanded porphyrins
on oxidation.12 Hence, formation of expanded por-
phyrins occurs only with electron-deficient aryl aldehy-
des (table 3).16
Ar
3.5 Non-covalent interaction of porphyrinoids
with pristine graphene in organic solvents
GO
Ar-CHO
N
H
5
HN
NH
The absorption spectroscopy and fluorescence is
an important technique to study the non-covalent
interactions of porphyrinoids with planar organic
molecules.28–30 Porphyrins are known to interact with
various carbon materials, such as graphite, fullerenes
and carbon nanotubes (CNTs), through π-stacking
between their electron–abundant aromatic cores and
6
7
Ar = a) C6F5
b) 2,6-Cl2C6H3
c) 2,4,6-Me2C6H2
d) C6H5
Figure 1. Synthesis of dipyrromethane using solid
graphite oxide.
Reactant Conc.
67mM
8
Ar-CHO
10
catalyst/DDQ
6
Ar = a) C6F5
b) 2,6-Cl2C6H3
c) 2,4,6-Me2C6H2
d) C6H5
11
Scheme 4. Synthesis of porphyrinoids in the presence of sulfonated graphene 4 using dipyrromethane.

1734
Sweta Mishra et al.
Table 4. Reaction of 5-aryl dipyrromethane and different aryl aldehyde in o-dichlorobenzene
using catalyst 4 [a]
Entry
Dipyrromethane
Aldehyde
Product Distribution (% Yield) [b],[c]
1
2
3
7a
7b
7a
6a
6b
6b
8a (16), 10a (28), 11a (10)
8b (19), 10b (10), 11b (traces)
8ab (4), 10ab (6)
[a] Reaction was conducted under inert atmosphere, reaction time: 2 h, rt. [b] 8: porphyrin, 10:
hexaphyrin, 11: octaphyrin. [c] Isolated yields.
Figure 2. Effect of catalyst loading (sulfonated graphene
4) on the yield on hexaphyrin as listed in Table 1.
Figure 4. Absorption spectral changes of [28]hexaphyrin
10a on addition of graphene in o-DCB.
(o-DCB), N-methylpyrrolidine (NMP) and 1,2,4-
trichlorobenzene, is responsible for graphite exfoliation
and stabilization of individual sheets of graphene.33
3.5a Absorption spectra measurements (table 5):
The UV-Visible spectra of aromatic porphyrins with
the addition of graphene, show a minor decrease in
intensity with no new absorption bands indicating
very small ground state interaction (figure S21).34 28π
hexaphyrin (anti-aromatic) 10a, showed red shifted
soret bands from 598 to 607 nm with increased inten-
sity in o-DCB with addition of graphene (figure 4).
This red shifting indicated J-type aggregate of anti-
aromatic hexaphyrin with graphene. This type of
result was also observed with anti-aromatic pentaphyrin
(figure 5). Such type of non-covalent interaction may
be used in the development of sensors, detection of
heavy metals and other newer materials. The non-
Figure 3. Yield of hexaphyrin from recycling runs of sul-
fonated graphene 4 as listed in Table 1.
conjugated surfaces of the carbon materials.31,32 covalent interaction of porphyrinoids with graphene
The energy of non-covalent adsorption of solvents sheets resulted into the formation of supramolecular
such as dimethylformamide (DMF), o-dichlorobenzene dyads.35

Synthesis of porphyrinoids by sulfonated graphene
1735
Table 5. Absorption Spectral measurements of various porphyrinoids with pristine graphene.
Absorption peak
(without addition)
Absorption peak
(after addition)
Entry
porphyrinoid
solvent
1
2
3
Porphyrin 8a (Fig S21)
N-fused Pentaphyrin 9a (Fig 5)
[26]Hexaphyrin 10a (Fig 4)
o-DCB
o-DCB
o-DCB
NMP
o-DCB
o-DCB
418
462
572
605
572
598
418 (minor difference in intensity)
465 (increase in intensity)
573
601 (increase in intensity)
4
4
[26]Hexaphyrin 10b (Fig S22)
[28]Hexaphyrin 10a (Fig 4)
577 (decrease in intensity + new peak at 605)
607 (increase in intensity)
Table 6. Fluorescence spectral measurements of various
porphyrinoids with pristine graphene.
Fluorescence
quenching
Entry
Porphyrinoid
Solvent
1
2
3
4
8a
8b
8c
8d
o-DCB
o-DCB
o-DCB
o-DCB
30%
30%
40%
50%
Figure 5. UV-Visible spectra of [24] N-fused pentaphyrin
9a (1 µM) on increasing addition of exfoliated graphene
(0.25mg. mL−1) in o-DCB.
Figure 7. Quenching of the steady-state fluores-
cence intensities of 5,10,15,20-tetra-(2’,3’,4’,5’,6’-
pentafluorophenyl)porphyrin 8a (1 µM) on increasing
addition of exfoliated graphene (0.25 mg. mL−1) in o-DCB,
λex = 418 nm.
porphyrins show remarkable changes on the addition of
pristine graphene solution in o-DCB. The intensity of
the fluorescence bands were quenched up to 50% on
addition of graphene in case of porphyrins with elec-
tron donating substituents at meso position (figure 6),
whereas those with electron withdrawing substituent
(figure 7, S23), 15–30% quenching was observed. The
Figure 6. Quenching of the steady-state fluorescence
intensities of 5,10,15,20-tetraphenylporphyrin 8d (1 µM) on
increasing addition of exfoliated graphene (0.25 mg. mL−1
)
in o-DCB, λex= 420 nm.
3.5b Fluorescence spectra measurements (table 6): quenching of fluorescence could be ascribed to the π–π
Unlike the absorption spectra, fluorescence spectra of interaction of porphyrin and graphene.

1736
Sweta Mishra et al.
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The high catalytic activity and turnover number in the
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The cross-condensation of dipyrromethane with alde-
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by C-C bond formation and oxidation reactions cat-
alyzed by sulfonated graphene. The use of heteroge-
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Supporting Information
Characterization of catalysts and synthesized com-
pounds using TEM, SEM, Raman, FTIR, powder
1
XRD, TGA, UV-Vis, H NMR, 13C NMR and ESI-
MS have been provided. Titration studies using UV-Vis
and steady state fluorescence spectroscopy have been
given. Figures S1–S23 and other results are reported in
Supplementary Information which is available at www.
ias.ac.in/chemsci.
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Acknowledgements
The authors gratefully acknowledge the Council of Sci-
entific and Industrial Research (CSIR), New Delhi, Uni-
versity Grants Commissions (UGC), New Delhi, for
financial assistance and USIC, University of Delhi, for
material characterization.
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