Facile synthesis, photophysical and electrochemical redox properties of octa- and tetracarboxamidophenylporphyrins and the first example of amido-imidol tautomerism in porphyrins

Article abstract of DOI:10.1016/j.dyepig.2016.10.053

5,10,15,20-tetrakis(4′-carboxamidophenyl)porphyrin (1) and 5,10,15,20-tetrakis(3′,5′-dicarboxamidophenyl)porphyrin (2) have been synthesized in excellent yields and characterized by various spectroscopic techniques and cyclic voltammetric studies. Notably, 1 and 2 exhibited amido-imidol tautomerism in DMSO-d₆. The imido tautomer ([sbnd]C(OH)[dbnd]NH) was stabilised in DMSO-d₆ at 293?K while the same was converted into amido form ([sbnd]CONH₂) at high temperature (418?K). This is a first example of amido-imidol tautomerism in porphyrins. The moderate electron withdrawing nature of imidol groups at meso-phenyl rings lead to 80–95?mV anodic shift in their first ring reduction potential whereas 50–110?mV anodic shift in first ring oxidation potential as compared to that of H₂TPP.

Full text of DOI:10.1016/j.dyepig.2016.10.053

Dyes and Pigments 139 (2017) 651e657
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Facile synthesis, photophysical and electrochemical redox properties
of octa- and tetracarboxamidophenylporphyrins and the first example
of amido-imidol tautomerism in porphyrins
Pinky Yadav, Muniappan Sankar*
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247667, India
a r t i c l e i n f o
a b s t r a c t
5,10,15,20-tetrakis(40-carboxamidophenyl)porphyrin (1) and 5,10,15,20-tetrakis(30,50-dicarboxamidophenyl)
porphyrin (2) have been synthesized in excellent yields and characterized by various spectroscopic tech-
niques and cyclic voltammetric studies. Notably, 1 and 2 exhibited amido-imidol tautomerism in DMSO-d6.
The imido tautomer (eC(OH)]NH) was stabilised in DMSO-d6 at 293 K while the same was converted into
amido form (eCONH2) at high temperature (418 K). This is a first example of amido-imidol tautomerism in
porphyrins. The moderate electron withdrawing nature of imidol groups at meso-phenyl rings lead to 80
e95 mV anodic shift in their first ring reduction potential whereas 50e110 mV anodic shift in first ring
oxidation potential as compared to that of H2TPP.
Article history:
Received 8 September 2016
Received in revised form
12 October 2016
Accepted 12 October 2016
Available online 30 December 2016
Keywords:
Amide porphyrins
Amido-imidol tautomerism
Fluorescence
© 2016 Elsevier Ltd. All rights reserved.
Electrochemistry
1. Introduction
most interesting non-covalent interactions as it showed real
promises in different fields such as catalysis, medicinal chemistry,
Porphyrin, 18-
p
aromatic macrocycle is a most significant
and molecular recognition [8,25,26]. Hydrogen bonding is a spe-
cific, directional, reversible and strong noncovalent bonding in-
teractions and a useful approach in supramolecular assembly.
Usually, functional groups having hydrogen atom(s) such as
eCOOH, eOH, eNH2, eCONH2 and eNHCONH2 located at the pe-
riphery of organic molecules, are commonly used in the construc-
tion of supramolecular assemblies. This allows their effective
interaction with an electron-rich atom such as N, O and F that acts
as a Lewis base. In recent years, chemists have paid much interest
towards crystal engineering of supramolecular self-assemblies of
tetraarylporphyrins by hydrogen bonding and coordination, or
tessellated by external metal ion or organic ligand bridging auxil-
iaries [27e32]. In recent years, meso-carboxyphenylporphyrins and
meso-pyridylporphyrins were turned out as a useful basic unit for
the construction of multiporphyrin arrays, metal organic frame-
work (MOF) and supramolecular capsules. The pyridyl porphyrins
exhibit a high tendency to react through their peripheral N-sites
with a variety of transition metal ion connectors and yielded hybrid
organic-inorganic coordination networks with varying topology
and porosity. The peripheral eCOOH and pyridyl group exhibit a
high tendency to construct self-assemblies, yielding multi-
porphyrin networks and frameworks in crystalline solids of varying
topology and dimensionality. Metal insertion into the porphyrin
core allows another dimension to the binding ability of this ligand
pigment found in nature, having four pyrrole units joined through
methine carbons. The four pyrrole nitrogens are positioned at the
four corners of the square and are ideal for coordination to metal
ions. Porphyrin has been successfully utilized across a wide range of
research disciplines due to their numerous profitable properties
such as attractive absorption and emission properties, strong
aromaticity and rich metal coordination chemistry [1e5]. Porphy-
rins and their metal complexes were used in catalysis [6e10], dye-
sensitized solar cells (DSSC) [11e14], photodynamic therapy (PDT)
[15,16], molecular sensors [17,18], nonlinear optics (NLO) [19e21],
and the construction of multiporphyrinic arrays [22e24] due to
their prominent absorption in the UVevis region and interesting
physicochemical properties. The construction of supramolecular
architectures from small organic or metal-organic compounds via
non-covalent interactions, such as hydrogen bonding and metal
coordination is highly useful in shape-selective catalysis, enantio-
meric separation and molecular storage. In recent years, the
development of new types of intermolecular interactions has been
grown tremendously. In particular, hydrogen bonding is one of the
* Corresponding author.
E-mail address: sankafcy@iitr.ac.in (M. Sankar).
http://dx.doi.org/10.1016/j.dyepig.2016.10.053
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

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P. Yadav, M. Sankar / Dyes and Pigments 139 (2017) 651e657
along the equatorial and axial directions [32]. Notably, benzanilide
exhibited temperature and solvent dependent amido-imidol
tautomerism [33,34]. To the best of our knowledge, such type of
tautomerism is not known in porphyrin chemistry. Keeping this in
mind, we aim to synthesize carboxamido substituted porphyrins (1
and 2, Chart 1) which have shown the interesting amido-imidol
tautomerism and also one can construct porphyrin-based supra-
molecular assemblies. Herein, we demonstrate the facile synthesis,
photophysical and electrochemical redox properties and tempera-
ture dependent amido-imidol tautomerism of tetra- and octaamide
substituted porphyrins (1e2).
2. Experimental section
2.1. Chemicals
All required chemicals were purchased from Alfa Aesar. Tetra/
octa-carboxyphenylporphyrins were synthesized by modified
literature methods [35,36].
2.2. Instrumentation
UVeVisible and fluorescence spectra were recorded using Cary
CONH2
H2NOC
CONH2
CONH2
CONH2
H2NOC
H2NOC
NH
N
N
NH
N
N
CONH2
H2NOC
HN
HN
H2NOC
CONH2
CONH2
1
2
Chart 1. Molecular structures of synthesized porphyrins (1e2).
CONH2
COOH
NH
N
NH
N
1. SOCl2
CONH2
H2NOC
COOH
HOOC
2. NH3, H2O
N
HN
HN
N
CONH2
1
COOH
H2NOC
CONH2
HOOC
COOH
CONH2
H2NOC
COOH
HOOC
NH
N
N
NH
N
N
1. SOCl2
2. NH3, H2O
HN
HN
CONH2
H2NOC
COOH
HOOC
H2NOC
CONH2
HOOC
COOH
2
Scheme 1. Synthesis of tetra/octa-carboxamidophenylporphyrins (1e2) from their corresponding carboxyphenylporphyrins.
Table 1
Electronic spectral and photophysical data of 1 and 2 (lmax, nm (ε ꢀ 10ꢁ3, L molꢁ1 cmꢁ1) in DMSO at 298 K.
Por
labsorption, nm (ε ꢀ 10ꢁ3 L molꢁ1 cmꢁ1
)
lem (nm)
ɸf
t
(ns)
kr (106 sꢁ1
)
knr(107 sꢁ1
)
H2TPP
1
2
417(513), 515(23.9), 549(5.01), 590(1.66), 647(1.58)
419(123), 515(5.0), 550(2.0), 590(1.11), 645(0.80)
421(303), 516(11.0), 550(2.3), 590(1.50), 645(0.52)
654, 718
654, 718
653, 718
0.110
0.089
0.069
11.4 (91%)
11.7 (96%)
11.4 (93%)
9.65
7.61
6.05
7.81
7.79
8.17
ɸf ¼ quantum yield (reference, H2TPP in toluene 0.11);
t
¼ fluorescence lifetime; kr ¼ radiative rate constant; knr ¼ non-radiative rate constant.

P. Yadav, M. Sankar / Dyes and Pigments 139 (2017) 651e657
653
100 spectrophotometer and Hitachi F-4600 spectrofluorometer,
respectively. All 1H NMR measurements were performed using
JEOL-400 MHz spectrometer in DMSO-d6. Electrochemical mea-
surements were carried out using CHI-620E electrochemical
workstation. A three electrode system made up of Pt-button
working electrode, Ag/AgCl reference electrode and a Pt-wire
counter electrode was used. The porphyrin concentration was
maintained approximately 1 mM throughout the experiment. All
measurements were performed in DMF which was purged with Ar
gas, using 0.1 M TBAPF6 as the supporting electrolyte.
2.3. Synthesis of tetra- and octaacid substituted porphyrins
Tetramethylester (0.593 g, 0.70 mmol) or octaethylester
porphyrin (0.834 g, 0.70 mmol) was dissolved in 100 mL THF in a
two neck round bottom flask. To this, 10 mL of aq. solution of KOH
(300 equivalents) was added and refluxed for 16 h at 75 ꢂC. After
completion of the reaction, THF was removed using rotary evapo-
rator and the resulting residue was treated with 2N HCl (150 mL)
which resulted green precipitate, filtered and washed with water
Fig. 1. Optical absorption spectra of 1 and 2 in DMSO at 298 K.
Fig. 2. Variable temperature 1H NMR spectra of 1 in DMSO-d6 from 293 K to 418 K.

654
P. Yadav, M. Sankar / Dyes and Pigments 139 (2017) 651e657
(50 mL ꢀ 5). The protonated porphyrin was neutralized with pyr-
idine (15 mL). Then, the pyridine was removed by rotary evapora-
tion and the resulting residue was washed with water (50 mL ꢀ 3)
and dried under vacuum. The crude porphyrin was recrystallized
from CHCl3 and acetone mixture (3:7, v/v). The yield was found to
be 0.54 g (98%) for the corresponding tetraacid and 0.680 g (100%)
for octaacid.
1: UV/Vis (DMSO): lmax (nm) (ε ꢀ 10ꢁ3 L molꢁ1 cmꢁ1) 417(513),
515(23.9), 549(5.01), 590(1.66), 647(1.58). 1H NMR in DMSO-d6:
d
(ppm) 8.82 (s, 8H, b-pyrrole-H), 8.33 (s, 4H, OeH of imidol), 8.28
(s, 16H, meso-phenyl-H), 7.63 (s, 4H, NeH of imidol), ꢁ2.98 (s, 2H,
NeH). ESI-MS (m/z): found 787.837 [MþH]þ, calcd. 787.843. Anal.
calcd. for C48H34N8O4: C, 73.27; H, 4.36; N, 14.24%. Found: C, 73.40;
H, 4.58; N, 14.17%.
2: UV/Vis (DMSO): lmax (nm) (ε ꢀ 10ꢁ3 L molꢁ1 cmꢁ1) 421(303),
516(11.0), 550(2.3), 590(1.50), 645(0.52). 1H NMR in DMSO-d6:
2.4. Synthesis of tetra- and octaamide porphyrins (1 and 2)
d
(ppm) 8.91e8.86 (m, 20H, meso-phenyl-H and b-pyrrole-H), 8.32
(s, 8H, OeH of imidol), 7.66 (s, 8H, NeH of imidol), ꢁ2.89 (s, 2H,
NH). ESI-MS (m/z): found 959.818 [MþH]þ, calcd. 959.943. Anal.
calcd. for C52H39N12O8.5: C, 64.52; H, 4.06; N, 17.36%. Found: C,
64.59; H, 4.17; N, 17.45%.
Tetraacid (0.35 g, 0.443 mmol) or octaacid (0.428 g, 0.443 mmol)
was taken in a 50 mL RB flask. To this, distilled SOCl2 (2 mL,
0.0274 mol) was added dropwise and refluxed for 1 h. Then excess
amount of SOCl2 was removed by vacuum distillation. To this, cold
NH4OH (6 mL) was added slowly till the precipitate was formed.
The precipitate was filtered and washed with water (50 mL ꢀ 3).
The crude porphyrins were recrystallized from CH3OH and acetone
mixture (2:8, v/v). Yield was found to be 98% (0.34 g, 0.432 mmol)
for 1 and 97% (0.411 g, 0.428 mmol) for 2.
3. Results and discussion
Tetra- and octa-carboxyphenylporphyrins were synthesized in
quantitative yields and characterized by various spectroscopic
Fig. 3. Variable temperature 1H NMR spectra of 2 in DMSO-d6 from 293 K to 418 K.

P. Yadav, M. Sankar / Dyes and Pigments 139 (2017) 651e657
655
techniques [35,36]. The carboxy substituted porphyrins were
treated with SOCl2 in order to get the corresponding acid chlorides
which reacted with aqueous ammonia to yield amide porphyrins
(1e2) as shown in Scheme 1. The newly synthesized porphyrins
were characterized by UVevis, fluorescence and 1H NMR spectro-
scopic techniques, mass spectrometry and electrochemical studies.
The optical absorption spectra of 1 and 2 were recorded in
DMSO at 298 K. Table 1 lists the absorption spectral data of these
porphyrins in DMSO. 1 and 2 exhibited a marginal red-shift in Soret
band (B band) at 419 and 421 nm, respectively as compared to
H2TPP (417 nm) whereas Q-bands weren't much influenced (Fig. 1).
The marginal shift in optical absorption spectra of 1e2 with respect
to H2TPP is possibly due to the large distance of amide groups from
the porphyrin aromatic core. There is no shift in the emission
spectra of 1 and 2 as compared to H2TPP as shown in Fig. S1 in the
supporting information (SI). The emission intensity of 1 and 2 is
considerably decreased with respect to H2TPP (Fig. S1 in the SI)
while maintaining the same concentration for all leading to lower
quantum yields (Table 1) and follows the order: H2TPP > 1 > 2. The
emission spectral data, quantum yields, radiative rate constants (kr)
and non-radiative rate constants (knr) of 1 and 2 are listed in Table 1.
The fluorescence quantum yields and radiative rate constants of 1
and 2 were decreased as compared to H2TPP whereas the increase
in non-radiative rate constants suggest a weak intramolecular
charge transfer interaction between the porphyrin core and meso-
phenyl groups containing imidol moieties.
disappeared which represents the coalescence temperature. Finally
at 418 K, a broad singlet at 7.45 ppm with 16 protons intensity was
appeared which corresponds to amido (eCONH2) functionality. On
the other hand, a multiplet (8.91e8.86 ppm range) at 293 K which
corresponds to o,p-protons of meso-phenyl rings and b-protons was
boarded at 418 K without shifting its peak position.
Over all, 1H NMR studies clearly suggest that 1 and 2 exhibit
imidol form at room temperature and amido form at high tem-
perature. To the best of our knowledge, this is a first example which
shows temperature dependent amido-imidol tautomerism in
porphyrins.
3.2. Electrochemistry
To probe the influence of imidol moieties on porphyrin
p-sys-
tem, we have carried out the cyclic voltammetric studies of these
porphyrins in DMF at 298 K. The cyclic voltammograms of H2TPP, 1
and 2 were shown in Fig. 4. Table 2 lists the comparative electro-
chemical redox potentials (vs. Ag/AgCl) of imidol porphyrins in
DMF. These porphyrins exhibited one irreversible oxidation and
two reversible reductions. The moderate electron withdrawing
nature of imidol groups is reflected in cyclic voltammetric studies.
For example, the first ring oxidation of 2 is 60 mV anodically shifted
3.1. Variable temperature (VT) NMR studies
The synthesised porphyrin derivatives (1e2) were characterised
by 1H NMR spectroscopy in DMSO-d6 at 298 K. The 1H NMR spectra
of 1 and 2 were presented in Figs. S2 and S3 in the SI.
1 exhibited a singlet of
b-pyrrole protons at 8.82 ppm and
another singlet corresponds to meso-phenyl protons were observed
at 8.28 ppm in DMSO-d6 at room temperature as shown in Fig. S2 in
the SI. Two broad singlets of imidol eOH and eNH protons
observed with four protons intensity each at 8.33 and 7.63 ppm,
respectively. This observation confirms that 1 exists in imidol form
in DMSO at 298 K. In case of 2, a multiplet was observed in the
range of 8.91e8.86 ppm which corresponds to o,p-protons of meso-
phenyl rings and b-protons as shown in Fig. S3 in the SI. Further, the
observed two broad singlets at 8.32 and 7.66 ppm correspond to
imidol eOH and eNH protons (Fig. S3 in SI), respectively. This also
further confirms that 2 also exists in imidol form at room tem-
perature. The variable temperature (VT) 1H NMR spectra of 1 and 2
in DMSO-d6 are shown in Figs. 2 and 3, respectively. While
increasing the temperature from 293 K to 418 K, eOH protons
(signal b as indicated in Fig. 2) disappears and eNH (signal a) starts
to shift. The observed coalescence temperature was at 353 K. While
increasing from 353 K, a new signal starts appearing at 7.54 ppm
while no signal was observed for eOH and eNH. Notably at 418 K, a
broad singlet at 7.42 ppm with eight protons intensity corresponds
to amide (CONH2) functionality indicating that 1 exists in amido
form at high temperature. On the other hand, there is no shift in the
Fig. 4. Cyclic voltammograms of 1 and 2 in DMF containing 0.1 M TBAPF6 using Ag/
AgCl as reference electrode with a scan rate of 0.1 V/s at 298 K.
position of
b-pyrrole protons (8.85 ppm) whereas the signal
sharpens at high temperatures. Interestingly, the singlet (AB type)
at 8.32 ppm (at 293 K) which corresponds to o,m-protons of meso-
phenyl ring was converted to AB quartet at 418 K without changing
its position. These results clearly indicate that 1 exists in imidol
form at room temperature and exists in amido form at higher
temperatures in polar solvents such as DMSO-d6. Fig. 3 represents
the VT NMR spectra of 2 in DMSO-d6. As observed for 1, two broad
singlets at 7.66 and 8.32 correspond to eNH and eOH of imidol
moiety at 293 K. While increasing from 293K to 353 K, the singlets
observed for eNH and eOH of imidol moiety were completely
Table 2
Electrochemical redox data (vs Ag/AgCl) of amide porphyrin derivatives in DMF
containing 0.1 M TBAPF6 at 298 K.
Por
Oxidation (V)a
I
Reduction (V)
I
DE1/2 (V)
II
H2TPP
1
2
1.17
1.22
1.28
ꢁ1.00
ꢁ0.93
ꢁ0.91
ꢁ1.47
ꢁ1.33
ꢁ1.34
2.18
2.15
2.19
a
Data obtained from DPV.

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P. Yadav, M. Sankar / Dyes and Pigments 139 (2017) 651e657
Fig. 5. DFT calculations of 1 and 2 using B3LYP functional, 6-31G basis set and IEFPCM polarized method in DMSO.
as compared to 1 which intern further 50 mV anodically shifted
with respect to H2TPP (Table 2) and follows the order:
2 > 1 > H2TPP.
porphyrin chemistry. Further, these porphyrins exhibited anodic
shift in their 1st oxidation (
¼ 50e110 mV) and reduction
D
E
½
(DE
¼ 80e95 mV) potentials as compared to H2TPP due to mod-
½
Imidol substitution at meso-phenyl rings lead to 80e95 mV
anodic shift in their first ring redox potentials as compared to
H2TPP. The positive shift in first ring redox potentials of these
porphyrins, is interpreted in terms of electron withdrawing nature
of imidol groups on meso-phenyl ring. As observed from 1st ring
reduction potentials, imidol substitution at para-position of phenyl
ring leads to more anodic shift as compared to di-meta-substitution
due to resonance (-R) and inductive (-I) effects of para-substituent
whereas meta-substituent can show only inductive effect (-I).
Further, the electron withdrawing nature of imidol substituents is
also reflected in the 1st ring oxidation process of 1 and 2.
erate electron withdrawing nature of imidol moieties. These por-
phyrins can be utilized for the construction of supramolecular
assemblies in their imidol form and the attempts are in progress.
Acknowledgments
We sincerely thank Council of Scientific and Industrial Research
(01(2694)/12/EMR-II), Science and Engineering Research Board (SB/
FT/CS-015/2012) and Board of Research in Nuclear Science (2012/
37C/61/BRNS) for financial support. PY thanks CSIR, India for senior
research fellowship.
Appendix A. Supplementary data
3.3. DFT studies
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.10.053.
To examine the influence of imidol substituents on porphyrin
macrocycle, the DFT calculations have been carried out using B3LYP
functional and 6-31G basis set in DMSO with default IEFPCM
polarized method. 2 exhibited a quasi-planar geometry with mean
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