Synthesis, spectral, and electrochemical studies of electronically tunable β-substituted porphyrins with mixed substituent pattern

Article abstract of DOI:10.1021/ic501259g

Two new families of porphyrins with mixed substituent pattern, viz. 2-nitro-12,13-disubstituted-meso-tetraphenylporphyrins (H₂TPP(NO₂)X₂, X = Ph, phenylethynyl (PE), 2-thienyl (Th), Br, and CN) and 2-nitro-7,8,12,13,17,18-hexasubstituted-meso-tetraphenylporphyrins (H₂TPP(NO₂)X₆, X = Br, Ph, PE, and Th), and their metal (Co(II), Ni(II), Cu(II), and Zn(II)) complexes have been synthesized and characterized. H₂TPP(NO₂)X₆ exhibited remarkable red shift in the Soret band (45-70 nm) and longest wavelength band, Q_x(0,0) (65-90 nm), as compared to H₂TPP(NO₂). Single-crystal structures of MTPP(NO₂)X₂ (M = Zn(II) and Ni(II); X = Br, Ph, and PE) showed quasi-planar conformation, whereas H₂TPP(NO₂)Th₂ and NiTPP(NO₂)Br₆ exhibited moderate and highly nonplanar saddle-shape conformations, respectively. Further, DFT fully optimized geometries of H₂TPP(NO₂)X₂ and H₂TPP(NO₂)X₆ revealed moderate and severe saddle-shape nonplanar conformations, respectively. The imino proton resonances of H₂TPP(NO₂)X₆ are significantly downfield shifted (Δδ = 1.10-1.80 ppm) relative to H₂TPP(NO₂). Mixed substituted highly nonplanar porphyrins exhibited higher protonation and deprotonation constants relative to H₂TPPX₈ (X = Cl and Br). The redox tunability was achieved by introducing electron donor (Ph and Th) and acceptor (PE, Br, and CN) groups on the MTPP(NO₂) backbone. The unusual variation in the spectral and electrochemical redox properties of mixed substituted porphyrins are interpreted in terms of both an inductive and a resonance interaction of substituents on the porphyrin π-system as well as nonplanarity of the macrocycle.

Full text of DOI:10.1021/ic501259g

Article
pubs.acs.org/IC
Synthesis, Spectral, and Electrochemical Studies of Electronically
Tunable β‑Substituted Porphyrins with Mixed Substituent Pattern
Ravi Kumar and Muniappan Sankar*
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee - 247667, India
*
S Supporting Information
ABSTRACT: Two new families of porphyrins with mixed
substituent pattern, viz. 2-nitro-12,13-disubstituted-meso-tetra-
phenylporphyrins (H TPP(NO )X , X = Ph, phenylethynyl
2
2
2
(
PE), 2-thienyl (Th), Br, and CN) and 2-nitro-7,8,12,13,17,18-
hexasubstituted-meso-tetraphenylporphyrins (H TPP(NO )X ,
2
2
6
X = Br, Ph, PE, and Th), and their metal (Co(II), Ni(II),
Cu(II), and Zn(II)) complexes have been synthesized and
characterized. H TPP(NO )X exhibited remarkable red shift
2
2
6
in the Soret band (45−70 nm) and longest wavelength band,
Q (0,0) (65−90 nm), as compared to H TPP(NO ). Single-
x
2
2
crystal structures of MTPP(NO )X (M = Zn(II) and Ni(II);
2
2
X = Br, Ph, and PE) showed quasi-planar conformation, whereas H TPP(NO )Th and NiTPP(NO )Br exhibited moderate
2
2
2
2
6
and highly nonplanar saddle-shape conformations, respectively. Further, DFT fully optimized geometries of H TPP(NO )X and
2
2
2
H TPP(NO )X revealed moderate and severe saddle-shape nonplanar conformations, respectively. The imino proton
2
2
6
resonances of H TPP(NO )X are significantly downfield shifted (Δδ = 1.10−1.80 ppm) relative to H TPP(NO ). Mixed
2
2
6
2
2
substituted highly nonplanar porphyrins exhibited higher protonation and deprotonation constants relative to H TPPX (X = Cl
2
8
and Br). The redox tunability was achieved by introducing electron donor (Ph and Th) and acceptor (PE, Br, and CN) groups
on the MTPP(NO ) backbone. The unusual variation in the spectral and electrochemical redox properties of mixed substituted
2
porphyrins are interpreted in terms of both an inductive and a resonance interaction of substituents on the porphyrin π-system as
well as nonplanarity of the macrocycle.
INTRODUCTION
nitro groups are versatile and can be converted into several
■
11
other functional groups. β-Nitroporphyrins exhibited interest-
Porphyrinoids play a vital role in biological systems, and their
synthesis arouses continuing interest in biological, material, and
1
2a
12b
ing catalytic, photophysical, and nonlinear optical proper-
ties.
12c
1
Further, β-bromosubstituted tetraarylporphyrins are
inorganic chemistry. The advantage of using porphyrin ligands
useful substrates for nucleophilic aromatic substitutions with
is that they are conformationally flexible and can adopt a range
13a
13b
13c
cyanides,
thiolates,
and benzaldoximate
or in palla-
of nonplanar conformations needed for a variety of biological
1
,2
dium(0)-mediated cross-coupling reactions with arylboronic
functions. Porphyrins and their metal complexes were used in
1
3d−f
13g
13h
13i
3
4
acids,
tributyltin reagents.
reactions of carbon−carbon bond formation is widely used for
alkynes, 1 alkenes, and organozinc or organo-
catalysis, dye-sensitized solar cells (DSSC), photodynamic
3j−n
5
6
Modification utilizing Pd-catalyzed
therapy (PDT), molecular sensors, nonlinear optical (NLO)
7
8
materials, and sorbents due to their outstanding properties,
such as high chemical and thermal stability, strong absorption
in the visible region, and flexible architectural modification to
tailor physicochemical and optoelectronic parameters. The β-
functionalization of meso-tetraphenylporphyrins is of great
interest since the electronic properties of the porphyrin
13,14
synthesis of new porphyrin systems of various architectures.
The meso-substituted push−pull porphyrins have been widely
explored due to their relative ease of synthesis, whereas studies
on β-pyrrole mixed substituted or push−pull porphyrins are
15
very much limited due to lack of synthetic methodologies.
9
Introduction of sterically demanding substituents at the
macrocycle can be altered by small changes in the substituents.
periphery leads to a significantly nonplanar macrocycle
Moreover, the substituents at the β positions of the porphyrins
exert much larger steric and electronic effects on the porphyrin
16
conformation. Perhalo substitution also affects the photo-
physical and electrochemical properties of the porphyrin π-
1e,f,10
π-system
than substituents at the meso-aryl positions. The
17
system extensively. An increase in the number of substituents
enhances the steric crowding and thus induces enhanced steric
repulsive interaction among the peripheral substituents leading
redox properties of the natural and synthetic porphyrins
depend upon various factors such as core structure, the type
and location of electron-donating or -withdrawing substituents,
the electronic nature and oxidation state of the central metal
10
ion, and the type and number of bound axial ligands. The β-
Received: June 2, 2014
nitration of the porphyrin ring is of special interest since the
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
1
8
to the nonplanarity of the macrocycle. On the other hand, the
two-photon absorption (TPA) cross section of push−pull
Zn(II) porphyrin with donor and acceptor groups at the β or
meso position of the porphyrin ring exhibited a much larger
boronic acid, affording H TPP(NO )Ph with 71% yield. In
2
2
2
general, β-substituted 2-thienyl porphyrins were prepared using
13j−n
Stille coupling reactions
by reacting bromoporphyrins with
expensive tributylthienylstannane. We synthesized the same
porphyrins via Suzuki coupling reaction using inexpensive 2-
thienylboronic acid with yields of 50−60%. H TPP(NO )Br
1
9
σTPA value than that of homo-substituted porphyrins.
Development of new materials with enhanced NLO responses
is of great scientific interest due to their potential applications
in optical limiting devices, optical communications, and optical
data processing. Asymmetrically substituted porphyrins con-
stitute a promising group of NLO compounds due to high7
nonlinearities upon irradiation with high-intensity laser light.
The main advantage of porphyrins is their chemical versatility,
which provides a tool for tailoring the linear and NLO
2
2
2
was subjected to Stille coupling reaction using tributylpheny-
lethynylstannane in order to get H TPP(NO )(PE) . NiTPP-
2
2
2
(NO )Br was also reacted with CuCN in quinoline to afford
2
2
NiTPP(NO )(CN)2 with 61% yield, which was further
2
demetalated using H SO to get H TPP(NO )(CN) .
2
4
2
2
2
The CuTPP(NO ) was hexabrominated using 10 equiv of
2
freshly recrystallized NBS by refluxing in 1,2-dichloroethane at
7
,20
21c
properties for specific applications. One field of potential use
of NLO active porphyrinic dyes is broad-band optical limiting
80 °C for 16 h with 80% yield. Then H TPP(NO )Br was
2
2
6
obtained by acid demetalation of CuTPP(NO )Br under mild
2 6
7
e,20
(
OL),
which aims for protection of human eyes and
conditions, which was subjected to Suzuki cross-coupling
sensitive optical elements against accidental exposure to high-
energy laser beams. Herein, we focused on synthesizing the
mixed substituted porphyrins in order to tune the electronic
spectral and electrochemical redox properties of the porphyrin
π-system. In general, the syntheses and properties of porphyrins
reactions to afford H TPP(NO )Ph and H TPP(NO )Th
2
2
6
2
2
6
with good yields (60−70%). H TPP(NO )(PE) was obtained
2
2
6
via Stille coupling in 55% yield using a similar procedure to that
of H TPP(NO )(PE) . Attempts to prepare MTPP(NO )-
2
2
2
2
(CN) failed by reacting NiTPP(NO )Br with CuCN and end
6
2
6
21
with mixed antipodal β-substituents are largely unexamined.
up with degraded products. Metal complexes (Co(II), Cu(II),
and Zn(II)) were prepared by reacting free base porphyrins
with corresponding metal acetate hydrates (10 equiv) in
refluxing CHCl /CH OH mixture for 30 min, whereas Ni(II)
With these objectives, we report the synthesis, structural,
physicochemical, and electrochemical redox properties of two
new families of porphyrins with mixed substituent pattern, viz.
3
3
2
-nitro-12,13-disubstituted-5,10,15,20-tetraphenylporphyrins
complexes were synthesized using nickel acetate in refluxing
22a
(
H TPP(NO )X , X = Ph, phenylethynyl (PE), 2-thienyl (Th),
DMF. Synthesized porphyrins were characterized by UV−
2
2
2
1
Br, and CN) and 2-nitro-7,8,12,13,17,18-hexasubstituted-
,10,15,20-tetraphenylporphyrins (H TPP(NO )X , X = Br,
vis, fluorescence, H NMR, and mass spectroscopic techniques,
5
elemental analysis, and single-crystal X-ray studies. Detailed
synthetic procedures are described in the Supporting
Information.
2
2
6
Ph, PE, and Th), and their metal (Co(II), Ni(II), Cu(II) and
Zn(II)) complexes (Chart 1). To the best of our knowledge,
Crystal Structure Discussion. Crystallographic data of
Chart 1. Molecular Structures of Porphyrins with Mixed
Substituent Pattern
NiTPP(NO )Ph (Py), ZnTPP(NO )(PE) (CH OH), ZnTPP-
2
2
2
2
3
(NO )Br (CH OH), NiTPP(NO )Br , and H TPP(NO )-
2
2
3
2
6
2
2
(
Th) ·CH OH·H O are listed in Table S1, Supporting
2 3 2
Information. ORTEP views (top and side) of H TPP(NO )-
2
2
(
Th) and NiTPP(NO )Br are shown in Figure 1. The top
2 2 6
and side ORTEP views for MTPP(NO )X (M = Ni(II) and
2
2
Zn(II); X = Ph, PE, and Br) are shown in Figure S1,
Supporting Information. Selected average bond lengths and
bond angles of these five porphyrins are listed in Table S2,
Supporting Information. The observed bond distances and
bond angles in these porphyrins are similar to those reported in
2
d,e,22b,c
the literature.
The nonplanarity of the porphyrin core is
induced by the steric repulsion among the peripheral
substituents, which enforces the relief of the strain through
bond lengths and angles. The C−C bond distances of the β-
pyrrole carbons (C −C bond lengths) bearing three β-
1
the protonation and deprotonation studies, H NMR spectral
features, and electrochemical redox properties of β-pyrrole tri-
and heptasubstituted meso-TPPs with mixed substituent pattern
and their metal complexes have not been studied so far.
β
β
substituents in these porphyrins are longer than that of C −
β′
C
distances where antipodal pyrroles bear no substitution
β′
(
Table S2, Supporting Information). One would anticipate
RESULTS AND DISCUSSION
varying degrees of nonplanar conformations for mixed
substituted porphyrins, since they bear substituents of different
sizes which results in an increase in C −C −C angles with
■
Synthesis and Characterization. Two new families of β-
pyrrole mixed substituted porphyrins, viz. β-tri- and heptasub-
stituted meso-tetraphenylporphyrins (MTPP(NO )X , n = 2,
), were synthesized and characterized, Scheme 1. 2-Nitro-
meso-TPPs are versatile precursors in porphyrin chemistry due
β
α
m
concomitant decrease in the N−C −C and M−N−C angles
2
n
α
m
α
6
(Table S2, Supporting Information). The pyrrole groups of
H TPP(NO )(Th) are tilted alternatively up and down from
2
2
2
to their excellent stability toward demetalation/metalation
the mean plane formed by the porphyrin macrocycle, and the
β-substituents (nitro and 2-thienyl groups) on the opposite
pyrrole units are tilted toward one face of the porphyrin. The 2-
thienyl groups are oriented trans to each other in order to
minimize the repulsions between the lone pairs. The displace-
ment of atoms from the mean plane (Δ24) clearly shows the
2
1c
sequences and can be prepared in high yields (70−80%).
The regioselective bromination of 2-nitro-meso-TPP afforded 2-
nitro-12,13-dibromo-meso-TPP (H TPP(NO )Br ), which was
2
2
2
further utilized for Pd-catalyzed coupling reactions. H TPP-
2
(
NO )Br was subjected to Suzuki coupling using phenyl-
2 2
B
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Inorganic Chemistry
Article
Scheme 1. Synthetic Routes for Various Mixed Substituted Porphyrins
saddle conformation of the core (Figure 1). This is evident
from the mean plane displacement of the β-pyrrole carbons
Ni(II) ion is in square planar geometry and about 0.035 Å
above the mean plane. The longer M−N bond length as
compared to M−N′ in MTPP(NO )X (Table S2, Supporting
(
ΔC ) and the meso-carbons from the mean plane (Figure 1c).
β
2
n
The magnitude of the displacement of the β-pyrrole carbons
ΔC ) of H TPP(NO )(Th) was found to be ±0.75 Å, which
Information) is possibly due to the electron-withdrawing effect
of peripheral β-substitutents. It is known from the literature
that ZnTPPBr , ZnTPP(Ph) Br , NiTPPBr (CN) , and
(
β
2
2
2
is significantly higher than that reported for H TPPBr (ΔC =
2
2
β
4
4
4
4
4
±
Å).
0.209 Å) but lower than that of H TPPBr (ΔC = ± 0.903
NiTPP(Ph) (CN) structures exhibited longer M−N distances
2
6
β
4
4
2
1c
Notably, the inner core imino protons (NH) are
due to the presence of the electron-withdrawing groups, which
21b,e
hydrogen bonded with pyrrole nitrogens, which are evidenced
from the distance between the shortest N···N of 2.908 Å. The
nitro group of H TPP(NO )(Th) is interacting with the
reduce the electron density on imino nitrogens.
The methyl
group of the axially coordinated CH OH in ZnTPP(NO )-
3
2
(PE) shows some positional disorder. It is noteworthy that the
2
2
2
2
pyrrole moiety of another porphyrin unit with a distance of
Zn(II) ion is about 0.295 Å above the mean plane, which is
higher than that of ZnTPP(NO )Br (CH OH) complex (0.237
3
.217 Å exhibiting donor−acceptor interactions. Further, the
2
2
3
oxygen of the NO group has van der Waals interactions with
Å). Notably, five-coordinated Ni(II) ion in NiTPP(NO )-
2
2
the aromatic CH of thienyl and phenyl groups (O···C distances
of 3.3−3.6 Å). The steric crowding between the β-substituents
and the meso-phenyl rings as well as the supramolecular
interactions are responsible for the nonplanar conformation of
the porphyrin macrocycle in H TPP(NO )(Th) . Notably,
Ph (Py) is 0.397 Å above the mean plane of the porphyrin core.
Moreover, the N−M−N and N′−M−N′ angles (161−170°)
are deviated from 180°, indicating the nonplanar conformation
2
of the M−(N) core. The magnitude of the displacement of the
4
β-pyrrole carbons (ΔC = ± 0.16 Å) in NiTPP(NO )Ph (Py)
2
2
2
β
2
2
NiTPP(NO )Br6 has a severe saddle-shape conformation
and ZnTPP(NO )Br (CH OH) is slightly higher than that of
2
2 2 3
(
(
Figure 1d and 1e) with displacement of the β-pyrrole carbons
ZnTPP(NO )(PE) (CH OH) (ΔC = ± 0.072 Å). Hence, the
2
2
3
β
ΔC = ± 1.10 Å). This is further supported by the increment
above-mentioned metalloporphyrins (MTPP(NO )X ) show
β
2 2
in C −C −C angles (∼127°) with concomitant decrement in
quasi-planar conformations. Moreover, ZnTPP(NO )-
β
α
m
2
the N−C −C (∼123°) and M−N−C (∼125°) angles along
X (CH OH) (X = Br, PE) has dimeric structure through the
α
m
α
2
3
with larger C −C and C −C bond lengths (1.34−1.35 Å).
hydrogen bonding interactions between the coordinated
β
β
β′
β′
C
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Inorganic Chemistry
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Figure 1. ORTEP diagrams showing top and side views of H TPP(NO )(Th) (a and b) and NiTPP(NO )Br (d and e), respectively. Solvates are
2
2
2
2
6
not shown for clarity, and in side view, the β-substituents and meso-phenyl groups are not shown for clarity. (c and f) Displacement of porphyrin-
core atoms (in Angstroms) from the mean plane for H TPP(NO )(Th) and NiTPP(NO )Br , respectively.
2
2
2
2
6
methanol with the nitro group of the another porphyrin units
with the shortest O···O distance of 2.874 (5) Å. The closest
distance between the metalloporphyrin units is more than 3.40
Å, indicating that the steric crowding between the β-
substituents and the meso-phenyl groups is responsible for the
nonplanar conformation rather than the π−π interactions
between the macrocyclic π-systems. From single-crystal
structure analysis, it is observed that H TPP(NO )Th and
bond lengths and bond angles in B3LYP/LANL2DZ-optimized
geometries of mixed substituted porphyrins are similar to those
2
d,e,22b,c
reported in the literature.
The nonplanar conformation
of free base porphyrins arises by the tilting of the pyrrole rings
to prevent repulsive interactions among the substituents, and
these result in the increment of bond lengths of C −C as well
β
β
as in the bond angles of C −C −C with a concomitant
β
α
m
decrease in the bond angles of N−C −C . Herein, H TPP-
2
2
2
α
m
2
ZnTPP(NO )Br6 exhibit moderate and severe nonplanar
(NO )X systems exhibited about a 2−4° change in these
2
2
2
saddle-shape conformations, respectively. Unfortunately, we
were unable to crystallize other free bases and metal complexes
of mixed substituted porphyrins, and hence, we carried out full
geometry optimization using DFT studies.
angles, and this change is comparable to that of the saddle-
21e,24
shape CuTPP(Ph) (CH ) and H TPPTh structures.
4
3 4
2
4
Notably, H TPP(NO )X (X = Ph, PE, Br, and Th)
2
2
6
exhibited highly nonplanar severe saddle-shape conformational
DFT Calculations. The ground state geometries of mixed
features in comparison to H TPP(NO )X . The C −C bond
2
2
2
β′
β′
substituted free base porphyrins (H TPP(NO )X and H TPP-
lengths (1.391−1.417 Å) of the pyrroles bearing four β-
2
2
6
2
(
NO )X ) were optimized in the gas phase by DFT calculations
substituents in H TPP(NO )X systems are longer than the
2
2
2
2
6
23
using the B3LYP functional and LANL2DZ basis set. Figures
and S2−S4, Supporting Information, represent the fully
C −C distances (1.375−1.388 Å) of the antipodal pyrroles
β
β
2
bearing three β-substituents (Table S4, Supporting Informa-
tion). The increment in the bond angles of C −C −C with
optimized geometries of these porphyrins (top and side views)
as well as the deviation of core atoms from the porphyrin mean
plane. Selected average bond lengths and bond angles of
H TPP(NO )X and H TPP(NO )X are listed in the Tables
β
α
m
the concomitant decrement in the bond angles of N−C −C in
α
m
H TPP(NO )X systems were found to be significantly large
2
2
6
(4−8°) in these angles which are comparable to that of
2
2
2
2
2
6
2
d,e
21c
S3 and S4, Supporting Information, respectively. Observed
H TPPX (X = Br, Cl, and Ph) and H TPPBr structures.
2 8 2 6
D
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Inorganic Chemistry
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Figure 2. B3LYP/LANL2DZ-optimized geometries showing top as well as side views of H TPP(NO )(Th) (a and b) and H TPP(NO )Br (d and
2
2
6
2
2
6
e), respectively. In side view, the β-substituents and meso-phenyl groups are not shown for clarity. (c and f) Displacement of porphyrin-core atoms
(
in Angstroms) from the mean plane for H TPP(NO )(Th) and H TPP(NO )Br , respectively. Color codes for atoms: C, black; N, blue; O, red; S,
2 2 6 2 2 6
yellow; Br, brown.
a
Table 1. Optical Absorption Spectral Data of Mixed Substituted Porphyrins
porphyrin
B band(s), nm
426(5.34)
Q bands, nm
H TPP(NO )
526(4.21), 604(3.62), 664(3.95)
536(4.08), 623(3.43), 683(3.91)
536(4.01), 686(3.80)
2
2
H TPP(NO )Br
2
436(5.23)
2
2
H TPP(NO )Ph
2
439(5.09)
2
2
H TPP(NO )(PE)
2
444(5.38)
540(4.25), 583(3.99), 687(3.93)
540(4.19), 695(4.01)
2
2
H TPP(NO )Th
2
440(5.24)
2
2
H TPP(NO )(CN)
2
440(5.47)
541(4.14), 585(4.00), 645(3.79), 702(4.30)
628(3.98), 739(3.80)
2
2
H TPP(NO )Br
6
371(4.39), 468(5.11)
377(4.44), 470(5.11)
494(5.22)
2
2
H TPP(NO ) Ph
6
571(3.83), 624(3.75), 729(3.90)
586(4.29), 666(sh), 751(3.43)
589(3.61), 644(3.75), 754(3.40)
2
2
H TPP(NO )(PE)
6
2
2
H TPP(NO )Th
6
325(4.65), 481(4.90)
2
2
aValues in parentheses refer to log ε (ε in mol−1 cm−1).
Among all, H TPP(NO )Th exhibited higher nonplanarity
the β-pyrrole carbons (ΔC ) and the meso-carbons from the
2
2
6
β
(
ΔC = ±1.240 Å) as compared to all other porphyrins. The
mean plane follows the order H TPP(NO )(CN) < H TPP-
β
2
2
2
2
top and side views of H TPP(NO )Th are shown in Figure 2a
(NO )(PE)2 < H TPP(NO )Br ≈ H TPP(NO )Ph
2 2 2 2 2 6 2 2 6
<
2
2
6
2
2
2
2
2
2
2
and 2b, respectively. Figure 2c represents the displacement of
porphyrin-core atoms from the mean plane in Angstroms,
indicating the severe saddle-shape conformation. The 2-thienyl
groups substituted at the β-pyrrole carbons are oriented trans
to each other in order to minimize the lone pair repulsions as
seen in the crystal structures. The mean plane displacement of
H TPP(NO )(Th) < H TPP(NO )(PE) < H TPP(NO )Br
≈ H TPP(NO )Ph < H TPP(NO )(Th) , indicating the
2
2
6
2
2
6
varying degrees of nonplanarity in these mixed substituted
porphyrins. The electronic effects of the substituents and the
influence of nonplanarity (as evidenced from single-crystal X-
ray structures and DFT calculations) on the porphyrin π-
E
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Inorganic Chemistry
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Figure 3. Electronic absorption spectra of (a) H TPP(NO )Ph (n = 0, 2, and 6) and (b) NiTPP(NO )X (X = H, CN, PE, Th, and Ph) in CH Cl
2
2
2
n
2
2
2
at 298 K. Porphyrin concentration was maintained between 7 and 10 μM.
Figure 4. (a) Overlayed optical absorption spectra of CuTPP(NO ) (8.87 μM) and CuTPP(NO )(CN) (5.18 μM) in CH Cl at 298 K. (b)
2
2
2
2
2
Fluorescence spectra of H TPP(NO )X (X = Ph, PE, Th) in CH Cl at 298 K.
2
2
2
2
2
system are further examined by spectroscopic and electro-
chemical redox studies.
Q bands with molar extinction coefficients (ε) similar to those
27
of their corresponding MTPPs (D ).
4h
Electronic Spectral Studies. Optical absorption spectra of
porphyrins and their metal complexes are influenced by the
presence of peripheral substituents and core metal
H TPP(NO )X (X = Br, Ph, PE, and Th) exhibited large
2 2 6
red shifts in the Soret band (Δλ = 40−70 nm) and Q (0,0)
max
x
band (Δλ = 65−90 nm) relative to H TPP(NO ) due to the
max
2
2
1
,2,10−14,25
ions.
It is known that the nonplanar conformation
increase in the number of X, nonplanarity of the macrocycle,
and/or electronic nature of the substituents (Figure S6,
Supporting Information). The longest wavelength band
of the macrocycle induces an unusual red shift in their spectral
2
6,2g
properties.
Table 1 lists the absorption spectral data of
newly synthesized free base porphyrins in CH Cl at 298 K.
(Q (0,0)) of free-base porphyrins showed an interesting
2
2
x
Representative absorption spectra of H TPP(NO )Ph (n = 0,
trend in red shift and aligns in the following order:
2
2
n
2
, and 6) and NiTPP(NO )X are shown in Figure 3a and 3b,
H TPP(NO ) < H TPP(NO )Br < H TPP(NO )Ph
<
2
2
2
2
2
2
2
2
2
2
respectively. All free base derivatives (H TPP(NO )X , where n
H TPP(NO )(PE) < H TPP(NO )(Th) < H TPP(NO )-
2 2 2 2 2 2 2 2
2
2
n
=
0, 2, 6) exhibited a characteristic Soret band (B band) and
(CN) < H TPP(NO )Ph < H TPP(NO )Br < H TPP-
2 2 2 6 2 2 6 2
three Q bands, whereas H TPP shows one B band and four Q
(NO )(PE) < H TPP(NO )(Th) . The observed red shift of
2
2 6 2 2 6
bands. H TPP(NO ) exhibited red shifts in the Soret band
the Q (0,0) band is in accordance with the increment in the
2
2
x
(
Δλ = 9 nm) and Q (0,0) band (Δλ = 17 nm) relative to
number of substituents (H TPP(NO ) < H TPP(NO )X <
max
x
max
2
2
2
2
2
H TPP. H TPP(NO )X (X = Br, Ph, PE, Th, and CN)
H TPP(NO )X ) and the extent of nonplanarity of the
2
2
2
2
2 2 6
21c
showed further red shifts in the B band (Δλ = 10−20 nm)
porphyrin π-system as evidenced from the crystal structures
max
and Q (0,0) band (Δλ = 20−30 nm) relative to H TPP-
and DFT calculations. Further, among the same series
(H TPP(NO )X or H TPP(NO )X ), the electron-withdraw-
x
max
2
(
NO ) (Figure S5, Supporting Information). This is possibly
2
2
2
2
2
2
6
due to the inductive and/or conjugative interaction of the
substituents with the π-system as well as the nonplanarity of the
porphyrin macrocycle as evidenced from the crystal structure of
H TPP(NO )Th and the optimized geometries of H TPP-
ing and/or conjugatively resonating substituents exhibited a
large bathochromic shift. Further, mixed substituted H TPP-
2
(NO )(PE) showed a dramatic red shift in the B band (68
2
6
nm) and Q (0,0) band (87 nm) relative to H TPP(NO ). The
2
2
2
2
x
2
2
(
NO )X (vide supra). Table S5, Supporting Information, lists
spectral features of H TPP(NO )X with mixed substituent
2
2
2 2 6
28
the UV−vis spectral data of all metal complexes. Although the
metal complexes, MTPP(NO )X (M = Co(II), Ni(II), Cu(II),
and Zn(II)) bear lower symmetry, they exhibited a B and two
pattern are similar to those of β-octabromo- or β-octaphenyl-
29
TPPs. The red shift in the highly substituted porphyrins has
been ascribed to the nonplanarity of the porphyrin ring in
2
2
F
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1
Figure 5. Representative H NMR spectra of the imino proton region of (a) H TPP(NO )X (X = Br, PE, Ph, and Th) and (b) H TPP(NO )X (X
2
2
6
2
2
2
=
Br, PE, Ph, Th, and CN) in CDCl at 298 K.
3
combination with the inductive or conjugative interaction of
the substituents that are in direct conjugation with the π-
system.
of MTPP(NO )X in CH Cl at 298 K. H TPP(NO )X (X =
2 n 2 2 2 2 2
CN, Ph, PE, and Th) exhibited a more red-shifted emission
(75−110 nm) than that of H TPP in CH Cl . Notably, highly
2
2
2
Electronic absorption spectra of representative CuTPP-
red-shifted emission bands (Δλ = 160−170 nm) with feeble
max
(
NO )X (X = H, CN) are shown in Figure 4a. Introduction
fluorescence intensity were observed for H TPP(NO )X (X =
2
2
2
2
6
of two more electron-withdrawing groups on the other
PE and Th) in comparison to H TPP in CH Cl . The
2 2 2
opposite pyrrole positions of CuTPP(NO ) showed a bath-
representative emission spectra of MTPP(NO )X (M = 2H
2 n
2
ochromic shift in their spectral features with considerable
increment in molar absorptivity (ε). Generally, metal
complexes, MTPP(NO )X , exhibited a B band and two Q
and Zn(II); n = 0, 2, and 6, X = Th, Ph, CN, and PE) are
shown in Figures S7−S9, Supporting Information. The free-
base porphyrins exhibited an interesting trend in the red shift of
their corresponding emission bands and aligns in the following
order: H TPP < H TPP(NO )(CN) < H TPP(NO )(PE) <
2
2
bands with the extent of red shift of the B band being about 7−
2
0 and 10−30 nm in Q (0,0) transitions relative to their
x
2
2
2
2
2
2
2
corresponding MTPP(NO ). MTPP(NO )(CN) complexes
H TPP(NO )Ph < H TPP(NO )Th < H TPP(NO )(PE)
2 2 2 2 2 2 2 2 6.
2
2
2
showed considerable gain in the intensity of the longest
wavelength Q (0,0) band relative to the Q (1,0) band (Figure
The increasing order of red shift and the decrement in the
quantum yields (Table S6, Supporting Information) are in
accordance with the increasing order of nonplanarity of the
porphyrin macrocycle as evidenced from X-ray crystallography
and DFT calculations. The same trend was also observed for
Zn(II) complexes as expected. Notably, H TPP(NO )X (X =
x
x
4
a), and this is possibly due to stabilization of a relative to a
1u 2u
of MTPP(NO )(CN) , thereby increasing the transition
2
2
30
probability of a2u to e (π*) relative to a to e (π*).
g
1u
g
MTPP(NO )X showed a dramatic red shift of 30−70 nm in
2
6
2
2
6
the Soret band and 35−100 nm in the Q (0,0) band as
Ph, Th, and Br) failed to show emission spectrum due to
enhanced nonplanarity which reduces the singlet excited state
x
compared MTPP(NO ). As expected (Table S5), the bands are
2
more blue shifted with the increase in electronegativity of the
lifetime in comparison to planar porphyrins as seen in
3
1
32a−c
metal ion. Generally, MTPP(NO )(Ph) derivatives show
H TPP(Et) and H TPP(Ph) .
Further, quenching of the
2
n
2
8
2
8
blue-shifted absorption spectral features relative to MTPP-
NO )(PE) or MTPP(NO )(CN) derivatives. This is
emission in H TPP(NO )Br and H TPP(NO )Br is ascribed
2 2 2 2 2 6
(
to the combination of nonplanarity and heavy atom effect of
bromo groups that are in direct conjugation with the porphyrin
π-system. Similar behavior was reported for β-brominated
2
n
2
n
possibly due to the variable steric crowding and the electronic
nature of the substituent that induces conformational differ-
ences between the macrocycles as well as the substituent effects.
A pronounced red shift in the absorption spectral features of
the mixed substituted cyano, phenylethynyl, and bromopor-
phyrins is due to push−pull, extended conjugation, and/or
nonplanar distortion of the porphyrin ring. The red shift
exhibited by nonplanar porphyrins is also supported by
porphyrins, for example, MTPPBr exhibited a weak red-shifted
4
emission, while MTPPBr failed to show a singlet emissio-
8
28a,32d
n.
1
H NMR Studies. H TPP(NO ) exhibits characteristic
2
2
chemical shifts arising from the β-pyrrole (9.10−8.71 ppm)
9
a
1
and meso-phenyl (8.29−7.70 ppm) proton resonances.
H
26
theoretical studies.
NMR spectra of free base mixed substituted porphyrins
The synthesized free base and Zn(II) complexes of mixed
substituted porphyrins were characterized by fluorescence
spectroscopy to elucidate the role of substitution and the
effect of nonplanarity. The steady state emission spectra of
H TPP(NO )X (X = Ph, PE, and Th) in CH Cl is shown in
(H TPP(NO )X ) (X = Br, Ph, PE, Th, and CN; n = 0, 2,
2
2
n
and 6) exhibit resonances arising from meso-phenyls, β-pyrrole
proton(s), β-substituents (Ph or Th or PE), and imino
hydrogens (Figures S10−S29, Supporting Information). The
chemical shifts of the meso-phenyl protons of H TPP(NO )X
2
2
2
2
2
2
2
n
Figure 4b. Table S6, Supporting Information, lists emission data
are comparable to those observed for the corresponding
G
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Inorganic Chemistry
Article
−6
Figure 6. UV−vis spectral titration of H TPP(NO )Br (9 × 10 M) with (a) TFA and (b) TBAOH in toluene; (insets) corresponding Hill plots.
2
2
6
H TPP(NO ). The β-pyrrole resonances of H TPP(NO )X
2
that metal ion got inserted into the porphyrin ring. The β-
proton resonances for NiTPP(NO )X are marginally upfield
2
2
2
2
(
0
X = Br, Ph, Th, and PE) are marginally upfield shifted (∼0.1−
2
n
.2 ppm) than that of H TPP(NO ), whereas H TPP(NO )-
shifted (0.03−0.11 ppm), whereas in the case of ZnTPP-
(NO )X they are marginally downfield shifted (0.02−0.26
ppm) in comparison to their corresponding H TPP(NO )X
derivatives. The meso-phenyl protons resonances of MTPP-
(NO )X
2
2
2
2
(
CN) exhibited a 0.25 ppm downfield shift due to the strong
2
n
2
electron-withdrawing nature of cyano groups. The β-pyrrole
proton of H TPP(NO )X (X = Br, Ph, PE, and Th) is 0.47−
2
2
n
2
2
6
0
.80 ppm upfield shifted than H TPP(NO ). The meso-phenyl
2
n
are marginally upfield (0.05−0.43 ppm) correspond-
2
2
ing to those of free base derivatives. The integrated intensities
of the proton resonances of these mixed substituted porphyrins
are consistent with the proposed structures.
protons of H TPP(NO )X (X = Br, PE, and Th) is marginally
2
2
6
shifted in comparison to H TPP(NO ), whereas H TPP-
2
2
2
(
NO )Ph is 0.25−0.41 ppm upfield shifted possibly due to the
2
6
Protonation and Deprotonation Studies. To examine
the effect of mixed substitution on nonplanarity, we carried out
protonation and deprotonation studies in toluene using
trifluoroacetic acid (TFA) and tetrabutylammonium hydroxide
ring current effect of β-pyrrole phenyl protons. H TPP(NO )-
2
2
Ph exhibited characteristic upfield shifted β-phenyl protons in
6
the range of 6.90−6.47 ppm, which is attributed to the ring
current effect of β-pyrrole phenyl protons, whereas β-
phenylethynyl protons resonate between 7.18 and 7.36 ppm,
indicating phenyl protons are distant from the ring current. The
core imino protons, β-pyrrole protons, and β-thienyl C-3
protons of H TPPTh (n = 2 and 6) resonating at the
(
TBAOH), respectively. Figure 6 shows the UV−vis spectral
changes of H TPP(NO )Br while increasing the concentration
of TFA (0.33−6.29 × 10 M) and TBAOH (0.33−6.62 ×
1
2
2
6
−
5
−6
0
M). Table 2 lists the protonation and deprotonation
2
n
downfield region are possibly due to the inductive effect of 2-
Table 2. Protonation and Deprotonation Constants of Free
Base Mixed Substituted Porphyrins in Toluene at 298 K
thienyl groups, which leads to decrement in the ring current of
2
4b
the porphyrin π-system.
1
Figure 5 shows the H NMR spectra of the imino proton
TFA
TBAOH
slope
region of H TPP(NO )X and H TPP(NO )X in CDCl at
r2
r2
2
2
6
2
2
2
3
porphyrin
log β2
slope
log β2
2
98 K. The core imino protons of H TPP(NO )Br (−1.5
2
2
6
H TPP(NO )Br
6
11.29
11.74
11.21
8.47
2.32
1.99
2.12
2.01
2.10
2.01
2.50
1.91
1.95
2.64
2.18
0.93
0.88
0.84
0.85
0.97
0.99
0.98
0.98
0.97
0.99
0.93
11.54
2.05
0.94
2
2
ppm), H TPP(NO )(PE) (−1.43 ppm), H TPP(NO )Ph
2
2
6
2
2
6
H TPP(NO )Ph
6
2
2
(
−1.14 ppm), and H TPP(NO )(Th) (−0.82 ppm) are
2
2
6
H TPP(NO )Th
7.91
11.34
10.40
10.21
1.65
2.36
2.00
1.90
0.80
0.97
0.91
0.96
2
2
6
downfield shifted relative to H TPP(NO ) (−2.60 ppm) as
shown in Figure 5a. The imino proton resonances of the
2
2
H TPP(NO )PE
2
2
6
H TPPBr8
2
10.53
9.21
H TPP(NO )X series, H TPP(NO )(PE) (−2.53 ppm),
2
2
2
2
2
2
H TPPCl8
2
H TPP(NO )(CN) (−2.52 ppm), H TPP(NO )Ph (−2.31
2
2
2
2
2
2
H TPP(NO )Br
2
7.92
2
2
ppm), and H TPP(NO )(Th) (−2.24 ppm) are more
2
2
2
H TPP(NO )Ph
9.54
2
2
2
downfield shifted than H TPP(NO ), whereas H TPP(NO )-
2
2
2
2
H TPP(NO )Th
8.76
2
2
2
2
Br (−2.65 ppm) is slightly upfield shifted (Figure 5b). Further,
2
H TPP(NO )PE
2
8.65
2
the downfield shift of the resonances is due to substituent
effects and/or the nonplanar conformation as observed in the
X-ray crystal structures and optimized geometries. Non-
planarity of the macrocycle causes the broad features of the
imino protons to resonate at the downfield region relative to
H TPP(NO )CN
2
3.69
2
2
constants of various free base mixed substituted porphyrins.
Figure 6a shows the concomitant decrement in absorbance of
H TPP(NO )Br at 468 nm and rising of a new band at 494
33
that in planar porphyrins (Figure 5a). The reason for the
2
2
6
largest downfield shift of NH protons of H TPP(NO )(Th) is
nm while increasing the concentration of TFA. As protonation
proceeds, the multiple Q bands are disappearing and a new
single broad band grows at 753 nm accompanied by the red
2
2
6
enhanced nonplanarity and an inductive effect induced by
thienyl groups which leads to decrement in the ring current of
2
4b
the macrocycle.
shift of 16 nm in the Q (0,0) band. In all cases, we could obtain
x
1
H NMR spectra of MTPP(NO )X (M = Zn(II) and Ni(II);
diprotonated porphyrin species which is further confirmed by a
Hill plot having a slope of ∼2 as shown in Figures 6a, inset, and
2
n
n = 2 and 6) complexes are devoid of imino protons, revealing
H
dx.doi.org/10.1021/ic501259g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
S30, Supporting Information, and Table 2. All diprotonated
species of mixed substituted porphyrins are stable and showing
aromatic nature as evidenced by the UV−vis spectral features.
H TPP(NO )X (X = Br, Ph, and Th) exhibits ∼6−288-fold
Table 3. Electrochemical Redox Potentials (in V vs Ag/
AgCl) of Mixed Substituted Porphyrins in CH Cl
2
2
Containing 0.1 M TBAPF with a Scan Rate of 0.1 V/s at
6
298 K
2
2
6
higher protonation constants (β ) in comparison to H TPPX
2
2
8
ΔE
(X = Br, Cl) in toluene, which is ascribed to the combined
oxidation (mV)
reduction (mV)
(mV)
effect of mixed substitution and nonplanarity. The observed log
β values of H TPPX (X = Br, Cl) in toluene are comparable
porphyrin
I
II
I
II
2
2
8
21h
H TPP
1000
1100
1118
1009
1123
1007
1294
1045
1335
1286
1214
1130
1233
1132
1462
1288
1091
1185
1033
−1230
−874
−754
−849
−752
−800
−519
−642
−921
−659
−805
−1540
−1086
−824
−1028
−827
−925
−827
−973
−1354
−732
2230
1974
1872
1858
1875
1807
1813
1687
1725
1757
1611
to those observed in CH Cl
2
2
2.
2
H TPP(NO )
2
2
H TPP(NO )(PE) has shown a (∼5.52−18.6 × 10 )-fold
2
2
6
H TPP(NO )Br
2
2
2
lower β value which is ascribed to the electron-withdrawing
2
H TPP(NO )Ph
2
2
2
nature of the phenylethynyl groups and less nonplanar
H TPP(NO )(PE)
2
2
2
conformation as compared to H TPP(NO )X (X = Br, Ph,
2
2
6
H TPP(NO )Th
2
2
2
and Th). H TPP(NO )(CN) exhibited a much lower log β
2
2
2
2
H TPP(NO )(CN)
2
2
2
value (3.69) than that of H TPP(NO )X (X = Br, Ph, PE, and
2
2
2
a
a
a
a
a
a
a
H TPP(NO )Br
6
2
2
Th), which is attributed to the combined effect of the quasi-
a
a
a
H TPP(NO )Ph
6
804
2
2
planar structure (ΔC = ±0.5 Å and over all mean plane
β
H TPP(NO )(PE)
6
1098
2
2
deviation of porphyrin core, Δ24 = 0.234 Å) and strong
electron-withdrawing nature of the cyano and nitro sub-
stituents. H TPP(NO )X (X = Br, Ph, PE, and Th) were
a
H TPP(NO )Th
6
806
−1345
2
2
a
Refers to irreversible peak potential.
2
2
2
shown to have lower log β values (7.9−9.54) in comparison to
2
H TPPR (R = Me, Ph, Th, and Br) with log β values (9.03−
2
4
2
in Figure 7. Table S7, Supporting Information, lists the
electrochemical redox data (vs Ag/AgCl) of various metal
2
1h
9
.96) possibly due to the strong electron-accepting nature of
the nitro group. Figure 6b shows the concomitant decrement in
the absorbance of H TPP(NO )Br at 468 nm while increasing
complexes in CH Cl2 at 298 K. The mixed substituted
2
2
2
6
porphyrins exhibited two successive one-electron electro-
chemical oxidations and reductions, whereas Co(II) complexes
show first metal-centered redox behavior followed by a π-
system. Generally, the first oxidation of Co(II) porphyrins is a
metal-centered process and well documented in the literatur-
the concentration of TBAOH, and a new band rises at 512 nm.
As deprotonation proceeds the multiple Q bands of the neutral
compound disappear and a new band rises at 736 nm. The inset
shows the Hill plot with a slope of ∼2 and log β value of 11.54
2
10c,34a,b
which reveals the acidic nature of the NH protons due to the
electron-withdrawing nature of NO2 and Br substituents.
Notably, H TPP(NO )(PE) has a close log β2 value to
e.
Further, we carried out the oxidation of Co(II)
porphyrins to Co(III) porphyrins by adding aliquots of tert-
2
2
6
butyl hydroperoxide (TBHP) in CH Cl as chemical oxidant
2
2
H TPP(NO )Br , reflecting the electron-accepting nature of
which is reflected in the UV−vis spectral changes as shown in
the Supporting Information (Figure S32 and Table S8). The
effect of mixed β-substitution (tri and hepta) as well as the
effect of core metal ion on redox potentials of MTPP(NO )X
n
2
2
6
the substituents (nitro and phenylethynyl groups) and induced
nonplanarity by mixed substitution. Further, H TPP(NO )Th
6
2
2
has a lower deprotonation constant as compared to H TPP-
2
2
(
NO )(PE) and H TPP(NO )Br , which is interpreted in
(n = 0, 2 and 6; X = Br, CN, PE, Ph and Th; M = 2H, Co(II),
Ni(II), Cu(II) and Zn(II)) are shown in Figures S33−S36,
Supporting Information.
2
6
2
2
6
24b
terms of the conjugative effect of the 2-thienyl groups.
Under similar conditions, H TPP(NO )Ph was showing
2
2
6
incomplete deprotonation with split Soret bands (Figure
S31c, Supporting Information) even at a higher concentration
of the base. This is possibly due to the electron-rich nature of
the porphyrin core which prevents formation of dianionic
species. Moreover, at very high concentrations of TBAOH, the
reactant is in equilibrium with the anionic species and does not
move forward to completion. H TPP(NO )X (X = Br, Ph, PE,
The first ring oxidation potentials for MTPP(NO )X (X =
2
2
Br, Ph, PE, Th, and CN and M = 2H, Co(II), Ni(II), Cu(II),
and Zn(II)) span a range from 0.85 to 1.33 V, whereas the
reduction potential varies from −0.63 to −1.07 V (Tables 3 and
S7, Supporting Information). The first ring redox potentials of
MTPP(NO )(CN)2 have a 200−350 mV anodic shift in
2
2
2
2
comparison to MTPP(NO )Ph , which is attributed to the
2
2
Th, and CN) does not show any spectral changes upon4
addition of very high concentration of TBAOH (5.97 × 10−
M), which indicates the moderate nonplanar conformation
strong electron-withdrawing nature of the cyano substituents.
The metal-centered reduction potential of CoTPP(NO )(CN)
2
2
is 470 mV anodically shifted as compared to CoTPP due to the
strong electron-withdrawing nature of nitro and cyano groups.
The metal-centered redox potentials are altered according to
the electronic nature of the substituents rather than the
influence of nonplanarity. Notably, trimixed substituted
porphyrins show the following trend in their first oxidation
potentials: MTPP(NO )Ph ≤ MTPP(NO )Th < MTPP-
(
ΔC = ±0.67−0.73 Å) of the porphyrin π-system as compared
β
to H TPP(NO )X (ΔC = ±1.01−1.24 Å).
2
2
6
β
Electrochemistry. The redox potentials of the porphyrin π-
system are influenced by the nonplanar conformation, nature of
the substituents, and core metal ions. To determine the
influence of macrocyclic nonplanarity and the combined effects
of electron-donor and -acceptor groups at the β-pyrrole
positions, we examined the electrochemical redox properties
of these mixed substituted porphyrins by cyclic voltammetric
studies. Under similar conditions, the corresponding MTPP-
2
2
2
2
(NO ) < MTPP(NO )(PE) < MTPP(NO )Br < MTPP-
2
2
2
2
2
(NO )(CN) . This trend is in accordance with the electronic
2
2
nature of the substituents. The cathodic shift in the oxidation
potentials of MTPP(NO )Ph2 relative to MTPP(NO ) is
2
2
21e
(
NO ) and MTPP derivatives were also examined, and the
explained in terms of the electron-donating nature of the
phenyl groups. For the Hammett equation E1/2 = 2σρ for
MTPP(NO )X , the plot of the first ring oxidation and
2
electrochemical redox data (vs Ag/AgCl) are listed in Table 3.
The influence of different β-substituents on the redox
potentials for CuTPP(NO )X and CuTPP(NO )X are shown
2
2
reduction vs the Hammett parameter (σ ) of the substituents
2
2
2
6
p
I
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Inorganic Chemistry
Article
Figure 7. Cyclic voltammograms of (a) CuTPP(NO )X (∼1 mM) and (b) CuTPP(NO )X (∼1 mM) in CH Cl containing 0.1 M TBAPF using
2
2
2
6
2
2
6
Ag/AgCl as reference electrode with a scan rate of 0.10 V/s at 298 K.
3
5
(
X) in MTPP(NO )X was examined to delineate the role of
potential varies from −0.69 to −1.08 V, which shows the
2
2
X on the redox potentials, and they show a more linear trend in
reduction and oxidation (Figure S38, Supporting Information,
and Table 4). The reaction constant (ρ) for oxidation of
broader range in comparison to MTPP(NO )X . The metal-
2
2
centered reduction potentials of CoTPP(NO )X is 0.2−0.6 V
2
6
more anodically shifted than CoTPP, whereas oxidation
potentials are less shifted (0.04−0.2 V). This indicates that
the electron-withdrawing groups influence significantly the
metal-centered reduction rather than oxidation. For the
Hammett equation E1/2 = 6σρ for MTPP(NO )X , the
Table 4. Summary of Hammett Plots with Reaction
Constants (ρ) and Correlation Coefficients (r ) of First Ring
Redox Potentials (vs Ag/AgCl) for Various Mixed
Substituted Porphyrins
2
2
6
a
Hammett plots were constructed to delineate the role of X
on the potentials, and they show a linear trend in reduction,
whereas more scattered points in oxidation were observed with
first oxidation
first reduction
porphyrin
ρ (V)
r2
ρ (V)
r2
2
poor r values (Figure S38, Supporting Information, and Table
H TPP(NO )X
2
0.193
0.251
0.121
0.146
0.117
0.154
0.187
0.117
0.86
0.86
0.77
0.92
0.23
0.31
0.62
0.42
0.249
0.252
0.154
0.261
0.194
0.206
0.232
0.161
0.98
0.98
0.87
0.99
0.95
0.97
0.97
0.88
4). The scattered points of oxidation in the Hammett plot
indicates the nonplanar confirmation of macrocycle as
evidenced from DFT calculations. This is further supported
by downfield resonances of NH signals of H TPP(NO )X in
2
2
CuTPP(NO )X2
2
NiTPP(NO )X2
2
ZnTPP(NO )X2
2
2
2
6
1
H TPP(NO )X
2
2
6
H NMR spectroscopy. Figure 8 shows the cyclic voltammo-
2
2
CuTPP(NO )X6
grams of CuTPP(NO )X (X = Ph or Th; n = 0, 2, 6) bearing
2
n
NiTPP(NO )X6
β-pyrrole phenyl or thienyl substituents. The oxidation
potentials of CuTPP(NO )Ph is ∼0.38 V cathodically shifted
2
ZnTPP(NO )X6
2
6
a
MTPP(NO )X , where X = Br, CN, Ph, PE, and Th.
in comparison to CuTPP(NO ), whereas reduction is almost
2
n
2
unaltered, indicating that each phenyl groups shifts 60 mV
cathodic (Figure 8) due to the electron-donating nature of the
MTPP(NO )X is in the range 0.12−0.25 V, whereas for the
2
2
Ph groups and increased nonplanarity. Notably, CuTPP(NO )-
reduction it ranges from 0.12 to 0.19 V. The effects of
2
Th exhibited a 0.3 V cathodic shift in oxidation relative to
trisubstitution in CuTPP(NO )X on redox potentials and the
6
2
2
CuTPP(NO ), whereas a 0.12 V anodic shift in reduction was
HOMO−LUMO gap is shown in Figure S37, Supporting
Information. Almost the same trend was observed for
H TPP(NO )X and other metal complexes (Table 4). In the
2
observed. The observed cathodic shift in oxidation is ascribed
to the combined effect of the electron-releasing conjugative
nature of 2-thienyl groups and the highly nonplanar
conformation of the macrocycle, whereas an anodic shift in
reduction is due to the electron-withdrawing inductive effect of
2
2
2
MTPP(NO )X series, the first ring redox potentials varied
2
2
according to the electronic nature of the substituents with
marginal modulation in the HOMO−LUMO gap (∼0.2 V as
24b
compared to MTPP(NO )), which is in accordance with the
the 2-thienyl groups.
Notably, the first oxidation potentials of MTPP(NO )X (X
2 6
2
observed moderate nonplanar conformation of the MTPP-
(
NO )X series (vide supra).
Further, we carried out electrochemical studies of MTPP-
= Ph and Th) are more cathodically shifted (0.15−0.25 V),
while the reduction potentials are marginally cathodic (0.03−
0.06 V) relative to their corresponding MTPP(NO )X
2
2
(
NO )X in order to examine the effect of the number of
2
6
2
2
substituents and nonplanarity (Figures S34−S36, Supporting
Information). The first ring oxidation potentials of these
porphyrins ranges from 0.68 to 1.31 V, whereas the reduction
derivatives due to the nonplanar conformation of the
macrocycle which destabilizes the HOMO. The ρ value
(reactivity constant) for oxidation of MTPP(NO )X is in the
2
6
J
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Inorganic Chemistry
Article
Figure 8. Cyclic voltammograms of (a) CuTPP(NO )Ph and (b) CuTPP(NO )Th (n = 0, 2, and 6) in CH Cl containing 0.1 M TBAPF using
2
n
2
n
2
2
6
Ag/AgCl as reference electrode with a scan rate of 0.1 V/s at 298 K. Porphyrin concentration was maintained ∼1 mM.
Figure 9. Cyclic voltammograms of (a) MTPP(NO )Ph and (b) MTPP(NO )Ph (M = 2H, Co(II), Ni(II), Cu(II), Zn(II)) in CH Cl containing
2
2
2
6
2
2
0
.1 M TBAPF using Ag/AgCl as reference electrode at 298 K.
6
range of 0.15−0.26 V, whereas for the reduction it ranges from
.16 to 0.21 V. The higher ρ values for oxidation of
MTPP(NO )X6 as compared to MTPP(NO )X2 systems
following trend Zn(II) < Cu(II) < H < Ni(II) < Co(II) for
both tri- and heptasubstituted porphyrins according to the
2
0
difference in their electronegativity. In MTPP(NO )X series,
2
2
2
6
indicate the ease of oxidations due to enhanced nonplanarity
the first ring redox potentials varied significantly according to
the electronic nature of the substituents and enhanced
nonplanarity upon substitution which results in the dramatic
reduction in the HOMO−LUMO gap (0.54−0.66 V) as
referenced from MTPP (Figure 10). By means of unsym-
metrical substitution, we are able to tune the redox potentials
with dramatic decrement in the HOMO−LUMO gap. The
fairly broader range of σ values for the first ring redox potentials
of mixed substituted porphyrins is possibly due to the effect of
the core metal ion and the nature of the mixed substituents at
the β-pyrrole positions. As reported earlier, the oxidation
potentials are largely influenced by the substituent effect and
of the macrocycle. β-Substituted TPP series, viz. MTPP(NO )-
2
9
b,24a,34c,d
X , MTPP(NO )X , MTPPX (n = 1, 4),
exhibited ρ
2
2
6
n
values higher than that reported for the β-octabromo/chloro
36
MT(4-X-phenyl)P (ρ = 0.03−0.08 V) and MT(4-X-phenyl)P
37
series (ρ = 0.065 V). This is anticipated since substitution at
the β-pyrrole position is in direct conjugation with the
porphyrin π-system, so that higher ρ values were observed.
The effect of the core metal ion on the redox potentials of
the representative porphyrins MTPP(NO )Ph (n = 2 and 6; M
2
n
=
Co(II), Ni(II), Cu(II), and Zn(II)) is shown in Figure 9. The
first ring oxidation potentials of these porphyrins show the
K
dx.doi.org/10.1021/ic501259g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
from the redox potentials correlates fairly well with the
HOMO−LUMO gap calculated from the longest wavelength
band in the optical absorption spectra of these mixed
substituted porphyrins. Q (0,0) and ΔE of mixed tri- and
x
1/2
heptasubstituted porphyrins are of comparable energy relative
to β-tetra- and octasubstituted porphyrins (H TPPX , n = 4
2
n
and 8) as indicated in Table 5. These results clearly indicate
that the tri- and heptamixed substitution can compensate for
the effect of homo tetra- and octasubstituted porphyrins.
CONCLUSIONS
■
Two new families of novel mixed substituted porphyrins and
their metal complexes have been synthesized and characterized.
Crystal structure analyses and DFT fully optimized geometries
of MTPP(NO )X and MTPP(NO )X revealed moderate and
2
2
2
6
Figure 10. HOMO−LUMO variation of CuTPP(NO )X , where X =
2
6
highly nonplanar saddle-shape conformations, respectively.
These unsymmetrically substituted porphyrins exhibited a
PE, Br, Ph, and Th, in comparison to CuTPP(NO ) and CuTPP.
2
dramatic red shift in their B and Q (0,0) bands as compared
x
to MTPP(NO ) and MTPP. The dramatic downfield shift of
the NH proton resonances and the higher protonation/
2
nonplanarity of the porphyrin macrocycle while the reduction
18,38
potentials are independent of structural change.
The almost
deprotonation constants of H TPP(NO )X readily reflects
21c
2
2
6
planar structure of H TPPBr
is anticipated to show variable
2
2
the electronic effects of the mixed substituents (X) and
degrees of nonplanarity with an increase in size or shape of the
substituent X in MTPP(NO )X derivatives. The crystal
nonplanarity of the macrocycle as compared to H TPP(NO )-
2
2
2
6
X2. The fairly broad range of the first ring redox potentials
from 0.68 to 1.31 V for oxidation and from −0.64 to −1.10 V
for reduction) and σ values of MTPP(NO )X clearly indicate
structure of H TPPBr showed a considerable nonplanar
2
6
(
21c
conformation of the porphyrin ring. The ease of oxidation
in MTPP(NO )X relative to the corresponding MTPP(NO )-
2
6
2
6
2
the redox tunability achieved by means of unsymmetrical
substitution. Further, the HOMO−LUMO gap considerably
decreases as we increase the number of β-substituents from 3 to
X2 derivatives is ascribed to the electronic effects of the
substituents and nonplanar distortion of the macrocycle;
however, the reduction potentials are predominantly dependent
on the nature of the substituents.
7
, i.e., MTPP(NO )X to MTPP(NO )X . The remarkable red
2 2 2 6
shift in electronic spectral features, downfield shift of NH
protons, variation in protonation and deprotonation constants,
and significant shift in redox potentials of these porphyrins are
interpreted in terms of both inductive and resonance
interactions of substituents on the porphyrin π-system as well
The electrochemical redox potentials and UV−vis spectral
data of these unsymmetrically substituted free base porphyrins
were compared with the literature having symmetrical β-pyrrole
substituents H TPPX (X = Br, Ph, Th, and CN) and H TPPX
8
2
4
2
(X = Br, PE and Ph) derivatives as shown in Table 5. ΔE1/2
as nonplanarity of the macrocycle. ΔE1 obtained from the
/2
redox potentials as well as from spectral data (Q (0,0) band)
x
Table 5. Comparison of First Ring Redox Data (ΔE /2) with
1
suggests that the unsymmetrical tri- and heptasubstitution can
compensate for the effect of homo tetra- and octasubstitution
of porphyrins (H TPPX , n = 4 and 8). By means of β-pyrrole
Longest Wavelength Band Energy Q (0,0) of Various
x
Porphyrins in CH Cl at 298 K
2
2
2
n
mixed substitution on the porphyrin macrocycle we were able
to achieve the remarkable bathochromic shift in electronic
spectral bands, varying degrees of nonplanarity, higher
reactivity constants (ρ values), and tunable electrochemical
redox properties with a dramatic reduction in the HOMO−
LUMO gap. Currently, we are utilizing these mixed substituted
porphyrins for nonlinear optical studies, catalytic, and sensor
applications and will report on these studies in the near future.
B
Q (0,0)
Q (0,0)
ΔE
1/2
b
x
x
porphyrin
(nm)
(nm)
(eV)
(V)
ref
TPP(NO )(PE)2
444
439
436
440
440
687
686
683
695
702
1.80
1.80
1.82
1.78
1.77
1.87
1.85
1.87
1.80
1.81
a
2
TPP(NO )Ph2
a
a
a
a
2
TPP(NO )Br2
2
TPP(NO )Th2
2
TPP(NO2)
CN)2
(
TPP(PE)4
TPPPh4
452
434
436
445
449
494
470
468
481
506
468
469
414
690
677
685
701
729
751
729
739
754
761
724
743
646
1.80
1.83
1.81
1.77
1.70
1.65
1.70
1.67
1.64
1.63
1.71
1.67
1.92
1.90
2.02
1.84
1.94
1.66
1.75
1.72
1.68
1.61
1.71
1.84
1.64
2.23
a
39a
40a
24b
9b
a
ASSOCIATED CONTENT
■
TPPBr4
S
Supporting Information
TPPTh4
TPP(CN)4
*
Experimental section containing synthetic procedures, figures
1
TPP(NO )(PE)6
of optical absorption and emission spectra, H NMR spectra,
2
TPP(NO )Ph6
a
UV−vis titrations for protonation and deprotonation, cyclic
voltammetric studies, and XRD data of novel mixed substituted
porphyrins. This material is available free of charge via the
Internet at http://pubs.acs.org.
2
TPP(NO )Br6
a
2
TPP(NO )Th6
a
2
TPP(PE)8
TPPPh8
TPPBr8
TPP
39b
40b
28a
16f
AUTHOR INFORMATION
■
Corresponding Author
a
b
Refers this work. ΔE1 = Ioxidation − Ireduction
.
*E-mail: sankafcy@iitr.ac.in.
/2
L
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Inorganic Chemistry
Article
Notes
Phthalocyanines 2000, 4, 368−373. (c) Bonnett, R. Chem. Soc. Rev.
1
995, 19−33. (d) Gros, C. P.; Eggenspiller, A.; Nonat, A.; Barbe, J.-
The authors declare no competing financial interest.
M.; Denat, F. Med. Chem. Commun. 2011, 2, 119−125. (e) Brí
Kralova, J.; Cígler, P.; Kejík, Z.; Pouckova, P.; Vasek, P.; Moserova,
Martasek, P.; Kral, V. Bioorg. Med. Chem. Lett. 2012, 1, 82−84.
f) Kumar, D.; Mishra, B. A.; Shekar, K. P. C.; Kumar, A.; Akamatsu,
̌
za, T.;
́
́
̌
́
̌
́
I.;
ACKNOWLEDGMENTS
■
́
́
We are grateful for the financial support provided by the
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
(
K.; Kusaka, E.; Ito, T. Chem. Commun. 2013, 49, 683−685.
(6) (a) Zhang, J.; Li, Y.; Yang, W.; Lai, S.-W.; Zhou, C.; Liu, H.; Che,
C.-M.; Li, Y. Chem. Commun. 2012, 48, 3602−3604. (b) Rakow, N. A.;
Suslick, K. S. Nature 2000, 406, 710−713. (c) Suslick, K. S. MRS Bull.
(
2012/37C/61/BRNS). We sincerely thank the reviewers for
2
2
004, 29, 720−725. (d) Zhang, C.; Suslick, K. S. J. Am. Chem. Soc.
005, 127, 11548−11549. (e) Zhang, C.; Bailey, D. P.; Suslick, K. S. J.
their valuable suggestions to improve the manuscript. We thank
Ms. Neetu Singh and Prof. U. P. Singh, IIT Roorkee, for single-
crystal X-ray data collection, and Ms. Pinky Yadav, IIT
Roorkee, for DFT calculations. We are thankful to Dr. Koya
Prabhakara Rao, VFSTR University, and Dr. P. Ramesh,
Samsung Biomedical Research Institute, for suggestions related
to X-ray structure solutions. R.K. thanks the Ministry of Human
Resource Development (MHRD), India, for the fellowship.
Agric. Food Chem. 2006, 54, 4925−4931. (f) Suslick, K. S.; Rakow, N.
A.; Kosal, M. E.; Chou, J.-H. J. Porphyrins Phthalocyanines 2000, 4,
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8
07−413. (g) Musto, C. J.; Lim, S. H.; Suslick, K. S. Anal. Chem. 2009,
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