Synthesis and Redox Properties of Superbenzene Porphyrin Conjugates

Article abstract of DOI:10.1021/acs.inorgchem.0c01682

Superbenzene porphyrin conjugates find wide range of applications from nonlinear optical materials to semiconductors. Herein, we report the synthesis and characterization of 5,15-bis(3,5-di-tert-butylphenyl)-10,20-bis(pentaphenylphenyl)phenylporphyrin and its Zinc-metallated complex. Oxidative planarization of 5,15-bis(3,5-di-tert-butylphenyl)-10,20-bis(pentaphenylphenyl)phenylporphyrin and its metallated complex was carried out by using NOBF4 as an oxidizing agent. The formation of superbenzene porphyrin conjugates validates its Scholl type reactions. The laboratory-synthesized porphyrin conjugates were characterized experimentally using spectroscopic techniques such as 1H NMR, 13C NMR, electron spin resonance, and ultraviolet-visible spectroscopy for structural conformation. In addition, density functional theory calculations were carried out to validate the experimental results. The theoretical and experimental results show that the 4-(pentaphenylphenyl)phenyl ligand increases the stability, optical properties, and rate of planarization of synthesized porphyrins. The conjugates exhibited intense and distant electronic communication between two hexabenzocoronene sites, taking advantage of porphyrin as a ?-spacer. The ?-radical cation has also been found to be an intermediate in oxidative C-C bond formation. NICS calculations support such a conclusion.

Full text of DOI:10.1021/acs.inorgchem.0c01682

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Article
Synthesis and Redox Properties of Superbenzene Porphyrin
Conjugates
Kharu Nisa,§ Vikas Khatri,§ Sharvan Kumar, Smriti Arora, Sohail Ahmad, Anshu Dandia, M. Thirumal,
Hemant K. Kashyap,* and Shive M. S. Chauhan*
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ABSTRACT: Superbenzene porphyrin conjugates find wide range of applications from nonlinear optical materials to
semiconductors. Herein, we report the synthesis and characterization of 5,15-bis(3,5-di-tert-butylphenyl)-10,20-bis-
(pentaphenylphenyl)phenylporphyrin and its Zinc-metallated complex. Oxidative planarization of 5,15-bis(3,5-di-tert-butylphen-
yl)-10,20-bis(pentaphenylphenyl)phenylporphyrin and its metallated complex was carried out by using NOBF4 as an oxidizing agent.
The formation of superbenzene porphyrin conjugates validates its Scholl type reactions. The laboratory-synthesized porphyrin
1
conjugates were characterized experimentally using spectroscopic techniques such as H NMR, 13C NMR, electron spin resonance,
and ultraviolet−visible spectroscopy for structural conformation. In addition, density functional theory calculations were carried out
to validate the experimental results. The theoretical and experimental results show that the 4-(pentaphenylphenyl)phenyl ligand
increases the stability, optical properties, and rate of planarization of synthesized porphyrins. The conjugates exhibited intense and
distant electronic communication between two hexabenzocoronene sites, taking advantage of porphyrin as a π-spacer. The π-radical
cation has also been found to be an intermediate in oxidative C−C bond formation. NICS calculations support such a conclusion.
voltaic devices.12 A dye molecule that is well-suited to be a
partner for HBC is porphyrin. Because of their peculiar
INTRODUCTION
■
Synthesis of π-extended porphyrins has attracted immense
photophysical characteristics, high extinction coefficients in the
visible spectrum, porphyrins are widely used in light-harvesting
architectures. Wong et al. were the first to prepare a fluorenyl-
linked porphyrin−HBC conjugate, which was intended to be a
potential dye molecule for photovoltaic devices.13 Jux and co-
workers started to work with directly linked porphyrin−HBC
conjugates, which are expected to feature, compared to the
fluorenyl-bridged derivatives of Wong et al., enhanced
electronic interaction between both chromophores. Taking a
interest owing to their unique optical and electronic proper-
ties,1 which render them highly valuable for a variety of
applications such as near-infrared (NIR) dyes,2 organic
semiconductors,3 and nonlinear optical materials.4 Various
aromatic hydrocarbons, including benzene,5 naphthalene,1c,6
pyrene,7 azulene,8 corannulene,9 and coronene,10 have been
fused to the meso and β positions of the porphyrins. In
addition, fusion of larger polycyclic aromatic hydrocarbons
(PAHs) with the core porphyrin has remained challenging.
Hexa-peri-hexabenzocoronene (HBC) with 42 sp2-hybridized
carbon atoms has been extensively investigated as an
archetypal nanographene.11 HBC is a redox active species, its
redox potential can be tuned by making use of π-spacers. Also,
the electronic characteristics of HBC can be regulated, by the
attachment of donor, acceptor, or redox active functional
groups. Furthermore, combinations of chromophores and
HBCs were investigated for applications in organic photo-
Received: June 8, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01682
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Scheme 1. Synthetic Procedure for the Synthesis of 1a−1d and the Radical Cations of 1a and 1c
cue from the Scholl14 reaction, which is one of the oldest C−C
bond-forming reactions and has been extensively utilized for
intramolecular oxidative cyclodehydrogenation of various
polybenzenoid aromatic hydrocarbons, a porphyrin moiety
with a 4-(pentaphenylphenyl)phenyl (PPP) ligand at the meso
positions was synthesized and designed (as shown in Scheme
1). The transformation, commonly known as the “Scholl
reaction”, turns a flexible, nonconjugated PPP moiety into a
planarized, aromatic PPP unit, which significantly influences
the characteristics of porphyrins. For example, due to an
electronic coupling between the two π-systems, a broadening
and bathochromic shift of the ultraviolet visible (UV−vis)
absorption bands of the porphyrin is observed. The toroidal
delocalization and propeller-like conformation of the PPP
ligand enables π−π interaction between the peripheral
aromatic rings and is suitable candidate for oxidative coupling
and constructing light-harvesting architectures.15−20 In addi-
tion, hexaarylbenzenes serve as highly luminescent compounds,
suitable for organic blue light-emitting diodes,21 and serve as
unique model compounds for graphite.22 Nevertheless,
polycyclic aromatic hydrocarbons with increasing molecular
weights face solubility problems. Porphyrin architectures based
on PPP scaffolds meso-connected to porphyrins were first
reported in 2002, and they show properties similar to those of
light-harvesting antenna LH2 in purple bacteria.23,24 As PPP
and related compounds are emerging as versatile building
blocks in organic synthesis, much effort is being invested in the
development of novel synthetic strategies.25−29 In this paper,
we report the synthesis of trans-5,15-bis(3,5-di-tert-butylphen-
yl)-10,20-bis(pentaphenylphenyl)phenylporphyrin and its con-
version into 5,15-bis(3,5-di-tert-butylphenyl)-10,20-bis-
(hexabenzocoronene)porphyrin. Furthermore, we aim to
B
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study the role of metal (Zn) in the core of superbenzene
porphyrin conjugates. Zinc(II) porphyrin derivatives have been
reported as versatile photosensitizers for applications in dye-
sensitized solar cells (DSSCs) with a high electric power
conversion efficiency.30 Here we have introduced 3,5-di-tert-
butyl phenyl groups at the meso position of porphyrin to
enhance the solubility and the thermodynamic and kinetic
stability of the porphyrin moiety. Our investigations also
include computational and experimental studies that aim to
uncover reasonable mechanistic paths for the conversion of 1a
into 1b and 1c into 1d.
The UV−vis absorption spectra of 1a, 1a+•, 1c, and 1c+• in
DCM revealed interesting features. The Soret peak of purple-
colored porphyrin was shifted from 417 to 445 nm along with
the appearance of a new band in the range of 670 nm,34 which
confirms the formation of the radical cation. In addition, this
change in color was also observed upon addition of NOBF4.
Similarly, the radical cation34 of 1c shows a shift in the Soret
band from 413 to 415 nm along with the appearance of peaks
in the range of 545 and 579 nm (Figure 1 and Figure S15).
RESULTS AND DISCUSSION
■
The synthesis of the targeted conjugates was carried out by the
synthesis of precursor 4-hexaphenylbenzaldehyde (5) as
reported previously31 (Figure S1). The targeted porphyrin
conjugates were synthesized by condensation between (5) and
3,5-(di-tert-butylphenyl)dipyrromethane (6) in dry dichloro-
methane (CH2Cl2) followed by oxidation with 2,3-dichloro-
5,6-dicyanobenzoquinone (DDQ) giving porphyrin 1a as the
major product in 19% yield. The trans structure is favored due
to steric constraints imposed by the relatively rigid PPP
porphyrin moieties. The subsequent metalation of porphyrin
1a with excess Zn(OAc)2·2H2O in dry CH2Cl2 gave porphyrin
1c in quantitative yield. To shed light on the chemical
transformation of 1a into 1b and 1c into 1d, a metal free
oxidant NOBF4 was used as an oxidizing agent.32 It was found
that porphyrins 1a and 1c form π-radical cations (as shown in
Scheme 1). The radical cations were synthesized by using one-
electron oxidant NOBF4 (1.0 equiv) and characterized by
electron spin resonance (ESR) spectroscopy. To our delight,
when an excess of NOBF4 (10.0 equiv) was added to 1a and
1c, oxidative coupling took place to form superbenzene
porphyrin conjugates 1b and 1d, respectively.
Figure 1. UV−vis spectra of 1a and 1c in DCM before and after the
addition of 1.0 equiv of NOBF4 (5 × 10−5 M).
Figure 2 illustrates the UV−vis−NIR spectra of 1b and 1d.
The hexabenzocoronene structure causes a more profound
The 1H and 13C NMR spectra of 1a in CDCl3 and C2D2Cl4
are shown in Figures S2−S4. At room temperature, macrocycle
1a exhibits two doublets at 6.59 and 6.81 ppm corresponding
to aryl protons and two multiplets in the range of 7.50 and 7.10
ppm. The resonances of the hydrogen atoms corresponding to
the PPP part of 1a appear more downfield at 8.53 ppm. These
signals are ascribed to eight protons attached directly to
porphyrin. In CDCl3, the resonance of the NH peak is
1
observed at −2.64 ppm. The H and 13C NMR spectra and
NOESY spectrum of 1b show a pronounced downfield shift at
9.97 ppm corresponding to four HBC protons. In addition,
resonances of β-pyrrolic protons in the case of 1b appear as a
1
Figure 2. Comparison of the electronic absorption spectra of 1b and
1d in CDCl3 (2 × 10−6 M).
doublet at 8.96 ppm (see Figures S5−S7). The H NMR
spectrum of 1c shows a slightly downfield shift in chemical
shift values compared to those of 1a as shown in Figure S8.
1
The H and 13C NMR spectra of 1d also revealed interesting
impact on the electronic system of the porphyrin. Interestingly,
the spectral shapes of the Q-band shape are considerably
distorted for symmetry-broken fused porphyrins 1d, whereas
that of centrosymmetric 1b restores a typical vibrational
structure common to normal porphyrins. Intense Q-bands in
features; a pronounced downfield peak at 11.44 ppm
corresponding to four HBC protons was observed, which
reveals that NOBF4 has cyclized the porphyrin33 moiety. Metal
oxidants like FeCl3 result in demetallation of Zn superbenzene
porphyrin conjugates. The β-pyrrolic protons resonate at 8.96
ppm as shown in Figures S9 and S10.
Mass spectrometric analysis of 1a and 1b confirmed the
exact composition with molecular ion peaks at m/z 1750.9433
and 1727.2075, respectively (see Figures S11 and S12,
respectively). Similarly, the mass spectra of 1c and 1d show
peaks at m/z 1822.9546 and 1806.1051, respectively (see
Figures S13 and S14, respectively).
1
1d at λmax = 1702 nm (ε = 1.12 × 105 M−1 cm−1) (Eox1 − Ered
= 0.37 eV)35 and two unequal Q-bands at 2249 and 2389 nm
are indicative of IVCT displayed by 1d. The strength of the
electronic interaction between the two optical electrodes in the
ground state is reflected in the distortion in the Q-bands of the
porphyrin conjugate. The extent of interaction of the HBCs via
the porphyrin π-surface increases in metallated superbenzene
porphyrin conjugates rather than free base superbenzene
C
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porphyrin conjugates. The metal coordination slightly distorts
the planarity of the porphyrin core, thereby decreasing the
flexibility of the porphyrin core.36
The redox properties of 1a−1d were investigated by CV and
DPV (Table 1). The CV studies demonstrated an irreversible
(368 nm) or the porphyrin (417 and 420 nm) unit of the
conjugates show that when the HBC unit is excited, emission
from the HBC is completely suppressed. We also notice that in
the case of 1b and 1d the emission at 741 nm is more intense
in 1d than in 1b, which is in agreement with our time-
dependent density functional theory (TD-DFT) studies. From
this, we can also deduce that porphyrin is the dominant partner
in 1b and hexabenzocoronene is the major dominant partner in
1d (see Figure S19).
Table 1. Summary of the Electrochemical Potentials for
Free Base and Metalated Porphyrin
oxidation (V)
reduction (V)
The ESR spectra of 1a+• and 1c+• showed signals at g values
of 2.0038 and 2.0035, respectively. The spin density
calculations indicated the major distribution of the spin over
the N atoms of the macrocycle and the meso positions of the
porphyrin unit (see Tables S2 and S3). In the case of the
H2TPP, the π-cation radical can be detected by its typical EPR
spectrum (g = 2.004),38 with a line width of 6 G. However, in
the case of 1a+•, the same is found at 3 G, the reason being that
in 1a+• the hyperfine coupling is not so prominent (Figure 3).
DFT Studies. To investigate the structural, electrochemical,
and spectral properties of porphyrin conjugates 1a−1d, DFT
studies were carried out. We also extended these studies to
understand the possible formation of radical cation inter-
mediates for porphyrins 1a and 1c. The B3LYP/LANL2DZ-
optimized structures of porphyrin conjugates 1a−1d for top
and side views in the ground state (S0) are presented in Figure
S18. The DFT studies revealed that the meso-substituted
porphyrins 1a and 1c adopt a geometry in which the PPP and
3,5-ditert-butylphenyl substituents are aligned in the plane of
the core porphyrin moiety. Because of the rigidity of PPP, the
core porphyrin moiety maintains its planarity with respect to
the “mean plane” defined by four meso carbon atoms.
Oxidative coupling of the C−C bond to form porphyrins 1b
and 1d results in the formation of a small dihedral angle with
the plane of the core porphyrin. The oxidative coupling
increases the level of conjugation between the core benzene
and PPP substituents. The increased level of conjugation of 1b
and 1d can be attributed to the frontier molecular orbital
(FMO) analysis.
ox
ox
ox
red
red
E1
E2
E3
E1
E2
1a
1b
1c
1d
0.95
−
−
−
−
−
−0.55
−0.71
−0.77
−0.77
−0.79
−1.34
−1.38
−
−
0.67
−0.40
1.48
1.02
1.57
type of curve like that of meso-substituted porphyrins. In
general, for porphyrin systems two oxidation peaks and two
reduction peaks are observed in cyclic voltammetry.37 As
observed for 1a and 1b, the CV and DPV plots exhibited
differences in their oxidation and reduction peaks (see Figures
S16 and S17, respectively). In the case of 1a, two one-electron
reduction waves are observed at Epc values of −0.55 and −0.79
V, corresponding to one oxidation wave at an Epa of 0.95 V. CV
and DPV plots of 1b demonstrate two quasi-reversible
reduction waves corresponding to the sequential two-step
electron transfer processes. The reductions are observed at Epc
values of −0.71 and −1.34 V for a scan rate of 0.10 V/s. The
difference (Ered11/2 − Ered21/2) of the split reduction waves, 0.9
V, indicates strong electronic interaction and formation of
mixed valent species. The two reductions are assigned to the
stepwise formation of a porphyrin π-anion radical and dianion,
respectively. 1c exhibited two reduction peaks at −0.77 and
−1.38 V corresponding to two oxidation peaks at 0.67 and 1.48
V, respectively. A second irreversible reduction of compound
1c is located at an Epc of −1.38 V. The higher peak current
seen for this process and the lack of a reverse peak on the
return scan suggest a multielectron transfer. The CV plot of 1d
exhibited a one-electron reduction wave at −0.77 V
corresponding to three one-electron oxidation waves at
−0.40, 1.02, and 1.57 V with a potential difference of 0.55 V
To understand the stability, reactivity, and electronic
properties of different porphyrin conjugates and their radical
cation intermediates, FMOs and their energies were calculated.
Comparisons of energies of the orbitals with the electron
density distribution for all of the porphyrin conjugates and
their radical cation intermediates at the DFT/B3LYP level of
theory are presented in Figures 4 and 5 and Figures S20 and
S21. The analysis of the selected FMOs reveals that the highest
between the second and third oxidation potential (ΔEox3‑ox2
)
indicating intense electronic communication and revealing the
formation of mixed valent species (MV) (see Figure S17).
The emission spectra of HBC-porphyrins 1b and 1d for
different excitation wavelengths that excite either the HBC
Figure 3. Experimental ESR spectra of 1a+• (left) and 1c+• (right) in DCM at 295 K. Microwave frequency of 9.92 GHz, microwave power of 8.195
mW, receiver gain of 2.00 × 103, modulation frequency of 100 kHz, and modulation amplitude of 3 G.
D
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Figure 4. Molecular energy and orbital diagrams of porphyrins 1a (left) and 1b (right) calculated using the B3LYP/LANL2DZ level of DFT.
Figure 5. Molecular energy and orbital diagrams of porphyrins 1c (left) and 1d (right) calculated using the B3LYP/LANL2DZ level of DFT.
occupied molecular orbitals (HOMOs) and lowest unoccupied
molecular orbitals (LUMOs) of 1a and 1b are localized either
at the core porphyrin moiety or at the PPP ligand. From
Figures 4 and 5, it is evident that upon metallation of the free
base porphyrin with Zn, the HOMO−LUMO band gap
increases and the extent of delocalization of the electron
density decreases. However, the formation of a radical cation
intermediate leads to a decrease in the HOMO−LUMO gap in
comparison to those of 1a and 1b, as depicted in Figure 4. This
decrease of the band gap in radical cation intermediates
increases the extent of delocalization of electrons. We can
conclude from the band gap that the higher the band gap, the
lower the extent of delocalization of electrons and vice versa.
Thus, formation of the radical cation intermediate governs the
oxidative planarization of the PPP substituent. From Figure 4,
one can see that the band gap (2.61 eV) remains the same for
both 1a and 1b. Also, the difference in energy between the
HOMOs of 1b and 1a suggests electronic communication
between two π-systems, which in turn is responsible for
efficient charge transfer between the core porphyrin and HBC
moieties.39 One can see from the frontier energy diagram that
the band gap increases when porphyrin is metallated with Zn
and the band gap decreases in the 1c+• radical cation. Kadish
and co-workers suggested that the HOMO−LUMO band gap
of the porphyrins is partially dependent on the nature of the
central metal ion, the substituents on the ring, and the
planarity of the ring.40 It is evident from the energy level
diagram that the HOMO−LUMO gap for zinc porphyrins was
slightly larger than that of free base porphyrin.40 LUMOs of all
of the porphyrin conjugates are concentrated in the core
porphyrin as predicted by the Goutermann four-orbital
model41 (see Figure 4). The FMOs of the radical cations of
1a and 1c were also analyzed (compiled in Figures S20 and
S21). It is found that the HOMO−LUMO band gap decreases
via the formation of radical cations, which in turn governs the
oxidative coupling of the C−C bond to form superbenzene
porphyrin conjugates, i.e., 1b and 1d, respectively. HOMO−1
E
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Scheme 2. Proposed Mechanism for the Conversion of 1a into 1b
and HOMO−2 of the radical cations of 1a and 1c are localized
on the PPP ligand.
supports the idea that the cyclization proceeds through a
radical cation mechanism30,45 (Figure S22).
The NICS (1)zz values of 1a showed a pronounced decrease
in its radical cation 1a+• at point C, i.e., sextuple benzene. In
addition, at point E, the NICS values are different; thus, NICS
computations favor the idea that the radical cation is
delocalized over the polycyclic aromatic moiety in accordance
with the proposed mechanistic pathway (Scheme 2). Similarly,
the radical cation of the metallated complex of porphyrin
conjugates shows a reversal in aromaticity at point E. The
sextuple benzene at point C showed a slight decrease in 1c+•
from that of 1c, which shows a notable difference from the
values of 1a and 1a+• (Figure S23).
The 10 lowest-energy electronic transitions of all of the
selected porphyrin conjugates were computed with TD-DFT
on the basis of the optimized ground-state geometries. The
simulated UV−vis spectra for porphyrin conjugates 1b and 1d
are shown in Figure S19, and the calculated transition energies,
oscillator strengths, and molecular orbital excitations for the
most relevant transitions of the electronic absorption bands of
porphyrins are listed in Table S4. TD-DFT calculations
showed absorptions in the range of 450−600 nm, and these
bands are in sharp contrast to Q-bands of porphyrins, which
are weaker, narrower, and located at shorter wavelengths.42
The UV−vis spectrum of 1b shows two Q-bands at 588 and
545 nm, whereas that of 1d shows one at 552 nm. These
absorptions are assigned as charge transfer (CT) transitions
from the hexabenzocoronene-based HOMO to LUMO or
LUMO+1, the π*-based orbitals of the porphyrin moiety. This
kind of lowest-lying CT transition implies that the HOMO is
chiefly based on the two hexabenzocoronene moieties and that
there is electronic communication in the MV state43 in 1d.
More importantly, the oscillator strength (which is the
measure of the intensity of the absorption peaks) of the Q-
band in 1d is increased compared to that of 1b. This indicates
that the introduction of zinc metal increases the oscillator
strength of the Q-band. The broadening and red-shift of the
absorption bands together with an increase in the intensity of
the Q-bands are promising strategies for determining the
inadequate light harvesting properties of the zinc superbenzene
porphyrin conjugates. This behavior may assist in the design of
new porphyrin dyes that improve the light harvesting ability.
We note that the TD-DFT calculations reproduce well the
experimental results of the electronic spectra.
CONCLUSION
■
In summary, two (pentaphenylphenyl)phenylporphyrin con-
jugates and their cyclized superbenzene porphyrin conjugates
have been synthesized using NOBF4 as an oxidizing agent.
NOBF4 has been proven to be a versatile reagent and is going
to replace oxidants such as FeCl3 for Scholl reactions, which
obviate problems of using a large excess of oxidant and
chlorination. NICS calculations revealed that planarization is
promoted between two expanded aromatic moieties. Taking
advantage of the beneficial characteristics of hexabenzocor-
onene as redox sites, we have quantified the IVCT bands of
MV superbenzene porphyrin conjugates. MV superbenzene
porphyrin conjugates possess an exceptionally small HOMO−
LUMO gap (λmax) at 1702 nm (Eox1 − Ered1 = 0.37 eV). These
results suggest that superbenzene porphyrin conjugates may
prove to be useful materials for light harvesting and charge
transport in photovoltaic devices. These types of molecules can
be targeted to understand the device performance in
optoelectronics.
Thereafter, nucleus-independent chemical shift calcula-
tions44 were performed at different positions in the π-planes
of the porphyrin rings to evaluate the aromatic nature of
porphyrins 1a and 1b and to gain insight into the conversion of
1a into 1b. The NICS (1)zz values, at points F and G in the
case of 1a and 1b, are drastically decreased in the plane of
porphyrin and pyrrolic amine, indicating the appearance of
global aromaticity (Figure S22). This indicates that all pyrrolic
rings sustain diamagnetic ring currents. A notable difference
can be seen at points C, E, K, and L in 1b adjacent to sextuple
benzene where the NICS values are positive. These values
indicate the appearance of local paratropic ring current. The
EXPERIMENTAL SECTION
■
Materials. Chemicals were from Spectrochem (India), Earth
(India), or Thomas Baker (India) and used as received. The reactions
for the synthesis of the radical cation of 5,15-bis(3,5-di-tert-
butylphenyl)-10,20-bis(pentaphenylphenyl)porphyrin and its Zn-
metallated complexes (1a+• and 1c+•) were carried out in a glovebox.
Thin layer chromatography (TLC) was carried on aluminum plates
coated with silica gel mixed with a fluorescent indicator from Merck
(Germany). Silica gel chromatography was performed on silica gel
60−120 mesh (spherical, neutral) purchased from Ekta Marketing
Corp. Dry solvents (DCM), UV−visible grade acetonitrile, and
chloroform were also purchased from Ekta Marketing Corp. 3,5-(Di-
tert-butylphenyl)dipyrromethane was prepared by following an earlier
report.46
sextuple carbon in 1a has a more negative NICS value in
comparison to that of the sextuple carbon in 1b. Similarly,
NICS (1)zz values for 1a at points D and E indicate a slightly
nonaromatic to antiaromatic nature. This corollary further
Characterization. NMR (1H and 13C) spectra were recorded on a
Jeol 400 MHz spectrometer in C2D2Cl4 and CDCl3 with TMS as a
standard. Molecular mass spectra of porphyrins were recorded with an
F
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Agilent mass spectrometer (Q-TOF B.05.01, B5125.1). UV−vis−NIR
spectra were recorded on a PerkinElmer (850 B) luminescence
spectrometer and a UV−vis−NIR Shimadu-2500 spectrometer. All of
the UV−vis spectra were recorded using UV grade chloroform. All
UV−vis measurements were carried out in a 1 cm quartz cuvette with
a 1 cm optical path length. Fluorescence was measured with
PerkinElmer NS-55S spectrofluorophotometer with a detector slit
width of 10 nm and a scan speed of 300 nm/min at room
temperature. The ESR spectra were recorded on a Bruker EPR 083
CS spectrometer operating at a frequency of 9.4 GHz with 100 kHz
frequency modulation and 1.0 G amplitude modulation. Electro-
chemical CV experiments and square wave voltammetry (SWV) were
carried out by using a μ Autolab M204 instrument having 438 NOVA
2.1 software consisting of a standard three-electrode arrangement with
platinum wire as the counter electrode, glassy carbon as the working
electrode, and Ag/AgCl or saturated calomel as the reference
electrode. All electrodes were cleaned prior to use. The Pt electrode
was cleaned in HNO3 at 50 °C for 10 min. Ag/AgCl was kept in HCl
for 10 min and then washed with ethanol. The glassy carbon electrode
was cleaned using 0.05 μm alumina powder followed by sonication in
ethanol and water for 15 min, and then the electrode was dried in an
inert atmosphere. All of the electrochemical measurements were taken
in Ar-purged solvents with n-Bu4NPF6 as the supporting electrolyte at
21 0.1 °C. CV studies of 1a−1d were performed in degassed DCM
under an Ar atmosphere at 0.05 mM, and the scan rate for the
measurements was typically 100 mV/s. DPV was carried out keeping
a peak amplitude of 25 mV, a peak width of 0.15, a pulse period of 0.5
s, and an increment E of 40 mV.
Computational Method. The quantum chemical calculations for
different porphyrin conjugates were carried out using DFT with the
B3LYP (Becke-3-parameters, Lee−Yang−Parr)47,48 hybrid exchange
correlation functional, which uses correction for both gradient and
correlation. For all of the systems, the electronic configurations of all
of the elements were described using the LANL2DZ49 basis set as
implemented in Gaussian 09W.50 For the radical cations, ground-state
geometry optimization and the spin density distribution were
calculated at the UB3LYP/LANL2DZ level of theory. Global
minimum energy configurations for all of the porphyrin conjugates
were confirmed by harmonic frequency calculations. The absence of
an imaginary frequency confirmed that all of the porphyrin conjugates
reached their minimum energy configuration. Natural bond orbital
(NBO) analysis and natural population analysis (NPA) were also
performed to calculate the partial charges and spin densities on the
atoms of the porphyrin.51 Vertical excitation energies were obtained
using the same level of theory for all of the porphyrin conjugates.
NICS calculations were performed at GIAO-B3LYP/LANL2DZ
computational levels for distances from the ring plane of porphyrin
from 0.0 to 1.0 Å.52 The 0.0 Å distance from the ring plane represents
NICS(0), and similarly, the 1.0 Å distance represents NICS(1). The
TD-SCF calculations were performed to identify the excited states for
all of the conjugates.53 The singlet and 10 excited states were
calculated for the superbenzene porphyrin conjugates.
Tolan as the precursor. In a round-bottom flask, tetraphenylcyclo-
pentadienone (460 mg, 1.2 mmol) and 1-formyldiphenylacetylene
(247 mg, 1.2 mmol) were added to diphenylether (6 mL) and the
whole reaction mixture was refluxed for 36 h under nitrogen. Upon
completion of the reaction, the purple color faded to brown, the
reaction mixture was cooled to room temperature, and with dropwise
addition of methanol the solid product precipitated, was filtered under
suction, and was purified by column chromatography: off-white solid;
1
64% yield; Rf = 0.67 (1:9 ethyl acetate/hexane); mp >250 °C; H
NMR (400 MHz, CDCl3-D) δ 9.78 (s, 1H, CHO), 7.42 (d, 2H, Ar),
7.11 (d, 2H, Ar), 6.91 (m, 25H, Ar); 13C NMR (400 MHz, CDCl3-D)
δ 191.0, 134.3, 130.4, 129.2, 127.6; ESI-MS 562.23.
Synthesis of 5-(3,5-Di-tert-butylphenyl)dipyrromethane. To
a stirred solution of 3,5-di-tert-butylbenzaldehyde (10 mmol, 3.342 g)
and pyrrole (30 mmol, 2.07 mL) was added trifluoroacetic acid (1.5
mmol, 171.03 μL) at room temperature. The reaction mixture was
stirred for half an hour, and the reaction progress was monitored by
thin layer chromatography. The white solid obtained was dipyrro-
methane: white solid; 95% yield; Rf = 0.33 (1:1 ethyl acetate/
petroleum ether); 1H NMR (400 MHz, chloroform-D) δ 7.87 (s, 2H,
pyrrolic-NH), 7.67 (d, 2H, β pyrrolic-H), 7.02 (s, 2H, Ar), 6.17 (m,
2H, β pyrrolic-H), 5.36 (s, 1H, meso-CH), 1.38 (s, 18H, tert-butyl).
Synthesis of 5,15-Bis(3,5-di-tert-butylphenyl)-10,20-bis-
(pentaphenylphenyl)phenylporphyrin (2HP-PPP) (1a). In a
three-neck round-bottom flask, dipyrromethane of 3,5-(di-tert-
butylphenyl)benzaldehyde (619 mg, 1.8 mmol) and 1-(4-
formylphenyl)pentaphenylbenzene (1013 mg, 1.8 mmol) were
dissolved in dry dichloromethane (150 mL) and ethanol (5 mL).
The reaction mixture was purged with argon for 15 min, and then
CF3COOH (20 μL, 0.014 mmol) was added. The completion of the
reaction was monitored by TLC, followed by addition of DDQ (230
mg, 1 mmol). The reaction mixture was stirred for 15 min, and the
product was purified by column chromatography. The second major
spot isolated was trans-A2B2 porphyrin, which was characterized by
1
UV−vis spectrometry, H NMR spectrometry, and ESI-MS spectros-
copy: purple solid; 19% yield; Rf = 0.50 (3:2 CHCl3/petroleum
ether); 1H NMR (400 MHz, C2D2Cl4) δ 8.93 (s, 8H, β-pyrrolic), 8.53
(d, 8H, J = 4.3 Hz), 8.07 (s, 3H), 7.73 (s, 9H), 7.10−7.33 (m, 26H,
PPPH), 7.04 (m, 13H), 6.83−6.89 (d, 6H, Ar), 1.35 (s, 18H, tert-
butyl), 1.29 (s, 18H, tert-butyl), −2.64 (s, 2H, NH) (the peak at −2.6
ppm is seen when spectra are recorded in CDCl3); 13C NMR (400
MHz, C2D2Cl4) δ 150.6, 136.6, 129.0, 121.8, 65.8, 35.0, 31.7; UV−vis
(CH2Cl2, room temperature) λmax (nm) [ε (M−1 cm−1)] 417 [6000],
509 [500].
Synthesis of 5,15-Bis(3,5-di-tert-butylphenyl)-10,20-bis-
(pentaphenylphenyl)phenyl Zincporphyrin (ZnP-PPP) (1c).
To the DCM solution of 1a (100 mg, 0.057 mmol) was added an
excess of zinc acetate Zn(OAc)2·2H2O. The completion of the
reaction was seen by monitoring the thin layer chromatogram of 1a
and the change in color from purple to dark pinkish: pinkish solid;
90% yield; Rf = 0.75 (3:4 CHCl3/petroleum ether); 1H NMR
(C2D2Cl4) δ 8.92 (s, 8H, β-pyrrolic-H), 8.02 (d, 3H, J = 16 Hz), 7.87
(s, 10H, Ar), 7.39 (d, 5H, J = 8 Hz), 7.27 (m, 18H), 7.09 (s, 5H),
7.07 (m, 8H), 7.06 (m, 10H, Ar−H), 6.71 (d, 3H, J = 12 Hz, Ar),
1.29 (s, 18H, tert-butyl), 1.19 (s, 18H, tert-butyl); experimental M+
1822.96; UV−vis (CH2Cl2, room temperature) λmax (nm) [ε (M−1
cm−1)] 413 [14000], 545 [322].
Synthesis of Precursors of 1a. 1-Formyldiphenylacetylene
(Tolan)54 was synthesized in the laboratory by the reaction of 4-
bromobenzaldehyde (272 mg, 1.46 mmol) with phenylacetylene (0.2
mL, 2 mmol) in the presence of piperidine (0.3 mL, 3 mmol) as the
base and palladium bis-triphenylphosphine (14.0 mg, 0.02 mmol) as
the reagent in a catalytic amount. The reaction mixture was heated in
a preheated oil bath at 70 °C for 10 min. The organic layer was
collected and then washed with acid [15% (v/v) HCl] until the
washings were neutral. The organic layer was washed again with
water, dried over Na2SO4, and purified by column chromatography
yielding pure substituted diphenylacetylene: yellow solid; 230 mg
Synthesis of 1b. To a stirring solution of 1a (49 mg, 0.028 mmol,
1 equiv) in CH2Cl2 solvent (20 mL) was added NOBF4 (32.48 mg,
0.28 mmol, 10 equiv) at 25 °C. The reaction mixture was vigorously
stirred until full conversion of the starting material was determined by
TLC. The reaction was slowly quenched with a saturated NaHCO3
solution (5 mL), and the mixture extracted with CHCl3 (15 mL). The
combined organic layers were dried over Na2SO4, and the reaction
mixture was dried with slow evaporation of the solvent at room
temperature: green solid; 90% yield; Rf = 0.60 (3:1 CHCl3/petroleum
1
yield (80%); Rf = 0.5 (1:9 ethyl acetate/hexane); H NMR (400
MHz, chloroform-D) δ 9.9 (s, 1H, CHO), 7.80−7.82 (s, 2H, CH),
7.61−7.63 (s, 2H, CH), 7.32−7.34 (m, 3H, CH); 13C NMR (400
MHz, CDCl3-D) δ 191.1, 135.3, 131.6, 129.4, 128.8, 128.3, 88.4; ESI-
MS 206.9.
Synthesis of Pentaphenylphenylbenzenecarbaldehyde
(PPP-CHO). PPP-CHO was synthesized in the laboratory using
1
ether); H NMR (CDCl3-D) δ 9.97 (s, 4H, HBC), 8.94 (d, 4H, β-
pyrrolic-H, J = 2.8 Hz), 8.44 (d, 4H, J = 4 Hz), 8.18 (d, 5H, J = 12
Hz), 7.88 (d, 4H, J = 16 Hz), 7.41 (d, 6H, J = 12 Hz), 7.1 (m, 11H),
6.98 (d, 6H, J = 12 Hz, Ar−H), 1.35 (s, 18H, tert-butyl), 1.27 (s, 18H,
G
https://dx.doi.org/10.1021/acs.inorgchem.0c01682
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
tert-butyl); 13C NMR (CDCl3) δ 151.7, 136.4, 129.8, 124.3, 123.3,
118.0, 55.6, 50.8, 31.3, 29.9, 19.0, 13.0; experimental M+ 1727.21;
UV−vis−NIR (CDCl3, room temperature) λmax (nm) [ε (M−1
cm−1)] 447 [124445], 676 [42860], 1723 [84845], 2287 [753000],
2491 [152900]; fluorescence λmax = 436, 481, 487, 554 (excitation at
417 nm), 524 (excitation at 445 nm), 740 (excitation at 368 nm) nm.
Synthesis of 1d (same as 1b). To a stirring solution of 1c (50.8
mg, 0.028 mmol, 1 equiv) in CH2Cl2 solvent (20 mL) was added
NOBF4 (10 equiv, 32 mg, 0.28 mmol) at 25 °C. The reaction mixture
was vigorously stirred until full conversion of starting material was
determined by TLC. The reaction was slowly quenched with a
saturated NaHCO3 solution (5 mL), and the mixture extracted with
CHCl3 (15 mL). The combined organic layers were dried over
Na2SO4, and the reaction mixture was dried with slow evaporation of
the solvent at room temperature: yellowish solid; 90% yield; Rf = 0.80
Sharvan Kumar − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Smriti Arora − Department of Chemistry, University of Delhi,
New Delhi 110007, India
Sohail Ahmad − Department of Chemistry, University of Delhi,
New Delhi 110007, India
Anshu Dandia − Department of Chemistry, University of Delhi,
New Delhi 110007, India
M. Thirumal − Department of Chemistry, University of Delhi,
New Delhi 110007, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01682
1
Author Contributions
(3:1 CHCl3/petroleum ether); H NMR (CDCl3-D) δ 11.4 (s, 4H,
§K.N. and V.K. contributed equally to this work.
HBC), 8.9 (d, 4H, β-pyrrolic-H, J = 5.2 Hz), 8.45 (d, 4H, J = 3.2 Hz,
Ar−H), 8.09 (s, 12H), 7.92 (d, 4H, J = 4.5 Hz), 7.76 (d, 4H, J = 2.4
Hz), 7.62 (d, 6H, J = 2.4 Hz), 7.11 (d, 6H, Ar−H, J = 11.2 Hz), 1.34
(s, 18H, tert-butyl), 1.22 (s, 18H, tert-butyl); 13C NMR (CDCl3) δ
157.0, 150.5, 129.9, 120.9, 123.4, 123.3, 119.9, 119.4, 53.9, 32.1, 29.9,
22.9, 14.4; experimental M+ 1806.11; UV−vis−NIR (CDCl3, room
temperature) λmax (nm) [ε (M−1 cm−1)] 417 [37955], 523 [22570],
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
UGC and DST India are acknowledged for financial assistance.
V.K. thanks the University Grants Commission (UGC) of
India and Indian Oil Corp. Ltd. for financial support.
1702 [112875], 2249 [268065], 2389 [231310]; fluorescence λmax
=
413, 433, 492 (excitation at 420 nm), 512 (excitation at 415 nm), 742
(excitation at 368 nm) nm.
Synthesis of the Radical Cation of 1a. To a solution of 1a in
CH2Cl2 solvent was added nitrosonium tetrafluoroborate (1 mg,
0.0086 mmol, 1 equiv). After this, the solution was added to 15 mg of
porphyrin 1a (15 mg, 0.0085 mmol, 1 equiv). The purple color of the
porphyrin-containing solution turned dark green as the reaction
proceeded. The reaction was performed in a glovebox under nitrogen
atmosphere conditions. The radical cation that formed was
characterized by ESR spectroscopy.
Synthesis of the Radical Cation of 1c. To a solution of 1c in
CH2Cl2 solvent was added nitrosonium tetrafluoroborate (1 mg,
0.0086 mmol, 1 equiv). After this, the solution was added to 15.6 mg
of porphyrin 1c (15.6 mg, 0.0086 mmol, 1 equiv). The pinkish color
of the porphyrin-containing solution became yellowish as the reaction
progressed. The reaction was performed in a glovebox under nitrogen
atmosphere conditions. The radical cation that formed was
characterized by ESR spectroscopy.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01682.
Methods and details, synthetic procedures, and spectra
of compounds (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Shive M. S. Chauhan − Department of Chemistry, University of
Delhi, New Delhi 110007, India; Email: smschauhan@
chemistry.du.ac.in, nylabhat.bn@gmail.com
Hemant K. Kashyap − Department of Chemistry, Indian
Institute of Technology Delhi, New Delhi 110016, India;
orcid.org/0000-0001-9124-2918; Email: hkashyap@
chemistry.iitd.ac.in
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New Delhi 110007, India; orcid.org/0000-0003-2300-
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Vikas Khatri − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
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