Optical properties and electronic structures of axially-ligated group 9 porphyrins

Article abstract of DOI:10.1142/S108842461550073X

A series of group 9 metal tetra-(p-tolyl)-porphyrin (M(ttp), M = Co(II), Rh(III), Ir(III)) complexes with axial phenyl substituents have been synthesized and characterized. An aryl bromide cleavage reaction of transition metal complexes was used to prepar

Full text of DOI:10.1142/S108842461550073X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–10
DOI: 10.1142/S108842461550073X
Published at http://www.worldscinet.com/jpp/
Optical properties and electronic structures of axially-ligated
group 9 porphyrins
Bei-Bei Wanga, Huiping Zuob, John Mack*c◊, Poulomi Majumdarc, Tebello Nyokongc◊,
Kin Shing Chan*b◊ and Zhen Shen*a◊
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures,
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
b Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
cDepartment of Chemistry, Rhodes University, Grahamstown 6140, South Africa
Received 4 May 2015
Accepted 24 May 2015
ABSTRACT: A series of group 9 metal tetra-(p-tolyl)-porphyrin (M(ttp), M = Co(II), Rh(III), Ir(III))
complexes with axial phenyl substituents have been synthesized and characterized. An aryl bromide
cleavage reaction of transition metal complexes was used to prepare the complexes from Co(ttp), Rh(ttp)Cl
and Ir(ttp)COCl, respectively. Magnetic circular dichroism (MCD) spectroscopy and TD-DFT calculations
have been used to study trends in the optical spectra and electronic structures. The effect of introducing
different para-substituents on the phenyl substituents was examined. During fluorescence emission studies,
phosphorescence was observed for the Ir(III) complexes in the near infrared (NIR) region.
KEYWORDS: metal porphyrins, MCD spectroscopy, TD-DFT calculations, phosphorescence.
ground and excited state degeneracy information that
INTRODUCTION
cannot be derived from analysis of UV-visible absorption
Metalloporphyrins (Fig. 1) are a class of tetrapyrrole
spectra and theoretical calculations alone [11]. The MCD
macrocycles whose inner pyrrole protons are replaced
spectroscopic technique provides valuable information
by a metal ion. The synthesis, optical spectroscopy
abouttheelectronicstructuresofmetalloporphyrins,when
and electrochemistry of metalloporphyrins have been
combined with the time-dependent density functional
investigated for decades [1]. Naturally occurring metallo-
theory (TD-DFT) calculations. A series of diamagnetic
porphyrins such as chlorophyll [2], vitamin B12 [3] and
group 9 complexes was selected for study, since the closed
heme [4], which play an important role in biological
shell d6 configurations of the M(III) ions result in novel
processes, have inspired research into the synthesis
optical properties. Ir(III) octaethylporphyrins and their
of novel metalloporphyrins and their applications in
tetra-benzo-fusedanalogswithaxialligationwithpyridine
catalytic oxidation [5, 6], photocatalytic oxidation [7, 8],
and N-(n-butyl)imidazole rings have been reported to
sensors and the process of the chemical bonds cleavage
have significantly different photophysical properties
[9, 10]. In this work, a series of transition metal porphyrin
from those of other group 9 metal porphyrin complexes
complexes were synthesized and studied by magnetic
due to the closed shell diamagnetic d6 configurations and,
circular dichroism (MCD) spectroscopy, which is based
in contrast with most transition metal complexes, they
on the differential absorbance of left and right circularly
have been reported to phosphoresce in the near infrared
polarized light, and provides key information about the
(NIR) region [12]. In this study, the para-substituents of
axial phenyl substituents of group 9 metal porphyrins are
varied by using the aryl bromide (Ar-Br) bond cleavage
◊ SPP full member in good standing
reaction of transition-metal complexes (Fig. 1) that has
been reported previously by the Chan group [13–15] to
study the effect on the optical properties and electronic
*Correspondence to: Zhen Shen, email: zshen@nju.edu.cn;
Kin Shing Chan, email: ksc@cuhk.edu.hk; John Mack, email:
j.mack@ru.ac.za
Copyright © 2015 World Scientific Publishing Company

2
B.-B. WANG ET AL.
R1
R2
Co1: M = Co, R1 = H, R2 = H
Co2: M = Co, R1 = CH3, R2 = H
Rh1: M = Rh, R1 = CH3, R2 = H
Rh2: M = Rh, R1 = CH3, R2 = F
Rh3: M = Rh, R1 = CH3, R2 = NO2
Ir1: M = Ir, R1 = CH3, R2 = F
Ir2: M = Ir, R1 = CH3, R2 =Br
Ir3: M = Ir, R1 = CH3, R2 = OMe
N
N
N
N
M
R1
R1
R1
Fig. 1. The structures of the synthesized group 9 metalloporphyrins
structures. Despite many decades of sustained research
interest in metalloporphyrins and improvements in the
application of the MCD technique, the spectroscopic
and theoretical analysis of the electronic structures of
metalloporphyrins with axial ligands has been limited.
Preparation of 5,10,15,20-tetra(p-tolyl)porphyrinato)-
rhodium(III)-phenyl (Rh1). Rh(ttp)Cl (20.0 mg, 0.024
mmol), K2CO3 (34.0 mg, 0.246 mmol), and t-BuOH
(0.12 mL, 1.264 mmol), and PhBr (0.026 mL) were
dissolved in 2.0 mL of degassed benzene, and the mixture
was heated at 120°C under N2 for 18 h. Excess benzene
was then removed by vacuum distillation and the residue
was purified by column chromatography on silica gel
eluted with CH2Cl2/hexane (1:1) to give Rh(ttp)Ph as a
red solid.Yield 19.8 mg (82%). Rf = 0.50 (CH2Cl2/hexane
(1:1)). 1H NMR (300 MHz; CDCl3): d, ppm 0.26 (d, 2H),
2.69 (s, 12H), 4.76 (dt, 2H), 5.23 (t, 1H), 7.52 (t, 8H),
8.00 (dd, 4H), 8.06 (dd, 4H), 8.76 (s, 8H). 13C NMR
(75 MHz; CDCl3): d, ppm 21.88, 120.68, 123.21, 124.22,
127.76, 129.11, 131.98, 134.11, 134.52, 137.62, 139.43,
143.39. HRMS (FAB): m/z 848.2360.
Preparation of 5,10,15,20-tetra(p-tolyl)porphyrinato)-
rhodium(III)-p-fluorophenyl(Rh2).Rh(ttp)Cl(20.0mg,
0.024 mmol), K2CO3 (34.0 mg, 0.246 mmol), t-BuOH
(0.12 mL, 1.264 mmol), and 4-bromofluorobenzene
(0.028 mL, 0.248 mmol) were dissolved in 2.0 mL of
degassed benzene and the mixture was heated at 120°C
under N2 for 8 h. Excess benzene was then removed
by vacuum distillation, and the residue was purified by
column chromatography on silica gel eluted with CH2Cl2/
hexane (1:1) to give Rh(ttp) C6H4(p-F) as a red solid.
Yield 12.4 mg (60%). Rf = 0.58 (hexane/CH2Cl2 (2:1)).
1H NMR (300 MHz; CDCl3): d, ppm 0.16 (dd, 2H), 2.69
(s, 12H), 4.55 (dd, 1H), 7.54 (d, 2H), 7.99 (dd, 4H), 8.02
(dd, 4H), 8.74 (s, 8H). 19F NMR (376 MHz; C6H5CF3): d,
ppm 62.84 (s, 1F). HRMS (FAB): m/z 866.2287.
EXPERIMENTAL
Synthesis
Preparation of 5,10,15,20-tetra(p-tolyl)porphyrinato-
cobalt(III)-phenyl (Co1). Co(ttp) (20.0 mg, 0.028 mmol),
PhBr (290.0 mL, 2.8 mmol), KOH (15.6 mg, 0.28 mmol)
and t-BuOH (130.0 mL, 1.37 mmol) were dissolved in
2.0 mL of degassed benzene, and the mixture was heated at
150°C under N2 for 3 h. Excess benzene was then removed
by vacuum distillation and the crude product was purified by
column chromatography over alumina eluted with CH2Cl2/
hexane (1:2) to give Co(ttp)Ph. Yield 19.8 mg (79%). Rf =
0.84 (CH2Cl2/hexane (1:2)). 1H NMR (400 MHz, CDCl3):
d, ppm 0.27 (d, 2H), 2.66 (s, 12H), 4.67 (t, 2H), 5.26
(t, 1H), 7.49 (d, 8H), 7.93 (s, 8H), 8.83 (s, 8H). 13C NMR
(100 MHz; CDCl3): d, ppm 21.7, 121.4, 122.0, 123.0,
127.7, 132.6, 133.0, 133.7, 137.5, 138.8, 145.5. HRMS
(FAB): m/z 804.2682.
Preparation of 5,10,15,20-tetraphenylporphyrinato-
cobalt(III)-phenyl (Co2). Co(tpp) (20.0 mg, 0.03 mmol),
PhBr (314.0 mL, 2.982 mmol), KOH (16.6 mg, 0.296 mmol)
and t-BuOH (140.0 mL, 1.474 mmol) were dissolved in
2.0 mL of degassed benzene, and the mixture was heated at
150°C under N2 for 2 h. Excess benzene was then removed
by vacuum distillation and the crude product was purified by
column chromatography over alumina eluted with CH2Cl2/
hexane (2:1) to give Co(tpp)Ph. Yield 24.4 mg (55%). Rf =
0.68 (CH2Cl2/hexane (2:1)). 1H NMR (400 MHz; CDCl3): d,
ppm 0.28 (d, 2H), 4.70 (t, 2H), 5.28 (d, 1H), 7.71 (m, 12H),
8.03 (s, 8H), 8.83 (s, 8H). HRMS (FAB): m/z 748.2016.
Preparation of 5,10,15,20-tetra(p-tolyl)porphyrinato-
rhodium(III)-p-nitro-phenyl (Rh3). Rh(ttp)Cl (20.0 mg,
0.024 mmol), K2CO3 (34.0 mg, 0.123 mmol), t-BuOH
(0.12 mL, 1.264 mmol), and 4-bromonitrobenzene
(50.0 mg, 0.248 mmol) were dissolved in 2.0 mL of
degassed benzene (1.0 mL) and the mixture was heated
at 120°C under N2 for 24 h. Excess benzene was then
removed by vacuum distillation and the residue was
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OPTICAL PROPERTIES AND ELECTRONIC STRUCTURES OF AXIALLY-LIGATED GROUP 9 PORPHYRINS
3
purified by column chromatography on silica gel eluted
Materials and equipment
with CH2Cl2/hexane (1:1) to give Rh(ttp)C6H4(p-NO2) as a
red solid. Yield 17.2 mg (80%). Rf = 0.68 (hexane/CH2Cl2
(1:1)). 1H NMR (400 MHz; CDCl3): d, ppm 0.37 (d, 2H),
2.70 (s, 12H), 5.61 (d, 2H), 7.55 (t, 8H), 8.00 (d, 4H),
8.06 (d, 4H), 8.83 (s, 8H). 13C NMR (176 MHz; CDCl3):
d, ppm 117.68, 123.13, 124.95, 127.69, 129.13, 132.07,
133.88, 134.26, 137.72, 138.78, 143.02. HRMS (FAB):
m/z 893.2230.
Unlessotherwisespecified,allreagentswerepurchased
from commercial suppliers and used directly without
further purification. The hexane used for chromatography
was distilled over anhydrous calcium chloride. Benzene
was distilled over sodium and stored in a Teflon-capped
tube under nitrogen gas before use. All reactions were
carried out in a Teflon screw capped tube wrapped with
aluminum foil to prevent undesired photochemical
reactions. Analytical thin-layer chromatography (TLC)
was performed on pre-coated silica gel 60 F254 plates.
Products were purified by chromatography on silica gel
(Merck, 70–230 and 230–400 mesh) or neutral alumina
(Merck, 90 active neutral, 70–230 mesh). 1H NMR
spectra were measured on Bruker DPX-300 (300 MHz)
and AvanceIII 400 (400 MHz) spectrometers, while
13C NMR spectra were recorded on Bruker DPX-300
(75 MHz), AvanceIII 400 (100 MHz), and AvanceIII
Preparation of 5,10,15,20-tetra(p-tolyl)porphyrinato-
iridium(III)-p-fluorophenyl (Ir1). Ir(ttp)(CO)Cl (19.8 mg,
0.021 mmol), K2CO3 (59 mg, 0.43 mmol) and 1-bromo-
4-fluorobenzene (4.1 mg, 2.6 mL, 0.024 mmol) were
dissolved in 2.0 mL of degassed benzene, and the mixture
was heated at 200°C under N2 for 20 h. Excess benzene
was then removed under vacuum distillation and the crude
product was purified by column chromatography over
alumina eluted with CH2Cl2/hexane (1:1) to give Ir(ttp)
C6H4(p-F) as a deep brown solid. Yield 20.3 mg (99%).
1
700 (176 MHz) spectrometers. H NMR spectra were
referenced internally to the residual proton resonance
1
Rf = 0.42 (CH2Cl2/hexane (1:1)). H NMR (300 MHz;
CDCl3): d, ppm 0.44 (dd, 2H), 2.68 (s, 12H), 4.54 (dd,
2H), 7.51 (d, 8H), 7.97 (d, 4H), 8.02 (d, 4H), 8.58 (s, 8H).
13C NMR (75 MHz; CDCl3): d, ppm 21.7, 86.7, 110.0 (d),
124.1, 127.6, 129.0 (d), 131.6, 133.7, 134.2, 137.5, 138.7,
143.0, 158.4 (d). HRMS (FAB): m/z 956.2855.
with tetramethylsilane (TMS, d, ppm 0.00), while CDCl3
(d, ppm 77.16) was used as the internal standard for 13
C
NMR spectroscopy. 19F NMR spectra were recorded on a
VarianXL-400spectrometerat376MHz. High-resolution
mass spectrometry (HR-MS) data were measured on a
ThermoFinnigan MAT 95 XL mass spectrometer in fast
atom bombardment (FAB) mode using 3-nitrobenzyl
alcohol matrix as the solvent or in electrospray ionization
(ESI) mode using MeOH/CH2Cl2 (1:1) as the solvent.
UV-visible absorption and luminescence spectra were
recorded on a Shimadzu UV-3600 spectrophotometer
and a Hitachi 4600 spectrofluorimeter. MCD spectra
were measured with a JASCO J-725 spectrodichrometer
equipped with a permanent magnet producing fields of
up to 1.5 T (tesla) with both parallel and anti-parallel
fields. The conventions recommended by Piepho and
Schatz [16] for the three Faraday terms, A1, B0 and C0,
that describe the intensity of MCD spectra are used
throughout.
Preparation of 5,10,15,20-tetra(p-tolyl)porphyri-
nato-iridium(III)-p-bromophenyl (Ir2). Ir(ttp)(CO)
Cl (21.4 mg, 0.023 mmol), K2CO3 (64 mg, 0.46 mmol)
and 1,4-dibromobenzene (6.0 mg, 0.025 mmol) were
dissolved in 2.0 mL of degassed benzene and the mixture
was heated at 200°C under N2 for 36 h. Excess benzene
was then removed under vacuum distillation and the crude
product was purified by column chromatography over
alumina eluted with CH2Cl2/hexane (1:1) to give Ir(ttp)
C6H4(p-Br) as a deep brown solid. Yield 17.9 mg (76%).
1
Rf = 0.52 (CH2Cl2/hexane (1:2)). H NMR (300 MHz;
CDCl3): d, ppm 0.37 (d, 2H), 2.68 (s, 12H), 4.85 (d, 2H),
7.52 (d, 8H), 7.99 (d, 4H), 8.02 (d, 4H), 8.59 (s, 8H). 13C
NMR (75 MHz; CDCl3): d, ppm 21.7, 94.1, 114.5, 124.0,
126.0, 127.7, 130.6, 131.6, 133.7, 134.2, 137.5, 138.6,
142.9. HRMS (FAB): m/z 1016.2055.
Preparation of 5,10,15,20-tetra(p-tolyl)porphyri-
nato)-iridium(III)-p-bromoanisole (Ir3). Ir(ttp)(CO)
Cl (20.0 mg, 0.022 mmol), K2CO3 (60 mg, 0.43 mmol)
and 4-bromoanisole (4.5 mg, 3.0 mL, 0.024 mmol) were
dissolved in 2.0 mL of degassed benzene and the mixture
was heated at 200°C under N2 for 9 h. Excess benzene
was then removed under vacuum distillation and the
crude product was purified by column chromatography
over alumina eluted with CH2Cl2/hexane (1:2) to give
Ir(ttp)C6H4(p-OMe) as a deep brown solid.Yield 18.6 mg
(89%). Rf = 0.45 (CH2Cl2/hexane (1:2)). 1H NMR
(300 MHz; CDCl3): d, ppm 0.46 (d, 2H,), 2.68 (s, 12H),
2.79 (s, 3H), 4.41 (d, 2H), 7.51 (d, 8H), 8.00 (d, 8H), 8.56
(s, 8H). 13C NMR (75 MHz; CDCl3): d, ppm 21.7, 54.1,
83.3, 109.3, 124.1, 127.6, 128.6, 131.5, 133.7, 134.2,
137.4, 138.8, 143.1, 153.8. HRMS (FAB): m/z 968.3057.
RESULTS AND DISCUSSION
Synthesis of metalloporphyrins
In order to synthesize the target group 9 metallo-
porphyrins, Co(por), Rh(ttp)Cl and Ir(ttp)COCl were
first synthesized using the literature procedures [17–19]
and an Ar-Br bond cleavage reaction [13–15] was then
used to prepare the target products by introducing phenyl
rings as axial ligands with differing para-substituents.
Co(tpp) did not react with PhBr until KOH was added to
the mixture. t-BuOH was then added as a phase transfer
reagent to increase the solubility of KOH in benzene,
so the reaction rate could be significantly increased.
When the loading of t-BuOH exceeds 50 equivalents,
Copyright © 2015 World Scientific Publishing Company
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B.-B. WANG ET AL.
Table 1. Ar-Br bond cleavage with Co(por)
Theoretical calculations
Geometry optimizations were carried out for the
group 9 complexes by using the B3LYP functional with
SDD basis sets (Fig. 2). The optical spectroscopy of
porphyrins can be described in terms of perturbations to
an ML = 0, 1, 2, 3, 4, 5, 6, 7, 8 sequence of MOs
2-
por
Time, h
Yield, %
associated with the parent C16H16 perimeter for the 16
ttpa
tppb
3
2
Co1 79%
Co2 55%
atom 18 p-electron system of the inner ligand perimeter
[20–23]. Michl [20–23] demonstrated that when the
symmetry of aromatic and heteroaromatic p-systems is
lowered by perturbations to the structure the alignments
of the nodal patterns of the four frontier p-MOs of the
a ttp = 5,10,15,20-tetra(p-tolyl)porphyrin. b tpp = 5,10,-
15,20-tetraphenylporphyrin.
parent perimeter are retained. The HOMO and LUMO
2-
16
of the parent C16H perimeter for porphyrins have ML =
Table 2. Ar-Br cleavage reactions with Rh(ttp)Cl
4 and 5 properties, respectively. This gives rise to
the B (or Soret) and Q-bands in the 400–450 and 500–
600 nm regions, respectively, based on allowed DML =
1 and forbidden DML = 9 transitions of Gouterman’s
4-orbital model [24]. Michl introduced an a, s, -a and -s
nomenclature for MOs (Fig. 3) whether there is a nodal
plane (a and -a) or antinodes (s and -s) aligned with
the y-axis of the porphyrin p-system [20–23]. TD-DFT
calculations were carried out by using the CAM-
B3LYP functional with SDD basis sets. The CAM-
B3LYP functional includes a long-range correction
of the exchange potential, which incorporates an
increasing fraction of Hartree–Fock (HF) exchange as
the interelectronic separation increases and is suitable
for complexes where there are bands with significant
charge transfer character as is likely to be the case
with d6 metal complexes. The dxz and dyz MOs are
predicted to lie close in energy to the a and s MOs of the
p-system, while the dz2 MO are predicted to lie within
2 eV of the LUMO, so there is scope for metal-to-ligand
charge transfer (MLCT) and d → d transitions in the
visible region.
FG
Time, h
Yield, %
p-H
18
8
Rh1 88%
Rh2 60%
Rh3 80%
p-F
p-NO2
24
Table 3. Ar-Br cleavage reactions with Ir(ttp)COCl
FG
Time, h
Yield, %
p-F
20
36
9
Ir1 99%
Ir2 76%
Ir3 89%
Optical spectroscopy
p-Br
p-OMe
The absorption spectra of the Co(ttp)Ph Co1 and
Co(tpp)Ph Co2 are shown in Fig. 4 and the energies of
the main spectral bands are tabulated in Table 1. Only
a very minor blue shift of the B-band is observed for
Co2 relative to Co1. The methyl para-substituents on
the phenyl rings are weakly electron-donating groups,
and only have a minor effect on the electron density
of the porphyrin ring and hence on the energies of the
porphyrin p-system MOs. The absorption spectra of
the Rh(ttp)Ph Rh1, Rh(ttp)C6H4(p-F) Rh2 and Rh(ttp)
C6H4(p-NO2) Rh3 (Fig. 4 and Tables 4 and 5) on the
one hand, and Ir(ttp)C6H4(p-F) Ir1, Ir(ttp)C6H4(p-Br)
Ir2 and Ir(ttp)C6H4(p-OMe) Ir3 on the other hand,
exhibited almost no difference in the positions of the Q
and B-bands of the Rh(III) and Ir(III) complexes, since
the para-substituents of the axial ligands do not have a
significant effect on the relative energies of the frontier
p-MOs of the porphyrin p-system. Since the ligands lie
however, the yield of Co(ttp) starts to decrease. These
reaction conditions are also applicable to Co(tpp). The
more electron deficient Co(tpp) has a faster reaction
rate and lower yield than Co(ttp) (Table 1). Rh(ttp)Cl
was found to react very slowly in benzene with PhBr in
the absence of base at 120°C. When a series of bases
(KOAc, K2CO3, KOH/t-BuOH) were added, the reaction
time shortened significantly. KOH was selected, since the
highest yields were obtained. 4-Bromofluorobenzene and
4-bromonitrobenzene also react with Rh(ttp)Cl to give
the target products in good yields (Table 2) and theAr-Br
cleavage rates for Ir(ttp)(CO)Cl were found to be greatly
enhanced by the presence of base (Table 3). K2CO3 was
finally selected in this regard based on a consideration of
both the reaction rate and the yield.
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OPTICAL PROPERTIES AND ELECTRONIC STRUCTURES OF AXIALLY-LIGATED GROUP 9 PORPHYRINS
5
Fig. 2. MO energies of the group 9 metal porphyrins. The a, s, -a and -s MOs of the porphyrin p-system are highlighted with thicker
gray lines with black triangles used to denote the a and -a MOs, and black circles used to denote the s and -s MOs. Asterisks (gray
for dyz and black for dxz) and crosses (gray for dz2 and black for dxy) are used to denote MOs associated primarily with the d orbitals
of the central metal ion. Gray diamonds are used to denote the HOMO-LUMO gaps and are plotted against a secondary axis
Fig. 3. Angular nodal patterns and energies for the a, s, -a and -s MOs of Co1 at an isosurface of 0.02 a.u. Michl introduced the a,
s, -a and -s nomenclature deopending on whether there is a nodal plane (a and -a) or antinodes (s and -s) aligned with the y-axis of
the porphyrin -system [20–23]. Frontier MOs with significant dxz, dyz and dz2 character are also shown
Copyright © 2015 World Scientific Publishing Company
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B.-B. WANG ET AL.
Fig. 4. Absorption and MCD spectra of the group 9 metal porphyrins in DCM. TD-DFT spectra calculated using the CAM-B3LYP
functional with SDD basis sets are plotted against a secondary axis. The Q and B-bands of the porphyrin p-system are highlighted
with large black diamonds (Table 5), while other pp* states are denoted with gray diamonds. Light gray and black circles are used
to highlight bands associated primarily with MLCT and d → d transitions, respectively. Vertical dashed lines are used to highlight
the energy values from right to left corresponding to 400, 500 and 600 nm
in an orthogonal arrangement relative to the p-system,
the para-substituent only has an inductive rather than a
mesomeric effect. Michl has demonstrated that inductive
substituent effects, which depend primarily on changes
in the s-bonding framework, affect the a, s, -a and -s
MOs to similar extents and hence are not expected to
have a significant effect on the HOMO-LUMO values
[22]. In contrast, the most intense absorption bands in
the Q-band regions of the Co(ttp)Ph Co1, Rh(ttp)Ph Rh1
and Ir(ttp)(p-F) Ir1 spectra range from 511 to 527 nm
(Fig. 4 and Table 4). When MCD spectra are recorded,
however, it becomes clear that the main electronic Q(0,0)
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OPTICAL PROPERTIES AND ELECTRONIC STRUCTURES OF AXIALLY-LIGATED GROUP 9 PORPHYRINS
7
Table 4. UV-visible absorption data for the group 9 metal porphyrins
Toluene
DCM
Acetonitrile
l, nm (e × 105)
l, nm (e × 105)
l, nm (e × 105)
Qvib
B(0,0)
Qvib
B(0,0)
Qvib
B(0,0)
Co1
Co2
Co(ttp)Ph
Co(tpp)Ph
528 (0.223)
528 (0.024)
520 (0.333)
520 (0.158)
520 (0.572)
508 (0.238)
509 (0.123)
507 (0.197)
411 (2.143)
409 (0.224)
414 (2.981)
415 (1.358)
415 (4.975)
408 (3.202)
408 (1.665)
409 (2.553)
527 (0.1671)
525 (0.027)
519 (0.303)
519 (0.2991)
520 (0.5710)
511 (0.222)
512 (0.143)
509 (0.254)
410 (1.751)
408 (0.271)
413 (2.740)
413 (2.718)
413 (5.446)
406 (3.207)
407 (2.036)
407 (3.780)
528 (0.172)
527 (0.022)
528 (0.160)
530 (0.152)
529 (0.368)
534 (0.096)
536 (0.065)
535 (0.109)
408 (1.979)
407 (0.233)
416 (1.899)
416 (1.771)
417 (4.091)
423 (1.968)
423 (1.296)
425 (1.976)
Rh1 Rh(ttp)Ph
Rh2 Rh(ttp)C6H4(p-F)
Rh3 Rh(ttp)C6H4(p-NO2)
Ir1
Ir2
Ir3
Ir(ttp)C6H4(p-F)
Ir(ttp)C6H4(p-Br)
Ir(ttp)C6H4(p-OMe)
bands lie in the weak tail of absorption intensity to the
red of these bands. The MCD spectra of porphyrins
with radially symmetric tetraphenylporphyrins are
normally dominated by derivative-shaped pseudo-A1
terms [11]. When there is only a minor lifting of the
orbital degeneracy of the LUMO of the p-systems, as
is the case with the series of group 9 metal porphyrins
in this study due to the presence of an axial ligand
(Fig. 2), the Faraday A1 terms are replaced with coupled
pairs of Gaussian-shaped oppositely-signed B0 terms
that can be viewed as pseudo-A1-terms. On this basis
the main Q(0,0) bands of the Co(III), Rh(III) and Ir(III)
complexes lie at ca. 555, 553 and 580 nm, respectively.
The trends predicted in the HOMO-LUMO gaps (Fig. 2)
broadly reflect this.
It should be noted, that the trend observed in the band
energies (Fig. 4) does not match that of the predicted
HOMO-LUMO gaps and calculated Q(0,0) band
energies (Fig. 2 and Table 5). When 1a, 2a and 3a are
considered, there is a destabilization of the LUMO (Fig.
2) that is probably related to the differing sizes of the
ions and hence their overlap with the lone pairs of the
pyrrole nitrogens and changes in the degrees of mixing
with the dxz and dyz MOs, which also have eg symmetry
under D4h symmetry [24]. In contrast, the B-band
of the Ir(III) complexes lie to the blue of those of the
Co(III) complexes as would be anticipated based on the
TD-DFT calculations. The most likely explanation for
the discrepancy in this regard is that the configurational
interaction between the main pp* and MLCT or dd states
is not accurately predicted in the Q-band region. The
Gaussian-shaped Faraday B0 terms in the 415-480 nm
region of the MCD spectra of Co1 and Co2 (Fig. 4) are
consistent with the presence of the MLCT bands predicted
by the TD-DFT calculations. The absence of derivative-
shaped signals in the MCD spectra of Rh1, Rh2 and Rh3
with crossover points that correspond to the maxima of the
intense absorption bands at ca. 414 nm, is also consistent
with this. It is noteworthy that the envelope of absorption
intensity in the B-band region of the spectra of the Ir(III)
complexes are considerably less broadened than those of
the corresponding Co(III) complexes. This is consistent
with the marked differences in the energies predicted for
the d → d transitions in the TD-DFT calculations (Fig.
4). There is a pseudo-A1 term at 408 nm in the spectrum
of Ir3, with a broad negative Faraday B0 term centered at
440 nm associated with an MLCT transition. These bands
overlap significantly in the spectra of Ir(ttp)C6H4(p-F)
Ir1 and Ir(ttp)C6H4(p-Br) Ir2. A marked destabilization
of the dxz MO of Ir(ttp)C6H4(p-OMe) Ir3 is predicted,
which lies coplanar with the phenyl p-system, and hence
provides more scope for an interaction with the electron-
donating p-OMe substituent of Ir3. In contrast, a marked
stabilization is predicted for this MO when an electron-
withdrawing p-NO2 is introduced into the structure of
Rh(ttp)C6H4(p-NO2) Rh3.
Luminescence spectra
The luminescence spectra of Ir(ttp)C6H4(p-F) Ir1,
Ir(ttp)C6H4(p-Br) Ir2, and Ir(ttp)C6H4(p-OMe) Ir3
are shown in Fig. 5. The phosphorescence quantum
yields are 0.55% for Ir1, 3.12% for Ir2 and 1.4% for
Ir3, respectively upon excitation at 405 nm. Emission
intensity was not detected for the Co(III) and Rh(III)
complexes under the same experimental conditions.
This is not surprising, since luminescence has only been
reported previously for group 8 d6 metal porphyrins
[25–27] and for Ir(III) octaethylporphyrins and their
tetra-benzo-fused analogs axially ligated with pyridine
and N-(n-butyl)imidazole rings [12]. The emission that is
observed in the NIR region demonstrates that the Ar-Br
bond cleavage reactions of transition-metal complexes
can be used to prepare a wider range of Ir(III) porphyrin
complexes that are potentially suitable for practical
applications such as photodynamic therapy, oxygen
detection, organic light-emitting diodes, photocatalysis,
photovoltaics and upconversion.
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 7–10

8
B.-B. WANG ET AL.
Table 5. TD-DFT spectra of the B3LYP optimized geometries of the complexes calculated with the CAM-B3LYP
functional and 6-31G(d) basis sets
b
Banda
#
Calcd.c
Expd
Wavefunction=e
Co(ttp)Ph – (Co1)
—
Q
1
4
—
—
—
—
—
Ground state
19.2
19.2
27.4
27.5
521
520
365
363
(0.00)
(0.00)
(1.14)
(1.14)
18.0
555
33% s → -a; 29% a → -s; 17% s → -s; 15% a → -a; …
21% s → -s; 20% a → -a; 17% s → -a; 15% a → -s; …
48% a → -a; 40% s → -s; …
5
B
9
24.4
410
10
45% a → -s; 39% s → -a; …
Co(tpp)Ph – (Co2)
—
Q
1
2
—
—
—
—
—
Ground state
19.1
19.2
27.4
27.5
521
520
365
364
(0.01)
(0.01)
(1.18)
(1.14)
18.0
555
44% s → -a; 39% a → -s; …
30% s → -s; 27% a → -a; …
47% a → -a; 39% s → -s; …
43% a → -s; 29% s → -a; …
3
B
9
24.6
408
10
Rh(ttp)Ph – (Rh1)
—
Q
1
2
—
—
—
—
—
Ground state
19.8
19.8
27.9
28.0
506
504
358
357
(0.01)
(0.01)
(1.17)
(0.88)
18.1
554
54% s → -s; 43% a → -a; …
52% s → -a; 45% a → -s; …
48% a → -a; 39% s → -s; …
33% a → -s; 28% s → -a; 21% dyz → -s; …
3
B
10
11
24.2
413
Rh(ttp)C6H4(p-F) – (Rh2)
—
Q
1
2
—
—
—
—
—
Ground state
19.8
19.8
27.9
27.9
506
504
358
358
(0.01)
(0.01)
(1.21)
(1.19)
18.1
553
55% s → -s; 44% a → -a; …
53% s → -a; 46% a → -s; …
45% a → -a; 37% s → -s; …
42% a → -s; 36% s → -a; …
3
B
9
24.2
413
10
Rh(ttp)C6H4(p-NO2) – (Rh3)
—
Q
1
2
—
—
—
—
—
Ground state
19.8
19.8
28.0
28.0
506
504
358
357
(0.01)
(0.01)
(1.21)
(1.31)
18.1
552
54% s → -s; 44% a → -a; …
52% s → -a; 46% a → -s; …
48% a → -a; 40% s → -s; …
49% a → -s; 43% s → -a; …
3
B
10
11
24.1
413
Ir(ttp)C6H4(p-F) – (Ir1)
—
Q
1
2
—
—
—
—
—
Ground state
20.5
20.6
28.5
28.6
487
485
351
350
(0.01)
(0.00)
(1.21)
(1.31)
17.2
580
52% s → -s; 46% a → -a; …
50% s → -a; 48% a → -s; …
47% a → -a; 42% s → -s; …
47% a → -s; 46% s → -a; …
3
B
10
11
24.6
406
Ir(ttp)C6H4(p-Br) – (Ir2)
—
Q
1
2
—
—
—
—
—
Ground state
20.5
20.6
28.5
28.6
487
485
350
350
(0.01)
(0.00)
(1.24)
(1.32)
17.3
578
52% s → -s; 46% a → -a; …
50% s → -a; 48% a → -s; …
47% a → -a; 42% s → -s; …
47% a → -s; 46% s → -a; …
3
B
10
11
24.6
407
(Continued)
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 8–10

OPTICAL PROPERTIES AND ELECTRONIC STRUCTURES OF AXIALLY-LIGATED GROUP 9 PORPHYRINS
9
Table 5. (Continued)
Ir(ttp)C6H4(p-OMe) – (Ir3)
—
Q
1
3
—
—
—
—
—
Ground state
20.6
20.7
28.5
28.5
485
484
351
350
(0.00)
(0.00)
(1.20)
(1.29)
17.2
580
51% s → -a; 46% a → -s; …
48% a → -a; 48% s → -s; …
46% a → -s; 41% s → -a; …
47% a → -a; 45% s → -s; …
4
B
10
11
24.5
407
a Band assignment described in the text. b The number of the state assigned in terms of ascending energy within the TD-DFT
calculation. c Calculated band energies (103.cm-1), wavelengths (nm) and oscillator strengths in parentheses (f). d Observed
energies (103.cm-1) and wavelengths (nm) in Fig. 4. e The wave functions based on the eigenvectors predicted by TD-DFT. One-
electron transitions associated with Michl’s perimeter model [20–23] are highlighted in bold. H and L refer to the HOMO and
LUMO, respectively.
Nos. 2013CB922101 & 2011CB808704), the National
Natural Science Foundation of China (No. 21371090),
and the Natural Science Foundation of Jiangsu
Province (BK20130054) to ZS, the China-South Africa
joint research program (CS08-L07) to ZS and JM, and
a CSUR grant (93627) from the NRF of South Africa
to JM. The theoretical calculations were carried out at
the Centre for High Performance Computing in Cape
Town.
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A series of group 9 metal tetra(p-tolyl)porphyrins
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reaction used to introduce the phenyl substituents and
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Financial support was provided by the Major State
Basic Research Development Program of China (Grant
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J. Porphyrins Phthalocyanines 2015; 19: 9–10

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