Trends in the optical and redox properties of tetraphenyltetraphenanthroporphyrins

Article abstract of DOI:10.1142/S1088424612500885

The results of TD-DFT calculations for a series of tetraaryltetraphenanthroporphyrins containing para-substituents with differing electron donating and accepting properties are compared to the observed optical and redox properties and Michl's perimeter mo

Full text of DOI:10.1142/S1088424612500885

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 833–844
DOI: 10.1142/S1088424612500885
Published at http://www.worldscinet.com/jpp/
Trends in the optical and redox properties of tetraphenyl-
tetraphenanthroporphyrins
John Mack†a◊, Kevin Lobbb, Tebello Nyokong*b◊, Zhen Shen*c◊ and
Nagao Kobayashi*a◊
a Department of Chemistry, Graduate School of Science, Tohoku University, 980-8578 Sendai, Japan
b Department of Chemistry, Rhodes University, 6140 Grahamstown, South Africa
c State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
Dedicated to Professor Tebello Nyokong on the occasion of her 60th birthday
Received 15 January 2012
Accepted 22 March 2012
ABSTRACT: The results of TD-DFT calculations for a series of tetraaryltetraphenanthroporphyrins
containing para-substituents with differing electron donating and accepting properties are compared to
the observed optical and redox properties and Michl’s perimeter model is used as a conceptual framework
for analyzing the results.
KEYWORDS: tetraphenyltetraphenanthroporphyrins, Michl’s perimeter model, TD-DFT calculations,
MCD spectroscopy.
form sapphyrins, hexaphyrins, octaphyrins, etc. [8],
co-planar polymerization of porphyrins [9], the expansion
INTRODUCTION
Red-shifted porphyrinoid chromophores have been
the focus of considerable research interest in recent
of the p-system with peripheral fused rings [10, 11] and
conformational distortions due to steric crowding of
years because of their potential use as photosensitizers in
peripheral substituents [12, 13].
photodynamic therapy (PDT) [1], as fluorescent probes
Although fused ring expansion of the ligand at the
for the recognition of cationic or anionic analytes [2] and
as dyes for applications in nonlinear optics [3], optical
limiting [4], opto-electronic devices [5] and solar energy
conversion [6]. A red shift of the absorption and emission
b-pyrrole positions is probably the most obvious strategy
for modifying the electronic and optical properties of the
p-system, only limited red shifts have been observed upon
substitution with fused benzene rings [10]. Spence and
bands of porphyrins is usually achieved by expanding the
Lash [14] demonstrated that the introduction of peripheral
p-conjugation system. A number of different approaches
have been adopted in this regard over the last decade,
including the introduction of meso-alkynyl substituents
[7], an increase in the number of core pyrrole rings to
acenaphthalene rings has a significantly larger impact on
the energies of the main pp* transitions and that further
red shifts are observed when phenyl substituents are
added at the meso-carbons to form the deeply saddled
tetraphenyltetraacenaphthoporphyrin (TPTANP) ligand.
Shen and coworkers subsequently prepared a series of
even more deeply saddled meso-tetraaryltetraphenanthr-
oporphyrin (TPTP) ligands [15, 16] (Scheme 1), which
resulted in a larger red shift of the Q and B bands
relative to the spectra of tetraphenylporphyrin (TPP)
and TPTANP. Mack et al. [17] demonstrated, based on
an application of Michl’s perimeter model [18] to the
magnetic circular dichroism (MCD) spectroscopy and
SPP full member in good standing
*Correspondence to: Tebello Nyokong, email: t.nyokong@
ru.ac.za, tel: +27 46-603-8801, fax: +27 46-622-5109; Zhen
Shen, email: zshen@nju.edu.cn, tel: +86 25-8368-6679, fax:
+86 25-8331-4502 and Nagao Kobayashi, email: nagaok@m.
tohoku.ac.jp, tel/fax: +81 22-795-7719
†Current address: Department of Chemistry, Rhodes University,
6140 Grahamstown, South Africa
Copyright © 2012 World Scientific Publishing Company

834
J. MACK ET AL.
C
N-CH2CO2Et
KOH
DBU THF
(CH2OH)2
170 °C
CO2Et
N
H
N
H
NO2
1
2
3
BF3·Et2O
dry CH2Cl2
-50 °C
ArCHO
Ar
Ar
NH
NH
NH
HN
HN
N
Triethylamine
DDQ
Ar
Ar
Ar
Ar
N
HN
Ar
Ar
5
4
Scheme 1. Scheme for the preparation of 5a (Ar = C6H5), 5b (Ar = 4-Me-C6H4), 5c (Ar = 4-F-C6H4), 5d (Ar = 4-Cl-C6H4), 5e
(Ar = 4-Br-C6H4), 5f (Ar = 4-I-C6H4), 5g (Ar = 4-NC-C6H4), 5h (Ar = 4-O2N-C6H4) and 5i (Ar = 4-Me2N-C6H4)
theoretical calculations of seventeen radially symmetric
zinc porphyrinoids, that key trends observed in the spectral
properties of porphyrinoids are largely determined by the
relative energies of the four frontier p-MOs associated
with Gouterman’s 4-orbital model [19]. Recent studies on
core modified tetrabenzoporphyrins [20] and a-phenylated
phthalocyanines [21] have demonstrated that the results
of DFT geometry optimizations are very similar to those
obtained by X-ray crystallography when steric hindrance
between peripheral fused rings results in severe ligand
folding due to the lack of conformational flexibility.
Trends observed in the experimental data for TPTPs
can, therefore, be readily compared to those predicted in
theoretical calculations for B3LYP optimized geometries
even in the absence of X-ray structures. In this paper the
utility of TD-DFT calculations for predicting the optical
and redox properties of porphyrinoid structures, which
differ with regard to their peripheral fused ring and aryl
meso-substituents, is assessed.
The deep saddling distortion of the p-system predicted
in the B3LYP geometry optimization for 5i (Fig. 1), tilts
the meso-phenyl rings almost into the plane of the four
pyrrole nitrogens facilitating conjugation between the
meso-groups and the inner perimeter of the p-system
[16]. As a result, even subtle electronic effects such as
those associated with the halogen substituent series, 5c–5f
(Scheme 1), were found to alter the optical spectroscopy
to a readily observable extent. A strong correlation was
observed for the compounds with electron withdrawing
substituents when the B band maxima of free base TPTPs
were plotted against the Hammett parameter sp [16]. In
this paper, a detailed analysis of TD-DFT calculations for
various fused-ring-expanded porphyrinoid compounds
and a series of eight TPTP compounds containing
different para-substituents on the phenyl groups is
reported, which provides further insights into the optical
and redox properties of these compounds.
EXPERIMENTAL
Theoretical calculations
Theoretical calculations were carried out using the
G03W software package [22] on the Perkin computer
cluster in the Department of Chemistry at Rhodes
University in Grahamstown, South Africa. The B3LYP
functional was used with 6-31G(d) basis sets for both the
geometry optimizations and the TD-DFT calculations
for the fused-ring-expanded porphyrinoid compounds,
5a–5e and 5g–5i, while 3-21G* basis sets were used
for 5f.
ELECTRONIC STRUCTURE OF PORPH-
YRINOIDS
MCD spectroscopy [23] has been used to definitively
identifythemainelectronicbandsofaseriesofTPTPs[16].
Copyright © 2012 World Scientific Publishing Company
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TRENDS IN THE OPTICAL AND REDOX PROPERTIES OF TETRAPHENYLTETRAPHENANTHROPORPHYRINS
835
Fig. 1. The B3LYP-optimized geometry of 5i exhibit a deep saddled conformation when viewed along the z- (LEFT) and y- (RIGHT)
axes
In this paper, the MCD spectral data are used to validate
theoretical descriptions of the electronic structures
provided by calculated TD-DFT and ZINDO/s spectra.
The analysis of the MCD spectra is based on intensity
mechanisms described by the Faraday A1, B0 and C0 terms
[23]. The application of TD-DFT to the three Faraday
terms is still under development so MCD spectra can
not currently be calculated using commercial software
packages such as the Gaussian and Amsterdam Density
Functional software suites. Older theoretical approaches
for describing the electronic structures of porphyrinoids,
such as Gouterman’s four orbital model [19] and Michl’s
perimeter model [18], can still be used to provide a
conceptual framework for validating the results of
theoretical calculations, however. Moffitt [24] and Michl
[18] demonstrated that the relative intensities of the
major electronic absorption bands of aromatic p-systems
can be successfully described in terms of perturbations
to the structure of a high-symmetry parent hydrocarbon
Michl’s terminology [18]) there is a mixing of the
allowed and forbidden properties of the Q and B bands
and a marked intensification of the Q band, which as a
consequence can become the dominant spectral feature
for some porphyrinoids such as the phthalocyanines
[25]. When DHOMO ≈ DLUMO the Q bands remain
relatively weak, since the orbital angular momentum
(OAM) properties of the parent hydrocarbon perimeter
are retained and absorption intensity in the Q band region
is based primarily on vibrational borrowing from the
allowed B(0,0) bands [26].
Michl [18] introduced an a, s, -a and -s terminology
for the four MOs derived from the HOMO and LUMO of
the parent perimeter so that p-systems of porphyrinoids
with differing molecular symmetry and with different
relative orderings of the four frontier p-MOs in energy
terms can be readily compared. One MO derived from
the HOMO of the parent perimeter and another derived
from the LUMO have nodal planes which coincide with
the yz-plane and are referred to, respectively, as the a and
–a MOs, while the corresponding MOs with antinodes
on the yz-plane are referred to as the s and –s MOs. The
HOMO level of 4N+2 p-electron systems have 2N nodal
planes, while the LUMO levels have 2(N+1) effectively
forming an octagon or a decagon on the 16 atom inner
perimeter (Fig. 2). The D4h symmetry of the porphyrin
dianion dictates that the –a and –s MOs are degenerate
since the ML = 5 nodal patterns differ along the x- and
y-axes only by being rotated by 90° with respect to each
other.
2-
(C16H16 in the case of metal tetrapyrrole porphyrinoid
complexes or C18H18 for free base compounds). The
nodal patterns of the p-system MOs are retained even
when the symmetry of the cyclic perimeter is modified.
As a consequence of this there is an ML = 0, 1, 2, 3,
4, 5, 6, 7, 8 sequence in ascending energy terms
in the p-MOs of metal porphyrinoids in an analogous
manner to the ML = 0, 1, 2, 3 p-MO sequence that
forms the basis of the aromatic properties of benzene.
Within the band nomenclature of Gouterman’s 4-orbital
model [19], there is an electronically allowed B transition
and a forbidden Q transition linking the frontier ML =
4, 5 p-MOs based, respectively, on DML = 1 and
9 transitions. When a structural perturbation results in
a marked lifting of the degeneracy of the MOs derived
from the HOMO and/or LUMO of the parent perimeter
(referred to as the DHOMO and DLUMO values within
RESULTS AND DISCUSSION
The B3LYP optimized geometries calculated
with 6-31G(d) basis sets for 5a–5e, 5g and 5h have
geometries similar to that of 5i (Fig. 1 and Table 1),
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 835–844

836
J. MACK ET AL.
Fig. 2. In Michl’s perimeter model [18] the nodal properties of the frontier p-MOs on the inner perimeter of porphyrinoid ligands
can be understood based on a C16H162- parent hydrocarbon. Gray and black are used to denote the phase changes in the isosurfaces of
the MO at 0.04 a.u. The 2N nodal planes of the MOs derived from the HOMO of a 4N+2 parent perimeter typically form octagons
on the perimeter with the planes running through alternating atoms, while the 2(N+1) nodal planes of the LUMO level MOs
form a decagon in which the nodal planes, with the exception of those lying on either the x- or y-axis, do not coincide with atom
positions. When there is a symmetry lowering perturbation such as fused ring expansion or core modification, the nodal patterns are
determined by the alignment of the plane of symmetry in the yz-plane. Michl introduced a, s, -a and –s nomenclature to describe the
four frontier p-MOs in this context based on whether there is a nodal plane (a and -a) or an antinode (s and -s) on what corresponds
to the ML = 0 in Fig. 3. Similar nodal patterns can be observed in the nodal patterns of the four frontier p-MOs of 5a [16]. Once the
alignment of the nodal planes has been clearly defined the effect of different structural perturbations can be readily conceptualized
on a qualitative basis through a consideration of the relative size of the MO coefficients on each atom on the perimeter
Table 1. Key parameters predicted for the major spectral bands
in the TD-DFT and ZINDO/s calculations of 5a, 5b and 5g–5i
since the deep saddling of the ligand is determined
primarily by the steric crowding caused by the fused
phenanthrene ring moieties and the aryl groups on the
meso-carbons. Michl’s perimeter model [18] can be
used to rationalize how different peripheral fused rings
affect the HOMO-LUMO band gap and the DHOMO
and DLUMO values, and hence the spectral properties
(Fig. 3). The effect of adding fused rings to the periphery
of the porphyrin ligand can be readily understood
with reference to the effect of the symmetry lowering
structural perturbations on the relative energies of four
key frontier p-MOs primarily associated with the 16
atom 18 p-electron cyclic polyene on the inner ligand
perimeter. The perimeter model approach often enables
accurate predictions about key aspects of the electronic
structure and optical spectroscopy to be made based
on a qualitative consideration of the alignment of the
nodal planes of the four frontier p-MOs. Under the D2h
symmetry of porphyrin compounds the alignment of
the 2N nodal planes of the HOMO level is defined for
symmetry reasons by the mirror planes running through
the pyrrole nitrogens, and this is used to define the
identity of the ML = 0 MO in Fig. 3. Once the alignment
of the nodal planes has been defined the effect of various
structural perturbations on the MO energies can be
readily determined.
The a and s MOs of porphyrin compounds remain
accidentally near degenerate. The net stabilizing effect
on the s MO due to the increased electronegativity of the
four pyrrole nitrogens at nodes of the a MO is matched
by a stabilizing effect on the a MO related to the presence
of the four peripheral C2H2 bridges at nodes of the s MO
(Fig. 4). There is a strong bonding interaction between
the inner ligand perimeter portion of the a MO of and
a
b
c
d
e
f
P
4.22
4.24
4.19
4.16
4.16
4.15
4.15
4.15
4.19
4.10
4.24
4.08
4.06
4.09
4.09
4.11
4.10
4.10
4.10
4.10
4.20
4.14
8.52
8.50
8.36
6.90
6.81
6.81
6.81
6.81
6.79
6.77
6.79
8.52
8.53
8.32
6.87
6.78
6.80
6.80
6.80
6.80
7.01
6.74
6.88
—
TPP
TPhen
5a
6.93 15.56
6.80
—
6.75 15.36
6.72 15.36
6.72 15.29
6.72 15.29
6.72 15.28
6.72 15.31
6.73 15.37
6.73 15.40
5b
5c
5d
5e
5g
5h
5i
Copyright © 2012 World Scientific Publishing Company
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TRENDS IN THE OPTICAL AND REDOX PROPERTIES OF TETRAPHENYLTETRAPHENANTHROPORPHYRINS
837
a stabilizing interaction introduced along one
axis is balanced by a destabilizing interaction
along the other so a low DLUMO value is
retained. The marked destabilization of the
a MO leads to a narrowing of the HOMO-
LUMO band gap and the introduction of a
significant DHOMO value, so there is a red
shift and intensification of the Q band. Only
a relatively minor red shift is observed for
the B band due to the interaction between
the B and higher energy pp* states [17].
Radially symmetric fused-ring-expansion
of the porphyrin ligand with acenaphthalene
moieties to form tetraacenaphthoporphyrin
(TANP) has a markedly different effect on the
relative energies of the four frontier p-MOs.
A destabilization of both the a and s MOs
is anticipated, since there are antibonding
interactions between the b-carbons of the
pyrrole moieties and the two carbon atoms of
Fig. 3. Michl’s perimeter model [18] for C16H126- (LEFT). The circle represents
a diagrammatic representation of the clockwise and counterclockwise motion
of p-system electrons on the inner ligand perimeter generating the ML value
for each complex p-MO. The alignment and magnitude of the magnetic
moments induced by the electron motion within each p-MO (CENTER) can
be predicted based on the right hand rule for solenoids and LCAO calculations.
The moments aligned along the z-axis with or against the applied field are
shown diagrammatically [18]. Right and left handedness is defined within
the peripheral C10H6 bridging units to which
they are attached, but this is relatively minor
in magnitude, since the MO coefficients on
classical optics looking towards the light source of the CD spectrometer (lcp
and rcp = left and right circularly polarized light)
the fused acenaphthalene rings are relatively
what can be viewed as four antibonding ethylene MOs
on the ligand periphery. Michl [18] has demonstrated that
the effect of the introduction of bridging atoms on the
MO energies is determined primarily by the number of
atoms in the bridge. When the bridge consists of an even
number of atoms the four frontier p-MOs of the parent
perimeter are retained as the four frontier p-MOs of the
expanded p-system. The size and relative sign phases
of the MO coefficients on the perimeter atom where
the bridge is attached and the first atom of the bridge
determine the ordering of the a, s, -a and –s MOs. In
the case of metal complexes, symmetry considerations
dictate that the -a and -s MOs remain degenerate,
since only the degeneracy of the ML = 2, 4, 6 MOs
is lifted on moving from D16h to D4h symmetry (Fig. 4).
The introduction of protonated pyrrole nitrogens along
the y-axis of free base compounds results in a slight
lifting of the degeneracy of the LUMO level. In the –a
and –s MOs, the interactions with the peripheral ethylene
bridges along the x- and y-axes are broadly similar, so
only a minor lifting of the degeneracy of the LUMO is
anticipated based primarily on the relative size of the
MO coefficients on the protonated and non-protonated
pyrrole nitrogens.
low. In contrast, there is a marked stabilization of the -a
and -s MOs since there is a strong bonding interaction
along the y- and x-axis, respectively, involving relatively
large MO coefficients. The DHOMO value of TANP is
therefore significantly lower than that predicted for TBP
and there is a markedly narrower HOMO-LUMO band
gap. This accounts for the unusually large red shift of the
Q and B bands reported by Lash and others [10, 14], and
the spectral properties which in other regards are broadly
similar to those of the parent porphyrin compound
since DHOMO ≈ DLUMO ≈ 0. A similar pattern is
observed when fused phenanthrenes are added to form
tetraphenanthroporphyrin (TPhen) but the stabilizations
of the -a and -s MOs are not as large as those observed
for TANP so a larger HOMO-LUMO band gap is,
therefore, predicted. When phenyl substituents are added
at the meso-carbons to form tetraphenyltetrabenzopor-
phyrin (TPTBP), tetraphenyltetraacenaphthoporphyrin
(TPTANP) and TPTP there is a destabilization of the s
MO due to the large coefficients on these atoms, which
typically results in a decrease in the DHOMO value
and the observed Q band intensity and a red shift of
both the Q and B bands (Figs 4 and 5). No significant
destabilization of the a MO is anticipated, since the meso-
carbons lie on nodal planes. The destabilization of the s
MO of TPTANP results in a soft MCD chromophore with
DHOMO ≈ DLUMO ≈ 0. A somewhat larger DHOMO
value is predicted for TPTPs (Fig. 5), resulting in a more
pronounced red shift and greater Q band intensity than is
observed with TPTANPs [16, 17].
When fused benzene rings are added to the porphyrin
ligand in a radially symmetric manner to form
tetrabenzoporphyrin (TBP), there is an antibonding
interaction between the a MO and the C4H4 bridge moiety
at all eight positions of attachment resulting in a marked
destabilization of the MO energy (Figs 4 and 5). In
contrast there is a slight stabilization of the s MO due to a
weak bonding interaction. The energies of the -a and -s
MOs of free base TBP remain largely unchanged because
It has long been known that the optical properties of
tetraarylporphyrin compounds can be fine-tuned using
the Hammett parameter of a para-substituent (sp) by
Copyright © 2012 World Scientific Publishing Company
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838
J. MACK ET AL.
Fig. 4. MO energies of the frontier p-MOs of radially symmetric fused-ring-expanded porphyrinoids in TD-DFT calculations
(bottom) carried out with the B3LYP functional using 6-31G(d) basis sets. The a and –a MOs with nodal planes on the protonated
pyrrole nitrogens (1au and 1b2g*) are denoted with light gray triangles, while the s and –s MOs (1b1u and 1b3g*) with significant MO
coefficients on these atoms are denoted by black squares. The nodal patterns of the a, s, -a and –s MOs of, from bottom to top,
porphyrin (P), tetrabenzoporphyrin (TBP), tetraacenaphthoporphyrin (TANP), tetraphenanthroporphyrin (TPhen) at an isosurface
of 0.01 a.u. (TOP). The imino protons are aligned with the y-axis. Gray and black are used to denote the phase changes in the
isosurfaces. Similar nodal patterns are observed for the corresponding tetraphenylporphyrinoids [17]. The average HOMO-LUMO
band gap values taking into account all four MOs derived from the HOMO and LUMO of the parent perimeter are denoted by dark
gray diamonds and are plotted against a secondary axis (CENTER). The trends in the band gap values closely mirror those observed
in the experimental (black diamonds) and calculated (light gray squares) energies of the Q and B bands (BOTTOM)
Copyright © 2012 World Scientific Publishing Company
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TRENDS IN THE OPTICAL AND REDOX PROPERTIES OF TETRAPHENYLTETRAPHENANTHROPORPHYRINS
839
Fig. 6. The observed Q and B band wavelengths and DEo-r
values in eV are denoted by black triangles, diamonds and
circles, respectively, and are plotted vs. 4sp [30]. Redox
potentials for the oxidation and reduction steps relative to the
saturated calomel electrode are plotted as light gray squares
against a secondary axis
-COOCH3, -Cl, -CN and –NO2 substituents. A similar
trend was recently reported for H2(p-X) TPTPs [16]
(Fig. 6). A particularly large interaction is anticipated in
this context, since increased steric hindrance rotates the
phenyl groups almost into the plane of the inner perimeter
of the p-system (Fig. 1). Kadish and Morrison [28]
reported that the closest agreement with the redox value
trend was observed with electron accepting substituents,
while compounds with electron donating –CH3 and
–NMe2 substituents were found to lie off the trendline.
When DHOMO ≈ DLUMO ≈ 0, as is predicted to be the
case for TPTPs, and there are close to fully forbidden
Q bands and fully allowed B bands based on the
DML = 1 and 9 properties of the Q and B transitions,
Fig. 5. The effect of structural perturbations on the DHOMO
it is reasonable to expect that the redox properties of the
and DLUMO values predicted by B3LYP (LEFT) and
aryl substituents will modify the energies of both the
by ZINDO/s (RIGHT) calculations for the same set of
Q and B bands based on the changes in the separation
B3LYP optimized geometries for 5a–5i and free base
of the first oxidation and reduction potentials (DEo-r),
which determine the magnitude of the HOMO-LUMO
band gap.
porphyrin (P), tetraphenylporphyrin (TPP), tetraben-
zoporphyrin (TBP), tetraphenyltetrabenzoporphyrin (TPTBP),
tetraacenaphthoporphyrin (TANP), tetraphenyltetraacena-
phthoporphyrin (TPTANP), tetraphenanthroporphyrin (TPhen)
and tetraphenyltetraphenanthroporphyrin (TPTP). In the light
gray shaded areas where DLUMO > DHOMO a +/- MCD sign
sequence is anticipated [17, 18] for the Q band in ascending
energy terms, while a -/+ sign sequence is anticipated in the
unshaded areas where DHOMO > DLUMO
When the energies predicted for the four frontier
p-MOs of the H2(p-X)TPTP compounds are plotted
(Fig. 7), the energies for the 5i MOs do not lie on the
trend line observed in the energies for the compounds
with electron withdrawing substituents, however. This
is not surprising, since Kadish and Morrison [28] only
reported a correlation between the redox potentials and
the sp of the substituents for TPPs containing para-
substituents with electron accepting properties (Fig. 6).
Marked differences are observed in the spectra of 5b and
5i relative to those of 5a and 5f (Fig. 8). In both cases,
there is a marked intensification of the Q band region,
which may be related in part to increased vibrational
band intensity caused by the –CH3 and –N(CH3)2 groups.
An unprecedented red shift of the Q band is observed
for 5i based on the strong electron donating properties
carrying out the synthesis with different benzaldehydes
so that meso-aryl groups with strongly electron
withdrawing or donating properties are introduced into
the p-system. Linear plots can often be obtained for E
½
vs. 4sp for the first and second oxidation and reduction
potentials of structurally related porphyrins [27]. For
example, in the mid-1970s, Kadish and Morrison [28]
reported linear trends in the redox values of free base
H2(p-X)TPP compounds with -OCH3, –CH3, -H, -F,
Copyright © 2012 World Scientific Publishing Company
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840
J. MACK ET AL.
and LUMO are destabilized to an equal extent.
This pattern is predicted for 5a–5g in B3LYP
calculations with 6-31G(d) basis sets (Fig. 7).
Michl has demonstrated that when a strong
mesomeric effect is introduced, as is the case with
the –NO2 groups in 5h and the –N(CH3)2 groups
in 5i, the presence of the substituent can have
markedly differing effects on the energies of the
four frontier p-MOs, based on differing extents
of mixing with MOs that are associated primarily
with the 2pz AOs on the substituents. Where 5i is
concerned, this mixing is most significant in the
a and s MOs, since they lie closer in energy to
the MOs introduced by the substituents [17]. The
destabilization which results from this is most
marked in the case of the s MO due to the large
MO coefficients on the meso-carbons (Fig. 4), and
this causes a decrease in the HOMO-LUMO band
gap relative to 5a and TPP and hence a marked red
shift of the Q and B bands (Fig. 8). The increase
in the predicted DHOMO value is consistent with
the marked intensification of the Q(0,0) bands. An
electron-withdrawing mesomeric interaction is
anticipated for 5h in which the largest interaction
is expected to be with the –a and –s MOs due to the
presence of low-lying unoccupied MOs associated
with the substituents. Since these MOs lack the
radial symmetry of the a and s MOs (Fig. 2), a
reasonably similar interaction is anticipated.
The –a and –s MOs both have significant MO
coefficients at the four meso-carbons, so there is
less scope for a significant DLUMO value.
The results of the TD-DFT calculations for
the H2(p-X)TPTP compounds are somewhat
problematic (Fig. 8). In the experimental data,
a single intense pair of coupled Faraday B0
terms is consistently observed in the B band
region, corresponding to the most intense band
in the absorption spectrum. In the calculated
TD-DFT spectra, however, the most intense
band in the B band region is usually predicted
to lie significantly to the blue of the initial set of
bands that are associated primarily with the four
frontier p-MOs (Fig. 8), due to configurational
interaction between the lower energy B state and
a state associated primarily with the 2a1 → –s
one-electron transition (Table 2). The 2a1 MO has
nodal patterns which are similar to those of the s MO,
which in group theory terms is the 1a1 MO. There is an
even more marked discrepancy between the calculated
and observed spectra of 5h, where there are low-lying
unoccupied MOs associated with the –NO2 substituents
(Fig. 7). In contrast in the ZINDO/s spectra calculated
using the same set of B3LYP geometries, the bands
associated with transitions from the a and s MOs to
the –a and –s MOs are consistently predicted to be the
most intense in the B band region of the spectra. Similar
Fig. 7. The energies of the frontier p-MOs predicted by TD-DFT
(TOP) and ZINDO/s (BOTTOM) calculations plotted against the
Hammett parameter (sp) [30]. Black squares denote the s and –s MOs
and large light gray triangles are used for the a and –a MOs. Small
squares denote p-MOs introduced by the para-substituents, while open
diamonds are used to denote MOs associated with the ligand p-system.
Gray diamonds are used to highlight the 2a1 MO which has nodal
patterns that are similar to those of the s MO, which in group theory
terms is the 1a1 MO. The HOMO-LUMO band gap values are plotted
against a right hand axis using large dark gray diamonds with labels
denoting the type of para-substituent in each case
of the –N(CH3)2 groups. DFT and INDO/s calculations
predict that the introduction of more strongly electron
donating groups at the para-positions of the phenyl
groups, destabilizes the four frontier p-MOs (Fig. 7).
As would be anticipated on this basis, negative shifts are
observed in both the oxidation and reduction potentials
(Fig. 6). The inductive effect associated primarily with
s-bonding is not expected to significantly alter the size
of the HOMO-LUMO band gap, since the HOMO
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TRENDS IN THE OPTICAL AND REDOX PROPERTIES OF TETRAPHENYLTETRAPHENANTHROPORPHYRINS
841
Fig. 8. Absorption and MCD spectra of 5a, 5b and 5f–5i recorded in 99:1 (v/v) CHCl3:ethanol in CHCl3 at 298 K. The coupled pairs
of overlapping Faraday B0 terms associated with the Q and B bands can be viewed as pseudo-A1 terms arising from transitions into
close lying excited states that would be orbitally degenerate if a central metal were present. TD-DFT (LEFT) and ZINDO/s (RIGHT)
spectra are plotted against a right hand axis using green diamonds and blue squares, respectively. The Q and B bands associated
primarily with one-electron transitions between the a, s, –a and –s MOs are highlighted with black diamonds and light gray squares
issues have been encountered in the calculated spectra of
other porphyrinoids [16, 17]. For example, two intense
bands are predicted in the UV region of D4h symmetry
metal porphyrin complexes, while only one intense A1
term is typically observed in the experimental spectra.
Nemykin has demonstrated that the size of the Hartree
Fock component added to the exchange-correlation
functional to form hybrid functionals such as B3LYP has
a significant effect on the relative energies of the highest
lying occupied MOs in calculations carried out on
phthalocyanines and that this can have a significant effect
on the calculated spectra [29]. The separation of the 1a1
and 2a1 MOs is significantly larger within the ZINDO/s
calculation (Fig. 7), and there are smaller DHOMO values
so there is less configurational interaction between the Q
and B and higher pp* states (Table 2).
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J. Porphyrins Phthalocyanines 2012; 16: 841–844

842
J. MACK ET AL.
Table 2. Key parameters predicted for the major spectral bands in the TD-DFT and ZINDO/s calculations of 5a, 5b and 5g–5i.
Bandb Obs.c
TD-DFTa
ZINDO/sa
Calcd.d
nm (f)
Percentage contributionse
Calcd.d
nm (f)
Percentage contributionse
a → -a a → -s s → -a s → -s 2a1 → -s
a → -a a → -s s → -a s → -s 2a1 → -s
Qx
Qy
Bx
By
820 821 (0.08)
778 747 (0.10)
588 559 (0.19)
567 538 (0.61)
20
0
0
25
0
0
66
0
73
0
0
0
1016 (0.01)
885 (0.01)
550 (1.40)
537 (1.85)
436 (0.77)
39
0
0
48
0
0
46
0
54
0
0
0
5a
5b
5g
5h
49
0
2
34
0
50
0
33
0
8
58
0
9
0
45
0
46
0
0
pp*
—
492 (0.63)
19
0
4
54
4
6
66
x
Qx
Qy
Bx
By
820 821 (0.08)
781 747 (0.10)
598 559 (0.19)
576 538 (0.61)
21
0
0
23
0
0
68
0
73
0
0
0
1017 (0.02)
887 (0.02)
549 (1.46)
541 (1.84)
429 (0.68)
36
0
0
45
0
0
49
0
56
0
0
0
57
0
4
23
0
53
0
32
0
7
62
0
8
0
47
0
44
0
0
pp*
—
493 (0.63)
12
0
3
68
3
5
67
x
Qx
Qy
Bx
By
855 830 (0.06)
797 778 (0.13)
619 607 (0.29)
591 578 (0.50)
26
0
0
26
0
0
66
0
71
0
0
0
1010 (0.01)
888 (0.02)
558 (1.53)
544 (1.99)
442 (0.78)
42
0
0
46
0
0
48
0
50
0
0
0
50
0
6
29
0
47
0
36
0
8
53
0
9
0
46
0
44
0
0
pp*
—
520 (0.69)
13
0
5
61
4
6
67
x
Qx
Qy
By
Bx
866 863 (0.08)
767 817 (0.14)
618 663 (0.12)
593 636 (0.14)
19
0
0
20
0
0
69
0
73
0
0
0
986 (0.03)
892 (0.07)
577 (1.33)
560 (1.80)
493 (0.57)
1040 (0.05)
31
0
0
32
37
0
0
49
30
0
45
0
0
0
53
0
3
33
0
0
0
11
0
43
0
3
0
39
0
35
12
60
pp*
—
566 (0.66)
17
19
0
6
51
0
0
0
40
0
x
Qx
902 913 (0.18)
0
0
73
32
0
0
Qy
By
Bx
834 834 (0.27)
601 552 (0.50)
573 547 (0.57)
0
70
0
13
0
72
0
0
2
0
0
0
10
0
902 (0.05)
555 (1.56)
552 (1.87)
428 (0.62)
0
0
41
57
0
53
29
0
0
0
0
3
5i
73
0
4
51
2
39
4
0
pp*
—
493 (0.38)
6
0
81
0
0
35
x
a
Details are provided for TD-DFT and ZINDO/s calculations of 5a, 5b and 5g–5i carried out using the same set of B3LYP
geometries. bThe band assignment is described in the text. cThe wavelengths of the band centers of the Faraday B0 terms observed
for the Q(0,0) and B(0,0) bands in the MCD spectra of 5a, 5b and 5g–5i. d The calculated wavelength of the Q and B bands and the
most intense pp* band to the red of the B bands with the oscillator strengths provided in parentheses. The data are arranged in five
labeled rows for each compound. eThe predicted percentage contribution from each one-electron transition between the four frontier
p-MOs derived from the HOMO and LUMO of the parent C16H126- perimeter and the one-electron transition between the 2a1 MO of
the ligand p-system and the –s MO.
the parent perimeter (DHOMO ≈ DLUMO ≈ 0) is an
important factor where the B band is concerned since
this limits the amount of configurational interaction with
higher energy pp* states. Further marked modifications
of the HOMO-LUMO band gap are predicted when
para-substituents are introduced to the phenyl groups on
the meso-carbons, which have a strong mesomeric effect.
In contrast, when the inductive effect is the dominant
factor as is the case with 5a–5g, the energies of the a, s,
–a and –s MOs are predicted to be modified to a similar
extent by the electron withdrawing or donating properties
CONCLUSION
Michl’s perimeter model provides a conceptual
framework which can be readily used to analyze
and predict trends observed in experimental and
calculated optical spectra. The red shifts observed
for the Q bands of tetraphenyltetraphenanthro- and
tetraphenyltetraacenaphthoporphyrin compounds are
due primarily to the marked stabilizations of the -a
and -s MOs relative to the electronic structure of TPP.
The retention of OAM properties similar to those of
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TRENDS IN THE OPTICAL AND REDOX PROPERTIES OF TETRAPHENYLTETRAPHENANTHROPORPHYRINS
843
of the para-substituents of the aryl groups. The TD-DFT
calculations of TPTPs appear to be problematic due
to an overestimation of the degree of configurational
interaction between one of the B excited state and a
higher lying pp* states. This provides further evidence
that the results of TD-DFT calculations can not be lined
up uncritically against the experimental spectra and
that detailed studies of trends in the MCD spectra and
TD-DFT spectra of structurally related porphyrinoids
are often required to definitively assign the major bands
within the optical spectra.
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Acknowledgements
This work was supported by the Department of Science
and Technology (DST), Japan and the National Research
Foundation (NRF), South Africa through a DST/NRF
Chairs Initiative for Professor of Medicinal Chemistry
and Nanotechnology, the National Natural Science
Foundation of China (No. 20971066 to Z.S.), National
Basic Research Program of China (No. 2011CB808704
to Z.S.) and a Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan (No. 20108007 (p-space) to N.K.)
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