Control of the orbital derealization and implications for molecular rectification in the radical anions of porphyrins with coplanar 90° and 180° β,β′-fused extensions

Article abstract of DOI:10.1021/jp076406g

Through-porphyrin electronic communication is investigated using linear-type and corner-type bis(quinoxalino)porphyrins in free-base form and their Zn^II, Cu^II, Ni^II, and Pd^II derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent β,β'-pyrrolic positions; they were synthesized from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)- porphyrin-2,3,12,13- and -2,3,7,8-tetraone, respectively, by reaction with 1,2-phenylenediamine. The degree of electron spin derealization into the fused rings in the π-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry, UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the π-radical anions show that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions are mediated through the central porphyrin unit, these interactions being sufficient to guarantee throughmolecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the π-radical anion becomes confined so that one quinoxaline group is omitted from spin derealization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.

Full text of DOI:10.1021/jp076406g

556
J. Phys. Chem. A 2008, 112, 556-570
Control of the Orbital Delocalization and Implications for Molecular Rectification in the
Radical Anions of Porphyrins with Coplanar 90° and 180° â,â′-Fused Extensions
Wenbo E,† Karl M. Kadish,*,† Paul J. Sintic,‡ Tony Khoury,‡ Linda J. Govenlock,‡
Zhongping Ou,† Jianguo Shao,† Kei Ohkubo,§ Jeffrey R. Reimers,‡ Shunichi Fukuzumi,*,§ and
Maxwell J. Crossley*,‡
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, School of Chemistry, The
UniVersity of Sydney, NSW 2006, Australia, and Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
ReceiVed: August 9, 2007; In Final Form: October 11, 2007
Through-porphyrin electronic communication is investigated using “linear-type” and “corner-type” bis-
(quinoxalino)porphyrins in free-base form and their ZnII, CuII, NiII, and PdII derivatives. These compounds
are porphyrins with quinoxalines fused on opposite or adjacent â,â’-pyrrolic positions; they were synthesized
from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrin-2,3,12,13- and -2,3,7,8-tetraone, respectively, by
reaction with 1,2-phenylenediamine. The degree of electron spin delocalization into the fused rings in the
π-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry,
UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density
functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the π-radical anions show
that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions
are mediated through the central porphyrin unit, these interactions being sufficient to guarantee through-
molecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner
nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the π-radical
anion becomes confined so that one quinoxaline group is omitted from spin delocalization. This indicates the
appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically
controlled rectification.
Introduction
not involve covalent linkages between adjacent components, the
architecture being established and maintained by an apoprotein
matrix with specific sites for binding the chromophores.
Interchromophore communication therefore essentially involves
through-space interactions.
Metalloporphyrins and their derivatives have attracted much
interest with respect to their applications in the area of functional
molecular devices.1-6 Much of the interest in these compounds
derives from their structural homology and similar redox
properties to macrocycles found in biological systems, which
are responsible for life-sustaining processes such as long-range
electron transfer and the transport of oxygen.7-9 Indeed, the
charge-separation and electron-conduction properties of the
photosynthetic reaction center (PRC) establish the paradigm for
“molecular electronics” and the concept of “molecular wires”.
The PRC and the associated light-harvesting systems consist
of arrays of porphinoid compounds at the porphyrin, chlorin
(dihydroporphyrin), bacteriochlorin, and isobacteriochlorin (tet-
rahydroporphyrin) levels of oxidation, depending on the organ-
ism and role being played by the particular chromphore.10 In
these systems, the chromophores are held fairly rigidly but in a
variety of relative orientations between successive units, de-
pending on where they are in the chain of events. The relative
orientations and interchromophore distances presumably have
evolved to maximize energy capture and transfer, transduction
of the energy to charge, or conduction of electrons away from
the PRC. These arrays are self-assembled systems and so do
Synthetic analogues of these “molecular electronic” natural
systems have been constructed but without the apoprotein
matrix, and therefore, they necessarily have covalent linkages
between the chromophores to maintain the molecular architec-
ture.2,6,11,12 As a consequence, the extent of coupling between
the chromophores in synthetic systems will be in part dependent
on the nature of the molecular linkages as through-bond
communication becomes a possibility.4 Many different archi-
tectures can result, depending on the position or region of the
porphyrin nucleus that is elaborated. Meso- and â-positions of
the macrocycle have been used as attachment points,13-15 and
â,â’-pyrrolic,16,17 â,meso,18 and â,meso,â faces19 have been
regions for fusion of annulated rings (Figure 1). In addition,
extension into the third dimension is possible through chelation
to coordinated metal ions.
Arrays have been reported in which porphyrins are laterally
extended in a linear fashion from opposite sides on the periphery,
with the ultimate goal being their utilization as molecular
wires.20-23 The electronic communication in synthetic porphyrin
systems that are electronically coupled through bonds, in
principle, can be modified or switched. Such switching has been
demonstrated in a linear coplanar bis-porphyrin that incorporated
* Authors to whom correspondence should be addressed. E-mail:
kkadish@uh.edu;
chem.usyd.edu.au.
fukuzumi@chem.eng.osaka-u.ac.jp;
m.crossley@
† University of Houston.
‡ The University of Sydney.
§ Osaka University.
10.1021/jp076406g CCC: $40.75 © 2008 American Chemical Society
Published on Web 01/03/2008

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 557
Results and Discussion
Synthesis of Bisquinoxalinoporphyrins. The bisquinoxali-
noporphyrins were prepared by the reaction of porphyrin-
tetraones with 1,2-phenylenediamine (6). Linear bisquinoxali-
noporphyrin (QPQ)2H (3a) was readily prepared in 95% yield
by reaction of linear tetraone 527 with an excess of 1,2-
phenylenediamine 6 (Scheme 1a). (QPQ)2H (3a) has a D2h
element of symmetry that is clearly evident by the signal pattern
in the 1H NMR spectrum. The spectrum contained a 72-proton
doublet for the tert-butyl protons, a four-proton triplet and an
eight-proton doublet for the aryl protons of the meso phenyl
rings, and a downfield four-proton doublet due to the â-pyrrole
protons. This doublet collapsed to a singlet upon irradiation of
the broad singlet upfield of tetramethylsilane due to the loss of
the 4J coupling to the inner NH protons. This indicates that the
aromatic pathway in this molecule is essentially locked in the
tautomer as drawn in Figure 3a, where the inner NH protons
reside on the non-annulated pyrrolic rings.33 This confirms that
(QPQ)2H (3a) has a bacteriochlorin-like aromatic pathway. The
UV-vis spectrum of (QPQ)2H (3a) shows a broad red-shifted
Soret band (449 nm (log ꢀ ) 5.41)) and Q-bands that tail to
higher wavelengths when compared to non-annulated porphy-
rins.
Figure 1. Positions for ring extension and ring fusion on the porphyrin
periphery.
a quinonoid unit that could be modified by both redox and
chemical means.24 Few systems have been prepared that
incorporate multidirectionality,25-27 a concept that might be
important for high-density architectures and for use in gating
mechanisms.28-30 The well-known chemistry at the â-pyrrolic
periphery of a porphyrin macrocycle provides the necessary
synthetic control to allow for multidirectional extension,27,31 an
example of which is shown in Figure 2 where both linear and
90° geometries are included to create a proposed array. Also
highlighted in this figure are the key molecular components,
where two quinoxaline groups are fused to the porphyrin
macrocycle in either a linear or a corner fashion.
Likewise, corner bisquinoxalinoporphyrin (PQ2)2H (4a) was
readily prepared in 94% yield by the condensation of corner
1
tetraone 7 and 1,2-phenylenediamine 6 (Scheme 1b). The H
NMR spectrum of (PQ2)2H (4a) shows a C2V symmetry pattern
in which tert-butyl protons and aryl protons on the meso ring
all give rise to signals in a 1:2:1 ratio, respectively, and the
four â-pyrrolic protons appear as a downfield AB quartet. The
In this paper, electronic communication through the porphyrin
ring is investigated by focusing on these bisquinoxalinopor-
phyrin molecular arrangements. The structures of the newly
investigated macrocycles are shown in Chart 1 and are given
the notation (QPQ)M 3 for the “linear” bisquinoxalinoporphyrins
and (PQ2)M 4 for the “corner” bisquinoxalinoporphyrins (M )
2H, ZnII, CuII, NiII, or PdII).32
C
2V symmetry pattern in the 1H NMR spectrum of (PQ2)2H (4a)
is an indication that the inner NH protons of the porphyrinoid
ring are in fast exchange on the NMR time scale, and this is
4
supported by the lack of resolved J coupling between the
â-pyrrolic and the inner NH protons, confirming the isobacte-
riochlorin-like character of (PQ2)2H (4a). In fact, the inner NH
protons have experienced a downfield shift as a result of the
lowering aromaticity associated with the isobacteriochlorin ring
structure and appear as a broad singlet at -0.55 ppm. At low
temperature (220 K) in CDCl3, this broad singlet begins to split
The doubly ring-annulated bisquinoxalinoporphyrins (QPQ)M
(3) and (PQ2)M (4) are quite valuable in that an investigation
of their fundamental electronic properties, in particular the
degree of communication between two quinoxaline units upon
electron transfer, should provide insight into the functionality
and potential applications of these molecules when used as
components in molecular wires. The (QPQ)M (3) component
has already been shown to facilitate electron transfer through a
super-exchange mechanism in giant multiporphyrin arrays that
were designed as close distance and orientation models of the
PRC.12,33 Therefore, an understanding of the electronic properties
of (QPQ)M (3) and (PQ2)M (4) upon one-electron reduction is
desirable. In particular, a knowledge of the degree of spin
distribution on the two fused quinoxaline units is important to
control the electron coupling term in porphyrin-based molecular
wires that might contain these components. Herein we report
how the extent of communication between the two quinoxaline
units in a bisquinoxalinoporphyrin can be controlled by changes
in either the nature of the central substitution (protons versus
metal(II) ion) in conjunction with the geometry of the two fused
quinoxaline groups, being opposite to each other in one case
and at adjacent sites of the porphyrin in the other. Electrochemi-
cal and UV-vis spectroelectrochemical techniques are used to
ascertain the electronic character and possible applications of
these systems. The present study provides valuable insight into
the rational design of porphyrin-based molecular wires for
applications as chemically controlled molecular conductors and
rectifiers.
1
in the H NMR spectrum as the exchange process becomes
observable on the NMR time scale, and the inequivalence of
the inner NH protons of (PQ2)2H (4a) becomes apparent. This
indicates that the aromatic delocalization pathway in this
molecule alternates between two degenerate forms as shown in
Figure 3b with any given â-â bond in (PQ2)2H (4a) being
alternately aromatic or ethylenic depending on which tautomer
is present.34
The metalation properties of (QPQ)2H (3a) and (PQ2)2H (4a)
were investigated by heating a solution of the porphyrin with
the appropriate metal salt in a suitable solvent mixture that
enhanced the solubility of reagents (Scheme 1). It was found
that (PQ2)2H (4a) was much easier to metalate than its linear
isomer, (QPQ)2H (3a); for example, quantitative zinc(II)
chelation of (PQ2)2H (4a) was achieved by heating at reflux a
mixture of a chloroform-methanol (2:1) solution of the
porphyrin and zinc(II) acetate dihydrate for 3 min, whereas 18
h were required to complete the metalation of (QPQ)2H (4a)
under identical conditions. Using conditions recently reported
for the metalation of linear and corner porphyrin-tetraones,31
the other metalations of (QPQ)2H (3a) and (PQ2)2H (4a) were
achieved in almost quantitative yield. The general UV-vis
spectral patterns for (QPQ)M (3) and (PQ2)M (4) resemble those
of other compounds with bacteriochlorin and isobacteriochlorin

558 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
Figure 2. Proposed molecular wire with linear and corner bisquinoxalinoporphyrin components colored in blue (Ar ) 3,5-di-tert-butylphenyl).
CHART 1
0.10 V/s). The latter result indicates that fusion of a second
quinoxaline group onto (PQ)2H (2a) has little or no effect on
the highest occupied molecular orbitals (HOMOs) of the free-
base bisquinoxalinoporphyrins.
Figure 5 illustrates cyclic voltammograms for Zn(II) com-
plexes of the related mono- and bisquinoxalinoporphyrins (PQ)-
Zn (2b), (PQ2)Zn (4b), and (QPQ)Zn (3b) in CH2Cl2 containing
0.10 M TBAP. Also shown in this figure is a cyclic voltam-
mogram of (P)Zn (1b) obtained under the same conditions. As
seen in the figure, the reversible potential for the first oxidation
of (PQ)Zn (2b; E1/2 ) 0.72 V) is the same as that of (P)Zn (1b;
E1/2 ) 0.72 V). This indicates that fusion of a quinoxaline group
onto the porphyrin ring has no effect on the HOMO of (PQ)-
Zn. The first reversible oxidation potentials of (PQ2)Zn (4b)
and (QPQ)Zn (3b) are positively shifted by only 50 and 40 mV,
respectively, as compared with that of (PQ)Zn (2b). Thus, fusion
of a second quinoxaline group onto (PQ)Zn has a small effect
on the HOMO of zinc bisquinoxalinoporphyrins as is the case
for free-base bisquinoxalinoporphyrins. As shown in Table 1,
the E1/2 values for the first oxidation of other metalated
compounds (QPQ)M (3a-e) and (PQ2)M (4a-e) with the same
metal ion are also identical within experimental error; i.e., the
difference in potentials between the two series of compounds
with the same metal ions is only 10-20 mV.
In contrast to the first oxidation, the reversible potential for
the first reduction of (QPQ)Zn (3b) in CH2Cl2 (E1/2 ) -1.14
V) is positively shifted by 170 mV from the E1/2 for the first
one-electron reduction of (PQ)Zn (2b) in the same solvent. This
difference in reduction potentials between (PQ)Zn and (QPQ)-
Zn also contrasts with what is observed for the free-base
derivatives with the same macrocycle, i.e., between (PQ)2H (2a)
and (QPQ)2H (3a) where the difference in E1/2 amounts to only
100 mV as seen in Figure 4. However, the positive shift in E1/2
for the first reduction of (PQ2)Zn (4b) is only 20 mV from (PQ)-
Zn (2b), whereas the positive shift for the corresponding free-
base derivatives is 160 mV (Figure 4). Thus, there is a significant
difference between the free-base and metal(II) bisquinoxali-
noporphyrins upon one-electron reduction, as highlighted above
and shown in Table 1 for other metal(II) derivatives.
structures, respectively.35,36 In the case of (QPQ)M (3b-e), the
Soret bands are split and shifted toward the red, and there is an
increase in the lower-energy Q-band intensities. For (PQ2)M
(4b-e), λmax occurs near 420 nm, and the Q-bands are tailed to
higher wavelengths. Unlike the free-base analogues, the metallo-
bisquinoxalinoporphyrins (QPQ)M (3b-e) and (PQ2)M (4b-
e) cannot exist in equilibrating tautomeric forms.37
These optimized metalation conditions for (QPQ)2H (3a) and
(PQ2)2H (4a) provide increased synthetic flexibility in preparing
functional multiporphyrin arrays, which will enable further redox
manipulation in complex â-pyrrolic fused arrays by having more
control of the desired synthetic objectives (vide infra).
Electrochemistry of Free-Base and Metalated Bisquinox-
alinoporphyrins. Cyclic voltammograms of the mono- and
bisquinoxalinoporphyrins (PQ)2H (2a), (QPQ)2H (3a), and
(PQ2)2H (4a) in CH2Cl2 containing 0.1 M tetra-n-butylammo-
nium perchlorate (TBAP) are shown in Figure 4. The E1/2 for
the first reduction of (QPQ)2H (3a) is positively shifted by 100
mV from E1/2 for the first reduction of (PQ)2H (2a). A larger
∆E1/2 value is obtained between reduction of (PQ)2H (2a) and
(PQ2)2H (4a) (160 mV), while at the same time the irreversible
oxidations of (QPQ)2H (3a) (Ep ) 1.02 V) and (PQ2)2H (4a)
(Ep ) 1.00 V) are located at almost the same potential as that
for the reversible oxidation of (PQ)2H (2a) (for a scan rate of
Similar differences in E1/2 for the one-electron reduction of
the mono- and bisquinoxalinoporphyrins are observed for the
other metalated compounds in CH2Cl2 (Table 1). The “linear-
type” bisquinoxalinoporphyrins, (QPQ)M (3), are in all cases
easier to reduce by 120-200 mV as compared to the (PQ)M
(2) derivatives with the same central metal ions, while the
“corner-type” bisquinoxalinoporphyrins, (PQ2)M (4), are easier
to reduce by only 20-60 mV as compared to (PQ)M (2) (Table
1). Also, as seen in Table 1, (QPQ)Cu (3c) and (QPQ)Pd (3e)
undergo two reversible and well-separated one-electron oxida-

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 559
a
SCHEME 1
a Reagents and conditions: i, CHCl3, stir; ii, conditions for metal chelation as described in the Experimental section.
tions to stepwise give π-cation radicals and dications, but these
ring-centered oxidations are overlapped in potential for (QPQ)-
Ni (3d), thus giving an overall two-electron conversion of the
initial compound to what is most likely a porphyrin π-dication.
Overlapped oxidations have been reported for a number of Ni-
(II) porphyrins with other macrocycles,39,40 and this behavior
is not specific to the Ni(II) bisquinoxalinoporphyrin complexes.
HOMO-LUMO Gap. A summary of the experimentally
measured HOMO-LUMO gaps of the different metal(II)
bisquinoxalinoporphyrins in CH2Cl2 is given in Table 1 along
with the measured values for (PQ)M (2). The HOMO-LUMO
gaps for the free-base derivatives were not quoted as the first
oxidations were irreversible and distorted by a chemical reaction.
These gaps are listed as ∆E1/2 in Table 1 and decrease in the
following order: (PQ)M (2) > (PQ2)M (4) > (QPQ)M (3) for
complexes with the same central metal ion. This result reflects
mainly the change in the lowest unoccupied molecular orbital
(LUMO) energy because the HOMO energy remains nearly the
same in this series. Addition of a second quinoxaline group to
(PQ)M (2) at an adjacent â-pyrrolic site to give (PQ2)M (4)
has only a minor effect on the electrochemistry of the porphyrin
macrocycle. In contrast, the significant decrease of the HOMO-
LUMO gap upon going from (PQ)M (2) to (QPQ)M (3) (on
the order of 200 mV) can be accounted for by a stabilization of
the porphyrin LUMO upon addition of the second fused
quinoxaline group and also indicates that placement of the two
quinoxaline groups in a linear fashion will have a profound
effect on reduction potentials.
UV-Vis Spectroelectrochemistry. The UV-vis spectral
changes obtained during reduction of (QPQ)2H (3a) and (PQ2)-
2H (4a) in a thin-layer cell are shown in Figure 6. The broad
absorption band at 700-800 nm and the disappearance of the
Q-bands at 500-700 nm upon addition of one electron are both
consistent with formation of a porphyrin π-radical anion. In the
case of (PQ2)2H (4a), the diagnostic porphyrin π-radical anion
band is split into two absorption maxima at 740 and 786 nm,
and this can be accounted for by a lowering of symmetry from
Figure 3. Aromatic delocalization pathway in (a) (QPQ)2H (3a), which
is essentially locked in a single tautomer, and (b) (PQ2)2H (4a) where
the tautomers are degenerate and rapidly interconvert (Ar ) 3,5-di-
tert-butylphenyl).

560 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
Figure 4. Cyclic voltammograms of (a) (PQ)2H (2a), (b) (QPQ)2H (3a), and (c) (PQ2)2H (4a) free-base compounds in CH2Cl2, 0.1 M TBAP.
Figure 5. Cyclic voltammograms of (a) (P)Zn (1b), (b) (PQ)Zn (2b), (c) (PQ2)Zn (4b), and (d) (QPQ)Zn (3b) in CH2Cl2, 0.1 M TBAP.
D2h for 3a to C2V for 4a. UV-vis spectra for radical anions of
the metalated derivatives also exhibit an absorption band that
can be assigned to the porphyrin π-radical anion. Typical
examples of these spectra are shown in Figure 7 for (QPQ)Cu
(3c) and (PQ)Cu (2c), respectively. The λmax value of the linear
bisquinoxalinoporphyrin (QPQ)Cu (3c) radical anion (760 nm)

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 561
TABLE 1: Half-Wave or Peak Potentials (V vs SCE) and HOMO-LUMO Gap (∆E1) of (PQ)M (2), (QPQ)M (3), and (PQ2)M
(4) in CH2Cl2 Containing 0.1 M TBAP
oxidationa
reduction
M
second
first
∆E1 (V)
first
second
third
fourth
PQ (2)c
QPQ (3)
PQ2 (4)
2H
0.99 (1e)
0.72 (1e)
0.97 (1e)
0.95 (2e)
1.06 (1e)
2.12
2.03
2.16
2.07
2.22
-1.13
-1.31
-1.19
-1.12
-1.16
-1.33
ZnII
CuII
NiII
PdII
1.10d (1e)
1.30d (1e)
-1.75d
-1.72d
-1.59d
-1.62d
1.46d (1e)
2H
1.02d (1e)
0.76 (1e)
0.91 (1e)
0.92 (2e)
1.01 (1e)
-1.03
-1.14
-0.99
-1.00
-1.00
-1.32d
-1.42
-1.40
-1.40
-1.43
ZnII
CuII
NiII
PdII
0.93 (1e)
1.04 (1e)
1.90
1.90
1.92
2.01
1.41 (1e)
2H
1.00d (1e)
0.77 (1e)
0.92 (1e)
0.93 (2e)
0.99 (1e)
-0.97
-1.29
-1.13
-1.07
-1.14
-1.23
-1.60d
-1.73d
-1.68d
-1.76d
-1.66d
-1.78d
-1.91d
-1.96d
ZnII
CuII
NiII
PdII
1.08d (1e)
1.03 (1e)
2.06
2.05
2.00
2.13
-1.58d
-1.58d
-1.64d
-1.56d
1.38 (1e)
-1.90d
a Numbers of transferred electrons are given in parentheses. b ∆E1 is the electrochemical HOMO-LUMO gap. c Taken from ref 38. d Peak potential
at a scan rate of 0.1 V/s for irreversible reaction.
Figure 7. UV-vis spectral changes obtained during the first reduction
of (a) (QPQ)Cu (3c) and (b) (PQ)Cu (2c) in CH2Cl2 containing 0.1 M
TBAP. Applied potential: Eapp) -1.15 and -1.40 V (vs SCE) for 3c
and 2c, respectively.
and (PQ)Cu (2c).42 The spectral data indicates that the HOMO
orbital is mainly localized on the porphyrin macrocycle without
delocalization onto the quinoxaline unit (vide supra).
Figure 6. UV-vis spectral changes obtained during the first reduction
of (a) (QPQ)2H (3a) and (b) (PQ2)2H (4a) in CH2Cl2 containing 0.1
M TBAP. Applied potential: Eapp) -1.25 and -1.40 V (vs SCE) for
Electronic Communication between the Two Quinoxaline
Units. To determine whether the two quinoxaline moieties (Q)
can communicate with each other through the central porphyrin
in the radical anion forms of (PQ2)M and (QPQ)M, electron
spin resonance (ESR) spectra were measured for the radical
anions of the 2H and Zn derivatives.31,32 The radical anions of
(PQ)M (2), (QPQ)M (3), and (PQ2)M (4) (M ) 2H and Zn)
were produced by photoinduced electron transfer from dimeric
1-benzyl-1,4-dihydronicotinamide ((BNA)2)43 to the quinoxali-
noporphyrins in MeCN as shown in Scheme 2. (BNA)2 acts as
a unique two-electron donor to produce 2 equiv of the radical
anions of electron acceptors, because BNA• formed upon
dissociative electron-transfer oxidation of (BNA)2 can transfer
an additional electron to electron acceptors.44
3a and 4a, respectively.
is significantly red-shifted as compared with that of the radical
anion for the monoquinoxalinoporphyrin (PQ)Cu (2c) (690 nm).
This indicates more delocalized spin over the porphyrin and
two quinoxaline units in the (QPQ)Cu (3c) radical anion as
compared with the (PQ)Cu (2c) radical anion.41
In contrast to the spectral features of the radical anions, the
radical cations of the bisqunoxalinoporphyrins exhibit broad
absorption bands with absorption maxima similar to those of
the monoquinoxalinoporphyrins. The spectra of these porphyrin
π-radical cations are similar to each other, irrespective of the
central metal ion. Typical examples of the spectral changes that
occur during oxidation are shown in Figure 8 for (PQ2)Cu (4c)

562 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
groups (Q) of the bisquinoxalinoporphyrin radical anions largely
agree with the ratios of the DFT-calculated spin densities as
shown in Table 2.45
The ESR spectrum of radical anions of (PQ)2H (2a) obtained
under photoirradiation of 2a with (BNA)2 is shown in Figure
9a. Hyperfine coupling constants of [(PQ)2H]•- determined from
this spectrum by computer simulation (see Experimental Section)
(Figure 9b) indicate that the unpaired electron is delocalized
on both the porphyrin and the quinoxaline units with a P/Q
nitrogen-atom ratio of 69:31. This is similar to the nitrogen-
atom spin-density ratio of 80:20 predicted by the DFT calcula-
tion (Figure 9c). However, as the calculations predict that the
porphyrin carbon atoms take a disproportionate fraction of the
unpaired electron, the calculated total spin-density ratio of 90:
10 (Table 2) indicates that the radical is porphyrin-dominated.
The DFT calculations also predict that the splitting between
the porphyrin-based and the quinoxaline-based orbitals over
which the charge is delocalized is 0.57 eV, the electronic
coupling that drives delocalization is on the order of 10% ×
0.57 ) 0.06 eV. Such coupling would be sufficient to facilitate
electron transport through one of these units if it were
incorporated into a single-molecule conduction device. It should
also be noted that no hyperfine splitting is observed for the free-
base protons. This indicates that the tautomer of [(PQ)2H]•- is
locked in a position where the protons are attached to pyrroles
that are not fused to the quinoxaline ring as is the case for neutral
(PQ)2H (2a) (vide supra).
Figure 8. UV-vis spectral changes obtained during the first oxidation
of (a) (PQ2)Cu (4c) and (b) (PQ)Cu (2c) in CH2Cl2 containing 0.1 M
TBAP. Applied potential: Eapp) 1.00 and 1.10 V (vs SCE) for 4c and
2c, respectively.
Similarly the ESR spectrum of the symmetric linear molecule
[(QPQ)2H]•-, shown in Figure 10a together with its computer
simulation spectrum (Figure 10b) and calculated spin distribu-
tions (Figure 10c), indicates that coupling occurs between the
porphyrin and the quinoxaline units. This radical anion has D2h
symmetry when aN values for N(21) and N(23) should be the
same. In such a case, however, the center line of the ESR signals
would be the strongest to afford odd number lines. The apparent
even number ESR lines observed in Figure 10a can only be
reproduced by introducing inequivalent aN values for N(21)
and N(23) as shown in Figure 10b. Such a break of the D2h
symmetry may be caused by the ion pair formation with the
counter cation (BNA+).45 The extent of spin delocalization in
the molecule [(QPQ)2H]•-, P/Q1/Q2 ) 88:6:6, appears less than
that seen for [(PQ)2H]•-. This again reflects a significant
interaction as the DFT-calculated separation between the
interacting orbitals is 0.58 eV.
TABLE 2: DFT-Calculated Total and Nitrogen-Atom
Spin-Density Ratios P/Q1/Q2 and P/Q/Q, in Comparison with
ESR-Observed Nitrogen-Atom Hyperfine Coupling Constant
Ratios for Atoms in the Porphyrin Macrocycle (P) and the
Quinoxaline Groups (Q) of the Radical Anions
nitrogen-atom contributions
total
molecule
ESR observed DFT calculated DFT calculated
The ESR spectrum of the asymmetric corner molecule
[(PQ2)2H]•- obtained under photoirradiation of 4a with (BNA)2
(Figure 11a) looks very different from either that of [(PQ)2H]•-
or its linear isomer, [(QPQ)2H]•-. The total spin-density ratio,
calculated to be P/Q1/Q2 ) 72:5:24 (Table 2 and Figures 11b
and 11c), indicates that the porphyrin interacts more strongly
with one of the quinoxaline groups than the other.
(PQ)2H 2a
69:31
49:0:51
80:10:10
66:34
48:34:18
47:27:27
80:20
51:5:44
82:9:9
62:38
48:30:22
66:17:17
90:10
72:5:24
88:6:6
76:24
73:14:13
74:13:13
(PQ2)2Ha (4a)
(QPQ)2Ha (3a)
(PQ)Zn (2b)
(PQ2)Zn (4b)
(QPQ)Zn (3b)
a Structures for the corner (PQ2) and linear (QPQ) species are shown
above the table.
ESR results showed that the porphyrin inner-ring hydrogen
tautomer is observed to be locked, as indicated by the hyperfine
interactions arising from just one pyrrole proton, confirming
1H NMR spectral analysis. It is this symmetry-breaking effect
that distinguishes the corner isomer from its linear counterpart:
The pyrrole with an attached proton is more susceptible to
reduction, bringing its energy into resonance with a porphyrin
orbital. This effect is manifested by the large positive shift (160
mV) in the one-electron reduction potential of (PQ2)2H as
compared with that of (PQ)2H (Figure 4).
The hyperfine splitting of the observed radical anions
combined with density functional theory (DFT) simulation of
the calculated spin-density distribution provides valuable in-
formation on the degree of communication between two
quinoxaline units, and the results obtained are summarized in
Table 2. This table shows the observed ratio of total nitrogen-
atom hyperfine coupling constants partitioned into contributions
from each quinoxaline ring and the porphyrin ring, the corre-
sponding ratio of calculated nitrogen-atom spin densities, and
the total DFT-calculated spin density for each of the fragments.
The observed nitrogen-atom hyperfine coupling constant ratios
for atoms in the porphyrin macrocycle (P) and the quinoxaline
In contrast to what is observed for the free-base bisquinox-
alinoporphyrins (4a and 3a), the ESR spectra of the radical

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 563
Figure 9. (a) ESR spectra of radical anions of (PQ)2H (2a) obtained under photoirradiation of 2a with (BNA)2. (b) Computer simulation spectrum
with (c) hyperfine coupling constants and the distribution of spin densities calculated using DFT.
Figure 10. (a) ESR spectrum of the radical anion of (QPQ)2H (3a) obtained under photoirradiation of 3a with (BNA)2. (b) Computer simulation
spectrum with (c) hyperfine coupling constants and the distribution of spin densities calculated using DFT.
anions of both zinc bisquinoxalinoporphyrins (4b and 3b)
together with the computer-simulated spectra45 and the DFT-
calculated spin densities indicate that there is significant
communication between the two quinoxaline units on the
molecule as shown in Figures 12 and 13. The enhanced coupling
for the linear molecule (QPQ)Zn (3b), which increases the
delocalization from P/Q1/Q2 ) 88:6:6 for (QPQ)2H (3a) to 74:
13:13, arises as zinc metalation destabilizes the porphyrin

564 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
Figure 11. (a) ESR spectrum of the radical anion of (PQ2)2H 4a obtained under photoirradiation of 4a with (BNA)2. (b) Computer simulation
spectrum with (c) hyperfine coupling constants and the distribution of spin densities calculated using DFT.
SCHEME 2
LUMO orbital, bringing it more into resonance with the
quinoxaline orbitals. This effect is also observed for the
monoquinoxalinoporphyrins (PQ)2H and (PQ)Zn. More pro-
foundly, however, metalation of the corner molecule removes
the inner NH protons that break the symmetry in the free-base
analogue so that (PQ2)Zn (4b) and (QPQ)Zn (3b) have very

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 565
Figure 12. (a) ESR spectrum of the radical anion of (a) (PQ2)Zn (4b) obtained under photoirradiation of 4b with (BNA)2. (b) Computer simulation
spectrum with (c) hyperfine coupling constants and the distribution of spin densities calculated using DFT.
similar spin densities. Hence, metalation eliminates the qui-
noxaline-orbital differentiation found for the free-base analogue
of the corner molecule.
slightly from those in Table 2 as this table shows spin densities
evaluated for the radical anion species. A four-orbital model
has been fitted to reproduce the orbital energies and percentages,
considering one orbital included for each quinoxaline as well
as two porphyrin orbitals. The resulting pure-component orbitals
and their interaction energies are shown in Figure 15. All
quinoxaline-porphyrin interaction strengths are significant,
ranging from 0.16 to 0.26 eV. For (PQ2)Zn (4b), the orbital
diagram corresponds to that for a tunneling barrier, allowing
current to pass bidirectionally. For (PQ2)2H (4a), the orbital
diagram corresponds to an Aviram-Ratner rectifier;46 as when
the potentials of the contact leads match the energy levels of
the quinoxalines to which they are connected, current will flow;
however, a large tunneling barrier is presented to current flow
at reverse bias.
Corner Bisquinoxalinoporphyrins as Potential Molecular
Rectifiers. Table 2 and Figures 11 and 12 indicate that the
properties of the LUMO of the free-base and zinc(II) corner
bisquinoxalinoporphyrins, (PQ2)2H (4a) and (PQ2)Zn (4b), are
quite distinct. This will result in different electrical behavior
when either compound is used in through-molecule electron
transport by attaching the peripheral quinoxaline groups to
external molecular wires or metallic leads. However, such
conductivity arises not just from single orbitals, and therefore,
an understanding of the properties of the nearby orbitals is also
required. Shown in Figure 14 are the four lowest-energy
unoccupied orbitals evaluated for the neutral zinc and free-base
corner bisquinoxalinoporphyrins, along with their energies and
the associated percentage localizations on the porphyrin and
quinoxaline fragments. These percentages for the LUMOs differ
It can be seen that for through-porphyrin conduction using
the Aviran-Ratner mechanism electrons must be injected

566 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
Figure 13. (a) ESR spectrum of the radical anion of (QPQ)Zn (3b) obtained under photoirradiation of 3b with (BNA)2. (b) Computer simulation
spectrum with (c) hyperfine coupling constants and the distribution of spin densities calculated using DFT.
Figure 14. Four lowest-energy unoccupied orbitals for (PQ2)Zn (4b, top) and (PQ2)2H (4a, bottom) showing the orbital energy and the percentage
of each orbital on the fragments P/Q1/Q2.
through the extended π-system attached to the pyrrolic group
that is not protonated on the inner periphery of the macrocycle
and leave via the extended π-system attached to the protonated
pyrrolic group. To use this motif in a molecular device, it will
be necessary to control the position of the inner hydrogens, along
the x-axis or along the y-axis, depending on which direction it
is desired for the flow of electrons, prior to reduction of the
rectifying unit. It is envisaged that this can be accomplished by
“locking” tautomerism through asymmetric substitution of the
non-annulated pyrrolic rings of the porphyrin. This type of bias
has been shown in simple tetraphenylporphyrins in earlier
studies.34 Regioselective incorporation of the rectifying unit into
a “molecular wire” with an inbuilt redox gradient that will also
contain external attachment sites should be straightforward using

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 567
pressure chemical ionization (APCI) mass spectra were recorded
on a ThermoQuest Finnigan LCQ DECA instrument. High-
resolution electrospray ionization Fourier transform ion cyclo-
tron resonance (HR-ESI-FT/ICR) spectra were acquired at the
Research School of Chemistry, The University of New South
Wales, on a Bruker Daltonics BioAPEX II FT/ICR mass
spectrometer equipped with a 4.7 T MAGNEX superconducting
magnet and an Analytica external ESI source. Electrospray
ionization high-resolution mass spectrometry (ESI-HRMS) was
performed on a VG Quattro II triple quadrupole mass spec-
trometer at the Research School of Chemistry, Australian
National University.
Figure 15. Isolated-component orbital energies and couplings (in eV)
for corner bisquinoxalinoporphyrins (PQ2)Zn (4b) and (PQ2)2H (4a)
fitted to reproduce the DFT-calculated orbital energies are percentage
localizations shown in Figure 14.
methodologies developed for the construction of charge-shift
compounds that model the PRC.12,47,48
Column chromatography was routinely carried out using the
gravity feed column technique on Merck silica gel type 9385
(230-400 mesh). Analytical thin-layer chromatography (TLC)
analyses were performed on Merck silica gel 60 F254 precoated
sheets (0.2 mm). Alumina refers to Merck aluminum oxide 90
active neutral I, type 1077. All commercial solvents were
routinely distilled prior to use. Light petroleum refers to the
fraction of bp 60-80 °C. Ethanol-free chloroform was obtained
by drying distilled chloroform over calcium chloride and passing
through a column of alumina immediately prior to use. Where
solvent mixtures were used, the proportions are given by
volume. Analytically pure samples of 3a-e and 4a-e were
obtained by recrystallization from a chloroform-methanol
solution.
Conclusions
This study indicates that the (QPQ)Zn (3b) macrocycle can
be used to facilitate electron transport through a molecular wire
with significant interaction of up to 0.25 eV between the
porphyrin and the quinoxaline LUMOs. Also, the (PQ2)Zn (4b)
macrocycle can be used to facilitate a 90° bend in a molecular
wire, having very similar electron-conductivity properties to that
of its linear counterpart. Electrochemical analysis indicates that
the Cu(II), Ni(II), and Pd(II) derivatives may act similarly. For
the free-base analogues, however, the electrical properties of
the unit are significantly altered. For the linear macrocycle, the
primary effect is to slightly reduce the coupling, but the breaking
of the molecular symmetry forced by the internal proton
arrangement leads to (PQ2)2H (4a) having the ability to act as
a potential Aviram-Ratner rectifier,46 passing electrons only
when they enter from the direction of the non-protonated pyrrolic
ring. Although any rectifier requires electrical contacts in the
solid state, the degree of electron spin delocalization into the
fused rings in the π-radical anions of the free-base and
metal(II) bisquinoxalinoporphyrins elucidated in this study
provides valuable chemical basis for design of the architecture
of a chemically controlled rectifier.
5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:12,13-b′′]porphyrin (3a). 2,3,12,13-Tetraoxo-5,10,15,-
20-tetrakis(3,5-di-tert-butylphenyl)bacteriochlorin (5)27 (130 mg,
0.116 mmol) and o-phenylenediamine (6) (85.0 mg, 0.786
mmol) were dissolved in dichloromethane (20 mL) and stirred
at room temperature for 10 min. The solvent was removed under
vacuum, and the crude residue was purified by chromatography
over silica (chloroform-light petroleum; 1:3). The major front
running brown band was collected and evaporated to afford 5,-
10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:
12,13-b′′]porphyrin (3a) (106 mg, 72%) as a purple microcrys-
talline solid, mp >300 °C. IR (CHCl3): 3383w, 3058w, 3028w,
3019s, 2964s, 2903m, 2867m, 1594s, 1495w, 1477m, 1428w,
1393w, 1363m, 1347w, 1299m, 1248m, 1223m, 1207m, 1199w,
1154m, 1132w, 1108s, 1009w, 946w, 926w, 901m cm-1. UV-
vis (CHCl3): 351 (log ꢀ 4.57), 398sh (4.90), 449 (5.41), 495sh
(3.82), 533 (4.38), 570sh (3.75), 614 (4.14), 668 (4.12) nm. 1H
NMR (400 MHz, CDCl3) δ: -2.48 (2 H, s, inner NH); 1.49
(72 H, s, t-butyl H); 7.73-7.77 (4 H, m, quinoxaline H); 7.82-
7.87 (4 H, m, quinoxaline H); 7.94 (4 H, t, J 1.8 Hz, Hp); 7.99
(8 H, d, J 1.8 Hz, Ho); 9.09 (4 H, d, J 1.8 Hz, â-pyrrolic H).
MS (ESI) (m/z): 1267.9 (M+ requires 1267.8). Anal. calcd. for
C88H98N8: C, 83.37; H, 7.79; N, 8.84. Found: C, 83.26; H,
7.89; N, 8.62.
Experimental Section
General Procedure. Melting points were recorded on a
Reichert melting point stage and are uncorrected. Microanalyses
were performed by the Campbell Microanalytical Laboratory,
University of Otago, New Zealand. Infrared spectra were
recorded on Perkin-Elmer model 1600 and FTIR-8400S SHI-
MADZU Fourier transform infrared spectrophotometers, as
solutions in the stated solvents. Intensity abbreviations used
are: w, weak; m, medium; s, strong; br, broad. UV-vis spectra
were routinely recorded on a Cary 5E UV-vis-near-IR
spectrophotometer in chloroform that was deacidified by filtra-
tion through an alumina column. 1H NMR spectra were recorded
on a Bruker DPX-400 (400 MHz) spectrometer. Samples were
dissolved in CDCl3, and the chloroform peak at 7.26 was used
as an internal reference unless otherwise stated. Deuterated
solvents were used as received except for deuterated chloroform,
which was deacidified by filtration through a plug of alumina.
Signals were recorded in terms of chemical shifts (in ppm),
relative integral, multiplicity, coupling constants (in Hz), and
assignment, in that order. The following abbreviations for
multiplicity are used: s, singlet; d, doublet; dd, doublet of
doublets; t, triplet; m, multiplet; br, broad; ABq, AB quartet; d
of ABq, doublet of AB quartet.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:12,13-b′′]porphyrinato}zinc(II) (3b). 5,10,15,20-Tet-
rakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]-
porphyrin (3a) (59.0 mg, 0.0465 mmol) and zinc(II) acetate
dihydrate (70.0 mg, 0.319 mmol) were dissolved in a solution
of chloroform (40 mL) and methanol (20 mL). The mixture
was then heated at reflux for 18 h. The solvent was removed
under vacuum, and the residue was purified by column
chromatography over silica (dichloromethane-light petroleum;
1:1). The front running green band was collected, and the solvent
was removed to afford {5,10,15,20-tetrakis(3,5-di-tert-butylphe-
nyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrinato}zinc(II) (3b)
(56.0 mg, 90%) as a green microcrystalline solid, mp
>300 °C. IR (CHCl3): 3057w, 2020m, 3016m, 2964s, 2903m,
Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectra were recorded on a VG TofSpec
spectrometer. Electrospray ionization (ESI) and atmospheric

568 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
2866m, 1593s, 1477m, 1464w, 1429w, 1393w, 1362m, 1340w,
1312w, 1300w, 1248w, 1215s, 1204s, 1169m, 1113w, 1036w,
949m cm-1. UV-vis (CHCl3): 345 (log ꢀ 4.63), 405 (5.04),
465 (5.25), 556 (4.01), 582 (3.83), 601 (4.07), 637 (4.18), 655
(4.71) nm. 1H NMR (400 MHz, CDCl3) δ: 1.50 (72 H, s, t-butyl
H); 7.78-7.82 (4 H, m, quinoxaline H); 7.90-7.93 (4 H, m,
quinoxaline H); 7.95 (4 H, t, J 1.8 Hz, Hp); 7.99 (8 H, d, J 1.8
Hz, Ho); 9.14 (4 H, s, â-pyrrolic H). MS (ESI) (m/z): 1332.0
(M+ requires 1331.2). MS (MALDI-TOF) (m/z): 1332 (M+
requires 1331). (ESI-HRMS C88H96N8Zn requires 1329.712.
Found: [M + H]+ 1329.716.). Anal. calcd. for C88H96N8Zn +
2CH3OH: C, 77.48; H, 7.51; N, 8.05. Found: C, 77.42; H,
7.22; N, 8.25.
chloride (37.0 mg, 0.209 mmol), and sodium acetate (33.0 mg,
0.402 mmol) were dissolved in a solution of toluene (10 mL)
and glacial acetic acid (18 M, 10 mL). The mixture was then
heated at reflux for 1 h. Dichloromethane (30 mL) was added
to the mixture, and the organic phase was separated, washed
with water (200 mL), sodium carbonate solution (10%, 2 ×
150 mL), and water (200 mL), dried over anhydrous sodium
sulfate, filtered, and evaporated to dryness. The residue was
purified by column chromatography over silica (dichloromethane-
light petroleum; 1:2), the front running green purple band was
collected, and the solvent was evaporated to give 5,10,15,20-
tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]-
porphyrinato}palladium(II) (3e) (44.0 mg, 97%) as a purple
needle solid, mp >300 °C. IR (CHCl3): 3057w, 2964s, 2903m,
2868m, 1595s, 1495w, 1477m, 1460w, 1420w, 1393w, 1362s,
1327w, 1304w, 1248m, 1215m, 1204s, 1178m, 1134w, 1118w,
1045m, 957m cm-1. UV-vis (CHCl3): 351 (log ꢀ 4.87), 395
(4.91), 452 (5.23), 531 (4.08), 546sh (3.92), 686 (4.13), 597sh
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:12,13-b′′]porphyrinato}copper(II) (3c). 5,10,15,20-
Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-
b′′]porphyrin (3a) (105 mg, 0.0830 mmol) and copper(II) acetate
monohydrate (214 mg, 1.07 mmol) were dissolved in a solution
of chloroform (80 mL) and methanol (40 mL). The mixture
was heated at reflux for 4 h. The solvent was removed under
vacuum, and the residue was washed through a plug of silica
(dichloromethane-light petroleum; 1:2). The major green band
was collected, and the solvent was removed to afford {5,10,-
15,20-tetrakis(3,5-di-tert-butylphenyl)-bisquinoxalino[2,3-b′:12,-
13-b′′]porphyrinato}copper(II) (3c) (100 mg, 91%) as a dark
green microcrystalline solid, mp >300 °C. IR (CHCl3): 3054w,
3025s, 2991w, 2962s, 1595s, 1493w, 1477m, 1426w, 1394w,
1363s, 1299m, 1248w, 1236w, 1226s, 1200w, 1176m, 1117m,
1008m, 950m, 900w cm-1. UV-vis (CHCl3): 346 (log ꢀ 4.73),
398 (5.01), 459 (5.27), 548 (4.02), 567sh (3.94), 608sh (4.08),
618 (4.14), 655 (4.49) nm. MS (ESI) (m/z): 1329.7 (M+ requires
1329.3). MS (MALDI-TOF) (m/z): 1330 (M+ requires 1329).
Anal. calcd. for C88H96N8Cu: C, 79.50; H, 7.28; N, 8.43.
Found: C, 79.90; H, 7.20; N, 8.08.
1
(4.04), 631 (4.82) nm. H NMR (400 MHz, CDCl3) δ: 1.48
(72 H, s, t-butyl H); 7.77-7.81 (4 H, m, quinoxaline H); 7.84-
7.86 (4 H, m, quinoxaline H); 7.92-7.93 (12 H, m, Ho, Hp);
8.97 (4 H, s, â-pyrrolic H). MS (ESI) (m/z): 1372.0 (M+
requires 1372.2). (HR-ESI-FT/ICR Found: [M + H]+ 1371.6928.
C88H96N8Pd requires 1371.6895.) Anal. calcd. for C88H96N8-
Pd: C, 77.03; H, 7.05; N, 8.17. Found: C, 76.55; H, 6.99; N,
8.06.
5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:7,8-b′′]porphyrin (4a). A mixture of 2,3,7,8-tetraoxo-
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)isobacteriochlorin (7)27
(54 mg, 0.0481 mmol) and o-phenylenediamine (6) (15.7 mg,
0.145 mmol) were dissolved in dichloromethane (10 mL) and
stirred at room temperature for 10 min. The reaction mixture
was evaporated to dryness, and the crude residue wsa purified
by chromatography over silica (dichloromethane-light petro-
leum; 1:2). The front running brown band was collected, and
the solvent was removed under vacuum to give 5,10,15,20-
tetrakis(3,5-di-tert-butylphenyl)-bisquinoxalino[2,3-b′:7,8-b′′]-
porphyrin (4a) (56.0 mg, 94%) as a dark purple microcrystalline
solid, mp >300 °C. IR (CHCl3): 3312w, 3058w, 3025s, 3011w,
2964s, 2903m, 2866m, 1595s, 1551w, 1476m, 1426w, 1393w,
1363s, 1297m, 1248m, 1233m, 1226s, 1211s, 1194w, 1157w,
1112m, 1000w, 984w, 933w cm-1. UV-vis (CHCl3): 314sh
(log ꢀ 4.49), 341 (4.60), 395sh (4.91), 416 (5.07), 456 (4.94),
482 (5.04), 559 (4.27), 595 (4.20), 626 (4.05), 678 (3.43) nm.
1H NMR (400 MHz, CDCl3) δ: -0.55 (2 H, br, s, inner NH);
1.45 (18 H, s, t-butyl H); 1.49 (36 H, s, t-butyl H); 1.53 (18 H,
s, t-butyl H); 7.79-7.82 (5 H, m, aryl H); 7.84-7.87 (2 H, m,
quinoxaline H); 7.89-7.93 (10 H, m, quinoxaline H, Ho, Hp);
8.02 (1 H, t, J 1.7 Hz, Hp); 8.06 (2 H, d, J 1.9 Hz, Ho); 8.68
and 8.72 (4 H, ABq, J 4.7 Hz, â-pyrrolic H). MS (ESI) (m/z):
1267.9 (M+ requires 1267.8). Anal. calcd. for C88H98N8: C,
83.4; H, 7.8; N, 8.8. Found: C, 83.6; H, 8.1; N, 8.4.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:12,13-b′′]porphyrinato}nickel(II) (3d). 5,10,15,20-Tet-
rakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]-
porphyrin (3a) (42.0 mg, 0.0331 mmol) and nickel(II) acetate
dihydrate (56.0 mg, 0.263 mmol) were dissolved in dissolved
in a solution of toluene (7 mL) and glacial acetic acid (18 M,
7 mL). The mixture was then heated at reflux for 18 h.
Dichloromethane (30 mL) was added to the mixture, and the
organic phase was separated, washed with water (200 mL),
sodium carbonate solution (10%, 2 × 150 mL), and water (200
mL), dried over anhydrous sodium sulfate, filtered, and evapo-
rated to dryness. The residue was purified by column chroma-
tography over silica (chloroform-light petroleum; 1:2), the front
running green band was collected, and the solvent was removed
to afford 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinox-
alino[2,3-b′:12,13-b′′]porphyrinato}nickel(II) (3d) (39.3 mg,
90%) as a green microcrystalline solid, mp >300 °C. IR
(CHCl3): 2967s, 2906m, 2868m, 1595s, 1478m, 1393w, 1364s,
1249m, 1179m, 1041w cm-1. UV-vis (CHCl3): 361 (log ꢀ
4.83), 404 (4.80), 465 (5.14), 551 (3.94), 618 (4.03), 662 (4.35)
nm. 1H NMR (400 MHz, CDCl3) δ: 1.46 (72 H, s, t-butyl H);
7.76 (8 H, d, J 1.6 Hz, Ho); 7.77-7.79 (4 H, m, quinoxaline
H); 7.83-7.86 (8 H, m, quinoxaline H, Hp); 9.01 (4 H, s,
â-pyrrolic H). MS (ESI) (m/z): 1324.1 (M+ requires 1324.5).
Anal. calcd. for C88H96N8Ni: C, 79.80; H, 7.31; N, 8.46.
Found: C, 80.07; H, 7.26; N, 8.55.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:7,8-b′′]porphyrinato}zinc(II) (4b). A solution of 5,-
10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:
7,8-b′′]porphyrin (4a) (40.6 mg, 0.0320 mmol) and zinc(II)
acetate monohydrate (28.0 mg, 0.128 mmol) was dissolved in
a solution of chloroform (30 mL) and methanol (15 mL), and
the mixture was heated at reflux for 3 min. The solvent was
removed under vacuum, and the residue was passed through a
plug of silica (dichloromethane-light petroleum; 1:1). The front
running green band was collected, and the solvent was removed
to give pure {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bis-
quinoxalino[2,3-b′:7,8-b′′]porphyrinato}zinc(II) (4b) (42.0 mg,
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:12,13-b′′]porphyrinato}palladium(II) (3e). 5,10,15,-
20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-
b′′]porphyrin (3a) (42.0 mg, 0.0331 mmol), palladium(II)

Radical Anions of Porphyrins
J. Phys. Chem. A, Vol. 112, No. 3, 2008 569
99%) as a dark green microcrystalline solid, mp >300 °C. IR
(CHCl3): 3059w, 3023s, 3014m, 2964s, 2903m, 2867m, 1594s,
1516w, 1477m, 1427w, 1392m, 1362s, 1345m, 1298m, 1248m,
1234m, 1222m, 1212m, 1170m, 1117m, 1093w, 1070w, 1036w,
1004m, 951m, 924w cm-1. UV-vis (CHCl3): 340 (log ꢀ 4.60),
405sh (4.94), 423 (5.04), 465 (4.84), 494 (4.95), 539sh (3.89),
requires 1325). Anal. calcd. for C88H96N8Ni: C, 79.8; H, 7.3;
N, 8.5. Found: C, 79.8; H, 7.5; N, 8.1.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:7,8-b′′]porphyrinato}palladium(II) (4e). 5,10,15,20-
Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]-
porphyrin (4a) (80.0 mg, 0.0631 mmol), palladium(II) chloride
(70.0 mg, 0.395 mmol), and sodium acetate (79.0 mg, 0.963
mmol) were dissolved in a solution of toluene (13 mL) and
glacial acetic acid (18 M, 13 mL). The mixture was then heated
at reflux for 5 min. Dichloromethane (30 mL) was added to
the mixture, and the organic phase was separated, washed with
water (200 mL), sodium carbonate solution (10%, 2 × 150 mL),
and water (200 mL), dried over anhydrous sodium sulfate,
filtered, and evaporated to dryness. The crude mixture was
purified by column chromatography over silica (dichloromethane-
light petroleum; 1:3). The front running red band was collected,
and the solvent was removed to give 5,10,15,20-tetrakis(3,5-
di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]porphyrinato}-
palladium(II) (4e) (86.0 mg, 99%) as a red microcrystalline
solid, mp >300 °C. IR (CHCl3): 3022m, 3014m, 3007w, 2964s,
2903m, 2868m, 1595s, 1537w, 1495w, 1477m, 1462w, 1427w,
1393w, 1364s, 1300m, 1248m, 1234m, 1217s, 1203m, 1178w,
1121w, 1018m, 1005w cm-1. UV-vis (CHCl3): 332 (log ꢀ
4.68), 347 (4.69), 390sh (4.77), 423 (4.94), 452 (4.87), 476
(4.93), 564 (4.39), 599 (4.54) nm. 1H NMR (400 MHz, CDCl3)
δ: 1.44 (18 H, s, t-butyl H); 1.48 (36 H, s, t-butyl H); 1.52 (18
H, s, t-butyl H); 7.78-7.92 (17 H, m, quinoxaline H, Ho, Hp);
8.03 (1 H, t, J 1.5 Hz, Hp); 8.04 (2 H, d, J 1.6 Hz, Ho); 8.80
and 8.88 (4 H, ABq, J 4.9 Hz, â-pyrrolic H). MS (ESI) (m/z):
1372.7 (M+ requires 1372.2). MS (MALDI-TOF) (m/z): 1373
(M+ requires 1372). (HR-ESI-FT/ICR Found: [M + H]+
1371.6890. C88H96N8Pd requires 1371.6895.) Anal. calcd. for
C88H96N8Pd + CH3OH: 76.13; H, 7.18; N, 7.98. Found: C,
76.09; H, 7.07; N, 8.07.
1
586sh (4.30), 596 (4.41), 629 (3.99) nm. H NMR (400 MHz,
CDCl3) δ: 1.48 (18 H, s, t-butyl H); 1.51 (36 H, s, t-butyl H);
1.53 (18 H, s, t-butyl H); 7.79-7.81 (5 H, m, quinoxaline H,
Hp); 7.87-7.94 (12 H, m, quinoxaline H, Ho, Hp); 8.04 (1 H,
t, J 1.6 Hz, Hp); 8.09 (2 H, d, J 1.6 Hz, Ho); 8.88 and 8.91 (4
H, ABq, J 4.5 Hz, â-pyrrolic H). MS (ESI) (m/z): 1331.8 (M+
requires 1331.2). Anal. calcd. for C88H96N8Zn: C, 79.4; H, 7.3;
N, 8.4. Found: C, 79.1; H, 7.5; N, 8.0.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:7,8-b′′]porphyrinato}copper(II) (4c). 5,10,15,20-Tet-
rakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]por-
phyrin (4a) (470 mg, 0.371 mmol) and copper(II) acetate
monohydrate (556 mg, 2.79 mmol) in a solution of chloroform
(80 mL) and methanol (40 mL) were heated at reflux for 10
min. The solvent was removed under vacuum, and the residue
was passed through a plug of silica (dichloromethane-light
petroleum; 1:3). The front running green band was collected,
and the solvent was removed to afford {5,10,15,20-tetrakis(3,5-
di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]porphyrinato}-
copper(II) (4c) (485 mg, 98%) as a dark green microcrystalline
solid, mp >300 °C. IR (CHCl3): 3069w, 3046w, 3020s, 2964s,
2904m, 2868m, 1595s, 1529w, 1492w, 1477m, 1427w, 1393w,
1363s, 1349m, 1296m, 1248m, 1228m, 1222s, 1176s, 1116w,
1072w, 1034w, 1008w cm-1. UV-vis (CHCl3): 328sh (log ꢀ
4.56), 339 (4.61), 397sh (4.79), 419 (4.94), 429sh (4.91), 462
(4.78), 488 (4.88), 584 (4.28), 621 (4.21) nm. MS (ESI) (m/z):
1329.8 (M+ requires 1329.3). MS (MALDI-TOF) (m/z): 1329
(M+ requires 1329). Anal. calcd. for C88H96N8Cu: C, 79.5; H,
7.3; N, 8.4. Found: C, 79.5; H, 7.4; N, 8.1.
Electrochemical Measurements. Cyclic voltammetry was
carried out by using an EG&G Princeton Applied Research
(PAR) 173 potentiostat/galvanostat. A homemade three-
electrode cell was used for cyclic voltammetric measurement
and consisted of a platinum button or glassy carbon working
electrode, a platinum counter electrode, and a homemade
saturated calomel reference electrode (SCE). The SCE was
separated from the bulk of the solution by a fritted glass bridge
of low porosity which contained the solvent/supporting elec-
trolyte mixture. UV-vis spectroelectrochemical experiments
were performed with a home-built thin-layer cell that had a light
transparent platinum net working electrode. Potentials were
applied and monitored with an EG&G PAR model 173
potentiostat. Time-resolved UV-vis spectra were recorded with
a Hewlett-Packard model 8453 diode array spectrophotometer.
Absolute CH2Cl2 from Aldrich Co. was used as received without
further purification. TBAP was purchased from Sigma Chemical
or Fluka Chemika Co., recrystallized from ethyl alcohol, and
dried under vacuum at 40 °C for at least 1 week prior to use.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino-
[2,3-b′:7,8-b′′]porphyrinato}nickel(II) (4d). 5,10,15,20-Tet-
rakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b′′]por-
phyrin (4a) (44.0 mg, 0.0347 mmol) and nickel(II) acetate
tetrahydrate (30.0 mg, 0.141 mmol) were dissolved in a solution
of toluene (10 mL) and glacial acetic acid (10 mL), and the
mixture was heated at reflux for 1 min. Dichloromethane (30
mL) was added to the mixture, and the organic phase was
separated, washed with water (200 mL), sodium carbonate
solution (10%, 2 × 200 mL), and water (200 mL), dried over
anhydrous sodium sulfate, filtered, and evaporated to dryness.
The crude mixture was purified by column chromatography over
silica (dichloromethane-light petroleum; 1:3). The front running
green band was collected, and the solvent was removed to give
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-
b′:7,8-b′′]porphyrinato}nickel(II) (4d) (45.5 mg, 99%) as a green
microcrystalline solid, mp >300 °C. IR (CHCl3): 3059w, 3018s,
2964s, 2904m, 2867m, 1595s, 1477w, 1427w, 1392w, 1364m,
1298w, 1247m, 1224s, 1177w, 1119w, 1016w, 952w cm-1
.
ESR Measurements. Radical anions of the quinoxalinopo-
rphyrins were prepared by the photoreduction of the compounds
with (BNA)245 in deaerated MeCN. Typically, a quinoxalinopo-
rphyrin was dissolved in CH2Cl2 (5.0 × 10-4 M in 300 µL)
containing (BNA)2 (2.5 × 10-4 M) and purged with argon for
10 min. The solution was bubbled with Ar gas through a syringe
that has a long needle. ESR spectra were recorded on a JEOL
JES-RE1XE spectrometer under irradiation of a high-pressure
mercury lamp (USH-1005D) by focusing at the sample cell in
the ESR cavity at 298 K. The magnitude of modulation was
chosen to optimize the resolution and signal-to-noise ratio of
UV-vis (CHCl3): 314sh (log ꢀ 4.48), 340sh (4.68), 355 (4.69),
399sh (4.69), 421sh (4.79), 436 (4.82), 466 (4.72), 491 (4.81),
584 (4.18), 622 (4.20) nm. 1H NMR (400 MHz, CDCl3) δ: 1.40
(18 H, s, t-butyl H); 1.43 (36 H, s, t-butyl H); 1.46 (18 H, s,
t-butyl H); 7.65 (2 H, d, J 1.7 Hz, Ho); 7.69-7.70 (5 H, m,
Ho, Hp); 7.73-7.77 (4 H, m, quinoxaline H); 7.79-7.82 (4 H,
m, Hp, quinoxaline H); 7.83 (1 H, d, J 1.7 Hz, Ho); 7.88 (1 H,
t, J 1.7 Hz, Hp); 7.91-7.94 (2 H, m, quinoxaline H); 8.65 and
8.78 (4 H, ABq, J 4.9 Hz, â-pyrrolic H). MS (ESI) (m/z): 1324.8
(M+ requires 1324.5). MS (MALDI-TOF) (m/z): 1325 (M+

570 J. Phys. Chem. A, Vol. 112, No. 3, 2008
E et al.
(25) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III.; Singh,
D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J.
Am. Chem. Soc. 1998, 120, 10001.
the observed spectra under nonsaturating microwave power
conditions. The g values were calibrated using a Mn2+ marker.
Calculations. Calculations of the properties of molecules
were performed using DFT with the B3LYP density functional49
and the 6-31G(d) basis set. All calculations were performed
using Gaussian 03.50 Graphical outputs of the computational
results were generated with the Gaussview software program
(version 3.09) developed by Semichem, Inc.51 The hyperfine
coupling constants were determined by computer simulation
using the Calleo ESR program (version 1.2) coded by Calleo
Scientific on a personal computer.
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Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., Grant No. E-680) and the Texas Advanced
Research program to K.M.K. under Grant No. 003652-0018-
2001 are gratefully acknowledged. This work was also partially
supported by a Discovery Research Grant (Grant No. DP0208776)
to M.J.C. and J.R.R. from the Australian Research Council and
a Grant-in-Aid (Grant No. 19205019) to S.F. from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
We thank the Australian Partnership for Advanced Computing
for provision of the computational resources.
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