Gold(III) porphyrins containing two, three, or four β,β′- fused quinoxalines. synthesis, electrochemistry, and effect of structure and acidity on electroreduction mechanism

Article abstract of DOI:10.1021/ic302380z

Gold(III) porphyrins containing two, three, or four β,β′- fused quinoxalines were synthesized and examined as to their electrochemical properties in tetrahydrofuran (THF), pyridine, CH₂Cl₂, and CH₂Cl₂ containing added acid in the form of trifluoroacetic acid (TFA). The investigated porphyrins are represented as Au(PQ₂)PF₆, Au(PQ₃)PF₆, and Au(PQ₄)PF₆, where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β′-pyrrolic position of the porphyrin macrocycle. In the absence of added acid, all three gold(III) porphyrins undergo a reversible one-electron oxidation and several reductions. The first reduction is characterized as a Au^III/Au^II process which is followed by additional porphyrin- and quinoxaline-centered redox reactions at more negative potentials. However, when 3-5 equivalents of acid are added to the CH₂Cl₂ solution, the initial Au^III/Au ^II process is followed by a series of internal electron transfers and protonations, leading ultimately to triply reduced and doubly protonated Au^II(PQ₂H₂) in the case of Au ^III(PQ₂)⁺, quadruply reduced and triply protonated Au^II(PQ₃H₃) in the case of Au ^III(PQ₃)⁺, and Au^II(PQ ₄H₄) after addition of five electrons and four protons in the case of Au^III(PQ₄)⁺. Under these solution conditions, the initial Au(PQ₂)PF₆ compound is shown to undergo a total of three Au^III/Au^II processes while Au(PQ₃)PF₆ and Au(PQ₄)PF₆ exhibit four and five metal-centered one-electron reductions, respectively, prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for reduction and UV-visible spectra of the electroreduced species are analyzed using linear free energy relationships.

Full text of DOI:10.1021/ic302380z

Article
pubs.acs.org/IC
Gold(III) Porphyrins Containing Two, Three, or Four β,β′-Fused
Quinoxalines. Synthesis, Electrochemistry, and Effect of Structure
and Acidity on Electroreduction Mechanism
Zhongping Ou,*,†,‡ Tony Khoury,§ Yuanyuan Fang,†,‡ Weihua Zhu,†,‡ Paul J. Sintic,§
Maxwell J. Crossley,*,§ and Karl M. Kadish*,‡
†School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
‡Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
§School of Chemistry, The University of Sydney, NSW 2006, Australia
S
* Supporting Information
ABSTRACT: Gold(III) porphyrins containing two, three, or
four β,β′-fused quinoxalines were synthesized and examined as
to their electrochemical properties in tetrahydrofuran (THF),
pyridine, CH2Cl2, and CH2Cl2 containing added acid in the
form of trifluoroacetic acid (TFA). The investigated
porphyrins are represented as Au(PQ2)PF6, Au(PQ3)PF6,
and Au(PQ4)PF6, where P is the dianion of the 5,10,15,20-
tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxa-
line group fused to a β,β′-pyrrolic position of the porphyrin
macrocycle. In the absence of added acid, all three gold(III)
porphyrins undergo a reversible one-electron oxidation and
several reductions. The first reduction is characterized as a
AuIII/AuII process which is followed by additional porphyrin-
and quinoxaline-centered redox reactions at more negative potentials. However, when 3−5 equivalents of acid are added to the
CH2Cl2 solution, the initial AuIII/AuII process is followed by a series of internal electron transfers and protonations, leading
ultimately to triply reduced and doubly protonated AuII(PQ2H2) in the case of AuIII(PQ2)+, quadruply reduced and triply
protonated AuII(PQ3H3) in the case of AuIII(PQ3)+, and AuII(PQ4H4) after addition of five electrons and four protons in the case
of AuIII(PQ4)+. Under these solution conditions, the initial Au(PQ2)PF6 compound is shown to undergo a total of three AuIII/
AuII processes while Au(PQ3)PF6 and Au(PQ4)PF6 exhibit four and five metal-centered one-electron reductions, respectively,
prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction
was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in
nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for
reduction and UV−visible spectra of the electroreduced species are analyzed using linear free energy relationships.
A positive linear shift in reduction potentials with increase in
the number of fused quinoxaline (Q) groups has been well-
documented upon going from simple metalloporphyrins to
derivatives of mono- and “linear” bis-quinoxalinoporphyrins,
with the magnitude of the shift depending upon the type and
oxidation state of the central metal ion and the specific site of
electron transfer.3−8,11−16 A much smaller shift in macrocycle-
centered reduction potentials was reported upon going from
the PQ to corner PQ2 quinoxalinoporphyrins with redox
inactive centers,13 but it was not known if the same trend would
be observed for metal-centered reactions, nor was it known
how the addition of three or four Q groups to the gold(III)
porphyrins would be reflected in E1/2 values for the multiple
reductions of these compounds in nonaqueous media. This is
INTRODUCTION
■
The electroreduction of quinoxalinoporphyrins can involve the
central metal ion, the conjugated porphyrin macrocycle, or the
fused quinoxaline group(s), depending upon the specific
compound and the experimental conditions.1−8 The measured
redox potentials will depend in each case upon the site of
electron transfer, the type and oxidation state of the central
metal ion, the number of fused quinoxaline groups, the specific
electron-donating or electron-withdrawing substituents on the
macrocycle,3,4,6−8 and the type and number of axial ligands.9
Protonation of the quinoxaline nitrogen atoms can also have a
significant effect on the electrochemical behavior of these
compounds as was demonstrated for derivatives containing
both electrochemically inert Sn(IV), Cu(II), or Ni(II) central
metal ions7 and redox active central metal ions such as Co(II),4
Mn(III),10 Ag(III),11 and Au(III).12
Received: October 31, 2012
Published: February 20, 2013
© 2013 American Chemical Society
2474
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
Chart 1. Structures of Gold(III) Quinoxalinoporphyrins
quinoxaline group fused to the β,β′-pyrrolic positions of the
porphyrin macrocycle (Chart 1).
Scheme 1. Published Mechanism for the Reduction of
AuIII(PQ)+ in Non-Aqueous Media Containing TFA12
Gold(III) quinoxalinoporphyrins have attracted a great deal
of interest, in part because they are easy to reduce and can
undergo multiple electron additions.2−4,6,7 The first reduction
involves an AuIII/AuII conversion in all cases while the second
and third one-electron reductions are proposed to be porphyrin
ring-centered electron transfers. Further reductions are also
possible at the fused quinoxaline group(s) and are often
observed at more negative potentials.5,17 Gold(III) quinox-
alinoporphyrins are also of interest since the mono- and bis-
quinoxaline derivatives were recently shown to undergo
multiple AuIII/AuII process in acidic solutions of CH2Cl2 or
PhCN,12 as shown in Scheme 1 for the case of Au(PQ)PF6.
investigated in the present study for three newly synthesized
gold(III) porphyrins which are represented as Au(PQ2)PF6,
Au(PQ3)PF6, and Au(PQ4)PF6, where P is the dianion of the
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a
Table 1. Half-Wave Potentials (V vs SCE) of AuIII(PQn) PF6 (n = 0−4) in CH2Cl2, Pyridine, and THF, 0.1 M TBAP
oxidation
reduction
third
potential separation (ΔE1/2,V)
solvent
CH2Cl2
cpd
first
first
second
fourth
fifth
1red-2red
2red-3red
1ox-1red
ref
P
1.59
1.54
1.48
1.48
1.36
1.24
−0.64
−0.47
−0.26
−0.31
−0.17
−0.06
−0.52
−0.35
−0.17
−0.24
−0.11
−0.01
−0.40
−0.20
−0.02
−0.07
0.09
−1.15
−0.97
−0.84
−1.05
−0.90
−0.97
−1.08
−0.87
−0.71
−0.97
−0.82
−0.86
−1.10
−0.88
−0.70
−0.95
−0.76
−0.80
0.51
0.50
0.58
0.74
0.73
0.91
0.56
0.52
0.54
0.73
0.71
0.85
0.70
0.68
0.68
0.88
0.85
0.97
2.23
2.01
1.74
1.79
1.53
1.30
3, 12
3, 12
12
tw
tw
tw
2
a
PQ
−1.82
−1.64
−1.55
0.85
0.80
0.50
0.60
0.42
0.68
0.82
0.85
0.51
0.60
0.46
0.67
0.77
0.83
0.50
0.61
0.48
a
QPQ
PQ2
PQ3
PQ4
P
a
a
−1.82
−1.66
−1.53
−1.50
−1.39
−1.76
−1.69
−1.56
−1.48
−1.44
−1.32
−1.77
−1.65
−1.53
−1.45
−1.37
−1.28
−1.66
Pyridine
PQ
3
QPQ
PQ2
PQ3
PQ4
P
tw
tw
tw
tw
2
a
a
b
−1.85
−1.68
−1.76
a
b
−1.97
−1.76
c
a
THF
1.89
2.29
1.97
1.62
1.61
1.54
1.49
a
a
a
a
a
a
PQ
1.77
−2.22
−2.04
−1.97
−1.85
−1.79
3
a
a
a
a
a
QPQ
PQ2
PQ3
PQ4
1.60
−2.22
−2.26
−2.05
−2.01
tw
tw
tw
tw
1.53
1.45
1.32
0.17
a
b
c
Irreversible peak potential at a scan rate of 0.10 V/s. tw = this work. Two overlapping one-electron reductions. Potentials vs Ag/AgCl.
2475
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
To determine if additional proton induced metal-centered
reductions might occur with further increase in the number of
Q groups, the three newly synthesized compounds were
therefore examined not only as to their electrochemical
properties in tetrahydrofuran (THF), pyridine, and CH2Cl2
but also in CH2Cl2 containing added acid in the form of
trifluoroacetic acid (TFA). The effect of the number of Q
groups on the redox potentials and protonation reactions is
discussed, and an overall reduction/oxidation mechanism is
then proposed. For simplification, the examined Au(III)
porphyrins are represented in the manuscript as Au(PQ2)+,
Au(PQ3)+, and Au(PQ4)+.
RESULTS AND DISCUSSION
■
Electrochemistry of AuIII(PQ2)PF6, AuIII(PQ3)PF6, and
AuIII(PQ4)PF6. The electrochemistry of Au(PQ2)+, Au(PQ3)+,
and Au(PQ4)+ was initially carried out in CH2Cl2, pyridine, and
THF containing 0.1 M TBAP as supporting electrolyte. The
half-wave potentials are summarized in Table 1 which also
includes data for previously investigated gold(III) porphyrins
having zero, one, or two linear β,β′-fused quinoxaline groups on
the macrocycle.2,3,12 Cyclic voltammograms of the six
AuIII(PQn)PF6 complexes analyzed in the present study are
illustrated in Figure 1 for the reductions in CH2Cl2 containing
0.1 M TBAP. As seen in the figure, and also previously
reported,12 AuIII(P)+, AuIII(PQ)+, and AuIII(QPQ)+ undergo
two or three one-electron reductions in CH2Cl2 (Figure 1a),
while four or five reductions are observed for AuIII(PQ2)+,
Au(PQ3)+, and AuIII(PQ4)+ under the same solution conditions
(Figure 1b).
The first one-electron reduction of AuIII(PQn)+ is metal-
centered,3,12 and this was verified by electron spin resonance
(ESR) in the present study for the product of singly reduced
AuIII(PQ3)+ and AuIII(PQ4)+ which show typical Au(II) signals
at g = 2.06 (Supporting Information, Figure S1). The reversible
half wave potentials for the AuIII/AuII process range from −0.64
to 0.17 V depending upon the solvent (CH2Cl2, Py, or THF)
and number of Q groups (0 to 4) fused to the macrocycle (see
Table 1). The first oxidation of the six AuIII(PQn)+ complexes is
reversible in CH2Cl2 and involves a one-electron abstraction
from the conjugated π-ring system, with E1/2 values ranging
from 1.59 to 1.24 V (see Table 1). The absolute potential
difference between the reversible first reduction of the six
Au(III) porphyrins at the central metal ion and the reversible
first oxidation at the conjugated macrocycle (ΔE1/2) range from
2.23 to 1.30 V in CH2Cl2 and is smallest in the case of
Au(PQ3)+ (1.53 V) and Au(PQ4)+ (1.30 V). This separation is
solvent dependent, as seen by the data in THF, where
reversible oxidations are seen only for the PQ2, PQ3, and PQ4
derivatives, leading to ΔE1/2 values of 1.61, 1.54, and 1.49,
respectively.
Figure 1. Cyclic voltammograms of Au(III) quinoxalinoporphyrins
which undergo (a) 2−3 reductions and (b) 4−5 reductions in CH2Cl2
containing 0.1 M TBAP.
Table 1). This unusually large potential separation between
formation of the Au(II) porphyrin π-anion radical and dianion
contrasts markedly with what is known to occur for ring-
centered reductions of all other metalloporphyrins with M(II)
centered metals, where ΔE1/2 values of 0.4 to 0.5 V are
generally observed.1 This might at first suggest a different site of
electron transfer in the case of the PQ and QPQ derivatives,
but the situation becomes more complex when also analyzing
potentials for the first three reductions and first oxidation of the
PQ2, PQ3, and PQ4 compounds. This analysis is given in Figure
2 where correlations between half-wave potentials (E1/2) and n,
the number of Q groups, are presented. As seen in the figure,
the first and third reductions of AuIII(PQn)+ shift positively in
potential by 0.14 to 0.15 V per added Q group (an easier
reduction) upon going from n = 0 to n = 4 while the first
The second and third reversible reductions of Au(P)+,
Au(PQ)+, and Au(QPQ)+ have been shown to occur at the
conjugated π-ring system to give AuII π-anion radical and
dianions under all solution conditions. The potential difference
between the AuIII/AuII process and the first ring-centered
reduction of the three newly synthesized quinoxalinoporphyrins
depends slightly upon solvent, with measured ΔE1/2 values
ranging from 0.50 to 0.58 V in CH2Cl2 or pyridine and from
0.68 to 0.70 V in THF. Much larger potential separations are
seen between the second and the third one-electron reductions
of Au(PQ)+ and Au(QPQ)+ in the above listed solvents where
the ΔE1/2 values range from 0.77 to 0.85 V (see Figure 1 and
2476
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
Figure 2. Plots of redox potentials of AuIII(PQn)+ in CH2Cl2 vs the
number of quinoxaline groups, where the black filled circles represent
of P, PQ, and QPQ and the red open circles represent PQ2, PQ3, and
PQ4.
Scheme 2. Proposed Mechanism for First Three Reductions
of AuIII(PQ2)+, AuIII(PQ3)+, and AuIII(PQ4)+ in CH2Cl2
Figure 3. UV−visible spectra of (a) AuIII(P)+, AuIII(PQ)+,
AuIII(QPQ)+ and (b)AuIII(PQ2)+, AuIII(PQ3)+, and AuIII(PQ4)+ in
CH2Cl2, 0.1 M TBAP.
oxidation shifts negatively by 0.09 V with the change in
structure, that is, an easier oxidation occurs as the number of
fused Q groups on the molecule is increased.
Changes in the measured E1/2 values for oxidation and
reduction of a given compound in the AuIII(PQn)+ series upon
going from AuIII(P)+ to AuIII(PQ4)+ will depend upon a
number of factors, the most important of which are substituent
effects of the fused Q groups on the electroreduction or
electrooxidation site, changes in the size of the conjugated π-
system, and changes in planarity of the macrocycle with
increase in number of Q groups or change of solvent. A
progressively easier reduction would be expected to occur upon
increasing the number of electron-withdrawing Q groups, and
an easier reduction would also be expected to occur if the size
of the conjugated π-system were increased upon going from
AuIII(P)+ to AuIII(PQ4)+. In the case of oxidation, a harder
electron abstraction process (more positive potential) would be
expected with increase in electron-withdrawing effect of the Q
substituents but an easier oxidation would result with an
increase in size of the porphyrin π-ring system. An increased
nonplanarity of the porphyrin macrocycle might also lead to an
easier oxidation as has been demonstrated for a number of
compounds.1,18−21
The shift of E1/2 with increase in number of Q groups for the
second reduction of AuIII(PQn)+ to form an AuII π-anion radical
is a combination of two trends. The first involves the P, PQ,
and QPQ derivatives (solid points in Figure 2) where half-wave
potentials linearly shift in a positive direction with increase in Q
(slope = 0.16 V) while the second involves AuIII(PQ2)+,
AuIII(PQ3)+, and AuIII(PQ4)+ (open points in Figure 2) where
there is little to no significant shift in E1/2. This suggests a
change in mechanism or change in charge delocalization
between the two series of compounds. Interestingly, the
separation in E1/2 between the first and third reductions of
AuIII(PQn)+ complexes remains relatively constant at 1.33 to
1.38 V for all of the compounds except PQ2 which displays a
smaller separation of 1.24 V in CH2Cl2.
This relatively constant potential separation between the first
and third reductions further emphasizes the unexpected trends
in E1/2 for the second reduction as a function of the number of
Q groups on the macrocycle. As seen in Figures 1 and 2, the
2477
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
Figure 4. Plots of energy for the Soret and two visible bands of
AuIII(PQn)+ vs the number of Q groups on the macrocycle for P, PQ,
and QPQ (black solid points) and PQ2, PQ3, and PQ4 (red open
points).
Figure 6. Comparisons of the UV−visible spectra of AuII(PQn),
ZnII(PQn), and PdII(PQn) (n = 3 and 4) in CH2Cl2 containing 0.1 M
TBAP.
second macrocycle-centered reduction of AuII(PQ2)+,
AuIII(PQ3)+, and AuIII(PQ4)+ become increasingly more
difficult as compared to the first reduction, the absolute
difference in potential between these two processes being 910
mV for AuIII(PQ4)+ as compared to 500 mV for the same two
electron transfer reactions of AuIII(PQ)+. This result is
consistent with an increased negative charge being located on
the macrocycle after the first metal-centered reduction of
AuII(PQ2)+, AuIII(PQ3)+, and AuIII(PQ4)+ and might be
accounted by a transfer of charge between the Au(II) central
metal ion and the porphyrin macrocycle as shown in Scheme 2.
In contrast to the second reduction, a linear relationship is
observed in the plot of E1/2 vs the number of β,β′-fused Q
substituents for all other reactions of AuIII(PQn)+ (Figure 2).
Similar relationships between E1/2 and n are also seen for
reductions and oxidations of the AuIII(PQn)+ complexes in
pyridine and THF (Supporting Information, Figure S2), thus
indicating that there is little to no solvent effect on the potential
shifts.
Spectra of Neutral and Reduced Compounds. UV−
visible spectra of the different AuIII(PQn)+ compounds in
CH2Cl2 are shown in Figure 3. The P, PQ, and QPQ
derivatives (Figure 3a) are each characterized by a single sharp
Soret band at 415, 435, and 450 nm, respectively, while the
PQ2, PQ3, and PQ4 derivatives (Figure 3b) display three
relatively broad Soret bands as seen in the figure. The energy of
the most intense Soret band, labeled as I in Figure 3, and that of
the two visible bands, labeled as II and III, all shift toward
longer wavelengths as a function of the number of Q groups on
AuIII(PQn)+. This relationship is shown in Figure 4 where a plot
of Soret band energy vs the number of Q groups shows one
linear relationship for all six compounds and that of the two
Figure 5. Thin-layer UV−visible spectral changes of AuIII(PQ2)PF6, AuIII(PQ3)PF6, and AuIII(PQ4)PF6 in CH2Cl2 containing 0.1 M TBAP during
the reductions at the indicated potential.
2478
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
is formed (Figure 5c) while the visible bands are blue-shifted by
9−18 nm during this one-electron addition process. The three
Soret bands of AuIII(PQ3)+ are also red-shifted by 5−13 nm
while the Q bands exhibit a 14−23 nm blue-shift during the
first controlled potential reduction at −0.20 V (Figure 5b).
Several isosbestic points are seen, indicating the lack of
spectrally detectable intermediates in the AuIII/AuII transition.
The final spectra of singly reduced AuIII(PQ3)+ and AuIII(PQ4)+
are similar in CH2Cl2 to that of unreduced PdII and ZnII
quinoxalinoporphyrins containing the same number of Q
groups. This comparison is shown in Figure 6 and further
confirms the formation of gold(II) porphyrins after a one-
electron reduction of the neutral compound.
The second and third reductions of AuIII(PQn)+ are proposed
to occur at the porphyrin π-ring system, and the decreased
intensity of the Soret bands in the final spectra after reduction
are consistent with this assignment. Examples of the spectral
changes during the conversion of AuII(PQn)+ to its π-anion
radical form are shown in Figure 5 for derivatives of PQ2, PQ3,
and PQ4.
Electrochemistry of AuIII(PQ2)PF6, AuIII(PQ3)PF6, and
AuIII(PQ4)PF6 in the Presence of Acid. The electrochemistry
of AuIII(PQ2)+, AuIII(PQ3)+, and AuIII(PQ4)+ was also carried
out in CH2Cl2 containing H+ in the form of TFA. Examples of
cyclic voltammograms are shown in Figure 7 and illustrate the
significant differences in electrochemistry which result after the
addition of TFA to solution. For all three quinoxalinoporphyr-
ins, the first AuIII/AuII reduction (at E1/2 = −0.31, −0.17, or
−0.06 V in the absence of acid) becomes irreversible, and this
process is followed at more negative potential by additional
irreversible metal-centered reductions as was previously
reported for AuIII(PQ)+ and AuIII(QPQ)+ in acidic nonaqueous
media.12 AuIII(PQ2)+ undergoes two irreversible reductions
followed by a reversible AuIII/II process at a scan rate of 0.1 V/s
while AuIII(PQ3)+ and AuIII(PQ4)+ undergo three and four
irreversible redox processes, respectively, followed at more
negative potentials by a reversible AuIII/AuII transition. The
sequential irreversible reductions are all assigned as Au(III)-
centered electron transfers as shown in Scheme 3.
The potentials for each reduction of the quinoxalinoporphyr-
ins in acidic CH2Cl2 are given in Schemes 1 and 3 and also
summarized in Table 2 for three series of compounds,
AuIII(PQn)+, AuIII(PQnHn)+, and AuII(PQnHn). The first series
of compounds are unprotonated, and the last two are fully
protonated.
Several key points are evident from the data in Table 2, the
most important of which is that the reversible potentials for
reduction of the fully protonated Au(III) quinoxalinoporphyr-
ins to their Au(II) forms are virtually independent of the
number of Q groups where E1/2 varies over the narrow range of
−0.70 to −0.73 V for the PQn derivatives with n = 2, 3, or 4
(see also last reaction in lower part of Scheme 3).
Figure 7. Cyclic voltammograms of (a) AuIII(PQ2)+, (b) AuIII(PQ3)+,
and (c) AuIII(PQ4)+ in CH2Cl2 containing 0.1 M TBAP in the absence
and presence of 2.5, 3.5, or 4.5 equiv of added TFA.
visible band shows two linear relationships, one for P, PQ, and
PQ2 and the other for PQ2, PQ3, and PQ4 (Figure 4).
To evaluate possible changes in the site of electron transfer
with increasing number of Q groups, the electrode reactions of
AuIII(PQ2)+, AuIII(PQ3)+, and AuIII(PQ4)+ were monitored in
CH2Cl2, 0.1 M TBAP by UV−visible spectroelectrochemistry.
Spectral changes obtained during the first two reductions of
these three compounds are shown in Figure 5.
Similar evolutions in UV−visible spectra are seen during the
first metal-centered reduction of the three porphyrins in
CH2Cl2. For example, the three Soret bands of neutral
AuIII(PQ4)+ are red-shifted by 4−10 nm as the Au(II) complex
Similar reduction potentials are also seen for reduction of the
partially protonated compounds in the PQ2, PQ3, and PQ4
series. For example, reduction of the mono-Q protonated
compound, Au(PQ3H)+, is located at Epc = −0.38 V while an
almost identical potential (Epc = −0.36 V) is seen for the
reduction of Au(PQ4H2)+. Similar reduction potentials are also
seen for AuII(PQ2H) (−0.50 V), Au(PQ3H2)+ (−0.56 V), and
Au(PQ4H3)+ (−0.54 V).
Finally, the potential for reduction of the fully protonated
AuII derivatives of PQ2, PQ3, and PQ4 ranges from −1.28 V for
AuII(PQ2H2) to −1.35 V for AuII(PQ4H4) and varies only
2479
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
Scheme 3. Proposed Reduction/Oxidation Mechanism of Au(PQ2)+, Au(PQ3)+, and AuIII(PQ4)+ in CH2Cl2 Containing 2.5, 3.5,
or 4.5 equiv of TFA
Further reduction of AuIII(PQ3H)+ at −0.40 V in the acid
containing CH2Cl2 solution gives a porphyrin product with a
Soret band at 429 nm and a Q-band at 562 nm (Figure 9b).
This spectrum is assigned to AuIII(PQ3H2)+ and is almost
identical to the spectrum of AuIII(PQ)+ which has bands at 435
and 588 nm in CH2Cl2.12 The reduction of AuIII(PQ3H2)+ at
−0.60 V (Figure 9c) leads to AuIII(PQ3H3)+ as a final porphyrin
product (λ = 409, 520, and 640 nm) and this is followed by
another AuIII/AuII process at −0.80 V (Figure 9d) to give the
Au(II) product AuII(PQ3H3). The UV−visible spectrum of the
fully protonated Au(II) porphyrin is characterized by a Soret
band at 420 nm and resembles the spectrum of both AuII(P)
(420 nm)12 and AuII(PQ4H4) (419 nm) (see Figure 10).
Finally the reduction of AuII(PQ3H3) at −1.40 V (Figure 9e)
results in a decreased intensity Soret band, consistent with the
formation of an Au(II) porphyrin π-anion radical under the
given solution conditions.
Table 2. Reduction Potentials (V vs SCE) of AuIII(PQn)+,
AuIII(PQnHn)+, and AuII(PQnHn) in CH2Cl2, 0.1 M TBAP
with added TFA
a
unprotonated
AuIII(PQn)+
−0.64
fully protonated
AuIII(PQnHn)+ AuII(PQnHn) reference
number of Q
0
1
−0.64
−0.60
−0.72
−0.70
−0.71
−0.73
−1.15
−1.11
−1.19
−1.28
−1.32
−1.35
12
12
12
tw
tw
tw
b
−0.36
−0.29
c
b
2
d
2
−0.31
−0.20
−0.11
3
4
a
1.5, 2.5, 2.5, 3.5, 4.5 equiv of TFA added in solution for compounds
with n = 1−4, respectively; potentials for PQ compound measured in
b
PhCN. Irreversible peak potential at a scan rate of 0.10 V/s. tw = this
c
d
work. The QPQ derivative. The PQ2 derivative.
UV−visible spectra obtained before and after the first five
reductions of AuIII(PQ4)+ in CH2Cl2 containing 4.5 equiv of
TFA are shown in Figure 10. The stepwise products of the first
four reductions and coupled chemical reactions are represented
as AuIII(PQ4Hx) where x = 1−4, and the prevailing reduction
mechanism is shown in Scheme 3. The maximum Soret band
intensity of the Au(III) porphyrins is stepwise blue-shifted from
483 nm for x = 0 (the unprotonated porphyrin) to 405 nm for
x = 4 (the fully protonated Q4H4 derivative).
In summary, the electrochemical and spectroelectrochemical
properties of AuIII(PQ3)+ and AuIII(PQ4)+ were examined in
CH2Cl2 containing 0.1 M TBAP. Each porphyrin undergoes
one metal-centered reduction to form the corresponding Au(II)
porphyrin, and this reaction is followed by two porphyrin ring-
centered reductions and one or two Q-centered reductions.
Different electrochemical behavior is seen for AuIII(PQ3)+ and
AuIII(PQ4)+ as compared to the related P, PQ, and QPQ
derivatives. Negative shifts in reduction potentials are seen
upon going from AuIII(QPQ)+ to AuIII(PQ3)+ to AuIII(PQ4)+
while positive shifts are observed upon going from the P to PQ
to QPQ derivatives under the same solution conditions. All of
the Au(III) quinoxalinoporphyrins in the presence of TFA will
undergo multiple AuIII/AuII processes because of an internal
slightly as a function of the number of Q groups on the
compounds. (see Figure 7)
Spectroelectrochemistry of AuIII(PQ2)+, AuIII(PQ3)+,
and AuIII(PQ4)+ in the Presence of TFA. Thin-layer UV−
visible spectroelectrochemistry was carried out in CH2Cl2
containing 0.1 M TBAP and added TFA to elucidate UV−
visible spectra of the PQ2, PQ3, and PQ4 derivatives in each
stepwise reduced and protonated forms. Spectral changes
obtained during the multiple one-electron reductions of
AuIII(PQ2)+ and AuIII(PQ3)+ are shown in Figures 8 and 9,
and the final UV−visible spectrum after each one electron
reduction of AuIII(PQ4)+ are shown in Figure 10. The reactants
and products of the AuIII/AuII processes are given in Scheme 3.
The unreduced Au(III) quinoxalinoporphyrins are unproto-
nated in CH2Cl2 solutions containing less than 20 equiv of
added acid as evidenced by the UV−visible spectra. In contrast,
protonation readily occurs for the electroreduced species, and
this leads to characteristic shifts in the UV−vis spectra. For
example, after potential reduction of AuIII(PQ3)+ at −0.25 V in
CH2Cl2 containing 3.5 equiv of TFA (Figure 9a), the Soret
band shifts from 466 to 452 nm and the Q-band shifts from 654
to 600 nm as AuIII(PQ3H)+ is generated as a product of the
one-electron reduction and following chemical reaction.
2480
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
Figure 9. Thin-layer UV−visible spectral changes of Au(PQ3)+ during
reductions at (a) −0.25, (b) −0.40, (c) −0.60, (d) −0.80, and (e)
−1.40 V in CH2Cl2 containing 0.1 M TBAP and 3.5 equiv of added
TFA.
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 electrolyte mixture.
Thin-layer UV−visible spectroelectrochemical experiments were
performed with a home-built thin-layer cell which has a light
transparent platinum net working electrode. Potentials were applied
and monitored with an EG&G PAR Model 173 potentiostat. Time-
resolved UV−visible spectra were recorded with a Hewlett-Packard
Model 8453 diode array spectrophotometer. High purity N2 from
Trigas was used to deoxygenate the solution and kept over the
solution during each electrochemical and spectroelectrochemical
experiment.
Chemicals. Dichloromethane (CH2Cl2) was obtained from Aldrich
Co. and used as received for electrochemistry and spectroelec-
trochemistry experiments. Tetra-n-butylammonium perchlorate
(TBAP) was purchased from Sigma Chemical or Fluka Chemika
Co., recrystallized from ethyl alcohol, and dried under vacuum at 40
°C for at least one week prior to use.
Synthesis. Hexafluorophosphate{5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)bisquinoxalino[2,3-β′:7,8-β″]porphyrinato}aurate(III),
AuIII(PQ2)PF6 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl) bisquino-
xalino[2,3-β′:7,8-β″]porphyrin (41.0 mg, 0.0316 mmol), potassium
tetrachloroaurate(III) (37.0 mg, 0.0979 mmol), and sodium acetate
(61.0 mg, 0.744 mmol) were dissolved in a solution of toluene (12
mL) and glacial acetic acid (18 M, 12 mL). The reaction mixture was
heated at reflux for 10 min. The mixture was then diluted in
dichloromethane (30 mL) and washed with water (2 × 100 mL),
sodium carbonate solution (10%, 2 × 100 mL), water (2 × 100 mL),
dried over anhydrous sodium sulfate, filtered, and the filtrate
evaporated to dryness. The residue was dissolved in chloroform (12
Figure 8. .Thin-layer UV−visible spectral changes of Au(PQ2)+ during
reductions at (a) −0.30 (b) −0.55, (c) −0.90, and (d) −1.35 V in
CH2Cl2 containing 0.1 M TBAP and 2.5 equiv of added TFA.
electron transfer and protonation of the fused Q group(s) on
the porphyrin macrocycle which regenerates a new Au(III)
porphyrin with a reduced and protonated quinoxaline group.
The number of multiple AuIII/AuII reactions observed is one
more than the number of Q groups on the compound. Thus,
AuIII(PQ)+ and AuIII(QPQ)+ undergo two and three metal-
centered reductions,12 while AuIII(PQ3)+ undergoes four metal-
centered reductions and AuIII(PQ4)+ exhibits five AuIII/AuII
processes in the presence of acid. The electrochemically
initiated protonation of the Q-group changes not only the
redox potentials of the compounds but also their UV−visible
properties as well as the conjugation between the porphyrin
ring and the quinoxaline units.
EXPERIMENTAL SECTION
■
Instrumentation. Cyclic voltammetry was carried out at 298 K
using an EG&G Princeton Applied Research (PAR) 173 potentiostat/
galvanostat. A homemade three-electrode cell was used for cyclic
voltammetric measurements and consisted of a platinum button or
2481
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
w, 2964 s, 2905 m, 2866 m, 1595 s, 1491 w, 1477 m, 1464 w, 1431 w,
1394 w, 1300 w, 1238 m, 1186 w, 1122 w, 955 w cm−1. λmax (CHCl3)
(log ε): 332 (4.61), 410 (4.77), 434sh (4.76), 460 (5.10), 561 (4.22),
597 (4.26) nm. 1H NMR (400 MHz, CDCl3): δ 1.45 (18 H, s, t-butyl
H); 1.48 (36 H, s, t-butyl H); 1.53 (18 H, s, t-butyl H); 7.86 (2 H, d, J
1.8 Hz, Ho); 7.88−8.01 (15 H, m, quinoxaline H, Ho, Hp); 8.04 (2 H,
d, J 1.8 Hz, Ho); 8.10 (1 H, t, J 1.8 Hz, Hp); 9.12 and 9.22 (4 H, ABq, J
5.1 Hz, β-pyrrolic H). 31P NMR (162 MHz, CDCl3): δ −146.33 (1 P,
h, J 714 Hz, PF6). 19F NMR (282 MHz, CDCl3): δ −75.13 (6 F, d, J
714 Hz, PF6). Mass spectrum (ESI) (m/z): 1462.3 ([M - PF6]+
requires 1461.7).
Hexafluorophosphate{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
trisquinoxalino[2,3-β′:7,8-β″:12,13-β‴]-porphyrinato}aurate(III),
AuIII(PQ3)PF6 {5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)trisquino-
xalino[2,3-β′:7,8-β″:12,13-β‴]porphyrin22,23 (42.0 mg, 0.0306
mmol), potassium tetra-chloroaurate(III) (59.7 mg, 0.158 mmol)
and sodium acetate (56.0 mg, 0.683 mmol) were dissolved in a
solution of toluene (7 mL) and glacial acetic acid (18M, 7 mL). The
reaction mixture was heated at reflux for 1 h. The mixture was then
diluted in dichloromethane (30 mL) and washed with water (2 × 100
mL), sodium carbonate solution (10%, 100 mL), water (2 × 100 mL),
dried over anhydrous sodium sulfate, filtered, and the filtrate
evaporated to dryness. The residue was dissolved in chloroform (12
mL) and stirred with a saturated solution of potassium hexafluor-
ophosphate (1.13g, 6.112 mmol) in water (12 mL) for 18 h. The
mixture was then diluted in dichloromethane (40 mL) and washed
with water (2 × 100 mL), dried over anhydrous sodium sulfate,
filtered, and the filtrate evaporated to dryness. The mixture was then
purified by column chromatography over silica (chloroform/methanol;
100:4). The polar green band was collected, and the solvent was
removed to afford pure hexafluorophosphate{5,10,15, 20-tetrakis(3,5-
di-tert-butylphenyl)trisquinoxalino[2,3-β′:7,8-β″:12,13-β‴]-
porphyrinato}aurate(III) (50 mg, 96%) as a green solid, mp >300 °C.
IR (ν, cm−1, CHCl3): 3060w, 2964s, 2905s, 2868m, 1595s, 1491w,
1477m, 1462w, 1393w, 1362s, 1302w, 1248m, 1234s, 1190m, 1161w,
999w. UV−vis (λ, nm, CHCl3): 312sh (log ε 4.61), 335 (4.67), 400sh
(4.66), 415 (4.67), 467 (4.90), 573sh (3.98), 613 (4.18), 653 (4.33).
1H NMR (400 MHz, CDCl3): δ 1.46 (36 H, s, t-butyl H); 1.48 (36 H,
s, t-butyl H); 7.86 (4 H, d, J 1.7 Hz, Ho); 7.90 (4 H, d, J 1.8 Hz, Ho);
7.91−8.05 (14 H, m, quinoxaline H, Hp); 8.08 (2 H, t, J 1.8 Hz, Hp);
9.26 (2 H, s, β-pyrrolic H). 19F NMR (282 MHz, CDCl3): δ −70.35 (6
F, d, J 714 Hz, PF6). MS (ESI): 1564.9 ([M-PF6]+ requires 1564.8).
HR-MALDI-FTICR found: [M-PF6]+ 1563.76201 requires 1563.7630.
Hexafluorophosphate{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
tetrakisquinoxalino[2,3-β′:7,8-β″:12,13-β‴:17,18-β‴′]porphyrinato}-
aurate(III), AuIII(PQ4)PF6 5,10,15,20-tetrakis(3,5-di-tert-butyl-
phenyl)tetrakisquinoxalino[2,3-β′:7,8-β″:12,13-β‴:17,18-β‴′]-
porphyrin23 (36.7 mg, 0.0249 mmol), potassium tetrachloroaurate(III)
(44.0 mg, 0.116 mmol), and sodium acetate (49.0 mg, 0.597 mmol)
were dissolved in a solution of toluene (7 mL) and glacial acetic acid
(18 M, 7 mL). The reaction mixture was heated at reflux for 15 min.
The mixture was then diluted with dichloromethane (40 mL) and the
organic phase washed with water (2 × 100 mL), sodium carbonate
solution (10%, 100 mL), water (2 × 100 mL), dried over anhydrous
sodium sulfate, filtered, and the filtrate evaporated to dryness. The
residue was dissolved in chloroform (12 mL) and stirred with a
saturated solution of potassium hexafluorophosphate (1.13 g, 6.11
mmol) in water (12 mL) for 18 h. The mixture was then diluted in
dichloromethane (40 mL) and washed with water (2 × 200 mL), dried
over anhydrous sodium sulfate, filtered, and the filtrate evaporated to
dryness. The mixture was then purified by column chromatography
over silica (chloroform/methanol; 100:4). The polar brown green
band was collected, and the solvent revolved to afford pure
hexafluorophosphate{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
tetrakisquinoxalino-[2,3-β′:7,8-β″:12,13-β‴:17,18-β‴′]porphyrin}-
aurate(III) (44.0 mg, 98%) as a green solid, mp > 300 °C. IR (ν, cm−1,
CHCl3): 3061w, 2964s, 2903m, 2866m, 1597m, 1560w, 1553w,
1514w, 1491m, 1477m, 1460w, 1435w, 1393w, 1362s, 1352m, 1335w,
1302w, 1258s, 1248s, 1236s, 1161s, 1128w, 1082m, 1043w. UV−vis
(λ, nm, CHCl3): 320sh (log ε 4.78), 340 (4.81), 410 (4.91), 482
Figure 10. UV−visible spectra of AuIII(PQ4)+ and its stepwise
reduction products in CH2Cl2 containing 0.1 M TBAP and 4.5 equiv
of added TFA.
mL) and stirred with a saturated solution of potassium hexafluor-
ophosphate (0.600 g, 0.326 mmol) in water (12 mL) for 18 h. The
mixture was then diluted in chloroform (40 mL) and washed with
water (2 × 200 mL) and dried over anhydrous sodium sulfate, filtered,
and the filtrate evaporated to dryness. The mixture was then purified
by column chromatography over silica (chloroform/methanol; 100:3).
The polar green band was collected and the solvent removed. The
product was redissolved in chloroform (10 mL) and stirred with a
saturated solution of potassium hexafluorophosphate (0.702 g, 0.381
mmol) in water (10 mL) for 18 h. The mixture was then washed with
water (2 × 200 mL) and dried over anhydrous sodium sulfate, filtered,
and the filtrate evaporated to dryness. The green purple band was
collected and evaporated to afford pure hexafluorophosphate-
{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-β′:7,8-
β″]porphyrinato}aurate(III) (35.0 mg, 69%) as a black solid, mp >300
°C. Found: C, 65.28; H, 6.22; N, 6.74. C88H96AuF6N8P requires C,
65.74; H, 6.02; N, 6.74%. (HR-MALDI-FTICR Found: [M - PF6]+
1461.73991. C88H96AuN8 requires 1461.74180). νmax (CHCl3): 3061
2482
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483

Inorganic Chemistry
Article
1
(4.91), 621 (4.19), 675 (4.77). H NMR (400 MHz, CDCl3): δ 1.47
(72 H, s, t-butyl H); 7.88 (8 H, br s, Ho); 7.96−7.99 (8 H, m,
quinoxaline H); 8.03−8.06 (8 H, m, quinoxaline H); 8.07 (4 H, br s,
Hp). 19F NMR (282 MHz, CDCl3): δ −76.95 (6 F, d, J 715 Hz, PF6).
MS (ESI) 1667.1 ([M-PF6]+ requires 1666.9). (HR-MALDI-FTICR
Found: [M-PF6]+ 1665.78766 requires 1665.78539).
(15) Stintic, P.; E, W.; Ou, Z.; Shao, J.; Mcdonald, J.; Cai, Z.; Kadish,
K. M. Phys. Chem. Chem. Phys. 2008, 10, 268−280.
(16) Ou, Z.; E, W.; Shao, J.; Burn, P.; Craig, S.; Robin, W.; Kadish, K.
M.; Crossley, M. J. J. Porphyrins Phthalocyanines 2005, 9, 142−151.
(17) Crossley, M. J.; Sintic, P. J.; Walton, R.; Reimers, J. R. Org.
Biomol. Chem. 2003, 1, 2777−2787.
(18) Kadish, K. M.; Li, J.; Van Caemelbecke, E.; Ou, Z.; Guo, N.;
Autret, M.; D’Souza, F.; Tagliatesta, P. Inorg. Chem. 1997, 36, 6292−
6298.
ASSOCIATED CONTENT
* Supporting Information
■
S
(19) D’Souza, F.; Zandler, M. E.; Tagliatesta, P.; Ou, Z.; Shao, J.; Van
Caemelbecke, E.; Kadish, K. M. Inorg. Chem. 1998, 37, 4567−4572.
(20) Tagliatesta, P.; Li, J.; Autret, M.; Van Caemelbecke, E.; Villard,
A.; D’Souza, F.; Kadish, K. M. Inorg. Chem. 1996, 35, 5570−5576.
(21) Kadish, K. M.; D’Souza, F.; Villard, A.; Autret, M.; Van
Caemelbecke, E.; Bianco, P.; Antonini, A.; Tagliatesta, P. Inorg. Chem.
1994, 33, 5169−5170.
(22) Khoury, T.; Crossley, M. J. New J. Chem. 2009, 33, 1076−1086.
(23) Khoury, T.; Crossley, M. J. Chem. Commun. 2007, 46, 4851−
4853.
ESR spectra of singly reduced gold porphyrins in CH2Cl2 at 77
K. Plots of reduction potentials of AuIII(PQn)PF6 in THF and
pyridine containing 0.1 M TBAP vs the number of quinoxaline
groups. Plots of reduction potentials of AuII(PQn) in CH2Cl2
containing 0.1 M TBAP with and without added TFA vs the
number of quinoxaline groups. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zpou2003@yahoo.com (Z.O.), m.crossley@chem.
usyd.edu.au (M.J.C.), kkadish@uh.edu (K.M.K.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by grants from the Robert A. Welch
Foundation (K.M.K., Grant E-680) and the Natural Science
Foundation of China (Grant 21071067). Support was also
provided by a Discovery Research Grant (DP0208776) from
the Australian Research Council to M.J.C.
REFERENCES
■
(1) Kadish, K. M.; Van Caemelbecke, E.; Rotal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 8, pp 1−114.
(2) Kadish, K. M.; E, W.; Ou, Z.; Shao, J.; Sintic, P. J.; Ohkubo, K.;
Fukuzumi, S.; Crossley, M. J. Chem. Commun. 2002, 356−357.
(3) Ou, Z.; Kadish, K. M.; E, W.; Shao, J.; Sintic, P. J.; Ohkubo, K.;
Fukuzumi, S.; Crossley, M. J. Inorg. Chem. 2004, 43, 2078−2086.
(4) Zhu, W.; Sintic, M.; Ou, Z.; Sintic, P. J.; McDonald, J. A.;
Brotherhood, P. R.; Crossley, M. J.; Kadish, K. M. Inorg. Chem. 2010,
49, 1027−1038.
(5) Ohkubo, K.; Garcia, R.; Sintic, P. J.; Khoury, T.; Crossley, M. J.;
Kadish, K. M.; Fukuzumi, S. Chem.Eur. J. 2009, 15, 10493−10503.
(6) Sintic, P. J.; E, W.; Ou, Z.; Shao, J.; McDonald, J. A.; Cai, Z.;
Kadish, K. M.; Crossley, M. J.; Reimers, J. R. Phys. Chem. Chem. Phys.
2008, 10, 515−527.
(7) Ou, Z.; E, W.; Zhu, W.; Thordarson, P.; Sintic, P. J.; Crossley, M.
J.; Kadish, K. M. Inorg. Chem. 2007, 46, 10840−10849.
(8) Kadish, K. M.; E, W.; Sintic, P. J.; Ou, Z.; Shao, J.; Ohkubo, K.;
Fukuzumi, S.; Govenlock, L. J.; McDonald, J. A.; Try, A. C.; Cai, Z.;
Reimers, J. R.; Crossley, M. J. J. Phys. Chem. B 2007, 111, 8762−8774.
(9) Hutchison, J. A.; Sintic, P. J.; Crossley, M. J.; Nagamura, T.;
Ghiggino, K. P. Phys. Chem. Chem. Phys. 2009, 11, 3478−3489.
(10) Armstrong, R. S.; Foran, G. J.; Hough, W. A.; D’Alessandro, D.
M.; Lay, P. A.; Crossley, M. J. Dalton Trans. 2006, 40, 4805−4813.
(11) Fukuzumi, S.; Ohkubo, K.; Zhu, W.; Sintic, M.; Khoury, T.;
Sintic, P. J.; E, W.; Ou, Z.; Crossley, M. J.; Kadish, K. M. J. Am. Chem.
Soc. 2008, 130, 9451−9458.
(12) Ou, Z.; Zhu, W.; Fang, Y.; Sintic, P. J.; Khoury, T.; Crossley, M.
J.; Kadish, K. M. Inorg. Chem. 2011, 50, 12802−12809.
(13) E, W.; Kadish, K. M.; Sintic, P.; Khoury, T.; Govenlock, L.; Ou,
Z.; Shao, J.; Ohkubo, K.; Reimers, J.; Fukuzumi, S.; Crossley, M. J.
Phys. Chem. A 2008, 112, 556−570.
(14) Ou, Z.; Zhu, W.; Sintic, P.; Fang, Y.; Crossley, M.; Kadish, K. M.
J. Porphyrins Phthalocyanines 2012, 16, 5−6.
2483
dx.doi.org/10.1021/ic302380z | Inorg. Chem. 2013, 52, 2474−2483