Androgynous porphyrins. Silver(II) quinoxalinoporphyrins act as both good electron donors and acceptors

Article abstract of DOI:10.1021/ja801318b

The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert- butylphenyl)porphyrinato}silver(II), {5, 10, 15, 20-tetrakis(3,5-di-tert- butylphenyl)quinoxalino[2,3-b′]porphyrinato}silver(II), {5, 10, 15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:7,8-b″] porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) bisquinoxalino[2,3-b′:12,13-b″]porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin π-radical cations and Ag(I) porphyrin π-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by π-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the β-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.

Full text of DOI:10.1021/ja801318b

Published on Web 06/28/2008
Androgynous Porphyrins. Silver(II) Quinoxalinoporphyrins Act
as Both Good Electron Donors and Acceptors
Shunichi Fukuzumi,*,† Kei Ohkubo,† Weihua Zhu,‡ Maxine Sintic,§ Tony Khoury,§
Paul J. Sintic,§ Wenbo E,‡ Zhongping Ou,‡ Maxwell J. Crossley,*,§ and
Karl M. Kadish*,‡
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan,
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, and
School of Chemistry, The UniVersity of Sydney, NSW 2006, Australia
Received February 22, 2008; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; m.crossley@chem.usyd.edu.au; kkadish@uh.edu
Abstract: The metal-centered and macrocycle-centered electron-transfer oxidations and reductions
of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis
spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are
{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-bu-
tylphenyl)quinoxalino[2,3-b′]porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisqui-
noxalino[2,3-b′:7,8-b′′]porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisqui-
noxalino[2,3-b′:12,13-b′′]porphyrinato}silver(II). The first one-electron oxidation and first one-electron
reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal
oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap,
determined by the difference in the first oxidation and first reduction potentials, decreases by introduction
of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous
character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and
good electron acceptors, something not previously observed in other metalloporphyrin complexes.
The second one-electron oxidation and second one-electron reduction of the compounds both occur
at the porphyrin macrocycle to produce Ag(III) porphyrin π-radical cations and Ag(I) porphyrin π-radical
anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are
shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a
positive direction as compared to the same electrode reactions of the porphyrin without the fused
quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical
anions by π-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at
the ꢀ-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin
macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables
them to act as both good electron donors and good electron acceptors.
Introduction
the oxidation potentials of metalloporphyrins are lower than
those of free-base porphyrins.3–6 When metalloporphyrins act
as good electron donors, they are not able to act as good electron
acceptors because the HOMO-LUMO gap of most metallopor-
phyrins is generally constant for macrocycle-centered reactions,
Porphyrins contain an extensively conjugated two-dimen-
sional π-system, which makes them suitable for efficient electron
transfer, as the uptake or release of electrons results in minimal
structural and solvation change upon electron transfer.1 Por-
phyrin macrocyclic ligands can tightly bind numerous metal
ions, many of which are redox active and can thus undergo one
or more metal-centered reductions or oxidations.2 Metallopor-
phyrins have normally been used as good electron donors, in
particular for photoinduced electron transfer reactions because
(3) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) Wasielewski,
M. R.; Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M. P. Sol. Energy
Mater. Sol. Cells 1995, 38, 127.
(4) (a) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA,
2000; Vol. 8, pp 153-190. (b) Gust, D.; Moore T. A.; Moore, A. L.
In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Germany, 2001; Vol. 3, pp 272-336.
† Osaka University, SORST, JST.
‡ University of Houston.
§ University of Sydney.
(1) Fukuzumi, S. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
8, pp 115-152.
(2) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, CA, 2000; Vol. 8; pp 1-114.
(5) Fukuzumi, S.; Imahori, H. In Electron Transfer in Chemistry; Balzani,
V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 2, pp 927-
975.
(6) Fukuzumi, S.; Guldi, D. M. In Electron Transfer in Chemistry; Balzani,
V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 2, pp 270-
337.
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 9451–9458 9451

A R T I C L E S
Fukuzumi et al.
and thus one observes either a facile oxidation or a facile
reduction but not both for the same compound.1–6
are significantly lower than those of the related Ag(II) porphyrins
having the same electronic configuration. Thus, (TPP)AgII
(TTP2- ) dianion of tetraphenylporphyrin) is a relatively good
electron donor as indicated by its low oxidation potential (0.59
V vs SCE), but it is not a good electron acceptor because its
reduction potential is too negative to act in this capacity (-1.01
V vs SCE).29 On the other hand, introducing strongly electron-
withdrawing groups such as NO2 onto the four meso-phenyl
substituents of the tetraphenylporphyrin molecule to give ((p-
NO2)TPP)Ag will lead to a positive shift of the reduction
potential (to E1/2 ) -0.83 V vs SCE in CH2Cl2), and the
porphyrin then becomes a stronger electron acceptor; at the same
time the compound becomes a weaker electron donor because
its oxidation potential is also shifted positively, in this case to
0.78 V vs SCE29 from the value of 0.59 V in the absence of
the electron-withdrawing substituents.
The potentials for oxidation or reduction of a metallopor-
phyrin and the magnitude of the compound’s HOMO-LUMO
gap can also be modulated by fusing quinoxaline groups at the
ꢀ-pyrrole positions of the porphyrin macrocycle in either a linear
or corner fashion.30,31 For example, the HOMO-LUMO gap
of the doubly ring-annulated bisquinoxalinoporphyrin (QPQ)Zn
is lowered to 1.90 eV as compared to its free-base analogue in
the absence of the metal ion, (QPQ)H2 (2.14 eV).31 (QPQ)Zn
can then act as a relatively good electron donor with its oxidation
potential of 0.76 V, but it is still not a good electron acceptor
because its reduction potential remains too negative, being
-1.14 V vs SCE.31
In contrast to what happens in the case of the Zn(II) por-
phyrins, the introduction of quinoxaline groups fused to an
Ag(II) porphyrin macrocycle will give derivatives that are both
easier to reduce and easier to oxidize than the zinc quinoxali-
noporphyrins, since in this case the electron additions and
abstractions both occur at the metal center. This is described in
the present article, where it is shown that Ag(II) quinoxali-
noporphyrins have a unique androgynous character in that they
can act as both good electron donors and good electron acceptors
at the same time. The investigated compounds are shown in
Chart 1 and were characterized in nonaqueous media by
electrochemistry, UV-vis spectroelectrochemistry, and EPR
spectroscopy. The unique androgynous character of Ag(II)
quinoxalinoporphyrins is demonstrated in the photoinduced
electron transfer reactions with a Zn(II) porphyrin and tetram-
ethyl-p-benzoquinone, which afford both the oxidized and
reduced species at the same time in contrast to the case of
“simple” Ag(II) porphyrins without quinoxaline units where this
does not occur. Such an androgynous character of Ag(II)
quinoxalinoporphyrins provides new insights into and a rationale
for the construction of biomimetic charge-separated and storage
ensembles using the same compound acting as both an electron
donor and an electron acceptor.
Metal-centered reductions of metalloporphyrins generally
occur at potentials more positive than for reduction at the
conjugated macrocycle,2 and when this occurs the porphyrin
can act as a good electron acceptor, one example being Au(III)
porphyrins,7,8 which have frequently been used as electron
acceptor components in linked donor-acceptor molecules.9–14
However, the same Au(III) porphyrins will not act as good
electron donors because the metal center cannot be further
oxidized and electron abstraction from the conjugated macro-
cycle occurs only at very positive potentials,7,8 far beyond what
is needed for a good electron donor.
In contrast to Au(III) porphyrins, the Ag(II) complexes
undergo both oxidation and reduction involving the metal center
to produce Ag(III) and Ag(I) porphyrins before electron transfer
reactions at the porphyrin macrocycle.15–22 Like Cu(II) por-
phyrins, a d9 configuration exists for the central metal ion of
Ag(II) porphyrins, but the site of oxidation in the two series of
related metal complexes is different in that the first one-electron
oxidation of the Cu(II) porphyrins leads to a π-radical cation
rather than generating a Cu(III) porphyrin with an unoxidized
macrocyclic ring.15,23–28 This is because its dx -y orbital energies
2
2
(7) Kadish, K. M.; E, W.; Ou, Z.; Shao, J.; Sintic, P. J.; Ohkubo, K.;
Fukuzumi, S.; Crossley, M. J. Chem. Soc., Chem. Commun. 2002,
356.
(8) Ou, Z.; Kadish, K. M.; E, W.; Shao, J.; Sintic, P. J.; Ohkubo, K.;
Fukuzumi, S.; Crossley, M. J. Inorg. Chem. 2004, 43, 2078.
(9) Harriman, A.; Sauvage, J.-P. Chem. Soc. ReV. 1996, 25, 41.
(10) (a) Chambron, J. C.; Chardon-Noblat, S.; Harriman, A.; Heitz, V.;
Sauvage, J. P. Pure Appl. Chem. 1993, 65, 2343. (b) Chambron, J.-
C.; Collin, J.-P.; Dalbavie, J.-O.; Dietrich-Buchecker, C. O.; Heitz,
V.; Odobel, F.; Solladie, N.; Sauvage, J.-P. Coord. Chem. ReV. 1998,
178-180, 1299.
(11) Blanco, M.-J.; Consuelo Jime´nez, M.; Chambron, J.-C.; Heitz, V.;
Linke, M.; Sauvage, J.-P. Chem. Soc. ReV. 1999, 28, 293.
(12) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J.-P.; Dixon, I. M.;
Sauvage, J.-P.; Williams, J. A. G. Coord. Chem. ReV. 1999, 190-
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(13) Fukuzumi, S.; Ohkubo, K.; E, W.; Ou, Z.; Shao, J.; Kadish, K. M.;
Hutchison, J. A.; Ghiggino, K. P.; Sintic, P. J.; Crossley, M. J. J. Am.
Chem. Soc. 2003, 125, 14984.
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W.; Ou, Z.; Shao, J.; Kadish, K. M.; Crossley, M. J.; Fukuzumi, S.
Chem. Phys. 2006, 326, 3.
(15) Godziela, G. M.; Goff, H. M. J. Am. Chem. Soc. 1986, 108, 2237.
(16) Kadish, K. M.; Davis, D. G.; Furhop, J.-H. Angew. Chem., Int. Ed.
Engl. 1972, 11, 1014.
(17) Fuhrhop, J.-H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. Soc. 1973,
95, 5140.
(18) Antipas, A.; Dolphin, D.; Gouterman, M.; Johnson, E. C. J. Am. Chem.
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(20) Scheidt, W. R.; Mondal, J. U.; Eigenbrot, C. W.; Adler, A.;
Radonovich, L. J.; Hoard, J. L. Inorg. Chem. 1986, 25, 795.
(21) For Ag(III) complexes of corroles, see: Bru¨ckner, C.; Barta, C. A.;
Brinas, R. P.; Krause Bauer, J. A. Inorg. Chem. 2003, 42, 1673.
(22) For Ag(III) complexes of carbaporphyrins and N-confused porphyrins,
see: (a) Furuta, H.; Ogawa, T.; Uwatoko, Y.; Araki, K. Inorg. Chem.
1999, 38, 2676. (b) Muckey, M. A.; Szczepura, L. F.; Ferrence, G. M.;
Lash, T. D. Inorg. Chem. 2002, 41, 4840.
Experimental Section
Chemicals. Absolute dichloromethane (CH2Cl2) and pyridine
were purchased from Aldrich Co. and used as received without
(23) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982.
(24) Fuhrhop, J.-H. Struct. Bonding 1974, 18, 1.
(25) Scholz, W. F.; Reed, C. A.; Lee, J. L.; Scheidt, W. R.; Lang, G. J. Am.
Chem. Soc. 1982, 104, 6791.
(29) Kadish, K. M.; Lin, X. Q.; Ding, J. Q.; Wu, Y. T.; Araullo, C. Inorg.
Chem. 1986, 25, 3236.
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2057.
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Fukuzumi, S.; Govenlock, L. J.; McDonald, J. A.; Try, A. C.; Cai,
Z.-L.; Reimers, J. R.; Crossley, M. J. J. Phys. Chem. B 2007, 111,
8762.
(27) For singly oxidized copper(II) phthalocyanine, see: Gardberg, A. S.;
Doan, P. E.; Hoffman, B. M.; Ibers, J. A. Angew. Chem., Int. Ed.
2001, 40, 244.
(31) E, W.; Kadish, K. M.; Sintic, P. J.; Khoury, T.; Govenlock, L. J.; Ou,
Z.; Shao, J.; Ohkubo, K.; Reimers, J. R.; Fukuzumi, S.; Crossley, M. J.
J. Phys. Chem. A 2008, 112, 556.
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J. Am. Chem. Soc. 1987, 109, 2644.
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Androgynous Porphyrins
A R T I C L E S
Chart 1
MAGNEX superconducting magnet and an analytical external ESI
source. 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). All commercial solvents were routinely distilled
before 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 it through a column of alumina
immediately before use. Where solvent mixtures were used, the
proportions are given by volume.
Synthesis. The silver(II) porphyrins were prepared in high yield
from their corresponding free-base analogues34 by treatment with
silver(II) acetate. IR and UV-vis spectra showed patterns consistent
with metal(II) porphyrins, and high-resolution mass spectra analysis
confirmed the molecular formula of each complex. The detail for
synthesis and analytical data are shown in Supporting Information
S1-S3.
EPR Measurements. EPR spectra of silver(II) porphyrins were
recorded on a JEOL JES-RE1XE spectrometer. The singly oxidized
products were produced by a one-electron oxidation of the silver(II)
porphyrins with [Fe(bpy)3]3+ (bpy ) 2,2′-bipyridine) in CH2Cl2.32
EPR spectra of the singly oxidized species were measured at 298
and 77 K. The magnitude of modulation was chosen to optimize
the resolution and signal-to-noise (S/N) ratio of the observed spectra
under nonsaturating microwave power conditions. The g values and
hyperfine coupling constants were calibrated using an Mn2+ marker.
Calculations. Theoretical calculations of the properties of
molecules were performed using density functional theory (DFT)
with the B3LYP density functional35 and the 3-21G* basis set. All
calculations were performed using Gaussian 03.36 Graphical outputs
of the computational results were generated with the Gauss View
software program (version 3.09) developed by Semichem, Inc.37
Laser Flash Photolysis Measurements. Measurements of
nanosecond transient absorption spectrum were performed according
to the following procedure. The laser system was provided by
UNISOKU Co., Ltd. A deaerated PhCN solution containing the
silver porphyrin and photosensitizer [(P)Zn or tetramethyl-p-
benzoquinone] was excited by Nd:YAG laser (Continuum, SLII-
10, 4-6 ns fwhm) with 10-30 mJ/pulse. Time courses of the
transient absorption spectra were measured by using a continuous
Xe lamp (150 W) and an In GaAs-PIN photodiode (Hamamatsu
2949) as a probe light and a detector, respectively. The output from
the photodiodes and a photomultiplier tube was recorded with a
digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). The
transient spectra were recorded using fresh solutions in each laser
excitation. All experiments were performed at 298 K. The solution
was deoxygenated by argon purging for 15 min before the mea-
surements.
further purification. Benzonitrile (PhCN), obtained from Fluka
Chemika Co. or Aldrich Co., was distilled over phosphorus
pentaoxide under vacuum before use. Tetra-n-butylammonium
perchlorate (TBAP) was purchased from Sigma Chemical Co. or
Fluka Chemika Co., recrystallized from ethanol, and dried under
vacuum at 40 °C for at least one week before use. Tris(2,2′-bipyridyl)-
iron(III) hexafluorophosphate, [Fe(bpy)3](PF6)3, was prepared by
adding 3 equiv of 2,2′-bipyridine to an aqueous solution of ferrous
sulfate and then by oxidizing the iron(II) complex with lead dioxide
in an aqueous H2SO4 solution, followed by the addition of KPF6.32
Electrochemistry. Cyclic voltammograms were obtained with
an EG&G Princeton Applied Research (PAR) 173 potentiostat/
galvanostat. A homemade three-electrode cell was used 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 that
contained the solvent/supporting electrolyte mixture. UV-visible
spectroelectrochemical experiments were performed with a home-
built thin-layer cell that had a light transparent platinum networking
electrode.33 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.
Results and Discussion
General Procedure. Melting points were recorded on a Reichert
melting point stage and were uncorrected. Infrared spectra were
recorded on an FTIR-8400S Shimadzu Fourier transform infrared
spectrophotometer, as solutions in the stated solvents. Intensity
abbreviations used are: w, weak; m, medium; s, strong; br, broad.
UV-visible spectra were routinely recorded on a Cary 5E
UV-vis-NIR spectrophotometer in chloroform that was deacidified
by filtration through an alumina column. High-resolution electro-
spray ionization Fourier transform ion cyclotron resonance (HR-
ESI-FT/ICR) spectra were acquired at the Research School of
Chemistry, Australian National University, on a Bruker Daltonics
BioAPEX II FT/ICR mass spectrometer equipped with a 4.7 T
EPR Spectra of Silver(II) Porphyrins. The EPR spectra of
{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}sil-
ver(II) [(P)Ag] and {5,10,15,20-tetrakis(3,5-di-tert-butylphe-
nyl)quinoxalino[2,3-b′]porphyrinato}silver(II) [(PQ)Ag] in
CH2Cl2 are shown in Figure 1a,c together with the computer
simulation spectra in Figure 1b,d. The hyperfine splitting
patterns due to the Ag nuclei (I ) 1/2) and four equivalent
nitrogens, each of which has I ) 1, are virtually the same as
earlier reported for Ag(II) porphyrins with other macrocyles.38–40
(35) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(36) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004. For full list of authors, see Supporting
Information S9.
(37) Dennington, R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.;
Gilliland, R. GaussView; Semichem: Shawnee Mission, KS, 2003.
(38) Kneubu¨hl, F. K.; Koski, W. S.; Caughey, W. S. J. Am. Chem. Soc.
1961, 83, 1607.
(32) Fukuzumi, S.; Nakanishi, I.; Tanaka, K.; Suenobu, T.; Tabard, A.;
Guilard, R.; Van Caemelbecke, E.; Kadish, K. M. J. Am. Chem. Soc.
1999, 121, 785.
(33) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 1489.
(34) Crossley, M. J.; Sintic, P. J.; Walton, R.; Reimers, J. R. Org. Biomol.
Chem. 2003, 1, 2777–2787.
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A R T I C L E S
Fukuzumi et al.
Figure 1. EPR spectra of (a) (P)Ag and (c) (PQ)Ag in CH2Cl2. The
computer simulation spectra are given with a(Ag) and a(4N) values in (b)
and (d), respectively.
Figure 3. Calculated spin densities and frontier molecular orbitals with R
(left) and ꢀ (right) spins of (a) (P)Ag and (b) (PQ)Ag (energy is given in
electronvolts).
di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-
b′′]porphyrinato}silver(II) [(QPQ)Ag] in CH2Cl2 along with the
computer simulation spectra of the two compounds. The a(Ag)
and a(4N) values of the two bisquinoxalinoporphyrins, (PQ2)Ag
and (QPQ)Ag, are slightly smaller than those of the parent
compound (P)Ag and the monoquinoxaline derivative (PQ)Ag. The
EPR results in Figures 1 and 2 are well-explained by the DFT
calculations.
Figure 3 shows the SOMO of (P)Ag and (PQ)Ag with the
spin densities [top columns of (a) and (b), respectively] together
with the lower occupied orbitals of R and ꢀ spins (see
Experimental Section for calculation method). The calculated
spin densities on the Ag and pyrrole nitrogens are virtually the
same for (P)Ag and (PQ)Ag, where 41% of the spin is on the
Ag(II) ion. The SOMO in each case involves the metal-centered
orbital. The HOMO-1 is solely a macrocycle-centered orbital
(a1u type orbital) and is significantly lower in energy than the
SOMO. This suggests that the first electron-transfer oxidation
of (P)Ag and (PQ)Ag should occur at the metal center and the
second at the porphyrin π-ring system, and this is exactly what
is observed as discussed in later sections of this article.
Figure 4 shows the calculated spin densities for the frontier
molecular orbitals of (PQ2)Ag and (QPQ)Ag. The unoccupied
molecular orbitals are given in Supporting Information S4. The
calculated spin densities on the Ag and pyrrole nitrogens of
(PQ2)Ag and (QPQ)Ag are slightly smaller than those of (P)Ag
and (PQ)Ag (see EPR spectra in Figure 2). In contrast to the
case of (P)Ag and (PQ)Ag (Figure 3), the SOMO energy
involving the metal-centered orbitals of (PQ2)Ag and (QPQ)Ag
are quite close to the HOMO-1 (a1u) energy of the macrocycle-
Figure 2. EPR spectra of (a) (PQ2)Ag and (c) (QPQ)Ag in CH2Cl2. The
computer simulation spectra are given with a(Ag) and a(4N) values in (b)
and (d), respectively.
The a(Ag) and a(4N) values of (PQ)Ag are also identical within
experimental error to those of (P)Ag.
Figure 2 shows the EPR spectra of {5,10,15,20-tetrakis(3,5-di-
t e r t - b u t y l p h e n y l ) b i s q u i n o x a l i n o [ 2 , 3 - b ′ : 7 , 8 -
b′′]porphyrinato}silver(II)[(PQ2)Ag] and {5,10,15,20-tetrakis(3,5-
(39) MacCragh, A.; Storm, C. B.; Koski, W. S. J. Am. Chem. Soc. 1965,
87, 1470.
(40) Brown, T. G.; Hoffman, B. M. Mol. Phys. 1980, 39, 1073.
9
9454 J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008

Androgynous Porphyrins
A R T I C L E S
Figure 5. Cyclic voltammograms of (a) (P)Ag, (b) (PQ)Ag, (c) (PQ2)Ag,
and (d) (QPQ)Ag in pyridine, 0.1 M TBAP.
process of (QPQ)Ag and the one-electron reduction of (P)Ag
(Ered) is harder by 340 mV than the reversible Ag(II)/Ag(I)
process of (QPQ)Ag.
Figure 4. Calculated spin densities and frontier molecular orbitals with R
(right) and ꢀ (left) spins of (a) (PQ2)Ag and (b) (QPQ)Ag (energy is given
in electronvolts).
As shown in Figures 3 and 4, the SOMO energy decreases
with increased delocalization on the porphyrin ligand orbital.
The decrease in energy follows the order: (P)Ag < (PQ)Ag <
(PQ2)Ag < (QPQ)Ag, and this parallels the positive potential
shift in the first reduction of the compounds as illustrated in
Figure 5. Such a trend was earlier reported for other metalated
quinoxalinoporphyrins with electroinactive metal ions.30,31,43
With regard to the oxidation, the first electron is removed
from the SOMO, and the Eox value is shifted in a positive
centered orbital. It should be noted that the ordering of the a1u-
and a2u-type orbitals changes with increasing substitution. The
actual site of electron transfer in each redox reaction of (P)Ag,
(PQ)Ag, (PQ2)Ag, and (QPQ)Ag was examined experimentally
by electrochemistry, UV-vis spectroelectrochemistry, and EPR
spectroscopy (vide infra), and this is discussed in the following
sections.
direction with decreasing SOMO energy, as also occurs for Ered
.
The SOMO energy decreases with increasing delocalization at
the porphyrin ligand orbital. The smaller degree of solvation
due to delocalization results in a positive shift of the redox
potentials. The more significant change in Ered values as
compared to Eox upon introduction of one or two quinoxaline
moieties onto the porphyrin ring suggests that solvation has a
less pronounced effect on the oxidation as compared to the case
of the reduction.
Redox potentials were also measured in the nonbonding
solvent CH2Cl2. The solvent window for observing oxidations
is much extended over that of pyridine and two oxidations could
be observed, but the reductions are ill-defined in this solvent
and were not further investigated in CH2Cl2 because of the
presence of coupled chemical reactions and demetalation of the
electrogenerated Ag(I) in the absence of a coordinating solvent.29
Cyclic voltammograms illustrating the two reversible oxida-
tions in CH2Cl2 are given in Figure 6, and the measured half-
Electrochemistry of Silver(II) Porphyrins. Figure 5 compares
cyclic voltammograms for the oxidation and reduction of the
investigated compounds in pyridine. As seen in the figure, each
Ag(II) porphyrin undergoes one oxidation and two reductions
within the potential range of the pyridine solvent.41,42 The
addition of one or two quinoxaline groups to the porphyrin
macrocycle results in a positive shift of all potentials as
compared to oxidation or reduction of the parent (P)Ag
compound. The magnitude of the potential shift upon adding
one or two fused quinoxaline groups to the macrocycle is much
larger for reductions than that for oxidations as seen, for
example, by a comparison of (P)Ag and (QPQ)Ag in pyridine.
The (P)Ag undergoes a one-electron reduction and one-electron
oxidation at -0.94 and +0.64 V, while the linear bisquinoxaline
derivative is reduced and oxidized at -0.60 and +0.81 V under
the same solution conditions. Thus, the one-electron oxidation
of (P)Ag (Eox) is easier by 170 mV than the Ag(II)/Ag(III)
(41) The reduction wave of free base H2P is also seen because of partial
demetalation associated with the reduction of (P)Ag at-1.1 V in Figure
5a; see refs43 and.44 For photoinduced demetalation of (TPP)Ag under
reducing conditions, see: Kunkely, H.; Vogler, A. Inorg. Chem.
Commun. 2007, 10, 479.
(43) (a) Ou, Z.; E, W.; Shao, J.; Burn, P. L.; Sheehan, C. S.; Walton, R.;
Kadish, K. M.; Crossley, M. J. J. Porphyrins Phthalocyanines 2005,
9, 142. (b) 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. (c) Ou, Z.; E, W.; Zhu, W.; Thordarson,
P.; Sintic, P. J.; Crossley, M. J.; Kadish, K. M. Inorg, Chem. 2007,
46, 10840.
(42) Giraudeau, A.; Louati, A.; Callot, H. J.; Gross, M. Inorg. Chem. 1981,
20, 769.
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A R T I C L E S
Fukuzumi et al.
Figure 7. Thin-layer UV-visible spectral changes during the Ag(II)/Ag(I)
process in pyridine containing 0.1 M TBAP for (P)Ag at a controlled
potential of -1.10 V vs SCE.
to 0.83 V for (PQ)Ag and then to 0.63-0.65 V for (PQ2)Ag
and (QPQ)Ag. This trend is consistent with the electrogenerated
porphyrin radical cation being stabilized by extended π-delo-
calization because of the quinoxaline moieties as indicated by
the SOMO-1 in Figures 3 and 4, and this is facilitated by a
strong association of the doubly oxidized porphyrin with the
perchlorate anion of the supporting electrolyte.
Site of Electron Transfer in Silver(II) Porphyrins. The site
of electron transfer after oxidation or reduction of a given
metalloporphyrin can in many cases be identified by measuring
UV-vis spectra of the electrogenerated products.2 In the current
study, the spectral changes were in almost all cases reversible
upon electrooxidation or electroreduction, and an example of
these changes is shown in Figure 7 for the electroreduction of
(P)Ag in a thin-layer cell containing pyridine, 0.1 M TBAP.
As seen from the figure, the sharp Soret band due to (P)AgII at
430 nm and the Q-band at 544 nm both disappear as the Ag(I)
form of the compound is generated. The singly reduced
porphyrin has a split Soret band with decreased intensity at λ
) 423 and 462 nm and a Q-band at 653 nm. The electron
addition to (P)Ag is reversible on the spectroelectrochemical
time scale, and the original spectrum of the unreduced porphyrin
could be obtained by application of a positive applied potential
to regenerate the starting compound. Similar spectral changes
were reported earlier for the electrochemical reduction of
(TPP)AgII to give [(TPP)AgI]- in DMSO.29
Although the observed spectral changes for the first one-
electron reduction of (P)Ag are reversible in pyridine, the second
electron addition is not reversible, presumably due to demeta-
lation of AgI as described in the literature29 for [(TPP)AgI]-
and its phenyl-substituted derivatives. In contrast, a demetalation
is not observed after the first or second reductions of the
investigated quinoxalinoporphyrins in pyridine on the spectro-
electrochemical time scale, and this is documented by the
reversible current-voltage curves in Figure 5.
Figure 6. Cyclic voltammograms of (a) (P)Ag, (b) (PQ)Ag, (c) (PQ2)Ag,
and (d) (QPQ)Ag in CH2Cl2, 0.1 M TBAP.
Table 1. Half-Wave Potentials (V vs SCE) for Oxidation and
Reduction of Silver(II) Porphyrins in Pyridine and CH2Cl2
Containing 0.1 M TBAP
solvent
macrocycle
2nd ox
1st ox
1st red
2nd red
pyridine
P
a
a
a
a
1.58
1.50
1.40
1.40
0.64
0.74
0.80
0.81
0.58
0.67
0.75
0.77
-0.94
-1.77b
PQ
PQ2
QPQ
P
PQ
PQ2
QPQ
-0.78
-1.59
-0.65
-0.60
c
c
c
c
-1.47
-1.54
c
c
c
c
CH2Cl2
a Reaction occurred beyond the positive potential limit of the solvent.
b Peak potential measured at a sweep rate of 0.10 V/s. c Reactions
ill-defined because of coupled chemical reactions and demetalation in
this solvent.
wave potentials in both solvents are summarized in Table 1.
The potentials for the first one-electron oxidation are similar in
the bonding and nonbonding solvents although the reactions are
slightly easier in CH2Cl2 because of a stronger ion-pairing
interaction of the electrogenerated Ag(III) porphyrins with the
perchlorate anion from the supporting electrolyte. It should also
be pointed out that potentials for the second one-electron
oxidation are shifted in a negative direction with an increase in
the number of fused quinoxaline groups on the molecule as
compared to a positive shift in E1/2 for the first reduction process
(Figure 6). The negative potential shift in the second oxidation
was not observed for previously investigated M(II) quinoxali-
noporphyrins31 and leads to a decrease in the potential separation
between the two oxidations from 1.00 V in the case of (P)Ag
Thus, a fusion of one or two quinoxaline groups to the
porphyrin giving PQ, QPQ, and PQ2 leads not only to easier
reductions but also to an enhanced stability of the singly and
doubly reduced species which are assigned, respectively, to an
Ag(I) porphyrin in the first electron addition and then to an
Ag(I) porphyrin π-anion radical in the second. The absolute
potential separation between these two processes ranges from
0.80 to 0.89 V (Figure 5) and is as expected based on similar
assignments of electron transfer site in a large number of iron
and cobalt porphyrins,2 the most well-studied of which are
(TPP)Fe and (TPP)Co.
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9456 J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008

Androgynous Porphyrins
A R T I C L E S
Figure 9. Thin-layer UV-vis spectral changes during controlled potential
oxidation of (PQ)Ag in CH2Cl2 containing 0.1 M TBAP at (a) 0.80 and (b)
1.60 V.
Figure 8. Thin-layer UV-vis spectral changes of (QPQ)Ag in pyridine,
0.2 M TBAP during potential reduction (a) at -0.80 V vs SCE to give an
Ag(I) porphyrin and (b) during the following porphyrin ring-centered
reduction at -1.60 V vs SCE.
The enhanced stability of the electroreduced quinoxalinopo-
rphyrins enabled measurements of their UV-vis spectra in a
thin-layer cell and examples of the spectral changes which
occurred during electron transfer for (QPQ)Ag in pyridine are
given in Figure 8a,b. During the first reduction, the Soret band
of (QPQ)Ag is shifted from 466 to 451 nm, while the Q-band
absorptions at 635 nm for the neutral compound disappear as a
new band grows in at 668 nm (Figure 8a). No loss of Soret
band intensity occurs after the first one-electron reduction of
(QPQ)Ag, but this is not the case after the second electron
addition at an applied potential of -1.60 V. This spectrum is
illustrated in Figure 8b and is diagnostic for formation of a
porphyrin π-radical anion in that it possesses a much decreased
Soret band intensity and broad absorption band in the long
wavelength region of the spectrum.
The spectra of singly and doubly reduced (PQ)Ag and
(PQ2)Ag were also electrogenerated in a thin-layer cell. The
spectral changes under application of an appropriate applied
reducing potential in pyridine are shown in Figures S5 and S6,
respectively.
The first and second one-electron oxidations of all four
compounds are also reversible in CH2Cl2 (Figure 6). An example
of the resulting spectral changes during the Ag(II)/Ag(III)
process and further oxidation to give the Ag(III) π-cation radical
is shown in Figure 9 for the case of (PQ)Ag in CH2Cl2, 0.1 M
TBAP.
Figure 10. Thin-layer UV-vis spectral changes in CH2Cl2 containing 0.1
M TBAP for (QPQ)Ag during the oxidation process at a controlled potential
of (a) 0.90 and (b) 1.60 V.
oxidized species should have Soret and Q bands which are both
significantly decreased in intensity. The π-cation radical spec-
trum should also possess a diagnostic broad absorption band in
the long wavelength region of the spectrum; this is exactly what
is observed after the second oxidation of (PQ)Ag, as shown in
Figure 9b.
UV-vis spectra for the singly and doubly oxidized forms of
(QPQ)Ag and (PQ2)Ag in CH2Cl2 were also obtained, and these
spectra are shown in Figures 10 and S7. The first one-electron
oxidation occurs at the central metal to generate the Ag(III)
form of the porphyrin. These reactions are observed for all four
investigated porphyrins, irrespective of the increased porphyrin
π-extension upon going from P to PQ and then to PQ2 or QPQ.
It should be noted that no triplet EPR signal is observed at 77
K for the singly oxidized (PQ2)Ag or (QPQ)Ag which are
produced after a one-electron oxidation with [Fe(bpy)3]3+ (bpy )
2,2′-bipyridine) (Supporting Information S8).44
The first one-electron abstraction from (PQ)Ag leads to a shift
in wavelength maxima for the Soret and Q bands of the
compound, but there is not a big change in intensity of the bands
as the Ag(III) form of the porphyrin is generated in solution.
This is expected for an M(II)/M(III) process of the porphyrin
and contrasts with what would be observed during formation
of a porphyrin π-cation radical where the spectrum of the singly
Androgynous Characteristics of (QPQ)Ag. ThesmallHOMO-
LUMO gap of (QPQ)Ag (Figure 5) makes it possible for
(QPQ)Ag to act as both an electron donor and an electron
acceptor in photoinduced electron-transfer reactions. This is
exemplified in the photoinduced electron transfer from (QPQ)Ag
9
J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008 9457

A R T I C L E S
Fukuzumi et al.
Figure 12. Transient absorption spectra of (QPQ)Ag (3.0 × 10-4 M; black
line) and (P)Ag (3.0 × 10-4 M; red line) with tetramethyl-p-benzoquinone
(2.5 × 10-3 M) obtained by nanosecond laser flash photolysis in deaerated
PhCN at 298 K at 1.0 ms after laser excitation (λ ) 355 nm).
Figure 11. Transient absorption spectrum of (QPQ)Ag (3.0 × 10-4 M)
with (P)Zn (2.0 × 10-5 M) obtained by nanosecond laser flash photolysis
in deaerated PhCN at 298 K at 300 µs after laser excitation (λ ) 440 nm).
electron transfer from (QPQ)Ag and (P)Ag to the triplet excited
state of Me4Q occurs efficiently to produce the oxidized species
([(QPQ)Ag]+ and [(P)Ag]+) and the reduced species (Me4Q•-).
to the triplet excited state of {5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)porphyrinato}zinc, 3[(P)Zn]*. The photoexcitation
of a deaerated PhCN solution (P)Zn and (QPQ)Ag affords both
([QPQ]Ag)+ (λmax ) 481 and 643 nm, cf. Figure 10) and
[(QPQ)Ag]- (λmax ) 451 and 668 nm, cf. Figure 8) as shown
in Figure 11. The one-electron reduced species [(QPQ)Ag]- is
In the case of (P)Ag, electron transfer from Me4Q•- (Eox
)
-0.85 V vs SCE)47 to (P)Ag (Ered ) -0.94 V vs SCE) is
endergonic, and thereby [(P)Ag]- cannot be formed. In contrast,
electron transfer from Me4Q•- to (QPQ)Ag (Ered ) -0.60 V
vs SCE) is thermodynamically allowed to occur to produce
[(QPQ)Ag]-. Thus, (QPQ)Ag indeed has an androgynous
character by being able to act as both a good electron donor
and a good electron acceptor. Such a unique androgynous
character of (QPQ)Ag opens new opportunities to use the same
compound as both an electron donor and an electron acceptor
moiety in charge-separation ensembles.
3
formed by photoinduced electron transfer from [(P)Zn]* (Eox
) -0.81 V vs SCE)45 to (QPQ)Ag (Ered ) -0.60 V vs SCE),
whereas the one-electron oxidized species [QPQ)Ag]+ is formed
by subsequent electron transfer from (QPQ)Ag (Eox ) 0.77 V
vs SCE) to [(P)Zn]•+ (Ered ) 0.77 V vs SCE) in PhCN.43a,46
Although the driving force of electron transfer from (QPQ)Ag
to [(P)Zn]•+ is zero, the large excess of (QPQ)Ag relative to
[(P)Zn]•+ results in completion of the electron-transfer reaction.
In contrast, the photoexcitation of a deaerated solution of (P)Zn
and (P)Ag affords only 3[(P)Zn]*, and neither electron transfer
from 3[(P)Zn]* to (P)Ag nor from (P)Ag to 3[(P)Zn]* occurred,
because both electron-transfer processes are thermodynamically
infeasible.
Acknowledgment. This work was partially supported by a
Grant-in-Aid (No. 19205019) and a Global COE program, “the
Global Education and Research Center for Bio-Environmental
Chemistry” from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. The support of the Robert A. Welch
Foundation (K.M.K., Grant E-680) and the Texas Advanced
Research program (K.M.K., Grant No. 003652-0018-2001) is also
gratefully acknowledged. This work was also partially supported
by a Discovery Research Grant (DP0208776) to M.J.C. from the
Australian Research Council.
The photoexcitation of a deaerated PhCN solution of Me4Q
and (QPQ)Ag also affords both [(QPQ)Ag]+ and [(QPQ)Ag]-
as shown in Figure 12. In contrast, the photoexcitation of a
deaerated solution of Me4Q and (P)Ag affords only [(P)Ag]+
(λmax ) 440 nm) (red line in Figure 12). In this case, the
absorption band due to Me4Q•- (λmax ) 450 nm) is overlapped
with the band due to [(P)Ag]+. In both cases, photoinduced
Supporting Information Available: Details of synthesis and
analytical data (S1-S3), unoccupied molecular orbitals of
(PQ2)Ag and (QPQ)Ag (S4), spectroelectrochemical data
(S5-S7), and EPR data (S8). Full list of authors for ref 36 (S9).
This material is available free of charge via the Internet at http://
pubs.acs.org.
(44) For the formation of a triplet in silver(II) porphyrins in the solid state,
see: Marumoto, K.; Takahashi, H.; Tanaka, H.; Kuroda, S.; Ishii, T.;
Kanehama, R.; Aizawa, N.; Yamashita, M. J. Phys.: Condens. Matter
2004, 16, 8753.
(45) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito,
O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607.
(46) Although the free energy change of electron transfer from (QPQ)Ag
to [(P)Zn]•+ is zero, the much larger concentration of (QPQ)Ag as
compared to that of [(P)Zn]•+ allows the electron transfer to undergo
completion.
JA801318B
(47) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Org. Chem. 1984, 49,
3571.
9
9458 J. AM. CHEM. SOC. VOL. 130, NO. 29, 2008