Unexpected change in charge transfer behavior in a cobalt(II) porphyrin-fullerene conjugate that stabilizes radical ion pair states

Article abstract of DOI:10.1021/ja048983d

Two cobalt(II) porphyrin-C₆₀ malonate-linked conjugates, the mono-connected Co1 and the bis-connected trans-2 isomer Co3, have been synthesized for the first time either by direct cyclopropanation with the precursor malonate Co4 or by metalation of the bisadduct H₂3. For the investigation of the interaction between the porphyrin donor and fullerene acceptor within these dyads, electrochemical and photophysical investigations have been carried out. Compared to Zn3 and trans-2 bisadduct 7, the first reduction of the fullerene moiety within Co3 becomes easier (40 mV in dichloromethane and 20 mV in benzonitrile), indicating significant interactions between the π-system of the fullerene and the d-orbitals of the central Co atom. Compared to the Co complexes 9, Co4, and Co1, the first oxidation of Co3 is considerably shifted to more positive potentials, if benzonitrile instead of dichloromethane is used as solvent. At the same time, the oxidation is no longer centered on the Co(II) center but on the porphyrin macrocycle, as corroborated by spectroelectrochemistry. A similar solvent dependence was observed in transient absorption spectroscopic measurements. In toluene, benzonitrile and anisole photoinduced electron transfer within Co3 leads to the formation of a charge-separated state Co(II)P^.+-C₆₀^.- with a lifetime of 560 ± 20 ns in benzonitrile, whereas in other solvents such as THF, nitrobenzene, ortho-diclorobenzene, and tert-butylbenzene the formation of a Co(III)P-C₆₀^.- as transient was detected, which is, however, short-lived (860 ± 40 ps in THF) and exhibits charge recombination dynamics that are in the Marcus inverted region. Particularly important is the fact that the electronic coupling (V) in Co(III)P-C ₆₀^.- is 18 cm^-1 substantially smaller than the V value of 313 cm^-1 in ZnP^.+-C60^.-.

Full text of DOI:10.1021/ja048983d

Published on Web 08/03/2004
Unexpected Change in Charge Transfer Behavior in a
Cobalt(II) Porphyrin-Fullerene Conjugate That Stabilizes
Radical Ion Pair States
Liam R. Sutton,† Michael Scheloske,† Kristina S. Pirner,† Andreas Hirsch,*,†
Dirk M. Guldi,*,‡,+ and Jean-Paul Gisselbrecht*,§
Contribution from the Institut fu¨r Organische Chemie, Friedrich-Alexander-UniVersita¨t
Erlangen-Nu¨rnberg, Henkestrasse 42, D-91054 Erlangen, Germany, Notre Dame Radiation
Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, Institute for Physical
Chemistry, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058
Erlanger, Germany, and Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
UniVersite´ Louis Pasteur, UMR CNRS no. 7512, 4, rue Blaise Pascal, 67000 Strasbourg, France
Received February 23, 2004; E-mail: Dirk.M.Guldi.1@nd.edu
Abstract: Two cobalt(II) porphyrin-C60 malonate-linked conjugates, the mono-connected Co1 and the bis-
connected trans-2 isomer Co3, have been synthesized for the first time either by direct cyclopropanation
with the precursor malonate Co4 or by metalation of the bisadduct H23. For the investigation of the interaction
between the porphyrin donor and fullerene acceptor within these dyads, electrochemical and photophysical
investigations have been carried out. Compared to Zn3 and trans-2 bisadduct 7, the first reduction of the
fullerene moiety within Co3 becomes easier (40 mV in dichloromethane and 20 mV in benzonitrile), indicating
significant interactions between the π-system of the fullerene and the d-orbitals of the central Co atom.
Compared to the Co complexes 9, Co4, and Co1, the first oxidation of Co3 is considerably shifted to more
positive potentials, if benzonitrile instead of dichloromethane is used as solvent. At the same time, the
oxidation is no longer centered on the Co(II) center but on the porphyrin macrocycle, as corroborated by
spectroelectrochemistry. A similar solvent dependence was observed in transient absorption spectroscopic
measurements. In toluene, benzonitrile and anisole photoinduced electron transfer within Co3 leads to the
formation of a charge-separated state Co(II)P•+-C60•- with a lifetime of 560 ( 20 ns in benzonitrile, whereas
in other solvents such as THF, nitrobenzene, ortho-diclorobenzene, and tert-butylbenzene the formation
•-
of a Co(III)P-C60 as transient was detected, which is, however, short-lived (860 ( 40 ps in THF) and
exhibits charge recombination dynamics that are in the Marcus inverted region. Particularly important is
the fact that the electronic coupling (V) in Co(III)P-C60•- is 18 cm-1 substantially smaller than the V value
of 313 cm-1 in ZnP•+-C60
.
•-
of the solar spectrum.3 Over the course of recent years they
emanate as light harvesting building blocks in the construction
of molecular architectures. Their high electronic excitation
energy, typically exceeding 2.0 eV, powers a strongly exergonic
electron transfer, which subsequently intercedes the conversion
Introduction
In the photosynthetic reaction center of purple bacteria,
photoinduced charge separation over a distance of several
nanometers takes place by a series of electron-transfer steps
between noncovalently linked porphyrin-like donors and quinone
acceptors that are held together in a protein matrix.1 A great
deal of effort has been devoted to covalently linked donor and
acceptor systems as photosynthetic model systems, in attempts
to efficiently generate long-lived charge-separated states.2
Fullerenes and porphyrins are molecular architectures ideally
suited for devising integrated, multicomponent model systems
to transmit and process solar energy. The rich and extensive
absorptions (i.e., π-π* transitions) seen in porphyrinoid
systems, the pigments of life, hold particular promise for
increased absorptive cross sections and, thus, an efficient use
(1) (a) McDermott, G.; Priece, S. M.; Freer, A. A.; Hawthornthwaite-Lawless,
A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517.
(b) Barber, J. Nature 1988, 333, 114. (c) Supramolecular Chemistry;
Balzani, V., de Cola, L., Eds.; NATO ASI Series; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1992. (d) The Photosynthetic
Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: New
York, 1993.
(2) (a) Carter, F. L. Molecular Electronic Devices, Dekker: New York, 1987.
(b) Photoinduced Electron Transfer; Fox, M. A., Channon, M., Eds.;
Elsevier: Amsterdam, 1988. (c) Electron Transfer in Chemistry; Balzani,
V., Ed.; Wiley-VCH: Weinheim, Germany, 2001. (d) Newton, M. D. Chem.
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Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (g)
Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (h) Gould, I. R.; Farid,
S. Acc. Chem. Res. 1996, 29, 522. (i) Balzani, V.; Juris, A.; Venturi, M.;
Campagna, S.; Serroni, S. Chem. ReV. 1996, 96, 759. (j) Willner, I. Acc.
Chem. Res. 1997, 30, 347. (k) Piotrowiak, P. Chem. Soc. ReV. 1999, 28,
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Wiley: Chichester, 2000.
† Institut fu¨r Organische Chemie, Friedrich-Alexander-Universita¨t Er-
langen-Nu¨rnberg.
‡ University of Notre Dame.
+ Institute for Physical Chemistry, Friedrich-Alexander-Universita¨t Er-
langer-Nu¨rnberg.
§ Universite´ Louis Pasteur.
9
10370
J. AM. CHEM. SOC. 2004, 126, 10370-10381
10.1021/ja048983d CCC: $27.50 © 2004 American Chemical Society

Cobalt(II) Porphyrin−Fullerene Conjugates
A R T I C L E S
between light and chemical and electrical energy. On the other
hand, implementation of C60 as a three-dimensional electron
acceptor holds great expectations because of their small
reorganization energy in electron-transfer reactions and has
exerted a noteworthy impact on the improvement of light-
induced charge separation.4 In particular, the delocalization of
chargesselectrons or holesswithin the giant, spherical carbon
framework (diameter > 7.5 Å) together with the rigid, confined
structure of the aromatic π-sphere offers unique opportunities
for stabilizing charged entities.5
The coupling of porphyrins and fullerenes is a theme of
considerable current interest, given the potential applications
of the photochemistry peculiar to the resulting systems in
photovoltaic devices and optical limiting and molecular elec-
tronics.6 Methods exist for synthesizing porphyrins with an
almost unlimited range of functionality and regiochemistry
(albeit sometimes in complex mixtures and low yield);7 together
with the emerging diversity of reliable and regioselective
methods for functionalizing fullerenes,8 the possibilities for
tuning photochemical properties in such conjugates are numer-
ous, to say the least.
Our efforts in this field have so far started with Zn(II) tetraaryl
porphyrins bearing one or two malonyl esters connected via
ethylene or propylene spacers to phenyl groups.9 These com-
pounds can then be appended to C60 using our modification10
of Bingel’s cyclopropanation reaction11 at the [6,6] bonds of
the fullerene to give mono- or bis-connected porphyrin-C60
conjugates such as Zn1, Zn2, and Zn3 (Figure 1). Although
eight distinct [6,6] bonds are in principle available for a second
malonate addition, the steric requirements of the porphyrin unit
tend to enforce extreme regioselectivity of bis-additions through
a tether-direction effect, leading to the equatorial, e-isomer Zn2
from the cis-porphyrin Zn5 and trans-2 isomer Zn3a from the
trans-porphyrin Zn6 (Figure 1).9f Our photochemical investiga-
tions have revealed that excitation of the porphyrin leads to the
formation of charge-separated states, which decay by charge
recombination in the Marcus-inverted region.9 This behavior is
modulated by the connectivity and flexibility of the conjugates
and can be further modified by the exploitation of axial metal-
ligand interactions to build simple complexes or supramolecular
aggregates.9
The impact that a redox-active metal center, such as transition
metals, exerts on the stabilization of radical ion pairs in
metalloporphyrin-electron acceptor conjugates is hardly ever
addressed.3 Ultrafast excited-state deactivation of the metal-
loporphyrin, due to efficient spin-orbit coupling, is largely
responsible for this scarce. The success in probing, for example,
cobalt(II) porphyrins as photoexcited state electron donors
depends on overcoming the fast excited state deactivation. An
obvious strategy mandates an intramolecular charge separation
event that is substantially fast. Considering that the thermody-
namics for charge separation in porphyrin-fullerene conjugates
are in the top region of the Marcus parabola, the effective rate
constant remains a function of the electronic coupling matrix.
The unique topology in the 3a series ensures strong coupling
with values that peak around 436 cm-1, which consequently
leads to rate constants faster than 1012 s-1 9f
.
We report here the synthesis of Co(II)-containing analogues
of Zn1 and Zn3, namely Co1 and Co3, as novel photosynthetic
model systems. A major incentive for probing cobalt porphyrins
stems from their enzymatic catalytic reactivity, such as the
function of coenzyme B12.12 We then present our observations
of the photochemical and redox behavior of the resulting
compounds in comparison with a number of model molecules,
showing that alteration of the metal contained within the
porphyrin has a profound effect on the physicochemical
properties of porphyrin-fullerene conjugates. Most outstand-
ingly, depending on the nature of the oxidation, that is, metal-
versus ligand-centered process, charge recombination can be
suppressed by up to 3 orders of magnitude.
(3) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: New York, 1999.
(4) For recent reviews on fullerene-containing donor-acceptor ensembles and
fullerene anions, see: (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9,
537. (b) Prato, M. J. Mater. Chem. 1997, 7, 1097. (c) Martin, N.; Sanchez,
L.; Illescas, B.; Perez, I. Chem. ReV. 1998, 98, 2527. (d) Imahori, H.; Sakata,
Y. Eur. J. Org. Chem. 1999, 2445. (e) Guldi, D. M. Chem. Commun. 2000,
321. (f) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695. (g) Reed,
C. A.; Bolskar, R. D. Chem. ReV. 2000, 100, 1075. (h) Gust, D.; Moore,
T. A.; Moore, A. L. J. Photochem. Photobiol., B 2000, 58, 63. (i) Gust,
D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (j) Guldi,
D. M.; Martin, N. J. Mater. Chem. 2002, 1978-1992.
(5) (a) Guldi, D. M.; Neta, P.; Asmus, K.-D. J. Phys. Chem. 1994, 98, 4617.
(b) Guldi, D. M.; Asmus, K.-D. J. Am. Chem. Soc. 1997, 119, 5744. (c)
Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.; Okada,
T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545.
(6) (a) Fukuzumi, S.; Guldi, D. M. Electron Transfer in Chemistry 2001, 2,
270-337. (b) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo,
C.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617-6628.
(c) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22-36.
(7) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, CA, 2000; Volume 1: Synthesis and Organic
Chemistry, and Volume 2: Heteroporpyhrins, Expanded Porphyrins and
Related Macrocycles.
Results and Discussion
Syntheses. We chose to synthesize our Co(II) porphyrin-
fullerene adducts either via the Zn(II) analogues, demetalation,
and Co(II) insertion or via direct coupling of a Co(II)-containing
porphyrin with C60, taking into account a number of factors:
(a) the free base intermediates represent a useful target in their
own right, for examination of physical properties and as a
starting point for the insertion of a wide range of metal cations,
(b) Bingel additions of metalated porphyrins to fullerenes
proceed in higher yield than those of free base porphyrins, (c)
demetalation of the Zn(II) adducts was expected to be straight-
forward and proved to be so, (d) Co(II) is paramagnetic,
presenting characterization difficulties when using it in inter-
mediate stages, and (e) the direct route is the least complicated.
Free base porphyrins H24, H25, and H26 were synthesized
by the statistical condensation of 2 equiv of pyrrole, 1 equiv of
benzaldehyde, and 1 equiv of malonyl-substituted benzaldehyde
under high dilution conditions (ca. 0.1 M benzaldehyde) under
various well-established sets of conditions, followed by oxida-
tion with excess 2,3-dichloro-5,6-dicyano-1,4-diquinone (DDQ)
(8) (a) Hirsch, A. Top. Curr. Chem. 1999, 199, 1-65. (b) Diederich, F.;
Kessinger, R. Acc. Chem. Res. 1999, 32, 537-545. (c) Diederich, F.;
Kessinger, R. Templated Org. Synth. 2000, 189-218.
(9) (a) Dietel, E.; Hirsch, A.; Eichhorn, E.; Rieker, A.; Hackbarth, S.; Roder,
B. Chem. Commun. 1998, 1981-1982. (b) Dietel, E.; Hirsch, A.; Zhou, J.;
Rieker, A. J. Chem. Soc., Perkin Trans. 2 1998, 1357-1364. (c) Camps,
X.; Dietel, E.; Hirsch, A.; Pyo, S.; Echegoyen, L.; Hackbarth, S.; Roder,
B. Chem.-Eur. J. 1999, 5, 2362-2373. (d) Guldi, D. M.; Luo, C.; Da
Ros, T.; Prato, M.; Dietel, E.; Hirsch, A. Chem. Commun. 2000, 375-
376. (e) Guldi, D. M.; Luo, C.; Prato, M.; Dietel, E.; Hirsch, A. Chem.
Commun. 2000, 373-374. (f) Guldi, D. M.; Luo, C.; Prato, M.; Troisi, A.;
Zerbetto, F.; Scheloske, M.; Dietel, E.; Bauer, W.; Hirsch, A. J. Am. Chem.
Soc. 2001, 123, 9166-9167.
(12) (a) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437. (b)
Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int.
Ed. Engl. 1994, 53, 1537. (c) Frey, P. Chem. ReV. 1990, 90, 1343. (d)
Buckel, W.; Golding, B. T. Chem. Soc. ReV. 1996, 25, 329.
(10) Camps, X.; Hirsch, A. J. Chem. Soc., Perkin Trans. 1 1997, 1595-1596.
(11) Bingel, C. Chem. Ber. 1993, 126, 1957-1959.
9
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A R T I C L E S
Sutton et al.
Figure 1. Porphyrin-fullerene adducts. M ) H2, Zn, or Co.
or p-chloranil. The reaction mixture was concentrated, and
excess methanol was added to precipitate the porphyrins. The
precipitate was obtained by vacuum filtration, and the individual
porphyrins were isolated by column chromatography on silica
gel with 5 v/v % EtOAc in dichloromethane as eluent and
recrystallization by addition of MeOH to concentrated dichlo-
romethane solutions of the appropriate bands. Yields were
statistical and reasonable, at around (in order of increasing
retention time) 5% for H24, 2% H26, and 4% H25, giving
quantities of the order of 100-1000 mg of porphyrin per
reaction.
with yields that were almost quantitative. Co(II) could alterna-
tively be inserted at this stage by the use of Co(OAc)2 in
refluxing THF over 6 h.
Using the modified Bingel reaction,10 we were able to effect
the mono- and bis-additions in good yield. The bis-addition can
be regarded as macrocyclization; high dilution of the reagents
(1:1 C60/porphyrin, DBU, and I2 in toluene), stirring at room
temperature for 16 h, and purification by chromatography
resulted in yields of around 50% Zn3 (see also ref 9).
Zn(II) was quantitatively removed from the adducts Zn1 and
Zn3 by treatment of 25-mL toluene solutions with excess 3:1
v/v TFA/MeOH. Addition of acid resulted in the immediate
formation of dark green solutions, which after stirring at room
Zn(II) was then conveniently inserted into the free base
porphyrins using excess Zn(OAc)2 in CH2Cl2/MeOH mixtures,
9
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Cobalt(II) Porphyrin−Fullerene Conjugates
A R T I C L E S
Table 1. Redox Potentials Observed in CH2Cl2 + 0.1 M Bu4NPF6
on a Glassy Carbon Working Electrode by Cyclic Voltammetry at a
Sweep Rate of 0.1 Vs-1
Potentials in Volts vs Fc+/Fca
compd
E
Red4′°
E
Red3′°
E
Red2′°
E
Red1′°
-1.05
-1.80 +0.34 +0.65
E
Ox1′°
E
Ox2′°
EOx3′°
7
8
9
Co4
-2.30 -1.95 b -1.40
-1.35 +0.28 +0.53 +0.79
-1.34 +0.35 +0.60 +0.85
-1.10 +0.34 +0.69
-1.80b
Zn3 -2.05b -1.90c -1.51
Co1
Co3
-1.42 (2 e-) -0.99 +0.37 +0.61 +0.85
-1.78b -1.57 (2 e-) -1.01 +0.40 +0.82 +0.96
a
b
Formal redox potential E′° ) (Epa + Epc)/2. Peak potentials for
c
irreversible electron transfers. Quasi reversible electron exchange: ∆Ep
)120 mV at V ) 0.1 V/s.
Figure 2. Absorption spectra of CoTt-BuPP (9), Co3, and trans-2-C62-
(COOEt)4 (7) in toluene.
compounds under the same experimental conditions, the redox
behavior of the zinc(II) and cobalt(II) porphyrin-fullerene
conjugates was interpreted. All redox potentials are summarized
in Tables 1 and 2.
temperature for 15 min were washed with 4 × 25 mL of H2O,
resulting in dark purple-brown color in the organic phase.
Evaporation of the organic solvent and drying in vacuo gave
the free base adducts H21 and H23.
Electrochemical Investigation of Zn3. In benzonitrile, the
cyclic voltammetry (CV) of Zn3 (Figure 3) gives rise to three
well-resolved one-electron oxidation processes. The reversibility
of these processes was examined by testing their peak currents
(Ip) and peak potentials (Ep) at variable scan rates (V).
Importantly, the corresponding peak potentials are constant up
to scan rates of 2 V/s, while the peak current ratio Ipa/Ipc is
close to unity at any V. The peak potential difference, that is,
∆Ep ) Epc - Epa always remains close to 57 mV (i.e., up to 1
V/s), and the Ip vs V1/2 relationship shows a linear correlation
with an intercept that cuts through the origin.
On the reduction side, a series of reduction steps were
developed. Clearly, the first reduction is a one-electron reversible
transfer, as suggested by the peak characteristics evolution with
scan rates. The second reduction behaves as an irreversible
electron transfer at sweep rates lower than 0.1 V/s. At sweep
rates exceeding 1 V/s, the electron transfer becomes, however,
reversible. The evolution of the peak characteristics is in
agreement with an ECirrev mechanism, namely, a reversible
electron transfer followed by an irreversible chemical reaction.
Two additional reduction steps were observed at, respectively,
-1.82 and -2.03 V, which revealed characteristics of either
quasi-reversible or reversible electron-transfer processes.
In dichloromethane, only two one-electron reversible oxida-
tions were observed, while the third one was masked by the
electrolyte discharge. The first and second reductions have
characteristics of a reversible one-electron step for sweep rates
above 0.5 V/s, whereas at sweep rates below 0.5 V/s the second
reduction exhibits a trend toward irreversibility. Overall, the
redox behavior is similar in both solvents.
The monoadduct Co1 was obtained by direct cyclopropana-
tion of C60 with Co4. Co(II) was inserted into H23 by the action
of excess Co(OAc)2 in THF under reflux. The progress of the
reaction was monitored by TLC, although differences in Rf
values of free base and Co(II) adducts were small. The metalated
Co3 was formed rather more slowly: 24 h were required before
all of the starting material was consumed, presumably because
one face of the porphyrin was completely blocked by fullerene.
Co1 and Co3 were purified by column chromatography or
filtration through a silica gel plug followed by recrystallization.
The resulting compounds were pure by TLC, and microanalysis
and mass spectrometry revealed molecular ion peaks. NMR was,
however, complicated by the paramagnetism of d7 Co(II): 1H
spectra were downfield-shifted and broadened, most markedly
those peaks corresponding to protons nearest to the metal center.
Absorption Characteristics. The ground-state absorption
spectra of the investigated conjugates Co1 and Co3 are
indicative of the divalent cobalt oxidation state. In particular,
the maxima around 535 nm are good matches to a cobalt(II)
porphyrin reference (CoTt-BuPP (9)) lacking a C60 moiety. This
supports the view that linking a strong electron acceptor, such
as C60, to the cobalt(II) porphyrin has no direct effect on the
cobalt’s oxidation state. However, the close proximity of the
porphyrin and fullerene π-systems gives rise to through-space
interactions, whose effects lead to red-shifts of the Soret and
Q-band transitions. In the 700-800 nm region (see Figure 2),
the spectra of Co1 and Co3 reveal a new broad band, which
we assign to a charge-transfer transition between the porphyrin
donor and the electron accepting fullerene. The presence of the
additional band therefore confirms electronic interaction between
the two chromophores in the dyads.
A closer inspection of Table 1 and a comparison with the
individual reference compounds leads to the following conclu-
sions: The redox behavior of Zn3 comprises first and second
oxidation processes that occur on the zinc porphyrin, while a
third oxidation, only observed in benzonitrile, removes an
electron from the C60 core. On the other hand, the two first
reduction processes correspond to the formation of radical anion
and dianion of the fullerene. Because of the instability of the
dianion, as commonly observed for C60 bisadducts,14 several
Electrochemical and Spectroelectrochemical Investiga-
tions. Electrochemical investigationsscyclic voltammetry (CV)
and rotating disk voltammetry (RDV)swere carried out in
benzonitrile and dichloromethane containing 0.1 M Bu4NPF6
as supporting electrolyte. With the help of the corresponding
building blocks, namely, trans-2-C62(COOEt)4 (7),13 ZnTPP
(8), and CoTt-BuPP (9), which were studied as reference
(14) Kessinger, R.; Gomez-Lopez, M.; Boudon, C.; Gisselbrecht, J. P.; Gross,
M.; Echegoyen, L.; Diederich, F. J. Am. Chem. Soc. 1998, 120, 8545-
8546.
(13) Hirsch, A.; Lamparth, I.; Karfunkel, H. R. Angew. Chem. 1994, 106, 453-
455; Angew. Chem., Int. Ed. Engl. 1994, 33, 437-439.
9
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A R T I C L E S
Sutton et al.
Table 2. Redox Potentials Observed in PhCN + 0.1 M Bu4NPF6 on a Platinum Working Electrode by Cyclic Voltammetry at v ) 0.1 Vs-1
Potentials in Volts vs Fc+/Fca
compd
ERed4′°
ERed3′°
ERed2′°
ERed1′°
EOx1′°
EOx2′°
EOx3′°
EOx4′°
7
8
9
Co4
-2.06
-1.91b
-1.36
-2.20
-0.95
-1.80
-1.35
-1.34
+1.21
+1.56b
+0.75
+0.67
+0.71
+0.35
0.0
+0.95
+0.99
Epa ) +0.09
Epc ) -0.21
+0.37
+0.06
+0.28
Zn3
Co1
Co3
-2.03
-2.12
-1.90c
-1.82
-1.92b
-1.77c
-1.40
-1.02
-0.92
-0.93
+0.77
+0.72
+0.78
+1.26
+0.98
+1.03
-1.41 (2 e-)
-1.50d
+1.22
+1.27
a
b
c
d
Formal redox potential E′° ) (Epa + Epc)/2. Peak potentials for irreversible electron transfers. Small amplitude not well resolved steps. Two
overlapping one-electron reductions separated by 60 mV.
Figure 3. Cyclic voltammetry (V ) 0.1V/s) of ZnTPP and Zn3 in PhCN
+ 0.1 M Bu4NPF6. Hair line: CV of ZnTPP (8); bold line: CV of Zn3;
dashed line: CV of Zn3, reverting the scan at -1.6 V.
electrogenerated electroactive species are present in solution.
This complication renders the assignment of the reduction
processes in the more negative region virtually impossible.15
Electrochemical Investigation of Co1a and Co3a. Five
oxidation steps were noted in cyclic voltammetric experiments
with Co1 and Co3 in benzonitrile (Figures 4 and 5). The first
oxidation is quasi-reversible, since the peak separation ∆Ep (i.e.,
Epc - Epa) varied with scan rate. Commonly, this process is
assigned to a metal-centered redox oxidation, that is, formation
of Co(III).16 The quasi-reversible behavior is not entirely
unexpected because of changes in the axial coordination, which
are likely to occur during the metal oxidation. Steps two and
three correspond to reversible one-electron oxidations of the
macrocylic ligand. On the contrary, the fourth and fifth one-
electron oxidations involve the C60 moiety.17,18
Figure 4. Cyclic voltammetry (V ) 0.1 V/s) of Co1 in benzonitrile and
0.1 M Bu4NPF6 for different anodic (top) or cathodic (bottom) switching
potentials.
Two well-defined reversible reduction steps were observed
on the reductive side. Whereas the first reduction involves just
a one-electron transfer, the second reduction implicates a
(15) The resulting CV is not a simple addition of the CVs of 7 and 8, because
of site-site interactions and Coulombic repulsions between highly reduced
species.
(16) (a) Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2260-2267. (b)
Kadish, K. M. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.;
Wiley & Sons: New York, 1986; Vol. 34, p 443. (c) Kadish, K. M.; Lin,
X. Q.; Han, B. C. Inorg. Chem. 1987, 26, 4161-4167. (d) Lin, X. Q.;
Boisselier-Cocolios, B.; Kadish, K. M. Inorg. Chem. 1986, 25, 3242-3248.
(e) Huet, D.; Gaudemer, A.; Boucly-Goester, C.; Boucly, P. Inorg. Chem.
1982, 21, 3413-3419.
(17) (a) Cardullo, F.; Seiler, P.; Isaacs, L.; Nierengarten, J. F.; Haldimann, R.
F.; Diederich, F. HelV. Chim. Acta 1997, 80, 343-371. (b) Nierengarten,
J. F.; Habicher, T.; Kessinger, R.; Cardullo, F.; Diederich, F.; Gramlich,
V.; Gisselbrecht, J. P.; Boudon, C.; Gross, M. HelV. Chim. Acta 1997, 80,
2238-2276.
(18) Guarr, T. F.; Meier, M. S.; Vance, V. K.; Clayton, M. J. Am. Chem. Soc.
1993, 115, 9862-9863.
Figure 5. Cyclic voltammetry (V ) 0.1 V/s) of Co1 (bold line) and Co3
(hair line) in benzonitrile and 0.1 M Bu4NPF6.
sequence of two electrons. The characteristics of the latter
electron exchange depend, however, on the conjugate. For Co1
the peak potential difference is close to 57 mV, with a peak
current approximately twice what is seen for the first reduction
peak. It is important to note that the peak shape reveals
9
10374 J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004

Cobalt(II) Porphyrin−Fullerene Conjugates
A R T I C L E S
characteristics of two overlapping one-electron transfers, oc-
curring, however, at nearly identical potentials. The formal redox
potential difference between the two one-electron transfers is
close to 37 mV, which corroborates the existence of two
independent redox centers.19 In Co3 the two overlapping
reductions are separated by 60 mV. Comparison with the redox
potentials of the precursors clearly indicates that the two
overlapping reduction steps in Co3 involve the one-electron
reduction of the C60 moiety and the first one-electron reduction
of the cobalt porphyrin moiety.20
The CVs in dichloromethane are similar to those described
for benzonitrile as a solvent. However, because of interferences
with the electrolyte discharge the number of reduction and
oxidation processes is restricted. In particular, only the first three
oxidations, all involving the cobalt porphyrin moiety, were
observed. On the reductive side, only two reduction steps were
registered. Again, the second step implicates two overlapping
one-electron steps, one occurring at the C60 moiety and one at
the CoP moiety.
To summarize, the studied porphyrin-fullerene conjugates
clearly show that the resulting CVs are not just the simple
addition of the CVs of the two reference building blocks. The
observed differences are due to “site-site” interactions. When
comparing Zn3 and Co3 with trans-2-C62(COOEt)4 (7), the
first fullerene reduction in Zn3 renders more difficult in both
solvents (dichloromethane: 50 mV; benzonitrile: 70 mV),
whereas for Co3, the first reduction of C60 becomes easier
(dichloromethane: 40 mV; benzonitrile: 20 mV). Such a
potential shift to more positive potentials may result from
electrostatic interactions in Co3. Indeed, similar potential shifts
have been observed for fullerene-dibenzo[18]crown-6 conju-
gates in the presence of potassium salts.21
Spectroelectrochemical Investigations. To identify the
electron-transfer sites for the CoTt-BuPP (9) and Co3, the
electrochemical studies were complemented by spectroelectro-
chemical investigations in dichloromethane and in benzonitrile.
In dichloromethane, 9 and Co3 give rise to similar spectral
evolutions during the first oxidation step. On the basis of the
literature, we assigned these data to a metal-centered electron
transfer.16
Figure 6 illustrates that the spectroelectrochemistry performed
in benzonitrile leads for 9 and Co3 to different spectral
evolutions. During the first oxidation of 9 the initial bands at
418 and 532 nm disappeared, while new bands appeared at 442,
555, and 592 nm. These spectral features are characteristics of
a Co(III) complex and are in agreement with previous data
observed in the same solvent by Wolberg.22 The spectrum
registered after the completion of the second oxidation with
maxima at 426, 439(sh), 647, 740, 846, and 941 nm is that of
a radical cation.
Figure 6. Spectra of the cobalt(II) species (line), the one-electron oxidized
species (bold line), and the two one-electron oxidized species (dashed line)
obtained during spectroelectrochemical oxidation of 9 (upper) and Co3
(lower) in benzonitrile and 0.1M Bu4NPF6.
in the large decrease of the Soret band. Tentatively, we assigned
this process to the generation of the corresponding radical cation
Co3•+. Additional support for our hypotheses was borrowed
from previous observations related to the radical cations of
CuOETPP23 and (Im)2NiOETPP.24 The drastic spectral differ-
ences observed during the first oxidation step of the cobalt
porphyrin Co3 in benzonitrile indicate the generation of the
radical cation and not that of the cobalt(III) species, since the
Co(III) spectral characteristics were not observed at all. Oxida-
tion at the plateau potential of the second oxidation step, which
corresponds to an overall exchange of two electrons, yielded a
spectrum whose characteristics (i.e., maxima at 388, 432,
For Co3, the initial bands at 423 and 533 nm vanished during
the electrolysis performed at the plateau potential of the first
oxidation. Simultaneously, new bands at 391, 443, and 559 nm
developed. The drastic differences, relative to 9, are seen mainly
(19) (a) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.
Soc. 1978, 100, 4248-4253. (b) Ammar, F.; Save´ant, J. M. J. Electroanal.
Chem. 1973, 47, 215-221.
(20) Further reductions are not well resolved for Co3a (Figure 5), whereas for
Co1a two further reductions are observed as shown in Figure 4.
(21) Bourgeois, J. P.; Seiler, P.; Fibbioli, M.; Pretsch, E.; Diederich, F.;
Echegoyen, L. HelV. Chim. Acta 1999, 82, 1572-1595.
(23) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.; Smith, K.
M.; Fajer, J. J. Am. Chem. Soc. 1994, 116, 8582-8592.
(24) Renner, M. W.; Barkigia, K. M.; Melamed, D.; Gisselbrecht, J. P.; Nelson,
N. Y.; Smith, K. M.; Fajer, J. Res. Chem. Intermed. 2002, 28, 741-759.
(22) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982-2991.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10375

A R T I C L E S
Sutton et al.
466(sh), 630, and 694 nm) are typical of a dicationic species.
It has to be mentioned that for 9 and Co3, the initial spectra
could be recovered quantitatively by stepwise reduction of the
generated dicationic species.
In conclusion, despite the similar electrochemical behavior
for 9 and Co3 in benzonitrile, the spectroelectrochemical
investigations clearly indicate that the oxidation of Co3 corre-
sponds to the following mechanism:
Co(II)P f Co(II)P•+ + e-;
Co(II)P•+ f Co(III)P2+ + e-
whereas for 9, the first one-electron oxidation generates the
expected cobalt(III) complex
Co(II)P f Co(III)P+ + e-
Figure 7. Picosecond transient absorption spectrum (visible-near-infrared
part) recorded with a 100 ps upon flash photolysis of dyad Co3 (∼5.0 ×
10-5 M) at 532 nm in deoxygenated THF, indicating the Co(III)P features
(λmax ) 560 and 590 nm) and C60 π-radical anion features (λmax ) 900
nm) - Co(III)P-C60•-. Insert depicts the decay of the Co(III)P features at
560 nm.
Photophysical Investigations. Because of the paramagnetism
of the 3d7 electronic configuration, the excited states of
cobalt(II) porphyrin are rapidly deactivated. Despite the low-
spin character (s ) 1/2) of CoP, the good overlap between the
transition metal d orbitals and the macrocylic lone pairs is
responsible for directing the deactivation from the metal center
to the porphyrin’s π-π* transitions. The emission spectra reveal
an eminent impact of the strong deactivation. In room-
temperature experiments, neither fluorescence nor phosphores-
cence was seen, in contrast to the high fluorescence quantum
yields (Φ ) 0.11) of the free base porphyrin (H2P) analogue.25
The fact that even phosphorescence is abolished prompts us to
the fact that all possible excited states are equally rapid
deactived. In summary, the fast excited-state deactivation of
cobalt(II) porphyrin prevents assessing the charge separation
kinetics on the basis of fluorescence spectroscopy. Likewise,
time-resolved fluorescence decay measurements failed to indi-
cate any noteworthy emission.
Similar to the spectrum of CoP-C60 (i.e., Co1 and Co3), no
notable light emission was recorded in the 550-1000 nm region.
Please note that the analogous zinc (Zn3) and free base
conjugates (H23) both give rise to emissive charge-transfer
features with quantum yields as high as 10-3 in the free base
case. The lack of appreciable charge-transfer emission, espe-
cially for Co3, is observed in several solvents with a wide range
of polarity (i.e., toluene, THF, and benzonitrile).
Considering the electrochemical analysis of Co1 and Co3 in
dichloromethane and benzonitrile, we estimated the energy for
the charge-separated state, that is, a one-electron oxidized
porphyrin and a one-electron reduced fullerene, to be around
1.4 and 1.3 eV, respectively. The lack of emissive features
hampers, however, the exact determination of the excited-state
energies. The long wavelength absorption in the ground-state
spectrum, which reflects the vertical up transition across the
ground state-singlet excited state energy gap, at ∼535 nm
allows us to deduce only an upper limit of the singlet excited
state (∼2.0 eV). Further support for this energetic positioning
can be lent from the data established for the analogous zinc
and free base porphyrins, which are at ∼2.0 eV (singlet excited
state) and ∼1.5 eV (triplet excited state).26 Thus, under strict
thermodynamic considerations and neglecting the short-lived
nature of photoexcited CoP (see below) a photoinduced electron
transfer evolving between the excited electron donating
cobalt(II) porphyrins and C60 as the electron acceptor is
energetically feasible.
To probe and visualize the electron transfer in photoexcited
Co1 and Co3, we performed time-resolved transient absorption
spectroscopy following short laser excitation (i.e., 18 ps or 8
ns) of the porphyrin chromophore at 532 nm. For the reference
9, we monitored in picosecond pump-probe experiments a very
short-lived transient. Spectroscopic features of this intermediate
in the 500-760 nm range are transient bleaching at 530 nm
flanked by new maxima at 500 and 555 nm. The lifetime is
very short and our ∼18 ps time resolution only allows an upper
limit estimation of ca. 30 ps. This value is in satisfying
agreement with previous reports.27 It should be pointed out that
the new spectrum disappeared completely about 50 ps after the
pulse. Also, no residual transient features were registered on
the micro- and millisecond time scale.
The transient absorption changes associated with Co3 in THF
are drastically different. The 500-960 nm region is depicted
in Figure 7. For comparison, we have added the transient seen
for the zinc conjugate Zn3 in Figure S1. Immediately after the
picosecond pulse a transient species is seen that displays a set
of maxima at 560 and 590 nm. Both features resemble the
characteristics of a square-planar 3d6 configuration found upon
metal-center oxidation of cobalt(II) porphyrin by electrochemi-
cal22 and pulse radiolytical means.28 This is in sharp contrast
to the ligand-centered oxidation process, which was observed
earlier for Zn3 and free base analogues. Further in the red,
•-
around 900 nm, a maximum is seen that resembles C60 seen
in a typical trans-2 adduct such as 7. Thus, we conclude that
the energetically low-lying radical pair is constituted by a Co-
•-
(III)P-C60 state as a product of an ultrafast photoinduced
electron transfer. The formation is buried within our experi-
mental time resolution, that is, <18 ps. To shed light onto the
electron-transfer kinetics in Co3, the zinc conjugate Zn3 proved
(25) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker: New York, 1993.
(26) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem.
Soc. 2000, 122, 6535.
(27) Rodriguez, J.; Kirmaier, C.; Holton, D. J. Am. Chem. Soc. 1989, 111, 6500.
(28) Neta, P.; Grebel, V.; Levanon, H. J. Phys. Chem. 1981, 85, 2117.
9
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Cobalt(II) Porphyrin−Fullerene Conjugates
A R T I C L E S
Figure 8. Picosecond transient absorption spectrum (visible-near-infrared
part) recorded with a 100 ps upon flash photolysis of dyad Co3 (∼5.0 ×
10-5 M) at 532 nm in deoxygenated benzonitrile, indicating the Co(II)P
Figure 9. Nanosecond transient absorption spectrum (visible-near-infrared
part) recorded with a 50 ns upon flash photolysis of dyad Co3 (∼5.0 ×
10-5 M) at 532 nm in deoxygenated anisole, indicating the Co(II)P π-radical
π-radical cation features (λmax ) 600 nm) - Co(II)P•+
.
cation features (λmax ) 680 nm) and C60 π-radical anion features (λmax
)
900 nm) - Co(II)P•+-C60•-. Insert depicts the decay of the Co(II)P
π-radical cation features at 680 nm.
to be helpful. Most importantly, the redox features of Zn3 and
Co3 are in close resemblance and ensure similar thermodynamic
driving forces. Steady-state fluorescence experiments suggest
that in Zn3 the electron transfer occurs with lower to subpico-
second rates and, thus, sufficiently fast to compete with the
intrinsic deactivation of the cobalt(II) porphyrin in Co3 (i.e.,
less than 30 ps).12
Both fingerprints (i.e., ∼560 and 900 nm) were employed as
reliable probes to determine the lifetime of the associated
Co(III)P-C60•- state in Co3. The decay curves were well-fitted
by a single exponential decay component. In particular, lifetimes
that are 860 ( 40 ps were derived from the decays of the
oxidized donor and reduced acceptor absorption at 560 and 900
nm, respectively.
In benzonitrile, the photoreactivity of Co3 changed drastically.
•-
At a time delay, when the Co(III)P-C60 maxima were
recorded in THF, the rather sharp transient of cobalt(III)
porphyrin is replaced by a very broad absorption that covers
most of the 500-750 nm range and exhibits a maximum at
600 nm. In fact, the spectrum as shown in Figure 8 bears close
resemblance with a one-electron oxidized ligand product,
π-radical cation. Around 900 nm, we found the characteristic
fingerprint of the fullerene π-radical anion. Thus, the spectro-
scopic analysis suggests formation of a Co(II)P•+-C60•- radical
pair, contrasting the observation in THF. The decay of the
Figure 10. Picosecond transient absorption spectrum (visible part) recorded
with a 100 ps upon flash photolysis of dyad Co3 (∼5.0 × 10-5 M) at 532
nm in deoxygenated toluene (λmax ) 565 nm), benzonitirle (λmax ) 605
nm), and anisole (λmax ) 660 nm), indicating the Co(II)P π-radical cation
features -Co(II)P•+. Insert depicts the stability of the Co(II)P π-radical
cation features in anisole at 660 nm.
A reference experiment with toluene helped to shed light onto
the electron-transfer aspect. In this aromatic solvent, we found
spectroscopic evidence for Co(II)P•+-C60•-. Likewise, in
benzene and anisole the long-lived photoproduct, with lifetimes
close to 1 µs, showed the characteristic marker of the
Co(II)P•+. The nanosecond spectrum (i.e., recorded with a time
delay of 100 ns) for Co3 in anisole is depicted in Figure 9.
Common to all these solvents is their aromatic character.
Interestingly, the Co(II)P•+ band is sensitively effected by the
solvent, with peaks at 565 nm (toluene), 605 nm (benzonitrile),
and 660 nm (anisole) (Figure 10). This trend suggests that the
polarizability of the solvent plays a key role in determining the
electron-transfer pathway. Since the ground-state absorption
spectra reveal a notable perturbation of the electron density,
namely, charges are moved, at least partially, from the electron
donor (i.e., Co(II)P) to the electron acceptor (i.e., C60), an
electron deficient porphyrin is left behind. In the case of
π-electron rich solvents, individual solvent molecules may
undergo a type of π-stacking with the porphyrin chromophore.
•-
Co(II)P•+-C60 radical pair, at 600 (Co(II)P•+) and 900 nm
(C60•-), is best fitted by a monoexponential rate law, from which
we derive a lifetime for the radical pair of 560 ( 20 ns (!) in
benzonitrile.
The difference in electron-transfer reactivity, that is, involve-
ment of either Co(II)P•+, which brings about a very long-lived
radical pair, or Co(III)P and a short-lived radical pair, raises
the question about the factors that are responsible for these
effects. It should be noted that both solvents are coordinating
ones that might interact with the free axial ligand sphere of the
cobalt(II), but have substantially different polarity. Thus, we
probed DMF, another coordinating solvent, whose polarity is
close to that of benzonitrile. Surprisingly, the product of the
photoinduced charge-transfer event is again the short-lived (0.15
•-
ns) Co(III)P-C60 state. This rules out the solvent polarity
effect and also the solvent coordination to the central metal of
the porphyrin.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10377

A R T I C L E S
Sutton et al.
Once this kind of complex is formed and a photoinduced
electron transfer transfers the charges completely to the C60 core,
where the electron is adequately delocalized, the positive charge
can now be delocalized over the entire porphyrin macrocycle,
extending into those solvent molecules that form the π-stacked
complex. Consequently, charge separation lifetimes on the order
of microseconds are noted, 3 orders of magnitude longer than
•-
that seen for Co(III)P-C60
.
In addition, complementary studies were performed with
cyclohexanecarbonitrile and o-dichlorobenzene/p-nitrobenzene
as solvents. In cyclohexanecarbonitrile, the nonaromatic ana-
logue of benzonitrile, we found, not unexpectedly, the short-
lived Co(III)P-C60•- state. Similarly, however, in o-dichloro-
benzene and p-nitrobenzene, spectroscopic evidence also points
to the formation of Co(III)P-C60•-. The radical pair lifetimes
are between 0.4 and 0.21 ns. The discrepancy between the
aromatic solvents raises the question if next to their compara-
tively high polarizabilities favoring π-π-stacking interactions
with the porphyrin chromophore additional parameters might
be operative and that influence the preferred formation of
Figure 11. Plot of [kBT ln kBET + (∆GBET°/2)] versus (∆GBET°)2 for dyad
Co3.
Now we turn to Co1. Because of the flexibility, which is
associated with the donor-acceptor connection in Co1, different
structural conformers are likely to be present in solution. An
important aspect in control over orientation and structure are
charge-transfer interactions, as they may prevail between the
donor and acceptor moieties. Earlier we proposed that electron
transfer in flexible dyads involves generation of a photoexcited
intramolecular exciplex.30 Formation of this critical intermediate
takes typically several hundred of picoseconds. Considering the
short-lived nature of photoexcited cobalt(II) porphyrins (i.e., less
than 30 ps), an intramolecular electron transfer evolving from
the latter is rather unfeasible. In line with this assumption, 532-
nm excitation resulted only in the typical porphyrin deactivation.
About 50 ps after the laser pulse the spectrum disappeared
completely.
Co(II)P•+-C60 versus Co(III)P-C60•-. Steric effects, for
•-
example, should be considered, which we tested with tert-
butylbenzene. Since this case also yielded the short-lived
•-
Co(III)P-C60 state, we conclude tentatively that a compli-
cated interplay between polarizability, steric bulk, and probably
additional effects is responsible for this rather unusual behavior.
The lifetimes of the charge-separated states (i.e., Co(III)P-
C60•-) can be correlated for Co3 with the solvent polarity. The
dependence of kCR (τ ) 1/kCR) versus free energy changes
indicates stabilizing effects for the charge-separated state at
higher -∆GCR° values. These are clear attributes of the
“Marcus-inverted” region, the highly exergonic region (-∆G°
> λ), where the electron-transfer rates start to decrease with
increasing free energy changes.29 To add another low-polarity
data point, we studied chloroform and found a lifetime for
An alternative approach to initiate an electron transfer in Co1
is the utilization of the fullerene excited states. In fact, the singlet
(1.76 eV) and triplet (1.5 eV) excited states of C60 derivatives
•-
are both powerful oxidants4f to form either Co(II)P•+-C60
•-
•-
Co(III)P-C60 of 1.1 ns. To quantify the driving force
or Co(III)P-C60 for which we determined energies around
dependence on the rate constants (kCR), the semiclassical Marcus
equation was employed. From the best fits of the driving force
dependence on charge recombination rates, the following values
for the reorganization energies (λ) and electronic couplings (V)
were derived: λ ) 0.84 eV and V ) 18 cm-1. Please note that
the electronic coupling element is substantially smaller than that
in the ZnP•+-C60•- case, where a similar analysis yielded a V
value of 313 cm-1. To confirm the small electronic coupling
element, we transferred the semiclassical Marcus equation into
a linear expression, eq 1.
1.4-1.3 eV. The well-known excited-state properties of metha-
nofullerenes shall be discussed first, since they emerge as
reference points for the interpretation of the features expected
upon 355-nm illumination of dyad Co1. The singlet excited
state, displaying a distinctive singlet-singlet transition around
900 nm, undergoes a quantitative intersystem crossing (ISC) to
yield the long-lived triplet manifold, for which maxima are noted
at 360 and 720 nm.
Detecting the instantaneous grow-in (i.e., 18 ps) of the 900-
nm absorption affirms the successful C60 excitation in Co1.
Instead of seeing, however, the slow ISC dynamics, the singlet-
singlet absorption decays in the presence of Co(II)P with
accelerated dynamics. The singlet excited-state lifetimes, as they
were determined from an average of first-order fits of the time-
absorption profiles at various wavelengths (850-950 nm) are
310 ps (toluene), 230 ps (THF), and 130 ps (benzonitrile).
Spectroscopically, the transient absorption changes, taken after
the completion of the decay, bear close resemblance with those
seen for Co3. In particular, we found distinctly different products
in THF and benzonitrile. In THF, the new transients reveal
strong maxima at 560 and 590 nm, which match those of the
one-electron oxidized Co(III)P. On the other hand, in benzoni-
1/2
4π3
∆GET°
kBT ln kET
+
) kBT ln
V2
-
2
(
[
)
2
]
h λkBT
(∆GET°)2
4λ
λ
-
(1)
4
A plot of [kBT ln kBET + (∆GBET°/2)] versus (∆GBET°)2 gave
a linear correlation, as shown in Figure 11. Analysis of this
plot afforded a reorganization energy of 0.75 eV and an
electronic coupling of 15 cm-1
.
(29) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (b)
Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111; Angew. Chem.
1993, 105, 1161.
(30) Guldi, D. M.; Luo, C.; Swartz, A.; Scheloske, M.; Hirsch, A. Chem.
Commun. 2001, 1066-1067.
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Cobalt(II) Porphyrin−Fullerene Conjugates
A R T I C L E S
activity, that is, formation of Co(III)P-C60 or Co(II)P•+-
C60•-, the aromatic character of the solvent certainly plays the
major role. Appreciable π-stacking with the porphyrin chro-
mophore is the overall driving force for stabilizing the long-
•-
•-
lived Co(II)P•+-C60 state. But polarizability of the solvent
is not the sole nature of the stacking. At this stage, it is evident
that a combination of steric and other effects also contributes
to the facilitation of this interesting phenomenon. Importantly,
depending on the nature of the oxidation, that is, metal- versus
ligand-centered process, charge recombination can be suppressed
by up to 3 orders of magnitude, reaching almost into the
microsecond time domain. Considering the close donor-
acceptor separation, namely, van der Waals contacts, the
lifetimes are promising, although not comparable to previous
findings, where separations are as large as 50 Å.6b
Experimental Section
Figure 12. Picosecond transient absorption spectrum (visible part) recorded
with a 100 ps upon flash photolysis of dyad Co1 (3.0 × 10-5 M) at 532
nm in deoxygenated THF (solid line) and benzonitrile (dashed line).
Synthesis Protocols and Spectroscopic Data. Chemicals. C60 was
obtained from Hoechst AG/Aventis and separated from higher fullerenes
by a plug filtration.31,32 All analytical reagent-grade solvents were
purified by distillation. Dry solvents were prepared using customary
literature procedures.33 trans-2-C62(COOEt)4 (7),13 ZnTPP (8),2
CoTt-BuPP (9),23 and Zn39a were synthesized according to literature
procedures in ref 4a.
Thin-Layer Chromatography (TLC). Riedel-de-Hae¨n silica gel F254
and Merck silica gel 60 F254. Detection: UV lamp, H3[P(Mo3O10)4]/
Ce(SO4)2/H2SO4/H2O bath, KMnO4/H2O and iodine chamber.
Flash Chromatography (FC). ICN Silica 32-63, 60 Å; typical
parameters for column diameter, loading, optimum eluant mixtures,
eluant flow rate etc. were selected from the literature.34
trile the key feature of the new product is a broad maximum in
the visible region around 600 nm, which resembles the one-
electron oxidized Co(II)P π-radical cation, Co(II)P•+, seen in
Co3. The spectral differences of the two spectra, namely, that
of Co(III)P and that of Co(II)P•+, are illustrated in Figure 12.
Concomitant with the Co(III)P and Co(II)P•+ features in the
visible, we noticed a near-infrared maximum at 1040 nm,
corroborating the one-electron reduction of the electron accept-
•-
ing fullerene, C60
.
Thus, in flexible dyad Co1, formation of the radical pairs
develops only as a result of the rapid decay of the fullerene
singlet excited state, upon 355-nm excitation, while in dyad Co3
even short-lived nature of photoexcited cobalt(II) porphyrins,
which is less than 30 ps, was shown successfully to initiate an
intramolecular electron transfer.
High-Performance Liquid Chromatography (HPLC). Shimadzu
liquid chromatograph LC-10AT with system controller SCL-10AVP,
preparative liquid chromatographs LC-8A, diode array detector, auto
injector, refractive index detector and UV/vis detector, selection valve,
and fraction collector. Analytical columns: Nucleosil 5 µm, 200 × 4
mm, Macherey-Nagel; Gromsil 100 Si, NP1, 5 µm, 200 × 4 mm;
Rexchrom Buckyclutcher 10 × 250 mm, Regis; and Nucleogel GFC
500-5, Macherey-Nagel. Preparative columns: Nucleosil 5 µm, 250
× 21 mm, Macherey-Nagel; Grom-Sil 100 Si, NP1, 5 µm, 250 × 20
mm; Nucleogel GFC 500-10, Macherey-Nagel; Buckyclutcher 250
× 21 mm.
Conclusions
Compared to Zn3 and trans-2 bisadduct 7 the first reduction
of the fullerene moiety within Co3 becomes easier by 40 mV
in dichloromethane and 20 mV in benzonitrile, indicating
significant interactions between the π-system of the fullerene
and the d orbitals of the central Co atom. Comparison of the
first oxidation step of the cobalt complexes (i.e., 9, Co4, and
Co1) reveals an interesting trend. Whereas in dichloromethane
the first oxidation occurs at similar potentials (+0.35 ( 0.05
V), in benzonitrile the first oxidation occurs at about 0.00 (
0.05 V, except for Co3, whose potential is much more positive
(+0.28 V). Such a large potential shift to positive potentials
denotes in benzonitrile a destabilization of the cobalt(III)
oxidation state in Co3 compared to all other studied cobalt
porphyrins and therefore a stabilization of the radical cation
character. All these trends, as well as the peculiar behavior
observed by spectroelectrochemistry for the first oxidation step
of Co3, are in agreement with the generation of a cobalt(II)
radical cation. Such a behavior has never been observed
previously and is in agreement with the photophysical results
observed in benzonitrile.
NMR Spectra. JEOL JNM EX 400 and JEOL JNM GX 400 (1H:
400 MHz, 13C: 100.5 MHz), Bruker Avance 300 (1H: 300 MHz, 13C:
75,4 MHz), Bruker Avance 400 (1H: 400 MHz, 13C: 100.5 MHz).
The chemical shifts are given in parts per million (ppm) relative to
SiMe4 (TMS). The resonance multiplicities are indicated as s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad
resonances).
UV/Vis Spectra. Shimadzu UV-3102 PC, UV-vis-NIR scanning
spectrophotometer; absorption maxima λmax are given in nanometers.
IR Spectra. Bruker FT-IR Vector 22, KBr pellets or thin film (NaCl
plates), νj values in cm-1
.
Mass Spectra. Micromass Zabspec, FAB (LSIMS) mode (3-
nitrobenzyl alcohol) and ESI mode; Varian MAT 311A EI mode.
Porphyrins H24, H25, and H26. Typical Procedure. 3-(2-(Meth-
oxymalonyloxy)ethoxybenzaldehyde (6.65 g, 25.0 mmol), benzaldehyde
(2.53 cm3, 25.0 mmol), pyrrole (3.48 cm3, 50.0 mmol), and tetraphen-
ylphosphonium chloride (58 mg, 0.15 mmol) were dissolved in CH2-
Cl2 under N2, and the solution was purged with N2 for 10 min. Boron
(31) Reuther, U. Ph.D. Dissertation, University of Erlangen-Nu¨rnberg, Germany,
2002.
In photoinduced electron-transfer experiments, two different
oxidation products were found besides C60•- in the radical ion
pair state, that is, either a metal-centered Co(III)P or a ligand-
centered Co(II)P•+. In light of the drastic changes in photore-
(32) Isaacs, L.; Wehrsig, A.; Diederich, F. HelV. Chim. Acta 1993, 76, 1231.
(33) Perrin, D. D.; Amarego, W. L. F. Purification of Laboratory Chemicals,
3rd ed.; Pergamon Press: Oxford, 1988.
(34) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-2925.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10379

A R T I C L E S
Sutton et al.
trifluoride etherate (630 µL, 5.0 mmol) was added, and the solution
was stirred for 1 h under exclusion of light. p-Chloranil (9.25 g, 37.6
mmol) was then added, and the solution was heated at reflux for 1 h.
The mixture was separated by multiple passage through flash chro-
matographic columns (SiO2, CH2Cl2/EtOAc 20/1). The first porphyrin
fraction contained tetraphenylporphyrin and was followed by monoester
H24. The third and fourth bands contained bis-functionalized porphyrins
H26 and H25, respectively.
MS (FAB, NBA): m/z ) 832 [M+]. FT-IR (KBr): ν˜/cm-1 ) 2964,
1734, 1684, 1647, 1637, 1596, 1575, 1541, 1508, 1488, 1348, 1309,
1261, 1191, 1178, 1146, 1103, 1069, 892, 794, 748, 714, 699.
1′-Methoxycarbonyl-1′-{2-[3-[5-[10,15,20-triphenylporphyrinato-
cobalt(II)]]]-phenoxy}ethoxycarbonyl-1,2-methano[60]fulleren Co1.
A mixture of 320 mg (0.385 mmol) of Co porphyrin Co4, 278 mg
(0.385 mmol) of C60, and 98 mg (0.385 mmol) of I2 were dissolved in
200 cm3 dry toluene under nitrogen protection, and subsequently 150
µL (1.04 mmol) of DBU was added. After being stirred for 48 h room
temperature, the reaction solution was cautiously concentrated on a
rotatory evaporater, and the residue was chromatographically purified
by means of FC chromatography (silica, toluene/EtOAc 10:1). Pure
Co1 was obtained as an orange-colored substance.
Co1 (160 mg, 0.10 mmol, 26.8%): 1H NMR (400 MHz, CDCl3): δ
) 3.80 (s, 3H, OCH3), 5.04 (s, 2H, CH2), 5.13 (s, 2H, CH2), 8.70 (s,
1H, ArH), 9.16-9.64 (m, 9H, ArH), 10.10 (br, 1H, ArH), 11.20-12.60
(br, 8H, ArH), 14.70 (br, 8H, â-CH); 13C NMR (100.5 MHz): δ )
162.19, 162.78 (C)O), 159.01, 141.69, 133.49, 130.20, 130.05, 129.55,
117.62 (arom. C), 68.53, 67.94 (C60 sp3), 65.18 (CH2), 53.69 (OCH3);
UV/Vis (CH2Cl2): λmax/nm (log ꢀ) ) 258 (5.01), 325 (4.62), 413.5
(4.82), 536.5 (3.96); MS (FAB, NBA): m/z ) 1545 [M+]; FT-IR
(KBr): ν˜/cm-1 ) 1763, 1595, 1573, 1487, 1451, 1427, 1349, 1265,
1229, 1205, 1177, 1109, 1094, 1071, 1003, 956, 834, 796, 752, 700,
667, 580.
1
H24 (484 mg, 0.63 mmol, 5.0%): H NMR (400 MHz, CDCl3): δ
) 8.84 (m, 8 H, â-CH), 8.21 (AA′BB′, 6 H, ArH), 7.85 (d, 1 H, ArH),
7.78 (s, 1 H, ArH), 7.75 (m, 9 H, ArH), 7.63 (t, 1 H, ArH), 7.30 (d, 1
H, ArH), 4.57 (t, 3J ) 4.64 Hz, 2 H, CH2), 4.36 (t, 3J ) 4.64 Hz, 2 H,
CH2), 3.68 (s, 3 H, OCH3), 3.44 (s, 2 H, CO-CH2-CO), -2.57 (br.
s, 2 H, NH). 13C NMR (100.5 MHz, CDCl3): δ ) 166.76, 166.50
(CdO), 156.77, 143.63, 143.06, 142.13 (arom. C), 134.55 (arom. CH),
131.15 (â-CH), 128.17, 127.72, 127.61, 126.68, 121.12 (arom. CH),
120.28, 119.48, 118.73 (meso-C), 114.20 (arom. CH), 65.86, 63.85
(CH2), 52.54 (OCH3), 41.17 (CO-CH2-CO). UV/vis (CH2Cl2): λmax
/
nm (lg ꢀ) ) 399 (sh, 4.87), 418 (5.65), 514 (4.21), 549 (3.77), 591
(3.63), 645 (3.42). MS (MALDI-TOF): m/z ) 775 (MH+). FT-IR
(KBr): ν˜/cm-1 ) 3316, 3054, 3024, 2950, 1736, 1596, 1575, 1471,
1440, 1349, 1178, 1152, 964, 801, 730, 700.
1
H25 (449 mg, 0.48 mmol, 3.8%): H NMR (400 MHz, CDCl3): δ
) 8.87 (m, 8 H, â-CH), 8.21 (AA′BB′, 4 H, ArH), 7.85 (d, 2 H, ArH),
7.79 (s, 2 H, ArH), 7.72 (m, 6 H, ArH), 7.60 (t, 2 H, ArH), 7.28 (d, 2
H, ArH), 4.52 (t, 3J ) 4.63 Hz, 4 H, CH2), 4.31 (t, 3J ) 4.63 Hz, 4 H,
CH2), 3.65 (s, 6 H, OCH3), 3.42 (s, 4 H, CO-CH2-CO), -2.42 (br.
s, 2 H, NH). 13C NMR (100.5 MHz, CDCl3): δ ) 166.72, 166.45
(C)O), 156.73, 143.54, 142.04 (arom. C), 134.51 (arom. CH), 131.17
(â-CH), 128.12, 127.70, 127.59, 126.66, 121.12 (arom. CH), 120.32,
119.51 (meso-C), 114.12 (arom. CH), 65.78, 63.77 (CH2), 52.48
(OCH3), 41.10 (CO-CH2-CO). UV/vis (CH2Cl2): λmax/nm (lg ꢀ) )
399 (sh, 4.82), 418 (5.58), 514 (4.16), 548 (3.79), 591 (3.66), 647 (3.49).
MS (MALDI-TOF): m/z ) 934 (MH +); FT-IR (KBr): ν˜/cm-1 ) 3316,
3055, 3025, 2951, 2925, 2853, 1736, 1596, 1576, 1472, 1438, 1336,
1262, 1151, 964, 803, 733, 701.
Metal-Free trans-2 Dyad H23. To a solution of 50 mg (0.030 mmol)
of dyad Zn3 in 25 mL of toluene, a 3% methanolic solution of
trifluoroacetic acid was added. The reaction mixture was stirred at room
temperature for 10 min. After being neutralized with aqueous NaHCO3
solution and extracted with EtOAc, the organic layer was washed with
H2O, dried over Na2SO4, and concentrated in vacuo. A quantity of 48
mg (0.029 mmol, 96.6%) of the product was isolated by flash
chromatography (silica, CH2Cl2/EtOAc 20:1).
1H NMR (400 MHz, CDCl3): δ ) 8.71 (m, 8 H, â-CH), 8.27 (d, 2
H, ArH), 8.17 (d, 2 H, ArH), 7.96 (d, 2 H, ArH), 7.80 (t, 2 H, ArH),
3
7.71 (m, 8 H, ArH), 7.39 (d, 2 H, ArH), 5.17 (dt, J ) 5.50 Hz, 2 H,
3
CH2), 4.77 (dt, J ) 5.50 Hz, 2 H, CH2), 4.52 (t, J ) 5.50 Hz, 4 H,
1
CH2), 3.93 (s, 6 H, OCH3), -3.00 (s, 2 H, NH). 13C NMR (100.5 MHz,
CDCl3): δ ) 163.63, 163.55 (CdO), 156.72 (arom. C), 147.75, 147.43,
145.66, 144.96, 144.93, 144.47, 144.15, 143.89 (fullerene sp2 C), 143.67
(arom. C), 142.86, 142.71, 142.42, 142.30 (fullerene sp2 C), 142.13
(arom. C), 141.95, 141.80, 141.63, 141.60, 141.51, 141.30, 141.09,
141.00, 140.90, 139.90, 139.69, 139.16, 138.32, 138.28, 137.99, 137.83
(fullerene sp2 C), 134.33, 134.15 (arom. CH), 132.39 (â-CH), 127.85,
127.64, 126.92, 126.71, 126.58 (arom. CH), 120.43, 119.53 (meso-C),
115.36 (arom. CH), 70.69, 70.02 (fullerene sp3 C), 67.33, 65.05 (CH2),
53.85 (OCH3), 49.09 (CO-C-CO). UV/vis (CH2Cl2): λmax/nm (log
ꢀ) ) 240 (5.05), 260 (5.07), 317 (4.67), 407 (sh, 4.77), 427 (5.34),
518 (4.09), 553 (3.75), 594 (3.64), 648 (3.40). MS (MALDI-TOF):
m/z ) 1652 (MH+). FT-IR (KBr): ν˜/cm-1 ) 3057, 3023, 2951, 2922,
1752, 1597, 1575, 1472, 1434, 1238, 800, 729, 701, 527.
H26 (227 mg, 0.24 mmol, 1.9%): H NMR (400 MHz, CDCl3): δ
) 8.86 (m, 8 H, â-CH), 8.21 (AA′BB′, 4 H, ArH), 7.85 (d, 2 H, ArH),
7.78 (s, 2 H, ArH), 7.75 (m, 6 H, ArH), 7.62 (t, 2 H, ArH), 7.28 (d, 2
H, ArH), 4.55 (t, 3J ) 4.64 Hz, 4 H, CH2), 4.34 (t, 3J ) 4.64 Hz, 4 H,
CH2), 3.68 (s, 6 H, OCH3), 3.44 (s, 4 H, CO-CH2-CO), -2.57 (br. s,
2 H, NH). 13C NMR (100.5 MHz, CDCl3): δ ) 166.74, 166.47
(CdO), 156.77, 143.59, 142.08 (arom. C), 134.53 (arom. CH), 131.08
(â-CH), 128.16, 127.74, 127.63, 126.69, 121.14 (arom. CH), 120.25,
119.62 (meso-C), 114.20 (arom. CH), 65.86, 63.81 (CH2), 52.52
(OCH3), 41.16 (CO-CH2-CO). UV/vis (CH2Cl2): λmax/nm (lg ꢀ) )
400 (sh, 4.90), 418 (5.65), 514 (4.26), 550 (3.85), 590 (3.74), 645 (3.58).
MS (MALDI-TOF): m/z ) 934 (MH +). FT-IR (KBr): ν˜/cm-1 ) 3316,
3055, 3024, 2951, 2924, 2853, 1736, 1596, 1576, 1472, 1438, 1335,
1260, 1150, 972, 803, 733, 701.
5-{3-[3-(Methoxymalonyloxy)ethoxy]phenyl}-10,15,20-tri-
phenylporphyrinato-cobalt(II) Co4. To a solution of 900 mg (1.9
mmol) of porphyrin H24 in 200 cm3 THF, 443 mg (1.8 mmol)
Co(OAc)2*4H2O was added, and the mixture was heated for 3 h under
reflux. The completing of the reaction was controlled by TLC.
Subsequently, the solution was cautiously evaporated, and the residue
was purified by flash-chromatography using an increasing solvent
polarity gradient (silica, CH2Cl2/EtOAc 20:1 to 4:1).
Trans-2 Dyad Co3. H23 (33 mg, 0.020 mmol) and Co(OAc)2 (10
equiv) were dissolved in THF (10 cm3) in a 25-cm3 round-bottomed
flask and heated to reflux with stirring under N2 for 24 h, until TLC
control (SiO2, toluene/EtOAC 20:1) showed complete consumption of
free base adduct starting material. The reaction mixture was concen-
trated on a rotary evaporator and filtered through a silica gel plug (CH2-
Cl2). A dark brown solid was obtained upon evaporation and recrys-
tallization (CH2Cl2/MeOH) of the colored product fraction.
1
Co4 (150 mg, 0.80 mmol, 10%): H NMR (400 MHz, CDCl3): δ
(30 mg, 0.018 mmol, 88%): Found: C, 78.81; H, 2.36; N, 3.28%
1
) 3.63 (s, 3H, OCH3), 3.66 (s, 2H, COCH2CO), 5.11 (br, 2H, CH2),
5.49 (br, 2H, CH2), 9.32 (br, 1H, ArH), 9.75 (br, 4H, ArH), 9.95 (br,
6H, ArH), 12.71 (br, 2H, ArH), 13.11 (br, 6H, ArH), 15.94 (br, 8H,
â-CH). 13C NMR (100.5 MHz): δ ) 166.90 (CdO), 160.15, 158.29,
156.80, 141,13, 134.79, 131.47, 130.85, 130.27, 127.63, 117.33, 98.56
(arom. C), 67.13, 64.50 (CH2), 53.04 (OCH3), 41.31 (COCH2CO). UV/
vis (CH2Cl2): λmax/nm (log ꢀ) ) 346 (4.12), 405.5 (4.88), 532 (3.90).
(C116H40CoN4O10: C, 81.55; H, 2.36; N, 3.28%); %. H NMR (400
MHz, CS2/CDCl3): (paramagnetic!) δ ) 14.77 (br. s, 8 H, â-CH), 12.94
(br. s, 2 H, ArH), 12.47 (br. s, 2 H, ArH), 11.26 (br. s, 2 H, ArH), 9.81
(br. s, 2 H, ArH), 9.56 (br. s, 2 H, ArH), 9.24 (br. s, 6 H, ArH), 8.85
(br. s, 2 H, ArH), 5.21 (br. s, 4 H, CH2), 5.00 (br. s, 4 H, CH2), 3.62
(br. s, 6 H, OCH3). UV/vis (CH2Cl2):
λmax/nm (lg ꢀ) ) 262
(5.08), 319 (4.71), 419 (5.12), 530 (4.13). MS (FAB, NBA): m/z )
9
10380 J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004

Cobalt(II) Porphyrin−Fullerene Conjugates
A R T I C L E S
1707 (M+), 720 (C60+). FT-IR (KBr): ν˜/cm-1 ) 3450, 2955, 1751,
1600, 1578, 1433, 1352, 1262, 1240, 1178, 1109, 1075, 1006, 798,
702, 527.
Photochemistry. Picosecond laser flash photolysis experiments were
carried out with 532-nm laser pulses from a mode-locked, Q-switched
Quantel YG-501 DP Nd:YAG laser system (pulse width 18 ps, 2-3
mJ/pulse). Nanosecond laser flash photolysis experiments were per-
formed with laser pulses from a Quanta-Ray CDR Nd:YAG system
(532 nm, 6 ns pulse width) in a front face excitation geometry. The
photomultiplier output was digitized with a Tektronix 7912 AD
programmable digitizer. Fluorescence lifetimes were measured with a
laser strope fluorescence lifetime spectrometer (Photon Technology
International) with 337-nm laser pulses from a nitrogen laser fiber-
coupled to a lens-based T-formal sample compartment equipped with
a stroboscopic detector. Details of the laser strobe systems are described
on the manufacturer’s web site, http://www.pti-nj.com. Emission spectra
were recorded with a SLM 8100 spectrofluorometer. The experiments
were performed at room temperature. A 570-nm long-pass filter in the
emission path was used to eliminate the interference from the solvent
and stray light for recording the fullerene fluorescence. Each spectrum
was an average of at least five individual scans and was corrected by
using the correction function supplied by the manufacturer, by
subtraction of the photomultiplier dark counts signal.
Electrochemistry. All compounds were studied in CH2Cl2 + 0.1
M Bu4NPF6 and in benzonitrile + 0.1 M Bu4NPF6, respectively. The
electrochemical measurements were carried out at room temperature
(20 °C) in CH2Cl2 containing 0.1 M Bu4NPF6 in a classical three-
electrode cell. The electrochemical cell was connected to a computerized
multipurpose electrochemical device (Autolab, Eco Chemie BV, The
Netherlands) controlled by a GPES software (v. 4.7) running on a PC
computer. The working electrode was a glassy carbon (GC) disk
electrode (diameter: 3 mm) used either motionless for cyclic voltam-
metry (V )10 mV s-1 to 10 V s-1) or as a rotating disk electrode.
The auxiliary electrode was a platinum wire, and the reference
electrode was an aqueous Ag/AgCl/KCl (sat.) electrode. All potentials
are referred to the ferrocene/ferrocenium (Fc+/Fc) couple used as the
internal standard. Under our experimental conditions, ferrocene was
oxidized at +0.40 V versus Ag/AgCl. The accessible potential domain
ranged from +1.4 to -2.4 V versus Fc+/Fc. CH2Cl2 (Merck, spectro-
scopic grade) was dried over molecular sieves (4 Å) and stored under
argon. Bu4NPF6 (Fluka, electrochemical grade) was used as received.
The electrolyte was degassed by bubbling argon through the solution
for at least 5 min, and an argon flow was kept over the solution during
measurements.
Studies carried out in benzonitrile were made in a glovebox under
drastic exclusion of water and oxygen (less than 2 ppm) as described
elsewhere.36 The classical three-electrode cell was connected to a
Princeton Applied research potentiostat 263, controlled by a Powerlab/
Echem electrochemical system (ADInstruments, USA). The working
electrode was a platinum disk (2 mm in diameter), the auxiliary
electrode a platinum wire, and the pseudo reference electrode was also
a platinum wire. Ferrocene, used as an internal standard, was introduced
to the solution at the end of the study. All given potentials are therefore
given versus ferrocene in agreement with the IUPAC recommendation.37
Spectroelectrochemical experiments were carried out as described
elsewhere.36
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft (SFB 583 Redoxaktive Metallkom-
plexe-Reaktivita¨tssteuerung durch Molekulare Architekturen),
the EU (RTN networks “WONDERFULL” and “FAMOUS”),
and the Office of Basic Energy Sciences of the U.S. Department
of Energy. This is contribution NDRL-4536 of the Radiation
Laboratory.
Supporting Information Available: Picosecond transient
absorption spectrum recorded with a 100 ps flash photolysis of
dyad Zn3. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA048983D
(35) Hashino, M.; Sonoki, H.; Miyazaki, Y.; Iimura, Y.; Yamamoto, K. Inorg.
Chem. 2000, 39, 4850.
(36) Bley-Escrish, J.; Gisselbrecht, J. P.; Vogel, E.; Gross, M. Eur. J. Inorg.
Chem. 2002, 2829-2837.
(37) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461-466.
(38) Guldi, D. M.; Imahori, H.; Tamaki, K.; Kashiwagi, Y.; Yamada, H.; Sakata,
Y.; Fukuzumi, S. J. Phys. Chem. 2004, 108, 541-548.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10381