trans-2 addition pattern to power charge transfer in dendronized metalloporphyrin C60 conjugates

Article abstract of DOI:10.1021/ja9029686

Coordinating different transition metalssmanganese(III), iron(III), nickel(II), and copper(II) - by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C₆₀ was attached as a ground-state electron acceptor. S

Full text of DOI:10.1021/ja9029686

Published on Web 06/16/2009
trans-2 Addition Pattern to Power Charge Transfer in
Dendronized Metalloporphyrin C60 Conjugates
Fabian Spa¨nig,† Michaela Ruppert,‡ Jo¨rg Dannha¨user,‡ Andreas Hirsch,*,‡ and
Dirk M. Guldi*,†
Department of Chemistry and Pharmacy and Interdisciplinary Center of Molecular Materials
(ICMM), Friedrich-Alexander UniVersita¨t Erlangen Nu¨rnberg, Egerlandstrasse 3,
91058 Erlangen, Germany, and Department of Chemistry and Pharmacy and Interdisciplinary
Center of Molecular Materials (ICMM), Friedrich-Alexander UniVersita¨t Erlangen Nu¨rnberg,
Henkestrasse 42, 91054 Erlangen, Germany
Received April 14, 2009; E-mail: guldi@chemie.uni-erlangen.de; andreas.hirsch@chemie.uni-erlangen.de
Abstract: Coordinating different transition metalssmanganese(III), iron(III), nickel(II), and copper(II)sby
a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C60 was attached
as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that
is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence
measurements provided support for mutual interactions between the redox-active constituents in the ground
state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the
occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling
in the range of 300 cm-1. Photophysical meansssteady-state/time-resolved fluorescence and transient
absorption measurementssshed light on the excited-state interactions. To this end, we have added pulse
radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e.,
fullerene) characteristics. π-π stacking of the excited state electron donor and the electron acceptor is
key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to
power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair
states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to
depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It
is important that the product of charge recombination, namely tripmultiplet excited states versus ground
state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation
and charge recombination) and the free energy change for the underlying reaction reveals a parabolic
dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm-1
closely resembling that seen for the zinc(II) and free base analogues.
)
Introduction
tion, the greatest attention has been paid to systems in which
porphyrins are linked to C60 by a variety of flexible, rigid, and
semirigid linkers.9,10
C60 and porphyrins are molecular architectures ideally suited for
devising integrated, multicomponent model systems to transmit and
process solar energy. The implementation of C60 as a three-
dimensional electron acceptor11 holds great expectations on account
One of the ultimate goals of molecular electronics is the
rational design of electron donor-acceptor conjugates/hybrids
capable of light-induced charge-separation over long distances
on the molecular scale. In this context, ultrafast charge-transfer
systems will likely figure prominently in proposed photodriven
molecular-scale rectifiers and emerging technologies that focus
on light-modulated data storage and retrieval.1-4
There has been considerable interest in electron donor-acceptor
systems, in which C60 serve as electron and energy acceptors
upon photoexcitation.5-8 While a large variety of such systems
have been synthesized and subjected to photophysical investiga-
(6) Guldi, D. M. The Small Reorganization Energy of Fullerenes. In
Fullerenes: From Synthesis to Optoelectronic Properties; Guldi, D. M.,
Martin, N., Eds.; Kluwer Academic Publishers: Norwell, MA, 2002;
p 237.
(7) Guldi, D. M.; Kamat, P. V. Photophysical Properties of Pristine
Fullerenes and Fullerene-Containing Donor-Bridge-Acceptor Systems.
In Chemistry, Physics and Technology; Kadish, K. M., Ruoff, R. S.,
Eds.; John Wiley and Sons: New York, 2000; p 225.
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† Egerlandstrasse 3.
‡ Henkestrasse 42.
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1997.
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(10) Bracher, P. J.; Schuster, D. I. Electron Transfer in Functionalized
Fullerenes. In Fullerenes: From Synthesis to Optoelectronic Properties;
Guldi, D. M., Martin, N., Eds.; Kluwer Academic Publishers: Norwell,
MA, 2002; p 163.
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9378 J. AM. CHEM. SOC. 2009, 131, 9378–9388
10.1021/ja9029686 CCC: $40.75  2009 American Chemical Society

Redox-Active C60-Metalloporphyrin Conjugates
A R T I C L E S
of its small reorganization energy in electron transfer reactions,12-15
which has exerted a noteworthy impact on the development of C60-
based materials for applications involving light-induced charge-se-
paration.5,6,8,16-27 Due to van der Waals attraction between C60
and porphyrins, electron donor-acceptor conjugates/hybrids tend
to adopt conformations, where these moieties achieve close spatial
proximity. To this end, several groups have successfully constructed
doubly linked conjugates, in which C60 and porphyrins are brought
into close proximity (i.e., π-π stacking).28-38 In fact, the center-
to-center distance is in the order of 6.5-7.0 Å. Charge transfer,
upon excitation, in such systems is extremely fast, occurring in
the picosecond time domain, usually to the exclusion of intramo-
lecular energy transfer. Notable, in systems with larger center-to-
center distance, both charge transfer and energy transfer occur.
Then, the polarity of the solvent emerged as a crucial parameter
in regulating the competition between the two processes.39,40
We have become interested in how redox-active metal centers,
such as transition metals, affect the nature and dynamics of an
intramolecular electronic dialogue. For instance, the choice of
paramagnetic metals creates a particularly interesting scenario, since
augmented electronic coupling is expected to enhance charge
transfer processes. However, such aspects have hardly ever been
addressed.30,41-43 Ultrafast excited state deactivation of the met-
alloporphyrins, due to efficient spin-orbit coupling and mixing with
d-states, is largely responsible for this rareness. The success in
probing, for example, metalloporphyrins–with manganese, iron,
nickel, cobalt, and copper–as photoexcited state electron donors
depends on overcoming the fast excited state deactivations. An
obvious strategy would involve an intramolecular charge separation
event that is substantially fast. One requisite is that the charge
separation is activationless, namely in the top region of the Marcus
parabola. Here, the effective rate constant remains a function of
the electronic coupling matrix. The electronic coupling of C60 and
porphyrins is a theme of considerable current interest, given the
potential applications of the photochemistry peculiar to the result-
ing systems in optoelectronics.44-46 The unique topology in
the aforementioned π-π stacks affords electronic couplings in the
order of several hundred cm-1. This, in turn, translates into rate
constants that exceed 1012 s-1 and that should compete with the
intrinsically fast deactivation seen in several metalloporphyrins.38
Now, in this paper, we report the synthesis and physico-
chemical features of an entire family of redox-active architec-
tures that are built on dendronized C60/porphyrins electron
donor-acceptor conjugates.
Results and Discussion
Synthesis and Structural Characterization. The porphyrin
core offers the possibility to complex metal atoms by the four
central nitrogen centers, which allows for the introduction of
additional redox-activity, especially, if transition metals are used.
Moreover, the central metal atoms can in principle be used for
the binding of axial ligands. Both dyads H2-1 and H2-2 were
used as ligands for Cu, Ni, Co, Fe, and Mn. For the insertion
of the metal ions in general a solution of H2-1 or H2-2 was
heated with the corresponding metal chlorides, acetates or
acetylacetonates and subsequently purified by column chromato-
graphy.47-49 In this way, the synthesis of the cobalt(II)-dyads
Co(II)-1 and Co(II)-2 was accomplished by refluxing a solution
of the free base dyads in CHCl3 with an excess of
Co(OAc)2 ·4H2O for a period of 4 h. The nickel(II)-dyads
Ni(II)-1 and Ni(II)-2 were generated by the thermal treatment
of the dyads with Ni(II) acetylacetonate in toluene for 3 h. The
corresponding iron(III)- and manganese(III)-dyads Fe(III)Cl-
1, Fe(III)Cl-2 and Mn(III)Cl-1, Mn(III)Cl-2, respectively,
were generated using FeCl2 ·4H2O and MnCl2 ·4H2O as metal
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Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263,
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(17) Diederich, F.; Gomez-Lopez, M. Chem. Soc. ReV. 1999, 28, 263.
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(24) Mateo-Alonso, A.; Guldi, D. M.; Paolucci, F.; Prato, M. Angew. Chem.,
Int. Ed. 2007, 46, 8120.
(25) D’Souza, F.; Ito, O. Coord. Chem. ReV. 2005, 249, 1410.
(26) Guldi, D. M.; Rahman, G. M. A.; Sgobba, V.; Ehli, C. Chem. Soc.
ReV. 2006, 35, 471.
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Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams,
R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257.
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Gisselbrecht, J.-P. J. Am. Chem. Soc. 2004, 126, 10370.
9
J. AM. CHEM. SOC. VOL. 131, NO. 26, 2009 9379

A R T I C L E S
Spa¨nig et al.
Scheme 1. Insertion of Different Metal Atoms for Synthesis of M-1 and M-2a
a (i) M ) Cu: Cu(OAc)2, MeOH, CH2Cl2, r.t., 83%; M ) Ni: Ni(acac)2, toluene, rf, 75%; M ) Co: Co(OAc)2 ·2H2O, CHCl3, rf, 87%; M ) Fe: FeCl2 ·4H2O,
lutidine, EtOH, CHCl3, rf, 98%; M ) Mn: MnCl2 ·4H2O, lutidine, EtOH, CHCl3, rf, 61%; (ii) M ) Cu: Cu(OAc)2, MeOH, CH2Cl2, r.t., 99%; M ) Ni:
Ni(acac)2, toluene, rf, 99%; M ) Co: Co(OAc)2 ·2H2O, CHCl3, rf, 77%; M ) Fe: FeCl2 ·4H2O, lutidine, EtOH, CHCl3, rf, 86%; M ) Mn: MnCl2 ·4H2O,
lutidine, EtOH, CHCl3, rf, 55%.
Scheme 2. Synthesis of Nondendronized Dyads M(III)-3 via
Precursor H2-3a
salts dissolved in CHCl3/MeOH and lutidine as base. The
insertion of copper(II) was carried out by stirring a solution of
Cu(OAc)2 ·2H2O in methanol with the dyads dissolved in
CH2Cl2 for 3 h at room temperature to yield Cu(II)-1 and
Cu(II)-2. The subsequent purification was accomplished by
column chromatography on silica and recrystallization from
n-pentane. All dyads were obtained in very good yields ranging
from 55% to 99%. In the case of iron(III)- and manganese(III)-
dyads Fe(III)Cl-1, Fe(III)Cl-2 and Mn(III)Cl-1, Mn(III)Cl-
2, a ligand exchange of the axial chloride by hydroxide was
observed after chromatography, which was reverted by washing
twice with 2 M HCl, followed by recrystallization from
n-pentane (Scheme 1).
For a comparison of the redox properties we prepared also
the nondendronized analogues of M-1 and M-2. For this
purpose, the metal-free dyad H2-330 was used as starting material
for the synthesis of Fe(III)Cl-3 and Mn(III)Cl-3 (Scheme 2).
The synthesis of M(II)-3 (M ) Cu, Ni), however, was carried
out by the initial metalation of the free base porphyrin H2-428
to give M(II)-4. The subsequent cyclopropanation of C60
afforded the dyads M(II)-3 (M ) Cu, Ni), which after
purification Via column chromatography on silica were obtained
in yields ranging from 37 to 46% (Scheme 3).
a (i) M ) Fe: FeCl2 ·4H2O, lutidine, EtOH, CHCl3, rf, 76%; M ) Mn:
MnCl2 ·4H2O, lutidine, EtOH, CHCl3, rf, 63%.
active molecules; see Table 1. Thus, we performed CV in
CH2Cl2 containing 0.1 M (Bu)4NBF4 as supporting electrolyte
and ferrocene as an internal standard. A glassy carbon working
electrode, a platinum counter electrode, and a silver reference
electrode were used. In addition, the potentials were verified
by means of differential pulse voltammetry.
The redox potential for the nondendronized Zn(II)-3 and
Co(II)-3 are taken from our previous work.30 The first oxidation
processes (Eox1) occur at 0.40 and 0.37 V for Zn(II)-1 and
Zn(II)-2, respectively, to yield [ZnIIP]•+. [ZnIIP]2+, on the other
hand is formed at 0.78 V (Eox2). In the region of reduction
As references for photophysical measurements, porphyrin-
metal complexes of Cu, Ni, Mn and Fe were required and
synthesized according to the aforementioned metal insertion
procedures using dyads M-1 and M-2. (Scheme 4).
Electrochemistry. In general, cyclic voltammetry (CV) pro-
vides valuable insight into mutual interaction between redox-
processes at -1.09 (Ered1), -1.49 (Ered2) and -2.04 V (Ered4
)
relate to the reduction of C60 to the corresponding radical anion,
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Redox-Active C60-Metalloporphyrin Conjugates
A R T I C L E S
Scheme 3. Synthesis of Nondendronized Dyads M(II)-3 via Precursor H2-4a
a (i) M ) Cu: Cu(OAc)2, MeOH, CH2Cl2, r.t., 78%; M ) Ni: Ni(acac)2, toluene, rf, 82%; (ii) C60, I2, DBU, toluene, r.t., 46% for M ) Cu and 37% for
M ) Ni.
Scheme 4. Synthesis of metalloporphyrin-precursors M-5a
a (i) M ) Cu: Cu(OAc)2, MeOH, CH2Cl2, r.t., 97%; M ) Ni: Ni(acac)2, toluene, rf, 95%; M ) Fe: FeCl2 ·4H2O, lutidine, EtOH, CHCl3, rf, 81%; M )
Mn: MnCl2 ·4H2O, lutidine, EtOH, CHCl3, rf, 85%.
dianion and trianion of C60, respectively. The reduction of the
porphyrin (Ered3) is discernible at -1.86 V for Zn(II)-1 and at
-1.88 V for Zn(II)-2.
2), Ered2 (-1.60 V for Ni(II)-3, -1.54 V for Ni(II)-1, and -1.55
V for Ni(II)-2), and Ered4 (-1.86 V for Ni(II)-1 and -1.97 for
Ni(II)-2).
Co(II)-1 and Co(II)-2 show three oxidation processes at Eox1
) 0.39 V, Eox2 ) 0.85 V, and Eox3 ) 1.04 V to lead to [CoIIIP],
[CoIIIP]2+, and [CoIIIP]3+. On the reductive side several redox
processes are noted. Whereas the C60 reduction involves
processes at Ered1 (-1.01 V for Co(II)-3, -0.98 V for Co(II)-
1, and -1.02 V for Co(II)-2), Ered3 (-1.57 V for Co(II)-3,
-1.46 V for Co(II)-1, and -1.48 V for Co(II)-2), and Ered5
(-1.78 V for Co(II)-3, -1.83 V for Co(II)-1, and Co(II)-2),
generating the C60 radical anion, C60 dianion and C60 trianion
the [CoIIP] reductions are detectable at Ered2 ) -1.33 V and at
Ered4 ) -1.70 V.30
In the case of Ni(II)-3, Ni(II)-1 and Ni(II)-2 the first
oxidation–at Eox1 ) 0.55 V for Ni(II)-3 and Eox1 ) 0.59 V for
Ni(II)-1 and Ni(II)-2–and second oxidation–at Eox2 ) 0.89 V
for Ni(II)-3 and Eox2 ) 0.91 V for Ni(II)-1 and Ni(II)-
2–generate [NiIIP]•+ and [NiIIP]2+, respectively. NiP reduction
The first anodic redox processes in Cu(II)-3, Cu(II)-1, and
Cu(II)-2 (Eox1 ) 0.49 V for Cu(II)-3 and about Eox1 ) 0.53 V
for Cu(II)-1 and Cu(II)-2) correspond to the formation of
[CuIIP]•+. In line with this, the second anodic redox process at
Eox2 ) 0.89 V for Cu(II)-3, Eox2 ) 0.93 V and Eox2 ) 0.99 V
for Cu(II)-1 and Cu(II)-2 is assigned to the second oxidation
to afford [CuIIP]2+. Formation of [CuIIP]•- is only observable
for Cu(II)-1 and Cu(II)-2 at around Ered3 ) -1.77 V.51-53 The
reductive processes involving C60 are detectable at Ered1 (-1.14
V for Cu(II)-3, -1.10 V for Cu(II)-1 and -1.06 V for Cu(II)-
2), Ered2 (-1.61 V for Cu(II)-3, -1.53 V for Cu(II)-1, and
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Commun. 2000, 373.
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Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 7090.
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F. Chem.sEur. J. 2000, 6, 1629.
at Ered3 ) -1.90 V for Ni(II)-3 and approximately at Ered3
)
-1.78 V for Ni(II)-1 and Ni(II)-2 generate [NiIIP]•-.50 The C60
reduction processes of the fullerene moiety are detectable at
Ered1 (-1.15 V for Ni(II)-3, -1.12 V for Ni(II)-1, and Ni(II)-
(37) Guldi, D. M.; Luo, C.; Da Ros, T.; Prato, M.; Dietel, E.; Hirsch, A.
Chem. Commun. 2000, 375.
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M.; Dietel, E.; Bauer, W.; Hirsch, A. J. Am. Chem. Soc. 2001, 123,
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9
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A R T I C L E S
Spa¨nig et al.
Table 1. Redox Potentials of Metalloporphyrin-Fullerene Dyads
the strongest shifts are seen between the nondendritic derivatives
and the first generation analogue. We find for the cobalt
analogues a shift for Eox3 of 80 mV for Co(II)-1 and Co(II)-2.
In Fe(III)Cl-1 and Fe(III)Cl-2 the Eox1 potentials shift about
50 mV to a more positive value relative to Fe(III)Cl-3. Cu(II)-1
and Cu(II)-2 reveal the most significant changes. In fact, Eox1
is shifted by 40 and 60 mV for Cu(II)-1 and Cu(II)-2,
respectively. Likewise, Ered1 and Ered2 are moved about 40 to
80 mV to less negative potentials. The reductive potentials of
Cu(II)-2 are shifted to less negative values and is therefore
showing a quite different behavior than the electron donor
acceptor conjugate discussed before, where the redox potential
of the second generation derivative was hardly shifted. The
nickel(II)- and manganese(III)-electron donor acceptor conjugate
show no significant change in the formal potential.
It is safe to conclude that the dendritic electron donor acceptor
conjugate are more difficult to be oxidized and easier to be
reduced when compared with the nondendronized analogues.
In our case the influence that the dendritic branches inflict is
moderate, because we have chosen a meta-substituted topology.
In line with previous work,59,60 we consider that the slight shifts
in the copper and iron electron donor acceptor conjugate relate
to the ability of these metal atoms to coordinate the dendron
and, in turn, to alter the axial ligation and the redox properties.
In the second generation derivatives, two opposing effects might
play a role. On one hand, the bulky dendrimer is coordinated
to the metal ions, affecting the redox behavior in the mentioned
manner. On the other hand, an electron-rich microenvironment
is created. This would impose changes on the redox potential
in the opposite direction, namely showing a hindered reduction
and an easier oxidation.
versus Fc+/Fca
Ered5 [V] Ered4 [V] Ered3 [V] Ered2 [V] Ered1 [V] Eox1 [V] Eox2 [V] Eox3 [V]
Zn(II)-330
Zn(II)-1
Zn(II)-2
Co(II)-330
Co(II)-1
Co(II)-2
Zn
Co
-
-
-2.05 -1.90 -1.51 -1.10 0.34 0.69
-2.04 -1.86 -1.45 -1.06 0.40 0.76
-2.04 -1.88 -1.49 -1.09 0.37 0.76
-
-
-
-
-1.78
-
-
-1.57 -1.01 0.40 0.82 0.96
-1.83 -1.70 -1.46 -1.33 -0.98 0.39 0.85 1.04
-1.83 -1.72 -1.48 -1.34 -1.02 0.38 0.85 1.03
Ni Ni(II)-3
Ni(II)-1
-
-
-
-
-
-
-
-1.90
-
-1.60 -1.15 0.55 0.89
-
-
-
-
-
-
-
-
-
-
-
-
-1.86 -1.77 -1.54 -1.12 0.59 0.91
-1.97 -1.79 -1.55 -1.12 0.59 0.91
Ni(II)-2
Cu Cu(II)-3
Cu(II)-1
-
-
-1.61 -1.14 0.49 0.89
-1.88 -1.79 -1.53 -1.10 0.53 0.93
-1.81 -1.76 -1.49 -1.06 0.55 0.99
Cu(II)-2
Fe Fe(III)Cl-3
-
-1.62 -1.12 -0.81 0.66
-
-
-
-
-
-
Fe(III)Cl-1 -1.98 -1.80 -1.51 -1.08 -0.78 0.71
Fe(III)Cl-2 -1.91 -1.82 -1.51 -1.10 -0.80 0.71
Mn Mn(III)Cl-3
-
-
-
-1.20 -0.75 0.72
Mn(III)Cl-1 -1.90 -1.63 -1.46 -1.21 -0.76 0.73
Mn(III)Cl-2 -1.91 -1.62 -1.45 -1.19 -0.78 0.70
a Electrochemical analysis was carried out in CH2Cl2 containing 0.1
M (Bu)4NBF4 as supporting electrolyte and ferrocene as an internal
standard with a glassy carbon working electrode, a platinum counter
electrode, and a silver reference electrode.
-1.49 V for Cu(II)-2), and Ered4 (-1.88 V for Cu(II)-1 and
-1.81 for Cu(II)-2).
The first oxidation of Fe(III)Cl-3 is at Eox1 ) 0.66 V, while
those of Fe(III)Cl-1 and Fe(III)Cl-2 are at Eox1 ) 0.71 V.
Although an exact assignment is rather difficult, ligand oxidation
to afford [FeIII(Cl)P]•+ seems more likely.54,55 In addition,
several reduction processes are detected including the one-
electron reduction of FeIII to FeII around -0.80 V and the one-
electron reduction of FeII to FeI at approximately Ered4 ) -1.81
V.56 Furthermore, we detected several reductions of C60 at Ered2
(-1.12 V for Fe(III)Cl-3, -1.08 V for Fe(III)Cl-1, and -1.10
V for Fe(III)Cl-2), Ered3 (-1.62 V for Fe(III)Cl-3, -1.51 V
for Fe(III)Cl-1 and Fe(III)Cl-2), and Ered5 (-1.89 V for
Fe(III)Cl-1 and -1.91 V for Fe(III)Cl-2).
The only oxidation of Mn(III)Cl-3, Mn(III)Cl-1, and
Mn(III)Cl-2 happens at Eox1 ) 0.72, 0.73, and 0.70 V,
respectively, to form most likely [MnIII(Cl)P]•+.57,58 On the
reductive side, reductions of Mn(III)Cl-3, Mn(III)Cl-1, and
Mn(III)Cl-2 are observed at Ered1 ) -0.76 V and Ered4 ) -1.63
V. Once again the electrochemistry of C60 is apparent in the
form of three reduction processes at Ered2 (-1.20 V for
Mn(III)Cl-3, -1.21 V for Mn(III)Cl-1, and -1.19 V for
Mn(III)Cl-2), Ered3 (-1.46 V for Mn(III)Cl-1 and -1.45 V
for Mn(III)Cl-2) and Ered5 (-1.90 V for Mn(III)Cl-1 and -1.91
V for Mn(III)Cl-2).
Electronic Absorption Spectra. When comparing the ground-
state absorption features of the different metalloporphyrins in
the references and in the corresponding donor-acceptor con-
jugates notable differences were noted. Most significantly, new
absorptions develop in a range, where neither the different
metalloporphyrins nor C60 is known to absorb, namely beyond
700 nm.61-63 The fact that the corresponding maxima tend to
red-shift with increasing solvent polarity leads us to postulate
a redistribution of charge density in the ground state, that is,
from the electron-donating metalloporphyrins to the electron
accepting C60. Further evidence for this charge transfer hypoth-
esis came from appreciably shifted maxima, especially those
of the metalloporphrins. In accordance with the footnote listed
in Table 2 we derived the electronic coupling matrix elements
(V) for all of the electron donor-acceptor conjugates with values
that range from 275 to 520 cm-1
.
Fluorescence. Figure 1 shows the fluorescence spectra of H2-
5, H2-1, and H2-2, recorded in THF solutions with matching
absorption at the excitation wavelength of 392 nm and/or 512
nm. For H2P (H2-5), we note the well-known emission features,
including maxima at 650 and 716 nm, fluorescence quantum
yields of 0.1164,65 and fluorescence lifetimes of 9.8 ns.66 In H2-
It should be noted that as the dendrimer generation increases
small differences emerge between the formal potentials. Hereby,
(40) Liddell, P. A.; Sumida, J. P.; Macpherson, A. N.; Noss, L.; Seely,
G. R.; Clark, K. N.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem.
Photobiol. 1994, 60, 537.
(41) Guldi, D. M.; Nuber, B.; Bracher, P. J.; Alabi, C. A.; MacMahon, S.;
Kukol, J. W.; Wilson, S. R.; Schuster, D. I. J. Phys. Chem. A 2003,
107, 3215.
(48) Borovkov, V. V.; Lintuluoto, J. M.; Inoue, Y. Synlett 1999, 61.
(49) Buchler, J. W.; Eikelmann, G.; Puppe, L.; Rohbock, K.; Schneehage,
H. H.; Weck, D. Liebigs Ann. Chem. 1971, 745, 135.
(42) Guldi, D. M.; Zilbermann, I.; Gouloumis, A.; Vazquez, P.; Torres, T.
J. Phys. Chem. B 2004, 108, 18485.
(43) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,
Eds.; Academic Press: New York, 1999.
(50) Wolberg, A.; Manassen, J. Inorg. Chem. 1970, 9, 2365.
(51) Godziela, G. M.; Goff, H. M. J. Am. Chem. Soc. 1986, 108, 2237.
(52) Kadish, K. M.; Guo, N.; Van Caemelbecke, E.; Paolesse, R.; Monti,
D.; Tagliatesta, P. J. Porphyrins Phthalocyanines 1998, 2, 439.
(53) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982.
(54) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1981, 20, 1348.
(55) Phillippi, M. A.; Shimomura, E. T.; Goff, H. M. Inorg. Chem. 1981,
20, 1322.
(44) Fukuzumi, S.; Guldi, D. M. Electron Transfer Chem. 2001, 2, 270.
(45) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22.
(46) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata,
Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617.
(47) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
9
9382 J. AM. CHEM. SOC. VOL. 131, NO. 26, 2009

Redox-Active C60-Metalloporphyrin Conjugates
A R T I C L E S
Table 2. Electronic Coupling, Reorganization Energies, Charge
Separation, and Charge Recombination Dynamics in Different
Solvents and the Corresponding Driving Forces
c
Va [cm-1
]
λ [eV] τCS [ps] τCR [ps] -∆G0CS [eV] -∆G0CR [eV]
H2-1
PhMe
PhCl
THF
PhCN
DMF
150
130
60
40
10
2200
1500
860
380
115
0.14
0.21
0.24
0.27
0.28
1.75
1.68
1.65
1.62
1.61
300
0.29
0.29
H2-2
PhMe
PhCl
THF
PhCN
DMF
160
130
60
40
10
2200
1500
860
440
130
0.14
0.21
0.24
0.27
0.28
1.75
1.68
1.65
1.62
1.61
300
520
Zn(II)-1
Figure 1. Steady state fluorescence of H2-5 (black spectrum), H2-1 (red
spectrum), and H2-2 (red spectrum) excited at 512 nm with matching
absorption of the excitation wavelength in deaerated THF (OD512 ) 0.11).
PhMe
PhCl
THF
PhCN
DMF
5
4
2
1
1
370
210
134
40
0.44
0.51
0.54
0.57
0.58
1.56
1.49
1.46
1.43
1.42
25
quantum yield of 0.04 and a fluorescence lifetime of 2.4 nssin
excellent agreement with values found in the literature.32,64,67
The quenching factor, in both Zn(II)-1 and Zn(II)-2 (Figure
S1), was determined to be 597.
None of the MnP (Mn(III)Cl-5), FeP (Fe(III)Cl-5), CoP
(Co(II)-5), NiP (Ni(II)-5), and CuP (Cu(II)-5) nor the corre-
sponding donor-acceptor conjugates revealed appreciable por-
phyrin centered fluorescence. A likely rationale involves the
paramagnetic nature of the transition metals. To this end, a good
overlap between the d orbitals of the metal and the nitrogen
lone pairs of the porphyrins triggers ultrafast excited state
deactivations.30,68
Zn(II)-2
PhMe
PhCl
THF520
PhCN
DMF
5
4
2
1
1
375
210
140
40
0.44
0.51
0.54
0.57
0.58
1.56
1.49
1.46
1.43
1.42
25
Mn(III)Cl-1
THF
PhCN
290
290
0.61
0.61
1
1
53
46
0.06
0.10
0.35d
0.31d
Mn(III)Cl-2
PhMe
THF
PhCN
1
1
1
60
50
45
0.01
0.11
0.15
0.40d
0.30d
0.26d
Turning to the 750-850 nm range additional emission
features are seen with quantum yields in the order of 10-5-10-4.
These are mirror images of the charge transfer absorption and,
thus, relate to a charge transfer emission. In H2-1 and H2-2
solvent dependent maxima evolve at 805 nm, for example, in
THF at room temperature; see Figure S2. Interestingly, these
features evolve as well in those electron donor-acceptor
conjugates that bear transition metals.
Fe(III)Cl-1
1
PhCN
PhCN
365b
365b
0.55b
0.55b
15
15
0.17
0.17
0.23d
0.23d
Fe(III)Cl-2
1
Ni(II)-1
PhMe
THF
PhCN
8
6
3
115
80
0.38
0.48
0.52
0.28d
0.18d
0.14d
275
275
405
405
0.39
0.39
0.33
0.33
60
The strong charge transfer emissions imply, besides large
electronic coupling matrix elements, slow radiative decays and
Ni(II)-2
PhMe
THF
PhCN
8
6
3
115
80
0.38
0.48
0.52
0.28d
0.18d
0.14d
60
(56) Kadish, K. M.; D’Souza, F.; Van Caemelbecke, E.; Villard, A.; Lee,
J. D.; Tabard, A.; Guilard, R. Inorg. Chem. 1993, 32, 4179.
(57) Carnieri, N.; Harriman, A. Inorg. Chim. Acta 1982, 62, 103.
(58) Kelly, S. L.; Kadish, K. M. Inorg. Chem. 1982, 21, 3631.
(59) Giraudeau, A.; Callot, H. J.; Jordan, J.; Ezhar, I.; Gross, M. J. Am.
Chem. Soc. 1979, 101, 3857.
Cu(II)-1
PhMe
THF
PhCN
1
1
1
>3000
1600
1200
0.40
0.50
0.54
1.73
1.63
1.59
Cu(II)-2
(60) Kadish, K. M.; Morrison, M. M.; Constant, L. A.; Dickens, L.; Davis,
D. G. J. Am. Chem. Soc. 1976, 98, 8387.
PhMe
THF
PhCN
1
1
1
>3000
1600
1200
0.40
0.50
0.54
1.73
1.63
1.59
(61) Tkachenko, N. V.; Lemmetyinen, H.; Sonoda, J.; Ohkubo, K.; Sato,
T.; Imahori, H.; Fukuzumi, S. J. Phys. Chem. A 2003, 107, 8834.
(62) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori,
H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124,
8067.
a The electronic coupling (V) was calculated via the following
equation out of the ground state absorption in THF. V ) (2.06 ×
10-2 ꢀ(εjmaxν˜maxν˜1/2))/(RC-C). RC-C is the center-to-center distance (6.16
Å), εmax is the extinction coefficient of the CT band, νmax is the energy,
and ν1/2 is the half width of the CT transition. As noted, the electronic
coupling is in all cases distinct. b Determined in THF. c Charge
recombination to the ground state. d Charge recombination to the lowest
metalloporphyrin tripmultiplet excited state.
(63) Imahori, H.; Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.; Lem-
metyinen, H.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105,
1750.
(64) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.;
Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem.
1996, 100, 15926.
(65) Lee, W. A.; Graetzel, M.; Kalyanasundaram, K. Chem. Phys. Lett.
1984, 107, 308.
1, and H2-2 the H2P fluorescence is quenched 470 times without,
however, affecting the overall fluorescence spectrum. The
quantum yields for H2-1 and H2-2 (2.3 × 10-4) correlates with
fluorescence lifetime of 0.25 ps, respectively. ZnP (Zn(II)-5),
exhibit fluorescence maxima at 600 and 660 nm, a fluorescence
(66) Tamaki, K.; Imahori, H.; Sakata, Y.; Nishimura, Y.; Yamazaki, I.
Chem. Commun. 1999, 625.
(67) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am.
Chem. Soc. 2000, 122, 6535.
(68) The Porphyrin Handbook; Kadish, K. M., van Camelbecke, E., Royal,
G., Eds.; Academic Press: San Diego, 2000; Vol. 8.
9
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A R T I C L E S
Spa¨nig et al.
small reorganization energies. In fact, the inner (λV ) (λabsorption
+ λemission)/2) and outer reorganization energies (λS ) -λemission
- ∆G0CR) were derived with the help of the charge transfer
absorption (λabsorption) and emission maxima (λemission). These led
to total reorganization energies that range from 0.29 eV (H2-1)
to 0.61 eV (Mn(III)-1)ssee Table 2.
Transient Absorption Measurements. To characterize the
transients formed upon photoexcitation and upon photodeacti-
vation of either the porphyrins (420 nm) or C60 (387 nm), we
carried out transient absorption measurements. They were
typically performed following femtosecond laser excitation (i.e.,
387 or 420 nm) in solvents of different polarity (i.e., toluene,
THF, benzonitrile, and DMF).
The visible range of H2-5 is in the excited state dominated
by intense transitions. Minima as a complement to the ground
state absorption evolve at 516, 549, 590, and 648 nm, while
maxima are seen at 450, 532, 567, 620, and 674 nm.67,69 Further
out in the red a broad, but nevertheless weak, transient
maximizes at around 1050 nm. Within the first few picoseconds
(i.e., ∼2 ps), higher lying excited states relax to the lowest
vibrational state of the first singlet excited state of H2P.
Commencing with this internal conversion, an intersystem
crossing starts to set in, which governs the photophysics of H2-5
over the course of ∼10 ns. The accordingly formed triplet
excited state of H2-5 reveals a characteristic peak around 780
nm. From here it is only the oxygen sensitive triplet excited
state that is generated, for which in the absence of molecular
oxygen a lifetime of 380 µs was determined.
When turning to H2-1 or H2-2 the visible regions are
dominated by H2P centered transitions, while the excited state
properties of C60 are observed in the near-infrared region. With
a delay of 2 ps all of these features (i.e., H2P and C60) transform
into the signatures of the C60 radical anion and the H2P radical
cation with characteristic bands at 900 nm and between 550
and 850 nm, respectively. The C60 radical anion features of
mono- and bis-adducts have been elucidated in great details and
appear for the trans-2 adduct with a maximum at 900 nm.70
The H2P radical cation, on the other hand, needed independent
characterization by means of pulse radiolysis, where the
corresponding species were obtained upon one electron oxidation
in either oxygenated or deoxygenated solutions of dichlo-
romethane. As a matter of fact, the radiolytic signature (i.e.,
transient maximum between 550 and 850 nm) closely resembles
that seen in the current photolytic experiments run with H2-1
or H2-2. With this spectroscopic confirmation in hands we
analyzed the time absorption profiles in terms of charge
separation and charge recombination kinetics. The lifetimes (see
Figure 2) are in THF 60 (i.e., charge separation) and 860 ps
(i.e., charge recombination). By increasing (i.e., benzonitrile)
or decreasing (i.e., toluene) the solvent polarity the driving forces
for charge separation and charge recombination change. This
affects the lifetimes in benzonitrile with values of 40 and 380
ps, while in toluene the lifetimes are 150 and 2100 ps.
Figure 2. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm, 100 nJ) of H2-2
(∼10-6 M) in argon saturated THF with several time delays between 0 and
3000 ps at room temperature; see legend for time evolution. (Bottom) Time
absorption profiles at 515, 630, and 900 nm of the spectra shown above,
monitoring the charge separation and charge recombination.
0.51 eV lower-lying triplet excited state.32,67,71 An intersystem
crossing with an efficiency of 0.88 powers this conversion. The
correspondingly formed triplet, with a maximum at 860 nm, is
oxygen sensitive and typically lasts for tens of microseconds
(i.e., 44 µs).
At early times the transients formed upon exciting Zn(II)-1
and Zn(II)-2 are practically identical to those seen in the case
of ZnP (Zn(II)-5); see Figure 3. After a time delay of 1 ps a
new broad transition starts to develop. The latter include maxima
in the visible range between 580 and 800 nm and in the near-
infrared range at 900 nm. A spectral comparison with previous
pulse radiolytic investigations helps to assign the set of visible
maxima to the ZnP radical cation,32,67 while the 900 nm band,
as already seen, is ascribed to the C60 radical anion.70 In THF,
charge separation and charge recombination take place with 2
and 140 ps, respectively. However, the lack of notable ZnP
triplet excited state formation suggests that the charge separated
states recombine directly to the ground state. Using toluene
resulted in longer lifetimes (i.e., 375 ps), while in benzonitrile
shorter lifetimes (i.e., 80 ps) were determined for the charge
separated state. Charge separation is similarly affected with 5
ps in toluene and 1 ps in benzonitrile.
When turning to ZnP (Zn(II)-5), the following features form
instantaneously. A minimum, due to Q-band bleaching, is
generated at 550 nm followed by maxima in the visible region
at 500 nm, as well as between 570 and 740 nm and in the near-
infrared region between 900 and 1000 nm. Within 2.4 ns, these
singlet excited state features transform into those of the
Next, we turned to MnP (Mn(III)Cl-5) and Mn(III)Cl-1/
Mn(III)Cl-2. Important is the nature of the ground state, 5S0.72
(69) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111,
6500.
(71) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito,
O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607.
(72) Yan, X.; Kirmaier, C.; Holten, D. Inorg. Chem. 1986, 25, 4774.
(70) Guldi, D. M.; Asmus, K.-D. J. Phys. Chem. A 1997, 101, 1472.
9
9384 J. AM. CHEM. SOC. VOL. 131, NO. 26, 2009

Redox-Active C60-Metalloporphyrin Conjugates
A R T I C L E S
Figure 3. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm, 100 nJ) of Zn(II)-2
(∼10-6 M) in argon saturated THF with several time delays between 0 and
500 ps at room temperature; see legend for time evolution. (Bottom) Time
absorption profiles at 555, 640, and 900 nm of the spectra shown above,
monitoring the charge separation and charge recombination.
Figure 4. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm, 150 nJ) of Mn-
(III)Cl-2 (∼10-6 M) in argon saturated PhCN with several time delays
between 0 and 3 ns at room temperature; see legend for time evolution.
(Bottom) time absorption profiles at 480, 550, and 900 nm of the spectra
shown above, monitoring the charge separation and charge recombination.
Directly following the laser pulse (Figure S3) MnP reveals
minima at 570 and 620 nm as a kind of negative imprint of the
ground state absorption. In addition, maxima are seen from 430
to 550 nm and beyond 645 nm.72 Implicit is the rapid formation
of a tripquintet (5T1) excited state. The initially formed 5S1 state,
on the other hand, was not observable, since the S1 to T1
transfer happens essentially within the detection limit of our
instrumental resolution as a consequence of efficient coupling
to the tripquinted state.72 Within the time window of the next
30 ps two deactivation processes compete: the spin allowed
recombination to the ground state (5S0) and spin conversion to
the tripheptet excited state (7T1).73 The recombination to the
ground state is heavily favored by very good overlaps between
the porphyrin (π, π*) and the manganese (d, d*) orbitals,as well
as by interactions with several charge transfer states.72,73 7T1,
formed in minor amounts, decays within 70 ps to the ground
state.
Figure 4 documents that the features developing for Mn-
(III)Cl-1 and Mn(III)Cl-2 differ from those of Mn(III)Cl-5.
Particularly important is the red region with a broad transition
that extends from 650 nm all the way to 1200 nm. A closer
look discloses the signature of the C60 radical anion, again, at
880 nm,70 together with another maximum at 695 nm, which is
tentatively assigned to the MnP radical cation (Figure S4). The
small blue-shift is likely to result from interactions with the
manganese. In the blue region, on the other hand, the imprint
of the ground state absorption is noticeable. Overall, a good
spectral resemblance with the features seen during the pulse
radiolysis study, that is, the one-electron oxidation and one-
electron reduction of MnP and C60, respectively, confirms the
charge transfer mechanism. Charge recombination populates the
triplet excited state and takes place within 50 ps in THF. On
the other hand, varying the solvent polarity from toluene to
benzonitrile led to a trend that tracks those already discussed
for H2-1, H2-2, Zn(II)-1, and Zn(II)-2. In brief, the lifetimes
decrease from 60 (i.e., toluene) to 45 ps (i.e., benzonitrile).
The transient absorption features of FeP (Fe(III)Cl-5) (Figure
S5) include in the visible region maxima at >490, 540, and 632
nm and ground state bleaching at 570 nm.74 In the near-infrared
region of the spectrum a maximum around 900 nm is discernible
directly after the laser excitation. Please note the resemblance
of this feature with the triplet markers in general. The
instantaneously formed 6S1 decays, like in Mn(III)Cl-5, rapidly
to the triphexet (6T1) state, again facilitated by an orbital and
spin allowed process.74 Following the formation of 6T1, recom-
bination to the ground state and internal conversion to other
tripmultiplets compete in the next 6 ps. Once more, the good
overlap between porphyrin (π, π*) and metal (d, d*) orbitals
together with possible charge transfer states lead to an intrinsi-
cally fast deactivation of the triphexet. Afterward, the tripoctet
(8T1) excited state follows a relaxation pathway to generate the
ground state within 25 ps.74
5
5
(74) Cornelius, P. A.; Steele, A. W.; Chernoff, D. A.; Hochstrasser, R. M.
(73) Harriman, A. J. Chem. Soc., Faraday Trans. 1 1981, 77, 369.
Chem. Phys. Lett. 1981, 82, 9.
9
J. AM. CHEM. SOC. VOL. 131, NO. 26, 2009 9385

A R T I C L E S
Spa¨nig et al.
Figure 5. (Top) differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm, 150 nJ) of Fe(III)Cl-2
(∼10-6 M) in argon saturated benzonitrile with several time delays between
0 and 50 ps at room temperature; see legend for time evolution. (Bottom)
Time absorption profiles at 470, 513, and 900 nm of the spectra shown
above, monitoring the charge separation and charge recombination.
Figure 6. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm, 100 nJ) of Ni(II)-2
(∼10-6 M) in argon saturated THF with several time delays between 0 and
500 ps at room temperature; see legend for time evolution. (Bottom) time
absorption profiles at 527, 676, and 907 nm of the spectra shown above,
monitoring the charge separation and charge recombination.
In Fe(III)Cl-1 and Fe(III)Cl-2 (Figure 5), we note that
essentially commencing with the excitation the aforementioned
excited state features transform into a new transient. The spectral
features of this newly formed transient, namely a broadband
red-shifted with regard to the long wavelength absorption (i.e.,
600-850 nm), are similar to the radiolytically formed FeP
radical cation, which is shown in Figure S6. This suggests a
rapid intramolecular charge transfer to oxidize FeP. Detecting
around 880 nm the fingerprint of the C60 radical anion70
completes also in this case the characterization of the charge
separated state. Again, the small blue-shift, relative to Zn(II)-
2, is likely to result from interactions with the metal center.
The charge separated states recombines within 15 ps to afford
the triplet excited state rather than the ground state.
The transient absorption for NiP (Ni(II)-5) at around 1 ps
shows excited state bleaching at 520 nm and excited state
absorption in the 430-500 nm region and above 540 nm (Figure
S7). Subsequently, the transient feature narrow and blue shift
within the next 20 ps and finally deactivate to the ground state
within 200 ps. The individually resolved steps are formation of
a singlet excited state, very fast intersystem crossing to form a
hot T1 state, a vibrational relaxation to the relaxed T1 excited
state and deactivation to the ground state.69,75
for this assignment came from pulse radiolytic investigations
(Figure S8) which resulted in a differential absorption maximum
that ranges from 540 to 730 nm. Kinetically, 6 and 80 ps were
determined for the charge separation and charge recombination
in THF. In line with the Marcus inverted behavior, charge
recombination takes values of 115 ps in toluene (a stabilization)
and 60 ps in benzonitrile (a destabilization). Charge separation,
on the other hand, slows down in toluene and accelerates in
benzonitrile.
CuP (Cu(II)-5) is different: Upon photoexcitation of the
ground (2S0) state, triplet excited state features grow in
instantaneously giving rise to excited state bleaching at 540 nm
and excited state absorption from 430 to 530 nm followed by
a maximum at 825 nm.41,69 This is documented in Figure S9.
In particular, the latter one is a known characteristic for triplets
of many porphyrins (i.e., free base porphyrins and metallopor-
phyrins). In fact it is a tripdoublet (2T2) state that is formed
rapidly, in very high quantum yields of about 88% ( 2%76,
from a singdoublet precursor, since the underlying conversion
is orbital and spin allowed. Now 2T2 deactivates via a series of
competing channels: these include internal conversion yielding
2T1, intersystem crossing to the tripquartet (4T2 or 4T1) state or
In Figure 6 the same singlet excited state features, as they
were seen for Ni(II)-5, develop directly after the laser excitation
of Ni(II)-1 and Ni(II)-2. Now, the very fast intersystem crossing
(20 ps) competes with the charge separation. In fact, after about
2 ps features of the NiP radical cation and of the C60 radical
anion70 were seen at 650 and 900 nm, respectively. Confirmation
(75) Zhang, X.; Wasinger, E. C.; Muresan, A. Z.; Attenkofer, K.; Jennings,
G.; Lindsey, J. S.; Chen, L. X. J. Phys. Chem. A 2007, 111, 11736.
(76) Pineiro, M.; Carvalho, A. L.; Pereira, M. M.; Gonsalves, A. M.; d,
A. R.; Arnaut, L. G.; Formosinho, S. J. Chem.sEur. J. 1998, 4, 2299.
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Redox-Active C60-Metalloporphyrin Conjugates
A R T I C L E S
evolve from the C60 singlet excited state (1.79 eV).77 It is also
notable that the two singlet excited states may interact via
transduction of singlet excited state energy. The correspondingly
formed charge separated state recombines competitively to the
lower lying H2P triplet excited state (1.4 eV)67 or to the ground
state. Evidence for a possible C60 triplet excited state (1.5
eV)70,78 formation was not observed during our measurements.
In Zn(II)-1 and Zn(II)-2 charge separation that commences
from the instantaneously formed ZnP singlet excited state (2.04
eV)32,67 is strongly exothermic (0.76 eV). Such a driving force
places the underlying charge transfer into the top region of the
Marcus parabola, where it would be activationless. Even a
charge separation that would commence with the C60 singlet
excited state (1.79 eV) is still fairly exothermic with 0.51 eV.
The charge recombination takes an interesting twist. In polar
media the only exothermic pathway is the direct recovery of
the singlet ground state. On the other hand, in nonpolar media,
formation of the ZnP triplet excited state is thermodynamically
within reach.
In Mn(III)Cl-1 and Mn(III)Cl-2 the deactivation upon
photoexcitation follows closely that of Mn(III)Cl-5. The initially
formed singquinted excited state 1*S5 (2.01 eV)79 decays within
0.1-0.5 ps, mainly via internal conversion to the ground state.80
The other competing processes are: The deactivation via the
charge separated state at 1.9 eV in Mn(III)Cl-1 and Mn(III)Cl-
2, or the spin allowed intersystem crossing to the tripquinted
excited state (1*T5)(1.80 eV), or the spin forbidden intersystem
crossing to the triphepted excited state (1*T7)(1.60 eV).73,79,81,82
Considering the weak driving force for charge separation of
0.1 eV in THF, charge separation occurs only in low yields.
Notably, even the use of more polar media fails in improving
the situation, 0.15 eV in benzonitrile. The dominant deactivation
is the internal conversion like in Mn(III)Cl-5.
Figure 7. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm, 100 nJ) of Cu(II)-2
(∼10-6 M) in argon saturated THF with several time delays between 0 and
500 ps at room temperature; see legend for time evolution. (Bottom) Time
absorption profiles at 542, 650, and 900 nm of the spectra shown above,
monitoring the charge separation and charge recombination.
The energetics of Fe(III)Cl-1 and Fe(III)Cl-2 bear close
resemblance to what has been seen for Mn(III)Cl-1 and
Mn(III)Cl-2. The initially formed singhexet excited state (1*S6),
located at 1.94 eV79, deactivates in toluene to the at 1.78 eV73
located triphexet excited state (1*T6) in less than 1 ps. Overall,
such an acceleration is based on strong orbital interactions
(d-d*, d-d, CT, π-π*) and overlaps. The triphexet excited
state (1*T6) recovers either directly or via the 0.24 eV73,79 lower
lying tripocted excited state to the ground state. In benzonitrile,
the charge separated state is located at 1.77 eV and, in turn,
0.01 eV below the triphexet excited state. In fact, charge
separation competes in Fe(III)Cl-1 and Fe(III)Cl-2 with the
formation of the triphexet excited state. Charge recombination,
on the other hand, to the tripocted excited state is located with
0.23 eV in the Marcus normal region.
deactivation to 2S0, which is the ground state.41,69 Notable, these
deactivation pathways are solvent dependent, 545 ps in ben-
zonitrile, 250 ps in THF and >3 ns in non-coordinating solvents.
Coordinating solvents affect the orbital energies of, for example,
the charge transfer state and tend to quench the triplet excited
states.41
In stark contrast to the CuP reference, Cu(II)-1 and Cu(II)-2
are both subject to very fast charge separation processes in THF
and benzonitrile (Figure 7). The time window during which the
charge separation occurs is about 1 ps. While in THF a weak
transient for a competing triplet excited state was observable
in addition to the charge separated state, in benzonitrile just
the signature of the charge separated state with maxima at 650
and 900 nm occurred. Pulse radiolytic oxidation, conducted with
CuP (Figure S10), further confirmed these characteristics, that
is, a transient maximum between 550 and 800 nm. Lifetimes
of the charge separated state were 1.6 and 1.2 ns in THF and
benzonitrile, respectively. In THF, the minor formed triplet
excited states deactivate within 160 ps. In toluene, on the other
hand, the lack of evidence for any charge transfer reaction
suggests that this deactivation channel is shut down for Cu(II)-1
and Cu(II)-2.
In Ni(II)-5 the initially formed singlet excited state 2*S (3.11
eV)83 decays ultrafast79,83,84 to form a hot triplet excited state
3(d, d). In the next step an intermolecular redistribution of energy
(77) Guldi, D. M.; Hauke, F.; Hirsch, A. Res. Chem. Intermed. 2002, 28,
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1543.
Energetics. The H2P singlet excited state is located in H2-5,
H2-1, and H2-2 at 1.89 eV.67 Once formed, several different
deactivation channels are feasible. One of them is charge
separation, at least for H2-1 and H2-2, to yield the corresponding
charge-separated state (1.53 eV). The latter may, in fact, also
(79) Brookfield, R. L.; Ellul, H.; Harriman, A. J. Chem. Soc., Faraday
Trans. 2 1985, 81, 1837.
(83) Patchkovskii, S.; Kozlowski, P. M.; Zgierski, M. Z. J. Chem. Phys.
2004, 121, 1317.
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G.; Muresan, A. Z.; Lindsey, J. S. J. Am. Chem. Soc. 2007, 129, 9616.
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A R T I C L E S
Spa¨nig et al.
through vibrational cooling results in a relaxed triplet excited
state, which is followed by ground-state recovery.75 In Ni(II)-1
and Ni(II)-2, the first step is assigned to a very fast charge
separation, which benefits from a good orbital overlap. The
charge separated state, located at 1.72 eV in THF, recombines
afterward to the relaxed triplet excited state. The driving force
for the charge recombination to the triplet excited state, which
should be located between 1.5 and 1.6 eV (0.7 eV below the
singlet excited state83), is about 0.18 eV.
Before examining the deactivation pathways that occur in
Cu(II)-1 and Cu(II)-2, the processes taking place in Cu(II)-5
have to be considered briefly. The instantaneously formed
singdoublet excited state, located around 2.13 eV,73,79,85 decays
rapidly in Cu(II)-5 in less than 350 fs69,86,87 to the tripdoublet
excited state which is located between 1.70 and 1.64 eV (1.70,76
1.66,73 1.64 eV for tripdouble-tripquartet equilibrium).41 Next,
the triplet and CT states equilibrate via a spin-forbidden
intersystem crossing to the 0.07 eV lower lying tripquartet
excited state (1.59 eV).73,79 The tripquartet excited state
deactivates afterward to the singdoublet ground state. In
Cu(II)-1 and Cu(II)-2, the charge separated state is located
marginally below that of the tripdoublet excited state at 1.63
eV. Nevertheless, strong interactions as they prevail between
the porphyrin (π-π*) orbitals and the copper (d-d*) orbitals
accelerate the charge separation. Taking the driving force for
the charge recombination to the tripdoublet excited state into
consideration, we estimate values of around 0.04 eV. In contrast,
recovery of the ground state is strongly exergonic and, thus,
placed deeply in the Marcus inverted region. Important is that
shifting the energy of the charge separated state–by, for instance,
varying the solvent polarity–above the tripdoublet state (toluene)
or below the tripquarted state (PhCN), charge separation remains
to be fast. In all of the cases the charge separated state
recombines to the ground state as it is reflected by the
experimental data. In other words, the driving force for the
charge separation is located in the top region of the Marcus
parabola, while the driving force for the charge recombination
is deeply in the Marcus inverted region.
Figure 8. Driving force (-∆G0ET) dependence of the rate constant for
charge separation (open circles) and charge recombination (full circles) in
Mn(III)Cl-1, Mn(III)Cl-2 (blue symbols), Fe(III)Cl-1, Fe(III)Cl-2 (orange
symbols), Co(II)-3 (grey symbols),30 Ni(II)-1, Ni(II)-2 (green symbols),
Cu(II)-1, Cu(II)-2 (red symbols), Zn(II)-1, Zn(II)-2 (brown symbols), H2-
1, and H2-2 (black symbols).
fast deactivation of the excited states in these metalloporphyrins.
The lifetimes of the rapidly and efficiently generated radical
ion pair states were found to depend on (i) the solvent polarity
and (ii) the metal species. To this end, they are as short as 15
ps when a tripmultiplet excited state is formed and reach to
values that are beyond 3000 ps when the ground state is
regenerated. Most important is, however, that a correlation
between the charge transfer rate (i.e., charge separation and
charge recombination) and the free energy change for the
underlying reaction reveals a parabolic dependence, as illustrated
in Figure 8, with parameters of the reorganization energy (0.84
eV) and electronic coupling (70 cm-1) closely resembling those
seen for the zinc(II)/free-base analogues studied in recent work
and those derived from the charge transfer absorptions/charge
transfer emissions. This particular finding is of a great value,
since it assists in establishing the Marcus relationship not only
by varying the solvent polarity but also by changing the metal
species. In other words it overcomes the drawbacks as they stem
from largely different outer reorganization energies (i.e., solvent
contributions) when screening toluene, orthodichlorobenzene,
benzonitrile, etc., while emphasizing the role of the internal
reorganization energies (i.e., structural contributions) in C60-
based electron donor-acceptor conjugates/hybrids.
Conclusion
In summary, we report here on the synthesis and charge
transfer features of an entire new family of redox-active electron
donor-acceptor architectures. In particular, we have developed
dendronized metal porphyrins attached to C60 via a trans-2
addition pattern by choosing Newkome-type dendrons of first
and second generations bound in the meta positions of the two
free phenyl rings. Most importantly, we have succeeded for the
first time in probing metalloporphyrins bearing manganese(III),
iron(III), nickel(II), and copper(II). A key feature is the close
distance, π-π stacking, that separates the excited state electron
donor from the electron acceptor. In fact, this π-π stacking
motif emerged as a powerful means to overcome the intrinsically
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft through SFB583.
Supporting Information Available: Steady-state fluorescence
spectra of Zn(II)-5, Zn(II)-1, and Zn(II)-2; charge transfer
absorption and emission spectra of H2-1 and Cu(II)-1; dif-
ferential absorption spectra, time absorption profiles, pulse
radiolytic oxidation of the metalloporphyrin references Mn-
(III)Cl-5, Fe(III)Cl-5, Ni(II)-5, and Cu(II)-5; and experimental
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
(85) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1.
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Chem. Soc. 1997, 119, 12578.
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