Design, synthesis, and photophysical studies of a porphyrin-fullerene dyad with parachute topology; charge recombination in the marcus inverted region

Article abstract of DOI:10.1021/ja038676s

As part of a continuing investigation of the topological control of intramolecular electron transfer (ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has been synthesized. A symmetrical strap, attached to or

Full text of DOI:10.1021/ja038676s

Published on Web 05/25/2004
Design, Synthesis, and Photophysical Studies of a
Porphyrin-Fullerene Dyad with Parachute Topology; Charge
Recombination in the Marcus Inverted Region
David I. Schuster,*, Peng Cheng, Peter D. Jarowski, Dirk M. Guldi,*
†
†
†
,‡
‡
§
§
|
Chuping Luo, Luis Echegoyen, Soomi Pyo, Alfred R. Holzwarth,
|
|
|
Silvia E. Braslavsky, Ren e´ M. Williams, and Gudrun Klihm
Contribution from the Department of Chemistry, New York UniVersity,
New York, New York 10003, and Radiation Laboratory, UniVersity of Notre Dame,
Notre Dame, Indiana 46556, Department of Chemistry, Clemson UniVersity,
Clemson, South Carolina 29634, and Max Planck Institute for Bioinorganic Chemistry
(formerly Strahlenchemie), D 45413 M u¨ lheim, Germany
Received September 23, 2003; E-mail: david.schuster@nyu.edu; guldi.1@nd.edu
Abstract: As part of a continuing investigation of the topological control of intramolecular electron transfer
(ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has
been synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing meso
positions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structures
1
13
3
were confirmed by H, C, and He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containing
dyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectra
of the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating that
there is no significant ground-state electronic interaction between the component chromophores. Molecular
modeling studies reveal that the lowest energy conformation of the dyad is not the C2v symmetrical structure,
but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two
π-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMR
data, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swinging
back and forth from one side of the fullerene to the other. The extensive fluorescence quenching in both
the free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process,
11
-1
kET ≈ 10
s , generating porphyrin radical cations and C60 radical anions, detected by transient absorption
spectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitude
in these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs in
the Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases.
Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formed
only with the free base dyad in toluene, where triplet formation from the charge-separated state is competitive
with decay to the ground state. The photophysical properties of the P-C60 dyads with parachute topology
are very similar to those of structurally related rigid π-stacked P-C60 dyads, with the exception that there
is no detectable charge-transfer absorption in the parachute systems, attributed to their conformational
flexibility. It is concluded that charge separation in these hybrid systems occurs through space in
unsymmetrical conformations, where the center-to-center distance between the component π-systems is
minimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systems
-
1
between 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm
.
Introduction
a significant period of time. In bacterial systems, which are most
2,3
thoroughly understood, photoinduced electron transfer (PET)
occurs in a photosynthetic reaction center, in which the relevant
small organic molecules, namely four bacteriochlorophylls, two
Artificial photosynthesis has been the impetus behind much
research since the earliest days of modern photochemistry. The
heart of photosynthesis is the absorption of sunlight and the
subsequent conversion of that excitation energy into chemical
potential energy by inducing charge separation that persists for
1
(
2) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.;
Academic Press: New York, 1993.
(
3) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40.
(b) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K., Smith,
K. M., Guilard, R., Eds.; Academic Press: New York, 1999; pp 153-190.
†
New York University.
Radiation Laboratory.
Clemson University.
Max Planck Institute.
‡
(
c) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (d) Balzani, V.; Moggi,
§
L.; F. Scandola, F. In Supramolecular Photochemistry; Balzani, V., Ed.;
Dordrecht, 1987; pp 1-28. (e) Gust, D.; Moore, T. A.; Moore, A. L. Acc.
Chem. Res. 1993, 26, 198. (f) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163.
|
(
1) Ciamician, G. Science 1912, 36, 385.
10.1021/ja038676s CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 7257-7270
9
7257

A R T I C L E S
Schuster et al.
bacteriopheophytins, two quinones, and one carotenoid polyene,
are embedded in a protein environment, which serves to
maximize the rate and efficiency of charge separation (CS) and
2
,3
minimize the possibilities of charge recombination (CR).
A
multistep electron transfer sequence along an energy gradient
generates a transmembrane CS state, with a quantum yield of
nearly unity, which preserves a significant fraction of the energy
of the initial excited state. Because the positive and negative
charges are separated by the thickness of the lipid bilayer, rapid
charge recombination, which would waste the stored energy as
heat, is precluded.
The simplest photosynthetic model systems involve interac-
tion in solution of a photochemically active pigment and an
additional electron donor (D) or acceptor (A), in which either
of the following reactions might occur
Figure 1. C60-porphyrin dyads 1 and 2.
electron donors. At the time this study was initiated some years
ago, the two dyads which had been studied most thoroughly
were 1 and 2, in their free base (M ) H2) and Zn-complexed
(M ) Zn) forms (see Figure 1). In both systems, the chro-
mophores are linked by bicyclic bridges created via a Diels-
Alder reaction. Many of the fundamental properties of P-C60
dyads were revealed in these seminal studies, which will be
8
,9
very briefly summarized here.
+
-
D* + A f D + A f D + A
Spectral data and molecular modeling on 1 and 1-Zn indicate
+
-
that there is some interaction between the two chromophores
D + A* f D + A f D + A
8
in their ground states. The lifetimes of the porphyrin excited
1
singlet states ( P*), from time-resolved fluorescence measure-
While much fascinating electron transfer (ET) chemistry has
been uncovered by such studies, these simple systems suffer
from several crucial problems. Since bimolecular reaction
kinetics in solutions are limited by diffusion, rate constants are
1
ments, are e7 ps in toluene. The substantial quenching of P*
1
is due to singlet-singlet energy transfer (EnT) from PZn* (2.1
1
1
eV) and P* (1.9 eV) to C60. The lifetime of PZn- C60* is ∼5
ps in toluene, while the singlet lifetime of a C60 model
compound lacking the porphyrin is 1.2 ns. This secondary
1
0
-1 -1
on the order of 10
of the lowest excited singlet states of typical pigments (typically
10 ns), and their generally poor solubility precludes efficient
M
s
at best. The very short lifetimes
•
+
•-
quenching is ascribed to ET to yield PZn -C60 with k ≈ 2 ×
<
1
1
-1
1
1
0
s . In toluene, P- C60* does not undergo ET due to
ET from taking place. As a result, efficient intermolecular PET
usually involves a triplet rather than a singlet excited state of
the donor or acceptor component. A further, and ultimately quite
serious, problem is that donor-acceptor (DA) separation and
orientation are not fixed in fluid solution.
An obvious approach to achieve efficient and rapid ET is to
covalently link donor and acceptor moieties, and indeed many
types of donor-spacer-acceptor compounds have been syn-
unfavorable thermodynamics. Instead, it decays by intersystem
3
crossing (IC) to P- C60*. In the more polar solvent benzonitrile,
1
•+
•-
P- C60* undergoes ET to give P -C60 with a rate constant
1
1
-1
of ∼2 × 10 s .
At about the same time, Imahori and co-workers studied
9
P-C60 dyads 2 and 2-Zn and obtained quite similar results. In
general, it has been found that the dominant decay mode of
photoexcited P-C60 dyads in nonpolar solvents (such as toluene
and benzene) is energy transfer (EnT) to give an energetically
lower-lying C60 excited singlet state, while in more polar
solvents such as THF and benzonitrile, rapid PET occurs to
5
thesized and studied. Early examples imitated photosynthetic
reaction centers by using porphyrins as the pigments and
covalently linked quinones as electron acceptors. Although such
6
PQ dyads do a good job of mimicking the gross features of the
photoinduced charge separation step of the photosynthetic
reaction center, back ET is typically quite fast, resulting in very
short-lived CS states.
Recently, there has been a great deal of interest in C60 as the
ultimate electron acceptor in multistep intramolecular PET
reactions. Several comprehensive reviews of the literature of
donor-C60 dyads and larger hybrids (triads, tetrads, pentads, etc)
incorporating a wide variety of electron donors and linkers have
been published.3 Not surprisingly, the greatest attention has
been given to model systems in which porphyrins (P) are the
•
+
•-
8-10
yield the P -C60 charge-separated state.
A critically
important characteristic of P-C60 dyads is that the C60 moiety
promotes photoinduced charge separation but retards the CR
process. This observation has been ascribed to the small
reorganization energy (λ) of C60 compared to λ for small electron
1
1,12
•-
acceptors such as quinones.
Since the charge in C60 is
spread over the whole C60 framework, the charge density is
reduced at each carbon, which in turn reduces the solvent
reorganization energy (λs). Since the rigid framework of C60
a,7
•-
remains essentially unperterbed in C60 , the internal reorganiza-
(
8) 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.
(
4) Diesenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol.
1
984, 180, 385. Yeates, T. O.; Korniya, H.; Chirino, A.; Rees, D. C.; Allen,
J. P.; Feher, G. Proc. Natl. Acad. Sci. U.S.A. 1988, 88, 7993.
(9) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada,
T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11 771.
(10) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore,
A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1997, 119, 1400.
Kuciauskas, D.; S. Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust,
D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. J. J. Phys. Chem. 1996, 100,
15926. Carbonera, D.; Di Valentin, M.; Corvaja, C.; Agostini, G.;
Giacometti, G.; Liddell, P. A.; Kuciauskas, D.; Moore, A. L.; Moore, T.
A.; Gust, D. J. Am. Chem. Soc. 1998, 120, 4398.
(11) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.; Okada,
T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545.
(12) For a review, see Guldi, D. M. In Fullerenes: From Synthesis to
Optoelectronic Properties; Guldi, D. M., Martin, N., Eds.; Kluwer
Academic Publishers: Dordrecht, 2002; pp 237-265.
(
5) Verhoeven, J. W.; Dirkxm L. P.; Deboer, T. J. Tetrahedron Lett. 1966,
4
399.
(
6) For a review, see Gust, D.; Moore, T. A. Top. Curr. Chem. 1991, 159,
02. Kong, J.; Loach, P. A. In Frontiers of Biological Energetics: From
1
Electrons to Tissues; Dutton, P. L., Leigh, J. S., Scarpa, H., Eds.; Academic
Press: New York, 1978; Vol. 1, 73. Tabushi, I.; Koga, N.; Yanagita, M.
Tetrahedron Lett. 1979, 257.
(7) (a) Martin, N.; S a´ nchez, L.; Illescas, B.; P e´ rez, I. Chem. ReV. 1998, 98,
2
527. Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519. Guldi, D.
M. Chem. Commun. 2000, 321. (b) Bracher, P. J.; Schuster, D. I. In
Fullerenes: From Synthesis to Optoelectronic Properties; Guldi, D. M.,
Martin, N., Ed.; Kluwer Academic Publishers: Dordrecht, 2002; pp 163-
2
12.
7258 J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004

Porphyrin-Fullerene Dyad with Parachute Topology
A R T I C L E S
tion energy (λi) is also small. Consequently, the CS rate is
positioned near the peak of the Marcus parabolic curve relating
kET to -∆G°ET, that is, near the point where -∆G°ET ) λ. Since
the thermodynamic driving force -∆G°CR in systems involving
13
fullerenes is >λ, CR is pushed into the Marcus inverted region,
which enhances the lifetime of the CS state. This makes donor-
C60 systems of particular interest as photosynthetic mimics and
as components of photovoltaic devices.
Design of Novel C60-Porphyrin Dyads
Figure 2. Face-to-face topology of porphyrin-C60 hybrids.
Both theory and experiment indicate that donor-acceptor
distance and relative orientation play a crucial role in controlling
the rate of PET, and consequently the efficiency of charge
separation (CS) and the lifetime of the CS state. For Gust and
Imahori’s systems,8 molecular modeling suggests that, when
structurally possible, the dyads adopt the face-to-face conforma-
tion shown below, due to π-π interaction between the two
chromophores. This phenomenon is now known to be general,
and is attributed to strong van der Waals attraction between
ultimately unsuccessful, as mixtures of isomeric C60 bis-adducts
were obtained which resisted separation by column chroma-
tography on silica or HPLC. At precisely this time, Diederich
and Hirsch independently reported the successful preparation
of two dyads if this type, namely, 3 and 4, shown below.1
,9
9,20
porphyrin and C60 moieties, in the solid state as well as in
solution.1
4,15
Such conformations facilitate through-space dia-
logue between the donor and the acceptor, as demonstrated by
efficient and rapid quenching of porphyrin fluorescence and
•
+
•-
generation of C60 excited states (by EnT) or PZn -C60 CS
states (by ET). To get more insight into the influence of
molecular topology on PET, we became interested in the design
and synthesis of P-C60 dyads in which the π-systems would
be structurally forced into a face-to-face arrangement.
To synthesize these compounds, tethers with terminal malonate
moieties were attached at meta positions of the 5,15-phenyl rings
of a tetraphenylporphyrin (TPP) precursor. Diederich’s dyad 3
1
9
was obtained as a trans-1 bis-adduct, and Hirsch’s dyad 4 as
2
0
a trans-2 bis-adduct, from two Bingel-Hirsch reactions. As a
result of these reports, we abandoned our plans to prepare dyads
of type A, and instead focused our attention on the synthesis
and photophysics of a parachute-shaped C60-porphyrin dyad of
type B, namely compound 5, shown below. Our efforts were
successful, in part due to experience gained in attempts to
Two types of compounds have such structural topology,
namely C60-porphyrin cyclophanes A and parachute-shaped
C60-porphyrin dyads B. These are depicted schematically in
Figure 2, where the circle represents a C60 moiety and the square
represents a porphyrin. The design of the C60-porphyrin cyclo-
phane A is based on the fact that C60 can undergo controlled
bis-addition using appropriate ligands (vide infra), whereas in
the parachute-shaped dyad B, a strap controls the relative
orientations of the C60 sphere and the porphyrin. Such com-
pounds can be prepared using the versatile Bingel-Hirsch
reaction, involving reaction of C60 with appropriate porphyrin-
linked malonates in the presence of CBr4 and DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene) as the base.1
21
prepare type A dyads.
Results
Synthesis of the Parachute-Shaped C60-Porphyrin Dyad
. The central concept in the synthesis of dyad 5 was to anchor
5
C60 to a malonate moiety inserted in a strapped porphyrin
6,17
Our efforts to prepare dyads of type A from strapped
1
8
porphyrins synthesized using Baldwin’s methodology were
(
13) Marcus, R. A. J. Chem. Phys. 1956, 24, 966.
(
14) Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.; Chaker,
L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am. Chem.
Soc. 1999, 121, 10 487. Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll,
B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc.
derivative. The sequence of reactions for synthesis of such a
porphyrin is shown in Scheme 1. Salicylaldehyde reacted with
3-bromopropanol in water to yield 2-(3-hydroxypropoxy)-
benzaldehyde 6 in 73% yield. Coupling of 6 to malonyl
dichloride gave bis-benzaldehyde 7 in 75% yield. Acid-catalyzed
1
999, 121, 7090.
(15) Schuster, D. I.; Jarowski, P. D.; Kirschner, A. N.; Wilson, S. R. J. Mater.
Chem. 2002, 12, 2041.
J. AM. CHEM. SOC.
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VOL. 126, NO. 23, 2004 7259

A R T I C L E S
Schuster et al.
Scheme 1. Scheme for Synthesis of Parachute Dyad 5
condensation of 7 with pyrrole 822 afforded strapped-bis-
solids with high melting points (>300 °C), and are quite soluble
in relatively nonpolar organic solvents such as CH2Cl2, CHCl3,
and toluene, but not in polar solvents such as methanol and
diethyl ether. After several washings with diethyl ether, the
purity of the dyads according to TLC analysis was sufficient
for structural characterization and photophysical studies.
Characterization of C -Porphyrin Dyad 5. The structure
(dipyrromethane) 9 in 65% yield. After debenzylation by
catalytic hydrogenolysis, the tetraacid 10 was reacted im-
mediately with trimethyl orthoformate in CH2Cl2, using trichlo-
roacetic acid as catalyst in the presence of zinc acetate, to give
the desired strapped-porphyrin 11 containing the central mal-
onate moiety in 15% yield. The crude reaction mixture of free-
base porphyrin 11 and the corresponding zinc complex 11-Zn
was quantitatively converted to 11-Zn by further treatment with
60
of the C -porphyrin dyad 5 was verified by spectral data,
6
0
1
13
3
including H NMR, C NMR, He NMR, UV-vis, and high-
1
1
Zn(OAc)2 in a mixed solvent of CH2Cl2 and MeOH. In H NMR
resolution mass spectra. As there are no protons on C , the H
60
spectra, the CH2 protons of the malonate moiety of 11 were
shifted upfield to δ 0.61 ppm compared to δ 3.30 in the
precursor 9, confirming that the strap resides over the aromatic
porphyrin ring system.
Finally, the strapped porphyrin malonate 11 was attached to
C60 via standard Bingel-Hirsch cyclopropanation, using CBr4
and DBU.17 Treatment of the reaction mixture with zinc acetate
in refluxing CH2Cl2/MeOH gave the zinc porphyrin-C60 dyad
NMR spectrum of 5 in CDCl exhibits the expected features of
the porphyrin and the strap with correct integration ratios. The
3
spectrum of 5 is very similar to that of the strapped porphyrin
11 except for the disappearance of the two protons (δ 0.61) on
the methylene carbon between the two carbonyl groups of the
1
3
malonate. The C NMR spectrum of 5 shows 9 resonances for
3
the sp carbon atoms, 24 of the 28 expected resonances in the
2
region δ 97.0-160.0 for the sp carbon atoms of the C60 moiety
5
-Zn, isolated as a dark-purple crystalline solid in 25% yield
and the porphyrin, as well as a resonance (δ 162.0) for the
malonate carbonyl groups (for details, see Supporting Informa-
tion). This spectrum clearly demonstrates that dyad 5 has
average C2V symmetry on the NMR time scale (see Discussion
below).
using preparative TLC (silica gel, CH2Cl2/MeOH 25:1). The
free-base dyad 5 was obtained by brief treatment of a CH2Cl2
solution of 5-Zn with 5% aqueous HCl, followed by neutraliza-
tion with aqueous NaHCO3. Both 5 and 5-Zn are crystalline
The existence of porphyrin and C60 moieties in dyad 5 was
confirmed by its UV-vis spectrum, where strong absorption
bands attributable to both moieties were observed. A high-
resolution MALDI-TOF mass spectrum shows a molecular peak
at m/z ) 1621.5 (calcd. 1621.8), confirming that all the pieces
are in place.
(
16) Bingel, C. Chem. Ber. 1993, 126, 1957. Hirsch, A. The Chemistry of the
Fullerenes; Georg Thieme Verlag: Stuttgart, 1994; Wilson, S. R., Schuster,
D. I., Nuber, B., Meier, M. S., Maggini, M., Prato, M., Taylor, R. In
Fullerenes: Chemistry, Physics and Technology; Kadish, K. M., Ruoff,
R. M., Ed.; Wiley-Interscience: New York, 2000; pp 147-156.
17) Camps, X.; Hirsch, A. J. Chem. Soc., Perkin Trans. 1 1997, 1595.
18) Baldwin, J. E.; Crossley, M. J.; Klose, T.; Orear, E. A.; Peters, M. K.
Tetrahedron 1982, 38, 27. Baldwin, J. E.; Cameron, J. H.; Crossley, M. J.;
Dagley, I. J.; Hall, S. R.; T. Klose, T. J. Chem. Soc., Dalton 1984, 1739.
Baldwin, J. E.; Perlmutter, P. Top. Curr. Chem. 1984, 121, 181.
19) Bourgeois, J. P.; Diederich, F.; Echegoyen, L.; Nierengarten, J.-F. HelV.
Chim. Acta 1998, 81, 1835.
(
(
3
Recently, He NMR, has been widely used in the character-
2
3
(
(
ization of fullerene compounds. Saunders, Cross, and co-
workers at Yale discovered that NMR-active 3He can be
20) Dietel, E.; Hirsch, A.; Eichhorn, E.; Rieker, A.; Hackbarth S.; Roder, B.
Chem. Commun. 1998, 1981.
(21) Cheng, P.; Wilson, S. R.; Schuster, D. I. Chem. Commun. 1999, 89.
22) Barton, D. H. R.; Kervagoret, J.; Zard, S. Z. Tetrahedron 1990, 46, 7587.
(23) Saunders, M.; Jim e´ nez-V a´ zquez, H. A.; Cross, R. J.; Mroczkowski, S.;
(
Freedberg, D. I.; Anet, F. A. I. Nature 1994, 367, 256.
7260 J. AM. CHEM. SOC.
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Porphyrin-Fullerene Dyad with Parachute Topology
A R T I C L E S
introduced into fullerene cages by heating them under high
pressures of 3He.24 3He trapped inside the fullerene cages
experiences a different magnetic environment from free He.
Table 1. E1/2 (V) Values for the Reduction Processes as an
Average of the Cathodic and Anodic Peak Potentialsa
3
compd
E1
E2
E3
E4
E5
b
d
Thus, a characteristic signal at -6.3 ppm is observed for
11
-2.01 (88)
b
3
3
11-Zn -2.18 (94)
He@C60 relative to gaseous He used as the internal standard,
c
-1.09(71) -1.45(76) -1.90b,e(103) -2.00b,e(128) -2.25 (66)
b
5
5
2
3
3
set to 0 ppm. In general, any product made from He-labeled
C60 yields a single sharp peak in its He NMR spectrum, since
-Znc -1.09(77) -1.45(76) -1.91(156) -2.28(83)
-2.41f
3
-1.96b(73)
13
b
altering the π-bonding structure of C60 affects the local magnetic
13-Zn -2.21 (105)
c
3
12
-1.06(80) -1.42(78) -1.88(136) -2.28(88)
field inside the cage and induces substantial He chemical shifts.
While typical [6,6] methanofullerenes such as Bingel adduct
a
+
b
All values are relative to a Fc/Fc internal reference. irreversible
3
24
c
d
1
2 (below) show a He NMR resonance at δ -8.1, dyad 5
processes. At low temperature (-35 °C). Values in parentheses are ∆Epp
(mV). e Overlapped peak. Peak potential from OSWV.
f
3
prepared from He@C60 shows a peak at δ -8.5 relative to
3
He gas (see Supporting Information). The upfield shift of 0.4
Table 2. E1/2 (V) Values for the Oxidation Processes as an
ppm is attributed to the shielding effect of the porphyrin ring
current.
Average of the Cathodic and Anodic Peak Potentialsa
HOMO−LUMO (∆E1/2)c
compd
E
1
E
2
1
1
0.27(80)b
0.08(53)
0.30(60)
0.13(56)
0.30(58)
0.06(68)
0.56*(116)
0.39*(54)
0.56(56)
2.28
2.26
2.30
2.41/2.54
2.26
2.28
1
5
5
1
1
1-Zn
e
-Zn
4
0.39(56)
d
0.49 (66)
d
4-Zn
0.37 (74)
a
b
All values are relative to a Fc/Fc+ internal reference. Values in
parentheses are DEpp (mV). For porphyrin subunits. d Irreversible pro-
c
cesses. e From OSWV data.
0.1 M tetra-n-butylammonium hexafluorophosphate as support-
ing electrolyte. These standard electrode potential data are
summarized in Tables 1 and 2, together with comparison data
for unstrapped porphyrin diols 13 and 13-Zn.
The cyclic voltammogram of free-base C60-porphyrin dyad
5
shows two reversible reductions with half-wave potentials
+
Confident that we had parachute-type porphyrin-C60 dyads
and 5-Zn in hand, these materials were subjected to a series
(E1/2) at -1.09 and -1.45 V (vs Fc/Fc ) based on the fullerene
core and two overlapping reductions (E1/2 ) -1.90 and -2.00
V), one corresponding to the fullerene and the other to the
porphyrin, respectively (see Figure S1, Supporting Information).
The reduction at -1.90 V is probably fullerene centered based
on the results observed for model compounds 11 and 12. The
CV of zinc dyad 5-Zn shows three reversible reductions at
5
of steady state and time-resolved photophysical studies.
Electrochemical and Photophysical Properties of the
C60-Porphyrin Dyads 5 and 5-Zn
The physical and photophysical properties of the topologically
+
novel dyads 5 and 5-Zn were investigated by using various
methods, including cyclic voltammetry, UV-vis absorption,
fluorescence emission, time-resolved fluorescence emission,
time-resolved transient absorption, and singlet oxygen measure-
ments. For comparison data, all measurements were also
performed on porphyrin model compounds 11 and 11-Zn as
well as C60 model compound 12. Some of these data were
reported in preliminary form in 1999.26
Electrochemical Studies. To estimate the energies of charge-
separated states formed by PET, the electrode potentials for
oxidation and reduction of the porphyrin and the functionalized
C60 moiety, respectively, were determined by cyclic voltammetry
-1.09, -1.45 and -1.91 V (vs Fc/Fc ) for the fullerene core.
Osteryoung Square Wave Voltammograms (OSWV) at low
temperature (-35 °C) allowed better assignments (Figure S2).
Compound 5 shows four reduction processes corresponding to
the fullerene moiety (the first three and the fifth) and one
reduction process (the fourth) corresponding to the porphyrin
th
th
moiety. Due to significant cathodic shifts of the 4 and 5
reduction processes for 5-Zn, it is not possible to state
definitively whether these reductions are centered on the
porphyrin moiety. Thus, while the OSWV observed for 5 is
close to the sum of those observed for 11 and 12, that for 5-Zn
is not the sum of those for 11-Zn and 12. The half-wave
potentials for the reductions of compounds 5 and 5-Zn corre-
sponding to the fullerene moieties are more negative than those
of the model compound 12 by ∼30 mV. The values of the
reduction potentials for 5 are close to those reported for trans-1
(CV). For the potentiometric measurements, a BAS 100 W
electrochemical analyzer (Bioanalytical System) was used (a 3
mm glassy carbon working electrode Ag/AgCl electrode as
reference electrode, calibrated against the ferrocene/ferrocinium
redox pair); the solvent used was dichloromethane containing
1
2
19
C60 bisadduct 14 (E
1/2
) -1.09 V and E
1/2
) -1.47 V).
The potentials for oxidation of 5, 5-Zn, 11, and 11-Zn are
listed in Table 2. Generally, the zinc porphyrin compounds (E1/2
(
24) Saunders, M.; Jim e´ nez-V a´ zquez, H. A.; Cross, R. J.; Mroczkowski, S.;
Gross, M. I.; Giblin, D. E.; Poreda, R. J. J. Am. Chem. Soc. 1994, 116,
2
193.
)
+0.08 V for 11-Zn and E1/2 ) +0.13 V for 5-Zn) are
(
25) Saunders, M.; Cross, R. J.; Jim e´ nez-V a´ zquez, H. A.; Shimshi, R.; Khong,
A. Science 1996, 271, 1693.
considerably easier to oxidize than the corresponding zinc-free
porphyrin compounds (E1/2 ) +0.27 V for 11 and E1/2 ) +0.30
V for 5) (see Figure S3). The first potential for oxidation of
(
26) Schuster, D. I.; Cheng, P.; Wilson, S. R.; Prokhorenko, V.; Katterle, M.;
Holzwarth, A. R.; Braslavsky, S. E.; Klihm, G.; Williams, R. M.; Luo, C.
P. J. Am. Chem. Soc. 1999, 121, 11 599.
J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004 7261

A R T I C L E S
Schuster et al.
Figure 3. Absorption spectra of dyad 5 (1) and model compounds 12 (2) and 11 (3) in toluene solution (left) and CH2Cl2 solution (right); all concentrations
-
6
were 5 × 10 M.
Figure 4. Absorption spectra of compounds 5 (1), 12 (2) and 11-Zn (3) in toluene solution (left) and CH2Cl2 solution (right); all concentrations were 5 ×
-
6
1
0
M.
+
1
1-Zn (+0.08 V vs. Fc/Fc ) is unusually low compared with
Steady-State Absorption Spectra The absorption spectra of
dyad 5 and 5-Zn in toluene (nonpolar solvent) and CH Cl (polar
solvent) are essentially a linear combination of the absorption
spectra of porphyrin reference compounds 11 and 11-Zn and
C60 reference compound 12 (Figures 3 and 4). In toluene, λmax
for the Soret band of the porphyrin in the free-base dyad 5 is at
that of zinc meso-tetraphenylporphyrin (+0.29 V) and zinc
octaethylporphyrin (+0.18 V).27 This could arise as a conse-
quence of an interaction between the carbonyl lone pairs and
the porphyrin ring, which would increase the electron density
in the latter. Structural effects sometimes result in easier
oxidation, as seen for ring distortions of short-chain basket-
handle porphyrins2 and those induced by steric interaction from
bromines at porphyrin â positions.29 To assess the role of steric
effects in the strapped porphyrin malonates 11 and 11-Zn, the
electrochemical data were compared with those for unstrapped
porphyrin-diols 13 and 13-Zn. No significant differences were
observed, indicating that the molecular conformation in 11 has
little effect on the oxidation processes. The two lowest potentials
for oxidation of compounds 5 and 5-Zn are ascribed to the
porphyrin moiety. The half-wave potential for the first oxidation
shifted to a more positive value than that of the model
compounds by ∼30 mV for 5 and ∼50 mV for 5-Zn.
2
2
4
14 nm, and the Q-bands appear at 510, 543, 581, and 633 nm
8
(Figure 3, left), whereas the zinc dyad 5-Zn features maxima
at 417, 543, and 579 nm (Figure 4, left). In more polar solvents
such as CH2Cl2, the absorption maxima of the dyads and the
model porphyrins are blue shifted by 1-5 nm relative to those
in toluene. These spectra suggest that there is no significant
electronic interaction between the porphyrin and fullerene
moieties in the ground state (however, see Discussion).
Steady-State Fluorescence Spectra. The fluorescence emis-
sion spectra of the dyads and model compounds were measured
in both toluene and CH2Cl2. Figure 5 shows the emission spectra
of dyad 5 and porphyrin reference 11 in toluene (top) and CH2-
Cl2 (bottom) solutions having equal absorbance at the excitation
wavelength (580 nm). Porphyrin 11 shows fluorescence maxima
at 635 and 698 nm with a fluorescence quantum yield ΦF of
(
27) Kadish, K. M.; Guo, N.; Caemelbecke, E. V.; Froiio, A.; Paolesse, R.;
Monti, D.; Tagliatesta, P.; Boschi, T.; Prodi, L.; Bolletta, F.; Zaccheroni,
N. Inorg. Chem. 1998, 37, 2358. Kadish, K. M.; Shiue, L. R. Inorg. Chem.
1
982, 21, 3623. Fuhrhop, J.-H.; Kadish, K. M.; Davis, D. G. J. Am. Chem.
Soc. 1973, 95, 5140.
0
.13, determined relative to that of 5,10,15,20-tetraphenylpor-
(
28) Kadish, K. M.; D’Souza, F.; Villard, A.; Autret, M.; Caemelbecke, E. V.;
Bianco, P.; Antonini, A.; Tagliatesta, P. Inorg. Chem. 1994, 33, 5169.
D’Souza, F.; Zandler, M. E.; Tagliatesta, P.; Ou, Z.; Shao, J.; Caemelbecke,
E. V.; Kadish, K. M. Inorg. Chem. 1998, 37, 4567.
29) Ravikanth, M.; Reddy, D.; Misra, A.; Chandrashekar, T. K. J. Chem. Soc.,
Dalton Trans. 1993, 1137. Reddy, D.; Ravikanth, M.; Chandrashekar, T.
K. J. Chem. Soc., Dalton Trans. 1993, 3575.
30
phin (TPP), ΦF ) 0.11. C60 model compound 12 shows
-4 31
typically weak fluorescence at 715 and 798 nm. (ΦF ≈ 10 ).
(
The porphyrin emission of dyad 5 is efficiently quenched by
the attached C60 in both toluene and CH2Cl2 by a factor of at
7262 J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004

Porphyrin-Fullerene Dyad with Parachute Topology
A R T I C L E S
Figure 5. Steady-state fluorescence emission spectra of dyad 5 (1) and porphyrin model compound 11 (2) in toluene solution (left) and CH2Cl2 solution
-
6
(
right) with excitation at 580 nm; concentrations were 5 × 10 M.
Table 3. Fluorescence Lifetimes of Dyads
fluorescence lifetimes τ
benzene
0.014 (0.904)
1
the lifetime of PZn* in dyad 5-Zn is shortened by a factor of
(ns)a,b
∼200 compared with that for the model zinc porphyrin 11-Zn
f
(1.8 ns).
compd
THF
Time-resolved fluorescence studies carried out in tetrahy-
5
0.023 (0.819)
0.08 (0.158)
0.013 (0.99)
11.3 (0.907)
1.8 (0.94)
0.057 (0.095)
drofuran (THF) gave quite similar results. For dyad 5, decay
components with lifetimes of 23 and 80 ps were observed, with
amplitudes of 0.82 and 0.16, respectively. The major component
is the porphyrin S1 state, whereas the latter is most likely the
fullerene S1 state. For 5-Zn, a single fluorescing species was
5
1
1
-Zn
1
1-Zn
0.009 (0.99)
13 (0.98)
1.81 (0.94)
a
λexc ) 550 nm, λobs ) 700 nm for 5 and 11, λobs ) 587 nm for 5-Zn
b
and 11-Zn. Numbers in parentheses are relative amplitudes.
1
detected with a lifetime of 13 ps, attributed to PZn*, more than
1
00-fold shorter than for model porphyrin 11-Zn.
least 70, whereas no emission from the fullerene moiety was
observed. Similarly, the fluorescence of 5-Zn is strongly
quenched by a factor of ∼100 compared to that of 11-Zn in
both toluene and CH2Cl2 (not shown).
Transient Absorption Studies. The studies at Notre Dame
were designed to detect nonemitting intermediates produced
upon photoexcitation of 5 and 5-Zn. Picosecond flash excitation
-
5
at 532 nm of ∼10 M solutions of dyad 5-Zn in toluene and
Time-Resolved Fluorescence Studies. Studies carried out
at M u¨ lheim by the single-photon counting method gave
information concerning the dynamic behavior of the excited
states of dyads 5 and 5-Zn. These data are summarized in Table
benzonitrile afford the time-absorption profiles shown in Figure
•+
6
nm)
. Formation of porphyrin radical cations PZn (λmax ≈ 670
8-12
on excitation of dyad 5-Zn at 532 nm occurs within
the time resolution of the apparatus (20-40 ps) in both nonpolar
3
.
•-
(toluene) and polar (THF, PhCN) solvents. Although C60 (λmax
A solution of the free-base dyad 5 in benzene was excited at
50 nm, and emission decay profiles were collected at 700 nm.
8-12
≈
1040 nm)
was not directly detected due to experimental
5
limitations at the time of these studies, it is assumed that this
An analysis yielded two exponential decay components with
lifetimes of 14 and 57 ps. The 14 ps component has a large
amplitude (0.90) at the detection wavelength (700 nm), whereas
porphyrin 11 emits strongly at this wavelength with a lifetime
of 13 ns. Therefore, the dominant fluorescing species in the
•+
species is formed along with PZn . Similar studies with dyad 5
(data not shown) also reveal transient absorption for the
porphyrin radical cation in the 500-760 nm region, again
signaling formation of P -C60 . Thus, extremely rapid electron
transfer to give a charge-separated radical ion pair is the domi-
nant decay pathway of P* in 5 and of PZn* in 5-Zn, in both
nonpolar and polar solvents. The CS states in turn decay by
BET to regenerate ground-state starting materials, except for
the free base dyad 5 in toluene where the C60 triplet state (λmax
≈
•+
•
1
case of free base 5 is P*, whose lifetime is shortened by a
1
1
factor of ∼1000 in benzene compared to 11, which lacks the
C60 moiety. This is consistent with the strong quenching of
porphyrin fluorescence in dyad 5 detected in the steady-state
experiments (see above). The 57 ps minor component to the
fluorescence decay has not been definitively identified. It could
31
720 nm) is formed with a quantum yield of 0.22 (see below).
The CR dynamics were measured by the rate of decay of the
1
1
be P- C60*, produced by singlet-singlet ET from P*-C60,
porphyrin radical cation absorption at 650 nm. These data for
and 5-Zn in six different solvents are collected in Table 4.
which would be expected to decay by intramolecular ET to give
5
•
+
•- 7b,8-12
the lower lying charge-separated (CS) state PZn -C60
.
There is clearly a strong dependence of the lifetimes of the CS
ion-pair states of both 5 and 5-Zn on solvent polarity. The trend
is very clear: the lifetimes of the CS states decrease with
increasing solvent polarity, i.e., the rates of BET increase as
solvent polarity increases. Since stabilization of the CS state
by solvation decreases the thermodynamic driving ∆G°CR for
Similar experiments were carried out with dyad 5-Zn in
benzene. Kinetic traces were collected at 587, 644 and 704 nm.
The fluorescence decay was well-fitted with a single-exponential
decay function with a lifetime of 9 ps, assigned to the excited
ZnP moiety. As can be seen from these more quantitative data,
charge recombination, BET for 5 and 5-Zn must be occurring
(
30) Lee, W. A.; Graetzel, M.; Kalyansundaram, K. Chem. Phys. Lett. 1984,
07, 308.
31) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
11-13
in the Marcus inverted region.
icant finding in the Discussion.
We will revisit this signif-
1
(
J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004 7263

A R T I C L E S
Schuster et al.
Table 5. Quantum Yields of Singlet Molecular Oxygen Formation
from the Dyads 5 and 5-Zn as Well as the Porphyrin References
11 and 11-Zn
singlet oxygen quantum yields Φ a
∆
b
compds
CH Cl
2 2
toluene
5
<0.01
<0.01
0.96 ( 0.09
0.80 ( 0.08
0.22 ( 0.02
<0.01
0.77 ( 0.08
0.76 ( 0.08
5
1
1
-Zn
1
1-Zn
a
λexc ) 532 nm, λobs ) 1270 nm, TPP (Φ∆ ) 0.67 ( 0.14) in toluene
used as reference. b Corrected for differences in refractive index and O2( ∆g)
radiative lifetime differences between the samples and the reference.
1
Figure 6. Time-resolved difference absorption spectra of ∼10-5
M
solutions of dyad 5-Zn (a) in toluene at 100 ps intervals from from -100
to 2200 ps, and (b) in benzonitrile at 50 ps intervals of from -50 to 400
ps, after excitation at 532 nm.
Table 4. Lifetimes of the Radical Pair Charge Separated States of
a
Dyads 5 and 5-Zn
Figure 7. Bent and symmetric conformations of dyad 5. The more stable
structure is (a) on the left.
relative static
permittivity, ꢀ b
5 radical pair
lifetime [ns]
5-Zn radical pair
lifetime [ns]
solvent
toluene
tetrahydrofuran
dichloromethane
o-dichlorobenzene
benzonitrile
r
2.38
7.58
8.93
9.93
25.20
36.71
3.5
1.015
0.099
0.095
0.095
0.069
0.056
Discussion
0.314
0.232
0.201
0.155
0.107
Conformations of C60-Porphyrin Dyad 5. The distance
between the electron donor and acceptor in a D-A system is
one of the key factors that controls the feasibility and kinetics
of electron and energy transfer. This distance is regulated by
the conformational properties of the system. Molecular modeling
studies (InsightII 97.2, Discover 3) on the parachute-shaped
dimethylformamide
a
Excitation at 532 nm; Detection at 650 nm. b Values from Murov, S.
L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, Second Edition;
Marcel Dekker: New York, 1993; and CRC Handbook of Chemistry and
Physics; CRC Press: Boca Raton, Florida.
1
5
C60-porphyrin dyad 5 reveal that in its lowest-energy
conformation, Figure 7a, the porphyrin does not reside above
C60 in a structure with C2V symmetry, Figure 7b, as previously
Quantum Yields of Singlet Oxygen Production. It is well
documented that C60 and its derivatives photosensitize the
formation of singlet molecular oxygen, O2( ∆g), via triplet-
triplet energy transfer, with an efficiency approaching 1.0.
Thus, studies of O2( ∆g) production are useful in probing the
generation of triplet states upon excitation of porphyrin-C60
dyads. The quantum yields (Φ∆) for formation of O2( ∆g) from
the dyads 5 and 5-Zn and porphyrin reference compounds 11
and 11-Zn were determined at M u¨ lheim by measurements of
the zero-time amplitudes of the O2( ∆g) luminescence decay at
270 nm compared with 5,10,15,20-tetraphenylporphine (TPP)
Φ∆ ) 0.67 ( 0.14 in toluene)33 as the standard. All solutions
were air-saturated and the excitation wavelength was 532 nm.
As shown in Table 5, Φ∆ for both 5 and 5-Zn in CH2Cl2 is <
.01; however, in toluene, Φ∆ for 5 is 0.22, whereas that for
-Zn remains <0.01.
2
1,26
thought.
Rather, in its most stable conformation, the
1
porphyrin ring system lies on the side of the C60 sphere, with a
center-to-center distance rcc (distance from the center of the
porphyrin to the center of the fullerene) of ∼7.8 Å compared
with ∼10 Å in conformation (b). This rcc value corresponds to
3
2
1
1
an edge-to-edge distance r of ∼4.2 Å, which is close to van
ee
der Waals contacts. The bent unsymmetrical conformation (a)
is stabilized by attractive van der Waals forces between the
porphyrin and C60 moieties, as previously proposed, and as
observed in many types of P-C60 dyads.
conformation (a), protons at C10 and C20 on the porphyrin ring
occupy different chemical environments, and should therefore
be differentiable by H NMR. Only a single peak for these two
protons is observed at δ 10.33 at room temperature. The 13
NMR spectrum of 5 is also consistent with a C2V symmet-
rical structure. Thus, either the computational predictions are
1
4
1
8
,9,34,35
In the bent
1
(
1
C
0
5
(
32) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F.
N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95,
(34) D. Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.;
Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd,. P. J. Phys. Chem. 1996,
100, 15926.
1
1. Bensasson, R. V.; Bienvenue, E.; Dellinger, M.; Leach, S.; Seta, P. J.
Phys. Chem. 1994, 98, 3492.
(33) Ogilby, P. R.; Foote, C. S. J. Am. Chem. Soc. 1982, 104, 2069.
(35) Schuster, D. I. Carbon 2000, 38, 1607.
7264 J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004

Porphyrin-Fullerene Dyad with Parachute Topology
A R T I C L E S
incorrect, or dyad 5 is conformationally mobile, with the
porphyrin moiety swinging from one side of the fullerene sphere
to the other, through the high energy C2V-conformation (b). This
could be confirmed from studies of the temperature dependence
Table 6. Free Energy Changes and Energy Transfer Rate
Constants in THF and Toluenea
E100b
E200b
∆G°ET
(eV)
∆GCR
(eV)
k
c
k
CR
c
ET
compds
solvents
(eV)
(eV)
(1010 s-1
)
(109 s-1
)
1
5
THF
1.96 1.80 -0.82 -1.15c
0.09
2.12 1.80 -1.15 -0.97
toluene 2.12 1.80 -0.81 -1.31
4.3
7.1
7.7
3.2
0.29
of its H NMR spectrum. Since ET generally occurs most rapidly
d
e
toluene 1.95 1.80 -0.47
THF
when the donor (porphyrin) and the acceptor (C60) are closest
5-Zn
10
0.98
7-10
together,
this process should be fastest in the bent conforma-
d
11
tion of 5. To date, attempts to obtain X-ray quality crystals of
a
The energy calculations are based on the lowest energy-bent conforma-
5
and 5-Zn have been unsuccessful.
b
1
2
tion (a) in Figure 7. E 00 and E 00 are energies of the 0-0 transition
between the S1 and S0 states for the porphyrin and C60, respectively. kET
Energetics of Charge Separation and Charge Recombina-
tion in Dyads 5 and 5-Zn. To assess the feasibility of electron
transfer between the photoexcited porphyrin and C60 in the dyads
c
-
1
-1
-1 d
) τf - τf (ref) , and kCR ) τcs . Derived from the fluorescence data in
e
benzene (Table 3). Charge recombination gives C60 triplet states as well
as ground state dyad 5.
5
and 5-Zn, the free energy change associated with this process
36
+•
was estimated using eq 1. E°(D /D) is the standard electrode
cation and C60 radical anion, respectively. Thus, using 0.30 V
as the standard electrode potential for oxidation of the porphyrin,
-1.09 V as the standard electrode potential for reduction of
C60, 1.95 eV as the excitation energy of the porphyrin, and 7.8
Å for the average center-to-center distance between the por-
phyrin and C60, ∆G°ET for dyad 5 in toluene was estimated to
be -0.47 eV. Similarly, ∆G°ET ) -0.81 eV was obtained for
-Zn in the same solvent, again assuming ET occurs in the bent
conformation. Accordingly, the charge-separated states P -
C60 and PZn -C60 reside at 1.48 and 1.31 eV above the
ground states, respectively. These data are summarized in Table
potential of the donor cation
+•
-•
2
∆
G° /eV ) e[E°(D /D) - E°(A/A )] - E - e /ꢀ r (1)
ET 00 r
-
•
radical resulting from the electron transfer, E°(A/A ) is the
standard electrode potential of the acceptor (both in V and
relative to the same reference electrode), ꢀr is the relative
medium static permittivity of the solvent (i.e., the dielectric
constant), e is the elementary charge, and r is the center-to-
center distance between the ions.
5
•+
•
-
•+
•-
Photoinduced electron transfer (PET) should occur most
rapidly in the lowest energy bent conformation (a) of dyad 5.
This conformation was therefore utilized for the free energy
calculation. The value of ∆G°ET for electron transfer between
the porphyrin S1 state and C60 in the dyad 5 in CH2Cl2 was
calculated as follows. The energy (E00) of the porphyrin lowest
excited singlet state of 5 in CH2Cl2 was estimated as 1.96 eV
relative to the ground state, from the average of the longest
wavelength absorption band and shortest wavelength emission
band of the porphyrin moiety. The average center-to-center
distance r between the porphyrin and the C60 moiety in the bent
conformation of 5 was estimated to be 7.8 Å from molecular
6
.
Accordingly, PET is thermodynamically favored in nonpolar
as well as polar solvents for both the free base dyad 5 and its
zinc analogue 5-Zn, which is rare if not unprecedented for
porphyrin-C60 dyads. The energy of the fullerene S1 state is 1.80
eV, based on the absorption and emission spectra of the fullerene
reference compound 12, while the energy of the T1 state of C60
is 1.57 eV. These energies were used to construct the diagrams
in Figures 8 and 9, which show the energy levels estimated for
all proposed intermediates and associated interconversion
pathways.
Kinetics of Photoinduced Electron Transfer and Back
Electron Transfer. The various pathways for decay of the
singlet states of the dyad 5 and 5-Zn can be discussed with
reference to Figures 8 and 9. No convincing evidence was
obtained for the intermediacy of discrete P- C60* states in the
decay processes of P*-C60, although this cannot be strictly
excluded. If C60 singlet excited states were formed, then we
would expect intersystem crossing to C60* to compete with
ET, but C60 triplets were not detected by transient absorption
in solvents other than toluene, and O2( ∆g) formation was
observed only for dyad 5 in toluene. Under these conditions,
the lifetime of P*-C60 is given by eq 3,
31
15
modeling. Thus, in CH2Cl2, where ꢀr ) 9.08, the Coulombic
2
+
term e /ꢀrr ) 0.21 eV. From cyclic voltammetry, E°(D /D) for
-
•
the porphyrin ) 0.30 V and E° (A/A ) for the fullerene )
1.09 V, both vs Fc/Fc . From these data, a value of ∆G°ET )
0.78 eV was obtained for 5 in CH2Cl2. Accordingly, the
charge-separated state P -C60 lies 1.18 eV above the ground
state in CH2Cl2. A similar calculation for the zinc dyad 5-Zn
gives ∆G°ET ) -1.11 eV. Since the excitation energy E 00 for
+
-
1
-
1
•+
•-
3
1
•
+
•-
5
-Zn is 2.12 eV, the CS state PZn -C60 in CH2Cl2 lies 1.01
1
eV above the ground state.
For 5 and 5-Zn in THF, which has a relative permittivity ꢀr
1
)
7.58, similar to that of CH2Cl2 (ꢀr ) 9.08), the values of
∆
-
G°ET from the porphyrin S1 state to C60 were estimated to be
0.82 eV and -1.15 eV, respectively,
1
/τ ) k + kf
(3)
f
ET
For calculation of ∆G°ET for dyads 5 and 5-Zn in a nonpolar
where τf is the lifetime of the porphyrin lowest singlet excited
state.
solvent, such as toluene, the modified relationship in eq 2 was
3
6,37
employed,
using the redox potentials measured in CH2Cl2
The fluorescence decay rate constant kf is estimated as 8.8
+
•
-•
2
7 -1
∆
G° /eV ) e[E°(D /D) - E (A/A )] - E - e /ꢀ r -
× 10 s from the 11.3 ns lifetime of the lowest singlet excited
ET
red
+
00
r
2
-
state of the model porphyrin 11. Thus, In THF, where τf ) 23
(
e /2)(1/r + 1/r )(1/8.93 - 1/ꢀ ) (2)
r
10 -1
ps, kET ) 4.3 × 10
s
(Table 6), and the quantum yield of
where ꢀr is the relative permittivity of toluene (2.38), and r+
photoinduced electron transfer Φet is ∼1.0.
-
We conclude that intramolecular quenching by C60 of the
porphyrin S1 state in dyads 5 and 5-Zn in CH2Cl2 as well as
5-Zn in toluene occurs entirely by electron transfer (ET). If
(
4.8 Å) and r (5.6Å) are the ionic radii of porphyrin radical
(
36) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 529.
J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004 7265

A R T I C L E S
Schuster et al.
Figure 8. Energy states of dyad 5 in tetrahydrofuran (left) and toluene (right).
Figure 9. Energy states of dyad 5-Zn in tetrahydrofuran (left) and toluene (right).
1
energy transfer (EnT) to give P- C60* were involved, efficient
Previously studied free-base C60-porphyrin dyads and zinc-
bound C60-porphyrin dyads show two different patterns of
3
intersystem crossing would occur to generate P- C60*, which
in turn would sensitize formation of O2( ∆g). In toluene, the
production of O2( ∆g) (Φ∆ ) 0.22 ( 0.02) upon excitation of
is attributed to the following sequence of events: (1) ET from
the porphyrin to the C60 moiety to give the CS state; (2) back
ET to give C60*, in competition with regeneration of the dyad
ground state; (3) EnT from C60* to ground-state oxygen ( O2)
1
7-11,34,35
photophysical behavior.
The dominant decay mode of
photoexcited P-C60 dyads in nonpolar solvents (such as toluene
or benzene) is energy transfer (EnT) to give a lower-lying
fullerene singlet excited state, while in polar solvents such as
1
5
3
•+
•-
benzonitrile, rapid photoinduced ET occurs to yield P -C60
3
3
charge-separated states, occasionally in competition with some-
what slower singlet-singlet EnT to give fullerene singlet excited
states. For ZnP-C60 dyads, PET usually dominates in nonpolar
1
38
to give O2( ∆g). These results are consistent with the data
obtained from time-resolved transient absorption studies, where
C60 triplets were detected only upon excitation of 5 in toluene.
We assume that kET for 5 in toluene is approximately the same
7
b
as well as polar solvents. Such behavior was shown by dyads
1 and 2, as discussed earlier. In the case of 1, it was concluded
that through-space rather than through-bond electronic transfer
10
-1
as in benzene, i.e., 7.1 × 10 s . Similar kinetic data were
1
10
obtained for 5-Zn, as shown in Table 6, except that O2( ∆g) is
dominates.
not formed under any conditions investigated.
It is useful to compare the results for dyads 5 with those for
dyads 15, where competition between EnT and ET as a
function of solvent polarity was also observed.
The magnitude of kET in both 5 and 5-Zn, approaching 1011
34
-1
s , reflects the enforced proximity of the porphyrin to C60 and
effective π, π-interactions. The fact that the charge separation
rates are so large, and are very similar in benzene (toluene)
and the much more polar THF, suggests that ET for 5 and 5-Zn
is occurring near the peak of the Marcus parabola.1
2,13
The CS states of 5 and 5-Zn decay by charge recombination
(CR) to their ground states, except for 5 in toluene where C60
triplet states are formed with a quantum efficiency of about 0.22.
The lifetimes of the CS ion-pair states determined by the decay
of porphyrin radical cation absorption at 650 nm were used to
determine the rate constants kCR shown in Table 6. The strong
dependence of solvent polarity on the CS ion pair lifetimes of
both 5 and 5-Zn shown in Table 4 indicate strongly that BET
12,13,26
(CR) occurs in the Marcus inverted region.
Perhaps the most significant finding in connection with the
7266 J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004

Porphyrin-Fullerene Dyad with Parachute Topology
A R T I C L E S
present study is that values of τf in toluene for dyads 15, 20-
to be ∼8.5 ps, corresponding to a quenching rate constant of
1
1 -1
2
2 ps, are significantly longer than those of 5, ∼14 ps, which
is consistent with smaller values of rcc for 5 (∼7 Å) than for 15
9.9 Å). For 15 in toluene, it was concluded34 that PET is
1.2 × 10 s , of the same order of magnitude as for 5-Zn (13
ps). Transient absorption experiments on 4 demonstrate that ET
•+
•-
(
prevails in all solvents, yielding the ZnTPP -C
CS state.
6
0
negligible, as fluorescence quenching was attributed entirely to
singlet-singlet EnT. In contrast, we conclude that for 5 ET
dominates, even in toluene. The number of bonds separating
the P and C60 moieties is much greater in 5 (10) than in 15 (2),
but 5 is able to adopt a folded conformation (see Figure 7a),
which is not accessible by 15. Thus, through-space electronic
interactions should dominate in the case of 5, whereas only
through-bond interactions are possible with 15. The fact that
rates of charge separation for 5 in THF (∼8 ps) and for 15 in
PhCN (4-6 ps) are comparably fast suggests to us that
topological differences between the two systems appear to have
less influence on ET than on singlet-singlet EnT.
The fact that ET rather than EnT dominates in the decay of the
porphyrin singlet excited state of 4 was attributed to π-stacking
of the two chromophores, which enhances electron exchange
over a dipole-dipole energy transfer process, just as we propose
above for 5. The product of charge recombination of the CS
state of 4 in all solvents except toluene is the singlet ground
state. In toluene, a more complex decay process of the CS state
of 4 (energy 1.54 eV) ensues, in which the first step is BET to
give the ZnTPP triplet (energy 1.53 eV) followed by triplet-
3
triplet EnT to give the fullerene triplet, Zn-TPP- C60 (1.50
eV). New time-resolved EPR studies demonstrate that a similar
3
8
pathway is followed by 5-Zn in toluene. In more polar
solvents, where the CS state is energetically stabilized, charge
recombination to give porphyrin and fullerene triplets is
thermodynamically prohibitive. The fact that the lifetimes of
the charge-separated state of 4 in THF (τ ) 385 ps) and CH2-
Cl2 (τ ) 122 ps) are significantly larger than those in the more
polar solvents dichloroethane (τ ) 61 ps) and benzonitrile (τ
The observation of PET upon photoexcitation of zinc dyad
5
-Zn in both polar (THF, PhCN) and nonpolar (benzene,
8
,10
toluene) solvents fits the pattern observed for dyad 1b
not 15b, where PET occurs only in polar solvents. The
magnitude of kET in both 5 and 5-Zn, approaching 10 s , is
similar to that in 1a and 1b, which suggests that strong π,
π-interactions occur between porphyrin and C60 in both systems.
The behavior of free-base dyad 5 is especially remarkable, in
but
3
4
1
1
-1
)
38 ps), shows that BET is occurring in the Marcus inverted
1
region, where larger values of the thermodynamic driving force
that ET prevails in the deactivation of P* in both polar and
1
2,13,41
-∆G°ET lead to slower rates (kCR).
nonpolar solvents, which is unprecedented. We attribute this to
through-space interactions, since through-bond interactions
Thus, there is a very close similarity between the photo-
(
involving 10 bonds) are highly unlikely. Estimates of the
physical behavior of π-stacked dyads 5-Zn and 4, except for
one very important difference: the pronounced red-shifts
observed in the UV-vis spectra of 4 compared with appropriate
model compounds, and the absence of such spectral shifts in
the case of 5 and 5-Zn. Indeed, we have observed substantial
red shifts in absorption spectra of P-C60 dyads with flexible
polyether linkers, where low energy conformations are readily
accessible in which the two chromophores are in close proximity
energies of CS states from electrochemical data support the
hypothesis that PET in 5 is exergonic in toluene as well as THF
(see Table 6 and Figures 8 and 9). Because of the special
topology of this dyad, intramolecular ET by electron exchange
is much faster than singlet-singlet EnT by a F o¨ rster dipole-
dipole mechanism. In toluene, where the CS state of 5 (1.48
eV) is destabilized so that it lies energetically close to the
3
39
P- C60* state (∼1.57 eV), decay to the latter with a spin flip
42
(
rcc ≈ 7-8 Å). In the case of 4, subtraction of the UV-vis
becomes energetically feasible. For 5 in toluene, there is no
absorption spectra of model compounds from the spectrum of
1
3
evidence that implicates P- C60* or P*-C60 states as precur-
sors to P- C60*, in contrast to the pathways proposed for dyads
1
4
revealed a new broad band in the 700-800 nm region,
3
assigned to a charge transfer (CT) transition between the
porphyrin donor and the fullerene acceptors, confirming ground-
state electronic interaction between the chromophores.41 From
the CT spectra the electronic coupling element V for 4 in toluene
8
,10,34,38
and 15.
Our findings with compounds 5 and 5-Zn, presented in
preliminary form in 1999,25 are completely in line with those
reported more recently for the π-π-stacked ZnP-C60 dyad
-
1
was estimated to be is 436 cm . The failure to observe similar
spectral features for 5-Zn, which otherwise possesses strikingly
similar photophysical properties, can be understood in terms of
the differences in conformation of the two systems. The fullerene
and porphyrin chromophores in 4 are rigidly held in a face-to-
face topology, optimizing π-stacking and van der Waals
interactions, whereas the flexibility of the linker in 5 permits
the porphyrin ring system to move back and forth between
conformations (see Figure 7a) in which van der Waals interac-
tions are maximized, but are not permanently maintained. Thus,
we attribute the absence of spectral shifts and CT absorption in
5-Zn, despite rapid PET in both polar and nonpolar solvents,
to the conformational flexibility in this system. Recent time-
resolved EPR data at low temperatures also are consistent with
4
0,41
4
.
In this C2-symmetric dyad, the trans-2 linkage to the
fullerene and the length of the tether enforces a face-to-face
orientation of the two chromophores. As in dyad 5-Zn, there
are 10 bonds joining the porphyrin and fullerene moieties. The
ZnTPP fluorescence in 4 is very strongly quenched in all
solvents studied (toluene, THF, dichloromethane, dichloro-
ethane, and benzonitrile). Fluorescence lifetimes were estimated
(
37) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer; VCH-
Wiley: New York, 1993.
(38) Recent time-resolved electron paramagnetic resonance studies indicate that
this process is more complicated: BET from the charge separated state of
3
5
C
-Zn in toluene first gives the lower lying porphyrin triplet state [ ZnP*-
3
60] which then undergoes triplet-triplet EnT to yield ZnP- C60*: Galili,
T.; Regev, A.; Levanon, H.; Schuster, D. I.; Guldi, D. M., submitted for
publication.
(
39) Kuciauskas, D.; Liddell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. J.
Am. Chem. Soc. 1998, 120, 10880.
38
(
40) Guldi, D. M.; Hirsch, A.; Scheloske, M.; Dietel, E.; Troisi, A.; Zerbetto,
F.; Prato, M. Chem. Eur. J. 2003. Guldi, D. M.; Luo, C.; Prato, M.; Dietel,
E.; Hirsch, A. Chem. Commun. 2000, 373.
PET from the bent conformation of 5-Zn. We hope to confirm
(
41) Guldi, D. M.; Hirsch, A.; Scheloske, M.; Dietel, E.; Troisi, A.; Zerbetto,
(42) MacMahon, S.; Schuster, D. I., Unpublished results, quoted in MacMahon,
S., Ph.D. Dissertation, New York University, 2003.
F.; Prato, M. Chemistry Eur. J. 2003, 9, 4968.
J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004 7267

A R T I C L E S
Schuster et al.
2
Figure 11. Plot of [kBT ln kBET + (∆G°BET/2)] vs (∆G°BET) for dyad 5
Figure 10. Plot of intramolecular electron-transfer rate constants in dyads
(
solid line) and dyad 5-Zn (dashed line).
5
and Zn-5 vs the thermodynamic driving force (-∆G°ET) The line
-
1
represents the best fit (λ ) 0.80 eV and V ) 9.86 cm ).
Despite the assumptions, the two approaches are in good
this proposal by studies of the temperature dependence of
spectral and photophysical properties of dyads topologically
related to 5.
Interpretation of Kinetics of Electron Transfer Processes
in Dyads 5 and 5-Zn According to Marcus Theory. As
concluded earlier, the thermodynamic driving force for PET
agreement. The λ values for 5 and 5-Zn are slightly larger than
those typically found for P-C60 dyads. For example, a
reorganization energy of 0.59 eV was reported in a recent study
involving several ZnP-C60 dyads with a series of spacers.43
Larger reorganization energies are obtained when ground-state
electronic interactions between the two chromophores prevail,
(
charge separation) in dyads 5 and 5-Zn places this process near
4
0,41
as with π-stacked dyad 4.
Thus, we propose that the
12,13
the peak of the Marcus parabola.
Back electron transfer
reorganization energy can be used as an indicator of the degree
of electronic interaction between the two chromophores.
The electronic coupling matrix elements V derived from the
plot in Figure 11 are lower than that found for the π-stacked
(charge recombination), for which values of -∆G°CR lie
between 0.65 and 1.48 eV, places this process in the inverted
region of the Marcus parabola, i.e., the highly exergonic region
(
-∆G° > λ), where the ET rates decrease with increasing free
4
0,41
dyad 4,
in line with the lack of CT features for 5 and 5-Zn,
11-13,41
energy changes.
A Marcus plot for dyads 5 and 5-Zn
in both the ground and excited states. Despite this, the BET
rates in 4 and 5 are similar, which suggests electronic interac-
tions exist in 5 and 5-Zn which are not discernible spectroscopi-
cally.
was constructed from the ET rates, measured in several different
solvents, and the values of the thermodynamic driving force
(
-∆G°ET), neglecting differences in the solvent polarity and
solvent reorganization energy (see Figure 10). Values for
∆G°BET were calculated for these dyads in ortho-dichloroben-
-
Experimental Section
zene, benzonitrile, and DMF following the procedure sum-
marized in eq 1. From this plot, the total reorganization energies
General Procedures. Unless specified, all reagents and solvents were
purchased from commercial sources and were used as received.
He@C60 was obtained from Professor M. Saunders and Dr. A. Khong
at the Chemistry Department of Yale University. Dry dichloromethane
and toluene were freshly distilled over calcium hydride.
(
λ) and electronic coupling matrix elements (V) for dyads 5
3
-1
and 5-Zn are estimated to be 0.8 ( 0.1 eV and 9.8 ( 1.0 cm ,
respectively.
We then analyzed the driving force dependence on the rate
constants, by applying eq 4, which can be transformed into the
linear expression shown in eq 5, where kB is the
Moisture-sensitive liquids and solutions were transferred by syringes
and introduced into reaction vessels through rubber septa. All the
reactions were stirred magnetically unless noted otherwise. The progress
of the reactions was monitored by thin-layer chromatography (TLC)
whenever possible. TLC was performed using precoated glass plates
(Silica gel 60, 0.25 mm thickness) containing a 254 nm fluorescent
indicator. Spots were visualized using one of the following tech-
niques: (a) 254 nm ultraviolet irradiation (UV); (b) insertion of the
plates into a chamber containing a mixture of iodine crystals and silica
gel. Flash chromatography was performed using ICN Silica 40-63 60
Å silica gel.
1H and 13C NMR spectra were obtained on either 200 or 300 MHz
Varian Gemini NMR spectrometers. NMR samples were prepared in
3
deuteriochloroform (CDCl ). All chemical shifts are reported relative
to an internal standard of tetramethylsilane. Steady-state absorption
spectra were measured on a Genesys 5 UV-vis spectrophotometer.
Fluorescence emission spectra were determined on a Perkin-Elmer LS
50 luminescence spectrophotometer.
1
/2
2
3
(
∆G°ET + λ)
4
π
2
kET
)
V exp -
(4)
(
2
)
[
]
4
λkBT
h λk T
B
∆
G°
ET
kBT ln kET
+
)
2
2
3
1/2
(
∆G° )
4
π
2
λ
4
ET
kBT ln
V
-
-
(5)
2
[
(
)
]
4λ
h λk T
B
Boltzmann constant.13 Plots of [kBT ln kBET + (∆G°ET/2)] versus
∆G°ET) for BET give a linear correlation for both dyads, as
2
(
shown in Figure 11. From this plot, the reorganization energy
(
0
λ) and electronic coupling (V) values obtained for dyad 5 are
.71 eV and 4.8 cm , respectively. A similar linear correlation
-1
(
43) Imahori, H.; Yamada, H.; Guldi, D. M.; Endo, Y.; Shimomura, A.; Kundu,
S.; Yamada, K.; Okada, T.; Sakata, Y.; Fukuzumi, S. Angew. Chem., Int.
Ed. Engl. 2002, 41, 2344.
for 5-Zn afforded a slightly smaller λ-value, 0.61 eV, and V )
-
1
5
.6 cm .
7268 J. AM. CHEM. SOC.
9
VOL. 126, NO. 23, 2004

Porphyrin-Fullerene Dyad with Parachute Topology
A R T I C L E S
Benzyl 3,4-diethylpyrrole-2-carboxylate (8). To a solution of
The organic phase was dried (Na
2 4
SO ) and concentrated to dryness in
3-acetoxy-4-nitrohexane (11.34 g, 60 mmol) and benzyl isocyanoacetate
2 2
vacuo. The residue was dissolved in CH Cl (20 mL) and heated at
(
11.37 g, 65 mmol) in 40 mL of THF was added DBU (19.44 mL, 130
reflux. A solution of zinc acetate (1.0 g) in methanol (5 mL) was added
and heating was continued for 5 min. The mixture was concentrated to
mmol). The resulting mixture was stirred at room temperature for 16
h. The reaction mixture was then poured into water, and extracted with
dryness in vacuo and chromatographed on silica gel (CH
2 2
Cl with 1%
CH
with Na
2
Cl
2
. The combined organic layer was washed with H
2
O and dried
Et N) to yield the desired zinc porphyrin 11 (138 mg, 12% yield).
3
1
2
SO and then concentrated. Purification by column chroma-
4
H NMR (200 MHz, CDCl ): δ -1.81 (2H, br), 0.62 (2H, s), 1.02
3
tography (silica gel, toluene) gave the desired product (10.6 g, 69%
yield).
(4H, m), 1.36 (12H, t), 2.10 (12H, t), 2.45 (4H, t), 2.92 (4H, m), 3.20
(4H, m), 3.60 (4H, t), 4.21 (8H, q), 7.08 (2H, d), 7.57 (2H, t), 7.80
1H NMR (200 MHz, CDCl
): δ 1.20 (3H, t, J ) 8.8 Hz), 1.24 (3H,
t, J ) 7.4 Hz), 2.44 (2H, q, J ) 8.8 Hz), 2.76 (2H, q, J ) 7.4 Hz),
.30 (2H, s), 6.64 (1H, d), 7.3-7.4 (5H, m), 8.90 (1H, br). 13C NMR
50 MHz, CDCl ): δ 14.94, 15.58, 18.00, 18.22, 65.72, 118.34, 119.68,
27.02, 128.16, 133.09, 136.53, 161.33.
-(3-Hydroxypropoxy)benzaldehyde (6). Salicylaldehyde (6.0 g,
9.1 mmol) was added dropwise to a solution of NaOH (2.0 g) in H
13
3
(2H, t), 8.57 (2H, d), 10.42 (2H, s). C NMR (50 MHz, CDCl ): δ
3
164.32, 159.64, 147.15, 145.65, 144.58, 143.71, 134.39, 132.90, 129.92,
119.74, 114.67, 112.34, 97.25, 65.41, 61.94, 38.95, 27.65, 21.37, 19.88,
18.73, 17.79.
5
(
1
3
C60-Porphyrin Dyad 5. To a solution of zinc porphyrin 11 (166
2
mg) and C60 (620 mg, 5 eq) in toluene (350 mL) under nitrogen was
4
2
O
added DBU (107 mg, 4 eq) and CBr (70 mg, 1.2 eq). The resulting
4
(80 mL), followed by dropwise addition of 3-bromo-1-propanol (5.6
mixture was stirred overnight. Chromatography (silica gel, CH Cl with
2
2
g, 40.3 mmol). The mixture was heated on a steambath for 12 h and
allowed to cool overnight. The solution was made strongly alikaline
3
1% Et N) gave the desired dyad 5 (72 mg, 25% yield).
1H NMR (200 MHz, CDCl
3
): δ 1.11 (4H, m), 1.22 (12H, t), 1.97
(
5
pH ≈ 10) by the addition of NaOH and extracted with CH
0 mL). The combined organic phase was washed with brine (2 × 50
mL), dried over Na SO , filtered and concentrated to yield 2-(3-
2
Cl
2
(4 ×
(
(
(
12H, t), 2.16 (4H, t), 2.72 (4H, m), 3.18 (4H, m), 3.80 (4H, t), 4.09
8H, q), 7.13 (2H, d), 7.46 (2H, t), 7.80 (2H, t), 8.57 (2H, d), 10.33
13
2
4
2H, s). C NMR (75 MHz, CDCl
3
): δ 161.53, 159.83, 145.44, 145.06,
hydroxypropoxy)-benzaldehyde as an oil. Purification by column
chromatography (silica gel, EtOAc:petroleum ether ) 1:1) gave a clear
oil (5.28 g, 73% yield).
144.98, 144.90, 144.53, 144.39, 144.29, 143.58, 142.85, 142.75, 142.67,
141.95, 141.61, 141.09, 140.62, 138.06, 134.03, 131.75, 130.45, 120.21,
1
1
13.46, 112.83, 97.27, 65.01, 63.37, 32.35, 27.89, 20.78, 19.91, 18.47,
1H NMR (200 MHz, CDCl
2H, t), 4.10 (2H, t), 6.90 (2H, m), 7.42 (1H, m), 7.65 (1H, m).
): δ 2.00 (2H, m), 3.66 (1H, s), 3.76
3
3
7.48. He NMR (500 MHz, 1-methylnaphthalene/CDCl
): δ -8.5.
) (ꢀ/dm mol cm ) 259 (115 600), 328
3
13
(
C
3
-1
-1
UV-vis: λmax (nm, CH
2
Cl
2
NMR (50 MHz, CDCl ): δ 190.63, 161.39, 136.58, 129.83, 128.53,
3
(
44 400), 412 (138 000), 510 (10 000), 544 (6400), 577 (7600). MALDI-
1
25.04, 121.04, 113.02, 66.42, 59.82, 32.47.
Bis-3-(2-formylphenoxy)propyl Malonate (7). To a solution of
-(3-hydroxypropoxy)benzaldehyde 6 (5.5 g, 30.6 mmol) in dry CH
(100 mL) at 0 °C was added a solution of malonyl dichloride (2.2
g, 15.3 mmol) in 5 mL of CH Cl , followed by addition of triethylamine
5.1 g, 50 mmol). The resulting mixture was stirred at 0 °C for 1 h.
+
+
TOF (m/z): 1621.3 (M ), calc. 1621.8 (M ). Fluorescence (λexc ) 580
nm) λmax (CH Cl ) 635, 698 nm.
2
2
2
2
-
Laser Flash Photolysis Experiments. 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
Cl
2
2
2
(
(18 ps pulse width, 2-3 mJ/pulse). Nanosecond Laser Flash Photolysis
The solution was then washed with water, dilute aqueous HCl, saturated
aqueous sodium bicarbonate, and again with water. The organic phase
was dried (Na SO ) and concentrated in vacuo. Purification by column
2 4
experiments were performed with laser pulses from a Quanta-Ray CDR
Nd:YAG system (532 nm, 6 ns pulse width) in a front face excitation
geometry.
chromatography (silica gel, CH
2
Cl
2
:EtOAc ) 100:7) gave the desired
(1∆
Singlet Molecular Oxygen Emission Measurements. O
2
g
)
product 7 (5.0 g, 76%).
decays were measured upon excitation of air-saturated solutions of
compounds 5, 5-Zn, 11, and 11-Zn with 8-ns laser pulses from the
second harmonic of a Nd:YAG laser at 532 nm. The detector was a
1H NMR (200 MHz, CDCl
): δ 2.20 (4H, m), 3.40 (2H, s), 4.12
4H, t), 4.35 (4H, t), 7.00 (4H, m), 7.51 (2H, t), 7.80 (2H, d), 10.49
3
(
(
13
2H, s). C NMR (50 MHz, CDCl
3
): δ 189.75, 136.35, 128.94, 121.41,
2
liquid N -cooled Ge-diode (Northcoast EO817FP) with a time resolution
1
12.92, 65.32, 62.67, 41.96, 29.02.
of 200 ns. The decay signals of 20 shots were averaged. Similar data
were collected for the toluene solution of the reference (5,10,15,20-
tetraphenylporphyrine, TPP). The absorbances of sample and reference
Malonate-Strapped Dipyrromethane 9. A solution of dialdehyde
(4.35 g, 10.2 mmol) and pyrrole 8 (10.53 g, 41.0 mmol) in absolute
7
ethanol (30 mL) was heated to reflux under nitrogen and concentrated
hydrochloric acid (0.24 mL) was added. The reaction mixture was
heated under reflux for 5 h, neutralized with concentrated NH OH, and
4
solutions were A532 ) 0.1 ( 0.002. The ratio of the slopes of the zero-
1
time extrapolated amplitude of the O
2 g
( ∆ ) single-exponential decay
vs the total laser energy per pulse (at low energy density where linearity
concentrated in vacuo. The crude product was purified by column
chromatography (silica gel, petroleum ether:EtOAc 3:1) to give the
dipyrromethane 9 as a yellow foam (10.0 g, 69% yield).
is observed) for sample and reference solutions affords the ratio of
1
quantum yields of O
the reference. The O
2
( ∆
g
) production for the compound studied and
44
1
2 g 2 2 ∆
( ∆ ) lifetime in all CH Cl solutions was τ )
1H NMR (200 MHz, CDCl
2H, t), 4.10 (2H, t), 6.90 (2H, m), 7.42 (1H, m), 7.65 (1H, m).
): δ 2.00 (2H, m), 3.66 (1H, s), 3.76
3
90 ( 3 µs and in toluene it was 30 ( 2 µs. Saturating the solutions
13
(
C
with O did not change the results.
2
NMR (50 MHz, CDCl ): δ 190.63, 161.39, 136.58, 129.83, 128.53,
3
1
25.04, 121.04, 113.02, 66.42, 59.82, 32.47.
Malonate-Strapped Octaethylporphyrin 11. A mixture of dipyr-
romethane 9 (2.84 g, 2.0 mmol), 10% Pd/C (0.75 g) and triethylamine
dry) (0.8 g) in dry THF (200 mL) was stirred under hydrogen until
Acknowledgment. The NYU group is grateful to the National
Science Foundation for support of this study under grants CHE-
009789 and CHE-9712735. The electrochemical studies at
Clemson and Miami were also supported by the National
Science Foundation. Studies at Notre Dame were supported by
the U. S. Department of Energy. S.E.B., A.R.H, and their co-
workers at M u¨ lheim thank Prof. Kurt Schaffner for his support.
This is contribution NDRL-4504 from the Radiation Laboratory.
(
hydrogen uptake was complete. The solution was filtered through Celite
and concentrated to dryness. The resultant foam of 10 was dissolved
in dry CH Cl (800 mL) and trimethyl orthoformate (4.60 mL, 41.9
2 2
mmol) and trichloroacetic acid (19.60 g, 120.0 mmol) were added. The
reaction mixture was stirred in the dark, protected from moisture, for
5
h and zinc acetate (1.0 g) in dry methanol (15 mL) was added. The
(
44) Wilson, S. R.; Yurchenko, M. E.; Schuster, D. I.; Yurchenko, E. N.;
Sokolova, O.; Braslavsky, S. E.; Klihm, G. J. Am. Chem. Soc. 2002, 124,
1977.
mixture was stirred for a further 48 h in the dark. The reaction mixture
was washed with water, saturated sodium bicarbonate and water again.
J. AM. CHEM. SOC.
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A R T I C L E S
Schuster et al.
3
Supporting Information Available: Cyclic voltammagrams
TOF mass spectrum of dyad 5 (Figure S7); He NMR spectrum
of dyad 5. This material is available free of charge via the
Internet at http://pubs.acs.org.
and Osteryoung aquare wave voltammograms of compounds
1
5
, 5-Zn, 11, 11-Zn, and 12 (Figures S1 to S3); 200 MHz H
NMR spectra of compounds 5, 9, and 11 (Figures S4 and S5);
1
3
5
0 MHz C NMR spectrum of dyad 5 (Figure S6); MALDI-
JA038676S
7270 J. AM. CHEM. SOC.
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VOL. 126, NO. 23, 2004