Modulating charge separation and charge recombination dynamics in porphyrin-fullerene linked dyads and triads: Marcus-normal versus inverted region

Article abstract of DOI:10.1021/ja003346i

Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C₆₀ dyad (H₂P-C₆₀), zincporphyrin-C₆₀ dyad (ZnP-C₆₀), ferrocene-zincporphyrin-C₆₀ triad (Fc-ZnP-C₆₀), ferrocene-freebase porphyrin-C₆₀ triad (Fc-H₂P-C₆₀), and zincporphyrin-freebase porphyrin-C₆₀ triad (ZnP-H₂P-C₆₀). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc⁺) and the C₆₀ radical anion (C₆₀^?-) pair in the Fc-ZnP-C₆₀ triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 μs. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-ΔG⁰_ET) of the electron-transfer rate constants (k_ET). Hereby, the semilogarithmic plots (i.e., log k_ET versus-ΔG⁰_ET) lead to the evaluation of the reorganization energy (γ) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: γ = 0.66 eV and V = 3.9 cm^-1 for ZnP-C₆₀ dyad and γ = 1.09 eV and V = 0.019 cm^-1 for Fc-ZnP-C₆₀, Fc-H₂P-C₆₀, and ZnP-H₂PC₆₀ triads. Interestingly, the Marcus plot in Fc-ZnP-C₆₀, Fc-H₂P-C₆₀, and ZnP-H₂P-C₆₀ has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence, of V (damping factor: β_CR = 0.58 A^-1) is deduced which depends primarily on the nature of the bridging molecule.

Full text of DOI:10.1021/ja003346i

J. Am. Chem. Soc. 2001, 123, 2607-2617
2607
Modulating Charge Separation and Charge Recombination Dynamics
in Porphyrin-Fullerene Linked Dyads and Triads: Marcus-Normal
versus Inverted Region
,
⊥
‡
,†
†
§
Hiroshi Imahori,* Koichi Tamaki, Dirk M. Guldi,* Chuping Luo, Mamoru Fujitsuka,
,§
‡
,⊥
Osamu Ito,* Yoshiteru Sakata, and Shunichi Fukuzumi*
Contribution from the Department of Material and Life Science, Graduate School of Engineering,
Osaka UniVersity, CREST, Japan Science and Technology Corporation, Suita, Osaka 565-0871, Japan,
The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihoga-oka,
Ibaraki, Osaka 567-0047, Japan, Radiation Laboratory, UniVersity of Notre Dame, Notre Dame,
Indiana 46556, and Institute for Chemical Reaction Science, Tohoku UniVersity, Katahira, Aoba-ku,
Sendai 980-8577, Japan
ReceiVed September 11, 2000. ReVised Manuscript ReceiVed January 2, 2001
Abstract: Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined
in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient
absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a
homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations
and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60),
ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and
zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-
+
separated state of all the investigated systems, namely, that of ferrocenium ion (Fc ) and the C60 radical anion
•
-
(
C60 ) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and
reveals a lifetime as long as 16 µs. Determination of CS and CR rate constants, together with the one-electron
redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving
0
force dependence (-∆G ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots
0
(
i.e., log kET versus -∆G ET) lead to the evaluation of the reorganization energy (λ) and the electronic coupling
matrix element (V) in light of the Marcus theory of electron-transfer reactions: λ ) 0.66 eV and V ) 3.9
-
1
-1
cm for ZnP-C60 dyad and λ ) 1.09 eV and V ) 0.019 cm for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H P-
2
C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear
evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The
-
1
coefficient for the distance dependence of V (damping factor: âCR ) 0.58 Å ) is deduced which depends
primarily on the nature of the bridging molecule.
Introduction
long-distance CS can only be attained in multistep ET systems.
In particular, in triad, tetrad, and pentad structures distantly
The primary challenge in artificial photosynthesis lies in the
design of photoinduced electron transfer (ET) systems, which,
upon photoexcitation, give rise to a long-lived charge-separated
state in high quantum yields. An important design consideration
merging from the natural process is that photosynthesis utilizes
relays of multistep ETs, which facilitate separating radical ion
pairs over long distances and, thereby, successfully retarding
energy-wasting charge recombination (CR).
Along this line considerable efforts have been directed toward
the preparation of novel donor-acceptor systems with the
purpose being to prolong the lifetime of the charge-separated
state, while, simultaneously, optimizing the efficiency of charge
(
1) (a) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer;
Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part D, pp
03-393. (b) Wasielewski, M. R. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, pp 161-
06. (c) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (d) Bard, A. J.; Fox,
M. A. Acc. Chem. Res. 1995, 28, 141.
3
2
(
2) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) Gust, D.;
Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (c) Gust, D.;
Moore, T. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 8, pp 153-
90. (d) Gust, D.; Moore, T. A.; Moore, A. L. Pure Appl. Chem. 1998, 70,
189.
1
2
(3) (a) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.;
Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J. Am. Chem. Soc. 1987, 109,
258. (b) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (c) Verhoeven,
3
1
-6
separation (CS). Despite the extreme difficulties, connected
with the design and synthesis of rigidly spaced ensembles, a
J. W. In Electron Transfer; Jortner, J., Bixon, M., Eds.; John Wiley &
Sons: New York, 1999; Part 1, pp 603-644. (d) Jordan, K. D.; Paddon-
Row, M. N. Chem. ReV. 1992, 92, 395.
*
Address correspondence to these authors. E-mail: imahori@
(4) (a) Chambron, J.-C.; Chardon-Noblat, S.; Harriman, A.; Heitz, V.;
Sauvage, J.-P. Pure Appl. Chem. 1993, 65, 2343. (b) Harriman, A.; Sauvage,
J.-P. Chem. Soc. ReV. 1996, 26, 41. (c) Blanco, M.-J.; Jim e´ nez, M. C.;
Chambron, J.-C.; Heitz, V.; Linke, M.; Sauvage, J.-P. Chem. Soc. ReV. 1999,
28, 293. (d) Kurreck, H.; Huber, M. Angew. Chem. Int. Ed. Engl. 1995, 34,
849. (e) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759. (f) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995,
95, 2725.
ap.chem.eng.osaka-u.ac.jp; guldi.1@nd.edu; ito@icrs.tohoku.ac.jp; fukuzumi@
ap.chem.eng.osaka-u.ac.jp.
⊥
Department of Material and Life Science, Graduate School of
Engineering, Osaka University.
‡
The Institute of Scientific and Industrial Research, Osaka University.
Radiation Laboratory, University of Notre Dame.
Institute for Chemical Reaction Science, Tohoku University.
†
§
1
0.1021/ja003346i CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/23/2001

2608 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Imahori et al.
separated donor-acceptor radical ion pairs are generated as a
result of an initial CS followed, in succession, by several charge
shift (CSH) reactions along well-tuned redox gradients.
pointed out recently by Mataga,11b however, the bell-shaped
driving force dependence of the intramolecular CR rates
including both the normal and inverted regions for donor-
acceptor linked systems has yet to be demonstrated.
In this context, an intrinsic property of the 3-dimensional
electron acceptor C60, namely, its small reorganization energy
For intermolecular ET dynamics, the inverted region has
8a,11b,14,15
(
λ) associated with ET reactions, evokes a number of important
rarely been observed.
This infrequent manifestation has
6
-8
consequences.
Most importantly, CS and CR processes are
been rationalized in terms of the distribution of donor-acceptor
11b
accelerated and decelerated, respectively, relative to comparable
systems in which, however, 2-dimensional acceptor moieties
are used (i.e., p-benzoquinone). Therefore, efficient stepwise
CS in a fullerene-containing triad, along a well-designed redox
gradient, can be realized regardless of the solvent environment,
whereas this takes place only occasionally in conventional
distance on the energy gap. In particular, enlarging the driving
0
force, especially into the highly exergonic region (-∆G ET .
0), results in an increase in λ, especially in the solvent
reorganization energy (λs). It should be emphasized that this,
9
in turn, governs the high CS rates. In addition, the Marcus
theory hypothesizes that λs also increases with intermolecular
1
-6
9
systems.
It should be mentioned that these effects are the
donor-acceptor distance. Taking these two effects into account
small reorganization energy and its consequences resulting from
the fullerene’s unique structure and symmetry, which are
ultimately responsible for its high degree of its delocalization
and structural rigidity.6
it seems plausible that observation of the inverted region in
intermolecular ET systems is extremely difficult.1 Such a
change in λ, as a function of separation distance, awaits
experimental confirmation. An appealing approach for ac-
complishing this would be to determine λ values, associated
with special sets of molecules assembled so that each set would
consist of an identical homologous series of donor-acceptor
pairs but with each set having a different characteristic intramo-
lecular donor-acceptor separation. Such an experimental strat-
egy for the evaluation of ET parameters at different donor-
acceptor separations necessitates the design of a minimum of
two homologous donor-acceptor linked series (i.e., dyads and
triads). In these ensembles the redox potentials of donor-
acceptor moieties should be changed without affecting the
relative distance.
1b
-8
The Marcus theory of ET provides a valuable guide for
controlling and optimizing the efficiency of CS versus CR. In
particular, the rates of CR can be markedly slowed by shifting
them deep into the inverted region of the Marcus parabola,
0
where the driving force (-∆G ET) is larger than the total
reorganization energy (λ) of ET.9,10 Extensive efforts have been
directed toward establishing the driving force dependence of
1
1-17
the ET rates,
and thereby probing the inverted region of
8
a,11b,12-15
the Marcus curve in donor-acceptor couples.
As
(
5) (a) Osuka, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69,
7
97. (b) Maruyama, K.; Osuka, A.; Mataga, N. Pure Appl. Chem. 1994,
In the present study a novel family of dyad and triad systems
was designed by combining the following building blocks
(electron donor) in various orders: H2P (P ) tetraphenylpor-
phyrin dianion), ZnP, and ferrocene (Fc) with the electron
accepting C60 (see Figure 1). Thus, the above criteria, namely,
different donor-acceptor separations and/or different donor
strengths, are unequivocally given to probe the reorganization
energies and the electronic coupling matrix elements in fullerene-
containing ensembles. We report herein the intramolecular CR
processes in both the normal and inverted regions of the Marcus
curve, using a series of ferrocene-porphyrin-fullerene triads
6
6, 867. (c) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143.
(6) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Imahori,
H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445. (c) Guldi, D. M. Chem.
Commun. 2000, 321. (d) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000,
3
3, 695. (e) Fukuzumi, S.; Imahori, H. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2000, in press.
7) (a) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi,
S.; Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263,
45. (b) Tkachenko, N. V.; Guenther, C.; Imahori, H.; Tamaki, K.; Sakata,
(
5
Y.; Fukuzumi, S.; Lemmetyinen, H. Chem. Phys. Lett. 2000, 326, 344. (c)
Imahori, H.; Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.; Lemmetyinen,
H.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105, 1750.
(8) (a) Guldi, D. M.; Asmus, K.-D. J. Am. Chem. Soc. 1997, 119, 5744.
(b) Guldi, D. M.; Neta, P.; Asmus, K.-D. J. Phys. Chem. 1994, 98, 4617.
1
8-20
19
Fc-ZnP-C60
and Fc-H2P-C60, and zincporphyrin-free-
(9) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus,
2
1
R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (c) Marcus, R. A.
Angew. Chem. Int. Ed. Engl. 1993, 32, 1111.
base porphyrin-fullerene triad ZnP-H2P-C60 (Figure 1) with
equal edge-to-edge distances (Ree) of 30.3 Å. In addition, these
triads will be compared to a zincporphyrin-fullerene dyad ZnP-
(10) (a) Bixon, M.; Jortner, J. In Electron Transfer; Jortner, J., Bixon,
M., Eds.; John Wiley & Sons: New York, 1999; Part 1, pp 35-202. (b)
Ulstrup, J.; Jortner, J. J. Chem. Phys. 1975, 63, 4358.
2
1-23
C60 (Ree ) 11.9 Å).
Hereby, we highlight the immediate
(11) (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 7, 259. (b) Mataga,
impact on the reorganization energy and the electronic coupling
matrix element that stem from fine-tuning the nature and the
separation distance of the intervening spacer. In sum, the present
study provides an unambiguous dependence of ET rate constants
on both the distance and driving force in a series of homologous
donor-acceptor arrays.
N.; Miyasaka, H. In Electron Transfer; Jortner, J., Bixon, M., Eds.; John
Wiley & Sons: New York, 1999; Part 2, pp 431-496.
(12) (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. Soc.
1
984, 106, 3047. (b) Closs, G. L.; Miller, J. R. Science 1988, 240, 440.
13) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B.
J. Am. Chem. Soc. 1985, 107, 1080.
14) (a) Gould, I. R.; Moody, R.; Farid, S. J. Am. Chem. Soc. 1988,
10, 7242. (b) Gould, I. R.; Farid, S. Acc. Chem. Res. 1996, 29, 522.
15) (a) Mataga, N.; Kanda, Y.; Okada, Y. J. Phys. Chem. 1986, 90,
880. (b) Asahi, T.; Mataga, N. J. Phys. Chem. 1991, 95, 1956.
16) (a) Harrison, R. J.; Pearce, B.; Beddard, G. S.; Cowan, J. A.; Sanders,
(
(
1
(
(17) (a) DeGraziano, J. M.; Liddell, P. A.; Leggett, L.; Moore, A. L.;
Moore, T. A.; Gust, D. J. Phys. Chem. 1994, 98, 1758. (b) Osuka, A.; Noya,
G.; Taniguchi, S.; Okada, T.; Nishimura, Y.; Yamazaki, I.; Mataga, N.
Chem. Eur. J. 2000, 6, 33.
(18) Fujitsuka, M.; Ito, O.; Imahori, H.; Yamada, K.; Yamada, H.; Sakata,
Y. Chem. Lett. 1999, 721.
(19) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.
J. Phys. Chem. B 2000, 104, 2099.
(20) Imahori, H.; Tamaki, K.; Yamada, H.; Yamada, K.; Sakata, Y.;
Nishimura, Y.; Yamazaki, I.; Fujitsuka, M.; Ito, O. Carbon 2000, 38, 1599.
(21) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am.
Chem. Soc. 2000, 122, 6535.
3
(
J. K. M. Chem. Phys. 1987, 116, 429. (b) Asahi, T.; Ohkohchi, M.;
Matsusaka, R.; Mataga, N.; Zhang, R. P.; Osuka, A.; Maruyama, K. J. Am.
Chem. Soc. 1993, 115, 5665. (c) Harriman, A.; Heitz, V.; Sauvage, J.-P. J.
Phys. Chem. 1993, 97, 5940. (d) Khundkar, L. R.; Perry, J. W.; Hanson, J.
E.; Dervan, P. B. J. Am. Chem. Soc. 1994, 116, 9700. (e) Heitele, H.;
P o¨ llinger, F.; H a¨ berle, T.; Michel-Beyerle, M. E.; Staab, H. A. J. Phys.
Chem. 1994, 98, 7402. (f) Macpherson, A. N.; Liddell, P. A.; Lin, S.; Noss,
L.; Seely, G. R.; DeGraziano, J. M.; Moore, A. L.; Moore, T. A.; Gust, D.
J. Am. Chem. Soc. 1995, 117, 7202. (g) H a¨ berle, T.; Hirsch, J.; P o¨ llinger,
F.; Heitele, H.; Michel-Beyerle, M. E.; Ander, C.; D o¨ hling, A.; Krieger,
C.; R u¨ ckemann, A.; Staab, H. A. J. Phys. Chem. 1996, 100, 18269. (h)
Tsue, H.; Imahori, H.; Kaneda, T.; Tanaka, Y.; Okada, T.; Tamaki, K.;
Sakata, Y. J. Am. Chem. Soc. 2000, 122, 2279.
(22) Yamada, K.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.
Chem. Lett. 1999, 895.
(23) Imahori, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Sakata, Y.;
Fukuzumi, S. J. Phys. Chem. A. 2001, 105, 325.

Porphyrin-Fullerene Linked Dyads and Triads
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2609
Figure 1. Structures of molecular triads, dyads, and references used in this study.
of component chromophores making up these molecules. This
supports the notion that there is a lack of electronic interaction,
long- or short-range, between the individual chromophores in
their ground state configuration. Principally similar superposi-
tions of component spectra were found in THF and DMF
solutions.
Results and Discussion
Synthesis and Characterization. The synthesis and char-
acterization of all the compounds (Fc-ZnP-C60, Fc-H2P-C60,
ZnP-C60, H2P-C60, ZnP-H2P-C60, ZnP-Fc, H2P-Fc, ZnP-
CONH-ref, ZnP-NHCO-ref, Fc-NHCO-ref, and C60-ref (Fig-
ure 1)) are described in details in the Supporting Information
One-Electron Redox Potentials and ET Driving Force. An
accurate determination of the driving force (-∆G0ET) for all
the intramolecular ET processes required measuring the redox
potentials of the reference chromophores (ZnP-CONH-ref,
(pp S1-S7). It is important to note that the absorption spectra
of Fc-ZnP-C60, Fc-H2P-C60, ZnP-C60, H2P-C60, and ZnP-H2P-
C60 in benzonitrile are reasonable superpositions of the spectra

2610 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Imahori et al.
0
Table 1. One-Electron Redox Potentials (E ) of Porphyrin,
polarity, because of the relatively large Ree values (>11 Å)
+
a
Ferrocene, and C60 References in Various Solvents (vs Fc/Fc )
18-23
employed.
THF
) 7.58)
benzonitrile
(ꢀ ) 25.2)
DMF
) 36.7)
Photodynamics of Porphyrin-Fullerene Linked Systems.
Time-resolved transient absorption spectra, following picosecond
and nanosecond laser pulses, were employed to examine the
photodynamics of ZnP-C60, H2P-C60, Fc-ZnP-C60, Fc-H2P-
C60, and ZnP-H2P-C60. Concisely, in THF, benzonitrile, and
(
ꢀ
s
s
(ꢀ
s
compd
E0ox/V E0red/V E0ox/V E0red/V E0ox/V E0red/V
ZnP-CONH-ref
0.40 -1.96
.73
0.35
.69
-0.02
0.34 -1.82
0.71
0.30 -1.85
0.67
0.29 -1.80
0.55
0.25 -1.84
0.51
0
•+
•-
ZnP-NHCO-ref
Fc-NHCO-ref
b
DMF the spectral characteristics of ZnP and C60 were found
0
for the ZnP-C60 and ZnP-H2P-C60 ensembles, while Fc-ZnP-
-0.01
-0.01
+
•-
C60 and Fc-H2P-C60 triads gave rise to those of Fc and C60
C
60-ref
-1.02
-
-1.04
-1.45
-0.92
-1.38
2
4-32
(see below).
To monitor the intramolecular ET dynamics,
1.57
the absorption of the one-electron reduced form of the electron
a
The redox potentials were measured by differential pulse voltam-
NPF as a
Not detectable
•-
acceptor (C60 ) was analyzed in the near-infrared region (NIR)
metry in THF, benzonitrile, and DMF with 0.1 M n-Bu
4
6
-1
b
around 1000 nm.
supporting electrolyte with a sweep rate of 10 mV s
.
within a sweep range of 0 to -2.0 V.
Photophysics of Porphyrin-Fullerene Linked Dyads. (a)
ZnP-C60: Time-resolved transient absorption spectra of ZnP-
C60 were measured by pico- and nanosecond laser photolysis.
For instance, picosecond time-resolved absorption spectra, as
recorded typically for a benzonitrile solution of ZnP-C60
ZnP-NHCO-ref, Fc-NHCO-ref, and C60-ref) in various sol-
vents. The differential pulse voltammetry was performed in
THF, benzonitrile, and DMF solutions containing the same
supporting electrolyte (i.e., 0.1 M n-Bu4NPF6). For example,
the peak positions of ZnP-NHCO-ref (see Supporting Informa-
tion (p S10)), which correspond to the first and the second
reversible oxidation of the porphyrin moiety, shift progressively
to a more negative direction as the solvent polarity indicated
by the dielectric constant (ꢀs) was changed in the following
order: THF (ꢀs ) 7.58) < benzonitrile (ꢀs ) 25.2) < DMF
(
24) (a) Mart ´ı n, N.; S a´ nchez, L.; Illescas, B.; P e´ rez, I. Chem. ReV. 1998,
9
8, 2527. (b) Prato, M. J. Mater. Chem. 1997, 7, 1097. (c) Diederich, F.;
G o´ mez-L o´ pez, M. Chem. Soc. ReV. 1999, 28, 263. (d) Sun, Y.-P.; Riggs,
J. E.; Guo, Z.; Rollins, H. W. In Optical and Electronic Properties of
Fullerenes and Fullerene-Based Materials; Shinar, J., Vardeny, Z. V.,
Kafafi, Z. H., Eds.; Marcel Dekker: New York, 2000; pp 43-81. (e) Guldi,
D. M.; Kamat, P. V. In Fullerenes; Kadish, K. M., Ruoff, R. S., Eds.; John
Wiley & Sons: New York, 2000; Chapter 5, pp 225-281. (f) Fukuzumi,
S.; Guldi, D. M. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001, in press.
(ꢀs ) 36.7) (Table 1). Negative shifts in the oxidation potentials
in increasingly more polar solvents are principally rationalized
by a stronger solvation, which the resulting radical cations and
dications of the porphyrin moieties experience. Similar negative
shifts were noted for the first and second oxidation steps in
ZnP-CONH-ref (Table 1). On the contrary, the first one-
electron oxidation potential of Fc-NHCO-ref remains nearly
constant, despite the substantial increase in solvent polarity. The
last reference (i.e., C60-ref) exhibits positive shifts for the
underlying first and second one-electron reduction potentials,
which are parallel to the preferential stabilization of the electron
relative to the radical anion with the increase in solvent polarity.
Table 1 summarizes all the redox potentials of the investigated
references.
(
25) (a) 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. (b) 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. (c) Gust, D.; Moore, T. A.; Moore,
A. L. Res. Chem. Intermed. 1997, 23, 621. (d) 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. (e) 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.
(f) Kuciauskas, D.; Liddell, P. A.; Moore, T. A.; Gust, D. J. Am. Chem.
Soc. 1998, 120, 10880. (g) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson,
T. E.; Weghorn, S. J.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D.
J. Am. Chem. Soc. 1999, 121, 8604. (h) Kuciauskas, D.; Liddell, P. A.;
Lin, S.; Stone, S. G.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem.
B 2000, 104, 4307.
(26) (a) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem.
Soc. 1995, 117, 4093. (b) Lawson, J. M.; Oliver, A. M.; Rothenfluh, D. F.;
An, Y.-Z.; Ellis, G. A.; Ranasinghe, M. G.; Khan, S. I.; Franz, A. G.;
Ganapathi, P. S.; Shephard, M. J.; Paddon-Row, M. N.; Rubin, Y. J. Org.
Chem. 1996, 61, 5032. (c) Williams, R. M.; Koeberg, M.; Lawson, J. M.;
An, Y.-Z.; Rubin, Y.; Paddon-Row, M. N.; Verhoeven, J. W. J. Org. Chem.
0
The driving forces (-∆G ET(CR) in eV) for the intramolecular
•
-
CR processes from C60 radical anion (C60 ) to zincporphyrin
•
+
+
•+
•-
radical cation (ZnP ) or ferricenium ion (Fc ) in ZnP -C60
,
+
•-
+
•-
•+
•-
Fc -ZnP-C60 , Fc -H2P-C60 , and ZnP -H2P-C60 were
calculated by eq 1, where e stands for the elementary charge.
1
996, 61, 5055. (d) Bell, T. D. M.; Smith, T. A.; Ghiggino, K. P.;
Ranasinghe, M. G.; Shephard, M. J.; Paddon-Row, M. N. Chem. Phys. Lett.
0
0
ox
•+
0
red
•-
1
997, 268, 223.
27) (a) Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Bioorg. Med. Chem.
996, 4, 767. (b) Baran, P. S.; Monaco, R. R.; Khan, A. U.; Schuster, D.
-
∆G
) e[E (D /D) - E (A/A )]
(1)
ET(CR)
0
(
1
•+
In short, [E ox(D /D)] is the first one-electron oxidation
potential of the donor (zincporphyrin or ferrocene) moiety, while
E red(A/A )] refers to the first one-electron reduction potential
of the acceptor (C60) moiety in THF, benzonitrile, and DMF.
The accordingly calculated -∆G ET(CR) values are listed in
Tables 2-4. Furthermore, the driving forces (-∆G ET(CS) in eV)
I.; Wilson, S. R. J. Am. Chem. Soc. 1997, 119, 8363. (c) 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. J. Am. Chem.
Soc. 1999, 121, 11599. (d) Fong, R., II; Schuster, D. I.; Wilson, S. R. Org.
Lett. 1999, 1, 729. (e) Wilson, S. R.; Schuster, D. I.; Nuber, B.; Meier, M.
S.; Maggini, M.; Prato, M.; Taylor, R. Fullerenes; Kadish, K. M., Ruoff,
R. S., Eds.; John Wiley & Sons: New York, 2000; Chapter 3, pp 91-176.
0
•-
[
0
0
(28) (a) Nierengarten, J.-F.; Schall, C.; Nicoud, J.-F. Angew. Chem. Int.
for the intramolecular CS processes (ZnP-C60, Fc-ZnP-C60, Fc-
H2P-C60, and ZnP-H2P-C60) were determined by:
Ed. 1998, 37, 1934. (b) Bourgeois, J.-P.; Diederlich, F.; Echegoyen, L.;
Nierengarten, J.-F. HelV. Chim. Acta 1998, 81, 1835. (c) Armaroli, N.;
Diederlich, F.; Dietrich-Buchecker, C. O.; Flamigni, L.; Marconi, G.;
Nierengarten, J.-F.; Sauvage, J.-P. Chem. Eur. J. 1998, 4, 406. (d) Armspach,
D.; Constable, E. C.; Diederlich, F.; Housecroft, C. E.; Nierengarten, J.-F.
Chem. Eur. J. 1998, 4, 723. (e) Camps, X.; Dietel, E.; Hirsch, A.; Pyo, S.;
Echegoyen, L.; Hackbarth, S.; R o¨ der, B. Chem. Eur. J. 1999, 5, 2362. (f)
Armaroli, N.; Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.; Diederlich, F.
Chem. Eur. J. 2000, 6, 1629. (g) van Hall, P. A.; Knol, J.; Langeveld-
Voss, B. M. W.; Meskers, S. C. J.; Hummelen, J. C.; Janssen, R. A. J. J.
Phys. Chem. A 2000, 104, 5974. (h) Eckert, J.-F.; Nicoud, J.-F.; Nieren-
garten, J.-F.; Liu, S.-G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali,
L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467.
0
0
-∆G
) ∆E0-0 + ∆G
(2)
ET(CS)
ET(CR)
Hereby, ∆E0-0 is the energy of the 0-0 transition energy
gap between the lowest excited state and the ground state. The
0
-
∆G ET(CS) values are given in Tables 2-4. It should be noted
that the Coulombic terms in the present donor-acceptor systems
are negligible, especially in solvents with moderate or high

Porphyrin-Fullerene Linked Dyads and Triads
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2611
Table 2. ET Rate Constants (kET) for CS and CR and the Driving Force (-∆G0ET) in ZnP-C60 and H
P-C60
ET/s-1,b
2
solvent
initial statea
final statea
-∆G0ET/eV
k
Φc
1
•+
•-
10,d
1
THF
ZnP*-C60 (2.07 eV)
ZnP -C60 (1.42 eV)
0.65
0.33
0.08
1.42
0.66
0.37
0.12
0.15
1.38
0.30
0.85
1.21
k
k
k
k
k
k
k
k
k
k
k
k
ET(CS1) ) 1.3 × 10
ET(CS2) ) 5.1 × 10
ET(CS3) ) 1.6 × 10
Φ
Φ
Φ
CS1( ZnP*) ) 0.96
1
•+
•-
8
7
5
1
(ꢀ
s
) 7.58)
ZnP- C60* (1.75 eV)
ZnP- C60* (1.50 eV)
ZnP -C60 (1.42 eV)
CS2( C60*) ) 0.40
3
•+
•-
3
ZnP -C60 (1.42 eV)
CS3( C60*) ∼ 1
•
+
•-
ZnP -C60 (1.42 eV)
ZnP-C60
ET(CR1) ) 3.7 × 10
1
•+
•-
9
1
benzonitrile
ZnP*-C60 (2.04 eV)
ZnP -C60 (1.38 eV)
ET(CS1) ) 9.5 × 10
ET(CS2) ) 5.5 × 10
ET(CS3) ) 1.5 × 10
ET(CS4) > 1.5 × 10
Φ
Φ
Φ
Φ
Φ
Φ
Φ
CS1( ZnP*) ) 0.95
1
•+
•-
8
1
(ꢀ
s
) 25.2)
ZnP- C60* (1.75 eV)
ZnP -C60 (1.38 eV)
CS2( C60*) ) 0.42
3
•+
•-
7
3
ZnP- C60* (1.50 eV)
ZnP -C60 (1.38 eV)
CS3( C60*) ∼ 1
3
•+
•-
7
3
ZnP*-C60 (1.53 eV)
ZnP -C60 (1.38 eV)
CS4( ZnP*) ∼ 1
•
+
•-
6,e
f
e
ZnP -C60 (1.38 eV)
ZnP-C60
ET(CR1) ) 1.3 × 10
CS(total) ) 0.85 (0.99)
1
•+
•-
9,e
1
e
H
2
P*-C60 (1.89 eV)
H
2
P -C60 (1.59 eV)
ET(CS1) ) 5.2 × 10
ET(CS1) ) 1.3 × 10
CS1( H
2
P*) ) 0.98
1
•+
•-
10,d
1
DMF
ZnP*-C60 (2.06 eV)
ZnP -C60 (1.21 eV)
CS1( ZnP*) ) 0.96
•
+
•-
6
(ꢀ
s
)36.7)
ZnP -C60 (1.21 eV)
ZnP-C60
ET(CR1) ) 1.8 × 10
a
The energy of each state relative to the ground state is given in parentheses. b The kET values for CS from ZnP* and C60* were determined
1
1
from the fluorescence lifetimes by using the following equation: kET ) (1/τ(ZnP-C60)) - (1/τ(ZnP-CONH-ref or C60-ref)). The other kET values
•
-
c
for CS and CR were determined by analyzing the rise or decay of C60 at 1000 nm. The efficiencies (Φ) for each deactivation pathway were
estimated on the basis of Schemes 1 and 2. From ref 22. e From ref 21. The total quantum yield of CS based on the comparative method. The
d
f
corresponding value obtained from the kinetic data is given in parentheses.
(absorption ratio of ZnP:C60 ) 77:23 at 532 nm), are displayed
in Figure 2. In detail, the differential spectrum taken immediately
after the laser pulse (50 ps delay) is characterized by the
bleaching of the porphyrin Q-band absorption at 560 and 600
nm and the stimulating emission from the zinc porphyrin singlet
1
excited state ( ZnP*) at 650 nm. At a delay time of 200 ps, a
new transition around 1000 nm grows-in, accompanied by
another, but much broader absorption around 650 nm, which
1
importantly differs from the spectral features of the ZnP* and
3
21
triplet excited state ( ZnP*). Based on our previous assign-
ment,1
8,21,23,30,31
we ascribe the former and the latter bands to
Figure 2. Picosecond time-resolved absorption spectra of ZnP-C60 at
a time delay of 50 (thin line) and 200 ps (thick line) excited at 532 nm
(
29) (a) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem.
(absorption ratio of ZnP:C60 ) 77:23) in argon-saturated benzonitrile.
Soc. 1997, 119, 974. (b) Polese, A.; Mondini, S.; Bianco, A.; Toniolo, C.;
Scorrano, G.; Guldi, D. M.; Maggini, M. J. Am. Chem. Soc. 1999, 121,
The spectra are normalized at the Soret band for comparison.
3
446. (c) Maggini, M.; Guldi, D. M.; Mondini, S.; Scorrano, G.; Paolucci,
Scheme 1. Reaction Scheme and Energy Diagram for
ZnP-C60 in Benzonitrile
F.; Ceroni, P.; Roffia, S. Chem. Eur. J. 1998, 4, 1992. (d) Guldi, D. M.;
Luo, C.; Prato, M.; Dietel, E.; Hirsch, A. Chem. Commun. 2000, 373. (e)
Guldi, D. M.; Luo, C.; Da Ros, T.; Prato, M.; Dietel, E.; Hirsch, A. Chem.
Commun. 2000, 375. (f) Mart ´ı n, N.; S a´ nchez, L.; Herranz, M. A.; Guldi,
D. M. J. Phys. Chem. A 2000, 104, 4648. (g) Luo, C.; Guldi, D. M.; Maggini,
M.; Menna, E.; Mondini, S.; Kotov, N. A.; Prato, M. Angew. Chem. Int.
Ed. 2000, 39, 3905. (h) Guldi, D. M.; Maggini, M.; Menna, E.; Scorrano,
G.; Ceroni, P.; Marcaccio, M.; Paolucci, F.; Roffia, S. Chem. Eur. J. 2001,
in press.
(30) (a) Imahori, H.; Hagiwara, K.; Akiyama, T.; Taniguchi, S.; Okada,
T.; Sakata, Y. Chem. Lett. 1995, 265. (b) Imahori, H.; Sakata, Y. Chem.
Lett. 1996, 199. (c) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.;
Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc.
1
996, 118, 11771. (d) Akiyama, T.; Imahori, H.; Ajavakom, A.; Sakata, Y.
•
- 33
the C60 radical anion (C60
)
and the zinc porphyrin radical
•
+ 34
cation (ZnP ), respectively. In accordance with these results
we propose the occurrence of a photoinduced ET, evolving from
ZnP* to the C60 and, in turn, creating the ZnP -C60 state.
1
•+
•-
The energy levels in benzonitrile, as extracted from Table 2,
are shown in Scheme 1 to illustrate the different relaxation
pathways of photoexcited ZnP-C60. Similar transient absorption
spectra, specifically, the spectral fingerprints of ZnP• and C60
+
•-
,
were obtained in THF and DMF.
M. V. J. Phys. Chem. B 1999, 103, 8864. (b) Thomas, K. G.; Biju, V.;
Guldi, D. M.; Kamat, P. V.; George, M. V. J. Phys. Chem. A 1999, 103,
(33) (a) Greaney, M. A.; Gorun, S. M. J. Phys. Chem. 1991, 95, 7142.
(b) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L.
B. F.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 4364. (c) Kato, T.;
Kodama, T.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru,
H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 180, 446. (d) Gasyna,
Z.; Andrews, L.; Schatz, P. J. Phys. Chem. 1992, 96, 1525.
1
0755. (c) Tkachenko, N. V.; Rantala, L.; Tauber, A. Y.; Helaja, J.;
Hynninen, P. H.; Lemmetyinen, H. J. Am. Chem. Soc. 1999, 121, 9378. (d)
Tkachenko, N. V.; Vuorimaa, E.; Kesti, T.; Alekseev, A. S.; Tauber, A.
Y.; Hynninen, P. H.; Lemmetyinen, H. J. Phys. Chem. B 2000, 104, 6371.
(
e) Montforts, F.-P.; Kutzki, O. Angew. Chem. Int. Ed. 2000, 39, 599. (f)
D’Souza, F.; Deviprasad, G. R.; Rahman, M. S.; Choi, J.-p. Inorg. Chem.
999, 38, 2157.
(34) (a) Fuhrhop, J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91, 4174.
(b) Chosrowjan, H.; Taniguchi, S.; Okada, T.; Takagi, S.; Arai, T.;
Tokumaru, K. Chem Phys. Lett. 1995, 242, 644.
1

2612 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Imahori et al.
Table 3. ET Rate Constants (kET) for CS, CR, and CSH and the Driving Forces (-∆G0ET) in Fc-ZnP-C60
solvent
initial statea
final statea
-∆G0ET/eV
k
ET/s-1,b
Φc
1
•+
•-
10,d
8
1
THF
Fc- ZnP*-C60 (2.07 eV)
Fc-ZnP -C60 (1.42 eV)
0.65
0.33
0.13
0.42
1.00
0.66
0.37
0.12
0.15
0.23
k
k
k
k
k
k
k
k
k
k
ET(CS1) ) 1.3 × 10
ET(CS2) ) 2.3 × 10
ET(CS5) ) 2.4 × 10
Φ
Φ
Φ
Φ
Φ
Φ
Φ
Φ
Φ
Φ
CS1( ZnP*) ) 0.95
1
•+
•-
1
(ꢀ
s
)7.58)
Fc-ZnP- C60* (1.75 eV)
Fc-ZnP -C60 (1.42 eV)
CS2( C60*) ) 0.23
1
+
•-
8,d
9
1
Fc- ZnP*-C60 (2.07 eV)
Fc -ZnP -C60 (1.94 eV)
CS5( ZnP*) ) 0.02
•
+
•-
+
•-
Fc-ZnP -C60 (1.42 eV)
Fc -ZnP-C60 (1.00 eV)
Fc-ZnP-C60
ET(CSH1) ) 1.4 × 10
CSH1 ∼ 1
+
•-
5,e
f
Fc -ZnP-C60 (1.00 eV)
ET(CR2) ) 2.7 × 10
CS(total) ) 0.74
1
•+
•-
9
1
benzonitrile
Fc- ZnP*-C60 (2.04 eV)
Fc-ZnP -C60 (1.38 eV)
ET(CS1) ) 9.5 × 10
ET(CS2) ) 5.1 × 10
ET(CS3) ) 3.2 × 10
ET(CS4) > 3.2 × 10
CS1( ZnP*)) 0.90
1
•+
•-
8
6
6
1
(ꢀ
s
)25.2)
Fc-ZnP- C60* (1.75 eV)
Fc-ZnP -C60 (1.38 eV)
CS2( C60*)) 0.40
3
•+
•-
3
Fc-ZnP- C60* (1.50 eV)
Fc-ZnP -C60 (1.38 eV)
CS3( C60*) ) 0.99
3
•+
•-
3
Fc- ZnP*-C60 (1.53 eV)
Fc-ZnP -C60 (1.38 eV)
CS4( ZnP*) ) 0.99
1
+
•-
1
Fc- ZnP*-C60
Fc -ZnP -C60
(1.81 eV)
Fc -ZnP-C60 (1.03 eV)
Fc-ZnP-C60
Fc-ZnP-C60
ET(CS5)
8
CS5( ZnP*)
(
2.04 eV)
)5.5 × 10
)0.05
CSH1 ∼ 1
•
+
•-
+
•-
9
Fc-ZnP -C60 (1.38 eV)
0.35
1.38
1.03
0.91
k
k
k
k
ET(CSH1) ) 2.8 × 10
Φ
•
+
•-
6
Fc-ZnP -C60 (1.38 eV)
ET(CR1) ) 1.3 × 10
+
•-
5,e
4,e
f
Fc -ZnP-C60 (1.03 eV)
ET(CR2) ) 1.3 × 10
ET(CR2) ) 6.3 × 10
Φ
Φ
CS(total) ) 0.82 (0.99)
CS(total) ) 0.36
+
•-
f
DMF
(ꢀ
s
Fc -ZnP-C60 (0.91 eV)
Fc-ZnP-C60
)36.7)
a
The energy of each state relative to the ground state is given in parentheses. b The kET values for CS from ZnP* and C60* were determined
1
1
from the fluorescence lifetimes by using the following equation: kET ) (1/τ (Fc-ZnP-C60 or ZnP-Fc)) - (1/τ(ZnP-Fc or ZnP-CONH-ref or
•
-
•+
C
60-ref)). The other kET values for CS, CR, and CSH were determined by analyzing the rise and decay of C60 at 1000 nm and the decay of ZnP
c
d
e
at 660 nm, respectively. The efficiencies (Φ) for each deactivation pathway were estimated on the basis of Scheme 3. From ref 19. From ref
f
1
8. The total quantum yield of CS based on the comparative method. The corresponding value obtained from the kinetic data is given in parentheses.
Table 4. ET Rate Constants (kET) for CR and the Driving Forces (-∆G0ET) in Fc-H
P-C60 and ZnP-H
P-C60
ET/s-1,b
2
2
solvent
THF
initial statea
final statea
ZnP-H
-∆G0ET/eV
k
Φ
CS(total)c
•
+
•-
4
ZnP -H
2
P-C60 (1.37 eV)
2
P-C60
1.37
k
ET(CR2) ) 2.9 × 10
Φ
CS(total) ) 0.26
(
ꢀ
s
) 7.58)
+
•-
5
benzonitrile
Fc -H
2
P-C60 (1.03 eV)
Fc-H
ZnP-H
Fc-H
ZnP-H
2
P-C60
P-C60
P-C60
P-C60
1.03
1.34
0.91
1.17
k
k
k
k
ET(CR2) ) 1.2 × 10
ET(CR2) ) 4.8 × 10
ET(CR2) ) 5.3 × 10
ET(CR2) ) 5.0 × 10
Φ
Φ
Φ
Φ
CS(total) ) 0.25
CS(total) ) 0.40
CS(total) ) 0.27
CS(total) ) 0.21
•
+
•-
4,d
4
(
ꢀ
s
) 25.2)
) 36.7)
ZnP -H
2
P-C60 (1.34 eV)
2
+
•-
DMF
Fc -H
2
P-C60 (0.91 eV)
2
•
+
•-
4
(ꢀ
s
ZnP -H
2
P-C60 (1.17 eV)
2
a
b
The energy of each state relative to the ground state is given in parentheses. The kET values for CR were determined by analyzing the decay
•
-
c
d
of C60 at 1000 nm. The total quantum yield of CS based on the comparative method. From ref 21.
Table 5. Fluorescence Lifetimes (τ) of ZnP-C60, Fc-ZnP-C60, and
the Reference Compounds in THF, Benzonitrile, and DMFa
lifetimes (τ)/ns
λ
s
obs [THF
) 7.58)]
λ
obs [benzonitrile
(ꢀ ) 25.2)]
λobs [DMF
(
ꢀ
s
(ꢀ
s
) 36.7)]
610 nm
0.075b
compd
ZnP-C60
ZnP-CONH-ref 2.1
60-ref
610 nm 720 nm 610 nm 720 nm
b
0.075
0.78
0.10
2.0
0.76
b
b
2.0
C
1.3
1.0
1.3
0.78
c
Fc-ZnP-C60
ZnP-Fc
0.074
0.095
0.95
c
1.4
a
ex ) 410 nm. b From ref 22. c From ref 19.
λ
Next we measured the fluorescence lifetimes (τ) of ZnP-
C60, ZnP-CONH-ref, and C60-ref with a time-correlated single-
photon-counting apparatus by using a 410 nm excitation
Figure 3. Nanosecond time-resolved absorption spectra of ZnP-C60
in argon-saturated benzonitrile excited at 532 nm (absorption ratio of
ZnP:C60 ) 77:23). The delay times between the excitation and
measurement are indicated on the left. The time profile of absorbance
at 1000 nm is shown as the inset.
(absorption ratio (ZnP-C60) of ZnP:C60 ) 90:10 in benzonitrile).
These experiments allowed probing the decay dynamics of the
photoexcited fullerene and porphyrin cores separately. Therefore,
the fluorescence decay was monitored at 610 and 720 nm,
3
0c
relating to the emission of the porphyrin (λmax ) 605, 655 nm)
and the C60 (λmax ) 720 nm)30c moiety, respectively. In general,
the fluorescence decay curves were well fitted by a single-
exponential decay component (see Table 5).
state and the C60 singlet excited state ( C60*). The ET rate
1
constant (kET(CS2); see Supporting Information (p S8)) and the
•
+
•-
1
efficiency of ZnP -C60 formation (ΦCS2( C60*)) from the
1
8 -1
On the basis of the fluorescence lifetimes (λobs ) 610 nm) in
C60* were determined as 5.5 × 10 s and 0.42 in benzonitrile,
9
-1
benzonitrile, the ET rate constants (kET(CS1) ) 9.5 × 10 s ;
respectively (Table 2).
1
Formation of C60 (1000 nm) and ZnP•+ (broad absorption
around 650 nm) was further substantiated (Figure 3) by a set
of complementary nanosecond experiments. Interestingly, in
•-
see Supporting Information (p S8)) for CS from ZnP* and the
1
•+
•-
efficiency (ΦCS1( ZnP*) ) 0.95) of ZnP -C60 formation were
determined. Nearly the same kET(CS1) values were found in THF
(
1
0
-1 22
10 -1 22
•-
•+
1.3 × 10 s ) and DMF (1.3 × 10 s ) (Table 2). Since
addition to the C60 and ZnP characteristics, we also noted
3
there is considerable excitation of the fullerene core that cannot
be avoided, we would like to emphasize that CS, and, thereby,
evidence for the fullerene triplet state ( C60*), as inferred by
the 700 nm absorption maximum. In line with this fullerene
triplet formation, the time profile at 1000 nm due to C60 reveals
2
1
•+
•-
•-
generation of ZnP -C60 , also occurs between the ZnP ground

Porphyrin-Fullerene Linked Dyads and Triads
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2613
7
a rise and a decay component with rate constants of 1.5 × 10
Scheme 2. Reaction Scheme and Energy Diagram for
H2P-C60 in Benzonitrile
6
-1
and 1.3 × 10 s , respectively (Figure 3, inset). Considering
the energy diagram in a polar solvent (shown for benzonitrile
in Scheme 1), photoinduced CS evolving from the fullerene
triplet state is thermodynamically exothermic and thus likely
to occur. In accordance with this conclusion, based on energet-
ics, is the fact that the rise in absorbance at 1000 nm ((kET(CS3))
7
-1
of 1.5 × 10 s ) matches exactly the C60 triplet decay at 700
3
nm. Given the intrinsic rate constant for the decay of C60*
4
-1 21
(
kT1 ) 4.0 × 10 s ), the efficiency of formation of the
3 3
charge-separated state from C60* (ΦCS3( C60*); see Supporting
Information (p S8)) is nearly unity (Table 2).
Since the energy level of the porphyrin triplet excited state
ZnP*: 1.53 eV) is slightly higher than that of the corre-
(3
21
3
21
sponding fullerene triplet state ( C60*: 1.50 eV), the charge-
Scheme 3. Reaction Scheme and Energy Diagram for
Fc-ZnP-C60 in Benzonitrile
3
separated state may also be generated via the transient ZnP*
7
-1
through ET with a rate constant (kET(CS4)) of >1.5 × 10 s .
4
-1 21
Taking the decay rate constant (kT2 ) 2.3 × 10 s ) of a
zinc-meso-tetraphenylporphyrin (ZnTPP) triplet excited state into
account, the efficiency of formation of the charge-separated state
3
3
from ZnP* (ΦCS4( ZnP*); see Supporting Information (p S8))
should also be close to unity (Table 2).
On the basis of the quantum yields of porphyrin (ΦISC2-
(1
ZnP*) ) 0.88) and C60 (ΦISC1( C60*) ) 0.98) triplet excited
21
1
21
•
+
•-
state formation, the total efficiency of ZnP -C60 formation
ΦCS(total)(ZnP-C60)) from the initial excited states in benzonitrile
(
35
is estimated to be 0.99 (Table 2). This appraisal is consistent
with the high quantum yield for CS of 0.85 determined from
the nanosecond time-resolved transient spectra (monofunction-
7
-1
•
-
-1
-1,36
with a rate constant of 2.2 × 10 s , generating, however,
alized C60 ((ꢀ1000nm ) 4700 M cm
) see Supporting
3
Information).37
eventually the porphyrin triplet state ( H P*) rather than the
2
2
1
1
ground state. The CS rate starting from the C60* (kET(CS2))
The resulting charge-separated state recombines to regenerate
the singlet ground state. From the decay kinetics at 1000 nm a
could not be determined by the fluorescence lifetime measure-
ments because of the interference caused by the strong H2P
6
-1
rate constant (kET(CR1)) of 1.3 × 10 s is deduced for a
benzonitrile solution.21 Remarkably, this rate is nearly 4 orders
emission (λmax(em) ) 655, 720 nm), which overlaps extensively
with the much weaker fullerene emission (λmax(em) ) 720 nm).
of magnitude slower than the fastest occurring CS, namely, that
1
1
9
-1
The unquenched H P- C * (1.75 eV) undergoes an intersystem
stemming from ZnP* (kET(CS1) ) 9.5 × 10 s ). Such a fast
CS, combined with a slow CR, in the ZnP-C60 dyad in polar
solvents is in clear contrast to conventional porphyrin-quinone
and -diimide linked dyads, where the CR rates are even faster
2
60
3
crossing to H2P- C60* (1.50 eV), which then decays either to
the ground state or to the H2P*-C60 state (1.40 eV).
Photophysics of Porphyrin-Fullerene Linked Triads. (a)
Fc-ZnP-C : The energy levels in benzonitrile, which are
expected to be of significance for the photoinduced ET reactions
in Fc-ZnP-C60, are taken from the data summarized in Table 3
and are diagrammed in Scheme 3.
Figure 4a depicts, as a representative example, the picosecond
absorption spectra of Fc-ZnP-C60 in a benzonitrile solution
(absorption ratio of Fc:ZnP:C60 ) 1:75:24 at 532 nm). Generally,
the spectral behavior of Fc-ZnP-C60 is similar to that discussed
above for ZnP-C60 (Figure 2). From this analogy we may
already gather that again two major CS pathways prevail, which
3
21
than the CS rates in polar solvents.2
,13,16
60
The photodynamical behavior of the ZnP-C60 dyad in THF
and DMF is similar to that described in benzonitrile (Table 2).
A possible explanation for this analogy is based on the
corresponding energy levels. In particular, the energies of the
1
3
excited states (i.e., ZnP* (2.04-2.07 eV), ZnP* (1.53 eV),
1
3
C60* (1.75 eV), and C60* (1.50 eV)) are substantially higher
than the energy of the charge-separated state in THF (1.42 eV),
benzonitrile (1.38 eV), and DMF (1.21 eV). This, in turn,
guarantees high driving forces for the associated CS and CR
processes.
•
+
•-
produce in both cases Fc-ZnP -C60 (Scheme 3). First, a fast
1
step occurs, which involves CS from ZnP* to C60 (kET(CS1) )
(b) H2P-C60: A similar picture can be summarized for the
9
-1
1
9
.5 × 10 s , ΦCS1( ZnP*) ) 0.90), and second, a slower
H2P-C60 dyad in benzonitrile (Scheme 2). CS from the freebase
1 8 -1
1
reaction between ZnP and C60* (kET(CS2) ) 5.1 × 10 s , ΦCS2-
porphyrin singlet excited state ( H2P* (1.89 eV)) to C60 in
1
•
+
•-
( C60*) ) 0.40). The rate constants (kET(CS1) and kET(CS2)) and
benzonitrile occurs to yield H2P -C60 (1.59 eV) with a rate
•
+
•-
1
9
-1
21
the efficiencies of Fc-ZnP -C60 formation from the ZnP*
constant (kET(CS1)) of 5.2 × 10 s (Table 2). In contrast to
1
1
1
(
ΦCS1( ZnP*)) and from the C60* (ΦCS2( C60*)) were deter-
mined as described for ZnP-C60 (see Supporting Information
p S9)). The fluorescence lifetime measurements of the ZnP-
Fc reference and ZnP-CONH-ref in benzonitrile (Table 5)
indicate, however, that yet another CS may occur, namely, from
the ZnP-C60 dyad, the resulting charge-separated state decays
1
(
35) ΦCS(total)(ZnP-C60) ) (absorption ratio of ZnP) × [ΦCS1( ZnP*)
(
1
3
1
+
+
0
ΦISC2( ZnP*) × ΦCS4( ZnP*)] + (absorption ratio of C60) × [ΦCS2( C60*)
1 3
ΦISC1( C60*) × ΦCS3( C60*)] ) 0.77 × [0.95 + 0.05 × 0.88 × 1] +
.23 × [0.40 + 0.60 × 0.98 × 1] ) 0.99.
36) Luo, C.; Fujitsuka, M.; Huang, C.-H.; Ito, O. Phys. Chem. Chem.
Phys. 1999, 1, 2923.
1
+
•-
(
the ferrocene to the ZnP* to produce Fc -ZnP (kET(CS5) )
8
-1
5
.5 × 10 s ; See Supporting Information (p S9)). The CS5 is
(
37) The difference in the ΦCS(total) values, obtained from the compara-
slower by a factor of ∼1/20 than the analogous process
tive method and the kinetic analysis, may largely stem from the experimental
uncertainty in the extinction coefficients of the transient species.
9
-1
involving C60 in Fc-ZnP-C60 (kET(CS1) ) 9.5 × 10 s ). Thus,

2614 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Imahori et al.
3
21
sponding fullerene triplet state ( C60*: 1.50 eV), the charge-
3
separated state may also be generated via the transient ZnP*
6
-1
through ET with a rate constant (kET(CS4)) of >3.2 × 10 s ,
as in the case of ZnP-C60. Taking the decay rate constant
4
-1 21
(
kT2 ) 2.3 × 10 s ) of a zinc-meso-tetraphenylporphyrin
(
ZnTPP) triplet excited state into account, the efficiency of
3
3
formation of the charge-separated state from ZnP* (ΦCS4( ZnP*))
should also be 0.99 (see Supporting Information (p S9)).
+
•-
Assuming that the efficiency of Fc -ZnP-C60 formation
1
+
•-
(
ΦCS5 × ΦCSH2) from ZnP* via Fc -ZnP -C60, which is a
minor CS pathway, is unity and that the overall absorption of
+
the ferrocene moiety is negligible, the total efficiency of Fc -
ZnP-C60 formation (ΦCS(total)) from the initial excited states,
as determined from the kET and ΦCS values in Table 3, is 0.99
Table 3). This is consistent with the quantum yield obtained
from the transient absorption spectra (ΦCS(total) ) 0.82).
•-
3
9
(
36,37
Thus, even in benzonitrile the Fc-ZnP-C60 triad produces, upon
photoexcitation, a long-lived charge-separated state (7.5 µs) with
an extremely high efficiency (0.99). This can be understood
reasonably by the fact that C60 has a small reorganization energy
in ET, which, in turn, assists in accelerating CS and CSH
reaction, while decelerating CR processes.
Figure 4. (a) Picosecond time-resolved absorption spectra of Fc-ZnP-
60 at delay time of 40 (thin line) and 100 ps (thick line) excited at
32 nm (absorption ratio of Fc:ZnP:C60 ) 1:75:24) in argon-saturated
C
5
benzonitrile. The spectra are normalized at the Soret band for
comparison. (b) Time profile of absorbance at 660 nm. The solid line
A similar photodynamical behavior was noted in THF and
DMF solutions (Table 3). For instance, the CSH rate constant
•
+
-1
9
-1
is a simulated curve using (τ(ZnP )) ) 2.8 × 10 s
.
(
kET(CSH1)) and the efficiency of the CSH (ΦCSH1) in THF were
9
-1
found to be 1.4 × 10 s and nearly unity, respectively.
+
•-
this deactivation pathway to generate Fc -ZnP -C60 (ΦCS5-
(
•-
By analyzing the decay kinetics of the C60 fingerprint at
1
ZnP*) ) 0.05; see Supporting Information (p S9)) is rather
negligible, although the charge-separated state could then
undergo an exothermic charge shift (CSH) (-∆G ET(CSH2) )
.78 eV) to generate Fc -ZnP-C60 efficiently.
+
•-
1
000 nm, the ET rate constants for CR in Fc -ZnP-C60
5
5
(
kET(CR2)) were determined as 2.7 × 10 (THF), 1.3 × 10
0
4
-1
18
(benzonitrile), and 6.3 × 10 s (DMF). The total quantum
+
•-
0
yields of CS (ΦCS(total)) in Fc-ZnP-C60 were found to be 0.74
in THF and 0.36 in DMF by the comparative method from the
corresponding transient absorption spectra (Table 3).36
While nanosecond transient absorption studies following laser
•
-
excitation of Fc-ZnP-C60 reveal unambiguously the C60
fingerprint (ca. 1000 nm), as in the case of ZnP-C60 (Figure
•
+
(b) Fc-H2P-C60: As a consequence of the poor signal-to-
noise ratio, picosecond time-resolved transient absorption studies
with Fc-H2P-C60 in benzonitrile were unsuccessful with respect
3
), none of the ZnP attributes (650 nm) could be detected on
this time scale (see Supporting Information (p S11)). Taking
the small molar absorption coefficient of the ferricenium ion
•-
-
1
-1
38
to any assignment of a C60 fingerprint (ca. 1000 nm). Instead,
nanosecond time-resolved transient absorption spectra of Fc-
H2P-C60 (absorption ratio of Fc:H2P:C60 ) 1:86:13 at 532 nm)
in benzonitrile were recorded (see Supporting Information
(p S12)). The spectral features of Fc-H2P-C60 are quite different
from those of Fc-ZnP-C60. In particular, a characteristic
(
λmax ∼ 800 nm (ꢀ ∼ 1000 M cm )) into account, it is
•
+
•-
concluded that the transient Fc-ZnP -C60 state undergoes a
subsequent and rapid CSH to generate the final Fc -ZnP-C60
+
•-
state. The CSH rate constant (kET(CSH1)) in benzonitrile (see
9
Supporting Information (p S9)) was determined as 2.8 × 10
-1
6
-1
s
by using the kET(CR1)(ZnP-C60) (1.3 × 10 s ) value in
3
•+ •+ -1
absorption appears around 700 and 800 nm, due to C * and
Table 2 and monitoring the decay of the ZnP ((τ(ZnP ))
60
3
21
9
-1
2
H P*, respectively, in addition to the weak absorption around
1000 nm originating from C60 . Absorption due to the freebase
porphyrin radical cation (H2P (∼700 nm)) could not be
)
2.8 × 10 s ) around 650 nm (Figure 4b). The efficiency of
•-
•
+
•+
•-
CSH from the Fc to the ZnP in Fc-ZnP -C60 (ΦCSH1) (see
•
+
40
Supporting Information (p S9)) was found to be nearly unity
detected within the time domain of >10 ns.
(Table 3).
The time profile of Fc-ZnP-C60 at 1000 nm displays a rise
In reference to the energy level diagram shown in Scheme
4, the deactivation pathways of photoexcited Fc-H2P-C60 in
benzonitrile can be proposed as follows. First, an initial ET to
and a decay component with underlying rate constants of
6
5
-1
3
.2 × 10 and 1.3 × 10 s , respectively (see Supporting
1
Information (p S11)). A closer inspection of the energy level
diagram in a polar solvent (Scheme 3; benzonitrile) reveals that
it is energetically feasible to generate Fc-ZnP -C60 from
C60 (CS1) follows the instantaneous population of Fc- H2P*-
•
+
•-
41
C60 (1.89 eV), to yield, in turn, Fc-H P -C
2
60
(1.59 eV).
•
+
•-
The resulting charge-separated state reacts essentially via three
different decay pathways with respective rate constants, namely,
3
C60*. This process is ascribed to the rise in absorbance at 1000
3
3
nm from which we derived a rate constant (kET(CS3)) of 3.2 ×
d1 2 60 d2 2 60
(i) (k ) to Fc- H P*-C (1.40 eV), (ii) (k ) to Fc-H P- C *
6
-1
+
•-
1
0 s . Taking the above facts into account, the efficiency
(1.50 eV), and (iii) (kET(CSH1)) to Fc -H P-C
(1.03 eV).
2
60
3
regarding the formation of the charge-separated state from C60*
3
(
ΦCS3( C60*)) (see Supporting Information (p S9)) can be
(39) ΦCS(total)(Fc-ZnP-C60) ) (absorption ratio of ZnP) × {[ΦCS1-
1 3
(1
+
[
ZnP*) + ΦISC2( ZnP*) × ΦCS4( ZnP*)] × [kET(CSH1)/(kET(CR1) + kET(CSH1))]
approximated to be 0.99 (Table 3).
1
1
ΦET(CS5)( ZnP*) × ΦET(CSH2)( ZnP*)} + (absorption ratio of C60) ×
1 1 3
Since the energy level of the porphyrin triplet excited state
ΦCS2( C60*) + ΦISC1( C60*) × ΦCS3( C60*)] × [kET(CSH1)/(kET(CR1) +
(3
ZnP*: 1.53 eV) is slightly higher than that of the corre-
21
kET(CSH1))] ) 0.76 × {[0.90 + 0.05 × 0.88 × 0.99] × 1 + 0.05 × 1} +
0
.24 × [0.40 + 0.60 × 0.98 × 0.99] × 1 ) 0.99.
(38) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28,
(40) Gasyna, Z.; Browett, W. R.; Stillman, M. J. Inorg. Chem. 1985,
2
459.
24, 2440.

Porphyrin-Fullerene Linked Dyads and Triads
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2615
Scheme 4. Reaction Scheme and Energy Diagram for
Fc-H2P-C60 in Benzonitrile
(ca. 1000 nm) (Table 4).21 While in benzonitrile the ET rate
(kET(CR2)) for CR is somewhat slower than that in DMF (5.0 ×
4
-1
4
1
0 s ); it was found to be, however, faster in THF (2.9 × 10
-
1
s ). The total quantum yields of CS in ZnP-H2P-C60 (ΦCS-
(
total)) were estimated from the transient absorption spectra36
as 0.26 in THF and 0.21 in DMF, respectively (Table 4). In
1
addition, CS2 was also found to occur from H2P to C60* (1.75
•
+
•-
eV) to generate ZnP-H2P -C60 (1.63 eV), followed by a
•
+
•- 21
subsequent CSH1 to lead to ZnP -H2P-C60
.
Driving Force Dependence of Photoinduced CS and CR
Rates. The lifetimes (τCR) of the charge-separated states (i.e.,
•
+
•-
+
•-
+
•-
•+
ZnP -C60 , Fc -ZnP-C60 , Fc -H2P-C60 , and ZnP -H2P-
C60
•
- 42
) can be correlated with the solvent polarity. In the case
of ZnP-C60 (τ ) 2.7 (THF), 0.78 (benzonitrile), 0.57 µs (DMF))
and ZnP-H2P-C60 (τ ) 34 (THF), 21 (benzonitrile), 20 µs
(DMF)) the lifetimes clearly decrease as the solvent polarity
increases (Table 6). However, since the driving force of CR
decreases (Tables 2 and 4) with an increase in the solvent
polarity, the observed trends in the lifetimes show that the
associated ET rate constants are in the inverted region of the
Scheme 5. Reaction Scheme and Energy Diagram for
ZnP-H2P-C60 in Benzonitrile
1-7,11b,13,16
Marcus parabola:
in other words, as the driving force
decreases the rate constants of CR increase (the lifetimes of
the charge-separated states decrease). In contrast, the lifetimes
+
•-
of the Fc -ZnP-C60 ion pairs (τ ) 3.7 (THF), 7.5 (benzoni-
+
•-
trile), 16 µs (DMF)) and the Fc -H2P-C60 ion pairs (τ ) 8.3
benzonitrile), 19 µs (DMF)) grows gradually from the mod-
erately to the strongly polar solvents, indicating that these CR
rates are in the normal region of the Marcus curve (Table 6).44
In this case, as the driving force for CR increases, the rate
constants for CR also increase. In general, the driving forces of
the CR processes in ZnP -H2P-C60 (1.17-1.37 eV) and
ZnP -C60 (1.21-1.42 eV) are markedly larger than those in
the corresponding Fc -ZnP-C60 and Fc -H2P-C60 pairs in
the normal region (0.91-1.03 eV). Thus, these thermodynamic
differences support the above assignment regarding the CR
kinetics being either in the normal or inverted region of the
Marcus parabola.
(
43
1
CS from the Fc-H2P- C60* (CS2) could not be confirmed
by fluorescence lifetime measurements, because of the extensive
•+
•-
overlap between the freebase porphyrin emission (λmax(em) )
•+
•-
6
55, 720 nm) and the C60 emission (λmax(em) ) 720 nm). This
+
•-
+
•-
is, in essence, similar to the H2P-C60 case mentioned above.
However, the energy levels in Scheme 4 show that a photoin-
duced CS reaction even between the fullerene singlet excited
state and the porphyrin (CS2) is a possibility that would produce
•
+
•-
1
Fc-H2P -C60 . In contrast, the unquenched Fc-H2P- C60*
1.75 eV) undergoes an intersystem crossing (ISC1) to the Fc-
To quantify the driving force dependence on the ET rate
constants (kET), eq 3 was employed, where V is the electronic
coupling matrix element, kB is the Boltzmann constant, h is the
(
3
H2P- C60* (1.50 eV), which decays either to the ground state
or the Fc- H2P*-C60 state (1.40 eV). The ET rate constant for
CR2 (kET(CR2)) and the total quantum yields for CS (ΦCS(total))
in Fc-H2P-C60 were determined from the transient spectra as
3
45
Planck constant, and T is the absolute temperature. Figure 5
0
shows the logkET versus -∆G ET plots for the different donor-
3
6
acceptor arrays.
5
-1
4
-1
1
.2 × 10 s and 0.25 in benzonitrile and 5.3 × 10 s and
0
.27 in DMF, respectively (Table 4).
1/2
0
2
3
(
∆G + λ)
4
π
2
ET
(
c) ZnP-H2P-C60: We reported the photophysics of ZnP-
H2P-C60 in benzonitrile elsewhere in depth (Scheme 5).
Briefly, an initial singlet-singlet energy transfer (EN) from
kET
)
V exp -
(3)
2
1
(
2
)
[
]
4λkBT
h λk T
B
1
10 -1
ZnP* (2.04 eV) to H2P (1.89 eV) (kEN2 ) 1.5 × 10 s ) is
-
1
The best fits of eq 3 provide λ ) 0.66 eV and V ) 3.9 cm
followed by a sequential ET relay evolving from the ZnP-
4
6,47
for ZnP-C60.
It should be noted that in ZnP-C60 the CS
1
•+
•-
H2P*-C60 (CS1) to yield ZnP-H2P -C60 (1.63 eV) and
•+
•-
subsequently ZnP -H2P-C60 (1.34 eV) via CSH1 in competi-
tion with the back ET to the ground state (kET(CR1)) and the
porphyrin and C60 triplet excited states (kd1 and kd2). Thereby,
the individual rate constants kET(CS1) and kET(CSH1) were 7.0 ×
(42) The time-absorption profiles were fitted as a single-exponential
decay, which, in turn, allows excluding any intermolecular ET processes
under the present conditions.
(43) Osuka, A.; Nakajima, S.; Maruyama, K.; Mataga, N.; Asahi, T.;
Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc. 1993,
115, 4577.
9
9
-1
21
1
0 and 2.2 × 10 s , respectively. The final charge-separated
(
44) The CR rates in donor-acceptor linked dyads are known to increase
state, formed in moderate quantum yields (0.40), decays directly
1-7,16,17
with increasing solvent polarity,
whereas the CR processes in donor-
4
to the ground state with a rate constant (kET(CR2)) of 4.8 × 10
acceptor linked triads are subject to the reverse trend.43 The latter case has
-1
•-
s
as analyzed from the decay kinetics of the C60 absorption
been ascribed to a superexchange mechanism, in which CR takes place
supposedly via the intermediate charge-separated state. However, both trends
were seen in the CR processes of Fc-ZnP-C60/Fc-H2P-C60 (normal region)
and ZnP-H2P-C60 (inverted region), despite the fact that the energy
difference between the final charge-separated state and the intermediate
state in ZnP-H2P-C60 is smaller than that in Fc-ZnP-C60/Fc-H2P-C60. Thus,
the superexchange mechanism can be ruled out in the present case.
(45) (a) Winkler, J. R.; Gray, H. B. Chem. ReV. 1992, 92, 369. (b)
McLendon, G. Acc. Chem. Res. 1988, 21, 160.
(
41) The fluorescence lifetime of H2P-Fc in THF indicates photoinduced
ET from the ferrocene to the H2P* to produce H2P -Fc (1.66 eV).
1
•-
+
19
1
7 -1
However, the ET from the ferrocene to the H2P* (7.4 × 10 s ) is slower
1
8
-1 19
by a factor of ∼1/8 than that from the H2P* to the C60 (6.1 × 10 s ).
+
•-
Thus, the deactivation pathway to generate Fc -H2P -C60 is also minor,
although the charge-separated state would undergo CSH to the C60 to
generate Fc -H2P-C60 eventually.
+
•-

2616 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Imahori et al.
Table 6. Lifetime (τCR) and Rate Constants (kET) for Charge Recombination in Fullerene-Based Dyad and Triads
THF (ꢀ ) 7.58) benzonitrile (ꢀ ) 25.2)
s
s
DMF (ꢀ
s
) 36.7)
ET/s-
1
τ
CR/µs
k
ET/s-1
τ
CR/µs
k
ET/s-1
compds
τCR/µs
k
•
+
•-
5
5
0.78a
7.5
6
5
5
4
6
4
4
4
ZnP -C60
2.7
3.7
c
3.7 × 10
2.7 × 10
c
2.9 × 10
1.3 × 10
1.3 × 10
1.2 × 10
4.8 × 10
0b.57
16
19
20
1.8 × 10
6.3 × 10
5.3 × 10
5.0 × 10
•
+
•-
b
b
Fc -ZnP-C60
•
+
•-
Fc -H
2
P-C60
P-C60
8.3
•
+
•-
4
a
ZnP -H
2
34
21
a
From ref 21. b From ref 18. c Not determined.
that the reorganization energy of fullerenes in ET is indeed
6
-8
exceptionally low.
On the other hand, for the triad systems (i.e., Fc-ZnP-C60/
Fc-H2P-C60/ZnP-H2P-C60) the following values were ex-
-
1
tracted: λ ) 1.09 eV and V ) 0.019 cm . Consequently, the
λ value in the ZnP-C60 dyad is 0.43 eV smaller than that of the
Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads.49 Such a
difference in λ values between molecular dyads and triads is
9
consistent with the Marcus theory, which predicts an increase
of reorganization energy (0.37 eV) with an increase in the
donor-acceptor distance from 11.9 Å for the dyad to 30.3 Å
5
0
for the triads. Thus, the λ values in the present systems are
sensitive to the distance where the λ values are rather insensitive
to the solvent polarity (THF, benzonitrile, and DMF) at the fixed
46,49
distance.
Interestingly, the λ value of the triad systems (1.09
eV) is located between the driving force of the CR process in
+
•-
+
•-
•+
the Fc -ZnP-C60 /Fc -H2P-C60 (normal region) and ZnP -
•
-
H2P-C60 pairs (inverted region). This, in turn, provides a
reasonable explanation for the opposite dependence of the
lifetimes on the solvent polarity between the two systems (vide
supra).
It should be noted here that the V-value in Fc-ZnP-C60, Fc-
-
1
H2P-C60, and ZnP-H2P-C60 (V ) 0.019 cm ) is quite small,
Figure 5. Driving force (-∆G0ET) dependence of intramolecular ET
rate constants in ZnP-C60 (CS, open circle; CR: open square), Fc-
-1
as compared to those in ZnP-C60 (V ) 3.9 cm ) as well as
-
1 1-6,16,17
conventional dyads (∼1-100 cm ).
The distance
2
ZnP-C60 (CR, solid circle), Fc-H P-C60 (CR, solid triangle), and ZnP-
dependence of electronic couplings, V, is frequently described
H
2
P-C60 (CR, solid square). The curves represent the best fit to eq 3
-
1
by eq 4:
2
(ZnP-C60: λ ) 0.66 eV, V ) 3.9 cm ; Fc-H P-C60, Fc-ZnP-C60, and
-
1
ZnP-H P-C60: λ ) 1.09 eV, V ) 0.019 cm ).
2
2
2
V ) V exp(-â R )
(4)
0
CR ee
processes from the singlet and triplet excited states of the C60,
the singlet excited state of the porphyrin, and the CR processes
are located in the normal, top, and inverted regions of the
Marcus parabola, respectively.48 The reorganization energy (λ)
in ZnP-C60 (0.66 eV) is smaller than those reported previously
where V0 is a maximal electronic coupling and âCR is a decay
coefficient (damping factor) that depends primarily on the nature
of the bridge molecule. The âCR value is calculated to be 0.58
-
1
Å , which is within a boundary of nonadiabatic ET reactions
16
for porphyrin-quinone and zincporphyrin-freebase porphy-
-1
for saturated hydrocarbon bridges (0.8-1.0 Å ) and unsaturated
17
rin linked dyads, which typically are in the range between
.8 and 1.2 eV. This comparison supports our earlier hypothesis
-
1 51
phenylene bridges (0.4 Å ).
0
(
46) Recently Osuka et al. reported 1,4-phenylene-bridged zincporphy-
rin-freebase porphyrin dyads with similar reorganization energies in THF
λ ) 0.82 eV) and DMF (λ ) 0.84 eV) despite the large difference in the
Conclusion
(
In summary, highly efficient photosynthetic ET has been
realized in a porphyrin-fullerene linked system, i.e., ferrocene-
zincporphyrin-C60 triad, in which the long-lived charge-
separated state (up to 16 µs) can be produced with an extremely
17b
solvent polarity. This was rationalized in terms of the small reorganization
energies of porphyrins leading to a delocalization of the positive and negative
charges. The delocalization in C60, provided by its large three-dimensional
π system, suggests that the reorganization energy in the present systems is
not susceptible to changes in the solvent polarity (i.e., THF, benzonitrile,
and DMF). Accordingly, reorganization energies of the dyad and triads are
reasonably assumed to be similar in THF, benzonitrile, and DMF.
(49) Considering the similarity in the intervening spacers between the
ferrocene and the C60 in Fc-H2P-C60 and Fc-ZnP-C60 and the zincporphyrin
and the C60 in ZnP-H2P-C60, the V values in the two systems may well be
identical. Since a charge of ferrocene is protected from solvation due to
the bulky cyclopentadienyl rings, both the triads are assumed to give similar
small λ values. This is consistent with the fact that ferrocenes are frequently
employed as an internal standard in electrochemical measurements. Por-
phyrins are also known to exhibit small λ values; see: Fukuzumi, S. In
The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, CA, 2000; Vol. 8, pp 115-151.
(
47) The semiclassical Marcus treatment including contribution of the
nuclear tunneling could not be applied to the present systems, since such
analysis made the fitting worse, as compared to the present analysis.
(
encompasses data for CS from different excited states and CR to the ground
state, i.e., ET rates for different reactions which take place between different
electronic states. Excitation and redox experiments with C60 indicate that
the nuclear coordinates of the fullerene singlet ground, singlet excited, and
reduced states are similar. Since porphyrins and fullerenes have small
48) The best fits of eq 3 for ZnP-C60 suffers from the fact that it
(50) The difference in the λ values (∆λ ) 0.37 eV) is estimated from
•
+
•-
2
2
reorganization energies and the Coulombic interaction in ZnP -C60
the following equation: ∆λ ≈ (ꢀ /n )[(1/Ree(dyad)) - (1/Ree(triad))] )
2
1
9
(
Ree ) 11.3 Å) is negligible, the electronic coupling as well as the
intramolecular reorganization energies may be similar. This justifies plotting
the CS and CR rates on the same curve.
7.2 × [(1/(11.9)) - (1/(30.3))].
(51) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992, 114,
6227.

Porphyrin-Fullerene Linked Dyads and Triads
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2617
high quantum yield (nearly unity). This can be rationalized by
the small reorganization energy of C60 in multistep ET processes,
where forward ET is accelerated and back ET is decelerated. It
should be noted here that both the normal and inverted regions
of the Marcus parabola have been observed for intramolecular
CR processes in the covalently linked triads. In addition, we
have successfully shown the dependence of the ET rate constants
on both the distance and driving force in a series of homologous
porphyrin-fullerene linked systems. The fundamental informa-
tion on ET obtained here will be helpful for the design of
artificial photosynthetic systems.
the Science and Technology Agency, Japan (No. 12310), and
the Office of Basic Energy Sciences of the Department of
Energy, USA. H.I. thanks the Sumitomo Foundation for
financial support. This is document NDRL-4260 from the Notre
Dame Radiation Laboratory.
Supporting Information Available: Experimental details
including synthetic procedures and characterization (pp S1-
S7), mathematical derivations for ET rate constants and forma-
tion efficiencies of each state (pp S8-S9), differential pulse
voltammograms of ZnP-NHCO-ref in THF, benzonitrile, and
DMF (p S10), and nanosecond transient absorption spectra of
Fc-ZnP-C60 (p S11) and Fc-H2P-C60 (p S12) in benzonitrile
Acknowledgment. This work was supported by Grant-in-
Aids for COE Research and Scientific Research on Priority Area
of Electrochemistry of Ordered Interfaces and Creation of
Delocalized Electronic Systems from the Ministry of Education,
Science, Sports and Culture, Japan, a Millennium Project from
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA003346I