Sequential energy and electron transfer in an artificial reaction center: Formation of a long-lived charge-separated state

J. Am. Chem. Soc. 2000, 122, 6535-6551

6535

Sequential Energy and Electron Transfer in an Artificial Reaction

Center: Formation of a Long-Lived Charge-Separated State

C. Luo,^†D. M. Guldi,*^,†H. Imahori,*^,‡K. Tamaki,^§and Y. Sakata^§

Contribution from the Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556,

Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,

2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, and The Institute of Scientific and Industrial Research,

Osaka UniVersity, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan

ReceiVed NoVember 9, 1999. ReVised Manuscript ReceiVed February 9, 2000

Abstract: A novel molecular triad, representing an artificial reaction center, was synthesized via linking a

fullerene moiety to an array of two porphyrins (i.e., a zinc tetraphenyl porphyrin (ZnP) and a free base tetraphenyl

porphyrin (H₂P)). In this ZnP-H₂P-C₆₀triad, the ZnP performs as an antenna molecule, transferring its singlet

excited state energy to the energetically lower lying H₂P. In benzonitrile, this energy transfer (k ) 1.5 × 10¹⁰

s^-1) is followed by a sequential electron-transfer relay evolving from the generated singlet excited state of

•-

H₂P to yield ZnP-H₂P^•+-C₆₀and subsequently ZnP^•+-H₂P-C₆₀with rate constants of 7.0 × 10⁹s^-1

and 2.2 × 10⁹s^-1, respectively. The final charge-separated state, formed in high yield (0.4), gives rise to a

remarkable lifetime of 21 µs in deoxygenated benzonitrile and decays directly to the singlet ground state. In

contrast, in nonpolar toluene solutions the deactivation of the porphyrin chromophores (ZnP and H₂P) takes

place via singlet-singlet energy transfer leading to the fullerene singlet excited state. This stems from the

unfavorable free energy changes for an intramolecular electron-transfer event in toluene from the singlet excited

state of H₂P to the adjacent fullerene acceptor.

Introduction

selectivity and control of electron transfer of the photosynthetic

reaction center has not yet been reached in these model systems.

With the advent of fullerenes, a new three-dimensional

electron acceptor became available that displays appealing

characteristics.^22,23Most important, fullerenes have a remarkably

low reorganization energy, associated with their electron transfer

(ET) reaction, which results from the rigid, confined structure

of the carbon network.^24,25Small resonance-Raman shifts²⁶and

small Stokes shifts,²⁷noticed in reduction and excitation

experiments, point to the similarity of the fullerene nuclear

configurations in the excited, reduced, and ground state. These

effects are beneficial to decelerate the energy-wasting back-

electron transfer (BET)²⁸relative to 2-dimensional electron

acceptors, which in general have less rigid structures and, thus,

higher reorganization energies than 3-dimensional fullerenes,

The principal aspects of the primary photochemical energy

conversion process in photosynthesis have been thoroughly

investigated in recent years, starting with the isolation and full

characterization of the photosynthetic reaction center.¹In

particular, there is a sequence of events that separate charges

across a membrane within a nanosecond.^2-4These events are a

fast energy transfer from the photoexcited antenna molecule to

the photosynthetic reaction center followed by two successive

one-electron reduction steps.^5,6It should be noted that one of

the key factors, to achieve a near-unity quantum efficiency for

the charge separation, implements small reorganization energies

for the initial photoinduced electron transfer from the special

pair bacterial chlorophyll to ubiquinone.⁷In this context, a vast

number of artificial systems were designed with the purpose of

mimicking these primary events.^8-21However, the exquisite

(12) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26,

198.

* Author to whom correspondence may be sent. E-mail: guldi.1@nd.edu.

^†University of Notre Dame.

(13) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.

(14) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18.

(15) Sutin, N. Acc. Chem. Res. 1983, 15, 275.

(16) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141.

(17) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163.

(18) Harriman, A.; Kubo, Y.; Sessler, J. L. J. Am. Chem. Soc. 1992,

114, 388.

^‡Graduate School of Engineering, Osaka University. E-mail: imahori@

ap.chem.eng.osaka-u.ac.jp.

^§The Institute of Scientific and Industrial Research, Osaka University.

(1) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol.

Biol. 1984, 180, 385.

(2) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawethornthwaite-

Lawless, A. M.; Paiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995,

374, 517.

(3) Stowell, M. H. B.; McPhillips, T. M.; Rees, D. C.; Soltis, S. M.;

Abresch, E.; Feher, G. Science 1997, 276, 812.

(4) Beratan, D. N.; Onuchic, J. N.; Winkler, J.; Gray, H. Science 1992,

258, 1740.

(5) Vermaas, W. F. J. Science 1994, 263, 545.

(19) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143.

(20) Galoppini, E.; Fox, M. A. J. Am. Chem. Soc. 1997, 119, 5277.

(21) Blanco, M.-J.; Jimen´ez, M. C.; Chambron, J.-C.; Heitz, V.; Linke,

M.; Sauvage, J.-P. Chem. Soc. ReV. 1999, 28, 293.

(22) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R.

E. Nature 1985, 318, 162.

(23) Fullerenes and Related Structures; Hirsch, A., Ed.; Springer: Berlin,

Vol. 199, 1999.

(6) Steffen, M. A.; Lao, K. Q.; Boxer, S. G. Science 1994, 264, 810.

(7) Moser, C. C.; Keske, J. M.; Warncke, K.; Farid, R. S.; Dutton, P. L.

Nature 1992, 355, 796.

(8) Maruyama, K.; Osuka, A. Pure Appl. Chem. 1990, 62, 1511.

(9) Gust, D.; Moore, T. A. Science 1989, 244, 35.

(10) Gust, D.; Moore, T. A. AdV. Photochem. 1991, 16, 1.

(11) Gust, D.; Moore, T. A. Top. Curr. Chem. 1991, 159, 103.

(24) Guldi, D. M.; Asmus, K.-D. J. Am. Chem. Soc. 1997, 119, 5744.

(25) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.;

Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545.

(26) McGlashen, M. L.; Blackwood, M. E.; Spiro, T. G. J. Am. Chem.

Soc. 1993, 115, 2074.

(27) Guldi, D. M.; Asmus, K.-D. J. Phys. Chem. A 1997, 101, 1472.

(28) Marcus, R. A. J. Phys. Chem. 1956, 24, 966.

Published on Web 06/30/2000

6536 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

and this makes them ideal candidates for model systems.^29-34

This is of great interest for modulating the rates and yields of

charge separation versus charge recombination, in particular,

in relation to conventionally employed acceptors, such as

quinones A and B in the reaction center. Consequently,

fullerenes emerged as ideal candidates for novel model donor-

acceptor systems.

Molecular calculations regarding the electronic configuration

of fullerenes have revealed that in C₆₀an energy gap of ∼1.8

eV separates a 3-fold degenerate LUMO from a lower lying

5-fold degenerate HOMO.^35,36The small energy gap underlines

the remarkable electron-accepting features of these carbon

spheres. In line with this, a low redox potential (associated with

the first reduction step) of -440 mV versus SCE is reported,

for example, in dichloromethane solutions.³⁷

By far the largest number of C₆₀-based donor-acceptor

systems studied to date are based on employing porphyrins as

antenna molecules to ensure the efficient light capture in the

visible region of the spectrum.^29-34,38-45One of the first reported

C₆₀-based donor-acceptor systems is H₂P-C₆₀and the analogue

ZnP-C₆₀dyads, in which the pigments are linked via an unusual

bicyclic spacer.^38aThe fluorescence lifetime measurements

suggested photoinduced ET in these systems. First direct

evidence for ET in donor-linked fullerenes also stems from time-

resolved transient absorption studies regarding a ZnP-C₆₀dyad,

in which the two chromophores are separated by a phenyl

spacer.^44a

The role played by the spacer is not just limited to structural

concerns, but more importantly, its chemical nature influences

the electronic communication between the electron acceptor (i.e.,

fullerene) and photoexcited electron donor (i.e., ZnP). As a

leading example for a short-spaced dyad, the π-π stacked

porphyrin-fullerene dyad (ZnP-C₆₀) should be cited,^39awhose

ET and BET dynamics helped to establish the following

fundamental aspect:^45bThe presence of a “Marcus-inverted”

region for BET was found, based on (i) the photoactivation of

an intramolecular ET in a variety of solvents and (ii) the

formation of a highly energetic charge-separated state. Similar

observations stem from a series of parachute-shaped dyads (e.g.,

ZnP-C₆₀and H₂P-C₆₀)^42eas well as ZnP-C₆₀dyads with a

variety of phenyl spacers.^25,44b

One approach to slowing down charge recombination is to

increase the distance between the donor and acceptor by means

of using spacer units of increasing size.⁴⁰For example, a long-

range photoinduced electron transfer was reported in a ZnP-

(29) Mart´ın, N.; Sa´nchez, L.; Illescas, B.; Pe´rez, I. Chem. ReV. 1998,

98, 2527.

(30) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537.

(31) Prato, M. J. Mater. Chem. 1997, 7, 1097.

(32) Diederich, F.; Gomez-Lopez, M. Chem. Soc. ReV. 1999, 28, 263.

(33) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445.

(34) Guldi, D. M. Chem Commun. 2000, 321.

(35) Haddon, R. C. Acc. Chem. Res. 1988, 21, 243.

(36) Haddon, R. C. Science 1993, 261, 1545.

C₆₀dyad, in which the two redox active moieties are separated

by a saturated norbornylogous bridge nine σ bonds in length.^40a

The lifetime of the radical ion pair in the “norbornylogous”

bridged dyad measures an impressively 420 ns^40aand clearly

discloses a significant improvement relative to the short-spaced

analogues. (e.g., the π-π stacked ZnP-C₆₀: 38 ps in

benzonitrile^45b).

(37) Echegoyen L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593.

(38) (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) Gust, D.; Moore, T. A.; Moore,

A. L. Pure Appl. Chem. 1998, 70, 2189. (f) 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. (g) Imahori, H.; Cardoso,

S.; Tatman, D.; Lin, S.; Noss, L.; Seely, G. R.; Sereno, L.; de Silber, J. C.;

Moore, T. A.; Moore, A. L.; Gust, D. Photochem. Photobiol. 1995, 62,

1009. (h) 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.

(39) (a) Dietel, E.; Hirsch, A.; Eichborn, E.; Rieker, A.; Hackbarth, S.;

Ro¨der, B. Chem. Commun. 1998, 1981. (b) Dietel, E.; Hirsch, A.; Zhou, J.;

Rieker, A. J. Chem. Soc., Perkin Trans. 2 1998, 1357. (c) Camps, X.; Dietel,

E.; Hirsch, A.; Pyo, S.; Echegoyen, L.; Hackbarth, S.; Ro¨der, B. Chem.

Eur. J. 1999, 5, 2362.

(40) (a)Bell, T. D. M.; Smith, T. A.; Ghiggino, K. P.; Ranasinghe, M.

G.; Shephard, M. J.; Paddon-Row, M. N. Chem. Phys. Lett. 1997, 268,

223. (b) Paddon-Row, M. N. Fullerene Sci. Technol. 1999, 7, 1151. (c)

Shephard, M. J.; Paddon-Row: M. N. Aust. J. Chem. 1996, 49, 395.

(41) (a) Nierengarten, J.-F.; Schall, C.; Nicoud, J.-F. Angew. Chem., Int.

Ed. 1998, 37, 1934. (b) Nierengarten, J.-F.; Oswald, L.; Nicaud, J.-F. Chem.

Commun. 1998, 1545. (c) Bourgeois, J.-P.; Diederich, F.; Echegoyen, L.;

Nierengarten, J.-F. HelV. Chim. Acta 1998, 81, 1835. (d) Armaroli, N.;

Diederich, F.; Echegoyen, L.; Habicher, T.; Flamigni, L.; Marconi, G.;

Nierengarten, J.-F. New J. Chem. 1999, 23, 77.

(42) (a) Safonov, I. G.; Baran, P. S.; Schuster, D. I. Tetrahedron Lett.

1997, 38, 8133. (b) Baran, P. S.; Monaco, R. R.; Khan, A. U.; Schuster, D.

I.; Wilson, S. R. J. Am. Chem. Soc. 1997, 119, 8363. (c) Fong, R.; Schuster,

D. I.; Wilson, S. R. Org. Lett. 1999, 1, 729. (d) Cheng, P.; Wilson, S.;

Schuster, D. I. Chem. Commun. 1999, 89. (e) 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.

(43) (a) Helaja, J.; Tauber, A. Y.; Abel, Y.; Tkachenko, N. V.;

Lemmetyinen, H.; Kilpela¨inen, I.; Hynninen, P. H. J. Chem. Soc., Perkin

Trans. 1 1999, 2403. (b) Tkachenko, N. V.; Rantala, L.; Tauber, A. Y.;

Helaja, J.; Hynninen, P. H.; Lemmetyinen, H. J. Am. Chem. Soc. 1999,

121, 9378.

The above-summarized dyad concept (i.e., ZnP-C₆₀) was

further developed by linking a carotenoid polyene (C) to a H₂P-

C₆₀dyad, constituting the first molecular triad published (e.g.,

C-H₂P-C₆₀).^38d,hIn this system the spatial separation between

the individual components is, however, rather short and, in turn,

limits the stability of the charge-separated state. A similar

linkage was chosen for the design of an artificial reaction center,

namely, the sequence of energy and electron transfer. In

particular, in an elaborated C₆₀-based hexad, (ZnP)₃-ZnP-

H₂P-C₆₀, star-shaped aligned porphyrins (i.e., four ZnP linked

to a H₂P) serve as a multichromophoric antenna system.^38f

In the present work we developed a concept in which we

have linked the fullerene acceptor covalently to an array of two

porphyrin moieties, namely, a tetraphenyl porphyrin free base

(H₂P) and the corresponding zinc complex (ZnP), to form a

novel artificial reaction center, as shown in Figure 1. Impor-

tantly, the center-to-center distances in ZnP-H₂P-C₆₀were

estimated to be 18.0 and 36.1 Å for the H₂P/C₆₀and ZnP/C₆₀

(44) (a) Imahori, H.; Hagiwara, K.; Akiyama, T.; Taniguchi, S.; Okada,

T.; Sakata, Y. Chem. Lett. 1995, 265. (b) Imahori, H.; Hagiwara, K.; Aoki,

M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J.

Am. Chem. Soc. 1996, 118, 11771. (c) Imahori, H.; Yamada, K.; Hasegawa,

M.; Taniguchi, S.; Okada, T.; Sakata, Y. Angew. Chem. Int. Ed. Engl. 1997,

36, 2626. (d) Higashida, S.; Imahori, H.; Kaneda, T.; Sakata, Y. Chem.

Lett. 1998, 605. (e) Tamaki, K.; Imahori, H.; Nishimura, Y.; Yamazaki, I.;

Shimomura, A.; Okada, T.; Sakata, Y. Chem. Lett. 1999, 227. (f) Imahori,

H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.;

Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Bull.

Chem. Soc. Jpn. 1999, 72, 485. (g) Fujitsuka, M.; Ito, O.; Imahori, H.;

Yamada, K.; Yamada, H.; Sakata, Y. Chem. Lett. 1999, 721. (h) Yamada,

K.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. Chem. Lett. 1999,

895.

(45) (a) Da Ros, T.; Prato, M.; Guldi, D. M.; Alessio, E.; Ruzzi, M.;

Pasimeni, L. Chem. Commun. 1999, 635. (b) Guldi, D. M.; Luo, C.; Prato,

M.; Dietel, E.; Hirsch, A. Chem. Commun. 2000, 373. (c) Guldi, D. M.;

Luo, C.; Prato, M.; Da Ros, T.; Dietel, E.; Hirsch, A. Chem. Commun.

2000, 375. (d) Da Ros, T.; Prato, M.; Guldi, D. M.; Ruzzi, M.; Pasimeni,

L. Submitted for publication.

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6537

Figure 1. Structure of ZnP-H₂P-C₆₀triad.

Scheme 1

separations, respectively, using CPK modeling. The ZnP chro-

mophore is believed to act herein as the antenna molecule, which

transfers its singlet excited-state energy to the adjacent H₂P

chromophore. Furthermore, a sequential electron-transfer relay

will evolve from the generated singlet excited state of H₂P,

injecting an electron into the electron-accepting fullerene, to

yield ZnP-H₂P^•+-C₆₀^•-and subsequently ZnP^•+-H₂P-C₆₀

.

•-

Earlier some of us reported on the time-resolved fluorescence

studies of this system, which revealed a rapid quenching of the

antenna fluorescence in the reference dyads ZnP-C₆₀and

H₂P-C₆₀(Scheme 3) and also in the molecular triad ZnP-

H₂P-C₆₀.⁴⁶Now, we wish to present the complete characteriza-

tion of the transient intermediates associated with the quenching

of the porphyrin emission by means of transient absorption

spectroscopy. We also provide the rate constants for all of the

individual events involved, starting from the initial quenching

processes to the decay kinetics of the final photoproducts.

Results and Discussion

Synthesis:⁴⁶The preparation of ZnP-H₂P-C₆₀and the

reference dyads ZnP-H₂P, H₂P-C₆₀and ZnP-C₆₀was

carried out as shown in Schemes 1-3. Important synthetic

intermediates 4 and 5 were prepared by the condensation of

dipyrromethane 1 with 3,5-di-tert-butylbenzaldehyde in the

presence of BF₃OEt₂, followed by base hydrolysis of 2 and 3,

respectively (Scheme 1). The key features for the synthesis of

ZnP-H₂P-C₆₀and ZnP-H₂P involve selective insertion of

the central zinc atom into the diporphyrin and its protection

from the demetalation. Therefore, after completion of the

insertion all of the procedures were carried out under neutral

or basic conditions. The free-base porphyrin carboxylic diacid

4 was converted to the corresponding bis(acid chloride) 6 by

treatment with SOCl₂. Amino zincporphyrin 7 was synthesized

from the corresponding freebase porphyrin, whereas 4-tert-

butyldimethylsiloxymethylaniline 9 was obtained by protection

of the hydroxymethyl group in 4-nitrobenzyl alcohol and

subsequent reduction of the nitro group in 8 (Scheme 2). Cross-

condensation of 6 with 7 and 9 in benzene in the presence of

pyridine afforded a mixture of porphyrins. The desired dipor-

phyrin 10 was isolated in 34% by tedious chromatographic

separation. Deprotection of tert-butyldimethylsilyl group in 10

with n-Bu₄NF gave 11, which was oxidized using activated

MnO₂to yield 12. ZnP-H₂P-C₆₀was obtained by 1,3-dipolar

cycloaddition using 12, N-methylglycine, and C₆₀in toluene in

79% yield. ZnP-H₂P was synthesized from 6, 7, and 4-hexa-

decylaniline by the same method as described for ZnP-H₂P-

C₆₀(Scheme 2). H₂P-C₆₀was obtained by condensation of 5

with 13, followed by acid hydrolysis of 14 and 1,3-dipolar

cycloaddition using 15, N-methylglycine, and C₆₀in toluene

(Scheme 3). Preparation of ZnP-C₆₀was carried out by

treatment of H₂P-C₆₀with Zn(OAc)₂. The single chromophore

references ZnP, H₂P, and C₆₀-Ar were also prepared (Figure

2). Their structures were verified by spectroscopic analyses

including ¹H NMR and MALDI-TOF mass spectra (see

Experimental Section).

The absorption spectrum of ZnP-H₂P-C₆₀in benzonitrile

is almost a linear combination of the spectra of ZnP, H₂P, and

C₆₀-Ar, indicating no significant interactions among the three

chromophores in the ground state. In principle, the absorption

features of the two porphyrin chromophores in the visible region

are much stronger than that of the fullerene moiety.

Electrochemical Properties and Thermodynamic Free

Energy Changes. The redox potentials of ZnP-H₂P-C₆₀,

ZnP-H₂P, ZnP-C₆₀, H₂P-C₆₀, ZnP, H₂P, and C₆₀-Ar were

measured by differential pulse voltammetry in benzonitrile using

(46) Tamaki, K.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.

Chem. Commun. 1999, 625.

6538 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

Scheme 2

Figure 2. Structures of ZnP, H₂P, and C₆₀-Ar references.

+0.29 V; E_1/2(H₂P^•+/H₂P) ) +0.49 V; E_1/2(C₆₀/C₆₀^•-) ) -1.05

V vs Fc/Fc⁺), implying the weak electronic coupling among

the three moieties. The difference in the substituents in ZnP-

H₂P-C₆₀and H₂P may be responsible for the shift in the first

oxidation potential of the freebase porphyrin moieties. Tables

1 and 2 and Scheme 4 show the energy levels of the locally

excited singlet states of the three moieties (i.e., ZnP, H₂P, and

C₆₀) and the free-energy changes. The energies of the 0-0

transition (∆E_0-0) between the S₁and the S₀state were

determined by averaging the energies of the corresponding (0,

0) peaks in the fluorescence and the absorption bands. Free

energy changes, -∆G_CSand -∆G_CR, were calculated by the

Rehm-Weller and Born equation.^47,48E_OXand E_REDare the

half-wave potentials of the one-electron oxidation step of the

porphyrin and one-electron reduction step of C₆₀, respectively,

in benzonitrile. ∆G_Sis the correction term for the effects of

solvent polarity as well as the Coulombic energy between donor

radical cation and acceptor radical anion. In benzonitrile the

Born equation may estimate the correct values for ∆G_S, whereas

in toluene that was tentatively used due to the overestimation

of the nonpolarity of toluene. The center-to-center distances

(R_D-A) were determined from CPK modeling using CAChe

(version 3.7 CAChe Scientific, 1994).

Scheme 3

Time-Resolved Absorption Studies. To characterize the

transient intermediates involved in the rapid deactivation of the

metalloporphyrins, a number of fast spectroscopic techniques

were employed. In particular, transient absorption measurements

were carried out following either pico- or nanosecond laser

excitation (337 or 532 nm) of the ZnP-H₂P-C₆₀triad and

the corresponding reference compounds in solvents of different

polarity. These data are further complemented by time-resolved

fluorescence lifetime measurements, published recently.⁴⁶The

pump wavelength of 532 nm was chosen in these experiments,

to guarantee the excitation into both the zinc porphyrin and free

base porphyrin (π-π*) visible transitions, namely, the Q-bands,

rather than excitation of the fullerene moiety.

ZnP and H₂P. Eighteen picosecond excitation of either the

free base tetraphenyl porphyrin (H₂P) or the corresponding zinc

complex (ZnP) resulted in characteristic absorption changes in

the 500-810 nm range. In particular, a net decrease of the

absorption was observed around 540 nm, a region, which is

dominated by strong ZnP and H₂P ground-state absorption. This

suggests consumption of the porphyrin as a result of converting

the porphyrin singlet ground states to the corresponding singlet

excited states, ¹*(π-π*)ZnP and ¹*(π-π*)H₂P. An immediately

formed absorption accompanies this porphyrin bleaching. For

1

example, the *(π-π*)H₂P exhibits a new absorption band

between 660 and 730 nm, a region in which the ¹*(π-π*)ZnP

absorbs, however, very weakly. The singlet excited-state absorp-

0.1 M n-Bu₄NPF₆as supporting electrolyte. The results are

summarized in Table 1. The potentials of ZnP-H₂P-C₆₀

(+0.30, +0.59 V, -1.04, vs Fc/Fc⁺) can be roughly explained

by the sum of ZnP, H₂P, and C₆₀-Ar (E_1/2(ZnP^•+/ZnP) )

(47) Weller, A. Z. Phys. Chem. 1982, 132, 93.

(48) Gaines, G. L. I.; O’Neil, M. P.; Svec, W. A.; Niemczyk M. P.;

Wasielewski, M. R. J. Am. Chem. Soc. 1991, 113, 719.

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6539

Table 1. Redox Potentials^aand Thermodynamic Driving Force in Fullerene-Porphyrin References, Dyads, and Triad

free energy change

redox potential

benzonitrile

toluene

H₂P^•+/H₂P^•+ZnP^•+/ZnP C₆₀/C₆₀

-∆G_CS-∆G_CR1-∆G_CSH-∆G_CR2-∆G_CS-∆G_CR1-∆G_CSH-∆G_CR2

•-

b

c

b

c

[V]

[eV]

0.12

[eV]

ZnP-H₂P-C₆₀

H₂P-C₆₀

ZnP-C₆₀

ZnP-H₂P

H₂P

0.59

0.55

0.30

-1.04

0.29

0.33

0.69

1.60

1.56

1.35

0.28

1.32

-0.56

-0.52

-0.13

2.46

2.42

2.21

2.34

0.34

0.30

0.59

0.49

ZnP

0.29

C₆₀-Ar

-1.05

^aRedox potentials were measured in benzonitrile (0.1 M Bu₄NPF₆, vs Fc/Fc⁺) by differential pulse voltammetry with a sweep rate of 10 mVs^-1

,

using a glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl (saturated KCl) reference electrode. ^b-∆G_CSand

-∆G_CR1denote the free-energy changes for an electron transfer from ¹*(π-*π)ZnP/¹*(π-*π)H₂P to C₆₀, and from C₆₀^•-to ZnP^•+/H₂P^•+, respectively.

•-

^c-∆G_CSHand -∆G_CR2denote the free-energy changes for an electron transfer from ZnP to the H₂P^•+, and from C₆₀to ZnP^•+, respectively. (see

refs 44b, 47, 48). -∆G_CS) ∆E_0-0- (-∆G_CR); -∆G_CR) E_OX- E_RED+ ∆G_S; ∆G_S) e²/(4πꢀ₀)) [(1/(2R₊) + 1/(2R_-) - 1/R_D-A) 1/ꢀ_S- (1/(2R₊)

+ 1/(2R_-)) 1/ꢀ_R]; E_OX) E_1/2(D^•+/D); E_RED) E_1/2(A/A^•-); R₊) radius donor (5 Å); R_-) radius acceptor (4.4 Å); R_D-A) center-to-center

distance; ꢀ_S) dielectric constant of solvent used for photophysical studies; ꢀ_S) 2.38 (toluene) and ꢀ_S) 25.2 (benzonitrile) ꢀ_R) dielectric

constant of solvent used for measuring the redox potentials; ∆E_0-0) excited-state energy of chromophore.

Table 2. Energies, Lifetimes, and Quantum Yields of Fullerene-Porphyrin References, Dyads, and Triads in Toluene

energy:

singlet excited

state [eV]

lifetime:

singlet excited

state

energy:

triplet excited

state [eV]

lifetime:

triplet excited

state

quantum yield:

triplet excited

state

compd

*H₂P

*ZnP

*C₆₀-Ar

1.90

2.08

1.75

1.90

1.75

10.1 ns

2.5 ns

1.8 ns

67 ps

1.40

1.53

1.50

380 µs

44 µs

25 µs

0.82

0.88

0.98

0

0.83

0.54

0

*H₂P-C₆₀

H₂P-*C₆₀

*H₂P-C₆₀

1.8 ns

1.50

1.40

4.2 µs

240 µs

a

*ZnP-H₂P

2.08

1.90

2.08

1.75

2.08

1.90

1.75

79 ps

10.1 ns

58 ps

1.8 ns

67 ps

87 ps

1.8 ns

ZnP-*H₂P

1.40

1.50

380 µs

25 µs

0.76

*ZnP-C₆₀

ZnP-*C₆₀

0.86

0

*ZnP-H₂P-C₆₀

ZnP-*H₂P-C₆₀

ZnP-H₂P-*C₆₀

ZnP-*H₂P-C₆₀

1.50

1.40

4.2 µs

240 µs

0.71

0.46

a

^aProduced via intramolecular energy transfer.

Scheme 4

and ZnP chromophores in THF with values of 9.8 ns and 2.0

ns, respectively.⁴⁶

ZnP

H₂P

9

^hν8 ¹*ZnP f ³*ZnP

^hν8 ¹*H₂P f ³*H₂P

(1)

(2)

9

Intersystem crossing is the predominant fate of these singlet

excited states (see Table 2).^50,51The accordingly formed triplet

excited state of H₂P reveals a characteristic peak around 780

nm (Figure 3, spectrum a), while the absorption of the analogous

ZnP is markedly shifted to the red (λ_max) 860 nm) (Figure 3,

spectrum b). Again, the derived triplet lifetimes of 380 µs

(³*(π-π*)H₂P) and 44 µs (³*(π-π*)ZnP) match those values

reported earlier for a free base tetraphenylporphyrin and the

corresponding zinc complex, respectively.⁴⁹The energy levels

of the singlet and triplet excited states, as derived from steady-

state fluorescence and phosphorescence measurements are

summarized, together with the associated lifetimes, in Table 2.

The triplet quantum yields in these reference porphyrins were

determined to be 0.82 and 0.88 for the free base and zinc

complex, respectively, resembling already published values.⁴⁹

In the presence of molecular oxygen, the triplet excited states

of both porphyrins experience a concentration-dependent de-

activation process with oxygen concentration varying be-

tween 0.9 and 4.5 mM. Bimolecular rate constants of 6.6 ×

tion gives rise to singlet lifetimes of 10.1 ns and 2.5 ns for H₂P

and ZnP, respectively. These are in excellent agreement with

previously reported values and confirm the negligible effects

stemming from the functionalization of the porphyrin periphery

on the singlet excited-state properties.^45a,49-51They also reveal

a good agreement with the fluorescence lifetimes of the H₂P

(49) (a) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of

Photochemistry; Marcel Dekker Inc.: New York, 1993. (b) Hoffman, M.

Z.; Bolletta, F.; Moggi, L.; Hug, G. L. J. Phys. Chem. Ref. Data 1989, 18,

219.

(50) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989,

111, 6500.

(51) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin

Complexes; Academic Press: London, 1992.

3

10⁸M^-1s^-1and 1.06 × 10⁹M^-1s^-1for *(π-π*)ZnP and

6540 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

annihilation and ground-state quenching and is quite short, ∼25

µs, under our standard conditions. It should be added that these

features are consistent with the general trend noticed for various

mono-functionalized fullerene derivatives.²⁷

C₆₀-Ar

9

^hν8 ¹*C₆₀-Ar f ³*C₆₀-Ar

(3)

In summary, the above features (i.e., of reference compounds

ZnP, H₂P, and C₆₀-Ar) are easily detectable markers for the

following intramolecular transfer reactions. It is expected that

the use of these probes will lead to a better understanding of

functionalized fullerene and porphyrin derivatives and, in

addition, that they will serve as a data set for the covalently

linked, fullerene-containing porphyrin dyads and triad.

Figure 3. Differential absorption spectra obtained 50 ns upon

nanosecond flash photolysis (532 nm) of ∼10^-5M solutions of H₂P in

nitrogen saturated benzonitrile (a) (s), ZnP in nitrogen saturated

toluene (b) (- - -), ZnP-H₂P dyad in nitrogen saturated benzonitrile

(c) (- - -).

ZnP-H₂P. Transient absorption changes, recorded upon 532

nm laser excitation of the ZnP-H₂P dyad, are almost super-

1

imposable with those recorded for the *(π-π*)H₂P. Specifi-

cally, a set of minima, which correspond to the Q-band

absorption (in toluene: 520, 556, 597, and 650 nm), is further

accompanied by a new transient absorbing between 660 and

730 nm (Figure 4). Time profiles, taken at various wave-

lengths, indicate, however, that the singlet excited state (i.e.,

¹*(π-π*)H₂P) is formed in two distinct steps. The faster process

lies essentially within the apparatus resolution (20 ps), while

the second, slower component has a rise time of 79 ps (1.3 ×

10¹⁰s^-1) in toluene and 85 ps (1.2 × 10¹⁰s^-1) in benzonitrile.

Complementary time-resolved fluorescence measurements of

the ZnP-H₂P dyad revealed dual fluorescence. In particular,

a rapid deactivation of the ¹*(π-π*)ZnP fluorescence (605 nm)

with an associated rate constant of 2.2 × 10¹⁰s^-1, resembling

the kinetics of the transient absorption measurements. On the

other hand, the ¹*(π-π*)H₂P fluorescence (720 nm) shows no

indication for a faster decay.⁴⁶

Table 3. Reactivity of Photolytically Generated Species in

Fullerene-Porphyrin Dyads and Triad with Molecular Oxygen

compd

reactive species

solvent

k_OXYGEN[M^-1s^-1]

ZnP

H₂P

*ZnP

*H₂P

*C₆₀-Ar

ZnP-*C₆₀

ZnP^•+-C₆₀

*H₂P-C₆₀

ZnP-*H₂P

benzonitrile

toluene

benzonitrile

toluene

benzonitrile

toluene

6.6 × 10⁸

1.06 × 10⁹

9.1 × 10⁸

1.0 × 10⁹

9.8 × 10⁸

9.4 × 10⁸

1.1 × 10⁹

1.08 × 10⁹

9.1 × 10⁸

9.2 × 10⁸

4.6 × 10⁷

C₆₀-Ar

ZnP-C₆₀

H₂P-C₆₀

ZnP-H₂P

•-

ZnP-H₂P-C₆₀ZnP-*H₂P- C₆₀

toluene

ZnP-H₂P-C₆₀ZnP^•+-H₂P-C₆₀benzonitrile

•-

It is interesting to note that the absorption ratio at 695 nm,

stemming from the faster (Figure 4, 25 ps time delay) and the

slower process (Figure, 150 ps time delay), is approximately

1:1. This is in line with the nearly identical singlet ground-

state absorption of the ZnP and H₂P moieties in ZnP-H₂P at

the excitation wavelength (532 nm). Also, the weak absorption

features of the initially formed ¹*(π-π*)ZnP should be noted.

Accordingly, it is safe to ascribe the secondary contribution to

originate from an intramolecular singlet-singlet energy transfer

between the ¹*(π-π*)ZnP (2.04 eV) and the energetically

lower-ying ¹*(π-π*)H₂P (1.89 eV). A similar conclusion was

reached from the time-resolved fluorescence measurements.⁴⁶

³*(π-π*)H₂P, respectively, were measured in the present study

(Table 3). Analogous to previous studies we ascribe the

intermolecular reaction to the following photosenitization

process of cytotoxic singlet oxygen:^50,51

³*ZnP + O₂f ZnP + ¹*O₂

³*H₂P + O₂f H₂P + ¹*O₂

(2a)

(2b)

C₆₀-Ar. The photophysics of the fullerene reference (C₆₀-

Ar, see Figure 2) can be summarized as follows: The singlet

excited-state absorption of C₆₀-Ar gives rise to a marked

singlet-singlet absorption around 890 nm, whose lowest

vibronic state has an energy of 1.75 eV. The latter was

determined in a complementary fluorescence measurement.

Once generated, this state is subject to a rapid and quantitative

intersystem crossing process to the energetically lower-lying

triplet excited state (1.50 eV).²⁷In the case of the currently

employed fullerene reference (C₆₀-Ar), this intersystem cross-

ing takes place with a time constant of 5.5 × 10⁸s^-1, governed

by a large spin-orbit coupling.⁵²The spin-orbit coupling makes

this process much faster than those noticed in most 2-dimen-

sional aromatic hydrocarbons. However, not only is the rate

remarkable, but also the overall efficiency of this process is

impressive, affording a triplet quantum yield close to unity

(Φ_TRIPLET) 0.98).⁵²The triplet-triplet absorption spectrum

reveals a maximum in the visible region (700 nm), accompanied

by another transition in the ultraviolet (UV) region (360 nm).

The triplet lifetime of C₆₀-Ar is dominated by triplet-triplet

ZnP-H₂P

9

^hν8 ¹*ZnP-H₂P f ZnP-¹*H₂P

(4)

According to the Coulombic energy transfer theory a singlet-

singlet energy transfer is proportional to the square of the

transition dipole moments and the inverse sixth power of the

donor-acceptor separation (R_D-A). The distance dependence

of this dipole-dipole mechanism can be simplified as follows

considering the deactivation of the photoexcited chromophore

1

(i.e., *(π-π*)ZnP; k_D) 4.0 × 10⁸s^-1).⁵³

ln(k_{EnergyTransfer}/k_D) ≈ 6 ln(R_D-A

)

(5)

At the given donor-acceptor separation of 18 Å, the rapid

and efficient energy transfer from the ¹*(π-π*)ZnP to the H₂P

is in agreement from the Coulombic energy-transfer theory,

(53) Turro, N. J. Modern Molecular Photochemistry; University Science

(52) Foote, C. S. Top. Curr. Chem. 1994, 169, 347.

Books, Sausalito, CA, 1978.

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6541

Figure 4. Differential absorption spectra obtained upon picosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-H₂P dyad in nitrogen

saturated toluene with a time delay of 0 ps (O), 25 ps (•), and 150 ps (×).

which, in its essence, predicts that the dipole-dipole mechanism

is mostly effective up to separations of ∼20 Å.

toluene and 0.15 eV in benzonitrile. Only electron-donating and

electron-withdrawing substituents at the ZnP and H₂P, respec-

tively, render the free-energy change for an intramolecular

electron transfer thermodynamically more feasible, yielding a

ZnP^•+-H₂P^•-radical pair.⁵⁵

Complementary nanosecond experiments, performed with a

similar ZnP-H₂P dyad solution, revealed a maximum at 780

nm (see Figure 3 c) and a lifetime of 380 µs. These are again

unambiguous attributes of the ³*(π-π*)H₂P. The overall triplet

quantum yields, as measured with the comparative method (i.e.,

using H₂P as a standard), are 0.76 (toluene) and 0.74 (benzo-

nitrile), are slightly lower than the one found for the H₂P

reference compound (Φ_TRIPLET) 0.82). These properties

confirm our tentative assignment of a singlet-singlet energy-

transfer mechanism controlling the deactivation of the photo-

excited ZnP in the ZnP-H₂P dyad.

ZnP-C₆₀. The two porphyrin-fullerene dyads, ZnP-C₆₀

and H₂P-C₆₀, are expected to undergo intramolecular electron

transfer, as shown earlier in porphyrin-fullerene dyads that

carried different linking blocks.^38-45This assumption is, fur-

thermore, supported by calculations regarding the thermody-

namic driving force (-∆G_CS) within the framework of the

dielectric continuum model (see Table 1).^47,48However, in the

event that nonpolar toluene is used as a solvent, the efficiency

of electron transfer versus the competing energy transfer will

depend strongly on the separation distance and may alter the

mechanism. Considering the distance dependence on electron

Similar to the reference compound (see eq 2b), admitting

molecular oxygen to the dyad solution led to a rapid deactivation

3

of the *(π-π*)H₂P state with bimolecular rate constants of

transfer (eq 6 with k_D) 4.0 × 10⁸s^{-1 53}

) we can expect that at

9.1 × 10⁸M^-1s^-1and 1.08 × 10⁹M^-1s^-1in toluene and

benzonitrile, respectively (see Table 3). It should be added that

singlet oxygen is the product of this bimolecular reaction.⁵⁴In

conclusion, the spatially separated ZnP-H₂P is subject, upon

excitation of the ZnP chromophore, to an efficient long-range

intramolecular energy transfer. Similar conclusions stem from

previous reports on chemically different ZnP-H₂P dyads.⁵⁵

short donor-acceptor separations (∼3 Å) electron-transfer

prevails.^45bL is the electronic factor, characterizing the overlap

of orbitals involved in the electron-transfer reactions.⁵³At larger

spatial separations, such as in the current dyads and triad (18

Å), it is likely that energy transfer is the dominant process.

ln(k_{Electron Transfer}/k_D) ≈ 2R_D-A/L

(6)

In the contributed system, the free-energy changes (-∆G_CS

)

for an intramolecular electron-transfer event is -0.61 eV in

Following a 18 ps laser pulse of a benzonitrile solution of

the ZnP-C₆₀dyad, differential absorption changes disclose the

instaneous formation of the ZnP singlet excited state (Figure 5,

30 ps). The two minima observed at 565 and 605 nm correspond

well to the slightly shifted Q-band absorption, due to the

complexation of the coordinating solvent (e.g., benzonitrile).^45a,d

These features, in combination with the broad absorption band

in the red (>620 nm) demonstrate the excitation of the ZnP.

The lifetime of the ¹*(π-π*)ZnP is, however, impacted by the

presence of the covalently attached electron acceptor (i.e.,

fullerene). Formation of a new transient develops as a result of

the rapid decay (k ) 1.1 × 10¹⁰s^-1), instead of the much slower

intersystem crossing (4.0 × 10⁸s^-1) to the triplet excited state.

(54) Ro¨der, B. Biophys. Chem. 1990, 35, 303.

(55) (a) Gust, D.; Moore, T. A.; Moore, A.; Leggett, L.; Lin, S.;

DeGraziano, J. M.; Hermant, R. M.; Nicodem, D.; Craig, P.; Seely, G. R.;

Nieman, R. A. J. Phys. Chem. 1993, 97, 7926. (b) DeGraziano, J. M.;

Liddell, P. A.; Leggett, L.; Moore, A.; Moore, T. A.; Gust, D. J. Phys.

Chem. 1994, 98, 1758. (c) Yang, S. I.; Lammi, R. K.; Seth, J.; Riggs, J. A.;

Arai, T.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem.

B 1998, 102, 9426. (d) Baraka, M. E.; Jano, J. M.; Ruhlmann, L.; Giraudeau,

A. Deumie, M.; Seta, P. J. Photochem. Photobiol. A 1998, 113, 163. (e)

Jensen, K. K.; Van Berlkom, S. B.; Kajanus, J.; Martensson, J.; Albinsson,

B. J. Phys. Chem. A 1997, 101, 2218. (f) Osuka, A.; Marumo, S.; Mataga,

N.; Taniguchi, S.; Okada, T.; Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki,

K. J. Am. Chem. Soc. 1996, 118, 155. (g) Gust, D.; Moore, T. A.; Moore,

A.; Kang, H. K.; DeGraziano, J. M.; Liddell, P. A.; Seely, G. R. J. Phys.

Chem. 1993, 97, 13637. (h) Kilsa, K.; Kajanius, J.; Martensson, J.;

Albinsson, B. J. Phys. Chem. B 1999, 103, 7329.

6542 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

Figure 5. (a) Differential absorption spectra obtained upon picosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-C₆₀dyad in

nitrogen saturated benzonitrile with a time delay of 0 ps (•), 30 ps (O), and 250 ps (×). (b) Time-absorption profile at 502 nm, displaying the initial

1

formation of the *(π-*π)ZnP chromophore and its subsequent transformation via an intramolecular electron transfer to the charge-separated

radical pair (ZnP^•+-C₆₀^•-) (∼250 ps after the pulse).

The new transient absorbs predominantly in a region red-shifted

relative to the porphyrin Q-bands (Figure 5, 250 ps, see kinetic

time trace).^44a,bThis suggests involvement of the ZnP π-radical

cation in an intramolecular electron transfer, occurring between

the photoexcited ZnP donor and the fullerene acceptor.

It is notable that molecular oxygen has a meaningful impact

on the lifetime of the charge-separated state. Reoxidation of

the fullerene π-radical anion and, in turn, the possible formation

of O₂may take place.^58,59This assumption is reasonable

•-

considering our measured redox potential for the first reduction

step of the fullerene moiety (E_1/2(C₆₀/C₆₀^•-) ) -1.04 V vs

Fc/Fc⁺) and E_1/2for the O₂/O₂^•-couple (-0.9 V vs Fc/Fc⁺). In

particular, we derived from an oxygen-dependent decay of the

fullerene π-radical anion absorption an intermolecular rate

(56) Guldi, D. M.; Hungerbuehler, H.; Janata, E.; Asmus, K. D. J. Chem.

Soc., Chem. Commun. 1993, 6, 84.

(57) Kato, T.; Kodama, T.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki,

S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 180,

446.

hν

•-

ZnP-C₆₀

9

8 ¹*ZnP-C₆₀f ZnP^•+-C₆₀

(7)

With the assistance of complementary nanosecond experi-

ments (Figure 6) the presence of the one-electron-reduced

fullerene, exhibiting a characteristic absorption maximum at

1000 nm, was also confirmed.^56,57These experiments (i.e.,

measurements at 1000 nm) enabled us, furthermore, to determine

the lifetime of the charge-separated ZnP^•+-C₆₀^•-pair (τ ) 580

ns) and the quantum yield as well (Φ ) 0.85) in deoxygenated

benzonitrile solutions.

(58) Ohlendorf, V.; Willnow, A.; Hungerbuhler, H.; Guldi, D. M.; Asmus,

K.-D. J. Chem. Soc., Chem. Commun. 1995, 759.

(59) Yamakoshi, Y.; Sueyoshi, S.; Fukuhara, K.; Miyata, N.; Masumizu,

T.; Kohno, M. J. Am. Chem. Soc. 1998, 120, 12363.

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6543

To follow up on this argument and to investigate the presumed

singlet-singlet energy transfer, we probed the growing-in of

the fullerene singlet excited state and related it to the deactiva-

tion of the photoexcited ZnP chromophore. This measurement

is possible because the absorption of the ¹*(π-π*)ZnP is weak

in the wavelength region where the predominant ¹*C₆₀absorp-

tion is located (around 890 nm).^27,60Indeed, the ZnP-C₆₀dyad

shows, upon laser irradiation in toluene, the growing-in of the

fullerene singlet excited state absorption. More importantly the

formation kinetics of the fullerene singlet (1.7 × 10¹⁰s^-1) were

1

virtually identical to those of the decay of the *(π-π*)ZnP

absorption around 500-650 nm (1.6 × 10¹⁰s^-1).

ZnP-C₆₀9

^hν8 ¹*ZnP-C₆₀f ZnP-¹*C₆₀f ZnP-³*C₆₀

(9)

Figure 6. Differential absorption spectra obtained 50 ns upon

nanosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-

C₆₀dyad in nitrogen saturated benzonitrile (s) and ZnP-H₂P-C₆₀

triad in nitrogen saturated benzonitrile (s s s).

In line with this energy-transfer mechanism, the differential

absorption changes, recorded immediately after a 20 ns pulse,

showed the spectral features of the fullerene triplet excited state.

Specifically, two maxima located at 360 and 700 nm and a low-

energy shoulder around 800 nm were observed.^27,60,61aAlso the

high quantum yield of the fullerene triplet (Φ_TRIPLET) 0.86)

should be noted. Our analysis clearly shows the photosensiti-

zation effect of the porphyrin antenna. This enables the efficient

fullerene triplet generation, even in a wavelength region where

the fullerene absorption is relatively weak (ꢀ_{532 nm}≈ 2000 M^-1

cm^-1). No spectral evidence, in terms of the fullerene π-radical

anion absorption at 1000 nm,^56,57was observed that would be

in support of a possible electron-transfer mechanism.

constant of 9.4 × 10⁸M^-1s^-1. This value is close to a diffusion-

controlled process (i.e., k_diff) 5.0 × 10⁹M^-1s^-1) as computed

from Smoluchowski’s theory of diffusion-controlled reactions

(with k_diff) 8RT/3η × 10³; η_{(benzonitrile)}) 1.24 [10^-3Pa × s]).⁴⁹

ZnP^•+-C₆₀^•-+ O₂f ZnP^•+-C₆₀+ O₂

(8)

•-

In toluene, two distinct follow-up reactions come next after

the completion of the initially formed ¹*(π-π*)ZnP state. The

first process is a fast decay, which is essentially completed

around 100 ps after the laser pulse with a rate constant of 1.7

× 10¹⁰s^-1(see 500-550 nm region). In contrast, the second,

slower step is a growing-in process with a time constant of 1.8

ns (k ) 5.5 × 10⁸s^-1) (see 650-750 nm region). This

observation is in clear contrast to the benzonitrile case that

revealed only a rapidly occurring electron transfer. Spectral

analysis of the two transients formed in toluene, for example,

after 100 and 4000 ps (Figure 7), shows surprisingly no

resemblance with the ZnP π-radical cation absorption (see

Figure 5, 250 ps). Instead, the earlier formed species has a broad

and structureless transition in the 550-750 nm range (Figure

7, 100 ps). These characteristics are similar to those noted for

the fullerene singlet excited-state absorption.^27,60The slower

produced transient (Figure 7, 4000 ps) possesses the charac-

teristics of the fullerene triplet excited state, for example, a

maximum at 700 nm. Consequently, we reach the conclusion

that a long-range singlet-singlet energy transfer, by means of

a dipole-dipole mechanism (see eq 5) governs the deactivation

of the photoexcited ZnP.

H₂P-C₆₀. In benzonitrile solutions, the singlet-singlet

absorption of the ¹*(π-π*)H₂P, formed directly upon photoly-

sis, decays rapidly. The derived rate constant of 5.2 × 10⁹s^-1

is in sharp contrast to the much slower occurring intersystem

crossing rate (9.9 × 10⁷s^-1), observed for the reference H₂P

complex (see Tables 2 and 4). The spectral features of the newly

formed transient, that is, a broad band red-shifted with regard

to the Q-band absorption, are similar to the radiolytically formed

π-radical cation of the H₂P (not shown). This suggests a rapid

intramolecular electron transfer to yield the H₂P^•+-C₆₀^•-state.

The lower singlet excited-state energy (1.89 eV) and, in addition,

the less favorable oxidation potential (E_1/2(H₂P^•+/H₂P) ) +0.55

V vs Fc/Fc⁺) of the free base porphyrin, relative to the ZnP

dyad (E_1/2(ZnP^•+/ZnP) ) +0.34 V vs Fc/Fc⁺), have a marked

impact on the electron-transfer dynamics. Despite the identical

separation, for example, a center-to-center distance of 18.0 Å,

-∆G_CSfor an electron transfer is only +0.33 eV (H₂P-C₆₀),

in comparison to a value of +0.69 eV for the ZnP-C₆₀dyad

(see Table 1).

Full spectral characterization of the charge-separated state was

performed with the help of complementary nanosecond meas-

urements. Photolysis of a similar H₂P-C₆₀solution led to a

maximum at 1000 nm, which is a clear indication for the

Also, the thermodynamic argument (see Table 1), that is, the

calculated driving force (-∆G_CS), contradicts an electron-

transfer event in toluene. Although -∆G_CSis clearly smaller

(-0.13 eV), relative to the benzonitrile case (+0.69 eV), the

existence of the fullerene π-radical anion.^56,57This C₆₀

•-

fingerprint allowed us to determine the lifetime of the associated

charge-separated state to be ∼45 ns. The latter decays via

1

decay of the *(π-π*)ZnP component is evidently faster in

3

another intermediate to the *(π-π*)H₂P, characterized by a

toluene. The large -∆G_CSvalue relates primarily to the lowering

of the energy of the charge-separated state in the polar solvent

and the stabilization resulting therefrom. In summary, the

energetics and the observed rate indicate an energy transfer

process in toluene.

transition maximizing around 780 nm (similar to Figure 3,

spectrum a). The 780 nm band resembles those recorded for

the H₂P and ZnP-H₂P references. In deoxygenated solutions

(61) (a) Guldi, D. M.; Maggini, M. Gazz. Chim. Ital. 1997, 127, 779.

(b) Martin, N.; Sanchez, L.; Illescas, B.; Gonzalez, S.; Herranz, M. A.;

Guldi, D. M. Carbon 2000, in press. (c) Guldi, D. M.; Gonzalez, S.; Martin,

N.; Anton, A.; Garin, J.; Orduna, J. J. Org. Chem. 2000, 65, 1978-1983..

(60) Guldi, D. M.; Maggini, M.; Scorrano G.; Prato, M. J. Am. Chem.

Soc. 1997, 119, 974.

6544 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

Figure 7. Differential absorption spectra obtained upon picosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-C₆₀dyad in nitrogen

saturated toluene with a time delay of 0 ps (b ), 30 ps (O), 100 ps (•), and 4000 ps (×).

generated fullerene triplet excited state (e.g., H₂P-³*C₆₀) gives

Table 4. Center-to-center Distances and Rate Constant for

Intramolecular Electron Transfer in Fullerene-Porphyrin Dyads and

Triad

rise to an overall efficiency of 83%, only a moderate 54% were

3

determined for the final product (e.g., *H₂P-C₆₀).

compd

solvent

R_D-A[Å] k_ET[s^-1

]

k_BET[s^-1

]

^hν8 ¹*H₂P-C₆₀f H₂P-¹*C₆₀f

H₂P-³*C₆₀f ³*H₂P-C₆₀(10)

H₂P^•+-C₆₀

benzonitrile

18.0

36.1

5.2 × 10⁹1.3 × 10⁷

1.1 × 10¹⁰1.3 × 10⁶

7.0 × 10⁹

•-

H₂P-C₆₀

9

ZnP^•+-C₆₀

•-

ZnP-H₂P^•+-C₆₀

ZnP^•+-H₂P-C₆₀

2.2 × 10⁹4.7 × 10⁴

ZnP-H₂P-C₆₀. Stimulated by the above findings, we

pursued the design of a molecular array, in which the free base

porphyrin separates the zinc porphyrin from the electron

acceptor. This strategy should, in principle, allow a charge shift

from the one-electron oxidized H₂P π-radical cation (E_1/2(H₂P^•+/

H₂P) ) +0.59 V vs Fc/Fc⁺) to the better electron donor ZnP

3

the *(π-π*)H₂P gives rise to a lifetime of 231 µs, decaying

to the singlet ground state. At this point we cannot offer any

reasonable rationale for the nature of the transient intermediate.

However, a similar ET behavior was reported in a C-C₆₀dyad

(C ) carotenoid polyene),^38gC-H₂P-C₆₀triad,^38dand tetra-

thiafulvalene-C₆₀dyad^61b,61cwhere BET proceeds mainly via

formation of the carotenoid triplet excited state (C-C₆₀and

C-H₂P-C₆₀)^38d,gand fullerene triplet excited state (tetra-

thiafulvalene-C₆₀).^61b,c

(E_1/2(ZnP^•+/ZnP) ) +0.30 V vs Fc/Fc⁺), prior to the charge-

recombination of the initially formed ZnP-H₂P^•+-C₆₀

.

•-

Benzonitrile. The whole dynamic range (e.g., between -50

and 4000 ps), in terms of differential absorption changes,

recorded upon 532 nm excitation of a ZnP-H₂P-C₆₀benzo-

nitrile solution, is depicted in Figure 8. The initial events are

virtually identical to those described above for the diporphyrin

reference dyad (ZnP-H₂P): Two successive steps are associ-

1

In toluene solutions the decay of the *(π-π*)H₂P moiety

of H₂P-C₆₀is subject to a similar change in mechanism as

described above for the ZnP-C₆₀dyad. Specifically, a rapid

singlet-singlet energy transfer (1.5 × 10¹⁰s^-1) takes place to

yield the fullerene singlet excited state. This is followed by an

intersystem crossing (5.6 × 10⁸s^-1) to the energetically lower-

lying fullerene triplet (similar to Figure 7). In the ZnP-C₆₀

dyad the fullerene triplet excited state (i.e., ZnP-³*C₆₀) is lowest

state available and, in turn, decayed to the singlet ground state.

Now, in the case of the current H₂P-C₆₀dyad, the energy of

the ³*(π-π*)H₂P state (1.40 eV), is located between the

fullerene triplet (1.50 eV) and the singlet ground state. As a

1

ated with the formation of the *(π-π*)H₂P, which then may

inject an electron into the fullerene moiety. These are (i) direct

532 nm excitation of the H₂P chromophore and (ii) rapid

intramolecular singlet-singlet energy transfer (k ) 1.5 × 10¹⁰

s^-1) originating from the ¹*(π-π*)ZnP (Figure 9). Importantly,

the measured energy transfer rate closely resembles the dynam-

ics of the ZnP-H₂P dyad (see above) and the time-resolved

fluorescence measurements.⁴⁶

consequence, an additional, but rather slow (k ) 2.4 × 10⁵s^-1

)

An interesting fact is that even in the case of the molecular

triad, the ratio of the direct excitation and the energy transfer is

nearly 1:1 as measured by the growing-in of the transient around

695 nm (see also the discussion regarding the energy transfer

in the photoexcited ZnP-H₂P dyad). This is in compliance with

the nearly equal absorbance of the two chromophores (i.e., ZnP

and H₂P) at the excitation wavelength (532 nm).

triplet-triplet energy transfer from the fullerene triplet to the

porphyrin is energetically permitted. Considering the center-

to-center distance of 18.0 Å, the much slower rate constant,

relative to the singlet-singlet energy transfer (∼10¹⁰s^-1), can

be rationalized by the different mechanisms, namely, dipole-

dipole (see eq 5) versus exchange (see eq 6), which are operative

in this system.⁵³In addition, the triplet quantum yields further

underlines the noted differences. While the intermediately

The initial excitation energy is almost quantitatively (∼75%,

see ZnP-H₂P) transferred to the H₂P chromophore. After the

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6545

Figure 8. Differential absorption spectra obtained upon picosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-H₂P-C₆₀triad in

nitrogen saturated benzonitrile with time delays of between -50 and 4000 ps.

completion of this energy transfer process, the sequence of two

follow-up reactions was noticed (see kinetic time trace of Figure

10). The faster one has a time constant of 7.0 × 10⁹s^-1(see

Table 4). Based on the kinetic similarity with the electron

transfer in the H₂P-C₆₀dyad, we ascribe this reaction

intermolecular rate constant of k ) 4.6 × 10⁷M^-1s^-1was

derived. This leads to the conclusion that C₆₀^•-is quantitatively

reoxidized by oxygen. Since the reduction potential of the

fullerene moiety (-1.04 V vs Fc/Fc⁺) is not changed relative

the one in the ZnP-C₆₀dyad, the following reduction of

molecular oxygen (E_1/2(O₂/O₂^•-) ) -0.9 V vs Fc/Fc⁺) appears

thermodynamically reasonable:

tentatively to the formation of the H₂P^•+-C₆₀pair in the

•-

molecular triad. Spectral characteristics of the porphyrin π-radi-

cal cation, particularly, in the red region, corroborated this

hypothesis (Figure 10, 500 ps).

ZnP^•+-H₂P-C₆₀^•-+ O₂f ZnP^•+-H₂P-C₆₀+ O₂(12)

•-

The energy of the ZnP-H₂P^•+-C₆₀state (1.60 eV) lies

•-

sufficiently above that of the long-range separated pair, namely,

Comparing the individually determined rate constants for (i)

an intramolecular electron transfer in benzonitrile (7.0 × 10⁹

s^-1) and (ii) an energy transfer in toluene (1.5 × 10¹⁰s^-1, see

below) leads to the hypothesis that in benzonitrile a competition

between the two transfer processes is operative. A thermody-

ZnP^•+-H₂P-C₆₀(1.32 eV). In principle the noted energy

•-

difference should be enough to promote a subsequent charge

shift from the H₂P^•+to the ZnP. Indeed, the spectral features,

recorded 3000 ps after the pulse (Figure 10, 3000 ps), are

reminiscent to the charge-separated state in the ZnP-C₆₀

reference dyad and, therefore, support the energetics argument.

From the growing-in kinetics of the ZnP π-radical cation at 650

nm we were able to derive a rate constant for this secondary

namic consideration (-∆G_CS) 0.29 eV; -∆G_{EnergyTransfer}

)

0.13 eV) suggests, however, a minor contribution of the

competing energy transfer mechanism. In good agreement with

the thermodynamic reasoning, no spectral evidence for the

participation of an energy transfer mechanism was deduced from

our picosecond-resolved transient absorption measurements.

To shed further light on this alternative electron-transfer

pathway, generation of the fullerene singlet excited state, which

would be the result of the intramolecular energy transfer, was

deemed necessary. An electron transfer starting from the

photoexcited fullerene was probed via a 337 nm excitation

experiment, where the fullerene moiety absorbs the main fraction

of the excitation light. In fact, the transient absorptions,

measured after a 20 ns laser pulse are essentially superimposable

to those of the 532 nm excitation. In particular, features of the

oxidized ZnP (600-750 nm) and the reduced fullerene moiety

(1000 nm) confirm the following electron-transfer scenario:

337 nm (benzonitrile):

electron transfer of 2.2 × 10⁹s^-1

features of both radical species, namely, the oxidized ZnP^•+

.

Complementary nanosecond experiments revealed the spectral

•-

(600-750 nm) and the reduced C₆₀(1000 nm) (Figure 6).

The long-range charge-separated pair is formed with a quantum

yield of 0.4. Due to the large spatial separation of 36.1 Å the

ZnP^•+-H₂P-C₆₀^•-decays with a remarkable lifetime of 21 µs.

The charge recombination process is dominated by a mono-

exponential recovery of the singlet ground-state rather than

population of a porphyrin triplet excited state. This observation

corresponds well with the energetic argument (1.32 eV).

532 nm (benzonitrile):

ZnP-H₂P-C₆₀^-hν8 ¹*ZnP-H₂P-C₆₀f

ZnP-¹*H₂P-C₆₀f ZnP-H₂P^•+-C₆₀^•-f

ZnP-H₂P-C₆₀^-hν8 ZnP-H₂P-¹*C₆₀f

ZnP^•+-H₂P-C₆₀(11)

•-

ZnP-H₂P^•+-C₆₀^•-f ZnP^•+-H₂P-C₆₀(13)

•-

The lifetime of the ZnP^•+-H₂P-C₆₀radical pair, opened

•-

the opportunity for a reaction with molecular oxygen. Admission

of air or pure oxygen to the triad solution resulted in a markedly

accelerated recovery of the fullerene ground state. From

analyzing the lifetime as a function of oxygen concentration an

The determination of the quantum yield for the charge-

separation showed surprisingly a higher value in the 337 nm

than in the 532 nm experiment. This can be rationalized, at least

in part, by the sequence of energy- and electron transfers

6546 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

Figure 9. (a) Differential absorption spectra obtained upon picosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-H₂P-C₆₀triad

in nitrogen-saturated benzonitrile with a time delay of 0 ps (•), 25 ps (O), and 100 ps (×). (b) Time-absorption profiles at 502 and 650 nm,

1

displaying the intramolecular energy transfer from the initially excited *ZnP-H₂P-C₆₀to ZnP-¹*H₂P-C₆₀, (completed around 100 ps after the

pulse) followed by an intramolecular electron-transfer yielding ZnP-H₂P^•+-C₆₀(∼300 ps after the pulse).

•-

involved upon excitation of the ZnP and H₂P chromophores in

the ZnP-H₂P-C₆₀triad, which proceed with a quantum yield

less than unity. On the other hand, excitation of the fullerene is

nearly quantitative.

the sum of the two reference dyads in toluene, namely, energy

transfer from ¹*(π-π*)ZnP to H₂P (ZnP-H₂P) and from ¹*(π-

π*)H₂P to C₆₀(H₂P-C₆₀). The initial energy transfer event (k

) 1.5 × 10¹⁰s^-1) between the two porphyrin moieties was again

1

confirmed by the two-step growing-in of the *(π-π*)H₂P

The conclusion of these experiments is that the formation of

the charge-separated radical-pair (ZnP-H₂P^•+-C₆₀^•-and con-

sequently ZnP^•+-H₂P-C₆₀^•-) upon 532 nm excitation is

independent of the pathway, namely, either direct electron

transfer from the ¹*(π-π*)H₂P to the fullerene or, alternatively,

energy transfer from the ¹*(π-π*)H₂P to the fullerene, followed

by an electron transfer.⁶²

component around 695 nm (see above ZnP-H₂P).

(62) A similar consideration applies for the two precursor dyads (i.e.,

H₂P-C₆₀and ZnP-C₆₀). In nonpolar toluene rapid and efficient energy

transfer governs the deactivation of the initially formed ¹*(π-π*)H₂P and

¹*(π-π*)ZnP states with rate constants of (1.5 × 10¹⁰s^-1) and (1.7 ×

10¹⁰s^-1), respectively. An additional electron transfer starts, however, to

compete with this energy transfer in polar benzonitrile. With the help of an

337 nm excitation experiments (e.g., fullerene excitation) it was confirmed

that the ¹*C₆₀is potent enough to activate the intramolecular electron transfer

from the fullerene end.

Toluene. In nonpolar toluene the mechanism changed

dramatically: The reactivity of the triad is now comparable to

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6547

Figure 10. (a) Differential absorption spectra obtained upon picosecond flash photolysis (532 nm) of ∼10^-5M solutions of ZnP-H₂P-C₆₀triad

in nitrogen saturated benzonitrile with a time delay of 0 ps (O), 500 ps (•), and 3000 ps (×). (b) Time-absorption profile at 505 and 650 nm (i.e.,

maximum difference between H₂P^•+and ZnP^•+absorption), displaying the charge-shift from the ZnP-H₂P^•+-C₆₀^•-(∼500 ps after the laser pulse)

to ZnP^•+-H₂P-C₆₀(∼5000 ps after the laser pulse).

•-

Since the energy of the charge-separated ZnP-H₂P^•+-C₆₀

is unfavorably raised in toluene (2.46 eV), relative to the

benzonitrile solution (1.60 eV), energy transfer is expected to

prevail over electron transfer. Indeed, singlet-singlet energy

transfer from the ZnP-¹*H₂P-C₆₀(1.90 eV) to the ZnP-H₂P-

¹*C₆₀(1.75 eV) governs the fate of the photoexcited chro-

mophore.

the decay of the ¹*(π-π*)ZnP, according to the distance (R_D-A

)

•-

dependence of the dipole-dipole mechanism (see eq 5).⁵³Thus,

the quantitative quenched ZnP fluorescence rules out the

energetically feasible concerted long-range energy transfer

instead of the stepwise transfer (i.e., from the ZnP to the H₂P

and finally to the C₆₀). In fact, the experimental data disproves

unequivocally the occurrence of a slow, concerted energy-

transfer mechanism.

532 nm (toluene):

The fullerene singlet excited-state undergoes intersystem

crossing to the triplet excited state, with a rate of 5.5 × 10⁸s^-1

which was derived from the growing-in dynamics of the

characteristic fullerene triplet excited-state absorption at 700

nm. Considering the fact that the intersystem crossing in

fullerenes is near unity,⁵²it is evident that the loss in efficiency

ZnP-H₂P-C₆₀

9

^hν8 ¹*ZnP-H₂P-C₆₀f

ZnP-¹*H₂P-C₆₀f ZnP-H₂P-¹*C₆₀(14)

A direct energy-transfer route from the ZnP to the fullerene,

over the estimated distance of 36.1 Å, should be comparable to

6548 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

Figure 11. Energy diagram of ZnP-H₂P-C₆₀triad in toluene and benzonitrile.

(fullerene Φ_TRIPLET) 0.71) corresponds well to the product,

¹*ZnP-H₂P-C₆₀/ZnP-¹*H₂P-C₆₀(Φ ) 0.92) and ZnP-

¹*H₂P-C₆₀/ZnP-H₂P-¹*C₆₀(Φ ) 0.83) transformations. The

fullerene triplet excited state decays finally via a slow and

inefficient triplet-triplet energy transfer to the lowest lying state

in the system (³*(π-π*)H₂P; 1.40 eV).

charge-injection into the fullerene core, and (iii) transportation

of the hole, this artificial reaction center reproduces the natural

system very well. Also its productiveness outperforms a recently

published antenna-reaction center complex.^38f

The higher efficiency of the current system, relative to

previously investigated dyad and triad systems³⁸cannot be

explained on the basis of the thermodynamic redox properties

of the species involved. One argument, which may, however,

be forwarded, could be the fairly large separation existing

between each of the redox active couples in ZnP-C₆₀(i.e.,

ZnP/C₆₀) and ZnP-H₂P-C₆₀(i.e., ZnP/H₂P and H₂P/C₆₀) of

approximately 18 Å. In principle, this separation guarantees an

efficient electron relay along the vertical energy gradient before,

however, charge recombination takes over to dominate the fate

of the first couple (i.e., ZnP-H₂P^•+-C₆₀^•-). A general comment,

regarding the charge-injection from the photoexcited chro-

mophore into the fullerene, should be made at this point. The

small free energy change associated with the H₂P/C₆₀couple,

relative to that of the ZnP/C₆₀couple is, at least, in part a major

limitation of the system presented. Currently, we are pursuing

work toward improving the initial charge-injection step.

Conclusions

In conclusion, we demonstrated the sequential energy- and

electron-transfer events in a supermolecular triad (lower part

of the diagram in Figure 11) and, thus, a system that mimics

the primary steps in natural photosynthesis. Furthermore, the

lifetime of the charge-separated radical pair of 21 µs is

promising and evokes further studies, for example, incorporation

into heterogeneous systems.⁶³It should be noted that the lifetime

•-

of the charge-separated radical pair, namely, ZnP^•+-H₂P-C₆₀

implies a significant advancement not only with respect to C₆₀-

based dyads but to C₆₀-based triads as well.^38-45Considering

the overall efficiency of 40% for (i) funneling light from the

antenna chromophore (i.e., ZnP) to the H₂P chromophore, (ii)

(63) (a) Imahori, H.; Yamada, H.; Ozawa, S.; Ushida, K.; Sakata, Y.

Chem. Commun. 1999, 1165. (b) Imahori, H.; Yamada, H.; Nishimura, Y.;

Yamazaki, I.; Sakata, Y. J. Phys. Chem. B, 2000, 104, 2099-2108.

In comparison to the previously published ZnP-P34-C₆₀

dyad,^44bthe charge-separated ZnP^•+-C₆₀^•-state even in ZnP-

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6549

Nd:YAG system (532 nm, 20 ns pulse width) in a front face excitation

geometry. A Xe lamp was triggered synchronously with the laser. A

monochromator (SPEX) in combination with either a Hamamatsu R

5108 photomultiplier or a fast InGaAs-diode was employed to monitor

transient absorption spectra.

C₆₀reveals a marked stabilization. Two structural differences

in ZnP-P34-C₆₀vs ZnP-C₆₀are obvious. First, and far most

important, the amide linkage, connecting the two redox-active

centers is reversed. Second the different fullerene functional-

ization leads to the incorporation of additional C-C bond in

ZnP-C₆₀. No doubt, the earlier reasoning appears to be of

higher significance with regard to accelerating charge separation

and to decelerating charge recombination.

The quantum yields were measured using the comparative method.

In particular, the strong fullerene triplet-triplet absorption (ꢀ₇₀₀

)

nm

16 100 M^-1cm^-1; Φ_TRIPLET) 0.98)⁶⁶served as probe to obtain the

quantum yields for the CS state, especially for the fullerene π-radical

anion (ꢀ₁₀₀₀) 4700 M^-1cm^-1).⁶⁷The quantum yields of ZnP and

On the other hand, in toluene solutions (upper part of the

diagram in Figure 11) energy was transferred sequentially over

a distance of 36.1 Å.

nm

H₂P were determined relative to a zinc tertraphenylporphyrin standard

(Φ_TRIPLET) 0.88).

Synthesis and Characterization. 1.^68,69A solution of 4-methoxy-

carbonylbenzaldehyde (9.88 g, 60.2 mmol) and pyrrole (42.0 mL, 605

mmol) was degassed by bubbling with nitrogen for 1 h, then

trifluoroacetic acid (0.12 mL, 1.6 mmol) was added. The solution was

stirred for 4 h at room temperature. The mixture was diluted with

benzene (300 mL), washed with 0.1 M sodium hydroxide aqueous

solution and water successively, and dried over anhydrous sodium

sulfate. The solvent and the unreacted pyrrole were removed under

reduced pressure at room temperature. The resulting red oil was

dissolved in a minimal amount of chloroform and purified by flash

column chromatography on silica gel (benzene/ethyl acetate/triethyl-

amine ) 40/1/0.4 f 20/1/0.2). The main first phase was collected and

evaporated under reduced pressure. Washing the residue with hexane

and subsequent filtration afforded 1 as a pale grayish yellow solid (7.12

g, 25.4 mmol, 42% yield). Analytical data were identical with the

reported data.⁶⁹

2.^68,70,71A solution of 1 (7.08 g, 25.2 mmol) and 3,5-di-tert-

butylbenzaldehyde⁷²(5.50 g, 25.2 mmol) in chloroform (2150 mL)

was degassed by bubbling with nitrogen for 1 h. The reaction vessel

was shielded from ambient light. Then boron trifluoride diethyl etherate

(3.20 mL, 25.3 mmol) was added as one portion. The solution was

stirred for 2 h at room temperature under nitrogen atmosphere. To the

reddish black reaction mixture was added p-chloranil (9.32 g, 37.9

mmol), and the resulting mixture was stirred overnight. Triethylamine

(10.6 mL, 76.1 mmol) was added, and then the reaction mixture was

concentrated. Flash column chromatography on silica gel with chlo-

roform as the eluent gave a mixture of five porphyrins including 2 and

3.⁷³Subsequent flash column chromatography on silica gel (hexane/

toluene ) 2/1 f 1/3) yielded the desired porphyrin from the mixture

as the third band. Reprecipitation with chloroform-methanol afforded

2 as a vivid reddish purple solid (1.75 g, 1.83 mmol, 14% yield): mp

> 300 °C; IR (KBr) 3316, 2955, 1722, 1607, 1592, 1476, 1434, 1362,

1278, 1248, 1113, 1022, 973, 916, 797, 764, 720 cm^-1; ¹H NMR (270

MHz, CDCl₃) δ -2.74 (br s, 2H), 1.52 (s, 36H), 4.11 (s, 6H), 7.80 (s,

2H), 8.07 (s, 4H), 8.31 (d, J ) 8 Hz, 2H), 8.43 (d, J ) 8 Hz, 2H), 8.79

(d, J ) 5 Hz, 2H), 8.90 (d, J ) 5 Hz, 2H); FABMS m/z 955 (M +

H⁺).

A different aspect of this study is concerned with the reactivity

of the photoproducts (e.g., triplet excited states and charge-

separated radical pairs) toward oxygen. All the triplet excited

states generated (i.e., ³*(π-π*)H₂P, ³*(π-π*)ZnP and

³*C₆₀)^52,54,64,65form singlet oxygen (¹O₂) with rate constants

that are close to a diffusion controlled process and high

efficiencies. Similarly, the two radical pairs formed (i.e., the

short-lived ZnP^•+-C₆₀and the much longer lived ZnP^•+-

•-

H₂P-C₆₀^•-) are subject to a markedly slower reaction with

molecular oxygen, yielding, however, the persistent superoxide

radical anion (O₂^•-).^58,59Although the nature of the latter oxygen

species differs from singlet oxygen, the product of the triplet

excited-state quenching, its cytotoxicity should be considered.

Thus, both reaction pathways, for example, energy- and electron

transfer, result in reactive intermediates that, upon reaction with

molecular oxygen, generate cytotoxic species.

Experimental Section

General. Melting points were determined on a Yanaco melting point

apparatus and were uncorrected. IR spectra were obtained on a Perkin-

Elmer System 2000 FT-IR. ¹H NMR spectra were measured on a JEOL

JNM-EX270 or a JEOL JNM-LA400 using tetramethylsilane as internal

standard. Fast atom bombardment mass (FABMS) and matrix-assisted

laser desorption/ionization (MALDI) time-of-flight mass (TOFMS)

spectra were obtained on a JEOL JMS-M600 and a Shimadzu/Kratos

KOMPACT MALDI I, respectively. Electron ionization mass (EIMS)

spectra were measured on a JEOL JMS-DX300. UV-visible absorption

spectra were recorded on a Shimadzu UV-3100PC. Elemental analyses

were performed on a Perkin-Elmer model 240C elemental analyzer.

Differential pulse voltammetry (DPV) was performed on a BAS CV-

50W using a Ag/AgCl as a reference electrode and ferrocene/

ferrocenium as an external standard.

Merck no. 5554 silica gel 60 F₂₅₄and Fuji Silysia BW-300 were

used for TLC and silica gel flash column chromatography, respectively.

All solvents and chemicals for the synthesis were of reagent grade

quality, purchased commercially, and used without further purification

except noted below. Pyrrole, dimethylformamide (DMF), benzene,

pyridine, and toluene were distilled from calcium hydride and stored

over molecular sieves. Tetrahydrofuran (THF) was distilled from

benzophenone ketyl. Benzonitrile for use in the electrochemical studies

was distilled from calcium hydride. Palladium on charcoal (5%) was

obtained from Wako Pure Chemical Industries, Ltd. Tetrabutylammo-

nium fluoride THF solution (1 M) was purchased from Tokyo Chemical

Industry Co., Ltd. Activated manganese dioxide (<5 µm) was obtained

from Aldrich Chemical Co., Inc.

4. A mixture of 2 (500.7 mg, 524.2 µmol) in 240 mL of THF-ethanol

(1:1) and potassium hydroxide (2.69 g) in water (24 mL) was refluxed

for 7.5 h. After cooling, the solvent was evaporated, the residue diluted

with 300 mL water, and the desired porphyrin dipotassium salt filtered.

Acidification (pH 2) of an aqueous suspension of the dipotassium salt

with concentrated hydrochloric acid and subsequent filtration gave 4

as a vivid reddish purple powder (473.2 mg, 510.4 mol, 97% yield):

mp > 300 °C; IR (KBr) 3325, 3000 (br), 2964, 1683, 1606, 1476,

1423, 1362, 1313, 1284, 1248, 1178, 973, 917, 796, 765, 716 cm^-1

;

¹H NMR (270 MHz, CDCl₃/DMSO-d₆) 3/2) δ -2.82 (br s, 2H), 1.54

Photophysical Measurements. For flash-photolysis studies, the

fullerene concentrations were prepared to exhibit an optical density of

at least 0.2 at 532 nm, the wavelength of irradiation. Picosecond laser

flash photolysis experiments were carried out with 532-nm laser pulses

from a mode-locked, Q-switched Quantel YG-501 DP ND:YAG laser

system (pulse width ∼18 ps, 2-3 mJ/pulse). The white continuum

picosecond probe pulse was generated by passing the fundamental

output through a D₂O/H₂O solution. Nanosecond laser flash photolysis

experiments were performed with laser pulses from a Qunta-Ray CDR

(66) Luo, C.; Fujitsuka, M.; Watanabe, A.; Ito, O.; Gan, L.; Huang, Y.;

Huang, C.-H. J. Chem. Soc., Faraday Trans. 1998, 94, 527.

(67) Luo, C.; Fujitsuka, M.; Huang, C.-H.; Ito, O. Phys. Chem. Chem.

Phys. 1999, 1, 2923.

(68) Lee, C.-H.; Lindsey, J. S. Tetrahedron 1994, 50, 11427.

(69) Bru¨ckner, C.; Zhang, Y.; Rettig, S. J.; Dolphin, D. Inorg. Chim.

Acta 1997, 263, 279.

(70) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;

Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827.

(71) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828.

(72) Newman, M. S.; Lee, L. F. J. Org. Chem. 1972, 37, 4468.

(73) Tamiaki, H.; Suzuki, S.; Maruyama, K. Bull. Chem. Soc. Jpn. 1993,

66, 2633.

(64) Guldi, D. M.; Asmus, K.-D. Radiat. Phys. Chem. 1999, 56, 449.

(65) DaRos, T.; Prato, M. Chem. Commun. 1999, 663.

6550 J. Am. Chem. Soc., Vol. 122, No. 28, 2000

Luo et al.

(s, 36H), 7.82 (s, 2H), 8.05 (s, 4H), 8.30 (d, J ) 8 Hz, 2H), 8.43 (d,

J ) 8 Hz, 2H), 8.83 (d, J ) 5 Hz, 2H), 8.91 (d, J ) 5 Hz, 2H); MALDI-

TOFMS (positive mode) m/z 927 (M + H⁺).

toluene/ethyl acetate/triethylamine ) 100/0.5/0.5 f 100/1/0.5). Re-

precipitation from chloroform-acetonitrile afforded 10 as a deep reddish

purple powder (157.2 mg, 72.8 µmol, 34% yield): mp > 300 °C; IR

(KBr) 2962, 1673, 1645, 1592, 1524, 1495, 1474, 1368, 1317, 1248,

5. A mixture of 3 (2.5 g, 2.5 µmol) in 500 mL of 2-propanol and

potassium hydroxide (14 g) in water (125 mL) was refluxed for 6 h.

After cooling, the mixture was acidified with concentrated hydrochloric

acid and extracted with chloroform. The organic extract was washed

with saturated sodium bicarbonate aqueous solution and dried over

anhydrous sodium sulfate; the solvent was then removed under reduced

pressure. Flash column chromatography on silica gel (chloroform, then

chloroform/methanol ) 9/1) afforded 5 as a vivid reddish purple powder

(1.9 g, 1.9 mmol, 76% yield): mp > 300 °C; IR (KBr) 3317, 3000

(br), 2963, 1694, 1593, 1475, 1423, 1394, 1363, 1267, 1248, 973, 915,

801, 761, 731, 715 cm^-1; ¹H NMR (270 MHz, CDCl₃) δ -2.68 (br s,

2H), 1.52 (s, 18H), 1.53 (s, 36H), 7.80 (m, 3H), 8.08 (d, J ) 2 Hz,

2H), 8.09 (d, J ) 2 Hz, 4H), 8.39 (d, J ) 8 Hz, 2H), 8.54 (d, J ) 8

Hz, 2H), 8.81 (d, J ) 5 Hz, 2H), 8.91 (s, 4H), 8.92 (d, J ) 5 Hz, 2H);

FABMS m/z 995 (M + H⁺).

1

1074, 1000, 974, 798, 757, 716 cm^-1; H NMR (270 MHz, CDCl₃) δ

-2.68 (br s, 2H), 0.15 (s, 6H), 0.98 (s, 9H), 1.53 (s, 36H), 1.55 (s,

54H), 4.79 (s, 2H), 7.42 (d, J ) 8 Hz, 2H), 7.74 (d, J ) 8 Hz, 2H),

7.79 (s, 1H), 7.81 (s, 2H), 7.83 (s, 2H), 8.12 (s, 11H), 8.17 (d, J ) 8

Hz, 2H), 8.24 (d, J ) 8 Hz, 2H), 8.32-8.48 (m, 9H), 8.83 (d, J ) 5

Hz, 2H), 8.90 (d, J ) 5 Hz, 2H), 8.95 (d, J ) 5 Hz, 2H), 8.98 (d, J )

5 Hz, 2H), 9.02-9.06 (m, 8H); MALDI-TOFMS (positive mode) m/z

2158 (M + H⁺).

11.⁷⁴To a solution of 10 (50.3 mg, 23.3 µmol) in THF (15 mL) was

added 0.070 mL of tetrabutylammonium fluoride (1 M in THF), and

then the mixture was stirred for 3.5 h at room temperature under

nitrogen atmosphere. The solvents were removed under reduced

pressure. Flash column chromatography on silica gel (toluene/ethyl

acetate ) 9/1) and subsequent reprecipitation from chloroform-

acetonitrile afforded 11 as a deep reddish purple powder (38.5 mg,

18.8 µmol, 81% yield): mp > 300 °C; IR (KBr) 2962, 1682, 1592,

7.^44bA solution of zinc acetate dihydrate (5.00 g, 22.8 mmol) in

methanol (100 mL) was added to a solution of 5-(4-aminophenyl)-

10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin^44b(501.6 mg, 519.0

µmol) in chloroform (400 mL) and refluxed for 1 h. After cooling, the

reaction mixture was washed with saturated sodium bicarbonate aqueous

solution and water successively and dried over anhydrous sodium

sulfate, and then the solvent was evaporated. Flash column chroma-

tography on silica gel with toluene as the eluent and subsequent

reprecipitation from benzene-methanol afforded 7 as a deep red purple

powder (506.4 mg, 491.8 µmol, 95% yield): mp > 300 °C; IR (KBr)

3385, 2961, 1592, 1476, 1426, 1393, 1362, 1339, 1286, 1247, 1219,

1515, 1494, 1477, 1363, 1314, 1248, 1000, 973, 799, 757, 717 cm^-1

;

¹H NMR (400 MHz, CDCl₃) δ -2.68 (br s, 2H), 1.53 (s, 36H), 1.55

(s, 54H), 4.68 (d, J ) 5 Hz, 2H), 7.41 (d, J ) 8 Hz, 2H), 7.73 (d, J )

8 Hz, 2H), 7.79 (s, 1H), 7.81 (s, 2H), 7.84 (s, 2H), 8.08 (s, 1H), 8.11-

8.13 (m, 10H), 8.17 (d, J ) 8 Hz, 2H), 8.24 (d, J ) 8 Hz, 2H), 8.33-

8.35 (m, 4H), 8.38 (d, J ) 8 Hz, 2H), 8.43 (d, J ) 8 Hz, 2H), 8.47 (s,

1H), 8.82 (d, J ) 5 Hz, 2H), 8.89 (d, J ) 5 Hz, 2H), 8.95 (d, J ) 5

Hz, 2H), 8.98 (d, J ) 5 Hz, 2H), 9.03 (s, 4H), 9.06 (d, J ) 5 Hz, 2H),

9.07 (d, J ) 5 Hz, 2H); MALDI-TOFMS (positive mode) m/z 2044

(M + H⁺).

1

1179, 1000, 929, 899, 882, 823, 798, 757, 718 cm^-1; H NMR (270

MHz, CDCl₃plus a drop of pyridine-d₅) δ 1.51 (s, 18H), 1.51 (s, 36H),

7.02 (d, J ) 8 Hz, 2H), 7.75 (t, J ) 2 Hz, 3H), 7.97 (d, J ) 8 Hz, 2H),

8.05 (d, J ) 2 Hz, 6H), 8.88-8.90 (m, 6H), 8.94 (d, J ) 5 Hz, 2H);

FABMS m/z 1028 (M + H⁺).

12. To a solution of 11 (38.4 mg, 18.8 µmol) in chloroform (20

mL), was added at one portion activated manganese dioxide (384.2

mg, 4.42 mmol), and then the suspended mixture was stirred for 2 h at

room temperature under nitrogen atmosphere. The catalyst was removed

by filtration, and the filtrate was concentrated under reduced pressure.

Flash column chromatography on silica gel (toluene/ethyl acetate )

50/1) and subsequent reprecipitation from chloroform-acetonitrile

afforded 12 as a deep reddish purple powder (29.9 mg, 14.6 µmol,

78% yield): mp > 300 °C; IR (KBr) 2961, 1694, 1591, 1515, 1495,

8.⁷⁴To a solution of 4-nitrobenzyl alcohol (5.65 g, 36.9 mmol) in

DMF (15.0 mL), were added tert-butylchlorodimethylsilane (8.84 g,

58.7 mmol) and imidazole (8.29 g, 122 mmol), and then the reaction

mixture was stirred for 2.5 h at room temperature under nitrogen

atmosphere. The solvent was removed under reduced pressure. Flash

column chromatography on silica gel (hexane/toluene ) 25/75/0.3)

afforded 8 as a colorless solid (9.87 g, 36.9 mmol, 100%): mp 30.5-

32.5 °C; IR (KBr) 2929, 2859, 1601, 1520, 1494, 1471, 1416, 1338,

1288, 1264, 1202, 1099, 1011, 940, 840, 779, 734, 673, 640 cm^-1; ¹H

NMR (270 MHz, CDCl₃) δ 0.13 (s, 6H), 0.96 (s, 9H), 4.83 (s, 2H),

7.48 (d, J ) 8 Hz, 2H), 8.19 (d, J ) 8 Hz, 2H); EIMS (rel intensity)

m/z 267 (M⁺, 70), 211 ([M - C₄H₈]⁺, 100); Found: C, 58.64; H, 7.92;

N, 5.05%. Calcd for C₁₃H₂₁NO₃Si: C, 58.39; H, 7.92; N, 5.24%.

9. A solution of 8 (3.32 g, 12.4 mmol) in THF (100 mL) was

hydrogenated over 5% palladium on charcoal (584.9 mg) at room

temperature and under atmospheric pressure until TLC analysis

indicated complete reduction of 8. The catalyst was removed by

filtration, and the filtrate was concentrated under reduced pressure to

give 9 as colorless oil (2.94 g, 12.4 mmol, 100%): IR (neat) 3449,

3362, 2955, 2929, 2857, 1625, 1519, 1471, 1376, 1257, 1213, 1174,

1

1477, 1362, 1312, 1247, 1220, 1000, 973, 798, 717 cm^-1; H NMR

(270 MHz, CDCl₃) δ -2.68 (br s, 2H), 1.53 (s, 36H), 1.55 (s, 54H),

7.79 (s, 1H), 7.81 (s, 2H), 7.84 (s, 2H), 7.93 (br s, 4H), 8.10-8.16 (m,

12H), 8.22-8.43 (m, 12H), 8.81 (d, J ) 5 Hz, 2H), 8.88 (d, J ) 5 Hz,

2H), 8.95 (d, J ) 5 Hz, 2H), 8.97 (d, J ) 5 Hz, 2H), 9.02 (s, 4H),

9.04-9.06 (m, 4H), 9.98 (s, 1H); MALDI-TOFMS (positive mode)

m/z 2042 (M + H⁺).

ZnP-H₂P-C₆₀.⁷⁵A solution of 12 (40.7 mg, 19.9 µmol), C₆₀(67.5

mg, 93.7 µmol), and N-methylglycine (83.3 mg, 935 mmol) in toluene

(75 mL) was refluxed under nitrogen atmosphere until TLC analysis

showed no detection of 12. The reaction mixture was poured on the

top of a flash chromatography column (silica gel) which was washed

with toluene as an eluate to remove unreacted C₆₀. Then the column

was washed with toluene/ethyl acetate (100/1), eluting the desired

product. Reprecipitation from benzene-acetonitrile afforded ZnP-H₂P-

1

1072, 1006, 837, 777, 666 cm^-1; H NMR (270 MHz, CDCl₃) δ 0.07

C₆₀as a dark grayish purple powder (43.8 mg, 15.7 µmol, 79% yield):

(s, 6H), 0.92 (s, 9H), 3.60 (br s, 2H), 4.62 (s, 2H), 6.65 (d, J ) 8 Hz,

2H), 7.10 (d, J ) 8 Hz, 2H); High resolution (HR) EIMS calcd for

[C₁₃H₂₃NOSi]⁺: 237.1549, found: 237.1544.

mp > 300 °C; IR (KBr) 2953, 1682, 1591, 1515, 1495, 1471, 1361,

1312, 1247, 1220, 1000, 973, 798, 716, 527 cm^-1; ¹H NMR (270 MHz,

CDCl₃) ꢀ -2.84 (s, 2H), 1.53 (s, 90H), 2.67 (br s, 3H), 3.67 (br s, 1H),

4.31 (br s, 1H), 4.50 (br s, 1H), 7.61 (br s, 2H), 7.79 (s, 3H), 7.81 (s,

2H), 7.96 (d, J ) 8 Hz, 2H), 8.04 (s, 4H), 8.09 (s, 6H), 8.20-8.31 (m,

8H), 8.42 (d, J ) 8 Hz, 2H), 8.48 (d, J ) 8 Hz, 2H), 8.70 (br s, 2H),

8.77 (d, J ) 4 Hz, 2H), 8.88-8.90 (m, 6H), 9.01-9.03 (m, 8H);

MALDI-TOFMS (positive mode) m/z 2794 (M + H⁺), (negative mode)

m/z 721 (C₆₀^-); UV-vis λ_max(log ꢀ) (THF) 350 (sh, 4.73), 361 (sh,

4.71), 404 (sh, 5.16), 420 (sh, 5.81), 427 (6.01), 485 (3.90), 516 (4.43),

555 (4.55), 597 (4.26), 649 (3.78), 704 (2.7) nm.

10.⁴⁶A solution of 4 (200.7 mg, 216.5 µmol), thionyl chloride (0.310

mL, 4.25 mmol), and pyridine (0.30 mL) in benzene (30.0 mL) was

refluxed for 2 h under nitrogen atmosphere. The excess reagent and

solvents were removed under reduced pressure, and the residue was

redissolved in a mixture of benzene (30 mL) and pyridine (1.0 mL).

To the reaction mixture were added 7 (222.1 mg, 215.7 µmol) and a

solution of 9 (50.5 mg, 212.7 µmol) in benzene (1.0 mL). The solution

was allowed to stir overnight at room temperature under nitrogen

atmosphere. TLC showed three products (R_f) 0.80, 0.43, and 0.15,

chloroform) and the second band was separated by flash column

chromatography on silica gel (toluene/triethylamine ) 100/0.5, then

ZnP-H₂P. The dyad was synthesized from 4 (86.9 mg, 93.7 µmol),

7 (96.5 mg, 93.7 µmol), and 4-hexadecylaniline (30.2 mg, 95.1 µmol)

(75) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,

(74) Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190.

9798.

Formation of a Long-LiVed Charge-Separated State

J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6551

by the same method as that described for 10. ZnP-H₂P was crystallized

as a deep reddish purple powder from benzene-acetonitrile (28.5 mg,

12.7 µmol, 14% yield): mp > 300 °C; IR (KBr) 2962, 2926, 2855,

1683, 1592, 1515, 1495, 1476, 1394, 1362, 1313, 1247, 1220, 1000,

973, 798, 717 cm^-1; ¹H NMR (270 MHz, CDCl₃) δ -2.69 (br s, 2H),

0.89 (t, J ) 7 Hz, 3H), 1.27-1.80 (m, 118H), 2.65 (t, J ) 7 Hz, 2H),

7.27 (d, J ) 8 Hz, 2H), 7.68 (d, J ) 8 Hz, 2H), 7.81-7.83 (m, 5H),

8.08 (s, 1H), 8.12 (s, 10H), 8.18 (d, J ) 8 Hz, 2H), 8.25 (d, J ) 8 Hz,

2H), 8.33-8.49 (m, 9H), 8.84 (d, J ) 5 Hz, 2H), 8.90 (d, J ) 5 Hz,

2H), 8.96 (d, J ) 5 Hz, 2H), 8.98 (d, J ) 5 Hz, 2H), 9.02-9.06 (m,

8H); MALDI-TOFMS (positive mode) m/z 2244 (M + H⁺); UV-vis

λ_max(log ꢀ) (THF) 356 (sh, 4.51), 368 (4.54), 404 (sh, 5.13), 420 (sh,

5.80), 427 (5.99), 486 (3.76), 516 (4.40), 556 (4.52), 597 (4.23), 649

(3.74) nm.

2H), 8.32 (d, J ) 8 Hz, 2H), 8.76 (d, J ) 5 Hz, 2H), 8.86 (s, 6H), 8.90

(br s, 1H); FABMS m/z 1847 (M + H⁺), 720 (C₆₀⁺); UV-vis λ_max

(log ꢀ) (THF) 366 (4.65), 405 (sh, 5.03), 420 (5.75), 484 (3.78), 516

(4.33), 550 (4.09), 593 (3.82), 649 (3.76) nm.

ZnP-C₆₀.^44g,hA saturated solution of zinc acetate dihydrate in

methanol (3.5 mL) was added to a solution of H₂P-C₆₀(70 mg, 38

µmol) in chloroform (35 mL) and refluxed for 30 min. After cooling,

the reaction mixture was washed with saturated sodium bicarbonate

aqueous solution and water successively, dried over anhydrous sodium

sulfate, and then the solvent was removed under reduced pressure. Flash

column chromatography on silica gel with chloroform as the eluent

and subsequent reprecipitation from chloroform-methanol afforded

ZnP-C₆₀as a dark grayish brown powder (60.8 mg, 0.0319 mmol,

84% yield): mp > 300 °C; IR (KBr) 2962, 1687, 1592, 1515, 1362,

1314, 1247, 1001, 823, 797, 717, 527 cm^-1; ¹H NMR (CDCl₃) δ 1.51

(s, 54H), 2.60 (s, 3H), 3.44 (br s, 1H), 3.91 (br s, 1H), 4.34 (br s, 1H),

7.49 (br s, 2H), 7.77 (m, 3H), 7.97 (d, J ) 8 Hz, 2H), 8.03 (m, 6H),

8.18 (d, J ) 8 Hz, 2H), 8.32 (d, J ) 8 Hz, 2H), 8.86 (d, J ) 5 Hz,

2H), 8.93 (s, 4H), 8.96 (d, J ) 5 Hz, 2H), 8.97 (br s, 1H); FABMS

m/z 1910 (M + H⁺), 721 (C₆₀⁺); UV-vis λ_max(log ꢀ) (THF) 405 (4.80),

426 (5.88), 490 (3.52), 519 (3.70), 557 (4.41), 597 (4.05), 637 (sh,

2.8) nm.

15. A solution of 5 (500 mg, 0.519 mmol), thionyl chloride (0.38

mL, 5.2 mmol), and pyridine (1 mL) in benzene (100 mL) was refluxed

for 1 h under nitrogen atmosphere. The excess reagent and solvents

were removed under reduced pressure, and the residue was redissolved

in a mixture of benzene (20 mL) and pyridine (1 mL). To the reaction

mixture was added 13⁷⁶(215 mg, 1.04 mmol). The solution was stirred

for 18 h at room temperature under nitrogen atmosphere, and then the

solvent was evaporated. Flash column chromatography on silica gel

with benzene as the eluent gave a mixture of 14 and 15. The mixture

was redissolved in a mixture of chloroform (50 mL), trifluoroacetic

acid (20 mL), and 5% sulfuric acid (15 mL). After stirring overnight

at room temperature, the mixture was poured onto 100 mL of water

and extracted with chloroform. The organic extract was washed with

saturated sodium bicarbonate aqueous solution and water successively,

dried over anhydrous sodium sulfate, and evaporated. Flash column

chromatography on silica gel with chloroform as the eluent and

subsequent reprecipitation from benzene-methanol afforded 15 as a

deep purple powder (270 mg, 0.253 mmol, 49% yield): mp > 300 °C;

IR (KBr) 3318, 2963, 2719, 1695, 1592, 1515, 1475, 1411, 1363, 1305,

ZnP.^44cZnP was synthesized from H₂P^44b(43.0 mg, 42.6 µmol) by

the same method as that described for ZnP-C₆₀. ZnP was crystallized

as a deep purple microcrystalline solid from benzene-acetonitrile (36.3

mg, 33.9 µmol, 80% yield): mp > 300 °C; IR (KBr) 3318 (br), 2962,

1674, 1593, 1515, 1394, 1362, 1288, 1247, 1220, 1181, 1069, 1000,

1

929, 823, 798, 718 cm^-1; H NMR (CDCl₃) δ 1.52 (s, 54H), 2.32 (s,

3H), 7.47 (s, 1H), 7.78 (m, 3H), 7.85 (d, J ) 8 Hz, 2H), 8.08 (m, 6H),

8.18 (d, J ) 8 Hz, 2H), 8.95 (d, J ) 4 Hz, 2H), 8.99 (d, J ) 4 Hz,

2H), 9.00 (s, 4H); FABMS m/z 1071 (M + H⁺); UV-vis λ_max(log ꢀ)

(THF) 360 (sh, 4.03), 385 (sh, 3.90), 405 (4.69), 426 (5.83), 494 (sh,

3.17), 519 (3.55), 557 (4.36), 597 (4.02) nm.

1

1247, 1164, 972, 915, 801, 732 cm^-1; H NMR (270 MHz, CDCl₃) δ

C₆₀-Ar. C₆₀-Ar was prepared from C₆₀, 3,5-di-tert-butylbenz-

aldehyde, and N-methylglycine following the same procedures as

described previously.^44f

-2.70 (br s, 2H), 1.55 (s, 54H), 7.79 (t, J ) 2 Hz, 3H), 8.01 (s, 4H),

8.07 (d, J ) 2 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.28 (d, J ) 7 Hz,

2H), 8.33 (br s, 1H), 8.41 (d, J ) 7 Hz, 2H), 8.79 (d, J ) 5 Hz, 2H),

8.91 (s, 4H), 8.92 (d, J ) 5 Hz, 2H), 10.01 (s, 1H); FABMS m/z 1098

(M + H⁺).

Acknowledgment. Part of this work was supported by the

Office of Basic Energy Sciences of the Department of Energy.

This is document NDRL-4175 from the Notre Dame Radiation

Laboratory). Part of this work was supported by Grants-in-Aid

for COE Research and Scientific Research on Priority Areas of

Electrochemistry of Ordered Interfaces (No. 11118247 to H.I.)

and Priority Area of Creation of Delocalized Conjugated

Electronic Systems (No. 10146103 to Y.S.) from Ministry of

Education, Science, Sports and Culture, Japan. Y.S. and H.I

thank the Mitsubishi and Sumitomo Foundation for financial

support.

H₂P-C₆₀. The dyad was synthesized from 15 (128 mg, 0.12 mmol),

₆₀(259 mg, 0.36 mmol), and N-methylglycine (320 mg, 3.60 mmol)

C

by the same method as that described for ZnP-H₂P-C₆₀. H₂P-C₆₀

was isolated as a brownish black powder from chloroform-methanol

(155.7 mg, 0.0844 mmol, 70% yield): mp > 300 °C; IR (KBr) 2962,

1686, 1593, 1515, 1476, 1423, 1363, 1313, 1247, 973, 914, 800, 729,

1

527 cm^-1; H NMR (270 MHz, CDCl₃) δ -2.82 (br s, 2H), 1.52 (s,

54H), 2.63 (s, 3H), 3.58 (d, J ) 10 Hz, 1H), 4.13 (s, 1H), 4.46 (d, J

) 10 Hz, 1H), 7.55 (br s, 2H), 7.78 (s, 3H), 7.98 (d, J ) 8 Hz, 2H),

8.03 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 8.18 (d, J ) 8 Hz,

(76) Imahori, H.; Azuma, T.; Ajavakom, A.; Norieda, H.; Yamada, H.;

Sakata, Y. J. Phys. Chem. 1999, 103, 7233.

JA993959Z

Article Doi

Sequential energy and electron transfer in an artificial reaction center: Formation of a long-lived charge-separated state

DOI: 10.1021/ja993959z

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ja993959z

Full text of DOI:10.1021/ja993959z

Products guided by the article

R&D Labs maybe for 1099596-62-9

Relevant to this article

Hot Product