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electron to the quinone (Q) resulting a charge-separated state consisting
of a cationic radical P'+ and an anionic Q'~ . The lifetime of P'+ - Q'~
is usually very short because of the back-reac-tion. The electron rapidly
returns to the porphyrin molecule, losing energy as heat, and the ground
state of the molecule is restored.
Kong and Loach described the first reaction-center mimic to have a
covalently bound porphyrin-quinone linkage.7874 Shortly thereafter,
Tabushi and coworkers designed a porphyrin-quinone molecule in which the
quinone is bound to the porphyrin via an amide linkage.80 These early
systems outlined the basic conditions on which later systems dramatically
improved. An extensive collection of porphyrin-quinone model compounds
have been generated for the purpose of investigating long-lived charge-
separated states. This overview will not be comprehensive,
Figure 39. Gust and Moore's triad (top) and pentad (bottom) and
Wasielewski's triad (middle). Reprinted with permission from Moore, T.
A.; Gust, D.; Mathis, P.; Mialocq, J. C.; Chachaty, C.; Bensasson, R. V.;
Land, E. J.; Doisi, D.; Liddell, P. A.; Lehman, W. R.; Nemeth, G. A.;
Moore, A. L. Nature 1984, 307, 630.
Chou et al.
but instead will discuss a few illustrative examples. Interested readers
are encouraged to consult an excellent review by Wasielewski on this
Substantial success has been made in using these complexes for
biomimetic studies. In the 1980s, Gust and Moore showed that the back
reaction could be slowed substantially using a "triad" molecule composed
of a tetraphenylporphyrin covalently linked to both a carotenoid and a
quinone.82 ~ 86 The slowing results from the formation of the charge-
separated state in which the two radical ions are separated by a neutral
porphyrin (Figure 39). Carotenoid porphyrin-quinone molecules possess a
long-lived ion-pair lifetime on the microsecond timescale. For
comparison, a typical donor-acceptor excited state lives for only several
hundred picoseconds due to efficient charge recombination. Wasielewski
and coworkers designed an octaethylporphyrin triad to affect a long-lived
charge-separated state also based on the multistep principles found in
carotenoid systems. The donor and acceptor distance was controlled by
triptycene-derived quinone and amine groups. Electron transfer from N,
/V-dimethylaniline (D) to P + was competitive with charge recombination
in the D-P + -Q" state; therefore, a long-lived charge-separated state
was observed. Gust and Moore incorporated the successful multi-step
strategy in a pentad comprised of carotenoid-dipor-phyrin-diquinone
subunits. The pentad adopted an extended
linear shape about 80 A in length. Irradiation at 650 nm produced the
porphyrin first-excited-singlet state, which underwent a series of
complex electron-transfer events. The final charge-separated state D' 1 -
P4 -Q' was long lived and formed with a quantum yield that approached
In the early 1990s, Osuka developed elaborate covalently linked
porphyrin dimers bound to a quinone electron acceptor, tethered via an
aromatic hydrocarbon linkage.87'88 The structural and functional aspects
found in reaction-center chromophores were modeled using stacked
porphyrins that roughly mimic special pair, at least in general structure
(Figure 40). Because Osuka's system featured octa-alkylated porphyrins as
the primary donor, the optically well-defined P+ cation intermediate
absorption at 670 nm provided a suitable marker to monitor transients.
The anthracene-pillared coplanar system had charge-separated-state
lifetimes that survived for only several hundred picoseconds. One
disadvantage to this system is the large separation between the porphyrin
faces in the porphyrin pair relative to that found in the photosynthetic
reaction-center special pair.
Osuka has also designed a system comprised of a zinc porphyrin dimer
(DP), a zinc porphyrin monomer (ZnP), and a pyromellitimide (pm) in place
of the quinone (Figure 41).89 The pyromellitimide anion absorbs at
Figure 40. Osuka's stacked porphyrin models for the special pair.
41 / Porphyrin Materials Chemistry
Figure 41. Osuka's triads, diporphyrin-porphyrin- pyromellitimide.
Figure 42. Molecular triad of diarylporphyrin(P)-carotenoid polyene (C)-
fullerene (C60). Reprinted with permission from Liddell, P. A.;
Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore,
T. A.; Gust, D. /. Am. Chem. Soc. 1997, 7 7 9, 1400.
715 nm, which is spectrally resolved from the porphyrin absorption bands.
Upon photoexcitation in THF at 532 nm, these triads give DP + -ZnP-pm "
ion pairs with lifetime of
2.4 ^s in relatively low quantum yield. By replacing the intermediate Zn
porphyrin with the nonmetalated free base,90 reasonable functional
similarity to the photosynthetic reaction center was obtained. An