Photoinduced intrarotaxane electron transfer between zinc porphyrin and [60]fullerene in benzonitrile

Article abstract of DOI:10.1002/anie.200390188

A through-space superexchange mechanism appears to be responsible for charge recombination (CR) following photoexcitation and charge separation (CS) in rotaxanes containing zinc porphyrin and C₆₀ moieties (see picture), as suggested by the low activation energy of this process. Furthermore, the charge-separated state is relatively long-lived.

Full text of DOI:10.1002/anie.200390188

Angewandte
Chemie
Electron Transfer in Rotaxanes
benzene to give rotaxane 6 in 69% yield. Finally, 6 was
converted into zinc porphyrin rotaxane 7 in 82% yield by
treatment with zinc acetate. The rotaxanes were characterized
by satisfactory NMR and FAB-MS spectra corresponding to
their structures. To our knowledge, these are the first rotaxane
porphyrin C60 conjugates, although such a pseudorotaxane
has been reported.[12] On the other hand, some fullerene-
containing catenanes have also been reported.[13]
Photoinduced Intrarotaxane Electron Transfer
between Zinc Porphyrin and [60]Fullerene in
Benzonitrile**
Nobuhiro Watanabe, Nobuhiro Kihara, Yoshio Furusho,
Toshikazu Takata,* Yasuyuki Araki, and Osamu Ito*
The steady-state absorption spectrum of 7 is shown in
Figure 1; the absorption bands at 420 and 500 600 nm are
In the photosynthetic center of bacteria, charge separation
proceeds by a series of electron-transfer steps between
porphyrin(P)-like components and quinones that are spatially
oriented in proteins. Numerous synthetic models for cova-
lently connected donor acceptor systems have been pro-
4]
posed,[1 based on coordinative bonds between donor and
acceptor.[5] Recently, photoinduced electron transfer between
donors and acceptors in rotaxanes was reported.[6,7] Rotax-
anes containing ZnP (donor) and AuP (acceptor) show
photoinduced superexchange forward electron transfer be-
tween noncovalently linked chromophores.[8] Imahori et al.
reported that a ZnP C60 linked dyad system shows a relatively
long lifetime of the charge-separated state, in addition to high
charge-separation efficiency.[3] We envisaged that the combi-
nation of a ZnP (donor), C60 (acceptor), and a rotaxane
skeleton could mimic the photosynthesis reaction center.
Herein we report on the ZnP/C60 rotaxane 7, designed as shown
in Scheme 1, in which through-space forward and backward
electron transfer processes between the ZnP and C60 moieties
and a relatively long charge-separation lifetime are expected.
Rotaxane 4 was synthesized by the hydrogen-bond-
assisted method for sec-amide-based rotaxanes.[9,10] Thus,
the condensation of isophthaloyl dichloride (2) and 5,10,15-
tris(3,5-di-tert-butylphenyl)-20-(4-aminophenyl)porphyrin
(3)[11] was carried out in chloroform in the presence of
macrolactam 1[9] at 08C to afford 4 in 17% yield. The
conversion of 4 into the corresponding zinc porphyrin
rotaxane 5 was achieved by treatment with zinc acetate in
refluxing chloroform. The Diels Alder reaction of 4 and five
equivalents of C60 was carried out in refluxing 1,2-dichloro-
Figure 1. Steady-state absorption spectrum of 7 (1î10À6 m) in PhCN.
Inset: steady-state absorption spectrum (solid line) and fluorescence
spectrum (dashed line) of 7 (1î10À5 m).
mainly the Soret and Q bands, respectively, of ZnP moiety 5.
The absorption bands of the C60 moiety appear at 700 nm and
below 350 nm. The absorption spectrum of 7 is almost a
superimposition of those of 5 and C60. Figure 1 also shows the
fluorescence spectrum of 7 above 600 nm; the fluorescence
peaks at 620 and 670 nm are attributed to the ZnP moiety 5,
while the fluorescence peak of C60, which is expected to
appear at 725 nm, may be hidden in the fluorescence bands of
the ZnP moieties. The fluorescence intensity of 7 was quite
weak compared to that of 5 in benzonitrile. Furthermore, an
enhancement in the fluorescence of the C60 moiety at 725 nm
was not observed, and this suggests that energy transfer from
the excited singlet state of the ZnP moieties to the C60 moiety
may not have taken place. These observations indicate that
electron transfer predominantly takes place from the excited
singlet state of the ZnP moieties to the C60 moiety.
[*] Prof. Dr. T. Takata, N. Watanabe, Dr. N. Kihara, Dr. Y. Furusho
Department of Applied Chemistry
Graduate School of Engineering
Osaka Prefecture University
The time profile of the fluorescence of rotaxane 7 shows
biexponential decay in benzonitrile (Figure 2); the lifetimes
of the ZnP moieties in 7 [tf1 = 93 ps (80%) and tf2 = 1560 ps
(20%)] were evaluated by the curve-fitting method. The tf1
value is small relative to that of 5 [tfr = 1790 ps (100%)], while
the tf2 value remained similar to that of tfr. From these
lifetimes, the quantum yield FCS and rate constant kCS for
charge separation were calculated to be 0.95 and 1.0 î 1010 sÀ1,
respectively, by using the ordinal equations [FCS = 1/tfr/
(1/tfrÀ1/tf1) and kCS = (1/tfrÀ1/tf1).[3]
1-1 Gakuen-cho, Sakai-shi, Osaka 599-8531 (Japan)
Fax: (+81)72-254-9910
E-mail: takata@chem.osakafu-u.ac.jp
Prof. Dr. O. Ito, Dr. Y. Araki
Institute of Multidisciplinary Research for Advanced Materials
Tohoku University, CREST (JST)
Katahira, Aoba-ku, Sendai, Miyagi 980-8577 (Japan)
Fax: (+81)22-217-5608
E-mail: ito@tagen.tohoku.ac.jp
[**] This work was partly supported by the Priority-Area-Research by the
Ministry of Education, Science, Technology, Sports and Culture of
Japan and by JSPS Research Fellowships for Young Scientists (N.W.)
which is greatly acknowledged.
Figure 3 shows the nanosecond transient absorption
spectra of rotaxane 7 in benzonitrile, observed by 532 nm
laser excitation (6 ns laser pulse). The transient absorption
band at 1000 nm observed immediately after laser pulse (6 ns)
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
Scheme 1. Synthesis of 4 7.
was assigned to the anion radical of the C60 moiety; the
rotaxane 7, which have longer lifetimes than the charge-
separated state.
absorption bands in the visible region, centered around
700 nm and 850 nm, were attributed to the triplet states of the
C60 and ZnP moieties, respectively, with which the cation
radical of the ZnP moieties may overlap.[3] These observations
confirm that charge separation takes place immediately after
laser exposure. The transient absorptions at 1000 ns were
mostly attributed to the triplet states of C60 and ZnP in
The inset time profile in Figure 3 shows the decay of the
radical anion of C60 at 1000 nm. The decay obeys first-order
kinetics with a rate constant of 5.5 î 106 sÀ1, which is the
charge-recombination rate constant kCR. This value corre-
sponds to a lifetime tRIP of 180 ns for the charge-separated
state (radical ion pair). This tRIP value is significantly larger
Figure 2. Time profiles of the fluorescence of rotaxane 7 (1î10À4 m)
and ZnP moiety 5 (1î10À4 m) in PhCN. Excitation wavelength was
410 nm.
Figure 3. Nanosecond transient absorption spectra of 7 (1î10À4 m)
*
observed by 532 nm laser excitation in PhCN after 100 ( ) and
*
1000 ns ( ). Inset: trace of the C60 radical anion at 1000 nm.
682
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Angewandte
Chemie
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Figure 4. Energy diagram for charge separation (CS) and charge recombi-
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than those reported for covalently connected dyad molecules;
for example, C60 dimethylaniline (< 1 ns),[14] C60 b-carotene
(< 1 ns),[15] and C60-TTF (2 ns).[16] Rotaxane 7 has a tRIP value
similar to that of covalently linked C60 ZnP (770 ns).[3]
The energy diagram for the charge separation and
recombination processes after photoexcitation is shown in
Figure 4. The ion-pair state was evaluated from the first
oxidation potential of the ZnP moiety (0.29 V vs ferrocene/
ferrocene+) and the first reduction potential of the C60 moiety
(À1.1 V vs ferrocene/ferrocene+) of rotaxane 7 in benzoni-
trile. The shortening of fluorescence lifetimes indicates that
charge separation takes place mainly from the excited singlet
state of the ZnP moiety to produce a radical ion pair. The rate
of this charge-separation process was as high as 1010 sÀ1. The
slow recombination rate suggests that this process lies in an
inverted region far more negative than the charge-separation
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[12] M. V. MartÌnez-DÌaz, N. S. Fender, M. S. RodrÌguez Morgade,
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perature dependence; from the semiempirical Marcus equa-
tion,[3] the free-energy of activation DGC°R was evaluated to be
57 meV, which is somewhat smaller than for conventional
dyad systems; for example, DG°CR of retinyl C60 is 160 meV.[17]
In general, a rather small DG°CR is expected for through-space
electron transfer, while a considerably higher DG°CR is
anticipated for through-bond electron transfer. Thus, the
small DG°CR observed for rotaxane 7 suggests that charge
recombination takes place by through-space electron transfer
with a superexchange mechanism.[7] The DGC°R value of
57 meV for 7 is slightly larger than that expected for the
complete through-space electron transfer system; hence, this
slight increase in DG°CR of 7 can be attributed to the flexibility
of the relative configuration of the ZnP and C60 moieties. The
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the retinyl C60 dyad (V= 0.34 cmÀ1 in PhCN).[17] This differ-
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distance smaller in 7 (center-to-center distance rcc ꢀ 10 ä)
than in retinyl C60 dyad (rcc = 11.4 ä), but also that 7 has a
more suitable orientation for electron transfer. Further
studies on electron transfer features are in progress.
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[17] M. Yamazaki, Y. Araki, M. Fujitsuka, O. Ito, J. Phys. Chem. A
2001, 105, 8615.
Received: August 5, 2002 [Z19881]
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