Electron transfer through-space or through-bonds? A novel system that permits a direct evaluation [4]

Full text of DOI:10.1021/ja010711c

10744
J. Am. Chem. Soc. 2001, 123, 10744-10745
Electron Transfer Through-Space or
Through-Bonds? A Novel System that Permits a
Direct Evaluation
Takashi Arimura,*,† Seiji Ide,† Yasuhiro Suga,†
Takuya Nishioka,† Shigeo Murata,† M. Tachiya,§
Toshihiko Nagamura,‡ and Hiroshi Inoue‡
Photoreaction Control Research Center, AIST
Tsukuba Central 5, Tsukuba 305-8565, Japan
Shizuoka UniVersity, Johoku
Hamamatsu 432-8011, Japan
ReceiVed March 19, 2001
ReVised Manuscript ReceiVed August 9, 2001
As to the conformational control, the 1,3-alternate conformer
1 was heated in 1,1,2,2-tetrachloroethane at reflux temperature
(140 °C) for 12 h, but isomerization of 1 did not take place
Considerable debate within the electron-transfer modeling
community continues to focus on how specific protein pathways
might, or might not, influence long-range biological electron-
transfer events.1,2 One way this critical issue is being addressed
is through the synthesis and study of simple, covalently linked
model systems.3 In general, these models typically consist of a
photodonor covalently attached to one or more electron acceptors
in a manner whereby the electron-transfer efficiency is finely
tuned by controlling the through-bond distance separating the
chromophores. However, among these models there are few, if
any, capable of a conformation-allowed direct through-space, as
opposed to through-bond, electron-transfer quenching process.4
It is well-known the calix[4]arene moiety exists in four conforma-
tions in solution.5 These distinct conformers provide a unique
opportunity for the examination of two mechanisms for photo-
induced electron transfer, that is, through-bond and through-space.
In this paper, therefore, we wish to report a new calixarene-based
donor-acceptor system, 1,3-alternate conformer 1, in which the
calix[4]arene serves to juxtapose a pyromellitimide acceptor near
the porphyrin photodonor plane. This provides a new calixarene-
based supramolecular system in which through-space donor-to-
acceptor electron transfer is observed upon photoexcitation.
Synthesis of 1,3-alternate-5-formyl-17-nitro-25,27-dipropoxy-
26,28-bis(3,5-di-tert-butylbenzyloxy)calix[4]arene 4 was com-
municated previously.6 Its elaboration into the porphyrin- and
pyromellitimide-substituted calixarene derivative 1 is shown in
Scheme 1. The cone-calix[4]arene derivative 2 and 5-phenyl-15-
(5-(25,27-dihydroxy-26,28-dimethoxy-17-nitrocalix[4]arene))-
2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, 3, were also
prepared in a similar manner.7
1
(confirmed by H NMR). Thus, n-propyl and benzyl groups are
bulky enough to inhibit the oxygen-through-the-annulus rotation.
On the other hand, cone conformer 2 gave two peaks for phenolic
OH groups at 7.48 and 7.80 ppm. This demonstrates that cone
conformer 2 is stabilized by intramolecular hydrogen bonds which
serve to separate donor and acceptor by ∼15 Å.8 Interestingly,
the cone conformation of 2 is essentially unperturbed over the
temperature range of the experiment (-30 to +60 °C).
The fluorescence spectra of 1, 2, and 3 in benzene (5.0 × 10-6
M) exhibit maxima at 585 and 640 nm with excitation at 400
nm. Furthermore, one can directly compare these fluorescence
intensities, because the absorbance of each is similar at this
wavelength. In this instance, the fluorescence intensity of 2 is
95% of that for the control compound 3. Apparently, the pyromel-
litimide group (Im) in 2 is too far removed from the porphyrin,
ca. 15 Å (as judged by CPK models) for the occurrence of any
fluorescence quenching of the porphyrin excited state.9 In contrast,
the fluorescence intensity of 1 is only 3.5% of that for 3. This
means Im in 1 efficiently quenched the fluorescence of ZnP*.10
(3) (a) Joran, A, D.; Leland, B. A.; Geller, G. G.; Hopfield, J. J.; Dervan,
P. B. J. Am. Chem. Soc. 1984, 106, 6090-6092. (b) Kumar, K.; Lin, Z.;
Waldeck, D. H.; Zimmt, M. B. J. Am. Chem. Soc. 1996, 118, 243-244. (c)
Roest, M. R.; Verhoven, J. W.; Schddeboom, W.; Warman, J. M.; Lawson, J.
M.; Paddon-Row, M. N. J. Am. Chem. Soc. 1996, 118, 1762-1768. (d) 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, 223-228. (e) Kuciauskas,
D.; Liddel, P. A.; Hung, S. C.; Lin, S.; Stone, S.; Seely, G. R.; Moore, A. L.;
Moore, T. A.; Gust, D. J. Phys. Chem. B 1997, 101, 429-440. (f) Imahori,
H.; Yamada, K.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2626-2629. (g) 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, 1400-1405. (h) Reek, J. N.; Rowan, A. E.;
de Gelder, R.; Beurskens, P. T.; Crossley, M. J.; de Feyter, S.; de Schryver,
F.; Nolte, R. J. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 361-363. (i) Jolliffe,
K. A.; Bell, T. D. M.; Ghiggino, K. P.; Langford, S. J.; Paddon-Row, M. N.
Angew. Chem., Int. Ed. 1998, 37, 916-919. (j) Han, H.; Zimmt, M. B. J. Am.
Chem. Soc. 1998, 120, 8001-8002.
(4) Bell, T. D. M.; Jolliffe, K. A.; Ghiggino, K. P.; Oliver, A. M.; Shephard,
M. J.; Langford, S. J.; Paddon-Row, M. N. J. Am. Chem. Soc. 2000, 122,
10661-10666 and references therein.
(5) Gutsche, C. D. In Calixarenes, Monographs in Supramolecular
Chemistry; Stoddart, J. F., Ed.; The Royal Society of Chemistry: London,
1989; Vol. 1.
(6) Arimura, T.; Ide.; Nishioka, T.; Sugihara, H.: Murata, S.; Yamato, T.
J. Chem. Res. (s) 2000, 234-236.
(7) Characterization data for all new compounds is included in the
Supporting Information.
* To whom correspondence should be addressed. E-mail: takashi-
arimura@aist.go.jp.
† Photoreaction Control Research Center, AIST.
§ AIST.
‡ Shizuoka University.
(1) (a) Closs, G. L.; Miller, J. R. Science 1988, 240, 440-447. (b) Moser,
C. C.; Keske, J. M.; Warncke, K.; Farid, R. S.; Dutton, P. L. Nature 1992,
355, 796-802. (c) Beratan, D. N.; Onuchic, J. N.; Winkler, J. R.; Gray, H.
B. Science 1992, 258, 1740-1741. (d) Pelletier, H.; Kraut, J. Science 1992,
258, 1748-1755. (e) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. (f)
Gust, D.; Moore, T. A. Acc. Chem. Res. 1993, 26, 198-205. (g) Paddon-
Row, M. N. Acc. Chem. Res. 1994, 27, 18-25.
(2) (a) Harriman, A.; Kubo, Y.; Sessler, J. L. J. Am. Chem. Soc. 1992,
114, 388-390. (b) de Rege, P. J. F.; Williams, S. A.; Therien, M. J. Science
1995, 269, 1409-1413. (c) Hunter, C. A.; Hyde, R. K. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1936-1939. (d) Sessler, J. L.; Wang, B.; Springs, S. L.;
Brown, C. T. Electron and Energy Transfer Reactions in Non-Covalently
Linked, Supramolecular Model Systems. In ComprehensiVe Supramolecular
Chemistry; Murakami, Y., Ed.; Pergamon Press Ltd: UK, 1996; Vol. 4,
Chapter 9, pp 311-335 and references therein. (e) Arimura, T.; Brown, C.
T.; Springs, S. L.; Sessler, J. L. Chem. Commun. 1996, 2293-2294. (f) Deng,
Y.; Roberts, J. A.; Peng, S.-M.; Chang, C. K.; Nocera, D. G. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2124-2127. (g) Sessler, J. L.; Sathiosatham, M.;
Brown, C. T.; Rhodes, T. A.; Wiederrecht, G. J. Am. Chem. Soc. 2001, 123,
3655-3660. (h) Arimura, T.; Ide, S.; Sugihara, H.; Murata, S.; Sessler, J. L.
New J. Chem. 1999, 23, 977-979.
(8) Additional information more relevant to the closest distances between
the donor and the acceptor of 1 and 2 were obtained by molecular modeling
(MM2) in the Supporting Information.
(9) When the pyromellitimide derivative 6 (5.0 × 10-6 M) was added to
a benzene solution of 3 (5.0 × 10-6 M), the fluorescence intensity of 3 was
reduced to 98%. Steady-state fluorescence quenching studies and time-resolved
fluorescence studies for 2 and 3 are provided in the Supporting Information.
(10) The shortest edge-to-edge separation of the chromophores of 1 is ca.
5 Å, whereas the shortest bonded connection between the chromophores of 1
and 2 is 19 bonds or roughly 27 Å (as judged by CPK models).
10.1021/ja010711c CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/04/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 43, 2001 10745
Scheme 1
anion (Im)- were observed. Figure 1 shows the femtosecond time-
resolved transient absorption spectra for 1, 1,3-alternate con-
former, in benzene solution. At 25-ps the spectrum shows
bleaching of the ZnP Q-bands, that is, at 545 and 580 nm, and a
distinct transient absorption band centered at 456 nm, both of
which appear instantaneously within the excitation flash. The
decay of this absorption band, which is due to the Sn r S1
absorption of ZnP,12 is monoexponential. As this absorption band
decays, a sharp absorption band typical of (Im)- and centered at
727 nm appears with a rise time of 14 ps and decays in 230 ps.
The spectrum in the 600-700 nm region contains substantial
absorbance for the (ZnP)+ centered at 660 nm and exhibits a
monoexponential decay. On the basis of previous work,13 we
determined the rate constants for electron-transfer reaction in 1
as: kCS ) 7.3 × 1010 s-1 and: kCR ) 3.1 × 109 s-1 on the basis
of the decay rates from S1 to (ZnP)+-(Im)- and from (ZnP)+-
(Im)- to S0.
As yet, it is not clear whether electron transfer occurs through-
space, through-solvent, or by a combination of the two. We con-
clude that a direct through-space (-solvent), as opposed to through-
bond, process accounts for the observed electron-transfer quench-
ing process in this calixarene based model system. In any event,
the present work serves to show that calixarenes can serve as
useful building blocks in the construction of through-space (-sol-
vent) donor-acceptor electron-transfer systems. Experiments
designed to elucidate the exact electron-transfer mechanism are
in progress.
Figure 1. Femtosecond time-resolved transient absorption spectra of 1
in benzene solution excited at 400 nm. The delay times from the exciting
pulse are indicated.
Acknowledgment. This work was supported by the COE development
program grant from the Science and Technology Agency. We thank Dr.
Christopher T. Brown for his helpful discussions.
Supporting Information Available: Details of the experimental
procedure and characterization data for all reaction products (PDF).
This material is available free of charge via the Internet at http://pubs.
acs.org.
In other words, the fluorescence of 1 is quenched in part by an
intraensemble electron transfer (i.e.. through-space) process and
not via a through-bond mechanism, because the bonds connecting
the porphyrin and the Im are exactly the same for 1 and 2.
Further support for the above conclusion was obtained from
femtosecond time-resolved transient absorption studies (400 nm
laser excitation).11 First, the transient absorption spectrum of the
cone conformer 2 was monitored over the range of 550-800 nm.
However, no absorption peaks for the radical cation (ZnP)+ and
JA010711C
(12) Osuka, A.; Nakajima, S.; Maruyama, K.; Mataga, N.; Asahi, T.;
Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc. 1993,
115, 4577-4589.
(13) Osuka, A.; Nakajima, S.; Maruyama, K.; Mataga, N.; Asahi, T. Chem.
Lett. 1991, 1003-1006.
(11) See Supporting Information for a full description of these experiments.