C O M M U N I C A T I O N S
Figure 2. Fluorescence decay spectra of 2 at 650 nm upon 410 nm
excitation in benzonitrile with no additive (black line) and with 20-fold
sodium perchlorate (red line).
Figure 4. Transient absorption spectra of 2 in Ar-saturated benzonitrile
with 20-fold sodium perchlorate under 532 nm laser photolysis.
to the triplet porphyrin. Obviously, the transient absorption spectra
support marked suppression of the intramolecular electron transfer
of 2 by complexation.
In conclusion, we have demonstrated that in the porphyrin-sexi-
thiophene-fullerene triad 2 photoinduced intramolecular electron
transfer can be completely controlled by complexation/decomplex-
ation of a sodium cation in the crown ether ring. Various types of
on/off switchable molecular architectures have been actively
studied,9 and most of the molecular systems cleverly take advantage
of photochemically controlled electron transfer.10 The present triad
2 represents a novel switchable system responding to a metal ion.
In this system, it is notable that the polyether-bridged sexithiophene
can indeed function as a complexation-gated molecular wire.
Figure 3. Transient absorption spectra of 2 in Ar-saturated benzonitrile
under 532 nm laser photolysis.
This means that a fast electron transfer occurs from the singlet
excited state of the porphyrin to the fullerene, through the
oligothiophene.2 However, the fluorescence of 2 is enhanced by
addition of excess sodium perchlorate. Evidently, complexation with
a sodium cation depresses the intramolecular electron transfer.
Furthermore, addition of the same amount of 15-crown-5 restores
the original fluorescence spectrum of 2. These two-way fluorescence
changes could be repeated at least several times.
Figure 2 shows the fluorescence time profiles of 2. Simulation
analysis of the black line observed for no additive indicated that
the photophysical decay mechanism of the porphyrin consists of
two paths with fluorescence lifetimes τf ) 105 (89%) and 1080 ps
(11%). The predominant fast decay path is assigned to intramo-
lecular electron transfer from the porphyrin chromophore to the
fullerene. In the presence of 20-fold sodium perchlorate, the time
profile changed to the red line, with only a slow single decay path
with τf ) 2580 ps. Evidently, the fast intramolecular electron
transfer is completely suppressed by complexation.
Acknowledgment. This research was supported by a Grant-in-
Aid of Scientific Research (No. 17205006) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Experimental procedures and
characterization data of all new compounds and Figure S1. This material
References
(1) (a) Otsubo, T.; Aso, Y.; Takimiya, K. Bull. Chem. Soc. Jpn. 2001, 74,
1789-1801. (b) Otsubo, T.; Aso, Y.; Takimiya, K. J. Mater. Chem. 2002,
12, 2565-2575.
(2) (a) Ikemoto, J.; Takimiya, K.; Aso, Y.; Otsubo, T.; Fujitsuka, M.; Ito, O.
Org. Lett. 2002, 4, 309-311. (b) Nakamura, T.; Fujitsuka, M.; Araki, Y.;
Ito, O.; Ikemoto, J.; Takimiya, K.; Aso, Y.; Otsubo, T. J. Phys. Chem. B
2004, 108, 10700-10710.
(3) Xiao, X.; Nagahara, L. A.; Rawlett, A. M.; Tao, N. J. Am. Chem. Soc.
2005, 127, 9235-9240.
(4) (a) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1993, 115, 12214-
12214. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV.
2000, 100, 2537-2574.
(5) (a) Sessler, J. L.; Johnson, M. R.; Lin, T.-Y.; Creager, S. E. J. Am. Chem.
Soc. 1988, 110, 3659-3661. (b) Osuka, A.; Maruyama, K.; Mataga, N.;
Asahi, T.; Yamazaki, I.; Tamai, N. J. Am. Chem. Soc. 1990, 112, 4958-
4959. (c) Helm, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1991,
113, 4325-4327.
(6) Pedersen, C. J.; Frensdorff, H. K. Angew. Chem., Int. Ed. Engl. 1972, 11,
16-25.
(7) Luo, C.; Fujitsuka, M.; Watanabe, A.; Ito, O.; Gan, L.; Huang, Y.; Huang,
C.-H. J. Chem. Soc., Faraday Trans. 1998, 94, 527-532.
(8) Matsumoto, K.; Fujitsuka, M.; Sato, T.; Onodera, S.; Ito, O. J. Phys. Chem.
B 2000, 104, 11632-11638.
(9) (a) Lukas, A. S.; Wasielewski, M. R. In Molecular Switches; Feringer,
B. L., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Chapter 1, pp 1-35.
(b) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines;
Wiley-VCH: Weinheim, Germany, 2003.
(10) (a) Tsuchiya, S. J. Am. Chem. Soc. 1999, 121, 48-53. (b) Endtner, J. M.;
Effenberger, F.; Hartschuh, A.; Port, H. J. Am. Chem. Soc. 2000, 122,
3037-3046. (c) Liddell, P. A.; Kodis, G.; Moore, A. L.; Moore, T. A.;
Gust, D. J. Am. Chem. Soc. 2002, 124, 7668-7669. (d) Liddell, P. A.;
Kodis, G.; Andre´asson, J.; de la Garza, L.; Bandyopadhyay, S.; Mitchell,
R. H.; Moore, T. A.; Moore, A. L.; Gust, D. J. Am. Chem. Soc. 2004,
126, 4803-4811. (e) Terazono, Y.; Kodis, D.; Andre´asson, J.; Brune,
A.; Hartmann, T.; Du¨rr, H.; R. H.; Moore, T. A.; Moore, A. L.; Gust, D.
J. Phys. Chem. B 2004, 108, 1812-1814. (f) Straight, S. D.; Andre´asson,
J.; Kodis, G.; Bandyopadhyay, S.; Mitchell, R. H.; Moore, T. A.; Moore,
A. L.; Gust, D. J. Am. Chem. Soc. 2005, 127, 9403-9409.
The nanosecond transient absorption spectra of 2 give definitive
information on the decay mechanism. Figure 3 shows the absorption
spectra of 2 at 0.1 and 1.0 µs, after the porphyrin moiety was
selectively excited by 532 nm laser photolysis. In the 0.1 µs spec-
trum, a characteristic band due to C60•- is observed at around 1040
nm.7 In addition, the two absorption bands are observed at around
1250 and 800 nm, assignable to 6T•+.8 In contrast to the 1250 nm
band, the 800 nm band remains after 1.0 µs, so the 800 nm band
must be overlapped by other bands presumably due to triplet por-
phyrin and fullerene species. This result is reasonably explained by
the photophysical decay mechanism of 1 speculated in our previous
paper:2b 1Po*-6T-C60 first undergoes an intramolecular electron
transfer to Po•+-6T-C60•-, which is followed by a fast hole shift
to the energetically more stable charge separated state Po-6T•+-
C60•-, and then deactivation occurs via charge recombination.
As shown in Figure 4, the transient absorption spectra of 2 in
the presence of 20-fold sodium perchlorate are completely different
from that obtained without sodium perchlorate. The specific bands
•-
due to both C60 and 6T•+ are no longer observed, and only a
strong band due to the triplet porphyrin species is observed at 780
nm. The singlet excited porphyrin must take a simple decay path
JA055648W
9
J. AM. CHEM. SOC. VOL. 127, NO. 44, 2005 15373