52
J. Am. Chem. Soc., Vol. 121, No. 1, 1999
Tsuchiya
Attempts were therefore made, using related electron-rich
porphyrin (donor) and electron-deficient porphyrin (acceptor),
to confirm the electron-transfer phenomenon by the fluorescence
quenching arising from intermolecular electron transfer. The
electron-rich porphyrin and electron-deficient porphyrin used
in this study are 5,10,15,20-tetrakis(2′,4′,6′-trimethylphenyl)-
porphyrin (11) and 5,10,15,20-tetrakis(2′,4′,6′-trimethylphenyl)-
cis-isomer produced by photoisomerization is smaller than that
of the trans-isomer, and this fluorescence quenching is attributed
to the intramolecular electron-transfer interaction between the
two porphyrin Zn complexes or the two porphyrins. Addition-
ally, though some decomposition of diporphyrin Zn complex 3
and its free base analogue 4 was observed under Xe lamp
irradiation, the spectroscopic data of these compounds (1 and
2) containing the porphyrin ring with eight fluorine atoms at
2
,3,7,8,12,13,17,18-octafluoroporphyrin (15). Since 11 is more
7-10,29
electron rich compared with diporphyrin 2, porphyrin 11 as the
electron donor is expected to afford more distinct evidence about
the electron transfer. In fact, when the acetonitrile solution (1
the â-position
did not give any evidence of decomposition,
thereby meeting an important requirement for practical
6
f,11
applications.
The discovery that intramolecular electron
-
5
×
10 M) of electron-rich porphyrin 11 and the acetonitrile
transfer between the porphyrin rings is casued by photocon-
trolled isomerization is worthy of note. These facts clearly
suggest that new diporphyrin Zn complex 1 and its free base
analogue 2 are particularly useful in the development of
photocontrolled molecular electronics, such as molecular
-
5
solution (1 × 10 M) of electron-deficient porphyrin 15 were
mixed, a 38% decrease of fluorescence intensity compared with
the original fluorescence intensity was observed. This result
suggests the formation of an electron-transfer complex (charge-
transfer complex) between electron-rich porphyrin 11 and
electron-deficient porphyrin 15 or the electron transfer between
these species. This decrease of fluorescence intensity caused
by the intermolecular electron transfer would be further evidence
that the smaller fluorescence intensity of cis-isomer arises from
the electron transfer.
30-32
switches,
and are the ideal materials to enable the construc-
tion of practical systems. In addition to being a useful material
for molecular electronics, new compounds 1 and 2 might also
serve as a potential model for the photosynthetic electron-
1
transport system, because the product from photoinduced
intramolecular electron transfer is very stable at room temper-
ature.
Other evidence of electron transfer is to confirm the existence
of the porphyrin radical pair, which is generated by the electron
transfer. As shown in Table 3, the ratios of quantum yields of
the diporphyrin system for the starting compounds (monomer)
are 0.2 and 0.109. Thus, there is the possibility that the radical
of the cis-isomer is explored by spectroscopic methods. Por-
phyrin radicals have been investigated by UV-visible spectrum
measurements. For example, UV-visible spectra of the radicals
Experimental Section
Materials. All solvents and reagents were purchased commercially.
Dry solvents were distilled from CaH
photoirradiation experiments was distilled from CaH
2
or LiAlH
4
. Acetonitrile for
under an argon
2
atmosphere and stored over molecular sieves. Other solvents were used
II
as received. Iron powder was converted to Fe Br
2
by a literature
procedure33 and Fe Br was used to prepared the iron(III) complexes
II
of porphyrin metal complexes (anion and cation) were found
2
to exhibit broad absorption from 600 to 800 nm.2
4,25
We also
of new porphyrins and diporphyrins.
found this broad absorption in the UV-visible spectrum of the
π cation radical of dodecaarylporphyrin Fe complex.2 Just after
photoirradiation, the UV-visible spectrum of the cis-isomer of
diporphyrin Zn complex 1 in CH3CN exhibited broad absorption
Physical Measurements. NMR spectra (JEOL GX270) and UV-
visible spectra (Hitachi 340) were collected routinely. The extinction
coefficient for the diporphyrin Zn complex 1 in dichloromethane at
room temperature has been determined at the Soret band. The
fluorescence spectra were collected by using a Shimazu RF-5300PC
6
(550-800 nm) centered at 685 nm, the maximum of which was
34
and a JASCO FP-770 under an argon atmosphere at room temperature.
obscured. This broad absorption was independent of temperature
from 25 to 0 °C. This absorption observed in the UV-visible
spectrum may be due to the existence of stable radicals of
diporphyrin Zn complex.27 On the other hand, the trans-isomer
did not show any absorption in the same region. Thus, these
experimental results27 would support the above explanation that
intramolecular electron transfer from the electron-rich porphyrin
Zn complex moiety to the electron-deficient porphyrin Zn
complex moiety in one molecule occurs.
The relative fluorescence quantum yields of diporphyrin Zn complex
(29) (a) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; Gray, H. B.
Science 1994, 246, 1311. (b) Takeuchi, T.; Gray, H. B.; Goddard, W. A.,
III. J. Am. Chem. Soc. 1994, 116, 1311. (c) Birbaum, E. R.; Schaefer, W.
P.; Labinger, J. A.; Bercaw, J. E.; Gray, H. B. Inorg. Chem. 1995, 34,
1751. (d) Hoffman, P.; Meunier, B. New J. Chem. 1992, 16, 559. (e)
Meunier, B. Chem. ReV. 1992, 92, 1411. (f) Lyons, J. E.; Ellis, P. E., Jr.
Catal. Chem. 1991, 8, 45. (g) D’Souza, F.; Villard, A.; Caemelbecke, E.
V.; Franzen, M.; Boschi, T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem.
1993, 32, 4042. (h) Sheldon, R. A., Ed. Metalloporphyrins in Catalytic
Oxidations; Marcel Dekker: New York, 1994. (i) Ochsenbein, P.; Mandon,
D.; Fischer, J.; Weiss, R.; Austin, R.; Jayaraj, K.; Gold, A.; Terner, J.;
Bill, E.; Muther, M.; Trautwein, A. X. Angew. Chem., Int. Ed. Engl. 1993,
32, 1437. (j) Mandon, D.; Ochsenbein, P.; Fischer, J.; Weiss, R.; Jayaraj,
K.; Austin, R. N.; Gold, A.; White, P. S.; Brigaud, O.; Battioni, P.; Mansuy,
D. Inorg. Chem. 1992, 31, 2044. (k) Bartoli, J. F.; Brigaud, O.; Battioni,
P.; Mansuy, D. J. Chem. Soc., Chem. Commun. 1991, 440. (l) Dolphin, D.;
Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251.
Conclusion
Azobenzene-linked diporphyrin Zn complex 1 and its free
base analogue 2 showed photoisomerization and subsequent
thermal isomerization, and this isomerization process (trans-
cis and cis-trans) was reversible. The noteworthy characteristics
of compounds 1 and 2 are that the fluorescence intensity of the
(30) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian,
D. F. J. Am. Chem. Soc. 1996, 118, 3996.
(
(
24) Groves, J. T.; Watanabe, Y. J. Am. Chem. Soc. 1988, 110, 8443.
25) (a) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J.
(31) (a) Huck, N. P. J. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun.
1995, 1095. (b) Goulle, V.; Harrimen, A.; Lehn, J. M. J. Chem. Soc., Chem.
Commun. 1993, 1034.
(32) (a) Fabbrizzi, L.; Zpoggi, A. Chem. Soc. ReV. 1995, 197. (b) Bissell,
R. A.; de Silva, A. P.; Gunaratne, H. Q. N.; Lynch, P. L. M.; Maguire, G.
E. M.; Sandanayake, K. R. A. S. Chem. Soc. ReV. 1992, 187. (c) Knorr,
A.; Daub, J. Angew. Chem. Int. Ed. Engl. 1995, 34, 2664.
Am. Chem. Soc. 1970, 92, 3451. (b) Closs, G. L.; Closs, L. E. J. Am. Chem.
Soc. 1963, 85, 818. (c) Dolphin, D., Ed. The Porphyrins; Academic Press:
New York, 1978.
(
26) Tsuchiya, S. J. Chem. Soc., Chem. Commun. 1991, 716.
(27) To obtain further evidence about this, the preliminary ESR measure-
ments of diporphyrin Zn complex 1 in butyronitrile were carried out at
room temperature. The ESR spectrum of the cis-isomer after photoirradiation
showed the broad peak, suggesting the existence of radicals.3f,25a,26,28 This
broadening is probably due to intramolecular dipolar-dipolar and exchange
interaction among the radicals.
(33) Winter, G. Inorg. Synth. 1973, 14, 101.
(34) The relative fluorescence quantum yields of compounds (1, 2, 5,
and 9) were summarized in Table 3. The measurements of the relative
fluorescence quantum yields of other diporphyrin Zn complexes and
diporphyrins have been tried by using several methods (Ogawa, S.; Tsuchiya,
S. Chem. Lett. 1996, 709). The results of these measurements will be
published in the future.
(28) Mitomo, S.; Tsuchiya, S.; Sen oj , M.; Tokita, S. Mol. Cryst. Liq.
Cryst. 1998, 312, 263.