Scheme 1. Possible Electron Transfer and Singlet Oxygen
Mechanism of Oxygenation of DPA Sensitized by Eo-Ru
-•
Figure 3. ESR spectra of the DMPO-O2 radical adduct
generated from an oxygen-saturated DMSO solution with DMPO
(20 mM) upon irradiation of 532 nm laser: (A) Eo-Ru 0.2 mM),
(B) addition of 5 mM of p-benzoquinone to A.
Eo-Ru and DMPO (5,5′-dimethyl-1-pyrrodine N-oxide) at
532 nm, which is identical to the DMPO-O2 (H) radical
adduct.6 The same signal was not observed in the absence
of either oxygen or Eo-Ru or in the presence of EoEt or
RB under the same conditions. When a quencher of O2- (p-
benzoquinone)7 was introduced to the system, the DMPO-
O2(H) signal disappeared. These results indicate generation
presence of different sensitizers. Rose Bengal is a typical
sensitizer of singlet oxygen with a quantum yield of Φ1O2
) 0.76.2a The reaction induced by RB should be more
efficient than that by Eo-Ru (Φ1O2 ) 0.54) if both reactions
are singlet oxygen mechanism. However the reaction sen-
sitized by Eo-Ru is much faster than those sensitized by
Rose Bengal (RB), EoEt/Ru mixture (1:1, molar ratio), or
EoEt. The percentages of converted DPA are 41%, 10%,
8%, and 7% for Eo-Ru, RB, EoEt/Ru mixture, and EoEt,
respectively, upon irradiation for 16 min. This result indicates
that singlet oxygen is not the predominant mechanism for
the photooxygenation induced by Eo-Ru. Further, the photo-
oxygenation of different anthracene derivatives in different
solvents induced by Eo-Ru was investigated. Apparently,
polar solvent like acetonitrile strongly increases the reaction,
whereas nonpolar solvent dramatically slows down the
reaction. After irradiation for 15 min, the percentage of con-
verted DPA and 9,10-dimethylanthracene (DMA) are 39%,
96% in acetonitrile and 4.6%, 42.4% in benzene, respectively.
Also, an electron-donating group in the 9,10 positions
accelerates the reaction, whereas an electron-accepting group
in the 9,10 positions decreases the reaction rate. The reaction
is very fast for DMA and 9-methlyl-anthracene (MA), with
no observable reaction for 9-anthraldehyde (CHO-AN) and
9-anthracenecarboxylic acid (COOH-AN), and there is a slow
reaction for anthracene (AN), 9,10-dichloroanthracene (DCA),
and 9,10-dibromoanthrcne (DBA). The reaction rate de-
creases DMA > MA > DPA > AN > DCA ≈ DBA >
CHO-AN ≈ COOH-AN. NMR and MS data prove that the
isolated product from the reaction of DPA is a peroxide of
9,10-dipenylanthracene. All of these results suggest that an
electron transfer mechanism is probably involved. To verify
the existence of an electron-transfer mechanism, an experi-
ment capturing superoxide anion was performed.
-
of O2 following irradiation of the solution of Eo-Ru and
also suggest that an electron transfer exists in the photooxy-
genation of anthracene derivatives induced by Eo-Ru. On
the basis of the results above, an electron-transfer photo-
oxygenation reaction mechanism is proposed (Scheme 1).
That the initial photoinduced intramolcular electron-transfer
process is responsible for the improved photosensitizaton of
eosin in the dyad of Eo-Ru can also be concluded.
The strong photosensitization ability and broad absorption
range in the visible region of Eo-Ru suggest that this
composite sensitizer has potential application in many areas
such as photoinduced reaction, photocatalysis, and photo-
synthesis and related solar conversion. Since superoxide
anion probably involves a phototherapy process,8 this
compound may find application in photodynamic therapy.
Acknowledgment. The authors thank Dr. Bojie Wang
for the measurement of the quantum yield of singlet oxygen
of eosin in Eo-Ru and the national science foundation of
china for the financial support.
Supporting Information Available: Experimental pro-
cedures, synthesis and spectral data for compounds. This
material is available free of charge via the Internet at
OL0353924
(6) John, R.; Harbor, M.; Hair, L. J. Phys. Chem. 1978, 82, 1397-
1399.
(7) Maning, L. E.; Kramer, M. K.; Foote, C. S. Tetrahedron Lett. 1984,
84, 2523-2526.
(8) Selman, S. H.; Hampton, J. A.; Morgan, A. R.; Keck, R. W.; Balkany,
A. D.; Skalkos, D. Photochem. Photobiol. 1993, 57, 681-685.
A typical 3 × 2 × 2 ESR signal (Figure 3) was obtained
upon irradiation of an oxygen-saturated DMSO solution of
Org. Lett., Vol. 5, No. 20, 2003
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