An unusual photosensitizer: Dyad of eosin-tris(2,2′-bipyridine)Ru(II)

Article abstract of DOI:10.1021/ol0353924

(Matrix presented) A dyad of eosin and tris(2,2′-bipyridine)Ru(II) was prepared, and its photophysical properties were investigated. The photosensitization of eosin is greatly enhanced by introduction of tris(2,2′-bipyridine)Ru(II), which is verified via

Full text of DOI:10.1021/ol0353924

ORGANIC
LETTERS
2003
Vol. 5, No. 20
3709-3711
An Unusual Photosensitizer: Dyad of
Eosin Tris(2,2 -bipyridine)Ru(II)
−
′
Bingwen Jing,* Manhua Zhang, and Tao Shen
Center for Molecular Science, Institute of Chemstry, Chinese Academy of Sciences,
Beijing 100080, P. R. China
bij3@lehigh.edu
Received July 24, 2003
ABSTRACT
A dyad of eosin and tris(2,2
eosin is greatly enhanced by introduction of tris(2,2
′
-bipyridine)Ru(II) was prepared, and its photophysical properties were investigated. The photosensitization of
-bipyridine)Ru(II), which is verified via photooxygenation of anthracene derivatives. The
′
electron-transfer mechanism of photosensitization is also discussed.
Polypyridine ruthenium complexes1 and xanthenes2 have
been widely used in many areas such as photochemistry,
electrochemistry, photoelectrochemistry, photocatalysis, ar-
tificial photosynthesis and related solar energy conversion,
laser dyes, fluorescence depolarization diagnostic devices,
photomedicine, molecular probes, and quantum counters. The
interest in such compounds is very active. Polypyridine
ruthenium, especially tris(2,2′-bipyridine)Ru(II), has an
absorption maximum in the range of 400-500 nm, and eosin,
a xanthene, strongly absorbs the light in the range of 500
nm to 600 nm with a very small extinction coefficient below
500 nm. Combination of these two chromophores would be
expected to produce some new properties compared to the
individual compounds since the dyad of eosin and Ru
possesses several advantages. First, the complementary
absorption spectra of eosin and Ru provide an extended
absorption range for the dyad, which would favor the
collection of light. Second, an intramolecular electron transfer
from eosin to Ru would take place upon the excitation of
eosin moiety or Ru moiety according to the Rehm-Weller
equation3 (free energy changes, ∆G ) -0.33 and -0.12 eV,
respectively, calculated from the data listed in Table 1).
Furthermore, since Ru(I) is relatively more stable than many
organic species,1a a relatively long-lived photoinduced charge
separation state (Eo+•-Ru-) would be expected. This is
useful for the dyad to take part in the secondary reaction
after an initial photoinduced electron transfer. This com-
munication reports the unusual photosensitization of a novel
dyad: eosin-tris(2,2′-bipyridine)Ru(II) (Eo-Ru).
* To whom correspondence should be addressed. Present address:
Department of Chemistry, Lehigh University, 6 East Packer Ave., Bethle-
hem, PA 18015.
(1) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
von Zelewsky, A. Coord. Chem. ReV. 1985, 84, 85-277. (b) Balzani, V.;
Juris, A.; Venturi, M. Chem. ReV. 1996, 96, 759-833. (c) O’Regan, B.;
Gratzel, M. Nature 1991, 353, 737-40. (d) Jing, B.; Zhang, H.; Zhang,
M.; Lu, Z.; Shen, T. J. Mater. Chem. 1998, 8, 2055-2060.
(2) (a) Moser, J.; Gratzel, M. J. Am. Chem. Soc. 1984, 106, 6557-6564.
(b) Jing, B.; Zhang, D.; Zhu, D. Tetrahedron Lett. 2000, 41, 8559-8563.
(c) Islam, S.; Konishi, T.; Fujitsuka, M.; Ito, O.; Nakamura, Y.; Usui, Y.
Photochem. Photobiol. 2000, 71, 675-680. (d) Aguilera, O. V.; Neckers,
D. C. Acc. Chem. Res. 1989, 22, 171-177. (e) Yu, Q.; He, J.; Shen, T. J.
Photochem. Photobiol., A 1996, 97, 53-56. (f) Sun, L.; Yang, Y.; He, J.;
Shen, T. Dyes Pigments 1995, 28, 275-279.
10.1021/ol0353924 CCC: $25.00
© 2003 American Chemical Society
Published on Web 08/30/2003

Table 1. Photophysical and Electrochemical Data of Model
Compounds and Dyada
2+
item
EoEt
Ru(bpy)3
Eo in Eo-Ru
λabs (nm)
λem (nm)
ΦF
ΦIST
ΦIC
538
562
0.41
0.39
0.20
454, 462b
620
542
561
0.003
0.54
0.20
0.257
0.04d
∼1.0d
0.00d
ΦET
t (ns)
3.68
760
0.362 (47%), 3.14 (53%)
ES/ET (eV) 2.31/1.90c
2.75/2.12d
Eox/Ered(V) 0.80/-1.04c 1.26/-1.26d
a λabs ) absorption wavelength, λem ) fluorescence wavelength; ΦF, ΦIST
,
ΦIC ) fluorescence quantum yield, intersystem crossing quantum yield,
and internal conversion quantum yield, respectively; ΦET ) electron transfer
quantum yield statically and dynamically; t ) lifetime of fluorescence; ES/
ET ) singlet/triplet energy; Eox/Ered ) oxide/reduction potential. b In Eo-
Ru. c Cited from ref 2e. d Cited from ref 1a.
Figure 2. Absorbance of DPA at 392 nm versus irradiation time
under light >510 nm in the presence of different sensitizers. [DPA]
≈ 1 × 10-4 M; [sensitizer] ≈ 2 × 10-6 M. A0 and A: absorbance
before and after irradiation, respectively.
The preparation of Eo-Ru was accomplished by esteri-
fying the carboxyl group of eosin with 4-bromomethyl-4′-
methyl-2,2′-bipyridine, followed by a coordination substi-
tution reaction with Ru(bpy)2Cl2‚H2O.
Generally, the quantum yield of singlet oxygen is ap-
proximately equal to the quantum yield of intersystem
crossing (ΦISC). Compared to the intersystem crossing
quantum yield of EoEt (ΦISC ) 0.39), the heavy atom Ru in
Eo-Ru enhances the intersystem crossing of eosin from 0.39
2+
to 0.54. Although both EoEt (ΦF ) 0.41) and Ru(bpy)3
(ΦF ) 0.04) have strong photoluminescence, very weak
emission (ΦF ) 0.003) was observed in Eo-Ru when the
Eo moiety was selectively excited at 540 nm and no emission
was measured for the Ru moiety, excited at 450 nm. In the
dyad, the fluorescence lifetime of the eosin moiety exhibits
a double exponential decay, 3.14 ns (54%) and 0.362 ns
(46%), respectively. The long-lived component comes from
the ground-state static electric interaction, whereas the shorter
one can be attributed to the intramolecular photoinduced
electron transfer from the excited eosin moiety to the Ru
moiety as well as the intramolecular “heavy atom effect”.
The total quantum yield of dynamic and static electron
transfer (ΦET) from the excited Eo moiety to Ru moiety is
ΦET ) 1 - ΦF - ΦISC - ΦIC ) 1 - 0.003 - 0.54 - 0.20
) 0.257; here ΦIC ) 0.20 was cited from the literature5 and
assumed no change in Eo-Ru. The lack of emission of Ru
moiety can be caused by triplet-triplet energy transfer from
the excited Ru moiety to eosin moiety and electron transfer
from eosin moiety to excited Ru moiety. Both processes are
thermodynamically allowed with a free energy change of
-0.22 and -0.12 eV, respectively. The photophysical and
electrochemical data of EoEt, Ru(bpy)32+, and Eo-Ru are
partly listed in Table 1.
Figure 1. Absorption spectra of dyad of Eo-Ru and model EoEt
2+
in acetonitrile: (s) Eo-Ru; (‚‚‚) EoEt; (- - -) Ru(bpy)3
.
The absorption spectrum of Eo-Ru (Figure 1) is charac-
terized by a simple combination of Ru(bpy)32+ (Ru) and eosin
ethyl-ester (EoEt). The metal to ligand charge transfer
(MLCT) band of Ru and absorption band of Eo in the visible
region in Eo-Ru have a red shift of 8 and 4 nm, respectively,
in acetonitrile compared to their individual components. This
suggests the existence of a ground state static electric
interaction between positively charged Ru and negatively
charged eosin ethyl ester, which is similar to the fluorescence
quenching of other xanthenes by viologen.2f The quantum
yield of generating singlet oxygen (Φ1O2) of eosin in Eo-
Ru is 0.54, directly measured through the time-resolved
The photosensitization of eosin was found greatly en-
hanced by introduction of the Ru complex, which was
verified by the photosensitization oxygenation of 9,10-
diphenylanthracene (DPA). Figure 2 shows the decrease of
characteristic absorption of DPA at 392 nm in oxygen-
saturated acetonitrile upon irradiation (>510 nm) in the
1
emission of ∆gO2 in the near infra region (1270 nm).4
(3) Karanos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449.
(4) Scurlock, R. D.; Ogilby, P. R. J. Phys. Chem. 1987, 91, 4599-4602.
(5) Zhao, Z.; Shen, T.; Xu, H.-J. J. Photochem. Photobiol., A 1990, 52,
47-53.
3710
Org. Lett., Vol. 5, No. 20, 2003

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
http://pubs.acs.org.
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
3711