The Journal of
Physical Chemistry
Registered in US. Patent Office 0 Copyright, 1981, by the American Chemical Society
VOLUME 85, NUMBER 10
MAY 14, 1981
LETTERS Studies on Man-Made Photoreaction Centers and Electron Relay Systems. 1. Enhancement of Charge Separation by the Aid of Aligned Viologen Units in Combination with Ruthenium Complexes' Taku Matsuo,* Tetsuo Sakamoto, Kelsuke Takuma, Katsuhlko Sakura, and Tatsuya Ohsako Department of Organic Synthesis, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 8 12, Japan (Received: Februay 18, 1981)
Tris(2,2'-bipyridine)ruthenium(II) complexes, Ru(bpy)az+,were covalently linked to viologen units to afford a model of man-made photoreaction centers. The emission from the ruthenium complex in the photoreaction of centers was almost completely quenched by the linked viologen units. The relative quantum yields (areel) the photoinduced reduction of viologen units in the presence of the electron donor EDTA were 1-2% of those for the free Ru(bpy)32+-methylviologen system in aqueous solutions. The a,,, values, however, remarkably increased in the combined system which consisted of the photoreaction center and the electron relay systems of aligned viologen units (micelle,polymer, and polysoap). The effect was attributed to the enhancement of charge separation of the photoproduced primary ion pair due to electron injection from the photoreaction centers into the electron relay system and the succeeding electron migration.
Introduction The tris(2,2/-bipyridine)ruthenium(II) complex, Ru(bpy)?+, has been well studied as a photosensitizer for splitting water into dihydrogen and dioxygen. Various viologen homologues (4,4/-bipyridinium ion, V2+) were frequently used as the electron acceptor, as well as the electron mediator from the photoexcited ruthenium complex to a metal colloid to produce molecular hydrogen."
The following factors are important to improve the catalytic efficiencies for photochemical hydrogen generation in an aqueous system involving Ru(bpy)z+ and a metal colloid: (1)The electron-transfer quenching of the photoexcited Ru(bpy),2+ by viologen derivatives should proceed with high efficiency. ( 2 ) The photoproduced redox pair, Ru(bpy)t+ and V+., must be quickly separated before it is annihilated by back electron transfer. Efficient electron-transfer quenching of photoexcited species, as Dointed out above. mav be achieved bv the use of a Dhotosensitizer with a covalently linked electron acceptor. Porphyrine complexes with covalently linked quinones ,
(1) Contribution No. 620 from the Department of Organic Synthesis, Faculty of Engineering, Kyushu University. (2) T. Nishijima, T. Nagamura, and T. Matauo, J.Polym. Sci., Polym. Lett. Ed., in press. (3) J. M. Lehn and J. P. Sawvage, Nonu. J. Chim., 1, 449 (1977). 0022-3654/81/2085-1277$01.25/0
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(4) J. Kiwi and M. Griitzel, J . Am. Chem. SOC.,101, 7214 (1979).
@ 1981 Amerlcan Chemical Soclety
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The Journal of Physical Chemistry, Vol. 85, No. 10, 1981
Letters
were synthesized for this purpose and their characteristics were reported.6p6 Such a system may be defined as a man-made photoreaction center analogous to photosynthesis. In the present paper, the intramolecular electron-transfer quenching of ruthenium complexes by closely located viologen group as shown in compounds 1-3 were
&)
QQ
R
50 0
600
700
R
Wavelength ( n m )
compared with the intermolecular quenching of standard system (4 and 5). The charge separation of photoproduced redox pairs, on the other hand, could be enhanced by the use of an electron relay system as supplied by aligned viologen units in micelles and polymer chains.' In this paper, polysoap with pendant viologen units (PS-V2+CI6,8) and CTAC micelles R - N ~ N - R
-CH2
8
PS-@C16
incorporating amphipathic viologen (2CI2VLf,7) were used as the electron relay system in combination with the above-described man-made photoreaction center.
Results and Discussion The emission spectra of 2C3V2+C2-Ru2+ (1, in methanol) and PS-V2+-Ru2+ (3, in water) are extremely weak in comparison with those of the corresponding reference ruthenium complexes (4, 5) under the same conditions (Figure 1). It is clear that the emissions of the ruthenium complexes in 1 and 3 were strongly quenched by the covalently linked viologen units. The same results were obtained with 2CI6V2+C2-Ru2+ (2) in CTAC micelles or methanol. Quenching efficiencies of the ruthenium complexes are summarized in Table I, where the emission intensities (I) (5) I. Tabushi, N. Koga, and M. Yanagita, Tetrahedron Lett., 267 (1979). (6) J. Dalton and L. R. Milgrom, J.Chem. SOC.,Chem. Commun., 609 (1979). (7)K.Takuma, T. Sakamoto,T. Nagamura, and T. Matsuo, J.Phys. Chem., in press.
Flgure 1. Emission spectra of ruthenium complexes. (A) R ~ ( b p y ) ~ ~ + (4) excited at 450 nm in a(S"("us solutions (2 X lo6 M). (B) RUCl2B2+ (5) and (C) 2C3V2+C2-Ru2 (1) or 2Cd6V2+C2-Ru2+(2) excited at 470 nm In methanol solutions (2 X 10- M). The emission intensity of PS-V2+-Ru2+ (3) is negligibly small.
TABLE I: Inter- and Intramolecular Quenching of the Luminescence from RuZ+Complexes by Viologen Units Rua+complex viologen unit III, E,, % MVZ+ a O.4gc 51 &!$~~~2i4/3) ps-vz+a oc 100 2C,VZ+C,-R~Z+ (1) 2C,Vz+ 0.055d 94.5 0.006d 99.4 2C,6Vz+C,-R~Z+ (2) 2Cl6V2* [V" unit] = 1 x M. [V2+unit] = 4 x M. I , is the luminescence intensity of Ru(bpy),,+ in aqueous solution. I , is the luminescence intensity of RuC,,B2+in methanol solution, The concentration of Ru units is M in all cases. 2x ~~
~~
of the ruthenium complexes in the presence of viologen units are compared with the standard intensity (lo)obtained in the absence of the viologen units. For ruthenium complexes with covalently bonded viologen unih, Iovalues were assumed to be the same as that of the corresponding reference ruthenium complex without viologen units (4 or 5). The concentration of viologen units was adjusted to 1X M for R ~ ( b p y ) (4) ~ ~and + PS-V2+-Ru2+ (3) systems, while 4 X 10" M viologen units were present in 2C3V2+C2-Ru2+(1) and 2C16V2+C2-RU2+(2) systems. The quenching efficiency (E,) in Table I is defined by the following equation: E, = (10- I ) / l o (1) The efficiency of the intramolecular quenching process is clearly higher than that of the intermolecular process. The same results were reported for porphyrines and quinones, as mentioned a b o ~ e . ~ ? ~ Photosensitized reductions of the viologens were carried out in the presence of EDTA as an electron donor. Aqueous solutions containing EDTA, ruthenium complex, and viologen were deoxygenated with argon, and were irradiated with visible light (A > 420 nm). The reduced products from the viologen units were followed by their absorptions at 603 nm (e = 10500 M-'cm-' for the monome# and at 535 nm (t = 6910 M-l cm-l for the dimer)?
The Journal of Physical Chemlstty, Vol. 85, No.
Letters
Scheme I
TABLE 11: Relative Quantum Yields ( Q r e l ) of Photoreduced Viologen Unit and Charge Separation Efficiency (Eos,rel)c system
photoinduced reduction system
EDTA
6v
I
EDTA ox
@re1 Ecs.re1
1 Ru(bpy),l+, M P a 1 0 0 100 2 2C,V2+C,-Ru2+ b 2 1 3 2C16Vz+C,-R~2+/CTACb 1 0.6 4 2C16V2+C,-R~z+/(CTAC + 2C1,V2+)a 6 3 6 2C16V2+C,-R~2+ + PS-V1+-C16" 56 28 [Vz+unit] = 1 X M. [Vz+u n i t ] = 4 x loF5M. [Ru2+unit] = 2 x M, [CTAC] = 1 x lo-, M, [ E D T A ] = 1 x lo-' M. a
corresponds to the efficiency of separating the photoproduced, primary ion pair to a distance so that back electron transfer would not take place with ease. The experimental values for @ral and the estimated EcsJdvalues are listed in Table 11. The E,,rel values are normalized to that for R ~ ( b p y ) , ~ + - M v ~system. + The E, values for 2C3V2+C2-Ru2+(1) and 2Cl6V2+C2-Rui' (2, in CTAC micelle) are extremely small (0.5 and 1%of the reference system, respectively). In both cases, Ep,values are extremely large as shown in Table I. Then it is most likely that the primary ion pairs are lost by the back electron transfer (eq 3-5). The Easelvalues considerably increase
*Ru2+-V2+
-
RuS+-V+.
Ru3+-V+. + Ru2+-V2+ RuS+-V+. + V2+ + RuS+-V2+ + V+.
+
RuS+-V2+ EDTA
(3)
(4) (5)
(6)
Ru2+-V2++ EDTA,
(7) when the photoreaction centers are used in combination with electron relay systems such as CTAC-2C12V2+mixed micelles (system 4, a 5-fold increase) and Ps-v2+-c16 polysoap (system 5, a 26-fold increase). A remarkable increase is also observed in PS-V2+-Ru2+ polymer (system 6, a 55-fold increase), where the photoreaction center is directly attached to the electron relay system on the polymer chain. The enhancement of the charge separation may be ascribed to electron injection from the photoreaction center to the electron relay systems as given by eq 6 and Scheme I. The injected electron will migrate among the viologen units in the relay system as previously In the meantime, Ru3+complex in the photoreaction center will be reduced by EDTA (eq 7), and the photoproduced viologen radicals survive without suffering +
5m
c
h
m 0,
c
rd
e'
The relative quantum yields for the viologen unit reduction were evaluated from the initial rates of formation of the viologen cation radical (V+-). The overall quantum yield for the reduced viologen unit (V+.) is comprised of two efficiencies: one for electron-transfer quenching (E,) and the other for the charge separation process (E,) (eq 2). The charge separation efficiency, as defined by eq 2, arel= constant x (EqEaJel) (2)
hv *Ru2+-V2+ Ru2+-V2+ +
IO, 198 1 1279
(8) E. M. Kosower and J. L. Cotter, J. Am. Chem. SOC.,86, 5524 (1964).
(9) The molar absorptivity (e) of the viologen radical dimer was determined from the absorption spectra of the vi010 en polymer reduced by excess sodium hydrosulfite. Under this confition, the electronic spectra was completely in agreement with that of the radical dimer (A, = 372 and 535 MI), and the absorption due to the monomer (A, = 603 nm) was hardly observed.
LU
-..
"24
a
"I
2,
24
...-..@..".
1 E
.-
n
e-
E
0
......
a1
I
-
......
-I
forward reaction
----• backward reaction
back electron transfer. Then the charge separation efficiency is expected to increase in comparison to the case without the electron relay system. The above expectation is fulfilled by the use of the three different electron relay systems: micelle, polymer, and polysoap as found in Table 11. One may suspect, however, that the charge separation efficiency increases with the enrichment of the reductant (EDTA dianion) to the polycation of the electron relay systems. The same type of EDTA enrichment should be expected with the CTAC micellar surface, where 2CI6V2+C2-Ru2+ is being incorporated. However, the Ea,rel value of the system (system 3 in Table 11) is essentially equal to that of 2C3V2+C2-Ru2+ in aqueous solution (system 2 in Table 11). Therefore, the enrichment of the EDTA concentration should not be so important to affect the charge separation efficiency of the photoreaction center. In conclusion, it is strongly emphasized that electron relay systems are efficient enough to suppress the rapid back electron transfer process in the photoreaction center which consists of a ruthenium complex and covalently linked viologen units. The charge separation efficiency will be further enhanced by appropriate modification of electron relay systems. Acknowledgment. The authors are grateful for financial support by the Grant-in-Aid for Scientific Research (No. 505046 and No. 575543) from the Ministry of Education of Japan. The study was also supported in part by a Grant-in-Aid from the Mitsubishi Foundation for the Contribution of Scientific Research.
