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DOI: 10.1039/A906816C (Paper) Phys. Chem. Chem. Phys., 2000, 2, 109-114

Photo-oxidation of 1,3-cyclopentadiene using partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes

Masahiro Suzuki*a, Oliver Bartelsb, Robert Gerdesb, Gerhard Schneiderb, Dieter Wöhrleb, Günter Schulz-Ekloffc, Mutsumi Kimuraa, Kenji Hanabusaa and Hirofusa Shirai*a
aDepartment of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano, 386-8567, Japan. E-mail: smasa@giptc.shinshu-u.ac.jp
bUniversität Bremen, Institut für Organische und Makromolekulare Chemie, Fachbereich 2, NW II, P.O. Box 330 440, Bremen, 28334, Germany
cUniversität Bremen, Institut für Angewandte und Physikalische Chemie, Fachbereich 2, NW II, P.O. Box 330 440, Bremen, 28334, Germany

Received 23rd August 1999, Accepted 5th November 1999

First published on UnassignedUnassigned22nd December 1999

Abstract

Photo-oxidation of cyclopenta-1,3-diene (CP) with singlet oxygen using tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)32+] and polymer-bound ruthenium(II) complexes as photosensitizers was investigated in oxygen-saturated ethanol. The polymer-bound ruthenium(II) complexes consist of a bis(2,2′-bipyridine)ruthenium(II) complex coordinated with two imidazolyl residues on partially quaternized poly(1-vinylimidazole) with hexyl groups (C6RuQPIm) and hexadecyl groups (C16RuQPIm) as the alkyl side chains. The photo-oxidation of CP using the polymer photosensitizers effectively occurred in a comparable manner to the Ru(bpy)32+ system: the degree of reaction, being given by the ratio of reacted CP and initial CP concentrations, was high. In particular, all of the CP added was oxidized in the C16RuQPIm system even when the CP concentration was low. This was attributed to the concentration of CP species into the heterogeneous reaction field formed by the polymer photosensitizers. The Ru(bpy)32+ photosensitizer showed excellent stability and was repeatedly able to be used for the photo-oxidation reaction. During the repeated experiments, the reaction activity for the polymer photosensitizer systems gradually decreased because the polymer photosensitizers changed to monochloro complexes [CnRu(Cl)QPIm (n=6 or 16)] in which one imidazolyl residue was substituted by a chloride ion. However, these polymer photosensitizers also had excellent stability. Compared with other photosensitizers, such as Rose Bengal and zinc(II) phthalocyaninetetrasulfonic acid, the stability of the ruthenium(II) complex photosensitizers was excellent.

Introduction

Singlet molecular oxygen, O2(1Δg), plays an important role in both natural and artificial photochemical processes. The quantum yields for O2(1Δg) generation have been measured in water and organic solvents for many photosensitizers.1–8 O2(1Δg) is believed to be the initial agent in the photodynamic therapy of cancer (PDT).9–16 Furthermore, in contrast to ground state molecular oxygen, O2(1Δg) has found considerable utility since it can undergo photo-oxidation reactions with a wide variety of electron-rich molecules such as sulfides,17,18 thiols,19,20 phenols21–23 and other organic compounds.24–29

One of the most reactive substrates for the endoperoxide formed by the photo-oxidation reaction is cycloalka-1,3-dienes, and many studies have been reported using various photosensitizers such as Rose Bengal, Methylene Blue, chlorophyll, porphyrins and phthalocyanines, which have a strong absorption in the visible region.30–37 Some peroxides obtained in the photo-oxidation of 1,3-dienes with singlet oxygen are of interest either as pharmaceuticals (e.g. ascaridole30) or as model compounds for biochemical studies (e.g., prostaglandin biosynthesis38). These model compounds have been prepared directly from fatty acid precursors39 or from cyclopenta-1,3-dienes followed by reduction of the remaining C–C double bond.40 For most preparative purposes, however, the photo-oxidation reactions have been designed in such as way that either in situ or subsequent treatment of the peroxides formed initially leads to synthetically valuable building blocks for organic synthesis. Furthermore, the endoperoxides are versatile starting materials for further transformations. Nucleophilic ring opening is another possibility for selective transformations that include reductions to diols, oxidations to dicarbonyl products and rearrangements to hydroxy carbonyl compounds, epoxy carbonyl compounds, bisepoxides, ene diones, etc.

The tris(2,2′-bipyridine)ruthenium(II) complex [Ru(bpy)32+] is a well known photosensitizer for water oxidation and photo-reduction reactions.41–44 In addition, this complex can produce O2(1Δg) with high quantum yield,8,45,46 comparable to those with Rose Bengal, Methylene Blue, porphyrins and phthalocyanines. On the other hand, we have reported that partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes (RuQPIms) are excellent photosensitizers for quenching of viologens,47 photosensitized charge separation48,49 and photoinduced hydrogen generation.50–52 However, these polymer photosensitizers have not been applied to the photo-oxidation reaction. In this paper, we report the first application of RuQPIms as photosensitizers to the photo-oxidation of cyclopenta-1,3-diene (CP) with singlet oxygen in ethanol and the effects of the alkyl side chain length. We selected two RuQPIms with a hexyl group (C6RuQPIm) and a hexadecyl group (C16RuQPIm) as the alkyl side chains because they are more stable for visible light irradiation than the non-quaternized ruthenium(II) complex-containing poly(10-vinylimidazole). Tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)32+], is used as a low molecular weight model compound.

Experimental

Materials

All reagents used for the syntheses of complexes were of analytical-reagent grade and were used without further purification. Ru(bpy)32+ was prepared according to the literature.53 Partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes (RuQPIms) were prepared by quaternization of poly(1-vinylimidazole)-bound ruthenium(II) complex (RuPIm)54 with hexyl bromide or hexadecyl bromide in methanol.47 The structure is shown in Fig. 1. Cyclopenta-1,3-diene (CP) obtained by distillation of dicyclopentadiene just before use was used in all measurements owing to its instability.
Structure of partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes.
Fig. 1 Structure of partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes.

Measurements

UV–Vis absorption spectrum measurements were carried out using a Perkin-Elmer Lambda 9 spectrophotometer. The photo-oxidation experiments were performed at 25°C in 50 ml of oxygen-saturated ethanol containing the photosensitizer and CP using an automatic apparatus as described previously.17,19 The oxygen consumption was followed continuously during stirring and irradiation with visible light from halogen lamp (250 W). After bubbling of the ethanol solution containing the photosensitizer for 10 min with oxygen, various concentrations of the CP were added. The reactions were monitored by measuring the oxygen consumption during the irradiation period. The initial rate of the photo-oxidation reaction was calculated from the initially linear slope of the graph of oxygen consumption vs. irradiation time. The stability of these photosensitizers was followed by visible spectroscopy. The absorbances of the photosensitizers in the reaction solutions, which were 450 nm for Ru(bpy)32+ and 490 nm for RuQPIms, before and after photo-oxidation were compared. Furthermore, the stability was also evaluated by repeated experiments. The repeated experiments were performed as follows. After bubbling of an ethanol solution of 5.0×10−5 M photosensitizer for 10 min with oxygen, CP (2.5×10−2 M) was added and then the oxygen consumption with visible light irradiation was monitored during the irradiation period. The reaction solution was bubbled with oxygen for 10 min again, and after the addition of 2.5×10−2 M CP, the oxygen consumption was monitored. This procedure was repeated. In all systems, the experimental errors were within 0.5%.

Results

When an oxygen saturated ethanol solution containing photosensitizer and CP was irradiated with visible light, the oxygen in the reaction cell was consumed (Fig. 2). Since the oxygen was not consumed in the absence of the photosensitizer and the quenching reaction of the photoexcited photosensitizers with CP did not take place under an argon atmosphere, the CP would be oxidized by singlet oxygen generated by energy transfer from the photoexcited photosensitizer to triplet molecular oxygen. The total amount of oxygen consumed was ca. 1.11×10−3 mol for Ru(bpy)32+ and ca. 1.25×10−3 mol for these polymers. Since this reaction cell contains 1.25×10−3 mol of CP, the CP reacts with an equivalent of singlet oxygen generated by photoexcited photosensitizers for the polymer systems.
Oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M photosensitizer and 2.5×10−2 M CP with visible light irradiation for Ru(bpy)32+, C6RuQPIm and C16RuQPIm systems.
Fig. 2 Oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M photosensitizer and 2.5×10−2 M CP with visible light irradiation for Ru(bpy)32+, C6RuQPIm and C16RuQPIm systems.

Ru(bpy)32+ as photosensitizer

Fig. 3 shows the oxygen consumption in ethanol solution containing various concentrations of Ru(bpy)32+ and 2.5×10−2 M CP during visible light irradiation, together with the dependence of the initial reaction rate on the concentration of Ru(bpy)32+. For all concentrations of Ru(bpy)32+ in this experiment, the total amount of oxygen consumed was 1.11×10−4 mol, although the reaction time when saturation of the oxygen consumption was reached was different. The degree of reaction, given by the molar ratio of initial CP and reacted CP concentrations,55 was ca. 90%. The initial reaction rate evaluated from the linear slopes of oxygen consumption with time linearly increased with increase in the concentration of Ru(bpy)32+, and then remained constant above 4.0×10−5 M of Ru(bpy)32+. Therefore, subsequent experiments were carried out with [Ru(bpy)32+]=5.0×10−5 M.
Oxygen consumption in oxygen-saturated ethanol solution containing various concentrations of Ru(bpy)32+ and 2.5×10−2 M CP with visible light irradiation: [Ru(II )]=1, 4.94×10−5; 2, 4.01×10−5; 3, 3.05×10−5; 4, 2.00×10−5; 5, 9.79×10−6; 6, 4.73×10−6 and 7, 2.12×10−6 M. Inset: dependence of initial reaction rate on concentration of Ru(bpy)32+.
Fig. 3 Oxygen consumption in oxygen-saturated ethanol solution containing various concentrations of Ru(bpy)32+ and 2.5×10−2 M CP with visible light irradiation: [Ru(II )]=1, 4.94×10−5; 2, 4.01×10−5; 3, 3.05×10−5; 4, 2.00×10−5; 5, 9.79×10−6; 6, 4.73×10−6 and 7, 2.12×10−6 M. Inset: dependence of initial reaction rate on concentration of Ru(bpy)32+.

Fig. 4 shows the oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M Ru(bpy)32+ and various concentrations of CP during visible light irradiation, together with the dependence of the initial reaction rate on the concentration of CP. The reaction rates, the total amount of oxygen consumed and degree of reaction are summarized in Table 1. The photo-oxidation reaction was completed within 15 min for all systems. The total amount of oxygen consumed and the degree of reaction increased with increase in the initial concentration of CP. The degree of reaction at the low CP concentrations was very low; in particular, that at [CP]=7.5×10−3 and 5.0×10−3 M was ca. 62 and 54%, respectively. The reaction rate increased linearly with increase in the concentration of CP, and then remained constant above 2.0×10−2 M CP.


Oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M Ru(bpy)32+ and various concentrations of CP with visible light irradiation: [CP]=1, 2.5×10−2; 2, 2.0×10−2; 3, 1.5×10−2; 4, 1.0×10−2; 5, 7.5×10−3 and 6, 5.0×10−3 M. Inset: dependence of initial reaction rate on concentration of CP.
Fig. 4 Oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M Ru(bpy)32+ and various concentrations of CP with visible light irradiation: [CP]=1, 2.5×10−2; 2, 2.0×10−2; 3, 1.5×10−2; 4, 1.0×10−2; 5, 7.5×10−3 and 6, 5.0×10−3 M. Inset: dependence of initial reaction rate on concentration of CP.
Table 1 Initial reaction rate (R) and total amount of oxygen consumed (OC) for photo-oxidation of CP using the photosensitizersa
Ru(bpy)32+C6RuQPImC16RuQPIm
   
[CP]/10−2MR/10−6 mol s−1OC/10−4 molR/10−6 mol s−1OC/10−4 molR/10−6 mol s−1OC/10−4 mol
 
a Concentration of photosensitizers is ca. 5.0×10−5 M.b Degree of reaction (%).
0000000
0.500.831.34(53.6b)0.892.21(88.4b)1.592.5(100b)
0.751.422.32(61.9b)1.463.53(94.1a)1.853.75(100b)
1.002.054.21(84.2b)1.674.53(90.6b)1.875.02(100b)
1.502.526.12(81.6b)1.697.49(99.9b)1.937.51(100b)
2.002.668.14(81.4b)1.6710.1(100b)1.979.99(100b)
2.502.7711.1(88.8b)1.4412.44(100b)1.9612.49 (100b)

Polymer photosensitizer systems

Fig. 5 shows the oxygen consumption in oxygen-saturated ethanol solution containing 2.5×10−2 M CP and various concentrations of the polymer photosensitizer, together with the dependence of the initial reaction rate on the concentration of the ruthenium(II) complex. For all concentrations of ruthenium(II) complex in these experiments, the total amount of oxygen consumed was ca. 1.25×10−4 mol and the degree of reaction was almost 100%. The initial reaction rate increased linearly with increase in the ruthenium(II) complex concentration, and then remained constant above 4.0×10−5 M Ru(II) complex for all systems. Therefore, subsequent experiments were carried out with [Ru(II)]=5.0×10−5 M.
Oxygen consumption in oxygen-saturated ethanol solution containing various concentrations of (A) C6RuQPIm or (B) C16RuQPIm and 2.5×10−2 M CP with visible light irradiation: [Ru(II )]=1, 5.03×10−5; 2, 4.04×10−5; 3, 3.05×10−5; 4, 2.03×10−5; 5, 9.88×10−6; 6, 4.94×10−6 and 7, 2.08×10−6 M. Inset: dependence of initial reaction rate on concentration of the ruthenium(II ) complex.
Fig. 5 Oxygen consumption in oxygen-saturated ethanol solution containing various concentrations of (A) C6RuQPIm or (B) C16RuQPIm and 2.5×10−2 M CP with visible light irradiation: [Ru(II )]=1, 5.03×10−5; 2, 4.04×10−5; 3, 3.05×10−5; 4, 2.03×10−5; 5, 9.88×10−6; 6, 4.94×10−6 and 7, 2.08×10−6 M. Inset: dependence of initial reaction rate on concentration of the ruthenium(II ) complex.

Fig. 6 shows the oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M Ru(II) complex and various concentrations of CP during visible light irradiation, together with the dependence of the initial reaction rate on the concentration of CP. The photo-oxidation reaction was completed within 20 min and the degree of reaction was very high for these systems. In particular, the C16RuQPIm systems demonstrated 100% reaction even when the CP concentration was low. The reaction rate depends significantly on the length of the alkyl side chain. For the polymer systems, the reaction rate increased up to [CP]=1.0×10−2 M and then remained almost constant.


Oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M polymer photosensitizer and various concentrations of CP with visible light irradiation: [CP]=1, 2.5×10−2; 2, 2.0×10−2; 3, 1.5×10−2; 4, 1.0×10−2; 5, 7.5×10−3; and 6, 5.0×10−3 M. Inset: dependence of initial reaction rate on concentration of CP.
Fig. 6 Oxygen consumption in oxygen-saturated ethanol solution containing 5.0×10−5 M polymer photosensitizer and various concentrations of CP with visible light irradiation: [CP]=1, 2.5×10−2; 2, 2.0×10−2; 3, 1.5×10−2; 4, 1.0×10−2; 5, 7.5×10−3; and 6, 5.0×10−3 M. Inset: dependence of initial reaction rate on concentration of CP.

Stability of photosensitizers

The stability of the photosensitizers was evaluated by repeated experiments on the photo-oxidation reaction and the absorption spectra before and after irradiation. Fig. 7 shows the repeated experiments on the photo-oxidation reaction of CP using (A) Ru(bpy)32+, (B) C6RuQPIm and (C) C16RuQPImas photosensitizers at [Ru(II)]=5.0×10−5 M and [CP]=2.5×10−2 M. During the repeated experiments using Ru(bpy)32+ photosensitizer, the reaction was completed in all cycles within 15 min and the total amount of oxygen consumed was ca. 1.11×10−4 mol; in addition, the initial reaction rate did not change. Furthermore, the absorption spectrum measurements showed that the absorbance and maximum wavelength did not change during the repeated experiments; therefore, the Ru(bpy)32+ photosensitizer was very stable.
Repeated experiments of photo-oxidation reaction of CP using (A) Ru(bpy)32+, (B) C6RuQPIm and (C) C16RuQPIm as photosensitizers at [Ru(II )]=5.0×10−5 M and [CP]=2.5×10−2 M, with the number of runs in ethanol on the abscissa. The irradiation time of one cycle is 15 min for the Ru(bpy)32+ system and 20 min in 1st to 9th cycles and 10th and 11th cycles for polymer systems.
Fig. 7 Repeated experiments of photo-oxidation reaction of CP using (A) Ru(bpy)32+, (B) C6RuQPIm and (C) C16RuQPIm as photosensitizers at [Ru(II )]=5.0×10−5 M and [CP]=2.5×10−2 M, with the number of runs in ethanol on the abscissa. The irradiation time of one cycle is 15 min for the Ru(bpy)32+ system and 20 min in 1st to 9th cycles and 10th and 11th cycles for polymer systems.

On the other hand, for the polymer photosensitizer systems, the initial reaction rate and the amount of oxygen consumption after irradiation for 20 min decreased with repeated light on-off cycles. Up to the fourth cycle, the amount of oxygen consumed was the same, indicating that the reaction was completed in 20 min. From the fifth to tenth cycles, however, the amount of oxygen consumed decreased, and the reaction was incomplete. When the irradiation was carried out for 30 min, the amount of oxygen consumed was the same in the first and second cycles. In the differential absorption spectra after irradiation for various cycles,56 the absorbance around 510 nm increased with repeated light on-off cycles.

Discussion

Ru(bpy)32+ and polymer photosensitizer systems

Fig. 8 shows the dependence of the initial reaction rate on the concentration of photosensitizer (A) and CP (B) for Ru(bpy)32+ and polymer photosensitizer systems. The initial reaction rate for the Ru(bpy)32+ system is similar to that for the polymer systems at a low photosensitizer concentration, whereas it is larger at a high photosensitizer concentration. The Ru(bpy)32+ photosensitizers do not interact with each other under the present experimental conditions. In contrast, it is known that these photosensitizers undergo an inter-polymer interaction between the alkyl side chains at high concentration,57–60 hence, these polymer photosensitizers would form a heterogeneous reaction field where the photo-oxidation reaction takes place. In the low photosensitizer concentration range, the photo-oxidation reaction for both Ru(bpy)32+ and polymer systems takes place in the homogeneous reaction field, consequently leading to similar reaction rates. In the high photosensitizer concentration range, the photo-oxidation reaction occurs in the homogeneous field for the Ru(bpy)32+ system, but the polymer photosensitizers provide a heterogeneous field for the reaction, giving a low initial reaction rate for the polymer systems. Furthermore, the inactivation of part of the ruthenium(II) complexes is a very important factor. With increasing concentration of the ruthenium(II) complex, the luminescence intensity increases linearly, and reaches a saturation value at high concentration, probably owing to self-quenching between the ruthenium(II) complexes.
(A) Dependence of initial reaction rate on concentration of ruthenium(II ) complex at [CP]=2.5×10−2 M. (B) Dependence of initial reaction rate on concentration of CP at [Ru(II )]=5.0×10−5 M: (●) Ru(bpy)32+; (■) C6RuQPIm; (▲) C16RuQPIm.
Fig. 8 (A) Dependence of initial reaction rate on concentration of ruthenium(II ) complex at [CP]=2.5×10−2 M. (B) Dependence of initial reaction rate on concentration of CP at [Ru(II )]=5.0×10−5 M: (●) Ru(bpy)32+; (■) C6RuQPIm; (▲) C16RuQPIm.

As shown in Fig. 8(B), the CP concentration dependence of the initial reaction rate for the Ru(bpy)32+ system is similar to the photosensitizer concentration dependence, but not for the polymer systems. This is the reason why these systems differ in the reaction field. The initial reaction rate for the Ru(bpy)32+ system is higher than that for the polymer systems at high CP concentration, but is lower than that for the polymer systems at low CP concentration. In particular, the initial reaction rate for C16RuQPIm system at [CP]=5.0×10−3 M is double that for the Ru(bpy)32+ system. This result is attributed to a concentration effect induced by incorporation of the CP species into the heterogeneous field formed by these polymer photosensitizers, as illustrated in Scheme 1.


Tentative representation of heterogeneous reaction field formed by polymer photosensitizers.
Scheme 1 Tentative representation of heterogeneous reaction field formed by polymer photosensitizers.

The heterogeneous reaction field, in which the alkyl side chains are closely packed, would have solvophobic properties, and is able to concentrate CP at low CP concentration. Consequently, the C16RuQPIm system demonstrates a high reaction rate at low CP concentration. Furthermore, the initial reaction rate is almost constant regardless of the CP concentration even when the latter increases, because the amount of CP which the heterogeneous reaction field can incorporate is limited. In addition, the fact that the degree of reaction for the polymer photosensitizer systems is almost 100% can be also explained by the same reasoning.

Effect of alkyl side chain length

Although the polymer photosensitizers form a heterogeneous reaction field, the solvophobic properties and the amount of CP incorporated into the field depend significantly on the length of the alkyl side chain of the polymers. The C6RuQPIm photosensitizer with short alkyl side chains forms a heterogeneous reaction field which has a low solvophobicity and a small size owing to electrostatic repulsion between the polymer chains and the small van der Waals interaction between the alkyl side chains. In contrast, the heterogeneous reaction field formed by the C16RuQPIm photosensitizer has a high solvophobicity and a large size. The steric hindrance of the long alkyl side chains decreases the electrostatic repulsion and increases the size of the field. In addition, the long alkyl side chains, which induce strong van der Waals interactions, bring about high solvophobicity, therefore leading to an increase in the amount of CP incorporated into the field.

Stability of photosensitizers

As mentioned above, the Ru(bpy)32+ photosensitizer demonstrates excellent stability. During repeated experiments on the photo-oxidation reaction, the initial reaction rate and saturated oxygen consumption hardly change. Furthermore, no change in the absorption spectra with visible light irradiation is observed.

Fig. 9 shows the change in the absorption spectrum of C16RuQPIm during the repeated experiments, together with differential absorption spectral change.61 Clearly, a red shift of the absorption maxima is observed, and the absorbance at 510 nm in the differential spectra increases, which indicates that these polymer photosensitizers change to the different complexes. It is well known that the bis(2,2′bipyridine)chloro-N-methylimidazolylruthenium(II) complex, [Ru(bpy)2(MeIm)(Cl)]2+, has an absorption maximum at 512 nm and luminescence maxima at 690 nm in methanol.54 Con sidering the fact, it appears that the polymer photosensitizers change to monochloro complexes [CnRu(Cl)QPIm (n=6 or 16)] in which one imidazolyl residue is substituted by a chloride ion that is the counter ion of the ruthenium(II) complex.62 Therefore, the decrease in reaction activity is caused by the formation of CnRu(Cl)QPIm. Although the CnRu(Cl)QPIm photosensitizers have a low reaction activity comparable to that of the original complexes (CnRuQPIms), they demonstrate the same maximum oxygen consumption as that of CnRuQPIms when the irradiation time is varied up to 30 min. Moreover, it is confirmed by the repeated experiments that the CnRu(Cl)QPIm photosensitizers also have excellent stability for the photo-oxidation reaction.


Change in absorption spectrum of C16RuQPIm during the repeated experiments. Inset: differential absorption spectral change.
Fig. 9 Change in absorption spectrum of C16RuQPIm during the repeated experiments. Inset: differential absorption spectral change.

Comparison with other photosensitizers

In order to make a comparison with other photosensitizers, the photo-oxidation reaction was carried out using Rose Bengal (RB) and zinc(II) phthalocyaninetetrasulfonic acid (ZnPTS) as the photosensitizers. These photosensitizers demonstrate excellent activity for the photo-oxidation reaction. The initial reaction rates are much larger than those of ruthenium(II) complex photosensitizers under the same conditions in the ruthenium(II) complex photosensitizer systems,63 and the reaction is completed within 10 min even when the RB and ZnPTS concentrations are 2.5×10−6 M. As shown in Fig. 10, however, the stability of these photosensitizers is very low: more than 50% of RB and ZnPTS has decomposed after the reaction. This is the reason why RB and ZnPTS are attacked by singlet oxygen.64 Furthermore, the degree of reaction for RB and ZnPTS is ca. 85%, which is similar to that of Ru(bpy)32+. These results indicate that the Ru(bpy)32+ and polymer-bound ruthenium(II) complexes have excellent stability comparable to that of RB and ZnPTS, although the reaction activity is low.
Absorption spectra of RB (dotted lines) and ZnPTS (solid lines) before and after the reaction at [photosensitizer]=2.5×10−6 M and [CP]=2.5×10−2 M.
Fig. 10 Absorption spectra of RB (dotted lines) and ZnPTS (solid lines) before and after the reaction at [photosensitizer]=2.5×10−6 M and [CP]=2.5×10−2 M.

Conclusion

We have carried out the photo-oxidation of cyclopenta-1,3-diene (CP) with singlet oxygen using Ru(bpy)32+ and polymer-bound ruthenium(II) complexes (C6RuQPIm and C16RuQPIm) as the photosensitizers in oxygen-saturated ethanol. These ruthenium(II) complex photosensitizers have excellent stability comparable to those of other photosensitizers such as Rose Bengal and ZnPTS, and can be repeatedly used for photo-oxidation. The polymer photosensitizer systems can oxidize CP more effectively than the Ru(bpy)32+ system. In particular, the C16RuQPIm system oxidizes all of the CP added even when the CP concentration is low, i.e., these polymer photosensitizers are effective at low concentrations of CP. This is attributed to the fact that the polymer photosensitizers form a heterogeneous reaction field in which CP species can be concentrated. Furthermore, these polymer photosensitizers can be separated and recovered from the reaction solution using a simple treatment, by precipitation through addition of acetone. In addition, the CnRu(Cl)QPIms return to the original complexes, CnRuQPIms, on refluxing in methanol.

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Footnote

† Research Fellow of the Japan Society for the Promotion of Science (JSPS Research Fellow).
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