Photoinduced electron transfer viologen

K. A. Jolliffe, T. D. M. Bdl, K. P. Ghiggino, S. J. Langford, M. N. Paddon-Row, Efficient Photoinduced Electron Transfer in a Rigid U-Shaped Tetrad Bearing Terminal Porphyrin and Viologen Units , Angew. Chem, Int. Ed. 1998,37, 916-919. 

With the aim of mimicking, on a basic level, the photoinduced electron-transfer process from WOC to P680+ in the reaction center of PSII, ruthenium polypyridyl complexes were used (182-187) as photosensitizers as shown in Fig. 19. These compounds are particularly suitable since their photophysical and photochemical properties are well known. For example, the reduction potential [Rum(bpy)3]3+/-[Run(bpy)3]2+ (bpy = 2,2 -bipyridine) of 1.26 V vs NHE is sufficiently positive to affect the oxidation of phenols (tyrosine). As traps for the photochemically mobilized electron, viologens or [Co(NH3)5C1]2+ were used. 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

The effect of temperature on the photoinduced electron transfer from [Ru(bpy)3]2+ to methyl viologen solubilized in cellophane has been investigated 98 K The first-order rate constant which depends exponentially on the distance between the reactants shows a non-Arrhenius type of behavior in the temperature interval from 77 to 294 K. This phenomenon, previously found to be of great importance in biological systems, is quantitatively interpreted in terms of a nonadiabatic multiphonon non-radiative process. 

Figure 7. Photoinduced electron transfer in monolayer systems with the cyanine dye CY as donor and the viologen derivative SV as acceptor. Relative fluorescence intensity of the donor monolayer vs. donor density at constant acceptor density. Bars Donor and acceptor at the same interface, density of A, o(A) = 0.01 nm". Circles donor and acceptor at different interfaces, distance 2.3 nm, o(A) = 0.43 nm-. ...

Photoinduced electron transfer rates can be considerably reduced when the counterion X- is changed from chloride to bromide. Charge transfer between the cationic part of a molecule and the bromide ion may be responsible to the reduction of photoinduced electron-transfer rates. Such a counterion effect on the photoinduced electron transfer and the reverse process has been demonstrated for examples of porphyrin-viologen-linked compounds (Mitsui et al. 1989). 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

Figure 27. Schematic representation of the reconstituted Mb electron acceptor composite via electrostatic interaction. The photoinduced electron transfer occurs from the reconstituted zinc Mb to electron acceptor such as methyl viologen, anthraquinone disulfonate, cytochrome c within the stable complex.

Excitation of the complexes leads to photoinduced electron transfer from the excited ruthenium polypyridyl site to the viologen acceptor. The Ru2+ site is restored through electron transfer from the TEOA or back-electron transfer from the bipyridine, while the viologen is oxidized by the electrode, thus generating the photocurrent. As illustrated in Figure 5.51, this mechanism is supported by experiments in which the electron acceptor 4ZV (see Figure 5.50) reduced the... 

In summary, the stereoselectivity was certainly observed in the photoin-duced electron transfer reactions of chiral ruthenium(II) complexes with chiral viologen and Co(III) complexes. However, not only the photoinduced electron transfer reaction but also the charge separation in the encounter complex and the reverse reaction between the ruthenium(III) complex and Co(acac)2 + acac participate in the stereoselection. In the reactions between the ruthenium(II) and Co(III) complexes, the energy transfer also contributes to the quenching reaction, which makes difficult the observation of stereoselectivity in the quenching reaction. 

These bis-viologenes were also applied to the stereoselective photoinduced electron transfer reactions with ZnMb [68]. Though the quenching reaction of... 

An example of such a system is dyad 34, consisting of a zinc porphyrin (Pzn) linked to a viologen-like moiety, pyridylpyridinium (V+), which is also a reasonably good electron acceptor (-0.71 V versus SCE) [189]. Excitation of the porphyrin yields Pzn-V+, which decays by photoinduced electron transfer to give Pzn" -V ... 

Another example of modulation of the electronic interaction between two metal units is schematically represented by compound 41 in Scheme 2(d), for which upon complexation of the vacant 2,2 -bipyridine a perturbation of the luminescence properties of the ruthenium moiety was reported [100]. Methylation caused a quenching of the emission, most likely due to photoinduced electron transfer from the terminal chromophores to the central viologen-type unit. 

Laser-photoinduced electron transfer in the three tetraphenylporphyrin-bound viologens 34—36 in reversed micelles led to radical pairs whose chemically induced dynamic electron polarization (CIDEP) spectra at room temperature proved dependent on the length of the spacer. 37... 

The matrix molecules of the mixed dye and acceptor monolayers have been omitted for clarity. Absorption spectrum of the viologen radical formed by the photoinduced electron transfer under nitrogen atmosphere, measured with an assembly of 5 dye-acceptor units with molar ratios of dye arachidate = 1 20 and acceptor to arachidate = 1 10. 

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