Methylviologen radicals

Figure 5. An idealized mechanism of photoinduced electron transfer from CdS conduction band to methylviologen (MV +)( resulting in formation of methylviologen radical cation (MV,+). The colloidal CdS particle as represented, was generated at the inside surface of the DHP vesicle. Its exact location is based on fluorescence quenching experiments (Figure 5). Inserts oscilloscope trace showing the formation of MV by the absorbance change at 396 nm, after a laser pulse at 355 nm.

Figure 18. Time-resolved diffuse reflectance spectra 12-150 ps after excitation from 532 nm laser pulse for Ru(bpy)3 -(CH2)5-MV- on mordenite. Inset shows the decay of methylviologen radical followed at 400 nm.

The results obtained in a comprehensive study of the oxidation of Eu(II) by the title oxidants have been recently reported by Schwane and Thompson (1989). These investigators compared the rates of reactions of Eu(II) with those determined for Fe(Il), SOj and the methylviologen-radical cation. They concluded that the predominant factor that characterizes the reaction rates is not the free energy change of the reaction. 

The formation of SOi in the reaction of reduced cytochrome P-450 with bisulfite in an aqueous solution near pH 7 has also been observed [26]. The reaction was very slow, with the E8R signal due to 802 increasing over a period of several hours. This reaction was probably not with H8O3", but rather with the small amount of free SO2 in the solution. This would be consistent with the observation that whereas 8O2 is reduced by the methylviologen radical anion with a near diffusion-controlled rate constant, 1.2 x 10 L mol" s", H8O3" reacts slowly, if at all with that radical anion [25]. 802" may also be an important intermediate leading to the formation of elemental sulfur in the radiolysis of aqueous 8O2 solutions [27]. 

Methylviologen radicals have been generated via the reaction of with 2-propanol and the rate constants for the reaction of MV" with pentaamminecobalt(III) complexes of pyridine, bipyridine, and their derivatives measured.Possible mechanisms are considered. 

Dai S., Sigman M.E., Burch E.L. Preparation of a photochromic glass doped with methylviologen radical cation via a sol-gel process. Chem. Mater. 1995 7 2054-2057 del Monte F., Levy D. Formation of fluorescent rhodamine B J-dimers in sol-gel glasses induced by the adsorption geometry on the silica surface. J. Phys. Chem. B 1998 102 8036-8041 Dunn B., Mackenzie J.D., Zink J.I., Stafsudd O.M. Solid-state tunable lasers based on dye-doped sol-gel materials. Proc. SPIE vol. 1328 Sol-Gel Optics 1990 1328 174-182 Dunn B., Zink J.I. Probes of pore environment and molecule-matrix interactions in sol-gel materials. Chem. Mater. 1997 9 2280-2291... 

The added electron is delocalized on the monovalent radical ion to which it is reduced (3). There is no general agreement on the molecular representation of the reduced stmcture. Various other viologen compounds have been mentioned (9,12). Even a polymeric electrochromic device (15) has been made, though the penalty for polymerization is a loss in device speed. Methylviologen dichloride [1910-42-5] was dissolved in hydrated... 

For the QPh-x-MV2+ system, the methylviologen cation radical (MV K) generated by laser photolysis decayed with a rate constant of kb = 3.2 x 108 M-1 s-1. This relatively strong retardation of the back ET is due to the electrostatic repulsion of MV + by the polycation [76]. 

Another type of dimer is that which consists of two radical molecules stacked on each other in a n-n interaction. Such dimers have been observed e.g., with 9-ethylphenazyl radical, tetramethyl-p-phenylenediamine cation radical (167), 7,7,8,8-tetracyanoquinodimethane radical anion (168), methylviologen cation radical (169), and l-alkyl-4-carbomethoxypyridinyl radicals (170). Attempts have been reported (170, 171) to interpret the electronic spectra of dimers of this kind by MO calculations. 

Work by Harbour, Chow and Bolton (1974) on the spin adducts of superoxide (or HOO )13 with nitrones paved the way for a number of investigations of superoxide and hydroperoxyl radical chemistry. Harbour and Bolton (1975) used DMPO to trap superoxide formed by spinach chloroplasts in the presence of 02. The signal strength was greatly enhanced when methylviologen was present, consistent with the hypothesis that this bis-pyridinium dication accepts an electron from the primary acceptor of photoprotein I, and then transfers it to molecular oxygen. 

Studies in cation-radical chemistry began with works by Weiss (1941) and Michaelis et al. (1941). However, intense investigations in this field started after instrumental methods emerged (ESR and optical spectroscopy in particular). Optical spectroscopy revealed a very intriguing case where direct sublimation of methylviologen (l,l -dimethyl-4,4 -dipyridinium) dichloride provokes the formation of methylviologen cation-radical monochloride. This phenomenon was established by Poizat et al. (1984), but remains unexplained. 

It can be seen that the coupling of the formation and decay processes increases with the width of the flash. In an intermediate case, the time dependence of the absorbance change will have the functional form of a double exponential, A exp(—f/i ) + B exp (—tlx"). One lifetime will be close to the lifetime of the transient species and the other to the lifetime of the pump. In the most unfavorable conditions, the functional form will be a single exponential with nearly the lifetime of the pump. The determination of the lifetime of a transient species formed by the decay of transformation of an excited state offers a similar difficulty. The reduction of methylviologen, MV2 +, by the metal to ligand charge transfer (MLCT) state19 of the Re(I) complex and the reoxidation of the produced radical, MV +, are illustrated in Equations 6.57-6.59. 

A more significant change obtains in the presence of an electron-transfer reagent such as N-methylviologen. In this case 3 is formed in 21% yield together with 6a-fluoro-4-cholestenone-3 (8%) and 6,6-difluoro-4-cholestenone-3 (13%). Use of a four-fold excess of reagent 1 increases the yield of 3 to 33 %. Apparently two radical pathways are possible, both involving ArIF-. 

In an important step to mimic the natural photosystem, tyrosine residues tethered to a Rubpy sensitizer as in 15 have been shown to reduce the Rum center obtained after oxidative quenching with methylviologen or [Co(NH3)5C1]2+.187 Formation of the resulting tyrosyl radical is a proton coupled process and it has been shown to be a concerted process in which the reorganization energy associated with deprotonation can be tuned by H-bonding and pH.188191 Similar results are observed for tyrosyl residues tethered to Re(I)diimine based chromophores.192... 

Here, MV " is methylviologen and the reaction rate of each process is denoted above. The MFEs on the formation of free radicals (Te) was measured by photostationary illumination in a continuous flow system. The observed relative MFEs (AR B)=[Ye(B)- Ye(PTY]/ Ye OTY) on the yield of photoinduced MV" radical with the four complexes (la-ld) is depicted in Fig. 12-13. This figure shows that the Te value of each complex decreases with increasing B from 0 T to 15 T and that it is almost saturated above 15 T. It is noteworthy that the AR(E) value decreases regularly for every replacement of a bipyridine ligand by a phenanthroline one. 

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