Methyl viologen chemical reactions

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

However, in certain cases, the rate of electron uptake by a particular species just happens to be slow. For example, electron transfer between the methyl viologen radical cation (MV ) and hydrogen peroxide has a rate constant of 2.0 (mol dm ) s , while the reaction between MV and just about any other chemical oxidant known is so fast as to be dijfusion-controlled. The reason for this is simply not known at the present time. 

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... 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

Few reactions of electrically neutral organic compounds by photoelectro-chemical means have been reported, and reduction has been observed only when oxygen has been specifically removed [91]. Reduction of a cationic organic substrate by conduction band electrons can occur, as in the case of methyl-viologens, for example [91, 92]. Chlorinated organic acids which have no oxidizable hydrogen, such as trichloroacetic acid were found to degrade, albeit in small yields [90]. This was attributed to oxidation via the valence holes which is known as a photo-KoIbe process (Scheme 33). 

MV2+ acceptors and SCN electron donors in solution [43], Colloidal semiconductor particles, typically of ca. 10-100 nm diameter, in aqueous sols may be treated as isolated microelectrode systems. Steady-state RRS experiments with c.w. lasers can be used to study phototransients produced at the surfaces of such colloidal semiconductors in flow systems [44], but pulsed laser systems coupled with multichannel detectors are far more versatile. Indeed, a recent TR3S study of methyl viologen reduction on the surface of photoex-cited colloidal CdS crystallites has shown important differences in mechanism between reactions occurring on the nanosecond time scale and those observed with picosecond Raman lasers [45]. Thus, it is apparent that Raman spectroscopy may now be used to study very fast interface kinetics as well as providing sensitive information on chemical structure and bonding in molecular species at electrode surfaces. 

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect >99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

---