Cation radicals: photochemically induced electron-transfer reactions between bipyridylium dications and formate, oxalate, and benzilate anions

Abstract

Aqueous solutions of the herbicidally active bipyridylium salts, paraquat dichloride and diquat dichloride, containing formate, oxalate, and benzilate anions undergo photoreduction to the bipyridylium cation radicals with high quantum efficiencies. Photoactivity occurs with light of wavelengths longer than can be absorbed by either oxidant or reductant, taken separately, and it is concluded that charge-transfer interaction between bipyridylium ion and carboxylate ion is the major activating influence. The three carboxylate anions are effective quenchers for the fluorescence of diquat suggesting a possible additional mechanism for photoreduction of the bipyridylium salts via interaction with their excited singlet states. SEVERAL types of bipyridylium salts have been shownto have interesting herbicidal properties 1 and the most important of these are exemplified by paraquat dichloride (I) (1,l '-dimethyl-4,4'-bipyidylium dichloride) and diquat dibromide (111) (6,7-dihydrodipyrido[1,2a : 2',1'-c]pyrazinedi--ium dibromide). Herbicidal activity appears to depend, in part, on the ease of (reversible) one-electron reduction of (I) and ( 111) to form stable but air-sensitive cation-radicals ( 11) and (IV) respectively.2 One-electron reduction of bipyridylium salts inay be achieved electro~hemically,~ by a variety of chemical reducing and also, as we have recently shown,6 by photolysis in the presence of primary and secondary alcohols. A particularly interesting observation from the latter type of reaction was that photoexcited bipyridylium dications react with alcohols by a primary electron-transfer process leading, in turn, to development of bipyridylium salts and their cation-radicals as new types of radical scavenger^.^^^

IONIC STRENGTH EFFECTS ON THE GROUND STATE COMPLEXATION and TRIPLET STATE ELECTRON TRANSFER REACTION BETWEEN ROSE BENGAL and METHYL VIOLOGEN

“…(1) are important for our complexation studies. Although values for K , of 0.1 and 0.7 rnol dm-3 have been reported by Barnett et al (1973) and Ebbesen and Ferraudi (1983), the value of Kz in water does not appear to have been determined, We found that MV+(CI-)2 behaves as a strong electrolyte over the concentration range 2 X W2-5 X rnol dmP3, with a molar conductivity of 253 (k 2) S cmz mol-' at infinite dilution which drops to 220 (k 2) S cmz mol-' at a concentration of 1 x rnol dm-3. These molar conductivities are consistent with full dissociation of the viologen dichloride to M V + and 2C1-, and because we observed no deviations from the behaviour expected of a strong electrolyte, K2 must be greater than about 0.2 mol dm-3, which puts it in the same range as K 1 .…”

Pulsed-laser flash and continuous photolysis of ion-pair complexes of methyl viologen and ethylenediamine tetra-acetic acid in aqueous solution

“…This result suggests that the optimum structure of the aggregate, regarding the value of 4, exists at these concentrations. It could be argued that reaction (13), involving an intramolecular transformation, is relatively independent of aggregate structure, whilst reaction (12), being intermolecular in nature, depends subtly on the distance between the geminate pair within the aggregate. Because the highest values of @(MV'+) occur at 20mmoldm-3 MV2+ and 0.10moldm-3 EDTA, we focus our attention on the behaviour of the system under these conditions in order to understand further the properties of the species within the optimum aggregate structure.…”

“…The positive dependence of @(MV'+) on temperature suggests, although it does not prove, that + has a positive temperature coefficient; the dependences of # and 7 on temperature could be opposite, but offsetting. If d, indeed, has a positive temperature coefficient and the rate constants for reactions (12) and ( 13) can be written in the form of the usual Arrhenius expression [ k = A exp(-E,/RT)], then + = (AI3/Al2) exp(-AEIRT), where AE = EaI3 -Ea12 and Ea13 > EaI2. The unusual aspect of the results presented here then becomes the effect of the excitation wavelength on the temperature dependence and one would not expect the temperature coefficients of r) and Ea13 to exhibit such an effect.…”

The reactions of paraquat and divalent metal ions with humic acid: Factors influencing stoichiometry

“…Early work using a wide variety of phenolic compounds revealed charge transfer complexes with paraquat, for which the formation constants were small (Ledwith and Woods, 1970;White, 1969;Ebbeson and Ferraudi, 1983;Barnett et al, 1972;Kuczynski et al, 1984). The synthetic polyelectrolytes poly(styrene sulphonate), poly (vinyl sulphonate) and polymethacrylate were all found to bind paraquat by simple electrostatic attraction, in the same way that they bind alkaline earth metal ions (Baumgartner et al, 1987;Kobayashi and Niki ;.…”