Photochemistry of Methyl Viologen in Aqueous and Methanolic Solutions

Article abstract of DOI:10.1021/j100242a028

The photochemistry of MV(2+)(Cl(1-))2 (methyl viologen; paraquat; 1,1'-dimethyl-4,4'-bipyridinium dichloride) has been investigated in both H2O and CH3OH.It is shown that the observed photochemistry occurs via the ground-state charge-transfer complexation with the halide counterion(s).In H2O, the radical pair MV(1+).Cl2(1-). is formed with a quantum yield of 0.2 from the dichloride complex (K_2Cl=1.4 M^-1) with a rate > 1E8 s ^-1.The MV(1+).Cl2(1-). formation and decay is pH independent (1.5 to 9), the kinetics of the disappearance being second order.When the Cl(1-) counterions are replaced by Br(1-) (K_2Br= 1.8 M^-1) or by I(1-) (K_2I= 3.2 M^-1) the quantum yield drops dramatically (<0.01).By comparison with Cs(1+) it is concluded that an intracomplex heavy-atom effect is observed.In methanol, where the end product is MV(1+).Cl(1-), it is shown that the radical pair MV(1+).Cl. must be an intermediate in the formation of the initial methoxy radical and MV(1+)., the observed quantum yield of the latter being strongly dependent on the chloride concentration.Furthermore, in agreement with previous suggestions in the literature, the results indicate that a weak methyl viologen chloride-methanol complex is involved in the MV(2+)(Cl(1-))2 photochemistry in methanol.