Mechanism of formation of the MV center dot+ radical during the UV excitation of methylviologen

The UV illumination of diluted aqueous solutions of methylviologen (MV2+) l eads to the formation of the methylviologen radical (MV.+), without the use of microheterogeneous materials or sacrificial electron donors. In the abs ence Of O-2 the MV.+ radical remains in solution for hours. The formation m echanism of MV.+ was investigated employing the laser flash photolysis tech nique. The hydrated electron (e(aq)(-)) band and the characteristic bands o f the MV.+ radical at 395 and 605 nm were detected in the MV2+ transient ab sorption spectra. This indicates that (1) MV2+ is ionized at 266 nm and (2) the e(aq) (-) + MV2+ reaction is a source of the MV.+ radical. The rate co nstant for this reaction was measured at pH 6.5 to be (3.8 +/- 0.1) x 10(10 ) M-1 s(-1) and its magnitude is in good agreement with literature values. The ionization quantum yield of 0.16 was determined for MV2+ using KI as a reference actinometer. The photodestruction quantum yield of MV2+ was calcu lated to be (8.7 +/- 0.5) x 10(-3) molecules/quantum. The formation quantum yield of MV.+ was measured at the two absorption maxima of the radical, ph i (395 nm) = (4.8 +/- 0.5) x 10(-3) and phi (605 nm) = (5.1 +/- 0.3) x 10(- 3) molecules/quantum. (C) 2001 Elsevier Science B.V. All rights reserved.