Flash Photolysis and Electron Spin Resonance Study of p-Chloranil

Abstract

The photochemistry of p-chloranil in dioxan and in isopropyl alcohol was re-investigated by a combination of flash-optical e.s.r.-rotating sector techniques. The transient photoradical is identified as the p-tetrachlorobenzosemiquinone neutral radical based on e.s.r. and kinetic evidence. The radicals decayed by self-disproportionation with a second order rate constant of approximately 8.2 x 108 M-1 s~l in dioxan and about 1.7 x 108 M-1 s-1 in isopropyl alcohol. The combined optical e.s.r. data yielded the following optical molar extinction coefficients for the neutral semiquinone radicals in dioxan: 7.3 x 103 M-1 cm-1 (420 nm); 7 7 x 103 M'1 cm~l (435 nm) and 6.0 x 1000 M-1 cm-1 (448 nm).