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 × 10⁸ M^–1 s^–1 in dioxan and about 1.7 × 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 semi-quinone radicals in dioxan: 7.3 × 10³ M^–1 cm^–1(420 nm); 7.7 × 10³ M^–1 cm^–1(435 nm) and 6.0 × 10³ M^–1 cm^–1(448 nm).