Triplet state properties of dichlorobenzenes

The triplet-triplet absorption spectra have been determined by laser flash photolysis for ortho, meta and para dichlorobenzene. The wavelength of maximum absorption λ_max was 290 nm for both the ortho and meta isomers and was 310 nm for the para isomer. The triplet decay was studied in cyclohexane and in methanol. It was found to be a function of the initial triplet concentration and the rate constants for triplet-triplet annihilation and the first-order decay were obtained. The relative triplet yields in methanol were determined by energy transfer to anthracene, and absolute quantum yields were estimated. From this and comparison with literature data for the photodecomposition quantum yields, it was concluded that the photochemically active state in methanol is the excited singlet.