Laser flash photolysis of chlorobenzene in polar solvents

Abstract

The laser flash photolysis of chlorobenzene at 266 nm was studied in methanol (MeOH) and other polar solvents. The triplet state properties in MeOH were determined: φ T = 0.7 ± 0.2; λ max = 305 nm; ϵ 305 = 6500 ± 800 M −1 cm −1; τ = 715 ± 20 ns. In the presence of LiCl a long-lived transient absorption with λ max = 350 nm was observed and it was assigned to Cl -rad 2. Part of the absorption was formed immediately after the laser pulse and a slow growth was also observed in the following microsecond. This was interpreted as being partly due to a singlet state very fast CCl bond homolysis and partly due to a triplet-mediated generation of Cl·. Also the characteristic absorption of solvated electrons was found in MeOH. A photo-ionization quantum yield of 0.03 ± 0.01 was estimated. The triplet lifetime was determined in various solvents and it was found to decrease with increasing dielectric constant. The photolysis mechanism of chlorobenzene is discussed in terms of a singlet state photosubstitution reaction with the radical cation as an intermediate, and a singlet and triplet homolytic CCl bond breakage.