Photoionization of β-carotene via electron transfer from excited states to chlorinated hydrocarbon solvents. A picosecond transient absorption study

Abstract

Time-resolved photoinduced (λ_exc 266, 355 and 532 nm) formation of the radical cation of β-carotene in the chlorinated solvents n-butylchloride, dichloromethane, 1,2-dichloroethane, chloroform and carbon tetrachloride, was studied with 5–10 ps time resolution. The radical cation is formed via electron transfer from the electronically excited solute to the solvent and is characterized by an absorption band with maximum at 900–950 nm. The process is efficient (ϕ > 0.12) and fast (t < 10 ps), which excludes involvement of the triplet state. It is suggested that the electron transfer proceeds from either the vibrationally excited singlet 2A⁻_g (l  2) or from the “forbidden” 1B⁻_u state. The formation times of the carotene radical cation in the chlorinated hydrocarbon solvents are on the order of their diffusion times.