Photolysis of 1-bromo-2,2-bis(p-methoxyphenyl)ethene in solution

Abstract

UV irradiation (λ_inc= 254 nm) of 1-bromo-2,2-bis(p-methoxyphenyl)ethene (AAHVB) in acetonitrile solution results in its decomposition: ϕ(–AAHVB)= 0.18 both in the absence and presence of O₂. Apart from the isomers [(E)-1-bromo-1,2-bis(p-methoxyphenyl)ethene (HAAVB) and (Z)-1-bromo-1,2-bis(p-methoxyphenyl) ethene (AHAVB)] the main photoproducts are 1,2-bis(p-methoxyphenyl)ethine (P-2) and HBr (ϕ≈ 0.1). Flash photolysis studies in acetonitrile, dichloromethane and n-hexane solution at 266 nm revealed that P-2 is formed with a halflife of less than a few ns via a singlet route involving vinyl cations which rapidly rearrange and split off protons. Part of the vinyl cations decay via a different route forming an ionic species with an optical absorption band at ca. 350 nm. The decay of the latter is not correlated with the formation of P-2. This ionic species probably results from an interaction of vinyl cations with solvent or AAHVB. AAHVB quenches triplet states of various compounds X (e.g. benzophenone, chrysene, benzil, pyrene) provided that E_T(X) > 200 kJ mol^–1. The energy transfer from these compounds to AAHVB does not result in the formation of P-2 or vinyl cations.