Excitation wavelength dependence of the Z → E one-way isomerization of 4-halostilbene radical cations in excited states

Excitation of (Z)-4-halostilbene radical cations ((Z)-XSt+; X = F, Cl, Br) around 500 or 700 nm induced their rapid one-way isomerization to the corresponding (E)-radical cations, as observed in two-color pulsed laser photolysis of (Z)- and (E)-XSt in acetonitrile containing 9,10-dicyanoanthracene and biphenyl as electron-transfer sensitizers. The efficiencies of isomerization and quenching by biphenyl as electron donor are discussed on the basis of the electronic configurations and energy surfaces of the relevant excited states.