One-way isomerization and internal rotation in anthrylethylenes in the excited triplet state

1-(2-Anthryl)-2-(bromophenyl)ethylene and 1-(2-anthryl)-2-(4-methoxyphenyl)ethylene undergo cis→trans one-way isomerization in the excited triplet state through an adiabatic process from the cis-triplet to the trans-triplet states. The trans-isomers of these compounds undergo one-way internal rotation in the excited triplet state with an activation barrier of }7 kcal mol^?1 and a frequency factor of }10¹² s^?1, while no internal rotation takes place in the excited singlet state.