Excited-state dynamics of trans- and cis-azobenzene after UV excitation in the ππ* band

Transient absorption measurements on cis- and trans-azobenzene after UV excitation in the pipi* band are presented and compared to data obtained after VIS excitation in the npi* band. The data show a two-step process, where after a fast motion on the S-2 potential energy surface the molecule relaxes to the S-1 state and in both cases (trans --> cis and cis --> trans) the isomerization reaction involves large-amplitude motion on the S-1 potential energy surface.