Sub-picosecond time-resolved absorption spectroscopy of a push–pull type p,p′-substituted trans-azobenzene

Relaxation dynamics of an azobenzene derivative with push–pull type substituents trans-(4-methoxyphenylazo)-4′-nitrobenzene (trans-MNAB) upon the S2 ← S0 excitation in acetonitrile was studied by sub-picosecond time-resolved transient absorption spectroscopy which exhibited a fast formation of the lowest excited singlet (S1) state within 1 ps and a vibrational cooling of the ground state (S0). All the events of the S1 state completed within <5.0–6.0 ps, suggesting that isomerization of trans-MNAB proceeds mainly via the S1 state. The absence of remarkable substituent effect on the dynamics of the excited states strongly suggested the absence of the rotational cis–trans isomerization mechanism via the S2 state.