Relaxation dynamics of an azobenzene derivative with push-pull type substituents trans-(4-methoxyphenylazo)-4'-nitrobenzene (trans-MNAB) upon the S-2 <-- S-0 excitation in acetonitrile was studied by sub-picosecond time-resolved transient absorption spectroscopy which exhibited a fast formation of the lowest excited singlet (S-1) state within I ps and a vibrational cooling of the ground state (S-0). All the events of the S-1 state completed within <5.0-6.0 ps, suggesting that isomerization of trans-MNAB proceeds mainly via the S, 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 S-2 state. (C) 2004 Elsevier B.V. All rights reserved.