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

Abstract

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 1 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 1 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.