Ultrafast Excited State Dynamics of trans-4-Aminoazobenzene Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast excited state dynamics of trans-4-aminoazobenzene (trans-4-AAB) in ethanol was investigated by femtosecond transient absorption spectroscopy. After being excited to the S2 state by 400 nm, trans-4-AAB decays from the S2 state to the hot S1 state by internal conversion with time constant of ∼70 fs. The photoisomerization through inversion mechanism on the S1potential energy surface and the internal conversion from the S1 state to the hot S0 state are observed, respectively. The average timescale of these two decay pathways is ∼0.7 ps. And the vibrational cooling of the hot S0 state of cis- and trans-4-AAB occur with time constants of ∼4 and ∼13 ps, respectively. Furthermore, the ultrafast intersystem crossing process are also observed. The timescale of intersystem crossing from the S2 state to the T4 state is about 480 ps while from the S1 state to the T2 state is ∼180 ps.