Effect of potential energy gap between the n-π* and the π-π* state on ultrafast photoisomerization dynamics of an azobenzene derivative

We considered the trans --> cis photoisomerization dynamics of the S-2-excited azobenzene derivatives in terms of the potential energy gap between the n-pi* (S-1) and the pi-pi* (S-2) state. The photoisomerization dynamics of trans-4-aminoazobenzene (trans-4-AAB) which has an energy gap (3000-4000 cm(-1)) rather smaller than that of trans-azobenzene (similar to10000 cm-1) due to an amino-substitution was investigated using UV-vis transient absorption spectroscopy. The recorded transient absorption spectra of the (S-2) excited trans-4-AAB in ethanol and heptanol indicated that the pi-pi* state of the trans-4-AAB decays to the pi-pi* state with a time constant of 0.2 ps. Following this process, the n-pi* state decays to the ground state via two reaction pathways with time constants of 0.6 and 1.9 ps, and finally vibrational cooling of the ground state occurs with - 15 ps. The state decay dynamics of the pi-pi* excited trans-4-AAB showed good agreement with that of the pi-pi* excited trans-azobenzene. On the other hand, the n-pi* state decay dynamics of the pi-.pi* excited trans-4-AAB was apparently different from that of the pi-pi* excited trans-azobenzene, and it corresponds well to that of the trans-azobenzene directly excited to the n-pi* state. In addition, in the photoisomerization of the pi-pi* excited trans-4-AAB, we could not observe the solvent dependence of the n-pi* state lifetime which has been reported in the photoisomerization of the pi-pi* excited trans-azobenzene. On the basis of the potential energy diagram of azobenzene, we considered that the similarity of the n-pi* state decay dynamics observed for the excited trans-4-AAB and the pi-pi* excited trans-azobenzene originated from the small potential energy gap between the and the n-pi* states of 4-AAB. These results indicated that not only the photoexcitation condition (pi-pi* excitation or n-pi* excitation), but also the potential energy gap between the pi-pi* and the n-pi* states is an important factor which determines the trans - cis photoisomerization dynamics of azobenzene derivatives.