Femtosecond/picosecond time-resolved spectroscopy of trans-azobenzene:: Isomerization mechanism following S2(ππ*)←S0 photoexcitation

Photoisomerization dynamics and the electronic relaxation process of trans-azobenzene after the S-2(pipi(*))<--S-0 photoexcitation were investigated in solution by femtosecond and picosecond time-resolved spectroscopy (UV-visible absorption, Raman, and fluorescence). Femtosecond time-resolved absorption spectrosocopy was per-formed to observe the transient absorption of the S-2 and S-1 states. Immediately after photoexcitation, a very broad transient absorption peaked at 475 and 600 nm was observed. This transient absorption decayed repidly within 0.5 ps, and this ultrafast component was attributed to the Sn<--S-2(pipi(*)) absorption. After the decay of the S-2 state, a transient absorption showing peaks at 410 nm and 500 nm was observed, which was ascribable to the S-1 state. This transient absorption is similar to the S-n<--S-1 absorption that is observed after S-1<--S-0 photoexcitation. Picosecond time-resolved Raman measurements were carried out to obtain information about the molecular structure of azobenzene in the S-1 state. The NN stretching frequency in the S-1 spectrum was determined with use of N-15-substituted azobenzene, and it was found that the NN stretching frequency in the S-1 state is very close to that in the S-0 state (1428 cm(-1) in the S-1 and 1440 cm(-1) in the S-0). This fact indicated that the NN bond retains a double bond character in the S-1 state. A strong similarity was also found between the S-1 and S-0 Raman spectra, The double bond nature of the NN bond as well as the similarity between the S-1 and S-0 Raman spectra indicates that the observed S-1 state has a planar structure around the NN bond. The Raman data indicate that the observed S-1 state is not a twisted excited state that appears during the rotational isomerization, but is the excited state that is populated during the S-2-->S-1-->S-0 relaxation process while retaining a planar molecular structure. Anti-Stokes Raman measurements were performed to obtain information about the vibrational relaxation process. The anti-Stokes Raman spectra showed that the S-1 state was highly vibrationally excited. It was also observed that the hot bands due to the S-0 state appear after the decay of the S-1 state and these S-0 hot bands disappear with a time constant of similar to16 ps in hexane. Femtosecond time-resolved and steady-state fluorescence measurements were performed and they revealed that the S-2-->planar S-1 relaxation process is the major relaxation pathway following S-2 photoexcitation. The quantum yield of the S-2-->planar S-1 electric relaxation was evaluated by comparing the intensity of the S-2 and S-1 fluorescence, and it was found to be almost unity. A series of time-resolved spectroscopy demonstrated that the S-2 rotational isomerization mechanism, which had been believed so far, does not exist. It has been clarified that the isomerization occurs in the S-1 state after S-2-->S-1 relaxation. Consequently, it is concluded that the isomerization of azobenzene takes place in the S-1 state by inversion in cases of both S-2 and S-1 photoexcitation.