Femtosecond/Picosecond Time-Resolved Spectroscopy of trans- Azobenzene: Isomerization Mechanism Following S2(ππ*) ← S0Photoexcitation

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Abstract
Photoisomerization dynamics and the electronic relaxation process of trans-azobenzene after the S2(ππ*) ← S0 photoexcitation were investigated in solution by femtosecond and picosecond time-resolved spectroscopy (UV-visible absorption, Raman, and fluorescence). Femtosecond time-resolved absorption spectrosocopy was performed to observe the transient absorption of the S2 and S1 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 ← S2(ππ*) absorption. After the decay of the S2 state, a transient absorption showing peaks at 410 nm and 500 nm was observed, which was ascribable to the S1 state. This transient absorption is similar to the Sn ← S1 absorption that is observed after S1 ← S0 photoexcitation. Picosecond time-resolved Raman measurements were carried out to obtain information about the molecular structure of azobenzene in the S1 state. The NN stretching frequency in the S1 spectrum was determined with use of 15N-substituted azobenzene, and it was found that the NN stretching frequency in the S1 state is very close to that in the S0 state (1428 cm-1 in the S1 and 1440 cm-1 in the S0). This fact indicated that the NN bond retains a double bond character in the S1 state. A strong similarity was also found between the S1 and S0 Raman spectra. The double bond nature of the NN bond as well as the similarity between the S1 and S0 Raman spectra indicates that the observed S1 state has a planar structure around the NN bond. The Raman data indicate that the observed S1 state is not a twisted excited state that appears during the rotational isomerization, but is the excited state that is populated during the S2 → S1 → S0 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 S1 state was highly vibrationally excited. It was also observed that the hot bands due to the S0 state appear after the decay of the S1 state and these S0 hot bands disappear with a time constant of ∼16 ps in hexane. Femtosecond time-resolved and steady-state fluorescence measurements were performed and they revealed that the S2 → “planar” S1 relaxation process is the major relaxation pathway following S2 photoexcitation. The quantum yield of the S2 → “planar” S1 electric relaxation was evaluated by comparing the intensity of the S2 and S1 fluorescence, and it was found to be almost unity. A series of time-resolved spectroscopy demonstrated that the S2 rotational isomerization mechanism, which had been believed so far, does not exist. It has been clarified that the isomerization occurs in the S1 state after S2 → S1 relaxation. Consequently, it is concluded that the isomerization of azobenzene takes place in the S1 state by inversion in cases of both S2 and S1 photoexcitation.