Picosecond time-resolved Raman study of trans-azobenzene

The electronic and vibrational relaxation of photoexcited tuans-azobenzene was investigated in solution by picosecond time-resolved Raman spectroscopy. Picosecond time-resolved Raman spectra were measured with the probe wavelength at 410 nm, which is in resonance with a transient absorption appearing after the S-2(pi pi*) <-- S-0 photoexcitation. Several transient Raman bands assignable to the S-1 state were observed immediately after photoexcitation. The lifetime of the S-1 state showed a significant solvent dependence, and it was determined as similar to 12.5 ps in ethylene glycol and similar to 1 ps in hexane. Time-resolved anti-Stokes Raman measurements were also carried out for hexane solutions to obtain information about vibrational relaxation process. The anti-Stokes spectra showed that the observed S-1 state was highly vibrationally excited. In addition, several anti-Stokes Raman bands due to the S-0 state were observed after the decay of the S-1 state, indicating that the vibrationally excited So state was generated after electronic relaxation in hexane. The lifetime of vibrationally excited So azobenzene was evaluated as similar to 16 ps by the analysis for the intensity change of the anti-Stokes NN stretch band. The assignment of the NN stretch band in the S-1 spectrum was made by using N-15-substituted azobenzene, and it was clarified that the NN stretching frequency in the Si state is very close to that of the S-0 state (1428 cm(-1) in S-1 and 1440 cm(-1) in S-0). The high NN stretching frequency in the S-1 state indicated that the NN bond retains a double bond nature in the S-1 state. Vibrational assignments for the other S-1 Raman bands were made by one-to-one correspondence between the S-1 and S-0 bands. The double bond nature of the NN bond and the high similarity between the S-1 and the S-0 Raman spectra indicated that the observed S-1 state has a planar structure around the NN bond. The obtained Raman data seemed to suggest that the inversion in the S-1 state takes part also in the isomerization following S-2(pi pi*) photoexcitation. The relaxation process of photoexcited trans-azobenzene as well as its photoisomerization mechanism is discussed on the basis of the observed Raman data.