Picosecond Time-Resolved Raman Study of <i>trans</i>-Azobenzene

Abstract

The electronic and vibrational relaxation of photoexcited trans -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 (ππ*) ← 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 ∼12.5 ps in ethylene glycol and ∼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 S 0 state was generated after electronic relaxation in hexane. The lifetime of vibrationally excited S 0 azobenzene was evaluated as ∼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 15 N-substituted azobenzene, and it was clarified that the NN stretching frequency in the S 1 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 (ππ*) photoexcitation. The relaxation process of photoexcited trans -azobenzene as well as its photoisomerization mechanism is discussed on the basis of the observed Raman data.