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 S2(ππ*) ← S0 photoexcitation. Several transient Raman bands assignable to the S1 state were observed immediately after photoexcitation. The lifetime of the S1 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 S1 state was highly vibrationally excited. In addition, several anti-Stokes Raman bands due to the S0 state were observed after the decay of the S1 state, indicating that the vibrationally excited S0 state was generated after electronic relaxation in hexane. The lifetime of vibrationally excited S0 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 S1 spectrum was made by using 15N-substituted azobenzene, and it was clarified that the NN stretching frequency in the S1 state is very close to that of the S0 state (1428 cm-1 in S1 and 1440 cm-1 in S0). The high NN stretching frequency in the S1 state indicated that the NN bond retains a double bond nature in the S1 state. Vibrational assignments for the other S1 Raman bands were made by one-to-one correspondence between the S1 and S0 bands. The double bond nature of the NN bond and the high similarity between the S1 and the S0 Raman spectra indicated that the observed S1 state has a planar structure around the NN bond. The obtained Raman data seemed to suggest that the inversion in the S1 state takes part also in the isomerization following S2(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.