Time-Resolved Impulsive Stimulated Raman Studies of l,1'-Binaphthyl in the Excited State:Low-Frequency Vibrations and Conformational Relaxation

摘要

The conformational relaxation of excited-state 1,1 '-binaphthy 1 in solution was investigated by using femtosecond time-resolved absorption and time-resolved time-domain Raman spectroscopy.Femtosecond time-resolved absorption measurements confirmed that the first transient (the "unrelaxed" S_1 state) is generated in a subpicosecond time scale after photoexcitation and that it undergoes a relaxation with a time constant of 16 ps (in cyclohexane) to form the second transient (the "relaxed" S_1 state).Time-domain Raman experiments provided low-frequency Raman data of the relaxed S_1 state for the frequency region of 0-400 cm~(-1).The obtained Raman data of the relaxed S_1 state were significantly different from the low-frequency So spectrum in solution.The relaxed S_1 spectrum resembles the spectrum of the crystalline form that has a dihedral angle of 69 deg,whereas the So Raman spectrum in solution coincides well with that of the crystalline form having a dihedral angle of 103 deg.The time-domain Raman measurements provided vibrational data that support a conformational change occurring in 1,l'-binaphthyl in the S_1 state.