Time-resolved impulsive stimulated Raman studies of 1,1′-binaphthyl in the excited state:: Low-frequency vibrations and conformational relaxation

The conformational relaxation of excited-state 1, 1'-binaphthyl 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, 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, state for the frequency region of 0-400 cm(-1). The obtained Raman data of the relaxed S, state were significantly different from the low-frequency So spectrum in solution. The relaxed S, spectrum resembles the spectrum of the crystalline form that has a dihedral angle of 69degrees, whereas the So Raman spectrum in solution coincides well with that of the crystalline form having a dihedral angle of 103degrees. The time-domain Raman measurements provided vibrational data that support a conformational change occurring in 1,1'-binaphthyl in the S-1 state.