Excited-state dynamics of 8'-apo-beta-caroten-8'-al and 7',7'-dicyano-7'-apo-beta-carotene studied by femtosecond time-resolved infrared spectroscopy.

We present infrared transient absorption measurements of the substituted apocarotenoids 8'-apo-beta-caroten-8'-al (I) and 7',7'-dicyano-7'-apo-beta-carotene (II) in the S(1) excited states by one-photon excitation (1PE) and two-photon excitation (2PE). 1PE populates the higher S(2) state, which converts to the S(1) state via rapid internal conversion, and 2PE populates the S(1) state directly. The 1PE-prepared population returns to the ground state on a approximately 19 ps time scale in I and a approximately 2.0 ps time scale in II. Distinct vibrational spectra and dynamics are observed from the 2PE-prepared S(1) state for both I and II: the symmetric C=C stretching vibration around 1500 cm(-1) in the S(1) state is several-fold increased in strength over other C=C stretching modes compared to the 1PE case, and long-lived absorptions are observed even after all the excited-state populations have decayed. The lifetimes of the S(1) state prepared by 2PE are slightly shorter (approximately 17 ps for I and approximately 1.7 ps for II) than the 1PE values. It is proposed that 1PE and 2PE lead to the population of different conformational minima of the S(1) potential surface. These two minima do not communicate on the time scale of the S(1) lifetime and have different relaxation channels on the ground-state surface.