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

Yoonsoo Pang, Matthew A. Prantil, Aaron J. van Tassle, Garth A. Jones, Graham R. Fleming

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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 similar to 19 ps time scale in I and a similar to 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 (similar to 17 ps for I and similar to 1.7 ps for II) than the 1PE values. It is proposed that I PE 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.
Original languageEnglish
Pages (from-to)13086-13095
Number of pages10
JournalThe Journal of Physical Chemistry B
Issue number39
Publication statusPublished - 2009

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