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2012 | 59 | 1 | 5-9
Article title

Excited-state dynamics of overlapped optically-allowed 1Bu+ and optically-forbidden 1Bu- or 3Ag- vibronic levels of carotenoids: Possible roles in the light-harvesting function

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Pump-probe spectroscopy after selective excitation of all-trans Cars (n = 9-13) in nonpolar solvent identified a symmetry selection rule of diabatic electronic mixing and diabatic internal conversion, i.e., '1Bu+-to-1Bu- is allowed but 1Bu+-to-3Ag- is forbidden'. Kerr-gate fluorescence spectroscopy showed that this selection rule breaks down, due to the symmetry degradation when the Car molecules are being excited, and, as a result, the 1Bu+-to-3Ag- diabatic electronic mixing and internal conversion become allowed. On the other hand, pump-probe spectroscopy after coherent excitation of the same set of Cars in polar solvent identified three stimulated-emission components (generated by the quantum-beat mechanism), consisting of the long-lived coherent cross term from the 1Bu+ + 1Bu- or 1Bu+ + 3Ag- diabatic pair and incoherent short-lived 1Bu+ and 1Bu- or 3Ag- split incoherent terms. The same type of stimulated-emission components were identified in Cars bound to LH2 complexes, their lifetimes being substantially shortened by the Car-to-BChl singlet-energy transfer. Each diabatic pair and its split components appeared with high intensities in the first component. The low-energy shifts of the 1Bu+(0), 1Bu-(0) and 3Ag-(0) levels and efficient triplet generation were also found.
Physical description
  • Christina R, Miki T, Kakitani Y, Aoyagi S, Koyama Y, Limantara L (2009) Energies and excited-state dynamics of 1Bu+, 1Bu- and 3Ag- states of carotenoids bound to LH2 antenna complexes from purple photosynthetic bacteria. Chem Phys Lett 480: 289-295.
  • Furuichi K, Sashima T, Koyama Y (2002) The first detection of the 3Ag- state in carotenoids using resonance-Raman excitation profiles. Chem Phys Lett 356: 547-555.
  • Kakitani Y, Miki T, Koyama Y, Nagae H, Nakamura R, Kanematsu Y (2009) Vibrational relaxation and internal conversion in the overlapped optically-allowed 1Bu+ and optically-forbidden 1Bu- or 3Ag- vibronic levels of carotenoids: Effects of diabatic mixing as determined by Kerr-gate fluorescence spectroscopy. Chem Phys Lett 477: 194-201.
  • Li C, Miki T, Kakitani Y, Koyama Y, Nagae H (2007) Negligible shift of 3Ag- potential in longer-chain carotenoids as revealed by a single persistent peak of 3Ag- → 1Ag- stimulated emission followed by 3Ag- ← 1Ag- transient-absorption. Chem Phys Lett 450: 112-118.
  • Miki T, Kakitani Y, Koyama Y, Nagae H (2008) Stimulated emission from the 1Bu-(0) level and the 1Bu+(0) +1Bu-(1 and 2) diabatic levels upon excitation to the 1Bu+(0) level in neurosporene and spheroidene. Chem Phys Lett 457: 222-226.
  • Sutresno A, Kakitani Y, Zuo P, Li C, Koyama Y, Nagae H (2007) Presence and absence of electronic mixing in shorter-chain and longer-chain carotenoids: Assignment of the symmetries of 1Bu- and 3Ag- states located just below the 1Bu+ state. Chem Phys Lett 447: 127-133.
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  • Zuo P, Sutresno A, Li C, Koyama Y, Nagae H (2007) Vibrational relaxation on the mixed vibronic levels of the 1Bu+ and 1Bu- states in all-trans-neurosporene as revealed by subpicosecond time-resolved, stimulated emission and transient absorption spectroscopy. Chem Phys Lett 440: 360-366.
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