• excitation energy transfer;
  • femtosecond spectroscopy;
  • molecular assembly;
  • photosynthesis


Ultrafast excited state dynamics of carotenoids in solution and bound to pigment–protein complexes have been investigated by femtosecond pump-probe spectroscopic measurements. Possible excitation energy transfer (EET) pathways between carotenoids and bacteriochlorophylls and their efficiency depend strongly on the conjugation length of carotenoids. In the case of Rhodobacter sphaeroides 2.4.1, dual EET channels from carotenoid to bacteriochlorophyll (S2[RIGHTWARDS ARROW] Qx and S1[RIGHTWARDS ARROW] Qy) upon excitation of carotenoid were observed. In the case of Rhodospirillum rubrum S1, on the other hand, the EET pathway of S1[RIGHTWARDS ARROW] Qy upon excitation of carotenoid was closed, whereas reverse energy transfer from bacteriochlorophyll to carotenoid was clearly observed upon excitation of bacteriochlorophyll. The role of carotenoids in the light-harvesting complexes is discussed in terms of their conjugation length.