Carotenoid-Bacteriochlorophyll Energy Transfer in LH2 Complexes Studied with 10-fs Time Resolution

In this report, we present a study of carotenoid-bacteriochlorophyll energy transfer processes in two peripheral light-harvesting complexes (known as LH2) from purple bacteria. We use transient absorption spectroscopy with ≈10 fs temporal resolution, which is necessary to observe the very fast energy relaxation processes. By comparing excited-state dynamics of the carotenoids in organic solvents and inside the LH2 complexes, it has been possible to directly evaluate their energy transfer efficiency to the bacteriochlorophylls. In the case of okenone in the LH2 complex from Chromatium purpuratum, we obtained an energy transfer efficiency of ηET2 = 63 ± 2.5% from the optically active excited state (S2) and ηET1 = 61 ± 2% from the optically dark state (S1); for rhodopin glucoside contained in the LH2 complex from Rhodopseudomonas acidophila these values become ηET2 = 49.5 ± 3.5% and ηET1 = 5.1 ± 1%. The measurements also enabled us to observe vibrational energy relaxation in the carotenoids’ S1 state and real-time collective vibrational coherence initiated by the ultrashort pump pulses. Our results are important for understanding the dynamics of early events of photosynthesis and relating it to the structural arrangement of the chromophores.