Femtosecond dynamics of the forbidden carotenoid S₁ state in light-harvesting complexes of purple bacteria observed after two-photon excitation

Time-resolved excited-state absorption intensities after direct two-photon excitation of the carotenoid S₁ state are reported for light-harvesting complexes of purple bacteria. Direct excitation of the carotenoid S₁ state enables the measurement of subsequent dynamics on a fs time scale without interference from higher excited states, such as the optically allowed S₂ state or the recently discovered dark state situated between S₁ and S₂. The lifetimes of the carotenoid S₁ states in the B800-B850 complex and B800-B820 complex of Rhodopseudomonas acidophila are 7 ± 0.5 ps and 6 ± 0.5 ps, respectively, and in the light-harvesting complex 2 of Rhodobacter sphaeroides ≃1.9 ± 0.5 ps. These results explain the differences in the carotenoid to bacteriochlorophyll energy transfer efficiency after S₂ excitation. In Rps. acidophila the carotenoid S₁ to bacteriochlorophyll energy transfer is found to be quite inefficient (φ(ET1) <28%) whereas in Rb. sphaeroides this energy transfer is very efficient (φ(ET1) ≃80%). The results are rationalized by calculations of the ensemble averaged time constants. We find that the Car S₁ → B800 electronic energy transfer (EET) pathway (≃85%) dominates over Car S₁ → B850 EET (≃15%) in Rb. sphaeroides, whereas in Rps. acidophila the Car S₁ → B850 EET (≃60%) is more efficient than the Car S₁ → B800 EET (≃40%). The individual electronic couplings for the Car S₁ → BChl energy transfer are estimated to be approximately 5-26 cm^-1. A major contribution to the difference between the energy transfer efficiencies can be explained by different Car S₁ energy gaps in the two species.