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 S-1 state are reported for light-harvesting complexes of purple bacteria. Direct excitation of the carotenoid S-1 state enables the measurement of subsequent dynamics on a fs time scale without interference from higher excited states, such as the optically allowed S-2 state or the recently discovered dark state situated between S-1 and S-2. The lifetimes of the carotenoid S-1 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 approximate to 1.9 +/- 0.5 ps. These results explain the differences in the carotenoid to bacteriochlorophyll energy transfer efficiency after S-2 excitation. In Rps. acidophila the carotenoid S-1 to bacteriochlorophyll energy transfer is found to be quite inefficient (phi(ET1) <28%) whereas in Rb. sphaeroides this energy transfer is very efficient (phi(ET1) approximate to 80%) The results are rationalized by calculations of the ensemble averaged time constants. We find that the Car S-1 --> B800 electronic energy transfer (EET) pathway (approximate to 85%) dominates over Car S-1 --> B850 EET (approximate to 15%) in Rb. sphaeroides, whereas in Rps. acidophile the Car S-1 --> B850 EET (approximate to 60%) is more efficient than the Car S-1 --> B800 EET (approximate to 40%). The individual electronic couplings for the Car S-1 --> 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 S-1 energy gaps in the two species.