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

Peter J. Walla, Patricia A. Linden, Chao Ping Hsu, Gregory D. Scholes, Graham R. Fleming

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)10808-10813
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume97
Issue number20
DOIs
StatePublished - Sep 26 2000

All Science Journal Classification (ASJC) codes

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