Exciton diffusion and relaxation in methyl-substituted polyparaphenylene polymer films.

Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units.

Direct energy transfer from the peripheral LH2 antenna to the reaction center in a mutant of Rhodobacter sphaeroides that lacks the core LH1 antenna.

The light-harvesting apparatus of the photosynthetic bacterium Rhodobacter sphaeroides is composed of a peripheral LH2 complex which directs excitation energy to the LH1/reaction center core. The puf BA genes encoding the LH1 polypeptides have been deleted, producing a photosynthetically-competent strain which contains LH2 and reaction centers. Time-resolved absorption and fluorescence measurements demonstrate that energy is efficiently transferred from LH2 to the reaction center, despite the absence of LH1. Energy trapping takes place in 55 +/- 5 ps at room temperature, compared to the result for the wild-type strain of 60 +/- 5 ps. At 77 K, the results for the mutant and wild type are 75 +/- 5 and approximately 35 ps, respectively; the slower time in the mutant is attributed to the small differences in antenna/reaction center contacts and relative distances that are bound to exist as a consequence of LH1 and LH2 being assembled from different alpha- and beta-polypeptides. Measurements with closed reaction centers provided new information on the nature of fast energy transfer within the B850 pigments of LH2. We conclude that the absorption band is inhomogeneously broadened, and the fast (approximately 10 ps) lifetime observed in the 847-857-nm region is interpreted as very rapid (1-5 ps) hopping of the excitation energy from high-energy to low-energy pigments within the B850 absorption band. Time-resolved anisotropy studies demonstrate that energy-transfer events within B850 occur on a subpicosecond to picosecond time scale.(ABSTRACT TRUNCATED AT 250 WORDS)