Two-photon study on the electronic interactions between the first excited singlet states in carotenoid-tetrapyrrole dyads |
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Authors: | Liao Pen-Nan Pillai Smitha Gust Devens Moore Thomas A Moore Ana L Walla Peter J |
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Affiliation: | Technische Universita?t Braunschweig, Institute for Physical and Theoretical Chemistry, Department for Biophysical Chemistry, Hans-Sommer-Strasse 10, 38106 Braunschweig, Germany. |
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Abstract: | Electronic interactions between the first excited states (S(1)) of carotenoids (Car) of different conjugation lengths (8-11 double bonds) and phthalocyanines (Pc) in different Car-Pc dyad molecules were investigated by two-photon spectroscopy and compared with Car S(1)-chlorophyll (Chl) interactions in photosynthetic light harvesting complexes (LHCs). The observation of Chl/Pc fluorescence after selective two-photon excitation of the Car S(1) state allowed sensitive monitoring of the flow of energy between Car S(1) and Pc or Chl. It is found that two-photon excitation excites to about 80% to 100% exclusively the carotenoid state Car S(1) and that only a small fraction of direct tetrapyrrole two-photon excitation occurs. Amide-linked Car-Pc dyads in tetrahydrofuran demonstrate a molecular gear shift mechanism in that effective Car S(1) → Pc energy transfer is observed in a dyad with 9 double bonds in the carotenoid, whereas in similar dyads with 11 double bonds in the carotenoid, the Pc fluorescence is strongly quenched by Pc → Car S(1) energy transfer. In phenylamino-linked Car-Pc dyads in toluene extremely large electronic interactions between the Car S(1) state and Pc were observed, particularly in the case of a dyad in which the carotenoid contained 10 double bonds. This observation together with previous findings in the same system provides strong evidence for excitonic Car S(1)-Pc Q(y) interactions. Very similar results were observed with photosynthetic LHC II complexes in the past, supporting an important role of such interactions in photosynthetic down-regulation. |
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