Two-Photon Study on the Electronic Interactions between the First Excited Singlet States in Carotenoid-Tetrapyrrole Dyads

Electronic interactions between the first excited states (S-1) of carotenoids (Car) of different conjugation lengths (84-11 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 oft 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.