Two-photon study on the electronic interactions between the first excited singlet states in carotenoid - Tetrapyrrole dyads

Electronic interactions between the first excited states (S₁) 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 S₁ - chlorophyll (Chl) interactions in photosynthetic light harvesting complexes (LHCs). The observation of Chl/Pc fluorescence after selective two-photon excitation of the Car S₁ state allowed sensitive monitoring of the flow of energy between Car S₁ and Pc or Chl. It is found that two-photon excitation excites to about 80% to 100% exclusively the carotenoid state Car S₁ 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 S₁ → 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 S₁ energy transfer. In phenylamino-linked Car - Pc dyads in toluene extremely large electronic interactions between the Car S₁ 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 S₁ - 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.