Singlet-singlet energy transfer mechanisms in covalently-linked fucoxanthin- and zeaxanthin-pyropheophorbide molecules

Two carotenoids, fucoxanthin and zeaxanthin, were covalently attached to each of five different pyropheophorbides. Singlet-singlet energy transfer within these ten carotenopyropheophorbide compounds was measured by femtosecond transient absorption spectroscopy and steady-state fluorescence excitation spectroscopy. In all five compounds containing fucoxanthin, energy transfer was found to occur from the higher-lying fucoxanthin S ₁ state to the lower-lying pyropheophorbide S ₁ state with 12-44% efficiency. The multiple saturated bonds separating the π systems of the fucoxanthin and pyropheophorbide molecules, the fact that the fucoxanthin S ₁ mutually implies S ₀ transition is partially allowed, and the good agreement between experimental and calculated energy transfer rates suggest that the Coulomb (Forster) mechanism is more important than the electron exchange (Dexter) mechanism for singlet-singlet energy transfer in these compounds. In contrast, all five zeaxanthin-containing compounds showed no clear evidence of energy transfer from the zeaxanthin S ₁ state to the pyropheophorbide S ₁ state. This is consistent with placing the zeaxanthin S ₁ state energy level slightly below that of all the pyropheophorbides examined here. However, energy transfer efficiencies of up to 15% were observed from the zeaxanthin S ₂ state to the pyropheorbide S ₁ state. These results suggest that several energy transfer mechanisms may operate simultaneously when carotenoid- chlorophyll distances are short.