Singlet−Singlet Energy Transfer Mechanisms in Covalently-Linked Fucoxanthin− and Zeaxanthin−Pyropheophorbide Molecules

• Published in: Journal of the American Chemical Society 1997-07, Vol.119 (27), p.6407-6414
• Main Authors: Debreczeny, Martin P, Wasielewski, Michael R, Shinoda, Satoshi, Osuka, Atsuhiro
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja970594e

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 S1 state to the lower-lying pyropheophorbide S1 state with 12−44% efficiency. The multiple saturated bonds separating the π systems of the fucoxanthin and pyropheophorbide molecules, the fact that the fucoxanthin S1 ↔ S0 transition is partially allowed, and the good agreement between experimental and calculated energy transfer rates suggest that the Coulomb (Förster) 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 S1 state to the pyropheophorbide S1 state. This is consistent with placing the zeaxanthin S1 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 S2 state to the pyropheophorbide S1 state. These results suggest that several energy transfer mechanisms may operate simultaneously when carotenoid−chlorophyll distances are short.