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Synthesis and Excited‐state Photodynamics of a Chlorin–Bacteriochlorin Dyad—Through‐space Versus Through‐bond Energy Transfer in Tetrapyrrole Arrays

2008; Wiley; Volume: 84; Issue: 3 Linguagem: Inglês

10.1111/j.1751-1097.2007.00258.x

1751-1097

C. Muthiah, Hooi Ling Kee, James R. Diers, Dazhong Fan, Marcin Ptaszek, David F. Bocian, Dewey Holten, Jonathan S. Lindsey,

Photosynthetic Processes and Mechanisms

Abstract Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne‐linked chlorin–bacteriochlorin dyad ( FbC‐pe‐FbB ). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Q y band of the chlorin constituent (675 nm) of FbC‐pe‐FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of ∼(5 ps) −1 and efficiency of >99%. The excited bacteriochlorin resulting from the energy‐transfer process in FbC‐pe‐FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long‐lived (5.4 ns) Q y excited state that exhibits a significant fluorescence quantum yield (Φ f = 0.19). Förster calculations are consistent with energy transfer in FbC‐pe‐FbB occurring predominantly by a through‐space mechanism. The energy‐transfer characteristics of FbC‐pe‐FbB are compared with those previously obtained for analogous phenylethyne‐linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular‐orbital characteristics of the energy donor and acceptor constituents. The electron‐density distributions in the frontier molecular orbitals provide insights into the through‐bond electronic interactions that can also contribute to the energy‐transfer process in the different types of dyads.