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

Authors


*Corresponding author emails: holten@wustl.edu (Dewey Holten), David.Bocian@ucr.edu (David F. Bocian), jlindsey@ncsu.edu (Jonathan S. Lindsey)

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 Qy 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) Qy excited state that exhibits a significant fluorescence quantum yield (Φf = 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.

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