Picosecond dynamics of energy transfer in porphyrin-sapphyrin noncovalent assemblies

The picosecond dynamics of noncovalent ensembles for energy transfer based on anion chelation are reported. The photoactive noncovalent complexes are assembled via salt-bridge formation between carboxyl-containing porphyrin photodonors and a monoprotonated pentapyrrolic sapphyrin acceptor. These complexes are formed with a K(a) of ca. 10³ M^-1 upon mixing the receptor and substrate in their respective free-acid and free-base forms in CD₂Cl₂ (as judged by ¹H NMR spectroscopic means). Upon irradiation at 417 nm, singlet-singlet energy transfer from the porphyrin to the sapphyrin subunit takes place readily with energy transfer dynamics that are consistent with a Forster-type mechanism. The title systems thus appear to be prototypic of a new kind of noncovalent energy transfer modeling that is predicated on the use of anion chelation.