Interfacing tetrapyridyl-C60 with porphyrin dimers via π-conjugated bridges: artificial photosynthetic systems with ultrafast charge separation

Abstract

We report on the synthesis, characterization and photophysical properties of a donor–bridge–acceptor supramolecular hybrid system, consisting of a tetrapyridyl fullerene derivative (C₆₀-tpyr) as electron acceptor, with the four pyridyl groups as part of oligophenyleneethynylene/phenylenevinylene bridges, and zinc porphyrin dimers (ZnP)₂ as electron donor species. Based on the metal-to-ligand coordination between the zinc metal centers of (ZnP)₂ and the four pyridyl entities of C₆₀-tpyr, a strong binding constant (5 × 10⁵ M⁻¹) for the formation of C₆₀-tpyr·[(ZnP)₂]₂ was evidenced. Insights into the electronic interactions between the photoactive (ZnP)₂ units and C₆₀-tpyr emanated from complementary physicochemical assays, which were further supported by theoretical calculations. Notably, the absorption and emission titration assays revealed strong interactions between the electron donor and acceptor species within C₆₀-tpyr·[(ZnP)₂]₂, both in the ground and excited state. Moreover, femtosecond and nanosecond laser photolysis transient absorption measurements were performed and provided solid evidence for intramolecular electron transfer processes derived from the singlet excited state of (ZnP)₂ to C₆₀-tpyr. Comparison with systems in which either four monomeric zinc porphyrins (ZnP) were complexed with C₆₀-tpyr or a (ZnP)₂ was coordinated with a dipyridylfullerene revealed the beneficial role of C₆₀-tpyr in increasing the lifetime of charge-separation.