Supramolecular triads of free-base porphyrin, fullerene, and ferric porphyrins via the "covalent-coordinate" binding approach: Formation, sequential electron transfer, and charge stabilization

Supramolecular triads composed of free-base porphyrin, fullerene, and ferric porphyrin were constructed by using "covalent-coordinate" binding strategy. For this, a free-base porphyrin was covalently linked to a fullerene entity bearing a pyridine ligand, and was subsequently utilized to coordinate ferric porphyrins bearing different peripheral substituents. The newly formed triads were characterized by spectral methods and the structures were deduced from DFT B3LYP/3-21G(*) methods. Free-energy calculations performed with use of the redox potential and emission data suggested the occurrence of sequential electron transfer from singlet excited freebase porphyrin to the covalently linked fullerene, followed by an electron transfer from fullerene anion radical to ferric porphyrin, ultimately generating free-base porphyrin cation radical and ferrous porphyrin as the electron-transfer products, anticipating the generation of long-lived charge-separated species as a consequence of distant separation between the oxidized and reduced species. Time-resolved emission and nanosecond transient absorption techniques were used to obtain kinetic and spectral evidence of electron transfer. Attempts were made to obtain the lifetime of the final charge separated species by monitoring the decay of H ₂P ^.+ at 620 nm. Lifetimes of the order of 20 μs were obtained; however, they were found to be overlapped with the long-living triplet states of porphyrin of similar lifetimes at the monitoring wavelength.