Mimicking The Photosynthetic Special Pair With Aluminum(III) Porphyrin Homodimer – Fullerene (C₆₀) Supramolecular Assemblies Capable of Long-Lived Charge Separation

The axial-bonding ability of aluminum(III) porphyrin has been employed to construct two covalently linked homodimer systems. The center-to-center distance between the two porphyrins, which are connected via an oxalic bridge is ∼6.90 Å. Unlike most other meso-substituted porphyrins, which have aromatic substituents, the Al porphyrins were decorated with propyl or heptyl side groups to tune their electronic properties. Further, the Lewis acidity of the Al center was exploited to coordinate the Lewis base, imidazole, appended to the electron acceptor, C₆₀, to construct a self-assembled supramolecular homodimer-C₆₀ donor-acceptor conjugate. Steady state optical studies show that the exciton coupling between porphyrins is weak, and time-resolved electron paramagnetic resonance (TREPR) measurements in the glass phase indicate that the triplet state of the dimer remains localized. When C₆₀ is coordinated to the monomers and dimers, femtosecond and nanosecond transient absorption studies and TREPR measurements show that electron transfer between the porphyrin and fullerene occurs. Despite the weak coupling between the two porphyrins of the dimer, the presence of a second porphyrin enhances both the yield and lifetime of the charge-separated state. Remarkably, the resulting charge-separated lifetimes found to be between 2–5 µs in the investigated reaction center mimics.