High Potential Zinc Porphyrin-Fullerene Dyads: Formation and Electron Transfer Studies

Supramolecular dyads of photosensitizing halogenated zinc porphyrins (and halogenated zinc phthalocyanines) and redox active fulleropyrolidine derivatives with pyridine or imidazole ligands were constructed based on metal-ligand axial coordination approach. Spectroscopic characterization accompanied by steady-state absorption and fluorescence studies confirmed the high binding affinity of the high potential zinc porphyrin towards fullerene ligands that was also supported by computational studies. Qualitative Benesi-Hildebrand plots allowed us to calculate the association constant, K of the supramolecular dyads. The redox states of the donor and acceptor entities in the dyads were established from electrochemical investigations using cyclic and differential pulse voltammetry techniques. Free energy calculations using Weller’s approach suggested exergonic photoinduced electron transfer process for all of the studied systems. Pump-probe transient absorption studies at the femtosecond and nanosecond time scale confirmed the formation of cation and anion radical ions in the donor-acceptor supramolecular dyads.  The kinetics of charge separation and charge recombination evaluated from the transient absorption studies revealed occurrence of ultrafast electron transfer events.