The ability of high oxidation potential zinc porphyrins acting as electron donors in photoinduced electron-transfer reactions is investigated. Donor−acceptor dyads were assembled via metal−ligand axial coordination of either pyridine or phenylimidazole functionalized fulleropyrrolidine with zinc porphyrin functionalized with different numbers of halogen substituents on the meso-aryl rings. Optical absorption studies on complex formation revealed relatively higher binding constants. Efficient quenching of fluorescence was observed for the newly assembled dyads, revealing their ability to undergo photoinduced events. Differential pulse voltammetry studies were performed to understand the structure−activity relationships with respect to the electron deficient nature of the porphyrins and to utilize these data to estimate free-energy change for charge-separation and charge-recombination processes. The absolute value of free-energy change for charge separation was found to be lower for halogenated porphyrins with higher oxidation potentials expecting to form high-energy radical ion pairs. Using femtosecond transient techniques, evidence for charge separation and kinetics of charge separation and recombination were obtained in toluene. The kinetic data obtained by analyzing the time profiles of the radical ions revealed occurrence of ultrafast charge separation and relatively slower charge recombination processes in the dyads. Notably, electron-transfer rates did not exactly follow the trends predicted based on Marcus theory of electron transfer. Donor− acceptor geometry and populating the triplet excited states of the sensitizers during charge recombination are considered to be possible reasons for this behavior.
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.
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