Donor-acceptor conjugates derived from cobalt porphyrin and fullerene via metal-ligand axial coordination: Formation and excited state charge separation

Two types of cobalt porphyrins, viz., meso-tetrakis(tolylporphyrinato)cobalt(II), (TTP)Co (1), and meso-tetrakis(triphenylamino porphyrinato)cobalt(II), [(TPA)4P]Co, (2) were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine, ImC60 to form a new series of donor-acceptor constructs. A 1:2 complex formation with ImC60 was established in the case of (TTP)Co while for [(TPA)4P]Co only a 1:1 complex was possible to positively identify. The binding constants K1 and K2 for step-wise addition of ImC60 to (TTP)Co were found to be 1.07 × 105 and 3.20 × 104 M-1, respectively. For [(TPA)4P]Co:ImC60, the measured K1 values was found to be 6.48 × 104 M-1, slightly smaller than that observed for (TTP)Co. Although both cobalt porphyrins were non-fluorescent, they were able to quench the fluorescence of ImC60 indicating occurrence of excited state events in the supramolecular donor-acceptor complexes. Electrochemistry coupled with spectroelectrochemistry, revealed the formation of cobalt(III) porphyrin cation instead of a cobalt(II) porphyrin radical cation, as the main product, during oxidation of phenyl imidazole coordinated cobalt porphyrin. With the help of computational and electrochemical results, an energy level diagram was constructed to witness excited state photo-events. Competitive energy and electron transfer from excited CoP to coordinated ImC60, and electron transfer from Im1C60*to cobalt(II) porphyrin resulting into the formation of PCoIII:ImC60•-charge separated state was possible to envision from the energy diagram. Finally, using femtosecond transient absorption spectroscopy and data analysis by Glotaran, it was possible to establish sequential occurrence of energy transfer and charge separation processes. The lifetime of the final charge separated state was 2 ns. A slightly better charge stabilization was observed in the case of [(TPA)4P]Co:ImC60 due to the presence of electron rich, peripheral triphenylamine substituents on the cobalt porphyrin.