Self-assembled porphyrin-C60 and porphycene-C60 complexes via metal axial coordination

Noncovalently linked electron donor-acceptor complexes consisting of either zinc tetraphenylporphyrin, (TPP)Zn, or zinc octaethylporphycene, (OEPc)Zn, as donor and pyridine appended C-60, py similar to C-60, as acceptor, via axial coordination of zinc, are reported. The UV-visible and H-1 NMR spectral studies reveal 1:1 complex formation between the donor and acceptor entities. The determined K values follow the oxidation potential of the employed zinc tetrapyrroles, and the calculated thermodynamic parameters reveal stable complexation. The singlet emission studies show efficient quenching of the investigated zinc tetrapyrrole emission upon axial coordination of pyridine appended C-60. Free-energy calculations indicate that the photoinduced electron transfer from the singlet excited zinc tetrapyrrole to C-60 is exergonic, and the estimated rates of electron transfer are found to be (2.4 +/- 0.3) x 10(8) s(-1) for the (TPP)Zn-py similar to C-60 complex and (2.2 +/- 0.3) x 108 s(-1) for the (OEPc)Zn-py similar to C-60 complex, respectively.