Strong inhibition of singlet oxygen sensitization in pyridylferrocene-fluorinated zinc porphyrin supramolecular complexes

A supramolecular pyridylferrocene (PyFc)-fluorinated zinc porphyrin (ZnTFPP) dyad system was developed, and its photodynamics was investigated in dichloromethane and acetonitrile. The nitrogen of PyFc coordinates to the metal center of zinc porphyrin to form a supramolecular donor-acceptor complex. Formation of the pyridylferrocene-zinc porphyrin supramolecule was probed by ¹H NMR and UV-vis spectroscopy. The one-electron reduction potential of ZnTFPP in dichloromethane is shifted to a negative direction by 0.08 V due to formation of the supramolecular complex with PyFc. The binding constant of ZnTFPP^.- with PyFc in the supramolecular complex determined from the potential shift is much smaller than the binding constant of the neutral form, ZnTFPP. The fluorescence of ZnTFPP is strongly quenched by electron transfer from PyFc to the singlet excited state (¹ZnTFPP*) in the supramolecular complex of ZnTFPP with PyFc. The lifetime of ¹ZnTFPP* is also shortened significantly in the supramolecular complex. No transient absorption spectrum due to the charge-separated state (ZnTFPP^.-PyFc⁺) or the triplet excited state (³ZnTFPP*-PyFc) was detected even at the picosecond time scale due to the ultrafast back electron transfer from ZnTFPP^.- to PyFc⁺ to yield the ground state supramolecular complex rather than the triplet excited state. This leads to strong inhibition of singlet oxygen sensitization of the zinc porphyrin by formation of the supramolecular complex.