Submillisecond-lived photoinduced charge separation in inclusion complexes composed of Li⁺@C₆₀ and cyclic porphyrin dimers

Lithium ion encapsulated [60]fullerene (Li⁺@C₆₀) is included within a free base and nickel complex of a cyclic porphyrin dimer (M-CPD_Py, M = H₄ and Ni₂) to afford supramolecules (Li⁺@C₆₀࣪M-CPD_Py) in a polar solvent (benzonitrile) with the association constants of 2.6 × 10⁵ M⁻¹ and 3.5 × 10⁵ M⁻¹, respectively. From the electrochemical analysis, the energies of the charge-separated (CS) states are estimated to be 1.07 eV for Li⁺@C₆₀࣪H₄-CPD_Py and 1.20 eV for Li⁺@C₆₀࣪Ni₂-CPD_Py. Both values are lower than the triplet excited energies of the fullerene and porphyrin. Upon the photoexcitation at the Q-band of the porphyrin chromophore of Li⁺@C₆₀࣪H₄-CPD_Py, electron transfer from the triplet excited state of the free base porphyrin to Li⁺@C₆₀ occurs to produce the CS state. Li⁺@C₆₀࣪Ni₂-CPD_Py also undergoes photoinduced electron transfer to produce the CS state. The lifetimes of the resulting CS states are 0.50 ms for Li⁺@C₆₀࣪H₄-CPD_Py and 0.67 ms for Li⁺@C₆₀࣪Ni₂-CPD_Py. These remarkably long CS lifetimes are the best values ever reported for non-covalent porphyrin-fullerene supramolecules in solution and are attributable to the lower CS energies than the triplet energy of each chromophore.