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

Abstract

Lithium ion encapsulated [60]fullerene (Li+@C60) is included within a free base and nickel complex of a cyclic porphyrin dimer (M-CPDPy, M = H4 and Ni2) to afford supramolecules (Li+@C60⊂M-CPDPy) 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+@C60⊂H4-CPDPy and 1.20 eV for Li+@C60⊂Ni2-CPDPy. 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+@C60⊂H4-CPDPy, electron transfer from the triplet excited state of the free base porphyrin to Li+@C60 occurs to produce the CS state. Li+@C60⊂Ni2-CPDPy also undergoes photoinduced electron transfer to produce the CS state. The lifetimes of the resulting CS states are 0.50 ms for Li+@C60⊂H4-CPDPy and 0.67 ms for Li+@C60⊂Ni2-CPDPy. 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.