Supramolecular formation of Li⁺@PCBM fullerene with sulfonated porphyrins and long-lived charge separation

Lithium-ion-encapsulated [6,6]-phenyl-C₆₁-butyric acid methyl ester fullerene (Li⁺@PCBM) was utilized to construct supramolecules with sulfonated meso-tetraphenylporphyrins (MTPPS^4-; M=Zn, H₂) in polar benzonitrile. The association constants were determined to be 1.8×10⁵m^-1 for ZnTPPS^4-/Li⁺@PCBM and 6.2×10⁴m^-1 for H₂TPPS^4-/Li⁺@PCBM. From the electrochemical analyses, the energies of the charge-separated (CS) states were estimated to be 0.69 eV for ZnTPPS^4-/Li⁺@PCBM and 1.00 eV for H₂TPPS^4-/Li⁺@PCBM. Upon photoexcitation of the porphyrin moieties of MTPPS^4-/Li⁺@PCBM, photoinduced electron transfer occurred to produce the CS states. The lifetimes of the CS states were 560 μs for ZnTPPS^4-/Li⁺@PCBM and 450 μs for H₂TPPS^4-/Li⁺@PCBM. The spin states of the CS states were determined to be triplet by electron paramagnetic resonance spectroscopy measurements at 4 K. The reorganization energies (λ) and electronic coupling term (V) for back electron transfer (BET) were determined from the temperature dependence of kBET to be λ =0.36 eV and V=8.5×10^-3 cm^-1 for ZnTPPS^4-/Li⁺@PCBM and λ =0.62 eV and V=7.9×10^-3 cm^-1 for H₂TPPS^4-/Li⁺@PCBM based on the Marcus theory of nonadiabatic electron transfer. Such small V values are the result of a small orbital interaction between the MTPPS^4- and Li⁺ @PCBM moieties. These small V values and spin-forbidden charge recombination afford a long-lived CS state.