Supramolecular Formation of Li + @PCBM Fullerene with Sulfonated Porphyrins and Long‐Lived Charge Separation

Lithium‐ion‐encapsulated [6,6]‐phenyl‐C 61 ‐butyric acid methyl ester fullerene (Li + @PCBM) was utilized to construct supramolecules with sulfonated meso ‐tetraphenylporphyrins (MTPPS 4− ; M=Zn, H 2 ) in polar benzonitrile. The association constants were determined to be 1.8×10 5   M −1 for ZnTPPS 4− /Li + @PCBM and 6.2×10 4   M −1 for H 2 TPPS 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 H 2 TPPS 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 H 2 TPPS 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 k BET 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 H 2 TPPS 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.