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

Abstract

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 1.8×10(5) M(-1) for ZnTPPS(4-) /Li(+) @PCBM and 6.2×10(4) M(-1) for H2 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 H2 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 H2 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 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 H2 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.