Synthesis and electronic properties of C60–o-quinodimethane adducts

Abstract

Several C₆₀–o-quinodimethane adducts having various aromatic rings have been synthesized by the simple thermal reaction of C₆₀ with dihydrocyclobutaarenes. According to MM2 calculations, the most stable conformation of the cyclohexene ring in these adducts is not the half chair, but the half boat. On the basis of variable temperature NMR results, its inversion rate was found to depend on the attached aromatic rings, mainly due to the steric effect. The absorption spectra in the visible region were substantially identical for all the adducts, exhibiting a weak and broad band around 700 nm with vibrational structures. The fluorescence spectra due to the C₆₀ moiety of the adducts were also similar to each other and independent of the excitation wavelength. The fluorescence excitation spectra were in good agreement with the corresponding absorption spectra. No fluorescence could be observed from the attached aromatic rings even in the case of pyrene, because of the quenching by the intramolecular S_1(pyrene)–S_o(C60) energy transfer. The similar transient absorption spectra of the C₆₀ adducts were ascribed to the T_nâ†�T₁ absorption of the C₆₀ moiety. These results show that little intramolecular electronic interaction between the C₆₀ and aromatic moieties takes place in the adducts.