SYNTHESIS AND ELECTRONIC-PROPERTIES OF C-60-O-QUINODIMETHANE ADDUCTS

Several C-60-o-quinodimethane adducts having various aromatic rings have been synthesized by the simple thermal reaction of C-60 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 C-60 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-0(C60) energy transfer. The similar transient -S absorption spectra of the C-60 adducts were ascribed to the T-n-->T-1 absorption of the C-60 moiety. These results show that little intramolecular electronic interaction between the C-60 and aromatic moieties takes place in the adducts.