Phthalocyanine–C60 Fused Conjugates Exhibiting Molecular Orbital Interactions Depending on the Solvent Polarity

Abstract

New covalently C60-connected zinc phthalocyanine (ZnPc) derivatives have been synthesized by utilizing successive cycloaddition reactions of C60 with a ZnPc derivative containing a pyridazine moiety employing Komatsu's method in reaction of C60 with phthalazine. The UV/Vis absorption spectrum of the fused conjugate (5) shows red shifts from the corresponding absorption of ZnPc derivative (8), indicating interactions between the ZnPc and C60 moieties. The DFT calculations under non-polar medium predict that the HOMO and LUMO of 5 localize on the ZnPc moiety, whereas LUMO+1 localizes on the C60 moiety, which reasonably explain the magnetic circular dichroism (MCD) and absorption spectra in toluene. Electrochemical redox potentials of 5 in polar solvents indicate the first-oxidation potential arises from the ZnPc moiety, whereas the first reduction potential is associated with the C60 moiety, suggesting the LUMO localizes on the C60 moiety in polar solvent. This reversal of the LUMO is supported by the ZnPc-fluorescence quenching with a nearby C60 moiety in benzonitrile, which leads to the charge-separation via the excited singlet state of the ZnPc moiety. In toluene on the other hand, such a ZnPc-fluorescence quenching owing to the photoinduced charge separation is not observed as predicted by the DFT-calculated LUMO on the ZnPc moiety.