Electronic and electrochemical properties as well as flowerlike supramolecular assemblies of fulleropyrrolidines bearing ester substituents with different alkyl chain lengths

A series of alkyl (methyl, ethyl, propyl, butyl) benzoate ester substituted fulleropyrrolidine derivatives (FP1-FP4) were synthesized and their electronic and electrochemical properties were investigated by means of absorption spectra, electronic structure calculation, and cyclic voltammetry (CV), respectively. The LUMO-HOMO energies and energy gaps of fullerene derivatives were estimated by the first reduction potential measured with CV combined with absorption spectra, which are consistent with those obtained from density functional theory (DFT) calculations. It was found that all fulleropyrrolidines showed very similar absorption spectra, orbital energies and redox behaviors, which are comparable with those of well-known phenyl-C61-butyric acid methyl ester (PCBM). The flowerlike supramolecular architectures obtained from the self-assembly of FP1-FP4 in chloroform-alcohol mixture solvents were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). A lamellar structure with a d-spacing of 1.92-2.02 nm that depends on the molecular size, corresponding to the thickness of a bilayer structure, suggested a face-to-face conformation of the substituent of C-60 and an interdigitation of the bare C-60 side packing. These fulleropyrrolidines have high C-60 content, are energetically PCBM-like, and are capable of forming complex flowerlike architectures, which provide fundamental insights into molecular design toward advanced fullerene materials.