Synthesis and photophysical properties of ferrocene-oligo(benzoateethynylene)- fulleropyrrolidines dyads and triads. Implications in photovoltaic cells

A series of fulleropyrrolidines-conjugated bridge-ferrocene or triazene oligomers were selectively synthesized by the Sonogashira reaction by applying the step-by-step approach. The bridge is constituted by 1, 2 and 3 benzoateethynylene units (BzE) and bears triazene (Et2N3) or ferrocene as terminal groups affording the C60-2PEN3 and C60-3PEN3 dyads and C60-1PEFe, C60-2PEFe and C60-3PEFe triads. DQF-COSY, HETCOR, 1H and 13C NMR and the MALDI-TOF characterization clearly confirmed the expected molecular structure. The absorption spectra of the fulleropyrrolidine oligomers do not match the sum of the individual spectra: N-methylfulleropyrrolidine (NMF) and BzEs, suggesting electronic interaction between the two moieties in the ground state. The fluorescence of the BzE is strongly quenched after functionalization with NMF, which could be indicative of energy or electron transfer from the triazene or ferrocene as electron donor to the fulleropyrrolidine electron acceptor through the π-bridge. The latter process was confirmed by cyclic voltammetry. The strength of the electron-accepting group gets to increase anodically the oxidation potential, or decrease cathodically the reduction potential in the order C60-pyrrolidine > benzoate. The character of the HOMO in the series is defined by the electron-donating ferrocene or triazene moiety, whereas the character of the LUMO is mainly determined by the electron-accepting group and is further supported by theoretical calculations. Photovoltaic devices presented low efficiencies, due to the absorption range of the oligomers being out of the maximum solar irradiance and the inhomogeneous organization in the films.