Photoinduced electron transfer in branched bis(ferrocenylacetylene)-C ₆₀ systems: Influence of the nature of conjugation

Four new branched 3,4- and 3,5-bis(ferrocenylacetylene)-phenyl-C ₆₀ molcules, in which the bis-ferrocene donor moieties and C ₆₀ acceptor moiety are covalently linked with phenylacetylene linkage through pyrrolidino and pyrazolino rings on the C₆₀ moiety, as abbreviated to be (Fc)₂≡Ph-NMPC₆₀ (o- and m-) and (Fc)₂≡Ph-PzC₆₀ (o- and m-), respectively, have been prepared in multistep procedures using microwave irradiation, as source of energy. The HOMO-LUMO gaps smaller than 1.25 eV were experimentally determined by cyclic voltammetry (CV) and Osteryoung square-wave voltammetry (OSWV) for (Fc)₂≡Ph-PzC₆₀ and (Fc)₂≡Ph- NMPC₆₀. In both polar and non-polar solvents, a photoinduced charge-separation (CS) process efficiently takes place in (Fc) ₂≡Ph-NMPC₆₀ and (Fc)₂≡Ph-PzC ₆₀ from the singlet excited state of C₆₀ ( ¹C₆₀*), as confirmed by picosecond-time-resolved emission spectroscopy. The CS states were confirmed by nanosecond-transient absorption spectroscopy. The lifetimes of the CS states were evaluated to be ca. 10 ns, which are shorter than those of the similar dyads with phenylenevinylene linkage, suggesting higher electron-hole conductivity through the phenylacetylene linkage.