Effects of extension or prevention of π-conjugation on photoinduced electron transfer processes of ferrocene-oligothiophene-fullerene triads

Photoinduced electron-transfer processes of alkyl-inserted ferrocene - trimethylene - oligothiophene - fullerene (Fc-tm-nT-C₆₀) linked triads and directly linked ferrocene - oligothiophene - fullerene(Fc-nT-C ₆₀) triads were investigated using time-resolved fluorescence and transient absorption spectroscopic methods. In nonpolar solvent, the energy-transfer (EN) process occurred from ¹nT* to C ₆₀ for both triads, without forming the charge-separated (CS) state. In polar solvent, the initial CS state, Fc-tm-nT^•+-C ₆₀^•-, was formed via Fc-tm-nT-¹C ₆₀* after the EN process from ¹nT* by photoexcitation of the nT moiety and after direct photoexcitation of the C ⁶⁰ moiety. For Fc-tm-nT^•+-C₆₀ ^•-, the positive charge shifted from the nT^•+ moiety to the Fc moiety, producing the final CS state, Fc^•+-tm- nT-C₆₀^•-, which lasted for 22-330 ns by changing nT from 4T to 12T. For Fc-nT-C₆₀ in polar solvent, the CS state, in which the radical cation is delocalized on both Fc and nT moieties ((Fc-nT) ^•+-C₆₀^•-), was formed immediately after direct photoexcitation of the nT and C₆₀ moieties. The lifetimes of (Fc-nT)^•+-C₆₀^•- were estimated to be 0.1-50 ns by changing nT from 4T to 12T. The longer lifetimes of Fc^•+-tm-nT-C₆₀^•- than those of (Fc-nT)^•+-C₆₀^•- are caused by the insertion of the trimethylene chain to prevent the π-conjugation between the Fc and nT moieties. The lifetimes for Fc^•+-tm-nT-C ₆₀^•- and (Fc-nT)^•+-C ₆₀^•- are prolonged by changing nT from 4T to 12T. For the charge-recombination process of Fc^•+-tm-nT-C ₆₀^•-, the damping factor was evaluated to be 0.10 Å^-1. For (Fc-nT)^•+-C₆₀ ^•-, the oxidation potentials of the nT moieties control the electron-transfer process with reflecting stabilization of the radical cations of the nT moieties.