Effects of Extension or Prevention of π-Conjugation on Photoinduced Electron Transfer Processes of Ferrocene−Oligothiophene−Fullerene Triads

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Abstract
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-C60) triads were investigated using time-resolved fluorescence and transient absorption spectroscopic methods. In nonpolar solvent, the energy-transfer (EN) process occurred from 1nT* to C60 for both triads, without forming the charge-separated (CS) state. In polar solvent, the initial CS state, Fc-tm-nT•+-C60•-, was formed via Fc-tm-nT-1C60* after the EN process from 1nT* by photoexcitation of the nT moiety and after direct photoexcitation of the C60 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-C60•- and (Fc-nT)•+-C60•- are prolonged by changing nT from 4T to 12T. For the charge-recombination process of Fc•+-tm-nT-C60•-, 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.

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