Elongation of lifetime of the charge-separated state of ferrocene - Naphthalenediimide-[60]fullerene triad via stepwise electron transfer

Photoinduced electron-transfer processes of a newly synthesized rodlike covalently linked ferrocene - naphthalenediimide - [60]fullerene (Fc - NDI - C ₆₀) triad in which Fc is an electron donor and NDI and C ₆₀ are electron acceptors with similar first one-electron reduction potentials have been studied in benzonitrile. In the examined Fc - NDI - C ₆₀ triad, NDI with high molar absorptivity is considered to be the chromophore unit for photoexcitation. Although the free-energy calculations verify that photoinduced charge-separation processes via singlet- and triplet-excited states of NDI are feasible, transient absorption spectra observed upon femtosecond laser excitation of NDI at 390 nm revealed fast and efficient electron transfer from Fc to the singlet-excited state of NDI ( ¹NDI ^*) to produce Fc ⁺ - NDI ^•- - C ₆₀. Interestingly, this initial charge-separated state is followed by a stepwise electron transfer yielding Fc ⁺ - NDI - C ₆₀ ^•-. As a result of this sequential electron-transfer process, the lifetime of the charge-separated state (τ _CS) is elongated to 935 ps, while Fc ⁺ - NDI ^•- has a lifetime of only 11 ps. (Figure presented)