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Free Energy Dependence of Photoinduced Electron Transfer in Octathiophene-Diimide Dyads.

Donor-acceptor dyads consisting of octathiophene (T8) paired with three (di)imide acceptors (naphthalene diimide (NDI), benzene diimide (BDI) and naphthalimide (NI)) were synthesized and probed for their photoinduced forward electron transfer (ET) and charge recombination kinetics by using ultrafast transient absorption spectroscopy. The three acceptors have different electron affinity, leading to variation in the energy of the charge-separated state and the driving force (ΔG) for forward ET and charge recombination. Analysis of the TA spectra and kinetics allows assignment of rates for forward ET and charge recombination for each of the oligomers. Electrochemistry and photoluminescence spectroscopy are used to determine the ΔG values for the ET processes. For two of the oligomers (T8NDI and T8BDI) the rates for forward ET and charge recombination are very rapid (k > 3 x 1010 s-1). By contrast for the third oligomer (T8NI) the rates for both processes are considerably slower (k < 5 x 109 s-1). Analysis of the rate/free energy correlation for the series of oligomers reveals generally good agreement with Marcus semi-classical theory. The rapid dynamics for T8NDI and T8BDI are explained as arising due to the processes occurring near the barrierless region (-ΔG ~ λ), or slightly into the Marcus inverted region (-ΔG > λ). The slower dynamics for T8NI are explained as arising because the forward ET is weakly exothermic, whereas charge recombination is deep into the inverted region. This study is the first to produce experimental results that match a full Marcus bell-shaped curve with ET rates in the normal, barrrierless and inverted regions in dyads based on -conjugated oligomer donor.