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

Donor-acceptor dyads consisting of octathiophene (T) 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 (TA) spectroscopy. The three acceptors have different electron affinities, 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 (T and T), the rates for forward ET and charge recombination are very rapid (k > 3 × 10¹⁰ s^-1). By contrast, for the third oligomer (T), the rates for both processes are considerably slower (k < 5 × 10⁹ s^-1). Analysis of the rate/free energy correlation for the series of oligomers reveals generally good agreement with the Marcus semiclassical theory. In all of the oligomers, the ET reactions are nonadiabatic, in part, due to weak coupling caused by out-of-plane twisting of the phenylene spacer that lies between the T segment and the (di)imide acceptors. The rapid ET dynamics for T and T 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 T 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 a π-conjugated oligomer donor.