Charge separation and (triplet) recombination in diketopyrrolopyrrole-fullerene triads

Synthesis and photophysics of two diketopyrrolopyrrole-based small band gap oligomers, end-capped at both ends with C-60 are presented. Upon photoexcitation of the oligomer, ultrafast energy transfer to the fullerene occurs (similar to 0.5 ps), followed by an electron transfer reaction. Femtosecond transient absorption has been used to determine the rates for charge separation and recombination. Charge separation occurs in the Marcus normal region with a time constant of 18-47 ps and recombination occurs in the inverted regime, with a time constant of 37 ps to 1.5 ns. Both processes are faster in o-dichlorobenzene (ODCB) than in toluene. Analysis of the charge transfer rates by Marcus-Jortner theory leads to the view that the positive charge must be located on the thiophene/dithiophene unit closest to the fullerene. Approximately 14% of the charge transfer state was found to recombine into the low-lying triplet state of the oligomer for the smaller system in ODCB.