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 C60 are presented. Upon photoexcitation of the oligomer, ultrafast energy transfer to the fullerene occurs (~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.