Structural Flexibility Slows Down Charge Transfers in Diaminoterephthalate-C₆₀ Dyads

In recent years, research has emphasized the significant influence of vibronic couplings on charge separation in donor-acceptor molecules and thin films. Diaminoterephthalates (DATs), known for their versatile optical and electronic properties, are intriguing donor materials. Efficient, vibronically assisted charge separation in DAT-acceptor dyads has been suggested. This study therefore investigates charge separation dynamics in DAT-linker-fullerene dyads in a polar solvent by using transient absorption spectroscopy. The results demonstrate photoinduced electron transfers with close to unity efficiency, occurring within 10-80 ps for benzene and biphenyl linkers. Surprisingly, there is a lack of orientational alignment between the donor and acceptor units, indicating a notable conformational flexibility that hinders electron transfer. Time-dependent density functional calculations support this finding. The obtained insights are crucial for developing and comprehending charge-transfer dyads under strong vibronic coupling conditions.