Symmetry-breaking charge separation in perylene diimide trimers: effects of aggregation and solvent polarity

Symmetry-breaking charge separation (SB-CS) is a fundamental process in natural photosynthetic systems, serving as the primary trigger for electron transfer and ultimately leading to the formation of a charge separated state. This mechanism has also been leveraged in optoelectronic devices to minimize energy loss and improve solar energy conversion efficiency. However, in organic solar cell (OSC) studies, SB-CS has predominantly been observed in polar environments, and the roles of molecular aggregation in modulating this process remain unclear. Herein, we investigated the influence of aggregation and solvent polarity on the SB-CS of the perylene diimide trimer (PDI-III) in different solvents. Steady-state absorption and fluorescence spectroscopy reveal that PDI-III exhibits a biphasic aggregation behavior depending on solvent polarity, with stronger aggregation occurring in both nonpolar and highly polar solvents than in solvents of intermediate polarity. Femtosecond transient absorption spectroscopy and time-resolved infrared spectroscopy indicate that SB-CS also emerges in toluene, with an extent that lies between those observed in chloroform and acetone. Further analysis suggests that in toluene, intermolecular aggregation strengthens π-π interactions and electronic coupling, thereby enabling SB-CS even in the absence of substantial solvent polarity. In acetone, intramolecular aggregation, together with strong solvent polarity, leads to more efficient SB-CS than in chloroform. Collectively, these results establish a clear mechanistic framework for how aggregation and solvent polarity govern SB-CS in PDI-III, offering guiding principles for minimizing energy loss while maintaining high photocurrent in next-generation OSCs.