Photoinduced Symmetry Breaking-Charge Separation in the Aggregated State of Perylene Diimide: Effect of Hydrophobicity

The aggregation of perylene diimide (PDI) strongly affects the efficiency of organic optoelectronic devices. Here, a simple N,N′-bis(6-undecyl)perylene-3,4,9,10-bis(dicarboximide) (PDI) was prepared. The steady-state absorption spectrum revealed that PDI showed a monomer state in dimethylformamide (DMF). In contrast, a binary DMF/water (25–50% volume percentage of water) solvent showed an aggregation state due to the hydrophobic effect. Further, we examined the excited state relaxation dynamics of both monomeric and aggregated states of PDI using transient absorption (TA) spectroscopy. We found that the aggregated state of PDI in binary DMF/water solvent with the volume percentage of water at 30% strongly coupled with each other due to the hydrophobic effect, which is favorable to the intermolecular symmetry-breaking charge separation (SB-CS) process. Notably, the TA spectroscopy reveals that the SB-CS state is disfavored for the simple flat structure of monomeric PDI in DMF solvent. Therefore, our work provided the photophysical understanding of PDI aggregation caused by hydrophobicity. Since the hydrophobic effect is significant for the aggregation of PDI, it is possible to change the hydrophobicity by controlling the molecular structure of the PDI derivative, to achieve a more effective charge separation state.