Discrete component model to explain the photophysics of bay-functionalized perylene bisimide derivatives

Abstract

Bay functionalization of perylene bisimides (PBIs) is a widely used strategy to tailor their electronic structure. However, most bay-functionalized PBIs are symmetric, bearing identical substituents. Here, we present an asymmetrically functionalized PBI with distinct groups at the bay positions. The introduction of an aldehyde intermediate at the bay positions enables stepwise formation of benzimidazole (Imi) and dicyanoethylene (DCE) moieties. In the resulting asymmetric mono-Imi-mono-DCE-PBI, Imi and DCE act as electron-donating and electron-withdrawing groups, respectively. Comparing its spectral features with symmetric di-Imi-PBI and di-DCE-PBI reveals that its absorption spectrum is dominated by the DCE group, while emission is governed by the Imi group. The photoluminescence quantum yield (PLQY) of this asymmetric PBI lies between di-Imi-PBI and di-DCE-PBI. Furthermore, acid titration of di-Imi-PBI yielded another asymmetric PBI through one-sided protonation. Unlike mono-Imi-mono-DCE-PBI, this partially protonated derivative shows the lowest PLQY value compared with its symmetric counterparts. Theoretical calculations provided molecular orbital energy levels of the PBI core and bay substituents. Applying a discrete component model, we rationalized the photo-induced charge transfer (PICT) behavior. This analysis shows that additional electron transfer from the PBI core to the benzimidazolium (Imi⁺) moiety in the protonated derivative accounts for the pronounced fluorescence quenching observed.