On the Planarity of Heterocyclic Bay‐Substituted Perylenediimides: Insight into the Fluorescent Photoinduced Electron Transfer Sensing Framework

Near‐infrared fluorescent pH indicators based on perylenediimide were designed and synthesized from 1,7‐dibromo‐3,4,9,10‐perylenetetracarboxylic dianhydride. The molecules were imidized with N , N ‐dimethylethylenediamine and bay‐substituted with four seven‐membered heterocyclic rings of azetidine, pyrrolidine, morpholine, and azepane. Purposely constructed in a modular receptor–spacer–fluorophore–spacer–receptor format, the fluorescent pH indicators possess photoinduced electron transfer (PET) and internal charge transfer mechanisms with turn‐on emission switching in the presence of acid. Counterintuitively, the dibrominated intermediate with two heavy atoms is more brightly emissive than the heterocyclic bay‐substituted compounds. Closer inspection of the heterocyclic compounds by density functional theory calculations reveals a non‐planar bent or twisted geometry between the two naphthyl subunits. These findings highlight a notable exception to a cherished PET sensing platform, while equally providing future opportunities for the modulation of the emission by conformational tuning.