Ionization and Anion-π⁺ Interaction: A New Strategy for Structural Design of Aggregation-Induced Emission Luminogens

Recent years have witnessed the significant role of anion-π⁺ interactions in many areas, which potentially brings the opportunity for the development of aggregation-induced emission (AIE) systems. Here, a new strategy that utilized anion-π⁺ interactions to block detrimental π-π stacking was first proposed to develop inherent-charged AIE systems. Two AIE-active luminogens, namely, 1,2,3,4-tetraphenyloxazolium (TPO-P) and 2,3,5-triphenyloxazolium (TriPO-PN), were successfully synthesized. Comprehensive techniques such as single-crystal analysis, theoretical calculation, and conductivity measurement were used to illustrate the effects of anion-π⁺ interactions on the AIE feature. Their analogues tetraphenylfuran (TPF) and 2,4,5-triphenyloxazole (TriPO-C) without anion-π⁺ interactions suffered from the aggregation-caused emission quenching in the aggregate state, demonstrating the important role of anion-π⁺ interactions in suppressing π-π stacking. TriPO-PN was biocompatible and could specifically target lysosome in fluorescence turn-on and wash-free manners. This suggested that it was a promising contrast agent for bioimaging.