Acridine-based macrocyclic fluorescent sensors: Self-assembly behavior characterized by crystal structures and a tunable bathochromic-shift in emission induced by H₂PO₄^-via adjusting the ring size and rigidity

In this paper, a series of novel acridine derived bisbenzimidazolium macrocyclic fluorescent sensors were designed and synthesized. X-ray crystal structures demonstrated the self-assembly behavior of these cyclophanes in the solid state driven by hydrogen bond and π-π interactions. Anion binding studies of these sensors revealed a significant effect of the macrocyclic size and rigidity for H₂PO₄^- sensing via the obvious turn-on as well as bathochromic-shift in fluorescence emission. Different cavity size or rigidity of the sensors showed different bathochromic-shifts (from 36 to 126 nm) in fluorescence emission induced by H₂PO ₄^-, which resulted in significant color changes of fluorescence from blue to orange red, orange, green and blue-green respectively. The unique fluorescence response toward H₂PO₄^- may be attributed to H₂PO₄^--induced assembly of sensors forming the excimer between two acridine rings to a different extent.