A modular approach to obtain benzimidazolium-urea-based, fluorophore-appended macrocyclic receptors was developed. This class of receptors could be used as selective ratiometric fluorescent sensors for H2PO4(-) due to the synergistic binding effect of benzimidazolium and urea moieties.

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 H2PO4(-), 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.

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