Sensitive fluorescence probes for dihydrogen phosphonate anion based on calix[4]arene bearing naphthol-hydrazone groups

t-Butyl and t-pentylcalix[4]arenes bearing two 2-naphthol-1-hydrazone groups at the lower rim were synthesized, and showed excited-state intermolecular proton transfer fluorescent signal with basic anion. They are more sensitive to dihydrogen phosphate anion than to fluoride anion, although the latter has stronger basicity. Compared with t-butylcalix[4]arene bearing two 2-naphthol-1-hydrazone groups, t-pentylcalix[4]arenes derivative has a larger fluorescent difference between dihydrogen phosphate and fluoride anion. This finding may be used to analyze dihydrogen phosphate anion in the presence of fluoride anion and provide a new approach for designing fluorescence probes that are highly selective for H2PO4−.