In order to improve the water-solubility of 4-(2'-hydroxyphenyl)-pyrimido[1,6-a]benzimidazole and 4-(2'-hydrx- yphenyl-4'-isopropoxy)-pyrimido[1,6-a]benzimidazoles, they were sulfonated in 98% H₂SO₄. Sodium 4-(2'-hydroxyphenyl)- pyrimido[1,6-a]benzimidazole-5-sulfonate (L1), sodium 4-(2'-hydroxyphenyl)-pyrimido[1,6-a]benzimidazole-3',5',10'-three- sulfonate (L2), sodium 4-(2',4'-dihydroxyphenyl)pyrimido[1,6-a]benzimidazol-5-sulfonate (L3) were obtained and indentified by the method of NMR, IR and MS. Fluorescence spectra of L1, L2 and L3 were detected, and they have the ESIPT effect with a large Stokes shift (175～225 nm). The structures of these compounds could be complex with Al³⁺, and the fluorescence spectra of Al³⁺ in L1, L2 and L3 were analyzed. The results showed that L2 exhibited good sensitivity selectivity to Al³⁺. The fluorescence of L2 was strengthened dramatically in the presence of Al³⁺. Other metal ions, such as Cu²⁺, Hg²⁺, K⁺, Mg²⁺, Na⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Fe³⁺ induced negligible fluorescence strengthening for L2 under the same conditions. When the concentration (0～20 μmol/L) of Al³⁺ was increased, the fluorescence intensity of L2 (10 μmol/L) was also increased. The Job’s plot and fluorescence titration indicated that the stoichiometry of L2-Al³⁺ was 1:2 and the detection limit of Al³⁺ was 0.073 μmol/L. Moreover, L2 was used for the visual detection of Al³⁺.

Key words: coordinate, aluminum ions, fluorescent probe, excited state intramolecular proton transfer (ESIPT), visual detection