Reversible pH-responsive fluorescence of water-soluble polyfluorenes and their application in metal ion detection

A novel water-soluble conjugated polymer poly{(4,4'-azobenzene)-2,7-[9,9-bis(6'-N,N,N,-trimethylammonium)hexyl fluorene]dibromide} (PFAB) has been designed and synthesized via Suzuki cross-coupling the fluorene units and azobenzene units. Through simple photoreduction, the azo group of the nonfluorescent PFAB to hydrazine group using UV light, polyfluorene PFAB-L with turn-on fluorescence in aqueous solution is obtained. The optical measurements illustrate that the generation of the flexible hydrazine group induces face-to-face arrangement of phenyl-fluorene-phenyl moieties. Therefore, the excimer formation of phenyl-fluorene-phenyl moieties was induced in PFAB-L. And the fluorescence of PFAB-L can be controlled through modulating the protonation of the -NH-NH- group in solution with different pH. The pH-responsive property is reversible. Moreover, the Fe(3+) ions can selectively quench the fluorescence of the PFAB-L. This new polymer PFAB-L could be used for selective and sensitive sensing Fe(3+) ions in aqueous solution.