Water-soluble anionic conjugated polymers for metal ion sensing: effect of interchain aggregation

Three sulfonato-containing fluorene-based anionic water-soluble conjugated polymers, which are specially designed to link fluorene with alternating moieties such as bipyridine (P1), pyridine (P2), and benzene (P3) have been synthesized via the Pd-catalyzed Sonogashira-coupling reaction, respectively. These polymers had good solubility in water and showed different responses for transition metal ions with different valence in aqueous environments: the fluorescence of bipyridinecontaining P1 can be completely quenched by addition of all transition metal ions selected and showed a good selectivity for Ni ²⁺; the pyridine-containing P2 had a little response for monovalent and divalent metal ions while showed good quenching with the addition of trivalent metal ions (with a special selectivity for Fe³⁺); P3 had responses only for the trivalent metal ions within the ionic concentration we studied. After investigation of the UV-vis absorption spectra, PL emission spectra, DLS, and fluorescence lifetime of P1-P3 in aqueous solution when adding transition metal ions, we found that the different spectrum responses of these polymers are attributed to the different coordination ability of the units linked with fluorene in the main chain. The energy or electron-transfer reactions were the main reason for fluorescence quenching of P1 and P2. On the other hand, interchain aggregation caused by trivalent metal ions lead to fluorescence quenching for PS and also caused partly fluorescence quenching of P1 and P2. These results revealed the origin of ionochromic effects of these polymers and suggested the potential application for these polymers as novel chemosensors with higher sensing sensitivity in aqueous environments.