Conjugated Polymer with Benzimidazolylpyridine Ligands in the Side Chain: Metal Ion Coordination and Coordinative Self-Assembly into Fluorescent Ultrathin Films

The synthesis and characteristic properties of a new conjugated copolymer with poly(phenylene-alt-fluorene) main chain and 2,6-bis(1'-methylbenzimidazolyl)pyridine (bip) ligands attached to the main chain via flexible spacer groups are described. The copolymer was prepared upon Pd-catalyzed Suzuki coupling of the 2,7-bispinacolatoboron ester of 9,9-dihexylfluorene and a 1,4-dibromobenzene derivative carrying two omega-bip-substituted alkoxy groups in the 2- and 5-position. The yield was 88%. The polymer is readily soluble in common organic solvents and exhibits a strong blue fluorescence with maximum at 416 nm; the fluorescence quantum yield in toluene is 85%. UV/vis titration of the polymer with zinc chloride in toluene/methanol (99:1 v/v) and ferrous perchlorate in toluene/methanol (24:1 v/v) indicates formation of 2:1 ligand:metal ion (bis)complexes. Complex formation is accompanied by ion specific color changes (ionochromism) and quenching of the ligand fluorescence. The backbone fluorescence is partially retained. The coordinative interactions between the ligand-substituted polymer ("polytopic ligand") and the divalent metal ions (Zn(II), Cu(II)) can be used for layer-by-layer assembly of organized films on solid supports. Multiple sequential adsorption of metal salts and polymer leads to coordinative supramolecular assembly of the cross-linked polymer-metal ion complex on the substrate, the thickness being controlled by the number of adsorption steps applied. Since the fluorescence of the polymer backbone is not completely quenched by the ligand-metal ion interaction, the films exhibit a bluish luminescence. The new materials might be useful for metal ion sensing and for preparation of fluorescent coatings.