Polyiminofluorene with conjugated benzimidazolylpyridine substituent groups: optical properties, ionochromism and coordinative self-assembly into electrochromic films

Abstract

Synthesis and characteristic properties of polyiminofluorene P1 with 2,6-bis(1′-methylbenzimidazolyl)pyridine (bip) ligands attached to the N-atoms of the backbone via conjugated 4-ethinylenephenylene spacer units are reported. P1 was obtained upon Pd-catalyzed amination of 2,7-dibromo-9,9-dihexylfluorene with 4-(2-(2,6-bis(1′-methylbenzimidazolyl)pyridinyl)-ethinyl)aniline. MALDI-TOF spectrometry shows that oligomers up to the tetradecamer are formed, the hexamer being the most abundant species. P1 is soluble in organic solvents such as xylene, toluene, or chlorobenzene forming yellow solutions with blue, green or yellow fluorescence depending on the polarity of the solvent. The fluorescence quantum yield in xylene is 41%. UV/Vis titration of the polymer with zinc chloride in toluene–methanol (49 : 1) indicates formation of 2 : 1 ligand–metal (bis) complexes and 1 : 1 mono complexes. Complex formation with various divalent transition metal ions is accompanied by ion specific colour changes (ionochromism) and quenching of the polymer fluorescence. The coordinative interactions between the bip ligands and metal ions such as Zn(II) or Cu(II) have been used for layer-by-layer assembly of ultrathin films on solid supports. Sequential adsorption of metal ions and P1 leads to formation of ultrathin coordination polymer network films. Due to ion specific metal-to-ligand charge transfer the P1/Zn(II) films are orange, while P1/Cu(II) films are purple. If anodically oxidized up to 750 mV vs. FOC, P1/Zn(II) and P1/Cu(II) films change colour into blue due to oxidation of the polymer backbone. Colour changes are reversible. Films prepared upon 12 dipping cycles exhibit switching times from the neutral to the oxidized state of 300 to 500 ms, the contrast being up to 53%. SEM studies indicate a homogeneous surface, which becomes slightly inhomogeneous after electrochemical cycling.