Organometallic polymers based on fluorene-bridged bis-benzimidazolylidene via direct ligand-metal coordination: Synthesis, characterization and optical properties

To develop new organometallic polymers we synthesized a novel fluorene-bridged benzimidazolium salt (BNM) by successive Suzuki coupling reaction and quaternization. Organometallic polymers of platinum (PMPt) and palladium (PMPd) were prepared via direct ligand-metal coordination. Chemical structures of the polymers were characterized by the ¹H NMR and FT-IR spectra, and elemental analysis. Their glass transition (195°C, 196°C) and thermal decomposition temperatures (287°C, 289°C) were comparable, as determined by DSC and TGA techniques, respectively. Their LUMO and HOMO energy levels were -2.26 eV, -2.29 eV and -5.76 eV, -5.78 eV, respectively, as estimated from the onset reduction and oxidation potentials obtained from cyclic voltammograms. Optical properties of the organometallic polymers were studied by absorption and photoluminescence (PL) spectra. The PL spectral intensity in solution was enhanced obviously upon increasing the HCl concentration, while that of absorption spectra remained almost unchanged. The PL emission enhancement is probably due to diminished intersystem crossing in the presence of a protonic acid.