Spectroscopy and Imaging

A series of zinc(II) bis-terpyridine complexes (see picture) is investigated by means of DFT calculations combined with Bader's quantum theory of atoms in molecules. Raman spectroscopy experiments and studies of the electro-optical properties of the complexes in solution and the solid state are also performed to examine their potential as new emissive materials in light-emitting devices. A series of nine zinc(II) complexes containing substituted 4′-phenyl-2,2′:6′,2′′-terpyridines as ligands is synthesized and fully characterized. The ground-state structures of four examples are calculated by means of DFT and their structural features are confirmed by experimental Raman spectroscopy. Special focus is placed on the degree of π-electron delocalization between the terpyridine unit and the attached phenyl moiety. Applying Bader’s quantum theory of atoms in molecules (QTAIM) and visualizing the electron-density distribution by intermolecular Δρ plots reveals an increase in ellipticity—and therefore π-electron delocalization—for phenylvinyl-substituted derivatives compared to phenylethynyl-substituted ones. Experimentally, this is verified by spectroscopic means, because an increase in ellipticity goes along with a pronounced decrease of the HOMO–LUMO energy band gap. Overall, the lateral π-conjugated substituents are found to strongly influence the electro-optical properties of the complexes. In solution, the color of emission can be modulated from violet to cyan (425–487 nm) and high quantum yields (ΦPLup to 0.60) are observed. Thin solid films of the complexes in a matrix of poly(methyl methacrylate) have been inkjet-printed, and their photophysical behavior (bright emission, ΦPLup to 0.30) reveals their potential as new emissive materials for applications in light-emitting devices.