Synthesis, Characterization, and Electro‐Optical Properties of Zn^II Complexes with π‐Conjugated Terpyridine Ligands

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 pi-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 Deltarho plots reveals an increase in ellipticity-and therefore pi-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 pi-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 (Phi(PL) up 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, Phi(PL) up to 0.30) reveals their potential as new emissive materials for applications in light-emitting devices.