1,10‐Phenanthroline‐Based Octahedra Induced by Protonation of a Nitrogen Atom: Structures and Emission Properties

Abstract

Although a few metal‐free octahedral compounds based on nitrogen‐containing ligands are known, the realization of mixed ligand compounds remains difficult. To obtain an improved understanding of this class of compounds, two new octahedral compounds, [(tmphenH)(phen)]PF₆, and [(tmphenH)(tmphen)]PF₆ (where tmphen=3,4,7,8‐tetramethyl‐1,10‐phenanthroline and phen=1,10‐phenanthroline), were synthesized. The relationship between the emission properties, as evaluated using experimental measurements and theoretical calculations, and structures of the protonated compounds was studied. In addition, differences between the atomic charge distributions and geometries in the ground and singlet excited states were investigated using density functional theory calculations. Both [(tmphenH)(phen)]PF₆ and [(tmphenH)(tmphen)]PF₆ formed hydrogen‐bonded octahedrons in acetonitrile solution, even in the excited state. However, the hydrogen bonding in [(tmphenH)(phen)]PF₆ was weak and thermal vibrations decreased the emission intensity. In contrast, the additional methyl substituents in [(tmphenH)(tmphen)]PF₆ suppressed thermal vibrations, resulting in a higher emission intensity.