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

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. 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. A systematic evaluation of the structure-emission property relationship revealed that both compounds formed hydrogen-bonded octahedrons in solution, even in the excited state.image