Syntheses, X‐Ray Crystal Structures, Emission Properties and DFT Calculations of Monoprotonated Polypyridines

Monoprotonated compounds [(L)HPF6] were prepared by the reaction of (L=bpy, phen, dpphen, bqn and ppy) with concentrated HCl in water. Monoprotonated pyridine rings are hydrogen bonded intramolecularly to the adjacent pyridine ring and intermolecularly to the adjacent PF6− in compounds. These hydrogen bonds restrain the nonradiative decay to produce intense emission. Density functional theory was applied to interpret the planarity in compounds. The attachment of one proton to the nitrogen in [(dpphen)HPF6] and [(bqn)HPF6] leads to the strong emission in acetonitrile (Φ = 0.046 and 0.097, respectively). In particular, the attachment of one proton to the ppy nitrogen results in exhibiting a strong emission with a large quantum yield (Φ = 0.264). Monoprotonated pyridine rings are hydrogen bonded intramolecularly to the adjacent pyridine ring and intermolecularly to the adjacent PF6‐ in compounds. These hydrogen bonds restrain the nonradiative decay to produce intense emission. In particular, the attachment of one proton to the nitrogen in [(bqn)HPF6] leads to the strong emission in acetonitrile (Φ =0.097). Density functional theory was applied to interpret the emission properties in compounds.