Ruthenium Complexes of Easily Accessible Tridentate Ligands Based on the 2‐Aryl‐4,6‐bis(2‐pyridyl)‐s‐triazine Motif: Absorption Spectra, Luminescence Properties, and Redox Behavior

Abstract

A family of tridendate ligands 1 a–e, based on the 2-aryl-4,6-di(2-pyridyl)-s-triazine motif, was prepared along with their hetero- and homoleptic Ru^II complexes 2 a–e ([Ru(tpy)(1 a–e)]²⁺; tpy=2,2′:6′,2′′-terpyridine) and 3 a–e ([(Ru(1 a–e)₂]²⁺), respectively. The ligands and their complexes were characterized by ¹H NMR spectroscopy, ES-MS, and elemental analysis. Single-crystal X-ray analysis of 2 a and 2 e demonstrated that the triazine core is nearly coplanar with the non-coordinating ring, with dihedral angles of 1.2 and 18.6°, respectively. The redox behavior and electronic absorption and luminescence properties (both at room temperature in liquid acetonitrile and at 77 K in butyronitrile rigid matrix) were investigated. Each species undergoes one oxidation process centered on the metal ion, and several (three for 2 a–e and four for 3 a–e) reduction processes centered on the ligand orbitals. All compounds exhibit intense absorption bands in the UV region, assigned to spin-allowed ligand-centered (LC) transitions, and moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region. The compounds exhibit relatively intense emissions, originating from triplet MLCT levels, both at 77 K and at room temperature. The incorporation of triazine rings and the near planarity of the noncoordinating ring increase the luminescence lifetimes of the complexes by lowering the energy of the ³MLCT state and creating a large energy gap to the dd state.