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

A family of tridendate ligands 1a-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 2a-e ([Ru(tpy)(1a-e)](2+); tpy = 2,2':6',2"-terpyridine) and 3a-e ([(Ru(1a-e)(2)](2+)), respectively. The ligands and their complexes were characterized by H-1 NMR spectroscopy, ES-MS, and elemental analysis. Single-crystal X-ray analysis of 2a and 2e demonstrated that the triazine core is nearly coplanar with the non-coordinating ring, with dihedral angles of 1.2 and 18.6degrees, 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 2a-e and four for 3a-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 (MLCT)-M-3 state and creating a large energy gap to the dd state.