Extending the luminescence lifetime of ruthenium(II) poly(pyridine) complexes in solution at ambient temperature

A set of ruthenium(II) poly(pyridine) complexes has been synthesized in which a central diethynylated pyrene moiety separates the 2,2'-bipyridine- and 2,2': 6',2-terpyridine-based terminals. The mononuclear complex, having only the 2,2'-bipyridine ligand coordinated with the metal cation, and the corresponding binuclear complex show remarkably similar luminescence properties in deoxygenated acetonitrile solution at room temperature. Two emission bands are evident in the spectrum. These bands appear to be in thermal equilibrium over the temperature range 0-60degreesC but only a single emitting species is seen in a frozen glass at 77 K. The phosphorescence lifetimes are significantly longer than those associated with the parent complexes under the same experimental conditions but, unlike most other metal complex-pyrene dyads, the luminescence yield is extremely sensitive to the presence of trace amounts of molecular oxygen. The analogous compound having two ruthenium(II) tris(2,2'-bipyridine)-based terminals shows comparable behaviour. Allowing for all of the measured photophysical and electrochemical properties, it is concluded that the triplet manifold has the metal-to-ligand, charge-transfer state localised on the metal complex in equilibrium with an intramolecular charge-transfer state involving the pyrene and a coordinated poly( pyridine) group. The latter state lies at lower energy in a polar solvent and controls the photophysics. At low temperature, only the metal-to-ligand, charge-transfer triplet is observed.