Structural and spectroscopic studies on Pt.Pt and pi-pi interactions in luminescent multinuclear cyclometalated platinum(II) homologues tethered by oligophosphine auxiliaries

The synthesis and structural, spectroscopic, and electrochemical properties of a series of trinuclear tridentate cyclometalated platinum(II) complexes tethered by bis(diphenylphosphinomethyl)phenylphosphine (dpmp) have been studied and compared with their mono- and binuclear homologues and a propeller-like congener. The X-ray crystal structures of several derivatives show the presence of a variety of intramolecular Pt.Pt, pi-pi, and C-H...O(crown ether) and intermolecular pi-pi interactions. The trinuclear complexes display strong absorption in the 400-600 nm region and show intense red to near-infrared phosphorescence with microsecond lifetimes in fluid and glassy solutions and in the solid state. These emissions are generally assigned as (3)MMLCT [dsigma-->pi(CNN)] in nature. The close similarities between the emission energies in acetonitrile solution and in the solid state at 298 K indicate that comparable Pt.Pt and pi-pi configurations are maintained in both media, and hence a relationship between the photophysical behavior of these lumophores and their solid-state structural features is proposed. The tendencies of the absorption and emission energies to red-shift from mono- to linearly tethered bi- and trinuclear Pt(II) species are evident. A light-emitting electrochemical cell using a trinuclear Pt(II) derivative as emitter has been demonstrated.