Co‐Ligand Involvement in Ground and Excited States of Electron‐Rich (Polypyridyl)Pt II Complexes

The electronic absorption and luminescence properties of electron‐rich organometallic platinum( II ) complexes with aromatic α‐diimine ligands [(α‐diimine)PtR 2 ] (R = alkyl, alkynyl, Ph or Mes, Mes = 2,4,6‐trimethylphenyl) were examined. The crystal and molecular structures of [(bpy)PtMe 2 ], [(tap)PtPh 2 ] and [(tmphen)PtMes 2 ] were determined from single‐crystal XRD and discussed in view of intermolecular interactions that influence the emission behaviour. The combination of theoretical calculations (DFT) and spectroscopic techniques (absorption and emission) revealed the character of electronic transitions and excited states. In particular they showed that the co‐ligand involvement in the frontier orbitals is small for R = Me, but large in case of R = alkynyl, Ph or Mes. Depending on the chelate ligand and its substitution pattern, the emission efficiencies of the complexes cover a wide range from strong emission in fluid solution at ambient temperature to very weak emission in rigid matrices (glasses) at low temperatures with no emission at ambient temperature. The emission lifetimes, determined at low temperature in solvent glasses, mainly depend on the chelate ligand and not on the co‐ligands. The emission quantum yields of the complexes in solution at room temperatures are much larger for R = Mes than for R = Ph or Me. The emission bands for the complexes with unsaturated co‐ligands show slight structuring at low temperatures in contrast to the alkyl‐substituted ones. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)