N^N^C platinum (II) complexes based on phenyl-pyridin-2-ylpyrimidine ligands: synthesis, electrochemical and photophysical properties

Abstract

A series of luminescent chloro- and alkynyl-platinum (II) complexes containing various tridentate cyclometalated ligands derived from phenyl-pyridin-2-ylpyrimidine and alkynyl ligands has been successfully synthesized and characterized. Their electrochemical, electronic absorption and luminescence properties have been experimentally investigated and supported by (Time-Dependent) Density Functional Theory ((TD)-DFT) calculations. Electrochemical studies show the presence of a ligand-centered reduction originating from the cyclometalating N^N^C ligands, as well as a major ligand-centered oxidation process (N^N^C or alkynyl) with minor contributions of the metal. Most of the complexes exhibits yellow-to-red luminescence at room-temperature in diluted solutions, in frozen matrices and in the solid state. Electron-withdrawing groups on the alkynyl-ligand induce a red shift of the emission band with increased quantum yield in solution. The emission is also sensitive to the position of the pyridyl fragment on the pyrimidine core: complexes with electron-poorer 4-pyridin-2-ylpyrimidine ligands exhibit red-shifted emission and decreased quantum yield compared to their 2-pyridin-4yl analogues. In solid state, chloro-platinum complexes without tBu bulky group exhibit red-shifted excimer emission.