The Binuclear Organoplatinum(II) Complexes with Diphosphine Bridging Ligands; Synthesis and Optical Properties

Cyclometalated organometallic compounds have been extensively investigated by many research groups because of their catalytic reactivity in organic synthesis and diverse medical and industrial applications. However, most applications are restricted to mononuclear cyclometalated organoplatinum(II) complexes, in which different kinds of electronic transitions, such as metal to ligand charge transfer (MLCT), ligand-centered (LC), and ligand to ligand charge transfer (L′LCT), are involved in any related emission operation. Also, the molecular interactions in platinum(II) complexes lead to significantly red-shifted long-wavelength emission, which is of considerable interest in near-infrared (NIR) absorbing and emitting organic materials.The binuclear platinum complexes have drawn attention because of their particular photophysical properties. Metal−metal to ligand charge transfer (MMLCT) transitions exist in binuclear platinum(II) complexes when the Pt···Pt distance is less than 3.5 Å (the sum of two platinum van der Waals radius), leading to marked red shifts compared to the corresponding emission peaks of mononuclear platinum complexes. Although mononuclear platinum(II) complex units tend to interact with each other in the solid state and in concentrated solutions, Pt···Pt interactions are generally not strong enough to emerge in a diluted solution at room temperature. Moreover, even in the solid state, fine control of the Pt···Pt interactions is rather difficult for self-assembled mononuclear platinum(II) complexes.From this viewpoint, double-decker platinum(II) complexes with well-defined Pt···Pt distances, built by bridging ligands, are very attractive. Furthermore, Pt···Pt distances can be precisely controlled using the appropriate bridging ligands. The formation of MMLCT transitions is due to charge transfer between a full Pt−Pt dσ* orbital and a partially empty ligand π* orbital on the cyclometalated ligand to accomplish a significantly red-shifted spectrum. In most cases, to design these systems, double bridging ligands with N^N, N^S, and N^O coordinating atoms were used, but the complexes with single P^P bridging ligands, such as bis(diphenylphosphino)methane (dppm), are still rare.The aim of this presentation is to introduce several new class of emissive cyclometalated platinum(II) complexes with a bridging dppm ligand. The luminescence properties of binuclear complexes with bridging dppm were studied; the influence of bridging dppm on its metal−metal interactions and their emission were explored.