Photophysics and Photochemistry of Metal Complexes with Open Coordination Sites

Abstract

Luminescent transition metal complexes have been one of the most exciting areas of research as evidenced by the extensive literatures showcasing their rich, diverse photoluminescence properties as well as potent impactful applications in materials science, photo-catalysis and medicines. For decades, substantial efforts have been devoted to the design of strong-field ligands with high structural rigidity for reducing excited state structural distortion, and versatile ligand scaffolds for tuning of excited state properties such as intersystem crossing efficiency, emission energy and excited state redox properties of transition metal complexes. In this lecture, the recent progress in the development of luminescent d⁸ and d¹⁰ metal complexes including that of platinum, palladium and gold with emission energies spanning from blue to deep red spectral region is discussed. The planar structure and open coordination sites of these complexes allow for metal-metal closed-shell interaction and substrate binding at the metal site in both the ground state and excited states. New classes of binuclear Pt(II) and Pd(II) complexes with short intramolecular metal-metal contacts and displaying metal-metal bonded excited states have been prepared. By using appropriate ligands, some of these binuclear Pd(II) complexes can be engineered to display long lived emissive ³MMLCT excited states with decent emission quantum yields in solutions at room temperature. By applying a living supramolecular polymerization approach through varying the counteranion, the preparation of sequential phosphorescent multiblock supramolecular Pt(II) and Pd(II) copolymers has been realized in one or more dimensions. Doping small amounts of Pt(II) complex into Pd(II) assemblies significantly improves phosphorescence efficiency, providing a new strategy for preparing luminescent metal-organic supramolecular materials. With the incorporation of spatially separated donor-acceptor pairs, the Au(III) complexes can be engineered to display highly efficient thermally activated delayed fluorescence (TADF) as the predominant emission origin with emission quantum yields of up to 94 % and lifetimes < 2 μs at room temperature. Due to the high sensitivity of the emission properties of d⁸ metal complexes to their local-environment, some of these metal complexes find useful applications in sensing of bio-molecular protein targets of relevance to cancer development. Recent progresses on the applications of luminescent Pt(II) and Au(I) complexes in photo-redox catalysis for organic synthesis and in the fabrication of high performance OLEDs are also discussed in this lecture. .