Excited States of d6 and d8 Transition Metal Complexes

Abstract

The electronic excited states of some platinum(II), rhenium(I) and ruthenium(II) complexes with diimine ligands were studied with spectroscopic and computational tools. This type of complex can form charge-transfer (CT) excited states which are potentially useful for solar energy applications. fac-Re(CO)3(N^N)L] complexes are discussed first. The complexes form luminescent CT states where an excited electron is localized on the N^N ligand. Resonance Raman spectroscopy shows how the origin of this excited electron can change from ligand-based to metal-based depending on donating ligand L. Variations in the N^N ligand in Re(I) complexes are also studied. Time-resolved infrared spectroscopy shows how complexes with 2-pyridyl-1,2,3-triazole (pytri) ligands form excited states with a smaller degree of charge separation than is typically seen. [Ru(pytri)(bpy)2]2+ complexes are shown to form CT states which are comparable to those of [Ru(bpy)3]2+ where an excited electron is localized on a bpy ligand. However, the pytri complexes have unusually short excited state lifetimes due to rapid formation of metal-centered triplet states (3 MC) which are characterized using density functional theory (DFT) calculations. A series of square-planar Pt(II) complexes containing pytri ligands with various donor ligands (chloride, phenylacetylide, catechol and benzenedithiol) are examined next. The ligand donor strength can control whether the complex is luminescent by adjusting the relative energies of the 3MC and CT states. Resonantly enhanced Raman bands associated with the pytri ligand are very consistent across the series. The catecholate and dithiolate complexes have an unexpected ordering of their CT absorption bands which is not predicted by DFT calculations. Finally, complexes in which an organic electron donor is appended are discussed. This brings about an additional CT state which adds an additional layer of complexity to the systems studied in previous chapters. The light-harvesting ability of the complexes is increased but their photophysical properties also change drastically.