Ligand-dependent excited state behaviour of Re(I) and Ru(II) carbonyl-diimine complexes

The relations between the structure and excited state properties of ReQ(CO)(3)(alpha-diimine) and Ru(E)(E')(CO)(2)(alpha-diimine) complexes (axial ligand E, E' = halide, alkyl, benzyl, metal fragment) are unravelled and discussed in detail. For example, it is shown how the increasing pi-donor strength of an axial ligand, such as a halide changes the character of the lowest excited state from MLCT to LLCT. On the other hand, the presence of a covalently bound axial ligand in the coordination sphere introduces a sigma pi* lowest excited state that involves an excitation of an electron from the metal-ligand sigma-bonding orbital to the pi* orbital of the alpha-diiamine. While the orbital parentage of the lowest excited state-MLCT, LLCT, sigma pi* or IL-is mostly determined by the axial ligand(s), its detailed properties (energy, lifetime, reactivity, decay mechanism) are dependent on both the axial and diimine ligands. Depending on the molecular structure and the medium, the excited state behaviour of these complexes ranges from a strong, long-lived emission to a very fast photochemical homolysis of a metal-ligand bond. Photochemical and photophysical properties of the complexes with different types of the lowest excited state are explored and pertinent structural effects discussed. It is shown how the excited state properties of the Re and Ru carbonyl-diimine complexes can be controlled by a judicious choice of the axial and diimine ligands and by the medium. These relations can be employed to design new functional molecular photonic materials, e.g. sensitisers, luminophores, photocatalysts, radical photoinitiators, luminescent probes or sensors. (C) 1998 Elsevier Science S.A. All rights reserved.