Tuning the spectroscopic and electrochemical properties of polypyridyl bridged mixed-metal trimetallic ruthenium(II), iridium(III) complexes: a spectroelectrochemical study

Abstract

A series of polypyridyl bridged trimetallic complexes of the type {[(bpy) 2Ru(BL)] 2IrCl 2} 5+ (where BL = 2,3-bis-2′-pyridylpyrazine (dpp), 2,3-bis-2′-pyridylquinoxaline (dpq) or 2,3-bis-2′-pyridylbenzoquinoxaline (dpb); bpy = 2,2′-bipyridine) have been prepared and their synthesis, characterization and spectroelectrochemical analysis are reported within. These complexes are of interest in that they contain two visible light absorbing centers covalently coupled to a known catalytically active central metal site. The trimetallic complexes show absorbances throughout the visible region of the spectrum and exhibit many electrochemical processes within the acetonitrile solvent window. All the systems studied possess a ruthenium based oxidative process as well as four bridging ligand based reductions, followed by iridium and bipyridine based reductive processes. The relative energy of the ruthenium based dπ highest-occupied molecular orbital (HOMO) remains constant for this series of trimetallic complexes. The energy of the lowest lying bridging ligand based π* orbital, the lowest-unoccupied molecular orbital (LUMO), however, shifts to more positive potentials when dpq or dpb are substituted for dpp. This gives rise to a lowest energy absorption, Ru(dπ) → BL(π*) metal-to-ligand charge transfer (MLCT) transition, which can be tuned to lower energy as a function of bridging ligand from dpp to dpq to dpb. Through the synthetic variation of bridging ligand orbital energy and the use of spectroelectrochemical studies, it has been possible to elucidate the nature of the complex spectroscopy and electrochemistry of these supramolecular complexes. The dpp and dpq bridged systems emit in fluid solution at room temperature and their emission energies and lifetimes have been determined.