Utilization of substituted polyazine bridging ligands to tune the spectroscopic and electrochemical properties of bimetallic ruthenium complexes

Abstract

The polyazine bridging ligands Cl 2dpq and Me 2dpq (where Cl 2dpq=6,7-dichloro-2,3-bis(2′-pyridyl)quinoxaline and Me 2dpq=6,7-dimethyl-2,3-bis(2′-pyridyl)quinoxaline) have been synthesized and their electrochemical and spectroscopic properties studied. Cl 2dpq is easier to reduce than the unsubstituted dpq (dpq=2,3-bis(2′-pyridyl)quinoxaline) ligand by 250 mV, while Me 2dpq is harder to reduce than dpq by 180 mV. These two substituted dpq ligands, along with dpp, dpq and dpb (dpp=2,3-bis(2′-pyridyl)pyrazine and dpb=2,3-bis(2′-pyridyl)benzoquinoxaline), give a series of five polyazine bridging ligands which provide a similar coordination environment to metals. In addition, this series of ligands makes possible the systematic variation of the energy of the lowest unoccupied molecular orbital (LUMO), the bridging ligand based π*. The relative energy of the π* orbitals is dpp>Me 2dpq>dpq>Cl 2dpq>dpb. The new monometallic and bimetallic systems of the form [(bpy) 2Ru(BL)] 2+ and [(bpy) 2Ru]2(BL) 4+ (where BLMe 2dpq and Cl 2dpq) have been synthesized and their spectroscopic and electrochemical properties studied. In addition, the previously studied systems with dpp, dpq and dpb as the bridging ligand have been prepared and their properties are reported herein for comparison. The metal complexes become easier to reduce as a function of bridging ligand with E 1/2(reduction) for dpp<Me 2dpq<dpq<Cl 2dpq<dpb. The energy of the lowest metal-to-ligand charge transfer (MLCT) band, Ru(dπ)→BL(π*), as well as the emission energy, shift to lower energy as a function of bridging ligand with E abs or E em for dpp>Me 2dpq>dpq>Cl 2dpq>dpb. These results indicate that it is possible to tune the spectroscopic and electrochemical properties of multimetallic complexes through the incorporation of substituent groups on polyazine bridging ligands.