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Electronic Tuning of Ruthenium Complexes by 8-Quinolate Ligands


Corresponding author email: (Claudia Turro)


A series of Ru(II) complexes were synthesized with the deprotonated forms of the ligands 8-hydroxyquinoline (quo) and 5-NO2-8-hydroxyquinoline (5-NO2-quo) as analogs to the prototypical complex [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine). Electrochemistry, spectroscopy and density functional theory calculations were utilized to investigate the electronic tuning of the occupied t2g-type orbitals of the metal center with variation in the ligation sphere. The maximum of the lowest energy absorption of complexes containing one, two and three 8-quinolate ligands progressively redshifts from 452 nm in [Ru(bpy)3]2+ to 510 nm in [Ru(bpy)2(quo)]+, 515 nm in [Ru(bpy)(quo)2] and 540 nm in [Ru(quo)3] in water. This bathochromic shift results from the increase in energy of the occupied t2g-type orbital across the series afforded by coordination of each subsequent quo ligand to the Ru(II) center. Time-dependent density functional theory calculations along with electrochemical analysis reveals that the lowest energy transition has contributions in the highest occupied molecular orbital from both the quo ligand and the metal, such that the lowest energy transition is not from an orbital that is purely metal-centered in character as in [Ru(bpy)3]2+.