Electronic Tuning of Ruthenium Complexes by 8‐Quinolate Ligands

Abstract

A series of Ru(II) complexes were synthesized with the deprotonated forms of the ligands 8-hydroxyquinoline (quo(-)) and 5-NO(2)-8-hydroxyquinoline (5-NO(2)-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 t(2g)-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 t(2g)-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+).

Unusually Short-Lived Solvent-Dependent Excited State in a Half-Sandwich Ru(II) Complex Induced by Low-Lying 3MC States

“…Ruthenium complexes attract broad interest in a variety of different fields, e.g., dye-sensitized solar cells, − photocatalysts, − as molecular sensors, − or as photoactivated medical agents. − This impressive breadth of applications roots in the variety of excited states which are accessible upon photoexcitation, including metal-to-ligand charge transfer (MLCT), metal-centered (MC), ligand-to-ligand charge transfer (LLCT), and intraligand (IL) states . The properties of these states can be easily tuned by ligand design. − Generally, the lifetime of the excited states is (comparably) long due to efficient intersystem crossing (ISC) from the initially populated singlet to the more stable triplet states. − …”