Redox and spectroscopic orbitals in Ru(II) and Os(II) phenolate complexes

Area of Science:

• Inorganic Chemistry
• Materials Science
• Photochemistry

Background:

• Phenolate-bound metal complexes offer tunable electronic properties.
• Ruthenium and osmium complexes are investigated for their potential in solar energy applications.
• Understanding frontier molecular orbitals (HOMOs and LUMOs) is crucial for designing efficient light-harvesting materials.

Purpose of the Study:

• To investigate the electronic structure and properties of novel phenolate-bound ruthenium and osmium complexes.
• To determine the origin of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs).
• To assess the impact of metal (Os, Ru) and ancillary ligands (2,2-bipyridine, 2,2-biquinoline) on electronic and photophysical properties.

Main Methods:

• Detailed spectroscopic (UV-Vis, resonance Raman) and electrochemical studies were performed.
• Dynamic electrochemistry was employed to assess the stability of oxidized complexes.
• Computational methods were used to analyze orbital contributions and mixing.

Main Results:

• The synthesized [M(L-L)2(box)](PF6) complexes exhibit rich spectroscopy extending into the near-infrared and good photostability.
• First oxidation is metal-based (MII/III), while a second irreversible wave is phenolate-based.
• Complex stability follows the order: [Ru(biq)2(box)]+ < [Ru(bpy)2(box)]+ < [Os(bpy)2(box)]+.
• A discrepancy between spectroscopic (phenolate HOMO) and electrochemical (metal HOMO) assignments suggests significant metal-ligand orbital mixing, particularly in biquinoline complexes.

Conclusions:

• These phenolate-bound complexes show promise as solar sensitizers.
• Electronic properties are significantly influenced by the choice of metal and ligands.
• Metal-ligand orbital mixing plays a critical role in the observed electronic transitions and redox behavior.