Electronic tuning of ruthenium complexes by 8-quinolate ligands

Area of Science:

• Inorganic Chemistry
• Photochemistry
• Computational Chemistry

Background:

• Ruthenium(II) complexes, such as [Ru(bpy)(3)](2+), are important in photochemistry.
• 8-hydroxyquinoline derivatives offer tunable electronic properties for ligand design.

Purpose of the Study:

• To synthesize and characterize novel Ru(II) complexes incorporating 8-hydroxyquinoline ligands.
• To investigate the electronic effects of varying the number of 8-hydroxyquinoline ligands on Ru(II) complex properties.
• To understand the origin of spectral shifts in these complexes.

Main Methods:

• Synthesis of Ru(II) complexes with 8-hydroxyquinoline (quo(-)) and 5-NO(2)-8-hydroxyquinoline (5-NO(2)-quo(-)) ligands.
• Electrochemical and spectroscopic analyses to study electronic properties.
• Density Functional Theory (DFT) and time-dependent DFT (TD-DFT) calculations for electronic structure and transitions.

Main Results:

• A progressive red-shift in the lowest energy absorption maximum was observed with increasing numbers of quo(-) ligands, from 452 nm in [Ru(bpy)(3)](2+) to 540 nm in [Ru(quo)(3)](-).
• DFT calculations indicated an increase in the energy of the occupied t(2g)-type orbitals across the series.
• TD-DFT and electrochemical data revealed that the lowest energy transition involves contributions from both the quo(-) ligand and the Ru(II) metal center.

Conclusions:

• The electronic properties and photophysical behavior of Ru(II) complexes can be effectively tuned by modifying the ligation sphere with 8-hydroxyquinoline derivatives.
• The observed bathochromic shift is attributed to the destabilization of the metal's occupied t(2g) orbitals upon coordination of successive quo(-) ligands.
• The lowest energy electronic transition in these complexes is not purely metal-centered, indicating significant ligand contribution.