Bulky ligands protect molecular ruby from oxygen quenching

Area of Science:

• Inorganic Chemistry
• Photochemistry
• Materials Science

Background:

• Chromium(III) complexes exhibit near-infrared phosphorescence from doublet states (2E/2T1).
• High photoluminescence quantum yields and long lifetimes are observed in the absence of oxygen.
• The prototype molecular ruby, [Cr(ddpd)2]3+, shows luminescence quenched by triplet oxygen (3O2) via Dexter energy transfer.

Purpose of the Study:

• To enable luminescence applications of molecular rubies in solution under aerobic conditions.
• To explore sterically demanding N,N'-dimethyl-N,N'-dipyridyl-pyridine-2,6-diamine (ddpd) ligands for shielding chromium(III) centers from oxygen.
• To broaden the applicability of highly emissive chromium(III) complexes.

Main Methods:

• Steady-state and time-resolved photoluminescence spectroscopy.
• Single crystal X-ray diffraction for structural analysis.
• Quantum chemical density functional theory (DFT) calculations.
• Stern-Volmer analyses of photoluminescence quantum yields and excited state lifetimes under inert and aerobic conditions.

Main Results:

• Sterically demanding ddpd ligands were synthesized and characterized.
• Optimal shielded chromium(III) complexes retained photoluminescence in air-saturated acetonitrile.
• Photoluminescence quantum yields up to 5.1% and excited state lifetimes of 518 µs were achieved in aerobic conditions.
• Ligand design effectively shields the chromium(III) center from oxygen quenching.

Conclusions:

• Steric shielding of chromium(III) centers by bulky ligands is a viable strategy to mitigate oxygen quenching.
• This approach significantly broadens the potential applications of phosphorescent chromium(III) complexes in aerobic environments.
• Further development of ligand design can enhance the performance of molecular rubies for luminescence applications.