Sensitization Pathways in NIR-Emitting Yb(III) Complexes Bearing 0, +1, +2, or +3 Charges

Area of Science:

• Coordination Chemistry
• Photophysics
• Lanthanide Complexes

Background:

• Macrocyclic ligands are crucial for stabilizing metal ions.
• Carbostyril chromophores are used for lanthanide sensitization.
• Tuning charge and donor groups impacts complex properties.

Purpose of the Study:

• Synthesize Yb(III) complexes with macrocyclic ligands and carbostyril chromophores.
• Investigate the effect of ligand donor groups (carboxylate vs. carbamide) and overall charge on complex properties.
• Analyze Yb(III)/Yb(II) reduction potentials and Yb(III) luminescence sensitization.

Main Methods:

• Synthesis of Yb(III) complexes with 1,4,7,10-tetraazacyclododecane derivatives.
• Paramagnetic 1H NMR spectroscopy to determine coordination geometry.
• Cyclic voltammetry to measure reduction potentials.
• Photoluminescence spectroscopy to quantify sensitization and quantum yields.

Main Results:

• Coordination geometry was largely unaffected by carboxylate/amide substitution.
• Yb(III)/Yb(II) reduction potentials shifted with complex charge, becoming easier to reduce for more positive charges.
• Carbostyril excitation led to Yb(III) luminescence in all complexes.
• Photoinduced electron transfer (PeT) from carbostyril to Yb(III) was observed, evidenced by decreased carbostyril fluorescence.
• Luminescence quantum yields were generally constant, except for a +3 charged complex with methylated amides, which showed enhanced emission.
• NH vibrations quenched Yb(III) emission, and faster PeT did not improve sensitization efficiency due to competing phonon-assisted energy transfer (PAEnT).

Conclusions:

• Ligand design offers control over Yb(III) complex properties, including reduction potential.
• Photoinduced electron transfer and phonon-assisted energy transfer play significant roles in Yb(III) sensitization.
• Despite challenges like NH vibrational quenching, Yb(III) complexes remain promising luminescent materials.