Seeking Brightness in Molecular Erbium-Based Light Upconversion.

Area of Science:

• Coordination Chemistry
• Materials Science
• Photophysics

Background:

• Dye-sensitized lanthanide-doped nanoparticles advance near-infrared (NIR) to visible-light upconversion.
• Molecular coordination complexes face challenges due to cationic sensitizers limiting lanthanide activator binding.
• Previous molecular designs had large sensitizer-activator distances, reducing energy transfer efficiency.

Purpose of the Study:

• To design and synthesize a novel molecular complex for efficient intramolecular light upconversion.
• To overcome electrostatic penalties and reduce sensitizer-activator distances in molecular systems.
• To enhance energy transfer upconversion (ETU) efficiency for lanthanide complexes.

Main Methods:

• Synthesis of a compact ligand ([L2]+) with a single sulfur connector.
• Preparation of nine-coordinate molecular adducts ([L2]Er(hfac)3]+) in solution.
• Detailed photophysical studies, including energy transfer upconversion (ETU) mechanism analysis.

Main Results:

• Successfully synthesized quantitative amounts of [L2]Er(hfac)3]+ at millimolar concentrations.
• Reduced sensitizer-activator distance by 40% to approximately 0.7 nm.
• Achieved a three times improved ETU mechanism and unprecedented brightness (Bup = 2.0(1) × 10^-3 M^-1·cm^-1) for NIR to visible light upconversion.

Conclusions:

• The novel compact ligand design effectively counterbalances electrostatic penalties, enabling efficient metal complexation.
• Close proximity of cyanine sensitizer and Erbium activator boosts the heavy atom effect, enhancing ETU.
• This molecular system offers a promising platform for bright NIR-to-visible light upconversion applications.