Tunable Optical Molecular Thermometers Based on Metallacrowns.

Area of Science:

• Coordination Chemistry
• Materials Science
• Nanotechnology

Background:

• Lanthanide complexes are promising for optical thermometry due to their sharp emission lines.
• Controlling the thermal dependence of lanthanide emission is crucial for developing accurate molecular thermometers.
• Ligand energy levels play a significant role in energy transfer processes within lanthanide complexes.

Purpose of the Study:

• To investigate the influence of ligand energy levels on the thermal behavior of lanthanide emission.
• To synthesize and characterize novel lanthanide-based metallacrowns for potential nanothermometer applications.
• To establish structure-property relationships governing the thermal sensitivity of lanthanide luminescence.

Main Methods:

• Synthesis and characterization of Ln₂Ga₈L₈^'L₈^″ metallacrowns using various lanthanides (Gd³⁺, Tb³⁺, Sm³⁺) and hydroxamic acid ligands.
• Spectroscopic analysis to determine ligand-centered singlet (S₁) and triplet (T₁) energy levels.
• Evaluation of thermal dependence of lanthanide emission intensity and quantum yields across a temperature range (150-350 K).

Main Results:

• The energy difference between ligand triplet states (T₁) and excited lanthanide states (Sm^{3+ 4}G_5/2, Tb^{3+ 5}D₄) significantly controls thermal emission dependence via back energy transfer.
• Small S₁-T₁ energy gaps (< 3760 cm^-1) introduce intersystem crossing mechanisms affecting thermal behavior.
• A mixed Tb₂moshi₈^'/Sm₂moshi₈^' system achieved high relative thermal sensitivity (> 3%/K at 225 K), demonstrating a functional optical thermometer.

Conclusions:

• Ligand energy level engineering is a viable strategy for tuning the thermal sensitivity of lanthanide-based molecular thermometers.
• The developed metallacrowns exhibit tunable thermal responses suitable for various temperature sensing applications.
• Mixing different lanthanide complexes offers a pathway to create multi-range optical thermometers with enhanced performance.