Efficient crystallization-induced emission in fluorenyl-tethered carboranes

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Fluorenyl-tethered o-carborane (Fl-Cb) derivatives are novel molecular assemblies.
• Understanding their solid-state properties is crucial for materials applications.

Purpose of the Study:

• To synthesize and characterize novel fluorenyl-tethered o-carborane (Fl-Cb) derivatives.
• To investigate their photophysical properties, particularly in solution and aggregated states.
• To explore the phenomenon of crystallization-induced emission.

Main Methods:

• Synthesis of mono- and di-aryl Fl-Cb derivatives from alkynes and B10H12(Et2S)2.
• X-ray crystallography to determine solid-state structures.
• Photophysical measurements including emission quantum efficiency (Φ) in solution and aggregated states.
• Theoretical calculations to understand electronic properties.

Main Results:

• Successful synthesis of Fl-Cb derivatives in ~60% yields.
• Di-aryl Fl-Cb showed rigid head-to-tail or tail-to-tail packing; mono-aryl showed loose packing.
• Strong emission quenching in solution, weak emission in mixed solvents (Φ=0.11).
• Significant crystallization-induced emission observed in di-aryl Fl-Cb (Φ up to 0.95).
• Broadened excitation spectra indicated orbital degeneracy in crystals.

Conclusions:

• Fl-Cb derivatives exhibit distinct packing behaviors based on aryl substitution.
• Aggregation leads to enhanced, albeit weak, emission in solution.
• Di-aryl Fl-Cb compounds demonstrate excellent crystallization-induced emission.
• Orbital degeneracy and ordered crystalline aggregates are key factors for this phenomenon.