Interlayer Electronic Decoupling Unlocks Giant Birefringence in π-Conjugated Metal-Organic Frameworks.

Area of Science:

• Materials Science
• Optics
• Crystallography

Background:

• Giant birefringence is crucial for advanced polarization optics but challenging to achieve.
• Organic π-conjugated molecules have high polarizability but are limited by dense π-π stacking, hindering optical anisotropy.
• Metal-organic frameworks (MOFs) offer ligand packing control, but 3D MOFs have high symmetry and 2D MOFs suffer from eclipsed stacking, limiting birefringence.

Purpose of the Study:

• To overcome the limitations of existing materials and achieve giant birefringence.
• To develop a novel strategy for designing highly birefringent MOFs.
• To enhance optical anisotropy in materials for polarization optics.

Main Methods:

• Anion-induced coordination competition strategy (AICCS) using inorganic anions (SO₄²⁻, NO₃⁻) and HHTP ligands.
• Competitive coordination with La³⁺ to induce in-plane slip of HHTP layers.
• Fabrication of MOFs with engineered slip-stacking to decouple electronic states.

Main Results:

• Successfully disrupted dense π-π stacking in MOFs through engineered slip-stacking.
• Achieved record-high birefringence values: Δn = 1.1 in LaHHTP-SO₄ and Δn = 1.3 in LaHHTP-NO₃.
• Demonstrated significant enhancement from the parent LaHHTP (Δn = 0.12).

Conclusions:

• AICCS is a versatile strategy for designing next-generation anisotropic optical crystals.
• Engineered slip-stacking effectively decouples interlayer electronic states, liberating π-system polarizability.
• This work provides a pathway to materials with giant birefringence for advanced optical applications.