Tuning Birefringence in Supramolecular Mg-PDA Frameworks: Modulating Bonding and Layer Connectivity

Area of Science:

• Materials Science
• Crystallography
• Supramolecular Chemistry

Background:

• Development of novel materials with significant optical properties is crucial for advanced photonic applications.
• Magnesium-organic frameworks offer tunable structures and properties for diverse functionalities.
• Understanding structure-property relationships in hydrogen-bonded and coordination frameworks is key to material design.

Purpose of the Study:

• To synthesize and characterize novel Mg-PDA supramolecular frameworks.
• To investigate the optical properties, particularly birefringence, of the synthesized compounds.
• To evaluate their potential for UV applications based on optical anisotropy and UV cutoff.

Main Methods:

• Hydrothermal synthesis of Mg-PDA crystals.
• Single-crystal X-ray diffraction for structural determination.
• Optical microscopy and spectroscopic measurements for birefringence and UV cutoff determination.
• Theoretical calculations to support experimental findings.

Main Results:

• Two novel Mg-PDA frameworks, [Mg(H2O)6][(C7H4NO4)·H2O]2 (1) and Mg(C7H3NO4)(H2O)2 (2), were successfully prepared.
• Compound 2 exhibits a 2D neutral layer structure with coplanar π-conjugated pyridine rings, leading to enhanced optical anisotropy.
• Compound 2 displays giant birefringence (Δn = 0.44 @ 546 nm), significantly higher than compound 1 (Δn = 0.34 @ 546 nm) and commercial α-BaB2O4 (Δn = 0.122 @ 546 nm).
• Both compounds possess short-wave UV cutoff edges (279 nm for 1, 273 nm for 2).

Conclusions:

• The structural differences between compounds 1 and 2 directly influence their optical anisotropy and birefringence.
• Compound 2 demonstrates superior birefringent properties among UV Mg-containing crystals, making it a promising candidate for UV photonic devices.
• The Mg-PDA system provides a viable platform for designing high-performance optical materials.