Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Hybrid organic-inorganic materials offer tunable optoelectronic properties.
• Polar materials are crucial for nonlinear optics and photonics applications.
• Lead halide perovskites and related structures are actively researched for their unique characteristics.

Purpose of the Study:

• To synthesize and characterize novel polar hybrid lead halide phases: (MDA)Pb2I6 and (MDA)Pb2Br6.
• To investigate their photophysical properties, including second harmonic generation (SHG) and luminescence.
• To understand the structure-property relationships governing their optical responses.

Main Methods:

• Single crystal growth of methylenedianiline lead iodide and bromide.
• X-ray diffraction for crystallographic analysis (noncentrosymmetric space group Fdd2).
• Photophysical characterization including SHG measurements and temperature-dependent fluorescence spectroscopy.

Main Results:

• Both (MDA)Pb2I6 and (MDA)Pb2Br6 crystallize in the polar Fdd2 space group, exhibiting oriented molecular dipoles.
• The bromide phase, (MDA)Pb2Br6, shows a stronger SHG response and distinct fluorescence around 630 nm below 75 K.
• (MDA)Pb2I6 exhibits a weaker SHG signal and minimal luminescence, attributed to iodide ion polarizability screening.

Conclusions:

• The polar nature of these hybrid phases enables nonlinear optical phenomena like SHG.
• Differences in halide sublattice character significantly impact photophysical properties.
• Iodide ion polarizability likely screens local electric fields, reducing optical response in (MDA)Pb2I6 compared to (MDA)Pb2Br6.