Dipolar Correlations in Amphidynamic Crystalline Rotor Arrays

NONTECHNICAL SUMMARY:

Crystalline solids have all their molecules aligned in a precise periodic manner. One can visualize molecules coming together to form ordered chains and layers that stack closely together to make up a three-dimensional crystal. While it is known that molecules with arbitrary shapes pack so tight together that they become essentially static, it has been shown that some molecular shapes and framework architectures result in the formation of amphidynamic crystals.

These relatively new architectures possess static, crystal-forming elements, linked to molecular units that can be highly mobile. A class of amphidynamic crystal-former of particular relevance to this project contains molecules that have the structural elements of a toy compass: they have needle-like, electric and/or magnetic rotary units shielded in a box, known as dipoles, which spontaneously reorient to interact with each other and with strong external magnetic (or electric) fields.

These dipole-dipole interactions, or dipolar correlations, offer opportunities to explore materials properties based on the ability of individual dipoles to interact with each other in manners that depend on the way they are organized. While some ordered dipolar arrays have symmetries where all the dipoles point in the same direction, others form structures where all the dipoles cancel each other.

The first alignment mode generates macroscopically polar crystals, and the second one leads to the formation of macroscopically non-polar materials. With support from the Solid State and Materials Chemistry program in NSF’s Division of Materials Research, researchers at the University of California Los Angeles explore dipolar correlations in amphidynamic crystalline rotor arrays that can be used for the preparation of addressable polar and non-polar materials.

They investigate whether dipolar correlations, spontaneous polarization and the collective reorientation of a set of interacting dipoles have the potential for controlling a number of thermal, optical, and dielectric properties while providing a promising platform for the development of smart materials, for example for clean energy or semiconductor applications, and artificial molecular machines. The effective use of large arrays of macroscopic toy compasses to illustrate the behavior of molecular dipole arrays is developed into a classroom demonstration for high school students that can help chemistry instructors introduce concepts related to molecular structure, intermolecular forces, and emergent phenomena.

TECHNICAL SUMMARY:

With support from the Solid State and Materials Chemistry program in NSF’s Division of Materials Research, researchers at the University of California Los Angeles synthesize crystalline dipolar arrays based on metal organic frameworks with tetragonal, hexagonal and Kagome lattice architectures to explore the onset of emergent order. While tetragonal electric dipole arrays are expected to adopt antiferroelectric order, electric dipoles on hexagonal and Kagome lattices are expected to adopt ferroelectric alignment and macroscopic polarization.

The structural challenges to surmount go beyond the creation of the proper lattice symmetry: It is well known that collective dipole reorientation and efficient polarization require rotational energy barriers to be smaller than the orientation-dependent dipole-dipole interaction energies, which need to be greater than thermal energy to avoid individual Brownian motion. Designing materials that meet these characteristics at ambient temperature (e.g., 300 K) require amphidynamic crystals with rotary dipoles that (1) have no intrinsic electronic barriers for rotation, (2) exist in voids that are greater than their volumes of revolution to avoid steric barriers, and (3) possess large electric dipole moments that are close enough to interact with each other without interfering with their rotational motion.

The synthesized structures include metal organic frameworks with Zn and Zr nodes, dense carboxylate layers to avoid interpenetration, and linkers with large electric dipole moments plus a set of structures with TEMPO base free radical rotary arrays. The new materials with “freely” rotating dipoles are investigated to determine if they have large dielectric constants over a broad range of applied AC field frequencies as a result of the ability of their dipoles to align with an external field.

Different relaxation properties, however, are observed for samples where the alignment is ferroelectric or antiferroelectric. While ferroelectric samples have large susceptibilities and large macroscopic polarization, samples with antiferroelectric alignment require fields that overcome their tendency to maintain their antiferroelectric ground state.

The researchers also study the temperature dependence of the dielectric constant, which is expected to be weak for barrierless dipolar arrays. Research on amphidynamic crystals based on inertial dipolar arrays provides a unique opportunity to educate and train talented materials chemists from a wide range of backgrounds. Through this project, the PI supports women and students from underprivileged backgrounds in his research group.

By maintaining a supportive and creative environment he thereby fosters careers in materials science and in science education.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.