Stimuli-Responsive Porous Molecular Crystals

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Non-Technical AbstractAs part of this project, supported by the Solid State and Materials Chemistry program at NSF, Prof. Miljanic and his group develop and study new materials known as porous molecular crystals (PMCs). PMCs are solid crystals reminiscent of sponges: they have tiny pores, whose dimensions are comparable to those of molecules. In most currently known porous materials, pores are rigid in structure. In these new materials, pores are designed in a way which allows their shrinkage and expansion in response to changes in their environment. Specifically, the developed materials respond to high pressure and to exposure to gaseous chemicals, including environmental pollutants such as Freons and inhalation anesthetics. The resulting changes in pore dimensions are accompanied by changes in their optical properties (i.e. color under visible and ultraviolet light), and such modifications allow their use in the detection of environmental pollutants. Potential future applications of these pressure- and chemically responsive crystals are as shock absorbers, energy-storage materials, sensors, and materials that could catch-and-release chemicals-both harmful and useful ones-on demand. Additionally, societal benefits of this project include the education of a new generation of researchers at the interface of chemistry and materials science, and industry. Prof. Miljanic and coworkers have established a startup company which eventually could utilize the porous materials proposed in this work in the recycling of anesthetics. These efforts are complemented by curriculum enrichment at the University of Houston through the continued development of an upper-level undergraduate course on Energy and Sustainability, and the incorporation of 3D printing into chemistry curriculum.Technical AbstractThrough this project, supported by the Solid State and Materials Chemistry program at NSF, new classes of porous materials are prepared and studied. Porous molecular crystals (PMCs) are composed of discrete molecules which are held together in the solid state only by weak noncovalent interactions. These connections are flexible, allowing easy changes in the arrangement of PMCs constituents. As a result, it is anticipated that PMCs will be responsive to high pressures and guest entry into the pores, which the researchers are verifying experimentally. The pressure-based changes in the structures of PMCs are tracked through a combination of solid-state NMR spectroscopy, mercury intrusion porosimetry, and high-pressure crystallography at the Argonne National Laboratory. Because of their fluorescent character, such changes in pore dimensions are accompanied by visible and measurable changes in emission properties. This optical response can be used to prepare solid-state sensors for methanol and fluorinated refrigerants such as Freons, hydrofluorocarbons (HFCs), and hydrofluoroolefins (HFOs). As the final component of the work, new porous liquid crystals are synthesized and preliminarily studied. Their basis are shape-persistent macrocycles known as dehydrobenzannulenes, whose structures can sustain a columnar pore. Synthetic decoration of these columns with mesogenic groups translates into liquid crystalline behavior, which in turn is studied through a combination of methods involving both in-house and synchrotron techniques. Broader impacts of the proposed work are in the training of researchers at the interface of organic chemistry and materials science, utilization of PMCs as platforms for the recycling of anesthetics, and new curriculum offerings at the University of Houston, which include a course on Energy and Sustainability, and incorporation of 3D printing into chemistry instruction.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.