CAREER: Solid-state molecular motion, reversible covalent-bond formation, and self-assembly for controlling thermal expansion behavior

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Non-Technical SummaryMaterials used in real-world settings are frequently exposed to changes in temperature. For materials used in outdoor applications such as concrete, this is typically due to weather or seasonal changes. For materials used in devices such as computers or electronics, this is due to excess energy that results in heat. Thermal expansion is the response of a material to any change in temperature. The way a material responds to temperature impacts its ability to function. If the thermal expansion behavior of a material is not understood and controlled, failure or fracture is likely to occur as a result of temperature fluctuations. The chemical structures of the molecules and the bonds that hold a solid material together typically dictate the thermal expansion behaviors. The behaviors of materials like concrete are well-understood; however, analogous behaviors for organic (carbon)-based materials are more challenging to predict and design because these materials are held together by weaker forces. Organic materials are becoming more widely used in a variety of fields such as electronics. Balancing high material performance with ideal thermal expansion is critical to such applications. This CAREER project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, develops fundamental strategies for controlling the thermal expansion behaviors of organic materials. Specifically, thermal expansion is influenced through the use of dynamic groups, which respond to temperature changes by undergoing motion or by making and breaking the bonds that hold the material together. The strategies developed in this project are expected to influence the design and preparation of novel materials with predictable thermal expansion properties for use in technological applications that advance national prosperity. Integrated with the research plan is a holistic education, mentorship, and outreach program involving underrepresented groups at each education stage from middle school through graduate school. The activities include (1) development and implementation of an annual presentation on 'Thermal Expansion Around Us' at Tech Savvy – a STEM workshop for middle school girls, (2) a traveling lab experiment on molecular structures and solid-state properties for high school students in West Texas, and (3) a STEM career preparation workshop for upper-level undergraduate and graduate students.Technical SummaryThis CAREER project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, develops a fundamental understanding of thermal expansion (TE) behaviors in organic crystalline solids through synthesis of novel, dynamic solid-state materials with controllable and predictable TE behaviors. TE is the response of a material to a change in temperature. The chemical structures of the molecules and the interactions that hold the solid together typically dictate TE behavior. However, other mechanisms such as structural flexibility or motion can give rise to unexpected or unique TE. For inorganic or covalent network solids, intermolecular forces are strong, structural assembly is well-controlled in three dimensions, and TE is often predictable. On the other hand, purely organic molecular solids are held together in three dimensions by weaker, noncovalent interactions. Directing self-assembly of individual organic molecules into a solid structure with full control over the noncovalent interactions comprising all crystallographic dimensions is challenging. Noncovalent forces, motion, and flexibility all affect TE in organic molecular solids. Reliably directing, achieving, and controlling solid-state motion, self-assembly, and predicting their influence on TE remains challenging. This CAREER project develops fundamental knowledge and systematic strategies for controlling and tuning TE in organic molecular solids through (1) installation of functional groups capable of undergoing solid-state molecular motion and reliably turning motion on and off, (2) use of reversible solid-state covalent-bond-forming reactions to switch between large and near zero TE behaviors within a single solid, and (3) control over self-assembly of organic molecules in all three crystallographic dimensions using orthogonal noncovalent interactions. The work is expected to advance fundamental knowledge of solid-state motion, reactivity, self-assembly, and TE, and transform the design of functional solid-state materials that exhibit dynamic properties. The educational and outreach activities emphasize a STEM-powered approach to education and career preparation by engaging students in STEM activities at each stage of education from middle school through graduate school. This is achieved through (1) development and implementation of an annual presentation on 'Thermal Expansion Around Us' at Tech Savvy – a STEM workshop for middle school girls, (2) a traveling lab experiment on molecular structures and solid-state properties for high school students in West Texas, and (3) a STEM career preparation workshop for upper-level undergraduate and graduate students.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.