Solid-state behaviors of materials - Thermal expansion, molecular pedal motion, phase transitions, and conformational flexibility

Design of materials utilizing the principles of supramolecular chemistry and the study of structure-property relationship has gained an immense amount of importance in the past few decades. Understanding the robustness, strength and directionality of an intermolecular interaction is a vital criteria that needs to be addressed in order to generate an effective material with desired physicochemical behavior. Cocrystallization, a process of synthesizing multi-component solids using non-covalent interactions, has been proven to be a very useful technique to alter the solid-state behaviors as compared to the single components. One solid-state property that solely relies on internal composition and that we have focused primarily on is thermal expansion (TE), which is the tendency of the solids to undergo change in shape, area, and volume with respect to change in temperature as an external stimuli. Many molecules in the solid state tend to undergo molecular motions depending on the type of symmetry they exhibit. We focused on molecules similar to that of azobenzenes and stilbenes as they have tendency to undergo molecular pedal motion. Incorporating components with motion-capable groups in a multi-component solid can afford large TE behaviors if the pedal motion can be activated. Another aspect that can contribute and enhance TE behaviors is torsional flexibility. Molecules consisting of imine groups have the tendency to undergo flexibility and motion capability, hence can influence TE behavior of the overall solids. We synthesized and explored bis pyridyl imine solid that undergoes large volumetric expansion as compared to it hydrogen bonded cocrystals. Moreover, molecular pedal motion was turned on in the solid by crystallizing it with another torsional flexible molecule that contained a motion-capable group and hence higher expansion was achieved along that direction. The category of mixed cocrystals has been underdeveloped, and, we reported the first example of one-dimensional halogen bonded mixed cocrystals where direct tunability in TE behavior was achieved with respect to the percentages of motion-capable and motion-incapable groups.

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