Influential Intermolecular Interactions in Supramolecules
Influential Intermolecular Interactions in Supramolecules
Grace Huang Thomas Jefferson High School for Science and Technolog
With amazement, I watched the two color-coded curves describing temperatures of different alcohols gradually diverge from each other. Less than a minute ago, the two functions were overlapping with almost the same starting temperature. Just the difference of a single carbon backbone in two alcohols can cause a clear difference in their rate of evaporation. This small yet significant fact I learned when conducting the Intermolecular Forces (IMF) Lab piqued my curiosity. If the difference of three atoms can cause an obvious change in the property of a molecule, what would we observe with more atoms and stronger forces? With this thought, I discovered numerous research projects discussing the role of IMF in enhancing the properties of supramolecular materials, or structures composed of several molecules with IMF.
In 1873, Johannes Diderik van der Waals discovered one of the earliest and best known types of IMF, or bonding between molecules. However, the concept of supramolecular chemistry was first introduced by Lehn in 1990, or 117 years after the first discovery of IMF [4]. Such a time gap is likely related to an underestimate of the importance of IMF due to the presence of intramolecular forces, or bonding within a molecule. Intramolecular forces like ionic and covalent bonding are much stronger than IMF like hydrogen bonding and π-π stacking interactions. After experimentation suggested the importance of IMF, increasing amounts of research were conducted to utilize them in enhancing the properties of supramolecular materials, leading to countless applications in many different fields.
In South China University of Technology, researcher Lin and his team used IMF to construct a supramolecular nanocomposite to explore its advantageous properties [2]. The researchers hypothesized that using IMF as the interfacial binding force between nanosilica (silicon dioxide nanoparticles) and a matrix (where the polymer is embedded) will significantly improve the mechanical properties of the supramolecular composite. Ureido-pyrimidone (Upy) units can form quadruple hydrogen bonds. The researchers chose a variation of Upy that involves isophorone diisocyanate (IPDI) in its synthesis, Upy-IPDI, to be the “connector”. First, the researchers assembled a polymer in the matrix by end-capping polycarbonate diol with Upy-IPDI. Next, they obtained SiO2-Upy by grafting modified Upy-IPDI onto the surface of nanosilica. Then, the researchers blended the two materials both modified using Upy-IPDI. By the end of the blending process, 5 wt% (weight percent) of SiO2-Upy successfully bonded with the polymer in the matrix, forming a supramolecular composite. They then measured the tensile stress and Young’s modulus (both measures of strength and stiffness) of this material. To their surprise, simply 5 wt% of SiO2-Upy increased the tensile stress by 292% and Young’s modulus by 198%, indicating potential applications in engineering [2].
Another research team associated with MIT utilized IMF to improve the thermal conductivity of a conjugated polymer, a type of supramolecular material [5]. The researchers suggested low thermal conductivity may be due to low stability of the polymer. Some previous attempts to address this issue resulted in high thermal conductivity along the chains but low thermal conductivity in between chains due to weak van der Waals forces. The team decided to use a conjugated polymer with stable carbon-carbon double bonds along the chains and π-π stacking interactions in between chains. However, traditional conjugated polymers have a low thermal conductivity of approximately 0.2 W/m-K (Watts/meter-Kelvin, which measures thermal conductivity) due to polymer entanglements. To solve this problem, the research team composed the molecule using oxidative chemical vapor deposition (oCVD). This technique builds a thin film from bottom to top by first inserting raw materials in vapor form, then stimulating continuous and organized growth, forming π-π stacking in between polymers. The oCVD approach ensures the entangled chains have a rigid backbone composed of carbon-carbon double bonds, reducing the amount of distortion. The film constructed using this approach achieved better stability and more controlled distortion compared to traditional conjugated polymers, resulting in a record-high thermal conductivity of 2.2 W/m-K. Thus, their product may be used in solar cells that capture renewable energy [5].
Deng and his fellow researchers at Tanjing University of Technology investigated the contribution of IMF in stabilizing porous supramolecular frameworks at Tianjin University of Technology [1]. Hydrogen-bonded organic frameworks (HOF) recently appeared as a new class of crystalline porous materials. Past studies have demonstrated the involvement of π-π stacking interactions in stabilizing HOF in addition to hydrogen bonding. However, the contribution of π-π stacking remained unclear. The team proposed a porous framework stabilized using only π-π stacking interactions to answer this question. They assembled a supramolecular porous framework using zinc-centered structural units and π-π stacking interactions between different carbon rings. Both single-crystal x-ray diffraction analysis (XRD) and thermogravimetric analysis suggested the only force supporting the framework was π-π stacking interactions. Then, they investigated the thermal stability of this framework. XRD analysis displayed the stability of its structure from room temperature all the way to 80℃. When they studied the chemical stability, they found the porous framework stable not only when dissolved in common solvents (water, methanol, ethanol, and benzene, etc.) but also when placed in solutions of pH ranging from 3 to 12 (very acidic to very basic). These revealed contributions of π-π stacking interactions will significantly improve chemists’ understanding of supramolecular assembly [1].
A key application of IMF is energy conservation. For example, polymers possessing high thermal conductivity can improve heat dissipation in solar cells [5]. Furthermore, graphene oxide stiffened with IMF are used in electrodes for batteries, indicating applications in transportation [3]. Recently, the topic of energy conservation gained attention from the global scientific community. Many nonrenewable resources cannot be replaced at the rate at which they are consumed due to the increasing population. Improved technologies that utilize renewable energy more efficiently may be our solution. However, there are problems remaining yet to be solved. For example, Professor Espinosa at Northwestern University (personal communication, Feb. 24, 2020) explained, “One major remaining challenge is the scaling up of production for applications in transportation.”
If slightly altered IMF can produce a visible difference in the evaporation rate of an alcohol, then hundreds of thousands of these forces summed up can create a great impact, such as dramatically enhancing the properties of supramolecular materials. Similarly, if just a single footstep into the world of IMF led to awe-inspiring discoveries, then we await a spectacular landscape when scientists fully uncover the secrets behind IMF.
References
[1] Deng, J., Luo, J., Mao, Y., Lai, S., Gong, Y., Zhong, D., & Lu, T. (2020). π-π stacking interactions: Non-negligible forces for stabilizing porous supramolecular frameworks. Science Advances, 6(2). https://doi.org/10.1126/sciadv.aax9976
[2] Lin, F., Wang, R., Liu, L., Li, B., Ouyang, L., & Liu, W. (2017). Enhanced intermolecular forces in supramolecular polymer nanocomposites. Journal of the American Chemical Society,11(9), 690-703. https://doi.org/10.3144/expresspolymlett.2017.67
[3] Mao, L., Park, H., Soler-Crespo, R. A., Espinosa, H. D., Han, T. H., Nguyen, S. T., & Huang, J. (2019). Stiffening of graphene oxide films by soft porous sheets. Nature Communications, 10(3677). https://doi.org/10.1038/s41467-019-11609-8
[4] Qin, B., Yin, Z., Tang, X., Zhang, S., Wu, Y., Xu, J., & Zhang, X. (2020). Supramolecular polymer chemistry: From structural control to functional assembly. Progress in Polymer Science,100. https://doi.org/10.1016/j.progpolymsci.2019.101167
[5] Xu, Y., Wang, X., Zhou, J., Song, B., Jiang, Z., Lee, E. M. Y., . . . Chen, G. (2018). Molecular engineered conjugated polymer with high thermal conductivity. Science Advances, 4(3). https://doi.org/10.1126/sciadv.aar3031
