ICE | The Israel Chemist and Engineer

22 The Israel Chemist and Engineer Issue 1, September 2015, Tishrei 5776 Scientific Articles Chemistry by (Mechanical) Force Charles E. Diesendruck Schulich Faculty of Chemistry, Technion – Israel Institute of Technology Email: Abstract: Chemical changes caused by mechanical stress (mechanochemistry) is not a new field in materials science. A chapter dedicated to this subject can be found in older polymer physics books. However, only in recent years has mechanochemistry reappearedas a usefulmethodology in polymer chemistry and even synthetic organic chemistry. In this article, I review the scientific development of this important energy transduction process, that ultimately led to the development of mechanophores, molecules that selectively respond to mechanical stimuli in a productive way. Finally, I describe some recent work on polymer mechanochemistry and new mechanophores that were developed for different applications, such as using mechanochemical changes to study stress distribution in polymers, change reaction mechanisms or develop new mechanoresponsive materials. Charles Diesendruck received his B.Sc. in analytical and environmental chemistry from Ben-Gurion University of the Negev. After serving in the army and working a few years at Chemada Fine Chemicals, he returned to BGU to complete a M.Sc. and a Ph.D. in organometallic chemistry with Prof. N. Gabriel Lemcoff. Diesendruck was a postdoctoral fellow in materials chemistry at the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign, working at the Autonomous Materials Systems group with Prof. Jeffrey S. Moore. In October 2014, Diesendruck joined the Schulich Faculty of Chemistry at the Technion, doing research in the fields of polymer chemistry and mechanochemistry. Introduction Several energy sources are used to drive chemical reactions. Commonly, we use heat to bypass high-energy transition states. In photochemistry, we convert low energy molecules to high- energy reactants using photons. In electrochemistry, we use electrical potential to add or remove electrons and induce the chemical change. Even in reactions where simply mixing two reactants at room temperature leads to products, we make use of the chemical potential stored in high-energy reactants to drive the reaction towards lower energy products. A different approach that requires a bit of imagination is to use mechanical energy. How can we transmit mechanical energy to molecules? Atomic force microscopy (AFM), where we use a very small tip to attach and pull molecules from surfaces (Figure 1), is a possibility 1 . However, carrying out a reaction molecule by molecule does not look very practical if one wants to prepare large amounts of compounds. As in many other fields of science, we can look to nature to find some ideas of how to use mechanical force to induce a chemical change. One process that comes to mind is the production of petroleum. Over millions of years, organic remains from living beings are converted under high pressure and temperature to this very valuable liquid. However, waiting millions of years for a chemical process is not a practical option. A second example is how organisms convert physical signals into chemical responses. How do we feel touch? How do we hear by detecting vibrations of air? The body uses numerous approaches to detect these changes in the mechanical environment such as opening or closing of ion channels and changing the conformation of a protein, among others 3 . The chemical meaning of an ion channel opening or closing or of enzymes changing their activity due to change in conformation are examples of mechanochemical transformations: the formation or breaking of chemical bonds, supramolecular and covalent, through the utilization of mechanical force. Figure 1: Stretching molecules using atomic-force microscopy 2 . Reproduced from ref. 2. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.