Spiropyrans as color-generating mechanophores

Due to the size scale of the molecules they work with, chemists are normally limited to indirect means of inducing and controlling chemical reactions. Sometimes, the inherent reactivity and structure of the reactants and the properties of the products such as thermodynamic stability can be relied upon to give the desired product. In other cases, high activation barriers can slow or even completely prevent reactions from occurring. These barriers are commonly overcome by changing the reaction conditions through the use of heat or light. Alternatively, the barriers can be artificially lowered through the use of catalysts. Catalysts can also be used to select a single reaction pathway out of many possible pathways, and this method is commonly employed to modify product ratios and in the synthesis of chiral molecules. More direct methods of molecular level manipulation such as scanning tunneling microscopes or single molecule force experiments using atomic force microscopy or optical tweezers are recent developments. However, these methods are limited in their usefulness by the small number of molecules that can be affected at any one time. Combining these two concepts to allow for the direct mechanical manipulation of large numbers of molecules has the potential to produce unique reaction products and smart materials such as polymers that response to mechanical stimuli. Mechanically responsive polymers are already known, but they generally rely on the disruption or formation of non-covalent bonds. Expanding their scope to include covalent bond changes in force-sensitive molecules called mechanophores potentially allows a much wider range of polymer responses to be accessed. In an effort to show the feasibility of this concept, a series of spiropyran-based mechanophores were designed which were expected to undergo a 6-?? electrocyclic ring-opening reaction when covalently linked into a polymer and subjected to mechanical force. This ring-opening yields a colored merocyanine which would allow the location and progress of the mechanically-induced reactions to be followed. A modular synthesis was developed which allowed for easy access to a wide variety of mechanophore and control spiropyrans. Since demonstration of mechanical activity in bulk samples was desired, a high priority was placed on optimizing the length, yields, and cost of the synthetic routes. The resulting spiropyrans were then incorporated into a variety of polymers such as methyl acrylate, methyl methacrylate, and polyurethane. Testing of these putative mechanophores in solution and the solid-state confirmed their mechanical activity. Potential alternative pathways such as thermal activation were excluded using multiple control spiropyrans. These control molecules were also covalently linked to the polymer but in ways not expected to allow for the efficient transfer of mechanical force from the bulk to the cleavable bond. In addition to validating the mechanophore concept, these results also demonstrated the usefulness of spiropyrans as polymer force sensors, as color change was found to be correlated with areas of high stress or strain