Structure-Activity Relationships in Mechanophores with Latent Conjugation

Materials often fail as a result of the mechanical loads they experience during use. On the molecular level, forces within polymers are distributed unevenly throughout the material, and some polymer subchains experience greater stress than others. In some cases, the forces experienced by these overstressed subchains can trigger chain scission events. Chain scission in turn might nucleate the formation of a microcrack that subsequently propagates, ultimately leading to material failure. In recent years, force reactive functional groups, or mechanophores, have emerged as the basis of a potential strategy for combatting this destructive cascade. The strategy, known as activated remodeling via mechanochemistry (ARM), comprises embedding mechanophores along the polymer backbone or within cross-­‐‑links, so that otherwise destructive force within an overstressed subchain triggers a constructive, rather than a destructive, response. ARM functions in both solution and bulk to form remodeled polymer networks where the number of bonds formed exceeds the number of bonds broken under typically destructive mechanical conditions. It requires no additional external stimulus or energy input beyond the imposed shear and results in orders-­‐‑of-­‐‑magnitude increases in bulk moduli.

These demonstrations have spurred a range of important and fundamental questions about stress-­‐‑responsive remodeling, including how to dissect the complex interplay between material deformation, mechanophore activation, nascent cross-­‐‑link rupture, mechanochemically triggered cross-­‐‑link formation, and the impact of various stages of each on the mechanical properties and eventual failure of the material. The answers to these questions require new mechanophores that not only activate and then cross-­‐‑link efficiently, but that give clear spectroscopic signatures of their state so that the levels of both activation and cross-­‐‑linking can be measured in situ and in real time.

In this dissertation, we design and explore two families of mechanophores for use in the context of the ARM concept. The first family is based on a substituted cyclobutene scaffold, which undergoes a force-­‐‑induced electrocyclic ring-­‐‑opening reaction to unveil butadiene. In Chapter 2, we investigate the intrinsic stability of a variety of substituted cyclobutenes, and then utilize pulsed ultrasound to change electronic distributions and spectroscopic signatures via mechanically unveiling latent conjugation pathways. Furthermore, we show the potential ARM-­‐‑type utility of the cyclobutene mechanophore by using click chemistry to react the activated butadiene with 4-­‐‑phenyl-­‐‑1,2,4-­‐‑triazoline-­‐‑3,5-­‐‑dione (PTAD).

These studies motivated quantification of the mechanical reactivity of the cyclobutene system as a function of substitution. In Chapters 3 and 4, we use single-­‐‑ molecule force spectroscopy (SMFS) to pull individual polymers comprised of cyclobutene mechanophore repeating units, and measure the force required to mechanically induce the ring-­‐‑opening reaction on the time scale of several hundred

milliseconds. We show that changes in polymer attachment near a reacting benzocyclobutene mechanophore can have dramatic effects when pulling from cis handles, but not when pulling from trans handles. Additionally, we provide evidence that electronic effects further away from the cyclobutene ring can be tuned without significantly altering the force at which CBE mechanically ring-­‐‑opens. As demonstrated in Chapter 2, these electronic effects can still have substantial effects for altering conjugation pathways and unveiled reactivity in the mechanically ring-­‐‑opened butadiene product.

The second family of mechanophore investigated in this dissertation is based on the ring-­‐‑opening of an oxabicyclo[2.1.0’pentane (OBP) to reversibly generate a highly colored carbonyl ylide. In Chapter 5, we synthesize a dibromoaryl substituted OBP and characterize the carbonyl ylide generated from application of UV light or heating above 100 ̊C. The carbonyl ylide is highly reactive with dipolarophiles or in the presence of oxygen. Unfortunately, most derivatives are highly sensitive to trace amounts of acid and we were unable to incorporate the putative mechanophore in a polymer. Through our efforts, however, we were able to identify two stable, sulfur-­‐‑based OBPs that we utilize in Chapter 6 in single-­‐‑molecule conductance experiments. In these experiments, we observe no evidence of mechanophore activation as a function of break-­‐‑junction elongation, which suggests that the guiding principles used to understand force-­‐‑induced reactivity may not hold in systems of high confinement.

The final chapter of this dissertation describes an easy-­‐‑to-­‐‑implement science outreach demonstration featuring a mechanically and photochemically color-­‐‑changing polymer. The active polymeric material is a filled poly(dimethylsiloxane) (PDMS) elastomer that is covalently functionalized with spiropyran (SP), which is both a photochemical and mechanochemical switch. The material can be reversibly changed from colorless to dark purple by exposing to light from a blue laser pointer or providing a mechanical stimulus such as hitting the polymer with a hammer or dragging a blunt object across the surface. The keynote demonstration is a PDMS chemical-­‐‑drawing board that allows children to literally ‘write without ink’ using a laser pointer or a blunt stylus. Collectively, these demonstrations are suitable for various student groups, and encompass concepts in polymer and materials chemistry, photochemistry, and mechanochemistry. This demonstration has been successfully employed dozens of times in multiple universities across North America.