Expanding Molecular Designs for Mechanochromic Polymers

In the field of polymer mechanochemistry, mechanical force is harnessed to promote selective chemical transformations in the stress-sensitive molecules known as mechanophores. Mechanochromic mechanophores, which produce visible optical changes in polymeric materials in response to mechanical force, are particularly attractive for applications in force sensing, damage reporting, and patterning. This dissertation explores new molecular design strategies for mechanochromic polymers, elucidates fundamental understanding of mechanophore reactivity, and develops systems capable of responding to discrete stimuli and exhibiting multistage coloration. Unlike typical angular naphthopyrans that exhibit photochromic and mechanochromic behavior, constitutionally isomeric linear naphthopyrans and 2H-1-benzopyrans are commonly considered non-photochromic at ambient temperature, while their mechanochemical activity has not been studied. In Chapter 1, we demonstrate that linear naphthopyrans incorporating an electron-donating amine substituent do in fact undergo a ring-opening reaction upon photoirradiation with UV light and upon mechanical activation, resulting in pronounced photochromic and mechanochromic behavior. In Chapter 2, we report that the indole-fused 2H-1-benzopyran mechanophore undergoes a ring-opening reaction under force to generate a yellow-colored merocyanine dye, which is further reversibly transformed to a purple-colored dye upon the introduction of acid, enabling a multi-staged coloration. In contrast to conventional mechanochromic mechanophores, the mechanically gated photoswitching strategy, which decouples mechanochemical activation from the chromogenic response, enables mechanophores with high force-specificity and a high degree of modularity. Chapter 3 describes a mechanically gated 3H-anthra2,1-bpyran photoswitch that enables a three-stage coloration process. Mechanochemical activation of the photoinert mechanophore in the polymer generates a yellow-colored 3H-anthrapyran photoswitch, which can subsequently undergo a photochemical ring-opening reaction producing an orange-red colored merocyanine dye. Chapter 4 further expands this concept through the design of a naphthopyran mechanophore bearing a furan–maleimide Diels–Alder adduct whose active photochromism is regulated by mechanical force, providing a new approach to multistate stimuli-responsive polymers.