Mechanically Gated Generation of a 3H-Anthra[2,1-b]pyran Photoswitch Enabling Multicolor Switching

Mechanochromic mechanophores are a class of stress-sensitive molecules that undergo force-induced chemical transformations resulting in optical changes that enable direct visualization of stress and/or strain in polymeric materials. Unlike conventional mechanochromic mechanophores, the mechanically gated photoswitching strategy decouples the mechanochemical activation from the chromogenic response, enabling high force-specificity and modularity. However, previous designs are limited to diarylethene photoswitches, which typically provide only a simple binary (colorless-to-colored) response. Here, we report on a mechanically gated 3H-anthra[2,1-b]pyran photoswitch that enables a three-stage coloration process. Mechanochemical activation of the photoinert mechanophore generates a yellow-colored 3H-anthrapyran photoswitch via a formal retro-[4 + 2] cycloaddition reaction. Subsequent UV photoirradiation induces a ring-opening reaction to produce an orange-red colored merocyanine dye. This study extends the repertoire of polymer mechanochromism to enable multicolor behavior in response to both force and light and provides a promising platform for applications such as force-sensing, multicolor patterning, and molecular logic gates.