We introduce the concept of mechanochemically gated photoswitching. Mechanical regulation of a photochemical reaction is exemplified using a newly designed mechanophore based on a cyclopentadiene− maleimide Diels−Alder adduct. Ultrasound-induced mechanical activation of the photochemically inert mechanophore in polymers generates a diarylethene photoswitch via a retro-[4 + 2] cycloaddition reaction that photoisomerizes between colorless and colored states upon exposure to UV and visible light. Control experiments demonstrate the thermal stability of the cyclopentadiene−maleimide adduct and confirm the mechanical origin of the "unlocked" photochromic reactivity. This technology holds promise for applications such as lithography and stress-sensing, enabling the mechanical history of polymeric materials to be recorded and read ondemand.
Mechanophores are molecules that undergo productive, covalent chemical transformations in response to mechanical force. Over the last decade, a variety of mechanochromic mechanophores have been developed that enable the direct visualization of stress in polymers and polymeric materials through changes in color and chemiluminescence. The recent introduction of mechanochemically gated photoswitching extends the repertoire of polymer mechanochromism by decoupling the mechanical activation from the visible response, enabling the mechanical history of polymers to be recorded and read on-demand using light. Here, we discuss advances in mechanochromic mechanophores and present our design of a cyclopentadiene–maleimide Diels–Alder adduct that undergoes a force-induced retro-[4+2] cycloaddition reaction to reveal a latent diarylethene photoswitch. Following mechanical activation, UV light converts the colorless diarylethene molecule into the colored isomer via a 6π-electrocyclic ring-closing reaction. Mechanically gated photoswitching expands on the fruitful developments in mechanochromic polymers and provides a promising platform for further innovation in materials applications including stress sensing, patterning, and information storage.1 Introduction to Polymer Mechanochemistry2 Mechanochromic Reactions for Stress Sensing3 Regiochemical Effects on Mechanophore Activation4 Mechanochemically Gated Photoswitching5 Conclusions
Molecular force probes conveniently report on mechanical stress and/or strain in polymers through straightforward visual cues. Unlike conventional mechanochromic mechanophores, the mechanically gated photoswitching strategy decouples mechanochemical activation from the...
During the past two
decades, our understanding of mechanochemical
reactivity has advanced considerably. Nevertheless, an incomplete
knowledge of structure–activity relationships and the principles
that govern mechanochemical transformations limits molecular design.
The experimental development of mechanophores has thus benefited from
simple computational tools like CoGEF, from which quantitative metrics
like rupture force can be extracted to estimate reactivity. Furan–maleimide
(FM) and anthracene–maleimide (AM) Diels–Alder adducts
are widely studied mechanophores that undergo retro-Diels–Alder
reactions upon mechanical activation in polymers. Despite possessing
significantly different thermal stability, similar rupture forces
predicted by CoGEF calculations suggest that these compounds exhibit
similar mechanochemical reactivity. Here, we directly probe the relative
mechanochemical reactivity of FM and AM adducts through competitive
activation experiments. Ultrasound-induced mechanochemical activation
of bis-adduct mechanophores comprising covalently tethered FM and
AM subunits reveals pronounced selectivityas high as ∼13:1for
reaction of the FM adduct compared to the AM adduct. Computational
models provide insight into the greater reactivity of the FM mechanophore,
indicating a more efficient mechanochemical coupling for the FM adduct
compared to the AM adduct. The methodology employed here to directly
interrogate the relative reactivity of two different mechanophores
using a tethered bis-adduct configuration may be useful for other
systems where more common sonication-based approaches are limited
by poor sensitivity.
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