Controlling and Exploiting Spiropyran-based Mechanochromism

<p>When mechanical force is applied to synthetic materials, polymer chains become</p><p>highly strained, leading to bond scission and ultimately material failure. Over the last</p><p>decade or so, work in the field of polymer mechanochemistry has coupled this tension to</p><p>desired covalent chemical reactions. These functionalities, known as mechanophores,</p><p>react to unveil a new molecular structure and triggering a constructive response. This</p><p>strategy has been explored for a variety of purposes, including stress sensing, stress</p><p>strengthening, small molecule release, catalysis, and development of soft devices.</p><p>Additionally, the effect of force on a reaction coordinate, through biasing and probing</p><p>reaction pathways and trapping of transition states and intermediates, has been well–</p><p>studied experimentally and in theory. This work reports on understanding structure property</p><p>relationships for the spiropryan mechanophore and expanding our control of mechanochromism </p><p>from the single-molecule to device scale.</p><p>First, we report the effect of substituents on spiropyran derivatives substituted</p><p>with H, Br, or NO2 para to the breaking spirocyclic C− O bond using single molecule</p><p>force spectroscopy. The force required to achieve the rate constants of ~ 10 s−1 necessary</p><p>to observe transitions in the force spectroscopy experiments depends on the substituent,</p><p>with the more electron withdrawing substituent requiring less force. Rate constants at</p><p>375 pN were determined for all three derivatives, and the force coupled rate dependenc</p><p>eon substituent identity is well explained by a Hammett linear free energy relationship</p><p>with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic,</p><p>dissociative character. The methodology paves the way for further application of linear</p><p>free energy relationships and physical organic methodologies to mechanochemical</p><p>reactions.</p><p>The development and characterization of new force probes has enabled</p><p>additional, quantitative studies of force-coupled molecular behavior in polymeric</p><p>materials. The relationship between strain and color change has been measured for</p><p>these three spiropyran derivatives. The color appears at around the same strain and the</p><p>ratio of color intensities remains constant for all three derivatives. This result was not predicted by </p><p> previously reported computational work and motivates future studies of</p><p>force distribution within filled silicones.</p><p>On the material and device scale, we have utilized mechanochromism for soft</p><p>and stretchable electronics, which are promising for a variety of applications such as</p><p>wearable electronics, human− machine interfaces, and soft robotics. These devices,</p><p>which are often encased in elastomeric materials, maintain or adjust their functionality</p><p>during deformation, but can fail catastrophically if extended too far. Here, we report</p><p>new functional composites in which stretchable electronic properties are coupled to</p><p>molecular mechanochromic function, enabling at-a-glance visual cues that inform user</p><p>control. These properties are realized by covalently incorporating a spiropyran</p><p>mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that</p><p>a strain threshold has been reached. The resulting colorimetric elastomers can be molded</p><p>and patterned so that, for example, the word “STOP” appears when a critical strain is</p><p>reached, indicating to the user that further strain risks device failure. We also show that</p><p>the strain at color onset can be programmed through the layering of silicones with</p><p>different moduli into a composite. As a demonstration, we show how color onset can be</p><p>tailored to indicate a when a specified frequency of a stretchable liquid metal antenna</p><p>has been reached. The multi-scale combination of mechanochromism and soft</p><p>electronics offers a new avenue to empower user control of strain-dependent properties</p><p>for future stretchable devices.</p><p>Through the study of the reaction that converts spiropyran into merocyanine, we</p><p>are able to teach and connect a number of standard general chemistry course topics</p><p>while also introducing students to polymer concepts. By framing a number of different</p><p>concepts including molecular orbital theory, quantum mechanics, equilibrium,</p><p>hydrogen bonding, mechanical work, and polymer chemistry with the same reaction,</p><p>our goal is to allow students to see connections in seemingly disparate sections of</p><p>general chemistry.</p><p>The reactivity of a mechanically active functional group is determined by the</p><p>activation energy of the reaction (ΔG‡) and the force-coupled change in length as the</p><p>reaction proceeds from the ground to transition state (Δx‡). Finally, we report a combination</p><p> of both principles enhances the mechanochemical reactivity of epoxides:</p><p>placing alkenes adjacent to cis-epoxide mechanophores along a polymer backbone</p><p>results in ring-opening to carbonyl ylides during sonication, whereas epoxides lacking</p><p>an adjacent alkene do not. Upon release, tension-trapped ylides preferentially close to</p><p>their trans-epoxides in accordance with the Woodward-Hoffman rules. The reactivity of</p><p>carbonyl ylides is exploited to tag the activated species with spectroscopic labels for</p><p>force-induced cross-linking through a reaction with pendant alcohols. Even with alkene</p><p>assistance, mechanochemical reactivity remains low; single molecule force spectroscopy</p><p>establishes a lower limit for ring-opening ca. 1 sec-1 at forces of ~2600 pN.</p>

Author dc:creator

• Barbee, Meredith Hyatt

Advisor dc:contributor.advisor

• Craig, Stephen L

• Chemistry
• Polymer chemistry
• Organic chemistry
• mechanochromism
• polymer mechanochemistry
• spiropyran
• stress-responsive materials

Handle dc:identifier.uri

https://hdl.handle.net/10161/18687

OAI identifier oai:identifier

oai:dukespace.lib.duke.edu:10161/18687