Controlling and Exploiting Spiropyran-based Mechanochromism

Abstract

When mechanical force is applied to synthetic materials, polymer chains becomehighly strained, leading to bond scission and ultimately material failure. Over the lastdecade or so, work in the field of polymer mechanochemistry has coupled this tension todesired covalent chemical reactions. These functionalities, known as mechanophores,react to unveil a new molecular structure and triggering a constructive response. Thisstrategy has been explored for a variety of purposes, including stress sensing, stressstrengthening, small molecule release, catalysis, and development of soft devices.Additionally, the effect of force on a reaction coordinate, through biasing and probingreaction pathways and trapping of transition states and intermediates, has been well–studied experimentally and in theory. This work reports on understanding structure propertyrelationships for the spiropryan mechanophore and expanding our control of mechanochromism from the single-molecule to device scale.First, we report the effect of substituents on spiropyran derivatives substitutedwith H, Br, or NO2 para to the breaking spirocyclic C− O bond using single moleculeforce spectroscopy. The force required to achieve the rate constants of ~ 10 s−1 necessaryto observe transitions in the force spectroscopy experiments depends on the substituent,with the more electron withdrawing substituent requiring less force. Rate constants at375 pN were determined for all three derivatives, and the force coupled rate dependenceon substituent identity is well explained by a Hammett linear free energy relationshipwith a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic,dissociative character. The methodology paves the way for further application of linearfree energy relationships and physical organic methodologies to mechanochemicalreactions.The development and characterization of new force probes has enabledadditional, quantitative studies of force-coupled molecular behavior in polymericmaterials. The relationship between strain and color change has been measured forthese three spiropyran derivatives. The color appears at around the same strain and theratio of color intensities remains constant for all three derivatives. This result was not predicted by previously reported computational work and motivates future studies offorce distribution within filled silicones.On the material and device scale, we have utilized mechanochromism for softand stretchable electronics, which are promising for a variety of applications such aswearable electronics, human− machine interfaces, and soft robotics. These devices,which are often encased in elastomeric materials, maintain or adjust their functionalityduring deformation, but can fail catastrophically if extended too far. Here, we reportnew functional composites in which stretchable electronic properties are coupled tomolecular mechanochromic function, enabling at-a-glance visual cues that inform usercontrol. These properties are realized by covalently incorporating a spiropyranmechanophore within poly(dimethylsiloxane) to indicate with a visible color change thata strain threshold has been reached. The resulting colorimetric elastomers can be moldedand patterned so that, for example, the word “STOP” appears when a critical strain isreached, indicating to the user that further strain risks device failure. We also show thatthe strain at color onset can be programmed through the layering of silicones withdifferent moduli into a composite. As a demonstration, we show how color onset can betailored to indicate a when a specified frequency of a stretchable liquid metal antennahas been reached. The multi-scale combination of mechanochromism and softelectronics offers a new avenue to empower user control of strain-dependent propertiesfor future stretchable devices.Through the study of the reaction that converts spiropyran into merocyanine, weare able to teach and connect a number of standard general chemistry course topicswhile also introducing students to polymer concepts. By framing a number of differentconcepts including molecular orbital theory, quantum mechanics, equilibrium,hydrogen bonding, mechanical work, and polymer chemistry with the same reaction,our goal is to allow students to see connections in seemingly disparate sections ofgeneral chemistry.The reactivity of a mechanically active functional group is determined by theactivation energy of the reaction (ΔG‡) and the force-coupled change in length as thereaction proceeds from the ground to transition state (Δx‡). Finally, we report a combination of both principles enhances the mechanochemical reactivity of epoxides:placing alkenes adjacent to cis-epoxide mechanophores along a polymer backboneresults in ring-opening to carbonyl ylides during sonication, whereas epoxides lackingan adjacent alkene do not. Upon release, tension-trapped ylides preferentially close totheir trans-epoxides in accordance with the Woodward-Hoffman rules. The reactivity ofcarbonyl ylides is exploited to tag the activated species with spectroscopic labels forforce-induced cross-linking through a reaction with pendant alcohols. Even with alkeneassistance, mechanochemical reactivity remains low; single molecule force spectroscopyestablishes a lower limit for ring-opening ca. 1 sec-1 at forces of ~2600 pN.