Controlling and Exploiting Spiropyran-based Mechanochromism

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2019

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Abstract

When mechanical force is applied to synthetic materials, polymer chains become

highly strained, leading to bond scission and ultimately material failure. Over the last

decade or so, work in the field of polymer mechanochemistry has coupled this tension to

desired covalent chemical reactions. These functionalities, known as mechanophores,

react to unveil a new molecular structure and triggering a constructive response. This

strategy has been explored for a variety of purposes, including stress sensing, stress

strengthening, small molecule release, catalysis, and development of soft devices.

Additionally, the effect of force on a reaction coordinate, through biasing and probing

reaction pathways and trapping of transition states and intermediates, has been well–

studied experimentally and in theory. This work reports on understanding structure property

relationships 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 substituted

with H, Br, or NO2 para to the breaking spirocyclic C− O bond using single molecule

force spectroscopy. The force required to achieve the rate constants of ~ 10 s−1 necessary

to observe transitions in the force spectroscopy experiments depends on the substituent,

with the more electron withdrawing substituent requiring less force. Rate constants at

375 pN were determined for all three derivatives, and the force coupled rate dependenc

eon substituent identity is well explained by a Hammett linear free energy relationship

with 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 linear

free energy relationships and physical organic methodologies to mechanochemical

reactions.

The development and characterization of new force probes has enabled

additional, quantitative studies of force-coupled molecular behavior in polymeric

materials. The relationship between strain and color change has been measured for

these three spiropyran derivatives. The color appears at around the same strain and the

ratio of color intensities remains constant for all three derivatives. This result was not predicted by

previously reported computational work and motivates future studies of

force distribution within filled silicones.

On the material and device scale, we have utilized mechanochromism for soft

and stretchable electronics, which are promising for a variety of applications such as

wearable electronics, human− machine interfaces, and soft robotics. These devices,

which are often encased in elastomeric materials, maintain or adjust their functionality

during deformation, but can fail catastrophically if extended too far. Here, we report

new functional composites in which stretchable electronic properties are coupled to

molecular mechanochromic function, enabling at-a-glance visual cues that inform user

control. These properties are realized by covalently incorporating a spiropyran

mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that

a strain threshold has been reached. The resulting colorimetric elastomers can be molded

and patterned so that, for example, the word “STOP” appears when a critical strain is

reached, indicating to the user that further strain risks device failure. We also show that

the strain at color onset can be programmed through the layering of silicones with

different moduli into a composite. As a demonstration, we show how color onset can be

tailored to indicate a when a specified frequency of a stretchable liquid metal antenna

has been reached. The multi-scale combination of mechanochromism and soft

electronics offers a new avenue to empower user control of strain-dependent properties

for future stretchable devices.

Through the study of the reaction that converts spiropyran into merocyanine, we

are able to teach and connect a number of standard general chemistry course topics

while also introducing students to polymer concepts. By framing a number of different

concepts 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 of

general chemistry.

The reactivity of a mechanically active functional group is determined by the

activation energy of the reaction (ΔG‡) and the force-coupled change in length as the

reaction 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 backbone

results in ring-opening to carbonyl ylides during sonication, whereas epoxides lacking

an adjacent alkene do not. Upon release, tension-trapped ylides preferentially close to

their trans-epoxides in accordance with the Woodward-Hoffman rules. The reactivity of

carbonyl ylides is exploited to tag the activated species with spectroscopic labels for

force-induced cross-linking through a reaction with pendant alcohols. Even with alkene

assistance, mechanochemical reactivity remains low; single molecule force spectroscopy

establishes a lower limit for ring-opening ca. 1 sec-1 at forces of ~2600 pN.

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Barbee, Meredith Hyatt (2019). Controlling and Exploiting Spiropyran-based Mechanochromism. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/18687.

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