Controlling and Exploiting Spiropyran-based Mechanochromism
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.
Polymer chemistry
Organic chemistry
mechanochromism
polymer mechanochemistry
spiropyran
stress-responsive materials
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