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Highly Sensitive Built-In Strain Sensors for Polymer
Composites: Fluorescence Turn-On Response through
Mechanochemical Activation
Zhong’an Li, Ryan Toivola, Feizhi Ding, Jeffrey Yang, Po-Ni Lai, Tucker Howie,
Gary Georgeson, Sei-Hum Jang, Xiaosong Li, Brian D. Flinn,* and Alex K.-Y. Jen*
Dr. Z. Li, Dr. R. Toivola, J. Yang, P.-N. Lai, Dr. T. Howie,
Dr. S.-H. Jang, Prof. B. D. Flinn, Prof. A. K.-Y. Jen
Department of Materials Science and Engineering
University of Washington
Seattle, WA 98195, USA
E-mail: bflinn@u.washington.edu; ajen@u.washington.edu
Dr. F. Ding, Prof. X. Li, Prof. A. K.-Y. Jen
Department of Chemistry
University of Washington
Seattle, WA 98195, USA
Dr. G. Georgeson
Boeing Research & Technology
Seattle, WA 98108, USA
DOI: 10.1002/adma.201600589
and ease of detecting color changes.[3,4,24–27]
Fluorescent emis-
sion spectroscopy capable of providing in situ analysis through
simultaneous optical excitation and detection of a material sur-
face has been recognized as a preferred technique for sensing
mechanical strain/damage in polymeric materials in real-time.
Mechanochromic polymers are commonly prepared by dis-
persing force responsive molecules (mechanophores) into
a host polymer matrix in which optical response to external
stimuli can be induced by altering their molecular conforma-
tion.[28,29]
However, most of the conformational changes of
mechanophores require high levels of strain (ε) to activate,
often at the level of 5.0 or more.[30,31]
These result in an insuf-
ficient sensitivity to detect the barely visible impact damage
(BVID) due to low energy impact. For example, BVID in the
CFRE often occurs well at or below ε of 1.0, sometimes even
below ε of 0.2. Alternatively, force induced chemical reactions
of mechanophores covalently bonded to a polymeric matrix
constitute a more attractive route to sensitively couple applied
force with the optical response,[6]
due to their similar response
mechanisms to those found in nature.[32]
So far, well-defined
artificial mechanophores that translate mechanical force into
useful chemical reactions are still very limited,[8]
and the most
successful example is the spiropyran–merocyanine system,
however the force activated colored merocyanine structure
shows significant photosensitivity and it is not compatible
with common high performance polymers that prevent them
for a useful damage sensor.[9,33,34]
Therefore, it is imperative to
design better mechanophores to develop an effective fluores-
cent mechanochemical sensing system.
In this paper, we report the development of a new class of
mechanochromic molecules (M1 and M2, Figure 1) that can be
covalently linked to an epoxy thermoset network as built-in fluo-
rescent strain sensors through simple and facile chemistry. As
shown in Figure 1a, under mechanical deformation, the sensing
molecules in the epoxy matrix undergo a force-induced elimina-
tion reaction to regenerate the conjugated pathway between the
donor and the acceptor forming a dipolar structure with strong
intramolecular charge-transfer. This sensor system can be acti-
vated even at a very low deformation strain of ≈0.14 accompa-
nied with a fluorescence “turn-on” response, and is stable under
prolonged exposure to light, demonstrating the significant value
for practical applications of strain/damage sensing.
Within the scope of current mechanochromic chemistry,
few examples of force-induced regeneration of conjugation
have been reported.[6,8]
In addition to well-known ring-opening
reaction in spiropyan–merocyanine system, some other
Many technologically relevant material systems require
mechanical integrity and longevity. Usually, a significant
impact damage in structural materials can be inspected visu-
ally, but even such damage can be overlooked during routine
inspection of large structures. Hidden structural damage can
accumulate without readily revealing itself to the in-service
operator or consumer, leading to eventual catastrophic failure.
As a result, it is critical to map out the damage distribution of
a material surface in real-time before a catastrophic failure that
cannot be easily determined from traditional fractography and
failure analysis methods. This need is particularly necessary in
the polymer composites such as carbon fiber reinforced epoxy
(CFRE) widely applied in the automotive and aerospace indus-
tries to ensure safety and reduce cost of operation,[1,2]
because
structural damages of such composites are often barely visible,
making them even more difficult to detect than those occur in
metallic structures.[3,4]
For chemists and materials scientists, the macroscopic
mechanical damages can be considered as force induced struc-
tural transformations of molecular structures of polymer com-
posites. Therefore, the details of the molecular transformation
can be systematically correlated to applied forces based on
changes in numbers of complex physical and chemical prop-
erties, a study of mechanochemistry.[5–8]
Notable progresses
have been recently made in this field,[9–16]
owing to increased
mechanistic understanding[17–22]
and developed methodologies
that enable comprehensive evaluation of solid-state mecha-
nochemistry.[23]
Among many molecular responses to force
that can be measured, changes in optical properties such as
absorption and fluorescence (mechanochromism) are particu-
larly advantageous because of high sensitivity of fluorescence
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examples are based on chain scission induced regeneration of
conjugation in anthracene[35]
and tricinnamate[36]
derivatives.
Here, we report a new design strategy that couples the force
together with molecular optical properties based on the regener-
ation of the conjugation pathway in a dipolar structure. A dipolar
dye D1 was chosen as the building block for our mechanophores
(Figure 2) for its red-fluorescence (λmax = 635 nm and Φ = 4.9%
at 10−5
M in dichloromethane solution, Figure S1 (Supporting
Information)),[37]
which can avoid the interference with the auto
fluorescence of common epoxy composites (Figure S7, Sup-
porting Information). The most challenging aspect in designing
an organic molecule with efficient mechanochromic response
is to induce a substantial change in a molecular structure with
only delicate mechanical energy for activation. The balance lies
on the core of our molecular design as we speculate that the β
carbon of the double bond between a strong dialkylamine elec-
tron donor and electron acceptor ((3-cyano-4,5,5-trimethyl-5H-
furan-2-ylidene) malononitrile, TCF) in D1 is very electrophilic
to enable the Michael addition reaction of a nucleophile to the β
carbon, but the thermodynamic instability of the change caused
by this reaction may enable reversible elimination of the nucleo-
phile by a low level mechanical force.
To validate this design strategy, primary amine derivatives
such as diethylenetriamine (DETA), ethylenediamine (EN), and
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Figure 1. a) Schematic diagram of mechanochemical reaction, accompanied by pronounced mechanochromism. Upon application of compression
force, the pi-electron conjugation pathway is regenerated, recovering the original structure and optical properties of a dipolar dye. Images of the rec-
tangular plates prepared from a M1-DGEBA-DETA matrix were taken under ambient light (upper) and in a darkroom under 390 nm UV illumination
(lower), both before and after compression at a strain level of 0.49. b) Chemical structure of mechanophores (M1 and M2) with two primary amine
functional groups that can bind to the epoxy matrix covalently. c) Chemical structure of the control sample (M3) showing the absence of the function-
ality required to link the molecule to the epoxy matrix.
Figure 2. Schematic reaction mechanism for the reaction between D1 and diethylenetriamine (DETA), ethylenediamine (EN), N,N-dimethylethylamine
(DMEA).

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N,N-dimethylethylamine (DMEA) were selected as nucleophiles
since they are commonly used as curing agents for epoxides.
As shown in Figure 2, the primary amine functionalities were
proven to readily react with the β carbon of the double-bond
linkage in D1 as we initially hypothesized, thereby perturbing
the charge-transfer pathway to induce a clear color change from
red to colorless (Figure S2, Supporting Information), which rep-
resents a cessation of fluorescence. Covalently incorporating
mechanophores into a polymer matrix is essential in creating an
efficient built-in mechanical sensor as it promotes efficient trans-
duction of the mechanical energy necessary to drive a chemical
change.[9]
Besides, it also can resolve the
potential compatibility problems between the
probe molecule and polymer matrix without
causing degradation of key material proper-
ties like mechanical strength and fracture
toughness. From this standpoint, both M1
and M2 are thus designed to have two primary
amine groups that can be used to react with
epoxide groups covalently during the curing
process, to become part of the thermoset
network (Figure 1b). Molecule M3, which
has no binding sites for epoxide groups, has
been used as a control (Figure 1c). Through
applying strain to the epoxy matrix with M1
or M2 incorporated covalently, the linkage
point (C–N bond) of the nucleophilic substi-
tution can be selectively cleaved to regenerate
the π-electron conjugation pathway, which
therefore recovers the capacity of intramole-
cular charge-transfer and subsequently allows
a colorless, nonfluorescent, unconjugated
molecular form (“OFF” state) to transform
into a strongly fluorescent conjugated mole-
cular form (“ON” state) as shown in Figure 1a.
In general, the Michael addition reac-
tion includes two steps; the addition of a
nucleophile to the β carbon, followed by the
protonation of the α carbon (Figure S3, Sup-
porting Information). Initially, we predicted
that D1 could undergo both steps of the
typical Michael addition to afford a neutral
product (Figure S4, Supporting Informa-
tion). However, the results from the struc-
tural characterization with proton nuclear
magnetic resonance (1H NMR) spectra and
mass spectrometry (see the Supporting Infor-
mation for details) strongly indicated that
D1 only allowed the first step of the addition
reaction to afford a carbanion intermediate.
This can be attributed to the very strong
electron-withdrawing and delocalized TCF
acceptor that is capable of stabilizing the
anionic charge of this intermediate,[38] and
sequentially disfavoring the second protona-
tion step. The reverse of Michael addition is
called beta-elimination, and both addition and
elimination reactions proceed through a same
resonance-stabilized carbanion intermediate
(Figure S3, Supporting Information). The competition between
these two reactions is in an equilibrium determined by different
conditions, suggesting that the nucleophilic moieties as the
point of substitution in M1–M3 can also function as reasonably
good leaving groups with low activation barriers to undergo a
beta-elimination if exposed to mechanical stress.
To gain mechanistic insight into this mechanochemistry, we
have performed density functional theory calculations to study
the effect of external mechanical force on M1 and M2. The
force-induced elimination reaction was modeled by a relaxed
potential energy surface (PES) scan (Figure 3a) where the C–N
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Figure 3. a) Potential energy versus C–N bond (red color marked) distance for M1 and M2
modeled by a relaxed potential energy surface (PES) scan. The distance was elongated with a
stepwise change of 0.1 angstrom (Å) while the rest of the molecule was optimized at each step.
b) The activation energies and transition state geometry of M1.

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bond was elongated with a stepwise change of 0.1 angstrom
(Å) while the rest of the molecule was optimized at each step.
As the C–N bond lengthens, the potential energies of M1 and
M2 increase until they reach their maxima at ≈3.0 Å, leading to
a reaction activation barrier of ≈2.5 eV. To verify these results,
the transition states along the deformation pathway were also
studied (Figure 3b and Figure S5 (Supporting Information)).
As shown, the computed activation energies of the transition
states for M1 and M2 are found to be 2.5 and 2.4 eV, respec-
tively, consistent with the relaxed PES scan results. These data
were similar to those observed in the force activation of spiro
C–O bond cleavage in spiropyran–merocyanine system,[9]
and
thus provides a solid theoretical framework for the mechanical
scission of C–N bond in our molecular system.
To evaluate the mechanochromism experimentally, we have
selected bisphenol-A diglycidylether (DGEBA)-DETA as a
model epoxy polymer matrix into which we have incorporated
0.05 wt% of M1 or M2 during curing (Figure 4a). This matrix
was fabricated with a stoichiometric amine–epoxide ratio and
was cured at room temperature for 24 h. The resulting com-
posite samples were cut into rectangular plates for compressive
mechanical deformation (see the Supporting Information for
details). A typical set of true-stress–true-strain curves for com-
pression measurements is shown in Figure 4b.
The optical and mechanical properties of pristine DGEBA-
DETA without any probe molecules were first measured as a refer-
ence, and a negligible change in the absorption and fluorescence
under mechanical deformation was observed (Figure S6 and S7,
Supporting Information). However, when both M1 and M2 were
covalently incorporated into DGEBA-DETA, an obvious mecha-
nochromic response occurred during deformation, whereas no
color change was observed for the control sample (M3) without
any necessary chemical functionality to couple matrix deforma-
tion. The color and fluorescent images of M1-DGEBA-DETA
samples under varying compressive strains are given in Figure 4c
as an example. Color change from colorless to red-purple pro-
vides a turn-on response with an observable fluorescent emission
color change from blue-white to red even at a very low threshold
strain of –0.14. As strain increases, the corresponding color and
fluorescence change become more pronounced. We attribute the
high sensitivity to the resonance-stabilized carbanion structure
having a much lower activation barrier for C–N bond cleavage
(beta-elimination). Moreover, the very rigid, crosslinked epoxy
matrix around the reaction sites may also play a role in deter-
mining the level of threshold force.[20]
We have also studied the spectral properties of probes in
DGEBA-DETA plates at varying compressive strains. As shown
in Figure 5a,b, the initial emission spectra excited at 390 nm for
both samples with M1 and M2 exhibit two fluorescence peaks at
≈505 and ≈630 nm, corresponding to autofluorescence of epoxy
matrix and fluorescent emission of dipolar dye D1, respectively.
The weak peak at ≈630 nm appearing before compression indi-
cates that a small amount of either M1 or M2 has already been
activated during matrix curing. As strain increases, the emis-
sion intensities at 630 nm relative to that at 505 nm enhance
considerably. Since the emission of pristine DGEBA-DETA does
not change under deformation (Figure S7, Supporting Informa-
tion), the increased emission at 630 nm should be derived from
the mechanochemical activation of M1 and M2. Moreover, no
peak at 630 nm can be observed in the emission spectra of the
control sample M3 at any strain (Figure 5c), strengthening the
evidence that the color and fluorescent changes in samples with
M1 and M2 are caused solely by the applied external mechanical
force, rather than by heat or light. The corresponding emission
intensity ratio between 630 and 505 nm is plotted in Figure 5d
as a function of the applied true strain, indicating our mechano-
phores can serve as ratiometric sensors for the selective visualiza-
tion of the spatial distribution of strain in the material’s surface.
Moreover, the effect of dye concentration for M1 in the matrix
has been also studied (Figure S9, Supporting Information),
which shows a unimolecular activation at different concentra-
tions that is consistent with our proposed activation mechanism.
After deformation, the samples are stored under ambient
conditions, and ≈70% of fluorescence intensity at 630 nm
Adv. Mater. 2016, 28, 6592–6597
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Figure 4. a) Schematic procedure for incorporating M1 (0.05 wt%) into the epoxy thermoset network of DGEBA-DETA during curing. b) True-stress–
true-strain diagram for compression. c) Images of samples of M1-DGEBA-DETA with varying degrees of compressive strain (top row: ambient light
illumination; bottom row: under 390 nm UV illumination in a darkroom).

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due to mechanochemical activation could still be measured
after 500 h (Figure S10, Supporting Information), suggesting
the good temporal and photostability of D1. As a result, our
designed mechanochromism may enable more robust damage/
strain sensing compared to that of photosensitive spiropyran
mechanophores. However as shown in Figure S11 and S12
(Supporting Information), both M1 and M2 displayed dramatic
changes in color and fluorescent emission when exposed to
high heat (≈70 °C), illustrating their thermochromic nature. So
the kinetics of this irreversible thermal activation was studied by
examining time resolved measurements of fluorescent emission
at varying temperatures. Figure S13 (Supporting Information)
shows the emission intensity ratio between 630 and 505 nm
of M1-DGEBA-DETA measured over time during exposure to
temperatures of 35–50 °C. Using Arrhenius first-order kinetics
(see the Supporting Information for details),[39] the activation
energy (Ea) of the OFF–ON transition can be calculated to be
≈24 kcal mol−1 for M1, and ≈23 kcal mol−1 for M2, respectively.
In summary, we have developed a new molecular-design
strategy to engineer mechanophores for very sensitive built-
in strain sensors in epoxy composites based on force-induced
regeneration of conjugation in a highly fluorescent dipolar dye.
This regeneration process recovers the intramolecular charge-
transfer interaction between an electron donor and acceptor,
accompanied with a visible and stable change in absorption and
fluorescent emission. The sensing system based on this new
mechanochemistry is very promising for detecting barely visible
impact damages, given an ability of detecting a very low strain
level ε of 0.14. The ratiometric response could lead to a selective
visualization of the spatial distribution of strain exceeding a given
threshold. Moreover, it is worth noting that the functionality of
the primary amine on the mechanophores can be fine-tuned for
the general applicability of other polymer composites. Therefore,
our findings not only provide a novel molecular-design strategy
to mechanochromic chemistry but also develop efficient built-in
strain sensors for commercially relevant composite materials.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Z.L. and R.T. contributed equally to this work. This work was funded
by the Boeing Company Project Code B8LDL. Prof. Jen thanks the
Boeing-Johnson Foundation for its support. Prof. Li thanks the financial
support from the US National Science Foundation (CHE-1265945 and
DMR-1408617).
Received: January 31, 2016
Revised: April 6, 2016
Published online: May 17, 2016
Figure 5. a–c) Fluorescent emission spectra of mechanophores M1 (a), M2 (b), M3 (c), in DGEBA-DETA plates at varying compressive strains, which
have been normalized to the epoxy matrix autofluorescence at 505 nm. d) Plots of the emission intensity ratio between signals at 630 and 505 nm of
each sample at varying strains.

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