Multicolor Mechanofluorophores for the Quantitative Detection of Covalent Bond Scission in Polymers

Article abstract of DOI:10.1002/anie.202101716

The fracture of polymer materials is a multiscale process starting with the scission of a single molecular bond advancing to a site of failure within the bulk. Quantifying the bonds broken during this process remains a big challenge yet would help to understand the distribution and dissipation of macroscopic mechanical energy. We here show the design and synthesis of fluorogenic molecular optical force probes (mechanofluorophores) covering the entire visible spectrum in both absorption and emission. Their dual fluorescent character allows to track non-broken and broken bonds in dissolved and bulk polymers by fluorescence spectroscopy and microscopy. Importantly, we develop an approach to determine the absolute number and relative fraction of intact and cleaved bonds with high local resolution. We anticipate that our mechanofluorophores in combination with our quantification methodology will allow to quantitatively describe fracture processes in materials ranging from soft hydrogels to high-performance polymers.

Full text of DOI:10.1002/anie.202101716

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How to cite:
International Edition: doi.org/10.1002/anie.202101716
German Edition: doi.org/10.1002/ange.202101716
Mechanofluorophores
Hot Paper
Multicolor Mechanofluorophores for the Quantitative Detection of
Covalent Bond Scission in Polymers
Christoph Baumann, Maria Stratigaki, Silvia P. Centeno, and Robert Gçstl*
Abstract: The fracture of polymer materials is a multiscale
process starting with the scission of a single molecular bond
advancing to a site of failure within the bulk. Quantifying the
bonds broken during this process remains a big challenge yet
would help to understand the distribution and dissipation of
macroscopic mechanical energy. We here show the design and
synthesis of fluorogenic molecular optical force probes (me-
chanofluorophores) covering the entire visible spectrum in
both absorption and emission. Their dual fluorescent character
allows to track non-broken and broken bonds in dissolved and
bulk polymers by fluorescence spectroscopy and microscopy.
Importantly, we develop an approach to determine the absolute
number and relative fraction of intact and cleaved bonds with
high local resolution. We anticipate that our mechanofluor-
ophores in combination with our quantification methodology
will allow to quantitatively describe fracture processes in
materials ranging from soft hydrogels to high-performance
polymers.
undergo productive bond scission upon the application of
mechanical force (mechanophores), particularly those alter-
ing their optical properties, show potential in this context and
are hence investigated as force reporting probes in polymer
materials.[1–3] Notable representatives of these are spiro-
(thio)pyran[4–8] and its rhodamine derivatives,[9–11] dioxe-
tane,[12–14] persistent delocalized radicals,[15–18] and aggregach-
romic dyes.[19–21] However, the direct correlation of macro-
scopic deformation with microscopic and molecular events is
still challenging, yet fundamental in the field of polymer
science. Recently, initial steps have been taken to address this
challenge by using confocal laser scanning microscopy
(CLSM) in combination with fluorogenic mechanophores to
quantitatively describe the spatial distribution of stress, strain,
and mechanical energy dissipation within hydrogels[22,23] and
elastomers.[24] However, these approaches are limited by
either a complicated relationship between fluorescence signal
and the number of broken bonds[22] or a lack of feedback over
the number of intact bonds.[23,24]
Introduction
Here we report a new mechanofluorophore design that
allows tuning of excitation and emission wavelengths and is
fluorescent in its deactivated as well as its activated state[11,25]
with a large spectral separation between these states. The
nature of this fluorogenic mechanophore platform renders
the quantitative detection of bond scission by CLSM possible
with high spatial resolution. We use Diels–Alder (DA)
adducts[26–28] of p-extended anthracenes and maleimide as
the mechanoresponsive moiety because the alteration of their
fluorescence is detectable at low concentrations and inde-
pendent of time (Scheme 1a).[23,24,29–31] For this, we show the
synthesis of three new mechanofluorophores based on
a symmetric design (Scheme 1b): Firstly, we approximate
the conventional non-symmetric 9-p-extended anthracene
mechanophore I[29,30] with a central phenyl moiety a to prove
the working principle of the new design concept. We then
introduce dual fluorescent character by synthesizing aceno-
thiadiazole-based mechanofluorophores bearing either
a benzo[c][1,2,5]-thiadiazole (BTD, b) or naphtho[2,3-c]-
[1,2,5]thiadiazole (NTD, c) central moiety. These acenothia-
diazoles not only show a considerable bathochromic shift in
their excitation and emission wavelengths, but moreover
allow direct quantification of covalent bond scission events at
the fracture site. The mechanofluorophores are incorporated
into linear poly(methyl acrylate) (PMA) and crosslinked
poly(hexyl methacrylate) (PHMA) networks. The successful
force-induced retro DA reaction is confirmed in solution on
linear PMA chains via ultrasound irradiation and in the solid
state on fractured PHMA rubber networks. For the BTD
mechanofluorophores in PHMA samples, we exemplarily
The deterioration or fracture of materials submitted to
mechanical force is a molecular process progressing into
a macroscopic site of failure. Understanding such multiscale
phenomena is a nontrivial endeavor and hence requires the
development of advanced analytical tools. In macromolecular
materials, mechanical force beyond a certain threshold
ultimately leads to the scission of covalent bonds and hence
the qualitative and quantitative observation of this process is
a promising strategy to tackle this challenge. Molecules that
[*] C. Baumann, Dr. M. Stratigaki, Dr. S. P. Centeno, Dr. R. Gçstl
DWI—Leibniz Institute for Interactive Materials
Forckenbeckstr. 50, 52056 Aachen (Germany)
E-mail: goestl@dwi.rwth-aachen.de
C. Baumann
Institute of Technical and Macromolecular Chemistry, RWTH Aachen
University
Worringerweg 1, 52074 Aachen (Germany)
Dr. S. P. Centeno
Institute of Physical Chemistry, RWTH Aachen University
Landoltweg 2, 52074 Aachen (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101716.
ꢀ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution Non-Commercial
NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.
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Scheme 1. DA adducts of p-extended anthracenes and maleimide undergo a force-induced retro DA reaction. a) Traditional non-symmetric
mechanofluorophore design of previous work.[29,30] b) Synthetic pathway for modular, symmetric design of dual fluorescent mechanofluorophores
in this work.
display the workflow for quantification of covalent bond
scission via CLSM.
are widely used chromophores for conjugated polymers and
provide excellent p-conjugation and aromaticity.[33–38] To
identify the activated mechanofluorophores, for determina-
tion of molar absorptivities and fluorescence quantum yields,
and for concentration calibration for bond scission experi-
ments using CLSM, control compounds 6a–c (Figure S1)
resembling the activated mechanofluorophores were synthe-
sized.
Results and Discussion
The mechanofluorophores were obtained starting from
terminal alkyne 1, the synthesis of which we described
previously (Scheme 1b).[29,30] Subsequent Sonogashira-type
cross-coupling with dihalide precursors of the desired central
moiety 2a–c yielded dialcohols 3a–c. Depending on the
desired polymerization technique, the dialcohols were then
esterified either with a-bromoisobutyryl bromide to form the
bifunctional initiators 4a–c for controlled radical polymeri-
zation (CRP) or with methacryloyl chloride to yield cross-
linkers 5a–c for bulk free radical polymerization. Cu-cata-
lyzed CRP of initiators 4a–c gave linear telechelic polymer
chains PMAa (Mn = 52.3 kDa, ꢀM = 1.17), PMAb (Mn =
57.3 kDa, ꢀM = 1.20), and PMAc (Mn = 59.5 kDa, ꢀM =
2.15) bearing the mechanofluorophores within their center
(Figure S47–49). Incorporation of the mechanofluorophores
as crosslinkers into bulk materials was performed by thermal
bulk free radical polymerization of hexyl methacrylate with
tetraethylene glycol dimethacrylate (TEGDMA) and 5a–c to
yield rubber networks PHMAa–c, respectively.
We anticipated that the new symmetric design would only
lead to one retro DA reaction per mechanofluorophore under
force even though two DA adduct moieties were present. This
hypothesis was based on previous experiments that showed
that a mechanophore is only active when positioned in the
central region of the chain,[30] but not when in a terminal
position,[32] as would be the case after one scission event
occurred. To fully cover the visible spectrum with the
available emission wavelengths, we introduced a phenyl,
a BTD, and an NTD core, respectively. Both BTD and NTD
The force-induced retro DA reactions of polymers
PMAa–c were then investigated by irradiation with ultra-
sound using an immersion probe sonicator (20 kHz).[39]
Sonication experiments were accompanied by gel permeation
chromatography (GPC) via refractive index (RI) detector
showing the expected decrease of the initial high molar mass
signals and the emergence of new signals with ca. half of the
initial molar mass with progressing sonication time (Fig-
ure 1a, S53, S55, S57). UV/Vis spectroscopy over the course
of sonication revealed the emergence of bathochromically
shifted absorption bands relative to the non-activated me-
chanophores. Notably, the absorption spectrum of PMAa
after sonication was nearly identical to non-symmetrically
activated I[29,30] with a slight bathochromic shift attributed to
the additional alkyne moiety conjugated into the p-system
(Figure 1b, S54). Excitation at a wavelength of 405 nm
yielded the expected cyan fluorescence between 400–550 nm.
PMAb and PMAc were already fluorescent in their
deactivated state and their spectral features were comparable
to other alkynylated BTDs and NTDs reported in the
literature.[40] PMAb before sonication revealed spectral
characteristics analogous to the activated version of PMAa
with cyan fluorescence. During sonication of PMAb, an
absorption band at 455 nm emerged, approximately 75 nm
redshifted compared to the non-activated form of the
mechanophore (Figure 1c, S56). Fluorescence excitation at
this band at a wavelength of 454 nm resulted in a yellow
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constants were calculated to kPMAa,UV/Vis = 6.90 ꢀ 10À3 minÀ1
and kPMAb,UV/Vis = 4.42 ꢀ 10À3 minÀ1. As we could not obtain
a narrow disperse version of PMAc, scission rate constants
could not be determined reliably for this mechanophore. Yet,
together with the spectral features of the control compounds
6a–c prepared for each activated mechanophore (Figure S50–
52), these data strongly corroborate the successful and
reasonably selective force-induced retro DA reaction at the
center of the polymer chains where only one DA adduct
moiety is cleaved and the second remains intact.
Fluorescence quantum yields were determined on small
molecule initiators 4a–c for the non-activated and control
compounds 6a–c for the activated state in MeCN using an
Ulbricht sphere (Table S1 also for hexyl acetate and hexane).
The phenyl mechanophore was only fluorescent in the
activated state 6a and showed a value of FF = 0.72 (lexc
=
405 nm) that was comparable to the activated mechanophore
I.[29] The dual fluorescent BTD mechanophore displayed an
excellent quantum yield in the non-activated state 4b of FF =
0.84 (lexc = 405 nm) whereas the force-activated derivative 6b
dropped to FF = 0.09 (lexc = 457 nm). A similar trend was
observed for the NTD mechanophore where a good quantum
yield in the non-activated state 4c of FF = 0.50 (lexc = 496 nm)
was measured, but a low quantum yield of FF = 0.02 (lexc
=
601 nm) of the force-activated chromophore 6c. We attrib-
uted this to the solvatofluorochromism typically observed in
such donor-acceptor dye conjugates and generally ascribed to
intramolecular charge transfer (ICT).[44]
We then turned towards the PHMA elastomer networks,
in which the overall crosslinker amount was 1 mol% of which
0.02 mol% were mechanophores 5a–c. The low mechano-
phore content warrants sufficient detectability[23] while leav-
ing the mechanical properties indistinguishable from the
parent material.[24] Cube-shaped samples of ca. 2 ꢀ 2 ꢀ 2 mm
were obtained by cutting the respective PHMA samples and
the cut edges were studied by CLSM. The captured images
correspond to a top view on the xy-plane (Figure 2). Wave-
lengths of the excitation lasers (lexc) and scanning ranges (lem
were chosen based on the spectra depicted in Figure 1.
While micrographs of PHMAa, allowed localization of
bond scission events expectedly (Figure 3a), the advantage of
using dual fluorescent mechanophores became immediately
visible. For both PHMAb and PHMAc, the non-activated
)
Figure 1. Sonochemical scission of polymers PMAa–c. a) Decrease of
Mn over sonication time as determined by the RI detector of GPC
elugrams including linear fits. Absorption (solid line) and emission
spectra (dotted line) in MeCN before and after sonication of b) PMAa
(lexc =405 nm), c) PMAb (lexc =405 and 454 nm), and d) PMAc
(lexc =495 and 601 nm).
emission at 609 nm. PMAc was found to be even farther
redshifted with an absorption band at 496 nm and yellow
fluorescence at 558 nm in its deactivated form. Activation by
sonication yielded a new absorption band at 552 nm while
excitation at this band at the wavelength of 601 nm led to red
emission at 659 nm (Figure 1d, S58).
Over the course of the sonication, the apparent scission
rate constants k were determined from GPC but also from
UV/Vis measurements via the molar absorptivities of control
compounds 6a–c.[41–43] Both approaches led to comparable
values hinting towards a preferred chain scission at the
mechanophore, however, slightly underestimating the rate
constants by UV/Vis due to the insensitivity to non-specific
chain scission (Figure S53–62). The apparent scission rate
Figure 2. Schematic of a blade-cut sample, and observation of the
fluorescence at the respective fracture site (top view of xy-plane) with
63ꢁ objective by CLSM.
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mechanophores could be observed in a channel different from
that of the activated mechanophores due to their large
spectral separation allowing for clear identification of dam-
aged and undamaged areas (Figure 3b,c). While the majority
of non-activated mechanophores were localized within the
bulk of the samples, the activated mechanophores were in
proximity of the cut as product of covalent bond scission.
Note here that since the sample was firstly cut with a sharp
precision blade and then loaded onto the microscopy slide,
the two fluorescent modes at the fracture site portrayed the
remaining non-activated and activated mechanophores after
cutting rather than the PHMA sample before and after
damage was inflicted.
We then reasoned that the dual fluorescent character of
the BTD and NTD mechanophores would allow the locally
resolved determination of the ratio of non-activated to
activated mechanophore and, calibrated with the control
compounds, absolute quantification of the number of non-
broken and broken crosslinks.
For this, we performed quantitative image acquisition on
fractured PHMAb with CLSM in photon counting mode (see
Supporting Information for detailed procedure). Slicing these
micrographs horizontally in x-direction led to individual
intensity profiles for each pixel line. We hypothesized that due
to the inhomogeneous distribution of stress and strain during
the fracture of the networks, the number of activated
mechanophores varied locally within different areas of the
cut. To obtain a representative impression over the measured
area, we hence averaged the individual intensity profile slices
over the whole y-direction yielding an averaged plot profile
(Figure 4a). This showed that the fluorescence of the
Figure 3. Bright-field images (i), CLSM micrographs (ii, iii), and
composites (iv) of the edges of cut PHMA elastomer network samples.
a) PHMAa; ii) activated mode lexc =405 nm, lem =420–500 nm.
b) PHMAb; ii) non-activated mode lexc =405 nm, lem =420–500 nm,
iii) activated mode lexc =470 nm, lem =530–650 nm. c) PHMAc; ii)
non-activated mode lexc =495 nm, lem =530–650 nm; iii) activated
mode lexc =601 nm, lem =616–785 nm. Scale bar: 20 mm.
Figure 4. a) Plot profile of non-activated and activated BTD mechano-
phore at the fracture region of PHMAb (300ꢁ300 pixel). The number
of photons per pixel was vertically averaged over the length of the cut
and is shown in dependence of the horizontal distance around the cut.
b) Composite of 100ꢁ100 pixel CLSM micrographs of non-activated
and activated BTD mechanophore. Scale bar: 5 mm. c) Pixel-by-pixel
contour map showing the percentage of activated relative to remaining
non-activated BTD mechanophores calculated from panel b.
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activated BTD mechanofluorophore expanded over a region
of 30 pixels (15 mm) from the cut indicating the area affected
by the fracture process. Alongside, the non-activated mecha-
nofluorophore intensity plateaued at regions away from the
fracture site and then progressively decreased when reaching
the fracture site as the signal of the activated mechanofluor-
ophore emerged.
calculations, the confocal volume for the non-activated mode
was 0.5 mm3 (close to the typical 0.2 mm3),[50] while that for the
activated mode was 1.6 mm3. The measured concentrations
corresponded, thus, to an average of 1199 and locally up to
3115 activated, and 163441 non-activated mechanofluoro-
phores on average at the fracture site and 366028 non-
activated in the unaffected bulk per pixel. The obvious
inconsistent total number of mechanofluorophores per pixel
was rooted in the decreasing material density at the cut
region, since during fracture the free volume increased, and
the material thinned and frayed. Fçrster resonance energy
transfer (FRET) between non-activated (FRET donor) and
activated (FRET acceptor) BTD mechanophores was ruled
out as significant contributor to this phenomenon by calcu-
lating the FRET efficiency in PHMAb to 0.26% (detailed
calculations see Supporting Information).
Subsequently, these data allowed constructing an unpre-
cedented visual representation of the mechanochemical
activation process. By calculating the fraction of activated
with respect to remaining non-activated mechanofluoro-
phores per individual pixel, we not only provided qualitative
bond scission information, but a direct pixel-by-pixel quan-
tification visualization enabled by the dual fluorescent
character of the employed mechanofluorophores (Fig-
ure 4b,c).
To quantitatively follow fracture in an experimental setup
that offers more control than cutting with a blade, we hung
weights (85 g and 275 g) on supported notched rectangular
PHMAb samples inducing fracture by crack propagation at
different speeds (Movie S1) and visualized the extent of bond
scission in CLSM plot profiles (Figure S4). We observed that
a higher weight led to a faster crack propagation and
consequently to a higher fraction of activated mechanophores
highlighting the potential of this mechanophore system to
correlate microscopic bond scission events to the macroscopic
mechanical properties of the parent material (Figure S5).
To exclude that the non-uniform activation of the BTD
mechanofluorophore over the length of the cut is an artifact
of fabrication or measurements, we created a multi-plot
profile analysis macro to draw consecutive horizontal selec-
tions with a defined distance interval. Applying a region of
interest tool and statistics functions, we found that the
maximum recorded number of photons for the activated
mechanofluorophore locally reached 162, while the number
of photons of the non-activated mechanofluorophore neither
varied significantly away from nor at the fracture site
(exemplary cases in Figure S2). Crosslinked polymer net-
works are non-ideal, contain dangling chain ends and loops,[45]
and mechanophores are differently aligned regarding the
direction of macroscopic force application.[46–48] Hence, each
pixel volume contained a different number of active mecha-
nofluorophores. However, the above data allowed us to
exclude extreme non-uniformity in crosslinker distribution
within PHMAb pinpointing the variation in activation to
microscopic stress and strain distribution differences.
To quantify non-activated and activated mechanofluoro-
phores within fractured PHMAb, we then calibrated the
CLSM setup by using the photon-counting mode on solutions
of the control compounds (Table S2). We chose hexyl acetate
as solvent as its refractive index and physicochemical micro-
environment matches that of PHMA.[49] This yielded calibra-
tion curves for the number of photons per pixel as a function
of the concentration of the respective molecule (Table S3,
Figure S3). By applying the linear fit equations to convert
number of photons to concentrations (molLÀ1), and subse-
quent multiplication with the Avogadro constant to obtain the
number of BTD molecules per m3, we calculated number
densities of the activated mechanofluorophores at their peak Conclusion
intensities at the fracture site of 7.5 ꢀ 1020 mÀ3 on average,
reaching up to 1.9 ꢀ 1021 mÀ3 locally within the cut. The
number density of the non-activated mechanophores was
calculated on average to 3.3 ꢀ 1023 mÀ3 at the fracture site
where the activated mechanofluorophore peaked and to 7.3 ꢀ
1023 mÀ3 within the unaffected bulk. The values measured for
the non-activated BTD mechanophore matched the bulk
mechanophore number densities of 6.0 ꢀ 1023 mÀ3 estimated
from the synthesis parameters of PHMAb and those pre-
viously reported for comparable materials.[24] From the
0.02 mol% fraction of BTD mechanophore crosslinks we
calculated the overall crosslink density of the network to 7.2 ꢀ
1025 mÀ3, which was roughly consistent with a value of 6.0 ꢀ
1025 mÀ3 obtained from swelling experiments and the Flory-
Rehner equation (see Supporting Information for detailed
procedure).
We here presented the design and synthesis of a mechano-
fluorophore family based on Diels–Alder adducts of p-
extended anthracenes and maleimide. The central aromatic
moiety of these optical force probes was readily exchanged
with dihalide building blocks enabling access to redshifted
absorption and emission spectra as well as dual fluorescence.
The mechanically induced bond scission process was qual-
itatively observed in solution by fluorescence spectroscopy
and in rubber networks using confocal laser scanning
microscopy. In addition, the considerable spectral separation
of the emission of non-activated and activated mechanofluor-
ophores allowed the direct and localized quantification of
non-broken and broken bonds.
We anticipate that these mechanofluorophores will find
more application to analyze the behavior of polymer materi-
als under force in the future, because their dual fluorescent
nature allows for tracking of the non-fractured materials,
enables to spatially resolve quantification of damage events,
and is the basis for their optical tunability over the entire
To further quantify and directly compare the number of
mechanofluorophores per confocal volume, we proceeded to
z-scanning of sub-diffraction fluorescent beads (see Support-
ing Information for detailed procedure). According to these
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[15] T. Yamamoto, S. Kato, D. Aoki, H. Otsuka, Angew. Chem. Int.
visible spectrum by comparably straightforward synthesis. We
thus expect these mechanofluorophores to contribute to the
elucidation of phenomena related to force transduction and
dissipation as well as the fracture of materials ranging from
soft hydrogels to high-performance polymers.
Ed. 2021, 60, 2680 – 2683; Angew. Chem. 2021, 133, 2712 – 2715.
[16] T. Kosuge, X. Zhu, V. M. Lau, D. Aoki, T. J. Martinez, J. S.
Moore, H. Otsuka, J. Am. Chem. Soc. 2019, 141, 1898 – 1902.
[17] K. Ishizuki, D. Aoki, R. Goseki, H. Otsuka, ACS Macro Lett.
2018, 7, 556 – 560.
[18] K. Imato, A. Irie, T. Kosuge, T. Ohishi, M. Nishihara, A.
Takahara, H. Otsuka, Angew. Chem. Int. Ed. 2015, 54, 6168 –
6172; Angew. Chem. 2015, 127, 6266 – 6270.
Acknowledgements
[19] C. Lçwe, C. Weder, Adv. Mater. 2002, 14, 1625 – 1629.
[20] A. Lavrenova, D. W. R. Balkenende, Y. Sagara, S. Schrettl, Y. C.
Simon, C. Weder, J. Am. Chem. Soc. 2017, 139, 4302 – 4305.
[21] C. Calvino, A. Guha, C. Weder, S. Schrettl, Adv. Mater. 2018, 30,
1704603.
C. B. and R. G. are grateful for support by a Freigeist-
Fellowship of the Volkswagen Foundation (No. 92888). M. S.
and R. G. acknowledge financial support from an Exploration
Grant of the Boehringer Ingelheim Foundation. Parts of the
analytical investigations were performed at the Center for
Chemical Polymer Technology CPT, which was supported by
the European Commission and the federal state of North
Rhine-Westphalia (No. 300088302). Financial support is
acknowledged from the European Commission (EUSMI,
No. 731019). The authors wish to thank F. Meder, L. Witzdam,
T. Zensen, S. Folkerts, P. Marten, N. Mꢁnstermann, and D.
Korkut for their synthetic contributions. Open access funding
enabled and organized by Projekt DEAL.
[22] T. Matsuda, R. Kawakami, T. Nakajima, J. P. Gong, Macro-
molecules 2020, 53, 8787 – 8795.
[23] M. Stratigaki, C. Baumann, L. C. A. van Breemen, J. P. A. Heuts,
R. P. Sijbesma, R. Gçstl, Polym. Chem. 2020, 11, 358 – 366.
[24] J. Slootman, V. Waltz, C. J. Yeh, C. Baumann, R. Gçstl, J.
Comtet, C. Creton, Phys. Rev. X 2020, 10, 041045.
[25] M. Karman, E. Verde-Sesto, C. Weder, Y. C. Simon, ACS Macro
Lett. 2018, 7, 1099 – 1104.
[26] J. Li, T. Shiraki, B. Hu, R. A. E. Wright, B. Zhao, J. S. Moore, J.
Am. Chem. Soc. 2014, 136, 15925 – 15928.
[27] J. Li, B. Hu, K. Yang, B. Zhao, J. S. Moore, ACS Macro Lett.
2016, 5, 819 – 822.
[28] E. Izak-Nau, D. Campagna, C. Baumann, R. Gçstl, Polym.
Chem. 2020, 11, 2274 – 2299.
Conflict of interest
[29] R. Gçstl, R. P. Sijbesma, Chem. Sci. 2016, 7, 370 – 375.
[30] D. Yildiz, C. Baumann, A. Mikosch, A. J. C. Kuehne, A.
Herrmann, R. Gçstl, Angew. Chem. Int. Ed. 2019, 58, 12919 –
12923; Angew. Chem. 2019, 131, 13051 – 13055.
[31] E. Izak-Nau, D. E. Demco, S. Braun, C. Baumann, A. Pich, R.
Gçstl, ACS Appl. Polym. Mater. 2020, 2, 1682 – 1691.
[32] J. M. Lenhardt, A. L. Black Ramirez, B. Lee, T. B. Kouznetsova,
S. L. Craig, Macromolecules 2015, 48, 6396 – 6403.
The authors declare no conflict of interest.
Keywords: fluorescence · fracture · mechanochemistry ·
microscopy · polymers
[33] S. E. Tan, M. S. Sarjadi, Polym. Sci. Ser. B 2017, 59, 479 – 496.
[34] J. Du, M. C. Biewer, M. C. Stefan, J. Mater. Chem. A 2016, 4,
15771 – 15787.
[35] T. C. Parker, D. G. Patel, K. Moudgil, S. Barlow, C. Risko, J. L.
Brꢃdas, J. R. Reynolds, S. R. Marder, Mater. Horiz. 2015, 2, 22 –
36.
[36] Y. Wang, T. Michinobu, J. Mater. Chem. C 2016, 4, 6200 – 6214.
[37] R. Ratha, A. Singh, M. A. Afroz, R. K. Gupta, M. Baumgarten,
K. Mꢁllen, P. K. Iyer, Synth. Met. 2019, 252, 113 – 121.
[38] B. A. D. Neto, A. A. M. Lapis, E. N. Da Silva Jfflnior, J. Dupont,
Eur. J. Org. Chem. 2013, 228 – 255.
[39] G. Cravotto, E. C. Gaudino, P. Cintas, Chem. Soc. Rev. 2013, 42,
7521 – 7534.
[40] A. L. Appleton, S. Miao, S. M. Brombosz, N. J. Berger, S.
Barlow, S. R. Marder, B. M. Lawrence, K. I. Hardcastle,
U. H. F. Bunz, Org. Lett. 2009, 11, 5222 – 5225.
[41] T. Sato, D. E. Nalepa, J. Appl. Polym. Sci. 1978, 22, 865 – 867.
[42] M. J. Kryger, A. M. Munaretto, J. S. Moore, J. Am. Chem. Soc.
2011, 133, 18992 – 18998.
[43] R. Stevenson, G. De Bo, J. Am. Chem. Soc. 2017, 139, 16768 –
16771.
[1] D. R. T. Roberts, S. J. Holder, J. Mater. Chem. 2011, 21, 8256 –
8268.
[2] F. Ciardelli, G. Ruggeri, A. Pucci, Chem. Soc. Rev. 2013, 42, 857 –
870.
[3] M. J. Jacobs, K. Blank, Chem. Sci. 2014, 5, 1680 – 1697.
[4] D. A. Davis, A. Hamilton, J. Yang, L. D. Cremar, D. Van Gough,
S. L. Potisek, M. T. Ong, P. V. Braun, T. J. Martꢂnez, S. R. White,
J. S. Moore, N. R. Sottos, Nature 2009, 459, 68 – 72.
[5] J. Li, C. Nagamani, J. S. Moore, Acc. Chem. Res. 2015, 48, 2181 –
2190.
[6] G. R. Gossweiler, T. B. Kouznetsova, S. L. Craig, J. Am. Chem.
Soc. 2015, 137, 6148 – 6151.
[7] H. Zhang, F. Gao, X. Cao, Y. Li, Y. Xu, W. Weng, R. Boulatov,
Angew. Chem. Int. Ed. 2016, 55, 3040 – 3044; Angew. Chem.
2016, 128, 3092 – 3096.
[8] Y. Pan, H. Zhang, P. Xu, Y. Tian, C. Wang, S. Xiang, R. Boulatov,
W. Weng, Angew. Chem. Int. Ed. 2020, 59, 21980 – 21985; Angew.
Chem. 2020, 132, 22164 – 22169.
[9] M. J. Teng, X. R. Jia, X. F. Chen, Y. Wei, Angew. Chem. Int. Ed.
2012, 51, 6398 – 6401; Angew. Chem. 2012, 124, 6504 – 6507.
[10] Z. Ma, M. Teng, Z. Wang, S. Yang, X. Jia, Angew. Chem. Int. Ed.
2013, 52, 12268 – 12272; Angew. Chem. 2013, 125, 12494 – 12498.
[11] Z. Wang, Z. Ma, Y. Wang, Z. Xu, Y. Luo, Y. Wei, X. Jia, Adv.
Mater. 2015, 27, 6469 – 6474.
[44] P. Rietsch, S. Sobottka, K. Hoffmann, P. Hildebrandt, B. Sarkar,
U. Resch-Genger, S. Eigler, ChemPhotoChem 2020, 4, 668 – 673.
[45] L. W. Hill, Prog. Org. Coatings 1997, 31, 235 – 243.
[46] B. A. Beiermann, S. L. B. Kramer, J. S. Moore, S. R. White, N. R.
Sottos, ACS Macro Lett. 2012, 1, 163 – 166.
[12] E. Ducrot, Y. Chen, M. Bulters, R. P. Sijbesma, C. Creton,
Science 2014, 344, 186 – 189.
[47] A. D. N. Celestine, B. A. Beiermann, P. A. May, J. S. Moore,
N. R. Sottos, S. R. White, Polymer 2014, 55, 4164 – 4171.
[48] B. A. Beiermann, S. L. B. Kramer, P. A. May, J. S. Moore, S. R.
White, N. R. Sottos, Adv. Funct. Mater. 2014, 24, 1529 – 1537.
[13] Y. Chen, A. J. H. Spiering, S. Karthikeyan, G. W. M. Peters,
E. W. Meijer, R. P. Sijbesma, Nat. Chem. 2012, 4, 559 – 562.
[14] Y. Yuan, W. Chen, Z. Ma, Y. Deng, Y. Chen, Y. Chen, W. Hu,
Chem. Sci. 2019, 10, 2206 – 2211.
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
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These are not the final page numbers!

Angewandte
Research Articles
Chemie
[49] G. Scott, Properties of Polymers. Their Correlation with Chem-
ical Structure; Their Numerical Estimation and Prediction from
Additive Group Contributions, Elsevier, Amsterdam, 1992.
[50] K. Jespersen, Y. Zausjitsyn, S. Westenhoff, T. Pullerits, A.
Yartsev, O. Inganäs, V. Sundstrçm, Fluorescence of Supermole-
cules, Polymers, and Nanosystems, Springer, Berlin, Heidelberg,
2008.
Manuscript received: February 3, 2021
Revised manuscript received: March 23, 2021
Accepted manuscript online: March 30, 2021
Version of record online: && &&
,
&&&&
&&&&
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Mechanofluorophores
C. Baumann, M. Stratigaki, S. P. Centeno,
R. Gçstl*
&&&& — &&&&
Multicolor Mechanofluorophores for the
Quantitative Detection of Covalent Bond
Scission in Polymers
A new platform of dual fluorescent
mechanofluorophores that display cova-
lent bond scission during material
breakage by changing their optical prop-
erties was synthesized. The absorption
and emission characteristics were tuned
to cover the entire visible spectrum.
Using photon quantitative confocal fluo-
rescence microscopy, these new mecha-
nofluorophores enabled the quantifica-
tion of bond scission events with mm
resolution.
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!