Mechanochromic Polymers That Turn Green Upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism

Article abstract of DOI:10.1002/chem.201800194

Mechanochromic polymers, that is, polymers sensitive to mechanical impact, promise great potential for applications in damage sensors. In particular, radical-type mechanochromic polymers, which produce colored radical species in response to mechanical str

Full text of DOI:10.1002/chem.201800194

A Journal of
Accepted Article
Title: Mechanochromic Polymers that Turn Green upon the
Dissociation of Diarylbibenzothiophenonyl: The Missing Piece
toward Rainbow Mechanochromism
Authors: Kuniaki Ishizuki, Hironori Oka, Daisuke Aoki, Raita Goseki,
and Hideyuki Otsuka
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To be cited as: Chem. Eur. J. 10.1002/chem.201800194
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Mechanochromic Polymers that Turn Green upon the
Dissociation of Diarylbibenzothiophenonyl: The Missing Piece
toward Rainbow Mechanochromism
Kuniaki Ishizuki, Hironori Oka, Daisuke Aoki, Raita Goseki, and Hideyuki Otsuka*[a]
Abstract: Mechanochromic polymers, i.e., polymers sensitive to
(a) DABBFꢀ
tBu
tBu
mechanical impact, promise great potential for applications in
damage sensors. In particular, radical-type mechanochromic
polymers, which produce colored radical species in response to
mechanical stress, may enable not only the visualization of
mechanical stress, but also its quantitative evaluation by
electron paramagnetic resonance analysis. In this work, a
radical-type mechanochromic polymer that exhibits a color
OH
OH
tBu
tBu
O
O
Mechanical
stressꢀ
O
O
O
O
O
O
O
O
O
O
tBu
tBu
HO
HO
tBu
tBu
change from white to green upon dissociation of
diarylbibenzothiophenonyl moiety at the mid-point of
a
a
(b) TASNꢀ
OH
OH
MeO
O
MeO
O
polystyrene chain is presented, and its mechanochromic
behavior examined. Mechanochromic materials that show a
variety of colors (‘rainbow colors’) in response to mechanical
stress were prepared by simply mixing radical-type
mechanochromic polymers of primary colors.
Mechanical
stressꢀ
CN
NC
CN
NC
O
OMe
O
OMe
HO
HO
Mechanoresponsive materials have recently received increasing
interest.[1–4] In particular, mechanochromic polymers, which
exhibit color changes upon exposure to mechanical stress, have
attracted much attention in the field of materials science, as
they are able to detect damage in materials, thus potentially
preventing serious accidents by providing the chance to easily
identify damaged parts and replace or repair them.[4-18] The
main strategy for the design of mechanochromic polymers
involves the incorporation of mechanochromophores, i.e.,
optically mechanoresponsive moieties, into polymer chains. A
variety of mechanochromic polymers have been developed
since Sottos and co-workers reported the first mechanochromic
spiropyran unit embedded in poly(methyl acrylate), which
endows this commodity plastic with mechanochromic
properties.[4] However, the combinatorial generation of color in
mechanochromism remains challenging, as the number of
molecules able to act as mechanochromophores is still limited. If
color-controlled mechanochromism could be realized, it would
be possible to detect damage and even recognize both the type
and intensity of the mechanical stimulus via differences in the
resulting colors.[19–22] Since such a mechanoresponsive behavior
is mainly limited to small molecules, the development of
macromolecules, e.g., polymers, with mechanochromic
properties is highly desirable, as it would allow the facile design
(c) DABBTꢀ
Mechanical
stressꢀ
O
O
S
S
S
S
O
O
Figure 1. Chemical structures of (a) DABBF, (b) TASN, and (c) DABBT
before and after applying mechanical stress.
of materials with properties fit for that purpose. Herein, we report
preliminarily work on the combinatorial generation of color with
mechanochromic polymers by mixing three different
mechanochromic polymers that exhibit differently colored
radicals in response to mechanical stimuli of similar magnitude.
The ultimate advantage of radical-type mechanochromic
polymers is not only the qualitative visualization of the
mechanical stress, but also its quantitative evaluation by
electron paramagnetic resonance (EPR) spectroscopy, both in
solution and the solid state. We have already reported two
radical-type
mechanochromic
polymers
that
bear
diarylbibenzofuranone (DABBF) or tetraarylsuccinonitrile (TASN)
moieties (Figure 1a, b).[7,23–27] Homolytic cleavage of the central
carbon–carbon bond in the DABBF and TASN units in response
to mechanical stress affords arylbenzofuranone (blue) and
diarylacetonitrile radicals (pink), respectively. In these molecular
systems, the regeneration of the carbon–carbon bonds via
coupling reactions of the generated radicals has also been
confirmed. Remarkably, the mechanoresponsiveness of the
DABBF and TASN mechanochromophores was strongly
enhanced after the incorporation in polymer chains. Since the
two radical-type mechanochromophores display different colors,
we envisioned that the development of a green radical-type
mechanochromophore would complete the set of three primary
colors, which would allow a subsequent development of rainbow
mechanochromism. Maeda et al. have reported that the
[a]
K. Ishizuki, Dr. D. Aoki, Dr. R. Goseki, and Prof. Dr. H. Otsuka
Department of Chemical Science and Engineering,
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
E-mail: otsuka@polymer.titech.ac.jp
H. Oka
Department of Organic and Polymeric Materials,
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
arylbenzothiophenonyl
diarylbibenzothiophenonyl (DABBT; Figure 1c) by homolytic
(ABT)
radical,
generated
from
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201800194
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cleavage in response to mechanical stress, exhibits green
color.[28,29] We thus focused on the DABBT skeleton and
designed DABBT derivatives for the insertion in a polymer chain.
Since the responsiveness of mechanochromophores can be
amplified upon incorporation in a polymer chain, a DABBT-
centered polymer seemed an appropriate prospective for a
mechanochromic material exhibiting green color upon exposure
to mechanical stress. In this work, DABBT was employed as the
O
O
Cl
OH
O
pyridine
S
S
CH2Cl2
r.t.
O
HO
O
DABBT-diol
O
N3
n
MeO
O
mechanochromophore for
polymer, given that this color represents the missing piece
toward the development of rainbow mechanochromism.
rational combination of such colored radical-based
a green-colored mechanochromic
O
O
CuBr, Cu(0), PMDETA
S
S
O
O
DMF
r.t.
O
O
A
O
DABBT-dialkyne
mechanochromic polymers, all of which are sufficiently stable in
air to be easily handled, could potentially allow the control of the
mechanochromism, the generation of a spectrum of colors, and
a quantitative evaluation of mechanical stress.
O
OMe
O
O
S
N
N
O
N
O
To incorporate the DABBT skeleton into a polymer chain,
a DABBT derivative with two hydroxy groups (DABBT-diol;
Scheme 1) was designed and synthesized in eight steps from
ethyl-2-(4-methoxyphenyl)acetate (Scheme S1). The product
was fully characterized by 1H NMR, 13C NMR, and IR
spectroscopy, as well as mass spectrometry measurements, the
results of which support the successful synthesis of DABBT-diol
(Figures S19–S21). To investigate whether DABBT-diol could
act as a green mechanophore, shear forces were applied to
DABBT-diol at room temperature in air. The pale-yellow
DABBT-diol powder turned to yellow/green after grinding
(Figure 2a). EPR studies of the powder before and after grinding
in a ball mill (30 Hz, 20 min) revealed a strong signal after
grinding, while only a weak signal was observed before grinding
(Figure 2c). The g value (2.005) confirmed that DABBT-diol
undergoes, in response to the shear forces, a homolytic bond
cleavage under concomitant formation of green radicals. To
obtain further insight into the dissociation behavior of DABBT-
diol, variable-temperature EPR and UV-vis spectroscopy
measurements were carried out in solution. Based on the results
of the EPR analysis and a van’t Hoff plot (Figure S30–S32), the
bond dissociation energy of the central C–C bond in DABBT-
diol (87.9–92.0 kJ/mol) was estimated to be very similar to
those of DABBF (85.4–104.8 kJ/mol)[27] and TASN (92.9
kJ/mol).[25] High-temperature (70 ºC) UV-vis measurements of a
1,4-dioxane solution of DABBT-diol revealed an absorption
maximum at 452 nm, whose complementary color is green,
n
O
O
N
N
N
S
O
O
MeO
n
O
PS-DABBT-PS
Scheme 1. Synthesis of DABBT-dialkyne and PS-DABBT-PS.
(a)ꢀ
OH
OH
O
Grindingꢀ
O
O
O
S
S
S
S
1 cmꢀ
O
O
O
O
HO
HO
(b)ꢀ
Grindingꢀ
O
O
O
O
S
S
S
S
1 cmꢀ
O
O
O
O
(c)ꢀ
DABBT-diol before grindingꢀ
ꢁꢀ
DABBT-diol after grindingꢀ
ꢁꢀ
demonstrating
good
correlation
with
the
observed
PS-DABBT-PS before grindingꢀ
ꢁꢀ
mechanochromism in the bulk. On the other hand, DABBF and
TASN derivatives display absorption maxima at 640 and 543 nm,
whose complementary colors are blue and pink, respectively
(Figure S33).
PS-DABBT-PS after grindingꢀ
ꢁꢀ
Prior to the incorporation of DABBT-diol into a polymer
chain, two ethynyl groups were added to DABBT-diol by
reaction with 5-hexynoyl chloride to afford DABBT-dialkyne.
Since mechanochromic polymers with relatively high glass
transition temperatures (Tg) prevent the recombination of the
activated radicals obtained upon cleavage of the
mechanochromophore, polystyrene was selected as the matrix
polymer to which the DABBT skeleton was attached.[25,26] In the
presence of CuBr as a catalyst, a click reaction of azide-
terminated polystyrene (Mn = 6 kDa, PDI = 1.18) and DABBT-
dialkyne afforded the corresponding linear polymer with a
DABBT unit in the middle of the chain as a white powder (PS-
DABBT-PS; Scheme 1). PS-DABBT-PS was characterized by
1H NMR spectroscopy and gel permeation chromatography
325
326
327
328
329
330
Magnetic field / mTꢀ
Figure 2. (a) DABBT-diol and (b) PS-DABBT-PS before and after grinding.
(c) EPR spectra of DABBT-diol and PS-DABBT-PS before and after
grinding.
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(GPC). The 1H NMR spectrum of PS-DABBT-PS (Figure S27)
exhibits the characteristic signals of the DABBT skeleton, while
the GPC profile shows a unimodal peak, whose molecular
weight is approximately twice that of the precursor (the azide-
terminated polystyrene; Figure S28), indicating the successful
synthesis of PS-DABBT-PS (Mn = 11 kDa, PDI = 1.13). The
mechanochromic
properties
of
PS-DABBT-PS
were
preliminarily evaluated by a grinding test (30 Hz, 20 min). After
grinding, a color change from white to green was clearly
observed (Figure 2b). The incorporation of polystyrene did not
only result in a color change of the corresponding DABBT
derivative from yellow to white before grinding, but also from
yellow/green to bright green after grinding. Although the green
color of the ground polymer was stable even after one day, the
color immediately vanished and returned to white upon heating
beyond the Tg of PS-DABBT-PS (90 ºC) (Figures S29 and S34),
or upon addition of a good solvent such as THF, which
increases the molecular mobility. An EPR analysis of PS-
DABBT-PS showed that the radical content increased with
grinding (g ≈ 2.005) in a similar fashion to that of ground
DABBT-diol (Figure 2c). These results clearly confirm the
mechanical selective scission of the central C–C bond of the
DABBT unit at the center of the polymer chain. The dissociation
ratio of DABBT in PS-DABBT-PS after grinding (~8.2%) was
estimated to be more than 40 times that of DABBT-diol (0.2%),
clearly demonstrating the enhancement of the mechanochromic
properties after incorporation into the polymer chains.[30–33] In
general, PS-DABBT-PS was found to exhibit a behavior that is
similar to that of other mechanochromic polymers bearing
DABBF and TASN units as mechanochromophores.[25,26]
Polystyrene-based mechanochromic polymers bearing
TASN and DABBF units that connect their polymer chains (PS-
DABBF-PS: Mn = 11 kDa, PDI = 1.14; PS-TASN-PS: Mn = 12
kDa, PDI = 1.10) were synthesized in a similar manner using the
corresponding TASN and DABBF derivatives.[25,26] The three
radical-type mechanochromic polymers, PS-DABBT-PS, PS-
DABBF-PS, and PS-TASN-PS were mixed and ground by ball
milling (30 Hz, 5 min). As shown in Figure 3, the mixture
containing PS-DABBT-PS and PS-DABBF-PS (1:1, w/w)
exhibited a color change from white to dark green after grinding,
which is a color that is different to those of the individual
polymers. This color change can be explained by the fact that
the individual polymers in the mixture absorb light of different
wavelengths and reflect or transmit other colors, as observed in
the case of colored paint. Similarly, the mixture of PS-DABBT-
PS and PS-TASN-PS (1:1, w/w) underwent a color change from
white to orange, while a mixture of PS-DABBF-PS and PS-
TASN-PS (1:1, w/w) exhibited a color change from white to
purple. Furthermore, the color of a mixture of all three polymers
(1:1:1, w/w/w) changed after grinding from white to beige, the
analogue of black when working with colored paint. These color
Figure 3. Photographs of PS mixtures containing DABBF, TASN, and
DABBT before (top) and after (bottom) grinding.
changes
were
also
observed
after
grinding
the
mechanochromophores themselves, i.e., the DABBT, DABBF,
and TASN derivatives without the polymer chains, showing the
corresponding color changes derived from the dissociation of the
mechanochromophores after grinding under similar conditions.
Such a rainbow mechanochromism may potentially be applied to
a wide variety of polymeric materials.
In conclusion, a novel mechanochromic polymer that
exhibits the green color of ABT radicals in response to shear
forces was successfully synthesized from DABBT. The
mechanochromic properties of DABBT were amplified upon
incorporation of polymer chains with a relatively high Tg. Rational
combinations with other mechanochromic polymers showed
different colors in response to similar shear forces, and thus
allowed the development of rainbow mechanochromism by
simply mixing these polymers. Since the mechanochromophores
can be inserted in different polymer chains, we intend to report
in the near future the application of such high-performance
materials with rainbow mechanochromism and defined
mechanical properties in damage sensors able to discriminate
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both the type and intensity of mechanical stimuli by the resulting
color.
Acknowledgements
This work was supported by KAKENHI grants 17H01205 (H.O.)
and 15K17907 (R.G.) from the Japan Society for the Promotion
of Science (JSPS) and the ImPACT Program of the Council for
Science, Technology, and Innovation (Cabinet Office,
Government of Japan). We thank Prof. Takashi Ishizone (Tokyo
Institute of Technology, Japan) for his help with the variable-
temperature UV-vis measurements.
Keywords: polymer reactions • chromism • mechanochemistry •
mechanochromism • radical
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Rainbow mechanochromism:
A
that
Kuniaki Ishizuki, Hironori Oka, Daisuke
Aoki, Raita Goseki, and Hideyuki
Otsuka*
mechanochromic
polymer
exhibits green color upon the
dissociation of
a
Page No. – Page No.
diarylbibenzothiophenonyl unit at the
mid-point of its polymer chain is
Mechanochromic Polymers that
Turn Green upon the Dissociation of
Diarylbibenzothiophenonyl: The
Missing Piece toward Rainbow
Mechanochromism
presented.
mechanochromism was achieved by
mixing three different radical-type
mechanochromic
exhibit blue, pink, and green color
upon exposure to shear forces.
Rainbow
polymers
that
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