Multicolor Mechanochromism of a Polymer/Silica Composite with Dual Distinct Mechanophores

Article abstract of DOI:10.1021/jacs.8b13310

The development of a multicolor mechanochromic polymer/silica composite is achieved by using two distinct types of mechanochromophores. The multicolor mechanochromism of the composite containing diarylbibenzofuranone in silica-rich domains and naphthopyran in the polymer-rich domain is observed. The obtained composite shows blue, green, and orange colors according to the intensity of applied mechanical stimuli, solvent addition, and lapse of time. This unique multicolor mechanochromic behavior is evaluated by solid-state UV-vis absorption spectroscopy, ab initio steered molecular dynamics simulations, and computed minimum energy paths on force-modified potential energy surfaces. The unique mechanochromism is attributed to the difference in properties, activated colors, and domain locations between the two mechanochromophores.

Full text of DOI:10.1021/jacs.8b13310

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Communication
Multicolor Mechanochromism of a Polymer/Silica
Composite with Dual Distinct Mechanophores
Takahiro Kosuge, Xiaolei Zhu, Vivian M. Lau, Daisuke Aoki,
Todd J. Martínez, Jeffrey S. Moore, and Hideyuki Otsuka
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Jan 2019
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Multicolor Mechanochromism of a Polymer/Silica Composite with
Dual Distinct Mechanophores
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†,‡
§
‡,∥
†,⊥
§
Takahiro Kosuge, Xiaolei Zhu, Vivian M. Lau, Daisuke Aoki, Todd J. Martinez, Jeffrey S.
Moore,
*,‡,∥
*,†, ⊥
Hideyuki Otsuka
†
Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo
‡
152-8550, Japan, Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United
§
States, Department of Chemistry and the PULSE Institute, Stanford University, Stanford, California 94305, United States,
∥
Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois
⊥
61801, United States, Research Institute of Polymer Science and Technology, Tokyo Institute of Technology, 2-12-1
Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Supporting Information Placeholder
ABSTRACT: The development of
a
multicolor
applied
mechanochromic polymer/silica composite is achieved
by using two distinct types of mechanochromophores.
The multicolor mechanochromism of the composite
containing diarylbibenzofuranone (DABBF) in silica-
rich domains and naphthopyran (NP) in the polymer-rich
domain is observed. The obtained composite shows
blue, green, and orange colors according to the intensity
of applied mechanical stimuli, solvent addition, and
lapse of time. This unique multicolor mechanochromic
behavior is evaluated by solid-state UV-vis absorption
spectroscopy, ab initio steered molecular dynamics
simulations, and computed minimum energy paths on
force-modified potential energy surfaces. The unique
mechanochromism is attributed to the difference in
properties, activated colors, and domain locations
between the two mechanochromophores.
Mechanochromic materials change color change
when mechanical stimuli are applied, which can be
utilized for stress detection and failure monitoring and
1
prevention. If the materials exhibit multicolor
mechanochromism
(mechanochromic
properties
showing two or more color changes in response to types
or intensity of applied mechanical force), they can be
utilized for multimodal analysis of material destruction
and stress sensing. Several multicolor mechanochromic
systems have been developed so far, including structural
Figure 1. (a) Chemical structures of DABBF, NP, and their
activated states. (b) Reverting behavior from the two
activated states.
2
3-5
color, supramolecular systems, and combination of
6
spiropyran and pyrene or combination of rhodamine
to
a
mechanochromophore-linked polymer, the
7-9
and pyrene. Introducing a mechanochromophore, i.e.,
mechanochromophore in the polymer chain is activated,
which results in a color change of the material. Several
types of mechanochromophores showing different colors
10
a color-changing mechanophore, into polymers is one
of the most effective approaches for obtaining
mechanochromic materials. When mechanical force is
1
1-29
with different mechanisms have been reported.
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However, most of them display a simple color change absorption spectroscopy. The most important feature of
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between two colors upon exposure to the mechanical
stimulus and rarely display multicolor change except for
the three-point anchored rhodamine mechanophore.
the combination is that they have significantly different
reverting properties: ABF radicals cleaved from DABBF
recombine very quickly in high-mobility regions such as
30
Since there are many mechanochromophore molecules
reported, combining of mechanochromophores is a very
in a low-glass transition temperature (T ) polymer
g
matrix in the bulk state, or in a solvent-swollen polymer
32-38
useful
mechanochromic materials. However, reports on
polymer-based materials with multiple
approach
for
designing
multicolor
matrix.
On the other hand, MC, which can be
reverted back to the NP state by visible light irradiation,
has a somewhat long life time and is detected visually
mechanochromophores are very limited, given that
rational molecular design of mechanophore systems is
still challenging. Recently, we developed a polymer-
even in a high-mobility environment low-T polymer
matrix; MC is able to retain its colored state even in a
solvent-swollen polymer matrix (Figure 1(b)).
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based material exhibiting multicolor mechanochromism
31
by using two radical-type mechanochromophores.
A
characteristic of radical-type mechanophores is their
mechano-sensitivity to the mobility in polymer domains
because they are in equilibrium with the corresponding
dissociated colored-radical species. Dissociated radicals
quickly recombine in high-mobility domains, resulting
in no detectable color change or fast color fading.
Therefore, their combination with other mechanophores
whose mechanochromic properties are insensitive to
mobility is expected to greatly contribute to the
development of
a
wide variety of multicolor
mechanochromic materials and to create their synergy
effect.
To demonstrate multicolor mechanochromism based
on the combination of a dividing radical-type and
another type of mechanochromophore, we focused on a
polymer/silica composite as the matrix since it has both
the soft domain of the polymer and the hard domain of
the silica. The rigid framework of the silica hard
domain, especially the interface between the hard and
soft domains, maximize radical-type mechanophore
performance because of their moderately limited
3
2
mobility. Furthermore, the mechanical properties of
the polymer/silica composite can be tuned by simply
changing feed ratios of their precursors.
Here, we report a vivid three-color changing
mechanoresponsive polymer/silica composite based on
two
mechanistically
distinct
types
of
Figure 2. Synthesis scheme of the composite, an optical
photograph of a sample piece, a TEM image, and
illustration of the two mechanophores' locations in the
sample.
mechanochromophores with different properties. We
chose diarylbibenzofuranone (DABBF) as the dividing
radical-type mechanophore and naphthopyran (NP) as
the non-dividing isomerization-type mechanophore. As
shown in Figure 1(a), the central C–C bond in DABBF
is mechanically cleavable to generate the corresponding
blue-colored arylbenzofuranone (ABF) radicals,
NP can be mechanically converted to the orange-colored
merocyanine (MC) form by isomerization.
mixture of blue and orange activated species is expected
to show a green color. Furthermore, the colored
As shown in Figure 2, the composite containing
DABBF units at the interface and NP units at the
polymer domain (DABBF-NP Composite) was
prepared from DABBF alkoxysilane derivative (1) and
poly(butyl acrylate) with dimethoxymethylsilyl groups
at both chain ends and NP at the center of polymer chain
3
2-38
and
1
4
The
(
2) by using a sol-gel method as reported in our previous
activated species have different absorbance peaks (λmax
)
3
2
paper. The obtained sample was a pale-yellow film,
and its morphology was observed by transmission
electron microscopy (TEM). Both spherical silica
in the visible region; i.e., the λmax values of ABF radicals
are 550 and 650 nm, and the λmax value of MC is 440
nm, indicating that they can be distinguished by UV-vis
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domains and polymer matrix are observed as shown in
Figure 2. The phase separation structure is similar to that
in
our
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Figure 3. (a) Optical photographs and plausible illustrations showing mechanochromophores' activation states in the
pristine, gently kneaded, strongly kneaded, and dichloromethane–added samples, and the sample left under visible light
for 20 min after dichloromethane addition, and (b) their solid-state absorbance spectra. (c) Optical photographs and
plausible illustrations showing mechanochromophores' activation states in the pristine and strongly kneaded samples,
and the sample left under visible light after strong kneading for 30 min and 24 h, and (d) their solid-state absorbance
spectra. The absorbance spectra were normalized by the absorbance intensity at 800 nm.
previous report.32
To gain a better understanding of the competing
mechanochemical processes of DABBF and NP
isomerization, we performed ab initio steered molecular
dynamics (AISMD) simulations and computed
minimum energy paths (MEP) on force-modified
To investigate the mechanochromic behavior of the
obtained composite, the DABBF-NP composite was
kneaded gently in a mortar. Immediately, its color
changed from pale yellow to blue, indicating as
expectedthat DABBF units were activated (Figure 3(a)
2
9, 39-40
potential energy surfaces (FMPES)
of these two
species. External forces of equal magnitudes were
applied to the terminal atoms (Figure 4). The directions
of the two force vectors were kept collinear to remove
translational and rotational components of the forces.
The electronic component of the potential energy
surfaces was computed at UB3LYP level of theory with
(
1)–(2)). This is confirmed by the change in the solid-
state UV-vis spectra before and after gentle kneading.
An increase in the absorption peak intensities at 550 and
6
50 nm originating from ABF radicals was observed
(Figure 3(b) (1)–(2)). With further strong kneading, the
color of the composite changed to green due to the
activation of NP units (Figure 3(a) (2)–(3)) with the
increase in the absorbance around not only 550 and 650
nm, but also around 440 nm, which is attributed to the
absorbance of MC (Figure 3(b) (2)–(3)). These
phenomena indicate that DABBF units at the interface of
the silica-rich domain were activated first, followed by
the activation of NP in the polymer domain.
6
-31g* basis set (Details of the computational setup are
described in SI). Firstly, to identify the reaction
mechanisms under mechanochemical conditions,
AISMD simulations were performed under external
driving forces for both DABBF and NP. Under
sufficient force, DABBF undergoes ultrafast fission to
yield a pair of radicals on the picosecond timescale. At 3
nN, all 10 AISMD trajectories resulted in bond fission
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forming a pair of ABF radicals. Four possible cis/trans for NP. Although the bond fission reaction barrier for
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isomers exist for the MC product resulting from NP
isomerization. Although these conversion pathways
exhibit significantly different barriers without force, all
four pathways were found to be feasible under load (SI).
After 12 ps simulation at 3 nN, out of 10 AISMD
trajectories, 6 gave double bond cis-trans isomer,
whereas the other 4 were nonreactive. The cis-trans
isomer is the main product at picosecond time scale.The
product geometries obtained from AISMD at high forces
and the geometries of NP and DABBF were optimized
at forces ranging from 0 to 3 nN. Using these geometries
as end points, minimum energy paths (MEP) were
optimized. Enthalpy barriers obtained from these MEPs
are plotted in Figure 5.
DABBF is slightly lower than that of NP, rapid
mechanochemical transformations are expected for both
species at the 1.5 nN force level which is easily achieved
under kneading conditions. The low force magnitude
needed for the reactions to take place and the small
difference between the barriers of the two species
suggest that there may be a little influence of the barrier
difference of the two units. Rather, differences in
internal force distributions in the material domains are
the more likely cause for the sequential activation of
DABBF and NP.
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Additionally, the composite was treated with a
swellable organic solvent. After adding dichloromethane
(DCM) to the strongly kneaded green colored sample,
the composite was swollen and its color immediately
changed to orange, indicating the recombination of ABF
radicals at the interface (Figure 3(a) (3)–(4)). In the
solid-state UV-vis absorbance spectra, the absorption
peaks at 550 and 650 nm disappeared while the
absorbance around 440 nm remained almost unchanged
(Figure 3(b) (3)–(4)). Then, after 20 min under ambient
light, the orange color faded, indicating that MC
reverted to the NP state (Figure 3(a) (4)–(5)). In the
solid-state UV-vis spectra, the absorbance around 440
nm disappeared (Figure 3(b) (4)–(5)). These phenomena
indicated that solvent addition induces rapid
recombination of ABF radicals; however, the reversion
from MC to NP is relatively slow.
Figure 4. Geometry and external force alignment for (a)
DABBF, (b) pair of ABF radicals, (c) NP, and (d) MC
The composite captures its stress history. Materials
that report both the peak load and load history provide a
mechano-sensor capable of mechanochemical history
recording. After 30 min under ambient light without any
solvent, a strongly kneaded green sample changed to
blue, which is caused by the reversion of MC to NP
(Figure 3(c) (2)–(3)). This is supported not only by
visual confirmation but also by the disappearance of the
absorbance around 440 nm (Figure 3(d) (2)–(3)). Then,
the blue color slowly faded due to the recombination of
ABF radicals (Figure 3(c) (3) ~ (4)). In the solid-state
UV-vis spectra, the intensity of the absorption peaks at
550 and 650 nm decreased slowly (Figure 3(d) (3)–(4)).
These bulk observation indicates that the recombination
of ABF radicals is slow in the low-mobility silica-rich
interface because it is highly affected by molecular
mobility. As a result, MC showed relatively fast
reversion to NP.
(cis-trans isomer).
In summary, we have demonstrated the multicolor
mechanochromism of the polymer/silica composite with
dual distinct mechanochromophores introduced into
different domains, i.e., a dividing type blue color-
showing DABBF mechanochromophore at mobility
limited interface and non-dividing type orange color-
showing NP mechanochromophore in the soft domain.
The composite showed blue, green, and orange color
according to the strength of the mechanical stimuli,
Figure 5. DABBF and NP reaction barriers on FMPES.
Although the barrier for bond fission in DABBF is
significantly lower than that of NP in the absence of
applied forces, the DABBF/NP bond fission reaction
barriers become more similar with increasing external
force. At an external force of 1.5 nN, the reaction
barriers are 8.3 kcal/mol for DABBF and 15.1 kcal/mol
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solvent addition, and lapse of time owing to the
differences in mechanoresponsivity, reversion reaction,
and domain location between the two
mechanochoromophores. The present approach can be
utilized for developing more sophisticated
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ASSOCIATED CONTENT
5
.
Wang, L.; Wang, K.; Zou, B.; Ye, K.; Zhang,
Supporting Information.
The Supporting Information is available free of charge on
the ACS Publications website.
Synthesis and characterization data; details on NMR, IR,
AISMD simulations, and MEPs.
H.; Wang, Y., Luminescent chromism of boron
diketonate crystals: distinct responses to different
stresses. Adv. Mater. 2015, 27 (18), 2918-22.
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AUTHOR INFORMATION
Corresponding Authors
7
.
Teng, M. J.; Jia, X. R.; Chen, X. F.; Wei, Y., A
*jsmoore@illinois.edu (J.S.M.)
*otsuka@polymer.titech.ac.jp (H.O.).
dipeptide-based multicolored-switching luminescent
solid material: when molecular assemblies meet
mechanochemical reaction. Angew. Chem. Int. Ed. 2012,
Notes
5
1 (26), 6398-401.
The authors declare no competing financial interest.
8
. Ma, Z.; Teng, M.; Wang, Z.; Yang, S.; Jia, X.,
ACKNOWLEDGMENT
Mechanically induced multicolor switching based on a
single organic molecule. Angew. Chem. Int. Ed. 2013, 52
The authors thank Mr. Ryohei Kikuchi, National
University Corporation Tokyo Institute of Technology
Center for Ascended Materials Analysis, for the TEM
measurements, Dr. Maxwell J. Robb and Ms. Abigail J.
Halmes for their advice on the synthesis. This work was
supported by KAKENHI grant 17H01205 (H.O.) from
the Japan Society for the Promotion of Science (JSPS).
T.K. acknowledges the financial support for JSPS
(47), 12268-72.
9. Ma, Z.; Wang, Z.; Meng, X.; Ma, Z.; Xu, Z.;
Ma, Y.; Jia, X., A Mechanochromic Single Crystal:
Turning Two Color Changes into a Tricolored Switch.
Angew. Chem. Int. Ed. 2016, 55 (2), 519-22.
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Li, J.; Nagamani, C.; Moore, J. S., Polymer
mechanochemistry: from destructive to productive. Acc.
Chem. Res. 2015, 48 (8), 2181-90.
Research
(No.16J07264). V.M.L. thanks the Arnold and Mabel
Beckman Foundation for Beckman-Brown
Fellowships
for
Young
Scientists
1
1.
Davis, D. A.; Hamilton, A.; Yang, J.; Cremar,
a
L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun,
P. V.; Martinez, T. J.; White, S. R.; Moore, J. S.; Sottos,
N. R., Force-induced activation of covalent bonds in
mechanoresponsive polymeric materials. Nature 2009,
Interdisciplinary Postdoctoral Fellowship. T.J.M. and
X.Z. acknowledge supported by the Office of Naval
Research through the MURI on Photomechanical
Material Systems (ONR N00014-18-1-2624).
4
59 (7243), 68-72.
1
2. Gossweiler, G. R.; Hewage, G. B.; Soriano, G.;
Wang, Q. M.; Welshofer, G. W.; Zhao, X. H.; Craig, S.
L., Mechanochemical Activation of Covalent Bonds in
Polymers with Full and Repeatable Macroscopic Shape
Recovery. ACS Macro Lett. 2014, 3 (3), 216-219.
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