Regioisomer-Specific Mechanochromism of Naphthopyran in Polymeric Materials

Article abstract of DOI:10.1021/jacs.6b07610

Transformation of naphthopyran into a colored merocyanine species in polymeric materials is achieved using mechanical force. We demonstrate that the mechanochemical reactivity of naphthopyran is critically dependent on the regiochemistry, with only one pa

Full text of DOI:10.1021/jacs.6b07610

Communication
pubs.acs.org/JACS
Regioisomer-Specific Mechanochromism of Naphthopyran in
Polymeric Materials
Maxwell J. Robb,†,‡ Tae Ann Kim,†,§ Abigail J. Halmes,†,‡ Scott R. White,†,∥ Nancy R. Sottos,*,†,§
and Jeffrey S. Moore*,†,‡
‡
§
†The Beckman Institute for Advanced Science and Technology, Department of Chemistry, Department of Materials Science and
∥
Engineering, and Department of Aerospace Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801,
United States
S
* Supporting Information
diastereomers being more reactive than trans diastereomers.12
Similarly, stereoisomers of gem-dihalocyclopropane13 and
ABSTRACT: Transformation of naphthopyran into a
benzocyclobutene13a,14 mechanophores react with different
colored merocyanine species in polymeric materials is
achieved using mechanical force. We demonstrate that the
mechanochemical reactivity of naphthopyran is critically
dependent on the regiochemistry, with only one particular
substitution pattern leading to successful mechanochem-
ical activation. Two alternative regioisomers with different
polymer attachment points are demonstrated to be
mechanochemically inactive. This trend in reactivity is
accurately predicted by DFT calculations, reinforcing
predictive capabilities in mechanochemical systems. We
rationalize the reactivity differences between naphthopyran
regioisomers in terms of the alignment of the target C−O
pyran bond with the direction of the applied mechanical
force and its effect on mechanochemical transduction
along the reaction coordinate.
threshold forces that favor activation of the cis diastereomers.
A few reports have also highlighted the importance of
regiochemistry. Varying the polymer attachment points on
spiropyran results in different threshold forces for activation.15
Computational studies also indicate that a mechanically
facilitated [4 + 2] cycloreversion of the 1,2,3-triazole moiety
is predicted for the 1,5-regiosomer, whereas the 1,4-regioisomer
undergoes homolytic cleavage of bonds adjacent to the triazole
ring for experimentally relevant models.16 Mechanical force has
been demonstrated to suppress the reactivity of an anthracene−
maleimide adduct with a particular substitution pattern by
increasing the reaction energy barrier,17 while a similar
phenomenon has also been observed for the cyclobutane-1,3-
dione moiety.18 Beyond the structure of the mechanophore
itself, the geometry of the attached polymers also affects the
reactivity.19 A so-called backbone lever-arm effect, for instance,
significantly enhances mechanical activity by providing a greater
mechanical advantage.20
olymer mechanochemistry is an emerging field of research
Pin which mechanical forces are harnessed to promote
selective chemical transformations.1 Polymers serve the critical
role of transducing a mechanical load to a particular covalent
bond within a mechanochemically active molecule, called a
mechanophore.2 Mechanochromic molecular force probes
provide a convenient route for visually identifying critical stress
and/or strain in a material as well as recording its mechanical
history. In addition to the extensively studied spiropyran−
merocyanine system,3 several other mechanochromic mecha-
nophores have been developed, including diarylbibenzofur-
anone4 and hexaarylbiimidazole,5 which fragment under
mechanical stress to generate colored free radicals. The
mechanochromic behavior of a rhodamine-based mechano-
phore6 and spirothiopyran7 has also recently been demon-
strated. In contrast to the large deformations typically required
to achieve mechanophore activation in the bulk,3,8 the
development of mechanophores for epoxy-based thermoset
polymers9 has led to mechanochromic composite materials that
activate at relatively low strain thresholds.
Here we introduce a new mechanochromic mechanophore
based on naphthopyran, specifically the 3H-naphtho[2,1-
b]pyran skeleton (Scheme 1). Similar to spiropyran, naph-
thopyrans are well-known photochromic molecules that
reversibly transform into colored merocyanine species under
UV light via a 6π electrocyclic ring-opening reaction.21 We
investigate the mechanochemical activation of three different
Scheme 1. Transformation of Naphthopyran into a Colored
Merocyanine Species Is Accomplished Using Mechanical
a
Force
Despite the growing repertoire of covalent mechanophores,10
a more complete understanding of structure−mechanochemical
activity relationships will drive further innovation.11 Molecular
geometry, in particular, has a significant influence on
mechanochemical reactivity. For example, cyclobutane-based
mechanophores are sensitive to stereochemistry, with cis
a
The numbering system of 3H-naphtho[2,1-b]pyrans is illustrated.
Received: July 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b07610
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
naphthopyran regioisomers in bulk polymeric materials and
find that only one regioisomer exhibits mechanochromic
properties, validating predictions from density functional theory
(DFT) calculations. A geometrical analysis provides insight into
the regioisomer-specific reactivity of naphthopyran and
reinforces mechanophore design.
DFT calculations using the simple computational technique,
constrained geometries simulate external force (CoGEF),22
were initially performed to investigate the mechanochemical
reactivity of naphthopyrans (Figure 1). Three different
a hydroxyethyl group improved the efficiency of the
naphthopyran synthesis while maintaining a reactive handle
for subsequent functionalization. In a second step, naphthopyr-
ans were functionalized with 4-pentenoic anhydride to facilitate
their covalent incorporation into polydimethylsiloxane
(PDMS),23 which is an easily accessible polymeric testing
platform (Scheme 2). Control experiments confirmed that the
Scheme 2. Preparation of PDMS Materials with Covalent
Incorporation of Three Different Naphthopyran
Regioisomers and a Monofunctional Control
Figure 1. DFT calculations (CoGEF) for three naphthopyran
regioisomers predict regioisomer-specific mechanochemical reactivity.
Cleavage of the C−O pyran bond and transformation to the
merocyanine species is predicted only for the naphthopyran
regioisomer substituted at the 5-position. Calculations were performed
at the B3LYP/6-31G* level of theory.
double bond of the pyran ring is unreactive toward Pt-catalyzed
hydrosilylation, as expected (see the SI for details). In addition
to the three bis(alkene)-functionalized naphthopyran re-
gioisomers (NP5, NP8, and NP9), a naphthopyran control
molecule containing a single vinyl group for covalent
attachment within a PDMS network was prepared. Since
mechanical force is not transferred across the molecule, the
control distinguishes between reactivity that is mechanical in
nature or originating from an alternative (e.g., thermal)
mechanism.
We began by investigating PDMS films covalently incorpo-
rating the three bis(alkene)-functionalized naphthopyran
regioisomers and the monofunctional control (Figure 2). The
materials, which were prepared with an approximately 1.5 wt %
loading of each naphthopyran, are clear and very slightly yellow
in color. Consistent with the CoGEF calculations, stretching
PDMS films incorporating NP5 connected at the 5-position
causes the gauge region of the material to turn orange-yellow in
color, suggesting mechanochemical transformation of the
naphthopyran into the merocyanine species (see the movie in
the SI). Under ambient conditions, the color fades relatively
quickly (on the order of several minutes) as the merocyanine
form is converted back into the naphthopyran. Importantly, no
regioisomers were studied, varying the position of the methoxy
group on the naphthalene ring. Starting from the equilibrium
geometry of each molecule, the distance between the methyl
groups was increased, and the constrained geometry was
optimized at distinct intervals of elongation. Interestingly, the
naphthopyran substituted at the 5-position exhibited selective
cleavage of the C−O pyran bond and successful transformation
to the merocyanine species with an estimated rupture force of
4.1 nN. In direct contrast, however, the naphthopyran
regioisomers substituted at the 8- and 9-position were both
predicted to undergo homolytic cleavage of one of the methyl
ether bonds at a significantly greater force of 6.3 nN.
Intrigued by the computational results, we set out to examine
the mechanical activation of the naphthopyran series in bulk
polymeric materials. Naphthopyrans were readily synthesized
via the acid-catalyzed reaction between appropriately sub-
stituted 2-naphthols and 1-(4-hydroxyphenyl)-1-phenylprop-2-
yn-1-ol (see the Supporting Information (SI) for details). Initial
monosubstitution of the dihydroxynaphthalene substrates with
B
DOI: 10.1021/jacs.6b07610
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Journal of the American Chemical Society
Communication
NP5, corresponding to the absorption peak (λmax) of the
merocyanine species obtained after irradiation with UV light.
The absorbance continued to increase asymptotically while the
specimen was maintained under mechanical stress. In contrast,
the absorption spectra of materials containing the two other
naphthopyran regioisomers, NP8 and NP9, did not change
over the course of the experiment. The absorption spectrum of
the control specimen also did not change under mechanical
stress, confirming that mechanical force was indeed responsible
for the color change in the material incorporating NP5.
In order to rationalize the regioisomer-specific mechano-
chemical reactivity of naphthopyran, we evaluated the geo-
metrical constraints imposed on the molecule under mechanical
stress. Similar to the lever-arm effect, which originates from
more efficient chemomechanical coupling through a properly
aligned polymer backbone,20 we posited that the orientation
between the externally applied force and the C−O pyran bond
is important for efficient mechanochemical transduction along
the reaction coordinate. From the highly constrained molecular
structures obtained from CoGEF calculations immediately prior
to bond rupture, the angle between the vector of applied
tension and the vector representing the C−O pyran bond was
calculated for each regioisomer (Figure 4). The vector
Figure 2. Photographs of PDMS materials incorporating naphthopyr-
ans after application of mechanical force (tension) and UV light. Only
the PDMS material containing naphthopyran substituted at the 5-
position (NP5) changes color upon stretching. Scale bar = 1 cm.
color change is observed upon mechanical stretching of the
PDMS materials incorporating either of the other regioisomers
NP8 and NP9 or the naphthopyran control. Illumination with
365 nm UV light, however, causes all of the PDMS materials to
change color, confirming the presence of the naphthopyran
molecules.
Mechanical testing combined with in situ visible absorption
measurements enabled further characterization of the mecha-
nochemical properties of the materials (Figure 3). PDMS
Figure 4. Geometrical evaluation of constrained naphthopyran
structures obtained from CoGEF calculations at displacements
immediately prior to bond cleavage. The directionality of the applied
force is approximated by the vector between the terminal oxygen
Figure 3. Visible absorption of PDMS materials incorporating
naphthopyran regioisomers and a monofunctional control under
mechanical force. Tensile specimens were uniaxially stretched (time =
0 min) to achieve a relatively constant true stress after initial relaxation.
Absorbance values recorded at the λmax corresponding to each
merocyanine species demonstrate the regioisomer-specific mechano-
chromism of NP5.
⎯⎯→
atoms (AB, yellow arrow), while a second vector describes the C−O
⎯⎯→
pyran bond (CD, red arrow).
representing the applied force was approximated by the
coordinates of the methoxy oxygen atoms of the elongated
structures (see the SI for details). For the mechanically active
naphthopyran regioisomer substituted at the 5-position, the
relatively shallow angle of 31° between these two vectors
reflects a good alignment of the C−O pyran bond with the
directionality of the external force. For the other two
naphthopyran regioisomers substituted at the 8- and 9-position,
this angle is significantly wider at 55° and 65°, respectively.
This trend is consistent with an alternative description of
mechanochemical coupling11 based on the calculated elonga-
tensile specimens incorporating each naphthopyran molecule
were prepared in a similar fashion as the films. The specimens
were uniaxially deformed in tension to achieve a relatively
constant true stress ranging between 2.4 and 2.8 MPa after
initial relaxation. The mechanical stress was maintained during
the course of the experiment while visible absorption spectra
were recorded every 30 min (see Figure S1). An immediate
increase in the absorbance around 440 nm was observed upon
mechanical deformation of the PDMS material incorporating
C
DOI: 10.1021/jacs.6b07610
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(7) Zhang, H.; Gao, F.; Cao, X.; Li, Y.; Xu, Y.; Weng, W.; Boulatov,
R. Angew. Chem., Int. Ed. 2016, 55, 3040−3044.
tion that accompanies the ring-opening reaction for each
regioisomer (see the SI for details).
(8) (a) Lee, C. K.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N.
R.; Braun, P. V. J. Am. Chem. Soc. 2010, 132, 16107−16111. (b) Black,
A. L.; Orlicki, J. A.; Craig, S. L. J. Mater. Chem. 2011, 21, 8460−8465.
(c) Beiermann, B. A.; Davis, D. A.; Kramer, S. L. B.; Moore, J. S.;
Sottos, N. R.; White, S. R. J. Mater. Chem. 2011, 21, 8443−8447.
(d) Lenhardt, J. M.; Black, A. L.; Beiermann, B. A.; Steinberg, B. D.;
Rahman, F.; Samborski, T.; Elsakr, J.; Moore, J. S.; Sottos, N. R.; Craig,
S. L. J. Mater. Chem. 2011, 21, 8454−8459. (e) Kingsbury, C. M.; May,
P. A.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R. J. Mater.
Chem. 2011, 21, 8381−8388. (f) Sottos, N. R. Nat. Chem. 2014, 6,
381−383.
In summary, we have demonstrated that naphthopyran is a
new mechanophore capable of color generation in polymeric
materials using mechanical force. Notably, only the naph-
thopyran regioisomer substituted at the 5-position exhibits
mechanochromic behavior, while regioisomers substituted at
the 8- and 9-position are mechanochemically inactive. This
trend in reactivity was also accurately predicted by DFT
calculations (CoGEF). We attribute the mechanochemical
activity of the naphthopyran regioisomer substituted at the 5-
position to the better alignment of the C−O pyran bond with
the direction of the externally applied mechanical force along
the reaction coordinate. Further research will investigate the
influence of structural modifications to the naphthopyran
mechanophore in order to tune the color and control the fading
properties.
(9) (a) Li, Z.; Toivola, R.; Ding, F.; Yang, J.; Lai, P.-N.; Howie, T.;
Georgeson, G.; Jang, S.-H.; Li, X.; Flinn, B. D.; Jen, A. K.-Y. Adv.
Mater. 2016, 28, 6592−6597. (b) de Luzuriaga, A. R.; Matxain, J. M.;
́
Ruiperez, F.; Martin, R.; Asua, J. M.; Cabanero, G.; Odriozola, I. J.
Mater. Chem. C 2016, 4, 6220−6223.
(10) Li, J.; Nagamani, C.; Moore, J. S. Acc. Chem. Res. 2015, 48,
2181−2190.
ASSOCIATED CONTENT
* Supporting Information
(11) Brown, C. L.; Craig, S. L. Chem. Sci. 2015, 6, 2158−2165.
(12) (a) Kryger, M. J.; Munaretto, A. M.; Moore, J. S. J. Am. Chem.
Soc. 2011, 133, 18992−18998. (b) Kean, Z. S.; Niu, Z.; Hewage, G. B.;
Rheingold, A. L.; Craig, S. L. J. Am. Chem. Soc. 2013, 135, 13598−
13604.
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b07610.
Experimental details, synthetic procedures, and NMR
and visible absorption spectra (PDF)
Movie showing the color change of NP5 upon stretching
(13) (a) Wang, J.; Kouznetsova, T. B.; Niu, Z.; Ong, M. T.;
Klukovich, H. M.; Rheingold, A. L.; Martinez, T. J.; Craig, S. L. Nat.
Chem. 2015, 7, 323−327. (b) Dopieralski, P.; Ribas-Arino, J.; Marx, D.
Angew. Chem., Int. Ed. 2011, 50, 7105−7108.
(14) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.;
Baudry, J.; Wilson, S. R. Nature 2007, 446, 423−427.
(15) Gossweiler, G. R.; Kouznetsova, T. B.; Craig, S. L. J. Am. Chem.
Soc. 2015, 137, 6148−6151.
(16) (a) Smalø, H. S.; Uggerud, E. Chem. Commun. 2012, 48,
10443−10445. (b) Jacobs, M. J.; Schneider, G.; Blank, K. G. Angew.
Chem., Int. Ed. 2016, 55, 2899−2902.
(17) Konda, S. S. M.; Brantley, J. N.; Varghese, B. T.; Wiggins, K. M.;
Bielawski, C. W.; Makarov, D. E. J. Am. Chem. Soc. 2013, 135, 12722−
12729.
a PDMS specimen (MPG)
AUTHOR INFORMATION
■
Corresponding Authors
*jsmoore@illinois.edu
*n-sottos@illinois.edu
Notes
The authors declare no competing financial interest.
(18) Groote, R.; Szyja, B. M.; Leibfarth, F. A.; Hawker, C. J.;
Doltsinis, N. L.; Sijbesma, R. P. Macromolecules 2014, 47, 1187−1192.
(19) (a) Klukovich, H. M.; Kean, Z. S.; Ramirez, A. L. B.; Lenhardt, J.
M.; Lin, J.; Hu, X.; Craig, S. L. J. Am. Chem. Soc. 2012, 134, 9577−
9580. (b) Church, D. C.; Peterson, G. I.; Boydston, A. J. ACS Macro
Lett. 2014, 3, 648−651.
ACKNOWLEDGMENTS
■
This work was supported in part by the National Science
Foundation (DMR 1307354). M.J.R. gratefully acknowledges
the Arnold and Mabel Beckman Foundation for a Beckman
Institute Postdoctoral Fellowship.
(20) (a) Klukovich, H. M.; Kouznetsova, T. B.; Kean, Z. S.; Lenhardt,
J. M.; Craig, S. L. Nat. Chem. 2013, 5, 110−114. (b) Wang, J.;
Kouznetsova, T. B.; Kean, Z. S.; Fan, L.; Mar, B. D.; Martínez, T. J.;
Craig, S. L. J. Am. Chem. Soc. 2014, 136, 15162−15165.
(21) Van Gemert, B. In Organic Photochromic and Thermochromic
Compounds; Crano, J. C., Guglielmetti, R. J., Eds.; Plenum Press: New
York, 1999; Vol. 1, pp 111−140.
(22) Beyer, M. K. J. Chem. Phys. 2000, 112, 7307−7312.
(23) Gossweiler, G. R.; Hewage, G. B.; Soriano, G.; Wang, Q.;
Welshofer, G. W.; Zhao, X.; Craig, S. L. ACS Macro Lett. 2014, 3,
216−219.
REFERENCES
■
(1) (a) Beyer, M. K.; Clausen-Schaumann, H. Chem. Rev. 2005, 105,
2921−2948. (b) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.;
Sottos, N. R.; White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755−
5798. (c) Ribas-Arino, J.; Marx, D. Chem. Rev. 2012, 112, 5412−5487.
(2) (a) Ribas-Arino, J.; Shiga, M.; Marx, D. J. Am. Chem. Soc. 2010,
132, 10609−10614. (b) Dopieralski, P.; Anjukandi, P.; Ruckert, M.;
̈
Shiga, M.; Ribas-Arino, J.; Marx, D. J. Mater. Chem. 2011, 21, 8309−
8316. (c) May, P. A.; Munaretto, N. F.; Hamoy, M. B.; Robb, M. J.;
Moore, J. S. ACS Macro Lett. 2016, 5, 177−180. (d) Schaefer, M.; Icli,
B.; Weder, C.; Lattuada, M.; Kilbinger, A. F. M.; Simon, Y. C.
Macromolecules 2016, 49, 1630−1636.
(3) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough,
D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martínez, T. J.; White, S.
R.; Moore, J. S.; Sottos, N. R. Nature 2009, 459, 68−72.
(4) (a) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.;
Takahara, A.; Otsuka, H. Angew. Chem., Int. Ed. 2015, 54, 6168−6172.
(b) Imato, K.; Kanehara, T.; Ohishi, T.; Nishihara, M.; Yajima, H.; Ito,
M.; Takahara, A.; Otsuka, H. ACS Macro Lett. 2015, 4, 1307−1311.
(5) Verstraeten, F.; Gostl, R.; Sijbesma, R. P. Chem. Commun. 2016,
̈
52, 8608−8611.
(6) Wang, Z.; Ma, Z.; Wang, Y.; Xu, Z.; Luo, Y.; Wei, Y.; Jia, X. Adv.
Mater. 2015, 27, 6469−6474.
D
DOI: 10.1021/jacs.6b07610
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