Highly sensitive mechano-controlled luminescence in polymer films modified by dynamic CuI-based cross-linkers

Article abstract of DOI:10.1039/c9cc08354e

Dynamic Cu^I-based mechanophores used as cross-linkers in polybutylacrylates enable highly sensitive detection of mechanical stress even at small strain (<50%) and stress (<0.1 MPa) values via reversible changes in luminescence intensity. Such s

Full text of DOI:10.1039/c9cc08354e

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Pages 1–166
ChemComm
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Julia R. Khusnutdinova et al.
Highly sensitive mechano-controlled luminescence in
I
polymer films modified by dynamic Cu -based cross-linkers

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Highly sensitive mechano-controlled
luminescence in polymer films modified by
dynamic Cu -based cross-linkers†
Cite this: Chem. Commun., 2020,
6, 50
I
5
Received 25th October 2019,
Accepted 18th November 2019
a
a
a
Ayumu Karimata, ‡ Pradnya H. Patil, ‡ Eugene Khaskin,
a
b
c
Sebastien Lapointe,
Robert R. Fayzullin,
Pavlos Stampoulis and
´
DOI: 10.1039/c9cc08354e
a
Julia R. Khusnutdinova
*
rsc.li/chemcomm
I
Dynamic Cu -based mechanophores used as cross-linkers in poly- recovery of the mechanophore’s original state. Such drawbacks
butylacrylates enable highly sensitive detection of mechanical hinder the development of practical mechanical stress probes that
stress even at small strain (o50%) and stress (o0.1 MPa) values can directly visualize subtle stress changes repeatably in real-time.
via reversible changes in luminescence intensity. Such sensitivity is Although some progress has been made in developing molecular
6
superior to previously reported systems based on classical organic mechanophores that do not require covalent bond scission,
7
mechanophores and it allows for direct visualization of mechanical including our previous report, these systems typically require
stress by imaging methods.
significant strain (4100%, e.g. stretching to more than two-fold of
the original length during a tensile test) and stress (several MPa)
Mechanoresponsive materials are a class of ‘‘smart’’ materials to observe a response. In general, the development of a mechano-
that demonstrate specific property changes induced by sensitive system with fast, reversible and sensitive response
mechanical stimuli, for example, showing self-reporting or remains a tremendous opportunity, as well as a challenge.
I
self-healing properties. Given the widespread use of synthetic
Our group has recently reported a Cu iodide complex with a
polymers for engineering applications, the design of mechano- macrocyclic pyridinophane ligand covalently incorporated into
responsive polymers capable of self-reporting mechanical a linear polyurethane chain that shows gradual photolumines-
7
stress is of high importance not only for preventing the cence intensity changes in response to tensile stress (Scheme 1a).
materials’ failure, but also for the better understanding of However, that initial system suffered from fast degradation under
polymer response to mechanical force. An interesting approach air, low photoluminescence quantum yield (PLQY), and it also
to the design of diverse mechanosensitive materials includes required stretching to several times of the sample’s initial
the use of mechanophores covalently incorporated into length (strain 4100%) in order to observe the response, mean-
polymer samples, which can change their spectroscopic proper- ing an overall low sensitivity and an inefficient transmission of
1
ties in response to mechanical force. The majority of com- mechanical force.
I
monly used mechanophores are based on organic molecules
In the current work we report new, photoluminescent (NHC)Cu
complexes covalently incorporated into polybutylacrylate as a cross-
2
3
4
such as spiropyran, 1,2-dioxetane, diaryldibenzofuranone,
5
and others, in which scission of a weak covalent chemical linker. As a result, cross-linked polybutylacrylate (cPBA) samples
bond is caused by mechanical force. Intrinsically, the requirement Cu1-cPBA and Cu2-cPBA (Scheme 1b) demonstrate a highly
for such covalent bond scission, and its associated structural sensitive response to mechanical stress even at small strain
reorganization, often leads to an irreversible response, or a slow (o50%) and stress (o0.1 MPa) values. Such sensitivity is
unprecedented when compared to many currently known
stress-responsive polymers containing organic-based mechano-
phores. This system enables direct visualization of mechanical
stress via imaging methods. In addition, we achieved good air-
stability as the samples showed only minor degradation after
several days under air.
We propose that the emission intensity increases in
response to mechanical force, due to restricting the mobility
of the cross-linker which contains the fluxional, Cu-based
mechanophore. Thus, these systems represent a new type of
mechanophore in which the mechanism of response is not
a
Coordination Chemistry and Catalysis Unit, Okinawa Institute of Science and
Technology Graduate University, 1919-1 Tancha, Onna-son, Okinawa, 904-0495,
Japan. E-mail: juliak@oist.jp
b
Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center,
Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russia
JEOL RESONANCE Inc., Musashino, Akishima, Tokyo, 196-8558, Japan
Electronic supplementary information (ESI) available: Experimental procedures
c
†
and characterization. CCDC 1903712 (Cu4), 1903713 (Cu6), 1937851 (Cu3), and
937852 (Cu5). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9cc08354e
1
‡
These authors contributed equally.
5
0 | Chem. Commun., 2020, 56, 50--53
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ligand as a cross-linker. PBA was selected for this study due to
its widespread use in industry, its suitable mechanical proper-
ties such as elasticity, and the possibility to further tune a wide
range of properties by a rich choice of monomers. Cross-linked
sample Cu1-cPBA was prepared by photo-initiated radical poly-
merization using butyl acrylate and bis(acrylate)-functionalized
Bn
I
ligand L1 (1 mol%), followed by incorporation of ( NHC)Cu
Bn
by dipping the film into a solution of ( NHC)CuCl precursor
(Scheme S5, ESI†). Cu2-cPBA was prepared by the analogous
procedure using acrylate-functionalized ligand L2 (Scheme S6,
8
ESI†). PLQYs of Cu1-cPBA and Cu2-cPBA were determined to
be 0.075 and 0.27, respectively.
The mechanical properties of Cu1-cPBA and Cu2-cPBA were
investigated using a tensile testing machine under an argon
atmosphere. The representative stress–strain (S–S) curves are
given in Fig. S31 (ESI†) showing good sample elongation (323%
for Cu1-cPBA, 476% for Cu2-cPBA).
To investigate the mechanoresponsive properties of the new
cross-linked Cu1-cPBA film, it was stretched uniaxially and at
the same time the luminescence spectrum during stretching
was measured by monitoring the film’s central area. As the
Cu1-cPBA film was being stretched, its luminescence intensity
increased (Fig. 1a). The plot of integrated luminescence inten-
sity vs. tensile strain (Fig. 1b) shows that luminescence inten-
sity gradually increased even when the strain was less than 50%
and the tensile stress was less than 0.1 MPa. Such sensitivity is
much higher than that found in the majority of previously
reported organic mechanophore systems, which typically require
I
Scheme 1 Cu -based mechanophores and model systems.
caused by the cleavage, or the formation of a covalent bond,
letting us achieve a fast and reversible response. Our mecha-
6
nistic proposal is supported by the study of model compounds
Cu3–6 (Scheme 1c), which display good correlation between
complex fluxionality and their non-radiative decay rate, as well
as their PLQY.
Before attempting to make cross-linkers and incorporating
them into a polymer, we optimized the synthesis of model com-
2–6
stress of several MPa to reliably observe a response.
The film was stretched and released three times from 0 to
00% of strain, showing only a slight photoluminescence (PL)
1
R
plexes Cu3–6 containing the N4 ligand, with its steric hindrance
modified by the R substituent (R = Me, n-Pr, i-Pr, t-Bu). The
complexes were synthesized by reacting the corresponding macro-
R
Bn
cyclic ligands N4 with ( NHC)CuCl (1,3-dibenzylbenzimidazoyl-
-ylidene)copper(I) chloride, followed by the counterion exchange
with KPF . These complexes were isolated and characterized by
NMR, IR, UV-vis spectroscopies, single-crystal X-ray diffraction
2
6
8
(
Fig. S72, ESI†), and elemental analysis. Their dynamic behavior
8
in solution was studied in detail by NMR (vide infra) and was
R
I
9
found to be similar to previously described ( N4)Cu I complexes.
Photophysical properties of Cu3–6 are summarized in Table 1.
We then set out to prepare cross-linked polybutylacrylate (cPBA)
samples using the bis(acrylate) functionalized pyridinophane
R
Bn
6 2 2
Table 1 Photophysical properties of [( N4)Cu( NHC)]PF in CH Cl at
a
2
98 K
b
c
d
À4 À1
e
À4 À1
Complex R
l
r
max [nm] PLQY t [ms] k 10 [s ] knr 10 [s ]
Cu3
Cu4
Cu5
Cu6
Me 567
0.06
0.19
0.30
8.20 0.732
7.59 2.50
9.98 3.00
11.5
10.7
7.01
1.56
n
Pr 561
Pr 551
i
t
Bu 549
0.72 18.0
4.00
a
b
c
Excitation at 380 nm. Emission maximum. Emission lifetime. Fig. 1 (a) Emission spectra of Cu1-cPBA during stretching. (b) Plot of
d
e
Radiative decay rate constant. Non-radiative decay rate constant.
integrated photoluminescence intensity vs. strain of Cu1-cPBA.
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cross-linker mechanically mixed with 1 wt% of model compounds
Cu4 and Cu6. The control samples with the complexes mechanically
incorporated into hexamethylene diacrylate-cross linked polybutyl-
acrylate showed insignificant changes in spectroscopically detected
emission intensity in response to tensile stress. The imaging
analysis also confirmed that there are no noticeable emission
intensity changes in response to stretching in the control
samples with mechanically incorporated complexes Cu4 and
Cu6 (Fig. S34–S35, ESI†). Thus, covalent incorporation of the
mechanophore as a cross-linker is required to clearly observe
4e
sensitive mechanoresponsive behavior.
Fig. 2 Photoluminescence intensity of Cu1-cPBA during repeated
stretching (strain varies from 0 to 100%).
To investigate their air-stability, films of Cu1-cPBA and
Cu2-cPBA were exposed to air for several days and the PLQYs were
recorded (see Fig. S29, ESI†). Both polymers showed good air-
8
intensity decrease when compared to the original value stability with only slight decrease of PLQY (3–4%) after 4 days under
II
measured during the first cycle. The luminescence enhancement air, presumably due to oxidation to trace Cu with oxygen. Thus,
was observed in a reproducible and reversible manner over more while not ideal, the strategy of introducing a carbene moiety makes
than 30 cycles of applied stress (Fig. 2). Furthermore, an additional the copper mechanophores suitable for practical applications.
intensity enhancement in response to tensile elongation was also
To shed light on the mechanoresponse mechanism, we
observed up to the film’s breaking point (Fig. S21, ESI†). Up to a analyzed the dynamic behavior and photophysical properties
00% strain value, the maximum wavelength and the shape of of model complexes Cu3–6. All these complexes show isomer-
8
2
the emission spectrum remains unchanged, indicating that no ization in solution that involves conformational fluxionality of
R
significant structural changes occur upon stretching.
the macrocyclic ligand N4 (Scheme 2). Variable temperature
A similar mechanoresponse which showed a measurable (VT) NMR studies in CD Cl solution show that as the steric
2
2
R
emission intensity increase at low strain and stress values hindrance of the N4 ligand increases, fluxionality of the
8
was also observed for Cu2-cPBA (Fig. S23, ESI†).
complexes decreases leading to slower isomerization. Interest-
A sensitive response and good PLQY suggest that it should be ingly, PLQY in CH Cl solution increases from Cu3 to Cu6,
2
2
possible to directly visualize applied mechanical stress via optical correlating with the increase of steric hindrance and an inversely
imaging. Accordingly, we monitored the luminescence intensity proportional decrease in fluxionality, accompanied by a blue shift
change in the Cu1-cPBA and Cu2-cPBA samples by a CCD camera of the emission peak (Table 1). Notably, this trend shows very good
during repeated cycles of stretching and release under UV-light correlation with the gradual decrease of the non-radiative decay
irradiation in the 0–200% elongation range. To our delight, lumi- rate constant knr from Cu3 to Cu6, showing that suppression of
nescence intensity smoothly and steadily increased in response non-radiative decay is likely the main reason for PLQY’s increase.
sample stretching (Fig. 3 and Fig. S33, ESI†). This response was
Based on these observations and by analogy with other photo-
I
quick and reversible, and upon releasing the sample to its original luminescent Cu complexes reported in the literature, we propose
length, PL intensity decreased accordingly. A movie showing real- that the increase of PLQY in more rigid model systems is due to the
time changes in the emission brightness in response to stress is suppression of Jahn–Teller distortions in the excited state in the
supplied in the ESI.†
presence of sterically hindered ligands. A similar effect was reported
I
We performed a control experiment to investigate whether for substituted phenanthroline Cu complexes, where ligand sterics
10
the emission intensity changes are caused by mechanical force were varied. By analogy, we propose that applied mechanical
that is directly transmitted to the cross-linker, or if it’s influ- force restricts cross-linker mobility, making it responsible for the
enced by a constrained environment. A reference sample of observed increase of emission intensity upon elongation.
cross-linked cPBA was prepared using hexamethylene diacrylate as a
Fig. 3 Imaging analysis of Cu1-cPBA during stretching: images of a film
during stretching at 0% (top), 100% (middle), 200% (bottom) strain.
R
+
Scheme 2 Fluxional behavior of [( N4)Cu(NHC)] complexes.
5
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R
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2
tion suppression that was proposed for an analogous system.
In summary, we report a highly sensitive mechanophore based
R
+
on the ( N4)Cu(NHC) complex, which was incorporated as a cross-
linker in polybutylacrylate. This system shows a highly sensitive
response to mechanical force, allowing for the detection of
mechanical stress changes even at small strain and stress values,
and it enables the direct visualization of mechanical stress. The
covalent incorporation of the Cu-based mechanophore as a cross-
linker is necessary to observe of a mechanoresponse. Such high
sensitivity is superior to many currently reported system based on
classical mechanophores such as spiropyrans, diaryldibenzofura-
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The mechanism of mechanoresponse is
7
8
9
different from commonly used organic-based mechanophores as
it does not involve covalent bond cleavage/formation, but is based
on restricting the mobility of the fluxional Cu complex and thus
suppressing non-radiative decay as confirmed by our studies of the
model systems. Considering the high sensitivity and good air
stability of these systems, they may find use in real-time visualiza-
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and properties of coordination compounds.
This work was supported by JSPS KAKENHI Grant Number
JP18K05247. We thank Dr Takuya Miyazawa, Dr Kieran Deasy
1
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This journal is ©The Royal Society of Chemistry 2020
Chem. Commun., 2020, 56, 50--53 | 53