News Letter(October, 2021)

SPACC ニュースレター(2021 年 10 月号)

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1. 研究紹介「Triboluminescence of pyridinophane CuI (NHC) complexesin crystals and polymers」
Okinawa Institute of Science and Technology
Coordination Chemistry and Catalysis Unit
Ayumu Karimata, Julia R. Khusnutdinova

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Triboluminescence of pyridinophane CuI (NHC) complexesin crystals and polymers
Okinawa Institute of Science and Technology
Coordination Chemistry and Catalysis Unit
Ayumu Karimata, Julia R. Khusnutdinova
First observation of triboluminescence of transition metal complex in amorphous polymer film.

Triboluminescence (TL), which is also called mechanoluminescence (ML) or fractroluminescence (FL), is aphenomenon in which light emission is caused by crushing, grinding, or rubbing materials1-4. In more generalterms, triboluminescence is a way of converting mechanical energy directly to light. Currently,
triboluminescent materials attract attention as components for damage sensors 1,2, self-powered displays3,pressure-sensitive devices. etc. Despite its long history (first example of TL was recorded by Bacon in 1605),
the detailed mechanism of this fascinating phenomenon remains a subject of debate until now, and search for
new TL materials and their applications remains an active area of research. Among many types of TL
materials, europium and terbium complexes have been widely studied, however, the use of rare earth metalbased materials has certain disadvantages due to their high cost and low availability. At the same time, there
are only a few reports of inexpensive transition metal complexes that show TL in the crystalline state,
however, no examples of transition metal-based amorphous materials (such as amorphous polymer films)
were known until recently1-5.
Photoluminescent CuI complexes have been studied extensively for OLED, photo-catalysis, and chemicalsensors because copper is a cheap and abundant metal. Although there are many reports on mechanochromic
behavior of photoluminescent CuI complexes in crystalline state, examples of triboluminescent CuI
complexes are extremely limited and it is difficult to single out a particular family of copper complexes where
this phenomenon could be systematically studied by variation of substituents or counteranions 6.
Our research group has synthesized and studied a new series of CuI complexes bearing a flexible
macrocycling ligand, pyridinophane (RN4) in order to create new stimuli-responsive polymer materials7.
During these studies, we found that [(RN4)CuI(NHC)]X complexes (NHC = N-heterocyclic carbene) show
good air stability and high photoluminescence quantum yield (PLQY). Furthermore, we demonstrated earlier
that when such complexes are covalently incorporated as cross-linkers into an elastic polymer chain, thephotoluminescence intensity changes in response to simple film mechanical stretching/release in a sensitiveand repeatable manner (< 0.1 MPa and < 100% in strain). Thus, these systems showed potential in thevisualization of mechanical stress distribution in the polymer by observing changes in photoluminescenceintensity using simple imaging method in real time using a CCD camera 8. However, this approach required(i) covalent attachment of the metal complex to the polymer and (ii) using external excitation source (e.g.
commercial “blacklight” UV lamp) to observe emission via photoluminescence.
Serendipitously, during these studies we noticed that the entire family of six [(RN4)CuI(NHC)]X complexes
Cu1-6 with variable ligands and counteranions (Scheme 1) also show intense TL simply by grinding thecrystals with a spatula or glass rod, which can be visible even by a naked eye in air. Detailed investigation of
the photophysical properties of these complexes showed that they have good PLQY of 0.660.83 in crystals
and 0.510.79 in poly(methylmethacrylate) (PMMA) films. Interestingly, Cu14 with different
counteranions (PF6-, CF3SO3-, CF3COO-, BPh4-) show
similar PLQY, indicating the effect of counter anion inthe crystals is not significant. The photographs of thecrystals Cu16 producing light emission by grinding
them under air are shown in Figure 1. We thenattempted to capture the moment of generation of lightfrom the crystal by using high speed camera. The
crystal of Cu5 was compressed by the two glass plates
under air and the photographs show light emission on
the cracks in the crystal

Scheme1.
Chemicalstructuresof[(RN4)CuI(NHC)]X (Cu16) (Bn = benzyl, Mes =mesityl, Dipp = 2,6-diidopropyl phenyl)

during the fracture (Fig 2).

Table 1. TL properties and space group for crystalline and PMMA (1 wt%) of
Cu1–6 under a nitrogen.

The emission imaging ofCu

Anionmax in crystal

max in PMMA

Space groupa

the TL taken by high speed

Cu1

PF6544526

Pca21, NC

Cu2

OTf539530

Pca21, NC

Cu3

CF3Ac536

536

Pca21, NC

Cu4

BPh4540527Pbca, CCu5

PF6536552

P21/n, C

Cu6PF6527

543

P212121, NC

camera from the side view
clearly confirmed that TL
is generated inside crystal,
notfromthecontact

surface (Fig 3).
Oneofdiscussed

mechanism of TL is that

aC: Centrosymmetric, NC: Non-centrosymmetricstrong electric field isgenerated on the cracked
surface of a piezoelectric(non-centrosymmetric)
crystal when the crystal is

Figure 1. Photographs of TL in crystal of (a) Cu1, (b) Cu2, (c) Cu3, (d) Cu4,(e) Cu5, and (f) Cu6. The crystals in glass tube were ground under air.

broken, which leads to excitation of a molecule in the crystal1-5. However, there are many reports on TL in
non-piezoelectric crystals thus consistent with the idea that TL may be generated via various mechanisms.
Our study with crystals of Cu16 show that both centrosymmetric and non-centrosymmetric crystals show

intense TL (Table 1). Powder X-ray diffraction patterns of ground crystals were consistent with the simulated
patterns obtained from single crystal of Cu16, indicating that packing structure does not change bymechanical grinding. Strong molecular interactions such as  stacking or dipole interaction are not present
in the crystals.

Figure 2. Photographs of TL of crystal of Cu5 taken by high speed camera (view from top). The crystal (size:ca. 2 mm) was compressed between two glass plates under air.

Figure 3. Emission imaging for TL of the crystal of Cu5 (size: ca. 2 mm) that was compressed between twoglass plates under air (view from side).
Based on clearly observed photoluminescencefrom polymer-blended complexes, we decided
to examine if triboluminescence phenomenon
will also be observed in the polymer films.
Application of easily prepared polymer films

Figure 4. Photographs of TL of PMMA film (diameter: ca.
5.5 cm) containing 10 wt% of (a) Cu1, (b) Cu5, (c) Cu6.
The films were rubbed with a glass rod under Ar gas.

and casted as coatings on the surface issignificantly more attractive for possible
practical applications of such materials indevice fabrication as compared to using
purely crystalline materials. Surprisingly,
when small amountsof Cu16were

physically blended into rigid polymer films(e.g. PMMA) simply by dissolving andcasting from solution, such films also exhibit

TL intensity (a.u.)that can be easily prepared by simple mixing

N2ArHeCO2Vacuum
SF6400 500 600 700 800 900
Wavelength (nm)Figure 5. TL spectrum of Cu5 in PMMA film (1 wt%) under
different gas of Air, N2, Ar, He, CO2, SF6, and vacuum (0.5 Torr).

visible TL when rubbed with a glass or plastic rod, even without tearing or rupturing the film. The
photographs of TL of 10 wt% PMMA films are shown in Figure 4. Powder X-ray diffraction analysis as well
as fluorescence microscopy images confirmed the absence of crystalline phase and nano/microparticles in
the PMMAs with 1 wt% and 10 wt% of Cu16. Previous studies on TL of photoluminescent molecules inpolymers have not revealed the generation of TL from amorphous state, or have shown TL from crystals
dispersed in polymers6,9-12. To the best of our knowledge, this is the first observation of TL of transition metalcomplex in amorphous polymer film. Red-shift of TL compared to photoluminescence was also confirmed,
which can be ascribed to different environment on the rubbed surface of PMMA. To get insight into the origin

of TL in PMMA film, TL spectra of Cu5 in PMMA (1 wt%) were measured under different gas flow including
nitrogen, argon, helium, carbon dioxide, sulfur hexafluoride, as well as under vacuum (0.5 Torr). TL ofCu16 from PMMAs are not observed in air, which is probably due to quenching of excited state by oxygenor high humidity. A broad emission band of Cu5 was observed along with sharp intense peaks that can beascribed to gas plasma emission of nitrogen, argon, or helium (Figure 5). This is sharp contrast to TL in thecrystals where no discernible gas discharge lines were observed. Unexpectedly, under CO2 gas and undervacuum (0.5 Torr), the emission spectra showed only the TL peak of Cu5 without the gas discharge peaks.
Interestingly, emission was extremely weak in SF6 gas. This result implies that triboelectrification is likely
involved in the observed TL properties of Cu-containing polymer films leading to generation of the electricfield and gas discharge, which eventually causes photoluminescence of the copper complex blended into thefilm. More importantly, we have shown that this may be observed in the absence of the crystalline phaseusing a simple Cu complex. Our discovery will provide new opportunity of organometallic compounds for awide range of applications including mechanical sensors capable of visualizing mechanical actions onpolymer materials via photoluminescence, and mechano-responsive materials such as self-glowing materials.
Detailed studies including screening of various luminophores and polymers for effective TL in polymers aswell as mechanistic investigation are ongoing.
References
1) I. Sage and G. Bourhill, J. Mater. Chem., 2001, 11, 231-245.
2) J.-C. G. Bünzli, K.-L. Wong, Journal of Rare Earths 2018, 36, 1-41.
3) Y. Zhuang, and R. J. Xie, Adv. Mater. 2021, 2005925
4) Y. Xie and Z. Li, Chem, 2018, 4, 943-971.
5) S. Mukherjee and P. Thilagar, Angew. Chem. Int. Ed., 2019, 58, 7922-7932.
6) A. İncel, C. Varlikli, C. D. McMillen, M. M. Demir, The Journal of Physical Chemistry C 2017, 121, 11709-11716.
7) P. H. Patil, G. A. Filonenko, S. Lapointe, R. R. Fayzullin and J. R. Khusnutdinova, Inorg. Chem., 2018, 57, 10009-10027.
8) A. Karimata, P. H. Patil, E. Khaskin, S. Lapointe, R. R. Fayzullin, P. Stampoulis and J. R. Khusnutdinova, Chem. Commun.,
2020, 56, 50-53.

9) N. Takada, J. Sugiyama, R. Katoh, N. Minami and S. Hieda, Synth. Met., 1997, 91, 351-354.
10) A. İncel, M. Emirdag-Eanes, C. D. McMillen and M. M. Demir, ACS Appl. Mater. Interfaces, 2017, 9, 6488-6496.
11) R. S. Fontenot, W. A. Hollerman, K. N. Bhat, M. D. Aggarwal and B. G. Penn, Polym. J. (Tokyo, Jpn.), 2014, 46, 111-116.
12) T. M. George, M. J. Sajan, N. Gopakumar and M. L. P. Reddy, J. Photochem. Photobiol., 2016, 317, 88-99
Acknowledgements
We thank Dr. Pradnya H. Patil, Dr. Sébastien Lapointe, Dr. Eugene Khaskin, Dr. Julio Manuel Barros Jr., Dr.
Olga Gladkovskaya, and Dr. Robert R. Fayzullin (Arbuzov Institute of Organic and Physical Chemistry,Russia) for their contributions to this project and helpful discussions. We also thank Research Support
Division at OIST including Instrumental Analysis Section and Engineering Section. This study wasfinancially supported by JSPS KAKENHI Grant Number JP18K05247.

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松村 有里子4. 今後の行事予定及び事務局からのお知らせ

The 27th International SPACC

The 28th International SPACC

Symposium (SPACC27)

Symposium (SPACC28)

場所: 高知工科大学永国寺キャンパス

場所: 台湾

会期: 2022 年（時期未定）

会期: 2023 年（時期未定）（再延期しました）（再延期しました）

担当: 小廣和哉 (高知工科大)担当: 天尾 豊 (大阪市立大学)

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松本健司 (高知大学)(National Taiwan University)

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シンポジウム世話人：
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編集後記
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