Tunable mechanochromic luminescence of 2-alkyl-4-(pyren-1-yl)thiophenes: Controlling the self-recovering properties and the range of chromism

Article abstract of DOI:10.1039/c9cc06406k

Controlling the behavior of mechanochromic luminescence (MCL) based on rational design principles is an outstanding challenge. Herein, an unprecedented self-recovering MCL that manifests in a large shift of the emission maximum (～200 nm) has been achieved for 2-alkyl-4-(pyren-1-yl)thiophenes upon introducing long alkyl chains and mixing with N,N′-dimethylquinacridone.

Full text of DOI:10.1039/c9cc06406k

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
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Tunable mechanochromic luminescence of 2-alkyl-4-(pyren-1-
yl)thiophenes: Controlling the self-recovering properties and the
range of chromism
Received 00th January 20xx,
Accepted 00th January 20xx
Minako Ikeya, Genki Katada and Suguru Ito*
DOI: 10.1039/x0xx00000x
Controlling the behavior of mechanochromic luminescence (MCL) required in order to develop a comprehensive understanding
based on rational design principles is an outstanding challenge. and rational design guidelines for self-recovering MCL dyes.
Herein, an unprecedented self-recovering MCL that manifests in a
Another critical challenge is the ability to tune the range of
large shift of the emission maximum (~200 nm) has been achieved the mechanical-stimuli-induced shift of the maximum emission
for 2-alkyl-4-(pyren-1-yl)thiophenes upon introducing long alkyl wavelength (em). The em values of almost all hitherto
chains and mixing with N,N′-dimethylquinacridone.
reported MCL dyes are < 150 nm.1 Only very few luminophores
exhibit a shift of the maximum emission wavelength of more
than 150 nm upon manual grinding.3 For organic
luminophores, Enomoto et al. reported an exceptionally large
Mechanochromic luminescence (MCL) describes the reversible
color change of solid-state emission upon exposure to
mechanical stimuli, and MCL materials have recently attracted
considerable interest owing to their promising potential as
next-generation smart materials such as mechanosensors,
wearable devices, and anti-counterfeiting paintings.1 This
interest is reflected in the publication of more than 500
articles on MCL systems in the past decade. Nevertheless, the
rational design guidelines for MCL dyes with specific
characteristics remain scarce.
Control over the recovery process of MCL is one great
challenge in order to advance the development of MCL dyes.
Usually, the mechanically switched emission color of MCL dyes
reverts to the original color upon applying a second external
stimulus such as heating or exposure to solvents. For several
luminophores, the rare occurrence of self-recovering MCL has
been reported, i.e., the mechanically changed state
autonomously reverts to the original state at room
temperature in the absence of a second external stimulus.2
Such self-recovering MCL is typically attributed to the
mechanical-stimuli-induced amorphization of crystalline
luminophores followed by spontaneous recrystallization.
Although it has been suggested in some reports that the self-
recovering property is due to an increased molecular mobility
em (~300 nm) value of aminobenzopyrenoxanthene.3d
A
large em (197 nm) of a Bodipy derivative was reported by
Ajayaghosh et al.3c However, reasonable predictions of as well
as control over em of an MCL dye remain so far impossible.
Considering the practical applications of MCL dyes, the
development of a general method to achieve large em values
is particularly desirable, especially because such MCL materials
facilitate the detection of mechanical stimuli by distinct
spectral changes. We have recently demonstrated a significant
improvement of poor MCL properties of an organic dye simply
by mixing with another dye that does not exhibit MCL
properties.4 In the
after
introducing
suitable
substituents
into
the
luminophores,2g,i,j,r,s,t,u further systematic studies are still
Fig. 1 Schematic illustration of the tuning of the MCL behavior.
(a) Typical MCL of a single-component dye with short alkyl
groups; (b) self-recovering MCL of a single-component dye
with long alkyl groups (partial self-recovery was observed in
this study); (c) high-contrast MCL of a two-component dye
Department of Chemistry and Life Science, Graduate School of Engineering
Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama
240-8501, Japan. E-mail: suguru-ito@ynu.ac.jp
†Electronic Supplementary Information (ESI) available: Synthetic details, spectral
data, X-ray diffraction analyses, theoretical calculations, DSC analyses, PXRD data,
and movie of MCL. CCDC reference numbers: 1947301 (1a), 1947302 (1b),
1947303 (1c), 1947304 (1e), and 1947305 (1f). See DOI: 10.1039/x0xx00000x.
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with short alkyl groups; (d) high-contrast self-recovering MCL of molten amorphous 1a (em = 480 nm; Fig. S9a), prepared by
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DOI: 10.1039/C9CC06406K
of a two-component dye with long alkyl groups.
heating 1a above its melting point followed by cooling to room
Table 1 Maximum emission wavelengths and melting point of
2-alkyl-4-(pyren-1-yl)thiophenes 1a–f.
Me
O
1a:R = CH3
1b:R = C2H5
N
1c:R = n-C3H7
1d:R = n-C4H9
1e:R = n-C5H11
1f :R = n-C6H13
Compd.
Crystalline powder 418 414 413 417 416 414
em (nm)a
Just after grinding 480 482 478 486 481 485
em (nm)b
5 s after grinding
em (nm)c
Melting point (°C)d 92
1a
1b
1c
1d
1e
1f
N
O
Me
R
S
DMQA

Fig. 2 Chemical structure of 2-alkyl-4-(pyren-1-yl)thiophenes
1a–f and N,N′-dimethylquinacridone (DMQA).

480 482 410, 422, 422, 410,
475 477 473 473

previous study, the em value (13 nm) of a 3,3′-dipyrenyl-2,2′-
bithiophene derivative increased to 135 nm for a two-
component mixed dye that also contained N,N′-
dimethylquinacridone (DMQA). The extension of the MCL
range is due to the OFF/ON switching of the energy transfer
from dipyrenylbithiophene to DMQA, which is triggered by the
mechano-induced amorphization of the crystalline mixture.
Herein, we report the rational tuning of the MCL properties
of 2-alkyl-4-(pyren-1-yl)thiophenes 1 (Fig. 1 and 2). The
recovery behavior of these dyes can be controlled by the
length of alkyl group (Fig. 1a and 1b), whereas the MCL range
can be expanded by mixing with DMQA (Fig. 1a and 1c). By
combining these two concepts, we accomplished an
unprecedented self-recovering MCL that exhibits a high-
contrast emission color change between violet and orange
(em ≈ 200 nm) for two-component dyes that consist of 1
with long alkyl chains and DMQA (Fig. 1d).
92
73
33
59
59
a Maximum emission band of crystalline powdered sample. b Maximum
emission band of ground sample measured immediately after grinding.
c
Maximum emission band and of ground sample measured 5 s after
grinding. Shoulder emission band (shorter wavelength) is also shown
for 1c–f. d Melting point of crystalline powdered sample.
2-Alkyl-4-(pyren-1-yl)thiophenes 1a–f, which contain a linear
alkyl substituent that varies in length from methyl (1a: R = CH3)
to hexyl (1f: R = n-C6H13), were synthesized via the Suzuki-
Miyaura coupling of pyrene-1-boronic acid and the
corresponding 2-alkyl-4-bromothiophenes (Scheme S1). The
MCL properties and melting points of crystalline samples of
1a–f are summarized in Table 1. Crystalline samples of 1a (R =
CH3) exhibit blue emission (em = 418 nm) under UV light (ex
=
365 nm). Density functional theory (DFT) calculations on 1a at
the B3LYP/6-31G(d) level of theory showed that the HOMO
and LUMO of 1a are located mainly on the pyrene ring (Fig. S6
and Table S1). In addition, a single-crystal X-ray diffraction
analysis of 1a revealed the absence of intermolecularly stacked
pyrene rings (Fig. S1). The blue emission of the crystalline
samples should accordingly be attributed to the monomer
emission of the pyrenyl group.
Upon grinding crystalline 1a with a spatula, the emission
color changed to light blue (em = 480 nm; em = +62 nm; Fig.
3a and 3c). Since the maximum excitation wavelength of
ground 1a shifted bathochromically compared to that of
crystalline 1a (Fig. S7a), the light blue emission can be
reasonably assigned to the emission from a static excimer of
intermolecularly stacked pyrenyl groups.5 The mechanically
changed state is stable at room temperature, and the original
blue emission can be recovered by heating to 80 °C or by
treatment with organic solvents (e.g., hexane or CH3CN)
followed by natural evaporation. Powder X-ray diffraction
(PXRD) analyses showed that the diffraction peaks of a
crystalline powder of 1a decreased after grinding and
increased after heating (Fig. 3e). Moreover, the emission band
Fig. 3 Photographs (a and b), fluorescence spectra (c and d),
and PXRD patterns (e and f) for the MCL of 1a and 1f.
Excitation wavelength is 365 nm.
temperature, was observed in the same region as that of
ground 1a. Accordingly, the MCL of 1a should proceed via a
typical crystalline-to-amorphous phase-transition mechanism.1
Although crystalline samples of the derivatives 1b–f also
exhibit blue emission (em = 413–417 nm), their MCL
properties strongly depend on the length of the alkyl
substituent (Table 1, Fig. S8). The MCL behavior of 1b (R =
C2H5) was almost identical to that of 1a. The emission color of
2 | Chem. Commun., 2019, 55, 1-4
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crystalline 1b (em = 414 nm) shifted bathochromically to light orange-emissive states, and therefore 1a/VDieMw AQrtAicle Oanlninde
DOI: 10.1039/C9CC06406K
blue (em = 482 nm, em = +68 nm) upon grinding, and 1b/DMQA exhibit MCL between violet and orange (Fig. 4a, S14,
reversed to the original blue after heating or exposure to and S15). PXRD patterns of as-prepared 1a/DMQA were
solvents. On the other hand, 1c–f, which contain longer alkyl almost identical to those of crystalline DMQA, indicating that
groups, exhibited partial self-recovery of the emission bands the as-prepared state contains amorphous 1a and crystalline
after grinding. Photographs, fluorescence spectra, and PXRD DMQA (Fig. S20a). Weak PXRD peaks other than those of
patterns for the MCL of 1f (R = n-C6H13) as a representative DMQA were observed for 1a/DMQA after grinding, and their
example are shown in Fig. 3. Upon grinding crystalline 1f, the intensities increased after heating. These peaks are in good
emission color changed to light blue (em = 485 nm; em = +71 agreement with those of crystalline 1a obtained by heating a
nm; Fig. 3b and 3d), which is almost identical to that of molten ground sample after melting.6 Since 1a and DMQA should exist
amorphous 1f (Fig. S9f). Interestingly, a slight hypsochromic as independent crystals in the violet-emissive state, the violet
shift (em = –12 nm) and an increase of a shoulder peak (em emission of 1a/DMQA should most likely be attributed to the
= 410 nm), which is in the same region as the emission of blue emission of crystalline 1a and a weak red emission of
crystalline 1f, were observed in the fluorescence spectrum crystalline DMQA. Moreover, the as-prepared and ground
measured after keeping the ground sample at room states of 1a/DMQA should contain perfectly amorphous and
temperature (~ 25 °C) for 5 s (Fig. 3d). A PXRD analysis partially crystalline 1a, respectively. Therefore, the green and
indicated that this state contains portions of crystalline 1f, orange emission of 1a/DMQA should be assigned to the
indicating a partial recovery of the initial crystalline state from emission of monomeric DMQA and intermolecularly
the ground amorphous state (Fig. 3f). In contrast to the ground interacting DMQA, respectively, similar to the previously
state of 1a and 1b, the partially recovered state of 1f was reported two-component system.4
stable even when heated below the melting point of 1f, and
In the case of 1c/DMQA, the orange-emissive state (em =
treatment with organic solvents (e.g., hexane or CH3CN) was 608 nm) spontaneously changed to another state at room
required to recover the original blue-emissive state (Fig. 3b temperature within 30 s (Fig. S16). This state showed two
and 3f). Similarly, 1c (R = n-C3H7), 1d (R = n-C4H9), and 1e (R = emission maxima at 407 and 608 nm, and further changed to a
n-C5H11) exhibited partially self-recovering MCL (Fig. S8b–d), violet-emissive state (em = 407 nm) upon heating to 60 °C.
i.e., the introduction of propyl or longer alkyl groups is PXRD analyses revealed that the intermediate state with two
effective to induce self-recovering properties.
emission maxima contains partially recrystallized 1c (Fig. S20c).
Differential scanning calorimetry (DSC) measurements of This behavior is similar to the partial self-recovery of single-
molten 1a and 1b showed their cold-crystallization transition component 1c. Although the exposure to organic solvent is
temperatures (Tc) at 59 °C and 48 °C, respectively (Fig. S11). In
contrast, a Tc was not observed in the DSC measurements of
molten 1c–f, and their glass-transition temperatures (Tg) were
observed below room temperature (1c: –19 °C; 1d: –23 °C; 1e:
–26 °C; 1f: –28 °C; Fig. S12). These results suggest high
molecular mobility of ground amorphous 1c–f at room
temperature, which would at least in part explain the self-
recovering properties of 1c–f. Although the partially recovered
states of 1c, 1e, and 1f reverse to the initial crystalline states
upon exposure to solvents (Fig. S10), 1d required
recrystallization from methanol to recover the original
Fig. 4 (a) Photographs for the MCL of 1a/DMQA. Photographs
(b) and fluorescence spectra (c) for the MCL of 1f/DMQA.
Excitation wavelength is 365 nm.
crystalline state, which is probably due to the low crystallinity
of 1d as evident from its low melting point (33°C).
We then attempted to expand the MCL range of 1a–f by
mixing with DMQA. We used slight modifications for a
previously reported procedure,4 and prepared two-component
mixtures of 1a–f/DMQA by evaporating a CH2Cl2 suspension of
1:1 molar mixtures of 1a–f and DMQA followed by melting on
a glass plate at 120 °C and cooling to room temperature. The
as-prepared mixtures exhibited green emission (em ≈ 540 nm)
in all cases, and the emission color changed to orange (em
≈
610 nm) upon grinding (Fig. S13). Remarkably, the behavior of
the orange-emissive states of 1a–f/DMQA depends on the
length of the alkyl chain in 1a–f.
The orange-emissive states of 1a/DMQA (em = 616 nm) and
1b/DMQA (em = 604 nm) changed to violet-emissive states
(1a: 413 nm; 1b: 408 nm) upon heating to 80 °C. These violet-
emissive states respond to mechanical stimuli and afford
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Fig. 5 PXRD patterns for DMQA (red), as-prepared 1f/DMQA
(green), 1f/DMQA after grinding (violet), and 1f after
recovering (blue).
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C9CC06406K
required for the single-component system, 1c in 1c/DMQA
recrystallized only by heating, probably because the presence
of crystalline DMQA facilitates the recrystallization of 1c. On
the other hand, the orange-emissive state of 1d/DMQA could
not be changed to the violet-emissive state by heating or
exposure to organic solvents, which is probably due to the low
crystallinity of 1d (Fig. S17 and S20d).
Notes and references
1
For recent reviews, see: (a) M. Kato, H. Ito, M. Hasegawa and
K. Ishii, Chem.-Eur. J., 2019, 25, 5105; (b) J. Zhao, Z. Chi, Y.
Zhang, Z. Mao, Z. Yang, E. Ubba and Z. Chi, J. Mater. Chem. C,
2018, 6, 6327; (c) X. Huang, L. Qian, Y. Zhou, M. Liu, Y. Cheng
and H. Wu, J. Mater. Chem. C, 2018, 6, 5075; (d) Y. Sagara, S.
Yamane, M. Mitani, C. Weder and T. Kato, Adv. Mater., 2016,
28, 1073; (e) Z. Ma, Z. Wang, M. Teng, Z. Xu and X. Jia,
ChemPhysChem, 2015, 16, 1811.
Most notably, the self-recovering MCL between violet and
orange was achieved for 1e/DMQA and 1f/DMQA with longer
alkyl groups (Fig. 4b, 4c, and S18). The orange-emissive states
of these two-component dyes (1e: 611 nm; 1f: 616 nm)
spontaneously changed to violet-emissive states at room
temperature (1e: 412 nm; 1f: 422 nm). Upon grinding these
violet-emissive states, the emission maxima were shifted by
~200 nm to orange and self-recovered to violet within 30 min
(1e/DMQA; Fig. S18 and S20e) or 20 s (1f/DMQA; Fig. 4b, 4c
and Movie S1). Subsequently, we examined the reproducibility
of this self-recovering MCL for 1f/DMQA, which turned out to
be fully reversible and repeatable (Fig. S19). Interestingly, the
recovery time can be controlled by the temperature. When a
glass plate with 1f/DMQA was placed on an ice plate (0 °C),
the ground orange-emissive sample required 1.5 h to recover
the original violet emission. This temperature dependence
suggests that the fast self-recovering behavior of 1f/DMQA
under room temperature should be attributed to the relatively
low activation energy of crystallization compared to other
derivatives. As shown in the PXRD patterns of 1f/DMQA (Fig.
5), 1f in the two-component dye recrystallized immediately
after grinding the as-prepared sample. It should also be noted
here that the recrystallization of 1e and 1f is probably
facilitated significantly by the presence of crystalline DMQA
compared to the single-component systems, where only
partial self-recovery was observed.
In summary, we have systematically established control over
the MCL properties of 2-alkyl-4-(pyren-1-yl)thiophenes 1. The
introduction of longer alkyl groups imparts self-recovering
properties, while the MCL range and the ease of the self-
recovery can be improved by mixing with DMQA.
Consequently, fast self-recovering MCL with a high-contrast
emission color change has been accomplished for 1f/DMQA
(em = 194 nm), which is, to the best of our knowledge, the
first example of a self-recovering MCL that can switch its
maximum emission wavelength by more than 150 nm.2,3 The
present design principles can be expected to be applicable to
other luminophores, which should accelerate the development
of self-recovering MCL dyes and their practical applications in
e.g. rewritable papers and temporal pressure sensors.
2
Only a limited number of MCL dyes (<5%) exhibits self-
recovering properties; for details, see: (a) P. Alam, N. L. C.
Leung, Y. Cheng, H. Zhang, J. Liu, W. Wu, R. T. K. Kwok, J. W.
Y. Lam, H. H. Y. Sung, I. D. Williams and B. Z. Tang, Angew.
Chem., Int. Ed., 2019, 58, 4536; (b) K. K. Kartha, V. S. Nair, V.
K. Praveen, M. Takeuchi and A. Ajayaghosh, J. Mater. Chem.
C, 2019, 7, 1292; (c) R. Zhao, L. Zhao, M. Zhang, Z. Li, Y. Liu, T.
Han, Y. Duan and K. Gao, Dyes Pigm., 2019, 167, 181; (d) F.
Zhao, Z. Chen, G. Liu, C. Fan and S. Pu, Tetrahedron Lett.,
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RSC Adv., 2017, 7, 43845; (f) T. Butler, A. S. Mathew, M.
Sabat and C. L. Fraser, ACS Appl. Mater. Interfaces, 2017, 9,
17603; (g) S. Ito, T. Taguchi, T. Yamada, T. Ubukata, Y.
Yamaguchi and M. Asami, RSC Adv., 2017, 7, 16953; (h) H.
Wu, C. Hang, X. Li, L. Yin, M. Zhu, J. Zhang, Y. Zhou, H. Ågren,
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3
(a) B. Hupp, J. Nitsch, T. Schmitt, R. Bertermann, K. Edkins, F.
Hirsch, I. Fischer, M. Auth, A. Sperlich and A. Steffen, Angew.
Chem., Int. Ed., 2018, 57, 13671; (b) T. Seki, N. Tokodai, S.
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and H. Ito, J. Am. Chem. Soc., 2017, 139, 6514; (c) S.
Cherumukkil, S. Ghosh, V. K. Praveen and A. Ajayaghosh,
Chem. Sci., 2017, 8, 5644; (d) M Tanioka, S. Kamino, A.
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This work was partly supported by JSPS KAKENHI Grant
Number 18H04508 in Grant-in-Aid for Scientific Research on
Innovative Areas “Soft Crystals: Area No. 2903”. The authors
are grateful to Mr. Shinji Ishihara (Instrumental Analysis
Center, Yokohama National University) for carrying out the
elemental analyses.
4
S. Ito, G. Katada, T. Taguchi, I. Kawamura, T. Ubukata and M.
Asami, CrystEngComm, 2019, 21, 53.
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5
6
For a review, see: F. M. Winnik, Chem. Rev., 1993, 93, 587.
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DOI: 10.1039/C9CC06406K
Similar PXRD patterns were also observed for the MCL of
1b/DMQA (Fig. S20b).
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