Tuning the lifetime of transient phases of mechanochromic gold isocyanide complexes through functionalization of the terminal moieties of flexible side chains

Article abstract of DOI:10.1246/cl.170482

The lifetime of the transient phases of luminescent mechanochromic gold isocyanide complexes could be tuned through the modification of the termini of flexible side chains. Upon grinding, these complexes initially showed an emission color change from blue to yellow. The yellow emission spontaneously changed to blue or green, but with different lifetimes of the yellow-emitting transient phases depending on the substituents. Details of the mechano-induced structure changes were also described.

Full text of DOI:10.1246/cl.170482

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Tuning the Lifetime of Transient Phases of Mechanochromic Gold Isocyanide Complexes
through Functionalization of the Terminal Moieties of Flexible Side Chains
Tomohiro Seki,* Kentaro Kashiyama, Shiki Yagai, and Hajime Ito*
Advance Publication on the web June 23, 2017
doi:10.1246/cl.170482
© 2017 The Chemical Society of Japan
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1
Tuning the Lifetime of Transient Phases of Mechanochromic Gold Isocyanide Complexes
through Functionalization of the Terminal Moieties of Flexible Side Chains
Tomohiro Seki,*1 Kentaro Kashiyama,1 Shiki Yagai,2 and Hajime Ito*1
1Division of Applied Chemistry & Frontier Chemistry Center, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628,
Japan
2Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan
E-mail: seki@eng.hokudai.ac.jp, hajito@eng.hokudai.ac.jp
of their mechanochromic properties.5,6 Interestingly, complex
The lifetime of the transient phases of luminescent
mechanochromic gold isocyanide complexes could be tuned
1 possessing flexible tetraethylene glycol (TEG) side chains
through the modification of termini of flexible side chains.
(Figure 2) exhibited unprecedented mechanochromism:7
Upon grinding, these complexes initially showed an emission
upon mechanical stimulation, a blue-emitting pristine sample
of 1 (1B) initially formed a yellow-emitting transient phase
1Y before spontaneously transforming to green-emitting
phase 1G over 2 s (Figure 2). X-ray diffraction (XRD)
analysis revealed that 1B and 1G are two different crystalline
phases and both have alternating layer structures composed
of rigid luminophoric sublayers and flexible TEG sublayers.
In contrast, 1Y was found to be amorphous phase. Although
more than ten mechanochromic compounds that
spontaneously recover the original phase from their ground
“transient” phases have been reported (i.e., C1 → [T] → C1,
Figure 1b)8, 1 was the first mechanochromic compound that
exhibits crystal-to-crystal phase transition mediated by a
short-lived “transient” phase (C1 → [T] → C2, Figure 1b).
color change from blue to yellow. The yellow emission
spontaneously changed to blue or green but with different
lifetime of the yellow-emitting transient phases depending on
the substituents. Details of the mechano-induced structure
changes were also described.
When solid-state emission colors of organic or
inorganic compounds changes upon grinding (C → G, Figure
1a), such
mechanochromism
a
phenomenon is called luminescent
(hereafter, referred to as
“mechanochromism”).1 Mechanochromic compounds are
promising as sensors and memory devices.2 Emission color
changes of mechanochromic compounds generally originate
from molecular rearrangement induced by mechanical
stimulation.3 In most cases, intermolecular interactions that
influence emission properties are changed, which induces
,4
emission color changes.1 Once formed, the ground phases
are generally stable under ambient conditions for a long
period (Figure 1a) because no further molecular
rearrangement occurs.
Figure 2. Structures and photographs of powder samples of 1, 2, and 3
taken under UV illumination.
Here, we tune the lifetime of transient phases of gold
isocyanide complexes through functionalization of the
terminal groups of their flexible TEG side chains. We
prepared new compounds 2 and 3 possessing rigid tolyl (2)
and flexible hexyl groups (3). Both 2 and 3 show
mechanochromism involving short-lived transient phases.
The transient phase of 2 spontaneously transforms into a new
phase (similar to 1), whereas that of 3 transforms into the
original phase (self-recovery). The lifetimes of the transient
phases of 1, 2, and 3 are evaluated by emission spectroscopy,
Figure 1. Schematic representations of a) typical mechanochromism and
b) mechanochromism involving a transient phase.
We have intensively studied the luminescent
mechanochromism of a series of aryl gold isocyanide
complexes.5,6 These aryl gold isocyanide complexes are
highly sensitive to mechanical stimulation and their subtle
structural modification often leads to remarkable alteration

2
Figure 3. Emission spectra of a) 2 and b) 3 upon mechanical
stimulation. Excitation wavelength = 365 nm. The red arrow in a)
indicates the spectral changes during the spontaneous phase transition.
and the crystal structures of 2 and 3 are analyzed by XRD to
elucidate the mechanism of the transient mechanochromism.
New complexes 2 and 3 were prepared according to a
modified reported procedure.7 Reprecipitation of 2 in CH2Cl2
and MeOH (or hexane) gave white powder 2B exhibiting
blue emission under UV illumination. Similarly,
blue-emitting 3 (3B) was obtained through a similar
reprecipitation procedure. NMR spectra of 2B and 3B in
CDCl3 indicate that the powder samples do not contain
solvent molecules (Figure S1).
To investigate the mechanism of mechanochromism of
2 and 3, powder XRD measurements were conducted. The
pristine powder of 2B exhibited a diffraction pattern
featuring many intense peaks (Figure S3), indicating its
crystalline nature. Ground powder 2G was also found to be
crystalline powder from its XRD pattern (Figure S3).
Importantly, the peak position of the diffraction pattern of
2G is different from that of 2B. This clearly indicates that the
molecular arrangements of 2B and 2G are different from
each other, excluding the possibility of the self-recovery
from the transient phase 2Y; i.e., C1 → [T] → C2 (Figure 1b).
Although the powder XRD pattern of short-lived 2Y could
not be measured in this study, 2Y has molecular
arrangements different from those of 2B and 2G considering
the previously reported result of 1 whose yellow-emitting
transient phase has proven an amorphous phase.7,10 For
complex 3, we compared the powder XRD patterns of 3B
and 3B′. Unground 3B and ground 3B′ gave almost identical
diffraction patterns (Figure S3), which indicates that they are
crystalline phases with essentially the same molecular
arrangements. Accordingly, the blue-emitting original phase
must be recovered from the transient phase 2Y,11 which is in
line with the results of emission spectroscopy (C1 → [T] →
C1, Figure 1b).
Complexes
2 and 3 showed mechanochromism
mediated by transient phases. Upon grinding blue-emitting
2B, its emission color initially changed to yellow because of
the formation of 2Y (Figure 2). Under ambient conditions,
the emission color of 2Y spontaneously transformed to green
over 150 s because of the formation of 2G (Figure 2). This
observation demonstrates that 2 displays mechanochromism
including the transient 2Y phase similar to 1; i.e., C1 → [T]
→ C2 (Figure 1b). The mechanochromism of 2 was
evaluated by steady-state and time-resolved emission
spectroscopy (Figure 3a). Upon excitation at 365 nm, 2B
exhibited a broad emission spectrum with an emission
maximum wavelength λem,max of 472 nm (blue line in Figure
3a). The absolute emission quantum yield Φem and average
9
emission lifetime τav of 2B are 0.095 and 42.7 μs,
respectively (Figure S2 and Table S1). Upon grinding 2B, a
bathochromic shift was initially observed and the resulting
2Y has λem,max = 538 nm (yellow line in Figure 3a). Upon
We next evaluated the lifetimes of the transient phases
of 1–3 at room temperature by probing emission intensity.
As mentioned above, 1Y, 2Y, and 3Y are short-lived
transient intermediates that spontaneously transform to
different phases with different time ranges (Figure 2). To
evaluate their lifetimes, emission intensities at the λem,max of
1Y, 2Y, and 3Y were plotted as a function of time just after
mechanical stimulation (Figure 4). The emission intensity of
1 decreased abruptly after mechanical simulation, indicating
the rapid transformation of 1Y into 1G after mechanical
stimulation ceased. Conversely, 2Y took 100 s to make its
emission intensity half, indicating the longer lifetime of the
transient phase 2Y. The emission spectrum of 3Y could not
be detected because of the very rapid transformation of 3Y
into 3B′ (lifetime: < 1 s).
standing 2Y under ambient conditions,
a
gradual
hypsochromic shift was observed over 150 s (yellow to green
lines in Figure 3a). λem,max of 2G is 522 nm and Φem is 0.14,
and τav is 18.1 μs (Figure S2 and Table S1). These spectral
features are similar to those of 1.7
For complex 3, mechanical stimulation of blue-emitting
pristine phase 3B initially afforded yellow-emitting phase 3Y
(Figure 2). This transient phase 3Y immediately transformed
to blue-emitting 3B′ (< 1 s, Figure 2). Emission spectra of
3B and 3B′ showed similar structured emission bands with
peaks at 447 and 466 nm (Figure 3b; see Table S1 for
photophysical properties). The spectral similarity implies the
self-recovery of the original phase from the transient phase
3Y; i.e., C1 → [T] → C1 (Figure 1b). The emission spectrum
of 3Y could not be detected because of its short lifetime. For
2B and 3B, increasing temperature causes emission color
changes at 125 and 85 °C, respectively, different from those
induced by mechanical stimulation, probably because of
chemical decomposition.
Figure 4. Decay profiles for the intermediate species monitored by
emission intensity at 564 nm for 1 (circles) and 538 nm for 2 (squares).
Inset shows a magnified view of the short time range.

3
Figure 5. Single-crystal structures and photographs (insets, taken under UV light) of 2B (a–c) and 2G (d–f).
The different lifetimes of the transient phases 1Y, 2Y,
and 3Y can be rationalized by considering the character of
the terminal substituents on their side chains. On one hand,
the tolyl group of 2 can form stronger intermolecular
interactions compared to a methyl group of 1 due to effective
π-π stacking. Such π-π stacking formed in the transient 2Y
phase possibly could slow the reorganization of the
molecular arrangement. On the other hand, the side chains of
3 do not have such aromatic groups. Upon grinding 3B, a
subtle reorganization of molecules takes place to form
yellow-emitting 3Y, which cannot be effectively stabilized
by interactions between side chains. As a result, the rapid
self-recovery of 3B may occur.
arrangement (Figure 5c). A green-emitting single crystal of
2G was prepared from acetone at −25 °C and crystallized in
monoclinic space group P21/c (Table S2). The luminophore
segments of 2G adopt a conformation that slightly deviates
from the perpendicular alignment, with θd = 19.39° (Figure
5d). Molecules in 2G adopt an S-shaped conformation,
taking advantage of the flexible TEG moieties (Figure 5d).13
The intermolecular nearest Au···Au distance of 2G is
5.3253(4) Å, indicating there are no aurophilic interactions.
However, two-fold Au···π interactions (3.378 Å, red dotted
lines) are observed with a neighboring molecule, forming
open-ended polymeric chains (Figure 5e).14 In addition, an
alternating layer structure containing rigid gold isocyanide
sublayers and flexible TEG layers is formed (Figure 5f),
similar to previously reported 1B and 1G.7 It is supposed that
the large difference of the molecular packing of 2B and 2G
(Figure 5) also contributes to the long lifetime of 2Y.
A blue-emitting single crystal of 3 was also obtained by
repeated recrystallization (Figure 6). Recrystallization of 3
from a CH2Cl2/MeOH mixture at −25 °C afforded the
blue-emitting single crystal. The experimentally obtained
powder XRD pattern of 3B was completely reproduced from
the resulting single crystal of 3 (Figure S4). The single
crystal of 3B crystallized in triclinic space group P-1 (Table
S2). The central gold isocyanide moiety has a twisted
conformation with θd = 80.62°. The molecules in 3B adopt a
straight conformation similar to those in 2B; however, the
terminal hexyl groups exhibit disorder (Figure 6a).
Molecules form two-fold Au···π interactions (3.403 Å, red
dotted lines in Figure 6b) with neighboring molecules to
construct polymeric chains. The nearest Au···Au distance is
5.4166(8) Å, indicating there are no aurophilic interactions.
We successfully grew two single crystals of
2
corresponding to 2B and 2G (Figure 5). A blue-emitting
single crystal of 2 was prepared from CH2Cl2/hexane at room
temperature and crystallized in triclinic space group P-1
(Table S2). From this single-crystal pattern, a simulated
powder XRD pattern was generated that was consistent with
the powder XRD pattern of 2B (Figure S4). The central
luminophore moiety has a flat conformation: the dihedral
angle between an isocyanide benzene ring and
tetrafluorobenzene rings on Au (θd) is 3.22° (Figure 5a). In
addition, the entire molecules including flexible TEG
segments adopt a rather straight conformation (Figure 5a).
Together with two neighboring molecules, each molecule in
2B forms two aurophilic interactions with an Au···Au
distance of 3.2666(4) Å (red dotted lines in Figure 5b),6,12
forming open-ended polymeric chains. The molecular
packing of previously reported 17 features alternating layers
consisting of rigid luminophore sublayers and flexible TEG
sublayers (vida supra). However, the molecular packing of
2B does not contain such a segregated layer-by-layer

4
D. Hashizume, E. Nakamura, Chem. Lett. 2011, 40, 576–578; (h) S.
J. Choi, J. Kuwabara, Y. Nishimura, T. Arai, T. Kanbara, Chem.
Lett. 2012, 41, 65–67.
[4] (a) K. Ariga, T. Mori, J. P. Hill, Adv. Mater. 2012, 24, 158–176; (b)
K. Ariga, T. Mori, J. P. Hill, Chem. Sci. 2011, 2, 195.
[5] (a) T. Seki, Y. Takamatsu, H. Ito, J. Am. Chem. Soc. 2016, 138,
6252–6260; (b) K. Sakurada, T. Seki, H. Ito, CrystEngComm 2016,
18, 7217–7220; (c) M. Jin, T. Seki, H. Ito, Chem. Commun. 2016,
52, 8083–8086; (d) K. Kawaguchi, T. Seki, T. Karatsu, A. Kitamura,
H. Ito, S. Yagai, Chem. Commun. 2013, 49, 11391–11393; (e) T.
Seki, M. Jin, H. Ito, Inorg. Chem. 2016, 55, 12309–12320.
[6] (a) A. L. Balch, Gold Bull. 2004, 37, 45–50; (b) P. Pyykkö, Angew.
Chem. Int. Ed. 2004, 43, 4412–4456; (c) V. W. Yam, E. C. Cheng,
Chem. Soc. Rev. 2008, 37, 1806–1813; (d) M. J. Katz, K. Sakai, D.
B. Leznoff, Chem. Soc. Rev. 2008, 37, 1884–1895; (e) H.
Schmidbaur, A. Schier, Chem. Soc. Rev. 2008, 37, 1931–1951; (f) X.
He, V. W.-W. Yam, Coord. Chem. Rev. 2011, 255, 2111–2123; (g)
H. Schmidbaur, A. Schier, Chem. Soc. Rev. 2012, 41, 370–412.
[7] S. Yagai, T. Seki, H. Aonuma, K. Kawaguchi, T. Karatsu, T. Okura,
A. Sakon, H. Uekusa, H. Ito, Chem. Mater. 2016, 28, 234–241.
[8] (a) G. R. Krishna, M. S. R. N. Kiran, C. L. Fraser, U. Ramamurty, C.
M. Reddy, Adv. Funct. Mater. 2013, 23, 1422–1430; (b) S. Ito, T.
Yamada, T. Taguchi, Y. Yamaguchi, M. Asami, Chem. Asi. J. 2016,
11, 1963–1970; (c) T. Wang, N. Zhang, K. Zhang, J. Dai, W. Bai, R.
Bai, Chem. Commun. 2016, 52, 9679–9682; (d) D. Tu, P. Leong, Z.
Li, R. Hu, C. Shi, K. Y. Zhang, H. Yan, Q. Zhao, Chem. Commun.
2016, 52, 12494–12497; (e) P. S. Hariharan, V. K. Prasad, S. Nandi,
A. Anoop, D. Moon, S. P. Anthony, Cryst. Growth Des. 2017, 17,
146–155; (f) L. Bu, M. Sun, D. Zhang, W. Liu, Y. Wang, M. Zheng,
S. Xue, W. Yang, J. Mater. Chem. C 2013, 1, 2028–2035; (g) P. S.
Hariharan, N. S. Venkataramanan, D. Moon, S. P. Anthony, J. Phys.
Chem. C 2015, 119, 9460–9469.
Figure 6. Single-crystal structure of 3B. A set of disordered segments of
hexyl moieties is highlighted in light blue.
In summary, the lifetimes of the transient phases of aryl
gold isocyanide complexes could be tuned by modifying the
terminal structure of the TEG side chains. The new
complexes 2 and 3 possessing tolyl and hexyl groups,
respectively, at the side chain termini, showed unique
mechanochromism involving transient amorphous phases.
Furthermore, the lifetimes of the transient phases depend on
the structure of side chain termini: the rigid tolyl groups
slowed the spontaneous phase transition of 2, while the
flexible hexyl groups facilitated the spontaneous phase
transition of 3. XRD analyses also provided detailed
structural information about 2 and 3, and the markedly
different molecular arrangements in 2 upon grinding reflect
the slow reorganization from the transient 2Y phase. This
work provides new insights into the structure–property
relationship of a unique class of mechanochromic gold
complexes.
2
[9] τav is defined as: Σ(Anτn ) / Σ(Anτn).
[10] The transient phases of self-recoverable mechanochromic
compounds were reported to be amorphous: see reference 8.
[11] The molecular arrangement of 3Y has not been determined yet.
[12] Previously reported blue-emitting 1B exhibited aurophilic
interactions: see reference 7.
[13] As a preliminary result, a structurally related compound also
displayed molecular arrangements featuring an S-shaped
conformation: see Figure S5 and S6 and Table S3.
Supporting
Information
is
available
on
http://dx.doi.org/10.1246/cl.******.
References and Notes
[14] H. Ito, T. Saito, N. Oshima, N. Kitamura, S. Ishizaka, Y. Hinatsu,
M. Wakeshima, M. Kato, K. Tsuge, M. Sawamura, J. Am. Chem.
Soc. 2008, 130, 10044–10045.
[1] (a) Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605–610; (b) A. L. Balch,
Angew. Chem. Int. Ed. 2009, 48, 2641–2644; (c) Z. Chi, X. Zhang,
B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, J. Xu, Chem. Soc. Rev.
2012, 41, 3878–3896; (d) X. Zhang, Z. Chi, Y. Zhang, S. Liu, J. Xu,
J. Mater. Chem. C 2013, 1, 3376–3390; (e) C. Jobbágy, A. Deák,
Eur. J. Inorg. Chem. 2014, 2014, 4434–4449; (f) Z. Ma, Z. Wang,
M. Teng, Z. Xu, X. Jia, ChemPhysChem 2015, 16, 1811–1828; (g)
T. Seki, H. Ito, Chem. Eur. J. 2016, 22, 4322–4329; (h) Y. Sagara, S.
Yamane, M. Mitani, C. Weder, T. Kato, Adv. Mater. 2016, 28,
1073–1095; (i) P. C. Xue, J. P. Ding, P. P. Wang, R. Lu, J. Mater.
Chem. C 2016, 4, 6688–6706.
[2] (a) H. Sun, S. Liu, W. Lin, K. Y. Zhang, W. Lv, X. Huang, F. Huo,
H. Yang, G. Jenkins, Q. Zhao, W. Huang, Nat. Commun. 2014, 5,
3601; (b) K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa,
H. Sato, Y. Shimoikeda, S. Yamaguchi, J. Am. Chem. Soc. 2013,
135, 10322–10325.
[3] (a) Y. Sagara, T. Komatsu, T. Ueno, K. Hanaoka, T. Kato, T.
Nagano, J. Am. Chem. Soc. 2014, 136, 4273–4280; (b) Y. Sagara, T.
Mutai, I. Yoshikawa, K. Araki, J. Am. Chem. Soc. 2007, 129,
1520–1521; (c) J. Wu, Y. Cheng, J. Lan, D. Wu, S. Qian, L. Yan, Z.
He, X. Li, K. Wang, B. Zou, J. You, J. Am. Chem. Soc. 2016, 138,
12803–12812; (d) G. Zhang, J. Lu, M. Sabat, C. L. Fraser, J. Am.
Chem. Soc. 2010, 132, 2160–2162; (e) S.-J. Yoon, J. W. Chung, J.
Gierschner, K. S. Kim, M.-G. Choi, D. Kim, S. Y. Park, J. Am.
Chem. Soc. 2010, 132, 13675–13683; (f) B. J. Xu, Y. X. Mu, Z.
Mao, Z. L. Xie, H. Z. Wu, Y. Zhang, C. J. Jin, Z. G. Chi, S. W. Liu,
J. R. Xu, Y. C. Wu, P. Y. Lu, A. Lien, M. R. Bryce, Chem. Sci.
2016, 7, 2201–2206; (g) H. Tsuji, G. M. O. Favier, C. Mitsui, S. Lee,