A Screening Approach for the Discovery of Mechanochromic Gold(I) Isocyanide Complexes with Crystal-to-Crystal Phase Transitions

Article abstract of DOI:10.1021/jacs.6b02409

Mechanoinduced phase transitions of emissive organic crystalline materials have received much attention. Although a variety of such luminescent mechanochromic compounds have been reported, it is challenging to develop mechanochromic compounds with crystal-to-crystal phase transitions in which precise structural information about molecular arrangements can be obtained. Here, we report a screening approach to explore mechanochromic compounds exhibiting a crystal-to-crystal phase transition. We prepared 48 para-substituted (R¹) phenyl[para-substituted (R²) phenyl isocyanide]gold(I) complexes designated R¹-R² (six R¹ and eight R² substituents) and then performed three-step screening experiments. The first screening step was selection of emissive complexes under UV light, which gave 37 emissive R¹-R² complexes. The second screening step involved evaluation of the mechanochromic properties by emission spectroscopy. Twenty-eight complexes were found to be mechanochromic. The third screening step involved preparation of single crystals, reprecipitated powders, and ground powders of the 28 mechanochromic R¹-R² complexes. The changes in the powder diffraction patterns of these complexes induced by mechanical stimulation were investigated. Two compounds exhibited a crystal-to-crystal phase transition upon mechanical stimulation, including the previously reported H-H complex. Single crystals of the as-prepared and ground forms of the newly discovered CF₃-CN complex were obtained. Density functional theory calculations indicated that the mechanoinduced red-shifted emission of CF₃-CN is caused by formation of aurophilic interactions. Comparison of the crystal structures of CF₃-CN with those of the other complexes suggests that the weaker intermolecular interactions in the as-prepared form are an important structural factor for the observed mechanoinduced crystal-to-crystal phase transition.

Full text of DOI:10.1021/jacs.6b02409

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Article
A Screening Approach for the Discovery of Mechanochromic Gold(I)
Isocyanide Complexes with Crystal-to-Crystal Phase Transitions
Tomohiro Seki, Yuki Takamatsu, and Hajime Ito
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b02409 • Publication Date (Web): 12 Apr 2016
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A Screening Approach for the Discovery of Mechanochromic
Gold(I) Isocyanide Complexes with Crystal-to-Crystal Phase
Transitions
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Tomohiro Seki,*a Yuki Takamatsu,a and Hajime Ito*a
Division of Applied Chemistry and Frontier Chemistry Center (FCC), Faculty of Engineering, Hokkaido University,
Sapporo, Hokkaido 060-8628, Japan
ABSTRACT: Mechano-induced phase transitions of emissive organic crystalline materials have received much attention.
Although a variety of such luminescent mechanochromic compounds have been reported, it is challenging to develop
mechanochromic compounds with crystal-to-crystal phase transitions in which precise structural information about mo-
lecular arrangements can be obtained. Here, we report a screening approach to explore mechanochromic compounds
exhibiting a crystal-to-crystal phase transition. We prepared 48 para-substituted (R1) phenyl[para-substituted (R2) phenyl
isocyanide]gold(I) complexes R1–R2 (six R1 and eight R2 substituents) and then performed three-step screening experi-
ments. The first screening step was selection of emissive complexes under UV light, which gave 37 emissive R1–R2 com-
plexes. The second screening step involved evaluation of the mechanochromic properties by emission spectroscopy.
Twenty-eight complexes were found to be mechanochromic. The third screening step involved preparation of single crys-
tals, reprecipitated powders, and ground powders of the 28 mechanochromic R1–R2 complexes. The changes in the pow-
der diffraction patterns of these complexes induced by mechanical stimulation were investigated. Two compounds exhib-
ited a crystal-to-crystal phase transition upon mechanical stimulation, including the previously reported H–H. Single
crystals of the as-prepared and ground forms of the newly discovered CF3–CN complex were obtained. Density functional
theory calculations indicated that the mechano-induced red-shifted emission of CF3–CN is caused by formation of au-
rophilic interactions. Comparison of the crystal structures of CF3–CN with those of the other complexes suggests that the
weaker intermolecular interactions in the as-prepared form are an important structural factor for the observed mechano-
induced crystal-to-crystal phase transition.
A literature survey of the crystal structure changes of
reported mechanochromic compounds indicates that
INTRODUCTION
crystal-to-amorphous phase transitions often take place
Luminescent mechanochromism is a phenomenon
where solid and liquid crystalline materials change their
photoluminescence properties upon applying mechanical
stimulation such as grinding, ball-milling, and crushing.1
These emission color changes are caused by changes in
intermolecular interaction patterns through changes in
the molecular arrangement in crystalline and liquid-
crystalline materials. The optical responses of mecha-
nochromic compounds reveal their potential applicability
to sensing and recording devices. One of the pioneering
luminescent mechanochromic compounds is the zinc
complex reported by Kanesato in 2005.2,3 In the last dec-
ade, the number of reports on luminescent mecha-
nochromic organic and organometallic compounds has
increased markedly, and more than 300 reports have now
been published. Our group first contributed to this re-
search field in 2008, where we reported a novel aryl(aryl
upon applying mechanical stimulation to alter their emis-
sion properties (Figure 1a). This is intuitively understand-
able because mechanical forces are noncoherent and ran-
dom stimuli on solids, and can therefore alter the integri-
ty of ordered molecular arrangements. Several of our pre-
viously reported complexes also show luminescent mech-
anochromism caused by crystal-to-amorphous phase
transitions upon mechanical stimulation.4,6 For such
amorphous ground forms, in many cases, it is difficult to
investigate existing intermolecular interaction patterns,
which are important to obtain insight into the underlying
mechanism of mechanochromism. In contrast, for mech-
anochromic compounds exhibiting crystal-to-crystal
phase transitions (Figure 1b), it is possible to obtain de-
tailed crystal structures both before and after changes in
the emission color by preparing the corresponding single
crystals. However, such mechanochromic compounds are
rare: <10% of reported mechanochromic compounds show
crystal-to-crystal phase transitions.5a,5c,7 In addition, de-
tailed analysis of mechanical-stimulation-induced crystal
structure changes will help to clarify why macroscopic
mechanical stimulation initiates effective perturbation in
isocyanide)gold complex with
a reversible mecha-
nochromic response.4 Subsequently, mechanochromism
of structurally related gold isocyanide complexes has been
reported.5
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microscopic molecular arrangements, which is currently
structures, and change in properties in response to me-
unknown.8
chanical stimulation, of aryl(aryl isocyanide) gold com-
plexes strongly depend on the substituent patterns. We
thus believe that systematic investigation of this family of
complexes could uncover mechanochromic compounds
with desired properties and behavior.
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Figure 1. Schematic representations of typical solid-state
structure changes of mechanochromic compounds (repre-
sented by black rectangles) upon applying mechanical stimu-
lation.
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Figure 2. Structures of the 48 R1–R2 complexes.
Controlling the properties or behavior of mecha-
nochromic compounds is challenging. Rational molecular
design of mechanochromic compounds has hardly be-
gun.5d,9 Therefore, production of new mechanochromic
compounds strongly relies on serendipitous discovery or
derivatization of known mechanochromic compounds.
For newly developed mechanochromic compounds, it is
difficult to control their phase-transition behavior; i.e.,
change the profiles of crystal structures or optical proper-
ties. This is because mechanochromic behavior are relat-
ed to the types of crystalline structures, and their control
or engineering is known to be challenging.10 Development
of new strategies to produce mechanochromic com-
pounds with desired properties and phase-transition be-
havior may advance investigation of mechanoresponsive
luminescent materials.
Here, we present a screening approach to discover
mechanochromic compounds with desired behavior. We
attempt to discover mechanochromic compounds with
crystal-to-crystal phase transitions, which are an uncom-
mon type of mechanochromic compound. The screening
approach proposed in this work is based on three steps, as
schematically illustrated in Figure 3. We prepare 48
gold(I) isocyanide complexes R1–R2 by systematically
changing the R1 and R2 substituents (Figure 2). We first
screen the emission properties of these 48 R1–R2 com-
plexes under ultraviolet (UV) light (first screening, Figure
3), and find 37 emissive complexes. We then evaluate the
emission color changes upon grinding by a spectroscopic
technique (second screening, Figure 3). This gives 28 R1–
R2 complexes with prominent luminescent mecha-
nochromism. To evaluate the profiles of the crystal struc-
ture changes of the 28 mechanochromic R1–R2 complexes
upon mechanical treatment, we perform powder and sin-
gle-crystal X-ray diffraction (XRD) analyses (third screen-
ing, Figure 3). In addition to the previously reported H–H
complex, we identify a new mechanochromic complex,
CF3–CN, exhibiting a crystal-to-crystal phase transition
when its emission color changes upon mechanical stimu-
lation. We further investigate the crystal structures, emis-
sion, and mechanochromic properties of the CF3–CN
complex to understand the mechanism of mecha-
nochromism.
We have intensively investigated aryl gold isocyanide
complexes exhibiting a variety of intriguing mecha-
nochromic properties.4−6,7e For example, a phenyl(phenyl
isocyanide)gold complex (Figure 2, R1 = R2 = H)5a,5e and
phenyl(dimethylphenyl isocyanide)gold complex5b,5e show
mechano-induced single-crystal-to-single-crystal phase
transitions with clear emission color changes, indicating
the potential of this class of gold complexes. Both mole-
cules also show typical mechanochromism upon strong
grinding by ball-milling through crystal-to-crystal phase
transitions (Figure 1b).5a,7e Although the molecular struc-
tures of these complexes are similar, the phase transition
characteristics of these two complexes are con-
trasting.5a,5b,5e,11 This indicates that the mechanochromic
properties, including the emission properties, solid-state
Figure 3. Schematic representation of the three-step screening approach used to identify mechanochromic
compounds with crystal-to-crystal (C-to-C) phase transitions.
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RESULTS AND DISCUSSION
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Synthesis
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We prepared 48 para-substituted phenyl(para-
substituted phenyl isocyanide)gold(I) complexes R1–R2
from fourteen precursors (Scheme 1). The R1–R2 complex-
es were prepared by the reaction of para-substituted phe-
nyl zinc compounds 1-R1 (R1 = OMe, Me, H, Cl, CF3, and
CN) and chloro(para-substituted phenyl isocyanide)
gold(I) complexes 2-R2 (R2 = NMe2, OMe, Me, H, Cl, CF3,
CN, and NO2) in tetrahydrofuran (THF) at 0 °C (Scheme 1,
see Supporting Information for more detail). The isolated
yields of the R1–R2 complexes ranged from 29% to 92%.
The 1-R1 compounds were prepared by the reaction of the
corresponding para-substituted phenyl Grignard reagents
and ZnBr2, except for 1-CF3 and 1-CN compounds (see
Supporting Information). 1-CF3 and 1-CN were prepared
according to the procedure reported by Knochel, in which
para-trifluoromethylphenyl iodide or para-cyanophenyl
iodide, respectively, was reacted with Zn powder in the
presence of LiCl.12 The 2-R2 compounds were prepared
according to reported procedures5a from commercially
available para-substituted aniline derivatives (see Sup-
porting Information).
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Scheme 1. Synthesis of the 48 R1–R2 complexes.
Mechanochromic Properties
As a first screening step, we observed the photolumi-
nescence of powder samples of the 48 R1–R2 complexes.
Eleven of the complexes were not emissive both before
and after mechanical stimulation. We used as-prepared
solid samples of the 48 R1–R2 complexes as their un-
ground forms, and their photographs were taken under
UV illumination. We then ground the samples using a
ball-mill at a rate of 4600 rpm for 15–30 min. Photographs
of the resulting ground forms were then taken under UV
illumination. These photographs are shown in Table 1, in
which the powders on the left- and right-hand sides of
each panel correspond to the as-prepared (unground) and
ground samples of the R1–R2 complexes, respectively
(hereafter, the ground forms of the R1–R2 complexes are
referred to as R1–R2ground). The ground and unground
Table 1. Photographs of the 48 R1–R2 complexes under UV illumination. In each panel, unground and ground
powders are on the left- and right-hand sides, respectively.
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forms of R1–R2 complexes containing NMe2 and NO2
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groups as the R2 moiety (R1–NMe2 and R1–NO2 complex-
es), excluding Cl–NMe2, do not show photoluminescence
in the visible region (left- and right-hand columns of Ta-
ble 1). These complexes are mostly nonemissive, probably
because of quenching through photoinduced electron
transfer.13 Thus, we conclude that these eleven complexes
do not have mechanochromic properties. We used the
other 37 R1–R2 complexes in the second screening step.
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We next evaluated the mechanochromic proper-
ties of the 37 luminescent R1–R2 complexes by naked-eye
observation under UV light and emission spectroscopy
(second screening, Figure 3). The photographs in Table 1
reveal that most of these R1–R2 complexes show clear
emission color changes upon mechanical stimulation. We
also measured excitation and emission spectra of the un-
ground and ground forms of the R1–R2 complexes (all of
the spectra are summarized in Figure S1–S6 and Table S1).
Based on the photographs under UV illumination (Table
1) and spectral shifts in the emission spectra of the as-
prepared and ground R1–R2 forms (Figure 4 and S1–S6), 28
complexes show mechanochromic properties. For exam-
ple, the H–OMe complex is mechanochromic and shows
an emission color change from blue to green upon ball-
milling (Table 1). The emission maximum of the H–OMe
complex upon excitation at 365 nm exhibits a red-shift
from 429 to 522 nm (Figure 4a). As another example, the
OMe–CN complex shows a green-to-yellow emission col-
or change upon grinding (Table 1) and the emission max-
imum red-shifts by 60 nm (from 532 to 592 nm, Figure
4b). Similarly, the other 26 R1–R2 complexes show emis-
sion color changes upon mechanical stimulation (Figure
S1–S6).14 Nine of the R1–R2 complexes, OMe–OMe, OMe–
Me, OMe–H, Cl–NMe2, Cl–OMe, Cl–Me, Cl–H, Cl–Cl,
and Cl–CF3, show only very small spectral changes upon
grinding (Figure S1 and S4). For example, the spectral
changes of Cl–Me and OMe–OMe are shown in Figure 4c
and d, respectively, in which the spectra are nearly the
same. Indeed, emission color changes were not observed
by the naked eye (Table 1). Thus, we determined that the-
se nine complexes are not mechanochromic.15 The re-
maining 28 mechanochromic R1–R2 complexes were in-
vestigated in the third screening step.
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Figure 4. Emission spectra of (a) H–OMe, (b) OMe–CN, (c)
Cl–Me, and (d) OMe–OMe following excitation at 365 nm.
For (a), blue and green lines are the emission spectra of the
unground and ground forms, respectively. For (b), green and
yellow lines are the emission spectra of the unground and
ground forms, respectively. For (c) and (d), colored and black
lines are the emission spectra of the unground and ground
forms, respectively.
Crystal Structure Changes
For the 28 mechanochromic R1–R2 complexes, we
performed a third screening: comparison of their molecu-
lar arrangements upon mechanochromism using XRD. To
determine the mechanochromic R1–R2 complexes with
crystal-to-crystal phase transitions, we compared the
powder XRD (PXRD) patterns before and after mechani-
cal stimulation. If the R1–R2 complexes show crystal-to-
crystal phase transitions, the PXRD patterns before and
after mechanical stimulation-induced emission color
changes will be different. However, PXRD patterns of un-
ground forms are difficult to obtain because unground
forms are sensitive to mechanical stress applied during
the measuring procedure of PXRD analysis. The mecha-
no-sensitive nature of our complexes prevented placing
the solid samples uniformly on substrates by pushing or
pressing. Instead, in this study, we decided to use simu-
lated powder patterns after single-crystal XRD analyses as
the PXRD patterns of the unground forms of R1–R2. For
R1–R2ground, we can easily obtain diffraction patterns by
typical PXRD measurements because these samples are
mechanically insensitive.
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Figure 5. Crystal structures of the 27 single-crystal R1–R2 complexes. The insets show photographs of the single crystals taken
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under UV illumination.
Recrystallization to prepare single crystals of the un-
ground forms of the R1–R2 complexes involved layering
MeOH or hexane as a poor solvent on top of a solution of
each R1–R2 complex in CH2Cl2 (typically, 10 mg R1–R2 in 1
mL CH2Cl2). Storing in a freezer for approximately 12 h
gave crystal samples at the interface between the two sol-
vents. After repeated trials, we obtained 27 single crystals
for the 28 mechanochromic R1–R2 complexes (Table S2).
These single crystals were all of suitable quality for single-
crystal analysis, as shown in Table S3 (R1 values from
3.22% to 9.90%). For the CN–H complex, single-crystal
analysis could not be performed because the crystals were
too small and thin (Table S2). Instead, we prepared a re-
precipitated powder of CN–H from CH2Cl2 and hexane
and its PXRD pattern was recorded to gather information
about the crystalline structure of the unground phase.
The emission spectra of the single crystals and reprecipi-
tated powders of the 28 complexes are almost the same as
those of the powder samples (Figures S8–S12 and Tables
S2 and S4). This indicates that the molecular arrange-
ments of the R1–R2 single crystals and reprecipitated
powders are the same as those of the powder samples.16
The space groups of these single crystals are mostly P-1
(nineteen single crystals), except for OMe–CF3 belonging
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to P21/n, OMe–CN, CF3–CN, and CN–CN belonging to
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P21/c, and H–Me and CF3–Cl belonging to C2/c. In the
single-crystal structures (see the six molecules represent-
ed by the space-filling model in Figure 5), head-to-tail
stacking with π-π stacking arrangements to form 1-D col-
umns are typically observed. In some single crystals, such
as H–Me and H–H, molecules have head-to-tail stacking
with CH/π interactions (Figure 5). However, the mole-
cules in CF3–CN have a stacking-like arrangement with
rotational displacement, indicating the presence of weak
intermolecular interactions.17
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Careful comparison of the simulated and PXRD
patterns of the ground forms shows that most of the
mechanochromic R1–R2 complexes undergo crystal-to-
amorphous phase transitions upon mechanical stimula-
tion. For example, the simulated pattern of unground
CF3–Cl and the PXRD pattern of CF3–Clground are shown in
Figure 6a. Clearly, the intensity of the diffraction pattern
of CF3–Clground is low and different from that of the un-
ground form, indicating that CF3–Clground is an amor-
phous phase. As another example, the diffraction patterns
of Me–Cl are compared in Figure 6b. The peak positions
of the diffractions of Me–Clground (black line in Figure 6b)
match well with those of the unground form (blue line in
Figure 6b) even though the remaining peak intensity is
relatively high. This may be caused by either the absence
of a crystal structure change upon grinding or by the co-
existence of the original crystal structure and a partially
formed amorphous phase in the ground form. The former
possibility is less likely than the latter because solid-state
emission properties are typically strongly connected to
the molecular arrangements in solid samples.18,19 Instead,
we consider that Me–Cl shows a crystal-to-(partial)
amorphous phase transition when mechanical stimula-
tion changes the emission properties. Indeed, mecha-
nochromic compounds without complete phase trans-
formation have been reported.7e,19,20 As shown in Figure
S13–S19, out of 28 R1–R2 complexes, 26 R1–R2 complexes
show this type of change in their PXRD patterns upon
grinding.21 Only the CF3–CN and H–H complexes do not
fit into this category, which will be considered in the fol-
lowing paragraph.
Figure 6. Simulated powder patterns of unground forms
derived from single crystals (upper lines) and PXRD patterns
of ground powders (lower lines) of (a) CF3–Cl, (b) Me–Cl,
and (c) CF3–CN complexes.
In addition to the previously-reported H–H com-
plex,5a,5e we discovered that CF3–CN shows a crystal-to-
crystal phase transition upon applying mechanical stimu-
lation. Figure 6c depicts the simulated powder pattern of
a CF3–CN single crystal (green line) and PXRD pattern of
the corresponding ground form (black line). The ground
form of CF3–CN exhibits an intense diffraction pattern in
which the peak positions are different from those in the
pattern of the unground form. This indicates that CF3–CN
undergoes a crystal-to-crystal phase transition upon me-
chanical grinding. As illustrated in Figure S15d, the previ-
ously reported H–H complex5a also exhibits different dif-
fraction patterns before and after mechanical stimulation.
These observations indicate that these two complexes
show mechanochromism based on crystal-to-crystal
phase transitions. Further discussion of the structural
characteristics of these two crystals and comparison with
other crystals is provided below. There are fewer than 20
reports of this type of crystal structure change for mecha-
nochromic compounds.5a,5c,7
Mechanochromism of the CF3–CN Complex
The changes in the optical properties of the CF3–CN
complex (Figure 7a) induced by grinding were investigat-
ed in more detail. Because the optical properties of the
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H–H complex have already been reported in detail,5a we
only investigated the CF3–CN complex here. As shown in
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To determine the origin of the red-shifted emission
band of the CF3–CN complex upon grinding, the changes
in the crystal structure were investigated in detail. Two
different single crystals that correspond to the unground
and ground forms of a material can provide molecular-
level information about its crystal-to-crystal phase transi-
tion. Preparation of a single crystal of the unground form
of CF3–CN has already been described. Because this single
crystal shows green emission (Figure 8a, i), hereafter, it
will be referred to as CF3–CNG. The structural information
obtained from the single crystal of CF3–CNG represents
that of the unground powder form of CF3–CN because
they show similar characteristics in their excitation and
emission spectra (Figure 7c and d, respectively). CF3–CNG
crystallizes in monoclinic P21/c space group [R1 = 9.22%,
wR2 = 20.62%, GOF = 1.174, a = 16.125(2) Å, b = 22.172(3) Å,
c = 7.618(1) Å, α = γ = 90°, β = 95.115(4)°, Z = 8, V =
2712.8(7) Å3, Dcalc = 2.302 g cm−3] (Table S3). Molecular
layers are observed along the ab plane (Figure 8a, iv). Ap-
proximately perpendicular to this layer, molecules of CF3–
CNG are arranged with slight rotational displacement
without typical head-to-tail orientations (Figure 8a, ii and
iii). Along this direction, molecules in the lattice only
form very weak CH/π interactions (distance between H
atom and centroid of the phenyl ring: 2.908 Å). Other
typical intermolecular interactions in related single crys-
tals of gold isocyanide complexes do not exist, such as π-π
and aurophilic interactions.5a,5b,24 It should be noted that
the shortest Au···Au distance is 3.706 Å, which is much
longer than the sum of the van der Waals radii of the Au
atoms, indicating the absence of aurophilic interactions in
CF3–CNG.
Figure 7b, the CF3–CN complex displayed mecha-
nochromism in which the green-emitting unground form
transformed to a yellow-emitting ground powder after
applying mechanical stimulation.22,23 Figure 7c and d show
excitation and emission spectral analyses of the CF3–CN
complex obtained before and after mechanical stimula-
tion, respectively. Before grinding, the CF3–CN complex
exhibits a broad emission band centered at 510 nm (Fig-
ure 7d). After grinding, CF3–CNground shows a 72-nm red-
shift and the emission maximum is at 582 nm. The excita-
tion spectra of the CF3–CN complex also show red-shifted
bands following mechanical stimulation. The unground
CF3–CN complex shows a broad excitation spectral band
up to 420 nm (green solid line in Figure 7c). After grind-
ing, CF3–CNground shows an excitation spectrum extending
to 500 nm (yellow solid line in Figure 7c). We also con-
firmed that the average emission lifetime decreased (τav:
from 47.1 to 0.756 µs) and the emission quantum yield
increased (Φem: from 7.86 to 23.1%) upon mechanical
stimulation (Figure S23 and Table S5).
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Figure 7. (a) Structure of the CF3–CN complex. (b) Photo-
graphs of the unground and ground forms of the CF3–CN
complex taken under UV illumination. (c) Excitation spectra
of powder samples of CF3–CN before (solid green line, λem
=
522 nm) and after (solid yellow line, λem = 582 nm) applying
mechanical stimulation, and single crystals of CF3–CNG
(dashed green line, λem = 510 nm) and CF3–CNY (dashed yel-
low line, λem = 580 nm). (d) Emission spectra of powder sam-
ples of CF3–CN before (solid green line) and after (solid yel-
low line) applying mechanical stimulation, and single crys-
tals of CF3–CNG (dashed green line) and CF3–CNY (dashed
yellow line). λex is 365 nm, except for CF3–CNG (λex = 370 nm)
Figure 8. (i) Photographs taken under UV light and (ii)–(iv)
single-crystal structures of (a) CF3–CNG and (b) CF3–CNY.
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We also prepared a yellow-emitting single crystal of
Page 8 of 12
1
CF3–CNY by very slow crystallization (>1 day). Comparison
of the powder patterns (Figure S25) and excita-
tion/emission spectra (Figure 7c and d) of CF3–CNY and
CF3–CNground indicates that the single crystal of CF3–CNY
corresponds to its ground form CF3–CNground. CF3–CNY
crystallizes in orthorhombic Pna21 space group [R1 =
4.82%, wR2 = 13.84%, GOF = 1.063, a = 9.3279(4) Å, b =
19.4405(8) Å, c = 15.1865(8) Å, α = β = γ = 90°, Z = 8, V =
2753.9(2) Å3, Dcalc = 2.268 g cm−3] (Table S3). Different
from CF3-CNG, layer-like arrangements are not observed:
the molecular arrangements of CF3–CNY shown in Figure
8b iv diverge from one flat plane. Four derived molecules
of CF3–CNY are shown in Figure 8b ii and iii; they are al-
most randomly arranged without typical stacking ar-
rangements. However, aurophilic interactions connect
these molecules with Au···Au distances of 3.119 or 3.281 Å.
All of these results indicate that upon mechanochromism
of the CF3–CN complex, a crystal-to-crystal phase transi-
tion occurs and aurophilic interactions are formed in the
daughter phase.
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Figure 9. Frontier molecular orbitals and the corresponding
Density functional theory (DFT) calculations based
on the single-crystal structures of CF3–CNG and CF3–CNY
reveal that the red-shifted emission of CF3–CN upon
grinding is caused by the formation of new ordered crys-
talline structures containing aurophilic interactions. We
performed time-dependent DFT (TD-DFT) calculations
(B3LYP/SDD) of CF3–CNG and CF3–CNY based on the te-
tramers derived from the single-crystal structures (Figure
9). The results indicate that the lowest unoccupied mo-
lecular orbital (LUMO) energy levels of CF3–CNG and
CF3–CNY are very similar (−3.13 and −3.10 eV, respectively).
In contrast, the energy levels of the highest occupied mo-
lecular orbital (HOMO) are remarkably different (−6.88
and −6.30 eV, respectively). Destabilization of the HOMO
energy level is typically observed in gold complexes with
formation of aurophilic interactions.25 The singlet excita-
tion energy levels of CF3–CNG and CF3–CNY obtained by
TD-DFT calculations (B3LYP/SDD) also qualitatively
match the experimental values (Figure S26). This indi-
cates the underlying mechanism of luminescent mecha-
nochromism of CF3–CN: mechanical stimulation induces
phase transition to a new crystal phase with aurophilic
interactions, which induces the red-shifted emission band.
We have previously reported structurally related aryl-
gold(I) isocyanide complexes that exhibit red-shifted
emission upon grinding because of the formation of new
crystal structures with aurophilic interactions.5a,5c This
mechanistic insight about the mechanochromism of CF3–
CN was obtained because single crystals of the unground
and ground forms were able to be grown.
energy potentials obtained by TD-DFT calculations of te-
tramers derived from the single-crystal structures of CF3–
CNG and CF3–CNY.
Structural Characteristic for Crystal-to-Crystal Phase
Transitions
Through the present screening approach, we identi-
fied one structural characteristic of mechanochromic
compounds with crystal-to-crystal phase transitions. As
examples of molecular arrangements of unground R1–R2,
the single-crystal structures of CN–Me and CF3–CNG are
shown in Figure 10. CN–Me forms 1-D columns through π-π
stacking interactions. Such molecular arrangements are
often observed in other single crystals of R1–R2 (Figure 5).
In contrast, CF3–CN (CF3–CNG) has molecular arrange-
ments with a slight rotational displacement around the
axis along the column direction. As a result, only a very
weak CH/π interaction (distance between H atom and
centroid of a phenyl ring: 2.908 Å) exists in CF3–CNG, and
the rotational displacement prevents formation of other
intermolecular interactions (π-π and aurophilic interac-
tions). We carefully checked existing intermolecular in-
teractions (CH/π, π-π, and aurophilic interactions) in the
single-crystal structures of the 27 unground forms; the
results are summarized in Table S6. Compared with CF3–
CNG, the other 26 R1–R2 single-crystal structures have
more prominent intermolecular interactions, including
CH/π, π-π, and/or aurophilic interactions, between
neighboring molecules (Table S6). This suggests that the
packing arrangement of CF3–CNG with relatively weak
intermolecular interactions may be suitable for the mech-
ano-induced phase transition to another crystal phase
with stronger intermolecular interactions, such as CF3–
CNY with strong intermolecular interactions including
aurophilic ones. These findings indicate that if such meta-
stable crystalline arrangements with weak intermolecular
interactions can be designed, we should be able to pro-
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We prepared 48 gold isocyanide complexes R1–R2 by
duce mechanochromic compounds with crystal-to-crystal
reacting six para-substituted phenyl metal precursors 1-R1
and eight chloro(para-substituted phenyl isocyanide)
gold(I) 2-R2. The screening approach to find crystal-to-
crystal phase transitions involved three screening steps.
The first screening involved observing the photolumines-
cence of the complexes, and 37 R1–R2 complexes were
found to be emissive. The second screening involved ap-
plying mechanical stimulation to the 37 R1–R2 complexes,
and 28 R1–R2 complexes were mechanochromic. For the
28 R1–R2 complexes, single crystals, reprecipitated pow-
ders, and ground powders were prepared. We evaluated
the changes in their diffraction patterns before and after
mechanical stimulation (third screening). We found that
two complexes, including the previously reported H–H
complex, exhibited mechanochromism with crystal-to-
crystal phase transitions. The mechanochromic properties
of the second complex CF3–CN were investigated in de-
tail. Because we were able to obtain accurate information
about the molecular arrangements of the unground and
ground forms of CF3–CN, we found that formation of au-
rophilic interactions induced red-shifted emission upon
mechanical stimulation. By comparing the unground
crystal structure of CF3–CN and those of the other com-
plexes, weaker intermolecular interactions were found in
unground CF3–CN than in the other complexes, which
would be an important factor affecting its mechano-
induced crystal-to-crystal phase transition. The insight
obtained in this study will be useful for future design of
this type of material. The present screening approach
could be expanded to ortho- and meta-substituted gold
isocyanide analogues, which would lead to development
of mechanochromic compounds with unique properties,
such as mechano-triggered single-crystal-to-single-crystal
phase transitions.5a,5b,5e,11 This work presents a promising
screening approach for the discovery of mechanochromic
materials with desired phase transitions/properties.
1
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phase transitions. One such example is a metastable dipo-
lar amphiphile recently published by the research group
including us.7d It should be noted that this is not the only
structural requirement of mechanochromic compounds
with crystal-to-crystal phase transitions because previous-
ly reported H–H5a,5e does not have this type of single-
crystal structure (Figure 5). The unground phase of H–H
forms head-to-tail dimers in the single crystal with prom-
inent CH/π interactions (2.815 Å).
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Figure 10. Comparison of the single-crystal structures of
CN–Me, displaying π-π stacking interactions, and CF3–CNG
showing rotational displacement.
The ground phases of both CF3–CN and H–H com-
plexes, which show crystal-to-crystal phase transitions,
contain aurophilic interactions and are thermodynamically
more stable than the corresponding unground phases. One
similarity between the crystal structures of CF3–CN and
H–H is the formation of aurophilic interactions in the
ground phases. Because aurophilic interactions are as
strong as hydrogen-bonding interactions,25 they can pro-
vide thermodynamic energy gain in the crystalline lattice
after mechano-induced phase transitions of CF3–CN and
H–H. Indeed, differential scanning calorimetry analyses
confirmed that the phase transitions of CF3–CN (Figure
S24) and H–H5a,e produced thermodynamically more sta-
ble phases. This is contrast to our previously reported
gold complex displaying a mechano-induced crystal-to-
amorphous phase transition in which mechanical stimula-
tion produced a thermodynamically less stable phase.4 In
this study, we found eight R1–R2 complexes that form
aurophilic interactions in the unground phases (Table S6).
These eight phases may be thermodynamically stable
enough that there are no other crystal structures that are
more stable; therefore, these complexes show crystal-to-
amorphous phase transitions upon mechanochromism.
These findings may indicate that thermodynamically sta-
ble molecular arrangements in unground phases of mech-
anochromic compounds are not suitable for crystal-to-
crystal phase transitions.
ASSOCIATED CONTENT
Supporting Information. Spectra, X-ray crystal graphics,
thermal analyses, quantum chemical calculation data and
other additional information. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
seki@eng.hokudai.ac.jp; hajito@eng.hokudai.ac.jp
Present Addresses
Kita 13 Nishi 8 Kita-ku, Sapporo, Hokkaido, 060-8628, Japan
Author Contributions
All of the authors have given their approval to the final ver-
sion of the manuscript.
ACKNOWLEDGMENT
This work was financially supported by the MEXT (Japan)
program “Strategic Molecular and Materials Chemistry
through Innovative Coupling Reactions” of Hokkaido Uni-
versity; Building of Consortia for the Development of Human
SUMMARY
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Page 10 of 12
Resources in Science and Technology, "Program for Fostering
Researchers for the Next Generation"; and by JSPS KAKENHI
Grant Number 15H03804, 15K13633, and 26810042. We also
thank the Frontier Chemistry Center Akira Suzuki “Laborato-
ries for Future Creation” Project, Hokkaido University.
Des. 2003
Chem. Rev. 2014
(11) Seki, T.; Ito, H. Chem. Eur. J. 2016
(12) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.;
Knochel, P. Angew. Chem. Int. Ed. 2006 45, 6040–6044.
,
3
, 873–885; (e) Cruz-Cabeza, A. J.; Bernstein, J.
1
2
3
4
5
6
7
8
,
114, 2170–2191.
,
22, 4322–4329.
,
(13) The excited states of luminophores containing ami-
no- and nitrobenzene rings are often quenched through
acceptor- and donor-excited photoinduced electron trans-
fer, respectively, because of the strong electron-donating
and -withdrawing abilities of the substituents. For an ex-
ample of quenching of luminophores containing amino-
benzene groups, see: (a) Seki, T.; Yagai, S.; Karatsu, T.;
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emission colors different from those shown in Table 1.
(16) That the single crystals and reprecipitated powders
correspond to unground phases is also supported by their
mechanochromic nature: mechanical stimulation of these
single crystals and reprecipitated powders changes their
emission colors and the corresponding ground powders
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the ball-milling time was extended. For example, the re-
sults obtained for H–CF3 are shown in Figure S20. Diffrac-
tion intensities of H–CF3 gradually decreased when ball-
milling time was prolonged from 15 to 60 min without
newly emerged diffraction peaks. This result indicates that
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phase and some amorphous domains, and that the con-
tent of amorphous domains increased upon prolonged
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should be responsible for the new emission properties of
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change and DSC analysis (Figure S24). This observation
PXRD patterns of its unground form. Comparison of the
1
2
3
4
5
6
7
8
diffraction patterns of unground and ground phases of
CN–H indicates that this complex shows a crystal-to-
amorphous phase transition (Figure S19a). We also con-
firmed that that reprecipitated powder indeed shows
mechanochromism: its emission color changed from
green to yellow.
indicates that the yellow-emitting phase of CF3–CN (cor-
responding to its ground phase) is thermodynamically
more stable than the green-emitting one. The yellow-
emitting phase of CF3–CN does not show a thermally in-
duced reverse phase transition back to the original phase,
which was confirmed by observation of photolumines-
cence and DSC analysis (Figure S24).
(22) The solid-state emission properties of CF3–CN are
very different from those of its monomeric state in CH2Cl2
solution (Figure S21 and S22 and Table S5). This indicates
that intermolecular interactions in the crystalline lattices
of CF3–CN strongly affect the optical properties of this
(24) Seki, T.; Kurenuma, S.; Ito, H. Chem. Eur. J. 2013, 19,
9
16214–16220.
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.
,
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1931–1951.
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