Trimethylsilyl Group Assisted Stimuli Response: Self-Assembly of 1,3,6,8-Tetrakis((trimethysilyl)ethynyl)pyrene

Article abstract of DOI:10.1021/acs.organomet.6b00781

Substitution of 1,3,6,8-tetraethynylpyrene with sterically bulky trimethylsilyl groups enabled four modes of molecular packing, two polymorphs (triclinic system (Y-form) and "loose" crystal Col_ho (O_K-form)), a "rigid" liquid crystalline phase Col_ho (O_C-form), and an amorphous phase (O_A-form), which emitted fluorescence at different wavelengths under UV light. The four phases could be interconverted by physical stimuli such as heating, grinding by mortar and pestle, and exposing to the vapor of organic solvents.

Full text of DOI:10.1021/acs.organomet.6b00781

Article
pubs.acs.org/Organometallics
Trimethylsilyl Group Assisted Stimuli Response: Self-Assembly of
1,3,6,8-Tetrakis((trimethysilyl)ethynyl)pyrene
Feng Xu,† Takanori Nishida,† Kenta Shinohara,† Lifen Peng,† Makoto Takezaki,† Takahiro Kamada,‡
Haruo Akashi,‡ Hiromu Nakamura,§ Kouki Sugiyama,§ Kazuchika Ohta,§ Akihiro Orita,*,†
and Junzo Otera†
‡
†Department of Applied Chemistry and Biotechnology and Research Institute of Natural Sciences, Okayama University of Science,
1-1 Ridai-cho, Kita-ku, Okayama 700 0005, Japan
§Smart Material Science and Technology, Interdisciplinary Graduate School of Science and Technology, Shinshu University, 1-15-1
Tokida, Ueda 386 8567, Japan
S
* Supporting Information
ABSTRACT: Substitution of 1,3,6,8-tetraethynylpyrene with sterically bulky trime-
thylsilyl groups enabled four modes of molecular packing, two polymorphs (triclinic
system (Y-form) and “loose” crystal Colho (OK-form)), a ”rigid” liquid crystalline phase
Colho (OC-form), and an amorphous phase (OA-form), which emitted fluorescence at
different wavelengths under UV light. The four phases could be interconverted by
physical stimuli such as heating, grinding by mortar and pestle, and exposing to the
vapor of organic solvents.
material, which enabled a response to various physical stimuli
in the solid state (Scheme 1). Compound 1 was composed of
INTRODUCTION
■
Because trialkyl(aryl)silyl groups exhibit attractive steric and
electronic effects, they play a pivotal role in organic synthesis
and organic materials.1 For instance, the bulky trialkylsilyl
groups are used as protecting groups of terminal ethynes and
hydroxyl groups, and the steric bulkiness of the silyl group can
be tuned by changing the substituents on the silicon atom to
realize selective protection and deprotection.1c The electron-
withdrawing effect of the silyl groups stabilizes α-carbanions
and facilitates Peterson olefination.1d This electronic effect is
enhanced by substituting phenyl group(s) on the silicon atom,
and regioselective hydroalumination and carbolithiation were
achieved in 3-hydroxy-1-silylpropynes.1e Although a number of
applications of silicon chemistry have been reported so far, a
new silicon reagent is still being developed.1f,g Silicon-
containing organic materials have also attracted great attention.
Substitution of fluorophores with trialkylsilyl groups enhanced
the emission intensity,1h and in silole, a participation of the
antibonding orbital of the C−Si bond in the butadiene 4π
system stabilized the LUMO of silole to achieve a small
HOMO−LUMO band gap.1i,j In order to expand the utility of
silicon chemistry in organic materials, we planned to take
advantage of the steric bulkiness of trialkylsilyl group in the
control of morphologies of a silicon-substituted fluorophore
which possesses stimuli-responsive properties in the solid
state.2 So far several stimuli-responsive pyrene derivatives have
been reported. These pyrenes underwent phase transitions
when external stimuli emerged (for instance, cubic to columnar
phase2f and columnar assembly to amorphous2g), and their
fluorescent properties were tuned in accordance with the
resulting molecular assemblies. We therefore designed 1,3,6,8-
tetrakis((trimethylsilyl)ethynyl)pyrene (1) as a fluorescent
Scheme 1. Structure and Plausible (Meta)stable States of 1
an expanded π system (tetraethynylpyrene) and four bulky
substituents (trimethylsilyl groups). In the solid state of 1, the
expanded pyrene π system would enable intense fluorescence,
and its fluorescent color could be tuned by a change in the
molecular assembly and/or the formation of a pyrene excimer.3
As has been well documented in crystals, organic molecules
tend to pack tightly to fill spaces; therefore, we envisaged that 1
would align linearly to achieve efficient π−π stacking between
disciform pyrene-containing π systems (forms A−C in Scheme
1).4,5 Although forms A−C might be discriminative stable/
metastable states of 1, if free rotation of the bulky trimethylsilyl
groups enabled smooth slippage of the pyrene cores,
interconversion of 1 among A−C would be triggered easily
Received: October 10, 2016
© XXXX American Chemical Society
A
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by physical stimuli, leading to a change in the fluorescent color
and intensity.6,7
Herein, we report that 1 shows four (meta)stable states, the
Y-, OC-, OA-, and OK-forms, which exhibit different fluorescent
colors. The four states of 1 can be reversibly interconverted by
physical stimuli such as heating, shearing, and exposure to
organic solvents.
Surprisingly, in the solid state, four assembly modes which
were different in morphology were observed, as summarized in
Table 1. When 1 was subjected to recrystallization and
reprecipitation, two different types of solventless forms were
obtained in accordance with the solvents used: yellow prisms
(Y-form) by recrystallization from acetone and orange narrow
needles (OC-form) by reprecipitaion from hexane.9 The Y- and
OC-forms exhibited different UV−vis absorption profiles
(diffuse reflectance method), showing absorption termini at
550 and 620 nm, respectively (Table 1 and Figure 2). When the
Y- and OC-forms were ground with a mortar and pestle, an
orange powder bearing an absorption terminus at 605 nm (OA-
form) was obtained.
When the the OA-form was heated over 90 °C, the OA-form
was transformed to another orange powdery state, the OK-
form, the absorption terminus of which was observed at 620
nm.10 On irradiation with UV light, all of the solid Y-, OC-, OA-,
and OK-forms emitted strong fluorescence, and the Emax values
were observed at 524 nm (ΦF = 0.66), 600 nm (0.49), 586 nm
(0.57), and 594 nm (0.17), respectively (Figure 2). The OC-,
OA-, and OK-forms exhibited rather long fluorescence lifetimes
(>48 ns) diagnostic of the formation of an excimer in
photoexcitation.11 Because the Y-form emitted fluorescence
at a wavelength longer than that in solution and exhibited a
shorter lifetime (2.2 ns), we concluded that an intermolecular
interaction such as exciton coupling would occur in the
emission of the Y-form.
RESULTS AND DISCUSSION
■
Synthesis and Structure Elucidations. First, 1 was
synthesized from commercially available pyrene in two steps:
the tetrabromination of pyrene and successive Sonogashira
coupling of the resulting tetrabromide with (trimethylsilyl)-
acetylene (Scheme 2).8 Compound 1 was easily purified by
column chromatography on silica gel using hexane as the eluent
and, after evaporation, was isolated as an orange powder in 89%
yield.
Scheme 2. Synthesis of 1
In order to shed light on the morphologies of 1 in the four
solid states, all of the forms were subjected to X-ray diffraction
analyses. Recrystallization of 1 from acetone afforded single
crystals of the Y-form suitable for X-ray structure analysis. The
crystallographic data and ORTEP drawing of 1 are shown in
Table 1 and Figure 3, respectively.12 The Y-form was depicted
as form B in Scheme 1 and exhibited a triclinic crystal system
Compound 1 showed no clear melting point or decom-
position up to 270 °C, and thermogravimetric analysis (TGA)
demonstrated that 1 underwent thermal decomposition above
296.0 °C (Figure S29 in the Supporting Information). In Figure
1 are shown normalized UV−vis absorption (CH2Cl2, 1.0 ×
(P1 (No. 2)) with the center of symmetry residing on the
̅
midpoint of the biphenyl unit. In the crystal of the Y-form, only
half-planes of the pyrene cores were overlapped to the
proximate molecule, and one of the three methyl groups on
each silyl group was inserted between silyl groups of the
proximate molecule. Because of the insufficient overlapping of
the proximate molecules, Y-form could not form an excimer in
the photoexcited state, leading to the monomeric emission on
irradiation by UV light.
When a powder X-ray diffraction (XRD) analysis was
undertaken, the OC-form demonstrated clear reflection peaks
(peaks 1−10) at room temperature (Figure 4a). This result
indicated a Colho arrangement of 1, depicted as form A in
Scheme 1. The sharp peaks 1−9 were observed at 16.0, 9.2, 8.0,
6.1, 5.4, 4.6, 4.5, 4.1, and 3.7 Å with a reciprocal d spacing ratio
of 1:√3:2:√7:3:√12:√13:4:√19 that corresponded to
(100), (110), (200), (210), (300), (220), (310), (400), and
(320) reflections, respectively. The reflections indicate a two-
dimensional hexagonal lattice (a = 18.5 Å) and showed good
agreement with the simulated pattern. Peak 10 in the high-angle
region was assigned to a stacking distance (h = 3.37 Å), and this
distance was appropriate as an intracolumnar distance of closely
packed aromatic rings.13 Although we assigned the OC-form as
a discotic columnar mesophase, the OC-form showed no
spontaneous fluidity as for conventional discotic liquid crystals
and could be isolated as a crystal-like solid by reprecipitation at
room temperature. Even at higher temperature, the OC-form
showed no fluidity14 and decomposed at 296 °C. In Figure 4a,
the OC-form exhibited sharp diffractions, as did the crystalline
Figure 1. UV−vis absorption and photoluminescence spectra of 1 in
CH2Cl2 (1.0 × 10−6 and 1.0 × 10−2 M).
10−6 M) and photoluminescence spectra (CH2Cl2, 1.0 × 10−6
and 1.0 × 10−2 M). In CH2Cl2, 1 indicated the largest
absorption band at 439 nm (ε = 58 × 104 L mol−1 cm−1).
When 1 was irradiated with UV light in CH2Cl2 (1.0 × 10−6
M), a strong emission was observed (Emax 441 nm, ΦF = 0.90).
The high fluorescence quantum yield indicated that efficient
expansions of the π system and the silyl groups play pivotal
roles in enhancing the emission in 1.8 In contrast to this, when
UV light was irradiated to a 10−2 M CH2Cl2 solution of 1,
broad excimer emission was recorded at 557 nm. This result
demonstrated that 1 could undergo an intermolecular
interaction despite the steric hindrance of trimethylsilyl groups.
B
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Table 1. Selected Physical Properties of Y-, OC-, OA-, and OK-Forms
Y-form
OC-form
orange needle
OA-form
orange powder
OK-form
description
yellow prism
orange powder
A’
form in Scheme 1
B
A
C
preparation
recryst from acetone
reprecipitated from hexane
“rigid” mesophase, Colho
18.5
shearing of Y-, OC-, or OK-form
heating of OA-form
“loose” crystal, Colho
18.8
a
phase, space group (No.) or phase
crystal, P1 (2)
amorphous
̅
a (Å)
6.316(2)
10.023(3)
15.378(5) (c)
70.431(9)
81.290(12)
80.513(13)
ca. 3.34
b (Å)
c or h (Å)
α (deg)
β (deg)
γ (deg)
3.37 (h)
3.47 (h)
b
d (Å)
3.37
3.47
c
UV (nm)
550
620
605
620
d
e
PL (nm) /ΦF
524/0.66
2.2
600/0.49
48.6
586/0.57
70.8
594/0.17
66.8
f
τ (ns)
g
kr/knr (106 s−1
)
300/155
10.1/10.5
8.1/6.1
2.6/12.4
a
b
c
d
e
Phase nomenclature: Colho = hexagonal ordered columnar mesophase. distance between pyrenes. Absorption terminus. Emax
. Absolute
f
g
fluorescence quantum yield. Emission lifetime. See the Supporting Information.
Figure 2. UV−vis absorption and photoluminescence spectra of 1 in
Y-, OC-, OA-, and OK-forms.
Figure 3. Space-filling model of Y-form 1.
Figure 4. XRD patterns recorded at 20 °C for (a) OC-form, (b) OA-
form, (c) OK-form, and (d) Y-form.
Y-form (Figure 4d), and this result also diagnosed the crystal-
like high 2D and 1D regularities of 1 in the OC-form. The
“rigid” discotic columnar mesophase similar to that of the
crystal was reported in octaphenoxy-substituted phthalocyani-
nato copper(II) derivatives.15 The XRD pattern of the OA-form
(the ground samples of Y- or OC-forms) showed a significant
decrease in peak intensities (Figure 4b). The XRD pattern
indicated that a phase conversion to amorphous form occurred
on grinding (forms A and B to form C in Scheme 1). In
contrast, when the OA-form was heated on a glass plate at 260
°C for 150 s, an amorphous to columnar phase transition (OA-
form to OK-form) occurred, and clear reflection peaks (peaks
1−11) were recorded at room temperature (Figure 4c). The
diffractions (peaks 1−4 and 7−10) observed at 16.34, 9.35,
8.11, 6.16, 4.65, 4.52, 3.74, and 3.47 Å could be ascribed to the
(100), (110), (200), (210), (220), (310), (320), and (001)
reflections, respectively, of the hexagonal columnar phase
(Colho, a = 18.8 Å, h = 3.47 Å) which is depicted as form A′ in
Scheme 1.
However, this phase (OK-form) exhibited several diffractions
which were inconsistent with Colho at 5.51, 5.11, and 3.42 Å
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(peaks 5, 6 and 11). These reflections may be attributed to
three-dimensionality. When peak 11 was ascribed to the (101)
reflection, the spacing could be calculated from the values of a
= 18.8 and h = 3.47 Å to be 3.39 Å, which is almost the same as
the observed value of 3.42 Å. Therefore, the OK-form of 1 was
assigned to a crystal form bearing a Colho molecular
arrangement having some three-dimensional regularity.
Although both OK- and OC-forms indicated Colho arrangement,
the OK-form showed a longer molecular distance h diagnostic
of the weaker π−π interaction between adjacent molecules 1: h
= 3.37 for the OC-form and 3.47 Å for the OK-form. In XRD
patterns (Figure 4a,c), the OK-form indicated peaks somewhat
broader than those of the OC-form,16 and this phenomenon
also demonstrated the loose arrangement of 1 in the OK-form.
From these results, we concluded that although the OK-form is
“crystalline” in form, it would have higher free energy at room
temperature than the “rigid liquid crystalline” OC-form because
of the looseness of the molecular arrangement in OK-form.17
Thermochromism. The thermal properties of the four
forms of 1 were investigated in terms of differential scanning
calorimetry (DSC) by heating each form to 270 °C, and all of
the thermochromic phase-transition sequences are summarized
in Figure 5.
Figure 7. Photoluminescence spectra of OC-forms which were
obtained by reprecipitation from hexane and by thermal phase
transition from Y-form. The inset shows the color change of Y-form
upon heating to 250 °C.
resulting orange form was consistent with that of the OC-form
which was obtained by reprecipitation from hexane (Figure 7).
This result demonstrated that the Y-form would undergo a
thermochromic phase conversion to OC-form upon heating.
The Y−OC transition enthalpy ΔH was estimated as 38.0 J/g
from the DSC curve, suggesting that the Y-form was
thermodynamically more stable than the OC-form at room
temperature.
When a virgin sample of the OA-form was heated, an
ambiguous exothermic profile was recorded on the drifting
baseline at 73.2 °C in the first run, and the phase transition
from OA- to OK-form proceeded sluggishly (Figure 6b).
Although in this OA−OK conversion, which was promoted by
heating at 250 °C for 24 h, no change was observed in color
and appearance, the emission maximum was slightly shifted
from 586 to 594 nm with a large decrease in fluorescence
quantum yield (ΦF = 0.57 to ΦF = 0.17) (Figure 8).
Figure 5. Thermochromic phase transition sequences for the Y-, OC-,
OA-, and OK-forms.
In the DSC profile of the Y-form, a clear endothermic peak
was observed at 166.4 °C only in the first heating (Figure 6a).
Both in the following cooling process and in the second cycle
no peak was observed (Figure S9 in the Supporting
Information). When it was heated, the Y-form changed rapidly
from yellow to orange (Figure 7, inset). The fluorescence of the
Figure 8. Photoluminescence spectra of OK-form which was obtained
by thermal phase transition from OA-form.
Vapochromism. Because the Y- and OC-forms were
prepared selectively by recrystallization from acetone and
reprecipitation from hexane, respectively, vapochromic phase
conversion was investigated by exposing each form to a vapor
of acetone or hexane.
When the OC-form on the glass plate was exposed to acetone
vapor at 30 °C, the color gradually changed from orange to
yellow (Figures 9a,b). Upon irradiation of the resulting yellow
form with UV light, green emission was observed at 525 nm
(Figures 9b,c). This emission profile was identical to that of the
Y-form which was obtained by recrystallization from acetone.
DSC analysis of the yellow form indicated a profile similar to
Figure 6. DSC thermograms of (a) Y-form and (b) OA-form.
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Figure 9. (a) The OC-form before exposure to acetone vapor under
room light (left) and under UV irradiation (right). (b) The Y-form
undergoing phase conversion from the OC-form by exposure to
acetone vapor under room light (left) and under UV irradiation
(right). (c) Photoluminescence spectrum of the Y-form undergoing
phase conversion from the OC-form.
Figure 11. Photoluminescence spectra of OC-forms which were
obtained by reprecipitation from hexane and by vapochromic phase
transition from OA- and OK-forms.
that of the Y-form and exhibited an exothermic peak at 167 °C
(Figure S18 in the Supporting Information). These results
showed that the OC-form underwent acetone-promoted
vapochromic phase conversion leading to the Y-form. Because
the crystal shapes remained intact during the phase conversion,
the phase conversion could be regarded as crystal to crystal
conversion.
and OK-forms. When the Y-form was sheared with a mortar
and pestle at room temperature, the color changed rapidly from
yellow to orange (Figure 12, inset). When the OC- and OK-
Vapochromic phase conversions occurred from the other O-
forms, as shown in Figure 10. When OA- and OK-forms were
Figure 12. Photoluminescence spectra of OA-forms which were
obtained by shearing Y-, OC- and OK-forms. (inset: color change of Y-
form by shearing).
Figure 10. Vapochromic phase transition sequences for the Y-, OC-,
OA-, and OK-forms.
forms were subjected to grinding on the mortar, the orange
color remained intact, and a remarkable color change was not
observed. Photoluminescence analyses of the resulting three
orange forms which were thus obtained by shearing Y-, OC-,
and OK-forms exhibited similar profiles (Emax = 586 nm), which
were assignable to the amorphous form OA-form (Figure 12).
The mechanochromic phase conversions are summarized and
shown in Figure 13.
Fluorescence Lifetime. Fluorescence decays of 1 in
CH2Cl2 and Y-, OC-, OA-, and OK-forms were measured
using an apparatus including a femtosecond laser system
(Figure 14).
exposed to acetone vapor overnight, they underwent a color
change from orange to yellow to furnish the Y-form, as
observed in the phase conversion of the OC-form. The yellow
forms thus obtained from OA- and OK-forms also showed
emission and DSC profiles which were identical with those of
the Y-form which was derived by recrystallization from acetone
(Figure S16 in the Supporting Information). On the other
hand, when the OA- and OK-forms were exposed to hexane
vapor at 30 °C overnight, there was no noticeable change in
color; however, analyses of the photoluminescence revealed
that the OA- and OK-forms were completely converted to the
OC-form (Figure 11). In sharp contrast, the hexane-promoted
phase conversion from Y- to OC-form was not observed: when
the Y-form was exposed to hexane vapor at 30 °C overnight,
the Y-form remained yellow, and no change was observed in
the fluorescence spectrum. The low reactivity of the Y-form in
hexane-promoted phase conversion could be explained in terms
of thermodynamic stability of the Y-form. Because the Y-form
is more stable than the OC-form at room temperature as DSC
analysis indicated, hexane vapor could not rearrange the Y-form
array of 1 to the hexagonal molecular array of the OC-form,
which was thermodynamically unfavorable.
Mechanochromism. Mechanochromic phase conversion
was investigated by providing a mechanical stimulus to Y-, OC-,
Figure 13. Mechanochromic phase transition sequences for the Y-,
OC-, OA- and OK-forms.
E
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524 nm (ΦF = 0.66) for the Y-form and 600 nm (0.49) for the
OC-form. Grinding of the two forms with a mortar and pestle
furnished the orange amorphous solid OA-form. The OA-form
emitted fluorescence at 586 nm (ΦF = 0.57) under UV light.
When it was heated, the OA-form underwent rapid phase
conversion to the OK-form, which exhibited emission at 594
nm (ΦF = 0.17) while the Y-form led to the OC-form. The
single-crystal X-ray diffraction analysis of the Y-form revealed
that, in the Y-form, only half-planes of 1 were overlapped with
the proximate molecule, leading to monomeric photolumines-
cence. The shorter fluorescence lifetime (τ = 2.2 ns) also
indicated the monomeric emission of the Y-form. Although the
XRD analyses demonstrated the hexagonal columnar arrange-
ment of the OC- and OK-forms, the former was characterized as
a “rigid” liquid crystalline form, and the latter as a “loose”
crystalline form. Because rather long fluorescence lifetimes (τ >
48 ns) were recorded for the OC-, OA-, and OK-forms, their
emissions were assigned as excimeric photoluminescence. In
the solid state of 1 were also observed vapochromic phase
transitions. Exposing the OC-, OA-, and OK-forms to acetone
vapor provided the Y-form, and exposing the OA- and OK-
forms to hexane yielded the OC-form. Assuming grinding
would enable a phase transition to a higher energy level of
phase, the correlations between the free energy and temper-
ature can be depicted in the order of OA-, OK-, OC-, and Y-
forms (Figure 15).18 Further investigation of expanded π
systems with bulky trialkyl(aryl)silyl substituents and their
applications as organic materials are underway.
Figure 14. Fluorescence decay profile of 1 in CH2Cl2 (1.0 × 10−6 M)
and Y-, OC-, OA-, and OK-forms.
As shown in Figure 14, 1 in CH2Cl2 and Y-form showed
rather short fluorescence lifetimes (<4 ns), while OC-, OA-, and
OK-forms had longer lifetimes (>48 ns). The short lifetimes
and the emission maxima of 1 in CH2Cl2 and Y-form recorded
at shorter wavelengths (441 and 524 nm, respectively) were
ascribable to monomeric emission. Although the X-ray single-
crystal analysis disclosed that the adjacent molecules 1 in the Y-
form were located at 3.34 Å, only halves of the adjacent
molecules 1 were overlapped with each other, and therefore,
the formation of a photoexcited excimer was prohibited, leading
to the predominant monomeric photoluminescence of the Y-
form. In contrast to this, the rather longer lifetimes which were
observed in a series of orange forms, OC-, OA-, and OK-forms,
diagnosed that they would emit excimeric photoluminescence.
These three orange forms showed their emission maxima at
wavelengths longer (580−600 nm) than that of the Y-form,
and this result also might support the excimer formation of
OC-, OA-, and OK-forms in photoexcitation.
The reaction rate constants for radiative (kr) and non-
radiative processes (knr) were calculated by the following
equations and are summarized in Table 1.
kr = ΦF/τ
knr = (1 − ΦF)/τ
The Y-form showed large reaction constants of radiative and
nonradiative processes (>1.0 × 108 s−1), while the OC-, OA-,
and OK-forms had rather smaller values (<1.3 × 107 s−1).
Although the OK-form demonstrated an emission lifetime
comparable to that of the OA-form (66.8 and 70.8 ns,
respectively), the OK-form emitted fluorescence in a remark-
ably smaller fluorescence quantum yield (ΦF = 0.17 and 0.57,
respectively). The low quantum yield of the OK-form could be
explained in terms of the smaller radiative (2.6 × 106 s−1) and
larger nonradiative reaction constant (12.4 × 106 s−1).17
Figure 15. G−T diagram of the Y-, OC-, OA-, and OK-forms.
EXPERIMENTAL SECTION
■
General Procedure. Unless otherwise stated, all reactions were
carried out under an argon atmosphere. Anhydrous tetrahydrofuran
(THF) was purchased and used without further purification. Toluene
and i-Pr2NH were distilled from CaH2 prior to use. All NMR spectra
were recorded at ambient temperature (ca. 20 °C) on JEOL LAMBDA
300 and 500 and ECS-400 NMR spectrometers and referenced to the
internal standard tetramethylsilane. MALDI-TOF MS was recorded on
a BRUKER autoflex speed instrument. UV−vis absorption and
emission spectra were measured in degassed solvents at 25 °C. The
absolute photoluminescence quantum yield (ΦF) was measured by an
integrated sphere system. The phase transition sequences for 1 were
revealed by using a polarizing optical microscope (Nikon ECLIPSE
E600 POL) equipped with a heating plate (Mettler FP82HT hot stage,
Mettler FP-90 Central Processor) and a differential scanning
calorimeter (Shimadzu DSC-50). The decomposition temperatures
were measured with a Rigaku Thermo plus TG 820 thermogravimetric
analyzer. The identification of the mesophases was performed by using
CONCLUSION
■
By introducing the steric bulky trimethylsilyl groups to an
expanded π system, 1,3,6,8-tetraethynylpyrene, we succeeded in
the synthesis of the stimuli-responsive fluorescent compound 1.
When the ethynylpyrene derivative 1 was subjected to
recrystallization/reprecipitation, Y- and OC-forms were ob-
tained: recrystallization of 1 from acetone provided yellow
crystalline prisms (Y-form), and reprecipitation from hexane
gave orange needles (OC-form). The Y- and OC-forms showed
not only different colors under room light but also different
fluorescent properties by irradiation with UV light; emission at
F
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a small-angle X-ray diffractometer (Bruker Mac SAXS System: Cu Kα
radiation) equipped with a heating plate (Mettler FP82HT hot stage,
Mettler FP-90 Central Processor). Crystals of Y-form were analyzed
by X-ray diffraction. A selection of crystal, measurement, and
refinement data is given in the Supporting Information. Diffraction
data were collected on a Rigaku Saturn724 diffractometer using
multilayer mirror monochromated Mo Kα radiation. Data were
collected and processed using CrystalClear (Rigaku).19 The structure
was solved with SIR2004.20 Isotropic and full-matrix anisotropic least-
squares refinements were carried out using SHELXL-97.21 The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
refined using the riding model. Compound 1 was prepared according
to the reported procedure.8a
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■
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H.; Maeda, T.; Mizuno, K. Molecules 2012, 17, 5108 and references
therein. (i) Tamao, K.; Yamaguchi, S.; Uchida, M.; Izumizawa, T.;
Furukawa, K. In Organosilicon Chemistry IV. From Molecules to
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Trans. 1998, 3693.
1,3,6,8-Tetrakis((trimethysilyl)ethynyl)pyrene (1).8a To pyr-
ene (1.01 g, 5.0 mmol) in nitrobenzene (15 mL) was added bromine
(3.52 g, 22.0 mmol) dropwise at room temperature, and the mixture
was stirred at 120 °C for 4 h. The reaction mixture was filtered, and
the residue was washed with methanol (50 mL) and acetone (50 mL).
After it was dried under the reduced pressure, 1,3,6,8-tetrabromopyr-
ene was obtained in a pure form (2.54 g, 4.91 mmol, 98% yield).
1,3,6,8-Tetrabromopyrene (250.0 mg, 0.48 mmol) was suspended in
Et3N (10 mL) and toluene (3 mL), and PdCl2(PPh3)2 (68 mg, 0.10
mmol), CuI (36 mg, 0.19 mmol), and Ph3P (50 mg, 0.19 mmol) were
added under an argon atmosphere. To the mixture was added
(trimethylsilyl)ethyne (280.0 mg, 2.88 mmol) at 60 °C, and the
mixture was stirred at 80 °C overnight. The reaction mixture was
diluted with CH2Cl2 and washed with water. The organic layer was
dried over MgSO4, and the solvents were removed under reduced
pressure. The crude product was subjected to column chromatography
on silica gel (hexane) and reprecipitation from CH2Cl2/MeOH to give
1 in a pure form (250.8 mg, 89% yield).
(2) For reviews, see: (a) Ichimura, K. Chem. Rev. 2000, 100, 1847.
(b) Ikeda, T. J. Mater. Chem. 2003, 13, 2037. (c) Ikeda, T.; Mamiya, J.;
Yu, Y. Angew. Chem., Int. Ed. 2007, 46, 506. (d) Wu, J.; Pisula, W.;
Mullen, K. Chem. Rev. 2007, 107, 718. (e) Binnemans, K. Chem. Rev.
̈
2005, 105, 4148. For selected papers on mechanochromic
fluorophores, see: (f) Sagara, Y.; Kato, T. Angew. Chem., Int. Ed.
2008, 47, 5175. (g) Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J.
Am. Chem. Soc. 2007, 129, 1520. (h) Ito, H.; Saito, T.; Oshima, N.;
Kitamura, N.; Ishizaka, S.; Hinatsu, Y.; Wakeshima, M.; Kato, M.;
Tsuge, K.; Sawamura, M. J. Am. Chem. Soc. 2008, 130, 10044.
(i) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605. (j) Dong, Y. Q.; Lam,
J. W. Y.; Tang, B. Z. J. Phys. Chem. Lett. 2015, 6, 3429. For selected
papers of multistimuli-responsive fluorophores, see: (k) Dou, C.;
Chen, D.; Iqbal, J.; Yuan, Y.; Zhang, H.; Wang, Y. Langmuir 2011, 27,
6323. (l) Shan, X.-C.; Jiang, F.-L.; Chen, L.; Wu, M.-Y.; Pan, J.; Wan,
X.-Y.; Hong, M.-C. J. Mater. Chem. C 2013, 1, 4339. (m) Yagai, S.;
Okamura, S.; Nakano, Y.; Yamauchi, M.; Kishikawa, K.; Karatsu, T.;
Kitamura, A.; Ueno, A.; Kuzuhara, D.; Yamada, H.; Seki, T.; Ito, H.
Nat. Commun. 2014, 5, 4013. (n) Sun, H.; Liu, S.; Lin, W.; Zhang, K.
Y.; Lv, W.; Huang, X.; Huo, F.; Yang, H.; Jenkins, G.; Zhao, Q.;
Huang, W. Nat. Commun. 2014, 5, 3601.
(3) Although trimethylsilyl groups are rather bulky, the pyrene 1
showed the formation of excimer in a 10−2 mol/L CH2Cl2 solution
(Figure 1). See also the Supporting Information for 10−3−10−5 mol/L
UVvis absorption and photoluminescence profiles. For selected
reviews of pyrene excimers, see: (a) Modern Molecular Photochemistry;
Turro, N. J., Ed.; University Science Books: Mill Valley, CA, 1991;
Chapter 5.. (b) Hu, J-Y.; Yamato, T. In Organic Light Emitting Diode−
Material, Process and Devices; Ko, S. H., Ed.; InTech: Rijeka, 2011;
Chapter 2.
1
1: orange powder; H NMR (400 MHz, CDCl3) δ 0.39 (s, 36H),
8.31 (s, 2H), 8.60 (s, 4H). Y-form and OC-form were obtained by
recrystallization from acetone and reprecipitation from hexane,
respectively. OA-form was obtained by grinding Y-, OC-, and OK-
forms with a mortar and pestle. OK-form was obtained by heating OA-
form at 150 °C for 2 h.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00781.
Experimental details, including synthetic procedures,
absorption and fluorescence spectra, quantum yields,
XRD patterns, and selected X-ray crystallographic data
(PDF)
(4) Molecular Crystals and Molecules, Kitaigorodsky, A. I., Ed.;
Academic Press: New York, 1973.
X-ray crystallographic data (CIF)
(5) Crystal structures and polymorphs of pyrenes have been
reported: (a) Fabbiani, F. P. A.; Allan, D. R.; Parsons, S.; Pulham,
C. R. Acta Crystallogr., Sect. B: Struct. Sci. 2006, 62, 826. (b) Feng, Q.;
Wang, M.; Dong, B.; He, J.; Xu, C. Cryst. Growth Des. 2013, 13, 4418.
For polymorphs of benzo[a]pyrene: Contag, B. Naturwissenschaften
1978, 65, 108 For mechanochromic pyrene liquid crystal: see ref 2a..
(6) It has been reported that the rotational bulky groups served as
“soft groups” leading to mesophases: (a) Ohta, K.; Shibuya, T.; Ando,
M. J. Mater. Chem. 2006, 16, 3635. (b) Takagi, Y.; Ohta, K.;
Shimosugi, S.; Fujii, T.; Itoh, E. J. Mater. Chem. 2012, 22, 14418.
(c) Hachisuga, A.; Yoshioka, M.; Ohta, K.; Itaya, T. J. Mater. Chem. C
2013, 1, 5315.
(7) The mesophase of tetrakis(bis((trialkylsilyl)ethynyl)phenyl-
ethynyl)pyrene was reported: Hirose, T.; Shibano, Y.; Miyazaki, Y.;
Sogoshi, N.; Nakabayashi, S.; Yasutake, M. Mol. Cryst. Liq. Cryst. 2011,
534, 81.
(8) (a) Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu,
H.; Inouye, M. Chem. - Eur. J. 2006, 12, 824. (b) Shao, G.; Orita, A.;
Nishijima, K.; Ishimaru, K.; Takezaki, M.; Wakamatsu, K.; Gleiter, R.;
Otera, J. Chem. - Asian J. 2007, 2, 489.
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.O.: orita@dac.ous.ac.jp.
ORCID
Akihiro Orita: 0000-0001-8684-2951
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Numbers
JP16H01164 in Middle Molecular Strategy and JP15K05440,
the Okayama Prefecture Industrial Promotion Foundation and
Grant for Promotion of OUS Research Projects. AO is grateful
to Professor Motohiro Akazome, Chiba University and
Professor Hajime Maeda, Kanazawa University for the kind
measurement of XRD and the helpful discussion, respectively.
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DOI: 10.1021/acs.organomet.6b00781
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(9) In the Supporting Information is shown a relation of the two
forms obtained and solvents used in recrystallization/reprecipitaion.
(10) Grinding of the OK-form led to a phase transition to the OA-
form.
(11) (a) Somerharju, P. Chem. Phys. Lipids 2002, 116, 57.
(b) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99,
2039.
(12) CCDC 1490712 (Y-form) contains supplementary crystallo-
graphic data.
(13) It is reported that the disk-shaped aromatic compounds would
be packed with an interplane distance of 3.4−3.6 Å: Bouas-Laurent,
H.; Durr, H. Pure Appl. Chem. 2001, 73, 639.
̈
(14) When the OC-form was heated, microscopic observation
showed no change in the orange needles of the OC-form up to 265 °C
(Figure S32 in the Supporting Information), and DSC analysis showed
neither exothermic nor endothermic change up to 300 °C, indicative
of no phase transition (Figure S9 in the Supporting Information,
second cycle).
(15) Octakis(m-alkoxyphenoxy)-substituted phthalocyaninato cop-
per(II) derivatives were isolated as green liquid crystals by
reprecipitation from ethyl acetate. They also showed no fluidity as a
mesophase and were transformed to isotropic liquids when heated at
180 °C: Ichihara, M.; Suzuki, A.; Hatsusaka, K.; Ohta, K. J. Porphyrins
Phthalocyanines 2007, 11, 503.
(16) When the OK-form was heated to 265 °C, XRD analyses
demonstrated that the peaks 5, 6, 10, and 11 of the OK-form were
shifted to the lower angle region (Figure S33 in the Supporting
Information). However, neither spontaneous fluidity nor phase
transition to a mesophase was observed (Figure S34 in the Supporting
Information).
(17) The rather low fluorescence quantum yield of the OK-form (ΦF
= 0.17) might be explained by nonradiative relaxation induced by the
“loose” arrangement of 1 in the OK-form.
(18) Because the OA- and OK-forms were transformed to Y- and OC-
forms by exposure to the vapors of organic solvents, we concluded
that the Y- and OC-forms would be placed below the OA- and OK-
forms in the G−T diagram.
(19) Pflugrath, J. W. Acta Crystallogr., Sect. D: Biol. Crystallogr. 1999,
55, 1718.
(20) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.;
Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Spagna, R.
J. Appl. Crystallogr. 2005, 38, 381.
(21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112.
H
DOI: 10.1021/acs.organomet.6b00781
Organometallics XXXX, XXX, XXX−XXX