Tricolor mechanochromic luminescence of an organic two-component dye visualization of a crystalline state and two amorphous states

Article abstract of DOI:10.1039/c8ce01698d

Solid-state emissive dyes with mechanochromic luminescence (MCL) properties change their emission colors upon exposure to mechanical stimuli. During the recent rapid advances in the area of MCL, considerable efforts have been focused on elucidating the MCL behavior of single-component dyes that can switch their solid-state emission between two colors. Herein, a novel strategy to achieve tricolor MCL is presented, which is based on mixing two organic dyes that exhibit poor MCL or no MCL properties, respectively. We propose that the tricolor MCL of the two-component dye should be attributed to the transition of the fraction with poor MCL properties from a crystalline to two amorphous states, specifically a perfectly amorphous and a proto-crystalline amorphous state. The present system should have advantages in terms of the availability of MCL dyes and the variety of potential combinations of two-component dyes. By making a use of the underlying design principle developed in this study, a wide variety of new MCL systems should be accessible, which should also accelerate the investigation of practical applications of MCL dyes.

Full text of DOI:10.1039/c8ce01698d

Showcasing research by Suguru Ito, Genki Katada,
Tomohiro Taguchi, Izuru Kawamura, Takashi Ubukata, and
Masatoshi Asami from Yokohama National University, Japan
As featured in:
Tricolor mechanochromic luminescence of an organic
two-component dye: visualization of a crystalline state
and two amorphous states
A two-component mixture of dipyrenylbithiophene
and N,N′-dimethylquinacridone exhibited a tricolor
mechanochromic luminescence (MCL), which should
be attributed to the transition of dipyrenylbithiophene
from a crystalline to two amorphous states. In contrast,
dipyrenylbithiophene showed a poor MCL between two colors.
See Suguru Ito et al.,
CrystEngComm, 2019, 21, 53.
rsc.li/crystengcomm
Registered charity number: 207890

CrystEngComm
View Article Online
View Journal | View Issue
PAPER
Tricolor mechanochromic luminescence of an
organic two-component dye: visualization of a
crystalline state and two amorphous states†
Cite this: CrystEngComm, 2019, 21,
53
*
Suguru Ito,
Genki Katada, Tomohiro Taguchi, Izuru Kawamura,
and Masatoshi Asami
Takashi Ubukata
Solid-state emissive dyes with mechanochromic luminescence (MCL) properties change their emission
colors upon exposure to mechanical stimuli. During the recent rapid advances in the area of MCL, consid-
erable efforts have been focused on elucidating the MCL behavior of single-component dyes that can
switch their solid-state emission between two colors. Herein, a novel strategy to achieve tricolor MCL is
presented, which is based on mixing two organic dyes that exhibit poor MCL or no MCL properties, re-
spectively. We propose that the tricolor MCL of the two-component dye should be attributed to the transi-
tion of the fraction with poor MCL properties from a crystalline to two amorphous states, specifically a per-
fectly amorphous and a proto-crystalline amorphous state. The present system should have advantages in
terms of the availability of MCL dyes and the variety of potential combinations of two-component dyes. By
making a use of the underlying design principle developed in this study, a wide variety of new MCL systems
should be accessible, which should also accelerate the investigation of practical applications of MCL dyes.
Received 7th October 2018,
Accepted 19th November 2018
DOI: 10.1039/c8ce01698d
rsc.li/crystengcomm
achieving such multicolor MCL lie in the mechanism of typi-
Introduction
cal MCL dyes. The emission-color change of
a single-
Mechanochromic luminescence (MCL) refers to a reversible
shift of the solid-state emission maximum of a luminescent
dye induced by the exposure to a mechanical stimulus (e.g.
shearing, grinding, or pressure). Luminophores with MCL
properties have attracted increasing interest owing to their
potential applications in e.g. mechanosensors, security inks,
and wearable technologies.1 Most notably, current research
on MCL has been focused exclusively on single-component
solid-state emissive dyes,2–4 and little is known about the
mechanoresponsive behavior of two-component dyes.5 In all
previously reported two-component MCL systems, two kinds
of organic dyes with good mechanoresponsive properties have
been mixed in order to achieve tricolor MCL, which is diffi-
cult to realize using single-component dyes.6 As organic dyes
with good MCL properties are relatively rare, it would be de-
sirable if poor MCL properties of organic dyes could be im-
proved upon mixing with another organic fluorophore that ex-
hibit no MCL properties. However, such systems have not yet
been established. In addition, the biggest challenges of
component MCL dye between two colors typically arises from
a transition of the molecular arrangement from an ordered
crystalline state to a disordered amorphous state.2,4 In some
cases, the crystalline state alters the degree of its crystallinity
or changes to another crystalline state within a polymorphic
relationship (Fig. 1a).3 Polymorphism is a well-known phe-
nomenon for organic crystals and the MCL properties of poly-
morphic crystals are well documented.7 In contrast, poly-
amorphism of organic compounds has only recently been
investigated in the field of pharmaceuticals.8 The difference
of emission properties between polyamorphic states has been
overlooked so far, although the emission-color change based
on polyamorphism should hold the promise of realizing
multicolor MCL. Herein, we report that a mixture of two or-
ganic dyes with poor MCL and no MCL properties, respec-
tively, exhibits MCL between three colors with a large shift
(Δλem ≤ 135 nm) of the emission maximum. We propose that
the emission-color change between three colors should be at-
tributed to a transition of a fraction of the MCL dye between
one crystalline and two amorphous phases (Fig. 1b).
Department of Advanced Materials Chemistry, Graduate School of Engineering,
Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-
8501, Japan. E-mail: suguru-ito@ynu.ac.jp
Results and discussion
For the purpose of developing a new organic dye with MCL
properties, we designed 5,5′-dihexyl-3,3′-diIJpyren-1-yl)-2,2′-
bithiophene (1), which is expected to form a mechanical-
† Electronic supplementary information (ESI) available: Experimental details,
spectral data, DSC thermograms, and fluorescence lifetime decays. See DOI:
10.1039/c8ce01698d
This journal is © The Royal Society of Chemistry 2019
CrystEngComm, 2019, 21, 53–59 | 53

View Article Online
Paper
CrystEngComm
The original blue emission could be recovered upon heating
the ground sample in a thermostatic oven to 110 °C (Fig. 2b).
In addition, molten amorphous 1 (B-M; bithiophene-molten),
prepared by heating B-C or B-G to 150 °C and subsequent
cooling to room temperature, exhibited a blue-green emis-
sion (λem = 500 nm). This emission spectrum is almost identi-
cal to that of B-G prepared by grinding B-C or B-M (Fig. 2b
and Fig. S1†). Powder X-ray diffraction (PXRD) analyses of
these samples revealed that the diffraction peaks of crystal-
line B-C disappeared for B-M and B-G (Fig. S2†). Therefore,
the MCL of 1 between B-C and B-G should be attributed to a
typical crystalline-to-amorphous transition. However, the
shift of the emission maxima is only 13 nm. This is probably
due to the fact that the conformational change of the mole-
cule is, contrary to our initial expectations, relatively small
between the crystalline and amorphous states. We speculated
that in the presence of another fluorophore an on–off
switching of energy transfer from 1 to another fluorophore
could be triggered by the transition of 1 from the crystalline
to the amorphous states, leading to a significant shift in the
solid-state emission maxima upon grinding.
Fig. 1 Schematic illustration for the MCL behavior of organic dyes. (a)
Typical MCL: MCL between crystalline and amorphous states, or
between two crystalline states (A and B). (b) This work:
a two-
component dye with tricolor MCL properties by mixing two organic
dyes with poor MCL and no MCL properties, respectively.
stimuli-responsive intramolecular stacking between the two
pyrenyl rings. Dipyrenylbithiophene 1 was synthesized via a
Suzuki–Miyaura coupling of pyrene-1-boronic acid with previ-
For that purpose, N,N′-dimethylquinacridone (DMQA,
Fig. 3a) was selected as the dye to be mixed with 1, since the
red dye DMQA exhibits an absorption band around 500 nm
in the solid state, which is in the same region as the maxi-
mum emission wavelength of B-G (Fig. S3†). A 1 : 1 molar
mixture of 1 and DMQA on a glass plate was molten on a hot
plate at 150 °C and then cooled to room temperature. The
ously reported 3,3′-dibromo-5,5′-dihexyl-2,2′-bithiophene
2
(Fig. 2a).9 In the presence of 20 mol% of PdIJPPh3)4, a mixture
of dibromide 2 with pyrene-1-boronic acid in 1,4-dioxane/
aqueous K2CO3 (2 M) was stirred for 12 h at 95 °C to give 1 in
41% yield as a pale-yellow crystalline powder (B-C; bithio-
phene-crystal) after reprecipitation from ethyl acetate. Al-
though the preparation of single crystals suitable for X-ray
1
diffraction analysis was unsuccessful, a H NMR analysis of 1
in CDCl3 suggested the existence of intramolecular stacking
between the two pyrenyl groups of 1.10 B-C exhibited a blue
fluorescence with a maximum wavelength (λem) at 488 nm.
Upon grinding B-C with a spatula, the resulting B-G (bithio-
phene-ground) phase exhibited a bathochromically shifted
solid-state emission maximum at 501 nm (Δλem = 13 nm).
Fig. 3 (a) Structural formula of DMQA and photographic images for its
emission in toluene and in the solid state. (b) Tricolor MCL of a two-
component dye prepared from 1 and DMQA. (c) Normalized fluores-
cence spectra of BD-C, BD-M, and BD-G. (d) DSC thermograms for
BD-C, BD-M, and BD-G. Tm, Tg, and Tc values are noted near the cor-
responding peaks and steps.
Fig. 2 (a) Synthesis and (b) MCL of dipyrenylbithiophene 1.
54 | CrystEngComm, 2019, 21, 53–59
This journal is © The Royal Society of Chemistry 2019

View Article Online
CrystEngComm
Paper
mixed dye BD-M (bithiophene/DMQA-molten) exhibited a
green emission with two emission maxima at 531 nm (stron-
ger) and 576 nm (weaker) (Fig. 3b and c), which are different
from the solid-state emission maxima of 1 [λem = 488 nm
(crystalline) and 500–501 nm (amorphous); Fig. 2] and DMQA
(λem = 648 nm; Fig. 3a). When BD-M was scratched and
ground with a spatula, a bathochromically shifted orange
emission (λem = 605 nm) was observed from the resulting BD-
G (bithiophene/DMQA-ground) state. Upon heating to 150 °C,
BD-G was retro-converted into the green-emissive BD-M, i.e.,
this two-component dye exhibits MCL. The transformation
between BD-M and BD-G is fully reversible and could be re-
peated more than five times (Fig. S4a†). Differential scanning
calorimetry (DSC) measurements of BD-M and BD-G revealed
a difference in thermal behavior (Fig. 3d). In the DSC ther-
mogram of BD-M, a glass-transition temperature (Tg) was ob-
served at 33 °C. For BD-G, on the other hand, an exothermic
cold-crystallization transition temperature (Tc) was observed
at 104 °C, followed by an endothermic peak that corresponds
to the melting point (Tm) at 124 °C.
Fig. 5 Schematic illustration for the proposed mechanism of tricolor
MCL in the (a) BD-C, (b) BD-M, and (c) BD-G states.
owing to the low fluorescence quantum yield of red-emissive
DMQA (Fig. 5a; Table 1, ΦF < 0.01). A slight hypsochromic
shift in the maximum emission wavelength of BD-C (λem
=
As the DSC analysis of BD-G showed a cold-crystallization
peak at 104 °C, BD-G was heated in an oven to 110 °C. Inter-
estingly, the emission color of the sample changed to blue
(λem = 470 nm) after heating. This blue-emissive BD-C
(bithiophene/DMQA-crystallized) state could be transformed
into the orange-emissive BD-G upon grinding. In other words,
the two-component dye exhibited MCL between BD-C and
BD-G together with a large shift of the maximum emission
wavelength (Δλem = 135 nm) (Fig. 3b and c). This transforma-
tion is also fully reversible and could be repeated more than
five times (Fig. S4b†). The DSC thermogram of BD-C showed
Tm = 127 °C (Fig. 3d) and when BD-C was heated to 150 °C,
the two-component dye returned to the initial BD-M state
(Fig. 3b).11
Powder X-ray diffraction (PXRD) analyses of BD-C, BD-M,
and BD-G provided mechanistic insights into the tricolor
MCL of the mixture of 1 and DMQA. The PXRD pattern of
BD-C was in good agreement with a superposition of those of
1 (B-C) and DMQA, indicating that 1 and DMQA exist as inde-
pendent crystals in BD-C (Fig. 4a–c). Therefore, the blue
emission of BD-C should be attributed to the emission of 1
470 nm) compared to B-C (λem = 488 nm) would be explained
by the absorption of the emission from 1 into DMQA. On the
other hand, the observed diffraction pattern for BD-M was in
good agreement with that of DMQA and diffraction peaks at-
tributable to 1 were not observed (Fig. 4d). Similarly, the
PXRD analysis of BD-G revealed that amorphous 1 and crys-
talline DMQA exist therein (Fig. 4e). Thus, significant differ-
ences between the PXRD patterns of BD-M and BD-G were
not observed although their emission colors vary substan-
tially. The fluorescence spectrum of BD-M was similar to
those of a green-emissive toluene solution of DMQA (λem
=
525 nm) and DMQA-doped organic light-emitting diodes (λem
= 530–540 nm),12 although the emission color of DMQA is
red (λem = 648 nm) in the solid state (Fig. 3a, Fig. S5†). While
the PXRD analysis suggested the existence of crystalline
DMQA in BD-M, some portion of DMQA would be dissolved
as monomer in the fraction of amorphous 1, and the green
emission of BD-M should most likely be rationalized in terms
of an emission of dissolved DMQA that results from an en-
ergy transfer from 1 to dissolved DMQA (Fig. 5b). A signifi-
cantly higher fluorescence quantum yield (ΦF = 0.016) and a
longer area-weighted mean fluorescence lifetime (<τF> = 10.6
ns) were observed for BD-M compared to red-emissive solid-
state DMQA (ΦF < 0.01; <τF> = 3.0 ns), supporting the
Table 1 Maximum wavelengths of the solid-state emission (λem), fluores-
cence quantum yield (ΦF), and area-weighted mean fluorescence lifetime
(<τF>) of the two-component dyes (BD-C, BD-M, and BD-G) and DMQA
<τF>c [ns]
b
Sample
λema [nm]
ΦF
BD-C
470
531
595
648
0.011
0.016
0.027
5.1
10.6
10.5
3.0
BD-M
BD-G
DMQA
<0.01
a
b
Fig. 4 PXRD patterns for B-C (a), DMQA (b), BD-C (c), BD-M (d), and
BD-G (e).
Excited by UV light (365 nm). Measured using an integrating
P
P
sphere. c <τF> = (An·τn2)/ (An·τn); cf. Fig. S10 and Table S1.
This journal is © The Royal Society of Chemistry 2019
CrystEngComm, 2019, 21, 53–59 | 55

View Article Online
Paper
CrystEngComm
energy transfer from 1 to dissolved DMQA. As the BD-G state
heating and others may not.4a A relatively high fluidity of the
supercooled liquid state of 1 with two hexyl groups would
prevent the formation of ordered alignments on heating,
which might be one reason for the no crystallization of mol-
ten 1 (BD-M and B-M) on heating. The observed emission
color of DMQA (green) in BD-M should thus be attributed to
the perfectly amorphous and highly mobile 1, which should
act like a hydrocarbon solvent to dissolve some portion of
DMQA. The exposure of BD-M to mechanical stimuli would
induce short-range alignments of 1, and the resulting BD-G
should thus contain proto-crystalline amorphous 1 (Fig. 5c).
Meanwhile, the highly-ordered crystalline structure of 1 in
BD-C should be destroyed to afford BD-G which contains
proto-crystalline amorphous 1. The proto-crystallinity of BD-G
also exhibits a higher fluorescence quantum yield (ΦF
=
0.027) and longer area-weighted mean fluorescence lifetime
(<τF> = 10.5 ns) than red-emissive DMQA, the energy trans-
fer from amorphous 1 to dissolved DMQA should also occur
in BD-G (Table 1). It should also be noted that both a primary
emission peak (λem = 531 nm) and a shoulder peak (λem = 576
nm) of BD-M should be assigned as monomer emissions of
DMQA because the same emission peaks were observed from
monomer DMQA in solutions (Fig. S5†). In addition, upon
changing the molar ratio of 1 and DMQA from 1 : 1 to 4 : 1, a
hypsochromic shift of the emission maximum and a increase
of the shoulder peak around 530–540 nm were observed for
BD-G (Fig. S6†). This hypsochromic shift caused by increas-
ing the ratio of less-polar 1 should be explained by the
solvatofluorochromic property of DMQA (Fig. S5†).13 Accord-
ingly, the emission of BD-G should be assignable to the
bathochromically-shifted monomer emission of DMQA
(Fig. 3c and Fig. 5b and c).
is supported by the facts that BD-G exhibits
a cold-
crystallization process at 104 °C and no diffraction peaks at-
tributable to long-range order. It would be reasonable to ex-
plain that the facile crystallization of BD-G on heating is due
to the presence of some short-range alignments of 1. The
maximum emission wavelength of BD-G is longer than that
of BD-M. This difference could be explained by the fact that
the nonpolar-solvent-like effect of proto-crystalline amor-
phous 1 is weaker than that of perfectly amorphous 1. As
there are partially ordered domains of 1 in the proto-
crystalline amorphous state, DMQA should be less
surrounded by 1, which is supported by the results of the
solid-state 13C NMR analyses (Fig. S8†) and the emission
spectra of BD-G prepared from different mixing ratios (Fig.
S6†). Accordingly, the intermolecular interaction between
DMQA molecules should be larger in BD-G than in BD-M,
which leads to the bathochromically-shifted fluorescence ow-
ing to the solvatofluorochromic nature of DMQA (Fig. 5c and
S5†).
In the solid-state 13C NMR spectra of BD-C, BD-M, and BD-
G, a considerable difference of the chemical shift values was
observed between BD-C, which includes crystalline 1, and BD-
M and BD-G, both of which include amorphous 1 (Fig. S8†).
More importantly, a significant difference was observed in
the alkyl region of the solid-state 13C NMR spectra of BD-M
and BD-G, i.e., the signals of the hexyl groups of 1 in BD-M (δ
= 31.38, 23.13, 14.66 ppm) were upfield shifted relative to
those of BD-G (δ = 31.93, 23.58, 14.98 ppm). This result sug-
gests that the hexyl groups of 1 in BD-M more efficiently sur-
round DMQA and are thus exposed to a stronger shielding ef-
fect from DMQA than those in BD-G.
The differences observed in the DSC analyses and 13C
NMR spectra suggest that the fraction of bithiophene 1
should exist in different amorphous states in BD-M and BD-
G. Judging from the thermal behavior and the PXRD pattern
of bithiophene 1 in the absence of DMQA (Fig. 2, Fig. S2 and
S9†), the B-M and B-G states should correspond to the frac-
tion of 1 in BD-M and BD-G states, respectively. However, vir-
tually identical emission spectra were observed for B-M and
B-G. In other words, upon addition of DMQA, the two amor-
phous states of 1 can be visualized by the difference in the
emission color.
The two amorphous states of 1 are considered to be a per-
fectly amorphous and a proto-crystalline amorphous state, re-
spectively.8,14 The perfectly amorphous state refers to a
completely disordered state without short- and long-range or-
der, whereas the proto-crystalline amorphous state refers to a
partially ordered state that exhibits some short-range order,
albeit in the absence of crystallinity. The concept of perfectly
and proto-crystalline amorphous states provides a reasonable
explanation for the difference in the thermal behavior and
emission color between BD-M and BD-G. In the molten
sample BD-M, 1 should exist in the perfectly amorphous state
as the molecular alignment of 1 would be completely disor-
dered upon melting (Fig. 5b). It has been reported that some
molten samples of organic MCL dyes may crystallize on
Conclusions
In summary, a tricolor mechanochromic luminescence (MCL)
that exhibits the large shift in the solid-state emission maxi-
mum has been realized by mixing dipyrenylbithiophene 1,
which exhibits poor MCL properties, and DMQA, which does
not exhibit MCL properties. This study thus is, to the best of
our knowledge, the first example of significantly improved
MCL properties of an organic dye by adding an organic dye
without MCL properties. The origin of the tricolor switch
should be attributed to the fraction of 1 in this two-
component dye. Importantly, the difference between the two
amorphous states of 1 that could not be characterized by
PXRD analysis has been clearly distinguished as the differ-
ence in emission color. The basic concept of this study, i.e.,
the mixing of mechanochromic and non-mechanochromic
fluorophores has advantages in terms of the availability of
MCL dyes and the variety of potential combinations of two-
component dyes and should thus generate a variety of new
MCL systems with unique properties. Eventually, this should
lead to the development of new fundamental insights and
practical applications of MCL-active dyes. Further studies on
56 | CrystEngComm, 2019, 21, 53–59
This journal is © The Royal Society of Chemistry 2019

View Article Online
CrystEngComm
Paper
other two-component MCL systems are currently in progress
in our laboratory.
was extracted with ethyl acetate three times, and the com-
bined organic layer was washed with water and brine, and
dried over anhydrous Na2SO4. After removal of the solvent un-
der reduced pressure, crude pyrene-1-boronic acid was used
directly in the next step.
Experimental
General
The mixture of pyrene-1-boronic acid and 3,3′-dibromo-
5,5′-dihexyl-2,2′-bithiophene (2, 3.65 mmol, 1.8 g) in 1,4-diox-
ane (50 mL) and 2 M aqueous K2CO3 solution (9 mL) was
degassed under ultrasonic irradiation. To the mixture was
added PdIJPPh3)4 (0.70 mmol, 813 mg), and the mixture was
further degassed under ultrasonic irradiation. After the mix-
ture was stirred at 95 °C for 12 h, water and dichloromethane
were added to the mixture. The organic layer was separated
and the aqueous layer was extracted with dichloromethane
three times. The combined organic layer was washed with wa-
ter and brine, and dried over anhydrous Na2SO4. After re-
moval of the solvent under reduced pressure, crude product
was purified with silica-gel column chromatography (hexane/
toluene = 8 : 1) followed by washing with hexane, and
reprecipitated from ethyl acetate to give 5,5′-dihexyl-3,3′-
diIJpyren-1-yl)-2,2′-bithiophene (1) in 41% yield (1.1 g) as pale
yellow powder.
All air-sensitive experiments were carried out under an atmo-
sphere of argon unless otherwise noted. IR spectra were
recorded on a Nicolet iS10 FT-IR spectrometer. 1H and 13C
NMR spectra were recorded on a Bruker DRX-500 spectrome-
ter (500 MHz for 1H and 126 MHz for 13C) using tetra-
methylsilane (0 ppm for 1H and 77.00 ppm for 13C) as an
internal standard. Solid-state 13C NMR spectra were recorded
on a Bruker Avance III 600 MHz spectrometer (151 MHz for
13C) equipped with an E-free probe for 4.0 mm outer diame-
ter rotors. The samples for solid-state 13C CP-MAS (magic an-
gle spinning) NMR measurements were packed into a 4.0
mm o.d. zirconia NMR rotor. 13C CP-MAS NMR experiments
were performed at MAS frequency 11.0 kHz and temperature
298 K. Spinal-64 proton high power decoupling15 of 78 kHz
was employed during each 13C acquisition. 13C chemical
shifts were externally referenced to the methyl carbon reso-
nance of adamantane at 38.48 ppm (TMS at 0.0 ppm).16
A
5,5′-Dihexyl-3,3′-diIJpyren-1-yl)-2,2′-bithiophene (1).
miniature fiber-optic spectrometer (FLAME-S-XR1-ES, Ocean
Optics) and a handy UV lamp (365 nm, LUV-6, AS ONE) were
used for the measurements of mechanochromic lumines-
cence. Fluorescence and diffuse reflectance spectra were mea-
sured on a JASCO FP-8300 fluorescence spectrometer. The ab-
solute fluorescence quantum yields were determined using a
100 mm ϕ integrating sphere JASCO ILF-835. Fluorescence
lifetime measurements were recorded on a nanosecond visi-
ble/near-infrared fluorescence spectrometer (VITA). The
curve-fitting analyses of fluorescence lifetime decays were car-
ried out by using Origin 2018 (OriginLab). Powder X-ray dif-
fraction (PXRD) measurements were performed on a Rigaku
SmartLab system using CuKα radiation. DSC data were
recorded on a Seiko Instruments DSC-6100 equipped with a
liquid nitrogen cooling unit (heating rate: 10 °C min−1). Melt-
ing points were determined on a Stuart melting point appara-
tus SMP3 and uncorrected. High-resolution mass spectrum
(HRMS) was recorded on a JEOL JMS-700 mass spectrometer
(EI). Silica gel 60 N (spherical, neutral, 63–210 μm) was used
for column chromatography. LC analyses were done on silica-
gel 60 F254-precoated aluminum backed sheets (E. Merck).
3,3′-Dibromo-5,5′-dihexyl-2,2′-bithiophene (2) was synthesized
according to the literature procedure.9 Other reagents were
commercially available.
41% yield; pale yellow solid; Mp. 125–126 °C; IR (KBr): vmax
3036, 2927, 2825, 1907, 1602, 1585, 1509, 1488, 1465, 1376,
1328, 1243, 1195, 1176, 1150, 1115, 1093, 1074, 961, 949, 930,
889, 839, 823, 792, 751, 717, 683 cm−1 1H NMR (500 MHz,
;
CDCl3, 283 K): δ (ppm) 8.07 (d, J = 7.6 Hz, 1H, Hr′), 7.92 (t, J
= 7.6 Hz, 1H, Hs′), 7.81 (d, J = 7.6 Hz, 1H, Ht′), 7.65 (d, J = 8.3
Hz, 1H, Hm), 7.64 (d, J = 8.8 Hz, 1H, Hp′), 7.58 (d, J = 7.3 Hz,
1H, Hr), 7.54 (t, J = 7.3 Hz, 1H, Hs), 7.51 (d, J = 8.5 Hz, 1H,
Ho), 7.47 (d, J = 7.3 Hz, 1H, Ht), 7.45 (d, J = 8.5 Hz, 1H, Hp),
7.44 (d, J = 8.3 Hz, 1H, Hl), 7.15 (d, J = 8.8 Hz, 1H, Ho′), 7.07
(d, J = 7.7 Hz, 1H, Hm′), 7.04 (d, J = 9.3 Hz, 1H, Hw′), 7.01 (d,
J = 9.3 Hz, 1H, Hv), 7.00 (d, J = 7.7 Hz, 1H, Hl′), 6.96 (d, J =
9.3 Hz, 1H, Hw), 6.89 (d, J = 9.3 Hz, 1H, Hv′), 6.63 (s, 1H,
Hh), 6.52 (s, 1H, Hh′), 2.84–2.77 (m, 4H, Hf, Hf′), 1.75–1.67
(m, 4H, He, He′), 1.42–1.36 (m, 4H, Hd, Hd′), 1.33–1.28 (m,
8H, Hb, Hb′, Hc, Hc′), 0.92–0.87 (m, 6H, Ha, Ha′); 13C NMR
(126 MHz, CDCl3, 283 K): δ (ppm) 145.3 (Cg′), 145.1 (Cg),
138.9 (Ci), 137.5 (Ci′), 131.53 (Ck′), 131.52 (Ck), 130.9 (Cq′),
130.5 (Cu′), 130.2 (Cj′), 130.05 (Cj), 130.01 (Cq), 129.73 (Cn),
129.67 (Cu), 129.3 (Cn′), 129.0 (Ch′), 128.4 (Ch), 127.8 (Cx),
127.4 (Cl), 127.2 (Cx′), 126.8 (Cl′), 126.45 (Co′), 126.44 (Cp′),
Synthesis of 5,5′-dihexyl-3,3′-diIJpyren-1-yl)-2,2′-bithiophene
(1). To a stirred solution of 1-bromopyrene (9.37 mmol 2.63
g) in THF (40 mL), a hexane solution of n-BuLi (2.6 M, 14.1
mmol, 6.50 mL) was added dropwise through a syringe at
−78 °C. To the mixture was added dropwise triisopropyl bo-
rate (18.7 mmol, 3.52 g) at −78 °C, and the reaction mixture
was gradually warmed to room temperature and stirred for
24 hours. To the reaction mixture was added 2 M aqueous
HCl, and the organic layer was separated. The aqueous layer
This journal is © The Royal Society of Chemistry 2019
CrystEngComm, 2019, 21, 53–59 | 57

View Article Online
Paper
CrystEngComm
126.3 (Co), 126.2 (Cp), 125.7 (Cv′), 125.6 (Cv), 125.2 (Cs′),
124.8 (Cs), 124.38 (Cw′, Ct′), 124.35 (Cm), 124.22 (Cz′), 124.17
(Cr′, Cr), 124.1 (Cy′), 123.8 (Ct), 123.6 (Cw), 123.5 (Cy), 123.4
(Cm′), 123.1 (Cz), 31.5 (Cc′, Cc), 31.4 (Ce), 31.3 (Ce′), 30.2 (Cf),
30.1 (Cf′), 28.8 (Cd), 28.7 (Cd′), 22.6 (Cb′, Cb), 14.2 (Ca′, Ca);
HRMS-EI (m/z): [M]+ calcd for C52H46S2, 734.3041; found,
734.3029.
2 For recent examples of single-component dyes that exhibit
MCL based on crystal-to-amorphous transitions, see: (a)
S. A. Sharber, K.-C. Shih, A. Mann, F. Frausto, T. E. Haas,
M.-P. Nieh and S. W. Thomas, Chem. Sci., 2018, 9, 5415;
(b) B. Li, K. Seth, B. Niu, L. Pan, H. Yang and H. Ge,
Angew. Chem., Int. Ed., 2018, 57, 3401; (c) D. T. Walters,
R. B. Aghakhanpour, X. B. Powers, K. B. Ghiassi, M. M.
Olmstead and A. L. Balch, J. Am. Chem. Soc., 2018, 140,
7533; (d) L. Wilbraham, M. Louis, D. Alberga, A. Brosseau,
R. Guillot, F. Ito, F. Labat, R. Métivier, C. Allain and I.
Ciofini, Adv. Mater., 2018, 30, 1800817; (e) Y. Takeda, T.
Kaihara, M. Okazaki, H. Higginbotham, P. Data, N. Tohnai
and S. Minakata, Chem. Commun., 2018, 54, 6847; ( f ) T.
Seki, K. Ida and H. Ito, Mater. Chem. Front., 2018, 2, 1195;
(g) H. Mori, K. Nishino, K. Wada, Y. Morisaki, K. Tanaka
and Y. Chujo, Mater. Chem. Front., 2018, 2, 573; (h) B. Xu,
H. Wu, J. Chen, Z. Yang, Z. Yang, Y.-C. Wu, Y. Zhang, C.
Jin, P.-Y. Lu, Z. Chi, S. Liu, J. Xu and M. Aldred, Chem.
Sci., 2017, 8, 1909; (i) T. Seki, K. Kobayashi and H. Ito,
Chem. Commun., 2017, 53, 6700; ( j) Y. Jiang, G. Li, W.
Che, Y. Liu, B. Xu, G. Shan, D. Zhu, Z. Su and M. R. Bryce,
Chem. Commun., 2017, 53, 3022; (k) B. Y.-W. Wong, H.-L.
Wong, Y.-C. Wong, V. K.-M. Au, M.-Y. Chan and V. W.-W.
Yam, Chem. Sci., 2017, 8, 6936; (l) F. Wang, C. A. DeRosa,
M. L. Daly, D. Song, M. Sabat and C. L. Fraser, Mater.
Chem. Front., 2017, 1, 1866; (m) M. Okazaki, Y. Takeda, P.
Data, P. Pander, H. Higginbotham, A. P. Monkman and S.
Minakata, Chem. Sci., 2017, 8, 2677.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was partly supported by JSPS KAKENHI Grant Num-
ber 18H04508 in Grant-in-Aid for Scientific Research on Inno-
vative Areas “Soft Crystals: Area No. 2903”, by the Tonen Gen-
eral Research Foundation, and by the CASIO Science
Promotion Foundation. The authors are grateful to Drs.
Hiroyuki Matsuzaki and Yusuke Okabayashi for the measure-
ment of the fluorescence lifetime values (National Metrology
Institute of Japan, National Institute of Advanced Industrial
Science and Technology). The authors also thank Ms. Masayo
Ishikawa (Suzukakedai Materials Analysis Division, Technical
Department, Tokyo Institute of Technology) for EI-HRMS
analyses.
Notes and references
1 For recent reviews, see: (a) Y. Sagara, S. Yamane, M. Mitani,
C. Weder and T. Kato, Adv. Mater., 2016, 28, 1073; (b) Z. Ma,
Z. Wang, M. Teng, Z. Xu and X. Jia, ChemPhysChem,
2015, 16, 1811; For seminal examples on MCL dyes, see: (c)
Y. Sagara, T. Mutai, I. Yoshikawa and K. Araki, J. Am. Chem.
Soc., 2007, 129, 1520; (d) J. Kunzelman, M. Kinami, B. R.
Crenshaw, J. D. Protasiewicz and C. Weder, Adv. Mater.,
2008, 20, 119; (e) H. Ito, T. Saito, N. Oshima, N. Kitamura, S.
Ishizaka, Y. Hinatsu, M. Wakeshima, M. Kato, K. Tsuge and
M. Sawamura, J. Am. Chem. Soc., 2008, 130, 10044; ( f ) T.
Abe, T. Itakura, N. Ikeda and K. Shinozaki, Dalton Trans.,
2009, 711; (g) Y. Ooyama, Y. Kagawa, H. Fukuoka, G. Ito and
Y. Harima, Eur. J. Org. Chem., 2009, 5321; (h) G. Zhang, J.
Lu, M. Sabat and C. L. Fraser, J. Am. Chem. Soc., 2010, 132,
2160; (i) S. Perruchas, X. F. L. Goff, S. Maron, I. Maurin, F.
Guillen, A. Garcia, T. Gacoin and J.-P. Boilot, J. Am. Chem.
Soc., 2010, 132, 10967; ( j) X. Zhang, Z. Chi, H. Li, B. Xu, X.
Li, W. Zhou, S. Liu, Y. Zhang and J. Xu, Chem. – Asian J.,
2011, 6, 808; (k) X. Zhang, Z. Chi, J. Zhang, H. Li, B. Xu, X.
Li, S. Liu, Y. Zhang and J. Xu, J. Phys. Chem. B, 2011, 115,
7606; (l) S.-J. Yoon and S. Y. Park, J. Mater. Chem., 2011, 21,
8338; (m) Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen,
H. Lv, S. Wen, B. Li, L. Ye, B. Zou and W. Tian, Angew.
Chem., Int. Ed., 2012, 51, 10782; (n) K. Nagura, S. Saito, H.
Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda and
S. Yamaguchi, J. Am. Chem. Soc., 2013, 135, 10322; (o) M.
Krikorian, S. Liu and T. M. Swager, J. Am. Chem. Soc.,
2014, 136, 2952.
3 For recent examples of single-component dyes that exhibit
MCL between different crystalline states, see: (a) M. Jin, T.
Sumitani, H. Sato, T. Seki and H. Ito, J. Am. Chem. Soc.,
2018, 140, 2875; (b) K. K. Neena, P. Sudhakar, K. Dipak and
P. Thilagar, Chem. Commun., 2017, 53, 3641.
4 We have previously reported several single-component or-
ganic dyes with MCL properties: (a) S. Ito, T. Taguchi, T.
Yamada, T. Ubukata, Y. Yamaguchi and M. Asami, RSC Adv.,
2017, 7, 16953; (b) S. Ito, T. Yamada and M. Asami,
ChemPlusChem, 2016, 81, 1272; (c) S. Ito, T. Yamada, T.
Taguchi, Y. Yamaguchi and M. Asami, Chem. – Asian J.,
2016, 11, 1963.
5 (a) Z. Ma, Y. Ji, Z. Wang, G. Kuang and X. Jia, J. Mater.
Chem. C, 2016, 4, 10914; (b) H.-J. Kim, D. R. Whang, J.
Gierschner, C. H. Lee and S. Y. Park, Angew. Chem., Int. Ed.,
2015, 54, 4330.
6 Only few single-component dyes exhibit tricolor MCL. For ex-
amples, see: (a) X. Wu, J. Guo, Y. Cao, J. Zhao, W. Jia, Y.
Chen and D. Jia, Chem. Sci., 2018, 9, 5270; (b) W. Yang, C.
Liu, S. Lu, J. Du, Q. Gao, R. Zhang, Y. Liu and C. Yang,
J. Mater. Chem. C, 2018, 6, 290; (c) Z. Ma, Z. Wang, X. Meng,
Z. Ma, Z. Xu, Y. Ma and X. Jia, Angew. Chem., Int. Ed.,
2016, 55, 519; (d) Y. Sagara and T. Kato, Angew. Chem., Int.
Ed., 2011, 50, 9128, See also ref. 2m and 4b.
7 For a review, see: (a) R. Tan, S. Wang, H. Lan and S. Xiao,
Curr. Org. Chem., 2017, 21, 236; For recent examples, see: (b)
C. Ge, J. Liu, X. Ye, Q. Han, L. Zhang, S. Cui, Q. Guo, G. Liu,
Y. Liu and X. Tao, J. Phys. Chem. C, 2018, 122, 15744; (c)
58 | CrystEngComm, 2019, 21, 53–59
This journal is © The Royal Society of Chemistry 2019

View Article Online
CrystEngComm
P. S. Hariharan, G. Parthasarathy, A. Kundu, S. Karthikeyan,
Paper
11 As the melting point of DMQA (286 °C, decomp.) is above
150 °C, only the fraction of bithiophene 1 should be
liquefied upon heating to 150 °C.
12 (a) H. Liu, F. Yan, W. Li, B. Chu, W. Su, Z. Su, J. Wang, Z.
Hu and Z. Zhang, Appl. Phys. Lett., 2010, 96, 083301; (b) Z.
Su, W. Li, B. Chu and S. Wu, Thin Solid Films, 2011, 519,
2540.
13 Another possible explanation for the bathochromically-
shifted emission of BD-G is the partial quenching of the
emission by the absorption of DMQA. However, this explana-
tion would be less probable because there is only a slight
difference between the solid-state absorption spectra of BD-
M and BD-G (Fig. S7†).
14 D. Q. M. Craig, V. L. Kett, J. R. Murphy and D. M. Price,
Pharm. Res., 2001, 18, 1081.
15 B. M. Fung, A. K. Khitrin and K. Emolaev, J. Magn. Reson.,
2000, 142, 97.
16 R. K. Harris, E. D. Becker, S. M. Cabral de Menezes, P.
Granger, R. E. Hoffman and K. W. Zilm, Pure Appl. Chem.,
2008, 80, 59.
Y. Sagara, D. Moon and S. P. Anthony, Cryst. Growth Des.,
2018, 18, 3971; (d) C. Arivazhagan, P. Malakar, R. Jagan, E.
Prasad and S. Ghosh, CrystEngComm, 2018, 20, 3162; (e) H.
Zhu, S. Weng, H. Zhang, H. Yu, L. Kong, Y. Zhong, Y. Tian
and J. Yang, CrystEngComm, 2018, 20, 2772.
8 (a) G. Zografi and A. Newman, J. Pharm. Sci., 2017, 106, 5;
(b) Y. N. Thi, K. Rademann and F. Emmerling,
CrystEngComm, 2015, 17, 9029.
9 X. Guo, S. Wang, V. Enkelmann, M. Baumgarten and K.
Müllen, Org. Lett., 2011, 13, 6062.
10 Dipyrenylbithiophene 1 should form intramolecular stacks
of pyrene groups as shown in Fig. 2a, since the 1H NMR
signals of the two non-equivalent pyrenyl groups in 1 are up-
field shifted relative to those of pyrenyl groups of reported
pyrenylthiophene derivatives. For 1H NMR spectra of
pyrenylthiophene derivatives, see: (a) M. Ashizawa, K.
Yamada, A. Fukaya, R. Kato, K. Hara and J. Takeya, Chem.
Mater., 2008, 20, 4883; (b) Y. Aso, T. Okai, Y. Kawaguchi and
T. Otsubo, Chem. Lett., 2001, 30, 420.
This journal is © The Royal Society of Chemistry 2019
CrystEngComm, 2019, 21, 53–59 | 59