Color Tuning of the Aggregation-Induced Emission of Maleimide Dyes by Molecular Design and Morphology Control

Article abstract of DOI:10.1002/chem.201501519

Aggregation-induced emission (AIE)-active maleimide dyes, namely, 2-p-toluidino-N-p-tolylmaleimide, 3-phenyl-2-toluidino-N-p-tolylmaleimide, 2-p-thiocresyl-3-p-toluidino-N-p-tolylmaleimide, and 2,3-dithiocresyl-N-arylmaleimides, were synthesized by facile

Full text of DOI:10.1002/chem.201501519

DOI: 10.1002/chem.201501519
Full Paper
&
Dyes/Pigments
Color Tuning of the Aggregation-Induced Emission of Maleimide
Dyes by Molecular Design and Morphology Control
Hiroaki Imoto,[a] Kohei Kizaki,[a] Seiji Watase,[b] Kimihiro Matsukawa,[b] and Kensuke Naka*[a]
Abstract: Aggregation-induced emission (AIE)-active malei-
mide dyes, namely, 2-p-toluidino-N-p-tolylmaleimide, 3-
phenyl-2-toluidino-N-p-tolylmaleimide, 2-p-thiocresyl-3-p-tol-
uidino-N-p-tolylmaleimide, and 2,3-dithiocresyl-N-arylmalei-
mides, were synthesized by facile synthetic procedures. The
dyes show intense emission in the solid state, and emission
colors were controlled from green (lmax =527 nm) to orange
(lmax =609 nm) by varying the substituents at the 2- and 3-
positions of the maleimide and the packing structures in the
solid state. 2,3-Disubstituted maleimide dyes effectively un-
derwent redshifts of their emission wavelength. Further-
more, some of the dyes exhibited mechanochromism and
polymorphism, and their emission properties were dramati-
cally dependent on the morphology of the solid samples.
The mechanisms of the emission behaviors were investigat-
ed by X-ray diffraction. The substituent of the nitrogen atom
of the maleimide ring affected the intermolecular interac-
tions and short contacts, which were observed by single
crystal X-ray crystallography, to result in completely different
emission properties.
Introduction
tions. The maleimide skeleton has attracted much attention for
luminescent materials because of its simple and versatile mo-
lecular structure.[10,11] Several works reported 2,3-diaryl malei-
mides showing various emission colors in solution[10] and lumi-
nescent polymers from 2,3-disubstituted maleimide mono-
mers.[11] However, there is no research on AIE of maleimide
dyes. Recently, we have developed AIE-active amino maleimide
derivatives[12,13] that can be synthesized in a one-pot process
(Scheme 1).[13] Amino maleimide derivatives such as 2-p-toluidi-
Solid-state emission is a key property for developing lumines-
cent materials.[1] Much research has been carried out to ach-
ieve intense emission in the solid state.[2] Aggregation-induced
emission (AIE) is very effective strategy to accomplish this
goal.[3] After the landmark research of Tang and co-workers in
2001,[4] various kinds of AIE-active dyes have been reported,
such as tetraphenylsilole,[4,5] tetraphenylethylene,[6] and so on.[7]
The basic strategies for the molecular design of solid-state
emission are restriction of non-emissive deactivation due to
molecular motions and suppression of concentration self-
quenching. For example, excitons of tetraphenylsilole are deac-
tivated by rotation of the phenyl groups and twisting of the
C=C bond in a good solvent, but these molecular motions are
frozen in the solid state.[8] In addition, steric hindrance of pro-
peller-shaped molecular structures inhibits concentration self-
quenching.
Scheme 1. Synthesis of amino maleimide 1.[13]
On the other hand, most such dyes require complicated syn-
thetic routes starting from commercially available com-
pounds.[9] Facile synthetic methods for AIE dyes are still desired
not only in academic research, but also for industrial applica-
no-N-p-tolylmaleimide (1) exhibit green emission under UV irra-
diation in the solid state, but no emission is observed in good
solvents. Free rotation of the secondary aryl amine deactivates
the excitons by a non-emissive pathway in good solvents. The
molecular motion is restricted in the solid state, and X-ray crys-
tallography revealed an edge-to-face packing structure that
prevents self-quenching. These dyes are potential candidates
for solid-state emission materials because of their availability
and designability. Color tuning of AIE-active maleimide dyes is
required to expand their versatility for applications.
[a] Dr. H. Imoto, K. Kizaki, Prof. K. Naka
Faculty of Molecular Chemistry and Engineering
Graduate School of Science and Technology
Kyoto Institute of Technology
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
E-mail: kenaka@kit.ac.jp
Our previous theoretical study on 2-aryl amino maleimides
indicated that a luminescent center is localized at the five-
membered maleimide ring and the secondary aryl amine.[13]
Hence, the 2- and 3-positions of the maleimide rings are
thought to be dominant for the emission color of the malei-
[b] Dr. S. Watase, Dr. K. Matsukawa
Osaka Municipal Technical Research Institute
1-6-50 Morinomiya, Joto-ku, Osaka 536-8553 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501519.
Chem. Eur. J. 2015, 21, 12105 – 12111
12105
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
mide dyes. Herein, 2,3-disubstituted amino maleimides and
their thioether analogues were investigated to achieve longer
emission wavelengths than those of the 2-aryl amino malei-
mides we reported.[12] These designed maleimide derivatives
seemed to show AIE activity because free rotation around the
secondary amine and/or the thiol could deactivate the excitons
effectively in solution. We also found that some of the present
maleimide dyes displayed unique emission behaviors derived
from polymorphism[14,15] and mechanochromism.[14,16] This is
the first study on color tuning of AIE-active maleimide dyes on
the basis of molecular design and morphology control.
tion of triethylamine lead to a yellowish precipitate of 5 after
stirring at room temperature for 16 h.[19]
Compound 2 was converted to 2,3-dichlo-N-arylmaleimides
6a–6c with one equivalent of p-substituted aniline in toluene
under reflux for 4 h.[20] The same synthetic procedure as that of
5 resulted in 7a–7c. It is noteworthy that purification only re-
quired washing with methanol, except for 4. The chemical
structures of the newly synthesized compounds were con-
firmed by 1H and 13C NMR spectroscopy and high-resolution
mass spectrometry.[21]
Optical properties
The optical properties of the maleimide dyes are summarized
in Table 1. The absorption spectra were measured in solution
(c=1.0”10À4 m in dichloromethane). The absorption maxima
were redshifted by introduction of substituents in the 2- and
3-positions in comparison to 1 (lmax =386 nm). The phenyl
group of 4 and the thiocresyl group of 5 caused redshifts of
the absorption maxima relative to 1 by 14 and 21 nm, respec-
tively, probably because the
Results and Discussion
Synthesis of maleimide dyes
We synthesized 3-phenyl-2-toluidino-N-p-tolylmaleimide (4), 2-
p-thiocresyl-3-p-toluidino-N-p-tolylmaleimide (5), and 2,3-di-
thiocresyl-N-arylmaleimides 7a–7c by facile synthetic proce-
dures (Scheme 2).
electron-donating thiocresyl and
phenyl groups raised the HOMO
levels and thus narrowed the
HOMO–LUMO gaps. The thiocre-
syl group was more effective in
narrowing the HOMO–LUMO
gap than the phenyl group.
Thus, longer wavelengths of the
absorption maxima of 7a–7c
were attained in comparison to
5 by changing the substituents
from toluidyl to thiocresyl. Elec-
tron-donating or electron-rich
substituents at the 2- and 3-po-
sitions are reported to cause red-
shifts
of
the
absorption
maxima.[10] On the other hand,
the wavelengths of the absorp-
tion maxima of 7a–7c were ap-
proximately same because the
N-substituents[22] of 7a–7c had
little effect on HOMOs and
LUMOs (Supporting Information,
Figure S18).
Scheme 2. Synthesis of maleimide derivatives from 2.
All the maleimide derivatives
studied here showed no emis-
sion in good solvents such as di-
One-pot transformation of 2,3-dichloromaleic anhydride (2)
into monosubstituted imide 3-p-toluidino-2-chloro-N-p-tolylma-
leimide (3) proceeded with two equivalents of p-toluidine in
toluene under reflux for 24 h. Subsequently, Pd-catalyzed
Suzuki–Miyaura coupling reaction[17] of 3 and phenylboronic
acid at 908C for 5 h led to 4 after purification by silica-gel chro-
matography and recrystallization from dichloromethane and
methanol.[18] Nucleophilic substitution of 3 with p-thiocresyl
was performed in diethyl ether at 08C, and subsequent addi-
chloromethane, chloroform, and THF because of free rotation
about flexible amine and/or thiol bonds in the luminophores
in solutions. However, they showed luminescence in the solid
state due to restriction of rotation, which suggests that they
have AIE activity. The solid samples for measurement of photo-
luminescence (PL) spectra were prepared by recrystallization.
In the case of 7a, three types of crystals were obtained (7a1–
7a3). The emission wavelengths of the 2,3-disubstituted malei-
mide dyes (lmax =553–609 nm) were redshifted from that of
Chem. Eur. J. 2015, 21, 12105 – 12111
www.chemeurj.org
12106
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
The emission color of 4 was reversibly changed by external
mechanical stimulus.
Table 1. Optical properties of the maleimides.
[a]
Compound
labs,sol
Solid state
To explore the difference in morphology between these
states, XRD analysis was carried out (Supporting Information,
Figure S17). The XRD pattern of 4cry showed defined diffraction
peaks, whereas that of 4grd showed only a halo peak, which re-
veals amorphous character of 4grd. The defined peaks were re-
covered in 4sol, though the diffraction pattern was different
from that of 4cry. Crystalline-to-amorphous and amorphous-to-
crystalline transitions caused blue- and redshift of the PL spec-
tra, respectively. Such a color change due to the phase transi-
tion suggests that the maleimide ring as a luminescent center
must be strongly affected by close contacts to adjacent mole-
cules. Since conformational flexibility of the thiol can induce
unspecified contacts among molecules on external stimulus,
various intermolecular interactions formed as local structure
may lead to mechanochromism. In addition, mechanochrom-
ism was observed only in 4, whereas the emission colors of
other dyes were not changed by grinding. Therefore, the rigid
and bulky phenyl group at the 3-position regulated formation
of a robust hydrogen-bond network, as was observed in 1.[12]
As a result, mechanical stimulus can cause crystalline-to-amor-
phous transition. The detailed mechanism of the mechano-
chromism is under investigation.
[b]
[c]
Color
Shape
lPL,solid
FPL,solid
1
4
5
386[d]
400
yellow
yellow
yellow
orange
yellow
orange
orange
yellow
precipitate
cubic
fiber
cubic (7a1)
needle (7a2)
needle (7a3)
cubic
527[d]
553
578
591
555
606
609
562
0.07[d]
0.03
407
0.02
<0.01
<0.01
0.14
<0.01
0.03
7a
430
7b
7c
430
435
needle
[a] Absorption maxima in dichloromethane (c=1.0”10À4 m). [b] PL maxi-
mum wavelength in the solid state. [c] Absolute quantum yield in the
solid state. [d] Ref. [13].
1 (lmax =527 nm). Generally, as the substituents of the 2- and
3-positions became more electron rich, the emission wave-
length became longer; this tendency is in good accordance
with their absorption wavelengths. The emission wavelengths
of 4 and 5 were shifted from that of 1 (lmax =527 nm) to 553
and 578 nm, respectively. As described previously, the emission
colors are controlled by the substituents on the 2- and 3-posi-
tions.[10,11] The emission properties of 7a–7c were quite differ-
ent due to the effect of the N-substituents, despite the similar
absorption spectra in solution. In addition, we found that 4
showed mechanochromism; emission color reversibly changed
between green and yellow on mechanical stimulus and treat-
ment with dichloromethane. Emission properties were affected
by not only chemical structures, but also by packing struc-
tures.
Polymorphism
Polymorphism of 7a was examined. Solid 7a was obtained as
an orange precipitate after the reaction, and subsequent re-
crystallization from dichloromethane and methanol gave mix-
ture of cubic orange crystals of 7a1 and needlelike yellow crys-
tals of 7a2. Besides, needlelike orange crystals of 7a3 were ob-
tained by recrystallization from dichloromethane and methanol
under relatively dilute conditions. We confirmed that all the
solid samples 7a1–7a3 were composed of the same compound
Mechanochromism
The PL spectra and emission behavior of 4 are shown in
Figure 1. The emission color of 4 after recrystallization from di-
chloromethane and methanol (4cry) was different from that of
1
by H and 13C NMR spectroscopy. For further verification, 7a2
was dissolved in dichloromethane, and recrystallization was
performed by adding methanol under dilute conditions. Even-
tually, needlelike orange crystals of 7a3 were isolated.
Each crystal form of 7a exhibited different emission wave-
lengths and quantum yields. Thus, 7a3 exhibited more inten-
sive emission at longer wavelength (lmax =606 nm, FPL =0.14)
than 7a1 (lmax =591 nm, FPL <0.01) and 7a2 (lmax =555 nm,
FPL > 0.01). Therefore, it is thought that the crystal packing
strongly affected their emission properties. We carried out XRD
analysis to compare the morphologies of 7a1–7a3. The diffrac-
tion patterns of 7a1–7a3 clearly differed from each other
(Figure 2, left). This indicates that 7a can form three types of
polymorph that have different emission behaviors in the solid
state (Figure 2, right). Flexibility of the two thioether bonds
probably allowed for various types of induced packing.
Figure 1. PL spectra (left) and photographs (right) of solid 4cry, 4grd, and 4sol
under UV irradiation at 365 nm.
the sample ground in a mortar (4grd). The yellow emission
(553 nm, FPL =0.03) of 4cry changed to green emission
(525 nm, FPL =0.03) of 4grd. Solid 4grd was dissolved in di-
chloromethane, and subsequently the solvent was removed to
obtain a dried residue (4sol). The emission maximum wave-
length of 4sol was 545 nm (FPL =0.04), similar to that of 4cry.
Crystallographic study
For further understanding of the effects of packing structure,
we performed single-crystal XRD of 7b and 7c (Figure 3).[23]
They showed different solid-state emission properties, al-
Chem. Eur. J. 2015, 21, 12105 – 12111
www.chemeurj.org
12107
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
The steric hindrance of the methoxyl group of 7b possibly in-
hibits the intermolecular face-to-face p–p stacking. The molec-
ular packing seemed to be a key factor in the emission proper-
ties of 7b and 7c, which were caused by the different intermo-
lecular interactions. The above-described formation of intermo-
lecular p–p stacking and CH–p interactions is not sufficient to
explain the difference in their emission properties.
We focused on the local surroundings of a maleimide lumi-
nophore in the crystals to explain the emission behaviors of
7b and 7c (Figure 4). The maleimide ring of 7b interacts with
Figure 2. XRD patterns (left) and photographs (right) of 7a1–7a3 under UV
irradiation at 365 nm.
Figure 4. Surroundings of the luminescent centers (dashed lines) of a) 7b
and b) 7c in the crystalline state.
Figure 3. Molecular packing in the crystal structures of a) 7b and b) 7c with
ellipsoids at 50% probability.
four neighboring molecules in the crystal through hydrogen
bonds (2.5–2.7 Š) between the electron-rich C=O and electron
poor CÀH bonds, whereas that of 7c interacts with two mole-
cules (2.6–2.7 Š). Such short contacts can lower the electron
density of the maleimide ring, and may reduce the HOMO–
LUMO gap, causing the redshift of the absorption and PL spec-
tra. In fact, the absorption edges of the diffuse-reflection spec-
tra of 7b and 7c were observed around 590 and 560 nm, re-
spectively (Supporting Information, Figure S19), contrary to the
trend of the wavelengths of their absorption maxima in solu-
tions. In the solid-state, the peak maximum of 7b was also ob-
served at longer wavelength (609 nm) than that of 7c
(562 nm). Thus, it suggests that the number of short contacts
in the local area near the luminophore is strongly related to
the emission properties.
though they had approximately the same UV/Vis absorption
maxima in solution (Table 1). Suitable single crystals for XRD
analysis were prepared by slow mixing of methanol with di-
chloromethane solutions, and 7b and 7c crystallized in the tri-
ꢀ
clinic P1 and monoclinic P21/c space groups, respectively. Both
compounds showed intramolecular p–p stacking of the S-sub-
stituted benzene rings,[24] and the average distances between
the two benzene rings were approximately 3.4 and 3.2 Š, re-
spectively. DFT calculations (Supporting Information, Fig-
ure S18) revealed that the LUMOs of 7b and 7c are located
mainly at the five-membered maleimide rings, which are
aligned in a nonparallel manner that restricts concentration
self-quenching in the crystals. In contrast to the similar intra-
molecular interactions, the intermolecular interactions of 7b
and 7c were substantially different. In the crystal packing of
7b, intermolecular edge-to-face CH–p interaction of the S-sub-
stituted benzene rings was observed, and the average distance
from the protons to the mean plane of the S-substituted ben-
zene rings is approximately 3.0 Š. In the crystalline packing of
7c, on the other hand, the S-substituted benzene rings interact
with the N-substituted benzene rings, and the average dis-
tance between the two benzene rings is approximately 3.5 Š.
Conclusion
We have described methodologies for color tuning of AIE-
active maleimide dyes by molecular design and morphology
control. The luminophore is located at the five-membered ma-
leimide ring, and its electronic state can be controlled by the
substituents in the 2- and 3-positions. Hence, the introduction
of thiocresyl, toluidyl, and phenyl groups in the maleimides
Chem. Eur. J. 2015, 21, 12105 – 12111
www.chemeurj.org
12108
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
tion effects. A correction for secondary extinction[27] was applied.
The structures were solved by heavy-atom Patterson methods[28]
and expanded by using Fourier techniques.[29] Some non-hydrogen
atoms were refined anisotropically, and the rest were refined iso-
tropically. Hydrogen atoms were refined by using the riding model.
The final cycle of full-matrix least-squares refinement on F2 was
based on observed reflections and variable parameters. In the case
of the crystalline product recrystallized from acetone, the final
cycle of full-matrix least-squares refinement on F was based on ob-
served reflections and variable parameters. All calculations were
performed with the CrystalStructure[30,31] crystallographic software
package. Crystal data and further information on XRD data collec-
tion are summarized in Tables S1 and S2 of the Supporting Infor-
mation.
drastically changed their emission colors. In addition, packing
structures strongly affected the emission properties. We found
that
4 exhibits mechanochromism; grinding the crystals
caused a blueshift, and treatment with dichloromethane led to
recovery of the original emission. Solid 7a showed three types
of polymorphism, and the emission properties depended on
the crystal packing. The packing structures in the solid state
were varied by means of the N-substituted benzene ring to
give completely different emission properties despite the simi-
lar absorption spectra in solution. Different modes of intermo-
lecular interactions were observed in 7b and 7c. The local sur-
roundings of the maleimide ring probably caused their differ-
ent emission behaviors, though prediction of packing struc-
tures is difficult.
This study suggests that AIE-active maleimides are promis-
ing candidates as multicolor luminescent dyes, stimuli-respon-
sive materials, and so on. Their simple molecular structures
and facile synthetic procedures are attractive for practical ap-
plications. Expansion to full-color emission, other functions of
maleimide dyes, and detailed mechanisms of their unique
properties are under research.
Synthesis
3-p-Toluidino-2-chloro-N-p-tolylmaleimide (3): The synthetic pro-
cedure followed the literature.[20] A toluene solution (20 mL) of 2
(0.784 g, 4.69 mmol) and p-toluidine (1.00 g, 9.34 mmol) was
heated to reflux for 24 h. After the reaction, the solvent was re-
moved in vacuo, and the residue was washed with methanol to
give
3
as yellow solid 0.812 g (2.49 mmol, 53%). 1H NMR
([D6]DMSO, 400 MHz): d=9.88 (s, 1H), 7.28–7.11 (m, 8H), 2.35 (s,
3H), 2.30 ppm (s, 3H). The solubility was too low to measure the
13C NMR spectrum.
Experimental Section
Materials
3-Phenyl-2-toluidino-N-p-tolylmaleimide (4): The synthetic proce-
dure followed the literature.[19] A saturated aqueous solution
(6.5 mL) of NaHCO3 was added to a solution of 3 (0.479 g,
1.47 mmol), phenylboronic acid (0.225 g, 1.85 mmol), and
[PdCl2(PPh3)2] (86.4 mg, 0.123 mmol) in toluene (15 mL) and etha-
nol (10 mL). The reaction mixture was heated at 908C under N2 at-
mosphere for 5 h. After cooling to room temperature, distilled
water (30 mL) was added, and the organic layer was extracted with
EtOAc (50 mL”2). The combined organic layers were washed with
brine (50 mL”3), and dried over MgSO4. After filtration, the sol-
vents were removed in vacuo. The residue was subjected to short-
column chromatography on silica gel with hexane/EtOAc=20/
1 (Rf =0.9) as eluent and recrystallization from dichloromethane
and methanol to give 4 as a yellow solid (76 mg, 0.206 mmol,
14%). 1H NMR (CDCl3, 400 MHz): d=7.37–7.27 (m, 5H), 7.15–7.07
(m, 5H), 6.85 (d, J=8.4 Hz, 2H), 6.58 (d, J=8.4 Hz, 2H), 2.40 (s, 3H),
2.23 ppm (s, 3H). The solubility was too low to measure the
13C NMR spectrum. HRMS (FAB) calcd for C24H20N2O2 [M]+:
368.1525; found: 368.1519.
Toluene, methanol, triethylamine, ethanol, diethyl ether, ethyl ace-
tate, hexane, and dichloromethane were purchased from Nacalai
Tesque, Inc. Dichloro maleic anhydride (2), p-toluidine, p-anisidine,
p-nitroaniline, p-toluenethiol, [PdCl2(PPh3)2], distilled water, silica
gel (Wakogel C-200), and magnesium sulfate anhydrous (MgSO4)
were purchased from Wako Pure Chemical Industry, Ltd. Phenylbor-
onic acid was purchased from Tokyo Chemical Industry Co., Ltd. All
commercially available chemicals were used without any purifica-
tion. 2-p-Toluidino-N-p-tolylmaleimide (1) was prepared by follow-
ing our previous paper.[13]
Measurements
1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on
a Bruker DPX-400 spectrometer in CDCl3 and [D6]DMSO with Me4Si
as internal standard. High-resolution mass spectra were obtained
on a JEOL JMS-SX102A spectrometer. UV/Vis spectra were recorded
on a Jasco spectrophotometer V-670 KKN. Emission spectra were
obtained on an JASCO fluorescence spectrophotometer FP-8500.
XRD data were recorded on a Smart Lab (Rigaku) with CuKa radia-
tion (l=1.5406 Š) in q/2q mode at room temperature. The 2q
scans were collected at 0.018 intervals, and the scan speed was
28minÀ1 in 2q.
2-p-Thiocresyl-3-p-toluidino-N-p-tolylmaleimide (5): An Et2O solu-
tion (10 mL) of
3 (99.7 mg, 0.305 mmol) and p-toluenethiol
(44.8 mg, 0.361 mmol) was stirred at 08C for 5 min, and subse-
quently triethylamine (3.85 g, 38.0 mmol) was added. After stirring
at room temperature for 16 h, the solvent was removed in vacuo,
and the residue was washed with methanol to give 5 as yellow
solid (96 mg, 0.232 mmol, 76%). 1H NMR (CDCl3, 400 MHz): d=
7.41–6.88 (m, 13H), 2.38 (s, 3H), 2.35 (s, 3H), 2.26 ppm (s, 3H). The
solubility was too low to measure the 13C NMR spectrum. HRMS
(FAB) calcd for C25H22N2O2S [M]+: 414.1402; found: 414.1406.
XRD of single-crystalline products
Single crystals were mounted on glass fibers with epoxy resin. In-
tensity data were collected at room temperature on a Rigaku
RAXIS RAPID II imaging-plate area detector with graphite-mono-
chromated MoKa radiation. The crystal-to-detector distance was
127.40 mm. Readout was performed in the 0.100 mm pixel mode.
The data were collected at room temperature to a maximum 2q
value of 55.08. Data were processed by the PROCESS-AUTO[25] pro-
gram package. An empirical or numerical absorption correction[26]
was applied. The data were corrected for Lorentzian and polariza-
2,3-Dichlo-N-p-tolylmaleimide (6a): The synthetic procedure fol-
lowed the literature.[20] A toluene solution (15 mL) of 2 (2.00 g,
12.1 mmol) and p-toluidine (1.29 g, 12.0 mmol) was heated to
reflux for 4 h. After the reaction, the solvent was removed in
vacuo, and the residue was washed with methanol to give 6a as
a
colorless solid 2.18 g (8.52 mmol, 71%). 1H NMR (CDCl3,
400 MHz): d=7.27 (d, J=8.8 Hz, 2H), 7.19 (d, J=8.4 Hz, 2H),
Chem. Eur. J. 2015, 21, 12105 – 12111
www.chemeurj.org
12109
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2.38 ppm (s, 3H); 13C NMR (CDCl3, 100 MHz): d=162.1, 138.9, 133.5,
130.0, 127.9, 126.0, 21.2 ppm.
Angew. Chem. Int. Ed. 2007, 46, 4273; Angew. Chem. 2007, 119, 4351;
e) C.-H. Zhao, A. Wakamiya, S. Yamaguchi, Macromolecules 2007, 40,
3898; f) M. Shimizu, K. Mochida, T. Hiyama, Angew. Chem. Int. Ed. 2008,
47, 9760; Angew. Chem. 2008, 120, 9906; g) W.-Y. Lai, R. Xia, Q.-Y. He,
P. A. Levermore, W. Huang, D. D. C. Bradley, Adv. Mater. 2009, 21, 355.
[2] Recent papers, see: a) R. Yoshii, A. Hirose, K. Tanaka, Y. Chujo, J. Am.
2,3-Dichlo-N-p-anisoylmaleimide (6b): A toluene solution (20 mL)
of 2 (1.00 g, 6.02 mmol) and p-anisidine (0.742 g, 6.03 mmol) was
heated to reflux for 4 h. After the reaction, the solvent was re-
moved in vacuo, and the residue was washed with methanol to
give 6b as a colorless solid (1.26 g, 4.64 mmol, 77%). 1H NMR
(CDCl3, 400 MHz): d=7.23 (d, J=8.8 Hz, 2H), 6.98 (d, J=8.8 Hz,
2H), 3.84 ppm (s, 3H); 13C NMR (CDCl3, 100 MHz): d=162.3, 159.7,
133.5, 127.6, 123.1, 114.7, 55.5 ppm.
Chem. Soc. 2014, 136, 18131; b) R. Yoshii, A. Hirose, K. Tanaka, Y. Chujo,
ˇ
ˇ
Chem. Eur. J. 2014, 20, 8320; c) P. Galer, R. C. Korosec, M. Vidmar, B. Sket,
J. Am. Chem. Soc. 2014, 136, 7383; d) M. V. Ramakrishnam Raju, H.-C.
Lin, Org. Lett. 2014, 16, 5564; e) A. Goel, A. Sharma, M. Rawat, R. S.
Anand, R. Kant, J. Org. Chem. 2014, 79, 10873; f) H. Braunschweig, S.
Ghosh, J. O. C. Jimenez-Halla, J. H. Klein, C. Lambert, K. Radacki, A. Stef-
fen, A. Vargas, J. Wahler, Chem. Eur. J. 2015, 21, 210.
2,3-Dichlo-N-p-nitrophenylmaleimide (6c):
A toluene solution
(15 mL) of (2.00 g, 12.0 mmol) and p-nitroaniline (1.66 g,
2
[3] Reviews, see: a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun. 2009,
4332; b) D. Ding, K. Li, B. Liu, B. Z. Tang, Acc. Chem. Res. 2013, 46, 2441;
c) R. Hu, N. L. C. Leung, B. Z. Tang, Chem. Soc. Rev. 2014, 43, 4494; d) J.
Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang, B. Z. Tang, Adv. Mater. 2014,
26, 5429.
[4] J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X.
Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem. Commun. 2001, 1740.
[5] Z. Li, Y. W. Dong, J. W. Y. Lam, J. Sun, A. Qin, M. Haüßler, Y. P. Dong,
H. H. Y. Sung, I. D. Williams, H. S. Kwok, B. Z. Tang, Adv. Funct. Mater.
2009, 19, 905.
[6] W. Z. Yuan, S. Chen, J. W. Y. Lam, C. Deng, P. Lu, H. H.-Y. Sung, I. D. Wil-
liams, H. S. Kwok, Y. Zhang, B. Z. Tang, Chem. Commun. 2011, 47, 11216;
Y. Liu, S. Chen, J. W. Y. Lam, P. Lu, R. T. K. Kwok, F. Mahtab, H. S. Kwok,
B. Z. Tang, Chem. Mater. 2011, 23, 2536; J. Huang, X. Yang, J. Wang, C.
Zhong, L. Wang, J. Qin, Z. Li, J. Mater. Chem. 2012, 22, 2478.
[7] a) S. Kamino, Y. Horio, S. Komeda, K. Minoura, H. Ichikawa, J. Horigome,
A. Tatsumi, S. Kaji, T. Yamaguchi, Y. Usami, S. Hirota, S. Enomoto, Y.
Fujita, Chem. Commun. 2010, 46, 9013; b) M. Nakamura, T. Sanji, M.
Tanaka, Chem. Eur. J. 2011, 17, 5344; c) T. Hirose, K. Higashiguchi, K. Mat-
suda, Chem. Asian J. 2011, 6, 1057; d) K. Kokado, Y. Chujo, J. Org. Chem.
2011, 76, 316; e) R. Yoshii, A. Nagai, K. Tanaka, Y. Chujo, Chem. Eur. J.
2013, 19, 4506.
[8] a) Y. Dong, J. W. Y. Lam, A. Qin, J. Liu, Z. Li, B. Z. Tang, J. Sun, H. S. Kwok,
Appl. Phys. Lett. 2007, 91, 011111; b) A. Qin, C. K. W. Jim, Y. Tang, J. W. Y.
Lam, J. Liu, F. Mahtab, P. Gao, B. Z. Tang, J. Phys. Chem. B 2008, 112,
9281.
[9] For example, the synthetic procedure of tetraphenylsilole is in accord-
ance with the following paper cited in ref. [4]; E. Braye, W. Hubel, I. Ca-
plier, J. Am. Chem. Soc. 1961, 83, 4406.
[10] H.-C. Yeh, W.-C. Wu, C.-T. Chen, Chem. Commun. 2003, 404.
[11] a) K. Onimura, M. Matsushima, K. Yamabuki, T. Oishi, Polym. J. 2010, 42,
290; b) M. Nakamura, K. Yamabuki, T. Oishi, K. Onimura, J. Polym. Sci.
Part A J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 4945.
[12] T. Kato, K. Naka, Chem. Lett. 2012, 41, 1445.
[13] K. Kizaki, H. Imoto, T. Kato, K. Naka, Tetrahedron 2015, 71, 643.
[14] Q. Qi, J. Zhang, B. Xu, B. Li, S. X.-A. Zhang, W. Tian, J. Phys. Chem. C
2013, 117, 24997.
12.0 mmol) was heated to reflux for 4 h. After the reaction, the sol-
vent was removed in vacuo, and the residue was washed with
methanol to give 6c as a colorless solid (0.55 g, 1.92 mmol, 16%).
1H NMR (CDCl3, 400 MHz): d=8.36 (d, J=9.2 Hz, 2H), 7.67 ppm (d,
J=9.2 Hz, 2H); 13C NMR (CDCl3, 100 MHz): d=161.2, 146.7, 136.2,
134.2, 125.7, 124.7 ppm.
2,3-Dithiocresyl-N-p-tolylmaleimide (7a): An Et2O solution (10 mL)
of 6a (0.300 g, 1.17 mmol) and p-toluenethiol (0.296 g, 2.39 mmol)
was stirred at 08C for 5 min, and subsequently triethylamine
(0.247 g, 2.44 mmol) was added. After stirring at room temperature
for 16 h, the solvent was removed in vacuo, and the residue was
washed with methanol to give 7a as a yellow solid (0.465 g,
1
1.08 mmol, 92%). H NMR (CDCl3, 400 MHz): d=7.23 (d, J=8.4 Hz,
4H), 7.17 (s, 4H), 7.09 (d, J=8.0 Hz, 4H), 2.33 ppm (s, 9H); 13C NMR
(CDCl3, 100 MHz): d=165.7, 139.0, 137.8, 136.1, 132.7, 129.9, 129.6,
128.8, 125.8, 125.5, 21.3, 21.1 ppm; HRMS (FAB) calcd for
C25H21NO2S2 [M]+: 431.1014; found: 431.1007.
2,3-Dithiocresyl-N-p-anisoylmaleimide (7b): An Et2O solution
(10 mL) of 6b (0.499 g, 1.83 mmol) and p-toluenethiol (0.474 g,
3.82 mmol) was stirred at 08C for 5 min, and subsequently triethyl-
amine (0.380 g, 3.76 mmol) was added. After stirring at room tem-
perature for 16 h, the solvent was removed in vacuo, and the resi-
due was washed with methanol to give 7b as an orange solid
1
0.786 g (1.76 mmol, 96%). H NMR (CDCl3, 400 MHz): d=7.24–7.18
(m, 6H), 7.09 (d, J=8.0 Hz, 4H), 6.89 (d, J=9.2 Hz, 2H), 3.77 (s, 3H),
2.33 ppm (s, 6H); 13C NMR (CDCl3, 100 MHz): d=165.8, 159.0, 139.0,
136.0, 132.7, 129.9, 127.4, 125.5, 124.1, 114.3, 55.5, 21.3 ppm; HRMS
(FAB) calcd for C25H21NO3S2 [M]+: 447.0963; found: 447.0972.
2,3-Dithiocresyl-N-p-nitrophenylmaleimide (7c): An Et2O solution
(10 mL) of 6c (99.5 mg, 0.347 mmol) and p-toluenethiol (88.7 mg,
0.714 mmol) was stirred at 08C for 5 min, and subsequently trie-
thylamine (90 mg, 0.089 mmol) was added. After stirring at room
temperature for 16 h, the solvent was removed in vacuo, and the
residue was washed with methanol to give 7c as a yellow solid
[15] a) Z. Zheng, Z. Yu, M. Yang, F. Jin, Q. Zhang, H. Zhou, J. Wu, Y. Tian, J.
Org. Chem. 2013, 78, 3222; b) H. Unesaki, T. Kato, S. Watase, K. Matsuka-
wa, K. Naka, Inorg. Chem. 2014, 53, 8270.
[16] a) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, J. Xu, Chem.
Soc. Rev. 2012, 41, 3878; b) X. Zhang, Z. Chi, Y. Zhang, S. Liu, J. Xu, J.
Mater. Chem. C 2013, 1, 3376.
[17] N. Miyaura, S. Suzuki, J. Chem. Soc. Chem. Commun. 1979, 866.
[18] K. Onimura, M. Matsushima, M. Nakamura, T. Tominaga, K. Yamabuki, T.
Oishi, J. Polym. Sci. Part A J. Polym. Sci. Part A: Polym. Chem. 2011, 49,
3550.
1
154 mg (0.333 mmol, 96%). H NMR (CDCl3, 400 MHz): d=8.23 (d,
J=8.8 Hz, 2H), 7.62 (d, J=9.2 Hz, 2H), 7.26 (d, J=7.2 Hz, 4H), 7.12
(d, J=8.0 Hz, 4H), 2.35 ppm (s, 6H); 13C NMR (CDCl3, 100 MHz): d=
164.6, 145.9, 139.5, 137.3, 136.5, 132.9, 130.0, 125.3, 124.9, 124.3,
21.3 ppm; HRMS (FAB) calcd for C24H18N2O4S2 [M]+: 462.0708;
found: 462.0716.
[19] M. P. Robin, P. Wilson, A. B. Mabire, J. K. Kiviaho, J. E. Raymond, D. M.
Haddleton, R. K. O’Reilly, J. Am. Chem. Soc. 2013, 135, 2875.
[20] W. H. Watson, G. Wu, M. G. Richmond, J. Chem. Crystallogr. 2003, 33,
983.
Keywords: aggregation · dyes/pigments · luminescence ·
mechanochromism · polymorphism
[21] Compounds 3–5 did not have enough solubility to perform 13C NMR
analysis. For details, see Experimental Section and Supporting Informa-
tion.
[22] The term “N-substituent” is used in the sense of “substituent on the N
atom of the maleimide ring” for the sake of simplicity.
[1] a) K.-T. Wong, Y.-Y. Chien, R.-T. Chen, C.-F. Wang, Y.-T. Lin, H.-H. Chiang,
P.-Y. Hsieh, C.-C. Wu, C. H. Chou, Y. O. Su, G.-H. Lee, S.-M. Peng, J. Am.
Chem. Soc. 2002, 124, 11576; b) Z. Xie, B. Yang, F. Li, G. Cheng, L. Liu, G.
Yang, H. Xu, L. Ye, M. Hanif, S. Liu, D. Ma, Y. Ma, J. Am. Chem. Soc. 2005,
127, 14152; c) C.-H. Zhao, A. Wakamiya, Y. Inukai, S. Yamaguchi, J. Am.
Chem. Soc. 2006, 128, 15934; d) A. Wakamiya, K. Mori, S. Yamaguchi,
[23] Detailed crystal data are listed in Tables S1 and S2 in the Supporting In-
formation. CCDC 1058130 (7b) and 1058131 (7c) contain the supple-
Chem. Eur. J. 2015, 21, 12105 – 12111
www.chemeurj.org
12110
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif..
the Crystallography Laboratory, University of Nijmegen: Nijmegen, The
Netherlands, 1999.
[29] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, S. Garcia-
Granda, R. O. Gould, J. M. M. Smits, C. Smykalla, PATTY, The DIRDIF Pro-
gram System; Technical Report of the Crystallography Laboratory, Uni-
versity of Nijmegen: Nijmegen, The Netherlands, 1992.
[30] CrystalStructure 3.8: Crystal Structure Analysis Package; Rigaku Americas:
The Woodlands, TX, 2000.
[24] The term “S-substituted benzene ring” is used in a sense of “benzene
ring attached to the maleimide ring through a sulfide bond” for the
sake of simplicity.
[25] PROCESS-AUTO, RAXIS data-processing software; Rigaku Corporation:
Tokyo, Japan, 1996.
[31] J. R. Carruthers, J. S. Rollett, P. W. Betteridge, D. Kinna, L. Pearce, A.
Larsen, E. Gabe, CRYSTALS Issue 11; Chemical Crystallography Laborato-
ry: Oxford, UK, 1999.
[26] T. Higashi, FS ABSCOR, Program for Absorption Correction; Rigaku Corpo-
ration: Tokyo, Japan, 1995.
[27] A. C. Larson in Crystallographic Computing (Ed.: F. R. Ahmed), Munks-
gaard, Copenhagen, 1970, pp. 291–294.
[28] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de Gelder, R.
Israel, J. M. M. Smits, The DIRDIF-99 Program System; Technical Report of
Received: April 19, 2015
Published online on July 14, 2015
Chem. Eur. J. 2015, 21, 12105 – 12111
www.chemeurj.org
12111
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim