Arylaminomaleimides as a new class of aggregation-induced emission-active molecules obtained from organoarsenic compounds

Article abstract of DOI:10.1246/cl.2012.1445

Heating 1,4-dihydro-1,4-diarsininetetracarboxylic acid dianhydride with excess amounts of aniline and toluidine provided 3-anilino-N-phenylmaleimide and 3-p-toluidino-N-p-tolylmaleimide, respectively, which showed aggregation-induced emission (AIE) proper

Full text of DOI:10.1246/cl.2012.1445

doi:10.1246/cl.2012.1445
Published on the web November 3, 2012
1445
Arylaminomaleimides as a New Class of Aggregation-induced Emission-active
Molecules Obtained from Organoarsenic Compounds
Takuji Kato and Kensuke Naka*
Department of Chemistry and Materials Technology, Graduate School of Science and Technology,
Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585
(Received June 28, 2012; CL-120693; E-mail: kenaka@kit.ac.jp)
Heating 1,4-dihydro-1,4-diarsininetetracarboxylic acid dian-
hydride with excess amounts of aniline and toluidine provided
3-anilino-N-phenylmaleimide and 3-p-toluidino-N-p-tolylmale-
imide, respectively, which showed aggregation-induced emis-
sion (AIE) properties. Not only are the present aminomaleimide
derivatives simple-structured AIE-active molecules, but they
also exhibit pH responsive properties without introducing
additional functional units.
temperature in comparison with our present reaction.4 The
chemistry of organoarsines has been developed and their use in
organic synthesis arouses much interest.5 For example, arsonium
ylides prepared from triphenylarsine and other organoarsines are
stronger nucleophiles than the corresponding phosphonium
ylides.6 A transition-metal-catalyzed aryl-aryl exchange reaction
between metal-bound aryl and arsenic-bound aryl compounds
is an attractive strategy for the synthesis of organoarsenic
compounds.7 However, the number of reports disclosing the use
of organoarsines in organic synthesis has been rather limited
until now. This is because most organoarsenic compounds are
prepared from arsenic chlorides or arsenic hydrides and require
extreme caution in handling because of their volatility and
toxicity. We have developed the facile synthesis of a series of
1,4-dihydro-1,4-diarsinine derivatives as cyclic organoarsenic
compounds starting from methylarsonic acid, a nonvolatile
organoarsenic compound.8
Recently, luminophores displaying aggregation-induced
emission (AIE) have attracted attention as promising materials
for optical and electronic devices,1 because high loading of dyes
in a matrix can be achieved without self-quenching to increase
light-emitting efficiency.2 After Tang and co-workers developed a
series of propeller-shaped molecules that are nonemissive in
solution and are induced to emit efficiently in the solid state,2a
several examples of AIE-active dyes have been reported.
Researchers have realized that restricted intramolecular vibra-
tional and rotational motions in the solid state are responsible for
AIE phenomena, and their occurrence in sterically crowded
molecules has been presented. In solution, intramolecular rotation
is active and serves as a relaxation channel for decay of the
excited state. The extremely large fluorescence enhancement in
the solid state is attributed not only to the spatial confinement
effect, but also to the formation of specific supramolecular
stacking architecture associated with the unique electronic and
geometric characteristics of the designed molecules.
Maleimide is an electron-deficient heterocyclic ring that
is similar to phthalimide, naphthalimide, and peryleneimide
derivatives, which are known to be n-type semiconducting
materials for organic transistors. A series of maleimide-based
fluorophores exhibit a large variation of emission spectra
spanning the entire visible range.3 Emission intensity of these
dyes, however, decreases with increasing solution concentration.
Only some limited maleimide-based dyes show solid-state
luminescence.3b However, no AIE-type maleimide-based mole-
cules have been reported until now. Here, we found that the
aminomaleimide derivatives show AIE properties as well as
essential pH responsive properties. No luminescence properties
have been reported for the aminomaleimide derivatives, since
they were regarded as having fungistatic, herbicidal, insecticidal,
and antitumor activity.4
The reaction of 1,4-dihydro-1,4-diarsininetetracarboxylic
acid dianhydride (1)8c with aniline or toluidine at moderate
temperature produced the corresponding 1,4-dihydro-1,4-diarsi-
ninetetracarboxylic acid diimides 2. However, 3-anilino-N-
phenylmaleimide (3a) and 3-p-toluidino-N-p-tolylmaleimide
(3b) were obtained by heating 1 with an excess amount of
aniline at 130 °C for 7 h and toluidine at 150 °C for 9.5 h,
respectively (Scheme 1, Path A). After the excess amines were
removed under reduced pressure, the residues were recrystallized
to give yellow crystalline products, 3a and 3b in 39 and 42%
yield, respectively. Formation of the maleimde structures was
1
confirmed by H and 13C NMR spectra, and elemental analysis
(Figures S1 and S2).14 Both structures were further confirmed by
X-ray crystallography (Figure S3).9,14
Aminomaleimide derivative 3a also formed in 30% yield by
heating N,N¤-diphenyl-1,4-dihydro-1,4-diarsininetetracarboxylic
acid diimide (2a) with excess amounts of aniline. This result
suggests that the reaction of 1 with excess amounts of amines
occurred via formation of 2. This reaction pathway is supported
by the fact that 3-p-toluidino-N-phenylmaleimide (3c) can be
Me
As
Me
As
O
O
O
O
R1
2 equiv
NH2
R1
R1
N
O
N
O
As
As
O
O
O
O
Me
Me
2
1
2
1
excess
R
NH2
excess
O
R
NH2
R1
We also found that the aminomaleimide derivatives were
unexpectedly obtained by heating a 2,3-bisarsenic-substituted
maleic anhydride derivative, i.e., 1,4-dihydro-1,4-diarsininete-
tracarboxylic acid dianhydride (1), with excess amounts of
arylamines. Some aminomaleimide derivatives can be synthe-
sized by the reactions of acetylenedicarboxylic acid methyl
esters with excess amounts of amines at relatively high
(Path A)
(Path B)
O
R1
N
O
R2
N
O
R1
3a: R1 = H
3b: R1 = CH3
3a: R1 = R2 = H
N
N
H
H
3c: R1 = H, R2 = CH3
Scheme 1. Synthesis of arylaminomaleimides.
Chem. Lett. 2012, 41, 1445-1447
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1446
Figure 1. Normalized emission (black; -ex = 400 nm, red;
-
-
ex = 400 nm) and excitation spectra (black; -em = 505 nm, red;
em = 521 nm) of 3a (black) and 3b (red) in the solid state at
Figure 2. Emission spectra of 0.8 mM 3a in water/THF under
335 nm irradiation.
room temperature.
obtained by the reaction of 2 and toluidine in 39% yield
(Scheme 1, Path B). The reaction of Path B is regarded as
hydroamination at the 2,3-position of the maleimide unit in 2.10
Although the mechanism of the present reaction is unknown, it
is possible that the As-C bond, which is weaker than the N-C
bond, triggers the exchang of As-C=C for N-C=C, allowing
the hydroamination to proceed.11
The emission properties of 3a were studied in solutions of
THF and water with different water/THF ratios, as 3a is highly
soluble in THF, but not in water. Although the emission
spectrum of 3a in a solution that was 56 vol % water was
practically a flat line parallel to the abscissa (Figure 2), emission
was observed when the solution was 58 vol % water. The
emission was enhanced when the solutions were 58 to 66 vol %
water and these solutions emitted green light of 493 nm. This
result clearly indicates that 3a is AIE active. The emission
maximum wavelength in solution (-max = 493 nm) is slightly
blue-shifted relative to that in the crystalline state.
Arylaminomaleimide 3a (2.6 mg, 0.98 mmol) was dissolved
in toluene (2.6 mg) and the toluene solution was added to
ethylene-vinyl acetate copolymer (EVA, 10 mg) to obtain a
coating solution. The coating solution was cast on a Si substrate,
heated at 70 °C, and baked at 130 °C for 20 min to form a pale-
yellowish, self-standing transparent film. Although the coating
solution showed no luminescence, the film exhibited very
intense green luminescence at 497 nm (Figure S6).14 The
maximum luminescence wavelengths of the crystals, film, and
aggregate were 505, 497, and 493 nm, respectively. The lower
energy excitation maximum of the film was observed at 445 nm,
which was blue-shifted from that of the crystalline compound at
460 nm (Figure S7).14 The planarity of the C(12) aryl ring
relative to the maleimide ring might governed the HOMO-
LUMO band gap energy.
The present aminomaleimides contain secondary amino
moiety. The luminescence is expected to be turned “on” at
high pH and “off” at low pH, because of ionization and
deionizaion under acidic and basic conditions, respectively.
When an aqueous 35% HCl solution (0.1 mL) was added to
the emissive 66 vol % water/THF solution of 3a, the emission
immediately decreased and the solution was nonemissive
within 30 min (Figures S8 and S9).14 In the aqueous medium
with low pH, 3a was converted to the salt form which is
soluble in water, and hence, nonemissive and transparent. No
emission was observed for the solid sample obtained after
treatment with HCl. Thus, the aminomaleimide derivatives are
not only simple-structured AIE molecules, but they also
exhibit pH responsive properties without introducing addi-
tional functional units.13
Both 3a and 3b were recrystallized from methanol to give
yellow crystals. The crystalline solids of 3a and 3b exhibited
very intense green (-max = 505 nm) and yellow (-max = 521 nm)
luminescence at room temperature (Figure 1). However, no
emission was observed for solutions in solvents such as CHCl3
and THF. The solid-state structures of 3a and 3b were
determined by X-ray crystallography (Figure S3).14 The crystal
structure of 3b was previously reported4f and our crystal data are
in agreement with the reported data. In 3a, the three rings are
planar with the C(1) aryl ring twisted 44.3° from the N(1)
maleimide ring, while the C(11) aryl ring is twisted 19.3° from
the heterocyclic ring. In 3b, the three rings are planar with the
C(1) aryl ring twisted 42.6° from the N(1) maleimide ring, while
the C(12) aryl ring is twisted 12.7° from the heterocyclic ring.
Because of the steric hindrance between the C(1) aryl ring
and the N(1) maleimide ring, the planar conformation of the
aminomaleimide moiety prevents them from suffering emission
quenching due to ³-³ stacking in the solid state. The emission
maximum wavelength for 3b is red-shifted relative to that for 3a,
because of its more planar structure. The torsion angle between
the maleimide ring and the C(12) aryl ring for 3a is larger than
between the maleimide ring and C(11) ring for 3b.
To better understand the photophysical properties of the
present aminomaleimide derivatives, the HOMO and LUMO of
3a and 3b were calculated by using the B3LYP/6-31G(d)
method. The optimized geometries12 of 3a and 3b suggested that
the conformations of the aminomaleimides are planar and the
HOMOs of both molecules have significant orbital density at the
same moieties, impying that ³-conjugation is extended to the
maleimide rings and the arylamine substituents (Figure S4).14
In contrast, the LUMOs of both molecules are dominated by
orbitals from the maleimide rings. These electron distributions
reveal intramolecular charge transfer in the aminomaleimide
moieties of 3a and 3b.
Chem. Lett. 2012, 41, 1445-1447
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1447
In summary, we found that the arylaminomaleimide
derivatives were obtained by hydroamination of 2,3-bisarsenic-
substituted maleic anhydride derivatives, i.e., 1,4-dihydro-1,4-
diarsininetetracarboxylic acid dianhydride (1), with arylamines.
While they are practically nonluminescent when dissolved in
solution, they are induced to emit intensely upon aggregate
formation, clearly showing their AIE properties. They also
exhibited pH responsive properties without introducing addi-
tional functional units. The present AIE-active maleimide-based
fluorophores should be good candidates for spectral conversion
films for solar cells and agricultural uses, since high loading of
the dyes can be achieved to increase efficiency. Although the
mechanism of the present reaction is unknown at the moment,
the As-C bond, which is weaker than the N-C bond, possibly
triggers the exchange of As-C=C for N-C=C, allowing
hydroamination to proceed. Detailed studies of the reaction
mechanism and synthesis of aminomaleimide derivatives with
different structures are underway to gain an understanding of the
chemistry of the present organoarsenic compounds. We expect
that our work will stimulate the field of organoarsenic chemistry.
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3
4
This work was supported by a Grant-in-Aid for Scientific
Research (No. 20350054) from the Ministry of Education,
Culture, Sports, Science and Technology, Government of Japan,
and partially supported by the Asahi Glass Foundation. We
thank Drs. K. Matsukawa and S. Watase of the Osaka Municipal
Technical Research Institute for X-ray crystallographic analysis.
We also thank Mrs. K. Ebara and M. Ishikawa of Nissan
Chemical Industries, Ltd. for theoretical calculation.
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© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/