One-pot synthesis and structure∑property relationship of aminomaleimides: Fluorescence efficiencies in monomers and aggregates easily tuned by switch of aryl and alkyl

Article abstract of DOI:10.1021/acs.joc.6b02706

Organic fluorophores have attracted great interest owing to their wide applications. They usually contain an electron-conjugated system with an aromatic moiety and show high emission in dilute solutions but weaker or even no emission upon aggregation. Her

Full text of DOI:10.1021/acs.joc.6b02706

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Article
One-Pot Synthesis and Structure-Property-Relationship of
Aminomaleimides: Fluorescence Efficiencies in Monomers
and Aggregates Easily Tuned by Switch of Aryl and Alkyl
Qiuhua Zhu, Ziwei Ye, Weijie Yang, Xiaotie Cai, and Ben Zhong Tang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02706 • Publication Date (Web): 21 Dec 2016
Downloaded from http://pubs.acs.org on December 24, 2016
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One‐Pot Synthesis and Structure‐Property‐Relationship of
Aminomaleimides: Fluorescence Efficiencies in Monomers and
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Aggregates Easily Tuned by Switch of Aryl and Alkyl
Qiuhua Zhu,*a Ziwei Ye,a Weijie Yang,a Xiaotie Caia and Ben Zhong Tang*b
aSchool of Pharmaceutical Sciences, Southern Medical University, 1838 Guangzhou Avenue North, Guangzhou 510515,
China
bDepartment of Chemistry, State Key Laboratory of Molecular Neuroscience, Institute of Molecular Functional Materials,
The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
ABSTRACT: Organic fluorophores have attracted
great interest owing to their wide applications. They
usually contain an electron conjugated system with
aromatic moiety and show high emission in dilute
solutions but weaker or even no emission upon
aggregation. Here, a simple one-pot three-component reaction (3CR) (method I) for the synthesis of various di- and
mono-substituted aminomaleimides (DAMIs and MAMIs) has been developed, and the reported 3CR (method II) has
been found to be efficient only for the synthesis of MAMIs with R2=alkyl. 12 AMIs were designed and synthesized
for investigation of the influence of structures on their optical properties in monomers and aggregates. It was found
that alkyl MAMIs, alkyl DAMIs and aryl AMIs/DAMIs show very different fluorescence efficiencies in different
solvents, and only MAMIs with butyl and oleyl show similar high emissions in powders to those in nonpolar
solutions. Single-crystal structures indicate that their fluorescence efficiencies in aggregates mainly correlate with
molecular packing modes. The efficient synthesis method, the sensitive fluorescence-on-off response to protic
solvents or polar solvents, and the unusual high emissions of AMI without any aromatic moiety in both monomer and
aggregates are expected to attract great interest in the fields of application and theory.
INTRODUCTION
Organic fluorophores have attracted great interest owing to their wide applications. They generally contain rigid
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aromatic groups and are highly emissive in dilute solutions but less or even no emissive upon aggregation,1which is
frequently referred to as the aggregation-caused quenching (ACQ) effect. Since fluorophores are usually used in
relative concentrated solutions or solid states, the ACQ effect becomes a thorny problem in many practical
applications. Therefore, different methods have been developed to overcome the effect such as increasing distance by
introduction of bulky substituents,2-4 controlling J-aggregation5-7 and cross-dipole molecular packing modes.8 In 2001,
a propeller-like fluorophore, 1-methyl-1,2,3,4,5-pentaphenylsilole, was found to be not emissive in solutions but
strongly emissive upon aggregation, which was termed as aggregation-induced emission (AIE).9 The AIE
characteristics overcome the ACQ effect fundamentally. For the last decade, AIE compounds have showed to be of
great advantages in many areas such as OLEDs10,11 chemo/biosensors,12-16 and bioimaging.17,18 Experimental results
prove that AIE fluorophores generally possess twist molecular structures with aromatic rotors that efficiently prevent
π- stacking.19-22 In spite of their comprehensive applications, the AIE compounds are limited to the application in
solutions because they are not emissive in monomers. Therefore, the fluorophores with high fluorescence efficiency
in both dilute solutions and aggregates are expected to have wider scope of applications and still remain an interesting
topic. For example, professor Yuan et al have tried to obtain high fluorescence efficiencies in both solutions and
aggregates via conjugation-induced rigidity in twisting molecules.23
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Maleimides, a class of organic fluorophores with many interesting properties such as tunable emission
wavelength,24 sensitivity to solvent polarity,24,25 high selective reactivity to thiol,26,27 have been used as OLED
materials,28,29 metal ion sensors,30,31 pH sensor,32 cell imaging dyes,26,27,33 etc. Recently,
2-arylamino-N-arylmaleimides have been found to have AIE characteristics32,34 and mechanochromic luminescence.35
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All reported maleimides contain aromatic moieties except two alkylaminomaleimides and four
alkylaminobromomaleimides reported in 2015 by O’Reilly group.36 However, they only studied the optical properties
of these aminomaleimids (AMIs) in different solutions.
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In our previous research on the multi-component reactions (MCR) of but-2-ynedioate, primary amine, aldehyde
for the synthesis of hetereocycles,37,38 we obtained a byproduct 2-butylamino-N-butylmaleimide (2But in Scheme 3),
whose bright cyan light on thin-layer chromatography plate under daylight, like a fluorescent stick, impressed us
deeply. However, we did not pay any attention to the optical properties but the synthetic methodology37-39 until we
developed a convenient five-component reaction for the synthesis of a series of novel C6-unsubstituted
tetrahydropyrimidines (THPs) with useful AIE characteristics.40 Interestingly, we found that THPs possess
unconventional intramolecular through-space electronic conjugation22 and can be used as fluorescent sensors for
copper ion,41 the critical micelle concentration (CMC) of ionic surfactants42,43 and temperature.44 In the process of
the investigation into the application of THPs as fluorescent sensors, we found that although AIE THPs possess the
advantage of highly sensitive fluorescence-on-off change, it is difficult to prepare samples for optical measurements
because they are used as suspensions41-43or solid states44 whose sizes influence sample stabilities41-43 or optical
properties44 and hence are needed to be controlled with suitable methods, which increase experimental procedures
and difficulties. At this point, an assumption came upon to our mind that if a fluorophore can be used as
fluorescence-on-off sensor in solution, it will be advantageous both in the high sensitivity of fluorescence-on-off
change and in the convenience and simplicity of sample preparation. The simple molecular structure and strong
solid-state fluorescence of 2But excited our interest so that we raised the questions: Can 2But be used as a
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fluorescence-on-off sensor in solution? Does it show AIE, ACQ or high emission in both monomers and aggregates?
What is the influence of substituents on the optical properties of AMIs? To answer these questions, we synthesized
aminomaleimides (AMIs) with aryl and alkyl substituents and studied their optical properties in different solvents and
powders via steady-state absorption, excitation and emission spectra as well as fluorescence quantum yields (ΦF).
Here, the results are reported.
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RESULTS AND DISCUSSION
Design and synthesis
AMIs can generally be synthesized from 3-halofuran-2,5-dione (1) via route I45,46 or from dimethyl
but-2-ynedioate (2) via route II,34,47-50 (Scheme 1). Route I contains a two-step46 and a three-step reactions45 that can
afford both various mono-substituted-aminomaleimides (MAMIs) and disubstituted-aminomaleimides (DAMIs).
However, the two-step46 and three-step reaction45 in route I can afford only 8-35% of total yields. Route II includes a
two-step reaction. In our previous work,38 we studied the influence of solvents on the reaction of 2 and primary
amines (R1NH2) and found that protic and aprotic solvents, respectively, favour the formation of cis- and
trans-enamines, but only a mixture of cis- and trans-enamines (cis/trans=97:3) was obtained because they were
inter-convertible during processes of separation and purification. The reaction of 2 and R1NH2 (R1 = alkyl or aryl
without strong electron-withdrawing groups such as NO2-), acting as an exothermic reaction, can be conducted at
room temperature in different solvents in yields higher than 98%. The two-step reactions in route II can proceed in
one-pot still with good to excellent yields.34,47,48,50,51 The one-pot reaction can proceed successfully at room
o
temperature when R2 is alkyl,50,51 but can do only at temperature higher than 180 C when R2 is aryl without
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electron-withdrawing groups and R1 are aryl.34,47,48 The synthesis of MAMIs with R1-R2 = alkyl-aryl and DAMIs via
the one-pot reaction has not been reported. In this study, we tried the synthesis under different conditions, but failed.
Then, we optimized solvents and temperature, and found that both MAMIs and DAMIs could be synthesized by the
one-pot three-component reaction (3CR) in good yields (method I depicted in Scheme 2). With the highly efficient
3CR in hand, we synthesized MAMIs with R2=aryl and DAMIs via the 3CR in Scheme 2 and MAMIs with R1=alkyl
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via the reported 3CR51 in Scheme 3.
O
R2
N
H
Route I
OH
Br
(Cl)
O
O
N
O
O
-H2O
O
R2
R2
R2NH2
R3
N
or
O
O
N
HN
R1
O
O
R1
Br
(Cl)
N
O
R2
O
DAMIs
MAMIs
Br
(Cl)
1
Route II
O
O
O
N
R2
R1NH2
R2NH2
HN
R1
O
O
R1
NH
MAMIs
(R1-R2 alkyl-aryl)
2
2
H
1
2
R
N
2
O
O
O
O
cis- and trans-enamines
Scheme 1 Two conventional routes for the synthesis of MAMIs and DAMIs
Method I
O
O
O
R1R3NH
Toluene
N
R2
R2NH2
1 eq.
N
R2
or
R1
O
or R1NH2
R1
110 oC, 6 h
N
N
Br
1.5 eq.
O
H
O
R3
O
MAMIs
DAMIs
1.1 eq.
O
O
O
O
N
N
N
N
N
N
O
O
NH
O
O
NH
Pid-But
Pid-Ph
But-Ph
2Ph
85%a
80%
75%
78%
a
a certain amuout (0.5 times of the amout of toluene) of N,N-dimethylformamide was needed
to disolve intermediate for the synthesis of 2Ph
Scheme 2 Synthesis of DAMIs or MAMIs with R2=aryl via the developed 3CR in this work
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Method II
O
O
O
O
O
N
R2
R1NH2
1 eq.
R2NH2
EtOH, rt, 48 h
60-88%
R1
N
H
4 eq.
(R2 = alkyl)
O
1 eq.
MAMIs
O
Ole
HN
O
O
O
N
N
N
Ole
N
N
H
N
H
N
O
O
H
O
O
Cyc-But
O
2But78%
69%
2Ole
2Me60%
73%
9
O
O
O
N
O
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N
O
N
N
HN
N
N
H
H
N
O
O
H
Ph-But 70%
88%
Phc-Me 72%
Phc-But
78%
But-Phc
Ole:

Scheme 3 Synthesis of MAMIs with R2=alkyl via the reported 3CR51
Optical properties of 2But in different solvents and aggregate sizes
The optical properties of 2But in different solvents were investigated firstly. As depicted in Figure 1, 2But
shows an obvious red shift in absorption (λab) (39 nm) (Figure 1b) and emission peaks (λem) (85 nm) (Figure 1a and
1c) from nonpolar cyclohexane (CH) to polar water. Meantime, the fluorescence was quenched in water. The detailed
optical data of 2But are listed in Table 1. Compared with the optical data of 2But in CH, polar aprotic solvents THF
and MeCN displayed small influence on λab (red-shift of 12 and 20 nm, respectively) and ΦF (1.2 and 1.6 times lower,
respectively), and almost no influence on Stokes shift (s) (red-shift of only 4 and 5 nm, respectively). However,
protic solvents ethanol (EtOH) and water caused sharp decrease of ΦF (23 and 700 times lower, respectively), distinct
red-shift of λab (23 and 39 nm, respectively) and changes in s (30 and 46 nm, respectively). The fluorescence
quenching as well as the large changes in λab and s are expected to be caused by hydrogen bonding between protic
solvent and 2But as reported many fluorophores containing carbonyl group.52-55
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H2O
Cycl
THF
MeCN
EtOH
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b
)
CH
357
365
368
0.09
0.06
0.03
0.00
THF
MeCN
EtOH
H2O
345
39 nm
384
320
360
400
440
Wavelength / nm
c)
445
CH
461
320
240
160
80
THF
MeCN
EtOH
H2O
85 nm
470

5
498
40
530
0
420
480
540
600
Wavelength / nm
Figure 1 Optical properties of 2But in cyclohexane (CH), tetrahydrofuran (THF), acetonitrile (MeCN), ethanol
(EtOH) and water (H2O) (2.00  105 M). (a) 2But in different solvents under daylight (upper) and UV light (365 nm)
(lower), (b) Absorption spectra and (c) Emission spectra (excited at peak marked in Figure 1b, excitation and
emission slit widths: 3 and 3 nm).
Table 1 Optical properties of 2But in solutions and aggregates.
b
Solution or
aggregate
CH
εa
λab
λem
Δs
ΦF
/cm‒ M‒
/nm
345
357
365
/nm
445
461
470
/nm(cm-1)
100(6514)
104(6319)
105(6121)
/%
1
1
4165
70
THF
3915
62/63c
44
MeCN
4040
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EtOH
H2O
5955
3220
368
498
530
480
476
472
130(7094)
146(7174)
54 (2641)
81(4308)
92(5129)
3
384
0.1
50 c
56 c
69 c
Crystal-L
Crystal-S
Powder
a
426 d
395 d
380 d
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Molar absorption coefficient of 2But in solution (2.00  105 M).
Relative quantum yields with
b
9,10-diphenylanthracence (DPA) in ethanol (5.00  106 M) as standard (ΦF=0.95)56 (excited at 350 nm). cAbsolute
quantum yield determined via calibrated integrating sphere (excited at 350 nm). d Energy-lower excitation peak in
Figure 2b.
The optical properties of 2But in solids were investigated to understand the influence of aggregation on its
optical properties. Since the fluorescence efficiencies of conventional organic fluorophores correlate with aggregate
sizes,57-61 the ΦF values of 2But in different aggregate sizes were measured. As shown in Figure 2, when aggregate
size decreases from large crystal to powder (ground crystals) (Figure 2a), 2But shows a blue shift of 47 nm in λem
(left spectra in Figure 2b) but a blue shift of only 8 nm in λem (right spectra in Figure 2b). Meanwhile, the ΦF
increases from 50 to 69 % (Table 1). The fluorescence quantum yield (69%) of 2But in powder is almost the same as
that (70%) in CH, that is, the fluorescence efficiencies of 2But in aggregates and monomers are almost the same. The
distinct blue shift in excitation spectra of 2But with decrease of aggregate sizes is because Coulombic interaction
energies between molecules become smaller, resulting in the wider band gaps.58,62,63 The slight changes in the λem and
ΦF with aggregate size might mainly arise from the reabsorption of energy-higher fluorescence64 because the Stokes
shift (53 nm) of 2But in crystals is much smaller than that (92 nm) in powder.
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crystal-L
-S
powder
a)
4
5
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7
10 mm
1 mm
2 mm
8
9
b)
1.2
0.8
0.4
0.0
crystal-L
crystal-S
powder
380 427 472480
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19
20
320
400
480
560
640
Wavelength / nm
Figure 2 Optical properties of 2But in different sizes of aggregates. (a) photos taken under daylight (upper) and
UV light (365 nm) (lower), (b) excitation (left) and emission (right) spectra. L and S stand for large and small,
respectively.
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Optical properties of other AMIs in different solvents and powder
2But possesses high emission in both monomers and aggregates, and its high emission is quenched in protic
solvents. What about the optical properties of other AMIs with aliphatic or aromatic substituents? To answer this
question, we designed and synthesized other 11 AMIs (molecular structures are shown in Scheme 2 and 3). The
optical properties of all synthesized AMIs in different solvents and powder are listed in Table 2. It can be seen that
substituents show great influences on the ΦF values of AMIs in different solvents. According the substituent influence
on ΦF, AMIs can be classified into three types: dialkyl MAMIs (entries 17), trialkyl DAMIs (entry 8) and aryl AMIs
(entries 912). The fluorescence efficiencies of dialkyl MAMIs are highin nonpolar solvent CH (ΦF = 64–72%),
moderate in polar solvents THF and MeCN (ΦF = 34–69%) and very low in protic solvents EtOH and Water
(3–0.03%). They have almost the same ab and em values in CH, which proves that their emissions arise from the
same aminomaleimide skeleton. However, only 2Ol and 2But are highly emissive in powder (ΦF =62 and 69%), and
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other dialkyl MAMIs display much lower fluorescence efficiencies (ΦF =1834%). In other word, only MAMIs with
long chain alkyl demonstrate high emission in both monomers and aggregates, and other dialkyl MAMIs show ACQ
effect.
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Table 2 Optical properties of AMIs in solvents and powder
Solution
/powdera
c
εb/cm1·M1
Entry
AMIs
λab/nm
λem/nm
ΦF /%
1
2But
CH
4165
3915
4040
5955
3220
345
357
365
368
384
380d
346
357
445
461
470
498
530
472
446
464
70
62
44
3
THF
MeCN
EtOH
Water
powder
CH
0.1
69 e
72
53
2
2Ol
5655
7100
THF
359
472
MeCN
7040
36
499
491
485
448
462
473
496
529
483
EtOH
Waterf
powder
CH
9675
1950
365
362
440 d
343
356
357
361
374
437 d
2
3
62 e
65
45
26
2
3
2Me
5305
7850
7660
7805
7490
THF
MeCN
EtOH
Water
powder
0.1
34 e
4
Cyc-But
CH
3860
6100
5545
6105
345
358
363
367
445
463
472
500
71
48
34
2
THF
MeCN
EtOH
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Water
2575
379
535
0.03
440 d
344
357
358
361
381
434 d
344
357
359
364
380
433 d
344
357
360
367
377
438
370
380
382
388
405
464 d
370
379
380
382
449 d
366
486
448
463
470
495
526
486
446
460
473
496
527
488
446
461
470
497
527
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EtOH
Water
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Phc-But
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Pid-Phc
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Water
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aGround crystals or as-prepared solid. Molar absorption coefficient of AMIs in solution (2.00  105 M).
cRelative quantum yields with 9,10-diphenylanthracence (DPA) in ethanol (5.00  105 M) as standard (ΦF = 0.95)56
b
d
e
(excited at 350 nm). Energy-lower peak in excitation spectrum. Absolute quantum yield determined via calibrated
integrating sphere (excited at 350 nm). f 2Ol is not completely soluble in water. ne: no emission.
Alkyl DAMI Pid-Phc (entry 8) is highly emissive (ΦF=88%) in CH but almost no emissive in aprotic polar and
protic solvents. Compared with dialkyl MAMI But-Phc in CH (entry 5), Pid-Phc shows 26 nm of red shift in ab and
18% increase in ΦF, which means that the substitution of hydrogen on N by alkyl increases the electron conjugation
and hence leads to a red shift in ab and enhancement in ΦF. In addition, the emission of Pid-Phc is quenched not
only by protic solvents but also by polar aprotic solvents. The moiety “-N-C=C-C=O” in AMIs, a typical electron
donor and acceptor structure, usually takes place twisted intramolecular charge transfer (TICT) and leads to
fluorescence quenching in polar solvent.65-68 Therefore, the fluorescence quenching of Pid-Phc in polar solvents is
expected to be caused by TICT. The reason why the fluorescence of alkyl MAMIs is not quenched in polar solvent
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might be that the intramolecular hydrogen bond (HB) between hydrogen on N and adjacent carbonyl oxygen prevents
the rotation of C-N bond in -N-C=C-C=O moiety, that is, prevents the TICT effect. Although the ΦF of Pid-Phc is
high to 88% in CH, its ΦF in powders is only 21%, which means that it displays stronger ACQ effect.
Aryl AMIs (entries 912) display very weak or almost no emission in different solvents owing to nonradiative
transition that is caused by phenyl rotation as common AIE compounds with aromatic rotors.9,20,22 Compared with
2But in CH (entry 1), Ph-But (entry 10) and But-Ph (entry 11) show 21 and 7 nm of red shifts in ab, respectively,
which indicates that phenyl at different positions could enhance the electron conjugation to varied extents. The ΦF
values of aryl MAMIs 2Ph, Ph-But and But-Ph in powder are 17, 39 and 38%, respectively, and aryl DAMI is not
emissive (entry 12). That is, aryl MAMIs show AIE effect and aryl DAMI is not emissive in both monomers and
aggregates.
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Single-crystal structures of alkyl and aryl AMIs
The excitation peaks of AMIs in powders are distinctly red-shifted (35–105 nm of red shift) compared with their
ab values in CH (Table 2), which indicates the red-shifted emissions in powder originate from aggregates rather than
molecular planarization.69 The formations of dimers,70 excimers,71-73 short-range ring interactions22 and J-aggregate
(aggregate that molecules are arranged in head-to-tail)5,74,75 usually cause such red shifted emissions in aggregates.
To understand the red-shifted emission of AMIs in aggregates, the single crystal structures of alkyl MAMIs (2But,
Cyc-But and Phc-Me) and DAMI (Pid-Phc), aryl MAMI Ph as well as aryl/alkyl MAMI Ph-But were investigated.
(All single crystal structures are measured in this work except that of Ph,34 and the main crystallographic data of
these AMIs are shown in Table S1 in the Supporting Information.).
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From the molecular packing on bc plane shown in Figure 3a–d, it can be seen that the molecules of dialkyl
MAMIs (2But, Cyc-But and Phc-Me) and aryl/alkyl MAMI Ph-But are connected as dimers via two N-H…O
(2.082.28 Å, 160169) HBs (marked as green a) and the dimers are connected bytwo weaker C-H…O (2.472.54
Å, 142177) (market as green b). Weak intramolecular HBs could be observed (marked as green c and d). The
dimers are packed as two parallel stairs with central ring coplane and the nitrogen atom of central ring on opposite
site (DS1 and DS2 represent two parallel stairs packed via dimers). Although all these MAMIs show similar
dimer-packed parallel stairs, the short-range ring interactions with the distance (r) between ring centres shorter than 5
Å are different. The short-range ring interactions between stairs exist in all these MAMIs, but the interactions along
stair direction are observed only in 2But. Compared with the λab (345 and 344 nm, respectively) and ΦF (71 and 70%,
respectively) values of Cyc-But and Phc-Me in CH, 2But in CH has almost the same λab (345 nm) and ΦF (70%)
values. However, 2But in powders are obviously different from Cyc-But and Phc-Me in powders in terms of the
values of λex (380 nm vs. 440 and 433 nm, respectively) and ΦF (69% vs. 34 and 18%, respectively). Since they have
almost the same optical properties in monomers and similar molecular packing modes (dimer-packed parallel stairs),
the distinctly different optical properties between them are assumed to come from the different intermolecular
interactions along stair direction.
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Figure 3 Molecular alignments of 2But (CCDC 1430703), Cyc-But (CCDC 1513071), Phc-Me (CCDC
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1513072), Ph-But (CCDC 1513073), Ph (CCDC 938824)34 and Pid-Phc (CCDC 1430704) in single crystals. (a) (e)
front view (upper) and side view (lower). Hydrogen atoms in the side view were omitted for clarity. The molecules in
Figure 3e are marked in different colours for clarity and indication of different molecular conformations.
The molecular arrangement on bc plane of diaryl MAMI 2Ph (upper in Figure 3e) is different from those of
dialkyl or alky/aryl MAMIs. The molecules on bc plane are connected as layers rather than dimers via intermolecular
N-H…O and C-H…O. Since four different conformations exist in single crystals, the two kinds of intermolecular
HBs are different in different layers (N-H…O in layer 1 and 2: 2.12 Å, 162 and 2.13 Å, 159, marked as a1 and a2,
respectively; C-H…O in layer 1 and 2: 2.42 Å, 130 and 2.43 Å, 132, marked as b1 and b2). From side view (lower
in Figure 3e), it can be seen that the molecules are paced as zigzag lines (ZLs) via short-range ring interactions
between the central rings with r=4.08 and 4.03 Å. The dihedral angle between the central rings along the zigzag lines
is equal to 18 with the nitrogen atom of central ring on opposite site. In addition, there are five weak C-H… bonds
(2.64–2.97 Å, 132–144) between the adjacent molecules. All molecules of 2Ph in aggregates are connected as a net
by intermolecular HBs and short-range ring interactions. Since the distance r along zigzag lines is near 4 Å, which
means the existence of strong π-π interactions, the large red-shift emission of 2Ph is expected to originate mainly
from the zigzag line arrangement.
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The molecular packing of DAMI Pid-Phc is shown in Figure 3f. Without hydrogen on N atom, that is, without
intermolecular N-H…O, the distance r between the central rings becomes shorter (3.87 Å) and forms slipped parallel
dimmers,76,77 which will lead to excimer formation and hence fluorescence decrease or quenching.1 This explains
distinct fluorescence decrease of Pid-Phc in powder (ΦF=25%) compared to that in CH (ΦF=88%) and the almost
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noemssion of Pid-Ph in powder.
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The single crystal structures of these AMIs indicate that intermolecular N-H…O and - stacking interaction are
two important factors influencing molecular packing of AMIs. For dialkyl or aryl/alkyl MAMIs, the intermolecular
hydrogen bond N-H…O is the main factor, which leads to formation of hydrogen-bonded dimmers that are packed as
parallel stairs with r>4.56 Å; for diaryl MAMIs, intermolecular - stacking interaction becomes the main factor,
which leads to --packed zigzag arrangement with r = 4.03 and 4.08 Å; and for DAMIs without hydrogen on
nitrogen atom, that is, without intermolecular N-H…O, - stacking interaction leads to the formation of --packed
slipped parallel dimers with r = 3.87 Å. These molecular packing modes and the r values show great influence on the
fluorescence efficiencies of AMIs. Only the parallel stair-like arrangement packed via hydrogen-bonded dimers with
r = 4.931 Å in 2But gives high solid-state emission.
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CONCLUSION
In this study, we have investigated into the synthesis and structure-property-relationship of AMIs. A 3CR based
on 3-halofuran-2,5-dione (method I) has been developed for the synthesis of DAMIs and MAMIs with different
substituents in the overall yields (75–85%) much higher than those (8–35%) of the reported two-step or three-step
reactions in route I. In addition, we found that the reported 3CR based on dimethyl but-2-ynioaedte (method II) is
suitable for the synthesis of N-alky MAMIs but limited to the synthesis of DAMIs and N-aryl MAMIs. Furthermore,
12 AMIs were designed and synthesized for investigation of the relationship between structures and optical properties.
Our experimental results indicate that the fluorescence efficiencies of AMIs in monomers and aggregates can be
easily tuned by switch of alkyl and aryl: Alkyl AMIs show high emission in both monomers and aggregates or ACQ
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effect, while aryl and aryl-alkyl AMIs display almost no emission in both monomers and aggregates or AIE effect.
The high emissions (ΦF up to 88%) of AMIs in monomers, proving to originate from aminomaleimide skeleton, will
be quenched in protic solvents by hydrogen bonding effect for alkyl MAMIs, in polar solvents by TICT effect for
alkyl DMIs, or in different solvents by the rotations of aryl for aryl and aryl/alkyl AMIs. The red-shifted emissions of
AMIs in aggregates as compared to those in monomers mainly correlate with intermolecular N-H…O and -
stacking interactions that cause formation of dimer-packed parallel stair-like arrangements, zigzag lines or
π-π-stacked dimers. The simple synthesis method, high emission in both monomers and aggregates, and sensitive
response to protic and polar solvents of AMIs are useful for practical applications. The further investigation into the
applications of AMIs as bio- and chemical fluorescent sensors are underway in our laboratory.
EXPERIMENTAL SECTION
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Materials and Instruments
All the chemicals used in this study were obtained from commercial suppliers and used without further
purification. For measurement of the optical spectra of samples, spectroscopically pure solvents and doubly distilled
water were used. All melting points were taken on amicro melting point apparatus and are uncorrected. 1H NMR (400
13
MHz) and C NMR (100.6 MHz) spectra were recorded on a 400 MHz NMR spectrometer using CDCl3 as solvent
and TMS as internal standard. Low resolution and high resolution mass spectra were obtained using electrospray
ionization (ESI) and quadrupole-linear ion-trap techniques. IR spectra were obtained as liquid films on potassium
bromide pellets with a Fourier transform-infrared spectroscopy. Photos were taken by Canon PC1356. The absorption
as well as the fluorescence excitation and emission properties were analyzed by an UV-Vis spectrophotometer
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(measurement wavelength range: 190–100 nm; wavelength accuracy: ±2 nm) and a spectrofluorophotometer (150 W
Xenon lamp; measurement wavelength range: 220–750 nm; wavelength accuracy: ±1.5 nm; S/N ratio:150; slit width:
1.5/3/5/10/15/20), respectively. The absolute fluorescence quantum yields were measured using calibrated integrating
sphere.
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General procedure for the one-pot synthesis of N-alky1 MAMIs (entries 1-7 and 10 in Table 2)
The intermediate enamines were prepared by the addition reaction of dimethyl but-2-ynedioate and primary
amine as described in our previous work.38 That is, the mixture of EtOH (6 mL), dimethyl but-2-ynedioate (284 mg, 2
mmol) and primary (2 mmol) were kept at room temperature for 5–10 min for the synthesis of intermediate enamines.
Then, to the reaction mixture, primary aliphatic amine (8 mmol) was added. After the reaction was completed (stirred
at room temperature for 48 h), the product mixtures were purified by column chromatography with petroleum
ether-ethyl acetate (6:1) as eluent to afford the desired products (60–88% yields).
General procedure for the one-pot synthesis of N-aryl MAMIs (entries 9 and 11 in Table 2) and DAMIs
(entries 8 and 12 in Table 2)
Bromomaleic anhydride (389 mg, 2.2 mmol) and primary amine (2 mmol) were added into 4 mL of tolurene in
sequence, kept at rt for 10 min and then at 110 oC for 6 h. Then, to the reaction mixture, primary or secondary amine
(3 mmol) was added. The reaction mixture was stirred until the temperature was reduced to room temperature, and
then the solution was removed by compressed air. Then crude product was separated by column chromatography with
petroleum ether-ethyl acetate (6:1) as eluent to afford the desired products (75–85% yield).
Preparation of 2But in different aggregate sizes
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Different sizes of crystals of 2But were prepared by recrystallization from dichloromethane/n-hexance solution
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at room temperature for different time.
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Preparation of AMIs in different solvents (2.00×10‒ M)
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Tetrahydrofuran stock solutions of 12 aminomaleimides (AMIs) with a concentration of 1 mM were prepared,
respectively. 100 μL of the stock solution was then transferred to 5 mL volumetric flasks, with tetrahydrofuran
removed by N2 and then the flasks filled to the mark with corresponding solvents for preparation of different
solutions of 12 AMIs except their tetrahydrofuran solutions. The tetrahydrofuran solutions of 12 AMIs were prepared
by adding 100 μL of tetrahydrofuran stock solutions into 5 mL volumetric flask, and then filling the flasks to the
mark with tetrahydrofuran.
Structure Characteristics of 12 AMIs in Table 2
2-butylamino-N-butylmaleimide (2But): 78% (350 mg, 1.56 mmol) yield, light yellow solid, mp=
81.081.3°C; IR (KBr): νmax = 3321, 3097, 2955, 2933, 2866, 1689, 1636, 1448, 1035, 783 cm-1; 1H NMR (400 MHz,
CDCl3) δ = 5.34 (s, 1H), 4.80 (s, 1H), 3.48 (t, J = 7.2 Hz, 2H), 3.213.16 (m, 2H), 1.711.52 (m, 4H), 1.491.28 (m,
4H), 0.98 (t, J = 7.2 Hz, 3H); 0.94 (t, J = 7.6 Hz, 3H) ppm; 13C NMR (101 MHz, CDCl3) δ = 172.6, 167.6, 149.2, 83.7,
43.9, 37.3, 30.8, 30.5, 20.0, 13.6 ppm; MS (ESI): m/z 225 (M+H+, 100), 449 (2M+H+, 95) and HR-ESI-MS for
C12H20N2O2 ([M+Na]+) Calcd: 247.1417; Found: 247.1414. The compound has CAS Registry Number (457959-70-5)
but no reference on it has been found by SciFinder.
2-oleylamino-N-oleylmaleimide (2Ol): 69% (845 mg, 1.38 mmol) yield, yellow solid, mp=62.562.6°C;
IR (KBr): νmax = 3477, 3334, 2955, 2915, 2849, 1692, 1643, 1469, 1099, 1266, 742 cm-1; 1H NMR (400 MHz, CDCl3)
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δ = 5.375.33 (m, 4H), 4.76 (s, 1H), 3.43 (t, J = 7.2 Hz, 2H), 3.14 (q, J = 6.8 Hz, 2H), 2.00 (s, 5H), 1.631.53 (m,
6H), 1.25 (s, 47H), 0.890.85 (t, J = 6.4 Hz, 6H) ppm; 13C NMR (101 MHz, CDCl3) δ = 172.6, 167.6, 149.2, 123.0,
129.9, 83.7, 44.3, 37.6, 32.5, 31.9, 29.3, 27.2, 26.8, 22.6, 14.1 ppm; MS (ESI): m/z 635 (M+H+, 10), 301 (100) and
HR-ESI-MS for C40H72N2O2 ([M+H]+) Calcd: 613.5667; Found: 613.5654 .
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2-methylamino-N-methylmaleimide (2Me): 60% (168 mg, 1.2 mmol) yield, yellow solid, mp =
147.2147.4°C; 1H NMR (400 MHz, CDCl3) δ = 5.43 (s, 1H), 4.84 (s, 1H), 2.98 (s, 1H), 2.95 (d, J=5.6 Hz, 3H) ppm;
1
13C NMR (101 MHz, CDCl3) δ = 172.5, 167.4, 150.4, 84.1, 30.5, 23.4 ppm; H and 13C NMR spectra are
consistent with the previous literature51. 2-cyclohexylamino-N-butylmaleimide (Cyc-But): 73% (365 mg, 1.46
mmol) yield, light yellow solid, mp= 73.874.1°C; IR (KBr): νmax = 3327, 3021, 2930, 2858, 1698, 1633, 1452, 1439,
1379, 1335, 1169, 1108, 1049 cm-1; 1H NMR (400 MHz, CDCl3) δ = 5.34 (d, J = 6.8, 1H), 4.77 (s, 1H), 3.47 (t, J =
7.2, 2H), 3.18–3.15 (m, 1H), 2.02–1.99 (m, 2H), 1.82–1.53 (m, 5H), 1.42–1.24 (m, 7H), 0.93 (t, J = 7.2, 3H) ppm; 13C
NMR (101 MHz, CDCl3): δ = 172.7, 167.8, 147.9, 83.4, 53.1, 37.3, 31.9, 30.8, 25.3, 24.4, 19.9, 13.6 ppm; MS (ESI):
m/z 635 (M–H+, 30), 75 (100) and HR-ESI-MS for C14H22N2O2 ([M+H]+) Calcd: 251.1754; Found: 251.1769.
2-butylamino-N-benzylmaleimide (But-Phc): 78% (403 mg, 1.56 mmol) yield, light yellow solid, mp =
1
107.2107.6°C；IR (KBr): νmax = 3323, 3117, 2923, 1739, 1701, 1617, 1402, 1053, 696, 654 cm-1; H NMR (400
MHz, CDCl3) δ = 7.38–7.27 (m, 5H), 5.35 (s, 1H), 4.84 (s, 1H), 4.65 (s, 2H), 3.20–3.15 (m, 2H), 1.67–1.58 (m, 2H),
1.44–1.39 (m, 2H), 0.97 (t, J = 7.2 Hz, 3H) ppm; 13C NMR (101 MHz, CDCl3): δ = 172.3, 167.2, 149.3, 136.8, 128.5,
+
83.9, 44.0, 41.0, 30.5, 19.9, 13.6 ppm; MS (ESI): m/z 259 (M+H+, 90), 534 (2M+NH4 , 100) and HR-ESI-MS for
C15H18N2O2 ([M+H]+) Calcd: 259.1441; Found: 259.1439. The compound has CAS Registry Number (607692-63-7)
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2-benzylamino-N-butylmaleimide (Phc-But): 88% (454 mg, 1.76 mmol) yield, yellow solid, mp =
121.3121.9°C; IR (KBr): νmax = 3332, 3125, 2953, 2862, 1749, 1698, 1620, 1444, 1026, 781, 698 cm-1; 1H NMR
(400 MHz, CDCl3) δ = 7.42–7.18 (m, 5H), 5.67 (s, 1H), 4.86 (s, 1H), 4.35 (d, J = 5.6 Hz, 2H), 3.48 (t, J = 7.2 Hz,
2H), 1.611.54 (m, 2H), 1.38–1.28 (m, 2H), 0.94 (t, J = 7.6 Hz, 3H) ppm; 13C NMR (101 MHz, CDCl3) δ = 172.5,
167.5, 148.8, 135.7, 128.9, 128.2, 127.7, 85.3, 48.4, 37.8, 30.7, 19.9, 13.6 ppm; HR-ESI-MS for C15H18N2O2
([M+H]+) Calcd: 259.1441; Found: 259.1445. The compound has CAS Registry Number (607692-56-8) but no
reference on it has been found by SciFinder.
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2-benzylamino-N-methylmaleimide (Phc-Me): 72% (311 mg, 1.44 mmol) yield, yellow solid, mp =
151.2151.3°C; 1H NMR (400 MHz, CDCl3) δ = 7.38–7.29 (m, 5H), 5.69 (s, 1H), 4.88 (s, 1H), 4.37(d, J = 5.6 Hz,
2H), 2.99 (s, 3H) ppm; 13C NMR (101 MHz, CDCl3) δ = 172.4, 167.6, 149.0, 135.6, 129.0, 128.2, 127.6, 85.4, 48.4,
23.4 ppm; 1H and 13C NMR spectra are well correspondent to the previous literature.50
2-(piperidin-1-yl)-N-benzylmaleimide(Pid-Phc): 75% (405 mg, 1.5 mmol) yield, light yellow solid, mp=
107.3107.8°C; IR (KBr): νmax =2919, 2851, 1701, 1652, 1618, 1540, 1455, 1399, 697, 654 cm-1; 1H NMR (400 MHz,
CDCl3) δ = 7.457.22 (m, 5H), 4.91 (s, 1H), 4.64 (s, 2H), 3.66 (b, 4H), 1.68 (b, 6H) ppm; 13C NMR (101 MHz,
CDCl3): δ = 170.6, 167.0, 150.4, 137.1, 128.5, 128.3, 127.4, 87.9, 48.4, 40.9, 25.5, 23.9 ppm; MS (ESI): m/z 271
+
(M+H+, 50), 558 (2M + NH4 , 100) and HR-ESI-MS for C16H18N2O2 ([M+H]+) Calcd: 271.1441; Found: 271.1435.
2-phenylamino-N-phenylmaleimide (2Ph): 85% (449 mg, 1.7 mmol) yield, yellow solid, mp = 245.4245.8°C;
1H NMR (400 MHz, CDCl3) δ = 7.52–7.20 (m, 11H), 5.70 (s, 1H) ppm; 13C NMR (101 MHz, CDCl3) δ = 171.2,
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139.3139.9°C; IR (KBr): νmax = 3313, 3121, 2956, 2863, 1756, 1686, 1638, 1446, 1421, 1183, 748, 694 cm-1; H
NMR (400 MHz, CDCl3) δ = 7.44–7.16 (m, 6H), 5.52 (s, 1H), 3.56 (t, J = 7.2 Hz, 3H) 1.66–1.59 (m, 2H), 1.39–1.34
(m, 2H), 0.96 (t, J = 7.2 Hz, 3H) ppm; 13C NMR (101 MHz, CDCl3): δ = 172.8, 168.3, 142.3, 138.4, 129.7, 124.4,
118.7, 88.9, 37.7, 30.7, 20.0, 13.6 ppm; MS (ESI): m/z 245 (M+H+, 5), 102 (100) and HR-ESI-MS for C14H16N2O2
([M+H]+) Calcd: 245.1285; Found: 245.1299.
2-butylamino-N-phenylmaleimide (But-Ph): 78% (381 mg, 1.56 mmol) yield, yellow solid, mp =
138.5138.8°C; 1H NMR (400 MHz, CDCl3) δ = 7.48–7.32 (m, 5H), 5.53 (s, 1H), 4.99 (s, 1H), 3.26 (q, J = 7.2 Hz,
2H), 1.73–1.66 (m, 2H), 1.51–1.42 (m, 2H), 1.00 (t, J = 7.2 Hz, 3H) ppm; 13C NMR (101 MHz, CDCl3) δ = 171. 0,
166.3, 149.0, 131.9, 128.9, 127.2, 125.8, 84.2, 44.1, 30.5, 20.0, 13.6 ppm; H and 13C NMR spectra are well
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2-(piperidin-1-yl)-N-phenylmaleimide(Pid-Ph): 80% (410 mg, 1.6 mmol) yield, light yellow solid, mp =
107.8108.1°C; IR (KBr): νmax = 3101, 2923, 2851, 1749, 1697, 1648, 1612, 1448, 1383, 766, 700 cm-1; 1H NMR
(400 MHz, CDCl3) δ = 7.477.32 (m, 5H), 5.07 (s, 1H), 3.74 (b, 4H), 1.74 (b, 6H) ppm; 13C NMR (101 MHz, CDCl3)
δ = 169.8, 166.1, 149.9, 131.9, 128.8, 127.2, 126.4, 88.32, 77.3, 77.2, 76.9, 76.6, 25.6, 23.9 ppm; MS (ESI): m/z 279
(M+Na+, 40), 102 (100) and HR-ESI-MS for C15H16N2O2 ([M+H]+) Calcd: 257.1285; Found: 257.1278; H and 13C
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NMR spectra are well correspondent to the previous literature.80
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ASSOCIATED CONTENT
*Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI:
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Table S1, 1H and 13CNMR spectrum copies of 12 AMIs in Table 2, thermal ellipsoid plots and "cif" files of five
new crystal structures.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zhuqh@smu.edu.cn and tangbenz@ust.hk
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Ian D. Williams and Herman H. Y. Sung for support on collection and analysis of some XRD data.
This work was supported by the Science and Technology Program of Guangdong Province (2015A010105015),
the National Natural Science Foundation of China (21272111) and the National Basic Research Program of China
(973 Program; 2013CB834701).
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