Carbazole-based diphenyl maleimides: Multi-functional smart fluorescent materials for data process and sensing for pressure, explosive and pH

Article abstract of DOI:10.1016/j.dyepig.2016.06.015

A series of carbazole-based diphenyl maleimide dyes with different N-substituent were synthesized through an Ullmann reaction in high yields. The dyes exhibit an intense emission in the solid state for their twisted conformation fixed by weak intermolecular interactions, resulting in the hindrance of the internal rotations. Their solid-stated emission wavelength and intensity are sensitive to external stimuli (like heat, pressure and vapor), due to a crystal-to-crystal or crystal-to-amorphous phase transition. Based on the multi-stimuli-responsive fluorescence properties of the N-benzyl analogue, two kinds of applications were explored. One is rewritable data recording processes including solvent writing and vapor writing. The other is sensing for low hydrostatic pressure (0-25 MPa). Interestingly, nanoparticles of the dyes formed in aqueous solution could effectively detect explosive and pH changes. A 15 ppm concentration of picric acid quenched the fluorescence of the N-methyl analogue with the detectable limit calculated to be 0.13 ppm. The N-methyl analogue also showed a different emission color and intensity over the pH range 12-14, which originated from the hydrolysis of the dye molecules in the strongly basic solution.

Full text of DOI:10.1016/j.dyepig.2016.06.015

Dyes and Pigments 133 (2016) 345e353
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Carbazole-based diphenyl maleimides: Multi-functional smart
fluorescent materials for data process and sensing for pressure,
explosive and pH
,
**
Xiaofei Mei a, Kexi Wei b, Guixiu Wen b, Zhengde Liu a, Zhenghuan Lin b
,
, b, *
Zhonggao Zhou a, Limei Huang a, E Yang b, Qidan Ling a
a College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, China
b Fujian Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou 350007, Fujian, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of carbazole-based diphenyl maleimide dyes with different N-substituent were synthesized
through an Ullmann reaction in high yields. The dyes exhibit an intense emission in the solid state for
their twisted conformation fixed by weak intermolecular interactions, resulting in the hindrance of the
internal rotations. Their solid-stated emission wavelength and intensity are sensitive to external stimuli
(like heat, pressure and vapor), due to a crystal-to-crystal or crystal-to-amorphous phase transition.
Based on the multi-stimuli-responsive fluorescence properties of the N-benzyl analogue, two kinds of
applications were explored. One is rewritable data recording processes including solvent writing and
vapor writing. The other is sensing for low hydrostatic pressure (0e25 MPa). Interestingly, nanoparticles
of the dyes formed in aqueous solution could effectively detect explosive and pH changes. A 15 ppm
concentration of picric acid quenched the fluorescence of the N-methyl analogue with the detectable
limit calculated to be 0.13 ppm. The N-methyl analogue also showed a different emission color and in-
tensity over the pH range 12e14, which originated from the hydrolysis of the dye molecules in the
strongly basic solution.
Received 10 April 2016
Received in revised form
6 June 2016
Accepted 8 June 2016
Available online 13 June 2016
Keywords:
Diarylmaleimides
Piezochromism
Data recording
pH
Nanoparticles
Explosive
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
limited for the difficulties in changing the molecular structures and
emission in the solid state.
Information processes including recording, storage and security
technologies of data has become an integral part of our life.
Recently, smart luminescent materials responsive to external
stimuli have attracted increasing interest for their potential appli-
cation in the processing of data record and storage [1e5]. Particu-
larly, organic multi-stimuli-responsive smart materials showing
piezochromic, vapochromic and thermochromic luminescence in
the solid state can change their emission colors or intensity in
response to an external grinding (or pressing, shearing), fuming
and heating [6e16]. The tunable solid-state fluorescence makes
them become good candidates for application as biological probes
[17], chemical sensing [18e20] and optical devices [21,22], besides
data processing. However, such smart organic materials are quite
It is generally known that an effective approach to control solid-
state luminescence is to alter the molecular packing mode and/or
conformation [23e29]. As a result, the factors affecting the aggre-
gated state and conformation would impose a significant effect on
the solid-state emission of materials. Three main factors should not
be ignored: bulky aromatic groups, heteroatoms (N, O, S or F) and
donor-acceptor (D-A) structure. Firstly, large and bulky groups can
result in a twisted structure and restrict intramolecular rotation,
which is positive for high fluorescent quantum yield [30e32].
Secondly, heteroatoms always generate weak intermolecular in-
teractions rigidfying the molecular conformation and locking the
molecular rotation, like hydrogen bonding and van der waals forces
[33e35]. Lastly, intramolecular charge transfer caused by D-A
structure of molecules can make their luminescent properties
sensitive to the external environment [36]. However, the molecules
simultaneously possessing these three features are very rare.
* Corresponding author. College of Materials Science and Engineering, Fujian
Normal University, Fuzhou 350007, Fujian, China.
3,4-Disubstituted
arylmaleimide
dyes
have
attracted
** Corresponding author.
E-mail addresses: zhlin@fjnu.edu.cn (Z. Lin), qdling@fjnu.edu.cn (Q. Ling).
http://dx.doi.org/10.1016/j.dyepig.2016.06.015
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

346
X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
considerable attentions in organic light-emitting diode (OLED)
[37e40] and anion sensing [41,42] for their good performance in
luminescence. However, due to the interaction of dipole-dipole and
dibromophenylmaleimide, were obtained from commercial sup-
pliers and were used without further purification unless otherwise
noted. 3,4-Dibromophenylmaleimide was synthesized according to
a previously reported method [40]. Thin layer chromatography was
performed on G254 plates of Qingdao Haiyang Chemical. Column
chromatography was performed on Sorbent Technologies brand
silica gel (40e63 mm, Standard grade).
High resolution MALDI and ESI mass spectra were recorded on
Bruker microflex LRF and micrOTOF II spectrometer, respectively.
NMR spectra were measured in CDCl3 on a Bruker Ascend 400 FT-
NMR spectrometer; 1H and 13C chemical shifts were quoted rela-
tive to the internal standard tetramethylsilane. Melting points were
measured using a SGW X-4 melting point apparatus. Fourier
transform infrared (FT-IR) spectra were recorded on a Nicolet 5700
FT-IR spectrometer. Differential scanning calorimetry was done on
a Perkin-Elmer DCS-7 instrument in the temperature range of
30e310 ꢀC under flowing N2 gas at a heating rate of 10 ꢀC/min.
UVevis spectra were obtained on a Shimadzu UV-2600 spectro-
photometer. The PL spectra were probed on a Shimadzu RF-5301PC
fluorescence spectrophotometer. The thermal annealing processes
were carried out in an oven. All photographs were recorded on a
Canon Powershot G7 digital camera under UV light (365 nm).
Powder X-ray diffraction (PXRD) data were collected using a PAN-
p-p stacking, most of arylmaleimides show stronger fluorescence
in solution than in solids. Regardless of the increasing interest in
the photophysical properties of arylmaleimides [43e47], their
solid-state fluorescence is rarely studied in the reported literature.
Due to the electron-deficiency of maleimide, a pair of cross-
intercepted dipoles would be formed, if the C (3) and C (4) posi-
tions of maleimide are symmetrically substituted by bulky aryl
groups with an electron-releasing ability. Additionally, the het-
eroatoms in arylmaleimides, like O and N, are expected to
dramatically affect the molecular packing modes by weak in-
teractions. Following the strategy, we have recently reported a
L-shaped and D-p-A-p-D type molecule, 3,4-bis(4-(9-carbazolyl)
phenyl)-N-methylmaleimide (BCPMM) with strong AIEE and CIEE
activity [48]. Its morphology-dependent fluorescence tuned by
external stimulus, could be utilized to construct complex sequential
logic systems. Therefore, arylmaleimides are potential stimuli-
responsive materials with tunable solid-state fluorescence [49,50].
In this paper, to examine whether the multi-stimuli-responsive
fluorescence is a general phenomenon for 3,4-bis(4-(9-carbazolyl)
phenyl)maleimides, to investigate how the different substituted
groups affect the emission, and to explore sensing application of
their nanoparticles, a new route was adopted to synthesize a series
of 3,4-bis(4-(9-carbazolyl)phenyl) maleimide dyes (5a-d, Scheme
1) with different N-substituent groups in high yields. Considering
that the size, rigidity, heteroatom and electronic effect of N-sub-
stituents would affect the packing structure and properties of
solids, methyl, benzyl, pentafluoro benzyl and tert-butoxycarbonyl
group were chosen as N-substituents. The solid-state luminescence
properties of 5 and the pH and explosive sensing of 5a nano-
particles were studied thoroughly. The results revealed that 5
exhibited morphology-dependent fluorescence that could be tuned
by external stimulus, and good sensing for pH and explosive, and
that their luminescent and sensing properties were influenced by
groups of different bulk and by heteroatoms. To the best of our
knowledge, smart fluorescent materials with responsiveness to
heat, vapor, pH, explosives and low hydrostatic pressure are rarely
reported.
alytical X-ray Diffractometer (X’Pert3 Powder) with Cu Ka radia-
tion. The simulated patterns were obtained from single-crystal X-
ray analysis. The fluorescence lifetime and absolute FF values of
solution and solid were measured using an Edinburgh Instruments
FLS920 Fluorescence Spectrometer with
a 6-inch integrating
sphere. The surface morphology of materials was analyzed by high
voltage transmission electron microscope on a JEM-2010 (JEOL).
The single crystals of 5b and 5c were mounted on a glass fiber
for the X-ray diffraction analysis. Data sets were collected on an
Agilent Technologies SuperNova Single Crystal Diffractometer
equipped with graphite monochromatic Cu-K
a
radiation
(
l
¼ 1.54184 Å). The single crystals were kept at 290.3 K for 5b and
100.0 K for 5c during data collection. The structures were solved by
SHELXS (direct methods) and refined by SHELXL (full matrix least-
squares techniques) in the Olex2 package. All non-hydrogen atoms
were refined anisotropically displacement parameters, and all
hydrogen atoms in the ideal positions attached to their parent
atoms.
Molecular Simulations were performed on Gaussian 09 pro-
gram. Geometry at ground state was fully optimized by the density
functional theory (DFT) method with the Becke three-parameter
hybrid exchange and the Lee-Yang-Parr correlation functional
(B3LYP) and 6-31G* basis set.
2. Experimental section
2.1. General information
All of the reagents and solvents used, except for 3,4-
Scheme 1. Synthetic routes of 5.

X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
347
2.2. Synthesis
FT-IR (KBr, cmꢁ1) 3052, 2928, 2850, 1769, 1711, 1600, 1509, 1478,
1450, 1402, 1358, 1228, 1005, 960, 913, 832, 750, 723; 1H NMR
2.2.1. General procedure for the synthesis of compound 4
(400 MHz, CDCl3)
d
8.14 (d, J ¼ 7.7 Hz, 4H), 7.86 (d, J ¼ 8.5 Hz, 4H),
In 25 mL round bottom flask, a mixture of compound 3 (1.63 g,
4 mmol), potassium tert-butoxide (0.94 g, 8.4 mmol), and DMF
(10 mL) was added and stirred for 1 h at 0 ꢀC, then bromide
(10 mmol) was added quickly into the mixture at this temperature.
After stirring for 10 h at room temperature, the mixture was poured
into water. It was extracted for several times with dichloromethane,
and then dried over anhydrous MgSO4. After the solvent was
removed under reduced pressure, the crude product was purified
by silica gel column chromatography to give the green product.
7.69 (d, J ¼ 8.5 Hz, 4H), 7.51 (d, J ¼ 8.2 Hz, 4H), 7.42 (t, J ¼ 7.3 Hz, 4H),
7.31 (t, J ¼ 7.3 Hz, 4H), 5.02 (s, 2H); 13C NMR (100 MHz, CDCl3)
d
169.38, 146.86, 140.15, 139.70, 135.45, 131.53, 129.90, 128.79,
126.71, 126.14, 123.97, 123.77, 121.21, 120.54, 120.44, 109.76, 29.69;
HRMS (ESI) m/z [M]þ calcd 759.1945, found 759.1908.
2.2.1.7. 3,4-Bis(4-(carbazol-9-yl)phenyl)-N-(2-(tert-butoxy)-2-
oxoethyl)maleimide (5d). Red powder; Yield 90%; mp 277 ꢀC; FT-IR
(KBr, cmꢁ1) 3054, 2925, 2856, 1768, 1745, 1709, 1598, 1512, 1479,
1449,1409,1367,1335,1315,1275,1230,1155, 940, 835, 744, 722; 1H
NMR (400 MHz, CDCl3) 8.21 (d, J ¼ 7.7 Hz, 4H), 7.88 (d, J ¼ 7.9 Hz,
4H), 7.73 (d, J ¼ 8.0 Hz, 4H), 7.62 (d, J ¼ 8.2 Hz, 4H), 7.49 (t, J ¼ 7.7 Hz,
4H), 7.37 (t, J ¼ 7.4 Hz, 4H), 4.49 (s, 2H), 1.55 (s, 9H); 13C NMR
2.2.1.1. 3,4-Bis(4-bromophenyl)-N-benzylmaleimide
(4b). Light
green powder; Yield 78%; mp 160 ꢀC; FT-IR (KBr, cmꢁ1) 3054, 2925,
2850, 1701, 1600, 1512, 1478, 1583, 1448, 1432, 1399, 1344, 1069,
1008, 923, 841, 821, 698; 1H NMR (400 MHz, CDCl3)
d
7.50 (d,
J ¼ 8.5 Hz, 4H), 7.47e7.42 (m, 2H), 7.38e7.29 (m, 7H), 4.79 (s, 2H);
13C NMR (100 MHz, CDCl3)
169.83, 136.17, 135.32, 132.08, 131.36,
(100 MHz, CDCl3) d 170.06, 166.54, 139.48, 138.89, 135.21, 131.40,
126.30, 126.24, 126.02, 123.20, 120.41, 120.26, 109.71, 83.22, 39.84,
d
27.87; HRMS (ESI) m/z [MþK]þ calcd 732.2265, found 732.2273.
128.89, 128.78, 128.03, 127.18, 124.88, 42.17; HRMS (ESI) m/z
[MþNa]þ calcd 517.9367, found 517.9343.
3. Results and discussion
2.2.1.2. 3,4-Bis(4-bromophenyl)-N-pentafluorobenzylmaleimide (4c).
Green powder; Yield 72%; mp 176 ꢀC; FT-IR (KBr, cmꢁ1) 3106, 2928,
2856, 1774, 1714, 1583, 1445, 1399, 1349, 1329, 1128, 1003, 831; 1H
3.1. Synthesis
We have reported the synthesis of 5a (i.e. BCPMM) in the pre-
vious paper [48] with the method shown in Scheme 1 (Route A). In
this route, 3,4-dibromomaleimide was employed as raw material
and firstly N-methylated with iodomethane in acetone to get
compound 2. Then, compound 2 was coupled with 4-(9H-carbazol-
9-yl) phenylboronic acid by a Suzuki coupling reaction to produce
the resulting compound 5a. Due to the large steric hindrance, the
separated yield of Suzuki coupling reaction is low, which results in
a 15% yield of 5a over the two reactions. To improve the yields of
diaryl maleimide dyes, a new synthetic route (Route B) was adopted
to replace Route A. In Route B, the 5a-d were synthesized through
the substitution of 3,4-bis(4-bromophenyl)maleimide and the Ull-
mann reaction of carbazole using copper iodide as catalyst, and
trans-1,2-cyclohexanediamine as ligand. The synthetic route is
recommended for high total yields of 60e78%. Pure resulting
products were obtained as a yellow powder after silica gel column
chromatography using DCM/petroleum ether (1:1) as the eluent.
The structure of the new diarylmaleimide dyes was confirmed by
1H NMR, 13C NMR and high-resolution mass spectrometry.
NMR (400 MHz, CDCl3)
d
7.51 (d, J ¼ 8.6 Hz, 4H), 7.34 (d, J ¼ 8.5 Hz,
4H), 4.91 (s, 2H); 13C NMR (100 MHz, CDCl3)
d
168.92, 146.81,
144.28, 138.80, 135.42, 132.16, 131.31, 126.81, 125.17, 109.01, 29.99;
HRMS (ESI) m/z [M]þ calcd 584.8998, found 584.9024.
2.2.1.3. 3,4-Bis(4-bromophenyl)-N-(2-(tert-butoxy)-2-oxoethyl)mal-
eimide (4d). Yellow-green powder; Yield 87%; mp 161 ꢀC; FT-IR
(KBr, cmꢁ1) 2987, 2925, 2856, 1743, 1710, 1587, 1486, 1422, 1396,
1370, 1229, 1154, 1070, 1013, 936, 829, 751; 1H NMR (400 MHz,
CDCl3)
d
7.49 (d, J ¼ 6.4 Hz, 4H), 7.35 (d, J ¼ 6.4 Hz, 4H), 4.29 (s, 2H),
1.48 (S, 9H); 13C NMR (100 MHz, CDCl3)
d
169.48, 166.31, 135.68,
132.10, 131.36, 127.09, 124.95, 83.01, 40.07, 28.03; HRMS (ESI) m/z
[M]þ calcd 541.9579, found 541.9571.
2.2.1.4. General procedure for the synthesis of compound 5. To a
25 mL round bottom flask was added carbazole (1 g, 6 mmol), 4
(1 mmol), potassium carbonate (0.84 g, 6 mmol), copper iodide
(95 mg, 0.5 mmol). The solid mixture was purged with N2 before
adding anhydrous dioxane (10 mL). The mixture was stirred at
room
temperature
for
15
min,
then
( )-trans-1,2-
3.2. Photophysical properties in solution
diaminocyclohexane (57 mg, 0.5 mmol) was added rapidly. The
mixture was heated to reflux overnight then cooled to ambient
temperature and then filtered. The crude product was purified by
silica gel column chromatography with mixed solvent dichloro-
methane (DCM)/petroleum ether (1:2, v/v) to give the pure
product.
In DCM solution, the maleimide dyes 5a-d exhibit similar ab-
sorption and emission properties (Fig. 1). It indicates that the
spectral properties of these compounds in solution are not influ-
enced much by changing the N-substituent on maleimide. In their
absorption spectra, there are three major absorption bands in the
range of 250e500 nm region derived from
pep* transitions. The
2.2.1.5. 3,4-Bis(4-(carbazol-9-yl)phenyl)-N-benzylmaleimide
(5b).
lower energy ones at about 330 nm and 420 nm are assigned to the
transitions involving both carbazole and maleimide moieties,
because neither of them alone displays an absorption at wave-
lengths longer than 325 nm. The lowest energy one at 420 nm
exhibits a charge-transfer character. Each molecule of 5 contains a
pair of cross-intercepted dipoles, which consist of an electron
donor (carbazole) and an electron acceptor (maleimide carbonyl).
Electron transition from donor to acceptor thus generates a charge-
separated state, corresponding to a red emission at 615e630 nm. A
solvent effect can be seen in both the absorption and emission
spectra (Fig. S1eS4), with relevant spectroscopic data summarized
in Table S1. To further understand the charge transfer transition,
quantum chemical calculations of compound 5a-d were performed
Yellow powder; Yield 79%; mp 298 ꢀC; FT-IR (KBr, cmꢁ1) 3060,
2927, 2850, 1763, 1699, 1600, 1512, 1478, 1449, 1399, 1344, 1359,
1336, 1228, 1172, 1019, 917, 833, 749, 722, 699; 1H NMR (400 MHz,
CDCl3)
d
8.17 (d, J ¼ 7.8 Hz, 4H), 7.95e7.81 (m, 4H), 7.69 (dd, J ¼ 8.4,
1.3 Hz, 4H), 7.54 (d, J ¼ 8.3 Hz, 6H), 7.39 (m, 11H), 4.92 (s, 2H); 13
C
NMR (100 MHz, CDCl3)
d 170.29, 140.24, 139.47, 136.30, 135.42,
131.55, 128.95, 128.82, 128.05, 127.12, 126.70, 126.14, 123.76, 120.50,
120.43, 109.81, 42.26.; HRMS (ESI) m/z [M]þ calcd 669.2416, found
669.2429.
2 . 2 .1. 6 . 3 , 4 - B i s ( 4 - ( c a r b a z o l - 9 - y l ) p h e n y l ) - N - p e n t a -
fluorobenzylmaleimide (5c). Orange powder; Yield 87%; mp 282 ꢀC;

348
X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
1.0
0.8
0.6
0.4
0.2
0.0
UV of 5a
UV of 5b
UV of 5c
UV of 5d
PL of 5a
PL of 5b
PL of 5c
PL of 5d
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 1. Normalized absorption (UV) and fluorescence (PL) spectra of 5 in DCM solution.
by density functional theory (DFT) method at the B3LYP/6-31G(d)
level. The calculated electron distributions of their HOMO and
LUMO are very similar (Fig. S5). The electron density of the HOMO
of 5a-d is mainly populated on the two benzene and carbazole
moieties (donor), while their electron density of the LUMO is
largely localized on the central maleimide moiety (acceptor).
Therefore, light excitation will clearly induce a charge-transfer
transition from the donor unit to the acceptor. The energy levels
of the HOMO and the LUMO of 5a-d are listed in Table S2. Their
calculated HOMO-LUMO energy gap is in the range of
2.54 eVe2.67 eV, which is very close to the optical band gap
(2.43 eVe2.51 eV). The electronic structure and orbital energy of
5a-d can readily account for the similarity of their optical proper-
ties in solution.
Fig. 2. (a) Photos of 5b-d solid under UV light and (b) their normalized fluorescence
spectra.
Table 1
Experimental data of photophysical properties of 5 in DCM solution and solid state.
a
Compound
States
lem, nm
FF, %
tav,
ns
kr,b nsꢁ1
knr,
c nsꢁ1
5a
Solution
YC1
YC2
RC
Solution
solid
Solution
solid
Solution
5d-Y
626
553
557
636
629
553
633
567
629
565
654
4
72
80
23
6
78
1
65
2
3.28
7.3
9.37
6.7
2.8
7.09
2.5
7.54
2.2
6.18
7.42
0.012
0.099
0.085
0.034
0.021
0.112
0.02
0.086
0.027
0.121
0.061
0.293
0.038
0.021
0.115
0.336
0.031
0.38
0.046
0.432
0.041
0.074
3.3. Emission in solid state
5b
5c
5d
After column chromatography, pure crystals of 5a-d with strong
and differently colored luminescence were obtained by recrystal-
lization from CH2Cl2/hexane or CHCl3/hexane. Interestingly, it was
found that 5d could be obtained in two kinds of crystal form: red
emissive 5d-R (natural evaporation) and yellow emissive 5d-Y
(rapid evaporation). The different emission spectra of 5b, 5c, 5d-Y
and 5d-R are shown in Fig. 2. For 5a, three kinds of polymorphs RC,
YC1 and YC2 have been found in our previous work [48]. For a
comparison, their photophysical parameters in the solid state and
DCM solution are listed in Table 1. The maximum emission wave-
length of 5b, 5c, 5d-Y and 5d-R appears at 553 nm, 567 nm, 565 nm
and 654 nm with quantum yields of 78%, 65%, 75% and 45%,
respectively. Compared with their solution, 5b-d crystals exhibit
huge increase in fluorescent quantum yields, due to their larger
radiative rate (kr) in the solid state. For example, the quantum yield
of 5c in solid is 65 times higher than that in DCM solution. Thus, all
of 5b-d crystals show a notable aggregation-induced enhanced
emission (AIEE) activity, like 5a.
It is important to analyze the molecular conformation and
packing modes of 5 to understand their solid-state emission. The
crystals of 5b and 5c being of high quality can be subjected to
single-crystal X-ray diffraction (SXRD). Both of 5b and 5c crystals
adopt the same crystal system (monoclinic) and space group (P 21/
n) [51]. Their molecular conformation and packing are shown in
Fig. 3. In the single molecule of 5b, one of carbazole ring slightly
deviates from the central maleimide ring with 1.0ꢀ of dihedral
angle, but the other one twists at a larger angle (87.2ꢀ). The dihedral
75
45
5d-R
a
Average lifetimes.
The radiative rate constant kr
The non-radiative rate constant knr ¼(1-FF)/ tav
b
c
¼
FF/ tav.
.
angles between maleimide plane and two carbazole planes are
11.7ꢀ and 89.9ꢀ in 5c, respectively. Due to the twisted conformation
of 5b and 5c, there is no specific packing affecting their photo-
physical properties in their crystals, such as H/J-aggregation, or p-p
stacking. The high quantum yields of 5b and 5c are attributed to the
existence of weak intermolecular interactions that would fix the
conformation, suppress the internal rotations, and block the non-
radioactive relaxation (Fig. S6 and S7). The main weak in-
teractions include CH/O (2.53 Å, 2.64 Å and 2.61 Å in 5b) and
CH/F (2.60 Å, 2.85 Å and 2.70 Å in 5c) hydrogen bonding, and CH
…
p (2.97 Å in 5b, 2.64 Å and 2.62 Å in 5c) interactions. Thus, like 5a
[48], it is molecular conformation not intermolecular packing
determining optical properties of 5b and 5c in solids. Unfortunately,
the size of the crystals of 5d-Y and 5d-R is too small to be detected
by SXRD. Based on the SXRD results of 5a-c, it can be inferred that
the planarity of 5d-R should be better than that of 5d-Y, resulting in
the red-shifted emission of 5d-R, relative to 5d-Y.

X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
349
Fig. 3. Molecular structures and packing structures of 5b (a) and 5c (b). The hydrogen
atoms in some pictures have been omitted for clarity.
3.4. Smart multi-stimuli-responsive luminescence
Due to the intense solid-state emission of 5b-d, their responsive
behavior in luminescence under various external stimuli has
aroused our interest. As shown in Fig. 4, initial 5d-Y with yellow
fluorescence emission (lmax ¼ 565 nm) was transformed into a red
solid with red emission (lmax ¼ 656 nm) by fuming in acetone
vapor for 24 h. The power X-ray diffraction patterns (PXRD) indi-
cate that the red solid has the same packing mode as 5d-R, and that
the transforming from 5d-Y into 5d-R is a crystal-to-crystal phase
transition (Fig. S8). Once 5d-Y and 5d-R were ground by using a
pestle in a mortar for 1 min, an orange solid emitting at 610 nm was
obtained. The orange emissive ground solids could be changed into
5d-R upon by fuming in acetone vapor for 3min, which could be
confirmed by the similar and intense diffraction peaks of 5d-R and
the fumed ground 5d (Fig. S8). In contrast, the ground 5d affords
very few diffraction peaks in its PXRD pattern. It indicates that the
conversion from 5d-R or 5d-Y into ground 5d was originated from
the transition of packing structure from orderly crystalline phase to
an amorphous state. Using the process combined with grinding and
fuming, the time of crystal-to-crystal phase transition from 5d-Y
into 5d-R was greatly reduced.
Due to the existence of solvent in the 5b crystals, heating 5b at
180 ꢀC for 5 min would cause a loss of the solvent in the crystals,
because differential scanning calorimetry (DSC) analysis shows a
wide endothermic peak corresponding to the solvent evaporation
before 150 ꢀC (Fig. S9). Although the heated 5b solid also exhibits a
similar green emission (lmax ¼ 556 nm) as 5b (Fig. 5a), PXRD results
indicate that its packing mode is different from that of 5b
(Fig. S10a). 5b and the heated 5b solid could be transformed into an
orange solid (lmax ¼ 601 nm) by grinding in a mortar with a pestle.
After fumed in acetone vapor for 5 min or heated to 150 ꢀC for about
3 min, the orange solid fully recovered their original green lumi-
nescence. This cycle could be repeated, allowing the powder to
switch between the two different emission colors. No obvious
peaks were found in the PXRD pattern (Fig. S10a), which indicates
the crystal structure of the heated 5b solid was destroyed and
recovered during the cycle of grinding and fuming.
Fig. 4. (a) Photos of switching between 5d-Y and 5d-R upon external stimuli, taken
under UV light. (b) The emission spectra of 5d-Y, fumed 5d-Y, ground 5d and fumed
ground 5d.
and S10b). Therefore, all of 5b-d can switch their emission between
different states under external stimuli, like 5a. It illustrates that the
multi-stimuli-responsive fluorescence is a general phenomenon for
this kind of diarylmaleimiedes, whatever the nature of the N-group.
3.5. Data process and sensing for pressure
According to these significant fluorescence switching features of
5b, two rewritable data recording processes were designed by
solvent writing and vapor writing (Fig. 6). The first one employed a
quartz plate coated uniformly with 5b as the ‘paper’, on which a
word ‘DCM’ was written by using a Chinese brush pen and CH2Cl2
solvent as ink, and observed for their weak orange fluorescence.
Then the word ‘DCM’ was easily erased by heating or fuming with
solvent vapor. The other one used the ground quartz plate as the
background, on which a word ‘THF’ was written by using a pen
loaded with CH2Cl2 vapor as the ink. In contrast to the orange-
emitting background, the word ‘THF’ showed intense green emis-
sion, and can be simply erased by grinding. The two data recording
processes could be repeated and switched through external stimuli.
In order to further understand the piezochromic behavior, the
effect of applied pressure on the luminescence of 5b was investi-
gated. As shown in Fig. 7, as the applied pressure is increased from
0 to 25 MPa, the fluorescence emission of the 5b gradually shifts
toward the longer wavelength side, and its intensity dramatically
declines by 87%. The observed changes of both the emission in-
tensity and wavelength of 5b under hydrostatic pressure are similar
as those under grinding, implying that the piezochromic emission
of 5b under different mechanical stimuli comes from the same
origin. Compared with several reported materials possessing pie-
zochromic behavior under the pressure of 3e10 GPa [52e54], 5b
5c also shows similar piezochromic and vapochromic fluores-
cence as 5d and 5b under grinding and fuming with vapor (Figs. 5b

350
X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
Fig. 7. Fluorescence Photos (a) and emission spectra (b) of 5b solid under different
hydrostatic pressures (0e25 Mpa).
shows distinctive changes in emission under the pressure less than
25 MPa. This result indicates that 5b will be promising sensor for
low hydrostatic pressure.
3.6. Nanoparticles and sensing for explosive (picrates) and pH
The generally poor solubility of organic fluorescent materials in
water solution greatly limits their application in the aqueous
environment. Thus, the development of organic fluorescent nano-
particles in aqueous solution is very important and preferred for
practical application. DMF was selected as a good solvent, added
H2O (bad solvent) to generate the nanoaggregates of 5. Fig. S11
shows emission spectra of 5d in DMF/H2O mixtures (1 ꢂ 10ꢁ5 M)
with different fractions of water. It can be seen that the fluorescent
intensities of 5d do not monotonously change with increasing the
water fraction. Compound 5d is almost non-emissive when the
amount of water increases from 0 to 30%. After the water fraction is
more than 30%, the fluorescent intensity of 5d begins to increase,
reaching maximum value in the solution of 70% water content,
accompanied by a blue shift of the spectra in comparison with that
in pure DMF. It indicates that 5d begins to form the aggregates in
DMF-water (70%: 30%). Further increasing the water fraction in the
mixed solvent to 90% causes a gradual decrease in the emission
intensity of 5d. Similar phenomena were found for 5a and 5b in
DMF-water (Fig. S12 and S13), but different phenomena were
observed for 5c in the mixture solution (Fig. S14). The emission
intensity of 5c is intensified in the mixture solution when the water
content increases from 70 to 90%. This increase in intensity may be
attributed to different aggregates with different emission proper-
ties formed in the DMF-water mixture. The morphology of nano-
particles of 5d in the DMF/H2O mixtures (4: 6 v/v) with different
fraction of 70% water content was analyzed by Transmission Elec-
tron Microscope (TEM). The TEM photos (Fig. S15) show the formed
nanoparticles are spherical, and they are not of uniform size,
ranging from 50 nm to 160 nm.
Fig. 5. Fluorescent photos and spectra of 5b (a) and 5c (b) under different external
stimuli.
Fig. 6. Photos of rewritable date recording processes including solvent writing and
vapor writing (lex ¼ 365 nm).

X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
351
Detection of explosives is receiving extensive interest because of
its homeland security and anti-terrorism implications. Picric acid
(PA) is often employed as model explosive among the nitroaromatic
compounds because of its relative stability on trial. In addition, PA is
very toxic, because the concentration of PA reached 5 ppm in the
water can be fatal. The nanoparticles of 5a-d in DMF/water solution
were used to detect the different concentration of PA in the water.
Interestingly, with the concentration of PA increasing in the
aqueous solution containing nanoparticles of 5a-d, the intensity of
emission reduces gradually (Fig. 8 and S16). However, when the
concentration of PA reaches 15 ppm, the levels of fluorescence
quenching of 5a-d are different. The degree of the fluorescence
quenching of 5a is most remarkable, reaching 90% (Fig. 8b). The
Stern-Volmer plots of 5a-d are linear and give quenching constants
of 0.5453, 0.1198, 0.1144 and 0.1403 ppmꢁ1 respectively (Fig. S17).
There is no spectral overlap between the absorption of PA and the
emission of 5a nanoparticles (Fig. S18), which indicates that the
quenching effect of PA is not originated from the energy transfer
from 5a to PA. The electron transfer from the excited state of 5a to
the ground state of PA should be the main quenching pathway for
the LUMO (ꢁ3.89 eV) of PA locating between the HOMO (ꢁ5.66 eV)
and LUMO (ꢁ2.99 eV) of 5a. Due to the lower steric hindrance of
methyl group in 5a, the accessibility of 5a molecules for PA is larger
than that of 5b-d, resulting in its larger fluorescent quenching upon
addition of PA. The detection limit of 5a for PA was calculated to be
0.13 ppm by the previously reported method [55]. The response
time of 5a for PA sensing was investigate in the range of 5 se300 s
Fig. S19 indicates that the interaction of 5a with PA was almost
completed in 5 s.
was sensitive to the pH in aqueous solution (Fig. 9). When the pH
value is less than 12, the fluorescent intensity changes over a
relatively narrow pH range. However, the peak suddenly disappears
at pH ꢃ 12, accompanied by the growth of a new emission band at
475 nm. The ratio of the fluorescent intensity at 608 nm and 475 nm
(I608/I475) presents a 3.1 ꢂ 104 fold decrease from 31 at pH 6 to
0.001 at pH 14. The absorption spectra of the solution upon pH at
11e13 shown in Fig. S20 indicate that the peak at 420 nm corre-
sponding to the charge transfer from phenylcarbazole to maleimide
disappears. Consequently, the fluorometric change in pH ꢃ 12
should be attributed to the hydrolysis of 5a to yield 5aH (Fig. S21). It
can be confirmed by the MALDI mass of 5a nanoparticles at pH 7
and 12, where a peak at m/z 657.884 is assigned to [5aH þ NaH]þ
with the loss of the peak at m/z 592.202 belonging to [5a-H]þ
(Fig. S21). Due to the hydrolysis of 5a in the strongly alkaline con-
dition, the recyclability of 5a for pH sensing is poor. When 5a
nanoparticles at pH 14 was acidized to be neutral, the emission
band at 608 nm couldn’t recovers, as indicated in Fig. S22.
4. Conclusions
In summary, we have obtained a series of carbazole-based
diphenyl maleimides 5a-d with strong solid fluorescence via an
Ullmann reaction in high yields. All these compounds exhibit
remarkable AIEE characteristics in the polar solvent. The mal-
eimides show similar photophysical properties in solution, due to
the similar electron density of the HOMO and LUMO. The results of
single-crystal X-ray diffraction illustrated that the weak intermo-
lecular interactions fixed the twisted conformation and suppressed
the internal rotations, resulting in their intense emission in the
solid state. Very differently from 5b and 5c, but similarly to 5a, two
crystals of 5d cultured from the same solvent with different crys-
tallization speed display intense yellow fluorescence (5d-Y) and red
fluorescence (5d-R), respectively. Interestingly, 5d molecules pre-
sent a crystal-to-crystal phase transition from 5d-Y into 5d-R
pH is an important parameter for chemical reactions and
physiological function associated with cellular behavior. When
these novel materials explored in the pH sensing, it was found that
the intensity of the emission peak at 608 nm of 5a nanoparticles
Fig. 8. Fluorescent photos (a) and emission spectra (b) of 5a nanoparticles in DMF/
water solution (4: 6 v/v) before and after addition of PA. Concentration of 5a: 4 mM;
Fig. 9. Fluorescent photos (a) and emission spectra (b) of 5a nanoparticles in DMF/
water solution (4: 6 v/v) at different pH values (lex ¼ 365 nm). Inset: Plots of I608/I475
versus pH value, where I608 and I475 are peak intensity at wavelength of 608 nm and
475 nm respectively.
lex ¼ 401 nm. Inset: Linear equation plots of I0eI versus [PA] in DMF/water mixture (4:
6 v/v); I ¼ peak intensity and I0 ¼ peak intensity at [PA] ¼ 0 ppm.

352
X. Mei et al. / Dyes and Pigments 133 (2016) 345e353
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On the basis of the significant fluorescence switching features of 5b,
two rewritable data recording processes were designed by solvent
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upon pH 1e11 to the weak blue emission at 475 nm upon pH 12e14.
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This work was supported by the National Natural Science
Foundation of China (21374017 and 21574021), Specialized
Research Fund for the Doctoral Program of Higher Education
(20093223110002 and 20123503120002), Program for Innovative
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