Highly sensitive and selective fluorescent sensor for copper(II) based on salicylaldehyde Schiff-base derivatives with aggregation induced emission and mechanoluminescence

Article abstract of DOI:10.1039/c7nj01186e

Three new Schiff-base derivatives with aggregation-induced emission (AIE) characteristics have been designed and successfully synthesized. Their photophysical properties in solution, aqueous suspension and the solid state were investigated systematically.

Full text of DOI:10.1039/c7nj01186e

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Highly sensitive and selective fluorescent sensor for copper(II) based
on salicylaldehyde Schiff-base derivatives with aggregation induced
emission and mechanoluminescence
Xiaoli Yang, Wenyan Zhang, Zihan Yi, Hao Xu, Jun Wei, Lingyun Hao1
College of Material Engineering, Jinling Institute of Technology, Nanjing 211169, P. R. China
Keywords:
Salicylaldehyde Schiffꢀbase, Aggregation induced emission, Cu2+ detection, Mechanofluorochromism
Abstract
Three new Schiffꢀbase derivatives with aggregationꢀinduced emission (AIE) characteristics have
been designed and successfully synthesized. Their photophysical properties in solution, aqueous
1
suspension and solid states were investigated systematically. Three compounds showed obvious πꢀπ*
transition mixed with intramolecular charge transfer (ICT) properties in UVꢀvis spectra, which were
evidenced by spectral analysis and theoretical calculations. The compounds exhibited evident AIE
characteristic with relatively high fluorescence quantum yield in the solid state (ΦF=15%ꢀ21%).
Moreover, the morphologies and sizes of particles in water/CH3CN solutions were studied using a
transmission scanning electron microscope (TEM) and dynamic light scattering (DLS). The
crystallographic data for A3 suggests that the existence of intramolecular hydrogen bonding
interactions restricted intramolecular rotation. Interestingly, A3 showed fluorescence turnꢀon sensing
towards Cu2+ via chelation enhanced fluorescence in a CH3CNꢀH2O (4 : 1, v/v) solution. It has fast
response time and a linear range of 0ꢀ20 µM for Cu2+ ions with a detection limit of 2.3 ×10ꢀ7 mol Lꢀ1. In
addition, A3 also exhibits a piezofluorochromic response.
Corresponding author.
Eꢀmail address: hly_jit@163.com.

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1. Introduction
Copper ions, the third most abundant metal ions, play significant roles in various biological
processes, and their homeostasis are critical for the metabolism and development of living organisms.1
However, unregulated concentration of Cu2+ ions will cause critical health and ecological issues.2ꢀ3 For
example, excess intake of Cu2+ ions can lead to serious diseases including Alzheimer’s, Parkinson’s,
Menke’s and Wilson’s diseases.4ꢀ5 On the other hand, deficiency of copper causes growth failure and
deterioration of the nervous system.6 Copper is also a significant metal pollutant due to its widespread
use in life science, medicine, chemistry and biotechnology. Therefore, the determination of copper ions
in various samples has been an important topic in the area of environmental protection and healthy life.
Compared with other techniques, the fluorescent chemosensors have attracted great attention due
to their great advantages including the high selectivity, sensitivity, specificity, low detection limit and
realꢀtime monitoring with fast response.7ꢀ8 Up to now, many fluorescent probes for Cu2+ have been
reported and even extended to applications in biological and environmental fields.9ꢀ20 Nevertheless,
some fluorescent molecules have a tendency towards aggregation and become weakly or even non
emissive. Such notorious phenomenon is called “aggregationꢀcaused quenching” (ACQ), which greatly
limits the applications as efficient fluorescent chemosensors. Aggregationꢀinduced emission (AIE)
materials that can solve the problem of ACQ have been paid great attention since their discovery in
2001. The AIEꢀactive materials offer a unique platform for scientists to construct fluorescent sensors.
To date, a large variety of AIEꢀactive fluorescent sensors have been developed and utilized as
biosensors.21ꢀ23
It is well known that Schiffꢀbase has advantages of relatively simple synthetic procedures and
easily coordinating with metal ions.24ꢀ25 Moreover, a number of fluorescence sensors based on
Schiffꢀbase derivatives bearing AIE/AIEE behavior have been reported.26ꢀ29 Recently, a few AIE
Schiffꢀbase derivatives were used as fluorescent sensors for Cu2+ ions.30ꢀ33 Herein, we synthesized three
new Schiffꢀbase derivatives modified by different electronꢀdonating and electronꢀwithdrawing group
(A1 to A3), such as carbazole, fluoren and naphthalimide, which were adopted as terminal substituents.
In addition, the molecules are large conjugated systems with twisted skeleton conformations that may
show good AIE performance. Their spectroscopic properties in solution, AIE behavior and the utility as

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fluorescent sensor for Cu2+ were investigated. The results display that all compounds exhibit
remarkable AIE property with yellow to red fluorescence emission and A3 was proved to be a Cu2+
fluorescent sensor, whereas A1 and A2 showed inferior selectivity to Cu2+. A3 showed fast response of
fluorescence sensing towards Cu2+ in a CH3CNꢀH2O (4 : 1, v/v) solution. It has a linear range of 0ꢀ20
µM for Cu2+ ions with a detection limit of 2.3 ×10ꢀ7 mol Lꢀ1. In addition, A3 also exhibited an obvious
piezofluorochromic response. Such mechanochromism was reversible upon the treatment of grinding
and fuming with CH2Cl2. The powder XRD studies showed that the mechanofluorochromic nature is
generated through crystallineꢀamorphous phase transformation under the stimulus of external force.
2. Experimental
2.1 Reagents
All solvents and reagents for synthesis were purchased from Aldrich and Energy Chemical
Regent Co. Ltd. Unless otherwise noted, all materials used in this work were commercially available
without any further purification. HPLC grade absolute acetonitrile and distilled deionized water were
used throughout the experiment. Silica gel (200–300 mesh) used for chromatography was purchased
from Sinopharm Chemical Reagent Co. Ltd. TLC was performed by using commercially prepared
100ꢀ400 mesh silica gel plates (GF254) and visualization was effected at 254 nm and 365 nm. Stock
solutions of A1-A3 (5.0×10ꢀ4 M) were prepared in spectroscopy grade acetonitrile. Stock solutions
(0.01 M) were prepared by FeCl3·6H2O, AlCl3·6H2O, CoCl2·2H2O, Pb(NO3)2, CuSO4·5H2O,
NiSO4·6H2O, CdCl2, AgNO3, MnCl2·4H2O, Zn(NO3)2·6H2O, BaCl2·2H2O and CrCl3·6H2O in
deionized water.
2.2 Synthesis and Characterization
All of the target compounds were characterized by proton and carbon nuclear magnetic resonance
spectroscopy (1H NMR, 13C NMR) and matrixꢀassisted laser desorption/ ionization time of flight mass
1
spectrometry (MALDIꢀTOF). H NMR (400 MHz) and 13C NMR (100 MHz) spectra were collected
1
using CDCl3 as the solvent. H NMR spectra are reported as followed: chemical shift in ppm (δ)
relative to the chemical shift of TMS at 0.00 ppm, integration, multiplicities (s=singlet, d=doublet,
t=triplet, m=multiplet). 13C NMR chemical shifts reported in ppm (δ) relative to the central line of
triplet for CDCl3 at 77 ppm. Mass spectra were obtained on an autoflex speed MALDIꢀTOF mass
spectrometer. UVꢀVis spectra were obtained on Uꢀ3310 UV spectrophotometer. Fluorescence spectra

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were recorded on a HitachiꢀFꢀ4500 fluorescence spectrophotometer. Dynamic light scattering (DLS)
experiments were conducted using a NanoPlusꢀ3 dynamic light scattering particle size analyzer.
Transmission electron microscopy (TEM) data were obtained on a FEI Tecnai G2 SꢀTwin. Single
crystal Xꢀray diffraction analyses were carried out on a Rigaku Saturn 724 CCD diffractometer with
Mo Kα radiation (λ = 0.71073 Å) at room temperature.
2.3. General synthetic procedure
Scheme 1. Synthetic route to compounds A1ꢀA3.
General synthetic procedure of compounds A1ꢀA3 is as follows.
1 mmol 4ꢀN,Nꢀ
dimethylaminoaniline and 1 mmol corresponding salicylaldehyde derivatives (Fig. S1ꢀS3) were added
to 20 mL anhydrous ethanol. The mixture was stirred and heated to 80 oC for 4 h. The reaction mixture
was allowed to cool to room temperature and concentrated in vacuo. The residue was purified by
column chromatography with petroleum/DCM. After dried under reduced pressure, products were
obtained in high yields (67ꢀ79%).
4ꢀ(9ꢀbutylꢀ9Hꢀcarbazolꢀ3ꢀyl)ꢀ2ꢀ(((4ꢀ(dimethylamino)phenyl)imino)methyl)phenol (A1). 1H
NMR (CDCl3, 400 MHz): δ ppm 13.75 (s, 1H), 8.75 (s, 1H), 8.27 (s, 1H), 8.15 (d, J =4 Hz, 1H), 7.68
(d, J =4 Hz, 3H), 7.45 (m, 3H), 7.33 (d, J =4 Hz, 2H), 7.23 (d, J =2 Hz, 1H), 7.11 (d, J =4 Hz, 1H),
6.77 (d, J =4 Hz, 2H), 4.34 (t, J =8 Hz, 2H), 3.01(s, 6H), 1.87 (m, 2H), 1.43 (m, 2H), 0.96 (t, J =8 Hz,
3H). 13C NMR (CDCl3, 100 MHz): δ ppm 164.37, 164.17, 161.35, 165.58, 150.16, 146.21, 136.47,
133.38, 132.68, 132.54, 131.17, 130.87, 130.18, 129.36, 128.82, 127.75, 126.82, 123.02, 122.30,
121.56, 119.88, 117.55, 112.68, 40.54, 30.25, 20.42, 13.87. ESIꢀHRMS: m/z calc. for [C31H31N3O+H]+:
462.25399, found 462.25351. Elemental analysis calculated for C31H31N3O: C, 80.66; H, 6.77; N, 9.10;

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Found: C, 80.67; H, 6.75; N, 9.07.
4ꢀ(9,9ꢀdibutylꢀ9Hꢀfluorenꢀ2ꢀyl)ꢀ2ꢀ(((4ꢀ(dimethylamino)phenyl)imino)methyl)phenol (A2). 1H
NMR (CDCl3, 400 MHz): δ ppm 13.84 (s, 1H), 8.75 (s, 1H), 7.74 (q, J1 =4 Hz, J2 =2 Hz, 2H), 7.65 (d, J
=2 Hz, 2H), 7.55 (d, J =4 Hz, 1H), 7.52 (s, 1H),7.30ꢀ7.36 (m, 5H), 7.11 (d, J =4 Hz, 1H), 7.11 (d, J =4
Hz, 1H), 6.77 (d, J =4 Hz, 2H), 3.02(s, 6H), 2.01 (t, J =8 Hz, 4H), 1.02ꢀ1.07 (m, 4H), 0.64ꢀ0.70 (m,
3H). 13C NMR (CDCl3, 100 MHz): δ ppm 160.40, 160.34, 157.53, 157.17, 151.48, 150.88, 150.01,
143.39, 140.84, 140.43, 139.96, 139.31, 137.47, 137.00, 132.58, 130.90, 129.88, 126.93, 126.79,
125.37, 122.89, 122.27, 120.56, 119.87, 119.65, 117.49, 112.77, 55.08, 40.61, 40.29, 26.01, 23.10,
13.89. ESIꢀHRMS: m/z calc. for [C36H40N2O+H]+: 517.32134, found 517.32080. Elemental analysis
calculated for C36H40N2O: C, 83.68; H, 7.80; N, 5.42; Found: C, 83.65; H, 7.83; N, 5.44.
2ꢀbutylꢀ6ꢀ(3ꢀ(((4ꢀ(dimethylamino)phenyl)imino)methyl)ꢀ4ꢀhydroxyphenyl)ꢀ1Hꢀbenzo[de]isoqui
1
nolineꢀ1,3(2H)ꢀdione (A3). H NMR (CDCl3, 400 MHz): δ ppm 14.10 (s, 1H), 8.70 (s, 1H), 8.64 (d, J
=4 Hz, 2H), 8.32 (d, J =4 Hz, 1H), 7.72 (t, J =8 Hz, 2H), 7.50 (s, 1H), 7.47 (d, J =4 Hz, 1H), 7.18 (d, J
=4 Hz, 1H), 6.76 (d, J =4 Hz, 2H), 4.22 (t, J =8 Hz, 2H), 3.02(s, 6H), 1.71ꢀ1.75 (m, 2H), 1.45ꢀ150 (m,
2H), 1.00 (t, J =8 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ ppm 159.84, 157.77, 149.96, 140.94,
139.69, 137.17, 133.18, 131.62, 131.07, 129.99, 125.78, 124.73, 122.25, 120.41, 119.92, 118.85,
118.29, 117.42, 112.80, 108.85, 42.92, 40.62, 31.19, 20.60, 13.91. ESIꢀHRMS: m/z calc. for
[C31H29N3O3+H]+: 492.22817, found 492.22803. Elemental analysis calculated for C31H29N3O3: C,
75.74; H, 5.95; N, 8.55; Found: C, 75.77; H, 5.93; N, 8.52.
3. Results and discussion
3.1. Synthesis of A1-A3
The synthetic routes for Schiffꢀbase derivatives were shown in Scheme 1. Compounds 3, 4 and 5
were synthesized according to the literatures.34ꢀ36 Firstly, compound 2 was prepared by compound 1
and bis(pinacolato)diboron in the presence of 1,1´ꢀ[bis(diphenylphosphino)]ferrocene palladium(II)
dichloride and potassium acetate in anhydrous 1,4ꢀdioxane with a yield of 82% .37 Then S1ꢀS3 were
obtained via suzuki reaction between compounds 2 and 3(4,5) catalyzed by tetrakispalladium in
relatively high yields (81% for S1, 88% for S2 and 83 % for S3). Finally, S1ꢀS3 were allowed to react

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with 4ꢀN,Nꢀdimethylaminoaniline in dry EtOH to give corresponding Schiffꢀbase derivatives(A1ꢀA3),
respectively. A1ꢀA3 are soluble in CH2Cl2, CHCl3, toluene and DMSO, but show poor solubility in
alcohols (such as methanol and ethanol) and aliphatic hydrocarbon solvents (such as cyclohexane and
nꢀhexane) at room temperature. All target molecules were purified by column chromatography on silica
gel, and unambiguously characterized by 1H and 13C NMR, MALDIꢀTOF mass spectrometry.
3.2. UV-vis absorption and fluorescent emission spectra in solutions
(a)
A1
A2
A3
A1
A2
A3
60000
40000
20000
0
(b)
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
300
400
500
600
Wavelength (nm)
Wanglength/nm
Fig. 1 (a) UVꢀvis absorption spectra and (b) normalized emission spectra of A1, A2 and A3 in CH3CN at a concentration of
1×10ꢀ5 M. The excitation wavelength was 397 nm for A1, 390 nm for A2, 350 nm for A3.
The UVꢀvis absorption and fluorescent emission spectra of A1, A2 and A3 in CH3CN solution are
presented in Fig. 1, and their optical characteristics are summarized in Table 1. In Fig.1a, it is clear that
A1 exhibited two main absorption bands located at 281 nm and 395 nm. The absorption band at 281
1
nm could be attributed to the π–π* transition of the conjugated πꢀsystem due to large molar extinction
1
coefficients, while the absorption band at 395 nm is corresponded to the π–π* transition and the
intramolecular charge transfer (ICT) transitions, which is in line with the salicylaldehyde derivatives
reported previously.38ꢀ40 For A2, three absorption bands are observed around 283 nm, 318 nm and 393
nm, whose assignment is similar with A1. Only one absorption band at 390 nm for A3 could be seen,
which could be attributable to 1π–π* transition mixed ICT transition and bigger molar extinction
coefficients of A3 than those of A1 and A2. Furthermore, minor solvatochromic effect of the major
absorption bands and positive solvatochromic effect of the lowꢀenergy bands (Fig. S4ꢀS6) support the
1
1π–π* and 1ICT assignments for these compounds. The π–π*/1ICT transition assignment of the lowest
energy absorption band is also supported by the density functional theory (DFT) calculations, which
will be discussed in the following section.

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Normalized emission spectra of compounds A1, A2 and A3 in CH3CN at a concentration of
1×10ꢀ5 M are illustrated in Fig. 1b. The emission properties of these compounds are influenced
significantly by the different aryl substituents and excitation of three compounds at room temperature
results in blue to orange luminescence. The emission of A1 and A2 bearing electronꢀdonating
components with the low quantum yields (ΦF=0.01 and 0.007) in CH3CN solution is bathochromic
shift compared with that of A3 containing electronꢀwithdrawing substituent. Furthermore, the emission
characteristics of compounds A1, A2 and A3 in a variety of solvents were investigated at room
temperature in Fig. 2. The emission of A1 had less dependence on the solvent polarity, suggesting that
weak ICT occurred. A similar outcome for compounds A2 was observed in Fig.S7. However, the
fluorescence spectra of A3 displayed two emission peaks in nonpolar solvent. Longꢀwavelength
emission was quenched with increasing the polarity of solvents, which indicates its emission state
originating from ICT state, whereas the shortꢀwavelength emission peak could be assigned as the
1πꢀπ* of naphthalimides group.
Hexane
(a)
Hexane
Toluene
DCM
1.0
0.8
0.6
0.4
0.2
0.0
(b)
1.0
0.8
0.6
0.4
0.2
0.0
Toluene
DCM
THF
THF
CH CN
CH CN
3
3
500
550
600
650
700
750
400
450
500
550
600
650
Wavelength (nm)
Wavelength/nm
Fig. 2 Normalized emission spectra of A1 (a) and A3 (b) in different solvents (1×10ꢀ5 M).
Table 1. Photophysical data for A1, A2 and A3
Compd.
λ (nm)a
εmax(10 4L·molꢀ1·cmꢀ1)
λmax (nm)
Φ
Eg/eV
solution/powder
281,356,395
283,318,393
389
5.8,2.5,2.8
3.2,3.8,2.6
3.3
a586/ b 597
a 573/ b 591
a 439/ b 574
0.01/0.21
0.007/0.19
0.002/0.15
3.46
3.62
3.68
A1
A2
A3

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a Recorded in 1×10ꢀ5 M CH3CN solution at R.T. b Recorded powder at R.T. c Calculated using DFT at the B3LYP/6ꢀ31G* level.
A1
A3
A2
Fig. 3 Contour plots of electron density distribution and energy values of HOMO and LUMO for A1–A3.
To understand nature of the ground electronic state and the lowꢀlying singlet excited electronic
state, the density functional theory (DFT) calculations were carried out by Gaussian 09 program using
DFT/B3LYP/6ꢀ31G* method. Fig. 3 shows the electron density distribution of the HOMO and LUMO
for compounds A1, A2 and A3. The πꢀelectrons in the LUMOs for A1, A2 and A3 are mainly localized
on the N,Nꢀdimethylaniline part, while their distributions of HOMOs are affected significantly by aryl
substituents. The HOMOs are mainly distributed on the N,Nꢀdimethylaniline part and salicylaldimine
motifs for A1, and the fluorene and salicylaldimine motifs for A2, as well as the naphthalimide
segment for A3. Therefore, the HOMOꢀLUMO transition dominantly contributing configuration to the
1
1
1
S1 state should be ascribed to πꢀπ* characters with a little of ICT feature for A1 and πꢀπ* /1ICT
characters for A2, respectively. However, the HOMOꢀLUMO transition of A3 should be corresponded
to 1ICT characters with some 1πꢀπ* feature. The calculated energy values and relative energy band gaps
(Eg) are presented in Fig. 3 and Table 1. The values of HOMOꢀLUMO gap decrease when
salicylaldimine core was introduced by the carbazole or fluorene, causing a slight red shift of the
1πꢀπ*/1ICT band compared with that modified by the naphthalimides substituent. These results are
consistent with our analysis of the lowꢀenergy absorption in the UVꢀvis absorption spectra. Moreover,
three compounds adopt twisted spatial conformation at their optimized lowest energy states, which
prevents close molecular packing in the solid state. Dihedral angles between the terminal
electronꢀdonating or withdrawing section and salicylaldimine motifs are 37.2° for A1, 36.8° for A2 and
51.9° for A3, respectively. Therefore, the introduction of different aryl substituents also influences

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molecular packing modes in the solid state and may endow them with AIE behaviors.
To further study the aggregation characteristics of compounds A1, A2 and A3, solid state
fluorescence emission spectra were measured. As shown in Fig. 4, the solid fluorescence spectra of
A1, A2 and A3 showed emission peaks at 597 nm, 591 nm and 575 nm, respectively. The values of
quantum efficiencies (ΦF) of the solids were determined to be 21%, 19% and 15% , respectively, which
are influenced distinctly by the aryl substituents. The relative high ΦF values make them potential
candidates in OLEDs.
A1
A2
A3
1.0
0.8
0.6
0.4
0.2
0.0
500
550
600
650
700
750
Wavelength (nm)
Fig. 4 The normalized emission spectra of A1, A2 and A3 as solid powders.
3.3. Aggregation induced emission (AIE) properties
To explore the aggregationꢀinduced emission of A1, A2 and A3, we used anhydrous CH3CN as
good solvent and water as poor solvent, which are commonly used in the study of AIE behavior. The
nanoꢀaggregates of A1, A2 and A3 were prepared by the precipitation method, adding water to a
CH3CN solution of the three compounds. The concentration of them in aqueous CH3CN with different
CH3CNꢀwater ratios was kept at 1×10ꢀ5 M. As expected, all these compounds present the AIEE and
AIE character. As example, the corresponding emission spectra and digital photo of compound A1 in
CH3CNꢀH2O mixture solution with different H2O fractions are shown in Fig. 5. A1 emitted fairly weak
orange emission in pure CH3CN peaked at 589 nm, and the Φf was only 0.01. It exhibited variable
emission in CH3CNꢀwater mixture solutions with different water fractions, which directly elucidates
the AIE process. When fw was below 60%, weak orange emission was gradually quenched. However,
the compound aggregated and an obvious enhancement of luminescence intensity was observed in the
70% (v/v) CH3CNꢀwater mixture. The PL intensity in a CH3CNꢀwater mixture with fw = 80% was

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about 21ꢀfold higher than that in pure CH3CN solution. When fw reached 90% in CH3CNꢀwater mixture
systems, the PL intensity decreased slightly. The fluorescent images of the solvent mixtures present a
clear view demonstrating the AIE effect in Fig. 5b. Similar trends of emission spectra for A2 and A3 in
a CH3CNꢀwater mixture are observed (Fig. S8ꢀS9).
0%
(a)
800
600
400
200
0
10%
20%
30%
40%
50%
60%
70%
80%
90%
50
20
15
10
5
Water fraction in CH CN/Vol%
3
40
50 60
70 80
0
10 20 30
500
550
600
650
700
750
Wavelength/nm
Fig. 5 (a) The fluorescence spectra of A1 in CH3CNꢀH2O mixture solution with different H2O fractions (vol., 0–90%). (b) Plot of
I/I0 vs. water content of the solvent mixture, where I0 is the PL intensity in pure CH3CN solution and the digital photo of A1 in
CH3CNꢀH2O mixture solution under irradiation of UV lamp at 365 nm.
Since the three compounds were hydrophobic, their nanoparticles began to aggregate as the
water fraction increased. To further investigate the morphological properties of all compounds in
aggregated state, transmission electron microscopy (TEM) and dynamic light scattering (DLS) were
used to measure the samples (90% water and 10% CH3CN, c = 1.0×10ꢀ5 M). As shown in Fig. 6,
spherical aggregates in the CH3CNꢀwater mixture emerge with the increasing proportion of water for
all compounds. Moreover, the average particle diameter determined by DLS data for each compound is
191 nm for A1, 204 nm for A2, 212 nm for A3, respectively. These results reveal the existence of
nanoꢀaggregates in high water content. Therefore, we deemed that the weakꢀemissive nature of the
three compounds in CH3CNꢀH2O systems with low fw might be ascribed to twisted intramolecular
charge transfer (TICT) activity that dominates the emission intensity and quenches the light
emission .41,42 However, the aggregation of particles restricted the intramolecular rotation (RIR) process
at high fw in CH3CNꢀwater, which facilitated the radiative decay and resulted in an enhanced
fluorescence emission.

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Fig. 6 TEM images (aꢀc) and particle size distributions (dꢀf) of A1–A3 in CH3CNꢀwater mixtures (1: 9, v/v) at a concentration of
1×10ꢀ5 M .
3.4 Crystal structure
Fig. 7 (a) Molecular structure and dihedral angle between aromatic rings of A3. (b) Crystal packing mode of A3 by weak πꢀπ
interactions.
Crystal structure is important for understanding the optoelectronic properties and molecular
packing. Single crystal of compound A3 was obtained by slow evaporation from the ethanol solution at

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room temperature. The relevant crystallographic data are summarized in Table S1ꢀS2. Compound A3
crystallizes in the triclinic system (space group Pꢀ1 (2)) and each unit cell contains two molecules. As
shown in Fig. 7a, the dihedral angles between the naphthalimide ring (P1) and the middle
salicylaldimine ring (P2), and the N,Nꢀdimethylaniline ring (P3) are 58.857o (P1ꢀP2), 3.831o (P2ꢀP3),
respectively. It is worth noting that all the atoms except hydrogen are nearly in the same plane in P2
and P3 due to intramolecular OH•••N hydrogen bond, which is consistent with the early report.27 In the
packing structure, the A3 molecules are stacked in an antiparallel fashion (head to tail) through weak
π–π interactions between the P1 (ca. 3.49 Å and 0.58 Å). The molecules after aggregation restrict the
molecular rotation where the enol conformation is favorable in ground state because of intramolecular
hydrogen bonding. After excitation, the enol form gets isomerized into keto form and hence results a
bright emission .43,44 Thus, the emission enhancement of A3 in solid state/aggregated state is ascribed
by restricted intramolecular rotation (RIR). Similarly, the reason for AIE behavior of A1 and A2 may
be identical with that of A3.
3.5 Recognition towards metal ions
The sensor properties of three Schiffꢀbase derivatives for metal ions were evaluated in the
CH3CNꢀH2O (4 : 1, v/v) solution. The selectivity of probes A1ꢀA3 towards Cu2+ was firstly evaluated
by employing the metal salts of Fe3+, Al3+, Co2+, Pd2+, Cu2+, Ni2+, Cd2+, Ag+, Mn2+, Zn2+, Ba2+and Cr3+
through fluorescence spectra. A1 and A2 show enhanced fluorescence with numerous metal ions (Fig.
S10ꢀS11), whereas A3 is a specific sensor for Cu2+. As shown in Fig. 8a, the emission centered at 463
nm exhibited unconspicuous change on the addition of several kinds of metal ions except in the case
of Cu2+. Upon the addition of 10 equiv Cu2+, the intensity of original emission band remarkably
increased within 5 s, which resulted in a significant cyan emission under the 365 nm UVꢀlamp (Fig. 8c).
In contrast, the other metal ions showed minor changes in fluorescence intensity when they were
introduced into the solutions of probe. Subsequently, competition experiment was performed by adding
10 ꢁM Cu2+ to the solution of A3 (10 ꢁM) in the presence of other coexisting metal ions. As shown in
Fig. 8b, the PL intensity of A3 in the presence of Cu2+ are almost unaffected by the addition of most
frequently coexisting metal ions. Namely, the fluorescent response of A3 for Cu2+could be hardly
interfered by other cations.

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1.0
0.8
0.6
0.4
0.2
0.0
(a)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ag(l)
Ba(ll)
Cu(ll)
Cd(ll)
Mn(ll)
Ni(ll)
Pb(ll)
Zn(ll)
Al(lll)
Cr(lll)
Fe(lll)
Co(ll)
A3 + metal ions
A3 + metal ions + Cu(ll)
(b)
400
450
500
550
600
650
700
Wavelength (nm)
A3 Fe Al Co Pb Cu Ni Cd Ag Mn Ba Cr Zn
Fig. 8 (a) Fluorescence spectra of A3 in CH3CNꢀH2O (4 : 1, v/v) solution upon adding of 10 equiv. different metal ions. (b) The
interference tests of A3ꢀCu2+ in CH3CN–H2O (4 : 1, v/v) solution. The low bars represent A3 (1.0 × 10 ꢀ5 M) with cations (1.0
ꢀ5
×10ꢀ5 M) except for Cu2+; the high bars represent A3 (1.0×10 M) with cations (1.0 ×10 ꢀ5 M) upon the subsequent addition of
Cu 2+ (1.0×10 ꢀ5 M). (c) Image of A3 upon adding of 10 equiv. different metal salts in water solution under excitation of UV light
at 365 nm. From left to right: blank, Fe3+, Al3+, Co2+, Pd2+, Cu2+, Ni2+, Cd2+, Ag+, Mn2+, Ba2+, Cr3+, Zn2+.
The quantitative analytical behavior of A3 in CH3CNꢀH2O (4 : 1, v/v) solution for determination
of Cu2+ was repeated three times and averaged during titration with different concentrations of Cu2+
from 0 to 2.2 equivalents (Fig. 9a).45 With the concentration of Cu2+ increased, the fluorescence
intensity of A3 at 463 nm increased steadily. Moreover, an excellent linear relationship was observed
between I463 and the concentration of Cu2+ in the range of 0ꢀ20 ꢁM with a correlation coefficient of R2
= 0.9904, which demonstrated the potential utility of A3 in the CH3CNꢀH2O (4:1, v/v) solution for
quantitative determination of Cu2+. By plotting the PL intensity changes as a function of concentration,
the detection limit of A3ꢀCu2+ based on the DL =3σ/S (σ is the standard deviation of blank
measurements, and S is the slope between the fluorescence versus Cu2+ concentration）(Fig. 9b), is
calculated to be about 2.3 × 10ꢀ7 M, which is much lower than that recommended by WHO in drinking
water (30 ꢁM). Furthermore, to gain an idea for the design of new Cu2+ chemosensors, specific features
of A3 were compared with some reported Schiffꢀbase chemosensors (Table S3). A3 exhibited
appealing analytical features such as relatively wide linear range and low detection limit.

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100
80
60
40
20
0
2.2 eq.
(b)
100
80
60
40
20
0
(a)
2
R =0.9904
y=41.38x+10.90
0
0.0
0.5
1.0
1.5
2.0
2.5
400
450
500
550
600
650
700
2+
-5
[
Cu
]
/10 M
Wavelength (nm)
Fig. 9 (a) Fluorescence titration spectra of A3 in the presence of different concentrations of Cu2+(0ꢀ2.2 equiv.) in CH3CNꢀH2O (4 :
1, v/v) solution. (b) Determination of the detection limit based on PL intensity (463 nm) of A3 with Cu2+.
Moreover, in order to explore the interaction and binding mechanism of A3 and Cu2+, the
fluorescence spectroscopy and MS spectrum were conducted. The result of the Job's plot experiment
showed a 2 : 1 stoichiometry complexation between A3 and Cu2+ (Fig. 10a), which was further
confirmed by mass spectrometry analysis. The ion mass spectrum indicated that an ion peak at m/z
1044.5134 was assignable to [A3+ Cu2++H]+(Fig. 10b). Based on the above findings, we propose that
the reaction mechanism in this system may proceed through the route depicted in Fig. 10c : Cu2+
bonding takes place with nitrogen of imine groups and oxygen of phenolic hydroxyl group.
60
(a)
40
20
0.0
0.2
0.4
0.6
0.8
1.0
2+
2+
[Cu ]/([A3]+[Cu ])
(c)
N
O
N
O
Cu2+
O
N
O
N
O
OH
N
N
O
O
C4H9
Cu
C4H9
C4H9
N
O
N
N
Fig. 10 (a) Job's plot of A3 and Cu2+ (464 nm). The total concentration of A3 and Cu2+ was 1.0
×
10ꢀ5 M. The experiments were
carried out at room temperature in CH3CNꢀH2O (4 : 1, v/v) solution. (b) MS spectrum of A3 in the presence of Cu2+.(c) Proposed

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binding mode of A3 with Cu2+
.
The PL spectra of A3 were recorded in CH3CNꢀbuffer mixture (1:9, v/v) with different pH
solution (pH=1ꢀ14) (Fig. S12). The emission spectra at lower pH (1ꢀ10) showed a very weak PL
intensity, whereas PL enhancement was observed with pH>10 because the phenolic group can be
deprotonated and form the phenoxide ion.
3.6 Mechanofluorochromic properties
1.2
610
initial sample
grinding sample
fuming sample
(a)
Initial sample
Ground sample
Fumed sample
(c)
1.0
0.8
0.6
0.4
0.2
0.0
600
590
580
570
500
550
600
650
0
2
4
Wavelength/nm
Cycle Numbers
Initial sample
Ground sample
Fumed sample
(d)
5
10
15
20
25
30
2
θ/deg
Fig. 11 (a) Normalized fluorescent spectra of A3 in different solidꢀstates: original, grinding and fuming (λex = 400 nm). (b)
Photo of A3 color changes after grinding and fuming stimuli under UV light at 365 nm. (c) Cycle numbers for reversible
grindingꢀfuming. (d) Powder Xꢀray diffraction patterns of the corresponding samples.
The mechanochromic materials with tunable emission have attracted considerable attention due to
their potential applications as sensors, security inks, optical recording and memory devices.46ꢀ48 The
twisted conformations of the molecules may generate mechanofluorochromic properties for them.
Therefore, we scratched three compounds with a metal spatula directly. Pristine sample of A3 exhibited
mechanochromic luminescent behavior in response to external stimuli (Fig. 11). When the sample was

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ground or scratched, its yellow emission band located at 573 nm turned dramatically orangeꢀred
emission (ca. 594 nm) with the maximum emission redꢀshifted by ca. 21 nm. Furthermore, the grinding
solid was exposed to CH2Cl2 vapor at room temperature and the fluorescence color of the grinding state
was almost converted to the original color. The grindingꢀfuming processes can be performed reversibly
many times. The redꢀshift observed in the ground solid powder may be attributed to conformation
planarity in the molecular packing process. Grinding gave relatively large effect on the quantum yield
(ΦPL) at room temperature (the pristine sample: ΦPL = 0.15; the ground sample: ΦPL = 0.07) , which
indicates that A3 molecules may enhance the intermolecular π–π interactions.
To further study the mechanochromic and vapochromic properties of A3 in the initial and ground
states, powder Xꢀray diffraction (PXRD) measurements in different solid states were performed. As
shown in Fig. 11d, the initial sample of A3 exhibited several sharp and strong peaks, indicating that the
packing of molecules is in a wellꢀordered mode. In contrast, many diffraction peaks in the ground
sample weakened or disappeared, which indicates that the ordered structure has been destroyed and
amorphous structure formed after grinding. Then the diffraction pattern of the fumed sample is similar
to that of the initial sample in terms of the intensity and shape of diffraction peaks, which implies that
molecules are repacked in the wellꢀordered mode. Therefore, phase transformation between crystal and
amorphous states under external grinding induces a significant mechanochromic behavior.
Conclusion
In summary, we successfully synthesized three new Schiffꢀbase derivatives with
aggregationꢀinduced emission (AIE) characteristics. All compounds exhibited evident AIE
characteristic with high fluorescence quantum yield in the solid state. The crystallographic data for A3
indicated that the existence of intramolecular hydrogen bonding interactions restricted intramolecular
rotation. In a CH3CNꢀH2O ( 4 : 1, v/v) solution, A3 showed highly sensitive and selective fluorescent
response for Cu2+, which could be applied to various industrial and environmental monitoring fields.
More importantly, A3 also exhibited reversible mechanochromic luminescence with mechanical
stressꢀinduced spectral shift of 21 nm. It has been confirmed that the transformation between the
crystalline and amorphous states is responsible for the reversible mechanofluorochromic behavior upon
external stimuli. The present study provides valuable information for designing materials with AIE,

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Cu2+ sensor and mechanofluorochromic properties based on Schiffꢀbase.
Acknowledgements
This work was supported by Research Fund for the Doctoral Program of Jinling Institute of
Technology (jitꢀ2012ꢀ27), Natural Science Foundation of Jiangsu Province (BK20130095), and the
‘Qing Lan’ Project Foundation of Jiangsu Province. The authors declare that we have no potential
conflict of interest.
References
：
1. D. J. Thiele and J. D. Gitlin, Nat. Chem. Biol., 2008, 4, 145ꢀ147.
2. E. L. Que, D. W. Domaille and C. J. Chang, Chem. Rev., 2008, 108, 1517ꢀ1549.
3. B. Sarkar, Chem. Rev., 1999, 99, 2535ꢀ2544.
4. Y. K. Jang, U. C. Nam, H. L. Kwon, I. H. Hwang and C. Kim, Dyes Pigm., 2013, 99, 6ꢀ13.
5. X. Zhang, M. Kang, H. Choi, J. Y. Jung, K. M. K. Swamy, S. Kim, D. Kim, J. Kim, C. Lee and J.
Yoon, Sens. Actuators, B, 2014, 192, 691ꢀ696.
6. B. Schleper and H. J. Stuerenburg, J. Neurol., 2001, 248, 705ꢀ706.
7. J. Li, D. Yim, W. D. Jang and J.Yoon, Chem. Soc. Rev., 2017, 46, 2437ꢀ2458.
8. Y. B. Ding, W. H. Zhu, and Y. S. Xie, Chem. Rev., 2017, 117 , 2203ꢀ2256.
9. Y. Fu, C. Ding, A. Zhu, Z. Deng, Y. Tian and M. Jin, Anal.Chem., 2013, 85, 11936ꢀ11943.
10. J. Huang, M. Liu, X. Ma, Q. Dong, B. Ye, W. Wang and W. Zeng, RSC Adv., 2014, 4,
22964ꢀ22970.
11. Y. Chen, C. Zhu, J. Cen, J. Li, W. He, Y. Jiao and Z. Guo, Chem. Commun., 2013, 49, 7632ꢀ7634.
12. Z. Jiang, S. Tian, C. Wei, T. Ni, Y. Li, L. Dai and D. Zhang, Sens. Actuators, B, 2013, 184,
106ꢀ112.
13. N. Aksuner, E. Henden, I. Yilmaz, A. Cukurovali. Dyes Pigm., 2009, 83, 211ꢀ217.

New Journal of Chemistry
Page 18 of 21
View Article Online
DOI: 10.1039/C7NJ01186E
14. Sellamuthu, S. Sankarasekaran, R. Rajendran, K. Anjali A. and S. M. Partha, Dalton Trans., 2012,
41, 13330ꢀ13337.
15. J. T. Yeh, W. C. Chen, S. R. Liu and S. P. Wu, New. J. Chem., 2014, 38, 4434ꢀ4439.
16. G. J. Tae, J. N. Yu, J. L. Jae, M. L. Myoung, Y. L. Sun and K. Cheal, Sens. Actuators, B, 2015, 211,
498ꢀ506.
17. M. Naveen, K. G. Vinod, Sens. Actuators, B, 2015, 210, 408ꢀ417.
18. B. K. Kyung, K. Hyun, J. S. Eun, K. Sumi, N. Insup and K.Cheal, Dalton Trans., 2013, 42,
16569ꢀ16577.
19. L. L. Yang, W. J. Zhu, M. Fang, Q. Zhang, C. Li, Spectrochim. Acta A Mol. Biomol. Spectrosc.,
2013, 109, 186ꢀ192.
20. G. J.Tae , J. N.Yu, J. L. Jae, M. L. Myoung, Y. L. Sun and K.Cheal, New J. Chem., 2015, 39,
2580ꢀ2587.
21. Y. Hong, J. W. Lam, B. Z. Tang, Chem. Commun., 2009, 40, 4332ꢀ4353.
22. R. T. Kwok, C. W. Leung, J. W. Lam, B. Z. Tang, Chem. Soc. Rev., 2015, 44, 4228ꢀ4238.
23. H. Wang, E. Zhao, J. W. Lam, B. Z. Tang, Mater. Today., 2015, 112, 365ꢀ377.
24. P. A. Vigato, S. Tamburini, L. Bertolo, Coord, Chem. Rev., 2007, 251, 1311ꢀ1492.
25. A. D. Garnovskii, A. L. Nivorozhkin, V. I. Minkin, ChemInform., 1993, 126, 1ꢀ69.
26. P. S. Song, X. T. Chen, Y. Xiang, L. Huang, Z. J. Zhou, R. R. Wei and A. J. Tong, J. Mater. Chem.,
2011, 21, 13470ꢀ13475.
27. Q. Feng, Y. Y. Li, L. L. Wang, C. Li, J. M. Wang, Y. Y. Liu, K. Li, H. W. Hou, Chem. Commun.,
2016, 52, 3123ꢀ3126.
28. W. Y. Fang, G. B. Zhang, J. Chen, L. Kong, L. M. Yang, H. Bi, J. X. Yang, Sens. Actuators. B,
2016, 229, 338ꢀ346.
29. L. Q. Yan, T. T. Qing, R. J. Li, Z. W. Wang, Z. J. Qi, RSC. Adv., 2016, 6, 63874ꢀ63879.

Page 19 of 21
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01186E
30. T. Sanji, M. Nakamura and M. Tanaka, Tetrahedron Lett., 2011, 52, 3283ꢀ3286.
31. M. Shellaiah, Y.ꢀH. Wu, A. Singh, M. V. Ramakrishnam Raju and H.ꢀC. Lin, J. Mater. Chem. A,
2013, 1, 1310ꢀ1318.
32. M. Yang, Y. Zhang, W. Zhu, H. Wang, J. Huang, L. Cheng, H. Zhou, J. Wu and Y. Tian, J. Mater.
Chem. C, 2015, 3,1994ꢀ2002.
33. D. Li, J. J. Li, Y. Duan, B. T. Zhao and B. M. Ji, Anal. Methods, 2016, 8, 6013ꢀ6016.
34. N. Chuard, K. Fujisawa, P. Morelli, J. Saarbach, N. Winssinger, P. Metrangolo, G. Resnati, N.
Sakai, and S. Matile, J. Am. Chem. Soc., 2016, 138, 11264ꢀ11271.
35. L. F. Lai, C. L. Ho, Y. C. Chen, W. J. Wu, F. R. Dai, C. H. Chui, S. P. Huang, K. P. Guo, J. T. Lin,
H. Tian, S. H. Yang, W. Y. Wong, Dyes Pigm., 2013, 96, 516ꢀ524.
36. D. Gudeika, J. V. Grazuleviciusa, D. Volyniuk, B. Rita, J. Gytis, M. Arunas, G. Alytis, J. Saulius,
Dyes Pigm., 2015, 114, 239ꢀ252.
37. J. Lee, J. Park, V. S. Hong, Chem. Pharm. Bull., 2014, 62, 906ꢀ914.
38. R. Zhang, M. Gao, S. Bai, B. Liu, J. Mater. Chem. B., 2015, 3, 1590ꢀ1596.
39. J. L. Yang, J. Rui, X. Xu, Y. Q. Yang, J. Su, H. J. Xu, Y. Y. Wang, N. Sun and S. Wang, RSC. Adv.,
2016, 6, 30636ꢀ30641.
40. J. B. Sun, W. H. Mi, P. C. Xue, J. Y. Zhao, L. Zhai, R. Lu, Dye. Pigm., 2017, 136, 633ꢀ640.
41. X. L. Zhu, R. Liu, Y. Li, H. Huang, Q. Wang, D. F. Wang, X. Zhu, S. S. Liu and H. J. Zhu, Chem.
Commun., 2014, 50, 12951ꢀ12954.
42. Y. Li, Y. Wu, J. Chang, M. Chen, R. Liu and F. Y. L, Chem. Commun., 2013, 49, 11335ꢀ11337.
43. A. Douhal, F. Lahmani, A. H. Zewail, Chem. Phys., 1996, 207, 477ꢀ498.
44. P. Alam, V. Kachwal, I. R. Laskar, Sens. Actuators. B, 2016, 228, 539ꢀ550.
45. I. J. Chang, K. S. Hwang, S.K. Chang. Dyes Pigm., 2017, 137, 69ꢀ74.

New Journal of Chemistry
Page 20 of 21
View Article Online
DOI: 10.1039/C7NJ01186E
46. X. Q. Zhang, Z. G. Chi, J. Y. Zhang, H. Y. Li, B. J. Xu, X. F. Li, S. W. Liu, Y. Zhang, and J. R. Xu,
J. Phys. Chem. B., 2011, 115, 7606ꢀ7611.
47. M. J. Teng, X.R. Jia, S. Yang, X. F. Chen, Y. Wei, Adv. Mater., 2012, 24, 1255ꢀ1261.
48. J. W. Chung, Y. You, H. S. Huh, B. K. An, S. J. Yoon, S. H. Kim, S. W. Lee, and S. Y. Park, J. Am.
Chem. Soc., 2009, 131, 8163ꢀ8172.

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The present study provides valuable information for designing materials with AIE, Cu2+ sensor
and MFC properties based on Schiff-base.