Schiff base derivatives containing heterocycles with aggregation-induced emission and recognition ability

Article abstract of DOI:10.1039/c3tc32591a

Three new Schiff base derivatives possessing vagarious blue aggregation-induced emission (AIE) characteristics in tetrahydrofuran (THF)-water were synthesized. Their photophysical properties in solution, aqueous suspension, film and crystalline states, along with their relationships, were comparatively investigated. The crystallographic data for dye 1 and dye 3 indicated that the existence of multiple intermolecular hydrogen bonding interactions restricted intramolecular vibration and rotation. Moreover, the sizes and growth processes of particles with different water fractions were studied using a scanning electron microscope (SEM) and dynamic light scattering (DLS). The results showed that the small homogeneous particles, assembled in an ordered fashion with appropriate water contents, exhibited distinct AIE behavior. Moreover, in a THF-H₂O (4:1, v/v) solution of HEPES (20 mM) buffer, dye 2 showed fluorescence turn-on sensing towards Cr³⁺ and Al³⁺via chelation enhanced fluorescence (CHEF). The 2:1 stoichiometries of the sensor complexes (dye 2-Cr³⁺ and dye 2-Al ³⁺) were calculated from Job plots based on fluorescence titrations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3tc32591a

Journal of
Materials Chemistry C
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PAPER
Schiff base derivatives containing heterocycles with
aggregation-induced emission and recognition
ability†
Cite this: J. Mater. Chem. C, 2014, 2,
2684
a
ab
a
c
Lianke Wang, Jun Zheng, Hongping Zhou, a Jieying Wua
*
*
Gang Liu, Mingdi Yang,
and Yupeng Tiana
Three new Schiff base derivatives possessing vagarious blue aggregation-induced emission (AIE)
characteristics in tetrahydrofuran (THF)–water were synthesized. Their photophysical properties in
solution, aqueous suspension, film and crystalline states, along with their relationships, were
comparatively investigated. The crystallographic data for dye 1 and dye 3 indicated that the existence of
multiple intermolecular hydrogen bonding interactions restricted intramolecular vibration and rotation.
Moreover, the sizes and growth processes of particles with different water fractions were studied using a
scanning electron microscope (SEM) and dynamic light scattering (DLS). The results showed that the
small homogeneous particles, assembled in an ordered fashion with appropriate water contents,
exhibited distinct AIE behavior. Moreover, in a THF–H2O (4 : 1, v/v) solution of HEPES (20 mM) buffer,
dye 2 showed fluorescence turn-on sensing towards Cr3+ and Al3+ via chelation enhanced fluorescence
(CHEF). The 2 : 1 stoichiometries of the sensor complexes (dye 2–Cr3+ and dye 2–Al3+) were calculated
from Job plots based on fluorescence titrations.
Received 31st December 2013
Accepted 28th January 2014
DOI: 10.1039/c3tc32591a
www.rsc.org/MaterialsC
become strong emitters when aggregated as powders or nano-
particles or fabricated as solid thin lms. In the past ten years,
Introduction
AIE/AIEE mechanisms involving restriction of intramolecular
rotation (RIR),6 twisted intramolecular charge transfer (TICT),7
cis–trans isomerisation,8 planarity and rotation ability, have
been reported, etc.9 However, it seems that some of these
mechanisms are conicting, and none of them can be used
universally.10 Currently, there is still much work to do to clarify
AIE behavior from the perspectives of molecular packing and
electronic structure.
In our study, we synthesized a new series of Schiff bases with
triphenylamine and benzimidazole groups (dyes 1–3). Struc-
turally, triphenylamine can not only increase the solubility of
the molecule, but also enhance the extent of electron delocal-
ization and electron donating ability. Benzimidazole and C]N
double bond groups are intended to enrich the p-electron
density and increase the extent of p-electron delocalization of
the system. The molecules are large conjugated systems with
twisted skeleton conformations, which may show good AIE
characteristics. We aim to reveal the AIE features of these Schiff
bases and their structure–property relationships. The spectro-
scopic properties of these compounds in solution and crystal
states were investigated to elucidate the mechanism of
enhanced emission in the aggregation state. Moreover, the
development of uorescent sensors for chemical species of
biological and environmental signicance is currently an
attractive eld for scientists.11 Among uorescence sensors,
Schiff bases12 have simple synthetic procedures and are used in
Organic light-emitting materials have led to a revolution in the
eld of optoelectronic devices and sensing. The design and
synthesis of efficient light emitters in the solid and aggregate
states are important in the development of advanced optical
and biomedical devices, such as organic light-emitting diodes
(OLEDs) and uorescent biosensors.1 However, most lumines-
cence suffers from the problem of aggregation-caused quench-
ing (ACQ)2 due to strong p–p stacking interactions in extended
p-conjugated systems. This is a typical problem for common
organic chromophores and limits their applications in the real
world, which require materials to be solids or lms.3
At the beginning of this century, Tang4 and Park,5 et al.
reported several material systems that exhibit aggregation-
induced emission (AIE) and aggregation-induced enhanced
emission (AIEE) properties. AIE/AIEE luminescent materials are
practically non-luminescent or weak emitters in solution, but
aCollege of Chemistry and Chemical Engineering, Anhui University and Key Laboratory
of Functional Inorganic Materials Chemistry of Anhui Province, Hefei 230601, P. R.
China. E-mail: zhpzhp@263.net; Fax: +86-551-63861279; Tel: +86-551-63861279
bSchool of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei
230601, P. R. China. Fax: +86-551-63828106; Tel: +86-551-63828150
cCenter of Modern Experimental Technology, Anhui University, Hefei 230039, P. R.
China
† Electronic supplementary information (ESI) available: Fig. S4–S18, Tables S1–S3.
CCDC 932875 and 941787. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3tc32591a
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many optical sensors, as well as AIE applications.13 Considering mmol) were added to 50 mL ethanol under an atmosphere of
Schiff base compounds containing pyridine groups, we evalu- nitrogen. The mixture was reuxed for 1 h, cooled to room
ated the recognition abilities of three compounds towards a temperature, ltered, and concentrated in vacuo. 1.0 g of grey
range of metal ions. Dye 2 was proven to be an efficient uo- white solid was obtained by recrystallization from ethanol,
rescent sensor for metal ions.14
yield: 95%. 1H NMR (400 MHz, d6-DMSO, TMS): d 12.13 (s, 1H),
7.94–7.96 (d, 2H, J ¼ 8.4 Hz), 7.34–7.37 (t, 4H, J ¼ 7.8 Hz), 7.24–
7.26 (s, 1H), 7.07–7.12 (q, 6H, J ¼ 6.8 Hz), 7.01–7.03 (d, 2H, J ¼
8.8 Hz), 6.62 (s, 1H), 6.48–6.52 (s, 1H), 4.91 (s, 1H), 4.51 (s, 1H).
13C NMR (100 MHz, d6-DMSO, TMS): d 149.06, 148.04, 146.61,
144.53, 129.64, 127.00, 124.61, 123.80, 123.70, 121.91, 111.74. IR
(KBr, cmꢀ1): 3455, 3382, 3196, 3051, 2598, 1632, 1576, 1488,
1333, 1277, 1172. ESI-MS (m/z): [M]+ calcd for C25H20N4 376.19;
found 376.15.
Experimental section
Materials and apparatus
4-Nitro-o-phenylenediamine, triphenylamine formaldehyde,
palladium acetate, 2-pyridinecarboxaldehyde, 3-pyridine-
carboxaldehyde and 4-pyridinecarboxaldehyde were purchased
from Aladdin; the other chemicals were available commercially
and used without further purication. IR spectra were recorded
with an FT-IR spectrometer (KBr discs) in the 4000–400 cmꢀ1
region. NMR spectral measurements were carried out at
400 MHz for 1H NMR and 100 MHz for 13C NMR, using DMSO-
d6 as a solvent. Chemical shis were reported in parts per
million (ppm) relative to internal TMS (0 ppm), and coupling
constants were reported in Hz. Splitting patterns were described
as singlet (s), doublet (d), triplet (t), quartet (q), or multiplet (m).
The mass spectra were obtained on an LTQ Orbitrap XL mass
spectrometer or an autoex speed MALDI-TOF/TOF mass
spectrometer. The one-photon absorption (OPA) spectra were
recorded on a UV-265 spectrophotometer. The one-photon
excited uorescence (OPEF) spectral measurements were per-
formed using a Hitachi F-7000 uorescence spectrophotometer.
OPA and OPEF of dyes 1–3 were measured in THF at a
concentration of 10 mM. The quartz cuvettes used were of 1 cm
path length. Dynamic light scattering (DLS) measurements were
conducted on a Delsa PN A54412AB Nano Submicron Grain
Particle Size Analyzer. The scanning electron microscopy (SEM)
studies were performed using a Hitachi S-4800 scanning elec-
tron microscope. One drop of the solution or suspension was
placed on a silicon slice, which was then dried in air.
Synthesis of dye 1. 2-Pyridinecarboxaldehyde (0.06 g, 0.55
mmol) was added dropwise to a solution of 5-amino-2-triphenyl-
aminobenzimidazole (0.19 g, 0.5 mmol) dissolved in triethylamine
(20 mL) and THF (2 mL). The solution was stirred for 2 h at
room temperature. The reaction mixture was ltered, and a yellow
1
solid (0.21 g) was obtained, yield: 88%. H NMR (400 MHz, d6-
DMSO, TMS): d 12.85–12.86 (d, 1H, J ¼ 4.0 Hz), 8.71–8.73 (d, 2H,
J ¼ 5.2 Hz), 8.19–8.21 (d, 1H, J ¼ 8.0 Hz), 8.05–8.08 (d, 2H, J ¼ 8.0
Hz), 7.95–7.98 (q, 1H, J ¼ 8.0 Hz), 7.64–7.67 (d, 1H, J ¼ 8.0 Hz),
7.52–7.54 (s, 1H), 7.48 (s, 1H), 7.36–7.40 (t, 4H, J ¼ 9.8 Hz),
7.26–7.31 (q, 1H, J ¼ 8.0 Hz), 7.12–7.16 (q, 6H, J ¼ 8.4 Hz), 7.04–
7.07 (d, 1H, J ¼ 8.4 Hz). 13C NMR (100 MHz, d6-DMSO, TMS): d
158.77, 158.50, 154.36, 152.43, 152.28, 149.59, 148.84, 146.57,
145.16, 144.86, 144.59, 143.51, 136.95, 135.59, 134.480, 129.75,
127.69, 125.28, 124.97, 123.99, 121.47, 118.86, 117.19, 116.34,
111.34, 110.49, 103.51. IR (KBr, cmꢀ1): 3414, 1593, 1544, 1399,
767, 686. MALDI-TOF m/z: [M]+ calcd for C31H23N5 465.11;
found 465.05.
Synthesis of dye 2. 3-Pyridinecarboxaldehyde (0.06 g, 0.55
mmol) was added dropwise to a solution of 5-amino-2-
triphenylaminobenzimidazole (0.19 g, 0.5 mmol) dissolved in
triethylamine (20 mL) and THF (2 mL). The mixed solution was
stirred for 2 h at room temperature. The reaction mixture was
ltered, and a yellow solid (0.20 g) was obtained, yield: 85%. 1H
NMR (400 MHz, d6-DMSO, TMS): d 12.82–12.83 (d, 1H, J ¼ 4.0
Syntheses
Synthesis of 5-nitro-2-triphenylaminebenzimidazole. Tri- Hz), 9.09–9.10 (d, 1H, J ¼ 4.0 Hz), 8.82–8.83 (d, 1H, J ¼ 4.0 Hz),
phenylamine formaldehyde (2.72 g, 10 mmol) and KI (0.2 g, 12 8.69–8.70 (d, 1H, J ¼ 4.0 Hz), 8.33–8.36 (d, 1H, J ¼ 4.0 Hz), 8.05–
mmol) were added to a solution of 4-nitro-o-phenylenediamine 8.07 (d, 2H, J ¼ 8.4 Hz), 7.61–7.66 (t, 1H, J ¼ 8.4 Hz), 7.53–7.58 (s,
ꢁ
in 30 mL of DMF. The mixture solution was reuxed at 150 C 1H), 7.51(s, 1H), 7.36–7.42 (q, 5H, J ¼ 8.8 Hz), 7.23–7.36 (q, 1H,
for 120 h. The reaction was monitored by TLC. Aer the reaction J ¼ 8.0 Hz), 7.14–7.16 (q, 6H, J ¼ 8.8 Hz), 7.05–7.07 (d, 2H, J ¼ 8.8
was completed, the solution was cooled, added to water (500 Hz). 13C NMR (100 MHz, d6-DMSO, TMS): d 162.29, 156.60,
mL), ltered, and puried by column chromatography with 156.27, 152.35, 152.12, 151.51, 150.27, 150.22, 148.80, 145.86,
ꢁ
petroleum (b.p. 60–90 C)–ethyl acetate (4 : 1) to give 3.94 g of 145.53, 144.59, 143.19, 135.56, 134.75, 134.19, 131.84, 129.75,
1
yellow solid, yield: 95%. H NMR (400 MHz, d6-DMSO, TMS): d 127.66, 124.94, 123.98, 123.10, 121.50, 118.75, 117.26, 116.19,
13.36 (s, 1H), 8.41 (s, 1H), 8.07–8.12 (q, 3H, J ¼ 8.8 Hz), 7.70– 110.17, 103.50, 103.40. IR (KBr, cmꢀ1): 3406, 3058, 1610, 1594,
7.72 (d, 1H, J ¼ 8.0 Hz), 7.31–7.41 (t, 4H, J ¼ 7.8 Hz), 7.14–7.19 1488, 1326, 1284, 694. MALDI-TOF m/z: [M]+ calcd for C31H23N5
(q, 6H, J ¼ 7.6 Hz), 7.04–7.06 (d, 2H, J ¼ 8.8 Hz). 13C NMR (100 465.11; found 465.11.
MHz, d6-DMSO, TMS): d 149.69, 146.32, 142.38, 129.74, 128.20,
Synthesis of dye 3. 4-Pyridinecarboxaldehyde (0.06 g,
125.29, 124.29, 121.50, 120.75, 117.64. IR (KBr, cmꢀ1): 3051, 0.55 mmol) was added dropwise to a solution of 5-amino-2-tri-
1642, 1593, 1471, 1343, 1172. ESI-MS (m/z): [M]+ calcd for phenylaminobenzimidazole (0.19 g, 0.5 mmol) dissolved in
C
25H18N4O2 406.15; found 406.14.
Synthesis of 5-amino-2-triphenylaminebenzimidazole. 5- room temperature. The reaction mixture was ltered, and a
amino-2-triphenylaminobenzimidazole (1.22 g,
mmol), yellow solid (0.20 g) was obtained, yield: 85%. 1H NMR
hydrazine hydrate (15 mL) and palladium acetate (0.02 g, 0.8 (400 MHz, d6-DMSO, TMS): d 12.88–12.89 (d, 1H, J ¼ 4.0 Hz),
20 mL of methanol. The mixed solution was stirred for 24 h at
3
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Table 1 Crystallographic data for dye 1 and dye 3
Compound
Dye 1
Dye 3
Empirical formula
Formula weight
Crystal system
Space group
C
62H50N10O3
C31H24N5O1
482.55
Monoclinic
P21/c
23.904(5)
9.753(5)
11.323(5)
98.054(5)
2613.18(19)
4
983.12
Monoclinic
P21/c
16.824(5)
27.111(5)
11.546(5)
91.858(5)
5264(3)
4
˚
a [A]
˚
b [A]
˚
c [A]
b [ꢁ]
3
˚
V [A ]
Scheme 1 Synthetic routes to target dyes 1–3.
Z
T [K]
293(2)
1.241
0.079
0.99–25.50
38 998
9789
298(2)
1.226
0.077
1.00–25.00
18 153
4610
Dcalcd [g cmꢀ3
]
2-pyridinecarboxaldehyde, 3-pyridinecarboxaldehyde or 4-pyr-
idinecarboxaldehyde. The structures of the product molecules
are conrmed by 1H NMR, 13C NMR, mass spectral data
(Fig. S4–S18†) and X-ray crystallographic analysis.
m [mmꢀ1
q range [ꢁ]
Total no. reections
]
No. unique reections
No. parameters rened
894
342
R1
wR2
GOF
0.0726
0.2111
1.042
0.0536
0.1857
1.164
Photophysical properties
As shown in Fig. 1, dyes 1–3 display similar absorption spectra,
which indicate that the position of the nitrogen atom in the
molecule has almost no effect on the absorption. The relevant
spectroscopic data are summarized in Table S1.† In addition,
the absorption spectra show two distinct absorption peaks,
which are located at ꢂ290 nm (l1) and ꢂ374 nm (l2); the former
is attributed to p–p* electronic transitions caused by the tri-
phenylamine,16 whereas the latter is likely ascribed to p–p*
electronic transitions and intramolecular charge transfer (ICT)
processes.17
8.808 (s, 1H), 8.75–8.76 (d, 2H, J ¼ 4.0 Hz), 8.06–8.08 (d, 2H, J ¼
8.0 Hz), 7.88–7.89 (d, 2H, J ¼ 4.0 Hz), 7.27–7.32 (d, 1H, J ¼ 4.0
Hz), 7.47–7.55 (d, 1H, J ¼ 4.0 Hz), 7.36–7.40 (q, 4H, J ¼ 7.4 Hz),
7.27–7.32 (t, 1H, J ¼ 4.0 Hz), 7.12–7.16 (t, 6H, J ¼ 8.6 Hz), 7.05–
7.07 (d, 2H, J ¼ 8.0 Hz). 13C NMR (100 MHz, d6-DMSO, TMS): d
156.560, 156.65, 152.52, 152.45, 151.41, 150.47, 150.35, 148.84,
145.86, 145.53, 144.59, 143.19, 135.56, 134.75, 134.19, 131.84,
129.75, 127.66, 124.94, 123.98, 123.10, 121.50, 118.75, 117.26,
116.19, 110.17, 103.87. IR (KBr, cmꢀ1): 3406, 3058, 1610, 1594,
1488, 1326, 1284, 694. MALDI-TOF m/z: [M]+ calcd for C31H23N5
465.11; found 465.22.
Aggregation-induced emission
The UV-vis spectra of dye 3 (10 mM) with different water contents
have been recorded and are shown in Fig. 2a; we can see that
dye 3 shows two absorption peaks in dilute THF solution. As the
water fraction (fw) increases to 90%, the peak at ꢂ290 nm
disappears gradually, and the peak at ꢂ370 nm increases a little
in intensity and is slightly blue-shied. The photoluminescence
(PL) spectra of dye 3 in THF–water mixtures with different water
contents show that the PL intensity is very weak, and almost no
X-ray crystallography
Single crystal data were collected on a Siemens Smart 1000 CCD
diffractometer. Determination of unit cell parameters and data
collections were performed with Mo Ka radiation (l ¼
˚
0.71073 A). Unit cell dimensions were obtained from least-
squares renements, and all structures were solved by direct
methods using SHELXS-97.15 The non-hydrogen atoms were
located in successive difference Fourier syntheses. The nal
renement on F2 was performed using full-matrix least-squares
methods with anisotropic thermal parameters for non-
hydrogen atoms. The hydrogen atoms were added in theoretical
positions, and the processing parameters for dye 1 and dye 3 are
shown in Table 1.
Results and discussion
Materials
Scheme 1 shows the synthetic routes of the three compounds
(dyes 1–3). The experimental details and structural character-
ization data are described in the Experimental section and ESI.†
The key synthetic steps of the three compounds are aldehyde–
amine condensation reactions. The Schiff bases can be facilely
Fig. 1 Absorption spectra of dyes 1–3 in six organic solvents of
obtained from 5-nitro-2-triphenylaminobenzimidazole and different polarities at a concentration of 1.0 ꢃ 10ꢀ5 mol Lꢀ1
.
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uorescence lifetime gradually increases; when fw ¼ 90%, the
weighted mean time for dye 3 (3.21 ns) is much longer than that
in pure solvent (1.78 ns). Dye 1 and dye 2 show similar results
(Table S2†). This long lifetime indicates the presence of new
aggregation species or couplings.19
As shown in Fig. 4a, with gradual addition of water to the
THF, the maximum emission wavelength of dye 3 is gradually
red-shied by about 40 nm, which is the same as the situation
in different polar solvents (Fig. 4b). This is a typical TICT effect
arising from increased solvent polarity, here achieved by
increasing the water fraction. Similar spectral changes can also
be observed for dye 1 and dye 2 (Fig. S1 and S3†).
Fig. 2 (a) UV absorption spectra and (b) PL spectral changes of dye 3
(1.0 ꢃ 10ꢀ5 mol Lꢀ1) in THF–water mixtures with different water
fractions.
change is observed as fw increases from 0% to 20% (Fig. 2b).
However, a signicant enhancement of the uorescence is
observed in THF–water mixtures with fw > 20%, accompanied by
a small red-shi. The PL intensity in a THF–water mixture with
fw ¼ 80% is about 37.0 times higher than that in pure THF
solution. When fw ¼ 90%, the PL intensity in the THF–water
mixture decreases. Similar spectral changes are observed for
dye 1 and dye 2 (Fig. S2†), thus we use dye 3 as an example to
explain the spectral changes.
Crystal structures
In order to better understand the mechanism of AIE, we obtained
single crystals of dye 1 and dye 3. The relevant crystallographic
data are summarized in Table 1. The dihedral angles between the
benzimidazole ring (P2) and the terminal pyridine ring (P1), and
the triphenylamine-based phenyl ring (P3) are 45.22ꢁ (PA1–PA2),
9.46ꢁ (PA2–PA3), 32.27ꢁ (PB1–PB2) and 26.09ꢁ (PB2–PB3) for dye 1,
and 1.38ꢁ (P1–P2) and 1.23ꢁ (P2–P3) for dye 3.
Dyes 1–3 are nearly non-emissive in THF solution, and their
uorescence quantum yield values (FF) are <0.1%; they become
stronger emitters in the aggregate state with FF values of up to
39–47%. The experimental errors from sample concentrations
and instruments are estimated to be ꢄ10%. The PL decay
dynamics of the compounds were studied by a time-resolved
technique (Table S2†). The uorescence decay behavior of pure
THF solutions of dyes 1–3 with the same concentrations as the
THF–water mixtures ts well with the double-exponential
(Fig. 3), which was obtained by tting the time-resolved uo-
rescence curves based on the following double-exponential
function:
As shown in Fig. 5 and 6, the water molecules are bound
tightly by dye 1 or dye 3 molecules through multiple O–H/N
and O/N bonds between the N atoms of the imidazole and
pyridine groups and the H and O atoms of H2O, with distances
˚
˚
˚
˚
˚
˚
of 2.113 A, 2.153 A, 2.278 A, 2.361 A, 2.483 A and 2.697 A for dye
˚
˚
˚
1, and 1.922 A, 2.236 A and 2.776 A for dye 3. Moreover, Fig. 5
and 6 show that dye 1 and dye 3 are also restricted by several
˚
kinds of C–H/p hydrogen bonds (d ¼ 3.467 A for dye 1, d ¼
˚
˚
2.848 A and 3.299 A for dye 3). The interactions x the benz-
imidazole, pyridine and triphenylamine groups, which further
y ¼ A1eꢀt/s + A2eꢀt/s
1
2
The values of A1 and A2 represent the fractions of molecules
in each environment.18 With increasing water fraction, the
Fig. 4 (a) The change in the emission maximum peak of dye 3 (1.0 ꢃ
10ꢀ5 mol Lꢀ1) in THF–water mixtures with different water fractions. (b)
PL spectra of dye 3 in six organic solvents (1.0 ꢃ 10ꢀ5 mol Lꢀ1).
Fig. 3 Fluorescence lifetimes of dyes 1–3 (1.0 ꢃ 10ꢀ5 mol Lꢀ1) in THF– Fig. 5 (a) Molecular structure of dye 1; (b) the 1D chain of dye 1 formed
water mixtures with different water fractions.
by O–H/N and C–H/p interactions.
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Fig. 6 (a) Molecular structure of dye 3; (b) the 1D chain of dye 3
formed by O–H/N, O/N and C–H/p interactions.
Fig. 8 Particle size distributions of dye 2 in THF–water mixtures with
50%, 60%, and 90% water fractions.
prevents free rotation around the C–C single bonds. Therefore,
the molecules become more rigid. We can infer that the intra-
molecular motion is strongly restricted on increasing the water
fraction in THF solutions of the dyes. As for the very weak PL
intensities of dyes 1–3 in the lm and crystalline states, the
causes are still not very clear. It is likely due to the formation of
excimers20 or other morphologies of the aggregates.
dynamic light scattering (DLS) to examine the morphological
structures and average diameters of dyes 1–3 at different volume
fractions (Fig. 7 and 8, and Table S3†). The SEM image of dye 2
in a THF–water mixture with fw ¼ 70% shows small homoge-
neous crystalline particles with an average diameter of 193 nm
by DLS. However, when fw reaches 90%, larger asymmetric
crystalline particles are formed with an average diameter of 659
nm. Moreover, with changing time, the aggregates form smaller
homogeneous particles stepwise, as shown in Fig. 9. This
phenomenon suggests that the diameters of the particles are
correlated with the THF–water ratio. This result gives direct
evidence of molecular aggregation during emission enhance-
ment; smaller homogeneous particles in THF–water mixtures
favor uorescence emission.21
Scanning electron microscopy (SEM) and dynamic light
scattering (DLS)
In order to further probe the enhanced emission of dyes 1–3 in
THF–water, we used scanning electron microscopy (SEM) and
Electronic structures
To better understand the photophysical properties of the
present luminogens, theoretical calculations on their energy
levels were performed. The molecular orbital amplitude plots of
the highest occupied molecular orbitals (HOMOs) and the
Fig. 7 SEM images of dyes 1–3 in THF–water mixtures at a concen-
tration of 1.0 ꢃ 10ꢀ5 M with different water fractions: (a) dye 1 in THF–
water (40 : 60, v/v); (b) dye 1 in THF–water (10 : 90, v/v); (c) dye 2 in
THF–water (30 : 70, v/v); (d) dye 2 in THF–water (10 : 90, v/v); (e) dye 3 Fig. 9 Particle size distributions of dye 2 at 70% water fraction at times
in THF–water (20 : 80, v/v); (f) dye 3 in THF–water (10 : 90, v/v).
of 10 min, 20 min, and 30 min.
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Journal of Materials Chemistry C
Fig. 10 Energy levels and electron density distributions of the frontier
molecular orbitals of dyes 1–3.
Fig. 13 (a) Job's plot of dye 2 and Al3+ (¼ 370 nm). The total concen-
tration of dye 2 and Al3+ was 1.0 ꢃ 10ꢀ5 M. The experiments were
carried out at room temperature in THF. (b) Levenberg–Marquardt
lowest unoccupied molecular orbitals (LUMOs) of dyes 1–3 are
shown in Fig. 10. The electrons of the compounds' HOMOs are
conned mainly on the triphenylamine (TPA) and benzimid- algorithm plot of the dye 2–Al3+ complex at room temperature in THF.
azole moieties, while the LUMOs+1 are distributed mainly on
the benzimidazole moiety.
(c) Job's plot of dye 2 and Cr3+ (¼ 370 nm). The total concentration of
dye 2 and Cr3+ was 1.0 ꢃ 10ꢀ5 M. The experiments were carried out at
room temperature in THF. (d) Levenberg–Marquardt algorithm plot of
the dye 2–Cr3+ complex at room temperature in THF.
Recognition towards metal ions
We evaluated the sensor properties of these triphenylamine-
substituted benzimidazole-based conjugated Schiff bases. The
cation binding properties of the chemosensors were studied by
employing the nitrate salts of Ag+, Ba2+, Ca2+, Cd2+, Co2+, Cu2+,
K+, Hg2+, Pd2+, Mg2+, Li+, Na+, Ni2+, Zn2+, Fe3+, Cr3+, Al3+, Ce2+,
Mn2+, Se2+ and Fe2+. By studying the dyes comparatively, dye 2 is
a specic sensor for Cr3+ and Al3+. However, dye 1 and dye 3
show enhanced uorescence with numerous ions. As shown in
Fig. 11, dye 2 shows better selectivity for Cr3+ and Al3+; the PL
intensities of dye 2 + Cr3+ and dye 2 + Al3+ increase 10- and 11-
Fig. 11 Fluorescence spectra of dye 2 (1.0 ꢃ 10ꢀ5 M, THF) upon
addition of 1 equiv. of different nitrate salts in 4 : 1 (v/v) THF–water
HEPES buffer solution.
Fig. 14 (a) The results of interference tests on dye 2–Al3+ in THF–H2O
(4 : 1, v/v). The low bars represent dye 2 (1.0 ꢃ 10ꢀ5 M) with cations (1.0
ꢃ 10ꢀ5 M) except for Al3+; the high bars represent dye 2 (1.0 ꢃ 10ꢀ5 M)
with cations (1.0 ꢃ 10ꢀ5 M) upon the subsequent addition of Al3+ (1.0
ꢃ 10ꢀ5 M). (b) The results of interference tests on dye 2–Cr3+ in THF–
H2O (4 : 1, v/v). The low bars represent dye 2 (1.0 ꢃ 10ꢀ5 M) with
Fig. 12 Changes in the fluorescence spectrum of dye 2 in THF–H2O cations (1.0 ꢃ 10ꢀ5 M) except for Cr3+; the high bars represent dye 2
(4 : 1, v/v) HEPES buffer solution (20 mM) upon titration with aqueous (1.0 ꢃ 10ꢀ5 M) with cations (1.0 ꢃ 10ꢀ5 M) upon the subsequent
solutions of Al3+ (a) and Cr3+ (b).
addition of Cr3+ (1.0 ꢃ 10ꢀ5 M).
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fold, respectively. To further understand the interactions in dye
2–Cr3+ and dye 2–Al3+, the uorescence spectra of dye 2 in THF–
H2O (4 : 1, v/v) solutions of HEPES buffer (20 mM) titrated with
Al3+ and Cr3+ were obtained. As shown in Fig. 12, the PL
intensity increased progressively. To understand the nature of
the interactions between the sensor and the metal ions, Job's
plot tests were carried out. As seen in Fig. 13, the results indi-
cated 2 : 1 stoichiometric complexation of dye 2 with Al3+ and
Cr3+ in buffer solution. In addition, the association constants
for Al3+ and Cr3+ are 1.27 ꢃ 105 and 1.01 ꢃ 105, respectively.
According to the Levenberg–Marquardt algorithm, considering
the inuence of C]N and pyridine, the structures of dyes 1–3
exhibit good chelation activity. Therefore, these results can be
explained by chelation enhanced uorescence.22
2 (a) S. W. Thomass III, G. D. Joly and T. M. Swager, Chem. Rev.,
2007, 107, 1339–1386; (b) C. T. Chen, Chem. Mater., 2004, 16,
4389–4400; (c) J. B. Birks, Photophysics of Aromatic Molecules,
Wiley & Sons, London, 1970.
3 (a) E. A. Jares-Erijman and T. M. Jovin, Nat. Biotechnol., 2003,
21, 1387–1395; (b) H. Saigusa and E. C. Lim, J. Phys. Chem.,
1995, 99, 15738–15747; (c) Y. Q. Dong, J. W. Y. Lam,
A. J. Qin, Z. Li, J. Z. Sun, H. H. Y. Sung, I. D. Williams and
B. Z. Tang, Chem. Commun., 2007, 40–42.
4 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740–1741; (b) J. Chen, C. C. W. Law,
J. W. Y. Lam, Y. Dong, S. M. F. Lo, I. D. Williams, D. Zhu
and B. Z. Tang, Chem. Mater., 2003, 15, 1535–1546; (c)
To verify the selectivity of the present approach towards Al3+
and Cr3+ analysis, control experiments were conducted under
the same experimental conditions as those used for Al3+ and
Cr3+. As shown in Fig. 14, dye 2 recognized Al3+ and Cr3+ with
little interference.
Z. Li, Y. Q. Dong, B. X. Mi, Y. H. Tang, M. Haussler,
¨
H. Tong, Y. Q. Dong, J. W. Y. Lam, Y. Ren, H. H. Y. Sung,
K. S. Wong, P. Gao, I. D. Williams, H. S. Kwok and
B. Z. Tang, J. Phys. Chem. B, 2005, 109, 10061–10066.
5 B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem.
Soc., 2002, 124, 14410–14415.
6 (a) Y. P. Li, F. Li, H. Y. Zhang, Z. Q. Xie, W. J. Xie, H. Xu, B. Li,
F. Y. Z. Shen and L. M. Hanif, Chem. Commun., 2007, 231–233;
(b) Q. Zeng, Z. Li, Y. Dong, C. Di, A. Qin, Y. Hong, Z. Zhu,
C. K. W. Jim, G. Yu, Q. Li, Z. A. Li, Y. Liu, J. Qin and
B. Z. Tang, Chem. Commun., 2007, 70–71; (c) Z. P. Yu,
Y. Y. Duan, L. H. Cheng, Z. L. Han, Z. Zheng, H. P. Zhou,
J. Y. Wu and Y. P. Tian, J. Mater. Chem., 2012, 22, 16927–16932.
7 (a) R. R. Hu, E. Lager, A. Aguilar-Aguilar, J. Z. Liu,
J. W. Y. Lam, H. H. Y. Sung, I. D. Williams, Y. C. Zhong,
K. S. Wong, E. Pena-Cabrera and B. Z. Tang, J. Phys. Chem.
C, 2009, 113, 15845–15853; (b) Y. Qian, M. M. Cai,
L. H. Xie, G. Q. Yang, S. K. Wu and W. Huang,
ChemPhysChem, 2011, 12, 397–404; (c) Y. Qian, M. M. Cai,
X. H. Zhou, Z. Q. Gao, X. P. Wang, Y. Z. Zhao, X. H. Yan,
W. Wei, L. H. Xie and W. Huang, J. Phys. Chem. C, 2012,
116, 12187–12195.
8 Z. Q. Xie, B. Yang, G. Cheng, L. L. Liu, F. He, F. Z. Shen,
Y. G. Ma and S. Y. Liu, Chem. Mater., 2005, 17, 1287–1289.
9 (a) Y. Yamaguchi, Y. Matsubara, T. Ochi, T. Wakamiya and
Z. I. Yoshida, J. Am. Chem. Soc., 2008, 130, 13867–13869; (b)
A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891–
4932; (c) R. Kabe, H. Nakanotani, T. Sakanoue, M. Yahiro
and C. Adachi, Adv. Mater., 2009, 21, 4034–4038; (d)
M. Shimizu, H. Tatsumi, K. Mochida, K. Shimono and
T. Hiyama, Chem.–Asian J., 2009, 4, 1289–1297.
Conclusions
In summary, we have obtained three triphenylamine-
substituted benzimidazole-based conjugated Schiff bases.
Crystal structure, SEM and DSL analyses revealed that small
homogeneous particles resulting from different water fractions
play an important role in AIE behavior. In addition, in THF–H2O
(4 : 1, v/v) HEPES (20 mM) buffer solution, dye 2 exhibits a
strong affinity for Al3+ and Cr3+, with the uorescence being
signicantly enhanced compared to other metal ions.
Acknowledgements
This work was supported by the Program for New Century
Excellent Talents in University (China), the Doctoral Program
Foundation of the Ministry of Education of China
(20113401110004), the National Natural Science Foundation of
China (21271003, 21271004 and 51372003), the Ministry of
Education, the Natural Science Foundation of Education
Committee of Anhui Province (KJ2012A024), the 211 Project of
Anhui University, the Higher Education Revitalization Plan
Talent Project (2013) and the Ministry of Education Funded
Projects Focus on Returned Overseas Scholar.
10 (a) Y. M. You, H. S. Huh, K. S. Kim, S. W. Lee, D. H. Kim and
S. Y. Park, Chem. Commun., 2008, 3998–4000; (b) Z. Zhen,
H. P. Zhou, G. Y. Xu, Z. P. Yu, X. F. Yang, L. H. Cheng,
L. Kong, Y. Feng, J. Y. Wu and Y. P. Tian, Tetrahedron,
2012, 68, 6569–6574.
Notes and references
1 (a) K. Komatsu, Y. Urano, H. Kojima and T. Nagano, J. Am.
Chem. Soc., 2007, 129, 13447–13454; (b) W. B. Cai,
D. W. Shin, K. Gheysens, O. Cao, S. X. Wang, S. S. Gambhir
and X. Y. Chen, Nano Lett., 2006, 6, 669–676; (c) 11 (a) S. W. Thomas, G. D. Joly and T. M. Swager, Chem. Rev.,
S. R. Forrest, Nature, 2004, 428, 911–918; (d) Y. Q. Tian,
C. Y. Chen, C. C. Yang, A. C. Young, S. H. Jang, W. C. Chen
and A. K. Y. Jen, Chem. Mater., 2008, 20, 1977–1987; (e)
W. B. Cai, D. W. Shin, K. Chen, O. Gheysens, Q. Cao,
2007, 107, 1339–1386; (b) P. Jiang and Z. Guo, Coord. Chem.
Rev., 2004, 248, 205–229; (c) R. Martinez-Mannez and
F. Sancenon, Chem. Rev., 2003, 103, 4419–4476; (d) Q. Zhao,
F. Li and C. Huang, Chem. Soc. Rev., 2012, 41, 3210–3244.
S. X. Wang, S. S. Gambhir and X. Y. Chen, Nano Lett., 2006, 12 (a) V. Bhalla, R. Tejpal, R. Kumar and A. Sethi, Inorg. Chem.,
6, 669–676.
2009, 48, 11677–11684; (b) D. Maity, A. K. Manna,
2690 | J. Mater. Chem. C, 2014, 2, 2684–2691
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
D. Karthigeyan, T. K. Kundu, S. K. Pati and T. Govindarju, 17 (a) S. J. Chung, T. C. Lin, K. S. Kim, G. S. He, J. Swiatkiewicz,
Chem.–Eur. J., 2011, 17, 11152–11161; (c) S. Sumalekshmy
and C. J. Fahrni, Chem. Mater., 2011, 23, 483–500; (d)
D. Maity and T. Govindaraju, Inorg. Chem., 2011, 50,
P. N. Prasad, G. A. Baker and F. V. Bright, Chem. Mater., 2001,
13, 4071–4076; (b) H. J. Lee, J. Sohn, J. Hwang and S. Y. Park,
Chem. Mater., 2004, 16, 456–465.
11282–11284; (e) J. F. Jang, Y. Zhou, J. Yoon and J. S. Kim, 18 R. Yan, J. W. Y. Lam, Y. Q. Dong, B. Z. Tang and K. S. Wong,
Chem. Soc. Rev., 2011, 40, 3416–3429. J. Phys. Chem. B, 2005, 109, 1135–1140.
13 (a) M. Kumar, R. Kumar and V. Bhalla, Org. Lett., 2011, 13, 19 L. Li, Y. P. Tian, J. X. Yang, P. P. Sun, J. Y. Wu, H. P. Zhou,
366–369; (b) K. H. Chen, J. S. Yang, C. Y. Hwang and
M. Feng, Org. Lett., 2008, 10, 4401–4404; (c) L. Basabe-
Desmonts, D. N. Reinhoudt and M. Crego-Calama, Chem.
S. Y. Zhang, B. K. Jin, X. J. Xing, C. K. Wang, M. Li,
G. H. Cheng, H. H. Tang, W. H. Huang, X. T. Tao and
M. H. Jiang, Chem.–Asian J., 2009, 4, 668–703.
Soc. Rev., 2007, 36, 993–1017; (d) J. M. Kim, S. J. Min, 20 Y. Q. Dong, J. W. Y. Lam, A. J. Qin, Z. Li, J. Z. Sun,
S. W. Lee, J. H. Bok and J. S. Kim, Chem. Commun., 2005,
3427–3429.
H. H. Y. Sung, I. D. Williams and B. Z. Tang, Chem.
Commun., 2007, 40–42.
14 (a) X. Y. Zhang, X. Q. Zhang, B. Yang, S. Q. Wang, M. Y. Liu, 21 J. Wang, J. Mei, R. Hu, J. Z. Sun, A. Qin and B. Z. Tang, J. Am.
Y. Zhang, L. Tao and Y. Wei, RSC Adv., 2013, 3, 9633–9636; (b) Chem. Soc., 2012, 134, 9956–9966.
X. Q. Zhang, X. Y. Zhang, S. Q. Wang, M. Y. Liu, L. Tao and 22 (a) S. Goswami, D. Sen and N. K. Das, Org. Lett., 2010, 12,
Y. Wei, Nanoscale, 2013, 5, 147–150.
856–859; (b) K. Soojin, Y. N. Jin, Y. K. Ka, H. K. Jin,
K. K. Hee, N. Seong-Won, H. K. So, P. Sungsu, K. Cheal
and K. Jinheung, Inorg. Chem., 2012, 51, 3597–3602; (c)
R. Joseph and C. P. Rao, Chem. Rev., 2011, 111, 4658–
4702.
15 G. M. Sheldrick, SHELXTL V5.1 Soware Reference Manual,
Bruker AXS, Inc., Madison, Wisconsin, USA, 1997.
16 H. B. Xiao, C. Mei, Y. C. Wang, H. Li, S. X. Qian, H. Y. Yin and
Z. S. Xu, Mater. Chem. Phys., 2011, 130, 897–902.
This journal is © The Royal Society of Chemistry 2014
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