Aggregation-induced and crystallization-enhanced emissions with time-dependence of a new Schiff-base family based on benzimidazole

Article abstract of DOI:10.1039/c3tc32551b

A new Schiff-base family containing [4-(1H-benzimidazole-2-yl)-phenyl]-bis- (4-ethoxy-phenyl)-amine has been synthesized through a condensation reaction. All the derivatives possess properties of aggregation-induced and crystallization-enhanced emission (AIE and CEE), which show time-dependent characteristics at a concentration of 10 μM and are studied in detail by scanning electron microscope (SEM) and transmission electron microscope (TEM). Different aggregation forms and the growth of crystals of the compounds, could be responsible for the notably different degrees of fluorescence enhancement.

Full text of DOI:10.1039/c3tc32551b

Journal of
Materials Chemistry C
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PAPER
Aggregation-induced and crystallization-enhanced
emissions with time-dependence of a new
Schiff-base family based on benzimidazole†
Cite this: J. Mater. Chem. C, 2014, 2,
3686
a
ab
a
a
Yang Wang, HongPing Zhou, Jun Zheng,c
*
*
Yuanle Cao, Mingdi Yang,
Xiuzhen Zhang,a Jieying Wu,a Yupeng Tiana and Zongquan Wud
A new Schiff-base family containing [4-(1H-benzimidazole-2-yl)-phenyl]-bis-(4-ethoxy-phenyl)-amine
has been synthesized through a condensation reaction. All the derivatives possess properties of
aggregation-induced and crystallization-enhanced emission (AIE and CEE), which show time-dependent
characteristics at a concentration of 10 mM and are studied in detail by scanning electron microscope
(SEM) and transmission electron microscope (TEM). Different aggregation forms and the growth of
crystals of the compounds, could be responsible for the notably different degrees of fluorescence
enhancement.
Received 3rd January 2014
Accepted 10th February 2014
DOI: 10.1039/c3tc32551b
www.rsc.org/MaterialsC
previously attributed to the restricted intramolecular rotation
(RIR) mechanism. Although the mechanism of AIE or CEE has
Introduction
been studied for a period of time, the number of the AIE or CEE
materials is quite limited, for example, silole-based compounds
and arylethene derivatives.6 To further enlarge the family of
specic AIE-active compounds, it is necessary to carry out more
extensive investigations in this eld.
Organic uorescent materials have attracted intensive interest
because of their potential applications in organic light-emitting
diodes (OLEDs), uorescent sensors, etc.1 However, many dyes
emit strongly when dissolved in good solvents, but become
weakly luminescent when fabricated into solid lms or aggre-
gated in poor solvents.2 This phenomenon is notoriously known
as aggregation-caused quenching (ACQ).
The ACQ effect has greatly limited the applications of
organic luminescent materials and has driven researchers to
seek anti-ACQ materials with higher efficiency in the aggregated
state than in the dissolved state. Several anti-ACQ materials
were reported by Tang et al.3 and Park et al.4 in 2001 and 2002,
respectively, and these substances are termed aggregation-
induced emission (AIE) and aggregation-induced enhancement
emission (AIEE) materials. Some of these materials show a
special phenomenon of AIE or AIEE in aqueous mixtures in the
crystal state form, which has been designated as crystallization-
enhanced emission (CEE).5 Many crystalline AIE materials have
been found to exhibit high uorescence efficiencies compared
to their amorphous counterparts. The AIE or CEE effect was
Recently, the synthesis and application of triphenylamine
(TPA) derivatives as emissive materials have been of great
interest for chemists and material scientists due to their charge-
transport properties, and thermal stability.7 TPA derivatives,
however, usually give a strong emission in organic solvent, but
suffer from the notorious effect of ACQ in the condensed phase.
Benzimidazole, a conjugated compound with good bioactivity,
is usually used as a medical intermediate.8 The combination of
two compounds may lead to the generation of a new system with
enhanced conjugation and bioactivity. A small aromatic ring
attached to benzimidazole by a single bond may activate the
radiative transition channel to some extent, which could endow
the compounds with AIE or AIEE properties.
We are interested in and have worked on the exploration of
new AIE or CEE systems.9 In this work, we synthesise six novel
TPA-substituted benzimidazole-based conjugated Schiff bases.
During the study, we discovered that the compounds emit
uorescence in the solvent mixtures with low water contents
containing crystals, but become non-luminescent, even in the
mixtures with a high water content containing homogeneous
nano-particles, which can be attributed to the AIE or CEE
process. The intramolecular motions of the compounds remain
active in the amorphous phase and can be suppressed by crystal
formation, thus causing the novel CEE effect. The AIE and CEE
phenomena of the compounds are accompanied by time-
dependent intensity enhancement. This behavior may be
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
dSchool of Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R.
China
† Electronic supplementary information (ESI) available: Fig. S1–S10, Tables S1–S3.
See DOI: 10.1039/c3tc32551b
3686 | J. Mater. Chem. C, 2014, 2, 3686–3694
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induced by the constantly changing size and morphology of the
nanoparticles under the impact of solvent, such as water.10
Results and discussion
Synthesis
The synthetic routes to compounds 1–6 and their intermediates
are depicted in Scheme 1. The detailed procedures for the
syntheses of the intermediates and nal products are described
in the experimental section. The targeted compounds 1–6 were
obtained by the condensation of an aldehyde and amine.
Absorption and photoluminescence properties
Fig. 1 and 2 show the absorption and uorescence spectra of
compounds 1 and 5 (the others are in Fig. S1 and S2†) in
different polar solvents (dichloromethane (DCM), tetrahydro-
furan (THF), ethanol, acetonitrile, N,N-dimethyl formamide
(DMF) and dimethylsulfoxide (DMSO)).
As seen in Fig. 1 and Table S1,† the polarity of the solvents
had little effect on the absorption wavelengths of compounds 1
and 5 (2–4, 6 in Fig. S1†), but exerted a great inuence on the
photoluminescence (PL) emission (1 and 5 in Fig. 2, 2–4, 6 in
Fig. S2†). For example, the maximum absorption wavelength
(la) of compound 1 is located at 373–381 nm, however, the
maximum emission wavelength (le) varied from 437 nm to 470
nm. The la and le (Table S1†) of 1–3 in different solvents are
almost the same, which reveals that the different location of the
N in pyridine has no obvious effect on the maximum emission
wavelengths of 1–3.
Fig. 1 Absorption spectra of compounds 1 and 5 in different polar
solvents. Concentration of the solutions: 10 mM.
density functional of B3LYP/6-31G(d). The optimized geome-
tries and HOMO (Highest Occupied Molecular Orbital)/LUMO
(Lowest Unoccupied Molecular Orbital) plots of 1–6 are illus-
trated in Fig. 3. All of the molecules adopt twisted non-planar
conformations at the terminal aromatic ring, which is favorable
for active intramolecular rotations of the aromatic ring in pure
To better understand the photophysical properties of the
compounds, we performed theoretical calculations with the
Scheme 1 Synthetic routes to compounds 1–6 and photographs of Fig. 2 Fluorescence spectra of compounds 1 and 5 in different polar
their solids taken under visible and UV light.
solvents (10 mM).
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Fig. 3 Energy level and electron density distribution of the frontier
orbitals of compounds 1–6.
solutions. In spite of the values of the band gap (DEg) differing
(Table S2†), the compounds have very similar maximum
absorption and emission wavelengths in the same solvents,
indicating the potential inuence of molecular rotation on
band gaps in the excited state.
Obviously, the electron clouds of the HOMOs and LUMOs of
compounds 1–6 are dominated by the triphenylamine section
and aromatic ring at the ends of the molecules, respectively,
which means that their absorption and emission primarily stem
from the ICT (intramolecular charge transfer) transitions.
However, the emission behavior of compounds 1–6 in solvents
as described above is strikingly different from that of conven-
tional ICT systems, which are nonuorescent in highly polar
solvents, but intensely emissive in nonpolar solvents. Thus the
formation of aggregation may restrain the ICT process in the
solution mixture to some extent, which is helpful for the light
emission.11
Fig. 4 UV absorption spectra of 1 and 5 in water–THF mixtures with
different volume fractions of water after water was injected for 1 h (10
mM).
90% water contents), indicating that during the self-assembly of
the emitting species the states of aggregation may have
changed. The same tendency can also be found in the absorp-
tion spectra of the other compounds (Fig. S3†).
AIE and CEE phenomena
The luminescence behavior of the water–THF mixtures with
different water contents aer water was injected for 24 hours is
shown in Fig. 5 (2–4, 6 in Fig. S4†). In pure THF, the emissions
of the compounds are very weak and virtually invisible.
However, strong blue and cyan emission can be exhibited in the
solvent mixtures with 30% and 60% water contents, respec-
tively. The absorption and emission behaviors of 1 in the pure
THF and THF–water mixtures may be understood as follows. In
the isolated state, that is the molecule completely dissolved in
good solvent, the active intramolecular rotations of the terminal
aromatic ring effectively deactivated its excited species, hence
the faint light emission can be observed. Since the formation of
aggregation can rigidify the molecular conformation and thus
block the nonraditive relaxation channel, the excited state
decays radiatively. As a result, 1 becomes highly emissive in the
aggregation state. As the proportion of water further increases
to a certain degree (90%), no uorescence can be observed again
(insets in Fig. 5 and S4†). All of the compounds show an
emission enhancement at a low water fraction, but a decrease at
a high water fraction, which exhibits a down-top-down “L”
pattern. This phenomenon has oen been observed in some
compounds with AIE properties, but the reasons remain
unclear.13 To better understand the luminescence behavior and
mechanism of the compounds in solvent mixtures, we carried
out the following study in detail.
The dilute THF solutions of 1–6 (1 ꢀ 10ꢁ5 mol Lꢁ1) were
transparent and displayed very faint lights with emission
maximums peaking in the region of 450 nm when excited at 380
nm at room temperature (Table 1).
The absorption spectra of 1 and 5 in the THF–water mixtures
are shown in Fig. 4. The spectrum prole was virtually
unchanged when a low water fraction was injected into the THF
solution. However, the absorption curves of 1–6 in the THF–
water mixtures with a relatively high water fraction ($60%)
decayed further from zero, even in the long wavelength region,
indicative of the existence of aggregative species in these solvent
mixtures.12 The maximum absorption wavelength of
1
undergoes a blue-shi from 381 nm (in pure THF solvent) to 361
nm (at 50% water fraction), and then a red-shi to 373 nm (at
Table 1 Fluorescence quantum yields (QY) obtained after water was
injected for 24 hoursa
QY
fw ¼ 0%
fw ¼ 30%
fw ¼ 60%
fw ¼ 90%
1
2
3
4
5
6
<0.1%
<0.1%
<0.1%
<0.1%
<0.1%
<0.1%
37.2%
26.6%
27.7%
28.0%
25.4%
35.3%
17.8%
11.3%
7.9%
12.1%
12.2%
14.3%
5.4%
1.8%
1.9%
<0.1%
1.9%
0.3%
As can be seen in Table 1, the uorescence quantum yield
(FF) of compounds 1–6 with a water fraction of 30% are higher
a
fw: water fraction.
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Fig. 6 Time-dependent changes of 1 and 5 in the PL peak intensity
(10 mM).
Fig. 5 PL spectra of the dilute solutions of 1 and 5 in water–THF
mixtures with different volume fractions of water (excitation wave-
length ¼ 365 nm) after water was injected for 24 hours (10 mM). The
insets show the emission images of 1 and 5 in pure THF, as well as in
solvent mixtures with 30%, 60% and 90% water contents taken under
365 nm UV illumination at room temperature (10 mM), respectively.
enhanced with time. The uorescent intensity enhancement of
1–6 with 30% water fractions increased from 32, 17, 14, 2, 32,
0.4 times to 102, 35, 62, 10, 90, 9 times aer water was added for
24 hours. Fig. 7 shows the photographs of a solution mixture
than that with the water fractions of 60% and 90%. Hence, the taken under the UV illumination with 50% water contents. As
addition of water may effectively inuence the conformation of the time goes from 0 min, to 30 min to 50 min, the uorescence
molecules to some extent. The similar quenched emission and intensity shows an enhancement of 30 times and 45 times
low uorescence efficiency (Table 1) of 1–6 in pure THF and compared with that of 0 min, respectively.
mixtures with a high water contents (>80%) suggests that they
may possess properties of AIEE.14
SEM and TEM observation
To further prove our hypothesis, we followed the time course
of spectral evolution of 1–6 (1, 5 in Fig. 6, 2–4 and 6 in Fig. S5†)
in mixtures with different water contents. Interestingly, the
emission intensity of a prepared aqueous mixture changed
constantly with time at room temperature. As depicted in Fig. 6,
the PL spectra of compounds 1 and 5 show a remarkable
enhancement in the emission intensity with low water frac-
tions. In contrast, almost no changes in the PL spectra of the
compounds are observed in the aqueous mixtures with over
80% water contents, even aer the mixture has been allowed to
stand for as long as 24 h. As the compounds have a good
solubility in a low hydration mixture, there are hardly any
nanoparticles available within a short time period. However, as
time passes, the molecules may gradually self-assemble to
The scanning electron microscopy (SEM) images of 1, 5 and 6 in
the THF–water mixtures with low and high water contents at
different times are shown in Fig. 8, which show the existence of
aggregation in the solvent mixture. For 1 with 50% water
contents (Fig. 8a), the particle size was about 200–300 nm. A
globular particle of 1 with 70% water content aer water was
injected for 1 hour (Fig. 8b) can be observed in a form of fusion.
Aer water was injected for 2.5 hours, the molecules grew into
crystal particles, giving
a
time-dependent uorescence
enhancement.15 On the other hand, in the solvent mixture with
90% water content, the molecules may disperse to amorphous
nanoparticles. From the absorption with no trailed peak of
compounds 1–6 in a low water fraction in Fig. 4 and the obvious
enhancement of emission in the mixture of low water faction
(<30%), one can nd that despite there being no tailed peak in
Fig. 7 Photographs of 1 in the THF/H2O mixtures with 50% water
the mixture with a low water contents, the emission can still be contents at different times taken under UV illumination (10 mM).
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Fig. 8 SEM of 1 ((a) 50%, 3 h, (b) 70%, 1 h and (c) 70% 2.5 h), 5 ((d) 50%, 2
h and (e) 70%, 3 h) and 6 ((f) 30%, 30 min, (g) 30%, 3 h and (h) 90%, 30
min) formed in the THF–water mixtures containing different water
contents in different time ranges after the water was injected.
Fig. 9 TEM images and ED patterns of crystalline (a–c) and amor-
phous nanoaggregates of 1 formed in the THF–water mixtures con-
taining 30% (a), 50% (b), 70% (c) and 90% (d) water contents after water
was injected for 12 hours, 2 hours, 1 hour and 24 hours, respectively.
larger rods with the shape of rounded strips, with widths of
ꢂ200 nm and various lengths ranging from 200–600 nm.
Similar to 1, 5 has a square structure aer water was injected for
2 hours with 50% water content (Fig. 8d), indicating some kind
of specic arrangement of molecules. 5 shows irregular round
strips (Fig. 8e) aer water was injected for 3 h with 70% water
content, which is similar to 1 (Fig. 8c). Particles of 6 with 30%
water content change from an amorphous form (Fig. 8f) with a
diameter of nearly 150 nm aer water was injected for 30 min to
a crystalline form (Fig. 8g) with a diameter of 300–400 nm aer
water was injected for 3 h. Initially only a small portion of the
compound molecules probably cluster together to form tiny
nanoparticles. The larger portion of the compound molecules
remaining in the solvent mixture then gradually deposits onto
the initially formed nanoparticles in a way similar to recrystal-
lization. The aggregations of 6 with 90% water content aer
water was injected for 30 min (Fig. 8h) can be observed in the
form of globes. Almost no change in the diameters and
morphology can be observed, even aer water was injected for
24 h in the solvent mixtures (Fig. S6†).
water contents changes from low to high, the quantity of
diffraction spots changed from more to less. Since there is only
a dim diffraction ring observed in Fig. 9d, the nanoparticles are
thus amorphous in nature.
The different rate of uorescence intensity enhancement of
compounds 1–6 in their solvent mixtures with the different
water contents (Fig. 6 and S5†) may be attributed to the different
speeds of crystallization related to their different solvent envi-
ronments.17 As shown in Fig. S8,† only two diffraction spots can
be observed aer water was injected for 40 min, however, many
of them appeared (Fig. S8b) aer 1 hour and 20 minute, which
may account for the time-dependent enhancement of uores-
cence emission.
Different properties in different states
As shown in Fig. S7,† aggregates with an average diameter of
ꢂ670 nm formed in the aqueous mixture with 70% water To gain further insight into the CEE properties of compounds
content aer water was injected for 30 min and grew up to 1–6, we obtained a series of PL measurements in powder, lm,
ꢂ1091 nm aer water was injected for 3 hours. In comparison to the pure solution and mixed solutions with fw ¼ 30% (Fig. 10
the size of the aggregates of 6 at 70% water contents, no obvious and S9†). As we can see from Table 2, compound 1 shows the
change in the diameters can be observed, despite the interval of value of lem as being in the order of the pure solution < the
23.5 hours, which is in accordance with the initial speculation. solution mixture (fw ¼ 30%) < lm < powder. The same order
The transmission electron microscopy (TEM) images of 1 in can also be observed in 2–4, 6. (a) In the powder state, the
the THF–water mixtures with different water contents (30%, molecules adopted the arrangement of having relatively good
50%, 70% and 90%) are shown in Fig. 9. The insets of Fig. 9(a–d) planarity, which offers the molecules relatively strong charge
show the electron diffraction (ED) patterns of the aggregates delocalization. (b) In the pure solution, the compounds are
formed in the THF–water mixtures with 30%, 50%, 70% and molecularly isolated without any interactions between the
90% water contents, respectively. The dark regular structures in adjacent molecules. Hence, molecules may distort without the
the photographs are the beam stop used to protect the detector intermolecular restriction, giving an emission of blue-shi. (c)
from the intense main undiffracted beam. A series of clear The molecules in the lm state have an emission wavelength
diffraction spots surrounding the main undiffracted beam can similar to that in the powder state, which is quite different from
be observed in Fig. 9a, indicating that the aggregates are crys- the wavelength in the pure solution. It is probably because the
talline or that the compound molecules are packed in an concentration of the solution may increase as the solvent
ordered fashion.16 In the mixtures with low water fractions, gradually evaporates, and majority of the isolated molecules
molecules of 1 may cluster together slowly in an ordered fashion agglomerate again.18 (d) The curves of the solution mixture
to form crystal-like aggregations. When the water fraction show a middle location in contrast to the other three states,
becomes high (>80%), its molecules may aggregate instantly to which indicated that some weak forces may be formed aer
form globular particles with no diffraction spots (Fig. 9d). As the water was injected to restrict the rotation of the aromatic
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Fig. 11 PL lifetime spectra of 1 and 5 in pure solution, solution
mixtures with 30% water content and 60% water content (10 mM).
Fig. 10 Photoluminescence spectra of 1 and 5 in powder, film, pure
THF solution and the solvent mixture with 30% water contents (10 mM)
after the water was injected for 24 h.
and 1.73 ns, respectively). The same tendency can also be
found in the other ve compounds, which implies that the
formation of crystalline aggregates restricts the rotation and
vibration of the groups in the molecules, so that longer
emission lifetimes were detected.21 The relatively low water
contents may make the molecules more conducive to self-
organization. The lowest lifetimes were observed at 60%
water content, which may be due to the combined action of
the increasing solvent polarity and the amorphous
aggregation.22
Table 2 Maximum emission wavelength of 1–6 in different statesa
lem/nm
1
2
3
4
5
6
Powder
Film
THF
531
516
437
458
—
—
436
458
537
531
442
459
523
513
437
459
472
520
443
460
548
544
438
458
Mixtures
a
No signal of emission in powder and lm was obtained for 2.
Conclusions
rings,19 giving a red-shi emission in contrast to the location in
the pure solution. As to the abnormal blue-shi of 5 in the
powder state, the reason may be that its molecules have rela-
tively larger aromatic rings, which enforces distortion of the
molecules to a large extent to avoid complanation.20
In summary, we have designed and synthesized six molecules
(1–6) with different terminal aromatic rings. All the
compounds are almost non-emissive when dissolved in pure
THF, but become strongly emissive when aggregated in
aqueous solvents. Efficient emission enhancement can be
achieved with relatively low water contents (#60%) solutions
containing crystalline phase, while low or even no emission is
realized in the amorphous phase formed in solutions with
high water contents (>60%). When the water ratio reaches 20%
(1 and 5) or 30% (2–4, 6) aer water was injected for 24 hours,
the PL intensity reached its maximum value. All compounds in
the solvent mixtures with 30% and 60% water contents exhibit
enhanced blue and cyan uorescence emissions with time-
dependence, respectively. The decreasing diffraction spots in
aqueous mixtures with the increasing water contents suggests
that compounds 1–6 are CEE-active materials. Evidently, a
simple manipulation of the composition mixture or a small
variation in the assembling environment may lead to the big
change in the emission efficiency of 1–6.
The time-resolved uorescence measurements of 1 and 5
are shown in Fig. 11 (others in Fig. S10†), and the detailed
data of the uorescence decay curves of 1–6 are listed in Table
S3.† The experimental errors are estimated to be ꢃ13% from
the sample concentrations and instruments. The lifetimes of
1–6 in different water fractions are obtained by monitoring at
the monomer emission. The decay behavior of 1–6 is in a
double-exponential manner in the solution mixture obtained
by monitoring at the monomer emission. The lifetimes of 1–6
with the water fraction of 0% is contributed to by the two
lifetime species, while the lifetime of 1–6 in the water frac-
tions of 30% and 60% is almost entirely from the contribu-
tion of the longer lifetime species. As shown in Table S3,† the
weighted mean lifetime of 1 in 30% water fraction (3.15 ns) is
longer than that of 1 in 0% and 60% water fractions (2.14 ns
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under reduced pressure and the mixture was puried by chro-
matography on silica gel using petroleum ether/ethyl acetate
(50 : 1, v/v) as the eluent, giving white crystals b, 7.8 g, yield:
43.6%. 1H-NMR (400 MHz, (CD3)2CO), d (ppm): 7.18–7.14 (t, J ¼
7.6 Hz, 2H), 7.02–7.00 (d, J ¼ 8.4 Hz, 4H), 6.86–6.71 (m, 7H),
4.04–3.99 (m, 4H), 1.37–1.34 (t, J ¼ 7.0 Hz, 6H).
Experimental
General information
All chemicals were available commercially. All the chemicals
were used directly without further purication. THF was HPLC
grade from BODI Organic Company and was used as received.
Synthesis of c. POCl3 (2.3 mL) was added dropwise to freshly
distilled DMF (2.8 mL) which was stirred in a ask (150 mL) with
ice water bath cooling. Aer 30 min, the solution become sticky
and was stirred a little longer to yield a reddish salt resembling
ice. Then the salt was combined with a portion of b (5.0 g, dis-
solved in 15 mL chloroform) to yield a reddish solution. Then the
reaction mixture was heated at 65 ꢄC for 10 h. The reaction was
monitored by TLC. Aer the reaction was completed, it was
concentrated. The black mixture was dissolved in dichloro-
methane and then poured into a large amount of water. Na2CO3
solution (40%) was added to adjust the pH of the mixture to 7–8.
The mixture was extracted three times with dichloromethane and
the organic extracts were dried over MgSO4. Aer removing
solvents under reduced pressure, the mixture was puried by
chromatography on silica gel using petroleum ether/ethyl acetate
(40 : 1, v/v) as the eluent, giving a yellow oily product 5.21 g, yield:
96.15%. 1H-NMR (400 MHz, (CD3)2CO), d (ppm): 9.76 (s, 1H),
7.67–7.65 (d, J ¼ 8.4 Hz, 2H), 7.19–7.17 (d, J ¼ 8.8 Hz, 4H), 6.98–
6.96 (d, J ¼ 8.4 Hz, 4H), 6.78–6.76 (d, J ¼ 8.8 Hz, 2H), 4.07–4.02
(m, 4H), 1.38–1.35 (t, J ¼ 7.0 Hz, 6H).
Instruments
The NMR spectra were recorded on a 400 MHz NMR instru-
ment. Chemical shis were reported in parts per million (ppm)
relative to internal TMS (0 ppm) and coupling constants in
hertz. Splitting patterns were described as singlet (s), doublet
(d), triplet (t) or multiplet (m). The IR spectra were recorded
with a Nicolet FT-IR NEXUS 870 spectrometer (KBr discs). The
UV-vis absorption spectra were recorded on a UV-3100 spec-
trophotometer. The mass spectra were obtained on an autoex
speed MALDI-TOF/TOF mass spectrometer. The scan electron
microscopy (SEM) and transmission electron microscopy (TEM)
studies were performed using a Hitachi S-4800 scanning elec-
tron microscope and JEOL JEM-100SX transmission electron
microscope. Dynamic light scattering (DLS) measurements were
conducted on a Malvern Zeta Nano-ZS90 Particle Size Analyzer.
Fluorescence measurements were carried out using an Edin-
burgh FLS920 uorescence spectrometer equipped with a 450 W
Xe lamp and a time-correlated single-photon counting (TCSPC)
card. For the time-resolved uorescence measurements, the
uorescence signals were collimated and focused onto the
entrance slit of a monochromator with the output plane
equipped with a photomultiplier tube. The absolute photo-
luminescence quantum yield (FF) values of the mixture with
different water fractions (0%, 30%, 60%, 90%; 1 ꢀ 10ꢁ5 M) were
determined using an integrating sphere.
Synthesis of d. A DMF solution containing 4-nitro-benzene-
1,2-diamine (1.3 g, 8.3 mmol) and KI (0.55 g) was added to a
DMF solution containing 4-[bis-(4-ethoxyphenyl)-amino]-benz-
aldehyde (3 g, 8.3 mmol). The mixture was then reuxed by
stirring for 16 h. Aer the reaction was completed, the reaction
mixture was cooled to room temperature and poured into water
to yield a large amount of precipitate. Then the precipitate was
ltrated and washed with water three times. The crude product
was puried by chromatography on silica gel using petroleum
Preparation and characterization
Synthesis of a. Sodium hydroxide (24.0 g, 0.6 mol) and 4- ether/ethyl acetate (5 : 1, v/v) as the eluent, giving a red
iodo-phenol (120.0 g, 0.54 mol) were crushed together in powdered solid 2.9 g, yield: 71%. 1H-NMR (400 MHz, DMSO-D6),
batches with a pestle and mortar. Then bromoethane (300 mL) d (ppm): 13.30 (s, 1H), 8.45–8.28 (d, J ¼ 7.2 Hz, 1H), 8.11–8.06 (t,
was added to this white powder. To this solution, cesium J ¼ 8.6 Hz, 1H), 8.00–7.98 (m, 2H), 7.75–7.62 (m, 1H), 7.15–7.13
hydroxide (10 g) in 20 mL of DMF and 3 drops of 18-crown-6 (d, J ¼ 8.4 Hz, 4H), 6.97–6.95 (d, J ¼ 8.8 Hz, 4H), 6.82–6.80 (d, J ¼
were added and stirred for 1 h at room temperature. Then the 8.8 Hz, 2H), 4.05–4.00 (m, 4H), 1.35–1.32 (t, J ¼ 6.8 Hz, 6H); MS
reaction mixture was heated at 60 ꢄC for 78 h. The mixture was (MALDI-TOF): m/z 493.575 [(M ꢁ 1)+, calcd 493.195].
concentrated and cooled to room temperature (RT) aer the
Synthesis of e. Hydrazine hydrate (19 mL) was added drop-
reaction was completed by monitoring with thin-layer chro- wise to ethanol (50 mL) containing bis-(4-ethoxyphenyl)-[4-(5-
matography (TLC). Much water was added and the mixture was nitro-1H-benzoimidazol-2-yl)-phenyl]-amine (1.9 g, 3.84 mmol).
extracted with dichloromethane (100 mL) three times. The Then the mixture was reuxed by stirring for 20 min and 0.15 g
organic extracts were dried over MgSO4. Aer removing the Pd(OAc)2ꢁwas added to the mixture. Aer stirring for one more
solvents under reduced pressure, a red oily liquid was obtained hour, the solution started to become a yellow transparent
(128.4 g, yield: 94.9%).
solution. The reaction mixture was then cooled to room
Synthesis of b. Aniline (5.0 g, 54 mmol) and 1-ethoxy-4-iodo- temperature and concentrated. A yellow solid was obtained
1
benzene (46.0 g, 185 mmol) were added to 1,2-dichlorobenzene (1.456 g, 3.14 mmol), yield: 81.6%. H-NMR (400 MHz, DMSO-
(200 mL) in a three-neck ask. Potash (17.94 g, 130 mmol) and D6), d (ppm): 12.01 (s, 1H), 7.86–7.84 (d, J ¼ 8.4 Hz, 2H), 7.23–
copper powder (8.34 g, 130 mmol) were slowly added to the 7.22 (d, J ¼ 7.6 Hz, 1H), 7.08–7.06 (d, J ¼ 8.4 Hz, 4H), 6.93–6.91
mixture aer stirring for a short moment in an atmosphere of (d, J ¼ 8.8 Hz, 4H), 6.80–6.78 (d, J ¼ 8.8 Hz, 2H), 6.62 (s, 1H),
nitrogen. Three drops of 18-crown-6 were added to the mixture. 6.49–6.47 (d, J ¼ 7.6 Hz, 1H), 4.87 (s, 2H), 4.03–3.98 (m, 4H),
ꢄ
The mixture was reacted at 180 C for 12 h. Aer the reaction 1.34–1.31 (t, J ¼ 6.8 Hz, 6H); MS (MALDI-TOF): m/z 463.936 [(M
was completed, it was cooled to RT. The solvents were removed ꢁ 1)+, calcd 463.221].
3692 | J. Mater. Chem. C, 2014, 2, 3686–3694
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Synthesis of compounds 1–3. Compound 1: 0.3 g (0.43 mmol)
Journal of Materials Chemistry C
obtained through vacuum ltration (0.32 g), yield: 84%. 1H-
NMR (400 MHz, DMSO-D6), d (ppm): 12.69(s, 1H), 8.72(s, 1H),
7.95–7.97 (d, J ¼ 8.0 Hz, 4H), 7.61–7.35 (m, 5H), 7.21–7.17 (t, J ¼
8.2 Hz, 1H), 7.13–7.10 (d, J ¼ 8.8 Hz, 4H), 6.96–6.94 (d, J ¼ 8.8
Hz, 4H), 6.83–6.80 (d, J ¼ 8.8 Hz, 2H), 4.05-4.00 (m, 4H), 1.35–
1.32 (t, J ¼ 6.8 Hz, 6H); 13C-NMR (100 MHz, DMSO-D6), d (ppm):
158.66, 155.63, 152.38, 150.38, 149.88, 145.87, 139.09, 136.35,
134.24, 133.03, 131.65, 128.75, 128.45, 127.52, 122.71, 120.69,
119.24, 117.55, 115.5, 63.17, 14.68; IR (KBr, cmꢁ1): 3408, 2980,
1607, 1505, 1473, 1449, 1390, 1288, 1238, 1193, 1166, 1112,
1046, 962, 921, 830; MS (MALDI-TOF): m/z 553.396 [(M + 1)+,
calcd 553.253]. Compound 5: 0.5 g (0.72 mmol) 2-{4-[bis-(4-
2-{4-[bis-(4-ethoxyphenyl)-amino]-phenyl}-1H-benzoimida zol-5-
ylamine was added to a benzene solution (10 mL) and the
mixture was stirred at 35 ꢄC for a moment. Then a mixed solution
of 0.046 g (0.43 mmol) pyridine-2-carbaldehyde and two drops of
glacial acetic acid were added to the mixture and the solid was
dissolved quickly. The mixture was reacted for 15 h and then
cooled to room temperature. A faint yellow powder was obtained
1
through vacuum ltration (0.15 g), yield: 63.1%. H-NMR (400
MHz, DMSO-D6), d (ppm): 12.80 (s, 1H), 8.73 (s, 2H), 8.22–8.20 (d,
J ¼ 8.0 Hz, 1H), 8.01–7.99 (d, J ¼ 8.4 Hz, 2H), 7.97–7.93 (t, J ¼ 7.6
Hz, 1H), 7.65–7.63 (d, J ¼ 8.4 Hz, 1H), 7.53–7.48 (m, 2H), 7.29–
7.28 (d, J ¼ 6.8 Hz, 1H), 7.11–7.09 (d, J ¼ 8.4 Hz, 4H), 6.94–6.92 (d,
J ¼ 8.4 Hz, 4H), 6.84–6.82 (d, J ¼ 8.4 Hz, 2H), 4.02–3.97 (m, 4H),
1.34–1.30 (t, J ¼ 6.8 Hz, 6H); 13C-NMR (100 MHz, DMSO-D6), d
(ppm): 159.06, 156.25, 155.01, 153.36, 160.57, 150.18, 145.41,
139.64, 137.53, 128.17, 125.84, 121.53, 121.13, 118.06, 116.08,
63.75, 15.28; IR (KBr, cmꢁ1): 3379, 3150, 2976, 1611, 1546, 1506,
1472, 1447, 1393, 1284, 1241, 1194, 1167, 1115, 1048, 961, 923,
830; MS (MALDI-TOF): m/z 552.760 [(M ꢁ 1)+, calcd 552.248].
Compound 2: The synthesis of 2 was similar to that of 1. Yield:
84.1%. 1H-NMR (400 MHz, DMSO-D6), d (ppm): 12.76 (s, 1H), 9.10
(s, 1H), 8.82 (s, 1H), 8.70–8.69 (d, J ¼ 4.4 Hz, 1H), 8.35–8.33 (d, J ¼
7.6 Hz, 1-H, d-H), 8.00–7.98 (d, J ¼ 8.0 Hz, 2H), 7.62 (s, 1H), 7.56–
7.43 (m, 2H), 7.25–7.24 (d, J ¼ 6.8 Hz, 1H), 7.11–7.09 (d, J ¼ 8.4
Hz, 4H), 6.94–6.92 (d, J ¼ 8.4 Hz, 4H), 6.84–6.81 (d, J ¼ 8.4 Hz,
2H), 4.02–3.97 (m, 4H), 1.34–1.31 (t, J ¼ 6.8 Hz, 6H); 13C-NMR
(100 MHz, DMSO-D6), d (ppm): 156.93, 156.24, 153.15, 152.11,
150.87, 150.53, 146.14, 139.66, 135.29, 132.48, 128.14, 124.60,
121.19, 118.11, 116.09, 63.76, 15.28; IR (KBr, cmꢁ1): 3396, 3165,
2976, 1607, 1546, 1505, 1474, 1448, 1393, 1288, 1240, 1193, 1167,
1115, 1048, 962, 923, 828; MS (MALDI-TOF): m/z 552.248 [(M ꢁ
1)+, calcd 552.346]. Compound 3: The synthesis of 3 was also
similar to that of 1. Yield: 63.1%. 1H-NMR (400 MHz, DMSO-D6), d
(ppm): 12.76 (s, 1H), 8.80 (s, 1H), 8.75–8.74 (s, J ¼ 4.8 Hz, 2H),
7.98–7.96 (d, J ¼ 8.4 Hz, 2H), 7.88–7.87 (d, J ¼ 4.4 Hz, 2H), 7.63–
7.62 (d, J ¼ 6.8 Hz, 1H), 7.51–7.45 (m, 1H), 7.28-7.26 (d, J ¼ 7.6 Hz,
1H), 7.13–7.11 (d, J ¼ 8.4 Hz, 4H), 6.96–6.94 (d, J ¼ 8.4 Hz, 4H),
6.83–6.80 (d, J ¼ 8.4 Hz, 2H), 4.05–4.00 (m, 4H), 1.35–1.32 (t, J ¼
6.8 Hz, 6H); 13C-NMR (100 MHz, DMSO-D6), d (ppm): 157.23,
156.25, 153.47, 150.95, 150.60, 145.44, 143.46, 139.63, 128.15,
122.64, 121.09, 118.09, 116.07, 63.75, 15.26; IR (KBr, cmꢁ1): 3414,
3145, 2977, 1602, 1550, 1504, 1476, 1448, 1394, 1286, 1239, 1192,
1167, 1115, 1045, 960, 922, 823; MS (MALDI-TOF): m/z 552.248
[(M ꢁ 1)+, calcd 552.030].
ethoxyphenyl)-amino]-
phenyl}-1H-benzoimidazol-5-ylamine
was added to methanol solution (10 mL) and the mixture was
stirred at 35 ꢄC for a moment. Then a mixed solution of 0.112 g
(0.72 mmol) 2-naphthaldehyde and one drop of glacial acetic
acid was added to the mixture and the solid was dissolved
quickly. The reaction was monitored through TLC. Not until 4
hours aerwards had the mixture completely reacted. A
yellowish white powder was obtained through vacuum ltration
1
(0.55 g), yield: 84.8%. H-NMR (400 MHz, DMSO-D6), d (ppm):
12.722 (s, 1H), 8.889 (s,1H), 8.41 (s, 1H), 8.20–8.18 (d, J ¼ 8.4 Hz,
1H), 8.05–7.97 (m, 5H), 7.61–7.43 (m, 4H), 7.27–7.25 (d, J ¼ 8.0
Hz, 1H), 7.13–7.11 (d, J ¼ 8.8 Hz, 4H), 6.96–6.94 (d, J ¼ 8.8 Hz,
4H), 6.83–6.81 (d, J ¼ 8.8 Hz, 2H), 4.04–4.01 (m, 4H), 1.35–1.32
(t, J ¼ 6.8 Hz, 6H); 13C-NMR (100 MHz, DMSO-D6), 155.63,
152.43, 149.89, 139.09, 134.1, 132.74, 128.64, 128.40, 127.82,
127.53, 126.77, 123.58, 121.51, 120.68, 117.54, 115.51, 63.16,
14.68; IR (KBr, cmꢁ1): 2973, 1609, 1547, 1503, 1475, 1445, 1390,
1284, 1238, 1195, 1167, 1117, 1046, 962, 924, 827; MS (MALDI-
TOF): m/z 603.417 [(M + 1)+, calcd 603.268]. Compound 6: the
synthesis of compound 6 was almost the same as 5. Yield: 98%.
1H-NMR (400 MHz, DMSO-D6), d (ppm): 13.50–13.43 (d, J ¼ 27.6
Hz, 1H), 12.77–12.76 (d, J ¼ 5.2 Hz, 1H), 9.04–9.03 (d, J ¼ 3.2 Hz,
1H), 7.98–7.96 (d, J ¼ 8.4 Hz, 2H), 7.70–7.63 (m, 2H), 7.52–7.49
(t, J ¼ 7.2 Hz, 1H), 7.42–7.38 (t, J ¼ 8 Hz, 1H), 7.31–7.28 (d, J ¼
8.4 Hz, 1H), 7.13–7.10 (d, J ¼ 8.4 Hz, 4H), 7.00–6.94 (m, 6H),
6.83–6.81 (d, J ¼ 8.8 Hz, 2H), 4.05–3.99 (m, 4H), 1.35–1.32 (t, J ¼
6.8 Hz, 6H); 13C-NMR (100 MHz, DMSO-D6): 160.2, 155.65,
152.82, 150.00, 144.71, 139.04, 134.44, 132.39, 130.72, 127.55,
122.26, 120.45, 119.45, 119.00, 117.49, 116.48, 115.51, 63.17,
14.67; IR (KBr, cmꢁ1): 2974, 1609, 1571, 1503, 1449, 1392, 1280,
1239, 1194, 1166, 1145, 1112, 1047, 963, 920, 827; MS (MALDI-
TOF): m/z 569.416 [(M + 1)+, calcd 569.247].
Synthesis of compounds 4–6. Compound 4: 0.3 g (0.43
mmol) 2-{4-[bis-(4-ethoxyphenyl)-amino]-phenyl}-1H-benzoimi-
dazol-5-ylamine was added to a methanol solution (10 mL) and
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 Natural Science
Foundation of Education Committee of Anhui Province
(KJ2012A024), the 211 Project of Anhui University, Higher
Education Revitalization Plan Talent Project of (2013) and the
Ministry of Education Funded Projects Focus on Returned
Overseas Scholar.
ꢄ
the mixture was stirred at 35 C for a moment. Then a mixed
solution of 0.046 g (0.43 mmol) benzaldehyde and one drop of
glacial acetic acid was added to the mixture and the solid was
dissolved quickly. The reaction was monitored by TLC. Not until
9 hours aerwards had the mixture completely reacted. Then
the solvent was removed under vacuum. DMF (5 mL) was put
into the product. The mixture was put into water (80 mL) aer
the product was fully dissolved. A yellowish white powder was
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Journal of Materials Chemistry C
Paper
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3694 | J. Mater. Chem. C, 2014, 2, 3686–3694
This journal is © The Royal Society of Chemistry 2014