Quinoacridine derivatives with one-dimensional aggregation-induced red emission property

Article abstract of DOI:10.1021/la202755z

A new series of acceptor-donor-acceptor (A-D-A) type quinoacridine derivatives (1-3) with aggregation-induced red emission properties were designed and synthesized. In these compounds, the electron-withdrawing 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile groups act as electron-accepting units, while the alkyl-substituted conjugated core acts as electron-donating units. The restriction of intramolecular rotation was responsible for the AIE behavior of compounds 1-3. All compounds were employed as building blocks to fabricate one-dimensional (1-D) organic luminescent nano- or microwires based on reprecipitation or slow evaporation approaches. Morphological transition from zero-dimensional (0-D) hollow nanospheres to 1-D nanotubes has been observed by recording SEM and TEM images of aggregated sates of compound 2 in THF/H ₂O mixtures at different aging time. It was demonstrated that the synthesized compounds with different lengths of alkyl chains displayed different wire formation properties. The single-crystal X-ray analysis of compound 2 provided reasonable explanation for the formation of 1-D nano- or microstructures.

Full text of DOI:10.1021/la202755z

Article
pubs.acs.org/Langmuir
Quinoacridine Derivatives with One-Dimensional Aggregation-
Induced Red Emission Property
Iqbal Javed, Tianlei Zhou, Faheem Muhammad, Jianhua Guo, Hongyu Zhang,* and Yue Wang*
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R.
China
S
* Supporting Information
ABSTRACT: A new series of acceptor−donor−acceptor (A−D−A) type
quinoacridine derivatives (1−3) with aggregation-induced red emission
properties were designed and synthesized. In these compounds, the
electron-withdrawing 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile
groups act as electron-accepting units, while the alkyl-substituted
conjugated core acts as electron-donating units. The restriction of
intramolecular rotation was responsible for the AIE behavior of
compounds 1−3. All compounds were employed as building blocks to
fabricate one-dimensional (1-D) organic luminescent nano- or microwires
based on reprecipitation or slow evaporation approaches. Morphological
transition from zero-dimensional (0-D) hollow nanospheres to 1-D
nanotubes has been observed by recording SEM and TEM images of
aggregated sates of compound 2 in THF/H2O mixtures at different aging
time. It was demonstrated that the synthesized compounds with different lengths of alkyl chains displayed different wire
formation properties. The single-crystal X-ray analysis of compound 2 provided reasonable explanation for the formation of 1-D
nano- or microstructures.
these highly ordered functional materials often possess
intriguing optical and electronic properties. In particular,
materials exhibiting red emission in their aggregated or solid
state might be useful in the field of efficient OLEDs, biosensors,
and biolables.12,13 However, the assembly of highly efficient red
emitting organic nano/microwires is still an issue as most of red
emissive organic molecules were provided with aggregation-
caused quenching (ACQ) characteristic in solid state. The
ACQ effect has greatly limited the scope of the device
applications of organic luminophors. In 2001, Tang and co-
workers proposed the concept of aggregation-induced emission
(AIE),14−17 the phenomenon in which compounds are
nonemissive in solution form but become emissive in the
aggregated state. Since this pioneering work, a great deal of
effort has been invested in developing organic compounds with
AIE properties and understanding the mechanism of the AIE
phenomenon. The studies on AIE opened a new route to
develop highly efficient luminescent organic materials in solid
state, especially for the high performance organic electro-
luminescent (EL) materials, which usually suffer from the
severe ACQ effect. Therefore, the design and synthesis of
organic conjugated molecules that simultaneously possess AIE
property and 1-D assembly feature will lead to the generation of
1D organic nano/micro materials with high fluorescent
INTRODUCTION
■
Micro- and nanoscale materials such as nanowires, nano-
particles, and nanorods represent attractive building blocks for
the fabrication of functional micro- and nanoscale devices.1 The
quantum size effect of micro- and nanostructured inorganic
semiconductors could induce new optical and electronic
properties compared with those of common bulk materials.2
Especially, one-dimensional (1-D) nanostructures have at-
tracted great attention due to the high aspect ratio which leads
to fascinating physical and chemical properties.3 The 1-D
assembly of organic functional molecules on various substrates
offers a useful strategy for the construction of well-defined
functional nanoscale materials which are potential candidates of
active components in gas sensor, organic light-emitting diodes
(OLED), organic field effect transistor (OFET), optical
waveguide fibers, photodetectors, and solar cells, etc.4−9
Recently, it was demonstrated that some π-conjugated small
organic molecules could be employed as building blocks to
assemble supramolecular 1-D nanostructures.10 However, the
controlling and understanding of anisotropic aggregation of
small organic molecules used for constructing 1-D micro- and
nanomaterials with well-defined structures remains a chal-
lenge.11 To obtain desirable organic nanomaterials for optical
and electronic applications, focus should be on the design and
synthesis of functional organic building blocks and develop-
ment of 1-D self-assembly.
Received: July 18, 2011
1-D luminescent nano/microwires formed by π-conjugated
organic molecules are attracting a lot of attention due to that
Revised: December 4, 2011
Published: December 7, 2011
© 2011 American Chemical Society
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accommodate the solution sample. Stock solutions (10−4 M) of
compounds in THF were primarily prepared. An aliquot of the stock
solution was then transferred to a 10 mL volumetric flask, into which
an appropriate volume of THF and water was added dropwise under
vigorous stirring to furnish 10−5 M solutions with different water
contents (0−75 vol %). The PL spectra were immediately recorded
once the solutions were prepared. The low-temperature PL measure-
ments were performed by using a 5 mm diameter quartz tube that was
placed in quartz-walled Dewar flask filled with liquid nitrogen (77 K).
The PL quantum yields (ΦF) of solutions were measured and
calculated by employing rhodamine B as standard (ΦF = 0.50 in
ethanol) according to the literature.19 The solid-state PL quantum
yields (ΦF) measured on Maya2000 Pro CCD spectrometer with an
integrating sphere.
efficiency. Recently, we have successfully fabricated luminescent
nano/microwires based on quinacridone derivatives (QA).18
QA derivatives are highly emissive in dilute solutions, however,
the produced nano/microwires exhibited very low fluorescent
quantum yields (0.19−2.0%) due to the ACQ effect. To pursue
strong luminescence organic nano/microwires, the exploration
of QA-based molecules which retain 1-D assembly feature and
have AIE property have been performed. In the present study,
we designed a new series of red emitting AIE active compounds
(1−3) with 1-D assembly properties by incorporating two
electron-withdrawing 2-(3,5-bis(trifluoromethyl)phenyl)-
acetonitrile units at the carbonyl position of alkyl-substituted
QA (Scheme 1 and Figure 1). To deeply understand the
X-ray Crystallography. Single crystal suited for X-ray structural
analysis was obtained by slow diffusion of petroleum ether into the
chloroform solution of 2. Diffraction data were collected on a Rigaku
R-AXIS RAPID diffractometer (Mo Kα radiation, graphite mono-
chromator) in the Ψ rotation scan mode. The structure determination
was done with direct methods by using SHELXTL 5.01v and
refinements with full-matrix least-squares on F2. The positions of
hydrogen atoms were calculated and refined isotropically.
Scheme 1. Synthesis Procedure of 2-(3,5-
Bis(trifluoromethyl)phenyl)acetonitrile Derivatives 1−3
Preparation of Nano- and Microstructures. Organic nano-
structures were prepared by using so-called reprecipitation method.
The compounds were dissolved in a good solvent THF, resulting in a
solution with concentration of 2 × 10−3 M. Then 200 μL of the
solution was injected into 1 mL of vigorously stirred bad solvent water
at 35 °C. One drop of suspension was put onto a silicon wafer and
copper grid on different time intervals for SEM and TEM examination.
The 1-D micromaterials were prepared by injecting 500 μL of alcohol
or chloroform into a 500 μL solution of compounds in THF and
casting a drop of solution sample immediately after mixing on glass or
silicon substrate followed by slow evaporation typically over 10 h at
ambient temperature. FESEM and fluorescence microscopic images
were recorded at ambient temperatures.
Materials. Quinacridone was purchased from Tokyo Kasei Kogyo
Co. 3,5-Dimethylaniline, 1-bromobutane, 1-bromohexane, and 1-
bromooctane were obtained from Acros Organics. Diethyl-2,5-
dihydroxy-1,4-dicarboxylate was purchased from Aldrich. The
chemicals were used directly without further purification. The CnQA
(n = 4, 6, 8) were synthesized according to the similar method
reported in the literature.20
Figure 1. Acceptor−donor−acceptor type quinacridine derivatives.
relationship between the molecular structure and properties, we
systematically studied the optical, AIE, 1-D assembly properties,
and morphology transition characteristics of these compounds.
The 1-D self-assembling behavior of the compounds 1−3 has
been further supported by single-crystal structure analysis.
Synthesis of 1. To a flask containing 2-(3,5-bis(trifluoromethyl)-
phenyl)acetonitrile (0.70 mL, 4 mmol) and triethylamine (TEA) (1.2
mL, 8.5 mmol) dissolved in toluene (20 mL), titanium chloride
(TiCl4) (0.6 mL, 5.4 mmol) dissolved in 5 mL of toluene was added
dropwise over 15 min by addition funnel under a nitrogen atmosphere.
The addition funnel was rinsed with 3 mL of toluene, and then N,N′-
di(n-butyl)quinacridone (C4QA) (0.424 g, 1 mmol) was added by a
powder addition funnel. The funnel was rinsed with 3 mL of toluene.
The reaction mixture was heated to reflux for 8 h. After cooling to
room temperature, the reaction mixture was filtered and the filtrate
obtained was dried and purified by column chromatography using
silica gel with dichloromethane:diethyl ether (50:1 v:v) as eluent to
give 0.491 g of 1 (yield: 55%). 1H NMR (CDCl3, ppm) (500 MHz): δ
8.47 (s, 1 H), 7.70−7.76 (m, 3 H), 7.46 (t, 1 H), 7.35 (d, 1 H), 6.88
(d, 1 H), 6.66−6.75 (m, 1 H), 4.42 (t, 1 H), 3.27−3.37 (m, 1 H), 2.08
(t, 1 H), 1.70−1.75 (m, 1 H), 1.62 (t, 1 H), 1.18 (t, 3 H), 0.84−0.95
(m, 1 H). MS: m/z 894.26 [M]+. Anal. Calcd for C48H34 F12N4
(894.26): C, 64.43; H, 3.83; N, 6.26. Found: C, 64.57; H, 3.98; N,
6.21. Mp 215 °C.
EXPERIMENTAL SECTION
■
Instrumentation. NMR spectra were recorded on Bruker
AVANCE 500 MHz spectrometer with tetramethylsilane as the
internal standard. Element analyses and mass spectra were measured
on Flash EA 1112 and GC/MS mass spectrometers, respectively. UV−
vis absorption spectra were recorded using a PE UV−vis lambdazo
spectrometer. To determine the effect of scattering on absorption, the
integrated sphere (IS) (an accessory of UV−vis lambdazo
spectrometer) was used to measure the absorption spectra. Particles
size was determined by dynamic light scattering measurements using
Zetasizer Nano-ZS Malvern instruments model ZEN3600. Photo-
luminescence (PL) spectra were collected on a Shimadzu RF-5301PC
spectrophotometer. The microscopy images of as-prepared nano/
microwires were obtained on an Olympus BX51 fluorescence
microscope. FESEM images were acquired on a JSM 6700F field
emission scanning electronic microscope. FETEM images were
acquired on a FEI Tecnai G2 F20 S-Twin D573 field emission
transmission electronic microscope operated at 200 kV.
Synthesis of 2. 2-(3,5-Bis(trifluoromethyl)phenyl)acetonitrile
(0.70 mL, 4 mmol) reacted with N,N′-di(n-hexyl)quinacridone
(C6QA) (0.480 g, 1 mmol) according to the procedure described
1
for the synthesis of 1 to yield 0.285 g (30%) of 2. H NMR (CDCl3,
ppm) (500 MHz): δ 8.46 (s, 1 H), 7.68−7.77 (m, 3 H), 7.45 (t, 1 H),
7.34 (t, 1 H), 6.86 (d, 1 H), 6.64−6.75 (m, 1 H), 4.42 (t, 1 H), 3.26−
3.35 (m, 1 H), 1.99−2.10 (m, 1 H), 1.45−1.68 (m, 4 H), 1.25 (t, 2 H),
1.00 (t, 2 H), 0.83−0.95 (m, 2 H), MS: m/z 950.30 [M]+. Anal. Calcd
Sample Preparation for Photoluminescence Measurements.
Small quartz cell with dimensions 0.2 × 1.0 × 4.5 cm3 was used to
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for C52H42 F12N4 (950.30): C, 65.68; H, 4.45; N, 5.89. Found: C,
66.09; H, 4.60; N, 5.93. Mp 184 °C.
Synthesis of 3. 2-(3,5-Bis(trifluoromethyl)phenyl)acetonitrile
(0.70 mL, 4 mmol) reacted with N,N′-di(n-octyle)quinacridone
(C6QA) (0.536 g, 1 mmol) according to the procedure described
1
for the synthesis of 1 to yield 0.553 g (55%) of 3. H NMR (CDCl3,
ppm) (500 MHz): δ 8.45 (s, 1 H), 7.68−7.75 (m, 3 H), 7.44 (t, 1 H),
7.32 (t, 1 H), 6.88 (d, 1 H), 6.63−6.72 (m, 1 H), 4.42 (t, 1 H), 3.24−
3.37 (m, 1 H), 2.00−2.04 (m, 1 H), 1.65 (t, 2 H), 1.60 (t, 2 H), 1.13−
1.54 (m, 6 H), 0.91 (t, 3 H), MS: m/z 1006.30 [M]+. Anal. Calcd for
C56H50 F12N4: C, 66.79; H, 5.00; N, 5.56. Found: C, 67.01; H, 4.85; N,
5.49. Mp 172 °C.
RESULTS AND DISCUSSION
■
Synthesis. Scheme 1 outlines the synthetic route of the
quinacridine-cored A−D−A type compounds 1−3. The
precursor alkyl-substituted quinacridones were synthesized
according to a standard procedure.20 The soluble alkyl-
substituted quinacridones were treated with 2-(3,5-bis-
(trifluoromethyl)phenyl)acetonitrile to form a new series of
quinacridine derivatives which were purified by short column
with moderate isolate yields (33−55%). These compounds are
soluble in common organic solvents, such as toluene,
chloroform, and tetrahydrofuran (THF), etc. The chemical
structures of 1−3 were verified by 1H NMR, mass
spectrometry, and element analysis.
Absorption Properties. The absorption spectra of all the
compounds were measured both as thin films on quartz and in
CHCl3 solution. As shown in Figure 2, these compounds have
Figure 3. PL spectra of 1 (a), 2 (b), and 3 (c) with different water
fractions and fluorescence emission image (0 and 50% H2O) under
500 nm excitation.
weak that almost no PL signals were recorded when excited at
500 nm. However, dramatic enhancement in luminescence was
observed when water as a nonsolvent is added into the THF
solution. Since the compounds were insoluble in water,
increasing the water fraction in the mixed solvent could change
their existing forms (the relative distance between the
molecules and geometry of compounds in the solution state)
from a solution or well-dispersed state in THF to the
aggregated particles in the aqueous THF. The enhanced
fluorescence intensity has been attributed to aggregation;
therefore, the compounds 1−3 were characterized as AIE
active. Changes in the PL peak intensity versus water fraction of
the mixture for all compounds are plotted as an inset in Figure
3. As the water fraction was increased from 0 to 75%, the
fluorescence intensity has increased many folds. It is worthy to
note that this enhancement was not the same for all
compounds. To have a quantitative picture, we have estimated
the fluorescence quantum yields (ΦF) of the THF solutions,
THF/water mixtures, and solid thin films of the compounds
Figure 2. Absorption spectra of compounds 1−3 in solutions and thin
films.
identical absorption spectrum profile in solution with the same
maximum absorption wavelength (λabs = 572 nm), suggesting
that the alkyl chain length does not affect the electronic
structure of the conjugated backbones. The λabs for 1 (571 nm),
2 (575 nm), and 3 (576 nm) in solid thin film are slightly
different from each other, suggesting that the alkyl chain may
affect the molecular aggregation state, which in turn slightly
affects absorption property of thin films. Notably, the spectral
profile in the solid state is quite similar to that of solution and
the numbers of absorption bands are the same. Hence, one to
one correspondence of the absorption bands between the
solution and solid state spectra is possible, indicating that the
molecular nature is well preserved even in the solid state.
Aggregation-Induced Emission. The photoluminescence
(PL) spectra of 1−3 in solution and aggregated states are
shown in Figure 3. The emission from the THF solution is so
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using rhodamine B as a standard. The fluorescence quantum
yield (ΦF) for 1, 2, and 3 in THF are 2.1 × 10−4, 2.3 × 10−4,
and 2.6 × 10−4, respectively, which has significantly increased to
8.9 × 10−3, 9 × 10−2, and 9.5 × 10−2 in a THF/H2O (1:1 v:v)
mixture and 1.0 × 10−2, 9.9 × 10−2, and 1.0 × 10−1 in solid
films.
nonradiative channels and populate the radiative decay. As
molecular rotations can be thermally activated, the effect of
molecular rotations on emission behavior can be appraised by
examining the temperature-dependent emission spectra.
The temperature-dependent PL spectra of dilute solutions of
compounds 1−3 in THF were measured (Figure 4). It is noted
Effect of Polar Structural Units on AIE. The molecules
without polar structural units excluded the involvement of J-
and H-aggregates effect in their PL proceses.21,22 Such AIE
molecules emits in the short wavelength region and emits blue
light in the aggregated state due to the absence D−A
interactions in the systems.23,24 Although alkyl-substituted
QA derivatives are not AIE active, their electron-donating
feature and twisted structure after substituting with 2-(3,5-
bis(trifluoromethyl)phenyl)acetonitrile units make QA mole-
cules are useful central core for constructing organic molecules
with AIE property. Compounds 1−3 with QA skeleton as a
donor unit and 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile
as acceptor units make them AIE active. Generally, the emission
wavelength and color of AIE molecules without polar units are
scarcely affected by solvent polarity. Molecules with intra-
molecular charge transfer nature like compounds 1−3 can
bathochromically shifted their fluorescence with increasing
solvent polarity due to that polar solvents alter their ground
and excited states and narrow their energy gaps.25,26
Concentration Dependence of the Aggregation-
Induced Emission. The concentration-dependent emission
property of compound 2 in THF/H2O (1:1 v:v) mixture has
been studied (Figure S1). Upon concentration increasing, the
emission intensity decreased significantly and the emission
maximum displayed a red-shift. Compound 2 was unsolvable in
H2O, the increase of concentration of 2 could reduce the
solvating power, which resulted in enhancement of aggregation
and increase of the particle size. As the particle size increase, the
emission intensity should decrease and the emission maximum
shift to longer wavelength.27 To support above explanation, the
particle size measurement has been performed. The dynamic
light scattering (DLS) experimental results showed that the
average diameters of particles for different concentrations were
340 nm (1 × 10−4 M), 375 nm (1.5 × 10−4 M), 415 nm (2 ×
10−4 M), 463 nm (2.5 × 10−4 M), and 601 nm (3 × 10−4 M).
For the solution systems with large size organic particles, their
emission and absorption properties may be influenced by the
scattering of the organic particles.28 The emission spectra
presented in Figure S1 have been obtained based on the
excitation wavelength of 500 nm. Therefore, the scattering of
excitation light has no effect on the emission spectra that
appears beyond 600 nm. The concentration-dependent
absorption spectra that have been recorded by integrated
sphere and normal transmission method are presented in
Figure S2. There is no significant effect of scattering on the
absorbance, except for the 3 × 10−4 M THF/H2O mixture. For
the absorption spectra measured by integrated sphere, the effect
of scattering can be eliminated.
Figure 4. Emission of 1 (a), 2 (b), and 3 (c) in THF at variable
temperature.
that at room temperature all compounds have very faint
emission or almost no emission. At −196 °C, these compounds
show intense emission peak due to the blockage of molecular
rotations. As the temperature is increased, the emission
intensity tend to decrease, but there is a dramatic abrupt
decrease in the emission intensity at the melting point of the
solvent (−108 °C) due to free molecular rotations which
deactivate the excited specie through the nonradiative way. The
extent of decrease at the melting temperature was not the same
for all compounds. The PL intensities of 1, 2, and 3 at −196 °C
Effect of Temperature on Molecular Rotations. It is
known that that rotational energy relaxation can nonradiatively
deactivate the excited specie and result in weakening the
corresponding fluorescence.29 In the dilute solutions, the active
molecular rotations may effectively decimate the excited exciton
through molecular collision with solvent molecules, thus
making the fluorescent molecules nonemissive. In the solid
aggregates, the stacking forces among the aromatic fluoro-
phores may restrict the molecular rotations which block the
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are 9-, 18-, and 19-fold higher than those at −108 °C. The
extent of decrease seems to be closely related to the steric effect
of QA skeleton. As compounds 2 and 3 have a longer alkyl
chain length than 1, so they have more steric effect making the
rotation difficult at low temperatures.
Morphology Transition from 0-D Nanostructures to
1-D Microwires. It is demonstrated that molecular structures
containing π-conjugated rigid core and soft alkyl chains could
be suitable to prepare 1-D nanostructures for the application of
photoelectric nanodevices.30 The molecular structure shown in
Figure 1 reveals that all molecules with π-conjugated rigid cores
and two alkyl chains may have one-dimensional self-assembly
property. Figure 5 shows the morphological transition of the
structure (Figure 5e). The SEM images of the sample prepared
after 30 min of aging in the combined solvent showed large-
scale 1-D nanostructures, which indicates the 1-D growing
tendency of the vesicles (Figure 5f,g). When the samples were
kept at ambient conditions for 1 day, the globular nanotubes
have transformed into long microwires with diameter of 350
nm (Figure 5h). The only difference between samples with the
two different morphologies is that they underwent different
aging time, so we can infer that the 1-D nanostructures were
grown from the initially formed vesicles. To clarify the structure
of the vesicles and nanotubes, the transmission electron
microscope (TEM) measurements (Figure S3) have been
performed and provided enough evidence for above dis-
cussions.
At this stage the mechanism of the formation of vesicles
remains poorly understand. A possible explanation for this
phenomenon is that for the compounds 1−3 segregation by
high solvophobic nature of the trifluoromethyl groups may
actuate the molecules assemble into layered structures and
finally close to form vesicles.31 The tube formation progress in
this study may be similar to the “curvature strain releasing”
process proposed by Rowan and Nolte which explain the fusion
of PS-PIAT from small vesicles to larger ones.32 There is large
number of membrane defects and high curvature in the initially
formed vesicles. These membrane defects results in the fusion
of vesicles during which curvature energy is released which
results in thermodynamically more stable tubular structure. As
THF erode the membrane, therefore, it plays an essential role
in the tubular formation. This might be proved by the fact that
no morphology transition would happen if the vesicles were
collected from the suspension and the solvents were allowed to
evaporate.
1D Self-Assembly. Beside the above-mentioned reprecipi-
tation process, the solution evaporation approach was also
employed to fabricate compounds 1−3 molecules into
luminescent wires. The experimental results demonstrated
that solution evaporation process leaded to the formation of
luminescent wires with larger diameters than reprecipitation
approach. Size controllable well-defined 1-D microwires have
been obtained by injecting 500 μL of alcohol into a 500 μL
THF solution (1 × 10−3 M) and casting a drop of sample on
silicon substrate followed by slow evaporation typically over 10
h at ambient temperature. Fluorescence microscopic images
and SEM images revealed that 1-D microwires with different
length and diameter have been generated for all compounds. It
is worth mentioning that the short alkyl chain alcohol with fast
evaporation process leads to a random morphology with
irregular size and shape. Interestingly, relatively uniform
ultralong microwires with high aspect ratio were obtained
when n-butanol or n-pentanol was injected into THF solution
due to slow evaporation of the solvents. The experimental
results demonstrated that the evaporation of the THF/n-
butanol solution of compound 1 led to the formation of
luminescent wires with small aspect ratio (diameter of 2 μm
and length up to about 200 μm) (Figure 6). Compounds 2 and
3 with longer alkyl chain could assemble into more uniform and
elongated microwires. Compound 2 generated rodlike micro-
wires with a diameter of 4 μm and length of more than 500 μm
(Figure 7). It is worth noting that microwires generated by the
slow evaporation of THF/n-pentanol and CHCl3/n-butanol
solutions of compound 2 were more elongated rodlike wires
with length of more than 1000 μm and diameter of 4 μm.
Interestingly, compound 3 has formed highly luminescent
Figure 5. (a−c) SEM images of large-scale vesicles collected
immediately after injecting solution in THF into well-stirred H2O.
(d−g) SEM images of intermediates of morphology transition. (h)
SEM final image of 1D microwire.
compound 2 in THF/H2O mixture solution. The nanoparticles
with opening holes on their surfaces have been observed when
the sample was collected on the silicon wafers immediately after
injecting 200 μL of THF solution (1 × 10−3 M) into 1 mL of
well-stirred water (Figure 5a,b). The diameters of the hollow
spheres range from 300 to 530 nm. It was revealed that the
vesicles were not separately dispersed, and their edges were
cohered. The membranes of these vesicles were shared, and
those between fused vesicles were eroded. This behavior
suggested a growing tendency of the vesicles. To further
investigate this trend, we prepared the samples after aging in
the combined THF/H2O solvent system for 10 min and
collected the intermediate states of the morphology transition.
SEM images of the intermediates showed that the vesicles were
arranged one-by-one in a linear way (Figure 5c,d). Some of the
membranes between the vesicles started forming a tubular
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from chloroform/pentane, and nanoaggregates in THF/H2O
(1:1 v:v) mixture, it was found that the nano/microwires
displayed the shortest emission maximum and nanoaggregates
showed the longest one. The emission maximum of powder
sample located between those of nano/microwires and
nanoaggregates. The powder sample exhibited obviously
weaker emission compared with nano/microwires and nano-
aggregates. The nanoaggregates generated in THF/H2O
mixture often have normally both amorphous and crystalline
phases,33 which may allow the molecules adopt multiconfigura-
tions. On the other hand, in the nano/microwires all of the
molecules adopt the same configuration or very similar
configuration. Therefore, microwires showed the shortest
emission maximum. The molecular configuration and packing
feature of the powder sample may lie between nano/microwires
and nanoaggregates. Shoulder peaks are observed for the
powders and nano/microwires. The shape of the spectrum of
the nanoaggregates is somewhat skewed, which can be
attributed to strong light absorption by the red tail of the
absorption band.
Figure 6. Fluorescence microscopic images (a) and FESEM images
(b) of microwires of compound 1 prepared by the slow evaporation
process.
Crystal Structure. To further explore AIE and 1-D self-
assembly properties of the present A−D−A type compounds,
molecular conformation as well as intermolecular interactions
in the solid state are really important. All the compounds were
subjected to single crystal growth; however, only compound 2
produced crystals that could be subjected to X-ray diffraction.
As depicted in Figure 9a, the crystal structure demonstrates that
Figure 7. Fluorescence microscopic images (a) of microwires of
compound 2 prepared from the THF/n-butanol solution and (c)
CHCl3/n-butanol solution. FESEM images (b) of microwires of
compound 2 prepared from the THF/n-butanol solution and (d)
CHCl3/n-butanol solution by the slow evaporation process.
ultralong and thin 1-D microwires with a diameter of 700 nm
and length of several millimeters (Figure 8).
Figure 9. Molecular structure and intermolecular hydrogen-bonding
interactions (a) and 1-D molecular packing structure constructed by
the hydrogen bonds of outer trifluoromethylphenyl groups (b).
Figure 8. Fluorescence microscopic images (a) of microwires of
compound 3 prepared from the THF/n-butanol solution and (c)
prepared from THF/n-pentanol by the slow evaporation process;
FESEM images (b) of microwires prepared from THF/n-butanol and
(d) prepared from the THF/n-pentanol by the slow evaporation
process.
the central quinacridine core is heavily bent from the planar
conformation and the whole molecule has a highly twisted
structure. This twisted structure is stabilized by multiple
intermolecular hydrogen-bonding interactions (totally 11
hydrogen bonds for each molecule) including C−H(CH2)···N-
(core), C−H(Ph)···N(CN), C−H(CH2)···F, and C−H-
(Ph)···F. The hydrogen bond interaction involving 2-(3,5-
bis(trifluoromethyl)phenyl)acetonitrile interlocks the free
rotations of 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile
along the bond between olefinic carbon and phenyl carbon,
Effect of Morphology on Luminescent Properties.
Comparing the emission behavior (Figure S4) of nano/
microwires generated from chloroform/n-butanol solution of
2 by the slow evaporation process (for instance), powder
sample (a mixture of crystals and amorphous solid) formed
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Article
which reduces the nonradiative decay pathways through
interactions among the neighboring molecules. It is also
important to note that core skeleton adopts nonplanar
geometry and rather buckled shape which inhibits face-to-face
molecular stacking, too. Therefore, the abundant intermolecular
interactions can enhance the rigidity of these compounds and
may be an advantage to the emission of aggregation state.
The intermolecular interactions of the outer 3,5-bis-
(trifluoromethyl)phenyl groups, which may determine the
self-assembly feature of the bulk sample, make molecules stack
into 1-D molecular columns, as shown in Figure 9b. Such a
packing structure implies that these types of molecules have a
natural tendency to aggregate into linear 1-D structures. The
single-crystal structure of compound 2 therefore provides a
rational explanation for the formation of 1-D microfibers. To
evaluate the molecular structure and packing structure of the
self-assembly structures, X-ray diffraction (XRD) patterns of
microwires constructed by compound 2 and that of single
crystal of compound 2 calculated from single crystal data were
compared (Figure S5). The XRD patterns of the microwires are
similar to the XRD pattern of single crystal which suggests that
molecular packing of the fibers is analogous to that in a single
crystal of 2.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (50773027 and 50733002), the Major
State Basic Research Development Program (2009CB939700),
and 111 Project (B06009).
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CONCLUSIONS
■
Quinacridine derivatives 1−3 synthesized by introducing 2-
(3,5-bis(trifluoromethyl)phenyl)acetonitrile at the carbonyl
position of alkyl-substituted quinacridones exhibited aggrega-
tion-induced-emission (AIE) and 1D self-assembly properties.
It was found that 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile
units in these derivatives play a vital role in 1-D self-assembly of
the molecules. All these as-synthesized AIE-active compounds
are red emitting in the aggregated state. These compounds are
also luminescent at low solution temperature, indicating that
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ASSOCIATED CONTENT
■
S
* Supporting Information
XRD pattern of compound 2, emission spectra of compound 2
at different concentration in THF/H2O mixture, emission
spectra of amorphous powder, nano/microwires and nano-
aggregates of compound 2, TEM images of morphology
transition, and crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
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Corresponding Author
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*Fax: +86-431-85193421. E-mail: hongyuzhang@jlu.edu.cn
(H.Z.); yuewang@jlu.edu.cn. (Y.W.).
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