Aggregation-induced emission enhancement materials with large red shifts and their self-assembled crystal microstructures

Article abstract of DOI:10.1039/c1ce05035d

Two simple intramolecular charge transfer compounds (BzBMN and BzFAN), derived from 2-benzyloxy-4-diethylaminobenzaldehyde, have been synthesized. Unlike typical AIEE molecules, the enhanced emission of these compounds in the solid state is accompanied by large red shifts in emission wavelength. Absorption and photoluminescence (PL) spectra of the compounds in solution, film and the X-ray single crystal structures were investigated to understand the mechanism of enhanced emission in the solid state. The results indicate that the bathochromic shift excimer-like emissions originated from dimers induced by the dipole-dipole interaction and long-range ordered arrangement (e.g. J-aggregation), and the enhanced emission is attributed to the restriction of intramolecular rotations caused by multiple intermolecular hydrogen bonding interactions. In addition, it was also observed that BzBMN and BzFAN can facilely form single crystal microbelts and microtubes by a simple reprecipitation method. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1ce05035d

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PAPER
Aggregation-induced emission enhancement materials with large red shifts
and their self-assembled crystal microstructures†
a
Qing Dai, Weimin Liu, Lintao Zeng, Chun-Sing Lee, Jiasheng Wu and Pengfei Wang
a
a
b
a
a
*
*
Received 10th January 2011, Accepted 14th April 2011
DOI: 10.1039/c1ce05035d
Two simple intramolecular charge transfer compounds (BzBMN and BzFAN), derived from 2-
benzyloxy-4-diethylaminobenzaldehyde, have been synthesized. Unlike typical AIEE molecules, the
enhanced emission of these compounds in the solid state is accompanied by large red shifts in emission
wavelength. Absorption and photoluminescence (PL) spectra of the compounds in solution, film and
the X-ray single crystal structures were investigated to understand the mechanism of enhanced emission
in the solid state. The results indicate that the bathochromic shift excimer-like emissions originated
from dimers induced by the dipole–dipole interaction and long-range ordered arrangement (e.g. J-
aggregation), and the enhanced emission is attributed to the restriction of intramolecular rotations
caused by multiple intermolecular hydrogen bonding interactions. In addition, it was also observed that
BzBMN and BzFAN can facilely form single crystal microbelts and microtubes by a simple
reprecipitation method.
(AIE) by Tang et al., or aggregation-induced emission
enhancement (AIEE) by Park et al. It offers another possible
solution for the ACQ problem and indeed some AIEE active
molecules have been shown to be promising emitters in high
efficiency optoelectronic devices.4c,5b,7 However, materials with
AIEE properties are limited and one common feature of these
AIEE molecules is that they contain multiple aromatic rings
and/or polycyclic aromatic hydrocarbon skeletons.8 Moreover,
most of them emit blue or green light, while red AIEE emitters
are relatively rare.2b,9
Introduction
Organic materials with a strong solid-state emission have
received great attention for their important applications in the
fundamental research field of solid-state photochemistry and in
the applied field of optoelectronic devices.1 However, for most
organic fluorophores, the effects of aggregation-caused
quenching (ACQ) have limited their large-scale applications.2
Many efforts have been made to overcome the ACQ problems,
such as the introduction of bulky substituents onto the original
chromophores or doping them into matrix materials to reduce
the extent of aggregation.3 However, unfortunately, fluo-
rophores with high emission in the solid state are still rare.
Recently, a few fluorophores, including silole derivatives,4
1,1,2,2-tetraphenylethene (TPE) derivatives,5 and 1-cyano-
trans-1,2-bis-(4-methylbiphenyl)ethylene (CN-MBE) deriva-
tives,6 have been found to show significant enhancement in
their light emission upon aggregation or in the solid state. This
unusual phenomenon was named aggregation-induced emission
On the other hand, 1D nano/microstructured materials have
been considered to be important building blocks for miniaturized
optoelectronic functional devices.10 To the best of our knowledge
only a few articles report the synthesis of red emitting single
crystal 1D organic microbelts or microtubes by the self-assembly
of simple molecules.11 Therefore, it is intriguing to develop 1D
nanostructures of highly efficient red emitting AIEE materials
with simple (few aromatic rings) molecular structures.
In this paper, we prepared two intramolecular charge-transfer
(ICT) compounds (BzFAN and BzBMN) with simple structures
based on 2-benzyloxy-4-diethylaminobenzaldehyde with for-
mylacrylonitrile or malononitrile as the electron-acceptor. These
molecules show a good AIEE character and a tunable orange or
red emission. The absorption and PL spectra of the compounds
in solution, nanosuspensions, film, and their crystal structures
were investigated to elucidate the mechanism of enhanced
emission in the aggregation state. Moreover, 1D single crystal
microbelts and microtubes with high emitting intensity were also
prepared by simple reprecipitation method from ethanol/water
mixtures. The morphologies and the formation mechanisms of
microtubes were also studied and discussed.
aKey Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing, 100190, People’s Republic of China.
E-mail: wangpf@mail.ipc.ac.cn; Fax: +86-10-62554670; Tel: +86-10-
82543435
bCenter of Super-Diamond and Advanced Films (COSDAF), Department
of Physics and Materials Science, City University of Hong Kong, Hong
Kong SAR, China. E-mail: apcslee@cityu.edu.hk
† Electronic supplementary information (ESI) available: UV-vis and
emission spectra in solution, crystal data of BzBMN and BzFAN.
CCDC reference numbers 806623 and 806624. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1ce05035d
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suspension.14 By using an integrating sphere, we further
measured the absolute PL quantum yields of the crystals of
BzBMN and BzFAN, which are 0.11 and 0.13, showing 112-fold
and 137-fold enhancement, respectively. These show that
BzBMN and BzFAN are AIEE active. Fig. 1c–d show fluores-
cence images of BzBMN and BzFAN in the ethanol solutions,
nanoparticle suspensions (95% water) and solid crystal states
under UV light. It can be seen that the suspensions have similar
emission to the crystals, which indicates that they might have
similar microstructures in the aggregates.
Results and discussion
Synthesis and structural characterization
As shown in Scheme 1, both BzBMN and BzFAN can be
synthesized via two simple steps. Compound 2 was synthesized
with 88% yield from 4-(diethylamino)salicyl-aldehyde (1). Then 2
was reacted with corresponding active hydrogen compounds
through the Knoevenagel reaction and the residues underwent
acid dissociation reaction to give the crude products: BzBMN or
BzFAN. The final products were purified by recrystallization in
ethanol with high yields (BzBMN, 85% and BzFAN, 45%,
respectively) and were characterized by nuclear magnetic reso-
nance (NMR) and high resolution mass spectroscopy (HR MS).
The corresponding emission spectra of BzBMN and BzFAN
in aqueous ethanol with different ethanol/water ratios are shown
in Fig. 2. For solvents with low water contents (<70%), the PL
emissions of the compounds are weak and dominated by emis-
sion from isolated molecules between 450 and 500 nm. Only
when the water fraction is more than 70% do the new emission
bands from the aggregates appear and the PL intensities enhance
swiftly with the increase of water fraction (insets of Fig. 2).
Photophysical properties in solutions
Table 1 lists the absorption and PL peaks of BzFAN and
BzBMN in different solvents and their corresponding quantum
yields. The absorption peaks (labs) of BzFAN and BzBMN range
from 420 to 445 nm and show a strong dependence on the solvent
polarity. The spectrum exhibits vibrational structure in nonpolar
hexane, while it is slightly broadened and becomes structureless
in a polar solvent, such as ethanol (Fig. S1a and S2a†).
Concurrently with the spectral red shift, the fluorescence
quantum yield (Ff) is also gradually reduced with an increase in
solvent polarity. In a highly polar ethanol solution, for example,
Mechanisms of emission enhancement
To determine the mechanism of enhanced emission, we measured
the PL spectra of BzBMN in an ethanol/methanol solution (4/1
by volume) at low temperature (Fig. 3). The PL intensities are
almost the same at temperatures from room temperature to 166
K (the solvents remain in liquid state). When the solution is
frozen at 77 K, the PL shows a dramatic increase in intensity and
negligible shift in peak wavelength (lem 476 nm). As there is no
intermolecular interaction between BzBMNs in the frozen dilute
solution, the PL spectrum represents the characteristics of iso-
lated molecules. The emission enhancement of the BzBMN
solution at 77 K is attributed to the restriction of intramolecular
rotation in such a rigid solid microenvironment, which shuts
down the nonradiative decay. The above results show that the PL
spectrum of BzBMN at 77 K is obviously different from that in
ethanol/water (v/v ¼ 5/95) mixtures. Thus, we can speculate that
the AIEE of BzBMN does not originate from isolated molecules
but from intermolecular aggregates.
fluorescence is almost quenched for both monomers (Ff
¼
0.00098 and 0.00095 for BzBMN and BzFAN, respectively). This
behavior is the typical characteristic of an ICT compound.12 The
very low fluorescence quantum yields of BzFAN and BzBMN in
monodisperse states might be attributed to the fast free rotations
of the diethylamino, benzyloxy, and malononitrile moieties,
which relaxes the excited states by nonradiative decay process.13
Aggregation-induced enhanced emission behaviors
Diluted solutions of BzBMN or BzFAN in ethanol give weak PL
with an emission maximum (lem) of 480 nm or 485 nm (Fig. 1).
By contrast, in ethanol/water (v/v ¼ 5/95) solutions, BzBMN and
BzFAN give bright emissions, peaked at 582 and 603 nm,
respectively. As water is a poor solvent for BzBMN and BzFAN,
it can be envisioned that the hydrophobic molecules would
cluster together to form aggregates in the aqueous mixtures at
such a high water contents (95%). This is further evidenced by the
observation of level-off tails in the long-wavelength region of the
absorption spectra due to the Mie effect of the nanoparticles
Fig. 4 and 5 show the molecular structures of BzBMN and
BzFAN in single crystals determined by X-ray diffraction (Table
S1 lists the lattice parameters and structural details†). The
dihedral angles for BzBMN and BzFAN between the planes of
N2C19N3 (N2C19O2) and the phenyl core are 7.5ꢀ and 5.3ꢀ,
respectively, which show small distortions. This result indicates
good planarity along the p-conjugated system in the two mole-
cules, although large twist angles (BzBMN 8.78ꢀ and BzFAN
43.50ꢀ) between the benzyloxy moiety and central phenyl core
exist. Fig. 4b and 5b show how the two neighbor molecules are
packed into centrosymmetric anti-parallel (head-to-tail) dimers.
Since BzBMN and BzFAN are donor-p-acceptor (D-p-A)
dipole molecules, the dipole interaction places the ‘‘A’’ moiety of
one molecule above the ‘‘D’’ moiety of another molecule.
Besides, p–p stacking interactions may make a contribution to
the anti-parallel stacking. Further examination shows that the
molecules in the dimer slip over each other along the direction of
the long molecular axis (‘‘slipped-parallel’’ structure). The slip-
ped structure not only avoids the maximum face-to-face stacking
that causes the quenched emission, but also favors the dipole–
dipole interactions in these ICT compounds. The interplanar
Scheme 1 Synthetic Route of BzBMN and BzFAN.
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Table 1 Absorption and emission characteristics of BzBMN and BzFAN in different states
BzBMN
BzFAN
a
labs (nm)
b
lem (nm)
a
labs (nm)
b
lem (nm)
Sample state
Ff (10ꢁ3
)
Ffc (10ꢁ3
)
In hexane
In toluene
In THF
In ethanol
In methanol
Crystal
422
437
441
442
443
440
468
473
477
483
567
1.17c
1.09c
1.00c
0.98c
0.97c
110d
420
434
438
445
445
462
464
470
475
480
617
1.15c
1.10c
1.01c
0.95c
0.85c
130d
a
b
c
Absorption maximum. Emission maximum. Quantum yield (ꢂ 10%), estimated using 9,10-diphenylanthracene(Ff ¼ 90% in cyclohexane) as the
d
standard. Absolute quantum yield, determined with an integrating sphere. The spectra were measured in dilute solution with solutes concentration
of 20 mM. Excitation wavelength: 410 nm.
Fig. 2 PL spectra of BzBMN and BzFAN (50 mM) in ethanol/water
(v/v) mixtures with different water contents. The insets show the changes
in the PL intensity of BzBMN (580 nm) and BzFAN (600 nm) vs. the
volume fraction of water in the ethanol/water mixtures.
Fig. 1 Absorption (black) and PL (red) spectra of (a) BzBMN and (b)
BzFAN in ethanol (dash line) and ethanol/water (5/95, v/v) (solid line)
distances found in other systems.15 In this circumstance, the p–p
mixtures with concentration kept at 50 mM. Excitation wavelength: 445
stacking interaction is weakened. Thus, we can conclude that
nm. (c), (d) Fluorescent images of BzBMN and BzFAN in ethanol (50
dipole–dipole interactions are the main driving forces for dimer
mM), ethanol/water (5/95, v/v) and in the form of solid crystals upon
formation. The strong dipole interaction is sufficient to cause
intermolecular excitonic coupling,16 which causes the excimer-
like emission. Fig. 4e and 5e show that the BzBMN and BzFAN
stack into uncontinuous structures with the dimer as the basic
building blocks:17 a larger interplanar spacing and slip angle
excitation with a 365 nm light source.
ꢀ
ꢀ
ꢀ
spacings of the dimer molecules are 3.573 A (slip angle, 28.08 )
ꢀ
for BzBMN and 3.643 A (slip angle, 40.90 ) for BzFAN, which
ꢀ
ꢀ
between different dimer blocks (3.809 A, 45.11 for BzBMN and
are larger than the representative p–p stacking interaction
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Fig. 3 PL spectra of BzBMN in ethanol/methanol solution (4/1, v/v) (50
mM) at different temperatures.
Fig. 5 Crystal structures of BzFAN: (a) single molecule structure. (b)
Top view of antiparallel dipole arrangement. (c) Side view of antiparallel
dipole arrangement and C–H/p, C–H/N hydrogen bonds interactions.
(d) C–H/O hydrogen bonds interactions. (e) The crystal stacking images
induced by hydrogen bonds interactions. Hydrogen atoms have been
omitted for clarity.
arrangement, such as J-aggregate,18 so we can consider that
J-type aggregates are formed when larger aggregates are formed.
Time-resolved fluorescence measurements were performed to
prove the existence of excitonic couplings. The fluorescence
decay curves of BzBMN and BzFAN with different excitation
Table 2 Fluorescence lifetime of BzBMN and BzFAN in ethanol solu-
tion and crystal powdera
Sampleb
lex (nm)
s1 (ns)
s2 (ns)
c2
B-sol
B-cry
445
445
500
445
445
500
< 0.1
1.16 (0.04)
1.37 (0.02)
< 0.1
2.33 (0.47)
4.60 (1.0)
30.1 (0.96)
33.1 (0.98)
1.173
1.125
Fig. 4 Crystal structures of BzBMN: (a) single molecule structure. (b)
Top view of the antiparallel dipole arrangement. (c) Side view of the
antiparallel dipole arrangement and C–H/p hydrogen bonds interac-
tions. (d) C–H/N hydrogen bonds interactions. (e) The crystal stacking
images induced by hydrogen bond interactions. Hydrogen atoms have
been omitted for clarity.
F-sol
F-cry
5.74 (0.53)
1.132
1.043
a
Relative weights of lifetime are given in parentheses. The detection
wavelength was 580 nm for BzBMN and 600 nm for BzFAN,
respectively, except for in solution dispersed state that was 485 nm.
b
ꢀ
ꢀ
Abbreviations: B-sol ¼ BzBMN in ethanol solution, B-cry ¼ BzBMN
4.125 A, 44.84 for BzFAN) are observed for both compounds.
The more slipped stacking further weakens the p–p interaction
and is in favour of the formation of a long-range ordered
crystal powder. F-sol ¼ BzFAN in ethanol solution, F-cry ¼ BzFAN
crystal powder.
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wavelengths are shown in Fig. S3† and the detailed data are listed
in Table 2. In the good solution, the lifetime was obtained by
monitoring at the monomer emission 485 nm. The decay of the
monomer is too fast to be measured with our equipment (the
limit is 0.1 ns). It should be less than 10 ps according to the result
of p-N,N-dialkylaminobenzylidenemalononitriles, which have
a similar main structure.13 In their crystalline forms, the two
compounds show obviously longer lifetimes when monitoring at
580 and 600 nm. The long lifetime indicates the existence of new
aggregation species or excitonic couplings. In addition, when
excited at 445 nm or less, a double exponential fitting is more
suitable for the decay curves of both compounds. When excited
at 500 nm, a single lifetime model gives a better fitting for
BzFAN. Although it is still a double exponential fitting for
BzBMN, the contribution of the longer lifetime species obviously
increases. The above results can be explained as follows: at
a shorter excitation wavelength of 445 nm both high and low
energy species or excitonic couplings are excited, however, at
a longer excitation wavelength of 500 nm, only the low energy
species or excitonic couplings with longer lifetime are active.
Fig. 4 and 5 also show that multiple hydrogen bonding inter-
actions are involved in the crystals. For BzBMN, two kinds of
interaction forces exist in the crystal (Fig. 4c–d). The phenyl ring
of one molecular benzyl moiety interacts with the benzyl
hydrogen of the neighboring column molecule through a C–H/
increasing concentrations of BzBMN in PMMA. It shows a weak
emission peak at 483 nm, similar to that in ethanol, at very low
doping concentrations (e.g. 1 wt %), which indicates that the
molecules of BzBMN are monodispersed in the PMMA films.
However, when the doping concentration is more than 5% wt,
BzBMN exhibits dual fluorescence. The emission band can be
resolved into two components, a short-wavelength emission
band at ꢃ483 nm from the monomer and a long-wavelength
emission band at ꢃ567 nm from the dimer (and other larger
aggregates at higher concentrations).19 The long-wavelength
emission intensity is also increased with increasing doping
concentration. The emission wavelength is much shorter than
that in the single crystal or nanoparticles. The reason for this
phenomenon might be that the aggregates of BzBMN in the film
are amorphous or lacking the long-range ordered arrangements,
such as J-aggregation, as in the crystals.18 The emission of
BzFAN in the PMMA film shows similar behavior (Fig. S4†). At
a low doping concentration (1 wt %), its monomer emission
appears at 485 nm, whereas with high doping concentration, the
emission is intensified and red-shifted.
The crystal growth and self-assembling process
Time-dependent absorption and PL spectra were measured to
reveal the crystal growth and self-assembly process of BzBMN
and BzFAN in ethanol/water mixture (5/95) (Fig. 7). As shown in
Fig. 7a, when the ethanol solution is injected into water, the
absorption is peaked at 431 nm with a shoulder around 458 nm
initially. With the time elapsing, the relative intensity of the
shoulder peak gradually increases and it has become the main
absorption band after 12 min. The blue shifted absorption band
of 431 nm might mainly originate from the anti-parallel dimer
structure. The new peak appearing at around 458 nm indicates
the formation of aggregates larger than the dimer structure.
Moreover, the maximum absorption wavelength gradually red
shifts as time proceeds, suggesting the formation of long-range
ordered arrangements with an extended p-electron conjugation
system20 (see discussion below). These results are also confirmed
ꢀ
p hydrogen bond (2.907 A). In addition, a C–H/N hydrogen
ꢀ
bond (2.692 A) exists between the cyano moiety and the para-
position benzyl hydrogen of the neighboring molecular layer.
For BzFAN, there are three kinds of hydrogen bonding inter-
actions in the crystal (Fig. 5c–d). A C–H/p hydrogen bond
ꢀ
(3.113 A) formed between the terminal hydrogen of ethyl moiety
and the phenyl core of the benzyl moiety connects the molecules
ꢀ
of two adjacent columns together. Two C–H/N (2.673 A, 2.733
ꢀ
ꢀ
A) and one C–H/O (2.704 A) hydrogen bonds exist between
different molecular layers. These hydrogen bonding interactions
restrict the rotations of the benzyl and malononitrile or nitrile
acetaldehyde moiety, which enhances the emission efficiency of
the BzBMN and BzFAN in aggregation state.
Thin solid films of BzBMN and BzFAN in PMMA were also
prepared by spin-coating to further investigate the AIEE
phenomenon. Fig. 6 shows the changes of the PL spectra with the
Fig. 7 Time-dependent absorption (a), (c) and PL (b), (d) spectra of
BzBMN and BzFAN molecules (50 mM) in ethanol/water (5/95) mixtures.
Excited wavelength: 445 nm.
Fig. 6 PL spectra of BzBMN in a PMMA film with different doping
concentrations (excited at 445 nm).
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by the PL spectrum (Fig. 7b). Comparing to that of the monomer
emission, the emission band of 567 nm is broad and structureless
and shows a large red shift, which may originate from a dimer or
small aggregates. Then the emission wavelength is further red-
shifted as the crystals grow. The emission peak of 582 nm can be
attributed to larger aggregates with an ordered crystal lattice
structure. After 18 min, the emission peak wavelength remains
unchanged but its intensity decreases due to the precipitation of
nanoparticles. According to the spectral data, we consider that
BzBMN might undergo a self-assembly process in ethanol/water
mixtures with high water contents. When BzBMN molecules are
injected into water, they would cluster together due to their
hydrophobicity, producing some tiny crystal nuclei (small
aggregates). With the growing of nuclei, larger aggregates such as
J-aggregates are formed, which result in further red shift in its PL
emission. BzFAN has a similar evolution trend as BzBMN in
water/ethanol mixture with several differences in the details: (1)
the blue-shifted absorption band at around 435 nm is not the
maximum peak in the initial stage (Fig. 7c). This is attributed to
ꢀ
a larger interplanar spacing (3.643 A) between the BzFAN dimer
ꢀ
than that of BzBMN (3.553 A), weakening the p–p interaction
between the dimer molecules; (2) the absorption wavelength of
dissolved monomer at 461 nm remains essentially unchanged and
the intensity is reduced more seriously due to the faster
consumption of free BzFAN molecules in the mixed solvents;20
(3) the intensity of a newly formed peak in the range of 483–503
nm is also reduced quickly as a result of faster precipitation
(Fig. 7c). We can also find evidence in the reduced emission
intensity in the red-shift range before it shifts to 600 nm (Fig. 7d)
due to faster precipitation.
Fig. 8 SEM micrographs of BzBMN (a, c, e) and BzFAN (b, d, f) at
several characteristic stages: (a) and (b) the immediately precipitated
nanoparticles, (c) and (d) the nanostructures at around 1 h after injection,
(e) and (f) almost completely grown structures at least 6 h after injection.
Scanning electron microscopic (SEM) studies were also carried
out to study the crystal growth process (Fig. 8). Upon injection
of the ethanol solution of BzBMN or BzFAN into water (5/95,
v/v), small nanoparticles varying from 0.5 to 1 mm form imme-
diately. Larger ordered crystal structures (microbelt for BzBMN
and microtube for BzFAN) form 1 h later. Finally, almost
perfect single crystal structures with smooth outer surfaces are
observed 6 h later. The microbelts are 2–5 mm wide with length
varying form ca. 15 to 40 mm. The microtubes are ca. 1.5–2 mm
wide with length ranged from 10 to 50 mm. Such morphologic
evolution corresponds well to the time-resolved spectra data.
As the tubular structure has many unique properties, the
formation mechanism of the BzFAN nanotubes was further
investigated. As mentioned, BzFAN molecules immediately
aggregate into nanoparticles (Fig. 8b) when the ethanol solution
is injected into water. The nanoparticles then grow into a sheet or
belt-like structure (Fig. 9b–c), which is commonly observed in
the organic nano/microtube formation process.21 These sheets or
belts begin to curl up to give tubular structures (see black arrows
in Fig. 9b–c) with the sheet broadening. A seaming process then
occurs to give an open-ended rectangular hollow tube (Fig. 9d–
h). Based on morphological features of the BzFANs nano-
structures (Fig. 9b–h), such as the sharp ends and multilayer
structures in Fig. 9d–e, we consider that the growth process of
tubular crystals may also involve a self-assembly process, in
which the initial sheets may serve as template and nucleation sites
for the formation of secondary belt-like nanostructures perpen-
dicular to the initial sheets. These eventually lead to the forma-
tion of the rectangular tubes. The edges along the tube axes (see
black arrows in Fig. 9h–i) of completed-growth tubes also indi-
cate that a self-assembly process is involved in the growing
process.
Fig. 9 SEM micrographs of BzFAN (a) microtubes containing different
incomplete growth structure, (b) black arrows: belt structures, (c) black
arrow: a sheet-like plane with one end has curled up, (d) and (e) the
intermediate tubular structures composed by several sheets or belts, (f)–
(h) defects in the open-ended and side feature, (i) microtubes of complete
growth. Scale bars: 1 mm.
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mixture was poured into water and extracted with ethyl acetate
(4 ꢄ 30 mL). The combined organic extracts were washed with
brine and dried (Mg2SO4). The crude product was purified by
column chromatography (petroleum ether/ethyl acetate ¼ 4 : 1)
to give 2 (1.3 g, 88% yield). Mp: 70–71 ꢀC; 1H NMR (CDCl3, 400
MHz, ppm) d: 1.154 (t, 6H, J ¼ 7.1 Hz), 3.372 (q, 4H, J ¼ 7.1
Hz), 5.174 (s, 2H), 6.071 (d, 1H, J ¼ 1.9 Hz), 6.290 (dd, 1H, J1 ¼
8.9 Hz, J2 ¼ 1.9 Hz), 7.325–7.458 (m, 5H), 7.734 (d, 1H, J ¼ 8.9
Hz), 10.246 (S, 1H). HR MS (EI), m/z: 283.1576 (M+, Calcd for
C18H21NO2, 283.1572).
Conclusion
In summary, we have synthesized two compounds (BzBMN and
BzFAN) with good yields and investigated their AIEE proper-
ties. Unlike typical AIEE active molecules, the two compounds
have
a simple molecular skeleton (2-benzyloxy-4-diethy-
laminobenzaldehyde) and few aromatic rings. It can be observed
that the emission wavelength shows a great red shift and the
intensity increases significantly in the solid state compared to
that of their mono-disperse states. Crystal structure analysis
revealed that dimers induced by dipole–dipole interaction and
long-range ordered arrangements (e.g. J-aggregation) are
responsible for the red shifted emissions in the crystals and
nanosuspensions. Besides, intermolecular hydrogen bonding
interactions, which restrict the intramolecular rotations and
block the nonradiative processes, result in the enhanced emis-
sions. It was also observed that BzBMN and BzFAN can easily
assemble into single crystalline microbelts and microtubes
respectively in ethanol/water mixtures.
Synthesis of 2-(2-(benzyloxy)-4-(diethylamino)benzylidene)
malononitrile (BzBMN)
To a 50 mL flask with two necks, 2 (0.5 g, 1.77 mmol) and
ethanol (10 mL) were added. After 2 was dissolved, malononitrile
(0.13 g, 1.95 mmol) and several drops of triethylamine were
ꢀ
added. The mixtures were stirred at 60 C for about 20 min to
produce some orange-red precipitates. After cooling, the
precipitates were filtrated. The crude products were recrystallized
with ethanol and gave some orange-red crystal powders (0.53 g,
90% yield). Mp: 160–161 ꢀC; 1H NMR (CDCl3, 400 MHz, ppm)
d: 1.166 (t, 6H, J ¼ 7.1 Hz), 3.401 (q, 4H, J ¼ 7.1 Hz), 5.135 (s,
2H), 6.045 (d, 1H, J ¼ 1.9 Hz), 6.336 (dd, 1H, J1 ¼ 9.3 Hz, J2 ¼
2.3 Hz), 7.358–7.436 (m, 5H), 8.074 (s, 1H), 8.266 (d, 1H, J ¼ 9.3
Hz); 13C NMR (CDCl3, 100 MHz, ppm) d: 160.9, 154.3, 150.9,
135.8, 130.7, 128.9, 128.5, 127.3, 117.2, 116.0, 109.7, 105.8, 94.2,
70.6, 68.6, 45.2, 12.6; HR MS (EI), m/z: 331.1688 (M+, calcd for
C21H21N3O, 331.1685).
Experimental
Materials and instruments
3, 3-diethoxypropionitrile, malononitrile, potassium tert-but-
oxide were purchased from Alfa Aesar Co. Other reagents were
purchased from Beijing Chemical Regent Co. All of them were
used directly without further purification. All solvents used for
spectra measurement are of chromatographic grade. Ethanol was
distilled from magnesium under dry nitrogen immediately prior
to use for spectra measurement. 1H NMR and 13C NMR spectra
were measured on a Bruker Avance P-400 spectrometer using
CDCl3 as the solvent and tetramethylsilane (TMS) as an internal
reference. HR MS spectra were recorded on a Waters GCT
Premier mass spectrometer. UV-vis spectra and fluorescence
spectra were obtained with Hitachi U-3010 and F-4500 spec-
trophotometers, respectively. Time-resolved fluorescence lifetime
experiments were performed by the time-correlated single-
photon counting (TCSPC) sysrem with Edinburgh Instruments
FLS920 spectrophotometers. Fluorescence quantum yields (Ff)
of BzBMN and BzFAN in solutions were estimated by using 9,
10-diphenylanthracene as the reference (Ff ¼ 1.0). The absolute
fluorescence quantum yields of the crystals are measured with an
integrating sphere. Field-emission scanning electron microscopy
(FE-SEM) images were recorded on a Hitachi S-4300 instrument
operated at an acceleration voltage of 10 kV.
Synthesis of (E)-3-(2-(benzyloxy)-4-(diethylamino)phenyl)-2-
formyl-acrylonitrile (BzFAN)
Under a nitrogen atmosphere, 2 (1.0 g, 3.5 mmol) in 10 mL THF
was added to a stirred mixture of potassium tert-butoxide (0.59 g,
5.3 mmol) and 3, 3-diethoxypropionitrile (0.60 g, 4.2 mmol) in 20
mL of THF. The mixture was stirred at 60 ꢀC for 15 h. After the
reaction was completed, 30 mL of water was added to the solu-
tion and the mixture was extracted with ethyl ether (3 ꢄ 30 mL).
The organic phase was washed with brine and was evaporated
under reduced pressure to gain reddish brown oils. Then, 20 mL
of hydrochloric acid solution (1M) was added and the solution
was refluxed for 1 h. After cooling, the reaction mixture was
neutralized with alkali solution and was extracted with ethyl
ether. The combined organic phase was washed with saturated
brine, and dried with MgSO4. The concentrated mixture was
purified by column chromatography (petroleum ether/ethyl
acetate ¼ 6/1). The crude product was recrystallized with
ethanol-water and gave red crystalline solids (0.60 g, 51% yield).
Mp: 168–169 ꢀC. 1H NMR (CDCl3, 400 MHz, ppm) d: 1.169 (t,
6H, J ¼ 7.1 Hz), 3.41 (q, 4H, J ¼ 7.1 Hz), 5.185 (s, 2H), 6.102 (d,
1H, J ¼ 2.3 Hz), 6.377 (dd, 1H, J1 ¼ 9.3 Hz, J2 ¼ 2.3 Hz), 7.260–
7.422 (m, 5H), 8.283 (s, 1H), 8.447 (d, 1H, J ¼ 9.3), 9.426 (s, 1H);
13C NMR (CDCl3, 100 MHz, ppm) d: 188.6, 161.5, 154.4, 151.4,
136.1, 131.9, 128.9, 128.4, 127.3, 117.1, 109.8, 105.9, 102.2, 94.4,
70.7, 45.2, 12.6; HR MS (EI), m/z: 334.1685 (M+, calcd for
C21H22N2O2, 334.1681).
Single crystals of the two compounds were prepared by slowly
vaporizing their tetrahydrofuran (THF) solutions in a hexane
atmosphere. The diffraction experiments were carried out on
a Rigaku Saturn 724 diffractometer equipped with a Mo-Ka
source at about 170 K. Crystal structures were solved with direct
methods and refined with a full-matrix least-squares technique,
using the SHELXS software package. Mercury (CSD software)
was used for crystal structure visualization.
Synthesis of 2-(benzyloxy)-4-(diethylamino)benzaldehyde (2)
Benzyl bromide (1.36 mL, 6.24 mmol) was added to a stirred
mixture of 1 (1.0 g, 5.2 mmol) and K2CO3 (0.86 g, 6.24 mmol) in
30 mL of acetonitrile. After stirring at 70 ꢀC for 4 h, the reaction
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 4617–4624 | 4623

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B. Z. Tang, J. Phys. Chem. B, 2007, 111, 11817; (b) M. Wang,
G. Zhang, D. Zhang, D. Zhu and B. Z. Tang, J. Mater. Chem.,
2010, 20, 1858; (c) Z. Li, Y. Q. Dong, J. W. Y. Lam, J. Sun,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant no. 21073213, 60778033), the
Knowledge Innovation Program of the Chinese Academy of
Sciences, the National High Technology Research and Devel-
opment Program of China (863 Program) (grant no.
2008AA03A327), and the National Basic Research Development
Program of China (973 Program) (grant no. 2009CB623703).
€
A. Qin, M. Haußler, Y. P. Dong, H. H. Y. Sung, I. D. Williams,
H. S. Kwok and B. Z. Tang, Adv. Funct. Mater., 2009, 19, 905.
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