Starburst triarylamine donor-acceptor-donor quadrupolar derivatives based on cyano-substituted diphenylaminestyrylbenzene: Tunable aggregation-induced emission colors and large two-photon absorption cross sections

Article abstract of DOI:10.1002/chem.201002821

In this work, we have developed a new class of aggregation-induced emission (AIE) active compounds, in which three electron-donating diphenylamine, phenothiazine, or carbazole groups are connected to the 1, 4-positions of the benzene through bis(α-cyano-4-diphenylaminostyryl) conjugation bridges to form three triarylamine quadrupolar derivatives (3a-c). Their one-and two-photon absorption properties have been investigated. The two-photon absorption (2PA) cross sections measured by the open-aperture Z-scan technique were determined to be 1016, 1484, and 814 GM for 3a-c, respectively. From this result, the high 2PA properties of these molecules are attributed to the extended π system and enhanced intramolecular charge transfer from the starburst triarylamine to the cyano group. Moreover, cyano-substituted diphenylamine styrylbenzene (CNDPASB)-based compounds are very weakly fluorescent in THF, but their intensities increase by almost 230, 70, and 5 times, respectively, in water/THF (v/v 90%) mixtures, in which they exhibit strongly enhanced red, orange, and deep yellow fluorescence emissions, respectively. This result indicates that the intramolecular vibration and rotation of these dyes is considerably restricted in nano-aggregates formed in water, leading to significant increases in fluorescence. It was found that the color tuning of the CNDPASB-based compounds could be conveniently accomplished by changing the starburst triarylamine donor moiety. Multilayer electroluminescence devices with TPBI (2,2′, 2′′-(benzene-1,3,5-triyl)-tri(1-phenyl-1H-benzimidazole)) electron-transporting layers have been made, with 3a and 3c as a non-doping red-yellow emitter. The preliminary results for these multilayer devices show a maximum efficiency of 0.25°, and electroluminescence (EL) wavelengths around 568 nm. The excellent 2PA and AIE properties of these compounds make them potential materials for biophotonic applications.

Full text of DOI:10.1002/chem.201002821

FULL PAPER
DOI: 10.1002/chem.201002821
Starburst Triarylamine Donor–Acceptor–Donor Quadrupolar Derivatives
Based on Cyano-Substituted Diphenylaminestyrylbenzene: Tunable
Aggregation-Induced Emission Colors and Large Two-Photon Absorption
Cross Sections
Bing Wang,[a] Yaochuan Wang,[b] Jianli Hua,*[a] Yihua Jiang,[a] Jinhai Huang,[a]
Shixiong Qian,[b] and He Tian*[a]
Abstract: In this work, we have devel-
oped a new class of aggregation-in-
duced emission (AIE) active com-
pounds, in which three electron-donat-
ing diphenylamine, phenothiazine, or
carbazole groups are connected to the
1, 4-positions of the benzene through
bis(a-cyano-4-diphenylaminostyryl) con-
jugation bridges to form three triaryl-
transfer from the starburst triarylamine
to the cyano group. Moreover, cyano-
substituted diphenylamine styrylben-
zene (CNDPASB)-based compounds
are very weakly fluorescent in THF,
but their intensities increase by almost
230, 70, and 5 times, respectively, in
water/THF (v/v 90%) mixtures, in
which they exhibit strongly enhanced
red, orange, and deep yellow fluores-
cence emissions, respectively. This
result indicates that the intramolecular
vibration and rotation of these dyes is
considerably restricted in nano-aggre-
gates formed in water, leading to sig-
nificant increases in fluorescence. It
was found that the color tuning of the
CNDPASB-based compounds could be
conveniently accomplished by changing
the starburst triarylamine donor
moiety. Multilayer electroluminescence
devices with TPBI (2,2’,2’’-(benzene-
1,3,5-triyl)-tri(1-phenyl-1H-benzimid-
azole)) electron-transporting layers
have been made, with 3a and 3c as a
non-doping red–yellow emitter. The
preliminary results for these multilayer
devices show a maximum efficiency of
0.25%, and electroluminescence (EL)
wavelengths around 568 nm. The excel-
lent 2PA and AIE properties of these
compounds make them potential mate-
rials for biophotonic applications.
ACHTUNGTRENNUNGamine quadrupolar derivatives (3a–c).
Their one- and two-photon absorption
properties have been investigated. The
two-photon absorption (2PA) cross sec-
tions measured by the open-aperture
Z-scan technique were determined to
be 1016, 1484, and 814 GM for 3a–c,
respectively. From this result, the high
2PA properties of these molecules are
attributed to the extended p system
and enhanced intramolecular charge
Keywords: aggregation
chemistry · fluorescence · organic
light-emitting diodes triaryl-
amines · two-photon absorption
· electro-
·
AHCTUNGTRENNUNG
Introduction
2PA process, the strategy for the design of molecules with
large 2PA cross sections (s) has been investigated both ex-
perimentally and theoretically.[6,7] The designs include sym-
metrical donor–conjugated-bridge–donor (D-p-D), accept-
or–conjugated-bridge–acceptor (A-p-A) and donor–conju-
gated-bridge–acceptor–conjugated-bridge–acceptor–conju-
gated-bridge–donor (D-p-A-p-A-p-D), or asymmetrical
donor–conjugated-bridge–acceptor (D-p-A) arrangements.
The enhancement of s has been correlated with intramolec-
ular charge transfer from the terminal donor groups to the p
bridge.[8] In addition, the magnitude of s can be controlled
through the modification of the molecular structure in such
a way as to affect the amount of intramolecular charge
transfer. Specifically, s increases when the conjugation
length of the p system is increased, and when electron ac-
ceptors (A) are attached as side groups to the p bridge, to
form molecules of general structure D-A-D.[9] Many efforts
have been made to develop novel 2PA materials by the
modification of D/A groups or branching symmetry. Cur-
rently, 2PA materials with multibranched triarylamine are
attracting great interest, and have become the focus of in-
Materials displaying high two-photon absorption (2PA) have
attracted considerable interest recently, owing to their po-
tential application in two-photon dynamic therapy,[1] three-
dimensional optical data storage,[2] up-converted lasing,[3]
optical power limiting materials,[4] bioimaging,[5] and so
forth. For the full exploitation of the great potential of the
[a] B. Wang, Prof. Dr. J. Hua, Y. Jiang, J. Huang, Prof. Dr. H. Tian
Key Laboratory for Advanced Materials
Institute of Fine Chemicals and Department of Chemistry
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (P.R. China)
Fax : (+86)21-64252758
Fax : (+86)21-64252756
E-mail: jlhua@ecust.edu.cn
tianhe@ecust.edu.cn
[b] Dr. Y. Wang, Prof. Dr. S. Qian
Department of Physics, Fudan University
Shanghai 200433 (P.R. China)
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tensive research in the area of 2PA materials for their good
electron-donating and transporting capabilities,[10] which can
enhance the 2PA cross section owing to the increase in chro-
mophore density.[11,12]
show unique enhanced emission rather than a fluorescence
quenching in the solid state.[22] In this work, we connect
three electron-donating dipheylamine, phenothiazine, or car-
bazole groups to the 1,4-positions of the benzene through
bis(a-cyano-4-diphenylaminostyryl) conjugation bridges, to
form three new quadrupolar-type D-p-A-p-A-p-D deriva-
tives (3a–c, respectively). It could be expected that mole-
cules with such structures containing multi-electron-donat-
ing end-cappers and cyano acceptor groups should have ex-
cellent 2PA values, as well as AIE properties.
Many dyes with large 2PA were designed and synthesized.
However, most of 2PA dyes are hydrophobic, and their fluo-
rescence is often quenched on aggregation, although they
may show high fluorescence efficiency in solutions, which
greatly limits their applications, and especially their biopho-
tonic applications.[13] For biophotonic applications, it is nec-
essary that the 2PA dyes are soluble or dispersible in water,
and that they remain highly fluorescent in aqueous media.
Therefore, for example, with the use of in vivo imaging,
these superiorities play an important role as they lead to a
finer resolution, a better in-depth tissue penetration, and a
decrease in cell damage.[14] Tang and co-workers recently ob-
served a novel phenomenon that overcomes fluorescence
quenching in the aggregation, which they called aggrega-
tion-induced emission (AIE). They synthesized a series of
molecules such as silole, tetraphenylethene, and their deriva-
tives, and these were induced to emit intensely by aggregate
formation.[15] They also performed a series of tests to modu-
late, externally and internally, the restriction of the intramo-
lecular rotation (RIR) process, which is a main cause of the
AIE effect.[16] In view of this, some special 2PA dyes were
designed and synthesized, which not only have a large two-
photon activity, but also overcome fluorescence quenching
in the aggregation. Prasad and co-workers have reported ag-
gregation-enhanced fluorescence and two-photon absorption
in nano-aggregates due to the hindering of molecular inter-
nal rotation.[17] Recently, our group designed and synthe-
sized multibranched triarylamine end-capped triazines that
exhibit enhanced 2PA and AIE properties.[18] However,
there are still few reports on dyes with high 2PA and AIE.
Triphenylamine and carbazole have been widely used in
opto- and electroactive materials, due to their good elec-
tron-donating and transporting capabilities, as well as their
special propeller starburst molecular structure.[19] Phenothia-
zine is also a well-known heterocyclic compound with elec-
tron-rich sulfur and nitrogen heteroatoms, and it is a poten-
tial hole-transport semiconductor in organic devices, pre-
senting unique electronic and optical properties.[20] Recently,
2PA materials with triarylamine as the electron donor have
aroused great interest and become the focus of intensive re-
search in this field.[10] Moreover, starburst triarylamine moi-
eties as electron donors have better electron-donating and
transporting capabilities, which may mean that the dyes
have quite large two-photon activity because of the highly
extended p conjugation and the appropriate intramolecular
charge transfer (ICT); the introduction of the starburst triar-
ylamine moiety is thus expected to exhibit the AIE phenom-
enon.[18,21] On the other hand, cyano-substituted compounds
show good optical and electrical properties due to their high
electron affinities. Molecules with an electron-withdrawing
cyano group on the central phenylene ring are strongly fluo-
rescent and have higher s values.[9b] In particular, some
cyano-substituted compounds have been reported which
Results and Discussion
Synthesis: The synthetic route to cyano-substituted diphe-
nylamine styrylbenzene (CNDPASB)-based compounds is
depicted in Scheme 1. The three dyes were prepared by a
typical Knoevenagel condensation[23,24] of a triarylamine al-
dehyde (2a–c) with 2,2’-(1, 4-phenylene) diacetonitrile. The
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Triarylamine Quadrupolar Derivatives
FULL PAPER
Figure 1. Normalized one-photon absorption of 3a–c in THF: c 3a;
b 3b; g 3c.
solvent for the dyes. Figure 2a shows the corresponding
emission spectra of 3a in aqueous THF with different water/
THF ratios at a concentration of 1ꢁ10À5 m. The emission
from the solution of 3a in THF was so weak that almost no
photoluminescence (PL) signal was recorded. However, sol-
utions containing 50:50 (v/v) water/THF mixtures displayed
a dramatic enhancement of red luminescence. When the
water/THF ratio reached 90%, the PL intensity was boosted
to the maximum. However, the spectral profile showed little
change with the addition of water into a solution of 3a in
THF. Water is a not a solvent for the dye, and its molecules
must aggregate in solvent mixtures with a high water con-
tent. Apparently, the emission of 3a is induced by aggregate
formation, thus verifying its AIE nature. The starburst tri-
phenylamine is just like a rotor, and benzene is like a
stator.[16] In the dilute solution, rotations of multiple triphen-
Scheme 1. Synthetic route for compounds 3a–c.
important intermediate aldehydes (2a–c) were readily syn-
thesized using the Ullmann reaction between 4-[N,N-bis-(4-
iodophenyl)amino]benzaldehyde (1) and the appropriate ar-
ylamine, that is, diphenylamine, phenothiazine, and carba-
zole, respectively. The molecular structure and purity of
each dye were confirmed by NMR and IR spectroscopy and
MS, which all gave satisfactory spectroscopic data. For ex-
À1
À
À
ample, the carbonyl signal (1690 cm ) and CH2 CN signal
(2248 cmÀ1) were not observed on the IR spectra of
À
CNDPASB-based derivatives, but the =C CN signal
1
(2208 cmÀ1) appeared. In the H NMR spectra, the signals of
ACHTUNGTRENyNUNG lamine peripheries against the benzene core may effective-
carbonyl groups were also not observed at about 9.90 ppm,
ly deactivate its excited state nonradiatively, thus making it
non-emissive. On the other hand, it is supposed that the
cyano group in 3a is effective in reducing the parallel face-
to-face intermolecular interaction in the aggregated state,
which consequently induces the formation of J-aggregates
with enhanced fluorescence emission in the nanopartic-
les.[22a] Restriction of the intramolecular rotations in the ag-
gregates blocks the channel of nonradiative decay, hence
changing the dye to a strong emitter.[25] The fluorescence be-
havior of 3a is well visualized through fluorescence images
of the solution and nanoaggregate dispersions (see Figure 4a
later). Figure 2c shows the absorption of 3a in THF and in a
dispersion of the nano-aggregate form (90% water) at 1ꢁ
10À5 m. In contrast to the fluorescence enhancement, the ab-
sorption maximum of 3a is red-shifted by nano-aggregation,
without any notable sign of J-aggregation, implying that mo-
lecular stacking is neither too close nor well ordered within
the nano-aggregated structure.[22d] The spectral red shifts in-
dicate that planarization of the distorted conjugation is in-
duced by aggregation, due to the intermolecular steric
effect, yielding extended conjugation lengths for 3a. More-
over, the molecules have a branched structure, which may
À
À
and the signal of CH2 CN (3.65 ppm) could not be detect-
ed, which indicated that the Knoevenagel condensation re-
action had taken place. It is worth noting that the synthesis
of these CNDPASB-based derivatives is very simple, and
that different functional donor groups can be introduced
with ease.
One-photon absorption: One-photon absorption of the dyes
3a–c in THF at dilute concentrations are shown in Figure 1.
The absorption maxima (lmax) of 3a–c are located at 473,
443, and 435 nm, respectively. It is clear that the lmax of 3a
is red-shifted by 30 and 38 nm compared with those of 3b
and 3c, respectively, which is in agreement with the order of
the increase of electron-donating ability of the donors: N,N-
dimethylaniline>phenothiazine>carbazole.
AIE properties: All the CNDPASB-based derivatives are
soluble in common organic solvents such as THF, acetone,
and dichloromethane, but are insoluble in water. Stable
water dispersions of nano-aggregates of 3a–c were prepared
by the precipitation method, using THF as a water-miscible
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J. Hua, H. Tian et al.
sentative scanning electron microscopy (SEM) image of the
obtained nano-aggregates is shown in Figure 2d.
The best way to obtain a quantitative picture of the AIE
process is to measure the change in the fluorescence quan-
tum yield (FF) with the variation in the amount of added
water. However, such measurements are hampered by the
severe light-scattering effects of the dye nano-aggregates on
their absorption spectra, which makes the FF determination
inaccurate.[26] As an alternative, we estimated the integrals
of the PL spectra, which are utilized to make the compari-
son. As for 3a, there was almost no PL intensity in the solu-
tion for the 0–30% (v/v) water/THF mixture, but it started
to increase swiftly upon the addition of water up to 50% (v/
v). When more water is added, the dye emits more intensely,
with a 230-fold increase in the I/I0 ratio achieved at a water
fraction of 90% (v/v) compared 0% (v/v) (Figure 2b). The
trajectory of the I/I0 ratio for 3a suggests that the molecules
start to aggregate at a water fraction of >50%, and that the
size and population of the nano-aggregates (shown in Fig-
ure 2d) continue to increase as the water fraction is in-
creased.
Similar effects are observed for 3b. A dilute solution of
3b in THF gives very weak PL, but in an aqueous mixture
with 90% water it emits a strong orange luminescence with
a 70-fold increase in the I/I0 ratio (Figure 2e), which means
that there is an increase in nano-aggregations (Figure 2 f)
and that 3b also has AIE activity. However, 3c is different
from 3a and 3b. Although we can detect its deep yellow
fluorescence emission signal in THF, the photoluminescent
intensity also increases with the addition of water into the
solution of 3c in THF. Compound 3c exhibits aggregation-
induced emission enhancement (AIEE) activity. The solu-
tion for the 50:50ACTHNUTRGNEUNG(v/v) water/THF mixtures displays a dra-
matic enhancement of luminescence. When more water is
added to the dye, it emits more intensely, with a five-fold in-
crease in the I/I0 ratio achieved at a water fraction of 90%
(v/v) (Figure 2j), suggesting that more and more nanoparti-
cles are produced (Figure 2l). This result indicates that, by
changing the triarylamine donor moiety, the CNDPASB-
based compounds show AIE or AIEE performance, in
which they exhibit strongly enhanced red, orange, and deep
yellow fluorescence emissions.
Two-photon absorption properties: The 2PA cross sections
of the 3a–c were determined by a femtosecond open-aper-
ture Z-scan technique, according to the method described
previously.[12] Figures 3a,c,e show their data for the open-
aperture Z-scan, and the 2PA coefficient obtained by data
fitting. The 2PA cross section s can be calculated by using
the equation s=hnb/N0, in which N0 =NAC is the number
density of the absorption centers, NA is the Avogadro con-
stant, and C represents the solute molar concentration. The
values of the 2PA cross section (s) for 3a–c are 1016, 1484,
and 814 GM, respectively, at a wavelength of 800 nm. The
high 2PA properties of molecules 3a–c are attributed to the
extended p system and enhanced intramolecular charge
transfer (ICT) from the triarylamine to the cyano group.
Figure 2. a), e), i) PL spectra of 3a–c, respectively, in THF/water mix-
tures. b), f), j) Plots of PL peak intensity versus water content of the sol-
vent mixture for 3a–c, respectively. c), g), k) Absorptions of 3a–c, respec-
tively, in THF and in a dispersion of the nano-aggregate form (90%
water) at concentrations of 1ꢁ10À5 m. d), h), l) SEM images of 3a–c, re-
spectively, nano-aggregates prepared in THF/H2O (1:9 v/v) at concentra-
tions of 1ꢁ10À5 m.
reduce the intermolecular dipole coupling because of the in-
creased intermolecular distance in the aggregate. A repre-
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Triarylamine Quadrupolar Derivatives
FULL PAPER
overlap between the one- and two-photon excitation fluores-
cence indicates that the emissions result from the same ex-
cited state, regardless of the different modes of excitation.
Figures 4c,f,i show their nano-aggregates suspended in aque-
ous media, emitting intense red, orange, and deep yellow
light, respectively, when excited with near-IR light (lex =
800 nm) to absorb two photons.
Figure 3. a), c), e) Open-aperture Z-scan traces of 3a–c, respectively,
(scattered circles are the experimental data, and the straight line is the
theoretical fitted data). b), d), f) Two-photon fluorescence emission spec-
tra for 3a–c, respectively, in THF and in a dispersion of the nano-aggre-
gate form (90% water) at
800 nm.
a
concentration of 1ꢁ10À5 m, excited at
Owing to the electron-donating strength (diphenyl-
ACHTUNGTRENNUNG
Figure 4. a), d), g) Photos of 3a–c, respectively, in the THF/water mix-
tures with 0% and 90% water contents, taken under illumination with
UV light. Dye concentration: 10À5 m; excitation wavelength: 350 nm. b),
e), h) On/off fluorescence switching of 3a–c, respectively, on TLC plates
in chloroform vapor (left) and without vapor (right) under UV light
(365 nm) illumination at room temperature. c), f), i) TPF emission images
of 3a–c, respectively, in the THF/water mixture (0 and 90% water).
Electrochemical properties: Figure 5 shows the cyclic vol-
tammetry (CV) results of the compounds using 0.1m tetra-
butylammonium hexafluorophosphate as supporting electro-
lyte in acetonitrile, with platinum-button working electrodes,
a platinum-wire counter electrode, and an SCE reference
electrode. The SCE reference electrode was calibrated using
the ferrocene/ferrocenium (Fc/Fc+) redox couple as an ex-
ternal standard. The electrochemical properties, as well as
the energy level parameters of the CNDPASB-based com-
pounds, are listed in Table 1. As shown in Figure 5, 3a ex-
hibits two pairs of quasi-reversible redox peaks (0.77/0.67
and 0.93/0.86), corresponding to two triphenylamine units in
different chemical environments. Compounds 3b and 3c
each exhibit one pair of quasi-reversible redox peaks at
0.89/0.85 and 1.12/1.04, respectively. It can be speculated
from Figure 5 that the first half-wave potentials (Eox) of 3a–
c are 0.72, 0.87, and 1.08 V, respectively. Therefore, the
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J. Hua, H. Tian et al.
with a UV lamp at room temperature. The fluorescence
turns off reversibly in an atmosphere of dichloromethane
vapor. This may be due to the fact that solvation of the
vapor of a good solvent releases the interaction of solid mol-
ecules to a greater extent, and causes free rotation of the
single bonds of the AIE molecules, leading to non-emission.
The same behavior was found for 3b (Figure 4e). However,
3c with AIEE properties did not show this behavior (Fig-
ure 4h), so this compound may not be very suitable for on/
off fluorescence switching applications.
In addition, AIE materials can also show good perform-
ances in organic light-emitting diodes (OLEDs), in which
the most common materials are doped dyes. When these
materials are doped in high concentrations, self-quenching
of the fluorescence takes place, leading to weak lumines-
cence which affects the performance of the OLEDs. Fortu-
nately, AIE materials overcome the self-quenching of fluo-
rescence in the aggregation, and have great potential for ap-
plication in OLEDs.
To investigate the electroluminescent (EL) performance
of those dyes, we selected 3a and 3c, which were made into
multilayer emitting devices. Both 3a and 3c are used as
non-doped emitters, which are made into EL devices. The
detailed device architectures of the two devices are as fol-
lows: ITO/CFx/3a (40 nm) or 3c (28 nm)/TPBI (40 nm)/LiF
(1 nm)/Al (100 nm), in which indium tin oxide (ITO) was
used as the anode, CFx was the hole-injection layer, the dye
(3a/3c) was the light-emitting layer, TPBI (2,2’,2’’-(benzene-
1,3,5-triyl)-tri(1-phenyl-1H-benzimidazole)) was the elec-
tron-transporting layer, LiF (1 nm) was the electron-injec-
tion layer, and Al (100 nm) was used as the cathode.
Figure 5. Cyclic voltammograms of 3a–c in THF containing 0.1 molLÀ1 of
tetrabutylammonium hexafluorophosphate (nBu4NPF6).
Table 1. Electrochemical properties of 3a–c.
[c]
EHOMO[a] [eV]
E0À0[b] [eV]
ELUMO [eV]
lcutoff [nm][d]
3a
3b
3c
À5.42
À5.57
À5.78
2.10
2.27
2.38
À3.42
À3.40
À3.50
588
546
520
[a] EHOMO values were measured in acetonitrile with 0.1m tetrabutyl-
ammonium hexafluorophosphate (TBAPF6) as the electrolyte (working
electrode: Pt; reference electrode: SCE; calibrated with ferrocene/
ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
The detailed EL performances of 3a and 3c are summar-
wire). [b] E0À0 was estimated from the intercept of the normalized ab-
sorption and emission spectra. [c] ELUMO was estimated by subtracting
E0À0 from the HOMO. [d] Estimated from the absorption thresholds.
ized in Table 2. A red-light emission was observed at lmax
=
644 nm for 3a, with a full-width at half-maximum (FWHM)
Table 2. Performance of the EL devices made with 3a and 3c.
ground-state oxidation potential corresponding to the
HOMO energy levels are À5.42, À5.57, and À5.78 eV (vs.
vacuum), respectively, according to the equation HOMO=
[b]
[c]
[f]
l
[a] [nm] Von [V] hext
hc[d] [cdAÀ1
]
CIE[e] (x, y) hp [lmWÀ1
]
3a 644
3c 568
10.2
9.53
0.77 0.67
0.33 0.83
(0.61, 0.38) 0.22
(0.46, 0.50) 0.25
Àe
ACHTUNGTRENNUNG
(Eox+4.7).[27] It is clear that the HOMO levels of 3a are
[a] At 20 mAcmÀ2. [b] Turn-on voltage. [c] Maximum external quantum
efficiency. [d] Maximum current efficiency. [e] CIE space values (Com-
mission internationale de l’ꢂclairage or International Commission on Illu-
mination) at 20 mAcmÀ2. [f] Maximum power efficiency.
higher by 0.15 and 0.36 eV than those of 3b and 3c, respec-
tively, which is in agreement with the order of the increase
of electron-donating ability of the donors: N,N-dimethylani-
line>phenothiazine>carbazole. Generally, a stronger elec-
tron-donating ability of the donor results in a higher
HOMO energy level. It was found that the HOMO and
LUMO energy levels can be tuned conveniently by changing
the donor moiety, as confirmed by electrochemical measure-
ments.
of about 100 nm, whereas 3c showed a main EL peak at
568 nm with a FWHM of about 150 nm (Figure 6a). Com-
pound 3c exhibited a deep yellow emission, and the spectral
profile changed little with the increase of current density
(Figure 6b). Moreover, 3c showed a good stability of the
light color, whereas the performance of 3a in this respect
was worse. The current density and luminous intensity of 3a
and 3c increased with the increase of voltage. The luminous
intensity of 3c reached 858 cdmÀ2 at ꢀ20 V (Figure 6c).
The current efficiencies attained by the EL devices for 3a
and 3c were 0.67 and 0.83 cdAÀ1, respectively, equivalent to
power efficiencies of 0.22 and 0.25 lmWÀ1, respectively. The
Applications of AIE materials: It is well known that AIE
materials possess the on/off fluorescence switching property
in some organic vapors; it is therefore suggested that one of
their most important potential applications could be in the
photo-switching field as a chemical vapor sensor. As shown
in Figure 4b, a thin layer of 3a developed on a TLC silica
plate emits bright and deep red light under illumination
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Chem. Eur. J. 2011, 17, 2647 – 2655

Triarylamine Quadrupolar Derivatives
FULL PAPER
orange, and deep yellow light,
respectively, when excited with
near-IR light (lex =800 nm) to
absorb two photons. The result
indicates that the color tuning
of these CNDPASB-based com-
pounds can be conveniently ac-
complished by changing the tri-
arylamine donor moiety. The
nonlinear absorption cross sec-
tion of the 3b derivative is as
large as 1484 GM, so this fluor-
ogen is a promising candidate
for biosensing applications.
Experimental Section
Materials: THF was pre-dried over
4 ꢃ molecular sieves, and distilled
under an argon atmosphere from
sodium benzophenone ketyl immedi-
ately prior to use. DMF was refluxed
with calcium hydride and distilled
before use. All other reagents and
chemicals were obtained from com-
mercial sources (J&K, Aldrich), and
used without further purification.
Figure 6. a) EL spectra of the devices: c: ITO/CFx/3a (40 nm) and b: 3c (28 nm)/TPBI (40 nm)/LiF
(1 nm)/Al (100 nm); b) EL spectra of 3c with different current densities; 5.0 mA; & 20.0 mA; 50.0 mA;
~
!
&
100.0 mA;
"
250.0 mA; c) Current-density–voltage–luminance characteristics of the device: ITO/CFx/3c
&
(28 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm); currenty density; & luminance d) Current efficiency of 3c.
maximum external quantum efficiencies were 0.77 and
0.33% for 3a and 3c, respectively (Table 2). The poor effi-
ciencies of these two devices may be due to the imbalance
of the charge carrier injection. This could also be discussed
based on the energy levels of the LUMO and HOMO. The
HOMO and LUMO energy levels obtained from electro-
chemical and UV/Vis spectra are À5.42/À3.42 and À5.78/
À3.50 eV for 3a/3c, respectively. The HOMO levels of the
two compounds match well with the energy level of ITO
glass, which would favor hole injection from the anode.
However, the LUMO levels of the compounds, which act as
the electron trap, would not favor electron injection. This
may be why the performance of the multilayer devices was
poor. Although the performance of the electroluminescence
(EL) devices based on the CNDPASB derivatives reported
here is only modest, a similar AIE compound used for a
white EL device with high performance was recently report-
ed by the Ma group.[28] The optimization of the OLED devi-
ces made with 3a and 3c is still under investigation.
Instrumentation: 1H and 13C NMR spectra were recorded on a Bruker
AM-400 spectrometer using [D]chloroform as solvent and tetramethylsi-
lane (d=0 ppm) as the internal reference. Mass spectra were recorded
on an ESI mass spectrometer. FTIR spectra were recorded on a Nicolet
Magna-IR550 spectrometer. The UV/Vis spectra were measured with a
CARY 100 spectrophotometer. The fluorescence spectra were taken on a
Varian-Cary fluorescence spectrophotometer. The cyclic voltammograms
of compounds were obtained using a Versastat II electrochemical work-
station (Princeton applied research), using a normal three-electrode cell
with a Pt working electrode, a Pt wire counter electrode, and a regular
calomel reference electrode in saturated KCl solution. The 2PA cross sec-
tions were measured by a femtosecond (fs) open-aperture Z-scan tech-
nique according to a previously described method.[12] Two-photon excited
fluorescence (TPF) was achieved using femtosecond pulses with different
intensities at a wavelength of 800 nm. The repetition rate of the laser
pulses was 250 kHz, and the pulse duration was 80 fs.
Device fabrication: Prior to the deposition of organic materials, the
indium-tin-oxide (ITO)/glass was cleaned using a routine cleaning proce-
dure and pretreated with oxygen plasma, then coated with a polymerized
fluorocarbon (CFx) film. Devices were fabricated under about 10À6 Torr
base vacuum in a thin-film evaporation coater following a published pro-
tocol.[29] The current–voltage–luminance characteristics were measured
with a diode array rapid-scan system using a Photo Research PR650
spectrophotometer and a computer-controlled, programmable, direct-cur-
rent (DC) source. All measurements were carried out in an ambient at-
mosphere at room temperature.
Conclusions
4-ACTHNUTRGNEUG[N N,N-Di(4-iodophenyl)amino]benzaldehyde (1): 4-(N,N-Diphenylami-
no)benzaldehyde (8 g, 29.3 mmol), potassium iodide (9.7 g, 58.6 mmol),
acetic acid (170 mL), and distilled water (15 mL) were heated to 808C,
with stirring maintained for 1 h. Then, potassium iodate (9.4 g,
44.0 mmol) was added and the reaction mixture was stirred for 6 h. The
mixture was poured into distilled water to induce precipitation of the
crude product. The precipitate was filtered off and washed with petrole-
um to give 12.9 g (84%) of yellow solid. 1H NMR (CDCl3, 400 MHz,
TMS): d=9.84 (s, 1H), 7.71 (d, J=8.0 Hz, 2H), 7.63 (d, J=8.0 Hz, 4H),
7.05 (d, J=8.0 Hz, 2H), 6.89 ppm (d, J=8.0 Hz, 4H).
Three new starburst triarylamine derivatives, based on
cyano-substituted
diphenylamine
styrylbenzene
(CNDPASB), 3a–c, have been designed and synthesized
through a concise and environmentally friendly procedure.
They are non-emissive when dissolved in a “good” solvent
such as THF. However, their nano-aggregates suspended in
aqueous media (e.g., 9:1 v/v H2O/THF) emit intense red,
Chem. Eur. J. 2011, 17, 2647 – 2655
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2653

J. Hua, H. Tian et al.
4-
{N,N-Bis[4-(N,N-diphenylamino)phenyl]amino}benzaldehyde
(2a):
116.6, 115.1, 113.3 ppm; HRMS (ESI): m/z calcd for C96H62N8S4 [M]:
1455.4059; found: 1455.4066.
Compound 1 (3 g, 5.7 mmol), diphenylamine (2.9 g, 17.1 mmol), copper
powder (1.6 g, 25.7 mmol), potassium carbonate (6.76 g, 49.0 mmol), and
[18]crown-6 (11.1 mg, 0.042 mmol) were heated in 1,2-dichlorobenzene
(100 mL) at 1808C for 48 h under an atmosphere of argon. The inorganic
components were removed by filtration after cooling. Then the solvent
was distilled under reduced pressure, and the crude product was purified
by column chromatography on silica (CH2Cl2/petroleum ether=1:3, v/v)
to give a golden yellow solid. Yield: 63%; 1H NMR (CDCl3, 400 MHz,
TMS): d=()=9.77 (s, 1H,), 7.74 (d, J=9.0 Hz, 2H), 7.35 (t, J=8.0 Hz,
8H), 7.18 (d, J=9.0 Hz, 4H), 7.05–7.12 (m, 12H), 7.03 (d, J=9.0 Hz,
4H), 6.92 ppm (d, J=9.0 Hz, 2H); MS (EI): m/z calcd calcd for
C43H33N3O [M]: 607.3; found: 607.2.
2-{4-[1-Cyano-2-(4-
phenyl}-3-(4-{N,N-bis[4-(N-carbazolyl)phenyl]amino}phenyl)acrylonitrile-
AHCTUNGERTG(NNUN 3c): A mixture of compound 2c (400 mg, 0.66 mmol), 2,2’-(1,4-phenyle-
ACHTUNGTREN{NUNG N,N-bis[4-(N-carbazolyl)phenyl]amino}phenyl)vinyl]-
AHCTUNGTRENNUNG
ne)diacetonitrile (50 mg, 0.32 mmol), and potassium tert-butoxide
(360 mg, 3.2 mmol) was dissolved in methanol (50 mL). This mixture was
stirred for 24 h at reflux temperature under an atmosphere of argon.
Upon cooling, a yellow solid precipitate was filtered off, washed with
cold methanol, and purified by column chromatography on silica
(CH2Cl2/petroleum ether=1:1, v/v) to give a yellow solid. Yield: 37%;
1H NMR (CDCl3, 400 MHz, TMS): d=8.17 (d, J=8.0 Hz, 8H), 7.91–7.97
(m, 4H), 7.71–7.77 (m, 4H), 7.56–7.61 (m, 8H), 7.42–7.50 (m, 26H),
7.25–7.33 ppm (m, 12H); 13C NMR (CDCl3, 100 MHz, TMS): d=144.4,
139.8, 132.9, 130.1, 127.3, 125.5, 125.3, 125.0, 122.4, 119.4, 119.0,
108.8 ppm; HRMS (ESI): m/z calcd for C96H62N8 [M]: 1327.5176; found:
1327.5167.
4-
ACHTUNGTRENNUNG
pound
1
powder (2.2 g, 34.2 mmol), potassium carbonate (9 g, 65.4 mmol), and
[18]crown-6 (15 mg, 0.056 mmol) were heated in 1,2-dichlorobenzene
(100 mL) at 1808C for 48 h under an atmosphere of argon. The inorganic
components were removed by filtration after cooling. Then the solvent
was distilled under reduced pressure, and the crude product was purified
by column chromatography on silica (CH2Cl2/petroleum ether=1:3, v/v)
to give a golden yellow solid. Yield: 50%; 1H NMR (CDCl3, 400 MHz,
TMS): d=9.88 (s, 1H), 7.88 (d, J=9.0 Hz, 2H), 7.47 (d, J=4.0 Hz, 8H),
7.28 (d, J=9.0 Hz, 2H), 7.14 (d, J=8.1 Hz, 4H), 7.02–7.06 (m, 4H), 6.91–
6.95 (m, 4H), 6.44 ppm (d, J=8. 1 Hz, 4H); MS (EI): m/z calcd for
C43H29N3OS2 [M]: 667.2; found: 667.2.
Acknowledgements
This work was supported by NSFC/China (20772031), the National Basic
Research 973 Program (2006CB806200), the Fundamental Research
Funds for the Central Universities (WJ0913001), and the Scientific Com-
mittee of Shanghai (10520709700).
4-ACHTUNGTRENNUNG{N,N-Bis[4-(N-carbazolyl)phenyl]amino}benzaldehyde (2c): Compound
1 (3.5 g, 6.7 mmol), carbazole (3.3 g, 20 mmol), copper powder (1.92 g,
30 mmol), potassium carbonate (8 g, 58.0 mmol), and 18-crown-6 (13 mg,
0.049 mmol) were heated in 1,2-dichlorobenzene (100 mL) at 1808C for
48 h under an atmosphere of argon. The inorganic components were re-
moved by filtration after cooling. Then the solvent was distilled under re-
duced pressure, and the crude product was purified by column chroma-
tography on silica (CH2Cl2/petroleum ether=1:3, v/v) to give a yellow–
green solid. Yield: 33%; 1H NMR (CDCl3, 400 MHz, TMS): d=9.90 (s,
1H), 8.16 (d, J=8.0 Hz, 4H), 7.83 (d, J=8.0 Hz, 2H), 7.60(d, J=8.0 Hz,
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C43H29N3O [M]: 603.2; found: 603.2.
2-{4-[1-Cyano-2-(4-{N,N-bis[4-(N,N-diphenylamino)phenyl]amino}phe-
nyl)vinyl]phenyl}-3-(4-
ACHTUNGTRENNUNG
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phenyl)acrylonitrile (3a):
A
1.2 mmol), 2,2’-(1,4-phenylene)diacetonitrile (78 mg, 0.5 mmol), and po-
tassium tert-butoxide (560 mg, 5 mmol) was dissolved in methanol
(50 mL). This mixture was stirred for 24 h at reflux temperature under an
atmosphere of argon. Upon cooling, a red solid precipitate was filtered
off, washed with cold methanol, and purified by column chromatography
on silica (CH2Cl2/petroleum ether=1:1, v/v) to give a red solid. Yield:
31%; 1H NMR (CDCl3, 400 MHz, TMS): d=7.81 (d, J=8.2 Hz, 4H),
7.65–7.69 (m, 4H), 7.44–7.46 (m, 4H), 7.26–7.29 (m, 2H), 7.12–7.19 (m,
20H), 7.01–7.05 ppm (m, 36H); 13C NMR (CDCl3, 100 MHz, TMS): d=
143.6, 129.9, 129.0, 128.9, 128.2, 127.9, 127.3, 127.1, 126.0, 125.8, 125.0,
124.8, 123.5, 123.3, 121.9, 120.3, 119.0, 117.4 ppm; HRMS (ESI): m/z
calcd for C96H70N8 [M]: 1334.5723; found: 1334.5768.
2-{4-[1-Cyano-2-(4-
ACHTUNGTRENNUNG{N,N-bis[4-(N-phenothiazinyl)phenyl]amino}phenyl)-
vinyl]phenyl}-3-(4-{N,N-bis[4-(N-phenothiazinyl)phenyl]amino}phenyl)a-
ACHTUNGTRENNUNG
crylonitrile (3b): A mixture of compound 2b (500 mg, 0.75 mmol), 2,2’-
(1,4-phenylene)diacetonitrile (52 mg, 0.33 mmol), and potassium tert-but-
oxide (370 mg, 3.3 mmol) was dissolved in methanol (50 mL). This mix-
ture was stirred for 24 h at reflux temperature under an atmosphere of
argon. Upon cooling, an orange solid precipitate was filtered off, washed
with cold methanol, and purified by column chromatography on silica
(CH2Cl2/petroleum ether=1:1, v/v) to give an orange solid. Yield: 30%;
1H NMR (CDCl3, 400 MHz, TMS): d=7.86 (d, J=8.0 Hz, 4H), 7.63–7.68
(m, 4H), 7.45–7.50 (m, 2H), 7.26–7.39 (m, 20H), 6.98–7.00ACTHNUGTRNEUNG(m, 10H),
6.85–6.88 (m, 12H), 6.77–6.81 ppm (m, 10H); 13C NMR (CDCl3,
100 MHz, TMS): d=145.6, 144.4, 142.5, 137.7, 136.1, 132.4, 129.4, 128.7,
128.2, 127.6, 127.0, 126.6, 125.1, 124.9, 123.8, 121.7, 120.6, 120.0, 118.6,
2654
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2647 – 2655

Triarylamine Quadrupolar Derivatives
FULL PAPER
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Received: October 1, 2010
Published online: January 24, 2011
Chem. Eur. J. 2011, 17, 2647 – 2655
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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