Synthesis, aggregation-induced emission (AIE) and electroluminescence of carbazole-benzoyl substituted tetraphenylethylene derivatives

Article abstract of DOI:10.1016/j.dyepig.2019.107898

Aggregation-induced emission (AIE) materials offer a solution to obtain high fluorescence efficiencies in solid state which is desirable for optoelectronic devices and display. Here in this work, three new types of D-π-A-π-D AIE luminogens based on carbazole-benzoyl substituted tetraphenylethylene (TPE) derivatives were designed and successfully synthesized. Meanwhile, their photophysical properties as well as electroluminescence (EL) characteristics were studied. These luminogens show good AIE property with high solid-state fluorescence quantum yields. Furthermore, the emission wavelengths for these three luminogens, both in solutions and in solid state, exhibit negligible variation by increasing the number of donor group. All three luminogens show solvatochromism and negative solvatokinetic effect in different solvents. More interesting is that on increasing number of donor groups, their thermal and morphological stability present regular change by increasing the number of carbazole-benzoyl group. The performance of non-doped OLED device based on TPE-3BZ-Cz is superior than the other two luminogens, which has been proved by further electroluminescence evaluation.

Full text of DOI:10.1016/j.dyepig.2019.107898

DyesandPigments173(2020)107898
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, aggregation-induced emission (AIE) and electroluminescence of
carbazole-benzoyl substituted tetraphenylethylene derivatives
Zhan Yingying1, Yang Zhiwen1, Tan Jihua, Qiu Zhipeng, Mao Yuanyou, He Jia, Yang Qingdan,
Ji Shaomin∗∗, Cai Ning, Huo Yanping∗
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Aggregation-induced emission (AIE) materials offer a solution to obtain high fluorescence efficiencies in solid
state which is desirable for optoelectronic devices and display. Here in this work, three new types of D-π-A-π-D
AIE luminogens based on carbazole-benzoyl substituted tetraphenylethylene (TPE) derivatives were designed
and successfully synthesized. Meanwhile, their photophysical properties as well as electroluminescence (EL)
characteristics were studied. These luminogens show good AIE property with high solid-state fluorescence
quantum yields. Furthermore, the emission wavelengths for these three luminogens, both in solutions and in
solid state, exhibit negligible variation by increasing the number of donor group. All three luminogens show
solvatochromism and negative solvatokinetic effect in different solvents. More interesting is that on increasing
number of donor groups, their thermal and morphological stability present regular change by increasing the
number of carbazole-benzoyl group. The performance of non-doped OLED device based on TPE-3BZ-Cz is su-
perior than the other two luminogens, which has been proved by further electroluminescence evaluation.
Aggregation-induced emission
Photoluminescence
Electroluminescence
Organic light-emitting diodes
Carbazole-benzoyl substituents
1. Introduction
strongly emissive in aggregate formation [12]. Recently, It has become
a tendency to exploit AIE molecules into the fabrication of non-doped
It is well-known that organic luminescence molecules have captured
considerable attention in both commercial and scientific area owing to
its huge potential in applications of biomedical imaging, chemical
sensing, optoelectronic devices and display [1–6]. Among all these
fields, organic light-emitting diodes (OLEDs) is in particularly entitled
to have fastest growing rate both in market and research [7]. Typically,
OLEDs consist of multiple layer organic thin films fabricating by the
methods like spin-coating [8,9], ink-jet printing [10] and vacuum-de-
position [9,11] etc. Organic molecules aggregate naturally during the
device fabrication process. However, for conventional planar lumino-
gens, aggregation is detrimental to light emission as a result of the
aggregation-caused quenching (ACQ) effect [12–14]: emission in con-
densed state is much weaker or thoroughly quenched compared with
that in dilute solution, which seriously limits their applications [15]. To
overcome this problem, a new type of organic luminescence molecules
showing aggregation-induced emission (AIE) phenomenon was firstly
discovered by Tang and co-workers in 2001 [16]. In contrast to ACQ,
AIE molecules are usually weak or non-emissive in solutions but
OLEDs to improve the device efficiency and simplify the structure si-
multaneously [17–21]. To date, various of AIE active cores that have
been synthesized and reported such as siloles [22], pyrene [23,24],
tetraarylethenes [25], phosphindole oxide [26], tetraphenylethylene
(TPE) [27], thieno [3,2-b]thiophene S,S-dioxide (TTDO) [28] and so on.
Among them, TPE derivatives have grabbed more attention because of
their high solid-state fluorescence quantum yields, easy synthesis and
facile functionalization. In general, the main approach to develop TPE-
like based AIEgens is either to replace the phenyl rings of TPE by
polycyclic aromatic hydrocarbons or modify the phenyl rings with
different substituents. Both of them lead to the change of spatial
structure and electronic characteristics of the whole molecule. Up to
now, TPE-derivative-based OLEDs have made a tremendous break-
through in improvement of device efficiency, stability, and longevity
[29,30]. Unfortunately, problems still exist, due to the poor carrier
mobility of TPE core, to solve the issue, it is necessary to add carrier
transporting layer into the OLED device which may complicates the
device structure and increasing the cost. Thus it is of great importance
∗ Corresponding author.
∗∗ Corresponding author.
E-mail addresses: smji@gdut.edu.cn (S. Ji), yphuo@gdut.edu.cn (Y. Huo).
1 Yingying Zhan and Zhiwen Yang contributed equally to this work.
https://doi.org/10.1016/j.dyepig.2019.107898
Received 6 June 2019; Received in revised form 16 September 2019; Accepted 17 September 2019
Availableonline18September2019
0143-7208/©2019PublishedbyElsevierLtd.

Y. Zhan, et al.
DyesandPigments173(2020)107898
to develop more TPE-like based AIEgens with better carrier trans-
porting ability. In terms of this, D-π-A-π-D framework is regarded as an
efficient strategy for molecule designing to solve the problem [31–33].
In general, D-π-A-π-D framework constituted by two electron donor
moieties with π bridges coupled with an electron acceptor group. The
strong intramolecular charge transfer (ICT) process can lower the band
gap between the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) thus resulting in bath-
ochromic-shift of emission. And this phenomenon get facilitated with
strengthened ability of the electron donor/acceptor group [34–37]. Xue
et al. reported a novel TPE-based luminogens, with AIE properties and
the corresponding compound was used in OLED device. This new
compound shows a desirable deep-blue emission and excellent elec-
troluminescence which can be attributed to the carefully select of
donor, acceptor groups and bonding positions [38]. So it is essential to
induce proper donor and acceptor group that can improve the carrier
transporting ability of the molecule while prevent large bathochromic
emission shift simultaneously.
2.2. Theoretical calculations
The ground-state geometric optimization of all studied luminogens
were performed by density functional theory (DFT) and time-dependent
DFT (TDDFT) at the level of B3LYP/6-31G(d) using Gaussian 09 soft-
ware package and the compositions of molecular orbits were analyzed
using the Gauss View 5.0 software [45,46].
2.3. Synthesis
2.3.1. Synthesis of tetraphenylethene
Tetraphenylethene was synthesized according to a standard proce-
dure [47]. A suspension of benzophenone (1.67 g, 10 mmol), Zinc
(1.97 g, 30 mmol) in 100 mL dried THF was stirred under nitrogen at-
mosphere at 0 °C. TiCl4 (1.67 mL, 20 mmol) was injected into the
system slowly. After addition, the mixture was stirred at −10 °C for
30 min before warming to room temperature, and then stirred at 70 °C
for 12 h. After cooling down to room temperature, the reaction was
quenched by aqueous solution of potassium carbonate and extracted
with dichloromethane (3 × 50 mL). The organic phase was combined,
washed with brine, dried over anhydrous sodium sulfate and further
purified by column chromatography (silica gel, petroleum ether: ethyl
acetate = 20:1), to afford the desired product as white solid (1.56 g,
94%). 1H NMR (400 MHz, CDCl3) δ ppm: 7.02 (m, 8H), 7.09–7.11 (m,
12H).
Thus in this context, we designed and synthesized three new D-π-A-
π-D type luminogens TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz bearing
one, two, and three carbazole-benzoyl as substituents respectively. It is
known that carbazole is an ideal electron donor which can significantly
improve the hole-transporting ability of molecule [39,40]. Besides, its
rigid planar feature is favorable for forming twisted spatial conforma-
tion with the related molecule, which sometime generates twisted in-
tramolecular charge transfer (TICT) [41]. On one hand, this is bene-
ficial for the separation of HOMO and LUMO which endows them with
unique and interesting optical and optoelectronic properties, on the
other hand, TICT is believed to result in effective depression of close
packing and enhancement of emission quantum yields in the solid state
[42–44]. Moreover, the same number of benzoyl is induced as electron
acceptor between the TPE core and carbazole to prevent excessive
charge concentration on the TPE which is obviously deleterious to the
balance of carrier transporting. All the three luminogens show intensive
blueish-green emission in the neat film, and their electroluminescence
characteristics were also preliminarily evaluated.
2.3.2. Procedures for Friede-crafts acylation
1-(4-Fluoro-Phenylcarbonyl)-1,2,2-triphenylethylene (TPE-BZ-F):
Tetraphenylethene (300 mg, 1.0 mmol) and aluminum chloride
(160 mg,1.2 mmol) were added in dry CS2 (10 mL), then a solution of 4-
Fluorobenzoyl chloride (190 mg, 1.2 mol) was added dropwise with
stirring. The suspension mixture was stirred for 5 h at room tempera-
ture, under nitrogen atmosphere. After completion of the reaction, the
mixture was stirred with in an ice bath while water was cautiously
added. The reaction mixture was then extracted with dichloromethane
(3 × 30 mL). The organic phase was combined, washed with diluted
aqueous sodium hydroxide, dried over anhydrous sodium sulfate and
further purified by column chromatography (silica gel, petroleum ether:
ethyl acetate = 15:1) to afford the compound TPE-BZ-F as yellow solid
(380 mg, 80%). 1H NMR (400 MHz, CDCl3) δ 7.71–7.65 (m, 2H), 7.44
(s, 2H), 7.07–6.98 (m, 13H), 6.95 (dt, J = 6.0, 3.5 Hz, 6H). 13C NMR
(101 MHz, CDCl3) δ 194.84, 166.53, 164.01, 148.57, 143.21, 143.18,
143.08, 142.73, 139.88, 135.07, 134.00, 133.97, 132.58, 132.49,
131.61, 131.35, 131.33, 131.28, 129.55, 127.93, 127.76, 126.98,
126.83, 115.50, 115.28. LC-MS m/z: calcd for C33H23FO 455.17, [M +
H]+, Found 455.15.
2. Experiment section
2.1. Materials and characterization methods
All the reagents and solvents were obtained as analytical-grade from
commercial suppliers and used without further purification. The 1H and
13C NMR spectra were recorded on a Bruker AVANCZ II 400 MHz
spectrometer using deuterated chloroform (CDCl3) and dimethyl sulf-
oxide (DMSO) as solvent. Chemical shifts are reported in ppm relative
to the indicated residual solvent. All coupling constants (J values) are
reported in hertz (Hz). Mass spectra were obtained on a Thermo Fisher
TSQ Endura™ Mass spectrometer. Photoluminescence (PL) spectra, ab-
solute fluorescence quantum yields were measured with an Edinburgh
FLS980 spectrometer. All UV–vis absorption spectra of the samples
were recorded using Shimadzu UV-2700 UV–vis Spectrophotometer.
Differential Scanning Calorimeter (DSC) and Thermogravimetric
Analysis (TGA) were performed by NETZSCH STA449F5 Jupiter
Synchronous thermal analyzer. Dynamic light scattering (DLS) were
conducted on a Malvern Zetasizer NANO ZS. Cyclic voltammetry (CV)
was recorded at 0.1 V/s in a Autolab PGSTAT302 N electrochemical
workstation with a three-electrode cell, Ag/AgNO3 referenced against
ferrocene/ferrocenium (Fc/Fc+), 0.18 eV and glassy carbon electrode
as working electrode. Column chromatographic separations were car-
ried out on silica gel (200–300 mesh). TLC was performed by using
commercially prepared 100–400 mesh silica gel plates (GF254) and
visualization was effected at 254 nm.
1,1-Bis(4-Fluoro-Phenylcarbonyl)-2,2-diphenylethylene
(TPE-
2BZ-F): Synthesis steps are the same as for compound TPE-BZ-F:
Tetraphenylethene (300 mg, 1.0 mmol), aluminum chloride (267 mg,
2.0 mmol), 4-Fluorobenzoyl chloride (317 mg, 2.0 mmol) in dry CS2
(15 mL), 7 h-stirring at room temperature. A yellow solid (403 mg 70%)
was obtained after purification by column chromatography (silica gel,
petroleum ether: ethyl acetate = 13:1). After synthesis, single crystal
was prepared and single-crystal X-ray diffraction was employed to
confirm its configuration (Fig. S1, Supporting Information) 1H NMR
(400 MHz, CDCl3) δ 7.79 (dd, J = 8.5, 5.4 Hz, 4H), 7.56 (d, J = 8.0 Hz,
4H), 7.15 (dt, J = 13.2, 6.6 Hz, 14H), 7.05 (dd, J = 6.9, 2.7 Hz, 4H).
13C NMR (101 MHz, CDCl3) δ 194.74, 166.58, 164.05, 147.88, 147.75,
144.36, 142.64, 142.48, 141.52, 138.72, 135.58, 135.41, 133.84,
133.81, 132.57, 132.48, 131.31, 131.28, 131.24, 131.22, 129.68,
129.54, 128.13, 127.98, 127.32, 127.18, 115.54, 115.51, 115.32,
115.30. LC-MS m/z: calcd for C40H26F2O2 577.19, [M + H]+, Found
577.18.
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Y. Zhan, et al.
DyesandPigments173(2020)107898
1,1,2-Bis(4-Fluoro-Phenylcarbonyl)-2-phenylethylene (TPE-3BZ-
C83H53N3O3, 1140.41, [M + H]+, Found 1140.59.
F): Synthesis steps are the same as for compound TPE-BZ-F:
Tetraphenylethene (300 mg, 1.0 mmol), aluminum chloride (800 mg,
6.0 mmol), 4-Fluorobenzoyl chloride (950 mg, 6.0 mmol) in ni-
trobenzene (30 mL), 12 h-stirring at 80 °C. A yellow solid (420 mg 60%)
was obtained after purification by column chromatography (silica gel,
petroleum ether: ethyl acetate = 15:1). 1H NMR (400 MHz, CDCl3) δ
7.72 (dd, J = 8.5, 5.4 Hz, 6H), 7.51 (t, J = 7.4 Hz, 6H), 7.13–6.98 (m,
17H). 13C NMR (101 MHz, CDCl3) δ 194.65, 194.59, 194.56, 166.64,
164.11, 147.23, 147.09, 147.06, 143.10, 141.97, 140.31, 135.94,
135.77, 133.73, 133.70, 133.66, 133.63, 132.58, 132.49, 131.28,
131.21, 131.19, 129.83, 129.72, 129.69, 128.23, 127.69, 115.63,
115.61, 115.58, 115.41, 115.39, 115.36. LC-MS m/z: calcd for
2.4. Devices fabrication and measurements
Multilayer OELDs were fabricated by the vacuum-deposition
method. The Indium Tin Oxide (ITO)-coated glass substrates were ul-
trasonically cleaned with deionized water, acetone, detergent, deio-
nized water, and isopropyl. Then, the organic layers were deposited
onto it by high-vacuum (5 × 10−4 Pa) thermal evaporation. All organic
layers were deposited sequentially. Dipyrazino[2,3-f:2′,3′-h]quinoxa-
line-2,3,6,7,10,11-hexacarbonitrile (HAT-CN, 20 nm) was used as hole-
injecting layer. N,N-bis(naphthalene)-N,N-bis(phenyl)benzidine (NPB,
40 nm) was used as the hole-transporting layer. TPE-BZ-Cz, TPE-2BZ-
Cz, and TPE-3BZ-Cz (40 nm) were used as light-emitting layers. 1,3,5-
tri (1-phenyl-1H-benzo [d]imidazole-2-yl)phenyl (TPBi, 40 nm) was
used as the electron-transporting layer and LiF (1 nm)/Al (100 nm) was
used as the cathode. The electroluminescence spectra, the current
density-voltage characteristics and the current density-voltage-lumi-
nance curves characterizations of the OLEDs were recorded by Photo
Research SpectraScan PR-745 Spectroradiometer and Keithley 2450
Source Meter simultaneously.
C
47H29F3O3 699.21, [M + H]+, Found 699.24.
2.3.3. Procedures for nucleophilic substitution reaction
1-(4-Carbazol-Phenylcarbonyl)-1,2,2-triphenylethylene (TPE-BZ-
Cz): A mixture of carbazole (25 mg, 0.15 mmol) and t-BuOK (33.7 mg,
0.30 mmol) in anhydrous DMF (10 mL) was stirred at 65 °C for 10 min
in a round bottom flask, then TPE-BZ-F (47.5 mg, 0.1 mmol) was added.
The reaction mixture was heated to 115 °C and stirred for 15 h. After
completion of the reaction, the resulting mixture was cooled to room
temperature, poured into water and extracted with dichloromethane
(3 × 30 mL). The organic phase was combined, washed with diluted
aqueous sodium hydroxide, dried over anhydrous sodium sulfate and
further purified by column chromatography (silica gel, petroleum ether:
ethyl acetate = 10:1) to afford the compound TPE-BZ-Cz as yellow
solid (480 mg, 80%).1H NMR (400 MHz, CDCl3) δ ppm 8.15 (d,
J = 7.7 Hz, 2H), 8.00 (d, J = 8.1 Hz, 2H), 7.68 (dd, J = 14.0, 8.0 Hz,
4H), 7.51 (d, J = 8.3 Hz, 2H), 7.43 (t, J = 7.7 Hz, 2H), 7.32 (t,
J = 7.4 Hz, 2H), 7.22–7.02 (m,17H). 13C NMR (101 MHz, CDCl3), δ
195.17, 148.72, 143.20, 143.18, 143.09, 142.78, 141.46, 140.31,
139.90, 136.26, 135.01, 131.72, 131.56, 131.39, 131.34, 131.32,
131.27, 131.14, 129.66, 129.45, 127.92, 127.86, 127.74, 127.71,
126.98, 126.88, 126.82, 126.73, 126.26, 126.20, 123.83, 122.22,
120.57, 120.46, 109.80. LC-MS m/z: calcd for C45H31NO 602.24, [M +
H]+, Found 602.3.
3. Results and discussion
3.1. Synthesis and crystal description of compounds
The molecular structures and synthetic routes are illustrated in
Scheme 1 Luminogens TPE-BZ-F, TPE-2BZ-F, and TPE-3BZ-F were
synthesized via Friedel–Crafts acylation according to the literature
procedures [48] and afforded decent yields. Fluorine atoms were then
substituted by carbazole groups and luminogens TPE-BZ-Cz, TPE-2BZ-
Cz, and TPE-3BZ-Cz were obtained via a simple nucleophilic substitu-
tion reaction, the detailed procedures and characterization data are
given in the Experimental Section. To better understand the molecular
structure of the luminogens, single crystal of TPE-BZ-Cz (Fig. 1a) were
prepared by the method: slowly evaporate the solution of TPE-BZ-Cz in
a mixture of n-hexane and THF (1:1, v/v). As shown in Fig. 1b the
carbazole group is at the para position to the benzoyl moiety with a
dihedral angle of 40.408° and there is also a large torsion angles of
49.734° between the phenyl ring of benzoyl and the plane where the
carbonyl and adjacent C–C bonds are located. Such twisted structure
could induce charge separation of the HOMO and LUMO, and also ef-
fectively prevent the fluorescence quenching in solid state caused by
intermolecular stacking. Moreover, there are multiple interactions be-
tween the adjacent TPE-BZ-Cz molecules, including C–H⋯π, C–H⋯O,
C–H⋯N, π⋯π (Fig. S2a b, Supporting Information). It is reasonable to
say the adjacent TPE-BZ-Cz molecules can form dimers with parallel
configurations of carbazole group by intermolecular π⋯π interactions
due to the well-known carbazole's propensity for intermolecular inter-
actions through either fully overlapped (FO) or partially overlapped
(PO) co-facial dimers [49] (Fig. S2c, Supporting Information). The
collective effect of these weak interactions can rigidify molecular
structure and restrict intramolecular motions, which decrease the non-
radiative relaxation and improve the fluorescence efficiency in the solid
state [28,50,51].
1,1-Bis(4-Carbazol-Phenylcarbonyl)-2,2-diphenylethylene (TPE-
2BZ-Cz): Synthesis steps are the same as for compound TPE-BZ-Cz,
carbazole (50 mg, 0.30 mmol), t-BuOK (67.4 mg, 0.60 mmol), DMF
(10 mL), TPE-2BZ-F (650 mg, 0.1 mmol). The reaction mixture was
heated to 115 °C and stirred for 15 h, a yellow solid was obtained
(480 mg, 70%) after further purification by column chromatography
(silica gel, petroleum ether: ethyl acetate = 10:1). 1H NMR (400 MHz,
CDCl3) δ ppm 8.15 (d, J = 7.7 Hz, 4H), 8.02 (d, J = 7.9 Hz, 4H), 7.71
(d, J = 8.2 Hz, 7H), 7.53–7.39 (m, 7H), 7.35–7.07 (m, 20H). 13C NMR
(101 MHz, CDCl3), δ 195.14, 148.09, 147.97, 144.52, 142.69, 141.63,
141.59, 140.28, 138.78, 136.05, 135.98, 135.57, 135.40, 132.74,
131.76, 131.45, 131.34, 131.29, 131.26, 131.18, 129.86, 129.72,
129.57, 128.21, 128.03, 127.39, 126.28, 126.21, 123.85, 120.61,
120.48, 120.44, 115.16, 109.78. LC-MS m/z: calcd for C64H42N2O2,
871.32, [M + H]+, Found 871.3.
1,1,2-Tri(4-Carbazol -Phenylcarbonyl)-2-phenylethylene (TPE-
3BZ-Cz): Synthesis steps are the same as for compound TPE-BZ-Cz,
carbazole (90 mg, 0.50 mmol), t-BuOK (101.0 mg, 0.90 mmol), DMF
(10 mL), TPE-3BZ-F (870 mg, 0.1 mmol). The reaction mixture was
heated to 115 °C and stirred for 15 h, a yellow solid was obtained
(480 mg, 70%) after further purification by column chromatography
(silica gel, petroleum ether: ethyl acetate = 10:1). 1H NMR (400 MHz,
CDCl3) δ 8.33–8.01 (m, 13H), 7.86–7.70 (m, 11H), 7.59–7.40 (m, 9H),
7.38–7.15 (m, 20H). 13C NMR (101 MHz, CDCl3) δ 195.03, 194.97,
194.94, 147.48, 147.34, 147.33, 143.29, 142.10, 141.75, 141.71,
140.47, 140.26, 140.23, 135.99, 135.97, 135.93, 135.86, 135.81,
132.11, 131.89, 131.81, 131.50, 131.42, 131.32, 131.12, 130.10,
129.96, 129.77, 128.36, 127.82, 126.30, 126.25, 123.88, 120.75,
120.68, 120.66, 120.58, 120.54, 120.50, 109.81. LC-MS m/z: calcd for
3.2. Optical properties
The UV–visible absorption spectra and fluorescence spectra of TPE-
BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz both in solutions and solid state
were recorded at 298 K (Fig. 2). The key photophysical data are sum-
marized in Table 1. In THF solutions all luminogens display very similar
absorption characteristic with vibronic-structured band in the range of
280–300 nm which is corresponding to the π–π* transition, while a
weak band in the range of 340–360 nm which can be assigned to the
ICT transition from the carbazole donors to the benzoyl acceptors. The
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DyesandPigments173(2020)107898
Scheme 1. Synthetic routes of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz.
2BZ-Cz. This can be attributed to a minor section of local excited-state
(LE) transition due to the relatively weak ICT in the solution. But with
the increasing number of carbazole-benzoyl group, ICT property of
whole molecule is strengthened, which also benefits the conversion
from LE to charge transfer (CT) states. Consequently, the vibronic
structures of TPE-3BZ-Cz disappeared, a broad emission band with
peak at 430 nm was presented. Overall, the PL intensity is weak and the
fluorescence quantum yields ΦF is too low to detect. This may be mainly
ascribed to that twisted and rotatable conformation of TPE gives rise to
non-radiative intramolecular motion in the solution, which serves as
relaxation channels for excited states and makes them weak emitters.
However, when the luminogens aggregated in spin-coated films, all of
them showed enhanced emission with large bathochromic shift. As
shown in Fig. 2d, the emission bands of TPE-BZ-Cz, TPE-2BZ-Cz, and
TPE-3BZ-Cz appeared at 502 nm, 510 nm, and 509 nm, respectively.
The corresponding peak show 70 nm (TPE-BZ-Cz), 78 nm (TPE-2BZ-
Cz), and 79 nm (TPE-3BZ-Cz) bathochromic shift compared with those
in dilute solution. This phenomenon could be attributed to the
strengthened dipole-dipole interactions. In addition, with the PL in-
tensity enhanced, the ΦF of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz
also increased more than twice to 62.67%, 64.91%, and 66.71%, re-
spectively, demonstrating their AIE properties.
On account of their D-π-A-π-D structures and in order to further
evaluate their ICT effect, we recorded their absorption and fluorescence
spectra in different solvents. As described in Fig. 3a, b, c, the absorption
spectra of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz were barely in-
fluenced by the polarity of solvents. In stark contrast, their fluorescence
spectra behavior changed more clearly. In general, with the polarity
increasing, the fluorescence spectra for all luminogens show a sig-
nificant bathochromic shift, and the emission intensity were also en-
hanced (Fig. 3d, e, f). Such solvatochromic properties verified the
moderate ICT characteristics in these luminogens. As is well known,
there are two different effects for PL intensity of luminogens in polar
solvents, one is a positive solvatokinetic effect, ie, the fluorescence
Fig. 1. a) Crystal structure b) Twisted dihedral angles of TPE-BZ-Cz.
absorption spectra in neat films are similar to those in solutions. But
with the electron-donating ability increasing, a slight bathochromic
shift was observed. As for fluorescence spectra, in dilute THF solution,
TPE-BZ-Cz and TPE-2BZ-Cz exhibit fine vibronic structure with peaks
of 407, 432 nm with the shoulder peak at 451 nm for TPE-BZ-Cz and
with peaks of 407, 432 nm with the shoulder peak at 459 nm for TPE-
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DyesandPigments173(2020)107898
Fig. 2. UV–visible absorption spectra of TPE-BZ-Cz, TPE-2BZ-Cz and TPE-3BZ-Cz in (a) THF solutions (10−5 M) and (b) Spin-coated film, respectively. PL spectra in
(c) THF solutions (10−5 M) and (d) Spin-coated film, respectively. excited at 340 nm.
intensity decreases with increasing polarity of the solvent. While the
other one is a negative solvatokinetic effect, that is, the fluorescence
intensity increases with the polarity of the solvent [52,53]. Apparently,
the solvatochromic properties of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-
3BZ-Cz are in good agreement with negative solvatokinetic effect. It is
worthy to notice that the unexpected strong emission of TPE-3BZ-Cz
recorded in n-hexane could be attributed to the formation of aggregates
owing to its poor solubility in this solvent (Fig. S3, Supporting In-
formation). This viewpoint is also evidenced by the high photon counts
rate of dynamic light scattering (DLS), which is capable of reflecting the
average particle size in the solvent to a certain degree (Table S1, Sup-
porting Information). Generally, the photon counts rate stay low in
organic solvent but significantly increases on the addition of water,
which validates the fluorescence intensity variation is not caused by the
aggregate but the polarity of the solvent. The reason for negative sol-
vatokinetic effect could ascribe to the biradical charge transfer invol-
ving the un-bridged double bonds and the other cause could be related
to the proximity effect for compounds with n–π* and π–π* electron
configurations [54,55].
3.3. Aggregation-induced emission (AIE) studies
In order to explore the AIE activity of TPE-BZ-Cz, TPE-2BZ-Cz, and
TPE-3BZ-Cz, fluorescence spectra in THF/water mixture with different
water fractions (fw) were recorded (Fig. 4). Since all the three lumi-
nogens are almost insoluble in water, adding water to the well-dis-
solved system is beneficial for the formation of aggregates. As depicted
in Fig. 4d, for TPE-2BZ-Cz, TPE-3BZ-Cz, the PL intensity remained
weak as the water fractions under 40%, whereas it increases gradually
with the water fractions going up until reached its maximum at the
f
w = 99%. Situations were a little different with TPE-BZ-Cz, that is, the
PL intensity was not intensified until the water fractions over 80%.
Moreover, its maximum PL intensity at the fw = 99% is the weakest
one. The inset pictures of Fig. 4a–c provide a vivid description to it. In
Table 1
Photophysical properties of TPE-BZs.
TPE-BZs
λabs (nm)
λem (nm)
sola/filmb
Φc(%)
HOMO/LUMOe(eV)
Egf (eV)
Tg/Td (°C)
sola/filmb
sola/filmb
TPE-BZ-Cz
TPE-2BZ-Cz
TPE-3BZ-Cz
340/354
340/349
340/354
496/502
502/510
503/509
20.79/62.67
26.81/64.91
29.73/66.71
−5.74/-2.78
−5.71/-2.75
−5.70/-2.73
2.96
2.96
2.97
85.4/353.6
114.7/524.4
151.7/508.9
a)
In H2O/THF mixtures (10−5 M) with water fraction(fw = 99%).
b)
c)
e)
f)
Spin-coated film on quartz plate.
Fluorescence quantum yield, determined by a calibrated integrating sphere.
Determined by CV measurement in solutions.
Optical bandgap calculated from the onset of absorption spectrum.
5

Y. Zhan, et al.
DyesandPigments173(2020)107898
Fig. 3. UV–visible absorption spectra (a), (b), (c) and PL spectra (d), (e), (f) of TPE-BZ-Cz (a)(d), TPE-2BZ-Cz (b)(e), and TPE-3BZ-Cz (c)(f), in different solvents with
varying polarities. Concentration: 10−5 M. The PL spectra maximum of each solution was chosen as its excitation wavelength.
fact, the maximum PL intensity of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-
3BZ-Cz at the fw = 99% showed a regular change which is consistent
with the amount of carbazole-benzoyl substitutes. In molecularly dis-
solved solution, molecules like exist in a relative random way. In this
regard, the intramolecular torsion is relatively free which is favorable
to the non-radiative decay. In the light of the restriction of in-
tramolecular motion (RIM) mechanism, aggregates of hydrophobic
fluorescent material will be increased with increasing poor solvent
water which results in the strength of steric hindrance. Apparently,
Owing to the huge bulk of carbazole-benzoyl group, the intramolecular
rotation of phenyl rings of TPE is decelerated and this process is
strengthened with more carbazole-benzoyl attached to the phenyl rings
of TPE. Such restriction of intramolecular motion decreases the non-
radiative decay of excited state energy and it is more conspicuous at
higher concentration. Consequently, the excited state energy could
decay through a radiactive pathway, thus, leads to rapid increase of PL
intensity. These results further validate the AIE nature of TPE-BZ-Cz,
TPE-2BZ-Cz, and TPE-3BZ-Cz。
3.4. Theoretical calculations and electrochemical properties
To deeply realize the electronic structures of these new luminogens,
density functional theory (DFT) and time-dependent DFT (TDDFT)
calculations were carried out using B3LYP/6-31G (d) basis set. The
geometry optimized structures show highly twisted conformation,
which are favorable for active intramolecular rotations in solutions and
leading to faint emission. Fig. 5 describes the calculated frontier mo-
lecular orbitals of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz. For all
luminogens, the electronic distributions of LUMOs are mainly located
on TPE core plus the benzoyl acceptor moiety. Due to the strong elec-
tron-withdrawing ability of carbonyl part of benzoyl, the phenyl ring of
TPE that bonds with substitutes contributes more LUMOs. HOMOs are
mainly located on the carbazole donors, but for TPE-3BZ-Cz, there is
only one carbazole donor makes contributions to HOMOs, revealing
that the molecular conjugation for TPE-3BZ-Cz is very different with
that of TPE-BZ-Cz, and TPE-2BZ-Cz. Moreover, there is a small spacial
overlap between the HOMOs and LUMOs which gradually decrease
6

Y. Zhan, et al.
DyesandPigments173(2020)107898
Fig. 4. Photoluminescence (PL) spectra
of (a) TPE-BZ-Cz, (b) TPE-2BZ-Cz and
(c) TPE-3BZ-Cz in H2O/THF mixtures
(10 × 10−5 m) with different water
fraction(fw),
excitation
wave-
length = 340 nm. (d) Plots of PL in-
tensity versus the water fraction. (Inset:
photos of TPE-BZ-Cz, TPE-2BZ-Cz and
TPE-3BZ-Cz in THF/H2O mixtures
(fw = 0%, 80%, and 99%, respectively),
taken under 365 nm excitation of a UV
lamp).
with increasing the number of carbazole-benzoyl substituents. The In
general, the distributions of HOMOs and LUMOs of the luminogens
validate their ICT nature, which could also be visualized with the
electrostatic potential in Fig. 6, the positive region is mainly on the
donor moiety and the negative one on the acceptor moiety.
electrochemical behaviors of these luminogens were measured by cyclic
voltammetry (CV). The results show that TPE-BZ-Cz, TPE-2BZ-Cz, and
TPE-3BZ-Cz have reversible oxidation waves (Fig. 7.), demonstrating
the good electrochemical stability. The onset oxidation potentials
(Eonset) are 1.34 V, 1.31 V, and 1.30 V for TPE-BZ-Cz, TPE-2BZ-Cz, and
TPE-3BZ-Cz, respectively. According to the equation of
[HOMO = −(Eonset + 4.4) eV], the HOMO levels of TPE-BZ-Cz, TPE-
2BZ-Cz, and TPE-3BZ-Cz are calculated to be −5.74 eV, −5.71 eV, and
The calculated HOMO-LUMO gaps energy (Eg) for TPE-BZ-Cz, TPE-
2BZ-Cz, and TPE-3BZ-Cz are 3.52, 3.32 and 3.22 eV, respectively. To
further evaluate the practical HOMO and LUMO energy, the
Fig. 5. Energy levels of HOMO and LUMO, energy gaps (Eg), and electron cloud distributions of molecules TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz, calculated by
the B3LYP/6-31G (d) program.
7

Y. Zhan, et al.
DyesandPigments173(2020)107898
Fig. 6. Plots of the electrostatic potential
for TPE-BZ-Cz, TPE-2BZ-Cz and TPE-3BZ-
Cz with B3LYP/6-31G (d) on the isodensity
surface of 0.002. The color is coded as red
for strong negative whereas blue for strong
positive. (For interpretation of the refer-
ences to color in this figure legend, the
reader is referred to the Web version of this
article.)
Fig. 7. Cyclic voltammograms of luminogens TPE-BZ-Cz, TPE-2BZ-Cz and TPE-
3BZ-Cz.
Fig. 9. Device configurations along with the energy levels of the materials used
in devices.
−5.70 eV, respectively. Moreover, according to the equation of [LUMO
= (HOMO + Eg) eV], where Eg is the optical bandgap obtained from
the onset absorption, their LUMO levels are calculated to be
−2.78 eV、−2.75 eV and −2.73 eV for TPE-BZ-Cz, TPE-2BZ-Cz, and
TPE-3BZ-Cz, respectively.
151.7 °C respectively, which indicates that with the increasing of sub-
stituents, the luminogens are favorable to form stable and amorphous
film under the vacuum thermal evaporation. According to the TGA
thermograms, the thermal decomposition temperatures (Td) of TPE-BZ-
Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz are 353.6 °C, 524.4 °C and 508.9 °C
respectively, and correspond to a 5% weight loss under a nitrogen at-
mosphere, indicating that the luminogens with multi-substituents have
better thermal stability and are more suitable for the fabrication of
organic electroluminescent devices than the single-substituent lumi-
nogen. To verify this and further investigate the application of these
luminogens in electroluminescence, three non-doped OLED devices A,
3.5. Thermal properties and OLED performances
Thermal properties were studied using differential scanning calori-
metry (DSC) and thermogravimetric analysis (TGA) under nitrogen at-
mosphere. The glass transition temperatures (Tg) and decomposition
temperatures (Td) of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz are
given in Table 1 and Fig. 8. The glass transition temperatures (Tg) of
TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz are 85.4 °C, 114.7 °C and
Fig. 8. TGA (a) and DSC (b) curves of TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz, recorded under nitrogen at a heating rate of (a) 20 and (b) 10 °C/min. Tg is the
glass-transition temperature; Td is the decomposition temperature.
8

Y. Zhan, et al.
DyesandPigments173(2020)107898
Table 2
Electroluminescent data for devices A, B and C.
c
d
e
Emitter
λEL (nm)a
Von (V)b
CE (cd A−1
)
PE (lm W−1
)
Lmax (cd m−2
)
CIE (x, y)f
EQE (%)g
A
B
C
TPE-BZ-Cz
TPE-2BZ-Cz
TPE-3BZ-Cz
506
516
526
5.0
5.0
4.5
0.124
0.378
0.183
0.055
0.164
0.095
225.6
424.3
940.3
(0.24,0.39)
(0.32,0.42)
(0.31,0.47)
0.16
0.20
0.36
a
b
c
d
e
f
Maximal EL peak value.
V on is the voltage at 1 cd m−2
Maximum current efficiency.
Maximum power efficiency.
Maximum luminance.
.
Commission Internationale de I'Eclairage coordinates.
EQE maximum external quantum yield.
g
Fig. 10. (a) The EL spectra of the device A, B, and C at 4 V. and (b) the luminace-current density-voltage (L-J-V) characteristics, Inset, CIE 1931 chromaticity diagram
with coordinates of device A, B, and C.
B, and C were fabricated with the structure of ITO/HATCN(20 nm)/
NPB (40 nm)/TPE-BZ-Cz/TPE-2BZ-Cz/TPE-3BZ-Cz (20 nm)/TPBi
(40 nm)/LiF (1 nm)/Al (100 nm). The energy level diagram and mo-
lecular structures of each layer are illustrated in Fig. 9. In general speak,
device B shows better performance in current efficiency and power
π-A-π-D framework:TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz were
designed and successfully synthesized. These luminogens exhibit
blueish-green in both solutions and aggregated states. Due to the ex-
istence of TPE core, the significant AIE properties were overserved with
quantum yield over 60% in the neat film. Besides, their photophysical,
thermal, electrochemical and electroluminescent properties were stu-
died systematically. Owing to their D-π-A-π-D nature, these luminogens
show solvatochromism and negative solvatokinetic effect: the fluores-
cence spectra bathochromic-shifted and the emission intensity en-
hanced with the increase of solvent polarity. The non-doped devices of
TPE-BZ-Cz, TPE-2BZ-Cz, and TPE-3BZ-Cz showed greener emission
than that of neatfilms. With the best thermal stability and highest
quantum yield in neatfilm, device C of TPE-3BZ-Cz affords best per-
formance among them, but the overall efficiency is still at a low level
which worth further optimizing and investigating.
efficiency, the maximum values are 0.378 cd A−1, and 0.164 lm W−1
,
respectively (Table 2). The turn-on voltage and maximum luminance
(Lmax) for device B are 5.0 V and 424.3 cd m−2, which is not as good as
that in device
C with the corresponding datas are 4.0 V and
940.3 cd m−2. Device A shows poorest performance with lowest cur-
rent efficiency, power efficiency, and luminance. It is reasonable to say
such poor device efficiency may have connection with its poor thermal
stability. The EL emission peaks (Fig. 10a) of devices A, B, and C lo-
cated at 506 nm (CIEx,y = 0.24, 0.39), 516 nm (CIEx,y = 0.32, 0.42),
and 526 nm (CIEx,y = 0.31, 0.47), respectively. Key device performance
data are listed in Table 2. According to the luminace-current density-
voltage curves displayed in Fig. 10b, device C of TPE-3BZ-Cz shows the
best EL performance with a maximum external quantum efficiency
(EQE) of 0.36%, which is consistent with its highest quantum yield in
the neatfilm and best thermal stability when vacuum-evaporated to
amorphous film. This indicates the introduction of carbazole-benzoyl
into TPE core, to some degree, improves the carrier transporting ability
of the molecule in terms of three luminogens discussed in this paper.
Such viewpoint is also confirmed by the fact that TPE-3BZ-Cz possess
most carbazole-benzoyl substitutes and showing the best device per-
formance of all. However, it is clear to see that all devices suffer from
poor performance. Multiple reasons, especially the unsuitable device
structure, should be involved to explain the inferior performance.
Better device performance could be expected by further device en-
gineering.
Declaration of interests
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (61671162; 21975055; 21975053), the Key
Project of Educational Commission of Guangdong Province, China
(2017KZDXM025), and Guangdong Province Universities and Colleges
Young Pearl River Scholar Funded Scheme (2016).
Appendix A. Supplementary data
4. Conclusions
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.dyepig.2019.107898.
In summary, three novel TPE-derivative-based luminogens with D-
9

Y. Zhan, et al.
DyesandPigments173(2020)107898
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