A multifunctional luminescent network film electrochemically deposited from a new AIEE emitter for OLEDs and explosive detection

Article abstract of DOI:10.1016/j.orgel.2019.03.029

Electrochemical polymerization (EP) has attracted considerable attention because of its high material utilization, simple process and low cost. In this work, a new AIEE-active blue-emitting molecule, TCzDPAn, was synthesized. The multifunctional luminesce

Full text of DOI:10.1016/j.orgel.2019.03.029

Accepted Manuscript
A multifunctional luminescent network film electrochemically deposited from a new
AIEE emitter for OLEDs and explosive detection
Hongmin Hao, Haiyuan Luo, Aihua Yi, Cong Liu, Bingjia Xu, Guang Shi, Zhenguo Chi
PII:
S1566-1199(19)30138-7
DOI:
https://doi.org/10.1016/j.orgel.2019.03.029
ORGELE 5167
Reference:
To appear in: Organic Electronics
Received Date: 1 October 2018
Revised Date: 14 March 2019
Accepted Date: 14 March 2019
Please cite this article as: H. Hao, H. Luo, A. Yi, C. Liu, B. Xu, G. Shi, Z. Chi, A multifunctional
luminescent network film electrochemically deposited from a new AIEE emitter for OLEDs and explosive
detection, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.03.029.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
A multifunctional luminescent network film electrochemically deposited
from a new AIEE emitter for OLEDs and explosive detection
Hongmin Hao a, †, Haiyuan Luo a, †, Aihua Yi b , , Cong Liua , , Bingjia Xu a, Guang Shi a,
Zhenguo Chi c
a
Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of
Chemistry and Environment, South China Normal University, Guangzhou 510006, China
b
School of Mechanical Engineering, Dongguan University of Technology, Dongguan 523808，
China
c
Materials Science Institute, School of Chemistry and Chemical Engineering, SunYat-sen
University, Guangzhou 510275, China
Abstract :
Electrochemical polymerization (EP) has attracted considerable attention because of its high
material utilization, simple process and low cost. In this work, a new AIEE-active blue-emitting
molecule, TCzDPAn, was synthesized. The multifunctional luminescent network film, with
TCzDPAn as the building block, was conveniently prepared by EP. The EP film exhibits smooth
surface morphology and high thermal stability, which shows great potential as light-emitting
materials. In addition, the EP film shows very sensitive detection for 2,4,6-trinitrophenol (TNP) in
solution and vapor states, through the synergetic effect of photo-induced electron transfer(PET)
and fluorescence resonance energy transfer (FRET). Overall, the study indicates the potential of
Corresponding author Tel: +86-769-22861122 fax: +86-20-22861122;
E-mail address: yiaihua92751@163.com (A.Yi),
Corresponding author Tel: +86-20-39310251 fax: +86-20-39310183;
E-mail address: liucong_011@163.com (C.Liu),
† Hongmin Hao and Haiyuan Luo contributed equally to this work.
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the multifunctional EP film as OLEDs materials and fluorescent sensors, which is attributed to the
specific molecular features of TCzDPAn and the intrinsic properties of the EP film.
Keywords : aggregation-induced enhanced emission; electrochemical polymerization film;
organic light-emitting device; 2,4,6-trinitrophenol; fluorescent sensor
1. Introduction
Conventional organic luminophores generally exhibit partially or completely emission
quenching in the aggregate state. However, luminophores are often used in the forms of films in a
lot of practical applications. For example, films made of luminophores are used as active layer of
organic light-emitting diodes (OLEDs) in optoelectronics applications [1-2]. In fluorescent sensing
application, probes made of luminescent films are more practical than solution-based probes in
view of recyclability, stability and portability, extensive suitability in gas/vapor sensing, and
convenience in construction of the sensing devices. The phenomenon of aggregation-caused
quenching (ACQ) is common in organic systems, which acts as one of major obstacle for the
development of high-performance photoluminescent, electroluminescent materials and
fluorescence-film probes.
In contrast, aggregation-induced emission (AIE) and aggregation-induced enhanced emission
(AIEE) chromophores emit efficiently in the aggregated state, which may render organic
luminophores to emit intensely in the solid state. Since the first report by Tang and co-workers in
2001, AIE and AIEE materials have been widely tested in broad applications of light-emitting
diodes, fluorescence labeling materials, photonic devices, and chemo- and biosensing[1-2]
.
Fluorescent thin films are often prepared by spin-coating, electrospinning, and electrostatic
assembly. However, these traditional methods have some shortcoming yet to be solved, such as
high material waste, poor control of the morphology, and complicated processing[3-4]. In recent
years, electrochemical polymerization (EP) has been successfully used in the synthesis of
conducting polymers for the applications in organic electroluminescence[5-8], fluorescent
sensing[9-10], electrochromic devices[11], supercapacitors[12-13], etc. Compared with the traditional
methods, EP technology has the advantages of high material utilization, simple process, low cost
and good application prospects, especially in the preparation of the large-size patterned
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electroluminescent devices[11]. Recently, due to the cross-linked network structure and chemically
stable properties of EP films, they are used to construct the fluorescence-film probes. Jiang et.al.
and Zhang et.al. reported the construction of microporous polymer networks films through EP,
which exhibits good sensitive detection for TNP and 2, 4, 6-trinitrotoluene (TNT),
respectively[9-10]. However, there are still few reports on the application of EP films in good
performance OLEDs and fluorescent sensors.
In this study, a multifunctional EP film, with TCzDPAn as the building block, was prepared.
Its applications as active layer of OLEDs and explosive detection sensors were then investigated.
The blue-emitting material TCzDPAn, in which the anthracene core was decorated with two
triphenylethene groups and functionalized with four carbazole groups, has been designed and
synthesized as outlined in Scheme 1. Anthracene derivatives possess outstanding
photoluminescence and electroluminescence properties, which have been used widely as emitting
materials in OLEDs[14]. Triphenylethene is characterized by its AIE attribute[15]. The four
peripheral carbazole groups in TCzDPAn enable the formation of an insoluble interlocked
network film via simple and effective EP method, because carbazole group is a highly
electroactive group. The photophysical, electrochemical and thermal properties, as well as the
morphology of the EP films were characterized, and their applications as OLEDs materials and
explosive detection sensors were demonstrated. The results indicate that the EP film shows high
potential as a multifunctional material for OLEDs and fluorescent sensing, attributed to both the
specific molecular features of TCzDPAn and the intrinsic properties of the EP film.
2. Experimental section
2.1 Materials and analytical methods
The supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6, 98 %), was
purchased from Alfa Aesar, and it was recrystallized twice and dried for 24 h under vacuum
before use. Tetrahydrofuran (THF) was distilled in the presence of sodium benzophenone. All
other chemicals and solvents were of analytical grade and used as received.
1H and 13C NMR spectra were measured on a Mercury-Plus 300 spectrometer (CDCl3 as the
solvent and tetramethylsilane as the internal standard). Mass spectra (MS) were carried out on a
Thermo DSQ MS spectrometer. Elemental analysis(EA) was measured on a Vario elemental
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analysis cube (Vario EL cube). Fourier Transform Infrared (FT-IR) spectra were obtained on a
Bruker Vector 33 spectrometer (KBr pellet). UV-visible absorption spectra (UV) were determined
on a Shimadzu UV-1700 spectrophotometer. Fluorescence spectra (FL) were measured on a
Hitachi FL-2500 spectrometer. Glass transition temperatures (Tg) were determined by differential
scanning calorimetry (DSC) at a heating and cooling rate of 10 °C /min under N2 atmosphere
using a NETZSCH thermal analyzer (DSC-Q20) and recorded during the second heating run.
Thermogravimetric analyses (TGA) were carried out using a thermal analyzer (NETZSCH, TG
209F3) under N2 gas flow with a heating rate of 20 °C /min. The fluorescence quantum yields
(ΦFL) of the compound in THF or water/THF mixtures were evaluated using a Quantaurus-QY
(Hamamatsu). Atomic Force Microscope (AFM) images were recorded using a Seiko SPA 400
with an SPI 3800 probe station. The high-resolution transmission electron microscopy (HR-TEM)
experiment was performed using a JEOL model JEM-3010 at an acceleration voltage of 300 kV.
The cyclic voltammetric (CV) method was used to prepare EP films by using a standard
one-compartment, three-electrode electrochemical cell attached to an Electrochemical
Workstation (Model CHI760D, Shanghai Chenhua). The indium-tin oxide (ITO)-coated glass,
titanium plate and Ag/Ag+ non-aqueous electrode was used as the working electrodes, the counter
electrode, and the reference electrode respectively. All CV measurements were performed under
an inert argon atmosphere with TCzDPAn (1 mg/mL) and supporting electrolyte of TBAPF6 (0.1
M) in electrolyte solution of a mixed solvent of acetonitrile/dichloromethane (V/V=1:1). After the
EP process, the resulting EP film was washed with a mixture of acetonitrile and CH2Cl2 (V/V =
1/1) to remove any unreactive precursors and supporting electrolytes, and then dried in a vacuum
oven.
2.2 Device fabrication and testing
The ITO-coated glass substrates was cleaned, and then dried in an oven at 90°C. Poly(3,4
ethylenedioxythiophene) (PEDOT) was used as the hole-transporting layer and was coated from
water solutions on the ITO substrate, yielding a 40 nm thick layer after drying (ca. 150 °C; 20
min). For spin-coated devices, the active layer was coated from toluene solutions containing 10
mg/mL TCzDPAn to give a film with a thickness of 70 nm. For EP devices, the emitting layers of
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EP films were electropolymerized by the CV mode. Finally, CsF (1 nm) and Al cathodes (100 nm)
were thermally deposited onto the emitting-layers in a vacuum with a back pressure of 5×10-4 Pa.
The electroluminescent (EL) spectra were recorded using a PR-705 Spectroscan spectrometer. The
brightness-current-voltage characteristics were recorded simultaneously with the measurement of
the EL spectra by combining the spectrometer with a Keithley 2400 programmable electrometer.
All measurements were carried out at room temperature under ambient conditions.
2.3 Explosives detections
To explore the sensing properties of the EP films for the detection of nitroaromatic
explosives, the fluorescence spectra of the EP films were recorded by immersing EP films in
ethanol solutions of 2,4,6-trinitrophenol (TNP). To demonstrate its utility in vapor phase detection
of TNP, the fluorescence spectra of the EP films were recorded in the presence of TNP vapor as a
function of time. To explore the cycle performance, the EP film was immersed into the TNP
ethanol solution (0.1 mM), and then measured with fluorescence spectrophotometer. The resulting
film was then washed with ethanol, and heated under the vacuumed at 50 °C for 4 h to recover the
fluorescence signal. The rinsed film was reused in the next-round sensing, and the whole process
was repeated for multiple times. The Stern-Volmer constant (Ksv) was used to quantify the
quenching efficiency. For static quenching process, the fluorescence intensity as a function of
increasing quencher concentration([A]) was described by the Stern-Volmer equation I0/I-1 =
Ksv[A], where I0 and I were respectively the PL intensities in the absence and presence of the
quencher, and Ksv was the Stern -Volmer constant(M-1). Limit of detection (LOD) was calculated
by the formula of LOD = 3σ/m, where σ was the relative standard deviation, m was the slope of
the calibration curve.
2.4 Synthesis
2.4.1 Synthesis of compound 1
A solution of 1-bromo-4-(bromomethyl)benzene (5.15 g, 16.3 mmol) was stirred under
argon atmosphere at 140 °C for 15 min. Thereafter, triethyl phosphite (24.38 g, 146.7mmol) was
added and the mixture was stirred for 7 h. After the excess triethyl phosphite was removed by
vacuum distillation, the residue was cooled to room temperature. A colorless clear liquid was
obtained (5 g, 98 % yield). 1H NMR (500 MHz, CDCl3) δ 7.43 (d, J=7.9 Hz, 2 H), 7.17 (dd, J=8.5,
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2.5 Hz, 2 H), 4.06-3.96 (m, 4 H), 3.09 (d, J=21.7 Hz, 4 H), 1.25 (t, J=7.1 Hz, 6 H); MS (EIS),
m/z : 307.0093 ([M+H]+, calcd for C11H16BrO3P, 306.002).
2.4.2 Synthesis of compound 2
A solution of 9-(6-bromohexyl)-9H-carbazole (1.03 g, 3.1 mmol) and 4,4’-dihydroxy
benzophenone (0.31 g, 1.4 mmol) in DMF (20 mL) was stirred under an argon atmosphere at 60°C.
K2CO3 (0.26 g, 1.7 mmol) and NaI (0.18 g, 1.1 mmol) was added and the mixture was stirred for
48 h. After filtration, the filtrate was distilled under reduced pressure, and the crude residue was
recrystallized from ethanol. A white powder was obtained (0.48 g, 45% yield). 1H NMR (500
MHz, CDCl3) δ 8.09 (d, J = 7.6 Hz ,4H),7.73 (d, J = 8.8 Hz ,4H), 7.43 (d, J = 16.2,8H),7.22 (d, J
= 3.6 ,4H), 6.87(d, J = 8.8 Hz, 4H), 4.32 (t, J = 6.8 Hz, 4H), 3.97 (s, 4H), 1.92 (d, J = 14.0 Hz, 4H),
1.75 (d, J = 13.4 Hz, 4H), 1.55-1.38 (m, 8H); MS (MALDI-TOF), m/z: 713.401 ([M+H]+ calcd for
C49H48N2O3, 712.366).
2.4.3 Synthesis of compound 3
A solution of compound 1 (1.38 g, 4.5 mmol) dissolved in THF (45 mL) was stirred under an
argon atmosphere at 0 °C. Potassium tert-butoxide (0.05g, 0.4 mmol) was added quickly. After
fully dissolved, compound 2 (0.36 g, 0.5 mmol) was added and the mixture was stirred
continuously for 12 h at room temperature. The solvent was distilled by rotary evaporation after
the reaction was completed. The crude product was poured into water and extracted with
dichloromethane. The organic layer was dried over sodium sulfate (MgSO4). Following filtration
and evaporation of solvent under vacuum, the resulting crude product was purified on a silica gel
column chromatography by dichloromethane/petroleum ether (V/V=1:2) to yield compound 3 as a
light-yellow powder (0.41 g, 95 % yield). 1H NMR (500 MHz, CDCl3) δ 8.12 (dd, J=7.8, 0.5 Hz,
4 H), 7.51-7.40 (m, 8 H), 7.25-7.21 (m, 7 H), 7.08-7.04 (m, 2 H), 6.92-6.88 (m, 2 H), 6.83-6.78 (m,
4 H), 6.73 (s, 1 H), 4.34 (dd, J=15.7, 7.2 Hz, 4 H), 3.92 (td, J=6.4, 3.6 Hz, 4 H), 2.02-1.87 (m, 4
H), 1.84-1.68 (m, 4 H), 1.60-1.40 (m, 9 H); MS (MALDI-TOF), m/z: 864.318 ([M]+, calcd for
C56H53BrN2O2, 864.329).
2.4.4 Synthesis of compound 4
A solution of compound 3 (2.00 g, 2.31 mmol) in 1,4-dioxane(25 mL) was stirred under an
argon atmosphere, followed by the sequent addition of bis(pinacolato)diboron (21.76 g, 6.93
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mmol), potassium acetate (KOAc，0.91 g，9.24 mmol), and Pd(dppf)Cl2 (0.14 g,0.18 mmol). The
reaction mixture was gradually heated to 85 °C and stirred for 15 h. After that, the mixture was
cooled to room temperature. The dioxane was removed by rotary evaporation. The residue was
poured into a 5 % sodium hydroxide (NaOH) solution and extracted with CH2Cl2. The organic
layer was dried over MgSO4, and then filtered. Thereafter, the solvent was removed by rotary
evaporation. The resulting crude product was purified by column chromatography on silica gel
using dichloromethane/petroleum ether (V/V=1:1) as the eluent. Compound 4 was obtained as a
1
light yellow solid (1.79 g, 85 % yield). H NMR (500 MHz, CDCl3) δ 8.11 (d, J=7.6 Hz, 1 H),
7.57 (d, J=7.4 Hz, 1 H), 7.50-7.38 (m, 2 H), 7.23 (t, J=7.4 Hz, 1 H), 7.05 (dd, J=19.9, 7.9 Hz, 1
H), 6.80 (dd, J=10.9, 8.4 Hz, 1 H), 4.34 (dt, J=10.1, 7.2 Hz, 1 H), 3.92 (t, J=6.3 Hz, 1 H), 1.93 (dt,
J=20.9, 7.2 Hz, 1 H), 1.75 (dt, J=13.4, 6.5 Hz, 1 H), 1.50 (ddd, J=26.2, 18.2, 10.6 Hz, 3 H), 1.32
(s, 3 H); MS (MALDI-TOF), m/z: 913.617 ([M+H]+, calcd for C62H65BN2O4, 912.504).
2.4.5 Synthesis of TCzDPAn
The compound 4 (1.05 g, 1.15 mmol), toluene (60 mL), 2 M potassium carbonate aqueous
(K2CO3) solution (2 mL) and appropriate phase transfer agents (AliquatR 336) were added to
three-necked flask and stirred. Then, 9,10-dibromoanthracene (0.10 g ， 0.29 mmol),
tetrakis(triphenylphosphine)palladium (Pd(PPh3)4, (0.20 g，0.17 mmol) were added into the
reaction mixture in sequence under the anhydrous anaerobic condition. The mixture was refluxed
at 90 °C for 24 h under an argon atmosphere. After cooling to room temperature, reaction mixture
was poured into water and extracted with dichloromethane. The organic layer was dried over
MgSO4. Solvent was removed under reduced pressure and the residue was purified by column
chromatography on silica gel using dichloromethane/petroleum ether (V/V=1:1) as eluent,
afforded TCzDPAn (0.19 g, 74 % yield) as a white solid. 1H NMR (500 MHz, CDCl3 ) δ 8.12 (d,
J=7.7 Hz, 4 H), 8.07 (d, J=7.7 Hz, 4 H), 7.70 (dd, J=6.8, 3.2 Hz, 4 H), 7.48 (t, J=7.5 Hz, 4 H),
7.45-7.40 (m, 8 H), 7.37 (d, J=8.1 Hz, 4 H), 7.32 (dd, J=9.4, 3.5 Hz, 8 H), 7.25-7.15 (m, 16 H),
6.99 (s, 2 H), 6.89 (d, J=8.6 Hz, 4 H), 6.85 (d, J=8.7 Hz, 4 H), 4.34 (t, J=7.1 Hz, 4 H), 4.29 (t,
J=7.1 Hz, 4 H), 3.94 (q, J=6.3 Hz, 8 H), 1.98-1.86 (m, 8 H), 1.76 (dd, J=14.5, 9.4 Hz, 8 H),
1.59-1.41 (m, 20 H); 13C NMR (500 MHz, CDCl3) δ 140.45, 131.60, 131.07, 129.85, 129.31,
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128.90, 125.62, 125.58, 124.89, 122.87, 120.38, 120.33, 118.77, 118.73, 114.65, 114.18, 108.64,
67.99, 67.79, 67.69, 42.97, 42.91, 29.19, 28.95, 27.09, 25.96; MS (MALDI-TOF), m/z: 1747.961
([M]+, calcd for C126H114N4O4, 1748.900); Anal. Calc. for C126H114N4O4: C 86.56, H 6.57, N 3.20,
O 3.60; Found: C 86.82, H 6.71, N 3.12 %
3. Results and discussion
3.1 Synthesis
The strategy to synthesize TCzDPAn compound is outlined in Scheme 1. The TCzDPAn
molecule was prepared through palladium-catalyzed Suzuki coupling reaction between
9,10-dibromoanthracene and corresponding boric acid ester. All the intermediate and final
products were fully purified and characterized, and the result data satisfactorily corresponded to
the expected molecular structures. The chemical structure and composition of the target compound
1
TCzDPAn was characterized and confirmed by H-NMR, 13C-NMR, MS and EA, which are
included in the experimental section.
Scheme 1 The synthetic routes of TCzDPAn
3.2 Optical properties of TCzDPAn and the AIEE effects
Fig.1a shows the absorption spectra of TCzDPAn in THF solutions(10 µM). TCzDPAn
exhibits three major absorption bands at 295 nm, 305-354 nm and 370-430 nm, respectively (Fig.
1a). The absorption peak at the position of 295 nm, 331nm and 378 nm were attributed to the π–π*
transitions of carbazole, peripheral triphenylethene groups and anthracene, respectively[14]. To
determine the AIEE behavior of TCzDPAn, the changes of photoluminescence (PL) behaviors in
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the solution and aggregate states were recorded. The FL spectra of TCzDPAn in the water/THF
mixtures (10 µM) with different contents of water are shown in Fig.1b. Since the molecules are
completely insoluble in water, their existing forms will change from a solution or well-dispersed
state in the pure THF to the aggregated nanoparticles, with an increase of the water fraction in the
mixed solvent. The TCzDPAn in pure THF solution exhibits weak FL intensity and shows PL
peak at 453 nm. However, the fluorescent intensity is significantly enhanced when the water
fraction exceeded 50%. In the solvent mixture with 95% water, the PL intensity is 17-fold higher
than that in the pure THF solution and shows strong blue fluorescence under UV irradiation (inset
graph). Simultaneously, the fluorescence quantum yield (ΦFL) values of TCzDPAn roses from 5.4 %
in pure THF solutions to 46.3 % in the solvent mixture with 95% water. These results indicate that
TCzDPAn exhibits significant AIEE effect. The emission images of TCzDPAn in water/THF
mixtures with different water fractions are shown in Fig.S1. The compound exhibits stronger
emission in water/THF mixtures with high water fractions than that in pure THF solution, which
further demonstrating the AIEE-effects. The central anthracene core decorated with two
triphenylethene groups which has twisted configuration, increases structural bulk and
non-planarity, effectively diminishes its degree of intermolecular π-π stacking and enable the
TCzDPAn exhibit AIEE-active[1, 15]
.
Fig.1 (a) UV absorption spectra of TCzDPAn in pure THF solution(10 µM). (b) FL spectra of
TCzDPAn in water/THF mixtures(10µM). The inset depicts the changes of FL peak intensity and
emission images in water/THF mixtures with water fractions of 0 % and 95 % under 365 nm UV
illumination at room temperature, respectively
3.3 Preparation of the EP films
The EP films of TCzDPAn were produced on electrodes by the CV mode. The CV curves of
TCzDPAn recorded for 10 scan cycles with a potential range from -0.6 to 0.86 V provided in Fig.2.
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In the first cycle of the positive CV scan, the onset potential appeared at 0.76 and the oxidation
peak is observed at 1.13 V, which is attributed to the oxidation of carbazole[7] (Fig.S2). The fact
that carbazoles transform into cationic radicals during oxidation and then couple effectively with
each other to form dimeric carbazole cations has been verified by previous studies[9, 16]. During the
negative scan, an obvious reductive peak is detected at the potential of 0.45 V, which corresponds
to the formation of the neutral dimeric carbazyl from the dimeric carbazyl cation[21]. From the
second scan, a new peak appears at 0.71 V, assigned to the oxidation of the formed dimeric
carbazoles in the film. The current of each cycle rises gradually with increased scan cycle,
indicating that the polymerization of the monomers and the growth of the film on the electrode
upon successive CV scan. The thickness of network films can be controlled precisely by adjusting
the numbers of scan cycles and the oxidative potentials, by around 0.7 nm per cycle under the
potential range from -0.6 to 0.86 V (Fig.S3).
Fig.2 Cyclic voltammograms (-0.6~0.86 V) of TCzDPAn recorded for 10 scan cycles
3.4 Characterization of the EP film
The structure of the EP film and the formation of dimeric carbazole were evidenced by FT-IR
spectra (Fig.S4). The peak at 723 cm-1 and 749 cm-1 in TCzDPAn is attributed to the vibrational
bands of C-H bonds of the bisubstituted benzene groups of carbazole ring[8]. The newly generated
peak at 798 cm-1 in the EP film is attributed to the vibrational bands of C-H bonds of the
trisubstituted carbazole ring, which demonstrates the existence of dimeric cabazole[4]. As there are
four carbazole units in a TCzDPAn molecule, a cross-linked film is deposited by the
electrochemical coupling reaction of carbazole groups, the schematic shown in Fig. S4. This EP
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film is very stable and insoluble in many common solvents, which further demonstrating its
cross-linking structures.
The PL spectra of the TCzDPAn spin-coated film and the EP film deposited on ITO are
presented in Fig.3. The TCzDPAn spin-coated film emits blue luminescence with the peak of 457
nm, while that of EP films at about 500 nm. There is about 43 nm red-shifted of the PL spectra of
EP film comparing with that of spin-coated film. This phenomenon is also observed in other
fluorescent EP films owing to their cross-linked-network structures and the relative close packing
of chromophores in the EP films[3,5-6]
.
Fig.3 The PL spectra of spin-coated film and EP film of TCzDPAn on ITO
The thermal property of TCzDPAn and the EP film were examined by TGA and DSC
measurements. As display in Fig S5a, TCzDPAn and the EP film show good thermal stability and
Td (5% -weight-loss temperature measured by TGA) is 390°C and 408 °C, respectively. The good
thermal stability of TCzDPAn can be ascribed to the large molecular weight and rigid carbazole
peripheries. The higher level of thermal stability of the EP film comparing with TCzDPAn is
attributed to its crosslink structure. The glass transition temperature (Tg) of TCzDPAn is detected
at 75 °C, while such thermal transition is not detected in the EP film(Fig.S5b), suggesting that the
EP film has a very high degree of cross-linking. The DSC curves of TCzDPAn reveals no peaks
associated with crystallized or melting transitions, indicating that TCzDPAn exhibits a very low
tendency towards crystallization[15]. Based on the above results, TCzDPAn and its EP film have
excellent thermal stability which is beneficial for the application as photoelectric materials in
OLED devices.
3.5 Morphologic studies of the EP film
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The morphology of EP film was studied by AFM and TEM. As revealed by AFM, the EP
film exhibits a uniform surface and the root mean square roughness (RMS) is only 1.7 nm, which
is importance for its application in optoelectronic devices (Fig.4). The uniform microporous
structure with diameter of less than 1 nm of the EP film is observed directly by TEM (Fig.4). The
permanent structural micropores of EP film will make it a promising candidate of excellent
fluorescence-film probe.
Fig.4 AFM and TEM images of the EP film.
3.6 Electrochemical characterization
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) are related to the redox potentials. They are two important parameters for
electroluminescent materials on account of their relationship with hole/electron-injecting
capability [17-18]. The electrochemical properties of TCzDPAn and the EP film as shown in Fig.S6.
As the onset reduction potential of TCzDPAn cannot be observed clearly, the LUMO energy level
is calculated from the HOMO energy level and the energy band gap which was estimated from the
absorption edge (LUMO =HOMO-⊿Eg ). The calculated HOMO and LUMO energies of
TCzDPAn are -5.40 eV and -2.93 eV respectively. The HOMO and LUMO energies of the EP
film are calculated to be -5.17 eV and -2.34 eV, respectively, according to the equation
EHOMO/LUMO = -(Eonset(ox/red),FOC+4.8) eV[19]
.
3.7 Electroluminescence properties
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In order to investigate the potential application of TCzDPAn and EP film as the emitting
layers (EML) in OLEDs, undoped devices with simple configuration of ITO/PEDOT:PSS (40
nm)/Spin-coated film (~70 nm)/CsF (1 nm)/Al (100 nm) (Device I) and ITO/PEDOT:PSS
(40nm)/EP film (~70 nm)/CsF (1 nm)/Al (100 nm) (Device II) were fabricated. A well-known
hole injection material PEDOT:PSS was used to lower the leakage current, CsF was used as the
modificatory layer of the cathode Al, which could enhance the electron injection, and ITO coated
glass was used as the substrate and anode[20]. All the characterization data for devices were
summarized in Supporting Information Table S1. The turn-on voltage(Von), maximum
luminance(Lmax) and maximum luminescent efficiency(ηC,max) of Device I is 4.8 V, 1282 cd/m2
and 0.38 cd/A, respectively (Table S1). The electroluminescent device using the EP film as EML
displays a maximum luminance of 923 cd/m2 and a maximum luminance efficiency of 0.28 cd/A
with very low roll-off of the efficiency, which demonstrates the great potential in EL devices of
this new AIEE compound and its EP film. Device I emits blue light with a maximum at 468 nm
and CIE coordinates of (0.16, 0.24) (Fig.5c and Table S1). Device II exhibits blue-green emission
with a maximum at 520 nm, which is about 52 nm red-shifted in contrast with that of Device I,
attributed to the their cross-linked-network structures and the relative close packing of
chromophores in the EP films. Although the EL performance of these Devices are not high, it
demonstrates the great potential in EL devices of this new AIEE compound and its cross-linked
polymer network films fabricated by simple EP technology. The device performances may be
further improved by the configuration of the EL devices and the optimization of the EP conditions.
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Fig.5(a) Current-voltage-luminance characteristics of the OLEDs. (b) luminous efficiency–current
density characteristics of the OLEDs. (c) EL spectra of TCzDPAn and EP film
3.8 Explosive detection by EP film
The stable cross-linked structure and good fluorescence properties of the EP film inspired us
to explore their applications as fluorescence-film sensor for nitroaromatic explosives. The
2,4,6-trinitrophenol (TNP) that commercial available is used as a model explosive. The
fluorescence changes of EP films in response to different amount of TNP in ethanol were studied.
Their PL intensity decrease significantly with the increasing loading amount of TNP but with no
change in spectral profiles(Fig. 6a). The fluorescence quenching rate is 96.86% with addition of
0.1mM TNP. To further quantify the quenching efficiency, the Stern-Volmer plot of relative PL
intensity (I0/I-1) of EP film versus TNP concentration is investigated and shown in Fig. 6b. The
Stern-Volmer plot is almost linear at low concentrations, and gives a quenching constant (Ksv) of
1.14×105 M-1(Fig. 6b), which is found to be higher than many other fluorescent chemosensors for
TNP[18, 21-22]. The limit of detection calculated by the formula of LOD=3σ/m can reach 500 nM,
revealing the excellent sensitivity to TNP of the EP film. Afterward, the curve bent upward,
suggesting the super amplification effect upon increasing the TNP concentration. The nonlinear
nature of the Stern-Volmer plot of TNP may be due to the energy transfer process[23]. In addition,
the cross-linked-network microstructure and porous nature of the EP film ensures its stability and
the easy and fast migration of exciton, which is benefit for the enhancement of sensing
efficiency[18, 24]. Furthermore, the reusability of the EP film was examined (Fig. 6c).
Approximately the same state of fluorescence quenching is observed after each cycle, and the
fluorescence intensities of the EP film recover to a similar level. The EP film is reusable for TNP
detection, which may be due to the stable cross-linked network structure of EP film.
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Fig. 6 (a) Fluorescence emission spectra of EP film with the addition of different amounts of
TNP in ethanol solution(10 µM-100 µM). (b) Stern-Volmer plots of I0/I-1 versus TNP
concentration. I0 and I are the fluorescence intensities of the EP film in the absence and presence
of TNP, respectively. Inset: The fitting diagram at low concentration (0- 50µM). (c) The cyclic
performance of the EP film in the detection of 0.1mM TNP solution
The efficient system for vapor phase sensing of TNP is extremely challenging because TNP
has very low vapor pressure (7.4 × 10-7 mm Hg at 25 °C) compared to several other nitroaromatic
explosives (including TNT and DNT)[25]. To demonstrate the utility of EP film in detection of
TNP in vapor state, the fluorescence spectra of the EP films were recorded in the presence of
saturated TNP vapor (Fig. 7a). As shown in Fig. 7b, fluorescence quenching is observed
immediately by exposing EP film to saturated TNP vapor, and the fluorescence quenching is 48 %
in 100 s exposure, and reaches 70 % in 240 s. Thus, the EP film displayed high sensitivity to TNP
vapor.
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Fig. 7 (a) Time-dependent PL spectra of EP film upon exposure to TNP saturated vapor at room
temperature for 0-240 s. Film thickness: 60 nm; excitation wavelength: 365 nm. (b)
Time-dependent fluorescence quenching studies of EP film upon exposure to saturated vapor
phase of TNP.
3.9 Mechanism for TNP detection
Generally, the photo induced electron transfer or energy transfer between the sensor
molecules and the nitroaromatics account for the fluorescence response of sensors[21, 26]. The
conduction band of fluorescent sensors usually lies at higher energies than the LUMOs of
electron-deficient analytes, thus resulting in the electron transfer and leading fluorescence
quenching. The calculated LUMO energy level of TNP is -3.92 eV, which is much lower than that
of the EP film (-2.34 eV), thus facilitates the occurrence of the photoinduced electron transfer[18,
27]. The non-linear Stern-Volmer plot for TNP also suggests an energy transfer mechanism. To
further account for the intrinsic reason for the quenching behaviors, the UV spectra of TNP and
the PL spectrum of EP films were investigated. It can be seen that the absorption of TNP overlaps
with the emission of EP film in the range of 385 to 470nm, which suggests that energy transfer
quenching process may be one of the quenching pathway (Fig. 8). The resonance energy transfer
can occur from fluorophore to non-emissive analyte, especially at the high concentration when the
fluorophore and the analyte are close to each other, which dramatically enhance
fluorescence-quenching efficiency. Based on these results, the quenching mechanism of the EP
film dominated by both charge and energy transfer.
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Fig. 8 Overlap of the absorption spectrum of TNP and the emission spectrum of EP film.
4 Conclusions
A luminescent network film, with a new AIEE-activity blue-emitting molecule, TCzDPAn as
the building block, has been constructed conveniently by EP technology. The EP film exhibits
smooth morphology and high thermal stability, which demonstrates the great potential as EML in
EL devices. Additionally, the EP film is promising reusable sensitive sensors for solution and
vapor phase sensing of TNP due to good luminescent properties, cross-linked network structure
and microporous morphology. The Ksv is found as high as 1.14×105 M-1, which is comparable to
the best performances of the fluorescence-film probes for TNP. The quenching mechanism of the
EP film dominated by both charge and energy transfer. Overall, the study indicates the potential of
the EP film as OLEDs materials and fluorescent sensors.
Acknowledgements
This work was supported by the National Natural Science Foundation of China(Grant
numbers: 51203054) and Natural Science Foundation of Guangdong Province of China
(2017A030313318).
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Figure captions
Scheme 1 The synthetic routes of TCzDPAn
Fig.1. (a) UV absorption spectra of TCzDPAn in pure THF solution(10 µM). (b) FL spectra of
TCzDPAn in water/THF mixtures(10µM). The inset depicts the changes of FL peak intensity and
emission images in water/THF mixtures with water fractions of 0 % and 95 % under 365 nm UV
illumination at room temperature, respectively.
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Fig.2. Cyclic voltomogram (-0.6~0.86 V) of TCzDPAn recorded for 10 scan cycles.
Fig.3. The PL spectra of spin-coated film and EP film of TCzDPAn.
Fig.4. AFM and TEM images of the EP film.
Fig.5. (a) Current-voltage-luminance characteristics of the OLEDs. (b) luminous
efficiency–current density characteristics of the OLEDs. (c) EL spectra of TCzDPAn and EP film.
Fig. 6. (a) Fluorescence emission spectra of EP film with the addition of different amounts of TNP
in ethanol solution(10 µM-100 µM). (b) Stern-Volmer plots of I0/I-1 versus TNP concentration. I0
and I are the fluorescence intensities of the EP film in the absence and presence of TNP,
respectively. Inset: The fitting diagram at low concentration (0- 50µM). (c) The cyclic
performance of the EP film in the detection of 0.1mM TNP solution.
Fig. 7 (a) Time-dependent PL spectra of EP film upon exposure to TNP saturated vapor at room
temperature for 0-240 s. (b) Time-dependent fluorescence quenching studies of EP film upon
exposure to saturated vapor phase of TNP.
Fig. 8. Overlap of the absorption spectrum of TNP and the emission spectrum of EP film.
Supporting Information
A multifunctional luminescent network film electrochemically deposited
from a new AIEE emitter for OLEDs and explosive detection
Hongmin Hao a, Haiyuan Luo a, Aihua Yi b*, Cong Liu a *, Bingjia Xu a, Guang Shi a, Zhenguo Chi
c
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Fig. S1 The emission images of the compound in water/THF mixtures with different water
fractions under 365 nm UV illumination at room temperature
Fig.S2 Cyclic voltomogram (-0.6~1.5 V, 200 mV/s) of TCzDPAn(first and second cycle).
Fig. S3 Plot of the TCzDPAn EP film thickness versus the CV (-0.6~0.86 V and -0.6~0.83 V)
cycle number
Fig. S4 FT-IR spectra of TCzDPAn and EP film
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Fig. S5 (a) TGA curves of TCzDPAn and the EP film. (b) DSC curves of TCzDPAn and the EP
film recorded under nitrogen at a scan rate of 10 °C /min (second heating run)
Fig. S6 Electrochemical voltammograms measured for (a) TCzDPAn and (b) the EP film
Table S1 EL performances of OLEDsa
Device Von [V]
Lmax [cd/m2]
ηC,max [cd/A]
λEL [nm]
CIEx,y
I
4.8
1282
923
0.38
0.28
468
520
(0.16,0.24)
(0.30, 0.51)
II
4.2
aAbbreviations: Von=turn-on voltage at 1 cd/m2, Lmax=maximum luminance, ηC,max=maximum
current efficiency, λEL=EL maximum,. CIEx,y=Commission international de I’Eclairage
chromaticity coordinates collected at 2 mA.
Device configuration:
I : ITO/PEDOT:PSS (40 nm)/Spin-coated film (~70 nm)/CsF (1 nm)/Al (100 nm)
II : ITO/PEDOT:PSS (40 nm)/EP film (~70 nm)/CsF (1 nm)/Al (100 nm)
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Highlights
> A new AIEE-activity blue-emitting molecule, TCzDPAn, has been synthesized.
> Luminescent network film has been prepared by electrochemical polymerization (EP).
> The EP film demonstrated great potential as OLED materials and fluorescence sensor.