Efficient red fluorescent OLEDs based on aggregation-induced emission combined with hybridized local and charge transfer state

Article abstract of DOI:10.1016/j.dyepig.2020.108770

Limited by the energy gap law and large conjugate structures, red organic light-emitting diodes (OLEDs) have long been criticized for their efficiency problems. The aggregation-induced emission (AIE) effect and the reverse intersystem crossing process can lead to high photoluminescent quantum yields (PLQYs) and high exciton utilization efficiency (EUE), two key objectives for obtaining efficient OLEDs. In this work, we chose benzothiadiazole as the acceptor (A) and triphenylamine and phenothiazine as the donors (Ds) to design and synthesize a D–A–D red fluorescent molecule, PBTPA. Results of systematic photophysical measurements indicated PBTPA to exhibit AIE trending, manifesting as aggregation-induced enhanced emission and the characteristics of hybridized local and charge transfer (HLCT) states. These features facilitated high solid-state luminescence efficiency and high exciton utilization to be achieved. A non-doped device fabricated using PBTPA displayed a red electroluminescence peak at 656 nm, corresponding to the Commission International de L'Eclairage coordinates of (0.65, 0.32), and an external quantum efficiency of 1.62percent. By calculation, the EUE reached a value of 50percent. According to the results of theoretical calculations, the effective hot exciton channel between the second triplet (T₂) and the lowest singlet (S₁) was responsible for the high EUE. This EUE value is relatively good for this type of red OLEDs. Evidence indicates that combining AIE and HLCT has excellent potential to aid the discovery of new-generation highly efficient red OLEDs.

Full text of DOI:10.1016/j.dyepig.2020.108770

Journal Pre-proof
Efficient red fluorescent OLEDs based on aggregation-induced emission combined
with hybridized local and charge transfer state
Yuan Yu, Miao Cang, Wei Cui, Lei Xu, Runze Wang, Mizhen Sun, Huayi Zhou,
Wenjun Yang, Shanfeng Xue
PII:
S0143-7208(20)31467-4
DOI:
https://doi.org/10.1016/j.dyepig.2020.108770
DYPI 108770
Reference:
To appear in: Dyes and Pigments
Received Date: 25 June 2020
Revised Date: 5 August 2020
Accepted Date: 5 August 2020
Please cite this article as: Yu Y, Cang M, Cui W, Xu L, Wang R, Sun M, Zhou H, Yang W, Xue S,
Efficient red fluorescent OLEDs based on aggregation-induced emission combined with hybridized local
and charge transfer state, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108770.
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Graphic Abstract:

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Efficient red fluorescent OLEDs based on aggregation-induced emission
combined with hybridized local and charge transfer state
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Yuan Yu, Miao Cang, Wei Cui, Lei Xu, Runze Wang, Mizhen Sun, Huayi Zhou,
Wenjun Yang, Shanfeng Xue*
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Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(QUST), School of Polymer Science & Engineering, Qingdao University of Science &
Technology, 53-Zhengzhou Road, Qingdao 266042, China
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* Corresponding author: Shanfeng Xue, Email: sfxue@qust.edu.cn
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Abstract:
Limited by the energy gap law and large conjugate structures, red organic
light-emitting diodes (OLEDs) have long been criticized for their efficiency problems.
The aggregation-induced emission (AIE) effect and the reverse intersystem crossing
process can lead to high photoluminescent quantum yields (PLQYs) and high exciton
utilization efficiency (EUE), two key objectives for obtaining efficient OLEDs. In this
work, we chose benzothiadiazole as the acceptor (A) and triphenylamine and
phenothiazine as the donors (Ds) to design and synthesize a D–A–D red fluorescent
molecule, PBTPA. Results of systematic photophysical measurements indicated
PBTPA to exhibit AIE trending, manifesting as aggregation-induced enhanced
emission and the characteristics of hybridized local and charge transfer (HLCT) states.
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These features facilitated high solid-state luminescence efficiency and high exciton
utilization to be achieved. A non-doped device fabricated using PBTPA displayed a
red electroluminescence peak at 656 nm, corresponding to the Commission
International de L’Eclairage coordinates of (0.65, 0.32), and an external quantum
efficiency of 1.62%. By calculation, the EUE reached a value of 50%. According to
the results of theoretical calculations, the effective hot exciton channel between the
second triplet (T2) and the lowest singlet (S1) was responsible for the high EUE. This
EUE value is relatively good for this type of red OLEDs. Evidence indicates that
combining AIE and HLCT has excellent potential to aid the discovery of
new-generation highly efficient red OLEDs.
Keywords: D–A–D type red fluorescent molecule; aggregation-induced emission;
hybridized local and charge-transfer state; red fluorescent organic light-emitting
diodes; exciton utilization
1. Introduction:
Since Tang developed the first organic light-emitting diode (OLED) in 1987, these
devices have been at the center of a great deal of research enthusiasm.1 Due to the
high contrast they afford, as well as their active luminescence and fast response,
OLEDs are currently widely used in commercial applications.2-5 Notably,
photoluminescent quantum yield (PLQY) and exciton utilization efficiency (EUE) are
two key parameters defining the performance of OLED devices.6, 7
Devices that emit light in the wavelength perceived as the color red, one of the three
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primary colors, are still beset by many problems. In order to design red light-emitting
molecules, large conjugate structures are usually necessary; these types of molecules,
however, tend to planarize, which causes the light they emit to shift toward longer
wavelengths. Furthermore, planarization tends to promote strong intermolecular
interactions, including π–π stacking interactions, which eventually favor
aggregation-caused quenching (ACQ) and lead to a decrease in PLQY value.8-10 Red
light-emitting OLEDs are thus generally less efficient than their blue and green
light-emitting counterparts. In order to address this issue, and on the basis of the
aggregation-induced emission (AIE) mechanism proposed by Tang et al. in 2001,11
the propeller-shaped molecular design was introduced to restrain intermolecular
motions in the aggregated state and further limit the non-radiative transitions among
molecules.12-14 Over time, therefore, AIEgens like tetraphenylethenes (TPEs) have
expanded the AIE molecular library; importantly, such molecular species have been
utilized in many research fields, including those focusing on highly sensitive sensors,
intelligent response systems, and precision medicine.15-20 A molecule named
2TPA-BT-N-2P with a distinct AIE effect synthesized by Liao et al. in 2020 was
observed to display a PLQY value of 91%.12 Using the AIE concept in the design of
red light-emitting molecules can effectively suppress quenching during the
luminescence process, which helps increase the molecules’ PLQY.
According to the energy gap law, the inherent narrow bandgap of red light emitters
could bring about a substantial vibrational–rotational coupling between the ground
and excited states, resulting in a reduction of the radiative transition rates and low
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luminescence efficiency.21-23 In order to overcome this problem, more singlet excitons
need to be involved in the fluorescence emission; in other words, the exciton
utilization (EUE) must be improved. According to the traditional spin–statistics rule,
only 25% of the singlet excitons can participate in fluorescence emission.24, 25 This
limitation can however be overcome transforming large numbers of triplet excitons
into singlet excitons, via, for instance, the triplet–triplet annihilation (TTA)
mechanism,26 the thermally activated delayed fluorescent (TADF) mechanism,27-29
and the hot exciton mechanism.30, 31
In detail, in the TTA mechanism, the lowest singlet (S1) and a ground state (S0)
exciton are generated by the two lowest triplet (T1) excitons. Notably, the maximum
internal quantum efficiency of TTA is 62.5%.32 The TADF mechanism exploits the
small energy difference between T1 and S1 and the highly separated highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO); in
TADF, reverse intersystem crossing (RISC) is enabled, thereby ultimately maximizing
the use of triplet excitons.33 Nevertheless, the accumulation of long-life T1 excitons at
high current density or high brightness results in severe roll-off, an issue that has
always plagued TADF materials. The hot exciton mechanism is based on the RISC
process associated with the small energy gap between the high-lying triplet state and
its energetically similar singlet state; notably, hot exciton materials tend to be
characterized by a large energy gap between T1 and T2, which prevents the triplet
excitons from being lost to internal conversion (IC).34 Importantly, many hot exciton
compounds are also characterized by the coexistence of local luminescence (LE) state
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with high luminescence efficiency and charge transfer (CT) state with weak exciton
binding energy; these species are called hybridized local and charge transfer (HLCT)
materials.35-37 These types of materials can achieve high efficiency and low roll-off
under non-doping conditions, while affording relatively good color purity, so they
represent a suitable choice in the design of efficient red light emitters. In this context,
the hot exciton material TPANZP, reported by the Ma group, displayed an EUE value
of 93%, in conjunction with an emission peak at 664 nm.35
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In the present study, by rationally selecting donor (D) and acceptor (A), we designed
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and
synthesized
a
D–A–D
type
molecule
dubbed
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4-(7-(10-ethyl-10H-phenothiazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenyla
niline, namely, PBTPA (see Scheme 1). Benzothiadiazole has a good
electron-accepting capacity, and it is well-planarized; benzothiadiazole’s ΔET1–T2 (the
energy gap between T1 and T2) has a value of 0.97 eV, which is advantageous from
the standpoint of the construction of molecular species emitting long-wavelength light.
Triphenylamine and phenothiazine are strong electron donors. We have attached an
ethyl group to the N atom of phenothiazine in the hope of increasing steric hindrance,
and thus suppressing ACQ to a certain extent. We then performed systematic tests and
theoretical calculations on PBTPA. This molecule exhibited typical AIE and HLCT
characteristics. Furthermore, we fabricated a non-doped OLED device that relied on
PBTPA as the light-emitting layer. This device emitted red light at 656 nm with
Commission International de L’Eclairage (CIE) coordinates of (0.65, 0.32) and
external quantum efficiency (EQE) of 1.62%, with a EUE of 50%. These data indicate
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that the device displays a relatively good performance as a red emitter. Notably, the
subsequent doping of this structure improved the device’s efficiency. We conclude
that combining AIE with HLCT is an effective design approach to manufacturing
efficient red light-emitting OLEDs.
2. Experimental section
2.1 Materials
All raw materials were commercially purchased from Aldrich Chemical Co. or
Energy
Chemical
Co.,
China
(HATCN),
(TAPC),
dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine
1,3-bis(N-carbazolyl)benzene
(mCP),
9,10-bis(2-naphthyl)anthraces
(AND),
1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB), and lithium fluoride (LiF).
Tetrahydrofuran (THF) was distilled over metallic sodium and dimethylformamide
(DMF) over calcium hydride before use. The other organic solvents and reagents were
all commercially available analytical-grade products and used as received without
further purification.
2.2 Measurements
1H and 13C NMR spectra were recorded on a Bruker AC500 spectrometer at 500 MHz
and 125 MHz, respectively, when the compounds were dissolved in deuterated
chloroform-d (CDCl3) with tetramethylsilane (TMS) as the internal standard. The
MALDI-TOF-MS mass spectra were measured using an AXIMA-CFRTM plus
instrument. Differential scanning calorimetry (DSC) curves were determined on a
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Netzsch DSC (204F1) instrument at a heating rate of 10 min-1. Thermogravimetric
analysis (TGA) was performed on a Netzsch (209F1) thermogravimetric analyzer in a
nitrogen atmosphere (50 mL min-1) at a heating rate of 10 min-1. UV-vis absorption
and fluorescence spectra of solution and films were measured by the Hitachi U-4100
spectrophotometer and Hitachi F-4600 spectrophotometer, respectively. In preparation
for the AIE test, the final product was first dissolved in THF (10-4 M) and then mixed
with water to form solutions (10-5 M) of different volume ratios. Photoluminescence
quantum yield was carried out with FLS-980 spectrometer. The lifetimes of nondoped
films were measured on an Edinburgh FLS-980 with an EPL-375 optical laser. Cyclic
voltammetry (CV) was performed with a BAS 100W Bioanalytical System, using a
glass carbon disk (diameter = 3 mm) as the working electrode, a platinum wire with a
porous ceramic wick as the auxiliary electrode, and Ag/Ag+ as the reference electrode
standardized by the redox couple ferrocenium/ferrocene. All solutions were purged
with a nitrogen stream for 10 min before measurement. The procedure was performed
at room temperature, and a nitrogen atmosphere was maintained over the solution
during measurements.
2.3 Theoretical calculation.
The ground-state geometry was optimized at the level of B3LYP/6-31G(d, p), spatial
distributions of the HOMOs and LUMOs of the compound was obtained from the
optimized ground state structure. On the basis of the optimized configuration of the
ground state (S0), the high excitation energy levels of singlet and triplet states were
evaluated using TD-M06-2X/6-31G(d, p). In order to gain further insight into the
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character of excited states, natural transition orbitals (NTOs) were evaluated for S1 -S5
and T1 -T5 states.
2.4 Device fabrication.
A nondoped device was fabricated with the structure of indium-tin oxide (ITO)/
dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) (5 nm)/
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine (TAPC) (50 nm)/
PBTPA (20 nm)/ 1,3,5-Tris(3-pyridyl-3-phenyl)benzene (TmPyPB) (50 nm) / LiF (1
nm)/ Al (100 nm). In this structure, ITO and LiF/Al were used as the anode and
cathode, respectively. HATCN and TAPC served as the hole injection layer and the
hole transport layer, respectively. TmPyPB served as both an electron transport layer
and a hole blocking layer. The doped structure was indium-tin oxide (ITO)/
dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) (5 nm)/
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine (TAPC) (50 nm)/
1,3-bis(N-carbazolyl)benzene
mCP
(10nm)/
9,10-bis(2-naphthyl)anthraces
(AND):PBTPA 2% (20 nm)/ 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB) (45
nm)/ LiF (1 nm)/Al (100 nm). In this structure, mCP served as the electron-blocking
layer and ADN as the host material for doping.
ITO-coated glass with a sheet resistance of 15‒20 Ω cm−2 was used as the substrate.
Before device fabrication, the ITO glass substrates were cleaned with isopropyl
alcohol and deionized water, dried in an oven at 120 °C. After oxygen plasma
cleaning for 3 min and finally transferred to a vacuum deposition system with a base
pressure greater than 5 × 10-6 mbar for organic and metal deposition. The thickness of
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each deposition layer was monitored using a quartz crystal thickness/ratio monitor
(STM-100/MF, Sycon). The current-voltage-luminance characteristics were measured
by using a Keithley source measurement unit (Keithley 2400 and Keithley 2000) with
a calibrated silicon photodiode. The EL spectra were measured by a Spectrascan
PR650 spectrophotometer. EQEs were calculated from the luminance, current density,
and EL spectrum according to the literature.
2.5 Synthesis
The synthesis routes and molecular structures of PBTPA is depicted in Scheme 1.
Intermediates M1, M2 and final product PBTPA were synthesized by Suzuki-Miyaura
Cross-Coupling.38 The nuclear magnetic resonance data of the intermediate products
M1 and M2 (Figure S1) are included in the supporting information. The nuclear
magnetic resonance spectrometry (Figure S2), mass spectrometry (Figure S3) and
elemental analysis confirmed that we finally got the target product.
Synthesis of 10-ethyl-10H-phenothiazine (1)
A mixture of 10 H-phenothiazine (4.00 g, 17.6 mmol), sodium hydroxide (6.00 g, 150
mmol), bromoethane (2.50 g, 22.7 mmol), dimethyl sulfoxide (70 mL) was stirred at
room temperature for 24 hours under nitrogen atmosphere. The reaction mixture was
poured into water and extracted with dichloromethane for several times. The organic
phase was collected and dried over anhydrous MgSO4. After filtration and solvent
evaporation, the residue was purified via silica gel column chromatography using
petroleum ether/dichloromethane (10/1; v/v) as an eluent to give the product as a
white solid (4.80 g, 84.4%).1H NMR (500 MHz, Chloroform-d) δ 7.16 – 7.08 (m, 4H),
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6.89 (td, J = 7.5, 1.2 Hz, 2H), 6.85 (dd, J = 8.1, 1.1 Hz, 2H), 3.91 (q, J = 7.0 Hz, 2H),
1.40 (t, J = 6.9 Hz, 3H).
Synthesis of 3-bromo-10-ethyl-10H-phenothiazine (2)
A solution of NBS (3.13 g, 17.6 mmol) in anhydrous DMF (40 mL) was added to a
solution of 1 (4.00 g, 17.6 mmol) in anhydrous DMF (70 mL) by dropwise at
0
.Then the mixture reacted at room temperature for 8 hours under nitrogen
atmosphere. The reaction mixture was poured into water and extracted with
dichloromethane for several times. The organic phase was collected and dried over
anhydrous MgSO4. After filtration and solvent evaporation, the residue was purified
via silica gel column chromatography using petroleum ether/ethyl acetate (15/1; v/v)
as an eluent to give the product as a white solid. Then the white solid was
recrystallized from a mixture of ethyl acetate/methanol (1:3; v/v) and we got the
1
target product (3.80 g, 70.6%). H NMR (500 MHz, Chloroform-d): 1.44 (t, J ¼ 6.9
Hz, 3H,); 3.91 (q, 2H), 6.71 – 7.30 (m, 7H).
Synthesis
of
10-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-phenothiazine (M1)
A solution of 2 (3.00 g, 9.8 mmol), bis(pinacolato)-diboron (2.98 g, 11.73 mmol),
Pd(dppf)2Cl2 (0.14 g, 0.19 mmol) and KOAc (2.88 g, 29.35 mmol) in degassed
dioxane (50 mL) was stirred at 90 for 8 h under nitrogen atmosphere. The reaction
was quenched by adding deionized water, and the resulting mixture was extracted
with dichloromethane. The organic phase was dried over anhydrous magnesium
sulfate and concentrated in vacuum. The crude product was purified via silica gel
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chromatography using petroleum ether/dichloromethane (3:2; v/v) as the eluent to
give the desired compound as white solid powder (1.90 g, 55%). 1H NMR (500 MHz,
Chloroform-d) δ 7.64 – 7.55 (m, 2H), 7.17 – 7.06 (m, 2H), 6.89 (t, J = 7.5 Hz, 1H),
6.84 (d, J = 8.1 Hz, 2H), 3.91 (q, J = 7.0 Hz, 2H), 1.40 (t, J = 6.9 Hz, 3H).
Synthesis of 4-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenylaniline (M2)
A mixture of 4,7-dibromobenzo[c][1,2,5]thiadiazole (3.05 g, 10.38 mmol),
(4-(diphenylamino)phenyl)boronic acid (3.00 g, 10.38 mmol), potassium carbonate
(0.72 g, 5.21 mmol), THF (80 mL) and deionized water (20 mL), with Pd(PPh3)4
(0.12 g, 0.10 mmol) acting as catalyst was refluxed at 70 °C for 24 h under nitrogen
atmosphere. After the mixture was cooled down, water (35 mL) was added to the
resulting solution and the mixture was extracted with dichloromethane for three times.
The organic phase was dried over anhydrous magnesium sulfate and concentrated in
vacuum. After filtration and solvent evaporation, the liquid was purified by silica gel
chromatography using petroleum ether/dichloromethane (3:1; v/v) to afford orange
solid (M2) (2.00 g, 42%), Compound M2 was recrystallized from a mixture of
trichloromethane and methanol (3:1; v/v). 1H NMR (500 MHz, Chloroform-d) δ 7.90
(d, J = 7.6 Hz, 1H), 7.80 (d, J = 8.6 Hz, 2H), 7.55 (d, J = 7.7 Hz, 1H), 7.28 (d, J =
18.2 Hz, 6H), 7.18 (dd, J = 8.3, 3.3 Hz, 6H), 7.08 (t, J = 7.4 Hz, 2H).
Synthesis
of
4-(7-(10-ethyl-10H-phenothiazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenyla
niline (PBTPA)
A mixture of M1 (1.44 g, 3.91 mmol), M2 (1.80 g, 3.91 mmol), potassium carbonate
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(0.27 g, 1.95 mmol), THF (80 mL) and deionized water (20 mL), with Pd(PPh3)4
(0.05 g, 0.04 mmol) acting as catalyst was refluxed at 70 °C for 24 h under nitrogen
atmosphere. After the mixture was cooled down, water (35 mL) was added to the
resulting solution and the mixture was extracted with dichloromethane for several
times. The organic phase was dried over anhydrous magnesium sulfate and
concentrated in vacuum. After filtration and solvent evaporation, the liquid was
purified via silica gel chromatography using petroleum ether/dichloromethane (2:1;
v/v) to afford jacinth solid (PBTPA) (1.20 g, 51%), pure compound PBTPA was
recrystallized from a mixture of trichloromethane and methanol (2:1; v/v). 1H NMR
(500 MHz, Chloroform-d) δ 7.89 – 7.85 (m, 2H), 7.83 (dd, J = 8.5, 2.1 Hz, 1H), 7.75
(d, J = 2.1 Hz, 1H), 7.74 – 7.69 (m, 2H), 7.30 (dd, J = 8.6, 7.3 Hz, 4H), 7.23 – 7.11
(m, 8H), 7.07 (dd, J = 7.9, 6.5 Hz, 2H), 7.02 (d, J = 8.5 Hz, 1H), 6.95 – 6.87 (m, 2H),
4.01 (q, J = 7.0 Hz, 2H), 1.48 (t, J = 6.9 Hz, 3H).13C NMR (126 MHz, Chloroform-d)
δ 154.03 , 147.43 , 144.88 , 131.54 , 130.91 , 129.84 , 129.29 , 128.18 , 127.62 ,
127.37 , 127.27 , 127.10 , 124.84 , 123.89 , 123.24 , 122.85 , 122.45 , 115.01 , 114.83 ,
53.34 , 41.91 , 12.99. MALDI-TOF MS (mass m/z): 604.1 [M+]. Anal. calcd. for
C38H28N4S2: C, 75.47; H, 4.67; N, 9.26; S, 10.60. Found: C, 75.39; H, 4.682; N, 9.27;
S, 10.564.
3. Results and discussion
3.1 Synthesis and Characterization
The synthetic route and molecular structure of PBTPA are depicted in Scheme 1.
Detailed synthetic steps and NMR data of the intermediate products are included in
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the supporting information. Intermediates M1 and M2 and the final product PBTPA
were synthesized by Suzuki–Miyaura cross-coupling.38 NMR (Figure S2) and mass
spectrometry (Figure S3) data confirmed that we had obtained the target product.
3.2 Thermal properties and electrochemical properties
As part of the development of the fabrication process of OLEDs, ensuring that the
light emitter is thermally stable, from a structural and activity standpoint, is essential.
We measured PBTPA’s thermal stability by thermogravimetric analysis and
differential scanning calorimetry (DSC) conducted in a nitrogen atmosphere. As can
be evinced from the data reported in Figure 1a and listed in Table 1, the
decomposition temperature (Td, corresponding to 5% weight loss) for PBTPA was
410 °C. A high Td value implies that a molecule has excellent thermal stability, and it
is suitable to be employed in the vacuum deposition technique. According to the DSC
curves, the glass-transition temperature (Tg) of PBTPA was 119 °C. A Tg value this
high guarantees the emitter’s morphological stability under heating, which allows the
molecule to display a consistent activity.
In order to test the electrochemical properties of PBTPA, cyclic voltammetry
measurements were conducted. As can be evinced from the data reported in Figure 1b
and Table 1, PBTPA’s reduction potential was found to be −1.61 V and its oxidation
potential −0.38 V. The fact that the oxidation and reduction curves were reversible
indicated that the molecule could gain and lose electrons stably, which can use
relieved in OLED devices as a luminous layer. The values for the energy levels of the
HOMO and LUMO of PBTPA were calculated to be −4.98 eV and −3.10 eV,
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respectively, which means that the value of the HOMO–LUMO gap was 1.88 eV,
consistent with red light emission.
3.3 Photophysical Properties
In Figure 2a are reported the UV-vis absorption and photoluminescence (PL) spectra
of PBTPA in toluene ([PBTPA] = 1 × 10–5 M). Two absorption peaks were observed at
310 nm and 461 nm. The sharp, strong absorption peak at 310 nm is mainly due to the
π–π* transition of the phenothiazine and triphenylamine fragments in the molecular
skeleton. The absorption band of PBTPA in the 370–550 nm wavelength range is the
result of an intramolecular charge transfer (ICT). Notably, measuring the UV-vis
absorption spectra in solvents with different polarities (Figure S4) was not associated
with any significant changes in the spectrum’s shape and position, indicating that the
value of the ground-state dipole moment is very constant. The emission peak of
PBTPA was observed at 629 nm, and it was characterized by a PLQY value of 50.9%
in pure toluene solution.
The data in Figure 2b illustrate the solvation effect, whereby the peaks in the PL
spectrum of PBTPA shift in wavelength as a function of the solvent’s polarity. In
particular, the said spectrum red-shifted by 74 nm on going from hexane to
acetonitrile as solvent. The formation of a new energy balance between the solvent
and the excited molecule is essential for the mentioned red shift in the PL spectrum to
be observed. This solvatochromic effect indicated the presence of a CT component in
the excited state. Notably, the PLQY value for PBTPA decreased steeply from 88% in
hexane to 3.6% in acetonitrile. In detail, in the low-polarity hexane solution, the
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PLQY of PBTPA was relatively large. As the solvent polarity increased, however, the
PLQY value decreased to 50.9% in the medium-polarity toluene, and to 57.9% in
butyl ether. In the highly polar acetone and acetonitrile, PLQY decreased drastically
to 4.3% and 3.6%, respectively. This downward trend of PLQY in higher polarity
solutions was caused by an increasing CT component that competes with LE.
In order to further delve into the solvation effect, a Lipper–Mataga solvation model,
whereby the Stokes shift values are plotted against the polarity of the solvent, was
constructed. The calculated dipole moment of excited PBTPA is 11.2 D, and the
ground dipole moment has a value of 3.2 D. The dipole moment of the excited
molecule is thus larger than that of the ground state, and it is smaller than that of the
typical CT molecule DMABN (23 D)39; once again, the constant slope of the line
(Figure 2c) implied that the excited state of the molecule is a typical hybrid state
composed of both CT and LE.
The lifetime curve in Figure 2d indicated that PBTPA in film form is characterized by
a short fluorescence lifetime (4.7 ns). In fact, the lifetime curve presented a single
exponential decay. The lack of a fluorescence delay illustrated well the fact that the
molecule’s behavior does not conform to the TADF or the TTA mechanisms.
Employing equations Kr = Фf/τ and Knr = 1/τ − Kr, where Фf represents the molecule’s
fluorescence efficiency (16.2% for PBTPA in film) and τ represents the molecule’s
transient fluorescence lifetime (4.71 ns for PBTPA in film), we were able to calculate
PBTPA’s radiation rate (Kr = 3.4 × 107 s−1) and the same molecule’s non-radiation rate
(Knr = 17.8 × 107 s−1). Notably, the lifetime of PBTPA in the film phase (4.71 ns) was
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similar to its counterpart measured in acetonitrile solution (4.70 ns). However, the
film's PLQY value (16.2%) was more than four-fold higher than that of acetonitrile
(3.6%). This difference might be attributed to the luminescence of PBTPA in
acetonitrile solution is a single molecule process, in contrast to the thin film case,
where molecules are aggregated and exhibit an AIE effect, which increases PL
efficiency.
To explore the AIE characteristics, we confirmed the PL spectra of PBTPA in the
mixture solutions. Based the data reported in Figure 3a, b, it can be evinced that
PBTPA had an emission at 674 nm with a broad full width at half maximum in pure
THF. The PL intensity began to decrease as the water fraction (fw) in the solvent
increased. However, it increased dramatically after fw reached a value of 50%.
Interestingly, a red shift of 15 nm occurred in PBTPA’s emission spectrum from 80%
fw to 90% fw. This observation might be due to a certain degree of change occurring in
the molecular aggregation state of PBTPA when fw reaches the value of 90%, making
the molecular configuration more planar, so that the increase in PL intensity at 90% fw
was not apparent as the red shift occurred. The AIE trend for the molecule can be
called aggregation-induced enhanced emission. The low PL strength of PBTPA at fw
values below 50% can be attributed to the molecule was combinations of donors and
acceptor units, which may trigger a twisted intramolecular charge transfer leading to
emission annihilation, as the solution’s polarity increases.40, 41 According to the theory
of restricted intermolecular motion, hindrance to the free rotation of the benzene ring
in the propeller-like configuration of triphenylamine and inhibition of the
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conformation change of phenothiazine are the main causes of the AIE phenomenon.42
3.4 Theoretical Calculations
In order to verify that PBTPA’s molecular design conforms to the energy distribution
of typical HLCT materials, we performed time-dependent density functional theory
(TDDFT) calculations on PBTPA. The geometry configuration and frontier molecular
orbitals (FMOs) were optimized by B3LYP/6-31G (d, p) in the Gaussian09 package.
The natural transition orbits (NTOs) were obtained at the level of M062X/6-31G (d,
p). The optimized geometry configuration depicted in Figure 4a for this D–A–D
molecule includes a phenothiazine unit characterized by a dihedral angle of 40°. The
value for the twist angle of the phenothiazine and triphenylamine units adjacent to
benzothiadiazole in PBTPA is 33°. Importantly, this moderately distorted structure
favors the formation of CT states. PBTPA’s FMOs (Figure S6) are such that the
HOMO of the molecule is distributed horizontally throughout the molecule’s
backbone. By contrast, the LUMO is mainly localized vertically on the
benzothiadiazole unit and, to a lesser extent, on the adjacent aromatic rings. The
partial overlap of HOMO to LUMO transition suggested that PBTPA possesses both
CT character and π–π* transition of local luminescence.
In order to explore further the characteristics of HLCT, we investigated the excited
states of PBTPA. The NTOs showed the hole-particle distribution of the first five
singlets and the first five triplets (Figure S7) and corresponding energy level diagram.
As can be evinced from Figure 4a, the lowest-energy singlet (S1) of PBTPA exhibits
the orbital characteristics of LE and CT hybridization, with a certain degree of overlap
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between the holes and particles. The oscillator strength is 0.7135, which means that S1
is the primary energy level for radiation transition. The lowest-energy triplet (T1)
mainly showed the hybridization state, with the particles and holes concentrated on
benzothiadiazole, an arrangement that is not conducive to the establishment of a weak
exciton binding interaction. The results of the energy level calculations (Figure 4b)
indicated that the magnitude of ΔE(T1–S1) (0.84 eV) is not conducive to the formation
of the “cold exciton” channel for the TADF-type RISC process. The T2 state
presented a CT-dominated hybrid state, resulting in the ΔE(T2–S1) value to be 0.17 eV,
a small enough value to afford the possibility for the hot exciton RISC process from
T2 to S1 to occur. The value of ΔE(T1–T2) is 1.01 eV, which is substantial enough to
render KRISC more favorable in the process of competing with the IC rate KIC. In the
excited state, a T2→S1 “hot exciton” channel formed, which changes the spin
characteristics of some triplet excitons to singlet excitons for fluorescence emission.
The results of the theoretical calculations performed indicated, therefore, that the
molecular energy distribution of PBTPA displayed the characteristics that would
favor HLCT, which is expected to break the 25% quantum statistical limitation, thus
affording the means to construct an efficient OLED.
3.5 Electroluminescence Properties
Based on the previously discussed electrochemical and thermal stability of PBTPA,
we employed the vacuum deposition technique to fabricate a non-doped OLED
comprising this red light emitter. The structure of this OLED, which we dubbed
device
I,
is
indium-tin
oxide
(ITO)/
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dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) (5 nm)/
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine (TAPC) (50 nm)/
PBTPA (20 nm)/ 1,3,5-Tris(3-pyridyl-3-phenyl)benzene (TmPyPB) (50 nm)/ LiF (1
nm)/ Al (100 nm). In this structure, ITO and LiF/Al were used as the anode and
cathode, respectively. HATCN and TAPC served as the hole injection layer and the
hole transport layer, respectively. PBTPA was used as the emitting material layer.
TmPyPB functioned as both an electron transport layer and a hole blocking layer.
Details of the electroluminescence performance of device I are listed in Table 2.
Device I achieved a maximum EQE value of 1.62%, and it was characterized by an
emission peak at 656 nm (Figure 5a) with CIE coordinates of (0.65, 0.32) in the
red-light region, which close to the red primary color standard set by REC709 (0.64,
0.33). Device I exhibited a low efficiency roll-off, at 100 cd m-2, the EQE was 1.33%,
which was 82% of the maximum EQE.
We went on to explore the utilization of excitons in the electroluminescence process.
The current density–luminance relationships of device I, reported in Figure S8 c,
presents a linear correction rather than quadratic relationship, so we can conclude that
a TTA mechanism of P-type delayed fluorescence is not compatible with device I.43, 44
In order to calculate the specific exciton utilization, we introduced the equation ꢀEQE
=
ꢀout·ꢀrec·ꢀpl·ꢀs, where ꢀꢁ is the radiative exciton ratio, ꢀꢂꢃꢄ is the light outcoupling
efficiency (its value is generally assumed to be 20%), ꢀpl is the film
photoluminescence efficiency, and ꢀrec is the fraction of exciton formation of the
injected charge carriers, which reaches its ideal value of 100% only if holes and
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electrons are fully balanced and completely recombined to form excitons.
Calculations indicate the exciton utilization efficiency of device I to be 50%.
Therefore, device I exceeded the spin statistic limitation by 25%, allowing the goal of
efficient luminescence to be achieved.
As a way to improve the performance of turn-on voltage and maximum brightness of
the device, the doped device II, with an optimized structure, was assembled. Detailed
data for this species are reported in Table 2, Figure 5b and Figures S9. Device II
consisted
of
indium-tin
oxide
(ITO)/
dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) (5 nm)/
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine (TAPC) (50 nm)/
1,3-bis(N-carbazolyl)benzene mCP (10 nm)/ 9,10-bis(2-naphthyl)anthraces (AND):
PBTPA2% (20 nm)/ 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB) (45 nm)/ LiF (1
nm)/ Al (100 nm). In this structure, mCP served as the electron-blocking layer and
ADN as the host material for doping. The efficiency of this device improved
significantly in comparison with device I. In particular, device II was found to be
characterized by an EQE of 4.86%, with an emission peak at 604 nm and CIE
coordinates of (0.55, 0.43); moreover, the voltage at 1cd A−1 was reduced to 3.4 V.
Notably, although doping device II overcame the problem associated with carrier
transport imbalance and achieved a huge improvement in efficiency over device I, it
did so at the expense of color purity.
4. Conclusions
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We designed the D–A–D-structured, red light emitter PBTPA. The stable thermal and
electrochemical properties of PBTPA indicated the potential of this species to be used
to fabricate OLED devices. Solvatochromic measurements indicated the molecule to
display a significant ICT, leading to an increase in polarity; the Lippert–Mataga
model combined with data on the film single-component lifetime at the nanosecond
level demonstrated that the excited state S1 of the molecule is characteristic of HLCT.
PBTPA was also characterized by AIE. The non-doped device fabricated using
PBTPA achieved red light emission at 656 nm with CIE coordinates of (0.65, 0.32)
and EQE of 1.62%, and the exciton utilization reached a value of 50%. This device
thus displays a relatively good performance as red light emitting material. TDDFT
calculations indicated PBTPA to be characterized by a large ΔET1–T2 value (1.01 eV)
and a small ΔET2–S1 (0.17 eV) value, so that an active hot exciton channel between T2
and S1 can be formed. This design idea of AIE combined with HLCT demonstrates the
great potential for the manufacture of PBTPA-based efficient red-light fluorescent
emitters.
Acknowledgment
This work was supported by the National Science Foundation of China (No.
51873095 and 51673105), the Key Project of Higher Educational Science and
Technology Program of Shandong Province of China (No. J18KZ001), and the
Natural Science Foundation of Qingdao City of China (No. 16-5-1-89-jch). And open
Project of the State Key Laboratory of Supramolecular Structure and Materials of Jilin
University (SKLSSM-202032). We appreciate that the State Key Laboratory of
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Luminescent Materials and Devices of South China University of Technology
provides convenience for fabricating OLED devices.
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Captions
Scheme 1. Synthetic steps to product PBTPA: a) Dimethyl sulfoxide, NaOH,
1-bromoethane, 24 h; b) dimethylformamide, N-bromosuccinimide, overnight; c)
potassium acetate, Pd(dppf)2Cl2, 90 °C, 8 h, N2 protection; d) K2CO3, Pd(pph3)4,
THF/H2O, 70 °C, 24 h, N2 protection.
Figure 1. Thermal and electrochemical properties of PBTPA. a) Thermogravimetric
analysis and differential scanning calorimetry curves of PBTPA; b) Cyclic
voltammograms of PBTPA. Eg: energy gap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Td:
decomposition temperature (corresponding to 5% weight loss); Tg: glass-transition
temperature.
Figure 2. Photophysical properties of PBTPA. a) UV-vis absorption and
photoluminescence (PL) spectra of PBTPA in toluene solution (10–5 M). b) Solvation
28