Enhancing performance for blue TADF emitters by introducing intramolecular CH?N hydrogen bonding between donor and acceptor

Article abstract of DOI:10.1016/j.dyepig.2019.03.016

Three novel thermally activated delayed fluorescence (TADF) emitters, named MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM are designed and synthesized for fabricating blue emission devices. The position of nitrogen atom in pyridine ring is varied relative to the donor unit and the presence of CH?N hydrogen bonding in MCZ-P3-DTM is confirmed by theoretic simulation and ¹H NMR analysis. Due to the introduction of CH?N hydrogen bonding between donor and acceptor, the dihedral angles, photophysical properties, together with thermal stabilities, of the targeted compounds are regulated ingeniously. As a consequence, MCZ-P3-DTM displays an increase of the molar extinction coefficient obviously and offers a higher PLQY of 60.1%. The TADF device adopting MCZ-P3-DTM as emitter shows a maximum external quantum efficiency (EQE) of 12.1%, which is superior to those of MCZ-B-DTM and MCZ-P2-DTM, demonstrating the introducing of intramolecular hydrogen bonding interactions between donor and acceptor is an efficient strategy for future structural design of TADF emitters.

Full text of DOI:10.1016/j.dyepig.2019.03.016

Accepted Manuscript
Enhancing performance for blue TADF emitters by introducing intramolecular CH⋯N
hydrogen bonding between donor and acceptor
Fulong Ma, Yu Cheng, Xiuxuan Zhang, Xiaofei Gu, Yu Zheng, Kamvan Hasrat,
Zhengjian Qi
PII:
S0143-7208(19)30223-2
https://doi.org/10.1016/j.dyepig.2019.03.016
DYPI 7405
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 January 2019
Revised Date: 5 March 2019
Accepted Date: 6 March 2019
Please cite this article as: Ma F, Cheng Y, Zhang X, Gu X, Zheng Y, Hasrat K, Qi Z, Enhancing
performance for blue TADF emitters by introducing intramolecular CH⋯N hydrogen bonding between
donor and acceptor, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.03.016.
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ACCEPTED MANUSCRIPT
Enhancing performance for blue TADF emitters by introducing
intramolecular CH···N hydrogen bonding between donor and acceptor
a
a,b,∗
Fulong Ma , Yu Cheng, Xiuxuan Zhang, Xiaofei Gu, Yu Zheng, Kamvan Hasrat, Zhengjian Qi
a
College of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, China
b
Jiangsu Province Hi-Tech Key Laboratory for Biomedical Research, School of Chemistry and Chemical Engineering, Southeast
University, Nanjing, 211189, China
Abstract: Three novel thermally activated delayed fluorescence (TADF) emitters, named
MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM are designed and synthesized for fabricating
blue emission devices. The position of nitrogen atom in pyridine ring is varied relative to the
donor unit and the presence of CH···N hydrogen bonding in MCZ-P3-DTM is confirmed by
1
theoretic simulation and H NMR analysis. Due to the introduction of CH···N hydrogen bonding
between donor and acceptor, the dihedral angles, photophysical properties，together with thermal
stabilities, of the targeted compounds are regulated ingeniously. As a consequence, MCZ-P3-DTM
displays an increase of the molar extinction coefficient obviously and offers a higher PLQY of
6
0.1%. The TADF device adopting MCZ-P3-DTM as emitter shows a maximum external quantum
efficiency (EQE) of 12.1%, which is superior to those of MCZ-B-DTM and MCZ-P2-DTM,
demonstrating the introducing of intramolecular hydrogen bonding interactions between donor and
acceptor is an efficient strategy for future structural design of TADF emitters.
Keywords: intramolecular hydrogen bonding; high absorption coefficient; blue emission; TADF
1
. Introduction
Organic light emitting diodes (OLEDs) are drawing tremendous attention due to
their various advantages such as light weight, flexibility and quick response, have
become the most promising technology in the new-generation display field[1-7].
Under electrical excitation, conventional fluorescent OLEDs can only harvest singlet
excitons for light emitting, therefore limiting the internal quantum efficiency (IQE) to
2
5%. The phosphorescent materials can harvest both singlet and triplet excitions
through intersystem crossing (ISC) process by heavy atom effect to realize 100% IQE
∗
Corresponding author. E-mail address: qizhengjian@seu.edu.cn (Z. Qi).

ACCEPTED MANUSCRIPT
[
8-10]. However, the incorporation of noble metals has many disadvantages like cost,
environmental pollution and instability, limiting their extensive application. Recently,
TADF emitters can harvest both triplet and singlet excitons through reverse
intersystem crossing (RISC) process under thermal activation, have been considered
as a cheaper alternative to phosphorescent OLEDs for the next generation display. The
principle of design TADF molecule is separating the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the
molecule, and therefore the energy gap between T and S was narrowed to facilitate
1
1
the triplet excitons up-convert to singlet ones[11-19].
Generally, the TADF emitters are designed to employ a twist structure between
donor and acceptor for separating the HOMO and LUMO to achieve the minimum
∆
E [20, 21]. In the past few years, there are numerous high-efficiency TADF emitters
st
with skillfully selected donor and acceptor units adopting D-A structures have been
developed[22-24]. However, the twist structure between donor and acceptor with
limited orbital overlap often leads to a weak absorption along with a low
photoluminescence quantum yield (PLQY)[25, 26]. Hence, the method to develop
TADF emitters with requisite small ∆E , intensive absorption and high PLQY is
st
essential for high-performance OLEDs. To address these issues, increasing the rigidity
of the molecule is a valid way to obtain desirable quantum efficiencies. Thereby,
several molecular design means for increasing rigidity have already been exploited to
improve the device efficiency: (i) connecting dual emitting cores to form steric
hindrance[27, 28]; (ii) introducing steric methyl between donor and acceptor moiety
to enhance steric hindrance[29-31]; (iii) increasing molecular rigidity through ortho
position linkage between donor and acceptor units[32-34]. In addition to the above
means, incorporating hydrogen bonding interaction into the molecule structure also
seems to a concise and effective way to generate rigid molecule structure[35, 36].
Moreover, intramolecular hydrogen bonding interaction can restrict the rotation
between donor and acceptor to inhibit non-radiative transition process and enlarge
conjugation[37], resulting in high absorption coefficient and improved PLQY of the

ACCEPTED MANUSCRIPT
TADF emitters.
Based on the aforementioned strategy, three novel TADF molecules,
-(3,6-bis(4-methoxyphenyl)-9H-carbazol-9-yl)phenyl)(dibenzo[b,d]thiophen-2-yl)m
4
ethanone
(MCZ-B-DTM),
5
-(3,6-bis(4-methoxyphenyl)-9H-carbazol-9-yl)pyridin-2-yl)(dibenzo[b,d]thiophen-2-
yl)methanone
(MCZ-P2-DTM),
6
-(3,6-bis(4-methoxyphenyl)-9H-carbazol-9-yl)pyridin-3-yl)(dibenzo[b,d]thiophen-2-
yl)methanone (MCZ-P3-DTM) were designed and synthesized. In these three
compounds, the 3,6-bis(4-methoxyphenyl)-9H-carbazole is chosen as the donor
moiety due to its moderate donor ability which is suitable for constructing blue TADF
emitters. Comparative study of their thermal stabilities, photophysical properties and
device performances are carried out in detail. Because of the different position of
pyridine group connecting to the keto and carbazole derivatives, intramolecular
CH···N hydrogen bonding between donor and acceptor formed in MCZ-P3-DTM is
1
confirmed by theoretical simulation and H NMR analysis. Compared with
MCZ-B-DTM and MCZ-P2-DTM, MCZ-P3-DTM exhibits reduced dihedral angle
with increased absorption coefficient, thermal stability and PLQY. As a result,
MCZ-P3-DTM based sky-blue OLED achieves external quantum efficiency (EQE) of
1
2.1%.
2
. Experimental
2
.1 General information
All solvents and materials were purchased from commercial sources and used
1
13
without any further purification if not specifically mentioned. H NMR and C NMR
spectra were measured on a Bruker Advance 600MHz using Si(CH ) as internal
3
4
standard. Molecular masses were recorded on a LC-MS (Agilent 1260, Agilent
Technologies), UV-Vis absorption spectra and photoluminescence emission spectra of
the target compounds were measured on a UV-2600 (Shimadzu Corporation) and a
Fluoromax-4 (Horiba Jobin Yvon Inc) spectrophotometer, respectively. The solid PL

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quantum efficiency was obtained using Fluoromax-4 (Horiba Jobin Yvon Inc) with an
integrating sphere under nitrogen at room temperature. Thermo gravimetric analysis
(TGA) were performed with Netzsch simultaneous thermal analyzer (STA) system
-1
(
STA409 PC) under a dry nitrogen gas flow at a heating rate of 10℃ min .
Differential scanning calorimetry (DSC) curves were measured using a Netzsch
instrument (TA, Netzsch). Cyclic voltammogram was measured on CH Instruments,
Inc (Shanghai, China) 660E electrochemical work station in a solution of Bu NClO
4
4
−
1
(
0.1 M) in anhydrous acetonitrile at a scan rate of 50 mV s with a Pt wire and
Ag/AgCl electrodes as the counter electrode and the reference electrode, respectively.
The ferrocene was selected as internal standard and the molecule thin films coated on
ITO-coated glass were measured.
2.2 Device measurements and characterization
The glasses coated by indium tin oxide (ITO) with sheet resistance of 15 Ω per
square were cleaned with deionized water, acetone and ethanol for 15 minutes in
sequence before fabricating devices. Then the substrates were treated with UV-Ozone
for 20 minutes. Organic layers were deposited into the ITO-coated substances when
-4
the pressure reached ~ 4 × 10 Pa in vacuum chamber. The deposition rates of
−
1
−1
−1
functional layers were 1~2 Å s , 0.1 Å s and 6 Å s for organic materials, LiF and
−
2
Al, respectively. The active area was 3 × 3 mm controlled by a shadow mask. All the
tests were performed at room temperature in ambient condition. Current density and
luminance versus driving voltage curves and electroluminescent spectra were
measured by Keithley2400s Source Meter and Photo Research PR 655
spectrophotometer. External quantum efficiencies of the devices were calculated
assuming a Lambertian emission distribution.
2
2
.3 Materials
.3.1 Synthesis of (6-bromopyridin-3-yl)(dibenzo[b,d]thiophen-2-yl)methanone
(5Br-DTM)

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5
-bromonicotinic acid (1.5 g, 5.36 mmol) was dissolved in thionyl chloride (15 mL)
and refluxed for 3 h. Then, the excess thionyl chloride was removed under vacuum to
give 5-bromopicolinoyl chloride as yellow crystalline solid. Next, 5-bromonicotinoyl
chloride and dibenzothiophene (0.99 g, 5.36 mmol) were dissolved in dry
dichloromethane (30 ml), and then aluminum trichloride (0.85 g, 6.4 mmol) was
added slowly at 0ºC followed by stirring at room temperature for 6 h and refluxed for
3
h. Then, the reaction mixture was quenched through pouring into 200 mL ice-HCl
solution and extracted with dichloromethane for three times. The combined organic
phase was washed three times with water and dried over anhydrous MgSO . The
4
solvent was removed under reduced pressure and the residue was purified on a silica
gel column using petroleum ether/dichloromethane mixture (4:1, v/v) as eluent to
1
afford the product as yellow powder in 57% yield. H NMR (600 MHz, DMSO-d6) δ
[
ppm]: 8.97-8.91 (m, 2H), 8.96-8.92 (m, 2H), 8.48 (d, J = 7.1 Hz, 1H), 8.48 (d, J = 7.1
Hz, 1H), 8.39 (dd, J = 8.3, 2.3 Hz, 1H), 8.39 (dd, J = 8.3, 2.3 Hz, 1H), 8.22 (d, J = 8.4
Hz, 1H), 8.22 (d, J = 8.4 Hz, 1H), 8.13-8.07 (m, 2H), 8.12-8.08 (m, 2H), 8.03 (dd, J =
1
3
8
.4, 0.5 Hz, 1H), 7.57 (dtd, J = 21.4, 7.2, 1.2 Hz, 2H). C NMR (151 MHz, DMSO) δ
ppm]: 192.73, 153.70, 150.00, 144.29, 140.86, 139.38, 135.17, 135.14, 132.89,
29.04, 128.21, 126.55, 125.68, 125.19, 124.41, 123.73, 123.38, 123.00. MS (LC-MS)
m/z]: calcd for C H BrNO , 366.97; found, 367.96.
[
1
[
1
8
10
5
2
.3.2 Synthesis of (5-bromopyridin-2-yl)(dibenzo[b,d]thiophen-2-yl)methanone
(6Br-DTM)
6
Br-DTM was prepared via the same procedure to that of 5Br-DTM by using
6
-bromopicolinic acid (1.2 g, 2.8 mmol) instead of 5-bromonicotinic acid in 54%
yield.
1
H NMR (600 MHz, CDCl ) δ [ppm]: 8.85 (dd, J = 2.4, 0.6 Hz, 1H), 8.59 (d, J = 1.4
3
Hz, 1H), 8.22-8.19 (m, 1H), 8.16 (dd, J = 8.2, 2.4 Hz, 1H), 8.01-7.98 (m, 1H), 7.92
13
-
7.88 (m, 2H), 7.54 (qd, J = 6.8, 1.4 Hz, 3H). C NMR (150 MHz, CDCl ) δ [ppm]:
3
1
93.26, 154.97, 151.10, 145.16, 139.90, 139.78, 135.77, 134.88, 132.81, 132.40,

ACCEPTED MANUSCRIPT
1
27.76, 127.69, 125.10, 124.44, 123.49, 123.05, 123.03, 122.03. MS (LC-MS) [m/z]:
calcd for C H BrNO , 366.97; found, 366.80.
1
8
10
5
2
.3.3 Synthesis of (4-(3,6-bis(4-methoxyphenyl)-9H-carbazol-9-yl)phenyl)
dibenzo[b,d]thiophen-2-yl)methanone (MCZ-B-DTM)
,6-bis(4-methoxyphenyl)-9H-carbazole (758.9 mg, 2.0 mmol) and sodium hydride
60%, 72 mg, 3 mmol) were dissolved in dry DMF (15 ml) under nitrogen condition.
stirring 30 min at room temperature,
(
(
3
After
dibenzo[b,d]thiophen-2-yl(4-fluorophenyl)methanone (500 mg, 1.63 mmol) in dry
DMF (5 ml) was added, and then the reaction mixture was stirred at 110ºC for 12 h.
After cooling, adding cold water to the mixture slowly and extracted with
dichloromethane for three times. The solvent was removed under reduced pressure
and the residue was purified on a silica gel column using petroleum
ether/dichloromethane mixture (3:1, v/v) as eluent to afford the product as
1
yellow-green powder in 78% yield. H NMR (600 MHz, CDCl ) δ [ppm]: 8.73 (s, 1H),
3
8
.35 (d, J = 1.6 Hz, 2H), 8.29-8.26 (m, 1H), 8.16 (d, J = 8.4 Hz, 2H), 8.02 (d, J = 1.0
Hz, 2H), 7.94-7.91 (m, 1H), 7.82 (d, J = 8.4 Hz, 2H), 7.69-7.66 (m, 6H), 7.62 (d, J =
1
3
8
.5 Hz, 2H), 7.56-7.53 (m, 2H), 7.06-7.03 (m, 4H), 3.89 (s, 6H). C NMR (151 MHz,
CDCl3) δ [ppm]: 195.35, 158.83, 144.33, 141.69, 139.78, 136.32, 135.64, 135.14,
1
1
34.31, 133.99, 133.80, 131.97, 128.30, 128.12, 127.56, 126.13, 125.59, 124.98,
24.55, 123.57, 123.01, 122.72, 122.08, 118.52, 114.30, 110.17, 55.41. MS (LC-MS)
[
m/z]: calcd for C H NO S, 665.20; found, 666.80.
45 31 3
2
.3.4 synthesis of (6-(3,6-bis(4-methoxyphenyl)-9H-carbazol-9-yl)pyridin-3-yl)
dibenzo[b,d]thiophen-2-yl)methanone (MCZ-P2-DTM)
,6-bis(4-methoxyphenyl)-9H-carbazole (618.6 mg, 1.63 mmol), 5Br-DTM (500
mg, 1.36 mmol), sodium carbonate (18.8 mg, 0.136 mmol), Copper iodide(25.9 mg,
.136 mmol), and 1,10-phenanthroline (24.5 mg, 0.136 mmol) were dissolved in dry
DMF (20 ml), the mixture stirred at 160ºC for 12 h. After cooling, the mixture was
(
3
0

ACCEPTED MANUSCRIPT
poured into water and extracted with dichloromethane for three times. The solvent
was removed under reduced pressure and the residue was purified on a silica gel
column using petroleum ether/dichloromethane mixture (3:1, v/v) as eluent to afford
1
the product as yellow powder in 76% yield. H NMR (600 MHz, CDCl ) δ [ppm]:
3
9
.13 (d, J = 2.2 Hz, 1H), 9.07 (d, J = 1.2 Hz, 1H), 8.46 (d, J = 8.3 Hz, 1H), 8.37-8.32
m, 3H), 8.27 (ddd, J = 10.7, 7.1, 2.7 Hz, 2H), 8.01 (d, J = 8.4 Hz, 1H), 7.90 (dd, J =
.0, 3.0 Hz, 1H), 7.71-7.65 (m, 6H), 7.60 (d, J = 8.5 Hz, 2H), 7.53 (dd, J = 5.9, 3.1 Hz,
(
6
2
1
1
1
1
3
H), 7.05 (d, J = 8.7 Hz, 4H), 3.89 (s, 6H). C NMR (150 MHz, CDCl ) δ [ppm]:
3
92.12, 158.94, 153.08, 146.07, 144.85, 139.66, 139.49, 137.17, 135.52, 135.37,
34.62, 134.19, 134.05, 132.61, 129.08, 128.32, 127.39, 126.16, 125.86, 124.90,
24.89, 124.75, 122.94, 122.47, 122.09, 118.69, 114.33, 109.78, 55.42. MS (LC-MS)
[
m/z]: calcd for C H N O S, 666.20; found, 667.40.
44 30 2 3
2
.3.5 synthesis of (5-(3,6-bis(4-methoxyphenyl)-9H-carbazol-9-yl)pyridin-2-yl)
(dibenzo[b,d]thiophen-2-yl)methanone (MCZ-P3-DTM)
MCZ-P3-DTM was prepared via the same procedure to that of MCZ-P2-DTM by
using 6Br-DTM (500 mg, 1.36 mmol) instead of 5Br-DTM in 73% yield.
1
H NMR (600 MHz, CDCl ) δ [ppm]: 9.20 (d, J = 2.0 Hz, 1H), 8.72 (d, J = 0.9 Hz,
3
1
8
7
H), 8.47 (d, J = 2.4 Hz, 1H), 8.32 (d, J = 1.7 Hz, 2H), 8.28-8.25 (m, 1H), 8.12 (d, J =
.6 Hz, 2H), 8.05-8.00 (m, 2H), 7.93-7.89 (m, 2H), 7.71-7.67 (m, 6H), 7.54 (ddd, J =
1
3
.0, 5.0, 1.6 Hz, 2H), 7.05-7.03 (m, 4H), 3.89 (s, 6H). C NMR (150 MHz, CDCl ) δ
3
[
ppm]: 157.95, 150.49, 138.97, 137.59, 134.13, 134.01, 133.06, 127.28, 126.83,
26.65, 124.78, 124.73, 124.03, 122.46, 122.01, 121.96, 121.06, 117.24, 116.12,
13.31, 111.36, 54.40. MS (LC-MS) [m/z]: calcd for C H N O S, 666.20; found,
1
1
6
4
4
30
2
3
67.40.

ACCEPTED MANUSCRIPT
Scheme 1. Synthetic routes of MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM.
3
. Results and discussion
3
.1. Synthesis and characterization
The synthetic routes and chemical structures of MCZ-B-DTM, MCZ-P2-DTM and
MCZ-P3-DTM were illustrated in Scheme 1. The MCZ[38] and CP-BP-F[39] were
obtained according to the previous literature methods. The key intermediates
5
Br-DTM and 6Br-DTM were facilely prepared through Friedel-Crafts reactions. The
target compounds were synthesized using MCZ reacted with corresponding
intermediates by means of Ullmann coupling reactions. The final products were
purified through column chromatography before measurements and device fabrication
processes. The chemical structures of these three TADF emitters were confirmed by
1
13
H NMR, C NMR and mass spectrometry.
1
H NMR chemical shifts of the carbazole unit were influenced by the surrounding
environment toward the molecule structures. As shown in Figure S1, when the
carbazole unit was linked with phenyl in MCZ-B-DTM, the proximal proton of
carbazole unit appeared at δ 7.63 ppm. However, when the phenyl was replaced by
pyridine in MCZ-P3-DTM, the proton was shift to δ 8.47 by 0.84 ppm, which was due
to the introduction of intramolecular CH···N bonding between the carbazole unit and
pyridine moiety. Besides, there was no obvious CH···N bonding between donor and

ACCEPTED MANUSCRIPT
acceptor was found in MCZ-P2-DTM due to the nitrogen in pyridine ring was para to
the carbazole unit, and therefore the proximal proton signal occurred at δ 7.61 ppm,
which was similar to MCZ-B-DTM[35].
3
.2. Thermal analysis
In order to analyze the thermal properties of designed emitters, the
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were
-1
performed at a heating rate of 10℃min under nitrogen atmosphere. As shown in
Figure 1(c), all the compounds exhibited high decomposition temperatures (Td,
corresponding to 5% weight loss of initial value) around 400 to 434℃. Particularly,
MCZ-P3-DTM presented a T at 434℃, which was higher than MCZ-B-DTM and
d
MCZ-P2-DTM due to the increased rigidity by incorporating intramolecular hydrogen
bonding. The higher decomposition temperatures implied their outstanding thermal
stabilities, which mean that they were suitable for evaporation process during device
fabrication. Meanwhile, the glass transition temperature (T ) of the three emitters
g
were not detected from 25 to 200℃, which was beneficial to reduce the phase
separation rate of the host-gust system to form stable uniform films[40].
Table 1. Thermal and photophysical properties.
a
a
b
c
c
d
d
e
f
ꢀꢁ
/ꢀꢂ
λꢃꢄꢅ
λꢆꢇ
ϕꢈ
τ /τ
S /T /∆E
HOMO/LUMO
[eV]
ꢉ
ꢂ
1
1
ꢅꢊ
Compound
[
℃]
[nm]
[nm]
[%]
[ns]/[µs]
[eV]
MCZ-B-DTM
MCZ-P2-DTM
- /400.0
- /428.6
- /434.2
292,366
292,390
292,382
462
493
484
35.6
51.4
7.8/5.1
8.4/3.5
5.9/4.2
3.14/2.71/0.43
2.91/2.55/0.23
2.98/2.69/0.29
5.38/2.42
5.44/2.59
5.42/2.50
MCZ-P3-DTM
60.1
a
-1
Measured by DSC/TGA at a heating rate of 10℃min .
b
Measured in toluene.
c
Measured in thin films doped in mCP .
d
e
The prompt and delayed fluorescence lifetimes of the emitters doped in mCP (10%) under room temperature.
Obtained in toluene at 77 K.
fHOMO was obtained from the CV: HOMO = – (ꢋꢑꢒ + 4.8 ‒ ꢋꢓ/ꢔ,ꢕꢕꢖꢆꢗꢗꢘꢙꢆꢚꢆꢕ) eV, ferrocene was used as an
ꢌꢍꢎꢏꢐ
external reference for calibration (ꢋ
= + 0.45 eV vs. Ag/AgCl) in Bu NClO /CH CN; LUMO =
4 4 3
ꢓ/ꢔ,ꢕꢕꢖꢆꢗꢗꢘꢆꢚꢙꢆ
opt
HOMO + Eg
.

ACCEPTED MANUSCRIPT
3
.3. Photophysical properties
The photophysical properties of these three compounds were collected based on
UV-vis absorption, photoluminescence (PL) spectra and time-resolved transient PL, as
presented in Figure 1 (a), (b) and (d). Their photophysical data were summarized in
4
-1
-1
Table 1. MCZ-P3-DTM displayed an absorption coefficient of 5.75×10 M cm ,
4
-1
-1
which was higher than those of MCZ-B-DTM (4.95×10
M
cm ) and
4
-1
-1
MCZ-P2-DTM (4.44×10 M cm ), respectively. As anticipated for the molecular
design, the absorption coefficient was intensified due to the more planar molecular
structure with enhanced conjugation between donor and acceptor by introducing
intramolecular CH···N hydrogen bonding. Moreover, all the compounds showed two
types absorption band. The absorption located on 298 nm can be ascribed to the local
excited transition of their conjugated backbone, while broad absorption at the region
of 366-390 nm were ascribed to intramolecular charge-transfer transition (ICT) from
donor to acceptor units. In addition, the fluorescence emission maximum of
MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM were 463, 493 and 484 nm in
toluene, respectively. Compared with the parent MCZ-B-DTM, the observed
bathochromic shifts of MCZ-P2-DTM and MCZ-P3-DTM can be attributed to the
enhanced ICT effect, which was resulted from the increased electron-withdrawing
capacity of the acceptors through introducing additional pyridine. The PL of these
compounds were also measured in different solvents with different polarities which
displayed gradual red-shift from toluene (Tol) to N,N-dimethylformamide (DMF)
(
Figure S3 (a), (b) and (c)), indicating the electronic communication between the
donor and acceptor moiety toward solvatochromic effect[41]. As shown in Figure S3
d), the phosphorescence spectra at 77 K in toluene were obtained, the triplet energies
can be calculated from the highest peak of the emission. The S /T were 3.14/2.71,
(
1
1
2
.91/2.55 and 2.98/2.69 eV of MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM
respectively. Therefore, the ∆E were 0.43, 0.23 and 0.29 eV for MCZ-B-DTM,
st
MCZ-P2-DTM and MCZ-P3-DTM, respectively. Notable, the energy gaps between
singlet and triplet of MCZ-P2-DTM and MCZ-P3-DTM were smaller than

ACCEPTED MANUSCRIPT
MCZ-B-DTM due to the enhanced coupling between singlet and triplet after
introducing heteroatoms into the acceptor part.[42]. Moreover, the lower ∆E was
st
beneficial for the effective RISC process from triplet to singlet states.
-
5
Figure 1. Absorption (a) and fluorescence (b) spectra of these compounds in 2.0 × 10 M toluene at room
temperature (inset: photograph of PL spectra of the emitters in toluene under UV irradiation at 365 nm). (c) TGA
and DSC (inset) curves of MCZ-B-DTM, MCZ-P2-DTM and MCZ-P2-DTM. (d) Transient PL decay curves of 10
wt% emitter doped mCP thin films.
In order to testify the TADF characteristics of MCZ-B-DTM, MCZ-P2-DTM and
MCZ-P3-DTM, the transient PL decay of 10 wt% emitter doped mCP thin films were
measured under nitrogen. Figure 1 (d) displayed the transient PL decay of the
compounds and all of them exhibited bi-exponential decays. The fast decays were
originated from the prompt fluorescence with short lifetimes of 7.8 ns, 8.4 ns and 5.9
ns, respectively. The slow decays can be attributed to delayed fluorescence with long
lifetimes of 5.1 µs, 3.5 µs and 4.2 µs, respectively. Besides, as showed in Figure S2,
the transient PL decays of neat films were measured for certifying the TADF
characteristics. Similar to the doped films, all the compounds in neat films also
displayed bi-exponential decay including fast and slow decays. The fast and slow
decays of MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM were 7.5 ns/2.1 µs,

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8
.2ns/2.9 µs and 6.8 ns/2.6 µs, respectively. The transient PL decay of 10 wt% emitter
doped mCP thin films were also measured under different temperatures to further
certify their TADF characters. As can be seen from the Figure S4, the delayed
components increased gradually from 200 K to 300 K, suggesting that these
compounds displayed obvious TADF characteristics and can be applied as TADF
emitters. The time-resolved photoluminescence spectra of the TADF emitters in
toluene under nitrogen at room temperature were measured. As shown in Figure 2,
the prompt and delayed photoluminescence spectra were obtained with similar shapes
which identified the delayed emission stemmed from the singlet via reversed process.
Figure 2. Prompt and delayed photoluminescence spectra of MCZ-B-DTM (a), MCZ-P2-DTM (b) and
MCZ-P3-DTM (c) in toluene under nitrogen at room temperature.
For further exploring the emissive characteristics of the designed TADF emitters,
the absolute PL quantum yield (PLQY) of the 10 wt% emitter doped mCP thin films
were obtained through integrating sphere under nitrogen at room temperature. The
PLQY of the doped thin films were 35.6%, 51.4% and 60.1% of MCZ-B-DTM,
MCZ-P2-DTM and MCZ-P3-DTM, respectively. The MCZ-P3-DTM exhibited the
higher PLQY among these three compounds due to the introduction of CH···N

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hydrogen bonding which restricted the rotation between donor and acceptor and
increased the structural rigidity[42]. Whereas, in MCZ-P2-DTM, the position of
nitrogen was para to the donor group, and hence the molecule was more flexible than
MCZ-P3-DTM; thus, it showed a lower PLQY than MCZ-P3-DTM.[42]
3
.4. Electrochemical analysis
The electrochemical properties of these compounds were performed by cyclic
voltammetry (CV). As depicted in Figure 3, all compounds displayed clear oxidation
behaviors, which can be assigned to the oxidation of the MCZ moiety. Compared with
MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM displayed lower reduction onset
potentials, which can be assigned to the augment of electron-withdrawing capacity of
the acceptor moieties induced by introduction of pyridine ring. The HOMO energy
levels were calculated to be 5.38, 5.44 and 5.42 eV of MCZ-B-DTM, MCZ-P2-DTM
and MCZ-P3-DTM, respectively, using the energy level of ferrocene (Fc) (4.8 eV) as
a reference. The LUMO energy levels were estimated to be 2.42, 2.59 and 2.50 eV for
MCZ-B-DTM, MCZ-P2-DTM and MCZ-P3-DTM, respectively, which were
calculated from the HOMOs and Eg.

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Figure 3. Cyclic voltammograms of MCZ-B-DTM, MCZ-P2-DTM and MCZ-P2-DTM.
3
.5. Theoretical calculations
The ground-state geometries and the frontier molecular orbital cloud distributions
of these compounds were calculated using density functional theory (DFT) at the
B3LYP/6-31G(d) level to better understand the influence of hydrogen bonding for the
structures and electronic properties. The geometrical structures and calculated
electron cloud distributions were displayed in Figure 4. The torsions between the
donor and acceptor units were different among these three compounds. The dihedral
angles between donor and acceptor were both 50° of MCZ-B-DTM and
MCZ-P2-DTM, respectively, while the angle of MCZ-P3-DTM changed to 32°. This
observed difference can be explained when considering the nitrogen atom in the
pyridine ring and therefore the proximal C-H of MCZ locked the rotation between the
MCZ and pyridine hybrid acceptor. In addition, the CH···N non-bonding distance was
calculated to be 2.4 Å in MCZ-P3-DTM, revealing the strong CH···N bonding existed

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between the pyridine and MCZ[6, 9]. Besides, the bond length of C-N (between the
MCZ and pyridine or benzene rings) was 1.402 Å in MCZ-P3-DTM, which was short
than 1.414 and 1.410 Å of MCZ-B-DTM and MCZ-P2-DTM, respectively,
demonstrated that MCZ was more strongly bond with pyridine ring in MCZ-P3-DTM.
These results further confirmed the strong intramolecular CH···N bonding interaction
between the donor and acceptor in MCZ-P3-DTM.
On the other hand, The HOMOs were located on the MCZ moieties due to its
strong electron donating property. It was worth noting that the HOMOs electron cloud
distributed on the carbazole units simultaneously extended to the substitutive methoxy
benzene part on its 3,6-position, which was beneficial for improve quantum yield and
structural stability[43, 44]. However, subtle difference of the LUMO distributions in
three molecules can be observed due to the different components in acceptor units.
The LUMOs of the three emitters were mainly distributed on the central carbonyl.
Meanwhile, the electron cloud extended to the MCZ unit slightly in MCZ-P3-DTM.
Therefore, compared with MCZ-B-DTM and MCZ-P2-DTM, the orbital overlap of
HOMO and LUMO were increased in MCZ-P3-DTM, which was beneficial for
increasing quantum yield through enhancing the radiative processes[45].
Figure 4. Optimized molecule structures and HOMO/LUMO distributions calculated by using B3LYP/6-31g(d)
level.

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3
.6. Electroluminescent properties
To evaluate the electroluminescence (EL) performance of these materials as
emitters, TADF OLEDs were fabricated with the following commonly used device
structure: indium tin oxide (ITO)/TAPC (30 nm)/mCP: 10 wt% emitters (50
nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). TAPC and TmPyPB were applied as
the hole transporting layer (HTL) and the electron transporting layer (ETL). mCP with
a triplet of 2.9 eV which was higher than the TADF emitters served as a host to
construct the emitting layer (EML). The energy diagram of the devices and
electroluminescent (EL) characteristics were showed in Figure 5, and the
electroluminescent data were summarized in Table 2. The HOMO levels of the
emitters were shallow than TAPC (-5.5 eV), so it can served as effective hole trapping
sites for holes[36]. The EL spectra of the devices employed the three emitters were
similar to the PL spectra from the corresponding 10% emitter doped mCP thin films
(Figure S4 (d)), which implied that the devices with proper matching of energy levels
leading to complete energy transfer from the host to dopants. The device employed
MCZ-B-DTM as emitter displayed blue emission with the maximum peak at 467 nm
and Commission International de I’Eclairage coordinates at (0.15, 0.20). The
MCZ-P2-DTM and MCZ-P3-DTM based devices displayed sky-blue emission with
maximum peaks at 496 and 486 nm and Commission International de I’Eclairage
coordinates were (0.22, 0.44) and (0.19, 0.33), respectively. The turn-on voltages of
the devices showed similar value from 3.2 to 3.5 V. The maximum EQEs of the
devices were 7.5 and 9.7% of MCZ-B-DTM, MCZ-P2-DTM, respectively, whereas
the device employed MCZ-P3-DTM as dopant displayed a maximum EQE of 12.1%.
The relative high EQE of MCZ-P3-DTM based device mainly due to the higher
PLQY stemmed from the introduction of intramolecular CH···N interactions.

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Figure 5. (a) Energy diagram of the materials used in the devices; (b) Normalized EL spectra; (c) Current
density-voltage-luminance (J-V-L) curves; (d) EQE versus luminance curves.
Table 2. Electroluminescence characteristics of the devices.
a
b
c
λꢆꢇ
nm]
Vꢘꢚ
Lmax
ηext
ηc
ηp
CIE
Comound
-
2
-1
-1
[
[V]
[cd m ]
[%]
[cd A ]
[lm W ]
(x,y)
MCZ-B-DTM
MCZ-P2-DTM
467
496
484
3.4
3.5
3.2
4101
10380
13350
7.5
9.7
14.4
23.3
26.6
11.3
16.3
18.6
(0.15,0.20)
(0.22,0.44)
(0.19,0.33)
MCZ-P3-DTM
12.1
a
The maximum emission recorded at 5 V.
b
-2
Turn-on voltage measured at 1 cd m .
c
Commission International de I’Eclairage coordinates at 5 V.
4
. Conclusion
In summary, three novel TADF emitters (MCZ-B-DTM, MCZ-P2-DTM and
MCZ-P3-DTM) were designed and synthesized based on pyridine hybrid acceptors
and a donor of MCZ. The nitrogen atoms in pyridine rings were varied relative to the
donor units to establish a comparative study with the parent molecule MCZ-B-DTM.

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The formation of intramolecular CH···N interactions between donor and acceptor
improved the PLQY of MCZ-P3-DTM resulted from the increased molecular
structural rigidity. The device adopted MCZ-P3-DTM as emitter exhibiting a
maximum EQE and CE of 12.1% and 26.6 cd/A, respectively, which was higher than
those of MCZ-B-DTM and MCZ-P2-DTM. These results manifested that the
formation of intramolecular hydrogen interactions was a promised way for the design
of efficient blue TADF emitters.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the supporting from the Science and
Technology Achievements Transformation Special Fund of Jiangsu Province
(BA2017035), and Ministry of Science and Technology key research and
development plan (2018YFF0215204).
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ó
ó
ó
ó
Higher absorption coefficient achieved by introducing hydrogen bonding into the
molecule.
Formation of intramolecular CH···N hydrogen bonding between donor and
acceptor enhances the molecule rigidity with increased PLQY and EQE.
Comparative study is carried out by varied the position of nitrogen atom in
pyridine.
This strategy is promising to achieve high efficient blue TADF emitters.