Rational molecular design of novel host material combing intra- and intermolecular charge transfers for efficient solution-processed organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2020.108188

Two novel D-σ-A molecules bearing 2,4,6-diphenyl-1,3,5-triazine (TRZ) units as electron-accepting segment and 1,3-di (9H-carbazol-9-yl)benzene (mCP) or 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9′H-9,3':6′,9″-tercarbazole (TCz) as electron

Full text of DOI:10.1016/j.dyepig.2020.108188

Journal Pre-proof
Rational molecular design of novel host material combing intra- and intermolecular
charge transfers for efficient solution-processed organic light-emitting diodes
Guimin Zhao, Dan Liu, Wenwen Tian, Wei Jiang, Yueming Sun
PII:
S0143-7208(19)32756-1
DOI:
https://doi.org/10.1016/j.dyepig.2020.108188
DYPI 108188
Reference:
To appear in: Dyes and Pigments
Received Date: 26 November 2019
Revised Date: 2 January 2020
Accepted Date: 3 January 2020
Please cite this article as: Zhao G, Liu D, Tian W, Jiang W, Sun Y, Rational molecular design of novel
host material combing intra- and intermolecular charge transfers for efficient solution-processed organic
light-emitting diodes, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108188.
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CRediT author statement
Author name
Guimin Zhao
The relevant CRediT role
Resources; Software; Validation; Data Curation;
Roles/Writing – original draft; Writing - Review &
Editing;
Dan Liu
Formal analysis; Investigation; Project administration;
Wenwen Tian
Wei Jiang
Software;
Methodology;
Conceptualization;
Validation;
Visualization; Supervision;
Yueming Sun
Funding acquisition;

Two novel molecules mCP-o-TRZ and TCz-o-TRZ with D-σ-A motif, bearing TRZ
units as electron-accepting segment and mCP or TCz as electron-donating moiety,
were designed and analyzed, where the coexistence of inter-CT and intra-CT was
observed in mCP-o-TRZ material, while TCz-o-TRZ only showed the intra-CT effect.

Rational molecular design of novel host material combing intra- and
intermolecular charge transfers for efficient solution-processed orga
nic light-emitting diodes
Guimin Zhaoa, Dan Liua, Wenwen Tiana, Wei Jianga*, Yueming Suna
aJiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, Jiangsu
Engineering Laboratory of Smart Carbon-Rich Materials and Device, School of
Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, 211189,
P. R. China.
*Corresponding author: Wei Jiang, E-mail: jiangw@seu.edu.cn
ABSTRACT
Two novel D-σ-A molecules bearing 2,4,6-diphenyl-1,3,5-triazine (TRZ) units as
electron-accepting segment and 1,3-di(9H-carbazol-9-yl)benzene (mCP) or
9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9'H-9,3':6',9''-tercarbazole
(TCz)
as
electron-donating
moiety,
named
-1,3-
as
9,9'-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)
phenoxy)
phenylene)
bis(9H-carbazole) (mCP-o-TRZ) and 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)
phenoxy)) phenyl)- 9'H-9,3':6',9''-tercarbazole (TCz-o-TRZ), are designed and
synthesized. The oxygen atom is applied as a bridge between the various donor and
acceptor parts to realize the conjugation-forbidden linkage of the molecular backbone.
The photophysical properties, solvatochromism, theoretical calculations, thermal
properties and device performance are systematically investigated. As anticipated, the
mCP-o-TRZ molecule was endowed with intramolecular charge transfer (intra-CT)
and intermolecular charge transfer (inter-CT), while only intra-CT can be obtained in
the TCz-o-TRZ material. The solution-processed OLEDs employed mCP-o-TRZ as
host for the phosphorescence guest (tris(2-methylphenylpyridine) iridium
(III) )(Ir(mppy)3) realized a maximum EQE of 9.0 %, which is superior to that of
TCz-o-TRZ. The results demonstrate that the development of new D-σ-A molecule
equipped with dual CTs is a promising strategy for fabricating organic light-emitting

diodes.
Keywords: D-σ-A molecule, conjugation-forbidden linkage, intra-CT, inter-CT,
solution-processed OLEDs, phosphorescence
INTRODUCTION
Solution-processed organic light-emitting diodes (OLEDs) have attracted tremendous
attention because of their promising ability to facilitate the scientific and industrial
fabrication in flexible displays and large area solid state lighting [1-6]. 100% internal
quantum efficiency can be realized in phosphorescent OLEDs (PHOLEDs), which
have the ability to capture both singlet and triplet excitons [7-13]. The phenomenon of
the excited state quenching stemmed from the triplet−triplet annihilation should be
avoided as much as possible to increase the excitons utilization and achieve highly
efficient electroluminescence [14-16]. Hence, significant efforts have been made to
devised suitable host materials, such as polymer[17-19], dendrimer[10, 20, 21], and
bipolar host materials[22, 23], which restrain triplet states quenching, balance charge
transfer, and confine the excitons to enhance the performance of the
solution-processed phosphorescent OLEDs (PHOLEDs)[24].
It is well known that the following intriguing properties, such as high triplet energy
(ET), compatible highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels, balanced charge carrier transport properties
(bipolar property), should be considered into the design of the optimal host material.
In addition, designing host material with small triplet−singlet split (∆EST) was
illustrated to reduce the operating voltage of PHOLEDs effectively[25]. Even though
the bipolar host with D and A units was exploited to achieve intrinsically small ∆EST,
this also results in the complex molecular design [26-28]. The host material adopting
D-π-A structure possess intramolecular charge transfer (intra-CT), usually leading to
a low ET[29]. Subsequently, the material with D−σ−A motif has drawn researchers’
much attention in consideration of their distinctive properties [30, 31]. Adachi et al.
[32]firstly exploited a battery of molecules with a D−σ−A structure, which showed
obvious thermally activated delayed fluorescence (TADF) property, and excellent
devices performances when these molecules were used as emitter. Furthermore, the

intermolecular charge transfer (inter-CT) was observed by Zhang et al[31], where the
molecule (DMAC-o-TRZ) was fabricated by oxygen atom to interrupt the conjugation
between D (DMAC) and A (TRZ) moieties and the external quantum efficiency (EQE)
were up to 15.0 % and 14.7 % in doped-devices and non-doped devices, respectively.
Not long ago, dual CTs containing both intra-CT and inter-CT emission was obtained
by Zhang DD et al[30], where the material was employed not only as emitter but also
as a host for flour- and phos-guest, showing superior performance and low roll-off in
devices. Theoretically speaking, the relation between intra-CT and inter-CT should be
competitive, which maybe generate dual-channel energy transfer to utilize the
excitons. Therefore, the development of novel materials with D−σ−A structure is
significant for fabricating OLEDs; meanwhile, the relationship between molecular
structures and their charge transfer features should be further investigated.
In this work, we provided a concise but impactful design strategy of organic
molecules that dual CT processes can be realized, where an oxygen atom as the space
σ-junction to construct
a
D−σ−A motif. Two novel compounds,
phenoxy) -1,3- phenylene)
9,9'-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)
bis(9H-carbazole) (mCP-o-TRZ) and 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)
phenoxy)) phenyl)- 9'H-9,3':6',9''-tercarbazole (TCz-o-TRZ), were synthesized and
analyzed, where suitable D (mCP or TCz) and A (TRZ) moieties were connected to
break the conjugation linkage. This linkage can not only partially suppress the
intramolecular CT (intra-CT) transition but also form the intermolecular CT emission
(inter-CT) which also can be called as exciplex-type material[33, 34]. Specifically, the
intra-CT was obtained in mCP-o-TRZ and TCz-o-TRZ molecules, but the inter-CT
was only observed in mCP-o-TRZ. Furthermore, it can be found that dual CTs can
increase the performance of solution-processed OLEDs when the above materials
were used as host material with Ir(mppy)3 guest emitter. The solution-processed
OLEDs employed mCP-o-TRZ as host for the phosphorescence guest (Ir(mppy)3)
realized a maximum EQE of 9.0 %, which is superior to that of TCz-o-TRZ. The
results show that multichannel energy transfer can be achieved in single molecule and
contribute to the full utilization of carrier. This design strategy opens up a promising

strategy for exploiting host materials for solution-processed phosphorescence OLEDs.
EXPERIMENTAL SECTION
1. Synthesis
Besides otherwise indicated, all materials and solvents were purchased from
commercial suppliers and used without further purification. The detailed synthetic
routes of the target compounds are given in Scheme S1 and S2.
1,3,5-tribromobenzene, carbazole, 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine and
1-iodo-4-methoxybenzene were purchased.
1.1 The synthesis of 9,9'-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy) -1,3-
phenylene) bis(9H-carbazole) (mCP-o-TRZ).
The synthesis of 9,9'-(5-bromo-1,3-phenylene) bis(9H-carbazole) (Br-mCP)
Carbazole (6.6 g, 39.7 mmol), 1,3,5-tribromobenzene (5 g, 15.9 mmol), cuprous
iodine (0.3 g, 1.6 mmol), potassium phosphate (13.7 g, 64.5 mmol) and
1,10-phenanthroline (0.3 g, 1.6 mmol) were dissolved in 100 mL of 1,4-dioxane. After
the stirred in a nitrogen atmosphere for 24 h at 90 °C, the reaction mixture was cooled
to room temperature. Then, the mixture extracted with dichloromethane (100 mL × 3)
and dried over anhydrous MgSO4. Finally, the crude product was subjected to flash
1
column chromatography to give compound Br-mCP (6.3 g, 79 %). HNMR (600
MHz, CDCl3, δ): 7.33 (m, 4H), 7.46 (m, 4H), 7.53 (m, 4H), 7.81 (m, 1H), 7.87 (m,
2H), 8.16(d, 2H). MS (MALDI-TOF) [m/z]: Calcd for C30H19BrN2, 486.07; Found,
487.40. Anal. Calcd (%) for C31H22N2O: C, 73.93; H, 3.93; Br, 16.39; N, 5.75. Found;
C, 73.92; H, 3.89; Br, 16.41; N, 5.78. Melting point: 211.8 °C.
The synthesis of 9,9'-(5-methoxy-1,3-phenylene) bis(9H-carbazole) (mCP-OMe)
9,9'-(5-bromo-1,3-phenylene)bis(9H-carbazole) (5 g, 10.2 mmol) was dissolved in
DMF (50 mL) under N2 atmosphere, and then NaOMe/MeOH (40 wt%, 50 mL),
1,10-phenanthroline(0.3 g, 1.6 mmol), CuI (0.3 g, 1.6 mmol) and potassium carbonate
o
(8.5 g, 61.5 mmol) were added. The mixture was heated to 140 C for 24 h. After
cooling to room temperature, the solution was poured into 150 mL of water, extracted
with CH2Cl2（3×50 ml）, dried over MgSO4 and evaporated in vacuo. Afterwards, the
crude product was further purified using column chromatography to give mCP-OMe

as a white solid (5.26 g, 60 %). 1HNMR (600 MHz, CDCl3, δ): 3.97 (s, 3H), 7.27-7.28
(m, 2H), 7.33-7.37 (t, J= 6.6 Hz, 4H), 7.45-7.49 (m, 5H), 7.61 (d, J= 7.8 Hz, 4H), 8.18
(d, J= 7.8 Hz, 4H). MS (MALDI-TOF) [m/z]: Calcd for C31H22N2O, 438.17; Found,
438.53. Anal. Calcd (%) for C31H22N2O: C, 84.91; H, 5.06; N, 6.39; O, 3.65. Found; C,
84.85; H, 5.09; N, 6.38; O, 3.68. Melting point: 204.1 °C.
The synthesis of 9,9'-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy) -1,3- phenylene)
bis(9H-carbazole) (mCP-o-TRZ).
To a stirred solution of mCP-OMe (5 g, 11.5 mmol) in dehydrated dichloromethane at
0 °C was added BBr3 (6.46 g, 25.6 mmol). After the addition was completed, the
mixture was stirred for 3 h. The reaction was quenched by methanol, followed by
adding a saturated solution of NaHCO3. After cooling to room temperature, the
solution was poured into 150 mL of water, extracted with CH2Cl2, dried over MgSO4
and evaporated in vacuo. The product was obtained and proceeded directly to the next
step without additional purification. Then, the crude product (0.30 g, 1.07 mmol),
potassium tert-butoxide (0.35 g, 3.11 mmol) and anhydrous DMF 10 ml were added
to a roundbottom flask (50 mL) under nitrogen atmosphere. The mixture was stirred at
150 °C for 1 h, then TRZ-PhBr (0.54 g, 1.4 mmol) in 10 ml DMF was added and
stirred at 150 °C for 12 h. After completion of the reaction, water and
dichloromethane were added to the cooled mixture. The organic layer was separated,
dried over MgSO4, and concentrated in vacuo. Later, the residue solid was purified by
column chromatography to give the final product as a white powder (0.4 g,
51 %).1HNMR (600 MHz, CDCl3, δ): 7.30 (m, 4H), 7.50 (m, 4H), 7.59 (d, 4H), 7.67
(m, 8H), 7.73 (m, 3H), 8.25 (m, 4H), 8.72 (m, 4H), 8.87 (d, 2H). 13CNMR (150 MHz,
CDCl3, δ): 109.71, 116.00, 119.01, 123.74, 126.30, 128.65, 131.31, 136.18, 140.29,
140.58, 159.01, 159.96, 170.80, 171.63. MS (MALDI-TOF) [m/z]: Calcd for
C51H33N5O, 731.27; Found, 731.292. Anal. Calcd (%) for C51H33N5O: C, 83.70; H,
4.55; N, 9.57; O, 2.18. Found; C, 83.65; H, 4.53; N, 9.63; O, 2.19. Melting point:
144.1 °C.
1.2 The synthesis of 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy)) phenyl)-
9'H-9,3':6',9''-tercarbazole (TCz-o-TRZ).

The synthesis of 9'-(4-methoxyphenyl)-9'H-9,3':6',9''-tercarbazole (TCz-Ph-OMe).
A mixture of 9'H-9,3':6',9''-tercarbazole (1 g, 2 mmol), 1-iodo-4-methoxybenzene
(0.58 g, 2.5 mmol), trans-1,2-cyclohexanediamine (0.01 ml, 0.02 mmol), K3PO4 (1.1 g,
5 mmol), and CuI (0.05 g, 0.26 mmol) was dissolved in 1,4-dioxane and heated to
reflux under argon atmosphere for 24 h. Then the crude mixture was filtered, CH2Cl2
added, and washed with diluted aqua ammonia, 1 M hydrochloric acid, saturated salt
water, and water successively. The extracted organic layer was combined and
evaporated under vacuum to remove the organic solvent. Consequently, the pure
product can be obtained by recrystallization from ethyl acetate to give the product as a
white solid (0.96 g, 80 %). 1HNMR (600 MHz, CDCl3, δ): 3.99 (s, 3H), 7.24 (d, 2H),
7.27-7.34 (m, 4H), 7.40-7.45 (m, 8H), 7.58-7.62 (m, 4H), 7.65 (d, 2H), 8.19 (d, 4H),
8.30 (s, 2H). MS (MALDI-TOF) [m/z]: Calcd for C43H29N3O, 603.23; Found, 603.73.
Anal. Calcd (%) for C43H29N3O: C, 85.55; H, 4.84; N, 6.96; O, 2.65. Found; C, 85.45;
H, 4.83; N, 7.03; O, 2.69. Melting point: 147.8 °C.
The synthesis of 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy)) phenyl)-
9'H-9,3':6',9''-tercarbazole (TCz-o-TRZ).
A solution of TCz-Ph-OMe (0.8 g, 1.32 mmol) in dry (5 ml) CH2Cl2 was cooled to
0 , and BBr3 (0.25 ml 1 M solution in CH2Cl2, 2.5 mmol) was added dropwise. After
stirring for 2 h 0 , the reaction was carefully quenched with methanol, followed by
adding a saturated solution of NaHCO3. Then, the mixture was extracted with CH2Cl2,
washed with water, and dried with Na2SO4. The product (0.66 g) was obtained after
removing all solvent and drying in vacuum, proceeded directly to the next step
without additional purification subsequently. Then, the crude product (0.65 g, 1.08
mmol), potassium tert-butoxide (0.37 g, 3.3 mmol) and anhydrous DMF 20ml were
added to a roundbottom flask (100 mL) under nitrogen atmosphere. The mixture was
stirred at 150 °C for 1 h, then TRZ-PhBr (0.60 g, 1.54 mmol) in 10 ml DMF was
added and stirred at 150 °C for 12 h. After completion of the reaction, water and
dichloromethane were added to the cooled mixture. The organic layer was separated,
dried over MgSO4, and concentrated in vacuo. Finally, the residue solid was purified
by column chromatography to give the product as a greenish yellow powder (0.47 g,

50 %).1HNMR (600 MHz, CDCl3, δ): 7.29 (d, 4H), 7.35 (m, 2H), 7.42 (m, 8H), 7.46
(d, 2H), 7.56-7.71 (m, 10H), 7.76 (m, 2H), 8.17 (d, 4H), 8.29 (d, 2H), 8.79 (m, 4H),
8.88 (m, 2H). 13CNMR (150 MHz, CDCl3, δ): 109.69, 111.23, 118.77, 119.71, 119.79,
120.32, 123.16, 125.91, 128.68, 130.42, 131.21, 132.08, 136.20, 140.86, 141.76,
156.27, 160.65, 170.91, 171.64. MS (MALDI-TOF) [m/z]: Calcd for C63H40N6O,
896.33; Found, 896.346. Anal. Calcd (%) for C63H40N6O: C, 84.35; H, 4.49; N, 9.38;
O, 1.78. Found; C, 84.37; H, 4.50; N, 9.33; O, 1.80. Melting point: 181.3 °C.
2. General information
1HNMR and 13CNMR spectra were recorded on a BRUKER AMX 600 MHz and 150
MHz instrument relative to Si (CH3)4 as internal standard at room temperature.
Molecular masses were measured by matrix-assisted laser desorption-ionization
time-of-flight mass spectrometry (MALDI-TOF-MS) using a BRUKER DALTONICS
instrument, with α-cyano-hydroxycinnamic acid as amatrix. Absorption and
photoluminescence emission spectra were collected by a SHIMADZU UV-2600
spectrophotometer and a HORIBA FLUOROMAX-4 spectrophotometer, respectively.
Cyclic voltammetry was performed on a CHI750C voltammetric analyzer in
dichloromethane solutions (10-3 M) (oxidation process) at a scan rate of 100 mV s−1. A
platinum plate was acted as the working electrode, a silver wire was acted as the
pseudo-reference electrode, and a platinum wire was used as the counter electrode.
The tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene were selected as
supporting electrolyte and internal standard, respectively. AFM (Seiko Instruments,
SPA-400) was used to determine the film surface morphology. Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) curves were recorded
with a Netzsch simultaneous thermal analyzer (STA) system (STA 409PC) and DSC
2910 modulated calorimeter under a dry nitrogen gas flow at a heating rate of 10 °C
min−1. The melting points were measured by a SGW X-4 micromelting point
instrument at room temperature.
3. Device fabrication and measurements
The glass substrates were coated with indium tin oxide (ITO) and then sequentially
cleaned in an ultrasonic bath with deionized water, acetone and ethanol for three times.

Before device fabrication, the ITO substrate was treated in a UV−ozone oven for 30
min. A PEDOT: PSS layer (50 nm) was spin-coated onto the ITO surface and baked at
150
for 10 min to remove the residual water. Then, an EML with a thickness of
about 40 nm was spin-coated from a filtered 10 mg mL-1 1, 2-dichloroethane solution
onto the PEDOT: PSS layer and annealed at 80 for 30 min to remove the residual
solvent
under
nitrogen
atmosphere.
Sequentially,
40
nm
1,
3,
5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI) was vacuum-deposited as an
electron-transporting and hole-blocking layer. Finally, Cs2CO3 (2 nm) and Al (100 nm)
were evaporated acting as the cathode for the devices. The current density−voltage-
luminance characteristics, current efficiency and power efficiency were tested using a
Keithley 2636A Sourcemeter coupled with Si-potodiodes calibrated with PR655. The
EL spectra were collected with a Photo-Research PR-655 SpectraScan. All the
characterizations were performed at room temperature in ambient condition without
encapsulation. External quantum efficiencies of the devices were calculated assuming
a Lambertian emission distribution.
4. Theoretical Calculation
The density functional theory (DFT) with B3LYP/6-31G* basis was applied to
examine the geometrical and electronic properties of mCP-o-TRZ and TCz-o-TRZ,
which was set by same basis in Gaussian 09 software. The optimized configuration of
ground states (S0) from two molecule were conducted firstly. The singlet and triplet
energies of the compounds were predicted by the time-dependent DFT, and the energy
level distributions can also be visualized then.
RESULTS AND DISCUSSION
Detailed synthesis of aimed molecule, mCP-o-TRZ and TCz-o-TRZ, are illustrated in
Supporting Information. As shown in Scheme 1, mCP-o-TRZ and TCz-o-TRZ were
readily synthesized via the Williamson synthesis between the TRZ and two different
donor
moieties.
1,3-di(9H-carbazol-9-yl)benzene
(mCP)
and
9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9'H-9,3':6',9''-tercarbazole
(TCz) were chosen as D units, while triphenyltriazine (TriPh-TRZ) was conducted as
acceptor unit. The chemical structures were confirmed by nuclear magnetic resonance

(NMR) and mass spectroscopy (Figure S1 to S6). And it was further purified by silica
gel column chromatography and recrystallization before device fabrication.
Scheme 1. Synthesis routes of mCP-o-TRZ and TCz-o-TRZ.
Theoretical calculations and analysis
To better understand the molecular spatial structure and electronic properties of the
designed molecule, density functional theory (DFT) calculations at the
B3LYP/6-31G(d) Level was used to investigate geometry configuration and frontier
molecular orbitals. As shown in Figure 1, the optimized molecular structures can be

clearly observed that the HOMOs of mCP-o-TRZ and TCz-o-TRZ are mainly
dispersed over mCP and TCz, respectively, whereas the LUMOs are only offered by
TRZ. The above phenomenon indicates an ideal FMO distribution by the σ-junction,
which significantly suppresses the intramolecular interaction between D and A units.
The levels of the singlet (S1) and triplet (T1) were simulated based on the optimized
ground state by using the time-dependent density functional theory (TD-DFT)
calculations, the S1/T1 energy levels of mCP-o-TRZ and TCz-o-TRZ were determined
to be 3.29/2.96 eV and 3.10/2.99 eV, respectively. The high triplet energy (ET) may be
explained by the twisted space configuration of both molecules, which can avoid
energy leakage when these molecules are as host material. To better understand the
intramolecular charge transfer, the natural transition orbitals (NTOs) were calculated
to investigate their S0→S1 transition characters of these two molecules (see Fig. S7).
Analysis of the NTOs evidently assures that the S1 state of mCP-o-TRZ and
TCz-o-TRZ are dominated by CT characteristic. In view of the introduction of O atom
as space linkage between D and A units, the strong intra-CT effect was obtained from
carbazole to triphenyltriazine units. Conversely, TCz-o-TRZ exhibits a great extent
intra-CT of 99.18 % than 96.11 % of mCP-o-TRZ, where this difference in S1 state
characters can be explained well in terms of the molecular structures. The
electron-rich three carbazole substitutions in the TCz-o-TRZ molecule efficiently
increase the electron donating ability, resulting in the more obvious intra-CT effect
than two carbazole units in the mCP-o-TRZ molecule.

Figure 1. HOMO, LUMO distributions and 3D structure of the optimized ground state
calculated by B3LYP/6-31G(d) level for mCP-o-TRZ and TCz-o-TRZ.
Table 1 Photophysical properties and electrochemical properties of these compounds.
ꢆ
a
b
e
e
i
h
h
λꢀꢁꢂ
λꢃꢄ
ꢅ
ꢆ
ꢅꢇ
τꢈ /τꢇ
HOMOf/LUMOf Eꢆ S1 /T1 ∆Eꢂꢉ
c
c
Compound
[nm] [nm] [ ] [ ]
[ns]
[eV] [eV] [eV] [eV]
mCP-o-TRZ
TCz-o-TRZ
243
240
449 109.2 304.7 5.6/113.9
463 135.6 346.7 7.6/62.1
-5.9/-2.357 3.492.76/2.7430.017
-5.62/-2.17 3.192.68/2.6520.028
aMeasured in dichloromethane. bMeasured in neat film state. cMeasured by DSC/TGA at a heating
e
rate of 10 min-1. The prompt and delayed fluorescence lifetimes of the neat film at room
f
temperature. Obtained from the cyclic voltammograms. The oxidation potential of Fc/Fc+ was
g
ꢐꢑ
measured to be 0.4 V. HOMO = ‒ (4.8 + ꢊ
ꢒ 0.4) eV，LUMO = HOMO + Eg. Obtained
ꢋꢌꢍꢎꢏ
h
i
from the onsets of absorption spectra in neat film. Obtained at 78 K in neat film. Obtained from
the peak of flour-spectra in neat film.
Photophysical properties
The photophysical properties of the materials were essential feasibilities to analyze
the mechanism between the aimed molecular structure and the effect of inter/intra
interactions. The absorption (Abs), fluorescence (Fluor), and phosphorescence (Phos)

of mCP-o-TRZ and TCz-o-TRZ, were investigated in toluene first. As can be seen
from Figure 2a and 3a, no intermolecular interactions were detected, and only a similar
strong Abs-peak at 290 nm from mCP-o-TRZ and TCz-o-Trz was observed. The
fluorescence both showed dual emission, where the LE-emission occupied a
predominant position, while the strength of a redshift emission with broad and
structureless characteristic was weakened. The LE emission of mCP-o-TRZ and
TCz-o-TRZ match well with the emission spectra of the donor units, mCP-OMe and
TCz-Ph-OMe, respectively. On the contrary, the Phos-spectra of mCP-o-TRZ and
TCz-o-TRZ at 77 K with 0.1 ms delay were well resolved and revealed characteristic
vibrational structures, thus indicating that its lowest triplet origin from the LE
emission in toluene. This situation can also reflect a practical problem that intra-CT
was partially suppressed in mCP-o-TRZ and TCz-o-TRZ from the strong LE-emission.
The T1 energy of mCP-o-TRZ and TCz-o-TRZ calculated from the highest energy
peak of Phos-spectra is 2.76 eV and 2.36 eV, while the S1 energy of mCP-o-TRZ and
TCz-o-TRZ calculated from the highest energy peak of Flour-spectra is 3.03 eV and
2.74 eV, resulting in a ∆EST of 0.27 eV and 0.38 eV, in toluene, respectively.
Figure 2b and 3b displayed the PL properties of the mCP-o-TRZ and TCz-o-TRZ neat
film. Normally, no new absorption band can be observed from the Abs-spectrum of
the neat films, it stated clearly that no new species appeared in the ground state. What
we are interested in is that, in contrast with the phenomenon in toluene of
mCP-o-TRZ, a significant red-shifted Fluor-spectrum peaked at 449 nm was observed
for the neat film of mCP-o-TRZ. This issue, however, was a clear red-shifted
Fluor-spectrum peaked at 463 nm of TCz-o-TRZ in the neat film as a result of its
stronger electron-donating capabilities. The Phos-spectrum of the mCP-o-TRZ neat
film was also investigated with vibrational structures, indicating that the T1 of
mCP-o-TRZ in the neat film came from the LE state. By contrast, the Phos-spectrum
of the TCz-o-TRZ neat film showed broad featureless emissions, resulting from the
CT state emission in nature, which was different from the LE emission observed in
toluene. This CT can be assigned to the intra-CT as clearly discussed later. The S1 /T1
energies of mCP-o-TRZ and TCz-o-TRZ obtained from the peak of Fluor-spectrum

and Phos-spectrum are 2.76/2.743 eV and 2.68 eV/2.652 eV, respectively, leading to a
∆EST of 0.017 eV and 0.028 eV in the neat films, which was relatively smaller than
that observed in toluene. The small energy difference between the S1 and T1 excited
states was obtained due to the minimally spatial overlap, in which the effective
separation of their electron densities was induced by the pull-in of the σ-bridge.
Moreover, the triplet state of mCP-o-TRZ and TCz-o-TRZ was LE state and CT state
respectively, while the singlet states of mCP-o-TRZ and TCz-o-TRZ were both CT
state. The electronic coupling between the local triplet, (3LE) and the charge transfer
singlet states, (1CT) yielding interconversion of singlet and triplet states by the spin
orbit charge transfer mechanism is more crucial to minimize the singlet–triplet energy
gap (△Est). Hence, the △Est of TCz-o-TRZ was larger than that of mCP-o-TRZ[35].
More interestingly, the red-shifted emission observed in the neat film can be
explicated as the inter-CT emission and gradually increased polarity.
To deep understand the nature of the red-shifted emission, hence, Fluor-spectra of
mCP-o-TRZ and TCz-o-TRZ in different solvents were investigated. As shown in
Figure. 2c and 3c, with increased solvent polarity, the maximum emission peaks show
clear bathochromic shifts. Moreover, the PL spectra gradually broadened but LE
emission was weakened with the increased polarity of the solvents, which indicated
the obvious solvatochromic effects of mCP-o-TRZ. However, the PL spectra of
TCz-o-TRZ were various, where there was only LE emission in solvents with low
polarity. With the increased polarity of the solvents, the CT state emission gradually
appeared with similar intensity. Consequently, the intra-CT emission does exist in
mCP-o-TRZ and TCz-o-TRZ molecules. The above phenomenon analyzed in Figure
2c and 3c showed that this emission fundamentally resulted from the intra-CT states
of mCP-o-TRZ and TCz-o-TRZ in substance. At the same time, the intra-CT effect
can’t be eliminated via this molecular design strategy, where the σ-junction just plays
a weakening role.
To validate the emission of inter-CT state, the Fluor-spectrum of these neat films with
different doped-concentration in polymethyl methacrylate (PMMA) were measured.
As demonstrated in Figure 2c, at a low dopant concentration of 5 wt%, both emission

peaks from the LE state of mCP-OMe and the CT state of mCP-o-TRZ were observed,
which is consistent with that in solution. The LE emission was obvious at a dopant
concentration of 5-10 wt %, and distinctly decreased with increasing dopant
concentration and finally vanished at a dopant concentration of 80 wt % (Figure S8).
The CT emission peaks, on the other hand, are 1) gradually red-shifted from 409 to
432 nm with the dopant concentration increasing from 5 to 30 wt %, 2) fixed at 434
nm from 30 to 50 wt %, and 3) continued to be red-shifted to finally 449 nm from 50
to 100 wt %, as shown in Figure S6. Apparently, with a dopant concentration
gradually increasing from 30 wt%, the intermolecular interaction was gradually
enhanced compared to that of intramolecular interaction at 409-432 nm, which
occupied a dominant position in the neat film. As depicted in Figure 3c, only LE
emission can be observed at a low dopant concentration of 5 wt%, while the CT
emission peaks occurred at 10 wt % and gradually red-shifted from 440 to 460 nm
with the dopant concentration increasing from 10 to 30 wt %. Different from the
mCP-o-TRZ at the high dopant concentration, no movement in the CT emission peaks
of TCz-o-TRZ can be obtained (Figure S8). Even at the pure film, the CT emission
peak was same as that observed at the 30 wt %.
In view of the above discoveries, it can be summarized that, for mCP-o-TRZ, the red
shift of the intra-CT emission from 409 to 432 nm, as illustrated in Figure 2d, was
induced by the increased polarity with the increasing dopant concentration from 3 to
30 wt %. The red shift at high doped concentration of 50-100 wt % from 434-449 nm
was ascribed to the inter-CT emission between D and A units in neighboring
molecules. For the TCz-o-TRZ, however, only the red shift of intra-CT emission from
400 to 460 nm, as demonstrated in Figure 3d, was obtained. There was no finding that
the inter-CT occurred in the TCz-o-TRZ. In order to explore the diversity of the
molecules, the emission of the mixed films of 9,9'-(5-methoxy-1,3-phenylene)
bis(9H-carbazole)
and
triphenyltriazine
(1:1),
9'-(4-methoxyphenyl)-9'H-9,3':6',9''-tercarbazole and triphenyltriazine (1:1) were
analyzed, which were displayed in Figure S9. The exciplex emission around 410 nm
was clearly revealed in the film of mCP-OMe: TriPh-TRZ, while exciplex emission

can’t be found in the film of TCz-Ph-OMe: TriPh-TRZ
. In other words, the exciplex
can’t be formed in the film of TCz-Ph-OMe: TriPh-TRZ, which is consistent with the
PL properties, resulting in the only intra-CT emission. Nevertheless, exciplex
emission can be obviously observed in the film of mCP-OMe: TriPh-TRZ, and the
intra-CT and inter-CT can be found in the above analysis, which was a good
explanation about the occurrence of inter-CT emission, where detailed information
was summarized in Figure 4 and Figure S10.
Figure 2. (a) Abs-, Fluor-, and Phos-spectra of mCP-o-TRZ in toluene with a
concentration of 1 × 10−5 mol/L. (b) Abs-, Fluor-, and Phos-spectra of the
mCP-o-TRZ neat film. (c) Fluor-spectra of mCP-o-TRZ with a concentration of 1 ×
10−5 mol/L in different solvents. (d) Fluor-spectra of PMMA: mCP-o-TRZ-doped
films.

Figure 3. (a)Abs-, Fluor-, and Phos-spectra of TCz-o-TRZ in DCM with a
concentration of 1 × 10−5 mol/L. (b) Abs-, Fluor-, and Phos-spectra of the TCz-o-TRZ
neat film. (c) Fluor-spectra of TCz-o-TRZ with a concentration of 1 × 10−5 mol/L in
different solvents. (d) Fluor-spectra of PMMA: TCz-o-TRZ-doped films.
Figure 4. The charge transfer mechanism of mCP-o-TRZ and TCz-o-TRZ molecules.
The long-range coupled emission properties were further investigated by transient PL
measurements. The measured photoluminescence transient decay curves of the neat
and doped films were fitted by a double exponential function to analyze the PL decay

characteristics, which clearly exhibited prompt and delayed components in Figure 5.
When the films were pure with no ratio of Ir(mppy)3, they only had several
nanoseconds of lifetime with a single exponential decay trend. With the increased
doped ratio, the films presented two decay components, defined as prompt
fluorescence and phosphorescence. The long lifetime came from the T1 state emission
from Ir(mppy)3. The above results indicated that the energy was completely
transferred to phosphorescence guest, which showed that the guest was an ideal
emitter for the mCP-o-TRZ and TCz-o-TRZ host materials depicted later in devices.
All the detailed photophysical dates were listed in Table 1.
Figure 5. Transient PL decay spectra of Ir(mppy)3 doped into mCP-o-TRZ and
TCz-o-TRZ films in different doped ratio (3/5/8 wt%) at room temperature.
Thermal properties and film-forming ability of the materials
To investigate the thermal stability of the small molecules on the morphological
stability and device performance, the thermal properties of mCP-o-TRZ and
TCz-o-TRZ were measured by a thermogravimetric analyzer (TGA) and differential
scanning calorimeter (DSC) at a heating rate of 10
min-1 under a nitrogen
atmosphere. As shown in Fig. 6, both molecules displayed high decomposition
temperatures (Td, the temperature corresponding to 5 % weight loss of the initial value)
of more than 300 . When compared with mCP-o-TRZ (Td, 304.7 ), TCz-o-TRZ
displayed a higher value of 346.7 . Meanwhile, the glass transition temperature (Tg)
of TCz-o-TRZ was 135.6 , which was also higher than that of mCP-o-TRZ
(109.2 ). After that, the relevant values of Td and Tg are listed in Table 1. The high
Tg value assured that the materials as emitting layer can form a uniform amorphous
film during the later device fabrication process. The more excellent thermal stability

of TCz-o-TRZ demonstrated that the steric resistance was beneficial for increasing the
molecular rigidity by depressing the intermolecular D–A interactions.
In addition, the film-forming ability is also essential for the good performance of
solution-processed OLEDs. The spin-coated films were measured by atomic force
microscopy (AFM). According to the AFM images in Figure 7, the root-mean-square
(RMS) roughness of TCz-o-TRZ film was 0.455 nm, which was smaller than that of
mCP-o-TRZ film (0.585 nm). The TCz-o-TRZ with a dendritic structure of TCz, has
larger molecular weight than mCP-o-TRZ, raising the solubility in the
solution-processed OLEDs. These results indicated that mCP-o-TRZ and TCz-o-TRZ
possessed high thermal stability and good film-forming ability, which would benefit
the solution-processed device fabrication and operation.
Figure 6. TGA (a) and DSC (b) curves of mCP-o-TRZ and TCz-o-TRZ at a heating
rate of 10 min-1 under nitrogen.
Figure 7. AFM topographic images of (a) mCP-o-TRZ and (b) TCz-o-TRZ.
Electrochemical analysis of the materials
For the sake of determining experimentally the frontier molecular orbital energy
levels, the electrochemical properties of the materials were examined by cyclic
voltammetry (CV) in DCM. As shown in Fig. S11, the HOMO energy levels of

mCP-o-TRZ and TCz-o-TRZ were estimated to be -5.9 eV and -5.62 eV, respectively,
obtained from the onset of oxidation. The LUMO energy levels were calculated to be
-2.36 eV and -2.17 eV, respectively, using the following equation: ELUMO = EHOMO
+
Eg. The experimental data of the HOMO and LUMO levels were also listed in Table 1,
which matched well with the theoretical simulations described formerly. In addition,
when multiple CV scans were carried out, the oxidation potential gradually shifted to
lower potentials and the current obviously increased. The oxidation process of
mCP-o-TRZ and TCz-o-TRZ was not reversible, which can be ascribed to the
electrochemically active sites (3,6-positions of carbazole) being left unblocked and
provided the possibility of further chemical reaction. As having shown in the literature,
the active sites of carbazole derivatives (like mCP) play an essential role when the
compounds transport positive charge carriers in devices[36-38].
Electroluminescence properties
Given the obtained efficient energy transfer, the device was fabricated by adopting the
structure of indium tin oxide (ITO) / PEDOT: PSS (40 nm)/EML (60 nm)/TPBI (40
nm)/Cs2CO3 (2 nm)/Al (100 nm). The non-doped devices were firstly fabricated and
analyzed in Figure S12, which showed that the electroluminescent (EL) spectra only
exhibited
a
single
emission
at
whole
wavelengths.
Then,
the
tris(2-methylphenylpyridine) iridium (III) (Ir(mppy)3), was chosen as phosphorescent
dopant with a concentration of 3, 5, and 8 wt % of mCP-o-TRZ and TCz-o-TRZ,
named as A1-A3 and B1-B3, respectively. To verify the effect of energy transfer from
sky-blue hosts to phosphorescent guest, the PL-properties of these materials were
analyzed. As shown in Fig. S13, the UV of Ir(mppy)3 and the Fluor-spectra of
mCP-o-TRZ, TCz-o-TRZ in neat films had a large overlap, which made clear that the
energy was commendably transferred from the host materials to the guest. The energy
diagrams of the device, EL spectra, and current density–voltage–luminance (J–V–L)
and current efficiency–luminance (CE–L) characteristics were displayed in Figure 8.
Furthermore, the power efficiency–luminance (PE–L) curves were shown in Figure
S14. All devices only exhibited green emission from Ir(mppy)3 as shown in Fig. 8b,
once more demonstrating that the energy can be efficiently transferred to the guest.

Additionally, all the devices data were summarized in Table 2. The turn-on voltage for
these OLEDs was 5-6 V. The current density-voltage-luminance (J–V-L) curves of all
devices were similar because the devices were fabricated with the same procedures
and structures. The maximum EQE was 9.0 % for the device with mCP-o-TRZ, which
was much higher than that of TCz-o-TRZ- based device (4.5 %). This was because
mCP-o-TRZ had the superiority of intra-CT and inter-CT properties, while the
TCz-o-TRZ only has the intra-CT characteristics, where binary channels for energy
transferring contribute to the full utilization of carrier. The potential of mCP-o-TRZ as
an exciplex-type single-molecular host material[39, 40], was beneficial to the reduced
efficiency roll-off and energy leakage. With the increased concentration doped guest,
however, the performance of these devices was gradually weak, which maybe resulted
from that the guest-guest triplet-triplet annihilation (TTA) was a dominant process for
highly excited state degradation when the guest was enough and the spacing was
small. When the dopant concentration was low, a broad recombination region with
diluent exciton concentration was realized, further making efforts to suppress TTA
process and enhance the utilization of carrier.
Figure 8. (a) Energy diagram of the materials used in the devices; (b) Normalized EL
spectra in the doped devices; (c) Current density–voltage–luminance (J–V–L) curves;

(d) Current efficiency versus luminance curves.
Table 2 Electroluminescence characteristics of the doped devices.
a
Vꢓꢔ
CEmax
[Cd A-1]
Lꢄꢀꢕ
[cd m-2]
EQEmax
[%]
PEmax
[lm W-1]
CIEb
Device
[V]
Device A1
Device A2
Device A3
Device B1
Device B2
Device B3
5.6
4.9
5.6
5.3
6.3
5.4
27.7
8.5
6084
4556
4149
1019
1247
1049
9.0
2.7
2.3
4.5
1.2
2.4
14.5
4.5
3.0
7.4
1.3
3.3
(0.29, 0.63)
(0.30, 0.62)
(0.30, 0.62)
(0.30, 0.61)
(0.30, 0.62)
(0.31, 0.61)
7.2
14.1
3.8
7.5
b
aTurn-on voltage measured at 1 cd m-2. Commission International de I’Eclairage coordinates at
10 V.
CONCLUSIONS
In summary, we developed two novel D-σ-A molecules bearing mCP/TCz as
electron-donating moiety and TRZ units as acceptor, namely mCP-o-TRZ and
TCz-o-TRZ, to analyze efficient intermolecular and intramolecular CT transition for
solution-processed OLEDs. The oxygen atom was regarded as a bridge between D
and A units to break the conjugation of the molecule backbone and control the
distance to some extent in an intramolecular way, providing the possibility of the
formation of intra-CT or inter-CT. Consequently, mCP-o-TRZ coexisted the effect of
inter-CT and intra-CT, while TCz-o-TRZ only bring out the intra-CT through a series
of photophysical analysis. Furthermore, the devices employed mCP-o-TRZ as the host
materials for the phosphorescence guest (Ir(mppy)3) were constructured with a
maximum EQE of 9.0 %, which was superior to that of TCz-o-TRZ. These results
demonstrated that intermolecular CT transition can broaden the channel of energy
transfer effectively and accelerate the utilization of carriers which benefit to enhance
the performance of solution-processed OLEDs.
AUTHOR INFORMATION
Corresponding Author

*Email: jiangw@seu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for the grants from the National Natural Science Foundation of China
(21875036 and 51103023), the Fundamental Research Funds for the Central
Universities (2242016K41082), We are also thankful for the National Basic Research
Program (973 program, 2013CB932902), the Priority Academic Program
Development of Jiangsu Higher Education Institutions, Nanjing science and
technology committee (2014-030002).
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