Enhancing Spin-Orbit Coupling by Introducing Lone Pair Electron with

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Enhancing Spin-Orbit Coupling by Introducing Lone Pair Electron with p-Orbital Character in Thermally Activated Delayed Fluorescence Emitter: Photophysics and Devices Zhanxiang Chen, Fan Ni, Zhongbin Wu, Yuchen Hou, Cheng Zhong, Manli Huang, Guohua Xie, Dongge Ma, and Chuluo Yang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00937 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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The Journal of Physical Chemistry Letters

Enhancing Spin-Orbit Coupling by Introducing Lone Pair Electron with p-Orbital Character in Thermally Activated Delayed Fluorescence Emitter: Photophysics and Devices Zhanxiang Chen,† Fan Ni,‡ Zhongbin Wu,§ Yuchen Hou,† Cheng Zhong,*, † Manli Huang,† Guohua Xie,† Dongge Ma*,§ and Chuluo Yang*,†,‡

Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic

†

Materials, Wuhan University, Wuhan 430072, P. R. China ‡

College of Materials Science and Engineering, Shenzhen University, Shenzhen

518060, P. R. China §

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer

Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China.

ACS Paragon Plus Environment

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (C.Y.).

*E-mail: [email protected] (C.Z.).

*E-mail: [email protected] (D.M.).


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ABSTRACT: Reverse intersystem crossing (RISC) is the rate-limited step for the radiative process of thermally activated delayed fluorescence (TADF) materials, which is important to achieve ideal photoluminescence and electroluminescence efficiency. Herein, we propose a new strategy of introducing lone pair (n) electron with p orbital character to enhance spin-orbit coupling (SOC) for promoting RISC process. A proof-of-concept TADF molecule with p orbital lone pair, namely MoCz-PCN, was developed and three counterparts without any p lone pairs, namely DMAc-PCN, DPAc-PCN, and SpiroAc-PCN, were constructed for comparison. The experimental data revealed that MoCz-PCN exhibits ca. 1.9 times higher RISC rate than the counterparts, which can be ascribed to an enhanced SOC. Moreover, a significant increase in external quantum efficiency is observed in MoCz-PCN-based OLED device. These findings provide a feasible strategy to develop highly efficient TADF emitters by introducing lone pair (n) electron with p orbital character.

TOC GRAPHIC


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Reverse intersystem crossing (RISC), the non-radiative transition from the first triplet (T1) to the first singlet (S1) excited state, is ubiquitous and important in the photophysical process of thermally activated delayed fluorescence (TADF) emitters.1-5 It is well known that rapid RISC process in TADF-based organic light-emitting diodes (OLEDs) would improve operational stability due to the reduced exciton density and lessened detrimental triplet-triplet annihilation.6-8 According to Fermi’s Golden rule,9 the RISC rate constant (kRISC) can be given by 𝑘𝑅𝐼𝑆𝐶 ∝

|

⟨𝛹1|𝐻𝑆𝑂|𝛹2⟩ 2 ∆𝐸12

|

(1)

where Ψ is a specific wave function of each stable state (the initial state of the RISC process is given the subscript 1 and the second state of RISC is given the subscript 2), 𝐻𝑆𝑂 is the operator for spin-orbital coupling (SOC), and E is the associated energy of each state. The kRISC of a TADF molecule is influenced by two general rules: (1) the smaller the energy separation between the initial and the second state (△E12), the stronger the kRISC and (2) the stronger the magnitude of SOC ⟨𝛹1|𝐻𝑆𝑂|𝛹2⟩, the stronger the kRISC. For the first strategy, Adachi et al.10 and Xu et al.11 have proposed that the introduction of a second type of donor (D2) or acceptor (A2) into a D-A system with a lower △E12 can be beneficial for higher RISC. The strategy could boost the kRISC of TADF emitters by ca. 2.7-3.7 times. However, to the best of our knowledge, the second strategy to boost SOC for enhanced RISC in TADF materials is poorly studied.12 Noticeably, previous studies of TADF-based OLEDs had revealed that, by adding heavy atoms (e.g., Cu13 or Pb14) or carbonyl group,15-17 the SOC may be significantly enhanced which allowed for fast RISC form the T1 to the S1 and simultaneously efficient radiative decay form S1 to the singlet ground state (S0). But without heavy atoms or carbonyl group, do there exist other ways to enhance SOC in


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TADF materials? The effect of SOC can be visualized by rotating the magnetic moment (μS) of a free electron in p orbital of an atom experienced under the influence of the orbital moment (μL) (Figure S2).18-19 Because two lone pair (n) electrons on the oxygen are contained in a p orbital, somewhat stronger SOC than in pure organic compound containing only hydrogen, carbon and nitrogen may be anticipated. Based on the above understanding, we speculated that some TADF emitters with more p lone pairs could promote the RISC process.

Figure 1. (A) Potential energy curves for significantly different minimum for S0, S1 and T1 showing the Franck-Condon allowed transitions. The EVA and EVE are vertical absorption energy and vertical emission energy, respectively. The number of 1 and 1’ refer to the initial state of ISC and RISC. (B) The spin-orbit coupling between S1 and T1 states. (C) Chemical structures (upper), HOMO and LUMO distributions, and calculated singlet (S1) and triplet (T1) energy levels (lower) of MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN.


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To prove this hypothesis, organic compounds containing 3,6-dimethoxy-9H-carbazole (MoCz) and 9,9-dimethyl-9,10-dihydroacridine (DMAc) unit were chosen as donor models, because the highest occupied molecular orbital (HOMO) of MoCz (-5.11 eV) is close to that of DMAc (-5.12 eV)20 (Figure S3), beneficial for ruling out other possible factors affecting on kRISC. The donor of 2,7-dimethoxy-9,9-dimethyl-9,10-dihydroacridine (MoDMAc) with methoxy linked to DMAc was not selected, since its shallower HOMO level (-4.62 eV) (Figure S4) may induce obvious changes in photophysical characteristic, and thus it is not helpful to clarify the function of the lone-pair electron (Table S3). For the sake of comparison, we also investigated a couple

of

TADF

donor

units:

9,9-diphenyl-9,10-dihydroacridine

(DPAc)

and

10H-spiro[acridine-9,9'-fluorene] (SpiroAc). Four novel D-A type TADF emitters based on these donors

were

designed

and

prepared,

namely,

4-(4-(3,6-dimethoxy-9H-carbazol-9-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (MoCz-PCN), 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (DMAc-PCN), 4-(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (DPAc-PCN)

and

4-(4-(10H-spiro[acridine-9,9'-fluoren]-10-yl)phenyl)

-2,6-diphenylpyridine-3,5-dicarbonitrile (SpiroAc-PCN) (Figure 1C). As expected, theoretical calculations reveal that MoCz-PCN has stronger spin-orbit coupling (ca. 0.073 cm-1) than the other three (ca. 0.032 cm-1) when p orbital lone pair was introduced. And the corresponding RISC rate (kRISC) is up to the same order of magnitude as the intersystem crossing rate (kISC) (> 106 s-1). Furthermore, the singlet radiation rate constant (kr,S) is increased by 1.4- 3.7 folds (> 107 s-1). These well-managed excited state transition successfully endows a remarkable increment of


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the external quantum efficiency (EQE) in a green TADF OLED (up to ca. 1.5 times). Density functional theory (DFT) calculation at the PBE0-D3(BJ)/def2-SVP level was performed to obtain ground state electronic structures. As shown in Figure 1C, these emitters feature well-separated HOMO and LUMO distributions that are mainly located on the donor and acceptor units, respectively. The geometries in the excited states (S1, T1, and T2 state) were also optimized at the same level with TDDFT method. The EVA(S1) (vertical absorption energy), EVE(S1) and EVE(T1) (vertical emission energy) were calculated by employing the gap-tuned range-separated LC-ω*PBE functional with the def2-SVP basis set within the Tamm-Dancoff approximation (TDA) on the basis of the optimized S0, S1 and T1 geometries, respectively (Figure 1A). The natural transition orbital (NTO) analysis show that holes and particles of S1 state are mainly contributed by their HOMOs and LUMOs, respectively, revealing their charge transfer (CT)-predominant singlet excited states (1CT1, Figure S5). However, for the T1 excited state, the holes of these molecules are further delocalized to bridged phenyl and acceptor fragment to facilitate the hole-particle overlaps, verifying the increased locally excited (LE) components in the T1 state (3LE1), which can be further quantitatively evidenced by the larger integrals in the triplet hole-particle overlap (3〈ΨH|ΨP〉) than that of the singlet hole-particle overlap(1〈ΨH|ΨP〉) (Table 1). Meanwhile, we note that T2 is dominated by their HOMOs and LUMOs, which is assigned as a CT transition (3CT2). The efficient RISC from T1 to S1 state is demonstrated by MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN with singlet-triplet energy difference (△EST) values of 0.11, 0.34, 0.24, 0.17 eV, respectively (Figure 1C).


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Table 1. TDDFT results of MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN. 〈S1|HSO|T1〉 e

〈S1|HSO|T2〉 f

EVA(S1)g

EVE(S1)h

EVE(T1)h

△ESTi

(cm-1)

(cm-1)

(eV)

(eV)

(eV)

(eV)

0.0265

0.073

0.004

3.42

2.73

2.62

0.11

0.08

0.0007

0.028

0.009

3.25

2.95

2.61

0.34

0.5715

0.10

0.0012

0.032

0.005

3.39

2.84

2.60

0.24

0.5693

0.08

0.0008

0.027

0.004

3.18

2.77

2.60

0.17

μec

Compound

1a H P

3b H P

MoCz-PCN

0.0031

0.5730

0.49

DMAc-PCN

0.0031

0.5902

DPAc-PCN

0.0036 0.0034

SpiroAc-PC N

(Debye)

fSd

The FMO wavefunction overlaps (n〈ΨH|ΨP〉, n=1 for singlet and 3 for triplet) of the first adiabatic singlet (S1)a and triplet (T1)b excited states. cElectric transition dipole moment of the first vertical excited state. dOscillator strength of the first vertical excited state. eThe spin-orbit coupling between S1 and T1 states. fThe spin-orbit coupling between S1 and T2 states. Transition Energies (EVAg and EVEh) on S0, S1 and T1 geometries optimized by the same method. i△EST= EVE(S1)EVE(T1).

As shown in eq 1, the RISC rate is proportional to the square of the SOC matrix element. The SOC between S1 and T1 states were calculated with PySOC21 by considering that the three T1 substrates (m=1, 0, -1) degenerate, i.e.,

〈𝑆1|𝐻𝑆𝑂|𝑇1〉 =

2 ∑𝑚 = 0, ± 1〈𝑆1|𝐻𝑆𝑂|𝑇𝑚 1 〉 . The SOC

operator 𝐻𝑆𝑂 represents the interaction of the SOC, which can be evaluated by the expression:22 (2)

𝐻𝑆𝑂 = 𝜁𝑆𝑂𝑺𝑳~𝜁𝑆𝑂𝝁𝑺𝝁𝑳

where 𝜁𝑆𝑂 is the SOC constant. It’s easy to figure out that SOC is the interaction between the magnetic moment (μS) due to the electron’s spin angular momentum (S) and the magnetic moment (μL) owing to the electron’s orbital angular momentum (L). For a molecule lying in the xz plane, for instance, μL serves as a torque that tends to twist the in-plane px orbital (n) to the out-of-plane py orbital (π or π*) around the z-axis of rotation (Figure S2). Reciprocally, μS serve as a torque that tends to twist the spin vector orientation, thus transforming a triplet into a singlet


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in RISC. The SOC results for MoCz-PCN, DMAc-PCN, DPAc-PCN and SpiroAc-PCN are listed in Table S2. As T1 is the initial state of RISC (Figure 1A), the calculations were done at the T1 state minimum. Table 1 shows that S1 and T2 have similar electron configuration so that there is no angular momentum change and the corresponding spin-orbit coupling is nearly zero according to El Sayed rules.23 On the other hand, S1 and T1 have different electron configuration, which is accompanied with a change in angular momentum. As a result, the corresponding spin-orbit coupling is larger for the S1-T1 pairs. Due to the vanishing of the SOC between S1 and T2, the RISC is restricted between S1 and T1 states with effective SOC. The computed SOC matrix element value 〈S1|HSO|T1〉 of MoCz-PCN reaches ca. 0.073 cm-1, which is more than 2-folds of that of other acridine substitute molecules (ca. 0.032 cm-1) (Figure 1B and Table 1). Therefore, it can be expected that the introduction of p orbital lone pair leads to the RISC rate of MoCz-PCN more rapidly than other acridine substitute molecules.


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Figure 2. (A) UV-vis absorption and normalized fluorescence spectra of MoCz-PCN, DMAc-PCN, DPAc-PCN and SpiroAc-PCN in toluene (1 × 10-5 M) at room temperature. (B) Normalized phosphorescence (77K) spectra of MoCzPh, DMAcPh, DPAcPh, SpiroAcPh and PhPCN in oxygen-free toluene solution (1.0 × 10-5 M). (C) Photoluminescence (PL) transient decay curves of MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN doped into CBP films (8 wt%). (D) Dependence of SOC (〈S1|HSO|T1〉) on rate constants (k) and efficiencies (η) of the devices.

The

photophysical

properties

of

MoCz-PCN,

DMAc-PCN,

DPAc-PCN,

and

SpiroAc-PCN were measured to verify the theoretically calculated values. Figure 2A shows the absorption and emission spectra of the four emitters in toluene solutions. According to the CT absorption data, the experimental oscillator strength (f) of MoCz-PCN reaches 12.8, 3.3 and 3.9 folds of that of DMAc-PCN, DPAc-PCN, and SpiroAc-PCN, respectively. And the value is basically consistent with the trend of singlet oscillator strength (fS) and electric transition dipole moment (μe) of the first vertical excited state based on the optimal ground state from time-dependent DFT calculations (Table 1 and Table S1).24 The broad fluorescence spectra of MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN in solutions manifest their 1CT-featured

S1 states. Figure 2B shows the phosphorescence spectra of MoCzPh, DMAcPh,

DPAcPh, SpiroAcPh and PhPCN in oxygen-free toluene (1.0 × 10-5 M) at 77 K.25 The phosphorescence spectra of MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN closely resemble that of PhPCN, indicating that the 3LE1 of these four emitters are the same and could be estimated from that of PhPCN (Figure S8B). With the goal of application in devices, the photophysical properties of these compounds were investigated by doping them into


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4,4’-bis(carbazol-9-yl)biphenyl (CBP) (Figure S7). As estimated from the onset wavelengths of fluorescence and phosphorescence spectra, the △EST of MoCz-PCN (0.13 eV) is very close to that of DMAc-PCN (0.13 eV) and is smaller than that of DPAc-PCN (0.19 eV) and SpiroAc-PCN (0.17 eV) (Table S4).

Table 2. Photophysical characteristics of 8 wt%-doped CBP films. Componud

PLmax(nm)

PLQY(%)*

τp(ns)

τd(ns)

kr,S(1/s)

kISC(1/s)

kRISC(1/s)

MoCz-PCN

520

67/9

15.12

1056.86

4.4E7

7.8E6

1.1E6

DMAc-PCN

525

44/48

35.43

2892.42

1.2E7

1.6E7

6.7E5

DPAc-PCN

505

56/18

22.19

2223.07

2.5E7

1.1E7

5.9E5

SpiroAc-PCN

509

66/18

21.08

2119.38

3.1E7

1.0E7

6.0E5

*PLQY for prompt (left) and delayed (right) component.

All of the emitters doped into CBP host with 8 wt% exhibited radiative decay rate constants (kr,S) varied from 1.2 × 107 to 4.4 × 107 s-1 (Table 2). The delayed fluorescence lifetime (τd) of MoCz-PCN decreased with the introduction of p orbital lone pair, resulting in a shorter τd (1.1 μs) for MoCz-PCN than that for DMAc-PCN (2.9 μs), DPAc-PCN (2.2 μs) and SpiroAc-PCN (2.1 μs) (Figure 2C). The significant reduction of τd in MoCz-PCN could be attributed to its large kRISC (1.1 × 106 s-1), which is at the same order of magnitude as its kISC (7.8 × 106 s-1) and one order magnitude higher than the kRISCs of DMAc-PCN (6.7 × 105 s-1), DPAc-PCN (5.9 × 105 s-1) and SpiroAc-PCN (6.0 × 105 s-1). It is showed that kRISCs of these molecules are directly proportional to 〈S1|HSO|T1〉 (Figure 2D). As expected from eq 1, the kRISC of MoCz-PCN is increasing with the increase of the 〈S1|HSO|T1〉 value and rises by ca. 1.9 times (from 5.9 × 105 to 1.1 × 106 s-1) when the 〈S1|HSO|T1〉 goes up to ca. 0.073 cm-1 from ca. 0.027 cm-1.


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Figure 3. (A) The device structure and energy level diagram of the materials employed in the device. (B) The molecular structure of the used materials. (C) External quantum efficiency characteristics for devices based on MoCz-PCN, DMAc-PCN, DPAc-PCN and SpiroAc-PCN (with a doping concentration of 8 wt%). (D) Current density-voltage-luminance characteristics for devices.

Table 3. Electroluminescence characteristics of the devices. Vona

CEb

PEc

EQEd

CIEe

EUEf

[V]

[cd A-1]

[lm W-1]

[%]

(x, y)

[%]

MoCz-PCN

2.8

66.9, 55.3, 42.7

71.5, 29.5, 16.0

20.0, 16.6, 12.8

(0.38, 0.57)

87.7

DMAc-PCN

3.0

45.8, 38.7, 31.0

48.0, 20.2, 11.6

13.8, 11.7, 9.4

(0.39, 0.56)

50.0

DPAc-PCN

3.0

50.0, 28.5, 17.3

52.2, 14.4, 5.9

15.4, 8.7, 5.3

(0.30, 0.55)

69.3

SpiroAc-PCN

2.8

49.6, 38.2, 33.2

55.6, 20.7, 12.7

14.7, 11.5, 9.8

(0.34, 0.57)

58.3

Emitter

aThe

turn-on voltage at 1 cd.m-2. Maximum value, values at 1000 cd.m-2 and 5000 cd.m-2.

bCurrent

efficiency.

cPower

efficiency.

dExternal

quantum efficiency.

eThe

Commission


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Internationale de L’Eclairage coordinates recorded at 9.0 V. fCalculated exciton utilization efficiency.

To demonstrate the impact of an increased kRISC on OLED performance, the same device structures were utilized for the four emitters.26 In virtue of their suitable FMO energy levels for barrier-free carrier injection (Figure S9), we fabricated OLEDs with the architecture of ITO/MoO3 (10 nm)/TAPC (50 nm)/mCP (10 nm)/CBP doped with TADF dopants (8 wt%, 20 nm)/BPhen (45 nm)/LiF/Al. ITO and Al act as anode and cathode, respectively. Molybdenum trioxide (MoO3) and lithium fluoride (LiF) are employed as hole- and electron-injection layers, respectively. Di-[4-(N,N-ditolyl-amino)-phenyl]-cyclohexane (TAPC) and bathophenanthroline (BPhen) serve as the hole- and electron-transporting layers, respectively. 1,3-bis(N-carbazolyl) benzene (mCP) is a hole-transporting and exciton-blocking layer. MoCz-PCN, DMAc-PCN, DPAc-PCN or SpiroAc-PCN is doped into the CBP host with 8 wt% doping concentration to serve as the emitting layers. Figure 3A shows a schematic diagram of the energy levels of the fabricated devices and the chemical structure of the used materials. The doped CBP films of the four emitters show PL quantum yields (PLQYs) of 76%, 92%, 74%, and 84%, respectively (Table 2). The electroluminescence (EL) characteristics of OLEDs employing these emitters are shown in Figure 3 and the key EL performance data are summarized in Table 3. The current density-voltage-luminance (J-V-L) characteristics of OLEDs are shown in Figure 3D. The devices of MoCz-PCN, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN show low turn-on voltages (2.8, 3.0, 3.0 and 2.8 V, respectively). Figure 3C shows the EQE-brightness characteristics of the tested OLEDs. In contrast to those of acridine-substituted analogs, due to the introduction of p orbital lone pair, MoCz-PCN-based device comprehensively demonstrated


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improved device performance (Table 3). What’s more, the intensely maximum luminance (Lmax) of 43785 cd.cm-2 was obtained at 14.0 V. The maximum EL efficiencies of MoCz-PCN-based device are 66.9 cd.A-1 of current efficiency (CE), 71.5 lm.W-1 of power efficiency (PE) and 20.0% of EQE, demonstrating better performance than the other three emitter-based devices and revealing that the p orbital lone pair modification of the D-A system can be an effective approach for remarkably improving the device performance (Table 3). Assuming generation ratios of 0.25 and 0.75 for singlet and triplet excitons by electrical excitation, the EQE of TADF-OLEDs can generally be expressed as EQE = IQE × 𝜙𝑜𝑢𝑡 = (EUE × 𝜙𝑃𝐿) × 𝜙𝑜𝑢𝑡

(3)

Where EUE is electrically generated excitons utilization efficiency, which is directly determined by exciton allocation proportion for radiation transition27 and 𝜙𝑜𝑢𝑡 is the optical out-coupling efficiency. By assuming that the outcoupling efficiency 𝜙𝑜𝑢𝑡 is 30%, the device based on MoCz-PCN can realize the EUE of up to 87.7%, obviously higher than the value of 50.5%, 69.3% and 58.3% for the acridine-substituted emitters of DMAc-PCN, DPAc-PCN and SpiroAc-PCN, respectively. The relationship was illustrated based on the calculated 〈S1|HSO|T1〉 and the experimental maximum EQE and EUE for these molecules in devices in Figure 2D. We found that the maximum EQE are gradually enhanced to 20.0% from 13.8%, with the increase of 〈S1|HSO|T1〉 from ca. 0.027 cm-1 to ca. 0.073 cm-1. Similarly, the EUE (from 50.0% to 87.7%) is following the same trend. These results are attributed to the fact that the large 〈S1|HSO|T1〉 quickens the RISC process of T1 → S1 to harvest more triplet excitons and promotes the EQE. Figure S11 shows the linear relevance between the maximum EQE and the key transition parameters. The maximum EQE exhibits the positive correlation relation with kr,S and kRISC and the reverse proportion to kISC. There is no doubt that the preferable performance of


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MoCz-PCN-based device is just established on the basis of the overall optimization of its excited-state transitions. In conclusion, we demonstrate an effective strategy to optimize excited-state characteristics of TADF molecules through introducing p orbital lone pair. It is showed that enhanced SOC related to oxygen can facilitate RISC. This improved RISC may be ascribed to the lone pair (n) electrons that are on the oxygen atom. The introduction of p orbital lone pair gives rise to the nearly 2-fold increase of kRISC, which further endows EQE with a ca. 45% increase. This “p lone pairs-donor-acceptor” molecular strategy is proven to be a simple and efficient way for boosting OLED performance in this work and may guide the design of highly efficient TADF molecules.

ASSOCIATED CONTENT General information and methods for common measurements, syntheses, crystal analyses, and device fabrication; rate constants, calculation data, and data of solution-processed devices; SOC visualizations, FMOs distributions, thermal and electrical properties, supplementary photophysical and electroluminescent properties, 1H

NMR and 13C NMR spectra.

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 91833304, 91433201 and 51873160), the National Basic Research Program of China (973 Program 2015CB655002), Shenzhen Peacock Plan (KQTD20170330110107046) and the key Technological Innovation Program of Hubei Province (No. 2018AAA013).

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Enhancing Spin-Orbit Coupling by Introducing Lone Pair Electron with

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