Highly Efficient Nondoped Green Organic Light-Emitting Diodes with

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Highly Efficient Non-doped Green Organic Light-emitting Diodes with Combination of High Photoluminescence and High Exciton Utilization Chu Wang, Xianglong Li, Yuyu Pan, Shitong Zhang, Liang Yao, Qing Bai, Weijun Li, Ping Lu, Bing Yang, Shi-Jian Su, and Yuguang Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10129 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

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ACS Applied Materials & Interfaces

Highly Efficient Non-doped Green Organic Lightemitting Diodes with Combination of High Photoluminescence and High Exciton Utilization Chu Wang1‡, Xianglong Li2‡, Yuyu Pan1, Shitong Zhang1, Liang Yao1, Qing Bai1, Weijun Li1, Ping Lu1, Bing Yang*1, Shijian Su*2 and Yuguang Ma2 1

Dr. Chu Wang, Prof. B. Yang, Dr. Y.Y. Pan, Dr. S. T. Zhang, Dr. L. Yao, Dr. Q. Bai, Dr. W. J. Li State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun, 130012, P. R. China E-mail: [email protected]

2

X. L. Li, Prof. S. J. Su, Prof. Y. G. Ma 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 E-mail: [email protected]

KEYWORDS: OLEDs, photoluminescence efficiency, exciton utilization, hybridized local and charge-transfer state (HLCT), hot exciton

Abstract: Photoluminescence (PL) efficiency and exciton utilization efficiency are two key parameters to harvest high-efficiency electroluminescence (EL) in organic light-emitting diodes

ACS Paragon Plus Environment

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(OLEDs). But it isn’t easy to simultaneously combine these two characteristics (high PL efficiency and high exciton utilization) into a fluorescent material. In this work, an efficient combination was achieved through two concepts of hybridized local and charge-transfer (CT) state (HLCT) and “hot exciton”, in which the former is responsible for high PL efficiency while the latter contributes to high exciton utilization. Based on a tiny chemical modification in TPABZP, a green-light donor-acceptor molecule, CzP-BZP was designed and synthesized with this efficeient combination of high PL efficiency of ηPL = 75% in the solid state and maximal exciton utilization efficiency up to 48% (especially, the internal quantum efficiency of ηIQE = 35% substantially exceed 25% of spin statistics limit) in OLED. The non-doped OLED of CzP-BZP exhibited an excellent performance: a green emission with a CIE coordinate of (0.34, 0.60), a maximum current efficiency of 23.99 cd A-1, and a maximum external quantum efficiency (EQE, ηEQE) of 6.95%. This combined HLCT state and “hot exciton” strategy should be a practical way to design next-generation, low-cost, high-efficiency fluorescent OLED materials.

1. Introduction Originating from the pioneering work of Tang and Vanslyke1 in 1980s, the commercial applications of organic light-emitting diodes (OLEDs) are now becoming a reality, such as flat panel displays and solid-lighting.7-29 In order to improve the competitiveness of OLEDs, the high-efficiency, low-cost light-emitting materials become an urgent requirement as a core technology. First of all, in terms of high-efficiency, the external quantum efficiency ηEQE of OLED devices can be estimated according to the equation 1:

ηEQE = ηIQE ×ηout = ηrec ×ηPL ×ηS ×ηout

(1)


2

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where ηIQE is the internal quantum efficiency (IQE); ηout (~1/2n2) is the light out coupling efficiency (for glass substrates, n=1.5, ηout is estimated as ~20%); ηrec is the efficiency for electron-hole recombination, which could be assumed as 100% in an optimized device structure;38 ηPL is photoluminescence efficiency of the solid film; ηS is the exciton utilization efficiency in OLEDs, and it equals 25% (upper limit of spin statistics) for fluorescence emitter and 100% for phosphorescence one, respectively. Clearly, these four parameters of ηrec, ηPL, ηS and ηout are directly proportional to the ηEQE of OLEDs, that is to say, their respective maximizations are responsible to the maximization of ηEQE. In this paper, we will only focus on two key parameters ηPL and ηS, because both of them can be adjusted and optimized more effectively through the rational molecular designs. However, it remains a considerable challenge to simultaneously acquire their respective high efficiency of ηPL and ηS for the purpose of maximizing the ηEQE of OLEDs. On the one hand, ηPL is dependent on the ratio between radiative rate and non-radiative rate, and radiative rate is proportional to the transition dipole moment. The magnitude of transition dipole moment can be enhanced by increasing the orbital overlap, indicative of the locally-excited (LE) state or the extended π-conjugation. On the other hand, two main ways are usually responsible for harvesting high ηS (break through the spin statistics): one way is to enhance spin-orbit coupling by the introduction of metals with heavy atom effect, the ηs can be increased to nearly 100% in phosphorescent materials due to the unforbidden transition between triplet and singlet states, such as some iridium complexes.2-8 Nevertheless, some negative effects are still included, such as the high cost of noble metals, the lack of blue phosphorescent material, and the scarce resources of noble metals, etc. The other way is from delayed-fluorescence mechanism without the participation of heavy metal atom. Typically, two promising approaches have been proposed


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for triplet to singlet exciton up-conversion: thermally-activated delayed-fluorescence (TADF)9-12, 40

and triplet-triplet annihilation (TTA).13, 39, 41 TADF mechanism is more effective than TTA

because nearly 100% exciton could be harvested to emit light by the thermally-assisted reverse intersystem crossing (RISC) from T1 to S1 state, far superior to 62.5% of TTA maximum exciton utilization. Additionally, TADF mechanism does dramatically improve the overall EL efficiency and it is now also used as host material to enhance triplet exciton utilization.14 However, the doped devices have to be usually adopted in TADF emitters for high-efficiency EL devices, which is against the easy fabrication of the non-doped OLEDs in some extent. As for TADF mechanism, RISC (T1→S1) occurrence mainly depends on the degenerate singlet and triplet energy splitting (∆EST), which usually requires very small orbital overlap from charge-transfer (CT) state. But it is common knowledge that CT state is usually not a highly efficient radiative state, as a result of the spatial separation of transition orbitals.34, 35 So it is contradictory and difficult to essentially combine high ηPL with high ηS in the same fluorescent molecule, in view of the orbital overlap and the excited state character. Recently, some pure organic donor-acceptor (D-A) compounds with hybridized local and charge transfer (HLCT) state character exhibited an ultra-high ηS of nearly 100% in fluorescent OLEDs, which can be well rationalized using “hot exciton” mechanism.23-29 It tactfully combines two complementary characteristics: the low-lying LE-dominated HLCT state provides a high radiative transition rate for high ηPL, whereas the high-lying CT-dominated HLCT state is responsible for a high ηs through the enhanced RISC (Tn→Sm) process along “hot exciton” channel.24, 27-30 As a guarantee, an extraordinarily large energy gap (>1.0 eV) between T2 and T1 states greatly reduce the internal conversion rate of IC (T2→T1) according to the energy-gap law.4, 31 As a result, the “hot” RISC (T2→S1/S2) dominates the triplet exciton decay due to the


4

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more competitive RISC (T2→S1/S2) than IC (T2→T1), instead of “cold” RISC (T1→S1) in TADF mechanism. Therefore, “hot exciton” mechanism with HLCT state character can harmonize above dilemma to harvest the maximized ηEQE, as a result of a complementary coexistence between high ηPL and high ηs. What is more, the T1−T1 concentration quenching could be hopefully alleviated in a certain degree, due to the restricted T1 exciton formation. Thus, it is feasible to realize the highly efficient non-doped fluorescent OLEDs using “hot exciton” mechanism and HLCT principle. But at the moment, it is of utmost urgency to construct and expand “hot exciton” and HLCT state system to validate this novel mechanism. As

a

reference,

a

twisting

D-A

molecule

N,N-diphenyl-4-(7-

phenylbenzo[c][1,2,5]thiadiazol-4-yl)aniline (TPA-BZP, Figure 1a) showed a special excited state character in good accordance with “hot exciton” and HLCT model,29 which has been examined by solvatochromic experiment and quantum chemical calculation. Interestingly, it surely harvested a ηs of 42% in non-doped fluorescent OLED, breaking through 25% upper limit of spin statistics.4, 5, 6 However, the excessive CT component in S1 emissive state leads to a relative low ηPL (49% in n-hexane and 40% in solid film) in TPA-BZP. In order to maximize ηEQE of OLED, a fine modulation in excited state is of enough necessity to enhance ηPL in solid film, which means to strengthen LE component of S1 state while maintaining the existing ηs from “hot exciton” channel. In this contribution, we focus on a new molecule 4-(4-(9H-carbazol-9yl)phenyl)-7-phenylbenzo[c][1,2,5]thiadiazole (CzP-BZP, Figure 1a) from a tiny chemical modification on TPA-BZP. Scheme 1 is the synthesis route of CzP-BZP. Compared with TPABZP, the donor moiety triphenylamine (TPA) is replaced with a weaker electron-donating carbazole-phenyl (CzP) in CzP-BZP, which is expected to further decrease CT component as well as increase LE component in S1 HLCT state of CzP-BZP. Thus, if the ηPL of CzP-BZP film


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could be significantly enhanced relative to TPA-BZP, the simultaneous high ηPL and high ηS would give rise to the great improvement of ηEQE in CzP-BZP OLED, and further to validate the HLCT and “hot exciton” mechanism. Scheme 1. Synthesis route of CzP-BZP

i: Suzuki coupling, phenylboronic acid, Na2CO3 (2M aq), water and toluene, Pd(PPh3 )4, 95 °C, reflux for 72 hours under N2 atmosphere; ii: 1,4-dibromobenzene, toluene, CuI, 110 °C, reflux for 24 hours under N2 atmosphere; iii: nBut-Li, tetrahydrofuran, -78 °C, 3 hours under N2 atmosphere, then add 2-isopropoxy4,4,5,5-tetramethyl-1,3,2-dioxaborolane (C9H19BO3) drop by drop under the same environment, the mixture was stirring under room temperature for 48 h; iv: Suzuki coupling, Na2CO3 (2M aq), water and toluene, Pd(PPh3 )4, 95 °C, reflux for 72 hours under N2 atmosphere.

2. Results and Discussion 2.1. Molecular design 2.1.1 Ground state and excited state geometry


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As shown in Figure 1a, CzP-BZP is composed of CzP as the electron donor and benzo[c][1,2,5]thiadiazole (BZ) as the electron acceptor. In order to greatly improve the ηPL, the TPA moiety in TPA-BZP is replaced with a weaker donor moiety of CzP, which is aimed at decreasing CT component (or increasing LE component) in the emissive S1 state of CzP-BZP. The ground state (S0) and excited state (S1) geometries were optimized by DFT and TDDFT methods for CzP-BZP and TPA-BZP respectively, and their structure parameters were shown in Figure 1a and Table S2. For the ground state, the twist angle θ1 are 53˚ and 37˚ for CzP-BZP and TPA-BZP, respectively. Compared with TPA-BZP, the larger θ1 of CzP-BZP can be ascribed to the stronger repulsion between the two adjacent hydrogen atoms in carbazole (a planar and rigid unit) and bridging phenyl ring, as a result of the stronger rigidity of CzP than TPA. From ground state to excited state, the twist angle θ1 of CzP-BZP and TPA-BZP are increased to be 65˚ and 60˚, while the twist angle θ2 and θ3 of CzP-BZP show a decreasing trend similar to those of TPABZP, respectively. Besides for CzP-BZP and TPA-BZP, the bonds R1 are remarkably elongated by 0.025 Å and 0.036 Å respectively from S0 to S1, while both R2 and R3 exhibit a relatively small alteration from S0 to S1. As a comparison, the smaller change of geometry from S0 to S1 occurs in CzP moiety of CzP-BZP than that of TPA unit in TPA-BZP, which may facilitate the suppression of non-radiation for the enhancement of ηPL. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are mainly localized on CzP and BZP moieties, respectively (Figure 1b). This D-A architecture may benefit a bipolar material design with the balanced carrier transport property when it is used as the light-emitting layer, in which CzP acts as hole-transporting group and BZP as electron-transporting group, respectively. Compared with TPA-BZP, the delocalization of HOMO obviously decreases in CzP-BZP, indicating that the CzP is a weaker


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donor than the TPA. Actually, the HOMO level of CzP-BZP is decreased by 0.38 eV relative to that of TPA-BZP, corresponding to the weaker electron-donating ability of CzP than TPA.

Figure 1. (a) Molecular structures of CzP-BZP and TPA-BZP and their optimized geometries of ground state S0 and excited state S1; (b) HOMO and LUMO of CzP-BZP and TPA-BZP at the geometry of S0 state; (c) Hybridization between LE and CT states at the geometry of S1 state; (d) Excited state (singlet and triplet) energy diagram of CzP-BZP and TPA-BZP at the geometry of S0 state.

2.1.2 Hybridized local and charge-transfer (HLCT) state To better understand the nature of excited states, the natural transition orbital (NTO)30 was analyzed for the pictorial description in Figure 1c and Figure S7. For S1 and S2 states, NTO "hole" and "particle" of CzP-BZP are distributed somewhat similar to those in TPA-BZP. For instance, "hole" is nearly delocalized over the whole molecular backbone, while "particle" is mainly localized on the BZ and the bilateral phenyl rings. Compared with TPA-BZP, though a similar "particle" contour is maintained in CzP-BZP, the "hole" contour is obviously shrunk as a


8

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result of the weaker donor ability of CzP, indicating an enhanced LE component in S1 state as we expected. On the other hand, the oscillator strength of CzP-BZP (0.4681) in S1 state is higher than that of TPA-BZP (0.4369), which indicates that the higher ηPL could be expected in CzPBZP than in TPA-BZP. As for the reason, a simple chemical modification from TPA to CzP induces the increase of LE component in the S1 emissive state, together with the more rigid molecular skeleton. It is noteworthy that both S1 and S2 states exhibit a typical character of hybridized local and charge-transfer (HLCT) state, in which the major LE character of S1 state is in favor of the high ηPL in CzP-BZP. What is more, we can deduce the HLCT state formation in both CzP-BZP and TPA-BZP, as shown in Figure 1c. The S1 and S2 states are just the two hybrid-splitting states, arising from the two state coupling between the pure LE state of BZP and the pure CT state with CzP→BZP or TPA→BZP electron transition, on the premise of the energetic closeness and appropriate coupling strength between LE and CT. As a further investigation, the “hole” on CzP or TPA moiety are exactly in the opposite phase between S1 and S2 states, whereas the “particle” on BZP unit remain the same between S1 and S2 states for CzPBZP and TPA-BZP, respectively. This implied that the interstate hybridization coupling occurs through the positive and negative linear combination between LE and CT state wavefunctions (eq. 2). Observably, the pure CT level of CzP-BZP is upraised relative to that of TPA-BZP, as a result of the weaker donor of CzP than TPA, leading to the LE-dominated S1 state in CzP-BZP and the LE/CT balanced S1 state in TPA-BZP. As a result, CzP-BZP should exhibit the higher ηPL and the blue-shifted emission relative to TPA-BZP.

ψ S / S = cLE ⋅ψ LE ± cCT ⋅ψ CT 1

2

(2)

2.1.3 Hot exciton channel for reverse intersystem crossing (RISC)


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In order to utilize the triplet exciton energy as much as possible in fluorescent OLED, the RISC channel is artificially designed and constructed according to two important characteristics of excited state energy diagram: one is high-lying singlet and triplet CT state for the effective RISC channel from triplet to singlet state, due to a very small singlet-triplet energy splitting (∆EST); the other is a large energy gap between T1 and T2 to greatly restrict IC decay from T2 to T1 for the priority of RISC (T2→Sm). The first ten singlet and triplet states were estimated using TD-M06-2X/6-31+G** at the geometry of S0 state, as shown in Figure 1d. Obviously, a large energy gap occurs between T1 and T2 for both CzP-BZP and TPA-BZP, arising from the same BZ acceptor group,28, 29 and the energy gap between T1 and T2 of Czp-BZP is 1.40 eV, which is larger than TPA-BZP (1.17eV). Besides, a very small ∆EST is observed between S1 and T2 states facilitating RISC (T2→S1) process in both CzP-BZP and TPA-BZP, as a result of their HLCT state character (see supporting information). Thus, compared with TPA-BZP, CzP-BZP can be expected to achieve the enhanced ηPL and the maintained ηs simultaneously, and further to maximize the ηEQE of the fluorescent OLED, as a result of the increased LE component in S1 state and the very similar RISC (T2→S1) channel to TPA-BZP.


10

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Figure 2. (a) Degradation temperature (Td) of CzP-BZP (Td, determined from the point of weight loss of 95% in thermogravimetric curves); (b) Glass phase transition temperature (Tg) and melting point temperature (Tm) of CzP-BZP.

2.2 Electrochemical and Thermal Properties Based on the cyclic voltammetry (CV) measurement, HOMO and LUMO energy levels were obtained for CzP-BZP and TPA-BZP, respectively (Table 1 and Figure S6). Both CzP-BZP and TPA-BZP have almost the same LUMO level (-3.16 eV and -3.11 eV, respectively), which is attributed to their same acceptor BZ unit. Differently, the HOMO level of -5.51 eV in CzPBZP is much lower than -5.22 eV of TPA-BZP, corresponding to that of CzP and TPA group, respectively. The decreased HOMO level of CzP-BZP is originated from the weakened electrondonating ability of CzP relative to TPA. Above electrochemical measurement is in good agreement with the results from DFT calculation. The thermal properties of CzP-BZP and TPA-BZP were measured using TGA and DSC respectively under a nitrogen atmosphere (Table 1, Figure 2). For CzP-BZP, the thermal degradation temperature (Td, determined from the point of weight loss of 5% in TGA curves) is estimated to be 391oC, and there is no glass phase transition temperature (Tg). Comparing with TPA-BZP, the thermo stability of CzP-BZP is improved significantly due to the stronger rigidity of CzP than TPA, which surely benefits the device operative stability of OLEDs. Table 1. The electronic energy and thermal properties of CzP-BZP and TPA-BZP. Materials

Tga (˚C)

Tdb (˚C)

HOMOc (eV)

LUMOc (eV)

E gd (eV)

CzP-BZP

none

391

-5.51

-3.16

2.35

TPA-BZP

56

340

-5.22

-3.11

2.11

(a)

Tg is glass transition temperature; (b) Td is thermal-decomposition temperature at a weight percentage of 95%; (c) HOMO and LUMO levels were calculated using the oxidation onset potentials and the reduction onset potentials, respectively; (d) Eg = ELUMO –EHOMO.


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2.3 Photophysical Properties 2.3.1 UV-vis and PL Properties In Figure 3a, UV-vis and PL spectra show the absorption peak at 394 nm and 437 nm for CzP-BZP and TPA-BZP respectively, and their emission peak at 527 nm and 587 nm in THF solution, respectively. Compared with TPA-BZP, CzP-BZP exhibits obvious blue shift for either UV-vis (43 nm) or PL (50 nm) spectrum, which can be ascribed to the weakened donor ability of CzP relative to TPA, leading to the increased LE component or the decreased CT component in S1 emissive state. In addition, the full width at half maximum (FWHM) in absorption spectrum of CzP-BZP is obviously narrowed relative to that of TPA-BZP, also indicating the decrease of CT component in S1 state in good agreement with the NTO description for S0→S1 transition. Furthermore, the UV-vis and PL spectra were measured in vacuum-evaporated film (Figure 3a). In contrast to the TPA-BZP, CzP-BZP shows a significant blue shift for either UV-vis or PL spectra in solid film. The larger twist angle of θ1 in the ground state of CzP-BZP may weaken the intermolecular interactions and alleviate the molecular aggregation, also in favor of the increase of ηPL through the suppression of the aggregate-induced fluorescence quenching. This viewpoint is further confirmed based on a comparison of PL efficiency between doped (in PMMA matrix) and non-doped films in Figure S2. As a matter of fact, PL spectra of CzP-BZP shows much smaller shift from doped to non-doped films than that in TPA-BZP. Exactly, this weak molecular aggregation leads to a much higher ηPL = 75% of CzP-BZP in vacuum-evaporated film by an integrating-sphere photometer, while the ηPL is merely 45% in TPA-BZP film.


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Figure 3. (a) Normalized UV and PL spectra of CzP-BZP in THF (tetrahydrofuran) solution at the concentration of 10-5 mol L-1 with TPA-BZP reference (inset are the molecular structure of CzP-BZP and TPA-BZP). (b) Solvatochromic PL spectra of CzP-BZP and TPA-BZP. (c) Linear correlation of orientation polarization f of solvent media with the Stokes shift (va – vf) for CzP-BZP and TPA-BZP. (See Table S1 for data; the lines in the high- and low-polarity regions were fitted to the points with f ≥ 0.15 and f ≤ 0.15, respectively). (d) Lifetime measurement of CzP-BZP and TPA-BZP in ether using time-correlated single photon counting method under the excitation of a laser (378.8 nm) with 96.8 ps pulse width.

2.3.2. Solvatochromic Effects To better understand the intramolecular charge-transfer character in excited states, the solvent effect36, 37 was examined on CzP-BZP and TPA-BZP, respectively, as shown in Figure 3b. From low-polarity hexane to high-polarity acetonitrile, the fluorescence of CzP-BZP exhibited obvious solvatochromic effect with the increase of solvent polarity, and the total redshift of 66 nm is much smaller than that of TPA-BZP (94 nm), as well as a slight shift of 13 nm and 15 nm for CzP-BZP and TPA-BZP respectively in the absorption spectra (Figure S1).


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These shifts indicate that the low-lying excited state S1 of the CzP-BZP and TPA-BZP possesses a certain CT state characteristic.32, 33 Moreover, these shifts suggest that the proportion of CT character in S1 state of CzP-BZP is surely lower than that in TPA-BZP, vice versa, the proportion of LE character for CzP-BZP is clearly higher than that in TPA-BZP. Certainly, CzP-BZP should possess a much higher PL efficiency relative to TPA-BZP, as a result of the enhanced LE component in emissive state. As a matter of fact, the ηPL of CzP-BZP is always higher than that of TPA-BZP in the same solvent from low-polarity hexane to high-polarity acetonitrile (Table 2). With the increase of solvent polarity, though both CzP-BZP and TPA-BZP exhibit a decreasing ηPL, a slower decline was observed for CzP-BZP than that in TPA-BZP, indicative of the lower CT component in CzP-BZP than that in TPA-BZP. Moreover, the dipole moment (µe) of the S1 state can be estimated from the slope of a plot of the Stokes shift (va-vf) versus the solvent polarity function f, according to the Lippert–Mataga equation.30 As shown in Figure 3c, for CzP-BZP and TPA-BZP, they both show two independent slopes of two-section fitted lines, which hints the existence of two different characters of excitedstate. The dipole moment µe of CzP-BZP is calculated to be 18.54 D in high-polarity solvent and 8.57 D in low-polarity solvent. As a comparison, the µe of TPA-BZP is evaluated to be 18.41 D in high-polarity solvent, which is very close to the µe of CzP-BZP in high-polarity solvent. While in low-polarity solvent, the µe is estimated to be 13.72 D, which is much larger than 8.57 D of CzP-BZP. This smaller µe in low-polarity solvent can be attributed to an usual more LE-like state, while the large µe should be treated as a main CT-like state.30 In low-polarity solvent, the smaller µe of CzP-BZP relative to TPA-BZP can be ascribed to the weaker donor ability of CzP than TPA, corresponding to the higher LE proportion as we expected in molecule design of CzPBZP.


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Obviously, the solvatochromic experiments show the two-section linear relation between Stokes shift and solvent polarity, indicating that both CzP-BZP and TPA-BZP possess an intercrossed excited state of LE and CT: a bigger contribution from the CT state in high-polarity solvents (f ≥ 0.2), whereas a bigger contribution from the LE state in low-polarity solvents (f ≤ 0.1), and a totally intercrossed excited state of the LE and CT may occur in a moderate polarity between butyl ether and ethyl acetate, or at the polarity level near isopropyl ether. In this case, the hybridized local and charge-transfer (HLCT) state exactly forms due to the intercrossing and coupling between LE and CT states. Besides, the lifetime measurement reveals that this intercrossed excited state in different polar solvents should be a hybridized local and chargetransfer (HLCT) state, instead of two species state through a simple addition of LE and CT. As shown in Figure 3d and Table 2, in moderate-polarity ethyl ether solvent, the fitted line of CzPBZP and TPA-BZP decay curve give a mono-exponential lifetime of 7.20 ns and 7.46 ns respectively, determined by the time-correlated single photon counting method under the excitation of a laser (375.0 nm) with a 96.8 ps pulse width. The mono-exponential lifetime demonstrates that the intercrossed LE and CT in the moderate-polarity solvent formed as one hybridized HLCT state, which verifies the rationality of initial molecular design very well. The film state emission of CzP-BZP exhibits the closest wavelength to that in moderate-polarity solvents of ethyl ether (Figure S3), thus a HLCT state is believed to be formed in CzP-BZP film as well. With the lifetime and efficiency, the radiative transition rates (kr) and the non-radiative transition rates (knr) in ether can be estimated for CzP-BZP and TPA-BZP, respectively (Figure S4). Compared with TPA-BZP, the kr of CzP-BZP is increased by over 3 times, while the knr of CzP-BZP is decreased by about twice. This result is in good agreement with the purpose of our molecular design.


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Table 2. Photophysical properties of TPA-BZP and CzP-BZP in different solvents. a

solvent

∆f

Hexane 0.0012 Ether 0.167 THF 0.21 Aceton 0.305

νa (nm) 438 435 437 424

a

νf (nm) 526 558 578 619

TPA-BZP νa-νf cFWHM dηPL (cm-1) (nm) (%) 3820 66 49.2 5067 79 20.1 5582 80 10.5 7430 106 1.0

b

e

Lifetime (ns) 6.77 7.46 7.95 3.58

a

νa (nm) 394 392 394 382

a

νf (nm) 481 504 527 562

CzP-BZP νa-νf cFWHM (cm-1) (nm) 4591 65 5669 74 6405 77 8284 85 b

d

ηPL (%) 69.7 59.9 42.6 5.9

e

Lifetime (ns) 5.87 7.20 7.52 5.20

(a) νa and νf are the UV and PL peaks in different solvents, respectively. (b) Stokes shift in different solvents. (c) The Full Width at Half Maximum. (d) Measured vs quinine sulphate. (e) Measured at the peak wavelengths in different solvents.

2.4. Electroluminescence Properties 2.4.1 Device performance To investigate the EL performances, we constructed a non-doped device with a frequently used multilayered structure (Figure 4a): indium tin oxide (ITO)/PEDOT:PSS (40 nm)/N,N′-di-1naphthyl-N,N’-diphenylbenzidine

(NPB)

(40

nm)/4,4′,4″-tri(N-carbazolyl)-triphenylamine

(TCTA) (5 nm)/CzP-BZP (20 nm)/1,3,5-tri(phenyl-2-benzimidazolyl)-benzene (TPBi) (40 nm)/LiF (1 nm)/Al (100 nm), in which PEDOT:PSS was utilized as a hole-injecting layer, NPB and TCTA as hole-transporting and electron-blocking layers, and TPBi as an electrontransporting and hole-blocking layer. We also constructed a non-doped device with a frequently used multilayered structure for TPA-BZP: ITO/PEDOT:PSS(40 nm)/NPB(40 nm)/TCTA(5 nm)/TPA-BZP(30 nm)/TPBi(50 nm)/LiF(1 nm)/Al(100 nm). As shown in Figure 4b, the EL spectra originate from about 449 nm, peaked at about 538 nm and ends at about 707 nm. Its FWHM is 88 nm. CzP-BZP shows a big blueshift than TPA-BZP’s 588 nm and displays a saturated green emission with the CIE coordinate of CzP-BZP is (0.34, 0.60), which is stable enough along the increase of drive voltage from 4 V to 7 V. The variation of CIE coordinate with voltage is listed in Table S3 in Supporting Information. As we can see in Table S3, the fluctuation of CIE coordinate is quite small with the increase of drive voltage from 4V to 7V. The EL device of CzP-BZP exhibits an excellent performance with a maximum luminance of


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16531 cd m−2, maximum current efficiency (CE) of ~23.99 cd A−1, maximum power efficiency (PE) of ~16.38 lm W−1, and maximum EQE of ~6.95 %, as shown in Figure 4c and Figure 4d. The detailed data is listed in Table 3. When compared with TPA-BZP, the OLEDs performance of CzP-BZP is improved comprehensively. With full confidence, CzP-BZP is the one among the best performances of non-doped green OLEDs.

Figure 4. (a) Energy level diagram, (b) EL spectra at different drive voltages, (c) current efficiency-luminancepower efficiency characteristics, and (d) current efficiency-luminance-EQE characteristics of the non-doped EL device with CzP-BZP or TPA-BZP as emitter.

2.4.2 Exciton utilization


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According to the EL efficiency expression (eq. 1), assuming a light out-coupling efficiency of ηout ≈ 20 %, we transformed the ηEQE into the corresponding maximum internal quantum efficiency ηIQE of ~ 35 %.3 This value is surely to break through the upper limit of spin statistics 25% (singlet/triplet = 1/3) in fluorescent OLEDs. More accurately, considering PL efficiency (ηPL = 75%) of CzP-BZP film, a nearly ηS = 48% excitons (obtained from ηIQE/ηPL = 35%/75%) generated from electron-hole recombination were employed to emit in the light-emitting layer of CzP-BZP, which far exceeds the upper limit of 25% in spin statistics. As requested by molecular design, CzP-BZP substantially maintains an almost same exciton utilization efficiency of TPABZP (ηS = 42%). To further explore the essences responsible for the high exciton utilization in CzP-BZP device, the several different mechanisms are taken into account, such as TADF, TTA and “hot exciton”. Firstly, in Figure S9, the luminance versus current density curve of CzP-BZP behaves an approximate linear relation, instead of the saturation tendency, revealing that the enhanced ηs of CzP-BZP is not originated from TTA mechanism.13,

39, 41

Secondly, in CzP-BZP, TADF

mechanism can also be excluded due to the following experiments and theoretical analysis. On the one hand, the lifetime measurement displayed only mono-exponential decay of fluorescence, indicating that there exists no delayed emissive component at all, let alone the TADF. On the other hand, the S1-T1 energy splitting (∆EST) was estimated to be 1.40 eV, which is too large to succeed the effective RISC (T1→S1) in TADF. Finally, “hot exciton” mechanism is workable to be responsible for the high exciton utilization in CzP-BZP and TPA-BZP. As important evidence, very interestingly, the phosphorescence can’t be observed in CzP-BZP and TPA-BZP at low temperature of 77 K (Figure S5), which means that the T1 state can’t be populated upon the photo excitation. As for reasons, one possible reason is a large energy gap between S1 and T1


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from TDDFT calculation, which significantly retards the ISC (S1→T1) process; the other possible way is also excluded as a two-step sequential process of ISC (S1→T2) and IC (T2→T1), but a big enough energy gap between T1 and T2 leads to a seriously blocked IC (T2→T1), and in turn facilitates the more competitive RISC (T2→S1) over IC (T2→T1) (Figure S8). Besides, the non-doped EL device of CzP-BZP exhibits a very slow efficiency roll-off at high current density, which bypasses the T-T concentration quenching in TADF mechanism. Table 3. The data summary of OLED performance for CzP-BZP and TPA-BZP. Devicea

CEmax/CE100cd/m2/ CE1000cd/m2 [cd A-1]b

PEmax/PE100cd/m2/ PE1000cd/m2 [lm W-1]b

ηEQEmax/ηEQE100cd/m2/ ηEQE1000cd/m2 [%]b

CzP-BZP

23.99/15.48/13.18

16.38/10.84/8.82

6.95/4.47/3.81

34.8

538

0.34, 0.60

48

TPA-BZP

8.84/8.82/7.92

7.18/6.68/4.64

3.80/3.78/3.41

19.0

588

0.55, 0.45

42

ηIQE/max [%]b

EL λmax [nm]b

CIE (1931) [x, y]b

ηS [%]c

(a) The optimized device structures for different emitters are: ITO/PEDOT:PSS (40 nm)/NPB (40 nm)/CzPBZP (20 nm) or TPA-BZP (30 nm)/TPBi (40 nm)/LiF/Al; (b) CE, luminous efficiency; PE, power efficiency; ηEQE, external quantum efficiency (EQE); ηIQE/max, maximum internal quantum efficiency; EL, electroluminescence; ηIQE/max =ηEQE/max/ηout, light out-coupling efficiency ηout of 20%; (c) Estimated from ηIQE/max divided by ηPL, assuming ηrec of 100%.

3. Conclusion In summary, we designed and synthesized a green fluorescent D-A molecule CzP-BZP according to HLCT state principle and “hot exciton” model, for the purpose of the maximization in EL efficiency through the simultaneous harvest of high PL efficiency and high exciton utilization in fluorescent OLED. As a result of a simple chemical modification from TPA-BZP to CzP-BZP, a right HLCT emissive state was obtained with increased LE and decreased CT components, leading to the significantly enhanced ηPL from 45% to 75% in film. Due to a large energy gap between T1 and T2, BZ unit was used to construct the “hot exciton” RISC (T2→S1) channel for high exciton utilization (ηS = 48%). The CzP-BZP exhibited an excellent non-doped green fluorescent OLED performance: a green OLED with a CIE (0.34, 0.60), a maximum current efficiency of 23.99 cd A-1, and a maximum ηEQE of 6.95%. Our results further validate a


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practical strategy to design the next-generation low-cost, high-efficiency fluorescent OLED materials using HLCT state principle and “hot exciton” model. ASSOCIATED CONTENT Supporting Information. Experimental details, the solvatochromic Lippert-Mataga model, solvatochromic effect and supplementary photophysical properties, cyclic voltammogram, delayed photophysical in 2methyltetrahydrofuran under liquid nitrogen temperature, optimized configuration in ground state and lowest excited state, NTOs of singlet and triplet excited states, J-V-L curve of the nondoped EL device. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Bing Yang. E-mail: [email protected] Shijian Su. E-mail: [email protected] Author Contributions Dr. Chu Wang‡ and Dr. Xianglong Li‡ contributed equally to this work. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (51273078, 51473063, 91233113 and 91233116), the National Basic Research Program of China (973 Program: 2013CB834801 and 2015CB655003) and the Graduate Innovation Fund of Jilin University (Project 2014011). REFERENCES


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(1) Tang C.W.; VanSlyke S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913-915. (2) Ma Y. G.; Zhang H. Y.; Shen J. C.; Chen C.M. Electroluminescence from Triplet Metal— Ligand Charge-transfer Excited State of Transition Metal Complexes. Synth. Met. 1998, 94, 245-248. (3) Baldo M. A.; O'brien D. F.; You Y. A.; Shoustikov; Sibley S.; Thompson M. E.; Forrest S. R. Highly Efficient Phosphorescent Emission From Organic Electroluminescent Devices. Nature, 1998, 395, 151-154. (4) Baldo M. A.; Lamansky S.; Burrows P. E.; Thompson M. E.; Forrest S. R. Excitonic Singlettriplet Ratio in A Semiconducting Organic Thin Film. Appl. Phys. Lett. 1999, 60, 1442214428. (5) Cao Y.; Parker I. D.; Yu G.; Zhang C.; Heeger A. J. Improved Quantum Efficiency for Electroluminescence in Semiconducting Polymers. Nature, 1999, 397, 414-417. (6) Shuai Z. G.; Beljonne D.; Silbey R. J.; Brédas J. L. Singlet and Triplet Exciton Formation Rates in Conjugated Polymer Light-Emitting Diodes. Phys. Rev. Lett. 2000, 84, 131-134. (7) Adachi C.; Baldo M. A.; Thompson M. E.; Forrest S. R. Nearly 100% Internal Phosphorescence Efficiency in An Organic Light-Emitting Device. J. Appl. Phys. 2001, 90, 5048-5051. (8) Chiang C. J.; Kimyonok A.; Etherington M. K.; Griffiths G. C.; Jankus V.; Turksoy F.; Monkman A. P. Ultrahigh Efficiency Fluorescent Single and Bi-Layer Organic LightEmitting Diodes: The Key Role of Triplet Fusion. Adv. Funct. Mater. 2013, 2, 739-746. (9) Uoyama H.; Goushi K.; Shizu K.; Nomura H.; Adachi C. Highly Efficient Organic LightEmitting Diodes From Delayed Fluorescence. Nature, 2012, 492, 234-239.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(10) Zhang Q. S.; Li B.; Huang S.; Nomura H.; Tanaka H.; Adachi C. Efficient Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence. Nat. Photonics. 2014, 8, 326-332. (11) Tanaka H.; Shizu K.; Miyazaki H.; Adachi C. Efficient Green Thermally Activated Delayed Fluorescence (TADF) from A Phenoxazine-Triphenlytriazine (PXZ-TRZ) Derivative. Chem. Commun. 2012, 48: 11392-11394. (12) Zhang Q. S.; Li J.; Shizu K.; Huang S.; Hirata S.; Miyazaki H.; Adachi C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light-Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706-14709. (13) Zhen C. G.; Chen Z. K.; Liu Q. D.; Dai Y. F.; Shin R. Y. C.; Chang S. Y.; Kiffer J. Fluorene-Based Oligomers for Highly Efficient and Stable Organic Blue-Light-Emitting Diodes. Adv. Mater. 2009, 21, 2425-2429. (14) Zhang D. D.; Duan L.; Zhang D. Q.; Qiu Y. Highly Efficient and Color-stable Hybrid Warm White Organic Light-Emitting Diodes Using A Blue Material with Thermally Activated Delayed Fluorescence. J. Mater. Chem. C. 2014, 2, 8191-8197. (15) Zhu M. R.; Yang C. L. Blue Fluorescent Emitters: Design Tactics and Applications in Organic Light-Emitting Diodes. Chem. Soc. Rev. 2013, 42, 4963-4976. (16) Chou H. H.; Chen Y. H.; Hsu H. P.; Chang W. H.; Chen Y. H.; Cheng C. H. Synthesis of Diimidazolylstilbenes as n-Type Blue Fluorophores: Alternative Dopant Materials for Highly Efficient Electroluminescent Devices. Adv. Mater. 2012, 24, 5867-5871. (17) Kim R.; Lee S.; Kim K. H.; Lee Y. J.; Kwon S. K.; Kim J. J.; Kim Y. H. Extremely Deep Blue and Highly Efficient Non-doped Organic Light Emitting Diodes Using An Asymmetric Anthracene Derivative with A Xylene Unit. Chem. Commun. 2013, 49, 4664-


22

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4666. (18) Zhen C. G.; Dai Y. F.; Zeng W. J.; Ma Z.; Chen Z. K.; Kiffer J. Achieving Highly Efficient Fluorescent Blue Organic Light-Emitting Diodes Through Optimizing Molecular Structures and Device Configuration. Adv. Funct. Mater. 2011, 21, 699-707. (19) Zou Y.; Zou J. H.; Ye T. L.; Li H.; Yang C. L.; Wu H. B.; Ma D. G.; Qin J. G.; Cao Y. Unexpected Propeller-Like Hexakis(fluoren-2-yl)benzene Cores for Six-Arm Star-Shaped Oligofluorenes: Highly Efficient Deep-Blue Fluorescent Emitters and Good HoleTransporting Materials. Adv. Funct. Mater. 2013, 23, 1781-1788. (20) Liu C.; Fu Q.; Zou Y.; Yang C. L.; Ma D. G.; Qin J. G. Low Turn-on Voltage, HighPower-Efficiency, Solution-Processed Deep-Blue Organic Light-Emitting Diodes Based on Starburst Oligofluorenes with Diphenylamine End-Capper to Enhance the HOMO Level. Chem. Mater. 2014, 26, 3074-3083. (21) Gao Z.; Cheng G.; Shen F. Z.; Zhang S. T.; Zhang Y. N.; Lu P.; Ma Y. G. Highly Efficient Deep Blue Light Emitting Devices Based on Triphenylsilane Modified Phenanthro [9, 10d] Imidazole. Laser Photonics Rev. 2014, 8, L6-L10. (22) Gao Z.; Liu Y. L.; Wang Z. M.; Shen F. Z.; Liu H.; Sun G. N.; Yao L.; Lv Y.; Lu P.; Ma Y. G. High-Efficiency Violet-Light-Emitting Materials Based on Phenanthro [9, 10-d] imidazole. Chem. Eur. J. 2013, 19, 2602-2605. (23) Li W. J.; Liu D. D.; Shen F. Z.; Ma D. G.; Wang Z. M.; Fei T.; Yang B.; Ma Y. G. A Twisting Donor-Acceptor Molecule with an Intercrossed Excited State for Highly Efficient, Deep-Blue Electroluminescence. Adv. Funct. Mater. 2012, 22, 2797-2803. (24) Li W. J.; Pan Y. Y.; Xiao R.; Peng Q. M.; Zhang S. T.; Ma D. G.; Li F.; Shen F. Z.; Wang Y. H.; Yang B.; Ma Y. G. Employing~100% Excitons in OLEDs by Utilizing a


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

Fluorescent Molecule with Hybridized Local and Charge-Transfer Excited State. Adv. Funct. Mater. 2014, 24, 1609-1614. (25) Tang S.; Li W. J.; Shen F. Z.; Liu D. D.; Yang B.; Ma Y. G. Highly Efficient Deep-Blue Electroluminescence Based on the Triphenylamine-Cored and Peripheral Blue Emitters with Segregative HOMO–LUMO Characteristics. J. Mater. Chem. 2012, 22, 4401-4408. (26) Zhang S. T.; Li W. J.; Yao L.; Pan Y. Y.; Yang B.; Ma Y. G. Enhanced Proportion of Radiative Excitons in Non-doped Electro-Fluorescence Generated from An Imidazole Derivative With An Orthogonal Donor–Acceptor Structure. Chem. Commun. 2013, 49, 11302-11304. (27) Yao L.; Zhang S. T.; Wang R.; Li W. J.; Shen F. Z.; Yang B.; Ma Y. G. Highly Efficient Near-Infrared Organic Light-Emitting Diode Based on a Butterfly-Shaped Donor– Acceptor Chromophore with Strong Solid-State Fluorescence and a Large Proportion of Radiative Excitons. Angew. Chem. Int. Ed. 2014, 126, 2151-2155. (28) Pan Y. Y.; Li W. J.; Zhang S. T.; Yao L.; Gu C.; Xu H.; Yang B.; Ma Y. G. High Yields of Singlet Excitons in Organic Electroluminescence through Two Paths of Cold and Hot Excitons. Adv. Opt. Mater. 2014, 2, 510-515. (29) Li W. J.; Pan Y. Y.; Yao L.; Liu H. C.; Zhang S. T.; Wang C.; Shen F. Z.; Yang B.; Ma Y. G. A Hybridized Local and Charge-Transfer Excited State for Highly Efficient Fluorescent OLEDs: Molecular Design, Spectral Character, and Full Exciton Utilization. Adv. Opt. Mater. 2014, 2, 892-910. (30) Chaudhuri D.; Sigmund E.; Meyer A.; Rçck L.; Klemm P.; Lautenschlager S.; Schmid A.; Yost S. R.; Voorhis T. Van; Bange S.; Hçger S.; Lupton J. M. Metal-Free OLED Triplet Emitters by Side-Stepping Kasha's Rule. Angew. Chem. 2013, 125, 13691-13694.


24

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Förster,T. 10th Spiers Memorial Lecture. Transfer Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959, 27, 7-17. (32) Frisch M. J.; Trucks G. W.; Schlegel H. B.; Scuseria G. E.; Robb M. A.; Cheeseman J. R.; Scalmani G.; Barone V.; Mennucci B.; Petersson G. A.; Nakatsuji H.; Caricato M.; Li X.; Hratchian H. P.; Izmaylov A. F.; Bloino J.; Zheng G.; Sonnenberg J. L.; Hada M.; Ehara M.; Toyota K.; Fukuda R.; Hasegawa J.; Ishida M.; Nakajima T.; Honda Y.; Kitao O.; Nakai H.; Vreven T.; Peralta J. E.; Ogliaro F.; Bearpark M.; Heyd J. J.; Brothers E.; Kudin K. N.; Staroverov V. N.; Kobayashi R.; Normand J.; Raghavachari K.; Rendell A.; Burant J. C.; Iyengar S. S.; Tomasi J.; Cossi M.; Rega N.; Millam J. M.; Klene M.; Knox J. E.; Cross J. B.; Bakken V.; Adamo C.; Jaramillo J.; Gomperts R.; Stratmann R. E.; Yazyev O.; Austin A. J.; Cammi R.; Pomelli C.; Ochterski J. W.; Martin R. L.; Morokuma K.; Zakrzewski V. G.; Voth G. A.; Salvador P.; Dannenberg J. J.; Dapprich S.; Daniels A. D.; Farkas O.; Foresman J. B.; Ortiz J. V.; Cioslowski J.; Fox D. J. Gaussian 09. Revision B.01. Gaussian, Inc. Wallingford CT, 2009. (33) Martin R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775-4777. (34) Jagtap S. P.; Mukhopadhyay S.; Coropceanu V.; Brizius G. L.; Brédas J.; Collard D. M. Closely Stacked Oligo (Phenylene Ethynylene)s: Effect of π-Stacking on the Electronic Properties of Conjugated Chromophores. J. Am. Chem. Soc. 2012, 134, 7176−7185. (35) Grabowski Z. R.; Rotkiewicz K.; Rettig W. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899-4031. (36) Lippert E.; Lüder W.; Boos H. In Advances in Molecular Spectroscopy; Mangini, A., Eds.; Pergamon Press: Oxford, 1962.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

(37) Birks J. Photophysics of Aromatics Molecules; New York: Wiley, 1970. (38) Sheats J. R.; Antoniadis H.; Hueschen M.; Leonard W.; Miller J.; Moon R.; Roitman D.; Stocking A. Organic Electroluminescent Devices. Science, 1996, 273, 884-888. (39) Hu J. Y.; Pu Y. J.; Satoh F.; Kawata S.; Katagiri H.; Sasabe H.; Kido J. BisanthraceneBased Donor–Acceptor-type Light-Emitting Dopants: Highly Efficient Deep-Blue Emission in Organic Light-Emitting Devices. Adv. Funct. Mater. 2014, 24, 2604-2071. (40) Nakagawa T.; Ku S. Y.; Wong K. T.; Adachi C. Electroluminescence Based on Thermally Activated Delayed Fluorescence Generated by A Spirobifluorene Donor–Acceptor Structure. Chem. Commun. 2012, 48, 9580-9582 (41) Segal M.; Singh M.; Rivoir K.; Difley S.; Voorhis T. V.; Baldo M. A. Extrafluorescent Electroluminescence in Organic Light-Emitting Devices. Nat. Mater. 2007, 6, 374-378.

Table of Contents HLCT state principle and “hot exciton” model, which are used for design and synthesis a green fluorescent D-A molecule CzP-BZP leading to the significantly enhanced ηPL from 45% to 75% in film compared with TPA-BZP. CzP-BZP shows an excellent non-doped OLED performance: a green OLED with a CIE (0.34, 0.60), a maximum current efficiency of 23.99 cd A-1, and a maximum ηEQE of 6.95%.


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Highly Efficient Nondoped Green Organic Light-Emitting Diodes with

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