Nonsymmetrical Connection of Two Identical Building Blocks

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Non-Symmetrical Connection of Two Identical Building Blocks: Constructing D-A Molecules as Deep Blue Emitting Materials for Efficient Organic Emitting Diodes Zhiqiang Li, Chenglong Li, Yincai Xu, NIng Xie, Xuechen Jiao, and Yue Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00300 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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The Journal of Physical Chemistry Letters

Non-Symmetrical Connection of Two Identical Building Blocks: Constructing D-A Molecules as Deep Blue Emitting Materials for Efficient Organic Emitting Diodes Zhiqiang Li,† Chenglong Li,*,† Yincai Xu,† Ning Xie,† Xuechen Jiao*,‡ and Yue Wang*,† † State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China ‡ Department of Materials Science and Engineering, Monash University, Australian Synchrotron, Clayton, VIC, Australia Corresponding Author *E-mail: [email protected] (C.L.); *E-mail: [email protected] (Y. W.); *E-mail: [email protected] (X. J.).

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ABSTRACT. In this contribution, we proposed a strategy to construct deep blue emission molecules based on the concept of non-symmetrical connection of two identical π–conjugated groups. It was demonstrated that non-symmetrical connection strategy indeed resulted in the separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the formation of D-A structure. For D-A molecules constructed by two identical groups, the degree of charge transfer is weaker and deep blue emission is easily achieved. Two D-A molecules (PIpPI and PImPI) were synthesized employing diphenylphenanthroimidazole (PI) as building block. The non-symmetric connection of PI groups endows these molecules with a D-A feature that can result in bipolar transport property. The non-doped organic light-emitting diodes (OLEDs) with PIpPI and PImPI as emitter exhibit deep-blue emission and maximum external quantum efficiencies (EQEs) of 8.84% and 6.83%, respectively. TOC GRAPHICS

KEYWORDS molecular engineering; D-A molecule, deep blue electroluminescence; organic light-emitting diodes; bipolar transport materials


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The π-conjugated molecules with donor-acceptor (D-A) structure feature have been widely adopted to develop organic luminescent materials for the applications in organic light-emitting diodes (OLEDs).1 The rich and varied excited states endow D-A organic luminescent molecules with the potential to produce high performance OLEDs.2-3 Recently, great progresses have been made in the exploitation of highly efficient OLEDs based on thermally activated delayed fluorescence (TADF) molecules with D-A characteristic.4-5 It is well known that the classical and efficient approach to design and synthesize D-A molecules is bonding one electron deficiency group (such as benzonitrile, triazine, sulfonyldibenzene and so on) and another electron rich moiety (such as acridine, triphenylamine, carbazole and so on).6-7 In general, the design and syntheses of green or red emission organic molecules with desired photoluminescence (PL) and electroluminescence (EL) properties are easily achieved by rationally composing typical donor and acceptor.8-9 However, constructing blue emitters (especially deep blue emitters) based on combining widely used donor and acceptor moieties is often unsuccessful due to following issues: (i) most of typical D-A molecules have lower excited energy and display emission with relatively long wavelength; (ii) the emission spectra based on classical charge transfer excited states often exhibit broad profile and poor color purity.10 Therefore, so far highly efficient deep blue OLEDs with TADF molecules as emitter are very rare.11-12 Although some fluorescent emitter based OLEDs displayed deep blue EL spectra, their efficiencies were not desire.11, 13 In this context, we propose a concept of constructing unclassical luminescent D-A molecules based on non-symmetrical connection of two identical components without typical donor or acceptor characteristics, which is an effective strategy to develop deep blue emitters for applications in OLEDs. In our studies, some organic conjugated systems with non-symmetric structure feature were selected to fabricate D-A molecules. The functional components that were


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employed to construct D-A structures in this study are not the extensively used donor or acceptor groups. We demonstrated that for some organic conjugated systems non-symmetrical connection can result in that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) separately distributed on the two conjugated groups. Therefore, the D-A molecules were successfully built based on two identical moieties. Here, diphenylphenanthroimidazole (PI) was employed as building block to construct D-A molecules PIpPI and PImPI (Figure 1a). These molecules displayed strong emission and were applied to fabricate highly efficient deep blue OLEDs.

Figure 1. (a) Molecule structure of PIpPI and PImPI. (b) ORTEP drawing of PIpPI crystal structure (left) and packing of PIpPI in the crystal unit cell (right). PIpPI and PImPI were synthesized based on simple procedure with high yield (see Support Information). The single crystal structure of PIpPI was obtained and Figure 1b illustrats the molecular structure and packing mode of PIpPI in the crystal. The torsion angles between the bridged benzene ring and two PI group are 12˚ and 76˚, respectively. There are only C–H···π interations between adjacent PIpPI molecules, and no π···π interations or other interactions are found in single crystal. The distorted configuration of PIpPI and weak intermolecular interaction


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in the solid state can effectively suppress aggregation-caused quenching and will guarantee the emission efficiency.14 Theoretical calculations were carried out to get further insight into the geometrical and electronic structures of PIpPI and PImPI together with parent molecule PI as comparison (Figure 2a). According to DFT calculations, the HOMO and LUMO distributions of PI are localized on the whole π-conjugated system, and they largely overlap each other. Contrarily, although the HOMO and LUMO distribution profiles of PIpPI and PImPI resemble that of PI, they present significant spatial separation feature and distribute separately on the two PI moieties within the same molecule. Therefore, we successfully built unclassical D-A structures based on two identical functional groups based on non-symmetric connection strategy. In PIpPI and PImPI, the formation of adequate separation of HOMO and LUMO can be attributed to the nonsymmetrically twisted connection of two PI segments and ineffective conjugation of dual PIs linked together through a bridged benzene ring at the nitrogen atom. Consequently, the D-A molecules were achieved by combining two PI groups. The natural transition orbitals (NTO) of S1→S0 transition of PIpPI and PImPI are shown in Figure 2b.15 The overlap of hole and particle for PIpPI is larger than that of PImPI, so PIpPI has a stronger oscillator strength (f) than PImPI. Through these calculation results we can deduce that althought PIpPI and PImPI have an intramolecular charge transfer (ICT) transition character, the weak ICT derive from the same functional group and the small overlap of NTOs still can guarantee them with deep blue fluorescence and high photoluminescence quantum yield (PLQY).


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Figure 2. (a) The distribution of HOMOs and LUMOs of PIpPI, PI and PImPI; (b) NTOs of S1 →S0 transition (mesh: hole, transparent: particle); f represents oscillator strength. All calculations were carried out at B3LYP-D3/6-311(g,d) method. The UV/Vis absorption spectra and photoluminescence (PL) spectra of PIpPI and PImPI in dichlormethane solution (10-5 mol L-1) and films are shown in Figure 3. These compounds display similar absorption features. The absorption bands peaked at 360 nm are attributed to the ICT transition from the electron-donating PI moieties to the electron-accepting PI moieties. These transitions have a relatively high molecular absorption coefficient, which proves that S1← S0 transitions are very efficient in this systerm. The absorption bands in the range of 300 to 345 nm can be assigned to the localized π→π* transition of the PI moieties. PIpPI and PImPI exhibit intense deep blue emission with high PLQYs in toluene solution (Table S1). The PL spectra of PIpPI and PImPI in thin film is slightly red-shifted with respect to their solution states and PLQYs of their thin films are 0.79 for PIpPI and 0.51 for PImPI, respectively. The red-shift and decreased PLQYs in film state should be attributed to the intermolecular interactions in aggregation state. However, due to their distorted molecular configuration, the


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negative effects of aggregation are highly suppressed. Based on low-temperature phosphorescence (PH) measurements in toluene at 77 K, the triplet energy levels were estimated to be 2.50 eV for PIpPI and 2.52 eV for PImPI. Their singlet-triplet energy gaps (△EST) values are greater than 0.8 eV, suggesting that TADF behavior should be elimnated for these class molecules.5, 12 In addition, we also checked their PL spectra in different polarity solvents. As can be seen in Figure S1, their emission spectra only gradually broadened a little and displayed a slight redshift when the solvents changed from low-polarity hexane to high-polarity DMF, indicating that PIpPI and PImPI are different from common D-A molecules with prominent solvation effect.16-19 The PL spectra of PIpPI and PImPI showed clear vibrational structure feature. This phenomenon should be attributed to the unclassical D-A structures with strong rigidity skeleton, which can resulted in vibrational structure of PL spectra for some D-A moleculess.20 For conjugated molecules, the paraa-substitution is more conducive to CT process than meta-substitution. Therefore, PImPI show more blue-shifted emission compared to PipPI. Furthermore, the D-A molecules based on two identical functional groups may display different emission behavior compared with typic D-A molecuels.


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Figure 3. UV–Vis absorption, PL spectra (excitation at 360 nm) in CH2Cl2 solution (10-5 mol L1)

and in film state and PH spectra in toluene at 77 K of PIpPI and PImPI. PIpPI and PImPI both exhibit high decomposition temperatures about 500 ℃ and high glass-

transition temperature (Tg) more than 160 ℃ (Table S1 and Figure S2), which indicates that these two compounds have good thermal stability and can form homogeneous and stable films by thermal evaporation. The estimated HOMO and LUMO levels of these compounds are almost identical, that are -5.6 eV and -2.2 eV, respectively, determined from the cyclic voltammetry experiments (Figure S3). Single-carrier devices were also fabricated to check carriers transport properties of them. From Figure S4 we can see that the hole only devices and electron only devices of these compounds have nearly the same current density values at the same voltage, manifesting superior bipolar transport property. By employing the space-charge-limited current (SCLC) method the carrier mobilities of PipPI (hole: 2.89×10-6 cm2V-1S-1 and electron: 8.14×106

cm2V-1S-1) and PimPI (hole: 4.17×10-6 cm2V-1S-1 and electron: 9.58×10-6 cm2V-1S-1) were

estimated. Balanced carrier transport property of the OLED emission layer will broaden the exciton recombination region, thus reducing efficiency roll-off. We fabricated blue non-doped OLEDs by vacuum evaporation technique with the device structure of [ITO/NPB (40 nm)/TCTA (5 nm)/PIpPI or PImPI (20 nm)/TPBi (35 nm)/LiF (0.8 nm)/Al (100 nm)]. The energy-level diagram and molecular structures of the materials used in these devices are shown in Figure 4e, and the performance of these OLEDs are depicted in Figure 4a-c and Table 1. PIpPI based device showed Commission Internationale de L’Êclairage (CIE) coordinate of (0.15, 0.07), which is very close to the National Television System Committee (NTSC) blue standard, and the EL spectrum remain nearly unchanged even when peak luminance of 17560 cd/m2 was reached at a voltage of 9 V. PImPI based device has a


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deeper blue EL with CIE coordinate of (0.16, 0.05) and maximum luminance of 3362 cd/m2. The maximum EL external quantum efficiencies (EQEs) are 8.84% for PIpPI and 6.83% for PImPI, respectively. Both devices have relatively low efficiency roll-offs, and their EQE can still remain 7.70% and 4.25% at a brightness of 1000 cd m-2 due to the bipolar transport property of these two materials. To our knowledge, these results are among the highest values of non-doped deep blue OLEDs with similar CIE coordinate and fluorescent molecules as emitter.9, 13, 21

Figure 4. (a) EL spectra. (b) Current density–Voltage–Brightness (J–V–L) plots. (c) EQE– Brightness plots and of non-doped OLEDs based on PIpPI and PImPI. (d) EQE–Brightness plots of red and green PhOLEDs adopted PIpPI and PImPI as hosts. (e) Energy level diagram and molecular structure of materials used in this study.

It is well known that the internal quantum efficiency (IQE) limitation of EL is 0.25 for normal fluorescent emitters. Therefore, the theoretical limitation of EQE for PIpPI based OLED would be in the range of 4.0–5.9% estimated according to the PLQY of 0.79 and light out-coupling efficiency (ηout) of 20–30%. Obviously, the obtained EQE of PIpPI based deep blue OLED is


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beyond to the theoretical limitation. To understand the high EL efficiency mechanism of PIpPI and PImPI, some detail characterizations were performed. Although the brightness-current density plot at low-current-density region exhibits linear relationship (Figure S5) and there is also no delayed fluorescence component observed for room and low temperature lifetime measurements (Figure S6). The triplet-triplet annihilation (TTA) induced improvement of EQE can not be eliminated due to that TTA process is often very complex in OLEDs. 22-23 Recently, it has been demonstrated that horizontal dipole orientations of anisotropic emitting molecules can remarkably imprtove the ηout value and sequently promote the EQE of OLEDs,24-28 which may be the origin of high efficiency in our OLEDs. PIpPI and PImPI both have anisotropic molecular structure feature and their transition dipole moments of S1 states calculated by TD-DFT are along the molecular long axis (Figure S7), suggesting that the exciting dipole of PIpPI and PImPI may adopt a relatively horizontal orientation in films. To verify above guess, synchrotron-based wide angle X-ray scattering (GIWAXS) has been performed on PIpPI and PImPI thin films.29 The simutaneous appearance of lamellar (100) and π-π (010) diffraction along out-of-plane direction (Figure S8a-b) suggests that the crsytallites/aggregates within PIpPI an PImPI thin films adotp rolling-log distribution,30 where the backbone (molecular long axis: c-axis) is locked in parallel to the substrate. The crystallographic information experimentally obtained here predicted the relatively horizontal dipole orientation. To be more quatitative, the crystallite orientation distribution (COD) was also extracted from the 2D GIWAXS patterns (Figure S8c-d). While identical trend of diffraction intensity as a function of pole figure can be observed from PIpPI thin film, the COD profile along π-π direction from PImPI exhibit bimodal distribution. This is suggestive that PIpPI mainly adopts rolling-log distribution, while PImPI crsytallites adopt rolling-log and face-on


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orientation. The predominantly strong rolling-log distribution observed from PIpPI, compared with PImPI, correlates well with the higher EQE performance of PIpPI than PImPI. Furthermore, the lamellar packing and π-π stacking distances of 10.26 Å and 4.38 Å were also extraced from the GIWAXS measurement, respectively (Figure S9). The quantitative calculation revealed that the rolling-log percentages are 72.69% for PIPPI and 58.44% for PImPI (Figure S10), respectively, suggesting that most of PIpPI and PImPI molecules adopted horizontal dipole orientation within films. Figure S11 depicts the results of polar emission spectra. Upon the polarizer rotating, the emission intensity displayed obvious change and the maxinum and minimum ratios are 2.7 for PIpPI and 1.9 for PImPI, indicating again the strong anisotropy of their emitting dipole. The polar emission measurement results agree well with the rolling-log percentage caculations. Hence, the high efficiency of these deep blue OLEDs should be attributed to horizontal dipole orientations of their emitting layers. Recently, Ma and Yang demonstrated that some fluorescence OLEDs displayed high EQE based “hot exciton” or “hhybridized local and charge-transfer excited state” mechanisms.9,31-33 Li and Shuai reported a deep blue fluorescence OLED based on triplet–polaron-interaction-induced upconversion from triplet to singlet process that can break through the limitation of the simple spin-statistics of 25%.34 In PIpPI and PImPI based OLEDs abover processes may exist and improved the EQE. Moreover, triplet-triplet annihilation in the non-doped films should also be possible contribution to the high efficiency. Inspired by their bipolar transport properties and suitable triplet energy, we investigated the ability of PIpPI and PImPI as host for green and red phosphorescent OLEDs (PhOLEDs). The


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Table 1. Summary of OLEDs performances in this study. Host

Von

λEL

Lmax

CEmax

PEmax

EQE

CIE1931

[V][a]

[nm][b]

[cd m−2][c]

[cd A−1][d]

[lm W−1][e]

[%][f]

(x, y)[g]

PIpPI

–

3.0

428

17560

5.33

5.31

8.84/7.70

0.15, 0.07

PImPI

–

3.2

416

3362

2.21

2.28

6.83/4.25

0.16, 0.05

G1

PIpPI

2.7

516

71980

81.10

87.06

22.72/19.97

0.33, 0.62

G2

PImPI

2.7

516

89810

79.78

79.96

22.32/21.40

0.33, 0.62

R1

PIpPI

2.6

616

37610

17.92

20.65

16.08/13.66

0.65, 0.35

R2

PImPI

2.7

616

30320

17.52

19.70

16.09/13.80

0.65, 0.35

[a]

turn-on votage at 0.5 cd m-2. [b] EL peak wavelength at 100 cd m-2. [c] maximum luminance.

[d]

maximum current efficiency. [e] maximum power efficiency. [f] maximum EQE and EQE at 1000 cd m-2. [g] recorded at 100 cd m-2. configuration of PhOLEDs are [ITO/NPB (40 nm)/TCTA (5 nm)/EMLs (25 nm)/TPBi (35 nm)/LiF (0.8 nm)/Al (100 nm)]. The EMLs are PIpPI or PImPI:10 wt% Ir(ppy)3 for G1 and G2; PIpPI or PImPI:8 wt% (bt)2Ir(dipba) for R1 and R2, respectively (Figure 4e). The results of these PhOLEDs are depicted in Figure 4d, S12 and Table 1. Both G1 and G2 have a fairly low turn-on voltage of 2.7 V and a relatively high peak luminance. The maximum EQEs of G1 and G2 are 22.72% and 22.32%, respectively, and their efficiency roll-offs are quite low. These device performances are comparable to that of the green PhOLEDs based on Ir(ppy)3 with subtle design hosts reported to date.35-37 The device R1 and R2 also show a low turn-on voltage and a maximum EQE of more than 16% with low efficiency roll-offs, whose performances are superior to the result previously reported by our group.38-39 Moreover, PIpPI and PImPI also have a small excited state dipole moment (μES), which can relieve host dopant dipole-dipole


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interactions, explaining the reason of low efficiency roll-offs of PhOLEDs adopted them as host materials.40 In conclusion, we have developed an unique strategy to build deep blue emitting D-A molecules by non-symmetrically connecting two identical π-conjugated building blocks. It was demonstrated that rationally optimizing the bond mode between two same functional moieties can resulted in the efficient separation of HOMO and LUMO and the formation of unclassical DA molecules with deep blue emission property. Efficient deep-blue emitters PIpPI and PimPI, in which HOMO and LUMO display separate distribution feature, were successfully obtained by non-symmetrically connecting two PI moieties within one molecule. PIpPI and PImPI based OLEDs exhibited high EQE of 8.84% and 6.83%, respectively, which are among the highest values of non-doped deep blue OLEDs with similar CIE coordinate and fluorescent molecules as emitter. The anisotropic molecular structure characteristic of PIpPI and PImPI leaded to the horizontal dipole orientation in solid thin films and improved the EL performance. The well balanced carrier transport property of PIpPI and PImPI not only relieves their devices efficiency roll-off, but also make them favorable hosts for green and red PhOLEDs. The nonsymmetrical connection of two identical groups has great potential to develop new high performance deep blue EL materials. Supporting Information. The Supporting Information is available free of charge Additional details of the detailed experimental information, synthesis, graphs, crystallographic information of single crystals and calculations geometric coordinate (PDF) AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]; *E-mail: [email protected]; *E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21772064 and 51803069), National Basic Research Program of China (2015CB655003), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12020200). This work was performed in part on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. REFERENCES (1)

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Nonsymmetrical Connection of Two Identical Building Blocks

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