Dispiro and Propellane: Novel Molecular Platforms for Highly Efficient

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Dispiro and Propellane: Novel Molecular Platforms for Highly Efficient Organic Light-Emitting Diodes Xiang-Yang Liu, Xun Tang, Yue Zhao, Danli Zhao, Jian Fan, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15645 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Materials & Interfaces

Dispiro and Propellane: Novel Molecular Platforms for Highly Efficient Organic Light-Emitting Diodes Xiang-Yang Liu, †a Xun Tang,†a Yue Zhao,b Danli Zhao,a Jian Fan,*,a,c, and Liang-Sheng Liao,*,a,c a

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of

Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P.R. China. b

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of

Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210023, China. c

Institute of Organic Optoelectronics (IOO), JITRI, Suzhou, Jiangsu 215212, China.

KYEWORDS: Dispirocycles, Host materials, Phosphorescence, OLEDs, Propellane derivatives

ABSTRACT: The incorporation of spatially oriented aromatic motifs in rigid molecular platforms is of great interest for the design of organic electronic materials. These structures can create unusual packing patterns and charge transport properties in the solid state which are not possible for the simple planar structures. Herein, we showed that the novel dispiro and propellane motifs were successfully used as robust molecular platforms for the construction of host materials (TPA, Cz, SF, and SO). The propellane derivative with the three functional groups arranged in the staggered conformation was studied for the first time as the host for organic light-emitting diodes. The green and red PHOLEDs hosted by these dispiro and propellane derivatives exhibited excellent electroluminescence performance. Particularly, the red OLED hosted by the propellane-type SF achieved a maximum efficiency of 47.3 cd A-1, 40.2 lm W-1 and 26.6%, and 97.6 cd A-1, 77.8 lm W-1 and 27.0% for the green OLED without any light-out coupling enhancement. These results suggest that the dispiro and propellane molecular platforms have great potential in the construction of OLED materials.

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INTRODUCTION Since the organic light-emitting diode (OLED) was reported by C. W. Tang and S. A. VanSlyke in 1987,1 it has attracted extensive research interest due to its practical applications in full-color display and solid-state lighting. During the last three decades, much effort has been contributed to the exploration of efficient OLED materials and the optimization of device architecture. Fluorescent organic compounds were generally used in early OLEDs, where the internal quantum efficiency (IQE) was limited to 25%. The transition metal (such as Ir and Pt) complexes can realize the phosphorescence emission from the T1 to S0 via the strong spin-orbit coupling effect. Therefore, a unity IQE could be achieved in theory.2,3 Within an OLED device, the triplet-involved quenching process such as triplet–triplet annihilation (TTA) and triplet exciton–polaron quenching (TPQ)4-7 mainly took place in the emitting layer (EML), and it is difficult to reduce such a triplet excitons quenching process effectively solely by the device optimization. The host materials play an important role in phosphorescent OLEDs (PHOLEDs) as a dispersion matrix for the phosphor emitters. The hosts were typically decorated with different functional groups to support the independent charge transport and energy transfer channels, which could inhibit the charge induced decomposition process of triplet exciton.8-11 Therefore, developing high performance host materials is a crucial way to realized high-efficiency PHOLEDs.12-14 So far much attention has been focused on the fluorene-based materials owing to their good thermal-stability, wide bandgap and high fluorescence efficiency during host development.15-20 9,9'-spirobifluorene (SBF) derivatives where two fluorene units were connected together by a spiro linkage demonstrated high spectral stability due to their limited oxidation of the spiro-carbon.21-27 SBF normally showed high glass transition temperature due to their highly steric rigid structure and orthogonal configuration. In addition, it can preserve high triplet energy because the central sp3 hybridized carbon atom can break the conjugation between the two fluorine motifs. However, SBF compounds generally showed high ionization potential, which could lead to the relatively large hole-injection barrier.28 So the structural modifications were applied to tune the frontier orbital energy levels. For example, the electron-donating groups such as triphenylamine and carbazole were introduced into the spiro-structure to improve the hole injection and transport abilities (Scheme ACS Paragon Plus Environment

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1).29-33 Despite the wide use of the SBF derivatives and their analogues, the synthesis of new host materials with novel spatial framework, tunable highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels, and great diversity in terms of derivatization and functionalization is urgently required. Several different types of dispiro compounds (Scheme 2) have recently been pioneered by Poriel et al,31-41 and encouraging power efficiency and external quantum efficiency were obtained. On the other hand, propellane42-47 derivatives are also featured with rigid three-dimensional framework, where three-biphenyl substructures are located around the C(sp3)-C(sp3) hub in a staggered conformation. So far propellane derivatives have rarely been studied in OLEDs. For example, Katsuhiko et al. reported the application of hexabenzo-annulated [4.4.4]propellane as hole-transport material.47 In this context, a new type of dispiro host (TPA) was synthesized by the reaction of (2-(diphenylamino)phenyl)lithium

with

10ʹH-spiro[fluorene-9,9ʹ-phenanthren]-10ʹ-one (Figure 1). Interestingly, when the flexible (2-(diphenylamino)phenyl)lithium was replaced by the relatively rigid species, (2-(9H-carbazol-9-yl)phenyl)lithium

and

(2-(phenothiazin-10-yl)phenyl)lithium,

propellane derivatives (Cz, SF and SO) were obtained. This work represented the first report of a propellane motif as the building block for host materials. All the four materials

demonstrated

excellent

thermal

stabilities,

suitable

triplet

and

HOMO/LUMO energy levels, thus they could serve as hosts for green and red PHOLEDs. The green and red OLED with bis(2-phenylpyridine) iridium (III) (acetylacetonate) (Ir(ppy)2(acac)) and bis(2-methyldibenzo-[f,h]-quinoxaline) iridium (III) (acetylacetonate) (Ir(MDQ)2(acac)) as green and red dopants were fabricated, respectively, and the devices exhibited excellent electroluminescence performance. The red OLED based on TPA showed a maximum efficiency of 40.2 cd A-1, 37.7 lm W-1 and 24.0%, and 47.3 cd A-1, 40.2 lm W-1 and 26.6% for SF-based red device. The green OLED hosted by SF exhibited a maximum efficiency of 97.6 cd A-1, 77.8 lm W-1 and 27.0%.

EXPERIMENTAL SECTION General Information



The NMR spectra, mass spectra, UV-vis absorption, PL and phosphorescent spectra, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV) of the hosts were performed with the same methods as reported in the literature.11 The OLED device fabrication and characterization were carried out with the reported procedures.48-50

Syntheses of Materials: 10'H-spiro[fluorene-9,9'-phenanthren]-10'-one 10'H-spiro[fluorene-9,9'-phenanthren]-10'-one was prepared according to method reported in reference 41,45. A mixture of 9H-fluoren-9-one, (1.73 g, 9.6 mmol) and triethyl phosphate (TEP, 4.65 g, 28 mmol) was heated at 170 oC (bath temperature) for 24 h with stirring. After cooling down room temperature, the orange solid was filtered and washed with hot benzene, cold CH3CN, and cold EtOAc to give the target compound (2.3 g, 70%). 1H NMR (400 MHz, Chloroform-d) δ 8.19 (d, J = 8.1 Hz, 1H), 8.09 (d, J = 8.0 Hz, 1H), 7.99 (d, J = 7.7 Hz, 1H), 7.79 (d, J = 7.7 Hz, 3H), 7.47 – 7.32 (m, 4H), 7.17 (t, J = 7.6 Hz, 2H), 7.11 – 7.00 (m, 3H), 6.61 (d, J = 7.9 Hz, 1H). MALDI-TOF-MS: m/z: calcd for [C26H20O+H]+: 344.413; found: 344.136. Anal. Calcd for C26H20O (%): C 90.67, H 4.68; found: C 90.48, H 4.75.

Compounds TPA, Cz, and SF were prepared according to the literature procedure.44 TPA White solid (yield, 65%). 1H NMR (400 MHz, DMSO-d6) δ 8.41 (d, J = 8.0 Hz, 1H), 8.28 (d, J = 7.9 Hz, 1H), 7.63 – 7.45 (m, 7H), 7.41 (t, J = 7.6 Hz, 1H), 7.34 –7.28 (m, 3H), 7.18 (d, J = 7.9 Hz, 1H), 7.01 – 6.96 (m, 3H), 6.75 (s, 2H), 6.60 – 6.17 (m, 8H), 5.75 (m, 2H) ppm.

13

C NMR (151 MHz, Chloroform-d) δ 145.55, 141.22, 139.45,

138.09, 137.73, 135.14, 134.60, 134.22, 133.87, 132.75, 129.95, 128.59, 127.82, 127.63, 127.44, 127.31, 126.91, 126.52, 126.17, 125.90, 125.74, 125.39, 124.59, 124.45, 124.19, 124.02, 123.67, 122.92, 120.63, 119.96, 119.38, 117.69, 112.47, 58.96, 58.63 ppm. MALDI-TOF-MS: m/z: calcd for [C44H29N+H]+: 571.723, found: 571.286. Anal. Calcd for C44H29N (%): C 92.44, H 5.11, N 2.45; found: C 92.58, H 5.35, N


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2.52. SF White solid (yield, 62%). 1H NMR (400 MHz, Chloroform-d) δ 7.81 (d, J = 7.8 Hz, 1H), 7.70 (d, J = 7.8 Hz, 1H), 7.63 (d, J = 7.7 Hz, 1H), 7.47 (d, J = 7.7 Hz, 1H), 7.35 – 7.28 (m, 3H), 7.16 (t, J = 7.6 Hz, 1H), 7.12 – 7.00 (m, 5H), 6.98 – 6.83 (m, 6H), 6.74 – 6.70 (m, 2H), 6.68 – 6.53 (m, 4H), 6.40 (d, J = 7.9 Hz, 1H), 6.27 (d, J = 8.0 Hz, 1H) ppm. 13C NMR (151 MHz, Chloroform-d) δ 156.30, 145.50, 141.15, 140.99, 139.86, 139.38, 137.95, 137.90, 137.73, 135.13, 134.61, 134.22, 134.07, 132.84, 132.56, 129.99, 128.56, 128.14, 127.86, 127.69, 127.57, 127.49, 127.42, 127.35, 126.97, 126.93, 126.45, 126.22, 125.90, 125.75, 125.37, 124.93, 124.48, 124.44, 124.25, 124.10, 123.75, 122.95, 122.82, 122.75, 122.55, 121.81, 120.41, 120.15, 119.70, 119.64, 118.67, 117.24, 117.11, 113.14, 111.63, 110.07, 108.16, 58.97, 58.72 ppm. MALDI-TOF-MS: m/z: calcd for [C44H27NS+H]+: 601.767, found: 601.148. Anal. Calcd for C44H27NS (%): C 87.82, H 4.52, N 2.33; found: C 87.94, H 4.36, N 2.47.

Cz White solid (yield, 70%). 1H NMR (400 MHz, DMSO-d6) δ 8.06 (d, J = 7.7 Hz, 1H), 7.88– 7.83 (m, 2H), 7.76 (t, J = 7.5 Hz, 2H), 7.52 (d, J = 8.0 Hz, 1H), 7.48 – 7.41 (m, 3H), 7.30 (t, J = 7.6 Hz, 1H), 7.26 – 7.19 (m, 3H), 7.15 – 7.03 (m, 3H), 7.02 – 6.95 (m, 4H), 6.87 – 6.80 (m, 3H), 6.77 (d, J = 7.8 Hz, 1H), 6.57 (d, J = 7.7 Hz, 1H), 6.44 (d, J = 8.1 Hz, 1H), 6.33 (d, J = 7.7 Hz, 1H) ppm. 13C NMR (151 MHz, Chloroform-d) δ 147.96, 143.83, 141.95, 141.66, 140.72, 137.64, 136.16, 134.12, 133.60, 131.65, 131.32, 130.51, 128.56, 128.03, 127.04, 126.60, 126.02, 124.26, 122.45, 119.03, 118.08, 113.13, 67.14, 55.18 ppm. MALDI-TOF-MS: m/z: calcd for [C44H27N+H]+: 569.707, found: 569.251. Anal. Calcd for C44H27N (%): C 92.76, H 4.78, N 2.45; found: C 92.89, H 4.46, N 2.54.

SO To a solution of H5IO6 (1.21 g, 5.3 mmol) in acetonitrile (30 mL), SF (3 g, 5 mmol) was added followed by addition of pyridinium chlorochromate (PCC, 22 mg, 2 mol %) in two portions at room temperature. The reaction mixture was stirred at room



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temperature for 4 h. The reaction mixture was then diluted with EtOAc (100 mL) and washed with saturated aq. Na2S2O3 solution (100 mL), and water (100 mL). The organic phase was then dried over anhydrous Na2SO4 and concentrated at reduced pressure to give the sulfoxide as white solid (2.5 g, yield 80 %). 1H NMR (400 MHz, DMSO-d6) δ 8.48 (d, J = 8.1 Hz, 1H), 8.32 (d, J = 8.0 Hz, 1H), 7.95 – 7.93 (m, 1H), 7.89 (d, J = 7.8 Hz, 1H), 7.63 – 7.59 (m, 2H), 7.54 (d, J = 7.5 Hz, 1H), 7.47 – 7.38 (m, 3H), 7.31 (t, J = 7.7 Hz, 1H), 7.23 – 7.00 (m, 8H), 6.96 (t, J = 7.6 Hz, 1H), 6.84 – 6.79 (m, 2H), 6.62 – 6.58 (m, 3H), 6.14 – 6.02 (m, 2H) ppm.

13

C NMR (151 MHz,

Chloroform-d) δ 148.75, 145.35, 144.39, 143.85, 143.20, 142.61, 141.86, 141.52, 139.74, 139.65, 139.36, 139.29, 138.75, 138.57, 137.79, 137.71, 137.58, 136.96, 136.86, 135.90, 135.28, 134.70, 134.60, 134.24, 133.49, 133.37, 133.28, 132.18, 131.48, 131.36, 131.14, 130.78, 130.18, 129.63, 128.86, 128.63, 128.51, 128.31, 128.26, 128.08, 127.96, 127.88, 127.79, 127.59, 127.30, 127.07, 126.93, 126.76, 126.60, 126.48, 126.39, 125.96, 125.51, 124.89, 124.49, 123.83, 123.53, 123.19, 123.07, 122.98, 122.76, 122.69, 122.58, 122.18, 122.01, 121.86, 120.98, 119.56, 119.18,

119.08,

65.57,

59.81

ppm.

MALDI-TOF-MS:

m/z:

calcd

for

+

[C44H27NO2S+H] : 633.765, found: 633.487. Anal. Calcd for C44H27NO2S (%): C 83.39, H 4.29, N 2.21; found: C 83.46, H 4.12, N 2.14.

RESULTS AND DISCUSSION Preparation and Characterization The synthetic routes of TPA, Cz, SF, and SO were outlined in Scheme S1. The precursor pinacolone 10ʹH-spiro[fluorene-9,9ʹ-phenanthren]-10ʹ-one was prepared via the reaction of 9H-fluoren-9-one with triethyl phosphate at 170 oC.51 And then the reaction of corresponding aryl lithium salts with the pinacolone in THF, followed by an acid-catalyzed Wagner-Meerwein rearrangement/cyclization44 ，gave the target propellane products in good yields. The four hosts were fully characterized with 1

H/13C NMR spectroscopy, mass spectrometry, and elemental analysis (Supporting

information). The molecular structures of TPA, Cz, SF, and SO were confirmed by X-ray analysis (Figure 2). The single crystal of the hosts were obtained by slow diffusion method of ethanol into a dichloromethane (for TPA and Cz) and toluene solution (for and SF and SO). The central C(sp3)-C(sp3) bond in TPA (1.614 Å) is slightly longer than those in Cz (1.569 Å), SF (1.575 Å), and SO (1.584 Å), which


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could be due to the repulsion between the acridine and biphenyl group.52,53 Within the dispiro compound TPA, the functional groups around each spiro linkage were essentially perpendicularly to each other; while within the propellane-type Cz, SF, and SO, the functional groups are arranged in the staggered conformation along the central C(sp3)-C(sp3) bond. In TPA, 9,10-dihydro-phenanthrene motif is slightly twisted

due

to

the

steric

hindrance

between

biphenyl

and

acridine.

9,10-dihydro-acridine and the fluorine units are located on the neighboring C(sp3) atoms with the dihedral angle around 37º between these two motifs. In Cz, SF, and SO, the 9,10-dihydro-phenanthrene units are also non-coplanar with the dihedral angle between phenyl groups less than 30º. In SF, the dihedral angle between phenyl rings of phenothiazine group is 49º, which is much larger than that in its oxidized derivative SO (29º). The dispiro and propellane molecules are held together in the solid state via intensive intermolecular C-H···π interaction and C-H···O hydrogen bonding (for SO).

Thermal Analysis In a multilayer OLED, the film qualities (such as uniformity and homogeneity) of each layer have a significant effect on the device performance. Therefore, high glass transition temperatures (Tg) and good thermal stability are the prerequisite for the host materials. The hosts TPA, Cz, SF, and SO exhibited excellent thermal stability with high decomposition temperatures (Td, 5% weight loss) and Tg of 351/367/375/407oC and 145/172/192/191oC, respectively (Figure S1, Figure S2 and Table 1). In this context, the rigid platforms applied in the hosts benefits to realize their good thermal properties.

Photophysical Properties The room temperature UV-vis absorption spectra and photoluminescence (PL) spectra in dichloromethane solution and low temperature (77 K) phosphorescence (Phos) spectra in toluene solution of TPA, Cz, SF, and SO are shown in Figure S3. Their photophysical characteristics are summarized in Table 1. The four hosts showed intense absorption around 250 nm, which could be attributed to π-π* transitions. The absorption onset of TPA (373 nm) and SO (372 nm) showed red-shift as compared to those of SF (332 nm) and Cz (357 nm). As a result, TPA (3.32 eV) and SO (3.33 eV)



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exhibited small optic band gap (Eg) relative to SF (3.73 eV) and Cz (3.47 eV), which could be due to the strong electron-donating acridine group in TPA and the strong electron-withdrawing sulfonyl group in SO. There are red-shifts in the PL spectra of TPA and SF relative to these of Cz and SO, which could be attributed to the different electron-donating abilities of the aromatic amine groups (acridine > phenothiazine > carbazole > phenothiazine 5,5-dioxide, see Figure S4). As shown by the Phos spectra of the hosts measured in toluene, the highest vibronic band of TPA, Cz, SF, and SO are peaked at 487, 491, 500, and 487 nm, giving triplet energy (ET) of 2.55, 2.53, 2.48, and 2.55 eV, respectively. In addition, the Phos spectra of the hosts as neat films were also recorded at 77K (Figure S5). The ETs estimated from emission peaks were in the narrow range of 2.47-2.58 eV, which agreed well with the results measured in toluene matrix. Moreover, the natural transition orbitals (NTO)54,55 analysis of TPA, Cz, SF, and

SO

showed

the

triplet

states

are

mainly

distributed

on

the

9,10-dihydrophenanthrene part (Figure S6), which could be responsible for the observation of the similar triplet energies of TPA, Cz, SF, and SO. Considering that the ETs are not high enough for blue PHOLEDs, they could be suitable hosts for green and red PHOLEDs.

Electrochemical Properties and Theoretical Calculation In order to study the electrochemical behaviors of TPA, Cz, SF, and SO, cyclic voltammetry (CV) were used to determine the oxidation potentials via typically tri-electrode

configuration

with

tetrabutylammonium

hexafluorophosphate

(n-Bu4NPF6) as the supporting electrolyte and ferrocene as the internal standard in degassed N,N-dimethyl formamide (DMF) solution at room temperature (Figure S7). The HOMO energy levels of TPA (-4.98 eV), Cz (-5.54 eV), SF (-5.21 eV) and SO (-5.62 eV) can be calculated from the onset of the first oxidation waves. So TPA exhibited the highest HOMO energy level among the four hosts, which was ascribed to the strong electron-donating effect of acridine group. Considering that an electron-withdrawing group sulfonyl group was applied in SO, it’s not surprising that SO showed the lowest HOMO energy level. The LUMOs were determined by the energy difference between Egs and the corresponding HOMOs, which are -1.66 eV, -2.07 eV, -1.48 eV, and -2.29 eV for TPA, Cz, SF, and SO, respectively. To gain insight of the electronic structures of the hosts, DFT calculations were performed at a B3LYP/6-31G(d) level. The HOMO/LUMO distributions of TPA, Cz, SF, and SO are


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shown in Figure S8. The HOMOs were predominantly localized at the strong electron-donating groups (acridine, carbazole and phenothiazine). The LUMOs were mainly distributed on the biphenyl moieties. The HOMO energy levels for the hosts were in the order of SO (-5.69 eV) < Cz (-5.23 eV) < SF (-5.08 eV) < TPA (-4.86 eV) (Figure S8), which followed the same trend as the experimental CV results demonstrated. The SO exhibited the lowest HOMO energy level among the four hosts probably due to the presence of sulfonyl group.

Electroluminescent Properties Since TPA, Cz, SF, and SO exhibited excellent thermal properties and suitable triplet/HOMO/LUMO energy levels, green and red PHOLEDs were fabricated with the simple device configuration ITO/HAT-CN (10 nm)/TAPC (55 nm)/Host: 10 wt% Ir(ppy)2(acac) (20 nm)/TmPyPB (35 nm)/Liq (2 nm)/Al (120 nm) and ITO/HAT-CN (10 nm)/TAPC (45 nm)/TCTA (10 nm)/Host: 6 wt% Ir(MDQ)2(acac) (20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (120 nm), respectively; HAT-CN = 1, 4, 5, 8, 9, 11-hexaazatriphenylene-hexacarbonitrile, Liq = 8-hydroxyquinolinolato lithium, TAPC

=

1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane,

1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene)

and

TmPyPB

TCTA

= =

4,4′,4′′-tris(N-carbazolyl)triphenylamine.

As shown in Figure S10, the EL spectra of the green/red devices are identical when hosted by different compounds. The hole/electron injection property, the hole/electron mobility and the singlet–triplet energy difference of host materials play an important role in the determination of driving voltage. SO showed the highest driving voltage among the four hosts, which could be due to its poor hole and electron mobility. The driving voltages of the green and red devices were recorded at 200 cd m-2. The propellane-type SF-based devices display a very high external quantum efficiency (EQE) of 27.0% and 26.6% with the maximum current efficiency (CE) of 97.6 cd A-1 and 47.3 cd A-1 and power efficiency (PE) of 77.8 lm W-1 and 40.2 lm W-1 for green and red devices, respectively (Table 2). We believe that this result demonstrated the highest EQE for the single host-based red OLEDs with Ir(MDQ)2(acac) as the dopant, even higher than that of TADF-hosted device.56 In addition, Ir(ppy)2(acac)-doped



green OLEDs hosted by SF showed comparable device performance with the best reported results.57,58 Notably, SF-based green OLED demonstrated a very low effeciency roll-off at a brightness of 1000 cd m-2 with CE, PE, EQE of 97.4 cd A-1, 71.3 lm W-1 and 26.8%, respectively (Figure 3). The red OLED based on Cz also exhibited good device performance with a maximum efficiency of 32.6 cd A-1, 25.9 lm W-1, 20.5% (Figure 4). SO based red and green device showed very low efficiency with the EQEs of 11.3% and 10.6%, respectively. The dispiro-type TPA-based red OLED demonstrated a maximum efficiency of 40.2 cd A-1, 37.7 lm W-1 and 24.0%. Furthermore, another red OLED hosted by SF was fabricated with simple structure: ITO/HAT-CN (10 nm)/TAPC (45 nm)/TCTA (10 nm)/Host: 6 wt% Ir(piq)3 (20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (120 nm), and high EQE over 23% was achieved with the Commission International de I’Eclairage coordinates of (0.67, 0.33) (Figure S12). The hole-only devices (ITO/MoO3 (10 nm)/Host (100 nm)/MoO3 (10 nm)/Al (100 nm)) and

electron-only devices (ITO/TmPyPB (20 nm)/Host (100

nm)/TmPyPB (20 nm)/Liq (2 nm)/Al (100 nm)) revealed that SF showed high hole and electron current density at a given voltage as compared to the other hosts (Figure S14), which led to its superior device performance. On the other hand, the J-V curves of green and red devices (Figure S15) indicated that SO based devices demonstrated a low current density at a given voltage relative to the other host-based devices, which could account in part for the low performance of SO-based device. In addition, TPA showed a relatively higher HOMO energy level than these of green and red dopants, which made it a hole-trapper in the emitting layer (EML).59

CONCLUSION In conclusion, four novel host materials of dispiro-type TPA and propellane-type Cz, SF and SO were designed and synthesized with facile synthetic strategy. These compounds showed excellent thermal stability and suitable triplet energies. As a result, green and red PHOLEDs were fabricated with TPA, Cz, and SF, SO as host materials. The champion device based on propellane-type SF reached 27.0%, 26.6%, and 23.4% for Ir(ppy)2(acac), Ir(MDQ)2(acac), and Ir(piq)3-doped devices, respectively. The ACS Paragon Plus Environment

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dispiro-type TPA based device also exhibited good EL performance with the maximum

efficiency

of

40.2

cd

A-1,

37.7

lm

W-1

and

24.0%

for

Ir(MDQ)2(acac)-doped red device. These results indicate that the dispiro and propellane molecular platforms have a great potential to construct novel host materials for the highly efficient OLEDs.

ASSOCIATED CONTENT Supporting Information The CCDC numbers for TPA, Cz, SF and SO are 1559948, 1559951, 1574765 and 1574766, respectively. The electrochemical properties, theoretical calculation results and electroluminescent properties and other electronic formats are available in the supporting information.

AUTHOR INFORMATION Corresponding Authors *Fan Jian. E-mail: [email protected]. *Liang-Sheng Liao. E-mail: [email protected].

Author Contributions X.-Y. Liu and X. Tang contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge financial support from the National Key R&D Program of China (2016YFB0400703), the National Natural Science Foundation of China (21472135, 61307036), Natural Science Foundation of Jiangsu Province of China (BK20151216), and the 111 Project. This project is also funded by Collaborative Innovation Center of Suzhou Nano Science and Technology (CIC-Nano), Soochow University and by the Priority Academic Program Development of Jiangsu Higher Education Institutions



Page 12 of 25

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Scheme

1.

Structures

of

8H-indolo[3,2,1-de]acridine,

9H-fluorene,

10-phenyl-9,10-dihydroacridine,

9H-quinolino[3,2,1-kl]phenothiazine,

9H-quinolino[3,2,1-kl]phenothiazine

5,5-dioxide,

9H-quinolino[3,2,1-kl]phenoxazine and their spiro derivatives.


and


Scheme 2. Reported structures of dispiro compounds.


Page 20 of 25

Page 21 of 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


N

or

i)

S

S N

N

N

Li Li ii) H2SO4 , HOAc

Cz

Path a

SF H5IO6 , PCC, CH3CN, 0 oC to RT

O

Path b O SO N

i)

N

N Li

ii) H 2SO4, HOAc

TPA

SO

Figure 1. The synthetic routes for propellane (Path a) and dispirocycle (Path b) derivatives.



Figure 2. Crystal structures of TPA, Cz, SF, and SO.

Figure 3. CE–, PE–, and EQE–L curves of green devices.

Figure 4. CE–, PE–, and EQE–L curves of red devices.


Page 22 of 25

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Table 1. Physical properties of TPA, Cz, SF and SO. Abs λmaxa)

PL λmaxa)

Tgb)

Tdc)

Egd)

ETe)

HOMOf)

LUMOg)

[nm]

[nm]

[oC]

[oC]

[eV]

[eV]

[eV]

[eV]

145

351

3.32

2.55/2.48

-4.98

-1.66

172

367

3.47

2.53/2.47

-5.54

-2.07

192

375

3.73

2.48/2.53

-5.21

-1.48

191

407

3.33

2.55/2.58

-5.62

-2.29

Host

TPA

228, 249

Cz

228, 285, 350

SF

228, 265, 285

SO

228, 272, 340

418, 439, 464 (sh) 357, 370 (sh) 404, 430, 455 (sh) 372

a) Measured in dichloromethane solution at room temperature, sh = shoulder peak. b) Tg: Glass transition temperature. c) Td: Decomposition temperature. d) Eg: Band gap energies were calculated from the corresponding absorption onsets. e) ET: Measured in toluene glass matrix/neat film at 77 K. f) HOMO levels were calculated from CV data. g) LUMO levels were calculated from the HOMOs and Egs.



Page 24 of 25

Table 2. Electroluminescence characteristics of the green and red devices.

Devicea

Vb

ηCEc

ηPEc

EQEc

CIEd

[V]

[cd A-1]

[lm W-1]

[%]

[x, y]

Host

G1

TPA

3.6

35.6, 34.2

26.5, 24.9

9.9, 9.6

0.32, 0.64

G2

SF

3.6

97.6, 97.4

77.8, 71.3

27.0, 26.8

0.32, 0.64

G3

SO

4.2

37.3, 36.4

30.7, 22.7

10.6, 10.4

0.32, 0.64

G4

Cz

3.6

64.3, 63.7

52.7, 45.8

18.1, 18.0

0.32, 0.64

R1

TPA

3.7

40.2, 36.1

37.7, 24.6

24.0, 21.7

0.62, 0.38

R2

SF

4.0

47.3, 42.6

40.2, 27.6

26.6, 24.1

0.62, 0.38

R3

SO

5.8

17.0, 13.6

11.3, 5.9

11.3, 9.1

0.62, 0.38

R4

Cz

3.8

32.6, 32.3

25.9, 21.1

20.5, 20.3

0.62, 0.38

R5

SF

4.7

20.0, 16.5

15.8, 8.6

23.4, 19.3

0.67, 0.33

a) The notation 1-4 in devices G1-G4 and R1-R4 indicates the corresponding devices fabricated with TPA, SF, SO, and Cz as the host respectively. Device configuration: G1-G4: ITO/HAT-CN (10 nm)/TAPC (55 nm)/Host: 10 wt% Ir(ppy)2(acac) (20 nm)/TmPyPB (35 nm)/Liq (2 nm)/Al (120 nm); R1-R4: ITO/HAT-CN (10 nm)/TAPC (45 nm)/TCTA (10 nm)/Host: 6 wt% Ir(MDQ)2(acac) (20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (120 nm). Device R5 was using SF as host with following structure: ITO/HAT-CN (10 nm)/TAPC (45 nm)/ TCTA (10 nm)/Host: 6 wt% Ir(piq)3 (20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (120 nm). b) Voltages at 200 cd m-2. c) Efficiencies in the order of the maxima and at 1000 cd m-2. d) Commission International de I’Eclairage coordinates measured at 5 mA cm-2.


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Table of Contents


Dispiro and Propellane: Novel Molecular Platforms for Highly Efficient

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