Management of Triplet Energy and Charge Transport Properties of

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C: Energy Conversion and Storage; Energy and Charge Transport

Management of Triplet Energy and Charge Transport Properties of Hosts by CN Position Engineering Su Kyeong Shin, Kyung Hyung Lee, Ha Lim Lee, and Jun Yeob Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11328 • Publication Date (Web): 17 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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The Journal of Physical Chemistry

Management of Triplet Energy and Charge Transport Properties of Hosts by CN Position Engineering Su Kyeong Shin+, Kyung Hyung Lee+, Ha Lim Lee+, Jun Yeob Lee* School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea E-mail: [email protected] +Su Kyeong Shin, Kyung Hyung Lee, and Ha Lim Lee contributed equally.

Abstract Novel hosts strategically designed to have high triplet energy and bipolar carrier transport features were derived by substituting carbazole and CN modified carbazole at ortho position of the ortho connected biphenyl core with a CN functional group. The position of the CN unit in the biphenyl core was controlled and the effect of the CN position on the photophysical properties and device performances of the blue phosphorescent organic light-emitting diodes. The CN substitution position changed the dihedral angle between aromatic units of the host materials, which managed the singlet energy, triplet energy and carrier transport properties of the hosts. Four hosts were synthesized to study the effect of the CN substitution position, and the hosts with the CN unit at sterically hindered position increased the triplet energy, while the hosts with the CN unit at other positions with less steric hindrance improved charge transport properties. The management of the CN position could increase the triplet energy of the hosts over 3.0 eV, and the best blue phosphorescent organic light-emitting diode showed over 20% external quantum efficiency.

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Introduction Recently, the device performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) have been improved as the light-emitting properties of the blue phosphors have been upgraded by engineering the chemical structure of the Ir or Pt based phosphorescent emitters to suppress the non-radiative loss process.1-13 Additionally, the host material assisted the advances of the blue PHOLED device performances by fully harvesting triplet excitons of the blue phosphors.14-19 There have been several types of hosts for application in the blue PHOLEDs20-25, but the development of the blue hosts was rather challenging because of high triplet energy requirement to prevent reverse energy transfer from the blue phosphors to the hosts and good stability issue for long device lifetime. In general, bipolar hosts with good hole and electron transport properties in addition to the high triplet energy and material stability are preferred in the PHOLEDs for carrier balance.26-27 Therefore, the hosts for blue PHOLEDs are desired to have high triplet energy and bipolar charge transport character using a stable hole transport unit and an electron transport unit with high triplet energy.28-30 The hole transport unit is generally carbazole and several electron transport units are combined with the carbazole to afford the bipolar host for blue PHOLEDs. The electron transport units are representatively triazine, CN modified biphenyl, phosphine oxide, and so on.31 Although the phosphine oxide unit has been popular as the electron transport unit of the bipolar hosts, it has a limitation of poor stability.32 The triazine unit has an advantage of molecular stability, but relatively low triplet energy by the planar molecular structure is also a limit for application in the blue PHOLEDs. The CN modified biphenyl can be an alternative of the phosphine oxide and triazine type electron transport units in the design of the bipolar hosts because it is chemically stable and has high triplet energy.33 For example, the 9-(3'-(9H-carbazol-9-yl)-5-cyano-[1,1'-


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biphenyl]-3-yl)-9H-carbazole-3-carbonitrile and 3',5-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-3carbonitrile (mCBP-1CN) host with the CN modified biphenyl provided high external quantum efficiency (EQE) and improved lifetime.32,34 However, only a few CN modified biphenyl based hosts are covered in the literatures and the design rule of the hosts is not established. Therefore, it is needed to study the relationship between the chemical structure of the CN modified biphenyl derived hosts and device performances as well as the photophysical properties. Herein, we describe the synthesis of four bipolar hosts with the CN modified and orthoconnected biphenyl electron transport unit as a building block of the hosts. The four bipolar hosts were designed to have different CN positions in the molecular structure to clarify the design principle of the CN modified hosts. It was described that the substituent position of the CN unit can manage the triplet energy, carrier transport properties, and device EQE of the hosts. A high EQE over 20% was achieved in the blue PHOLEDs using the CN modified biphenyl derived host by engineering the CN position of the host.

Experiment General information Chemicals and reagents purchased from commercial suppliers were used without further purification. Tetrahydrofuran (THF), n-hexane, methylene chloride (MC) and hydrochloric acid were products of Duksan Co.. Carbazole, n-butyllithium solution and triethyl borate were supplied from Sigma Aldrich Co.. Cesium carbonate, N,N-dimethylformamide (DMF) and potassium carbonate were purchased from Daejung Chemical & Metal Co.. 9Hcarbazole-3-carbonitrile and tetrakis(triphenylphosphine) palladium(0) were purchased from P&H tech Co.. The 1-bromo-2-fluorobenzene and 3-bromo-2-fluorobenzonitrile reagents



were purchased from Alfa Co.. 2-Bromo-3-fluorobenzonitrile was purchased from CombiBlock Inc., 3-bromo-4-fluorobenzonitrile came from J&H CHEM Co., and 4-bromo-3fluorobenzonitrile was supplied from GOM Co.. Tetrahydrofuran was purified by distillation with sodium under a nitrogen atmosphere and then used in the reaction. All reactions and manipulations were performed under nitrogen. Analytical methods are in supporting information.

Synthesis 9-(2-Bromophenyl)-9H-carbazole Carbazole (5.00 g, 29.9 mmol) and cesium carbonate (29.23 g, 89.7 mmol) were added in a 200 ml pressure tube and filled with N,N-dimethylformamide (DMF, 70 ml). A liquid reagent of 1-bromo-2-fluorobenzene (9.81 ml, 89.7 mmol) was then injected and stirred at 160 °C using oil bath for 16 h. After confirming that the reaction was completed, the temperature was dropped to room temperature and the mixture was extracted with MC and deionized water. The organic layer was dehydrated with anhydrous magnesium sulfate, filtered and evaporated to remove the solvent. After further column chromatography purification using MC and HEX solvents, the final material was obtained as a white solid powder. Yield 73 % (7.03 g), 1H NMR (500 MHz, DMSO-d6): M 8.49 (d, 2H, J= 7.5 HZ), 8.19 (d, 1H, J=6.5 HZ), 7.88-7.81 (m, 2H), 7.77-7.72 (m, 3H), 7.63 (t, 2H, J= 7.5 HZ), 7.40 (d, 2H, J= 8.5HZ). MS (APCI) m/z 322.02 [(M + H)+].

(2-(9H-carbazol-9-yl)phenyl)boronic acid The starting material, 9-(2-bromophenyl)-9H-carbazole (3.00 g, 9.31 mmol), was added to the two-neck round-bottomed flask and the reactor was maintained under a nitrogen atmosphere. Purified THF (100 ml) was injected into the flask and the flask was immersed in


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a stainless steel bowl kept at a low-temperature condition using dry ice and acetone. After about 2 h, a 2.5 M concentration of n-butyllithium solution (7.41 ml, 18.60 mmol) was carefully poured dropwisely. After 3 h, additional triethyl borate (4.74 ml, 27.90 mmol) was injected. The reaction mixture was stirred for about 14 h and then quenched with a mixture of hydrochloric acid and deionized water. After filtering out the reaction mixture, it was extracted with MC and water, and then the organic layer was evaporated. The product was obtained as a white powder. Yield 89% (2.40 g), 1H NMR (500 MHz, DMSO-d6): M 7.90 (d, 2H, J=8.0 HZ), 7.42-7.34 (m, 2H), 7.24 (t, 1H, J=7.5 HZ), 7.16 (t, 3H, J=8.0 HZ), 7.10 (t, 3H, J=8.5 HZ), 6.83 (t, 1H, J=8.0 HZ), 6.57 (d, 1H, J=7.0 HZ), 2.0 (s, 1H). MS (APCI) m/z 288.25 [(M + H)+].

2'-(9H-carbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-2-carbonitrile (2-(9H-carbazol-9-yl)phenyl)boronic

acid

(3.00

g,

10.4

mmol)

and

2-bromo-3-

fluorobenzonitrile (2.50 g, 12.4 mmol) were placed in the two-neck round-bottomed flask and dissolved in THF (100 mL) under a nitrogen atmosphere. A solution of potassium carbonate (5.75 g, 41.60 mmol) in distilled water (50 mL) was then added to the solution. Finally, tetrakis(triphenylphosphine) palladium(0) (0.60 g, 0.52 mmol) was added to the flask and then the reactants were refluxed for 20 h at 310

. After cooling the flask, the mixture inside

the flask was extracted with MC and deionized water. The organic material in the MC layer was purified by column chromatography and was obtained as a white powder. Yield 66% (2.50 g), 1H NMR (500 MHz, DMSO-d6): M 8.35 (d, 1H, J=7.5 HZ), 8.30 (d, 1H, J=8.0 HZ), 8.06-8.00 (m, 3H), 7.91(d, 1H, J=8.0 HZ), 7.71-7.54 (m, 5H), 7.50-7.41 (m, 3H), 7.24 (t, 1H, J=8.25 HZ). MS (APCI) m/z 363.52 [(M + H)+].

9-(2'-(9H-carbazol-9-yl)-6-cyano-[1,1'-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile



(CNCzCN1) The purified 2'-(9H-carbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-2-carbonitrile (1.40 g, 3.80 mmol), 9H-carbazole-3-carbonitrile (1.03 g, 5.40 mmol) and cesium carbonate (4.95 g, 15.2 mmol) were placed in a 100 ml pressure tube and filled with N,N-dimethylformamide (DMF, 30 ml). The reactants were stirred for about 18 h at 160 °C using an oil bath. After the reaction, the mixture was extracted with MC and deionized water. Finally, after further column chromatography purification and sublimation, a white solid compound was obtained. Yield 85% (1.72 g), 1H NMR (500 MHz, CDCl3): M 8.34 (d, 1H, J=8.0 HZ), 8.26 (t, 1H, J=8.75 HZ), 8.10 (d, 1H, J=8.50 HZ), 8.03 (d, 1H, J=7.0 HZ), 7.79 (d, 1H, J=7.50 HZ), 7.67 (t, 1H, J=7.25 HZ), 7.59 (t, 1H, J=7.25 HZ), 7.46 (t, 1H, J=7.75 HZ), 7.40 (d, 1H, J=8.0 HZ), 7.24-7.03 (m, 7H), 6.97 (d, 1H, J=8.5 HZ), 6.89 (t, 1H, J=7.25 HZ), 6.83 (d, 1H, J=8.5 HZ). 13C

NMR (125 MHZ, CDCl3): M 142.7, 141.2, 141.1, 140.3, 140.2, 136.3, 135.8, 135.2, 134.4,

133.3, 131.8, 131.4, 129.4, 129.1, 128.9, 127.2, 126.2, 125.9, 125.4, 124.2, 123.8, 123.7, 122.7, 121.8, 121.5, 121.2, 120.7, 120.2, 120.1, 119.1, 117.2, 111.7, 109.9, 109.4, 102.7. MS (m/z): found, 534.1843 ([FAB+]); Calcd. for C38H22N4, 534.1844.

2'-(9H-carbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-3-carbonitrile The synthetic method 2'-(9H-carbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-3-carbonitrile was the same as that of 2'-(9H-carbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-2-carbonitrile except that 3bromo-4-fluorobenzonitrile was used instead of 2-bromo-3-fluorobenzonitrile. After column and sublimation purification, a white final product was obtained. Yield 65% (2.30 g), 1H NMR (500 MHz, DMSO-d6): M 8.35 (d, 1H, J=7.5 HZ), 8.30 (d, 1H, J=8.0 HZ), 8.06-8.00 (m, 3H), 7.91(d, 1H, J=8.0 HZ), 7.71-7.54 (m, 5H), 7.50-7.41 (m, 3H), 7.24 (t, 1H, J=8.25 HZ). MS (APCI) m/z 363.74[(M + H)+].


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9-(2'-(9H-carbazol-9-yl)-5-cyano-[1,1'-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzCN2) The synthetic procedure of CNCzCN2 was the same as that of CNCzCN1 except the 2'-(9Hcarbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-3-carbonitrile starting material. Final product was a white powder after column and sublimation purification. Yield 58% (1.72 g), 1H NMR (500 MHz, CDCl3): M 8.68 (s, 1H), 8.14 (d, 1H, J= 8.0 HZ), 7.84 (d, 1H, J= 8.0 HZ), 7.65 (t, 1H, J= 7.75 HZ), 7.41 (s, 1H), 7.24 (d, 2H, J= 8.5 HZ), 7.00 (d, 2H, J= 8.0 HZ), 5.54 (s, 2H).

13C

NMR (125 MHZ, CDCl3): M 139.5, 138.8, 135.6, 135.5,

135.0, 133.7, 130.9, 129.9, 129.3, 128.5, 127.2, 125.9, 123.9, 123.7, 123.0, 121.8, 120.7, 118.8, 112.3, 102.7. MS (m/z): found, 534.1844 ([FAB+]); Calcd. for C38H22N4, 534.1844.

2'-(9H-carbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-4-carbonitrile 2'-(9H-carbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-4-carbonitrile was synthesized according to the synthetic method of 2'-(9H-carbazol-9-yl)-6-fluoro-[1,1'-biphenyl]-2-carbonitrile except that 4-bromo-3-fluorobenzonitrile (2.08 g, 10.4 mmol) was used instead of 2-bromo-3fluorobenzonitrile. Final compound was a white powder. Yield 62% (1.95 g), 1H NMR (500 MHz, DMSO-d6): M 8.36 (d, 2H, J=8.0 HZ), 8.02-7.92 (m, 4H),7.64-7.59 (m, 3H), 7.53 (t, 2H, J=7.25 HZ), 7.47 (d, 1H, J=8.0 HZ), 7.41 (d, 2H, J=8.5 HZ), 7.26-7.21 (m, 2H). MS (APCI) m/z 363.43[(M + H)+].

9-(2'-(9H-carbazol-9-yl)-4-cyano-[1,1'-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzCN3) The synthetic procedure of CNCzCN3 was the same as that of CNCzCN1 except the 2'-(9Hcarbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-4-carbonitrile starting material. Final product was a white powder after column and sublimation purification.



Yield 57% (1.68 g), 1H NMR (500 MHz, CDCl3): M 8.35 (s, 1H), 8.17 (s, 1H, J= 6.5 HZ), 8.08 (d, 2H, J= 8.0 HZ), 7.67-7.64 (m, 2H), 7.42 (t, 1H, J= 7.0 HZ), 7.02 (d, 2H, J= 8.0 HZ). 13C

NMR (125 MHZ, CDCl3): M 136.6, 136.0, 135.6, 134.7, 133.1, 132.8, 131.0, 129.3, 128.6,

127.2, 125.9, 124.0, 123.8, 121.7, 120.8, 118.1, 112.9, 102.6. MS (m/z): found, 534.1851 ([FAB+]); Calcd. for C38H22N4, 534.1844.

2'-(9H-carbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-3-carbonitrile The synthetic procedure of 2'-(9H-carbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-3-carbonitrile is similar to that of 2'-(9H-carbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-2-carbonitrile except the 3bromo-2-fluorobenzonitrile (2.50 g, 12.4 mmol) starting material. A white solid compound was obtained as a product. Yield 81% (3.03 g), 1H NMR (500 MHz, DMSO-d6): M 8.34 (d, 2H, J= 8.0 HZ), 8.02-7.92 (m, 4H), 7.65 (t, 2H, J= 7.0 HZ), 7.59-7.51 (m, 3H), 7.42 (d, 2H, J= 8.5 HZ), 7.25 (t, 1H, J= 7.75 HZ), 7.00 (t, 1H, J= 7.75 HZ). MS (APCI) m/z 363.80[(M + H)+].

9-(2'-(9H-carbazol-9-yl)-3-cyano-[1,1'-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzCN4) The synthetic procedure of CNCzCN4 was the same as that of CNCzCN1 except the 2'-(9Hcarbazol-9-yl)-2-fluoro-[1,1'-biphenyl]-3-carbonitrile starting material. Final product was a white powder after column and sublimation purification. Yield 61% (1.33 g), 1H NMR (500 MHz, CDCl3): M 8.51 (s, 1H), 8.02-7.97 (m, 3H), 7.92 (t, 1H, J= 8.0 HZ), 7.59 (t, 1H, J= 7.5 HZ), 7.93 (t, 1H, J= 7.5 HZ), 7.01 (s, 2H). 13C NMR (125 MHZ, CDCl3): M 141.3, 140.2, 136.4, 135.7, 135.5, 131.1, 129.4, 129.1, 127.6, 126.0, 125.7, 124.0, 123.9, 122.7, 122.1, 120.6, 116.0, 103.0. MS (m/z): found, 534.1849 ([FAB+]); Calcd. for C38H22N4, 534.1844.


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Device fabrication The device structure using the CNIm dopant is as follows. BPBPA:HATCN(40 nm:30%)/BPBPA

(10

nm)/PCZAC

(10

nm)/CNCzCN1,

CNCzCN2,

CNCzCN3,

CNCzCN4:CNIm (30 nm:10%)/DBFTRZ (5 nm)/ZADN (20 nm)/LiF (1.5 nm)/Al (200 nm). The emitting layers were CNCzCN1:CNIm, CNCzCN2:CNIm, CNCzCN3:CNIm and CNCzCN4:CNIm and CNIm doping concentration was 10 wt%. HATCN is 1, 4, 5, 8, 9, 11hexaazatriphenylene hexacarbonitrile, BPBPA is 1 1 1R1R&" " E;, ,R& 2 2

?&( (R&/

PCZAC

is

?&(& F&;, ,R&

9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-

dihydroacridine, DBFTRZ is 2,8-bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]furan and ZADN

is

2-[4-(9,10-Di-naphthalen-2-yl-anthracene-2-yl)-phenyl]-1-phenyl-1H-

benzimidazole. The device structure of the FIrpic devices was ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/oCBP (10 nm)/CNCzCN1, CNCzCN2, CNCzCN3, CNCzCN4:Firpic (25 nm:5%)/TSPO1 (5 nm)/TPBi (20 nm)/LiF (1.5 nm)/Al (200 nm). TAPC is ( (R& cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine], TSPO1 is called diphenyl[4(triphenylsilyl)phenyl]phosphine oxide and TPBi is # #R #S&;, 6 B&

3

"

?&" !;,&2

&

1-H-benzimidazole). All layers used in the experiments were deposited by vacuum thermal evaporation at a high pressure of 3.0

10-7 torr. The material was thermally evaporated and

then the device was protected from moisture and oxygen by encapsulation using a glass lid in a glove box under a nitrogen atmosphere. Electrical characterization of the device was performed using a Keithley 2400 source meter and optical characteristics were analyzed using a CS 2000 spectrometer.

Results and discussion




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carbazole-3-carbonitrile produced the four final products. All compounds were obtained at a reasonable yield over 50%. Column choromatography, recrystallization, and sublimation were used to purify the materials. Chemical identification methods such as 1H and

13C

nuclear magnetic resonance spectrometer and high resolution mass spectrometer were used to verify the chemical structure.

H N

F F H N

Br

Br DMF, Cs2CO3

N

n-BuLi, triethylborate THF, -78

B(OH)2 N

N

F

NC Br

K2CO3, Pd(0), THF, DW

NC

CN

NC

DMF, Cs2CO3

N

N

CN

CNCzCN1

H N

F

CN

F

N

Br

NC

K2CO3, Pd(0),THF, DW

CN N

CN

N

DMF, Cs2CO3

CNCzCN2

H N

Br F

CN

N

F

NC

CN

CN K2CO3, Pd(0), THF, DW

N

CN

DMF, Cs2CO3

N

CN

CNCzCN3 H N

Br F F

N CN

NC K2CO3, Pd(0), THF, DW

N

CN

CN

DMF, Cs2CO3

N

CN

CNCzCN4

Scheme 1. Synthetic scheme of CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4.

Frontier orbitals of the four hosts were computed using the B3LYP 6-31G* basis set of the Gaussian 09 Software. The computation results of the hosts are shown in Figure 2. CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 showed similar highest occupied molecular orbital (HOMO) distribution localized in the carbazole unit. The HOMO was weakly extended to the phenyl unit of the CN modified biphenyl core. However, the lowest unoccupied molecular orbital (LUMO) dispersion was dissimilar in the four hosts. Although the main LUMO dispersing part was the CN modified carbazole, the LUMO was mostly



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confined in the CN modified phenyl unit of the core structure in the CNCzCN1, CNCzCN2, and CNCzCN4. However, the LUMO of the CNCzCN3 was rather widely distributed over the whole CN modified biphenyl core structure.

HOMO

LUMO

CNCzCN1

CNCzCN2

CNCzCN3

CNCzCN4

Figure 2. HOMO and LUMO distribution of the CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 hosts.

In order to understand the origin of the different molecular orbital pictures of the hosts, the


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geometrical structure of the hosts was compared. Figure 3 shows the dihedral angle of the molecules obtained from optimized geometry of the hosts. The dihedral angle between two phenyl units of the CN substituted biphenyl and that between carbazole unit and the phenyl unit of the biphenyl core were mostly compared. The dihedral angle between two phenyl units of the biphenyl core was in the order of CNCzCN4 (76 o)>CNCzCN1 (71 o)>CNCzCN2 (68 o)>CNCzCN3 (48 o). The CNCzCN4 and CNCzCN1 compounds with the CN unit next to the phenyl linkage of the biphenyl core possessed large dihedral angle due to steric effect by the CN substituent. The CNCzCN2 showed relatively small dihedral angle due to less steric hindrance than CNCzCN1, but the spatial overlap of the CN unit with the carbazole unit imparted intermediate dihedral angle. In the case of the CNCzCN3, the CN had little effect on the steric hindrance and did not distort the two phenyl units to a large extent. The angle between carbazole or CN modified carbazole and the biphenyl core was very high in the CNCzCN3, while it was relatively small in other hosts with the order of 1 3 1,U 1 3 1(T 1 3 1#. Instead of large distortion between two phenyl units of the CN modified biphenyl core, the carbazole and CN carbazole unit were oriented perpendicular to the phenyl plane. From the dihedral angle of the hosts, it can be understood that the frontier orbital calculation results showing wide dispersion of the LUMO in the CNCzCN3 are due to small dihedral angle between two phenyl units.35




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the hosts according to CN position can be correlated with the degree of conjugation of the CN modified biphenyl core and charge transfer (CT) character of the molecules. One clear feature observed in the fluorescence and phosphorescence spectra is the difference of the origin of the emission. The fluorescence spectra reflect CT emission from CT singlet excited state, while the phosphorescence spectra are mostly local emission from local triplet excited state. This indicates that the triplet energy would be dominated by the degree of conjugation correlated with the dihedral angle, while the singlet energy would be governed by the CT character as well as the degree of conjugation. The order of the triplet energy was in agreement with the dihedral angle of the biphenyl core structure. The triplet energy of the CNCzCN3 was lower than that of other hosts due to small dihedral angle between two phenyl units and para orientation of the CN unit from the phenyl unit. The CNCzCN1 and CNCzCN4 displayed high triplet energy by large distortion of the biphenyl core structure. The phosphorescence spectra of the CNCzCN1 and CNCzCN4 suggest that the origin of the phosphorescence is emission from local triplet excited state with weak contribution from CT triplet excited state. The stabilization of the CT triplet excited state was clearly observed in the CNCzCN3 by strong CT character, which was also a key contributor to the low triplet energy of the CNCzCN3. Figure 5 shows the correlation of the triplet energy with the dihedral angle of the hosts. In the case of the singlet energy, it was affected by CT character as well as the dihedral angle. Among the four hosts, the singlet energy of the CNCzCN1 and CNCzCN2 did not agree with the dihedral angle. The singlet energy of the CNCzCN1 was smaller than that of CNCzCN2 in spite of large dihedral angle between two phenyl units and it can be explained by strong CT character of the CNCzCN1. The CT character was estimated from the solvatochromic effect of the hosts in different solvents with different polarity.36-37 The solvent dependent photoluminescence (PL) spectra of the hosts are presented in Figure 6.




Page 16 of 33

Page 17 of 33

Figure 4. Fluorescence and phosphorescence spectra of CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4.

3.2

Triplet energy (eV)

3.1

3

2.9

2.8

40

50

60

70

80

Dihedral angle (degree)

Figure 5. The triplet energy of the hosts according to dihedral angle of the molecules.

300

Hexane

Intensity (arb. unit)


Hexane Toluene THF

CNCzCN1

400

500

600

300

Wavelength (nm)

Toluene THF

CNCzCN2

400

500

Hexane


Hexane

300

Toluene THF

CNCzCN3

400

600

Wavelength (nm)


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


500

600

300

Wavelength (nm)

Toluene THF

CNCzCN4

400

500

Wavelength (nm)


600


Page 18 of 33

Figure 6. Solvent dependent PL spectra of CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4.

The HOMO and LUMO levels of the hosts were estimated by characterizing electrochemical oxidation and reduction using electrochemical measurements. Cyclic voltammetry (CV) data of the hosts are shown in Figure S2 in supporting information. The HOMO levels of the CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 hosts were -6.09, 6.04, -6.06 and -6.10 eV, respectively. As the HOMO dominating hole transport unit is carbazole in all hosts, the HOMO was similar in the four hosts. The LUMO levels of the CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 hosts were -2.90, -2.87, -2.99, and -2.91 eV, respectively. Except for the CNCzCN3, all hosts displayed similar LUMO levels by the CN modified biphenyl core. In the case of CNCzCN3, the LUMO was relatively deep by the para oriented CN unit relative to the phenyl unit. Therefore, the HOMO-LUMO gap was small in the CNCzCN3. All material characterization data are in Table 1. Thermal characterization of the hosts was performed using differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA) because the host should be stable at elevated temperature during operation and during evaporation process. The stability at high temperature during operation can be confirmed by glass transition temperature from DSC and the processing stability can be checked by thermal decomposition temperature from TGA. DSC and TGA data of the hosts are in Figure S3 and S4 in supporting information. As the difference of the four hosts was only the CN position without any change of the building blocks, the thermal properties were not different. The glass transition temperature was in the range from 130 oC to 137 oC and the thermal decomposition temperature defined as the temperature at 5 wt% weight loss was 400 ~ 410 oC. All hosts showed superior thermal stability by reaching high glass transition temperature over 130


oC

and thermal

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of the single carrier devices reflects carrier mobility characteristics of the hosts in the absence of the energy barrier difference. The four hosts can be grouped into high J group (CNCzCN2 and CNCzCN3) and low J group (CNCzCN1 and CNCzCN4) in the hole only device data. They also can be classified into high J group (CNCzCN2), mid J group (CNCzCN3 and CNCzCN4) and low J group (CNCzCN1) according to the J of the electron only devices. The J of the blue PHOLEDs followed the grouping of the J of the hole only devices, indicating that hole plays a determining role of in the device operation of the blue PHOLEDs. This trend can be interpreted by the higher J of the hole only devices than that of the electron only devices. As the blue PhOLEDs in this work are hole dominant devices, the J of the hole only devices governed the J of the blue PhOLEDs in spite of energy barrier for carrier injection as shown in the energy level diagram (Figure S6). The HOMO levels were not critical to the J because they are similar in the four hosts. Although the LUMO levels were dissimilar, they were not of great importance to the J because the J of the electron only device was low. . The EQE of the blue PHOLEDs are plotted against the L in Figure 7(b). The EQE also can be categorized into high EQE group (CNCzCN1 and CNCzCN4) and low EQE group (CNCzCN2 and CNCzCN3), and it was closely correlated with the J of the hole only device. The EQE of the blue PhOLEDs is mostly dominated by carrier balance when triplet excitons of the phosphors are not quenched by the host. The four hosts of this work can harvest triplet excitons of the CNIm because of high triplet energy over 2.90 eV higher than that of the phosphor for energy transfer, indicating that carrier balance is the dominant parameter determining the EQE. As the electron carrier density was low in the four hosts, the hole carrier density dictated the carrier balance. The carrier balance is improved when the hole density and electron density are similar. Therefore, high EQE was observed in the low J group in the hole only device.


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and (0.15,0.26), respectively.

CNCzCN1


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 33

380

CNCzCN2 CNCzCN3 CNCzCN4

480

580

680

780

Wavelength (nm)

Figure 9. Electroluminescence spectra of CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 devices.

Lifetime test of the blue PHOLEDs was also performed at a constant current operation mode. The lifetime measurement results measured at 200 cd/m2 are presented in Figure 10. In the lifetime test, the CNCzCN2 was better than other hosts and the other three hosts performed similarly.39 The lifetime trend can be correlated with the molecular structure of the four hosts because the degree of distortion affects the bond dissociation energy of the chemical bonds. In other works, the effect of bond dissociation energy on the chemical stability and lifetime was described.40 Additionally, the bond dissociation energy calculation results of a phenyl substituted 9-phenylcarbazole at ortho position according to the dihedral angle between carbazole and phenyl unit show that the bond dissociation energy is reduced at large dihedral angle due to disruption of the

conjugation (Figure S7). Therefore, the dihedral angle of the


Page 23 of 33

hosts is an important parameter to the device lifetime of the hosts. The structural feature of the CNCzCN2 host is the relatively small dihedral angle between phenyl units and carbazole derived units along with small angle between two phenyl units. The CNCzCN2 was less distorted than other hosts, which extended the lifetime of the CNCzCN2 devices. The short lifetime of the CNCzCN3 is due to the large dihedral angle between the phenyl units and carbazole units, while that of CNCzCN1 and CNCzCN4 is caused by overall large dihedral angle of the molecules.

110 CNCzCN1 CNCzCN2 CNCzCN3 CNCzCN4

100

Luminance (%)

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


90 80 70 60 50 40 0

50

100

150

200

Time (h) Figure 10. Lifetime data of the of CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 devices at 200 cd/m2.

Blue PHOLEDs with FIrpic instead of CNIm were also developed to monitor the EQE change. In the FIrpic devices, the device structure was constructed to inject electrons without any energy barrier because large energy barrier would decrease electron density in the emitting layer. In particular, the higher hole density than electron density of the four hosts



based devices prompted us to design the devices with increased electron density. The EQE data of the FIrpic devices (5% doping concentration) without any energy barrier for electron injection for high electron density are shown in Figure 11. The EQE trend was totally different in the FIrpic devices. The high hole density group (CNCzCN2 and CNCzCN3) showed higher EQE than the low hole density group (CNCzCN1 and CNCzCN4). As shown in the energy level diagram in Figure S8, the electron injection is efficient in the FIrpic devices, which balanced carriers in the host with high hole density, resulting in high EQE in the high hole density hosts. The maximum EQE of the FIrpic doped CNCzCN3 devices was higher than 20%. Therefore, the CN position engineered hosts can be applied in various device structures for high EQE in the blue PHOLEDs. All device data are summarized in Table 2.

25

Quantum efficiency (%)

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 33

20 15 10 CNCzCN1 CNCzCN2 CNCzCN3 CNCzCN4

5 0 1

10

100

Luminance

1000

10000

(cd/m2)

Figure 11. EQE-luminance data of the FIrpic doped of CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4 devices at a doping concentration of 5%.

Table 2. Summarized device data of the CNCzCN1, CNCzCN2, CNCzCN3, and CNCzCN4


Page 25 of 33 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


devices.

Device CNIm doped CNCzCN1 CNIm doped CNCzCN2 CNIm doped CNCzCN3 CNIm doped CNCzCN4 FIrpic doped CNCzCN1 FIrpic doped CNCzCN2 FIrpic doped CNCzCN3 FIrpic doped CNCzCN4 a)

EQE a) [%] [1000cd/ [Max m2] ]

LE b) [lm/W] [1000cd/ [Max m2] ]

J c) [Cd/A] [1000cd/ [Max m2] ]

Color coordinate

13.7

17.7

16.2

33.1

24.2

31.6

(0.15,0.26)

8.7

13.3

11.1

24.5

15.4

23.4

(0.15,0.25)

8.5

10.3

10.8

19.2

15.2

18.4

(0.16,0.26)

13.4

16.1

14.6

25.8

23.9

28.7

(0.15,0.26)

14.5

15.7

19.1

25.5

27.2

29.0

(0.14,0.31)

18.9

19.5

28.4

35.0

37.7

39.0

(0.14,0.33)

18.4

21.0

27.1

36.4

35.6

40.5

(0.14,0.32)

16.2

16.5

21.6

27.3

31.6

31.8

(0.14,0.32)

External quantum efficiency. b) Power efficiency. c) Current efficiency.

Conclusions In conclusion, high triplet energy hosts derived from an ortho connected CN modified biphenyl core structure were developed by changing the CN position in the core structure. The engineering of the CN position could control the triplet energy and charge transport properties of the hosts. A high triplet energy over 3.00 eV and high EQE in the devices were realized using the CN modified biphenyl derived host design by ortho connection of all moieties building the backbone structure. Moreover, the hosts with the different CN positions could be appropriately selected and used in various device structures due to diversified charge transport properties.



Acknowledgements This work was supported by Nano Materials Technology Development Program (2016M3A7B4909243) through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning.

Supporting Information Available This information is available free of charge via the Internet at http://pubs.acs.org Instrumental analysis Calculated singlet energy of the hosts with methyl substituent instead of CN. Cyclic voltammetry data of the hosts. DSC thermograms of hosts Thermogravimetric analysis data of hosts Chemical structures of CNIm and FIrpic. Energy level diagram of the 10% CNIm doped CNCzCN1, CNCzCN2, CNCzCN3, CNCzCN4 devices. Bond dissociation energy according to dihedral angle Energy level diagram of the 5% FIrpic doped CNCzCN1, CNCzCN2, CNCzCN3, CNCzCN4 hosts devices.

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