Thermally Activated Delayed Fluorescence Behavior Investigation in

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Thermally Activated Delayed Fluorescence Behavior Investigation in the Different Polarity Acceptor and Donor Molecules Joon Beom Im, Raju Lampande, Gyeong Heon Kim, Ju Young Lee, and Jang Hyuk Kwon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10854 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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The Journal of Physical Chemistry

Thermally Activated Delayed Fluorescence Behavior Investigation in the Different Polarity Acceptor and Donor Molecules Joon Beom Im, Raju Lampande, Gyeong Heon Kim, Ju Young Lee*, Jang Hyuk Kwon* Department of Information Display, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemoongu, Seoul, 130-701, Republic of Korea. ABSTRACT To investigate the influence of intramolecular charge transfer (CT) characteristic, rigidity and polarity of a molecule on photo-physical properties, we designed and synthesized two types of thermally activated delayed fluorescence (TADF) emitters with malononitrile and acrylonitrile moieties as an electron accepting units. Their photo-physical properties such as singlet-triplet energy split (∆EST), photoluminescence quantum yields (PLQYs), and electronic structures were theoretically and experimentally evaluated. Among the synthesized materials, the emitters with acrylonitrile moiety as an acceptor and phenoxazine, dimethylacridine and tert-butylcarbazole as a donor revealed small ∆EST values, good PLQYs, and efficient TADF performances. In contrast, the malononitrile derivatives demonstrated high ∆EST values, very low PLQYs, and relatively poor TADF performances even though they have strong intramolecular CT characteristics and high polarity. We found that high molecular polarity and strong intramolecular CT characteristic are not essential factors for attaining good TADF performances over molecular rigidity.

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1.

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INTRODUCTION

In recent years, organic light emitting diodes (OLEDs) have gained significant interest in display and solid state lighting applications because of its superior performances as well as low cost and eco-friendly approach.1-4 However, to attain further enhancement in efficiency and performances, new efficient emitters needed to be developed. Typically, a fluorescent molecule collects 25% of the singlet excitons and indicates only 25% of the internal quantum efficiency (IQE) under electrical excitation, whereas 75% of the non-emissive triplet excitons are deactivated as heat.5-8 Thus, significant research works have been done to utilize the nonemissive triplet excitons.5,6,9 The most effective technique to utilize both single and triplet excitons is by containing heavy metals into the emitter, which improves the phosphorescence efficiency via spin orbit coupling.10,11 Typically, the phosphorescent materials can attain ideally 100 % IQE by harvesting 25% singlet and 75% triplet excitons.9-12 The main disadvantage of these phosphorescent materials is their high cost because it usually comprises nonrenewable heavy metal elements.13 Recently, thermally activated delayed fluorescence (TADF) materials have attracted a significant attention in electroluminescent devices due to its promising alternative to attain high exciton formation efficiency.12-14 Normally, TADF materials have a small energy difference between lowest singlet (S1) and triplet (T1) excited states (∆EST) to enable fast up-conversion from T1 to S1. This small ∆EST and high reverse intersystem crossing (RISC) rate enables TADF materials to attain up to 100% IQE.13,14 Previously, it has been reported that the TADF properties strongly influence by the degree of spatial separation of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) and molecular rigidity to keep similar geometry between S0 and S1 state.13,15-22 Efficient TADF emitter also have small ∆EST almost


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close to zero with strong RISC, high photoluminescence quantum yields (PLQYs) and low nonradiative decay rate constant. Additionally, well separated HOMO and LUMO should have partial overlap to lead an efficient charge transfer characteristic, which results in small ∆EST with good electronic transition from electron donor to acceptor. Therefore, several TADF emitters with efficient acceptor unit such as triazine,15,17,19,23-26 cyano,16,27-30 sulfone,18,31 heptazine,20 oxadiazole,21 triazole,21 ketone,22 and 1,4-diazatriphenylene,32 have been reported recently, however there are almost no reports available on the effect of intramolecular charge transfer (CT) characteristic and molecular rigidity on the TADF properties. Investigation of such molecular properties in the TADF molecules is very essential to develop future efficient TADF emitters. To achieve high PLQYs, insertion of rigid moiety and proper steric hindrance approach between donor and acceptor moieties are effective for the development of efficient TADF molecules. In this paper, two new acceptor groups, malononitrile and acrylonitrile were introduced as electron accepting moieties in the TADF emitters. These moieties are not yet been reported as an electron acceptor in the TADF emitters. By using these two acceptor units and diverse donor moieties with different donor ability, a series of TADF emitters were designed and synthesized. The

synthesized

emitters

are

2-(bis(4-(3,6-di-tert-butyl-9H-carbazol-9-

yl)phenyl)methylene)malononitrile

(2CN-tCz),

2-(bis(4-(9,9-dimethylacridin-10(9H)-

yl)phenyl)methylene)malononitrile

(2CN-Ac),

2-(bis(4-(1H-indol-1-yl)phenyl)meth-

ylene)malononitrile

(2CN-Ind),

3,3-bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)acrylon-

itrile (CN-tCz), 3,3-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)acrylonitrile (CN-Ac), 3,3bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)acrylonitrile (CN-PXZ) as shown in Scheme 1. Herein, we report the effect of two different acceptor moieties in combination with diverse donor


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units on the photo-physical properties of emitters and also elaborate how TADF performances are affected by rigidity and intramolecular CT characteristics of the molecules.

2.

EXPERIMENTAL SECTION

2.1 Synthesis To synthesize the TADF emitters, all reagents and solvents were purchased from Aldrich Chemical Co. and TCI. The synthetic routes of the emitters are shown in Scheme 1. Synthesis of 2-(bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)methylene)malononitrile (2CN-tCz) A solution of bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)methanone (3.2 g, 4.3 mmol) and malononitrile (2.8 g, 43.0 mmol) in pyridine (40 ml) was stirred at 90° C for 17 hours. The reaction mixture was then concentrated and diluted with CH2Cl2. The resulting solution was washed with water, dried over MgSO4, filtered through silica gel, and concentrated in vacuo. The residue was recrystallized from CH2Cl2/MeOH to afford the titled compound (2.9 g, 86 %) as yellow solid. 1H NMR (CDCl3, 400 MHz): δ 8.17 (m 4H), 7.82 (m, 8H), 7.56 (m, 8H), 7.43-7.50 (m, 6H), 7.52 (m, 8H), 1.50 (s, 36H);

13

C NMR (CDCl3, 125 MHz) δ 172.57, 144.21, 142.92,

138.35, 133.39, 132.51, 126.02, 124.27, 124.08, 116.61, 114.28, 109.46, 80.93, 34.89, 32.03; Elemental analysis Calc. for C56H56N4: C, 85.7; H, 7.2; N, 7.1. Found: C, 85.7; H, 7.3; N, 7.4.

Synthesis of 2-(bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)methylene)malononitrile (2CNAc) A procedure similar to that used for 2CN-tCz was followed but with bis(4-(9,9-dimethylacridin10(9H)-yl)phenyl)methanone (4.0 g, 6.7 mmol) instead of bis(4-(3,6-di-tert-butyl-9H-carbazol-9-


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yl)phenyl)methanone. The title compound was obtained as a red solid (4.1 g, 95 %). 1H NMR (400 MHz, CDCl3): δ 7.75 (m, 4H), 7.55 (m, 4H), 7.51 (dd, J=7.6, 1.6 Hz, 4H), 7.11 (m, 4H), 7.05 (m, 4H), 6.56 (dd, J=8.0, 1.2 Hz), 1.69 (s, 12H);

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C NMR (CDCl3, 125 MHz) δ 172.52,

146.52, 140.39, 133.81, 133.20, 133.14, 128.54, 126.55, 125.33, 122.26, 116.29, 114.27, 80.68, 36.55, 30.35; Elemental analysis Calc. for C46H36N4: C, 85.7; H, 5.6; N, 8.7. Found: C, 85.6; H, 5.6; N, 8.9.

Synthesis of 2-(bis(4-(1H-indol-1-yl)phenyl)methylene)malononitrile (2CN-Ind) A procedure similar to that used for 2CN-tCz was followed but with bis(4-(1H-indol-1yl)phenyl)methanone (3.2 g, 7.8 mmol) instead of bis(4-(3,6-di-tert-butyl-9H-carbazol-9yl)phenyl)methanone. The title compound was obtained as a yellow solid (3.1 g, 87 %). 1H NMR (400 MHz, CDCl3): δ 7.75 (m, 2H), 7.73 (m, 10H), 7.44 (d, J=3.2 Hz, 2H), 7.32 (d, J=7.6 Hz, 2H), 7.26 (d, J=7.6 Hz, 2H), 6.80 (d, J=3.2 Hz, 2H);

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C NMR (CDCl3, 125 MHz) δ 172.38,

143.90, 135.35, 133.00, 132.53, 130.10, 127.24, 123.50, 123.28, 121.68, 121.46, 114.21, 110.69, 105.86, 80.77; Elemental analysis Calc. for C32H20N4: C, 83.5; H, 4.4; N, 12.2. Found: C, 83.5; H, 4.4; N, 12.7.

Synthesis of 3,3-bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)acrylonitrile (CN-tCz) To an ice bath cooled stirred solution of diethyl (cyanomethyl)phosphonate (1.1 mL, 7.0 mmol) in THF (10 mL) was added a solution of lithium bis(trimethylsilyl)amide (1.0 M in THF, 7.0 mL, 7.0 mmol) and stirred for 40 minutes. The resulting solution was then added to a solution of bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)methanone (3.0 g, 4.1 mmol) in THF (24 mL). The reaction mixture was stirred at room temperature for 16 hours and quenched with water. The


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resulting mixture was then extracted with t-BuOMe and then CH2Cl2. The combined extracts were dried over MgSO4 and concentrated in vacuo. The residue was recrystallized from CH2Cl2/MeOH to afford the titled compound (2.9 g, 93 %) as white solid. 1H NMR (CDCl3, 400 MHz): δ 8.18 (m, 4H), 7.80 (d, J = 8.4 Hz, 2H), 7.77 (d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.4 Hz, 2H), 7.65 (d, J = 8.4 Hz, 2H), 7.47-7.53 (m, 8H), 5.94 (s, 1H), 1.50 (s, 36H); 13C NMR (CDCl3, 125 MHz) δ 161.44, 143.66, 143.52, 140.70, 140.27, 138.86, 138.75, 136.90, 134.97, 131.34, 130.13, 126.48, 126.43, 123.90, 123.84, 117.98, 116.52, 116.43, 109.45, 109.29, 95.28, 34.86, 32.07; Elemental analysis Calc. for C55H57N3: C, 86.9; H, 7.6; N, 5.5. Found: C, 86.7; H, 7.7; N, 5.7.

Synthesis of 3,3-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)acrylonitrile (CN-Ac) A procedure similar to that used for CN-tCz was followed but with bis(4-(9,9-dimethylacridin10(9H)-yl)phenyl)methanone (2.4 g, 4.0 mmol) instead of bis(4-(3,6-di-tert-butyl-9H-carbazol-9yl)phenyl)methanone. The title compound was obtained as a yellow solid (2.2 g, 88 %). 1H NMR (CDCl3, 400 MHz): δ 7.73 (d, J = 8.4 Hz, 2H), 7.59 (d, J = 8.4 Hz, 2H), 7.44 (d, J = 8.4 Hz, 2H), 7.41 (d, J = 1.6 Hz, 2H), 7.39 (d, J = 1.6 Hz, 2H), 7.37 (d, J = 8.4 Hz, 2H), 6.87-6.98 (m, 8H), 6.29 (dd, J = 8.0, 1.2 Hz, 2H), 6.23 (dd, J = 8.0, 1.2 Hz, 2H), 5.22 (s, 1H), 1.62 (s, 12H);

13

C

NMR (CDCl3, 125 MHz) δ 161.35, 143.90, 143.32, 140.71, 140.62, 138.31, 136.54, 132.23, 131.78, 131.74, 131.00, 130.58, 130.50, 126.59, 126.49, 125.47, 125.34, 121.14, 121.05, 117.64, 114.25, 114.19, 96.39, 36.13, 31.19, 31.14; Elemental analysis Calc. for C45H37N3: C, 87.2; H, 6.0; N, 6.8. Found: C, 87.2; H, 6.1; N, 7.0.

Synthesis of 3,3-bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)acrylonitrile (CN-PXZ)


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A procedure similar to that used for CN-tCz was followed but with bis(4-(10H-phenoxazin-10yl)phenyl)methanone (3.0 g, 5.5 mmol) instead of bis(4-(3,6-di-tert-butyl-9H-carbazol-9yl)phenyl)methanone. The title compound was obtained as a yellow solid (2.8 g, 89 %). 1H NMR (CDCl3, 400 MHz): δ 7.76 (d, J = 7.6 Hz, 2H), 7.62 (d, J = 7.6 Hz, 2H), 7.53 (d, J = 7.6 Hz, 2H), 7.46 (d, J = 7.6 Hz, 2H), 6.00 (m, 6H), 6.74 (m, 6H), 6.67 (m, 4H), 5.94 (s, 1H);

13

C NMR

(CDCl3, 125 MHz) δ 160.98, 144.07, 141.66, 141.08, 138.48, 136.75, 133.97, 133.87, 132.35, 131.49, 131.41, 131.10, 123.45, 123.35, 121.95, 121.85, 117.44, 115.84, 115.74, 113.43, 113.32, 96.71; Elemental analysis Calc. for C39H25N3O2: C, 82.5; H, 4.4; N, 7.4 O, 5.6. Found: C, 82.6; H, 4.4; N, 7.6; O, 5.4.

2.2 Material characterization 1

H and

13

C NMR spectra were determined using Bruker Avance 400 NMR spectrometer. The

DSC was measured using TA Instruments DSC Q2000. Elemental analyses were quantified using Thermoscientific FLASH 2000. UV-vis absorption and photoluminescence (PL) spectra were measured using SCINCO S-4100 spectrometer and JASCO FP8500 spectrometer, respectively. The transient PL decay of thin film was recorded using a Quantaurus-Tau fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics Co.) in N2 filled atmosphere. Electrochemical analyses of the synthesized TADF emitters were inspected using cyclic voltammetry (CV). CV was performed using EC epsilon electrochemical analysis equipment. To measure the CV characteristics of TADF emitters, platinum wire and synthesized material on 50 nm thick ITO/glass substrate (1 cm x 3 cm) and Ag wire in 0.01 M AgNO3 were used as counter, working and reference electrodes, respectively. The 0.1 M tetrabutyl ammonium perchlorate (Bu4NClO4) in acetonitrile solution was used as a supporting electrolyte. Using an


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internal ferrocene/ferrocenium (Fc/Fc+) standard, the potential values were converted to the saturated calomel electrode (SCE) scale. The optical band-gap was determined by using the edge of absorption spectra. The LUMO level of each material was calculated from the optical band gap and HOMO value.

2.3 Device fabrication and characterization To fabricate TADF OLEDs, Indium-Tin-Oxide (ITO) coated glass substrates were successively cleaned in ultrasonic bath with acetone and isopropyl alcohol. After cleaning, nitrogen was used to dry the substrates and to remove residual solvents followed by UV-ozone treatment for 10 minutes. All organic layers and metal cathode were deposited on pre-cleaned ITO substrates in vacuum evaporation system under vacuum pressure of ~1 x 10-7 Torr. The deposition rate of all organic layers was about 0.5 Å/s. Especially, the deposition rate of LiF and Al were maintained at 0.1 Å/s and 5 Å/s, respectively. After the deposition process, all devices were encapsulated using glass cover and UV curable resin inside the nitrogen glove box. The emission area was 4 mm2 for all the samples studied in this work. The current density versus voltage (J-V) and luminance versus voltage (L-V) characteristics of fabricated OLED devices were measured with Keithley 2635A SMU and Konica Minolta CS-100A, respectively. EL spectra and Commission Internationale de l’Eclairage(CIE) 1931 color coordinate were obtained using Konica Minolta CS-2000 spectroradiometer.

2.4 Computational data To determine the molecular orbitals, and the energy levels of the TADF emitters, density functional theory (DFT/B3LYP) and time dependent density functional theory (TD-DFT/GGA)


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simulations were executed with the double numerical plus d-functions (DND) atomic orbital basis set. The molecular simulations were done using DMol3 module in Material studio 8.0 software package (Accelrys Inc., San Diego, California, United States).

3.

RESULT AND DISCUSSION

3.1. Molecular design and Synthesis To investigate the effect of the molecular rigidity and intramolecular CT characteristic of the TADF emitters on their performances, malononitrile and acrylonitrile groups as an electron acceptor were explored in combination with various donor moieties, carbazole, indole, acridine, and phenoxazine, which have different donating ability. The malononitrile group is a well-known strong electron acceptor moiety for red fluorescent dopant. Using this unit, we supposed to obtain highly efficient donor-acceptor-donor type TADF molecules. The acrylonitrile group considered as a relatively weaker acceptor than the malononitrile group was also examined for a comparison of TADF performances. From the photo-physical properties of two series of TADF emitters, we expected to get a valuable insight into the effect of electron accepting ability and molecular geometry on TADF performances. A synthetic scheme for new TADF molecules is shown in Scheme 1. The ketone starting materials were synthesized by the reported procedure.22 The malononitrile derivatives were obtained by the reaction of the ketones with malononitrile in the presence of pyridine. The acrylonitrile derivatives were prepared by the reaction of the ketones with lithium diethyl (cyanomethyl) phosphonate which was generated in situ by treating diethyl (cyanomethyl) phosphonate with lithium bis(trimethylsilyl)amide at 0 °C. The detailed procedure and structural characterization of the compounds are described in the Supporting Information.


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Scheme 1. Synthetic routes of TADF emitters

3.2. Estimation of photo-physical properties using molecular simulations Prior to synthesis of TADF emitters, the geometrical differences of the TADF emitters at ground (S0) and excited (S1) states were investigated from the geometrical optimizations of S0 and S1 states by using density functional theory (DFT) calculations with B3LYP (beck three parameter hybrid functional and Lee-Yang-Parr correlation functional) function implemented in DMOL3. We anticipated that molecular rigidity would play a vital role in improving PLQYs and TADF properties of the emitters. Therefore, to understand the effect of molecular rigidity on the TADF performances, the torsion angle and bond length calculations between both phenyl rings and donor units of the emitters (D-A-D structure) at S0 and S1 were performed using DFT simulation module.33 The acrylonitrile derivatives demonstrated a higher torsion angle at S0 and relatively smaller torsion angle change at S1. The torsion angles and bond lengths of both types of the emitters at S0 and S1 are given in Table 1. The torsion angles of 2CN-Ac were 61.2°, 57.1° at S0 and changed to 88.6°, 80.0° after excitation. Similarly upon excitation, the torsion angles of CN-Ac were changed from 66.5°, 59.3° to 88.9°, 89.7°, but the angle changes between S0 and S1 were smaller compared with 2CN-Ac. The torsion angle differences between S0 and S1 for 2CNAc (27.4°, 22.9°), 2CN-tCz (37.7°, 20.7°) were significantly higher than CN-Ac (22.4°, 30.4°), CN-tCz (26.0°, 22.7°). The malononitrile derivatives exhibit more planner molecular geometry


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due to large change in the torsion angle at S1 state. Such significantly large change in the torsion angle is ascribed to the strong electron withdrawing property of the malononitrile electron acceptor. Bond length calculations were also performed to gain more insight into the molecular geometry at S0 and S1. The malononitrile derivatives demonstrated the large bond length variations in the connected donor units (carbazole and acridine) and phenyl rings (Table 1). The bond length variations of both derivatives between S0 and S1 are 0.042, 0.019 Å (2CN-Ac), 0.042, 0.019 Å (2CN-tCz), 0.041, 0.009 Å (CN-Ac) and 0.037, 0.010 Å (CN-tCz), respectively. The torsion angle and bond length changes between S0 and S1 led to geometrical variations in the molecules. The geometrical changes for all designed TADF emitters are shown in Figure 1. The acrylonitrile derivatives present less geometrical changes than the malononitrile derivatives. These results indicate that the acrylonitrile derivatives have more rigid structure than the malononitrile derivatives. Normally, the high geometrical change between S0 and S1 can increase non-radiative decay process with energy transition to vibronic energy. Hence, we supposed that the malononitrile based emitters might have relatively low PLQYs due to their large geometrical changes.

Figure 1. Geometrical change between ground state (red line) and excited state (blue line) using DFT simulation.


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Additionally, we presumed that the molecular polarity could have a significant effect on the intramolecular CT characteristic. Similarly, such effect may alter the electronic states of the molecules, which lead to a change in the ∆EST. Therefore, the molecular polarity of both derivatives was evaluated from the dipole moment calculation using DFT module. The dipole moments of both derivatives at S0 and S1 as well as the differences of the dipole moment between S0 and S1 are summarized in Table 1. The dipole moments of the TADF emitters before and after excitation were estimated to be 4.79, 5.62 (2CN-Ac), 7.28, 8.11 (2CN-tCz), 5.60, 6.38 (2CN-Ind), 3.65, 4.09 (CN-Ac), 4.21, 4.59 (CN-tCz), and 2.84, 3.19 (CN-PXZ), respectively. These dipole moment values indicate that the polarity of the malononitrile derivatives is higher than that of the acrylonitrile derivatives, which is ascribed to the strong electron withdrawing ability of the malononitrile moiety. The acrylonitrile based emitters revealed moderately small changes of dipole moment between S0 and S1. Herein, higher dipole moment of the malononitrile derivatives clarified the large stabilization of the S1 state and strong intramolecular CT between donor and acceptor.

Table 1. Computational analysis using DFT calculation. a

a

Torsion angle (°)

b

Bond length (Å)

Dipole moment

Emitter S

0

a

S

1

S -S

S

S

S -S

S

S

S -S

0

1

0

1

0

1

0

1

0

2CN-Ac

61.2, 57.1 88.6, 80.0

27.4, 22.9

1.423, 1.414

1.465, 1.433

0.042, 0.019

4.79

5.62

0.83

2CN-tCz

40.5, 53.1 78.2, 73.8

37.7, 20.7

1.411, 1.398

1.453, 1.417

0.042, 0.019

7.28

8.11

0.83

2CN-Ind

35.9, 32.5 76.2, 72.9

40.3, 40.4

1.401, 1.400

1.460, 1.413

0.059, 0.013

5.60

6.38

0.78

CN-Ac

66.5, 59.3 88.9, 89.7

22.4, 30.4

1.423, 1.423

1.464, 1.432

0.041, 0.009

3.65

4.09

0.44

CN-tCz

42.0, 47.5 68.0, 70.2

26.0, 22.7

1.414, 1.413

1.451, 1.423

0.037, 0.010

4.21

4.59

0.38

CN-PXZ

67.4, 63.5 85.9, 86.2

18.5, 22.7

1.427, 1.428

1.467, 1.434

0.040, 0.006

2.84

3.19

0.35

1

b

Torsion angles and bond length between donor and bridge moieties, Vector length : fixed at 10, Density functional theory (DFT/B3LYP) and time dependent density functional theory (TD-DFT/GGA) calculations were executed with the double numerical plus d-functions (DND) atomic orbital basis set.


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Figure 2 shows the distribution of HOMO and LUMO for the TADF emitters. Here, all six molecules contain phenyl rings between acceptor and donor units to confine HOMO and LUMO in the respective donor and acceptor. The orbital overlap between the phenyl ring and donor unit is important because it has direct relationship with ∆EST.13 A small orbital overlap generally leads to a small ∆EST. Similarly, small exchange interaction integral of the HOMO and LUMO wave functions of a molecule is good for small ∆EST.34 As shown in Figure 2, the LUMOs for all compounds are dominantly located on the central electron accepting core, while the HOMOs are mainly localized on the donor moieties, because of the highly twisted geometry between the donor and acceptor constituents. The calculated HOMO, LUMO energies, energy bandgaps and T1 values are summarized in Table 2. The well separated HOMO and LUMO with partial overlap leads to a good CT characteristic, and small ∆EST for good TADF performances.35 The small overlapping between both HOMO and LUMO was obtained in the acrylonitrile derivatives because of large torsion angles between donor moieties and phenyl ring,. However, the malononitrile derivatives show relatively large overlap between donor and acceptor unit, resulting in relatively larger ∆EST and stronger intramolecular CT characteristics. The calculated ∆EST values are 0.33 (2CN-Ac), 0.32 (2CN-tCz), 0.45 (2CN-Ind), 0.19 (CN-Ac), 0.11 (CN-tCz), and 0.06 (CN-PXZ) eV, respectively. As expected from the orbital overlap results, ∆EST values of the malononitrile derivatives were found to be very large compared to the acrylonitrile derivatives. These calculated ∆EST values are based on the local triplet state.


13


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Figure 2. Spatial distribution of the HOMO and LUMO from DFT calculation.

3.3. Photo-physical and electrochemical properties The UV-Vis absorption and emission spectra of the synthesized TADF emitters in toluene solvents (10-5 M) are shown in Figure 3 and their comparative values of lowest-energy absorption as well as PL peaks are presented in Table 2. The malononitrile derivatives displayed weak absorption peaks at around 400-438 nm with respect to the electron donor ability of acridine, carbazole and indole, respectively, while the acrylonitrile derivatives with acridine, carbazole, and phenoxazine donor exhibited absorption peaks at relatively shorter wavelength of 360-405 nm. As reported normally weak absorption peak at the longer wavelength in TADF emitters could be assigned as intramolecular CT transition from donor to acceptor.36-38 Our these absorption bands also can be ascribed to the intramolecular CT from donor to acceptor moiety. To confirm the CT state and difference of stabilization of CT state, PL measurements were performed at room temperature and low temperature. The maximum fluorescence emission peaks of the malononitrile derivatives in toluene solvent (10-5 M) at room temperature (300 K) were attained at 649 (2CN-Ac), 566 (2CN-tCz) and 536 nm (2CN-Ind), respectively. On the other


14

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Figure 3. UV-vis absorption (red dash line), PL spectrum at 300K (green solid line) and 77K (blue solid line) in toluene 10-5 M solution. (a) 2CN-Ac, (b) 2CN-tCz, (c) 2CN-Ind, (d) CN-Ac, (e) CN-tCz and (f) CN-PXZ.

hand, CN-Ac, CN-tCz, and CN- PXZ exhibited maximum PL peaks (300 K) at 519, 450, and 574 nm, respectively. Similarly, the PL peaks of 2CN-Ac, 2CN-tCz, 2CN-Ind, CN-Ac, CN-tCz and CN-PXZ in toluene (10-5 M) at low temperature (77 K) were placed at 509, 499, 470, 456, 442 and 496 nm, respectively. The PL spectra of the acrylonitrile derivatives (CN-Ac, CN-tCz) at both low and room temperature displayed a substantial blue shift compared with the malononitrile derivatives (2CN-Ac, 2CN-tCz). In particular, 2CN-Ac and 2CN-tCz demonstrated significant differences of 140 nm and 67 nm between room and low temperature PL peak compared to CN-Ac (62 nm) and CN-tCz (10 nm), respectively. These differences are dedicated to the amount of stabilization of S1 state by CT character. Therefore, these results confirmed the strong stabilization of S1 state in malononitrile derivatives compared with acronitrile derivatives and these trends are also perfectly correlated with dipole moment values. On the other hand, smaller blue PL shift (from 300 K to 77 K) indicate the moderate stabilization of the acrylonitrile derivatives at S1 state. Additionally, upon photoexcitation both derivatives in toluene (10-5 M)


15


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Page 16 of 30

demonstrated a wide range of light emission from blue to red color due to the different electron donor ability of donor moiety and polarity of the molecules. The ∆EST values of our synthesized TADF emitters were evaluated to be 0.14 (2CN-Ac), 0.11 (2CN-tCz), 0.34(2CN-Ind), 0.03 (CN-Ac), 0.02 (CN-tCz) and 0.10 eV (CN-PXZ), respectively, using fluorescence (300 K) and phosphorescence (77 K) emission spectra (Table 2). The triplet energies of the TADF emitters were obtained from the onset of phosphorescence emission spectra (77 K) measurements (Figure S1). The replacement of strong electron acceptor moiety (malononitrile) with relatively weak electron acceptor moiety (acrylonitrile) in the emitters displayed a significant enhancement in triplet energy from 2.11 to 2.50 eV and 2.32 to 2.74 eV for CN-Ac and CN- tCz and wider energy bandgap because of the lower conjugation length and moderate electron withdrawing ability of acrylonitrile (Table 2). Theoretically calculated ∆EST, T1 and energy bandgap values show good agreement with experimental data. Table 2. Physical properties and DFT calculation results. HOMO b LUMO c Eg d T1 e ∆EST f (cal.) (cal.) (cal.) (cal.) (cal.) τP [ns] τd [µs] ΦPLg [%] [eV] [eV] [eV] [eV] [eV] 5.26 3.01 2.25 2.11 0.14 h 2CN-Ac 436 649 213 3.10 1.71 ND (5.32) (3.00) (2.32) (1.99) (0.33) 5.13 2.70 2.43 2.32 0.11 h 2CN-tCz 438 566 128 4.35 1.02 ND (5.18) (2.64) (2.54) (2.27) (0.27) 5.30 2.63 2.67 2.33 0.34 h 2CN-Ind 400 536 136 5.20 0.04 ND (5.48) (2.72) (2.76) (2.31) (0.45) 5.15 2.62 2.53 2.50 0.03 CN-Ac 373 519 146 5.09 8.01 31.34 (5.20) (2.68) (2.48) (2.23) (0.25) 5.14 2.38 2.76 2.74 0.02 CN-tCz 364 450 86 3.86 7.47 31.12 (5.18) (2.49) (2.69) (2.56) (0.24) 5.29 2.78 2.51 2.41 0.10 CN-PXZ 405 574 169 5.84 5.79 37.13 (5.39) (2.74) (2.65) (2.48) (0.17) a b c d Measured in Toluene. Determined from the onset of the oxidation potentials. Calculated from the HOMO and Eg. Measured from onset of absorption spectra. e Measured from the onset of the phosphorescence spectra. f Determined from time dependent PL spectrum measured in liquid nitrogen (77 K). g Photoluminescence quantum yields measured in toluene solution (10-5 M) using integrated sphere. h TADF characteristic was not detected. Emitters

λabs a [nm]

λPL a [nm]

Stokes shift [nm]

To investigate the effect of molecular geometry change between S0 and S1, Stokes shift values were evaluated for TADF emitters (Figure S2). The Stokes shift of 2CN-Ac was 213 nm,


16

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which was considerably larger compared to that of CN-Ac (146 nm). At the same time, 2CN-tCz exhibited a Stokes shift of 128 nm, which is exceedingly higher than CN-tCz (86 nm) (Table 2). The considerably large Stokes shift in the fluorescence of malononitrile derivatives is ascribed to huge variation in the dipole moment and big change in the molecular geometry between S0 and S1 (Table 1). In addition, these results also indicate that the malononitrile derivatives consume more energy at S1 state to change the molecular geometry compared with the acrylonitrile derivatives. Furthermore, to understand solvatochromic effect, fluorescent spectra of all TADF emitters were measured in different polarity solvents (Figure S3). The fluorescent spectra of relatively high polar malononitrile derivatives showed a strong solvatochromic effect with red shifted PL in highly polar solvents (Figure S3). However, the acrylonitrile derivatives which has lower polarity than the malononitrile derivatives, demonstrated less red shifted PL spectra (Figure S2). Such large red shift in the PL spectrum of the malononitrile derivatives in polar solvents is clear evidence of large stabilization of the S1 state and strong CT characteristic.39,40 Indeed, very strong CT characteristics and large stabilization of the S1 state is not beneficial for attaining excellent TADF properties.18,39 The large stabilization of the S1 state and strong CT characteristics can have significant variations in the molecular geometry between S0 and S1, resulting in increases vibrational coupling.18,39 In order to examine the delayed fluorescence behavior of the malononitrile and acrylonitrile derivatives, transient and time-resolved PL measurements were performed at room temperature (Figure 4). The relative prompt and delayed lifetime values of all six TADF emitters are shown in Table 2. The thin films of doped TADF emitters, 2CN-Ac, 2CN-tCz, 2CN-Ind, and CN-PXZ (6%), in wide bandgap and high triplet energy host, 1,3-bis(9-carbazolyl)benzene (mCP), and

CN-Ac and CN-tCz (6%) in bis[2-(diphenylphosphino)phenyl] ether oxide


17


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Page 18 of 30

(DPEPO) host, were prepared using vacuum evaporation process. The transient PL measurements at room temperature (300 K) showed the prompt fluorescence decay components of 2CN-Ac, 2CN-tCz, 2CN-Ind with lifetime in the range of 3.10 ns, 4.35 ns and 5.20 ns, respectively. However, delayed fluorescence lifetimes of 2CN-Ac, 2CN-tCz, and 2CN-Ind were not detected. This result can be attributed to the high vibronic coupling because of strong intramolecular CT characteristics. On the other hand, the prompt fluorescence components of the acrylonitrile derivatives, CN-Ac, CN-tCz, and CN-PXZ, showed lifetimes of 5.09 ns, 3.86 ns, and 5.84 ns, respectively, while the delayed exciton lifetimes were relatively well detected for instance 8.01 µs (CN-Ac), 7.47 µs (CN-tCz), and 5.79 µs (CN-PXZ), respectively, because of their small ∆EST and moderate CT characteristics. Herein, the PLQYs of the malononitrile derivatives was very low due to their relatively high molecular polarity and large geometrical changes, which leads to large ∆EST and large stabilization of the S1 state with strong CT characteristic (Table 2). In contrast, the acrylonitrile derivatives displayed comparatively higher PLQYs because of their improved delayed fluorescence characteristics. In summary, it is important to note that high molecular polarity and high intramolecular CT characteristic are not good for the efficient TADF performances. Hence, to attain excellent delayed fluorescence properties, balanced molecular polarity and moderate intramolecular CT characteristic including strong molecular rigidity are needed.


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Figure 4. Transient PL decay of (a) 2CN-Ac. Inset : 2CN-tCz and 2CN-Ind (b) CN-Ac, CN-tCz and CN-PXZ. (2CN-Ac, 2CN-tCz, 2CN-Ind and CN-PXZ doped 6 wt% in mCP film. CN-Ac and CN-tCz doped 6 wt% in DPEPO film.)

In order to investigate the HOMO energy levels of the synthesized TADF emitters, electrochemical analyses were performed using CV measurements. The CV characteristics of both derivatives are shown in Figure S4. The HOMO energy level of TADF emitters was obtained from the oxidation potential of TADF emitters using voltamograms, while, LUMO level was calculated from the energy bandgap and HOMO level. The HOMO, LUMO energy levels, and energy bandgaps of the synthesized TADF emitters are listed in Table 2. Indeed, the LUMO values obtained from the onset of reduction potential showed valid agreement with the above mentioned energy levels. The relative HOMO values of the malononitrile derivatives are correlated well with their counterparts. However, the acrylonitrile derivatives leads to lower LUMO levels due to their relatively moderate electron withdrawing characteristics compared with the malononitrile derivatives. The LUMO energy levels of CN-Ac, CN-tCz, and CN-PXZ were found to be 2.17, 2.38 and 2.78 eV, whereas for 2CN-Ac, 2CN-tCz, and 2CN-Ind they are 3.01, 2.70, 2.63 eV, respectively. The experimental HOMO, LUMO and energy gap values of both derivatives indicated good agreement with the theoretically calculated energy levels and energy bandgaps (Table 2).


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3.4. Electroluminescent properties By considering the good TADF performances of the acrylonitrile derivatives, multilayer OLEDs were fabricated to estimate their electroluminescence properties (Figure S5). The electroluminescent devices were fabricated with the following configurations: Device A: ITO (50 nm)/ HATCN (7 nm)/ NPB (125 nm)/TAPC (20 nm)/ mCP (10 nm)/ EML (25 nm)/ TSPO1 (5 nm)/ TPBi (20 nm)/ LiF (2.5 nm)/ Al (100 nm) Device B and Device C: ITO (50 nm)/ HATCN (7 nm)/ NPB (40 nm)/ TCTA (10 nm)/ EML (20nm)/ Bphen (35 nm)/ LiF (2.5 nm)/ Al (100 nm) Herein, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN), 4,4’-bis[N-(1-naphthyl)N-phenylamino]-1,1’-biphenyl methylphenyl)benzenamine]

(a-NPD), (TAPC)

and

4,4′-cyclohexylidenebis[N,N-bis(4-

1,3-bis(carbazol-9-yl)benzene

(mCP)

were

sequentially employed as hole-injection (HIL) and hole transporting layers (HTL). Similarly, diphenyl-4-triphenylsilylphenyl-phosphine oxide (TSPO1), 1,3,5-tris(N-phenylbenzimidazol-2yl)benzene (TPBi) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used as electron transport layers (ETL). Particularly, a thin layer of mCP with sufficiently high T1 was inserted between the interfaces of emitting layer (EML) and HTL in Device A to confine T1 excitons in the

blue

TADF

emitters

and

suppress

T1

exciton

quenching.22

The

bis[2-

(diphenylphosphino)phenyl] ether oxide (DPEPO) host with 40% doped CN-tCz and 4,4′-Bis(Ncarbazolyl)-1,1′-biphenyl (CBP) host with 5% doped CN-Ac (and CN-PXZ) emitters were employed as EML for Device A and Device B (Device C), respectively. The energy level diagrams of the fabricated device configuration and the chemical structures of organic materials are shown in Figure S5.


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Figure 5. Device performances of CN-Ac, CN-tCz and CN-PXZ. (a) J-V-L characteristics (b) EQE-current density characteristics and (c) EL spectra.

The J-V, L-V, current density versus external quantum efficiency (EQE) characteristics, and electro-luminance (EL) spectra of the optimized OLEDs are presented in Figure 5a, b, c. The OLEDs with CN-tCz, CN-Ac and CN-PXZ exhibited EL peaks at 480, 460 and 530 nm, which are almost similar with solid state PL spectra of the host-guest emissive layer (same as EML composition). Hence, this result confirms that the EL emissions were solely produced from the emitters themselves without any other energy losses (Figure S6). The CIE color coordinates of CN-tCz, CN-Ac, CN-PXZ based OLED devices were (0.15, 0.14), (0.16, 0.28), and (0.31, 0.54), which represent deep blue, sky blue and green color, respectively. The values of voltages, luminance, current and power efficiencies are summarized in Table.3. The deep blue OLED with CN-tCz revealed a maximum EQE of 4.01% and current efficiency of 3.40 cd/A. Similarly, Sky blue and green light emitting devices revealed a maximum EQE of 2.43% (CN-Ac), 2.40% (CNPXZ) and current efficiency of 1.50 and 1.30 cd/A, respectively. These efficiency values are relatively low compared to the previously reported TADF emitters because of their low PLQYs.18,19,41 Additionally, all OLEDs indicated severe EQE decrease with increasing current density. We assume that this efficiency roll-off at high current density is mainly attributed to


21


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excessive triplet excitons in the EML, which can cause exciton quenching by singlet-triplet or triplet-triplet annihilation.

Table 3. Electroluminescence properties. Turn-on Voltage a [V]

Driving voltage (at 500 cd/m2) [V]

Maximum Luminance [cd/m2]

CN-AC

3.2

5.2

CN-tCz

3.5

CN-PXZ

3.1

a

Maximum efficiency (at 1,000 cd/m2)

CIE 1931 (x, y) e

CE b [cd/A]

PE c [lm/W]

EQE d [%]

4,644

2.85 (2.75)

2.43 (1.98)

1.50 (1.49)

(0.16, 0.28)

8.5

1,164

3.40 (2.60)

3.05 (1.15)

4.01 (2.90)

(0.15, 0.14)

5.5

8,546

3.80 (3.70)

2.40 (2.10)

1.30 (1.28)

(0.31, 0.54)

Measured at 1 cd/m2, b Current efficiency, c Power efficiency, d External Quantum efficiency, e Measured at 1,000 cd/m2.

4. CONCLUSION In conclusion, we demonstrated the effect of molecular rigidity and intramolecular CT on the photo-physical properties of the new TADF emitters synthesized by using new electron acceptor moiety (malononitrile and acrylonitrile) and diverse electron donors (carbazole, indole, acridine, and phenoxazine) with variable donor ability. The malononitrile derivatives with high polarity exhibited relatively poor delayed fluorescence properties and very low PLQYs due to large changes in the molecular geometry and dipole moment between S1 and S0. These derivatives exhibited high intramolecular CT characteristics with increased vibrational coupling, and large stabilization of the S1 state. However, the acrylonitrile derivatives with balanced polarity demonstrated a moderate intramolecular CT characteristic and stabilization of the S1 state. Similarly, small changes in molecular geometry at S0 and S1 state resulted in high PLQYs and good delayed fluorescence properties. The electroluminescent devices with CN-Ac, CN-tCz, and


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CN-PXZ emitters showed maximum external quantum efficiency of 4.01, 2.43 and 2.40% with deep blue, sky and green light emission, respectively. We anticipate that moderate CT characteristic and molecular polarity as well as strong molecular rigidity will be crucial factors for the development of future efficient TADF materials.

ASSOCIATED CONTENT Supporting Information Available: Stock shift demonstration using absorption and PL spectrum, fluorescence spectra of TADF emitters in different solvents, cyclic voltammetry plot, device structure including chemical structure of used materials, structural characterization of TADF (CNMR and H-NMR). The supporting information is available free of charge via the internet at http://pubs.acs.org/

AUTHOR INFORMATION Corresponding Authors Prof. Jang Hyuk Kwon and Ju Young Lee, email: [email protected] and [email protected]

ACKNOWLEDGMENTS This

work

was

supported

by the

Human

Resources

Development

program

(no.

20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. And this work was also supported by the Industrial Strategic Technology Development Program of MKE/KEIT (10048317).


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Graphical Abstract


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Thermally Activated Delayed Fluorescence Behavior Investigation in

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