Quinolinylmethanone-Based Thermally Activated Delayed

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Quinolinylmethanone-Based Thermally Activated Delayed Fluorescence Emitters and the Application in OLEDs: Effect of Intramolecular H-bonding Vasudevan Thangaraji, Pachaiyappan Rajamalli, Jayachandran Jayakumar, Min-Jie Huang, Yu-Wei Chen, and Chien-Hong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22704 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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Quinolinylmethanone-Based Thermally Activated Delayed Fluorescence Emitters and the Application in OLEDs: Effect of Intramolecular H-bonding Vasudevan Thangaraji, Pachaiyappan Rajamalli, Jayachandran Jayakumar, Min-Jie Huang, Yu-Wei Chen, and Chien-Hong Cheng* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. KEYWORDS: quinoline, TADF, OLEDs, H-bonding, narrow band-width ABSTRACT: Three new quinoline TADF emitters, 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC, were designed and synthesized and the emitters show ΔEST as low as 0.07 eV and high PL quantum yield (PLQY) up to 98%. An electroluminescence device based on 2QPM-mDTC can reach high EQE over 24%. Compared with the reported TADF devices, the device shows narrow emission band width and high color purity. The excellent device performance is likely ascribed to the molecular design of 2QPM-mDTC containing an intramolecular H-bonding in the molecule.

Organic light-emitting diodes (OLEDs) have been a research attention for nearly 3 decades because they have applied in flatpanel display, flexible display, and thin-film lighting.1-3 Since 2012, OLEDs employing metal-free TADF emitters have evolved as an alternative for phosphorescence OLEDs.4-7 However, despite that the number of TADF emitters increases rapidly, very few of them can be considered for the actual use. One the frequently observed issues is the severe efficiency rolloff at a practical brightness level, the other is the poor color purity.8 Although many TADF devices show high efficiency at low brightness (1 cd/m2), the efficiency drops very rapidly at practical brightness level (100 cd/m2).9-10 In addition, most TADF molecules show broad emission spectrum with a full width at half maximum (FWHM) of nearly 100 nm.11 A molecule can harvest both singlet (S1) and triplet (T1) excitons for light emitting, if the triplet can undergo fast reverse intersystem crossing (RISC) to the singlet via thermal activation.12-13 The molecule with this property is called TADF emitter. A very small singlet (S1)-triplet (T1) energy gap (ΔEST) is required to realize fast RISC. In general, TADF molecules were designed to contain a donor (D) and acceptor (A) with highly twisted structure in order to achieve small ΔEST. Nonetheless, this type of molecules generally has large emission bandwidth and high roll-off of the device efficiency.1415 Therefore, the design strategy to have a highly twisted and rigid molecular structure should be attractive to achieve efficient TADF-based OLEDs with improved color purity and low efficiency roll-off.16 Although few attempts were made to overcome the existing problems in the TADF emitters, the issues are not completely addressed. In this work, we synthesized three new benzoylquinoline derived TADF emitters 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC (see Supporting Information for nomenclatures).17-18 The benzoylquinoline is used as the acceptor units, while carbazole and t-butyl carbazole unit are used as donor units. This donoracceptor TADF systems accomplishes high external quantum efficiencies (ηext) of 24%. The TADF device B based on 2QPMmDTC as the emitter shows reduced roll-off at a practical

brightness level and retain 90% of maximum EQE at 100 cd/m2 while EQE of 4QPM-mDTC (device C) is dropped to 74%. In addition, we have demonstrated the effect of the singlet charge transfer (1CT) and triplet local excited (3LE) state energy levels on the photoluminescence quantum yield and device performance. Furthermore, this device shows good color purity due to narrow emission induced by intramolecular H-bonding between phenyl and quinoline (acceptor) units. This design strategy opens-up a path to design a pure color TADF emitter with reduced roll-off. The Ullmann coupling was employed to prepare the present three molecules 2QPM-mDC, 2QPM-mDTC and 4QPMmDTC via the reaction of (3,5-dibromophenyl)(quinolin-2-yl) methanone with 9H-carbazole, and 3,6-di-tert-butyl-9Hcarbazole, and the reaction of (3,5-dibromophenyl)(quinolin-4yl) methanone with 3,6-di-tert-butyl-9H-carbazole (Scheme 1). The detailed synthetic procedures are given in SI. These compounds were further purified by vacuum sublimation; while the structures are confirmed by single crystal X-ray analyses, high-resolution mass spectrometry (HRMS), and 1H and 13C NMR spectra. In 2QPM-mDTC and 4QPM-mDTC, t-butyl group is used to protect the reactive C3 and C6 positions of the carbazole groups to increase the stability and to enhance the PLQY.17-18 In addition, the ketone-based acceptor unit is used to further enhance the stability because carbonyl unit is known to be more stable comparing with the widely used sulfone or phosphine oxide units.19 To see the HOMO and LUMO electron density distribution and the ΔEST, the time-dependent density functional theory (TD-DFT) calculation were performed for these molecules. Figure 1 shows that the LUMO is localized on the benzoylquinoline (or phenyl(quinolin-2-yl)methanone) unit due to the strong electron withdrawing nature of this moiety and the HOMO is mainly on the carbazole unit for 2QPM-mDC. In the case of 2QPM-mDTC and 4QPM-mDTC, the HOMO is mostly on one t-butyl carbazole group and HOMO-1 is on the other t-butyl carbazole. The DFT results are summarized in

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Tables S1-S3. The HOMOs and LUMOs are well separated and the separations indicate that the transition from HOMO to LUMO of these compounds are charge-transfer character and are likely responsible for the small calculated ∆EST values of 0.08, 0.08 and 0.11 eV for 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC, respectively. Noticeably, an intramolecular hydrogen bonding exists between the o-hydrogen on the central phenyl and the quinoline N atom with a distance of 2.3 Å in 2QPM-mDC and 2.2 Å in 2QPM-mDTC (Table S1-S2). The Hbonding is expected to restrict the intramolecular rotation, and leads to higher PLQY and color purity. The crystal structure of 2QPM-mDC and 2QPM-mDTC confirms the proposed structure. The H-bonding observed from DFT is confirmed by the single crystal structure. The quinoline nitrogen and an ortho hydrogen of the center phenyl unit reveal a distance of 2.4 Å in 2QPM-mDC and 2.3 Å in 2QPM-mDTC comparable to those from the DFT calculation (Figure 2). The results indicate that the strong tertiary butyl carbazole donor in 2QPM-mDTC increases the H-bonding strength compared to the weaker carbazole based donor in 2QPM-mDC. It is also noteworthy that similar H-bonding is not observed in 4QPM-DTC in both DFT calculation and single crystal (Figure 1-2). In addition, the twist angle between the donors (carbazole or t-butyl carbazole) and the center phenyl unit are calculated from the crystal structures. The twist angles (average) are 53°, 54°, and 54° for 2QPM-mDC, 2QPM-mDTC, and 4QPM-mDTC, respectively. The UV-vis absorption and emission spectra of these molecules in non-polar to polar solvents are depicted in Figures S1-S3. The absorption bands at 339, 344 and 342 nm are assigned as a π→π* transition for 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC, respectively. The corresponding weak broad bands at 383, 397 and 395 nm are attributed to the intramolecular charge transfer (ICT) absorption associated with the electron transfer from the carbazole or t-butyl carbazole groups to the benzoylquinoline moiety. Upon photoexcitation, 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC, emit sky blue light at 488 nm, green light at 510 nm and blueish green 494 nm, respectively, in toluene solution (Figure S4). These compounds display significant positive solvatochromism from non-polar to polar solvent; the emission shifts from 488 nm in toluene to 568 nm in DCM for 2QPM-mDC and from 469 nm in n-hexane to 589 nm in DCM for 2QPM-mDTC, and from 445 nm in n-hexane to 580 nm in DCM for 4QPM-mDTC. The observations support the DFT calculation results shown above. On the other hand, the phosphorescence spectra of these emitters (Figure S4) show a fine-structured emission in toluene at 77K. This indicates that their phosphorescence emissions are from a locally excited (LE) state of the molecule. We further analyze the natural transition orbitals (NTOs) to explore the nature of the first triplet excited state for quinolinylmethanonebased compounds.20 Apparently, the holes of three materials as shown in Figure S5 are mainly localized on the two carbazole groups. The particles of 2QPM-mDTC and 4QPM-mDTC are mostly located on one t-butyl carbazole, while the particle of 2QPM-mDC is distributed over the two electron-donating moieties. NTO analysis of the lowest triplet excited state suggests a predominant 3LE contribution from the carbazole backbone for quinolinylmethanone-based materials, consistent with the observed phosphorescence spectra.

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Scheme 1. Synthesis of 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC.

N

Br

N

CuI, 1,10-phenanthroline

Br

K2CO3, DMF, 160 oC

N

H N

O 2QPM-mDBr

R

O

R

R

R

R

N

R

R- H (2QPM-mDC) R- t-Bu (2QPM-mDTC)

N O

Br

CuI, 1,10-phenanthroline

N Br

tBu N

K2CO3, DMF, 160 oC

H N

O

tBu

4QPM-mDBr

tBu

tBu

tBu

N

tBu

4QPM-mDTC

Figure 1. Structures and molecular orbitals of 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC. (a)

(b)

(c)

Figure 2. Crystal structures of (a) 2QPM-mDC (b) 2QPM-mDTC and (c) 4QPM-mDTC.

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Table 1. Photophysical properties of 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC λabs

λem

λem

(nm)b

(nm)c

419

339, 383

2QPM-mDTC

433

4QPM-mDTC

421

Dopant

Td (°C)a

2QPM-mDC

LUMO

(nm)d

HOMO (eV)e

492

496

˗5.71

344, 397

510

518

342, 395

494

500

Eg (eV)g

ET (eV)h

ΔEST (eV)i

ΦPL (%)j

˗2.87

2.84

2.65

0.19

90

˗5.63

˗2.93

2.70

2.63

0.07

98

˗5.60

˗2.80

2.80

2.62

0.18

80

(eV)f

aT is the decomposition temperature determined by TGA. bMeasured in toluene at 1×10-5 M at RT. cFluorescence spectrum measured in d a co-doped thin film (mCBP:dopant (7 wt%), 30 nm) at 300 K. dPhosphorescence spectrum measured in the co-doped thin film at 77 K. eThe HOMO energy levels were determined from photoelectron spectroscopy. fCalculated from HOMO - E . gEstimated from the onset g of fluorescence spectrum measured in the co-doped thin film. hEstimated from the onset of phosphorescence spectrum measured in a codoped thin film. iES - ET = ΔEST, jAbsolute total PLQY evaluated for the 7 wt% dopant in mCBP films (30 nm) by an integrating sphere.

a) 1.0

Flu. at 300 K Phos. at 77 K

Intensity (a.u)

0.8 0.6 0.4 0.2 0.0 400

450

500

550

600

650

700

Wavelength (nm) b) 1.0

Flu. at 300 K Phos. at 77 K

0.8

Intensity (a.u.)

To investigate the photophysical property of these emitters, thin films (30 nm) containing dopant (7 wt%) in mCBP were prepared by vacuum deposition. The fluorescence and phosphorescence spectra of these doped films were measured at 300 K and 77 K, respectively. The singlet energy gaps for 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC in thin film are 2.84 eV, 2.70 eV and 2.80 eV, respectively, calculated from the onset of the fluorescence spectra at room temperature. The corresponding triplet energy gaps are 2.65 eV, 2.63 eV and 2.62 eV, respectively, are calculated from the onset of low temperature phosphorescence spectra at 77 K (see Figure 3). These values are summarized in Table 1. The ΔEST was calculated to be 0.19 eV for 2QPM-mDC, 0.07 eV for 2QPMmDTC, and 0.18 eV for 4QPM-mDTC. The small ΔEST values indicate that these emitters probably possess TADF property with efficient up-conversion from T1 to S1. The electrochemical property is investigated by photoelectron spectrometer (PES) and are shown in Figure S6. The HOMO levels were calculated to be -5.71 eV, -5.63 eV and -5.60 eV, respectively, for 2QPMmDC, 2QPM-mDTC and 4QPM-mDTC and obtained from the onset values in Figure S6. The LUMO levels were calculated from HOMO – Eg to be -2.87 eV (2QPM-mDC), -2.93 eV (2QPM-mDTC) and -2.80 eV (4QPM-mDTC). The measured PLQY of 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC are 4.0%, 6.5% and 5.5%, respectively, in oxygen-free toluene solution. In the presence of oxygen, the value decreases to 0.1% and 2.5% and 2.0% respectively for 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC. The results suggest that these molecules have TADF property, because the T1 state of molecules is readily quenched by the triplet ground state of oxygen instead of the RISC and reduction of the fluorescence intensity.17-18

0.6 0.4 0.2 0.0 400

500

600

Wavelength (nm)

c) 1.0

Flu. at 300 K Phos. at 77 K

0.8

Intensity (a.u)

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0.6 0.4 0.2 0.0

450

500

550

600

Wavelength (nm)

650

Figure 3. Fluorescence spectra at RT and phosphorescence spectra at 77 K of the thin film of 7 wt% 2QPM-mDC (a), 7 wt% 2QPMmDTC (b), and 7 wt% 4QPM-mDTC (c) doped in mCBP films (30 nm).

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Intensity (a.u.)

2QPM-mDTC 2QPM-mDC

0.1

0.01 0

200

400

600

800

1000

1200

Time (µs ) b) 1 300 K 250 K 200 K 100 K

2QPM-mDTC

Intensity (a.u.)

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

0.1

kRISC for 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC are 5.4 x 104 s-1, 2.7 x 105 s-1 and 1.9 x 104 s-1, respectively. The kRISC value of 2QPM-mDTC is one order of magnitude higher than those of 2QPM-mDC and 4QPM-mDTC due to 2QPM-mDTC retains high PLQY with small ΔEST value (0.07 eV) than the other compounds (Table 1), The DFT calculation also support the observation, fast kRISC of 2QPM-mDTC is mainly due to the energy difference between 1CT and 3LE. It is almost zero for 2QPM-mDTC and 0.05 eV for 4QPM-mDTC (Figure 5). It is known that the 3LE state is more crucial for up-converting the triplet excitons to the singlet state than the 3CT.26 Therefore, energy difference between 3LE and 1CT state are very important for RISC along with ΔEST values. The thermal stability of these emitters are measured by thermogravimetric analysis (TGA) under nitrogen atmosphere and the data are shown in Figures S8-S10. The results show 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC possess very high Td of 419 °C, 433 °C and 421 °C, respectively, at 5 wt% weight loss. The differential scanning calorimetry (DSC) measurement shows a Tg at 115 °C for 2QPM-mDC and no Tg point up to 350 °C for 2QPM-mDTC and 4QPM-mDTC due to high melting point of these compounds. Since the melting point is close to decomposition temperature and it is not possible to measure Tg for the 2QPM-mDTC and 4QPM-mDTC. The observed high thermal stability is important for the morphologic stability of these molecules in the thin films.

0.01 0

100

200

300

400

500

600

Time (µs )

Figure 4. Temperature-dependent transient PL curves of 7 wt % (a) 2QPM-mDC, 2QPM-mDTC and (b) 2QPM-mDTC (various temperature) doped in mCBP films (30 nm).

Remarkably, the PLQYs of 7 wt% 2QPM-mDC, 2QPMmDTC and 4QPM-mDTC doped in mCBP thin films (30 nm) measured by an integrating sphere under N2 atmosphere are 90%, 98% and 80% respectively. These thin-film PLQYs are much higher than those in the toluene solution. The enhancement is the result of solid-state enhanced emission due to the suppression of collisional and rotational quenching in the solid state.21-23 To further support the TADF property in codoped thin film, the transient PL characteristics was measured at various temperature. As shown in Figure 4, the slop of delayed emission component decreases as the temperature drops from 300 K to 100 K. The results can be rationalized by the decrease of the RISC rate from triplet back to the singlet as the temperature drops and suggests that 2QPM-mDTC is a TADF emitter. Transient Pl decay curves (at 300 K) consist of two components: one is the prompt emission with lifetimes of 23.6 ns, 16.1 ns and 11.3 ns and the second is the delayed emission with lifetimes of 362.0 s and 90.2 s, 357 s for 2QPM-mDC, 2QPM-mDTC (Figure 4) and 4QPM-mDTC (Figure S7), respectively. The phosphorescence spectrum of 2QPM-mDTC and 4QPM-mDTC exhibits structureless emission, while the 2QPM-mDC shows structured emission (Figure 3). It suggests that the phosphorescence emission is from local triplet excited state (3LE) for 2QPM-mDC and from charge transfer state (3CT) for 2QPM-mDTC and 4QPMmDTC.20 The PLQYs and decay lifetimes were used to calculate the rate constants of 2QPM-mDC, 2QPM-mDTC and 4QPMmDTC according to the reported method.24-25 The calculated

Figure 5. Schematic representation of singlet and triplet states of 2QPM-mDTC and 4QPM-mDTC based on DFT calculation.

To know the electroluminescence (EL) performance of these emitters, organic light-emitting devices A, B and C were fabricated using 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC as the dopants, respectively. The device architecture, energy level diagram and strucure of the compounds employed in the devices are shown in Figure S11. Devices A, B and C were fabricated with the following device structure: ITO/HAT-CN (10 nm)/TAPC (20 nm)/mCBP: 2QPM-mDC or 2QPM-mDTC or 4QPMmDTC (7 wt%) (30 nm)/TmPyPB (60 nm)/LiF (0.8 nm)/Al (100 nm) (see Figure S11 for details).17

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Table 2. EL performances of 2QPM-mDC, 2QPM-mDTC and 4QPM-mDTC as dopants Device

Vd (V)

EQE (%, V)

L (cd/m2, V)

CE (cd/A, V)

PE (lm/W, V)

CIE (x,y), V

max (nm)

A

3.5

17.5, 3.5

14866, 12.5

46.6, 3.5

41.6, 3.5

(0.19, 0.46), 8

498

B

3.5

24.0, 4.0

26447, 14.5

79.5, 4.0

62.3, 4.0

(0.25, 0.56), 8

514

C

3.8

16.2, 4.0

10025, 15.5

45.7, 4.0

40.8, 4.0

(0.20, 0.47), 8

502

cd/m2,

Vd is operating voltage at a brightness of 1 EQE is maximum external quantum efficiency, L is maximum luminance, CE is maximum current efficiency, PE is maximum power efficiency, CIE is Commission Internationale de l’Eclairage chromaticity coordinates and λmax is the wavelength of EL maximum.

The performances of A, B and C are shown in Figure 6 and Figure S12, and are listed in Table 2. Devices A and C show blueish green electroluminescence and EQEs of 17.5%, and 16.2%, respectively. Interestingly, device B shows very high EQE of 24% and green emission with good electroluminescence properties, which is better than the earlier reported green TADF devices.27-28 The current efficiency and power efficiency are 46.6 cd/A and 41.6 lm/W for device A and 79.5 cd/A and 62.3 lm/W for device B and 45.7 cd/A and 40.8 lm/W for device C. Moreover, devices A, B and C show maximum luminance of 14866 cd/m2, 26447 cd/m2, 10025 cd/m2 respectively, without applying light out-coupling technique. The EL spectra of devices A and C reveal blueish green emission band at 498 nm and 502 nm with CIE coordinates at 8 V of (0.19, 0.46) and (0.20, 0.47), respectively. B reveal a green emission band at 514 nm with CIE of (0.25, 0.56). The FWHM of devices A, B and C are 75 nm, 75 nm and 90 nm, respectively. As we expected from the DFT calculation and single crystal structure (vide supra), devices A and B show narrower emission bands (FWHMs of ~ 75 nm) than device C (FWHM ~ 90 nm). Devices A and B show much narrower emission bands than most of the reported TADF emitters (~100 nm).28-30 This result suggests that the intramolecular H-bonding is important to enhance the color purity of the TADF OLEDs. Furthermore, device B also exhibits lower efficiency roll-off relative to device C. Device B retains an EQE as high as 21.5% (10% off) at the practical brightness level of 100 cd/m2 and 15.5% (35% off) at 500 cd/m2. However, the EQE of device C droped quickly to 12.0% (26% off) at 100 cd/m2 and to 7.0% at 500 cd/m2 (57% off).

b) Device A Device B Device C

1.0 Intensity (a.u.)

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

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0.8 0.6

75 nm

75 nm

90 nm

0.4 0.2 0.0 400

500 600 Wavelength (nm)

700

Figure 6. (a) EQE vs Luminance of device A, B and C, and (b) electroluminescence spectra and measured FWHM of devices A, B and C.

In summary, we have reported three TADF emitters based on a new acceptor unit and these emitters concurrently possess low ΔEST of 0.07 eV and high PLQYs up to 98%. An electroluminescence device based on 2QPM-mDTC as the emitter reached a high EQE over 24% and current efficiency of 79.5 cd/A without any light out coupling technique. The power efficiency reached up to 62.3 lm/W and with a maximum brightness of 26447 cd/m2. Unlike most of reported TADF devices, devices A and B show high color purity due to intramolecular H-bonding. The results shed light on the future molecular design for high EQE and color purity of the TADF molecules.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: General information, computational details, TD-DFT calculation, Single-crystal packing, UV−vis, PL, PES, TGA, DSC, Transient PL, EL spectra, device performance, and an EL performance table (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT We thank the Ministry of Science and Technology of Republic of China (MOST 106-2119-M-007-020) for support of this research and the National Center for High-Performance Computing (account number: u32chc04) of Taiwan for providing computing time.

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