Molecular Design of Deep Blue Thermally Activated Delayed

Molecular Design of Deep Blue Thermally Activated Delayed Fluorescence Materials Employing a Homoconjugative Triptycene Scaffold and Dihedral Angle Tu...
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Cite This: Chem. Mater. 2018, 30, 1462−1466

Molecular Design of Deep Blue Thermally Activated Delayed Fluorescence Materials Employing a Homoconjugative Triptycene Scaffold and Dihedral Angle Tuning Wenliang Huang,† Markus Einzinger,‡ Tianyu Zhu,† Hyun Sik Chae,§ Soonok Jeon,¶ Soo-Ghang Ihn,¶ Myungsun Sim,¶ Sunghan Kim,¶ Mingjuan Su,† Georgiy Teverovskiy,† Tony Wu,‡ Troy Van Voorhis,*,† Timothy M. Swager,*,† Marc A. Baldo,*,‡ and Stephen L. Buchwald*,† †

Department of Chemistry, ‡Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Advanced Materials Lab, Samsung Advanced Institute of Technology, Mountain View, California 94043, United States ¶ Samsung Advanced Institute of Technology, Samsung Electronics Co., Suwon-si, Gyeonggi-do 16678, Korea S Supporting Information *

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rganic light emitting diodes (OLEDs)1 are an emerging alternative to traditional light sources in applications such as lighting and displays.2−4 Thermally activated delayed fluorescence (TADF) has recently emerged as a triplet harvesting mechanism for OLED development.5−8 The pioneering work of Adachi demonstrated that donor−acceptor (D−A)-type organic molecules could overcome the 25% internal quantum efficiency (IQE) limit imposed by the statistics of exciton formation9 by utilizing this TADF process.10−12 This discovery stimulated the rapid development of TADF materials6,8,13−16 with an emphasis on blue emitters.17−42 Computational approaches also evolved to complement experimental efforts.43−51 A large oscillator strength value (f) is required for efficient radiative decay of the first excited singlet state S1 to the ground state S0. In addition, a small singlet−triplet energy splitting (ΔEST) is crucial for enabling the repopulation of S1 from the first excited triplet state T1.44 Although for a compound to manifest a large f value requires substantial overlap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), a small ΔEST can be realized only when this overlap is diminished. Researchers have tried to resolve this dilemma by molecular design.7 It has been shown that modification of D−A type fluorophores provides a route to efficient TADF emitters.20,22,52−57 Herein, we report the successful transformation of a fluorophore CZ-TRZ into a series of deep blue TADF emitters by applying two strategies: (1) homoconjugation through incorporation of a triptycene scaffold and (2) manipulation of dihedral angles by methyl substitution (Figure 1). Although the parent molecule CZ-TRZ 1 emits deep blue light, most TADF molecules having this basic structure have substituents that shift the S1 emission to sky-blue or green.22−24,58−64 We surmised that homoconjugation could decrease ΔEST while maintaining a high S1 energy. Swager recently reported the preparation of yellow TADF emitters based on the triptycene framework.65 We hypothesized that the homoconjugation effect66,67 could be more generally applied if the triptycene scaffold could be incorporated into the carbazole donor as the triptycene-fused carbazole (TCZ). TCZ was synthesized in four steps by applying Pd-catalyzed C−N © 2018 American Chemical Society

Figure 1. Homoconjugation and dihedral angle tuning.

coupling and ring closure via intramolecular C−H activation (Scheme 1, i−iv).68 The triazine acceptors (TRZ) were readily Scheme 1. Syntheses of Compounds 2−6

coupled to aryl spacers, with varying methyl substituents, in one step (Scheme 1, v). Finally, palladium catalyzed N-arylation was employed to afford TCZ-TRZ (2), TCZ-TRZ(Me) (3), and other methyl substituted derivatives (4−6) (Scheme 1, vi). As a result of the structural rigidity of the triptycene scaffold, compounds 2−6 showed excellent thermal stability, exhibiting Tg and Td values higher than those of 1 (Table S3).69 Received: August 17, 2017 Revised: February 12, 2018 Published: February 13, 2018 1462

DOI: 10.1021/acs.chemmater.7b03490 Chem. Mater. 2018, 30, 1462−1466

Communication

Chemistry of Materials

compounds 1−3 (Table 2, column 4), in accord with the predictions obtained by TDDFT calculations (Table 1, column 4). We measured the PL quantum yields (PLQYs) of compounds 1−6 to evaluate their potential performance in OLED devices. In solution, compounds 2 and 4 exhibited high PLQY values comparable to those of 1, while the values for compounds 3, 5, and 6 were much lower (Table 2, column 3). This is consistent with the trend of oscillator strength values (Table 1, column 2). However, the PLQYs of compounds 2−6 in doped films are significantly lower than those in solution (Table 2, column 3). Time resolved photoluminescence measurements were performed on doped films (Figure 2), and the rate constants

We performed time-dependent density function theory (TDDFT) calculations to estimate the f and ΔEST values of compounds 1−6 (Table 1). 1 and 2 were calculated to have Table 1. TDDFT Calculation Results of Compounds 1−6 compound

f

S1/T1 (eV)

ΔEST (eV)

angle θ1/θ2 (deg)

1 2 3 4 5 6

0.315 0.276 0.081 0.243 0.073 0.053

3.05/2.70 2.95/2.66 2.88/2.76 2.99/2.72 2.94/2.83 2.99/2.90

0.346 0.292 0.116 0.271 0.108 0.089

51.5/0.9 51.6/0.8 69.5/0.4 51.5/25.1 69.6/26.9 69.1/34.1

similar values for f, implying that both should exhibit strong fluorescence. The estimated ΔEST of 2 (0.292 eV) is smaller than that of 1 (0.346 eV), while the S1 energy of 2 remains high at 2.95 eV. Substituting hydrogen with methyl on the phenylene ring at a position ortho to the carbazole results in sterically induced twisting.41,70 As the dihedral angle θ1, defined by the carbazole plane and the phenylene plane (Figure 1), changes from 51.6° in 2 to 69.5° in 3, the ΔEST of 3 is further reduced to 0.116 eV. However, when the phenylene ring is substituted at a position ortho to the triazine as in 4, the ΔEST stays almost unchanged at 0.271 eV despite an increase in dihedral angle θ2, defined by the triazine plane and the phenylene plane (Figure 1). As a result, the calculated values of f and ΔEST for 5 and 6 are similar to that of 3. The HOMOs and LUMOs of compounds 1−3 are depicted in Figure S1. While all LUMOs localized on the triazine are qualitatively similar, the HOMOs of 2 and 3 extend to the neighboring phenyl rings because of the homoconjugation effect,66,67 which reduces HOMO and LUMO overlap at the phenylene compared to 1. The overlap is further reduced in 3 because of the larger θ1. Steady-state absorption experiments were performed for compounds 1−6 (Figures S2−7). They all showed a broad absorption (330−400 nm) that was assigned to the intramolecular charge transfer transitions from the CZ or TCZ donor to the TRZ acceptor.63 Photoluminescence (PL) properties of the molecules were investigated in toluene and bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films (doped at 15 wt %). In solution, compounds 2−6 showed deep blue emission with maxima around 430 nm, compared to 416 nm of 1. The emission of doped films was slightly red-shifted with respect to the solution and exhibited emission maxima from 435 to 457 nm for compounds 2−6 compared to 441 nm for 1 (Table 2, column 2). The ΔEST values were estimated from the onsets of the room temperature and low temperature PL spectra and were found to be 0.30, 0.27, and 0.16 eV for

Figure 2. Transient decay of compounds 1−6 (measured in codeposited film (15 wt %) in DPEPO).

for prompt fluorescence (kF), intersystem crossing (kISC), and reverse intersystem crossing (kRISC) were calculated according to the literature method.6 For prompt fluorescence, compounds 1−6 showed similar decay lifetime ranging from 7.2 to 10.6 ns (Table 2, column 5). While 1 barely had any delayed emission, compounds 2−6 clearly exhibited delayed emission with lifetimes ranging from 37 to 58 μs (Table 2, column 6). The delayed component was more prominent in compounds with θ1 (3, 5, and 6) larger than that of the others (2 and 4). This observation is consistent with the TDDFT prediction: increasing dihedral angle θ1 will result in smaller ΔEST. During the revision of this manuscript, a similar dihedral angle tuning effect on the CZ-TRZ system was reported by Adachi et al.41 OLED devices employing compounds 2−6 as emissive dopants were tested in the following device structure (Figure 3a): ITO (160 nm)/(HAT-CN (10 nm)/NPB (50 nm)/mCP (10 nm)/DPEPO:dopant (15 wt %) (30 nm)/PPF (10 nm)/ PPF:Liq (30 nm)/Liq (1 nm)/Al (100 nm). We obtained maximum external quantum efficiencies (EQEs) of 10.4 and 11.1% in devices employing 2 and 3, respectively (Figure 3b). These values exceed the theoretical limit of typical fluorescence

Table 2. Photophysical Properties of Compounds 1−6 compound

λPL (nm) sola/filmb

ΦPLc,d

ΔESTe(eV)

τpb (ns)

τdb (μs)

kF (107 s−1)

kISC (106 s−1)

kRISC (102 s−1)

1 2 3 4 5 6

416/441 432/457 431/451 429/444 427/442 427/435

0.78(ND) 0.77(0.40) 0.60(0.32) 0.80(0.51) 0.46(0.30) 0.47(0.17)

0.30 0.27 0.16 0.12 0.14 0.18

7.2 9.0 10.6 9.8 10.5 10.0

ND 38 51 58 39 37

14 6.2 2.0 5.8 1.2 1.4

ND 2.4 10.1 4.5 7.9 7.7

ND 6.1 35.3 8.5 36.9 33.2

In toluene (1 × 10−5 M) at RT. bCodeposited film (15 wt %) in DPEPO. cAbsolute PLQY using an integrating sphere under N2 measured in toluene. dPLQY of doped film in parentheses. eΔEST estimated from onset. ND = not determined. a

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DOI: 10.1021/acs.chemmater.7b03490 Chem. Mater. 2018, 30, 1462−1466

Communication

Chemistry of Materials ORCID

Wenliang Huang: 0000-0003-0056-8256 Tianyu Zhu: 0000-0003-2061-3237 Timothy M. Swager: 0000-0002-3577-0510 Stephen L. Buchwald: 0000-0003-3875-4775 Funding

Research reported in this publication was funded by Samsung Electronics. M.E., T.Z., and T.W. and the theoretical modeling and device fabrication and characterization were supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Award DE-FG02-07ER46474). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sigma-Aldrich for the generous donation of XPhos and tBuXPhos ligands. We thank Drs. Michael Pirnot and Yiming Wang for advice on the preparation of this manuscript.

Figure 3. (a) Device structure and energy band diagram. (b) EQEs of 2 and 3 as a function of current density. (c) EL spectra of 2 (black) and 3 (red) with their location in the CIE color space at 50 cd/m2. (d) Current density and luminance as a function of applied voltage.

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materials and are 2.5 times higher than the 4% EQE for 1, providing additional evidence that these materials exhibit TADF. However, the devices using compounds 4−6 as emitters showed lower EQEs (Table S4). The inferior performance of those emitters was attributed to their weak absorption, low PLQYs, and rapid degradation under electrical excitation. The EQEs and CIE values for compound 2 and 3 at 50 cd/m2 were 3.4%, (0.159, 0.142) and 2.0%, (0.170, 0.179), respectively. The sever efficiency roll-off was attributed to the long excited state lifetime that caused degradation at high current density (Figure S13). This phenomenon is common for deep blue TADF emitters due to the high excited state energy;17,23,32 however, recent molecular design strategies31,32,42 and optimization of host−guest interaction39 alleviated the efficiency roll-off. In summary, we successfully transformed fluorescent emitter 1 to TADF emitters 2 and 3 with EQEs surpassing 10% while maintaining the deep blue emission. Our results indicated that the introduction of a homoconjugative triptycene scaffold enhanced the TADF properties by effectively reducing the overlap between HOMO and LUMO, but the dihedral angle tuning did not yield significant improvement of device performance in our study because it had conflicting effects in PLQYs and ΔEST. Further study of triptycene-fused carbazole for TADF materials in OLED devices is ongoing in our laboratories.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03490. Experimental and calculation details and characterization data, including NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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DOI: 10.1021/acs.chemmater.7b03490 Chem. Mater. 2018, 30, 1462−1466

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DOI: 10.1021/acs.chemmater.7b03490 Chem. Mater. 2018, 30, 1462−1466