Molecular Design of Deep Blue Thermally Activated Delayed

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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, SooGhang Ihn, Myungsun Sim, Sunghan Kim, Mingjuan Su, Georgiy Teverovskiy, Tony Wu, Troy Van Voorhis, Timothy M. Swager, Marc A. Baldo, and Stephen L. Buchwald Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03490 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Chemistry of Materials

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, CA, 94043, United States ¶ Samsung Advanced Institute of Technology, Samsung Electronics Co., Suwon-si, Gyeonggi-do 16678, Korea

Supporting Information Placeholder

ABSTRACT: Thermally activated delayed fluorescence (TADF) is an emissive mechanism with promising applications in the development of organic light-emitting diodes (OLEDs). Rational engineering of a donor-acceptor fluorophore through incorporation of a triptycene scaffold and methyl groups for dihedral angle tuning led to the discovery of a novel class of deep blue TADF emitters. OLED devices manufactured using these compounds as dopants exhibited maximum external quantum efficiencies (EQEs) up to 11.1% with emission maxima around 450 nm. Transient photoluminescence study and density functional theory calculations supported the importance of the proposed homoconjugative and dihedral angle effects.

OLEDs1 are an emerging alternative to traditional light sources in applications such as lighting and displays.2-4 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% 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 ∆EST can only be realized 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).

Figure 1. Homoconjugation and dihedral angle tuning. 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

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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 coupling and ring closure via intramolecular C-H activation (Scheme 1, i-iv).68 The triazine acceptors (TRZ) were readily 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 (See Table S3).69 Scheme 1. Syntheses of compounds 2–6. (iii) Pd-DavePhos-G3 2-Cl-1-BrC6H4, NaOtBu 1,4-dioxane, 80 ºC, 3 h

(i) HNO3/HOAc 70 ºC, 6 h (ii) Pd/C, H2 EtOH r.t., 12 h

Br R3

R2

R4

(v) nBuLi - 78 ºC, 1 h then - 78 ºC to r.t. 12 h

R1

R3

R2

R4 N

X = Br or I Cl N Ph

N N

TCZ

Br

X

Ph

Ph

(vi) Pd2(dba)3 XPhos or tBuXPhos NaOtBu, 1,4-dioxane 110 ºC, 16 h

N N

N

R4

N N

H N

(iv) Pd-PCy3-G3 K2CO3, PivOH DMA, 110 ºC, 16 h

TTC-NH2

TTC

R1

NH2

Ph 2 TCZ-TRZ: R1-4 = H; 3 TCZ-TRZ(Me): R1 = Me; 4 TCZ-TRZ(Me'): R2 = Me; 5 TCZ-TRZ(Me2p): R1,4 = Me; 6 TCZ-TRZ(Me2o): R1,2 = Me.

R2

R3

R1 N

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 similar values for f, implying that both should exhibit strong fluorescence. The estimated ΔEST of 2 (0.292 eV) was 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 ∆EST of 3 was further reduced to 0.116 eV. However, when the phenylene ring was 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

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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. Table 1. TDDFT calculation results of compounds 1−6. Cpd. 1 2 3 4 5 6

f 0.315 0.276 0.081 0.243 0.073 0.053

S1/T1 (eV) 3.05/2.70 2.95/2.66 2.88/2.76 2.99/2.72 2.94/2.83 2.99/2.90

ΔEST (eV) 0.346 0.292 0.116 0.271 0.108 0.089

Angle θ1/θ2 (°) 51.5/0.9 51.6/0.8 69.5/0.4 51.5/25.1 69.6/26.9 69.1/34.1

Steady-state absorption experiments were performed for compounds 1−6 (Figure 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 nm to 457 nm for compounds 2−6, compared to 441 nm of 1 (Table 2 Column 2). The ΔEST 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 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 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 for prompt fluorescence (kF), intersystem crossing (kISC), and reverse intersystem crossing (kRISC) were calculated according to the literature method6. For prompt fluorescence, compounds 1−6 showed similar decay lifetime ranging from 7.2 ns to 10.6 ns (Table 2, Column 5). While 1 barely had any delayed emission, compounds 2-6


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Chemistry of Materials Table 2. Photophysical properties of compounds 1−6. 2 -1 kF (107 s-1) Cpd λPL (nm) sola/filmb ΦPLc,d ΔESTe(eV) τpb (ns) τdb (μs) kISC (106 s-1) kRISC (10 s ) 1 416/441 0.78(ND) 0.30 7.2 ND 14 ND ND 2 432/457 0.77(0.40) 0.27 9.0 38 6.2 2.4 6.1 3 431/451 0.60(0.32) 0.16 10.6 51 2.0 10.1 35.3 4 429/444 0.80(0.51) 0.12 9.8 58 5.8 4.5 8.5 5 427/442 0.46(0.30) 0.14 10.5 39 1.2 7.9 36.9 6 427/435 0.47(0.17) 0.18 10.0 37 1.4 7.7 33.2 a In toluene (1 x 10-5 M) at RT. b Co-deposited film (15wt%) in DPEPO. c Absolute PLQY using an integrating sphere under N2 measured in toluene. d PLQY of doped film in parenthesis. e ΔEST estimated from onset. ND = Not Determined.

clearly exhibited delayed emission with lifetimes ranging from 37 μs to 58 μs (Table 2, Column 6). The delayed component was more prominent in compounds with larger θ1 (3, 5 and 6) than 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

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

OLED devices employing compounds 2−6 as emissive dopants were tested in the following device structure (Figure 3a): ITO (160nm)/(HAT-CN (10nm)/NPB (50nm)/mCP (10nm)/DPEPO:dopant (15 wt%) (30nm)/PPF (10nm)/PPF:Liq (30nm)/Liq (1nm)/Al (100nm). We obtained maximum 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 materials and are 2.5 times higher than the 4% EQE for 1,64 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 have alleviated the efficiency roll-off.

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 2 location in the CIE color space at 50 cd/m . (d) Current density and luminance as a function of applied voltage.

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. ASSOCIATED CONTENT Supporting Information. Experimental and calculation details and characterization data including NMR spectra are included in the Supporting Information.

AUTHOR INFORMATION Corresponding Authors



Correspondence should be addressed to [email protected], [email protected], [email protected] and [email protected]. Funding Sources

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 No. DEFG02-07ER46474). Notes A patent application based on this work has been filed.

ACKNOWLEDGMENT 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.

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SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis graphic and/or synopsis paragraph, see the Instructions for Authors on the journal’s homepage for a description of what needs to be provided and for the size requirements of the artwork. Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem, etc., that is discussed in the manuscript. Consult the journal’s Instructions for Authors for TOC graphic specifications.

Insert Table of Contents artwork here Deep Blue Fluorescence Emitter Large f, large ∆EST and high S1

Homoconjugation of Triptycene Scaffold

Deep Blue TADF Emitters

Dihedral Angle Tuning by Methyl Substituion

Moderate f, small ∆EST and high S1

N N

N N Ph

N =A N

Ph

CZ- TRZ 4% EQE

A

TCZ- TRZ & TCZ- TRZ(Me) 10.4 & 11.1% EQE

H Me

A


Molecular Design of Deep Blue Thermally Activated Delayed

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