Control of Dual Conformations: Developing TADF Emitters for Highly

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Control of Dual Conformations: Developing TADF Emitters for Highly Efficient Single-Emitter White Organic-Light Emitting Diodes Kai Wang, Yizhong Shi, Cai-Jun Zheng, Wei Liu, Ke Liang, Xing Li, Ming Zhang, Hui Lin, Silu Tao, Chun-Sing Lee, Xue-Mei Ou, and Xiaohong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08083 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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ACS Applied Materials & Interfaces

Control of Dual Conformations: Developing TADF Emitters for Highly Efficient Single-Emitter White Organic-Light Emitting Diodes Kai Wang, †,‡ Yi-Zhong Shi, ‡ Cai-Jun Zheng, *,† Wei Liu, ‡ Ke Liang, ‡ Xing Li, ‡ Ming Zhang, † Hui Lin, † Si-Lu Tao, † Chun-Sing Lee, *,§ Xue-Mei Ou, ‡ and Xiao-Hong Zhang*,‡ †

School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu, Sichuan 610054, P.R. China ‡

Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P.R. China § Department of Chemistry and Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, P.R. China KEYWORDS: White organic light-emitting diodes, Thermally activated delayed fluorescence, Spontaneous white emission, Conformational control, Dual conformations

ABSTRACT: In this work, we propose a novel concept to develop two fluorophors 2-(10H-phenothiazin-10-yl)thianthrene 5,5,10,10-tetraoxide (PTZ-TTR) and 2-(4-(10H-phenothiazin-10-yl)phenyl)thianthrene 5,5,10,10-tetraoxide (PTZ-Ph-TTR) showing dual conformations for highly efficient single-emitter white organic-light emitting diodes (WOLEDs). Both molecules exist in two stable conformations. Their nearly-orthogonal forms own lower energy levels and show thermally activated delayed fluorescence (TADF) characteristic, while their nearly-planar conformers possess higher energy levels and show only prompt fluorescence. These dual conformers were exploited for fabricating WOLEDs with complementary emission colors contributing by the two conformations. Moreover, the originally wasted triplet energy on the nearly-planar conformation can be transferred to the nearly-orthogonal one, then harvested via the TADF channel, realizing full exciton utilization. A PTZ-TTR-based single-emitter device exhibits standard white emission with a CIE coordinate of (0.33, 0.33) and a high color rendering index value of 92. On the other hand, the PTZ-Ph-TTR-based single-emitter device realizes an emission approaching warm white light and high maximum external quantum efficiency of 16.34%. These results demonstrate an alternative approach for designing high performance WOLEDs based on single TADF emitters.

INTRODUCTION With good potential for solid-state lighting applications, white organic light emitting diodes (WOLEDs) have drawn extensive attention and developed rapidly.1-2 White emission is typically formed with two3-4 (i.e. orange and blue) complementary or three2, 5-8 primary colors and thus generally require more than one emitters. Device designs using multiple emission layers (EMLs) 2, 5, 8-9 or single EML with multiple co-doped emitters3-4, 6-7, 10-11 are usually employed. However, the multi-EML architecture possesses complicated configurations,2, 5, 8 leading to high fabrication costs. The multi-dopant single-EML device architecture can simplify the device structure, 3-4, 6-7 but precise control on doping concentrations reduces device reproducibility. One possible approach for addressing the above issue is to develop single emitters which can give white emission in devices.12 Initial works were based on traditional fluorescence mechanism.13-17 Polymers and star-shaped dendrimers with multiple chromophores were first developed,13-14 however, they suffer from problems of difficult

synthesis and purification. In 2013, Liu et al. then reported a halochromic white-light small molecule realizing external quantum efficiency (EQE) of ~3.0%.15 In 2011, Kim et al. reported a white emission emitter based on excitedstate intramolecular proton transfer (ESIPT) systems exhibiting efficiencies of 3.10 cd A−1 and 2.20 lm W−1.16 Very recently, Zhang et al. developed an ESIPT molecule t-MTTH achieving EQE of 1.70%.17 Although plenty of molecules based on traditional fluorescence mechanism successfully realizing single-emitter white emission, EQEs of these devices are theoretically limited to 5%,18 which restricts their further developments. To address the efficiency issue, metal complexes with phosphorescence have been exploited.19-25 Platinum (II) complexes have been successfully reported for achieving white emission with complementary colors based on their monomers and excimers. In 2007, Kalinowski et al. developed PtL2Cl as a single dopant for realizing WOLEDs using excimer and exciplex emission and achieved a color rendering index (CRI) as high as 90 with a high EQE of ~7 %.25 In the same year, Williams et al. reported a Pt (II) based phosphor FPt for making a singleemitter WOLED with a high EQE of about 18%.22 In 2012, Kui et al. developed phosphorescent Pt(II) complexes con-

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taining tetradentate O^N^C^N ligands and achieved a EQE of 16.5% in WOLEDs, and 9.7% in polymer-based white devices, respectively.23 Nevertheless, the use of noble metal complexes also imply a high cost26-27 and it is thus of interest to exploit the possibility of developing pure organic emitters which can achieve single-emitter white emission with high exciton utilization efficiency. Recently, OLEDs based on thermally activated delayed fluorescence (TADF) can realize full exciton utilization without the assistance of rare metal,4-5, 26, 28-36 providing appropriate candidates to instead expensive phosphorescent emitters. In past five years, plenty of TADF emitters with various structures have been developed.37-43 Among them, some of TADF emitters were reported to have dual stable conformations, which was firstly found with phenothiazine (PTZ)-based TADF emitters.44 Most recently, our group further realized that TADF molecules constructed by all pseudoplanar segments such as 9, 9-dimethyl-9, 10dihydroacridine (DMAC) and phenoxazine (PXZ) would universally possess dual stable conformations and have the probability to exhibit dual emissions during excitations,45 suggesting the great potential in developments of dual-emission TADF emitters. Given on this point, in this work, we propose a novel concept to develop the specific TADF fluorophors with dual conformations as the single emitter for highly efficient WOLEDs.

Figure 1. Energy transfer diagram of WOLEDs based on single TADF emitters with dual conformations. As shown in Figure 1, such a novel fluorophor has a nearly-planar and a nearly-orthogonal conformations.44-45 The nearly-planar conformation exhibits a higher energy fluorescence without TADF characteristic; whereas, the nearly-orthogonal conformation shows a lower energy fluorescence with TADF characteristic. By adjusting the band gaps and relative distributions of the two conformations, the specific fluorophor can realize white emission with two complementary color fluorescence. More importantly, both the lowest singlet excited state (S1) and lowest triplet excited state (T1) of the nearly-planar conformation can be higher than those of the nearlyorthogonal one.44 The previously-wasted triplet excitons on the nearly-planar conformation can transfer to the nearly-orthogonal one, then radiate via the TADF channel, avoiding the exciton energy loss. Thus, it is possible to re-

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alize white emission as well as high exciton utilization by using these specific TADF fluorophors with dual conformations as the emitter in the devices. To characterize the key factors on development of such specific TADF emitters, two compounds 2-(10Hphenothiazin-10-yl) thianthrene 5,5,10,10-tetraoxide (PTZ-TTR) and 2-(4-(10H-phenothiazin-10-yl)phenyl) thianthrene 5,5,10,10-tetraoxide (PTZ-Ph-TTR) were newly developed and investigated together with two earlier reported emitters 2-(9,9-dimethylacridin-10(9H)yl)thianthrene 5,5,10,10-tetraoxide (DMAC-TTR)45 and 2(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)thianthrene 5,5,10,10-tetraoxide (DMAC-Ph-TTR)41. All four emitters can theoretically have dual stable conformations and possess TADF characteristics with their nearly-orthogonal conformations. With the different electron-donor (D) moieties, the bandgaps of four fluorophors can be well adjusted. Especially for nearly-orthogonal conformation, the emission color can be significantly varied from greenishyellow region with DMAC as D segment to orange with PTZ as D segment. Thus, only PTZ-TTR and PTZ-Ph-TTR can realize dual complementary color fluorescence (blue fluorescence from the nearly-planar conformation and orange fluorescence from the nearly-orthogonal conformation). Moreover, by introducing a phenyl ring between electronacceptor (A) and D segments, the interaction between D and A segments can be evidently reduced and result in a significant decline in the population of nearly-planar conformations. The relative distributions of dual conformations can thus be well tuned and lead different exciton distributions. Due to the lack of nearly-orthogonal conformers, PTZ-TTR shows a strong blue emission and a weak orange emission and PTZ-TTR-based WOLED realizes white light with a CIE coordinate of (0.33, 0.33) and a high CRI value of 92 at a brightness of 300 cd m-2. Whereas, with a high population of the near-orthogonal configuration with TADF, PTZ-Ph-TTR presents a much stronger orange emission as well as an inherently higher exciton utilization. As expected, PTZ-Ph-TTR-based WOLED exhibits a stable emission approaching warm white light and a high maximum forward viewing EQE of 16.34%. These results demonstrate an alternative approach for achieving single-emitter white emission with high exciton utilization by using TADF emitters with suitable dual conformations.

RESULTS AND DISCUSSION Molecular Design and Synthesis. The detailed molecular structures of PTZ-TTR, PTZ-Ph-TTR, DMAC-TTR and DMAC-Ph-TTR are shown in Figure 2. The D segments of non-planar group PTZ and pseudoplanar group DMAC lead four molecules to have the potential to possess dual stable conformations.44-45 And thianthrene 5,5,10,10-tetraoxide (TTR) was chosen as the A segment due to its appropriate energy levels.40-41 Particularly, as sulfur atom is much larger than the carbon atom, PTZ segment presents a natural crooked form, while DMAC generally exhibits a planar form without aromaticity.44, 46-47 Meanwhile, PTZ segment has stronger electron-donating ability than DMAC segment.26 Thus, the properties of PTZ-based and DMACbased compounds should be significantly varied, which


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will affect their capacity as the single emitter for highperformance WOLEDs. Moreover, connecting modes of D-A and D-phenyl-A types were also chosen to further optimize and discuss the influence of the interaction between the D and A segments.

Figure 2. The chemicial structures compounds and their relationships.

of

studied

Two DMAC-based molecules were synthesized according to literature. 41, 45 While two PTZ derivatives were synthesized as shown in Scheme 1. The intermediate 2bromothianthrene was firstly cyclized by benzene-1,2dithiol and 1,2,4-tribromobenzene via nucleophilic substitution. Then, the substitute tetraoxide was realized via oxidation by hydrogen peroxide. The target molecules were achieved by Buchwald-Hartwig cross coupling reaction for PTZ-TTR and Suzuki reaction for PTZ-Ph-TTR. The chemical structures of PTZ-TTR and PTZ-Ph-TTR were finally characterized and confirmed via nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Moreover, two compounds were further purified by sublimation before they were used for device fabrication.

H N S III

SH

Br I

S

Br

S

Br II

O O S

Br

S N

S O O PTZ-TTR

Br

+ SH

O O S

S O O VI

S N

H N S

O

Cl Cl

V

IV

O B

+

S O O

Br N S

I: Cs2CO3; IV:Pd(OAc)2, NaOtBu;

O O S

N

PTZ-Ph-TTR

S

II: H2O2/HOAc; V: bis(pinacolato)diboron,Pd(dba)2, KOAc;

formations to two molecules. These tendencies are similar with our previous estimations of DMAC-TTR45 and DMACPh-TTR (seen in Figure S1). Further theoretical calculations on the structural optimization show that two local minima are realized at the dihedral angles of ~10 o and ~80o for both PTZ-TTR and PTZ-Ph-TTR, corresponding to the nearly-planar conformation and nearly-orthogonal conformation, respectively. As the reported DMAC-TTR45, the nearly-planar conformation presents lower molecular energy in dual stable conformations for both the PTZ derivatives, indicating the nearly-planar conformation is more stable for both compounds.48 Specifically, with the addition of a phenyl ring, the interaction between D and A segments can be evidently weakened and results in a more stable nearly-orthogonal conformation. The energy differences between two conformations greatly decrease from 0.157 eV for PTZ-TTR to 0.026 eV for PTZ-Ph-TTR, and from 0.041 eV for DMAC-TTR to -0.063 eV for DMAC-PhTTR. According to Boltzmann distribution,46 the relative ratios between the nearly planar and orthogonal conformations for four compounds are listed in Table S1, which are 99.8% and 0.2% for PTZ-TTR, 73.6% and 26.4% for PTZ-Ph-TTR, 83.1% and 16.9% for DMAC-TTR, 7.9% and 92.1% for DMAC-Ph-TTR, respectively. For DMAC derivatives, the additive phenyl ring causes a minima reverse between these two conformers. DMAC-Ph-TTR molecule is the most stabilized in the nearly-orthogonal conformer, which is totally different from other three compounds. And the extremely small proportion of 7.9% will lead the influence of the nearly-planar conformation hard to be observed for DMAC-Ph-TTR, which is consistent with the previous report.41 From another perspective, we can also find that the energy differences between conformations would be adjusted in a wide range by changing the D segments from PTZ to DMAC (from 0.157 eV for PTZ-TTR to 0.041 eV for DMAC-TTR and from 0.026 eV for PTZ-PhTTR to -0.063 eV for DMAC-Ph-TTR). The relative proportions of the nearly planar conformers in PTZ-based molecules are evidently higher than that in DMAC-based molecules, which should be caused by the natural crooked form of PTZ segment has much more tendency to stabilize nearly-planar conformation comparing with more rigid pseudoplanar DMAC segment. Therefore, with these simple adjustments of changing the connecting modes and constituent groups, we can easily control the differences between conformational energies and adjust the conformational populations, which would further affect the energy transfer process and result in different emission spectra.

III: Pd(OAc)2, NaOtBu; VI: Pd(PPh3)4, Na2CO3.

Scheme 1. Synthesis routes of PTZ-TTR and PTZ-Ph-TTR

Theoretical calculations. Density functional theoretical (DFT) calculations were first performed on the energy surfaces of these molecules at ground states. As shown in Figure 3, both the PTZ derivatives exhibit approximate periodic change of 180o with two valleys at near 0o and 90o for twisting dihedral angle between PTZ and TTR (or phenylTTR) segments, predicting the existence of dual stable con-

As the newly developed compounds, we further calculated the distributions of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of two conformations for PTZ derivatives. As shown in Figure 3, in the nearly-planar conformations, HOMOs and LUMOs are greatly extended from their original domains to their counterparts leading to more HOMO-LUMO overlap and thus a larger singlettriplet energy difference (ΔEST). Correspondingly, the electron-donating abilities of PTZ and electron-withdrawing abilities of TTR segments are evidently weakened, resulting in significant deeper HOMO energy levels of -5.94 eV for PTZ-TTR and -5.52 eV for PTZ-Ph-TTR and shallower



LUMO energy levels of -1.92 eV for PTZ-TTR and -2.08 eV for PTZ-Ph-TTR than their origins in nearly-planar conformations. While in the nearly-orthogonal conformations, the HOMOs are mainly located on the PTZ segment with energy levels of -5.32 eV for PTZ-TTR and -5.12 eV for PTZPh-TTR, respectively, and the LUMOs are located mainly on TTR (or phenyl-TTR) segments with energy levels of -2.41 eV for PTZ-TTR and -2.39 eV for PTZ-Ph-TTR, respectively. The two frontier orbitals are separated sufficiently and are expected to have small ΔESTs.38 These results are very similar with the estimations of DMAC-TTR and DMAC-Ph-TTR in their corresponding conformations (seen in Figure S2). To confirm the prediction on ΔESTs, we further estimated the energy gaps of the two conformations for both emitters from the theoretical calculations. As listed in Table S1, the nearly-planar conformations are estimated to have S1 (T1) energy levels of 3.546 eV (2.965 eV) for PTZ-TTR and 3.080 eV (2.623 eV) for PTZ-Ph-TTR. Thus, the ΔESTs of the nearly-planar conformations are as large as 0.581 eV for PTZ-TTR and 0.457 eV for PTZ-Ph-TTR, matching our prediction and suggesting non-TADF characteristics in nearlyplanar molecules. In contrast, the S1 and T1 energy levels of the nearly-orthogonal conformations are estimated as 2.336 and 2.317 eV for PTZ-TTR and 2.369 and 2.364 eV for PTZ-Ph-TTR. Thus, the nearly-orthogonal conformations of PTZ-TTR and PTZ-Ph-TTR theoretically have extremely small ΔESTs of 0.019 and 0.005 eV, respectively, suggesting possible TADF characteristics.48 Moreover, both the S1 and T1 energy levels of the nearly-planar conformations are higher than that of the nearly-orthogonal conformations for all the studied molecules. Thus, the nonradiative triplet excitons on the nearly-planar conformers can transfer onto the nearly-orthogonal conformers, then radiate via the TADF channel, theoretically leading full exciton utilization.

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tion spectra in both dilute and high concentration toluene solution at were measured at room temperature. Besides the local excited (LE) absorption bands in the range below 330 nm for PTZ-TTR and 350 nm for PTZ-Ph-TTR, their absorption spectra are similar to DMAC-TTR reported with dual conformations and significantly differ from DMAC-PhTTR with only one weak intramolecular charge transfer (ICT) absorption observed in the range from 350 to 450 nm (seen in Figure S3b), suggesting both PTZ derivatives are characterized compounds with dual conformations.45 Specifically, the ones which could be obviously observed ranging from 330 to 400 nm for PTZ-TTR and from 330 to 420 nm for PTZ-Ph-TTR should be ascribed to the nearlyplanar conformations. Meanwhile, very weak ICT absorptions indistinctly observed in dilute solution can be clearly confirmed from the long-wavelength area until about 500 nm for PTZ-Ph-TTR and 530 nm for PTZ-TTR under high concentration (seen in Figure S3a), which should be attributed to the nearly-orthogonal conformations with the extremely low relative distributions. In dilute toluene solution at room temperature, most of intermolecular energy transfer is suppressed as well as the possible formation of excimer/exciplex and other aggregations. PTZ-TTR exhibits a strong emission peaked at 435 nm and an observable weak emission peaked around 635 nm; whereas, PTZ-PhTTR presents a strong emission peaked at 450 nm and an emission peaked at 600 nm with half the former intensity, which are consistent with the different relative distributions of dual conformations predicted from the theoretical calculations for two emitters. As a comparison, we measured the photoluminescence (PL) spectra of their microcrystals obtained with slow-growing conditions at room temperature, in which the specific unitary conformers may dominate. As shown in Figure S4c, both the emitters exhibit single emission peaks which can be distinguished from their corresponding spectra in solution. In particular, PTZTTR microcrystal exhibits a single emission peaked at 440 nm, which is consistent with the former emission in toluene and further confirmed to consist of the nearly-planar conformers via single-crystal X-ray measurement (seen in Figure S5). While PTZ-Ph-TTR microcrystal emits a single emission peaked at 587 nm, which is nearly identical with the latter emission area in a dilute mixed solution of 75 vol% toluene and 25 vol% hexane and should be attributed to the nearly orthogonal conformation. These results further

confirm the dual emission peaks from the corresponding dual conformations.

Figure 3. Theoretical calculations of the energy surfaces, HOMO (up) and LUMO (below) distributions of the nearly planar (blue boxes) and orthogonal (yellow boxes) conformations for (a) PTZ-TTR and (b) PTZ-Ph-TTR. Photophysical Properties. Photophysical properties of the studied compounds were performed to compare and further investigate the characteristics of their dual conformations. As shown in Figure 4a and S3a, their absorp-

Figure 4b shows the emission spectra of four studied compounds 5 wt% doped in 4,4’-bis(carbazol-9-yl) biphenyl (CBP) films. By exciting the host matrix at 330 nm at room temperature, most of excitons are initially generated from the host matrix and then transfer into the dopants. Meanwhile, exciton transfers between guest molecules in different conformations are expected to unavoidably happen.45 Thus, the emission intensities from highlying nearly-planar conformations would be evidently suppressed while the intensities of low-lying nearlyorthogonal ones could be significantly improved comparing the corresponding emission spectra in dilute toluene. DMAC-TTR, PTZ-Ph-TTR and PTZ-TTR respectively present emissions from nearly planar conformation peaked at 430 nm, 465 nm and 445 nm and emissions from nearly


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orthogonal conformation peaked at 535 nm, 548 nm and 570 nm; while DMAC-Ph-TTR exhibits a single emission peak at 500 nm, which could be attributed to nearly orthogonal conformation. From DMAC-TTR to PTZ-Ph-TTR and PTZ-TTR, the intensities from nearly planar conformations are increased in turn and even results in a reverse for PTZ-TTR, which should be ascribed to the differences of relative conformational distributions as well. In particular, in the nearly orthogonal conformations, the electrondonating ability of PTZ is significantly stronger than DMAC, and similar with the electron-donating segment PXZ. Thus, PTZ-TTR (or PTZ-Ph-TTR) shows significant bathochromic shift of 35 nm (or 48 nm) comparing with DMAC-TTR (or DMAC-Ph-TTR), and similar with the corresponding spectra of reported emitter PXZ-Ph-TTR (PXZDSO2) with the same components except replacing PTZ as PXZ41, indicating they were indeed from the nearly orthogonal confor-

mations. While in nearly planar conformation, comparing DMAC-TTR, PTZ-TTR only exhibit slight bathochromicshift of 9 nm. This is because the electron-donating ability of sulfur atom is evidently weakened due to its weak conjugation with phenyl rings for PTZ and only slightly higher than DMAC. This kind of differences in dual conformers would further benefit to differ the emission areas from dual conformations and help to realize complementary emissions in one single compound. In our studied four compounds, DMAC-based emitters exhibit greenish-yellow emission from their nearly-orthogonal conformations, thus only PTZ-TTR and PTZ-Ph-TTR can realize dual complementary color fluorescence (blue fluorescence from the nearly-planar conformation and orange fluorescence from the nearly-orthogonal conformation) and further investigations are mainly focused on these two PTZ-based emitters.

(a)

Absorption(a.u.)

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

Normalized Intensity (a.u.)

(b) 1.4 PTZ-Ph-TTR PTZ-TTR

1.0

Emission(a.u.)

DMAC-TTR PTZ-TTR

1.2

DMAC-Ph-TTR PTZ-Ph-TTR

1.0 0.8 0.6 0.4 0.2 0.0

300

400

500

400

600

450

500

550

600

Wavelength (nm)

Wavelength(nm)

(c)

(d) 2.0

2.0

Normalized 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


Flou. Phos.

1.5

1.0

0.5

Flou. Phos. 1.5

1.0

0.5

0.0

0.0 400

500

600

Wavelength(nm)

700

400

500

600

700

Wavelength(nm)

Figure 4. (a)Absorption and emission spectra (excited at 360 nm) of PTZ-TTR and PTZ-Ph-TTR measured in dilute toluene at room temperature; (b) emission spectra of four studied compounds respectively 5 wt% doped in CBP films by exciting host matrix at 330 nm at room temperature; fluorescence (black) and phosphorescence (red) spectra of (c) PTZ-TTR and (d) PTZ-Ph-TTR measured in dilute 2-MeTHF excited at 350 nm (upper) and measured 10 wt% doped in CBP excited at 450nm (bottom) at 77 K. For (c): dashed line is the extension line.



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Table 1. Summary of physical properties of PTZ-TTR and PTZ-Ph-TTR. Molecule PTZ-TTR PTZ-Ph-TTR DMAC-TTR DMAC-Ph-TTR

λfluo. a)

S1b)

T1c)

ΔESTd)

[nm]

[eV]

[eV]

[eV]

planar

428

3.14

2.76

0.38

orthogonal

551

2.64

2.54

0.10

Conformation

planar

441

3.20

2.63

0.57

orthogonal

552

2.38

2.37

0.01

planar

416

3.17

2.72

0.45

orthogonal

558

2.463

2.457

0.006

orthogonal

504

2.70

2.65

0.05

PLQY e) 11.04% 50.83% -

a)

Determined from the first energy peak of the fluorescence spectra at 77 K; b) Determined from starting position of the fluorescence spectra at 77 K; c) Determined from starting position of the phosphorescence spectra at 77K; d) ΔEST=S1-T1; e) Measured 10 % mixed in CBP in atmosphere.

To further suppress the intermolecular energy transfer and precisely study the nearly planar conformations, we excited the former strong ICT absorptions of two PTZ derivatives from nearly-planar conformers with 350 nm light source at 77 K in dilute 2-methyltetrahydrofuran (2MeTHF). The extremely low concentration and temperature will hinder the energy transfer in the system. As shown in Figure 4c and 4d, the fluorescence and phosphorescence from nearly planar conformations of two emitters are clearly observed. As expected, the S1 energy levels are only slightly narrower than DMAC-TTR. In particular, the S1 and T1 energy levels of nearly planar conformations are determined to be 3.14 and 2.76 eV for PTZ-TTR and 3.20 and 2.63 eV for PTZ-Ph-TTR, respectively. Large ΔESTs of 0.38 and 0.57 eV respectively for PTZ-TTR and PTZ-PhTTR indicate two nearly planar conformers are hard to realize TADF characteristics. Meanwhile, the position of the low-temperature phosphorescence could exclude the possibility of room-temperature phosphorescence for twopeak emissions. Due to the extremely weak latter ICT absorptions mainly attributed to the nearly-orthogonal conformers, it is hard to get distinguishable signals under the same condition. Thus, to precisely characterize the properties of nearly orthogonal conformations, we measured two emitters doped in CBP films with a high concentration of 10 wt% excited at 450 nm at 77 K. As the nearly-planar conformers have the ICT absorptions below 420 nm, the low-energy excitation at 450 nm would be mainly absorbed by the nearly-orthogonal conformers. Moreover, with a high doping concentration, a handful of excitons on the nearly-planar conformers can also transfer to the nearly-orthogonal ones and their influence could be negligible. Their S1 energy levels are respectively estimated to be 2.64 eV for PTZ-TTR and 2.38 eV for PTZ-Ph-TTR, significantly lower than their corresponding DMAC derivatives, indicating the huge influence of D segments on adjusting the energy level in nearly orthogonal conformers. Moreover, the ΔESTs are determined to be 0.10 and 0.01 eV for PTZ-TTR

and PTZ-Ph-TTR, respectively, significantly smaller than the nearly-planar conformations. Such small ΔESTs can cause effective reverse intersystem crossing (RISC) process lead to TADF. Transient PL decay in thin films of PTZTTR and PTZ-Ph-TTR in CBP were further measured to confirm their TADF characteristics (Figure S6). The deep blue emission around 427 nm for PTZ-TTR and 442 nm for PTZ-Ph-TTR show only prompt decays of 5.17 and 7.08 ns, respectively, indicating their non-TADF characteristic. Meanwhile, the long wavelength emission exhibits not only prompt decays of 8.30 and 6.77 ns, but also delayed decays of 12.29 and 4.11 μs, respectively at room temperature. Moreover, with the temperature increased from 200 to 300 K, delayed lifetime of both PTZ-TTR and PTZ-Ph-TTR doped film show significant decreases, indicating their TADF characteristic. We then measured the photoluminescence quantum yield (PLQY) of 10 wt% PTZ-TTR and PTZPh-TTR doped CBP films in atmosphere. As summarized in Table 1, the PLQY of PTZ-Ph-TTR has a high value of 50.83%, whereas PTZ-TTR only shows a low PLQY of 11.04%. Such difference on PLQYs first should be attributed to the enhancement of dipole moment owing to the additional phenyl bridge,49 on the second is caused by the higher triplet exciton utilization on higher ratio of the nearly-orthogonal conformations for PTZ-Ph-TTR. Electroluminescence Properties. To study the exciton transfer and utilization process during electrical excitation, we firstly use PTZ-TTR and PTZ-Ph-TTR individually as emitters with different doping ratios in devices with a structure ITO/ TAPC (35 nm)/ TCTA (10 nm)/ CBP: x wt% PTZ-TTR (or PTZ-Ph-TTR) (20 nm)/TmPyPb (45 nm)/ LiF (1nm)/Al, where ITO was indium tin oxide as anode; TAPC was 1,1-bis [4-[N,N-di(p-tolyl) amino]phenyl]cyclohexane as hole-transporting layer; TCTA was 4,4’,4’’-tris (carbazol9-yi) triphenylamine as blocking layer; TmPyPb was 1,3,5tri[(3-pyridyl)-phen-3-yl]benzene as electron-transporting layer and LiF/Al was used as cathode.


Page 7 of 13

Table 2. Summary of EL performance of the WOLEDs in this work.

Emitter

λem

[%/lm W-1/ cd A-1]

[V]

Max.

@102 cd m-2

@103 cd m-2

[nm]

CIE

CRI

PTZ-TTR

3.2

2.68/4.93/5.02

2.07/2.89/3.86

0.93/0.82/1.75

448, 584

(0.33, 0.33)a)

92a)

PTZ-Ph-TTR

3.45

16.34/41.75/45.21

13.82/29.29/38.25

11.04/19.58/30.55

456, 568

(0.41, 0.47)b)

64b)

Determined at the brightness of 300 cd m-2; b) Determined at the brightness of 104 cd m-2. (b)

(a)104 Power Efficiency (lm W-1)

103

10

100

2

10-1

101

10-2

100

PTZ-TTR PTZ-Ph-TTR 10-1

10-3 1

10

100

1000

300

104 250 3

10

200

2

10

150

101

100 50

100

PTZ-TTR PTZ-Ph-TTR 0

10-1 3

10000

4

5

Luminance (cd m-2)

7

8

9

(d)

100 cd m2 1000 cd m2

1.4

1.0

0.8

1.2

0.6

1.0

0.4

0.2

0.8 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

0.6 0.4 0.2

0.8 0.8

0.6

0.6

400

500

600

700

0.4

0.2

0.4

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

4V

0.2

6V

0.0

0.0

100 cd m2 1000 cd m2 10000 cd m2

PTZ-Ph-TTR

y

PTZ-TTR


1.6

6

Voltage(V)

1.8

y


(c)

Current Density (mA cm-2)

10

1

Luminance ( cd m-2)

a)

EQE/PE/CE

Von

Exernal Quantum Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

500

5V 7V 600

700

Wavelength (nm)

Wavelength (nm)

Figure 5. (a) PE-luminance-EQE curves, (b) luminance-voltage-current density curves and normalized EL spectra of (c) PTZ-TTR (insert: CIE variations from 100 to 1000 cd m-2) and (d) PTZ-Ph-TTR (insert: CIE variations from 100 to 10000 cd m-2 and observed emissions under different driving voltages) based WOLEDs. The variations of electroluminescence (EL) spectra are shown in Figure S7 and summarized in Table S2. For all these devices, the relative intensities of orange emission in EL spectra are similar with the corresponding spectra of reported PXZ-Ph-TTR 41 and universally higher than that in their corresponding PL ones, which could be ascribed to the utilizations of triplet excitons in the nearly-orthogonal conformations. At the same brightness, with doping ratios increased, the intensities of blue emission from the nearlyplanar conformation gradually decrease; whereas, the orange emission from nearly-orthogonal conformation correspondingly increases. For PTZ-TTR-based devices at a luminance of 300 cd m-2, with the doping ratio increasing from 3 to 50 wt%, the CIE coordinate varies from (0.25, 0.21) with strong blue emission to (0.49, 0.44) with only residual blue emission. As the limiting condition, the non-

doped PTZ-TTR-based device even exhibits a stable CIE coordinate of (0.52, 0.47) without any blue emission peak. However, due to the extremely low relative distributions of the nearly-orthogonal conformers, the doping ratios of TADF molecules stayed at extremely low values even at non-doped condition (assuming the distributions are the same with the calculated results in vacuum condition), which are far from enough for an effective RISC process. Thus, the maximum EQE of the devices are less than 3%, staying in the range of traditional fluorescence devices, suggesting the nearly-planar conformers with non-TADF characteristic dominate the whole exciton utilizations. Moreover, with the doping ratio further increased to higher than 20 wt%, significant EQE declines were observed, which should be ascribed to the influence of high exciton concentration quenching in high-lying nearly-planar con-



formers. While for PTZ-Ph-TTR, with a higher population of the nearly-orthogonal conformation, blue emission is always significantly lower than orange, even at an extremely lower doping ratio of 1.2 wt%. With the doping ratio increases, the intensity of blue emission totally disappears at a doping ratio of 10 wt%. Correspondingly, the EQEs of devices remained similar of ~16% at 1.2 and 5 wt% and showed slight decline at a doping ratio of 10 wt% due to the exciton concentration quenching effect. These results could further confirm the exciton transfer process from the nearly-planar conformation with high energy levels to the nearly-orthogonal conformer with lower energy levels, and the energy transfer is enhanced with increased doping ratio. With high populations of TADF molecules, effective RISC process would occur and resulting in highly efficient devices. We further try to decrease the doping ratio to 1 wt% to suppress the singlet exciton transfer between nearly-planar and nearly-orthogonal conformers, the emission from CBP host can be clearly observed and the device efficiency is significantly reduced because the exciton transfer between host and guest would become significantly incomplete. The doping ratios for WOLEDs are finally optimized as 14.7 wt% for PTZ-TTR and 1.2 wt% for PTZ-Ph-TTR. Table 2 summarizes the performance of the two devices. Both devices exhibit dual complementary emissions as shown in Figure 5c and 5d. The PTZ-TTR device exhibits an ideal standard white light with dual emission peaks of 448 and 584 nm, CIE coordinate (0.33, 0.33) and high CRI value of 92 at 300 cd m-2. However, due to the high concentration of nearly planar conformations and the lack of nearlyorthogonal conformers, strong singlet exciton transfer from nearly-planar conformers to nearly orthogonal ones would unavoidably happen and become an important process during the whole electron-excitation. The intensity of blue emission is significantly increased with a large CIE coordinate variation of (0.07,0.07) in range from 100 to 1000 cd m-2, because the nearly orthogonal conformers are gradually saturated. While for the PTZ-Ph-TTR device, due to the high triplet excitons utilizations together with the stronger energy transfer from the nearly planar to the nearly orthogonal conformers, blue emission of 456 nm is realized with an intensity of only about 20% to orange emission of 568 nm in the EL spectrum. An emission approaching warm white light is achieved with a CIE coordinate of (0.41, 0.47) and CRI of 64 at the brightness of 10,000 cd m-2. Moreover, since singlet exciton has a much shorter diffusion length than that of triplet excitons,8 the singlet exciton transfer from nearly-planar conformers to nearly-orthogonal ones can be well suppressed while the triplet exciton transfer would still effective happen between the two conformers with such a low total doping ratio. Singlet and triplet excitons can be well assigned to nearly-planar conformations and nearly-orthogonal ones. Thus, the CIE coordinate variation is only (0.01,0.02) in range from 100 to 10000 cd m-2, significant lower than PTZ-TTR based one. Figure 5a and b illustrates the power efficiency (PE)EQE-luminance and luminance-voltage-current density curves of the two devices. The device based on PTZ-Ph-

Page 8 of 13

TTR exhibited a low turn-on voltage of 3.45 V with a maximum EQE of 16.34%, a maximum PE of 41.75 lm W-1 and a maximum current efficiency (CE) of 45.21 cd A-1, respectively. Moreover, at the practical brightness of 100 and 1000 cd m-2, the EQEs remained high value of 13.82% and 11.04%, respectively. These results are even comparable to those phosphorescence-based single-emitter WOLEDs,19-25 indicating the excellent performance of PTZPh-TTR. Such a high exciton utilization can be explained by excellent exciton distributions and effective RISC process in nearly orthogonal conformations. Indeed, triplet excitons have a long diffusion length of about 100 nm8 so that those captured by the nearly-planar conformers can effectively transfer into the low-lying nearly-orthogonal conformers with TADF characteristic. In contrast, due to the lack of nearly orthogonal conformations, much inferior performance was realized in the PTZ-TTR-based device with the maximum EQE of 2.68%, the maximum PE of 4.93 lm W-1 and the maximum CE of 5.02 cd A-1, respectively. By designing single-emitter systems with dual conformational compounds, which have high PLQYs, efficient TADF characteristics and suitable conformational distributions, the qualities and efficiencies of white emission can be expected to be further increased.

CONCLUSION In summary, based on the TADF mechanism, we propose a novel concept for developing fluorophors with dual conformations for highly efficient single-emitter WOLEDs. In these fluorophors, the nearly-orthogonal conformers own lower energy levels and TADF characteristic, while the nearly-planar conformers possess higher energy levels and non-TADF characteristic. Thus, they can realize white emission with two complementary color fluorescence by adjusting the band gaps and relative distributions of the two conformations. Moreover, by comparing four emitters PTZ-TTR, PTZ-Ph-TTR, DMAC-TTR and DMAC-Ph-TTR which can theoretically exhibit dual stable conformations and TADF characteristics in their nearly-orthogonal conformations, the key factors to adjust the emission colors and intensities are founded and can be effectively controlled. Their bandgaps and relative distributions can be well adjusted by changing the electron-donor (D) moieties with different flexibilities. On the other hand, the relative distributions of dual conformations can be also well tuned by controlling the interaction between D and A segments. As a result, two novel TADF emitters PTZ-TTR and PTZ-PhTTR are expected to show spontaneous white light during EL excitation. With careful optimizations, WOLEDs based on PTZ-TTR exhibits standard white emission with a CIE coordinate of (0.33, 0.33) and a high CRI value of 92. A PTZ-Ph-TTR-based device realizes an emission approaching warm white light and high maximum forward-viewing external quantum efficiency of 16.34%. Moreover, due to the ideal exciton distributions and utilizations, only a slight CIE coordinate variation of (0.01,0.02) is observed in range from 100 to 10000 cd m-2. These results demonstrate a promising new approach for developing low cost and highperformance single-emitter WOLEDs.

EXPERIMENTAL SECTION


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Synthesis. All the reagents and chemicals in this study were purchased from commercially sources and used without further purification. Synthesis of 2-bromothianthrene 5,5,10,10-tetraoxide: A solution in 50 mL of sieve-dried N,N-Dimethylformamide (DMF) of 1.420 g (10 mmol) of 1,2-benzenedithiol and 3.11 g (10 mmol) of 1,2,4-tribromobenzene was stirred for 30 min with strict exclusion of air with 6.670 g (20.5 mmol) of Cs2CO3. The reaction was then heated at 150 °C for 3 h. After removal of most of the DMF by vacuum distillation, the residue was partitioned between water and CH2Cl2. Concentration of the nonaqueous phase was followed by crude chromatography in petroleum ether. The residue of removal of solvent from the main fraction was white solid (2.85 g, yield: 96.6 %). MS (EI) m/z: [M]+: calcd for C12H7BrS2 293.9; found, 293.9. Then it was oxidized by dissolution in 12 mL of acetic acid, addition of 6 mL of 30% H2O2, and warming slowly first for 1.5 h at 45 °C and then for 10 h at 75 °C until a large amount of insoluble matter was precipitated from the solvent. The cooled reaction was diluted with 800 mL of water, and the resulting solid was removed by filtration, washed with water, and dried, followed by crude chromatography in CH2Cl2.yielding 3.25 g of white solid (94%).1H NMR (400 MHz, Acetone-d6) δ 8.39 (s, 1H), 8.32 (dd, J = 5.6, 3.4 Hz, 2H), 8.24 (s, 2H), 8.08 (dd, J = 5.8, 3.3 Hz, 2H). MS (EI) m/z: [M]+: calcd for C12H7BrO4S2 357.89; found,359.89. Synthesis of 2-(10H-phenothiazin-10-yl)thianthrene 5,5,10,10-tetraoxide(PTZ-TTR): Toluene (20 ml) and tri-tertbutyl phosphine solution 10% in pentane (0.3 mL, 0.13 mmol) were added to a mixture of 2-bromothianthrene 5,5,10,10tetraoxide. (467 mg, 1.3 mmol), 10H-phenothiazine (325 mg,1.625 mmol), palladium acetate (15 mg, 0.065 mmol), and sodium tert-butoxide (156mg, 1.625mmol). With stirring, the suspension was heated at 90°C for 24 h under nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel using dichloromethane as the eluent to give a bright white and orange powder, with an 85.1% yield (540 mg). 1H NMR (400 MHz, DMSO-d6) δ 8.31 – 8.18 (m, 2H), 8.13 – 8.06 (m, 1H), 8.04 – 7.95 (m, 2H), 7.73 (ddd, J = 17.8, 7.9, 1.1 Hz, 4H), 7.62 – 7.51 (m, 3H), 7.44 (td, J = 7.7, 1.2 Hz, 2H), 7.31 (dd, J = 8.9, 2.6 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ 150.41, 140.58, 140.46, 140.42, 139.39, 135.42, 133.51, 133.02, 129.52, 128.62, 127.90, 127.88, 127.49, 127.37, 125.75, 125.33, 115.70, 109.92. MS (EI) m/z: [M]+: calcd for C24H15NO4S3 477.02; found, 477.02. Synthesis of 10-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)-10H-phenothiazine: Toluene (20 ml) and tri-tertbutyl phosphine solution 10% in pentane (2.4 mL, 1 mmol) were added to a mixture of 1-bromo-4-chlorobenzene (1.91 g, 10 mmol), 10H-phenothiazine (1 g, 5 mmol), palladium acetate (56 mg, 0.25 mmol), and sodium tert-butoxide (960mg, 10mmol). With stirring, the suspension was heated at 90°C for 24 h under nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel using dichloromethane as the eluent to give a white powder of 10-(4-chlorophenyl)-10H-phenothiazine, with a 58.2 % yield (900 mg). MS (EI) m/z: [M]+: calcd for C18H12ClNS 309.03; found, 309.03. Then the intermediate chloride (780 mg, 2.5 mmol) was added to a two-necked flask together with a

mixture of bis(pinacolato)diboron (660 mg, 2.6 mmol) bis(dibenzylideneacetone)dipalladium(0) (Pd(dba)2)(80 mg, 0.125 mol), potassium acetate (275 mg, 2.75 mmol) and 2dicyclohexylphosphino-2',6'-dimethoxybiphenyl (SPhos) (205 mg, 0.5 mol). With stirring, o-xylene (20 mL) was added and the suspension was heated at 150 °C for 24 h under nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel using dichloromethane as the eluent to give a white powder of 10-(4-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-10Hphenothiazine, with an 90 % yield (900 mg). 1H NMR (600 MHz, DMSO-d6) δ 7.90 (d, J = 8.2 Hz, 2H), 7.37 (d, J = 8.2 Hz, 2H), 7.24 – 7.17 (m, 2H), 7.04 (dd, J = 11.2, 4.3 Hz, 2H), 6.97 (dd, J = 17.2, 9.7 Hz, 2H), 6.42 (t, J = 11.6 Hz, 2H), 1.35 (s, 12H). MS (EI) m/z: [M]+: calcd for C24H24BNO2S 401.16; found, 401.16. Synthesis of 2-(4-(10H-phenothiazin-10yl)phenyl)thianthrene 5,5,10,10-tetraoxide (PTZ-Ph-TTR): Toluene (10 ml), water (5 mL) and ethanol (5 mL) were added to a mixture of 2-bromothianthrene 5,5,10,10-tetraoxide (467 mg, 1.3 mmol), 10-(4-(4,4,5,5-tetramethyl- 1,3,2-dioxaborolan -2-yl)phenyl) -10H-phenothiazine (1 g, 5 mmol), Tetrakis (triphenylphosphine) palladium(0) (75 mg, 0.065 mmol), and Na2CO3 (1.06 g, 10mmol). With stirring, the suspension was heated at 90°C for 24 h under nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel using dichloromethane as the eluent to give an orange powder of an 85 % yield (610 mg). 1H NMR (400 MHz, DMSO-d6) δ 8.51 (d, J = 1.5 Hz, 1H), 8.38 (dt, J = 9.4, 6.5 Hz, 4H), 8.15 – 7.97 (m, 4H), 7.50 (d, J = 8.5 Hz, 2H), 7.23 (dd, J = 7.6, 1.4 Hz, 2H), 7.13 – 7.04 (m, 2H), 7.00 (t, J = 7.4 Hz, 2H), 6.57 (d, J = 8.0 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 146.17, 140.10, 139.73, 139.52, 137.46, 134.67, 133.74, 133.63, 131.20, 129.32, 127.41, 127.02, 126.80, 126.01, 125.91, 124.82, 124.11, 123.92, 119.37. MS (EI) m/z: [M]+: calcd for C30H19NO4S3 553.04; found, 553.04.

ASSOCIATED CONTENT Supporting Information. Experimental details including general Information as well as device fabrication and characterization. Supplementary Figures and Table including theoretical calculations, absorption, fluorescence, phosphorescence and transient decay spectra, device optimizations. This material is available free of charge via the Internet at http://pubs.acs.org. The X-ray data were also deposited with the CCDC (entry 1862622).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (C.-J.Z.) E-mail: [email protected] (X.-H.Z.) E-mail: [email protected] (C.-S.L.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT



This work was supported by the National Natural Science Foundation of China (Grant No.51533005, 51773029, 51373190), the National Key Research & Development Program of China (Grant No. 2016YFB0401002), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project and Qing Lan Project, P.R. China.

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Control of Dual Conformations: Developing TADF Emitters for Highly

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