De Novo Design of Excited-State Intramolecular Proton Transfer

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De Novo Design of Excited-State Intramolecular Proton Transfer Emitters via a Thermally Activated Delayed Fluorescence Channel Kailong Wu, Tao Zhang, Zian Wang, Lian Wang, Lisi Zhan, Shaolong Gong, Cheng Zhong, Zheng-Hong Lu, Song Zhang, and Chuluo Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04795 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Journal of the American Chemical Society

De Novo Design of Excited-State Intramolecular Proton Transfer Emitters via a Thermally Activated Delayed Fluorescence Channel Kailong Wu,†,# Tao Zhang,‡ Zian Wang,† Lian Wang,& Lisi Zhan,† Shaolong Gong,*,† Cheng Zhong,† Zheng-Hong Lu,*,‡ Song Zhang,*,& Chuluo Yang*,†,# †

Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, People’s Republic of China. # Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, People’s Republic of China. ‡ Department of Physics, Yunnan Key Laboratory for Micro/Nano Materials and Technology, Yunnan University, Kunming 650091, People’s Republic of China. & State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China. ABSTRACT: Developing excited-state intramolecular proton transfer (ESIPT) emitters with high photoluminescence quantum yields (ΦPLs) and long fluorescence lifetimes in solid state remains a formidable challenge. In this study, we integrated the molecular design tactics of thermally activated delayed fluorescence (TADF) into ESIPT molecules with the goals of improving their ΦPLs and increasing their fluorescence lifetimes. Two proof-of-concept molecules, PXZPDO and DMACPDO, were developed by adopting symmetric D-π-A-π-D molecular architectures (where D and A represent donors and acceptors, respectively) featuring electron-donating phenoxazine or a 9,9-dimethyl-9,10-dihydroacridine moiety, an ESIPT core β-diketone and phenylene π-bridges. Both molecules exhibited sole enol-type forms stabilized by intramolecular hydrogen bonds and exhibited a unique and dynamic ESIPT character that was verified by transient absorption analyses. Endowed with distinct TADF features, PXZPDO and DMACPDO showed high ΦPLs of 68% and 86% in the film state, coupled with notable delayed fluorescence lifetimes of 1.33 and 1.94 µs, respectively. Employing these ESIPT emitters successfully achieved maximum external quantum efficiencies (ηexts) of 18.8% and 23.9% for yellow and green organic light-emitting diodes (OLEDs), respectively, which represent the state-of-the-art device performances for ESIPT emitters. This study not only opens a new avenue for designing efficient ESIPT emitters with high ΦPLs and long fluorescence lifetimes in solid state, but also unlocks the huge potential of ESIPT emitters in realizing highefficiency OLEDs.

1. INTRODUCTION Excited-state intramolecular proton transfer (ESIPT) is highly attractive because it is often associated with a large Stokes shift, dual emission, and spectral sensitivity to the surrounding environment.1 Endowed with these properties, ESIPT emitters have been the focus of much attention during the past few years for their wide application in various fields, such as fluorescence imaging, molecular logic gates, and UVabsorbers.2,3 Despite these successes, the practical employment of ESIPT emitters in optoelectronic devices, especially in organic light-emitting diodes (OLEDs),4 remains a formidable challenge because of various issues such as low photoluminescence quantum yields (ΦPLs), short fluorescence lifetimes, and unclear emissive mechanisms in solid state.4 The utility of ESIPT emitters in OLEDs normally leads to inferior external quantum efficiencies (ηexts) of below 1%.3 So far, there have only been a few reports of relatively efficient ESIPT emitters. For example, You et al. developed a series of 2-(2’hydroxyphenyl)oxazoles-based ESIPT molecules exhibiting deep-blue emission and moderate ΦPLs of ~40%, and achieved ηexts of up to 7.1%;5 Adachi and co-workers found that an old molecule, triquinolonobenzene, exhibited ESIPT and thermal-

ly activated delayed fluorescence (TADF) with a relatively high ΦPL of about 60% in film state, and successfully achieved an ηext of 14%.6 Despite these advances for good ESIPT emitters, the molecular design for efficient ESIPT emitters has rarely been explored. Meanwhile, the highest ηext of ESIPTbased OLEDs is still far away from the upper limit of OLEDs. It will be of important significance to develop new efficient ESIPT emitters for OLEDs. TADF emitters have attracted much interest during the past few years due to their unique ability to theoretically achieve 100% internal quantum efficiency (IQE).7,8 Numerous endeavors have recently been made to develop TADF emitters9-19 with nearly 100% IQEs20-22 and over 20% ηexts23-33 have been achieved for multicolor TADF OLEDs. In order to counteract the small singlet-triplet energy splitting (∆EST) and high ΦPL, TADF emitters usually possess a pre-twisted charge-transfer (CT) configuration in D-A or D-π-A systems, where D and A represent donors and acceptors, respectively.34-41 Endowed with these structural features, TADF emitters generally exhibit long delayed fluorescence (DF) lifetimes and high ΦPLs in solid state. These advantages of TADF emitters may provide a feasible approach to compensate for the flaws of traditional

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Figure 1. (a) Chemical structure, (b) Gaussian optimized structure, (c) HOMO/LUMO distributions of PXZPDO, PXZDMePDO, DMACPDO and DMACDMePDO. Single-crystal structure diagrams of (d) PXZPDO and (e) PXZDMePDO.

ESIPT emitters. Through incorporating design requirements for TADF features into ESIPT molecules, a new class of ESIPT emitters featuring TADF characteristics could be constructed.42 To validate our hypothesis, we designed and synthesized two proof-of-concept molecules, 1,3-bis(4-(10Hphenoxazin-10-yl)phenyl)-3-hydroxyprop-2-en-1-one (PXZPDO) and 1,3-bis(4-(9,9-dimethylacridin-10(9H)yl)phenyl)-3-hydroxyprop-2-en-1-one (DMACPDO) (Figure 1a), by adopting symmetric D-π-A-π-D molecular architectures. Within this architecture, an electron donor, phenoxazine (PXZ) or a 9,9-dimethyl-9,10-dihydroacridine (DMAC) moiety, was incorporated into an electron accepting β-diketone core via phenylene π-bridges. The symmetric molecular configuration was adopted to simplify emissive characteristics, while the D-π-A-π-D architecture guaranteed a strong intramolecular CT state and high ΦPL. The β-diketone core was selected for its strong electron-accepting ability,43-45 and its enol-type tautomer to promote a proton transfer (PT) process.46,47 Reference molecules, 1,3-bis(4-(10H-phenoxazin-10-yl)phenyl)-2,2dimethylpropane-1,3-dione (PXZDMePDO) and 1,3-bis(4(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,2-dimethylpropane1,3-dione (DMACDMePDO), were also synthesized, and had

dimethyl-substituted β-diketone cores in the same molecular architectures as PXZPDO and DMACPDO, respectively. As expected, PXZPDO and DMACPDO tended to exhibit enoltype forms stabilized by intramolecular hydrogen bonds (IHBs), accompanied with distinct ESIPT character verified by transient absorption analyses. Obvious TADF features were also observed for PXZPDO and DMACPDO, coupled with DF lifetimes of 1.33 and 1.94 µs, and high ΦPLs of 68% and 86% in film state, respectively. Consequently, PXZPDO and DMACPDO achieved record-high ηexts for ESIPT emitters of 18.8% and 23.9% for yellow and green OLEDs, respectively.

2. RESULTS AND DISCUSSION 2.1 DFT Calculations Generally, β-diketone derivatives exist as keto and enol forms (cis-enol) at room temperature. To understand different tautomeric structures and their influence on electronic properties, density functional theory (DFT) calculations were performed on these molecules. As shown in Figure 1b, the enoltype tautomers of PXZPDO and DMACPDO exhibited rigid planar configurations in their electron acceptor cores derived

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∆E (eV)

(a)

0.02

S1-T2

0.01

S2-T1

0.00 -0.1

A

0.0

0.1

center

B

Deviation (Å)

(b) 0.02

∆E (eV)

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


0.01

S1-T2 S2-T1

0.00 -0.1

A

0.0

center

0.1

B

Deviation (Å) Figure 2. Energy gaps between S1 and T2, S2 and T1 versus the position of hydroxyl hydrogen for (a) PXZPDO and (b) DMACPDO. Calculated at the PBE0/def2-SVP level of theory.

from 1.60 Å and 1.59 Å IHBs, and possessed lower ground and excited states compared to their ketone forms with twisted acceptor cores (Table S1). These results suggest that the enoltype tautomers of PXZPDO and DMACPDO were more stable in both ground and excited states. Comparatively, the reference molecules, PXZDMePDO and DMACDMePDO, only exhibited ketone-type forms because the dimethyl substitution in their β-diketone cores prohibited enol-type tautomer formation. Previous studies have revealed that CT and PT can coexist in enol-type tautomer of β-diketone derivatives containing a suitable electron donor.47 To determine whether it could occur in our newly-designed molecules, PXZPDO and DMACPDO, energy scanning of their excited states (S1, T1, S2, T2) versus the position of hydroxyl hydrogen were performed (Figure S1). For PXZPDO and DMACPDO, the maximum energy barriers of the PT processes were calculated to be ca. 0.04 eV, indicating that their PT processes can be rather active at room temperature. Frontier molecular orbital (FMO) analysis revealed that the HOMOs of these molecules were mainly located on PXZ/DMAC units, whereas their LUMOs mainly localized on β-diketone units and the adjacent phenylene-bridges (Figure 1c). This can be ascribed to the large torsion angles

between the PXZ or DMAC unit and the adjacent phenylene bridge of 74.8°/85.2° for PXZPDO, 73.0°/74.5° for PXZDMePDO, 89.5°/90.0° for DMACPDO and 83.9°/85.3° for DMACDMePDO. Such good FMO separations endow all molecules with small ∆ES1-T1s/∆ES2-T1s of 0.010/0.021 eV for PXZPDO, 0.021/0.026 eV for PXZDMePDO, 0.006/0.019 eV for DMACPDO and 0.014/0.020 eV for DMACDMePDO (Table S2)48, which could benefit reverse intersystem crossing (RISC) process from T1 to S1 (S2).49-53 Excited state analysis of PXZPDO and DMACPDO demonstrated that their S1 and T1 states mainly originated from the transitions of HOMO to LUMO, and S2 and T2 states mainly possessed the transitions of HOMO-1 to LUMO (Table S3). According to the El-Sayed rule,54,55 the ISC processes from the S1 to T1 for PXZPDO and DMACPDO are forbidden, due to their almost identical transition conformations. However, the small ∆ES2-T1 values of both emitters could endow PXZPDO and DMACPDO with efficient RISC processes from T1 to S2. As shown in Figure 2, the ∆ES2-T1 values of both emitters were dynamically changed during the active PT processes of PXZPDO and DMACPDO. It is worth noting that when the hydroxyl hydrogen was changed to the center position of two oxygen atoms, S2 highly mixed with S1 for the PT-active molecules of PXZPDO and DMACPDO. Meanwhile, the highly mixing of T1 and T2 also occurred in both emitters. In this sense, the deactivation processes of S1, S2, T1 and T2 can create a new RISC channel of T2→S1 for PXZPDO and DMACPDO. Consequently, the PT-active molecules of PXZPDO and DMACPD possessed much more efficient triplet-harnessing abilities when compared to the reference molecules with the static ∆EST values. These results suggested that active PT process in both emitters could promote the RISC processes of PXZPDO and DMACPDO, and thereby enhance their lightemitting properties after photonic and electrical excitations. 2.2 Synthesis and Characterization All compounds were produced by Pd-catalyzed C-N crossing-coupling reactions of PXZ or DMAC units with the corresponding dibromo-substituted β-diketone intermediates in high yields (Scheme S1).56,57 Their molecular structures were well characterized by 1H NMR, 13C NMR, mass spectrometry and elemental analysis. For PXZPDO and DMACPDO, obvious 1 H NMR signals belonging to the hydroxy proton from their enol-type structures were observed at a chemical shift of 16.85 and 16.93 ppm in CDCl3, and slightly shifted to 17.15 and 17.22 ppm in d6-DMSO, respectively (Figure S2). These results indicate that PXZPDO and DMACPDO exhibited dominant enol-type forms in solution, which could be stabilized by hydrogen bonds and the extended conjugation. To determine which type of hydrogen bond existed in these enol-type structures, the hydrogen bond acidities (A) were quantified using the following equation: A = 0.0065 + 0.133∆δ where ∆δ = δ(DMSO) – δ(CDCl3) is the difference in the chemical shifts between the two solvents.58 A values for PXZPDO and DMACPDO were estimated to be 0.046 and 0.045, respectively, which are much smaller than 0.1. These results established the existence of strong IHBs for PXZPDO and DMACPDO in solution. Furthermore, the completely symmetric 1H NMR spectra of enol-type PXZPDO and


Journal of the American Chemical Society Table 1. Physical properties of these compounds. λabs (nm)a

compound

a

λem (nm)a

Eg (eV)b

ES/ET (eV)c

∆EST (eV)d

τd (µs)e

Rp/Rd f

ΦPLg

PXZPDO

334/441

594

2.41

2.54/2.50

0.04

1.33

0.47/0.53

68%

PXZDMePDO

324/416

562

2.58

2.68/2.61

0.07

1.48

0.85/0.15

54%

DMACPDO

291/345/406

533

2.59

2.64/2.53

0.11

1.94

0.51/0.49

86%

DMACDMePDO

283/353/390

506

2.78

2.82/2.66

0.16

1.84

0.68/0.32

64%

-5

b

c

Measured in toluene (10 M). Obtained from the intersection of the normalized absorption spectra. Obtained from the onset of fluorescence spectra and phosphorescence spectra of these emitters doped into CBP films. dCalculated from ES and ET. eDF lifetimes of these emitters doped into CBP films at 300 K. fThe weighting factors for prompt and delayed fluorescence. gΦPLs of these emitters in CBP host under oxygen-free conditions at room temperature. Note: doping concentrations were 1 wt.% for PXZPDO and PXZDMePDO, 6 wt.% for DMACPDO and DMACDMePDO.

(a)

PXZPDO_Abs. PXZDMePDO_Abs. DMACPDO_Abs. DMACDMePDO_Abs.

1.2 1.0

Intensity (a.u.)

DMACPDO confirmed the existence of a fast PT process in their β-diketone cores. In contrast, the dimethyl substitution in the β-diketone cores made PXZDMePDO and DMACDMePDO display sole ketone-type forms in solution, assupported by their 1H NMR spectra. The single-crystal X-ray crystallographic analysis further identified molecular structures of PXZPDO and PXZDMePDO (see Table S4 for more detailed information). As shown in Figures 1d and 1e, PXZPDO existed as an enol-type form in a single crystal, and contained a planar acceptor core stabilized by an IHB of 1.85 Å, whereas PXZDMePDO exhibited a ketone-type configuration with a twisted acceptor core. Furthermore, both molecules take highly twisted structures, accompanied by nearly vertical dihedral angles between PXZ units and the adjacent phenylene bridges of 79.2° for PXZPDO and 88.2° for PXZDMePDO. These results are in close agreement with the results obtained from the DFT simulation. For practical application in OLEDs, organic molecules usually form amorphous films during the deposition process under high vacuum. To determine the existing form of PXZPDO in OLEDs, solid-state 1H NMR was measured for the sublimated film of PXZPDO. A broad shoulder peak with a chemical shift of 18.6 ppm was integrated to be 0.95 proton when defining the total NMR signal of PXZPDO as 26 protons (Figure S3), and could be assigned to the signal of hydroxy proton in the enol-type tautomer. These results clearly established the existence of a dominant enol form of PXZPDO in a sublimated film when used in OLEDs. 2.3 Photophysical Properties As seen in Figure 3a, these emitters showed two types of absorption bands simultaneously in toluene: strong absorption bands peaking at ca. 300 nm assigned to the π-π* transition of PXZ/DMAC groups, and weak absorption bands peaking in the range of 340-450 nm belonging to the CT transition from PXZ/DMAC units to β-diketone cores.59,60 Furthermore, all emitters exhibited structureless emissions with main peaks at 594, 562, 533 and 506 nm for PXZPDO, PXZDMePDO, DMACPDO and DMACDMePDO, respectively (Table 1). When compared to their corresponding reference molecules, PXZPDO and DMACPDO emissions exhibited bathochromic shifts of 32 and 27 nm that could be ascribed to their extended π-conjugations originating from planar acceptor configurations of PXZPDO and DMACPDO. Furthermore, PXZPDO displayed a red-shifted emission when compared with DMACPDO due to the stronger electron-donating capabilities of PXZ units than DMAC units. The photophysical properties

PXZPDO_Fl. PXZDMePDO_Fl. DMACPDO_Fl. DMACDMePDO_Fl.

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength (nm)

(b) 1.0

Intensity (a.u.)

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PXZPDO_Fl. PXZPDO_Ph.

0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0

PXZDMePDO_Fl. PXZDMePDO_Ph.

DMACPDO_Fl. DMACPDO_Ph.

DMACDMePDO_Fl. DMACDMePDO_Ph.

500

600

Wavelength (nm) Figure 3. (a) Normalized UV-Vis absorption and fluorescence spectra in toluene solutions (10-5 M). (b) Normalized photoluminescence (300 K) and phosphorescence (77 K) spectra of 1 wt.% PXZPDO/PXZDMePDO and 6 wt.% DMACPDO/DMACDMePDO doped into the CBP host.

of these emitters in film state were also investigated in a 4,4’di(9H-carbazol-9-yl)-1,1’-biphenyl (CBP) host (Figure 3b), which is a widely used host in OLEDs due to its bipolar charge-transporting ability and relatively high triplet energy of 2.56 eV. Notably, the doped films of these emitters displayed broad emissive profiles with main peaks at 552, 524, 524 and 497 nm for PXZPDO, PXZDMePDO, DMACPDO and DMACDMePDO, respectively, which matched well with the outcomes of these emitters in PMMA films (Figure S4). These results demonstrated that the emissive characteristics originated from these emitters rather than from species formed


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(a)

Delay times (ps):

0.2

0.15, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60 and 0.90.

∆OD

0.1 0.0 Delay times (ps):

0.2

1, 3, 7, 14, 19, 50 and 100.

0.1 0.0 460 480 500 520 540 560 580 600 620 640

Wavelength (nm)

(b) 0.2

Figure 5. Simplified schematic diagram depicting the excited state dynamics of PXZPDO and DMACPDO.

Delay times (ps): 0.03, 0.11, 0.20, 0.32, 0.53, 0.70 and 1.

0.1

∆OD

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


0.0 0.2

Delay times (ps): 0.5, 1, 3.5, 6, 12, 18, 23, 25, 58 and 100.

0.1

0.0 450

500

550

600

650

700

Wavelength (nm) Figure 4. Time-resolved TA spectra recorded following photoexcitation of (a) PXZPDO and (b) DMACPDO in toluene using 400 nm laser pulses.

between emitters and the CBP host. According to the onset wavelengths of fluorescence and phosphorescence spectra of these emitters in the CBP host, the ∆ESTs of PXZPDO, PXZDMePDO, DMACPDO and DMACDMePDO were estimated to be 0.04, 0.07, 0.11 and 0.16 eV, respectively. These values promote the effective up-conversion process from T1 to S1, and thus an effective TADF process could occur in these compounds. Femtosecond time-resolved differential transient absorption (fs-TA) spectra were performed to inspect excited state dynamics of PXZPDO and DMACPDO in toluene. According to their UV-Vis absorption spectra, PXZPDO and DMACPDO can be excited from the ground state to the S1 states upon excitation of 400 nm laser pulses. Both PXZPDO and DMACPDO exhibited two main transient absorption (TA) bands in the whole probe wavelength (Figure S5). As shown in Figure 4, both PXZPDO and DMACPDO exhibited strong TA bands in the range of < 500 nm, coupled with weak TA bands in the range of 500~600 nm for PXZPDO and > 550 nm for DMACPDO. Furthermore, their main TA bands were rapidly established, indicating that the absorption of the S1 states may mainly account for this phenomenon. In the sub 1ps domain, PXZPDO exhibited two fast rising TA bands peaking at ca. 470 and 540 nm. It is worth noting that the TA band centered at 470 nm of PXZPDO blue-shifted to ~ 460 nm within 300 fs. In combination with a large Stokes shift of 5840 cm-1 for PXZPDO in toluene, one could expect that an

ESIPT process could occur on the S1 state of PXZPDO. These results are in good agreement with the DFT simulation that the PT process leads to a small energy difference in S1 state of PXZPDO. Similarly, the S1 state of DMACPDO also experienced an ESIPT process within 500 fs. These results are basically consistent with the previous report by Chou et. al. that an ultrafast ESPIT process was demonstrated to occur in < 25 fs for o-HBDI and < 150 fs for PBT-NHTs.61,62 The relaxation dynamics of fs-TA spectra of both emitters (Figure S6) revealed that PXZPDO and DMACPDO exhibited double exponential decays consisting of fast and slow absorption components with the corresponding lifetimes of 3.1 ps and µs scale for PXZPDO, and 1.8 ps and µs scale for DMACPDO. According to the energy gap law, a small energy gap between the singlet and triplet states is beneficial for an efficient ISC. 63,64 Apparently, the small ∆ESTs of less than 0.2 eV for PXZPDO and DMACPDO could be easy to open a channel for the ISC process. In this sense, the fast decay components of both emitters may be attributed to the fast ISC process from S1 to T1; while their µs components could belong to the deactivation processes of their triplet states. To figure out this point, we measured the nanosecond time-resolved transient absorption (ns-TA) spectra of both emitters in toluene (Figure S7). Both emitters exhibited distinct positive absorption signals in the range of 390-510 nm for PXZPDO and 420-460 nm for DMACPDO, which can be assigned to the absorption of their triplet states. The consistence of these absorption regions in the fs-TA and ns-TA spectra for both emitters clearly established that the ESIPT procedures were directly coupled with or followed by the ISC process for PXZPDO and DMACPDO. Furthermore, the ground state bleaching was only observed at below 400 nm in the ns-TA spectra of both emitters. This result indicated that the ground state bleaching in the 400-500 nm region could not make a significant contribution to the absorption spectra of both emitters. Interestingly, the ns-TA spectra of both emitters displayed strong negative signals with the main peaks close to their fluorescence emission in toluene, accompanied with µs scale lifetimes. This manifested that the triplet states contributed to this fluorescence emission of both emitters via a RISC process. These results suggested that following the ESIPT and ISC process, the RISC process from the triplet state to the singlet states could occur in both emitters, and thereby result in the distinct DF emission. The simplified excited-state dynamics of PXZPDO and DMACPDO were depicted in Figure 5.



Figure 6. Transient PL spectra of (a) 1 wt.% PXZPDO and (b) 6 wt.% DMACPDO doped into CBP host from 100 to 300 K. Inset: Transient PL spectra of (a) PXZPDO and (b) DMACPDO in toluene (10-5 M) under aerated and degassed condition.

As shown in the insets of Figures 6 and S8, the transient PL spectra of these emitters in toluene under degassed condition displayed second-order exponential decays containing prompt components with lifetimes of 7.1 ns for PXZPDO, 13.3 ns for PXZDMePDO, 16.4 ns for DMACPDO and 22.6 ns for DMACDMePDO, and delayed components with lifetimes of 0.4 µs for PXZPDO, 0.6 µs for PXZDMePDO, 1.6 µs for DMACPDO and 1.0 µs for DMACDMePDO. In the presence of oxygen, however, these emitters exhibited single exponential decays with short lifetimes of 8.9, 8.2, 9.6 and 13.3 ns, respectively. These results demonstrated that triplet states of these emitters contributed to their emissive processes. Obvious signals with chemical shifts of 17.59 and 17.71 ppm (Figures S9 and S10) belonging to the hydroxy protons in d8-toluene denoted dominant enol-type forms of PXZPDO and DMACPDO in toluene. To explore the emissive nature of these emitters, transient PL spectra of these emitters in a CBP host (1 wt.% for PXZPDO and PXZDMePDO, 6 wt.% for DMACPDO and DMACDMePDO) were also conducted. These emitters exhibited double exponential decays consisting of prompt and delayed fluorescence components with corresponding lifetimes of 12.0 ns/1.33 µs for PXZPDO, 14.0 ns/1.48 µs for PXZDMePDO, 10.8 ns/1.94 µs for DMACPDO and 17.0 ns/1.84 µs for DMACDMePDO at room temperature. The gradually increased DF components of PXZPDO, PXZDMePDO, DMACPDO and DMAC-

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DMePDO from 100 to 300 K (Figures 6 and S8) also clearly established the distinct TADF nature of these emitters. These emitters in a CBP host achieved relatively high ΦPLs of 68% for PXZPDO, 54% for PXZDMePDO, 86% for DMACPDO and 64% for DMACDMePDO at 300 K. The higher ΦPLs of PXZPDO and DMACPDO than those of PXZDMePDO and DMACDMePDO could be attributed to their higher structure rigidity of enol-type acceptor cores that suppressed nonradiative processes. Moreover, the contributions of delayed (Φd) components to the ΦPLs for PXZPDO and DMACPDO were estimated to be 53% and 49%, respectively. Considering the lifetime data of PXZPDO and DMACPDO in the CBP host, the rate constants of ISC (kISC) and RISC (kRISC) were estimated to be 5.7 x 107 s-1 and 1.3 x 106 s-1 for PXZPDO, 5.2 x 107 s-1 and 8.8 x 105 s-1 for DMACPDO, respectively (Table S5). In comparison with the reference molecules, the kRISC values of ESIPT-active molecules were significantly enhanced, evidencing more efficient RISC processes in PXZPDO and DMACPDO with respect to their reference molecules.44 Because the RISC process plays a decisive role in TADF emission, one can expect that the active ESIPT process could boost the TADF emission of both emitters. These results are consistent with the DFT simulation that the active ESIPT process could promote the RISC processes and the related lightemitting process for PXZPDO and DMACPDO. The excellent emissive properties of PXZPDO and DMACPDO in film states clearly demonstrated the success of this design strategy. 2.4 Device Characterization The relatively high ΦPLs and distinct TADF nature of PXZPDO and DMACPDO inspired us to explore their application as emitters in OLEDs. To achieve this goal, a typical multilayer OLED structure was adopted (Figures 7a and 7b). This consisted of an indium tin oxide (ITO)/1,1-bis[(di-4tolylamino)phenyl]cyclohexane (TAPC) (30 nm)/4,4′,4′′-tri(Ncarbazolyl)triphenylamine (TCTA) (5 nm)/emissive layer (EMLs) (15 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB) (65 nm)/LiF (1 nm)/Al (100 nm),65,66 where LiF was the electron-injection layer, and TAPC/TCTA and Tm3PyPB acted as the hole- and electron-transporting materials, respectively. PXZPDO and DMACPDO were doped into the CBP host with an optimal doping concentration of 1 wt.% and 6 wt.% (Figure S11), respectively, as the EMLs. Such low doping concentrations can avoid detrimental exciton quenching processes, such as singlet-triplet annihilation (STA), triplet-triplet annihilation (TTA) and so on. We also fabricated the reference devices using PXZDMePDO and DMACDMePDO in the same device configurations with the corresponding doping concentrations to be used for comparison. As shown in Figure 7c, all devices exhibited similar current density-voltage characteristics with the same turn-on voltage of 3.4 V, which could be attributed to the similar HOMO/LUMO levels of these emitters. The PXZPDO- and DMACPDO-based devices displayed broad and intense emissions with main electroluminescent (EL) peaks at 560 and 536 nm, respectively, and favorable Commission International de l’Eclairage coordinates of (0.44, 0.53) and (0.36, 0.57) at a driving voltage of 7 V. The good consistencies of the EL and PL spectra of PXZPDO and DMACPDO in the CBP host support that their EL emissions originated from the enol-type tautomers of PXZPDO and DMACPDO. More excitingly, the


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Figure 7. (a) Energy level diagram of materials employed in the devices. (b) Molecular structures of materials used in the devices. (c) Current density and luminescence versus voltage characteristics for the devices. Inset: Electroluminescence spectra of the devices at 7 V. (d) External quantum efficiency versus luminance curves for the devices.

Table 2. Summary of EL data for the devices. device

concentration

Von (V)a

ηc (cd A-1)b

ηp (lm W-1)b

ηext (%)b

λem (nm)c

CIE (x,y)d

PXZPDO

1 wt.%

3.4

59.0/55.2/45.5

54.1/41.3/28.5

18.8/17.6/14.5

560

(0.44, 0.53)

PXZDMePDO

1 wt.%

3.4

39.6/37.1/31.5

35.2/26.5/19.0

12.2/11.4/9.7

544

(0.38, 0.56)

DMACPDO

6 wt.%

3.4

81.0/71.5/52.5

73.7/53.5/33.0

23.9/21.1/15.5

536

(0.36, 0.57)

DMACDMePDO

6 wt.%

3.4

43.8/36.1/24.8

40.5/27.0/15.6

14.6/12.0/8.3

518

(0.28, 0.53)

a

-2 b

The turn-on voltage recorded at a brightness of 1 cd m . The order of measured value: the maximum, then values at the luminance of 100 and 1000 cd m-2. cThe main EL emission peak at a driving voltage of 7 V. dCommission International de l’Eclairage coordinates recorded at 7 V.

PXZPDO-based device achieved a maximum current efficiency (ηc,max) of 59.0 cd A-1, a maximum power efficiency (ηp,max) of 54.1 lm W-1, and a maximum ηext (ηext,max) of 18.8%. Comparatively, the higher ΦPL and kRISC of DMACPDO gave its device better EL performance (a ηc,max of 81.0 cd A-1, a ηp,max of 73.7 lm W-1, and a ηext,max of 23.9%, Figure S12 and Table 2). These efficiencies are significantly higher than those of the PXZDMePDO-based (39.6 cd A-1, 35.2 lm W-1, 12.2%) and DMACDMePDO-based (43.8 cd A-1, 40.5 lm W-1, 14.6%) reference devices. The could be attributed to the much higher

ΦPL and boosted kRISC induced by the ESIPT process for PXZPDO and DMACPDO compared with their corresponding reference molecules. To the best of our knowledge, 18.8% and 23.9% ηexts not only represent state-of-the-art device performances for ESIPT emitters, but are also among the highest values for TADF OLEDs within a similar color gamut (Table S6). Moreover, at the practical luminance of 1000 cd m-2, the ηext of the PXZPDO-based device remained as high as 14.5%, accompanied by a slow efficiency roll-off of 23%; while the ηext of the DMACPDO-based device was still 15.5% with a



moderate efficiency roll-off of 35%. Remarkably, 18.8% and 23.9% ηexts far exceed the theoretical limit of conventional fluorescent OLEDs, clearly demonstrating an effective utilization of triplet excitons via a TADF channel. To determine how many single and triplet excitons are utilized in the EL procedure, ηr (exciton utilization efficiency) was estimated on the basis of the following equation: ߟୣ୶୲ = ߛߟ௥ Φ௉௅ ߟ௢௨௧ where ηout represents the light out-coupling efficiency, γ refers to the charge balance factor, and ΦPL is the ΦPL of the emission layer. Considering a ΦPL of 68% for PXZPDO and 86% for DMACPDO, the perfect charge balance (γ = 1) and relatively high ηout of 25-30% in the devices, the ηrs of PXZPDO- and DMACPDO-based devices were estimated to be 92-111% and 93-111%, respectively, indicative of nearly 100% harvesting of singlet and triplet excitons in the EL processes in these devices.

3. Conclusion In conclusion, we developed two proof-of-concept emitters using PXZPDO and DMACPDO that simultaneously exhibited dynamic ESIPT properties and distinct TADF features. Creating a TADF channel in ESIPT molecules endowed PXZPDO and DMACPDO with apparent DF lifetimes of 1.33 and 1.94 µs, as well as high ΦPLs of 68% and 86% in the film state, respectively. The use of PXZPDO and DMACPDO in OLEDs produced record-high ηexts of 18.8% and 23.9% for yellow and green OLEDs, respectively, based on ESIPT emitters, indicative of the success of our innovative strategy. Our TADF-featured ESIPT emitters overcame the shortcomings of traditional ESIPT emitters that exhibit short fluorescence lifetimes and low solid-state ΦPLs. This finding provides a new and feasible approach for the design of efficient ESIPT emitters, as well as their practical implementation in OLEDs.

4. EXPERIMENTAL SECTION Synthesis. The key intermediates, 1,3-bis(4-bromophenyl)3-hydroxyprop-2-en-1-one (DBrPDO) and 1,3-bis(4bromophenyl)-2,2-dimethylpropane-1,3-dione (DBrDMePDO), were synthesized according to the literatures.56,57 Standard procedures are used to dry the solvents. Unless otherwise stated, all reagents were used as received without further purification. Synthesis of 1,3-bis(4-(10H-phenoxazin-10-yl)phenyl)-3hydroxyprop-2-en-1-one (PXZPDO): A mixture of DBrPDO (1.14 g, 3 mmol), 10H-phenoxazine (1.30 g, 7.00 mmol), palladium acetate (34 mg, 0.15 mmol), tri-tert-butylphosphine tetrafluoroborate (135 mg, 0.45 mmol), sodium tert-butoxide (672 mg, 7.00 mmol) and 50 mL redistilled toluene was purged with argon for 30 min, and then stirred at 110 oC for 48 h. After cooling to room temperature, the reaction mixture was treated with brine, extracted with CH2Cl2, washed with brine, and dried over anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel using n-hexane/dichloromethane (1:1 by vol.) as the eluent to afford a red powder (1.49 g, 85% yield). 1H NMR (400 MHz, CDCl3, δ): 16.85 (s, 1H), 8.24 (d, J = 8 Hz, 4H), 7.52 (d, J = 8 Hz, 4H), 6.96 (s, 1H), 6.74-6.60 (m, 12H), 5.98 (d, J = 8 Hz, 4H). 13C NMR (100 MHz, CDCl3, δ): 184.9,

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144.0, 143.2, 135.3, 133.7, 131.3, 130.0, 123.3, 121.9, 115., 113.3, 93.6. MS (EI) m/z: [M]+ calcd for C39H26N2O4, 586.19; found, 586.45. Anal. Calcd for C39H26N2O4 (%): C 79.85, H 4.47, N 4.78; found: C 79.68, H 4.39, N 4.70. Synthesis of 1,3-bis(4-(10H-phenoxazin-10-yl)phenyl)-2,2dimethylpropane-1,3-dione (PXZDMePDO): A similar procedure to the preparation of PXZPDO was employed but using DBrDMePDO instead of DBrPDO. A yellow powder was obtained with 89% yield. 1H NMR (400 MHz, CDCl3, δ): 8.06 (d, J = 8 Hz, 4H), 7.36 (d, J = 8 Hz, 4H), 6.71-6.63 (m, 8H), 6.53 (t, J = 8 Hz, 4H), 5.82 (d, J = 8 Hz, 4H), 1.78 (s, 6H). 13C NMR (100 MHz, CDCl3, δ): 199.3, 144.1, 143.6, 135.3, 133.3, 131.8, 130.9, 123.3, 122.1, 115.8, 113.3, 59.8, 25.4. MS (EI) m/z: [M]+ calcd for C41H30N2O4, 614.22; found, 614.25. Anal. Calcd for C41H30N2O4 (%): C 80.11, H 4.92, N 4.56; found: C 79.94, H 4.86, N 4.52. Synthesis of 1,3-bis(4-(9,9-dimethylacridin-10(9H)yl)phenyl)-3-hydroxyprop-2-en-1-one (DMACPDO): A similar procedure to the preparation of PXZPDO was employed but using 9,9-dimethyl-9,10-dihydroacridine instead of 10Hphenoxazine. A yellow powder was obtained with 86% yield. 1 H NMR (400 MHz, CDCl3, δ): 16.93 (s, 1H), 8.28 (d, J = 8 Hz, 4H), 7.53-7.47 (m, 8H), 7.02-6.95 (m, 9H), 6.32 (d, J = 8 Hz, 4H), 1.70 (s, 12H). 13C NMR (100 MHz, CDCl3, δ): 185.0, 145.5, 140.5, 135.0, 131.4, 130.6, 129.9, 126.5, 125.4, 121.1, 114.3, 93.7, 36.1, 31.2. MS (EI) m/z: [M]+ calcd for C45H38N2O2, 638.29; found, 638.62. Anal. Calcd for C45H38N2O2 (%): C 84.61, H 6.00, N 4.39; found: C 84.51, H 5.92, N 4.33. Synthesis of 1,3-bis(4-(9,9-dimethylacridin-10(9H)yl)phenyl)-2,2-dimethylpropane-1,3-dione (DMACDMePDO): A similar procedure to the preparation of DMACPDO was employed but using DBrDMePDO instead of DBrPDO. A green powder was obtained with 85% yield. 1 H NMR (400 MHz, CDCl3, δ): 8.11 (d, J = 8 Hz, 4H), 7.47 (d, J = 8 Hz, 4H), 7.39 (d, J = 8 Hz, 4H), 6.98-6.92 (m, 8H), 6.25 (d, J = 8 Hz, 4H), 1.84 (s, 6H), 1.67 (s, 12H). 13C NMR (100 MHz, CDCl3, δ): 199.6, 146.1, 140.3, 134.7, 131.7, 131.5, 130.0, 126.4, 125.2, 121.5, 114.9, 59.8, 36.2, 30.6, 25.5. MS (EI) m/z: [M]+ calcd for C47H42N2O2, 666.32; found, 666.24. Anal. Calcd for C47H42N2O2 (%): C 84.65, H 6.35, N 4.20; found: C 84.55, H 6.34, N 4.16. Femtosecond and Nanosecond Time-resolved Transient Absorption Measurements. Details of the femtosecond laser system have been described elsewhere,67,68 and thus only a brief introduction is given here. The seed beam is generated by a commercial Ti:sapphire oscillator pumped by a CW second harmonic of a Nd:YVO4 laser, and then amplified by a Nd:YLF pumped regenerative amplifier to generate a 1 kHz pulse train centered at 800 nm of approximately 35 fs pulse duration with maximum energy of 1 mJ per pulse. Excitation at 400 nm is the second harmonic generation of the fundamental pulse obtained by a 0.5 mm-thickness BBO crystal, with pulse energy of ∼1 µJ at the sample position. A white light continuum, generated by focusing the 800 nm fundamental light on a 1 mm sapphire plate, is reflected from the front and back surfaces of a quartz plate to obtain probe and reference beams. The pump and probe pulses intersect in the sample at an angle of ∼4°, and the reference beam is transmitted through the sample at a different spot. The relative polarization of the


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pump and probe pulses is set to the magic angle for all the measurements. A linear translation stage is used to delay the probe beam to monitor the pump–probe dynamics. The resulting spectra are detected by a CCD camera (PI-MAX, 1024 × 256 pixel array) equipped with a spectrometer (Princeton, SpectraPro 2500i). The instrumental response function of the system, determined by cross correlation between excitation and probe pulses using the optical Kerr-gate method, is typically better than 250 fs. Nanosecond time-resolved transient absorption spectra were measured with a home-built laser flash photolysis system.69 The third harmonic (355 nm) of a QSwitched Nd:YAG laser (PRO-190, Spectra Physics) was used as the excitation source (pulse duration 8 ns, repetition rate of 10 Hz, pulse energy < 10 mJ per pulse). A 500 W Xenon lamp was used as the analyzing light, and passed through a flow quartz cuvette perpendicular to the pulsed laser. The optical absorption path length was 10 mm. A monochromator equipped with a photomultiplier (CR131, Hamamatsu) was used to record the transient absorption spectra within a wavelength range of 320-800 nm. The typical spectral resolution was less than 1 nm. A dynamic decay curve of the intermediate was averaged by multi-shots and recorded with an oscilloscope (TDS3052B, Tektronix). All the solutions were deoxygenated by purging with high purity argon (99.99%) for about 20 minutes prior to measurements.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. General information, syntheses, analytical data, DFT studies, photophysical studies, thermal studies, electrochemical studies, EL data and NMR spectra (PDF) X-ray crystallographic data for PXZPDO (CIF) X-ray crystallographic data for PXZDMePDO (CIF)

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundation of China (51573141, 91433201 and 11674355), the National Key R&D Program of China (2016YFB0401002), the National Basic Research Program of China (2015CB655002), the Shenzhen Peacock Plan (KQTD20170330110107046), and the Natural Science Foundation for Distinguished Young Scholars of Hubei Province (2017CFA033).

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De Novo Design of Excited-State Intramolecular Proton Transfer

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