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Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through-Space Charge Transfer Hiroyuki Tsujimoto,† Dong-Gwang Ha,‡ Georgios Markopoulos,† Hyun Sik Chae,∥ Marc A. Baldo,*,§ and Timothy M. Swager*,† †

Department of Chemistry, ‡Department of Materials Science and Engineering, and §Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ∥ Samsung Research America, 255 Main Street, Suite 702, Cambridge, Massachusetts 02142, United States S Supporting Information *

ABSTRACT: Emissive molecules comprising a donor and an acceptor bridged by 9,9-dimethylxanthene, were studied (XPT, XCT, and XtBuCT). The structures position the donor and acceptor with cofacial alignment at distances of 3.3−3.5 Å wherein efficient spatial charge transfer can occur. The quantum yields were enhanced by excluding molecular oxygen and thermally activated delayed fluorescence with lifetimes on the order of microseconds was observed. Although the molecules displayed low quantum yields in solution, higher quantum yields were observed in the solid state. Crystal structures revealed π−π intramolecular interactions between a donor and an acceptor, however, the dominant intermolecular interactions were CH···π, which likely restrict the molecular dynamics to create aggregation-induced enhanced emission. Organic light emitting devices using XPT and XtBuCT as dopants displayed electroluminescence external quantum efficiencies as high as 10%.

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INTRODUCTION The optimization of organic light emitting diode (OLED) technologies has challenged chemists since Tang and Van Slyke reported the first OLED in 1987.1 The promise of high efficiency OLEDs remains, with attractive advantages in lighting, smartphones, flat panel displays2 and emerging flexible displays. In a classical OLED, the internal quantum efficiencies (IQEs) are limited by an electron−hole to emitted photon conversion of 25% because nonemissive triplet excitons constitute 75%3,4 of the generated excited states. As a result, a maximum of 25% of excitons contribute to the IQE. To convert singlet and triplet excitons into emitted light, organometallic emitters based on platinum and iridium have been used as a result of the fact that their large spin−orbit coupling leads to efficient phosphorescent emission.5 Despite the high efficiency of phosphorescent OLED devices, the cost of these rare metals, the difficulty of creating robust blue emitters, and competing triplet−triplet annihilation remain as limitations in these systems. An attractive alternative strategy is to utilize thermally activated delayed fluorescence (TADF),6−18 which utilizes the up-conversion from triplet excitons to singlet states by reverse intersystem crossing (RISC). With low nonradiative rates and efficient singlet emission, in principle the TADF approach can result in devices with near 100% IQE.19−21 The majority of TADF molecular designs utilize conformational effects to twist donor and/or acceptor π systems from © XXXX American Chemical Society

coplanarity to minimize overlap of the HOMO and LUMO states. A low overlap limits the exchange energy gained in the triplet state and results in a small energy difference between singlet and triplet states (ΔEST)22 (Figure 1a) . However, many twisted TADF materials suffer from reduced quantum yields

Figure 1. (a) Twisted D−A single molecule, (b) D−A exciplex, (c) Ushaped space-through architecture (this work), and (d) molecular structures of XPT, XCT, and XtBuCT. Received: January 25, 2017

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DOI: 10.1021/jacs.7b00873 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 2. HOMO and LUMO orbital distributions and calculated bandgaps, singlet (S1), triplet (T1) energy levels, and oscillator strengths ( f) for XPT, XCT, and XtBuCT based on TD-DFT at the B3LYP functional and 6-31G* basis set.

Scheme 1. Synthesis of XPT, XCT, and XtBuCTa

a

Conditions; (a) R−H, K2CO3, 18-crown-6, activated copper bronze, 1,2-dichlorobenzene, reflux, 48 h; (b) (i) 2-iodo-4,6-diphenyl-1,3,5-triazine, nBuMgCl, −78 °C, THF, 10 min, (ii) ZnBr2LiCl, −78 °C, THF, 15 min, (iii) Pd2dba3 (5 mol %), CPhos (15 mol %), THF, reflux, 16 h.

resulting from π−π intermolecular interactions in the solid state.23 OLEDs based on TADF materials also still suffer from second-order luminescence quenching processes resulting in EQE roll-off at high brightness.24,25 We report herein a donor−acceptor (D−A) xanthene molecule as a new high efficiency TADF emitter designed to have intramolecular through space D−A π−π interactions.26 The through-space charge transfer is mediated by spatial π−π interactions because the donor and acceptor groups are placed in close proximity. In a through-space conjugation system, charge is transferred through aromatic π bonds linking between a donor and an acceptor. Our designs focus on a nonplanar molecular scaffold that cofacially organizes a donor and an acceptor at a well-controlled distance (Figure 1c). This structure allows small ΔEST, and the further restriction in the solid (aggregated) state provides for enhanced emission that can be considered aggregation induced

delayed fluorescence (AIDF). The introduction of various donor and acceptor combinations on the bridge structure allows us to modify the emission color and is compatible with both AIE and TADF properties.

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RESULTS AND DISCUSSION Calculations. In our designs, a donor and an acceptor are bridged through the 4 and 5 positions of a 9,9-dimethylxanthene scaffold, which generates an interchromophore spacing (assuming the π-systems to be aligned in parallel planes) of 4.7 Å.27 These designs were guided by time-dependent density functional theory (TD-DFT) based on the B3LYP functional and a 6-31G* basis. We examined three different donors with different electron donating characters to control the photoluminescence (PL) color (Figure 1d). The geometry optimizations in the gas phase revealed a U shape and cofacial intramolecular alignment of a donor and an acceptor with a B


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Figure 3. Crystal structures of (a) XPT (d = 3.423 Å), (b) XCT (d = 3.375 Å) and (c) XtBuCT (d = 3.299 Å). The d below each structure represents the distances shown by the black dashed lines.

Table 1. Physical Properties of XPT, XCT, and XtBuCT dopant

λabs (nm)a

λem (nm)a

QYsat.O2 (%)b

QYsat.N2 (%)b

τp (ns)c

τd (μs)d

λem (nm)e

QY (%)f

τd (μs)g

Tg (°C)h

Td (°C)i

HOMO (eV)j

LUMO (eV)k

Eg (eV)l

XPT XCT XtBuCT

272, 310 293, 341 298, 346

562 419 451

1.0 2.1 1.2

7.7 5.9 6.0

2.8 1.1 4.0

2.3 3.0 2.0

566 418 453

66

3.3

35

4.1

101 109 132

318 312 313

−4.99 −5.58 −5.49

−2.22 −2.39 −2.33

2.77 3.19 3.16

Measured in toluene (1 × 10−5 M) at room temperature. bEstimated in toluene using POPOP as the standard (Φ = 0.975 excited at 366 nm in cyclohexane) under saturated O2 or N2 at room temperature. cMeasured in toluene using POPOP as the standard (τ = 1.35 ns in ethanol) under saturated O2 at room temperature. dMeasured in toluene (1 × 10−5 M) under saturated N2 at room temperature. eMeasured in thin film at room temperature. fAbsolute total quantum yield evaluated using an integrating sphere: 10 wt % dopant doped in DPEPO under N2 at room temperature; g Measured in thin film under N2 at room temperature. hObtained from DSC measurement. iObtained from TGA measurement under N2. j Estimated from the oxidation potential in CH2Cl2 solution by cyclic voltammetry. kEstimated from HOMO + Eg. lEstimated from the onset of absorption spectra in CH2Cl2. a

copper bronze as the catalyst to generate monosubstituted intermediates (1a, 2a, and 3a). The acceptor unit was installed via Negishi cross-coupling reactions using Pd(0) with CPhos29 and (4,6-diphenyl-1,3,5-triazin-2-yl)zinc(II) bromide, which was prepared in situ via an iodide-magnesium exchange30 and followed by transmetalation. The final products were purified by column chromatography and characterized 1H NMR, 13C NMR, high resolution mass spectroscopy, and single crystal structure analysis. The crystal structures shown in Figure 3 confirm the cofacial arrangement of the donor and acceptor groups at distances (3.3−3.5 Å) that are shorter than the calculated geometries (3.8−4.5 Å). Physical Properties. The absorption and photoluminescence spectra of XPT, XCT, and XtBuCT are shown in Figures S1−S3 and summarized in Table 1. XPT has a broad absorption band ranging from 300−360 nm, which is attributed to ππ* transitions. Similar broader bands were observed around the same region for XCT and XtBuCT. The absorption spectra of XPT, XCT and XtBuCT are largely insensitive to the solvent polarity. In contrast, PL spectra showed strong dependence on solvent polarities. For example, the maximum of the XPT emission shifts from 524 nm in cyclohexane to 645 nm in acetone, which is indicative of a large increase in polarity in the excited state. Consistent with this deduction, the stronger the donor (XPT > XtBuCT > XCT), the greater the red shift of emission from nonpolar to polar solvent. Notably, the XPT emission exhibited a remarkably large Stokes shift from 214 nm in cyclohexane to 335 nm in acetone (524 and 645 nm emission relative to the 310 nm ππ* absorption) as a result of its strong intramolecular charge transfer (ICT) character. The excitation spectra of the compounds showed maxima at 313 nm

distance of 3.8−4.5 Å. The HOMO and LUMO orbital distributions, ΔEST, and oscillator strengths of these molecules are shown in Figure 2 and Table S1 of the Supporting Information (SI). The HOMO orbitals are localized over donor groups, and the LUMO orbitals are primarily distributed over the 2,4-diphenyl-1,3,5-triazine moiety. For XtBuCT, the LUMO orbitals have limited delocalization on to the phenyl ring of the xanthene backbone. The low computed HOMO/ LUMO orbital overlaps gave rise to small ΔEST values (1−8 meV), suggesting the prospects for rapid equilibration of the lowest triplet (T1) and singlet (S1) states. The low overlap leads to small oscillator strengths (0.00007−0.0005), however with suppression of competitive vibronic couplings facilitating nonradioactive decay28 highly efficient luminescence can be achieved. When ACRFLCN23 as a spiro-linked TADF example was calculated with TD-DFT (B3LYP,6-31G*), its ΔEST was 0.008 and f was 0.000. The result of a TD-DFT calculation of 4CzIPN9 as a typical covalently linked TADF showed that ΔEST was 0.128 and f was 0.064. Therefore, our molecules display an electronic energy tendency (smaller f and ΔEST) similar to the spiro-linked type TADF (ACRFLCN) as opposed to a classical TADF molecule (4CzIPN) as a result of the absence of the aromatic linkage between a donor and an acceptor. Synthesis and Characterization. Our 9,9-dimethylxanthene-based donor/acceptor chromophores were synthesized as shown in Scheme 1, and the details of all procedures and characterizations are given in the SI. The donor groups (phenothiazine for XPT, carbazole for XCT and 3,6-di-tertbutylcarbazole for XtBuCT) were installed on the 4-carbon position of xanthene via Ullmann reactions using an activated C


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Figure 4. (a) PL transient spectrum of XPT in toluene under saturated oxygen and saturated nitrogen at room temperature; concentration of XPT was 1 × 10−4, λex = 336 nm (b) PL spectra of XPT in THF/water mixture and the change of normalized PL peak intensity with different water fractions; concentration of XPT was 1 × 10−5 M, λex = 320 nm; inset: PL images of XPT with different water fractions under 365 nm UV light.

crystal packing of XPT suggests two possible modes of inter/ intra molecular interactions including intramolecular π−π and intermolecular CH···π interactions (Figure S9). The XPT has an intramolecular π−π interaction between the donor and acceptor at 3.3−3.5 Å, which allows for efficient CT without producing a strong ground state π interaction. Additionally, intermolecular CH···π interactions are observed in a unit cell at the distance of 2.7−2.9 Å that stabilize the crystal packing and restrict the intramolecular motion giving rise to AIE. The crystal packing structures of XCT and XtBuCT also showed intermolecular CH···π interactions that can assist in creating AIE behavior (Figures S10 and S11). The thermal properties of XPT, XCT, and XtBuCT were investigated by TGA and DSC (Figures S12 and S17). Thermal stability is necessary for creating stable films by chemical vapor deposition, and 5% weight loss was measured at temperatures between 310 and 320 °C and the materials displayed glass transition temperatures (Tg) between 101 and 132 °C. XtBuCT showed the highest Tg, which is attributed to the di-tert-butyl group. Cyclic voltammetry was performed to determine the relative oxidative energies of our xanthene TDAF materials (Figure S18), and the results are summarized in Table 1 and Table S3. Upon the basis of the oxidative waves in anodic sweep, the electrochemical HOMO levels are estimated to be −4.99 eV, −5.58 eV, and −5.49 eV for XPT, XCT, and XtBuCT, respectively. HOMO levels exhibited a decreasing trend (XCT > XtBuCT > XPT) which is consistent with the electrondonating characteristics (phenothiazine > di-tert-butylcarbazole > carbazole). XCT showed irreversible oxidative waves and was less stable than XtBuCT and XPT, which is expected as a result of oxidative coupling at the 3- and 6-positions that are para to the nitrogen atom of the carbazole.43 The optical bandgaps (Eg) were determined from the onset of the absorption band (Figure S19) in CH2Cl2 to be 2.77, 3.19, and 3.16 eV, and LUMO energy levels were then estimated from HOMO − Eg to be 2.22, 2.39, and 2.33 eV for XPT, XCT, and XtBuCT respectively. Through-Space Charge-Transfer. There have been previous efforts to create D−A exciplex TADF emitters (Figure 1b).8,44−47 However, these D−A exciplex structures typically display lower EQEs than the twisted D−A single TADF molecules. These systems have the advantage of very small ΔEST (usually less than 50 meV), however coevaporation of donor and acceptor molecules leads to a distribution of structures with uncontrolled intermolecular interactions and

for XPT, 306 nm for XCT, and 317 nm for XtBuCT in toluene. Conversely, the thin film forms gave less defined excitation spectra with larger intensity at lower wavelengths, which represents a composite of direct excitation and energy transfer at lower wavelengths (Figure S4). To probe the anticipated small energy gap of the T1 and S1 levels, photoluminescence quantum yields (PLQYs) and excited state lifetimes (τ) were measured in the presence and absence of triplet-quenching oxygen in toluene (Table 1, Figure 4a and Figure S5). When oxygen is excluded the PLQYs increased with XPT going from 1.0% → 7.7%, XCT increasing from 2.1% → 5.9%, and XtBuCT rising from 1.2% → 6.0%. Additionally, we observed higher quantum yields with XPT and XtBuCT from 7.7% to 65% and 6.0% to 35% in nitrogenbubbled toluene and in thin film under nitrogen, respectively. Under nitrogen, XPT displayed distinctive delayed (τd) 2.3 μs relaxation in addition to a prompt (τp) 2.8 ns relaxation. Similar relaxations were observed for XCT (τp = 1.1 ns, τd = 3.0 μs) and XtBuCT (τp = 4.0 ns, τd = 2.0 μs). After bubbling with oxygen, the delayed components were substantially decreased or not detectable. The delayed fluorescence was also observed in the solid state (τd = 3.3 μs for XPT, τd = 4.1 μs for XtBuCT) (Figure S6). These results with prompt and delayed emissive components are consistent with TADF behavior. We find that our xanthene with donor−acceptor substitution exhibited lower PLQY in solution, but showed an increase emission efficiency in solid state, a process that is known as aggregation induced emission (AIE). The AIE phenomenon was reported by Tang et al. in 2001,31 and is a highly desirable property in OLEDs because it promises devices that are much less sensitive to the precise concentration of dopants in the emissive layer.32−39 To investigate AIE in these materials, water is added to THF solutions as indicated in Figure 4b. For XPT, a relative PL intensity was 0.05 in THF, then the intensity of the PL was reduced and red-shifted at ∼50% by weight water (f w). At this stage XPT is still dissolved, and an increase in the charge transfer character lowers the quantum yield. Increasing f w to 80% produced a dramatic increase in a blue-shifted PL and for higher f w’s the PL continued to increase with it being 20 times higher than the initial THF solution (f w = 99). At such large water contents nanoclusters with less polar environments form and the emission blue-shifts and the restricted motion lowers the dynamics of the system thereby attenuating competing nonradiative transitions.40,41 Similar but less dramatic AIE behaviors (Figures S7 and S8) were observed for XCT and XtBuCT that contain higher rigidity carbazole rings.42 The D


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Figure 5. (a) OLEDs device structure, (b) current density−voltage−luminance characteristics of the devices, (c) EQEs of the OLEDs as a function of current density, and (d) EL spectrum of the devices at 1 mA/cm2.

maximum at 445 nm which is assignable to a 9PPT (D) emission. Conversely, spin-coated thin films of 1:1 9PPT (D) and TPTr (A) have a PL maximum at 552 nm with a shoulder at 445 nm, which are attributable to CT exciplex emission (1CT) and localized donor emission (1LE) respectively (Figure S24). The same behavior was observed for XCT (D-b-A) in comparison to solutions and thin films containing mixtures of 9PCz (D) and TPTr (A) (Figure S25). This behavior reveals (Figure S26) that only the 9,9-dimethylxanthene bridged chromophores display CT emission independent of the concentrations and solvent. Without the bridge, exciplexes are not formed between model D and A units in solution and the model D and A units are not capable of CT emissions. In thin films, the donor and acceptor constituents form exciplexes but the organization is not sufficient to prevent some residual emission from the D constituents. Mechanochromism. XPT displays notable mechanochromic properties similar to what has been observed for other AIE materials48 and some other D−A type chromophores39,49,50 (Figure S27). A single crystal of XPT obtained from vapor diffusion (pentane, in the presence of 1,2-dichlorobenzene vapor) technique exhibited green PL at 524 nm. However, crystals produced from solvent evaporation gave a slightly redshifted emission maximum at 536 nm. Upon grinding with a pestle and mortar the emission and excitation spectra changed, and the emission became yellow with a maximum PL at 569 nm (Figure S28a). The 536 nm green emission was restored by directing CH2Cl2 vapor at the sample or heating over the Tg temperatures previously determined. These processes are all reversible and the samples can be subjected to multiple cycles. Sublimated or spin-coated films of XPT display emission

low EQEs. The key element of our design is to produce excited state through-space charge-transfer (CT) in a single molecule. To probe the importance of the intermolecular character we conducted our control experiments using different architectures. Specifically, we compared bridged donor−acceptor compounds (D-b-A) XPT and XCT with mixtures of model donors (D) 9-phenylphenothiazine (9PPT) or 9-phenylcarbazone (9PCz) and the model acceptor (A) 1,3,5-triphenl2,4,6-triazine (TPTr). We have further studied the bridged donor−donor (D-b-D) compound XP2 (Figure S20). The absorption spectra of XPT, XP2, 9PPT, and TPTr are shown in Figure S21. XP2 and 9PPT exhibited similar absorption spectra and had absorption maxima at around 320 nm, which are attributable to the ππ* transition of the phenothiazine group. TPTr did not have a specific absorption over this region. Alternatively, XPT showed a distinctive broad absorption band around 300−330 nm, which is attributable to a D−A interaction. XP2 exhibited PL around 445 nm in various polar environments, and as expected for the D-b-D structure, it did not show solvatochromism (Figure S22). PLs of XPT (D-bA) and XCT (D-b-A), XP2 (D-b-D), 9PPT (D), and 9PCz (D) in toluene and thin film are shown in Figure S23. In toluene, XPT (D-b-A) had PL with a maximum at 562 nm, which is attributable to CT emission and more red-shifting than PL of XP2 (D-b-D) or 9PPT (D) with a maximum at 445 nm. TPTr (A) is not emissive, and is therefore absent in these spectra. In thin films, similar behavior is observed, however, XP2 (D-b-D) is less emissive in thin films than in solution and displayed a more red-shifted PL at 487 nm than observed for 9PPT (D) suggesting aggregated-induced quenching occurs. A 1:1 mixture of 9PPT (D) and TPTr (A) in toluene displays an emission E


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confirming that we are extracting light from both triplet to singlet excitons generated by charge recombination in the OLED devices. The molecular architectures reported provide a promising design for the development of further AIDF materials.

spectra similar to that of the ground solid (λem = 563−569 nm) (Figure S28b) and after exposure to CH2Cl2 vapor or heating over Tg, the green emission at 536 nm was generated. Upon the basis of this behavior the green emission appears more stable and DSC profiles of two different states are shown in Figure S29. Clearly, the green material is a single phase and shows only a melting transition. Upon initial heating, the yellow ground phases have a metastable state that undergoes an exothermic crystallization transition at about 130 °C in the first heating, which is associated with the transition to the green emitting phase. Once thermally equilibrated both materials have the same melting points. Powder XRD did not reveal any new sharp reflexes for the yellow solid that were not present in the green solid (Figure S30). Hence, our DSC and XRD investigations suggest that the yellow metastable phases are disordered or amorphous. Using ground XPT, reversible drawing and erasing can be readily performed by placing materials on a glass substrate (Figure S31). The yellow “MIT” and face images are written with pressure. Upon heating, the yellow “MIT” characters are completely erased and this reversible process can be conducted in succession many times. Device Fabrication and Evaluation. XPT and XtBuCT were used as emissive dopants in OLED devices created by thermal evaporation with the following architecture (Figure 5a): ITO (100 nm)/1,1-bis[4-[N,N′-di(p-tolyl)amino]-phenyl] cyclohexane (TAPC) (70 nm)/oxybis(2,1-phenylene))bis(diphenylphosphine oxide) (DPEPO): Dopant (10%) (30 nm)/DPEPO (2 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPb) (45 nm)/LiF (1 nm)/Al (100 nm). Figure 5b shows the current density−voltage−luminance characteristics of the devices. The XPT device showed lower turn-on voltage than the XtBuCT device as a result of its lower HOMO− LUMO gap. The electroluminescence (EL) spectra (Figure 5d) display peaks at 584 nm for XPT and 488 nm for XtBuCT devices that are red-shifted relative to the PL determined in toluene solution. Considering the solvatochromic properties of these compounds, this is likely caused by the polar nature of DPEPO. As shown in Figure 5c, the maximum EQE of XPT devices was 10%, which exceeds the theoretical limit of 6% expected for a simple fluorescent OLED. Therefore, we can conclude that XPT is efficiently converting triplet excitons into emission through the singlet pathway by the TADF mechanism. The maximum EQE of XtBuCT device was 4% and is likely limited by its lower PLQY of 35%, which is almost half of XPT (66%).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00873. Additional experimental synthesis, photophysical, electrochemical, thermal properties, computational data, XRD data, and device fabrication details (PDF) Crystallographic data for XPT (CIF) Crystallographic data for XCT (CIF) Crystallographic data for XtBuCT (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Timothy M. Swager: 0000-0002-3577-0510 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ms. Li Li (MIT) is acknowledged for mass spectrometry measurements. Dr. Peter Müller (MIT) and Dr. Jonathan Becker (MIT) are acknowledged for X-ray crystal measurements and powder XRD measurements. We are grateful to Samsung for financial support of this research. G.M. thanks the German Academic Exchange Service (DAAD) for a postdoctoral fellowship.

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REFERENCES

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CONCLUSIONS In summary, we have designed and synthesized three 9,9dimethylxanthene bridged D−A molecules bearing phenothiazine, carbazole or 3,6-di-tert-butylcarbazole as donor groups. The rigid placement of a donor and an acceptor into a cofacial arrangement at a distance of 3.3−3.5 Å produces quantitative formation of a charge transfer excimer structure. These “Ushaped molecules” exhibited delayed fluorescence in the absence of triplet-quenching oxygen in both solution and solid states, and hence, are characterized as TADF materials. These materials also showed enhanced quantum yields in the solid state and also displayed AIE behavior. The crystal structure analysis suggests that CH···π interactions promote a rigid environment that decreases nonradiative deactivation. The OLED devices using XPT as the emitter displayed electroluminescence with a 10% EQE, which was higher than the theoretical limit of simple fluorescent OLEDs, thereby F


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