Fluorinated dibenzo[ a , c ]-phenazine-based green to red thermally activated delayed fluorescent OLED emitters

Purely organic thermally activated delayed fluorescence (TADF) emitting materials for organic light-emitting diodes (OLEDs) enable a facile method to modulate the emission color through judicious choice of donor and acceptor units. Amongst purely organic TADF emitters, the development of TADF molecules that emit at longer wavelengths and produce high-eﬃciency devices that show low eﬃciency roll-oﬀ remains a challenge. We report a modular synthesis route that delivers three structurally related fluorinated dibenzo[ a , c ]-phenazine-based TADF molecules, each bearing two donor moieties with diﬀerent electron-donating strengths, namely 3,6-bis(3,6-di- tert -butyl-9 H -carbazol-9-yl)-10-fluorodi-benzo[ a , c ]phenazine ( 2DTCz-BP-F ), 3,6-bis(9,9-dimethylacridin-10(9 H )-yl)-10-fluorodibenzo[ a , c ]-phenazine ( 2DMAC-BP-F ) and 10,10’-(10-fluorodibenzo[ a , c ]phenazine-3,6-diyl)bis(10 H -phenoxazine) ( 2PXZ-BP-F ). They exhibit donor strength-controlled color-tuning over a wide color range from green to deep-red with photoluminescence maxima, l PL , of 505 nm, 589 nm, and 674 nm in toluene solution. OLED devices using these TADF materials showed excellent to moderate performance with an EQE max of 21.8% in the case of 2DMAC-BP-F , 12.4% for 2PXZ-BP-F and 2.1% with 2DTCZ-BP-F , and associated electroluminescence (EL) emission maxima, l EL , of 585 nm, 605 nm and 518 nm in an mCBP host, respectively.


Introduction
Among the emitting materials for use in organic light-emitting diodes (OLEDs), purely organic thermally activated delayed fluorescence (TADF) emitters have drawn intense interest in recent years as they enable devices to reach a theoretical internal quantum efficiency (IQE) of 100%. This is possible through efficient harvesting of both singlet and triplet excitons to produce light, the latter of which are converted to the former via reverse intersystem crossing (RISC). Organic TADF emitters do not contain scarce, noble metals that are extracted through environmentally damaging mining operations. Swift progress has been reported in the development of purely organic TADF emitters and now there are numerous examples of TADF OLEDs showing comparable efficiencies to phosphorescent devices. 1,2 RISC at ambient temperatures occurs in organic compounds that possess a small energy gap, DE ST , between the lowest-lying singlet state S 1 and triplet state T 1 , and show non-zero spinorbit coupling (SOC). 3 For this scenario to occur, there must be a spatial separation of the electron-donating unit accommodating the highest-occupied molecular orbital (HOMO) and the electron-accepting unit hosting the lowest-unoccupied molecular orbital (LUMO). 4 The implementation of this donor-acceptor molecular design produces a strong charge-transfer (CT) character of the S 1 state. 3 The design of TADF materials that emit at longer wavelengths poses some unique challenges for maintaining a high photoluminescence quantum yield (F PL ). The F PL is dependent on the rate constant of radiative decay processes such as fluorescence, but also nonradiative decay processes such as internal conversion (IC) and intersystem crossing (ISC). 5 In large, aromatic molecules, where the electronic relaxation lies within the rule of a weak coupling limit as reported by Englman and Jortner, 6 the rate constant of the nonradiative decay, k nr , is inversely proportional to the exponential of the optical energy gap DE opt . In contrast, the rate constant of the radiative decay, k r , is proportional to the cube of DE opt . 5,7,8 As the energy of the emissive excited state decreases, the influence of nonradiative decay increases exponentially because the vibronic coupling between the excited state and ground state is facilitated. The challenge of reducing losses due to vibrational quenching and other nonradiative decay pathways in TADF molecules emitting at longer wavelengths can be partially addressed by introducing rigidity into the molecular structure of the donor and acceptor units. Common acceptors for purely organic TADF emitters are aromatic ketones such as anthraquinones, naphthalimides, or heteroaromatic systems like quinoxaline and dibenzo[a,c]phenazine (BP). 9 These acceptors show deep LUMO levels of À3.4 eV, 10 À2.99 eV, 11 À2.81 eV, 12 and À2.90 eV, 13 respectively, that contribute to stabilizing the S 1 state and are therefore beneficial for use in the design of TADF emitters targeting longer wavelength regions.
Zhao and co-workers first reported TADF compounds bearing the BP acceptor, which exhibits a rigid, large p-conjugated system. 14 These compounds contain one to three donor moieties in the donor-acceptor or poly(donor)-acceptor strategy, which are commonly applied for TADF molecule design. The greater number of 9,9-dimethyl-9,10-dihydroacridine (DMAC) donors was expected to strengthen the intramolecular charge transfer (ICT) and lead to color-tuning from green to orange-red emission with electroluminescence maxima, l EL , of 560 nm, 576 nm, and 606 nm for devices featuring 1DMAC-BP, 2DMAC-BP, and 3DMAC-BP, respectively (Fig. 1). A maximum external quantum efficiency (EQE max ) of 22.0% was observed for the OLED device with 3DMAC-BP doped in mCBP (18 wt%) at 606 nm. By employing the stronger donor 10H-phenoxazine (PXZ), the l EL for the devices with 1PXZ-BP, 2PXZ-BP, and 3PXZ-BP were red-shifted to 590 nm, 606 nm, and 634 nm, respectively. The most efficient device with 1PXZ-BP as the emitter showed an EQE max of 26.3% (7 wt% doped in CBP). 13 Both Lee and coworkers as well as Wang and coworkers, have reported fluorosubstituted BP acceptors intending to strengthen the acceptor with the presence of the strongly inductively electron-withdrawing fluorine substituent. 15,16 Lee and coworkers reported the use of a fluorine substituent at the acceptor moiety in the ortho-position (FBPCNAc, Fig. 1) to the donor moiety. FBPCNAc is brightly luminescent in 1 wt% doped polystyrene film with l PL = 607 nm, a F PL of 79% and a delayed lifetime of t d = 11.1 ms. In the electroluminescent (EL) device, it showed an emission maximum of l EL = 597 nm, an EQE max of 23.8% and low efficiency roll-off. Wang and coworkers on the other hand, attached two fluorine substituents in 11-and 12-position to the BP acceptor on the opposite side of the donor moieties in 3-and 6-position (TAT-FDBPZ, Fig. 1) and observed that the introduction of the fluorine substituents led to a stronger ICT state and a red-shifted emission. The emission of the 20 wt% doped CBP films of the fluorinated TAT-FDBPZ is bathochromically shifted from l PL = 593 nm by 17 nm in comparison to its nonfluorinated analogue (l PL = 576 nm). However, this came at the cost of a slightly decreased F PL from 76% to 62%. The t d of TAT-FDBPZ is 1.51 ms, which is shorter than for the nonfluorinated analog where t d = 2.30 ms. The device based on TAT-FDBPZ showed l EL of 611 nm and an EQE max of 9.2%.
The three emitters were purified further by gradienttemperature sublimation. The chemical structure and purity of the three compounds were confirmed using 1 H, 13 C, and 19 F nuclear magnetic resonance (NMR) spectroscopy, highresolution mass spectrometry (HRMS), infrared spectroscopy, melting point analysis, and elemental analysis (EA). A single crystal suitable for X-ray diffraction analysis was obtained for 2DMAC-BP-F by evaporating a solution in deuterated benzene in an NMR tube at room temperature ( Fig. 2a). Analysis of the crystal structure of 2DMAC-BP-F showed that the fluorine atom is disordered about a mirror plane. The DMAC donor units are strongly twisted with a dihedral angle of 651 to the BP acceptor. The DMAC donors display an almost planar conformation with the quaternary carbon being pushed out of the plane by 0.11 Å while the two benzene rings of the DMAC are tilted towards each other by 41. Crystals of 2DTCz-BP-F were obtained by evaporation of a solution in deuterated chloroform in an NMR tube at room temperature (Fig. 2b). Four crystallographically independent molecules were found with the fluorine atoms disordered about a mirror plane and the donor units strongly twisted with an average dihedral angle of 46.61. Crystallographic data of both molecules are quoted in the ESI † (Table S1).

Theoretical calculations
The ground-state geometries of 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F were optimized using density functional theory (DFT) at the PBE0/6-31G(d,p) level of theory in the gas phase. 17,18 The excited state properties were calculated by time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation (TDA-DFT) based on the optimized ground-state geometries. 19 The calculated energy levels of the highest occupied molecular orbits (HOMOs) and lowest unoccupied molecular orbits (LUMOs) are presented in Fig. 3, and the results are summarized in Table S2 (ESI †). The dihedral angles between the donor and acceptor moieties were found to be around 48.61 and 47.61 for 2DTCz-BP-F, 87.91, and 90.11 for 2DMAC-BP-F and 85.91 and 72.21 for 2PXZ-BP-F, respectively. In comparison with the value obtained from the crystal structure, the dihedral angle between the DMAC and the BP-F groups in 2DMAC-BP-F (651) was found to be smaller than that theoretically calculated, while the average dihedral angle between the DTCz and the BP-F groups in 2DTCz-BP-F (46.61) was in good accordance with the calculated value. Due to the almost orthogonal conformations of 2DMAC-BP-F and 2PXZ-BP-F, the HOMO and LUMO distributions are localized on the donor and acceptor moieties, respectively, in both molecules, which results in small DE ST . The LUMOs of all three compounds are distributed over the BP-F acceptor core, while the HOMOs are

Electrochemistry
Cyclic voltammetry (CV) was performed to determine the HOMO and LUMO levels of the emitters. The oxidation and reduction potentials of the emitters were evaluated in Arsaturated dichloromethane (DCM) solution with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The values are reported versus standard calomel electrode (SCE). The results obtained from the CV measurements are summarized in Table 1.
As shown in Fig. 4, all three compounds show reversible oxidation and reduction processes. The main oxidation waves occur at 0.80 V, 1.00 V, and 1.32 V for 2PXZ-BP-F, 2DMAC-BP-F, and 2DTCz-BP-F, respectively. These are each assigned to the oxidation of PXZ, DMAC, and DTCz, and reflect the relative strength of the donors. 2DMAC-BP-F shows an additional minor oxidation wave at 0.77 V, which is characteristic of the redox behavior of DMAC-containing compounds. 24 The respective HOMO levels are À5.14 eV, À5.34 eV, and À5.66 eV for 2PXZ-BP-F, 2DMAC-BP-F, and 2DTCz-BP-F. The reduction waves occur at very similar potentials of À1.21 V, À1.19 V, À1.18 V for 2PXZ-BP-F, 2DMAC-BP-F, and 2DTCz-BP-F, respectively, and indicate that the electronic coupling between the donor and acceptor moiety is small.
The corresponding redox gaps, DE H-L , decrease from 2.50 V to 2.20 V and 2.01 V for 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F, respectively matches the HOMO-LUMO gap trend predicted by DFT calculations.

Photophysical properties
The UV-vis absorption spectra of the three emitters in dilute toluene are shown in Fig. 5a, and the photophysical properties are summarized in Table 2. All three compounds exhibit strong absorption bands at around 310 nm, which can be attributed to locally excited (LE) p-p* transitions of the donors and BP-F  moieties, respectively. [24][25][26] Weaker and broad absorption bands are observed from 410 to 520 nm, which are assigned to ICT transitions from the donor units to the acceptor core. 27 This latter band is more intense for 2DTCz-BP-F (l abs = 440 nm, 21 Â 10 3 M À1 cm À1 ) compared to those of 2DMAC-BP-F (l abs = 415 nm, 3 Â 10 3 M À1 cm À1 ) and 2PXZ-BP-F (l abs = 476 nm, 3 Â 10 3 M À1 cm À1 ) as the DTCz groups adopt a less twisted conformation, leading to greater conjugation and greater oscillator strength for the ICT transitions in 2DTCz-BP-F, values that are corroborated by the DFT calculations ( Fig. S12-S17, ESI †).
There is the expected shift to lower energies of the ICT band across the family of compounds that is aligned with increasing donor strength. All compounds exhibit unstructured and broad PL spectra in toluene (Fig. 5a), indicative of an excited state with strong ICT character, with peak maxima, l PL , at 505 nm, 589 nm, and 674 nm for 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F, respectively. Positive solvatochromism is observed for all compounds (Fig. 5a and Table S3, ESI †), which is consistent with the CT nature of the emissive excited state. The optical bandgaps, E g , calculated from the normalized absorption and emission spectra intersection point, are 2.60 eV, 2.32 eV, and 2.13 eV for 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F, respectively. Except for 2DTCz-BP-F (E g = 2.60 eV vs. E S1 theory = 2.73 eV), experimental E g for 2DMAC-BP-F (E g = 2.32 eV vs. E S1 theory = 2.16 eV) and 2PXZ-BP-F (E g = 2.13 eV vs. E S1 theory = 1.89 eV) were found to be larger than those calculated. The photoluminescence quantum yields, F PL , in degassed toluene solution of 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F are 51%, 30%, and 8%, respectively. These dropped to 49%, 21%, and 6% upon exposure to oxygen ( Table 2). The prompt fluorescence and phosphorescence spectra of all compounds in 2-MeTHF at 77 K were measured to determine the S 1 and T 1 energies from their respective onsets ( Fig. 5b and Fig. S18b, ESI, † Table 2 Table 2). The delayed emission is strongly quenched upon the exposure of oxygen, indicating accessible triplet states. The ICT band of 2DTCz-BP-F decays monoexponentially with t p of 6.6 ns, no delayed emission is observed for this compound.
We next measured the photophysical properties of all three compounds in an OLED-relevant host 3,3 0 -di(9H-carbazol-9-yl)-1,1 0 -biphenyl (mCBP) as this host matrix has sufficiently high triplet energy (T 1 = 2.84 eV) to confine the excitons onto the emitter. 28 The dopant concentration was varied from 1-10 wt% in doped film to optimize the F PL (Table S5, ESI †). The F PL of the 5 wt% 2DTCz-BP-F in mCBP doped film is 59.7% at l PL of 522 nm; 10 wt% 2DMAC-BP-F in mCBP doped film is 78.0% at l PL of 584 nm, and 1.5 wt% 2PXZ-BP-F in mCBP doped film is 58.0% at l PL of 611 nm under an N 2 atmosphere (Table 2). These F PL values decreased in air to 46.8% for 2DTCz-BP-F, 48.3% for 2DMAC-BP-F, and 47.3% for 2PXZ-BP-F. All three compounds show unstructured CT-based emission in mCBP doped film at room temperature, shown in Fig. S22a-c (ESI †). As shown in Fig. 6, all three compounds showed multiexponential decay kinetics with average prompt fluorescence lifetimes, average t p , of 4.3 ns, 19.6 ns, and 31.0 ns and average delayed emission lifetimes, average t d , of 10.15 ms, 90.6 ms and 1.83 ms at room temperature for 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F, respectively. The corresponding rate constants of intersystem crossing (k ISC ) for the three compounds in mCBP films are 1.09 Â 10 8 s À1 , 2.45 Â 10 7 s À1 , 1.52 Â 10 7 s À1 for 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F, respectively. The rate constants of reverse intersystem crossing (k RISC ) in mCBP films for 2PXZ-BP-F reached 2.41 Â 10 5 s À1 , a value much faster than 2DTCz-BP-F of 5.14 Â 10 1 s À1 , and 2DMAC-BP-F of 1.33 Â 10 4 s À1 , respectively. The relative intensities of the delayed PL increased with increasing temperature from 100 K to 300 K, thereby corroborating the TADF nature of the emission of these three compounds in the mCBP films. The extremely long lifetime and sharp decrease of the emission intensity at low temperature for the 2DTCz-BP-F doped mCBP film can be explained by the large DE ST (vide infra) and inefficient TADF.
There is an expectedly large DE ST of 0.30 eV for 2DTCz-BP-F, while the DE ST for 2DMAC-BP-F and 2PXZ-BP-F is much smaller at 0.11 eV, and 0.02 eV, respectively. The S 1 level of 2DTCz-BP-F in mCBP doped film (S 1 = 2.54 eV) is similar to the S 1 level of 2DTCz-BP-F in PMMA doped film (S 1 = 2.56 eV) and very close to the energy level of 2DTCz-BP-F in 2-MeTHF glass (S 1 = 2.64 eV), all of which indicates that the S 1 state in 2DTCz-BP-F is of mixed 1 LE and 1 CT character. The structured phosphorescence and triplet energy level of 2DTCz-BP-F does not change in different media such as PMMA (T 1 = 2.22 eV, Fig. S22d, ESI †), mCBP (T 1 = 2.24 eV, Fig. S22a, ESI †), and in 2-MeTHF glass (T 1 = 2.21 eV, Fig. 5b). Furthermore, these values match with the phosphorescence of the F-BP acceptor (2.26 eV) in 2-MeTHF glass (Fig. S18a, ESI †) and imply that the T 1 level of 2DTCz-BP-F has 3 LE character. The calculated DE ST value of 2DTCz-BP-F in PMMA is 0.34 eV and 0.30 eV in mCBP, values that render TADF inefficient. The weak emission band at 522 nm in the millisecond timescale spectra of 2DTCz-BP-F in mCBP and PMMA may be due to residual delayed fluorescence.
The S 1 levels of the other two emitters 2DMAC-BP-F and 2PXZ-BP-F in PMMA are 2.56 eV and 2.34 eV, which are significantly blue-shifted in comparison to those in mCBP doped films, for which the S 1 level for 2DMAC-BP-F is 2.40 eV, and for 2PXZ-BP-F is 2.22 eV. The T 1 level of 2DMAC-BP-F is 2.29 eV, very similar to that in PMMA (2.28 eV for 2DMAC-BP-F) and a value that aligns with the T 1 level of the BP-F acceptor in 2-MeTHF (2.26 eV, Fig. S18a, ESI †). However, the T 1 level of 2PXZ-BP-F in mCBP is 2.20 eV, which is stabilized from the value measured in PMMA at 2.28 eV (Fig. S22f, ESI † and Table 2). These results reveal that the T 1 state in 2DMAC-BP-F possesses dominant 3 LE character, while 2PXZ-BP-F shows mainly 3 CT character. The estimated DE ST of 2DMAC-BP-F is 0.11 eV, and of 2PXZ-BP-F is 0.02 eV in mCBP, while the DE ST in PMMA of 2DMAC-BP-F is 0.28 eV, and of 2PXZ-BP-F is 0.06 eV, which is suitably small for harvesting triplet excitons. Indeed, the small DE ST of 2DMAC-BP-F and 2PXZ-BP-F is a sign of an efficient TADF emitter for OLEDs.

Device characterization
OLED devices based on 2PXZ-BP-F, 2DMAC-BP-F and 2DTCz-BP-F were fabricated by vacuum deposition following a typical bottom-emitting OLED device architecture (Fig. 7a) that consists of indium tin oxide (ITO) /1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) (5 nm)/N,N 0 -di(1-naphthyl)-N,N 0diphenyl-(1,1 0 -biphenyl)-4,4 0 -diamine (NPB) (40 nm)/tris(4carbazoyl-9-ylphenyl)amine (TCTA) (10 nm)/emissive layer (20 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) (40 nm)/ LiF (0.6 nm)/Al (100 nm), where HATCN, NPB and TCTA play the role of hole injection layer (HIL), hole transportation layer (HTL) and electron blocker layer (EBL), respectively. TmPyPB acts both as electron transport layer (ETL) and hole blocking layer due to its deep HOMO (À6.7 eV), 29 and LiF acts as an electron injection layer (EIL). The molecular structures of the materials used in these OLEDs are shown in Fig. 7b. The emission layer (EML) comprises 1.5 wt% of 2PXZ-BP-F, 10 wt% 2DMAC-BP-F, or 5 wt% of 2DTCz-BP-F doped into mCBP, based on the doping study discussed above (Table S5, ESI †). The performance of the OLEDs is summarized in Table 3. Current density-voltage-brightness ( J-V-L) curves, EQE-luminance curves, and electroluminescence spectra (EL) are given in Fig. 7c. As shown in Fig. 7e, each EL spectrum is similar to that of the corresponding PL spectrum in the thin film with EL maxima, l EL , at 605 nm for 2PXZ-BP-F, 585 nm for 2DMAC-BP-F, and 518 nm for 2DTCz-BP-F. Similar to that observed by PL, the trend in emission energy follows that of increasing donor strength. The corresponding CIE coordinates are (0.55, 0.44), (0.51, 0.48) and (0.29, 0.58) for the devices with 2PXZ-BP-F, 2DMAC-BP-F and 2DTCz-BP-F, respectively. The turn-on voltage of the devices lies between 3.3 V to 3.7 V and is dependent on the energy gap between the HOMO of materials used in HTL and EML layers. The 2DMAC-BP-F based device showed the best overall performance with the highest maximum external quantum efficiency (EQE max ) of 21.8%, a maximum current efficiency (CE max ) of 59.7 cd A À1 , and maximum power efficiency (PE max ) of 55.4 lm W À1 (Table 3, Fig. 7d and Fig. S23 and S24, ESI †). The EQE max of the 2PXZ-BP-F-based device is 12.4% with CE max = 26.3 cd A À1 and PE max = 23.0 lm W À1 . The 2PXZ-BP-F-based device showed moderate roll-off efficiency, with the EQE at 100 cd m À2 at 9.3% and the EQE at 1000 cd m À2 at 6.3%. The 2DMAC-BP-F-based device, however, showed higher efficiency roll-off, with an EQE at 100 cd m À2 of 8.7% and an where A i is the preexponential for lifetime t i ). Prompt and delayed emissions were measured by TCSPC and MCS, respectively (l exc = 343 nm). e Photoluminescence quantum yields of thin films were determined using an integrating sphere (l exc = 305 nm or 340 nm) under N 2 atmosphere at 298 K. Values quoted inside the parentheses are in the presence of O 2 . EQE of 1000 cd m À2 at 3.3%. The maximum brightness of the 2PXZ-BP-F-based device reached 12 350 cd m À2 at an EQE of 2.3%. The relatively low efficiency roll-off in 2PXZ-BP-F originates in part from the low triplet exciton concentration due to the relatively short delayed lifetime (t d = 1.83 ms). 30,31 Notably, the 2PXZ-BP-Fbased device reached an EQE of 2.5% at 10 000 cd m À2 with an emission wavelength beyond 600 nm. Although 2DTCz-BP-F shows a high F PL of ca. 60% in the 5 wt% doped in mCBP, the device exhibits a low EQE max of 2.1%. As a result of the too high DE ST , the harvesting of triplet excitons in the 2DTCz-BP-F-based device is very inefficient as reflected in the very long delayed lifetime, which causes more triplet-triplet annihilation and triplet-polaron annihilation. Devices fabricated using MoO 3 as the HIL showed similar performance but reached lower luminance and low current density than the devices using HATCN (Table S6 and Fig. S25, ESI †).

Conclusions
This study reported a series of green-to-red-emitting fluorine-substituted dibenzo[a,c]phenazine-based (BP-F) TADF emitters. 2DTCz-BP-F, 2DMAC-BP-F, and 2PXZ-BP-F, which showed color tuning based on the choice of donor, emitting from green to deep-red.
The rigid and planar constituent groups with large steric hindrance between donor and acceptor units endow these emitters with high F PL vlues and suitably small DE ST . Among them, 2DMAC-BP-F exhibits the highest F PL , at 78%, a relatively small DE ST of 0.11 eV at 584 nm in 10 wt% doped mCBP, whereas 2PXZ-BP-F shows the smallest DE ST of 0.02 eV with shortest delay lifetime of 1.83 ms at 611 nm in 1.5 wt% doped mCBP. OLED devices using these TADF materials showed excellent performance with an EQE max of 21.8% in the case of 2DMAC-BP-F with l EL of 585 nm and 12.4% for 2PXZ-BP-F with l EL of 605 nm. The relatively low efficiency roll-off in 2PXZ-BP-F is due to the short delayed lifetime, making this material a very good TADF emitter for OLEDs in the family of devices that can reach brightness above 10 000 cd m À2 with an emission wavelength beyond 600 nm. These results demonstrate that simple modification of the BP acceptor with a fluorine substituent is an effective approach to design orange-red/red TADF emitters with devices that show high EQE and low-efficiency roll-off.