High‐Performance Orange–Red Organic Light‐Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl‐Containing Delayed Fluorescence Molecules

Abstract Developing orange to red purely organic luminescent materials having external quantum efficiencies (η exts) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (Φ PL), and large optical outcoupling efficiency (Φ out) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin‐11‐yl(phenyl)methanone, is created and employed for constructing orange–red delayed fluorescence molecules with various acridine‐based electron donors. The incorporation of benzoyl leads to red‐shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased Φ PLs, and the adoption of spiro‐structured acridine donors promotes horizontal dipole orientation and thus renders high Φ outs. Consequently, the state‐of‐the‐art orange–red organic light‐emitting diodes are achieved, providing record‐high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A−1, and 93.5 lm W‒1. By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.


Introduction
Organic light-emitting diodes (OLEDs) [1,2] have received continuous and intense research interests of academia and industry owing to their distinct merits that ensue promising applications in display and lighting devices. Maximizing device efficiencies, minimizing energy consumption, and reducing manufacturing cost are the fundamental prerequisites for massive commercialization of OLEDs, [3] which greatly rely on the exploration of robust light-emitting materials. Purely organic thermally activated delayed fluorescence (TADF) materials without any precious noble metals are currently in the spotlight because they are not only low-cost but also able to harvest both singlet and triplet excitons via reverse intersystem crossing (RISC) to boost electroluminescence (EL) efficiencies. [4][5][6][7] Constructing highly twisted electrondonor and acceptor (D-A) structures can bring about adequate separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), making energy splitting (∆E ST ) between the lowest singlet excited (S 1 ) state and the lowest triplet excited (T 1 ) state reduced to ensure rapid RISC and thus improve exciton utilization. [8][9][10][11][12] After a landmark breakthrough by Adachi's group, many efficient sky-blue, green, and yellow TADF materials have been developed thus far and their corresponding OLEDs have reached the highest external quantum efficiencies ( ext s) of around 38%. [13][14][15][16] In contrast, the device efficiencies of orange-tored TADF materials with EL peaks over 560 nm remain far behind. [17][18][19][20][21] In order to realize efficient long-wavelength EL emissions, strengthening intramolecular charge transfer (ICT) effect in a highly twisted D-A framework is widely used in the design of TADF materials. [22][23][24][25] But according to the Franck-Condon transition principle, [26] the separation of HOMOs and LUMOs as well as strong ICT effect often leads to low radiative decay rate and weak oscillator strength, thereby resulting in decreased photoluminescence (PL) quantum yields (Φ PL s). On the other hand, the horizontal dipole ratio (Θ // ) of the molecule should also be enhanced to break the limit of 20-30% optical outcoupling efficiency (Φ out ). [27,28] In fact, some orange-red TADF materials obtained by great efforts possess small ΔE ST s and high Φ PL s, but their EL efficiencies are less than satisfactory due to their low Θ // s, and vice versa. [29][30][31][32][33] It is virtually challenging to simultaneously achieve a small ∆E ST , a high Φ PL , and a large Φ out , especially for TADF materials with long-wavelength emissions. In consequence, there are only very limited reports on the efficient orange-red TADF OLEDs with highest ext s of around 30%. [3,[34][35][36][37] To break through the predicament, in this contribution, we propose a feasible molecular design for orange-red TADF materials and synthesize a series of tailor-made luminescent molecules DPPM-DMAC, DPPM-DPAC, DPPM-SFAC, DPPM-STAC, and DPPM-SXAC. In these molecules, various acridine derivatives 9,9-dimethyl-9,10-dihydroacridine (DMAC), 9,9-diphenyl-9,10dihydroacridine (DPAC), spiro[acridine-9,9'-fluorene] (SFAC), spiro[acridine-9,9'-thioxanthene] (STAC), and spiro[acridine-9,9'xanthene] (SXAC), are adopted as electron donors, in which the substituents at the 9-position of acridine are altered to optimize Θ // s. A large electron acceptor of dibenzo[a,c]phenazin-11-yl(phenyl)methanone (DPPM) is created by grafting benzoyl to dibenzo[a,c]phenazine (DP) to increase electron-withdrawing ability for generating longer-wavelength emissions. The presence of carbonyl group is also anticipated to enhance spin-orbit coupling (SOC) matrix elements [38][39][40] to facilitate RISC and benefit delayed fluorescence. For comparison, a control TADF molecule (DP-SXAC) containing a DP acceptor and SXAC donor is prepared and studied. The systematical experiments and theoretical calculations reveal that these DPPM-based molecules exhibit more efficient delayed fluorescence with longer emission wavelengths, higher Φ PL s, and shorter delayed lifetimes than DP-SXAC. The OLEDs employing DPPM-based molecules as emitters radiate orange-red lights and achieve a record-beating ext of 33.5%, much superior to that of DP-SXAC-based device (19.5%). The high Φ PL s, efficient RISC as well as large out s account for the outstanding EL performances of these DPPM-based molecules.

Result and Discussion
The molecular structures of DPPM-based molecules (DPPM-DMAC, DPPM-DPAC, DPPM-SFAC, DPPM-STAC, DPPM-SXAC), and the control molecule DP-SXAC without benzoyl are shown in Figure 1, and their detailed synthetic routes involving two steps are shown in Schemes S1 and S2 of the Supporting Information. Briefly, the key precursors DP-Br and DPPM-Br are synthesized by cyclization reactions of 3,6dibromo-phenanthrenequinone with o-phenylenediamine and (3,4-diaminophenyl)phenylmethanone, respectively, which are coupled with acridine-based donors via Buchwald-Hartwig C-N coupling reaction to produce target molecules in good yields. They are fully characterized by 1 H NMR, 13 C NMR, and highresolution mass spectrometry, and the results are in good agreement with the molecular structures. The detailed synthetic procedures and characterization data are described in the Supporting Information.
The thermal stability of these molecules is examined by thermogravimetric analysis. As shown in Figure S1 in the Supporting Information, they have excellent thermal stability with high decomposition temperatures (T d s, corresponding to 5% loss of initial weight) of 447°C for DPPM-DMAC, 493°C for DPPM-DPAC, 532°C for DPPM-SFAC, 542°C for DPPM-STAC, and 525°C for DPPM-SXAC. The T d s of DPPM-SFAC, DPPM-STAC, and DPPM-SXAC are obviously higher than those of DPPM-DMAC and DPPM-DPAC, indicating the spiro structure has positive effect on increasing thermal stability. Besides, the T d of DPPM-SXAC is also much higher than that of DP-SXAC (497°C), revealing that the introduction of benzoyl has improved the thermal stability of the molecule. On the other side, all of these molecules hold good electrochemical stability as well, as evidenced by the reversible oxidation and reduction processes in cyclic voltammetry measurement ( Figure S2, Supporting Information). According to the thresholds of the first oxidation and reduction waves, the HOMO and LUMO energy levels are calculated to be -5.28 and -3.46 eV for DPPM-DMAC, -5.42 and -3.55 eV for DPPM-DPAC, -5.36 and -3.51 eV for DPPM-SFAC, -5.41 and -3.51 eV for DPPM-STAC, -5.41 and -3.52 eV for DPPM-SXAC, and -5.41 and -3.24 eV for DP-SXAC. Compared with other molecules, DPPM-DMAC has a higher HOMO energy level owing to the stronger electron-donating ability of DMAC. [3,41] And the DPPMbased molecules exhibit lower LUMO energy levels than DP-SXAC, which is ascribed to the stronger electron-withdrawing ability of DPPM acceptor resulting from the additional carbonyl group.
To gain insights into the electronic structures, density functional theory calculation is carried out to simulate the optimized geometries at the ground state and frontier molecular orbital distributions. All the molecules adopt highly twisted geometries with large torsional angles over 80 o between donors and acceptors, facilitating the separation of HOMOs and LUMOs  and thus delay fluorescence. Due to the electron-withdrawing effect of carbonyl group, the HOMO and LUMO distributions become more separate in DPPM-SXAC than in DP-SXAC, as evidenced by the less electron cloud locating on the phenyl ring connected with acridine moiety, resulting in a much smaller ΔE ST (0.065 eV) of DPPM-SXAC than that of DP-SXAC (0.138 eV).
Moreover, the analysis on the hole and electron distributions of the excited states discloses that, for the S 1 state, the hole and electron are distributed separately on the donor and acceptor, respectively, indicative of the typical charge transfer (CT) state dominated by -* transition (Figure 2A). Different from the S 1 state, both hole and electron of the T 1 state are primarily centered on the acceptor, implying the predominant characteristic of localized excitation state governed by n-* and -* transition. The distinct difference in transition nature between S 1 and T 1 states is beneficial for the occurrence of RISC processes, rendering increased RISC rate. [22,[42][43][44] In contrast, the second triplet excited (T 2 ) and third triplet excited (T 3 ) states of both molecules are dominated by the CT characters. Theoretically, upconversion of CT-featured triplet states to CT-featured singlet states is difficult. [22] But T 2 and T 3 lie close to S 1 , with small energy level splitting and large SOC matrix elements between S 1 and T 2 , and S 1 and T 3 ( Figure 2B,C), suggesting these highly lying triplet excited states may contribute to spin-flip event as well. [45,46] Interestingly, the calculated SOC matrix element between S 1 and T 1 of DPPM-SXAC (0.082 cm -1 ) is larger than that of DP-SXAC (0.057 cm -1 ), benefiting from n-* transition of carbonyl group. [38][39][40] These results demonstrate the introduction of benzoyl can exert apparent positive effect on RISC by not only reducing ΔE ST but also enlarging SOC matrix element, leading to efficient delayed fluorescence with short delayed lifetimes. The promoted RISC is also favorable to suppress nonradiative decay of triplet excited states, and thus conducive to increasing PL efficiency.
DPPM-based molecules represent analogous absorption spectral profiles in dilute tetrahydrofuran (THF) solutions (Figure 3A). The intense absorption bands at around 400 nm are attributed to the -* transitions of the molecules, while the weak and broad absorption bands at about 450 nm are associated with ICT transitions from acridine-based donors to DPPM acceptor. They show orange to red PL emissions (568-603) with Φ PL s of 29-44% in dilute toluene solutions, which are greatly redshifted to 655-682 nm with decreased Φ PL s of 1-4% in THF solutions ( Figure S3 and Table S1, Supporting Information), due to strengthened ICT effect in polar solvent. On the contrary, DP-SXAC shows an absorption band at about 430 nm in THF from ICT transition, and greatly blue-shifted PL peaks at 532 in toluene and 598 nm in THF, due to its relatively weak ICT effect. On the other hand, the PL intensities in toluene can be apparently weakened by O 2 ( Figure S4, Supporting Information) . And the transient PL decay spectra of these molecules display obvious delayed components in O 2 -free toluene bubbled with N 2 , but the delayed components are reduced greatly in the presence of O 2 ( Figure S5, Supporting Information). These findings suggest that the triplet excited states are involved in PL emissions.
In THF solutions, DPPM-based molecules show faint PL emissions without distinct delayed fluorescence. By adding a poor solvent of water into their THF solutions, the PL peaks are progressively blue-shifted and PL intensities are enhanced greatly ( Figure 3C,D and Figure S6, Supporting Information). With the addition of a large amount of water, these luminogenic molecules form aggregates because of their hydrophobic nature, and the enhanced and blue-shifted PL emissions in aggregates are caused by the lowered matrix polarity and restricted intramolecular motions. [47][48][49] More interestingly, the PL lifetimes of these molecules get elongated apparently and the delayed components become more prominent at high water fractions, indicative of aggregation-induced delayed fluorescence phenomenon ( Figure 3E and Figure S7, Supporting Information), which is attributed to the promoted RISC by mainly impeding internal conversion of the excited states in condensed phase. [9] DPPM-based molecules show orange-red PL emissions with peaks at 557-587 nm in doped films with 4,4′-bis(carbazol-9yl)biphenyl (CBP) host, while DP-SXAC emits green PL emission peaking at 528 nm ( Figure 3B). DPPM-based molecules have similar Φ PL values of 86-94% in doped films, much higher than that of DP-SXAC (60%). The promoted RISC can reduce nonradiative decay of triplet excited states and thus could be accountable for the higher Φ PL of DPPM-SXAC than DP-SXAC. The ∆E ST s of DPPM-based molecules are calculated to be 0.06-0.25 eV from the onset of fluorescence and phosphorescence spectra at 77 K ( Figure S8, Supporting Information), smaller than that of DP-SXAC (0.34 eV). This finding is in good agreement with the calculation results. The transient PL decay spectra of these molecules in doped films display eminent double-exponential decay with prompt and delayed components ( Table 1 and Figure S9, Supporting Information). They possess better delayed fluorescence property than DP-SXAC. The delayed fluorescence lifetime of DPPM-SXAC (37 μs) is dramatically shorter than that of DP-SXAC (1991 μs), validating the significant positive impact of carbonyl group on the acceleration of RISC. The temperature-dependent transient PL decay spectra show that by increasing temperature from 77 to 300 K, the ratio of delayed fluorescence gradually rises ( Figure 3F and Figure S10 and Table S2, Supporting Information). The triplet excited states undergo RISC to up-convert to singlet excited states via thermal activation, verifying the TADF characteristics of these molecules.  The Θ // s of these molecules are investigated by angledependent p-polarized PL measurement. As shown in Figure 4, DPPM-STAC, DPPM-SXAC, and DPPM-SFAC exhibit high Θ // values of 80%, 82%, and 82%, indicating these molecules prefer horizontal dipole orientation. But DPPM-DPAC and DPPM-DMAC only have low Θ // s of 69% and 66%, respectively, close to those of purely isotropic chromophores. The low Θ // s are prob-ably caused by the major z component of transition dipole moment vectors, as revealed by the simulation of transition dipole moment in S 1 state (Figure 4). These results clearly demonstrate that the spiro-structured acridine derivatives can significantly promote horizontal dipole orientation. On the other hand, the direction of transition dipole moment vector of DPPM-SXAC is almost identical to that of DP-SXAC, and the Θ // of DPPM-  SXAC is also close to DP-SXAC (79%) ( Figure S11, Supporting Information), suggesting the introduction of benzoyl does not induce adverse effect on horizontal dipole orientation of these molecules.

Conclusion
In summary, a series of orange-red delayed fluorescence molecules DPPM-DMAC, DPPM-DPAC, DPPM-SFAC, DPPM-STAC, and DPPM-SXAC are successfully developed by fusing strong DPPM acceptor with acridine-based donors. The introduction of benzoyl not only strengthens ICT effect but also enhances SOC matrix element and decreases ΔE ST to promote RISC, leading to prominent delayed fluorescence with greatly red-shifted emission peaks, enhanced Φ PL s, and shortened delayed lifetimes. On the other hand, the horizontal dipole orientation of the molecule can be facilitated by creating a spirostructure at the 9-position of acridine, which is favored to improve Θ // s and Φ out s. And the presence of benzoyl induces no adverse effect on horizontal dipole orientation. This rational molecular engineering simultaneously endows DPPM-SXAC with efficient red-orange delayed fluorescence, a high Φ PL of 94%, and a large Φ out of 36% in CBP host. As a result, DPPM-SXAC realizes superb EL performance with ext,max , C,max , and P,max of up to 33.5%, 95.3 cd A −1 , and 93.5 lm W −1 , respectively. To the best of our knowledge, the outstanding EL efficiencies represent the best device performance among previously reported orangered TADF OLEDs. Moreover, the preliminary results also suggest that the presence of benzoyl can improve device stability to some extent. It is envisioned that the proposed molecular design will bring about more efficient TADF materials for high-performance OLEDs.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.