Acceptor-Donor-Acceptor π-Stacking Boosts Intramolecular Through-Space Charge Transfer towards Efficient Red TADF and High-Performance OLEDs

Organic push-pull systems featuring through-space charge transfer (TSCT) excited states have been disclosed to be capable of exhibiting thermally activated delayed fluorescence (TADF), but to realize high-efficiency long-wavelength emission still remains a challenge. Herein, we report a series of strongly emissive orange-red and red TSCT-TADF emitters having (quasi)planar and rigid donor and acceptor segments which are placed in close proximity and orientated in a cofacial manner. Emission maxima (λem) of 594−599 nm with photoluminescence quantum yields (PLQYs) of up to 91% and delayed fluorescence lifetimes of down to 4.9 μs have been achieved for new acceptor-donor-acceptor (A-D-A) molecules in doped thin films. The presence of multiple acceptors and the strong intramolecular π-stacking interactions have been unveiled to be crucial for the efficient low-energy TSCT-TADF emissions. Organic light-emitting diodes (OLEDs) based on the new A-D-A emitters demonstrated electroluminescence with maximum external quantum efficiencies (EQEs) of up to 23.2% for the red TSCT-TADF emitters. An EQE of 18.9% at the brightness of 1000 cd m−2 represents one of the highest values for red TADF OLEDs. This work demonstrates a modular approach for developing high-performance red TADF emitters through engineering through-space interactions, and it may also provide implications to the design of TADF emitter with other colours.


Introduction
The past decade has witnessed a booming research interest in seeking luminescent purely organic push-pull molecules that can harvest triplet excitons for organic light-emitting diode (OLED) applications owing to their advantages over noble metal complexes in terms of low cost. Rather than direct radiative decay of the lowest lying triplet excited state (T 1 ) to give phosphorescence, the upconversion of T 1 excitons to singlet ones (S 1 ) which subsequently radiate to give thermally activated delayed fluorescence (TADF) has been devised for organic molecules to circumvent their very slow phosphorescent decay processes [1,2]. It can be seen that a key to efficient TADF is a fast T 1 →S 1 reverse intersystem crossing (RISC) process which is executed by an electron spin-flip. According to the Fermi golden rule under the Condon approximation, RISC rate correlates proportionally with spin-orbit coupling matrix element (SOCME) between the coupling singlet and triplet excited states and inversely with their energy difference (ΔE ST ). Taking into account of the very small SOCME for purely organic molecules devoid of a heavy atom, a rapid RISC takes place only when the coupling singlet and triplet excited states are close in energy, that is, a trifling ΔE ST . To this end, it has been a consensus to design molecules with spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) which can effectively minimize the exchange interaction. Following this principle, efficient TADF emissions with widely tunable excited state properties including energies and lifetimes have been realized through connecting an electron donor (D) and an acceptor (A) in a twisted manner [3][4][5].
Remarkably, Kaji and coworkers reported a significantly boosted RISC rate for a triptycene-supported molecule by virtue of a vibronically coupled spin-orbit coupling (SOC) effect [30]. The space-confining effect has been exploited for greatly suppressing the nonradiative decay of TSCT excited states [31][32][33]. However, high-efficiency TSCT-TADF emissions have been mainly limited to the blue-to-green regime with only few reports on relatively weak red emissions [25,28]. Although various D-A combinations have been developed for widely tuning the colours of TBCT-TADF emitters, they may not be well suited for the design of TSCT-TADF emitters if their geometric structures do not favour through-space interactions [34]. In particular, the suppression of nonradiative decay is essential for efficient red emissions to offset the energy gap law [35,36]. Therefore, a molecular design that allows for simultaneous manipulations of all these parameters has become an imperative demand.
The cofacial donor-acceptor orientation has proven crucial for regulating the electronic communications and thus the excited state dynamics of push-pull dyads, which is an important topic in the studies of charge-separated states [37][38][39]. However, its implications on TADF properties have been scarcely explored. We recently demonstrated highly efficient green TADF and phosphorescence emissions by confining quasiplanar motifs to have a face-to-face orientation [40,41]. We envisioned that this molecular design leveraging on strong intramolecular π-stacking interactions would be able to break the efficiency limit on long wavelength TSCT-TADF emissions, as opposed to an edge-toface orientation. Herein, we designed four molecules, DPXZ-QX, DPXZ-DFQX, DPXZ-2QX, and DPXZ-2DFQX using planar dibenzo[a,c]phenazine (QX) and its fluorinated derivative (DFQX) as the acceptors (Figure 1) [42][43][44][45][46][47][48][49]. The quasiplanar O-bridged triphenylamine (DPXZ) was used as the donor [40,41,50]. For comparison, triphenylamine (TPA) and phenoxazine (PXZ) were used as the donors to prepare the control compounds TPA-QX and PXZ-QX. Single-crystal X-ray diffraction studies revealed substantial face-to-face alignment of the donor and acceptor and thus strong π-π interactions in the four molecules containing DPXZ. These four emitters exhibit orange-red to red TSCT-TADF emissions with photoluminescence quantum yields (PLQYs) of 70−91% in doped films and have demonstrated OLEDs (λ em = 588 -617 nm) with maximum external quantum efficiency (EQE) over 23%. In stark contrast, TADF properties were not observed for TPA-QX and PXZ-QX, revealing the crucial role of face-to-face orientation in promoting electronic communications for TSCT-TADF emission.

Results
2.1. Molecular Design, Synthesis, and Structures. Geometries of the donor and acceptor and their relative orientation constitute the two key factors steering the strength of π-π interactions. As shown in Figure S1, the two planes at orthopositions of fiveand six-membered rings are largely deviated from a parallel orientation. In these two cases, close contacts only exit between atoms near the bridge. Other prevailing bridges based on anthracene-and xanthene-type skeletons have been used for underpinning two planes in parallel [51]. However, only intramolecular π-π interactions between partial planes on the rigid anthracene can be found after significant torsion. Despite the shortening of distance between the anchorages in a bent geometry in xanthene, its flexibility is deleterious to emission efficiency [21]. In contrast, carbazole and fluorene derivatives can confine the two planes to be in proximity with a large degree of π-π overlap [31-34, 40, 41]. In addition, this kind of skeletons allows for a construction of sandwich-type molecular architecture in which intramolecular interactions can be further strengthened. More importantly, the presence of multiple donors/acceptors has proven capable of boosting RISC between states with different orbital configurations [52][53][54][55][56]. Illustration of the design concept from "edge-to-face" to sandwich-type "face-to-face" is shown in Figure 1(a), following which the emitters in this study have been developed (Figure 1(b)).
All target compounds were prepared by a three-step procedure (Supplementary Materials) and obtained as yellowto-orange powders. Their structures were characterized by 1 H and 13 C NMR spectroscopy, high-resolution mass spectrometry, elemental analysis, and single-crystal X-ray diffractions. Thermogravimetric analysis (TGA) revealed high decomposition temperatures (T d at a 5% weight loss) of 402−470°C for all the six compounds under Ar, among which the lowest thermal stability was observed for TPA-QX ( Figure S2).
All single crystals of the present compounds were grown by slow evaporation of their solutions in mixed dichloromethane (DCM)/hexane. The crystal data are compiled in Tables S1-S3 (Supplementary Materials). As depicted in Figures 1(c) and S3, the QX and DFQX in all molecules have a flat geometry and are tilted with respect to the carbazole plane (torsion angles: 48−70°). The short distances (ca. 3.5 Å) between the donor and acceptor segments in DPXZ-QX, DPXZ-DFQX, DPXZ-2QX, and DPXZ-2DFQX signify strong π-π stacking interactions. Of note, the attractive force buckles the DPXZ to allow for a parallel orientation of the participating Ph ring relative to the QX plane. The force also leads to an antiparallel orientation of the two QX/DFQX planes in DPXZ-2QX and DPXZ-2DFQX, endowing them with a C 2 symmetry. A pair of enantiomers are observed for the crystals containing DPXZ. Differently, there is no evident short π-π contact in TPA-QX and PXZ-QX. Instead, 2 Research C-H⋯π interactions result from the edge-to-face alignment of the donor and acceptor.
To validate the rational design in this work, the electronic structures of all molecules were studied through theoretical calculations using dispersion-corrected density functional theory (DFT) and time-dependent DFT (TDDFT) [57]. First, theoretical insights into the intramolecular π-π stacking interactions were gained by reduced gradient density (RGD) analysis of the optimized ground state (S 0 ) structure [58]. The spikes within ±0.02 a.u. of the sign (λ 2 )ρ value reveal the presence of noncovalent interactions which are the strongest in the A-D-A structures ( Figure S4). It can be clearly seen from the RDG isosurfaces that the noncovalent interactions are mainly confined to the space between the donor and acceptor in each molecule. As illustrated in Figure S5, the HOMO in the S 0 geometry for each molecule is predominantly localized on the donor (TPA, PXZ, or DPXZ) with a very minor contribution from the nonbonding p orbital of the N(carbazoyl) atom. The LUMO is localized on the QX/DFQX moiety. Natural transition orbital (NTO) analyses of their S 1 states show a TSCT nature (Figures 2 and S5). However, the T 1 states of TPA-QX and PXZ-QX are dominated by a local-excitation (LE) character of QX, in contrast to the TSCT nature of T 1 states for the others. This difference is caused by the destabilized 1,3 TSCT states when TPA or PXZ is used, which also result in relatively larger ΔE ST values. Therefore, an energy diagram that does not favour the TADF process is obtained for TPA-QX and PXZ-QX. Of interest, the presence of multiple acceptors imparts the sandwich-type  with key short C-π/π-π distances indicated.
3 Research emitters DPXZ-2QX and DPXZ-2DFQX dense excited states that should facilitate the RISC process as per the El-Sayed rule [52][53][54][55][56]. It is worth mentioning that the literature reports on engineering of multiple charge transfer excited states are majorly limited to D-A-D-type molecules with few exceptions of A-D-A congeners [59]. Importantly, the calculated oscillator strength (f) values of up to 10 -2 are significantly high among the TSCT transitions, implying strong electronic coupling between HOMO and LUMO in DPXZ-2QX and DPXZ-2DFQX.

Photophysical Properties.
The electronic spectra of all the cofacial D-A and A-D-A stacks were recorded in toluene at room temperature. The four precursors CQX, CDFQX, C2QX, and C2DFQX, which do not contain the D moiety (Supplementary Materials), were studied under the same conditions. As depicted in Figures 3(a) and S6, all of the compounds exhibit similar UV-Vis absorption spectral profiles. The intense absorptions in the region below 300 nm and in 340−410 nm are assigned to π-π * and charge transfer (carba-zole→QX/DFQX) transitions, respectively. Close inspection of the absorption spectra of the D-A and A-D-A compounds show additional broad absorption tails in the lowest energy regime with much lower intensities. They are attributed to direct charge transfer transitions from the donor (TPA, PXZ, or DPXZ) to QX/DFQX, evidencing the presence of appreciable through-space electronic interactions.
For assignment of the emitting states, the photoluminescence spectra of the four precursors were firstly examined to identify the excited state energy levels of fragments. As illustrated in Figures 3(b) and S7, vibronically structured emissions with peak maxima (λ max ) at 428−442 nm are observed for CQX, CDFQX, C2QX, and C2DFQX in nonpolar hexane, which are assigned to singlet LE states of the QX and DFQX moieties (termed 1 LE A where the subscript A denotes acceptor). The 1 TBCT states arising from carbazo-le→QX/DFQX transitions are proposed to lie at higher energy levels in hexane. Increasing solvent polarity switches on their 1 TBCT emissions in toluene (λ max = 464 -490 nm) and dichloromethane (λ max = 556 -586 nm). Taking DPXZ-QX as an example, new broad lower-energy emission bands appear at λ max = 571 and 614 nm in hexane and toluene (Figure 3(c)), respectively, in comparison with the emissions of its precursor CQX. These bands are assigned as 1 TSCT (DPXZ→QX) emissions. The coexistence of 1 LE A / 1 TSCT and 1 TBCT/ 1 TSCT emissions in hexane and toluene reveals incomplete internal conversion (IC), presumably due to weak coupling between the TSCT and    Figure S9, an enhancement of TSCT emission in the absence of air was observed in hexane, suggestive of the TADF characteristic.
The solid-state emission properties of all the D-A and A-D-A compounds were studied in a panel of hosts including polymethyl methacrylate (PMMA), 1,3-bis(N-carbazolyl) benzene (mCP), 9,9′-biphenyl-3,3′-diylbis-9H-carbazole (mCBP), 4,4′,4″-tris(carbazole-9-yl) triphenylamine (TCTA), and 2,2,2″-(1,3,5-benzinetriyl)-tris(1phenyl-1-H-benzimidazole (TPBI). All emitters in doped films display single emissions at room temperature ( Figure S10). By comparing with the emissions of their precursors ( Figure S11), the emissive states for all the compounds in the solid state are assigned as TSCT in nature. Enhanced through-space electronic coupling interactions are surmised to boost the IC processes, leading to the populations of lowest-lying TSCT states. Impressively, the PLQYs for the emitters containing DPXZ are determined to be 71−91% and 65−86% in mCP and mCBP under Ar, respectively (Tables 1 and S4). In contrast, TPA-QX and PXZ-QX show much lower PLQYs (Table 1). To study their emission mechanisms, phosphorescence spectra of all emitters in different hosts at 77 K were also recorded. As depicted in Figures 4 and S10, TPA-QX and PXZ-QX have similar phosphorescence energies which are assigned to 3 (Tables 1  and S5). The presence of two close-lying 3 CT states likely opens multiple RISC channels for DPXZ-2QX and DPXZ-2DFQX [52][53][54][55][56]. Notably, the delayed fluorescence lifetime is much shorter for DPXZ-DFQX than for DPXZ-QX. It has been established that an intervention of the 3 LE state between the 1 CT and 3 CT states can boost the RISC rate significantly [52][53][54][55][56]. A slight increase of the acceptor strength in DPXZ-DFQX engenders more stabilized 1,3 CT states between which the 3 LE regulation is more pronounced. This difference is minimized for DPXZ-2QX and DPXZ-2DFQX because of the presence of another CT state which can also boost the RISC.

Electrochemistry.
The electrochemical properties of all compounds were examined by cyclic voltammetry in DCM ( Table 1). As shown in Figure S13, two quasireversible oxidations were observed for each compound with halfpotentials (E 1/2 ) in the range of 0.77−0.92 and 1.32−1.44 V versus Ag/AgCl, respectively. The first couple is assigned to the oxidation of TPA, PXZ, or DPXZ. The second couple is the carbazole-centered redox process. Irreversible waves with onset potentials (E onset ) ranging from -1.41 to -1.31V (versus Ag/AgCl) were noted in the cathodic scan, corresponding to the reductions of the QX and DFQX moieties. By referring to the redox potential of Cp 2 Fe +/0 , the

Research
HOMO levels (ca. −5.1 eV) are estimated to be comparable for all molecules except TPA-QX (-5.25 eV). The deeper HOMO level for TPA-QX is consistent with the theoretical simulation and spectroscopic results. As expected, the presence of F atoms on the acceptor stabilizes the LUMOs for DPXZ-DFQX and DPXZ-2DFQX in comparison with DPXZ-QX and DPXZ-2QX. It is notable that DPXZ-QX has a slightly higher LUMO level than TPA-QX and PXZ-QX, likely due to stronger interactions between more planar DPXZ and QX.  Figure 5(b). The neat films of HAT-CN, TAPC, TCTA, POT2T, and ANT-BIZ act as hole-injection, hole-transporting, electron/exciton blocking, hole/exciton blocking, and electron-transporting layers, respectively. An additional layer of mCBP was also inserted adjacent to the emitting layer to confine excitons. Device characteristics are plotted in Figures S14-S17 and the representative data of DPXZ-QX and DPXZ-2QX in Figure 5. The key numerical device data are compiled in Table 2 (DPXZ-QX and DPXZ-2QX) and Table S6 (DPXZ-DFQX and DPXZ-2DFQX).    Table S7, the maximum EQE of 23.2% for DPXZ-2QX represents one of the highest efficiencies for red TADF emitters without using TPA and its substituted derivatives as the donor [43,46,47,59,[64][65][66][67]. It is noted DPXZ-DFQX and DPXZ-2DFQX have lower EQEs than the emitters without containing F atoms, presumably due to the self-quenching effect for the former. The higher concentration-sensitivity of device EQEs for DPXZ-DFQX and DPXZ-2DFQX also supports this proposition. Among the four examined emitters, DPXZ-QX shows the largest efficiency roll-off at high luminance, which should be mainly due to its much longer delayed fluorescence lifetime. Akin to a previous finding [40], a higher concentration was found to be beneficial for reducing efficiency roll-off at high brightness. Improved charge balance is proposed to be responsible for this characteristic. With a weak concentration-quenching effect and a short delayed fluorescence lifetime, the EQE of the device doped with 12 wt% DPXZ-2QX remains as high as 18.9% at the luminance of 1000 cd m -2 . It is remarkable that this performance is superior than most of the red TADF emitters, irrespective of a TBCT or TSCT process ( Figure 5 (h) and Table S7). For instance, despite a high maximum EQE over 30%, the value dramatically drops to 6.4% at the brightness of 1000 cd m -2 [68]. Therefore, the accomplishments herein demonstrate that a TSCT design integrating high molecular rigidity, strong intramolecular π-π interactions, and multiple donors/acceptors is viable to deliver high-performance red TADF devices.

Discussion
In summary, a molecular design of high-performance orange-red to red TADF emitters featuring intramolecular TSCT excited states has been demonstrated. The confining of rigid and (quasi)planar donor and acceptor(s) in a faceto-face orientation, which allows for strong intramolecular π-stacking interactions within the donor-acceptor pair, ensures concurrently boosted radiative charge transfer transition and suppressed nonradiative decay. Together with the regulation of the RISC rate by using multiple acceptors, an acceptor-donor-acceptor-type emitter with a sandwich configuration has been developed to deliver highperformance red OLEDs with a high external quantum efficiency and a small efficiency roll-off. This achievement substantiates the significance of control over the conformation and orientation of donor-acceptor for the design of high-efficiency TSCT-TADF emitters. The engineering of intramolecular cofacial π-stacking interactions provides a modular approach to the development of full-colour highperformance TADF emitters.

Data Availability
All data supporting the findings of this study are presented in the article and supplementary materials. Additional data are available from the corresponding author upon reasonable request.

Conflicts of Interest
The authors declare no conflict of interest regarding the publication of this article.

Supplementary Materials
Experiments including general materials and instrumentations. Methods of single crystal analysis, theoretical simulations, device fabrication, and measurement. Syntheses and characterizations of new compounds. Figure S1: illustration of TSCT molecule design and list of selected TSCT molecules in the literature report. Figure S2: thermal properties. Tables S1-S3: crystal data. Figure S3: crystal structures. Figure S4: noncovalent interaction analysis using RDG. Figure S5: analysis of electronic structures using DFT and  -S12 and Tables S4-S5: photophysical properties. Figure S13: cyclic voltammograms. Figures S14-S17 and Table S6: OLED data. Table S7: comparison of the present device performances with those in the literature reports. (Supplementary Materials)