Planar and rigid pyrazine based TADF emitter for deep blue bright organic light emitting diodes

Two blue thermally activated delayed fluorescence (TADF) emitters bearing di- tert -butyl carba-zoles as the electron donor groups and pyrazine ( DTCz-Pz ) or dipyrazine ( DTCz-Pz ) as the electron acceptor are presented. The DFT calculations predict DTCz-Pz and DTCz-DPz to possess high S 1 energies (3.19 eV and 3.08 eV, respectively), and relatively large D E ST values (0.52 eV and 0.56 eV, respectively). The closely layered intermediate triplet states between S 1 and T 1 , predicted by DFT calculations, are expected to facilitate the reverse intersystem crossing (RISC) and improve spin-vibronic coupling efficiency between the excited states even the relatively larger ΔE ST s. The ΔE ST s for DTCz-Pz and DTCz-DPz are 0.27 eV and 0.38 eV, and both molecules show high photoluminescence quantum yields (65%, and 70%, respectively) and the decay lifetimes show temperature dependence in a PPT host, which is consistent that both molecules are TADF emitters in PPT. The OLEDs based on DTCz-Pz exhibit deep blue emission with λ EL of 460 nm and CIE of (0.15, 0.16). The maximum external quantum efficiency (EQE max ) reaches 11.6%, with a maximum luminance (L max ) of up to 6892 cd m -2 , while the device based on DTCz-DPz exhibits sky blue emission with λ EL of 484 nm and CIE of (0.15, 0.30), an EQE max of 7.2%, and L max of 8802 cd m -2 .


Introduction
Recently, organic light-emitting diodes (OLEDs) using organic thermally activated delayed fluorescence (TADF) emitters have generated great interest as a cheaper alternative to phosphorescent OLEDs based on noble metal complexes. [1][2][3] Triplet excitons can be converted to singlet excitons 2 in TADF materials via reverse intersystem crossing (RISC) due to the presence of a very small singlet−triplet energy gap (ΔEST). This was realized by a molecular design that localizes the HOMO on a donor moiety and the LUMO on an acceptor moiety, usually incorporated within a highly twisted structure. Although a plethora of TADF emitters has been developed since the first examples of high-efficiency TADF OLEDs in 2012, [1] deep blue TADF OLEDs (CIE coordinates where x < 0.15, y < 0.20) remain underdeveloped, and their efficiencies and stabilities are still generally lower than sky blue and green TADF OLEDs. [4,5] Therefore, the development of alternative, highly efficient blue-emitting materials remains highly desired. Sensible molecular structure design as well as electron donor and acceptor selection are required in order to obtain deep blue emission.
aimed decreasing the electron affinity of the electron acceptor by replacing the non-bridging phenyl rings in TRZ with adamantyl groups. The molecule MA-TA exhibited a destabilized LUMO of -2.96 eV compared to -3.12 eV for DMAC-TRZ, and a correspondingly destabilized S1 level of 2.90 eV (in toluene) compared to 2.74 eV (in toluene) for DMAC-TRZ. [9,11] As a result, the OLED using MA-TA exhibited deep-blue emission with λEL= 465 nm and CIE coordinates of (0.15, 0.16), along with a high EQEmax of 22.1%. [9] Adachi et al. further pushed the color towards the deep-blue region by using a weaker electron donor in 3,6-dimethylcarbazole coupled with a methylphenylene bridge that, due to the presence of the methyl group, increasing the torsion angle of the donor, leading to reduced conjugation, smaller DEST and a higher S1 energy. [8] The molecule Cz-TRZ3 possesses a shallow LUMO of -2.71 eV (in DCM) and a high S1 energy of 3.10 eV (in toluene). [8] The OLEDs based on Cz-TRZ3 exhibited deep-blue emission with CIE coordinates (0.15, 0.10) and EQEmax of 19.2%. [8] On the other hand, Chou et al. found that by simply replacing 3 the triazine with pyrimidine can push the emission to higher energy as well. [12] The molecule T3 possesses a much destabilized LUMO energy of -2.37 eV (in MeCN) and an S1 energy of 2.95 eV (in toluene), while the device based on T3 exhibited blue emission with λEL= 465 nm and CIE coordinates of (0.17, 0.21) and an EQEmax of 11.8%. [12] Wang et al. further blue-shifted the emission by using bespoke acridine-carbazole fused donor, and the bulky donor makes the molecule 12AcCz-PM adopt a quasi-orthogonal conformation. [13] The S1 of 12AcCz-PM is 3.06 eV while the LUMO value is similar to T3 at -2.31 eV (in MeCN). [13] The OLEDs based on 12AcCz-PM exhibited deep blue emission with λEL= 438 nm and CIE coordinates (0.15, 0.06); however, the EQEmax was only 5.7% due in part to the large ΔEST (0.39 eV in DPEPO). [13] These examples in the literature have shown that triazine and pyrimidine acceptors are compatible with obtaining blue and deep blue TADF emitters and reasonably high efficiencies in OLEDs. The related N-heterocycle, pyrazine, has thus far not been explored for blue TADF emitter design, though the pyrazine has been used within an electron-acceptor design to tune its electron affinity and the corresponding energies of the emitters. [14][15][16] Recently, Duan et al. reported a series of blue TADF emitters bearing pyrazine as an acceptor and benzofuro-carbazoles or benzothieno-carbazoles as donor moieties. [17] Amongst the molecules in the study, BFCZPZ2 possessed a moderately small ΔEST (0.31 eV in PPT) and high photoluminescence quantum yield (PLQY) (91% in PPT). [17] The device based on BFCZPZ2 exhibited deep blue emission with λEL= 464 nm and CIE coordinates of (0.15,0.16), and the EQEmax reached 21.3%, but reduced to 5.1% at 10 mA/cm 2 . [17] This work showed the potential of pyrazine in the design of blue TADF materials.

Theoretical Calculations
The ground state (S0) geometries of DTCz-Pz and DTCz-DPz were optimized by Density functional theory (DFT) and the excited states and their electronic configuration were predicted by time-dependent DFT calculations using the Tamm-Dancoff approximation (TDA-DFT) at the PBE0/6-31G(d,p) level of theory in the gas phase. [18,19] The results are summarized in Figure 2.
The DFT modeling predicts a rather flat conformation with average dihedral angles between the  [14] In both emitters, the HOMOs are distributed across the entire molecule while the LUMOs are mainly localized on the acceptor cores. 5 The more strongly electron-accepting DPz in DTCz-DPz results in more stabilized HOMO and LUMO levels compared to those in DTC-Pz. Analogously, the S1 and T1 levels are predicted to be slightly stabilized in DTCz DTCz-DPz] distributed over the whole molecule. This is different from the S1 and T1 states, which are dominated by a hybrid CT and LE transition from HOMO to LUMO. The presence of an intermediate T2 state that is of different symmetry to S1 in both DTCz-Pz and DTCz-DPz will contribute to a more efficient TADF as RISC will be faster, facilitated by spin-vibronic coupling between T1 and T2 and enhanced spin-orbit coupling (SOC) between T2 and S1. [20,21] We calculated the |VSOC| 2 values as the average spin-orbital coupling matrix elements (SOCME) between their S1/T1 states and S1/T2 states based on their optimized excited-state structures, [22] which are shown in Table 1. The |VSOC| 2 value between S1/T1 for DTCz-Pz is 0.017 cm -2 and this increases to 0.517 cm -2 for S1/T2 due to the different orbital character of T2 compared to S1. Analogously, the |VSOC| 2 values for DTCz-DPz also increases from 0.003 cm -2 (between S1/T1) to 0.170 cm -2 (between S1/T2). The closely layered intermediate T2 state and high SOCME values between the S1/T2 states provide an indirect route for rISC to occur despite the relatively large DEST for DTCz-Pz and DTCz-DPz. 6 (Figures S1-S4), high-resolution mass spectrometry, melting point determination, 7 and elemental analysis (Figures S5-S6). The two emitters were purified by silica gel chromatography followed by temperature gradient vacuum sublimation, and the purity was verified by high performance liquid chromatography (HPLC) analysis (Figures S7-S8). The thermal properties of these emitters were determined by thermogravimetric analysis (Figure S13).  Table S1). For DTCz-DPz, TDA-DFT predicted a higher energy HLCT transition on 340 nm and a LE transition on the carbazole moiety at 300 nm ( Table S2). The oscillator strengths (f) for DTCz-Pz and DTCz-DPz are calculated to be 0.2514 and 0.5255, respectively, from the spectra, [24] which are consistent with the trend observed from the TDADFT calculations (0.2414 and  The time-resolved PL decays of these materials were measured in 10 -5 M toluene solution under nitrogen (Figure 5). In both compounds, the emission decays with bi-exponential kinetics.
There is a nanosecond prompt emission, which occurs from direct radiative depopulation of the S1 state with t1 of 6.0 ns (63.4%) and 2.9 ns (85.7%) for DTCz-Pz and DTCz-DPz, respectively.
There is a second component with a lifetime (τ2) of 93 ns (36.6%) and 121 ns (14.3%) for DTCz-Pz and DTCz-DPz, respectively. Delayed emission is not observed for these compounds in solution.   The absolute FPL values measured using an integrating sphere under argon and air for the toluene solutions, and co-doped films are summarized in Table 1. DTCz-DPz shows significantly higher FPL of 96% in degassed toluene than DTCz-Pz (69%), while the 7 wt% PPT doped films are similarly bright with FPL of 70% and 65% for DTCz-Pz and DTCz-Pz, respectively. a. S1= singlet state energy obtained from the onset of the prompt fluorescence spectra (1-100 ns) measured at 77 K with lexc = 343 nm; T1 = triplet state energy obtained from the onset of the phosphorescence spectra (1-10 ms) measured at 77 K with lexc = 343 nm; ΔEST = E(S1) -E(T1); FPL = photoluminescence quantum yield measured using an integrating sphere under nitrogen flow with lexc = 340 nm; τp = prompt fluorescence lifetime measured at room temperature with time window of 100 ns (lexc = 378 nm). τd = delayed fluorescence lifetime measured at room temperature with time window of 1-40 ms (lexc = 378 nm). All measurements were performed in co-doped PPT film (7 wt%). Normalized intensity / a.u.

Conclusions
We have synthesized two pyrazine-based emitters DTCz-Pz and DTCz-DPz bearing a mono and dipyrazine acceptors and di-tert-butyl carbazole as the donor group. These two materials show reasonably high photoluminescence quantum yields, ranging from 76 to 96%, in both toluene solution and doped PPT thin films. The transient PL decay results in the doped thin film confirm that