Pyrazine-Based Blue Thermally Activated Delayed Fluorescence Materials: Combine Small Singlet–Triplet Splitting With Large Fluorescence Rate

Metal-free thermally activated delayed fluorescence (TADF) emitters have emerged as promising candidate materials for highly efficient and low-cost organic light-emitting diodes (OLEDs). Here, a novel acceptor 2-cyanopyrazine is selected for the construction of blue TADF molecules via computer-assisted molecular design. Both theoretical prediction and experimental photophysical data indicate a small S1-T1 energy gap (ΔEST) and a relative large fluorescence rate (kF) in an o-phenylene-bridged 2-cyanopyrazine/3,6-di-tert-butylcarbazole compound (TCzPZCN). The kF value of 3.7 × 107 s−1 observed in a TCzPZCN doped film is among the highest in the TADF emitters with a ΔEST smaller than 0.1 eV. Blue TADF emission is observed in a TCzPZCN doped film with a short TADF lifetime of 1.9 μs. The OLEDs using TCzPZCN as emitter exhibit a maximum external quantum efficiency (EQE) of 7.6% with low-efficiency roll-off. A sky-blue device containing a derivative of TCzPZCN achieves an improved EQE maximum of 12.2% by suppressing the non-radiative decay at T1.


INTRODUCTION
Owing to the small energy gap ( E ST ) between the lowest singlet (S 1 ) and triplet (T 1 ) excited states, metal-free thermally activated delayed fluorescence (TADF) molecules can upconvert from their T 1 to S 1 by absorbing environmental thermal energy and then decay radiatively from the S 1 . Organic light-emitting diodes (OLEDs) employing TADF emitters can convert both singlet and triplet excitons into light with a theoretical yield up to 100% (Wex and Kaafarani, 2017;Yang et al., 2017;Cai and Su, 2018;, and have emerged as a new representation for highly efficient and low-cost OLEDs Li W. et al., 2014;Ai et al., 2018;Bian et al., 2018). A twisted donor-phenylene-acceptor (D-Ph-A) structure has been demonstrated to be an effective strategy for the design of TADF materials (Zhang et al., 2012). A number of efficient blue, green, and red TADF materials with small E ST have been developed by using this strategy in recent years (Uoyama et al., 2012;Wang et al., 2014Wang et al., , 2017Zhang et al., 2014a,b;Chen et al., 2016;Li et al., 2017;Yuan et al., 2017;Wu et al., 2018;Zhang D. et al., 2018).
Except for E ST , the value of fluorescence rate (k F ) for TADF emitters has attracted more and more attention in recent years, because it is a key for not only the quantum efficiency of the emitter but also the TADF lifetime and the device stability (Zhang et al., 2014a,b;. The D-Ph-A-type TADF emitters with small twisting angles between the neighboring planes can have high k F values but suffer from large E ST , which leads to significant efficiency roll-off in their devices (Zhang et al., 2012;Li et al., 2013;Hirata et al., 2015;Chen X.-K. et al., 2017). Although increasing the twisting angle can reduce E ST , the k F value also decreases due to the reduced overlap of the orbitals involved in the S 1 transition (Zhang et al., 2014a). Especially, for blue TADF emitters with large band gaps, large twisting angle cannot ensure a small E ST , because the molecules may have a low-lying triplet state localized at the D or A moieties (Zhang et al., 2014b). Overall, the difficulty of TADF material design is to have small E ST and high k F at the same time. To enlarge the ratio of k F to E ST , the D-A couple should be carefully selected, and the twisting geometry should be well-designed.

RESULTS AND DISCUSSION
The CT transition energy is significantly related to the electrondonating ability of the donor and the electron-withdrawing ability of the acceptor in a D-A molecule. To avoid a lowlying locally excited triplet state ( 3 LE) existing under the S 1 ( 1 CT), both donor and acceptor moieties should have a limited conjugation length, and the conjugation between donor and acceptor should be broken (Zhang et al., 2014b). The electronwithdrawing capability of pyrazine is weaker than that of 1,3,5triazine, which is an ideal acceptor for blue and green TADF materials. To enhance the electron-withdrawing capability of pyrazine, one to three cyano groups are attached onto the pyrazine ring in 2-phenylpyrazine, in which the phenyl ring is used as a π-bridge between the donor and acceptor moieties. The TABLE 1 | Computed vertical absorption energies (E VA ), zero-zero energies (E 0−0 ), lowest unoccupied molecular orbital energies (E LUMO ), and reduction potentials (E RED ) of cyano-substituted triazine, pyrimidine, and pyridine moieties.  (Mazur and Hipps, 1995) and calculated from the E LUMO with a correlation of E RED = 0.79 × (-E LUMO ) + 1.01 .

No
The structures of the molecules are shown in Table 1.
calculated zero-zero energy of T 1 [E 0−0 (T 1 )] and the reduction potentials (E RED ) of the substituted pyrazine fragments are listed in Table 1 and compared with those of TRZ (Mazur and Hipps, 1995;Huang et al., 2013;Wang D. et al., 2019). As shown in Figure 1, there is a roughly proportional relationship between E 0−0 (T 1 ) and E RED , i.e., reducing the conjugation length of a moiety generally decreases its electron-withdrawing capability.
The theoretical E 0−0 (T 1 ) of 3-phenylpyrazine-2-carbonitrile  (2.88 eV) is as high as that of TRZ (2.87 eV), while the electron-withdrawing capability of 1 (E RED = 2.94 eV) is even higher than that of TRZ (E RED = 2.63 eV) (Mazur and Hipps, 1995), indicating that 2-cyanopyrazine is a promising acceptor for blue and green TADF molecules. Using 3-phenylpyrazine-2-carbonitrile  as the π-bridge attached acceptor and 3,6-di-tertbutylcarbazole as the donor, two molecules TCzPZCN and 2TCzPZCN are designed (Figure 2A). TCzPZCN has only one 3,6-ditertbutylcarbazole donor group, which links to the acceptor group 2-cyanopyrazine via the ortho position of the phenylene (Ph) bridge (Wang R. et al., 2018). The ground-state geometry of TCzPZCN is optimized by DFT/B3LYP/6-31G * . The dihedral angle between carbazole donor and Ph-bridge is 69 • , while that between 2-cyanopyrazine acceptor and Ph-bridge is 55 • (Figure 2B). Such moderate dihedral angles allow a small overlap of the orbitals involved in the CT transition on the Ph-bridge but effectively break the conjugation between the donor and the acceptor. For 2TCzPZCN, there are two 3,6-di-tertbutylcarbazole groups attached to the ortho and meta positions of the Phbridge. Although the meta-linked carbazole and the Ph-bridge have a relatively small dihedral angle of 52 • , the meta linkage prevents the orbitals on the donor from extending to the acceptor ( Figure 2B). Using the K-OHF method, a semiempirical descriptor selection method based on time-dependent DFT , the vertical absorption energies (E VA ) of TCzPZCN and 2TCzPZCN are calculated to be 3.15 and 3.16 eV, respectively. Assuming that their absorption is a 0-1 transition, the commonest transition for TADF emitters in weak polar medium, the E 0−0 (S 1 ) values of TCzPZCN and 2TCzPZCN in toluene are evaluated to be 2.91 and 2.92 eV, respectively (Huang et al., 2013). The E ST and the oscillator strength (f ) are calculated to be 0.05 and 0.0157 for TCzPZCN, respectively, and 0.05 and 0.0184 for 2TCzPZCN, respectively.
The ratios of f to E ST are among the highest values for the TADF emitters ( E ST < 0.15 eV) calculated using the K-OHF method (Table S1). Using a rough relationship between the theoretical frontier orbital energies and the measured redox potentials , the oxidation potentials (E OX ) of TCzPZCN and 2TCzPZCN are calculated to be 5.46 and 5.41 V, respectively, in dichloromethane, while the E RED of TCzPZCN and 2TCzPZCN are calculated to be 2.90 and 2.98 V, respectively, in acetonitrile ( Table 2). The synthesis of TCzPZCN and 2TCzPZCN is described in the Supplementary Material. Their absorption and emission spectra in toluene and 10 wt% m-bis(N-carbazolyl)benzene (mCP) films are presented in Figure 3 and Figure S1. As shown in Figure 3A, the absorption shoulders in the wavelength region of 350-430 nm can be ascribed to the intramolecular charge-transfer (CT) transitions. The fluorescence (1-2 ns component) and phosphorescence (1-2 ms component) spectra in toluene at 77 K are all smooth and broad. The E ST values can be estimated from the energy difference between the fluorescence and phosphorescence peaks. The measured E ST of 0.07 eV for TCzPZCN and 0.06 eV for 2TCzPZCN are close to the theoretical values ( Table 2). From the onset of the fluorescence bands at room temperature (RT; Figure S1), the 0-0 energies of TCzPZCN and 2TCzPZCN in toluene are estimated to be 2.91 and 2.82 eV, respectively, which are also in good agreement with the above theoretical estimation. These two compounds emit brightly at 77 K but dimly at RT with photoluminescence quantum yields (PLQY) <0.10 ( Figure 3A inset and Table 2). In comparison to the emission spectra in solvent glass, those in the fluid solution ( Figure S1) exhibit a significant redshift, indicating a correlation between the serious non-radiative decay in RT toluene and the excited-state geometrical relaxation process.
The transient decay spectra in degassed toluene at RT are presented in Figure 3B. No obvious TADF component is observed in the microsecond time range. Besides the singleexponential nanosecond fluorescence decay, a quick decay in the picosecond time scale is recorded by using an ultrafast timecorrelated single photon counting (TCSPC). This fast decay could be resolved into two exponentially decaying components with the lifetimes (τ ) of 0.14 and 2.6 ns for TCzPZCN and 0.12 and 3.2 ns for 2TCzPZCN. It is reasonable to expect that the nonradiative decay rate (k nr ) is not a constant during the fluorescence decay process. It is known that the excited-state solvation and relaxation process can be completed in a few picoseconds in fluid solution (Castner et al., 1987;Kinoshita and Nishi, 1988), resulting in a very fast non-radiative decay via a so-called free rotor and loose bolt effects (Turro et al., 2010). If the radiative and non-radiative decay rates are constants in the total luminescence process, the k F value can be obtained by the following formula: where F and τ F are the PLQY and lifetime of the fluorescence component, respectively. Given that both the k F and k nr in the first few nanoseconds are the same as the values after that, the PLQYs of these two compounds in toluene will be significantly higher than the observed ones. Consequently, if we calculate k F from the measured F and the dominant nanosecond τ with Equation 1, the k F value will be considerably underestimated.
In 10-wt%-doped mCP films, TCzPZCN and 2TCzPZCN exhibit blueshifted emission peaks at 483 and 493 nm, respectively ( Figure 3C), with respect to that in toluene. Meanwhile, the PLQYs of TCzPZCN and 2TCzPZCN in the doped films increase to 0.47 and 0.44, respectively, owing to the suppression of the collision-induced non-radiative decay (Turro et al., 2010;Zhang et al., 2014a). However, it was previously demonstrated that there is enough free volume in amorphous organic semiconductor films (Sun et al., 2017). The largeamplitude excited-state distortion cannot be fully inhibited in the films, leading to the moderate PLQYs for these films. TADF decay can be observed from the doped films, with a short lifetime of 1.9 µs for TCzPZCN and 8.1 µs for 2TCzPZCN ( Figure 3C).
It is known that the solvation effect increases the separation of the electron and hole in a CT state (Sun et al., 2017; and consequently decreases the fluorescence rate. According to the time-resolved emission spectroscopy shown in Figure 3D, the solvation process in a doped mCP film can last for dozens of nanoseconds, which is much slower than that in fluid solutions (Deng et al., 2019). The fluorescence rate of a TADF emitter in organic thin films decreases gradually in this time region and therefore can have a higher average value than that in solution. Since the fluorescence decays in organic thin films are always best fit by multiple exponentials (Figure 3D), an average lifetime determined from the time the fluorescence intensity decays to 1/e of the initial value ( Table 2) is used to calculate the k F values. The k F value of TCzPZCN in doped mCP films is then calculated to be 3.7 × 10 7 s −1 , which is considerably higher than those of the TADF emitters having a E ST smaller than 0.1 eV ( Table S2). In comparison to TCzPZCN, 2TCzPZCN has a lower k F of 2.2 × 10 7 s −1 , probably because of the reduced distance between the charge centroids of the donor and acceptor orbitals (Figure 2). According to first-principles calculation, the square root of the CT transition rate is approximately proportional to the effective D/A separation distance and the orbital overlap integral (Phifer and McMillin, 1986;Zhang et al., 2014a).
The oxidation and reduction behaviors of TCzPZCN and 2TCzPZCN are measured by cyclic voltammetry in dichloromethane and acetonitrile, respectively ( Figure S2). From the onsets of the quasi-reversible redox couples, the vacuum-state-referenced E OX and E RED of TCzPZCN are determined to be 5.57 and 2.92 V, respectively, while those of 2TCzPZCN are determined to be 5.53 and 2.95 V, respectively. These potential values are all close to their  Six OLEDs containing TCzPZCN and 2TCzPZCN are fabricated using a very simple device structure, as shown in Figure 4A. The 30-nm-thick emissive layers of the six devices are TCzPZCN or 2TCzPZCN doped mCP films and their neat films (see Table 3). At a doping concentration of 10 wt%, both TCzPZCN-(1a) and 2TCzPZCN-based (2a) OLEDs display a sky-blue emission with a maximum at 480 nm ( Figure 4B). Devices 1a and 2a turn-on at 3.4 and 3.9 V, respectively, and reach a maximum luminance of 4579 and 6257 cd/m2 at 11 and 12 V, respectively ( Figure 4C). The maximum external quantum efficiencies (EQEs) of Devices 1a and 2a are found to be 7.1 and 12.2%, respectively (Figure 4D), which are both higher than the upper limit of the traditional fluorescent OLEDs (5%). Although the PLQYs of these two compounds are approximate in 10-wt%-doped mCP films, the maximum EQEs of their devices are quite different. The internal quantum efficiency (IQE) of a TADF OLED can approach the PLQY of the emitter only when the internal conversion from S 1 to S 0 is the principal deactivation pathway for the emitter (Zhang et al., 2014a). The maximum IQE for Device 1a is lower than the PLQY of the emissive layer (0.47) when a light out-coupling efficiency of 0.2-0.3 is assumed, indicating that the non-radiative decay at T 1 for TCzPZCN cannot be ignored in the doped films. In comparison to Device 2a, Device 1a shows a reduced EQE roll-off that can be attributed to the short TADF lifetime of 1.9 µs for TCzPZCN in doped film (Zhang et al., 2014a,b).
The influence of doping concentration on device performance is shown in Figures S3, S4 and Table 3. The performance of TCzPZCN-based OLEDs is rather insensitive to the doping concentration of the emissive layer owing to the highly twisted configuration of the emitter (Zhang et al., 2015;Cha et al., 2016;. The electroluminescence (EL) spectra and EQE-current density curves of 10-and 30-wt%doped devices almost coincide with each other respectively ( Figure S3). Even the undoped device exhibits similar EQEcurrent density characteristics in the current density range from 1 to 100 mA/cm 2 with a slightly broader EL spectrum in comparison to the 10-wt%-doped device. In contrast, increasing the doping concentration of 2TCzPZCN-based OLEDs produces clear redshifts of the EL spectrum and decreases the EQE maximum. The lower steric hindrance surrounding the metalinked carbazole may be responsible for the relatively strong intermolecular interaction between emitters in 2TCzPZCN doped films.

CONCLUSION
On the basis of a novel acceptor 2-cyanopyrazine, a type of blue emissive TADF molecule with small E ST (<0.1 eV) is successfully designed and synthesized. The fluorescence kinetics investigation indicates that the non-radiative decay rates of these molecules in toluene are far from constants. The ultrafast fluorescence decay (1/τ ) in the first hundred picoseconds after the excitation is related to a significant excited-state structural distortion. In doped mCP films, an o-phenylene-bridged 2-cyanopyrazine/3,6-di-tertbutylcarbazole compound (TCzPZCN) shows a fast TADF decay with a lifetime of 1.9 µs, as well as a high fluorescence rate of 3.7 × 10 7 s −1 that can be comparable to those of the TADF emitters having relatively large orbital overlap and E ST (>0.2 eV). Although the pyrazine-based TADF emitters in solid films exhibit only moderate quantum yields on PL and EL suffered by the structural distortion process, it is one step toward a TADF emitter with both small E ST and large k F . Additionally, we have demonstrated that 2-cyanopyrazine is a promising acceptor for the construction of blue TADF emitters. By suppressing the structural distortion induced non-radiative decay, efficient pyrazine-based TADF emitters with short TADF lifetime can be expected.

DATA AVAILABILITY
The datasets generated for this study are available on request to the corresponding author.