Color Tuning of Multi-Resonant Thermally Activated Delayed Fluorescence Emitters Based on Fully Fused Polycyclic Amine/Carbonyl Frameworks

Two novel p -extended amine/carbonyl-based multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been designed and synthesized. The two emitters are isomeric, comprised of nine fused rings and show green-yellow emission. Sym-DiDiKTa and Asym-DiDiKTa possess tert -butyl groups distributed in a symmetrical and asymmetrical fashion, respectively, which significantly impact the single-crystal packing structure. The two compounds possess similar singlet-triplet energy gaps, D E ST , of around 0.23 eV, narrowband emission characterized by a full width half maximum, FWHM, of 29 nm and a


Introduction
4][5][6][7] Unlike phosphorescent OLEDs that funnel the emitting excitons via the lowest-lying triplet excited state, TADF compounds convert triplet excitons into singlets via reverse intersystem crossing (RISC), [8][9] which is possible at ambient temperatures due to the small S1-T1 energy gap, DEST. 10 Compounds that possess a small DEST show a small overlap of the electron density between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).To adhere to this requirement, the great majority of organic TADF emitters are based on a highly twisted donor-acceptor architecture and emit from a long-range charge transfer (LRCT) state. 7This strategy, however, leads to molecules where there is a large degree of structural relaxation in the excited state [11][12] that manifests in broadband emission with full width half maximum (FWHM) values of around 70-100 nm. 13 This is unattractive to the display industry as the color purity of these devices is poor and color filters must therefore be used to meet the industry standards for red, green and blue pixels.2] Termed multiple resonance (or multiresonant) TADF (MR-TADF) emitters, these compounds possess small-to-moderate DEST values, narrowband emission (with FWHM typically < 30 nm) and limited positive solvatochromism due to the short-range charge transfer (SRCT) nature of the emissive S1. 22 Maximum external quantum efficiencies (EQEmax) as high as 34% for deep blue, 22 37% for sky-blue, 23 37% for green 24 and 36% for red 25 have been achieved for OLEDs containing MR-TADF emitters comprised of boron-nitrogen (B/N) doped polycyclic aromatic frameworks.Recently, the library of MR-TADF emitters has been expanded to contain boronoxygen (B/O)-based emitters 26 and carbonyl-nitrogen (C=O/N)-based emitters [14][15][16][17][18][19] as well as C=O/N/O-, 17,27 C=O/N/S- 17,28 and C=O/N/SO2-based 28 emitters.
][18][19][31][32] Recently, Yasuda et al. reported a linearly extended C=O/N-based emitter, QA-2 (Figure 1) displaying TADF in 3 wt% PPCz doped film, with a ΔEST of 0.19 eV. 27Similar to the design strategy used for v-DABNA, where the central ring is functionalized with meta-disposed B-π-B and N-π-N groups, QA-2 also contains meta-disposed C=O-π-C=O and N-π-N, which leads to an improvement of the TADF properties over the parent compound DiKTa, with a faster delayed lifetime (48 µs in 3 wt% PPCz and 93.3 µs in 5 wt% mCP, respectively).Despite the apparent increased conjugation length, a slightly blue-shifted λPL of 465 nm in toluene was observed for QA-2, compared to that of DiKTa (DEST of 0.15 eV and λPL of 473 nm in toluene). 15The OLED with QA-2 showed an EQEmax 19.0%.Our group has recently reported a helically chiral isomer of QA-2 that contains meta-disposed C=O-π-C=O and N-π-N skeleton.Enantiomers of Hel-DiDiKTa (Figure 1) 33 display circularly polarized luminescence (CPL), with a |gPL| of 4 × 10 -4 , but likewise, its λPL (473 nm) is similar to that of DiKTa.
The exploitation of para-disposed functional groups to generate p-extended skeletons is a powerful strategy to tune the photophysical properties of MR-TADF emitters to the red. 25,34 Yauda and co-workers have adopted this strategy where both the B-π-B and N-π-N-based groups are para-linked to construct red MR-TADF emitters such as BBCz-R (Figure 1) that exhibits a λPL of 615 nm and a narrow FWHM of 21 nm (0.07 eV) in dilute toluene solution. 30is strategy has also been used to generate B/O and B/S derivatives as shown in Figure 1. 35-compounds containing para-C=O-π-C=O and N-π-N frameworks have not as of yet been presented.In this context, expanding the chemical space explored in ketone-based MR-TADF emitters is desired.
Herein, we present two new isomeric p-extended C=O/N-based polycyclic aromatic compounds based on the annellation of two triangulene DiKTa units to form tetraketone structures.9]32 Sym-DiDiKTa is composed of nine fully fused rings with symmetrically arranged tert-butyl substituents while the tert-butyl groups are disposed asymmetrically in Asym-DiDiKTa.Both emitters show moderate DEST of around 0.23 eV, red-shifted emission at ~540 nm compared to DiKTa, a small FWHM of 29 nm and delayed lifetime, τd, of 1700 µs and 2100 µs for Sym-DiDiKTa and Asym-DiDiKTa, respectively.
Owing to their exciting properties, these compounds were employed as emitters in OLED devices, and to assess improvements in efficiency roll-off, they were also investigated as terminal emitters in hyperfluorescence (HF) OLEDs, which combine a narrowband emitter as terminal dopant with a co-deposited TADF emitter acting as an assistant dopant in a host matrix.Although MR-TADF OLEDs have achieved extremely high EQEmax, an issue generally encountered is the severe efficiency roll-off that is mainly caused by quenching mechanisms such as singlet-triplet annihilation (STA) and triplet-triplet annihilation (TTA) due to the accumulation of triplet excitons as a result of their typically slow reverse intersystem-crossing rate (kRISC). 17,37 ] In these devices, exciton harvesting is managed by the donor-acceptor TADF assistant dopant and these excitons are transferred to the terminal MR-TADF emitter via a Förster resonant energy transfer (FRET) mechanism.Emission from the terminal emitter then occurs, benefiting from the typically fast radiative decay and narrowband emission of the MR-TADF compound. 23To ensure efficient FRET, there must be an effective overlap between the emission spectrum of the assistant dopant and the absorption spectrum of the emitter.

Results and Discussion
The two emitters were obtained via a four-step linear sequence (Scheme 1a).
Compound 1 was obtained in good yield following a copper-catalysed Ullman coupling between the previously reported dimethyl 2,5-bis((4-(tert-butyl)phenyl)amino)terephthalate 41 and methyl 2-iodobenzoate.Quantitative saponification yielded the key intermediate 2, which then underwent a four-fold intramolecular Friedel-Crafts acylation of the in situ-prepared acyl chloride derivative in the presence of the Lewis acid AlCl3 to afford a mixture of two isomers, Sym-DiDiKTa and Asym-DiDiKTa, in 36% and 5% yield, respectively, after isolation by column chromatography.The compounds were further purified by gradient-temperature vacuum sublimation and their structure and purity were confirmed by a combination of NMR spectroscopy, high-resolution mass spectrometry, melting point determination and HPLC and elemental analyses.packing.In the case of Sym-DiDiKTa, adjacent molecules are partially superimposed, forming π-stacked chain along the c-axis with the molecules stacked in a slipped manner (Figure 2c and   d).In contrast, for Asym-DiDiKTa, adjacent molecules display a greater degree of overlap and more π-stacking (Figure 2g), resulting in symmetric pairs of adjacent molecules with the tert-butyl groups pointing in the opposite direction (Figure 2h).Additional π-stacking interactions link these to form π-stacked chains along the a-axis.The distance between the central ring (e) between two nearest molecules is 9.1422(9) Å for Sym-DiDiKTa, whereas for Asym-DiDiKTa it is only 3.4699(17) Å.We also obtained single crystals of Sym-DiDiKTa from slow evaporation of a CH2Cl2:MeOH solution (8:2 ratio).Under these conditions, the crystal structure is comprised π-stacked columns of molecules along the crystallographic aaxis (Figures S13a and b), with greater superimposition than in the previous case (distance between the central rings (e) of 3.8500(2) Å).This is achieved with the aid of MeOH molecules forming both strong and weak hydrogen bonds between and within the columns.
We have previously demonstrated that DFT methods are not appropriate for accurately predicting the excited state properties of MR-TADF materials. 42Explicit inclusion of secondorder electronic correlation effects using wavefunction-based methods such as Spin component Scaling second-order approximate Coupled-Cluster (SCS-CC2) calculations with the cc-pVDZ basis set addresses this problem, resulting in accurate DEST prediction for MR-TADF materials. 43The two isomers possess identical S1 and T1 energies of 2.93 eV and 2.69 eV, respectively, and thus a DEST of 0.24 eV (Figure 3), which is similar to previously reported ketone-containing MR-TADF materials. 44Compared to the parent compound DiKTa (DEST = 0.27 eV and S1 = 3.45 eV), there is a modest decrease in DEST and significant stabilization of S1.The smaller computed DEST in Sym-DiDiKTa to Asym-DiDIKTa relative to DiKTa is due to the increase in the π conjugation of these materials compared to DiKTa resulting in a lowering of the exchange energy, as has been documented in other extended MR-TADF systems, such as v-DABNA and OAB-ABP-1. 26,44 he predicted stabilization of S1 results from the nitrogen atoms being positioned para to each other, which positively reinforce their electron-donating character, as has been previously reported. 18,39 he oscillator strength associated with the S0-S1 transition is significant at 0.26 and 0.27 for Sym-DiDiKTa and Asym-DiDIKTa, respectively.The difference density pictures of S1 and T1 of each emitter highlight the alternating pattern of increasing and decreasing electronic density on neighbouring atoms that produce the emissive short-range charge transfer (SRCT) excited state, which is characteristic of MR-TADF materials.Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) recorded in degassed dichloromethane (Figures 4a and b) document a reversible reduction wave at Ered = -0.96and -0.94 V vs SCE for Sym-DiDiKTa and Asym-DiDiKTa, respectively.The oxidation wave of Sym-DiDiKTa is more reversible than that of Asym-DiDiKTa (both at 1.53 V vs SCE), revealing the impact of the regiochemistry that the tert-butyl groups play in terms of electrochemical stability.The corresponding HOMO and LUMO levels are nearly identical, at -5.87 eV for both compounds and -3.38 eV for Sym-DiDiKTa and -3.40 eV for Asym-DiDiKTa, respectively, which align well with the gas-phase DFT calculations at the PBE0/6-31G(d,p) level of theory (-5.94 eV and -2.69 eV, respectively, Table S11 and Figure S20).The experimentally determined HOMO levels of Sym-DiDiKTa and Asym-DiDiKTa are similar to that of DiKTa (-5.93 eV), while the LUMO levels are significantly stabilized (LUMO of DiKTa = -3.11eV). 15The electrochemical data are summarized in Table S11.The monomolecular photophysical properties of Sym-DiDiKTa and Asym-DiDiKTa were first studied in dilute toluene solution (Figures 4c and d).The UV-vis absorption spectra of both emitters in toluene are expectedly nearly identical, yet show significant differences compared to that of DiKTa in terms of the SRCT absorption band's wavelength (around 435 for DiKTa [14][15] but around 515 nm for Sym-DiDiKTa and Asym-DiDiKTa).The molar absorptivity, e, of the SRCT band for Sym-DiDiKTa and Asym-DiDiKTa is 26,325 M -1 cm -1 and 23,458 M -1 cm -1 , respectively, assigned to the transition to S1 according to the SCS-CC2 calculations (Figure 3).The e is somewhat larger than reported for DiKTa (e of DiKTa = 21,000 M -1 cm -1 ), 15 which is consistent with the trends in the calculated oscillator strength where f = 0.20 for DiKTa, compared to 0.26 and 0.27 for Sym-DiDiKTa and Asym-DiDiKTa, respectively.The absorption spectra of both Sym-DiDiKTa and Asym-DiDiKTa also show a high-energy shoulder at 485 nm (e = 11,680 and 10,238 M -1 cm -1 for Sym-DiDiKTa and Asym-DiDiKTa, respectively), which likely originates from a vibronic band and not a transition to a higher-lying singlet state given that computed energies of the S2 state in each of these two compounds is ca.0.48 eV higher in energy and the S0-S2 transition possesses negligible oscillator strength. 16There is also a high-energy, high-intensity band at 373 nm (e = 31,900 M -1 cm -1 and 28,527 M -1 cm -1 for Sym-DiDiKTa and Asym-DiDiKTa, respectively, Figures 4c and d), which is assigned to transitions to S4 and S5, for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Table S12) based on the comparison with the SCS-CC2 simulated absorption spectra (Figure S21 and Table S12).The difference density plots of these states (Figure S21) show that there is only minimal density situated on the central phenyl ring and more density situated on the carbonyl groups compared with the difference density pattern of the S1 state (vide supra.).
Both compounds display green emission in dilute toluene with emission maxima, lPL, of 540 nm and 541 nm for Sym-DiDiKTa and Asym-DiDiKTa, respectively, which are ca.90 nm red-shifted from that of DiKTa (lPL = 453 nm).This bathochromic shift is corroborated by the SCS-CC2 calculations. 19-20, 30, 46Both compounds display narrow PL spectra at room temperature (FWHM = 29 nm), and small Stokes shifts of ca. 25 nm, which confirms the small degree of geometrical reorganization in the excited state owing to their conformationally rigid structure (Figures 2c and d).The small degree of positive solvatochromism (Figure S15 and Tables S2 and S3) reflects the SRCT character of the emissive excited state. 15The S1 excited state energy levels were determined to be 2.26 and 2.25 eV, respectively, for Sym-DiDiKTa and Asym-DiDiKTa and the T1 excited state energy levels are identical at 2.02 eV, each obtained from the lPL of the respective prompt fluorescence and phosphorescence spectra in toluene glass at 77 K.We note that the nature of T1 and S1 are identical based on the difference density plots (Figure 3).The corresponding DEST values are 0.24 eV and 0.23 eV, respectively, for Sym-DiDiKTa and Asym-DiDiKTa.These values match with those predicted by SCS-CC2 calculations (DEST = 0.24 eV for both compounds).The PL quantum yield, FPL, in toluene is 70% under N2, which decreases to 64% in air for Sym-DiDiKTa and 53% under N2 and 50% in air for Asym-DiDiKTa.The FPL values were next measured for vacuum-deposited 1 wt% doped thin films in 1,3-bis(N-carbazolyl)benzene (mCP).The FPL values were determined to be 64% and 57% for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Table S5).The FPL values closely resemble those obtained in dilute toluene, which implies that non-radiative decay due to vibrations is not significant in these compounds.UV-vis absorption of CT transition; b) Prompt emission in toluene degassing with N2; c) Photoluminescence quantum yield in toluene relative to quinine sulfate in 1N H2SO4 (FPL = 54.6%);d) Full width at half maximum; e) Obtained using an integrating sphere under N2; f) Energy gap between S1 and T1 calculated from the difference of the peaks of the fluorescence and phosphorescence spectra in toluene glass at 77 K g) 1 wt% Sym-DiDiKTa and Asym-DiDiKTa doped in mCP.We next investigated the solid-state photophysical properties of spin-coated 1 wt% Sym-DiDiKTa and Asym-DiDiKTa doped thin films in mCP.The low doping concentration was selected to mitigate the potential for undesired aggregation in the films.As shown in Figures 5a and c, at 300 K, Sym-DiDiKTa and Asym-DiDiKTa show a similar emission profile with emission maxima, lPL, at 542 and 547 nm, respectively, values that are close to the prompt emission maximum in toluene.A small FWHM of 35 nm (0.13 eV) was calculated for both compounds in the films, which is slightly broader than the FWHM of 29 nm (0.12 eV) determined in toluene, indicating the influence of the matrix on the conformational stabilization of the excited states.The average τd values are 4.6 µs and 3.0 µs for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Figure 5, Table S7).We then compared the oxygen dependence of the PL spectrum at room temperature.The PL spectrum in air shows a small decrease in intensity compared to the spectrum under vacuum (Figures S17a and b), indicating a weak involvement of triplet excited states.The temperature-dependent PL spectra (Figures 5a and c) and temperature-dependent time-resolved PL decays (Figures 5b and d) of both emitters document the characteristic decrease in intensity and in the contribution of the delayed emission, respectively, with decreasing temperature.This could be due to the MR-TADF behavior only being apparent in a suitable host matrix due to exciplex-like host-emitter interactions. 28

LEC devices
The light-emitting electrochemical cell (LEC) can be comprised of solely air-stable materials and feature a very simple and robust device structure in the form of a single-layer active material sandwiched between two electrodes.0] These mobile ions redistribute when a voltage is applied, and enable p-type electrochemical doping of the organic semiconductor at the anode and n-type doping at the cathode.With time, these doping regions grow in size and make contact under the formation of a p-n junction.The fact that Sym-DiDiKTa displays highly reversible electrochemical oxidation and reduction behaviour in the cyclic voltammetry experiments (Figure 4a) suggests that it could be fit for the task of the emissive organic semiconductor in a LEC device.Figure 7a presents the steady-state EL spectrum recorded during the driving with a constant current density of 77 mA/cm 2 , with the lEL at 551 nm and the FWHM being 60 nm.
The broadening of the EL spectrum of the LEC device in comparison to the PL spectra in Figure 4b indicates the formation of exciplexes with the blend host.demonstrates that the ion mobility in the active material is high.

Conclusion
In summary, new amine/carbonyl-based MR-TADF materials were developed by an approach that expands the p-conjugated backbone to show green-yellow emission.The regio-

Figure 2 .
Figure 2. (a) Synthesis of Sym-DiDiKTa and Asym-DiDiKTa.(b) Schematic of molecular structure of Sym-DiDiKTa and Asym-DiDiKTa showing atom labels (double bonds omitted for clarity).(c & f) View showing Sym-DiDiKTa and Asym-DiDiKTa in the crystal structure (diagonal view).(d & g) Side view of the packing diagram of Sym-DiDiKTa and Asym-DiDiKTa.(e & h) Top view of the packing diagram of Sym-DiDiKTa and Asym-DiDiKTa.

Figure 6 .
Figure 6.(a) EQE versus current density curves of Devices I-III.(b) Electroluminescence spectra of Devices I-III at 500 cd/m 2 .

Figure 7 (
Figure4bindicates the formation of exciplexes with the blend host.Figure7(b) details the voltage and luminance transients recorded during the early stages of the constant-current driving.The observed initial decrease of the voltage is a characteristic indicator of conductivity-enhancing electrochemical doping, whereas the increase in luminance is in line with the gradual formation of a p-n junction, where electrons and holes can recombine efficiently into excitons.Importantly, these observations imply that the Sym-DiDiKTa emitter can be in situ electrochemically p-and n-type doped during LEC operation.This conclusion is further supported by the fact that the LEC device delivered a significant luminance of 300 cd/m 2 despite being equipped with an air-stable Al cathode in direct contact with the active emissive material.Finally, the luminance turn-on time is a direct indicator of the ion mobility in the active material,[52][53] and the comparatively fast turn-on time of less than 2 s to 100 cd/m 2

Figure 7 .
Figure 7. (a) The steady-state EL spectrum.(b) The temporal evolution of the luminance (left y-axis, solid black squares) and the drive voltage (right y-axis, open red circles) for the ITO/PEDOT:PSS /TCTA:POT2T: Sym-DiDiKTa:THABF4/Al LEC device.The LEC devices were driven by a constant current density of 77 mA/cm 2 .
tert-butyl groups was shown to significantly impact the single-crystal packing structure.With a DEST of around 0.23 eV in toluene, the narrowband emitters, FWHM, of 29 nm, display TADF activity with a τd of 1700 µs and 2100 µs for Sym-DiDiKTa and Asym-DiDiKTa, respectively.Application in OLEDs provided devices with electroluminescence maxima at around 544 nm, and an EQEmax of circa 10% which low value is attributed to the long delayed lifetimes of this class.Upon further studies in hyperfluorescence devices, an EQE of 19.9% was finally obtained, showing the promise of this class of molecules towards this strategy.We also show the first examples of a LEC device incorporating an MR-TADF emitter.With a lEL at 551 nm and a larger FWHM of 60 nm due to formation of exciplexes with the host, the device based on Sym-DiDiKTa delivered a significant luminance of 300 cd/m 2 and high ion mobility with a fast turn-on time of less than 2 s to 100 cd/m 2 .