Color tuning of multi-resonant thermally activated delayed fluorescence emitters based on fully fused polycyclic amine/carbonyl frameworks

Abstract

Two novel π-extended amine/carbonyl-based multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been designed and synthesized. The two emitters are isomeric, composed 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, ΔE_ST, of around 0.23 eV, narrowband emission characterized by a full-width at half-maximum, FWHM, of 29 nm and a photoluminescence quantum yield, Φ_PL, of 70% and 53% for the symmetric and asymmetric counterparts, respectively, in toluene. Investigation in OLEDs demonstrated that the devices with Sym-DiDiKTa and Asym-DiDiKTa displayed electroluminescence maxima of 543 and 544 nm, and maximum external quantum efficiencies (EQE_max) of 9.8% and 10.5%, respectively. The maximum EQE was further improved to 19.9% by employing a hyperfluorescence strategy. We further present the first example of a neutral MR-TADF emitter incorporated in a LEC device where Sym-DiDiKTa acts as the emitter. The LEC shows a λ_EL at 551 nm and FWHM of 60 nm with luminance of 300 cd m⁻² and a fast turn-on time of less than 2 s to 100 cd m⁻².

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10^th Anniversay statement

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Introduction

Organic light-emitting diodes (OLEDs) have now matured as a technology and are now integrated in efficient, flexible and ultra-high contrast next-generation displays.^1,2 Organic thermally activated delayed fluorescence (TADF) materials are increasingly viewed as an attractive alternative emitter class to phosphorescent complexes that contain noble metals as they do not contain scarce elements yet can likewise harvest both singlet and triplet excitons in the device to generate light at comparable efficiencies.^3–7 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, ΔE_ST.¹⁰ Compounds that possess a small ΔE_ST 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.⁷ This strategy, however, leads to molecules for which the conformational space is large 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. Hence, significant effort has been devoted in recent years to suppress vibrational and conformational relaxation within the emitter through the development of narrowband emitters.^14–20

An exciting molecular design strategy to achieve these requirements was introduced by Hatakeyama and co-workers and relies typically on p- and n-doped nanographenes.^21,22 Termed multiple resonance (or multiresonant) TADF (MR-TADF) emitters, these compounds possess small-to-moderate ΔE_ST 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 (EQE_max) as high as 34% for deep blue,²² 40% for sky-blue,²³ 35%²⁴ for green and 36% for red²⁵ 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 boron–oxygen (B/O)-based emitters²⁶ and carbonyl–nitrogen (C=O/N)-based emitters^14–19 as well as C=O/N/O-,^17,27,28 C=O/N/S-^{17, 29} and C=O/N/SO₂-based²⁹ emitters.

While the recent conceptual progress in MR-TADF emitter design has delivered a burgeoning number of efficient B/N-doped compounds that emit across the entirety of the visible spectrum,^30,31 few tactics have been advanced to exploit the promising class of C=O/N-based MR-TADF emitters, which are represented by the emitter DiKTa (Fig. 1).^{14–19,32,33} Recently, Yasuda et al. reported a linearly extended C=O/N-based emitter, QA-2 (Fig. 1) displaying TADF in 3 wt% PPCz doped film, with a ΔE_ST of 0.19 eV.²⁷ Similar 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 λ_PL of 465 nm in toluene was observed for QA-2, compared to that of DiKTa (ΔE_ST of 0.15 eV and λ_PL of 473 nm in toluene).¹⁵ The OLED with QA-2 showed an EQE_max of 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 (Fig. 1)³⁴ display circularly polarized luminescence (CPL), with a |g_PL| of 4 × 10⁻⁴, but likewise, its λ_PL (473 nm) is similar to that of DiKTa.

Fig. 1 Schematical representation of the molecular design discussed in this work.^{15,27,31,34,36,37,42}

The exploitation of para-disposed functional groups to generate π-extended skeletons is a powerful strategy to tune the photophysical properties of MR-TADF emitters to the red.^25,35 Yasuda 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 (Fig. 1) that exhibits a λ_PL of 615 nm and a narrow FWHM of 21 nm (0.07 eV) in dilute toluene solution.³¹ This strategy has also been used to generate B/O and B/S derivatives as shown in Fig. 1.^36,37 Remarkably, among the many MR-TADF emitters reported, comprehensive studies on 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 π-extended C=O/N-based polycyclic aromatic compounds based on the annellation of two triangulene DiKTa units to form tetraketone structures. Differently from QA-2 and Hel-DiDiKTa, the central ring in each of Sym-DiDiKTa and Asym-DiDiKTa is functionalized with para-C=O–π–C=O and N–π–N groups, these molecules represent the first ketone-based MR-TADF examples of their kind (Fig. 1).^17–19,33Sym-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 ΔE_ST of around 0.23 eV, red-shifted emission at ∼540 nm compared to DiKTa, a small FWHM of 29 nm and delayed lifetime, τ_d, of 4.6 ms and 3.0 ms 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 EQE_max, 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 (k_RISC).^17,38 Therefore, a potential solution to circumvent this issue is to use MR-TADF compounds in combination with an efficient exciton harvesting assistant dopant in a HF device.^39–41 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.³⁹ To 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 (Fig. 2a). 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)terephthalate⁴³ 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 analysis.

Single crystals were isolated following the gradient-temperature vacuum sublimation. Crystallographic data of Sym-DiDiKTa and Asym-DiDiKTa revealed that both molecules display very similar structures with each DiKTa unit showing a helical-like structure. The torsion angles generated by this helical section are similar (torsion angles C3–C2–N1–C23): 29.6(5)° for Sym-DiDiKTa and 27.9(3) and 28.1(3)° for Asym-DiDiKTa (Fig. 2c and f). Both compounds pack following a one-dimensional slipped π–π stacking motif with interplanar chains along the c-axis for Sym-DiDiKTa [centroid⋯centroid distance 3.5876(18) Å] and along the a-axis for Asym-DiDiKTa [centroid⋯centroid distances 3.4697(13)–3.7685(13) Å]. The disposition of the tert-butyl groups in the compounds influences their 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 (Fig. 2d and e). In contrast, for Asym-DiDiKTa, adjacent molecules display a greater degree of overlap and more π-stacking (Fig. 2g), resulting in symmetric pairs of adjacent molecules with the tert-butyl groups pointing in the opposite direction (Fig. 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 a-axis (Fig. S13a and b, ESI‡), 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.

Fig. 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 and f) View showing Sym-DiDiKTa and Asym-DiDiKTa in the crystal structure (diagonal view). (d and g) Side view of the packing diagram of Sym-DiDiKTa and Asym-DiDiKTa. (e and h) Top view of the packing diagram of Sym-DiDiKTa and Asym-DiDiKTa.

We have previously demonstrated that DFT methods are not appropriate for accurately predicting the excited state properties of MR-TADF materials.⁴⁴ Explicit inclusion of second-order 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 ΔE_ST prediction for MR-TADF materials.⁴⁵ The two isomers possess identical S₁ and T₁ energies of 2.93 eV and 2.69 eV, respectively, and thus a ΔE_ST of 0.24 eV (Fig. 3), which is similar to previously reported ketone-containing MR-TADF materials.⁴⁶ Compared to the parent compound DiKTa (ΔE_ST = 0.27 eV and S₁ = 3.45 eV), there is a modest decrease in ΔE_ST and significant stabilization of S₁. The smaller computed ΔE_ST 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.^26,46 The 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.^18,41 The 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 plots 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.⁴⁶

Fig. 3 Difference density picture and excited state energies of Sym-DiDiKTa (a) and Asym-DiDiKTa (b) calculated at SCS-CC2/cc-pVDZ, where red and green lines indicate S₁ and T₁ energies respectively of DiKTa, (isovalue = 0.001).

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) recorded in degassed dichloromethane (Fig. 4a and b) document a reversible reduction wave at E_red = −0.96 and −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 Fig. S20, ESI‡). 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 = −3.11 eV).¹⁵ The electrochemical data are summarized in Table S9 (ESI‡).

Fig. 4 Optoelectronic characterization of Sym-DiDiKTa and Asym-DiDiKTa: (a and b) cyclic and differential pulse voltammograms of Sym-DiDiKTa and Asym-DiDiKTa, respectively, in degassed CH₂Cl₂ with 0.1 M [ⁿBu₄N]PF₆ as the supporting electrolyte and Fc/Fc⁺ as the internal reference (Fc/Fc⁺ = 0.46 V vs SCE).⁴⁷ (c and d) Absorption (black line), steady-state PL spectra obtained in toluene at 300 K (blue line) and 77 K (red line; delay: 1 ns; gate time: 100 ns, λ_exc = 343 nm), and phosphorescence (Phos.; delay: 1 ms; gate time: 8.5 ms, λ_exc = 343 nm) spectra in toluene glass at 77 K (green olive line) of Sym-DiDiKTa and Asym-DiDiKTa, respectively.

The monomolecular photophysical properties of Sym-DiDiKTa and Asym-DiDiKTa were first studied in dilute toluene solution (Fig. 4c, d and Table 1). 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, ε, 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 (Fig. 3). The ε is somewhat larger than reported for DiKTa (ε 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 (ε = 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.¹⁶ There is also a high-energy, high-intensity band at 373 nm (ε = 31 900 M⁻¹ cm⁻¹ and 28 527 M⁻¹ cm⁻¹ for Sym-DiDiKTa and Asym-DiDiKTa, respectively, Fig. 4c and d), which is assigned to transitions to S₄ and S₅, for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Table S12, ESI‡) based on the comparison with the SCS-CC2 simulated absorption spectra (Fig. S21 and Table S12, ESI‡). The difference density plots of these states (Fig. S21, ESI‡) 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).

Table 1 Optoelectronic properties of Sym-DiDiKTa and Asym-DiDiKTa

	In toluene								In film
	λ abs /nm	ε ^a/M^−1 cm^−1	λ PL /nm	Φ PL in N2 (air)^c/%	FWHM^d/nm (eV)	S 1 /eV	T 1 /eV	ΔEST^f/eV	λ PL/nm	Φ PL/%	FWHM^d/nm (eV)
a 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 (ΦPL = 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.
Sym-DiDiKTa	373/485/515	31900/11680/26325	540	69 (65)	29 (0.12)	2.26	2.02	0.24	542^g	64	35 (0.14)^g
Asym-DiDiKTa	373/485/516	28527/10238/23458	541	53 (50)	29 (0.12)	2.25	2.02	0.23	547^g	57	35 (0.14)^g

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Both compounds display green emission in dilute toluene with emission maxima, λ_PL, of 540 nm and 541 nm for Sym-DiDiKTa and Asym-DiDiKTa, respectively, which are ca. 90 nm red-shifted from that of DiKTa (λ_PL = 453 nm). This bathochromic shift is corroborated by the SCS-CC2 calculations.^19,20,31,48 Both 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 (Fig. 2c and d). The small degree of positive solvatochromism (Fig. S15 and Tables S2, S3, ESI‡) reflects the SRCT character of the emissive excited state.¹⁵ The 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 λ_PL 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 (Fig. 3). The corresponding ΔE_ST 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 (ΔE_ST = 0.24 eV for both compounds). The PL quantum yield, Φ_PL, 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 Φ_PL values were next measured for vacuum-deposited 1 wt% doped thin films in 1,3-bis(N-carbazolyl)benzene (mCP). The Φ_PL values were determined to be 64% and 57% for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Table S5, ESI‡). The Φ_PL values closely resemble those obtained in dilute toluene, which implies that non-radiative decay due to vibrations is not significant in these compounds.

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 Fig. 5a and c, at 300 K, Sym-DiDiKTa and Asym-DiDiKTa show a similar emission profile with λ_PL 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. Another aspect to consider are concentration-dependent aggregation effects since the concentration of emitters at 1 wt% doping in the film is higher than that of the 10⁶ M solution. The average τ_d values are 4.6 ms and 3.0 ms for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Fig. 5 and Table S7, ESI‡). 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 (Fig. S17a and b, ESI‡), indicating a weak involvement of triplet excited states. The temperature-dependent PL spectra (Fig. 5a and c) and temperature-dependent time-resolved PL decays (Fig. 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.²⁹

Fig. 5 (a and c) Temperature-dependent PL study of 1 wt% of Sym-DiDiKTa and Asym-DiDiKTa, respectively, in mCP, λ_exc = 350 nm. (b and d) Temperature-dependent time-resolved PL decays for 1 wt% Sym-DiDiKTa and Asym-DiDiKTa, respectively, in mCP matrix (λ_exc = 378 nm).

OLED characterization

We fabricated vacuum-deposited OLEDs using Sym-DiDiKTa and Asym-DiDiKTa as the emitters. The following device configuration was used: indium tin oxide (ITO)-coated glass (100 nm)/TAPC (35 nm)/TCTA (10 nm)/mCP: 3 wt% of Sym-DiDiKTa or Asym-DiDiKTa (20 nm)/TmPyPB (30 nm)/Liq (2 nm)/Al (100 nm). 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) is the hole-transporting layer, 4,4′,4-tris(carbazol-9-yl)triphenylamine (TCTA) is used for electron-blocking and host layers, 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB) is the electron-transporting layer, and lithium 8-hydroxyquinolinolate (Liq) and aluminum are the electron injection and cathode layers, respectively. Fig. 6 and Table 2 summarize all the device characteristics. TADF-only devices based on Sym-DiDiKTa (Device I) and Asym-DiDiKTa (Device II) emitted green narrowband electroluminescence, λ_EL, at 543 and 544 nm, respectively, values that are consistent to their corresponding λ_PL. The FWHMs of both Devices I and II are 36 nm, which resulted in CIE coordinates of (0.362, 0.623) and (0.376, 0.613), respectively. The EQE_max of Device I is 9.8%, which is slightly lower than the 10.5% measured for Device II. We hypothesized that the moderate Φ_PL in the mCP-doped films limits the EQE_max values in Devices I and II. The moderate Φ_PL may be due to the long-lived delayed fluorescence that permits a greater probability that non-radiative decay processes such as triplet-polaron quenching and triplet–triplet annihilation will contribute to the decay of the excitons. Both devices suffered from severe efficiency roll-off issues, which in part are due to the long delayed fluorescence lifetimes of Sym-DiDiKTa and Asym-DiDiKTa. Moreover, the linear decrease in the EQE – current density curve indicates possible exciton-polaron annihilation processes, which can be ascribed to trap formation by the emitters.

Fig. 6 (a) EQE versus current density curves of Devices I–III. (b) Electroluminescence spectra of Devices I–III at 500 cd m⁻².

Table 2 Summary of device characteristics

Device	Dopant	V on/V^a	EQE/%^b	λ EL/nm^c	Max. brightness (cd m^−2)^d	FWHM/nm^c	CIE (x, y)^c
a Voltage at 1 cd m^−2. b Value at maximum, 100 cd m^−2 and 1000 cd m^−2. c Value at 500 cd m^−2. d Maximum brightness.
I	3 wt% Sym-DiDiKTa	3.5	9.8/1.8/0.8	543	4310	36	(0.359, 0.624)
II	3 wt% Asym-DiDiKTa	4	10.5/1.3/0.8	544	3989	36	(0.369, 0.616)
III	3 wt% Asym-DiDiKTa: 20 wt% 4CzIPN	3.2	19.9/9.9/6.4	548	53625	56	(0.397, 0.592)

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To improve the device performance, we implemented a hyperfluorescence (HF) strategy in Device III with the following device structure: indium tin oxide (ITO)-coated glass (100 nm)/HAT-CN (10 nm)/Tris-PCz (30 nm)/mCBP (5 nm)/mCBP: 20 wt% 4CzIPN: 3 wt% Asym-DiDiKTa (30 nm)/T2T (10 nm)/BPy-TP2 (40 nm)/Liq (2 nm)/Al (100 nm), where 1,4,5,8,9,11-hexaazatriphenyl-enehexacarbonitrile (HAT-CN) is the hole-injection layer, 9-phenyl-3,6-bis(9-phenyl-9Hcarbazol-3-yl)-9H-carbazole (Tris-PCz) is the hole-transporting layer, mCBP is used in the exciton-blocking and host layers, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) is a green TADF assistant dopant, 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) is the hole-blocking layer and 2,7-di(2,2′-bipyridin-5-yl)triphenylene (BPy-TP2) is the electron-transporting layer, and lithium 8-hydroxyquinolinolate (Liq) and Al are the electron injection and cathode layers, respectively. Gratifyingly, the device performance improved and an EQE_max of 19.9% was achieved along with an enhanced maximum brightness of 53625 cd m⁻². The HF device not only resulted in a better EQE, but also reduced the efficiency roll-off when compared to Device II. The λ_EL is red-shifted to 548 nm and there is a slightly larger FWHM of 56 nm. The broader FWHM may originate from an incomplete energy transfer from 4CzIPN to Asym-DiDiKTa.

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. This renders the LEC technology highly fit for scalable and cost-efficient printing and coating fabrication under ambient air.^49,50 The characteristic feature of LEC devices is the combination of mobile ions with the emissive organic semiconductor within the emissive layer.^51,52 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 (Fig. 4a) suggests that it could be fit for the task of the emissive organic semiconductor in a LEC device.

We fabricated and characterized LEC devices with the following configuration: indium tin oxide (140 nm)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (40 nm)/26DCzPPy (44 wt%):POT2T (44 wt%):Sym-DiDiKTa (4 wt%):THABF₄ (8 wt%) (100 nm)/Al (100 nm), where THABF₄ (tetrahexylammonium tetrafluoroborate) is the ionic liquid electrolyte that contributes the mobile ions, and 6-bis(3-carbazol-9-yl)pyridine (26DCzPPy):(1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl)tris(diphenylphosphine oxide) (POT2T) is a blend-host matrix, which was introduced to enable for the formation of a uniform solution-processed thin film and to suppress losses by exciton-polaron⁵³ and exciton-exciton quenching.

Fig. 7a presents the steady-state EL spectrum recorded during the driving with a constant current density of 77 mA cm⁻², with the λ_EL 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 Fig. 5a indicates the formation of exciplexes with the blend host. This conclusion is supported by the fact that the FWHM of the EL spectrum is essentially independent of the guest concentration for a guest concentration range of 1 to 8 wt% (Fig. S22, ESI‡). Fig. 7b 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,^54,55 and the comparatively fast turn-on time of less than 2 s to 100 cd m⁻² demonstrates that the ion mobility in the active material is high.

Fig. 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/26DCzPPy:POT2T: Sym-DiDiKTa:THABF₄/Al LEC device. The LEC devices were driven by a constant current density of 77 mA cm⁻².

Conclusion

In summary, new amine/carbonyl-based MR-TADF materials were developed by an approach that expands the π-conjugated backbone to show green-yellow emission. The regio-functionalization of the tert-butyl groups was shown to significantly impact the single-crystal packing structure. With a ΔE_ST of around 0.23 eV in toluene, the narrowband emitters, FWHM, of 29 nm, display TADF activity with a τ_d of 4.6 ms and 3.0 ms for Sym-DiDiKTa and Asym-DiDiKTa, respectively. Application in OLEDs provided devices with electroluminescence maxima at around 544 nm, and an EQE_max 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 λ_EL 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.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The St Andrews team would like to thank EPSRC (EP/P010482/1) and the Leverhulme Trust (RPG-2016-047) for financial support. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F. R. S.-FNRS) under Grant no. 2.5020.11. Y. O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). The authors also acknowledge Ms N. Nakamura and Ms K. Kusuhara for their technical assistance with this research. This work was supported financially by the JSPS Core-to-Core Program (grant number: JPJSCCA20180005), Kyulux Inc, the Swedish Research Council, the Swedish Energy Agency, and Bertil & Britt Svenssons stiftelse för belysningsteknik.

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Footnotes

† The research data supporting this publication can be accessed at https://doi.org/10.17630/f844ce22-cf6b-413d-ae79-70e2e38a1f74

‡ Electronic supplementary information (ESI) available: A summary of prior examples of carbonyl-containing MR-TADF emitters, experimental details, synthesis procedures and characterization data, NMR spectra, HRMS, HPLC, X-ray crystallographic data, supplemental photophysical data, electrochemical data, xyz coordinate file for computations. CCDC 2225501–2225503. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3tc00641g

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