Diindolocarbazole – achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units

Abstract

In this work we present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms. Based on an in silico design, the compound DiICzMes₄ possesses a red-shifted emission, enhanced photoluminescence quantum yield, and smaller singlet-triplet energy gap, ΔE_ST, than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the ΔE_ST when the optimized singlet and triplet geometries are used. Slow yet optically detectable reverse intersystem crossing contributes to low efficiency in organic light-emitting diodes using DiICzMes₄ as the emitter. However, when used as a terminal emitter in combination with a TADF assistant dopant within a hyperfluorescence device architecture, maximum external quantum efficiencies of up to 16.5% were achieved at CIE (0.15, 0.11). This represents one of the bluest hyperfluorescent devices reported to date. Simultaneously, recognising that MR-TADF emitters do not require acceptor atoms reveals an unexplored frontier in materials design, where yet greater performance may yet be discovered.

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New concepts

Thermally activated delayed fluorescence (TADF) compounds have generated tremendous interest over the past decade for use as emitters in organic light-emitting diodes (OLEDs). The conventional donor–acceptor (D–A) design presented in the literature shows broad and unstructured emission, resulting in poor colour purity in the devices. A sub-class of TADF materials, so-called multi-resonant TADF (MR-TADF) overcomes this issue, emitting with very narrow spectra. Compared to D–A TADF materials there are relatively few MR-TADF materials. This is in part due to the poor in silico modelling of these compounds. We recently reported how wavefunction-based methods were necessary to accurately model them. We have since exploited this computational methodology towards the design of a number of novel MR-TADF compounds. In previous MR-TADF designs, acceptor atoms or functionalities were essential components in the polycyclic aromatic hydrocarbon compounds to achieve TADF. In the present contribution we demonstrate conclusively that this is not the case and MR-TADF with no acceptor groups is possible. We report a tetramesitylated diindolocarbazole emitter that shows TADF. We further report one of the deepest blue hyperfluorescent OLEDs using this compound as a terminal emitter.

Introduction

The organic light-emitting diode (OLED) field has taken another step forward with the introduction of multiresonant thermally activated delayed fluorescent (MR-TADF) materials.¹ As with conventional donor–acceptor (D–A) TADF emitters, MR-TADF compounds possess suitably small singlet–triplet energy gaps (ΔE_ST) to permit triplet excitons to be up-converted to singlets by reverse intersystem crossing (RISC), unlocking considerably improved device efficiency in OLEDs^2–4 alongside applications in several other optoelectronic contexts.^5–8 RISC is achieved in D–A TADF materials through the reduction of the exchange integral by electronically decoupling the donor and acceptor moieties as a result of a highly twisted conformation,^2,4 with the HOMO situated on the donor and LUMO on the acceptor, combined with vibronic coupling between local and charge transfer triplet states to facilitate spin orbit coupling.⁹ Due to the conformational flexibility inherent in these classes of emitter, the charge-transfer emission bands are particularly broad, resulting in poor colour purity of the resulting OLEDs and extreme challenges in achieving deep-blue colour coordinates.¹⁰

For MR-TADF emitters the HOMO–LUMO separation and thus small ΔE_ST are achieved via a complementary pattern of the electron density distribution on adjacent atoms within the molecule¹ between HOMO and LUMO states, made possible by the incorporation of suitably positioned electron-donating and electron-withdrawing atoms (or functional groups). The reorganization of the electron density upon excitation is relatively localized, so that the lowest singlet and triplet excited states possess short-range charge transfer (SRCT) character.¹¹ The small exchange integral in MR-TADF compounds is best illustrated with difference density plots (Fig. 1). Seemingly paradoxical for charge-transfer states, there is also a suitably large overlap of the excited and ground state wavefunctions, leading to larger oscillator strengths for the S₁–S₀ transition and thus fast radiative decay rates, k_r. We note that the wavefunction will be of mixed locally excited (LE) and CT character, and in this case the LE contribution appears to dominate, thus coupling to the ground state is high but electron exchange energy remains sizeable. Combined with conformationally rigid structures these SRCT states confer a very narrow emission spectrum with full width at half maxima (FWHM) below 30 nm,¹ leading to much greater colour purity, which is required for high-definition displays and advantageous for achieving deep-blue emission.¹²

Fig. 1 Evolution of MR-TADF emitters including simplified difference density plots for each core.

MR-TADF materials DABNA-1 and DABNA-2 were first reported in 2016 by Hatakeyama and co-workers.¹³ These compounds contain a central accepting boron atom and para-disposed donating nitrogen atoms that achieve the desired alternating pattern of the electron density distribution. OLEDs employing DABNA-1 and DABNA-2 showed maximum external quantum efficiencies, EQE_max, of 13.5% and 20.2% with Commission Internationale de l’Éclairage, CIE, coordinates of (0.13, 0.09) and (0.12, 0.13), respectively. Low RISC rates (compared to contemporaneous D–A–D emitters)^14–17 in this early work resulted in severe efficiency roll-off though, with efficiencies at 1000 cd m⁻² not reported. A large number of other MR-TADF materials using this same design template have since been reported.^18–20

Dramatic improvements in both EQE_max and efficiency roll-off were recently reported for the extended system v-DABNA (Fig. S1, ESI‡).²¹ This compound contains two DABNA-1 units that are fused together, extending the π conjugation (Fig. 1), which results in a smaller ΔE_ST from 0.18 eV in DABNA-1 (in 1 wt% mCBP)¹³ to 0.02 eV (in 1 wt% DOBNA-OAr).²¹ OLEDs made with this emitter show an outstanding EQE_max of 34.4% at CIE (0.12, 0.11), assisted in part by spontaneous emitter alignment improving device optical outcoupling, as confirmed by angularly-resolved emission measurements in a later work.²² Further, due in part to its small ΔE_ST and class-leading RISC rates, the device shows superior efficiency roll-off, with an EQE at 1000 cd m⁻² of 26.1% (Table S1, ESI‡).

Another family of MR-TADF compounds, introduced by Zhang et al. in 2019,²³ relies on boron and nitrogen atoms to direct the electron density pattern, but these compounds incorporate fused donor units, such as in CzBN. The fused system results in increased electronic delocalisation, thus stabilizing S₁, which produces a red-shift in the emission (Fig. 1). The first series of emitters incorporated peripheral electron-withdrawing groups onto the CzBN core, which led to a further red-shift of the emission. OLEDs with 2F-BN, 3F-BN and 4F-BN showed EQE_max of 22.0%, 22.7% and 20.9%, respectively, at CIE of (0.16, 0.60), (0.20, 0.58) and (0.12, 0.48), representing the first examples of green-emitting MR-TADF OLEDs. Following this early work a range of materials based on this molecular design have been reported;^23–28 the OLED with BBCz-G showing the highest efficiencies, with an EQE_max of 31.8% at CIE of (0.26, 0.68).²⁹ This design was also used to produce the first examples of red-emitting MR-TADF compounds, BBCz-R²⁹ and R-(T)BN³⁰ (Fig. S1, ESI‡). The red-shifted emission in both examples results from the positioning of the donating nitrogen atoms, and thus the withdrawing boron atoms, para to each other, thereby inducing partial bonding and antibonding character and resulting in a smaller HOMO–LUMO gap, ΔE, and a much lower energy emissive S₁ state.³⁰ Devices with these three emitters showed EQE_max surpassing 20% at CIE coordinates of (0.67, 0.33)²⁹ and (0.72, 0.18).³⁰ Another route to colour tuning has been the addition of electron donating and withdrawing substituents on the CzBN core, with blue- or red-shifts of the emission possible depending on the position and number of additional donors and acceptors.^27,29,31,32

An alternative core unit based on DABNA-1 was presented firstly by Oda et al.,³³ where the positions of boron and nitrogen atoms are switched (ADBNA-Me-Mes and ADBNA-Me-Tip, Fig. 1 and Fig. S1, ESI‡). Having two boron and one nitrogen atoms results in a smaller ΔE compared to that of DABNA-1; however, there is minimal impact on ΔE_ST (Table S1, ESI‡). The resulting OLEDs showed CIE of (0.10, 0.27) and (0.11, 0.29) for ADBNA-Me-Mes and ADBNA-Me-Tip, respectively. The EQE_max for these devices are 16.2% and 21.4%, respectively, while efficiency roll-off is modest, with EQE₁₀₀ of 11.2% and 15.4%.

Another family of MR-TADF compounds contains carbonyl groups in lieu of boron atoms as the electron acceptor. The first example, QAO, reported in 2019, translated into devices with an EQE_max 19.4% at CIE (0.13, 0.18).³⁴ We showed that decoration of this core with mesityl groups, Mes₃DiKTa, can mitigate aggregation induced quenching (AIQ),²⁰ which is a common problem with these planar molecules. With this emitter, the OLED showed the highest EQE_max for this family of compounds of 21.1% at CIE of (0.12, 0.32). A phenyl-substituted structure, 3MTPTOAT, based on a related core, TOAT, which itself has previously been reported as a room temperature phosphorescent emitter,³⁵ was used as the emitter in an OLED that showed a very high EQE_max of 31.2%.³⁶ A range of emitters has now been reported incorporating carbonyl groups within the molecular design;^33,34,37 however, most of these emitters show relatively large ΔE_ST and the devices often show EQE_max values inferior to 20%. A full summary of the discussed literature emitters including structures, photophysical data and OLED device performances can be found in Fig. S1 and Table S1 (ESI‡).

Despite the excellent characteristics of MR-TADF emitters, the majority of MR-TADF emitters have a low k_RISC, with most around 10⁴ s⁻¹ (Table S2, ESI‡). The slow k_RISC has proved detrimental to device performance with most OLEDs using the MR-TADF compound as an emitter suffering from large efficiency roll-off.¹³ In the literature only two examples exist where k_RISC surpasses 10⁶ s⁻¹ (Fig. S2, ESI‡), m-CzBNCz²⁷ and BSBS-N1,³⁸ where k_RISC reaches 1.08 and 1.90 × 10⁶ s⁻¹, respectively. Even direct comparison between reported RISC rates from different research teams is challenging though, due to the plurality of reported methods for determining its value^39,40 and subtle yet important practical concerns.⁴¹ The MR-TADF emitters with the fastest k_RISC are nevertheless two orders of magnitude slower than the best performing D–A TADF emitter (Fig. S2, ESI‡). This was predicted by Northey and Penfold⁴² and experimentally shown by Stavrou et al.,⁴³ that the RISC mechanism in MR-TADF systems occurs through crossing between T₁ and an upper triplet state via reverse internal conversion. This involves closely-lying triplet states and requires new design rules for new chemical structures with optimal efficiency. A large factor in this apparent gap in RISC rates is that the chemical space explored for MR-TADF emitters remains small compared to the thousands of donor–acceptor TADF compounds reported. Furthermore, we recently demonstrated¹¹ that time-dependent DFT calculations, which are commonly used to predict the nature and the energies of the excited singlet and triplet states of D–A TADF compounds,⁴⁴ do not accurately predict these parameters for MR-TADF compounds, thus hindering computationally guided molecular design. We have shown repeatedly that coupled cluster calculations,^33,45–47 which include double excitation contributions, perform significantly more accurately, albeit at a higher computational cost.

Here, we apply the same coupled cluster methodology to guide the design of a new class of MR-TADF materials, which surprisingly do not require an electron-accepting functionality within the compound. Despite the lack of acceptor atom, a complimentary pattern of increasing and decreasing electron density is achieved for S₁ (but not necessarily for T₁) compared to S₀ in this class of emitters. DiICzMes₄ was also compared to two smaller reference emitters, ICz and ICzMes₃, with mesityl groups in DiICzMes₄ intended to suppress AIQ.³³ Compared to ICz and ICzMes₃, the expansion of the π-system in DiICzMes₄ ensures a further decrease of the HOMO–LUMO overlap and results in a much smaller ΔE_ST, reduced from 0.47 eV in ICz to 0.26 eV in DiICzMes₄ (in toluene). Further, there is a desirable increase in Φ_PL across the series from 37%, 56% and 67%, accompanied with a red-shift in the emission maximum, λ_PL, from 374 nm, 387 nm and 441 nm in 3 wt% PMMA films, for ICz, ICzMes₃ and DiICzMes₄, respectively, all in agreement with recent SCS-CC2 calculations for B/N-doped nanographenes.¹¹

Crucially, although the core DiICz structure decorated with ^tBu groups has recently been reported,^48,49 its identity as a TADF emitter – confirmed here by time-resolved photophysical measurements – was overlooked until recently.⁵⁰ An analogous non-fused tricarbazole-amine system (TCA_C4) had previously been shown to have a small singlet triplet gap, 0.21 eV, and gives moderate TADF via a reverse internal conversion (rIC), upper triplet state crossing mechanism.⁵¹ It is only recently that the MR-TADF mechanism has been elucidated to take place through a similar rIC mechanism in v-DABNA,⁴³ and presumably also other MR-TADF emitters. Nonetheless, in both previous reports of the DilCz structure the compound was presented as a purely fluorescent system (named pICz⁴⁸ and 5⁴⁹), with relatively large ΔE_ST of 0.29 eV. Recently, a similar derivative, tPBisICz, was introduced as a MR-TADF emitter, and the authors contended that RISC proceeds between T₂ and S₁.⁵⁰ The device showed an EQE_max of 23.1% at CIE (0.15, 0.05); however, efficiency roll-off was severe (Table S3, ESI‡) and this is likely due to the inefficient k_RISC of 1.4 × 10³ s⁻¹. Although the RISC rate for DiICzMes₄ is slow (similar to that of TCA_C4⁵¹), this work supports the existence of an entirely new subcategory of ‘acceptor-free’ MR-TADFs, which may yield improved performance in device applications in future.

Results and discussion

Modelling

Initial ground state optimisation followed by vertical excitation were performed at the SCS-CC2/cc-pVDZ level of theory. Indolocarbazole (ICz) has been frequently used by the TADF community, able to act as both a donor or acceptor⁵² depending on to the nature of the substituents. ΔE_ST was predicted to be 0.33 eV, which is high for TADF materials but rationalized by the different nature of S₁ and T₁ excited states. Indeed, S₁ displays a typical difference density pattern characteristic of a SRCT excited state while T₁ exhibits a locally excited (LE)-like pattern, with the latter more stabilized, hence the large ΔE_ST (Fig. 2). It has been inferred previously that extending the MR-TADF electronic delocalisation could be a viable strategy to decrease ΔE_ST at the same time as increasing the oscillator strength.¹¹ Based on this hypothesis, four derivatives of ICz were modelled, with differing patterns of the relative position of the nitrogen atoms: DiICz-m-1, DiICz-m-2, DiICz-p-1 and DiICz-p-2. Compared to the parent ICz, each of these four emitters has a stabilized S₁ state, decreasing from 3.78 eV for ICz to 3.58 eV, 3.57 eV, 3.36 eV and 3.32 eV for DiICz-m-1, DiICz-m-2, DiICz-p-1 and DiICz-p-2, respectively, the result of delocalization of the S₁ wavefunction (see Fig. 2). As previously reported for other MR-TADF emitters,³⁰ when the donating nitrogen atoms are located para to each other the red-shift is the largest. The para-derivatives here also have the smallest predicted ΔE_ST of 0.17 eV and 0.15 eV for DiICz-p-1 and DiICz-p-2, respectively, while ΔE_ST is 0.30 eV and 0.32 eV for DiICz-m-1 and DiICz-m-2.

Fig. 2 S₁ and T₁ difference density patterns, ΔE_ST, S₁ energy and oscillator strength, f, for the proposed DiICz units.

Of DiICz-p-1 and DiICz-p-2, DiICz-p-2 has a considerably larger oscillator strength of 0.15 compared to 0.01 in DiICz-p-1 and thus this motif was assessed as the most promising. Furthermore, we have previously demonstrated that addition of mesityl groups can mitigate AIQ,³³ which plagues MR-TADF materials⁴³ (and many other similar systems^53,54) owing to their planar and electron-rich geometries. With this in mind, we designed the mesityl derivative of ICz, ICzMes₃. In this compound the mesityl groups have the added benefit of reducing ΔE_ST (calculated for vertical transitions from the ground state geometry) from 0.33 eV to 0.21 eV. The decrease in ΔE_ST is essentially the result of preferential stabilization of S₁ while the energy of T₁ energy is only minimally affected (Fig. S31, ESI‡). The small stabilization of T₁ in ICzMes₃ can be explained by the absence of significant orbital contributions from the carbon atoms connecting the mesityl groups in the T₁ difference density pattern (Fig. S31, ESI‡). We also investigated the role that decoration with mesityl groups would play on the core structure of DiICz-p-2, which together form the target material DiICzMes₄. In DiICzMes₄ (Fig. S31, ESI‡); the mesityl substitution helps to reduce the predicted ΔE_ST from 0.15 eV to 0.13 eV for similar reasons as described for ICzMes₃. Due to the close energy of the LE T₁ and the SCRT T₂ states of the DiICz-p-2, substitution by the four mesityl groups allows inversion between the two. T₁ becomes SCRT in DiICzMes₄ possessing similar, yet slightly different character than S₁. The literature emitter tBisICz was also modelled using the same approach and the ΔE_ST of 0.14 eV was found to be larger. Further, the energy gap between S₁ and T₂ is also larger at 0.07 eV compared to 0.05 eV in DiICzMes₄. Owing to these moderate differences in the energy landscape of the excited states, DiICzMes₄ is expected to show improved RISC rates.

In contrast to previously investigated MR-TADF emitters, we see large changes when comparing ΔE_ST computed from vertical excitation from the ground state geometry and experiments due to the different nature of T₁ and S₁ states (see difference density plots in Fig. S29 and S30, ESI‡). In such a case, relaxation of the excited states could be key to reach quantitative agreement with the experiments. We thus optimized both the S₁ and T₁ states within the TDA using PBE0 functional and 6-31G(d,p) basis set, and computed the T₁ and S₁ excited state energies at the SCS-CC2/cc-PVDZ level of theory for ICz, ICzMes₃ and DiICzMes₄ as well as for three literature MR-TADF compounds, DABNA-1, BCzBN and DiKTa. Quantitative agreement with the experiments is reached with ΔE_ST increasing for ICz, ICzMes₃ and DiICzMes₄, to 0.59 eV, 0.45 eV and 0.29 eV (Fig. 3), respectively, caused by a larger relaxation energy of the T₁ state in line with a greater LE character for this state (Tables S9–S11, ESI‡). Interestingly, such an increase in ΔE_ST does not manifest for DABNA-1, BCzBN and DiKTa, wherein ΔE_ST is only shifted by a maximum of 0.04 eV, owing to the similar SRCT nature of T₁ and S₁ (see Tables S9–S11 and Fig. S30 and S32, ESI‡). The similar orbital character of S₁ and T₁ in many previous emitters, and the ones presented here, implies that RISC between these two states is not symmetry allowed according to El Sayed's rules.⁹ Thus, RISC must occur via a spin-vibronic mechanism involving intermediate triplet states lying between S₁ and T₁. Irrespective of the starting geometry, a close lying triplet state of different orbital type is present, whose involvement has been shown to contribute to the MR-TADF RISC mechanism.⁴³ Both smaller ΔE_ST and ΔE_T2T1 were observed, decreasing across the series from ICz, ICzMes₃ and DiICzMes₄. We observed again a decreased ΔE_ST upon incorporation of mesityl groups from 0.59 eV for ICz to 0.45 eV ICzMes₃. Unlike previously reported MR-TADF emitters that contain acceptor atoms/groups, for this class it is essential to optimise the excited states in order to achieve quantitative agreement with experimental ΔE_ST.

Fig. 3 Structures, excited state energies and difference density plots of each S₁ and T₁ for ICz (left panel), ICzMes₃ (central panel) and DiICzMes₄ (right panel) from excited state optimized geometry.

Synthesis

The materials were synthesised through a multistep reaction sequence outlined in Fig. 4. Carbazole was coupled to 2-bromofluorobenzene under S_NAr conditions at elevated temperatures in an excellent yield of 96%. Intramolecular oxidative ring closing using Pd(OAc)₂ afforded ICz in a good yield of 85%. Subsequent electrophilic bromination using NBS afforded intermediate ICzBr₃ in 79% yield, which was then decorated with mesityl groups using a Suzuki–Miyaura coupling reaction, producing ICzMes₃ in a good yield of 69%. A similar Suzuki–Miyaura coupling was employed to obtain CzMes₂ from dibromocarbazole in 62% following a literature procedure.⁵⁵ Intermediate 2 was obtained in 75% via an S_NAr reaction that proceeded at lower temperature (50 °C). Double oxidative cyclization using Pd(OAc)₂ generated DiICzMes₄ in 59% yields. Crystals of ICzMes₃ and DiICzMes₄ were grown from slow evaporation of methanol into a saturated solution of toluene over several days. Packing in ICzMes₃ is primarily governed by π–π stacking interactions between mesityl groups on adjacent molecules (Fig. S25, ESI‡). For DiICzMes₄ π–π stacking occurs between the mesityl group of one molecule and the DiICz core of an adjacent molecule. The ICz unit in both compounds was not perfectly flat (Fig. 4). Thermogravimetric analysis (TGA) of ICzMes₃ and DiICzMes₄ (Fig. S24, ESI‡) reveals good thermal stability for both compounds with T_d, the temperature representing 5% weight loss, of, respectively, 374 °C and 450 °C.

Fig. 4 Synthesis of (a) the emitters and crystal structures of (b) ICzMes₃ and (c) DiICzMes₄.

Optoelectronic characterization

The electrochemical properties were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in DCM for oxidation and DMF for reduction (Fig. 5a), with the electrochemical potentials reported versus SCE (Table 1). ICz showed irreversible oxidation and reduction waves with the former appearing to undergo polymerisation, which has been previously reported for ICz⁵⁶ and seen in other carbazole-containing emitters.⁵⁷ Addition of the mesityl groups in ICzMes₃ renders the oxidation pseudoreversible in a similar manner to what was previously observed for Mes₃DiKTa,³³ with E^ox at 1.45 V versus 1.43 V for ICzMes₃. Indeed, McNab et al. had demonstrated that the electrochemical instability of ICz is associated with dimer formation centred at the para positions.⁵⁶ There is likewise little change in the irreversible reduction waves with reduction potentials of these two compounds, E^red, at −2.21 V and −2.16 V for ICz and ICzMes₃, respectively. By contrast, both oxidation and reduction waves for DiICzMes₄ are largely reversible. The oxidation wave is cathodically shifted to 1.11 V while the reduction wave is anodically shifted to −1.92 V, both a reflection of the larger conjugation length of this molecule compared to ICz and ICzMes₃. This produced a significant reduction in the redox gap, ΔE_redox, in agreement with calculations, where the calculated ΔE decreases from 4.65 eV and 4.50 eV for ICz and ICzMes₃ to 3.86 eV in DiICzMes₄. The trends in HOMO and LUMO values are corroborated at the DFT level (Tables S5 and S12, ESI‡).

Fig. 5 Solution-state optoelectronic data. (a) CV (solid line) and DPV (dashed) where the anodic scan is in DCM and the cathodic scan is in DMF, with 0.1 M [nBu₄N]PF₆ as the supporting electrolyte and Fc/Fc⁺ as the internal reference (0.46 V for DCM and 0.45 for DMF vs. SCE);⁵⁸ (b) steady-state PL in toluene, at concentrations of 0.6–2.0 ×10⁻⁵ M, where λ_exc = 320 nm for ICz and ICzMes₃ and λ_exc = 380 nm for DiICzMes₄, and pictures of each in solution (ICz, ICzMes₃ and DiICzMes₄ left to right).

Table 1 Solution optoelectronic data of each emitter

Compound	λ abs (ε)^a/nm (×10^4 M^−1 cm^−1)	λ PL ^, (FWHM)/nm	Φ PL N2 (air)/%	S1^d/eV	T1^d/eV	ΔEST^e/eV	τ p /ns	HOMO^g/eV	LUMO^g/eV
a Toluene at 300 K. b Values in parentheses are the FWHM. c In degassed toluene measured with an integrating sphere under N2, values in parentheses are in air-saturated solution, λexc = 330 nm. d From the onset of the steady-state emission and phosphorescence in toluene glass at 77 K, λexc = 330 nm. e Energy difference between the onset of the steady-state and phosphorescence at 77 K. f λ exc = 355 nm. g The HOMO and LUMO energies were determined according to EHOMO/LUMO = −(E^oxonset/E^redonset + 4.8) eV.^59 h λ exc = 320 nm. i λ exc = 380 nm. j λ exc = 350 nm. k λ exc = 350 nm.
ICz	285 (31), 392 (10), 309 (6), 320 (7), 350 (6), 364 (9)	374 (21)^h	58 (30)	3.35	2.88	0.47	15	−5.79	−2.14
ICzMes3	291 (40), 300 (14), 318 (6), 330 (8), 363 (6), 379 (8)	387 (21)^h	66 (43)	3.24	2.85	0.39	22	−5.77	−2.19
DiICzMes4	302 (58), 307 (62), 316 (59), 345 (19), 365 (39), 410 (8), 431 (11)	441 (17)^i	70 (47)^j	2.83^k	2.57^k	0.26	41	−5.45	−2.43

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We next investigated the photophysical properties of the three emitters in solution. The UV-Vis absorption data in toluene (PhMe), 2-methyltetrahydrofuran (2-MeTHF), ethyl acetate (EtOAc), dichloromethane (DCM) and dimethylformamide (DMF) can be found in Fig. S33 and Tables S13–S15 (ESI‡). The polarity of the solvent had minimal impact on the absorption spectra, with nearly identical absorption maxima, λ_abs, and molar absorptivity values, ε, regardless of solvent. Using the representative data in toluene (Table 1), there is a high intensity, low energy band at 364 nm, 379 nm and 431 nm for ICz, ICzMes₃, and DiICzMes₄, respectively, assigned by calculations to a SRCT band; there is a second distinguishable band at smaller ε at 350 nm, 363 nm and 410 nm, respectively, that is likely due to a transition to a different vibronic level of the S₁ state based on the ca. 0.15 eV energy gap between these two bands. Both ICz and ICzMes₃ possess higher energy bands at 320 nm and 330 nm of similar ε, which we assign to transitions to the S₂ state. The similar ε values are captured at the SCS-CC2 level where both S₁ and S₂ have similar oscillator strengths, f, of 0.10 and 0.09 for ICz and 0.14 and 0.13 for ICzMes₃. A far greater oscillator strength of 0.66 is predicted for the transition to S₂ for DiICzMes₄ compared to that to S₁ (f = 0.21). Indeed, the band at 365 nm possesses a significantly larger ε of 39 × 10⁴ M⁻¹ cm⁻¹ compared to that at 431 nm (ε = 11 × 10⁴ M⁻¹ cm⁻¹), suggesting a greater degree of LE character for the transition associated with this band.

Minimal changes in emission energy and band shape were observed upon modulation of the solvent polarity (Fig. S33 and Tables S13–S15, ESI‡). Such behaviour is characteristic of MR-TADF emitters, which undergo emission from a SRCT excited state.¹¹ Owing to their rigid nature, the emission is narrow and the Stokes shifts are small (10, 8, and 10 nm, respectively, for ICz, ICzMes₃ and DiICzMes₄) reflecting the very small reorganisation energy between the ground and excited state. The corresponding FWHM for the PL spectra in toluene are 21 nm, 21 nm and 17 nm for ICz, ICzMes₃ and DiICzMes₄, respectively. There are low energy shoulders apparent in the steady-state PL of all three emitters. This shoulder is assigned to a vibronic shoulder (vide infra) (Fig. S34, ESI‡).

The energies of the singlet and triplet states, and hence, ΔE_ST, were determined based on the high-energy onset of the prompt fluorescence and phosphorescence spectra obtained at 77 K in toluene glass (Fig. S34, ESI‡). In all cases, the phosphorescence is very well vibrationally structured and characteristic of a carbazole moiety in strong contrast with respect to the fluorescence, supporting the different nature of the S₁ and T₁ excited states. There is a progressive decrease in ΔE_ST of 0.47 eV, 0.39 eV and 0.26 eV for ICz, ICzMes₃ to DiICzMes₄, respectively, a trend that is well reproduced by the SCS-CC2 calculations when considering the optimized excited state structures (Table S11, ESI‡). We simulated the vibronically resolved fluorescence and phosphorescence spectra for DiCz-p-2 (we omitted the mesityl groups from DiICzMes₄ to avoid spurious negative vibration modes) and obtain excellent agreement with the corresponding experimental spectra of DiICzMes₄ (see Fig. S35, ESI‡). The lower energy shoulder of the fluorescence spectrum observed experimentally is attributed to a vibronic transition based on the cross-comparison with the simulated one. This shoulder disappears with increasing concentration when aggregate emission begins to contribute significantly to the emission spectrum (Fig. S37d, ESI‡). Furthermore, the simulated vibronically-resolved phosphorescence spectrum is also in excellent agreement with the experiment. Interestingly, there is an enhanced vibronic intensity associated with high-frequency (1200–1600 cm⁻¹) vibrations in the phosphorescence spectrum in comparison to fluorescence spectrum. This reflects the more pronounced geometric relaxation taking place in T₁ compared so S₁, which translates into a larger adiabatic ΔE_ST in comparison to the vertical ΔE_ST (Table S11, ESI‡) and provides clear spectroscopic evidence for the different character of the S₁ and T₁ excited states. This behaviour is again in strong contrast with most of the MR-TADF emitters previously reported in the literature.

The solution photoluminescence quantum yields increase from 58%, 66% and 70% for ICz, ICzMes₃ to DiICzMes₄, respectively, again reflecting expected trends in the calculations.¹¹ Time-resolved PL decays revealed prompt CT lifetimes of 15.0 ns, 21.6 ns and 40.5 ns for ICz, ICzMes₃ and DiICzMes₄, respectively. A small contribution of delayed emission was observed for ICz, which was ascribed to originate from TTA (Fig. S34b, ESI‡), while no delayed emission was observed for either ICzMes₃ or DiICzMes₄ (Fig. S34d and f, ESI‡).

We next investigated the solid-state PL behaviour in a wide bandgap host, PMMA at 3 wt% doping of emitter. This and subsequent wide bandgap (high triplet energy) OLED hosts were selected to strongly exclude the possibility of guest-to-host triplet quenching in both optical and device investigations. The λ_PL are 377 nm, 391 nm and 442 nm for ICz, ICzMes₃ and DiICzMes₄, respectively, values that are modestly red-shifted compared to those in toluene. The ΔE_ST values are similar to those measured in toluene at 0.50 eV, 0.41 eV and 0.29 eV for ICz, ICzMes₃ and DiICzMes₄, respectively, and align with the calculated ΔE_ST using optimized excited state structures. The Φ_PL are similar to those in toluene at 37%, 58% and 67%, for ICz, ICzMes₃ and DiICzMes₄, respectively. Again, a red-shifted emission, a decreased ΔE_ST and an improved Φ_PL are observed across the series from ICz to ICzMes₃ and to DiICzMes₄ (Fig. S34, ESI‡). Owing to their large ΔE_ST and excessive S₁ energies, the photophysical properties of ICz and ICzMes₃ were not investigated in other hosts.

We next investigated the photophysical properties of DiICzMes₄ in mCP as this OLED-compatible host matrix has a suitably large T₁ energy of 2.9 eV.⁶⁰ The optimum doping concentration as a function of Φ_PL was determined (Fig. 6a and Table S16, ESI‡). No AIQ was observed up to 3 wt%, with Φ_PL maintained at 82%; beyond this concentration the Φ_PL decreases, with neat films showing a Φ_PL of 30% (Fig. 6a). The FWHM of a drop-cast 3 wt% doped film in mCP is larger at 40 nm; a low-energy shoulder increases in intensity with increasing doping, which we assigned to an emission from an aggregate (Fig. S37d, ESI‡). However, when films were spin-coated, the aggregate formation could be suppressed, with 3 wt% spin-coated films having a FWHM of 21 nm at λ_PL 451 nm. At this concentration the ΔE_ST is 0.26 eV, leading to a long τ_d of 433 μs but with a delayed emission suppressed at low temperatures (Fig. 6c). A similar behaviour exists when DiICzMes₄ is doped in DPEPO at 5 wt% where the delayed emission is no longer observed below 80 K (Fig. S38b, ESI‡). In the time-resolved measurements the spectra of the delayed emission match that of the prompt fluorescence, and thus can be assigned to emission from the S₁ state rather than any room temperature phosphorescence, which has been observed in other rigid systems⁶¹ (Fig. S37b, ESI‡). TTA was ruled out as the emission mechanism owing to the linear power dependence of the emission intensity (Fig. 6d).⁶²

Fig. 6 Solid-state photophysics of DiICzMes₄. (a) Φ_PL as a function of doping concentration, calculated using an integrating sphere. An exponential decay has been fitted to guide the reader, λ_exc = 350 nm; (b) steady-state (SS) emission spectra at RT (λ_exc = 330 nm) and 77 K and phosphorescence spectrum at 77 K in 3 wt% mCP films, (λ_exc = 350 nm); (c) temperature-dependent time-resolved PL decays in 3 wt% mCP films, λ_exc = 355 nm; (d) intensity dependence as a function of laser power in 3 wt% mCP, λ_exc = 355 nm.

The contribution of the delayed emission to the overall emission is often small in MR-TADF emitters, reflecting the efficient k^S_r (and small Φ_ISC) and the slow k_RISC. For instance, for DABNA-1 and DiKTa the Φ_d is around 4% for 1 wt% DABNA-1¹³ in mCBP and 1% for DiKTa in toluene.³³ For DiICzMes₄, the Φ_d is 1.2%, and thus k_RISC is slow in this emitter, at 1.8 × 10² s⁻¹ following the methodology of Masui et al.⁶³ This is substantially slower than most MR-TADF emitters, but similar to tPBisICz and tBisICz, which were reported as 1.4 and 0.14 × 10³ s⁻¹, respectively, in 1 wt% mCP:TSPO1 films.⁵⁰ In this work, neither ICz nor ICzMes₃ show TADF due to their too large ΔE_ST of 0.47 eV and 0.39 eV, respectively, measured in toluene glass; however, DiICzMes₄ shows weak (though unambiguous) TADF as its ΔE_ST of 0.26 eV is much smaller (Table 2).

Table 2 Solid-state photophysical properties

Compound^a	λ PL (FWHM)^b/nm	Φ PL /%	S1^d/eV	T1^d/eV	ΔEST^e/eV	τ p /ns	τ d /μs
a In 3 wt% doped PMMA films. b λ exc = 330 nm, where value in parentheses is FWHM. c Determined using an integrating sphere, λexc = 330 nm. d S1 and T1 determined from the onset of the steady-state and phosphorescence spectra, respectively, at 77 K, λexc = 330 nm. e Calculated from the energy difference between the energies of the S1 and T1 states at 77 K. f λ exc = 355 nm. g λ exc = 350 nm. h λ exc = 350 nm. i In 3 wt% doped mCP films.
ICz	377 (29)	37	3.38	2.88	0.50	N/A	N/A
ICzMes3	391 (28)	58	3.26	2.85	0.41	N/A	N/A
DiICzMes4	442 (20)^g	67	2.86^h	2.57^h	0.29	N/A	N/A
DiICzMes4	451 (22)^g	82^f	2.82^h	2.56^h	0.26	14	433

---

In anticipation of OLED applications, additional time-resolved emission decays were also collected for DiICzMes₄ in a wide range of suitably high-triplet OLED hosts (Fig. S38, ESI‡). For these experiments 10% loading in drop-cast films was used, improving the overall signal but also enhancing the emission detectable from red-shifted dimer or excimer species, as evident in the individual normalised spectra (contour plots, Fig. S38c, ESI‡). In line with the stationary emission spectra of MR-TADF materials in varying solvent polarity (Fig. S33, ESI‡), we observe only minor differenced in the time-resolved spectra and decays regardless of host.

Devices

Regioisomeric derivatives of DiICz have been reported in the form of 4,⁴⁹m-FLDID⁶⁴ and tDIDCz,⁶⁵ where each was described as a traditional fluorescent emitter. Large experimentally determined ΔE_ST values of 0.36 eV and 0.44 eV were reported for m-FLDID⁶⁴ and tDIDCz,⁶⁵ respectively, in frozen THF glass. The corresponding UV-emitting OLEDs showed EQE_max of 5.2% and 3.3% at CIE of (0.16, 0.03) and (0.16, 0.02), respectively. Having confirmed the previously overlooked though admittedly weak TADF activity of DiICzMes₄, its use as an emitter in OLEDs was assessed. Devices using a stack of ITO (HIL/anode) |NPB (HTL, 40 nm)|TSBPA (EBL, 10 nm)|DiICzMes₄:DPEPO 10% (EML, 30 nm)|DPEPO (HBL, 10 nm)|TBPi (ETL, 40 nm)|LiF (EIL, 1 nm)|Al (cathode, 100 nm) were fabricated (Fig. 7), with representative performance shown in Fig. 8. These results show that the low rate of RISC in DiICzMes₄ is insufficient to enable efficient triplet harvesting even at the lowest current densities (and corresponding lowest brightness, ∼10 cd m⁻²) investigated here. The resulting low EQE_max values are consistent with the DiICzMes₄ acting akin to a fluorescent dopant, only able to harvest singlet excitons for emission with an upper limit of EQE_max < 5%. This result is in-line with what was observed for previous acceptor-free rIC DF material TCA_C4.⁵¹ The OLED shows CIE coordinates of (0.15, 0.11). Despite the higher EQE_max observed for tPBisICz at very low brightnesses, the efficiency roll-off of that device was severe with the EQE at 100 cd m⁻² only about 5%. Comparing our OLED results at equivalent brightnesses reveals similar overall performance metrics with the previously reported work (Table S3, ESI‡).⁵⁰

Fig. 7 Device architecture and chemical structures of materials employed.

Fig. 8 OLED performance for different EMLs. (a) EL spectra of TADF D–A–D 35% (red), HF OLEDs 1 : 35 wt% (blue) in DPEPO host, absorption (black dotted) and PL (green) spectra of DiICzMes₄ 1 wt% in mCP for comparison. (b) CIE coordinates, where square (DiICzMes₄ only) and circle (HF) overlap, (c) JVL curves, and (d) EQE vs. current density, where fitting has been applied (dotted line) to guide the reader.

Additional devices using a different stack consisting of ITO|NPB (HTL, 40 nm)|mCBP (EBL, 10 nm)|DiICzMes₄:host X% (EML, 30 nm)|T2T (HBL, 10 nm)|T2T:LiQ 45% (EIL/ETL, 35 nm)|Al (cathode, 100 nm) were also fabricated. In these the concentration of the dopant was varied (5, 12, and 20 wt%) and different EML hosts additionally investigated: mCBP (hole transporting), DPEPO (electron transporting). Representative device performance and spectra are shown in Fig. S39 (ESI‡), with no significant improvement compared to the results in Fig. 8. With increasing concentration no difference was observed in current density–voltage–luminance (JVL) and EQE as a function of current density although a broadening in the electroluminescence (EL) spectrum was observed. The broadening is assigned to the dimer/excimer contribution as shown from the previous photophysical results (Fig. S37d and S38c, ESI‡).

In order to compensate for the low RISC rate of DiICzMes₄ we also applied it as a terminal emitter in hyperfluorescent OLEDs (HF OLEDs) with a D–A–D TADF co-host. In order to ensure good spectral overlap necessary for energy transfer, we employed a dimethylacridine-tetramethylthioxanthene-S,S-dioxide (identified as TADF in Fig. 7) based TADF previously reported to give high EQEs and blue emission [CIE of (0.15, 0.19)] in the same OLED stack.¹⁴ This D–A–D TADF was co-evaporated at 35% in the EML, alongside 1% DiICzMes₄ and bulk host DPEPO. The resulting OLEDs possessed good efficiency, with an EQE_max > 16% and CIE of (0.15, 0.11) enabled by triplet harvesting of the D–A–D co-host, while outputting narrow blue emission from the DiICzMes₄. The HF OLED showed relatively lower efficiency roll-off, offering a practical strategy to circumvent large efficiency roll-off resulting from inefficient k_RISC of the MR-TADF emitter (Fig. 8).

As our integrating sphere system is not sensitive to very low luminances, we do not observe the same high maximum EQEs (∼32%) previously reported using a similar hyperfluorescence approach with pICz.⁴⁸ However, comparing our device data at equivalent brightnesses reveals improved performance (Fig. S40, ESI‡), which we infer is due to the improved efficiency roll-off of our D–A–D co-host. Indeed, this performance at higher brightnesses is amongst some of the best reported for HF OLEDs at this colour coordinate (Table S18, ESI‡). The previously reported DPAc-DtCzBN:PPF co-host has a similar intrinsic maximum efficiency as ours, and with slightly blue shifted EL spectrum should also enjoy marginally improved FRET overlap with the MR-TADF emitter in the device. Despite the adequate FRET overlap in both devices, a subtle shoulder can still be observed in our EL spectra, indicating residual emission from the D–A–D co-host. As hyperfluorescence applications of MR-TADF emitters become increasingly popular to circumvent their low RISC rates,^22,66–68 engineering both their PL spectra (for ideal-blue emission), as well as their absorption spectra (for minimal Stokes shift, enabling broad compatibility with D–A–D TADF co-hosts)⁶⁹ take on equally important roles for applications. The latter of these can significantly alleviate the requirement for D–A–D co-hosts with deep blue EL, which remain challenging to design despite nearly a decade of intense global research in this direction.

We finally note that in both our hyperfluorescence devices and those previously reported, inclusion of the MR-TADF leads to significantly worse efficiencies at reasonable brightnesses compared to the D–A–D co-host alone. While the DiICzMes₄ would be expected to increase device performance due to spontaneously emitter dipole alignment and improved outcoupling,⁴⁸ other detrimental processes must also be at play to result in an overall detriment to performance. These may include charge trapping or Dexter transfer to the slow-RISC MR-TADF dopant, although these processes have proven to be incredibly challenging to even quantify by traditional means.⁷⁰ Therefore while the improvement in colour coordinate offered by the MR-TADF hyperfluorescence strategy is welcomed, it is clear that a deeper understanding of the relevant in operando mechanism and processes is required to unlock their full potential.

Conclusions

We have designed and investigated an MR-TADF compound that does not contain any explicit electron-acceptor units, opening a new design paradigm for MR-TADF emitters. SCS-CC2 calculations guided the design, confirming a strategy to coincidentally decrease ΔE_ST and improve oscillator strength with increasing electronic delocalization. Photophysical measurements revealed a reduced ΔE_ST and increased Φ_PL were observed in both solution and doped films for DiICzMes₄ compared to ICz and ICzMes₃. Although ΔE_ST was rather large at 0.26 eV in mCP, TADF was nonetheless observed in this and other solid-state hosts. Activation of TADF occurs through the involvement of higher-lying triplet states of different orbital types to S₁, resulting in non-negligible SOC.^42,43 Owing to inefficient RISC, simple guest–host OLEDs showed low efficiency, although hyperfluorescent devices achieved good EQE_max of 16.5%, at deep-blue colour coordinates (0.15, 0.11) with improved relative efficiency roll-off. Discovery of new regions of chemical space suitable for the development of MR-TADF emitters thus opens new paths towards understanding their optical properties and improving their performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The St Andrews team would like to thank 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, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. We acknowledge support from the European Union's Horizon 2020 research and innovation programme under the ITN TADFlife (GA 812872). Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant no. F.4534.21 (MIS-IMAGINE). D. B. is a FNRS Research Director. EZ-C is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089).

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Footnotes

† The research data supporting this publication can be accessed at https://doi.org/10.17630/915048c3-9d32-43ee-a18d-b49f95042a55

‡ Electronic supplementary information (ESI) available: ¹H and ¹³C NMR, and HRMS spectra of all new compounds and HPLC traces of all the emitters; computational data and coordinates; photophysical data; device data; crystallographic data for ICzMes₃ and DiICzMes₄. CCDC 2104486 and 2104487. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1mh01383a

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