Ortho‐Carborane Decorated Multi‐Resonance TADF Emitters: Preserving Local Excited State and High Efficiency in OLEDs

Abstract A novel class of o‐carboranyl luminophores, 2CB‐BuDABNA (1) and 3CB‐BuDABNA (2) is reported, in which o‐carborane moieties are incorporated at the periphery of the B,N‐doped multi‐resonance thermally activated delayed fluorescence (MR‐TADF) core. Both compounds maintain the inherent local emission characteristics of their MR‐emitting core, exhibiting intense MR‐TADF with high photoluminescence quantum yields in toluene and rigid states. In contrast, the presence of the dark lowest‐energy charge transfer state, induced by cage rotation in THF, is suggested to be responsible for emission quenching in a polar solvent. Despite the different arrangement of the cage on the DABNA core, both 1 and 2 show red‐shifted emissions compared to the parent compound BuDABNA (3). By utilizing 1 as the emitter, high‐efficiency blue organic light‐emitting diodes (OLEDs) are achieved with a remarkable maximum external quantum efficiency of 25%, representing the highest reported efficiency for OLEDs employing an o‐carboranyl luminophore as the emitter.


Introduction
Ortho-carborane (1,2-closo-C 2 B 10 H 12 ), an electron-deficient icosahedral boron cluster, has garnered considerable attention as a steric and electronic building block for constructing various luminophores over the past decade [1] due to its steric DOI: 10.1002/advs.202309016bulkiness and 3D electron delocalization through three-center, two-electron bonds. [2]1c,4] This electronic interaction between the carboranyl * orbital and the delocalized * orbital of an aromatic luminophore significantly influences the LUMO of the luminophores, thereby altering their excited-state properties.Consequently, o-carboranyl luminophores, especially those featuring a 2-R-substituent on the cage, predominantly exhibit broad CT emissions in the low-energy region.
In the context of developing novel o-carboranyl luminophores for optoelectronic materials applications, such as solid-state emitting [5] and stimuli-responsive materials, [6] it remains challenging to understand the role of o-carborane in controlling the photophysical properties of various luminophores.Since the electronic effects of o-carborane can vary depending on the -systems used, the choice of the parent -luminophore becomes a critical factor in tuning the excited-state properties.With this in mind, we decided to employ a multi-resonance induced thermally activated delayed fluorescence (MR-TADF) core as a new luminophoric -skeleton to create novel o-carboranyl TADF emitters (Figure 1, right).Due to the alternate localization of the highest occupied molecular orbital (HOMO) and LUMO on different atoms, resulting in a short-range CT (SRCT) transition, MR-TADF emitters typically exhibit narrowband emissions. [7]Therefore, one can anticipate that the electronic coupling between the o-carborane and the B,N-doped MR-emitting core may induce distinct excited-state properties compared to conventional o-carboranyl luminophores.Additionally, the introduction of ocarborane into either the HOMO-or LUMO-dominant positions may have a varying impact on the excited state.To unveil the effects of o-carborane on the photophysical and electroluminescent properties of MR-TADF emitters, we herein report two o-carborane-appended MR-TADF emitters, namely 2CB-BuDABNA (1) and 3CB-BuDABNA (2), along with a reference emitter, BuDABNA (3).We found that the inherent local emission characteristics of the MR-emitting core are retained but tuned through o-carborane substitution.Computational results also provide a rationalization of the observed photophysical properties of 1 and 2, including local emission, redshift in absorption and emission, and spectral quenching in a polar medium.Furthermore, the OLED devices incorporating 1 as the emitter achieve a high maximum external quantum efficiency (EQE max ) of 25.0%.This study represents the first example demonstrating excellent OLED performance by utilizing an o-carboranyl luminophore as the emitter.
Emitters 1 and 2 exhibited high thermal decomposition temperatures (T d5 = 382−400 °C, at 5 wt.% decomposition), significantly exceeding that of 3 (303 °C).The elevated stability can be attributed to the high chemical and thermal stability of the o-carborane cage, as well as the steric protection of the MR-core by the cage. 11B NMR spectroscopy confirmed the presence of both the trigonal boron atom of the MR-core ( 30−42 ppm) and carboranyl boron atoms ( −4 to −10 ppm) in 1 and 2. The crystal structure of 1 revealed that the two methyl groups on the carborane cages were oriented in the opposite direction with respect to the plane of the B,N-core, providing large steric protection(Figure 2).In contrast, the two carboranyl methyl groups in 2 were held on the same side of the plane.The carboranyl C−C bond distances (1.698(5) Å for 1 and 1.688(3)−1.693(3)Å for 2) were in the typical range for carboranyl luminophores.The large dihedral angles ( = C CB −C CB −C Ph −C Ph ) between the carborane and phenylene ring in 1 and 2 indicated a perpendicular orien-tation of the carboranyl C−C bond axis toward the phenylene ring.

Photophysical Properties
The photophysical properties of 1−3 were first investigated in toluene (Figure 3 and Table 1).The strong SRCT absorption in both 1 (451 nm) and 2 (476 nm) exhibited a redshift compared to reference 3 (442 nm).Inspection of the experimental HOMO and LUMO levels indicated that the LUMO stabilization by carborane substitution is greater than the HOMO stabilization in 1 and 2 when compared to the corresponding levels in 3 (Figure S7, Supporting Information).This finding suggests that, unlike the distinct positional electronic effects on either HOMO or LUMO in previous luminophores, [1i,k,9] o-carborane substitution on the MR-core has a more pronounced impact on LUMO stabilization.The PL spectra of all compounds exhibited narrowband emissions typical for B,N-doped MR-emitters.The deep blue emission peak of 3 (455 nm) underwent redshifts in 1 (467 nm) and 2 (493 nm) (Figure 3b).The significant redshift in 2 indicates a substantial stabilization of the singlet excited state (S 1 ) through carborane substitution at the 3-position.Notably, no ICT band other than the local emission was detected in 1 and 2, indicating that the carborane moiety does not alter the emissive SRCT states of the MR-core in toluene.The narrow full width at half maximum (FWHM) of the emission in 3 (22 nm) was maintained in 1 (22 nm) while it was slightly broadened in 2 (29 nm).The PL quantum yields (PLQY, Φ PL ) of 1 and 2 also remained high (76−80%).
In a polar THF medium, however, 1 and 2 became poorly emissive, whereas emitter 3 maintained a high PLQY (Table 1).1i,j,10] However, at 77 K, a broad emission band was concurrently observed for 1 in the lower energy region, whereas 2 did not exhibit it (Figures S12b,S13b, Supporting Information).This band could be attributed to the ICT state formed between the MR-core and the carboranyl * orbital.The absence of such a band in 2 suggests that the ICT state persists at low temperatures in the sterically hindered 1. Corroborating the weak emission in THF, both 1 and 2 exhibited aggregation-induced emission (AIE) behavior in a THF/water mixture (Figure 3c). [11]ith an increase in the water fraction in a THF solution, particularly over 80% water, insoluble aggregates began to form, displaying strong emissions.The nearly consistent emission peak wavelength and spectral broadening indicate the predominance of an identical emitting species in both the solution and aggregate states, ruling out the formation of low-energy aggregates.This prompted further investigation into the emission properties of 1−3 in a rigid matrix, such as PMMA (Figure S16, Supporting Information).Emitters 1 and 2 exhibited slightly blue-shifted emissions with narrowed FWHMs compared to those observed in THF.Remarkably, the PLQYs of both emitters were significantly boosted in PMMA films, with the PLQY of 1 (76%) nearly comparable to that of emitter 3.1i] The observation of the delayed component in the transient PL decay further implies the retention of the TADF property of the MR-emitting core in 1 and 2.

Theoretical Studies
To elucidate the electronic and photophysical properties of 1 and 2, computational studies based on time-dependent DFT (TDDFT) were conducted for all the compounds (see Supporting Information for computation details).The frontier molecular orbitals (FMOs) of all compounds in the ground state (S 0 ) were mainly localized on the DABNA backbone with alternating HOMO and LUMO distributions (Figure S18 and Table S2, Supporting Information).For both 1 and 2, the o-carborane moieties contributed more significantly to the LUMO than to the HOMO, resulting in a more efficient stabilization of the LUMO compared to the HOMO.Thus, their reduced HOMO−LUMO energy gap (E g ) by 0.06 and 0.31 eV compared to the E g of 3 leads to redshifts in the absorption and emission spectra of 1 and 2 relative to those of 3. The larger stabilization of the LUMO than the HOMO in 2 can be readily understood with o-carborane substitution at the LUMO-dominant 3-position (see Figure 1).However, the LUMO of 1 is unexpectedly more stabilized than its HOMO, despite ocarborane being introduced at the HOMO-dominant 2-position.
Fragment orbital (FO) analysis was conducted to unveil the electronic interactions leading to the formation of FMOs (Figure 4).As expected, strong electronic coupling between the MR-core and o-carborane was observed for the LUMO of 2. For 1, recognizable electronic couplings leading to the LUMO were found between the lowest unoccupied fragment orbitals (LUFOs) of MRcore and o-carborane, as well as the HOMO formed via strong electronic coupling.Particularly, the feasibility of a -orbital contribution to the LUMO of 1, as identified from the spatial distribution of -LUFO of MR-core (Figure 4b), which was not observed in 2, would be considered to extraordinarily stabilize the LUMO of 1 compared to its HOMO, leading to the reduction of the E g of 1.The natural transition orbital (NTO) analysis revealed predominant localization of the hole and particle NTOs of the S 1 and T n (n = 1 and 2) states on the MR-core (Figure S21 and Table S3, Supporting Information), confirming the preservation of SRCT character, i.e., LE at the MR-core, in the electronic transition processes of 1 and 2, as expected from the distribution of FMOs in the S 0 state.To examine the spectral quenching in a polar solvent medium, additional TDDFT calculations were performed for 1, which exhibited superior TADF performance compared to 2. In addition to the bright S 1 (S 1 -LE) state geometrically similar to the S 0 , we identified a dark S 1 (S 1 -CT) state dependent on the C CB −C CB bond length and the dihedral angle  (see Table S4 (Supporting Information) for their geometries).Their transition characters were elucidated by NTO analysis (Figure 5a; Table S5, Supporting Information).The negligible oscillator strength (f) of S 1 -CT indicates that the spectral quenching for 1 is closely related to a nonemissive relaxation process from S 1 -CT (Table S4, Supporting Information).The schematic potential energy surfaces (PESs), depending on  for one of the two carboranes, constructed using TDDFT results, reveal minima for the bright and dark S 1 states at ≈90°and ≈0°(≈180°), respectively (Figure 5b; Figure S23, Supporting Information).The spectral quenching can thus be understood with structural changes, i.e., the dihedral rotation of ocarborane accompanied with C CB −C CB bond elongation, from S 1 -LE to S 1 -CT (Figure S24, Supporting Information).The extent of the relative stability of S 1 -LE with respect to S 1 -CT at ≈90°d ecreases gradually as the polarity of the surrounding medium increases.In particular, the minimum energy in the PES of S 1 -LE is almost comparable to the maximum energy in the PES of S 1 -CT in the most polar THF medium.Consequently, the relative distribution of PES for S 1 -CT to that for S 1 -LE indicates that the energy barrier from S 1 -LE to S 1 -CT states can be interpreted as diminishing with the increasing polarity of the surrounding medium.Because the rotational PES was constructed via a series of single-point calculations by only changing the dihedral angle  from the optimized structure, it cannot precisely describe the reaction barrier required for the geometric change from S 1 -LE to S 1 -CT.Especially, the energy barrier due to C CB −C CB bond elongation was not addressed.Nevertheless, the experimental spectral quenching can be qualitatively rationalized with the gradual change of the PES of S 1 -CT with respect to S 1 -LE depending on the environmental polarity at the level of theory employed in PES evaluation.

Electroluminescent Properties
Finally, the o-carboranyl MR-TADF emitters were applied to blue-emitting OLEDs.We selected 1 as the emitter due to its excellent emissive properties.Prior to OLED fabrication,  S6, Supporting Information). [12]The 5 wt.% doped thin films of 1 and 3 in the host matrix exhibited narrowband emissions with FWHMs of 26 and 27 nm and peak wavelengths at 472 and 459 nm, respectively.Notably, the thin film of 1 showed a high PLQY of ≈93%, comparable to that of 3 (92%).The shorter delayed fluorescence lifetime ( d ) of 1 (69.1 μs) compared to that of 3 (92.6 μs) is mainly attributable to the smaller S 1 −T 1 gap (ΔE ST ) of 1 relative to that of 3 (0.14 vs 0.16 eV), as similarly observed in a PMMA film (Figure 3d; Figure S25b, Supporting Information).These results indicate the excellent MR-TADF properties of 1 in the host film.Next, the OLEDs with the following structure were constructed (Figure S27, Supporting Information): glass/indium-tin-oxide (ITO, 70 nm)/MoO 3 (1 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 30 nm)/tris(4-carbazoyl-9-ylphenyl)amine (TCTA, 10 nm)/ SiCzCz (10 nm)/SiCzCz:SiTrzCz2:emitter (70:30:x wt.%, 25 nm)/SiTrzCz2 (5 nm)/1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB, 40 nm)/LiF (1 nm)/Al (130 nm).As depicted in Figure 6, D1−D4 devices with a wide range of doping concentrations of emitter 1 (1, 2, 5, and 10 wt.%) exhibited blue electroluminescence (EL) with emission peaks at 468−472 nm.In particular, the narrow FWHM of 27−28 nm was maintained in the given doping range.These EL spectral features are consistent with the PL spectrum of the host film of 1 and can be partly attributed to the steric shielding of the MR-core by the cage.Remarkably, all devices exhibited high EQE max above 20%, owing to the high PLQY and TADF property of 1 (Table 2).1f,k,13] The efficiency is comparable to that of the control device (R3) employing 5 wt.% emitter 3 with an EQE max of 25.1%.However, the efficiency roll-off of D3 at high current density is smaller than that of the control device.This result demonstrates that the incorporation of o-carborane into the B,N-doped MR-emitting core allows us to effectively tune the emission color and reduce intermolecular interactions between the MR-cores, leading to maintaining the blue color purity with high EQE even at high doping concentrations.

Conclusion
In conclusion, we have demonstrated that the local emission characteristics of the parent -luminophores can be preserved while being tuned through o-carborane substitution.2CB-BuDABNA (1) and 3CB-BuDABNA (2), wherein methyl-ocarborane moieties were introduced at the periphery of the MRemitting core, were prepared.Both compounds exhibited intense MR-TADF in toluene and rigid states, with significant redshifts of their emission peaks compared to that of the parent compound BuDABNA (3).Utilizing 1 as the emitter, we achieved highly efficient blue OLEDs with an EQE max of 25%.This efficiency stands as unprecedented for OLEDs based on o-carboranyl luminophores as emitters.The findings of this study indicate that o-carborane can serve as a steric and electronic auxiliary for MR-TADF emitters, holding promise for the development of a novel class of o-carboranyl TADF materials.
a)  In oxygen-free toluene at 298 K (2.0 × 10 −5 m);b)  In oxygen-free THF at 298 K (2.0 × 10 −5 m); c) 5 wt.%-doped film in PMMA; d) Absolute PLQYs; e) Full width at half maximum of the PL spectrum; f) PL lifetimes of prompt ( p ) and delayed ( d ) decay components; g) ΔE ST = E S − E T .Singlet (E S ) and triplet (E T ) energies were estimated from the fluorescence and phosphorescence spectra at 77 K.

Figure 4 .
Figure 4. Fragment orbital (FO) diagrams for a) 3CB-BuDABNA (2) and b) 2CB-BuDABNA (1), which are constructed only using the significant electronic couplings between BuDABNA() and two CBs().The orbital energies (in eV) are presented in parentheses and the contributions (in %) of FOs to HOMO and LUMO are provided.The FOs of two CBs involved in the coupling with BuDABNA are omitted for clarity.(See Figures S19,S20(Supporting Information) for a full description of FO diagrams for 2 and 1, respectively).

Figure 5 .
Figure 5. a) NTOs (isovalue = 0.02 e Å −3 ) for bright and dark S 1 states of 1. b) Schematic diagram for the potential energy surfaces of bright and dark S 1 states of 1, depending on surrounding medium.

Figure 6 .
Figure 6.a) EL spectra of TADF-OLEDs (D1−D4) based on emitter 1 at different doping concentrations in the emitting layer and photograph of the working device.b) External quantum efficiency−current density (EQE−J) characteristics of D1−D4.