Peripheral mesitylene-modified sky-blue multi-resonance TADF emitter with enhanced horizontal orientation and suppressed aggregation-caused quenching achieving 38.6% external quantum efficiency

Abstract

Boron-based multiresonance thermally activated delayed fluorescence (MR-TADF) emitters combine intrinsically narrowband emission with near-unity internal quantum efficiency. However, their practical implementation in OLED displays remains limited by aggregation-caused quenching, long delayed fluorescence lifetimes, suboptimal horizontal emissive transition dipole moment ratio (Θ), and poor electrical stability. Among boron-based MR-TADF systems, carbazole-based CzBN has emerged as a representative and widely studied motif due to its facile chemical modifiability, which allows for the development of diverse derivatives aimed at alleviating these limitations. Here, we present MesCzBN, a sky-blue MR-TADF emitter featuring four mesitylene groups appended to the periphery of the CzBN core. The mesitylene “umbrella” sterically shields the MR core and promotes a Θ value of 84% and a high photoluminescence quantum yield of 100% in a carbazole-based host matrix. MesCzBN achieves an external quantum efficiency of 38.6%, a peak electroluminescence wavelength of 494 nm, and a narrow full width at half maximum of 28 nm in a hyperfluorescent device. Peripheral mesitylene modification thus provides a novel sterically wrapping strategy to enhance horizontal molecular orientation and device efficiency in OLEDs, serving as a superior replacement for the benchmark tCzBN with four tert-butyl groups.

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1. Introduction

Boron-based multiresonance thermally activated delayed fluorescence (MR-TADF) emitters have attracted considerable attention as promising candidates for next-generation, low-power consumption, ultra-high-definition organic light-emitting diode (OLED) displays, owing to their high photoluminescence quantum yield (PLQY), intrinsically narrow emission bandwidth, and internal quantum efficiency approaching 100%, enabled by efficient triplet-to-singlet upconversion via the TADF mechanism.¹ Compared with traditional donor–acceptor (D–A) type TADF materials,² MR-TADF materials offer superior color purity, making them highly desirable for display applications. Despite this progress, key challenges such as aggregation-caused quenching (ACQ), long delayed-fluorescence lifetime (τ_d), suboptimal horizontal emissive transition dipole moment (TDM) ratio (Θ), and poor electrochemical stability remain unsolved.

Among boron-based MR-TADF systems, carbazole-based CzBN³ has emerged as a representative and widely studied motif because of its facile chemical modifiability, which allows for the development of diverse derivatives. Although unmodified CzBN suffers from severe ACQ and poor horizontal molecular orientation in solid films, it serves as an ideal core structure for functionalization aimed at improving photophysical and device performance (Fig. 1).

Fig. 1 Chemical structures of CzBN derivatives.

To further enhance device performance, researchers have adopted sterically wrapping strategies to protect the planar MR core and suppress ACQ. For example, simple substitution of CzBN with four tert-butyl (tBu) groups yields tCzBN (originally reported as DtBuCzB^3a), which exhibits an external quantum efficiency (EQE) of 21.6% with EL emission peak wavelength (λ_EL)/full width at half maximum (FWHM) values of 488 nm/29 nm.^3a Alternatively, introduction of four tBu-phenyl groups produces DtBuPhCzB, achieving a higher EQE of 26.5% but with a broadened FWHM of 52 nm even at 3 wt% doping concentration. π-Expansion of the CzBN skeleton, however, leads to deterioration of color purity.

In recent years, additional bulky units have been incorporated into tCzBN derivatives to sterically shield the MR core and further enhance performance.^4–12 In 2022, Yang and co-workers introduced a 1,3-di(9H-carbazol-9-yl)benzene (mCP)-derived moiety into tCzBN to obtain BN-CP1, which achieved an EQE of 40% and Θ ratio of 93% with λ_EL/FWHM values of 496 nm/25 nm; even at a high doping concentration of 30 wt%, BN-CP1 maintained an EQE of 33.3% with minimal spectral shift.⁴ That same year, Duan and co-workers developed DCzBN by incorporating four tBu groups onto the mCP unit, yielding an EQE of 37.2% with λ_EL/FWHM values of 488 nm/24 nm.⁵ In 2023, Zhang and co-workers introduced a dimesitylborane unit into the tCzBN skeleton to produce BNB′-1, which exhibited an EQE of 40.3% and a Θ ratio of 98% with λ_EL/FWHM values of 539 nm/30.6 nm.⁶ Also in 2023, Yang and co-workers reported a phenoxazine-modified tCzBN derivative, CzBN3, which delivered an EQE of 42.3%, and Θ ratio of 95% with λ_EL/FWHM values of 487 nm/27 nm in a hyperfluorescent device.⁷ These studies clearly demonstrate that steric wrapping of the CzBN core represents a highly effective strategy for achieving high EQE and strong ACQ suppression without compromising color purity (Fig. S1 and Table S1).

To overcome the remaining limitations of tCzBN while retaining its compatibility, we set out to design a sterically enhanced analogue capable of simultaneously suppressing ACQ and maximizing horizontal emissive TDM ratio without compromising color purity. Building on these advances, we designed and synthesized MesCzBN, a novel sky-blue MR-TADF emitter featuring four bulky 1,3,5-trimethylphenyl (Mes) groups at the periphery of the CzBN skeleton as a replacement for tCzBN. This design resulted in a high horizontal emissive TDM ratio (Θ = 84%), effective suppression of ACQ, and an EQE of 38.6% with λ_EL/FWHM values of 494 nm/28 nm in a hyperfluorescent device. To clarify the specific advantages imparted by mesitylene substitution, three RCzBN derivatives such as CzBN, tCzBN, and MesCzBN were systematically compared under identical conditions in terms of photophysical parameters, horizontal molecular orientation, and OLED performance.

2. Results and discussion

2.1. Molecular design and quantum calculation of MesCzBN

As described above, CzBN is the most widely used MR-TADF core structure. By sterically shielding its large MR framework with bulky peripheral substituents, ACQ caused by π–π stacking can be effectively suppressed and Θ value can be increased. Three RCzBN derivatives—CzBN, tCzBN, and MesCzBN—are shown in Fig. 2(a) using Corey–Pauling–Koltun (CPK) models. As evident from these models, tCzBN is a derivative modified with four tBu groups on the peripheral MR framework. However, it lacks steric hindrance both sides of the π-surface, leaving the MR framework partially exposed. Note that peripheral substitution of tBu or Mes groups increases the molecular aspect ratio, resulting in a shift of the transition dipole moment: in CzBN, the dipole is oriented obliquely to the π-plane, whereas in tCzBN and MesCzBN it becomes more parallel to the plane. Ideally, installing bulky groups that sterically cover both sides of the π-surface should further suppress ACQ and promote horizontal alignment.

Fig. 2 (a) Chemical structures, the transition dipole moments (red arrow), surface areas, and aspect ratios of RCzBN derivatives. (b) HOMO and LUMO distributions and energy levels of RCzBN derivatives at TD-B2PLYP/cc-pVDZ//B3LYP/6-31G(d) level of theory (isovalue = 0.02). (c) UV-Vis absorption and PL spectra in a dilute toluene solution (1 × 10⁻⁵ M).

As such, we focused on Mes group. The methyl groups at the 1,3-positions (ortho position) of mesitylene can effectively cover both sides of the π-surface. Unlike tCzBN, where the π-core is slightly distorted due to the steric repulsion of tBu groups, MesCzBN adopts an almost planar conformation due to the more relaxed steric environment.

We also performed double-hybrid time-dependent density functional theory (DH-TDDFT) calculations. As reported by Kondo, DH-TDDFT provides improved accuracy in predicting the S₁ and T₁ excited states and the singlet–triplet energy gap (ΔE_ST) of MR-TADF molecules (Fig. 2(b)).¹³ Following previous reports, we used (c_c, c_x) = (0.40, 0.27) as the hybrid coefficients. The calculated S₁ energies of the RCzBN derivatives were in the range of 2.47–2.54 eV (corresponding to 488–502 nm), indicating sky-blue emission. Introduction of tBu or Mes groups at the CzBN periphery led to a decrease in S₁ energy by ∼0.07 eV, resulting in a red-shift of ∼10 nm in the emission wavelength. The T₁ state also decreased slightly by ∼0.04 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the RCzBN derivatives show that, in tCzBN, the incorporation of electron-donating alkyl groups results in HOMO/LUMO values becoming shallower. In contrast, in MesCzBN, the HOMO and LUMO levels remain essentially unchanged compared to the parent CzBN. This suggests that the mesityl groups are oriented orthogonally to the MR framework, interrupting π-conjugation and thereby exerting minimal influence on the electronic structure. All the calculation results are summarized in Table 1.

Table 1 Thermal and photophysical properties

Compound	MW	T g/Tm/Td5^a [°C]	Calculated S1/T1/ΔEST^b [eV]	S 1/T1/ΔEST^c [eV]	I p/Ea/Eg^d [eV]	λ PL/FWHM^e [nm]	τ d [μs]	PLQY^g [%]
a T g and Tm were measured using DSC, and Td5 was measured using TGA. b Calculated values by DH TD-DFT at B2PLYP/cc-pVDZ level of theory. c Calculated from the peak (onset) of fluorescent and phosphorescent spectra of dilute toluene solution (1 × 10^−5 M). d I p was measured using PYS, Eg was considered as the point of intersection of the normalized absorption spectra, and Ea was calculated using Ip and Eg. e PL emission peak wavelength/full width at half maximum of λPL. f Delayed fluorescence lifetime of 3 wt% emitter-doped mCBP film. g Photoluminescence quantum yield of dilute toluene solution (1 × 10^−5 M)/3 wt% emitter-doped mCBP film/3 wt% emitter-10 wt% 5CzBN-doped mCBP film.
MesCzBN	889.0	268/398/495	2.469/2.379/0.090	2.56/2.54/0.02 (2.66/2.66/0.00)	5.74/3.23/2.51	486/24	68.0	86/100/100
tCzBN	640.7	n.d./373/424	2.477/2.375/0.102	2.65/2.49/0.16^3c	5.32/2.83/2.49	484/24	95.5^3c	100/100/87
CzBN	416.3	n.d./303/431	2.535/2.419/0.116	2.70/2.55/0.15^3c	5.30/2.79/2.51	474/25	64.2^3c	84/100/83

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2.2. Synthesis, characterization, solubility, thermal properties, and photophysical properties in solution state

The novel MR-TADF emitter MesCzBN was readily synthesized via an amination reaction followed by a one-pot borylation starting from 3,6-dimesitylcarbazole (Scheme S1). The structure of the target compound was fully confirmed by ¹H NMR, ¹³C NMR, mass spectrometry, and elemental analysis (see SI, Fig. S2–S4).

Thermal properties were evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Fig. S5 and S6). MesCzBN exhibited a glass transition temperature (T_g) of 268 °C, indicating high thermal stability in the solid-state thin film. In contrast, no T_g was observed for CzBN and tCzBN, suggesting that both compounds are highly crystalline. The 5 wt% decomposition temperatures (T_d5) for all RCzBN derivatives exceeded 350 °C, indicating their suitability for vacuum deposition. Among them, MesCzBN showed the highest T_d5 value of 495 °C, presumably due to its high molecular weight (M_w = 889.0). All the thermal properties are summarized in Table 1.

The UV-vis absorption and photoluminescence (PL) spectra of RCzBN derivatives in dilute toluene solution (10⁻⁵ M) are shown in Fig. 2(c) and Fig. S7. MesCzBN exhibited a sharp emission peak with λ_PL/FWHM values of 486 nm/24 nm, which are nearly identical to those of tCzBN (484 nm/24 nm) and slightly red-shifted with a narrower bandwidth compared to CzBN (474 nm/25 nm). As mentioned in the quantum chemical calculation section above, DH-TDDFT calculations accurately predicted the emission wavelengths. The PLQY value of MesCzBN reached 86%, which is comparable to those of CzBN (84%) and tCzBN (100%). These results indicate that the introduction of mesityl groups produces effects comparable to those of tert-butyl groups in terms of emission wavelength, FWHM, and PLQY in dilute solution. The photophysical properties in dilute toluene solution are summarized in Table 1.

2.3. Solid-state photophysical properties, and molecular orientation

Next, we evaluated the solid-state photophysical properties of the RCzBN derivatives. The ionization potential (I_p) was measured from vacuum-deposited neat films using photoelectron yield spectroscopy (PYS), yielding values of 5.30 eV for CzBN, 5.32 eV for tCzBN, and 5.74 eV for MesCzBN (Fig. S8). The optical bandgap (E_g) was estimated from UV–vis absorption spectra, and the electron affinity (E_a) was calculated from the I_p and E_g values (Fig. S9). The resulting E_a values were 2.79 eV for CzBN, 2.83 eV for tCzBN, and 3.23 eV for MesCzBN. Among the RCzBN derivatives, MesCzBN exhibited both deeper I_p and E_a values by ∼0.4 eV compared to CzBN and tCzBN.

We then evaluated the PLQY values in doped films using mCBP, a wide-energy-gap host, to confine excitons of RCzBN derivatives. In 3 wt% RCzBN-doped mCBP films, high PLQY values were observed: 100% for CzBN, 100% for tCzBN, and 100% for MesCzBN. To further investigate ACQ behavior, we analyzed the dependence of PLQY and FWHM on doping concentration (Fig. S10–S12 and Table S2). Among the three, MesCzBN exhibited the most suppressed ACQ behavior, maintaining high PLQY and minimal broadening of FWHM at elevated doping concentrations. Notably, the concentration-dependent spectra exhibited features consistent with aggregate-state emission (Fig. S10–S12), a phenomenon broadly recognized for MR-TADF emitters.¹⁴RCzBN derivatives showed relatively high PLQY values (∼70%) in the 3–10 wt% doping range. Notably, MesCzBN retained a PLQY of 65% even at a high doping concentration of 25 wt%, approximately 1.3–1.7 times higher than those of CzBN and tCzBN. Moreover, the FWHM for MesCzBN increased only to 37 nm at 25 wt%, whereas CzBN and tCzBN exhibited larger FWHMs of 68 nm and 51 nm, respectively. The newly developed MesCzBN exhibited apparent TADF behavior (Fig. S13), and RCzBN derivatives exhibited long τ_d values of around ∼100 μs in mCBP without the use of sensitizer similar to the conventional boron-based MR-TADF emitters (Fig. S14–S16).

As previously described, peripheral substitution with bulky groups increases the molecular aspect ratio, altering the orientation of the transition dipole moment. In CzBN, the dipole is oriented obliquely relative to the π-plane, whereas in tCzBN and MesCzBN, it aligns more parallel to the plane (Fig. 2(a)). We quantified this effect by measuring the Θ values in RCzBN-doped mCBP films via angle-dependent photoluminescence analysis. MesCzBN exhibited a high Θ value of 84%, compared to 75% for tCzBN and 67% for CzBN (Fig. 3(a)–(c)). This enhanced molecular orientation is expected to directly improve light out-coupling efficiency. All the photophysical properties in the solid states are summarized in Table 1.

Fig. 3 PL intensities of 3 wt% (a) CzBN-, (b) tCzBN-, and (c) MesCzBN-doped mCBP films without 5CzBN at different angles. UV-Vis absorption spectrum of RCzBN and normalized PL spectrum of 5CzBN film. (d) CzBN, (e) tCzBN, (f) MesCzBN.

Collectively, these results demonstrate that peripheral mesitylene substitution provides an effective molecular design strategy for simultaneously suppressing ACQ and enhancing molecular orientation in MR-TADF-based OLEDs using this novel CzBN-derived core structure.

2.4. Hyperfluorescent OLED performance

Hyperfluorescent OLED devices¹⁵ were fabricated using RCzBN derivatives as terminal emitters and the blue TADF emitter 5CzBN¹⁶ as the sensitizer. As discussed in the previous section, RCzBN derivatives exhibit long τ_d values, which can cause significant efficiency roll-off at high luminance in OLEDs. To address this, we incorporated a TADF sensitizer to reduce the τ_d values. 5CzBN was selected as the sensitizer due to its good spectral overlap between its emission and the absorption of RCzBN (Fig. 3(d)–(f)). A three-component system comprising mCBP: 10 wt% 5CzBN: 3 wt% RCzBN exhibited high PLQY values of 83% for CzBN, 87% for tCzBN, and 100% for MesCzBN (Table 1 and Fig. S17–S19). The device structure was ITO (65 nm)/polymer buffer layer¹⁷ (20 nm)/TAPC (20 nm)/TCTA (10 nm)/mCBP: 10 wt% 5CzBN: 3 wt% RCzBN (20 nm)/TmPyPB¹⁸ (50 nm)/LiF (1 nm)/Al (100 nm)] (Fig. 4(a)). In this architecture, mCBP served as the host material, and TmPyPB functioned as the electron-transport layer (ETL), contributing to reduced driving voltages. The energy level diagram is shown in Fig. 4(b), and the electroluminescence (EL) spectra are presented in Fig. 4(c).

Fig. 4 (a) Chemical structures of materials and device structure of OLED in this study. (b) Energy diagram of the device. Device performance of the OLEDs: (c) EL spectra; (d) J–V–L characteristics; (e) EQE-L characteristics; (f) PE-L characteristics.

The MesCzBN-based device exhibited a λ_EL value at 494 nm, with Commission Internationale del Eclairage (CIE) coordinates of (0.09, 0.49) and a FWHM of 28 nm. Compared to the tCzBN-based device, the MesCzBN-based device displayed a slightly longer emission wavelength and a similar FWHM value. In contrast, the CzBN-based device showed a significantly broader FWHM of 46 nm, much wider than that observed in solution. The turn-on voltages for all RCzBN-based devices were approximately 3.2 V, suggesting similar carrier injection characteristics (Fig. 4(d)). The MesCzBN-based device achieved an EQE of 38.6%, which was significantly higher than those of the tCzBN (33.5%) and CzBN (25.8%) devices (Fig. 4(e)).

As the J–V characteristics were nearly identical in the luminance region below 100 cd m⁻², the differences in EQE can primarily be attributed to the Θ value rather than variations in carrier balance. Notably, similar trends in EQE values were also observed in non-hyperfluorescent devices fabricated without 5CzBN (Fig. S20, S21, and Table S3). Although we also fabricated devices without the use of the 5CzBN sensitizer for comparison, these traditional TADF OLEDs exhibited significantly lower EQE_max values (23.9–32.7%) and pronounced efficiency roll-off at high luminance. This is attributable to the long delayed fluorescence lifetimes (τ_d ∼100 μs) of RCzBN derivatives, which lead to severe triplet–triplet annihilation (TTA) and triplet–polaron quenching (TPQ) under high current density. In contrast, hyperfluorescent devices employing 5CzBN sensitizer successfully suppress these exciton losses via faster energy transfer, resulting in improved device performance. In addition, we prepared a device containing only the 5CzBN sensitizer, without any RCzBN terminal emitter. Although this sensitizer-only device displayed moderate efficiency (EQE_max = 26.9%) and stable operation, its electroluminescence spectrum was notably broader (FWHM = 70 nm), centered at 483 nm, reflecting insufficient color purity (Fig. S23). These results highlight the critical role of combining 5CzBN with RCzBN terminal emitters in achieving both high efficiency and narrowband emission. The hyperfluorescence strategy effectively addresses the limitations of both traditional TADF and sensitizer-only devices by enabling fast exciton harvesting and sharp spectral profiles. Owing to the high EQE and moderate turn-on voltages, the hyperfluorescent devices based on RCzBN derivatives exhibited high power efficiencies: 75.4 lm W^-1 for MesCzBN, 62.6 lm W^-1 for tCzBN, and 55.6 lm W^-1 for CzBN (Fig. 4(f)). All the hyperfluorescent OLED performance is summarized in Table 2. While the difference in EQE_max is primarily attributed to the higher Θ value, the close I_p alignment between TCTA and MesCzBN facilitates hole injection and improves carrier balance, which likely contributes to the reduced EQE roll-off. These results demonstrate that peripheral mesitylene modification represents a novel steric wrapping strategy that significantly enhances EQE in OLEDs, making MesCzBN a superior alternative to the benchmark tCzBN bearing four tert-butyl groups.

Table 2 Hyperfluorescent OLED performance

Compound	V on [V]	PEmax/CEmax/EQEmax^b [lm W^−1/cd A^−1/%]	PE100/CE100/EQE100^c [lm W^−1/cd A^−1/%]	PE1000/CE1000/EQE1000^d [lm W^−1/cd A^−1/%]	λ EL/FWHM^e [nm]	CIE (x,y)^f
a Voltage (V) at 1 cd m^−2. b Power efficiency (PE), current efficiency (CE), and external quantum efficiency (EQE) at maximum. c PE, CE, and EQE at 100 cd m^−2. d PE, CE, and EQE at 1000 cd m^−2. e EL emission peak wavelength/full width at half maximum of λEL. f CIE at 100 cd m^−2.
MesCzBN	3.20	75.4/80.0/38.6	62.4/76.0/36.6	40.9/58.0/28.0	494/28	(0.09, 0.49)
tCzBN	3.23	62.6/63.7/33.5	41.9/52.3/27.5	18.5/28.5/15.0	491/26	(0.09, 0.43)
CzBN	3.20	55.6/57.3/25.8	41.6/51.3/23.1	22.5/33.4/15.0	486/46	(0.14, 0.45)

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3. Conclusion

In summary, we have developed MesCzBN, a novel sky-blue MR-TADF emitter featuring peripheral mesitylene substitution as a steric wrapping strategy to overcome the limitations of benchmark tCzBN. The mesityl groups effectively suppress ACQ by shielding both sides of the MR framework, and simultaneously promote a high Θ value of 84% due to the enhanced molecular aspect ratio and planarity. MesCzBN exhibits excellent thermal stability, high PLQY in both solution and solid states, and reduced FWHM broadening at high doping concentrations. In hyperfluorescent OLEDs, the MesCzBN-based device achieved a maximum EQE of 38.6%, significantly outperforming tCzBN and CzBN analogues, with a power efficiency of 75.4 lm W^-1. The improved device performance is primarily attributed to enhanced horizontal molecular orientation rather than carrier balance. Our findings demonstrate that peripheral mesitylene modification provides an effective and practical design strategy for realizing high-efficiency, high-color-purity MR-TADF emitters. MesCzBN serves as a promising alternative to tCzBN for next-generation OLED applications and provides valuable insight into the molecular engineering of MR-TADF emitters.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5tc02609a

Acknowledgements

The authors thank Dr Yong-Jin Pu (RIKEN) for his valuable support with the quantum chemical calculations. We gratefully acknowledge the financial support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through the subsidy program for the development of advanced research infrastructure. H. S. also acknowledges partial financial support from JSPS KAKENHI (Grant number 23H02032 and 23K26725), and the Iwatani Naoji Foundation.

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