A B–N bond-doped multiple resonance emitter with extended π-conjugation for narrowband sky-blue OLEDs exhibiting low efficiency roll-off

Abstract

Incorporating B–N covalent bonds into the classic boron–nitrogen multiple resonance (B,N-MR) skeleton within a polycyclic heteroaromatic framework presents an effective approach for developing novel narrowband MR emitters. In this study, the synthesis process was simplified using a lithium-free one-pot borylation reaction, yielding a narrowband MR emitter, ICz[B–N], featuring an indolocarbazole segment to extend π-conjugation and induce a redshifted emission. The para-positioned nitrogen–π–nitrogen conjugation in the indolocarbazole segment exhibits enhanced electron-donating ability relative to the traditional carbazole moiety, while the rigidly extended MR backbone enables both spectral redshift and narrowband emission. The synthesized ICz[B–N] displayed sky-blue emission at 488 nm, representing a 42 nm redshift relative to B,N-MR blue emitter doping with the B–N bond, while maintaining a narrow spectral linewidth of only 19 nm. The corresponding sensitized OLED device achieved a peak brightness exceeding 70 000 cd m⁻², an external quantum efficiency of 15.0%, and minimal efficiency roll-off.

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1. Introduction

Incorporating heteroatoms into a polycyclic aromatic hydrocarbon (PAH) framework has attracted considerable attention.¹ Recent advancements in heteroatom doping of PAHs with different electronegativities have resulted in distinct frontier molecular orbital (FMO) distributions, triggering an interesting multiple resonance (MR) effect.² This effect significantly reduces the split energy (ΔE_ST) between the singlet and triplet states (S₁–T₁), facilitating up-conversion processes similar to those observed in conventional thermally activated delayed fluorescence (TADF) emitters.³ Additionally, it suppresses vibrational coupling and conformational relaxation between the ground and excited states, leading to narrow spectral linewidths and high photoluminescence quantum yields (Φ_PL), superior color purity, and high luminous efficiency.⁴ In 2016, Hatakeyama and co-workers first introduced the MR emitter DABNA-1 featuring B and N atoms, which exhibited a narrowband blue color at 460 nm with a full width at half maximum (FWHM) of 33 nm.^2b Building upon this work, BN-MR systems have developed rapidly, with researchers incorporating heteroatoms of varying electronegativities (e.g., B, –C=O, N, O, S, and Se) into PAHs, spanning the visible spectrum from blue to deep red.⁵ Moreover, strategies such as peripheral substituent modifications and π-conjugated framework extension have been widely employed to fine-tune energy levels and photophysical properties.^5e,6 However, for most BN-MR emitters reported to date, particularly those emitting in the blue to deep-blue region, the incorporation of peripheral substituents enhances intramolecular charge transfer (CT), which increases structural relaxation in the excited state and significantly broadens the FWHM as the emission wavelength shifts toward the red region.⁷ Therefore, developing molecular modification strategies that achieve red-shifted emission while maintaining narrow emission bandwidths remains a challenging yet critical task. In our previous work, we successfully addressed this challenge by replacing the carbazole group in the red emitter (BNO) and the deep-blue emitter (CzBNO) with rigid indolocarbazole fragments (5-phenyl-5,8-dihydroindolo[2,3-c]carbazole or 5-phenyl-5,7-dihydroindolo[2,3-b]carbazole), thereby obtaining BNNO and IDCzBNO emitters featuring extended MR frameworks.^6d,8 These emitters exhibited significantly red-shifted emission spectra with narrow FWHMs, indicating that π-conjugation extension within the molecular backbone is an effective strategy for tuning emission without compromising bandwidth.

Alternatively, PAHs containing readily accessible B–N covalent bonds have garnered increasing interest owing to their straightforward synthesis and the unique dipole structure of the B–N bond.⁹ Our group integrated the MR characteristics of B,N frameworks using facile synthetic approaches to develop a series of B–N bond-doped narrowband emitters.¹⁰ This method employs an amine-directed strategy, achieving high yields in a one-pot process without lithium reagents. Notably, the reported parent compound [B–N]N exhibits a deep-blue color at 442 nm in solution, with an FWHM of 19 nm.^10d,11 This underscores the importance of further modulating its excited-state energy levels and structural features to elucidate the relationship between the molecular structure and photophysical behavior.

Here, we utilize 5-phenyl-5,8-dihydroindolo[2,3-c]carbazole (23cICz) as a π-conjugated building block with multiple N atoms, which is embedded in an asymmetrical B–N bond-doped MR framework to develop a narrowband sky-blue BN-MR emitter, namely, ICz[B–N]. This emitter exhibits exceptional photophysical properties and device performance. The N–π–N structure within the 23cICz fragment enhances the electron-donating capacity of the framework, while its rigid planar conformation significantly extends the MR skeleton, thereby mitigating geometrical changes and vibrational relaxation. Compared to the parent [B–N]N, which is based on two carbazole fragments, ICz[B–N] achieves a red-shifted emission at 42 nm, with a peak wavelength of 484 nm, while preserving a narrow FWHM of 19 nm. This molecular design strategy for red-shifted emission significantly surpasses previously reported approaches that rely on peripheral substitution, which typically broadens the FWHM and diminishes color purity. Notably, the sky-blue OLED device based on ICz[B–N] achieved a peak external quantum efficiency of 15.0%, while retaining high performance with 13.3% and 12.9% at luminance levels of 1000 and 5000 cd m⁻², respectively.

2. Results and discussion

2.1. Material synthesis and thermal stability

The synthesis of the target compound ICz[B–N] involves a straightforward three-step procedure, as illustrated in Scheme 1. First, precursor 1a was synthesized via a copper powder-catalyzed Ullmann reaction between 2-bromo-4-iodo-1-methylbenzene (1.0 equiv.) and 5-phenyl-5,8-dihydroindolo[2,3-c]carbazole (1.0 equiv.). Subsequently, precursor 1a was subjected to a classical Suzuki–Miyaura cross-coupling reaction with the 3,6-di-tert-butyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole unit (1.0 equiv.), affording precursor 2a in an 82% yield. The final step involved a one-pot lithium-free electrophilic C(sp²)–H arene borylation reaction of precursor 2a using BBr₃ (2.0 equiv.) and Et₃N (4.0 equiv.) in o-DCB at 160 °C, smoothly producing ICz[B–N] in 94% yield. The compound was purified via column chromatography and vacuum sublimation to ensure high purity. ICz[B–N] was fully characterized and structurally identified using ¹H- and ¹³C-NMR spectroscopy and MALDI-TOF mass spectrometry, as detailed in the ESI.† Thermal stability was assessed via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Under a nitrogen atmosphere at 10 °C min⁻¹, TGA revealed a 5% weight loss at decomposition temperature (T_d) of 410 °C and DSC analysis confirmed that ICz[B–N] does not exhibit a detectable glass transition temperature (T_g) within the measured range of 25–280 °C (Fig. S1, ESI†), indicating the emitter's remarkable thermal stability. This high thermal stability is advantageous for ensuring the material's durability during evaporation processes. However, the glass transition temperature (T_g) was not observed within the tested temperature range.

Scheme 1 (a) Depicts the design strategy of expanding π-conjugation and the chemical structures. (b) The straightforward three-step reaction process of ICz[B–N].

2.2. Theoretical calculations

To gain insight into the electronic transitions of the B–N doped ICz[B–N], density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at the B3LYP/6-31G(d) level. As depicted in Fig. 1a, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of ICz[B–N] delocalized across the entire molecular framework. Hole and electron analysis using Multiwfn¹² indicates that the S₁ state transition exhibits characteristics of a hybridized state, predominantly involving short-range CT (SRCT), with a partially localized excitation (LE) on the central benzene ring of indolocarbazole segments. The results show that the S₀–S₁ transition is predominantly from the HOMO to the LUMO and associated with a significant oscillator strength (f = 0.2830), suggesting that the emitter has a high radiative transition rate. Additionally, root-mean-square displacement (RMSD) analysis of the S₀ and S₁ geometries reveals a remarkably small RMSD value for ICz[B–N] of 0.046 Å, closely matching that of the parent compound [B–N]N (0.047 Å). Subsequently, the vibrationally resolved emission spectrum of ICz[B–N] was simulated in toluene. As shown in Fig. 1b, ICz[B–N] emits blue light with an exceptionally narrow FWHM of approximately 15 nm.

Fig. 1 (a) The frontier molecular orbital distributions of ICz[B–N], depicting the hole (blue) and electron (green) isosurface for the S₁ state (isovalue of 0.02), and the optimized structures for the S₀ (red) and S₁ (blue) states. (b) Simulated emission spectra of ICz[B–N] (Gaussian broadening, a half-width at half-maximum of 300 cm⁻¹). (c) HR factors and λ for ICz[B–N] and parent [B–N]N at different vibrational frequencies.

To gain deeper insights into the impact of the π-extension indolocarbazole framework on the spectral linewidth during the radiation of ICz[B–N], we employed the MOMAP¹³ program to calculate the contributions of each vibrational mode to the Huang–Rhys (HR) factor as well as reorganization energy (λ) (Fig. 1c). The [B–N]N core was also studied for comparison and displayed an obvious vibrational mode attributed to a low-frequency twisting vibration at 72.95 cm⁻¹ (an HR value of 0.295), while the HR value for the structurally extended ICz[B–N] molecule at 70.37 cm⁻¹ is significantly lower at 0.096. The fusion of the indolocarbazole moiety not only weakens vibronic coupling in the high-frequency region (above 1000 cm⁻¹) but also produces two additional low-frequency twisting modes at 25.82 cm⁻¹ and 90.64 cm⁻¹ (Fig. S2, ESI†). The high-frequency vibrational modes are associated with atomic-level stretching behaviors involving bonds such as C–C and B–N. For [B–N]N, two dominant vibrational motions were identified at 1409.96 cm⁻¹ and 1655.93 cm⁻¹, exhibiting pronounced λ values of 66.16 and 68.5 cm⁻¹, respectively. Interestingly, ICz[B–N] with the extended indolocarbazole unit significantly suppresses these two vibrational modes (1385.73 cm⁻¹ and 1667.32 cm⁻¹), reducing the λ values to 31.51 cm⁻¹ and 31.33 cm⁻¹, respectively. The above results indicate that the rigid planar indolocarbazole structure effectively suppresses the structural deformation and various stretching vibrational modes of the B–N doped emitter, which is beneficial for achieving a narrow spectral linewidth.

2.3. Electrochemical and photophysical properties

Next, we investigated the electrochemical properties of ICz[B–N]via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) under forward scan conditions in degassed dichloromethane. As depicted in Fig. 2a, ICz[B–N] exhibits an irreversible redox couple, with DPV measurements revealing oxidation and reduction potentials of 0.28 V and −2.21 V, respectively, relative to the ferrocene/ferrocenium (Fe/Fc⁺) couple. These potentials enabled the estimation of HOMO/LUMO energy levels as −5.08 eV/−2.59 eV, respectively, yielding a bandgap of 2.49 eV. Optical absorption and emission profiles were recorded in a diluted toluene solution (1.0 × 10⁻⁵ M) (Fig. 2b). A well-defined absorption peak centered at 473 nm (λ_abs) with a large molar extinction coefficient (ε = 4.41 × 10⁴ M⁻¹ cm⁻¹) was observed, attributable to the HOMO → LUMO transition based on the computational results. Notably, ICz[B–N] exhibits a redshift of 42 nm compared to [B–N]N (λ_abs = 431 nm),^10d primarily due to the expanded indolocarbazole unit in its molecular framework, which modulates the FMOs and consequently narrows its optical bandgap. The PL spectrum of ICz[B–N] in toluene solution displays a bright sky-blue emission with a peak at 484 nm as well as a narrowband linewidth of 19 nm (0.10 eV), a 42 nm (0.24 eV) redshift compared to the blue emission of [B–N]N. The Stokes shift of ICz[B–N] is a mere 11 nm, similar to that of [B–N]N (11 nm). Furthermore, when ICz[B–N] is doped at 2.0 wt% into the conventional host 9-(3-(9H-carbazol-9-yl)phenyl)-9H-3,9′-bicarbazole (mCPBC),¹⁴ the resulting film emits at 488 nm along with a Φ_PL of 90% and a narrow FWHM of 24 nm.

Fig. 2 (a) The electrochemical behaviors measured via CV and DPV. (b) Optical absorption and emission spectra in solution. (c) Fluorescence and phosphorescence spectra (20 ms delay) of ICz[B–N] at 77 K. (d) The transient decay curve of ICz[B–N] in the doped film.

To elucidate the TADF characteristics of ICz[B–N], we initially examined its fluorescence and phosphorescence behavior in toluene at 77 K, with phosphorescence delayed by 20 ms, estimating the singlet (S₁) and triplet (T₁) energies to be 2.54 eV and 2.26 eV, respectively, leading to a ΔE_ST of 0.28 eV (Fig. 2c). This obtained ΔE_ST value is relatively larger than that of many conventional MR-TADF emitters, which probably originates from the enhanced HOMO and LUMO overlap across the central benzene ring of indolocarbazole and the B–N bond. The PL transient decay properties were investigated in the 2.0 wt% doped film. As shown in Fig. 2d, ICz[B–N] exhibits a characteristic of a bi-exponential decay profile, featuring both prompt (τ_PF) and delayed (τ_DF) fluorescence components. The fitted lifetimes (τ_PF and τ_DF) were determined to be 7.3 ns and 54.8 μs, respectively. The relatively short lifetime of ICz[B–N] allows for a higher rate of radiative transition from the S₁ state (k_r) at 9.2 × 10⁷ s⁻¹, as well as the rate constant for reverse intersystem crossing (k_RISC) from the T₁ state to the S₁ state, measured at 1.90 × 10⁴ s⁻¹. The detailed photophysical data of ICz[B–N] are presented in Table 1.

Table 1 A summary of the photophysical properties of ICz[B–N]

λ abs [nm]	λ PL [nm]	FWHM^b [nm]	Φ PL [%]	Φ PF [%]	Φ DF [%]	E ST [eV]	τ PF [ns]	τ DF [μs]	k r [10^7 s^−1]	k ISC [10^7 s^−1]	k RISC [10^4 s^−1]	HOMO/LUMO^h [eV]
a Maximum absorption value. b PL and FWHM in solution and the film. c PL quantum yield in the film. d Prompt (ΦPF) and delayed fluorescence (ΦDF) quantum yields. e S1/T1 energy splitting value. f Lifetimes of the prompt (τPF) and delayed (τDF) fluorescence. g Related rate constants. h HOMO and LUMO energy levels.
473	484/488	19/24	90	67	23	0.28	7.28	54.8	9.2	4.53	1.90	−5.08/−2.59

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2.4. Electroluminescence performance

Based on the narrow spectral bandwidth and exceptional photophysical properties of ICz[B–N], we further investigated its potential as an emissive dopant in OLEDs. The device structure consisted of ITO/HATCN (5 nm)/NPB (30 nm)/SiCzCz (10 nm)/SiCzCz: SiTrCz2: ICz[B–N] (30 nm)/SiTrCz2 (10 nm)/DPPyA (30 nm)/LiF (0.5 nm)/Al (150 nm), as shown in Fig. 3a. Within the device, HATCN, NPB, and DPPyA served as the hole injection layer (HIL), hole transport layer (HTL), and electron transport layer (ETL), respectively. SiCzCz and SiTrCz2 were combined in a 1 : 1 ratio to form a stable exciplex host, while also acting as the electron- and hole-blocking layers, respectively.¹⁵ The optimal doping ratio of ICz[B–N] was determined to be 2.0 wt% with 30.0 wt% 4TCzBN as a sensitizer. The structure of the functional material layers is shown in Fig. 3b.^10a,16

Fig. 3 (a) Depicts the OLED device architecture, and (b) shows the functional material structures within each layer. (c) The luminance–voltage–current density characteristics of the device. (d) and (e) EL profile and CIE coordinates at 1000 cd cm⁻², along with a picture illustrating the EL color. (f) Compares the EL spectra at varying brightness levels. (g) and (h) Plot the relationships between luminance and CE and PE, and EQE, respectively.

As illustrated in Fig. 3c, the ICz[B–N]-based device exhibited a low turn-on voltage (V_on) of 2.7 V and achieved a high luminance (L_max) exceeding 70 000 cd m⁻², suggesting efficient charge injection and transport within the device. It emitted bright sky-blue light at 488 nm with an FWHM of 24 nm (0.14 eV), a characteristic that closely matched the PL spectrum observed in the thin film (Fig. 3d). Its CIE chromaticity coordinate was (0.127, 0.417), corresponding to the sky-blue region (Fig. 3e). Furthermore, Fig. 3f illustrates the EL spectral characteristics of the device at different brightness levels, demonstrating that at high brightness (e.g., 5000 cd m⁻²), the EL spectrum exhibited almost no variation, with the peak wavelength remaining stable at around 488 nm, further confirming that the EL spectrum originates entirely from the emission of ICz[B–N]. Impressively, the device achieved a peak EQE (EQE_max) of 15.0%, a peak power efficiency (PE_max) of 27.7 lm W⁻¹, and a peak current efficiency (CE_max) of 31.7 cd A⁻¹ (Fig. 3g and h). The device also displayed negligible roll-off in efficiency, retaining EQEs of 13.3% and 12.9% at 1000 cd m⁻² and 5000 cd m⁻², respectively. A summary of the EL performance is provided in Table 2. These results support the strategy of employing a π-extended BN-MR molecular framework to effectively maintain narrow spectral emission and high device efficiency.

Table 2 OLED performance based on ICz[B–N]

λ EL [nm]	FWHM^b [nm eV^−1]	V on [V]	L max [cd m^−2]	EQEmax/1000/5000^e [%]	PE max/1000/5000^e [%]	CE max/1000/5000^e [%]	CIE (x,y)^f
a EL peak wavelength. b FWHM and corresponding energy. c Turn-on voltage (Von). d Maximum luminance (Lmax). e Maximum EQE, power efficiency (PEmax) and current efficiency (CEmax), and values recorded at 1000 and 5000 cd cm^−2. f CIE coordinates at 1000 cd cm^−2.
488	24/0.14	2.7	76690	15.0/13.3/12.9	27.7/16.6/11.9	31.7/27.5/26.6	(0.127, 0.417)

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3. Conclusions

We have developed a narrowband sky-blue MR-TADF emitter, ICz[B–N], by integrating indolocarbazole segments into an asymmetrical MR framework featuring a B–N bond. This emitter exhibits a sharp emission centered at 484 nm with a narrow FWHM of 19 nm in solution. The para-positioned N–π–N configuration of the indolocarbazole segment contributes to a substantially enhanced π-conjugation compared to conventional carbazole moieties. This extended π-conjugated plane effectively mitigates changes in molecular geometry and reduces vibrational relaxation in the excited-state configuration. This extended π-conjugated plane not only stabilizes the excited-state geometry but also suppresses vibrational relaxation, resulting in an emission redshift while retaining spectral sharpness. The sensitized OLED device incorporating ICz[B–N] delivers vivid sky-blue EL at 488 nm with an FWHM of 24 nm, a peak EQE of 15.0%, and minimal efficiency roll-off, maintaining EQE values of 13.3% and 12.9% at 1000 and 5000 cd m⁻², respectively. This amine-directed one-step borylation method, coupled with the π-conjugation extension strategy, provides unique insights into the development of novel B–N luminescent materials.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

This work was supported by the National Key Research and Development Program (Grant No. 2020YFA0715000), the National Natural Science Foundation of China (Grant No. 52222308, 22135004, and 52303253) and the Yunnan Fundamental Research Project (No. 202501CF070071).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01905b

‡ These authors contributed equally to this work.

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