Construction of a high color-purity deep-blue emitter based on an indolo[3,2,1-jk]carbazole center using a crossed long-short axis (CLSA) molecular design strategy

Abstract

Moving toward next-generation ultrahigh-definition and high-resolution displays, the development of high-performance blue organic light-emitting diodes (OLEDs) with emission matching the BT.2020 standard is essential and requires advancements in the molecular design strategy. Herein, the molecular design strategy of the crossed long-short axis (CLSA) is applied for the first time for the construction of indolo[3,2,1-jk]carbazole (ICz) for tuning emission color purity via the spectral narrowing effect, and the derivative CNICz-2BuCz exhibits expected optical performance with a full width at half maximum of 33 nm in solution and enhanced PLQY due to the introduction of peripheral tert-butyl modified carbazole groups. Owing to high-lying reverse intersystem crossing channels and a narrow emission characteristic, it exhibits excellent device performance with a maximum external quantum efficiency of 7.46% at CIE_y = 0.045, showcasing the great potential of the combination of the CLSA strategy and the ICz group in realizing efficient and narrow blue OLEDs.

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Introduction

Organic light-emitting diodes (OLEDs) have revolutionized display and lighting technologies, yet OLEDs capable of meeting deep-blue color gamut standards remain scarce.¹ In general, emissions with wavelengths ranging from 400 to 500 nm are defined as blue light, and color purity improves as the spectral linewidth narrows.² The Commission Internationale de l’Éclairage (CIE) color coordinates describe the light color of OLED devices more precisely by converting linear color coordinates into a three-dimensional red-green-blue (RGB) color space. Different RGB color gamut spaces can be obtained by varying the specified three primary colors and the definitions of white light, in which the color coordinates of (0.14, 0.08) are defined as blue light.³ The Broadcast Television Service 2020 (BT.2020) standard is widely regarded as the ideal next-generation color gamut standard, which is expected to cover 100% of the color gamut and lead the display field into the high resolution, in which the color coordinates of blue light are defined as (0.131, 0.046).⁴

In addition to a wavelength peak in the blue region and a narrow spectrum, high efficiency is required for high-performance blue materials; therefore, the regulation of the excited-state properties of materials is essential.⁵ Featuring a large dihedral angle between the donor and acceptor as well as separation of the locally excited (LE) and charge-transfer (CT) states, the crossed long-short axis (CLSA) is a promising material design strategy for high-performance blue materials.⁶ The CLSA strategy not only imparts a donor–acceptor structure for carrier injection but also activates high-lying reverse intersystem crossing (RISC) channels for breaking the limitation of exciton utilization efficiency (EUE). With weaker electron-donating ability and larger planarity, indolo[3,2,1-jk]carbazole (ICz) can inhibit structural relaxation and vibrational coupling of excited states and is an ideal luminophore for narrow emission.⁷ J. Y. Lee's group used ICz as the central element to design thermally activated delayed fluorescence (TADF) materials CNICCz and CNICtCz, in which CNICCz reached a maximum external quantum efficiency (EQE) of 12.4% at the emission of 449 nm but dropped to 6.4% at a brightness of 100 cd m⁻².⁸ The electroluminescence (EL) spectrum of CNICCz broadened to 56 nm and lost color purity due to the saturated charge transfer (CT) component. The group led by J. Y. Lee also constructed the violet molecule tDIDCz using two ICz units, which exhibits a full width at half maximum (FWHM) of 14 nm at 393 nm with an EQE of 2.8%.⁹ These results prove the potential of the high color purity of the ICz group and highlight the challenge of balancing efficiency and color purity.

In this work, ICz was introduced to the CLSA configuration as a central luminophore to increase the color purity, connecting tert-butyl carbazole groups to regulate the conjugation of the long axis. Introducing the benzonitrile group with an electron-withdrawing ability as a donor to form a charge-transfer interaction with the acceptor on the long axis improves the electron injection capability of the material (Fig. 1). The target molecule, CNICz-2BuCz, was designed and synthesized, exhibiting a blue emission with a peak at 413 nm and a narrow FWHM of 33 nm. Meanwhile, theoretical calculations revealed the hybridized local and charge-transfer (HLCT) characteristics of CNICz-2BuCz, accompanied by a high-lying reverse intersystem crossing (RISC) process. The prepared OLED device based on CNICz-2BuCz demonstrates a maximum EQE of 7.46% at 420 nm, maintaining a FWHM of 40 nm and CIE coordinates of (0.163, 0.045) simultaneously. These results demonstrate a reliable strategy for blue materials with high efficiency and high color purity, which is first reported for ICz as a CLSA construction center.

Fig. 1 Molecular design strategy for CNICz-2BuCz.

Results and discussion

Synthesis and characterization

The molecular structure and synthesis route of CNICz-2BuCz are shown in Scheme S1, SI, and the control molecule CNICz is also synthesized. The thermal stabilities of CNICz and CNICz-2BuCz are evaluated. Without glass transitions, the thermal decomposition temperature (T_d, weight loss 5%) of CNICz was 331 °C, and the T_d of CNICz-2BuCz was 534 °C. The electrochemical properties of CNICz and CNICz-2BuCz were tested using cyclic voltammetry (Fig. S1). According to the oxidation potential and reduction potential, the corresponding highest occupied molecular orbitals (HOMOs) of CNICz and CNICz-2BuCz are calculated to be −5.56 eV and −5.32 eV, and the lowest unoccupied molecular orbitals (LUMOs) are −2.32 eV and −2.44 eV, respectively. The reduced band gap of HOMO and LUMO is beneficial for charge carriers to recombine on CNICz-2BuCz.

Theoretical calculation

The optimized ground state (S₀) geometry and the frontier orbitals of CNICz and CNICz-2BuCz are obtained via density functional theory (DFT) calculations (Fig. 2). The optimized S₀ configuration of CNICz shows a twist angle of 42.8° between the ICz and the benzonitrile group. The HOMO of CNICz is distributed on the whole molecular skeleton, and the LUMO is localized on the ICz group. The optimized S₀ configuration of CNICz-2BuCz exhibits a torsion angle of 40.8° between the long axis and the short axis, ensuring conjugation along the long axis, and a torsion angle of 42.9° between the short axis, maintaining a weak interaction between the donor and the acceptor. The HOMO of CNICz-2BuCz is shifted to the entire long axis, and the LUMO is still localized on the ICz center. Additionally, the introduction of tert-butyl carbazole improves electron injection, which is consistent with the shallower HOMO of CNICz-2BuCz.

Fig. 2 The optimized S₀ geometry and the frontier orbitals of CNICz and CNICz-2BuCz.

In order to further understand the radiative transition process of CNICz-2BuCz, the natural transition orbital (NTO) and energy levels were calculated (Fig. S2). The CNICz-2BuCz showed a large energy gap (ΔE_ST = 0.46 eV) between the lowest singlet (S₁) and the lowest triplet (T₁); however, the ΔE_STs between the high-lying triplet states and the S₁ state are small, especially the ΔE_ST between S₁ and T₄ (0.04 eV), indicating that the up-conversion may appear in high-lying triplet states. The incomplete overlap of NTOs demonstrated the HLCT characteristic of both S₁ and T₄ states of CNICz-2BuCz.¹⁰ The spin–orbit coupling (SOC) constants (〈S₁H^SOT_n〉) of triplet states and S₁ were further calculated to locate the RISC channels (Table S1). As a result, the CNICz-2BuCz showed a SOC constant between T₄ and S₁ (〈S₁H^SOT₄〉) of 0.06 cm⁻¹, which is higher than the SOC constant between T₁ and S₁ (〈S₁H^SOT₁〉), indicating the existence of T₄ → S₁ transitions (Fig. S3).¹¹

Photophysical properties

The absorption and emission spectra of CNICz and CNICz-2BuCz in toluene solutions (10⁻⁵ M) are presented in Fig. 3, with relevant data summarized in Table 1. The absorption of CNICz peaking at 374 nm and 314 nm can be ascribed to the π–π* transition, and the absorption peaks of CNICz-2BuCz appear at 388 nm and 327 nm. CNICz shows an ultraviolet emission peaking at 383 nm with a FWHM of 23 nm. In contrast, CNICz-2BuCz shows a red shift to 413 nm with a FWHM of 33 nm, due to the introduction of tert-butyl carbazole. Both compounds exhibit single-exponential transient fluorescence lifetimes of 7.28 ns (CNICz) and 6.31 ns (CNICz-2BuCz), respectively, without obvious delay components. Estimated by the onset of fluorescence and phosphorescence spectra in toluene solution at low temperature (77 K), the S₁ and T₁ states of CNICz are calculated to be 3.35 eV and 2.87 eV, and the S₁ and T₁ states of CNICz-2BuCz are 3.16 eV and 2.76 eV, respectively. The large ΔE_S1T1 of CNICz-2BuCz is 0.40 eV, which is almost identical to the theoretical simulation of 0.46 eV. Diluting CNICz and CNICz-2BuCz into the host materials—2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF), both molecules exhibit slightly red-shift emissions peaking at 394 and 427 nm and improved photoluminescence quantum yield (PLQY) values of 46.9% and 54.2%, respectively (Fig. S4).

Fig. 3 UV-vis absorption and PL spectra (a) and (b), transient PL decay curves (c), fluorescence and phosphorescence spectra at 77 K (d) and (e), and solvatochromic Lippert–Mataga model (f) of CNICz and CNICz-2BuCz.

Table 1 Photophysical characteristics of CNICz and CNICz-2BuCz

	λ abs (nm)	λ em (nm)				η PL (%)		τ (ns)	S1/T1/ΔES1T1/ (eV)^c
		Soln^a	FWHM	Film^a	FWHM	Soln	Film
a Soln: toluene solution; film: doped film; λabs: absorption peak; λem: emission peak. b τ: fluorescence lifetime measured in toluene solution (10^−5 M). c S1, T1, and ΔES1T1 were calculated from fluorescence and phosphorescence spectra and was calculated from theoretical simulation.
CNICz	374	383	23	394	42	32.5	36.9	7.28	3.35/2.87/0.48/0.49
CNICz-2BuCz	388	413	33	427	43	46.9	54.2	6.31	3.16/2.76/0.40/0.46

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The characteristics of the excited states of CNICz and CNICz-2BuCz are further evaluated by the solvation effect (Fig. S5). As the solvent polarity increases, CNICz shows a small red shift of 6 nm, while CNICz-2BuCz shows a red shift of 10 nm, indicating the different characteristics of the S₁ state of the two molecules. According to the Lippert–Mataga equation, CNICz exhibits a linear relationship between the Stokes shift (v_a–v_f) and solvent orientation polarization (f), with an excited state dipole moment (μ_e) of 9.21 D, corresponding to the luminescent component in the LE state (Tables S2 and S3).¹² The CNICz-2BuCz exhibits two slopes in solvents with different polarity, where a small dipole moment (μ_e) of 9.79 D in low polarity solutions (f ≤ 0.145) corresponds to the LE component in the excited state, and a large dipole moment (μ_e) of 15.26 D in high polarity solutions (f ≤ 0.167) corresponds to the CT component in the excited state. The HLCT excited state characteristic of CNICz-2BuCz is consistent with the NTO simulations.

Electroluminescence performance

The electroluminescent (EL) performances of CNICz and CNICz-2BuCz were evaluated using OLED devices with the configuration of ITO (90 nm)/HATCN (5 nm)/TAPC (40 nm)/TCTA (5 nm)/mCP (5 nm)/EML (20 nm)/PPF (5 nm)/TPBi (40 nm)/LiF (1 nm)/Al (120 nm). The functional layers 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN), TAPC (1,1-bis[(di-4-tolylamino)phenyl]cyclohexane), tris[4-(carbazol-9-yl)phenyl]amine (TCTA), and TPBi (2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) work as hole injection, hole transport, hole buffer, and electron transport layers, respectively. Given the high T₁ energy levels of 9,9′-(1,3-phenylene)bis-9H-carbazole (mCP) (2.9 eV) and PPF (3.0 eV), they are adopted as exciton-blocking layers to confine excitons within the emitting material layer (EML) (Fig. 4, Fig. S6, and Table 2). The non-doped device of CNICz (device N1) shows intense UV emission peaking at 392 nm with a FWHM of 36 nm, and the corresponding turn-on voltage (V_on) and maximum EQE are 4.0 V and 1.90%, respectively. The non-doped device of CNICz-2BuCz (device N2) shows a blue emission peaking at 432 nm with a FWHM of 48 nm and a CIE_y exceeding 0.05. The V_on of device N2 decreases to 3.4 V, which can be attributed to the enhanced carrier injection properties. The maximum EQE of device N2 increased to 5.38% and is maintained at 5.05% at a brightness of 1000 cd m⁻². To further improve the EL performance by suppressing the π–π interactions in the molecular aggregation state, doped devices with different concentrations were prepared. The doped devices of CNICz (devices D11–D13) and CNICz-2BuCz (devices D21–D23) were prepared with EMLs of PPF: x wt% dopant, where x is 10, 20, and 30, respectively. The device D21 achieved the best performance, with blue emission peaking at 420 nm and a FWHM of 40 nm, and the corresponding L, CE, PE, and EQE reached 2107 cd m⁻², 2.46 cd A⁻¹, 2.14 lm W⁻¹, and 7.46%, respectively. Moreover, the CIE_y value of device D21 reached the ideal 0.045, which is very close to the 0.046 specified by the BT.2020 color gamut standard.

Fig. 4 (a) Device structure. (b) EL spectrum of devices at 10 mA cm⁻², (c) EQE–luminance characteristics of devices. (d) Luminance–voltage–current density characteristics of devices based on CNICz-2BuCz. (e) Measured horizontal transition dipole moment ratio of CNICz-2BuCz in the doped film. (f) Current density–luminance of device D21.

Table 2 Device performances of OLEDs

Device	λ EL (nm)	FWHM (nm)	V on (V)	L (cd m^−2)	CE^c (cd A^−1)	PE^d (lm W^−1)	EQE^e (%)	CIE^f (x, y)
a Turn-on voltage (L ≥ 1 cd m^−2). b Maximum luminance. c Maximum current efficiency. d Maximum power efficiency. e Maximum EQE/EQE at 100 cd m^−2/EQE at 1000 cd m^−2. f Commission Internationale de l'Éclairage, recorded at 10 mA cm^−2.
Emitter: CNICz
N1	392	36	4.0	742	0.34	0.26	1.90/1.70	(0.161, 0.043)
D11	388	23	4.4	131	0.16	0.12	2.89/1.41	(0.165, 0.026)
D12	388	27	4.0	278	0.27	0.21	3.13/2.01	(0.164, 0.031)
D13	390	31	3.8	491	0.40	0.33	2.95/2.20	(0.164, 0.041)
Emitter: CNICz-2BuCz
N2	432	48	3.4	5583	2.55	2.36	5.38/5.34/5.05	(0.157, 0.053)
D21	420	40	3.6	2107	2.46	2.14	7.46/5.16/3.72	(0.163, 0.045)
D22	424	42	3.4	3566	2.14	1.97	7.08/6.09/4.96	(0.158, 0.038)
D23	428	44	3.2	4560	2.34	2.30	6.97/6.50/5.56	(0.157, 0.040)

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The exciton utilization efficiency of device D21 is calculated using the formula EQE = γ × EUE × η_out × PLQY, where γ is the factor of the charge recombination efficiency (considered to be 100%). The horizontal dipole orientations (Θ_‖) of CNICz-2BuCz in the doped film (10 wt%) is evaluated to be 95.5%, and the optical coupling output efficiency (η_out) is calculated to be 36.0%. According to the measured PLQY in the doped film, the EUE is calculated to be 38.2%, exceeding the exciton utilization limit of traditional fluorescence materials. It is speculated that there are other triplet up-conversion channels in CNICz-2BuCz, due to the transient fluorescence decay curves excluding the T₁ → S₁ transition.¹³ The current density–luminance of the CNICz-2BuCz device shows a linear relationship, and the ratio of delayed emission to total emission (delay% = the ratio of delayed emission in total emission) calculated from the transient EL curve below 5.61%, eliminating the main contribution of triplet–triplet fusion (TTF) mechanism (Fig. S7 and Table S4).¹⁴ It can be concluded that the high-lying RISC channels (T₄ → S₁) mainly contribute to the high exciton utilization of CNICz-2BuCz.

Conclusion

Deep-blue materials with narrow emission are the way to achieve the CIE_y of 0.046 to meet the BT.2020 standard; however, effective molecular design strategies are rare. In this work, we present a reliable solution to obtain high-performance blue materials via the CLSA molecular design strategy. The introduction of the ICz group guarantees the narrow emissive characteristic of CNICz-2BuCz, and the CLSA strategy enables the effective regulation of excited state properties. Finally, CNICz-2BuCz showed good color purities in solution with a peak at 413 nm and a FWHM of 33 nm. In the doped device, CNICz-2BuCz also achieved a CIE_y value of 0.045, which is consistent with the BT.2020 standard. Furthermore, CNICz-2BuCz achieved an EUE of 38.2% and a high EQE of 7.46% owing to the high-lying RISC process. The above results demonstrate that incorporating the ICz group into the CLSA skeleton represents an effective strategy for regulating excited states and achieving high color purity.

Author contributions

Conceptualization: J. Lou, X. Guo, and H. Zhang; methodology: J. Lou, Y. Chen and H. Zhang; investigation: J. Lou and X. Guo; writing – original draft: J. Lou, and H. Zhang; writing – review & editing: H. Zhang, and Z. Wang; funding acquisition: Z. Wang and B. Z. Tang; resources: Z. Wang and B. Z. Tang; supervision: Z. Wang and B. Z. Tang.

Conflicts of interest

The authors declare no competing interests.

Data availability

The data supporting this article have been included as part of the SI. Scheme S1, Fig. S1–S7, and Table S1–S4. See DOI: https://doi.org/10.1039/d5tc02735g.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (52473173), Natural Science Foundation of Guangdong Province (2022B1515020084), Guangdong Basic and Applied Basic Research Foundation (2023B1515040003), Key Project of Yunnan Provincial Department of Science and Technology (202303AC100021), Independent Research Project of State Key Lab of Luminescent Materials and Devices (SCUT) (Skllmd-2024-10, Skllmd-2025-05), Science and Technology Program of Guangzhou (2023A04J0988) and Key-Area Research and Development Program of Guangdong Province (2024B0101040001).

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