The magic methyl effect of thermally activated delayed fluorescent emitters on blue organic light-emitting diodes

Abstract

A methyl group is a common substituent in medicinal chemistry. The introduction of methyl groups always results in a profound enhanced biological activity of pharmaceuticals, known as the magic methyl effect. Meanwhile, a methyl group is also widely used for the construction of organic materials in organic light-emitting diodes (OLEDs). In this study, we systematically study the methyl effect of blue thermally activated delayed fluorescent (TADF) emitters on the photophysical properties and device performance in OLEDs. Three new blue TADF emitters with different numbers of methyl groups, namely 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1, have been successfully designed and synthesized. It is found that the methyl group induces steric hindrance and greatly affects their photophysical, thermal, and TADF properties. Their emission maximum is gradually blue-shifted from 464 to 455 nm, simply by increasing the number of methyl groups. However, 3Me-HDT1 resulted in an unexpected low external quantum efficiency (EQE) of only 1%, in which 3Me-HDT1 decomposed upon device fabrication. Contrarily, the devices based on 1Me-HDT1 and 2Me-HDT1 result in high EQEs of up to 21.2% and 19.1%. Furthermore, applying 1Me-HDT1 for hyperfluorescent OLEDs leads to pure-blue electroluminescence at 471 nm, and a higher EQE of 26.2%, together with improved CIE_x,y of (0.13, 0.16).

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Chin-Yiu Chan

Chin-Yiu Chan is currently an assistant professor in the Department of Materials Science and Engineering and an affiliated assistant professor in the Department of Chemistry at City University of Hong Kong. After the completion of BSc in Chemistry at Chinese University of Hong Kong, he earned his PhD at the University of Hong Kong, under the supervision of Prof. Vivian Wing-Wah Yam. He was a full-time postdoctoral fellow and research associate professor in the group of Prof. Chihaya Adachi at Kyushu University. His research focuses on the design and synthesis of photo-functional materials and their applications.

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Introduction

The choice of different substituents for organic functional materials is crucial to determine their functional properties. A methyl group is a small, monovalent, and lipophilic unit which is extensively utilized in medicinal chemistry and significantly affects bioactivity. The effect of a methyl group in pharmaceuticals is known as the magic methyl effect.¹ Beyond medicinal chemistry, methyl groups are always found in organic functional materials, including luminescent materials,² photochromic materials,³ sensory materials,⁴etc.

Recently, luminescent materials with narrow emissions, able to realize a high color purity of photoluminescence (PL) or electroluminescence (EL), have attracted much attention. Particularly, ultra-high-definition (UHD) display panels with BT.2020 standard have to be manufactured with the help of narrow-emission materials.⁵ To fulfill the BT.2020 standard, multi-resonance (MR) emitters that show narrow emissions and thermally activated delayed fluorescence (TADF) properties represent the next-generation luminescent materials for UHD displays.^6–8 Unfortunately, the device stability based on MR-TADF materials is still unsatisfactory.⁹ It has been known that the device performance can be greatly enhanced by employing the hyperfluorescent (HF) strategy, in which a TADF-assistant dopant (AD) and an MR-TADF emitter are combined in the emitting layer (EML).^2,10–13 The dual-emitter approach in the HF system allows excitons to be transferred from the TADF-AD to MR-TADF emitters by the Förster resonance energy transfer (FRET) process.¹⁰ Eventually, the MR-TADF emitter emits light in HF-OLEDs. Since methyl groups can improve solubility, MR-TADF emitters with methyl groups have been widely reported in the literature.^14,15 However, in our recent study, the MR-TADF emitters with methyl groups resulted in worse device performance in pure-green HF-OLEDs.²

In contrast to pure-green HF-OLEDs, developing pure-blue or deep-blue HF-OLEDs is the most challenging goal. Although a rational molecular design of MR-TADF emitters is necessary, the most determining factor for efficient and stable HF-OLEDs is the TADF-AD. A good molecular design of TADF-ADs can significantly improve the efficiency, stability, or color purity in HF-OLEDs. To have good color purity, the overlapping between the emission spectrum of a TADF-AD and the absorption spectrum of an MR-TADF emitter is important. Stable and narrow emissive pure-blue HF-OLEDs with a stable sky-blue TADF assistant dopant (HDT1) and an MR emitter (v-DABNA) have been previously reported, in which the FRET efficiency is only 67%.¹¹ The moderate FRET efficiency is due to the unmatched redshifted emission from HDT1 that led to an insufficient spectral overlapping.^16–18 As a result, if we can structurally modify HDT1 with a more blue-shifted emission, then the color purity in the HF OLED is expected to be improved.

In this study, a series of blue TADF emitters based on HDT1 were designed and synthesized, namely 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1. By incorporating different numbers of methyl groups in blue TADF emitters, we have systematically studied their photophysical properties and corresponding OLED performance. 1Me-HDT1 and 2Me-HDT1 are thermally stable, but 3Me-HDT1 is found to be thermally unstable upon sublimation. TADF-only devices based on 1Me-HDT1 and 2Me-HDT1 achieved EQE_maxs of up to 21% with bluer emissions of 484 and 478 nm, respectively, when compared to that of the parent emitter (HDT1). Applying 1Me-HDT1 for HF-OLEDs leads to pure-blue electroluminescence at 471 nm and a higher EQE of 26.2%, together with a better CIE_x,y of (0.13, 0.16).

Results and discussion

Molecular design, synthesis, and DFT calculations

Although there are only a few reports of stable and efficient blue TADF emitters, both benzonitrile and triazine are common acceptors for constructing stable blue TADF emitters. For benzonitrile, by employing HDT1 in a hyperfluorescence (HF) device, stable and efficient HF-OLEDs with pure-blue emission have been demonstrated. Nonetheless, the incomplete FRET in the HF devices (∼67%) resulted in fair CIE coordinates (0.15, 0.20).¹¹ Therefore, to enhance the FRET in the HF-OLED, a blue-shifted emission of the benzonitrile-based TADF emitter is highly required. Based on the chemical skeleton of HDT1, the terphenyl group has been replaced by different functional groups, i.e., 2-methylphenyl, 2,6-dimethylphenyl, and 1,3,5-trimethylphenyl, to blueshift the emission (Fig. 1). 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 have been designed and synthesized via multi-step synthesis. All final products, except 3Me-HDT1, were purified by temperature-gradient sublimation under vacuum after column chromatography to obtain highly pure materials, which were then used to fabricate OLEDs by vacuum deposition. All three TADF emitters have been characterized by ¹H NMR spectroscopy (Fig. S1–S15, ESI†) and atmospheric-pressure chemical ionization-electron ionization (APCI-EI) mass spectrometry. Satisfactory elemental analyses have also been conducted. To investigate the differences in the geometric and optical properties of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1, quantum-chemical calculations were performed using time-dependent DFT (TD-DFT) calculations at the B3LYP/6-31G(d) level. The calculated energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the optimized geometries of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1, are shown in Scheme 1 and Fig. 1. The torsion angle between the para-substituent and the central benzonitrile unit in HDT1, i.e., reference compound, is 58°. However, the torsion angles of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 are found to be 60°, 71°, and 71°, respectively, indicating that the methyl groups on the phenyl ring generate steric hindrance. Similar to HDT1, the HOMOs of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 are almost identical, which delocalized throughout the two diphenylcarbazole units. The LUMOs of three emitters are delocalized over the central BN core and the para-substituent. Here, the extent of delocalization on the para-substituent is shrunk when the torsion angle is high. The DFT-calculated singlet–triplet energy gap (ΔE_ST) values of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 are found to be 0.08, 0.11, and 0.11 eV, respectively. The oscillator strength (f) calculated for three compounds is found to be 0.0519, 0.0643, and 0.0679, respectively. The relatively high f values ensure high photoluminescence quantum yields (PLQYs).

Fig. 1 Molecular structures of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 and their corresponding DFT calculations based on B3LYP/6-31G(d).

Scheme 1 Chemical structures of newly synthesized blue TADF emitters and their corresponding torsion angles.

Photophysical, thermal, and electrochemical properties

Studying photophysics of the newly synthesized emitters is essential, in which 1Me-HDT1, 2Me-HDT1, or 3Me-HDT1 was first dissolved in toluene solutions with a concentration of 10⁻⁵ M (Fig. 2 and Table 1). From the UV-vis absorption spectra, all emitters showed absorption bands from 300 to 450 nm. The absorption bands below 350 nm were assigned to the π–π* transition band of the emitters, whereas the absorption bands over 400 nm were assigned to the charge transfer (CT) absorption.¹⁹ After that, the solution sample was excited at 340 nm and the emission spectrum was recorded. It was found that a structure-less emission band was found at 464, 458, and 455 nm for 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1, respectively. The introduction of methyl groups induces steric hindrance which causes a higher torsion angle between the phenyl rings and central BN rings, i.e., 58° for terphenyl, 60° for methylphenyl, 71° for dimethylphenyl and 71° for trimethylphenyl. The higher torsion angle leads to a more localized LUMO on the emitter that results in a blue-shifted emission. On the other hand, although 2Me-HDT1 and 3Me-HDT1 possessed same dihedral angles, the additional methyl group in 3Me-HDT1 provided a positive inductive effect to the central benzonitrile core, leading to the most blue-shifted emission. Besides, high PLQYs of 92%, 95%, and 97% were recorded for 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1, respectively, in degassed toluene solutions. The singlet energy levels (S₁s) of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 are 2.90, 2.95, and 2.95 eV, respectively, while their corresponding triplet energy levels (T₁s) are 2.80, 2.82, and 2.74 eV.

Fig. 2 Photophysical properties of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 in toluene solution (10⁻⁵ M) and doped in the mCBP host with 20 wt% doping concentration.

Table 1 Experimental photophysical parameters of HDT1, 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 in solution and doped thin film

Material	In toluene						20 wt% in mCBP
	λ max (nm)	Φ Air (%)	Φ Ar (%)	ΔEST (eV)	τ p (ns)	τ d (μs)	λ max (nm)	Φ Ar (%)	Φ p (%)	Φ d (%)	τ p (ns)	τ d (μs)	k r (10^7 s^−1)	k ISC (10^7 s^−1)	k RISC (10^6 s^−1)
a PLQY in aerated toluene. b PLQY in degassed toluene. c Prompt intensity. d Delayed intensity. e Ref. 11.
HDT1	476	12	100	0.02	7.9	10.6	485	86	30	56	7.8	2.9	3.76	8.74	0.92
1Me-HDT1	464	13	92	0.10	7.3	7.0	477	89	25	64	6.0	3.3	4.17	12.5	1.03
2Me-HDT1	458	17	95	0.13	4.7	11.0	474	96	28	68	5.2	3.8	5.38	13.8	0.89
3Me-HDT1	455	10	97	0.07	4.6	9.0	473	83	27	56	4.6	3.3	5.87	15.9	0.86

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Apart from the ground-state properties, the excited-state properties are important. We examined the transient decay profiles of three emitters in toluene. All three emitters showed prompt and delayed lifetimes (τ_p and τ_d), which confirmed their TADF properties (Fig. 3). The τ_ps values of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were found to be 7.3, 4.7, and 4.6 ns, respectively, whereas the τ_ds values of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were found to be 7.0, 11.0, and 9.0 μs, respectively. The τ_ps and τ_ds of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were similar to those of HDT1, and this might be due to the similar molecular structures. The experimental ΔE_STs values of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were found to be 0.10, 0.13, and 0.07 eV, respectively, which are very close to the calculated values and are larger than that of HDT1 (0.02 eV).

Fig. 3 Transient decay profiles of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 in toluene solution (10⁻⁵ M) and doped in the mCBP host with 20 wt% doping concentration.

Apart from the photophysics in the solution state, the photophysical properties in doped films were also studied by simply doping 20 wt% of each emitter in a host material, 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP). To have a better comparison, all doped films were prepared using a spin-coating method. In the doped films, the emissions of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were found to be 477, 474, and 473 nm, respectively, all are slightly redshifted compared to those in toluene. The redshifted phenomenon can be attributed to the existence of host–guest interactions in the doped films, which have been commonly observed in doped films. On the other hand, the introduction of methyl groups does not lower the PLQYs when compared to that of parent HDT1 (83%), and 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 showed PLQYs of 86%, 96%, and 83%, respectively, under an argon atmosphere. The triplet exciton lifetime is important for determining the device stability. In TADF materials, the shorter delayed lifetime or a higher k_RISC will result in enhanced device stability. Therefore, the transient decay profiles were studied in the doped films. All three new sky-blue TADF emitters displayed similar prompt and delayed lifetimes to that of the parent HDT1. The rate constants were calculated according to the literature.^2,11 The calculated k_RISCs of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were 1.03 × 10⁶, 0.89 × 10⁶, and 0.86 × 10⁶ s⁻¹, respectively, which are all comparable to that of HDT1 (0.92 × 10⁶ s⁻¹). With a higher dihedral angle, the LUMO is more localized on the central benzonitrile core, which, in turn, decreases the spatial separation of the HOMO and LUMO, resulting in a larger ΔE_ST and a slightly lower k_RISC. Nonetheless, with comparable k_RISCs, it would be expected that the device performance of three new sky-blue emitters should be similar to that of an HDT1-based device.

Electrochemical and thermal properties

The electrochemical and thermal properties of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were studied and are shown in Fig. S16 and S17 (ESI†). In cyclovoltammetry, since the HOMOs are mainly localized on the diphenylcarbazole units in three emitters, all three TADF emitters display irreversible oxidation at 1 V, corresponding to the HOMO level of −5.8 eV. In contrast to oxidation, the larger dihedral angle in 3Me-HDT1 leads to a more localized electron distribution in the LUMO, thereby resulting in a reduction at −2.2 V, while in 1Me-HDT1 having a smaller dihedral angle, its reduction occurred at −2.1 V. Apart from the electrochemical study, the thermal properties of three emitters were studied by thermogravimetric analysis. In general, the decomposition temperature at 3% weight loss (T_d) is higher when the molecular weight of the molecule increases. However, when the number of methyl groups increases, the dihedral angle increases, which, in turn, results in a lower T_d. The T_ds values of 1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 were > 500, 500, and 488 °C, respectively.

OLED performance

OLEDs were fabricated with the following configuration: indium-tin-oxide (ITO)-coated glass (100 nm)/HAT-CN (10nm)/TrisPCz (30 nm)/mCBP (5 nm)/mCBP: 20 wt% of HDT1 (Ref-B) or 1Me-HDT1 (device A) or 2Me-HDT1 (device B) or 3Me-HDT1 (device C) (30 nm)/SF3-TRZ (10 nm)/SF3-TRZ: 30 wt% Liq (20 nm)/Liq (2 nm)/Al (100 nm) is used to confirm OLED characteristics. 1,4,5,8,9,11-Hexaazatriphenyl-enehexacarbonitrile (HAT-CN) is the hole-injection layer, 9-phenyl-3,6-bis(9-phenyl-9H-carbazol-3-yl)-9H-carbazole (TrisPCz) is the hole-transporting layer, mCBP is used for exciton-blocking and host layers, 2-(9,9′-spirobi[fluoren]-3-yl)-4,6-diphenyl-1,3,5-triazine (SF3-TRZ) is the electron-transporting layer, and 8-hydroxyquinolinolato-lithium (Liq) and Al are the electron injection and cathode layers, respectively (Fig. S18, ESI†). All the device characteristics are shown in Fig. 4 and Fig. S19 (ESI†), and Table 2. Devices A–C are fabricated based on an emitting layer (EML) consisting of 20 wt% of corresponding emitters doped in the mCBP host. The turn-on voltages of devices A and B were 3.2 and 3.4 V, respectively, similar to that of Ref-B (3.2 V). However, device C showed a higher turn-on voltage of 4.6 V. Regarding the external quantum efficiency (EQE), both devices A and B achieved a maximum EQE of 21.2% and 19.1%, respectively, slightly lower than that of Ref-B (22%). However, device C resulted in an unexpectedly low maximum EQE of only 0.8%. It can be ascribed to the decomposition of 3Me-HDT1 during device fabrication, in which its EL spectrum was largely different from its PL spectrum. The decomposed 3Me-HDT1 led to poor carrier transport properties within the device, given that the turn-on voltage is exceptionally high (4.6 V). Moreover, device C only showed a maximum luminance of 3850 cd m⁻² at 12 V, whereas devices A and B showed comparable maximum luminance values of 25 724 and 26 547 cd m⁻², respectively, similar to that of Ref-B (30 848 cd m⁻²). Despite the unsatisfactory device performance, device C displayed a blue-shifted electroluminescence of 475 nm, corresponding to CIE_x,y of (0.15, 0.26). We have tested the operation stability of device A, B and Ref-B at an initial luminance of 1000 cd m⁻². In contrast to the good device stability of Ref-B with the LT₉₅ value of 28 h, device stabilities of A and B resulted in shorter LT₉₅ values of 4 and 1 h, respectively (Fig. S19, ESI†). The worse device stabilities might be attributed to the blue-shifted emission (higher energy) of 1Me-HDT1 and 2Me-HDT1, which were confirmed by the photochemical stability measurements. Upon continuous UV irradiation of the degassed toluene solutions of 1Me-HDT1, 2Me-HDT1, and HDT1, it was found that the order of photochemical stability is HDT1 > 1Me-HDT1 > 2Me-HDT1, which was consistent with the device stability as well (Fig. S20, ESI†). Although the introduction of a methyl group can effectively blue-shift the emission, the resulting photochemical stability is lowered. In addition to the importance of photo-chemical stability, the bond dissociation energies of emitters in ground and excited states may also be another determining factor on device stability. It has been found that TADF emitter with a smaller dihedral angle between the donor and acceptor can stabilize weak bonds, such as C–N bonds.²⁰ In our study, 1Me-HDT1 with a smaller dihedral angle achieved longer device stability, compared to 2Me-HDT1 with a larger dihedral angle. This may be attributed to exciton stabilization by a smaller dihedral angle of the substituent group in 1Me-HDT1.

Fig. 4 OLED performance of four devices at 20 wt% doping concentrations. (a) EL spectra of four devices: (b) EQE versus luminance; (c) current efficiency versus luminance; (d) power efficiency versus luminance.

Table 2 Device performance of blue TADF/HF-OLEDs

Device	Dopant	Voltage^a (V)	L max (cd m^−2)	CE^c (cd A^−1)	PE^d (lm W^−1)	EQE^e (%)	λ EL (nm)	CIEx,y^g
a Turn-on voltage at 1 cd m^−2. b Maximum luminance at 12 V. c Maximum current efficiency. d Maximum power efficiency. e Values at 1, 100, and 1000 cd m^−2. f Electroluminescence maximum at 1000 cd m^−2. g Value at 1000 cd m^−2.
Ref-B	20 wt% HDT1	3.2/4.6/7.0	30 848	53.9	52.9	22/21/19	487	0.19, 0.40
A	20 wt% 1Me-HDT1	3.2/4.2/6.4	25 724	49.7	48.8	21.2/20.8/19.2	485	0.18, 0.37
B	20 wt% 2Me-HDT1	3.4/4.4/6.2	26 547	41.9	37.9	19.1/19.0/17.5	479	0.18, 0.34
C	20 wt% 3Me-HDT1 (decomposed)	4.6/—/—	3850	1.4	0.53	0.8/0.8/0.8	475	0.15, 0.26
A-HF	1 wt% v-DABNA: 20 wt% 1Me-HDT1	3.2/4.8/6.2	15 800	30.4	29.8	26.2/23.5/18.9	471	0.13, 0.16

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Nonetheless, with the improved color purity and high efficiency of 1Me-HDT1, HF-OLEDs have been fabricated by the combination of 1Me-HDT1 and v-DABNA as a TADF assistant dopant and a terminal emitter, respectively. The device performance is shown in Fig. 5 and tabulated in Table 2. The EQE of the HF-OLED was increased to 26.2%, which is higher than that of device A and Ref-B. The enhanced EQE originated from the high horizontal orientation of v-DABNA. Most importantly, with the HF device architecture, the color purity of the resulting HF-OLED is improved to (0.13, 0.16), which is better than the reported CIE_x,y of (0.15, 0.20) based on the parent HDT1. The FRET efficiency was found to be increased from 67% to 80% (Fig. S21, ESI†).¹¹ The better CIE_x,y is even closer to the blue color requirement of the BT.2020 standard (0.13, 0.05). We believe that a further molecular design on the assistant dopant and terminal emitter will improve device performance in HF OLEDs. The result here lays a foundation for the design of TADF assistant dopant or MR TADF materials in blue HF OLEDs.

Fig. 5 HF-OLED performance of the device based on 20 wt% of 1Me-HDT1 and 1 wt% of v-DABNA. (a) Device structure for the HF-OLED; (b) EQE versus luminance; (c) EL spectrum at 1000 cd m⁻²; (d) J–V–L curve.

Conclusion

In conclusion, it has been found that increasing the number of methyl groups at the para-substituent of the blue BN-based TADF emitter induced steric hindrance, thus resulting in a larger dihedral angle. The bluer TADF emitter is expected to be beneficial to HF-OLEDs, meaning better color purity can be achieved. 1Me-HDT1 and 2Me-HDT1 showed blueshifted electroluminescence (485 and 479 nm, respectively) and comparable maximum EQEs (21.2% and 19.1%, respectively). Unfortunately, 3Me-HDT1 was found to be thermally unstable upon thermal evaporation, thereby resulting in a poor EQE of only 1%. The methyl-induced steric bulkiness gave rise to poorer thermal stability, which eventually shortened the device stability.

Experimental section

All reagents were used as received from commercial sources and were used without further purification. 9H-Carbazole with a purity of 99.9% (without isomer) was purchased from Ushio Chemix Co., and 3,6-diphenyl-9H-carbazole with the purity of 99.7% and 98.0% was purchased from Suzhou Ge'ao New Material Co., Ltd. Chromatographic separations were carried out using silica gel (200–300 nm). The three materials investigated in this study were synthesized by following the procedures described below. All compounds were purified twice by temperature gradient vacuum sublimation. ¹H nuclear magnetic resonance (NMR) spectra were obtained in CDCl₃ with a Bruker Biospin Avance-III 500 NMR spectrometer at ambient temperature. Chemical shifts (δ) are given in parts per million (ppm) relative to tetramethylsilane (TMS; δ = 0) as the internal reference. Mass spectra were recorded in positive-ion atmospheric-pressure chemical ionization (APCI) mode on a Waters 3100 mass detector. Elemental analyses (C, H, and N) were carried out with a Yanaco MT-5 elemental analyzer. Toluene solutions containing these three materials (0.1 mM) were prepared to investigate their absorption and photoluminescence characteristics in the solution state. Thin-film samples (20 wt%-1Me-HDT1, 2Me-HDT1, and 3Me-HDT1 doped in an mCBP host with a thickness of ∼100 nm) were prepared on quartz glass substrates by spin coating to study their exciton confinement properties in the film state. Ultraviolet–visible absorption (UV-vis) and photoluminescence (PL) spectra were recorded on a PerkinElmer Lambda 950 KPA spectrophotometer and a JASCO FP-8600 spectrofluorometer. Absolute PL quantum yields were measured on a Quantaurus-QY measurement system (C11347-11, Hamamatsu Photonics) under nitrogen flow, and all samples were excited at 340 nm. The prompt and delayed PL spectra of the samples were measured under vacuum using a streak camera system (Hamamatsu Photonics, C4334) equipped with a cryostat (Iwatani, GASESCRT-006-2000, Japan). Cyclic voltammetry (CV) was carried out on a CHI600 voltammetric analyzer at room temperature with a conventional three-electrode configuration consisting of a platinum disk working electrode, a platinum wire auxiliary electrode and an Ag wire pseudo-reference electrode with ferrocenium–ferrocene (Fc⁺/Fc) as the internal standard. Argon-purged N,N-dimethylformamide was used as a solvent for oxidation scanning with tetrabutylammonium hexafluorophosphate (TBAPF₆) (0.1 M) as the supporting electrolyte. The cyclic voltammograms were obtained at a scan rate of 100 mV s⁻¹. Thermal gravimetry-differential thermal analysis (TG-DTA) was performed by Bruker TG-DTA 2400SA with a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere.

Quantum chemical calculations

All calculations were carried out using the Gaussian 16 program package. The geometries in the ground state were optimized via DFT calculations at the B3LYP/6-31G* level. TD-DFT calculations for the S₀ → S₁ and S₀ → T₁ transitions using the B3LYP functional were then performed according to the optimized geometries of the lowest-lying singlet and triplet states, respectively.

Device fabrication and measurements

The OLEDs were fabricated through the vacuum deposition of the materials at ca. 10⁻⁵ Pa onto indium–tin–oxide-coated glass substrates having a sheet resistance of ca. 15 Ω sq⁻¹. The indium–tin oxide surface was cleaned ultrasonically and sequentially with acetone, isopropanol, and deionized water, then dried in an oven, and finally exposed to ultraviolet light and ozone for about 10 min. Organic layers were deposited at a rate of 1–2 Å s⁻¹. Subsequently, Liq was deposited at 0.1–0.2 Å s⁻¹. The devices were exposed once to nitrogen gas after the formation of the organic layers to allow the fixing of a metal mask to define the cathode area. For all OLEDs, the emitting areas were determined by the overlap of two electrodes as 0.04 cm². The J–V–luminance characteristics were evaluated using a Keithley 2400 source meter and an absolute external quantum efficiency (EQE) measurement system (C9920-12, Hamamatsu Photonics, Japan). Device operational stability was measured using a luminance meter (CS-2000, Konica Minolta, Japan) at a constant DC current at room temperature.

Data availability

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

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by City University of Hong Kong (Project No. 9610637). The authors acknowledge Dr Rangani W. Weerasinghe and Ms Yanmei Hu for their help with device fabrication and photophysical measurements. The authors also acknowledge the funding support from JSPS KAKENHI International Leading Research (ILR) (23K20039) and Kyulux Inc.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01486g

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