N-Heterocyclic carbene-based tetradentate platinum(II) complexes for phosphorescent OLEDs with high brightness

Abstract

Organic light-emitting diode (OLED) is an attractive technology for display applications in high-end electronics. Tetradentate Pt(II) complexes are phosphorescence emitters of great importance for OLED fabrication. A novel series of N-heterocyclic carbene (NHC), benzocarbene (Ph/NHC) and pyridinocarbene (Py/NHC)-based tetradentate Pt(II) complexes with fused 5/6/6 metallocycles was designed and developed. Their electrochemical and photophysical properties were systematically studied through experimental and theoretical investigations, which were greatly affected by the NHC, Ph/NHC and Py/NHC. Pt(NHC-1) shows a dominant emission peak at 509 nm with a quantum efficiency of 40% and an excited lifetime of 1.8 μs in a 5 wt% doped PMMA film at room temperature. A Pt(NHC-1)-doped green OLED using 26mCPy as the host material demonstrated a very high L_max of 64 416 cd m⁻², which was a record-high value for the reported Pt(II) complex-based phosphorescent OLED. The OLED also achieved a peak EQE of 13.1% with small efficiency roll-off, which still maintained EQEs of 12.8%, 11.6% and 8.6% at a high brightness of 100, 1000 and 10 000 cd m⁻².

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Introduction

N-Heterocyclic carbenes (NHCs) are electrically neutral species possessing two nonbonding electrons, which may be spin-paired for the singlet state or spin-non-paired for the triplet state.¹ In 1968, Wanzlick² and Öfele³ almost simultaneously published their research on the isolation of the first NHC-based metal complex. By 1991, the first thermally stable and free NHC was successfully separated by Arduengo.⁴ After that, NHCs have been comprehensively investigated in mainstream and modern chemistry. They are widely used as organic catalysts⁵ and ligands in organometallic chemistry,⁶ main group chemistry⁷ and materials science.^8,9 In particular, luminescent NHC-based metal complexes have attracted great attention in the past decades, because of their potential for application as emitters in organic light-emitting diodes (OLEDs) for lighting and display, such as linear-type Pd(0), Au(I), Ag(I) and Cu(I) complexes,^10–18 square planar-type Pt(II)^19–36 and Pd(II)³⁷ complexes, and octahedron-type Ir(III)^38–50 and Fe(III) complexes.^51,52

Many NHC complex-based OLEDs demonstrated good device performances, such as high quantum efficiencies,^{13–18,21,29,33} high colour purities^21,29,33,35 and long device operational lifetimes.^{21,22,35,53–61} NHC-based tris-bidentate Ir(III) complexes were studied first, in particular, for many blue and deep-blue OLEDs.^38–40,48 Recently, through judicious molecular design, Chi, Chou and co-workers successfully developed a novel series of bis-tridentate NHC-based Ir(III) complexes for efficient blue and deep-blue OLEDs.^{42,44,46,47,49,50} Phosphorescent Pt(II) complexes were recognized to be potentially commercial emitters for efficient and stable OLED applications until tetradentate ligands were employed for the molecular design.^{21,22,53–61} In 2017, Credgington and co-workers demonstrated that cyclic (alkyl)(amino)-carbene (CAAC)-based linear Au(I) complexes could act as efficient emitters for solution-processed OLEDs with near-100% internal quantum efficiency (IQE).¹³ Two years later, Thompson and co-workers successfully eliminated the nonradiative decay in NHC-based linear Cu(I) and Ag(I) emitters and realized >99% quantum efficiencies and microsecond lifetimes.^14,17 Recently, we designed and developed the first series of NHC-based Pd(II) complexes for narrow-band deep-blue emitters with a dominant peak at 437–439 nm and a full-width at half-maximum (FWHM) of 34–48 nm.³⁷

Luminescence intensity is an important parameter of OLEDs, especially for lighting applications. In 2015, Forrest and co-workers found that NHC-based fac-Ir(pmp)₃ could simultaneously act as a phosphorescent deep-blue emitter, hole transporter and electron-blocking material, which enabled the OLED to achieve a very high brightness of >7800 cd m⁻².⁴⁰ Typically, Pt(II) complex-based deep-blue OLEDs exhibited maximum brightness (L_max) less than 10 000 cd m⁻², by comparison, green OLED doped Pt(II) emitters could achieve much higher brightness. Che and co-workers in 2019 showed that tetra-Pt-S1-doped green OLEDs realized a L_max of 49 600 cd m⁻² and a peak EQE of 16.6%.⁶² In 2020, we fabricated a green OLED using tetradentate Pt(ppy-1) as an emitter which achieved a L_max of 40 979 cd m⁻² and a peak EQE of 18.5%.⁶³ Recently, we developed a new tetradentate Pt(II) complex Pt(pbiz) and enabled green OLEDs to demonstrate a L_max of 55 481 cd m⁻² and a peak EQE of 21.6% using 2,6-di(9H-carbazol-9-yl)pyridine (26mCPy) as the host material.⁶⁴ However, issues still remain, such as heavy efficiency roll-off at high brightness. Thus, the design of stable phosphorescence emitters, which can efficiently luminesce at a large current density, is important for the potential commercial applications of display and lighting.

In the transition metal complexes discussed above, the NHC carbon typically is in the singlet state and adopts bent sp²-hybridization, the electron pair occupied sp² orbital possesses a strong σ-donating ability and bonds with a vacant d orbital of the metal, such as 5d_xz for Pt(II). Meanwhile, the vacant p_z orbital of the carbon has weak π-accepting ability and can bond with a completely filled d orbital of the metal, such as 5d_x²–y² for Pt(II), both of which enable the NHCs to be ideal ligands to form stable organometallic complexes for phosphorescence emitters (Fig. 1). In this work, we report a novel series of NHC, benzocarbene (Ph/NHC) and pyridinocarbene (Py/NHC)-based tetradentate Pt(II) complexes with fused 5/6/6 metallocycles (Fig. 1). The tert-butyl group was incorporated into the phenyl group to improve the solubilities and also prevent the intermolecular interaction, which might induce triplet–triplet annihilation (TTA). The electrochemical and photophysical properties of the Pt(II) complexes are systematically studied. The Pt(II) complexes show dominant emission peak of 495–505 nm with quantum efficiency of 5–55% in 5 wt% doped poly(methyl methacrylate) (PMMA) film at room temperature. The Pt(NHC-1)-doped green OLED using 26mCPy as the host material demonstrated a very high L_max of 64 416 cd m⁻² and a peak EQE of 13.1% with small efficiency roll-off.

Fig. 1 Chemical structures of NHC, Ph/NHC and Py/NHC-based tetradentate Pt(II) complexes studied in this work. ACz, aza carbazolyl group.

Results and discussion

The synthetic route of the NHC, Ph/NHC and Py/NHC-based tetradentate Pt(II) complexes is shown in Fig. 2. Pd(0)-catalyzed C–N bond cross-coupling of 1-(3-bromo-5-(tert-butyl)phenyl)-1H-imidazole or derivatives (1) with 9-(9H-carbazol-2-yl)-9H-pyrido[2,3-b]indole (2) using SPhos as the ligand and t-BuONa as the base in toluene afforded intermediate 3 in 57–76% yields, which was methylated with CH₃I and then underwent anion exchange with NH₄PF₆ to give the desired ligand(NHC-1), ligand(Ph/NHC) and ligand(Py/NHC) in 61–86% yields for the two steps. Ligand(NHC-2) could be similarly synthesized through Cu(I)-catalyzed C–O bond cross-coupling of 1(NHC-1) with 3-(9H-pyrido[2,3-b]indol-9-yl)phenol (4) using 2-picolinic acid as ligand and K₃PO₄ as the base to afford intermediate 5 in 72% yields, and then methylation and anion exchange in 65% yield. Pt(NHC-1), Pt(Ph/NHC), Pt(Py/NHC) and Pt(NHC-2) were synthesized by metalation of the corresponding ligands with Pt(COD)Cl₂ in the presence of NaOAc in 1-methoxy-2-(2-methoxyethoxy)ethane (DME) at 120 °C to form the desired sole Pt(II) complex in 16–73% yields. The metalation yield for the synthesis of Pt(NHC-2) (73%) was much higher than those of the others, because the flexible molecular geometry of the ligand(NHC-2) using oxygen atom as the linking group facilitates the coordination with the Pt(II) ions. The synthetic details are provided in the ESI.† The key intermediates 3 and 5, and all the ligands and the newly developed tetradentate Pt(II) complexes were confirmed by ¹H and ¹³C NMR spectroscopy, and high resolution mass spectroscopy (HRMS).

Fig. 2 The synthesis of NHC, Ph/NHC and Py/NHC-based tetradentate Pt(II) complexes. X = CH for Ph/NHC or X = N for Py/NHC.

All the Pt(II) complexes exhibit irreversible redox processes in N,N-dimethyl formamide (DMF) under a nitrogen atmosphere (Fig. S1, ESI†), which can be attributed to the square planar molecular geometry and the vacant d orbital of the Pt(II) complexes to be attacked by the nucleophilic solvent molecules.^33,64,65 The values of their redox potentials were measured by different pulsed voltammetry (DPV) relative to an internal reference ferrocenium/ferrocene (Cp₂Fe/Cp₂Fe⁺) (Fig. S2, ESI†) and listed in Table 1. The Pt(II) complexes show oxidation potentials in 0.29–0.48 V. Pt(NHC-1), Pt(Ph/NHC) and Pt(Py/NHC) have similar oxidation potentials with 0.29, 0.36 and 0.38 V, respectively, suggesting the oxidation processes dominantly took place at the PhCz and the metal moieties. However, Pt(NHC-2) has a much larger oxidation potential of 0.48 V compared to the other Pt(II) complexes, suggesting the oxidation process mainly occurred at the PhOPh and the metal moieties. The Pt(II) complexes exhibit reduction potentials in a small range between −2.33 and −2.39 V, demonstrating the reduction processes dominantly occurred at the ACz moieties. The differences of the redox potentials of the Pt(II) complexes indicate the NHC, Ph/NHC and Py/NHC moieties also have significant influence on their electrochemical properties.

Table 1 Dihedral angles, energy levels and electrochemical properties of tetradentate Pt(II) complexes

Metal complex	Dihedral angle^a	Calc. HOMO^b/LUMO^b/ΔEg^c [eV]	E ox [V]	E red [V]	HOMO^e/LUMO^f/ΔEg^c [eV]	E T1 [eV]
a Dihedral angle between terminal NHC and ACz planes on the basis of optimized S0. b From DFT calculations at the B3LYP/6-31G(d)/LANL2DZ level based on optimized S0. c ΔEg = LUMO–HOMO. d From different pulsed voltammetry (DPV) measurements (Fig. S2, ESI). e HOMO = −(Eox + 4.8) eV. f LUMO = −(Ered + 4.8) eV. g Triplet energy ET1 estimated from the peak of the phosphorescence emission spectrum at 77 K in 2-MeTHF.
Pt(NHC-1)	46.72	−4.59/−1.47/3.12	0.29	−2.39	−5.09/−2.41/2.68	2.58
Pt(Ph/NHC)	48.47	−4.64/−1.53/3.11	0.36	−2.33	−5.16/−2.47/2.69	2.58
Pt(Py/NHC)	48.15	−4.63/−1.59/3.04	0.38	−2.33	−5.18/−2.47/2.71	2.56
Pt(NHC-2)	49.19	−4.69/−1.57/3.12	0.46	−2.37	−5.28/−2.43/2.85	2.63

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Density functional theory (DFT) calculations^63,66–68 reveal that the tetradentate Pt(II) complexes show distorted planar molecular geometries with dihedral angles between the terminal NHC and ACz planes of 46.72°, 48.47°, 48.15° and 49.19° in their ground states respectively (Table S1 and Fig. S1). The bond length of Pt–C¹_NHC (2.082 Å) in Pt(NHC-1) is significantly shorter than that of the Pt–N¹_ACz (2.172 Å) in Pt(NHC-1), and the Pt–N_Py (2.200 Å) in Pt(ACzCCz-3)⁶⁷ which employed a similar tetradentate ligand (Table S1, ESI†). Similar results are also found in Pt(Ph/NHC), Pt(Py/NHC) and Pt(NHC-2), indicating the high bond strength of the Pt–C¹_NHC in the newly developed Pt(II) complexes. All the tetradentate Pt(II) complexes exhibit similar the highest occupied molecular orbital (HOMO) and HOMO−1 distributions, however, their lowest unoccupied molecular orbital (LUMO) and LUMO+1 distributions are significantly different (Fig. 3 and Fig. S3, ESI†). Pt(NHC-1), Pt(Ph/NHC) and Pt(Py/NHC) have the HOMO distributions on π_PhCz orbitals and d_Pt centers with energy levels of −4.59, −4.64 and −4.63 eV, respectively, and Pt(NHC-1) has the HOMO distribution on the π_PhOPh orbital and the d_Pt center with energy levels of −4.69 eV (Fig. 3). All the Pt(II) complexes show LUMO distributions on π_ACz orbitals with energy levels of −1.47to −1.59 eV besides Pt(Py/NHC), also having some distribution on π_Py/NHC (Fig. 3). The LUMO+1 distributions are mainly on the π_NHC–Ph orbitals for Pt(NHC-1) and Pt(NHC-2), and on the π_Ph/NHC and π_Py/NHC orbitals for Pt(Ph/NHC) and Pt(Py/NHC) respectively (Fig. 3 and Fig. S3, ESI†). These results indicate that the electron-withdrawing abilities are in the order ACz > Py/NHC > Ph/NHC > NHC. The energy levels for the Pt(II) complexes from the calculation results are well in agreement with the trend of their HOMO/LUMO levels from their redox potentials (Table 1).

Fig. 3 Density functional theory calculations of frontier orbitals for Pt(II) complexes based on optimized S₀ geometries. The H atoms were omitted for clarity.

The UV–vis absorption spectra of the tetradentate Pt(II) complexes at room temperature (RT) in dichloromethane (DCM) solution are illustrated in Fig. 4. Pt(NHC-1), Pt(Ph/NHC) and Pt(Py/NHC) show significantly stronger absorption in the region of less than 375 nm compared to Pt(NHC-2) due to the small conjugation in Pt(NHC-2). Between 375 and 475 nm, all the four Pt(II) complexes have similar absorption strength, but Pt(NHC-2) has a significantly large blue-shift compared to those of the other Pt(II) complexes. The above results reveal that, for all the Pt(II) complexes, the very strong absorption bands less than 375 nm are attributed to spin-allowed ¹(π → π*) transitions in the cyclometalating ligands. The absorption bands at 375–475 nm are identified as spin-allowed metal-to-ligand charge-transfer (¹MLCT) transitions. The absorption for the ³MLCT transitions could be also observed above 480 nm, which were structureless and very different from the previous reported Pt(II) complexes.^{29,30,33,63,66–68}

Fig. 4 Absorption spectra of the NHC, Ph/NHC and Py/NHC-based Pt(II) complexes in dichloromethane at room temperature.

The comparison of the emission spectra and transient decay spectra of the Pt(II) complexes under various conditions is shown in Fig. 5 and Fig. S4 (ESI†), and the data of their photophysical properties are listed in Table 2. At 77 K in 2-methyltetrahydrofuran (2-MeTHF), the Pt(II) complexes show vibronic emission spectral peaks at 472–484 nm indicating strong admixed ligand-centered (³LC) and mixed ³MLCT characters in their T₁ states.³³ Pt(NHC-2) has a much longer excited-state lifetime (τ) of 11.1 μs compared to those of the other Pt(II) complexes of 4.0–5.6μs, indicating the significant influence of the tetradentate ligands. At room temperature in dichloromethane, the Pt(II) complexes exhibit a significant red-shift of 58, 44, 40 and 55 nm for Pt(NHC-1), Pt(Ph/NHC), Pt(Py/NHC) and Pt(NHC-2), respectively. Importantly, they all show broad and structureless emission spectra indicating significantly enhanced ³MLCT (π_Phd_Pt* → π_ACz) characters in their T₁ states. By comparison, the NHC-based Pd(II) complexes typically show dominantly ³LC characters in their T₁ states because of their relatively weak SOC effect.³⁷ The quantum efficiencies (Φ_PL) of the Pt(II) complexes are relatively low (2–18%). However, in the doped PMMA films, the Φ_PL could be significantly enhanced even measured in air, and Pt(NHC-1) and Pt(NHC-2) demonstrated the Φ_PL of up to 40% and 55% respectively. This is attributed to the suppression of molecular vibrations and the oxygen exclusion by the film. The Pt(II) complex-doped films also show broad and structureless emission spectral peaks at 495–509 nm with a τ of 1.0–3.9 μs. The improved Φ_PL and short τ enable Pt(NHC-1) and Pt(NHC-2) to achieve radiative rate (k_r) of 2.22 × 10⁵ s⁻¹ and 1.41 × 10⁵ s⁻¹, indicating that they can act as ideal phosphorescence emitters for device fabrication.

Fig. 5 Comparison of photoluminescence spectra of (a) Pt(NHC-1), (b) Pt(Ph/NHC), (c) Pt(Py/NHC) and (d) Pt(NHC-2) under various conditions. 2-MeTHF, 2-methyltetrahydrofunan; RT, room temperature; DCM, dichloromethane; PMMA, polymethyl methacrylate.

Table 2 Photophysical properties of NHC, Ph/NHC and Py/NHC-based Pt(II) complexes

Complex	Emission at 77 K in 2-MeTHF		Emission at RT in DCM					Emission at RT in 5 wt% PMMA
	λ max [nm]	τ [μs]	λ max [nm]	τ [μs]	Φ PL [%]	k r [10^4 s^−1]	k nr [10^4 s^−1]	λ max [nm]	τ [μs]	Φ PL [%]	k r [10^4 s^−1]	k nr [10^4 s^−1]
a Measured in air using an integrating sphere. DCM, dichloromethane; 2-MeTHF, 2-methyltetrahydrofuran; PMMA, poly (methyl methacrylate).
Pt(NHC-1)	481	5.6	539	0.7	18	25.7	117.1	509	1.8	40	22.2	33.3
Pt(Ph/NHC)	481	4.5	525	1.4	2	1.4	70.0	505	1.3	19	14.6	62.3
Pt(Py/NHC)	484	4.0	524	1.8	2	1.0	54.6	506	1.0	5	5.0	95.0
Pt(NHC-2)	472	11.1	517	0.7	18	8.6	39.0	495	3.9	55	14.1	11.5

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The photoluminescence properties of the Pt(NHC-1) in 10 wt% doped host materials mCBP [3,3-di(9H-carbazol-9-yl)biphenyl] and 26mCPy [2,6-di(9H-carbazol-9-yl)pyridine] films were studied (Fig. 6). In mCBP film, Pt(NHC-1) shows a broad emission spectra at 519 nm with a relatively low Φ_PL of 23% and a τ of 1.1 μs. However, the Φ_PL can be significantly enhanced to 45% in 26mCPy film, and the emission spectrum shows a blue-shift with a dominant peak at 512 nm. The enhanced Φ_PL in 26mCPy enables Pt(NHC-1) to act as an emitter for OLED fabrication.

Fig. 6 Photoluminescence spectra of Pt(NHC-1) in 10 wt% doped mCBP and 26mCPy films.

To evaluate the electroluminescence properties of Pt(NHC-1), OLEDs were fabricated using mCBP (device I) and 26mCPy (device II) as host materials respectively. The device architecture was ITO/HATCN (10 nm)/TAPC (60 nm)/Pt(NHC-1):host (10%, 35 nm)/PPT (2 nm)/Bepp2:Li₂CO₃ (30 nm)/Li₂CO₃ (1 nm)/Al (100 nm), where HATCN was 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene and employed as a hole injection material, TAPC was 1,1′-bis[4-(di-p-tolylamino)phenyl]-cyclohexane and used as a hole-transporting material, and PPT was dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) and acted as a hole-blocking material for its deeper LUMO level (−6.6 eV) than those of the host materials mCBP (−6.2 eV) and 26mCPy (−6.0 eV), Bepp2 was bis [2-2-(hydroxyphenyl) pyridinato]beryllium and Bepp2:Li₂CO₃ was used as electron-transporting material^69–71 (Fig. 7a). The device performances are summarized in Table 3.

Fig. 7 (a) Energy level diagram and device structures of the Pt(NHC-1)-doped OLEDs. (b) EL spectra, (c) current density–voltage–luminance (J–V–L) characteristics, and (d) EQE versus current density plots of the Pt(NHC-1)-doped OLEDs. HTL: hole transporting layer; EML: emissive layer; HBL: hole blocking layer; ETL: electron transporting layer.

Table 3 EL performance of Pt(NHC-1)-doped OLEDs

Device	EML	V on (V)	λ EL (nm)	EQE^a (%)	L max (cd m^−2)	CIE (x, y)
a Maximum EQE, and EQE at 100, 1000 and 10 000 cd m^−2. Von: turn-on voltage at 1 cd m^−2. Lmax: maximum brightness. CIE: Commission Internationale de l’Eclairage.
Device I	10% Pt(NHC-1):mCBP	3.5	512	13.9/13.6/12.5/9.2	60 275	(0.288, 0.588)
Device II	10% Pt(NHC-1):26mCPy	3.9	509	13.1/12.8/11.6/8.6	64 416	(0.261, 0.575)

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Both device I and device II showed turn-on voltages (V_on) of 3.5 V and 3.9 V, respectively. They also exhibited sole and similar EL spectral peaks at 512 nm with CIE coordinates of (0.288, 0.588) and (0.261, 0.575) respectively (Fig. 8), device I had a slight red-shift of 3 nm compared to device II (Fig. 7b and Table 3). The distorted molecular geometry of the Pt(NHC-1) and the introduction of the tert-butyl group (Table S1, ESI†) prevents the intermolecular interaction between the dopant molecules, and no excimer or aggregation emission was observed, which is helpful for improving the colour purity of the OLEDs. Compared to the photoluminescence spectra of Pt(NHC-1), the electroluminescence of both device I and device II exhibited similar spectra with blue-shift of 7 nm and 3 nm, respectively. The maximum brightness (L_max) of 60 275 and 64 416 cd m⁻² could be achieved for device I and device II, respectively (Fig. 7c and Table 3). As far as we know, the L_max values of the device I and device II are record-high values for the reported Pt(II) complex-based phosphorescent OLEDs (Fig. 9 and Fig. S5, Table S3, ESI†).^{62,63,66,72–74} This result suggested the superiority of NHC-based Pt(II) complex as emitters for OLED applications because of the chemical stability. The high brightness facilitates the emitter for potential lighting application. Additionally, both the devices demonstrated small efficiency roll-off, the maximum EQE, and EQE at 100 and 1000 cd m⁻² were 13.9%, 13.6%, 12.5% and 13.1%, 12.8%, 11.6% for device I and device II, respectively, yet maintained EQEs of 9.2% and 8.6% at a high brightness of 10 000 cd m⁻² (Fig. 7d and Table 3). The TAPC, which possesses a higher HOMO level (−2.0 eV) than those of the host materials mCBP (−2.7 eV) and 26mCPy (−2.3 eV), and the hole-blocking material PPT can confine the electrogenerated triplet excitons inside of the emitting layer and achieve a high device performance. It is believed that the performance of the OLED can be further improved by using the state-of-the-art device structure.

Fig. 8 Electroluminescence colour coordinates on the CIE 1931 chromaticity diagram of the Pt(NHC-1)-doped OLEDs.

Fig. 9 (a) Chemical structures of representative Pt(II) complexes for green to yellow OLEDs. (b) The comparison of the maximum brightness of the representative Pt(II) complex-based green to yellow OLEDs, and the emissive layer (emitter/host) was marked for each OLED. mCBP: 3,3-di(9H-carbazol-9-yl)biphenyl; 26mCPy: 2,6-di(9H-carbazol-9-yl)pyridine; o-CzPy: 2,6-bis(2-(9H-carbazol-9-yl)phenyl)pyridine; m-TPAPy: 3,3′-(pyridine-2,6-diyl)bis(N,N-diphenylaniline); TCTA: 4,4′,4′′-tri (N-carbazolyl)triphenylamine.

Conclusions

In summary, a new series of NHC, Ph/NHC and Py/NHC-based tetradentate Pt(II) complexes with fused 5/6/6 metallocycles were designed and developed. Systematic experimental and theoretical studies indicate that the NHC, Ph/NHC and Py/NHC had a significant influence on the electrochemical properties, frontier orbitals and photophysical properties. The Pt(II) complexes show a dominant emission peak at 517–539 nm with a quantum efficiency of 2–18% in dichloromethane solution at room temperature. In a 5 wt% doped PMMA film at room temperature, the Pt(II) complexes exhibit significant blue-shift peaks at 495–505 nm and greatly enhanced quantum efficiencies of 5–55%. Pt(NHC-1)-doped green OLEDs were fabricated using mCPB and 26mCPy as host materials with small efficiency roll-off, which demonstrated very high L_max values of 60 275 and 64 416 cd m⁻² and peak EQEs of 13.9% and 13.1% respectively. This work could offer a valuable reference for the development of novel stable phosphorescent Pt(II) emitters, which can efficiently luminesce at large current density, for OLED fabrication for potentially commercial display and lighting applications.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant no. 21878276, 22178319 and 22138011) and the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-A2019013) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1tc05207a

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