Highly efficient orange OLEDs with narrow-emitting symmetric tetradentate platinum(II) complexes based on rigid steric hindrance aza-triptycene pyridazine ligands

Abstract

A series of symmetric tetradentate Pt(II) complexes with rigid steric hinderance aza-triptycene pyridazine ligands Pt-DPM, Pt-PIP and Pt-DPT were synthesized and fully characterized. These complexes exhibited excellent thermal stability with T_d values of 407–462 °C. All these complexes showed orange emission with emission maxima at 580–593 nm and short lifetimes of 3.37–3.82 μs. Most importantly, these complexes exhibit narrow full widths at half maximum of 38–45 nm in thin films. Furthermore, the introduction of an aza-triptycene skeleton with a Y-shaped rigid 3D molecular structure reduced bimolecular interactions, leading to a significant increase in the PLQYs (>88%) compared with the reference Pt(II) complex Pt-D. Solution-processed organic light-emitting diodes using three new Pt(II) complexes as emitters show excellent performance. The efficiencies are dramatically improved to 9.67%, 23.67 cd A⁻¹, and 26 670 cd m⁻² for the OLED based on Pt-DPM, and to 16.94%, 44.66 cd A⁻¹, and 54 918 cd m⁻² for that based on Pt-DPT, respectively. Due to the introduction of an aza-triptycene unit and isopropyl group with steric hindrance, the device exhibits good properties even at high doping concentrations. When a neat device based on Pt-DPT was adopted as the emitting layer, the non-doped orange device exhibited a high brightness of 10 873 cd m⁻² and a luminous efficiency of 9.56 cd A⁻¹ along with CIE coordinates of (0.59, 0.40). These results suggest that these materials have potential applications in OLEDs.

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1. Introduction

Phosphorescent transition metal complexes are capable of harvesting both singlet and triplet excitons and realizing a theoretical internal quantum efficiency of 100%. These desirable photophysical properties and promise as emitters for organic light emitting diodes (OLEDs) have generated enormous interest. Among these transition metal complexes, Ir(III)^1–3 and Pt(II) complexes^4–6 have attracted much attention in recent years because of their extraordinary photophysical properties.^7,8 Recently, rapid progress has been made in the development of phosphorescent materials such as bidentate,^9,10 tridentate,^11,12 and tetradentate^13–23 Pt(II) complexes due to the relatively low cost and stable performance of platinum salts. In these Pt(II) complexes, the symmetrical bidentate and tetradentate Pt(II) complexes with stronger Pt–C coordination bonds between the platinum metal center and ligand can lead to strong spin–orbit coupling, effectively weaken molecular vibrations and reduce non-radiative transitions.²⁴ These results can effectively improve the luminous efficiency and provide good thermal and chemical stability. In particular, tetradentate Pt(II) complexes have a highly rigid structure that can effectively reduce molecular vibrations and rotations, thereby minimizing non-radiative transitions and achieving high luminescence efficiency.^24,25

Recently, orange phosphorescent Pt(II) complexes have attracted significant interest because of their wide application in color displays and warm white light solid state lighting.^18,26 There are also many reports focused on the development of various Pt(II) complexes with high emission efficiency and good color purity. For example, Zhao et al.²⁷ reported two concentration-dependent tridentate Pt(II) complexes based on 1,3-bis (2-pyridyl) benzene. Yin et al.²⁸ introduced a chiral alkyl chain into the tridentate chelating ligand 2,6-bis(N-alkylbenzimidazol-2′-yl) benzene, and three chiral Pt(II) complexes were successfully designed and synthesized. These tridentate Pt(II) complexes have only one Pt–C coordination bond. It is not beneficial for luminous efficiency and stability. Furthermore, these complexes exhibited strong orange emission only at high concentrations in acetonitrile solution with a full width at half maximum (FWHM) of approximately 100 nm. Yang et al.²⁹ designed three orange-red dinuclear Pt(II) complexes with more Pt–C bonds. Due to the increased rigidity of the complex structure, the luminescence efficiency has been improved. The orange-red emitting device displayed the highest external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 11.2%, 21.3 cd A⁻¹, and 11.7 lm W⁻¹. Zhu's group³⁰ designed dinuclear orange Pt(II) complexes using donor–acceptor-type oxadiazole-thiol chelates as bridging ligands. With different doping concentrations, devices with different luminescent colors were fabricated with red emission at 614 nm (η = 8.7%), NIR emission at 716 nm (η = 5.1%) and white-light emission (η = 11.6%) in non-doped, doped and hybrid devices, respectively. However, due to the excellent planarity of the structure and easy stacking, the FWHMs of these materials almost exceed 100 nm. Wong et al.³¹ reported a trinuclear Pt(II) complex based on aryl benzothiazole ligands. Accordingly, the peak EQE, CE, and PE of the solution-processed orange OLED based on the trinuclear Pt(II) complex with a doping concentration of 9 wt% are 17.0%, 35.4 cd A⁻¹, and 27.2 lm W⁻¹, respectively. When the doping concentration is increased to 11 wt%, there is a significant decrease in device performance due to the concentration quenching effect of material luminescence. However, there are few research reports on orange-light-emitting devices with narrow emission based on tetradentate Pt(II) complexes. Hang et al.³² reported on a carbene-based tetradentate Pt(II) complex, which exhibited orange electroluminescence (EL) colors with the Commission Internationale de l'Eclairage (CIE) values of (0.55, 0.44) with a concentration of 20 wt%. The orange device can achieve an LT₇₀ longevity equal to 662 h. The Pt(II) complex exhibits a FWHM of only 38 nm in a dilute solution; however, at high doping concentrations, the FWHM of the device exceeds 100 nm.

Therefore, it is particularly important to design orange-light-emitting devices of tetradentate Pt(II) complexes with narrow emission, high color purity and high luminescence efficiency. The common strategy is to introduce steric hindrance groups to suppress concentration quenching.³³ The steric hindrance group can effectively suppress concentration quenching, but the introduced group also increases the non-radiative transitions caused by the rotation and vibration of certain bonds in the molecule, which to some extent reduces the luminescence performance of the materials. It is crucial to design and develop cyclometalated ligands that combine both steric hindrance units and rigid skeleton structures. The rigidity and unique three-dimensional configuration of the triptycene structure are conducive to increasing the stability of luminescent materials and reducing the non-radiative transition rate. The wheel-like three-dimensional structure of triptycene is bulky and induces steric hindrance, making π–π stacking difficult,  which prevents aggregation and thereby avoids luminescence quenching. So far, there have been a few studies on iridium complexes based on the tricyclic skeleton.^33–36 However, there have been no reports on its application in platinum complexes. In the preliminary work, a class of C^N=N bidentate cyclometalated ligands based on pyridazine and phthalocyanine were successfully developed, which easily form iridium(III) or Pt(II) complexes under mild conditions because the adjacent sp² N atoms in the C^N=N ligand have no steric hindrance H atom, which makes the bonding between the nitrogen atom and metal atom stronger.³⁷ Based on the excellent structural properties of aza-triptycene and the good luminescence properties of tris-cyclometalated iridium(III) complexes, in this study, the triptycene unit was successfully introduced into tetradentate ligands (Fig. 1). A series of new aza-triptycene-based cyclometalated ligands and corresponding tetradentate Pt(II) complexes have been synthesized.

Fig. 1 The design idea of this project.

2. Experimental

2.1. Materials and characterization

All commercial reagents were used without further purification, unless otherwise noted. N,N-Dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) were dried using standard procedures before use. Dichloromethane (DCM) suitable for electrochemical and spectral experiments was obtained by distillation over calcium hydride, and silica gel (200–300 mesh, Qingdao Haiyang) was used for column chromatography.

2.2. Preparation of ligands

Compounds ddcme, ddca, dda, and ddah have been reported in the literature.³³ The synthetic procedures of the reference Pt(II) complex Pt-D are provided in the ESI.†

12,15-Dichloro-9,10-dihydro-9,10-[4,5]epipyridazinoanthracene (ddcp). A mixture of ddah (6 g, 20 mmol) and phosphorus oxychloride (30 mL, 320 mmol) was added and the mixture was refluxed at 125 °C for 24 h under a N₂ atmosphere. After completion of the reaction, the solvent was then removed by rotary evaporation and the residue was poured into 400 mL of ice water and the pH was adjusted with dilute NaOH solution to weakly alkaline. The mixture was then stirred for a period of time until the precipitate was completely separated. The resulting precipitate was filtered off and purified by chromatography on silica gel with a mixture of petroleum ether (PE) and ethyl acetate (EA) (5 : 1, v/v) as the eluent to afford ddcp as a white solid in 59% yield. ¹H NMR (400 MHz, CDCl₃) δ = 7.55–7.49 (m, 4H), 7.15 (d, J = 17.2, 4H), 5.83 (s, 2H).

12-Chloro-15-(2,6-dimethylphenoxy)-9,10-dihydro-9,10-[4,5] epipyridazinoanthracene (cdpm). A mixture of ddcp (1.71 g, 5 mmol), 2,6-dimethylphenol (0.49 g, 4 mmol) and K₂CO₃ (1.38 g, 10 mmol) in DMF (30 mL) was stirred at 120 °C for 24 h under a N₂ atmosphere. Then, the reaction mixture was cooled to room temperature and poured into 500 mL of water with stirring. The precipitate was collected by filtration, washed with water and purified by chromatography on silica gel with PE and EA (5 : 1, v/v) as the eluent to afford cdpm as a white crystal. Yield: 84.0%. ¹H NMR (400 MHz, DMSO) δ = 7.73–7.60 (m, 4H), 7.19–7.08 (m, 7H), 6.32 (s, 1H), 6.10 (s, 1H), 1.97 (s, 6H).

12-Chloro-15-(piperidin-1-yl)-9,10-dihydro-9,10-[4,5] epipyridazinoanthracene (cpip). A mixture of ddcp (1.71 g, 5 mmol), piperidine (0.51 g, 6 mmol) and NaH (0.24 g, 10 mmol) in DMSO (30 mL) was stirred at 50 °C for 24 h under a N₂ atmosphere. Then, the reaction mixture was cooled to room temperature and poured into 500 mL of water under stirring. The precipitate was collected by filtration, washed with water and purified by chromatography on silica gel with PE and EA (5 : 1, v/v) as the eluent to afford cpip as a white crystal. Yield: 87%. ¹H NMR (400 MHz, CDCl₃) δ 7.54–7.40 (m, 4H), 7.15–7.01 (m, 4H), 5.79 (s, 1H), 5.56 (s, 1H), 3.45–3.24 (m, 4H), 1.94–1.82 (m, 4H), 1.72 (d, J = 5.7 Hz, 2H).

12-Chloro-15-(2,6-diisopropylphenoxy)-9,10-dihydro-9,10-[4,5]epipyridazinoanthracene (cdpt). A mixture of ddcp (1.71 g, 5 mmol), 2,6-diisopropylphenol (0.71 g, 4 mmol) and NaH (0.24 g, 10 mmol) in DMSO (30 mL) was stirred at 50 °C for 24 h under a N₂ atmosphere. Then, the reaction mixture was cooled to room temperature and poured into 500 mL of water with stirring. The precipitate was collected by filtration, washed with water and purified by chromatography on silica gel with PE and EA (5 : 1, v/v) as the eluent to afford cdpt as a white crystal. Yield: 55%. ¹H NMR (400 MHz, CDCl₃) δ 7.57 (dd, J = 8.4, 3.8 Hz, 4H), 7.27–7.24 (m, 1H), 7.23–7.12 (m, 6H), 6.06 (s, 1H), 5.88 (s, 1H), 2.89–2.71 (m, 2H), 1.26 (d, J = 6.3 Hz, 6H), 1.01 (d, J = 6.4 Hz, 6H).

3,3′-Oxybis(bromobenzene) (obb). A mixture of 3-bromophenol (8.6 g, 0.05 mol), 1,3-dibromobenzene (23.4 g, 0.1 mol), CuI (3.8 g, 0.02 mol), K₃PO₄ (21.25 g, 0.1 mol) and 2-picolinic acid (3.8 g, 0.03 mmol) in DMSO (10 mL) was stirred at 120 °C for 24 h under a N₂ atmosphere. Then, the reaction mixture was cooled to room temperature and poured into 500 mL of water with stirring. The precipitate was collected by filtration, washed with water and purified by chromatography on silica gel with PE as the eluent to afford obb as an oily product. Yield: 31.7%.¹H NMR (400 MHz, DMSO) δ 7.37–7.34 (m, 4H), 7.24 (dd, J = 2.6, 1.4 Hz, 2H), 7.04 (ddd, J = 5.2, 4.0, 2.3 Hz, 2H).

2,2′-(Oxybis(3,1-phenylene))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (obp). A mixture of obb (0.65 g, 2 mmol), bis(pinacolato)diboron (3.06 g, 12 mmol), KOAc (0.59 g, 6 mmol) and PdCl₂(dppf) (0.15 g, 0.2 mmol) in DMF (20 mL) was stirred at 80 °C for 24 h under a N₂ atmosphere. Then, the reaction mixture was cooled to room temperature and poured into 500 mL of water with stirring. The precipitate was collected by filtration, washed with water and purified by chromatography on silica gel with PE as the eluent to afford obp as an oily product. Yield: 71%. ¹H NMR (400 MHz, DMSO) δ 7.50–7.38 (m, 4H), 7.24–7.12 (m, 4H), 1.24 (d, J = 16.1 Hz, 24H).

A mixture of tetrahydrofuran (THF) (3 mL), H₂O (2 mL), PdCl₂(dppf) (0.04 g, 0.06 mmol), K₂CO₃ (0.083 g, 0.6 mmol), obp (0.084 g, 0.2 mmol), and the cyclometalated compound (0.6 mmol) was stirred in a N₂ atmosphere for 12 h at 80 °C. After cooling to room temperature, the solution was extracted with dichloromethane, and then the solvent was removed in vacuo. The residue was purified by column chromatography on silica gel using PE and EA (5 : 1, v/v) as the eluent to afford the desired ligand.

15,15′-(Oxybis(3,1-phenylene))bis(12-(2,6-dimethylphenoxy)-9,10-dihydro-9,10-[4,5]epipyridazinoanthracene) (ODPM). Yield: 53%.¹H NMR (400 MHz, CDCl₃) δ 7.64 (t, J = 7.8 Hz, 2H), 7.58–7.48 (m, 8H), 7.38 (d, J = 8.3 Hz, 2H), 7.18–7.02 (m, 14H), 6.95 (t, J = 7.5 Hz, 4H), 6.08 (s, 2H), 5.81 (s, 2H), 2.10 (d, J = 6.1 Hz, 12H).

15,15′-(Oxybis(3,1-phenylene))bis(12-(piperidin-1-yl)-9,10-dihydro-9,10-[4,5]epipyridazinoanthracene) (OPIP). Yield: 36%. ¹H NMR (400 MHz, CDCl₃) δ 7.66 (dd, J = 7.8, 1.7 Hz, 2H), 7.62–7.53 (m, 4H), 7.43 (t, J = 10.0 Hz, 6H), 7.07 (d, J = 7.1 Hz, 4H), 7.01 (td, J = 7.4, 2.1 Hz, 4H), 6.96–6.88 (m, 4H), 5.76 (s, 2H), 5.65 (s, 2H), 3.40 (s, 8H), 1.82 (d, J = 66.2 Hz, 12H).

15,15′-(Oxybis(3,1-phenylene))bis(12-(2,6-diisopropylphenoxy)-9, 10-dihydro-9,10-[4,5]epipyridazinoanthracene) (ODPT). Yield: 38%. ¹H NMR (400 MHz, CDCl₃) δ 7.62 (t, J = 7.9 Hz, 2H), 7.51 (dd, J = 18.4, 10.7 Hz, 8H), 7.35 (dd, J = 8.2, 1.4 Hz, 2H), 7.23–7.10 (m, 10H), 7.05 (dd, J = 10.6, 4.2 Hz, 4H), 6.94 (t, J = 7.0 Hz, 4H), 6.07 (s, 2H), 5.78 (s, 2H), 2.81 (dt, J = 13.7, 6.9 Hz, 4H), 1.27–1.19 (m, 12H), 0.98–0.87 (m, 12H).

2.3. Synthesis of complexes

A mixture of acetic acid (5 mL), K₂PtCl₄ (0.023 g, 0.055 mmol), tetrabutylammonium bromide (TBAB) (0.002 g, 0.005 mmol) and the cyclometalated ligand (0.05 mmol) was added to a Schlenk tube under an N₂ atmosphere. The reaction was stirred at room temperature for 12 h and then at 120 °C for 70 h. The reaction mixture was cooled to room temperature and poured into 50 mL of water. The residue was purified by column chromatography on silica gel using PE and DCM (1 : 1, v/v) as the eluent to afford the desired complexes.

Pt-DPM. Orange solid, yield: 28%. ¹H NMR (400 MHz, CDCl₃) δ 8.02 (d, J = 6.4 Hz, 2H), 7.49 (d, J = 6.5 Hz, 4H), 7.44 (t, J = 7.6 Hz, 2H), 7.33 (d, J = 8.1 Hz, 2H), 7.14 (d, J = 6.5 Hz, 4H), 7.02 (d, J = 6.3 Hz, 12H), 6.95–6.89 (m, 2H), 6.68 (s, 2H), 5.48 (s, 2H), 2.07 (s, 12H). ¹³C NMR (100 MHz, CDCl₃) δ 160.32, 158.77, 151.89, 148.51, 142.71, 142.40, 130.07, 129.28, 126.14, 126.08, 125.14, 124.61, 124.44, 124.27, 120.99, 47.32, 29.34, 22.67. MALDI-TOF-MS: m/z = 1111.434 (calcd. 1111.306 for [C₆₄H₄₄N₄O₃Pt] [M]⁺).

Pt-PIP. Range solid, yield: 18%. ¹H NMR (400 MHz, CDCl₃) δ 7.90 (d, J = 7.5 Hz, 2H), 7.50 (dd, J = 5.3, 3.2 Hz, 4H), 7.46–7.36 (m, 6H), 7.28 (d, J = 8.7 Hz, 1H), 7.12–7.00 (m, 8H), 6.60 (s, 2H), 5.63 (s, 2H), 3.46–3.28 (m, 8H), 1.83 (d, J = 68.8 Hz, 12H). ¹³C NMR (100 MHz, CDCl₃) δ 159.35, 158.98, 152.20, 145.49, 142.88, 126.01, 125.94, 124.53, 124.12, 124.04, 120.79, 50.16, 22.67. MALDI-TOF-MS: m/z = 1037.482 (calcd. 1037.338 for [C₅₈H₄₆N₆OPt] [M]⁺).

Pt-DPT. Orange solid, yield: 25%. ¹H NMR (400 MHz, CDCl₃) δ 7.98 (d, J = 7.4 Hz, 2H), 7.46 (t, J = 7.8 Hz, 6H), 7.35 (d, J = 8.0 Hz, 2H), 7.25–7.17 (m, 6H), 7.06–6.95 (m, 12H), 6.63 (s, 2H), 5.37 (s, 2H), 2.96 (dt, J = 13.7, 6.9 Hz, 4H), 1.06 (d, J = 6.5 Hz, 24H). ¹³C NMR (100 MHz, CDCl₃) δ 159.71, 152.30, 149.86, 148.21, 142.30, 126.02, 125.93, 125.50, 125.39, 124.84, 124.51, 124.48, 121.03, 117.91, 49.85, 47.43, 27.43. MALDI-TOF-MS: m/z = 1239.584 (calcd. 1239.463 for [C₇₃H₆₄N₄O₃Pt] [M]⁺).

3. Results and discussion

3.1. Synthesis and characterization

The synthetic routes for these complexes Pt-DPM (with a methyl group), Pt-PIP (with a piperidinyl group) and Pt-DPT (with an isopropyl group) are in Scheme 1. The key intermediate 3,6-dichloropyridazine derivatives were synthesized from anthracene by means of Diels–Alder cycloaddition,³⁸ cyclization with hydrazine hydrate and halogenation in the presence of phosphorus oxychloride. Then, mono-substituted functionalized triptycene pyridazine derivatives were conveniently prepared by Williamson ether synthesis or through N-alkylation reactions with 3,6-dichloropyridazine derivatives in the presence of an alkali in a polar solvent according to literature procedures.³⁹ The six-membered ring structure of pyridazine can remain relatively stable during the reaction process, making ring-opening or other coordination side reactions unlikely. Thus, the reaction can be carried out more specifically as expected, which is crucial for improving the reaction yield. The primary ligands ODPM, OPIP and ODPT were conveniently prepared from mono-substituted functionalized triptycene pyridazine derivatives and bispinacol borate of diphenyl ether using a classical Suzuki coupling reaction. Target tetradentate Pt(II) complexes Pt-DPM, Pt-PIP and Pt-DPT were synthesized with moderate yields (20–35%) through a direct reaction of K₂PtCl₄ and corresponding ligands under certain conditions. The structures of all intermediates and complexes have been characterized by ¹H/¹³C NMR and MALDI-TOF-MS (Fig. S9–Fig. S34†).

Scheme 1 Synthetic routes of the ligands and Pt(II) complexes. (i) DMAD; (ii) NaOH, MeOH/H₂O, HCl; (iii) (CH₃CO)₂O, CH₃COONa; (iv) N₂H₄H₂O, CH₃COOH; (v) POCl₃; (vi) K₃PO₄/CuI/DMSO/2-picolinic acid; (vii) PdCl₂(dppf)/KOAc/DMF; (viii) PdCl₂(dppf)/K₂CO₃/THF/H₂O; (ix) obp, PdCl₂(dppf)/K₂CO₃/THF/H₂O; and (x) K₂PtCl₄/CH₃COOH.

3.2 Thermal properties

In order to determine the thermal stability of these Pt(II) complexes, thermogravimetry analyses (TGA) were carried out under a N₂ atmosphere, as shown in Fig. 2. The 5% weight loss temperatures (T_d) are listed in Table 1. The thermal decomposition temperatures of these complexes all exceed 400 °C. From Fig. 2 and Table 1, we can observe that the T_d values of Pt-DPM, Pt-PIP and Pt-DPT are 407, 462 and 454 °C, respectively. The T_d values of Pt-DPM and Pt-DPT are lower than that of Pt-PIP, which is because the ether bond of aromatic ether units in complexes Pt-DPM and Pt-DPT easily breaks and decomposes when heated, while aromatic amine compounds formed after the substitution of piperidine are relatively more stable. The isopropyl group has higher steric hindrance than the methyl group, which makes Pt-DPT have better thermal stability than Pt-DPM. These aza-triptycene-based complexes were found to exhibit excellent thermal stability. These results indicated that these complexes are beneficial for the extension of the working life of OLEDs in practical applications.

Fig. 2 Thermogravimetric spectra of Pt(II) complexes.

Table 1 Physical properties of these Pt(II) complexes

Complex	Td ^a (°C)	λabs, max ^b (nm)	λPL, max ^b (nm)	λPL, max ^b (nm)	η^c (%)	τ^d (μs)	EHOMO^e (eV)	ELUMO^f (eV)	E^optg (eV)	T1^g (eV)	kr^h (×10^5 s^−1)	knr^h (×10^5 s^−1)
a Decomposition temperature of 5% weight loss.b Measured in DCM (10^−5 M) and in PMMA films at a conc. of 1 wt%.c PL quantum yields were measured in PMMA at a conc. of 1 wt%.d Lifetimes were recorded in PMMA at a conc. of 1 wt%.e Calculated from the empirical equation: EHOMO = −(EOX + 4.8 − Eox (Fc/Fc^+)) eV.f LUMO levels were calculated from the HOMO and E^optg. E^optg was estimated from the absorption edge.g Estimated from the highest energy peak of the phosphorescence spectra at 77 k.h (kr + knr) = 1/τ (μs), where τ is the emission lifetime. The radiative decay rate constant kr is calculated using η (%) = kr/(kr + knr).
Pt-DPM	407	263 344 412	584	585	92	3.82	−5.50	−3.15	2.35	2.10	2.41	0.21
Pt-PIP	462	270 334 405	593	588	88	3.43	−5.45	−3.14	2.31	2.11	2.57	0.35
Pt-DPT	454	285 340 410	580	580	96	3.37	−5.53	−3.17	2.36	2.12	2.85	0.12

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3.3. Photophysical properties

The UV-vis absorption spectra and photoluminescence (PL) spectra of Pt-DPM, Pt-PIP and Pt-DPT in dichloromethane solutions at room temperature are shown in Fig. 3. Two typical absorption regions are observed for all three Pt(II) complexes. The high energy strong absorption bands below 300 nm are attributed to the spin allowed ¹π–π* transition on the cyclometalating ligands (LC).^40,41 The relatively weak absorption bands from 300 nm to 430 nm are assigned to metal-to-ligand charge-transfer (MLCT) transitions involving both the cyclometalating ligands and the platinum metal ions, as well as spin–orbit coupling enhanced ³π–π* intra-ligand charge transfer (ILCT).⁴² This part of the absorption is characteristic of phosphorescent materials, and the strong spin–orbital coupling between ligands and the central metal allows the originally forbidden transition, which is an advantage for phosphorescent emission.⁴³ The structures of the three complexes are similar, and only the substituents are different, so the difference in the absorption edge of the three complexes is small. The absorption edges of these complexes are at 530–543 nm. Pt-PIP has the maximum absorption edge (543 nm) because the piperidinyl group is a strongly electron-donating unit. The calculated dihedral angle between the cyclometalated benzene ring and cyclometalated pyridazine ring is only 8 degrees, and the two rings are almost coplanar and have a great degree of conjugation. The electron cloud density of the molecule is increased, and E^opt_g is 2.31 eV (Fig. 4(b) and Table 1). The maximum absorption edges of Pt-DPM and Pt-DPT are smaller than that of Pt-PIP, and the absorption edge of Pt-DPT (530 nm) is the smallest. This may be due to the existence of two large isopropyl hindrance groups at positions 2 and 6 of the phenoxy substituent group, which reduces the coplanarity of the cyclometalated benzene ring and cyclometalated pyridazine ring, and the dihedral angle between the two rings increases to 16 degrees. Accordingly, the conjugation degree of the complex is reduced, the delocalization of electrons is restricted, and E^opt_g is increased to 2.36 eV (Fig. 4(b) and Table 1), resulting in a blue shift of absorption. This result is also consistent with the spectrogram in Fig. 3.

Fig. 3 UV-vis absorption and PL spectra of Pt(II) complexes in CH₂Cl₂ solution.

Fig. 4 (a) The cyclic voltammograms, and (b) theoretical (blue) and experimental (red) HOMO/LUMO energy levels of the Pt(II) complexes.

The PL spectra of these complexes in dichloromethane solution showed a strong orange phosphorescent emission. The emission peaks of these complexes were at 584, 593 and 580 nm for Pt-DPM, Pt-PIP and Pt-DPT, respectively (Table 1). The PL emission peak of complex Pt-D in dichloromethane solution is located at 595 nm (Table S1†). Compared with complex Pt-D, the emission peak of Pt-DPM undergoes a blue shift. This is because the introduction of the aza-triptycene structure increases the steric hindrance of molecular rigidity, resulting in a decrease in the degree of conjugation. The energy gap is increased to 2.35 eV (the energy gap of Pt-D is 2.04 eV) (Fig. 4(b) and Table 1). The increased energy required for electron transition causes the emission peak to shift towards the blue region. In addition, the emission peak of the complex Pt-DPT was more blue-shifted than those of the other two complexes due to the introduction of an isopropyl group with a certain steric hindrance compared with the methyl group, which further improved the steric resistance density, reduced the degree of conjugation, and inhibited the aggregation of molecules. At the same time, because the electron donating ability of the introduced isopropyl group is stronger than that of the methyl group, the energy gap increases to 2.36 eV (Fig. 4(b) and Table 1), causing an increase in the energy required for electron transition, resulting in a blue shift of its spectrum.

Broadband emission spectra with FWHMs exceeding 60 nm are typically observed in traditional fluorescent, phosphorescent, and thermally activated delayed fluorescence (TADF) materials under both photoluminescence and electroluminescence conditions. This broadening is attributed to vibrational coupling between the singlet ground state (S₀) and singlet/triplet excited states (S₁ or T₁), as well as charge-transfer (CT) interactions.⁴⁴ In general, emitters with rigid molecular structures have a single principal vibration stretching, which makes the FWHM narrower.⁴⁵ It has been reported that the spectra of phosphorescent metal complexes can be narrowed by introducing large rigid steric hindrance groups into the phosphorescent metal complexes to restrict the free relaxation of the ligands and inhibit the intermolecular interactions.^46–48 The FWHM of conventional metal Pt(II) complex phosphors is typically in the range of 40–100 nm. For example, Li et al. reported that the emission spectrum of the Pt(II) complex was significantly broadened with an FWHM of 85 nm at room temperature.⁴⁹ However, the FWHM can be significantly compressed by these novel ligand designs. The FWHM of the Pt(II) complex Pt-D is 43 nm (Fig. S3†). Due to the introduction of an aza-triptycene unit with a rigid Y-shaped 3D molecular structure, the FWHM values of Pt-DPM, Pt-PIP and Pt-DPT are 38 nm, 49 nm and 43 nm in DCM solution, respectively (Fig. S1†). Interestingly, the FWHM values of Pt-DPM, Pt-PIP and Pt-DPT are only 38 nm, 45 nm and 42 nm in PMMA films at a concentration of 1 wt%. Narrow FWHM values are a prerequisite for realizing high color purity OLEDs.⁵⁰ The CIE coordinates of Pt-DPM, Pt-PIP, and Pt-DPT were measured as (0.58, 0.41), (0.59, 0.39), and (0.57, 0.42), respectively, which are close to the ideal monochromatic points, indicating high color purity. These results demonstrate that these complexes are classified as narrow-band emitters. These results show that it is possible to obtain high efficiency and high color purity OLEDs with precisely controlled colors by introducing a rigid non-conjugated functional unit.

The phosphorescence spectra of these complexes at 77 K and room temperature were recorded on a PMMA film at a concentration of 1 wt% (Fig. S2†). At room temperature, the emission peaks of Pt-DPM, Pt-PIP and Pt-DPT are 585 nm, 588 nm and 580 nm, respectively. At 77 K, the emission peaks of Pt-DPM, Pt-PIP and Pt-DPT are 588 nm, 587 nm and 585 nm, respectively (Table S1†). The photoluminescence quantum yields (PLQYs) of these complexes doped in PMMA at a concentration of 1 wt% were determined. The PLQYs of Pt-DPM, Pt-PIP and Pt-DPT are 92%, 88% and 96%, respectively. Compared with the Pt(II) complex Pt-D with a tetradentate pyrazine ligand, whose PLQY is 73%, the luminescence efficiency is significantly improved without modifying the rigid aza-trienyl unit. This is mainly because of the introduction of an aza-triptycene unit with a high degree of structural rigidity and a minimally distorted molecular geometry that inhibits the free vibration of the molecule and reduces the non-radiative transitions. These factors lead to a significant improvement in luminescence efficiency. The triplet energy (T₁) levels were calculated according to the 77 K spectra (Table 1). These complexes show T₁ energy in the range of 2.10–2.12 eV, which follows the order Pt-DPM < Pt-PIP < Pt-DPT. Moreover, the transient PL decay curves (Fig. S4†) of these complexes display single exponential decay with fitting lifetimes of 3.82, 3.43 and 3.37 μs for Pt-DPM, Pt-PIP and Pt-DPT, respectively, and it was obvious that the emission of these complexes was phosphorescent in nature.

3.4 Electrochemical properties and theoretical calculations

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of these tetradentate Pt(II) complexes are of great significance to the design of OLED structures. Cyclic voltammetry with ferrocene as the internal standard was used to study the electrochemical properties of the three Pt(II) complexes and to calculate the HOMO and LUMO energy levels of the three Pt(II) complexes. According to the following formula: E_HOMO = −(E_ox + 4.8 − E_ox (Fc/Fc⁺)) eV, the HOMO energy level is obtained from the oxidation peak potential (E_ox). The energy gap E_g (eV) = hc/λ_abs = 1240/λ_abs value is estimated from the UV-vis absorption edge, and the LUMO energy level is determined according to the E_LUMO = E_HOMO + E_g equation. During the anodic oxidation process, the oxidation peaks of these complexes can be observed, and the E_ox values are 0.70 V, 0.65 V, and 0.73 V for Pt-DPM, Pt-PIP and Pt-DPT, respectively, which is attributed to the Pt(III)/Pt(IV) oxide pair in the metal center, in line with the reported cyclometalated Pt(II) complexes. It can be found that the oxidation potential of Pt-PIP with the strong electron withdrawing group piperidine unit on the pyridazine ring is lower than those of Pt-DPM and Pt-DPT. Due to the strong electron-donating ability of the piperidinyl group, electrons are more easily lost, making the compound more easily oxidized and resulting in a lower oxidation potential in the electrochemical oxidation reaction. The HOMO of the Pt(II) complex was located on the cyclometalated benzene ring and bridged oxygen atom. Compared with the Pt(II) complex Pt-D, due to the introduction of the aza-triptycene structural unit with rigid steric hindrance into Pt-DPM, the HOMO energy level decreases, resulting in a significant blue shift of the emission spectrum (Fig. 5 and Fig. S3†). Compared with the complexes Pt-DPM and Pt-DPT, due to the introduction of the strong electron donating properties of the piperidine unit and the substituted phenoxy units onto the cyclometalated pyridazine ring, the molecular electron cloud density increased and the HOMO energy level increased, resulting in a red shift of the spectra with Pt-PIP. Compared with the substituted phenoxy units, the piperidine group has a stronger electron donating ability, resulting in a more significant redshift in the emission spectrum of Pt-PIP. This result is consistent with the emission spectra of these complexes (Table 1). Moreover, according to the above formula, the HOMO and LUMO energy levels can be calculated. The HOMO energy levels are −5.50 eV, −5.45 eV and −5.53 eV, respectively (as shown in Table 1). It can be determined that the LUMO energy levels of Pt-DPM, Pt-PIP and Pt-DPT are −3.15 eV, −3.14 eV and −3.17 eV, respectively (as shown in Fig. 4). Due to the strong electron donating effect of piperidine, among the three Pt(II) complexes, Pt-PIP exhibits the highest LUMO energy level and the phenomenon of red shift is also the most obvious. In addition, the experimental value of the HOMO/LUMO energy level inferred from CV data is slightly lower than the theoretical value calculated using DFT, but the calculated trends of E_g, E_HOMO and E_LUMO are in good agreement with the experimental value, and the HOMO and LUMO levels have similar changing trends (as shown in Table 1 and Fig. 4(b)).

Fig. 5 Molecular orbital diagrams and the orbital composition analysis of the Pt(II) complexes.

In order to further study the photophysical and electrochemical properties of Pt(II) complexes, density functional theory (DFT) calculations were carried out to study the electron cloud state and orbital distribution of these Pt(II) complexes. As shown in Fig. 5, the HOMO was primarily localized at the Pt atom center (50.46%–53.38%) and the cyclometalated benzene rings (37.39%–38.46%), while the pyridazine ring of cyclometalated ligands (9.23%–11.46%) made a small contribution. On the other hand, the LUMO was located almost entirely over the cyclometalated pyridazine rings (77.76%–92.04%), along with minor contributions from the cyclometalated benzene rings (6.7%–21.6%) and the Pt(II) center (0.64%–1.28%). This shows that the introduction of electron-donating groups has a great influence on its HOMO energy level, and the distribution of HOMO energy levels on the cyclometalated benzene rings decreases, which will increase the energy gap. This result is consistent with the changing trend of emission spectra of the three complexes. Moreover, the electron clouds on Pt atoms of these aza-triptycene Pt(II) complexes with rigid steric hindrance are higher than those of other analogues by about 10%–30%.^22,23 The widespread distribution of electron clouds at the Pt atom center is critical for achieving high luminescence efficiency by efficient MLCT in the phosphorescent Pt(II) complexes, which enables the phosphorescent complexes to achieve high luminescence efficiency by efficient metal–ligand charge transfer (MLCT).

3.5 Electrophosphorescent properties

In order to evaluate the EL performance of these Pt(II) complexes, single emission layer (EML) devices with Pt-DPM, Pt-PIP and Pt-DPT as emitters were prepared with the structure of ITO/PEDOT : PSS (30 nm)/TCTA : T APC : PVK : OXD-7 : Pt(II) complexes x wt% (50 nm, x = 0.5–30)/TPBi (35 nm)/Ca (15 nm)/Ag (100 nm). The weight ratio of TCTA : TAPC : PVK : OXD-7 = 18 : 18 : 5 : 9. The energy level diagram and the molecular structure of the materials used in the device are shown in Scheme 2. In this device structure, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) with high conductivity is used as the hole transport layer (HTL), and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) is introduced as the electron transport layer (ETL) and hole blocking layer (HBL), which can effectively improve the balance of carriers and improve luminous efficiency. Tris(4-(9H-carbazol-9-yl)phenyl) amine (TCTA), 4,4′-(cyclohexane-1,1-diyl)bis(N-phenyl-N-(p-tolyl)aniline) (TAPC), poly(N-vinylcarbazole) (PVK) and 1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl) benzene (OXD-7) are used as the mixed host materials of Pt(II) complexes due to their suitable HOMO and LUMO energy levels. As shown in Scheme 2, the HOMO and LUMO energy levels of the three dopants are embedded between the HOMO and LUMO energy levels of the host material, indicating that carriers can be injected into the host material or directly captured by the Pt(II) emitter. The energy transfer between them is effective in the emission layer (EML). Pt(II) complex devices with different doping concentrations were prepared, and the detailed results are shown in the figures and tables in the manuscript and the ESI.†

Scheme 2 The device configurations and chemical structures of the materials used.

As can be seen in Fig. S5(a), Fig. S6(a) and Fig. S7(a),† at low doping concentrations, a weak emission at approximately 450 nm from the TAPC/PVK host was detected. When the doping concentration is not less than 1 wt%, the luminescence of the host materials disappears, indicating complete energy transfer between the host and guests. Interestingly, the EL spectra of these complexes are very stable within the concentration range of 5 wt%–30 wt%, with the values of Δx or Δy in Commission Internationale de l'Eclairage (CIEx, y) color coordinates not greater than 0.01 (Fig. S5(e), Fig. S6(e) and Fig. S7(e)).† The devices based on Pt-DPM, Pt-PIP and Pt-DPT had excellent spectral stability due to the introduction of non-conjugated rigid steric hindrance groups. This result can be attributed to the introduction of the aza-triptycene unit, which suppresses molecular packing and reduces intermolecular interactions.

To directly observe device performance, the devices of the three complexes Pt-DPM, Pt-PIP, and Pt-DPT are compared at their optimal doping concentrations of 1 wt%, 10 wt% and 5 wt% (named G1, G2 and G3), respectively. In Fig. 6, the EL spectra and the brightness–voltage–current density, current efficiency–brightness, and external quantum efficiency–brightness curves of G1, G2, and G3 are presented. The key performance parameters of the device are shown in Table 2. As shown in Fig. 6(a), the EL spectra of the three platinum complexes perfectly match the phosphorescence emission in dichloromethane solution at room temperature, indicating that the EL emission of the device originates from the Pt(II) emitter. Compared with the reference complex Pt-D (the device based on Pt-D shows a peak external quantum efficiency, current efficiency, and brightness of 4.35%, 1.97 cd A⁻¹, and 16 926 cd m⁻², respectively, in Fig. S8.†), these devices all exhibit better performance. For device G1, the L_max is 26 670 cd m⁻², the maximum current efficiency (η_{c, max}) is 23.67cd A⁻¹, and the EQE_max is 9.67%. For device G2, the L_max, η_{c, max} and EQE_max are 29 232 cd m⁻², 19.45 cd A⁻¹, and 9.93%, respectively. These results indicate the significant effect of the aza-triptycene group in improving device performance, and the underlying reason for the above phenomenon is that the introduction of an aza-triptycene unit with rigid steric hindrance can reduce intermolecular aggregation and effectively suppress luminescence quenching caused by an increased concentration. Impressively, device G3 achieved the best performance with maximum efficiencies of 44.66 cd A⁻¹, 26.97 lm W⁻¹ and 16.94%, respectively. The efficiency is almost four times that of the complex Pt-D and two times that of the complex G1, suggesting that isopropyl further inhibited the aggregation of molecules and significantly enhanced the luminous efficiency. As evidenced by Fig. 6(d) and Table 2, a significantly mitigated efficiency roll-off trend was observed in the EQE vs. luminance curves of G2 and G3 compared to G1. At a high luminance of 1000 cd m⁻², negligible degradation in current efficiencies and EQEs was observed relative to their peak values, with EQE maintained rates of 78.5%, 96.4%, and 94.6% for G1, G2, and G3, respectively. G2 and G3 have lower efficiency roll-off rates compared to G1, and the reason for the above phenomenon is that the introduction of an aza-triptycene unit with rigid steric hindrance can reduce intermolecular aggregation and effectively reduce the efficiency roll-off. Meanwhile, the enhancement of substituent rigidity and the increase in substituent steric hindrance have a more significant impact on suppressing the efficiency roll-off. Due to the introduction of an aza-triptycene unit with rigid steric hindrance, the device has good properties even at a high doping concentration. As can be seen from Table S2,† the devices based on Pt-DPM, Pt-PIP and Pt-DPT exhibit high-brightness values of 9639, 21 756 and 43 708 cd m⁻², maximum luminous efficiencies of 9.05, 12.57 and 28.92 cd A⁻¹ and high EQEs of 3.70, 6.42 and 10.97% at a high doping concentration of 30 wt%. Moreover, it can be seen from the performance parameters of the devices at different concentrations in Fig. S5–S7 and Table S2† that the devices based on Pt-DPM, Pt-PIP and Pt-DPT all show low roll-off. When the doping concentration ranges from 10 wt% to 30 wt%, the current efficiency of the device based on Pt-DPM drops from 16.52 cd A⁻¹ to 9.05 cd A⁻¹, the current efficiency based on Pt-PIP decreases from 19.45 to 12.57 cd A⁻¹, and the current efficiency based on Pt-DPT only decreases from 36.35 to 28.92 cd A⁻¹. The efficiency roll-off rates of Pt-DPM, Pt-PIP and Pt-DPT are 45.2%, 35.3% and 20.4% from 10 wt% to 30 wt%, respectively. The minimum efficiency roll-off of Pt-DPT emphasizes the effectiveness of rigid steric hindrance in suppressing aggregation and exciton quenching. The insensitivity of efficiency to changes in doping concentration implies that increasing steric hindrance can decrease intermolecular interactions and enhance the stability of the devices.

Fig. 6 Characteristics of the devices G1, G2 and G3. (a) The electroluminescence spectra at 8 V, (b) the luminance–voltage–current density (L–V–J) curves, (c) the current efficiency–luminance (η_c–L) curves and (d) the external quantum efficiency–luminance (η_E–L).

Table 2 Summary of device luminescence and efficiency data

Device (concentration)	λEL, max^a (nm)	Von^b (V)	Lmax^c (cd m^−2)	ηc, max^d (cd A^−1) (ηc, L1000^f)	EQEmax^e (EQEL1000^g)	CIE^h (x, y)
a EL at 8 V.b Turn-on voltage recorded at a luminance of 1 cd m^−2.c Maximum luminance.d Maximum current efficiency.e Maximum external quantum efficiency (EQE).f Current efficiency at 1000 cd m^−2.g EQE at 1000 cd m^−2.h Commission Internationale de l'Eclairage (CIE).
G1 (1%)	584	3.67	26 670	23.67 (18.60)	9.67 (7.59)	(0.58,0.41)
G2 (10%)	592	3.98	29 232	19.45 (19.45)	9.93 (9.57)	(0.59,0.39)
G3 (5%)	584	3.67	54 918	44.66 (42.26)	16.94 (16.03)	(0.57,0.42)

---

Given its excellent performance with a high doping concentration (30 wt%), the performance of non-doped devices based on Pt-DPT was further evaluated. The Pt-DPT device exhibited excellent performance even under non-doped conditions (Table S2†). Its non-doped device exhibited a high brightness of 10 873 cd m⁻², a current efficiency of 9.56 cd A⁻¹, a power efficiency of 6.48 lm W⁻¹ and an external quantum efficiency of 3.63%. The isopropyl structure effectively inhibits molecular aggregation, enhancing luminescence efficiency. This result clearly demonstrates the critical role of steric hindrance groups in enhancing OLED device performance. Their promising potential for practical OLED applications is highlighted by the excellent performance of these complexes.

4. Conclusions

In summary, three novel tetradentate Pt(II) complexes with rigid steric hindrance based on a triptycene pyridazine derivative were successfully developed. These complexes exhibit a very strong orange phosphorescence emission with high quantum yields of 0.88–0.96 in PMMA films. Most importantly, these complexes have narrow FWHM values of 38–45 nm in thin films. All these complexes also exhibit good thermal stability with T_d > 407 °C. Solution-processed organic light-emitting diodes using these new Pt(II) complexes as emitters show excellent performance. Compared with Pt-D, the efficiencies are dramatically improved to 9.67%, 23.67 cd A⁻¹, and 26 670 cd m⁻² for the OLED based on the sterically hindered aza-triptycene tetradentate Pt(II) complex Pt-DPM with methyl substitution, and to 16.94%, 44.66 cd A⁻¹, and 54 918 cd m⁻² for the OLED based on the sterically hindered aza-triptycene tetradentate Pt(II) complex Pt-DPT with isopropyl group substitution, respectively. At a high doping concentration of 30 wt%, the EL device based on the complex Pt-DPT exhibits a maximum luminous efficiency of 28.92 cd A⁻¹ with a high EQE of 10.97%. The non-doped device based on the complex Pt-DPT presents a luminous efficiency of 9.56 cd A⁻¹ and a high brightness of 10 873 cd m⁻². More importantly, the CIE coordinates of the devices based on the complex Pt-DPT are almost identical with different doping concentrations. The results show that the introduction of the rigid non-conjugated triptycene skeleton can effectively inhibit the red shift caused by aggregation at a high concentration, reduce self-quenching and dramatically improve the luminous efficiency of Pt(II) complexes.

Data availability

All relevant data are within the manuscript and its ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51774005, 21572001, 61675088, 61675089, and 62075103).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi01049g

‡ These authors contributed equally to this work.

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