A thermally activated delayed fluorescent platinum(II) complex for red organic light emitting diodes with high efficiencies and small roll-off

Abstract

Although phosphorescent Pt(II) complexes have shown great potential for organic light-emitting diodes (OLEDs), red phosphorescence with a high photoluminescence quantum yield (PLQY) and a short emission lifetime is still scarce because of the limitation on the radiative decay rate. Transition metal complexes have proved advantageous for emitting thermally activated delayed fluorescence (TADF), but TADF Pt(II) complexes have rarely been reported. Herein, we describe the design and synthesis of two red TADF Pt(II) complexes, namely Pt-tCzDBPZ (Pt1) and Pt2-tCzDBPZ (Pt2). By using a donor–acceptor type monodentate ligand, the complexes have metal-perturbed intraligand charge-transfer (MPICT) excited states featuring small singlet–triplet energy gaps (0.086–0.089 eV) and a moderately large spin–orbit coupling matrix element (∼10 cm⁻¹). The complexes exhibit red TADF emissions (ca. 630 nm) with high PLQYs of 82–92% and delayed fluorescence lifetimes of 1.51–1.63 μs. The vacuum-deposited red OLEDs based on Pt-tCzDBPZ show high external quantum efficiencies of up to 35.6% and small roll-offs. In spite of the significant heavy atom effect of Pt for facilitating phosphorescence, this study demonstrates a feasible design of Pt(II)–TADF emitters which can enable them to outperform phosphorescent Pt(II) complexes.

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Kai Li

Dr Kai Li received his BSc degree in chemistry from the University of Science and Technology of China in 2008. Then, he obtained his PhD degree at the University of Hong Kong in 2013 under the supervision of Prof. Chi-Ming Che. After a postdoctoral study in the University of Hong Kong from 2013 to 2018, he joined Shenzhen University as an assistant professor and was promoted to associate professor in 2022 and then to professor in 2024. His research interest is in the design, synthesis and applications of luminescent molecular materials with a particular focus on transition metal complexes.

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Introduction

Phosphorescent Pt(II) complexes have shown great potential for a wide range of applications including photocatalysis, bioimaging and photodynamic therapy, and optoelectronic devices.^1–4 The Pt atom has a large spin–orbit coupling (SOC) constant of 4481 cm⁻¹. Provided a substantial involvement of the Pt d orbital in the excited state process, the Pt(II) complexes have ultrafast intersystem crossing (ISC) of S₁ → T₁ and strongly allowed radiative decay of T₁ → S₀. In the context of organic light-emitting diodes (OLEDs), the highly phosphorescent Pt(II) complexes have been the most promising emitters as surrogates to Ir(III) complexes. For example, blue and green emitting Pt(II) complexes with photoluminescence quantum yields (PLQYs) close to 100% have been developed. When used as emitting dopants for OLEDs, these complexes have demonstrated high external quantum efficiencies (EQEs) over 20%. However, it is still challenging to achieve high-efficiency red phosphorescent Pt(II) complexes for OLEDs. On one hand, this is because of the inherent limitation of the energy gap law, which imposes a fast non-radiative decay (k_nr) via substantial electronic–vibrational coupling. There are only a few highly red phosphorescent platinum(II) complexes in which the excited state structural changes are suppressed by using rigid multidentate cyclometallating ligands.^1,2,5–7 In addition to the energy gap law, the PLQYs of red phosphorescent platinum(II) complexes are also limited by their slow radiative decay rates (k_r). The k_r values of the T₁ states of Pt(II) are usually on the order of 10⁴ s⁻¹, rendering them uncompetitive with the k_nr in the low emission energy region. As mentioned earlier, a large and rigid π-conjugated ligand is essential for attaining a redshifted emission and a reduced non-radiative decay. However, this ligand design may decrease the metal participation in the photophysical process, thereby resulting in a small k_r. In contrast, Ir(III) complexes have distinct electronic structures which have near degeneracy of the metal d orbitals and permit efficient mixing of the singlet and triplet excited states.^8,9

In recent years, it has been proved feasible to develop aggregates of Pt(II) complexes or dinuclear Pt(II) complexes as efficient red triplet emitters. As depicted in Scheme 1A, the presence of intermolecular or intramolecular Pt–Pt interactions in these systems generates low-energy triplet excited states with a metal–metal-to-ligand charge-transfer (³MMLCT) character.^10–18 With fast radiative decay rates, the ³MMLCT emissions have increased efficiencies and shorter lifetimes in comparison with the mononuclear Pt(II) emitters. The photophysical properties of the Pt(II) aggregates critically depend on the molecular packing, which is difficult to design and control. The dinuclear Pt(II) complexes have relatively large molecular weights, rendering the sublimation and vacuum deposition challenging.

Scheme 1 The design concept and molecular structures of (A) phosphorescent mono-/dinuclear Pt(II) emitters with metal–metal-to-ligand charge-transfer (MMLCT) excited states; (B) TADF dinuclear Pt(II) emitters; (C) TADF mono-/dinuclear Pt(II) emitters with metal-perturbed intraligand charge-transfer (MPICT) excited states with key optical properties and device performances.

In the past 15 years, the endeavors for developing OLED emitters have been mainly devoted to those exhibiting thermally activated delayed fluorescence (TADF).^19–23 TADF emitters are capable of harvesting the triplet excitons by converting them to singlet excitons via reverse intersystem crossing (RISC). Therefore, TADF OLEDs can deliver comparable performances as phosphorescent devices in term of a 100% internal quantum efficiency. To achieve efficient TADF emission, it is desirable to simultaneously have a sufficiently small energy gap (ΔE_ST) between the S₁ and T₁ states and a strong spin–orbit coupling, both of which benefit a fast RISC process. In this respect, the past few years have also witnessed a blossoming of metal TADF emitters for OLEDs owing to the large SOC constants of metal atoms. Indeed, a plethora of d¹⁰ coinage metal complexes have been reported to have efficient TADF.^24–32 For these complexes, ligand-to-ligand charge-transfer (LLCT) excited states can be facilely constructed by using appropriate ligands and the metal center plays a bridging role.^25,33–35 Moreover, the metal complexes can make use of the heavy atom effect of the metal to facilitate the RISC processes, giving rise to short delayed fluorescence lifetimes. Many of the TADF d¹⁰ coinage metal complexes have proved to be good emitters for OLEDs featuring high EQEs and very low efficiency roll-off. These achievements pave a way for the metals that have not achieved good phosphorescence performances. However, it has been perceived to be challenging to achieve TADF from Ir(III) and Pt(II) complexes which have been demonstrated to be capable of emitting efficient phosphorescence. This perception is probably based on the fact that the TADF and phosphorescence are competing processes for depleting the T₁ excited state.^36–41 However, given that the radiative decay rates of T₁ states of Pt(II) complexes hardly exceed 10⁴ s⁻¹, there is no doubt that TADF emission can be switched on for Pt(II) complexes. Recently, Pander et al. evidenced a TADF Pt(II) complex that shows both a high PLQY of 92% and a short lifetime of 2.3 μs (Scheme 1B).^36,37 They also observed TADF emission from other dinuclear Pt(II) complexes. Obviously, a pronounced charge-transfer character of their lowest-lying excited states is a key to realize TADF over phosphorescence. However, the tuning of the excited state for Pt(II) complexes supported by multidentate π-conjugated ligands is not easy, which limits the design and screening of TADF Pt(II) complexes.

In this work, we developed two Pt(II) complexes, namely Pt-tCzDBPZ (Pt1) and Pt₂-tCzDBPZ (Pt2), by using the metal-perturbed intraligand charge-transfer (MPICT) concept (Scheme 1C). Complexes Pt1 and Pt2 exhibit red TADF with PLQYs of 92% and 82% and lifetimes of 1.51 and 1.63 μs in doped films. The Pt1-based red OLED achieved a maximum EQE of 35.6%, which was maintained at 34.3% at 1000 cd m⁻². This work validates a feasible design of TADF metal complexes using metals having significant SOC constants.

Results and discussion

Synthesis and characterization

The structures of Pt1 and Pt2 are shown in Scheme 1C. The key design of the complexes is the use of a donor–acceptor (D–A) ligand in which the acceptor moiety is non-coordinating. For the ligand design toward red emission, the strong electron-accepting dibenzo[a,c]phenazine (DBPZ) was attached to the α-position of 3,6-di-tert-butylcarbazole (tCz). This D–A molecule in its N-deprotonated form acts as a monodentate amide ligand. Different from the most commonly reported Pt(II) complexes in which the Pt atom is embedded in the chromophore ligand,^{1–7,10–18,36,37} the Pt atom in this design only has a perturbing role by binding to carbazole. This coordination also significantly lifts the energy level of the highest occupied molecular orbital (HOMO) that is localized on the carbazolate moiety. The criteria for the choice of the pincer ligand include high rigidity and strong σ-donating capability. Further, a high-lying π* orbital is essential for the pincer ligand to avoid its participation in the lowest excited state, giving rise to an MPICT excited state. Both [(N^C^N)Pt]⁺ (N^CH^N = 1,3-di(pyridin-2-yl)benzene) and DBPZ have a planar geometry to form intramolecular π⋯π stacking interactions. It has been manifested that such a scaffold is highly beneficial for suppressing excited state structural changes, thus leading to high PLQYs.^41–43 The steric effect-induced torsion of DBPZ with respect to the carbazolate plane furnishes a small ΔE_ST. The synthesis of D–A precursors has been reported.⁴² The Pt(II) complexes were synthesized with yields of 70% and 72% by following a previous method.^31,41 The structures of both complexes were ascertained by various spectroscopic methods (multinuclear NMR, high resolution mass spectrometry) and elemental analysis (see details in the ESI†).

Single crystal X-ray diffraction analyses were also conducted on Pt1 and Pt2. The crystallographic data and key structural metrics are listed in Tables S1 and S2.† The bond distances of Pt–C (1.897–1.923 Å) and Pt–N_carbazole (2.120–2.154 Å) in Pt1 and Pt2 are comparable to those of [(N^C^N)PtCl] and [(bzimb)Pt(carbazolyl)] (bzimb = 2,6-bis(N-alkylbenzimidazol-2′-yl)benzene) in the literature (Fig. S1†).^44,45 As shown in Fig. 1A, the [(N^C^N)Pt]⁺ and DBPZ moieties in Pt1 have torsion angles of 66.6° and 66.3° with respect to the carbazolate plane. For Pt2, the torsion angles of the DBPZ with respect the two carbazolate bridges are 71.8° and 68.8°. Therefore, a good separation of the HOMO and the lowest unoccupied molecular orbital (LUMO) results for both complexes (vide infra). Note that there are strong intramolecular metal⋯π interactions (ca. 3.2 Å) and π⋯π interactions (3.1–3.6 Å) between [(N^C^N)Pt]⁺ and DBPZ in both Pt1 and Pt2. The noncovalent interactions are also revealed by the Hirshfeld partition (IGMH) analysis, as shown by the green regions between [(N^C^N)Pt]⁺ and DBPZ planes in both Pt1 and Pt2 (Fig. 1B).^46,47 On one hand, these attractive interactions are conceived to enhance the rigidity of the Pt(II) complexes by restricting the intramolecular motions. On the other hand, the close and coplanar alignment of the DBPZ plane can protect the metal center, as seen by the space-filling diagrams (Fig. S2†). The buried volumes (% V_bur, defined as the fraction of the ligand volume over the total volume of the sphere around the metal) were determined to be 83.6% and 92.7% for Pt1 and Pt2, respectively.^31,48 Therefore, the noncovalent interactions are conceived to have cooperative effects on improving the molecular stability by restricting the metal–ligand dissociation and shielding the metal center.

Fig. 1 (A) Perspective views of crystal structures of Pt1 and Pt2 with the dihedral angles between the planes of the corresponding rings. The short contacts of Pt⋯π and π⋯π in the single-crystal structures are measured. (B) Independent gradient model based on Hirshfeld partition (IGMH) analysis of the optimized structures of Pt1 and Pt2 showing the presence of multiple attractive noncovalent interactions.

Photophysical properties

The UV-Vis absorption spectra of the two free ligands and Pt(II) complexes were measured in toluene solutions at room temperature. Table 1 lists the numerical data for the Pt(II) metal complexes. As depicted in Fig. 2A, the intense absorptions peaking at ca. 280 and 300 nm observed for the ligands could be mainly attributed to π–π* transitions of the carbazolate and the acceptor moieties. In the visible region, relatively weaker absorption bands peaking at 405 and 420 nm are observed, which could be mainly assigned to the intraligand charge-transfer (ILCT) transitions from the carbazolate moiety to the lateral acceptor (DBPZ). Upon coordination with a metal ion, the ILCT bands show a large bathochromic shift to 525 nm, which is ascribed to the Pt–N bonding interaction that increases the HOMO energy level.⁴¹ It is worth noting that the absorption spectra of Pt1 and Pt2 bear close resemblance, especially in the 450–650 nm region. The fact that they have the same transition from the carbazole donor and DBPZ acceptor can explain the similarity. Like many reported complexes, Pt1 and Pt2 also show negative solvatochromic properties in their absorptions (Figure S3†). Upon photo-excitation at 450 nm, Pt1 and Pt2 show broad and featureless emission bands with maxima at 645 nm in degassed toluene at room temperature (Fig. 2B). Compared with the ligands, the emission maxima of Pt1 and Pt2 are also redshifted by 170 and 175 nm, respectively. The phosphorescent emissions of the ligands were also measured in frozen toluene (77 K) with a time gate of 25 ms. It is clear that the ligands have vibronically structured bands localized on the DBPZ moiety, resulting in large ΔE_ST values. Differently, the emission spectra of Pt1 and Pt2 are as broad as those at room temperature.

Table 1 Photophysical data of the Pt(II) complexes in different media

Complex	λ abs (nm) (ε/10^3 M^−1 cm^−1)	λ em.RT (nm)	λ em.RT (nm)	τ RT (μs)	Φ RT (%)	k r (10^5 s^−1)	k nr (10^5 s^−1)	λ max.77K (nm)	τ 77K (μs)
a Measured in dilute toluene (10^−5 M) at 298 K. b Measured in degassed toluene at 298 K (concentration = 10^−4 M; λex = 450 nm for Pt1 and Pt2). c Measured in 5 wt% PMMA at 298 K and 77 K (λex = 410 nm for Pt1 and Pt2). d The average emission lifetime of 5 wt% PMMA film. e The PLQY of 5 wt% PMMA film. f Calculated by kr = Φ/τ and knr = (1 − Φ)/τ for the 5 wt% PMMA film.
Pt1	315 (31), 387 (26), 402 (26), 525 (2.8)	645	632	1.51 (0.99 μs, 75%; 3.07 μs, 25%)	92	6.09	0.53	627	87.7 (6.59 μs, 16%; 43 μs, 46%; 176 μs, 38%)
Pt2	316 (29), 398 (21), 525 (2.6)	645	630	1.63 (0.90 μs, 69%; 3.26 μs, 31%)	82	5.03	1.10	623	70.6 (4.91 μs, 24%; 33.7 μs, 42%; 163 μs, 34%)

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Fig. 2 (A) Absorption spectra of the free donor–acceptor ligands and Pt(II) complexes in toluene (10⁻⁵ M) at 298 K. (B) Normalized emission spectra of the free donor–acceptor ligands and Pt(II) complexes in degassed toluene at 298 and 77 K (concentration = 10⁻⁴ M; λ_ex = 375 nm for L1 and L2, and λ_ex = 450 nm for Pt1 and Pt2). (C) Normalized emission spectra of the free donor–acceptor ligands and Pt(II) complexes in 5 wt% PMMA films at 298 and 77 K (λ_ex = 375 nm for L1 and L2, and λ_ex = 410 nm for Pt1 and Pt2). (D) Variable temperature photoluminescence decay characteristics of Pt1 in 5 wt% PMMA film (constant emission wavelength = 632 nm). Inset: Arrhenius fit of the k_TADF value versus temperature calculated using eqn (1) (ESI†) for the 5 wt% doped film of Pt1. (E) Boltzmann-type fit to the emission lifetimes of Pt1 calculated using eqn (2) (ESI†) in 5 wt% PMMA film at various temperatures. (F) Fractions of TADF and phosphorescence as a function of temperature for Pt1.

The photoluminescence properties of the Pt(II) complexes were carefully investigated in doped poly(methyl methacrylate) (PMMA) films. In previous studies, we have determined the large ΔE_ST values (ca. 0.5 eV) for the D–A ligands and confirmed that they exhibit only prompt fluorescence.^41–43 As shown in Fig. 2C, Pt1 and Pt2 exhibit intense red emissions with peaks at ca. 630 nm at room temperature. Importantly, the PLQYs are as high as 82% and 92%, on par with the Pt(II) aggregates with ³MMLCT excited states.^10,15 The transient photoluminescence characteristics show a second-order decay, giving two lifetimes both in the microsecond regime (Table 1). The weight-averaged lifetimes of Pt1 and Pt2 are 1.51 and 1.63 μs, respectively. Thus, a fast ISC can be inferred for the complexes because of the lack of prompt fluorescence. The second-order decay behavior is likely due to the presence of conformational isomers.²⁷ The large radiative decay rates for Pt1 (6.1 × 10⁵ s⁻¹) and Pt2 (5.0 × 10⁵ s⁻¹) point to a TADF mechanism. Further, temperature-dependent (77 to 297 K) emission lifetimes were measured (Fig. 2D). Upon cooling to 77 K, there is a 40- to 60-fold increase in the emission lifetime. It is necessary to point out that phosphorescence lifetimes may also increase with decreasing temperature, but mainly due to suppressed non-radiative decay rather than a variation of radiative decay. However, the substantial changes in radiative decay rates for Pt1 and Pt2 indicate a change of emission mechanism from TADF at room temperature to phosphorescence at 77 K.^{15,26,32,36,37} The steady-state photoluminescence spectra at these temperatures were also measured (Fig. S4†). The emission spectra of Pt1 and Pt2 remain broad and featureless throughout the temperature range of 77–298 K, revealing the charge-transfer character for both S₁ and T₁ states. Note that slight blueshifts in the emission spectra were observed upon cooling. This could be attributed to the suppressed excited state geometrical relaxation and/or restriction of triplet diffusion at low temperatures in the rigid matrix.^27,28,34,35 Following the method for analyzing metal TADF emitters,³⁵ the radiative decay rate lifetimes can be well fitted by an Arrhenius plot (ln k_TADFvs. 1/T). The ΔE_ST values were estimated to be 96 and 117 meV, which in conjunction with strong SOC support efficient TADF (Fig. 2D and S5, and Tables S3 and S4, ESI†). Moreover, under the assumption of a fast thermalization between the T₁ and the S₁ states, the plot of emission lifetimes as a function of temperature for Pt1 and Pt2 could also be fitted using a Boltzmann type equation (Fig. 2E and S6†),⁴⁹ giving ΔE_ST values of 86 and 89 meV, respectively. As depicted in Fig. 2F and S7,† the emissions of these complexes at low temperature (77 K) originate exclusively from T₁.⁵⁰ The contribution of emission from the S₁ state (TADF) increases as the temperature increases. At room temperature (298 K), TADF accounts for 97% of the total emission for Pt1 and Pt2, respectively. Therefore, the TADF mechanism is unambiguously confirmed for the Pt(II) complexes. It is also believed that other TADF Pt(II) complexes can be designed by using this tactic.

Transient absorption spectroscopy

The excited state nature and kinetics (λ_ex = 400 nm) of Pt1 have been investigated by femtosecond and nanosecond transient absorption (fs/ns TA) spectra in toluene. As depicted in Fig. 3, the spectral evolution of Pt1 mainly occurs in three stages. Firstly, the broad positive TA bands around 440–500 nm and 550–650 nm attributed to the excited state absorption (ESA) grow rapidly within 0.7 ps. Based on the characteristic absorptions of the DBPZ radical anion,⁵¹ this process is tentatively assigned to the generation of a charge-transfer state (carbazolate → DBPZ). Then, in the second stage (1.77–574 ps), the ESA peaks at 475 and 585 nm are further raised slightly whereas a mild decrease of the ESA peak at 640 nm was observed (Fig. 3B). Accordingly, this evolution observed is assigned to S₁ population transfer to the T₁ manifold by ISC, which can be supported by the subsequent very slow decay in the third stage (Fig. 3C). Three lifetime components could be extracted from the fs-TA decay trace at 595 nm by a tri-exponential function fitting, giving rise to time constants of 0.34, 184, and 4380 ps, respectively (Fig. 3D). The τ values of 0.34 and 184 ps are assigned to the population in the ICT state (S₁) and the S₁ to T₁ ISC, respectively. The ultralong lifetime and triplet nature of the ESA peak at 440 nm are proved by the ns-TA spectra and oxygen quenching experiment under 400 nm laser pulse excitation (Fig. S8†). A kinetic study at the selected wavelength of these fs-TA spectra revealed that there was a time constant of 159 ns for the triplet state decay which should correlate with the processes of RISC (T₁ → S₁) and ISC (T₁ → S₀). According to the results of fs/ns-TA experiments, the intersystem crossing rate constant (k_ISC) can be estimated to be 5.4 × 10⁹ s⁻¹ for Pt1, indicating the role of metal atom in accelerating the spin-flip process. The k_ISC of Pt1 is comparable to those of the reported d¹⁰ TADF metal complexes with k_ISC values in the ∼10⁹–10¹¹ orders of magnitude.^32,52,53 Note that the ultrafast S₁ structural relaxation or solvent reorientation (in view of the weak solvation capability) in the timescale of sub-to-several ps was not observed in our measurements. Furthermore, based on the value of K_eq for the T₁ ⇌ S₁ equilibrium (eqn (S4)†), the k_RISC was estimated to be 6.4 × 10⁷ s⁻¹, which is much larger than those of typical organic TADF molecules (k_RISC = 10⁴–10⁶ s⁻¹).^54,55

Fig. 3 (A–C) Femtosecond transient-absorption (fs-TA) spectra of Pt1 in toluene at different time delays acquired after femtosecond laser excitation at 400 nm (concentration = 0.1 mM). (D) Kinetic traces at 595 nm are shown.

Theoretical studies

Density functional theory (DFT)/time-dependent density functional theory (TDDFT) calculations were carried out on Pt1, Pt2 and the ligands using the PBE0 functional⁵⁶ and def2-SVP basis set.⁵⁷ DFT was employed to optimize the ground state geometries and TDDFT was used for single point calculations of excited states. The methods have been used previously for studying MPICT emitters.⁴¹ The calculations on the sandwich ligand were also performed, as has been done for the double layer ligand. For each ligand, the electronic transition of the S₁ state is mainly related to the HOMO → LUMO, where the HOMO is mainly distributed on the donor units (tCz) with some significant contributions from DBPZ, and the LUMO is predominantly localized on the DBPZ. Thus, the S₁ state of the ligands exhibits a charge-transfer transition with an admixture of locally excited (LE) character (Fig. S9 and S10†). The HOMO/LUMO levels are −5.75/−2.43 and −5.71/−2.46 eV for the ligands. The ΔE_ST values are up to 0.63–0.66 eV, which explains their fluorescence properties. As depicted in Fig. 4, the optimized ground state geometries of Pt1 and Pt2 show that the torsions of DBPZ with respect to tCz are from 48° to 57.7°. The LUMOs of both complexes display very similar distributions over DBPZ. However, there are notable differences in their HOMOs compared with the free ligands. In addition to the principal contribution from the tCz unit, the HOMO consists of 5d orbitals of the Pt(II) center. Thus, the S₁ and T₁ states involving the transition of HOMO → LUMO exhibit evident ILCT and non-negligible metal-to-ligand CT (MLCT) character. The energy levels of HOMO/LUMO are −4.70/−2.39 and −4.69/−2.36 eV for Pt1 and Pt2, respectively. According to TDDFT calculations based on their optimized S₀ geometries, the energies of S₁ states of Pt1 and Pt2 are 1.85 and 1.86 eV, respectively, which are very close to the corresponding T₁ energies (1.80 eV). Such small ΔE_ST values of less than 0.06 eV for both compounds are due to the highly separated HOMO and LUMO distributions (Fig. S9 and S10†). Besides, the calculated SOC matrix element (SOCME) values between S₁ and T₁ states are 9.83 cm⁻¹ for Pt1 and 10.18 cm⁻¹ for Pt2, which are significantly larger than those for ligands. Therefore, both the large SOC and small ΔE_ST boost the spin-flip process for efficient TADF for Pt1 and Pt2. It can be seen that the N^C^N ligand is not directly involved in the frontier molecular orbitals for Pt1 and Pt2 and only the metal atom plays a perturbing role in the excited state process. This electronic structure echoes the design concept of the MPICT excited state towards TADF emitters. Therefore, the N^C^N ligands can be replaced by other ligands. Moreover, this allows for the design of metal-TADF emitters using various metal ions.

Fig. 4 The optimized ground-state geometries, frontier molecular orbitals and excited state energy levels of Pt1 and Pt2.

Thermal and electrochemical properties

The thermal properties of Pt(II) complexes were investigated by thermogravimetric analysis (TGA) under a nitrogen atmosphere. As depicted in Fig. S11,†Pt1 and Pt2 begin to decompose at 390 °C and 355 °C (T_d: the temperature at which a weight loss of 5% is recorded), revealing their high thermal stability. The electrochemical properties of both Pt(II) complexes were examined using cyclic voltammetry in dichloromethane. As shown in Fig. S12,† the complexes show (quasi)reversible oxidation waves with half wave potentials (E^OX_1/2) of ca. 0.42 V versus Ag/AgCl. The almost identical oxidations for both Pt(II) complexes are assigned to the carbazolate ligand-centered processes. Notably, Pt2 shows a second (quasi)reversible oxidation with the E^OX_1/2 of 0.47 V. It is assigned to the redox process of the other carbazolate ligand. By referring to the redox couple of Fc⁺/Fc, the HOMO levels of the complexes were determined to be ca. −4.73 eV (Table S5†). By addition of the optical bandgaps (determined from absorption onsets), their LUMO levels were estimated to be from −2.55 to −2.64 eV. The HOMO and LUMO values are very close to those for Pd(II) analogues, again manifesting the sole perturbing role of the metal.

Electroluminescence

The electroluminescence properties of Pt1 were studied in vacuum-deposited OLEDs. Complex Pt2 was not examined because of its large molecular weight. As shown in Fig. 5A, the devices have a configuration of ITO/HAT-CN (5 nm)/TAPC (30 nm)/TCTA (15 nm)/mCBP (10 nm)/DMIC-TRZ:Pt1 (40 nm)/POT2T (20 nm)/ANT-BIZ (30 nm)/Liq (2 nm)/Al (100 nm). The EML is composed of 1,3-dihydro-1,1-dimethyl-3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)indeno-[2,1-b]carbazole (DMIC-TRZ) doped with Pt1 at concentrations of 1–10 wt%. Fig. 5A and B show the energy level diagram of the device and the chemical structures of the other materials. The key device data are summarized in Table 2. All devices exhibit red electroluminescence peaking at 610–633 nm (Fig. 5C). The redshifts with increasing doping concentration are likely due to the intermolecular interactions. Impressively, the device with a 1 wt% doping level shows a maximum EQE and current efficiency (CE) of 35.6% and 61.2 cd A⁻¹ (Fig. 5D). The maximum luminance exceeds 136 800 cd m⁻² (Fig. 5E). In particular, the EQE and CE values of Pt1 remain as high as 34.3% and 58.8 cd A⁻¹ at the brightness of 1000 cd m⁻², revealing a very low efficiency roll-off of 1.3%. At 10 000 cd m⁻², the efficiency roll-off is as small as 7.9%. To the best of our knowledge, the high EQE_max with small roll-off and CE values are among the best EL performances for red OLEDs (λ_EL > 600 nm) doped with either a TADF or a phosphorescent emitter (Fig. S13 and Tables S6 and S7†). Note that the EQE decreases when the doping concentration is increased, presumably due to aggregation-caused quenching. As shown in Fig. 5E, a high doping level leads to a lower current density, likely suggesting the charge-trapping mechanism that is also operative under electrical excitation. In the meantime, a maximum EQE of 27.3% for the 10 wt% doped device together with the CIE_x,y of (0.63, 0.36) still represents one of the state-of-the art red OLEDs without using Ir(III) complexes as the emitter or sensitizer (Fig. S13 and Tables S6 and S7†).

Fig. 5 (A) Device configuration and the energy level diagram. (B) Chemical structures of the materials used for device fabrication. (C) EL spectra of Pt1 recorded at 1000 cd cm⁻². (D) External quantum efficiency (EQE) versus luminance curves of Pt1. (E) Current density and luminance versus voltage characteristics of Pt1.

Table 2 OLED performances of Pt1 in vacuum-deposited OLEDs

Dopant ratio	λ max [nm]	L [cd m^−2]	CE^b [cd A^−1]		PE^c [lm W^−1]		EQE^d [%]		CIE^e [(x, y)]
			Max	at 1000 cd m^−2	Max	at 1000 cd m^−2	Max	at 1000 cd m^−2
a Maximum luminance recorded at 1000 cd m^−2. b Current efficiency at a maximum and 1000 cd m^−2. c Power efficiency at a maximum and 1000 cd m^−2. d External quantum efficiency at a maximum and 1000 cd m^−2. e CIE coordinates at 1000 cd m^−2.
1 wt%	610	136 800	61.2	58.8	60.7	42.0	35.6	34.3	0.58, 0.42
3 wt%	615	127 000	45.9	44.7	43.5	29.3	32.1	31.0	0.60, 0.40
5 wt%	624	113 600	39.1	37.2	36.0	22.5	30.8	29.5	0.61, 0.39
10 wt%	633	63 200	25.1	22.0	20.3	9.1	27.3	23.0	0.63, 0.36

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Conclusions

We have developed two red Pt(II)–TADF emitters featuring metal-perturbed intraligand charge-transfer (MPICT) excited states. The use of a donor–acceptor type monodentate ligand is key to the realization of TADF properties. The minor involvement of the Pt atom in the frontier molecular orbital (FMO) plays a crucial role in enhancing the spin–orbit coupling effect. The Pt(II) complexes show high PLQYs of up to 92% in doped thin films and short excited-state lifetimes of 1.51–1.63 μs. Temperature-dependent emission lifetime studies determined the ΔE_ST values to be 86–89 meV. The ISC rate was estimated to be on the order of 10⁹ s⁻¹ through an ultrafast transient absorption study. OLEDs doped with the mononuclear Pt1 exhibited a high EQE of 35.6% at 610 nm. The devices also showed very low efficiency roll-off at 1000 and 10 000 cd m⁻². This work demonstrates a rational design of Pt(II)–TADF complexes which may overcome the limitations on triplet radiative decay rates that are imposed on phosphorescent Pt(II) complexes.

Data availability

All data supporting this study are available from the article and ESI.†

Author contributions

J.-G. Yang synthesized the materials and analyzed the data with the help of J. Zhang. N. Li fabricated the device. X.-F. Song and J.-G. Yang performed theoretical calculations. J. Li and M.-D. Li performed the femtosecond transient-absorption (fs-TA) studies. K. Li conceived the original idea and wrote the manuscript. All authors contributed to the discussion of the results.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22322505, 22271196 and 22301226), the Natural Science Foundation of Hubei Province (2023AFB273), the Shenzhen Science and Technology Program (ZDSYS20210623091813040) and the Research Team Cultivation Program of Shenzhen University (2023DFT004). K. Li acknowledges support from the Department of Science and Technology of Guangdong Province (2019QN01C617).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2205684 and 2205685. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ta02301c

‡ Jian-Gong Yang and Nengquan Li contributed equally.

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