Anomalous redshifted emission in Pt(II) vs. Ir(III) complexes with identical ligands and the application of these complexes in high-efficiency OLEDs

Abstract

Conventionally, Ir(III) complexes typically show longer emission wavelengths compared to their Pt(II) complexes with identical ligands. In this work, a series of Pt(II) and Ir(III) complexes based on identical benzothiophene ligands were designed. Experimental results demonstrate that the Pt(II) complexes exhibit red-shifted emission compared to the Ir(III) complexes. This is caused by the lower T₁ energy level of the Pt(II) complexes. The device efficiency prepared by vapor deposition can reach up to 20.3% with very small efficiency roll-off, demonstrating superior performance among Ir(III) complexes bearing thiophene and related structures. Furthermore, the broader spectrum of Pt(II) complexes makes them suitable for applications in white light devices.

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Introduction

In recent years, transition metal complexes have attracted great attention as highly efficient phosphorescent emitters owing to their strong spin–orbit coupling (SOC) and the ability to harvest both singlet and triplet excitons.^1–11 Among them, d⁸ platinum(II) and d⁶ iridium(III) complexes represent two of the most important classes of coordination systems for optoelectronic applications.^12–26 The large spin–orbit coupling effect and theoretical 100% exciton utilization make them core luminescent materials in organic light emitting devices (OLEDs), where the fine-tuning of photophysical properties through ligand design has been a central strategy for achieving high performance.^27–31 In most reported studies, Ir(III) complexes within the same ligand frameworks typically exhibit longer wavelength emission compared to Pt(II) complexes due to their lower triplet state energy levels.^32–35

In this work, a series of Ir(III) and Pt(II) complexes based on the same ligand framework were designed and synthesized to achieve the modulation of the emission behavior. Unusually, the Pt(II) complexes exhibit longer wavelength emission than the Ir(III) complexes in the experiment, showing an anomalous redshift phenomenon. To understand the red-shifted emission of Pt(II) complexes within the same ligand framework, Pt(II) complexes have a lower triplet state (T₁). Furthermore, Pt(II) complexes show nearly planar arrangements with short Pt–Pt interactions, which enable more metal components to participate in charge transfer and enhance the SOC effect.³⁶ In addition, OLEDs fabricated by vapor deposition exhibited a maximum efficiency of up to 20.3%, with low efficiency roll-off, which exhibits promising performance among Ir(III) complexes with thiophene-derived ligands (Scheme 1).

Scheme 1 The wavelengths shown in this work differ from those of conventional Ir(III) and Pt(II) complexes with the same ligand.

Results and discussion

The synthetic routes and structures of the organic ligands and complexes are shown in Fig. 1 and Schemes S1–S3. Three similar ligands were synthesized through the Suzuki cross-coupling reaction. Synthesis of Pt(II) complexes: in the first step, 1.0 equivalent of K₂PtCl₄ and ligands react in a mixture of 2-ethoxyethanol and water to form a dimer, and in the second step, the dimer reacts with auxiliary ligands and t-BuOK in CH₂Cl₂ solution to yield Pt(II) complexes. In the first step of the synthesis of Ir(III) complexes, IrCl₃·nH₂O reacts with 2.0 equivalents of ligands in a mixture of tetrahydrofuran and water to form a dimer. The second step is identical to the second step in the synthesis of Pt(II) complexes. The chemical structures of the complexes were characterized by nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) (Fig. S1–S6). The structures of MBTQ-Ir and MBTQ-Pt were further confirmed by single-crystal X-ray diffraction.

Fig. 1 Molecular structures of Pt(II) and Ir(III) complexes and their PL wavelengths measured in CH₂Cl₂ at room temperature.

The molecular perspective and packing diagrams of complexes MBTQ-Ir and MBTQ-Pt are shown in Fig. 2, with crystallographic data provided in the SI (Tables S1–S4). The Pt(II) complex molecules adopt a highly planar conformation, with a key torsion angle (O1–O2–C21–N1) of approximately 0°. The Pt(II) complex molecules are arranged in a head-to-tail alternating manner (Fig. S7), with an intermolecular Pt–Pt distance of 3.603 Å, indicative of Pt–Pt interactions. These intermetallic interactions are conducive to metal-to-metal-to-ligand charge transfer, promoting the population of the excimeric state and enhancing the SOC effect through greater metal orbital involvement. Concurrently, this planar structure and short distance facilitate intermolecular π–π stacking, with a π–π interaction distance of 3.681 Å between adjacent molecules, contributing to the observed excellent quantum yields in the solid state. It should be noted, however, that such close packing can also induce strong intermolecular interactions, which may lead to broadening of the emission spectrum, accompanied by a reduction in color purity. Crystal packing analysis of MBTQ-Ir reveals weaker π–π stacking interactions (Fig. S8), with adjacent molecules exhibiting staggered face-to-face arrangements of cyclometallated ligands, at distances of 3.872 and 3.935 Å. This contrasts with the stronger Pt–Pt and π–π interactions observed in platinum analogues.

Fig. 2 Crystal structures of MBTQ-Ir and MBTQ-Pt, and selected bond lengths of metal center-related bonds. Hydrogen atoms are omitted for clarity.

As shown in Fig. 3a, the electronic absorption spectra of these complexes in dichloromethane exhibit a strong absorption band in the range of approximately 300–400 nm, which can be assigned to spin-allowed ligand-centered singlet π–π* transitions of the two distinct cyclometallated ligands. Additionally, a weak and broad absorption band is observed in the range of approximately 400–550 nm; this low-energy absorption band can be attributed to a mixed character involving metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) transitions.

Fig. 3 UV–vis absorption (a) and photoluminescence spectra of Ir(III) and Pt(II) complexes in the CH₂Cl₂ solution at room temperature (b)–(d).

The photoluminescence (PL) spectra of these complexes (Fig. 3) exhibited low-energy emissions in the range of 580–630 nm, accompanied by long lifetimes ranging from 0.39 to 1.10 μs. The large Stokes shifts and prolonged lifetimes suggest a triplet origin for these emissions. It is worth noting that, compared to Ir(III) complexes, Pt(II) complexes exhibit red-shifted emission. To rule out the possibility that the emission shift is caused by aggregation in the Pt(II) complex, we recorded its PL spectra in dichloromethane–methanol mixtures with different methanol fractions (f_w) (Fig. S13). As f_w increases, the complexes gradually aggregate, and aggregation quenching causes the phosphorescence intensity to gradually decay. However, no further red shift was observed in the emission, indicating that the red shift in Pt(II) complexes is due to their single-molecule emission.

At room temperature, the phosphorescence spectrum exhibits a broad-band characteristic (Fig. 3), consistent with typical ³MLCT behavior. When the temperature is lowered to 77 K (conducted in a 2-methyl-tetrahydrofuran glassy matrix to prevent aggregation), the spectrum shows a blue shift, and two distinct phosphorescence peaks are observed. The vibrational spacing observed in the 77 K phosphorescence spectrum of the complexes (approximately 1200–1400 cm⁻¹) highly correlates with the corresponding phosphorescence emission spacing of the ligand measured under identical conditions (approximately 1200–1350 cm⁻¹) (Fig. S11). This indicates that the triplet state retains the fundamental vibrational modes of the organic ligand framework. Furthermore, its vibrational electronic fine structure also indicates that the phosphorescence spectrum is attributable to emission from the ³π–π* state at the ligand center. DFT analysis confirms the dominant ³π–π* character and a minor ³MLCT effect, with both the hole and electron states exhibiting weak metallic characteristics. Additionally, the blue shift observed in the 77 K spectrum can be attributed to rigid color effects.^32,37,38

In the doped PMMA films (Fig. S12), low energy emissions were also observed, and the PL spectra were similar to the related phosphorescent emissions in the CH₂Cl₂ solutions; the emission lifetimes of these doped films were prolonged to 1.84–3.60 μs, also demonstrating their triplet parentage.

Notably, despite sharing the same ligand framework, the Pt(II) complexes (PBTQ-Pt, BTQ-Pt, and MBTQ-Pt) exhibit varying degrees of redshift compared to their Ir(III) complexes (PBTQ-Ir, BTQ-Ir, and MBTQ-Ir), and they also exhibit high quantum efficiency.

DFT and TD-DFT calculations at the B3LYP/6-31G(d,p) level were used to study the molecular conformation, FMO distribution, and excited-state energy levels of the polymer model compound. HOMOs are mainly distributed on the d orbitals of the metal and the C-related benzothiophene unit of the C^N ligand (Fig. S15), whereas the electron density of LUMOs is mainly concentrated on the more electron-poor N ligand fragment and shows a small amount of distribution on the metal center. This spatially separated distributional feature confers a pronounced metal-to-ligand charge transfer (MLCT) property, leading to a decrease in the excited state energy and thus a redshift in emission wavelengths. For all of these Pt(II) complexes, the NTO calculation results showed that hole → particle transitions made a dominant contribution (>93.27%) to their T₁ states (Fig. 4), while Ir(III) is less than 89.77%. In addition, according to Table S5, the calculations show that the HOMO → LUMO transitions make significant contributions to the T₁ states of PBTQ-Pt BTQ-Pt and MBTQ-Pt (84.69%, 88.47% and 87.65%), which are much higher than those of the corresponding Ir(III) complexes (46.27%, 70.67% and 71.31%); the T₁ state energy of Pt(II) complexes is lower than that of their Ir(III) analogs, and this trend is consistent with experimental observations.

Fig. 4 Natural transition orbital distribution patterns of S₀ → T₁ excitation for Pt(II) and Ir(III) complexes on the basis of their optimized T₁ geometries.

The lower T₁ state energies of Pt(II) complexes in the same ligand environment can be attributed to their molecular orbital structural features, and the T₁ states usually originate from charge transfer from the d orbitals of the metal centers to the π* orbitals of the ligands. Meanwhile, the HOMO and LUMO levels were estimated by cyclic voltammetry (CV) in CH₃CN solution (Table 1); the corresponding HOMO levels of PBTQ-Pt, BTQ-Pt and MBTQ-Pt were calculated to be −5.40, −5.45, and −5.45 eV, indicating deeper energy levels compared to Ir(III) complexes (−5.30, −5.31 and −5.28 eV). The electrochemical HOMO–LUMO gaps for BTQ-Pt, PBTQ-Pt and MBTQ-Pt were calculated to be 2.58, 2.59 and 2.66 eV, respectively, and those for BTQ-Ir, PBTQ-Ir and MBTQ-Ir were calculated to be 2.61, 2.61 and 2.67 eV. The HOMO–LUMO gap value of Pt(II) complexes are smaller than those of the corresponding Ir(III) complexes. Therefore, it is reasonable that Pt(II) complexes could show much deeper red emissions.

Table 1 Photophysical, HOMO/LUMO energy levels and thermal data of these complexes

	Absorption^a (nm)	Emission (FWHM)^a^,^b (nm)	PLQY^a^,^g	τ P ^a^,^b (μs)	HOMO/LUMO (eV)	T d ^e (°C)	K r/Knr ^f (10^5 s^−1)
a Measured at a concentration of ca. 10^−5 M in CH2Cl2 solution at room temperature. b Measured at doped PMMA films at room temperature. (The doping concentrations of Pt(II) complexes and Ir(III) complexes are 2% and 4%, respectively.) c HOMO and LUMO values from theoretical calculations. d HOMO and LUMO were calculated from CV curves measured in acetonitrile solution. e Temperature at 5% loss of weight. f K r = PLQY/τP. Knr = (1 − PLQY)/τP. g Measured at doped CBP films at room temperature. (The doping concentrations of Pt(II) complexes and Ir(III) complexes are 2% and 4%, respectively.)
PBTQ-Ir	277, 323, 451, 500	620 (70)/615 (72)	34.12/69.5	0.68/2.29	−5.20/−1.69^c	333	5.02/9.69
					−5.30/−2.69^d
PBTQ-Pt	256, 319, 384, 464, 482	631 (76)/627 (80)	59.11/61.8	1.10/3.60	−5.42/−1.82^c	303	5.37/3.72
					−5.40/−2.81^d
BTQ-Ir	300, 329, 468, 507	590 (54)/586 (61)	57.28/76.3	0.55/2.30	−5.04/−1.78^c	290	10.42/7.77
					−5.31/−2.7^d
BTQ-Pt	272, 287, 349,	600 (79)/586 (78)	19.00/65.4	0.82/1.84	−5.25/−1.93^c	295	2.32/9.88
	390, 456, 481				−5.45/−2.87^d
MBTQ-Ir	300, 343, 459, 499	580 (55)/578 (59)	39.07/78.9	0.39/2.14	−4.98/−1.70^c	364	10.02/15.62
					−5.28/−2.61^d
MBTQ-Pt	272, 286, 345,	590 (79)/583 (78)	19.80/75.2	0.85/2.69	−5.21/−1.87^c	308	2.33/9.44
	386, 448, 471				−5.45/−2.79^d

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In order to illustrate the electroluminescence properties of these complexes, OLEDs were prepared using these complexes as dopants based on the vaporization method (Fig. 5 and Fig. S17). First, the device structure used for materials is ITO/HAT-CN (10 nm)/TAPC (40 nm)/TCTA (5 nm)/CBP:Ir complex (4 wt%) or Pt complex (2 wt%) (20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). The doping concentration is further optimized, and the experimental results show that the Ir(III) complex performs best at a doping concentration of 4 wt%, while the Pt(II) complex has the highest efficiency at 2 wt%.

Fig. 5 Best EL performance of OLEDs based on the complexes: (a)–(c) EL spectra. (d) Maximum EQE summary of OLEDs based on Ir(III) complexes employing thiophene and benzothiophene with EL peaks in the range of 550–660 nm. (e) Curves of EQE vs. luminance at a doping concentration of 2.0 wt% (Pt(II) complexes) and 4.0 wt% (Ir(III) complexes). (f) J–V–L characteristics.

In terms of emission wavelength, the EL peaks of Pt(II) complex devices reached 632 nm, 589 nm and 580 nm (Table 2), which are significantly higher than those of Ir(III) complex devices (620 nm–576 nm), verifying the characteristics of Pt(II) complexes that emit longer wavelengths and tend to be red. Furthermore, as shown in Fig. S18, the EL spectra of the Pt(II) complexes remain virtually unchanged at different doping concentrations, demonstrating the absence of aggregation-induced redshift in the solid state. The EL emission peak positions of the devices show that the Pt(II) complexes have a significant redshift compared to the Ir(III) complexes, and this trend is consistent with the PL spectra as well as the theoretical calculations.

Table 2 Key EL performance of OLEDs based on these complexes

Devices	λ EL (nm)	V turn-on (V)	L max (cd m^−2)	EQE (%)	CE (cd A^−1)	PE (lm W^−1)	CIE (x, y)
PBTQ-Ir	620	4.5	18 000	17.3	13.4	7.4	(0.66, 0.33)
PBTQ-Pt	632	5.0	1228	9.1	4.84	3.04	(0.66, 0.32)
BTQ-Ir	584	5.0	35 020	19.9	40.74	20.11	(0.58, 0.42)
BTQ-Pt	589	4.5	174.6	13.7	21.96	13.8	(0.60, 0.40)
MBTQ-Ir	576	4.0	65 670	20.3	48.53	30.49	(0.55, 0.45)
MBTQ-Pt	580	4.0	2800	15.3	30.64	18.28	(0.56, 0.43)

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Among them, the device based on MBTQ-Ir exhibits the highest EQE (20.3%), which is consistent with the high PLQY observed in the film and represents a leading level among Ir(III) complexes employing thiophene and its derivatives (Fig. 5(d)), and the CE and PE of the device are 48.53 cd A⁻¹ and 30.49 lm W⁻¹, respectively. The efficiency roll-off is low; at a brightness of 1000 cd m⁻², the efficiency roll-off is only 1%. Based on Pt complexes, the best device performance was achieved at a doping concentration of 2% by MBTQ-Pt (15.3%), which was lower than that of the devices based on corresponding Ir(III) complexes. Ir(III) complexes exhibit excellent electroluminescence efficiency, while Pt(II) complexes show remarkable red-shifted emission properties, providing diverse molecular design strategies for realizing high-performance red OLEDs.

Conclusion

In summary, we have successfully synthesized three pairs of Pt(II) and Ir(III) complexes based on different benzothiophene ligands. Unlike the conventional point that Ir(III) complexes usually exhibit longer emission wavelengths under the same ligand conditions, all three pairs of complexes in this study show a red-shifted emission property of the Pt(II) complexes compared with the corresponding Ir(III) complexes. Further studies show that this phenomenon is mainly attributed to the lower T₁ state of Pt(II) complexes, which provides a new direction for expanding the molecular design strategy of red light-emitting materials. OLEDs based on these complexes were prepared by vapor deposition, in which the device with MBTQ-Ir as the light-emitting layer exhibited the highest EQE of 20.3%, which is in the leading position among the Ir(III) complexes with thiophene-containing structures that have been reported. It is worth mentioning that the broad emission spectral properties exhibited by the Pt(II) complexes provide a good material basis for realizing the construction of high-quality white light OLEDs.

Author contributions

X. Y. and Y. S.: conceptualization; G. Z., X. Y. and Y. S.: methodology; W. L., S. X., X. D. and A. Y.: validation; W. L., S. X., X. D., A. Y., S. L., J. X., Y. D.: formal analysis; W. L., S. X., X. D., A. Y.; S. L. and J. X.: investigation; X. Y. and Y. S.: resources; W. L., S. X., X. D. and A. Y.: data curation; W. L., S. X., X. Y. and Y. S.: writing – original draft preparation; W. L., S. X., X. Y. and Y. S.: writing – review & editing; W. L., S. X., X. D.; A. Y. and Y. D.: visualization; X. Y. and Y. S.: supervision; X. Y. and Y. S.: project administration; G. Z., X. Y. and Y. S.: funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support this article are available in the article and its supplementary information (SI). Supplementary information: experimental instrumentation, synthetic procedures, structural characterization data including NMR and MS data, theoretical calculations, thermodynamics data, and device performance data. See DOI: https://doi.org/10.1039/d5qi01849h.

CCDC 2454098 (MBTQ-Ir) and 2453791 (MBTQ-Pt) contain the supplementary crystallographic data for this paper.^39a,b

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22375158 and 22175137), the Key Research and Development Program of Shaanxi (2025CY-YBXM-148), and the Fundamental Research Funds for the Central Universities (xzy012023039 and xtr072024032). The characterization assistance from the Instrument Analysis Center of Xi'an Jiaotong University is also acknowledged.

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