Heteronuclear Pt^II–Pd^II dimers formation through ligands subtle tailoring

Abstract

The synthesis of heteronuclear Pt–Pd complexes has long presented a significant challenge, hindering both scientific exploration and industrial applications in electroluminescence and catalysis. This study reports the successful preparation and isolation of two Pt–Pd complexes, designated as 1 and 2, achieved by utilizing distinct cyclometallating C^N ligands. Both complexes were comprehensively characterized, with single crystal X-ray diffraction confirming Pt–Pd distances of 2.8557(5) Å and 2.8548(6) Å for 1 and 2, respectively. Furthermore, these complexes exhibited strong triplet emission at wavelengths of 656 nm and 669 nm, with photoluminescence quantum yields of 40.5% and 48.1% for 1 and 2, respectively. The potential of these complexes in electroluminescent applications was also explored, achieving maximum external quantum efficiencies (EQE_max) of 10.9% for 2 and 10.5% for 1. This inaugural synthesis of phosphorescent heteronuclear Pt–Pd complexes, featuring pronounced metal–metal interactions, not only provides a new synthetic strategy but also paves the way for further application-based studies of heteronuclear Pt–Pd complexes in OLEDs and photocatalysis.

---

Introduction

Binuclear Pt and Pd transition metal complexes with d⁸–d⁸ interactions have garnered attention for their intriguing structures and diverse applications, ranging from bioimaging,^1–3 photocatalysis,^4,5 OLEDs,^6–15 and optical sensors,^16–19 to photodynamic therapy.^20,21 Recently, double-layered square planar Pt–Pt complexes featuring cyclometallating ligands have been extensively studied. These complexes are particularly valued in OLED technology for their metal–metal bond-induced triplet red emission, characterized by metal–metal-to-ligand charge transfer (MMLCT), meeting the requirements for red phosphorescent materials.^22–25 Furthermore, these Pt complexes can form Pt-DNA adducts with nuclear DNA, which inhibits DNA replication and transcription, ultimately inducing cell apoptosis.^26–30 Conversely, palladium complexes with double-layered square planar cyclometallating ligands have primarily been developed as catalysts in organic syntheses. Notably, Ritter reported on bimetallic Pd(III) complexes that enhance palladium-catalyzed carbon–heteroatom bond formation.^31–36 Although studies on the photophysical properties of binuclear palladium complexes have been conducted by Che and Gray, no emission was observed from these complexes at room temperature until resent report of emissive binuclear palladium complexes, opening new avenues for their application in OLEDs.^37–39

The properties of binuclear Pt and Pd complexes are closely tied to their metal–metal interactions.^22,40–43 Despite a bond order of zero for these d⁸–d⁸ complexes, the significant bonding interactions still play a crucial role in their chemistry.^44,45 The Pt–Pt bonding interactions have been well-documented and verified through X-ray single crystal diffraction and Raman^46,47 spectroscopy. Similarly, Zhang has recently explored binuclear Pd complexes, investigating Pd–Pd bonding interactions using both XRD and Raman spectroscopy.²⁴ These reports indicated that the 5dz²–5dz² orbital overlap of Pt–Pt complexes and the 4dz²–4dz² orbital overlap of Pd–Pd complexes, resulting in versatile electronic transitions of binuclear molecules, and these complexes can be used as efficient emitters and catalysts.^45,48,49 Beyond these types of homonuclear Pt–Pt and Pd–Pd complexes, the heteronuclear Pt–Pd complexes and their chemistry could be even more interesting because of the versatile electronic transitions generated from the 5dz²–4dz² orbital overlapping. However, these theoretical existing complexes have never been reported due to the difficulties of preparing and isolating the target complexes from their by-products bearing the same organic ligands and undistinguishable physical and chemical properties.

In this article, we conceived a strategy to synthesize and isolate two heteronuclear Pt^II–Pd^II complexes through subtle tailoring cyclometallating ligands of phenyl pyridines. With the modification of one ligand, the synthetic condition did not change much. However, it resulted the easily separation of the target heteronuclear Pt–Pd complexes from the homonuclear Pt–Pt and Pd–Pd by-products. Two heteronuclear complexes of 1 and 2 were successfully isolated with yields of 24% and 31%, respectively. 1 and 2 were fully characterized using standard small molecule techniques of UV/visible absorption, cyclic voltammetry (CV), nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS), elemental analysis, Raman spectroscopy, and X-ray single crystal diffraction (XRD). The photophysical properties of two complexes were investigated, and 1 exhibited triplet emission at 656 nm with quantum yields Φ = 40.5%, and 2 showed triplet emission at 669 nm with Φ = 48.1% in crystalline state, respectively. Moreover, both 1 and 2 were used as emitting materials to fabricate OLED through vapor deposition process and 1-based OLED exhibited an EQE_max of 10.5% with CIE coordinates of (0.60, 0.38) and 2-based OLED having EQE_max = 10.9% with CIE coordinates of (0.62, 0.37), respectively.

Results and discussion

Syntheses and characterizations

The synthesis of bimetallic Pt–Pd complexes closely mirrors the method used for preparing homonuclear Pt–Pt complexes, albeit with minor modifications. Initially, the homonuclear Pt–Pt (R1, R2) and Pd–Pd (R3, R4) precursors^33,50 are prepared. Upon successfully obtaining these four starting materials, a mixture of two complexes (R1 with R3 and R2 with R4) is treated with five equivalents of the clamping ligand N,N′-diphenylformamidine (HDPhF) and four equivalents of potassium carbonate in 1,2-dichloroethane (DCE). This reaction is maintained at 90 °C for 24 h, forming the target heteronuclear bimetallic products, 1 or 2, along with by-products of the homonuclear bimetallic Pt–Pt and Pd–Pd complexes. Due to the different cyclometallating ligands used in the three complexes (1, a, and b), each was successfully isolated using silica gel column chromatography, yielding separate outputs of 24%, <1%, and 15%, respectively (Fig. 1). 2, which incorporates alternative cyclometallating ligands—2-[(1,1′-biphenyl)-3-yl]-4-phenylpyridine (bppy) for Pt and phenylpyridine (ppy) for Pd—was prepared with a yield of 31%. 1 and 2 were extensively characterized using a variety of techniques, including ¹H NMR, ¹³C NMR, ¹⁹⁵Pt NMR, high-resolution mass spectrometry (HRMS), Raman spectroscopy, elemental analysis, and thermogravimetric analysis (TGA). In the ¹H NMR spectra, 1 displayed two peaks at 8.44 and 8.17 ppm, attributed to the protons of the –NCHN– group within two clamping ligands of HDPhF (Fig. S44 and S47†). The discrepancy between these peaks suggests the absence of a C2 rotational symmetry axis in 1, contrasting with the homonuclear by-products a and b, which show single peaks at 8.38 ppm and 8.16 ppm, respectively. The chemical shifts are consistent with reported homonuclear Pt and Pd complexes bearing HDPhF ligands.^24,46,51 The ¹⁹⁵Pt NMR spectra, recorded using a JEOL 600 MHz spectrometer, showed a peak at −3179.00 ppm for 1 and −3156.92 ppm for 2 (Fig. 2f). The downfield shift in 2 is due to the additional phenyl rings in the bppy ligand, which reduce the shielding of the Pt atom. Comparing with binuclear or mononuclear platinum complexes, these shifts are in a reasonable position.^51–53 HRMS data for both complexes matched the theoretical calculations closely in both values and isotopic patterns. To verify Pt–Pd bonding interactions, Raman spectroscopy⁴⁶ was employed, revealing a Pt–Pd stretching vibration at 100 cm⁻¹, similar to the Pt–Pt stretching at the same frequency but more pronounced than the Pd-Pd stretching vibration at 85 cm⁻¹ (Fig. 2e). This indicates stronger Pt–Pd interactions than Pt–Pt and much stronger than Pd-Pd. Elemental analysis measurement confirmed that the experimental results were consistent with the simulations. Thermogravimetric analysis showed a decomposition temperature of 334 °C for 1 and 218 °C for 2, highlighting the greater thermal stability of 1. The decomposition temperature of 1 is almost the same as that of the binuclear platinum complex,^25,54,55 while 2 exhibits lower decomposition temperature probably ascribed to the differences in stability when Pt and Pd metals are connected to different ligands (Fig. S31 and S34–S39†).

Fig. 1 Synthetic strategy for heteronuclear Pt^II–Pd^II complexes.

Fig. 2 (a) Single crystal structures for 1 and 2; H atoms omitted for clarity; and thermal ellipsoids drawn at the 30% level. (b) Single crystal intramolecular/intermolecular π–π interactions for 1 and 2; H atoms omitted for clarity. (c) HR-MS spectra for 1 [M + H]⁺. (d) HR-MS spectra for 2 [M + H]⁺. (e) Raman spectra of 1 (red), 2 (blue). (f) ¹⁹⁵Pt NMR spectrum of 1 (red), 2 (blue) in deuterated chloroform.

X-ray crystallography

Single crystals of 1 (CCDC 2311651†) and 2 (CCDC 2311650)† (15 mg, 0.010 mmol) were grown by slow evaporation of a DCM/n-hexane mixture (1 : 1 v/v) in a 5 mL vial at room temperature. Both complexes crystallized in the orthorhombic system within the Pbca space group, featuring a double-layer square planar geometry. In 1, the Pt and Pd atoms are coordinated by ppy and bppy ligands, respectively, while in 2, the coordination is reversed (Fig. 2a). The Pt–Pd distances were determined to be 2.8557(5) Å for 1 and 2.8548(6) Å for 2, shorter than the distances in homonuclear Pt–Pt⁵⁶ and Pd–Pd complexes, which are 2.860 Å and 2.8644(2) Å, respectively, despite having identical cyclometallating ligand of ppy and clamping ligand of HDPhF, the Pt–Pd distances are shorter than both Pt–Pt and Pd-Pd distance. Considering the bigger ionic radius of Pt²⁺ than Pd²⁺, the shortest metal–metal distances of Pt–Pd in three Pt–Pt, Pt–Pd, Pd–Pd complexes indicated its strongest atomic interaction which also supported by the Raman spectra analysis (Fig. 2e).

Besides the intensive Pt–Pd interactions, 1 and 2 also demonstrate significant π–π interactions between the cyclometallating ligands bppy and ppy. Specifically, one intermolecular π–π interaction exists between two phenyl rings from different ppy ligands in each complex, with centroid–centroid distances of 3.762(6) Å. Additionally, 1 exhibits two intramolecular π–π interactions between the ppy and bppy within the same molecule, with centroid–centroid distances of 3.806(4) Å and 3.757(4) Å. Similarly, 2 shows intramolecular π–π stacking with centroid–centroid distance of 3.829(6) Å, and two intramolecular π–π stacking with centroid–centroid distance of 3.767(4) Å and 3.738(5) Å between ppy and bppy ligands (Fig. 2b). The abundant π–π interactions in the crystalline state align the flat ligands ppy and bppy nearly parallel to each other, with dihedral angles of 8.92(16)° for 1 and 8.45(18)° for 2.

Photophysical properties

The UV/visible absorption and emission spectra of 1 and 2 are illustrated in Fig. 3, with detailed photophysical data provided in Table 1. The absorption spectra of complexes 1 and 2 in CH₂Cl₂ at a concentration of 10⁻⁵ mol L⁻¹ are shown in Fig. 3a (dashed). Both 1 and 2 exhibit intense absorption bands in the high energy region at 268 nm, with molar extinction coefficients of 7.4 × 10⁴ for 1 and 8.2 × 10⁴ M⁻¹ cm⁻¹ for 2. These bands can be assigned to the intraligand charge transfer (ILCT) of the cyclometalated C^N ligands and N^N bridging ligands.^41,46 The absorption bands at 348 nm, with molar extinction coefficients ranging from 2.2 × 10⁴ to 3.1 × 10⁴ M⁻¹ cm⁻¹, are tentatively assigned to a mixture of ligand-to-ligand charge transfer (LLCT) from π_N^N to π_C^N* and metal-to-ligand charge transfer (MLCT) from Pt 5d to π_C^N*.^24,25,54,55 Lower energy absorption bands at 470 nm, with molar extinction coefficients of 2.1 × 10³ and 3.1 × 10³ M⁻¹ cm⁻¹, are metal–metal-to-ligand charge transfer with the nature of dσ*_(Pt–Pd) to π_C^N*.^25,55

Fig. 3 (a) UV/visible absorption spectra (dashed) of 1 (red), 2 (blue) in CH₂Cl₂ at room temperature (r.t.) and normalized emission spectra in air-free CH₂Cl₂ at r.t. (The right vertical axis of (a) and the left vertical axis of (b) share the same title). (b) Normalized emission spectra in different concentrations PMMA film of 1. (c) Normalized emission spectra in different concentrations PMMA film of 2. (d) Normalized emission spectra of 1 (red), 2 (blue) at crystalline state. (e) PL decay curves for 1 (red) and 2 (blue) in 2 wt% PMMA films at r.t. (f) Cyclic voltammograms of 1 (red) and 2 (blue), measured in acetonitrile containing 0.1 M tetra-n-butylammonium hexafluorophosphate. (g) Three-dimensional excitation-dependent emission spectra in 2 wt% PMMA film of 1. (h) Three-dimensional excitation-dependent emission spectra in 2 wt% PMMA film of 2. (i) Three-dimensional excitation-dependent emission spectra at crystalline state of 1.

Table 1 Photophysical properties^a and electrochemical parameters for 1 and 2 ^b

Complex	Absorption ^c	Medium (298 K)		Emission λmax/nm	φ em ^f %	τ (μs)	k r ^g	k nr ^g	E ox(V)	E HOMO ^h (eV)	E LUMO ^i (eV)	E g ^j (eV)
a The emission spectra were recorded with the excitation wavelength of 470 nm. b Scan rate = 100 V s^−1 in 0.1 M [n-Bu4N][PF6] (Pt electrode; E vs. Fc^+/Fc; 25 °C). Complexes were measured in dry CH2Cl2. c Electronic absorption band maxima (λabs) and molar absorption coefficients (log ε) in CH2Cl2 at r.t. d Measured in N2-degassed CH2Cl2 at 1 × 10^−5 mol L^−1. e Measured in 1 wt% 2 wt% 10 wt% 20 wt% PMMA films on quartz plate. f Phosphorescent quantum yield measured using an integrating sphere, the accurate value of PLQY is approximately 90%–110% of the measured value. g The radiative rate constant (kr) and the nonradiative rate constant (knr) were estimated using the following equations. kr (10^5 s^−1) = φ/τ, knr (10^5 s^−1) = (1 − φ)/τ. h HOMO energy levels calculated from CV data using ferrocene as an internal standard. i LUMO energy levels calculated from CV date and UV/visible absorption onset. j Optical band gap from the absorption spectra in degassed CH2Cl2, determined by Eg = hc/λonset.
1	268 (74 246), 348 (22 173), 470 (2109)	CH2Cl2 ^d		669	14.9	2.50	0.60	3.40	0.41/0.46, 0.89/0.98	−5.02	−2.38	2.64
		Crystalline state		656	40.5	2.53	1.60	2.35
		PMMA ^d	1 wt%	638	16.1	1.48	1.09	5.67
			2 wt%	644	27.3	2.50	1.09	2.91
			10 wt%	646	37.7	1.64	2.30	3.80
			20 wt%	652	37.3	2.49	1.50	2.52
2	269 (81 571), 348 (30 747), 470 (3132)	CH2Cl2 ^d		669	12.7	2.29	0.55	3.81	0.40/0.46, 0.94/1.05	−5.01	−2.37	2.64
		Crystalline state		669	48.1	2.19	2.20	2.37
		PMMA^e	1 wt%	640	13.7	4.20	0.33	2.05
			2 wt%	638	17.6	4.18	0.42	1.97
			10 wt%	643	31.7	3.04	1.04	2.25
			20 wt%	654	32.5	2.38	1.37	2.84

---

1 and 2 exhibited nearly identical structureless triplet emissions characteristic of MMLCT at 669 nm in N₂ nitrogen-degassed CH₂Cl₂ solution at room temperature (Fig. 3a). Specifically, the emission wavelength of 669 nm for 1 is positioned between the 733 nm of the homonuclear Pt–Pt complex a with ppy ligands and the 596 nm of the homonuclear Pd–Pd complex b with bppy ligands, demonstrating the nature of the π_CN* → dσ*_(Pt–Pd) emission. However, considering the non-emission of the homonuclear Pd–Pd complex c with ppy ligands under the same conditions, it is evident that 2 displays a 47 nm blue-shift relative to the homonuclear Pt–Pt complex with bppy ligands, as illustrated in Fig. S20† CH₂Cl₂.

In the crystalline state, the emission wavelengths for 1 and 2 are 656 nm and 669 nm, respectively, closely mirroring their solution-state emissions (Fig. 3d). Notably, 2 shows a 13 nm redshift compared to 1, which can be attributed to its shorter Pd–Pt distance in the crystalline state. The PL properties of 1 and 2 in PMMA at various dopant concentrations (1 wt%, 2 wt%, 10 wt%, and 20 wt%) were examined and depicted in Fig. 3b and c. As the doping concentration increases from 1 wt% to 20 wt%, the complexes exhibit slight emission red shifts, likely due to concentration-dependent π–π stacking formation. Additionally, compared to their solution and crystalline states, the blue-shifted emissions of 1 and 2 in PMMA-doped films suggest an extension of Pt–Pd distances in PMMA state.

1 and 2 exhibited short luminescent lifetimes (τ) ranging from 1.48 to 4.20 μs across three different media: degassed CH₂Cl₂, PMMA film, and crystalline state (Table 1). These lifetimes are comparable to those reported for dinuclear Pt–Pt complexes, which range from 1.4 to 2.4 μs,²² but are shorter than those for dinuclear Pd–Pd complexes, which span from 2.22 to 23.09 μs. Notably, the highest quantum yields for 1 and 2 in the crystalline state are 40.5% and 48.1%, respectively. PMMA film's peak PL quantum yields reach 37.7% for 1 and 32.5% for 2, while in degassed solution, the yields are 14.9% for 1 and 12.7% for 2. Comparing with the reported Pt–Pt complexes having PLQY close to unity^12,41,57 and our previous report of Pd–Pd complexes with PLQY of 70%,²⁴ one heteronuclear Pt–Pd complex exhibited moderate PLQY of 48%. Since this is the first report of synthesizing heteronuclear Pt–Pd complexes, it exhibited promising future for exploring stable Pt–Pd emitters with better PLQY. This finding suggests that substituting metal atoms in these dinuclear complexes presents an opportunity to develop even brighter luminescent materials, potentially surpassing the performance of their homonuclear counterparts.

Three-dimensional excitation-dependent emission spectra for 1 and 2 were recorded in three different media—degassed CH₂Cl₂, 2 wt% PMMA film, and crystalline state—at room temperature, as illustrated in Fig. 3g–i and ESI Fig. S13–S18.† Analysis of these spectra in the 2 wt% PMMA film reveals that both complexes display two prominent emission peaks. For 1, these peaks are labelled as G-1 and G-2 in Fig. 3g, occurring at excitation wavelengths of 350 nm and 520 nm, respectively. Similarly, for 2, the peaks are identified as H-1 and H-2 in Fig. 3h, also at excitation wavelengths of 350 nm and 520 nm. The mountain prominence (G-1) at the excitation wavelength of 350 nm for 1 is higher than that of the mountain prominence (G-2) at the excitation wavelength of 520 nm. 2 shows a lower mountain prominence (H-1) at the excitation wavelength of 350 nm compared to (H-2) at 520 nm. This indicates that the excitation at 350 nm with the nature of IL (π_CN → π_CN*) exhibits more efficient charge transfer than that at excitation of 520 nm with the nature of MMLCT (dz²* → π*) for complex 1. However, complex 2 exhibited efficient charge transfer at the excitation wavelength of 520 nm rather than the excitation wavelength of 350 nm. Given their UV/visible absorption spectra, the primary chromophore group in 1 is located at the bppy fragment, with high energy absorption (π_CN → π_CN*) around 350 nm. In contrast, the populated absorption of 2 shifts to MMLCT (dσ*_(Pt–Pd) → π_CN*), which absorbs at a lower energy around 520 nm. This difference is attributed to the molecular rearrangement with bppy and ppy ligands exchange in these two complexes. Although 1 and 2 exhibited different predominating absorption at 350 nm and 520 nm, respectively, they both exhibit nearly identical triplet emissions at 640 nm through the π_CN* → dσ*_(Pt–Pd) process. Both complexes show a continuous emission profile from 350 to 520 nm in the crystalline state due to intense π–π interaction normalizing their UV/visible absorption.

Electrochemistry

The electrochemical behaviours of 1 and 2 were analysed in CH₂Cl₂via cyclic voltammetry, with results displayed in Fig. 3f, Table 1 and Fig. S32, S33.† Both complexes exhibit two reversible oxidative peaks at 0.41/0.46 V and 0.89/0.98 V for 1, and 0.40/0.46 V and 0.94/1.05 V vs. ferricenium/ferrocene (Fc⁺/Fc) for 2, affirming their binuclear structures. The minor displacement in the oxidation peaks is linked to structural similarities. Oxidative peaks are tentatively assigned to the oxidation of the ppy and bppy chelated Pt and Pd cores. No reduction peak was observed down to −0.4 V vs. Fc⁺/Fc. The half-wave oxidation potentials (vs. Fc/Fc⁺) were used to determine the (HOMO energy levels, with the LUMO) levels calculated by subtracting the bandgap energy derived from UV/visible absorption spectroscopy. The calculated HOMO levels for 1 and 2 are −4.93 and −4.95 eV, respectively, with LUMO levels at −2.29 and −2.31 eV. Their similar HOMO and LUMO levels, estimated from comparable electrochemical and photophysical properties, are noteworthy.

Electroluminescence

Due to the good photophysical properties of these two complexes, they were used as emitting materials for OLED device fabrication using a vapor deposition process. The OLED device structure is as follows: ITO/HAT-CN (6 nm)/HAT-CN (0.3 wt%): TAPC (50 nm)/TCTA: 1 (X wt%) or 2 (X wt.%) (10 nm)/26DCzPPy: 1 (X wt%) or 2 (X wt%) (10 nm)/Tm3PyP26PyB (60 nm)/LiF (1 nm)/Al (100 nm), emitting reddish-orange light. Neat 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) serves as the hole injection layer (HIL), with di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) doped with HAT-CN as the hole transport/electron blocking layer (HTL/EBL). The emitting layer (EML) employs 4,4′,4′′-tri(N-carbazolyl) triphenylamine (TCTA) and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) as hosts, with 1 and 2 as dopants at varying concentrations (X = 2, 4, 6). 3,5-Tris(6-(3-(pyridin-3-yl)phenyl)pyridin-2-yl) (Tm3PyP26PyB) serves as the electron transport layer (ETL) due to its excellent electron mobility and a low-lying HOMO level (−6.5 eV). The cathode layers are comprised of LiF and aluminium.

As two reddish-orange emitting dopants, 1 and 2 were doped into TCTA and 26DCzPPy at different concentration ranges, respectively. At a lower concentration of 2 wt%, the emission of two hosts, TCTA and 26DCzPPy, in EL spectra of 1 (see Fig. 4d, shoulder peak at 402–469 nm) indicated an insufficient energy transfer from host to emitter, resulting in relatively low efficiency at a lower dopant concentration (see Table 2). As dopant concentration increased, host emission in the devices was eliminated, enhancing their efficiency. As depicted in Fig. 4g, for devices based on 2, no significant host emission was observed at any concentration, indicating effective energy transfer from the host to 2 and successful suppression of intermolecular interactions by the bulky ligands. Fig. 4e and h show that under a specific driving voltage, the current density of both devices decreased as the doping concentration reduced, suggesting that direct trapping may be the primary emission mechanism. Additionally, the luminance of both devices increased with higher doping concentrations. At a 4 wt% doping concentration in devices based on 1 and 2, maximum EQEs of 10.5% and 10.9% were achieved, respectively, with luminances rising to 6004 cdm⁻² and 6734 cdm⁻². The luminescence images of the OLED device are shown in Fig. S40 and S41.†

Fig. 4 (a) Proposed energy-level diagram. (b) Molecular structures of each layer. (c) Device structure (d) Normalized EL spectra on 1. (e) Current density–voltage–luminance (J–V–L) characteristics on 1. (f) EQE-luminance characteristics of OLEDs based on 1 with various concentrations. (g) Normalized EL spectra on 2. (h) Current density–voltage–luminance (J–V–L) characteristics on 2. (i) EQE-luminance characteristics of OLEDs based on 2 with various concentrations.

Table 2 Key performance metrics of OLED devices containing 1 and 2

Complex	Concentration	L ^a (cd m^−2)	EQE ^b (%)	CE ^b (cd A^−1)	PE ^b (lm W^−1)	CIE ^c (x, y)	λ max ^d (nm)	FWHM ^e (nm)
a Maximum luminance. b Maximum value. c CIE coordinates at 10 mA cm^−2. d Maximum peak of the EL emission at 100 cd m^−2. e FWHM of the EL emission peak at 100 cd m^−2.
1	2 wt%	3609	7.0	7.2	7.0	(0.57, 0.37)	640	120
	4 wt%	6004	10.5	10.0	10.1	(0.60, 0.38)	633	113
	6 wt%	6518	7.6	7.0	6.5	(0.62, 0.37)	640	118
2	2 wt%	4798	7.5	8.0	7.9	(0.59, 0.37)	636	109
	4 wt%	6734	10.9	10.0	10.7	(0.62, 0.37)	637	109
	6 wt%	7213	8.0	8.0	8.1	(0.62, 0.37)	638	109

---

Conclusions

We synthesized two heteronuclear Pt–Pd complexes, 1 and 2, for the first time. These complexes, featuring primary cyclometallating ligands of 2-[(1,1′-biphenyl)-3-yl]-4-phenylpyridine and phenylpyridine, along with two clamp ligands of N,N′-diphenylformamidine, were thoroughly characterized using standard small molecule techniques to confirm their structures. Their molecular architectures were elucidated via single crystal X-ray diffraction, revealing Pt–Pd distances of 2.8557(5) and 2.8548(6) Å for 1 and 2, respectively, indicating strong bimetallic interactions. Both complexes demonstrated robust photophysical properties with triplet emissions at 656 nm and 669 nm and quantum efficiencies of 40.5% and 48.1%, respectively. Their performance, comparable to homonuclear Pt–Pt and Pd–Pd complexes, underscores their potential in OLED applications. Despite initial trials exhibiting maximum external quantum efficiencies (EQE_max) of 10.5% with CIE coordinates of (0.60, 0.38) for 1 and 10.9% with CIE coordinates of (0.62, 0.37) for 2, these results mark the beginning of an ongoing exploration into the potential applications of heteronuclear bimetallic Pt–Pd complexes. With the formation heteronuclear Pt–Pd complexes bearing intensive Pt–Pd interactions, they are expected to be widely used in OLED and photocatalysis in the foreseeable future.

Data availability

Data supporting this study are included within the article and/or ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

Yuzhen Zhang acknowledges Prof. Liang Zhou at Changchun Institute of Applied Chemistry Chinese Academy of Sciences for providing the facility to perform OLED studies.

National Natural Science Foundation of China (22165002), Guangxi Natural Science Foundation (AD20238043, 2018GXNSFDA281002), Xiangsihu Young Scholars Innovative Research Team of Guangxi Minzu University (2020RSCXSHQN01).

References

1. S. Culham, P.-H. Lanoë, V. L. Whittle, M. C. Durrant, J. A. G. Williams and V. N. Kozhevnikov, Highly Luminescent Dinuclear Platinum(II) Complexes Incorporating Bis-Cyclometallating Pyrazine-Based Ligands: A Versatile Approach to Efficient Red Phosphors, Inorg. Chem., 2013, 52, 10992–11003.
2. Y. Zhu, K. Luo, L. Zhao, H. Ni and Q. Li, Binuclear platinum(II) complexes based on 2-mercaptobenzothiazole 2-mercaptobenzimidazole and 2-hydroxpyridine as brigding ligands: Red and near-infrared luminescence originated from MMLCT transition, Dyes Pigm., 2017, 145, 144–151.
3. A. S.-Y. Law, L. C.-C. Lee, K. K.-W. Lo and V. W.-W. Yam, Aggregation and Supramolecular Self-Assembly of Low-Energy Red Luminescent Alkynylplatinum(II) Complexes for RNA Detection, Nucleolus Imaging, and RNA Synthesis Inhibitor Screening, J. Am. Chem. Soc., 2021, 143, 5396–5405.
4. D. Gómez de Segura, A. Corral-Zorzano, E. Alcolea, M. T. Moreno and E. Lalinde, Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis, Inorg. Chem., 2024, 63, 1589–1606.
5. J.-J. Zhong, W.-P. To, Y. Liu, W. Lu and C.-M. Che, Efficient acceptorless photo-dehydrogenation of alcohols and N-heterocycles with binuclear platinum(II) diphosphite complexes, Chem. Sci., 2019, 10, 4883–4889.
6. Y. Sun, B. Liu, Y. Guo, X. Chen, Y.-T. Lee, Z. Feng, C. Adachi, G. Zhou, Z. Chen and X. Yang, Developing Efficient Dinuclear Pt(II) Complexes Based on the Triphenylamine Core for High-Efficiency Solution-Processed OLEDs, ACS Appl. Mater. Interfaces, 2021, 13, 36020–36032.
7. M. Z. Shafikov, R. Daniels, P. Pander, F. B. Dias, J. A. G. Williams and V. N. Kozhevnikov, Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs, ACS Appl. Mater. Interfaces, 2019, 11, 8182–8193.
8. B. Ma, P. I. Djurovich, S. Garon, B. Alleyne and M. E. Thompson, Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light-Emitting Diodes, Adv. Funct. Mater., 2006, 16, 2438–2446.
9. Z.-Q. Zhu, C.-D. Park, K. Klimes and J. Li, Highly Efficient Blue OLEDs Based on Metal-Assisted Delayed Fluorescence Pd(II) Complexes, Adv. Opt. Mater., 2019, 7, 1801518.
10. J. Song, H. Xiao, L. Fang, L. Qu, X. Zhou, Z.-X. Xu, C. Yang and H. Xiang, Highly Phosphorescent Planar Chirality by Bridging Two Square-Planar Platinum(II) Complexes: Chirality Induction and Circularly Polarized Luminescence, J. Am. Chem. Soc., 2022, 144, 2233–2244.
11. M. Mao, H. Li, K.-W. Lo, D. Zhang, L. Duan, S. Xu, Q. Wan, K. Tan, P. An, G. Cheng and C.-M. Che, Color-Tunable Organic Light-Emitting Diodes with Single Pt (O^N^C^N)-Dibenzofuran Emitter Exhibiting High External Quantum Efficiency of ≈30% and Superior Operational Lifetime, Adv. Mater., 2024, 36, 2311020.
12. M. Xue, T.-L. Lam, G. Cheng, W. Liu, K.-H. Low, L. Du, S. Xu, F.-F. Hung, D. L. Phillips and C.-M. Che, Exceedingly Stable Luminescent Dinuclear Pt(II) Complexes with Ditopic Formamidinate Bridging Ligands for High-Performance Red and Deep-Red OLEDs with LT97 up to 2446 h at 1000 cd m⁻², Adv. Opt. Mater., 2022, 10, 2200741.
13. K.-W. Lo, G. S. M. Tong, G. Cheng, K.-H. Low and C.-M. Che, Dinuclear PtII Complexes with Strong Blue Phosphorescence for Operationally Stable Organic Light-Emitting Diodes with EQE up to 23 % at 1000 cd m⁻², Angew. Chem., Int. Ed., 2022, 61, e202115515.
14. C.-M. Hung, S.-F. Wang, W.-C. Chao, J.-L. Li, B.-H. Chen, C.-H. Lu, K.-Y. Tu, S.-D. Yang, W.-Y. Hung, Y. Chi and P.-T. Chou, High-performance near-infrared OLEDs maximized at 925 nm and 1022 nm through interfacial energy transfer, Nat. Commun., 2024, 15, 4664.
15. S.-F. Wang, C.-C. Wu, P.-T. Chou, Y.-C. Kung, W.-Y. Hung, C.-J. Yu, C.-H. Yeh, F. Zhou, J. Yan and Y. Chi, Pt(II) phosphors with dual 1,6-naphthyridin-5-yl pyrazolate chelates and non-doped organic light emitting diodes, J. Mater. Chem. C, 2024, 12, 16608–16616.
16. H.-S. Lo, S.-K. Yip, K. M.-C. Wong, N. Zhu and V. W.-W. Yam, Selective Luminescence Chemosensing of Potassium Ions Based on a Novel Platinum(II) Alkynylcalix[4]crown-5 Complex, Organometallics, 2006, 25, 3537–3540.
17. K. M.-C. Wong and V. W.-W. Yam, Luminescence platinum(II) terpyridyl complexes—From fundamental studies to sensory functions, Coord. Chem. Rev., 2007, 251, 2477–2488.
18. M. J. Bryant, J. M. Skelton, L. E. Hatcher, C. Stubbs, E. Madrid, A. R. Pallipurath, L. H. Thomas, C. H. Woodall, J. Christensen, S. Fuertes, T. P. Robinson, C. M. Beavers, S. J. Teat, M. R. Warren, F. Pradaux-Caggiano, A. Walsh, F. Marken, D. R. Carbery, S. C. Parker, N. B. McKeown, R. Malpass-Evans, M. Carta and P. R. Raithby, A rapidly-reversible absorptive and emissive vapochromic Pt(II) pincer-based chemical sensor, Nat. Commun., 2017, 8, 1800.
19. H. Matsukawa, M. Yoshida, T. Tsunenari, S. Nozawa, A. Sato-Tomita, Y. Maegawa, S. Inagaki, A. Kobayashi and M. Kato, Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica, Sci. Rep., 2019, 9, 15151.
20. I. A. P. Linares, M. S. Uría, M. A. S. Graminha, B. A. Iglesias and A. M. A. Velásquez, Antileishmanial activity of tetra-cationic porphyrins with peripheral Pt(II) and Pd(II) complexes mediated by photodynamic therapy, Photodiagn. Photodyn. Ther., 2023, 42, 103641.
21. P. Sen and T. Nyokong, A novel axially palladium(II)-Schiff base complex substituted silicon(IV) phthalocyanine: Synthesis, characterization, photophysicochemical properties and photodynamic antimicrobial chemotherapy activity against Staphylococcus aureus, Polyhedron, 2019, 173, 114135.
22. B. Ma, J. Li, P. I. Djurovich, M. Yousufuddin, R. Bau and M. E. Thompson, Synthetic Control of Pt⋯Pt Separation and Photophysics of Binuclear Platinum Complexes, J. Am. Chem. Soc., 2005, 127, 28–29.
23. C. K. Zhou, Y. Tian, Z. Yuan, M. G. Han, J. Wang, L. Zhu, M. S. Tameh, C. Huang and B. W. Ma, Precise Design of Phosphorescent Molecular Butterflies with Tunable Photoinduced Structural Change and Dual Emission, Angew. Chem., Int. Ed., 2015, 54, 9591–9595.
24. L. Qiao, X. Kong, K. Li, L. Yuan, Y. Shen, Y. Zhang and L. Zhou, Phosphorescent PdII–PdII Emitter-Based Red OLEDs with an EQEmax of 20.52%, Adv. Sci., 2024, 11, 2404621.
25. P. Rajakannu, W. Lee, S. Park, H. S. Kim, H. Mubarok, M. H. Lee and S. Yoo, Molecular Engineering for Shortening the Pt⋯Pt Distances in Pt(II) Dinuclear Complexes and Enhancing the Efficiencies of these Complexes for Application in Deep-Red and Near-IR OLEDs, Adv. Funct. Mater., 2023, 33, 2211853.
26. L. Ma, L. Li and G. Zhu, Platinum-containing heterometallic complexes in cancer therapy: advances and perspectives, Inorg. Chem. Front., 2022, 9, 2424–2453.
27. E. R. Jamieson and S. J. Lippard, Structure, Recognition, and Processing of Cisplatin−DNA Adducts, Chem. Rev., 1999, 99, 2467–2498.
28. I. Russo Krauss, G. Ferraro and A. Merlino, Cisplatin–Protein Interactions: Unexpected Drug Binding to N-Terminal Amine and Lysine Side Chains, Inorg. Chem., 2016, 55, 7814–7816.
29. A. Casini and J. Reedijk, Interactions of anticancer Pt compounds with proteins: an overlooked topic in medicinal inorganic chemistry?, Chem. Sci., 2012, 3, 3135–3144.
30. S. Rottenberg, C. Disler and P. Perego, The rediscovery of platinum-based cancer therapy, Nat. Rev. Cancer, 2021, 21, 37–50.
31. D. C. Powers, E. Lee, A. Ariafard, M. S. Sanford, B. F. Yates, A. J. Canty and T. Ritter, Connecting Binuclear Pd(III) and Mononuclear Pd(IV) Chemistry by Pd–Pd Bond Cleavage, J. Am. Chem. Soc., 2012, 134, 12002–12009.
32. D. C. Powers, M. A. L. Geibel, J. E. M. N. Klein and T. Ritter, Bimetallic Palladium Catalysis: Direct Observation of Pd(III)−Pd(III) Intermediates, J. Am. Chem. Soc., 2009, 131, 17050–17051.
33. D. C. Powers, D. Benitez, E. Tkatchouk, W. A. Goddard III and T. Ritter, Bimetallic Reductive Elimination from Dinuclear Pd(III) Complexes, J. Am. Chem. Soc., 2010, 132, 14092–14103.
34. D. C. Powers, D. Y. Xiao, M. A. L. Geibel and T. Ritter, On the Mechanism of Palladium-Catalyzed Aromatic C−H Oxidation, J. Am. Chem. Soc., 2010, 132, 14530–14536.
35. D. C. Powers and T. Ritter, Bimetallic Redox Synergy in Oxidative Palladium Catalysis, Acc. Chem. Res., 2012, 45, 840–850.
36. D. C. Powers and T. Ritter, A Transition State Analogue for the Oxidation of Binuclear Palladium(II) to Binuclear Palladium(III) Complexes, Organometallics, 2013, 32, 2042–2045.
37. J. E. Bercaw, A. C. Durrell, H. B. Gray, J. C. Green, N. Hazari, J. A. Labinger and J. R. Winkler, Electronic Structures of PdII Dimers, Inorg. Chem., 2010, 49, 1801–1810.
38. C. Zou, J. Lin, S. Suo, M. Xie, X. Chang and W. Lu, Palladium(II) N-heterocyclic allenylidene complexes with extended intercationic Pd⋯Pd interactions and MMLCT phosphorescence, Chem. Commun., 2018, 54, 5319–5322.
39. Q. Wan, J. Yang, W.-P. To and C.-M. Che, Strong metal–metal Pauli repulsion leads to repulsive metallophilicity in closed-shell d8 and d10 organometallic complexes, Proc. Natl. Acad. Sci. U. S. A., 2021, 118, e2019265118.
40. H. J. Park, C. L. Boelke, P. H.-Y. Cheong and D.-H. Hwang, Dinuclear Pt(II) Complexes with Red and NIR Emission Governed by Ligand Control of the Intramolecular Pt–Pt Distance, Inorg. Chem., 2022, 61, 5178–5183.
41. M. Chaaban, Y.-C. Chi, M. Worku, C. Zhou, H. Lin, S. Lee, A. Ben-Akacha, X. Lin, C. Huang and B. Ma, Thiazol-2-thiolate-Bridged Binuclear Platinum(II) Complexes with High Photoluminescence Quantum Efficiencies of up to Near Unity, Inorg. Chem., 2020, 59, 13109–13116.
42. V. Sicilia, L. Arnal, D. Escudero, S. Fuertes and A. Martin, Chameleonic Photo- and Mechanoluminescence in Pyrazolate-Bridged NHC Cyclometalated Platinum Complexes, Inorg. Chem., 2021, 60, 12274–12284.
43. Q. Wan, W.-P. To, X. Chang and C.-M. Che, Controlled Synthesis of PdII and PtII Supramolecular Copolymer with Sequential Multiblock and Amplified Phosphorescence, Chem, 2020, 6, 945–967.
44. T. Theiss, S. Buss, I. Maisuls, R. López-Arteaga, D. Brünink, J. Kösters, A. Hepp, N. L. Doltsinis, E. A. Weiss and C. A. Strassert, Room-Temperature Phosphorescence from Pd(II) and Pt(II) Complexes as Supramolecular Luminophores: The Role of Self-Assembly, Metal–Metal Interactions, Spin–Orbit Coupling, and Ligand-Field Splitting, J. Am. Chem. Soc., 2023, 145, 3937–3951.
45. Y. Luo, Y. Cheng, D. Zhang, X. Mei, D. Tang, J. Hu and T. Luo, Controlling the Triplet Potential Energy Surface of Bimetallic Platinum(II) Complex by Constructing Structure–Property Relationship: A Theoretical Exploration, Inorg. Chem., 2023, 62, 2440–2455.
46. S. F. Wang, L.-W. Fu, Y.-C. Wei, S.-H. Liu, J.-A. Lin, G.-H. Lee, P.-T. Chou, J.-Z. Huang, C.-I. Wu, Y. Yuan, C.-S. Lee and Y. Chi, Near-Infrared Emission Induced by Shortened Pt–Pt Contact: Diplatinum(II) Complexes with Pyridyl Pyrimidinato Cyclometalates, Inorg. Chem., 2019, 58, 13892–13901.
47. Y. Ai, Y. Li, M. H.-Y. Chan, G. Xiao, B. Zou and V. W.-W. Yam, Realization of Distinct Mechano- and Piezochromic Behaviors via Alkoxy Chain Length-Modulated Phosphorescent Properties and Multidimensional Self-Assembly Structures of Dinuclear Platinum(II) Complexes, J. Am. Chem. Soc., 2021, 143, 10659–10667.
48. W.-P. To, Q. Wan, G. S. M. Tong and C.-M. Che, Recent Advances in Metal Triplet Emitters with d6, d8, and d10 Electronic Configurations, Trends Chem., 2020, 2, 796–812.
49. S. Roy, A. A. Lopez, J. E. Yarnell and F. N. Castellano, Metal–Metal-to-Ligand Charge Transfer in Pt(II) Dimers Bridged by Pyridyl and Quinoline Thiols, Inorg. Chem., 2022, 61, 121–130.
50. J. Liu, C.-J. Yang, Q.-Y. Cao, M. Xu, J. Wang, H.-N. Peng, W.-F. Tan, X.-X. Lü and X.-C. Gao, Synthesis, crystallography, phosphorescence of platinum complexes coordinated with 2-phenylpyridine and a series of β-diketones, Inorg. Chim. Acta, 2009, 362, 575–579.
51. P. Pinter, J. Soellner and T. Strassner, Metallophilic Interactions in Bimetallic Cyclometalated Platinum(II) N-Heterocyclic Carbene Complexes, Eur. J. Inorg. Chem., 2021, 2021, 3104–3107.
52. Y. Shen, X. Kong, F. Yang, H.-D. Bian, G. Cheng, T. R. Cook and Y. Zhang, Deep Blue Phosphorescence from Platinum Complexes Featuring Cyclometalated N-Pyridyl Carbazole Ligands with Monocarborane Clusters (CB11H12−), Inorg. Chem., 2022, 61, 16707–16717.
53. M. A. Esteruelas, S. Moreno-Blázquez, M. Oliván and E. Oñate, N,C,N-Pincers in Platinum Bimetallic Complexes: Influence of the Pincer and Bridging Ligands on the Metal–Metal Bond and the Photophysical Properties, Inorg. Chem., 2024, 63, 14482–14494.
54. V. Sicilia, J. Forniés, J. M. Casas, A. Martín, J. A. López, C. Larraz, P. Borja, C. Ovejero, D. Tordera and H. Bolink, Highly Luminescent Half-Lantern Cyclometalated Platinum(II) Complex: Synthesis, Structure, Luminescence Studies, and Reactivity, Inorg. Chem., 2012, 51, 3427–3435.
55. W. Xiong, F. Meng, C. You, P. Wang, J. Yu, X. Wu, Y. Pei, W. Zhu, Y. Wang and S. Su, Molecular isomeric engineering of naphthyl-quinoline-containing dinuclear platinum complexes to tune emission from deep red to near infrared, J. Mater. Chem. C, 2019, 7, 630–638.
56. C.-M. Che, M. Xue and T.-L. Lam, Dinuclear Platinum(II) Red Emitters For Oled Applications, 2022.
57. Y. Zhang, J. Miao, J. Xiong, K. Li and C. Yang, Rigid Bridge-Confined Double-Decker Platinum(II) Complexes Towards High-Performance Red and Near-Infrared Electroluminescence, Angew. Chem., Int. Ed., 2022, 61, e202113718.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures, NMR, MS, IR, UV/visible absorption, phosphorescent data, CV, TGA, X-ray, and OLED figures. CCDC 2311650 and 2311651. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02842b

‡ These authors contributed equally to this work.

---