Narrow-band circularly polarized red electroluminescence in trinuclear clusters

Abstract

Through the elaborate design of an ethynyl-naphthalimide ligand with a phenylethylamino group as chiral origin, we report the synthesis, characterization, photophysical and electroluminescence properties of a pair of chiral PtAu₂ trinuclear clusters with high-efficiency circularly polarized narrow-band red emission. As demonstrated by experimental and theoretical studies, the phosphorescence of the trinuclear cluster originates from the intraligand (³IL) triplet state of ethynyl-naphthalimide having a chiral benzylamino moiety. The R/S-PtAu₂ clusters exhibit significant circularly polarized luminescence (CPL) properties with an asymmetry factor of approximately ±1 × 10⁻³. Organic light-emitting diodes (OLEDs) based on chiral PtAu₂ clusters demonstrate circularly polarized electroluminescence (CPEL) performance with an asymmetry factor of approximately ±7 × 10⁻⁴ and an external quantum efficiency (EQE) of 13.7%. The devices exhibit narrow-band red emission and high color purity with CIE 1931 coordinates of (0.68, 0.32) and a full width at half maximum (FWHM) of 29 nm. Both circular dichroism (CD) and CPL studies demonstrate the excellent chiral absorption and emission characteristics of trinuclear cluster enantiomers in the ground state as well as in the excited state.

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Introduction

Circularly polarized luminescence (CPL) exhibits tremendous potential applications in various fields such as optical materials, 3D displays, optical sensors, and optoelectronic devices.^1–5 Traditionally, one can attain circularly polarized light by converting various colored lights through a polarizer and a quarter-wave plate, but the conversion process suffers from brightness loss and complicated device structures. Compared with traditional physical methods, chiral luminescent materials directly produce various circularly polarized lights. Furthermore, circularly polarized organic light-emitting diodes (CP-OLEDs) based on chiral phosphors are promising for applications in next-generation high-definition 3D displays in commercial fields.^6–8 Ideal CPL materials have the characteristics of large asymmetry factors (g_PL) and high luminescence quantum yields. In recent years, scientists have made great efforts for the development of various chiral luminescent materials,^9–15 including chiral conjugated polymers, chiral transition metal complexes, and chiral TADF materials. Among them, chiral transition metal coordination phosphors simultaneously utilize singlet and triplet excitons, theoretically achieving an internal quantum efficiency (IQE) of 100%. Thus, the strategy based on chiral metal coordination phosphors to make CP-OLEDs has been one of the best approaches for the development of next-generation high-definition 3D display technology.^16–22

Compared to the widely used phosphors based on mononuclear metal complexes, phosphorescent materials based on oligonuclear metal clusters have several advantages.^23–25 For instance, many oligonuclear metal clusters exhibit highly efficient phosphorescence owing to facile intersystem crossing (ISC) and excellent molecular rigidity. They are mostly accessible under mild synthetic conditions and are sufficiently stable and robust. Furthermore, the phosphorescence properties are readily tunable through ligand modification and metal substitution. However, metal cluster phosphors frequently exhibit characteristics of multifold emissive origins from metal-centered (MC), intraligand (IL), metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), and ligand-to-ligand charge transfer (LLCT) triplet states. The multifold radiative triplet states lead to broad emission bands due to the superimposition of multiple emission spectra with the full width at half-maximum (FWHM) exceeding 60 nm, making it extremely challenging to achieve high color purity. With the continuous growth in demand for ultra-high resolution and definition displays, high color purity CP-OLEDs are crucial for full-color displays.²⁶ To achieve the goal of high-definition 3D displays, it is necessary to develop highly efficient phosphors that simultaneously possess narrow-band emission and circularly polarized luminescence properties.

To address the issue of broad emission band owing to the existence of multiple emission origins in metal cluster phosphors, it is necessary to attain a single emission spectrum through effective energy transfer from various triplet states to the lowest-lying triplet state. In this work, taking advantage of ethynyl-naphthalimide ligands having chiral benzylamino moieties for the design of trinuclear PtAu₂ clusters, we have successfully achieved highly efficient narrow-band CPL with an FWHM of 25–29 nm, a luminescence efficiency of 72%, and a g_PL of ±1 × 10⁻³, as well as superior CP-OLEDs with a g_EL of ±7 × 10⁻⁴ and an external quantum efficiency (EQE) of 13.7%.

Results and discussion

Synthesis and characterization

Using an R/S-α-phenylethylamino moiety as chiral origin, we elaborately designed rigid-plane ethynyl-naphthalimide ligands R/S-Me₃SiC≡CNaph (ESI†). With them as ligands, we synthesized a pair of mononuclear platinum(II) enantiomers R/S-Pt(PPh₃)₂(C≡CNaph)₂ (R-Pt and S-Pt) by the reactions of R/S-HC≡CNaph with Pt(PPh₃)₂Cl₂ in the presence of NEt₃ and under the catalysis of CuI. Using R/S-Pt as synthetic precursors, we were able to obtain dTolmp-linked PtAu₂ trinuclear cluster enantiomers (Scheme 1), R/S-[PtAu₂(dTolmp)₂(C≡CNaph)₂] (R-PtAu₂1a, S-PtAu₂1b) in 80% yield by the reaction of R/S-Pt, dTolmp, Au(tht)Cl, and NH₄ClO₄ in a 1 : 2 : 2 : 2 molar ratio at ambient temperature.

Scheme 1 R-PtAu₂ (1a) and S-PtAu₂ (1b) trinuclear clusters.

We determined the structures of R-PtAu₂ (1a) and S-PtAu₂ (1b) cluster enantiomers by single crystal X-ray diffraction. Fig. 1 depicts the perspective views of PtAu₂ clusters 1a (Fig. 1a) and 1b (Fig. 1b) with their respective R and S configurations due to chiral α-phenylethylamino groups. The PtAu₂ cluster arises from one Pt and two Au atoms bound to double dTolmp and two deprotonated HC≡CNaph ligands through σ-bonding. The trans-arranged Pt(C≡CNaph)₂ moiety is supported by double dTolmp ligands through the middle P atoms, whereas the P atoms at both ends are bound to two Au centers in linear mode. The trinuclear cluster Au–Pt–Au (179.339(18)° for 1a and 179.52(5)° for 1b) exhibits a quasi-linear arrangement with Pt at the middle and Au at two sides. The Pt⋯Au distance (2.90–2.94 Å) is significantly smaller than the sum of van der Waals radii for Pt and Au, indicating a strong Pt–Au intermetallic interaction. The significant d⁸–d¹⁰ intermetallic interaction together with four five-membered rings greatly enforces the stability of the PtAu₂ cluster. The middle P atoms of two dTolmp coordinate with the Pt center in trans-orientation, while the P atoms at both ends are bound to two Au centers in quasi-linear mode, respectively. Two trans-arranged ethynyl-naphthalimide ligands coordinate to the Pt center to give a square-planar coordination geometry composed of trans-arranged C₂P₂ donors. Each Au center is bound to two P donors in a quasi-linear arrangement with the P–Au–P angle of 172.2(2)–174.6(2)°. The PtC₂P₂ plane is nearly co-planar to a naphthalimide plane with only 12.6° of the dihedral angle. Noticeably, steric hindrance from double dTolmp and ethynyl-naphthalimide ligands provides excellent protection or shield for the PtAu₂ cluster from non-radiative relaxation in the triplet state and has a positive impact on the phosphorescence efficiency (vide infra).

Fig. 1 Perspective views of trinuclear cluster enantiomers 1a (R-PtAu₂, (a)) and 1b (S-PtAu₂, (b)), plotted from X-ray crystallography. Hydrogen atoms and phenyl groups at dTolmp are omitted for clarity.

Photophysical and theoretical studies

For a comparison purpose and better assignment of the absorption and emission properties, the UV-Vis absorption and emission spectra of the ethynyl-naphthalimide ligand HC≡CNaph, mononuclear Pt precursor R-Pt(PPh₃)₂(C≡CNaph)₂ (R-Pt) and R-PtAu₂ cluster 1a in dichloromethane (1 × 10⁻⁵ M) solutions are depicted in Fig. 2.

Fig. 2 The UV-Vis absorption and photoluminescence spectra of HC≡CNaph, R-Pt(PPh₃)₂(C≡CNaph)₂ (R-Pt) and R-PtAu₂ cluster 1a in CH₂Cl₂ solutions (1 × 10⁻⁵ M) at ambient temperature.

The UV-Vis absorption of PtAu₂ clusters in dichloromethane solutions at below 300 nm is mainly from ligand-centered transitions. Absorption peaks of moderate intensity in the range of 300 to 380 nm are attributed to ethynyl-naphthalimide ligands. The strong and broad absorption band at 380–450 nm originates mostly from the ¹IL (intraligand) state of ethynyl-naphthalenediimide, mixed with ¹LLCT (ligand-to-ligand charge transfer) from ethynyl-naphthalenediimide to dTolmp, and ¹MC (metal-centered) character in the PtAu₂ cluster, as verified by TD-DFT studies (vide infra).

The emission of the ethynyl-naphthalimide ligand is peaked at 530 nm with a luminescence lifetime of 5.7 ns. The nanosecond scale lifetime confirms its fluorescence characteristics. In comparison, the mononuclear R-Pt precursor exhibits dual emission bands with a strong peak at 510 nm (τ_em = 2.8 ns) and a weak peak at 630 nm (τ_em = 6.2 μs). The emission band peaked at 510 nm having a lifetime similar to that of the ethynyl-naphthalimide ligand, implying that it is fluorescent. The emission band that peaked at 630 nm has a lifetime in the microsecond range, indicating its phosphorescence characteristics. Furthermore, the phosphorescence emission at 630 nm becomes progressively pronounced as the temperature gradually lowers. Obviously, the heavy atom effect of Pt in the mononuclear Pt precursor facilitates intersystem crossing from the lowest-lying singlet (S₁) state to the triplet (T₁) state so that it shows some extent of phosphorescence characteristics. However, since the energy level of the radiative triplet state is not much lower than that of the non-radiative d–d transition, triplet state deactivation is mostly conducted through a non-radiative process so that phosphorescence emission is quite weak.

In striking contrast, an R-PtAu₂ cluster 1a only displays phosphorescence emission peaked at 625 nm with a lifetime of 4.4 μs and a quantum yield of 28.0% without any fluorescence trace at 510–560 nm. Undoubtedly, the PtAu₂ cluster with significant Pt–Au interaction enhances the phosphorescence efficiency dramatically because the lowest-lying triplet state in the PtAu₂ cluster exhibits much lower energy than that of the non-radiative d–d transition, so that the deactivation process is predominated through phosphorescence radiation and it becomes strongly phosphorescent. On the other hand, compared with that in the mononuclear Pt precursor, the much higher phosphorescence efficiency of PtAu₂ clusters is also due to more facile intersystem crossing and better molecular rigidity upon the formation of PtAu₂ clusters through significant Pt–Au intermetallic contact.

Interestingly, whether mononuclear Pt precursors or trinuclear PtAu₂ clusters (Fig. 2) show the same narrow-band phosphorescence spectra centered at 625–630 nm with a full width at half maximum (FWHM) of 25–29 nm in both solutions and solid states, although the phosphorescence efficiency improves dramatically upon the formation of PtAu₂ clusters. Normally, such narrow-band emissions critical for high-definition displays can only be achieved for quantum dot materials or heteroatom polycyclic aromatic compounds.^27–30 Currently, commercially available OLED phosphors are mostly featured with broad-band emissions because of multiple emission origins in metal coordination phosphors. To address this issue, it is necessary to attain a single emission spectrum through an energy transfer approach so that the energy of multiple triplet states could be effectively transferred to the lowest-lying triplet state.²⁶ As depicted in Fig. 2, since the formation of PtAu₂ clusters does not induce an appreciable phosphorescence spectral change relative to that of the mononuclear Pt precursor, it is most likely that narrow-band phosphorescence originates from the lowest-lying triplet excited state centered at the ethynyl-naphthalenediimide ligand.

To clarify the phosphorescence origin and transition characteristics, we conducted TD-DFT studies on R/S-PtAu₂ clusters la and 1b in the ground state (S₀) and the lowest-lying triplet state (T₁), respectively. As indicated in Tables S2 and S3 (ESI†), since the hole and electron in the ground state concentrate mostly at ethynyl-naphthalimide ligands with much less population at dTolmp and metal centers, the electron migration involved in the lowest-energy absorption is localized at the ethynyl-naphthalimide ligands with a typical ¹IL transition, mixed with π (C≡CNaph) → π* (dTolmp) ¹LLCT and ¹MC characteristics. On the other hand, the hole and electron in the T₁ emission state focus entirely on the ethynyl-naphthalimide ligands as depicted in Fig. 3, so that the phosphorescence emission originates predominately from the ³IL transition within the ethynyl-naphthalimide ligand. Consequently, the phosphorescence origin in PtAu₂ clusters is totally from the ethynyl-naphthalimide ligand without the participation of dTolmp and metal centers. Nevertheless, the supporting effect of double dTolmp exerts a critical role in stabilizing PtAu₂ cluster structures and enhancing molecular rigidity so as to dramatically enhance the phosphorescence efficiency upon the formation of trinuclear clusters.

Fig. 3 The plots of holes and electrons in the T₁ emission transition state (isovalue = 0.04) for trinuclear clusters 1a (R-PtAu₂) and 1b (S-PtAu₂), obtained by the TD-DFT method at the PBE1PBE level.

Chiral-optical properties

Fig. 4 depicts the UV-Vis absorption and emission spectra together with the circular dichroism (CD) spectra of trinuclear clusters 1a (R-PtAu₂) and 1b (S-PtAu₂) in CH₂Cl₂ solutions, the CPL spectra in solid states, and the dependence of the g_PL factor on the wavelength.

Fig. 4 (a) The UV-Vis absorption and normalized emission spectra of trinuclear clusters 1a (R-PtAu₂) and 1b (S-PtAu₂) in CH₂Cl₂ solutions (1 × 10⁻⁵ M) at ambient temperature. (b) The CD spectra of trinuclear clusters 1a (R-PtAu₂) and 1b (S-PtAu₂) in CH₂Cl₂ solutions (1 × 10⁻⁵ M) at ambient temperature. (c) The CPL spectra of trinuclear clusters 1a (R-PtAu₂) and 1b (S-PtAu₂) in the solid state. (d) The g_PL factor versus the wavelength curves of trinuclear clusters 1a (R-PtAu₂) and 1b (S-PtAu₂).

The PtAu₂ cluster enantiomers display similar narrow-band red emission (Fig. 4a) peaked at ca. 625 nm (FWHM of 29 nm for 1a and 25 nm for 1b). The quantum yield is 28% for 1a and 27% for 1b in deaerated CH₂Cl₂ solutions at ambient temperature. The occurrence of vibronic-structured emission bands with a vibrational progression spacing of 1185 cm⁻¹ arises most likely from the typical vibration modes of the naphthalenediimide moiety in the ground state, which coincides perfectly with the phosphorescence characteristics of the ³IL triplet state.

In the CD spectra (Fig. 4b), enantiomers 1a and 1b show mirror-image circular dichroism signals at 250–450 nm in the ground state, which corresponds well to their optically active enantiomer characteristics. Moreover, we utilized CPL techniques to investigate the chiral properties of R- and S-PtAu₂ cluster enantiomers in the excited states. As depicted in Fig. 4c, PtAu₂ cluster enantiomers 1a and 1b show perfectly symmetrical and reversed CPL signals in the range of 550–750 nm. In contrast to a positive spectrum with a dissymmetry factor g_PL of 1.0 × 10^–3 for the R-PtAu₂ cluster 1a (Fig. 4d), S-PtAu₂ cluster 1b reveals a negative signal with a g_PL of −1.0 × 10^–3. Since CPL spectra coincide with their emission spectra, this demonstrates well the chiral-optical properties of enantiomers in the excited state.

Circularly polarized electroluminescence

Since R- and S-PtAu₂ cluster enantiomers display superior CPL characteristics as well as highly efficient narrow-band emission with excellent red chromaticity, they are ideal candidates as phosphors for red-emitting OLEDs through a solution process. We chose R-PtAu₂ cluster 1a to optimize solution-processed OLEDs. We used PEDOT:PSS and Poly-TPD as hole injection and hole transport materials to prepare the corresponding hole injection and hole transport layers through a spin-coating process using aqueous and chlorobenzene solutions, respectively.

To achieve better carrier balance and confine perfectly excitons in the emitting layer, we doped red-emitting 1a to blended host materials composed of a hole transport host and an electron transport host. OXD-7 having two bulky tert-butyl groups serves as an electron transfer host to prevent the crystallization of the PtAu₂ cluster in the emission layer. We screened commercially available hole transport materials such as TCTA, mCP and 2.6-DCzPPy, revealing that TCTA was better than other materials. Then we attempted to optimize doping ratios of PtAu₂, TCTA and OXD-7 and found that the optimal doping ratios were 10% PtAu₂, 45% TCTA, and 45% OXD-7. Finally, we improved the device by screening commercially available materials used for the electron transport layer (ETL) such as TmPyPB, TPBi and BmPyPB, demonstrating that the use of TmPyPB provided the best OLED performance. Then we attained the optimal device configuration ITO/PEDOT:PSS (50 nm)/Poly-TPD (15 nm)/10% PtAu₂:45% TCTA:45% OXD-7 (50 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). The device afforded a peak current efficiency of 14.5 cd A⁻¹, a power efficiency of 12.6 lm W⁻¹, and an EQE of 13.7%.

Fig. 5 illustrates the energy diagram of the materials used in device optimization as well as the electroluminescence behavior of the optimal device. Except for the slight enhancement of the vibronic-structured bands, electroluminescence spectra display narrow-band emission centered at 625 nm with an FWHM of 29 nm, which coincides well with the photoluminescence spectra in the corresponding emitting layers. The chromatic coordinates of electroluminescence are (0.68, 0.32) with perfect pure red light chromaticity, which is useful for high-definition displays. Furthermore, the dependence of the circularly polarized electroluminescence (CPEL) spectrum on the wavelength for the R-PtAu₂ cluster 1a corresponds well to that for the S-PtAu₂ cluster 1b except that the latter shows the same negative values. The dissymmetry factor g_EL of circularly polarized electroluminescence is 7.0 × 10⁻⁴ for R-PtAu₂ cluster 1a and −7.0 × 10⁻⁴ for S-PtAu₂ cluster 1b. Thus, solution-processed OLEDs demonstrate superior circularly polarized electroluminescence behavior due to the chiral characteristics of R- and S-PtAu₂ cluster enantiomers.

Fig. 5 (a) The energy diagram of the materials involved in solution-processed OLEDs based on 1a. (b) The current density–voltage–luminance characteristics in solution-processed OLEDs based on 1a. (c) The dependence of CE/EQE on current density for solution-processed OLEDs based on 1a. (d) The electroluminescence and photoluminescence spectra of solution-processed OLEDs based on 1a and 1b. (e) The CPEL spectra of solution-processed OLEDs based on 1a and 1b. (f) The g_EL factor versus wavelengths for 1a and 1b.

Conclusions

Through an elaborate ligand design by attaching a chiral center to the ethynyl-naphthalenediimide ligand, we prepared a pair of R- and S-PtAu₂ cluster enantiomers so as to achieve highly efficient narrow-band red emission with a FWHM of 25–29 nm and excellent CPL with a dissymmetry factor g_PL of ±1.0 × 10^–3. The experimental and theoretical studies indicate that circularly polarized narrow-band red emission stems from the ³IL triplet state of ethynyl-naphthalenediimide. Solution-processed OLEDs show narrow-band red emission with a FWHM of 29 nm and chromatic coordinates of (0.68, 0.32), which is perfect for high-definition displays. Due to the internal chiral characteristics of the enantiomers, R/S-PtAu₂ clusters exhibit reciprocally corresponding electroluminescent spectra with mutually converse CPEL signals and a dissymmetry factor of ±7.0 × 10⁻⁴. Anyway, this work provides a new strategy for achieving high-efficiency narrow-band circularly polarized red electroluminescence.

Experimental

General procedures and materials

All operations were conducted under a dry argon atmosphere using standard Schlenk techniques and vacuum-line systems unless otherwise specified. The solvents were dried, distilled, and degassed prior to use. Bis(di-o-tolylphosphino-methyl)phenylphosphine (dTolmp)³¹ was prepared by synthetic procedures described in the literature.

Synthesis of Pt(PPh₃)₂(C≡CNaph)₂ (Pt)

To a Schlenk flask were added Pt(PPh₃)₂Cl₂ (198 mg, 0.25 mmol), HC≡CNaph (244 mg, 0.75 mmol), CuI (1 mg), NEt₃ (2 mL), and CHCl₃ (40 mL). The solution mixture was stirred at 50 °C and the reaction was monitored by thin-layer chromatography. Two days later, the solvents were removed, and the brown residue was chromatographed on a silica gel column using dichloromethane–petroleum (v/v = 20 : 1) as an eluent, affording the product as a brown solid. Yield: 82%.

R-Pt

¹H NMR (600 MHz, CDCl₃): δ 8.33 (d, J = 7.2 Hz, 2H), 8.12 (d, J = 7.7 Hz, 2H), 7.82 (d, J = 5.6 Hz, 12H), 7.44 (d, J = 7.8 Hz, 4H), 7.36–7.27 (m, 24H), 7.20 (t, J = 7.4 Hz, 2H), 7.15 (t, J = 7.8 Hz, 2H), 6.49 (dt, J = 14.2, 7.4 Hz, 4H), 1.93 (d, J = 7.1 Hz, 6H). ³¹P NMR (243 MHz, chloroform-d): δ 19.6 (t, 3P, J_Pt–P = 1293 Hz).

S-Pt

¹H NMR (400 MHz, CDCl₃): δ 8.36 (dd, J = 7.3, 1.3 Hz, 2H), 8.14 (d, J = 7.8 Hz, 2H), 7.85 (dtd, J = 7.9, 5.9, 1.6 Hz, 12H), 7.49–7.45 (m, 4H), 7.40–7.28 (m, 24H), 7.24–7.15 (m, 4H), 6.52 (dd, J = 11.0, 7.5 Hz, 4H), 1.95 (d, J = 7.1 Hz, 6H). ³¹P NMR (243 MHz, chloroform-d): δ 19.6 (t, 2P, J_Pt–P = 1295).

Synthesis of [PtAu₂(dTolmp)₂(C≡CNaph)₂] (PtAu₂)

Under an argon atmosphere, dTolmp (41 mg, 0.073 mmol), Au(tht)Cl (23.4 mmol, 0.073 mmol), and NH₄ClO₄ (0.1 mg, 0.08 mmol) were stirred in CH₂Cl₂ (20 mL) at room temperature for 2 h. Then to the solution was added Pt(PPh₃)₂(C≡CNaph)₂ (50 mg, 0.0365 mmol). After stirring for 10 h, the solvent was removed under reduced pressure. The residue was chromatographed on a silica gel column using CH₂Cl₂–acetone (50 : 1) as an eluent to afford a PtAu₂ cluster as a yellow solid. Yield: 80%.

R-PtAu₂ (1a)

Anal. Calcd for C₁₁₆H₁₀₂O₁₂P₆N₂Cl₂Au₂Pt: C, 54.38; H, 4.01; N, 1.09. Found: C, 54.62; H, 3.83; N, 1.07. HRMS m/z (%): 1181.2552 (100) [M − 2ClO₄]²⁺ (calcd: 1181.2631). ¹H NMR (400 MHz, CDCl₃): δ 8.59 (s, 2H), 8.51 (t, J = 7.8 Hz, 4H), 8.16 (ddd, J = 28.6, 14.5, 7.0 Hz, 10H), 7.81 (t, J = 7.7 Hz, 2H), 7.54–7.44 (m, 8H), 7.35 (t, J = 7.5 Hz, 4H), 7.25–7.12 (m, 10H), 7.07–6.97 (m, 10H), 6.90 (t, J = 7.6 Hz, 4H), 6.70 (ddd, J = 30.8, 15.0, 8.2 Hz, 8H), 6.55 (t, J = 7.1 Hz, 2H), 5.36 (d, J = 12.6 Hz, 4H), 4.97 (p, J = 7.5 Hz, 4H), 2.15–2.09 (m, 12H), 2.02 (t, J = 8.4 Hz, 18H). ³¹P NMR (162 MHz, CDCl₃): δ 21.7–20.9 (m 4P, J_P–P = 32 Hz), 4.7–3.8 (m, 2P, J_P–P = 39 Hz, J_Pt–P = 1270 Hz). IR (cm⁻¹): 2092 w (C≡C).

S-PtAu₂ (1b)

Anal. Calcd for C₁₁₆H₁₀₂O₁₂P₆N₂Cl₂Au₂Pt: C, 54.38; H, 4.01; N, 1.09. Found: C, 54.12; H, 3.93; N, 1.09. HRMS m/z (%): 1181.2529 (100) [M − 2ClO₄]²⁺ (calcd: 1181.2631). ¹H NMR (600 MHz, CDCl₃): δ 8.59 (s, 2H), 8.52 (dd, J = 10.1, 7.6 Hz, 4H), 8.23–8.12 (m, 8H), 8.10 (s, 2H), 7.81 (t, J = 7.7 Hz, 2H), 7.54–7.51 (m, 4H), 7.47 (dd, J = 13.6, 5.1 Hz, 4H), 7.35 (t, J = 7.8 Hz, 4H), 7.25–7.13 (m, 10H), 7.05 (p, J = 10.5, 9.2 Hz, 8H), 6.98 (t, J = 7.5 Hz, 2H), 6.90 (t, J = 7.6 Hz, 4H), 6.79–6.75 (m, 4H), 6.71 (t, J = 7.6 Hz, 2H), 6.66 (t, J = 7.7 Hz, 2H), 6.56 (q, J = 7.1 Hz, 2H), 5.53–5.31 (m, 4H), 4.96 (dq, J = 13.8, 6.6 Hz, 4H), 2.15–2.09 (m, 12H), 2.04–1.99 (m, 18H). ³¹P NMR (162 MHz, CDCl₃): δ 21.5–20.9 (m 4P, J_P–P = 44 Hz), 5.0–4.4 (m, 2P, J_P–P = 38 Hz, J_Pt–P = 1257 Hz). IR (cm⁻¹): 2092 w (C≡C).

Device preparation

The OLED structure is ITO/PEDOT:PSS (50 nm)/Poly-TPD (15 nm)/host materials:PtAu₂ (50 nm)/ETL (40 nm)/LiF (1 nm)/Al (100 nm). Indium–tin oxide (ITO) glass substrates were successively cleaned by sonication in a glass detergent, acetone, isopropanol and deionized water, and finally treated with UV-ozone for 15 min. The hole injection layer (PEDOT:PSS) was spin-coated on the pre-processed substrates through a 0.22 μm filter and oven dried at 130 °C for 15 min to form a film of 50 nm thickness. The hole transport layer was prepared through a spin-coating process using a chlorobenzene solution of Poly-TPD, which was then annealed at 120 °C for 15 min to produce a film of 15 nm thickness. The emitting layer was spin-coated by using a CH₂Cl₂ solution (5 mg mL⁻¹) through a filter with blend host materials and a PtAu₂ complex. After that, the substrates were diverted to a vacuum chamber. A 40 nm electron-transporting layer and 1 nm LiF and 100 nm Al were evaporated consecutively at a base pressure of less than 4 × 10⁻⁴ Pa. Current density–voltage–luminance characteristics were measured on a Keithley 2400 source meter and a calibrated silicon photodiode. The electroluminescence (EL) spectra were recorded on a HORIBA Jobin–Yvon FluoroMax-4 spectrometer.

Physical measurements

UV-Vis absorption spectra were recorded on a UV-2600i spectrophotometer. Circular dichroism (CD) absorption spectra were recorded on a JASCO J-1500. Infrared spectra (IR) were recorded on a Bruker VERTEX 70 FT-IR spectrophotometer using an attenuated total reflectance method. High resolution mass spectrometry (HRMS) was performed on a Bruker Impact II Q-TOF mass spectrometer using dichloromethane and methanol mixtures as mobile phases. ¹H and ³¹P NMR spectra were performed on a Bruker Avance III 400 spectrometer with SiMe₄ and H₃PO₄ as internal and external references, respectively. Emission and excitation spectra and emissive lifetimes in degassed solutions, solid states and films were recorded on an Edinburgh analytical instrument (FLS-920 fluorescence spectrometer). Absolute quantum yields were determined by the integrating sphere (142 mm in diameter) using an Edinburgh FLS-920 spectrofluorophotometer. The circularly polarized luminescence spectra of PtAu₂ clusters, as well as the circularly polarized electroluminescence spectra of OLED devices, were recorded on a JASCO CPL-300. Thermogravimetric analysis was recorded on a NETZCH STA 449F3 instrument.

Crystal structural determination

The diffraction data of 1a were collected on an XtaLAB Synergy X-ray single crystal diffractometer using a Mo target (λ = 0.71073 Å). The diffraction data collection of 1b was performed on a Synergy custom X-ray single crystal diffractometer using a liquid metal jet light source Liquid MetalJet D2+, a high brightness liquid gallium anode target (λ = 1.34050 Å) and a silicon array photon direct reading detector. Data were corrected for absorption effects using the multi-scan method (SADABS).³² The structures were solved and refined using the Bruker SHELXTL software package, a computer program for automatic solution of crystal structures, and refined by the full-matrix least-squares method with ShelXle version 4.8.6, a Qt graphical user interface for SHELXL.³³ All non-hydrogen atoms were refined anisotropically, whereas the hydrogen atoms were generated geometrically and refined using isotropic thermal parameters.

Computational method

The calculations were implemented by using the Gaussian 16 program package.³⁴ The geometrical structures as isolated molecules in the ground state and the lowest-energy triplet state were first optimized, respectively, by the restricted and unrestricted density functional theory (DFT) method with the gradient corrected correlation functional PBE1PBE.^35,36 The initial structures were extracted from the single crystal structural data. During the optimization process, the convergent values of maximum force, root-mean-square (RMS) force, maximum displacement and RMS displacement were set by default. To analyze the absorption and emission transition properties, 80 singlet and 6 triplet excited-states were calculated, respectively, based on the optimized structures in the ground state and the lowest-energy triplet state to determine the vertical excitation energies by time-dependent density functional theory (TD-DFT)^37–40 with the same functional used in the optimization process. In the calculation of excited states, the polarizable continuum model method (PCM)⁴¹ with CH₂Cl₂ as a solvent was employed. The self-consistent field (SCF) convergence criteria of the RMS density matrix and maximum density matrix were set by default in the excited-state calculation. In these calculations, the Stuttgart–Dresden (SDD)⁴² basis set and the effective core potentials (ECPs) were used to describe the Pt and Au atoms, while other non-metal atoms of P, O, N, C and H were described by the all-electron basis set of 6-31G**. Visualization of the hole and electron plots was performed using GaussView. The contributions of fragments to the hole and electron⁴³ in the electronic excitation process were analyzed using the Multiwfn 3.3.8 program.⁴⁴

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (grants 92061202 and 21801242).

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Footnote

† Electronic supplementary information (ESI) available: Tables and figures giving additional photophysical and computational data, and X-ray crystallographic files in CIF format for 1a and 1b. CCDC 2348909 and 2348910. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi00973h

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