Heterobimetallic iridium^III–europium^III complex: the role of donor energy on sensitising the Eu^III ion

Abstract

In luminescent devices, the combination of red, green, and blue luminophores (RGB system) allows the creation of other colours in the electromagnetic spectrum, including the perception of white light. Among the three fundamental colours necessary to create the RGB system, normally, the red colour has received great attention compared with the others because red luminophores present lower Φ due to the energy gap law. Red emitters are easily obtained using Eu^III ion, however, although the red emitters based on the Eu^III ion have high colour purity, the Φ normally is lower compared with red emitters based on the Ir^III ion, even though the colour purity of the latter is not so high. Thus, to produce efficient red-emitting complexes, a strategy that has been developed is the synthesis of heterobimetallic d–f complexes to join the high Φ of the d-metal complexes with the high colour purity of the Eu^III ion. Herein, a novel dual bimetallic emitter was synthesised and fully characterised, the [{Ir(dfppy)₂(μ-bpdc)}₃Eu₂]Cl₃·nH₂O·mCH₃OH (dfppy is the 2-(2,4-difluorophenyl)pyridine cyclometallating ligand, and the ancillary ligand bpdc is the 2,2′bipyridine-3,3′-dicarboxylic acid). This complex was immobilised in a polymeric PMMA film presenting a high Φ value, 42.2 ± 4.2%, in the yellow spectral region. Its precursor, the novel green-emitting Ir^III complex, [Ir(dfppy)₂(bpdc)], showed 97.2 ± 9.7% of Φ after doped in PMMA matrix. Both films were used to coat a UV-LED chip to investigate the photophysical properties when these complexes are applied to create phosphor-converted LEDs. The bimetallic prototype exhibited higher radiant photostability after 18 h of operation compared with the heteroleptic Ir^III complex-doped film. For the first time, the lowest triplet energy (19 103 cm⁻¹) from an Ir^III complex that efficiently sensitises the Eu^III ion in an Ir^III–Eu^III heterobimetallic complex has been determined using voltage modulation in a UV-coated LED prototype, which can be used to engineer new pure red-emitting d–f bimetallic systems.

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1. Introduction

Screen devices such as televisions, cell phones, and computers, as well as public and domestic lighting, are responsible for a large share of energy consumption in developed countries,^1–3 which is directly connected to the low efficiency of these devices in converting electricity into light. In this scenario, the search for luminescent materials with high emission quantum yields (Φ), to reduce energy consumption has been incessant. Compounds based on metal complexes with a large spin–orbit coupling constant (ξ), such as Ir^III,⁴ Pt^II,⁵ Os^II,⁶ and Re^I,⁷ have been reported with high values of Φ's. However, these complexes have broad emission bands attributed to a metal-to-ligand charge transfer (MLCT), a ligand-centered (LC), or a hybrid (MLCT-LC) emissive state.⁸ As a consequence, the colour purity of d-metal complexes is quite low, which is a problem for imaging devices because higher colour purity enables the production of images with greater colour gamut and contrast. Therefore, a convenient strategy to increase the colour purity of luminophores is the use of lanthanide ions that exhibit narrow emission bands. In addition, their emission properties are less dependent on the chemical environment in which they are inserted.⁹

In luminescent devices, the combination of red, green, and blue luminophores (RGB system) allows the perception of all other colours, including white light.¹⁰ Among the three fundamental colours necessary to create the RGB system, red causes great apprehension in the scientific community because red luminophores normally exhibit lower Φ due to the energy gap law than the other two.¹¹

The red emission colour, in turn, can be easily obtained through the coordination of organic molecules to the Eu^III ion. Photophysical and energy transfer processes of Eu^III complexes have been deeply investigated,¹² allowing the construction of an energy map between the excited donor state of the ligand (D) and the excited acceptor state (A) of Eu^III ion.¹³ These studies report that the dominant energy transfer path is singlet state (S) → triplet state (T) → Eu^III, and thus, the only important parameter is considered to be the gap between the feeding triplet state of the ligand (D) and the excited states of the Eu^III ion.¹⁴ This process is called the antenna effect. Normally, the best energy transfer rates in monocentered Eu^III complexes are found when the triplet state has a slightly higher energy than the acceptor excited states of Eu^III ion. It is agreed that a safe energy difference between D and A states to avoid high energy back-transfer rates and promote high Φ is observed about 2500 and 3000 cm⁻¹ above the ⁵D₀.¹⁴ While ⁵D₀ is the main emitter state of the Eu^III ion, ⁵D₁ is the main acceptor state for most of the ligand classes, including β-diketones,¹⁵ exceptions to this rule are Schiff bases that sensitise the ⁵D₀ emitting state better,¹⁶ and polyaminocarboxylates, in which the best acceptor is the ⁵D₂ excited state.¹⁷ These rationalisations are obtained through the analysis of an enormous number of different complexes with ligands having different triplet energies, making this strategy time and resources consuming.

Although red emitters based on the Eu^III ion have high colour purity, the Φ is normally lower than that of red emitters based on Ir^III complexes. Strategically, the synthesis of heterobimetallic d–f complexes or d–f metal organic frameworks (MOFs) has been explored, combining the high Φ of the d-metal complexes with the high colour purity of the lanthanide^III ions. In this context, systems based on Ir^III complexes coordinated to Eu^III ions are the most investigated aiming for synergic results for a good luminophore.^18–20

In an Ir^III–Eu^III heterobimetallic complex, the Ir^III complex itself acts as a sensitising ligand, and the fundamental idea is that the triplet excited overpopulated state, resulting from the high spin–orbit coupling (SOC) of the Ir^III ion (ξ = 4430 cm⁻¹),²¹ transfers its energy to Eu^III ion.²² As in monocentered Eu^IIIcomplexes, in heterobimetallic Ir^III–Eu^III complexes, the sensitisation efficiency depends on the energy difference between D and A; however, for d–f systems, this energy gap still needs to be further studied.

Data from the literature reveal that the energy transfer efficiency between the D state of Ir^III complex and the A state of Eu^III ion is not so obvious, because in this case, the D state is also an efficient and long-lived emitter state. Consequently, in such systems, three different situations can occur, as depicted in Fig. 1. (i) When the D state is energetically lower than the emissive state of Eu^III ion, the sensitisation does not occur, as expected, and the Ir^III complex is not a good antenna for the Eu^III ion. This is the case of [{(ppy)₂Ir(μ-pmc)}₃EuCl₃] heterobimetallic complex reported by Lian et al.,²³ in which the D state is at approximately 16 666 cm⁻¹, and it is situated below the ⁵D₀ emitting level of the Eu^III ion (17 225 cm⁻¹),²⁴ accordingly only the emission from the Ir^III complex component is detected. By increasing the D energy state to be energetically higher than ⁵D₀, the sensitisation efficiency increases until it reaches its maximum. (ii) If the D state is higher but close to the A state, the energy might be transferred partially, characterising an emission spectrum with both emissive components, a broad band from Ir^III component, and narrow emission bands from Eu^III. (iii) If the D state is higher than the A state to avoid energy back-transfer, the emission spectrum will show only the narrow emission bands from Eu^III. A few heterobimetallic complexes with efficient energy transfer to the Eu^III ion have been reported, such as [{(dfppy)₂Ir(μ-phen5f)}₃EuCl]Cl₂ published by Chen et al.,²⁵ where the D state is centred at 20,408 cm⁻¹; Jiang et al.¹⁸ also reported a heterobimetallic complex with efficient energy transfer, i.e., the [{Ir(dfppy)₂(cbphen)}₃ClEu]Cl₂, in which the D state is situated at 19 230 cm⁻¹ at room temperature in dichloromethane solution. In both cases solely emission from the Eu^III component is observed. If the D-state energy is much higher than the ⁵D₀ energy, emission from both components will occur due to the poor energy transfer efficiency, and a spectrum-like situation (ii) is expected, which has been reported for several Ir^III–Eu^III bimetallic systems.^23,26,27 In such studies, D-state values above 21 690 cm⁻¹ make the sensitisation process inefficient. Systems based on multiple emissive states find several applications, such as white light for general lighting,²⁸ or in luminescent ratiometric probes for analyses in environmental²⁹ and biomedical fields.³⁰ However, a clear boundary between a situation in which there is only emission from the Eu^III component or a mixture of light from components has never been established. In this work, two films of PMMA with [Ir(dfppy)₂(bpdc)] heteroleptic complex (Ir-p), or [{Ir(dfppy)₂(μ-bpdc)}₃Eu₂]Cl₃·nH₂O·mCH₃OH heterobimetallic complex (Ir-p–Eu) were used to coat a UV-LED chip to create two phosphor-converted LED (PC-LED) prototypes. The results acquired by modulating the voltage applied to the Ir-p–Eu:LED prototype allowed us to establish the minimum energy required to solely observe the emission of Eu^III ions.

Fig. 1 Energy transfer between Ir^III complex and Eu^III in a bimetallic system.

2. Experimental

2.1. Chemicals

Methanol (CH₃OH, Synth, 99.5%), iridium^III chloride hydrate (IrCl₃·xH₂O, Aldrich, 99.99%), 2 ethoxyethanol (C₄H₁₀O₂, Aldrich, 99.0%), dichloromethane (CH₂Cl₂, Synth, 99.99%), 2-(2,4-difluorophenyl)pyridine (dfppy – C₁₁H₇F₂N, Aldrich, 97%), 2,2′-bipyridine-3,3′-dicarboxylic acid (bpdc – C₁₂H₈N₂O₄, Aldrich, 97%), and PMMA ([CH₂C(CH₃)(CO₂CH₃)]_n, Aldrich) were used as start reactants without any further purification.

2.2. Complex syntheses

Synthesis of the [Ir(dfppy)₂(bpdc)] complex (Ir-p). The iridium complex was synthesised according to a procedure described in the literature.³¹ The first step was cyclometalated iridium^III dimer synthesis using the Nonoyama route,³² [(dfppy)₂Ir(μ-Cl)₂Ir(dfppy)₂]. The IrCl₃·xH₂O was refluxed with 2.2 equivalents of the dfppy (2-(2,4-difluorophenyl)pyridine) cyclometalating ligand in a mixture of 2-ethoxyethanol and water (3 : 1) for 24 h. The second step was iridium complex synthesis. The ancillary ligand bpdc (2,2′ bipyridine-3,3′-dicarboxylic acid) (54.2 mg – 0.22 mmol) was dissolved in water, then a solution of [(dfppy)₂Ir(μ-Cl)₂Ir(dfppy)₂] (112.2 mg–0.09 mmol) in CH₂Cl₂ (5 mL) was added to the ancillary ligand solution, resulting in a biphasic system; then, CH₃OH (5 mL) was added resulting in a monophasic system. The mixture was stirred for 4 h at room temperature; after this time, the solvents were reduced by heating and the complex was precipitated by adding water, filtered, and dried in a dissector. Yield: 88.7% (78.7 mg). FTIR – ATR (v_max/cm⁻¹): 1602 (_υCOO⁻)_ass, 1572, 1558 (_υC=N)_pyridine, 1374 (_υCOO⁻)_sym. MALDI-TOF for C₃₄H₂₀F₄IrN₄O₄m/z found(calcd.) of 817.1(817.1), representing [[Ir(dfppy)₂(bpdc)] + H]⁺. UV-Vis (in DCM, 6.48 × 10⁻⁶ mol L⁻¹); λ_max 254 and ε_max 41203 mol⁻¹ dm³ cm⁻¹, others λ nm (ε, mol⁻¹ dm³ cm⁻¹): 300 (19444), 350 (6327), 417 (1234), 488 (308). Fluorescence: excitation λ_max (in aerated DCM solution, 6.48 × 10⁻⁶ mol L⁻¹) 488 nm, emission 555 nm, Stokes shift 67 nm, quantum yield (Φ_fl) 10.4%.

Synthesis of the heterobimetallic {[Ir(dfppy)₂(μ-bpdc)]₃Eu₂}Cl₃·nH₂O·nCH₃OH complex (Ir-p–Eu). A mass of 67.2 mg (0.082 mmol) of the iridium complex [Ir(dfppy)₂(bpdc)] was dissolved in 15 mL of methanol, and 3 mL of EuCl₃ ethanolic solution (0.054 mol L⁻¹) was added to the iridium complex solution. The solution was kept under stirring for 4 h. After this time, the system was heated to reduce the volume, and the complex was dried in a desiccator, Fig. 2. Yield: 61.1%. FTIR – ATR (v_max/cm⁻¹): 1596 (_υCOO⁻)_ass, 1572, 1558 (_υC=N)_pyridine, 1383 (_υCOO⁻)_sym. Elemental analysis; found(calcd.) C: 39.45%(39.39%), H: 2.87%(2.86%), N: 5.90%(5.30%) to C₁₀₄H₉₀Cl₃Eu₂F₁₂Ir₃N₁₂O₂₈ ({[Ir(dfppy)₂(μ-bpdc)]₃Eu₂}Cl₃·14H₂O·2CH₃OH), MALDI-TOF found(calcd.) [{[(dfppy)₂Ir(μ-bpdc)]₃Eu₂}Cl·13H₂O·2CH₃OH – (2Cl⁻ + H₂O)]²⁺m/z of 1541.2(1541.1). UV-Vis (in DCM, 4.62 × 10⁻⁶ mol L⁻¹); λ_max 254 and ε_max 56 277 mol⁻¹ dm³ cm⁻¹, others λ nm (ε, mol⁻¹ dm³ cm⁻¹): 300 (31168), 350 (10606), 417 (3246), 488 (1082). Fluorescence: excitation λ_max (in aerated DCM solution, 4.62 × 10⁻⁶ mol L⁻¹) 451 nm, emission Ir^III component 560 nm, Eu^III component 616 nm. Quantum yield (Φ_fl) 9.7%.

Fig. 2 Molecular structure of the complexes synthesised in this study. For clarity, water- and methanol-coordinated molecules were omitted in the representation of the bimetallic complex.

Preparation of PMMA films with Ir-p and Ir-p–Eu complexes. Solutions of the complexes in DCM were prepared and mixed with PMMA dissolved in DCM to obtain films containing 0.5 wt% of the complex. The mixture was stirred and heated to reduce the solvent volume to approximately 0.5 mL, and the mixture was then deposited on a 22 × 22 mm glass substrate and covered with a Petri dish in an atmosphere saturated with DCM vapour.
Preparation of PC-UV LED prototypes. The 0.5 wt% doped PMMA films were glued onto the surface of a UV-LED chip (λ_max = 395 ± 10 nm, Shenzhen Chang Long Technology Co., Ltd, type SMD) with cyanoacrylate and allowed to dry overnight. The emission spectrum of the LED prototype was acquired operating at 2.98 V and 54 mA, and the photostability was monitored for 18 h in a PerkinElmer model LS55 spectrometer using an emission slit width of 3 nm. To evaluate the functionality of the fabricated devices at different voltages, the LED prototype spectra were recorded by varying the voltage from 2.90 V to 3.05 V in intervals of 0.01 V. The power supply to turn on the LED was an ICEL PS- 1500 pack.

2.3. Instrumental

Absorption spectroscopy. The UV-Vis absorption spectra were obtained using a Shimadzu model UV1800 spectrometer, double beam, within the 900–200 nm spectral range and resolution of 1 nm. For this, solutions of both complexes at a concentration of 10⁻⁶ mol L⁻¹ were prepared in dichloromethane, ethanol, and acetonitrile.

Luminescence. Emission, excitation, lifetime decay curves, and the overall relative emission quantum yield of the Ir^III–Eu^III complex and the iridium^III complex (Φ) were measured in a PerkinElmer LS55 fluorimeter with a pulsed Xe lamp (9.9 W) as the source of excitation and an R928 PMT photomultiplier. The overall relative emission quantum yields were estimated using rhodamine 6G in ethanol as standard (Φ = 94%). Eqn (1) was applied where Φ_s is the emission quantum yield of the standard (s), I is the integrated area under the corrected emission spectrum, A is the absorbance at the excitation wavelength, and n is the refractive index.^33–35 The letters u and s represent the measured parameters for the sample and standard, respectively.

Intrinsic emission quantum yield (Φ^Eu_Eu). It was measured using the emission spectra of the heterobimetallic Ir^III–Eu^III complex and the lifetime decay of the ⁵D₀ emissive state of the Eu^III ion. The experimental lifetime (τ) of an excited state is determined by measuring the luminescence decay as a function of time, which is related to the time required for the emissive state to decay to 1/e of the initial intensity. The depopulation process is described by eqn (2), where the emission intensity is proportional to the number of photons in the excited state:

I = I₀·e^−t/τ (2)

Depopulation can occur by radiative (A_rad) or non-radiative (A_nrad) paths. The total decay rate (A_t = A_rad + A_nrad) is given by the inverse of the lifetime, as shown in eqn (3):³⁶

τ⁻¹ = A_rad + A_nrad (3)

The radiative decay rate is calculated by the sum of the photons emitted in each ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, and 4) set of transitions, as shown in eqn (4) and (5):

A_rad = A₀₀ + A₀₁ + A₀₂ + A₀₃ + A₀₄ (4)

A ₀₁ is the spontaneous decay rate referring to the magnetic dipole allowed transition ⁵D₀ → ⁷F₁, used as an internal standard because it is not influenced by the electric field over the system. S_01:0J and v_01:0J are the area and barycenter of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F_J transitions (J = 0, 2, 3, and 4), respectively. Using the radiative and nonradiative decay rates, it is possible to determine the intrinsic emission quantum yield (Φ^Eu_Eu) of the ⁵D₀ emissive state of the complex, as shown in eqn (6):³⁷

MALDI-TOF analyses. They were performed on a Bruker Daltonics autoflex III smart beam using a malono matrix. The complexes were dissolved in dichloromethane and then mixed with the matrix for analysis.

FTIR analyses. They were performed on a Bruker Invenio® model equipped with an ATR module composed of a diamond crystal. The measurements were recorded between 400 and 4000 cm⁻¹ with 2 cm⁻¹ of resolution and 120 scans. This equipment is placed in the Laboratório de Biossistemas (LBS) at FCT/UNESP in Presidente Prudente–SP, coordinated by Professor Aldo Eloizo Job.

Elemental analysis. They were obtained on a PerkinElmer CHN 2400 series ii.

Differential scanning calorimetry (DSC). Measurements were performed using TA Instruments model SDTQ600 equipment.

3. Results and discussion

3.1. Structural characterisation of the complexes

The zwitterionic heteroleptic Ir^III complex Ir-p, which was used as the ligand, was synthesised according to the method described in the literature.³¹ In this complex, dfppy is the 2-(2,4-difluorophenyl)pyridine cyclometallating ligand, and the ancillary ligand bpdc is the 2,2′-bipyridine-3,3′-dicarboxylic acid. The stoichiometry of Ir^III complex was confirmed by MALDI-TOF analysis, [M + H]⁺m/z found(calcd.) of 817.1(817.1) (Fig. S1, ESI†); FTIR (Fig. S2, ESI†) and UV-Vis (Fig. S3, ESI†) spectroscopies were also performed to complete the elucidation.

The heterobimetallic Ir-p–Eu complex was synthesised in methanolic solution with the aforementioned synthesised Ir^III complex and europium^III chloride (EuCl₃). The formation of the heterobimetallic complex was confirmed by elemental analysis, found(calcd.) C: 39.45%(39.39%), H: 2.87%(2.86%), N: 5.90%(5.30%) to C₁₀₄H₉₀Cl₃Eu₂F₁₂Ir₃N₁₂O₂₈ – [{Ir(dfppy)₂(μ-bpdc)}₃Eu₂]Cl₃·14H₂O·2CH₃OH. The stoichiometry of the Ir-p–Eu complex was confirmed by MALDI TOF analysis, in which the spectra display the molecular ion peak with the isotopic signature of the Ir^III and Eu^III ions, as shown in Fig. 3. In the Ir-p–Eu spectrum, it is possible to observe the molecular ion signal at 1541.2 m/z, which is assigned to the ionised structure ([{[(dfppy)₂Ir(μ-bpdc)]₃Eu₂}Cl·13H₂O·2CH₃OH – (2Cl⁻ + H₂O)]²⁺), with a chemical formula of C₁₀₄H₈₈Eu₂F₁₂Ir₃N₁₂O₂₇Cl²⁺ and a molar weight of 3081.91 g mol⁻¹. The chemical formula of the neutral species is C₁₀₄H₉₀Eu₂F₁₂Ir₃N₁₂O₂₈Cl₃ with a molar weight of 3170.82 g mol⁻¹. Signal peaks associated with other ionised species were detected at 1497.1 and 1480.1, these values indicate a lower number of water molecules associated with the complex. The FTIR spectra (Fig. S4, ESI†) show that the Ir^III complex is coordinated by the oxygen atoms of the carboxylate groups into the Eu^III ion. The antisymmetric-stretching of the carboxylic group, ν(COO⁻)_ass, shifted from 1601 cm⁻¹ to 1598 cm⁻¹, indicating a lower oscillator energy and now matches with C=N stretching. On the other hand, the symmetric-stretching, ν(COO⁻)_sym, shifted to higher energy, from 1374 cm⁻¹ to 1384 cm⁻¹. Other bands related to C=N and C=C bonds were not influenced by coordination with Ln^III ions, as shown in Fig. S4 (ESI†). Furthermore, water and methanol molecules can be in both coordinated and hydrated forms, as the coordination number of Eu^III ions can vary from 6 to 12. The elucidation of the structure of the Ir-p–Eu complex was based on the article published by Calefi et al.,³⁸ in which dinuclear Eu^III and Tb^III complexes with 2, 2′-bipyridine-4,4′-dicarboxylic acid, a ligand similar to bpdc used here, and the synthesis was also carried out via wet route. Normally, the bpdc ligand forms metal organic frameworks (MOF) when the compound is prepared by the hydrothermal route.³⁹ From the FTIR data, it was possible to assume that carboxylic groups from Ir-p moisture are coordinated by the monodentate binding mode to Eu^III, since the energy difference between ν_ass(COO⁻) and ν_sym(COO⁻) in Ir-p–Eu (Δ = 213 cm⁻¹) is higher than that measured for the ionic ligand⁴⁰ (Na₂bpdc: Δ = 205 cm⁻¹), Fig. S5 (ESI†). UV-Vis spectra were recorded, and they are presented in the ESI† Fig. S6. Both complexes showed absorption bands extending from the deep UV to visible region. The thermal stability of the complexes was determined by differential scanning calorimetry (DSC), which displays an exothermic event with onset at 368 °C for both complexes. These temperatures were attributed to the initial degradation of these complexes, probably starting at the Ir component, as shown in Fig. S7 (ESI†). The coordination of the Ir-p complex with the Eu^III ion did not influence the thermal stability of the final compound.

Fig. 3 MALDI-TOF spectrum of the Ir-p–Eu complex.

3.2. Luminescence characterisation

The luminescent behaviour of the complexes was analysed in an aerated solution of ethanol (EtOH), dichloromethane (DCM), and acetonitrile (ACN). The Ir-p–Eu was also analysed in the solid state. The emission spectra of Ir-p are dominated by a MLCT broadband in all three solvents analysed and are strongly influenced by solvent polarisation properties, as shown in Fig. 4(A). Ir-p can be excited in a wide range from 250 nm to over 500 nm, Fig. S8 (ESI†). The Ir–p–Eu complex emits from green to orange because of the reminiscent emission from the Ir–p complex, which partially sensitises the Eu^III, along with narrow emission bands of the ⁵D₀ → ⁷F_J (J from 0 to 4) transitions of Eu^III, Fig. 4(B). The maximum emission wavelength and the relative emission intensity of the MLCT band in both complexes change under the influence of the solvent, while the f–f transitions remain almost unchanged for the Ir–p–Eu complex, as expected. This shows that the sensitisation of the Eu^III ion is dependent on the environment. The spectral changes in the emission spectra of both samples reflect the displacement of the colour coordinates (CIE-1931 x; y), as follows: Ir-p in DCM: (0.496; 0.497), ACN: (0.500; 0.487) and EtOH: (0.424; 0.529); Ir-p–Eu in DCM: (0.564; 0.422), ACN: (0.599; 0.394), EtOH: (0.554; 0.434) and solid state: (0.500; 0.475). The relative emission quantum yield (Φ) was analysed for all tested solutions, and for both complexes, the same behaviour was observed greatest value being found in the DCM solution and the lowest in the EtOH solution. For Ir-p, the following values were found, DCM: 10.4%, ACN: 2.3%, and EtOH: 1.3%; for Ir-p–Eu, the following values were found: DCM: 9.7%, ACN: 6.6%, and EtOH: 4.4%. The method used to calculate the relative emission quantum yield is described in the Experimental Section, which was obtained from the UV-Vis and emission spectra (Fig. S9 and S10, ESI†). The lifetime (τ), Fig. S11 (ESI†), and intrinsic emission quantum yield (Φ^Eu_Eu) of the ⁵D₀ level of the Eu^III ion were determined, and the following values were found; DCM: τ = 0.477 ms, and Φ^Eu_Eu = 18.1%; ACN: τ = 0.567 ms, and Φ^Eu_Eu = 26.8%; EtOH: τ = 0.263 ms, and Φ^Eu_Eu = 10.1%, and in the solid state: τ = 0.237 ms, and Φ^Eu_Eu = 7.4%. The lifetime values were obtained using a monoexponential decay fit and are presented in Fig. S11 (ESI†).

Fig. 4 Emission spectra of Ir-p in (A) and Ir-p–Eu in (B). All emission spectra were acquired using 350 nm as the excitation wavelength. Emission and excitation slit of 10 nm and cutting filter of 390 nm.

To evaluate the potential application of Ir-p and Ir-p–Eu complexes as coatings on PC-LEDs, they were immobilised into PMMA films. The emission spectra of both films are shown in Fig. 5 compared to the emission spectra of the respective complexes in the DCM solution. By comparison, it is possible to observe that the immobilisation in the PMMA matrix resulted in a blue shift of the MLCT emission band of the Ir^III component due to the rigidochromic effect.⁴¹ The overall emission quantum yield of the doped PMMA films was measured using an integrated sphere, and the values were 97.2% (± 9.7%) for Ir-p and 42.2% (± 4.2%) for Ir-p–Eu doped films. The lower value found for the bimetallic-doped film evidences the strong influence of the PMMA matrix in the radiative and non-radiative pathways of Eu^III emission, since this ion is more susceptible to vibronic coupling or multiphonon relaxation.⁴² The measured Φ for the bimetallic system in dichloromethane solution was 9.7%, a value lower than the values already reported in the literature considering an efficient energy transfer between the Ir^III moiety and the Eu^III ion, in which pure red-emitting compounds were reported, such as the complex {[(dfppy)₂Ir(μ-phen5f)]₃EuCl}Cl₂, with 17% of Φ²⁵ or {[Ir(dfppy)₂(cbphen)]₃ClEu}Cl₂ and Ir(dfppy)₂(cbphen)Eu(TTA)₃ with 10% and 44%, respectively.¹⁸ It is a consensus that dual-emitting Ir^III–Eu^III bimetallic complexes have a lower Φ than pure red-emitting bimetallic complexes, such as the complexes published by W. Jiang et al.,²⁶ which Φ comprises between 11% and 28%; therefore, the bimetallic system presented here follows the default behaviour. Furthermore, it is important to mention that no article has reported the Φ of Ir^III–Eu^III bimetallic complexes after incorporation into polymeric films. Here, the immobilisation of the dyad in the PMMA matrix increased the Φ from 9.7% to 42.2%, indicating that incorporation into the PMMA film has boosted the Φ of the blend, qualifying it to be applied in coated UV-LEDs. All additional luminescent characterisations of the complexes can be found in the ESI,† from Fig. S8 to Fig. S12. From Fig. 4 (B) it is clear that the present system corresponds to a situation in which the triplet donor state of the Ir^III component does not completely sensitise the Eu^III ion; hence, dual emission is observed from both emitter states (³MLCT of the Ir^III component and ⁵D₀ of the Eu^III ion).

Fig. 5 Comparison between emission spectra measured in DCM solution and immobilised in PMMA film, (A) heteroleptic Ir-p complex and (B) bimetallic Ir-p–Eu complex.

3.3 PC-LED prototypes characterisation

The PMMA films were glued onto the surface of a UV-LED chip (λ_max = 395 ± 10 nm, Shenzhen Chang Long Technology Co., Ltd, type SMD) with cyanoacrylate and allowed to dry overnight. Then, luminescence measurements were performed to monitor the radiant stability of the fabricated prototypes. In this construction the UV light is generated by electroluminescence, which then photoexcites the top films, thus the PMMA films are excited by photoluminescence. Photographs of the fabricated prototypes in operation are shown in Fig. 6(A). The emission band centred at 390 nm presented in both prototypes is assigned to the UV emission of the chip itself used as the excitation source Fig. 6(A). The emission spectrum of the Ir-p:LED prototype is dominated by a broadband in the green spectral region (λ_max = 545 nm). The same broadband is observed for the Ir-p–Eu:LED prototype, however, the narrow emission bands of the Eu^III ion in the red spectral region are also present, similar to what was observed in solution, once again indicating a dual emission from the device Ir-p–Eu:LED as the Ir^III component can only partially transfer its energy to the Eu^III component. The radiant photostability of the prototypes was analysed at 2.98 V over an uninterrupted 18 h of operation, and the measured emission spectra are shown in Fig. S13 (ESI†). The radiant profile of the Ir-p–Eu:LED prototype closely follows the bare LED curve, indicating high photostability of the Ir-p–Eu complex. On the other hand, the Ir-p:LED curve indicates lower photostability, losing 28% of the initial emission area after 10 h of operation; thereafter, the emission area remained almost constant. As the bare UV-LED loses approximately 13% of its initial area, it is pertinent to say that the decrease in the total emission area for the Ir-p–Eu:LED prototype is largely due to the decrease in the efficiency of the commercial UV LED chip used as the excitation source. By monitoring the emission intensity of the Ir-p–Eu:LED prototype over time, it was possible to observe that both Ir-p and Eu^III emission components decrease; however, the Eu^III component decreases faster than the Ir-p broadband emission, as can be seen in the normalised emission spectra in Fig. 6(B), Moreover, the maximum emission wavelength of the Ir-p component shifts to a lower energy, and consequently, the energy transfer efficiency to the Eu^III ion decreases, leading to a lower efficient sensitisation. Thus, the decrease in the integrated emission area of the Ir-p–Eu:LED prototype is not only related to the degradation of the heterobimetallic complex, as expected. This may also be due to the decrease in the Eu^III sensitisation efficiency over the operation time, probably due to the heating of the doped film by the UV-LED chip. To attest to this behaviour, the temperature was monitored before and after 18 h of operation using a digital thermometer, and the temperature showed an increase of 1.5 °C, from 27.2 °C to 28.7 °C. The Ir-p:LED prototype also exhibited the same behaviour, shifting the emission towards a lower energy region over the operating time (Fig. S14, ESI†). In order to evaluate the functionality of the fabricated devices at different voltages, the LED prototype spectra were recorded by varying the voltage from 2.90 V to 3.05 V in intervals of 0.01 V, Fig. 6(A). For both prototypes, a shift in the maximum emission wavelength of the Ir-p component towards higher energy was observed as the applied voltage increases (λ_max 558 → 535 nm). In the Ir-p–Eu:LED, this phenomenon boosted the sensitisation of the Eu^III ion when the voltage was increased, which can be observed through the normalised spectra obtained for each applied voltage, Fig. 7(A). Similar shifting behaviour for the Ir-p:LED prototype can be seen in Fig. S15 (ESI†). Fig. 7(B) shows a graphic of the ratio between the area under the emission curve of the Ir^III component and under the emission narrow band assigned to ⁵D₀ → ⁷F₂ of the Eu^III ion in the Ir-p–Eu:LED device as a function of the maximum emission energy of the Ir^III component at the different tested voltages. A linear behaviour is obtained between 2.98 and 3.04 V. Below 2.98 V, the spectra exhibited low resolution and high noise/signal ratio (Fig. S16, ESI†), and for this reason, they were not considered. Above 3.04 V, the prototype emission saturates the detector, making it impossible to adequately quantify the photons. The emission areas of each component were determined using the deconvolution method for each emission spectrum obtained (Fig. S17–S23, ESI†). Through a linear fit, it was possible to propose eqn (7), which relates the Ir^III/Eu^III emission ratio as a function of the maximum emission energy of the Ir^III component (assuming it as the donor state), and thus calculate the minimum energy required for the total sensitisation of the Eu^III ion.Where is the ratio of the integrated areas under the Ir^III and Eu^III components and E_{Ir_max} is the energy of the maximum wavelength of emission for the Ir^III (D) component in the Ir-p–Eu:LED prototype, given in cm⁻¹. In this way, when is null, only the emission of the Eu^III component will be observed, and the energy necessary to satisfy this condition is 19 103 cm⁻¹, approximately 1876 cm⁻¹ above the Eu^III ion ⁵D₀ emitter level. This value is very close to those reported in the literature for total Eu^III ion sensitisation in both monocentred Eu^III 13 and heterobimetallic Ir^III–Eu^III complexes.²⁰

Fig. 6 (A) Emission spectra of the UV-LED chip used as the excitation source (in blue), of the Ir-p:LED prototype (in green), and of the Ir-p–Eu:LED prototype (in yellow). (B) Radiant emission spectra obtained at an interval of 1 h for the Ir-p–Eu:LED prototype. The insertion shows the energy diagram illustrating the energy decrease in the donor state of the Ir-p in the fabricated prototype over the operation time.

Fig. 7 (A) Emission spectra recorded by varying the applied voltage of the Ir-p–Eu:LED prototype, from 2.90 V to 3.04 V; (B) linear fit of the Ir^III/Eu^III emission area ratio as a function of the energy of the maximum emission band of the Ir^III component in the Ir-p–Eu:LED prototype.

Conclusions

A novel dual-emitting Ir^III–Eu^III bimetallic complex, [{Ir(dfppy)₂(μ-bpdc)}₃Eu₂]Cl₃·nH₂O·mCH₃OH, was synthesised and fully characterised. This complex was immobilised in a polymeric PMMA film and presented high Φ value, 42.2 ± 4.2% in the yellow spectral region. Its precursor, Ir-p, showed 97.2 ± 9.7% of Φ after doped in PMMA matrix. The Ir-p–Eu bimetallic complex was applied as a PC-LED and showed great radiant stability, losing only 15% of the initial intensity after 18 h of operation. However, the dominant emission wavelength suffered a redshift because heating over the operation time decreased the energy of the donor ³MLCT state of Ir^III component, decreasing the efficiency of the sensitisation process to the Eu^III ion. In addition, with this study, it was possible to determine the minimum energy of the ligand triplet donor level to observe only the red emission of the Eu^III ion in a heterobimetallic Ir^III–Eu^III complex, that is, 19103 cm⁻¹. With this result, it is possible to predict whether an iridium heteroleptic complex has enough energy to efficiently sensitise an Eu^III ion to produce pure red-emitting compounds.

Author contributions

Felipe da Silva Manrique Canisares: conceptualization, data curation, formal analysis, and writing: original writing – original draft. Renan Caike Silva: formal analysis, writing – original draft. Marian Rosaly Davolos: writing – review & editing, supervision. Ana Maria Pires: writing – review & editing, supervision. Sergio Antonio Marques de Lima: conceptualization, writing – review & editing, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful to the São Paulo Research Foundation (FAPESP, Grant No. 2019/26103-7 and Grant No. 2015/03400-5) for supporting this research. We also acknowledge the Brazilian agencies for the concession of scholarships CNPq (A.M.P. for Grant No. 309448/2021-2 and S.A.M.L. for Grant No. 308868/2022-6), and CAPES (RCS for Grant no. 88887.686158/2022-00 and F.S.M.C. for Grant No. 88887.341772/2019-00.), and Fulbright Commission.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nj03161f

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