White LEDs are known as the fourth generation of light sources after incandescent bulbs, fluorescent, and high-pressure gas discharge lamps. They offer high luminous efficiency, low energy consumption, long service life, low weight, environmental friendliness, and anti-vibration advantages^[1-3]. White LEDs are achieved mainly through light-converting techniques, which are low-cost and easy to realize industrially. One current production technology is to excite the yellow phosphor using an LED blue chip such that the two mixed radiations generate white light^[4-6]. However, the scarcity of red emission in the visible spectrum of artificial light leads to the white LED devices' low color rendering index^[7]. Furthermore, human eyes are especially sensitive to blue light, with excessive blue light stimulation causing visual fatigue and disorders of the circadian rhythm^[8]. To overcome the above drawbacks and improve the light color stability, Jung Han et al.^[9] proposed an alternative solution for manufacturing white light in September 1998, namely the “ultraviolet chip + red/green/blue tricolor phosphors” mode. In this mode, the luminous performance of white light depends strongly on the ratios of three primary color phosphors. At the same time, red emission plays a role in adjusting the color temperature and color rendering index. Near-ultraviolet (N-UV) chip-based white LEDs have received considerable attention regarding many applications, such as operating theatres, museums, health lighting, and plant-growth lighting, because of their more optimal color stability and fluorescence, as well as their better user experience and eye friendliness^[10-12].

Red light is vital in determining the color temperature and color rendering performance of white LED devices. Thus, new red phosphors have become one of the research hotspots in the white LEDs industry. Many scholars have developed Eu³⁺ and Tb³⁺ doped inorganic phosphors, which exhibit excellent luminous properties^[13-16]. Numerous researchers are committed to introducing rare earth complexes into polymer matrix composites in order to obtain rare-earth-doped polymer phosphors with high luminous efficiency because rare earth ions exhibit excellent luminescence properties^[17] , and polymer materials have significant advantages in machine-shaping, for example, lightweight, and low-cost^[18-19]. At present, Eu³⁺ ion is often used as a red-light activator because of the 5D₀/7F_J transitions (J = 0−4) of Eu³⁺ ion that leads to a characteristic red emission band around 610 nm^[20-22]. For example, Chang C H et al.^[23] designed and synthesized the complexes Eu(pzc)₃(phen) and Eu(mpzc)₃(phen) with an excitation band between 250 and 350 nm, the most intense emission band being a narrow peak centered at 614 nm. Matsushita A F Y et al.^[24] prepared a red light-emitting macromolecular composite Eu(PSA)Phen, which produced an excitation band between 275 nm and 375 nm and a main emission band of 610−630 nm (red emission) when excited at 348 nm.

The steady-state excitation and emission spectra for red-emitting europium (Eu) complexes tend to be narrow-band spectra with weak emission intensities. Their characteristic absorption peaks mainly concentrate around 340 nm, and they exhibit weak absorption of violet light at 400 nm, which does not match well with the output wavelength of commercial N-UV LED chips (350−420 nm)^[25-26]. In contrast, the excitation spectrum of iridium (Ir) complexes has a broadband structure, which can be effectively excited by UV and blue light, corresponding to a wide coverage of the emission spectrum. By synthesizing specially designed ligands for Ir complexes and modulating the coordination pattern, broadband emission spectra can be obtained in any range from the N-UV to the N-IR region, thus enabling sensitization of lanthanide ions^[27], including Eu ions.

This paper proposes a simple and practical solution to achieve broadband excitation of Eu(III) ion-doped polymer phosphors and broaden the application scope of red fluorescent polymeric materials. The proposed method involves synthesizing the Eu-Ir bimetallic complex using Ir complexes as the ligand for Eu ions, then preparing a red copolymer phosphor through free-radical polymerization. The paper then evaluates the material’s optical and thermal properties for potential application in N-UV LEDs.

Europium chloride (EuCl₃, 99.99%), 1,10-phenanthroline (Phen, 99%), bis[2-(4,6-difluorophenyl) pyridinato-C2,N] (picolinato)iridium(III) (FIrPic>99%, Luminescence Technology Corp.), and undecylenic acid (UA, 99%), ethanol ≥99.7%, methanol ≥99.5%, methyl-methacrylate (MMA≥99%), dimethyl sulfoxide (DMSO, AR), azodiisobutyronitrile (AIBN≥98%).

1 mmol EuCl₃, 2 mmol FIrPic, 1 mmol Phen, and 1 mmol UA, according to the substance content of 1∶2∶1∶1, were separately dissolved in 5 mL anhydrous ethanol for use. The UA ethanol solution was then injected into a 100 mL three-neck flask and stirred for about 10 minutes at 50 °C. Subsequently, the EuCl₃ ethanol solution was added drop by drop to the mix, which was neutralized to pH 4.5 with 1 mol/L NaOH ethanol solution. After further stirring for 30 minutes at 50 °C, the FIrPic ethanol solution was added to the blend, and a large amount of white precipitate became visible during the neutralization of the blended solution to pH 7 with 1 mol/L NaOH ethanol solution. Finally, the Phen ethanol solution was added to the reaction mixture via a dropper and then stirred for three more hours at 50 °C. At the end of the experiment, the precipitate was washed several times with ethanol and dried in an oven at 55 °C until a constant weight was reached. The white powder resulting from this process was Eu(FIrPic)₂(Phen)UA. The synthesis route of the proposed Eu-Ir bimetallic complex is shown in Figure 1.

The reactive complex Eu(FIrPic)₂(Phen)UA (0.05 g) and MMA (1.28 g) were completely dissolved in the DMSO solution (10 mL) using an ultrasonic bath. The resulting mixture was then poured into a 100 mL three-neck flask, where the reaction was carried out under a nitrogen atmosphere. Subsequently, 2 mL of AIBN ethanol solution (pure AIBN is 0.04 g) was added dropwise to the mixing solutions placed in the water bath at (78 ± 2) °C, and deoxygenation by nitrogen gas stopped when the reactive system became very viscous. After that, the closed system remained at 78 °C for 48 hours to ensure a complete reaction, and all reactants were immersed in 100 mL methanol at room temperature. After two hours, the white flocculent precipitate started to generate. Finally, the collected precipitate was filtered through three-layer filter paper and washed with DMSO solution three times to remove unreacted monomers and impurities. Then, the resulting products were dried in an oven at 60 °C for 48 hours until a constant weight was reached. After grinding the substance thoroughly, a white powder called poly(MAA-co-Eu(FIrPic)₂(Phen)UA), abbreviated as PM-Eu-Ir, was obtained. Figure 2 shows the specific synthesis route.

FT-IR spectra in the 4000~500 cm⁻¹ range were recorded in a KBr pellet using a Fourier transform infrared spectrometer (FTIR, Tensor 27, Bruker). The UV-vis absorption spectra were taken in methylene chloride (CH₂Cl₂, 10⁻⁴ mol/L) using a Hitachi-U3900 spectrometer. Fluorescence spectra were measured by a full-function steady-state fluorescence spectrometer (Fluoromax-4, Horiba) with a slit width of 5 nm, and the prepared phosphor was dissolved in 10⁻⁴ mol/L methylene chloride. Thermogravimetric Derivative thermogravimetry (TG-DTG) analysis and differential scanning calorimetry (DSC) curves on phosphorescent copolymer were conducted using a thermal analysis instrument (209F3, NETZSCH) with a heating rate of 10 °C/min under N_2. The fluorescence lifetime of the phosphorescent copolymer was measured in the solid state using a fluorescence spectrometer (FLS980, Edinburgh). The quantum efficiency (QE) was measured by an FLS-980 fluorescence spectrophotometer equipped with a 450 W Xe light source in integrating sphere mode. The morphology of the synthetic complex and prepared phosphorescent copolymer was observed using FE-SEM (JSM-6700F, JEOL). The ST-900 M photometer and Keithley 2400 digital source meter measured electroluminescence spectra. All measurements were made at room temperature unless otherwise stated.

The bimetallic complex Eu(FIrPic)₂(Phen)UA synthesized in this paper exhibits good solubility in some highly and moderately polar solvents, such as DMSO, DMF, CH₂Cl₂, CHCl_3, and THF, but poor solubility in water, methanol, and acetone. Furthermore, because of the linear macromolecule structure of the prepared PM-Eu-Ir, the red phosphorescent copolymer has excellent solubility in most polar solvents containing acetone, THF, CH₂Cl₂, CHCl_3, DMSO, and DMF at room temperature, which significantly improves the film-forming performance of organic phosphors.

The FT-IR spectra of the Ir complex FIrPic, synthetic Eu(FIrPic)₂(Phen)UA, and copolymer PM-Eu-Ir recorded by KBr tablets in the range 4000~400 cm⁻¹ are given in Figure 3 (color online). The corresponding characteristic peaks thereof are illustrated in Table 1. In Figure 3 (a), FIrPic as the first ligand shows two C-C stretching vibrations of the aromatic ring at 1346 and 1292 cm⁻¹ and the C-F stretching vibration at 1246 cm⁻¹. Besides, the C=N stretching vibration of FIrPic occurs at 1477 cm⁻¹, and two C-N stretching vibrations are found at 1165 and 1107 cm⁻¹. In addition, the absorption band at 1051 cm⁻¹ is ascribed to the in-plane CH bending vibration of the aromatic ring, and the absorption bands at 833, 762 and 700 cm⁻¹ are attributed to the out-of-plane CH bending vibrations in the 900−670 cm⁻¹ region.

Since the C=O and C-O in carboxyl ion (-COO-) are two essentially equivalent $ \mathrm{C}\text{＝}\mathrm{O} $ bonds and are strongly coupled to each other, the corresponding absorption band of the -COO- group in FIrPic is split into two parts: the asymmetrical stretching vibration νas(COO) at 1647 cm⁻¹ and the symmetrical stretching vibration νs(COO) at 1406 cm⁻¹. The Δν(as–s)(COO) of 241 cm⁻¹ is larger than 200 cm^−1[28] because FIrPic is a mono-coordinate complex. However, the bimodal characteristic absorption bands are almost invisible when the coupling between two CO bonds in carboxylate (-COO-) is weak. This suggests successful coordination between the first ligand FIrPic and Eu(Ⅲ) ions.

The overall vibration mode of the aromatic ring skeleton in the chosen complex FIrPic presents a series of sharp absorption peaks at 1601, 1563, and 1512 cm⁻¹ with different intensities. According to the hard/soft acid/base principle^[29], the rare-earth Eu ion is a classic hard acid with a coordination number of 8, which makes it easier to form stable complexes with hard-base ligands containing oxygen or nitrogen atoms. Therefore, compared with the original complex FIrPic, the vibration bands of the aromatic ring skeleton of the bimetallic Eu-Ir complex shift to lower frequencies (1612, 1543, and 1504 cm⁻¹) as the degree of conjugation increases. The absorption band at 1053 cm⁻¹ is caused by the in-plane CH bending vibration of the aromatic ring in the Eu-Ir complex.

Compared with the second ligand Phen, the out-of-plane C-H bending vibrations of the aromatic ring of the Eu-Ir complex are shifted from 864 and 739 cm⁻¹ to 785 and 710 cm⁻¹, respectively. The -C=N stretching vibration shifts from 1493 cm⁻¹ to 1414 cm⁻¹ because two Eu-N coordination bonds formed in the coordination of the Eu(III) ion with the second ligand, Phen, resulting in a reduced force constant of C=N^[30]. Hence, the corresponding absorption peak shifts to lower frequencies, indicating that the coordination between the second ligand Phen and Eu(Ⅲ) ions is successful.

The absorption peaks at 2925 and 2855 cm⁻¹ are related to the asymmetric CH₂ stretching vibration and symmetric CH₂ stretching vibration in the active ligand UA, respectively. When Eu(III) ions form a stable complex with UA, the relevant vibration peaks will shift towards higher frequencies because of the hyperconjugation effect; for example, the asymmetrical stretching vibration ν_as(CH₂) and symmetrical stretching vibration ν_s(CH₂) of Eu(FIrPic)₂(Phen)UA are shifted to 2930 and 2860 cm⁻¹.

The absorption band observed at 1711 cm⁻¹ is attributed to the C=O stretching vibration of carboxylic acid in UA. However, no C=O stretching vibration band is found in the FT-IR spectrum of Eu(FIrPic)₂(Phen)UA because the carboxyl group (COOH) reacts with the Eu ion to generate a carboxylate. Similarly, the C-OH out-of-plane bending vibration of carboxylic acid in UA is at 910 cm⁻¹ while the corresponding C-OH vibration of Eu(FIrPic)₂(Phen)UA is invisible due to the Eu-O bond formed in the coordination between C-OH group and the Eu ion, indicating that the active ligand UA is successfully coordinated with the Eu(Ⅲ) ion. Finally, the vibration frequency of 2974 cm⁻¹ is assigned to the =CH₂ symmetric stretching at the end of the double bond, and further polymerization can take place because of the C=C double bond of Eu(FIrPic)₂(Phen)UA.

As can be seen in Figure 3(b), the band shape of phosphorescent copolymer PM-Eu-Ir accords with that of complex Eu(FIrPic)₂(Phen)UA. The characteristic absorption peak at 1732 cm⁻¹ in copolymer PM-Eu-Ir is associated with the C=O stretching vibration mode in PMMA. Compared with the absorption peak at 2974 cm⁻¹ for Eu(FIrPic)₂(Phen)UA, there is no absorption peak corresponding to the C=C double bond in the FT-IR spectrum of PM-Eu-Ir as a result of successful copolymerization. Besides, the Eu-N stretching vibration band at 581 cm⁻¹ in PM-Eu-Ir is much weaker than that in Eu(FIrPic)₂(Phen)UA; this is because the Eu-Ir complex accounts for a smaller proportion of whole block copolymer macromolecules after the occurrence of copolymerization.

Figure 4 (color online) displays the UV-vis absorption spectra of Ir complex FIrPic, bimetallic complex Eu(FIrPic)₂(Phen)UA, and phosphorescent copolymer PM-Eu-Ir dissolved in CH₂Cl₂ solution (1×10⁻⁴ mol/L) at room temperature. The spectrum of complex Eu(FIrPic)₂(Phen)UA has two absorption bands at 275 nm and 344 nm in the near-UV region, which can be assigned to the absorption from the ligand Phen and the conjugated structures formed in the complex, respectively. Compared to Ir complex FIrPic, the highest absorption peak occurs at a longer wavelength (344 nm), and the corresponding absorption intensity is stronger than that of the peak at around 378 nm for FIrPic, which matches well with commercial 365 nm UV-LED chips.

The absorption spectrum for copolymer PM-Eu-Ir displays three clear absorption bands centered at 229, 269, and 340 nm. This demonstrates that the prepared copolymer-based red phosphor can be excited by UV light, such as commercial 365 nm chips. The absorption peak located at 229 nm is primarily attributed to the π-π* interactions between UA and Phen ligands in complex Eu(FIrPic)₂(Phen)UA. Moreover, the absorption bands around 269 and 340 nm are mainly ascribed to the conjugated double bonds in the benzene ring of the ligands. It can be concluded that little or no dissociation occurs during the polymerization between MMA and Eu(FIrPic)₂(Phen)UA, which remain a stable bimetallic ligand system. Thus, central Eu³⁺ ions can emit the characteristic fluorescence after absorbing the UV light^[31].

The FE-SEM micrographs of synthetic bimetallic complex Eu(FIrPic)₂(Phen)UA and the phosphorescent copolymer PM-Eu-Ir can be seen in Figure 5. There are significant differences between them: the Eu(FIrPic)₂(Phen)UA presents a long laminated structure with an average width of about 1 µm, while the micro-morphology of PM-Eu-Ir is a typical multilayer spatial network structure consisting of micro-size spherical aggregates, and the interior of each aggregate is composed of intertwined molecular chains, suggesting that the polymerization reaction is complete in this experiment, not a simple mixture of Eu-Ir complex monomers and linear polymer PMMA.

The thermal properties of the copolymer PM-Eu-Ir were investigated by differential scanning calorimetry (DSC), thermogravimetric (TG), and differential thermal gravimetric (DTG) analysis at a heating rate of 10 °C/min under N₂ atmosphere. As shown in Figure 6 (color online), the glass transition temperature (T_g) of copolymer PM-Eu-Ir was found to be 134 °C; a high T_g value is essential for maintaining good thermal stability of phosphors when encapsulated into LED devices. According to the ASTM E1641-2007 (Standard Test Method for Decomposition Kinetics by Thermogravimetric)^[32], the intersection of an extended baseline and line composed of a point of 5% weight loss and a point of 50% weight loss is defined as the onset decomposition temperature (T_onset), as illustrated in Figure 6(b), the T_onset of synthetic PM-Eu-Ir is 257 °C.

It seems obvious that no weight loss occurs between 100 °C and T_onset because there is no coordination water in copolymer PM-Eu-Ir. Then, the maximum decomposition rate of −1.7%/min appears at 379 °C with a weight loss of 74%. Eventually, complete decomposition of the sample is reached at 419 °C with a residue of 1.4wt%. More specifically, the TG curve of PM-Eu-Ir displays two weight loss steps. The first step in the range of 257~300 °C changes gently, and the weight loss of 12% is primarily related to the decomposition of small molecule ligands and precursor complexes from the copolymer side chain. The second step (310~410 °C) changes sharply, the maximum weight loss of 86% in this step is due to the pyrolysis of the main chain of the copolymer and the complete decomposition of complexes at higher temperatures. Besides, the above results illustrate that the prepared red-phosphorescent copolymer remains stable up to 257 °C, indicating PM-Eu-Ir has good thermal stability in a wide range (25~250 °C) and meets the temperature requirements for the LED emitting layer, given that the operating temperature of LED is generally below 150 °C^[33-34].

The excitation and emission spectra of synthetic bimetallic complex Eu(FIrPic)₂(Phen)UA and phosphorescent copolymer PM-Eu-Ir dissolved in the CH₂Cl₂ solution (1 × 10⁻⁴ mol/L) at room temperature are presented in Figure 7 (a) and 7 (b) (color online). The excitation spectrum for Eu(FIrPic)₂(Phen)UA complex exhibits a narrow band in the 350 to 420 nm range and two excitation wavelengths at 375 and 393 nm, respectively, which match well with commercial UV-LED chips. In particular, the peak located at 393 nm indicates that adding Ir complex increases the absorption intensity of Eu ions to violet 400 nm light irradiation, extending the applicability of prepared bimetallic complexes to UV-LED chips.

When Eu(FIrPic)₂(Phen)UA is excited by 365 nm UV light, its emission spectrum shows a multi-peak structure covering the yellow to red light visible region of the spectrum, and the five sharp emission peaks at 579, 592, 613, 652 and 700 nm are attributed to the ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transitions of the Eu(III) ion, respectively^[35-37]. Specifically, the ⁵D₀→⁷F₁ transition at 592 nm is a magnetic dipole transition less affected by surroundings, while the highest peak at 613 nm caused by ⁵D₀→⁷F₂ transition corresponds to the electric dipole transition susceptible to external electromagnetic fields^[38], its relevant CIE chromaticity coordinates (0.581, 0.311) are shown in Figure 7(c). As the Ir complex FIrPic emits bright green fluorescence under UV light excitation (see Figure 8, color online), the synthetic bimetallic complex Eu(FIrPic)₂(Phen)UA exhibits excellent red emission under UV light of the same wavelength, which suggests that the trivalent Eu ion can be efficiently sensitized by the addition of Ir complex, without affecting the emission characteristics of Eu³⁺ ions.

The excitation spectrum of the copolymer PM-Eu-Ir presented in Figure 7(b) displays an excitation band between 350 nm and 420 nm and a peak wavelength of 359 nm, indicating that PM-Eu-Ir can match well with 365 nm N-UV LED chips. Compared with bimetallic complex Eu(FIrPic)₂(Phen)UA, the excitation peak of copolymer PM-Eu-Ir shows a blue-shift from 393 to 359 nm. This may be because the nonluminous ground-state complex formed in copolymerization of Eu(FIrPic)₂(Phen)UA complex with MMA leads to static quenching, which typically changes the excitation wavelength for fluorescent molecules. Moreover, the PM-Eu-Ir exhibits a sharp and strong emission peak at 612 nm when excited at 365 nm, and its corresponding CIE coordinates (0.461, 0.254) are closer to the reddish-violet light region, as marked in Figure 7(c). Furthermore, five characteristic emission peaks at 583, 592, 612, 651, and 700 nm are also attributed to the ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transitions of Eu³⁺ ions, respectively.

The fluorescence-decay curve of Eu³⁺ ions in the copolymer PM-Eu-Ir and bimetallic complex Eu(FIrPic)₂(Phen)UA recorded at the excitation wavelength of 365 nm is displayed in Figure 9 (color online). Eu(III) ions show a reddish-violet emission at 612 nm due to the ⁵D₀ → ⁷F₂ transition, and no emission is observed from Ir(III) ions in the copolymer after being excited.

The decay curve for sample PM-Eu-Ir is fitted to the exponential equation (1) as follows:

$ I(t)=A+B_{1} \exp(-t/\tau_{1})+B_{2} \exp(-t/\tau_{2}) \quad, $ (1)

where τ₁ and τ₂ are the shorter and longer lifetime parameters, respectively. B₁ and B₂ are fitting constants. The average fluorescence lifetime of PM-Eu-Ir is calculated by displayed equation (2).

$ \langle \tau \rangle=\frac{B_1\tau_1^2+B_2\tau_2^2}{B_1\tau_1+B_2\tau_2}\quad. $ (2)

The lifetime of the copolymer phosphor PM-Eu-Ir is 634.54 µs with a fitting coefficient χ² of 1.383. The lifetime of bimetallic complex Eu(FIrPic)₂(Phen)UA is 1042.91 μs with the fitting coefficient χ² of 1.120. PM-Eu-Ir has a shorter fluorescent lifetime than Eu(FIrPic)₂(Phen)UA, resulting in a shorter copolymer response time.

In addition, the quantum efficiency (QE) of the phosphors was tested under 365 nm excitation, showing that the QE of Eu(FIrPic)₂(Phen)UA is 22.63%, which is slightly higher than that of other reported Eu³⁺-complex red phosphors^[39-40]. Moreover, the QE of PM-Eu-Ir is 30.68%, higher than that of Eu(FIrPic)₂(Phen)UA.

The electroluminescence (EL) spectra of the fabricated red LED lamp beads with PM-Eu-Ir at 3.5 V are presented in Figure 10(a) (color online). Apart from the main peak at 612 nm, we can see other characteristic peaks of Eu(III) ion emission. The EL spectral curve of the red LEDs is consistent with that of the copolymers. Besides, as shown in the subgraph of Figure 10(a), the CIE chromaticity coordinates of the tested red LEDs (0.480, 0.323) accord with that of the copolymer PM-Eu-Ir (0.461, 0.254). Fig. 10(b) (color online) displays the luminance–voltage characteristics of the original LEDs (365 nm UV chip) without PM-Eu-Ir and red LEDs fabricated with PM-Eu-Ir, which shows the maximum luminance (149800 cd/m²) of the red LEDs increases by 2.3 times compared to that (45920 cd/m²) of the original LEDs. These results indicate that PM-Eu-Ir has strong application potential in red LEDs.

In this paper, we propose a red-emitting copolymeric phosphor PM-Eu-Ir synthesized through radical polymerization using the complex Eu(FIrPic)₂(Phen)UA as the precursor and PMMA as the macromolecular ligand. The bimetallic Eu-Ir complex containing central Eu³⁺ ions and Ir complex FIrPic (the first ligand), Phen (the second ligand), and UA (the active ligand) were synthesized by a simple and effective method. The prepared copolymeric phosphor PM-Eu-Ir was shown to be soluble in common organic solvents at room temperature, which significantly improves the film-forming performance of phosphors used in large-area displays and the packaging of remote phosphor-converted LEDs. The micro-morphology of PM-Eu-Ir is a typical multilayer spatial network structure, indicating a complete polymerization reaction between Eu-Ir complex monomers and linear polymer PMMA. As far as prepared phosphorescent copolymer PM-Eu-Ir is concerned, it shows good thermal stability in the temperature range of 25~250 °C, a strong red emission at 612 nm when excited at 365 nm, and an average fluorescence lifetime of 634.54 μs. The fabricated LEDs with the copolymer PM-Eu-Ir display red light exhibit emission with a luminance of 149800 cd/m², meeting the requirements of commercial near-UV LED applications.