A Novel Polymethyl Methacrylate Derivative Grafted with Cationic Iridium(III) Complex Units: Synthesis and Application in White Light-Emitting Diodes

A novel orange-yellow-emitting polymethyl methacrylate derivative grafted with cationic iridium(III) complex units was synthesized and used as down-conversion luminescent materials in light-emitting diodes (LEDs). The polymer had a thermal decomposition temperature (Td) of 275 °C. With the temperature increasing from 20 to 100 °C, its photoluminescent intensity decreased to 76.8% with thermal quenching activation energy (Ea) of 0.2775 eV. A series of LEDs was fabricated by 460 nm blue GaN chips and the polymer blended in silicone at different concentrations. At 4.0 wt %, a cold white LED was obtained, the correlated color temperature (CCT) was 10,050 K, color rendering index (CRI) was 71.2, luminous efficiency (ηL) was 5.3 lm·w−1, and Commission Internationale de L’Eclairage (CIE) chromaticity coordinates were (0.30, 0.24). At 5.0 wt.%, the LED emitted neutral white light, its CCT was 4938 K, CRI was 75, ηL was 13.8 lm·w−1, and the CIE value was (0.34, 0.27). At 5.5 wt %, 6.0 wt %, 7.0 wt %, and 8.0 wt %, the LEDs all emitted warm white light; their CCTs were 3446, 3093, 2557, and 2337 K, respectively; their CRIs were 73.6, 71.8, 63.8, and 59.0, respectively; their ηL were 18.1, 16.3, 14.8, and 13.7 lm·w−1, respectively; and their CIE values were (0.36, 0.30), (0.40, 0.35), (0.45, 0.38), and (0.50, 0.42), respectively. At 9.0 wt %, the blue light of GaN chip was completely absorbed by the polymer and only the orange-yellow light of the polymer emitted. The results suggested the polymer was a promising orange-yellow-emitting phosphor candidate for white LEDs, especially for warm white LEDs.


Introduction
As the new generation solid-state light sources, white light-emitting diodes (LEDs) have some fascinating advantages such as high efficiency, energy-saving, long lifetime, small size, and environmentally friendly properties, and have been widely used in general illumination, full-color displays, liquid crystal display backlights, automotive headlamps, and so on [1][2][3]. At present, white LEDs are mainly fabricated by the combination of ultraviolet or blue chips and down-conversion luminescent materials [1][2][3][4][5][6]; relatively, the blue chips are more commonly used since the high brightness blue GaN chips were successfully contrived in the late 1980s and early 1990s [7,8]. For example, the most commonly commercial white LEDs are just fabricated by the combination

Materials and Equipments
All chemicals and reagents were purchased from Shanghai Titan Scientific Co. Ltd. (in Shanghai, China) and used without further purification unless otherwise stated. 1 H NMR spectra were recorded on a Bruker AV400 spectrometer operating at 400 MHz with tetramethylsilane (TMS) as internal standard. Mass spectra (MS) were obtained on a Bruker amaZon SL liquid chromatography mass spectrometer (LC-MS) with an electrospray ionization (ESI) interface using acetonitrile as matrix solvent. Infrared spectra were recorded using a Thermo Scientific Nicolet IS10 Fourier transform infrared (FTIR) spectrometer. Thermogravimetry (TG) was measured on a Netzsch STA449F3 thermal analyzer (NETZSCH-Gerätebau GmbH, Selb, Germany) at a heating rate of 10 • C·min −1 under N 2 . Photoluminescence (PL) spectra were obtained on a JobinYvon FL3-21 spectrofluorometer, and the temperature of the solid sample was controlled by a temperature controller (REX-C110, Kaituo Compressor Parts Co. Ltd., Dongguan City, China). Molecular weights and molecular weight distribution (M w /M n ) values of the polymer were determined by Waters 1515 gel permeation chromatograph (GPC) equipped with a Waters 2414 refractive index detector, calibrated using polystyrenes; tetrahydrofuran (THF) was used as an eluent at a flow rate of 1.0 mL·min −1 at 40 • C.

Synthesis of the Polymer (Compound 5)
A mixture of the cationic iridium(III) complex (0.50 g, 0.50 mmol), methyl methacrylate (1.65 g, 16.5 mmol), and 2,2'-azobis(2-methylpropionitrile) (AIBN, 0.080 g, 0.49 mmol) in anhydrous toluene (5 mL) was heated at 60 • C with stirring for 36 h under argon atmosphere. After cooling to room temperature, the resultant yellow solid was filtered and solved in tetrahydrofuran (THF, 5mL), and was then added drop by drop into methanol (100 mL), after which the polymer was separated out and filtered again. Such a dissolution and precipitation process was repeated three times. The purified polymer was dried under vacuum at 60 • C for 24 h. The number-and weight-averaged molecular weights (M n , M w ) of this polymer were estimated by GPC to be 14,007 and 18,229, respectively, with polystyrene as a standard.

Fabrication and Performance Measurements of LEDs
LEDs using the polymer as down-conversion luminescent materials were fabricated and measured. The polymer was blended in silicone at different mass ratios of 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0 wt%. Every mixture of the polymer in silicone was stirred homogeneously and coated onto 460 nm-emitting blue GaN chips until just the reflective cavities were filled up. All LEDs were dried and solidified at 150 • C for 3 h. The LEDs were all operated at 20 mA forward current, their electroluminescent spectra were recorded on a high accurate array spectrometer (HSP6000, HongPu Optoelectronics Technology Co. Ltd., Hangzhou City, China), and their performances were measured by an integrating sphere spectroradiometer system (Everfine PMS-50, Everfine Photo-E-Info Co., Ltd., Hangzhou, China).

Photoluminescence Property
The normalized photoluminescent excitation (Ex) and emission (Em) spectra of the cationic iridium(III) complex monomer (i.e., compound 4) in CH 2 Cl 2 solution (1 × 10 −5 mol·L −1 ), as well as those of the polymer in CH 2 Cl 2 solution (0.001 wt %), blended in silicone (2.0 wt %), and in powder are shown in Figure 1. In solution, the emission spectrum of the polymer was very similar to that of the cationic iridium(III) complex monomer, which suggests that luminescence of the polymer originated from the cationic iridium(III) complex units and that the cationic iridium(III) complex units were well dispersed in the polymer chain. Although the excitation spectra of the polymer were measured under three different conditions, they were in the same zone, and all located between 225 nm and 545 nm with the maximum excitation wavelength (λ ex,max ) of 337 nm. They all covered the blue light zone, which means the polymer can be excited by blue light from GaN chips (415−505 nm, λ em,max = 460 nm). However, in every excitation spectrum, relative to their maximum excitation zone around 337 nm, the relative intensities of the excitation spectra in blue light zone were different; the relative intensity of that in solution was the lowest, in silicone was the second, and in powder was the highest, mainly because of the sequential increasing of molecular interaction and π-π stacking with the increasing concentration of the polymer. Furthermore, mainly because of these reasons, the emission spectra of the polymer in CH 2 Cl 2 solution, in silicone, and in powder exhibited sequential red shifts; the maximum emission wavelengths were 560 nm, 568 nm, and 574 nm, respectively. Nevertheless, the emission spectra of the polymer under the three different conditions were all mainly located in 485−755 nm, and all of them were orange-yellow light, which contained some red light components. Theoretically, this polymer was beneficial for obtaining warm white light when it was used as luminescent materials in GaN-based white LEDs [34].

Photoluminescence Property
The normalized photoluminescent excitation (Ex) and emission (Em) spectra of the cationic iridium(III) complex monomer (i.e., compound 4) in CH2Cl2 solution (1 × 10 −5 mol·L −1 ), as well as those of the polymer in CH2Cl2 solution (0.001 wt %), blended in silicone (2.0 wt %), and in powder are shown in Figure 1. In solution, the emission spectrum of the polymer was very similar to that of the cationic iridium(III) complex monomer, which suggests that luminescence of the polymer originated from the cationic iridium(III) complex units and that the cationic iridium(III) complex units were well dispersed in the polymer chain. Although the excitation spectra of the polymer were measured under three different conditions, they were in the same zone, and all located between 225 nm and 545 nm with the maximum excitation wavelength (λex,max) of 337 nm. They all covered the blue light zone, which means the polymer can be excited by blue light from GaN chips (415−505 nm, λem,max = 460 nm). However, in every excitation spectrum, relative to their maximum excitation zone around 337 nm, the relative intensities of the excitation spectra in blue light zone were different; the relative intensity of that in solution was the lowest, in silicone was the second, and in powder was the highest, mainly because of the sequential increasing of molecular interaction and π-π stacking with the increasing concentration of the polymer. Furthermore, mainly because of these reasons, the emission spectra of the polymer in CH2Cl2 solution, in silicone, and in powder exhibited sequential red shifts; the maximum emission wavelengths were 560 nm, 568 nm, and 574 nm, respectively. Nevertheless, the emission spectra of the polymer under the three different conditions were all mainly located in 485−755 nm, and all of them were orange-yellow light, which contained some red light components. Theoretically, this polymer was beneficial for obtaining warm white light when it was used as luminescent materials in GaN-based white LEDs [34].

Thermal Stability and Thermal Quenching Property
Because of the heat treatment in the device fabrication and exothermic working process of LEDs, it is necessary to investigate the heat resisting property of luminescent materials, mainly including

Thermal Stability and Thermal Quenching Property
Because of the heat treatment in the device fabrication and exothermic working process of LEDs, it is necessary to investigate the heat resisting property of luminescent materials, mainly including the thermal stability and thermal quenching propery. The thermal stability of the polymer is investigated by thermogravimetry (TG) under N 2 and the resultant TG curve is shown in Figure 2. The TG curve showed a major mass loss after 275 • C, which means 275 • C was its thermal decomposition temperature (T d ). Such T d is enough to meet the heat-resistance requirement because LED devices are usually fabricated and work at a temperature below 150 • C [36].
Polymers 2019, 11, x FOR PEER REVIEW 6 of 11 investigated by thermogravimetry (TG) under N2 and the resultant TG curve is shown in Figure 2.
The TG curve showed a major mass loss after 275 °C, which means 275 °C was its thermal decomposition temperature (Td). Such Td is enough to meet the heat-resistance requirement because LED devices are usually fabricated and work at a temperature below 150 °C [36]. The photoluminescent intensity of luminescent materials usually decreases with the increasing temperature, which is known as the thermal quenching property. The photoluminescent spectra of the polymer excited by 460 nm blue light were measured with increasing temperature from 20 to 180 °C and are shown in Figure 3. From 20 to 180 °C, the photoluminescent intensities obviously and sequentially decreased with the increasing temperature. The relative intensities of the maximum emission wavelength (λem,max) at 573 nm were 100% (at 20 °C), 96.8% (at 40 °C), 89.7% (at 60 °C), 84.6% (at 80 °C), 76.8% (at 100 °C), 70.8% (at 120 °C), 62.6% (at 140 °C), 56.6% (at 160 °C), and 50.7% (at 180 °C), respectively. However, the shapes, emission wavelength bands, and λem,max of these spectra almost did not change, which suggested the light color of the polymer had high stability.   The photoluminescent intensity of luminescent materials usually decreases with the increasing temperature, which is known as the thermal quenching property. The photoluminescent spectra of the polymer excited by 460 nm blue light were measured with increasing temperature from 20 to 180 • C and are shown in Figure 3. From 20 to 180 • C, the photoluminescent intensities obviously and sequentially decreased with the increasing temperature. The relative intensities of the maximum emission wavelength (λ em,max ) at 573 nm were 100% (at 20 • C), 96.8% (at 40 • C), 89.7% (at 60 • C), 84.6% (at 80 • C), 76.8% (at 100 • C), 70.8% (at 120 • C), 62.6% (at 140 • C), 56.6% (at 160 • C), and 50.7% (at 180 • C), respectively. However, the shapes, emission wavelength bands, and λ em,max of these spectra almost did not change, which suggested the light color of the polymer had high stability. investigated by thermogravimetry (TG) under N2 and the resultant TG curve is shown in Figure 2.
The TG curve showed a major mass loss after 275 °C, which means 275 °C was its thermal decomposition temperature (Td). Such Td is enough to meet the heat-resistance requirement because LED devices are usually fabricated and work at a temperature below 150 °C [36]. The photoluminescent intensity of luminescent materials usually decreases with the increasing temperature, which is known as the thermal quenching property. The photoluminescent spectra of the polymer excited by 460 nm blue light were measured with increasing temperature from 20 to 180 °C and are shown in Figure 3. From 20 to 180 °C, the photoluminescent intensities obviously and sequentially decreased with the increasing temperature. The relative intensities of the maximum emission wavelength (λem,max) at 573 nm were 100% (at 20 °C), 96.8% (at 40 °C), 89.7% (at 60 °C), 84.6% (at 80 °C), 76.8% (at 100 °C), 70.8% (at 120 °C), 62.6% (at 140 °C), 56.6% (at 160 °C), and 50.7% (at 180 °C), respectively. However, the shapes, emission wavelength bands, and λem,max of these spectra almost did not change, which suggested the light color of the polymer had high stability.   The speed of thermal quenching also can be evaluated and represented by its activation energy (E a ) value; a higher E a means slower thermal quenching and higher thermal stability [16,28,37]. The E a can be obtained from the variant of Arrhenius equation [28]: where I and I o represent the relative photoluminescent intensity at the experimental temperature (T) and room temperature (20 • C, i.e., 293 K), respectively; A is a constant; and k B is Boltzmann's constant (8.617 × 10 −5 eV·K −1 ). The relative photoluminescent intensities (I) of the polymer at the above-mentioned different temperatures (T) are shown in Figure 4. The ln[(I o /I) − 1] versus 1/T is also shown in an inset in Figure 4. The data were well-fitted, then E a can be calculated by the slope value, that is, −(E a /k B ); which was equal to 0.2775 eV. In comparison with some reported orange-yellow or yellow phosphors in recent years [34,[37][38][39], this E a value was at an intermediate level, which suggested the polymer had high thermal stability and could be applied in LEDs. The speed of thermal quenching also can be evaluated and represented by its activation energy (Ea) value; a higher Ea means slower thermal quenching and higher thermal stability [16,28,37]. The Ea can be obtained from the variant of Arrhenius equation [28]: where I and Io represent the relative photoluminescent intensity at the experimental temperature (T) and room temperature (20 °C, i.e., 293 K), respectively; A is a constant; and kB is Boltzmann's constant (8.617 × 10 −5 eV·K −1 ). The relative photoluminescent intensities (I) of the polymer at the abovementioned different temperatures (T) are shown in Figure 4. The ln[(Io/I) − 1] versus 1/T is also shown in an inset in Figure 4. The data were well-fitted, then Ea can be calculated by the slope value, that is, −(Ea/kB); which was equal to 0.2775 eV. In comparison with some reported orange-yellow or yellow phosphors in recent years [34,[37][38][39], this Ea value was at an intermediate level, which suggested the polymer had high thermal stability and could be applied in LEDs.

Performances of the Polymer Used in LEDs
In order to investigate the applicability and efficacy of the polymer used as down-conversion luminescent materials in white LEDs, a series of GaN-based LEDs were fabricated using the polymer as down-conversion luminescent materials blended in silicone at different concentrations (x wt %, x = 4.0, 5.0, 5.5, 6.0, 7.0, 8.0, 9.0). The emission spectra of the LEDs are shown in Figure 5, and the performance data are listed in Table 1.

Performances of the Polymer Used in LEDs
In order to investigate the applicability and efficacy of the polymer used as down-conversion luminescent materials in white LEDs, a series of GaN-based LEDs were fabricated using the polymer as down-conversion luminescent materials blended in silicone at different concentrations (x wt %, x = 4.0, 5.0, 5.5, 6.0, 7.0, 8.0, 9.0). The emission spectra of the LEDs are shown in Figure 5, and the performance data are listed in Table 1.
Almost every spectrum consisted of two emission peaks. The left peaks mainly located at 400~500 nm, and its λ em, max was 460 nm, which can be assigned to the emission of blue GaN chips. The right peaks mainly located at 500~780 nm and were the emission of the polymer. With the increasing of the blending concentrations, the relative emission intensity of the polymer gradually raised and, at the same time, the emission of GaN chips gradually declined and disappeared at 9.0 wt %. Mainly because of the sequential enhancing of molecular interaction and π-π stacking with the increasing of the blending concentrations, the maximum emission wavelengths (λ em,max ) of the polymer showed a slight red shift, as listed in Table 1, from 4.0 wt % to 9.0 wt %, and the λ em,max changed from 583 nm to 590 nm.  Almost every spectrum consisted of two emission peaks. The left peaks mainly located at 400~500 nm, and its λem, max was 460 nm, which can be assigned to the emission of blue GaN chips. The right peaks mainly located at 500~780 nm and were the emission of the polymer. With the increasing of the blending concentrations, the relative emission intensity of the polymer gradually raised and, at the same time, the emission of GaN chips gradually declined and disappeared at 9.0 wt %. Mainly because of the sequential enhancing of molecular interaction and π-π stacking with the increasing of the blending concentrations, the maximum emission wavelengths (λem,max) of the polymer showed a slight red shift, as listed in Table 1, from 4.0 wt % to 9.0 wt %, and the λem,max changed from 583 nm to 590 nm.
As shown in Figure 6, from 4.0 wt % to 9.0 wt %, the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the LEDs showed the light color gradually changed from cold white light to neutral white light, to warm white light, and then to orange-yellow light. In detail, at 4.0 wt % (No. 1), the LED emitted cold white light, its correlated color temperature (CCT) was 10,050 K, and its CIE values were (0.30, 0.24). At 5.0 wt % (No. 2), the light color of the LED became neutral white  As shown in Figure 6, from 4.0 wt % to 9.0 wt %, the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the LEDs showed the light color gradually changed from cold white light to neutral white light, to warm white light, and then to orange-yellow light. In detail, at 4.0 wt % (No. 1), the LED emitted cold white light, its correlated color temperature (CCT) was 10,050 K, and its CIE values were (0. 30 Figure 6 and visually exhibited the above-mentioned changes. As for their color rendering index (CRI), as listed in Table 1, from No. 1 to No. 7, they increased first and then gradually become smaller. At 5.0 wt % (No. 2), the CRI was 75, which was the maximum value and higher than that of YAG:Ce, which was most commonly used in GaN-based white LEDs [16,28,34]. Their luminous efficiencies (η L ) also increased first and then gradually become smaller and reached the maximum value of 18.1 lm·w −1 at 5.5 wt % (No. 3). In contrast to similar work in past [16,28,34], especially relative to those of small molecular organic cationic iridium(III) complexes [34], the luminous efficiencies of such LEDs were a bit low, probably be caused by various factors; for example, the doping concentration of the cationic iridium(III) complex units was not optimal, the purity of the polymer needed to be further improved, or the devices needed to be further optimized.
absorbed by the polymer, this LED only emitted the orange-yellow light of the polymer, its CCT was 2051 K, and its CIE values were (0.30, 0.24). The working state photographs of No. 2 (neutral white light), No. 3 (warm white light), No. 4 (warm white light), No. 5 (warm white light), and No. 7 (orange-yellow light) are shown in Figure 6 and visually exhibited the above-mentioned changes. As for their color rendering index (CRI), as listed in Table 1, from No. 1 to No. 7, they increased first and then gradually become smaller. At 5.0 wt % (No. 2), the CRI was 75, which was the maximum value and higher than that of YAG:Ce, which was most commonly used in GaN-based white LEDs [16,28,34]. Their luminous efficiencies (ηL) also increased first and then gradually become smaller and reached the maximum value of 18.1 lm·w −1 at 5.5 wt % (No. 3). In contrast to similar work in past [16,28,34], especially relative to those of small molecular organic cationic iridium(III) complexes [34], the luminous efficiencies of such LEDs were a bit low, probably be caused by various factors; for example, the doping concentration of the cationic iridium(III) complex units was not optimal, the purity of the polymer needed to be further improved, or the devices needed to be further optimized.

Conclusions
A novel orange-yellow-emitting polymethyl methacrylate derivative grafted with cationic iridium(III) complex units was synthesized and successfully used as down-conversion luminescent materials in LEDs. The polymer had a thermal decomposition temperature of 275 °C, with the temperature increasing from 20 to 100 °C, and its photoluminescent intensity decreased to 76.8% with

Conclusions
A novel orange-yellow-emitting polymethyl methacrylate derivative grafted with cationic iridium(III) complex units was synthesized and successfully used as down-conversion luminescent materials in LEDs. The polymer had a thermal decomposition temperature of 275 • C, with the temperature increasing from 20 to 100 • C, and its photoluminescent intensity decreased to 76.8% with thermal quenching E a of 0.2775 eV. Under the excition of 460 nm blue GaN chips, the polymer can be used for obtaining cold/neutral/warm white LEDs, especially warm white LEDs. Overall, the best performances of the white LEDs were obtained when the polymer was blended in silicone at 5.5 wt %, the corresponding CRI was 73.6, CCT was 3446 K, the light color was warm white light, CIE values were (0.36, 0.30), and η L was 18.1 lm·w −1 .