Tunable white light emission of a large area film-forming macromolecular complex with a high color rendering index

The technological route of pre-polymerization and post-coordination was employed to obtain a rapid film-forming macromolecular complex emitting white light by simultaneously combining poly (styrene-co-glycidyl methacrylate) with three primary color complexes [r(R)/g(G)/b(B)]. The white light emission of the macromolecular complex was realized by tuning the ratios of the trichromatic complexes or altering excitation wavelengths. Upon excitation at 365 nm, macromolecular complex 5 with R:G:B = 1:10:1 exhibits three discrete characteristic peaks at 458 (blue emission), 543 (green emission), and 612 (red emission) nm. The corresponding CIE coordinates of (0.332, 0.337) are in close proximity to pure white light (0.330, 0.330). Macromolecular complex 3 (R:G:B = 1:8:0.5) achieves white emission being excited at the wavelength of 375 nm. Macromolecular complex 5 was melted at 140 °C, dropped on a 365 nm UV chip, and then cooled to room temperature to fabricate an LED device with CIE coordinates of (0.35, 0.34), CCT of 5306 K, and CRI of 95.2. The prepared macromolecular complex film further shows vivid color reproduction for real objects, confirming the high rendering index of the macromolecular complex. In addition, the macromolecular complex remains stable up to 272.6 °C, meeting the requirements of operating temperature for LEDs. This work establishes a rapid film-forming macromolecular complex with potential application in NUV-WLEDs.

RGB mode. In recent years, much effort has been devoted to improving the color rendering index (CRI) of phosphor. Unsatisfactorily, significant improvement in this property has not been reached and the CRI is mostly in the range of 80-85 [5][6][7].
Among the numerous kinds of phosphors, assemblies of lanthanide(Ш)-containing complexes have attracted considerable attentions for their effective and characteristic luminescence, with generally chosen red (Eu 3+ , Pr 3+ , Sm 3+ ), green (Tb 3+ , Er 3+ ), and blue (Dy 3+ , Be 2+ , Ce 3+ ) emissive ions [8][9][10][11][12]. What is worth noting is that most studies have focused on zinc(II) complexes [13] in the research field of blue light emission to replace beryllium(II) complexes, as the latter is too toxic to be applied in practice. Afterwards, small molecular complexes were introduced in the polymer matrix to avoid intrinsic shortcomings such as poor film-forming property in the process of preparation [14]. Wolff et al. [15] solved the film-forming problem for the first time by doping organic rare earth complexes into polymer matrix. Their research results indicated that the polymer matrix had fluorescence enhancement effect on rare earth ions. Since then, several polymers as doped substrates have been investigated [16][17][18]. However, there are two major drawbacks for doped polymer phosphors. One is the poor compatibility between rare earth complexes and polymers, which gives rise to non-uniform dispersion of complexes in polymer matrix. Another is energy transfer caused by the mutual excitation of rare earth ions. Compared with the doped system, bonding the luminescent ions directly into the polymer chain would overcome the above defects. Hence, many scholars paid their attentions to bonding polymer phosphors. Shunmugam et al. [19] reported a novel white emitting functionalized polymer chelating simultaneously with Eu 3+ , Tb 3+ , and Dy 3+ . Gao et al. [20] bonded naphthoic acid and benzoic acid onto the side chains of polystyrene by polymerization, and coordinated Eu 3+ or Tb 3+ ions onto acid-functionalized polystyrenes to prepare binary rare earth macromolecular complexes. However, the most notable problem of technological means-coordinating different rareearth ions simultaneously with the same functionalized polymer ligand-is a bad match of energy level between one of rare earth ions and molecular ligands. In other words, it is necessary to obtain precursor complexes with better match of energy levels before introduction into polymer matrix. Balamurugan et al. [21] developed a rare earth macromolecular complex emitting white light with CIE coordinates (0.28, 0.34) by coordinating different lanthanide complexes with carboxylic functionalized poly(mphenylenevinylene). Forster et al. [22] realized the coordination of carbonyl groups present in polycarbonate with rare-earths. Parra et al. [23] reported that Eu 3+ complex precursor could be immobilized by Eu-O interaction in the polymer matrix diglycidylmetacrylic (DGMA). The mentioned materials realized good matches of energy levels between precursor complexes and matrix, and demonstrated excellent film-forming properties. However, there are few reports about the polymer phosphors of large area film-forming and curing for direct encapsulation.
Herein, we prepared copolymer matrix PS-GMA by the radical polymerization of styrene and glycidyl methacrylate. Subsequently, unsaturated complexes of Eu 3+ , Tb 3+ , and Zn 2+ were coordinated with carbonyl present in the side chain of polymer host to obtain macromolecular complexes. We constructed a series of macromolecular complexes containing Eu 3+ , Tb 3+ , and Zn 2+ displaying a successive change of visible photoluminescence emission color (including white emission) by adjusting the ratio of tricolor monomers. Furthermore, we demonstrated that a UV converted WLED rapidly cured and fabricated with our macromolecular complexes without encapsulating materials, presented CIE chromaticity coordinates (0.33, 0.36), color temperature 5308 K, and color rendering index 95.2. All the results established that the high rendering index macromolecular complex is a material suitable for indoor illumination.

Synthesis of complexes Eu(TTA) 2 (Phen), Tb(p-BBA) 3 , Zn(BTZ)
The complexes were prepared by a method analogous to our earlier work [24]. Eu(TTA) 2 (Phen): A solution of 1 mmol EuCl 3 in 10 mL ethanol was added dropwise to a solution of 1 mmol Phen in 5 mL ethanol. The pH of the solution was neutralized to 4-5 with 1.0 mol L −1 sodium hydroxide ethanol solution. Then, 2 mmol TTA in 5 mL ethanol was added to the mixed solution. The pH of the solution was neutralized to 6-7 by adding 1.0 mol·L −1 sodium hydroxide ethanol solution and a large amount of precipitate appeared. The mixed solution was refluxed with stirring at 55 °C for 3 h. The precipitate was collected and repeatedly washed with ethanol. Finally, the product was obtained as solid by drying under vacuum at 80 °C for 24 h. Element analysis (calc.) C 45.11% (45.13%), H 2.86% (2.87%), N 3.33% (3.31%), S 7.54% (7.53%). Yield: 71.95%.
Tb(p-BBA) 3 : A solution of 1 mmol TbCl 3 in 10 mL ethanol was mixed with a solution of 3 mmol p-BBA in 5 mL ethanol. The mixed solution was heated in a water bath and refluxed with stirring at 55 °C for 3 h. The pH of the solution in the whole process was controlled to 6-7. Finally, the precipitate was collected, repeatedly washed with ethanol, and dried under vacuum at 80 °C for 24 h. Element analysis (calc.) C 59.37% (59.35%), H 5.14% (5.15%). Yield: 62.95%.
Zn(BTZ): A solution of 1 mmol ZnCl 2 in 10 mL of ethanol was added dropwise to a solution of 1 mmol BTZ in 5 mL ethanol. The pH of the solution was neutralized to 6-7 with 1.0 mol·L −1 sodium hydroxide ethanol solution. The mixed solution was heated in a water bath and refluxed with stirring at 55 °C for 3 h. The precipitate was collected and washed with absolute ethanol. Finally, the product was obtained as solid by drying under vacuum at 80 °C for 24 h. Elemental analysis (calc.) C 61.44% (61.43%), H 7.01% (6.99%), N 2.56% (2.55%), S 6.63% (6.61%). Yield: 72.49%.
The synthesis routes of the complexes are depicted in Fig. 1.

Synthesi
Tetrahydrofur necked flask a 30 min to rem was sealed a polymerizatio reaction produ times. Finally powder [25]. Edinburgh LFS-920 spectrometer. Electroluminescence (EL) spectra were obtained on a computer controlled PMS-50 UV-vis-near IR spectrometer with an integrating sphere.

IR spectra
IR spectra of copolymer PS-GMA and macromolecular complex PS-GMA-Eu-Tb-Zn are given in Fig. 4. The benzene ring vibrations of PS at 1604, 1492, and 1454 cm −1 are clearly identified in PS-GMA. The characteristic bands at 908 and 850 cm −1 are assigned to the epoxy group [26]. Additionally, the characteristic band at 1726 cm −1 is ascribed to the stretching vibration of C = O and the bands at 1182, 1124, and 1074 cm −1 are caused by the stretching vibration of C-O-C in the ester group. The above results indicate that copolymer PS-GMA has been synthesized. Furthermore, it is quite clear that the band shape and wavenumber of PS-GMA-Eu-Tb-Zn are basically in accordance with those of PS-GMA, which is due to the low content of complex. Nonetheless, there are some subtle differences between the copolymer PS-GMA and macromolecular complex PS-GMA-Eu-Tb-Zn. The characteristic bands in PS-GMA-Eu-Tb-Zn at 445 and 418 cm −1 are attributed to the stretching vibration of Eu-O in the complex Eu(TTA) 2 (Phen) and Tb-O in the complex Tb(p-BBA) 3 , respectively. Besides, the weak band at 472 cm −1 is caused by the interaction between metallic Zn 2+ and carbonyl group in the side chain of GMA chain segment [27]. All of these results imply that the three primary complexes have coordinated with the carbonyl group from the GMA chain segment in the polymer PS-GMA.

UV-vis absorption spectra
UV-vis absorption spectra of polymer PS-GMA and polymer PS-GMA-Eu-Tb-Zn in DMF are shown in Fig. 5(a). In comparison, UV-vis absorption spectra of complex Eu(TTA) 2 (Phen), Tb(p-BBA) 3 , and Zn(BTZ) are presented in Fig. 5(b). In polymer PS-GMA, the strong absorption peak centered at 271 nm originates from the K absorption band of the benzene ring in the PS segment. In polymer PS-GMA-Eu-Tb-Zn, the K absorption band of the benzene ring becomes stronger than that in PS-GMA, and this is due to enhancement of the n-π* absorption (270 nm) of C = N in the BTZ of complex Zn(BTZ) [28]. Moreover, two broad absorption peaks at 279 and 334 nm are observed in PS-GMA-Eu-Tb-Zn. The former mainly comes from the π-π* transition of the conjugate structure between the benzene ring and the carboxyl or carbonyl group in complex Tb(p-BBA) 3 [29]. The latter was ascribed to the joint effect of the absorption of the ketonic form of TTA in complex Eu(TTA) 2 (Phen) and intramolecular charge transition from the phenol ring to the benzothiazole unit in complex Zn(BTZ) [30]. The above analysis reveals that the complexes are effectively coordinated with the copolymer PS-GMA. Thermogravimetric (TG) analysis of polymer PS-PGMA-Eu-Tb-Zn in the temperature range of 100 °C to 900 °C was conducted at heating rate 10 °C/min under nitrogen. A first-order derivative curve (DTG) was fitted with the Origin 8.0 software package. The results are illustrated in Fig. 6. Apparently, the thermal decomposition process is divided into two stages. The first weight loss occurs at 272.6-413.0 °C with weight loss 86.2% and the weight loss rate reaches the maximum at 382.0 °C, occurring at this decomposition stage. In this stage, the decomposition changes gently probably owing to the decomposition of copolymers with low molecular weight and the partial removal of small molecule ligands and decomposition of the polymer skeleton chain. The second weight loss occurs at 418.6-545.2°C with a tiny weight loss of 6.8% attributed to complete decomposition of complexes, which further confirms the coordination between metal ions and ligands. The results suggest that the polymer keeps stable up to 272.6 °C. In view of operating temperatures of LEDs below 150 °C [31], the macromolecular complex is thermally stable for the requirement of LEDs.

Photoluminescence properties
The excitation and emission spectra of complexes Eu(TTA) 2 (Phen), Tb(p-BBA) 3 , and Zn(BTZ) in the solid state recorded at room temperature with slit width 1 nm are presented in Fig. 7. The excitation spectrum of Eu(TTA) 3 (see Fig. 7(a)) obtained by monitoring the emission of Eu(III) ion at 612 nm shows a broad absorption peak from 240 nm to 500 nm with a maximum at 325 nm, which is caused by π-π* transition of the TTA ligand [32]. Upon excitation at a wavelength of 365 nm, the emission spectrum of the complex exhibits characteristic emissions at 579, 590, 612, and 649 nm attributed to the 5 D 0 -7 F 0 , 5 D 0 -7 F 1 , 5 D 0 -7 F 2 , and 5 D 0 -7 F 3 transitions, respectively. Among them, the dominant peak at 612 nm results in pure red emission. In Fig. 7(b), the excitation spectrum of Tb(p-BBA) 3 monitored at 543 nm similarly shows a broad band from 240 nm to 400 nm centered at 362 nm due to intramolecular charge transfer in the p-BBA ligand [33]. Being excited at 365 nm, the complex Tb(p-BBA) 3 presents four characteristic emissions at 488, 542, 582, and 619 nm, ascribed to 5 D 4 -7 F 6 , 5 D 4 -7 F 5 , 5 D 4 -7 F 4 , and 5 D 4 -7 F 3 transitions of Tb 3+ , respectively. Obviously, pure green emission is the result of the dominant peak at 542 nm. The complex Zn(BTZ) shows a typical blue emission peak centered at 472 nm when excited at 365 nm ( Fig. 7(c)).
The excitation spectrum displays a broad band with maximum at 367 nm when monitored at 450 nm. The above results suggest that the complexes Eu(TTA) 2 (Phen), Tb(p-BBA) 3 , and Zn(BTZ) can be qualified as three primary colors to be mixed in proper proportion for white light emission upon excitation at 365 nm.  The emission spectra of polymers PS-GMA-Eu-Tb-Zn 1-8 with different molar ratios upon excitation wavelength at 365 nm are illustrated in Fig. 8. The corresponding CIE chromaticity coordinates are calculated from the emission spectra and marked in Fig. 9. All the polymers display blue, green, and red characteristic emissions attributed to the complexes Zn(BTZ),Tb(p-BBA) 3 , Eu(TTA) 2 (Phen). It is observed that the coordinates travel through the light pink area while shifting from pink to the white light zone. It's gratifying to see that the CIE chromaticity coordinates of samples 4, 5, 6, and 7 distribute in the white light zone, which are realized by changing the contents of the green and blue complexes. Among the coordinates of the white light region, (0.332, 0.337) of sample 5 is optimal under excitation at 365 nm, which is very close to pure white light at (0.330, 0.330), according to the 1931 CIE coordinate diagram. presents the atio 1:10:1 at only typical pe and 5 D 0 -7 F 3 tr ristic peaks at 4 b(p-BBA) 3   It is feasib light emissio chromaticity c altering the chromaticity excitation wav Fig. 11. Luminesce m the polymer btained white l ctrum of macro d in Fig. 12

Quantum yield and fluorescence lifetime
The quantum yield ( ) s η of the polymer with RGB ratio 1:10:1 at room temperature was also measured in 10 −3 mol·L −1 DMF solution and calculated according to Eq. (1) using Eu(TTA) 3 (phen) as reference. is the quantum yield of Eu(TTA) 3 phen in 10 −3 mol·L −1 DMF [37]. A, I and N denote the area of the emission spectrum, the absorbance intensity at the excitation wavelength, and the refractive index of the solvent, respectively. The refractive index N S is equal to N R under the condition of the same DMF solution used for measurement. The quantum yield ( ) s η of the macromolecular complex is calculated as 0.112.
As one of the significant indicators of luminescent performance, fluorescence lifetime was measured in the solid state at room temperature obtained by fitting luminescence decay data to the double exponential Eq. (2).
where τ 1 and τ 2 are the shorter and longer lifetime components, respectively. B 1 and B 2 are fitting constants. The average lifetime τ is given by Eq. (3).

WLED fa
In

Conclusions
In this work, a rapidly curing macromolecular complex has been developed using the technical route of pre-polymerization and post-coordination. White light emission was realized through two kinds of technical schemes (adjusting the ratio of R/G/B complexes or altering the excitation wavelength). Macromolecular complex 3 achieves white emission being excited at 375 nm. The macromolecular complex with the ratio of R:G:B = 1:10:1 exhibits three discrete characteristic peaks at 458 (blue emission), 543 (green emission), and 612 (red emission) nm, respectively. The corresponding CIE coordinates of (0.332, 0.337) are in close proximity to pure white light (0.330, 0.330). WLED fabricated with macromolecular complex 5 shows applicable CCT of 5306 K, excellent CRI of 95.2, and luminous efficiency of 103 lm/W being driven at 350 mA. The CIE coordinates from the EL spectrum are (0.35, 0.34), similar to the PL spectrum, maintaining the original luminous performance. The prepared macromolecular complex film further shows the vivid color reproduction of real objects, confirming the high rendering index of the macromolecular complex. The results suggest that the macromolecular complex saves the trouble of mixing with packaging adhesive and curing for long times in conventional LED encapsulation. In addition, the macromolecular complexes keep stable until 272.6 °C, meeting the requirements for the operating temperature (180 °C) of LEDs. Our findings demonstrate that the macromolecular complexes have potential applications in NUV-WLEDs.