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Local coordination, influence on synthesis and luminescent features of Eu ions in SiO-PMMA hybrid materials 3+

2

Filipe Augusto de Jesus, Barbara Vasconcelos Santana, Jose Mauricio Almeida Caiut, and Victor Hugo Vitorino Sarmento Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05208 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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Industrial & Engineering Chemistry Research

Local coordination, influence on synthesis and luminescent features of Eu3+ ions in SiO2–PMMA hybrid materials Filipe Augusto de Jesus1, Barbara Vasconcelos Santana1, José Maurício Almeida Caiut2 and Victor Hugo Vitorino Sarmento*1 1Department of Chemistry, Federal University of Sergipe, Av. Vereador Olímpio Grande s/n, Centro, Itabaiana, SE, Brazil 2Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040–901 Ribeirão Preto, SP, Brasil

Keywords: Hybrid materials, Eu3+ ions, luminescence, concentration quenching, clusters Abstract

Due to the tunability of properties demonstrated by organic–inorganic hybrid materials, they have attracted considerable attention in recent years. However, despite luminescence being one of the major research topics involving hybrids, SiO 2– PMMA system have been poorly explored. In this paper, Eu3+:SiO2–PMMA samples were prepared with different Eu3+/Si molar ratios in order to determine Eu3+ local coordination, the potential interferences of these ions on synthesis reactions and the optimized lanthanide concentration for improved luminescence output. Careful analysis of FTIR spectra showed the presence of Eu3+ ions coordinated to carbonyl groups in the hybrid matrix, while TG results, allied to FTIR, indicate dopant influence on the mechanisms of formation of polymeric chains. Photoluminescence results showed that, at higher lanthanide contents, the position of 5D0 → 7F0 band and spectroscopic parameters are modified, a clear signal of the formation of lanthanide clusters, which results in concentration quenching of luminescence.

1. Introduction For at least 20 years, organic–inorganic hybrid materials have been considered good hosts for luminescence emission from lanthanide ions. This kind of matrix conjugates properties of inorganic (high density, hardness, fragility) and organic (low density, plasticity, rubbery) matter1. Appropriate composition and synthesis control can create a multifunctional material with controlled properties suitable for many applications, such as biomaterials2, coatings3, in the medical4, optical5, electronic6 and electrochemical7 fields. Lanthanide ions present an intrinsic luminescence (from 4f–4f intraconfigurational transitions) with great color purity, high luminescence quantum yields and long-lived emission bands8. Hence, this combination of features from both organic-inorganic hybrids and lanthanide ions have attracted the attention of the scientific community and it has become a broad field of research 9–12. Several classes of organic-inorganic hybrid materials doped with lanthanide ions – mostly trivalent europium (Eu3+) – have been extensively explored, highlighting diureasils13,14 and siloxane-polyethyleneoxide15,16. Diureasils are hybrid materials composed of a siliceous backbone grafted to polymer segments at both ends by urea cross-

links. These hybrids doped with Eu3+ ions have been studied for a long time and important aspects for luminescent features, such as local coordination of dopant ions, energytransfer mechanisms and quantum efficiency/yield, were determined13,14. Similar analyzes were also performed for siloxane-polyethyleneoxide hybrids15,16. However, Siloxane– polymethylmethacrylate (SiO2–PMMA) hybrid materials, widely studied with regard to their structural/thermal properties17, action as flame-retardant18, bioactivity19,20 and anti-corrosive coatings21,22, have not been the focus of luminescence research. There have been only a few reports on the luminescent features of Europium compounds in SiO2–PMMA hybrid host23-25. Nevertheless, these papers are from different authors with distinct aims (behavior analysis of various Eu3+ complexes in this hybrid material). As a result there had been no systematic structural studies concerning Eu3+– doped SiO2–PMMA (Eu3+:SiO2–PMMA) samples until the first paper published by our group26. However, the works of Bian et.al.24 and Huang et. al.25 show that both inorganic and organic phases in SiO2–PMMA hybrids act together to give rise to a synergic effect that can enhance the luminescent

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properties displayed by this class of materials. Bian and coworkers24 compared pure polymer and SiO2–PMMA hybrid samples doped with an europium complex, and concluded that the hybrid is a better host because silicon networks disperse and confine dopant molecules, which minimize quenching processes. In turn, Huang and collaborators25 encapsulated a ternary europium complex in silica and SiO2–polymer (polymethylmethacrylate, polyvinylpyrrolidone or polyvinylbutyral) hybrid samples. Their results showed a better luminescence performance of the hybrids, which they attributed to the presence of polymeric chains at which Eu3+ coordinates, avoiding approximation to deleterious hydroxyls from silanol groups (–Si–OH) in silica networks. The behavior described in the works cited corroborates the standpoint that SiO2–PMMA hybrid materials constitute a promising host for lanthanide luminescence. Nevertheless, important parameters that can contribute or impair the performance of these materials still should be analyzed. Therefore, in this study, Eu3+:SiO2–PMMA samples, doped with different lanthanide concentrations, were prepared in order to determine Eu3+ local coordination, potential interferences of these ions on synthesis reactions and the optimized lanthanide concentration for improved luminescence output.

2. Experimental Methods 2.1 Preparation of Samples In order to synthesize Eu3+:SiO2–PMMA hybrid samples, 2 mL (8.4 mmol) of 3–(Trimethoxysilyl)propyl methacrylate (TMSM, 98 %, Sigma–Aldrich) were hydrolyzed by 0.53 mL of Hydrochloric Acid (HCl, Analytical Grade, 37 %, Isofar) solution (0.1 mol/L) in a vial tube. The resulting solution was stirred until complete homogenization, then an appropriate amount of Europium Chloride Hexahydrated (EuCl3.6H2O) alcoholic solution was added and the system kept under stirring for 5 minutes. In another vial, 0.92 mL (8.4 mmoL) of Methyl methacrylate (MMA, 98 %, Neon) was mixed with 0.0684 g (0.27 mmoL) of Benzoyl Peroxide (BPO, 75 %, Vetec) and stirred for 5 minutes. Both reagents were put together and stirred for 5 more minutes. Then, the obtained sol was transferred to eppendorfs® and dried on a stove at 60 °C for 24 hours. After drying, the monolithic body was mortified and thermally treated at 200 °C for 3 hours so as to enhance the luminescent properties of Eu3+ ions in these samples26. Five different volumes (0; 0.45; 1.05; 2.1 and 4.2 mL) of EuCl3.6H2O solution were used in order to obtain samples 3+ with 𝐸𝑢 ⁄𝑆𝑖 molar ratios of 0, 1.0, 2.5, 5.0 and 10.0 % (mol/mol), which were named Eu0, Eu1, Eu2.5, Eu5 and Eu10, respectively. 2.2 Characterization 2.2.1 Infrared Spectroscopy. The structure of the samples was investigated by Fourier Transform Infrared Spectroscopy (FTIR) performed on an IR Prestige–21 (Shimadzu) spectrometer with samples diluted in KBr pellets. Spectra were recorded in transmittance mode after

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16 scans in the range 4000–400 cm-1 with resolution of 2 cm-1. Origin® software was used to fit specific regions of the spectra so as, to separate the components of some bands and perform more accurate analyzes. A linear baseline was supposed at all points of the spectra where the absorption intensity was insignificant. The best results of the fit process were obtained using Gaussian functions and variations at bandwidth, frequency and intensity of each band component. 2.2.2 Thermogravimetry. Thermal stability and behavior of samples were studied by Thermogravimetric Analysis (TG) performed on a TGA Q50 thermobalance (TA Instruments) in an Argon Atmosphere with a flow of 60 mL/min in the sample and 10 °C/min heating rate from room temperature to 800 °C. Derivative Thermogravimetric Curves (DTG) were obtained using the software TA Universal Analysis® and fit with Origin® software through a method similar to that used for FTIR spectra. 2.2.3 Scanning Electron Microscopy. SEM micrographs were obtained in a Quanta 200F (FEI) microscope using a 30kV current and at low vacuum. Prior to analyses, the samples were metalized through deposition of a thin layer of Gold (Au) to improve electronic conduction of the powders. Different magnifications were employed to acquire images with distinct approximations to fully characterize the samples morphology. 2.2.4 Photoluminescence Spectroscopy. Photoluminescence spectra were acquired on a Fluorolog FL3–22 spectrofluorimeter (Jobin-Yvon) equipped with a R928 Hammamatsu photomultiplier and a 450 W Xe excitation lamp at room temperature. Excitation spectra were corrected to the Xe lamp intensity and spectrometer (excitation monochromator) response, while emission spectra were corrected only to spectrometer (emission monochromator) response. Measurements of emission decay were performed with the same equipment using a pulsed Xe lamp (3 µs) bandwidth source.

3. Results and discussion 3.1 Structural Characterization Analyzes of the structure of Eu3+:SiO2–PMMA hybrid materials were performed using FTIR and TG techniques to evaluate possible influences resulting from the presence of Eu3+ ions. Samples with increasing dopant amounts were prepared in order to extrapolate signals arising from the influence of Eu3+ and therefore, identify local coordination of these ions and potential interferences of them under synthesis reactions. Figure 1 presents the FTIR spectra, from 2000 to 400 cm-1, of Eu0, Eu1, Eu2.5, Eu5 and Eu10 samples. In the overall profile of all spectra the presence of an intense narrow band at 1728 cm-1 (assigned as 1) and a set of bands from about 1196 cm-1 to 1100 cm-1 (assigned as 7, 8, and 9) can be clearly seen. Band 1 is associated to C=O stretching vibrations27 and its occurrence is related to the significant presence of carbonyl groups, coming from structures formed from both MMA and TMSM, in the samples. In turn, bands 7 (~1196 cm-1), 8 (~1157 cm-1) and 9 (~1102 cm-1) are attributed, respectively, to Si–O–C, Si–O and Si–O–Si


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Figure 1. FTIR spectra of Eu0, Eu1, Eu2.5, Eu5 and Eu10 samples presented from 2000 to 400 cm-1.

stretching vibrations28. The presence of Si–O–Si groups in the samples, corroborated by band 10 at 688 cm-1 (arose from bending vibrations of these groups), indicates that hydrolysis and condensation reactions were successful in the formation of a silica network. On the other hand, Si–O– C groups in the samples indicates that TMSM molecules were not fully hydrolyzed, because in hydrolysis reactions these groups are replaced by Si–OH. This incompleteness of sol–gel reactions have already been reported28 and are acceptable, in the low level stated here, considering the simultaneous occurrence of hydrolysis, condensation and organic polymerization reactions, which makes it difficult for each to achieve its maximum yield. A similar discussion can explain band 2, at 1634 cm -1, attributed to C=C stretching vibrations. The presence of these groups confirms that polymerization reactions were also not completed and there are still TMSM and/or MMA monomeric units in the structure of samples. Besides the concurrence between simultaneous processes, sol–gel reactions begin before polymerization (only started after a “break” of polymerization thermal initiator at stove), which makes it likely that silica networks in formation hinder the mobility of MMA monomers and reduce the necessary free space for subsequent polymeric chain growth28. It should be highlighted, however, that the ratio between areas from 𝑆 1728 and 1634 cm-1 bands ( 𝐶=𝑂⁄𝑆 ) presented high 𝐶=𝐶 values, which indicates that most of the MMA precursor units were successfully polymerized. In the Eu0 sample, this parameter is equivalent to 100, whereas in doped samples it oscillates in the 8.3 – 10 range. These values indicate that Eu3+ ions have influence on polymerization reactions in SiO2–PMMA hybrids because there was a 10 fold decrease in the mentioned ratio in Eu3+–doped samples. An observed consequence of this behavior is the growing intensity of the bands 3 and 4 at, respectively, 1600 cm -1 and 1547 cm-1. These bands are both associated with vibrations in the bonds of coupled (C=O)…(C=C) groups29 and the increase in

Figure 2. A – Magnification of C=O band (band 1 at Fig.1), from 1680 to 1780 cm-1, of FTIR spectra from Eu0, Eu1, Eu2.5, Eu5 and Eu10 samples. B, C, D, E, F – Decomposition of C=O band of each spectrum in its components.

their relative intensities corroborates the presence of larger amounts of unsaturated groups in the samples with higher Eu3+ content. Lastly, bands 5 (at 1267 cm-1) and 6 (at 1245 cm-1), present in all spectra, are both attributed to the stretching vibrations of C–O ether groups. Like in the work of Bermudez and coworkers13, FTIR spectra were used to investigate coordination site(s) of Eu3+ ions at the host. It can be seen in Figure 1 that the dopant concentration increase did not change the overall shape of spectra, however, with magnification and fit of C=O band, presented in Figure 2, the broadening and slight modification of this signal shape became noticeable. Hence, deeper analyzes were performed through fitting and decomposing the band centered at 1728 cm-1 in its components. In the band of the Eu0 sample (undoped), the fitting process revealed the presence of three components at 1707 (assigned as III), 1731 (assigned as II) and 1757 cm -1 (assigned as I). This splitting of C=O band of sol–gel derived organic/inorganic hybrids has already been reported in the literature30,31. It can be explained by differences in the environment around the carbonyl groups. With different interaction strengths, C=O bonds with distinct degrees of freedom arise and varied absorption bands appear. Based on the works cited previously, it is possible to infer that



components with higher energy absorptions are associated to “free” C=O, that is, carbonyls that are not part of condensed structures where they interact with many groups. On the other hand, the lower the absorption energy, the higher the level of interactions between C=O and its environment. Thus, component I, at 1758 cm-1, can be attributed to “free” carbonyl groups in non-polymerized MMA and TMSM molecules, whose presence had already been predicted by C=C and Si–O–C bands. In turn, 1732 and 1707 cm-1 components are associated, respectively, to C=O without and with hydrogen bond interactions. The spectra of all doped samples presented one more component of C=O band at lower energies. The presence of this component, assigned IV, at about 1699 cm -1, indicates the influence of Eu3+ ions on the carbonyl environment, which makes sense considering the possibility of coordination with the oxygen in this group. Besides, the appearence of this component at lower energies indicates that these C=O groups are interacting stronger than by hydrogen bonds, which is in perfect agreement with the energy of these kinds of electrostatic interactions. Another interesting point in C=O(…Eu3+) component analysis concerns its overall contribution to the C=O band. Except for the component at Eu5 band, which seems to present an usual experimental oscillation, the higher the lanthanide concentration, the more representative the percentage of C=O(…Eu3+) component is. At Eu1, this component represents 7.1 % of the band total area, while at Eu2.5 and Eu10 the values are, respectively, 11.7 % and 14.1 %. This increase agrees with the rise in the dopant concentration, which provides more Eu3+ ions to coordinate with the C=O groups. Similar fitting procedures were carried out on bands which arose from ether (C–O–C) and silicon (Si–O–Si, Si–O– C, Si–O) groups. However, detailed view of these bands (not shown) did not present significant changes as the Eu 3+ concentration increased. Therefore, it is possible to conclude that at Eu3+:SiO2–PMMA hybrid nanocomposites, lanthanide ions are preferentially coordinated with carbonyl groups rather than ether-type oxygens. In order to evaluate the thermal behavior and decomposition of samples doped with different amounts of Eu3+ ions, TG analyzes were performed. First derivative of TG curves were calculated and fitted using Origin® software, which enabled more accurate discussion about simultaneous thermal events that occur in the 200 – 800 °C temperature range. These results are presented in Figure 3. TG curves (Figure 3A) demonstrated a similar weight loss profile for all the studied samples. Initially, there is a low–temperature thermal event associated to the desorption of water and solvent molecules remaining from synthesis reactions. Furthermore, this event could also be related to desorption of gases adsorbed at the pores of samples. The presence of this event is not observable in a second run with the same sample, which confirms its attribution. Further analysis showed that as Eu+3 concentration increases, this event involves larger weight percentages (from 2.5 % in Eu0 to 6.3 % in Eu10), and it occurs until higher temperatures (from 70 °C in Eu0 to 173 °C in Eu10). This behavior is not directly caused by the presence of Eu3+ ions, rather by the higher amount of ethanol (solvent of EuCl3.6H2O alcoholic solution) used in

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Figure 3. A – Thermogravimetric curves of Eu0, Eu1, Eu2.5, Eu5 and Eu10 samples registered from room temperature to 800 °C. B, C, D, E, F – Fit of derivative thermogravimetric curves in 200–600 °C temperature range.

the synthesis of samples with a larger concentration of lanthanide. Knowing that hydroxyl groups may represent a problem for europium–based luminescent systems, in subsequent studies the amount of solvent used will be minimal as, it has been demonstrated that these molecules remain in the structure of the samples even after 200 °C thermal treatment. After the first event finished, samples presented good thermal stability until around 200 °C, temperature from which a considerable weight loss occurs mainly related to polymeric chains decomposition. This demonstrates that 200 °C is the maximum suitable temperature to perform thermal treatment after gelation due to the fact, that at higher temperatures the sample composition is changed by thermal degradation. It was necessary to work with DTG curves for accurate analysis of the 200 – 800 °C temperature range, whose fit show the occurrence of four simultaneous events (assigned 1, 2, 3, and 4). The literature on PMMA thermal behavior32,33 reports that when heated to specific temperatures, it undergoes depolymerization according to the type of polymeric chain formed. By correlation with results presented by Kashiwagi and coworkers32, thermal event 1 is attributed to the random scissions of chains with vinyl unsaturated terminations, while 2 is associated to scissions of “head–to–tail” ends. Both types of polymeric chains are commonly formed in radical polymerization mechanism and, from the areas of these thermal events shown in Figure 3B–F, it is possible to infer the interference


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of Eu3+ ions and solvent amount on the termination step of polymerization reactions. The calculation of weight percentages decomposed by thermal event 1 demonstrates that until Eu5 sample there was little influence of Eu3+ ions on the formation of vinyl– terminated chains (39.0 – 46.1 %), however in the Eu10 sample (the highest Eu3+ concentration) the presence of these chains decreases dramatically to 13.2 %. At the same time, degradation of “head–to–tail” chains, in event 2, represents similar percentages to the Eu0 to Eu5 samples (9.0–17.0 %), while in Eu10 this value increases significantly to 33.5 %. This indicates the interference of the high Eu3+ concentration on the formation of polymeric chains, making it difficult to form vinyl-terminated chains, while making “head-to-tail” chains easier to form. It corroborates what has already been seen in the FTIR spectra, where the influence of Eu3+ ions on synthesis reactions was first noticed. In turn, thermal events 3 and 4 are associated to the removal of water molecules produced in the silanol condensation reactions, a typical process in silica networks, that may occur in a wide range of high temperatures 34,35. Mass loss percentages involved in both events are very close: 10.6 – 13.7 % for event 3 and 1.0 – 1.6 % for event 4. This indicate that the Eu3+ ions have no significant influence in the formation of inorganic networks, which corroborates the observations made on the FTIR spectra. Lastly, residue percentages increased from 26.1 % in Eu0 to 32.2 % in Eu10, which can be explained by the amount of Eu 3+ in the samples. At high temperatures, with decomposition of the polymeric chains where Eu3+ ions were coordinated, they bond to oxygen atoms to form thermally stable oxides. Thus, the larger the amount of lanthanide ions in the sample, the higher the oxide formation will be and, consequently, the higher the residue mass. SEM micrographs were obtained aiming to evaluate possible influences of Eu3+ content on the morphological features of the samples and are shown in Figure 4. Eu0 image presents a smooth surface with a homogeneous texture. On the other hand, the doped samples presented rough agglomerates on the surface, which becomes more occupied by these structures as dopant concentration is raised. In the micrograph of Eu10 sample (Figure 4E), the surface start to become rough at lanthanide content. So, the results confirm the influence of Eu3+ ions on the mechanisms of hybrid structure formation, which were already reported by FTIR and TG results.

Figure 4. SEM micrographs acquired for Eu0 (A), Eu1 (B), Eu2.5 (C), Eu5 (D) and Eu10 (E) samples.

Concerning band shape, CTB was clearly formed by at least two components centered around 273 and 280 nm, assigned 1 and 2 in Figure 5A. In previous studies with similar samples, the analysis of these CTB components were carried out and results were identical. Therefore it is

3.2 Luminescent Properties Luminescent properties of Eu1, Eu2.5, Eu5 and Eu10 samples were initially analyzed through its excitation spectra, presented in Figure 5. On account of the great intensity difference between the two ranges and to improve observation of the bands, spectra were split in parts A (250 – 320 nm) and B (320 – 600 nm). In the first, all samples displayed a broad charge–transfer band (CTB) related to the transfer of electrons from oxygen atoms in the hybrid host to Eu3+ ions36,37. The relative intensity of CTB was much higher than the other transitions, which suggests that it was the preferential pathway for the excitation of Eu 3+ ions.

Figure 5. Excitation spectra of Eu1, Eu2.5, Eu5 and Eu10 samples acquired monitoring 5D0 → 7F2 hypersensitive transition at around 612 nm and presented in the ranges 250– 3250 nm (A) and 320–600 nm (B).



known that they both lead to excitation of Eu3+ ions in close similar coordination sites and with the same emission features. Another CTB component, with low intensity, can also be well discerned in Eu1 spectrum, but its presence was less pronounced in Eu2.5, Eu5 and Eu10 spectra. In the spectral range presented in Figure 5B, it was possible to notice a broad band whose beginning, at around 315 nm, is overlapped by the far more intense CTB and extends to 450 nm. This band was related to electron–hole recombinations (EHRB), which can occur both on the siliceous backbone and in polymeric chains of the hybrids. This phenomenon is commonly reported in luminescence studies of silica-based hybrid materials38,39 and, according to Nunes et. al.39, is typical of donor–acceptor pairs and mediated by localized centers, such as oxygen-related defects in silica nanodomains. Narrow lines from 4f–4f transitions were observed in both A and B spectral ranges, highlighting 5L6 ← 7F0 (394 nm) and 5D2 ← 7F0 (464 nm)14. The relative intensity of these bands, compared to CTB or EHRB, rises as the dopant concentration increases. Because these transitions occur between electronic states in Eu3+ configuration, their intensities are closely related to the amount of these ions in the system, which explains the observed trend. In the spectrum of Eu10, the sample with the highest Eu3+ content, it was possible to clearly distinguish 5H4 ← 7F0 (3 – 318 nm), 5D4 ← 7F0 (4 – 362 nm), 5GJ ← 7F0 (5 – 376, 381 e 384 nm), 5D3 ← 7F0 (6 – 415 nm), 5D1 ← 7F0 (7 – 534 nm), 5D0 ← 7F0 (8 – 578 nm) and 5D0 ← 7F0 (9 – 590 nm)14. Due to its higher relative intensity, CTB was chosen as the pathway to excite Eu3+ ions, acquire emission spectra and register images of the samples emitting its characteristic light. The spectra were normalized and are presented in Figure 6A, along with the images registered during the emission (Figure 6B). All the intraconfigurational 5D0 → 7FJ (0 ≤ 𝐽 ≤ 4) transitions and similar profiles in the spectrum of every sample can be seen. The presence of bands from 5D0 → 7F0 transition, at around 578 nm, indicates that there were Eu 3+ ions at non-centrosymmetric sites in all the samples. Moreover, the relatively narrowness of the band in the spectra (45–54 cm-1) is a signal that the lanthanide ions are distributed in a set of similar symmetry sites, which is typical of amorphous hybrid materials8,40. Carlos, Malta and Albuquerque40 relate the energy of this transition to the nephelauxetic effect and, therefore, with the covalency degree of Eu3+ bonds to the ligands in their first coordination sphere. The analysis of the barycenter position showed a slight decrease from 17305 cm-1 (Eu1) to 17295 cm-1 (Eu10), which is indicative of the fact that, with a higher lanthanide content, the bonds between Eu 3+ ions and their first coordination shell have slightly larger covalency degrees. This result suggests changes in the coordination sites of Eu3+ ions as its content increases, which is also supported by the fall in full–width at half maximum (FWHM) of 5D0 → 7F0 bands (from 54.0 cm-1 for Eu1 to 44.8 cm-1 for Eu10). An important and useful parameter to analyze the symmetry of Eu3+ coordination sites is the ratio between the 𝑆 areas of 5D0 → 7F2 and 5D0 → 7F1 transitions ( 02⁄𝑆 )16,41. 01 The first occurs by the electric dipole mechanism and is

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Figure 6. A – Emission spectra, recorded at room temperature, of Eu1, Eu2.5, Eu5 and Eu10 samples acquired with excitation at the barycenter of CTB (λ𝑒𝑥𝑐 = 280 nm). B – Pictures registered for the four samples during emission.

hypersensitive, i.e. its intensity is strongly dependent on the environment around the Eu3+ ions, so that this transition tends to be more intense for these ions in low symmetry sites. In turn, the second one occurs by the magnetic dipole mechanism and its intensity does not depend on the site symmetry, so that it may be used as an internal pattern for 𝑆 intensity calculus41. 02⁄𝑆 values were 6.8, 6.2, 5.6 and 4.9 01 for Eu1, Eu2.5, Eu5 and Eu10, respectively. In general, Eu 3+ ions lie in a low symmetry environment, but the reduction observed at ratio values indicates an increase in the amount of lanthanide ions occupying more symmetric sites in samples with a higher dopant content. In order to evaluate this hypothesis, a deeper analysis was carried out for all spectra in the range of the 5D0 → 7F0 band. As this transition is degenerate, it should be


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composed of only one signal for each equivalent coordination site of Eu3+ ions. PeakFit® software was used in an attempt to decompose the band of this transition in different components. The results are presented in Figure 7. Figure 7 shows that each 5D0 → 7F0 band, in fact, was the sum of the contributions of at least three different peaks, which suggests the presence of a minimum of three coordination sites of Eu3+ ions with low symmetry in the samples. These data are demonstrated in Table 1, in which slight differences in the positions of peaks can be seen, with emphasis on the red shift of Peak 1. This is the main contribution to the displacement of the overall band. In turn, the values of area percentages of Peaks 2 and 3 follow, respectively, clear downward and upward trends. Through the analysis of 5D0 → 7F0 band of Eu3+ ions, it was possible to follow a change in the symmetry of the coordination sites, which agrees with the observed red-shift well and also with the intensity decrease of the 5D0 → 7F2 transition. This was already expected owing to the Eu3+ concentration rise in the matrix, what should promote the formation of clusters in this system. Clustering of lanthanide ions in sol-gel derived materials is a widely reported phenomenon which, has been investigated in several papers both using experimental and theoretical methods42,43. The formation of these clusters involves the generation of Eu–O–Eu bonds and these are detrimental to the overall luminescence features, given that they shorten the distance between the lanthanide ions, allowing cross-relaxation and energy–transfer between them. Costa et.al.42 have also reported, as another consequence of clustering, the loss of site selectivity, which was observed in the present study when emission spectra acquired by different excitation wavelengths (not shown) were analyzed.

Table 1 Barycenter position (cm-1) and Area Percentages (%) of the three peaks composing 5D0 → 7F0 bands of Eu1, Eu2.5, Eu5 and Eu10 samples. Peak 1

Peak 2

Peak 3

Center

Area

Center

Area

Center

Area

Eu1

17344

9.2

17307

82.4

17288

8.4

Eu2.5

17333

11.0

17307

75.1

17284

13.9

Eu5

17336

10.8

17310

68.9

17288

20.3

Eu10

17323

14.6

17302

59.8

17284

25.6

Despite the fact that all the results indicate the gradual formation of higher amounts of Eu3+ clusters as the dopant content rises, this hypothesis was still evaluated toward theoretical aspects through the calculation of a set of spectroscopic parameters. They were obtained from analysis of both emission spectra and decay curves (not shown). The main results were summarized in Table 2. Table 2 Set of parameters – 𝑨𝒓𝒂𝒅 , 𝝉𝒓𝒂𝒅 , 𝝉𝒆𝒙𝒑 , 𝒒, 𝑨𝒏𝒓𝒂𝒅 and Ω𝟐 – calculated from Eu1, Eu2.5, Eu5 and Eu10 emission spectra. Arad (ms −1 )

Anrad τrad (ms −1 ) (ms)

τexp (ms)

q (%)

Ω2 −20

(10

cm2 )

Eu1

0.53

2.33

1.88

0.35

18.6

11.9

Eu2.5

0.50

2.28

2.02

0.36

17.8

11.0

Eu5

0.46

2.48

2.18

0.34

15.6

9.94

Eu10

0.43

2.80

2.33

0.31

13.3

8.69

The changes observed for the Radiative Emission Rate (𝐴𝑟𝑎𝑑 ) and Non-radiative Emission Rate (𝐴𝑛𝑟𝑎𝑑 ) values were in agreement with the decline in the luminescence intensity by non-radiative decays from the 5D0 emission state, associated with the increase in the Eu3+ content in the hybrid system. This behavior can be associated to the formation of clusters42,43. Experimental lifetimes (𝜏𝑒𝑥𝑝 ), obtained from emission decay curves, decrease from Eu1 to Eu10, causing a considerable decrease in quantum efficiencies (𝑞). The trend followed by these values, a reduction from 18.6 % (Eu1) to 13.3 % (Eu10), agrees perfectly with previous discussions as, it proves that there is reduction in 5D0 emission efficiency with higher lanthanide content. The Judd–Ofelt Intensity Parameter (𝛺2 ) was calculated in order to analyze symmetry changes in its environment44. The significant change in 𝛺2 values agrees with 𝑆02 ⁄𝑆 variations and corroborates the influence of 01 lanthanide content on the symmetry of the Eu 3+ coordination sites.

4. Conclusions 5D0

7F0

Figure 7. Results of → band fit for Eu1 (A), Eu2.5 (B), Eu5 (C) and Eu10 (D) samples.

Synthesis of Eu3+:SiO2–PMMA hybrid samples with different dopant contents proved to be a good strategy to



analyze local coordination and its influence on the synthesis and luminescent features of these materials. The rise in the Eu3+ concentration interfered in some different aspects of the samples, such as the mechanisms of the formation of polymeric chains. It could be seen that the presence of higher amounts of lanthanide ions favored the termination step by combination and leads to more “head–to–tail” chains. Regarding the local environment of Eu3+ ions, carbonyl groups in the matrix were the preferred coordination site. However, it was observed that with larger Eu3+ contents changes occurred with the coordination in the higher symmetry sites. This was widely supported by the luminescence results, as a modification in the excitation profile of the samples, a red shift and a narrowing of 5D0 → 7F0 transition band and a decrease in the 𝑆02⁄ 𝑆01 , 𝐴𝑟𝑎𝑑 , 𝜏𝑒𝑥𝑝 , 𝑞 and 𝛺2 parameters. At higher Eu3+ concentrations the presence of lanthanide clusters is more pronounced, which affected the behavior of the spectroscopic parameters and impaired the luminescence performance of materials by cross–relaxation and energy–transfer mechanisms. In this context, the 3+ optimized dopant content was that achieved with 𝐸𝑢 ⁄𝑆𝑖 = 1 %, which minimized the formation of clusters. In short, it can be said that this work contributes to the comprehension of Eu3+ behavior in SiO2–PMMA hybrid materials, clarifying and discussing topics not yet studied. Furthermore, it opens a wide range of possible works to optimize the luminescent performance of these materials, to enable future applications.

AUTHOR INFORMATION Corresponding Author * Victor Hugo Vitorino Sarmento: [email protected]

ACKNOWLEDGEMENTS Authors would like to thank CNPQ, CAPES and FACEPE for their financial support, CETENE for the use of some equipment and Prof. Dr. Severino Alves Júnior (DQF–UFPE) for the use of the spectrofluorimeter.

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