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PAPER~ Nguyên T. K. Thanh, Xiaodi Su et al. Fine-tuning of gold nanorod dimensions and plasmonic properties using the Hofmeister effects

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Carboxyl-functionalized ionic liquids: synthesis, characterization and synergy with rare-earth ions Received 00th January 20xx, Accepted 00th January 20xx

Talita Jordanna de SouzaRamos, a Guilherme Henrique Berton,bTania Maria Cassol,b Severino Alves Júniora†

DOI: 10.1039/x0xx00000x www.rsc.org/

Herein, we describe for the first time room temperature ionic liquids (RTILs) and imidazole-based cations with appended carboxylic acids as terminals, which are directly derived from the anhydrides. All structures were designed to meet the following criteria: 1) easy preparation (up-scaling into kg scale possible); 2) melting point 1 ms, high quantum efficiency (47%), featured quantum yield (27%), and remarkable sensitization efficiency to ILs up to 83%, which suggests a synergistic coordination with lanthanide ions. We show that the materials obtained are potentially applicable for the construction of light-emitting electrochemical cells,owing tothe properties of their basic components, including ionic liquids (with high electrical conductivity and a similar structureto liquid crystals) and lanthanide ions (which have unprecedented photophysical properties).

Introduction Ionic liquids (ILs) are defined as molten organic salts. Most have melting points lower than 100 °C. It is estimated that there are as many as 1018 ILs available, including their binary and ternary mixtures.The sheer number of ILs provides enormous scope for industrial and scientific innovations.1 Room temperature ionic liquids (RTILs) are ILs with melting points below room temperature. An RTIL typically consists of nitrogen or phosphorous-containing organic cations and large organic or inorganic anions.2 Among the requirements for the study of ILs/RTILsare the developments of new structures, a database with all of the properties of the structures already synthesized, a comparison between commercial solvents and ionic liquids with potential solvent applications, and less costly synthetic methodologies.3 Beyond already well established applications as solvents or catalysts4, ionic liquids have been the

a. Laboratório

de Terras Raras, Departamento de Química, Centro de Ciências Exatas e da Natureza (CCEN), Universidade Federal de Pernambuco, Recife – PE, 50740-560, Brazil. b. Grupo de Líquidos Iônicos e Metais, Departamento de Química e Biologia, Universidade Tecnológica Federal do Paraná, São Francisco Beltrão – PR, 85601970, Brazil. † E-mail: [email protected] Electronic Supplementary Information (ESI) available: Details of Nuclear magnetic resonance, FTIR/Raman spectroscopy, Elemental/Thermophysical analysis, UV-vis and Photoluminescence spectrophotometry. See DOI: 10.1039/x0xx00000x

subject of reviews detailing their preparation, and their applications in polymer (polyILs) synthesis5,battery/fuel cell electrolytes6, biological activity and pharmaceuticals and medicine.7Other applications include their use in high performance luminophores8 or as coordinating ligands. ILs have several advantages over typical organic systems,including high solubility,structural flexibility,and thermal/chemical stability. An area not yet well explored is the photophysical properties of ILsinluminescence, where lanthanides arehighlighted ashigh-performingluminophores due to their luminescence features, such as quantum efficiencies of up to 100%9, narrow luminescence bands and long lifetimes.10

The introduction of rare-earth compounds into ionic liquids dates from 200611 and has aroused so much interest that there has been an emergence of a wide field of research focused specifically on soft materials.12,13This field of research was motivated by the potential of ILsto minimize vibration-induced deactivation processes usedto design a rigid metal-ion environment, free of high-energy vibrations and protecting the Ln3+ ion from solvent interaction. Soft materials favourably combine the properties of ionic liquids with unique optical properties of rare-earth compounds, such as a sharp emission band, long decay time inthe excited state, and a tunable emission from the UV to the infrared spectral region. Even with highlighted photophysical features,10luminophores formed with several types of oxo-ligands14,15,16coordinated to lanthanide ions do not show photostability. It is well known in the literature that carboxylate

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termini provide several coordination modes that favour the formation of three-dimensional structures.17The primary goal of our work was to develop a new family of ionic liquids and apply them to the development of photostable structures.

All technologies under development are based on solid state electroluminescent materials and belong to the general area of solidstate lighting (SSL). The main technologies being developed in SSL are light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), and light-emitting electrochemical cells (LECs). Light-emitting electrochemical cells (LECs)have emerged as an alternative option.18Generally, LECs have several advantages over OLEDs, such as a single-layer configuration, balanced carrier injection, low voltage operation and allowing the use of air-stable electrodes.19The luminescent materials in LECs are either luminescent polymers together with ionic salts, or ionic species.18The desired composition of LECs are materials that have properties such as high electrical conductivity and visible light emission. Furthermore, higher electroluminescent efficiencies are expected due to the well-known phosphorescent nature of the metal complexes. 20 Owing to these advantages, LECs based on metal complexes of Ir3+, Cu3+ and Ru3+ have attracted much attention in recent years.21,22,23Even with the unprecedented photophysical properties of lanthanide-ILcomplexes, the synthesis of materials potentially applicable for the construction of light-emitting electrochemical cells has not been reported. This report is a first example of four ionic liquids (ILs) formulated from anhydrides, imidazole-based cations with appended carboxylic acids as terminals. We examinethe potential of RTILs as building blocks for coordination chemistry with Ln(III). We focus on the structure–property relationship for luminescentionogelsfrom experimental results and a computational study of photoluminescence. Spectroscopic properties,such as intensity parameters Ωλ (λ = 2, 4 and 6), energy transfer (WET) and back – transfer (WBT) rates, radiative (Arad) and nonradiative (Anrad) decay rates, and quantum efficiency (η),of Eu(III)-compounds were computationallymodelled using the electronic and spectroscopic semi-empirical approach24 widely used in photoluminescence studies.25,26,27,28

Experimental Section All reagents were purchased from commercial sources as described in SI. Synthesis of ionic liquids and luminescent ionogels are given in detail in the supplementary information (ESI†). Synthesis of Ionic Liquids: Monocationic carboxylate ionic liquids (MC1-4) were prepared starting with 1Methylimidazolium and 1-Bromoethanol, as shown in Figure 1. Translucent and colourless crystals of Bromide 2-(1methylimidazolium-1-yl)-1-ethanol were obtained with up to 90% yield. For the synthesis of ILs, 2-(1-ethylimidazoly)-1ethanol bromide was added to acetonitrile and 4 mmols of the corresponding anhydride (succinic, phthalic, phenyl succinic or

3,3-dimethyl glutaric). The ionic liquids that were View obtained and Article Online DOI: 10.1039/C8TC00658J theiryieldswere: 2-(3-methylimidazolyl-1-yl)-etoxy-4oxobutanoic acid bromide (MC1), 80%; 2-(3-methylimidazolyl1-yl)-etoxy)-carboyl)-benzoic-phthalicacid bromide (MC2), 78%;4-(2-(3-methylimidazolyl-1-yl)-methyl)-4-oxo-2phenylbutanoic acid bromide (MC3), 75%; and 3,3-dimethyl-5(2-(3-methylimidazolyl-1-yl)-ethoxy)-5-xopentanoic acid bromide (MC4), 98%.The proposed mechanisms for the reactions are presented in the supplementary information (Figure S1).

(B)

(A)

(C)

Figure 1 (A) General synthesisprocedure used to prepare new monocationic carboxylate-ionic liquids 2- (3-methylimidazolyl-1-yletoxy-4-oxobutanoic acid bromide (MC1), 2-(3-methylimidazolion-1-yl)etoxy-carboyl benzoic-phthalic acid bromide (MC2), 4-(2-(3methylimidazolion-1-yl)-methyl)-4-oxo-2-phenylbutanoic acid bromide (MC3), and 3,3-dimethyl-5-(2-(3-methylimidazolyl-1-yl)-ethoxy)-5oxopentanoic acid bromide (MC4). Ionic liquid photographs were acquired under daylight (B) and UVA exposure(C).

Synthesis of Ln(MCx)3yH2O:Luminescent ionogels weresynthesized from a simple metathesis reaction between the IL and lanthanide salt. Photographs of ionogels under daylight and UV areavailable in Figure S2. Characterization: Structural, thermal and photophysical characterizations of ionic liquids and luminescent ionogels are described in the supplementary information (ESI†). Based on the theories developed by Malta29,30,we calculated the 4f-4f intensity parameters and emission quantum efficiency (η). Emission quantum efficiency was determined using Equation 1, where Arad is the radiative decay rate, given by the sum of the spontaneous emission coefficients (A J’s)Arad = ∑4J=1 AJ . The total radiative decay rate (Atotal) is given by Atotal = τ1, where τ is the lifetime for radiative decay.31 Finally, the nonradiative decay rate (Anrad) is given by Anrad = Atotal– Arad.

𝜼=

𝑨𝒓𝒂𝒅 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟏 𝑨𝒓𝒂𝒅 + 𝑨𝒏𝒓𝒂𝒅

Results and discussion Synthesis and structural characterization

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According to the methodology developed by our research group, four ionic liquids were obtained with high yields (75 to 98%). All ILs synthesized were translucent, colourless, and viscous at 25°C and 1atm (as seen in Figure S2). 1H NMR spectra (Figure S4) showbehaviour qualitatively similar to the 13C NMR spectra (Figure S5), and both are consistent with COSY/HSQC characterization results (Figures S6-9),suggesting the formation of the proposed structures for ILs. NMR spectra are shown in detail in ESI† (Tables S1-2). FTIR interpretation was performed according to Table S3 and describes the intensity and attribution of each stretch. As expected, functionalized-IL carboxyl groups can be identified by absorption bands at 1560 and 1484 cm−1, as shown in Figure S10, which are assigned to asymmetric stretching and symmetric stretching of the carboxylate group, respectively.32FTIR spectra of MC1-4 showed bands at ~1586 and 1560 cm−1,which are assigned to the imidazole ring.33Vibrational bands in the far-IV spectra (MC3>MC1>MC4. Changes in the alkyl chain length cause changes in the interactive forces, which result in decreased electrostatic attraction between the cation and anion.38Strong signals observed in the region at the high-frequency band (~128 cm-1) were enlarged due to a coupling with the peak associated with a wagging of the bonds in the N-C imidazolium ring39,and strong signals were also identified for all samples at ~171 cm-1. The Raman spectra of ILs are quite similar to others inthe 287476 cm-1 region, and at ~1428 cm-1; both are associated with the CH2(N)/CH3(N)CN is relative to the imidazolium structural stretch, as has been previously reported.40Small intensity peaks are identified in

the 408 cm-1 region associated with CH2(N), CH3and CH3(N)CH View Article Online DOI:at 10.1039/C8TC00658J bonds.41The O-C-O moiety has signals arranged ~588 cm-1 (relates to symmetric bending) and 1103 to 1328 cm-1 (relates to asymmetric bending).30Less significant peaks, and common to all samples, are observed at 788-888 cm-1 arising from the ρ 42 (CH2)rocking. Pronounced peaksat 1028-1054 cm-1, explained through C-O symmetricstrong peak,40and overlapped with ν(C-C), are observed in the MC1-MC4 structures.

Figure 2 Raman spectra for ionic liquids: (2- (3-methylimidazolyl-1-yl) etoxy-4-oxobutanoic acid bromide (MC1), 2-(3-methylimidazolion-1-yl)etoxy-carboyl benzoic-phthalic acid bromide (MC2), 4-(2-(3methylimidazolion-1-yl)-methyl)-4-oxo-2-phenylbutanoic acid bromide (MC3), and 3,3-dimethyl-5-(2-(3-methylimidazolyl-1-yl)-ethoxy)-5oxopentanoic acid bromide (MC4), at room temperature.

An interesting behaviour is identified in the 1554-1744 cm-1 region, related to CH3(N)HCH symmetric bending with CH2(N)/CH3(N)CN.40 For samples MC1 and MC4, these peaks are removed, while for samples MC2 and MC3, these peaks are closer and detached. This outcome can be explained by the similarities between the MC1/MC4 structures, which do not present phenyl, and thus allow a greater flexibility in the organic chain. The relatively broad band ranging from 2800 to 2963 cm-1, which are assigned to symmetrical and asymmetrical C–H stretching (δ(CH3)) from the imidazolium cation (fingerprint bands), can serve as a useful probe to reflect structural change.43 The subtle difference may be associated with the elongation of the MC4 organic chain compared to the MC1 structure. Whereas in samples with a phenyl structure (MC2/MC3), this peak is superimposed on the enlargement of nearby peaks. The band identified at ~ 2950 cm-1 refers to νas(CH3) and was identified in all samples. This peak is due to the presence of several CH3 terminals in the structures.44 Another signal common to all samples occurs at 3078-3089 cm-1, attributed to ν(CH), and at ~3096 cm-1, assigned to v (=(C-H)ring.44 Signals identified at 3159 cm-1 (MC1), 3168 cm-1 (MC4), and 3178 cm-1 (MC2/MC3) are associated with HC=CH ring, and ν(C-H).39 Figure 2 shows a strong bandcentred at 2959 cm-1, together with a small peak seen as a shoulder located at approximately 3433 cm-1. These peaks are associated with water molecules that interact preferentially with anions Br -···HOH···Br.34,45We compared the results obtained with ionic liquids used as


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ligands, ionogels and lanthanide salts (Figures S15-22). On the whole, in the luminescent materials we observed, all signals referring to the structures of the ionic liquids besides the emission of some electronic transitions to the trivalent ions, are already identified in Raman lines.46The intensities of the characteristic peaks tothe C=O bond,strong in Lis, became weaker in ionogels, accompanied by a shift to higher frequency. This outcome might have resulted from coordination of the IL with the metal in this moiety.47Raman characterization for Eu3+/Gd3+-ionogels is reported in detail in the ESI†. The assignments of several distinct Raman peaks of ionic liquids (MC1-4) and Gd3+/Eu3+-ionogels are listed in Tables S4 and S5, respectively. Melting Points

Melting points of the MC1 and MC4 ILs are 21°C and 20°C, respectively. These structures have aliphatic chains between the ester and the carboxylic acid termini, whereas MC2 and MC3ILs have the phenyl structure, which hinder structural packaging, and theirmelting points are 19 °C. The lowerstructural packaging observed to MC2 and MC3 are consistent with the presence of fewerintermolecular forces (such as H-bonds) and lower melting points.48

carboxylic acid (n → σ * transition).51This signal is superimposed with View Article Online DOI: 10.1039/C8TC00658J broad and intense bands associated with imidazole cation. At 227 nm (for MC2) and 220 nm (for MC3), the signal for absorption of the benzene ringcan be observed. The molar absorption coefficient of the compounds depends on the concentration, ranging from approximately 5 M-1 cm-1up to 35 M-1 cm-1. This outcome indicates the formation of a new species via intermolecular interactions taking place in solution. This formation is common behaviour for ionic liquids and has been previously studied.50 UV-Vis spectra of ionogels present similar behaviour to those observed in ILs (Figure S26). The bands identified at ~200 nm and ~280 nm (π; n → π* transitions) refer to imidazolium moiety and intermolecular interactions.50The complexes Eu(MC2)3(H2O)2 and Eu(MC3)3(H2O)2 correspond to ~237 nm bands associated with absorption of the benzene ring.In comparison with the free ligand, these peaks were displaced from 10 nm (for the MC2 complex) and 17 nm (for the MC3 complex) to regions of lower energy.Fig S26 shows the absorption spectra for ionogels with Eu3+. It’s noteworthy that ionogels are transparent in the visible range.

IL miscibility with several solvents

Miscibility of the 1-methylimizodalium carboxylate-ILs with protic, aprotic, polar and nonpolar solvents is reported in Table S6.

Thermal analysis Mechanisms of thermal degradation for ionic liquids and ionogels are discussed in detail in the Supplementary Information (Pages 28-29). It is noteworthy that the ionic liquids and the ionogels are stable up to 150 °C and 223 °C, respectively. TG profiles are reported in Figure S24 for ionic liquids (MC1-4) and Figure S25 for ionogels. Photophysical characterization Studies reported in the literature indicate that imidazoliumbased ionic liquids have significant absorption at approximately 260 nm.49 Figure 3 shows UV-Vis spectra of the elaborated ILs, where the signals for absorption of the imidazolium ring occurs at ~200 nm (broad and intense band) and a weaker and broader signal occurs at ~280 nm (π; n → π* transitions),indicating the formation of a new species via intermolecular interactions in solution.50To prove that the absorption is not due to any impurity, we adopted a strategy different from that of comparing absorption spectra from repeatedly purified IL samples (Figure S25). That the absorption is actually due to the imidazolium moiety is proved by starting with pure 1methylimidazole used in the synthesis of ionic liquids. UV-Vis spectra exhibited a band centred at ~205 nm assigned to the absorption of

Figure 3 Absorption spectra of new monocarboxylate ionic liquids 2-(3methylimidazolyl-1-yl) etoxy-4-oxobutanoic acid bromide (MC1), 2-(3methylimidazolion-1-yl)-etoxy-carboyl benzoic-phthalic acid bromide (MC2), 4-(2-(3-methylimidazolion-1-yl)-methyl)-4-oxo-2-phenylbutanoic acid bromide (MC3), and 3,3-dimethyl-5-(2-(3-methylimidazolyl-1-yl)ethoxy)-5-oxopentanoic acid bromide (MC4). Measurements were obtained with a cuvette of 1 cm pathlength and at ambient temperature.

Steady-state photoluminescence spectra (Figure 4) from ionic liquids were obtained from pure samples and without use of solvents. As observed in previous studies,50the excitation spectra of the ILs confirm the observation of the prominence of intermolecular interactions, with a main excitation maximum at ~323 nm and a very small component at ~300 nm, the contribution of which grows with increasing concentration. Excitation spectra reached a maximum approximately at 358 nm, 332 nm, and 339 nm, for MC1, MC2 and MC4 compounds, respectively. This excitation is related to the transition π → π*.The emission for a single excitation wavelength is found to consist of two components, as previously reported.49Ionic liquids, in particular those based on the imidazolium cation, should

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be more accurately considered as threedimensional networks of anions and cations linked together by weak interactions36, as recorded using UV-Vis/NMR measurements. The short wavelength emission (here, at ~430 nm) is due to the monomeric form of the imidazolium ion, and the long wavelength emission (~550 nm) is due to its well-known associated forms.36 Emissions are identified as intraligand transitions and assigned as π* → π. The Stokes displacements were calculatedto be: 79 nm for MC1, 104 nm for MC2, 96 nm for MC3, and 86 nm for MC4. We observed that the ionic liquids that have a benzene ring in their structure demonstrate a greater separation between the triplet levels, which causes a smaller overlap between the excitation-emission bands, and thus, a greater Stokes displacement.

Figure 4 Steady-state excitation (dotted line) and fluorescence (solid line) spectra for new monocarboxylate ionic liquids 2-(3methylimidazolyl-1-yl) etoxy-4-oxobutanoic acid bromide (MC1; black line), 2-(3-methylimidazolion-1-yl)-etoxy-carboyl benzoic-phthalic acid bromide (MC2; blue line), 4-(2-(3-methylimidazolion-1-yl)-methyl)-4oxo-2-phenylbutanoic acid bromide (MC3; green line), and 3,3-dimethyl5-(2-(3-methylimidazolyl-1-yl)-ethoxy)-5-oxopentanoic acid bromide (MC4; pink line). Measurements were obtained at ambient temperature.

The emission identified for Gd3+-complexes corresponds to the emission of the ionic liquid because there is no transfer of energy from the IL to Ln3+ ion. The Gd3+ emission (from the first excited state 6P ) has higher energy, typically 32 200 cm-1, corresponding to 310 7/2 nm.52 Emission and excitation spectra of Gd3+-ILs complexes are shown in Figures S27-S30. In comparison between these spectra with photoluminescence results from ionic liquids (MC1-4), itis noteworthy that there was a shift between the excitation and emission bands to lower wavelengths. The variation of the maximum emission (ΔλEm) and excitation (ΔλEx) were calculated, for each complex, indicating the modification of the ligand before coordination with the gadolinium. Ligand triplet state energy was estimated from the luminescence spectra of Gd(MCx) 3(H2O)y complexes, using methods previously reported.29Triplet level values and spectrum modifications (ΔλEm and ΔλEx) are set out in Table 1. We compared thediagrams of the singlet and triplet levels tothe ILs and forEu3+in Figure 5. All systems showed that the triplet level of the ionic liquid is between 23 000 and 30 000 cm-1. This fact is of

immense relevance in the elaboration of high View performance Article Online DOI: 10.1039/C8TC00658J luminescent systems, due to the formation of high energy phonons, which deactivate the non-radiative ions.46 Thus, the ligands reported in this work present a remarkable potential in synergistic coordination with the Eu3+ ion. This synergy is discussed below.

Table 1 Modification of the ligand on coordination with gadolinium presented by the calculations of the variation of the maximum emission (ΔλEm) and excitation (ΔλEx), and determination of the triplet level of the monocationic carboxylate-ionic liquid (M1-4). Complex

ΔλEx (nm)

ΔλEm (nm)

Triplet Level Energy of IL MC1-4 (cm-1)

Gd(MC1)3(H2O)3 Gd(MC2)3(H2O)2 Gd(MC3)3(H2O)2 Gd(MC4)3(H2O)3

24 11 21 16

16 38 54 39

23585 24978 26063 25797

Figure 5 Energy diagram (on the left) offree ligands: (2- (3methylimidazolyl-1-yl) etoxy-4-oxobutanoic acid bromide (MC1), (2- (3methylimidazolion-1-yl) etoxy) carboyl) benzoic-phthalic acid bromide (MC2), 4-(2-(3-methylimidazolion-1-yl)-methyl)-4-oxo-2-phenylbutanoic acid bromide (MC3), and 3,3-dimethyl-5-(2-(3-methylimidazolyl-1-yl)ethoxy)-5-oxopentanoic acid bromide (MC4). For comparison purposes,on the right is the diagram of energy levels forthe trivalent europium ion.

Excitation spectra obtained by monitoring the5D0→7F2 line (619 nm for Eu(MC1)3(H2O)3; 614 nm for Eu(MC2)3(H2O)2; 616 nm for Eu(MC3)3(H2O)2; and 617 nm for Eu(MC4)3(H2O)3). The excitation spectrum consists of a series of sharp absorption lines, positioned in the 320-580 nm region, which can be attributed to transitions within the 4fn configurations of Eu3+ions.10The most intense transition of the excitation spectrum indicates the wavelength at which the direct excitation in the ion should give the greatest contribution to photoluminescence of the compound. This wavelength is equal to 394 nm for Eu(MC1)3(H2O)3, Eu(MC2)3(H2O)2 and Eu(MC4)3(H2O)3, and 395 nm for Eu(MC3)3(H2O)2.Fig 6, and Figures S31-34show the corresponding emission spectra (red line) of the complexes obtained from direct ion excitation inEu3+ (transition 7F0→5L6). In these


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spectra, five emission bands can be seen, centred at 580, 595, 612, 657 and 702 nm, distinguished in all complexes, and assigned to the 5D →7F (J = 0-4) transitions in Eu3+.53 0 J

Figure 6 Excitation (black line;  em = 614 nm) and emission (red line;  ex = 394 nm) for Eu(MC2)3(H2O)2. All measurements were performed at steady state and at room temperature.

The emission spectrum (Figure S35) obtained from the excitation in the band of each ligand (358 nm for Eu(MC1)3(H2O)3; 332 nm for Eu(MC2)3(H2O)2; 325 nm for Eu(MC3)3(H2O)2; and 340 nm for Eu(MC1)3(H2O)3) does not show the ligand phosphorescence band between 392-450 nm. Excitation of the complexes in the ligand part (MC1-4) gives the same spectralcharacteristicsasthe ion-centred Eu3+ emission. Emission spectra of Eu(MC1)3(H2O)3showtransition 0-4 with relevant intensity, which can be explained by the greater polarizability of the MC1 ligand compared to the other ILs (MC2, MC3 and MC4).54The presence of donor groups or acceptors in the ligand structures modify the electron density around the ion and interfere with the position of the 5D0→5F0 transition.49 This transition was observed at 579 nm for Eu(MC1)3(H2O)3,Eu(MC3)3(H2O)2 and Eu(MC4)3(H2O)3, and at 577 nm for Eu(MC2)3(H2O)2. This outcome suggests that MC2 presents lower polarizability when compared to

the other ligands. The lower intensity of the 0-4View transition to Article Online DOI: 10.1039/C8TC00658J Eu(MC2)3(H2O)2 corroborates this fact. Less polarizable environments, in general, contribute to decrease the covalent character of the metal-ligand bonds in the complexes (nephelauxetic effect), which implies a lower displacement for 0-0 transition, and a lower intensity for the 0-4 transition.48The presence of the 5D0→5F0 transition indicates that the Eu3+ ion is located in a symmetry site of the type Cs, Cn, or Cnv. This fact, together with the mono-exponential profile of each mono-exponential system of the 5D0 excited state radiative decay curve of the Eu3+ ion (Figures S36-37), indicates that Eu3+ ions predominantly exhibit coordination environments with only one symmetrycentre.55Emission spectra for the complexes (Figures S31-34) show the multiplicities of the transitions, indicating that the point group for the environment of symmetry of the europium ion is around C2v.55 This outcome corroborates the hypothesis, represented by the 5D0→5F0 transition, that the chemical environment around Eu3+ must be of low symmetry.The fluorescence lifetimes were analysed throughout the spectral range of the europium ion (excitation at 7F0→5L6 and emission at 5D0→7F2),as shown in Figure S36. We also examined the decay profiles measuring the temporal behaviour of the fluorescence by exciting the organic part of the samplesand monitoring the fluorescence emission at 5D →7F (Figure S37). All fluorescence decay profiles could be best 0 2 fitted to a mono-exponential decay function, as shown in Figures S36-S37. The fluorescence lifetimes reported in this work are four times greater than those of luminescent gels, elaborated witheuropium and carboxylate ligands, recognized as high emission performance gels reported in the literature.56 We calculated the parameters of experimental intensities: Ω2 and Ω4, Einstein spontaneous emission coefficient (Arad), the nonspontaneous emission coefficient (Anrad), quantum emission efficiency (η), and finally, R0-2/0-1 and R0-4/0-1, following wellestablished models in the literature for thedetermination of the energy transfer of ligand-lanthanide,29the position and nature of excited states,57 and the intensity parameters.58Table 2 shows these results and shows the values of emission lifetimes from the 5D0in Eu3+.

Table 2 Experimental parameters of intensities: Ω2 and Ω4, Einstein spontaneous emission coefficient (Arad) and non-spontaneous emission coefficient (Anrad), quantum emission efficiency (η), R0-2/0-1, R0-4/0-1, and lifetime (τ) to elaborated complexes. Complex Ω2 Ω4 Arad Anrad η R0-2/0-1 R0-4/0-1 τ (ms) (10-20) (10-20) (s-1) (s-1) (%) ± 0.001 ms Eu(MC1)3(H2O)3 5.55 5.91 377.0 1377.9 21.5 3.16 1.52 0.5699 Eu(MC2)3(H2O)2

10.59

6.1

473.9

518.8

47.7

5.95

1.56

1.0072

Eu(MC3)3(H2O)2

8.61

7.78

451.4

524.4

46.3

4.84

1.98

1.0249

Eu(MC4)3(H2O)3

7.35

7.35

393.3

1082.0

26.7

4.15

1.87

0.6779

The disparity between non-radiative rates can be explained by observing the vibrational spectra in the infrared region of these complexes. FTIR spectra of Eu(MC1)3(H2O)3 and Eu(MC4)3(H2O)3 complexes show O-H (water-group) absorption bands at 3384 and 3393 cm-1, and the Eu(MC2)3(H2O)2 and Eu(MC3)3(H2O)2 complexes show lower intensity signals at 3476 and 3424 cm-1, respectively. The bands for the MC2/MC3 complexes are in a region of lower energy

with respect to the bands forthe MC1/MC4 complexes. The energy difference between 5D0 and 7F6 (for Eu3+ ion) is ~12300 cm-1. Therefore, there is a better resonance condition involving three phonons (3×3393 cm-1) in the case of the Eu(MC1)3(H2O)3/Eu(MC4)3(H2O)3 complexes, causing a considerable increase in the rate of non-radiative decay. As seen in Table 2, MC1/MC4 complexes show lower efficiency. TGA results for

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Eu(MC1)3(H2O)3/Eu(MC4)3(H2O)3showed 3 coordination water molecules. Whereas for Eu(MC2)3(H2O)2 and Eu(MC3)3(H2O)2, only 2 water molecules were observed, which corroborates the vibrational spectra (FTIR/Raman) and justifies the higher quantum efficiency presented by these complexes. An analysis of Table 2 can be understood from the separation of the structures of the ligand. For ligands with alkyl chains (Eu(MC1)3(H2O)3 and Eu(MC4)3(H2O)3),we observe a high nonradiative decay rate in relation to the radiative decay rate, the lowest quantum efficiency, and the shortest lifetime. These observations can be associated with the high incidence of nonradiative processes, as vibrational coupling of O-H oscillators from water molecules coordinated with Eu3+.Eu(MC2)3(H2O)2 and Eu(MC3)3(H2O)2had low nonradiative decay rates (Anrad) in relation to high radiative decay rates (Arad), higherquantum efficiency,and a lifetime beyond 1 ms. The Arad, and lifetimes, are the highest values reported for luminescent ionogels with europium ions reported in the literature..59,60,61 The values reported for the photophysical properties ofEu(MC2)3(H2O)2 and Eu(MC3)3(H2O)2can be explainedby theposition of the triplet level in the energy diagram of the ionic liquids resonant with the emitting level of Eu3+ (Figure 5). We observed that Eu(MC2)3(H2O)2showed greater efficiency compared to complexes with ligand structures similar to those proposed in this work.62We compared the luminescence properties of different europium complexes to evaluate the effect of alkyl chain length and flexibility. The ligandstructures of MC1 and MC4 did not present steric impediment, besides having a greater structural flexibility, which implies a greater absorption of water and a closer approximation between neighbouring ions. The self-quenching process between neighbouring europium complexes could cause nonradiative dissipation of excited states.13 The effective isolation and confinement of europium complexes inside more rigid structures, such as Eu(MC2)3(H2O)2 and Eu(MC3)3(H2O)2,were crucial toincreasingluminescence efficiency, which explains the higher values of quantum efficiency and lifetime associated with these systems. Due to the nature of the chemical environment for the intensity of 5D0→7F2 and5D0→7F1 transitions, their intensity ratio (R03+ 2/0-1) could be used to assess the asymmetry of the Eu coordination environment.63 A relatively higher ratio usually denotes a relatively higher degree of asymmetry and better luminescent monochromaticity.13The disparity in the values found forR0-2/0-1 and R0-4/0-1 represent the structural differences caused by the ionic liquids used as ligands. The high values for R0-2/0-1 can be explained by the ionic nature of the ligands that involve a load compensation through insidecoordination network. The fact that no load transfer band in the spectrum can be seensupports this explanation. The high values ofR0-2/0-1 suggest environments with low symmetry, as registered by the emission spectrum. The Judd-Ofelt parameterΩ2 reported for Eu(MC2)3(H2O)2 is similar to the values found in the literature for organic ligands with benzenescarboxylates.63,64 The disparity of results reported in the literature compared to other structures elaborated in this work (Eu(MC1)3(H2O)3, Eu(MC3)3(H2O)2 and Eu(MC4)3(H2O)3) indicate strong differences in the chemical environment for Eu 3+ ions between these compounds. These differences can be explained by

the benzene structure in the main chain (MC2) and View benzenehexaArticle Online 10.1039/C8TC00658J carboxylic acid.63 In MC3, the benzene ringDOI: is only a branch of the main chain. In Eu(MC1)3(H2O)3 and Eu(MC4)3(H2O)3,which show great differences between Ω2 values, there is no benzene ring. The parameter Ω4 reflects the stiffness of the chemical environment around the lanthanide ion. The lowest Ω4values were exhibited to by the coordination network Eu(MC3)3(H2O)2,reflecting the network effect on the structural rigidity of the material. Similar ligand structures, Eu(MC1)3(H2O)3 and Eu(MC4)3(H2O)3, show congruent results that indicate the large size of the structural difference caused by the absence of the benzene ring. Preliminary measures of absolute emission quantum yields are reported in Table S8. We calculated the sensitization efficiency for the ligand based on previously reported methods.65 The high quantum yield observed for these complexes seems to be due to the calculated high yield energy transfer from these ligand states to the quasi-resonant Eu3+ energy levels. Photostability of the Eu3+-complexes were evaluated by monitoring the emission/excitation spectra intermittently under continuous exposure to UV irradiation. Under UV exposure (UVA at 365 nm and UVB at 320 nm), the luminescence intensities of the intraconfigurational Eu3+ transitions suffer insignificant changes, as observed in Figures S38-39. Using the intensity of the 5D0-7F2 transition, we produced a graph that provides evidence of photostability for systems against UV radiation (Figure 7).

Figure 7 Photostability curves of Eu(MCx)3(H2O)yobtained by monitoring the intensity of the 5D0-7F2 transition of the Eu3+ ion. All curves were recorded at room temperature under irradiation with a 450 W Xenon lamp. Emission spectra were obtained at steady-state and at room temperature. Measurements were performed with the same sample over different absolute doses (0, 0.5 and 1.0 J) of UVA (365 nm) and UVB (320 nm) radiation. Complexes and wavelengths used in photoluminescence analysis: Eu(MC1)3(H2O)3, ex = 394 nm; Eu(MC2)3(H2O)2, ex = 394 nm; Eu(MC3)3(H2O)2, ex = 395 nm; and Eu(MC4)3(H2O)3, ex = 394 nm.

We studied photostability by comparing the measurements performed in the same sample over three years (2016, 2017 and 2018). The emission and excitation spectra are reported in Figure S41 for all coordination systems. Eu–complexes showed great chemical stability and photostability to produce exactly the same spectral


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emissions/excitations even three years after synthesis of the ionogels. The computational method24 employed in this work has already been well established in the literature and its main objective is to provide data for a detailed study of photoluminescence forionogels,25crystals,26composities27and general complexes.28This methodology presented consistent results for each of the elaborated complexes. All information pertaining to each structure and geometry optimization method are reported in the supplementary information (Table S15). In the determination of the coordination sphere of the complexes, we found very reliable results for which we have the lowest percentage errors that have been reported for this computational procedure.26,62The structures of the ionogels were optimized using theSparkle/PM7 model. These numbers of water molecules are in accordance with those predicted by

thermogravimetric analysis. Figure 8 shows a representative View Article Online DOI: 10.1039/C8TC00658J coordination sphere for Eu(MC2)3(H2O)2. Figures of coordination geometries forEu(MC1)3(H2O)3, Eu(MC3)3(H2O)2, and Eu(MC4)3(H2O)3complexes are available in the supplementary information (Figures S38-S41). As shown in Figure 8, the ligandcoordination comes from its carboxylate group in a chelating coordination, as predicted by FTIR analysis. The coordination polyhedron is organized in a distorted symmetry as a bicapped trigonal prism, which indicates that the arrangement is in point group symmetry C2V.66 This symmetry requires sp3d3f hybrids and are common coordination polyhedra for a coordination number of eight or nine.66 The proposed symmetry is consistent with the estimate from the emission spectrum of all compounds. The distances between the metal centre and the oxygen atoms from ionic liquid and coordinated water are shown in Tables S9-S11 in the ESI†.

Figure 8 Calculated geometry of the ground-state forEu(MC2)3(H2O)2 using the PM7 model. (A) The coordination environment of the Eu3+ ion and (B) its coordination polyhedron.

Spectroscopic properties as intensity parameters Ωλ (λ = 2, 4 and 6), energy transfer (WET) and back – transfer (WBT) rates, radiative (Arad) and nonradiative (Anrad) decay rates, quantum efficiency (η) and quantum yield (q) of europium compounds were calculated using semi-empirical models specialized in the study of photoluminescence of europium ions (LUMPAC).14All results are exhibited in Table 3 and are in agreement with those obtained experimentally, evidencing the accuracy of the computational methodology. Their outstanding photophysical properties inspired us to investigate the mechanisms of population transfer involved in the systems. Intramolecular energy transfer (ET) and back transfer (BT) were calculated, considering that the Eu3+ levels arose from the metal ion at an intermediate coupling.24 Theoretical data are reported for all complexes in Tables S12-S15.Energy transfer for Eu(MC1)3(H2O)3, Eu(MC2)3(H2O)2 and Eu(MC4)3(H2O)3 complexes follow a multipolar mechanism. For Eu(MC3)3(H2O)2, the profile suggests that the energy transfer occurs by an exchange

mechanism.67,68 The population diagrams of the complexes showed the synergy between carboxylate-ionic liquids with Eu3+ ions (Figures S50-54). We note that Eu3+/Gd3+-ionogels presented a similar coordination mode (as evidenced by FTIR and Raman spectra) and thermal stability until 223° C (Figures S23-ESI†) associated with high viscosity at room temperature. The interesting features of the soft materials include outstanding ionic conductivity due to a high content of ILs, easy coating on surfaces, and excellent luminescence properties (e.g., long lifetime, high colour purity, highlighted quantum efficiency, and photostability). These features might render them extremely valuable for various optical applications. The ionogels can also be entirely solution-processed, which is largely favourable to the development of low-cost and large-area lighting, or yet applicable for the composition of flexible displays in light emitting devices (LEDs), light-emitting electrochemical cells, and luminescent solar concentrators.

Table 3 Parameters of intensities: Ω2 and Ω4, the Einstein spontaneous emission coefficient (Arad) and the non-spontaneous emission coefficient (Anrad), the quantum emission efficiency (η), R0-2/0-1, R0-4/0-1and lifetime (τ) for complexes modelledusing Sparkle/PM7.

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Eu(MC1)3(H2O)3 Eu(MC2)3(H2O)2 Eu(MC3)3(H2O)2 Eu(MC4)3(H2O)3

Ω2 (10-20 cm2) 6.59 10.7 9.89 8.55

Ω4 (10-20 cm2) 4.02 5.98 5.01 5.41

Arad (s-1) 308.4 461.2 422.5 388.3

2

Conclusions In summary, we reported, designed and synthesized an imidazolium based-ionic liquid with appended carboxylic acids as termini, which were directly derived from anhydrides. The ionic liquids produced showthe following: high yields (up to 98%) and high purity (proven by NMR, elementary analysis and melting point); thermal stability up to 100 °C, associated with melting points below room temperature, chemical stability in water and common organic solvents, as demonstrated by miscibility tests; and featured luminescence with intensity on the order of 106 photons s-1. With these aforementioned characteristics, carboxyl-functionalized ILs showed triplet levels in the best possible region (23000 - 30000 cm1) for synergistic coordination with lanthanide ions. Our study demonstrates a complete analysis of the structureproperty relationship for each complex (Eu(MC1)3(H2O)3, Eu(MC2)3(H2O)2, Eu(MC3)3(H2O)2 and Eu(MC4)3(H2O)3). The coordination polyhedron is in bicapped trigonal prism symmetry, which is consistent with emission spectrum estimatesfrom all the compounds. The supramolecular ionogelshad high viscosity at room temperature, transparency, and thermal stability until223°C, and strong photoluminescence (>1 ms, η =48% and φ =27%). The extraordinary photophysical properties (presented in a detailed experimental and theoretical study) thus make these materials extremely valuable for optical applications such as light-emitting electrochemical cells. Our future research will focus on the fabrication of emitting devices based on Eu(MC1)3(H2O)3, Eu(MC2)3(H2O)2, Eu(MC3)3(H2O)2 and Eu(MC4)3(H2O)3 as thin films.

3 4 5

6 7 8 9 10

11 12 13 14 15

16

Acknowledgements This work was supported by FACEPE: APQ-0675-1.06/14. We gratefully acknowledge LAC-UFPE, Analytical Central (dQFUFPE) and CETENE for carryingout several analyses. We thankCAPES/CNPQ for financial support APQ-0549-1.06/17 INCT.

17 18 19

Notes and references ‡ The authors declare no competing financial interests.

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K. Dong, X. Liu, H. Dong, X. Zhang and S. Zhang, Chem. Rev.,

Anrad (s-1) 1446.3 531.6 553.3 1086.8

η (%) 17.6 46.4 43.3 26.3

R0-2/0-1DOI: 10.1039/C8TC00658J R0-4/0-1 4.01 6.52 6.02 5.20

1.23 1.84 1.54 1.67

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