Coupling photoluminescence and ionic conduction properties using

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Functional Nanostructured Materials (including low-D carbon)

Coupling photoluminescence and ionic conduction properties using the different coordination sites of ureasil-polyether hybrid materials Gustavo Palacio, Sandra Helena Pulcinelli, Rachid Mahiou, Damien Boyer, Geneviève Chadeyron, and Celso V. Santilli ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11149 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials Gustavo Palácio †,‡, Sandra H. Pulcinelli †, Rachid Mahiou ‡, Damien Boyer ‡, Geneviève Chadeyron ‡, Celso V. Santilli †* †

Chemistry Institute of the University of the State of São Paulo (IQ-UNESP), Araraquara, São Paulo, Brazil, 14801-350

‡

Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, F-63000 ClermontFerrand, France KEYWORDS: Organic-inorganic hybrid materials, photoluminescence, ionic conductivity, sol-gel.

ABSTRACT: In this paper, we demonstrate that each functional group of ureasil organic-inorganic hybrid (OIH) materials can act as a specific coordination site for a given active guest species, hence allowing the possibility of combining different functional properties. To illustrate this concept, the sol-gel process was used to produce di-urea crosslinked siloxane-polyethylene oxide (UPEO) and siloxane-polypropylene oxide (U-PPO) hybrid host frameworks with similar molecular weights (1900 and 2000 g.mol-1 for PEO and PPO, respectively), with Li+ and Eu3+ as active guest ions providing ionic conduction and photoluminescence (PL) properties, respectively. Comparison of Fourier transform infrared (FTIR) spectra and small angle X-ray scattering (SAXS) results for single-doped (using Li+ or Eu3+) and co-doped (using Li+ and Eu3+) U-PEO and U-PPO hosts showed that in every case, there was specific coordination of Eu3+ by the carbonyl group of the urea bridge and of Li+ by ether-type oxygen of the PEO and PPO chains. Optical analyses demonstrated that loading with Li+did not affect the luminescence properties of the Eu3+-loaded OIH. Although loading with Eu3+ had a small effect on ionic transport, co-doping with Li+ ions ensured macroscopic ion-conduction of the transparent and luminescent hybrid material. The results suggested that the combination of both properties in a transparent elastomeric material could be useful for the development of multifunctional devices. The results suggested that the combination of both properties in a transparent elastomeric material could be useful for the development of multifunctional polyelectrolytes applied in the field of the dual luminescent devices like photoelectrochromic smart windows.

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1. INTRODUCTION The preparation of multifunctional materials based on substances formed by different functional groups (moieties or chemical bonds) has become a popular way to design systems with advanced passive and active properties, which have emerging applications in different fields 1,2. For this purpose, organic-inorganic hybrids (OIH) are useful materials, since organic and inorganic moieties interact covalently as building blocks at the nanometric scale, combining the advantages of conventional inorganic oxides (thermal, mechanical, and structural stabilities) and organic polymers (specific functionality and high flexibility) 3,4. Furthermore, the hybrid backbone can be crosslinked by primary bonds, secondary bonds, entanglements, or semi-crystalline domains, depending on the chemical natures of the inorganic moieties and the organic polymers 5,6. Many of these primary and secondary interactions can be used to tune the nanoscopic structure and the macroscopic properties of the material, by incorporation of the appropriate guest substances in the OIH host 7,8. Siloxane-polyether provides the basis for a versatile OIH host in which the organic and inorganic moieties are linked by means of ureasil bridges 9. The multifunctionality of the ureasil polyether (U-PE) is achieved by the characteristics provided by each component, including: i) rigidity of the siloxane crosslinking nodes; ii) hydrogel (swelling) behavior resulting from the hydrophilicity and flexibility of polyethylene oxide (PEO) 10; iii) dimensional stability in the presence of moisture and in aqueous media (non-swellable), resulting from the hydrophobicity of polypropylene oxide (PPO) chains 11; iv) metal cation solvation by ether-type oxygen of the PE chains 12; v) complexation of cationic and anionic species by the resonant structure of the urea bridge 13; vi) transparency to UV and visible light resulting from the nanoscopic size of the organic and inorganic moieties 14; vii) intrinsic photoluminescence (PL) capacity due to the NH groups of the urea bridges and electron-hole recombination in the siloxane nodes, leading to broad emission in the blue/green spectral region when exposed to UV light 15. Due to its wide range of features, U-PE has potential applications in areas including the environment, human health and care, optics, and ionic materials 16,17,18,19,20. However, despite considerable research activity, few studies have explored the multifunctionality of the U-PE host achieved by incorporating different guest ions in order to prepare materials with two or more functional properties. The purpose of the present work was to demonstrate this concept by combining at least two different properties, namely photoluminescence (PL) and ionic conduction, by the co-incorporation of Eu3+ and Li+ into OIH materials based on U-PEO and U-PPO with similar molecular weights (1900 g mol-1 for PEO and 2000 g mol-1 for PPO). U-PEO and U-PPO loaded with Eu3+ ions are white emitters, due to overlap of the hybrid host intrinsic emission with the red color of the intra-4f straight lines of the guest cation 21. It is well accepted that lanthanide cations loaded in U-PE interact specifically with the carbonyl (Ocarb) of the urea linkage 13. On the other hand, in the case of U-PE loaded with alkaline cations, the host-guest interaction preferentially involves solvation by the ether-type oxygen atoms (Oeth) of the polymer chains 22. The mobility of the cations, facilitated by the segmental motion of the polyether chains, imparts liquidlike electrolyte behavior to U-PEO and U-PPO loaded with lithium salt and other alkaline salts 23. This ionic conduction

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process is hindered at high Li+ loading ([Oeth]/[Li] < 15), due to the formation of O-Li+-O crosslinking between ether oxygen atoms of different polymer chains, which increases chain rigidity, or to cation-anion interactions 24. This paper reports a comparative study of the nanostructural features, the specificity of the Li+ and Eu3+ cation coordination sites, and the PL and ionic conduction properties of unloaded, Li+- or Eu3+-loaded, and Li+ + Eu3+ double salt-loaded U-PPO and U-PEO organic-inorganic hybrids polymer electrolytes (OIH-PE) of class A2 25,26, keeping the concentrations of the metals at [Ocarb]/[Eu3+] = 3 and [Oeth]/[Li+] = 15. The main aim was to demonstrate the ease of combining different macroscopic properties by means of the coordination of active guest ions at different polar sites of the hybrid materials. In order to validate this concept, combination of the inherent optical properties of lanthanide cations and the ionic mobility of alkaline salt ions was chosen in order to impart two functional properties to the sol-gel-derived OIH material: photoluminescence and liquid-like ionic conduction. The multifunctional hybrid concept adopted here could be useful for the development of the dual luminescent devices with possible applications in lighting and displays. In this respect the Li+ + Eu3+ double loaded OIH polyelectrolyte could be envisaged as an electroluminescent or a photoluminescent layer acting as active controllers of the inside lighting and light color and/or as outdoor displays in a building window facade. 2. EXPERIMENTAL 2.1 Materials and synthesis The OIH materials were synthesized by the sol-gel method, according to a well-known procedure and using commercially available reagents 27,28. In brief, the ureasil (U) crosslinking agent IsoTrEOS (3-(isocyanatopropyl)triethoxysilane, SigmaAldrich, 95% purity, CAS #24801-88-5) was covalently bonded to both ends of the functionalized PEO (O,O′-bis(2aminopropyl) polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol, Jeffamine® ED-2003, CAS #65605-36-9) or PPO (O.O′-bis(2-aminopropyl)polypropylene oxide, Jeffamine® D-2000, CAS #9046-10-0) by reacting their terminal aminopropyl groups, at a molar ratio of 1:2. Typically, 4 g of IsoTrEOS and 16 g of Jeffamine® ED-2003 or D-2000 were stirred together in ethanolic solution under reflux for 6 h at 78 ºC, to prepare 20 g of the OIH precursor. The ethanolic solution containing the OIH precursor (EtO)3 Si(CH2)3 NH C(=O) NH CH (CH3) CH2 -(polyether) -OCH2 CH (CH3) NH C(=O) NH (CH2)3 -Si(OEt)3 was then stored. The second step of the methodology comprised the sol-gel reaction involving the Si(OEt)3 hydrolysis, generating silanol moieties, followed by a condensation reaction to form ureasil crosslinking nodes. The hydrolysis was started by adding a specific volume of water to satisfy the relationship [H2O]/[Si] = 4, together with 12 μL of HCl (10-2 mol.L-1) as acid catalyst, to 1.6 mL of the ethanolic solution containing the hybrid precursor (equivalent to 0.5 g of the solid hybrid precursor). This procedure resulted in the formation of a crosslinked network with two main polar oxygen sites (identified as I and II in Scheme 1), ascribed to ether-type oxygen (Oeth) and carbonyl-type oxygen (Ocarb), respectively. Formation of the OIH:Eu3+, OIH:Li+, and OIH:Eu3+Li+ complexes was carried out by the solubilization of 0.051 and 0.049 g of EuCl3.6H2O (Sigma-Aldrich, 99.9% purity, CAS #13759-92-7) and 0.176 and 0.127 g of CF3SO2NLiSO2CF3 (Sigma-Aldrich, 99.95%


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purity, CAS #90076-65-6) in the PEO and PPO ethanolic OIH precursor solutions, respectively. Around 0.5 g amounts of transparent solid xerogels were obtained after drying under vacuum at 100 °C for 1 h.

Scheme 1. Schematic diagram of the different OIH matrices, highlighting their two polar oxygen active sites (I: Oether; II: Ocarbonyl).

2.2 Characterizations Differential scanning calorimetry (DSC) measurements were carried out using a Q100 analyzer (TA Instruments). Small disks with diameter of 0.5 cm and weighing approximately 10 mg were cut from the OIH samples and placed in aluminum pans. The samples were first heated at a rate of 10 °C/min from room temperature to 100 °C, maintaining the final temperature for 10 min, in order to eliminate the residual water/ethanol, followed by cooling to −90 °C at a rate of 10 °C/min. A second heating scan from −90 °C to 100 °C, at 10 °C/min, was used to assess the thermal transitions. Nitrogen was used as the gas purge, supplied at a flow rate of 50 mL/min. The nanostructures of the hybrids were analyzed by small angle X-ray scattering (SAXS) measurements performed at the SAXS1 beamline of the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil). The beamline was equipped with a 2D Pilatus 300k detector located 910.9 mm from the sample, recording the image of the scattering intensity, I(q), as a function of the modulus of the scattering vector, q = 4π/λsin(ε/2), where ε is the X-ray scattering angle. The data were normalized considering the varying intensity of the direct X-ray beam, the detector sensitivity, and the sample transmission. The parasitic scattering intensity due to the cell windows and vacuum was subtracted from the total scattering intensity. Infrared spectra were recorded with a Nicolet 760 FTIR spectrometer operated with OMNIC software. Spectra for the OIH films (~50 μm) were obtained in transmission mode using 32 scans and 4 cm-1 resolution in the range from 400 to 4000 cm-1. The X-ray photoelectron spectroscopy (XPS) analysis was carried out at a pressure of less than 10-7 Pa using a commercial spectrometer (Scienta Omicron ESCA+) with a high-performance hemispheric analyzer (EAC2000) to confirm the nominal amount and to verify the local bonding structure of Eu3+ 4d loaded into OIH The Al Kα line was used (hυ = 1486.6 eV) and the analyzer pass energy for the high resolution spectra was set to 20 eV. Emission spectra (with excitation at 365 nm) were recorded at room temperature (25 °C) for U-PPO, U-PEO, and the Eu3+loaded and Eu3+Li+ double salt-loaded OIH materials. The measurements were performed using a Hamamatsu C9920-

02G PL-QY system equipped with a 150 W Xe lamp coupled to a monochromator for wavelength discrimination, an integrating sphere (Spectralon® coating, Ø = 3.3 inch), and a high sensitivity CCD camera for detection of the entire luminescence spectrum. The ionic conductivities of U-PPO, U-PEO, and the Li+loaded and Eu3+Li+ double salt-loaded OIH materials were measured at room temperature (25 °C), using a Solartron 1260 Impedance/Gain-Phase Analyzer in a frequency range from 1 MHz to 50 Hz, with an applied signal amplitude of 3 mV. The tests were performed using rectangular flat-surface monoliths about 2 mm thick and with electrical area of 3 mm2. The two parallel surfaces were painted with a leit-silver ink (SigmaAldrich, CAS #09937) to form the electrical contacts. Complex impedance plots of Z” as a function Z’ (Nyquist diagrams) were computed from the raw experimental data. The intersection of the imaginary impedance at low frequency with the real impedance axis corresponded to the ionic conductance of the samples, so the ionic conductivity could be calculated from knowledge of the sample thickness and the silvered area of the electrodes29. 3. RESULTS AND DISCUSSION 3.1 Effect of Eu3+ and Li+ loadings on thermal transitions The roles played by the nature of the metallic cation (Eu3+ or Li+) and the type of polyether macromer (PEO or PPO) in influencing the crystallinity and dynamic properties of the hybrid matrix were analyzed in order to demonstrate the selective coordination sites of the Eu3+ and Li+ cations. Quantitative information about the effects of these parameters was obtained from analyses of the reversible thermal transformations involved in the first order (crystallization ↔ melting) and second order (glass ↔ rubber) transitions of the crystalline and amorphous phases of the polyether. Both transitions are generally observed for semi-crystalline polymers, with the temperatures of crystallization (Tc), melting (Tm), and glass transition (Tg) changing according to the molecular weight of the polyether PEO chains 30. These two events characterizing the semi-crystalline state of the PEO were evidenced from the DSC curves recorded during heating of U-PEO and U-PEO:Eu3+ (Fig. 1). The changes in the heat fluxes, of around -48 °C (U-PEO) and -58 °C (U-PPO), evidenced the difference between the heat capacities (ΔCp) of the glass and rubber states for the glass transition of the amorphous polymer phase of the hybrid material. This feature, together with the endothermic peak observed for U-PEO at ~30 °C, corresponding to melting of the crystalline PEO, characterized the semi-crystalline nature of this hybrid31. On the other hand, the absence of the melting event characterized the amorphous nature of both the unloaded and loaded U-PPO. Comparison of the thermal events of the unloaded U-PEO and U-PPO with those for the OIH materials loaded with Li+ and Eu3+ provided evidence of the selective interaction of Li+ and Eu3+ with sites I and II (Scheme 1), respectively. Irrespective of the polyether, the Tg values were essentially invariant with the addition of Eu3+, indicating that both the PEO and PPO chain mobility and the inter-chain interactions of the amorphous polymer phase were not affected by the Eu3+ loading 24. Moreover, the endothermic melting event of UPEO remained apparent for U-PEO:Eu3+, revealing that the addition of Eu3+ did not cause any appreciable modification of the helical conformation of crystalline PEO. These features indicated that the Eu3+ atoms incorporated in the two OIH



2 mW/g

matrices were not coordinated with the ether-type oxygen atoms of the polyether chains. However, the Tg values showed significant increases, from -50 to -37 and -35 °C for the Li+-loaded and double salt-loaded UPEO, respectively, and from -58 to -40 and -36 °C for the Li+loaded and double salt-loaded U-PPO, respectively. This Tg upshift evidenced the increased rigidity of the polymer chains, attributed to the formation of inter- and intra-molecular interactions between the alkaline cations and ether-type oxygen atoms of the amorphous polymer phase 29. In addition, the absence of the endothermic peaks corresponding to the melting of crystalline PEO phase 32 in U-PEO:Li+ and UPEO:Eu3+Li+ indicated that the presence of the Li+ cations hindered PEO crystallization. In fact, the strong coordination of small cations by the PEO chains induces “crown ether” conformations, restraining the helical PEO conformations of crystalline polymers 12. These findings provided strong evidence of the interaction of Li+ with the polar ether-type oxygen atoms of the PEO and PPO chains (Site I, Scheme 1) 29. It should be noted that this type of coordination was not evidenced in the case of Eu3+ loading. 28 °C 20 °C

Tf

-37 °C

ether oxygen atoms, due to coordination of the Li+ cations to the PEO and PPO chains 34. It could be concluded from the area under each component that the proportion of noncomplexed chains followed the order: U-PPO:Li+ > UPPO:Eu3+Li+ > U-PEO:Eu3+Li+ ≈ U-PEO:Li+. The pronounced component at 1134 cm-1 for U-PPO:Li+ and U-PPO:Eu3+Li+, characteristic of the “free” amide ion, was attributed to the symmetric out-of-phase stretching mode of the SO2 group 34. The event at 1060 cm-1 was also ascribed to an amide band. With Li+ loading, the amide I band was upshifted from 1635 to 1657 cm-1, indicating (as discussed below) decreased intensity of hydrogen bond association, due to the formation of a disordered PEO-urea structure 35. Irrespective of the type of ureasil-polyether (U-PEO or UPPO), the νCOC vibration band remained essentially invariant after incorporation of Eu3+ (Fig. 2). This invariance was interpreted as a fingerprint of the absence of interactions between the Eu3+ cations and the ether-type oxygen atoms of the polyether chains. This explanation was supported by spectroscopic results reported for europium triflate 36, europium perchlorate 37, and europium nitrate 38 loaded onto low molecular weight U-PEO.

+

U-PEO:Li

(a)

3+

U-PEO:Eu 3+

+

U-PEO:Eu Li

U-PEO

Tg = -48 °C -40 °C +

U-PPO:Li

3+

U-PPO:Eu

-36 °C

3+

+

U-PPO:Eu Li

-25

U-PPO U-PPO:Li+ U-PPO:Eu3+ U-PPO:Eu3+:Li+

U-PPO

Tg = -58 °C -50

Absorbance (arb. units)

Heat Flow (arb.units)

-35 °C

EXO

0

25

50

75

100

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1800

1600

1500

1400

1300

1200

1100

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1000

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Figure 1. DSC curves for the unloaded (black curves), Eu3+loaded (red curves), Li+-loaded (blue curves), and Eu3+Li+-loaded (magenta curves) U-PEO (top) and U-PPO (bottom) hybrids.

3.2 Chemical environments of the cations Valuable information about the chemical environments of the Eu3+ and Li+ cations loaded into the OIH matrices was obtained from the FTIR spectra, in the regions of the C-O-C backbone stretching mode (νCOC, 1200-1000 cm-1) and the so-called amide vibrations (1800-1500 cm-1). Both of the unloaded OIH materials showed a broad band centered at around 1100 cm-1 (1091 cm−1 for U-PEO and 1109 cm-1 for UPPO) and a shoulder at about 1134 cm-1 (Figs. 2(a) and 2(b)), ascribed to the νCO vibration mode of uncomplexed (OCH2)based moieties and to the coupled vibration of the νCO and rCH2 modes, respectively 33. After Li+ loading, there were downshifts of the band from 1091 to 1073 and 1083 cm-1 for U-PEO:Eu3+Li+ and U-PEO:Li+, respectively, accompanied by decreases in intensity. In the case of the U-PPO:Li+-loaded samples, this band envelope was split into at least three components, two of them resulting from the downshift from 1109 cm-1 to 1084 and 1078 cm-1, for the U-PPO:Eu3+Li+ hybrid. Both of these events (downshift and band splitting) were attributed to a change in the local environment of the

1700

(b) Absorbance (arb. units)

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1800

1700

U-PEO U-PEO:Li+ U-PEO:Eu3+ U-PEO:Eu3+:Li+

1600

1500

1400

1300

1200

Wavenumber (cm-1) Figure 2. Room temperature FTIR spectra from 1700 to 950 cm-1 for (a) U-PPO and (b) U-PEO.


1550

1680

Absorbance 1600

Wavenumber / cm-1

1550 1636

1500 1700

1650

U-PPO:Eu3+

1600

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(d) U-PEO:Eu3+

1700

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1650

1600

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+

1657

1500

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:Li

1700

1550

2nd derivative

1650

1569

(f) U-PEO:Eu

3+

1600

1585

2 derivative

1567

:Li

1585

(e) U-PPO:Eu

1637

Wavenumber / cm

1500 1700

Chi^2=6.88240E+003

nd SS=4.81768E+005

1612

-1

1641

1550

1612

1600

+

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1639

1587 1639

1669 1650

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1660

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1633

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1653

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1660

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1563

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1600

1550

1500

Wavenumber (cm-1)

Figure 3. Gaussian fitting and 2nd derivative of the FTIR spectra in the amide region for (a) U-PPO, (b) U-PEO, (c) U-PPO:Li+, (d) UPEO:Li+, (e) U-PPO:Eu3+Li+, and (f) U-PEO:Eu3+Li+.

Dahmouche et al 39 used different spectroscopic techniques to study the dependence of the Eu3+ concentration on the number of repeated oxyethylene units and showed that for di-ureasils containing long organic segments with PEO molecular weight (MWPEO) of 2000 g.mol-1) and a low concentration of Eu3+ ([Oeth]/[Eu3+] ≥ 80), the cations mainly interacted with carbonyl-type oxygen of the urea crosslinks located at the organic/inorganic interface. In contrast, for di-ureasils with short organic segments (MWPEO = 600 g.mol-1) and a similar Eu3+ concentration, the ions were unable to disrupt the strong and ordered hydrogen-bonded urea-urea structures and the

preferential coordination sites were the ether-type oxygen atoms of the polymer chains. The most significant effect resulting from the Eu3+ loading of the OIH matrices was the appearance of a new band in the amide region (1800–1500 cm-1) 13, with contributions from amide I and amide II (Fig. 2). The amide I band envelope (at around 1640 cm-1) consisted of different C=O environments including “free” C=O groups 40 and ordered/disordered hydrogen-bonded aggregates of different strengths, involving NH from urea groups and oxygen atoms of ether and/or carbonyl groups. The amide II region (at around 1545 cm-1)



3.3 Effects of Eu3+ and Li+ loading on the nanostructure The effects of the different coordination sites of Eu3+ and Li+ on the nanostructural features of the OIH were examined by SAXS measurements. The SAXS curves of U-PPO and UPEO (Fig. 4) exhibited single broad peaks with maxima (qmax) at 1.64 and 1.89 nm-1, respectively, ascribed to the spatial correlation between the regularly spaced siloxane crosslinked nodes 45. After Eu3+ loading, the correlation peaks showed displacements to qmax of 1.46 and 1.65 nm-1 for U-PPO:Eu3+ and U-PEO:Eu3+, respectively. Accordingly, the average correlation distance between adjacent siloxane nodes (ξ = 2π/qmax) 46 increased from 3.80 to 4.30 nm for U-PPO:Eu3+ and from 3.30 to 3.80 nm for U-PEO:Eu3+. Increased correlation peak intensities were also observed for both the U-PEO:Eu3+ and U-PPO:Eu3+ hybrids. These features reflected increased electron density contrast between the inorganic and polymeric phases, as a consequence of the greater free volume of the polymeric matrix caused by the breaking of H-bonding associations (evidenced by the increase of ξ) and the presence of Eu3+ near the siloxane nodes 47. In contrast, when the OIH materials were loaded with Li+, the correlation peaks were upshifted, leading to decreases of ξ from 3.30 to 3.25 nm for U-PEO and from 3.80 to 3.20 nm for

qmax (nm-1) U-PPO U-PPO:Li+ U-PPO:Eu3+ U-PPO:Li+Eu3+

I(q) (arb. units)

was mainly associated with N-H in-plane bending vibration, which is sensitive to the chain conformation and intermolecular hydrogen bonding16. The individual components of the broad amide I-amide II envelope were obtained from least-square curve profile fitting based on the number and position of the constituent bands found using the established criteria of the second derivative of the spectrum 41. As shown in Fig. 3(b), this approach showed that the amide I envelope of the U-PEO spectrum presented three components, the first one (at 1636 cm-1) attributed to ordered urea-urea, the second one (at 1660 cm-1) to disordered urea-urea or disordered PEO/urea associations, and the last one (at 1680 cm-1) to non-hydrogen-bonded (free) carbonyl groups. The last component was not observed for U-PPO (Fig. 3(a)), probably due to the hydrophobic interactions of methyl groups (CH3) present along the PPO chains 6. For both hybrids, the amide II band showed only a single intense component at around 1550 cm-1. The structural modifications arising from the addition of Eu3+ can be seen in Figs. 3(c) and 3(d). The Gaussian fitting revealed complex deconvolution of the amide I region, with four components for U-PPO:Eu3+ and three for U-PEO:Eu3+. For both OIH matrices, the main amine I band was downshifted to 1612 cm-1, while the amine II band was upshifted to 1587 cm-1, indicating the rupture of disordered PEO-urea and PPO-urea associations with Eu3+ loading 27. Suppression of the component corresponding to the “free” C=O indicated the participation of carbonyl oxygen atoms in the Eu3+ coordination, giving rise to another C=O environment with a maximum at 1612 cm-1. Another important event was the development of an amide II contribution at 1587 cm-1, due to a change of the hydrogen bonding network involving the NH group of the urea unit, which was probably caused by interactions with the anions (Cl- and CF3SO3-). The presence of nitrogen atoms makes the protons of the urea linkage more electropositive than those of PEO, with stronger anion attraction favoring the NH-anion association 42. In the case of the double salt loading, the Gaussian fitting based on the second derivative of the spectrum (Figs. 3(e) and 3(f)) showed no abrupt change of the amide I contribution, due to the interaction of Li+ cations with Oeth. On the other hand, the structural changes caused by Eu3+ and Li+ co-doping were evidenced by the appearance of a second amide II contribution at ~1568 cm-1, associated with changes of the hydrogen bonding network involving the NH group of the urea unit, probably induced by interaction with the CF3SO3- anions. This complexation could be explained by the strong Lewis base character of both anions and the strong Lewis acid character of the NH group29, and was in good agreement with experimental and theoretical results reported elsewhere for the complexation of different anions on polyethers containing urea groups 43,44. The values of the Gaussian fitting parameters for undoped and Eu3+-doped U-PPO and U-PEO are provided in Table S1(a) and (b). It could be concluded from these results that the C=O groups actively participated in the complexation of Eu3+ ions, while the NH groups participated in complexation of the anions. Moreover, the additional participation of Cl- anions on the complexation of Eu3+ cations was evidenced from the fitted high resolution Eu 4d XPS spectra (Fig. S1 in the Supporting Information)

1.64 1.97 1.46 1.60

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U-PEO U-PEO:Li+ U-PEO:Eu3+ U-PEO:Eu3+Li+

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1.80 1.91 1.67 1.86

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Figure 4. Room temperature SAXS curves for the U-PPO (a) and U-PEO (b) multifunctional OIH materials.


10

a)

exc: 365 nm

368 nm

3+

U-PPO:Eu

Absolute Quantum Yield (%)

8

395 nm

3+

U-PPO:Eu 3+ + U-PPO:Eu Li

6

4

465 nm

2

400

450

500

450

F4

5

D0

C

5

NH NH

400

7

7

5

5

D0

D0

5

electron-hole O

7

7

R-SiO3-Si

F1

F0

7

 (nm)

D0

350

F3

300

D0

250

 nm

F2

0

500

550

600

650

700

exc: 365 nm

7

b)

F2

 (nm)

3+

5

F3

D0

7

5

 nm

D0

7

F0

D0

7

F1

7

F4

5

D0

U-PEO:Eu 3+ + U-PEO:Eu Li

R-SiO3-Si

5

D0

O electron-hole

5

U-PPO. This shrinkage was accompanied by decrease of the correlation peak intensity, evidencing decrease of the electron density contrast between the siloxane nodes and the polymeric matrix. This feature was a consequence of filling of the free volume of the polymer by Li+, providing further evidence of coordination of the cations by the ether-type oxygen atoms of the polyether chains 27. The simultaneous effects of Eu3+ and Li+ were clearly evidenced by the invariance of the correlation peak observed when the OIH was loaded with both cations. These features were in agreement with the complexation of Eu3+ by the carbonyl group of the amide function, as well as Li+ coordination by the ether oxygen of the polyether backbone, as indicated by the FTIR results (Figs. 2 and 3). 3.4 Bifunctional properties induced by Eu3+Li+ co-doping The photoluminescence properties of the Eu3+-loaded OIH samples were evaluated at room temperature by recording the emission spectra (PL, Figs. 5(a) and 5(b)) upon excitation at 365 nm, which corresponded to the maximum observed in the excitation spectrum (PLE) (insert in Fig. 5(a)). The broad band observed in the PLE was ascribed to the ligand-to-metal charge-transfer (LMCT) band, which occurs in the same energy region of the 7F0→5L6 and 7F0→5D3 transitions 48. The PL spectra of Eu3+ consist of a broad band between 380 and 580 nm and a series of narrow lines assigned to transitions from the first excited state, 5D0, to the 7F0–4 low energy levels of the Eu3+ 4f6 configuration 49. The broad PL band in the green-blue spectral range also observed for both the Eu3+ unloaded and Li+-loaded OIH (see picture in Fig. 5(c)) was ascribed to overlapping of the NH groups emission from urea bridges and that originating from electron-hole recombination in the siloxane nodes 50,51, as observed elsewhere for similar hybrids 52. The strongest emission, at about 615 nm and corresponding to the 5D0→7F2 Eu3+ transitions, was responsible for the red emission observed for both UPEO:Eu3+ and U-PEO:Eu3+Li+ (Fig. 5(c)). The observed intra-4f6 transitions were mainly of electric dipole (ED) nature, with the exception of 5D0→7F1, which is practically independent of the chemical environment. The narrow line at 582 nm, corresponding to the 5D0→7F0 transition53 present in all the OIH materials studied, revealed a disordered type complexation similar to that observed in oxide glasses54. On the other hand, the so-called “hypersensitive” 5,55 transition, 5D0→7F2, is a forced electric dipole (ED) transition forbidden by the selection rules. However, the 5D0→7F2 electric dipole transition becomes allowed for molecules without centers of inversion, because the Laporte’s rule is relaxed by the J-Mixing induced by odd parity terms of the Hamiltonian ligand field 50. The emission strength of this transition is sensitive to the surroundings of Eu3+ 56. For this reason, the ratio (R) between the intensities, I(5D0→7F2)/I(5D0→7F1), called the asymmetric ratio, is widely used as a criterion of the coordination state and the site symmetry for the Eu3+ ions. This hypersensitive ratio R and the Judd-Ofelt (J-O) Ω2 parameter have close values and reveal similar physical significance of the symmetric/asymmetric and covalent/ionic nature of the bonding between Eu3+ ions and the surrounding ligands 57. Thus, Ω2 (R) is a useful parameter for probing the asymmetry of the Eu3+ sites58,59. Lower Ω2 (R) values indicate greater asymmetry60. The calculated Ω2 (R) values (Table 1) were higher than those observed elsewhere for Eu3+ incorporated in several oxide glasses and polymers, including PEO and PPO21.

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (arb. units)

Page 7 of 12

C NH NH

400

450

500

550

600

650

700

 (nm)

(c)

Figure 5. Effect of Eu3+-doping (red) and Eu3+Li+ co-doping (magenta) on the emission spectra of the OIH hybrids: (a) U-PPO (insert: excitation spectra (PLE) of the U-PPO:Eu3+ complexes; maximum at 368 nm) and (b) U-PEO. (c) Pictures of the OIH matrices without (transparent) and with UV irradiation (λexc = 365 nm): blue (undoped and Li+-doped) and red (Eu3+-doped and Eu3+Li+-co-doped) emissions.

Table 1. Specific conductivity (σ), Judd-Ofelt parameter (Ω1,2) and FWHM of the 5D0 → 7F0 transition, for the U-PEO and UPPO OIH materials doped with different cations, at 25 ºC. OIH material

σ (Ω-1.cm)

Ω1,2

U-PEO U-PEO:Li+ U-PEO:Eu3+ U-PEO:Eu3+Li+

5.3 x10-6 2.1 x10-4 2.6 x10-5 6.2 x10-5

----3.3 3.0

5D → 7F 0 0 (FWHM x10-7 cm-1) ----1.8 2.3

U-PPO U-PPO:Li+ U-PPO:Eu3+ U-PPO:Eu3+Li+

5.0 x10-8 1.9 x10-5 --6.7 x10-6

----2.9 2.4

----1.6 1.7



transfer from water molecules absorbed into the hydrophilic PEO chains. This hypothesis also provided an explanation for the insulating-type behavior found for the hydrophobic UPPO:Eu3+. The U-PEO:Eu3+Li+ material exhibited an ionic conductivity value intermediate between those for UPEO:Eu3+ and U-PEO:Li+. Comparison between U-PPO:Li+ and UPPO:Eu3+Li+ showed a similar decrease of the ionic conductivity for the co-doped samples, which could be attributed to the greater tendency for ion-pair formation between Li+ and Cl-, relative to Li+ and CF3SO3-. This was expected from the high thermodynamic stability of LiCls (ΔHf = -408.27 kJ.mol-1), compared to the large “soft” CF3SO3anion (ΔHf of LiNO2 = -372 kJ.mol-1) 70, which has low iondipole stabilization energies, together with high solvation energies due to mutual polarizability 71.

4

+

U-PEO:Li 3+ U-PEO:Eu 3+ + U-PEO:Eu Li

(b) 4

3

-1

The larger Ω2 parameter was a good indication that the symmetry of the Eu3+ sites was distorted. The data (Figs. 5(a) and 5(b); Table 1) also showed that irrespective of the presence of Li+ cations in the OIH, the Eu3+ emission spectra presented similar spectral distributions and intensity ratios, indicative of very similar asymmetric coordination spheres 61. This behavior, which has been reported for other ureasilpolyether materials, could be explained by the rather low covalence of europium cations coordinated by the carbonyl groups of the urea bridges and by Cl- or CF3SO3- anions 62. Furthermore, the calculated values of the 5D0→7F0 full width at half maximum (FWHM), between 1.63 x 10-7 and 2.3 x 10-7 cm-1 (Table 1), suggested that the Eu3+ cations in all the OIH samples studied were located in a continuous distribution of closely similar sites. From the similarity of the PL spectra (Figs. 5(a) and 5(b)) and the parameters gathered in Table 1, it could be concluded that Eu3+ occupied similar coordination sites in the U-PEO and U-PPO matrices, and that the addition of Li+ in the co-doped OIH materials did not affect the photoluminescence properties of the europium. These results also indicated that the high energy absorbed by the ureasil linkage was efficiently transferred to the Eu3+ luminescence center (the “antenna effect”) 63,64, even in the presence of Li+ in the OIH matrix. The complex plane of impedance (Nyquist plot) of the Li+loaded OIH consisted of a semi-circle (or depressed arcs) at high frequency and an inclined tail (close to 45°) at the lowest frequencies (Fig. 6). The straight line slope characterized the linear diffusion processes of the ions at the OIH-electrode interface, in this frequency range 65. The semi-circles were incomplete at their highest frequency (left side), indicating that the spectral frequency utilized was not sufficiently high to follow the cation jump frequency. The semi-circle corresponded to the specific ionic transfer property of the OIH bulk electrolyte 66. The intersections of the semi-circles with the straight lines at the abscissa axes corresponded to the values of the specific electrical resistivity (ρ) of the OIH:Li+ polyelectrolyte. For U-PEO:Li+, the semi-circle in the high-frequency domain vanished and the impedance diagram was dominated by a tail in the entire frequency range. It has been reported that the high frequency semi-circle does not appear in impedance plots for ion-facilitated plasticized polymer electrolytes 67. This phenomenon confirmed that the rubber features of the OIH:Li+ monoliths behaved similarly to a liquid electrolyte 68, in agreement with the structural characterization. Table 1 shows the specific conductivity (σ) values of U-PEO and U-PPO, for different cation doping conditions. The highest room temperature conductivity was obtained for U-PEO:Li+ (Table 1), while the lowest value was found for UPPO:Eu3+, which showed electrical insulating-type behavior. Conversely, the conductivity found for U-PEO:Eu3+ was higher than for U-PPO:Li+, which was consistent with the lower mobility of the PPO chains expected from the smaller Tg values revealed by the DSC analyses (Fig. 1) 24. is similar to the maximum value observed69 for PEG:EuCF3SO3 for Eu3+ loading of [Oeth]/[Eu3+] = 65, which is equal to the ratio used here. For [Oeth]/[Eu3+] > 65 the concentration of free ionic carriers was found essentially independent of the amount of Eu3+ loading69, indicating that this cation have a marginal contribution on the macroscopic ionic conductivity. This low conductivity of U-PEO:Eu3+. could be explained by proton

-Im(Z) (.cm x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 12

2

1

0

0

1

2

-1

4

3

4

Re(Z) (.cm x 10 ) Figure 6. Nyquist impedance diagrams of the U-PPO (a) and UPEO (b) monoliths loaded with Li+, Eu3+ and Eu3+Li+, at room temperature.


Page 9 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based on the hard/soft acid-base principle 72, in which Li+ cations are hard bases and small anions that are hard to polarize, such as Cl-, are hard acids, Whang et al. analyzed the effect of lithium salt anions on polytetramethylene oxide glycol (PTMG). Large polyatomic CF3SO3- anions with delocalized charges are less hard and have weaker interactions with Li+, compared to Cl- 66. Nevertheless, U-PEO:Eu3+Li+ presented a fairly high ionic conductivity at room temperature, together with distinct photoluminescence properties resulting from the intrinsic optical characteristics of the OIH matrix together with the Eu3+ PL lines. Such features make these multifunctional materials excellent candidates for technological applications in optical-ionic devices. 4. CONCLUSIONS Ureasil-poly(ethylene oxide) (U-PEO) and ureasilpoly(propylene oxide) (U-PPO) hybrid materials with individual and multifunctional properties were easily prepared by the sol-gel route, with single doping with Eu3+or Li+, or coupled doping with Eu3+ and Li+. The preferential complexation of Eu3+ by the carbonyl group of the urea linkage, evidenced for single-doped U-PEO and U-PPO, was preserved in the Eu3+Li+ co-doped hybrid materials. In addition, the preferential solvation of Li+ by the ether type oxygen atoms of the polyether chains, observed for the singledoped U-PEO and U-PPO hybrids, was also evidenced for the co-doped materials. The similarity of the photoluminescence properties observed for the single-doped (Eu3+) and co-doped (Eu3+ and Li+) UPEO and U-PPO hybrids revealed that the light emission process was essentially unaffected by the presence of lithium ions. Analysis of the PL spectra indicated that the specific characteristics of the polymer moieties, such as the degree of crystallinity and hydrophilicity of PEO, or the hydrophobicity of PPO, did not affect the so-called “antenna effect”. For all the OIH materials studied (amorphous, semi-crystalline, hydrophilic, or hydrophobic), the energy absorbed by the ureasil linkage was efficiently transferred to the low covalence and asymmetric Eu3+ luminescence center. The ionic conductivity of the OIH material co-doped with Eu3+ and Li+ was lower than that for the OIH doped with Li+ alone, due to anion exchange resulting from the use of europium and lithium salts with different anions (chloride and triflate). Formation of the ion pair consisting of the small Li+ ion and the large triflate polyatomic anion is easier, compared to ionic interaction between Li+ and the small monoatomic chloride. The larger amounts of Li+ charge carriers and the higher mobility associated with the segmental motion of the linear PEO chain resulted in higher conductivity of the single-doped and co-doped U-PEO material. The findings of this work demonstrate that useful multifunctional properties can be obtained by the coordination of active guest ions at specific sites of organic-inorganic hybrid materials. In addition to the conjugation of photoluminescence and ionic conduction, this concept could be applied to a wide range of other functional properties. ASSOCIATED CONTENT Supporting Information Tables containing the FTIR Gaussian fitting values of the Amide I and Amide II regions for the OIH materials based on PPO and PEO polymers, undoped, Eu3+-doped and Eu3+Li+

double salt-loaded and fitted high resolution Eu3d XPS spectra with the quantitative elemental amount of Eu3+. AUTHOR INFORMATION Corresponding Author * Celso V. Santilli. E-mail: [email protected]

Orcid Celso V. Santilli: 0000-0002-8356-8093

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Brazilian agency CNPq (grants 142495/2013-1, 203031/2015-6, and 401679/2013-6). This work is also a collaborative research project of the members of National Institute on Advanced Eco-Efficient Cement-Based Technologies (FAPESP INCT 2014 50948-3; 465593/2014-3). The authors thank prof. V. Mastelaro (IFSC/USP) and P. Hammer (IQ/UNESP) for XPS analysis, and the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas for use of the SAXS1 beamline.

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Multifunctional O-I Hybrid Materials

Li+

Eu UV Excitation

3+


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Coupling photoluminescence and ionic conduction properties using

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