Urea Cross-linkages Research Articles

Di-urea cross-linked siloxane hybrid materials incorporating oligo(oxypropylene) and oligo(oxyethylene) chains

Novel bridged silsesquioxanes (BSs) incorporating short oligo(oxypropylene) or oligo(oxyethylene) chains bonded to a siliceous network via urea cross-links were synthesized by the sol–gel process from the organosilane precursors (CH3CH2O)3-Si-(CH2)3-NHC(=O)NH-R-NH(O=C)NH-(CH2)3-Si-(OCH2CH3)3, with R≡-[CH(CH3)CH2O]xCH2CH(CH3)- (x∼2.5 and 6.1) or R≡-(CH2)2O(CH2)2O(CH2)2-, respectively. The new BSs were identified by the notations d-U′(Y′) with Y′ = 400 and 230, and d-U(Y) with Y = 148, where d stands for di, and U/U′ denote the urea group. The hybrid materials were produced as transparent, amorphous, homogeneous, and soft monoliths, although d-U(148) presented some rigidity. In the three samples the siliceous framework is mainly composed of [–(CH2)Si(OSi)3] and [–(CH2)Si(OSi)2(OH)] substructures. The highest polycondensation degree of the siloxane network was found in the case of d-U′(400), pointing out the formation of a three dimensional framework. In the three materials the urea groups are extensively involved in the formation of strong ordered urea-urea hydrogen-bonded aggregates of different degree of order. The d-U′(230) BS is a promising candidate as mold material for micropatterning applications aiming the fabrication of smart coatings. The prospects for these materials in flexible electrochromic devices for smart windows, where they can act as outermost substrates, are also excellent.

Self-Structuring of Lamellar Bridged Silsesquioxanes with Long Side Spacers

Diurea cross-linked bridged silsesquioxanes (BSs) C(10)C(n)C(10) derived from organosilane precursors, including decylene chains as side spacers and alkylene chains with variable length as central spacers (EtO)(3)Si-(CH(2))(10)-Y-(CH(2))(n)-Y-(CH(2))(10)-Si(OEt)(3) (n = 7, 9-12; Y = urea group and Et = ethyl), have been synthesized through the combination of self-directed assembly and an acid-catalyzed sol-gel route involving the addition of dimethylsulfoxide (DMSO) and a large excess of water. This new family of hybrids has enabled us to conclude that the length of the side spacers plays a unique role in the structuring of alkylene-based BSs, although their morphology remains unaffected. All the samples adopt a lamellar structure. While the alkylene chains are totally disordered in the case of the C(10)C(7)C(10) sample, a variable proportion of all-trans and gauche conformers exists in the materials with longer central spacers. The highest degree of structuring occurs for n = 9. The inclusion of decylene instead of propylene chains as side spacers leads to the formation of a stronger hydrogen-bonded urea-urea array as evidenced by two dimensional correlation Fourier transform infrared spectroscopic analysis. The emission spectra and emission quantum yields of the C(10)C(n)C(10) materials are similar to those reported for diurea cross-linked alkylene-based BSs incorporating propylene chains as side spacers and prepared under different experimental conditions. The emission of the C(10)C(n)C(10) hybrids is ascribed to the overlap of two distinct components that occur within the urea cross-linkages and within the siliceous nanodomains. Time-resolved photoluminescence spectroscopy has provided evidence that the average distance between the siliceous domains and the urea cross-links is similar in the C(10)C(n)C(10) BSs and in oxyethylene-based hybrid analogues incorporating propylene chains as side spacers (diureasils), an indication that the longer side chains in the former materials adopt gauche conformations. It has also allowed us to demonstrate for the first time that the emission features of the urea-related component of the emission of alkylene-based BSs depend critically on the length of the side spacers.

Enhanced photoluminescence features of Eu 3+-modified di-ureasil-zirconium oxocluster organic–inorganic hybrids

In this work, a series of transparent di-ureasil hybrids containing different amounts of methacrylic acid modified zirconium tetrapropoxide (ZrMcOH) nanoclusters (5–85 mol%) and incorporating EuCl 3 and [Eu(tta) 3(H 2O) 2] (tta = thenoyltrifluoroacetonate) complex were prepared. These hybrids are multi-wavelength emitters due to the convolution of the host intrinsic emission (electron–hole recombinations occurring in siliceous and urea cross-linkages) Eu 3+ intra-4f 6 transitions. The ZrMcOH incorporation deviates the maximum excitation wavelength of the hybrid host intrinsic emission from the UV (365 nm) to the blue (420 nm) and enhances the absolute emission quantum yield from 6.0 ± 0.6% to 9.0 ± 0.9%, and contributes to an increase in the 5D 0 lifetime values, quantum efficiency due to a decrease in the non-radiative transition probability and OH groups coordinated to the Eu 3+ ions.

Terbium(III)-containing organic–inorganic hybrids synthesized through hydrochloric acid catalysis

Organic–inorganic hybrids incorporating Tb(acac) 3·3H 2O (where acac is acetylacetonate) were synthesized via conventional hydrolysis sol–gel reaction in the presence and absence of an acid catalyst (hydrochloric acid, HCl). The host framework of these materials, named di-ureasils, is formed by polyether-based chains grafted to both ends to a siliceous backbone through urea cross-linkages (–NHC(C O)NH–). Four different concentrations of HCl (0.5, 1.0, 1.5, and 2.0 mol L −1) were used as catalyst for hydrolysis reactions. The gelation time of the resulting materials depends on the HCl concentration varying between 5 and 20 min. The hybrids were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), 29Si and 13C magic-angle spin nuclear magnetic resonance (NMR) and photoluminescence spectroscopy. The emission quantum yield for the undoped hybrids lies between 8.7 and 10.6%, much higher than those obtained for analogous samples prepared via conventional hydrolysis in the absence of catalyst. For the Tb 3+-containing hybrids the lanthanide local environment was affected due to different synthetic conditions used. An increase in the 5D 4 lifetime values, relatively to that of the isolated Tb(acac) 3·3H 2O complex, is in good agreement with a smaller 5D 4 non-radiative transition probability in the hybrids suggesting the replacement of the Tb 3+coordinated water molecules by the oxygen atom of the carbonyl group of the di-ureasil host.

Photoluminescence of bulks and thin films of Eu3+-doped organic/inorganic hybrids

Abstract Organic–inorganic hybrids formed by polyether-based chains grafted to both ends to a siliceous backbone through urea cross-linkages ( NHC( O)NH ), named di-ureasil, have been used as host for incorporation of Eu3+ in the form of EuCl3. The bulks and the thin films, both optically transparent, were characterized by excitation, absorption and emission spectroscopy. Photoluminescence results point out that the Eu3+ ions occupy, at least, two distinct local environments. Besides, the processing method (thin films or bulks) has influence on the energy levels of the hybrid host probably due to the lower degree of organization of the thin films structure.

Organic–inorganic hybrid matrix doped with alkenyldiazenido complexes of molybdenum

New molecular systems based on conjugated groups, trans-[FMo(NNCHCHCHCHCHCH 2CH 3)(Ph 2PCH 2CH 2PPh 2) 2][BPh 4] have been synthesized and characterized. This complex was immobilized in hybrid organic–inorganic matrix obtained by sol–gel. The hybrid matrix of these organically modified silicates, classified as ureasil formed by poly(oxyethylene) chains [POE, (OCH 2CH 2) n ] grafted to siloxane domains by means of urea cross-linkages. All of the new materials were characterized by thermal analysis (DSC), surface analysis (XPS) and spectroscopic methods (FTIR and UV/vis), to gather information on the modifications introduced by the molybdenum complex in hybrid organic–inorganic matrix, as well as on the immobilized complex integrity. The immobilization of the metal complex in the hybrid matrix has kept their integrity, although some distortions can be observed caused by steric effects imposed by the structure of hybrid matrix or/and by interactions between the metal complex and some groups of the hybrid matrix.

Organically Modified Silica-Based Xerogels Derived from 3-Aminopropyltrimethoxysilane and 3-Isocyanatepropyltriethoxysilane through Carboxylic Acid Solvolysis

Organically-modified silica xerogels from 3-aminopropyltrimethoxysilane (APTES) and 3-isocyanatepropyltriethoxysilane (ICPTES) have been synthesized through carboxylic acid (formic acid, acetic acid and valeric acid) solvolysis. The resulting hybrid materials have been characterized by powder X-ray diffraction, mid-infrared spectroscopy, 29Si and 13C nuclear magnetic resonance, and photoluminescence spectroscopy. The results show that urea cross-links have been formed in these hybrids. The luminescence features depend on the selected carboxylic acids. For example, comparatively to the hybrids derived from formic and acetic acid solvolysis, valeric acid shows a red-shift of the emission features.

Iron Oxide and Oxide-Hydroxide Nanoparticles in Organic-Inorganic Matrices

Nanometric ferrihydrite, maghemite and magnetite particles formed within an organicinorganic hybrid matrix were obtained by the sol-gel process. In contrast to precipitation techniques, sol-gel process appears as suitable way to achieve size-controlled nanoscopic magnetic particles anchored in a hybrid structure. The hybrid matrix here reported, named di-ureasil, is composed of poly(oxyethylene) chains grafted to siloxane groups by means of urea cross-linkages. The formation of ferrihydrite particles was achieved incorporating iron nitrate during the sol-gel process, at low pH. The formation of maghemite takes place after the incorporation of a mixture of Fe3+ and Fe2+ ions and treatment with an ammonia solution, after the sol-gel process. Magnetite nanoparticles are formed after the incorporation of Fe2+ ions and treatment with ammonia at 80°C. The AC magnetic susceptibility shows thermal irreversibility with a blocking temperature TB≈13K and ≈25K depending on frequency for the ferrihydrite and maghemite particles, respectively. The magnetite nanoparticles are blocked at room temperature. Above the irreversibility the magnetization of ferrihydrite and maghemite follows a Langevin function modified with a linear term, as found in antiferromagnetic and ferrimagnetic particles.

FT-IR and Raman spectroscopic study of di-urea cross-linked poly(oxyethylene)/siloxane ormolytes doped with Zn 2+ ions

The interactions occurring in di-urea ( NHC( O)NH ) cross-linked poly(oxyethylene) (POE)/siloxane hybrids (di-ureasils) doped with zinc triflate (Zn(CF 3SO 3) 2) were investigated by Fourier Transform infrared (FT-IR) and Raman (FT-Raman) spectroscopies. Bonding of the Zn 2+ ions to the urea carbonyl oxygen atoms occurs in the entire range of compositions studied (∞ > n ≥ 1, where n, salt content, is the molar ratio of oxyethylene moieties per Zn 2+ ion). At n > 20 the incorporation of the guest cations progressively reduces the number of free C O groups. At n = 20 the saturation of the urea cross-links is attained and the Zn 2+ ions start to coordinate to the POE chains giving rise to the formation of a crystalline POE/Zn(CF 3SO 3) 2 complex. The latter process occurs at the expense of the destruction of the hydrogen-bonded POE/urea structures of the host di-ureasil structure. New hydrogen-bonded associations, more ordered than the urea–urea aggregates present in the non-doped matrix and including Zn 2+⋯O C coordination, emerge in parallel. “Free” and weakly coordinated CF 3SO 3 − ions, present in all the xerogels studied, appear to be the main charge carriers of the conductivity maximum of this family of ormolytes located at n = 60 at 30 °C. In materials with n ≤ 20 contact ion pairs, “cross-link separated” ions pairs and higher ionic aggregates appear. The data reported demonstrate that the behaviour of the di-ureasils doped with triflate salts depends on the type of cation.

Di-ureasil ormolytes doped with Mg 2+ ions: Part 2. Cationic and anionic environments

Fourier Transform mid-infrared and Raman (FT-IR and FT-Raman, respectively) spectroscopies were used to investigate the cation/polymer, cation/cross-link, cation/anion and hydrogen bonding interactions in sol–gel derived ormolytes doped with magnesium triflate (Mg(CF 3SO 3) 2). The framework of these xerogels (di-ureasils) contains poly(oxyethylene) (POE) segments with about 40.5 repeat units bonded at both ends to a siliceous backbone through urea cross-links. Materials with compositions ∞ > n ≥ 1 (where n is the ratio of oxyethylene moieties per Mg 2+ ion) were studied. FT-IR data revealed that, while the urea carbonyl oxygen atoms bond to the cations in all the compounds analyzed, the POE chains complex the Mg 2+ ions only at high salt content. Although the beginning of the POE/Mg 2+ interaction was detected spectroscopically at n = 10, thermal data obtained in Part 1 of this series of papers suggest that this threshold composition is probably located at lower salt content. Bonding of the Mg 2+ ions to the ether oxygen atoms of the POE chains is accompanied by the destruction of the urea/POE hydrogen-bonded aggregates formed in the d-U(2000) medium. Proofs of the existence of a crystalline POE/Mg(CF 3SO 3) 2 complex in samples with n ≤ 10 were found. “Free” and weakly coordinated CF 3SO 3 − ions, present in all the samples examined, appear to be the main charge carriers of the conductivity maximum of this family of ormolytes (i.e., d-U(2000) 20Mg(CF 3SO 3) 2). At n ≤ 40, along with those anionic species, another anionic configuration, attributed to contact ion pairs, emerges. Higher ionic aggregates are formed in di-ureasils with n ≤ 5.

Structure–photoluminescence relationship in Eu(iii) β-diketonate-based organic–inorganic hybrids. Influence of the synthesis method: carboxylic acid solvolysis versus conventional hydrolysis

Organic–inorganic hybrids incorporating Eu(nta)3·bpy (where nta and bpy stand for 1-(2-naphthyl)-4,4,4-trifluoro-1,3-butanedionate and 2,2′-bipyridine, respectively) were prepared either by acetic acid solvolysis or a conventional hydrolysis sol–gel route. The host framework of these materials, classed as di-ureasil, consists of a siliceous network grafted, through urea cross-linkages, to both ends of poly(ethylene oxide) chains. The resulting Eu(III)-based di-ureasils were investigated by small angle X-ray scattering, X-ray diffraction, Fourier transform mid-infrared spectroscopy, 29Si and 13C nuclear magnetic resonance, and photoluminescence spectroscopy, with particular attention paid to the effect of the adopted synthesis strategy on the relationship between structure and emission properties. The dimensions and the degree of condensation of the siloxane nanodomains depend noticeably on the synthesis route and the overall emission quantum yield decreases from 15 (conventional hydrolysis) to 6% (solvolysis route). The broad white-light emission typical of the di-ureasil host was not detected here suggesting, therefore, the activation of energy transfer channels between the hybrid host's emitting centres and the Eu(III) ions. As the first coordination shell of Eu(III) is essentially independent of the synthesis method employed, the significant decrease in the emission quantum yield for the di-ureasil prepared by acetic acid solvolysis might be explained by the interaction between the hybrid emitting centres and the nta ligand levels, favouring a larger non-radiative transition probability.

Effect of Concentration of the Diazoalcene Molybdenum Complex Immobilized in Ureasil Matrix

The complex trans-[FMo(NNCHCHCHCH2CH3)(Ph2PCH2CH2PPh2)2][BPh4], was dispersed in a hybrid matrix synthesized by a sol-gel process. The host matrix of the so-called ureasil is a network of silica to which oligopolyoxyethylene chains [POE, (OCH2CH2) n ] are grafted by means of urea cross-links. The free complex and sol-gel materials were characterized by thermal analysis (DSC), surface analysis (XPS) and spectroscopic methods (FT-IR and UV/Vis). The data gathered indicates that the molybdenum(IV) complex is immobilized in the host matrix, and it exhibits structural properties different from those of the free form. These differences could arise either from distortions caused by steric effects imposed by the structure of hybrid matrix or by interactions with the matrix. These materials shows potential applications in heterogeneous catalysis in mild conditions.

Matrix assisted formation of ferrihydrite nanoparticles in a siloxane/poly(oxyethylene) nanohybrid

Matrix-assisted formation of ferrihydrite, an iron oxide hydroxide analogue of the protein ferritin core, in a sol–gel derived organic–inorganic hybrid is reported. The hybrid network (named di-ureasil) is composed of poly(oxyethylene) chains of different average polymer molecular weights grafted to siloxane domains by means of urea cross-linkages and accommodates ferrihydrite nanoparticles. Magnetic measurements, Fourier transform infrared and nuclear magnetic resonance spectroscopy reveal that controlled modification of the polymer molecular weight allows the fine-tuning of the ability of the hybrid matrix to assist and promote iron coordination at the organic–inorganic interface and subsequent nucleation and growth of the ferrihydrite nanoparticles whose core size (2–4 nm) is tuned by the amount of iron incorporated. The polymer chain length, its arrangement and crystallinity, are key factors in the anchoring and formation of the ferrihydrite particles.

Structure of magnetic poly(oxyethylene)–siloxane nanohybrids doped with FeIIand FeIII

Hybrid organic–inorganic nanocomposites doped with FeIIand FeIIIions and exhibiting interesting magnetic properties have been obtained by the sol–gel process. The hybrid matrix of these ormosils (organically modified silicates), classed as di-ureasils and termed U(2000), is composed of poly(oxyethylene) chains of variable length grafted to siloxane groups by means of urea crosslinkages. Iron perchlorate and iron nitrate were incorporated in the di-ureasil matrices, leading to compositions within the range 80 ≥n≥ 10,nbeing the molar ratio of ether-type O atoms per cation. The structure of the doped di-ureasils was investigated by small-angle X-ray scattering (SAXS). For FeII-doped samples, SAXS results suggest the existence of a two-level hierarchical structure. The primary level is composed of spatially correlated siloxane clusters embedded in the polymeric matrix and the secondary, coarser level consists of domains where the siloxane clusters are segregated. The structure of FeIII-doped hybrids is different, revealing the existence of iron oxide based nanoclusters, identified as ferrihydrite by wide-angle X-ray diffraction, dispersed in the hybrid matrix. The magnetic susceptibility of these materials was determined by zero-field-cooling and field-cooling procedures as functions of both temperature and field. The different magnetic features between FeII- and FeIII-doped samples are consistent with the structural differences revealed by SAXS. While FeII-doped composites exhibit a paramagnetic Curie-type behaviour, hybrids containing FeIIIions show thermal and field irreversibilities.

Magnetic properties of Fe-doped organic–inorganic nanohybrids

We present a magnetic study of Fe-doped diureasils (siloxane-based networks to which poly(ethylene oxide)-based chains are grafted by urea cross linkages doped with Fe(II) or Fe(III) ions. Structural studies show that the Fe(II) ions interact mainly with the organic chain, whereas the incorporation of Fe(III) leads to the formation of iron-based nanoclusters, with radius increasing from 20 to 40 Å. Fe(II)-doped samples behave as simple paramagnets, with μeff=5.32 μB. Fe(III)-doped hybrids present antiferromagnetic interactions, with TN increasing with Fe(III) concentration up to 13.6 K for 6% doping. Thermal irreversibility was observed below ∼40 K and is stronger for higher concentrations. The coercive fields (HC) are of the order of 1000 Oe at 5 K. Hysteresis cycles are shifted to negative fields, revealing the presence of exchange anisotropy interactions with exchange fields (HE) of the order of 100 Oe. Both fields decrease rapidly with increasing temperature. We analyze this behavior in terms of the contribution of surface spin disorder to exchange anisotropy.

Tungsten hydride complex as a template in organic–inorganic hybrid materials

A tungsten hydride complex, [WH 2( η 2-OOCCH 3)(Ph 2PCH 2CH 2PPh 2) 2][BPh 4], was dispersed in a hybrid matrix synthesized by a sol–gel process. The host matrix of the so-called ureasil is a network of silica to which oligopolyoxyethylene chains [POE, (OCH 2CH 2) n ] are grafted by means of urea cross-links. The free complex and sol–gel materials were characterized by thermal analysis (DSC) and spectroscopic methods (FT-IR and UV/Vis). The data gathered indicate that the tungsten(IV) complex is immobilized in the host matrix, and it exhibits structural properties different from those of the free form. These differences could arise either from distortions caused by steric effects imposed by the structure of hybrid matrix or by interactions with the matrix.

Enhanced emission from Eu(III) β-diketone complex combined with ether-type oxygen atoms of di-ureasil organic–inorganic hybrids

Organic–inorganic hybrids, named di-ureasils and described by polyether-based chains grafted to both ends to a siliceous backbone through urea cross linkages, were used as hosts for incorporation of the well-known coordination complex of trivalent europium (Eu 3+) ions described by the formula [Eu(TTA) 3(H 2O) 2] (where TTA stands for thenoyltrifluoroacetone). By comparing with Eu 3+-doped di-ureasil without complex form the new materials prepared here enhanced the quantum efficiency for photoemission of Eu 3+ ions. The enhancement can be explained by the coordination ability of the organic counterpart of the host structure which is strong enough to displace water molecules in [Eu(TTA) 3(H 2O) 2] from the rare earth neighbourhood after the incorporation process. High intensity of Eu 3+ emission was observed with a low non-radiative decay rate under ultraviolet excitation. The quantum efficiency calculated from the decay of 5D 0 emission was 74%, which in the same range of values previously obtained for the most efficient Eu 3+ coordination compounds reported in literature. Luminescence, X-ray absorption and infrared absorption results considered together leads to a picture where the first coordination shell of Eu 3+ is composed of the 6 oxygen atoms of the 3 β-diketonate ligands and 2 ether-like oxygen atoms of the host.

Room temperature visible/infrared emission and energy transfer in Nd3+-based organic/inorganic hybrids

The photoluminescence features and the energy transfer processes of Nd3+-based siloxane-poly(oxyethylene) hybrids are reported. The host matrix of these materials, classed as di-ureasils, is formed by a siloxane backbone covalently bonded to polyether chains of two molecular weights by means of urea cross-links. The room-temperature photoluminescence spectra of these xerogels show a wide broad purple-blue-green band (350–570 nm), associated with the emitting centres of the di-ureasil host, and the typical near infrared emission of Nd3+ (700–1400 nm), assigned to the 4F3/2 → 4I9/2,11/2,13/2 transitions. Self-absorptions in the visible range, resonant with intra-4f3 transitions, indicate the existence of an energy conversion mechanism of visible di-ureasil emission into near infrared Nd3+ luminescence. The existence of energy transfer between the di-ureasil's emitting centres and the Nd3+ ions is demonstrated calculating the lifetimes of these emitting centres. The efficiency of that energy transfer changes both with the polymer molecular weight and the Nd3+ concentration.

Magnetic Sol-Gel Derived Poly(oxyethylene)- Siloxane Nanohybrids

AbstractThe magnetic and structural properties of sol-gel derived organic/inorganic nanocomposites doped with Fe(II), Fe(III), Nd(III) and Eu (III) ions are discussed. These hybrids consist of poly(oxyethylene)-based chains grafted onto siloxane nanodomains by urea cross-linkages. Small angle X-ray scattering data show the presence of spatial correlations of siloxane domains embedded in the polymer matrix. The magnetic properties of rare-earth doped samples are determined by single ion crystal-field-splitted levels (Eu3+ J=0; Nd3+ J=9/2) and the small thermal irreversibility is mainly associated to structural effects. Fe2+-doped samples behave as simple paramagnet with residual antiferromagnetic interactions. Fe3+-doped hybrids are much more complex, with magnetic hysterisis, exchange anisotropy and thermal irreversibility at low temperatures. Néel temperatures increase up to 14K for the highest (∼5.5%) Fe3+ mass concentration.

Structural modelling of Eu3+-based siloxane–poly(oxyethylene) nanohybrids

The modelling of the local structure of sol–gel derived Eu3+-based organic/inorganic hybrids is reported, based on Small-Angle X-ray Scattering (SAXS), photoluminescence and mid-infrared spectroscopy. The hybrid matrix of these organically modified silicates, classed as di-ureasils and termed U(2000) and U(600), is formed by poly(oxyethylene) (POE) chains of variable length grafted to siloxane domains by means of urea cross-linkages. Europium triflate, Eu(CF3SO3)3, was incorporated in the two di-ureasil matrices with compositions 400 ≥ n ≥ 10, n is the molar ratio of ether oxygens per Eu3+. The SAXS data for undoped hybrids (n = ∞) show the presence of a well-defined peak attributed to the existence of a liquid-like spatial correlation of siloxane rich domains embedded in the polymer matrix and located at the ends of the organic segments. The obtained siloxane particle gyration radius Rg1 is around 5 Å (error within 10%), whereas the interparticle distance d is 25 ± 2 Å and 40 ± 2 Å, for U(600) and U(2000), respectively. For the Eu3+-based nanocomposites the formation of a two-level hierarchical local structure is discerned. The primary level is constituted by strongly spatially correlated siloxane particles of gyration radius Rg1 (4–6 and 3–8 Å, errors within 5%, for U(600)nEu(CF3SO3)3, 200 ≥ n ≥ 40, and U(2000)nEu(CF3SO3)3, 400 ≥ n ≥ 40, respectively) forming large clusters of gyration radius Rg2 (≈75 ± 10 Å). The local coordination of Eu3+ in both di-ureasil series is described combining the SAXS, photoluminescence and mid-infrared results. In the di-ureasils containing long polymer chains, U(2000)nEu(CF3SO3)3, the cations interact exclusively with the carbonyl oxygens atoms of the urea bridges at the siloxane–POE interface. In the hybrids containing shorter chains, U(600)nEu(CF3SO3)3 with n ranging from 200 to 60, the Eu3+ ions interact solely with the ether-type oxygens of the polymer chains. Nevertheless, in this latter family of hybrids a distinct Eu3+ local site environment involving the urea cross-linkages is detected when the europium content is increased up to n = 40.