Urethane cross-linked poly(oxyethylene)/siliceous nanohybrids doped with Eu3+ ionsPart 1. Coordinating ability of the host matrix

Fourier transform mid-IR and Raman spectroscopies were extensively used to examine cation/polymer and cation/cross-link interactions, as well as hydrogen bonding, in Eu3+-doped sol–gel derived organic/inorganic materials (monourethanesils).

The hybrid framework of these xerogels contains short methyl end capped polyether segments covalently bonded to a siliceous backbone through urethane groups.

The cations were incorporated as europium triflate, Eu(CF3SO3)3.

Samples with compositions ∞ > n ≥ 10 (where n indicates the ratio of (OCH2CH2) moieties per lanthanide ion) were investigated.

The spectral data obtained provided unequivocal evidence that the cations coordinate exclusively to the urethane carbonyl oxygen atoms in compounds with 400 ≥ n ≥ 50.

The participation of the ether oxygen atoms of the polymer chains in the Eu3+ complexation process is initiated at n = 30.

The presence of a crystalline phase was detected at n = 10.

The variation of the glass transition temperature of the monourethanesils studied with salt concentration is in perfect agreement with these claims.

The beginning of the Eu3+–polyether interaction is accompanied by a breakdown of the hydrogen bonded aggregates of the matrix, as confirmed by photoluminescence spectroscopy.


The development of advanced nanoscale organic/inorganic frameworks with potential application in many fields has attracted considerable attention in the last few years.1

The combination of the hybrid strategy, that has inherited the advantages of the sol–gel chemistry, with the host–guest concept of polymer electrolytes2–4 has aroused special interest in the domain of optics.

In this context, a family of intensely luminescent nanohybrid xerogels with outstanding features was introduced.5–7

The hybrid host employed contains poly(oxyethylene), POE, chains of variable length grafted at both ends to a siliceous backbone through urea linkages (–NHC(O)NH–).

This framework (diureasil)8 was represented by U(Y) (Y = 2000, 900 and 600), where U indicates the urea group and Y denotes the average molecular weight of the organic precursor used.

The active constituents are in general Eu3+ ions, added as a triflate salt, Eu(CF3SO3)3, whose concentration may be taken to higher levels than those normally used in conventional composite materials.

The Eu3+-doped long chain diureasils, processed as highly transparent monoliths,6a,6b,9,10 are acceptable ion conductors6a,b,10b and behave like white-light phosphors.6a,b,9,11 Their most remarkable feature is the possibility of tuning their color along the Commission Internationale d'Éclairage chromaticity diagram by simply varying either the salt content or the excitation wavelength.9c

The multifunctionality of the diureasils containing Eu3+ ions derives from the coordinating ability of the hybrid matrix itself.6d,e,9,10 The activation of the coordinating sites of this POE/siloxane structure (the oxygen atoms of the urea carbonyl groups and the oxygen atoms of the polyether chains) can be easily controlled by changing either the salt content at constant chain length or the length of the organic segments at constant salt concentration.6d,e,10

In the present work urethane cross-linked xerogels doped with Eu(CF3SO3)3–a family of close analogs of the diureasils–will be thoroughly investigated with the primary goal of elucidating the structure/properties relationship.

The organic/inorganic host of these compounds (monourethanesils)12,13 is composed of short methyl end capped POE chains covalently bonded to a siliceous network through urethane moieties (–NH(CO)O–).

This matrix was designated as m-Ut(Y′) (Y′ = 2000, 750 and 350, corresponding to about 45, 17 and 7 (OCH2CH2) repeat units, respectively),12 where m denotes mono, Ut represents the urethane group and Y′ indicates the average molecular weight of the starting organic material.

The creation of monohybrids, through the reduction of the number of cross-links/polymer segment from two to one, allows to simplify the complicated structure of the diureasils.

Preliminary studies of the luminescence features of the Eu3+-doped monourethanesils suggested that the local chemical environment of the cation is similar to that found in the corresponding diureasil materials.

Thus, despite having poorer mechanical properties, the monourethanesils may be viewed as good model compounds of the diureasils in terms of cationic coordination.

Therefore, an in-depth characterization of the monourethanesil system will help to enlighten the complex behavior of the diureasils and clarify many questions that remain unanswered regarding the cationic and anionic environments in the salt-doped diurea cross-linked hybrids.

The systematic analysis that we have decided to perform on a series of m-Ut(750)-based monourethanesil samples containing a wide concentration range of Eu(CF3SO3)3 will be divided into two parts.

Part 1 will be devoted to the study of cation/polymer and cation/cross-link interactions.

Fourier Transform mid-IR (FT-IR) and Raman (FT-Raman) spectroscopies will be employed extensively to check if the methyl-terminated POE chains of m-Ut(750) participate in the complexation of the Eu3+ ions, since rich information regarding the cationic environment is provided by cation-induced changes in vibrational motions of conformation sensitive polymer bonds14–23 and in metal ion–oxygen stretching motions.18–23

The “amide I” and “amide II” regions24 will be also inspected to estimate the degree of hydrogen bonding in the monourethanesil materials25,26 and to determine the role of the urethane carbonyl oxygen atoms in the Eu3+ coordination mechanism.27

This work will be complemented with data retrieved from photoluminescence spectroscopy and differencial scanning calorimetry.

The scope of Part 2 will be to examine ionic association in the monourethanesil hybrids in order to gain additional insight into the local chemical environment of the lanthanide ions.

Cation–anion interactions will be evaluated through the analysis of characteristic modes of the triflate ion in the FT-IR and FT-Raman spectra of the same set of samples considered in Part 1.

Results and discussion

Vibrational spectroscopy

Cation/polymer interactions

Several vibrational modes of POE are extremely sensitive to the interaction of the polymer backbone with cations and can be thus employed as diagnostic tools to monitor the changes undergone by the polyether chains upon the addition of the guest salt.

Classically, two spectral regions are widely used in the field of polymer electrolytes to probe the coordination of the cations to the host polyether:4,14–23 (1) Skeleton CO stretching, ν(CO), region.

In this frequency interval (1200–1000 cm–1) the solvation of the cations by the heteroatoms of the polyether chains produces a distinct shift of the intense ν(CO) band to lower wavenumbers.

The magnitude of this shift depends on the strength of the interaction.

(2) CH2 rocking, ρr(CH2), region.

Between 1000 and 800 cm–1 the bands ascribed to a mixture of CH2 rocking modes and CC stretching vibrations (from 1000 to 900 cm–1)28 and the bands originated from CH2 rocking modes coupled with CO stretching vibrations (from 900 to 800 cm–1)28 are found.

Their frequency is strongly dependent on the –O–C–C–O– angle and hence on the polymer local conformation.

The room-temperature mid-IR spectra of the m-Ut(750)nEu(CF3SO3)3 family of materials analyzed in the present work are collected in Fig. 1.

The assignment of the main bands produced by the polymer chains of the same hybrid samples is given in Table 1.

Fig. 2 shows that in the ν(CO) region the IR spectrum of the non-doped monourethanesil matrix displays a peak around 1113 cm–1 and a shoulder at about 1143 cm–1 ascribed, respectively, to the CO stretching vibration mode and to the coupled vibration of the CO stretching and CH2 rocking modes (Table 1).

In the spectra of the m-Ut(750)nEu(CF3SO3)3 xerogels with 400 ≥ n ≥ 30, both the 1113 cm–1 feature, characteristic of non-coordinated oxyethylene moieties, and the 1143 cm–1 shoulder persist practically unchanged as increasing amounts of Eu(CF3SO3)3 are incorporated into the m-Ut(750) host (Fig. 2).

In the low-frequency side of the ν(CO) envelope, the only change worth noting in the spectra of the same Eu3+-doped compounds is the growth of a shoulder near 1068 cm–1 at n = 30 (Fig. 2, Table 1).

The further addition of Eu(CF3SO3)3 (i.e., n = 10) has, however, tremendous implications.

Fig. 2 and Table 1 clearly illustrate that the m-Ut(750)10Eu(CF3SO3)3 material gives rise to a series of extraordinary changes: (1) the very strong band centered at approximately 1113 cm–1 disappears; (2) the shoulder seen at about 1068 cm–1 in the spectrum of m-Ut(750)30Eu(CF3SO3)3 is transformed into a strong event at 1072 cm–1; (3) two new intense features are distinctly observed at 1115 and 1105 cm–1; (4) two new strong components appear at 1097 and 1057 cm–1, together with two shoulders located at 1123 and 1063 cm–1.

Bernson et al14. reported that in the case of the electrolytes formed with La(CF3SO3)3 and POE all the (OCH2CH2) moieties are virtually bonded to the cations at n = .914

The characteristic band attributed to La3+-coordinated ether oxygen atoms was a single event at 1073 cm–1.14

It is noteworthy that in the case of a POE-based system doped with lithium triflate, LiCF3SO3, a crystalline complex was formed for composition n = 3,17ai.e., a composition that in terms of anionic concentration corresponds to n = 9 for a trivalent cation.

Moreover, the frequency of a characteristic spectral feature of the Li+-based compound appeared at 1092 cm–1.17a In addition, in the spectra of the NH4CF3SO3-based polymer electrolytes with n < 817b and in that of the POE1KCF3SO3 crystalline compound,17c the broad ν(CO) band at 1102 cm–1 was replaced by two peaks at 1116 and 1107 cm–1.

Considering the remarkable coincidence between the spectral features just indicated and those of m-Ut(750)10Eu(CF3SO3)3, it seems logical to associate the new bands appearing in the ν(CO) region of the latter material to a salt-rich crystalline complex.

Further evidences of the formation of a crystalline phase in the m-Ut(750)10Eu(CF3SO3)3 compound may be found in the ρr(CH2) region.

Fig. 3 shows that the main feature of the IR spectra of the monourethanesil samples with ∞ > n ≥ 30 within the 980–900 cm–1 region is a peak at approximately 955 cm–1 and two shoulders at lower frequency (Table 1).

In this range of compositions the incorporation of increasing amounts of lanthanide salt to the hybrid matrix does not modify the intensity and position of these three spectral events.

In contrast, in the spectrum of m-Ut(750)10Eu(CF3SO3)3 the band envelope becomes considerably stronger and a new, intense sharp feature develops at 937 cm–1 (Fig. 3, Table 1), characteristic of crystalline high molecular weight poly(ethylene glycol)dimethyl ether, PEGDME.28c

Fig. 3 also reveals that the dominant feature of the compounds with ∞ > n ≥ 30 in the 900–800 cm–1 interval is a band centered at approximately 847 cm–1.

Several shoulders are also observed (Table 1).

Within this range of salt concentration the intensity and position of the bands remain essentially unaffected.

Again, the contour of the envelope is drastically modified in the spectrum of the xerogel with n = 10, the most remarkable change being the presence of a new sharp, medium intensity band located at 843 cm–1 (Fig. 3, Table 1).

This event was also assigned to crystalline PEGDME.28c Interestingly, the crystalline complexes POE1NH4CF3SO3 and POE1KCF3SO3 exhibit bands around 933 and 841 cm–117b and 936 and 843 cm–1,17c respectively.

Although less sensitive to alterations in the backbone conformation, the CH2 scissoring and CH3 deformation modes, which appear in the 1500–1400 cm–1 frequency interval, and the CH2 wagging modes, situated in the 1400–1300 cm–1 spectral range, can be also used to complement the conclusions drawn from the two spectral regions discussed above.

A close examination of Fig. 4 and Table 1 allow us to state that, between 1500 and 1300 cm–1, the spectra of the monourethanesil compounds with n > 10 closely resemble the spectrum of m-Ut(750).

It may be also immediately inferred that, in the same region, the spectral signature of the host hybrid is deeply modified at n = 10.

The most significant change detected between 1500 and 1400 cm–1 in the spectrum of the latter sample is the presence of a new band of medium intensity at 1467 cm–1 and several new shoulders (Fig. 4, Table 1).

In the CH2 wagging region of m-Ut(750)10Eu(CF3SO3)3 a significant enhancement of the shoulder located at 1360 cm–1 occurs (Fig. 4, Table 1).

The features at 1467 and 1360 cm–1 are also produced by crystalline PEGDME,28c thus corroborating the fact that in the most concentrated sample the disordered polyether chains of m-Ut(750) adopt regularly ordered conformations to bond to the Eu3+ ions.

The intense band situated at approximately 872 cm–1 in the Raman spectrum of the most concentrated xerogel (Fig. 5) represents another spectroscopic proof of the interaction between the Eu3+ ions and the ether oxygen atoms of the oligopolymer chains at this salt concentration.

The 872 cm–1 event is visible already in the Raman spectrum of m-Ut(750)30Eu(CF3SO3)3 as a very weak shoulder (Fig. 5).

The absence of this feature in the Raman spectra of the xerogels with lower salt content (Fig. 5) suggests that the participation of the oxyethylene moieties in the cation complexation process is initiated near the composition at which the saturation of the urethane cross-links is attained, i.e., at n = 17.

In analogy with previous authors,18–23 we will associate the 872 cm–1 band to a stretching vibration mode involving wrapping of the pendant oligopolyether segments of m-Ut(750) around the Eu3+ ion.

This mode, represented by ν(M–On) and usually termed as “oxygen breathing” mode, is typically seen in the Raman spectra of complexes of crown ethers and metal cations.29

On the basis of the model proposed by Papke et al.,18 it is accepted that the formation of the cation/ether oxygen complex favors a gauche conformation of the C–C bonds.

In this context it is fundamental to point out that a band indicative of the trans conformations of the –O–C–C–O– sequences30,31 and centered at about 810–805 cm–1 is distinctly discerned in the spectra of the doped xerogels with 400 ≥ n ≥ 30 (Fig. 5).

The drastic reduction of the intensity of this feature in the Raman spectrum of the m-Ut(750)10Eu(CF3SO3)3 material (Fig. 5) is consistent with the marked decrease in the relative amount of trans conformations that occurs at the expense of the increase of the proportion of gauche ones.

It should be stressed that these spectral results do not provide any information regarding the number of pendant oligopolyether chains of the monourethanesil structure that are used to coordinate each cation.

Powder diffraction studies demonstrated that in the crystalline complexes POE3LiX (where X = CF3SO332a,b and N(SO2CF3)2, bis(trifluoromethanesulfonate)imide32c), POE3NaClO432b and POE4MSCN (where M = K, Rb and NH4)32b each polymer chain forms an isolated one-dimensional coordination complex.

In all these structures the monovalent cations are located within the POE helix.

No ionic cross-linking was observed in any case.32a,b The structures were preserved on going from the crystalline to the amorphous state.32a,b There are no diffraction data available in the literature regarding POE-based complexes doped with Eu(CF3SO3)3.

However, considering that the ionic radius of Eu3+ lies between those of Li+ and Na+, it seems logical to assume henceforth that “POE–Eu3+ coordination” corresponds to the interaction of the lanthanide ion with several ether oxygen atoms of a single polymer chain.

A third pronounced effect discerned in the Raman spectra reproduced in Fig. 5 helps to substantiate the formation of cation/polyether bonds in the most concentrated monourethanesil.

It involves the growth of a series of bands between 925 and 885 cm–1 in the two most concentrated samples as the content of guest lanthanide salt is increased (Fig. 5).

The curve-fitting performed on this region (not shown) allowed us to conclude that these new events are situated at approximately 918, 911 (shoulder), 907, 902, 896 and 893 cm–1.

Furlani et al20. also found a band centered at about 906 cm–1 in the Raman spectra of PEG(400)-based electrolytes doped with Eu(N(SO2CF3)2)3.

We therefore conclude that the significant transformation of the spectral profiles in m-Ut(750)10Eu(CF3SO3)3 are caused by an increase in the local order of the polymer segments arising from their interaction with the Eu3+ ions and the formation of crystalline complexes.

It must be stressed that we cannot determine the extent of crystallization occurring in the system at n = 10.

However the changes observed in the above discussed spectra unambiguously prove the presence of crystalline domains.

Therefore we tentatively describe the m-Ut(750)10Eu(CF3SO3)3 xerogel as a material containing a significant fraction of crystalline phase(s).

Cation/cross-link interactions

We provided above strong arguments that support the claim that the cation coordination process that occurs in the m-Ut(750)-based xerogels containing Eu(CF3SO3)3 closely resembles that reported for the long chain diureasil parent analogs U(2000)nEu(CF3SO3)3.10,11a Similarly to what happens in the latter compounds, in the monourethanesils examined in the present study the pendant methyl end capped polyether segments start to solvate the guest lanthanide ions near the cross-link saturation composition.

Another noteworthy similarity found between both hybrid systems is the formation of a crystalline compound at high salt content.

It is important to note that the number of hydrogen donor sites present in the urethane and urea groups is not the same: the urethane linkage contains just one N–H group, whereas the urea moiety is composed of two N–H groups.

This implies that, while the CO moiety of a urethane group may form a single hydrogen bond with the N–H moiety of another urethane group, urea groups of neighboring molecules may be linked by means of planar bifurcated hydrogen bonds.33

As a consequence, the hydrogen bonding geometry of the urethane groups might be substantially different from that of the urea groups.

Such situation resulted for instance in the tighter packing of adjacent chains observed in bis-urea compounds with respect to the bis-urethane analogues.33a The distance between the carbonyl carbon atoms of two adjacent urea molecules and two adjacent urethane molecules in these materials was found to be 4.63 and 5.10 Å, respectively.33a The hydrogen bonding behavior in the essentially amorphous diureasils and monourethanesils should be, however, quite different from that occurring in the compounds reported in ref. 33a, which are highly crystalline.

Indeed, the IR spectra demonstrate the presence of a large number of “free” CO groups in the matrices based on U(2000) and on the investigated m-Ut(750), as will be shown below.

Hence, we do not expect a significant difference in hydrogen bond strengths in both hybrid systems.

To complete the description of the chemical environment around the lanthanide ions in the m-Ut(750)nEu(CF3SO3)3 hybrids, we will investigate in this section the spectral signature of these composites in the “amide I” and “amide II” regions, as it seems logical to presume that the Eu3+ ions will very likely bond to the carbonyl oxygen atom of the urethane cross-links in the monourethanesil xerogels with 400 ≥ n ≥ 50.

This study will enable us to assess in parallel the extent of hydrogen bonding in this family of xerogels.

Our analysis of the “amide I” and “amide II” bands of the Eu3+-doped monourethanesil compounds will take into account numerous spectroscopic studies of simple and segmented poly(urethane)s,25 of poly(urethane urea) copolymers26 and of lithium-doped polymer electrolytes based on a thermoplastic polyurethane27ac or a segmented polyether poly(urethane urea)27d host.

“Amide I” region

The “amide I” region of poly(urethane)s25 corresponds to the amide I region of polyamides.34

The amide I mode (or simply, the carbonyl stretching mode) is a highly complex vibration that is comprised of a major contribution from the CO stretching vibration and less important contributions from the C–N stretching and C–C–N deformation vibrations.34

As the CO stretching vibration is sensitive to the specificity and magnitude of hydrogen bonding, the amide I envelope consists of several distinct components reflecting different environments of CO groups, usually termed as associations, aggregates or structures.

Hence quantitative analysis is feasible in the amide I region only if the difference in the absorption coefficients of the non-bonded and bonded carbonyl bands is taken into account.

Band area determinations may be thus considered an adequate reflection of functional group concentration.25d,34

The “amide I” envelope of the salt-free m-Ut(750) matrix (Fig. 7) contains three distinct components centered at about 1750, 1720 and 1700 cm–1 (Fig. 8).

The event observed near 1750 cm–1 is inherent to urethane cross-links free from hydrogen bonding interactions, i.e., urethane linkages whose N–H and CO groups remain non-bonded (Scheme 1, A).25

On the basis of previous works,25 we propose that the 1720 cm–1 feature is related to the absorption of hydrogen-bonded CO groups in disordered aggregates (urethane–polyether association of Scheme 1, B) and that the component at 1700 cm–1 corresponds to the absorption of CO groups included in a much more ordered hydrogen-bonded aggregate (urethane–urethane association illustrated in Scheme 1, C).

Fig. 7 provides unequivocal evidence that the addition of the lanthanide salt to the m-Ut(750) hybrid matrix produces a series of pronounced changes in the “amide I” region.

In general terms, we may state that, as the amount of guest salt rises, the center of gravity of the whole “amide I” profile shifts toward lower frequencies, suggesting that the strength of the hydrogen bonds globally increases.

In the spectra of the monourethanesils with high salt concentration a new component located at 1665 cm–1 emerges.

With the increase of Eu(CF3SO3)3 content, while the intensity of this feature is markedly enhanced, that of the 1750 cm–1 mode, originated from free CO groups, is significantly reduced.

In the spectrum of the rich-salt material with n = 10 the 1665 cm–1 band becomes the most prominent event and the 1750 cm–1 peak disappears.

The spectral findings just described sustain the hypothesis that the Eu3+ ions coordinate to the carbonyl oxygen atoms of the urethane cross-links over the whole range of composition studied.

Another conclusion that may be immediately drawn from the spectra reproduced in Fig. 7 is that the band position of the three components of m-Ut(750) situated at about 1750, 1720 and 1700 cm–1 is totally unaffected by salt doping.

Van Heumen et al.27b also observed this effect in lithium triflate-based thermoplastic polyurethanes.

Because of the lability of hydrogen bonds, it was fundamental to study the evolution of the spectra of the monourethanesils as a function of temperature.

As the spectra recorded at three different temperatures (see Experimental section) are coincident in the “amide I” region, we may deduce that below 85 °C the disordered and ordered hydrogen-bonded aggregates of the m-Ut(750)nEu(CF3SO3)3 hybrids are totally insensitive to temperature, a proof of their stability.

To gather additional information of the modifications undergone by the characteristic modes of the “amide I” region of the monourethanesil xerogels upon inclusion of increasing amounts of the triflate salt, we performed curve-fitting in the 1800–1630 cm–1 frequency envelope.

The complex spectral profiles were decomposed into Gaussian bands.

The results obtained with three representative doped samples are illustrated in Fig. 8.

The composition dependence of the area of the bands at 1750, 1720, 1700 and 1665 cm–1 is shown in Fig. 9.

Fig. 9 allows to infer that the presence of a growing amount of Eu3+ ions in the monourethanesil system has several important consequences:

(1) From n = 400 to 30 the fraction of non-bonded CO groups is progressively reduced.

At n = 10 the 1750 cm–1 event vanishes completely, indicating that no CO groups are left free.

(2) In the 400 ≥ n ≥ 70 composition interval, the amount of hydrogen-bonded CO groups belonging to disordered regions increases.

At higher salt concentration, a considerable amount of these aggregates are, however, disrupted.

In the most concentrated xerogel only weak traces of the hydrogen-bonded associations B (Scheme 1) are found.

The drastic destruction of these urethane–polyether hydrogen-bonded structures at n = 10 derives most probably from the fact that, at this salt concentration, the polymer chains are being extensively required to interact with the cations and can no longer bond to the N–H groups of urethane cross-links via hydrogen-bonding.

(3) The area of the feature at 1700 cm–1 suffers an increase in hybrids with 400 ≥ n ≥ 90, but decreases rapidly upon further addition of Eu(CF3SO3)3.

This abrupt loss of band area may be interpreted as a solid indication that the introduction of the guest salt deeply perturbs the ordered aggregates C (Scheme 1) of the monourethanesil samples, presumably leading to their breakdown.

A minor proportion of such hydrogen-bonded structures remains at n = 10.

(4) In compounds with 70 ≥ n ≥ 30 a new component, that gradually grows with the increase of salt content, appears at approximately 1665 cm–1.

In the “amide I” spectral region of m-Ut(750)10Eu(CF3SO3)3 this band becomes dominant.

The trend demonstrated by the various components of the “amide I” envelope of the monourethanesils confirms that the most concentrated xerogel studied is subject to a tremendous structural transition as a result of the formation of the crystalline polyether/lanthanide salt complex.

The nature of the species that give rise to the intense 1665 cm–1 feature is unknown at this stage.

Because of its low frequency, we feel tempted to speculate that it is associated with highly ordered, strongly hydrogen-bonded aggregates, formed at the expense of the breakdown of practically all the previously existent ones (associations B and C of Scheme 2).

The regular increase of this band with the increase of salt content reveals that the new hydrogen-bonded aggregates are strongly coordinated to the Eu3+ ions.

It is instructive to comment that the interpretation proposed for the “amide I” region of the monourethanesil materials relies essentially on the fact that the host hybrid m-Ut(750) matrix is only slightly hygroscopic, a result of the utmost relevance, since it indicates that in the doped samples the main species responsible for the presence of water is the guest lanthanide salt, owing to the typical great water affinity of the Eu3+ species.

This explains why the almost complete removal of water from the salt-containing compounds could be easily accomplished through classical procedures, such as drying under vacuum at room temperature (see Experimental section), as confirmed by the low intensity of the characteristic broad OH stretching envelope in the high-frequency region of the FT-IR spectra reproduced in Fig. 1.

Thus any contribution of the water bending mode, which appears typically near 1640 cm–1 in pure water, to the 1665 cm–1 feature found in the “amide I” envelope of the m-Ut(750)nEu(CF3SO3)3 xerogels with n lower than 70 is considered to be negligible.

“Amide II” region

The “amide II” mode of poly(urethane)s is akin to the amide II feature of polyamides.34

The amide II band (which is often simply designated as the N–H in-plane bending, δi.p. NH, band) is a mixed mode containing a major contribution from the N–H in-plane bending vibration and minor contributions from the C–N stretching and C–C stretching vibrations.

It is sensitive to both chain conformation and intermolecular hydrogen bonding and adequately reflects the distribution of hydrogen bond strenghts.34

In the spectra of the m-Ut(750)nEu(CF3SO3)3 xerogels with ∞ > n ≥ 30 the “amide II” mode appears as a single broad, medium to weak band centered at approximately 1534 cm–1 (Fig. 7).

In the spectrum of m-Ut(750)10Eu(CF3SO3)3 the same feature undergoes a dramatic shift to 1568 cm–1 (Fig. 7).

In the whole range of salt concentration analyzed we do not detect any indication of the presence of separate contributions presumably originated from hydrogen-bonded associations of various degrees of order (Fig. 7).

Moreover, there is no evidence for a distinct free component.

The marked upshift of the “amide II” feature observed at n = 10 confirms that in this monourethanesil sample hydrogen bonding is much stronger than in the more dilute ones.11a

The coincidence of the “amide II” region of the spectra recorded at three different temperatures for each composition (see Experimental section) leads us to state that at temperatures lower than 85 °C the “amide II” mode of the monourethanesil is a temperature independent event.

This finding is consistent with the behavior of the materials in the “amide I” region.

Differential scanning calorimetry

Rich information regarding cation/polyether complexation may be obtained from DSC measurements.4

In polymer electrolytes the interaction between the cations and the ether oxygen atoms of the polymer chains is known to be accompanied by the formation of transient ionic cross-links that partially arrest the local motion of the adjacent solvating segments.

As a consequence the glass transition temperature, Tg, of the host polymer is shifted to higher temperatures.

This effect may be quite dramatic in the case of rich-salt compounds.

The dependence of the Tg of the m-Ut(750)nEu(CF3SO3)3 monourethanesils with composition is in accordance with the spectroscopic data presented above.

Fig. 6 clearly shows that, as expected, the Tg of the hybrid matrix (–48 °C) remains nearly constant in samples with n > 30.

This result confirms once more that the pendant oligopoly(oxyethylene) chains of m-Ut(750) are not involved in the coordination of the lanthanide ions in this range of composition.

In addition, the same plot provides conclusive evidence that in xerogels with n = 30 and 10, Eu3+/polymer bonding occurs, leading to a marked increase of the Tg of the polyether segments (17 and 76 °C, respectively) (Fig. 6).

Photoluminescence spectroscopy

The photoluminescence studies carried out give reliable support to the spectroscopic results discussed in Section 1.

The 14 K emission spectra obtained for the m-Ut(750)nEu(CF3SO3)3 monourethanesils with n = 200, 60 and 30 at the excitation wavelength that maximizes the cation luminescence intensity are reproduced in Fig. 10.

These spectra are composed of a series of yellow–red sharp lines-assigned to intra-4f transitions between the 5D0 and the 7F0–4 levels6a,9a,e-that overlap a large broad band in the purple–blue–green region.

In the case of the most dilute sample examined here (n = 200), an intra-4f self-absorption, attributed to the 7F0 → 5D2 transition, is also observed (marked with an asterisk in Fig. 10).

It was recently demonstrated that the broad emission which already appears in the emission spectra of the non-doped diureasil and diurethanesil host frameworks9d,11b may be expressed by the convolution of a long-lived emission from the NH groups of the urea and urethane bridges, respectively, with a short-lived component originated from electron–hole recombinations occurring in the siliceous domains.9d The maximum intensity energetic position of the hybrid host emission and its relative intensity with respect to the Eu3+ lines strongly depend on the amount of guest salt incorporated.

Fig. 10 shows, not only that the energy band of the m-Ut(750)200Eu(CF3SO3)3 xerogel is red-shifted with respect to those of the more concentrated compounds, but also that its intensity is greater than that of the ion lines.

We may also infer from Fig. 10 that an increase in the salt concentration from n = 200 to 30 induces an inversion in the relative intensity of the emission of the hybrid host and that of the Eu3+ ions, since the yellow–red lines become clearly more intense in the spectrum of the salt-rich material.

Nevertheless, variations on salt content do not affect the Eu3+ intra-4f lines:6a,9a,b,e the energy, shape or relative intensity of the observed transitions remain unchanged (Fig. 10).

These observations substantiate the presence of a similar cation local coordination site within the entire concentration range considered.

Furthermore, as the 5D0 → 7F2 forced electric-dipole transition is stronger than the 5D0 → 7F1 magnetic-dipole one, we are led to conclude that the point symmetry group of the Eu3+ ions in the m-Ut(750)-based monourethanesils does not have an inversion center,35 a finding that completely discards the occurrence of a first coordination sphere for the lanthanide ions exclusively composed of water molecules.36

This extremely important result is in perfect agreement with our claim that the local chemical environment of the cations in the m-Ut(750)nEu(CF3SO3)3 composites includes, apart from a minor number of oxygen atoms of water molecules, oxygen atoms from three different species: the urethane carbonyl moieties (in the whole range of salt composition analyzed here), the polymer oxyethylene units (in samples with n ≤ 30) and the triflate ions (see Part 2 of this series of papers).

The decrease in the relative intensity of the host broad band with respect to the Eu3+ lines observed as the amount of Eu3+ is increased from n = 200 to 30, already reported for similar organic/inorganic hybrids,9a,b,e,10a may be explained in terms of energy transfer between the emitting centers of the matrix (donors) and the lanthanide ions (acceptors).9e The energy transfer mechanism associated with the host emission component resulting from the photoinduced proton-transfer between urethane groups is enhanced as a consequence of the progressive suppression of the hydrogen bonds that occurs with the increase of salt content (as demonstrated in section 1), which leads to a delocalization of the proton, rendering the induced proton transfer between NH groups easier.

This effect is manifested in the emission spectra through a decrease in the relative host broad band intensity.


Monourethane cross-linked poly(oxyethylene)/siloxane hybrids incorporating a wide range of europium triflate, Eu(CF3SO3)3, concentration (∞ ≥ n ≥ 10, where n is the molar ratio of (OCH2CH2) repeat units per Eu3+ ion) were investigated by means of several techniques to study the cation/polymer, cation/cross-link and hydrogen bonding interactions.

The analysis of the specific vibrational modes of the POE chains, as well as of the “amide I” and “amide II” frequency envelopes, characteristic of the urethane cross-links, provided strong evidences that in these materials the coordination of the lanthanide ions takes place in the following way: (1) at n > 30 the cations interact exclusively with the carbonyl oxygen atoms of the urethane linkages; (2) at n = 30 the cations start to bond to the oxygen atoms of the polyether chains; (3) at n < 30 a dramatic breakdown of the matrix hydrogen-bonded aggregates occurs caused by cation entrapment; (4) at salt composition n = 10 a crystalline polymer/salt complex is formed.

Photoluminescence spectroscopy clearly confirmed conclusion (3).

The composition dependence of the glass transition temperature of the xerogels studied corroborated conclusion (4).

The role of the anion in the coordinating behavior of the monourethanesils will be examined in Part 2 of this series of papers.



3-Isocyanatepropyltriethoxysilane (ICPTES, Fluka) and poly(ethylene glycol) methyl ether (PEGME, Aldrich, average molecular weight = 750) were used as received. tetrahydrofuran (THF, Merck) and ethanol (CH3CH2OH, Merck) were stored over molecular sieves.

Europium(iii) trifluoromethanesulfonate (Eu(CF3SO3)3, Aldrich) was dried under vacuum at 90 °C for several days prior to being used.

High purity distilled water was used in all experiments

The preliminary stage of the preparation of the Eu3+-doped monourethanesils involved the formation of a covalent urethane linkage between the terminal hydroxyl group of a poly(ethylene glycol) methyl ether molecule containing about 17 oxyethylene units, PEGME(750), and the isocyanate group of an alkoxysilane precursor, ICPTES (Scheme 2).

The second stage of the synthesis implied the addition of water and ethanol, to start to hydrolysis and condensation reaction characteristic of the sol–gel chemistry (Scheme 2).

The nomenclature m-Ut(750)nEu(CF3SO3)3 was adopted to identify the final materials.

Xerogels with n = 400, 200, 90, 70, 60, 50, 30 and 10 were prepared.

An undoped sample, designated as m-Ut(750), was also obtained.

Step 1: synthesis of the monourethanesil precursor, m-UtPTES(750)

PEGME(750) (2.5 g, 3.3 mmol) was dissolved in THF (10 mL) by stirring.

ICPTES (0.823 mL, 3.3 mmol) was added to this solution in a fume cupboard.

The flask was then sealed and the solution stirred for 24 h at moderate temperature (≈70 °C).

The structure of the transparent polydisperse oil obtained in such conditions, designated as monourethanepropyltriethoxysilane (m-UtPTES(750)), was confirmed by 1H NMR, IR and HR-MS (FAB).1H NMR (400.13 MHz, CDCl3, 25 °C, TMS): δ 3.79 (q, 3J(H,H) = 7 Hz, 6H, Hd), 3.72–3.52 (m, 68H, Hg), 3.36 (s, 3H, Hf), 3.18–3.12 (m, 2H, Hc), 1.62–1.57 (m, 2H, Hb), 1.22–1.18 (m, 9H, He), 0.62–0.58 (m, 2H, Ha); IR (KBr): 1716, 1660 cm–1 (ν(CO)), 1533 cm–1 (δi.p.(NH)); HR-MS (FAB) M + H+: found 984.570282, calc.

984.577463 for C43H90NO21Si (H3C(OCH2CH2)16O(CO)NH(CH2)3Si(OCH2CH3)3).

Step 2: synthesis of the monourethanesils, m-Ut(750)nEu(CF3SO3)3

Ethanol (0.776 ml, 13.3 mmol), an appropriate mass of EU(CF3SO3)3 (Table 2) and water (90 µl, 5 mmol) were added to the m-UtPTES(750) solution prepared in the first stage of the synthesis procedure (molar proportion 1 ICPTES∶4 CH3CH2OH∶1.5 H2O).

The mixture was stirred in a sealed flask for 30 min and then cast into a Teflon mould which was covered with Parafilm and left in a fume cupboard for 24 h.

After a few hours gelation was already visible.

The mould was transferred to an oven at 60 °C and the sample was aged for a period of 4 weeks.

Transparent monolithic xerogels with a yellowish coloration were formed.

Differential scanning calorimetry

A disk section with a mass of approximately 40 mg was removed from the monourethanesil film, placed in a 40 µL aluminium can and introduced in a dessicator over phosphorus pentaoxide for at least 12 h at room temperature.

After this drying treatment the can was hermetically sealed and the thermogram of the sample was recorded with a DSC131 Setaram Differential Scanning Calorimeter equipped with a liquid-nitrogen cooling accessory.

The purge gas used was high purity nitrogen supplied at a constant 35 cm3 min–1 flow rate.

The sample was first cooled to –110 °C at 10 °C min–1 and then heated up to 150 °C at the same heating rate.

It was then quenched to –110 °C at 20 °C min–1 and finally heated to 150 °C at 10 °C min–1.

Fourier transform IR spectroscopy

FT-IR spectra were acquired under vacuum at room temperature and 40 and 85 °C using a Bruker IFS-66V Fourier transform spectrometer connected to an Oxford cryostat system.

The spectra were collected over the range 4000–400 cm–1 by averaging at least 150 scans at a resolution of 2 cm–1.

The temperature of the samples was held constant for about 25 min before the measurement was initiated.

Solid samples (2 mg) were finely ground, mixed with approximately 175 mg of dried potassium bromide (Merck, spectroscopic grade) and pressed into pellets.

Prior to recording the spectra the discs were kept in an oven under vacuum at 90 °C for several days in order to reduce the levels of adsorbed water.

To evaluate complex band envelopes and to identify underlying component bands of the spectra, the iterative least-squares curve-fitting procedure in the Peakfit37 software was used extensively throughout this study.

The best fit of the experimental data were obtained by varying the frequency, bandwidth and intensity of the bands.

Gaussian band shapes were employed.

Fourier transform Raman spectroscopy

The FT-Raman spectra were recorded at room temperature with a Bruker IFS-66 spectrometer equipped with a FRA-106 Raman module and a near-IR YAG laser with wavelength 1064 nm.

The spectra were collected over the 3200–300 cm–1 range at a resolution of 2 cm–1.

The accumulation time for each spectrum was 4 h.

Photoluminescence spectroscopy

The emission spectra were recorded at room temperature using a 0.25 m KRATOS GM-252 excitation monocromator and a 1 m 1704 SPEX Czerny–Turner spectrometer coupled to a Hamamatsu R928 photomultiplier.

A 300 W xenon arc lamp was used as excitation source.

All the spectra were corrected for the spectral response of the detector.