Molecular design of luminescent organic-inorganic hybrid materials activated by europium (III) ions

Solid State Sciences 3 (2001) 211– 222

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Molecular design of luminescent organic-inorganic hybrid materials activated by europium (III) ions Anne-Christine Franville *, Rachid Mahiou, Daniel Zambon, Jean-Claude Cousseins Laboratoire des Mate´riaux Inorganiques, UPRES A-6002, Universite´ Blaise Pascal and ENSCCF, F-63177 Aubie`re, France Received 9 October 2000; accepted 26 October 2000 Dedicated to Professor Michel Tournoux on the occasion of his retirement

Abstract Luminescent hybrid materials consisting in rare-earth (Eu3 + , Gd3 + ) organic complexes covalently attached to a silica-based network have been obtained by a sol– gel process. Four dicarboxylic acids with different aromatic subunits (dipicolinic acid, 4-phenyl-2,6-pyridinedicarboxylic acid, 4-(phenylethynyl)-2,6-pyridinedicarboxylic acid and 2,6-Bis(3-carboxy-1-pyrazolyl)pyridine) have been chosen as ligands for Ln3 + ions. They were grafted to 3-aminopropyltriethoxysilane (APTES) to give organically modified alkoxysilanes that were used as molecular precursors for the preparation of hybrid materials. Ln3 + first coordination sphere, composition of the siloxane matrix and connection between the organic and inorganic parts have been characterized by infrared spectroscopy, by 13C29Si solid-state NMR as well as by elemental analyses. UV excitation in the organic component resulted in strong emission from Eu3 + ions due to an efficient ligand-to-metal energy transfer. As compared to reference organic molecules, hybrid samples exhibited similar emission properties under UV excitation in addition to mainly unchanged excited states lifetimes. However, by direct excitation of the Eu3 + -5D0 energy level, the presence of two different site distributions were evidenced in the four hybrid compounds. Emission features related to each of these site distributions and their respective attribution were investigated. Variations in the relative emission intensities were observed according to the nature of the organic chromophore. These variations were discussed in relation to the ATE (Absorption-Transfer-Emission) mechanism and to the relative energy positions of the ligand and the rare-earth ions respectively. © 2001 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. Keywords: Class II hybrid materials; Sol–gel; Rare-earth organic complexes; Trivalent europium; Luminescence; Local probe; Phosphor

1. Introduction Lanthanide ions, especially trivalent europium and terbium, have been long used in phosphors materials because of their sharp and intense emission bands based on f-f electronic transitions [1]. Complexation * Correspondence and reprints. E-mail address: [email protected] (A.-C. Franville).

of these ions by organic ligands offers several advantages and in the past decade, a lot of work has been devoted to the design of luminescent lanthanide (III) organic complexes which can act as efficient LightConversion Molecular Device (LCMD) [2–5]. In these systems, light can be absorbed by the ligand and the absorbed energy may be transferred to the emitting metal ion (the so-called antenna effect), thus markedly increasing the absorption cross section [6]. In addition, organic ligands protect lanthanide ions

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from their environment (residual water or solvent molecules, etc…); this shielding effect considerably reduces non-radiative de-activation processes, arising both from coupling with the lattice vibrations and from concentration quenching [7]. Thus, europium organic complexes with dicarboxylic acids, b-diketones or macrocyclic ligands such as crown-ethers or cryptates for example are well known to exhibit high luminescence yields under UV excitation. However, use of these systems in optical devices is very limited due to their low thermal and mechanical resistances. Incorporation of rare-earth complexes in inorganic matrices has recently grown considerable interest as the resulting organic-inorganic materials are likely to combine the good photophysical properties of the organic component with the favorable thermal and mechanical characteristics of inorganic networks. In this area, most of the previous studies concern encapsulation of luminescent molecules in silica-based sol –gel matrices [7 – 18]. On one hand, such siloxane matrices are attractive considering their thermal stability and their optical transparency in the UV-visible range [19]; on the other hand, the sol –gel process has proved to be a very convenient route for the production of organic-inorganic hybrid materials [20 –22]. Sol – gel method is based on hydrolysis/condensation reactions [23] and its mild processing conditions allow the introduction of organic molecules. Moreover, a wide range of materials with flexible properties may be obtained by this method: composition, microstructure and shape of these hybrid materials can be tailored by modifying the nature of the two components which are associated, their relative concentration, as well as by adjusting the sol – gel conditions. Two classes of hybrid materials have been defined according to the type of bondings between the organic and inorganic parts [20]. Class I refers to hybrid compounds in which only hydrogen bondings or van der Waals forces exist between the two different parts. In class II hybrid materials, organic functionalities are covalently attached to the host matrix. Our recent work concerns class II hybrid materials in which luminescent rare-earth organic complexes are anchored to a siloxane matrix via SiC linkages [24,25]. The strategy to prepare such materials is to design new organoalkoxysilanes, which combine a chromophore unit that can efficiently absorb the incident light, chelating functions that can bind to the Ln3 + ions and trialkoxysilyl groups that act as

the inorganic network precursors. When rare-earth ions are introduced in silica-based networks obtained at low temperature (typically< 150 °C), the radiative lifetime of emission and the resulting luminescence intensity are usually quite low due to the large number of residual hydroxyl groups [26]. The choice of bulky aromatic ligands is therefore critical to decrease OH quenching as well as to isolate the emitting centers. Covalent attachment between organic and inorganic components seems beneficial to achieve high luminescence yields for the following reasons: i) leaching of the active molecules is avoided, ii) higher concentrations of dopant ions can be reached, iii) clustering of the emitting species is prevented and iv) materials with better homogeneity can be obtained. In this paper, the preparation and the luminescence properties of Eu3 + -activated organic-inorganic hybrid materials derived from four different dicarboxylic acids (see Fig. 1) will be described. By comparing the emission features under different excitation modes, it is possible to put forward some influence of the siloxane matrix on the intrinsic optical properties of the Eu3 + organic complexes. It appears that the effect of the inorganic network will partly depend on the coordination mode of the rare-earth ions.

2. Experimental Starting materials were purchased from Aldrich Chemical Co; modified alkoxydes were used as received while other reagents were purified according to the literature procedures [27]. All solvents were distilled before use by the standard methods [27]. Europium and gadolinium nitrates were obtained by heating the corresponding oxide (Rhoˆne-Poulenc, 99.99 %) in dilute nitric acid. 1 H and 13C NMR spectra were recorded in CDCl3 on a Bruker AC 400 spectrometer with tetramethylsilane (TMS) as internal reference. 29Si (59.6 MHz) and 13C (75.0 MHz) CPMAS-NMR experiments were conducted on a Bruker DSX 300 with recycle delays of 1 s and pulsewidths of 3.8 ms and 8.0 ms respectively (internal reference: TMS). Infrared absorption spectra were obtained on a Nicolet 5SXC FTIR spectrometer and elemental analyses were performed by the Service Central d’Analyse (CNRS, France).

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All optical measurements were performed on powdery materials at temperatures between 15 and 300 K. Sample cooling was provided by a closed He optical cryostat (Cryomech GB15). Diffuse reflectance spectra were measured in the 200 –800 nm range using MgO as reference on a Perkin-Elmer Lambda 2S spectrometer equipped with a RSA-PE20 reflectance accessory. Continuous excitation spectra in the UV-visible range (250 – 580 nm) were recorded with a Xe-lamp. Fluorescence and reson-

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nant excitation spectra were obtained using either a Jobin-Yvon LA04 nitrogen laser or a pulsed tunable visible laser operating with a mixture of rhodamine dyes (Rh590+Rh610), pumped by the second harmonic of a Continuum surelite-I Nd:YAG. KDP frequency doubler was added at the output of the dye laser to generate UV beam. The spectra were recorded using a Jobin-Yvon HR1000 monochromator and detected by a Hamamatsu R1104 photomultiplier. Data acquisition was obtained using a Keitley

Fig. 1. Formula of the starting dicarboxylic acids Ai (i= 1 – 4), of the derived silylated monomers Si (i= 1 – 4) and of the corresponding organic ligands Oi (i =1–4).

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Fig. 2. Reaction scheme for the preparation of Ln3 + -activated organic-inorganic hybrid materials (Ln3 + Eu3 + , Gd3 + ) from dicarboxylic acids.

DAS 1600 card controlled with a PC computer and boxcar integrator PAR 162/164 from EG&G is used to ensure a good signal to noise ratio. Luminescence lifetime measurements were made using a Lecroy 9310A digital oscilloscope. Relative fluorescence efficiencies were estimated at room temperature by using the 254 nm radiation provided by an Hg lamp. Appropriate filters were placed before and after the sample to avoid any signal induced by the scattered radiation. The fluorescence emitted in the overall solid angle was collected by means of a convergent lens and focused at the entrance slit of the HR1000 monochromator.

3. Synthesis and characterizations

3.1. Preparation of the materials Dipicolinic acid (A1) is commercially available while the other acids, the 4-phenyl-2,6-pyridinedicarboxylic acid (A2), the 4-(phenylethynyl)-2,6pyridinedicarboxylic acid (A3) and the 2,6-Bis(3-carboxy-1-pyrazolyl)pyridine (A4) were prepared according to the procedures described in refs [28], [29] and [30] respectively. The same reaction sketch depicted in Fig. 2 was used to obtained the silylated monomers Si (i= 1–4) from the corres-

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ponding acid. The four different dicarboxylic acids were first converted to acyl chlorides by refluxing in excess SOCl2 under argon for 6 hours. After isolation, the acyl chlorides were directly reacted with 3-aminopropyltriethoxysilane (APTES) in diethyl ether, in presence of pyridine. Details concerning the required experimental conditions for the coupling reaction have been given elsewhere [25]. Si (i= 1–4) monomers were recovered by filtration of the precipitated pyridinium chloride followed by removal of the solvents and were then dried under a vacuum line. IR, 1H and 13C NMR spectra relative to Si are in full agreement with their respective formula and evidence in each case completion of the grafting reaction with APTES [31]. As described in Fig. 2, hybrid materials were prepared from silylated monomers in several steps using a sol – gel process. Complexation was performed in ethanol by adding appropriate amounts of europium/gadolinium nitrate previously dissolved in alcohol. The Ln3 + /Si molar ratio is 1/3 for i= 1, 2, 3 and 1/1 for i= 4. In this sol was introduced hydrochloric acid as an acid catalyst for hydrolysis reactions. Gels were obtained after aging for 4–7 days under ambient conditions (gelation was carried out

in teflon beakers covered with holed parafilm) and are finally dried at 70 °C for 48 h after grinding. Undoped xerogels, i.e. that are not submitted to complexation with lanthanide salts, were also prepared from Si monomers using a similar procedure as above. In the following sections, Eu3 + (Gd3 + )-activated and undoped hybrid materials will respectively be noted EuHi (GdHi) and Hi with i= 1– 4. For comparison purpose, the organic ligands Oi (i= 1–4, see Fig. 1) were synthesized through amidation between the different Ai acids and N-butylamine. Eu3 + organic complexes that will afterwards be noted EuOi (i= 1–4) were obtained by refluxing a mixture of the corresponding Oi ligand and Eu(NO3)3, 6 H2O in acetonitrile for 2 h (in moles, Eu/O1 – 3 = 1/3 and Eu/O4 = 1/1). Complexes were precipitated in diethyl ether and the resulting white solids were filtrated, washed with the reaction solvent and dried overnight under vacuum. Elemental analyses gave satisfactory results in all cases and confirmed the predicted stoichiometry of the EuOi organic complexes.

3.2. Solid-state NMR spectroscopy 13

Fig. 3. Solid-state 13C CPMAS NMR spectra of Hi (i= 1 – 4) undoped hybrid xerogels.

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C and 29Si solid-state NMR experiments were performed on undoped hybrid materials due to the paramagnetic character of Eu3 + –Gd3 + ions. 13 C CPMAS NMR spectra relative to Hi (i=1–4) are presented in Fig. 3, together with assignment of the main peaks. These spectra show three peaks located at nearby 43, 23 and 11 ppm characteristic of the (CH2)3 aliphatic chains. Other signals are assigned to aromatic carbons while the peak at 164 ppm due to CO amide functions is also clearly evidenced in all spectra. Besides, no new signals were observed for hybrid xerogels as compared to the 13C NMR spectra relative to starting silylated monomers (not shown here). These results suggest that no modification of the organic component occurs during the sol –gel process under the experimental conditions used. Fig. 4 shows the 29Si CPMAS spectra recorded for Hi (i= 1–4). The broad signal located between –80 and –40 ppm is attributed to trifunctional CSiO3 groups while no tetrafunctional units whose chemical shifts are typically in the –110/ –80 ppm range are detected [32,33]. This proves that whole of the organic functionalities are covalently attached to the siloxane network via SiC bonds. Peaks due to T1,

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Fig. 4. Solid-state 29Si CPMAS NMR spectra of Hi (i= 1 – 4) undoped hybrid xerogels. Table 1 Relative amounts of T1, T2 and T3 species and condensation rates of these T units as assessed by 29Si solid-state NMR for Hi (i= 1–4) hybrid materials.

H1 H2 H3 H4

T1 (%)

T2 (%)

T3 (%)

C(T) (%)

5.2 10.7 7.5 14.7

55.7 51.4 45.7 46.7

39.1 37.9 46.7 38.6

78.0 75.7 79.7 74.6

T2 and T3 sites can be distinguished within the broad signal and their relative amounts can be quantitatively estimated after deconvolution [32]. Table 1 gives the relative proportions of the different silicon species and the resulting condensation rate C(T) of the siloxane matrix measured for all Hi samples. Except for H3, the main species are T2 ones exhibiting two SiOSi oxo bridges and all C(T) values are between 75 and 80 %. Using Si monomers as the only inorganic network precursors and the sol –gel processing conditions previously described, the amount of uncondensed silanol groups is not negligible. Besides, the above results do not reveal any difference in the condensation ability depending on the structure of the starting silylated monomer.

3.3. Infra-red spectroscopy and elemental analyses Formation of the complexes within the siloxane matrix and their stoichiometry have been assessed by

infrared spectroscopy and elemental analyses. It is noted that similar results were obtained for Eu3 + and Gd3 + -activated hybrid materials and, in the following section, only those concerning EuHi samples will be detailed. Table 2 gives the vibration frequencies and the assignments of the main absorption bands observed in the FTIR spectra relative to Hi and EuHi (i= 1–4). For all these hybrid materials, the broad signal observed between 1 150 and 980 cm – 1 is assigned to (SiOSi) asymmetric stretching vibrations while the sharp peak at 1 200 cm – 1 confirms the presence of SiC bonds (n(SiC)). Due to uncondensed silanol groups, an additional absorption peak located at 907 cm – 1 (nas(SiOH)) is also present. The band centered at about 3 400 cm – 1 and characteristic of n(OH) vibrations is always relatively intense and can be attributed either to SiOH groups or to residual water molecules. In addition, FTIR spectra show the typical absorption bands corresponding to the organic component. By comparing the spectra recorded for Hi and EuHi respectively, it appears clearly that each ligand coordinates to Ln3 + ions via their two carbonyl groups and the nitrogen atom of pyridine. This derives from the following observations: – In the case of EuHi samples, a single peak is detected for the n(CO) vibration, which is shifted to lower frequencies (Dn ca. 20 cm – 1) as compared to that observed for the undoped hybrid materials, indicating that all carbonyl groups coordinate to Eu3 + ions. Similarly, the increase in the d(CNH) bending vibrations is due to Eu3 + coordination by the amide functions [34,35]. – A peak located at around 415 cm – 1 is evidenced in EuHi spectra, which is related to the formation of EuN bonds (d(LnN)) [36]. EuHi samples were finally characterized by elemental analyses. The results collected in Table 3 reveal no significant difference between found and calculated (%Si/%Eu) ratios; this, together with FTIR results, proves that Si (i= 1, 2, 3) and S4 monomers give with Ln3 + ions 1/3 and 1/1 complexes respectively. The resulting coordination modes for the different EuHi compounds are schematically represented in Fig. 5. As shown, hybrid materials derived from acids A1, A2 and A3 exhibit a similar coordination sphere for Ln3 + ions. In these cases, complexation by the ligands leads to a 9-fold coordination (full coordination), that is favorable to protect Eu3 + ions from their environment. Formula

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Table 2 Vibration frequencies (in cm–1) and assignments of the main infrared absorption bands observed in the 4 000–400 cm–1 range for undoped (Hi) and Eu3+-activated (EuHi) hybrid samples (i =1–4).

n-(OH) n-(NH) n-(CHaro) nas-(CH2) ns-(CH2) n-(C C) n-(C= O) d-(CHaro) d-(CNH) d-(CH) n-(CC) (CH2)wag n-(SiC) nas-(SiOSi) n-(SiOH) n-(NO–3) d-(LnN)

H1

EuH1

H2

EuH2

H3

EuH3

H4

EuH4

3405 3322 3090 2937 2885 – 1660 1586 1543 1447 1312 1269–1243 1199 1119–1083 –1037 907

3400 3273 3091 2938 2888 – 1637 1596 1559 1459 1314 1279 1200 1102–1075 –1018 907 1435–1383 –720 415

3406

3403 3238 3081 2935 2880 – 1638 1606 1553 1502

3403

3390 3260 3081 2933 2882 2210 1639 1604 1553 1492–1442 1350–1311 1273–1239 1199 1098–1013

3408 3330 3090 2930 2887 – 1655 1607–1587 1558 1510–1448 1372 1293–1254 1198 1085–1050 –987 906

3387

3082 2929 2881 – 1660 1603 1542 1500–1446 1344 1275–1240 1199 1105–1075 –1003 902

1270 1200 1087–1014 902 1436–1383 –731 421

3080 2928 2884 2212 1661 1602 1539 1442 1348 1278–1236 1199 1113–1025 –996 902

901 1383–729

3095 2935 2890 – 1637 1584 1512

1200 1077–1048 –1010 903 1471–1383 –1307–738

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of the S4 monomer allows for Eu3 + a 5-fold coordination, including two nitrogen atoms of the pyrazole groups. This coordination sphere is completed by nitrate ions, as confirmed by the infrared vibration frequencies noted for these ions in the case of EuH4. Indeed, the two absorption bands observed at 1 471 and 1 307 cm – 1 are characteristic of chelated NO3– groups [36].

excited level of europium. On the other hand, excitation spectra of Eu3 + emission (lem = 616.0 nm) are dominated by a broad band which is assigned to a S0 “ S1 absorption transition within the organic chromophore (see Fig. 7); positions of the as-determined excitation maxima are given in Table 4. These first results are indicative of an efficient sensibilization of the metal centers luminescence by the organic ligands in the hybrid materials we have prepared.

4. Luminescence properties

4.1.1. UV excitation Emission properties of Eu3 + -activated hybrid compounds were first investigated at 15 K under UV excitation (337.1 nm). As presented in Fig. 8, emission spectra recorded for all EuHi (i= 1–4) samples by direct excitation of the organic component exhibit very similar features related to the typical 5D0 “ 7FJ

All the EuHi samples exhibit thermal stability up to 300 °C in air and are not modified under the pulsed UV laser beam corresponding to an energy of around 1 mJ (ca. 1019 photons.cm – 2) while the corresponding EuOi compounds are stable up to 150 °C and become brown under the previous pumping conditions [24].

4.1. Excitation and fluorescence spectra The main paths of energy transfer between the excited ligand and Ln3 + states are summarized in Fig. 6 [37 –39]. Independently of the excitation wavelength, fluorescence spectra recorded for EuHi are associated with the radiative transitions from the 5D0

Table 3 Eu/Si relative weight percentages as calculated and found by elemental analyses for EuHi (i= 1–4) hybrid materials.

EuH1 EuH2 EuH3 EuH4

%Si/%Eu calculated

%Si/%Eu found

1.11 1.11 1.11 0.37

1.06 1.03 1.07 0.35

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Fig. 5. Coordination mode of Ln3 + ions in hybrid materials obtained from S1S2S3 monomers (a) and S4 monomer (b). XH, C6H5 and C8H5 for S1, S2 and S3 respectively.

(J= 0–4) transitions. Relative intensities between the Dipolar Electric (DE) and Dipolar Magnetic (DM) radiative transitions (predominance of the 0–2 transition) together with the shape of the emission peaks (unresolved and quite broad) are characteristic of the amorphous character of the sol – gel derived hybrid materials. Fluorescence decays were measured at low temperature under the same UV excitation by monitoring the 5D0 “ 7F2 emission transition peaking at 616.0 nm. Decay profiles recorded for EuHi (i= 1–4) fit a monoexponential law with, however, an initial risetime. Risetime values are of the order of 2–4 ms and are connected to non-radiative de-excitation processes from the upper 5DJ energy levels of Eu3 + ions to the 5D0 emitting level. Lifetime values in the long time scale are collected in Table 5 and compared to those of the corresponding EuOi organic complexes. Lifetimes of the Eu3 + -5D0 excited state are slightly shortened in presence of the siloxane matrix but remain for all EuHi samples in the millisecond time scale. This shows that Eu3 + ions are efficiently protected by the organic ligands from the effect of residual OH groups. Decrease of the lifetime is however relatively more important in the case of EuH4; this can be connected with the more ‘‘opened’’ structure of this compound, which results in a less efficient shielding effect from the ligand than in the other hybrid materials.

4.1.2. Excitation in the 5D0 level 5 D0 ’ 7F0 excitation spectra recorded at 15 K by monitoring the overall 5D0 “ 7F2 transition for the four different hybrid compounds are shown in Fig. 9. Two contributions, that will afterwards be labeled A and B, are clearly observed in these spectra and are attributed to two different site distributions as the 5D0 ’ 7Fo transition is in principle a non-degenerate one. The respective frequencies and half-widths of these two bands were estimated by deconvolution of the overall spectra (assuming a sum of two gaus-

Fig. 6. Sensibilization mechanism of europium (III) luminescence in organic complexes. As presented on the left side, fluorescence quenching is observed when T1 transfers its energy to a LMCT state. The following abbreviations are used: A =absorption, F = fluorescence, P =phosphorescence, ISC =inter-system crossing, ET=energy transfer, BT =back-energy transfer, LMCT = ligand-to-metal charge transfer.

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are identical for EuH1, EuH2 and EuH3 samples whereas a shift towards higher frequencies is observed for both species in the case of EuH4. According to [40], the frequency of the Eu3 + -5D0 l 7F0 transition is correlated with the sum of the Table 5 Lifetimes of the 5D0 excited state (t in ms) and relative emission intensities (I) measured for EuHi (i= 1–4) samples under UV excitation. Radiative lifetimes recorded for EuOi (i= 1–4) complexes are also indicated as reference; I values were normalized to 100. [t] (hybrid materials) Fig. 7. Excitation spectra at 298 K of Eu3 + fluorescence (lem = 616.0 nm) in EuHi (i =1–4) hybrid materials. Table 4 Energy positions of the excitation maxima (lexc max) and of the singlet (S1) and triplet (T1) states in EuHi (i =1–4) hybrid materials (in nm (cm–1)). [lexc max] EuH1 EuH2 EuH3 EuH4

294 335 370 338

(34014) (29851) (27027) (29586)

[S1] 288 330 358 336

(34722) (30303) (27933) (29762)

EuH1 EuH2 EuH3 EuH4

[t] (organic complexes)

1.06 0.68 0.69 0.64

1.22 0.88 0.75 1.15

[I]

27.5 100 54.1 1.3

[T1] 369 434 465 396

(27100) (23041) (21505) (25252)

Fig. 9. 5D0 ’ 7F0 excitation spectra at 15 K of the overall 5D0 “ 7 F2 fluorescence recorded for EuHi (i= 1 – 4 from bottom to top) hybrid materials. Table 6 Energy positions (E in cm–1 (nm)) and half-widths (D1/2 in cm–1) of the 5D0 ’ 7Fo transition relative to A and B contributions respectively. [A contribution] Fig. 8. Low temperature fluorescence spectra of EuHi (i= 1 – 4 from bottom to top) hybrid materials under 337.1 nm excitation.

sian functions), and are gathered in Table 6. Halfwidth of the B contribution which is located on the high-energy side is about twice to that of the A contribution. Energy positions relative to A and B

[E] EuH1 EuH2 EuH3 EuH4

17218 17212 17218 17248

(580.79) (580.99) (580.79) (579.78)

D1/2 12.2 11.1 12.4 13.5

[B contribution] [E] 17240 17238 17239 17262

(580.05) (580.11) (580.08) (579.31)

D1/2 21.2 21.6 22.1 24.3

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Fig. 10. 5D0 ’ 7F0 excitation spectra at 15 K of the 5D0 “ 7F2 transition recorded for EuH1 after selection of the emission wavelength (lem = 613.5 nm).

nephelauxetic effects due to the coordinating groups. The previous observations are consistent with the Eu3 + coordination modes described in the above section: a similar 9-fold coordination in the case of EuH1 – 3 and a 5-fold coordination completed by nitrate ions in the case of EuH4 (nitrate groups are known to induce an increase of the 5D0 l 7F0 transition frequency [40]). No significant change in the excitation band shape, with respect to the emission wavelength, is detected by modifying the delay time after the laser pulse. However, variations in the intensity ratios between the A/B site distributions, are evidenced by monitoring specific emission wavelengths. Fig. 10 shows as an example an excitation spectrum relative

to EuH1 for which the A contribution is no more observed (lem = 613.5 nm). Selective excitation in the A and B bands produces two different emission spectra, indicating that the two Eu3 + site distributions are not connected by a site to site energy transfer. To illustrate this, details of the 15 K emission spectra relative to the 5D0 “ 7F1,2 transitions and recorded for EuH1 under the two different excitation wavelengths are presented in Fig. 11. Three Stark components can be clearly distinguished for the 5D0 “ 7F1 transition. A displacement of the barycenter of these components towards higher energies and a more important splitting of the 7F1 manifolds are observed in the case of the B-excited emission spectrum. These results can be respectively attributed to a decrease in the Ln3 + -ligand distances and to a stronger crystal field effect [41]. The shape of the 5D0 “ 7F2 transition suggests that B excitation leads to broader emission transitions; this is indicative of a wider site distribution for B. Besides, Aand B-excited emission exhibit pure exponential decays with a lower time constant in the case of B excitation (0.9 ms against 1.18 ms for A in the case of EuH1 for example). All the aforementioned remarks correlate the assumption that the B site distribution is related to Eu3 + ions containing OH groups in their direct environment. Coupling of Eu3 + ions with OH groups is well known to produce a broadening of the optical transitions, a blue shift of the electronic levels and an increase in the non-radiative de-excitation probabilities. Comparison of the spectra reported in Fig. 8 and 11 shows that only the A contribution is observed under UV excitation. This

Fig. 11. 5D0 “ 7F1 (left side) and 5D0 “ 7F2 (right side) emission spectra recorded at 15 K for EuH1 by excitation in the A (lexc = 580.8 nm) (a) and B (lexc = 580.0 nm) (b) bands. The three contributions observed for 5D0 “ 7F1 transitions arised from deconvolution of the emission spectra by sum of gaussian functions.

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may be explained by an efficient energy transfer, from T1 excited state to an EuOH charge transfer band in the case of B species. The role of this charge transfer state in de-excitation processes is not unambiguously established and is the subject of further investigations. In addition, it must be noted that the relative intensity of the B contribution in the 5D0 ’ 7 F0 excitation spectra of EuHi do not reflect the real concentration of Eu3 + centers directly coupled with OH groups, since the oscillator strength of the optical transition in EuOH is high enough to ensure an efficient fluorescence of such centers.

4.2. Relative emission intensities Relative emission intensities of EuHi (i = 1–4) materials were measured at room temperature from the intensity of the 5D0 “ 7F2 radiative transition, using a 254 nm excitation line. The obtained values are reported in Table 5 and are found to decrease in this order: EuH2 > EuH3 > EuH1 > EuH4. With respect to the sensibilization mechanism described in Fig. 6 [39], luminescence yields of Eu3 + organic complexes depend on the efficiency of the three following steps: absorption, energy transfer and fluorescence from Eu3 + emitting level. Under our experimental conditions, regarding the high photon density of the incident light, we can assume that all metal centers present in the illuminated area of the sample are excited in the samples. Variations in the emission intensities can in that case be explained by considering only the relative energy levels of the excited states of the ligand to the excited states of Eu3 + . Positions of the singlet (S1) and triplet (T1) states relative to each of the organic ligands were determined by studying the absorption/emission properties of Gd3 + hybrid materials [5]. S1 energy level was extracted from diffuse reflectance spectra recorded at room temperature on GdHi samples, assuming that absorption edges correspond to the 0 (S0)“ 0 (S1) transition. The broad emission band displayed in the blue region by gadolinium compounds under UV excitation (290.0 nm) at 15 K is assigned to phosphorescence from the triplet state; T1 level was then positioned considering the lowest emission wavelength (0–0 transition). The derived values for S1 and T1 energy positions are gathered in Table 4. Previous studies have established that the most efficient energy transfer occurs between the triplet state and Eu3 + -5D1 level (exchange mechanism) [42].

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In the case of EuH2 and EuH3, the low position of T1 is favorable to a ligand-to-metal energy transfer via the 5D1 state and results in high emission intensities. This intensity is slightly lower for EuH3 due to a possible back transfer from the 5D2 excited state of Eu3 + (ca. 21 500 cm – 1) to the triplet state which is located at almost the same energy in this compound. The important energy differences between T1 and 5D1 levels in the case of the two other samples can account for their relatively low emission intensities. Two additional phenomena may explain the very low intensity observed for EuH4: i) back-energy transfer from the resonnant 5L6 and 5D3 levels of Eu3 + to T1 and ii) effect of the OH groups which, as previously mentioned, is more sensitive in this material.

5. Conclusion In this study, various luminescent organic-inorganic hybrid materials have been prepared by grafting four different dicarboxylic acids to a siloxane network precursor (APTES). Complexation of rareearth ions by the silylated monomers was achieved in the sol, as confirmed by FTIR and elemental analyses; this way, the inorganic matrix was built around the organometallic moieties by hydrolysis/condensation reactions. This method of synthesis seems well adapted in order to retain the luminescence properties of the Eu3 + organic complexes in the derived hybrid materials; this was shown by studying the emission features of EuHi under UV excitation and by comparing them to those relative to EuOi. During this work, luminescence of Eu3 + ions has proved to be a useful local probe, in particular to investigate the possible interactions of the active species with the sol –gel matrix. The presence of two site distributions was evidenced in the studied hybrid compounds: most presumably, one is related to Eu3 + ions without OH groups in their first coordination sphere and the other to Eu3 + ions with OH groups in their direct environment. The relatively high emission intensities of EuHi, especially EuH2 and EuH3, materials (the luminescence intensity of the organic complex is retained up to 85 % in the hybrid samples), in addition to their improved thermal stability as compared to organic molecules (up to 300 °C in air) opens the possibility of making organic-inorganic hybrid phosphors usable for example in display or lighting devices while, until now, only pure inorganic compounds were used for such applications.

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