Paul Legentil

Fabrice Leroux

Sandrine Therias

Damien Boyer

Geneviève Chadeyron

Sulforhodamine B-LDH composite as a rare-earth-free red-emitting phosphor for LED lighting

Domestic lighting using white light-emitting diodes (WLEDs) requires a choice of materials to be eco-friendly as well as economically viable. Revisiting the optical properties of sulforhodamine B (SRB) with a new focus -designing rare-earth-free phosphors -is found here to be highly promising. Relying on a Layered Double Hydroxide (LDH) host, the dye molecules are stabilized, and the inorganic-organic assembly formed is dispersed into silicone. Homogeneous composite films present suitable and stable photometric parameters, making them appropriate candidates to replace red-emitting phosphors based on rare-earth elements.

Introduction

White light-emitting diodes (WLEDs) are undoubtedly the new generation of solid-state lighting sources, because of their excellent features such as high luminous efficiency, energy saving properties, long lifetime and absence of toxic mercury. Currently, the most popular approach to producing WLEDs consists in the combination of a blue chip, emitting between 450 to 480 nm, and YAG:Ce 3+ , a yellow phosphor. 1 The main drawback to this system arises from its weak emission in the red spectral range, yielding a poor colour rendering index (CRI) and a cool colour temperature. Therefore, a red-emitting phosphor is commonly added to this system to improve the colorimetric characteristics of the light. Numerous studies addressing the synthesis of red phosphors compatible with blue chips are reported in the literature. 2 These red phosphors must fulfil several requirements, such as a good stability under photonic and thermal stresses, in addition to a high emission quantum yield. Sulfides and nitrides doped with Eu 2+ are generally used as redemitting phosphors in commercial WLEDs, although they suffer from several drawbacks, including poor thermal stability, water sensitivity or high cost. Moreover, these red phosphors are formulated using rare earth elements (REE). The world's demand for REE, especially within the clean energy, military, and consumer electronics sectors, is projected to reach 200 000 tons per year by 2025, while the current annual supply is estimated at only 113 000 tons. Any attempt to increase REE supply at a high enough rate to meet the demand will encounter many economic, political, and environmental limitations.

3 Indeed, mining requires a high capital input due to the necessity to extract from new sites with their associated refining facilities. [START_REF] Humphries | Rare Earth Elements: The Global Supply Chain Congressional research service[END_REF] The REE mining process also produces large amounts of waste, which once released cause huge environmental damage. Economic issues are also related to political ones, since the lack of security in the supply chain creates significant instability in the REE market. Currently, over 95 % of the global REE supply comes from China.

In this context, it is now crucial to identify or design REE-free phosphors capable of delivering cost-effective light energy conversion, especially for commercial WLED applications, which represent a very large market. Among the alternatives to REE containing phosphors, efforts have been made to develop new materials, such as organic molecules or quantum dots (QD). The present study is focused on revisiting an organic molecule, sulforhodamine B (SRB), a commonly-used dye. This choice is guided by the fact that this common organic dye, belonging to the xanthene family, emits a red-orange light (λ em, max = 590 nm) upon green excitation (λ abs, max = 565 nm) in aqueous diluted solution. Moreover, the SRB molecules are able to absorb photons in a broad spectral range from 450 to 600 nm, which makes them compatible with blue commercial chips. Based on the optical properties of SRB, the use of SRB dye in a white LED is expected to emphasize the reddish contribution of the global emission spectra.

Until recently, most of the works published in the literature have been related to the use of SRB in cell cytotoxicity assays. [START_REF] Hasegawa | [END_REF][6][7] As far as LED applications are concerned, SRB molecules have never been investigated. Indeed, SRB exhibits interesting luminescence characteristics but only in diluted solutions, 8, 9 as most organic dyes, and is optically inactive in its solid state. Furthermore, it has already been reported that the straightforward dispersion of dye molecules into a polymer can cause them to aggregate, thus forming heterogeneous composite films presenting a tiny absolute photoluminescence quantum yield (PL QY abs ), 10 underlining the need to maintain the dye molecules apart from each other to avoid their optical

quenching. An elegant solution consists in immobilizing the SRB molecules in a host lattice like a layered double hydroxide (LDH) as previously reported. [START_REF] Yan | Sulforhodamine B Intercalated Layered Double Hydroxide Thin Film with Polarized Photoluminescence[END_REF] Pioneering research has underlined the benefit of confining photoactive species in a restricted space such as a two-dimensional host in terms of 1) their strong adsorption through host-guest interaction, to avoid possible migration and aggregation within a polymer, 2) their dispersion in the presence of co-adsorbates to avoid, as far as possible, guest-guest interactions.

12 Traditionally, LDH materials have found applications as precursors in catalysis and as scavengers in environmental applications, but have also generated renewed interest in the energy field 13, 14 as well as in biology. [15][16][17] , or QDs such as CdTe 20 or core-shell InP/ZnS. 21 The LDH structure could also be used as a host matrix in order to synthesise ultra-thin 2D carbon nitride (CN) in the interlamellar space; this material exhibits very high luminescence properties, compatible with WLED applications.

22

As mentioned above, only Yan et al. have already performed the intercalation of SRB in a LDH host matrix. [START_REF] Yan | Sulforhodamine B Intercalated Layered Double Hydroxide Thin Film with Polarized Photoluminescence[END_REF] However, several studies deal with the combination of SRB and different substrates. SRB molecules have been covalently bound to core PAMAM dendrimers which then offer blue emissive species, 23 whereas the SRB molecules are originally red-emitting dyes. SRB photoactive species have also been attached to silica spheres 24 and adsorbed onto different pore-size silicas 25 that were found to affect the absorption and emission properties of the dye with a bathochromic effect. When SRB molecules are supported on graphene oxide or boron nitride nanosheets, 26 an electron-transfer process occurs, producing a non-luminescent complex with the former substrate and rapid quenching for the latter. From this rapid survey, SRB molecules seem to offer preferentially highly and stable luminescence properties when entrapped in a 2D matrix such as LDH, rather than when combined on 1D or 3D substrates, where their luminescence is quenched or unsuitably shifted.

It is well known that the aggregation of dye molecules may cause the quenching of luminescence, arising from the dense molecular packing that forms possible guest-guest interactions which turn off the luminescence. Thus, the quantity of dye molecules dispersed onto or into a host structure should be as low as required by the optical properties. Experimentally, the dye molecules are dispersed with surfactant molecules such as dodecylsulfate (DS) or dodecylbenzenesulfonate (DBS) when using a LDH host structure, since the surfactant acts as a spacer enabling the dye molecules to be well accommodated and ensconced within the host galleries. Another aspect investigated here for LED applications concerns the protection of the photoactive molecules against possible thermal degradation and photonic stress occurring when used in the LED set-up.

In this study, the intercalation of SRB molecules into a Zn 2 Al LDH host structure was performed using the usual coprecipitation method. To dispose SRB molecules as far as possible from each other dodecylsulfate surfactant molecules (DS) were co-intercalated with SRB. Indeed, the surfactant increases gallery height, which facilitates the incorporation of the SRB molecules, and at the same time its long alkyl chains supply a non-polar environment, preventing the SRB molecules from aggregating. Optical characterizations of the LDH-DS-SRB were investigated using IR and UV spectroscopy, and PL QY abs , emission spectra and CIE chromaticity coordinates were also recorded. Luminescent composite films prepared from LDH-DS-SRB dispersed in silicone were combined with commercial blue chip and YAG:Ce phosphor to produce a warm white light. Finally, the stability under thermal and photonic stresses of such composite films was compared to that of a silicone/LDH-free SRB film to demonstrate the benefits of such a hybrid co-intercalated LDH-DS-SRB assembly. The pH was maintained at 8.5 by adding 0.25 M NaOH during the synthesis process. Coprecipitation was performed under nitrogen at 20 °C. The mixture was centrifuged at 5,000 rpm for 5 minutes; the sedimented solid on the bottom of the flask was washed several times with de-ionized water until a clear and transparent supernatant was obtained. The paste was recovered and dried overnight at room temperature to obtain a powder.

Experimental

Manufacture of Silicone/HDL-DS-SRB composite films

The LDH-DS-SRB powder was used to produce the silicone/hybrid composite material with different hybrid loadings. The two-component silicone elastomer (silicone Bluesil-RTV 141 A&B) was composed of a viscous liquid, called part A, cured by a polyaddition reaction with a catalyser, part B.

The silicone film was prepared as follows: a mixture of part A and part B (10 phr of part A) was homogenised using a mechanical mixer (Planetary Centrifugal vacuum Mixer "Thinky Mixer") for 10 min at 1200 rpm.

The silicone/hybrid composite material (20 wt. % LDH-DS-SRB and 80 wt. % silicone) was made by mixing the LDH powder (0.6 g) with part A of the silicone elastomer (2.18 g) using the mechanical mixer ("Thinky Mixer") for 10 min at 1200 rpm. Then the mixture obtained was passed through a rolling mill (EXAKT brand) composed of three rollers (spacing of 30 m between the first two rollers and 50 m between the last two rollers) to achieve a better dispersion of the LDH platelets by shearing and to obtain a homogeneous hybrid component. Part B was added at 10phr (0.22 g) and homogenised using the mechanical mixer for 10 min at 1200 rpm. The silicone film and the silicone/hybrid composite film were prepared by casting onto a Teflon surface using an Elcometer 4340 automatic film applicator. The knife blade height was set to 200 µm and the casting speed was 30 mm/s. This twocomponent silicone film was cured at 80 °C for 2 hours. After curing, the film thickness (90  5 m) was measured using an Elcometer 456 coating thickness gauge.

Characterisation X-ray diffraction

LDH powders were characterised by X-Ray Diffraction; the XRD patterns were recorded with a Philips X-Pert Pro diffractometer operating with Cu-Kα radiation (λ = 1.5418 Å). The data were collected in a 2θ range between 5° and 70° with a step size of 1°/min.

Thermogravimetric analysis

Thermogravimetric (TG) analyses were performed on a Setaram TGA 92 instrument with a linear heating rate of 5 °C.min -1 under air.

UV-Visible absorption

The UV-visible absorption spectra of the samples were recorded in the wavelength range of 200 to 800 nm with a UVvis spectrophotometer (SP-3000 Plus) equipped with an integrating sphere and UV-Probe software.

Luminescence

Quantum yield efficiencies and emission spectra were measured using a C9920-02G PL-QY integrating sphere measurement system from Hamamatsu Photonics. The setup consisted of a 150 W monochromatized Xe lamp, an integrating sphere (Spectralon coating, ⌀= 3.3 in.) and a high-sensitivity CCD camera.

Emission stability of films under LED irradiation

Reliability studies were carried out using a home-made setup consisting of a power-controlled blue LED emitting at 450 nm as the excitation source, and a HR4000 high resolution spectrometer (Ocean Optics) as the PL analyser. The samples were positioned on a heating element whose temperature was adjusted to 80 °C. The emission spectra of the composite films were collected every 20 min over a period of 24 h. Their area was integrated to obtain the total emission intensity. LED power was measured using a Scientech Model Mentor MA 10 with a MC2501 calorimetric head unit (25.4 mm aperture). Measurement was performed by centring the head unit over the LED source and measuring the LED power of light emitted through the aperture. The blue LED power was 183 mW. The LED power density expressed in W/m 2 was calculated by LED power (in watt) per unit surface of the sample (0.25 cm ). According to our experiment, the LED power density was 2480 W.m -2 for blue LED.

Results and discussion

Concentration screening of SRB in LDH matrix XRD pattern As mentioned in the introduction, this kind of photoactive molecule is known for its efficient incorporation into LDH matrices. 3,[START_REF] Yan | Sulforhodamine B Intercalated Layered Double Hydroxide Thin Film with Polarized Photoluminescence[END_REF] The dispersion provided by hosting photoactive molecules in LDH has already been demonstrated 10 to improve 1.2 nm the luminescent properties of dyes such as fluorescein. In the same vein, the hosting of SRB molecules in Zn 2 Al is expected to lead to a luminescent hybrid powder. Additionally, the DS spacer is here co-intercalated to further homogeneously disperse SRB molecules within the inter-lamellar space. The DS surfactant is known to hold apart the stacked layers that could subsequently be of interest when such a hybrid assembly is used as filler and dispersed into a polymer.

Firstly, the optimal DS/SRB ratio leading to the best luminescence properties was established through the absolute photoluminescence quantum yield values (PL QY abs ). To this aim several syntheses were performed, varying the molar ratio x =n SRB /(n DS +n SRB ) with 0.0005 < x < 0.02. The XRD patterns between 2 and 70° in 2θ of each LDH-DS-SRB sample made, as well as the template LDH-DS powder, are presented in Fig. 2. Series of harmonic peaks (00l) are well observed (Fig 2 . insert) and result in a basal spacing of 2.5 nm, in agreement with the literature for LDH/DS. 29 However, the SRB content is too small to be observed, the basal spacing being imposed by the spacer alone.

Absolute Photoluminescence Quantum Yields Variations in the absolute photoluminescence quantum yields (PL QY abs ) as a function of the excitation wavelengths are shown for each sample in Fig. 3a.

Three wavelengths are of interest: 365 nm and 480 nm wavelengths correspond to UV and blue commercial LED respectively, while 570 nm is the excitation wavelength leading to the best PL QY ab . The PL QY int decreases with an increase in SRB content, whereas Absorbance increases with the SRB amount for both excitation wavelengths, 480 and 385 nm. Depending on the excitation wavelength, since the PL QY abs is obtained by the multiplication of PL QY int (Fig. 3c) and Absorbance (Fig. 3d), a maximum of PL QY abs is reached for x SRB = 0.0005 with a value close to 60 % for λ exc = 570 nm, whereas for λ exc = 365 nm and 480 nm the maximum of PL QY abs corresponds to x SRB = 0.002. For λ exc = 480 nm, PL QY abs luminescence decreases when the SRB amount becomes higher than x = 0.002. This observation could be explained by a concentration quenching phenomenon, which is predominant when the concentration of SRB increases above a certain threshold. The same behaviour was evinced in the case of fluorescein molecules incorporated into LDH galleries. 10 Indeed, when intercalated SRB molecules are sufficiently close to each other in such a restricted space, they are prone to aggregation by π-stacking (parallel stacking) due to conjugated xanthene rings or by hydrogen bonding (head-to-tail). Xanthene dyes such as SRB or fluorescein are well-known 30 to interact mutually, resulting in luminescence quenching when the concentration reaches a critical value to form dimers, trimers, etc. Although the monomer yields luminescence properties, most aggregates led to the extinction of the optical properties due to non-radiative energy transfer between the dye molecules. 31 This phenomenon increases with the dye concentration in solution, and luminescence totally disappears for SRB powder. The restricted space supplied by the LDH host reduces the SRB molecular packing and acts as a dilute medium in dispersing the SRB dyes. An optimal value of SRB of x = 0.002 is found, corresponding to about 25 times less than the values previously reported; [START_REF] Yan | Sulforhodamine B Intercalated Layered Double Hydroxide Thin Film with Polarized Photoluminescence[END_REF] all the reported values were surprisingly high, corresponding here to the extinction of luminescence for the SRB molecules. All the excitation wavelengths studied result in an optimal PL QY abs for a very low concentration of photoactive molecules in the LDH inter-lamellar space. In all cases, a compromise between a high internal photoluminescence quantum yield (isolated SRB molecules in the inter-lamellar-space) while maintaining an appropriate absorbance (given for a significant amount of SRB in the LDH-DS-SRB samples) must be reached (Fig. 3).

Optical properties

In the following section, the optical properties of the LDH-DS-SRB sample with the optimized luminescence properties under blue excitation (480 nm), LDH-DS-SRB (x = 0.002), defined as LDS in composite are presented.

Absorption properties

In order to study the absorption of the SRB molecules in the LDH-DS-SRB powder, the absorption spectrum was recorded in solid state through an LDH-DS-SRB-KBr pellet. This was prepared by the dispersion of the pristine LDH-DS-SRB (0.01 % w ) in KBr powder. The UV-visible absorption spectrum obtained for the pellet is presented in Fig. 5 and compared to that of SRB in aqueous solution (10 -4 M). As reported by several authors such as K. Ray et al.

32

, V. Paolucci et al. 23 or Wittman et Penzkofer 33 , in dilute aqueous or alcohol solutions, the absorption spectrum of SRB is characterised by a typical profile, as shown in Fig. 5 (blue line). Indeed, the absorption band at 575 nm is linked to the S 0 -S 1 absorption of monomers, whereas the obvious shoulder at 535 nm comes from the S 0 -S 1 absorption of aggregates (dimer, trimer…) 34 . Regarding the UV-Visible absorption spectrum of LDH-DS-SRB powder, all the distinctive absorption bands of SRB molecules are observed. However, the monomer/aggregate ratio increases significantly, due to the presence of numerous aggregates in the LDH-DS-SRB powder, where the dilution of SRB is worse than in the aqueous solution (10 -4 M). Other absorption bands appear between 200 and 450 nm, and can be assigned to the absorption of photons from the fundamental state to more energetic S n excited states. 35 Furthermore, an increase in absorption between 200 and 250 nm is attributed to the absorption of Zn 2+ and Al 3+ cations from the LDH matrix. 36 Thus, absorption properties are not modified but less intense after SRB intercalation.

Emission spectra

The normalized emission spectra of LDH-DS-SRB powder and SRB in aqueous solution (10 -4 M) were recorded under blue excitation (480 nm) as shown in Fig. 6. The emission spectrum of SRB exhibits a broad and asymmetric band between 550 and 750 nm, with a maximum at 625 nm and a large shoulder at around 655 nm. As described by Yan et al. [4], these contributions are related to different environments for SRB molecules. The main peak located at the highest energy point corresponds to the SRB monomers, while the shoulder is attributed to the aggregated forms (dimers, trimers, ...). Compared to the LDH-DS-SRB matrix, the emission spectrum (Fig. 6, line red) has a similar profile. However, the fluorescence maximum is red-shifted (by 30 nm) and the ratio between the two main emission bands is modified. Indeed, the contribution to the longer wavelengths increases. We can conclude from these observations that the aggregated forms of SRB are in slightly greater quantity for SRB in the LDH matrix compared to SRB in dilute aqueous solution (10 -4 M).

These aggregated forms of SRB are responsible for non-radiative de-excitation processes that are accompanied by a decrease in light emission properties. Similar behaviour has been identified for the intercalation of fluorescein into a LDH host with the formation of J-type and H-type aggregates. 10 As shown in Fig. 6b), LDH-DS-SRB powder (x = 0.002) and SRB in solution both exhibit CIE colour coordinates associated with a red-orange emission. In accordance with its emission spectrum, the sample leads to a more reddish colouring than the SRB solution. The SRB content in the LDH host matrix is clearly shifting the coordinates (x;y) in the CIE 1931 plot of each sample (Fig. S1). An increase in SRB input tends to accentuate the red contribution of the emitted light under blue excitation. This behaviour is also observed when the SRB molecules are encapsulated in silica spheres 24 , due to aggregation effects, whereas when the SRB molecules are attached to PAMAM dendrimers, 23 the emission is then blue-shifted. In this latter case, the luminescence mechanism is modified because of electron transfers between the dye molecules and the core dendrimers.

Composite silicone/LDH-DS-SRB film (x = 0.002) To further assess the possible interest of the selected LDH-DS-SRB (x = 0.002) powder for a LED application, a composite film was prepared by dispersing the powder in a silicone elastomer (PDMS-type) matrix (according to the protocol described in the experimental part). A composite film, named Si-LDS, with a thickness of 90  5 m was obtained by mixing 20 wt. % LDH-DS-SRB (x = 0.002) and 80 wt. % silicone. The photographs of this composite film are reported on Fig. S2 under daylight and UV radiation (365 nm) showing the optical properties. The SEM image (Fig. S3) of the film confirms the homogenous dispersion of the particles (LDH-DS-SRB) in the elastomer. The emission spectrum of the Si-LDS composite film (Fig. 6a) was recorded under excitation at 480 nm. Comparison of this spectrum with that of the LDH-DS-SRB powder (x = 0.002) shows a shift towards shorter wavelengths. This phenomenon can be explained by a disaggregation effect of the LDH platelets, and consequently by an increase in the distance between SRB molecules induced by the rolling step during film preparation. Moreover, the contribution at the longest wavelengths previously attributed to the aggregated forms of SRB is less intense in the film than in the powder. Both observations agree with the fact that the fabrication process of the composite film allows the SRB molecules to disaggregate, thus providing an additional dilution effect by reducing the nonradiative energy transfers.

As a result, the chromaticity diagram exhibits a slightly more orange-coloured dot for Si-LDS film than for LDH-DS-SRB powder (Fig. 6b). PL QYabs of Si-LDS composite film PL QY abs of the Si-LDS composite film were recorded for excitation wavelengths from 300 to 600 nm (Fig. 7). The film exhibits the same values as for the LDH-DS-SRB (x = 0.002) powder, meaning that the dispersion in silicone did not modify photoluminescence behaviour. For the composite film, the decrease in absorbance by dispersing the LDH-DS-SRB (x = 0.002) powder is counter-balanced by a decrease in the non-radiative energy transfers, thus explaining why similar PL QY abs values are obtained (Fig. S4).

White light emission produced by the association of a blue LED chip and two silicone films

As shown previously, the emission spectrum of LDH-DS-SRB covers a wide wavelength range from yellow to red. It is of interest to extend the yellow emission of the commercial phosphor YAG:Ce (Fig. 8) to the red wavelength range. Thus the combination of a blue LED chip with these two phosphors, YAG:Ce and LDH-DS-SRB, is expected to yield a warmer white emission associated with a better CRI. In order to assess this combination, a YAG-Ce composite film with a 200 m thickness (labelled Si-YAG) composed of 43 wt. % commercial powder and 57 wt. % bi-component silicone matrix was produced. This loading rate was optimised in previous work. 37 Si-LDS and Si-YAG films were placed on a 450 nm commercial LED in remote-phosphor configuration as shown in Fig. 9.

The photometric parameters of three different configurations were recorded:

 Blue LED /Si-YAG (Fig. 9 b).



Blue LED/Si-YAG/Si-LDS (Fig. 9 c).



Blue LED/Si-LDS/Si-YAG (Fig. 9 d). The emission spectra of these three configurations, as well as a picture of the device, are presented in Fig. 9. For the first LED/Si-YAG configuration, part of the contribution of the blue LED is not absorbed by the YAG: Ce composite Si-YAG film, and is transmitted (around 425 nm); the composite film also emits a large contribution. The colour of the resulting light is yellow-green, which is confirmed by the CIE 1931 coordinates (Fig. S5). When the Si-LDS composite film is added above the Si-YAG film, the presence of the SRB increases the red-orange range contribution in the global emission spectrum (Fig. 9 c), and the photometric parameters of the resulting light are modified, as reported in Table 1. However, when the Si-LDS composite film is located between the blue LED chip and the Si-YAG film, the global spectrum emission of the system is slightly different. Indeed, as we have shown in Fig. 5, green radiations are strongly absorbed by the SRB molecules, whereas blue radiations are moderately absorbed. Thus, in the LED/Si-YAG/Si-LDS configuration, the Si-LDS composite film absorbs part of the green emission from the Si-YAG composite film, which reduces the contribution of the latter, while in the LED/Si-LDS/Si-YAG configuration the re-absorption phenomenon is minimised and the contribution of the Si-YAG composite film is relatively higher. The positions of CIE 1931 coordinates of these three configurations are presented in Fig. S5. The benefit of the SRB in the blue LED + YAG:Ce 3+ system is clearly evidenced, since the position is shifted towards the well-known «blackbody» 38 curve, which leads to warmer white light emission as reported in Table 1.

The colour temperature and the colour rendering index (CRI) were also recorded for each of the configurations shown in Fig. 9. It is found that both parameters are tuneable with respect to the film stacking order. Indeed, the configuration relative to Figure 9 c) yields a warm white with an excellent CRI. Furthermore, it underlines that a CRI close to 90 is possible to achieve, due to the strong contribution in the red spectral range provided by the Si-LDS film. Si-LDS composite film stability In order to demonstrate the benefit of the hybrid LDH-DS-SRB powder in white LED devices, the stability of the Si-LDS composite film was studied under thermal and photonic stresses. Fig 10 shows the variations in PL QY int (%) as a function of the excitation wavelength for Si-LDS films before and after thermal treatment for 24 h at 80 °C and 120°C. Under 480 nm excitation (wavelength of a blue commercial chip), the Si-LDS film heat-treated 24h at 120°C lost approximately 43 % of its initial PL QY int, while it remained unchanged after exposure at 80 °C. As shown by the interpretation of the UV-Visible and infrared spectra of a silicone film before and after thermal treatment at 120°C, the polymer itself is not damaged under such thermal stress (Fig. S6). Hence, we can conclude that the reduction in optical properties should be correlated to a modification of the environment of the SRB molecules. Indeed, as the temperature increases, the SRB molecules in the Si-LDS film tend to aggregate, which promotes non-radiative de-excitation processes and reduces optical performance. This phenomenon is highlighted by the emission spectra in Fig. 11. In order to assess the stability of the Si-LDS composite films in operating conditions close to those encountered in LED devices, these latter, as well as a composite film made with pristine SRB dye dispersed in a silicone matrix (Si-SRB), were irradiated by a powerful blue LED (450 nm, power: 2480 W.m -2

) for 1400 minutes at 80 °C in a black chamber. The amount of SRB was identical in both samples. Emission spectra were recorded every 20 minutes, and the variations in emission spectra were plotted as a function of the irradiation time in Fig. 12 for both composites. The photoluminescence properties of the Si-LDS composite are stable under blue LED irradiation, and the fluorescence emission intensity of the film remains at 100 % of its original value after the first 24 hours of irradiation. The Si-SRB composite, where the SRB molecules are not protected by the LDH matrix, exhibits a decrease in its luminescence features. After 2600 minutes, the fluorescence intensity had decreased by approximatively 40 %. Thus, the added value of the LDH host matrix is well demonstrated, as is the stability of the Si-LDS composite film under thermal (up to 80 °C) and photonic stresses.

Conclusions

This study underlines the ability of a LDH host structure to co-incorporate concomitantly two organic molecules, DS and SRB. DS acts as a spacer, increasing the interlamellar space of the LDH sheets, thus propping apart the 2D-structure as well as rendering the LDH intergalleries organophilic. Moreover, the DS molecules are found to supply a dilution effect for SRB molecules, since the large quantity of DS forces the SRB molecules to be distant from each other, limiting their guest-guest interaction and their potential optical quenching. Experimentally, after precise screening, an optimized SRB input of x = 0.002 is found to provide the hybrid LDH-DS-SRB with a suitable red emission under blue excitation.

The role of LDH in the production of the composite luminescent and homogeneous films is highlighted here. LDH platelets facilitate the dispersion of the SRB molecules in the silicone, while the SRB molecules tend to aggregate in the absence of LDH. Interestingly, Si-LDS films provide better photometric performances (CRI = 89.4 and Tc = 2700 K) for white LED by enriching the emission spectra in combination with a YAG:Ce 3+ composite film and a blueemitting LED. The composite films were found to be stable under photonic and thermal stresses applied to the LED device (100 % of the initial luminescence after one day of exposure), underlining clearly the stabilisation effect supplied by the LDH host matrix.

Finally, these findings open new perspectives for further improving LED technology. The stability of the fabricated composite films should now be investigated in depth to address the real potential of SRB molecules relying on an organomodified LDH toolbox.

11

 11

2

 2

Figure 1 .Figure 2 .

 12 Figure 1. Schematic representation of SRB dye in 2D (a) and 3D (b). SRB (Fig. 1 a) and b)) is an organic dye composed of a planar xanthene backbone with a benzenedisulfonate group.

Fig. 4

 4 Fig. 4 illustrates the variation in PL QY abs as a function of the SRB amount in the LDH host structure for λ exc = 480 nm.

Figure 3 .

 3 Figure 3. (a) Variation in PL QYabs as a function of the excitation wavelength for LDH-DS-SRB samples and variation in PL QYabs (b), internal photoluminescence quantum yield PL QYint (c) and Absorbance (d) as a function of the SRB content (x) for λexc = 480 nm and λexc = 365 nm.

Figure 4 .

 4 Figure 4. Variation in PL QYabs as a function of x for λexc = 480 nm.

1 Figure 5 .

 15 Figure 5. Absorption spectra of SRB in aqueous solution (10 -4 M) (blue line) and of the LDH-DS-SRB powder (x = 0.002) (red line).

Figure 6 .

 6 Figure 6. a) Emission spectra of the LDH-DS-SRB powder (x= 0.002) (red line), SRB in water solution (10 -4 M) (blue line) and the silicone Si-LDS film (purple line); b) CIE chromaticity coordinates of LDH-DS-SRB powder (x=0.002), SRB in water (10 -4 M) and Si-LDS composite film

Figure 7 .

 7 Figure 7. Variation in PL QYabs for the Si-LDS composite film and the LDH-DS-SRB (x = 0.002) powder.

Figure 8 .

 8 Figure 8. Emission spectra of LDH-DS-SRB (x = 0.002) powder and a commercial phosphor YAG:Ce (λexc = 480 nm).

Figure 9 .

 9 Figure 9. Emission spectra of a) commercial blue LED, and white LED prototypes designed by combinations of a commercial blue LED (450 n m) with b) Si-YAG and c) and d) Si-LDS composite films.

Figure 10 .

 10 Figure 10. Variations in PL QYint as a function of the excitation wavelength for Si-LDS film and Si-LDS films heat-treated at 80 °C and 120°C for 24 hours.

Figure 11 .

 11 Figure 11. Emission spectra of Si-LDS films before and after thermal treatment for 24h at 120°C (exc = 480 nm).

Figure 12 .

 12 Figure 12. Variations in emission spectra plotted as a function of the irradiation time for both composites Si-LDS (a) and Si-SRB (b). The «sawtooth»-like shape is explained by the variation in blue LED power over time.

procedure of LDH hybrid materials using the coprecipitation method

  MaterialsSulforhodamine B sodium salt C 27 H 29 N 2 NaO 7 S 2 and sodium dodecylsulfate CH 3 (CH 2 ) 11 OSO 3 Na were purchased from Sigma-Aldrich, and YAG:Ce 3+ from Phosphortech. Zn(NO 3 ) 2 .6H 2 O (purity 99.9+%), Al(NO 3 ) 3 .9H 2 O (purity 99.9+%) and NaOH (97%) were obtained from Sigma Aldrich. The two-component silicone elastomer, Bluesil RTV 141 part A and part B, was supplied by Elkem. The series of sulforhodamine B (SRB) -dodecyl sulfate (DS) LDH phases, Zn 2 Al-DS-SRB, called LDH-DS-SRB, was prepared by coprecipitation with various DS/SRB ratios. The synthesis of [Zn 2 Al 1 (OH) 6 ]

	Synthesis

+ [DS -] 1-x [SRB -] x •mH 2 O was performed using 50 mL of sulforhodamine B and dodecylsulfate aqueous solution (with de-ionized water). 50 mL of an aqueous solution of Zn 2+ (3.2 mmol) and Al +3 (1.6 mmol) was added dropwise over a period of 3 hours under magnetic stirring. For instance, to synthesize the [Zn 2 Al 1 (OH) 6 ] + [DS -] 0.999 [SRB -] 0.001 •mH 2 O sample, 1.6x10 -3 mmol and 1.5984 mmol were used for SRB salt and sodium dodecylsulfate, respectively.

Table 1 .

 1 (x:y) coordinates, correlated colour temperature (CCT) and colour rendering index (CRI) of the three presented configurations.

	Configuration	x : y	CCT (K)	CRI
			4440
	Blue-LED/Si-YAG	0.39 : 0.50		58.2
			neutral white
	Blue-LED/Si-YAG/Si-LDS	0.47 : 0.42	2700 warm white	89.4
	Blue-LED/Si-LDS/Si-YAG	0.42 : 0.45	3690 warm white	73.9

Acknowledgements

This work was supported by CPER DEFI MMASYF through its 2016 «MetaProfile» project. The authors would therefore like to thank the European Union, within the framework of the European Regional Development Fund (ERDF), and the Région Auvergne Rhône-Alpes, which co-funded this project.

Conflicts of interest

There are no conflicts to declare.


Paul Legentil

Fabrice Leroux

Sandrine Therias

Rachid Mahiou

Geneviève Chadeyron

Sandrine Thérias

Revisiting fluorescein and layered double hydroxide using a synergistic approach: A complete optical study

come   L'archive ouverte pluridisciplinaire

Introduction

In recent years, global warming on Earth has been causing major environmental problems. To respond to this growing concern, scientists orient their research towards sustainable solutions, and develop processes to save energy. Almost 20 % of the electricity consumption in the world is due to the lighting sector, explaining why light-emitting diodes (LED) have emerged to provide efficient and sustainable products replacing common lighting devices such as incandescent bulbs. However, commercial LEDs are manufactured using rare earth-based phosphors; these elements are expensive and not eco-friendly due to their extraction process and their life-cycle assessment. They might be replaced by organic fluorescent molecules that are cheaply available and sustainable. Even if these molecules exhibit high photoluminescent quantum yields, they nevertheless suffer from a lack of robustness in standard operating conditions, thus greatly limiting them as efficient phosphors in marketed lighting devices.

Fluorescein is a very common organic dye which has met with great interest for many years in a wide range of applications thanks to its biocompatibility, high molar absorptivity and excellent fluorescence quantum yield [START_REF] Jin | White light emission from a blue LED, combined with a sodium salt of fluorescein dye[END_REF].

Fluorescein is an organic molecule belonging to the xanthene family and is used as a probe in medical and biological applications (optical imaging, detection of analytes…) [START_REF] Spaide | Retinal vascular layers imaged by fluorescein angiography and optical coherence tomography angiography[END_REF][START_REF] Komatsu | Design and Synthesis of Highly Sensitive and Selective Fluorescein-Derived Magnesium Fluorescent Probes and Application to Intracellular 3D Mg2+ Imaging[END_REF][START_REF] Kaines | The Use of Ultra Wide Field Fluorescein Angiography in Evaluation and Management of Uveitis[END_REF]. Fluorescein is a red powder in the solid state, emitting a strong yellow-green light in liquid solution (in water at basic pH or ethanol for example) at low concentration (below 10 -3 M) and under UV-blue excitation. At higher concentrations, a quenching phenomenon appears to reduce its light emission, and the solid powder does not luminesce at all. In solution, the dye can take several prototropic forms depending on the pH [START_REF] Klonis | Spectral properties of the prototropic forms of fluorescein in aqueous solution[END_REF]. The dianionic form is the most luminescent one, and its contribution to the dye total emission exceeds the others. A shoulder on the edge of the main peak (emission of the dianionic form) may be observed, indicating the presence of the monoanionic form [START_REF] Rodrigues | Excited-State Proton Transfer of Fluorescein Anion as an Ionic Liquid Component[END_REF]. Indeed, the monoanionic form also exhibits luminescent properties but its emission is much weaker, and the global emission spectra are blue-shifted with a broadening of the emission band compared to that of the dianionic form.

The optical properties of fluorescein could also lead to an interesting future in lighting applications, and particularly in the LED-based field. Indeed, fluorescein emits green-yellow radiation under blue excitation (λ max exc = 490 nm) compatible with a blue LED chip to produce a modulable (warm or cold) white light emission for lighting applications. Some studies [START_REF] Jin | White light emission from a blue LED, combined with a sodium salt of fluorescein dye[END_REF][START_REF] Uthirakumar | Yellow light-emitting polymer bearing fluorescein dye units: Photophysical property and application as luminescence converter of a hybrid LED[END_REF] have already emphasized the potential of fluorescein to replace phosphors associated with blue LEDs in commercial devices such as rare-earth based common YAG:Ce. However fluorescein, like organic molecules, is very sensitive to external photonic and thermal stress, and its luminescence quickly breaks down in use [START_REF] Smith | The conservative behaviour of fluorescein[END_REF], which has so far been a limiting factor for further development.

To overcome this issue, one alternative is to protect the dye from quenching and instability (i.e. avoiding aggregation, thermal degradation, and so on) using a solid inorganic matrix, such as for example using Layered Double Hydroxides (LDH), an anion host structure. Recently, photoactive molecules intercalated into LDH layers have drawn much attention for the development of nanophotosensitizers for therapeutic uses [START_REF] Gao | Nano-photosensitizer based on layered double hydroxide and isophthalic acid for singlet oxygenation and photodynamic therapy[END_REF], stimuli-responsive intelligent materials and luminescent sensors [START_REF] Zhao | Niflumic Anion Intercalated Layered Double Hydroxides with Mechano-Induced and Solvent-Responsive Luminescence[END_REF][START_REF] Gao | Layered host-guest long-afterglow ultrathin nanosheets: high-efficiency phosphorescence energy transfer at 2D confined interface[END_REF]. Some studies [START_REF] Lee | Fluorescein dye intercalated layered double hydroxides for chemically stabilized photoluminescent indicators on inorganic surfaces[END_REF] reported the intercalation of fluorescein molecules into LDH. These materials present an interest, especially for catalysis and environmental applications [START_REF] Yan | Highly luminescent covalently bonded layered double hydroxide-fluorescent dye nanohybrids[END_REF][START_REF] Conterosito | High-Throughput Preparation of New Photoactive Nanocomposites[END_REF][START_REF] Lonkar | Nanohybrids of phenolic antioxidant intercalated into MgAl-layered double hydroxide clay[END_REF]. Their general chemical formula is: [M(II) 1-x M(III) x (OH) 2 ] x+ [A n-] x/n •mH 2 O where M(II) is a bivalent cation such as Zn 2+ , Mg 2+ , Fe 2+ , etc.; M(III) is a trivalent cation such as Al 3+ , Fe 3+ , etc. and A n-is an anion. The structure is layered and therefore anisotropic, built from cations and hydroxides having a net positive charge caused by the partial substitution of bivalent cations by trivalent cations. To ensure electroneutrality, anions are located in the interlayer space of the LDH with the presence of water molecules. Since the Fluorescein molecule is an anionic dye, it may be hosted within the LDH structure. The idea here is to possibly tether and stabilize fluorescein molecules while optimizing their luminescence properties. It also allows fluorescein to be isolated from the outside environment (O 2 from air) to avoid any accelerated oxidation and improve the dispersion of the fluorescein in a polymer to produce homogeneous composite films for LED-based devices. LDH-based fluorescent and phosphorescent thin films (TFs) elaborated by layer by layer (LBL) method have been reported in the literature [START_REF] Yan | A strategy to the ordered assembly of functional small cations with layered double hydroxides for luminescent ultra-thin films[END_REF][START_REF] Gao | Ordered assembly of hybrid room-temperature phosphorescence thin films showing polarized emission and the sensing of VOCs[END_REF][START_REF] Gao | Layer-by-layer assembly of long-afterglow self-supporting thin films with dual-stimuli-responsive phosphorescence and antiforgery applications[END_REF]. Herein, we focused on self-supporting thin luminescent composite films LDH-Fluorescein/silicone which have been prepared by solution casting onto a Teflon surface using an Elcometer 4340 automatic film applicator. Indeed, from the point of view of the organic dye, LDH inorganic filler can be viewed as an anti-aggregation and protective agent, while from the LDH side, the dye appears as an organo-compatibilizing agent to better resist polymer dispersion.

In this paper, the synthesis and characterization of a hybrid LDH-Fluorescein (LDH-F) material are presented, and its optical properties are thoroughly examined. Among the several LDH, the Zn-Al host matrix was chosen for this study as it displays a higher crystallinity compared to the more usual hydrotalcite Mg-Al for example, thus allowing a more accurate characterization by XRD of the hybrid materials as well as the filler upon polymer dispersion. LDH was used as host matrix to prevent aggregation of fluorescein dye, allowing to avoid the quenching of the luminescence and acting as a protective environment. Indeed, the usual passive LDH filler becomes here a photofunctional active filler. The LDH matrix creates a dilute environment for the dye that is normally observed in an aqueous solution: the LDH provides the fluorescein molecules with an environment which is suitable for them to luminesce well. Although, several studies [START_REF] Shi | Optical pH Sensor with Rapid Response Based on a Fluorescein-Intercalated Layered Double Hydroxide[END_REF][START_REF] Shi | Tunable photoluminescence properties of fluorescein in a layered double hydroxide matrix and its application in sensors[END_REF][START_REF] Shi | Tunable Photoluminescence Properties of Fluorescein in a Layered Double Hydroxide Matrix by Changing the Interlayer Microenvironment[END_REF] about nanohybrids LDH-Fluorescein have been reported for different applications a complete understanding of the mechanisms involved is scarcely reported so far. In this work, the optical properties were deeply investigated and a better understanding is highlighted to unravel mechanisms responsible of the luminescence of the solid powder LDH-F. Indeed, a better understanding permits here the mechanisms responsible for the luminescence to be identified. It is also demonstrated from an analysis of the photostability that LDH platelets stabilize the dye under potential operating conditions.

Experimental section

Materials

The two-component silicon polymer, Bluesil RTV 141 part A and part B, was supplied by Elkem. Zn(NO 3 ) 2 .6H 2 O (purity 99.9+%), Al(NO 3 ) 3 .9H 2 O (purity 99.9+%) and NaOH (97%) were obtained from Sigma Aldrich. The fluorescein disodium salt C 20 H 10 Na 2 O 5 was purchased from Alfa Aesar.

Synthesis procedure of LDH hybrid materials using the coprecipitation method

The fluorescein layered double hydroxide phase, Zn 2 Al-F, was prepared by the coprecipitation method with different fluorescein concentrations. The synthesis of [Zn 2 Al 1 (OH) 6 ] + [NO [START_REF] Komatsu | Design and Synthesis of Highly Sensitive and Selective Fluorescein-Derived Magnesium Fluorescent Probes and Application to Intracellular 3D Mg2+ Imaging[END_REF] -] 1-x [F 2-] x •mH 2 O was performed using fluorescein disodium salt in 50 mL of de-ionized water. 50 mL of an aqueous solution of Zn 2+ (3.4x10 -2 M) and Al +3 (1.7x10 -2 M) was added dropwise over a period of 3 hours under magnetic stirring. For instance, to make the [Zn 2 Al 1 (OH) 6 ] + [NO 3 -] 0.999 [F 2-] 0.0005 •mH 2 O sample, 5.2x10 -1 mmol of fluorescein disodium salt was used.

The pH was maintained at 10.5 by adding 0.25 M NaOH during the synthesis processes. Coprecipitation was performed under nitrogen at 20°C. The mixture was centrifuged at 5,000 rpm for 5 minutes; the sedimented solid on the bottom of the flask was washed several times with de-ionized water until a clear and transparent supernatant was obtained. A paste was recovered and stored at 5°C. A small amount was dried overnight at room temperature to obtain a powder and to determine the proportion of the dry extract.

Elaboration of Silicone/HDL-F resin composite films

The LDH-F (0.05 %) powder or the similar paste was used to elaborate the polymer/hybrid composite material with different loadings. The two-component silicone polymer (silicone Bluesil-RTV 141 A&B) was composed of a viscous liquid, called part A, cured by a polyaddition reaction with a catalyser, part B.

The silicone film was prepared as follows: a mixture of part A and 10 phr of part B was prepared, homogenised with a mechanical mixer (Planetary Centrifugal vacuum Mixer "Thinky Mixer") for 10 minutes at 500 rpm. Next, the final composite film was prepared by casting onto a Teflon surface using an Elcometer 4340 automatic film applicator. The knife blade height was set at 200 µm and the casting speed was 30 mm/s. This two-component silicone polymer was cured at 80°C for 2 hours. The film thickness was measured using an Elcometer 456 coating thickness gauge.

The preparation and the curing process for the composite films were the same as for the silicone film.

2.4. Characterisation 2.4.1. X-ray diffraction LDH powders were characterised by X-Ray Diffraction; the patterns were measured with a Philips X-Pert Pro diffractometer operating with Cu-Kα radiation (λ=1.5418 Å). The data were collected in a 2θ range of between 5° and 70° with a step size of 1°/min.

Elemental analyses CHNS

Elemental analyses of LDH powders LDH-F were performed to determine the composition of samples with a Flash 2000 CHNS analyzers (Thermo Electron).

Thermogravimetric analysis

Thermogravimetric (TG) analysis were performed on a Setaram TGA 92 instrument with a linear heating rate of 5 °C.min -1 under air.

Fourier-transformed infrared spectroscopy

The infrared spectra were recorded using a Nicolet 5700-FTIR spectrometer with Omnic software. Spectra were obtained using a summation of 32 scans and a resolution of 4 cm -1 . LDH and fluorescein powders were studied using the KBr pellet technique.

In the case of silicone/LDH-F composite films, calibration of the thickness of the nonphotodegraded samples was performed by normalising the absorbance at 2140 cm -1 in IR spectra, which corresponds to a vibration band of the (Si-H) bending mode relative to the silicone. All the spectra were compared with an absorbance of 0.240 at 2140cm -1 .

UV-Visible absorption

The UV-visible absorption spectra of the samples were recorded in the wavelength range of 200 to 800 nm with a UV-vis spectrophotometer (SP-3000 Plus) equipped with an integrating sphere and UV-Probe software.

Luminescence

Quantum yield efficiencies were measured using a C9920-02G PL-QY integrating sphere measurement system from Hamamatsu Photonics. The setup consisted of a 150 W monochromatized Xe lamp, an integrating sphere (Spectralon coating, ⌀= 3.3 in.) and a high-sensitivity CCD camera. Excitation and blue-excited emission spectra were recorded with a Jobin-Yvon apparatus consisting of a Xe lamp operating at 400 W coupled with two monochromators (Triax 550 and Triax 180) and a cryogenically cooled charge-coupled device (CCD) camera (Jobin-Yvon Symphony LN2 series) for emission spectra and Hamamatsu 980 photomultiplicator for excitation spectra. Excitation spectra were corrected for instrument response and Xe lamp intensity using sodium salicylate. The resolution of the system was better than 0.1 nm in both emission and excitation configurations. Luminescence decays were recorded using second-harmonic generation on a pulsed Nd:YAG OPO Ekspla NT342A laser (3-5 ns pulse duration, 10 Hz, 5 cm -1 line width, 0.3 mJ-20 mJ in the UV-blue). The emitted photons were detected at right angles to the excitation and analyzed via an Edinburgh FLS980 spectrometer (Czerny-Turner monochromator, 300 mm focal length, 1200 grooves mm -1 grating and minimum pass band of 0.1 nm) equipped with Hamamatsu R928P PMT (200-850 nm range).

Scanning electron microscopy (SEM)

Scanning electron microscopy was performed with a ZEISS Supra 55 VP scanning electron microscope in high vacuum at 20 kV using a back-scatter electron detector (QBSD).

Irradiation of films

Polymer and composite photodegradation under artificial aging conditions was performed in a SEPAP 12-24 unit (ATLAS) by UV-visible light irradiation (λ>300 nm) in presence of oxygen at 60°C (controlled by a Pt thermocouple). The device comprised four medium-pressure mercury lamps (Novalamp RVC 400W) situated vertically at each corner of the chamber. The samples were placed on a rotating carousel positioned in the centre of the chamber. Wavelengths below 300 nm were filtered by the glass envelope of the lamps.

Results and discussion

XRD characterisation

Diffractogram results

Figure 1 shows the XRD patterns of LDH-fluorescein powders synthetized by coprecipitation. The XRD patterns are shown as a function of the fluorescein relative input. The latter is expressed by the mole ratio between fluorescein and Al 3+ cations used during coprecipitation. Diffraction planes (00l) are the fingerprints of the layered structure of LDH [START_REF] Aisawa | Intercalation of amino acids and oligopeptides into Zn-Al layered double hydroxide by coprecipitation reaction[END_REF]. For the sample with the least amount of fluorescein (a), the positions of 2 = 10.1° and 20.2° (i.e. 0.78 nm and 0.39 nm) are characteristic of a nitrate LDH phase, and assigned to diffraction planes (003) and (006), respectively. As the amount of fluorescein increases, these peaks at 10.1° and 20.2° progressively disappear, and a series of harmonic peaks is shifted to a lower angle and located at 5.5°, 11°, 16.5°, and 22° for the sample with the highest fluorescein content. For the intermediate contents (fig. 1b, 1c, 1d and1e), the XRD patterns overlapped and correspond to a combination of nitrate and fluorescein phases which are mixed. These XRD peaks can be integrated to calculate the amount of fluorescein present in the interlayer space (inset fig 1). A simple methodology based on pole concentration is used here. Indeed, for sample (h), the experimental XRD pattern does not show the nitrate signal, and the ratio of the integration of the first and second harmonics (003)/(006) of the fluorescein signals can be calculated, resulting in a constant . For other samples for which a combination of nitrate and fluorescein is observed, the (006) peak of the fluorescein signal merges with the (003) peak of the nitrate signal. Using the constant and the integration of the merged signals, the ratio (003) Fluo /(003) nitrate may be calculated for all the samples studied. These results are shown in the inset in figure 1.

For the series of harmonic peaks, the one at 2 = 5.5° leads to a basal spacing of 1.61 nm and is ascribed to diffraction plane (003), such values agreeing well with the basal spacing observed for fluorescein intercalated between LDH layers according to either Costantino et al. [START_REF] Costantino | Surface Uptake and Intercalation of Fluorescein Anions into Zn-Al-Hydrotalcite. Photophysical Characterization of Materials Obtained[END_REF] or Tanaka et al. [START_REF] Tanaka | Synthesis and cellular uptake of fluorescein intercalated layered double hydroxide[END_REF]. Indeed, Costantino et al. showed that fluorescein can be intercalated in a Zn 2 Al LDH matrix by the ion-exchange method from a LDH-perchlorate in a fluorescein solution. As in the present study, several concentrations of fluorescein were used, resulting in the presence of a pristine LDH phase with a fluorescein LDH phase exhibiting a basal spacing of 1.65 nm. A slightly lower value of 1.58 nm is observed for MgAl LDH. In this case and similarly to the present study, Tanaka et al. performed fluorescein intercalation by coprecipitation and reported the setting up of two distinct fluorescein phases depending on the pH of the synthesis, leading to two interlayer distances of 0.78 and 1.58 nm. According to Tanaka et al., guest accommodation within the interlayer domain for the former is horizontal to the inorganic sheets, while it is vertical (perpendicularly tilted) for the latter. Elemental analysis of powders was performed for some samples (sample a, d and h) to determine the experimental fluorescein amounts remaining in the LDH powders after the synthesis. Microanalyses CHNS allows to find the mass fraction of C, N and H elements contained in each sample. Elemental analyses results are reported in table 1. Carbonate pollution in interlamellar space is likely as XRD patterns shown and has been taken into account in the calculations. Thus, the experimental formula can be easily calculated by using the experimental molecular mass Mw determined with thermogravimetric analysis (see fig. S1 in the Supporting Information). For instance, the experimental formula of the sample d obtained is: Zn 2 2+ Al 3+ (OH -) 6 (CO 3 2-) 0.26 (NO 3 -) 0.22 (Fluo 2-) 0.13 , 1.5 H 2 O in making assumption that the ratio Zn/Al = 2 is fixed, in this way x = 0.13 for fluorescein amount whereas the theoretical value expected was x = 0.15. In the same way, experimental formula for sample h is Zn 2 2+ Al 3+ (OH -) 6 (CO 3 2-) 0.09 (NO 3 -) 0.0.06 (Fluo 2-) 0.38 , 2.8 H 2 O whereas theoretical amount of fluorescein was x = 0.5. The experimental calculations were distorted when the amount of fluorescein was too weak (for the sample a) otherwise the experimental values are closed from the theoretical values. Thus, the coprecipitation method for intercalation fluorescein is well effective.

XRD interpretation of fluorescein intercalation

From the above interpretations, the attribution of the former value to an intercalated fluorescein LDH phase is questionable, since it corresponds to the basal spacing of the nitrate form. Tanaka et al. argued that the amount of fluorescein was 16.5 % of the total interleaved anions and that the amount of fluorescein adsorbed onto LDH carbonate platelets represented only 2.70 %. However, it is known [START_REF] Hibino | Acid Treatment of Layered Double Hydroxides Containing Carbonate[END_REF] that carbonate anions present a high affinity towards the LDH framework and that they are not easily displaced unless acidic conditions or calcination are employed. Although Costantino et al. also worked with different concentrations of fluorescein, only one unique basal spacing of 1.65 nm was observed, although the pH of the ion-exchange reaction was the same (pH = 8) as the one for which Tanaka et al. observed a horizontal distribution of the fluorescein molecules with a basal spacing of 0.78 nm. The interpretation of Tanaka et al. is also in discrepancy with the present results, showing a unique stacked structure for a fluorescein loading of 10 %, which is comparable to some extent to the Tanaka et al. experimental conditions. It is interesting to note that for the low fluorescein content, the nitrate phase is observed. However, for a loading of 10 % fluorescein, a carbonate phase is clearly evidenced by the saw-toothed structure around 60° and attributed to diffraction peaks (110) and (103). Interestingly, carbonates anions displaced the initially interleaved anions when the interlayered gap was not fully occupied by the same anions (b, c and d), making some possible paths available within the interlayer space for the carbonate anions to diffuse.

To conclude this structural discussion, the basal spacing at 0.78 nm is characteristic of the nitrate LDH phase, and does not correspond to horizontally accommodated fluorescein molecules, since the space available, 0.30 nm (knowing that the thickness of the LDH cationic sheets is 0.48 nm), is too small for the cumbersome guest molecule and the not strictly planar fluorescein molecule (Figure 2) to be correctly lodged in the interlamellar space; this in addition to the low charge matching, which is not reasonably in favour of such a horizontal accommodation.

Luminescence properties

Emission spectra interpretation

Fluorescein powder does not exhibit luminescence properties; while in aqueous media the fluorescein dye is characterized by a bright yellow-green emission under blue excitation (fig. 3a) [START_REF] Klonis | Spectral properties of the prototropic forms of fluorescein in aqueous solution[END_REF][START_REF] De | Environmental effects on the aggregation of some xanthene dyes used in lasers[END_REF][START_REF] Math | Dual fluorescence and laser emissions from fluorescein-Na and eosin-B[END_REF]. As shown in Fig 3b andc, a yellow green emission is recorded for HDL/fluorescein powder upon excitation at 450 nm, which is the wavelength corresponding to the emission of a commercial blue LED for a potential further association.

Fluorescence emission studies were undertaken using an excitation wavelength of 450 nm (corresponding to the emission of a commercial blue LED). The emission spectra of LDH-fluorescein (0.05 %, 1.25 % and 35%) powders are shown in fig. 3c. For comparison, the emission spectrum of fluorescein in aqueous solution (10 -7 M, pH 10) has been added (fig. 3a). As the amount of fluorescein in the LDH phase increases, fluorescence intensity decreases and the spectra show a gradual red shift (fig. 3c). A high concentration of fluorescein in the LDH phase leads to a luminescence quenching phenomenon (as seen in fig. 3c). The emission spectrum of the 0.05% LDH-Fluorescein sample shows a predominant peak at 520 nm with a shoulder at 556 nm. A similar trend in the emission spectrum is observed for fluorescein in diluted solution (fig. 3a). As described by De et al. [START_REF] De | Environmental effects on the aggregation of some xanthene dyes used in lasers[END_REF][START_REF] De | Spectroscopic studies with fluorescein dye-Protonation, aggregation and interaction with nanoparticles[END_REF] the main emission is ascribed to the dianion while the shoulder contains several contributions, arising from the emission of some species due to a neutral form of the dye or some dye aggregates. The deconvolution of the broad emission band by Gaussian functions (fig. 3a andb) defines three contributions, both for the fluorescein in solution and for the LDH-F powder. However, as shown in table 2, the position and the intensity of each contribution are slightly different for the two samples, which can be explained by a change in the environment of the fluorescein dye. Each contribution could be ascribed to three different fluorescent species. Thus, based on previous work, we can consider that fluorescein luminesces through three forms which can be attributed, for the aqueous solution, to the dianionic form as well as the monoanionic and the neutral forms [START_REF] De | Spectroscopic studies with fluorescein dye-Protonation, aggregation and interaction with nanoparticles[END_REF]. In this case, the dianionic form is the most luminescent form (red curve/solid line in fig. 3a), whereas the monoanionic form (green curve/dashed line in fig. 3a) and the neutral form (blue curve/dash-dot line) are red-shifted with a lower intensity. For the LDH-F sample, the green (dashed line) contribution intensity becomes higher than the red one (solid line) indicating that the monoanionic population is higher than the dianionic population. This result can be explained by introducing the concept of "aggregates". Aggregation depends on many parameters, such as the structure of the monomer but also the nature of the solvent (if the environment is liquid), temperature, pH values and ionic strength. According to the Kasha and El-Bayoumi report [START_REF] Kasha | The exciton model in molecular spectroscopy[END_REF], several absorption peaks are observed in a solution containing aggregates. A monomer molecule has a singlet excited state, while a dimer will have two levels: one with lower energy than the monomer singlet excited state and one at higher energy, according to the molecular exciton model. Figure 4 illustrates the mechanism resulting from the aggregation of two single fluorescein molecules. Depending on the orientation of the molecules in the aggregate, one of these levels will be a permitted absorption transition, which will be forbidden for the others. For a dye such as fluorescein, which belongs to the xanthene family, aggregation is due to conjugated rings through the π-stacking effect. When dipoles of each molecule are arranged in a parallel manner (fig. 4a), only the transition to the higher energy level is allowed, and hence the electronic absorption transition in the dimer will be blue-shifted in comparison to the monomer [START_REF] Eisfeld | The J-and H-bands of organic dye aggregates[END_REF]. For aggregates in which the monomers are arranged in a head-to-tail manner (fig. 4b), only a transition to the lowest split level is permitted. As a result, the emission spectrum is redshifted with respect to the monomer. As mentioned by S. Das et al., the head to tail arrangement cannot be observed in LDH-F. Based on these results, the different contributions of the emission spectrum in figure 3b can be detailed as follows: the main emission peak can be assigned to the dianionic form (monomer), while the peak at the shorter wavelength can be attributed to the dianionic dimer arranged in a parallel manner, and the peak at the higher wavelength (590 nm) corresponds to the monoanionic monomer.

To examine the fluorescein environment close to LDH platelets, the excitation spectra were recorded and studied.

Excitation spectra

The excitation spectra of the LDH-F sample were recorded by monitoring three emission wavelengths (fig. 5). It appears that the excitation spectra depend strongly on the monitored emission wavelength. In any case, the excitation bands were located in the range lying between 375 and 530 nm and exhibited three contributions corresponding to the different arrangements of the fluorescein dye. As previously stated, we can easily attribute the band close to 490 nm to the dianionic form, and the shoulder around 475 nm to the monoanionic form of the fluorescein. The broad band located at 400 nm can be ascribed to another fluorescein form. Indeed, according to Sjoback et al. [START_REF] Sjöback | Absorption and fluorescence properties of fluorescein[END_REF] this excitation band may arise from dimers constituted of two aggregated monomers in parallel stacking. This assumption is in agreement with the findings of Kasha and El Bayoumi, who expected a blue-shifted band [START_REF] Shi | Tunable Photoluminescence Properties of Fluorescein in a Layered Double Hydroxide Matrix by Changing the Interlayer Microenvironment[END_REF]. We also could not exclude the possibility that part of the fluorescein was adsorbed onto the surface of the LDH layers. To attempt to discard or retain this possibility, the excitation spectra of calcinated LDH-nitrate and LDH-carbonate with adsorbed fluorescein were recorded (fig. 5b andc, respectively). These samples were prepared from calcinated LDH-nitrate and LDH-carbonate dispersed in fluorescein solution (10 -6 M). In these conditions, the fluorescein dye was only adsorbed at the surface of the oxide obtained by the calcination of the LDH or of the layers in the case of the LDH-carbonate. In the absence of any intercalation, the same behaviour was observed, meaning that the environment between intercalated and adsorbed fluorescein molecules in the LDH (fig. 5a) is similar. Stacking onto the platelets is surmised at this stage.

Luminescence Decays

All the excitation spectra shown in Fig. 5 clearly exhibit three broad contributions peaking at around 400, 450 and 490 nm. The intensities of these bands differ from one sample to another, but remain quite constant in position when the monitored emission is modified. Selective excitation is difficult to elucidate, since the three contributions are close in position and overlapping. Considering the sample of interest (Fig. 5a), the excitation at around 490 nm seems to be the most useful for an analysis of the dynamic processes. This position fits the absorption maximum observed for fluorescein in the xanthene major form in solution, as reported by Zhang et al. [START_REF] Zhang | Fluorescence Properties of Twenty Fluorescein Derivatives: Lifetime, Quantum Yield[END_REF], which is around 496 nm. It is attributed to the 1 S 0  1 S 1 absorption transition. The time decay of the fluorescein in solution is reported to be around 3-4 ns and must be measured using a high-frequency laser with ps pulse duration. These possibilities are not available with our measurement apparatus. Despite this, luminescence decays can be recorded for the LDH-F sample, indicating that the LDH host modifies the decay time of the fluorescein since the time range of luminescence decays extends to the ms range, as observed in fig 6 . To record the luminescence decays, three wavelengths were considered, one in the middle of the broad emission band at 535 nm and the two others on the high and low edge energy sides at 485 and 605 nm.

The deconvolution method was used to fit the decays by double exponential form, one in the rise and the second in the tail of the decays using the function I(t)= A -B.exp (-t/1) + C.exp (-t/2) , where A corresponds to the contribution of the noise and B and C are respectively the contributions of the rise and the long decay with the  1 and  2 time constants. The reliability of the fits is higher than 96%, the A contribution to the overall luminescence decays is less than  4.10 -3 % and can be l considered as negligible. The derived  1 and  2 values are reported in table 3. All decays have the same shape and show no dependence on either emission or excitation wavelengths. We can state that the recorded luminescence, due to the long luminescence decay time, is inherently phosphorescent, arising from the triplet state of the fluorescein. Such consideration has been reported by D-J. Lougnot et al. [START_REF] Lougnot | Photoionization of fluorescein via excited triplet and singlet states[END_REF] which have estimated that absorption-emission-decaying 1 S 0  1 S 1 processes occur during the laser pulse when its duration is larger than the photoionization process. However, the transition emission arising from the 3 T 1 state should be accompanied by a red shift for the emission. To the best of our knowledge, only one paper reports the position of the 3 T 1  1 S 0 transition in the case of fluorescein, which lies at around 625 nm. Our measurements of the luminescence emissions show clearly that no such notable shift was observed between fluorescein in solution and that in LDH-F. Moreover, the luminescence decays of the fluorescein in solution exhibit the same shape than that of fluorescein intercalated in LDH.

Considering the decay results, the rise is always similar at around 700 ns. The nature of this rise was the subject of further experiments, notably by checking Time Resolved Spectroscopy. The long decay time constant is around 700 ns. The two values are similar, indicating that that the species present in the LDH-F are probably thermalized in terms of energy levels and exhibit mutual energy transfer. The increase in fluorescence lifetime in LDH-F compared to the reported results concerning fluorescein in solution is an indication that the photoluminescence quantum yield is reduced.

Absolute photoluminescence quantum yields

The absolute photoluminescence quantum yields (PL QY ab ) of LDH-F powder and paste samples with varying amounts of fluorescein (between 0.05% to 35% of the occupied interlamellar space) were recorded upon excitation in the 250-500 nm range, as shown in fig. 7. For all the samples (fig. 7a for powders and 7b for pastes), the PL QY ab evolution as a function of the excitation wavelength, leads to the same profiles. The absolute quantum yield of the fluorescein in water-based solution of 10 -4 mol. concentration, upon 450 nm excitation, is of about 65%. The emission spectrum of LDH-F (fig. 3b) is the same as the emission spectrum of fluorescein in solution (fig. 3a) under such excitation, indicating that the intercalation doesn't modify the main electronic structure of the emitting species. For the LDH-F the LDH layers to some extent confine the fluorescein molecules on the basis of host-guest interactions (such as ionic interactions), and the anionic state, molecular configuration and microenvironment of interlayer fluorescein may be totally different from the fluorescein solution.

The samples with a very small amount of fluorescein exhibit only photoluminescence properties. Indeed, the amount of fluorescein must be lower than 1 % in the LDH to record a PL QY ab higher than 5% for either powder or paste. For both samples, the best PL QY ab is reached for an excitation wavelength of 490 nm, corresponding to maximum fluorescein absorption. The maximum values of absolute quantum yield were measured for LDH-F (0.05 %) paste, with a value of 21.4 %. PL QY ab is higher for the paste sample than for the powder, and two assumptions can be suggested. The first concerns the effect of water in the paste; indeed, the paste produced after centrifugation contains roughly 80 wt. % of water and 20 wt. % of powder. In this case, the fluorescein molecules are in an environment that is close to that of a solution, explaining why luminescence is increased. The second possibility relies on the reflective index (RI). The RI of water is 1.33, against 1 for air; thus, the paste which contains water will absorb more photons. Accordingly, absorption is greater for the paste and its PL QY ab is improved.

The intercalation of fluorescein in such low concentrations (0.05 %) cannot be detected from XRD patterns, where only the nitrates are visible (as seen in fig. 1). Nevertheless, we can suppose, based on the emission and excitation spectra that the fluorescein molecules are in close proximity to the LDH, most probably adsorbed onto the LDH platelet surfaces.

We have demonstrated that the fluorescein dye must be present in the LDH in small quantities to provide luminescence properties. Such behaviour can be explained by a concentration quenching phenomenon, which is predominant when the concentration of fluorescein dye increases. When the fluorescein molecules are closed, they formed aggregates by π-stacking (parallel stacking) due to conjugated xanthene rings or by hydrogen bonding (head-to-tail) with carboxylic acid and alcohol groups [START_REF] De | Environmental effects on the aggregation of some xanthene dyes used in lasers[END_REF]. Fluorescein is well-known [START_REF] De | Spectroscopic studies with fluorescein dye-Protonation, aggregation and interaction with nanoparticles[END_REF] for self-quenching its luminescence when the concentration reaches a sufficiently high value to form these aggregates of monomer molecules such as dimers, trimers etc. Although the monomer exhibits good luminescence properties, all the aggregates led to an extinction of the optical properties due to non-radiative energy transfer between the dye molecules [START_REF] Das | Controlling J aggregation in fluorescein by bile salt hydrogels[END_REF]. This phenomenon increases with the dye concentration in solution, and for fluorescein powder no luminescence is observed. As a conclusion, we have shown that the lamellar space of the LDH host matrix can be considered as a dilution medium for the fluorescein dye, and plays the same dispersing role as that of an aqueous solution.

Composite film LDH-F/silicone

Luminescence properties

The LDH-F powder presenting the best absolute quantum yield efficiency value was used in this section. TEM (see fig. S2 in the Supporting Information) revealed that LDH-F powder consisted of pseudo-hexagonal nanosheets stacked on top of each other. The LDH-F (0.05 %) was dispersed in a silicone resin to prepare the composite films. The thickness of the composite films ranged from 100 to 120 µm. Composite film homogeneity as well as the random orientation dispersion of the LDH platelets has been shown by SEM (see fig. S3 in the Supporting Information). In order to get a composite film suitable for a commercial blue LED, the LDH/dye nanohybrids were dispersed in a polymer such as silicone. The PL QY ab of the composite films with variable amounts (between 10 % to 60 %) of LDH-F (0.05% powder), were recorded under excitation in the 250-500 nm range, as shown in fig. 8a. The best PL QYab was achieved on excitation at 490 nm for each composite film, and increased gradually with the quantity of LDH-F particles in the silicon matrix. This could be explained by an increase in the absorption of photons with the increase in filler loading. On the other hand, when the percentage of LDH-F particles was too high (> 40 wt. %), the mixture of powder and silicon became inhomogeneous and the composite films could not be hardened, due to excessive quantities of nanoparticles compared to the silicone resin. Thus, the percentage of LDH-F (0.05 %) particles dispersed within the silicone resin is fixed at 20 wt. % in the following. Moreover, the PL QY ab recorded for the homogenous composite film elaborated with a loading of LDH-F of 40 wt. % (PL QY ab =25%) is higher than that obtained for the powder and paste LDH-F (0.05 %) samples. The value of 25% for the PL QY ab remains low for general lighting LED but could be of interest for other LED applications such as display and anticounterfeiting for example. We can also add that the silicone matrix enables the creation of a dilution medium for the LDH-F powder, thus improving its luminescent properties.

LDH platelets exhibit a good compatibility with the silicone matrix thereby providing a homogeneous dispersion. Previous studies demonstrated an efficient dispersion of organo-modified LDH within PDMS type polymer [START_REF] Naime Filho | Percolated non-Newtonian flow for silicone obtained from exfoliated bioinorganic layered double hydroxide intercalated with amino acid[END_REF]. To confirm the interest of the LDH matrix in the dispersion of fluorescein in the silicone matrix, a composite (silicone/fluorescein 0.02% ) film was prepared. The amount of fluorescein therein was comparable to the amount of pure fluorescein contained in the composite silicone LDH-F (0.05 %) film with 20 wt. % of filler loading. This composite silicone/fluorescein 0.02% film emits no luminescence at all, as shown in figure 8a, thus confirming the action of LDH as a "solid solvent" in diluting the fluorescein, resulting in greater luminescent properties for the composite films. In this 2D organized structure, the fluorescein dye is intercalated in the interlayer gallery space. In this configuration, the fluorescein molecules are spaced away from each other and the quenching due to the aggregation is avoided.

The emission spectrum of the composite films is similar to that recorded for the powder. Indeed, as for the LDH-F (0.05 %) powder, the three contributions of fluorescein are observed (fig. 8b).

Photostability of LDH-F/silicone composite films

The photostability of the fluorescein chromophore dispersed into the composite films represents one of the key parameters for potential application in LED device. To the best of our knowledge, this paper is the first study of the phostostability of LDH-F/silicone composite films. Composite films with LDH-fluorescein and LDH-nitrate and pure silicone film were irradiated in accelerated ageing conditions (λ>300, 60°C) to study the potential effects that LDH and fluorescein could have on silicone photooxidation.

The chemical modifications during irradiation were monitored by IR spectroscopy. Fig. 9a presents the FTIR spectra of a pure silicone film (without filler) as a reference during irradiation under accelerated artificial conditions. The initial spectrum is typical of a silicon polymer like PDMS. The two intense bands at 2960 and 2075 cm -1 are relative to the (C-H) vibration mode of the silicone. The region between 1800 and 2200 cm -1 reveals IR bands corresponding to (Si-O) and (Si-H) vibration modes. The two bands near 1405 and 1445 cm -1 are ascribed to CH 3 symmetrical bending vibration. The IR spectrum of silicone during the aging process presents a decrease in two bands, one at 2140 cm -1 relative to the (Si-H) vibration mode and one at 905 cm -1 (Si-O-Si). In fig. 9b andc, both IR bands are shown after subtracting the spectra from the initial spectrum (t = 0). To distinguish the thermal curing effect from the photooxidation of the silicone polymer, a post-curing experiment was performed at 80°C for 264 hours on the composite films and the pure silicone film. FTIR spectra were then recorded before and after post-curing. Both FTIR spectra of the pure silicon film are presented in fig. 10. The IR spectrum after post-curing shows the decrease in absorbance of the two IR bands at 2140 cm -1 and 905 cm -1 . Thus, the modifications of these two bands are due to a post-curing phenomenon and not to a photooxidation process. Fig. 11a shows the spectra of a silicone/LDH-F composite film during irradiation, and Figs 11b andc, present subtracted spectra. Before irradiation, the IR spectrum of the silicone/LDH-F composite film shows a broad and intense band around 3500-3600 cm -1 , assigned to the (O-H) stretching of water molecules and hydroxyl groups from the LDH matrix. At 1605 cm -1 , water from the LDH is observed, as well as the characteristic band at 1350 cm -1 of nitrate in LDH. The metaloxygen vibrations of LDH can be noticed in the region of 1000-400 cm -1 . During irradiation, two IR bands are modified at 2140 cm -1 and 905 cm -1 , as observed in the case of the reference silicone film. Moreover, a new IR band at 1725 cm -1 develops upon irradiation for the silicone/LDH-F composite film. This new IR band can be ascribed to photodegradation products of the polymer matrix. The modification observed for the broad and intense band around 3500 cm -1 may be due to a change in the hydration of the samples during irradiation.

Variations in the absorbance of the IR bands at 2140 cm -1 , 1725 cm -1 and 905 cm -1 during irradiation were plotted as a function of time for the silicone film and both composite films, silicone/HDL-F (0.05%) and silicone/LDH-N (with nitrate ions intercalated), and are shown in Fig. 12. A decrease in absorbance for the two IR band at 2140 cm -1 and 905 cm -1 was observed for the three films (pure silicone and the two composite films with LDH-F and LDH-N), with approximately the same kinetic curves, due to a post-curing phenomenon.

At 1725 cm -1 , the same modification is observed during photoaging between the silicone/LDH-F composite and the composite film with LDH-N, both adopting a similar kinetic. The band at 1725 cm -1 in the pristine silicone film is also slightly visible (Fig. 9b). Thus, the formation of this IR band in the composite film could be explained by the photodegradation of the silicone polymer, where the presence of an LDH matrix (in both cases, LDH-F and LDH-N) has a pro-degradant effect.

These results show that in accelerated aging conditions, photodegradation of the silicone matrix is observed in presence of LDH, either with LDH-F or with LDH-N. This effect is not due to the presence of fluorescein molecules in the composite.

Conclusion

In this study, it has been demonstrated that fluorescein molecules could be intercalated in large amounts into an LDH host structure, adopting a vertical orientation. However, only extremely small amounts of fluorescein in the lamellar space enable a yellow luminescent powder to be obtained. Fluorescein accommodation is then not detected by XRD. Indeed, from the optical properties of the LDH-F hybrid material, several emission contributions relative to different species are explained by a model in which the dianionic monomer, the monoionic monomer and the dianionic dimer forms of the fluorescein coexist. Thus, LDH brings a "solid solvation effect" to the fluorescein. The LDH-F hybrid material can be dispersed in a polymer matrix such as silicone to yield a luminescent composite film. LDH enables a homogeneous film to be produced, which was not possible with fluorescein alone. From the point of view of the organic phosphor, LDH layers provide dispersion avoiding quenching and migration while for LDH, the dye supplies the photoactivity. The dispersion of the LDF-F filler in the silicone matrix does not affect the optical properties of the hybrid material, the PLQY being even higher than for the powder. From the photoaging study, it has been shown that the presence of LDH-F or LDH-N filler modifies polymer photostability during irradiation, and that this is due to the presence of LDH platelets rather than being caused by the fluorescein molecules. Fluorescein was used as a model to study the added-value of hybrid materials based on a phosphor and an LDH host matrix. In this vein LDH host efficiently the organic phosphor. These results showed the potentiality of LDH as photofunctional active filler and this approach can be extended to other phosphors such as organic dyes or quantum dots. ), after 264h of photooxidation λ>300 nm, 60°C and after post-curing treatment in oven at 80°C.

LDH

Figure 3 :Figure 5 :Figure 6 :

 356 Figure 3: Emission spectra at 20°C with excitation at 450 nm of: a) fluorescein in water (10 -7 M) with deconvolution in three contributions. b) LDH-fluorescein (0.05%) powder with deconvolution in three contributions. c) LDH-fluorescein powder with three loadings of fluorescein in LDH host (0.1 % ___ , 1.25 % __ _ __ and 35% _ _ _ )

Figure 7 :Figure 8 :

 78 Figure 7: Absolute quantum yield depending on the wavelength with different amounts of fluorescein (i :0.05 %, ii: 0.1 %, iii: 1.25 %, iv: 10 for paste samples (a) and powder samples (b)

Figure 9 :

 9 Figure 9 : FTIR spectra during photooxidation at λ>300 nm, 60°C, of a pristine silicone film : a) between 400 and 4000 cm -1 b)between 1650 and 2400 cm -1 (after subtraction of the initial spectrum) and c) between 885 and 950 cm -1 (after subtraction of the initial spectrum).

Figure 10 :

 10 Figure 10 : FTIR spectra of a pristine silicone film (between 1600 and 2400 cm -1

Figure 11 :

 11 Figure 11: FTIR spectra during photooxidation at λ>300 nm, 60°C, of a composite film silicone/LDH-F (0.05%) (with 20 %w of LDH-F) : a) between 400 and 4000 cm -1 b) between 1650 and 2400 cm -1 (after subtraction of the initial spectrum) and c) between 885 and 950 cm -1 (after subtraction of the initial spectrum).

Figure 12 :Figure S1 :

 12S1 Figure 12: Variations of absorbance at 2140, 1725 and 905 cm -1 as a function of irradiation time for the composite film silicone/LDH-F (0.05%), the composite film silicone/LDH-N and the pristine silicone film

sample C (wt %) N (wt %) H (wt %) Mw (g.mol -1 ) a

		0.84	4.6	3.0	449.84
	d	0.93	10.5	3.20	334.13
	h	0.21	21.4	3.8	431.43

Table 1 :

 1 Elemental analyses obtained from CHNS analysis and molecular mass obtained by TGA analysis of LDH samples a, d and h.

	Emission spectra	λ max (nm) E (cm -1 )	FMWH (cm -1 )
	Fluorescein in water (10 -7 M)	Red contribution (dash) Green contribution (dot) Blue contribution (dash-dot)	514 535 564	19437 18706 17736	926 1453 2239
	LDH-fluorescein (0.05%) powder	Red contribution (dash) Green contribution (dot) Blue contribution (dash-dot)	518 545 582	19288 18358 17171	1083 1707 2660

Table 2 :

 2 Results of the emission bands deconvolution by the sum of three gaussian functions fig.3a and b

Formatting of funding sources

This work was supported by CPER DEFI MMASYF through its 2016 «MetaProfile» project. Thus, the authors thank European Union in the framework of the European Regional Development Fund (ERDF), and Région Auvergne Rhône-Alpes, which co-funded this project.

Time constants

Fitting goodness τ 1 (ns) τ 2 (ns) Adjusted R 2