Niall O'toole

Constance Lecourt

Yan Suffren

Francois Toche

Rodica Chiriac

Nicole Gilon

François Bessueille

Arnaud Brioude

Erwann Jeanneau

Dominique Luneau

François Toche

Cédric Desroches
email: cedric.desroches@univ-lyon1.fr

Intercalation of a manganese(II)-thiacalixarene luminescent complex in layered double hydroxides: synthesis and photophysical characterization

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Harnessing energy, and in particular energy of the electromagnetic spectrum, is of enormous societal and scientific significance, and methods of incorporating photoactive species into functional materials must be studied and optimised. There are numerous ways and means for the creation of this type of functional material. One of them is the preparation of so-called hybrid materials composed of an organic part and an inorganic part, each with a specific functionality. 1 For several years we have been working on the coordination chemistry of thiacalixarene. 2 One major result has been the discovery of the strong luminescence, emitted at approximately 600 nm (excitation at 350 nm), shown by tetra-nuclear clusters of Mn II ions sandwiched between two p-tert-butylsulfonylcalixarene macrocycles (ThiaSO 2 ), of formula [(ThiaSO 2 ) 2 (Mn II ) 4 F]K. The ThiaSO 2 ligands were found to play the role of antenna in inducing this luminescence. Spectroscopic studies have demonstrated that interactions between the cation and the anionic cluster [(ThiaSO 2 ) 2 (Mn II ) 4 F] -have an influence on the luminescence A c c e p t e d M a n u s c r i p t quenching by energy transfer from the excited complex to O 2 . These studies have shown that when the anion-cation separation distance increases such that the coordination interaction disappears, the molecular system becomes extremely sensitive to oxygen. 2b To further study and exploit this phenomenon, we envision the intercalation of the luminescent [(ThiaSO 2 ) 2 (Mn II ) 4 F] -anionic complex into the interlayer space of inorganic positively charged layered double hydroxide (LDH) materials. 3 Layered double hydroxides (LDHs) are composed of positively charged metal hydroxide layers containing divalent and trivalent cations such as Mg 2+ and Al 3+ and interlayer anions. The general formula is [M 1-x M' x (OH) 2 ] x+ (A m-) x/ m nH 2 O, where M is the divalent metal, M' is the trivalent metal, x is the degree of substitution, A is an anion of charge m, and n is the number of interstitial water molecules found in the cell. LDHs are unusual in that most materials of this structural type contain negatively charged layers. [START_REF] Xue | Layered Double Hydroxides[END_REF] The practical applications of LDHs are as varied as the potential components of the material.

[Mg 1-x Al x (OH) 2 ] x+ (A m-) x/ m nH 2 O are ideal candidates for the inorganic matrix to host functional anions such as this luminescent manganese-thiacalixarene cluster anion (Figure 1). The anionic nature of this luminescent species allows it to act as the interlayer anion in an LDH, offering a simple route towards the creation of a luminescent hybrid material. between the positively charged hydroxide layers would allow us to evaluate the luminescence and oxygen sensibility shown by our system within such a matrix. The luminescence of these hybrid materials could also be modulated via various stimuli, such as pressure resulting from interlayer spacing and host-guest interactions. Insertion between the inorganic layers may also give rise to an ordered assemblage of the luminescent anions, allowing for materials demonstrating polarized photoemission properties.

In this paper, we report the synthesis and photophysical properties of LDHs materials doped with intercalated [(ThiaSO 2 ) 2 (Mn II ) 4 F] - of formula Mg 0.71 Al 0.29 (OH) 2 (NO 3 ) 0.288 ((ThiaSO 2 ) 2 (Mn) 4 F) 0.002 1. A red-orange luminescence is observed for compound 1 and has been attributed to luminescence of the complex sandwiched within the LDH, originating from the 4 T 1 (t 2g 4 e g 1 ) → 6 A 1 (t 2g 3 e g 2 ) d-d transition of the manganese(II) ions.

We show the acute sensitivity of this luminescence to the presence of O 2 , and conversely demonstrate that it remains stable to photo-oxidation, opening perspectives for its as application in oxygen sensing or singlet oxygen generation.

Results and discussion

Synthesis

Synthetic routes to LDH@[(ThiaSO 2 ) 2 (Mn II ) 4

F] materials

Due to the insolubility of the luminescent [(ThiaSO 2 ) 2 (Mn II ) 4 F]K complex in most of the solvents compatible with the preparation of LDHs, we moved towards a two-step synthesis which consists of firstly preparing the LDH host matrix followed by an anion substitution to introduce our anionic complex. This second step, as described by Hibino,6

A c c e p t e d M a n u s c r i p t

There are many different synthetic methods towards LDHs, in part due to the incredible variety which is possible for this structural family. Hydrothermal synthesis seemed to us to be the most efficient. The reaction mixture consists of a salt, usually nitrate, of each desired metal (here Mg(NO 3 ) 2 and Al(NO 3 ) 3 ) and an amine which acts as an ammonia precursor compound. 7 The principle is that as the temperature increases, the amine is hydrolysed releasing ammonia into the solution thus raising the pH and bringing about the precipitation of the product. This method works better when the trivalent ion involved is aluminium; the amphoteric nature of the Al 3+ ion makes it an effective centre for precipitation of the product in the fluctuating pH solution. and washed with deionised water at least three times. Because of the strength of the attraction between the carbonate ion and the interlayer space of the LDH, this species is substituted with nitrate anions before the complex intercalation takes place. 9 To this end, the LDH-carbonate is suspended in an aqueous acid -salt solution of concentration 0.005 mol.L -1 NaNO 3 and 1 mol.L -1 HNO 3 under a neutral atmosphere (Ar or N 2 ). After stirring for 16 hours at room temperature, the product is recovered by centrifugation and washed with degassed water. From the nitrate material direct anion exchange with the anionic [(ThiaSO 2 ) 2 (Mn) 4 F] -cluster can be carried out. This entire process is done under inert atmosphere to avoid contamination with CO 2 from the surroundings, which would form carbonate anions within the LDH layers. First the LDH-nitrate is suspended in formamide (0.02 g in 50 cm 3 of formamide). Osmotic pressure of the formamide molecules pushes apart the LDH layers causing exfoliation, eventually leading to total dissociation. The resulting suspension shows Tyndall scattering when a beam of light is shone through it. A dimethylformamide solution of the luminescent complex (0.025 mol.L -1 )

A c c e p t e d M a n u s c r i p t is added to the LDH suspension, which immediately becomes cloudy as the hydroxide nano-sheets reform around the luminescent centres. Generally, for this type of procedure, the guest anion is dissolved in formamide. Unfortunately, our complex is insoluble in formamide; solubility has thus far only been observed in DMF or DMSO. Moreover, the solvent DMF on its own induces a tightening of the hydroxide layers. For these reasons a solution of complex in a minimum of DMF is prepared and added slowly. The intercalated product is the removed from suspension by centrifugation, washed with DMF and water several times and obtained as off-white micro-powder. Variations of temperature, agitation, order of addition and so on were attempted for this reaction, to optimise the conditions. The following is a generalised synthesis for these compounds derived from this optimisation: 24 h stirring time at 25°C seems to be the most effective parameters.

The formula Mg 0.71 Al 0.29 (OH) 2 (NO 3 ) 0.288 ((ThiaSO 2 ) 2 (Mn) 4 F) 0.002 of 1 was obtained by elemental analysis and indicates that only a small amount of complex is embedded in LDH.

Thus, we can consider the anionic layers of these materials to be constituted of mostly nitrate ions, with comparatively few anionic [(ThiaSO 2 ) 2 (Mn) 4 F] -cluster. LDHs exhibit a high positive charge density on the main layers. For example, a ratio of trivalent to divalent metal of x = 1/3 corresponds to one charge per 50 Å 2 on each side of the layer, leading to one charge for each 25 Å 2 in the inter-lamellar domains. 10 Our cluster complex represents one negative charge for each 170 Å 2 . Thus, it is difficult to achieve electroneutrality with only these complexes inside the interlayer space. In the literature, the majority of calixarene derivatives inserted in LDH systems are sulfonated, with four negative charges per macrocycle. 11 Therefore, they have a higher negative charge density and can fully substitute nitrate ions. A similar case in the literature deals with a sulfonated thiacalixarene complex complexed with an Ag + ion. In this case, despite a head to tail arrangement of the macrocycles to increase the charge density per unit volume, the macrocycle layer is still supplemented by carbonate anions. Regarding the TGA curves, decomposition of LDHs occurred in three consequent steps. In Figure 2, the first step observed in the range of 50-200°C corresponds to solvent loss from internal gallery surfaces and the external non gallery surfaces. For Mg-Al-NO 3 LDH, the second step in the range of 200-400°C is mainly attributed to the dehydroxylation and the third step, observed above 400°C, arises due to the elimination of NO 3 -from the LDH interlayer space. 14 For 1, the second and third steps resemble those of Mg-Al-NO 3 LDH and it is difficult to identify the loss of mass specifically due to the complex. The DSC for Mg-Al-NO 3 LDH and compound 1 show three major peaks, the positions of which fit remarkably well with those in the DTG curve, indicating that they correspond to mass-loss processes. For Mg-Al-NO 3 LDH all peaks are endothermic, contrarily to the exothermic peaks of compound 1 which are attributed to decomposition (between 300 and 500°C

X-Ray Diffraction Characterization

Figure 3 shows the powder X-Ray Diffraction diagram of intercalate material compared with the diagrams for carbonate and nitrate Mg-Al layered double hydroxides. the X-Ray Diffraction patterns of compounds Mg-Al-CO3 (red curve) and Mg-Al-NO3 (blue curve) shows that for each compound, the products were obtained in a pure phase. These patterns correspond to those described in the literature. 3, 6b) We would expect to see peaks corresponding to the (003) and

(006) planes as with the carbonate and nitrate forms, shifted to a lower 2θ value as a result of the larger size of the complex anion creating a larger interlayer space. For the sample intercalated product 1 (Figure 3 systems. For example, we would expect the largest d value to correspond to the (003) plane, which would mean that the smaller distances would be exact divisions of this;

12.02 / 2 = 6.01 Å for the (006) plane, 12.02 / 3 = 4.00 Å for the (009). This is evidently not exactly the case. The peak forms are less sharp, with broader, less intense peaks than those seen for the nitrate and carbonate LDHs. This could be explained by the disorder introduced by the anion distribution within the LDH layers and/or by the size of fundamental crystallization domains. The crystal packing of [(ThiaSO 2 ) 2 (Mn) 4 F]K shows that [(ThiaSO 2 ) 2 (Mn) 4 F] -anions are linked through the coordinated potassium ion and sulphonyl group, making [(ThiaSO 2 ) 2 (Mn) 4 F]K a one-dimensional network with a distance between two cations of 12.1 Å (Figure 4). The charge balancing cation is found near the sulfonyl groups, in the plane

Counts/s 2θ ( )

A c c e p t e d M a n u s c r i p t

formed by the four manganese ions, indicating that the negative charge is concentrated in this region. The width of complex 1 is between 7.5 and 10.5 Å (see Figure S5 in Supporting Information) and a layer of Mg-Al hydroxide has a width of 4.8 Å. 15 The first observed distance derived from the X-Ray diagram, which is equal to 12.02 Å, could therefore correspond to the hypothesis presented in Figure 4 for which the anion [(ThiaSO 2 ) 2 (Mn) 4 F] -is sandwiched lengthwise by positive LDH layers. Figure 5 shows a comparison of the X-Ray Diffraction data obtained for the LDH hybrid product obtained after a stirring time of 24 h (Product 1, green curve), and after a stirring time of 72 h (product 2, violet curve). While their X-ray diffraction diagrams are similar, the elementary analysis and spectroscopy characterizations show that these two compounds are different. the [(ThiaSO 2 ) 2 (Mn) 4 F]K complex.

In the case of Al 3+ the reaction between [(ThiaSO 2 ) 2 (Mn) 4 F]K and aluminium(III) nitrate nonahydrate in DMF at room temperature yielded white rectangular block crystals, which were studied by single crystal X-Ray Diffraction. Figure 6 shows the structure obtained from this analysis (see Figure S6 and crystallographic data in Tables S1-S3 in Supporting Information). The metal core consists of four Al 3+ ions in an approximate square planar arrangement, linked by µ 2 -hydroxide ligands lying in this metal plane. Most generally clusters with four aluminium ions, bridged by hydroxide ions, adopt a cubane-type structure. Here, the aluminium cluster architecture obtained for the [Al 4 (ThiaSO 2 ) 2 ] complex is uncommon, especially for aluminium ions displaying an octahedral coordination geometry. 16 Each metal centre shows identical pseudo-octahedral coordination geometry, with two bonds to hydroxide ligands, two bonds to phenolic oxygen from the thiacalixarene and one to a sulfonyl oxygen, with a molecule of DMF completing the coordination sphere. Hydrotalcite and its nitrate derivative both crystallize in the trigonal-rhombohedral (R-3m, n° 166) lattice system. The symmetry arising from this crystalline class results in a hexagonal form of the particles. Figure S7 (in Supporting Information) shows SEM images of the crystalline form of the nitrated material, proving that there has been no (or minimal) deformation during the acid-salt treatment (LDHs can be vulnerable to attack by strong acid).

Figure 7 shows the hexagonal crystalline form for intercalated LDH 1 remains unchanged. We can therefore say that no major changes in terms of morphology (and by association therefore, perhaps in terms of symmetry class) have occurred.

Photophysical characterization of compound 1

The solid-state luminescence spectra have been recorded for compound 1 in the presence and absence of atmospheric O 2 (O 2 or N 2 atmosphere, respectively). Figure 8 shows the evolution of the excitation and emission spectra with respect to atmosphere. The solid-state excitation spectrum shows a UV band above 25000 cm -1 (below 400 nm) arising from the ligand-centered-absorption (-* transitions). To avoid quenching by oxygen (O 2 ), the A c c e p t e d M a n u s c r i p t luminescence was measured under N 2 atmosphere. The emission band maximum increased by around 400% and redshifted by -490 cm -1 (+20 nm) from 15870 cm -1 (630 nm) to 15380 cm -1 (650 nm) as shown in Figure 8. Successive emission measurements by changing atmospheres (N 2 and O 2 ) have revealed that the variation in luminescence intensity is reversible (schematized by the double arrow in figure 8). The study of this reversibility was again carried out on compound 1 after aging for one year under ambient atmosphere (Fig. S8). The same emission profile was observed, demonstrating the stability of this phenomenon over time. The emission spectra (λ em = 25000 -12500 cm -1 or 400 -800 nm) have been measured at variable excitation wavelengths ranging between 33333 -25000 cm -1 (λ exc = 300 -400 nm, Figure 9) under different atmospheres (N 2 and O 2 ). Under N 2 atmosphere, the emission spectra show a maximum band centered at 15380 cm -1 (650 nm), where spectra measured under an excitation of 340 -365 nm showed the strongest emission intensity. Under O 2 atmosphere, emission spectra show a maximum band centered at 15870 cm -1 (630 nm), and the excitation range corresponding to the most intense emission spectra was between 355 -365 nm. For the two sets of atmospheric parameters (N 2 and O 2 ), the emission band shows two maxima, one for an excitation lower than 300 nm (this maximum cannot be precisely determined on our experimental setup) and a second around 360 nm.

The photophysical properties of 1 have been also studied at variable temperature between 77 K and 293 K (Figures S9 andS10), and the excitation and emission spectra were found to be slightly sensitive to changes in the temperature. In this temperature range, a weak redshift of the energy band maximum E max of -340 cm -1 (+14 nm) from 15720 cm -1 to 15380 cm -1 (636 -650 nm) and an increase of the band width at half height F WHM of +330 cm -1 from 2350 cm -1 to 2680 cm -1 (+18 nm from 96 nm to 114 nm) of the broad band were observed.

Vibronic intensity gains lead to a broadening of the luminescence band, a trend which is typical of d-d (metal centered) luminescence transitions such as for Mn 2+ . 2b) The lifetime under N 2 atmosphere at 77 K has been estimated at 1.51(1) ms compared to 1.02(2) ms at 293 K. This increase of 50% is due to a reduction of the non-radiative effects as the temperature decreases.

Previous spectroscopic studies have demonstrated that an influence is exerted on this luminescence quenching (energy transfers from the excited complex to O 2 ) by interaction between the cation and the anionic cluster. These studies show that an extreme sensitivity to the presence of oxygen in the molecular system can be attained by increasing the anion-cation separation distance, and/or by removing the coordination interactions. Among the different molecular homologues studied in our previous work, compound 1 displays the greatest sensitivity to O 2 in the solid-state. We have previously demonstrated that in solution this complex can be converted into a mononuclear compound of Mn 3+ , via a process of electron transfer between the complex and molecular oxygen. This reaction is characterized by a color change from colorless to intense violet. In contrast, when compound 1 is dispersed in DMF and irradiated at 360 nm, we do not observe appearance of any purple color characteristic of the photo-bleaching of the inserted complex. Furthermore, when 1 is illuminated in DMF solution in the presence of triphenylphosphine, the latter compound is rapidly converted to

Conclusion

The strong luminescence, emitted at 600 nm (excitation at 360 nm) by the previously published tetra-nuclear clusters of Mn II ions and two ThiaSO 2 macrocycles (ThiaSO 2 = p-tert-butylsulfonylcalixarene), of formula [(ThiaSO 2 ) 2 (Mn II ) 4 F] -, motivates us to try to define the full potential of this molecular system. In this context we were interested in the sensitivity of the luminescence emission of this system to oxygen. Previous work has led to the conclusion that the interaction of the cation with the anionic complex is of paramount importance. To exploit this phenomenon, we have carried out intercalation of the luminescent [(ThiaSO 2 ) 2 (Mn) 4 F] -anionic complex into the interlayer space of inorganic positively charged

Figure 1 :

 1 Figure 1: Schematic representation of ThiaSO 2 /Mn II system intercalated in a layered double

12

 12

A c c e

  p t e d M a n u s c r i p tOn the basis of the charge density of the LDH layers and the molecular dimensions of [(ThiaSO 2 ) 2 (Mn II ) 4 F] -, we believe that the extent of intercalation is limited by the size of the complex and not by the LDH anion-exchange capacity. This phenomenon has already been encountered with low charge density molecular systems such as ruthenium-based complexes.13

Figure 2 :

 2 Figure 2: TGA (black curve) and DSC (blue curve) of Mg-Al-NO 3 LDHs (left) and

  , green curve), the diagram shows a succession of three broad peaks positioned at 7.34° (d = 12.02 Å), 14.96° (d = 5.91 Å) and 29.91 (d = 3.89 Å).

A c c e p t e d M a n u s c r i p tFigure 3 :

 3 Figure 3: Powder X-Ray Diffraction patterns of Mg-Al-CO 3 (red pattern), Mg-Al-NO 3 (blue

Figure 4 :

 4 Figure 4: Schematic of hypothesised potential phases formed by the intercalation of

Figure 5 :

 5 Figure 5: Overlay comparison of the X-Ray diffractograms obtained with an X-Ray source of

Figure 6 :

 6 Figure 6: Structure of tetra-aluminium complex of Al 4 (ThiaSO 2 ) 2 . Color key: C = grey capped

A c c eFigure 7 :

 7 Figure 7: SEM images of intercalated LDH product 1.

Figure 8 :

 8 Figure 8: Solid-state normalized excitation (λ em = 630 or 650 nm) and emission (λ ex = 360 nm)

A c c e p t e d M a n u s c r i p tFigure 9 .

 9 Figure 9. Solid-state relative emission spectra vs excitation wavelength (λ ex = 300 -400 nm) of

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t triphenylphosphine oxide as

  followed by GCMS. These observations are in agreement with a production of singlet oxygen under light by compound 1. Then, the compound 1 can be recovered by centrifugation and recycled for a new photo-reaction. The high sensitivity of compound 1 to oxygen could render it useful as a sensitizer, in oxygen sensing devices or in singlet oxygen generation reactions following type II photo-oxygenation mechanisms.

(

  layers of layered double hydroxide (LDH) materials. Utilizing the osmotic swelling of layered double hydroxides (LDHs, [Mg 1-x Al x (OH) 2 ] x+ (NO 3 -) x ) in formamide, the anionic luminescent [(ThiaSO 2 ) 2 (Mn II ) 4 F] -complex has been introduced between the positive layers of LDHs, leading to a doping of LDH. In the recovered LDH layers the hexagonal prism morphology of the precursor is well retained. The formula of the resulting compound 1 Mg 0.71 Al 0.29 (OH) 2 (NO 3 ) 0.288 ((ThiaSO 2 ) 2 (Mn) 4 F) 0.002 (H 2 O) 0.5 ) was determined by elemental and thermogravimetric analysis. The low doping rate is attributed to the low negative charge density (e/Å 2 ) of the complex. Basal spacing of 12.02 Å indicates that [(ThiaSO 2 ) 2 (Mn) 4 F] -is sandwiched across the interlamellar width by positive LDH layers. The photophysical properties of the compound 1 were studied in the solid-state. Compound 1 presents A c c e p t e d M a n u s c r i p t

  ). The complex has a basic formula of C 82 H 96 FMn 4 O 26 S 8 , leading to a general formula for compound 1 of Mg 0.71 Al 0.29 (OH) 2 (NO 3 ) 0.288 C 0.16 H 0.19 F 0.002 Mn 0.008 O 0.05 S 0.016 . The oxidation of the organic part of the compound is very exothermic and explains the exothermicity of the peaks observed on the DSC curve for compound 1.

  The formulae found are Mg 0.72 Al 0.28 (OH) 2 (NO 3 ) 0.28 for LDH-NO 3 , It has been suggested that ion-exchange reactions at relatively low pH favour leaching of M II ions from the layers, thus giving rise to higher values of x. It seems therefore that for compound 2, and for all compounds which are subjected to long stirring duration with [(ThiaSO 2 ) 2 (Mn) 4 F] -, the reaction conditions lead to leaching of Mg 2+ . A possible hypothesis to explain this is a competition between the three metal ions, Mg 2+ , Mn 2+ and Al 3+ for complexation with ThiaSO 2 . It appears that under these synthesis conditions, the ThiaSO 2 ligands extricate Mg 2+ ions from the inorganic hydroxide layers. Thus, the excess Al 3+ seen in compound 2 is likely in the form of oxo-hydroxide which is most probably amorphous and therefore not detected by XRD. To try to understand this hypothesis, we introduced to a DMF solution of a salt of either Mg 2+ or Al3+

	Mg 0.71 Al 0.29 (OH) 2 (NO 3 ) 0.288 ((ThiaSO 2 ) 2 (Mn) 4 F) 0.002 A c c e p t e d Mg 0.57 Al 0.43 (OH) 2 (NO 3 ) 0.425 ((ThiaSO 2 ) 2 (Mn) Counts/s 2θ ( ) 10 M a n u s c r i p t for compound 1 and 20 30

4 F) 0.005 for compound 2. The ratio Mg/Al is the same for LDH-NO 3 and compound 1, but differs for compound 2.

gaseous atmospheres using a range of excitation wavelengths. Extinction of emission in the presence of oxygen is reported, demonstrating the very high sensitivity of this compound to O 2 .

The LDH host matrix allows for the O 2 -sensitivity to remain similar to that observed in solution whilst avoiding photo-degradation, thereby opening perspectives for use in oxygen sensing or singlet oxygen generation.

Experimental Reagents and materials

All chemicals and solvents were used as received (solvents and chemicals: Aldrich) .

[(ThiaSO 2 ) 2 (Mn II ) 4 F]K compound was synthesized by the published procedure. 2a)

Synthesis of [Mg 2 Al(OH) 6 ](CO

The LDH hydrotalcite is synthesized by a hydrothermal method. Four Teflon lined autoclaves each containing 12 cm 3 of an aqueous solution of Mg(NO 3 ) 2 •6H 2 O (0.250 mol.L -1 ), Al(NO 3 ) 3 •9H 2 O (0.125 mol.L -1 ) and hexamethylenetetramine (0.325 mol.L -1 ) are heated to 150°C for 36 h. The resulting suspensions are collected together and centrifuged (8 500 rpm, 30 min) followed by subsequent washing in deionised water (3 x 25 cm 3 ) and ethanol

(1 x 20 cm 3 ) with recovery by centrifugation each time. The product is dried overnight and recovered as a white microcrystalline powder (1.1428 g, 4.05 mmol with respect to formula, yield = 67%

LDH)

On a Schlenk line under Argon flow, Mg-Al-CO 3 LDH (0.3120 g, 1.11 mmol) is suspended in 150 cm 3 of a solution of NaNO 3 (1.0 mol.L -1 ) and HNO 3 (5 mmol L -1 ). This A c c e p t e d M a n u s c r i p t mixture is agitated under Argon atmosphere at room temperature for 16 h, then is centrifuged (8 500 rpm, 30 min). The supernatant solution is decanted off and the solid material is washed with deionised water (3 x 50 cm 3 ) and 96% ethanol (1 x 40 cm 3 ). The product is dried under vacuum and recovered as a white microcrystalline powder (0.2650 g, 1.0 mmol, yield = 90% yield). P-XRD (2θ in °): 9.93, 19.95, 30.08, 34.74, 38.05, 40.47, 43.70. Formula was determined by TGA analysis and confirmed by elemental analysis (see Figure S1 in Supporting Information).

Synthesis of [Mg 0.71 Al 0.29 (OH) 2 ](NO 3 ) 0.288 ((ThiaSO 2 ) 2 (Mn) 4 F) 0.002 (H 2 O) 0.5 1: (Intercalation of luminescent material) Many trials concerning variations of temperature, agitation, order of addition and so on were carried out for this reaction, to optimise the synthesis conditions as follows.

Under inert atmosphere (Ar) the nitrated form Mg-Al-NO 3 LDH (0.02 g, 0.24 mmol i.e. 0.067 mmol of NO 3 -) is added to formamide (50 cm 3 ) and allowed to form a homogenous dispersion such that the suspension becomes translucent and demonstrates Tyndall light scattering. 0.2 g (0.1 mmol) of [(ThiaSO 2 ) 2 (Mn) 4 F]K dissolved in 4 cm 3 DMF is added dropwise to the solution of LDH with stirring. This mixture is agitated under inert atmosphere during 24 h. The suspension is then centrifuged (13 000 rpm, 5 min). The collected solid is washed with DMF (3 x 24 cm 3 ), degassed H 2 O (3 x 24 cm 3 ) and ethanol (1 x 24 cm 3 ). The product 1 is dried at 50°C under reduced pressure, and collected as a pale yellow luminescent powder (0.019 g, yield = 88%)). P-XRD (2θ in °): 7.34°, 14.96° and 29.91. Elemental analysis (mass %): [Al] = 16.6%; [Mg] = 36.5%; [Mn] = 0.74%. Formula was supported by TGA analysis (see Figure S2 in Supporting Information).

Synthesis of [Mg 0.57 Al 0.43 (OH) 2 ](NO 3 ) 0.425 ((ThiaSO 2 ) 2 (Mn) 4 F) 0.005 2. The preparation of compound 2 is identical to compound 1 except for the stirring time, which is 72 hours. P-XRD [Mn] = 0.146%.

Characterizations

Scanning Electron Microscopy (SEM) was performed on a Vega Tescan SEM microscope.

Elemental analysis was performed by ICP-OES (ICAP 6000 Thermo Fisher Scientific) for Al and Mg, and ICP-HRMS (ElementXR, Thermo Fisher Scientific) for trace Mn determination.

Quantitative measurements were obtained after a wet acid digestion (HNO 3 /HF mixture) of the powder.

Powder X-Ray Diffraction (P-XRD) was performed on a PANanlytical XpertPro MRD diffractometer with a Cu Kα1 radiation (λ = 1.54060 Å) used with 40 kV and 30 mA settings in θ/θ mode, reflection geometry.

For Single Crystal X-Ray Diffraction (SC-XRD), suitable crystals were selected and mounted on an Xcalibur kappa geometry diffractometer (Rigaku OD, 2015) equipped with an Atlas CCD detector and using Mo radiation (λ = 0.71073 Å). Intensities were collected at 150 K by means of the CrysalisPro software. [START_REF]CrysAlisPro Software system[END_REF] Reflection indexing, unit cell parameters refinement, Lorentz polarisation correction, peak integration and background determination were carried out with the CrysalisPro software. An analytical absorption correction was applied using the modelled faces of the crystal. [START_REF] Clark | [END_REF] The structures were solved by direct methods with SIR97 and the least square refinement on F 2 was achieved with the Crystalis software. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H in the range 0.93 -0.98 Å, and U iso (H) in the range 1.2 -1.5 times U eq of the parent atom), after which the positions were refined with riding constraints. For the experimental details see Supporting Information. CCDC-2010107 contains the supplementary crystallographic data for A c c e p t e d M a n u s c r i p t this paper. These may be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Spectroscopic measurements.

Solid-state excitation and emission spectra were recorded with a commercial fluorimeter Fluorolog 3 from Horiba Jobin Yvon Co, equipped with a Xe lamp 450 W, and a UV-visible photomultiplier (Hamamatsu R928, sensitivity 185 -900 nm). The excitation/emission spectra were realized on powder samples pasted directly onto copper plates using conductive silver glue. For the measurements realized at variable temperature (77 K -300 K), the samples were introduced into an OptistatCF liquid nitrogen cooled cryostat from Oxford Instruments Co. The emission spectra were corrected for the instrumental response function. Appropriate filters were utilized to remove the laser light, the Rayleigh scattered light and associated harmonics from the emission spectra. Luminescence lifetime (τ > 10 μs) have also been measured using this apparatus with a xenon flash lamp (phosphorescence mode) Lifetimes were averages of 2 or 3 independent determinations. The air pressure dependent solid-state luminescence spectra have been realized in a cryostat equipped with a primary pump.

For illumination experiments, LEDs of various wavelengths 360 and 400 nm were used. The LED were purchased from LED ENGINE Co and calibrated with a flame spectrometer from Ocean Optic with a nominal flux of 430 and 586 mW/cm 2 , respectively, at a working distance between the source and the sample of d = 10 mm.


Lucia Fanini

Christophe Piscart

Enzo Pranzini

Christian Kerbiriou

Isabelle Le Viol

Julien Pétillon

The extended concept of Littoral Active Zone considering soft sediment shores as social-ecological systems, and an application to Brittany (North-Western France

The extended concept of littoral active zone considering soft sediment shores as social-ecological systems, and an application to Brittany (North-Western France)

targeting research, perception, communication, sustainable use and management of the LAZ as a functional unit.

1.Study approach

The background information (paragraphs 2-5) was structured by gathering current paradigms and definitions around soft sediment -when concepts are still debated, versions presented are backed by references and related empirical data. The call for extension of the concept of sandy beach, opening options for landscape scale considerations, is here answered by considering salt marshes (motivations detailed in paragraph 6), hence the use of "soft sediment shores". Human-modified beaches are included in the background (paragraph 7), assuming that this is a most common condition, in a context where sandy shores are affected by human-driven phenomena -though to a different extent-at global scale. The social template of the Littoral Active Zone is described on such background, and defined physically in space and time to reach the depiction of its boundaries (paragraph 8). We finally provided detailed information for the case of Brittany (paragraph 9), utilizing the standard structure of a social-ecological system as two templates (the social and ecological one), inter-connected by adaptive management and ecosystem services. To this aim, information comprehends not only scientific papers but also grey literature and references to ongoing projects. Conclusions are an open invitation to consider the LAZ and its diverse components as the proper dimension to approach and manage systems connecting land and sea.

Current paradigms on sandy shores: patterns and processes across the Littoral Active Zone.

Sandy shores are ecotonal systems connecting land and sea. Their definition is slightly different depending on the discipline considered. For instance, in geomorphology the "beach" goes from the depth of closure to the upper limit of the runup under extreme waves, and the dune is excluded from the sandy shore definition sensu stricto [START_REF] Komar | Environmental controls on littoral sand transport[END_REF].In ecology, sandy shores consist of three entities along the land-sea axis: surf zones, beaches and dunes. Each one of them is characterized by distinct organism assemblages, tightly related to the physical constraints of the microhabitats they inhabit (McLachlan and Defeo 2017 and references within). A unifying concept is that the different zones along the land-sea continuum are connected by a constant exchange of energy and material, including boulders, pebbles, gravel, sand or silt (see [START_REF] Wentworth | A scale of grade and class terms for clastic sediments[END_REF] for sediment size categories).Hence the concept of Littoral Active Zone (LAZ) [START_REF] Tinley | Coastal Dunes of South Africa 109FRD[END_REF] was proposed to define a soft-sediment coastal system, with boundaries identified by the effects of the waves (seaward) and aeolian (landward) transportation of sediment. Sediment is in fact being A c c e p t e d M a n u s c r i p t exchanged across the geomorphic system of the LAZ, in a dynamic process. This led to the use of sediment budget [START_REF] Komar | Environmental controls on littoral sand transport[END_REF] for sandy shores management purposes: the concept of LAZ referred to the littoral geomorphology and proposed the management of a unit including both marine beach/surf zone and terrestrial dune ecosystems. However, such a conceptual framework has not been fully applied to the biotic component of the system, where multi-level exchanges (of energy, material and nutrients) occur along the same unit (after McLachlan and Defeo, 2017):

• Primary producers (surf phytoplankton, seagrasses, benthic microflora, pioneer and dune vegetation);

• Primary consumers (dominated by bacteria, peracaridans, annelids, mollusks, insects);

• Predators (dominated by birds, coleopterans, spiders, crabs)

• "Temporary" i.e. only spending within the LAZ a key stage of their life-cycle: planktonic larvae and juvenile stages; nesting or migrating birds; nesting turtles.

While research on sandy shores provided a solid background with respect to morphodynamics, current knowledge related to biotic components indicates that they are 1) mostly physically driven and 2) generally known in terms of patterns, but with a gap in knowledge concerning the processes shaping the patterns measured [START_REF] Defeo | Global patterns in sandy beach macrofauna: species richness, abundance, biomass and body size[END_REF].

Across-shore dimension.

Along the LAZ, at a dimension defined as "mesoscale" in ecology of sandy shores (i.e. "within beaches"), both water-breathing and air-breathing organisms can be found, following zonations which fluctuate in space and time linked to habitat availability [START_REF] Mclachlan | Zonation on sandy beaches[END_REF]. Indeed, the beach is the one entity of the LAZ in which both water-breathers and air-breathers cooccur [START_REF] Defeo | Patterns, processes and regulatory mechanisms in sandy beach macrofauna: a multi-scale analysis[END_REF], while surf zones are exclusively inhabited by water-breathers and dunes by air-breathers. [START_REF] Dugan | Beyond beach width: Steps toward identifying and integrating ecological envelopes with geomorphic features and datums for sandy beach ecosystems[END_REF] proposed the concept of "envelopes" for the beach entity, i.e. spatial units affected by temporal units varying daily-to-yearly. This explained macrofaunal patterns better than the mere "beach width" variable. [START_REF] Scapini | Behavioral adaptations of sandy beach macrofauna in face of climate change impacts: A conceptual framework[END_REF] remarked on the relevance of behavior in the macrofauna zonation patterns, with plasticity as the main adaptation to recover and maintain a suitable habitat. By extending the analysis to the whole LAZ, the characteristic mobility of substrate is the main biotic driver that shapes microhabitats and to which the fauna adjusts their behavioural responses.

Along-shore dimension.

A c c e p t e d M a n u s c r i p t

Each LAZ across-shore relates to an along-shore unit (perpendicular to the land-sea axis), which can be called the physiographic unit, sedimentary cell, or beach units. Along-shore dimension is defined by natural headlands or man-made structures overpassing the depth of closure, i.e. the depth to which the cross-shore sedimentary exchange is insignificant and which is identified where bathymetric profiles repeated over time are connected (closed) [START_REF] Hallermaier | Use for a calculated limit depth to beach erosion[END_REF]. By considering the dune as a part of the Active Zone, units can also overpass headlands (e.g. locations in which transgressive dunes have climbed escarpments and passed the headlands, [START_REF] Pye | Is ' re-mobilisation ' nature restoration or nature destruction? A commentary[END_REF]. Embayments and pocket beaches, with boundaries generating turbulence and rip currents at their edges, are easy to identify as units. Within units however, as e.g. in the case of extended beaches, exposure and longshore currents may affect the sand mobility [START_REF] Komar | Environmental controls on littoral sand transport[END_REF]; hence a beach can hence include a continuum from deposition to erosion, confusing the identification of units along-shore [START_REF] Carter | Coastal environments[END_REF]. Also mixed conditions such as sparse rocks present on sand substrates, affect the flow of energy and material within units. Dynamics become more relevant in the case of wrack inputs ashore, driving the spatio-temporal patterns of stranding. In ecology, given the large amount of studies focusing on sand-only conditions, conditions of periodical inputs of stranded material are often overlooked. However, they are important in terms of dynamics backing the biotic patterns measured ashore. In general, beach fauna patterns present land-like characteristics (McLachlan and Defeo, 2017 but also see [START_REF] Defeo | Coupling between macrofauna community structure and beach type: a deconstructive meta-analysis[END_REF], with single beaches ("ecological mesoscale") remaining an essential unit for analysis.

In summary, the paradigm of sandy beaches as physically-controlled environments, when applied to the mobility of the substrate as LAZ characteristic, highlights the following features as keys for resident biota: 1) Pore space for the development of rich interstitial fauna dominated by bacteria, protozoans and meiofauna. The interstitial system processes organic materials flushed into the sand and returning the inorganic nutrients to the sea [START_REF] Pearse | Ecology of sandy beaches at Beaufort, North Carolina[END_REF].

2) Incohesive substrate as a driver of burrowing mechanisms of macrofauna. "Dealing with substrate" defines essential traits of organisms inhabiting the surf zone [START_REF] Gibson | Macrofaunal burrowing: the medium is the message[END_REF]. On the supralittoral area, substrate and non-substrate modifiers can be found, whose presence largely depends on substrate granulometry and related water retention, as well as on allochthonous inputs (e.g. seagrasses mats) [START_REF] Pérès | Recherches sur les peuplements benthiques de la Méditerranée nordorientale[END_REF]Picard, 1958, Sassa and[START_REF] Sassa | Role of geoenvironmental dynamics in the biodiversity of sandy beaches and sandflats: the ecohabitat chart and its ecological implications[END_REF].

A c c e p t e d M a n u s c r i p t

Vertical dimension (depth).

The depth of sediment mixing is rather difficult to define, though it is generally considered from the physical point of view as the layer of sediment mixed by waves over a specific time interval. It depends on wave energy and sediment size, and influences the substrate-related activities of benthic fauna. Resident bivalves were estimated to completely re-work the first 5 cm of fine sediment, producing fecal pellets moved by the currents that caused high turbidity and a sediment loss of about 30 cm/year (Risk and Moffatt, 1977). The biotic feature which best describes the interaction with the vertical dimension is the biological trait of bioturbation that is functionally relevant for the ecosystem (see e.g. [START_REF] Van Der Linden | A biological trait approach to assess the functional composition of subtidal benthic communities in an estuarine ecosystem[END_REF]. While the relevance of burrowing for organisms inhabiting the benthic of marine and lower littoral was thoroughly investigated (see e.g. [START_REF] Gibson | Macrofaunal burrowing: the medium is the message[END_REF][START_REF] Che | It's tough to be small: dependence of burrowing kinematics on body size[END_REF], the upper littoral system was overlooked and only recently explored [START_REF] Sassa | Role of geoenvironmental dynamics in the biodiversity of sandy beaches and sandflats: the ecohabitat chart and its ecological implications[END_REF]Yang, 2019, Celentano et al., 2019).Specialist and generalist species can be defined with respect to burrowing patterns: substrate specialists such as cirolanid isopods are burrowing in fine sand only and within a narrow range of water saturation conditions [START_REF] Giménez | Longshore patterns of distribution of macroinfauna on a Uruguayan sandy beach: an analysis at different spatial scales and of their potential causes[END_REF], while generalists such as ghost crabs can display a range of shifts in zonation and behavior across the littoral and cope with severe impacts, e.g. those from off-road vehicle driving, by shifting burrowing behaviour [START_REF] Gül | Impacts of human disturbance on ghost crab burrow morphology and distribution on sandy shores[END_REF].Ecological categories can also be defined within single taxonomic groups based on their burrowing capabilities [START_REF] Lowry | Substrate dependent talitrid amphipods from fragmented beaches on the north coast of Crete (Crustacea, Amphipoda, Talitridae), including a redefinition of the genus Orchestia and descriptions of Orchestia xylino sp. nov. and Cryptorchestia gen. nov[END_REF]. Burrowers and non-burrowers can coexist on a soft littoral substrate, yet have a different reaction to substrate disturbances, such as wrack removal [START_REF] Fanini | Comparing methods used in estimating biodiversity on sandy beaches: Pitfall vs. quadrat sampling[END_REF].

6.The concept of LAZ extended to other interface systems: Salt marshes.

Salt marshes are intertidal environments also characterized by soft sediment. They have been included in recent conceptual frameworks regarding human-environment interactions on softsediment coastlines, namely as low-energy extreme environments (Dugan et al., 2018 and within).

Such conceptual extension would allow, as a consequence, the broadening of questions from mesoscale (in its ecological meaning) to landscape ecology [START_REF] Schlacher | Golden opportunities: a horizon scan to expand sandy beach ecology[END_REF].Sandbanks of salt marshes are also featuring the flood and drain of salt water, though with a higher material retention, sharing most abiotic and biotic characteristics of sandy beaches. The identification of a LAZ for salt marshes can be based on the features shared with sandy littoral zones, such as:

A c c e p t e d M a n u s c r i p t

1) The exchange of energy and material landwards and seawards. The low and high marsh behave in this respect as the beach and dune continuum. Salt marshes would represent systems with low-energy exposure (see also [START_REF] Dugan | Generalizing ecological effects of shoreline armoring across soft sediment environments[END_REF], low mixing of the substrate -a feature likely enhancing the role of biota in sediment mixing and making it available for tidal current transport. In tidal channels however, physical processes might be dominant.

2) The biotic components of salt marshes, representative of a sea-land interface with the same dominant categories of species found on sandy littorals. Assemblages are defined by abiotic filtering and biotic interactions, likely changing with saltmarsh succession [START_REF] Scharma | Ecosystem assembly rules: the interplay of green and brown webs during salt marsh succession[END_REF]. Where the flood frequency is high, marine species gain relevance (see e.g. Kneib 1984, Levin and[START_REF] Levin | Influence of vegetation and abiotic environmental factors on salt marsh invertebrates[END_REF], while high marshes are hosting more terrestrial fauna [START_REF] Pétillon | Influence of abiotic factors on spider and ground beetles communities in different salt-marsh systems[END_REF], whether specialist or temporary.

3) The effect of salinity: many terrestrial species like spiders display better survival and increased fitness under non saline conditions (see e.g. [START_REF] Foucreau | Effects of diet and salinity on the survival, egg laying and metabolic fingerprints of the ground-dwelling spider Arctosa fulvolineata (Araneae, Lycosidae)[END_REF]) while few taxa act like marine species, i.e. with enhanced survival and fitness when exposed to salt stress (see e.g. talitrid amphipods, [START_REF] Persson | Dispersal of Platorchestia platensis (Kröyer) (Amphipoda: Talitridae) along Swedish coasts: a slow but successful process[END_REF]. 4) Like beaches, salt marshes function as "islands", from both population (genetic diversity) and assemblage (species richness) points of view (see e.g. [START_REF] Desender | Age and size of European saltmarshes and the population genetic consequences for ground beetles[END_REF][START_REF] Rey | Immigration and Extinction of Salt Marsh Arthropods on Islands: An Experimental Study[END_REF]Strong 1983respectively). Furthermore, patches of vegetation within a landscape matrix dominated by another plant species also act as "islands within islands" [START_REF] Puzin | Contrasted responses of dominant ground-dwelling arthropods to landscape salt-marsh fragmentation[END_REF].

5) The provision of a range of ecosystem services, most relevant being: fish nursery, trophic enrichment for aquaculture, aesthetic value for real estate and attenuation of waves [START_REF] Adams | Salt marsh at the tip of Africa: Patterns, processes and changes in response to climate change[END_REF]. With respect to sandy beaches, salt marshes are typically less affected by direct human impacts such as tourism. Yet, they are subjected to a range of anthropogenic activities such as sheep grazing or mowing.

As low-energy systems, sand banks of salt marshes would indeed represent an extreme condition of dissipative beaches [START_REF] Mclachlan | Characterising sandy beaches into major types and states: Implications for ecologists and managers[END_REF], with lower turbulence and smaller substrate grain size allowing the most benign conditions to biodiversity and life-history traits of resident populations. An extended surf zone would allow for richer surf-fish assemblages. Finally, in a A c c e p t e d M a n u s c r i p t metapopulation perspective, they would act as sources like flat and dissipative beaches [START_REF] Defeo | Global patterns in sandy beach macrofauna: species richness, abundance, biomass and body size[END_REF] -as opposed to oceanic exposed, reflective beaches acting as sinks.

An opportunity to test the "sandy beach model" on other interface systems would increase the relevance of coastal studies, specifically addressing the role of environmental heterogeneity in granting refugia, resilience and diversity to the coastal system [START_REF] Schoeman | Climate-change impacts on sandy-beach biota: crossing a line in the sand[END_REF]. Even though merging fields which traditionally have been kept separated is challenging [START_REF] Orr | Towards a unified study of multiple stressors: divisions and common goals across research disciplines[END_REF], the relevance of LAZs is a suitable common denominator for the approach to different interface systems.

Social-ecological systems: human modified littorals.

The anthropogenic modification of the shoreline relates to different aspects. Coastal squeeze [START_REF] Defeo | Threats to sandy beach ecosystems: a review[END_REF][START_REF] Elliott | Chapter 1 -A Synthesis: What Future for Coasts, Estuaries, Deltas, and other Transitional Habitats in 2050 and Beyond?[END_REF] is the trend resulting from the combination of actions undertaken to first extend and then preserve the infrastructures and properties on the land-side, coastal erosion and sea-level rise on the sea side. Changes in the near future are expected to occur on and strongly affect the LAZ: urbanization, increase of population on the littoral, increasing demand for recreation, climate change-related phenomena, and will likely affect its patterns and processes. To address coastal squeeze, two classical actions are: 1) retreat and 2) defend [START_REF] Williams | The management of coastal erosion[END_REF]. Considering coastline retreat implies the knowledge of land uses behind the LAZ, i.e. a likely difficult ecosystem analysis at short-and mid-term scales (though, different perspectives based on species-environment models are proposed by Mendoza-Gonzales, 2013). As a general assumption, when the natural boundaries of the LAZ are preserved (particularly the sand reservoirs and the vegetation) the system can resist stress or recover from stress (resilience), acting as a natural buffer against storms and natural erosion (see e.g. [START_REF] Sutton-Grier | Future of our coasts: The potential for natural and hybrid infrastructure to enhance the resilience of our coastal communities, economies and ecosystems[END_REF].

"Defend" implies human interventions which are directly and irreversibly modifying the LAZ, for instance: hard infrastructures introducing fixed elements in a mobile sediment environment transform a sand coast into a rock one -this is also true for non-defense hard infrastructure, such as harbors, piers and artificial reefs for fishing. This should interrupt the continuum, defining new beach units in terms of both abiotic and biotic components [START_REF] Fanini | Effects of beach nourishment and groynes building on population and community descriptors of mobile arthropodofauna[END_REF][START_REF] Dugan | Generalizing ecological effects of shoreline armoring across soft sediment environments[END_REF], and beaches increasingly becoming man-made landscapes (a temporal follow-up provided by [START_REF] Nordstrom | Living with shore protection structures: a review[END_REF]. The extension of infrastructures on the sea side was defined as "ocean sprawl".

Hypotheses on multi-scale effects of ocean sprawl on soft sediment coastlines were formulated [START_REF] Heery | Identifying the consequences of ocean sprawl for sedimentary habitats[END_REF] yet they remain to be tested, especially regarding the effects on biota and A c c e p t e d M a n u s c r i p t dynamics such as e.g. connectivity. For instance, as the longitudinal connectivity (structural and functional) of LAZ biota and the role of marine wrack inputs are poorly studied, the research on the effects of man-made fragmentation is likely to be built on missing baselines. Finally, human intervention can relate to the stabilization of dunes via introduced plant species, see e.g. the use of Casuarina ssp. and related interference with sea turtles' nesting [START_REF] De Vos | Effect of introduced Casuarina trees on the vulnerability of sea turtle nesting beaches to erosion[END_REF] or the effects of the plant Ammophila arenaria replacing native species in Australia [START_REF] Bird | Coastal Geomorphology[END_REF].

Human activities occurring on the LAZ can also be periodical and/or with reversible effects, such as direct trampling and wrack removal. These kind of pressures are mostly related to recreation and are sharper in case of seasonal use of the resource beach. On one hand, these impacts are related to substrate changes and depletion of organic input, mainly affecting the fauna inhabiting or foraging on the beach and upper part of the LAZ, where wrack inputs are a relevant feature and sustain dynamics of faunal succession [START_REF] Dugan | Marine macrophyte wrack inputs and dissolved nutrients in beach sands[END_REF]. Supralittoral fauna can in fact actively reach the wrack when available, but also display opportunistic diet shifts given the patchy and discontinuous availability of the resource [START_REF] Olabarria | Succession of macrofauna on macroalgal wrack of an exposed sandy beach: effects of patch size and site[END_REF][START_REF] Colombini | Temporal and spatial use of stranded wrack by the macrofauna of a tropical sandy beach[END_REF].This latter aspect is however exacerbated by human actions(reviewed by [START_REF] Dugan | Loss of coastal strand habitat in southern California: the role of beach grooming[END_REF]. On the other hand, recreation is connected to a primary role of the landscape, including substrate characteristics as one important feature [START_REF] Pranzini | Nourishing tourist beaches[END_REF]. The presence of stranded material (paradigmatic are the cases of Posidonia banquettes in the Mediterranean Sea, and Sargassum in Mexico and Caribbean Islands) is perceived as negative for tourism and consequently local authorities/managers are actively removing stranded material, at least seasonally. It is worth remarking that along the range of impact affecting soft sediment coastlines as social-ecological systems, the sand bank of salt marshes would represent a lower impact level, allowing for a "compare and contrast" approach.

Though, regulations and management add a mismatching layer to the system, with the blanket term 'littoral' including the non-aligned definitions based on different disciplines, and encompassing a range of environments with specific features and processes [START_REF] Mclachlan | The ecology of sandy shores[END_REF].

LAZ as a relevant functional social-ecological unit.

A c c e p t e d M a n u s c r i p t

To summarize the concept of social-ecological unit and tie it to the LAZ, we apply the conceptual scheme proposed by [START_REF] Bretagnolle | Action-orientated research and framework: insights from the French long-term social-ecological research network[END_REF] (Figure 1).The reasons of identifying the LAZ as a socio-ecosystem are rooted in two backing concepts:

• The consideration of mobile substrates, physically defining the LAZ and common denominator across social and ecological template is a relevant background for such an integrated approach.

• The consideration of time as fourth dimension, with the identification of different temporal scales highly depending on the phenomena which should be observed. The proper selection of a time scale will allow an understanding of changes across both templates.

[Figure 1. Littoral Active Zone as social-ecological system (definition of the templates after [START_REF] Bretagnolle | Action-orientated research and framework: insights from the French long-term social-ecological research network[END_REF]. The boundaries of the system are considered in the three dimensions physically defined in this paper.

The social template of the LAZ is characterized by a strong land-bias. While there is no human resident on the LAZ, there is a physical direct use of the zone, as well as regulations in place to define such use. At the same time, citizens have been indicated as "missing layers" in marine spatial planning [START_REF] Martin | The missing layer: Geo-technologies, communities, and implications for marine spatial planning[END_REF], neglecting a fundamental component of the social template. Perceptions and attitudes of citizens are powerful drivers of the system. Hence, providing literacy and awareness can bring considerable change. However, when referring to the Littoral Active Zone as defined, this may indeed be a difficult background for the creation of awareness, in terms of both biota and physical processes. Beaches appear as piles of sand inhabited by cryptic and non-appealing key species (Dugan et al., 2010). Even high rates of endemism do not result as appealing as iconic species, probably because most animal species are composed of small and colorless (invertebrate) species [START_REF] Harris | Rich diversity, strong endemism, but poor protection: addressing the neglect of sandy beach ecosystems in coastal conservation planning[END_REF][START_REF] Leandro | Bias and perspectives in insect conservation: a European scale analysis[END_REF].

Iconic species are usually LAZ facilitative and were therefore found to be less responsive to changes than resident fauna [START_REF] Schoeman | Climate-change impacts on sandy-beach biota: crossing a line in the sand[END_REF], even though may be key for conservation [START_REF] Mclachlan | Sandy beach conservation and recreation: guidelines for optimising management strategies for multi-purpose use[END_REF].Another common issue is the perception of what component of the stranded material is really litter: attitudes towards stranded wrack are generating conflicts between conservation and recreation for decades. A few key drivers acting on the LAZ are summarized and represent potential conflicts between conservation vs. exploitation.

Tourism. Beyond the budgetary consideration applied in geomorphology (input and output estimates), square meters of substrate are a unit used for the estimate of surface available for A c c e p t e d M a n u s c r i p t seaside activities that can be given a market value. In California, for every $1 spent on beach nourishment in Miami, the federal government expects to collect $320 in tax revenues from beach tourism [START_REF] Houston | The economic value of beaches: a 2008 update[END_REF], while a conservative estimate of $587 million per year is estimated as value of beach visits in Queensland, Australia [START_REF] Rolfe | Valuing beach recreation across a regional area: The Great Barrier Reef in Australia[END_REF].In general, the "Big five" for beach attractiveness are: Water colour, Beach cleanliness, Safety, Facilities and Scenery (Anfuso et al., 2018). The choice of a given place as a tourist destination is driven by both domestic and international market pressures, hence mainly external to the LAZ. In this sense the use of Blue Flag is a most relevant example of driver, considering environmental quality along with social parameters (such as safety for swimmers) within the same set of desired conditions. The Blue Flag award was found to increase market competitiveness of beaches [START_REF] Dodds | Is blue flag certification a means of destination competitiveness? A Canadian context[END_REF].However, the concept is often confused with touristic promotion, and increasing information about underlying criteria would be required [START_REF] Lucrezi | Managing beaches and beachgoers: Lessons from and for the Blue Flag award[END_REF], but see McKenna et al., 2011). This stresses again the importance of citizen literacy and awareness in exploiting the full potential of an existing and well established tool.

Cultural value. This is a value often overlooked, in spite of being a potentially powerful driver of decision making. There is a clear context-dependency related to the cultural value of littoral active zones, depending on the history, social attitudes and size of population benefitting from it [START_REF] Daniel | Contributions of cultural services to the ecosystem services agenda[END_REF]. Processes modifying cultural assets, and specifically impacts (e.g. plastics, nourishment, urban sprawl) can be a leverage to engage citizens. Attitudes depend on cultural value and historical use of the beaches, with biased choices stemming from human expectations on sandy shores (mainly driven by recreation, real estate values, etc.), rather than on the balanced representation of the system. This finally leads to unpredictable long-term consequences [START_REF] Elliott | Estuarine, coastal and marine systems restoration: confusing management and science -a revision of concepts[END_REF].There is an increase in different programs (among them, citizen science programs -Earp and Liconti, 2020) and measures proposed by NGOs and land managers to re-connect people to the ecosystem, both with respect to its functioning as a whole (in the case of LAZ phenomena such as trophic network, erosion) and targeted conservation issues (ecosystem and/or species-focused approach). It is too early to describe LAZ-based experiences, as many of these actions are just starting, though platforms such as Ocean Best Practices and European Citizen Science are preparing the environment for sharing them.

Fishery. This category holds a commercial interest, which, around soft-sediment shores, is generally related to subsistence, recreational or commercial (listed in an increasing spatial scale of A c c e p t e d M a n u s c r i p t relevance), though they usually co-occur [START_REF] Mclachlan | Oceanography and marine biology: an annual review[END_REF]. While subsistence relates mainly to direct harvesting, e.g. of clam populations, and even when commercial, it is still related to the artisanal and local levels, commercial fishery extends from the surf zone to the pelagic one, with nursery areas for pelagic and demersal species. Lack of management plans encompassing the whole species-habitat creates severe impacts, see e.g. the sequential fisheries. These are spatially segregated fleets affecting different components of the life stage of one or more fish species [START_REF] Seijo | Fisheries bioeconomics: theory, modelling and management[END_REF].

Connected templates.

In the extended LAZ concept, we link the social template to the biophysical template with the adaptive management loop. In practice, in the governance of LAZ, there might be mismatches between ecological units and administrative/managerial ones. The latter ranges from international to national to local, and can be site-specific. An ubiquitous and transdisciplinary risk is the excessive consideration of recreation at the expenses of the other components. The perceived conflict "conservation vs. exploitation" is also expected to harshen in a context of climate change eroding the available resources. Even though scientific input has been used to set standards, there has been inadequate integration of costs of management and benefits of controls [START_REF] Mclachlan | The ecology of sandy shores[END_REF].For instance, eventual discrepancies between the environmental degradation perceived by the public and policy-makers with those identified by scientists will need to be identified and evened up via information sharing. Several countries have specific requirements for developing and implementing littoral protection; the immediate priority is to avoid further development on areas likely impacted by erosion in the future, measures are hence based on coastal setbacks, allowing for retreat, fixing a minimum distance from the shore for new buildings. There is a huge variability worldwide regarding this distance (from 5-15 m at Bahamas to 300 m in Denmark; though 100 m is the most common distance allowed. Depending on the geomorphology of the coastline, in some cases such as Australia elevation becomes the parameter to set limits on). Though, the way different references points are defined for "shore"e.g. low water mark; line of stable vegetation; etc. are adding further variability (see [START_REF] Simpson | Coastal setbacks in Latin America and the Caribbean: A study of emerging issues and trends that inform guidelines for coastal planning and development[END_REF].

The loop between biophysical template and social template is representing the challenge of ecosystem-based governance. Even though the set of ecosystem services provided by sandy littoral zones is broad, it is mostly localized and occurs within the LAZ, and tightly related with coastal management units [START_REF] Sardà | A proposed ecosystem-based management system for marine waters: linking the theory of environmental policy to the practice of environmental management[END_REF]. The fluid nature of the system is challenging the attempts to estimate and valuate ecosystem services, e.g. mapping and land cover changes, a powerful tool A c c e p t e d M a n u s c r i p t for (Burkhard et al., 2012) could not be applicable in a context of an always varying extension of the area, and mobile substrate. In particular, if the littoral area -or the LAZ as we proposed-is widely neglected in the international legislation as a management unit. Instruments for integrated approaches to standardise development and management include a triple bottom line (TBL) approach, a well-established framework with multiple stakeholder input used to account for social, economic, and environmental impacts of management options. This should produce ecologically meaningful and protective standards for any jurisdiction which could effectively be applied to the LAZ.

Application of LAZ extended concept to a coastal region: the case of Brittany

As a test for this approach, after structuring the information on biophysical and social templates around the concept of LAZ, we applied it to the coast of Brittany, filling with detailed information related to the templates and the LAZ of a so far overlooked case, such as European macrotidal shores. Questions to support further research are listed as part of the interfaces between templates (ecosystem services and adaptive management: Table 1).

The coastline of Brittany represents 33% of the French (excluding overseas) coastline. The study of its soft sediment shores is however overlooked. As mentioned above, this is not only a regional need, but also a requirement to test current ecological paradigms by adding inputs from macrotidal conditions (see [START_REF] Mclachlan | The ecology of sandy shores[END_REF][START_REF] Fanini | Advances in sandy beach research-Local and global perspectives[END_REF]. Indeed, the high variability and case-dependency of littoral ecology should warn against the use of "one size fits all", and the concept of LAZ as a unit should help in tackling the biophysical and social dimensions, in which phenomena can be observed and described reducing the bias due to mismatching scales.

The structure of information in biophysical and social templates (Figure1) allows for the identification of those features supporting the interfaces between templates, sustaining their inter-connection via a double loop. A set of actions is recommended to assist with the implementation of social-ecological governance, integrating templates along their interfaces:

Ecosystem services interface (from biophysical to social).

• Consider relevant beach units including substrate characteristics. The Breton coastline is heterogeneous with a mix of rocks and soft substrates as well as different tide types (from macrotidal regimen to tidal flats).A flexible yet biophysical-based unit such as the LAZ would be a meaningful framework for mapping management and related effects.

• Focus research on connectivity taking into account the conditions of macrotidal shores, the presence of cricks, and human modification of the coastline. Cases such as: species • Use the available knowledge and resources to define the role of wrack and of green tides on the LAZ. Answer questions related to the interactions between substrate (cricks, soft bottom)and stranded material interaction; test hypotheses (so far general such as e.g. in [START_REF] Trewhella | The Essential Guide to Beachcombing and the Strandline[END_REF] on the role in the trophic chain of amphibians and mammals (including bats and mustelids, so far overlooked in studies) foraging on beach wrack, and/or on local fauna successions in the dynamic process of stranded material decomposition (as reported by [START_REF] Colombini | Temporal and spatial use of stranded wrack by the macrofauna of a tropical sandy beach[END_REF] for tropical shores). Disentangling the temporal dimension of cast wrack and the fauna depending on it, would greatly support management actions.

• Consider salt marshes as extremes not only environmental (i.e. lowest energy soft sediment systems), but also in terms of social use (i.e. lowest human impact) and include them in analyses across a set of conditions, from open oceanic to sand banks of salt marshes.Brittany hosts one of the largest intertidal areas worldwide, with the Mont Saint-Michel Bay extending to Normandy on the East, and would be a paradigmatic example in this respect.

• Depict scenarios in terms of composition and quantity of seaweed beach wrack under global warming and eutrophication and seaweed exploitation conditions. Answer questions related to the shift in beach wrack management and in the definition of ecological synchronies (e.g. the co-occurrence of spring beach wrack deposition due to lifecycle of kelp and the refueling of birds during their pre-breeding migration).

Adaptive management (from biophysical to social).

• Consider the current littoral protection framework and the objectives of its establishment:

in France, all littoral areas are concerned against urbanization-artificialization by the "Loi Littoral" (law n°86-2, 3 January 1986). The law relating to the development, protection and development of the coast, known as the coastal law, is a French law which aims to regulate A c c e p t e d M a n u s c r i p t the development of the coast for the protection of the excesses of real estate speculation and to allow free public access to coastal paths. The limit to the building in a buffer strip of 100 m along the littoral is included within this legal framework. Beyond a setting against unbridled urbanization, the law mentions: "The implementation of a research and innovation effort focusing on the resources of the coast"; The sustainability of an aquatic economy: "the preservation and development of economic activities linked to proximity to water, such as fishing, marine farming, port activities, shipbuilding and repair and maritime transport"; The sustainability of a non-aquatic economy: "the maintenance or development, in the coastal zone, of agricultural or forestry activities, industry, crafts and tourism.". However, resources and implementation of these aims rely on other actors, which for Brittany are mainly (in terms of surface concerned): 1) the Natura 2000 network • Make best use of the impressive availability of data from LTERs and from specific groups (e.g. ornithologists). The information has to be communicated to the coastal zone users in the form of specific campaigns, focusing on the LAZ as a continuum and land-sea interface.

Specifically, raise awareness on the role of wrack and in particular of Ulva to sustain governance with respect to a debated topic and involving possible human health issues.

Within a vision of beach users engagement and promotion of conservation, implement actions targeting perception, e.g. investigating on the public awareness and attitudes towards LAZ resident fauna [START_REF] Leandro | Bias and perspectives in insect conservation: a European scale analysis[END_REF]; debunk incorrect perceptions (e.g.

beaches seen as piles of sand), and counteract the standardization of beach scenery (see [START_REF] Anfuso | Coastal Tourism Importance and Beach Users 'Preferences: The "Big Fives" Criterions and Related Management Aspects[END_REF], acknowledging and valuing the individual "personality" of each beach.

• Capitalize the existing citizen monitoring programs to provide standardized, high quality data at LAZ level. The example of California, where major changes in space and time were identified (e.g. Hubbard et al., 2014, reporting local extirpations) due to their long-term and large scale perspective, could be supported by fine-scale citizen monitoring actions.

The very same actions could also facilitate the integration of conservation issues in management of shoreline and ultimately the empowerment of volunteers (but see the aims of Plages Vivantes, a citizen science program of the National Museum of Natural History, Paris www.plages-vivantes.fr).

• Identify iconic species related to the LAZ (birds, including migrating and wintering birds, are better candidates, at least on a temporal base. One example of ongoing program is the specific regional action plan "PRA" "Gravelot à collier interrompu" (Charadrius alexandrinus) for which there is a specific (Brittany) regional action plan "PRA" www.bretagne-vivante.org/Nos-actions/Connaitre/Les-oiseaux/Especes-rares-etmenacees/Le-Gravelot-a-Collier-Interrompu/Un-plan-regional-d-action-pour-le-Gravelot-acollier-interrompu-en-Bretagne). The connection of iconic species with the significance ofLAZ for the completion of their life-cycle would allow for ad hoc campaigns maintaining a species-environment focus. Under a management perspective, the information related to iconic species is also a parameter proposed by [START_REF] Mclachlan | Sandy beach conservation and recreation: guidelines for optimising management strategies for multi-purpose use[END_REF] for the allocation of management priorities in the case of beach mixed use, hence this information would finally find an application outlet.

• Consider LAZ units for Driver-Pressure-State-Impact-Response studies and their outcomes as governance support (see e.g. Pirrone et al., 2005), keeping the environmental mesoscale as the appropriate level to efficiently detect DPSIR. The mosaic of geomorphologies and uses found in Brittany would in fact allow for the isolation of different phenomena at the same spatio-temporal scale.

• Sustain the current asset of littoral protection, understanding the underlying mechanisms which connect the range of tools available for sustainable management. This could reveal mismatches between theoretical and practical frameworks. With respect to the Blue Flag for example, 35 beaches (including one lake) and ten harbours were awarded in 2020 in Brittany (maps at https://bretagne-environnement.fr/dataset/communes-et-ports-labellis%C3%A9s-pavillon-bleu-en-bretagne/resource/895be2a9-3280-4f83-83af). However, most of the sites are not located in protected areas. This is likely due to the fact that Blue Flag awards in France are mainly perceived as advertising for mass tourisms. Plus, the award mechanism requires active application. As a result, only cities apply and eventually achieve the flag, while natural and protected areas do not apply, in order to avoid overtourism. Finally, because the Loi Littoral is already implementing beach quality since 1985, the connection between a further external driver such as the Blue Flag and environmental quality is weaker than in other contexts and the impact of Blue Flag remains related to the promotional aspects of tourism.

• The points listed above, the background information proceeding for this and its framing in

10.Conclusions

The use of the LAZ concept to define meaningful units as the research subject and its consideration in a context like Brittany revealed 1) important information to be retrieved as a baseline for soft sediment coastlines, and 2) directions for a potential extension of concepts, from ecological mesoscale to landscape scale. Firstly, the consideration of substrate characteristics defined the LAZ and was tightly intertwined with both biophysical and social templates. This consideration aligns with the call for broadening the use of the geodiversity concept [START_REF] Boothroyd | Old processes, new movements: the inclusion of geodiversity in biological and ecological discourse[END_REF], and is especially meaningful when considering physically-driven environments such as soft sediment shores. In practice, information from beach or salt-marsh unit level and including measurements of basic parameters such as beach profiles, granulometry and rocky patches can return extremely powerful integrated datasets.

Considering specific features with remarkable relevance both for biophysical and social template, a further recommendation -would be to retrieve baselines of biodiversity for beach units, to be compared with conditions of 1) allochthonous input, including perception and management from the social template and 2) spatio-temporal use of the resources by endemics and iconic species, including perception and management from the social template.

Regarding the up-scaling to landscape ecology, the LAZ concept would likely find a place within the integrated landscape approach framework as defined by [START_REF] Freeman | Operationalizing the integrated landscape approach in practice[END_REF]. In this respect, the conceptual extension to salt marshes is likely to open new insights into landscape ecology, supported by a gradient analysis and going beyond the "compare and contrast" approach. On the layout defined by such heterogeneity, the relevance and use of data from management (ranging from tourism exploitation to fish nurseries for commercial value to conservation as seen from the case of Brittany) would be maximized, with a shift to qualitative integrated data [START_REF] Alexander | Qualitative data sharing and synthesis for sustainability science[END_REF] for effective a) research on social-ecological systems and b) governance support.

A c c e p t e d M a n u s c r i p t

  s c r i p t distribution ranges (e.g. the coastal spider Halorates reprobus reported to have the Southern boundary of its distribution range in Brittany,[START_REF] Courtial | Breton versus British spiders: are they so different?[END_REF]; Crambe maritimum and its clustering along European coasts,[START_REF] Kadereit | Terrestrial growth and marine dispersal? Comparative phylogeography of five coastal plant species at a European scale[END_REF] and/or species disappearance (such as the carabid Eurynebria complanata and the earwig Labidura riparia,[START_REF] Courtial | Invertébrés continentaux du littoral sableux breton, poursuite de l'inventaire des dunes et des plages sableuses, évaluation de l'impact d'activités humaines et valorisation des résultats[END_REF]) could open paths to the study of structural and functional connectivity based on the heterogeneity of the coastline, backing the recorded patterns with their drivers, and provide inputs to conservation strategies.

  the social-ecological context would finally respond to the urgent need for specific legal instruments. In fact, even if several European Directives (e.g.2000/60/CE on the water quality; 79/409/CEE for avifauna; 92/43/CEE for habitat conservation; 2008/56/CE on the Marine Strategy Framework) are implemented, their framing in a social-ecological context would be essential for the sustainable management of littoral ecosystems.

Acknowledgements

The Brittany Region for its financial support throughout the Project 'BOOST Europe' IMPAIR (2019-2020) and the LTSER ZA Armorique for data sharing.

Salt marshes are present, along with embayments and exposed coastlines. "La Loi Littoral" in France prohibits since 1986 any new building outside the existing town in a buffer strip of 100m from the highest water level along sea and lake shores and is applied in Brittany.

The coastline presents "cricks", i.e. a mix of rocks and soft substrate. Cricks are likely to affect substrate mobility and dynamics in both across-and alongshore dimension of the LAZ.

They are also likely to provide microhabitats at the interface between rocky and soft sediment. though there is a lack of studies related to the food-web associated to stranded wrack as pulse resource, this is also hypothesized to be relevant to others terrestrial vertebrates such as toads and bats [START_REF] Trewhella | The Essential Guide to Beachcombing and the Strandline[END_REF].

Some beaches are periodically affected by green tides (Ulva)(Charlier et al.

2007

), which proliferation and accumulation is related to the conjunction of the natural confinement of some shallow water masses, despite the strong tidal movement, and a nitrate terrestrial load from agricultural origin (Menesguen 1992 but also see https://www.ceva-algues.com/en/ ).

Green tides occur during spring and summer and has been detrimental to recreational use of several beaches of Brittany and consequently in littoral tourism economy (Menesguen 1992).

The region has a high value related to tourism. Brittany is the forth region of