Daniel Cvejn

Sylvain Achelle
email: sylvain.achelle@univ-rennes1.fr

] Old

Ich Pytela

Jean-Pierre Malval

Arnaud Spangenberg

Nolwenn Cabon

Filip Bureš
email: filip.bures@upce.cz

Françoise Robin-Le Guen

Tripodal molecules with triphenylamine core, diazine peripheral groups and extended π π π π-conjugated linkers

Keywords: Diazines, Triphenylamines, Nonlinear Optics, Fluorescence, Intramolecular charge transfer

A series of five octupolar molecules with central triphenylamine donor and peripheral diazine electron-withdrawing groups has been prepared. These structures possess extended π-spacer comprising multiple bonds in a combination with 1,4-phenylene and 2,5-thienylene subunits.

Starting from tris(4-ethynylphenyl)amine, the key step in the preparation of the chromophores involves triple Sonogashira cross-coupling reaction. The photophysical properties of the compounds are described. A strong positive emission solvatochromism, typical for dyes featuring Intramolecular Charge Transfer (ICT), is observed, two-photon absorption (2PA) properties were also measured. Linear and nonlinear optical properties as well as

Introduction

During the past decade, organic molecules with a delocalized π-system of electron have been subject to numerous studies due to their prospective applications in field-effect transistor (OFETs), 1 organic light-emitting diodes (OLEDs) 2 and photovoltaic devices. [START_REF]UV/Vis and photoluminescence (PL) data of compounds 1-5 in Toluene (p S9)[END_REF] Among these systems, donor acceptor π-conjugated (D-π-A) materials, also called push-pull, represent a subclass with particular applications such as quadratic and cubic nonlinear optical (NLO) properties. [START_REF]Emission wavelength as a function of E T (30) for compounds[END_REF] Intrinsic NLO properties of organic push-pull molecules are mainly dictated by the extent of ICT, which is originated by the D-A interaction and can easily be illustrated by two limiting resonance forms (aromatic and quinoid/zwitterionic arrangements). 5 In contrast to ordinary linear D-π-A molecules, star-shaped molecules with D(-π-A) 3 or A(-π-D) 3 arrangements exhibit enhanced ICT and further peculiar properties due to their unique structures and manifold transitions. [START_REF][END_REF] Branching of a push-pull molecule helps to reduce aggregation and to maintain luminescence in the solid state. Star-shaped structures are also known to bring cooperative enhancement in terms of conjugation expansion and twophoton absorption (2PA) properties. [START_REF]1PA and 2PA spectra of compounds 1-5 (p S13) 8[END_REF] Among others, triphenylamine represent undoubtedly the most frequent precursor used for the construction of D(-π-A) 3 molecules and its derivatives have been widely applied as 2PA-active substances. Triphenylamine core is considered as a moderate electron donor with propeller-shaped arrangement and C 3 symmetry and, therefore, displays an octupolar feature. Acceptor terminated triphenylamine derivatives were described as excellent 2PA chromophores. 8 Such chromophores have found applications

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3 for 2PA fluorescent microscopy, 9 cellular bioimaging, 10 2PA-photodynamic therapy, 11 3D optical data storage, 12 microfabrication, 13 three-photon absorbers, 14 organic solar cells, 15 molecular glasses, 16 guest molecules for intercalation and metal-organic frameworks (MOF) 17 as well as various detectors and sensors. 18 Pyrimidine, pyrazine and quinoxaline (benzopyrazine) rings belong to the diazine heterocycles, which have been widely used as electron-withdrawing moieties in chargetransfer chromophores. 19 In particular, the 4,6-di(arylvinyl)pyrimidine motif has been synthetically well-established and push-pull molecules based on this scaffold delivered large 2PA cross-section. 20 Linear charge-transfer chromophores bearing diazine ring were investigated for their fluorescence, 21 2 nd order NLO 22 and 2PA properties. 23 Recently, Martin and coworkers have reported some star-shaped molecules with triphenylamine core and pyrimidine and pyridazine rings as peripheral groups and some of these tripodal molecules exhibited promising 2PA cross-section. 24 In general, the electron donor and acceptor as well as the π-conjugated bridge are integral parts of the D-π-A system and their optimization represents a key parameter to enhance the ICT and maximize the fluorescence intensity and 2PA cross-section. 5 Hence, the main purpose of the present article is to design and synthesize new star-shaped molecules 1-5 based on triphenylamine equipped with diazine peripheral acceptors and extended π-conjugated linkers (Chart 1). Influence of the length, planarity and composition of the π-linker as well as the nature of the diazine rings (nitrogen atom topology and further extension) on the photophysical and electronic properties will be thoroughly studied.

<Insert Chart 1.>

Results and discussion

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4 Preparation of star-shaped molecules. Arylvinyldiazines can easily be obtained by Knoevenagel condensation of corresponding aldehyde with commercially available methyldiazines. Vanden Eynde and co-workers described the synthesis of arylvinyldiazines using boiling aqueous 5 M NaOH in the presence of Aliquat 336 as a phase-transfer catalyst. 25 These conditions proved to be very efficient in the case of 4-methylpyrimidine and 2-methylquinoxaline and smoothly afforded bromo-substituted intermediates 6-9 in the yields of 54-79 % (Scheme 1). On the contrary, using 2-methylpyrazine and similar reaction condition provided only traces of the desired arylvinyldiazine 10. This is most likely due to a lower acidity of the hydrogens appended at the methyl group. However, potassium hydroxide in DMSO (dimsyl potassium) at room temperature proved to be a sufficient base to abstract 2-methylpyrazine hydrogens 26 and subsequent reaction with 5-bromothiophene-2carbaldehyde afforded compound 10 (Scheme 2). In this way, compound 10 was obtained in good yield and its purification was facile. Stereochemical outcome of the aforementioned Knoevenagel reactions was verified by 1 H NMR spectroscopy where all intermediates 6-10 showed pair of doublets with the coupling constants 3 J(H,H) about 16 Hz, which clearly evidences selective formation of (E)-configured compounds 6-10. 27 <Insert Scheme 1> <Insert Scheme 2> Arylvinyldiazines 6-10 were further used as coupling partners for the construction of target molecules 1-5 (Scheme 3). Threefold Sonogashira cross-coupling reaction with tris(4ethynylphenyl)amine (11) 8a carried out with Pd(0)-precatalysts, diisopropylamine, 1,4-dioxane and the temperature elevated up to 92 °C afforded star-shaped chromophores 1-5. 1 andS8. The analyses were carried out using low concentrated solutions (1.0 to 2.0 × 10 -5 M for UV/Vis spectra and 1.0 to 3.0 × 10 -6 M for PL spectra). As an example, the spectra for derivatives 1 and 4 in dichloromethane are shown in Figure 1. Under these conditions, self-absorption effects were not observed. All compounds proved to be photostable and none E to Z isomerization was observed under the given conditions.

Besides

<Insert Table 1> <Insert Figure 1> In dichloromethane, all target compounds exhibited absorption in visible region with the position of the longest-wavelength absorption maxima ( max ) appearing at 401 to 436 nm. In all cases a second particularly developed band of higher energy was also observed. Molecular extinction coefficients of 1-5 (ε = 74.5 to 108.3 mM -1 .cm -1 ) are approximately two to three times higher than that of corresponding linear molecules. 22a Depending on the compound used, the emission maxima were observed in the green to orange region ( max = 555-630 nm). Interestingly, fluorescence

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quantum yields are relatively high (Φ F = 0.37-0.67) even though the emission is red-shifted.

Nevertheless, except for compound 2, the fluorescence quantum yields are higher in toluene than in dichloromethane: a quantum yield up to 1.00 was observed for compound 1 in toluene. As expected for donor-acceptor functionalized π-conjugated molecules featuring the ICT, large Stokes shifts (up to 8340 cm -1 ) were observed, which indicates large differences (vibrational, electronic, geometric) between the Franck-Condon state and the emissive excited state.

When comparing the nature of the peripheral diazine ring, the most red-shifted emissions were observed for quinoxaline compounds 3 and 4, whereas the most hypsochromically shifted emission was observed for pyrazine substituted compound 5. A similar trend was observed previously. 22e The highest fluorescence quantum yield in dichloromethane is observed for pyrazine derivative 5.

In general, replacement of 1,4-phenylene moieties in compounds 1 and 3 by 2,5-thienylene rings (compounds 2 and 4) led to a red shift of both absorption and emission spectra. This is in accordance with the previous observation on linear push-pull chromophores. 28,5 However, in case of quinoxaline derivatives 3 and 4, in dichloromethane, a blue-shifted emission is observed for thienylene derivative 4.

In an effort to gain further insights into the photophysical process within these push-pull molecules, the absorption and emission behavior of the compounds 1-5 was studied in a variety of different aprotic solvents. While the positions of the longest-wavelength absorption maxima were not significantly affected, an increase in the solvent polarity led to bathochromic shifts of the emission maxima (Table 2). This behavior is generally observed for chromophores with ICT. 7d,9b,9d,21,22a As an example, the spectra recorded for compound 2 are shown in Figure 2. When going from the least polar hexane ( em = 468 nm) to toluene, dioxane, THF, CH 2 Cl 2 and acetone, the emission maxima was considerably red-shifted up to

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DMSO, the emission intensity was low and less detectable. When increasing the polarity, the emission spectra are less structured and emission bands are larger. To further illustrate strong emission solvatochromism of target star-shaped chromophores, Figure 3 shows the emission color of 4 in different solvents. Such behavior is consistent with a stabilization of the highly polar emitting excited states by polar solvents and has also been observed either for triphenylamine derivatives bearing electron-withdrawing groups 8b,9,29 or for diazines substituted with electron-donating substituents. 21,22a,22e,30 The emission solvatochromism represents one of the possible ways to evaluate ICT upon excitation in fluorescent molecules.

Hence, the emission maxima were plotted versus the E T (30) Dimroth-Reichardt polarity parameters 31 (see the Supporting Information). For compounds 1-3 are the slope of the regression line slightly higher than for their linear analogues indicating a branching effect that enhances the ICT. c,d,22c,e,33 These molecules appear therefore as potential colorimetric pH sensors. However, it should be noted that these color changes appear at higher concentration of TFA (10 -2 -10 -1 M) than for other diazine derivatives with shorter π-conjugated linker (10 -5 -10 -3 M). 34 On the other hand, the emission was totally quenched after treatment with acid, as is generally observed for amino electron donor substituted diazines. 21a,d,34 <Insert Figure 4 Frenkel exciton model, 34 all those spectral effects should be ascribed to a significant interbranch coupling within these multipolar chromophores which leads to a splitting of the initially degenerate single excited levels associated to each molecular synthon. 7d,13a,35 Such a coupling is consequently responsible for the presence of multiple electronic π-π* transitions within the longest wavelength of the 1PA band as previously observed for similar trigonal branched structures. 7d,13a,35a,36,36 It should be noted that the 2PA cross sections are clearly enhanced on going from 1,4-phenylene group used as 'electron relay' to a 1,2-thienylene one (compounds 1 and 3 versus 2 and 4). As expected, changing the pyrimidine end groups by pyrazine ones induces a noticeable decrease of δ max (chromophores 1 versus 5). However, 4.

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The first oxidation seems to be quasi-reversible, mostly one-electron process (irreversible for 4) and most probably involve the central triphenylamine donor. The first reduction is a reversible two-electron process that may involve mostly diazine acceptor moiety and the adjacent π-system. The first oxidation/reduction potentials were found within the range of 0.61 to 0.69 and -2. <Insert Table 4 >

Computational studies

The spatial and electronic properties of chromophores 1-5 were investigated using Gaussian W09 package 37 at the DFT level. The initial geometries of molecules 1-5 were estimated by PM3 method implemented in ArgusLab 38 and these were subsequently optimized by DFT HCTH/6-311+G(d) method. The energies of the HOMO and LUMO, their differences and ground state dipole moments were calculated with the DFT B3LYP/6-311++G(2d,p) and are summarized in Table 4.

The calculated energies of the HOMO and LUMO of 1-5 were found within the same range as electrochemically measured values as can be seen on the energy level diagram shown in Figure 7. Moreover, both energies correlate tightly and, therefore, the used DFT method can be considered as a reasonable tool for description of the electronic properties of 1-5 (see the SI).

<Insert Figure 7> Very similar trends as seen by the CV measurements can also be deduced from the DFT calculated energies of the HOMO and LUMO and their differences. The HOMO-LUMO gap can be reduced by replacement of the peripheral acceptor within the order of

5 > 1 > 2 > 3 M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

Conclusions

In summary, we have successfully synthesized and characterized a series of triphenylaminebased tripodal molecules bearing peripheral diazine electron-attracting groups. The key step of their synthesis involves threefold Sonogashira cross-coupling reaction between tris(4ethynylphenyl)amine (donor central part) and the corresponding bromoarylvinyldiazine derivatives (peripheral acceptors). These compounds exhibit absorption in the blue region and are highly luminescent with significant Stokes shifts. For all the compounds a strong emission solvatochromism was observed in a variety of non-polar solvents. This finding supports formation of a very polar excited ICT state. These materials exhibit 2PA properties, measured by 2PA excited fluorescence spectroscopy within the spectral range of 700 to 1020 nm. 2PA cross sections up to 427 GM were obtained. The HOMO and LUMO levels were estimated by

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12 cyclic voltametry measurements and DFT calculations. From all the performed measurements and calculations is apparent that derivatives 2 and 4, bearing 2,5-thienylene spacer, exhibit the highest 2PA cross section and the lowest HOMO-LUMO gap even if their solvatochromic ranges are lower than that of their 1,4-phenylene analogues. The pyrimidine and quinoxaline derivatives display similar 2PA cross sections but quinoxaline compounds showed lower HOMO-LUMO gap. In view of the current wide and general interest in novel 2PA absorbers, we believe that this joint study would be considered as a very useful guide in designing octopolar tripodal molecules bearing diazine peripheral acceptor groups.

Experimental Section

General. Compounds 6-8 were obtained according to reported procedures. 22a,22e Compound 11 was obtained from tris(4-iodophenyl)amine and TMSA by Sonogashira cross-coupling reaction according to reported procedure. 8a,40 All air-and moisture-sensitive reactions were carried out in flame-dried glassware subsequently cooled under nitrogen. NMR spectra were acquired at room temperature on a Bruker AC-300 spectrometer. Chemical shifts are given in parts per million relative to TMS ( 1 H, 0.0 ppm) and CDCl 3 ( 13 C, 77.0 ppm). Acidic impurities in CDCl 3 were removed by treatment with anhydrous K 2 CO 3 . High resolution mass analyses were performed at the "Centre Régional de Mesures Physiques de l'Ouest" (CRMPO, University of Rennes1) using a Bruker MicroTOF-Q II apparatus. IR spectra were recorded on a Perkin-Elmer spectrum1000 FTIR using KBr plates. UV/vis spectra were recorded with a UVIKON xm SECOMAM spectrometer using standard 1 cm quartz cells. Fluorescence spectra were recorded using Spex FluoroMax-3 Jobin-Yvon Horiba apparatus. Compounds were excited at their absorption maxima (longest-wavelength absorption band) to record the emission spectra. The Φ F values were calculated using a well-known procedure using 9,10diphenylethynylanthracene in cyclohexane as standard. 41 Stokes shifts were calculated by considering the lowest energetic absorption band.

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13

2PA measurement. The two-photon absorption (2PA) measurements were performed with femtosecond mode-locked laser pulse using a Ti: Sapphire laser (Coherent, Chameleon Ultra II: pulse duration: ~140 fs; repetition rate: 80 MHz; wavelength range: 680-1040 nm). A relative 2PEF method 42 was employed to measure the two-photon absorption cross-sections, δ. The measurements of 2PA cross-sections were performed relative to reference molecules (r) such as fluorescein 40,43 in water at pH = 11. The value of for a sample (s) is given by:

r S S S r r r r S S c S c S δ η η δ . Φ Φ =
Where S is the detected two-photon excited fluorescence integral area, c the concentration of the chromophores, and Φ is the fluorescence quantum yield of the chromophores. is the collection efficiency of the experimental set-up and accounts for the wavelength dependence of the detectors and optics as well as the difference in refractive indices between the solvents in which the reference and sample compounds are dissolved. The measurements were conducted in a regime where the fluorescence signal showed a quadratic dependence on the intensity of the excitation beam, as expected for two-photon induced emission. For the calibration of the two-photon absorption spectra, the two-photon excited fluorescence signal of each compound was recorded at the same excitation wavelength as that used for standards (i.e. λ exc = 782 nm for fluorescein). The laser intensity was in the range of 0.2-2 x 10 9 W/cm 2 .

The experimental error on the reported cross section is 15 %. 9): 2-Methylquinoxaline (720 mg, 5.00 mmol) and 5-bromothiphene-2-carbaldehyde (955 mg, 5.00 mmol) were added to a solution of NaOH (25 mL, 5M) with addition of approx. 100 mg (0.25 mmol) of Aliquat® 336. The resulting emulsion was heated up to reflux for 2 h. After cooling to 25 °C, the gummy-like precipitate was decanted, washed with water (2 × 25 mL) and dried in vacuo. The crude

(E)-2-(2-(5-Bromothiophen-2-yl)vinyl)quinoxaline (

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14 product was crystallized from DCM/n-heptane to afford 860 mg (54 %) of 9 as a brownish yellow solid. Mp: 107-109 °C 1 H-NMR (300 MHz, CDCl 3 ): (ppm) : 7.02 -7.07 (m, 3H), 7.73 (dquint, 2H, 3 J = 6.9 Hz, 4 J = 1.5 Hz), 7.89 (d, 1H, 3 J = 15.9 Hz), 8.04 (dt, 2H, 3 J = 6.9 Hz, 4 J = 1.5 Hz), 8.92 (s, 1H). 13 General procedure for Sonogashira cross-coupling reaction: A mixture of 4,4',4''tris(ethynyl)triphenylamine 11 (80 mg, 0,25 mmol) and the corresponding bromo derivative (0.91 mmol) was dissolved in a mixture of 1,4-dioxane (20 mL) and diisopropylamine (5 mL).

Nitrogen was bubbled through the mixture for 10 min whereupon [Pd(PPh 3 ) 4 ] (29 mg, 0.025

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mmol, 10 %) and CuI (9 mg, 0.05 mmol, 20 %) were added and the reaction mixture was stirred 92 °C for 12 h. The reaction was quenched with water (50 mL), extracted with DCM (2 × 100 mL) and the combined organic extracts were dried over MgSO 4 and the solvents were evaporated in vacuo. The crude product was purified by column chromatography with the indicated stationary phase and the solvent system.

Tris-{4-[4-((E)-2-pyrimidin-4-yl-vinyl)-phenylethynyl]-phenyl}-amine (1):

Tris-{4-[5-((E)-2-pyrimidin-4-yl-vinyl)-thiophen-2-ylethynyl]-phenyl}-amine (2):

The title compound was obtained from 7 following the general procedure for Sonogashira crosscoupling reaction and was purified by column chromatography (Al 2 O 3 , EtOAc). Reddishorange solid (61 mg, 28 %). Mp: 110 °C (dec). 1 H-NMR (300 MHz, CDCl 3 ): (ppm) : 6.83 (d,3H,3 J = 15.9,Hz),7.11 (d,6H,3 J = 8.7 Hz),7.19 (d,3H,3 J = 3.4 Hz),7.20 (d,3H,3 J = 3.4 Hz),3H),7.46 (d,6H,[START_REF]UV/Vis and photoluminescence (PL) data of compounds 1-5 in Toluene (p S9)[END_REF]

Tris-{4-[5-((E)-2-quinoxalin-2-yl-vinyl)-thiophen-2-ylethynyl]-phenyl}-amine (4):

The title compound was obtained from 9 following the general procedure for Sonogashira crosscoupling reaction and was purified by column chromatography (SiO Table 1. UV/Vis and photoluminescence (PL) data in CH 2 Cl 2 .

Table 2. Emission solvatochromism of diazine derivatives 1-5 in various aprotic solvents.

Table 3. Two-photon absorption photophysical properties of compounds 1-5 in dichloromethane. a All spectra were recorded at room temperature at c = 1.0 to 2.0 × 10 -5 M for absorption and c = 1.0 × to 3.0 × 10 -6 M for emission. b Fluorescence quantum yield (±10%) determined relative to 9,10-bisphenylethynyl-anthracene in cyclohexane (Φ F = 1.00). 1 c Calculated using less energetic absorption band.

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M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT Compd a UV/vis λ max , nm (ε,mM -1 •cm -1 ) PL λ max , nm Φ F b Stokes shift c cm -1

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  tripodal molecules 1-5, a formation of twofold and single cross-coupled products was could partially be suppressed by an excess of bromo derivatives 6-10.However, the relatively low isolated yields of 23-60 % are mainly caused by difficult separation of 1-5 from the side products by column chromatography (repeated purifications were required). All of the new materials exhibited good solubility in a variety of solvents, especially in THF and chlorinated solvents. The 1 H, 13 C NMR and HRMS data were consistent with the expected structures. <Insert Scheme 3> UV-vis and Fluorescence Spectroscopy. The UV-Vis and photoluminescence (PL) spectroscopic data of star-shaped molecules 1-5, measured in dichloromethane and in toluene at 25 °C, are presented in Tables

3 and 4 32 <Insert 3 >

 34323 22a When comparing the different diazine rings, quinoxaline derivatives exhibited the highest regression line slopes. A replacement of 1,4-phenylene by 2,5thienylene units in compounds 2 and 4 versus 1 and 3 tends to reduce the regression line slope and, therefore, chromophore 3 showed the highest regression line slope. When compared with star-shaped triphenylamine derivatives equipped with trifluoromethanesulfonate acceptors and similar π-conjugated linkers, the quinoxaline derivatives 3 and 4 showed very similar solvatochromic behavior.Dichloromethane solutions of compounds 1-5 underwent an important color change upon addition of trifluoroacetic acid (TFA) with an important bathochromic shift of the absorption spectra (Figures4-5). We had already observed this phenomenon, which is due to the protonation of the diazine ring that enhances its electron-withdrawing character and into the molecule. 21a,

> <Insert Figure 5 >

 5 Two photon absorption measurements. 2PA data of chromophores 1-5, measured by relative two-photon excited fluorescence (2PEF) method, are presented in Table3, and the 2PA spectra are shown in Figure6. According to our spectral resolution, all octupoles exhibit a broad 2PA band whose maximum absorption wavelength is slightly lower than twice λ max of the one photon absorption (1PA) band (see supporting information). The 2PA cross section (δ max ) are comprised between 158 and 427 GM. It is noteworthy that the thiophene-based derivatives 2 and 4 present another distinctive 2PA band around 940 nm. According to the

  (benzopyrazine) derivatives 3 and 4 gave similar δ max values in comparison with their pyrimidine analogues 1 and 2. In term of action cross section, defined by the product of 2PA cross section and fluorescence quantum yield, compound 2 showed the highest figure of merit with Φ×δ max = 248 GM. chromophores 1-5 was investigated by cyclic voltammetry in DMF-[Bu 4 N][PF 6 ] at a scan rate of 0.1 V/s. The working electrode was a glassy carbon disk, tungsten wire was used as the counter electrode and a non aqueous Ag/Ag + electrode was used as reference. A representative CV diagram of chromophore 5 is shown in the SI, and the voltammogram shows two oxidation and two reduction waves. The second oxidation and the second reduction are irreversible processes. The first oxidation/reduction potentials as well as the electrochemical gap Eg elchem and recalculated energies of the HOMO and the LUMO are presented in Table

  02 to -1.70 V, respectively. Whereas the first oxidation is affected by the structural changes in 1 and 3-5 only negligibly, replacement of the peripheral diazine acceptor moieties and composition of the π-linker have significantly influenced the first reduction potentials in 1-5 and the corresponding electrochemical gap. A comparison of electrochemical behavior of chromophores 1, 3 and 5 allows evaluation of pyrimidine, quinoxaline and pyrazine moieties. The quinoxaline strongest withdrawing property is M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 10 reflected by the most positive reduction potential and the lowest electrochemical gap being measured for 3. Replacement of 1,4-phenylene by 2,5-thienylene units in 1/2 and 3/4 shifted the first reduction positively, while the gaps of 3 and 4 are almost identical. Thus, chromophores 3/4 bearing the quinoxaline acceptor showed the lowest electrochemical gap as well as the most bathochromically shifted CT-bands, especially for 4 with 2,5-thienylene units. The same tendencies are observed for optical gap.

11> 4 .

 4 Chromophores 3 and 4 bearing the most electron withdrawing quinoxaline moieties possess the lowest calculated gaps of 2.54 and 2.31 eV. The latter is further reduced by the presence of 2,5-thienylene instead of 1,4-phenylene π-linker. Positions of the frontier molecular orbitals in 1-5 are visualized in Figure 8. The HOMO is localized on the central amino donor and adjacent alternating positions of the triphenylamine core, whereas the LUMO is spread over the peripheral diazine acceptors and the adjacent π-linker. Both frontier orbitals are clearly separated which further confirms CT character of chromophores 1-5. <Insert Figure 8 > Considering tripodal character and C3 symmetry group of each derivative 1-5, the LUMO is localized over one or two particular branche(s). The third branch is occupied by the LUMO+1. The HOMO as well as the HOMO-1 remained on the central amino donor. This is a common feature and has already been observed earlier for tripodal molecules based on triphenylamine. 8a,39 As expected, the calculated ground state dipole moments µ G are modest within the range of 0.08 to 4.79 D, which reflects tripodal character of chromophores 1-5.

  Scheme 1. Synthesis of bromo-substituted intermediates 6-9.

Figure 1 :

 1 Figure 1: Normalized UV/vis (solid lines) and emission spectra (dashed lines) of

Figure 2 .

 2 Figure 2. Normalized emission of compound 2 in different aprotic solvents.

Figure 3 .

 3 Figure 3. Fluorescence color changes for 3 in various solvents (from left to right: hexane,

Figure 4 :

 4 Figure 4: Color change of a DCM solution (c =10 -1 M) of 4 in the presence of TFA.

Figure 5

 5 Figure 5: UV/vis spectra of 2 in DCM (c = 6 10 -6 M) with and without addition of TFA (c = 10 -1 M).

Figure 6 :

 6 Figure 6: Two-photon absorption spectra of the chromophores 1-5 in dichloromethane.

Figure 7 :

 7 Figure 7: Energy level diagram showing electrochemical (black) and DFT calculated

Figure

  Figure 8: HOMO/HOMO-1 (red) and LUMO/LUMO+1 (blue) localization in 4 and

  central triphenylamine and peripheral diazine were prepared.A strong positive emission solvatochromism is observed.These materials exhibit two-photon absorption properties.

a

  All spectra were recorded at room temperature at c = 1.0 × 10 -5 M to 2.0 × 10 -5 M for absorption and c = 1.0 × 10 -6 M to 3.0 × 10 -6 M for emission. b Fluorescence quantum yield (±10%) determined relative to 9,10-Bis-phenylethynyl-anthracene in cyclohexane (Φ F = 1.00). i c calculated using less energetic absorption band. of solution of chromophore 5 in various solvents (c = 10 -5 M) taken under irradiation with a handheld UV lamp (365 nm). From left to right: Hexane, Toluene, dioxane, THF, DCM.

Table 4 . Electrochemical, a and DFT calculated data as well as optical gap for chromophores 1-5.

 4

  All potential are given vs. ferrocene; b E HOMO/LUMO = -(E 1/2(ox1/red1) + 4.8); 1 c Eg = E HOMO -E LUMO ; d Undistinguishable wave; e Irreversible peak, stand for Ep; f All data were

					ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT
	Compd Comp.	Hexane E T (30) a = 31.0 E T (30) a = 33.9 E T (30) a = 36.0 Toluene Dioxane λ max , nm λ max , nm λ max , nm Compound λ 1PA / nm λ 2PA / nm E T (30) a = 37.4 THF λ max , nm δ max / GM a E T (30) a = 40.7 CH 2 Cl 2 λ max , nm Φ×δ max / GM Acetone E T (30) a = 42.2 λ max , nm 1 401 760 256 141 E 1/2 (ox1) E 1/2 (red1) E HOMO E LUMO Eg elchem Eg optical E HOMO DFT E LUMO DFT Eg DFT / V a / V a / eV b / eV b / eV c / eV / eV f / eV f / eV f	MeCN E T (30) a = 45.6 λ max , nm µ G / D f
	1 2	1 2 3	441 468, 493 0.61 -1.91 2 458 3 -d -1.70	463 487 -5.41 422 484 413 -	479 501 -2.89 499 -3.10	800 2.52 780 -	539 551 2.63 576 2.53	427 239	577 587 -5.42 630 -5.32	-2.72 248 88 -2.74	-b 618 2.70 664 2.58	3.62 0.08	-b -b -b
	3	4 5	485, 516 432 0.63 -1.77 4	508, 535 448 436 -5.43	533 461 -3.03	760 2.40	580 513 2.57	416	617 551 -5.31	158 -2.77	635 581 2.54	4.79	-b 612
	4 5		a Dimroth-Reichardt polarity parameter,kJ⋅mol -1 . b no signal detected M A N U S C R I P T 5 404 740 158 106 a The experimental error on 2PA cross section is 15%. M 0.69 e -1.72 -5.49 -3.08 2.41 2.43 -5.23 -2.92 0.64 -2.02 -5.44 -2.78 2.66 2.71 -5.33 -2.44 A N U S C R I P T calculated by DFT method HCTH/6-311+G(d)//B3LYP/6-311++G(2d,p).	2.31 2.89	1.40 0.48
			A C C E P T E D A C C E P T E D

a 1 A. A. Isse, A. Gennaro, J. Phys. Chem. B, 2010, 114, 7894-7899.

I. B. Berlman, (1971) "Handbook of Fluorescence Spectra of Aromatic Molecules," Academic Press, N.Y.

H and 13 C-NMR spectra of chromophore 1 (300/75 MHz, CDCl 3 ).

H and 13 C-NMR spectra of chromophore

(300/75 MHz, CDCl

).

H and 13 C-NMR spectra of chromophore 3 (300/75 MHz, CDCl 3 ).

H and 13 C-NMR spectra of chromophore 4 (300/75MHz, CDCl 3 ).

H and 13 C-NMR spectra of chromophore 5 (300/75 MHz, CDCl 3 ).

H and 13 C-NMR spectra of intermediate 9 (300/75 MHz, CDCl 3 ).

H and 13 C-NMR spectra of intermediate 10 (300/75 MHz, CDCl 3 ).

1PA (blue) and 2PA (red) spectra (plotted against half of the excitation wavelength) of 1 in dichloromethane.1PA (blue) and 2PA (red) spectra (plotted against half of the excitation wavelength) of 2 in dichloromethane.

1PA (blue) and 2PA (red) spectra (plotted against half of the excitation wavelength) of 3 in dichloromethane.1PA (blue) and 2PA (red) spectra (plotted against half of the excitation wavelength) of 4 in dichloromethane.

1PA (blue) and 2PA (red) spectra (plotted against half of the excitation wavelength) of 5 in dichloromethane.

SUPPORTING INFORMATION

Star-shaped molecules with triphenylamine core, diazine peripheral groups and extended π π π π-conjugated linkers


Guillaume Grillet
email: guillaume.grillet@chu-rennes.fr

Nicolas Marjanovic
email: nicolas.marjanovic@chu-brest.fr

Jean-Marc Diverrez
email: marc.diverrez@chu-brest.fr

Pierre Tattevin
email: pierre.tattevin@chu-rennes.fr

Jean- Marc Tadié
email: jean-marc.tadie@chu-rennes.fr

Erwan L'her
email: erwan.lher@chu-brest.fr

Intensive care medical procedures are more complicated, more stressful, and less comfortable with Ebola personal protective equipment: A simulation study

Keywords: Ebola virus disease, Intensive care, Protective equipment, Stress, Workload, Task performance

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

To the editor Ebola virus disease (EVD) is a life-threatening condition. Appropriate management of organ failure, hemodynamic instability, and metabolic disorders significantly improves survival. This implies that life-saving procedures are undertaken in case of need, including endotracheal intubation, nasogastric tube placement and central venous catheter (CVC) insertion. The challenge is to provide high quality of care to patients with life-threatening EVD, under optimal safety conditions for health care workers, i.e. with reinforced personal protective equipment (PPE), ensuring that no exposure to patient blood or any other body fluid occur [START_REF] Shears | Ebola virus disease in Africa: epidemiology and nosocomial transmission[END_REF][START_REF] Cohen | Infectious diseases. When Ebola protection fails[END_REF][START_REF] Rouveix | Promoting the Safety of Healthcare Workers in Africa: From HIV Pandemic to Ebola Epidemic[END_REF]. We assessed the impact of Ebola PPE use on the performance of senior ICU physicians during common intensive care unit (ICU) procedures, and on the workload, in a simulation environment.

The study was performed in our simulation department. Thirteen volunteer senior ICU physicians performed orotracheal intubation and nasogastric tube placement on a simulation mannequin (Megacode Kelly Sim, Laerdal™, Stavanger, Norway), and CVC insertion on a dedicated echogeneous task trainer (CAE Healthcare™, St Laurent, QC, Canada). The use of ultrasound for CVC insertion was left to the discretion of the physician. Each participant had to complete all procedures twice, one with standard protection, and one with Ebola PPE, in an order determined by the randomization. The Ebola PPE consisted in a N95 Particulate Filtering Face Respirator, with large protective glasses, surgical hood, fluid resistant coveralls, boot covers, and two pairs of gloves, as recommended [START_REF]WHO updates personal protective equipment guidelines for Ebola response[END_REF]. All sequences were recorded using an HD video camera. Participants were equipped with a thoracic belt for the monitoring of heart rates and chest wall movements to record upper body tilt (Zephyr BioHarness 3 TM , Annapolis, USA). Procedural time was independently assessed by two reviewers, using a video tagging software (StudioCode TM , Marseille, France). Equipment ergonomics were assessed for each participant after the complete course, using a Likert scale

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

3 asking three questions for each procedure: i) stress during the task (from 1, major stress to 5, no stress); ii) equipment's ease of use (from 1, very difficult to 5, very easy), and iii) comfort (from 1, very uncomfortable to 5, very comfortable). The task workload was assessed using the National Aeronautics and Space Administration Task Load Index (NASA-TLX), a multifaceted tool for perceptual (subjective) workload evaluation [START_REF] Hart | Development of NASA-TLX (Task Load Index): Results of Empirical and Theoretical Research[END_REF], based on a weighted average of ratings on six subscales: mental demand, physical demand, temporal demand, performance, effort, and frustration. These steps provide a global score from 0 to 100, higher values indicating heavier workload. All tests were performed using Statview 5.0.1 (SAS Institute, Cary, NC, USA). Mann-Whitney and Kruskall Wallis non-parametric tests were used for procedural time analysis and Likert scale comparisons. Physiological parameters comparison used a Laird and Ware regression test with R [START_REF] Dean | Generalized linear mixed models: a review and some extensions[END_REF], and the lme4 library [START_REF] Zhang | On fitting generalized linear mixed-effects models for binary responses using different statistical packages[END_REF].

NASA-TLX data were compared using a non-parametric Wilcoxon test. Data are provided as median [interquartile ratio], unless specified otherwise.

Global success rate for the complete procedure course was 100% with standard protection, and 85% with Ebola PPE (p=0.48). Higher degrees of body tilt were measured with Ebola PPE, as compared to standard protection (p<0.05). In most cases, procedures were rated as easier, and more comfortable with standard protection, than with Ebola PPE (Table 1). Median global task load index was higher with Ebola PPE, as compared to standard protection, for orotracheal intubation (44.3 vs. 20.3, p=0.007), and nasogastric placement (38.9 vs. 25.6, p=0.008, Figure 1). For CVC insertion, global task load index was not significantly different for the whole group (58.6 vs. 37.6, p=0.182). However, differences were significant for the 7 physicians who performed ultrasound-guided procedures (54.9

[34.7-67.1] vs. 29.5 [14.1-50.1], p=0.028), but not for the 6 physicians who didn't use ultrasound (76 [52.9-84.9] vs. 46.3 [36-58.9], p=0.686). Physical demand was higher with Ebola PPE as compared to standard protection for nasogastric tube placement (2.5 [0.9-5. Although these findings are not unexpected given the constraints and discomfort of Ebola PPE reported by health care workers, the application of simulation procedures in this study provided evidence-based, and quantitative data, on the impact of currently recommended Ebola PPE on these life-saving procedures. This innovative model may be used to compare different combinations of Ebola PPE, and select those with lower impact on quality of care and workload, while still ensuring effective protection. Person-to-person transmission of Ebola virus primarily occurs through contact with infected patients' fluids [START_REF] Shears | Ebola virus disease in Africa: epidemiology and nosocomial transmission[END_REF][START_REF] Cohen | Infectious diseases. When Ebola protection fails[END_REF][START_REF] Rouveix | Promoting the Safety of Healthcare Workers in Africa: From HIV Pandemic to Ebola Epidemic[END_REF]. During ICU procedures, often performed in emergency, physicians are at risk for contamination [START_REF] Zhang | On fitting generalized linear mixed-effects models for binary responses using different statistical packages[END_REF][START_REF] Wolf | Severe Ebola virus disease with vascular leakage and multiorgan failure: treatment of a patient in intensive care[END_REF][START_REF] Markovic-Denic | Occupational exposure to blood and body fluids among health-care workers in Serbia[END_REF], and the use of a dedicated Ebola PPE is mandatory. All health care providers potentially involved must receive adequate training for correct use of Ebola PPE, including safety (i.e. no breaches in protection), and efficacy (i.e. being able to provide appropriate care with maximum ease, dexterity, comfort, and minimal stress) [START_REF] Rouveix | Promoting the Safety of Healthcare Workers in Africa: From HIV Pandemic to Ebola Epidemic[END_REF][START_REF]WHO updates personal protective equipment guidelines for Ebola response[END_REF]. Because the management of patients with EVD is a complex process, simulation plays a major role in the preparation of health care worker to anticipate the difficulties that may arise while taking care of patients suspected of, or confirmed with, EVD [START_REF] Gaba | Simulation as a critical resource in the response to Ebola virus disease[END_REF].

Our study has limitations. First, sample size was limited. However, this study was performed in a homogeneous group of senior, experienced ICU physicians. Second, the use of manikins, in a simulated environment, is merely a proxy for the actual clinical scenario. However, only simulated conditions enables to perform a standardized, randomized study, with prospective collection of an extensive set of data, and extensive post-hoc debriefing. We found that common ICU procedures are more complicated, more stressful, and less comfortable, with Ebola PPE. These necessary protections increase workload, and may be associated with increased risk of severe adverse events, either for the patient (procedure

  6 [0.4-0.9], p=0.022), and CVC insertion (3.6 [1.8-13.4] vs. 1.2 [0.4-2.5], p=0.009), but nor for orotracheal intubation.

failure, complications), or the physician (Ebola virus transmission). Ebola PPE should be evaluated by simulation and ergonomics studies as the one reported herein, to optimize the selection of Ebola PPE that would ensure both the safety of health care workers, and the quality of care for patients suspected of EVD.

Acknowledgment

The authors thank all the physicians from the French Medical Intensive Care Training Team of Western France, and the dedicated technicians from the simulation department of Brest University of Medicine, who made this study possible.

Transparency declaration

All authors: no potential conflict of interest.

Highlights

• This simulation study analyzed the impact of Ebola personal protective equipment (PPE) on ICU procedures Stress, Ease of use and comfort during the procedures are Likert scales from 1 to 5: For Stress during the task, 1 for major stress and 5 no stress; for ease of use, 1 very difficult and 5 very easy; for comfort, 1 very uncomfortable and 5 very comfortable.


Ridha Bellouz

Sami Kallel

Kamel Khirouni

Octavio Peña

Octavio Pena

Mohamed Oumezzine

Structural, Electrical conductance and complex impedance analysis of (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (0 x 0.20) perovskite

Keywords: Perovskite, X-ray diffraction, Impedance spectroscopy, AC conductivity

come

Introduction

The investigation of manganites with general formula Ln 1-x A x MnO 3 (Ln=La, Nd, Pr, etc., A=Sr, Ca, Ba, etc.) has attracted extensive attention in the last decades due to its special physical properties and potential application. These manganite materials exhibit remarkable physical properties including colossal magnetoresistance (CMR), charge ordering [START_REF] Tokura | For a review, see Colossal-Magnetoresistive Oxides[END_REF][START_REF]Charge Ordering and Related Properties of Manganese Oxides[END_REF] and metal-insulator transition and also have potential applications for magnetic recording and magnetic sensors [START_REF] Jin | Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films[END_REF][START_REF] Yang | Structural, transport, and magnetic properties in the Ti-doped manganites LaMn 1 K x Ti x O 3 (0 x 0.2[END_REF]. Most of the manganites presenting the CMR effect have a paramagnetic-ferromagnetic (PM-FM) transition at the Curie temperature. Their electrical resistivity shows a semiconductor behavior above T C and a metallic behavior below T C . The understanding of CMR phenomenon, metallic behavior and the strong FM interactions is generally based on the double exchange (DE) model [START_REF] Zener | Interaction between the d-Shells in the Transition Metals. II. Ferromagnetic Compounds of Manganese with Perovskite Structure[END_REF]. In this model, there is an exchange of electrons from neighbouring Mn 3+ to Mn 4+ ions through oxygen when their core spins are parallel and hopping is not favoured when they are antiparallel. However, it was suggested that the DE model is not enough to explain the CMR phenomenon. Some authors suggested that other factors such as Jahn Teller effect [START_REF] Millis | Double Exchange Alone Does Not Explain the Resistivity of La 1 x Sr x MnO 3[END_REF] and phase separation [START_REF] Lakshmi | Influence of sintering temperature and oxygen stoichiometry on electrical transport properties of La 0.67 Na 0[END_REF][START_REF] Rao | Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems[END_REF] are responsible for the behavior observed in manganite.

Several studies have been performed on the effect of substitution in the A-site by divalent elements (Sr, Ba…) [START_REF] Ewe | Electrical, magnetoresistance and magnetotransport properties of Nd 1 x Sr x MnO 3[END_REF][START_REF] Suard | Role of the A-site cations on the magnetic structures and transport properties in the Nd 0.7 Ba 0.3[END_REF]. In fact, doping at the rare-earth site indirectly affects the conduction mechanism with its repercussion on bandwidth and bond angle between adjacent manganese ions [START_REF] Millis | Lattice effects in magnetoresistive manganese perovskites[END_REF]. The electrical conduction in these materials has contributions from grains (bulk), grain boundaries, and electrode specimen interface [START_REF] Hodge | Impedance and modulus spectroscopy of polycrystalline solid electrolytes[END_REF][START_REF] Hsu | Interfacial and quantum well effects on ac magnetotransport of La 0.7 Sr 0.3 MnO 3 /La 1.4 Sr 1.6 Mn 2 O 7 composites[END_REF]. In order to understand the conduction behavior, it is necessary to separate the various contributions to the total observed resistance. Complex impedance spectroscopy (CIS) is an important and powerful tool to study defects, microstructure, surface chemistry and electrical properties of materials. For a polycrystalline sample, this technique enables us to separate the contributions of bulk, grain boundary and electrode in the impedance very easily [START_REF] Maiti | Complex-plane impedance analysis for semiconducting barium titanate[END_REF][START_REF] Sdiri | Electrical conductivity and dielectric analysis of the perovskite La 0.7 Ca 0.3 x K x MnO 3 (x=0.00x=0.00[END_REF].

Generally, Nd 0.7 Sr 0.3 MnO 3 perovskite undergoes a metal-semiconductor transition at the temperature T MS of 229-239 K, [START_REF] Yang | Spin-dependent electroresistance in Nd 0.67 Sr 0.33 MnOy[END_REF][START_REF] Ying | The effect of Ga doping in Nd 0.7 Sr 0.3 MnO 3 system[END_REF]. That is usually accompanied by a FM-PM transition at the Curie temperature T C . It was shown by many authors that physical properties of Nd 1-x Sr x MnO 3 can be considerably affected by the substitution of the Ln-site by various other elements such as Eu [START_REF] Venkataiah | Anomalous variation of magnetoresistance in Nd 0.67-y EuySr 0.33 Mn0 3 manganites[END_REF], La [START_REF] Vadnala | Influence of Nd Substitution by La in Nd 0.7 Sr 0.3 MnO 3 on Structural and Transport Properties for Sensing Applications[END_REF], Pr, Dy [START_REF] Ling | Effect of magnetism and average radius at A-site on T C in Nd 0.6 Ln 0.1 Sr 0.3 MnO 3 (Ln=La, Pr, Gd, Dy) system[END_REF], Gd [START_REF] Kundu | Evidence of electronic phase arrest and glassy ferromagnetic behaviour in Nd 0.4 Gd 0.3 /Sr 0.3 MnO 3 manganite comparative study between bulk and nanometric samples[END_REF]. Recently, by doping Y ions into Nd 0.7 Sr 0.3 MnO 3 , Khiem et al. [START_REF] Phan | Electron-spin-resonance study of Y-doped Nd 0.7 Sr 0.3 MnO 3 ceramics[END_REF] have found a decrease of T C and a transition from a metallic phase to an insulating phase when the Y content increased. Padmavathi et al. [START_REF] Padmavathi | Electrical behavior of some rareearth-doped Nd 0[END_REF] have found that the transition temperature T MI decreases with increasing <r A > as Nd is progressively substituted by Ce in Nd 0.33 Sr 0.33 MnO 3 .

The aim of this work is to study the structural and electrical properties (material impedance, electrical relaxation process, conductance behavior, etc.) of (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (0 x 0.20) system using complex impedance spectroscopy (CIS) technique. The effects of temperature and frequency have been investigated and a pronounced contribution of the grain boundary in this compound is demonstrated.

Experimental procedures

Polycrystalline samples of (Nd Finally, these pellets were quenched to room temperature. This step was carried out in order to preserve the crystalline structure at the annealing temperature. The structural characterization was done through X-ray diffraction measurements (XRD) using a ''Panalytical X'Pert Pro'' diffractometer with Cu K radiation (K =1.5406 Å). Data for the Rietveld refinement were collected in the range of 2 from 10 to 100° at room temperature with a step size of 0.017° and a counting time of 18 s per step. The structure refinement was carried out by the Rietveld analysis of the powder XRD data with the FULLPROF software [START_REF] Roisnel | Computer program FULLPROF[END_REF]. For the electrical measurements, the opposite sides of the sample were coated with conducting silver paint. The transport properties of the polycrystalline sample were examined by ac impedance spectroscopy using an Agilent 4294A over a frequency range from 40 Hz to 1 MHz at various steady temperatures (80-320 K). DC resistivity measurements were performed using the conventional four-probe method.

Results and discussion

Microstructure analysis

Fig. 1 shows typical XRD patterns performed at room temperature for x = 0.0 and 0.20 compounds. All samples were found to be single phase without any detectable impurity.

There is no segregation of CeO 2 oxide as observed in many works [START_REF] Padmavathi | Electrical behavior of some rareearth-doped Nd 0[END_REF][START_REF] Krishnamoorthy | Magnetic and magnetotransport properties of Ce doped nanocrystalline LaMnO 3[END_REF]. The structure refinement was performed in the orthorhombic setting of Pnma space group, in which the (Nd, Ce, Sr) atoms are at 4c (x, 0.25, z) position, Mn at 4b (0.5, 0, 0), O 1 at 4c (x, 0.25, z) and O 2 at 8d (x, y, z). We noticed that calculated diffraction patterns match well with measured ones. The difference observed between the intensities of the measured and calculated diffraction lines can be attributed to the existence of preferential orientation of the crystallites in the samples [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF]. Positions for the Bragg reflection are marked by vertical bars. Differences between the observed and the calculated intensities are shown at the bottom of the diagram.

The refinement results are listed in Table 1, which also reports the residuals for the weighted pattern R WP , the pattern R P , the structure factor R F and the goodness of fit .

It is well known that the tolerance factor T G [28] determines the crystal structure of ABO 3 perovskite. Only for T G close to unity is a cubic perovskite structure obtained. For T G 1, a tilt and rotation of the oxygen octahedra are obtained compensating for the misfit of the ionic radiis of the involved A and B cations. This can be seen from the definition of T G :

) O r B r ( 2 O r A r G t (1)
Where r A , r B and r O are respectively the average ionic radii of the A and B perovskite sites and oxygen anion. Usually, for (0.96 t G 1), the connecting pattern of the oxygen octahedra is rhombohedral, whereas it can be orthorhombic or monoclinic for lower values of T G . The values obtained of T G (see Table 1) suggest that our samples have the orthorhombic phase with perovskite structure (0.89 t G 0.96).

We can see in Table 1 that the lattice parameters and unit cell volume increase slightly with increasing Ce content. This slight increase can be directly related to the increase of average ionic radius of A site <r A > which is due to the larger ionic radius of Ce 3+ ion ( 3 Ce r =1.196Å and 3 Nd r = 1.15Å [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]). The average crystallites size of the materials have been evaluated using peak broadening technique and Scherrer's formula given by <D S > = k / cos , where <D S > is the average particle size, k is the particle shape factor (=0.89), is the wavelength of CuK radiation (=1.5406Å), is the full width at half maximum of the XRD peak and is the Bragg's diffraction angle of the peak. The as obtained <D S > are estimated to be mostly of 57, 69 and 76 nm for x = 0, x = 0.10 and x = 0.20 samples, respectively. This result shows that it has no appreciable difference in particle size for all samples.

Electrical conduction

The variation of resistivity with temperature T of the (Nd 1-x Ce x ) 0.7 Sr 0. In the low temperature (T < T MS ) metallic behavior, the electrical conduction is generally understood according to the DE theory. In this model, the Mn 3+ -O-Mn 4+ coupling produces conduction from the half-filled to the empty eg orbital. As known, the decrease of the resistivity when increasing temperature indicates the presence of semiconductor behavior where the conduction is thermally activated.

The resistivity can be also well fitted, at high temperature (T > T MS ), by the SPH model

) T B k E exp( AT T hopp (2)
where T is the absolute temperature, k B is the Boltzmann temperature, A is the preexponential factor and E hopp is the activition energy of conduction. The activation energies (E hopp ) thus obtained is summarized in Table 2. In many cases, the conduction process is related to the electron delocalization phenomenon that is present in this type of material (electron hopping of the eg electron from Mn 3+ to Mn 4+ via the O 2-orbital), which involves a tendency to form small polarons in the material.

G( ) =G DC +A n (3)
where G DC is the DC conductance, is the angular frequency, A is a pre-exponential factor dependent on temperature and n is the frequency exponent which depends on frequency and temperature. When the frequency is increased, the mean displacement of the charge carriers is reduced and thus the AC conductance of the sample follows the law G AC = A n . The frequency dependence of conductance suggests the hopping conduction.

Fig. 4 shows the variation of AC conductance as a function of frequency for all samples at 280 K. It is clear that the conductance decreases with increasing Ce concentration. This behavior is confirmed by the electrical resistivity measurements (Fig. 2). The values of G DC , the constant (A) and the exponent (n) for the three samples (x = 0, x=0.10 and x=0.20) are listed in Table 3. It can be seen in this table that G DC and n decrease while the A increases when increasing the Ce doping concentration. The variation of the exponent (n) can be expected if the polarizability of involved material depends on the energy barrier for a simple hopping process between two sites [START_REF] Pelaiz-Barranco | Phase transitions in ferrimagnetic and ferroelectric ceramics by ac measurements[END_REF].

The plot of ln(G DC T) versus 1000/T is shown in Fig. 5. At high temperatures, a linear variation was observed, which proves that conductance is dominated by thermally activated hopping of small polaron (SPH) and can be described by Mott and Davis law [35]:

) T k E Bexp(- T G B hopp DC (4)
Where B is the pre-exponential factor, E hopp is the activation energy of conduction, T is the absolute temperature and K B is the Boltzmann constant. The activation energies E hopp , as calculated from the slopes (Fig. 5), are listed in Table 2. It is worth noticing that the energy (E hopp ) estimated by both methods yields the same order of magnitude.

Complex impedance analysis

Impedance spectroscopy is an experimental technique for the characterization of electrical properties of electronic materials. It enables us to separate the real and imaginary components of the electrical parameters and hence provides a true picture of the materials properties.

In general, the complex impedance ) Z( under sinusoidal regime can be expressed as: indicates that the electrical processes in the material arise basically due to the contribution from bulk material [START_REF] Irvine | Electroceramics: Characterization by Impedance Spectroscopy[END_REF][START_REF] Selvasekarapandian | The ac impedance spectroscopy studies on LiDyO2[END_REF][38]. The diameter of these semicircles changes with temperature. The center of semicircle making an angle with ' Z -axis (The angle is between the Z' axis and the line which related the centre of circle with intersection of the semicircular arcs on the real axis) and is temperature-dependent reveals the non-Debye type relaxation process in the material [START_REF] Barsoukov | Impedance Spectroscopy Theory, Experiment and Applications[END_REF][START_REF] Intatha | Impedance Study of Giant Dielectric Permittivity in BaFe 0.5 Nb 0.5 O 3 Perovskite[END_REF]. These spectra were fitted using Zview software and the best fit is obtained when employing an equivalent circuit formed by a serial association of a grain resistance R g (bulk resistance) with a resistance R gb (grain-boundary resistance) associated in parallel with constant phase element impedance (Z CPE ).

jZ'' Z' Z ) ( (5) where
The CPE impedance (Z CPE ) is given by the following relation:

Where A 0 indicates the value of capacitance of the CPE element (expressed in Farad units), and is the factor exponent (0 < < 1).The factor represents the capacitive nature of the element [START_REF] Macdonald | Electrochim. Reflections on the history of electrochemical impedance spectroscopy[END_REF]: if =1, the element is an ideal capacitor and if =0, it behaves as a frequency independent ohmic resistor. A 0 and can be temperature dependent.

The values of the bulk (R g ) and the grain boundary (R gb ) resistances can be obtained from the intercept of the semicircle and real part of impedance ( ' Z ) axis. Accordingly, the intercept of the ' Z axis in the right (corresponding to the low frequency) is the sum of R g and R gb , while the intercept with the ' Z axis in the left (corresponding to the high frequency) stands for R g . We can reveal from Fig. 6 using Eq. 6 the real and imaginary components of the impedance related to the equivalent circuit:

2 )) 2 sin( 0 A gb (R 2 )) 2 cos( 0 A gb R ( 1
)) 2 cos( 0 A gb R (1 gb R Rg Z' (7) 2 )) 2 sin( 0 A gb (R 2 )) 2 cos( 0 A gb R ( 1
)) 2 sin( 0 A R ' Z' 2 gb (8)
The parameters (R g , R gb , A 0 and ) were obtained for each temperature after fitting the data by Eqs.( 7) and (8). Fig. 7 shows the variation of the resistance of grains and grain boundaries (inset) of the sample x=0.20 with temperature. The decrease in the values of R b and R gb (diameters of the semicircular arcs in figure 2) with temperature, above T MS , indicates the presence of thermally activated conduction mechanism in this system which is typical for the semi conductors. Generally two types of thermal activation are responsible for the reduction in resistive properties with temperature [42, 43]. In the case of band conduction, carrier density increases with rise in temperature, while in the case of hopping, carrier concentration is determined by the doping level and it is the carrier mobility which is thermally activated.

By contrast, the values of grain resistance R g and the values of grain boundary resistance (R gb ) below T MS increase with increasing temperature (metallic behavior). This result is in good agreement with the electrical resistivity. ) with frequency at different temperatures for x=0.20. The impedance value is typically higher in the low-frequency region, and then it decreases gradually with increasing frequency. The decrease in ' Z with the increasing frequency may be attributed to the presence of space charge polarization in the material. In fact, this phenomenon has been further verified by observing the coalesced behavior of real part of high frequency impedance for all measured temperatures. Similar behavior at lower frequencies was also observed by Sen et al. [START_REF] Sen | Impedance studies of Sr modified BaZr 0.05 Ti 0.95 O 3 ceramics[END_REF]. We can notice that ' Z increases with increasing temperature below T MS (inset of Fig. 8). However, above T MS , it decreases with the increasing of temperature. ) with frequency for some representative temperatures. The spectra are characterized by the appearance of peaks, such behavior indicates the presence of relaxation process in the system. Similar behavior has been reported for other manganites and different perovskite systems [START_REF] Cohn | Giant dielectric permittivity of electrondoped manganite thin films, Ca 1 x La x MnO 3 (0 x 0.03[END_REF][START_REF] Rahmouni | Chromium effects on the transport properties in La 0.7 Sr 0.3 Mn 1 x Cr x O 3[END_REF][START_REF] Sutar | Dielectric and impedance spectroscopy of Sr(Bi 0.5 Nb 0.5 )O 3 ceramics[END_REF] and it has been attributed to localized hopping of polarons between lattice sites with a characteristic timescale [START_REF] Lily | Impedance spectroscopy of (Na 0.5 Bi 0[END_REF]. From these peaks we deduce the value of relaxation frequency (fr) above T MS , and then we calculate the relaxation time using the relation of = 1/ (2 fr). It is observed that the value of relaxation time is found to be decreasing with the increase of temperature which represents semiconducting behavior of the sample. This semiconducting nature of the grains in ceramics is believed to be due to the loss of oxygen during high temperature sintering process [START_REF] Das | Impedance spectroscopy analysis of (Pb 0.93 Gd 0.07 )(Sn 0.45 Ti 0.55 ) 0.9825 O 3 ferroelectrics[END_REF]50]. As a result, the activation energy can be obtained by using the Arrhenius formula

) T k E exp( relax 0 0
, 0 is the pre-exponential factor, relax is the activation energy and B is the Boltzmann constant. The plot of the relaxation time ( ), as a function of inverse temperature (1000/T), is given in Fig. 10. The activation energy (E relax ) of the relaxation process and relaxation times at infinite temperature ( 0 ) were determined from the slope and intercept, respectively, of the linear least square fits shown as solid lines in Fig. 10 and the values are listed in Table 2. It is observed, from this table, that the activation energy for conduction is greater than relaxation energy .The activation energy for conduction (Ea) is the sum of both the creation of charge carriers and hopping free energy of charge carriers over long distances while the activation energy for relaxation is equal to the migration free energy of charge carriers and their hopping between the adjacent lattice sites. The difference between the conduction and relaxation activation energies may be attributed to the creation of free energy, which shows that the carrier concentration is temperature dependent [START_REF] Shah | Change of conduction mechanism in the impedance of grain boundaries in Pr 0.4 Ca 0.6 MnO 3[END_REF][START_REF] Komine | Dielectric properties in LaFe 0.5 Ga 0.5 O 3[END_REF].

Conclusion

In summary, we have investigated in this work, the structure and electrical transport properties of (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO3 (0 x 0.20) perovskites, using impedance spectroscopy technique over a wide range of temperature and frequency. The X-ray diffraction analysis revealed that all samples exhibit single perovskite with orthorhombic Pnma structure. The electrical investigation shows a metallic semiconductor transition with a metal-like conductivity below and semiconductor-like conductivity above a critical temperature T MS . Electrical conductivity analysis has indicated that conductance can be described by Jonscher universal power law. From the DC conductance study, electronic conduction is found to be dominated by thermally activated hopping of small polarons (SPH), such activation energy was also deduced from the electrical resistivity measured using a conventional four-probe method. Complex impedance analysis indicates that the electrical properties of the material are strongly dependent on temperature and frequency. The

3

 3 MnO 3 samples ( 0 x 0.20) was shown in Fig 2. The (T) curves reveal that all the samples undergo a metalsemiconductor transition with increasing temperature at T MS = 230, 192 and 186 K for x = 0.00, 0.10 and 0.20, respectively (T MS determined from the maximum of (d /dT). It can also be noted that the transition temperature T MS is dependent on Ce concentration. As cerium concentration is increased, the value of maximum resistivity increases and the T MS decreases. Similar behavior has been observed by D. Varshney and al. for La 0.7-x Ce x Ca 0.3 MnO 3 [30].

Fig. 3

 3 Fig. 3 shows the variation of AC conductance (G AC ) with frequency f at different temperatures for the (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 compounds (x=0 and x=0.20). These curves are frequency independent in the low frequency region (as shown by the plateau at low frequencies), followed by a sharp increase at high frequencies. In the plateau region, the conductance (G DC ) decreases with temperature (T <T MS ) indicating that the metallic behavior dominates. But the conductance increases with increasing temperature for T >T MS indicating the presence of semiconductor behavior. The variation of G AC with high frequencies occurs with changes in slope which suggests a superposition of different transport mechanisms in this frequency range. This behavior

Fig. 6 shows

 6 Fig.6 shows Nyquist plots (imaginary part of complex impedance ' ' Z vs real part of complex impedance ' Z ) for (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (x= 0.20) at several temperatures. The impedance spectrum is characterized by the appearance of some compressed semicircle arcs, which can be well traced with the increase of frequency. The presence of a single semicircular arc

Fig. 8

 8 Fig. 8 shows the variation of real part of impedance ( ] Re[ ' Z Z

Fig. 9

 9 Fig. 9 shows the variation of the imaginary part of impedance ( ] Im[ ' ' Z Z

Table 1 :Table 2 :Table 3 :

 123 Refined structural parameters of (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (0 x 0.20) at room temperature. Space group Pnma. V is the cell volume; B iso is the overall isotropic thermal parameter; TM-O the bond lengths between Mn and O and Mn-O-Mn are the bond angles, R wp , R p and R F are the agreement factors for the weighted profiles, the profiles and the structure factors; ² is the goodness of fit. The numbers in parentheses are estimated standard deviations to the last significant digit. Activation energy estimated from resistivity, conductance and relaxation plots for (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (x= 0, 0.10, 0.20) compounds. 0 is relaxation time at an infinite temperature. Values of the DC conductance, the constant (A) and the exponent (n), for (Nd 1-x Ce x ) 0,7 Sr 0,3 MnO 3 (x= 0, 0.10, 0.20) compounds, determined at the temperature T=280K.

Figures captionsFigure 1 .

 1 Figures captions

Figure 2 .

 2 Figure 2 . The temperature dependence of resistivity for (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 compounds (x = 0, 0.10 and 0.20 content). Solid lines are linear fits to data using SPH model.

Figure 3 .

 3 Figure 3. Variation of the AC conductance (G AC ) as a function of frequency at different temperatures 80-320 K for (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 samples. ((a) x = 0, (b) x = 0.20).

Figure 4 .

 4 Figure 4. Variation of the AC conductance (G AC ) versus frequency f of (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (x = 0, 0.10, 0.20) compounds at the temperature T = 280 K.

Figure 5 .

 5 Figure 5. Variation of the ln(G DC T) as a function of (1000/T) for (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (x = 0, 0.10, 0.20) samples.

Figure 6 .

 6 Figure 6. Complex impedance plots at given temperatures for (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 (x=0.20) compound.

Figure 7 .

 7 Figure 7. Variation of Rg and Rgb with temperature for the (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 sample [x = 0.20].

Figure 8 .

 8 Figure 8 . Variation of real part of the impedance (Z') of the (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 sample (x = 0.20) as a function of frequency for different temperatures.

Figure 9 .

 9 Figure 9. Variation of imaginary part of the impedance (Z'') of the (Nd 1-x Ce x ) 0.7 Sr 0.3 MnO 3 samples [x = 0.20] as a function of frequency for different temperatures.

Figure 10 .

 10 Figure 10. Variation of the ln( ) as a function of (1000/T) for the Ce-doped compounds.Solid lines are linear fits to data.

Fig. 1

 1 Fig. 1.

Fig. 2

 2 Fig. 2.

  Fig. 4.

Fig. 6

 6 Fig. 6.

  1-x Ce x ) 0.7 Sr 0.3 MnO 3 (0 x 0.20) have been prepared by conventional solid state reaction method. Stoichiometric ratio of Nd 2 O 3 , CeO 2 , SrCO 3 and MnO

[START_REF]Charge Ordering and Related Properties of Manganese Oxides[END_REF]

(previously dried at 800 K in order to remove any eventual water) were thoroughly mixed to get homogeneous powders, which were calcined at 1100 °C for 72 h. The calcined mixtures were then pressed into pellets (12 mm diameter and 2 mm thickness under 8 tons/cm 2 ) and sintered several times in air, with intermediate grindings, at 1400 K for 48 h.

Table 1 :

 1

			5.4593(1)	5.4644(2)	5.4674(3)
			7.7243(2)	7.7293(3)	7.7183(2)
			5.4553(1)	5,4598(1)	5.4690(2)
			230.05(2)	230.60(1)	230.79(3)
	(Nd/Ce/Sr)	x	0.0276(6)	0.0294(4)	0.0247(3)
	At.Positions	z	0.0036(5)	0.0038(1)	-0.008(7)
	(Nd/Ce/Sr)	B iso ( 2 )	1.20(2)	1.12(3)	0.89(4)
	(Mn)	B iso ( 2 )	0.51(5)	0.82(4)	0.39(1)
	O (1)	x	0.5012(4)	0.5055(1)	0.4879(2)
	At.Positions	z	0.0248(2)	0.0295(8)	0.0755(5)
	O (1)	B iso ( 2 )	0.82(6)	0.83(2)	0.52(2)
		x	0.5012(4)	0.5055(7)	0.4900(6)
	O (2) At.Positions	y	0.4879(2)	0.0295(6)	0.0686(3)
		z	0.24(6)	0.12(2)	-0.29(1)
	O (2)	B iso ( 2 )	1.47(3)	1.68(3)	1.30(2)
	d Mn-O1 (Å)		1.936(1)	1.940(2)	1.982(9)
	Mn-O1-Mn (°)		172.059(3)	169.80(2)	158.545(1)
	d Mn-O2 (Å)		1.8237(2)	1.720(1)	1.67(4)
	Mn-O2-Mn (°)		154.475(4)	156.4 (3)	167.313(3)
	d Mn-O (Å)		1.8615	1.830	1.826
			163.280	163.10	162.929

Mn-O-Mn (°)

Table 3 :

 3

	x	G DC	A	exponent n
	0	6.85E 2	4.715E 12	1.94
	0.10	6.15E 2	3.043E 13	1.88
	0.20	8.20E 3	1.043E 13	1.79

(%) 2.71

3.97 3.73

Acknowledgement

This work is supported by the Tunisian National Ministry of Higher Education, Scientific Research and the French Ministry of Higher Education, Scientific Research, within the frame work of collaboration Franco-Tunisian.

impedance spectrum is characterized by the appearance of semicircle arcs, well modeled in terms of the electrical equivalent circuit. The analysis of the temperature variation of the imaginary-part of the impedance has indicated that the observed relaxation process is thermally activated. From these obtained results, we have deduced the evidence of a hopping mechanism in the conductivity behavior. The activation energies obtained from the conductance is slightly higher than that from time relaxation analyses.