L Zarazúa-Villalobos

N Mary

J Soo-Hyun

K Ogawa

H Kato

Y Ichikawa

Microstructure and corrosion study of Fe-based bulk metallic glass obtained by Spark Plasma Sintering

Keywords: Spark plasma sintering, Metallic glass, Corrosion resistance

Due to the necessity of developing new materials with better anti-corrosive properties, bulk metallic glasses are the focus of many research projects. In this study, a metallic glass with Fe48Cr15Mo14C15B6Y2 composition was produced by high-pressure atomization spray and densified by Spark Plasma Sintering. Different sintering cycles, temperatures, and pressures were carried out to conserve the amorphous structure while increasing its density. The temperatures used for this study were below the glass transition temperature, and the uniaxial force was variated between 50 to 400 MPa. Amorphous samples with nearly 98 % of relative density were obtained. The effect of the sintering cycle used on the phase transformation and microstructure of the materials was studied. Additionally, the porosity and crystallinity were related to the corrosion resistance properties through electrochemical characterization.

Introduction

Metallic glasses are increasing interest in academic researches and industry. They are electronically and optically like ordinary metals, but the absence of crystals and defects as vacancies, dislocations, or grain boundaries increase their chemical, physical and mechanical properties [START_REF] Demetriou | Amorphous metals for hard-tissue prosthesis[END_REF]. These materials could be used in a wide range of fields. They can be used to develop magnetic devices, microsystems, or for aeronautical or biomedical applications. The absence of grain boundaries provides high mechanical strength and corrosion resistance, making these materials an ideal candidate to stand up to aggressive environments depending on their composition [START_REF] Wong | Degradation behavior and mechanical strength of Mg-Zn-Ca bulk metallic glass composites with Ti particles as biodegradable materials[END_REF][START_REF] Baulin | A new, toxic element-free Mg-based metallic glass for biomedical applications[END_REF][START_REF] Li | Recent advances in bulk metallic glasses for biomedical applications[END_REF].

Metallic glasses have been developed as powders, ribbons, and small pieces [START_REF] Wong | Degradation behavior and mechanical strength of Mg-Zn-Ca bulk metallic glass composites with Ti particles as biodegradable materials[END_REF][START_REF] Baulin | A new, toxic element-free Mg-based metallic glass for biomedical applications[END_REF][START_REF] Sharma | Mechanical crystallization of Fe-based amorphous alloys[END_REF][START_REF] Xu | Corrosion behavior and pitting susceptibility of in-situ Ti-based metallic glass matrix composites in 3.5 wt.% NaCl solutions[END_REF].

Coatings of metallic glass have also been studied by different deposition techniques [START_REF] Liu | Fe-based amorphous coatings: Structures and properties[END_REF][START_REF] Xie | Comparison of Microstructure and Tribological Properties of Plasma, High Velocity Oxy-Fuel and Detonation Sprayed Coatings from an Iron-Based Powder[END_REF][START_REF] Henao | Influence of Cold Gas Spray process conditions on the microstructure of Fe-based amorphous coatings[END_REF].

However, new processing techniques that allow manufacturing larger pieces of bulk metallic glasses (BMGs) are still needed. Spark plasma sintering (SPS) is an advanced sintering technique for consolidating powders into nanostructured bulk materials. It has been recently used in broader applications and has gained a strong reputation as a versatile method for solid-state processing of metals, ceramics, and composites. The ease in parameter manipulation in SPS makes this process beneficial for the preservation of amorphous structures. [START_REF] Singh | Microstructure and tribological behavior of spark plasma sintered iron-based amorphous coatings[END_REF]. Some studies have been performed to study the use of SPS in metallic glass processing [START_REF] Chu | Consolidation and mechanical properties of Cu 46Zr 42Al 7Y 5 metallic glass by spark plasma sintering[END_REF][START_REF] Ravi | Spark Plasma Sintering of Fe-Cr-Mo-P-B-C-Si Amorphous Alloy[END_REF]. They found that SPS can increase the specimens' size produced by a conventional copper mold casting in diameter and with a higher fracture strength.

One of the challenges to fabric BMG is to keep the amorphous structure on the materials. However, still unresolved if fully dense materials with some crystallinity percentage have higher corrosion resistance than a completely amorphous one with lower density.

Among the variety of BMGs, Fe-Cr-based bulk metallic glasses have attracted much attention because of superior corrosion resistance, high strength, and hardness. Like stainless steel, a concentration of Cr above 8 at.% leads to the growth of a passive film that protects the material from aggressive environments [START_REF] Waseda | Corrosion behaviour of metallic glasses[END_REF][START_REF] Duarte | Element-Resolved Corrosion[END_REF][START_REF] Wang | Effects of Cr contents in Fe-based bulk metallic glasses on the glass forming ability and the corrosion resistance[END_REF]. However, Fe-Cr-based BMGs exhibit almost no plastic strain and strain hardening even in compression when tested at room temperature, restricting their applicability [START_REF] Ni | High performance amorphous steel coating prepared by HVOF thermal spraying[END_REF]. In this study, SPS's effects on the densification, crystallinity, porosity, and corrosion properties of a Fe-Cr-Mo-C-B-Y metallic glass alloy are investigated to guide future SPS developments of Fe-Cr bulk metallic glasses.

Experimental

Metallic glass powder preparation

The appropriated amounts of high purity elemental materials (> 99.9 %) were used to produce the mother alloy with a final atomic composition of Fe48Cr15Mo14C15B6Y2. The 2 at.% of yttrium was added to the metallic composition because it enhanced the glass-forming ability [START_REF] Pang | Bulk glassy Fe-Cr-Mo-C-B alloys with high corrosion resistance[END_REF][START_REF] Wang | Influence of yttrium as a minority alloying element on the corrosion behavior in Fe-based bulk metallic glasses[END_REF]. The alloy was home-made by convection melting under an argon atmosphere, and the metallic glass powder was obtained by high-pressure spray atomization. After atomization, the powder was sieved until 43 µm mesh.

Spark Plasma Sintering parameters and characterization

Three different sintering cycles were selected to vary the samples' density while searching to conserve the amorphous phase. The first sintering cycle (S1) used a heating rate of 50 ℃/min until the final temperature (Tf) with 15 minutes of holding time at this temperature. Cycle two (S2) used an initial heating rate of 50 ℃/min until Tf-15 ℃ and a heating rate of 1℃/min until Tf. Cycle 3 (S3) used an initial heating rate of 50 ℃/min until Tf-30 ℃ and a heating rate of 1°/min until Tf.

The tests were performed in a commercial SPS (Fuji-SPS, japan) equipment using 7 g of powder, a tungsten carbide died of 15 mm of diameter, and Grafoil ® as a protective film between the died and the powder. The final temperature was fixed at 575 °C, below the glass transition temperature (Tg) of this alloy (577 °C); the uniaxial force used for the test was 50 MPa. It worth mentioning that the Tg of this alloy was measured at a heating rate of 10 °C/min and the heating rate during the sintering process starts at 50 °C/min, which causes an increase in the Tg according to the works of T. Paul et al. [START_REF] Paul | Viscous flow activation energy adaptation by isochronal spark plasma sintering[END_REF][START_REF] Paul | Kinetics of isochronal crystallization in a Fe-based amorphous alloy[END_REF].

The samples obtained by the three different sintering cycles were analyzed by XRD, SEM, and the density was measured by the Archimedes method. After the results of the three different sintering cycles were obtained, the one with better properties respecting the amorphous phase of the material was selected and used for the subsequent tests where the temperature and the uniaxial force were variated. All the samples obtained were analyzed by X-ray diffraction (XRD), and their densities were measured as well by the Archimedes' method. Table 1 reports the sample names and the SPS pressure and temperature used in this work. Note that the parameter selection is explained later in the document.

Electrochemical test

The electrochemical tests were performed in the samples obtained from the three different sintering processes and the sample obtained after the variables' optimization. Before the test, the samples' surface was polished until SiC 1200, and the roughness was measured by a 2D profilometer using the JIS1994 standard in a Surfcoder SE300 (Kosaka Lab, Japan). Average values were given for three profiles.

The electrochemical characterizations were carried out in a typical three-electrode glass cell.

The working electrode was the sample with an average surface of 1.10 cm² in contact with the solution. The real surfaces were determined by optical surface measurements (Keyence One Shoot microscope) before corrosion tests. A graphite rod (5 cm²) and a Mercury Sulfate Electrode (MSE, 0VMSE = +0.650 VENH) were used as a counter electrode and the reference electrode, respectively. Corrosion tests were performed in a naturally aerated sodium sulfate solution (10 g/L) at pH 6.5 (no buffer solution and room temperature). This solution was preferred to NaCl to avoid pitting, which can be sensitive to crystallinity, porosity, and the roughness of the materials. Electrodes were connected to an SP-300 potentiostat equipped with a low current probe (Biologic, France). Open circuit potential (OCP) was first recorded for 3 hours to ensure the potential stability and the passive film steady state.

Electrochemical impedance spectroscopy measurements were then performed at the OCP with a frequency range of 100 kHz to 10 MHz with 9 points per decade and signal amplitude of 5 Vrms. The electrochemical characterizations were completed by linear sweep voltammetry. A reduction step was performed from the OCP to -0.7 VMSE, and then the potential was reversed to -0.3 VMSE. The scan rate was fixed to 1 mV/s. Only the potentiodynamic curves from -0.7 VMSE to -0.3 VMSE are shown here since the forward and backward cathodic currents overlap.

Characterization techniques

The powder was analyzed by X-ray Diffraction (XRD) apparatus from 30 to 80 ° in 2θ/θ mode with a step of 0.5° /min using Cu-Kα radiation at 40 kV and 40mA. The granulometry analysis was performed in dry by laser diffraction using a Malvern equipment (LMS-3000).

The morphology of the particles was analyzed by SEM (Hitachi SU-70) coupled with an EDS probe (SDD Apollo XL). Differential scanning calorimetry (DSC) analysis was performed in a Rigaku apparatus at a heating rate of 10 °C/ min from 300 to 700 °C. After corrosion tests at the OCP, a surface analysis was conducted by XPS analysis (Quantum 2000 apparatus (ULVAC-PHI, Inc). A 100 µm spot monochromatic Al Kα source (1486.7 eV) in constantanalyzer-energy mode (pass energy 187.85 eV for wide scan and 58.7eV for regions) was selected to record the spectrum of Fe2p, Cr2p, and Mo3d. Details of the analysis are given later.

Results and discussion

Metallic glass powder, obtaining and Sintering.

The powder obtained by induction oven and gas atomization (Figure 1a) has mainly spherical morphology particles mixed with elongated particles typical of this processing method, as shown in figure 1a. After a sieving process, the particle size showed a lognormal distribution with an average particle size (Dv50) of 24 µm (Figure 1b). Particle size has been shown to affect the densification process during spark plasma sintering. Smaller particle sizes increase the contact areas and have stronger driving forces increasing the mass migration, which benefits the densification [START_REF] Cheng | Effect of particle size on densification of pure magnesium during spark plasma sintering[END_REF][START_REF] Shon | Effect of particle size distribution on microstructure and mechanical properties of spark-plasma-sintered titanium from CP-Ti powders[END_REF]. The mean particle size used in this work is similar to the smaller reported for other authors with the same composition [START_REF] Singh | Nanocrystallization in spark plasma sintered Fe48Cr15Mo14Y2C15B6bulk amorphous alloy[END_REF][START_REF] Harimkar | Microstructure and properties of spark plasma sintered Fe-Cr-Mo-Y-B-C bulk metallic glass[END_REF]. Even if sieving was performed, the particle distribution was relatively wide, as confirmed by a SPAM of 1.5. The X-ray diffractogram (Figure 1c) on the powder shows a significant and broad peak, characteristic of the amorphous structures [START_REF] Sorour | Densification and microstructure of Fe-Cr-Mo-B-C alloy fabricated by spark plasma sintering[END_REF].

The DSC test (Figure 1d) shows a glass transition temperature (Tg) of 577 °C and two exothermic peaks, which correspond to two crystalline temperatures Tx1 and Tx2, at 653 and 670 °C, respectively, and a supercooled liquid region (ΔT=Tx-Tg) of 76 °C. T. Paul et al. [START_REF] Paul | Initial stage densification during spark plasma sintering of Fe-based amorphous alloy powder: Analysis of viscous flow[END_REF] studied the sintering mass mechanism of a similar amorphous alloy powder by SPS. They found the densification´s onset, which is around 320 °C. The maximum densification rate attended was at ~ 585 °C retaining its amorphous structure. Additionally, they corroborate that the dominant mass transport mechanism during sintering is by viscous flow. On the other hand, A. Shing et al. [START_REF] Singh | In Situ Nanocrystallization-Induced Hardening of Amorphous Alloy Matrix Composites Consolidated by Spark Plasma Sintering[END_REF] sintered the same alloy at a temperature close to the Tg (570 °C).

They found that it leads to a notable atomic reordering and the formation of medium-rangeordering (MRO) clusters. However, this formation of nanocrystals could reduce the propagation of the shear bands during deformation, increasing the material's hardness. S.P.

Harimkar et al. [START_REF] Harimkar | Microstructure and properties of spark plasma sintered Fe-Cr-Mo-Y-B-C bulk metallic glass[END_REF] also reported crystallization during SPS at temperatures below the Tx and close to the Tg. Therefore, to avoid devitrification, temperatures below the Tg were used for this work.

Figure 2 shows the sintering cycles applied to the metallic glass powder to produce BMG samples. The cycles S1 and S2 almost overlap even if they do not exactly follow the same steps. For S1, the maximum temperature (575 °C) was reached in one step with a 50 °C/min heating rate. Because of the SPS's temperature control system (PID), the maximum temperature was overtaken for a short period (2 min). To avoid it, the cycle S2 was designed to reach (i) 560 °C with a heating rate of 50 °C/min and then (ii) 575 °C with a heating rate of 1 °C/min. For the cycle S3, the powder was heated up to 555 °C with a rate of 50 °C. Then, the temperature of 575 °C was reached with a 1 °C/min heating rate. With this procedure, the cycle S3 took 15 min more than S1 and S2. Note that the materials' sintering time has an important role since it can induce crystallization or crystal growth [START_REF] Harimkar | Microstructure and properties of spark plasma sintered Fe-Cr-Mo-Y-B-C bulk metallic glass[END_REF][START_REF] Graeve | The kinetics of devitrification of amorphous alloys: The time-temperature-crystallinity diagram describing the spark plasma sintering of Fe-based metallic glasses[END_REF]. The samples obtained after SPS had a diameter of ~14 mm and a thickness of ~10 mm. They were labeled as S1, S2, and S3, depending on the sintering cycled used.

XRD patterns on S1, S2, and S3 samples are reported in figure 3a. The diffractograms show the coexistence of amorphous and crystalline structures with a ratio that depends on the SPS cycle. Thus, S1 presents the highest degree of crystallization, while S3 remains mostly amorphous. According to figure 3a, S2 presents an intermediate behavior. For S1, two kinds of carbides precipitates were identified: (Fe, Cr)23 (B, C)6 (ICDD 98-061-2585) and C1Cr2Fe14 (ICDD 98-007-6800). The S2 diffractogram shows similar peaks as S1 characteristics of carbides but with lower intensity, suggesting a higher ratio of the amorphous phase. After cycle S3, the sample remains mostly amorphous with low-intensity peaks characteristics of chromium iron carbide C1Cr2Fe14. The samples' crystallinity percentage was calculated with the formula proposed by M. Ohlberg et al. [START_REF] Ohlberg | Determination of Percent Crystallinity of Partly Devitrified Glass by X-Ray Diffraction[END_REF] for partially devitrified glasses using the X-ray diffractograms. The crystallinity obtained in the samples is 70 %, 24 %, and 19 % for S1, S2, and S3, respectively, with a standard deviation of ±5 %.

The samples' density (Figure 3b) showed the highest value for sample S2 with 87 %, followed by the S1 sample with 85 % and S3 with 83 %. Even though the final temperature was the same for the three cycles, the sintering heating rate affects the material's activation energy. The sintering time of sample S3 is longer than the other samples, due to the very slow heating rate, but the crystallinity percentage of the sample is the lowest as well as the density (Figure 3). This phenomenon was also observed by L. H. Liu et al. [START_REF] Liu | Densification mechanism of Ti-based metallic glass powders during spark plasma sintering process[END_REF]; they found out that the behavior of viscous flows is different during SPS sintering; slow rates of heating increase the activation energy for mass transportation as it is demonstrated with a lower density in sample S3. Tanaji Paul et al. [START_REF] Paul | Prediction of heating rate controlled viscous flow activation energy during spark plasma sintering of amorphous alloy powders[END_REF] predicted the viscous flow activation energy dependence on the heating rate by the Arrhenius equation and the directional structural relaxation model, demonstrating a viscosity dependence on the heating rate at temperatures lower than the Tg. They modeled different heating rates and demonstrated that the activation energy decreases with the heating rate's increments.

Additionally, to this effect, it should be considered the effective time at the final temperature. Sample S1 had a holding time of 15 min at the maximum temperature, which can induce devitrification, and with it, an increase in the viscosity affecting the densification process.

Tanaji Paul et al. [START_REF] Paul | Crystallization Mechanism in Spark Plasma Sintered Bulk Metallic Glass Analyzed using Small Angle Neutron Scattering[END_REF] studied the effect of SPS annealing on a BMG Fe-based alloy's crystallization behavior with a similar composition to the one used in this work. They established that during the annealing time, the pre-existing nuclei grow into well-developed crystals, and the increase in the duration of isothermal annealing causes an increase in the volume fraction of all crystal sizes in the material. It implies that during the sintering cycle S1, crystals were formed during the heating process and grew during the holding time.

Figure 4 shows the SEM top surface images of the edges, the centers, and the cross-section of samples S1, S2, and S3. The micrographs of the edge top surface (Figure 4a) show that the particles are in an initial sintering stage where their morphology is almost as in the initial state suggesting local bonding mechanisms in the outer region. This area still is very porous.

In the center of the sample (Figure 4b), metallic glass particles merge more or less well as a cycle function. The morphology of the particles is no longer observed in samples S1 and S2.

However, in sample S3 their morphology is still distinguishable. However, they have already formed and grown necks during the sintering process. S3 sample have an evident porosity, which is not observed in S1 and S2 samples. Finally, in the cross-section micrographs (Figure 4c), it can be observed how the densification changes in each sample. In S1 sample it can be observed different degrees of sintering. It has areas where the particles are totally cohered and others where they just have necks formation. In the S2 sample, it is observed higher densification on the top than in the center. The particles in the top are cohered with some remaining porosity while in the center of the sample, still possible to observe the particles' morphology, and the sample is very porous. S3 sample shows a more homogeneous sintering degree between the top and the center, but it is more porous than samples S1 and S2.

The difference of densification between the edge and the center observed for all the samples in the micrographs (the centers are higher densified than the edges) and a difference in density from the top to the center is a direct consequence of the establishment of two gradients of temperature in the SPS mold. The first gradient is axial from the bottom to the top surface of the sample. The second gradient is radial from the center to the edge of the sample. Note that the gradient amplitude is modified by the diameter and thickness of the sample produced.

According to Tanaji et al. [START_REF] Paul | Densification and Crystallization in Fe-Based Bulk Amorphous Alloy Spark Plasma Sintered in the Supercooled Liquid Region[END_REF], for a similar composition of metallic glass as this work, the radial temperature gradient during SPS for a die of 15 mm is ~29 °C between the center and the die wall (where the thermocouple is located) when a heating rate of 100 °C/min is applied.

The ΔT is directly proportional to the heating rate; for a heating rate of 50 °C/min, the temperature gradient is ~14.5 °C.

On the one hand, S1 presents the highest degree of cohesion between the grain, a relative density of 85 %, and a crystallinity ratio of 70 %. On the other hand, SEM images on S3

show particles not entirely interconnected in the center of the sample, consistent with the low relative density (about 83 %) and a low degree of crystallinity (19 %). The sample S2 presents a compromise between the densification, the particle's cohesion, and the crystallinity degree.

Even if S2 presents an average behavior, further material optimization was performed from the sintering cycle S3. Indeed, the crucial point is to keep the amorphous structures of the alloys.

Optimization of BMG materials

In SPS, the applied pressure during the process aids in particle arrangement. It enhances the sintering kinetics, while the temperature enhances the atomic motion, and with it, the diffusion process, but in metallic glasses, also promotes crystallization. Note that the temperature should be above 535 °C of a Fe-alloy to activate particle deformation playing a role in the material densification [START_REF] Paul | Pressure controlled micro-viscous deformation assisted spark plasma sintering of Fe-based bulk amorphous alloy[END_REF]. Therefore, the final temperature was decreased at 560 ℃ to avoid crystallization, but the axial pressure of samples S4 was increased from 50 MPa to 100, 200, 300, and 400 MPa. Additionally, to reduce the axial thermal gradient, the powder used was decreased from 7 g to 3.5 g. X-ray diffractograms were recorded on the new samples with the conditions mentioned above. Figure 5a reports the X-ray pattern for each pressure. It is worth noticing that the crystalline phase observed previously at 575 °C and 50 MPa is no longer detected, with a maximum temperature of 560 °C. Therefore, a reduction of 10 °C of Tf is beneficial to keep the material's amorphous state. Whatever the pressure, only amorphous peaks are observed on diffractograms confirming that the higher sintering pressure does not affect the volume fraction of the amorphous phase until the temperature is carefully selected. Note that this trend may be alloy-dependent, as Maurya et al. [START_REF] Maurya | Effect of consolidation pressure on phase evolution during sintering of mechanically alloyed Al86Ni8Y6 amorphous powders via spark plasma sintering[END_REF] observed with the consolidation pressure of amorphous Al alloys by SPS.

Relative densities of the samples are reported as a function of the pressure in figure 5b. A linear increase in density is observed from 50 to 200 MPa. Above this pressure, the density stabilizes about 98 %. The improvement of the density with the pressure was already theoretically analyzed by Paul et al. [START_REF] Paul | Pressure controlled micro-viscous deformation assisted spark plasma sintering of Fe-based bulk amorphous alloy[END_REF] on Fe-based amorphous alloys. They stated that the compressive viscous flow deformation of the particles was pressure dependent. The increase in density is then obtained through plastic deformation, and the sliding of the grain boundaries occurs to a larger extent at higher pressures. The high density also results in the particle size distribution and the presence of small elements that easily fill the voids and reduce porosities [START_REF] Pellizzari | Influence of processing parameters and particle size on the properties of hot work and high speed tool steels by Spark Plasma Sintering[END_REF]. In figure 6, SEM images of the Fe-alloy BMG samples at different uniaxial pressures are shown. In the images, we can observe the effect of the pressure on the densification in the specimens at 50 and 100 MPa; the particles are slightly interconnected, the morphology of the particles does not yet present a change. At these parameters, the sintering process is in an early stage. The micrography of the sample at 200 MPa, shows a high increment in densification compared with the two previous samples; at this pressure, the particles emerged just with a remaining porosity between them. This also confirms the results obtained for the relative density (Figure 5b). The samples at 300 and 400 MPa showed an increase in densification with just a few remnant porosities on the surface. However, the 300 MPa sample has a denser surface, which is in coherence with the relative density results.

The S4-300 sample's cross-section showed defects in the specimen due to particle's detachment during the cutting and polishing process and maybe due to a low cohesive force between them. The cross-section of the sample also shows a homogeneous density through the sample.

The 15 °C reduction in the final temperature resulted beneficial in preserving the material's amorphous phase but significantly affected the sintering process. With the same sintering cycle and pressure as sample S3, sample S4-50 decreased in density from 83 % to 69 %. The increase in applied pressure proved to be effective in mitigating this effect and enabling to obtain higher densities.

OCP behavior and passive film composition

Electrochemical tests were performed to evaluate the crystallinity and the relative density influence on the Fe-Cr BMG´s corrosion resistance. These tests were performed on S1, S2, and S3 samples and compared with S4-300, in which any crystallization was observed by XRD and had the highest relative density and homogeneity of the produced samples. Note that reproducibility tests were carried out, and the results overlap well. Therefore, only the trend curves are reported below.

It must be noticed that the presence of a gradient density may affect the electrochemical response of the systems. Assuming no variation in the chemical composition along the sample diameter, the electrochemical reactivity of the surface should be affected by the roughness induced by the open pores at the surface; rough passive surfaces are more sensitive to localized corrosion than flat ones [START_REF] Jaffré | Effect of Mechanical Surface Treatments on the Surface State and Passive Behavior of 304L Stainless Steel[END_REF]. This phenomenon is increased if pores are interconnected. In the following, macroscopic electrochemical tests were performed, knowing that they may underestimate surface reactivity since a macroscopic behavior is recorded [START_REF] Akid | A comparison between conventional macroscopic and novel microscopic scanning electrochemical methods to evaluate galvanic corrosion[END_REF]. To further in the effect of edge-center porosity of samples, local electrochemical analysis should be applied, which does not fall under this paper's scope [START_REF] Jadhav | Review-The Use of Localized Electrochemical Techniques for Corrosion Studies[END_REF].

Figure 7a shows the Open Circuit Potential recorded on SPS samples in the aerated sulfate solution (neutral pH, room temperature, and stagnant condition). Whatever the sample, the steady-state potential is reached after 1 10 6 seconds of immersion. S1 presents the most anodic value at about -0.4 VMSE, followed by S2 and S3 at about -0.45 VMSE. Note that the difference of 50 mV might not be significant since a slight deviation of the potential is still detected for S2 and S3. In Figure 7a, S4-300 present the most active potential at about -0.52 VMSE. For this sample, the dropout of potential was confirmed on reproducibility tests and always occurred below 6 10 4 seconds. Note that this potential transient is not understood yet.

Yang et al. [START_REF] Yang | Effects of crystallization on the corrosion resistance of Fe-based amorphous coatings[END_REF] already reported such a trend focusing on the content of Mo, Cr, and Iron at the oxide state. For experimental confirmation, XPS analyses were performed on SPS samples after immersion at the OCP. Fe2p, Cr2p, and Mo3d were recorded. XPS peak software was used to fit the XPS spectra (Figure 8). Based on the literature [START_REF] Asami | ESCA study of the passive film on an extremely corrosion-resistant amorphous iron alloy[END_REF][START_REF] Asami | The X-ray photo-electron spectra ofseveral oxides of iron and chromium[END_REF] and open database La surface.com, Fe2p was split into two regions: 706-707 eV and 708-714 eV. The first region was assigned to Fe° metallic state, and the second region was attributed to Fe oxidic states. The Cr 2p peak consisted of three peaks at around 573-571 eV, 575-576 eV, and 577-578 eV for the metallic state, Cr3+, and Cr6+ oxidic states, respectively. For the Mo 3d spectra, six peaks were used. Two peaks between 227-228 eV and 230-231 eV corresponded to Mo° metallic state. Two other peaks at 228-229 eV and 233-234 eV were assigned to Mo 4+ oxidic state. Finally, the peaks between 231-232 eV and 234-236 eV were attributed to Mo 6+ oxidic state. Figure 8 illustrates the XPS fit results of the Fe 2p, Cr 2p, and Mo 3d peaks for S1 (70% of crystallinity) and S4 (1% of crystallinity). The atomic concentration of these metallic elements was calculated from the metallic and oxidic state areas, for Yang et al. [START_REF] Yang | Effects of crystallization on the corrosion resistance of Fe-based amorphous coatings[END_REF], the amorphous state of the material facts on the quantity of Cr-and Mo-oxides in the passive. Therefore, the quantity  was calculated from equation 1. Results of Equation 1 are reported in Table 2. They show an increase in the quantity of Cr-and Mooxide species on the surface with the crystallinity reduction of the samples. Consequently, the differences in passive currents on polarization curves and EIS experiments could be expected.

𝛽 = 𝐴 𝐶𝑟 2𝑝 +𝐴 𝑀𝑜 3𝑑 𝐴 𝐶𝑟 2𝑝 +𝐴 𝑀𝑜 3𝑑 +𝐴 𝐹𝑒 2𝑝 Equation 1.
With the large concentration of carbon in the material, the formation of MxCy carbides and intermetallic compounds were detected by X-ray diffraction (Figure 3). Depending on the thermal treatment, the carbide concentration reduces from S1 to S4. Precipitation of carbides is known to create a depletion area in Fe, Cr around. These places are then less covered by a stable and chrome-enriched film and are easily attacked. The oxidation state of Mo, Cr, and Fe suggests forming a complex oxide film with a chemistry that may depend on the degree of crystallinity and the Cr/Mo ratio [START_REF] Yang | Effects of crystallization on the corrosion resistance of Fe-based amorphous coatings[END_REF][START_REF] Ameer | Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions[END_REF]. This later information was not confirmed by our XPS analysis either because of the sample's roughness that affects the peak morphology or the immersion duration. Tafel's fit was performed on polarization curves for maker positions at a minimum of 80 mV of the corrosion potential. EC-Lab software was used to calculate the corrosion potential (Ecorr), the current density (Jcorr) at Ecorr, the Tafel constants ( and c), and the corrosion rate (Table 2).

Surface reactivity in sulfate solution

The corrosion potential, Ecorr, follows the trend observed on OCP. The highest cathodic potential is attributed to S4-300, which is the optimized materials in terms of amorphous phase and density. On the opposite, S1 presents the most anodic potential. For Yang et al. [START_REF] Yang | Effects of crystallization on the corrosion resistance of Fe-based amorphous coatings[END_REF], the crystallization of Fe-Cr metallic glass alloy results in stabilizing iron oxides instead of chromium or molybdenum oxides. As the surface chemistry change, modifications of the cathodic and anodic behavior are expected.

The cathodic reaction is under a mix charge transfer and mass transport limitation on Tafel's plots, as expected in a neutral, aerated, and stagnant solution. In table 2, the Tafel constants c are in the range of 120 to 90 mV/decade, corresponding to the hydrogen evolution reaction [START_REF] Conway | Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H[END_REF]. Also, one can notice a decrease of c with sample amorphization. This evolution is either due to hydrogen reduction or oxide film reduction [START_REF] Yang | Effects of crystallization on the corrosion resistance of Fe-based amorphous coatings[END_REF][START_REF] Hermas | A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H2SO4[END_REF]. For the former, crystallinity could play a role in the surface energy linked to the oxide layer's chemical composition and contribute to hydrogen reduction. For the latter, oxide reduction may occur during the cathodic polarization. Depending on the film chemistry, iron oxides may preferentially reduce compare to chromium or molybdenum oxides [START_REF] Oblonsky | A surface enhanced Raman spectroscopic study of the passive films formed in borate buffer on iron, nickel, chromium and stainless steel[END_REF][START_REF] Schmuki | In Situ X-Ray Absorption Near-Edge Spectroscopic Study of the Cathodic Reduction of Artificial Iron Oxide Passive Films[END_REF][START_REF] Boissy | Correlation between predictive and descriptive models to characterize the passive film -Study of pure chromium by electrochemical impedance spectroscopy[END_REF]. As the ratio of iron oxide/hydroxides is higher for S1 than S2-S3 and S4-300, the cathodic current is stronger.

Consequently, Ecorr shifts to anodic values.

A passive plateau is observed in the anodic domain for the samples except for S1 (current density at about 500 µA/cm²). The passivation domain or pseudo passivation is in the range of the corrosion potential and -0.35 VMSE. This behavior is in agreement with the literature, which reports that partial crystallization of an amorphous structure is detrimental for the nucleation and growth of a passive film as well as its protectiveness even if the passive film chemistry do not differ for the same material at the crystalline or amorphous state [START_REF] Asami | ESCA study of the passive film on an extremely corrosion-resistant amorphous iron alloy[END_REF]. Above -0.35 VMSE, the transpassive region starts with the selective dissolution of iron and chromium, as observed on stainless steel passive film [START_REF] Tanaka | Corrosion characteristics of Fe203-Cr203 artificial passivation films under potentiostatic control[END_REF]. The oxide nature also accounts for corrosion resistance. Less anodic current is required to maintain the steady-state passive film on amorphous materials than on semi-crystalline ones (Figure 7b and values of a in table 2).

Consequently, the corrosion rate can be mitigated when fully amorphous materials are produced.

After 3 hours of immersion, electrochemical impedance spectra (EIS) were recorded at the OCP. In figure 9, Nyquist diagrams are characterized by a pseudo-capacitive loop online with a passive film's growth in a sulfate solution. On the one hand, the semi-circle loop increases with the amorphous state of the bulk material, evidencing the beneficial effect of amorphous surfaces for corrosion resistance as reported for other chemical composition [START_REF] Wang | Effects of Cr contents in Fe-based bulk metallic glasses on the glass forming ability and the corrosion resistance[END_REF][START_REF] Scully | Corrosion and related mechanical properties of bulk metallic glasses[END_REF]. On the other hand, S1 is characterized by a constant phase of 45° for low frequencies values associated with reactions limited by diffusion processes [START_REF] Apreutesei | Binary Zr-Ni/Co metallic glass films: Role of the structural state on their durability[END_REF].

EIS diagrams were fitted using the two electrical equivalent circuits (EEC) reported in table 2. Both EEC are made with a constant phase element (CPE), Qdl, related to the space charge and double-layer capacitance of the passive layer parallel to a resistance, Rct, which expresses the charge transfer resistance associated with the faradic processes of oxidation and reduction (Equation 2). The CPE was preferred to the ideal capacitor (Cdl) because of the sample surface heterogeneities and roughness. Using Brugg's approximation, it is possible to estimate the space charge capacitance Csc with Equation 3. For S1, the diffusion process at low frequency is modeled by a Warburg semi-infinite diffusion element where σ1 is the Warburg coefficient (Eq .4). Note that W is added in serial with Rct. For both EEC, Re represents the electrolyte resistance.

𝑍(𝑓)

𝐶𝑃𝐸 = [𝑄 𝑑𝑙 . (2𝜋𝑓) n ] -1 Equation 2 1 2𝜋(𝑅 𝑡 .Q 𝑑𝑙 ) 1 n ⁄ = 1 2𝜋R.C 𝑑𝑙 Equation 3 𝑍(𝑓) 𝑊 = √2.𝜎 1 √𝑗.2.𝜋𝑓 Equation 4
Table 2 lists the EEC fit results, whereas the lines plotted in figure 9 show the fit results. Re is in the range of 50 to 150 Ohm cm² for all the tests, which is expected for a sulfate solution at 10 g/L. The crystallinity state induces a polarization resistance increase confirming the underneath microstructure's role in the passive film grown [START_REF] Apreutesei | Binary Zr-Ni/Co metallic glass films: Role of the structural state on their durability[END_REF][START_REF] Liu | Effect of microstructure on corrosion behaviours of a Ni-based metallic glass[END_REF][START_REF] Gervasi | Analysis of the impact of alloy microstructural properties on passive films formed on low-C 13CrNiMo martensitic stainless steels[END_REF][START_REF] Olsson | Passive films on stainless steels-chemistry, structure and growth[END_REF].

For Qdl and n, the results in table 2 show a significant difference between crystalline (i.e., S1) and quasi amorphous structures (i.e., S2, S3, and S4-300). Qdl is about a few hundred Fs (n-1) cm -2 for crystalline samples and drops to a few ten Fs (n-1) cm -2 for the degree of crystallinity lower than 30 %. Note that Cdl follows the same trend. On the one hand, the parameter n is about 0.6, which is characteristic of limited reactions through nano-micro porous structures [START_REF] Mokhtari | Corrosion resistance of porous ferritic stainless steel produced by liquid metal dealloying of Incoloy 800[END_REF][START_REF] Cooper | Simulated impedance of diffusion in porous media[END_REF][START_REF] Barcia | Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering[END_REF]. The transmission line model and its derivatives were proposed to consider the pore size geometry, distribution on the impedance in the absence of diffusion [START_REF] Wang | Impedance studies of nickel hydroxide microencapsulated by cobalt[END_REF][START_REF] Keiser | Abschätzung der porenstruktur poröser elektroden aus impedanzmessungen[END_REF]. Some works also proposed quantifying the porosity properties (number, length, radius) from the impedance data [START_REF] Barcia | Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering[END_REF][START_REF] Hitz | Experimental study and modeling of impedance of the her on porous Ni electrodes[END_REF][START_REF] Barcia | Further to the paper "application of the impedance model of de Levie for the characterization of porous electrodes[END_REF]. These approaches fall under this work scope as this behavior was only found for the most crystalline sample.

On the other hand, the parameter n is at approximately 0.8 for the three other samples. The deviation from the ideal capacitor (n=1) is attributed to the metallurgical defects (voids, vacancies) in the space charge volume and the surface morphology, which is not smooth since samples were only polished to SiC 4000. Morphological gradients also exist from the center to the samples' outer region (Figure 4) [START_REF] Boissy | Correlation between predictive and descriptive models to characterize the passive film -Study of pure chromium by electrochemical impedance spectroscopy[END_REF][START_REF] Marcelin | Electronic properties of passive films from the multi-frequency Mott-Schottky and power-law coupled approach[END_REF][START_REF] Cördoba-Torres | Relationship between the origin of constantphase element behavior in electrochemical impedance spectroscopy and electrode surface structure[END_REF]. Therefore, the bulks' crystallization and its consequences on the passive film structure are detrimental to its intrinsic resistance and not only regarding its nucleation and growth, as seen before.

Regarding the corrosion properties of the four Fe-based samples, there is a significant effect of SPS parameters on the surface reactivity. Whatever the sintering cycle selected, an oxide layer grows on the alloy surface. If operating conditions are suitable for amorphous structures, the Cr and Mo content in the oxide layer increases instead of Fe. The surface becomes passives and is characterized by a steady-state current in the passive range of a few µA/cm². The passive domain is similar to all samples regardless of the iron content in the film, suggesting that the oxide layer is not only formed by chromium oxide but a dual or duplex oxide mixing Cr, Mo, and Fe. The tuning SPS parameter also has a consequence on the degree of porosity. It must be noticed that porosity increases with the degree of crystallinity.

Porosity and, more generally, surface defects are prone to localized corrosion phenomena. In a chloride-free environment, the general dissolution behavior of materials is only characterized. The metallurgical study showed an absence of homogeneity in the porosity distribution along the sample's diameters. Indeed, the SPS process is more efficient on the samples' center than on edge. This gradient of defects also has a consequence on corrosion behavior. As the porosity increases, the corrosion resistance of the material changes. In addition to the chemical composition of the oxide layer, the porosity on the surface (i.e., roughness) and pore interconnection give suitable conditions for localized corrosion. If the oxide layer is not strong enough (no pure chromium oxide layer), dissolution can start more quickly. Further work should focus on the local electrochemical of SPS samples, comparing edge and center reactivities to go deep in the local effect of crystallinity, porosity on micro galvanic coupling with a denser and amorphous area of the sample.

Conclusions

Spark plasma sintering is a technique that facilitates the processing of amorphous alloys. The heating rate plays an important role in the activation energy; for super viscous materials, slow heating rates increase it, which was beneficial to conserve the amorphous phase in this case, but the material's densification was reduced. This effect can be compensated with the increment of the uniaxial pressure. In the SPS, the slow heating rates also reduce the temperature gradient allowing a homogeneous densification process.

Materials porosity reduces the corrosion resistance of the materials. However, for metallic glasses, more advantages have been found in preserving the material's amorphous phase than reaching higher densities. Table 2. Tafel fit on LSV curves for the four samples immersed in a neutral sulfate solution. The equivalent weight was equal to 26.56 g/eq, and the density was taken to 7.9 g/cm 3 .

0

Figure

  Figure 7b reports the typical linear sweep voltammetry curves after 3 hours at the OCP.

Figure 1 .

 1 Figure 1. Powder of Fe-base metallic glass, a) SEM observations, b) Size distribution by laser granulometry, c) XR-Diffraction and d) DSC.

Figure 2 .

 2 Figure 2.Sintering cycles S1, S2 and S3 with final temperature of 575 ℃. Axial pressure 50 MPa.

Figure 3 .

 3 Figure 3. a) X-ray diffraction patterns on BMG and b) density of Fe-base metal glass sintered by S1, S2 and S3 cycles at a maximum temperature of 575 ℃ (axial pressure 50 MPa).

Figure 4 .

 4 Figure 4. Top surface SEM images of sintered samples (axial pressure 50 MPa) S1, S2, and S3 a) at the edge, b) at the center and c) in cross-section.

Figure 5

 5 Figure 5. a) Difractogramas and b) Graph of relative density of Fe-base BMG sintered at 560 ℃ at different pressures.

Figure 6 .

 6 Figure 6. Top surface SEM images at the center of the sintered samples S4 at different uniaxial pressures (MPa) and cross-section of sample at 400 MPa.

Figure 7 a

 7 Figure 7 a) Open circuit measurements and b) linear polarization curves on S1, S2, S3, and S4-300 in 10 g/L Na 2 SO 4 solution (room temperature, stagnant conditions).

Figure 8 .

 8 Figure 8. XPS data and fitted Fe 2p, Cr 2p, and Mo 3d spectra for (a-c) S1 and (d-f) S4.

Figure 9 .

 9 Figure 9. Nyquist diagrams on four samples immersed in an aerated, stagnant sulfate solution. a) zoom for S1 sample (high crystallinity ratio) and b) comparison between samples with low crystallinity ratio. The dash lines are the results of the EEC fits.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

  L. Zarazúa-Villalobos, N. Mary, J. Soo-Hyun, K. Ogawa, H. Kato, Y. Ichikawa, Microstructure and corrosion study of Fe-based bulk metallic glass obtained by spark plasma sintering, Journal of Alloys and Compounds, Volume 880, 2021, 160399, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.160399.

Table 1 .

 1 List of samples and SPS pressure and temperature.

	Sample name	Maximum temperature	Maximum Pressure
	S1 -S2 -S3	575 °C	50 MPa
	* Differences are in the sintering cycle:
	S1 = Room temperature to 575°C at 50°C/min and 15 min of holding time
	S2 = Room temperature to 560°C at 50°C/min then to 575°C at 1°C/min
	S3 = Room temperature to 545°C at 50°C/min then to 575°C at 1°C/min
	S4-50	560	°C	50 MPa
	S4-100	560	°C	100 MPa
	S4-200	560	°C	200 MPa
	S4-300	560	°C	300 MPa
	S4-400	560	°C	400 MPa
	* Identical sintering cycle:
	S4 = Room temperature to 560°C at 50°C/min then to 560 °C at 1°C/min

https://doi.org/10.1016/j.jallcom.2021.160399.

Acknowledgments L. Zarazúa-Villalobos acknowledge CONACYT (Mexican Council of Science and Technology) for the fellowship granted. JSPS KAKENHI Grant Number JP19K05092 supported this work. Masatoshi Tanno (Tohoku University, Niche) is also acknowledged for the XPS analysis.

Samples


Corentin Pigot
email: corentin.pigot@univ-amu.fr

Sébastien Péralta

Thanh-Tuân Bui

Malek Nechab

Frédéric Dumur
email: frederic.dumur@univ-amu.fr

Push-pull dyes based on Michler's aldehyde: Design and characterization of the optical and electrochemical properties

Keywords: Push-pull dyes, solvatochromism, Michler's aldehyde, TNF, TCF, rhodanine

Fifteen dyes based on Michler's aldehyde used as the electron donating group and differing by the electron accepting groups have been designed and synthesized. Interestingly, all dyes showed a broad absorption extending over the visible range. Examination of the solvatochromism in twenty-three solvents of different polarities revealed these dyes to give remarkable linear correlations using the Kamlet-Taft solvent polarity scale or the SPP Catalan empirical scales. The different dyes were also characterized by photoluminescence spectroscopy as well as cyclic voltammetry. To get a deeper insight into the optical properties of the different dyes, theoretical calculations were also carried out.

Introduction

Push-pull dyes that comprise an electron donor connected to an electron acceptor by mean of a spacer are the focus of intense research efforts due to the facile tunability of their optical properties. [START_REF] Bureš | Fundamental aspects of property tuning in push-pull molecules[END_REF][START_REF] Kulhánek | Push-pull molecules with a systematically extended π-conjugated system featuring 4,5-dicyanoimidazole[END_REF] By acting on the electron donating and the electron accepting abilities of the two partners, position of the intramolecular charge transfer band can be easily modified and dyes absorbing from the near-UV-visible range to the infrared region can be easily prepared.

Jointly, broadness of the ICT band can also be enlarged by extending the π-conjugated spacer so that dyes with panchromatic absorptions are now accessible. [START_REF] Heyer | Panchromatic Push-Pull Dyes of Elongated Form from Triphenylamine, Diketopyrrolopyrrole, and Tetracyanobutadiene Modules[END_REF][START_REF] Gómez Esteban | Panchromatic Push-Pull Chromophores based on Triphenylamine as Donors for Molecular Solar Cells[END_REF][START_REF] Duan | Facile synthesis of bisdicyanovinylidene-end-capped push-pull molecules as panchromatic absorbers[END_REF] Benefiting from their intense absorption bands in the visible range, push-pull dyes thus constitute candidates of choice for applications such as dyes for non-linear optics, [START_REF] Huo | Novel nonlinear optical push-pull fluorene dyes chromophore as promising materials for telecommunications[END_REF][START_REF] Raimundo | Design and Synthesis of Push-Pull Chromophores for Second-Order Nonlinear Optics Derived from Rigidified Thiophene-Based π-Conjugating Spacers[END_REF][START_REF] Mohammed | Synthesis and characterisation of push-pull flavin dyes with efficient second harmonic generation (SHG) properties[END_REF] active layers for solar cells, [START_REF] Parsa | Designing Push-Pull Porphyrins for Efficient Dye-Sensitized Solar Cells[END_REF][START_REF] Yella | Molecular Engineering of Push-Pull Porphyrin Dyes for Highly Efficient Dye-Sensitized Solar Cells: The Role of Benzene Spacers[END_REF][START_REF] Farré | Synthesis and properties of new benzothiadiazole-based push-pull dyes for p-type dye sensitized solar cells[END_REF][START_REF] Magaldi | Carbazole-based material: synthesis, characterization, and application as hole transporting material in perovskite solar cells[END_REF][START_REF] Le | Synthesis, Thermal, Optical and Electrochemical Properties of Acridone and Thioxanthone Based Push-Pull Molecules[END_REF][START_REF] Magaldi | Carbazole Electroactive Amorphous Molecular Material: Molecular Design, Synthesis, Characterization and Application in Perovskite Solar Cells[END_REF][START_REF] Magaldi | Hole transporting materials for perovskite solar cells: molecular versus polymeric carbazolebased derivatives[END_REF] biological labelling [START_REF] Thooft | A Small Push-Pull Fluorophore for Turn-on Fluorescence[END_REF][START_REF] Karpenko | Push-pull dioxaborine as fluorescent molecular rotor: far-red fluorogenic probe for ligand-receptor interactions[END_REF] organic field effect transistors [START_REF] Jousselin-Oba | Synthesis, Aromaticity, and Application of peri-Pentacenopentacene: Localized Representation of Benzenoid Aromatic Compounds[END_REF][START_REF] Jousselin-Oba | Modulating the ground state, stability and charge transport in OFETs of biradicaloid hexahydro-diindenopyrene derivatives and a proposed method to estimate the biradical character[END_REF][START_REF] Jousselin-Oba | Excellent Semiconductors Based on Tetracenotetracene and Pentacenopentacene: From Stable Closed-Shell to Singlet Open-Shell[END_REF][START_REF] Jousselin-Oba | Synthesis and Properties of Benzo-Fused Indeno[2,1-c]fluorenes[END_REF][START_REF] Broman | Tetraceno[2,1,12,11-opqra]tetracene-extended tetrathiafulvaleneredox-controlled generation of a large PAH core[END_REF][START_REF] Sbargoud | Low Bandgap Bistetracene-Based Organic Semiconductors Exhibiting Air Stability, High Aromaticity and Mobility[END_REF][START_REF] Sbargoud | Diindeno[1,2-b:2′,1′n]perylene: a closed shell related Chichibabin's hydrocarbon, the synthesis, molecular packing, electronic and charge transport properties[END_REF] photochroms [START_REF] Tanaka | Synthesis and Photochromism of Spirobenzopyran Derivatives Bearing an Oxymethylcrown Ether Moiety: Metal Ion-Induced Switching between Positive and Negative Photochromisms[END_REF][START_REF] Frigoli | P-Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study[END_REF][START_REF] Ishiguro | Improved thermal stability in photochromism-based optically controllable organic thin film transistor[END_REF][START_REF] Frigoli | The Control of Photochromism of [3H]-Naphthopyran Derivatives with Intramolecular CH-π Bonds[END_REF][START_REF] Delbaere | Mechanistic understanding of the photochromism of a hybrid dithienylethene-naphthopyran system by NMR spectroscopy[END_REF] or as visible light photoinitiators of polymerization. [START_REF] Sun | Sunlight Induced Polymerization Photoinitiated by Novel Push-Pull Dyes: Indane-1,3-Dione, 1H-Cyclopenta[END_REF][START_REF] Pigot | Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization[END_REF][START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF][START_REF] Lalevée | New thioxanthone and xanthone photoinitiators based on silyl radical chemistry[END_REF][START_REF] Belon | Overcoming the oxygen inhibition in the photopolymerization of acrylates: A study of the beneficial effect of triphenylphosphine[END_REF][START_REF] Telitel | Soft Photopolymerizations Initiated by Dye-Sensitized Formation of NHC-Boryl Radicals under Visible Light[END_REF][START_REF] Lalevée | Recent advances in sunlight induced polymerization: role of new photoinitiating systems based on the silyl radical chemistry[END_REF][START_REF] Lalevée | N-Heterocyclic Carbene Boranes Accelerate Type I Radical Photopolymerizations and Overcome Oxygen Inhibition[END_REF][START_REF] Garra | Charge Transfer Complexes as Pan-Scaled Photoinitiating Systems: From 50 μm 3D Printed Polymers at 405 nm to Extremely Deep Photopolymerization (31 cm)[END_REF][START_REF] Fouassier | Three-component photoinitiating systems: towards innovative tailor made high performance combinations[END_REF] With aim at optimizing the synthetic routes of numerous organic compounds, recently, photoredox catalysis making used of pushpull dyes as photosensitizers has emerged as a promising approach which was applied to various chemical transformations. [START_REF] Ravelli | Dyes as Visible Light Photoredox Organocatalysts[END_REF][START_REF] Le | s-Tetrazine Dyes: A Facile Generation of Photoredox Organocatalysts for Routine Oxidations[END_REF][START_REF] Fidaly | Visible light photoredox organocatalysis: a fully transition metal-free direct asymmetric α-alkylation of aldehydes[END_REF][START_REF] Oliveira V Da | Organocatalysis: A Brief Overview on Its Evolution and Applications[END_REF] Considering that the intramolecular charge transfer (ICT) band constitutes the key element of push-pull dyes focusing the interest of numerous researchers, intensity and position of this absorption band can be easily modified by applying the following principles. Notably, at constant electron acceptor, a red-shift of the ICT band can be obtained by elongating the spacer between the donor and the acceptor, but also by improving the electron-releasing ability of the donor. [START_REF] Dietlin | Photopolymerization upon LEDs: new photoinitiating systems and strategies[END_REF][START_REF] Tehfe | Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry[END_REF][START_REF] Lalevée | Recent Developments of Versatile Photoinitiating Systems for Cationic Ring Opening Polymerization Operating at Any Wavelengths and under Low Light Intensity Sources[END_REF][START_REF] Tehfe | Long Wavelength Cationic Photopolymerization in Aerated Media: A Remarkable Titanocene/Tris(trimethylsilyl)silane/Onium Salt Photoinitiating System[END_REF][START_REF] Guillaneuf | Importance of the Position of the Chromophore Group on the Dissociation Process of Light Sensitive Alkoxyamines[END_REF][START_REF] Fouassier | Radical polymerization activity and mechanistic approach in a new three-component photoinitiating system[END_REF][START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF][START_REF] Quint | Visible-lightmediated α-phosphorylation of N-aryl tertiary amines through the formation of electrondonor-acceptor complexes: synthetic and mechanistic studies[END_REF][START_REF] Tehfe | Near UVvisible light induced cationic photopolymerization reactions: A three component photoinitiating system based on acridinedione/silane/iodonium salt[END_REF] Parallel to the red-shift, elongation of the spacer also mechanically induces an enhancement of the molar extinction coefficient by improving the oscillator strength. [START_REF] Kuhn | Oscillator Strength of Absorption Band in Dye Molecules[END_REF][START_REF] Feng | First Principles Design of Dye Molecules with Ullazine Donor for Dye Sensitized Solar Cells[END_REF] Among electron donors that inherently comprise an elongated spacer, cinnamaldehydes are appealing candidates. [START_REF] Coe | Helquat Dyes: Helicene-like Push-Pull Systems with Large Second-Order Nonlinear Optical Responses[END_REF] With aim at developing donors with electron donating abilities improved compared to that of 4dimethylaminobenzaldehyde, Michler's aldehyde i.e. 3,3-bis(4-(dimethylamino) phenyl)acrylaldehyde is a cinnamaldehyde derivative exhibiting a strong electron donating ability. This aldehyde was notably at the basis of numerous dyes used for the design of chromophores for solar cells or photoinitiators of polymerization. [START_REF] Francos | Butadiene dyes based on 3-(dicyanomethylidene)indan-1-one and 1,3-bis(dicyanomethylidene)indane: synthesis, characterization and solvatochromic behaviour[END_REF][START_REF] Jang | Oligophenylenevinylene-Functionalized Ru(II)-bipyridine Sensitizers for Efficient Dye-Sensitized Nanocrystalline TiO2 Solar Cells[END_REF][START_REF] Shandura | Substituted xanthylocyanines. II. Pyroninocyanines[END_REF][START_REF] Sun | Novel Push-Pull Dyes Derived from 1H-cyclopenta[b]naphthalene-1,3(2H)-dione as Versatile Photoinitiators for Photopolymerization and Their Related Applications: 3D Printing and Fabrication of Photocomposites[END_REF][START_REF] Sun | Free Radical Photopolymerization and 3D Printing Using Newly Developed Dyes: Indane-1,3-Dione and 1H-Cyclopentanaphthalene-1,3-Dione Derivatives as Photoinitiators in Three-Component Systems[END_REF][START_REF] Tehfe | New Push-Pull Dyes Derived from Michler's Ketone For Polymerization Reactions Upon Visible Lights[END_REF] In this work, a series of 15 dyes based on Michler's aldehyde have been prepared and examined for their photophysical properties (See Figure 1). Notably, a positive solvatochromism could be evidenced with several semi-empirical scales such as the Kamlet-Taft or the Catalan empirical scales. The different dyes were also examined for their electrochemical properties and theoretical calculations were carried out to support the experimental data. Besides, these dyes have been examined in five separated studies so that their optical properties were tested in different conditions. It has to be noticed that in this work, allylrhodamine (EA14) has been studied with ethylrhodanine (EA13) as electron acceptor precursors. Even if the replacement of an ethyl group by an allyl group is out of interest from the photophysical properties viewpoint, it can provide a unique opportunity to design dyes that can be used in the Future by photopolymerists. Indeed, Type II photoinitiators are extremely popular in photopolymerization and we wanted to anticipate future works of photopolymerists aiming at addressing the extractability issue. [START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF][START_REF] Kavalli | Comparison of pure epoxy vs. epoxy-anhydride photopolymerization[END_REF][START_REF] Kavalli | Ultrafast Epoxy-Anhydride Photopolyaddition Reaction[END_REF][START_REF] Kirschner | Silyl Glyoximides: Toward a New Class of Visible Light Photoinitiators[END_REF][START_REF] Topa-Skwarczyńska | Development of the first panchromatic BODIPY-based one-component iodonium salts for initiating the photopolymerization processes[END_REF][START_REF] Hammoud | Novel phenylaminebased oxime ester photoinitiators for LED-induced free radical, cationic, and hybrid polymerization[END_REF][START_REF] Zhang | Development of a Zeolite/Polymer-Based Hydrogel Composite through Photopolymerization for 3D Printing Application[END_REF][START_REF] Gencoglu | Benzophenone-Functionalized Oligo(Amido Amine)/Iodonium Salt Systems as Visible Light Photoinitiators[END_REF][START_REF] Zhang | Photopolymerization of Pollen Based Biosourced Composites and Applications in 3D and 4D Printing[END_REF][START_REF] Lai | N-Aryl glycines as versatile initiators for various polymerizations[END_REF][START_REF] Zeng | Cytotoxic and cytocompatible comparison among seven photoinitiators-triggered polymers in different tissue cells[END_REF][START_REF] Sprick | New hydrogen donors for amine-free photoinitiating systems in dental materials[END_REF]] EA1-EA15 have been selected as electron acceptors as these structures strongly differ by their electron-withdrawing ability.

Malononitrile (EA1) is a weak acceptor [START_REF] Ghanem | Synthesis of push-pull triarylamine dyes containing 5,6-difluoro-2,1,3-benzothiadiazole units by direct arylation and their evaluation as active material for organic photovoltaics[END_REF][START_REF] Choi | Exploiting the potential of 2-((5-(4-(diphenylamino)phenyl)thiophen-2yl)methylene)malononitrile as an efficient donor molecule in vacuum-processed bulkheterojunction organic solar cells[END_REF][START_REF] Kotowicz | Malononitrile derivatives as push-pull molecules: Structure -properties relationships characterization[END_REF][START_REF] Bogdanov | Synthesis, crystal structure studies and solvatochromic behaviour of two 2-5-[4-(di\-methyl\-amino)\-phen\-yl]penta-2,4-dien-1-yl\-idenemalono\-nitrile derivatives[END_REF] whereas EA8 is the strongest one of the series.

The others offer a gradient of electron-withdrawing ability. To end, methyl pyridinium (EA15) is commonly used as electron acceptor. [START_REF] Deligeorgiev | Styryl dyessynthesis and applications during the last 15 years[END_REF][START_REF] Narang | Characterization of a New Solvent-Sensitive Two-Photon-Induced Fluorescent (Aminostyryl)pyridinium Salt Dye[END_REF][START_REF] Wong | Simultaneous High-Throughput Conformational and Colloidal Stability Screening Using a Fluorescent Molecular Rotor Dye, 4-(4-(Dimethylamino)styryl)-N-Methylpyridinium Iodide (DASPMI)[END_REF] Therefore, considering the wide range of electron acceptor used, it was logical to include it in this study. Besides, we do agree that due to its betaine structure, its solvatochromism could be anticipated to be different from that of the other dyes.

Figure 1.

Chemical structures of push-pull dyes PP1-PP15 examined in this work.

Results and Discussion

Synthesis of the dyes

The fifteen dyes reported in this work have been prepared by a Knoevenagel reaction between Michler's aldehyde and the different acceptor precursors. However, depending on the electron acceptors used, different synthetic conditions had to be used. Typically, Knoevenagel reactions are carried out in basic conditions, using piperidine [START_REF] Dalessandro | Mechanism of the Piperidine-Catalyzed Knoevenagel Condensation Reaction in Methanol: The Role of Iminium and Enolate Ions[END_REF][START_REF] Van Beurden | The Knoevenagel reaction: a review of the unfinished treasure map to forming carbon-carbon bonds[END_REF] or diisopropylamine [START_REF] Pigot | Unprecedented Nucleophilic Attack of Piperidine on the Electron Acceptor during the Synthesis of Push-Pull Dyes by a Knoevenagel Reaction[END_REF] as the bases. Thus, in the case of EA1, EA3-EA6, EA9, EA10, EA12-EA15, piperidine was used as organocatalyst. Conversely, in the case of EA2, no base was used, the Knoevenagel reaction occurred spontaneously with the condensation of EA2 in ethanol. PP2 could be obtained in 85% yield. Similarly, by using a highly polar solvent such as DMF, EA11 could spontaneously condensate, enabling to get PP11 in 78% yield after 10 min. of reaction. Finally, in the case of EA7 and EA8, due to the remarkable stability of EA7 and EA8 anions in basic conditions, these anions are totally unreactive for Knoevenagel reactions so that acidic coupling conditions had to be used. The two dyes were prepared by refluxing Michler's aldehyde and EA7 or EA8 in acetic anhydride overnight, enabling to get PP7 and PP8 in 89 and 84% yields.

Overall, as shown in Scheme Finally, an original synthetic route was also developed for the synthesis of 5-bromo-1Hindene-1,3(2H)-dione EA4 even if the synthesis of this electron acceptor was previously reported in the literature. [START_REF] Omid | Montmorillonite KSF Clay as Novel and Recyclable Heterogeneous Catalyst for the Microwave Mediated Synthesis of Indan-1,3-Diones[END_REF][START_REF] Hark | Novel approaches toward ninhydrin analogs[END_REF] Notably, 4-bromophthalic anhydride first reacted with ethanol in acidic conditions, enabling to produce diethyl 4-bromophthalate in almost quantitative yield.

Then, in a second step, by mean of a Claisen condensation of the diester with ethyl acetate using NaH as the base, the intermediate sodium 5-bromo-2-(ethoxycarbonyl)-1,3-dioxo-2,3-dihydro-1H-inden-2-ide could be formed. Upon treatment under acidic conditions, the targeted 5bromo-1H-indene-1,3(2H)-dione EA4 could be obtained in 80% for the two steps (See Scheme 2).

Optical properties

Due to the good solubility of all dyes in chloroform, comparison of their optical properties was carried out in this solvent. It has to be noticed that the photoluminescence of the different dyes has not reported in this work, the dyes being not emissive at all. As shown in Figure 2, all dyes showed a broad absorption band in the visible range corresponding to the intramolecular charge transfer (ICT) band. As anticipated, the most blue-shifted absorption (λmax = 500 nm) was detected for PP1 exhibiting malononitrile as the electron acceptor. The most red-shifted absorption was found for PP8 exhibiting the strongest electron acceptor i.e.

EA8.

A maximum absorption located at 727 nm was found for this dye. Following this dye, the second most red-shifted absorption was logically found for PP7 (λmax = 698 nm), also bearing a tetracyano-substituted electron acceptor (EA7) but differing from EA8 by its polyaromaticity.

Interestingly, PP11 showed two ICT bands located in the visible range at 557 and 696 nm respectively and presence of these two ICT bands are consistent with previous results reported in the literature for dyes comprising 2,4,5,7-tetranitro-9H-fluorene EA11 as the acceptor. [START_REF] Noirbent | Dyes with tunable absorption properties from the visible to the near infrared range: 2,4,5,7-Tetranitrofluorene (TNF) as a unique electron acceptor[END_REF][START_REF] Noirbent | Recent Advances on Nitrofluorene Derivatives: Versatile Electron Acceptors to Create Dyes Absorbing from the Visible to the Near and Far Infrared Region[END_REF] Noticeably, all dyes exhibited a second intense absorption band close to the ICT band appearing as a second absorption band such as in the case of PP7 and PP8, or a shoulder such as in PP3 and PP4. Based on theoretical calculations later detailed in the manuscript but also in SI, this second absorption band detected at higher energy than the ICT band can be assigned to HOMO-1 => LUMO or HOMO => LUMO+1 transitions, depending on the dye. Especially, two absorption bands with high oscillator strength could be determined for all dyes. PP1-PP15 are also characterized by remarkable molar extinction coefficients and the results are summarized in Table 1. As shown in Figure 3, all dyes exhibited high molar extinction coefficients. The smallest molar extinction coefficient was found for PP11 comprising the TNF acceptor. A molar extinction coefficient of 28150 M -1 .cm -1 was found for this dye at 557 nm. The highest molar extinction coefficients were determined for PP7 and PP8

comprising respectively 2,2'-(1H-indene-1,3(2H)-diylidene)dimalononitrile EA7 and 2,2'-(1H- cyclo-penta[b]naphthalene-1,3(2H)-diylidene)dimalononitrile EA8
as the acceptor, the molar extinction coefficients peaking respectively at 97950 and 89200 M -1 .cm -1 for PP7 and PP8. Theoretical studies were also carried out to investigate the energy levels as well as the molecular orbitals (M.O.) compositions of the different dyes. DFT calculations were performed for all dyes at the B3LYP/6-311G(d,p) level of the theory using Gaussian 09 programs except for PP15 for which the LAND2Z basis set was used to determine the transitions involved in the different absorption peaks. Chloroform was used as the solvent model with a polarizable continuum model (PCM). [START_REF] Frisch | Gaussian 16 Rev. C.01[END_REF][START_REF] Lee | Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density[END_REF][START_REF] Becke | A new mixing of Hartree-Fock and local density-functional theories[END_REF][START_REF] Hehre | Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules[END_REF][START_REF] Scalmani | Continuous surface charge polarizable continuum models of solvation. I. General formalism[END_REF][START_REF] O'boyle | cclib: A library for package-independent computational chemistry algorithms[END_REF][START_REF] Tomasi | The IEF version of the PCM solvation method: an overview of a new method addressed to study molecular solutes at the QM ab initio level[END_REF] As shown in Table 1 and Figure 5, a blue-shift of the theoretical absorption spectra was found for all dyes compared to the experimental ones. Typically, a difference of about 30 nm was found between the experimental and the theoretical absorption maxima. This difference increased up to 105 nm, 110 nm and 160 nm for PP7, PP8 and PP11 respectively. Concerning PP11, mismatch between the experimental and theoretical values was already reported for TNF-based push-pull dyes and assigned to the difficulty to model the UV-visible absorption spectra of these dyes exhibiting two ICT bands. [START_REF] Noirbent | Dyes with tunable absorption properties from the visible to the near infrared range: 2,4,5,7-Tetranitrofluorene (TNF) as a unique electron acceptor[END_REF] Concerning PP7 and PP8, the mismatch between the experimental and theoretical absorption spectra can originate from the torsion induced by the presence of the two dicyanomethylene groups on EA7 and EA8, resulting from the steric hindrance generated by these groups with the electron donor. While examining the composition of the ICT band detected in the visible, as shown in Table 2, this absorption band is mainly associated to HOMO->LUMO, HOMO-1->LUMO and HOMO-> LUMO+1 transitions, depending on the dye. Even if a mismatch is observed between the experimental and theoretical positions of the ICT bands, a clear trend can be observed. Thus, by improving the electron deficiency of the electron acceptor, a red-shift of the absorption maxima can be observed, the most red-shifted absorption being observed for the strongest electron acceptor of the series, namely EA8. Second, within this series of dyes, if the HOMO level remains relatively constant, the electron donor being the same for all dyes, as anticipated, improvement of the electron withdrawing ability of the acceptors resulted in a stabilization of the LUMO energy level, decreasing from -2.61 eV for PP1 to -3.24 eV for PP8. Besides, another explanation can also be given. Indeed, the maximum absorbance wavelength depends on the degree of conjugation, especially when HOMO -> LUMO dominates the ICT. The HOMO-LUMO gap energy will decrease with the increase of the strength of the electronwithdrawing group, but also with increasing the number of electrons in the ICT molecular pathway. Both structural features increase the conjugation, reducing the HOMO-LUMO gap, and ultimately shifting the absorption band bathochromically. Therefore, the relative position of the maximum absorbance wavelength cannot be attached solely to the electron-withdrawing strength of the acceptor moiety, neglecting the fact that the number of electrons participating in the solvatochromic core of the dyes is not constant or nearly constant. Therefore, the results can be explained in terms of conjugation, and not only in terms of electron-withdrawing ability. For instance, as shown in Figure 6, the naphthalene subunit of PP8 has an essential participation in the ICT. While examining the electronic distribution of the HOMO and LUMO energy levels, a localization of the HOMO energy level mostly located onto the electron donor and a LUMO energy level mostly centered onto the electron acceptor was logically observed, as shown in Figure 6.

Solvatochromism

Push-pull dyes typically exhibit a strong sensitivity to the solvent polarity so that influence of this parameter on the optical properties was examined in twenty-three solvents of different polarities. Intramolecular character of the charge transfer process was confirmed by examining the variation of the optical density with the concentration. All dyes showed linear correlations upon dilution of the different solutions, excluding the presence of intermolecular interactions between dyes. [START_REF] Guerrini | Interplay between Intra-and Intermolecular Charge Transfer in the Optical Excitations of J-Aggregates[END_REF] To get a deeper insight into the solvatochromism, several empirical polarity scales have been developed and the Kamlet-Taft's, [START_REF] Kamlet | Linear solvation energy relationships. 23. A comprehensive collection of the solvatochromic parameters[END_REF] Catalan's, [START_REF] Catalán | On the ET (30), π*, Py, S', and SPP Empirical Scales as Descriptors of Nonspecific Solvent Effects[END_REF] Kawski-Chamma Viallet's, [START_REF] Kawski | Wellenzahl von elecktronenbanden lumineszierenden molecule[END_REF] Lippert-Mataga's, [START_REF] Lippert | Dipolmoment und Elektronenstruktur von angeregten Molekülen[END_REF] McRae Suppan's, [START_REF] Suppan | Solvent effects on the energy of electronic transitions: experimental observations and applications to structural problems of excited molecules[END_REF] Dimroth-Reichardt's, [START_REF] Reichardt | Solvatochromic Dyes as Solvent Polarity Indicators[END_REF] and Bakhshiev's [START_REF] Bakshiev | Universal intermolecular interactions and their effect on the position of the electronic spectra of molecules in two component solutions[END_REF] scales are well adapted for push-pull dyes exhibiting D-π-A structures. [START_REF] Bauer | Differentiating Between Dipolarity and Polarizability Effects of Solvents Using the Solvatochromism of Barbiturate Dyes[END_REF][START_REF] Jessop | Solvatochromic parameters for solvents of interest in green chemistry[END_REF] A summary of the optical properties is provided in Tables 3 and4. Interestingly, among the different polarity scales examined, good correlations could be obtained with the Kamlet-Taft and the Catalan polarity scales. As shown in Figure 7, remarkable linear correlations could be determined for PP7. The same holds true for all dyes except PP11 and PP12 for which lower squares for the correlation coefficients (R²) could be determined. In the case of PP15, no linear correlations could be obtained, irrespective of the solvent polarity scales (See Figure 8). This unusual behaviour for pyridinium-based dyes has previously been reported in the literature for betaine dyes and assigned to a modification of the relative electrophilicities of both the electron-pair donor and acceptor moieties with the solvent, so that irregular behaviours can be found for this family of dyes. [START_REF] Ooyama | Solvatochromism of novel donor-π-acceptor type pyridinium dyes in halogenated and non-halogenated solvents[END_REF][START_REF] Rezende | A general framework for the solvatochromism of pyridinium phenolate betaine dyes[END_REF][START_REF] Jacques | Negative solvatochromism of a series of pyridinium phenolate betaine dyes with increasing steric hindrance[END_REF] Besides, by using a multiparametric approach, better correlations could be obtained for PP15, since a correlation coefficient of 0.61 could be determined using the Catalan scale. The Catalan solvent polarity scale is also capable to discriminate between the solvent polarity/polarizability effect i.e. with the solvent polarity/polarizability (SPP) parameter and the solvent dipolarity effect, which is examined with the solvent dipolarity (SdP) parameter. Based on the squares of the correlation coefficients, better linear regressions were obtained with the SPP parameter than the SdP one, evidencing that the solvatochromism was mainly governed the polarity/polarizability of the solvents.

Catalan SPP parameter a)

Catalan SPP parameter a) Finally, sensitivity of the dyes to the solvent polarity was examined by plotting the variation of the HOMO-LUMO gaps vs. Taft parameters. As shown in Figure 9, similar slopes were found for all dyes, except PP12 comprising a TCF electron acceptor. In this case, a greater slope was found, evidencing a higher charge redistribution upon photoexcitation.

Electrochemical properties

Electrochemical properties of PP1-PP15 were measured in acetonitrile by cyclic voltammetry, at a scan rate of 100 mV.s -1 , using tetrabutylammonium hexafluorophosphate (0.1M) as the supporting electrolyte. The working, pseudo-reference and counter electrodes were platinum disk (Ø = 1 mm), Ag wire, and Au wire gauze, respectively. Ferrocene was used as an internal standard. Redox potentials of all dyes against the half-wave oxidation potential of the ferrocene/ferrocenium cation pair are given in Table 5. Along with the redox values, energy levels of the HOMO and LUMO orbitals are given, the values being determined by using the two equations established by Pommerehne et al. [START_REF] Pommerehne | Efficient two layer leds on a polymer blend basis[END_REF] Two cyclic voltammograms are also presented in Figure 10, the cyclic voltammogram of PP4 containing ferrocene used as reference for calibration. In this work, all dyes possess the same electron donating group but differ by the electron accepting moieties. As shown in Table 5, all dyes exhibited a one-electron oxidation process around 0.40 V/Fc range except for PP13 and PP14 that comprise a rhodanine electron accepting moiety. In this case, an oxidation potential lowered by ca 200 mV was detected. As shown by DFT calculations, a high electronic density is located on the sulfur atoms of the rhodanine moiety so that the first oxidation process occurs on the sulfur atoms of the acceptor and not on the electron donating moiety. This behavior has previously been reported in the literature for rhodanine-based dyes comprising an electron donor of similar structure. [START_REF] Pigot | Synthesis, optical and electrochemical properties of a series of push-pull dyes based on the 4,4-bis(4methoxy phenyl)butadienyl donor[END_REF] Conversely, this behavior is not observed for PP10, also comprising an electron accepting group bearing a sulfur atom. For all the other dyes, the similarity of the oxidation processes is consistent with an oxidation process centered on the dimethylamino group of the Michler's donor. Concerning the reduction process, logically, a decrease of the reduction potential with the electron accepting strength was observed. Thus, in the case of PP1 comprising a weak electron acceptor, the reduction was determined at -1.48 V/Fc. Conversely, 2,4,5,7-tetranitrofluorene (TNF) used in PP11 is among the strongest electron acceptor ever reported in the literature. [START_REF] Noirbent | Recent Advances on Nitrofluorene Derivatives: Versatile Electron Acceptors to Create Dyes Absorbing from the Visible to the Near and Far Infrared Region[END_REF] Due to its remarkable electron accepting ability, the dye could reduce at -0.89V/Fc. Similarly, 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) is a strong electron acceptor so that the reduction process of PP12 could be detected at -1.13 V/Fc. [START_REF] Noirbent | Synthesis, optical and electrochemical properties of a series of push-pull dyes based on the 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) acceptor[END_REF] Interestingly, upon introduction of cyano groups on the indanedione derivatives (PP5, PP7 and PP6 and PP8) a significant decrease of the reduction potentials was observed, consistent with an improvement of the electron-withdrawing ability of the corresponding electron acceptors. From the cyclic voltammograms, energy levels of the HOMO and LUMO orbitals could be estimated, and a comparison with the values determined experimentally and by theoretical calculations could be established. [START_REF] Cardona | Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications[END_REF] As shown in Figure 11, a good adequation between the experimental and the theoretical values of the HOMO energy levels of all dyes could be found, even if the values are slightly underestimated by cyclic voltammetry. A higher mismatch could be found between the experimental and the theoretical values of the LUMO energy levels. In all cases, position of the LUMO levels was overestimated by theoretical calculations compared to the experimental ones. This mismatch can be assigned to the electronic distribution of their LUMO orbitals. Indeed, for most of the dyes, a strong contribution of the electron donating moiety to the LUMO orbitals can be found. In the case of PP15, as shown in SI, a strong contribution of the pyridinium moiety is only found in LUMO+1, the LUMO orbital extending all over the molecule. Finally, a comparison between the optical, electrochemical and theoretical bandgaps could be established. As shown in Figure 12, a relatively good adequation between the optical and the theoretical bandgaps could be determined for all dyes. Conversely, the electrochemical bandgap is greatly underestimated. This underestimation can notably be assigned to the difficulty to clearly determine the onsets of the oxidation and the reduction peaks on the different voltammograms.

Conclusion

In this work, a series of push-pull dyes based on Michler's donor have been studied

Figure 2 .

 2 Figure 2. Normalized UV-visible absorption spectra of PP1-PP15 in chloroform.

Figure 3 .

 3 Figure 3. UV-visible absorption spectra of PP1-PP15 in chloroform.

Figure 4 .

 4 Figure 4. Classification of electron acceptors established on the basis of the experimental position of the absorption maxima.

Figure 5 .

 5 Figure 5. Theoretical UV-visible absorption spectra of PP1-PP15 in chloroform.

Figure 6 .

 6 Figure 6. Contour plots of the HOMO and LUMO energy levels for PP8 and PP11.

Figure 7 .

 7 Figure 7. Linear correlations obtained while using different polarity scales for PP7 Kamlet-Taft (a) and Catalan (b) and (c) empirical scales.

Figure 8 .

 8 Figure 8. Linear correlations obtained while using different polarity scales for PP15 Kamlet-Taft a) and Catalan b) and c) empirical scales.

Figure 9 .

 9 Figure 9. Variation of the HOMO-LUMO gaps of PP1-PP15 with the solvent polarity using Taft parameters (π*).

Figure 10 .

 10 Figure 10. Comparisons between the cyclic voltammograms of PP1 and PP4 measured in acetonitrile at a scan rate of 100 mV•s -1 , with tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) as the supporting electrolyte (In the CV curve of PP4, redox system detected at 0V correspond to ferrocene used as a reference).

2 . 22 a

 222 All potentials recorded in 0.1M TBAP/acetonitrile. EHOMO (eV) = -4.8 -Eox onset and ELUMO (eV) = -4.8 -Ered onset. b ΔEopt in chloroform. c ΔEopt in acetonitrile. d determined for the oxidation or the reduction peak due to the weak intensity of the peak not enabling to determine the position of the onset with accuracy.

Figure 11 .

 11 Figure 11. Comparisons between the frontier orbitals' energy levels estimated in chloroform by DFT calculations and those obtained by cyclic voltammetry in acetonitrile.

Figure 12 .

 12 Figure 12. Comparisons of the bandgaps determined by UV-visible absorption spectroscopy in chloroform, cyclic voltammetry in acetonitrile and by theoretical calculations in chloroform.

  using different techniques. Interestingly, by modifying the electron-withdrawing ability, dyes with absorption maxima located from 503 nm for PP1 to 727 nm for PP8 could be determined in chloroform. In this work, a new electron acceptor outperforming the reported structures has also been synthesized, namely EA8. Especially, EA8 showed higher electron-withdrawing ability than the well-established 2,4,7,8-tetranitrofluorene EA11. Considering that the same electron donor has been used for elaborating the different dyes, a scale of electron-withdrawing ability could be established between electron acceptors of different structures. Interestingly, all dyes exhibited a positive solvatochromism, what is classically observed for dyes possessing a strong electron donor such as Michler's aldehyde. Analysis of the energy bandgaps by electrochemistry, UV-visible absorption spectroscopy and theoretical calculations revealed a good adequation between the theoretical and optical bandgaps. Future works will consist in developing dyes of extended π-conjugation between the donor and the acceptor in order to get dyes strongly absorbing in the near-infrared range. EA8 being a new electron acceptor of unprecedented electron-withdrawing ability, future works will also consist in designing new dyes with this electron acceptor.

Scheme 2. Synthetic route to EA4, EA6 and EA8.

	Noticeably, among the fifteen electron-acceptors used in this study, only six electron
	acceptors i.e. EA1, EA2, EA9, EA10, EA13 and EA14 are commercially available. All the
	other electron acceptors were prepared according to procedures previously reported in the
	literature (See SI). An original electron acceptor had notably to be prepared, namely 2,2'-(1H-
	cyclopenta[b]naphthalene-1,3(2H)-diylidene)dimalononitrile EA8. This electron acceptor
	could only be obtained from EA3[87] at elevated temperature, by using butan-1-ol as the
	solvent for the Knoevenagel reaction. Indeed, all attempts to prepare this electron acceptor in
	similar conditions to that used for EA7, namely by using sodium acetate as the base and ethanol
	as the solvent failed, only producing EA6 as the unique reaction product. By using new reaction
	conditions, namely ammonium acetate[88-90] (that is sometimes used as the base for the
	synthesis of EA7) and butan-1-ol as the solvent, EA8 could be obtained in 38% yield (See
	Scheme 2).
	have previously been synthesized and investigated for their optical properties. Michler's
	aldehyde has been prepared according the procedure developed by our group in 2008.[86]
	Scheme 1. Synthetic routes to PP1-PP15.

1, PP1-PP15 could be obtained with acceptable reaction yields ranging from 75% for PP13 to 89% for PP7. It has to be noticed that among the fifteen dyes reported in this work, seven of them i.e. PP4, PP6, PP8-PP10, PP13, and PP14 have never been previously reported in the literature. Concerning the other dyes, PP1 and PP2 have only been examined as visible light photoinitiators of polymerization and not for their optical and photophysical properties.[61] PP3[83], PP5,[56] PP7,[56] PP11,[84] PP12,[85] and PP15[86]

Table 1 .

 1 Optical characteristics of the different compounds in chloroform and the values theoretically determined.

	chloroform
	compounds	PP1	PP2	PP3	PP4	PP5	PP6	PP7	PP8
	λ (nm)	503	571	606	587	543, 670	568, 697	572, 698	587, 727
	ϵ (M -1 .cm -1 )	36400	58800	75900	66600	28900, 77950	18350, 74700	40900, 97950	28900, 89200
	Δ E(eV)	2.46	2.17	2.05	2.11	1.85	1.78	1.78	1.70
	compounds	PP9	PP10	PP11	PP12	PP13	PP14	PP15
	λ (nm)	560	598	557, 693	674	523	527	572
	ϵ (M -1 .cm -1 )	55900	86900	28150, 58120	44400	53900	34200

Table 2 .

 2 Summary

of the simulated absorption characteristics in dilute chloroform of synthetized compounds. Data were obtained in chloroform solution. Position of the HOMO and LUMO energy levels determined by electrochemistry in acetonitrile are also given. Optical, electrochemical and theoretical bandgaps are given in eV.

Compounds EHOMO

(th) (eV) ELUMO (th) (eV) max (nm) Transitions EHOMO (el) (eV) ELUMO (el) (eV) ΔETH a (eV) ΔEET b (eV) ΔEopt c (eV)

					HOMO			LUMO
					Homo HOMO			Lumo LUMO
											ΔEopt
											d
											(eV)
	PP1	-5.53	-2.74	501	HOMO-1=>LUMO (53%)	-5.18	-3.50	2.79	1.68	2.46	2.51
	PP2	-5.39	-2.77	540	HOMO=>LUMO (89%)	-5.16	-3.55	2.62	1.70	2.17	2.23
	PP3	-5.41	-2.90	569	HOMO=>LUMO (93%)	-5.17	-3.55	2.51	1.62	2.05	2.08
	PP4	-5.44	-2.85	550	HOMO=>LUMO (88%)	-5.16	-3.53	2.59	1.63	2.11	2.14
	PP5	-5.49	-3.12	613	HOMO=>LUMO (91%)	-5.09	-3.60	2.37	1.49	1.85	1.87
	PP6	-5.50	-3.18	624	HOMO=>LUMO (93%)	-5.17	-3.70	2.32	1.47	1.78	1.86
	PP7	-5.60	-3.26	652	HOMO=>LUMO (91%)	-5.17	-3.90	2.34	1.39	1.77	1.78
	PP8	-5.60	-3.30	644	HOMO=>LUMO (95%)	-5.10	-3.65	2.30	1.45	1.70	1.70
	PP9	-5.46	-2.74	520	HOMO=>LUMO (75%)	-5.12	-3.59	2.72	1.53	2.21	2.27
	PP10	-5.50	-2.89	546	HOMO=>LUMO (85%)	-5.18	-3.67	2.61	1.51	2.07	2.10
	PP11	-5.54	-3.79	914	HOMO=>LUMO (98%)	-5.14	-4.02	1.75	1.35	1.79	1.82
	PP12	-5.42	-3.17	616	HOMO=>LUMO (94%)	-5.09	-3.88	2.25	1.21	1.84	1.92
	PP13	-5.24	-2.69	548	HOMO=>LUMO (98%)	-5.02	-3.67	2.55	1.35	2.37	2.42
	PP14	-5.25	-2.71	549	HOMO=>LUMO (98%)	-5.00	-3.73	2.54	1.27	2.35	2.41
	PP15	-5.16	-2.84	649	HOMO=>LUMO (84%)	-5.10	-3.63	2.32	1.47	2.17	2.22

a ΔETH in chloroform b ΔEET in acetonitrile c ΔEopt in chloroform. d ΔEopt in acetonitrile.

Table 3 .

 3 Summary of the optical properties of PP1-PP8 in solvents of different polarities.

	compounds	π* 1	PP1 2	PP2 2	PP3 2	PP4 2	PP5 2	PP6 2	PP7 2	PP8 2
	heptane	-0.08	461	n.s. 3	551	537	625	625	n.s. 3	689
	cyclohexane	0.00	465	525	555	543	630	633	666	766
	triethylamine	0.14	469	n.s. 3	561	546	638	n.s. 3	n.s. 3	n.s. 3
	diethyl ether	0.27	475	533	569	555	645	645	n.s. 3	734
	p-xylene	0.43	483	550	579	564	649	649	688	715
	diethyl carbonate	0.45	479	533	576	558	648	644	684	714
	AcOEt	0.54	483	550	581	567	656	653	687	721
	butanol	0.47	492	578	614	590	668	667	n.s. 3	727
	ethanol	0.54	497	578	616	590	667	668	n.s. 3	726
	toluene	0.54	487	550	584	567	653	652	682	716
	1,4-dioxane	0.55	481	544	578	560	648	647	n.s. 3	716
	THF	0.58	490	553	588	567	665	659	692	725
	diglyme	0.64	496	556	594	579	672	669	698	732
	acetone	0.71	492	562	594	576	666	667	697	727
	anisole	0.73	500	564	598	581	667	664	699	730
	acetonitrile	0.75	493	555	595	578	663	665	696	727
	chloroform	0.78	500	566	602	584	659	668	699	729
	1,2-dichloroethane	0.81	500	557	575	569	651	667	699	729
	dichloromethane	0.82	500	573	605	584	668	668	699	728
	dimethylacetamide	0.88	504	575	607	591	682	682	709	740
	DMF	0.87	505	569	610	591	680	679	706	740
	DMSO	1.00	514	583	620	600	687	686	713	743
	nitrobenzene	1.01	520	582	617	599	685	687	712	743

Table 4 .

 4 Summary of the optical properties of PP9-PP15 in solvents of different polarities.

	compounds	π* 1	PP9 2	PP10 2	PP11 2	PP12 2	PP13 2	PP14 2	PP15 2
	heptane	-0.08	511	549	n.s. 3	n.s. 3	481	483	n.s. 3
	cyclohexane	0.00	513	551	n.s. 3	583	485	487	n.s. 3
	triethylamine	0.14	520	562	650	587	490	492	n.s. 3
	diethyl ether	0.27	518	569	n.s. 3	602	495	502	497
	p-xylene	0.43	530	577	662	605	506	507	n.s. 3
	diethyl carbonate	0.45	527	573	648	605	500	504	506
	butanol	0.47	562	598	676	690	515	512	533
	AcOEt	0.54	534	580	665	614	503	507	504
	ethanol	0.54	562	598	676	697	514	517	529
	toluene	0.54	533	578	667	609	508	509	526
	1,4-dioxane	0.55	533	576	n.s. 3	605	504	506	510
	THF	0.58	537	583	676	627	512	512	516
	diglyme	0.64	541	591	682	652	515	520	518
	acetone	0.71	541	588	679	642	509	515	517
	anisole	0.73	546	590	696	653	519	522	535
	acetonitrile	0.75	546	591	681	644	512	515	515
	chloroform	0.78	562	597	697	673	522	526	558
	1,2-dichloroethane	0.81	553	593	694	684	521	523	586
	dichloromethane	0.82	557	595	697	679	520	524	576
	dimethylacetamide	0.88	555	504	696	687	532	532	514
	dimethylformamide	0.87	555	602	696	689	526	528	515
	DMSO	1.00	563	609	692	707	534	536	512
	nitrobenzene	1.01	567	606	722	706	538	541	565

1

Kamlet and Taft parameters 2 Position of the ICT bands are given in nm. n.s.

3

: not soluble.

Table 5 .

 5 Electrochemical characteristics of studied compounds PP1-PP15.

		Ered1 a	Ered1 onset	EOx1 a	Eox1 onset	EHOMO	ELUMO	ΔEET	ΔEopt b	ΔEopt c
		V/Fc	V/Fc	V/Fc	V/Fc	eV	eV	eV	eV	eV
	PP1	-1.48	-1.30	0.40	0.38	-5.18	-3.50	1.68	2.46	2.51
	PP2	-1.30	-1.25	0.40	0.36	-5.20 d	-3.50 d	1.70	2.17	2.23
	PP3	-1.30	-1.25	0.41	0.37	-5.17	-3.55	1.62	2.05	2.08
	PP4	-1.32	-1.27	0.39	0.36	-5.16	-3.53	1.63	2.11	2.14
	PP5	-1.35	-1.20	0.38	0.29	-5.09	-3.60	1.49	1.85	1.87
	PP6	-1.25	-1.10	0.44	0.37	-5.17	-3.70	1.47	1.78	1.86
	PP7	-1.02	-0.92	0.37	-	-5.17 d	-3.78 d	1.39	1.77	1.78
	PP8	-1.24	-1.15	0.40	0.30	-5.10	-3.65	1.45	1.70	1.70
	PP9	-1.30	-1.21	0.43	0.32	-5.12	-3.59	1.53	2.21	2.27
	PP10	-1.25	-1.13	0.46	0.38	-5.18	-3.67	1.51	2.07	2.10
	PP11	-0.89	-0.78	0.46	0.34	-5.26 d	-3.91 d	1.35	1.79	1.82
	PP12	-0.92	-1.05	0.37	0.29	-5.09	-3.88	1.21	1.84	1.92
	PP13	-1.39	-1.13	0.31	0.22	-5.02	-3.67	1.35	2.37	2.42
	PP14	-1.40	-1.24	0.30	0.20	-5.00	-3.73	1.27	2.35	2.41
	PP15	-1.32	-1.17	0.43	0.30	-5.10	-3.63	1.47	2.17

Kamlet and Taft parameters

Position of the ICT bands are given in nm. n.s.

: not soluble.

Acknowledgment

The authors thank Aix Marseille University and The Centre National de la Recherche Scientifique (CNRS) for financial supports. The Agence Nationale de la Recherche (ANR agency) is acknowledged for its financial support through the PhD grant of Corentin Pigot (ANR-17-CE08-0010 DUALITY project).