Polyaniline (PANI) is a candidate for electrocatalysis, and can be combined with metal nanoparticles to fabricate high-performance electrodes for electrochemical energy conversion and storage. However, its intrinsic properties appear to be dependent on the synthesis conditions so that from the majority of the reports, it is quite difficult to establish an overall performance trend. In this contribution, we report an extensive and systematic physicochemical and electrochemical screening of the potentiality of chemically synthesized PANI as an electrode material to provide an overall understanding of the effect of the entire synthesis conditions. We have integrated different methods (TGA-DSC, XRD, SEM, EDX, FTIR, BET, CHNS, XPS, CV, and EIS) to deeply examine the as-synthesized materials and interrogate their electrocatalytic efficiency towards hydrogen evolution reaction, which was chosen as a model reaction of critical importance for H 2 production from water splitting. It was found that all the synthesis parameters affect strongly the physical and electrochemical characteristics of the PANI-based materials. Specifically, XPS analysis contributed to identify the oxidation levels of the PANI samples on the basis of oxidizing agents. The outcomes provided by the study delineate a rational pathway for the further design and fabrication of PANI-based metal nanoparticles as advanced electrode materials.

Insights from the Physicochemical and Electrochemical Screening of the Potentiality of the Chemically Synthesized Polyaniline

Institut Européen des Membranes, IEM -UMR 5635, ENSCM, CNRS, Univ Montpellier, Montpellier, France [c] Institut Charles Gerhardt, UMR-5253, CNRS, ENSCM, Univ Montpellier, Montpellier, France [d] Université de Poitiers, IC2MP UMR-CNRS 7285, 4 rue Michel Brunet, B27 -TSA 51106, 86073 Poitiers Cedex 9, France Introduction Hydrogen (H 2 ) is becoming a cornerstone not only for various industrial processes as a feedstock, but also as an energy carrier involved in the energy conversion and storage scenarios through the electrolysis and fuel cells. [1][2][3] One of the most effective and alternative route for its production with high purity is the water electrolysis. 3,4 Noble metals, and in particular the Pt-Ru-Ir series, have been considered as ideal electrocatalysts for the water electrolysis by reducing the overpotential during the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), but the high cost and low earth abundance of these materials have limited their use in large-scale H 2 production technologies. 3,[5][6][7][8] In such context, it is necessary to synthesize cost-effective materials suitable for HER as an alternative to Pt group metals (PGMs) based electrocatalysts. Among the electrode materials playing a significant role in the performance of energy storage and conversion devices, conducting polymers combined with metallic species are good candidates to elaborate high-performance electrocatalysts due to their interesting structural and physical properties, taking advantage of each component. [9][10][11][12][13][14][15][16] Among those conducting polymers, polyaniline (PANI) [17][18][19] is one of the most investigated material because of its high specific capacitance due to multi-redox reactions, its good electronic properties due to protonation and its high availability. In electrochemistry, PANI can directly act as "electrode" or indirectly as a supporting material. To date, PANI has proven to be an ideal supporting matrix for the immobilization of metal nanoparticles, resulting in electrode materials with superior performance and dispersive ability to prevent agglomeration of metal catalysts. 11,12,[20][21][22][23][24][25][26][27][28] Traditionally, PANI is synthesized by chemical or electrochemical methods, resulting in a powder or a thin film with interesting electron transfer abilities during electrochemical oxidation and reduction reactions in (bio)electrocatalysis. 19,25,26,[29][30][31][32][33][34][35][36][37][38][39][40][41] As sketched in Figure 1, the polymer exists in three main oxidation states, well-identified by methods of cyclic voltammetry, a fully reduced state as leucoemeraldine base (LB), a semi-oxidized state as emeraldine base (EB) or emeraldine salt (ES) with an acid doping agent, and a fully oxidized state as pernigraniline base (PB). [42][43][44] The typical synthetic pathway for obtaining PANI involves the presence of an acid doping agent and an oxidizing agent that enables the polymerization of aniline (ANI, C 6 H 5 NH 2 , consists of a phenyl group attached to an amino group). The oxidizing agents, such as APS, K 2 Cr 2 O 7 , H 2 O 2 , KMnO 4 , KI, 17,[45][46][47] allow the elimination of a proton from the aniline monomer and the initiation of the polymerization through redox processes involving repetition of monomeric units. 48 The amount of the oxidizing agent during the polymerization of PANI influences the amount of radical cations and thus the polymerization yield; however, an excess can lead to excessive oxidation of PANI, while a deficit may be insufficient to obtain the polymer. 49 Thus, the control of the [oxidizing agent]/[ANI] ratio is of critical importance. The insulating form of PANI, namely emeraldine base, consists in two amine nitrogen atoms followed by two imine nitrogen atoms and is a blue color (Figure 1b). The acidic doping agent (normal acid or Lewis acid), such as HClO 4 , FeCl 3 , NH 4 BF 4 , HCl, HNO 3 , H 2 SO 4 , H 3 PO 4 , 20,44 allows the protonation of the imine nitrogen atoms of the emeraldine base to form the emeraldine salt (Figure 1c). It results in the generation of charge-carriers (a doubly charged bipolaron that spontaneously dissociates to give two polarons or radical cations) along the polymer backbone. 44,50 The hopping of the charge carriers through the polaron structure gives rise to a conducting polymer. [10][11][12] In addition to the influence of the amount and nature of the doping agent, the regular distribution of the counter-ions of the acid doping agent along the polymer backbone ensures a high structural regularity of the polymer backbone, and contributes to the increase of the charge carrier mobility. So, the doping process allows PANI to pass from insulator state to a conductive state with a crystalline (ordered) region responsible of its conductivity, dispersed in an amorphous (disordered) region. 29 The chemical synthesis parameters based on the oxidizing agent, acid doping agent and counter anions have a great influence on the oxidation state and the doping level of PANI, resulting in various final morphologies and properties. Extensive literature is available regarding the influence of these reaction parameters on the synthesis of PANI, however, the results are difficult to compare because of different conditions of preparation, and the works dealing with the effect of all the reaction parameters related to the electrochemical characteristics of PANI are rarely available. This work was motivated by the desire to determine the optimal reaction conditions during the chemical oxidative synthesis of PANI in order to develop a noble metal-free PANI matrix with consistent quality and high electrical conduction, and to obtain an effective support of electrocatalysts. Given the high importance of PGMs-free electrocatalysts development for H 2 production, we used HER as the model reaction in acidic media. For that purpose, this work presents a systematic investigation aiming to establish the influence of the synthesis variables such as the doping agent, the oxidant agent, and oxidant/monomer ratio on the morphological, electrical and electrochemical properties of the resulted PANI-based materials. The entire results reveal that all the aforementioned reaction parameters affect strongly at different levels the structural properties of the PANI materials, which in turn affect its efficiency towards HER in acidic media.

Experimental section

Chemicals and materials.

Aniline (ANI, 100%, Prolabo), hydrochloric acid (HCl, 37%, Carlo Erba), sulfuric acid (H 2 SO 4 , 98%, Sigma Aldrich), nitric acid (HNO 3 , Cheminova, 60%) and phosphoric acid (H 3 PO 4 , 50%, Panreac) were used as doping agents. Ammonium persulfate ((NH 4 ) 2 S 2 O 8 , 98%, Biochem), potassium permanganate (KMnO 4 , 99%, Fisher) and potassium dichromate (K 2 Cr 2 O 7 , 99%, Prolabo) were used as oxidizing agents for the chemical aniline polymerization. Isopropanol (99.5%, Sigma Aldrich) and Nafion ® suspension (5 wt%, sigma Aldrich) were used for the catalyst ink preparation. All chemicals were used without further purification, and electrolytic solutions used for polyaniline synthesis or for electrochemical characterizations were prepared freshly. All named ultrapure water (MQ) is provided by Milli-Q Millipore source (18.2 M cm at 20 °C).

Preparation of PANI-based materials

The synthesis conditions are listed in Table S1. Typically, PANI was synthesized by the chemical oxidative polymerization as follows: a defined amount of aniline (ANI) is dissolved in 100 mL of an acid doping agent (HCl, H 2 SO 4 , H 3 PO 4 or HNO 3 ) by stirring at room temperature. A solution consisting of 0.2 M of oxidizing agent ((NH 4 ) 2 S 2 O 8 , KMnO 4 , or K 2 Cr 2 O 7 ) dissolved in another 100 mL of the above acid was added dropwise to the solution of aniline with vigorous stirring for 2 h at 5 °C. After 3 min, the color of the solution changed from colorless to dark green, indicating the beginning of the polymerization. The resulting PANI powder was filtered and washed with ultrapure water until the filtrate was colorless, and dried in an oven at 40 °C for one day.

Methods for physicochemical characterizations of the materials

X-ray diffraction (XRD) patterns were recorded by a DR-X-PRO-MPD-Philips diffractometer with Cu K radiation at the wavelength of 1.5406 Å, the voltage and tube current being 40 kV and 40 mA, respectively. Spectra were collected in a 2θ range from 5 to 80° at a room temperature. FTIR spectra were recorded on a Nicolet iS50 FTIR Spectrometer in the range of 4000-400 cm -1 , with a resolution of 4 cm -1 . The measurements were carried out at room temperature on the basis of 1 mg of PANI and 200 mg of KBr compacted into thin pellets using hydraulic press (5 tons). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with a SDT Q600 TA instruments on crucible of aluminum containing ca. 6 mg of material, in the temperature range of 25 to 500 °C with a heating rate of 10 °C min -1 and under air atmosphere (100 mL min -1 ).The electrical conductivity was measured by four-point method at room temperature on pellets of the PANI (1 mm thickness and 13 mm diameter) using an Keithley Electrometers 2400 (Model LUCAS LABS, Pro 4-8400). The morphology of PANI powder was analyzed by scanning electron microscopy (SEM) on Hitachi S-4800 FEG, and the elemental analysis was obtained by energy dispersive X-ray spectroscopy (EDX) on the microscope ZEISS EVOHD 15, followed by 2D mapping of carbon (C) and nitrogen (N). The surface properties (porosity and the specific surface area (S BET )) were investegated by Brunauer-Emmett-Teller (BET) and distribution BJH measurements. N 2 adsorption-desorption isotherms were recorded at liquid N 2 temperature (77.3 K) using a Micromeritics ASAP 2020 instrument. CHNS analysis was performed on an Elementar vario MICRO cube. X-ray photoelectron spectroscopy (XPS) analyses (the analyzed surface and depth are of 300 × 700 µm 2 and 10 nm, respectively) were performed on a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic radiation source Al Mono (Al k =1486.6 eV) operating at 150 W (15 kV and 10 mA). Data were recorded at a step of 1 eV (transition energy of 160 eV) and 0.1 eV (transition energy of 20 eV) for the survey and high-resolution spectra, respectively. The measured binding energy (BE) was corrected on the basis of the energy of C1s at 284.6 eV by using the CASA XPS software for the peaks fitting.The quantification was carried out from the peak area after correction with a suitable sensitivity factor.

Catalytic ink preparation.

The catalytic ink was prepared by mixing 2 mg of PANI powder, 180 µL of isopropanol and 20 µL of Nafion ® suspension in a water ultrasonic bath for 20 min to form a homogenous ink.

Then, 4 µL of the ink was coated on the surface of well-polished glassy carbon rotating disk electrode (GC-RDE, 5 mm diameter, 0.2 mg cm -2 loading) and dried under N 2 atmosphere at room temperature for 5 min. The GC-RDE was first polished with alumina at 3 µm, 1 µm and 0.05 µm, washed with ultrapure water, and a sonication in pure water for 5 min in order to remove all traces of alumina.

Electrochemical characterization by cyclic voltammetry

The electrochemical characterization was conducted in a conventional three-electrode cell using the SP150 potentiostat (Biologic Science Instruments). Ag/AgCl/KCl (3 M) was used as reference electrode and was separated from the solution by a Haber-Luggin capillary tip. The potentials were calibrated to the reversible hydrogen electrode (RHE) according to the calibration relationship E(V vs RHE) = E(Ag/AgCl) + (0.208 + 0.059×pH). A slab of glassy carbon of about 12 cm 2 was used as a counter electrode. The working electrode consists of a GC-RDE modified with the PANI based catalyst, which was characterized in 0.5 M H 2 SO 4 (pH = 0.3) solution using cyclic voltammetry (CV) between 0.06 and 1.26 V vs RHE at 100 mV s -1 . The electrolyte solution was purged with N 2 over 20 min before measurements. All voltammograms are iR-free, i.e., corrected by the "potential drop" between the working and reference electrodes according to the relationship E real = E applied -R  ×I. R  was obtained by the electrochemical impedance spectroscopy (EIS).

Electrochemical performance.

The electrocatalytic behavior of PANI-based electrodes towards HER was evaluated by linear sweep voltammetry (LSV). Before electrocatalytic measurements, each electrode was firstly cycled by CV for 30 cycles between 0.06 and 1.26 V vs RHE in N 2 -saturated 0.5 M H 2 SO 4 , then LSV was conducted from the open circuit potential (OCP) to -1.024 V vs RHE at a scan rate of 5 mV s -1 and 900 rpm for 7 cycles. The Tafel slope value was calculated from the plot of the potential as a function of the current density in a logarithm-scale. The kinetic activity of the as-prepared electrodes towards HER was also estimated by EIS by scanning the frequencies from 200 kHz to 100 mHz (10 mV amplitude), at E appl = -0.6 V vs RHE.

Accelerated ageing experiments.

Durability tests for the as-modified electrodes were carried out by LSV between OCP and -1.024 V vs RHE at a scan rate of 5 mV s -1 and 900 rpm for 500 cycles.

Results and Discussion

Influence of the doping agent on the physical, morphological and electrochemical properties

Effect of the doping agent concentration

The proton-based acid doping of the emeraldine base segment results in the protonation of the imine nitrogen atom in the backbone chain and gives emeraldine salt form with positive (proton) and negative (anion) charges. Thus, we firstly hypothesized that the doping agent concentration should affect the electrical conductivity of the conducting polymer. We especially aimed to study the polymerization process when using HCl as the proton-based acid doping agent, as a starting point for the preparation of PANI. To this end, Cl -was chosen as the simplest and smallest counter-ion, in comparison to nitrate, sulfate and phosphate ones.

Hence, the effects were likely to be easily accessed. To study the effect of the acid doping agent concentration on the polymer's properties, different PANI materials have been synthesized by the chemical oxidative polymerization in different concentration of HCl from 0.30 to 1.00 M, referred to as PANI-HCl0.30, PANI-HCl0.5, PANI-HCl0.75, and PANI-HCl1.00. We have chosen APS as the oxidizing agent with the molar ratio R = [APS]/[ANI] = 2 for [ANI] = 0.1 M as the best compromise. To evaluate the influence of those HCl conditions on the crystallinity and subsequently the conductivity and catalytic properties, we firstly characterize the homemade PANI-based materials by XRD. Figure 2a shows that the as-synthesized PANI-HCl materials exhibit 4 peaks at 2θ values of 8.88, 15.27, 20.80 and 25.54°, corresponding to (001), (011), (100) and (110) crystal planes, respectively, thus confirming the successful synthesis of a crystalline form of PANI, in agreement with the litterature. 51,52 Accordingly, the first peak at 8.88° is assigned to the d-spacing between the N atoms of adjacent main chain and the doping agent, the peaks at 15.27° and 20.80° are attributed to the parallel periodicity to the chains, whereas the peak at 25.50°, more intense, is ascribed to higher presence of PANI chains with perpendicular periodicity. [53][54][55] PANI in the un-doped state (polyaniline emeraldine base) is amorphous in nature, 56 and its crystallinity increases with increasing concentrations of the doping agent due to the incorporation of counter-ion (herein Cl -) and hydrostatic forces with protons. 57 This behavior is evidenced by the presence of a wider peak around 20.80° for the sample PANI-HCl0.30, thus reflecting a more amorphous structure. Moreover, the peak at 8.88°, mainly observed for the PANI-HCl0.50 and PANI-HCl0.75 samples, comes from a shorter distance between the counteranion and the polymer chains, reflecting a higher order of PANI chains. 58 Besides, the values of β (full width at half-maximum), L V (crystalline size) and d-spacing, estimated using Scherer and Bragg formula Eqs. 1 and 2, respectively 55,59 (Table S2), confirm the better mobility of charge carriers along the polymer main chain in the sample PANI-HCl0.50, meaning that it should have a higher electrical conductivity. 60



   sin 2  hkl d [1]    cos k L V  [2]
Where (= 1.54060 Å) is the X-ray wavelength,  is the Bragg's angle, (in radian) is the full width at half-maximum (FWHM) of the peak, k is a constant (≈ 0.9), L V is the average crystallite size, d(hkl) is the inter-planar spacing of the crystallographic plane (hkl).

We next used FTIR spectroscopy to perform a deep characterization of the polymer materials. Figure 2b shows that increasing the HCl concentration results in spectra with a lower bands height in the region 2000-500 cm -1 , and broader bands in the region 3000-3800 cm -1 . The main peaks position of are resumed in Table S3. ), and 826 cm -1 (out-of-plane deformation of the bonds C-H of aromatic cycles). The shift of the bands corresponding to the quinoid and benzenoid rings to a lower wavenumber for the sample PANI-HCl0.50 confirms the higher degree of protonation of the material. 50 The profiles of TGA and DSC of PANI-HCl are reported in Figure 2c; the main characteristics are reported in Table S4. The upper temperature of 500 °C was fixed to avoid the combustion of carbon. The TGA curves reveal a common feature with a three-step mass loss for all samples.

The first weight loss at about 120 °C is associated with the evaporation of water molecules that are adsorbed and trapped between the hydroscopic polymer chains and requiring a much higher temperature to remove them. 64,65 The peaks position is 142; 143, 143 and 147 °C for PANI-HCl0.30, PANI-HCl0.50, PANI-HCl0.75 and PANI-HCl1.0, respectively. For PANI-HCl1.0, the phenomenon occurs at much higher temperatures presumably because of the agglomerated morphology nature of the particles constituting the materials, whereby the removal of those solvent molecules embedded in the PANI's network is much hard. This will be fixed by the SEM observations. The second weight loss between 200 °C and 350 °C is ascribed to the elimination of the residual doping agent and the decomposition of smaller polyaniline fragments, 66 and the third loss above 330 °C is attributed to the degradation of the main PANI chains. 67 From DSC curves in Figure 2c, it can be observed that the evolution of PANI-HCl samples is similar and display four peaks corresponding to the endothermic process that needs energy in each decomposition stage (values reported in Table S4). The endothermic peaks between 130-150 °C and 160-200 °C correspond to the structural rearrangement and moisture desorption. The third endothermic peak between 230 °C and 320

°C can be associated to a mix phenomenon of "de-doping process", exclusion of residual anions (Cl -) from the polyaniline structure and the polymer reticulation, while the change at 422 °C is attributed to the structural degradation of the main PANI backbone. 68 The polymer reticulation is basically an exothermic process; however, as in the present case many phenomena occur simultaneously, the reticulation would be hidden. Indeed, the prevalence of the entire endothermic parts may imply that the reticulation of the polymer does not occur.

The forth physical method used herein to investigate those PAN-based carbon materials was the N 2 adsorption-desorption. This technique allows the assessment of the textural properties of porous materials by using N 2 as a probing molecule. Figure 2d depicts the N 2 adsorption-desorption isotherms (which belongs to the type IV) of representative PANI-HCl samples obtained at [HCl] = 0.50, 0.75 and 1.00 M. The hysterisis loop at a higher relative pressure that is characteristic of capillary condensation indicates the presence of mesopores. Quantitative data are reported in Table 1 and Table S4. The evaluated BET surface was 23, 25 and 6 m 2 g -1 for PANI-HCl0.50, PANI-HCl-0.75 and PANI-HCl1.00, respectively. From the pore size distribution measured by the BJH method (Figures S1), the polymers exhibit a mesopourous structure. 69,70 The pore volume is 0.13, 0.16 and 0.06 cm 3 g -1

for PANI-HCl0.50, PANI-HCl-0.75 and PANI-HCl1.00, respectively. The lowest specific surface area and pore volume values for PANI-HCl1.00 reinforces the presumption from the previous TGA results, which were suggesting a possible presence of agglomerated particles that reduce the available exposed surface. The strong impact of HCl concentration on the electrical conductivity (σ) of the PANI-HCl samples can be observed in Figure 2d (inset), and gathered in Table 1. σ increases from 3.7×10 -4 to 0.25 S cm -1 for PANI-HCl0.30 to PANI-HCl0.50. This can be explained by the higher concentration of remaining protons capable of binding to N atoms of the imine units, thus promoting the formation of polarons/bipolarons between the polymer chains that are the charge carriers responsible for electrical conductivity of PANI. 71,72 For the concentration above 0.5 M HCl, the slight decrease in σ can be explained by a possible agglomerated morphology of the particles induced by the above excess protons binding to nitrogen atoms. From these physical characterizations of the HCldepend properties of PANI, it can be concluded that the doping process allows the polymer to The above studies show only the optimal conditions when these PANI-based materials are intended to be used as "raw material". To determine whether the conclusion could hold or not for the application in electrochemistry in aqueous solutions, we further electrochemically characterized them in H 2 SO 4 . Figure 2e shows the iR-free CV curves (2 nd and 30 th cycles)

over the potential window of -0.06 to 1.26 V vs RHE. Compared to the blank glassy carbon electrode, the CV profiles of the PANI-based materials exhibit two characteristic redox processes, which correspond to the well-described oxidation states of PANI. 67,70,73 The PANI-HCl1.00 exhibits the lowest current density response, which could be substantiated by the above BET measurements. The pair of redox peaks located at 0.56/0.37 V vs RHE is ascribed to the electrochemical transition from the fully reduced form of PANI (leucoemeraldine) to the partially oxidized state (emeraldine salt), whereas the second pair of redox peak at 0.94/0.77 V vs RHE is attributed to the transition between the emeraldine salt and the fully oxidized form of PANI (pernigraniline). 73 After 30 cycles, the CV profiles present a steadystate response with only one redox system with oxidation/reduction peaks located at around 0.67 and 0.52 V vs RHE, respectively. It is argued that this behavior corresponds to an irreversible morphological transformation in the polymer associated with the reorganization of the polymer chains and mobility of the anions among the cycles. 44,74 The electrocatalytic activity of the PANI-based materials for HER was evaluated by LSV and compared with pure GC-RDE support. It should be pointed out that the HER's mechanism on metal surfaces in acidic medium has been extensively studied, 75,76 and consists in three steps described by Eqs. 3-5. The Tafel slope value gives important information on the rate determining step (rds) in an electrochemical reaction. Indeed, the dominant reaction step in the HER process can be theoretically indicated by a Tafel slope. The latter can be obtained from experimental data of LSV or EIS and is broadly interpreted as the relation between the current density and the overpotential (dependence of steady-state current densities on a variety of overpotentials).

Volmer:

H + (aq) + e -→ H (ads) F RT b V  3 . 2  b V = 118.2 mV dec -1 at 25 °C [3]
Heyrovsky: H (ads) + H + (aq) + e -→ H 2(g)

 F RT b V    1 3 .
2 b H = 39.4 mV dec -1 at 25 °C [4] Tafel:

H (ads) + H + (aq) → H 2(g) F RT b T 2 3 . 2 
b H = 29.5 mV dec -1 at 25 °C [5] Where b V , b H and b T are the Tafel slopes (mV dec -1 ),  is the symmetry coefficient (typically,  = 0.5), F is the Faraday constant (96 485 C mol -1 ), R is the ideal gas constant (8.314 J K -1 mol -1 ), and T is the absolute temperature (273.15 + °C).

Figure 2f shows that pristine GCE exhibits a very limited catalytic activity, while the PANI-HCl0.30 exhibits a catalytic activity closed to the PANI-HCl0.50, despite its low electrical conductivity, probably due to the protonation of the nitrogen atoms of amines units of polyaniline that occurs in the strong acid H 2 SO 4 . The overpotential required for driving a cathodic current density of 10 mA cm -2 for PANI-HCl0.30, PANI-HCl0.50, PANI-HCl0.75

and PANI-HCl1.0 is 0.74, 0.77, 0.77 and 0.85 V, respectively. This confirms the low performance of the sample PANI-HCl1.00. These catalytic activities are also assessed by the charge transfer resistance (R ct ) values obtained from Nyquist plots in Figure 2g and the Tafel slopes in Figure 2h. The modelling of EIS data by a representative equivalent electrical circuit (EEC) embedded in Figure 2g, i.e., R Ω +Q CPE //R ct enables the determination of R ct and Q CPE (constant phase element). [START_REF] Lasia | Electrochemical Impedance Spectroscopy and its Applications[END_REF][START_REF] Orazem | Electrochemical Impedance Spectroscopy[END_REF] The lack of homogeneity and mass transport is modelled with a Q CPE while the metric of R ct (inversely proportional to rate constant k°, thus to the exchange current density j°) is the ability of the electron-transfer electrode reaction to drive a large current density with a less driving force (overpotential). [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] For a given electrode, the lower R ct is, the higher is the kinetic rate.

The results gathered in Table 1 show that R ct = 10, 20, 26 and 31  cm 2 for [HCl] = 0.30, 0.50, 0.75 and 1.00, respectively. So, even though the sample PAN-HCl0.30 has a reduced "raw electrical conductivity" about two order of magnitude lower, the use of 0.5 M H 2 SO 4 as the electrolyte enables to compensate the ionic conductivity.

Furthermore, the determined mechanism descriptor of Tafel slope 80 is b = 120-150 mV dec -1 . This suggests that the rds of is the Volmer reaction (for a symmetry coefficient of  = 0.49-0.39) characterized by the limitation by the protons adsorption. On the basis of both metrics of the charge transfer resistance and the Tafel slope, the PANI-HCl1.0 has the lowest performance that may be supported by the previous TGA-DSC and BET results.

The above studies suggest that there may be a drastic morphological difference between the PANI materials obtained with [HCl] ≤ 0.50 M and [HCl] ≥ 1.00 M. Hence to gain further insights into the morphology, SEM analysis was undertaken for representative samples synthesized with [HCl] = 0.30, 0.50 and 1.00. Figure 3 shows that the PANI powders present an irregular granular morphology. The increase in HCl concentration to 1.00 M during the chemical polymerization results in an increase in the particle size, leading to agglomeration that result in lower BET surface and electrocatalytic performance.

Effect of the ratio Oxidant/ANI

Having demonstrated the ability to use HCl to grow self-supported PANI particles, we sought to study the impact of APS more carefully. Indeed, given the important role of the oxidizing agent on the polymerization yield of PANI, its impact has been studied to modulate the catalytic efficiency. To these ends, the synthesis conditions consist in HCl as the doping agent at 0.5 M, APS as the oxidizing agent at [APS] = 0.2 M. Different PANI-based materials were synthesized using variable quantities of ANI through the ratio R = [APS]/[ANI] = 0.4-2.0 and referred to as PANI-R0.40, PANI-R0.5, PANI-R0.67, PANI-R1.00 and PANI-R2.00. The same set of methods was used to characterize the materials, XRD (Figure 4a), FTIR (Figure 4b), TGA-DSC (Figure 4c), N 2 adsorption-desorption isotherms and electrical conductivity (Figure 4d and Table 1). The profiles are similar to those of Figures 2a-d oxidation that produces shorter conjugated PANI chains. 49,[START_REF] Hammed | [END_REF] These results can be confirmed by XRD data (Figure 4a and Table S2) that reveal the higher crystallinity degree of the polymer PANI-HCl-R0.5. FTIR spectra (Figure 4b) do not show any significant structure change with the variation of the ratio Oxidant/ANI. Contrary to what has been observed by several authors, 17,44 in our case, the evaluation of "the oxidation degree augmentation" of PANI for increase amount of APS could not be highlighted from the area ratio of the characteristic quinoid and benzenoid peaks. The N 2 adsorption-desorption isotherms and BET analysis (Figure 4d, Table S4) indicate that S BET = 21, 26 and 23 m 2 g -1 for PANI-R0.50, PANI-R1.00 and PANI-R2.00, respectively. The pore volume is ca. 0.13 cm 3 g -1 for the three samples, meaning the same trends. At this stage of the study concerning the effect of the

Effect of the nature of the doping agent

The electrical conductivity of conducting polymers depends also on the charge carriers' mobility, which depends on the size of the counter-ion of the doping agent attached to polymer chain and the type of the formed polarons. Typically, a proton-based acid doping agent dissociates in water to generate protons and anions that could be inserted into the polymer matrix during the polymerization reaction and the positive charge is neutralized. 82 The acid dissociation constant and the counter-ion size may results in a difference in the protonation degree. During the synthesis, the anions having the smallest size easily diffused in the polymer matrix, thereby resulting in a higher doping level and high efficiency of the mobility process, which increase the conductivity. 83,84 In this context, the influence of the -d, while the results from the electrochemistry are displayed in Figures 6e-h. Among the different acids, PANI-HCl exhibits by far the highest electrical conductivity (Figure 6d, inset). This property was confirmed by the data deduced from XRD (Figure 6a) that reveal the higher crystallinity degree and lower d-spacing for PANI-HCl (Table S2). The FTIR spectra analysis (Figure 6b and Table S3) supports the higher degree of protonation for PAN-HCl with a significant shift of the bands corresponding to the quinoid and benzenoid rings to lower wavenumbers. 50 No observable changes were observed for the N-H vibration modes with respect to hydrogen bonding. The thermal behavior of polyaniline shows a significant desorbed amount of the moisture for PANI-HCl about 17% and an improved thermal stability for PANI-HNO 3 with a total weight loss of 33%. For PANI-H 2 SO 4 and PANI-H 3 PO 4 , a steeper weight loss is observed, suggesting a lower thermal stability of these polymers. Data collected from BET measurements by N 2 adsorption and desorption, and BJH methods (Figure 6d, Table S4, Figure S1) confirmed the mesoporous structure for all materials, and showed that PANI-HCl has the highest specific surface area of 21 m 2 g -1 , while PANI-H 2 SO 4 has the lowest value of 16 m 2 g -1 .

Figure 6e shows the voltammograms of those PANI-based materials, which are similar for the entire samples and depicts the two characteristic pairs of redox peaks, corresponding to the oxidation and reduction process when scanning towards the anodic and the cathodic potential, respectively. It can be well observed that PANI-HCl and PANI-H 3 PO 4 display higher current densities compared to PANI-H 2 SO 4 and PANI-HNO 3 . From the HER's LSV polarization curves (Figure 6f) and Nyquist impedance plots at E appl = -0.6 V vs RHE (Figure 6g), PANI-HCl exhibits the best activity towards HER with the lowest R ct of 15 Ω cm 2 , whereas PANI-H 2 SO 4 shows the lowest performance with R ct of 41 Ω cm 2 . This time we can deduce that there is little variation in the slope of Tafel (Figure 6h), b = 122-130 mV dec -1 that denotes that the rate limiting step is still the Volmer reaction of protons. The SEM images (Figure 7) show a granular morphology contacted with some nanofibers domains for PANI-HCl, and more compacted and plate shape morphologies for the samples PANI-H 2 SO 4 , PANI- (2.1×10 -3 S cm -1 ) and PANI-K 2 Cr 2 O 7 (2.6×10 -5 S cm -1 ), compared to PANI-APS (Figure 8d, inset). This behavior may be explained by the low protonation level during the oxidation process in the presence of K 8b, the increase of the oxidation degree of PANI with the different oxidant was not evaluated from the area ratio of the characteristic quinoid and benzenoid peaks, nevertheless, PANI-APS shows the higher degree of protonation with a significant shift of the main characteristic bands to lower wavenumbers. 50 From the thermal behavior of the three samples in Figure 8c, the weight loss process in the case of PANI-APS and PANI-KMnO 4 is characterized by three stages attributed to different structural changes in the polyaniline structure, as discussed above, while PANI-K 2 Cr 2 O 7 displays only two weight loss stages. The second step corresponding to the removal of doping agent has disappeared. The sample shows only 12%

weight loss between 120-275 °C (Table S4), because of the low protonation degree, in addition, the faster weight loss and the absence of thermal stability zone were observed. The N 2 adsorption-desorption isotherms and BET analysis (Figure 8d, Table 1, Table S4) indicate that S BET = 21, 11 and 2 m 2 g -1 for PANI-APS, PANI-K 2 Cr 2 O 7 and PANI-KMnO 4 , respectively. The pore volume is 0.13, 0.04 and 0.01 cm 3 g -1 for the same trend, meaning a significantly increased "exposed surface area" for the PANI obtained under the synthesis condition with APS as the oxidizing agent. At this stage of the study concerning the type of oxidizing agent to be used in order to achieve the highest electrical conductivity and most importantly the surface area; APS should be used as the oxidizing agent at 0.2 M.

We next combined CV and LSV of HER to establish the voltammetry profiles and the electrocatalytic performance of the different samples derived from the use of the three types of oxidizing agents. From Figures 8e-h, the influence of the oxidant is clearly obvious. While PANI-APS presents the two typical redox transitions of leucoemeraldine/emeraldine-base and emeraldine-salt/pernigraniline with higher current densities, both PANI-K 2 Cr 2 O 7 and PANI-KMnO 4 display only one pairs of redox peaks at 0.78/0.41, 0.78/0.48 V vs RHE, with a potential difference (E) of 0.37 and 0.30 V, respectively. Those large peak-to-peak differences can be interpreted as an increase of the irreversibility. The Figures 8e-f show that PANI-APS has the best activity towards HER (overpotential of 0.69 V at 10 mA cm -2 ) with the lowest R ct of 15 Ω cm 2 , while PANI-K 2 Cr 2 O 7 presents a very weak catalytic activity with Tafel slope value of 178 mV dec -1 . The combination of the SEM observations (Figure 9) and the measured BET surfaces area (Table 1) demonstrate that the oxidant agent has a strong impact on the structural morphology of the PANI. PANI-APS has a granular morphology with an open structure (S BET = 21 m 2 g -1 ), while the PANI-K 2 Cr 2 O 7 exhibits a spherical morphology with irregular size distribution (S BET = 11 m 2 g -1 ), and PANI-KMnO 4 is characterized by a dense morphology with a very low porosity (S BET = 2 m 2 g -1 ). Based on these results, we have considered APS as the best oxidizing agent for the synthesis of efficient PANI-based materials as "a potential catalyst" or "a supporting material of metallic nanoparticles" to be used in electrochemical applications. Conclusively, all those previous results show the ability to manipulate the synthesis conditions towards different polyaniline materials with different morphologies, electrical conductivity, electrochemical behaviors and electrocatalytic performance.

Compositional and surface screening

Having performed a thorough and step-by-step study of the synthesis conditions of PANI, we sought to study the materials composition more carefully. We specifically aimed to determine whether the theoretical chemical composition of the polyaniline is obtained or not and especially, the proportion of the different forms of PANI. For the atomic composition, we firstly utilized the EDX analysis. Figures 10a-h of representative materials qualitatively confirm the presence of C and N. The 2D maps confirm the successful synthesis of N-doped carbon materials with a uniform dispersion of carbon and nitrogen for each sample. As indicated in Figure 1, the theoretical atomic ratio C/N is 6. Quantitative EDX analysis in Figure 10i shows that an experimental proportion of 1 atom of N for 6 atoms of C is maintained in the entire PANI-based materials obtained from different conditions of [HCl] and [oxidizing agent]/[ANI]. Furthermore, EDX analysis of PANI-APS, PANI-K 2 Cr 2 O 7 and PANI-KMnO 4 samples showed the presence of S, Cr, and Mn species (not shown herein), which could also explain the observed morphology of the materials and their electrical and electrocatalytic properties. It is reported that sulfur improves the electrocatalytic activity of the HER. 85 On the contrary, Cr and Mn, whose oxide state within the current PANI is indisputable, exclude any catalytic effect. A further calcination step could enable obtaining a network of C, N and Cr/Mn that is more catalytically active, which is beyond the scope of the present study.

To deepen our knowledge about two of the key experimental parameters affecting drastically the electrocatalytic properties of PANI, we next carried out XPS analysis. Figure S2 shows the indexed survey XPS of the as-synthesized PANI-HCl0.5-R0.5-APS and PANI-HCl0.5-R1.0-APS to investigate the effect of [oxidizing agent]/[aniline] ratio, and of the PANI-HCl0.5-R0.5-K 2 Cr 2 O 7 to study the effect of the oxidizing agent. High-resolution C1s and N1s spectra are shown in Figure 11 and the extracted overall atomic composition is reported in Table S5. The presence of chlorine (0.5 at%) in both PANI materials synthesized with APS and its absence in the sample with K 2 Cr 2 O 7 will be explained below after the examination of the N1s level. On the upside, the presence of S (1.6 at%) for APS-based materials and chromium (2.7 at%) for the K 2 Cr 2 O 7 -based material was expected. The C/N atomic ratio of 6.4-7.0 obtained by XPS, which analyzes only a nanoscale domain (~10 nm) of the sample, confirms the previous EDX results and definitely supports the successful retention of the aniline-based blocks (6 atoms of C for 1 atom of N). Figures 11a-c display the different components of high-resolution XPS of C1s while quantitative atomic composition is reported in Table S6. The peak centered at ca. 284.6 eV belongs to the graphitic carbon (sp 2hybridized carbon atoms, C=C) 86,87 while that at ~285.6 eV can be associated with the formation of C-N chemical bonds as well as C-O function. [87][88][89] The peaks situated at 286.5

and 288.1 eV can be assigned to the carbonyl (C=O) and carboxylate (O-C=O) functions, respectively. For graphene-based materials, it is well-known that the peak at 290-291 eV belongs to the π → π* shake-up satellite peak. 87,90 However, looking closely at the possible structures of the PANI-based materials (Figure 1), this π → π* shake-up satellite can be split up to two peaks depending on the oxidation state of the PANI, herein at 289.6 and 292.0 eV for PANI-HCl0.5-R0.5-APS or PANI-HCl0.5-R0.5-APS (in agreement with the litterature 89 ).

A single peak is observed for PANI-HCl0.5-R0. eV. 89,[91][92][93][94][95][96][97] Additionally, the peak at 406. 1-406.6 eV is assigned to the π → π* shake-up satellite (N5) of the doublet presents on some nitrogen. 98,99 The atomic distribution of those nitrogen atoms into N1-N5 species is reported in Figure 12a and Table S7. By calculating the atomic ratio (N1+N3+N4)/(N1+N2+N3+N4), it is possible to further determine the oxidation state of the PANI. [95][96][97]100 More precisely, a value of 0.5 indicates an intermediate oxidation stage of nitrogen in the PANI (emeraldine form), a value closed to 1 signifies a higher concentration of quinoid groups and therefore a highly oxidized form (pernigraniline), and a ratio below 0.5 denotes the predominance of a highly reduced PANI (leucoemeraldine). As it can be also observed in Figure 12a, the evaluated ratio is 0.48, 0.41 and 0.66 for PANI-HCl0.5-R0.5-APS, PANI-HCl0.5-R1.0-APS, and PANI-HCl0.5-R0.5-K 2 Cr 2 O 7 , respectively.

In other words, the PANI obtained by using APS as an oxidizing agent is in the form of emeraldine while a pernigraniline form results from the use of K 2 Cr 2 O 7 . Interestingly, the polaron/bipolaron amount is 43, 38, and 23 at% for PANI-HCl0.5-R0.5-APS, PANI-HCl0.5-R1.0-APS, and PANI-HCl0.5-R0.5-K 2 Cr 2 O 7 , respectively. In other words, the high amount of the polaron species is accompanied by the presence of choline. It is well-known that these species are responsible of the electrical conductivity () of the polyaniline. [10][11][12] Thus, it is not surprising that from Figures 4d and8d, the values are  = 1.39, 0.64 and 2.6×10 -5 S cm -1 for PANI-HCl0.5-R0.5-APS, PANI-HCl0.5-R1.0-APS, and PANI-HCl0.5-R0.5-K 2 Cr 2 O 7 , respectively. Conclusively, this XPS study validates the previous physical and electrochemical data and supports the critical importance of the synthesis inputs.

The utilization of both EDX and XPS techniques does not allow accessing the amount of hydrogen atoms. Even if this was the case, a real composition would not be possible because those methods do not perform the entire bulk analysis quantitatively. So, we alternatively utilized the "CHNS analysis" to determine the real bulk composition of C, N and H (Table S8). The results in terms of C/N and C/H atomic ratios are reported in Figure 12b. It also reveals that the C/N ratio is about 6, while the C/H ratio is ca. 0.9, indicating the presence of a large amount of N and H atoms. However, the inevitable presence of water molecules adsorbed and trapped between polyaniline chains results in an overestimation of the H atoms in the polyaniline materials since the theoretical ratio was expected to be between 1.20 and 1.50 depending on the oxidation state of the polyaniline (Figure 1 and Table S7).

Accelerated ageing effect on the electrochemical properties

After the previous set of characterization, we finally sought to gain deeper insights about the behavior of the PANI upon the potential cycling. The electrochemical accelerated ageing procedures by potential/current cycling or by fixing potential/current enable to anticipate the behavior of a material during real use. Given the electrochemical behavior of polyaniline as previously noted by voltammetry, we used LSV to interrogate the durability of two representative PANI samples. Experiments were performed with PANI-R0.50 and PANI-R1.00 by scanning the electrode potential between OCP and -1.024 V vs RHE at a scan rate of 5 mV s -1 and 900 rpm for 500 cycles and by recording the potential of OCP 30 s between two cycles to reach a relative stabilized electrode after the removal of the H 2 bubbles from the electrode surface. The obtained results are reported in Figures 13a-b. It should perhaps be reiterated that the previous LSV of HER are those obtained for the 7 th LSV, in order to save time. The evolution of the overpotential required to reach the metric current density of 10 mA cm -2 (Figure 13a) is in agreement with the previous behavior of the CVs. suggesting stable HER activity in acid media. After 300 cycles, the overpotential is about 500 mV for the electrode PAN-R0.5, i.e., 150 mV lower than the electrode PANI-R1. This is in agreement with the previous results. It should be noted that the blank experiment of a glassy carbon electrode does not show any significant change of the activity. So, the observed positive change could be assigned to a structural change at the polymer itself. Further deeper postmortem scrutinizing could provide insights about the trend. In the literature, one of the methods identified for boosting the electrocatalytic activity of polyaniline towards the HER in an aqueous medium is the introduction of species such as Co and Ni, making it possible to reach a current density of 10 mA cm -2 with overpotential of 100-300 mV. 20,24,25 For instance, Feng et al. 25 have demonstrated that the multi-composition PANI/CoP material increases the hydrogen evolution electrocatalysis in 0.5 M H 2 SO 4 , whereby the PANI can effectively capture H + from hydronium ions to form protonated amine groups, having a higher positive charge density than those of hydronium ions and can be electro-reduced easily. Even though the concept is beyond the scope of the present work, the introduction of metals followed by the materials calcination should results in electrocatalysts with high performance. In this case, the knowledge provided by the present study on the oxidation state of polyaniline would be crucial. Indeed, the transformation at high temperatures of the latter into a nanostructured carbon network containing or not nitrogen depends strongly on how those nitrogen atoms were bound to the carbon as well as the type of carbon-carbon bounds in the parent structure.

Conclusion

The present work was aimed to provide a complete understanding of the effect of the synthesis conditions on the physical and electrochemical properties of the polyaniline (PANI) obtained from the conventional chemical polymerization. Different physicochemical (TGA-DSC, FTIR, XRD, SEM, EDX, CHNS, BET, and XPS) and electrochemical (voltammetry and impedance spectrometry) methods have been integrated to characterize the as-synthesized polyaniline materials and to interrogate the property-to-performance relationship. The electrocatalytic hydrogen evolution reaction (HER) in acidic media has been chosen as a model reaction. Indeed, the electrochemical production of H 2 , an essential feedstock for industrial processes and energy carrier for fuel cells, appears to be one of the sustainable power-to-X scenarios to significantly reduce the dependence on conventional energy sources based on fossil fuels. Our outcomes show that the morphology, the conductivity, and the electrochemical performance of the polyaniline are mainly determined by the synthesis conditions. Specifically, the proton-based acid doping agent and the oxidant affect the degree of crystallinity, the inter-chain separation, the oxidation state, the degree of doping and further the density of charge carriers and their mobility along the polymer backbone, which in turn affects the electrical properties of PANI. The XPS results show that the use of APS as an oxidizing agent leads to the formation of the emeraldine (the best [oxidizing agent]/[aniline] ratio being 0.5), while a pernigraniline form arises from the use of K 2 Cr 2 O 7 . After 500 cycles of the electrode potential cycling, the synthesized PANI using the ammonium persulfate (APS) as the oxidizing agent and HCl as the doping acid has a stable electrode potential of -0.5 V vs RHE at the current density of -10 mA cm -2 for a Tafel slope of ca. 120 mV dec -1 . The present study contributes to advances in the use of polyaniline, as an emerging conductive polymer for the design of cost-effective electrode materials for electrochemical energy conversion and storage applications. Specifically, it paves pathways for the development of efficient nanocatalysts for the electrocatalysis, whereby noble metal-free elements could be embedded in a matrix of an optimized conducting polymer to yield nitrogen and metals doped carbon nanostructured networks.

  Djara, a,b Yaovi Holade, b,z,* Abdelhafid Merzouki, a Nathalie Masquelez, b Didier Cot, b Bertrand Rebiere, c Eddy Petit, b Patrice Huguet, b Christine Canaff, d Sophie Morisset, d Teko W. Napporn, d,* David Cornu, b and Sophie Tingry b,z [a] Laboratoire de Physico-Chimie des Hauts Polymères (LPCHP), Université Ferhat Abbas, Sétif 1, Algeria [b]

  go from an insulating state to a semiconductor or conductive state. Conclusively, the best compromise for the starting points of [ANI] = 0.1 M and [APS]/[ANI] = 2 should be the use of HCl as the proton-based acid doping agent at the concentration of 0.5 M. This means that the molar ratio of HCl to ANI in the reactor should be 10:1.

5 and 10 .

 510 Since ANI was fixed to 0.1 M, the above concentrations correspond to a molar ratio [HCl]/[ANI] = 3, It can be concluded that the presence of HCl is necessary to trigger the chemical polymerization process. But, too high concentration of protons relative to ANI (about 10) results in their excessive binding to N of the imine units. This promotes the formation of polarons/bipolarons between the polymer chains, which obviously leads to the formation of larger particles and reduced porosity. Based on all these physico-chemical and electrochemical screenings, the experimental conditions [HCl] = 0.5 M has been chosen as the best compromise for further investigations.

  . The electrical conductivity (inset in Figure4d) measurements show that the materials obtained for a molar ratio [APS]/[ANI] < 0.5, for instance the PANI-HCl-R0.4, have low conductivity. This is probably because of the low polymerization yield due to the deficiency of APS for oxidizing all the aniline monomer, which mainly results in the formation of shorter conjugation length of the polymer. The optimal conductivity of 1.39 S cm -1 is reached for the optimal ratio [APS]/[ANI] = 0.5. However, unlike the previous study where a plateau had been reached for larger [HCl]/[ANI] molar ratio, too much excess of APS induces an obvious deterioration of the electrical conductivity (0.25 S cm -1 for [APS]/[ANI] = 2) due to the degradation by over-

[

  APS]/[ANI] molar ratio, the best experimental conditions should be [HCl] = 0.5 M, [APS] = 0.2 M, and [APS]/[ANI] = 0.5 through [ANI] = 0.4 M.Next, CV and LSV studies were carried out to probe the electrochemical and electrocatalytic properties of the samples. It should be mentioned that the optimized sample from the previous section is PANI-R2.00. Figure4ashows that the highest current density for the redox peaks is reached for the sample PANI-R0.50. This behavior is substantiated by the performance towards HER (Figures4f-g). PANI-HCl-R0.5 displays the best activity in terms of the overpotential at 10 mA cm -2 , decreasing from 0.77 to 0.69 V. The determined charge transfer resistance is R ct = 15, 53, 42 and 20 Ω cm 2 for PANI-R0.50, PANI-R0.67, PANI-R1.00 and PANI-R2.00, respectively. This results highlight the fast electron transfer ability of the sample obtained with a [APS]/[ANI] molar ratio of 0.5. As previously, the evaluated Tafel slope (Figure4h) of b = 110-131 mV dec -1 indicates that the rds is the Volmer reaction of the protons adsorption at the electrocatalytic surface. The SEM observations in Figure5for the materials obtained by [APS]/[ANI] = 0.5, 1 and 2 indicate that the aforementioned parameter affects marginally the morphology of the resulting polymers. This is consistent with the N 2 adsorption-desorption isotherms, in which the surface and pore volume are substantially the same. From the above investigations, we came with the conclusion that the optimal conditions should be [HCl] = 0.5 M, [APS] = 0.2 M, and [APS]/[ANI] = 0.5, i.e., [ANI] = 0.4 M and they have been chosen as the best compromise for the next step.

  nature of the proton-based acid doping agent on the physicochemical and electrochemical properties of polyaniline was carefully studied during the synthesis of the polyaniline in the presence of the strong acids, H 2 SO 4 , H 3 PO 4 , HNO 3 and HCl, commonly used for in the synthesis and doping of PANI. Based on the previous conclusions, we have synthesized PANI-based materials using APS as the oxidizing agent at [APS] = 0.2 M, R = [APS]/[ANI] = 0.5 (i.e. ANI at 0.4 M) and different doping agents at a concentration of 0.5 M. The obtained materials are referred to as PANI-HCl, PANI-HNO 3 , PANI-H 2 SO 4 and PANI-H 3 PO 4 . The physicochemical characterizations are reported in Figures 6a

H 3

 3 Figures 8a-d and electrochemical ones are displayed in Figures 8c-h. The crystallinity of the polymers has been examined by XRD and reported in Figure 8a. While PANI-APS is characterized by a partially crystalline structure with the typical peaks at 8.88°, 15.54°, 20.82° and 2θ = 25.50°, PANI-KMnO 4 and PANI-K 2 Cr 2 O 7 in contrast show broad Bragg diffraction peaks centered at 24.15° and 20.78°, respectively, which is characteristic of an amorphous structure. This result confirms the observed extremely low conductivity of PANI-KMnO 4

5 -K 2

 52 Cr 2 O 7 . This is in line with the chlorine result and can indicate the change in the oxidation state of the PANI. Specifically, a deep analysis of N1s (Figures11d-f) reveals four main components, known to be the imine (

Figure 1 (Figure 1 .

 11 Figure 1 (single column)

Figure 2 (Figure 2 .

 22 Figure 2 (double column)

Figure 3 .

 3 Figure 3. Overview (a, c, e) and near view (b, d, f) SEM micrographs of the as-synthesized PANI at different concentrations of [HCl]: (a, b) 0.3 M, (c, d) 0.50 M and (e, f) 1.00 M. Oxidizing agent = APS, R = [APS]/[ANI] = 2 and [ANI] = 0.1 M.

Figure 4 (Figure 4 .

 44 Figure 4 (double column)

Figure 5 (Figure 5 .

 55 Figure 5 (single column)

Figure 6 (Figure 6 .

 66 Figure 6 (double column)

Figure 7 .

 7 Figure 7. Overview (a, c, e, g) and near view (b, d, f, h) SEM micrographs of the assynthesized PANI with doping agents at 0.5 M: (a, b) HCl, (c, d) HNO 3 , (e, f) H 2 SO 4 and (g, h) H 3 PO 4 . Oxidizing agent = APS and R = [APS]/[ANI] = 0.5.

Figure 8 (Figure 8 .

 88 Figure 8 (double column)

Figure 9 .

 9 Figure 9. Overview (a, c, e) and near view (b, d, f) SEM micrographs of the as-synthesized PANI with oxidizing agents at the ratio R = [oxidizing agent]/[ANI] = 0.5 and [ANI] = 0.4 M. (a, b) APS, (c, d) K 2 Cr 2 O 7 , and (e, f) KMnO 4 . Doping agent = HCl at 0.5 M.

Figure 10 (Figure 10 .

 1010 Figure 10 (double column)

Figure 11 (Figure 11 .

 1111 Figure 11 (double column)

Figure 12 .

 12 Figure 12. (a) Surface atomic composition of the different types of N species as-determined from XPS of N1s. (b) CHNS analysis of materials: atomic ratios of C/N (Left y-axis) and C/H (Right y-axis).

Figure 13 (Figure 13 .

 1313 Figure 13 (single column)

  2 Cr 2 O 7 and KMnO 4 . The particularity of APS is its high redox

potential (2.01 V vs SHE) compared to the values of the redox potential of KMnO 4 and K 2 Cr 2 O 7 (1.51 and 1.33 V vs SHE, respectively), making it the best candidate for the chemical oxidative polymerization to yield PANI. From FTIR spectra in Figure

Table 1 .

 1 BET surface area, electrical conductivity (EC), and EIS analysis of the as-prepared PANI-based materials.

	Entry	BET S BET (m 2 g -1 ) (S cm -1 ) R  ( cm 2 ) R ct ( cm 2 ) Q CPE (µF s a-1 ) EC EIS	a
	PANI-HCl0.30_APS_R2	-	3.7×10 -4	1.4	10.3	1.6	0.90
	PANI-HCl0.50_APS_R2	23	0.25	1.6	20.1	1.6	0.92
	PANI-HCl0.75_APS_R2	25	0.18	1.3	26.2	1.2	0.92
	PANI-HCl1.00_APS_R2	6	0.15	1.3	31.4	1.2	0.91
	PANI-R0.50_HCl0.50_APS	21	1.39	2.4	15.2	1.8	0.90
	PANI-R0.67_HCl0.50_APS	-	0.75	2.2	41.5	1.8	0.94
	PANI-R1.00_HCl0.50_APS	26	0.64	2.1	17.5	1.8	0.94
	PANI-R2.00_HCl0.50_APS	23	0.25	1.6	20.1	1.6	0.89
	PANI-HCl_R0.50_APS	21	1.39	2.4	15.2	1.8	0.90
	PANI-HNO 3 _R0.50_APS	20	0.26	2.0	20.2	1.5	0.89
	PANI-H 2 SO 4 _R0.50_APS	16	0.50	2.0	41.5	1.4	0.91
	PANI-H 3 PO 4 _R0.50_APS	18	0.33	2.3	18.2	1.4	0.92
	PANI-APS_R0.50_HCl	21	1.39	2.4	15.2	1.8	0.90
	PANI-K 2 Cr 2 O 7 _R0.50_HCl	11	2.6×10 -5	2.0	345.0	1.4	0.90
	PANI-KMnO 4 _R0.50_HCl	2	2.1×10 -3	1.4	75.4	1.3	0.93

Acknowledgements

D. Razik gratefully acknowledges the financial support from Campus France for his stay at IEM Montpellier through the program PROFAS B+ of "algéro-français cooperation". This work at IEM Montpellier was supported by the CNRS Energy unit through the project "PEPS19-ELECTROFUEL".


José María

Andrés Castán

Clément Dalinot

Sergey Dayneko

Laura Abad Galan

Pablo Simón Marqués

Olivier Alévêque

Magali Allain

Olivier Maury

Ludovic Favereau

Philippe Blanchard

Pablo Simón Marqués

Gregory C Welch

Clément Cabanetos

Synthesis, characterization and use of benzothioxanthene imide based dimers

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Over the years, imide containing rylenes have demonstrated their potential as strong candidates for a variety of organic electronic applications due to their excellent thermal, chemical and photochemical stabilities, as well as intriguing electronic and redox properties. 1 Perylene and naphthalene diimides can be clearly identified as the beacons of this molecular family in terms of reported studies, chemical modifications and target applications (Figure 1). Motivated by the exploration and functionalization of original chemical structures, we have recently focused our attention on an overlooked and inexpensive vat dye, the benzothioxanthene imide (BTXI). Never used for organic electronic purposes and exclusively functionalized on the nitrogen atom of the imide group, we have reported the selective and efficient monobromination of its rylene core, affording the BTXI-Br (Figure 1), 3,4 and have opened the door to the design of new and original molecular architectures that have been successfully evaluated as active materials in organic photovoltaic (OPV) devices. 5,6 Consequently, to further explore the potential of this dye, we report herein the synthesis and use of three dimers to prepare the first BTXI-based organic light-emitting diodes (OLEDs). As depicted in scheme 1, target molecules stemmed from the common BTXI-Br derivative. The later was indeed either sulfonated (BTXI-SO2-Br) or engaged in a Suzuki-Miyaura crosscoupling reaction in order to be subsequently and efficiently mono-brominated at the bay position (Br-BTXI). Finally, dimers D1, D2 and D3 were prepared from the corresponding bromo derivatives, namely BTXI-Br, BTXI-SO2-Br and Br-BTXI respectively, under copper-catalyzed Ullmann reaction conditions. 7 In parallel, pallado-catalyzed cross coupling conditions have been applied, in a further attempt to improve the synthetic yields, particularly for D1 and D3, but were not found to be that much convincing (Scheme S1). Single crystals of all the dimers were prepared by slow evaporation techniques. In addition to confirming the structures, the latter highlighted, as expected, large dihedral angles between the constituting BTXI units of ca. 75.9, 75.6 and 60.2 for D1, D2 and D3 respectively (Figure 2). Such important twists might contribute to the good solubility of the derivatives and should profoundly modify the self-assemble properties compared to those of the BTXI monomer, and consequently tune the solid-state luminescence (vide supra). Comparison of crystal packing indicates that (i) BTXI forms a very regular headto-tail stacking with strong cofacial π-interactions (d= 3.675 and 3.816 Å, Figure S20); (ii) D1 and D2 crystallized as dimers with strong and moderate intermolecular π-interactions respectively (Figures S21 andS22); and D3 forms more complex hexameric toroidal structure with circular slipped π-interactions between two vicinal molecules (Figures S23-S25).

In parallel, the photophysical properties have been investigated in both diluted dichloromethane solution and in the solid state. Results are gathered in the Table 1. In solution, it appears that oxidation into sulfones (D2) induces a significant blue shift (of ca. 50 nm) of the long-wavelength absorption band (π-π* transitions) initially centered around 450 nm (D1, D3) (Figure S16 and S28), in agreement with the previously reported BTXI-SO2 derivative. 8 A similar effect is also observed in the emission. Importantly, compared to BTXI, 8 the dimerization results in a significant decrease of the luminescence quantum yield in solution (Table 1). This effect is explained by a more favoured intersystem crossing process of a non-emissive triplet state (note that a detailed photophysical study will be published elsewhere). When moving to solid state, the photophysics of the compounds are governed by the present intermolecular Dimer λabs (nm) Please do not adjust margins Please do not adjust margins interactions. For instance, the emission of BTXI is red shifted and completely quenched due to aggregation quenching as a result of the formation of the strong π-stacking intermolecular interactions described above (Figure S20). This observation is also valid for dimer D1 with only a 0.06 quantum yield. D2 shows a larger quantum yield of 0.14 that can be explained by the distortion induced by the sulphur oxidation reducing the πstacking intermolecular interactions. Finally, D3 presents a sharp emission at 580 nm strongly red-shifted (FWHM = 1442 cm -1 against 2348 cm -1 and 3145 cm -1 for D1 and D2, respectively), accompanied by the highest quantum yield of the series. This behaviour is in agreement with the slipped πinteractions observed in the crystal structure and suggests the formation of J-aggregates in the solid state that are frequently observed for the perylene diimide chromophores family (Figure 3). 9 The Cyclic voltammogram of D1, plotted in Figure 4, revealed the presence of two reversible and successive processes in both positive and negative regions attributed to the formation of stable cation /di-cation and anion/di-anion species respectively. While changing the grafting position (D3) induces a more visible splitting of the reversible processes with minor impact on their potentials (greater delocalization of charges), the conversion of the sulfurs into sulfones results in a drastic modification of the electrochemical signatures of D2. Indeed, on one side, the reversibility of the oxidation processes seems to be lost, while on the other side two reversible and a quasi-reversible split waves were recorded in the negative region. Thereafter, frontier molecular orbital energy levels were thus estimated from the onset of the first oxidation and reduction waves (Table 1). With almost similar LUMO levels, dimerization in the bay area (D3 vs D1) induces a slight destabilization of the HOMO level resulting in the reduction of the gap also highlighted during the UV-visible measurements. In stark contrast, oxidation of the sulfur into sulfone causes the stabilization of both frontier orbitals while increasing the band gap.

ε (M -1 cm -1 ) λem (nm) Φf a λem b (nm) Φf b Epa (V/Fc+/Fc) Epc (V/Fc+/
Complementarily investigated by computational chemistry, simulated frontier energy levels and UV-visible spectrum were also found to follow these experimental trends (Figure S26-28 and Table S2).

Hence, and as a further step into the characterization of these dimers and for a deeper structure-property relationship investigation, solution-processed OLED devices, of architecture: glass/ITO/PEDOT:PSS/Emitting layer (EML)/ZnO/Ag, were then fabricated. 10 The luminescent and commercially available conjugated polymer PFO, also known as F8, was used into the EMLs as a host material to improve the performances of small molecules-based OLEDs. 10,11 Table 2. OLEDs performance based on PFO and/or blends with dimers.

Tested under ambient conditions, it is noteworthy that all devices emitted light when connected to an external voltage, thus demonstrating, for the first time, up and running BTXIbased OLEDs. An overview of the main characteristics of the three BTXI-based OLEDs is summarized in Table 2 and their electroluminescence spectra (EL) and the Commission Internationale de l'Eclairage (CIE) color coordinates are shown in Figure 5. In agreement with the optical properties of the three dimers, different colors of emitted lights, from blue to orange, were recorded. These results highlight, once again, the ease of In addition, the EML layers were spin-casted from o-xylene which indicates that these materials can be easily transferred to develop flexible OLEDs by roll-to-roll coating into industrial scale. Moreover, the performance of these BTXI-based OLEDs could be improved by using a multilayer OLEDs structure. 12

Conclusions

The synthesis and characterization of three original BTXI-based dimers is reported herein. Subttle chemical modifications, namely the oxidation of the constituting sulphur into sulfone and the dimerization in the bay area, were investigated as well as their impact on the optical and electrochemical properties. Due to their good solubility and highly twisted structures, these dimers were finally and successfully used as emitting materials for the demonstration of the first BTXI-based OLEDs.

2

 2

Figure 1 .

 1 Figure 1. Structure of NDI, PDI, BTXI and BTXI-Br.

Scheme 1 .

 1 Scheme 1. Synthetic route to D1, D2 and D3.

Figure 2 .Figure 3 .

 23 Figure 2. X-ray structures of D1, D2 and D3.

Figure 4 .

 4 Figure 4. Cyclic voltammogram of D1 (top), D2 (middle) and D3 (bottom).

  tunability at different levels induced by simple chemical modifications. The redshift and changing of EL shape of BTXIbased OLEDs relative to the photoluminescence is a result of the aggregation of the dimers and various formation and relaxation of exciton mechanisms in OLED structure (under voltage) and "clean" films (under light excitation).

Figure 5 .

 5 Figure 5. Electroluminescence spectra (top) and CIE color coordinate spectra (above) of the D1 (black), D2 (red) and D3 (blue) based devices under external voltage.

Table 1 .

 1 Optical and electrochemical data recorded for

D1, D2 and D3. a in

  solution and b on powders.

  Fc) EHOMO (eV) ELUMO (eV) ∆Eelec (eV)

	BTXI	455	22000	502	0.99	579	0.02	0.94	-1.82	-5.63	-3.10	2.53
	D1	455	51000	509	0.19	534	0.06	1.00	-1.74	-5.64	-3.16	2.48
	D2	384	37000	450	0.29	460	0.14	1.47	-1.18	-6.07	-3.73	2.35
	D3	473	31000	530	0.34	580	0.18	0.86	-1.70	-5.54	-3.21	2.32

Acknowledgments

Authors thank the MATRIX SFR of the University of Angers. J.M.A.C. and P.S.M. thanks the European Union's Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement No.722651 (SEPOMO). The Région Pays de la Loire is also acknowledged for the grant of C. D. (Projet étoile montante SAMOA). The authors are also grateful to ANR (SADAM ANR-16-CE07-0015-01) for the financial support and PDF grant of L.A.G. S. D. thanks the MITACS program for financial support. G. C. W. thanks the University of Calgary.

Conflicts of interest

There are no conflicts to declare.

Statement of Contributions

JMA: synthesis, characterization and writing of the ESI, CD: synthesis and characterization, SD: device fabrication and testing, LAG and PSM: computational chemistry, OA: electrochemical characterization, MA: crystallographic characterization, OM: LAG's PI and discussions, PB, JMA/PSM's co-PI and discussions, GW: SD's PI and discussions, CC: CD's PI and JMA/PSM's co-PI, writing of the article


Clément Dalinot

Pablo Simón Marqués

José M Andrés Castán

Pierre Josse

Magali Allain

Laura Abad Galán

Cyrille Monnereau

Olivier Maury

Philippe Blanchard

Clément Cabanetos
email: clement.cabanetos@univ-angers.fr

Regioselective mono halogenation and homo/hetero dihalogenation of the benzothioxanthene monoimide

Keywords: selective halogenation, benzothioxanthene, organic chemistry, rylene

Through the preparation and characterization of five new derivatives, the regioselective mono and di, homo and hetero, halogenation of the benzothioxanthene (BTXI) core is demonstrated herein. All structurally solved by X-ray crystallography, these complementary functionalized building blocks open doors to the design of new symmetrical and asymmetrical π-conjugated systems based on this promising but still under-explored rylene.

Introduction

Over the past decades, the naphthalene (NDI) and perylene (PDI) diimides have emerged as reference building blocks in the field of organic electronics due to their excellent thermal and photochemical stability, synthetic tunability as well as specific electrochemical and high charge transport properties (Figure 1). [START_REF] Feng | Synthesis and Application of Rylene Imide Dyes as Organic Semiconducting Materials[END_REF][START_REF] Sun | Water-soluble perylenediimides: design concepts and biological applications[END_REF][START_REF] Zhang | Selfassembly, optical and electrical properties of perylene diimide dyes bearing unsymmetrical substituents at bay position[END_REF][START_REF] Kozma | Perylene diimides based materials for organic solar cells[END_REF][START_REF] Würthner | Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials[END_REF][START_REF] Liu | Non-fullerene small molecule acceptors based on perylene diimides[END_REF] Although belonging to the imide-containing rylene family, the benzothioxanthene monoimide (BTXI) has not triggered such research interest, until recently. Synthesized in the 70's and solely functionalized on the Nposition for post-grafting and/or solubility purposes, [START_REF] Grayshan | Heterocyclic derivalives of naphthalene-1,8-dicar boxylie anhydride. Part III. Benzo[k,l] thioxanthene-3,4-dicarboximides[END_REF][START_REF] Kadhim | A new intramolecular cyclisation reaction-I: Novel synthesis of benzo(k,1)thioxanthene-3,4-dicarboxylic anhydride and derived dyestuffs[END_REF][START_REF] Danko | Synthesis, photochemical stability and photo-stabilizing efficiency of probes based on benzothioxanthene chromophore and Hindered Amine Stabilizer[END_REF][START_REF] Mao | Benzothioxanthene dyes as fluorescent label for DNA hybridization: synthesis and application[END_REF][START_REF] Zhang | Formation and stabilization of the telomeric antiparallel G-quadruplex and inhibition of telomerase by novel benzothioxanthene derivatives with anti-tumor activity[END_REF] we indeed recently demonstrated the selective mono-bromination (Figure 1, BTXI-Br) and use of this new promising block to prepare new π-conjugated systems via conventional palladium catalyzed cross-coupling reactions for organic photovoltaic applications. [START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF][START_REF] Payne | Donor or Acceptor? How Selection of the Rylene Imide End Cap Impacts the Polarity of π-Conjugated Molecules for Organic Electronics[END_REF][START_REF] Dayneko | Ternary organic solar cells: using molecular donor or acceptor third components to increase open circuit voltage[END_REF] As a step towards new design principles, and therefore potential applications, we report herein a systematic halogenation study of the BTXI block to afford complementary mono-and di-halogenated derivatives.

Results and Discussion

Hence, and from our early reported procedure, [START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF] where the BTXI was efficiently mono-brominated in the presence of 1.1 equivalents of bromine (Table 1, entry 1), we started to investigate the reactivity of additional and safer sources of bromine atoms. To that end, the reactivity of the cheap and well-known Nbromosuccinimide (NBS) was first evaluated. It turned out that regardless of the reaction time (from few minutes to hours), temperature (room temperature or reflux) and nature of the solvent (chloroform or N,N-dimethylformamide), the entirety of the starting material was systematically recovered (Table 1, entries 2-5). However, while activating the NBS with ptoluenesulfonic acid in ethanol barely improved the conversion (Table 1, entry 6), [START_REF] Grolleau | Clean and Efficient Iodination of Thiophene Derivatives[END_REF] it was found that mechanochemistry can be an interesting tool to prepare the target BTXI-Br in high yield (Table 1, entry 7). [START_REF] Bose | Electrophilic aryl-halogenation using Nhalosuccinimides under ball-milling[END_REF] However, this solvent-free method requires a large excess of reactant (12 equivalents) to reach a full conversion of the starting material (Table 1, entry 9). In contrast, this was not the case for the dibromoisocyanuric acid since only 1.1 molar equivalents of this strong bromine source were indeed necessary to efficiently convert the BTXI into the target compound. Moreover, it is important to note that the reaction occurs under very mild conditions, i.e., at room temperature and overnight (Table 1, entry 10). However, considering the relative high price of this reagent, the use of a cheaper one but still known to be a strong and solid source of bromine was finally assessed, namely the pyridinium tribromide. When engaged under the previously described mild conditions, i.e., 1.1 equivalents at room temperature and overnight (Table 1, entry 11), a maximum conversion of 50% was reached (47% isolated), corresponding to a mixture of the target BTXI-Br and the starting material (BTXI). In an attempt to improve the reactivity, the conventional oil bath was directly replaced by microwave irradiations (Table 1, entry 12). Unfortunately, a wellbalanced blend of BTXI-Br and BTXI was once again recovered even after one hour of stirring at 100 °C. According to these observations, it was decided to double the amount of brominating agent (Table 1, entry 13) resulting in a full conversion, thus suggesting that the quantity of the pyridinium tribromide was the limiting parameter.

On the other hand, characterization of the crude by mass spectrometry also revealed the presence of a minor product attributed to a dibrominated species.

As an important building block for a multi functionalization of the BTXI core, the second insertion of a bromine atom was naturally investigated. To do so, the amount of pyridinium tribromide was significantly increased to 5, 10 and even 20 equivalents under the same heating conditions (Table 2, entries 1-3). It turned out that using a large excess of this brominating agent indeed generated a dibrominated species but with a maximum yield of 12%, the major product still being in these conditions the mono derivative, ie, the BTXI-Br.

Hence, owing to its high reactivity, 2.2 equivalents of dibromoisocyanuric acid were subsequently added to a solution of BTXI (Table 2, entry 4). After 16 hours under reflux and the total consumption of the starting material, only one spot migrated on the thin layer chromatography (TLC), corresponding, after purification, to a dibrominated compound. With a resulting yield of ca. 32%, the quantity of brominated agent was increased to 5.0 equivalents (Table 2, entry 5). In these conditions, although only a single dibrominated derivative was recovered from the purification, the synthetic yield dropped to 18%, resulting from the higher amount of polar by-products stuck at the start of the chromatography column. On the other hand, while the use of microwaves irradiation indeed reduce both the reaction time and the quantity of such by-products, these conditions systematically lead to a mixture of mono and dibrominated derivatives with a maximum conversion in Br-BTXI-Br reached with 2.2 equivalents of dibromoisocyanuric acid (Table 2, entry 6-7). Consequently, the BTXI was finally exposed to an increasing quantity of bromine and the solutions were refluxed overnight (Table 2, entries 8-10). While, in all cases, the starting material was systematically consumed, the full conversion into a dibrominated derivative was only reached with 20 equivalents (Table 2, entry 10). All isolated Br-BTXI-Br products were analyzed by [START_REF] Feng | Synthesis and Application of Rylene Imide Dyes as Organic Semiconducting Materials[END_REF] H NMR revealing a perfect superimposition of their spectra, thus confirming, in consistency with the TLC (same Rf = 0.6 in toluene), the generation of the same and unique dibrominated compound regardless of the nature of the brominating agent. Luckily, single crystals were successfully grown revealing the exact grafting of the second bromine atom at the 11 position as depicted in Figure 2. In parallel, the iodination of the BTXI was also investigated to extend the range of functionalization and reactivity (Table 3). Mild and green conditions, reported by Gohier and coworkers, [START_REF] Grolleau | Clean and Efficient Iodination of Thiophene Derivatives[END_REF] using N-iodosuccinimide in ethanol with a catalytic amount of ptoluenesulfonic acid, were first investigated (Table 3, entry 1).

Stirring the reaction mixture overnight at 60 °C successfully led to the formation of a new compound. Once isolated in good yield by column chromatography, the latter was subsequently analyzed by mass spectrometry and NMR experiments revealing the presence of a single iodine atom grafted on the constituting naphthyl moiety of BTXI, the exact position of which was, once again, solved by single crystal X-ray diffraction (Figure 3). As for BTXI-Br, the iodination occurs on the rylene core at the βposition with regards to the sulfur atom. In addition to changing the heating method (Table 3, entry 2), the full conversion of the BTXI in this new derivative, namely BTXI-I, was only achieved by increasing the quantity of N-iodosuccinimide (Table 3, entry 3). Interestingly, though a large excess of this reagent was used, no multi-halogenated compound was detected in the crude. In an attempt to generate such species, a stronger method based on the use of the iodic acid/iodine couple in a mixture of acetic and sulfuric acid, water and chloroform was considered and evaluated. After 16 hours of stirring at room temperature, TLCs highlighted the formation of two compounds (Table 3, entry 4). Once purified by simple column chromatography, the major product was found to be the above mentioned mono-iodinated BTXI (BTXI-I) while the second, isolated with a low yield, corresponds to a di-iodinated derivative. In stark contrast with Br-BTXI-Br where both bromine atoms are localized on the naphthyl ring, 1 H NMR experiments of the new I-BTXI-I suggested a functionalization of the constituting phenyl ring (Figure 4, purple tagged protons). While the integrity of the naphthyl constituting protons were recovered when comparing the spectra of both BTXI-I and I-BTXI-I, the multiplet centered at ca. 7.45 ppm, that comprises three hydrogen atoms, splits into a doublet (at 7.19 ppm) and a doublet of doublets (at 7.72 ppm) of one each. Moreover, the characteristic doublet centered at 8.5 ppm of I-BTXI-I, also attributed to a phenyl proton, appears significantly deshielded (compared to its BTXI-I analogue) thus supporting a close-by halogenation. Single crystal X-ray diffraction later confirmed these suppositions, thus revealing the exact position of the second iodine atom, corresponding to a functionalization in para of the sulfur atom (Figure 5). It turns out that the core of the expected derivative presents a higher degree of distortion compared to the isolated compound, induced by the bulkiness of the iodine atom, with a characteristic dihedral angle of 30° vs 8° respectively (carbon nuclei in pale yellow, Figure 6). Moreover, comparison of the corrected total energy of both regioisomers revealed a difference of ca 10 Kcal.mol -1 suggesting that the isolated I-BTXI-I is more thermodynamically favored than its expected counterpart (Figure SX). Hence, in a further attempt to improve the conversion into this new bis-functionalized derivative, the temperature of the reaction was increased to 90 °C for 24 h (Table 3, entry 5). While a larger amount of I-BTX-I was indeed isolated, these conditions also led to more unidentified and mostly insoluble by-products.

Scheme 1. Regioselective hetero dihalogenated derivatives reported herein.

Finally, to take it to the next level, these reactions were finally complementarily combined to afford hetero-and regiospecific dihalogenated derivatives (Scheme 1).

To do so, BTXI-Br was successfully iodinated in acidic conditions in the presence of iodic acid and iodine. As for I-BTXI-I, the iodine atom was found to be localized on the phenyl ring in para position with respect to the sulfur atom as evidenced by [START_REF] Feng | Synthesis and Application of Rylene Imide Dyes as Organic Semiconducting Materials[END_REF] H NMR and single crystal X-ray diffraction (I-BTXI-Br, Figure 7). In parallel, BTXI-I was first exposed to 20 equivalents of bromine. After reflux overnight, TLCs revealed the total consumption of the starting compound and generation of a single compound. However, once isolated and characterized, the latter turned out to be the dibrominated Br-BTXI-Br. Consequently, to avoid the halogen exchange reaction, the excess of bromine was replaced by only 1.1 equivalents of dibromoisocyanuric acid. First highlighted by TLCs, a new compound, whose Rf was found to be different from that of the Br-BTXI-Br, was successfully isolated in good yield after the total consumption of the starting material and purification of the crude. Mass spectrometry confirmed the presence of both a bromine and an iodine atom on this new BTXI derivative and 1 H NMR suggested a functionalization on the naphthyl ring as for Br-BTXI-Br (see SI). Luckily, crystals of good quality were successfully grown and analyzed by X-ray diffraction confirming both hypothesized structures (Figure 7).

Conclusion

In conclusion, the regioselective dihalogenation of the BTXI core is demonstrated herein. As a result, five new mono and dihalogenated derivatives were successfully prepared, isolated and characterized. Depending on the nature and source of the halogen atom, different positions on the rylene core are now fully and, above all, selectively and complementarily accessible (hetero dihalogenated derivatives) for post grafting purposes, thus opening doors to new designs of functional π-conjugated BTXI based molecules and polymers.

Experimental Section

Supporting Information

7

 7

Figure 1 .

 1 Figure 1. Structures of naphthalene diimide (NDI), perylene diimide (PDI), benzothioxanthene monoimide (BTXI) and the brominated derivative BTXI-Br.

Figure 2 .

 2 Figure 2. Molecular structure of the Br-BTXI-Br obtained by X-ray diffraction.

Figure 3 .

 3 Figure 3. Molecular structure of the BTXI-I obtained by X-ray diffraction.

Figure 4 . 1 H

 41 Figure 4. Comparison of BTXI, BTXI-I and I-BTXI-I 1 H NMR spectra recorded

Figure 5 .

 5 Figure 5. Molecular structure of the I-BTXI-I obtained by X-ray diffraction To rationalize this result, optimized geometries of the expected and isolated I-BTXI-I were investigated using the Gaussian09 suite of programs at B3LYP (6-31+G** for C, H, N, O, S and LANL2DZ for I) level of theory with the inclusion of Grimme's D2

Figure 6 .

 6 Figure 6. Optimized geometries of the isolated (left) and expected (right) I-BTXI-I

Figure 7 .

 7 Figure 7. X-ray structures of I-BTXI-Br (left) and Br-BTXI-I (right).

Table 1 .

 1 Experimental conditions evaluated in this study to mono-brominate the BTXI. R.T. = room temperature.

Table 2 .

 2 Experimental conditions evaluated for the preparation of a dibrominated BTXI.

		Entry	Source of Bromine atom	Number of equivalents	Solvent	Experimental conditions	Synthetic yield of BTXI/BTXI-Br and Br-BTXI-Br
		1	Pyridinium tribromide	5.0		dichloromethane	microwaves, 100 °C; 1 hour	0%/93%/5%
		2	Pyridinium tribromide	10.0		dichloromethane	microwaves, 100 °C; 1 hour	0%/85%/11%
		3	Pyridinium tribromide	20.0		dichloromethane	microwaves, 100 °C; 1 hour	0%/83%/12%
		4	Dibromoisocyanuric acid	2.2		N,N-dimethylformamide	reflux.; overnight	0%/0%/32%
	5	Dibromoisocyanuric acid	5.0	N,N-dimethylformamide	reflux.; overnight	0%/0%/18%
	6	Dibromoisocyanuric acid	2.2	N,N-dimethylformamide	microwaves, 100 °C; 1 hour	0%/41%/59%
	7	Dibromoisocyanuric acid	5.0	N,N-dimethylformamide	microwaves, 100 °C; 1 hour	0%/30%/41%
	8		Bromine	5.0		dichloromethane	reflux; overnight	0%/78%/19%
	9		Bromine	10.0		dichloromethane	reflux; overnight	0%/23%/73%
	10		Bromine	20.0		dichloromethane	reflux; overnight	0%/0%/98%

Table 3 .

 3 Experimental

conditions for the synthesis of the mono-and di-iodinated BTXI based derivatives (BTXI-I and I-BTXI-I). R.T. = room temperature.

Entry Source of Iodine atom Number of equivalents Solvent Experimental conditions Synthetic yield of BTXI/BTXI-I and I-BTXI-I

	1	N-iodosuccinimide	2.0	ethanol	p-toluenesulfonic acid; 60 °C.; overnight	30%/67%/0%
	2	N-iodosuccinimide	2.0	ethanol	p-toluenesulfonic acid;	12%/80%/0%
					microwave, 100 °C; 1
					hour
					p-toluenesulfonic acid;
	3	N-iodosuccinimide	4.0	ethanol	microwave, 100 °C; 1	0%/100%/0%
					hour
				acetic acid:sulfuric
	4	Iodine/Iodic acid	2.5/cat.	acid:water:CHCl3 (1:0.02:0.2:0.08	R.T.; overnight	0%/67%/3%
				/v:v:v:v)
	5	Iodine/Iodic acid	2.5/cat.	acetic acid:sulfuric	90 °C; overnight	0%/38%/19%
				acid:water:CHCl3
				(1:0.02:0.2:0.08
				/v:v:v:v)

Acknowledgements

Authors thank the MATRIX SFR of the University of Angers. J.M.A.C. and P.S.M. thanks the European Union's Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement No.722651 (SEPOMO). The Région Pays de la Loire is also acknowledged for the grant of C. D. (Projet étoile montante SAMOA). The authors are also grateful to ANR (SADAM ANR-16-CE07-0015-01) for financial support and the PDF grant of L.A.G.

Entry for the Table of Contents

The regioselective mono and di, homo and hetero, halogenation of the benzothioxanthene (BTXI), an overlooked sulphur containing rylene, is reported here. All structurally solved by X-ray diffraction, these newly functionalized BTXI cores open unprecedented doors to the design of new symmetrical and asymmetrical π-conjugated systems.