P-M Gueye

B Gómez-Ferrer

C Kaden

Cristelle Pareige
email: cristelle.pareige@univ-rouen.fr

Role of Ni, Si and P on the formation of solute-rich clusters under irradiation in Fe-Cr alloys

After irradiation of Fe-Cr alloys of low purity (model alloys of F-M steels), minor solute elements as P, Ni and Si have been shown to create solute clusters which significantly contribute to hardening and might be associated with small dislocation loops. In order to understand the role of each impurity on the formation of the nano-features formed under irradiation and the eventual synergies between the different species, Fe-15at.%Cr-X (X=Si, Ni, P, NiSiP) alloys of different composition have been ion irradiated and characterized using atom probe tomography. Irradiation were performed at 300 °C up to 2.5 dpa in four alloys: Fe15CrNi, Fe15CrSi, Fe15CrP and Fe15CrNiSiP. Influence of C atoms implanted during irradiation on the nanostructure evolution is also discussed. The study of the evolution of the nanofeatures formed under irradiation with the dose as a function of the composition highlights the role of P and C on the formation of the nano-clusters and confirm the radiation-induced nature of solute-rich clusters.

Introduction

The operation temperature window of ferritic/martensitic steels under irradiation is limited at high temperature by irradiation creep and at low temperature by radiation embrittlement. Part of the radiation embrittlement is due to radiation hardening caused by the formation of intragranular nanometric features. Two families of nanofeatures contribute to the hardening: (i) dislocation loops [START_REF] Hernández-Mayoral | Transmission electron microscopy investigation of the microstructure of Fe-Cr alloys induced by neutron and ion irradiation at 300 °C[END_REF][START_REF] Matijasevic | Effect of Cr on the mechanical properties and microstructure of Fe-Cr model alloys after n-irradiation[END_REF][START_REF] Dubinko | Effect of Si-Ni-P on the emergence of dislocations loops in Fe-9Cr matrix under neutron irradiation: TEM study and OKMC modelling[END_REF] and (ii) clusters and/or precipitates formed either by radiation-induced or by radiation-enhanced mechanisms [START_REF] Kuksenko | Cr precipitation in neutron irradiated industrial purity Fe-Cr model alloys[END_REF][START_REF] Bergner | Estimation of the solubility limit of Cr in Fe at 300 °C from small-angle neutron scattering in neutron-irradiated Fe-Cr alloys[END_REF][START_REF] Bachhav | Microstructural changes in a neutron-irradiated Fe-15 at.%Cr alloy[END_REF][START_REF] Jiao | Segregation behavior in proton-and heavy-ion-irradiated ferritic-martensitic alloys[END_REF]. These clusters and/or precipitates are often constituted by a variety of nanoobjects of different composition and structure, each of these objects having its own contribution to the hardening [START_REF] Gómez-Ferrer | Nano-hardening features in high-dose neutron irradiated Eurofer97 revealed by atom-probe tomography[END_REF][START_REF] Bergner | Application of a threefeature dispersed-barrier hardening model to neutron-irradiated Fe-Cr model alloys[END_REF][START_REF] Konstantinović | Mechanical properties of FeCr alloys after neutron irradiation[END_REF][START_REF] Monnet | Multiscale modeling of precipitation hardening: Application to the Fe-Cr alloys[END_REF]. In low-Cr Fe-Cr model alloys of ferritic/martensitic steels, minor elements as P, Ni and Si have been shown to impact the dislocation loop distribution (decrease in size and increase of number density) [START_REF] Dubinko | Effect of Si-Ni-P on the emergence of dislocations loops in Fe-9Cr matrix under neutron irradiation: TEM study and OKMC modelling[END_REF] and create solute-rich clusters (SRC) which significantly contribute to the hardening [START_REF] Bergner | Application of a threefeature dispersed-barrier hardening model to neutron-irradiated Fe-Cr model alloys[END_REF][START_REF] Monnet | Multiscale modeling of precipitation hardening: Application to the Fe-Cr alloys[END_REF]. The fact that the SRCs are observed in alloys with Ni, Si and P at impurity levels, the bell-shape1 dependence of their solute content with temperature [START_REF] Kuksenko | Cr precipitation in neutron irradiated industrial purity Fe-Cr model alloys[END_REF][START_REF] Pareige | Behaviour of P, Si, Ni impurities and Cr in self ion irradiated Fe-Cr alloys -Comparison to neutron irradiation[END_REF][START_REF] Bachhav | Microstructural changes in a neutronirradiated Fe-6at.%Cr alloy[END_REF] and the systematic positive RIS (radiation induced segregation) of Ni, Si and P at point defect sinks [START_REF] Messina | Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys[END_REF] are strong indications that their formation is induced by the irradiation and not thermodynamically enhanced. Comparison of the number density of TEM invisible dislocation loops obtained by Object Kinetic Monte Carlo (OKMC) simulations by Chiapetto et al. [START_REF] Chiapetto | Effect of Cr content on the nanostructural evolution of irradiated ferritic/martensitic alloys: an object kinetic Monte Carlo model[END_REF] with the number density of the SRCs measured by atom probe tomography (APT) [START_REF] Kuksenko | Cr precipitation in neutron irradiated industrial purity Fe-Cr model alloys[END_REF] strongly suggested that these objects are the same features and that the SRCs are associated to small point defect clusters. Recent OKMC simulations by Castin et al. [START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF] confirmed this hypothesis and highlighted the dominant role of the immobilized self-interstitial atoms clusters in the nucleation of the solute-rich clusters (SRCs). Since migrating P can be trapped by substitutional P atoms creating very stable complexes [START_REF] Domain | Diffusion of phosphorus in α-Fe: An ab initio study[END_REF][START_REF] Meslin | Theoretical study of atomic transport via interstitials in dilute Fe -P alloys[END_REF], it could be possible that these stable complexes act as a nuclei for the formation of the SRCs.

In order to understand the role of each minor element (namely Ni, Si and P) on the formation of the SRCs and the possible synergies between these species, but also to confirm the nature of the SRCs, four Fe-15at.%Cr-X (X=Si, Ni, P, NiSiP) alloys of different composition have been ion irradiated at 300 °C up to 2.5 dpa and characterized using atom probe tomography. The 3D distribution maps of the chemical species have been analyzed to characterize the evolution of the cluster´s characteristics, i.e. size, number density, composition and size distribution, with the dose and the minor element composition. Cr also forms clusters at the higher dose but treatment of these clusters will not be addressed in this paper. The Cr concentration being the same in all the alloys, differences in behaviour cannot be attributed to the formation of Cr-rich clusters.

Materials and experiments

Materials

Four different Fe-15at.%Cr-X model alloys (X = Ni, Si, P, NiSiP) (Table 1) were cast by OCAS in an induction vacuum furnace. From each lab cast, the pieces were heat treated in a pre-heated furnace at 1200°C for 1 h 30 min, subsequently hot rolled and finally air-cooled down to room temperature obtaining a fully ferritic microstructure (for further details see [START_REF] Pareige | MatISSE FP7-Fission-2013[END_REF]). Composition provided in Table 1 was measured by APT and isotopic overlaps were systematically corrected. The grain size of the alloys is very similar: 239 µm for Fe15CrNiSiP, 253 µm for Fe15CrP, 217 µm for Fe15CrSi and 240 µm for Fe15CrNi [START_REF] Pareige | MatISSE FP7-Fission-2013[END_REF].

Irradiation conditions

The samples (8-10 x 10 x 1 mm 3 ) were mirror polished before irradiation using mechanical polishing in several steps down to diamond suspension of 1 μm. Subsequently, the deformed surface layer was removed by low-temperature (~0°C) electropolishing using a solution of 98% of ethylen glycol monobutyl ether and 2% of perchloric acid.

The model alloys were irradiated with 5 MeV Fe 2+ ions at 300°C using a 3MV-Tandetron accelerator at the Ion Beam Center of HZDR, Dresden, Germany. The irradiation was designed in such a way that the nominal displacement damage was reached at a depth of 500 nm in order to avoid surface effect and the influence of the implantation peak [START_REF] Tissot | Influence of injected interstitials on α′ precipitation in Fe-Cr alloys under self-ion irradiation[END_REF]. The displacement damage and implantation profiles given in Figure 1 were calculated using SRIM (Stopping and Range of Ions in Matter) [START_REF] Ziegler | The Stopping and Range of Ions in Solids[END_REF][START_REF] Ziegler | [END_REF] with the ''Quick Kinchin-Pease calculation" mode following the recommendation of Stoller et al. [START_REF] Stoller | On the use of SRIM for computing radiation damage exposure[END_REF] and using a displacement threshold of 40 eV [24]. In order to guaranty a homogeneous lateral exposure over the whole set of samples the focussed ion beam was scanned over the area of the samples during the irradiation. The scanning frequencies were 1041 Hz and 1015 Hz in horizontal and vertical direction, respectively. The ion flux was measured continuously by means of Faraday cups (current measurement) and integrated to obtain the ion fluence. The ion flux was 1.3•10 11 cm -2 •s -1 corresponding to a damage rate of 5•10 -5 dpa/s at 500 nm depth. The irradiation temperature of 300°C was maintained by fixing the samples on a heating target. The temperature control was based on a thermocouple placed on the backside of one sample. The irradiation doses reached at 500 nm are 0.1, 0.5 and 2.5 dpa. Results obtained at 0.1 dpa for all alloys and at 0.5 dpa for Fe15CrNiSiP are already published in [START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF].

Figure 1: Displacement damage and injected Fe concentration profiles for the target dose 2.5 dpa at 500 nm with 5 MeV Fe-ions. In grey, the region investigated by APT.

Atom probe technique and data analysis methods

The APT samples of irradiated materials were prepared by lift-out of a chunk and subsequent annular milling using a Scanning Electron Microscope -Focus Ion Beam (SEM-FIB ZEISS Crossbeam X540). At least three APT tips from at least two different grains have been analysed for each condition. All the tips were prepared at a depth of (500 ± 100) nm from the surface in order to investigate the regions were the target doses are reached i.e. 0.1 dpa, 0.5 dpa or 2.5 dpa. The final milling was performed with a Ga beam energy of 2 kV in order to reduce implantation of Ga ions in the material. The maximum concentration of Ga found in the samples did not exceed 0.05 at%.

APT analyses of the alloys were conducted using a high-resolution local electrode atom probe, LEAP 4000 XHR (CAMECA), with a detector efficiency of 36%. The samples were set at a temperature of 55 K in order to avoid preferential evaporation of Cr atoms. The specimens were electrically pulsed with a pulse fraction of 20% of the DC voltage with a pulse repetition rate of 200 kHz. The detection rate was set up between 0.1% and 0.35%. 3D reconstructions were performed using IVAS software (CAMECA). The reconstruction factors have been adjusted for every analysed sample (i.e. every APT volume). The compression factor  has been derived from the crystallographic angles between poles observed on the desorption maps and the field factor (k) has been derived from the expected interplanar distance at the poles. The values used for  and k have ranged between 1.5-1.65 and 3.6 -5 (for tips on micro-coupons), respectively. APT samples of non-irradiated materials were electropolished. In that case, the field factor was between 2.1 and 3.3.

The data treatment was performed using the GPM APT data software for atom probe users developed by GPM, Rouen, France. The distribution of the minor solute elements (Ni, Si and P) in the matrix before and after irradiation has been quantified using a statistical tool based on the first nearestneighbour distance distributions (1NN method) [START_REF] Philippe | Clustering and nearest neighbour distances in atom-probe tomography[END_REF]. An iso-concentration method (ICM) [START_REF] Pareige | Behaviour of P, Si, Ni impurities and Cr in self ion irradiated Fe-Cr alloys -Comparison to neutron irradiation[END_REF][START_REF] Hyde | Analysis of Radiation Damage in Light Water Reactors: Comparison of Cluster Analysis Methods for the Analysis of Atom Probe Data[END_REF] for cluster identification has also been applied when clusters are clearly visible and can be identified by the tool. In this paper, only clusters of minor solute elements are characterized.

1NN method

The frequency distributions of the 1NN distances of homo-pairs (X-X) being either Ni, Si or P-have been measured experimentally for every APT volume in regions far away from field desorption lines and poles. To build the histograms the sampling size used is 0.05 nm. In order to increase statistics, averaging has been done over all the analysed volumes for a given irradiation condition for each alloy.

The experimental frequency distributions have been smoothed by averaging over 3 successive points and normalized to 100 (frequencies are thus in percentage). In case of random distribution of atoms, the probability density 𝑃(𝑟) to find two 1NN X-X atoms at a distance r within dr for diluted elements is given by [START_REF] Philippe | Clustering and nearest neighbour distances in atom-probe tomography[END_REF]:

𝑃 1𝑁𝑁 (𝑟) = 4𝜋𝑄𝐶 0 r 2 exp (- 4 3 𝜋𝑄𝐶 0 𝑟 3 ) (1)
Where Q is the detection efficiency of APT and C0 is the nominal concentration per unit of volume in the analysed volume. The position of these two peaks depends on the concentration of the phases. The higher the concentration, the shorter the P-P distances. In the following, this curve will be named 1NN. The red curve corresponds to the difference between the experimental P-P curve and the random 1NN curve (1NN). The peak at short P-P distance is associated to P clusters. The peak at large distance corresponds to P-P pairs in the matrix.

Cluster characterization

The iso-concentration method (ICM) is a cluster identification procedure based on three parameters [START_REF] Hyde | Analysis of Radiation Damage in Light Water Reactors: Comparison of Cluster Analysis Methods for the Analysis of Atom Probe Data[END_REF][START_REF] Lefebvre-Ulrikson | Atom Probe Tomography -Put Theory into practice[END_REF][START_REF] Dong | Atom Probe Tomography Interlaboratory Study on Clustering Analysis in Experimental Data the Maximum Separation Distance Approach[END_REF]: a concentration threshold (Cth), a minimum number of atoms (Nb) detected in the clusters and a distance d corresponding to the distance below which two atoms belong to the same cluster (d = 0.5 nm). The selection of the parameters Cth and Nb is made in such a way that ghost clusters (i.e. selection of stochastic fluctuations in the matrix) are avoided in a random solid solution of the same composition. Cth ensures that less than 0.01% of the filtered atoms in the random solution would have this concentration [START_REF] Hyde | Analysis of Radiation Damage in Light Water Reactors: Comparison of Cluster Analysis Methods for the Analysis of Atom Probe Data[END_REF]. Calculations are performed on 3D concentration maps built from sampling of APT volumes in small boxes of 1 nm in size [START_REF] Lefebvre-Ulrikson | Atom Probe Tomography -Put Theory into practice[END_REF]. Nb is chosen in order to identify zero clusters in the random solution for the selected Cth value. These parameters were estimated for each APT volume.

The important point is to ensure that the choice made allows the correct identification (in size and shape) of all the clusters visible on the 3D images. Cth depends on the concentration in solutes. It varies from 1.2 to 3.2 at.% and Nmin varies from 6 to 10 [START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF].

The number density (NV) of the identified clusters was determined by a simple ratio of the number of the observed clusters to the overall analysed volume. After application of cluster identification methods, clusters are often found to be surrounded by a matrix shell rich in Fe and Cr which are erroneously associated to the clusters. These shells are systematically removed using an erosion method before measuring the size. The radius of each cluster was calculated as indicated in equation ( 2) considering the clusters as spherical:

𝑅 𝑐𝑙𝑢𝑠 = √ 3𝑛𝑉 𝑎𝑡 4𝜋𝑄 3 (2)
with n the number of detected atoms in each cluster, Vat the Fe atomic volume (a0 3 /2 with a0 the lattice parameter) and Q the detector efficiency. This equivalent radius is calculated for every cluster and the average radius, R, is finally calculated. To measure the composition of the clusters, the following procedure was applied: erosion profiles were drawn by class size and atoms belonging to the core of the clusters were selected (i.e. atoms from the plateau so that the interface is not considered). This guarantees a correct measurement of the composition of the clusters [START_REF] Lefebvre-Ulrikson | Atom Probe Tomography -Put Theory into practice[END_REF][START_REF] Hatzoglou | Quantification of APT physical limitations on chemical composition of precipitates in Fe-Cr alloys[END_REF]. In order for the plateau to be visible, it is important to adapt the step size used to draw the profiles. A step size of 0.03 nm has been used. The mass spectrum is calculated with the selected atoms in order to calculate the composition of the clusters. Isotopic overlaps are corrected and background noise removed. The composition values were averaged over all the APT analysed volumes for a given condition.

Results

The 3D distributions of P, Si and Ni in Fe15CrP, Fe15CrSi, Fe15CrNi and Fe15CrNiSiP irradiated up to 2.5 dpa are presented Figure 3. At 0.1 dpa, only P maps reveal clustering. At 2.5 dpa, clusters are visible on P, Ni and Si maps. At this dose, comparison of Ni maps in Fe15CrNi and Fe15CrNiSiP shows a stronger clustering when Ni is associated to P and Si. This is revealed by the fact that more clusters are visible on the elemental map when the three species are altogether (Figure 3c). 1NN and 1NN distributions at 0.1 dpa are reported in Figure 4 for homo-pairs and in Figure 5 for hetero-pairs. When segregated dislocation lines are observed in the APT volumes, they are removed in order to only probe 1NN distributions associated to segregation at point defect clusters or dislocation loops. Comparison of 1NN curves at 0.1 dpa and before irradiation is provided in Figure 6. Before irradiation, slight deviations from the random distribution are observed. After irradiation, these deviations increased. Two clear maxima appear on the 1NN curves revealing the presence of clusters embedded in the diluted matrix. As already mentioned in [START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF], whereas no clustering is visible on the spatial distribution maps of Ni and Si at 0.1 dpa, the 1NN and 1NN curves show the presence of P, Ni and Si clustering.

Figure 5 shows that Ni, Si and P form clusters altogether. The 1NN distributions of P-P, Ni-Ni and Si-Si pairs at 2.5 dpa in all alloys are reported in Figure 7.

Comparison of 1NN curves in alloys with only one minor element to the alloys with the three of them shows that clustering of Ni, Si and P is stronger in alloys containing Ni+Si+P altogether: the amplitude of the peaks corresponding to the clusters is greater in the alloy containing the three minor solutes. Table 2 presents the evolution of the SRCs composition in Fe15CrNiSiP with the dose. Whereas P concentration is very high at 0.1 dpa, one notes a decrease at 0.5 and 2.5 dpa, whereas Ni and Si concentrations increase. At 2.5 dpa, dislocation loops are also visible on the 3D images (Figure 8). They are detected with the same filtering criteria as those used to detect SRCs (Cth, Nmin and d). Dislocation loops were identified by visual inspection of the individual objects i.e. when a hole is visible in the feature detected by ICM, it is considered as a dislocation loop. The size of each loop has been measured considering the outer limit of the segregated region, not the dislocation loop by itself as the dislocation loop is not visible with APT. Consequently, the size is probably overestimated. The number density, radius and composition are provided in Table 2. Only the largest loops are expected to be visible with TEM. Table 3 gives the C concentration before and after irradiation at 2.5 dpa. One can note a significant increase in C content after irradiation. Even if a share of the C concentration can be assigned to the FIB preparation (used to prepare irradiated sample, but not AR samples which were electropolished), the measured C concentration is mainly due to C contamination during irradiation. It appears that some carbon contamination is unavoidable in spite of the efforts to limit it [START_REF] Gigax | Beam-contamination-induced compositional alteration and its neutron-atypical consequences in ion simulation of neutroninduced void swelling[END_REF][START_REF] Taller | Multiple ion beam irradiation for the study of radiation damage in materials[END_REF]. This is confirmed by alone. The clusters of C atoms cannot come from FIB preparation which implies a homogeneous distribution of C. C clusters were not observed in Fe15CrNiSiP and Fe15CrP. We do not have any explanation for this observation at this stage.

Discussion

Clustering of Si, Ni and P has been detected in alloys containing only Ni, Si or P as well as in the alloy containing the three elements after ion irradiation. Clustering occurs at a dose as low as 0.1 dpa for all the species whatever the alloy as already reported in [START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF]. When the three species are present in the alloy, they form clusters altogether. Whereas this behaviour is clearly visible on 3D images of P, statistical treatment is needed to highlight the segregation of Ni and Si. 1NN curves and composition of SRCs show that P is the specie having the strongest segregation tendency as already mentioned in [START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF] and predicted by modelling [START_REF] Messina | Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys[END_REF][START_REF] Messina | Exact ab initio transport coefficients in bcc Fe-X (X=Cr,Cu,Mn,Ni,P,Si) dilute alloys[END_REF]. P atoms are the fastest diffusers [START_REF] Domain | Diffusion of phosphorus in α-Fe: An ab initio study[END_REF][START_REF] Meslin | Theoretical study of atomic transport via interstitials in dilute Fe -P alloys[END_REF] and present a strong positive flux coupling with vacancies and self-interstitials towards point defect sinks [START_REF] Messina | Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys[END_REF][START_REF] Meslin | Theoretical study of atomic transport via interstitials in dilute Fe -P alloys[END_REF][START_REF] Messina | Exact ab initio transport coefficients in bcc Fe-X (X=Cr,Cu,Mn,Ni,P,Si) dilute alloys[END_REF].

Because P mixed dumbbells are trapped by substitutional P atoms in the matrix [START_REF] Domain | Diffusion of phosphorus in α-Fe: An ab initio study[END_REF][START_REF] Meslin | Theoretical study of atomic transport via interstitials in dilute Fe -P alloys[END_REF], the immobilized dumbbells can act as nuclei of dislocation loops. Indeed, the experimental work of Hardouin-Duparc [START_REF] Hardouin-Duparc | Etude de la formation sous irradiation des amas de défauts ponctuels dans les alliages ferritiques faiblement alliés[END_REF] reports the increase in number density of dislocation loops in Fe-P under irradiation. These immobile complexes become sinks that could further be enriched by Ni, Si, Cr and P [START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF]. This trapping and the high diffusion coefficient of P result in a high P concentration of the clusters at 0.1 dpa (Table 2). The P concentration of the SRCs decreases at 0.5 and 2.5 dpa, while the SRCs are progressively enriched by the slower diffusers, namely Ni and Si, which have also been shown to have systematic positive RIS at point defect sinks [START_REF] Messina | Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys[END_REF].

Comparison of 1NN curves between alloys containing only one minor specie and the one obtained in the alloy containing the three of them (Figure 7) revealed that segregation level quantified by the 1NN graphs is the same in all the alloys at 0.1 dpa as already mentioned in [START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF] (similar amplitudes of the peaks associated with the clusters for all the alloys). However, at 2.5 dpa, it is no more the case. At this higher dose, clustering was stronger when the three species are altogether. The trapping effect of P mixed dumbbells by P explains this observation as it favours the creation of nuclei for SRCs. However, the fact that P clustering is also stronger in Fe15CrNiSiP than in Fe15CrP shows that Ni and Si also have an influence. As a conclusion, there is a synergistic effect of the three species favouring clustering, P certainly playing a determining role.

At 2.5 dpa in Fe15CrNi and Fe15CrSi, the clusters formed by Ni and Si are enriched in C. A question arises. Would Si and Ni cluster in the absence of C? As P atoms, C atoms are known to form immobile complexes, mainly with vacancies [START_REF] Chiapetto | Simulation of nanostructural evolution under irradiation in Fe-9%CrC alloys: An object kinetic Monte Carlo study of the effect of temperature and doserate[END_REF][START_REF] Jansson | The nanostructure evolution in Fe-C systems under irradiation at 560 K[END_REF]. Therefore, these immobile complexes become sinks that could further be enriched by Si in Fe14CrSi or by Ni in Fe14CrNi [START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF]. It is therefore possible that C ends up in the clusters, not because of affinity with Ni and Si, but with point defects, forming stable complexes with them, which are later enriched with Ni and Si solutes that are dragged there by single point-defects.

In Fe15CrNiSiP, both SRCs and dislocation loops enriched in Ni, Si, Cr and P were observed at 2.5 dpa. Their composition (Table 2) is very similar. This confirms that they are of similar nature. At 2.5 dpa, the number density of decorated point defect clusters is therefore equal to the sum of the number density of dislocation loops and SRCs, i.e. (4.9 ± 0.5) 10 23 m -3 .

Based on these experimental results, previous work [START_REF] Pareige | Behaviour of P, Si, Ni impurities and Cr in self ion irradiated Fe-Cr alloys -Comparison to neutron irradiation[END_REF][START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF] and on the strong affinity of these species to segregate at dislocation loops and dislocation [START_REF] Dubinko | Effect of Si-Ni-P on the emergence of dislocations loops in Fe-9Cr matrix under neutron irradiation: TEM study and OKMC modelling[END_REF][START_REF] Jiao | Segregation behavior in proton-and heavy-ion-irradiated ferritic-martensitic alloys[END_REF][START_REF] Gómez-Ferrer | Nano-hardening features in high-dose neutron irradiated Eurofer97 revealed by atom-probe tomography[END_REF][START_REF] Messina | Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys[END_REF][START_REF] Hatzoglou | Quantification of APT physical limitations on chemical composition of precipitates in Fe-Cr alloys[END_REF][START_REF] Hardouin-Duparc | Etude de la formation sous irradiation des amas de défauts ponctuels dans les alliages ferritiques faiblement alliés[END_REF][START_REF] Chiapetto | Simulation of nanostructural evolution under irradiation in Fe-9%CrC alloys: An object kinetic Monte Carlo study of the effect of temperature and doserate[END_REF][START_REF] Jansson | The nanostructure evolution in Fe-C systems under irradiation at 560 K[END_REF][38][39], the irradiation-induced nature of the SRCs as mentioned in [START_REF] Pareige | Behaviour of P, Si, Ni impurities and Cr in self ion irradiated Fe-Cr alloys -Comparison to neutron irradiation[END_REF][START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF][START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF] and the formation mechanism proposed in [START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF] are confirmed. SRCs are small decorated point defects clusters.

Conclusions

The effect of composition on SRC formation was studied based on a set of four Fe-15Cr alloys with different impurity contents (Fe15CrNiSiP, Fe15Cr-Ni, Fe15CrSi and Fe15CrP). These alloys were irradiated at 300 °C with 5 MeV Fe-ions up to 2.5 dpa.

Clustering of Ni, Si or P species has been detected from low doses (0.1 dpa) thanks to the 1NN data analysis. The clustering of P atoms is observable on the 3D atom distribution maps for all studied doses.

However, the clustering of Ni and Si species is only visually observed at high dose (2.5 dpa) but exist as shown by statistical tests.

A synergetic effect due to the presence of the three species has been evidenced, P certainly playing a determining role. The important role of P on early SRC formation is confirmed, in agreement with diffusion data [START_REF] Domain | Diffusion of phosphorus in α-Fe: An ab initio study[END_REF][START_REF] Meslin | Theoretical study of atomic transport via interstitials in dilute Fe -P alloys[END_REF] and the stronger dragging of P over the other elements as predicted by Messina et al. [START_REF] Messina | Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe-X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys[END_REF][START_REF] Messina | Exact ab initio transport coefficients in bcc Fe-X (X=Cr,Cu,Mn,Ni,P,Si) dilute alloys[END_REF].

At 2.5 dpa, dislocation loops are observed and exhibit similar enrichment levels as SRCs. The clusters are first enriched by P. Ni and Si arrive later at these sinks. These results are consistent with an irradiation-induced nature of the SRCs as mentioned in [START_REF] Pareige | Behaviour of P, Si, Ni impurities and Cr in self ion irradiated Fe-Cr alloys -Comparison to neutron irradiation[END_REF][START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF][START_REF] Gómez-Ferrer | On the role of Ni, Si and P on the nanostructural evolution of FeCr alloys under irradiation[END_REF] and the formation mechanism proposed by Castin et al. [START_REF] Castin | The dominant mechanisms for the formation of solute-rich clusters in low-Cu steels under irradiation[END_REF]

Figure 2

 2 Figure 2 presents an example of experimental 1NN P-P frequency distribution (green curve) in an irradiated Fe15CrNiSiP alloy compared with the 1NN random distribution of the same concentration of P (black curve). The subtraction of the random frequency distribution from the experimental one reveals two peaks (red curve): one associated with the clusters and one associated with the matrix.The position of these two peaks depends on the concentration of the phases. The higher the concentration, the shorter the P-P distances. In the following, this curve will be named 1NN.

Figure 2 :

 2 Figure2: Experimental 1NN P-P frequency distribution (green curve) in an irradiated Fe15CrNiSiP alloy compared with the 1NN random distribution for the same P concentration (black curve). The red curve corresponds to the difference between the experimental P-P curve and the random 1NN curve (1NN). The peak at short P-P distance is associated to P clusters. The peak at large distance corresponds to P-P pairs in the matrix.

Figure 3 :

 3 Figure 3: 3D-distribution maps of P (green), Ni (orange) and Si (blue) in ion irradiated Fe15CrP, Fe15CrSi, Fe15CrNi and Fe15CrNiSiP before irradiation (AR), at 0.1 dpa, 0.5 dpa and 2.5 dpa at 300 °C.

Figure 4 :

 4 Figure 4: Experimental and theoretical 1NN distributions and 1NN curve at 0.1 dpa at 300 °C for P-P pairs in (a) Fe15CrP and (b) Fe15CrNiSiP, Si-Si pairs in (c) Fe15CrSi and (d) Fe15CrNiSiP, Ni-Ni pairs in (e) Fe15CrNi and (f) Fe15CrNiSiP. The black distribution corresponds to the random distribution of atoms (binomial distribution). The green, blue and orange distribution are the experimental 1NN distributions of P-P, Si-Si and Ni-Ni, respectively.The red curve is the 1NN curve. The dotted lines correspond to the maximum level observed in alloys with only one specie and are reported on the graphs obtained for the alloy with the three species for comparison.

Figure 5 :

 5 Figure 5: Experimental and theoretical 1NN distributions and 1NN curve at 0.1 dpa for (a) P-Si pairs, (b) P-Ni pairs and (c) Ni-Si pairs in Fe15CrNiSiP

Figure 6 :

 6 Figure 6: 1NN curves for (a) P-P pairs, (b) Si-Si pairs and (c) Ni-Ni pairs in Fe15CrNiSiP before irradiation and at 0.1 dpa at 300 °C.

Figure 7 : 7 *

 77 Figure 7 : 1NN and 1NN distributions at 2.5 dpa for P-P pairs in (a) Fe15CrP and (b) Fe15CrNiSiP, Si-Si pairs in (c) Fe15CrSi and (d) Fe15CrNiSiP, Ni-Ni pairs in (e) Fe15CrNi and (f) Fe15CrNiSiP. The black curves correspond to the 1NN distribution of X-X pairs in the corresponding random alloys. Dashed-lines mark the maximum 1NN values measured in alloys with only one element. The dotted lines correspond to the maximum level observed in alloys with only one specie and are reported on the graphs obtained for the alloy with the three species for comparison.

Figure 8 :

 8 Figure 8: Fe15CrNiSiP irradiated at 2.5 dpa. Both SRCs and dislocation loops are visible. All Si, Ni and P atoms are presented. Fe and Cr atoms in SRCs are visible (concentration threshold used: Ni+Si+P > 3 at.%). Difference between (a) and (b) is only the orientation of the volume. Depending on the orientation of the loops in the volume, loops can appear as SRC in the image. The hole in the middle of the segregated zone is not always visible. A video is available in supplementary materials.

Figure 9

 9 which presents the 3D distribution maps of Si, Ni and C in Fe15CrSi and Fe15CrNi alloys irradiated at 2.5 dpa. These images together with 1NN and 1NN curves for C-Si and C-Ni pairs (Figure 9) reveal the co-segregation of Ni and Si with C, but do not exclude that Ni or Si or C form some clusters (a) (b)

Figure 9 :

 9 Figure 9: 3D-distributions of (a) Si and C atoms in Fe15CrSi and (b) Ni and C atoms in Fe15CrNi after irradiation at 2.5 dpa at 300 °C. 1NN and 1NN distributions at 2.5 dpa for (c) C-Si pairs in Fe15CrSi and in (d) C-Ni pairs in Fe15CrNi.

Table 1 -

 1 Intragranular compositions of the Fe-15at.%Cr-X (X = NiSiP, Ni, Si, P) model alloys measured by APT in at%. The uncertainties corresponds to the maximum of the difference between average concentration of X and the concentration of X measured in the different APT volumes.

	Short Name Cr (at%)	Ni (at%)	Si (at%)	P (at%)	Al (at%)	C (at%)	N (at%)
	Fe15CrNiSiP	14.8	0.098	0.42	0.065	0.056	0.011	0.03
	(G394)	± 0.4	± 0.012	± 0.05	± 0.024	± 0.004	± 0.014	± 0.02
	Fe15CrNi	15.55	0.099	0.013	0.003	0.070	0.010	0.08
	(G391)	± 0.11	± 0.001	± 0.013	± 0.001	± 0.001	± 0.004	± 0.02
	Fe15CrSi	15.06	0.006	0.422	0.002	0.068	0.006	0.02
	(G392)	± 0.18	± 0.001	± 0.019	± 0.001	± 0.001	± 0.02	± 0.03
	Fe15CrP	15.0	0.006	0.016	0.042	0.064	0.002	0.01
	(G393)	± 0.7	± 0.01	± 0.012	± 0.002	± 0.001	± 0.001	± 0.02

Table 3 :

 3 C concentration before irradiation (AR) and at 2.5 dpa.

		Fe15CrP	Fe15CrSi	Fe15CrNi	Fe15CrNiSiP
	AR	0.0020 ± 0.0003	0.006 ± 0.02	0.010 ± 0.009	0.01 ± 0.01
	2.5 dpa	0.07 ± 0.07	0.15 ± 0.04	0.11 ± 0.04	0.26 ± 0.04

At very low temperature point defects having very low mobility, no dragging of the solute towards the point defect sinks occurs. At high temperature back-diffusion is important removing concentration gradients. As a consequence, segregation is stronger at intermediate temperatures.

Acknowledgements

This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project). This research was partly funded by the Euratom's Seventh Framework Programme FP7/2007-2013 under grant agreement No. 604862 (MatISSE project) and contributes to the EERA (European Energy Research Alliance) Joint Programme on Nuclear Materials (JPNM). Experiments were performed on the GENESIS platform. GENESIS is supported by the Région Haute-Normandie, the Métropole Rouen Normandie, the CNRS via LABEX EMC and the French National Research Agency as a part of the program "Investissements d'avenir" with the reference ANR-11-EQPX-0020. The use of the HZDR Ion Beam Center facilities and the support by its staff is gratefully acknowledged.

Data Availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.


Tereza Horáčková

Manon H E Bousquet

Adrien Morice

Ugo Triballier

Gabriel Canard

Pavel Lhoták

Denis Jacquemin
email: denis.jacquemin@univ-nantes.fr

Simon Pascal
email: simon.pascal@cnrs.fr

Olivier Siri
email: olivier.siri@univ-amu.fr

Fully Zwitterionic Diaminobenzoquinonediimines Promoted by Cyanoaromatic N-substituents

2,5-Diamino-1,4-benzoquinonediimine (DABQDI) derivatives possessing two electron-withdrawing aromatic Nsubstituents can potentially exhibit a zwitterionic ground state in lieu of the expected canonical structure. It was previously shown that the use of nitroaromatics electron-withdrawing groups (EWG) could yield the quantitative formation of zwitterionic tautomer of DABQDI in polar solvents, while a mixture of canonical and zwitterionic forms was present in low polarity solvents. In this work, we report that the replacement of nitrocontaining EWG by weaker nitrile ones prevents the formation of canonical species in apolar solvents for the benefit of the zwitterionic tautomer. This counterintuitive observation is rationalized with theoretical calculations, which points out that the fully zwitterionic electronic structure of the nitrile-containing DABQDI arises from the absence of possible intramolecular hydrogen bonding between the cyanoaromatic Nsubstituent and the N-H proton of the bridge in the canonical form.

Introduction

π-Zwitterions are a class of organic molecules featuring conjugated cationic and anionic charges, providing them with unique electronic and optical properties allowing elaborating new materials for organic electronics and nonlinear optics [1][START_REF] Beverina | π-Conjugated Zwitterions as Paradigm of Donor-Acceptor Building Blocks in Organic-Based Materials[END_REF][START_REF] Lou | A Twist on Nonlinear Optics: Understanding the Unique Response of π-Twisted Chromophores[END_REF][4].

The design of ground-state π-zwitterions can be rather straightforward when considering squaraine [START_REF] Chen | Squaraine dyes for organic photovoltaic cells[END_REF][START_REF] Ilina | Squaraine Dyes: Molecular Design for Different Applications and Remaining Challenges[END_REF][START_REF] He | Squaraine Dyes for Photovoltaic and Biomedical Applications[END_REF] and parent zwitterionic polymethines [START_REF] Zheng | Striking Effect of Intra-versus Intermolecular Hydrogen Bonding on Zwitterions: Physical and Electronic Properties[END_REF][START_REF] Lynch | Croconaine Dyesthe Lesser Known Siblings of Squaraines[END_REF][START_REF] Nagao | Synthesis and properties of barbiturate indolenine heptamethinecyanine dyes[END_REF][START_REF] Taylor | Superprotonic conduction of intrinsically zwitterionic microporous polymers based on easy-to-make squaraine, croconaine and rhodizaine dyes[END_REF] that are omnipresent in the fields of photovoltaics, optical detection and bio-imaging.

However, expending the diversity of conjugated π-zwitterions is not trivial and only few strategies are emerging in the literature, notably by linking strongly electrondonating and electron-withdrawing groups (EWG) in twisted π-systems [START_REF] Lou | A Twist on Nonlinear Optics: Understanding the Unique Response of π-Twisted Chromophores[END_REF][START_REF] Shimizu | HOMO-LUMO Energy-Gap Tuning of π-Conjugated Zwitterions Composed of Electron-Donating Anion and Electron-Accepting Cation[END_REF] or in polycyclic heteroaromatic compounds such as quinacridine derivatives [START_REF] Pascal | Zwitterionic[END_REF][START_REF] Delgado | C-Functionalized Cationic Diazaoxatriangulenes: Late-Stage Synthesis and Tuning of Physicochemical Properties[END_REF], naphthalenediimides [START_REF] Kumar | Doubly zwitterionic, direduced, highly electron-rich, air-stable naphthalenediimides: redox-switchable islands of aromaticantiaromatic states[END_REF] and azoniadibenzo[a,j]phenalenides [START_REF] Arikawa | Azoniadibenzo[a,j]phenalenide: A Polycyclic Zwitterion with Singlet Biradical Character[END_REF], which often yields photophysical properties in red to near-infrared ranges. Another remarkable strategy towards π-zwitterions is based on the design of rigid polycyclic antiaromatic species that are prompt to form stable ground-state zwitterions (Figure 1), such as tetraphenylhexaazaanthracenes (TPHA) [START_REF] Hutchison | Tetraphenylhexaazaanthracene: A Case for Dominance of Cyanine Ion Stabilization Overwhelming 16π Antiaromaticity[END_REF][START_REF] Langer | 3,5,7,9-Tetraphenylhexaazaacridine: A Highly Stable, Weakly Antiaromatic Species with 16 π Electrons[END_REF][START_REF] Constantinides | Tetraphenylhexaazaanthracenes: 16π Weakly Antiaromatic Species with Singlet Ground States[END_REF][START_REF] Zissimou | Oxidation of Tetraphenylhexaazaanthracene: Accessing a Scissor Dimer of a 16π Biscyanine[END_REF], tetraazapentacenes (TAP) [START_REF] Wudl | Polyazaacenes: new tricks for old dogs[END_REF][START_REF] Koutentis | Regiospecific synthesis of 5,7-disubstituted quinoxalino[2,3-b]phenazines[END_REF][START_REF] Guevara-Level | First principles investigation of the spectral properties of neutral, zwitterionic, and bis-cationic azaacenes[END_REF] and triazino-phenazine hybrid derivatives [START_REF] Ioannou | Some cyclization reactions of 1,3diphenylbenzo[END_REF], bisdithiazoles [START_REF] Beer | Antiaromatic Bis(1,2,3dithiazoles) with Zwitterionic Ground States[END_REF][START_REF] Winter | Thermal conversion of a pyridine-bridged bisdithiazolyl radical to a zwitterionic bisdithiazolopyridone[END_REF] or benzoquinone dithiazoles (BDT) [START_REF] Mailman | The Power of Packing: Metallization of an Organic Semiconductor[END_REF][START_REF] Lekin | Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals[END_REF]. Besides, benzoquinone monoimine (BQMI) derivatives are the most simple quinoidal molecules that feature a ground state zwitterionic structure due to the formation of two polymethine subsystems: one trimethine cyanine being positively charged and one negatively charged trimethine oxonol [START_REF] Siri | Unprecedented zwitterion in quinonoid chemistry[END_REF][START_REF] Braunstein | A 6π + 6π Potentially Antiaromatic Zwitterion Preferred to a Quinoidal Structure: Its Reactivity Toward Organic and Inorganic Reagents[END_REF][START_REF] Yang | First transamination reactions for the one-pot synthesis of substituted zwitterionic quinones[END_REF]. The delocalization of the charges brings to the BQMI unique absorption properties in the visible range, and up to the NIR upon introduction of strong electron-donating N-substituents [START_REF] Bonneau | An Unprecedented Blue Chromophore Found in Nature using a "Chemistry First" and Molecular Networking Approach: Discovery of Dactylocyanines A-H[END_REF][START_REF] Ruiz | Small Panchromatic and NIR Absorbers from Quinoid Zwitterions[END_REF]. We recently described that 2,5-diamino-1,4-benzoquinonediimines (DABQDI, Figure 1) can adopt either canonical (uncharged) or zwitterionic forms following an intramolecular proton transfer from one 6π-electrons subunit to the other (Figure 2) [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF][START_REF] Pascal | Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles[END_REF]. Two main parameters can favour the zwitterionic tautomer of DABQDI, namely the presence strong EWG substituents on one of the two trimethine subunits, and a highly polar environment, both parameters logically allowing the easier formation of a charge separated structure. For instance, the introduction of two fluorodinitrobenzene strong EWG in DABQDI 1 (Chart 1) promotes a fully zwitterionic ground state of the quinone in polar solvents only, e.g., dichloromethane, acetone, and dimethylsulfoxide. Nevertheless, when the compound is dissolved in any solvent of lower polarity, a mixture of both canonical and zwitterionic tautomers is formed in solution. While this equilibrium found utility in the development of a colourful vapochromic probe for the detection polar volatile organic compounds [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF], further applications of these chromophores, notably in molecular electronics, would require the use of a material exclusively present in its zwitterionic form to guarantee the strong dipole moment that is beneficial for the development of interlayer molecular films in photovoltaic devices and transistors [START_REF] Liu | Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics[END_REF][START_REF] Mahmood | Tuning graphene transistors through ad hoc electrostatics induced by a nanometer-thick molecular underlayer[END_REF][START_REF] Luczak | The Quinonoid Zwitterion Interlayer for the Improvement of Charge Carrier Mobility in Organic Field-Effect Transistors[END_REF]. For this purpose, we investigate herein the replacement of nitro EWG in the original DABQDI 1 with nitrile functions in the compounds 2-4 (Chart 1). The introduction of the fluoroisophthalonitrile N-substituents impacts the electronic and photophysical properties of the chromophores and promotes the zwitterionic tautomer of quinones 2 and 3 that persists in various solvents, despite the fact that the cyano groups are widely considered as less electron-withdrawing than the nitro ones. These experimental findings are then rationalized through theoretical calculations.

Experimental

Analytical methods and apparatus

The melting points (M.P.) of the vacuum-dried powders were measured in open capillary tubes with a STUART SMP30 melting points apparatus and are uncorrected. NMR spectra were recorded on a JEOL ECS400 NMR spectrometer at room temperature (except when noted otherwise). 1 H and 13 C NMR chemical shifts are given in ppm (δ) relative to Me 4 Si with solvent resonances used as internal standard (CDCl 3 : 7.26 ppm for 1 H and 77.2 for 13 C). NMR peak assignments were confirmed using a DEPT-135 method. IR spectra were recorded on an Agilent Cary 630 FTIR equipped with an attenuated total reflectance (ATR) sampling. Electronic absorption spectra were recorded in spectroscopic grade solvents on a VARIAN CARY 50 SCAN spectrophotometer at room temperature with a 300 nm/min scan rate using 1 cm path quartz cuvettes. HRMS analyses were performed on a QStar Elite (Applied Biosystems SCIEX) or a SYNAPT G2 HDMS (Waters) spectrometers by the "Spectropole" of the Aix-Marseille University. These two instruments are equipped with an ESI or MALDI source, and a TOF mass analyzer.

Cyclic voltammetry (CV) data were recorded using a BAS 100 (Bioanalytical Systems) potentiostat and the BAS100W software (v2.3). All experiments were conducted under an argon atmosphere in a standard one-compartment using a three electrodes setup: a Pt working electrode (Ø = 1.6 mm), a Pt counter electrode and a leak-free Ag/AgCl reference electrode of 5 mm diameter. Tetra-n-butylammonium hexafluorophosphate ([ n Bu 4 ][PF 6 ]) was used as supporting electrolyte (10 -1 M), with a concentration of the electro-active compound of ca. 10 -3 M. The reference electrode was calibrated using ferrocene (E°(Fc/Fc + ) = 0.46V/SCE in DCM) [START_REF] Connelly | Chemical Redox Agents for Organometallic Chemistry[END_REF]. The scan rate was 100 mV/S. The solution was degassed using argon before recording each reductive scan, and the working electrode (Pt) was polished before each scan recording.

Materials and synthetic procedures

Compounds 1 [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF], 5 [START_REF] Lavaud | Fused bis-azacalixphyrin that reaches NIR-II absorptions[END_REF], and 6 [START_REF] Cho | Cool and warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as blue emitter and triplet host[END_REF] were prepared following previously reported protocols. All the reactions were carried out under argon atmosphere. Reagents were purchased from Sigma-Aldrich and used as received. When heating was required, oil bathes were used. Column chromatography were performed using Silica 60 M (0.04-0.063 mm). Synthesis of 7. Compounds 5 (100 mg, 0.230 mmol) and 6 (75 mg, 0.457 mmol) were dissolved in 10 mL of anhydrous acetonitrile under argon. The solution was cooled to 0 °C and degassed by argon bubbling for 15 min. Then, N,Ndiisopropylethylamine (0.24 mL, 1.38 mmol) was added dropwise to the solution at 0 °C. The reaction mixture was then stirred at 25 °C for 30 min and finally heated to reflux overnight. The crude solution was then evaporated under reduced pressure.

The residue was purified by column chromatography over silica gel using pure dichloromethane then dichloromethane with 1% of methanol as eluent to afford 7 in 60% yield (90 mg, 0.138 mmol) as a beige solid. Rf = 0.12 (SiO 2 , DCM). M. P.: 210-212 °C. ), found: 505.3452.

Theoretical calculations

All calculations have been carried out on structures using methyl groups instead of the longer alkyl chains and have been performed with Gaussian16 [START_REF] Frisch | Gaussian 16, revision A.03[END_REF], using default procedures and algorithms except when noted below. We have selected the PBE0 [START_REF] Adamo | Toward reliable density functional methods without adjustable parameters: The PBE0 model[END_REF] global hybrid functional for the ground-state calculations, and applied tighten thresholds for the energy (at least 10 -9 au) and geometry convergences (socalled tight threshold in Gaussian), as well as used an improved DFT integration grid (ultrafine grid). For all compounds, the ground-state forces were minimized with the 6-31G(d) (for early conformational screening) and 6-311++G(2d,2p) (for the presented results on the key structures) atomic basis sets. We subsequently verified the absence of imaginary frequencies by computing analytically the Hessian at the same level of theory. The excited-state calculations were performed with Time-Dependent Density Functional Theory (TD-DFT) considering 5-to-15 states determined with the CAM-B3LYP long-range corrected functional [START_REF] Yanai | A new hybrid exchange-correlation functional using the Coulombattenuating method (CAM-B3LYP)[END_REF] (more suited for charge-transfer states) and the same 6-311++G(2d,2p) atomic basis set. The solvent effects were modeled through the Polarizable Continuum Model (PCM) [START_REF] Tomasi | Quantum Mechanical Continuum Solvation Models[END_REF],

using the linear-response non-equilibrium model for the excited-state calculations.

The density difference plots representing the excited states have been drawn by computing the excited state density with the Z-vector approach. Charge-transfer parameters were determined using Le Bahers' model as implemented in Gaussian16

relaxed excited-state densities have been used [START_REF] Bahers | A Qualitative Index of Spatial Extent in Charge-Transfer Excitations[END_REF].

RESULTS AND DISCUSSION

Synthesis

The syntheses of the nitrile-containing targets were carried out via nucleophilic aromatic substitution of tetraaminobenzene ( 5) on 4,6-difluoroisophthalonitrile (6) in refluxing acetonitrile (Scheme 1). When one equivalent of 5 and two equivalents of 6

were used, the intermediate 7 was isolated in 60% yield after purification by silica gel column chromatography. To reach the hybrid compound 10 substituted with both nitrile-and nitro-containing aromatics, the synthesis of the intermediate 8 was envisaged following the reaction of equimolar quantities of 5 and 6. However, as a result of the spontaneous aerobic oxidation of 8 during the reaction workup, the quinone 4 was isolated as a brown solid, the structure of which was confirmed by the high-resolution mass spectrum analysis that shows molecular ion peak with m/z 505.3452, corresponding to [4+H] + . It is noteworthy that such [1+1] substitution adduct was rarely isolated previously using 1,5-difluoro-2,4-dinitrobenzene 9 as electrophile, notably due to the intramolecular NH … O 2 N bonds strongly favouring the [1+2] adduct (the aromatic precursor of 1) [START_REF] Chen | N-Substituted Azacalixphyrins: Synthesis, Properties, and Self-Assembly[END_REF]. The poor stability of 4 regarding oxidation was also previously noticed for tetraaminobenzene derivatives substituted with weak electron-withdrawing aromatics [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF][START_REF] Longevial | Stabilization of a 12-π electrons diamino-benzoquinonediimine tautomer[END_REF]. This observation is easily explained by the presence of a single fluoroisophthalonitrile moiety in 8 compared to the presence of two similar substituents in the more stable 7. Consequently, the synthesis of 10 was undertaken without isolation of 8, following a two steps in one pot strategy that afforded 10 in a moderate 39% yield, the first substitution being the limiting step. Attempts to promote the nucleophilic aromatic substitution by increasing the reaction time or changing the temperature did not improve the yield of 10. Finally, compounds 7 and 10 were readily oxidized in presence of two equivalents of p-chloranil to afford the quinones 2 and 3 in 78% and 91% yields, respectively, the green coloration of their powders indicating the presence of zwitterionic species (vide infra).

Scheme 1. Synthesis of quinones 2-4.

Electronic properties

The cyclic voltammograms of 1-4 were recorded in DCM and show irreversible first reduction processes at -0.84, -0.90 and -0.99 V vs. Fc/Fc + , for compounds 1, 3

and 2, respectively (Figure 3 and Table 1). This order follows the decrease in the electron-withdrawing strength of the N-substituents, the reduction being facilitated for the nitro-containing species. In this series, the oxidation waves are poorly shifted compared to 1, nevertheless reversible processes are found at ca. 0.68 V upon introduction of nitrile functions in 2 and 3, instead of the irreversible process observed for 1. The quinone 4 exhibits irreversible processes, the reduction and oxidation waves are respectively found at -1.29 and 0.49 V, suggesting a canonical tautomer, by comparison with the previously reported 1,3bis(trifluoromethyl)benzene-containing DABQDI derivative featuring a canonical ground state and irreversible redox processes found at -1.29 and 0.44 V Fc/Fc + in the same solvent [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF]. Irreversible peak potential at a scan rate of 100 mV s -1 . c ΔE = E pa -E pc expressed in mV.

When the two tautomers are close in energy, the canonical-zwitterionic equilibrium is impacted by the polarity of the environment in both solution and solid states [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF][START_REF] Pascal | Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles[END_REF]: the more polar the medium is, the more intense the band centred at 700 nm is.

This low energy hallmark transition is attributed to an intramolecular charge-transfer (ICT) between the negatively and positively charged moieties of the zwitterionic tautomer and any intensity increase observed on this band while increasing the solvent polarity implies an increasing proportion of the zwitterionic species, as illustrated with nitro-containing quinone 1 in Figure 4. Indeed, while increasing the solvent polarity, the lowest energy band of 1 undergoes a significant hyperchromic effect, with ε 705 ~ 2400 M -1 .cm -1 in toluene and ε 697 ~ 6900 M -1 .cm -1 in DCM.

Surprisingly, such shift is not observed for the nitrile-containing quinone 2 with ε ca. 7500 M -1 .cm -1 in these two solvents, apparently indicating the exclusive presence of the zwitterionic tautomer in solution in all solvents. This hints that the canonical tautomer is significantly less stable in 2 than 1, which might look counterintuitive as the nitro accepting strength exceeds its nitrile counterpart [START_REF] Hansch | A survey of Hammett substituent constants and resonance and field parameters[END_REF], and one would naively foresee the opposite behaviour, as the strongest acceptors should, in principle, stabilize the charge-separated zwitterionic isomer. Less surprisingly, 3

featuring mixed nitrile and nitro EWG exhibits an intermediate behaviour, the lowest energy band intensity varying between 3400-5300 M -1 .cm -1 , going from toluene to DCM. The spectrum of compound 4 does not show any band centred at 700 nm but a broad and weak shoulder between 500-700 nm, which indicates the predominance of the canonical form in solution irrespective of the solvent. This observation is in line with the brown powder coloration of 4 and the cyclic voltammogram of this compound that notably differs from the ones recorded for the ground state zwitterions 1-3. Nevertheless, the slight bathochromic shift of the lower energy band observed in DMSO (ε 600 ~ 800 M -1 .cm -1 ) could be attributed to the presence of a trifling portion of zwitterion. Finally, we note that the pH-dependent absorption properties of the quinones 2-4 revealed the possibility to generate cationic and anionic quinones in solution, as previously reported [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF] (see Supporting Information for details). To specifically assess the impact of the N-substituents, the electronic absorptions of 1-4 were compared in the least polar solvent (Figure 5). The quinone 4 only presents the canonical tautomer in toluene, the fluoroisophthalonitrile moiety being insufficiently electron-withdrawing to promote the zwitterionic form. In contrast, compounds 1-3 present the zwitterion signature in the red region and, surprisingly, the intensity of the band is sensibly increased upon replacement of the nitro functions by the nitrile ones. As stated above, such result is unexpected since the stronger electron-withdrawing nitroaromatic substituents would be expected to more efficiently promote fully zwitterionic species compared to the cyanoaromatic ones. To study the influence of the nitrile and nitro moieties on the canonical-zwitterionic equilibrium in more details, it was deemed necessary to carry out an in silico investigation.

Theoretical study

To obtain more insights into the experimental results, we have carried out theoretical calculations using DFT and TD-DFT, with a methodology detailed in the experimental section. This approach was used before for similar species [START_REF] Pascal | Controlling the canonical/zwitterionic balance through intramolecular proton transfer: a strategy for vapochromism[END_REF][START_REF] Pascal | Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles[END_REF].

One key aspect here is the energetic balance between the canonical and zwitterionic tautomers (Figure 2). Of course, DFT is not infinitely accurate with typical errors on relative free energies of similar species of ca. 1-3 kcal.mol -1 (which can lead to significant changes in Boltzmann populations). Nevertheless, we do trust that the trends in the series are correct. All our results are detailed in the SI and we focus here on the main aspects helping to grasp the unexpected measurements. First, for all structures TD-DFT calculations clearly confirm that the presence of the redshifted absorption at ca. 700 nm experimentally is a signature of the zwitterionic tautomer, none of the canonical forms absorbing in that region (see the SI).

We have already considered 1 in a previous work [START_REF] Pascal | Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles[END_REF], so we can likely be brief. In toluene and DMSO we found that the most stable zwitterionic structure is stabilized by 2.9 and 5.0 kcal.mol -1 as compared to its canonical counterpart. Considering the above-state error bar, this indicates that the zwitterion is almost exclusively present in DMSO, whereas a small portion of canonical tautomer is present in toluene. This is qualitatively consistent with the experimental spectrum displayed in Figure 4, which shows that the characteristic zwitterionic band is significantly more intense in the most polar solvents.

For 2, we performed a complete conformational search for both tautomeric forms.

In DMSO, the difference of free energies between the most stable zwitterion and canonical structures attains 6.8 kcal.mol -1 . The corresponding difference is 4.6 kcal.mol -1 in toluene. In other words, the zwitterionic tautomer is even more stabilized than in 1. This theoretical prediction fits experiment, as Figure 4 clearly suggests that the zwitterion is systematically present in 2 irrespective of the solvent, indicating that it is strongly favoured. However, as evoked previously, this result is surprising since the strongest accepting group (i.e., the nitro) should normally ease the formation of the charge-separated structure (i.e., the zwitterion) in the ground electronic state, so one would expect the canonical form to become relatively more stabilized in 2 than in 1. By examining the actual geometries predicted by DFT, we could rationalize this outcome. Indeed, as can be seen in Figure 6, in the canonical form of 1, a bifurcated hydrogen bond absent in the zwitterion is present [START_REF] Feldblum | Strength of a bifurcated H bond[END_REF][START_REF] Chipanina | Intramolecular and intermolecular bifurcated hydrogen bonds in 2-pyrrolyl-7-hydroxy-2-methylidene-2,3-dihydro-1H-inden-1-one[END_REF], as the transferred hydrogen atom is in close contact with one of the oxygen atom of the vicinal ortho nitro group. Such interaction is absent in where the transferred hydrogen atom is more than 2.8 Å away from the nitrogen atom of the cyano group. This extra stabilizing interaction, counterbalancing the purely electronic effect, is present in 1 but absent in 2, which likely explains why the formation of the canonical structure can be observed in the former compound and not the latter. This analysis is further strengthened by the results obtained for 3 (see the SI for details). Indeed, on the one hand, the most stable canonical form displays a hydrogen atom on the nitro side (the transfer on the nitrile side leads to a structure less stable by 0.7 kcal.mol -1 ), and, on the other hand, this structure is less stable than its zwitterionic counterpart by only 4.6 (2.6) kcal.mol -1 . These values are very similar to the one found for 1, hinting that 3 and 1 should have similar behaviour, which is indeed clear from Figure 5. Considering all computed conformers, this leads to a slight presence of the canonical form in the apolar solvent (see the SI).

Finally for 4, theory predicts a 98/2 (99/1) canonical/zwitterion blend in neutral DMSO (DCM), the predominance of the canonical tautomer being a logical consequence of the presence of only two accepting groups instead of four. As for the other compounds, TD-DFT predicts that the zwitterionic lowest absorption is both (much) more intense and redshifted as compared to the canonical one (see the SI), and therefore, the very weak absorption above 600 nm in the experimental measurements (Figure 4) is a signature of a tiny presence of zwitterion in the solution.

CONCLUSION

The synthesis of fluoroisophthalonitrile-containing DABQDIs was achieved and their spectroscopic properties revealed a fully zwitterionic structure in solution for 2 and, to a lesser extent, for the hybrid nitro/cyano compound 3, while the fluorodinitrobenzene-containing analogue 1 showed the presence of a mixture of zwitterionic and canonical species in solution. These results are counterintuitive at first glance since nitro EWG are stronger acceptor than cyano groups, the former should promote to a greater extent the zwitterionic tautomer. However, the

Figure 1 .

 1 Figure 1. π-zwitterions built around antiaromatic 6-membered rings.

Figure 2 .Chart 1 .

 21 Figure 2. Equilibrium between the canonical and zwitterionic tautomers in 2,5diamino-1,4-benzoquinonediimines (EWG = electron-withdrawing group).

1 H

 1 NMR (400 MHz, CDCl 3 ): δ = 7.75 (d, JH-F = 6.7 Hz, 2H, CH), 6.80 (s, 1H, CH), 6.29 (s, 2H, NH), 6.27 (d, JH-F = 11.5 Hz, 2H, CH), 5.98 (s, 1H, CH), 3.97 (t, J = 5.1 Hz, 2H, NH), 3.18 -3.13 (m, 4H, NH-CH2), 1.63 -1.60 (m, 4H, CH2), 1.32 -1.27(m, 20H, CH2), 0.89 (t, J = 6.9 Hz, 6H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl 3 ): δ = 166.7 (C, JC-F = 262.8 Hz), 155.2 (C, JC-F = 13.5 Hz), 146.8 (C), 138.9 (CH, JC-F = 3.7 Hz), 128.6 (CH), 115.1 (C), 113.1 (C), 109.9 (C), 100.6 (CH, JC-F = 23.4 Hz), 93.8 (C), 93.2 (CH), 91.6 (C, JC-F = 18.3 Hz), 43.5 (N-CH2), 31.9 (CH2), 29.5 (CH2), 29.4 (2 CH2), 27.3 (CH2), 22.8 (CH2), 14.3 (CH3). IR (neat, cm -1 ): ν = 3356, 3240, 3081, 3048, 2923, 2850, 2226, 1632, 1569, 1521, 1449, 1419, 1375, 1323, 1296, 1256, 1215, 1172, 1099, 938, 895, 836, 756, 722, 669. HRMS (ESI+) calculated for [M+H] + : 651.3730 (C 38 H 45 F 2 N 8 + ), found: 651.3731. Synthesis of 10. Compounds 5 (270 mg, 0.62 mmol) and 6 (100 mg, 0.609 mmol)were dissolved in 13 mL of anhydrous acetonitrile under argon. The solution was degassed by argon bubbling for 15 min. Then, N,N-diisopropylethylamine (0.64 mL, 3.67 mmol) was added dropwise to the solution. The reaction mixture was stirred at 25 °C for 5 min and then heated to reflux for 1.5 h. After the reaction mixture was cooled down to 25 °C, 1,5-difluoro-2,4-dinitrobenzene 9 (120 mg, 0.588 mmol) was added under argon and the reaction was continued at 25 °C overnight. Then, the solvent was evaporated under reduced pressure and the crude product was purified by column chromatography over silica gel using dichloromethane as eluent. The product 10 was isolated in 39% yield (169 mg, 0.245 mmol) as an orange solid. Rf = 0.34 (SiO 2 , DCM). M. P.: 204-206 °C. 1 H NMR (400 MHz, CDCl 3 ): δ = 9.27 (s, 1H, NH), 9.16 (d, JH-F = 7.8 Hz, 1H, CH), 7.75 (d, JH-F = 6.8 Hz, 1H, CH), 6.82 (s, 1H, CH), 6.52 (d, JH-F = 13.2 Hz, 1H, CH), 6.32 (s, 1H, NH), 6.30 (d, JH-F = 11.2 Hz, 1H, CH), 6.02 (s, 1H, CH), 4.00 (t, J = 5.2 Hz, 1H, NH), 3.92 (t, J = 5.4 Hz, 1H, NH), 3.19 -3.15 (m, 4H, NH-CH2), 1.65 -1.60 (m, 4H, CH2), 1.33 -1.27 (m, 20H, CH2), 0.90 -0.86 (m, 6H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl 3 ): δ = 166.7 (C, JC-F = 264.8 Hz), 160.0 (C, JC-F = 272.5 Hz), 155.1 (C, JC-F = 14.1 Hz), 150.1 (C, JC-F = 14.1 Hz), 146.8 (C), 146.1 (C), 138.9 (CH), 128.2 (CH), 128.0 (C), 127.9 (CH), 127.4 (C, JC-F = 9.7 Hz), 115.1 (C), 113.0 (C), 110.1 (C), 109.6 (C), 103.8 (CH, JC-F = 30.2 Hz), 100.6 (CH, JC-F = 30.2 Hz), 93.9 (C), 93.3 (CH), 91.6 (C, JC-F = 19.7 Hz), 43.61 (N-CH2),

13 C{ 1

 131 H} NMR (101 MHz, CDCl 3 ): δ = 165.5 (C, JC-F = 267.4 Hz), 159.1 (C, JC-F = 10.6 Hz), 158.3 (C, JC-F = 272.5 Hz), 157.1 (C, JC-F = 12.7 Hz), 151.6 (C, JC-F = 11.5 Hz), 150.3 (C), 149.6 (C), 140.95 (C), 138.7 (CH, JC-F = 2.8 Hz), 137.8 (C), 130.1 (C), 125.5 (CH), 115.9 (C), 113.1 (C), 111.5 (CH, JC-F = 23.3 Hz), 108.8 (CH, JC-F = 20.3 Hz), 104.9 (C), 95.3 (C, JC-F = 17.9 Hz), 87.0 (CH), 83.7 (CH), 44.4 (N-CH2), 43.

Synthesis of 4 .

 4 Compounds 5 (270 mg, 0.620 mmol) and 6 (100 mg, 0.609 mmol) were dissolved in 17 mL of anhydrous acetonitrile under argon. The solution was degassed by argon bubbling for 15 min. Then, N,N-diisopropylethylamine (0.63 mL, 3.62 mmol) was added dropwise to the solution. The reaction mixture was stirred at 25 °C for 5 min and then heated to reflux for 1.5 h. After the reaction mixture was cooled down to 25 °C, the solvent was then evaporated under reduced pressure.The crude residue was purified by column chromatography over silica gel using dichloromethane with 1% of methanol as eluent to afford 4 in 28% yield (85.4 mg, 0.169 mmol) as a brown solid. Rf = 0.21 (SiO 2 , DCM/MeOH, 99/1). M. P.: 123-125 °C. 1 H NMR (400 MHz, CDCl 3 ): δ = 7.88 (d, JH-F = 6.8 Hz, 1H, CH), 6.82 (d, JH-F = 10.0 Hz, 1H, CH), 5.98 (br s, 1H, NH), 5.35 (s, 1H, CH), 5.20 (s, 1H, CH), 3.50 (t, J = 6.9 Hz, 4H, N-CH2), 3.10 (t, J = 7.2 Hz, 4H, N-CH2), 1.75 -1.65 (m, 4H, CH2), 1.42 -1.25 (m, 20H, CH2), 0.90 -0.86 (m, 6H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl 3 ): δ = 165.2 (C, JC-F = 267.5 Hz), 161.1 (C, JC-F = 10.5 Hz), 157.33 (C, JC-F = 7.9 Hz), 157.31 (C), 153.1 (C, JC-F = 3.3 Hz), 152.2 (C), 138.7 (CH), 115.1 (C), 112.9 (C), 109.9 (CH, JC-F = 21.5 Hz), 103.9 (C), 96.

Figure 3 .Table 1 .

 31 Figure 3. Cyclic voltammograms of compounds 1-4 in DCM solutions containing 0.1 M of [ n Bu 4 N][PF 6 ].

Figure 4 .

 4 Figure 4. Electronic absorption solvatochromism of compounds 1-4 (ca. 10 -5 M).

Figure 5 .

 5 Figure 5. Comparison of the electronic absorption of compounds 1-4 in toluene.

Figure 6 .

 6 Figure 6. DFT structures of the most stable conformers of the zwitterionic (top) and canonical (bottom) tautomers of 1, 2, and 3 (from left to right) in DMSO. Significant hydrogen bonds (Å) are indicated. In the canonical 2, the shortest distance between the (transferred) hydrogen atom and the nitrogen of the cyano group is 2.86 Å.

13 C{ 1 H} NMR

  43.55 (N-CH2), 31.9 (CH2), 29.44 (CH2), 29.36 (CH2), 27.3 (CH2), 22.8 (CH2), 14.2

	1052, 908, 839, 811, 757, 674. HRMS (ESI+) calculated for [M+H] + : 649.3573
	(C 38 H 43 N 8 F 2 + ), found: 649.3570.
	(CH3). IR (neat, cm -1 ): ν = 3362, 3267, 3049, 2924, 2853, 2232, 2110, 1634, 1624,
	Synthesis of 3. Compound 10 (20 mg, 0.029 mmol) was dissolved in 1 mL of
	1580, 1524, 1480, 1454, 1413, 1375, 1334, 1296, 1261, 1215, 1169, 1127, 1054,
	938, 877, 836, 742, 713, 691, 671. HRMS (ESI+) calculated for [M+H] + : 691.3526 degassed chloroform and p-chloranil (16 mg, 0.066 mmol) was added to the solution.
	(C 36 H 45 N 8 O 4 F 2 + ), found: 691.3526. After the reaction mixture was stirred at 25 °C for 30 min, the solvent was
	evaporated under reduced pressure. The crude product was purified by preparative
	= 20.5 Hz), 105.2 (C, JC-F = 2.03 Hz), 95.1 (C, JC-F = 18.10 Hz), 86.53 (CH), 83.46
	(CH), 43.9 (N-CH2), 31.9 (CH2), 29.24 (CH2), 29.19 (CH2), 28.3 (CH2), 27.1 (CH2),
	22.7 (CH2), 14.2 (CH3). IR (neat, cm -1 ): ν = 3367, 3277, 3226, 2923, 2853, 2579,
	2223, 1606, 1591, 1545, 1503, 1453, 1421, 1329, 1290, 1268, 1230, 1146, 1087,

Synthesis of 2. Compound 7 (30 mg, 0.046 mmol) was dissolved in 1 mL of degassed chloroform and p-chloranil (12 mg, 0.049 mmol) was added to the solution.

After the reaction mixture was stirred at 25 °C for 30 min, the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography over silica gel using pure dichloromethane as eluent, affording compound 2 in 78% yield

(23.3

mg, 0.036 mmol) as a dark green solid. Rf = 0.13 (SiO 2 , DCM). M. P.: 183-185 °C. 1 H NMR (400 MHz, CDCl 3 ): δ = 8.64 (br s, 2H, NH), 7.82 (d, JH-F = 7.1 Hz, 2H, CH), 6.86 (d, JH-F = 11.1 Hz, 2H, CH), 5.46 (s, 1H, CH), 5.36 (s, 1H, CH), 3.43 (t, J = 7.2 Hz, 4H, N-CH2), 1.84 -1.77 (m, 4H, CH2), 1.49 -1.43 (m, 4H, CH2), 1.35 -1.25 (m, 16H, CH2), 0.90 -0.86 (m, 6H, CH3). (101 MHz, CDCl 3 ): δ = 165.5 (C, JC-F = 266.5 Hz), 159.2 (C, JC-F = 10.3 Hz), 157.7 (C), 150.2 (C), 138.7 (CH, JC-F = 3.4 Hz), 115.99 (C), 113.1 (C), 108.8 (CH, JC-F thin layer chromatography over silica gel using dichloromethane as eluent to afford 3 in 91% yield (18.1 mg, 0.026 mmol) as a dark green solid. Rf = 0.34 (SiO 2 , DCM). M. P.: 114-116 °C. 1 H NMR (400 MHz, CDCl 3 ): δ = 8.82 (d, JH-F = 7.8 Hz, 1H, CH), 8.56 (br s, 2H, NH), 7.82 (d, JH-F = 6.9 Hz, 1H, CH), 6.94 (d, JH-F = 12.1 Hz, 1H, CH), 6.82 (d, JH-F = 10.5 Hz, 1H, CH), 5.39 (s, 1H, CH), 5.38 (s, 1H, CH), 3.46 -3.40 (m, 4H, N-CH2), 1.84 -1.77 (m, 4H, CH2), 1.46 -1.25 (m, 20H, CH2), 0.90 -0.87 (m, 6H, CH3).

Acknowledgements

This work was supported from the grant of Specific university researchgrant no. A2 FCHT 2022 098, and by the Agence Nationale de la Recherche, in the frame of the SOCOOL project. We acknowledge the Centre National de la Recherche Scientifique and the ministère de l'enseignement supérieur et de la recherche. P.L. thanks the Czech Science Foundation (20-08667S) for financial support. This work used the computational resources of the GliCid/CCIPL computational mesocenter installed in Nantes.

computational study brings evidences that, while there is a possible interaction between the transferrable hydrogen atom of the quinone and the oxygen atom of the closest nitro function that relatively stabilises the canonical form in 1, the fully zwitterionic electronic structure of 2 arises from the absence of intramolecular hydrogen bonding between the closest nitrile group and the transferred H atom (i.e. absence of bifurcated H-bonding interactions). These results bring evidences that the use of cyanoaromatic EWG is relevant for the future design of stable and fully zwitterionic DABQDI-based architectures such as azacalixquinarene macrocycles.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at xxx.