Pablo Simón Marqués

José María

Andrés Castán

Laura Abad Galan

Magali Allain

Olivier Maury

Tangui Le Bahers

Philippe Blanchard

Clément Cabanetos

Laura Abad

Pablo Simón Marqués

Investigation of the K 4 [Fe(CN) 6 ] mediated mono-and bispalladium-catalyzed cyanation of the benzothioxanthene core

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

 Introduction

Over the decades, naphthalene and perylene imide derivatives have attracted considerable research attention and have emerged as iconic representatives of the rylene family. [START_REF] Nowak-Król | Progress in the synthesis of perylene bisimide dyes[END_REF][START_REF] Feng | Synthesis and Application of Rylene Imide Dyes as Organic Semiconducting Materials[END_REF][START_REF] Chen | Beyond perylene diimides: synthesis, assembly and function of higher rylene chromophores[END_REF][START_REF] Huang | Perylene-3,4,9,10tetracarboxylic Acid Diimides: Synthesis, Physical Properties, and Use in Organic Electronics[END_REF][START_REF] Guo | Imide-and Amide-Functionalized Polymer Semiconductors[END_REF] Conversely, and despite promising optical properties, the N-(alkyl)benzothioxanthene-3,4-dicarboximide (BTXI) remained almost unnoticed, [START_REF] Grayshan | Heterocyclic derivalives of naphthalene-1,8-dicar boxylie anhydride. Part III. Benzo[k,l] thioxanthene-3,4-dicarboximides[END_REF] in particular by the organic electronic community. Moreover, their exclusive functionalization on the N-position (imide) restricted their use in bio-related [START_REF] Mao | Benzothioxanthene dyes as fluorescent label for DNA hybridization: synthesis and application[END_REF][START_REF] Qian | Synthesis and properties of benzothioxanthene dicarboximide hydroperoxide: an efficient 'time-resolved' DNA photocleaver with long-wavelength[END_REF] and sensing applications. [START_REF] Zhi-Zhang | Fluorescence Sensor for Water Content in Organic Solvents Based on Covalent Immobilization of Benzothioxanthene[END_REF][START_REF] Danko | Synthesis, photochemical stability and photo-stabilizing efficiency of probes based on benzothioxanthene chromophore and Hindered Amine Stabilizer[END_REF] Recently, our group undertook an exploratory work aiming at demonstrating the efficient functionalization of this sulphur containing rylene by nitro groups [START_REF] Andrés Castán | Nitration of benzothioxanthene: towards a new class of dyes with versatile photophysical properties[END_REF] and halogen atoms [START_REF] Dalinot | Regioselective Monohalogenation and Homo/Hetero Dihalogenation of Benzothioxanthene Monoimide[END_REF] to i) prepare original functional -conjugated structures and ii) evaluate their potential in (opto)electronic devices (Figure 1). [START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF][START_REF] Andrés Castán | Synthesis, characterization and use of benzothioxanthene imide based dimers[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 further step toward the rationalization of the structureproperty relationships, we report herein the selective cyanation of this promising building block and the investigation of the impact of the nitrile groups on the electrochemical and photophysical properties. [START_REF] Delgado | Tuning the Charge-Transport Parameters of Perylene Diimide Single Crystals via End and/or Core Functionalization: A Density Functional Theory Investigation[END_REF][START_REF] Gao | Cyano-Substituted Perylene Diimides with Linearly Correlated LUMO Levels[END_REF][START_REF] Ahrens | Cyanated Perylene-3,4-dicarboximides and Perylene-3,4:9,10-bis(dicarboximide): Facile Chromophoric Oxidants for Organic Photonics and Electronics[END_REF]



Results and discussion As a starting point, the conventional Rosenmund-von Braun method was firstly considered. [START_REF] Koelsch | THE ROSENMUND-von BRAUN NITRILE SYNTHESIS1[END_REF][START_REF] Ellis | Cyanation of aromatic halides[END_REF] Despite the use of a large excess of copper(I) cyanide, polar solvents and high temperatures, no conversion of the starting material was observed. [START_REF] Ahrens | Cyanated Perylene-3,4-dicarboximides and Perylene-3,4:9,10-bis(dicarboximide): Facile Chromophoric Oxidants for Organic Photonics and Electronics[END_REF] Consequently, and considering the good reactivity of BTXI-Br in pallado catalyzed reactions, conditions reported by Beller were subsequently evaluated (Table 1, entry 1). Beyond the introduction of a catalytic system, namely the palladium(II) acetate/bis(diphenylphosphino)ferrocene (Pd(OAc) 2 /dppf), the toxic CuCN was replaced by the potassium hexacyanoferrate(II) (K 4 [Fe(CN) 6 ]) used in food industry as additive (E536). [START_REF] Yan | Palladium-Catalyzed Direct Cyanation of Indoles with K4[Fe(CN)6[END_REF][START_REF] Jia | Chelation-Assisted Palladium-Catalyzed Cascade Bromination/Cyanation Reaction of 2-Arylpyridine and 1-Arylpyrazole C-H Bonds[END_REF][START_REF] Gerber | Cyanation of Aryl Bromides with K4[Fe(CN)6] Catalyzed by Dichloro[bis{1-(dicyclohexylphosphanyl)piperidine}]palladium, a Molecular Source of Nanoparticles, and the Reactions Involved in the Catalyst-Deactivation Processes[END_REF] Nonetheless, while a total conversion was indeed monitored by thin layer chromatography, only a minor amount of target compound was recovered after purification. To avoid the degradation of the latter, temperature was first reduced but at the detriment of the conversion since only traces of BTXI-CN were isolated after 24 h (Table 1, entry 2). On the other hand, limiting the reaction time through the use of microwave irradiations was found to be an efficient and powerful tool to reach decent yields (Table 1, entry 3). In parallel, while replacing the dppf ligand by another chelating phosphine, such as the Xantphos (Table 1, entry 6), led to similar synthetic yields, the use of monodentate phosphine ligands turned out to be pernicious since they favor dehalogenation side-reactions (Table 1, Entries 4 and 5). Finally, to reduce the reaction time, the molar equivalents of both the nitrile source and the catalytic system were increased, resulting in excellent yield after only 30 minutes of µW irradiation (entry 7). It is important to note that these optimized conditions can be successfully transposed to gram scale synthesis and the purification procedure simplified by replacing the solvent consuming silica-gel chromatography by a simple filtration/precipitation of the crude (see experimental section).

Once the monocyanation of the BTXI in hands, attempts of difunctionalization were subsequently considered. To that end, the above-mentioned optimized conditions were applied to Br-BTXI-Br, ie, a dibrominated benzothioxanthene derivative (Scheme 1). In addition to requiring extra 30 minutes to reach the full consumption of the starting material, the latter was systematically converted into two new products that turned out to be dicyanated regioisomers according to high resolution mass spectrometry. NMR experiments first confirmed that the major compound can be assigned to the expected CN-BTXI-CN A (Scheme 1), ie, corresponding to the substitution of the two bromine atoms by nitrile groups (Figure 2). However, while the 1 H NMR spectra of both Br-BTXI-Br and CN-BTXI-CN A indeed show two singlets between 9.0 ppm and 8.5 ppm, resulting from the difunctionalization of the naphthyl block, the singlet at 8.9 ppm splits in two doublets for the minor compound (CN-BTXI-CN B), suggesting that one of the cyano group is localized on the lower phenyl ring. Carried out in parallel, NOESY NMR experiments highlighted the absence of cross relaxation between nearby bay protons that was eventually confirmed by X-ray diffraction on single crystals (Figure 2 and see SI).To rationalize this regio-isomerization, the cyanation reaction of Br-BTXI-Br was reperformed but in presence of deuterated water (Figure 3 and see SI). Integration of the corresponding 1 H NMR signals confirmed a deuterium-proton exchange estimated around 73% and 90% for CN-BTXI-CN A and B respectively. This simple but effective experiment suggests the formation of a five-membered palladacycle in the bay position generated after the oxidative addition step and subsequent elimination of HBr as reported on bi-aryl derivatives. [START_REF] Campo | Aryl to Aryl Palladium Migration in the Heck and Suzuki Coupling of o-Halobiaryls[END_REF] Furthermore, the presence of deuterated CN-BTXI-CN A also highlights the reversibility of the isomerization and that most of the catalytic turnovers pass through this palladacycle complex. Consequently, two possible catalytic pathways can be considered as depicted in Scheme 2. These features can be corre related to the electronwithdrawing nature of the cyano moieties resulting in the stabilization the radical anion and, a contrario, destabilization of the radical cation. [START_REF] Chen | Beyond perylene diimides: synthesis, assembly and function of higher rylene chromophores[END_REF][START_REF] Huang | Perylene-3,4,9,10tetracarboxylic Acid Diimides: Synthesis, Physical Properties, and Use in Organic Electronics[END_REF][START_REF] Ahrens | Cyanated Perylene-3,4-dicarboximides and Perylene-3,4:9,10-bis(dicarboximide): Facile Chromophoric Oxidants for Organic Photonics and Electronics[END_REF] Interestingly, while similar frontier energy levels were estimated for both BTXI-CN and CN-BTXI-CN B, the regio isomer CN-BTXI-CN A exhibits shallower HOMO and LUMO levels. This difference was also highlighted by UV-Visible spectroscopy. While a minor hypsochromic shift of the 400-500 nm centered absorption band was recorded for CN-BTXI-CN B, an appreciable bathochromic shift coupled to a significant hypochromic shift of the latter band was indeed monitored for the second regioisomer, namely CN-BTXI-CN A. These features reveal that the second cyanation, occurring at the two possible positions of the bay area, has either no or a significant impact on the energetics depending on the cyanation position.

Highly fluorescent, all compounds show analogue structured emission profiles that mostly follow the absorption trend. Once again related to the number and position of the nitrile groups, fluorescence quantum yields (Φ f ) ranging from 71% to 88% were measured for the series (vs 100% for the bare BTXI). Interestingly, these results were cross-tabulated singlet-oxygen generation efficiencies (Φ Δ ) revealing that the decrease of Φ f was counterbalanced by an increase of Φ Δ . The origin of this gradient singlet-totriplet Inter-System Crossing (ISC) might be correlated to a deformation of the -system. [START_REF] Galán | Theoretical and experimental investigation on the intersystem crossing kinetics in benzothioxanthene imide luminophores, and their dependence on substituent effects[END_REF] Corroborated by X-ray diffraction, the cyanation of the BTXI turns out to be an efficient strategy to break the planarity of the core since torsion angles associated with the bay area carbon atoms of 1°, 12° and 22° were measured for BTXI-CN, CN-BTXI-CN A and CN-BTXI-CN B respectively (see SI). These results were further confirmed by TD-DFT calculation (see SI), highlighting the notably large spin-orbit coupling computed for the dicyano compounds, in agreement with the deformation of their-system.

In parallel, nitrile moieties were also demonstrated as original and orthogonal protecting groups. As depicted in Scheme 3, the successive cyanation and nickel-catalyzed hydrogenolysis [START_REF] Patra | Nickel-catalyzed hydrogenolysis of unactivated carbon-cyano bonds[END_REF] of the highly reactive position in alpha of the sulfur atom, led to the successful synthesis of Th-BTXI, corresponding to the first example of a mono functionalized BTXI derivative in the bay position.

 Conclusion

In summary, we have explored herein the pallado-catalyzed cyanation of the benzothioxanthene imide rylene. Optimized on a mono-halogenated derivative using nontoxic nitrile sources, reac-tion conditio ns were subsequ ently applied to the dibr omo BTXI derivative resulting in the formation of two regio isomers.

Scheme 3. Illustration of the use of a cyano moiety as protecting group for the preparation of Th-BTXI

Once structurally solved, complementary hydrogen-deuterium exchange experiments were carried out to propose a credible reaction mechanism involving a five-membered palladacycle in the bay area. Characterization of this new series of BTXI revealed that the position of the cyano groups has a significant impact on the structural, electrochemical and photophysical properties. Moreover, the latter were also demonstrated as original orthogo- were added into a microwave reactor. Once sealed and purged under argon atmosphere, 3 mL of N,N-dimethylacetamide were added. The mixture was then irradiated in the microwave cavity with a pre-selected temperature from 100 o C to 140 o C. The reaction mixture was then cooled to room temperature and the solvent removed under vacuum. The crude was purified by column chromatography on silica gel using dichloromethane as the eluent to obtain a yellow solid (95%). Purification protocol B:

The crude was solubilized in dichloromethane and passed through a silica plug. The filtered solution was then concentrated under reduced pressure. The resulting solid was triturated in a methanol/dichloromethane mixture (4:1), filtrated and successively washed with hot methanol and distilled pentane to afford a yellow solid (86%).

BTXI-CN:

)-1H- thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione
(BTXI-Br) (500 mg, 1.11 mmol), K 4 [Fe(CN) 6 ] (407 mg, 1.11 mmol), sodium carbonate (175 mg, 1.11 μmol), palladium(II) acetate (50 mg, 0.22 mmol) and Xantphos (255 mg, 0.44 mmol). Once under inert atmosphere, 20 mL of N,N-dimethylacetamide were added and the reaction mixture were subsequently irradiated in the microwave cavity at 140 °C. After 1 h of reaction and cooled to room temperature, the two reaction mixture were mixed together. The solvent was then removed under vacuum and the product purified following either the purification protocol A (79% yield) or B (70% yield).

Synthesis of CN-BTXI-CN:

Anhydrous N,N-dimethylacetamide (3 mL) was added to a mixture of 5,11-dibromo-2-(pentan-3-yl)-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione (Br-BTXI-Br) (50 mg, 94.12 μmol), K 4 [Fe(CN) 6 ] (69 mg, 180.23 μmol), sodium carbonate (15 mg, 141.17 μmol), palladium(II) acetate (50 mg, 18.82 μmol) and Xantphos (22 mg, 37.65 μmol). The reaction mixture was then stirred for 1 h in the microwave cavity at 140 o C. The reaction mixture was cooled to room temperature and the solvent removed under vacuum. The crude was purified by silica gel column chromatography using dichloromethane as eluent to obtain 18 mg of CN-BTXI-CN A (45% yield) in the first fraction and 9 mg of CN-BTXI-CN B (23% yield) in the second fraction, both of them as a yellow solids.

CN-BTXI-CN A:

1 H-NMR (300 MHz, CDCl 3 ): δ 9.06-9.00 (m, 1H), 8.94 (s, 1H), 8.73 (s, 1H), 7.76 -7.64 (m, 3H), 5.06-5.00 (m, 1H), 2.31 -2.10 (m, 2H), 2.02 -1.83 (m, 2H), 0.88 (t, J = 7.5 Hz, 6H). [START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF] was added to a solution of BTXI-CN (50 mg, 0.13 mmol) in dichloromethane (5 mL). The reaction mixture was refluxed for 16 h before being washed with saturated Na 2 S 2 O 3 , water and brine. Once dried over MgSO 4 , the solvent was removed by rotary evaporation and the crude was subjected to silica gel column chromatography using toluene as eluent. 57 mg of the final product were obtained as an orange solid (95% yield). [START_REF] Nowak-Król | Progress in the synthesis of perylene bisimide dyes[END_REF]

Synthesis of Th-BTXI-CN:

To a mixture of Br-BTXI-CN (50 mg, 104.74 μmol) and tributyl(thiophen-2-yl)stannane (78 mg, 209,48 μmol) was added in degassed toluene (3 mL). Pd(PPh 3 ) 4 (6 mg, 5.24 μmol) was then added before refluxing the mixture for 16 h under inert atmosphere. The reaction was cooled to room temperature and the solvent removed under vacuum. Purification of the crude was performed by column chromatography on silica gel using dichloromethane as the eluent affording 50 mg of an orange solid (100% yield). 1 H-NMR (300 MHz, CDCl 3 ): δ 8.81 (s, 1H), 8.59 (s, 1H), 7.66 (dd, J = 8.4, 0.9 Hz, 1H), 7.55 (dd, J = 8.0, 1.1 Hz, 1H), 7.41 (dd, J = 5.1, 1.2 Hz, 1H), 7.34 (td, J = 7.7, 1.2 Hz, 1H), 7.21 (dd, J = 3.6, 1.2 Hz, 1H), 7.11 -7.03 (m, 2H), 5.08 -4.96 (m, 1H), 2.33 -2.12 (m, 2H), 2.00 -1.79 (m, 2H), 0.88 (t, J = 7.5 Hz, 6H). [START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF] (5 mg, 18.73 μmol) and PCy 3 (16 mg, 56.18 μmol) inside a glovebox under inert atmosphere. The tube was sealed and taken out from the glovebox before repeating three vacuum-hydrogen cycles. Then, degassed toluene (3 mL) and AlMe 3 (94 μL, 187.27 μmol, 2M in toulene) were added and the reaction was heated at 130 o C for 16 h. Thereafter, the mixture was cooled to room temperature and the crude filtrated through celite using ethyl acetate as eluent. The filtrate was concentrated before being purified by column chromatography using dichloromethane as eluent. Finally, 12 mg of the expected product were obtained as an orange solid (42% yield) whereas 5 mg of the starting material Th-BTXI-CN (18% yield) were recovered. (BTXI-Br) (100 mg, 0.22 mmol), tributyl(thiophen-2-yl)stannane (140 μL, 165 mg, 0.44 mmol) and Pd(PPh 3 ) 4 (13 mg, 11.05 μmol) were dissolved in dry and degassed toluene (7mL). The reaction mixture was then stirred and refluxed for 5 h under inert conditions. Once concentrated under vacuum, the crude was purified by column chromatography using dichloromethane as eluent thus affording 89 mg of an orange powder (88% yield). 1 H-NMR (300 MHz, CDCl 3 ): δ 8.61 (d, J = 8.2 Hz, 1H), 8.48 (s, 1H), 8.24 (d, J = 8.3 Hz, 1H), 8.22 -8.17 (m, 1H), 7.53 (dd, J = 5.1, 1.2 Hz, 1H), 7.43 -7.30 (m, 4H), 7.22 (dd, J = 5.1, 3.5 Hz, 1H), 5.06 (tt, J = 9.6, 5.8 Hz, 1H), 2.39 -2.14 (m, 2H), 2.01 -1.82 (m, 2H), 0.90 (t, J = 7.5 Hz, 6H). [START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF] Isotopic labeling experiments: Dicyanation reaction was carried out following the same procedure described above in presence of 100 μL of D 2 O. Both regioisomers were isolated in similar yields and analysed by 1 H NMR (Figure S34-36) and MS spectrometry (Figure S47-48) to evaluate the presence of deuterated species.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Figure 1 .

 1 Figure 1. Chemical structure of BTXI and related examples of extended -conjugated structures used in OLED and OPV devices.

22

 22

Scheme 1 .

 1 Scheme 1. Synthetic route to CN-BTXI-CN.

Figure 2 .

 2 Figure 2. Comparative 1 H NMR spectra of Br-BTXI-Br and the two isolated bis cyanated regio isomers (top) and NOESY NMR spectra of CN-BTXI-CN B (bottom).

Figure 3 .

 3 Figure 3. Comparison of 1 H NMR spectra of CN-BTXI-CN B (left) and CN-BTXI-CN A (right) prepared in presence (red) or absence (blue) of deuterated water. Hence, following the oxidative addition of Br-BTXI-CN to LnPd 0 , the resulting F OA intermediate can undergo a transmetalation reaction with the nitrile source to afford F TR 1 and therefore the "expected" CN-BTXI-CN A regioisomer through a final reductive elimination step (cycle A). In parallel, F OA can also be engaged in a remote C-H activation process leading to the formation of the five-member palladacycle RI (cycle B). With a reversibility demonstrated by the deuterium exchange experiments, the latter can go back to F OA or move towards F EX to achieve the observed isomerization in the bay area that will further generate F TR 2 and finally CN-BTXI-CN B via a transmetalation and subsequent a reductive elimination step. To support this proposed catalytic path, Density Functional Theory (DFT) calculation were performed in parallel (see SI). Consider-

  [START_REF] Nowak-Król | Progress in the synthesis of perylene bisimide dyes[END_REF] H-NMR (300 MHz, CDCl 3 ): δ 8.63 (s, 1H), 8.33 (d, J = 7.9 Hz, 1H), 7.59 -7.48 (m, 2H), 7.38 -7.26 (m, 2H), 7.18 -7.09 (m, 2H), 7.00 (dd, J = 5.1, 3.5 Hz, 1H), 6.92 (ddd, J = 8.5, 7.2, 1.5 Hz, 1H), 5.06 -4.92 (m, 2H), 2.28 -2.11 (m, 2H), 1.88 -1.78 (m, 2H), 0.82 (t, J = 7.5 Hz, 6H).[START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF] C NMR (75 MHz, CDCl 3 ): δ 144.5, 139.9, 137.0, 136.1, 133.3, 132.0, 129.9, 129.4, 129.1, 129.0, 128.0, 127.4, 127.3, 127.0, 126.9, 126.7, 126.3, 121.6, 57.5, 25.0, 11.3. HRMS (MALDI+): calculated for C 27 H 21 NO 2 S 2 455.1008; found 455.1015. Synthesis of BTXI-Th: 5-bromo-2-(pentan-3-yl)-1Hthioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione

  Hz, 1H).[START_REF] Josse | Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications[END_REF] C NMR (75 MHz, CDCl 3 ): δ 162.9, 162.6, 144.8, 140.8, 137.0, 132.9, 131.3, 131.1, 130.8, 129.2, 129.0, 128.0, 127.4, 127.2, 122.2, 119.2, 118.6, 115.8, 105.0, 58.2, 24.9, 11.4. HRMS (FAB-): calculated for C 24 H 17 BrN 2 O 2 S 476.0189; found 476.0981.

	1H), 7.56 -7.43 (m, 2H), 5.05 -4.93 (m, 1H), 2.20 (m, 2H), 1.97
	-1.82 (m, 2H), 0.87 (t, J = 7.4
	H-NMR (300
	MHz, CDCl 3 ): δ 8.93 (s, 1H), 8.76 (dd, J = 7.8, 1.8 Hz, 1H), 8.57
	(s, 1H), 7.60 (dd, J = 7.6, 1.8 Hz, 1H), 7.60 (dd, J = 7.6, 1.8 Hz,

  C NMR (75 MHz, CDCl 3 ): δ 163.7, 163.6, 146.2, 143.8, 138.8, 136.3, 132.9, 132.1, 131.5, 131.3, 129.9, 129.6, 128.7, 128.6, 128.4, 127.9, 127.8, 127.3, 127.2, 121.6, 119.2, 116.1, 104.7, 58.0, 25.0, 11.4. HRMS (MALDI+): calculated for C 28 H 20 N 2 O 2 S 2 480.0961; found.480.0968. Synthesis of Th-BTXI: An oven-dried screw-cap reaction tube was charged with Th-BTXI-CN (30 mg, 62.42 μmol), Ni(COD) 2

  C NMR (75 MHz, CDCl 3 ): δ 164.8, 140.1, 139.0, 136.8, 133.6, 132.4, 131.8, 129.9, 129.8, 128.2, 127.7, 127.6, 127.5, 127.2, 126.7, 126.6, 126.1, 125.9, 121.6, 119.5, 117.8, 57.4, 25.0, 11.4. HRMS (MALDI+): calculated for C 27 H 21 NO 2 S 2 455.1008; found 455.1009.

ACKNOWLEDGMENT

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 Project "étoiles montantes" SAMOA. L.A.G. acknowledges the French Agence Nationale de la Recherche for her grant (ANR SADAM ANR-16-CE07-0015-02).

AUTHOR INFORMATION

Corresponding Author *clement.cabanetos@univ-angers.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


Tatiana Ghanem

Tony Vincendeau

Pablo Simón Marqués

Hossein Amir

Sana Habibi

Ali Abidi

Sylvie Yassin

Jean Dabos-Seignon

Philippe Roncali

Clément Blanchard

Cabanetos

Amir Hossein Habibi

Sana Abidi

Ali Yassin

Sylvie Dabos- Seignon

Jean Roncali

Philippe Blanchard

Clément Cabanetos

Synthesis of push-pull triarylamine dyes containing 5,6difluoro-2,1,3-benzothiadiazole units by direct arylation and evaluation as active material for organic photovoltaics

come

Introduction

During the past two decades, the synthesis of active materials for organic solar cells (OSCs) has generated a considerable research effort. [START_REF] Riede | Organic Solar Cells-The Path to Commercial Success[END_REF][START_REF] Duan | Progress in Stability of Organic Solar Cells[END_REF][START_REF] Hong | Recent Advances in High-Efficiency Organic Solar Cells Fabricated by Eco-Compatible Solvents at Relatively Large-Area Scale[END_REF] The development of specifically designed conjugated polymers combined with the most recent non-fullerene acceptors has progressively led to OSCs with power conversion efficiencies (PCE) approaching 20 %, ie, close to those of silicon solar cells. [START_REF] Lin | 4 % Organic Solar Cells Using a High Ionization Energy Self-Assembled Monolayer as Hole-Extraction Interlayer[END_REF] Besides stability, the synthetic complexity, cost and scalability of active materials still remains a major problem for chemists involved in the development of active OPV materials. [START_REF] Riede | Organic Solar Cells-The Path to Commercial Success[END_REF][START_REF] Po | Beyond Efficiency: Scalability of Molecular Donor Materials for Organic Photovoltaics[END_REF] The introduction of molecules with unequivocal chemical structure as a possible alternative to polydisperse conjugated polymers around 2005 has represented a first attempt to address the problems of the cost and scalability of donor materials for OSCs. [START_REF] Roncali | Molecular Materials for Organic Photovoltaics: Small Is Beautiful[END_REF][START_REF] Venkateswararao | Small Molecules for Vacuum-Processed Organic Photovoltaics: Past, Current Status, and Prospect[END_REF][START_REF] Cabanetos | Arylamine Based Photoactive Push-Pull Molecular Systems: A Brief Overview of the Chemistry "Made in Angers[END_REF] However, with a few exceptions, the synthesis of most of these molecular donors (MDs) resorts to transitionmetal-catalyzed C-C cross-coupling reactions. [START_REF] Kudrjasova | A Direct Arylation Approach towards Efficient Small Molecule Organic Solar Cells[END_REF][START_REF] Okamoto | C-H Arylation Reaction: Atom Efficient and Greener Syntheses of π-Conjugated Small Molecules and Macromolecules for Organic Electronic Materials[END_REF] This strategy implies the halogenation and functionalization of complementary building blocks by organometallic functional groups. In recent years the direct arylation of C-H activated aromatic or heteroaromatic moieties has emerged, as an appealing alternative to conventional organometallic coupling reactions. [START_REF] Hong | Recent Advances in High-Efficiency Organic Solar Cells Fabricated by Eco-Compatible Solvents at Relatively Large-Area Scale[END_REF][START_REF] Bura | Direct (Hetero)Arylation Polymerization: Trends and Perspectives[END_REF][START_REF] Mainville | Direct (Hetero)Arylation: A Tool for Low-Cost and Eco-Friendly Organic Photovoltaics[END_REF] Although high conversion yields can be reach by means of careful optimizations of the reaction conditions, this method has been proven to be perfectly geared for the preparation of polymer and molecular donor materials. For instance, the synthesis of the triarylamine based push-pull TPA-T-DCV, (Figure 1) flagship of our group, was recently simplified by the use of the C-H activation of the commercially available 2-Thiophenecarboxaldehyde. [START_REF] Grolleau | Rapid and Green Synthesis of Complementary D-A Small Molecules for Organic Photovoltaics[END_REF] Obtained at gram-scale in only two steps from cheap commercially available starting materials, TPA-T-DCV presents interesting potentialities as donor material in planar and co-evaporated bulk heterojunction OSCs. These promising first results paved the way to a myriad of structural variations on both the donor (D) and acceptor (A) constitutive blocks with the double objective of a better structureproperties rationalization and optimization of the photovoltaic performances. [START_REF] Cabanetos | Arylamine Based Photoactive Push-Pull Molecular Systems: A Brief Overview of the Chemistry "Made in Angers[END_REF][START_REF] Jiang | Manipulation of the Band Gap and Efficiency of a Minimalist Push-Pull Molecular Donor for Organic Solar Cells[END_REF][START_REF] Labrunie | Pentaerythritol Based Push-Pull Tetramers for Organic Photovoltaics[END_REF][START_REF] Jiang | Manipulation of the Electronic and Photovoltaic Properties of Materials Based on Small Push-Pull Molecules by Substitution of the Arylamine Donor Block by Aliphatic Groups[END_REF] As shown by Wong and coworkers, the insertion of electron deficient (hetero)aromatic moiety such as benzothiadiazole (BzT) between the arylamine (D) and the dicyanovinyl group (A) represents a particularly interesting approach to finetune the ground-and excited-state characteristics of the D-A system. [START_REF] Lin | A Low-Energy-Gap Organic Dye for High-Performance Small-Molecule Organic Solar Cells[END_REF] However, the use of the BzT building generally requires the use of harsh electrophilic halogenation conditions leading to moderate yields, mixtures of products and/or unstable intermediates. In this context, direct (hetero)-arylation appears to be a method of choice to overcome these synthetic problems. Marder and co-workers, have already reported the synthesis of symmetrical and unsymmetrical diaryl flanked 5,6-difluoro-2,1,3-benzothiadiazole (BT2F) derivatives using this methodology. [START_REF] Zhang | Controllable Direct Arylation: Fast Route to Symmetrical and Unsymmetrical 4,7-Diaryl-5,6-Difluoro-2,1,3-Benzothiadiazole Derivatives for Organic Optoelectronic Materials[END_REF][START_REF] Zhang | C-H-Activated Direct Arylation of Strong Benzothiadiazole and Quinoxaline-Based Electron Acceptors[END_REF] However considering that difluoro derivative have never been prepared as active materials for organic photovoltaics, we report herein the synthesis, characterization and evaluation of the two push-pull derivatives illustrated in Figure 1. Hence, two different triphenylamine blocks were used to assess their impact on the electronic and charge transport properties of the molecules and therefore their photovoltaic efficiencies. While the methoxy groups of MD-OMe is expected increase the donor strength of the triphenylamine block, the two biphenyl side groups of MD-Ph were recently found to significantly increase the hole-mobility of some arylamine based D-A materials. [START_REF] Marqués | Effect of 4-Biphenyl Groups on the Charge Transport and Photovoltaic Properties of Arylamine Based Push-Pull Systems[END_REF]

Results and discussion.

Both push-pull were prepared according to the initial synthetic route depicted in Scheme 1.

Scheme 1. Synthetic route to MD-OMe and MD-Ph

Coupling conditions to afford arylamine functionalized unsymmetrical 5,6-difluoro-2,1,3benzothiadiazole derivatives 2 and 3, were first carried out and optimized with the 4-bromo-N,N-bis(4-methoxyphenyl)aniline. As shown in Table 1, the first attempt was performed in presence of Palladium(II) acetate and tri-tert-butylphosphonium tetrafluoroborate (PtBu 3 .HBF 4 ) under conventional oil bath heating (entry 1). Despite operating in relatively diluted conditions, only moderate yields were obtained mainly because of the major formation of the symmetrical arylamine end capped derivative (2a). The reduction of the number of equivalents of bromo derivative by a factor of two results in a slight improvement of the conversion yield (entry 2).In parallel, the replacement of the conventional oil bath by microwave irradiation (entry 3) leads to a further slight improvement of the yield but considerably shortens the reaction time. One hour of irradiation appears as a good compromise between the consumption of the starting material and the formation of the symmetrical byproduct (2a). Afterwards, the initial catalytic coupled was modified by replacing the tri-tertbutylphosphonium tetrafluoroborate either by the tri(o-tolyl)phosphine (entry 4) or the tricyclohexylphosphine tetrafluoroborate (entry 5). While similar conversion rates were monitored with the neutral ligand (entry 4), a descent synthetic yield reaching almost 80% was achieved with the bulkier charged analogue (entry 5). Applied to the biphenyl derivative, these conditions led to the target unsymmetrical 5,6-difluoro-2,1,3-benzothiadiazole based analogue 3 in similar good yields (78%). Each monoarylated products were then subsequently engaged in another cross-coupling reaction with the 2-bromo-5-(2,2-dicyanovinyl)thiophene. Even if the unsymmetrical functionalization of the BT2F core by direct arylation was successfully operated with electron rich moieties, low yields were achieved mainly due to the mild reactivity of the halogenated thiophene, that appeared too deactivated by the dicyanovinyl group. Consequently, and as compromise between reactivity and extra synthetic steps, carbaldehyde functionalized unsymmetrical 5,6-difluoro-2,1,3 benzothiadiazole derivatives 4 and 5 were prepared by coupling 2 and 3 with the 5-bromothiophene-2-carbaldehyde respectively (Scheme 2).

Scheme 2. Second Synthetic route considered to PP-OMe and PP-Ph through a Knoevenagel reaction

Though still modests, synthetic yields achieved were nonetheless significantly improved and are in consistency with early reported results. [START_REF] Zhang | Controllable Direct Arylation: Fast Route to Symmetrical and Unsymmetrical 4,7-Diaryl-5,6-Difluoro-2,1,3-Benzothiadiazole Derivatives for Organic Optoelectronic Materials[END_REF] As a further step toward simplification, attempts to prepare these carbaldehyde derivatives by a one-pot like method were thus carried out. To do so, BT2F was first reacted with bromo arylamine before adding the 5-bromothiophene-2carbaldehyde to the reaction mixture. After a second round of microwave irradiation, carbaldehyde 4 and 5 were successfully isolated with synthetic yields similar to those achieved with the above described two-step procedure. Finally, the later compounds were engaged in a Knoevenagel reaction with malononitrile to afford the corresponding final push-pull molecules. Despite full conversion confirmed in both cases by TLC, the biphenyl derivative was isolated in lower yield due to a limited solubility at room temperature, resulting in a strong interaction with the silica gel. The identity and purity of the final compound was confirmed by NMR spectroscopy and mass spectrometry (see SI).

Figure 2 shows the UV-Vis absorption spectra of the two compounds in dichloromethane (DCM) and as thin film on glass. The spectrum of both compounds shows three successive absorption bands with maxima at 332, 413 and 513 nm for the biphenyl compound MD-Ph in solution. Replacement of the biphenyl by a methoxy group produces, as expected, a small bathochromic shift of the maxima at longer wavelength (ca 14 nm) reflecting the higher electron donor effect of the methoxy group compared to a phenyl ring. This behavior results in a reduction of the optical bandgap estimated from the low-energy absorption onset of thin films from 1.86 to 1.68 eV. Comparison of the spectra of the two compounds shows that the introduction methoxy groups on the TPA block also increases the molar absorptivity of the visible absorption bands by almost a factor of two (Table 2). While the UV-Vis absorption spectrum of TPA-T-DCV and many of its parent systems usually exhibits a single transition in the visible region, attributed to an internal charge transfer (ICT) from the TPA moiety to the DCV group, the spectra of MD-OMe and MD-Ph shows two distinct intense absorption bands in the 400-600 nm region. In order to get further insights on this question, ground-state density-functional theory (DFT) geometry optimizations and densityfunctional theory (TD-DFT) calculations were carried out using B3LYP/6-311G* basis sets (see SI). As expected, these experiments first confirmed the ICT nature of the low energy contributions. Assigned to a HOMO->LUMO transition, calculations indeed showed that the electronic density of the HOMO levels is mainly localized on the arylamine moiety while the LUMOs are on the benzothiadiazole/DCV blocks (Figure 3). The absorption bands at ca 400 nm, were also found to have a charge-transfer character since they predominantly arise from a HOMO-1->LUMO transition (see SI). Moreover, the band gap reduction induced by the electron-donating methoxy groups (MD-OMe) compared to simple phenyl rings was found to originate from the significant destabilization of the HOMO level exceeding by far that of the LUMO ( HOMO = 0.19 eV vs  LUMO = 0.10 eV). Hence, photoelectron spectroscopy in air (PESA) measurements were subsequently performed and HOMO levels of -5.43 eV and -5.64 eV were calculated for MD-Ome and MD-Ph respectively. Once crosstabulated with UV-visible experiments, the corresponding LUMO levels were simply calculated by adding the optical band gap (film spectra) to the HOMO values thus confirming, as depicted in the figure 4, the computed predictions. Designed as potential donor molecular systems for organic solar cells, such difference in HOMO level might have a significant impact on the photovoltaic parameters, in particular on the open circuit voltage (V oc ). Consequently, simple air-processed bulk heterojunction (BHJ) solar cells using [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as electron acceptor material were fabricated. Active layers with different donor: acceptor (D/A) weight to weight (w/w) ratios were processed in air from chloroform solutions, and embedded in devices of architectures ITO/PEDOT:PSS/active layer/Ca/Al. The photovoltaic data measured under AM 1.5 simulated solar light illumination (100 mW•cm -2 ) are gathered in Table 3. The best power conversion efficiencies, of 1.35% and 2.09%, were reached with a 1:2 and 1:3 D/A ratio for MD-OMe and MD-Ph based devices respectively . Comparison of their corresponding current to tension (J-V) curves revealed that the better efficiency recorded in MD-Ph based OSCs mainly e from their higher short circuit current (Jsc) and open circuit voltage (Voc) values (Figure 5). While the latter parameter (Voc) can be correlated to the deeper HOMO level of the biphenyl derivative, MD-OMe was nonetheless characterized by better absorbing properties in the visible range. Interestingly and in agreement with the recorded Jsc values, external quantum efficiency measurements performed on the best devices also highlighted an improved contribution of the biphenyl derivative to the photocurrent compared to its methoxy counterpart (Figure 5). Hence, in an attempt to rationalize these results, morphologies of the active layers were first probed by atomic force microscopy (AFM). As illustrated in figure 6, smooth surface topographies with similar root mean square (RMS) parameters of ca 0.6 nm were observed in both cases, (Figure 7). Nonetheless, MD-Ph/ PC 71 BM blend exhibits nano domains of smaller size than those observed with the methoxy derivative which can be the harbinger of a better biphenyl induced selforganization of the donor molecule which can, in turn, be beneficial for the charge transport properties and therefore the Jsc parameter. Consequently, both optimized active layers were subsequently embedded in hole-only devices of architecture ITO/PEDO:PSS/donor/Au. Through the use of the space charge limited current (SCLCs) method, hole mobilities of ca 7.3x10 -5 and 8.3x10 -4 cm 2 V -1 s -1 were measured for MD-OMe and MD-BP respectively (see SI). Consequently, this order-of-magnitude difference might compensate the lower absorbing properties of the active layer and contribute to the higher Jsc value delivered by the MD-BP based devices.

Conclusion

In summary, two new triphenylamine based push-pull molecules containing 5,6-difluoro-2,1,3benzothiadiazole units have been synthesized. In order to simplify the synthesis procedure and limit the formation of potentially toxic by-products, two major steps of the synthesis were performed using optimized direct (hetero)-arylation conditions. As a step towards simplification, the synthetic route was further shortened by the use of a "one-pot" procedure without modification of the catalytic system nor purification of intermediates. The analysis of the electronic properties of the two compounds shows, in agreement with theoretical results, that the methoxy derivative presents a red-shifted absorption, a higher molecular absorption coefficient and leads to a material of smaller band gap. The evaluation of the photovoltaic properties of the two compounds as donor material in BHJ cells shows that in spite of less favorable optical properties, the biphenyl derivative leads to better performances due to the conjunction of a higher open-circuit voltage and better charge transport properties. Evaluated in simple air processed devices, these preliminary results pave the way to further optimization and device engineering to reveal the true potential of these accessible and greener materials.

Acknowledgment

The French ministry of Research is acknowledged for the PhD grant of T.G. (project MOSA). P.S.M. and A.H.A thanks the European Union's Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement No.722651 (SEPOMO).

Figure 1 .

 1 Figure 1. Structure of the early reported TPA-T-DCV and the two arylamine based push-pull studied herein, namely MD-OMe and MD-Ph

Figure 2 .

 2 Figure 2. UV-Vis absorption spectra of MD-OMe (blue) and MD-Ph (red) in DCM (left) and as thin films on glass (right).

Figure 3 .

 3 Figure 3. Optimized geometries and computed energy levels of MD-Ph (left) and MD-OMe (right)

Figure 4 .

 4 Figure 4. Energy level diagram estimated in solid state from PESA and UV-visible experiments

Figure 5 .

 5 Figure 5. Current density-voltage characteristics and EQE spectra of the best OMe-BT2F-DCV (blue)and Be-BT2F-DCV (red) based organic solar cells.

Figure 7 .

 7 Figure 7. AFM phase images (20x20 and 5x5m) of optimized OMe-BT2F-DCV and Be-BT2F-DCV based active layers.

Table 1 .

 1 Reactions conditions used for the coupling of 4-bromo-N,N-bis(4-methoxyphenyl)aniline and BT2F

	Entry	OMe-Br (eq)	Catalyst (mol %)	Ligand (mol %)	Heat source, temp./time	2 (% yield)
	1	1.0	Pd(OAc) 2	P t Bu 3 .HBF 4	Oil bath 120°C; 16h	20%
	2	0.5	Pd(OAc) 2	P t Bu 3 .HBF 4	Oil bath (120°C; 16h)	33%
	3	0.5	Pd(OAc) 2	P t Bu 3 .HBF 4	-wave (150°C; 1h)	36%
	4	0.5	Pd(OAc) 2	P(OMePh) 3	-wave (150°C; 1h)	40%
	5	0.5	Pd(OAc) 2	PCy 3 .HBF 4	-wave (150°C; 1h)	76%

Table 2 .

 2 Optical Data gathered from CH 2 Cl 2 solutions and on thin films spun casted on glass sheets

	Molecule	λ abs solution max (nm)	ε (M -1 cm -1 )	λ abs thin film max (nm)	opt (eV) a  g	HOMO (eV) b	LUMO (eV) a
		315	51800	325
	MD-OMe	412	44100	421		1.68	-5.43	-3.75
		527	41100	571
		332	42900	343
	MD-Phe	413	21900	423		1.86	-5.64	-3.78
		513	20200	552
	a estimated from the low-energy absorption onset of thin films ; b determined by photoelectron spectroscopy in air
			(PESA) ; c LUMO = HOMO-E g	opt

Table 3 .

 3 Photovoltaic data measured on 28mm 2 solar cells under AM 1.5 simulated solar light illumination at 100 mW•cm -2 .

	Molecular Donor	D:A ratio (w/w)	Voc (V)	Jsc (mA/cm²)	FF	PCE (%)
	Ome-BT2F-DCV	1:1	0.71	-3.18	0.33	0.74
	Ome-BT2F-DCV	1:2	0.85	-4.55	0.35	1.35
	Ome-BT2F-DCV	1:3	0.77	-4.73	0.33	1.20
	BP-BT2F-DCV	1:1	1.01	-4.56	0.33	1.52
	BP-BT2F-DCV	1:2	1.01	-5.47	0.34	1.88
	BP-BT2F-DCV	1:3	0.96	-6.43	0.34	2.09


Kumari Pooja

Zaccharie Ramzi

Chaithya Giliyar Radhakrishna

Philippe Ciuciu

Chaithya Gr

MC-PDNET: DEEP UNROLLED NEURAL NETWORK FOR MULTI-CONTRAST MR IMAGE RECONSTRUCTION FROM UNDERSAMPLED K-SPACE DATA

Keywords: MRI reconstruction, Deep learning, Multicontrast imaging

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Magnetic Resonance Imaging (MRI) is one of the most powerful imaging techniques used to probe soft tissues within the human body, especially those of the human brain. Included within a clinical MRI exam, the physician has the option to exploit several contrastweighted MR images of the same anatomy including T1, T2, T2 * , FLAIR, etc., to maximize diagnostic accuracy. The corresponding data is collected sequentially and the MR images reconstructed independently. In the interest of high-throughput and fast MR imaging, Compressed Sensing (CS) has been used for more than a decade to reduce scan times [START_REF] Lustig | Sparse MRI: The application of compressed sensing for rapid MR imaging[END_REF] but at the expense of longer iterative image reconstruction. CS MRI reconstruction has been successfully used in hundreds of papers with several significant contributions in the specific domain of multi-contrast (MC) image reconstruction [START_REF] Huang | Fast multicontrast MRI reconstruction[END_REF][START_REF] Lai | Sparse MRI reconstruction using multi-contrast image guided graph representation[END_REF][START_REF] Lai | Joint sparse reconstruction of multi-contrast MRI images with graph based redundant wavelet transform[END_REF].

This success is attributed to the capacity of viewing multiple imaging contrasts of the same underlying anatomy that share similar structures, as illustrated in Fig. 1 on a single brain. Leveraging this shared information in an optimal way, however, requires designing hand-crafted joint sparsity priors that go beyond the single sparsity currently in use in classical MR image reconstruction. Such advanced priors combine two ingredients: First, the choice of a sparse transform or representation (e.g. graph-based wavelet [START_REF] Lai | Sparse MRI reconstruction using multi-contrast image guided graph representation[END_REF][START_REF] Lai | Joint sparse reconstruction of multi-contrast MRI images with graph based redundant wavelet transform[END_REF]) and second, the definition of the most appropriate penalty term (e.g. group-LASSO [START_REF] Majumdar | Calibration-less multi-coil MR image reconstruction[END_REF] or OSCAR [START_REF] Howard | Simultaneous regression shrinkage, variable selection, and supervised clustering of predictors with oscar[END_REF][START_REF] Gueddari | Calibrationless oscarbased image reconstruction in compressed sensing parallel mri[END_REF]) to combine multi-contrast information. One method to address these issues is to learn the prior itself in a data-driven manner. Deep learning (DL) offers the ideal framework by which to achieve this goal and has actually already been successfully used for MR image reconstruction [START_REF] Yang | Deep ADMM-net for compressive sensing MRI[END_REF][START_REF] Zhu | Image reconstruction by domaintransform manifold learning[END_REF][START_REF] Hammernik | Learning a variational network for reconstruction of accelerated mri data[END_REF][START_REF] Schlemper | A Deep Cascade of Convolutional Neural Networks for MR Image Reconstruction[END_REF][START_REF] Lee | Deep residual learning for accelerated MRI using magnitude and phase networks[END_REF], notably see the benchmark paper published in [START_REF] Ramzi | Benchmarking MRI reconstruction neural networks on large public datasets[END_REF]. The existing literature on DL for MC-MRI reconstruction comprises two contributions. The first, called joint variational network (jVN) [START_REF] Polak | Joint multi-contrast variational network reconstruction (jvn) with application to rapid 2d and 3d imaging[END_REF], has basically extended the variational neural network originally introduced for single contrast multi-coil MR image reconstruction from Cartesian under-sampled data [START_REF] Hammernik | Learning a variational network for reconstruction of accelerated mri data[END_REF]. The second contribution entails a deep information sharing network to exploit the structural similarity across imaging contrasts [START_REF] Sun | A deep information sharing network for multi-contrast compressed sensing mri reconstruction[END_REF]. However, in this work the architecture is based on a cascade of networks and dense connections hence significantly affecting the memory footprint of the architecture on GPU resources.

In the present work, we propose an extension of the PD-Net architecture to MC-MRI reconstruction. This cross-domain architecture was initially introduced for tomographic image reconstruction [START_REF] Adler | Learned primal-dual reconstruction[END_REF], tested as such on MRI [START_REF] Cheng | Model learning: Primal dual networks for fast mr imaging[END_REF] and then further extended as XPD-Net to multi-coil MR image reconstruction [START_REF] Ramzi | Benchmarking MRI reconstruction neural networks on large public datasets[END_REF] 1 . As this architecture secured the second place at the 2020 brain fastMRI challenge [START_REF] Matthew J Muckley | Results of the 2020 fastMRI challenge for machine learning MR image reconstruction[END_REF], it naturally holds great promise for multi-contrast MR image reconstruction. In this work we introduce the MC-PDNet architecture that accumulates imaging contrasts as a supplementary channel dimension which allows us to share weights in the CNN across contrasts and limit the memory footprint. Moreover, for this proof of concept the focus will only pertain to single-coil 2D imaging and Cartesian data even though it could be easily extended to 3D imaging and for non-Cartesian readout following the ideas proposed in the NC-PDNet extension [START_REF] Ramzi | NC-PDNet: a density-compensated unrolled network for 2D and 3D non-Cartesian MRI reconstruction[END_REF]. Finally, we present the results of retrospective studies performed on 4-fold under-sampled magnitudeonly data extracted from the above described in-house database that was constructed for analyzing the effects of aging on the brain. On 8 unseen test subjects, we show how MC-PDNet provides improved image quality in comparison to single contrast DL architectures (PD-Net, U-Net).

MODELING

In this section, we introduce a basic formulation of CS MC-MRI reconstruction that will be reused afterwards in the DL framework for handling the multi-contrast data consistency term.

Multi-contrast CS MR image reconstruction

Under multi-contrast setting with C contrasts, the stacked MR images can be modeled by

X = (xc) C c=1 with xc ∈ C N ×N ∀c ∈ [1, C] being a 2D MR image per contrast.
Let M be the number of measurements in the Fourier domain with M N × N as we operate in the CS setting. Then the acquired k-space data yc associated with the c th contrast2 is given by the following idealized forward model:

yc = FΩ c xc, ∀c = 1 . . . , C (1)
where FΩ c is the undersampled fourier operator. In our case of Cartesian under-sampling with masks Ωc, we get:

FΩ c = ΩcF,
where F is the fast Fourier transform. Note that different undersampling masks may be used across the multiple imaging contrasts for boosting the signal-to-noise ratio or recovering high frequencies in a complementary manner.

Let Ψs be an orthonormal transform (e.g. a wavelet transform) that sparsifies xc in the sense that zc = Ψsxc is a sc-sparse vector, i.e. its support contains at most sc non-zeros coefficients. For the sake of simplicity, we assume that zc ∈ C n Ψ , ∀c and denote Z = [z1, z2 . . . , zC ] T ∈ C n Ψ ×C . A convenient approach to perform CS MC-MRI reconstruction is to seek the global minimizer of the following convex but non-smooth cost function, which was introduced in the calibrationless multi-coil MRI reconstruction [START_REF] Gueddari | Calibrationless oscarbased image reconstruction in compressed sensing parallel mri[END_REF]:

Z = argmin Z∈C n Ψ ×C 1 2 C c=1 yc -FΩ c Ψ * s zc 2 2 + λg(Z). (2)
This formulation combines the 2 multi-contrast data consistency term with a group-sparsity promoting term g(•). If we assume that the same sparsity structure exists across the multiple contrasts, the group Least Absolute Shrinkage and Selection Operator or group-LASSO penalty [START_REF] Majumdar | Calibration-less multi-coil MR image reconstruction[END_REF] can be used for g. However, such sparsity structure may vary across contrasts, leading to the use of a more flexible choice for g like OSCAR penalty term (Octagonal Shrinkage and Clustering Algorithm for Regression) [START_REF] Gueddari | Calibrationless oscarbased image reconstruction in compressed sensing parallel mri[END_REF]. In [START_REF] Gueddari | Calibrationless oscarbased image reconstruction in compressed sensing parallel mri[END_REF][START_REF] Condat | A primal-dual splitting method for convex optimization involving Lipschitzian, proximable and linear composite terms[END_REF], a primal-dual optimization method proposed by Condat-Vũ was used to solve [START_REF] Huang | Fast multicontrast MRI reconstruction[END_REF]. It relies on the closed form expression of the proximity operator of g, which is available in the case of group-LASSO and OSCAR regularization (see [START_REF] Gueddari | Calibrationless oscarbased image reconstruction in compressed sensing parallel mri[END_REF][START_REF] Condat | A primal-dual splitting method for convex optimization involving Lipschitzian, proximable and linear composite terms[END_REF] for details).

After convergence, the multi-contrast images can be reconstructed as follows: ∀c ∈ [1, C], xc = Ψ * s zc. However, CS MC-MRI reconstruction is more demanding in terms of computing time as compared single contrast reconstruction. Additionally, there is no guarantee that the hand-crafted prior for CS MC-MRI reconstruction is optimal for the set of imaging contrasts under study as no information about the data is injected. Although dictionary learning [START_REF] Ravishankar | Magnetic Resonance Image Reconstruction from Highly Undersampled K-Space Data Using Dictionary Learning[END_REF] has helped to tackle this limitation and build up data-informed priors, this is done at the cost of computing time.

The recent rise of large imaging databases and DL has facilitated the emergence of new MRI reconstruction algorithms which efficiently use GPU processing to speed up the reconstruction times at inference (i.e. test time) and improve the image quality at the same time. Hereafter, we detail how we can benefit from cross-domain DL architectures as the most promising approaches to combine the data consistency term involved in (2) with a learned denoiser that corresponds to the proximity operator of the prior g.

Multi-contrast PDNet : MC-PDNet

In our study, we have focused on the primal dual network (PDNet) which unrolls the Primal Dual Hybrid Gradient (PDHG) algorithm [START_REF] Chambolle | A First-Order Primal-Dual Algorithm for Convex Problems with Applications to Imaging[END_REF]. In this architecture, proximal operators are replaced by convolutional operators. Additionally, we carry a buffer between the different unrolled iterations (Nc = 12) rather than a single multicontrast image estimate. This buffer allows us to learn a nonlinear acceleration scheme as explained in [START_REF] Adler | Solving ill-posed inverse problems using iterative deep neural networks[END_REF].

In this work, we adapted the originally developed multi-coil XPDNet [START_REF] Ramzi | Benchmarking MRI reconstruction neural networks on large public datasets[END_REF] to MC-MRI. The coil dimension in the network was replaced with the contrast dimension. Hence, the input to our network is the stacked multi-contrast k-space data Y = (yi) C i=1 . We discarded the parts of the network related to the sensitivity map refinement. We carried out a vectorized Fourier transform on all data to alternate between the image space and k-space. We present this multi-contrast PDNet (MC-PDNet) in Fig. 2. The components of this network include:

• Buffer State: To include the buffer state for each contrast image, we have taken the outer dimension as buffer dimension. As input dimensions will be n sl × C × H × W × B dim with n sl the number of slices, C the number of contrasts, H and W the respective height and width of convolution filters, B dim will define the buffer size (here, B dim = 5). • Multi-contrast Fourier transform: Under-sampled FFT and adjoint FFT were applied to each contrast using the same mask Ωc = Ω, ∀c for the sake of simplicity while alternating between primal (i.e. image) and dual (i.e. k-space) domains. • Multi-contrast Data consistency: Here, we extended to multicontrast imaging the measurements residual as done for the primal-only version of the PDNet and in the winning solution of the fastMRI challenge [START_REF] Pezzotti | An Adaptive Intelligence Algorithm for Undersampled Knee MRI Reconstruction: Application to the 2019 fastMRI Challenge[END_REF]:

f (Y, Y k , Ω) = Y -MΩY k
where MΩ is the multi-contrast stacked masking operator and Y k is the full k-space data at the kth unrolled iteration of MC-PDNet. • Joint metrics for training: We have given equal weight to each contrast during the training stage. The metrics used for evaluating the networks are as Peak signal-to-noise ratio (PSNR) and structural similarity index (SSIM). They are computed over the 35 middle slices of each volume and then averaged across contrasts, that is the dynamic range (used for the computation of PSNR and SSIM) is computed on the whole volume. The parameters and definitions used are the same as in the fastMRI paper [START_REF] Zbontar | fastmri: An open dataset and benchmarks for accelerated mri[END_REF]. By doing so, the MC-PDNet architecture jointly learns from each contrast and improves the overall reconstruction of each contrast.

EXPERIMENTAL SETUP

Data

In this study, we used 2D MR imaging dataset from Senior protocol acquired on a clinical 3T MR system (Magnetom Prisma FIT , Siemens Healthcare, Erlangen, Germany) with a 64-channel head/neck Fig. 2. Description of MC-PDNet (reference from [START_REF] Adler | Learned primal-dual reconstruction[END_REF]): Input data g are retrospective multi-contrast under-sampled k-space data. Here, the red and blue boxes represent the primal and dual networks, respectively. h0 and f0 represent the initial k-space and image respectively which are set as zero-valued tensors. T stands for the multi-contrast Fourier operator while T * represents the inverse multi-contrast Fourier operator. Final output is the stacked multi-contrast reconstructed image. coil array at NeuroSpin, CEA Paris Saclay, with an in-plane resolution of 1mm and slice thickness of 2.5mm. The 2D MR image size was 256 × 256 and we had 70 slices per volume. In this study we used the FLAIR, T * 2 GRE and T2 TSE contrasts in each volunteer and we left T1 images aside sa they did not match in size. The full dataset had 65 volumes, which was split into training and validation as 80% / 20%. 8 additional volumes were used for testing. We used magnitude-only images in DICOM format due to the absence of raw k-space data to take complex-valued images. We retrospectively under-sampled the Fourier space of these images using a 1D variable density Cartesian sampling along the phase encoding direction (see Fig. 3). For this study, we used an acceleration factor (AF) of 4 (i.e. 25% of k-space data were used). We fully sampled the 8% in the center of k-space to preserve the contrasts and then under-sampled the higher frequencies randomly.

Training details

Our model was trained with batch size of 1 with mean absolute error (MAE) as loss, at a learning rate of 10 -3 using the ADAM optimizer. We used a learning rate scheduler which reduced the learning rate by a factor of 0.5 if the network didn't improve for 15 consecutive epochs. We trained our network for 100 epochs on a Quadro RTX8000 GPU with 48GB DRAM. The entire training process took us 30 minutes.

RESULTS

We compared the proposed MC-PDNet against (i) zero-filled inverse Fourier transform as a naive baseline, (ii) U-Net [START_REF] Ronneberger | U-net: Convolutional networks for biomedical image segmentation[END_REF] with similar filters and (iii) PDNet. Each of these networks was learned on each contrast separately. Such comparison aims to understand if the proposed architecture utilizes the complementary information conveyed by other imaging contrasts. We present the PSNR and SSIM scores for T * 2 GRE, T2 FLAIR and T2 TSE in Tab. 1, 2, 3 respectively. We observe that MC-PDNet clearly outperforms all the competing methods with SSIM scores reaching up to 0.98 as compared to 0.93-0.94 in U-Net and 0.97 in PDNet. Specifically, improved performance of MC-PDNet architecture over PDNet was particularly observed in PSNR. This strongly suggests that the trained multi-contrast network utilizes complimentary information conveyed by the other data/contrasts.

Qualitative results

To understand the performance of MC-PDNet visually, we present a single reconstructed slice and residual from FLAIR contrast in the test set in Fig. 4. We clearly observe that MC-PDNet provides the closest image to the ground truth as its residual map retains much lesser structures. This is also corroborated with its higher SSIM and PSNR scores of 0.9724 and 38.21, respectively.

CONCLUSION AND DISCUSSIONS

In this study, we proposed a novel extension of the PD-Net architecture, called MC-PDNet, to address multi-contrast MR image reconstruction. We found that such a network can exploit complementary information conveyed by other imaging contrasts through the multiple k-space data. The information is actually shared in the image domain where the MR images as stacked as multiple channels with common weights, thereby improving image quality. MC-PDNet was evaluated retrospectively in the Cartesian setting on 4fold under-sampled multi-contrast data. However future prospect will involve the extension to non-Cartesian acquisition by mimicking the NC-PDNet [START_REF] Ramzi | NC-PDNet: a density-compensated unrolled network for 2D and 3D non-Cartesian MRI reconstruction[END_REF] and possibly the validation on larger public databases (e.g. OASIS-3).

COMPLIANCE WITH ETHICAL STANDARDS

This Senior protocol involves a total of 100 healthy volunteers scanned longitudinally once a year to understand the dynamics of aging for human brain. The protocol has been approved by local and national ethical committees, and written consent was obtained from the volunteers.

Fig. 1 .

 1 Fig. 1. Multi-contrast MR images (FLAIR, T2 Turbo spin echo -TSE and T2 Gradient Recalled Echo) sharing similar structures.

Fig. 3 .

 3 Fig. 3. Sampling mask (256 × 256) used to generate under-sampled k-space data.

Fig. 4 .

 4 Fig. 4. Reconstruction result for T2 FLAIR image (35th slice, one of the subject from test set), first column is the ground truth, second column shows reconstructed image from each model and third column depicts the residual between ground truth and reconstructed image

Table 1 .

 1 Performances on 8 test individuals at 3T (T2* GRE).

	Model	PSNR-mean (std) / SSIM-mean	# params
		(std)
	Zero Filled	30.13 (1.45) / 0.8233 (0.023)	0
	UNET	35.34 (1.61) / 0.9401 (0.010)	481801
	PDNet	40.55 (2.37) / 0.9763 (0.007)	381936
	MC-PDNet	41.3 (1.32) / 0.9813 (0.004)	381936

Table 2 .

 2 Performances on 8 test individuals at 3T (T2 FLAIR).

	Model	PSNR-mean (std) / SSIM-mean	# params
		(std)
	Zero Filled	26.38 (1.04) / 0.717 (0.022)	0
	UNET	32.62 (0.94) / 0.9304 (0.008)	481801
	PDNet	38.52 (1.89) / 0.9747 (0.007)	381936
	MC-PDNet	41.71 (1.75) / 0.9846 (0.003)	381936

Table 3 .

 3 Performances on 8 test individuals at 3T (T2 TSE).

	Model	PSNR-mean (std) / SSIM-mean	# params
		(std)

Zero Filled 25.43 (0.96) / 0.6812 (0.017) 0 UNET 30.93 (1.65) / 0.929 (0.014) 481801 PDNet 39.05 (1.92) / 0.9829 (0.004) 381936 MC-PDNet 40.0 (2.41) / 0.9847 (0.006)

Here, X stands for a wildcard and means that XPD-Net can internally host any CNN in the k-space and image domains.

Here we ignore B 0 inhomogeneities artifacts and T 2 -decay effects.

ACKNOWLEDGMENTS

We have no acknowledgments to disclose.


Sofia Benouakta
email: sofia.benouakta@univ-lyon1.fr

Florin Hutu
email: florin-doru.hutu@insa-lyon.fr

Yvan Duroc

Passive UHF RFID Yarn For Temperature Sensing Applications

Keywords: RFID sensor, textile yarn, E-Thread®, RFID tag

This paper presents an Ultra High Frequency (UHF) Radio Frequency IDentification (RFID) temperature sensor integrated into a textile yarn. The tag system consists of a thermistor of which the resistance is sensitive to temperature variation. The theoretical working principle is detailed and validated using electromagnetic simulation. The UHF RFID temperature sensor is fabricated using the E-Thread® technology. Experimental results demonstrate the potential of the sensor for detecting a temperature variation comprised between 25°C and 70°C through a variation of the reading distance between 4.5 meters to 6.2 meters.

I. INTRODUCTION

The Radio Frequency Identification (RFID) technology became ubiquitous in our everyday life [START_REF] Duroc | RFID: A key technology for Humanity[END_REF]. RFID is used in a wide range of applications such as logistics, access control and ID cards. Passive RFID systems consist on a communication between two elements: a reader and a passive tag by using the backscattering technique [START_REF] Nikitin | Performance limitations of passive UHF RFID systems[END_REF].

More recently, passive RFID tags operating in the UHF RFID band have gained interest in the wearable concept. For example, the association of an RFID tag to an accessory or a clothing allows its identification from a distance that can reach several meters. However, identification is not the only capability of an RFID tag and several research works have demonstrated their potential of being employed as sensors [START_REF] Tedjini | Augmented RFID tags[END_REF]. Several examples could be found in the literature where the RFID tags are made by using different techniques: for instance, a temperature sensor tag fabricated using 3D printing technique [START_REF] Trangkanukulkij | A 3D printed flexible passive RFID for temperature sensing[END_REF]; a humidity sensor tag fabricated using the inkjet printing technique and Kapton as a sensitive material [START_REF] Virtanen | Inkjet-printed humidity sensor for passive UHF RFID systems[END_REF]; a strain sensor tag fabricated using adhesive conductive textile in order to associate the antenna to a textile substrate [START_REF] He | Clothing-integrated passive RFID strain sensor platform for body movement-based controlling[END_REF]. In these studies, it is worth to note that the tag and the object are considered as two individual elements.

The concept presented in [START_REF] Andia | Slenderly and conformable passive UHF RFID yarn[END_REF] consists of an RFID tag integrated into a textile yarn using the new assembling technology, i. e. E-Thread® [START_REF] Vicard | Cap for a chip device having a groove, device provided with said cap, assembly consisting of the device and a wire element, and manufacturing method thereof[END_REF]. The RFID yarn allows every object integrating it to be identified. Inspired from this design, [START_REF] Benouakta | New approaches for augmented UHF RFID textile yarn[END_REF] emphasized the potential of the RFID yarn for sensing applications. In this paper, we propose the design of a passive RFID yarn including the temperature sensing functionality. The proposed structure of the sensor tag uses a thermistor as sensitive element and exploits the impedance mismatching between the antenna and the chip to translate the temperature variation.

Section II details the configuration and the theoretical concept of the designed temperature sensor-tag yarn. Section III presents the design parameters and the main simulated characteristics. Section IV shows the experimental tests in order to validate the proposed concept. Finally, section V draws the conclusion and perspectives.

II. PRINCIPLE OF THE TEMPERATURE SENSOR-TAG YARN

A. Design and Configuration

The proposed sensor tag (Fig. 1) relies on the E-Thread® UHF RFID yarn structure presented in [START_REF] Andia | Slenderly and conformable passive UHF RFID yarn[END_REF]. The tag is composed of a radiating element having the length L 1 associated to an RFID chip [10]. The RFID chip is connected to a stub of length L 2 . Both the radiating element and the stub are forming a transmission line where the conductors are of radius a and a' respectively. The conductors are separated by a distance s (center to center). The stub is loaded by a thermistor that has as an electrical equivalent model a variable resistance R th . The thermistor replaces the short circuit employed to achieve the impedance matching between the antenna and the chip in the initial solution. Note that this allows to maintain a similar (and so, realistic) assembling process.

B. Theoretical Concept

The temperature variation creates a modification of the thermistor's resistance R th which impacts the antenna's input impedance Z ant . This modification causes an impedance mismatching between the tag's antenna and the tag's chip that can be represented by a transmission coefficient τ, expressed by [START_REF] Duroc | RFID: A key technology for Humanity[END_REF].

𝜏𝜏 = 4𝑅𝑅 𝑐𝑐ℎ𝑖𝑖𝑖𝑖 • 𝑅𝑅 𝑎𝑎𝑎𝑎𝑎𝑎 �𝑍𝑍 𝑐𝑐ℎ𝑖𝑖𝑖𝑖 + 𝑍𝑍 𝑎𝑎𝑎𝑎𝑎𝑎 � 2 (1)
where 𝑍𝑍 𝑐𝑐ℎ𝑖𝑖𝑖𝑖 = 𝑅𝑅 𝑐𝑐ℎ𝑖𝑖𝑖𝑖 + 𝑗𝑗𝑋𝑋 𝑐𝑐ℎ𝑖𝑖𝑖𝑖 is the complex impedance of the RFID tag's chip and 𝑍𝑍 𝑎𝑎𝑎𝑎𝑎𝑎 = 𝑅𝑅 𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑗𝑗𝑋𝑋 𝑎𝑎𝑎𝑎𝑎𝑎 is the complex impedance of the RFID tag's antenna.

In order to determine the transmission coefficient τ for a fixed chip impedance, the expression of Z ant has to be defined as a function of the thermistor resistance R th , and of the stub's length Therefore, using the transmission line theory, the loaded stub's impedance viewed at the center of the radiating element can be expressed by:

𝑍𝑍 𝑡𝑡ℎ = 𝑍𝑍 0 𝑅𝑅 𝑡𝑡ℎ + 𝑗𝑗𝑍𝑍 0 tan 𝜃𝜃 𝑍𝑍 0 + 𝑗𝑗𝑅𝑅 𝑡𝑡ℎ tan 𝜃𝜃 (2)
where 𝜃𝜃 = 2 𝜋𝜋 𝐿𝐿 2 ⁄ 𝜆𝜆 is the stub's electrical length (with the 𝜆𝜆 wavelength) and 𝑍𝑍 0 is the transmission line's characteristic impedance considered in free space. The characteristic impedance 𝑍𝑍 0 can be expressed depending on the transmission line's dimensions as: 𝑍𝑍 0 = 276 log 10 ( 𝑠𝑠 �𝑎𝑎𝑎𝑎 ′ ). Finally, the antenna's input impedance Z ant is the result of a shunt between the radiating dipole impedance Z a and the loaded stub's impedance and can be expressed as:

𝑍𝑍 𝑎𝑎𝑎𝑎𝑡𝑡 = α𝑍𝑍 𝑎𝑎 𝑍𝑍 𝑡𝑡ℎ α𝑍𝑍 𝑎𝑎 + 𝑍𝑍 𝑡𝑡ℎ ( 3
)
where 𝛼𝛼 = 𝑙𝑙𝑙𝑙(𝑠𝑠/𝑎𝑎′) [𝑙𝑙𝑙𝑙(𝑠𝑠/𝑎𝑎′) -𝑙𝑙𝑙𝑙(𝑎𝑎/𝑎𝑎′)] ⁄ is the step-up factor that equals 4 if 𝑎𝑎 = 𝑎𝑎′. It is important to note that when 𝑅𝑅 𝑡𝑡ℎ = 0 Ω, the sensor tag exhibits the same behavior as matched RFID tag using the short circuit (i.e., "gammamatch" of the initial solution).

III. SIMULATION AND CHARATERIZATION OF THE RFID

TEMPERATURE SENSOR-TAG YARN The dimensions of the temperature sensor-tag yarn have been optimized for an operating frequency in the European UHF RFID band. The final structure has the following dimensions: 𝐿𝐿 1 = 136 𝑚𝑚𝑚𝑚, 𝐿𝐿 2 = 23 𝑚𝑚𝑚𝑚, a = a ′ = 60 µ𝑚𝑚, 𝑠𝑠 = 0.44 𝑚𝑚𝑚𝑚.

The considered Negative Temperature Coefficient (NTC) thermistor [START_REF] Murata | NTC thermistors[END_REF] is characterized by a decrease of its resistance for an increase of the temperature.

In order to study the impact of the temperature on the impedance matching of the sensor-tag in free-space, the transmission coefficient described by ( 1) is employed. The values of this coefficient are derived from electromagnetic simulations using the full wave simulation tool CST Microwave Studio® 2018. The thermistor's resistance Rth is varied between 0 Ω and 22 Ω to describe a temperature variation comprised between 25°C (ambient temperature) and a temperature higher than 150°C. The latter condition allows to achieve the impedance matching case (i.e., short circuit at the thermistor level which brings at the chip's level an equivalent impedance which is the complex conjugate of the chip's impedance).

The obtained result shows the impedance mismatching as a function of temperature. Moreover, two resonances are observed. In the case of the matching condition (zero resistance), the first resonance is observed at 865 MHz and the second one at 1070 MHz. These two resonance frequencies may be the result of the presence of two conductors having the lengths L1 and L 2 .

IV. EXPERIMENTAL CHARACTERIZATION OF THE RFID TEMPERATURE SENSOR-TAG YARN

A. Fabricated Prototype

The designed RFID temperature sensor-tag yarn has been fabricated using the E-Thread® assembling process [START_REF] Trangkanukulkij | A 3D printed flexible passive RFID for temperature sensing[END_REF]. Fig. 3 shows the sensor-tag that is integrated within a textile yarn.

The dimensions in millimeters are:

136 × 23 × 0.44 × 0.06 (L 1 × L 2 × 𝑠𝑠 × a).

B. Experimental Setup

Fig. 4 illustrates the experimental setup employed in order to characterize the manufactured sensor-tag prototype. The RFID sensor-tag is placed within a box and the temperature inside it is varied using a hot-gun and monitored using a thermocouple. The performance of the sensor tag is measured using Tagformance Pro test bench from Voyantic [START_REF] Voyantic | Tagformance Pro[END_REF].

C. Evaluation in terms of Read Range

The read range is an important and common criterion employed in practice for the characterization of an RFID UHF communication performance. This parameter is directly dependent on the transmission coefficient between the tag's antenna and the tag's chip. In the present evaluation, we are choosing the read range in order to highlight the tag's temperature variation. More precisely, when the temperature of the environment containing the sensor-tag varies, the thermistor's resistance is varying also and consequently the transmission coefficient follows the variation as shown by Fig. 2. Finally, the reading range is impacted by the mismatch between the tag's antenna and the tag's chip. In practice, the activation power (i. e. the minimum power level of the signal provided by the reader which activates the tag) will vary. Indeed, when the temperature increases, the activation power will decrease because the transmission coefficient 𝜏𝜏 increases.

Using the described experimental setup, the reading range of the sensor-tag is measured for a temperature range comprised between 25°C and 70°C (not more, in order to protect the RFID chip from heating issues).

The reading range for four temperature values and in the frequency range from 700 MHz to 1200 MHz is shown in Fig. 5. At an ambient temperature of 25°C the sensor tag has the lowest reading range of 4.5 meters. The increase of the temperature increases the reading range until it reaches the value of 6.2 meters. This behavior is in coherence with the proposed concept consisting of the sensor-tag's impedance matching improvement as the temperature increases. V. CONCLUSION In this paper, a UHF RFID temperature sensor-tag has been proposed for an integration into a textile yarn using the E-Thread® technology. By employing electromagnetic simulations, the dimensions of the tag's antenna and of the stub were optimized in order to obtain an impedance matching at 865 MHz for a zero resistance condition. A first sensor-tag prototype has been fabricated and experimentally tested. It is demonstrated that the temperature has an impact on the reading range of the sensor-tag which validates the proposed concept. The RFID yarn integrating a temperature sensor has very promising applications for example for monitoring the temperature of a clothing wearer in a non-invasive way while maintaining the passivity of the RFID tag.

Fig. 1 .

 1 Fig. 1. Configuration of the temperature sensor-tag yarn.

Fig. 2 .

 2 Fig. 2. Simulated transmission coefficient versus frequency for different temperature valeurs.

Fig. 3 .

 3 Fig. 3. Fabricated RFID temperature sensor-tag using the E-Thread® technology.

Fig. 4 .

 4 Fig. 4. Sensor-tag characterization setup using the Tagformance Pro test bench from Voyantic.

Fig. 5 .

 5 Fig. 5. Read range of the sensor-tag measured for a temperature range of 25°C -70 °C.

ACKNOWLEDGMENT

We would like thank the Région Auvergne-Rhône-Alpes for funding RAFTING project. Thank you to our industrial partner Primo1D for their contribution in the manufacturing process.