Dr Thomas Biet

Dr Kévin Martin

Dr Jihane Hankache

Dr Nora Hellou

Andreas Hauser
email: andreas.hauser@unige.ch

Thomas Bürgi

Dr Nicolas Vanthuyne

Prof Tal M Aharon

Marco Caricato
email: mcaricato@ku.edu

Jeanne Crassous

Narcis Avarvari
email: narcis.avarvari@univ-angers.fr

Prof A Hauser

Prof. T Bürgi

Caricato

Triggering the emission by the helical turn in thiadiazole-helicenes

Keywords: chirality, helicenes, thiadiazoles, chiroptical properties, DFT calculations

Introduction of heterocycles in the helical skeleton of helicenes allows modulation of their redox, chiroptical and photophysical properties. Herein, we describe the straightforward preparation and structural characterization by single crystal X-ray diffraction of thiadiazole-[7]helicene, which has been resolved into (M) and (P) enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands has been structurally characterized and it shows the presence of both (M) and (P) isomers coordinated to the metal centre. The emission properties of the unprecedented heterohelicenes are highly dependent on the helical turn, as the [7]-and

[5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4% and 6.5%, respectively. DFT calculations suggest a quenching of the luminescence of enantiopure [7]helicenes through an intersystem crossing mechanism arising from the relaxed excited S1 state.

Introduction

Heterohelicenes constitute a particularly interesting family of the non-planar conjugated aromatic helical molecules known as helicenes, [1] as they combine the inherent properties of carbohelicenes such as strong optical rotation and circular dichroism (CD), [2] nonlinear optical properties, [3] self-assembly abilities towards supramolecular materials, [4] with the specificity of the heterocycle or the corresponding heteroatom. A large majority of the reports on heterohelicenes concern either the thiahelicenes, where thiophene units replace totally [5] or partially the benzene rings, [6] or azahelicenes containing pyridine rings, [7] together with their metal complexes. [8] However, other heterocyclic units such as carbazole, [9] pyrane, [10] xanthenium, [11] quinacridinium, [12] pyridinium, [13] phenoxazine, [14] phenothiazine, [15] phosphole, [16] dibenzofurane, [17] or the more exotic azaborine [18] and silole [19] have been included relatively recently in helical structures for different functions and properties. The presence of certain heterocycles in the helical skeleton either provides electroactive character, with the possibility to access chiroptical redox switches, or allows the modulation of the luminescence properties. Concerning the former, besides thiahelicenes [5d] and helquats, [13b] examples of electroactive helicenes which have been studied for the redox modulation of the CD signal are still rare in the literature [20] and include organometallic derivatives of [6]helicene, [21] tetrathiafulvalene-helicenes [22] and helicene-quinones. [23] Note that for a phenothiazine based double hetero [4]helicene a crystalline radical cation salt has been isolated and structurally characterized. [15] On the other hand, it is known that fluorescence quantum yields of carbohelicenes are generally very low due to a very efficient intersystem crossing (ISC) from singlet to triplet excited states, [24] yet the presence of heterocycles such as carbazoles [9] and siloles, [19] possessing themselves luminescent properties, strongly enhances the fluorescence emission of the corresponding heterohelicenes. Moreover, when thiahelicenes are fused with electron acceptor quinoxaline units, [6e] or upon oxidation of the sulfur atoms into sulfones, [25] high fluorescence quantum yields and circularly polarized luminescence are observed, as a consequence of an increased energy gap between the lowest singlet and triplet states. Interesting to note also is the stronger emission for an S-shaped double azahelicene when compared to the simple azahelicene congener. [7d] A particularly interesting heterocyclic unit for its emission properties and electron-acceptor character is benzothiadiazole (BTD), [26] which has been extensively used during the last decade in the design of materials for red light emission, [27] organic field-effect transistors (OFETs), [28] and photovoltaics, [29] or associated in donor-acceptor dyads with tunable luminescence. [30] However, thiadiazole units have been never fused to helicene scaffolds to the best of our knowledge. We describe herein the synthesis and the structural characterization of enantiopure thiadiazole-[7]helicene and its S-shaped double [4]helicene, together with the smaller congeners thiadiazole-[5]helicene and thiadiazole-benzanthracene, respectively, as first representatives of a new family of electron poor heterohelicenes. Their chiroptical and photophysical properties, supported by DFT calculations, are investigated. The coordinating character of these helical ligands is highlighted through the preparation and solid state structure of a copper (II) complex containing two thiadiazole-[5]helicene units.

Results and Discussion

Synthesis and solid state structural analysis

The synthesis of the thiadiazole-[7]helicene 1 and double [4]helicene 2 starts with the Wittig reaction between the in situ generated phosphorous ylide derived from the phosphonium salt 6 and the benzothiadiazole aldehyde 8, followed by the oxidative photocyclization [1] of the intermediate stilbene 9 obtained as mixture of cis and trans isomers (Scheme 1). [START_REF]Experimental details and characterization of compounds, X-ray data, CV, UV-vis, ECD, VCD, photophysical measurements and theoretical calculations details are provided in the Supporting Information[END_REF] Thiadiazole-[5]helicene 3 and thiadiazole-benzanthracene 4 have been prepared through a similar strategy, starting from the methyl-naphthalene phosphonium bromide 7 and the same aldehyde 8 as before.

Scheme 1. Synthesis of thiadiazole-helicenes 1-4.

Although the photocyclization reaction is usually highly regioselective with respect to both aromatic units connected via the double bond, [1] in the present case we observed selectivity only for the benzothiadiazole moiety, while both possible isomers are formed for the carbocyclic parts, i.e. [4]helicene and naphthalene. They correspond to cyclization at either 1,2-or 2,3-positions, affording, respectively, the hetero-[7]helicene 1 and the fused bis-[4]helicene 2, or hetero-[5]helicene 3 and the thiadiazole-benzanthracene 4 which were separated by column chromatography. Note that the same regioselectivity issue for the photocyclization was observed for the formation of [7]carbohelicene versus its S-shaped isomer. [START_REF] Talele | [END_REF] Since generally [4]helicenes display no particular steric hindrance and have very low racemization barriers, [33] only compound 1 was resolved into its M and P enantiomers by chiral HPLC, which afforded very good separation. [START_REF]Experimental details and characterization of compounds, X-ray data, CV, UV-vis, ECD, VCD, photophysical measurements and theoretical calculations details are provided in the Supporting Information[END_REF] Additionally, attempts to resolve the [5]helicene 3 in the same conditions failed, which is not unexpected as [5]helicene has a racemization barrier of around 24 kcal mol -1 , [34] confirmed by theoretical calculations, [33,35] and, moreover, replacement of benzene rings by five member ring heterocycles severely reduces this value. [36] The first eluted enantiomer for 1 (using a Chiralpak IF column, see SI for the conditions) was the dextrorotatory, with a specific optical rotation of   25 +23465±3% °cm 2 /dmol. [37] Although in the case of helicenes the P enantiomer is dextrorotatory, [37b,38] we could also confirm the absolute configuration for the two enantiomers of 1 by single crystal X-ray analysis. Both (M)-1 and (P)-1 crystallize in the orthorhombic chiral space group P2 1 2 1 2 1 , with one independent helicene molecule in general position in the asymmetric unit (Figure 1). The dihedral angle between the terminal rings of the helicene skeleton, defining the helical curvature (hc), amounts to 45°, a typical value for [7]helicenes. [39] The thiadiazole ring overlaps with the terminal benzene C21-C26 with a distance of 3.87 Å between the centroids of the two cycles and several short N1•••C distances, e.g. N1•••C22 2.90 Å and N1•••C21 3.39 Å, but also partially with the penultimate benzene C17-C22 as shown by the short N1•••C18 distance of 2.79 Å (see also Figure S4 and Tables S1-S2). The packing is governed by - interactions, with some relatively short intermolecular S•••C distances (Figure S5). [START_REF]Experimental details and characterization of compounds, X-ray data, CV, UV-vis, ECD, VCD, photophysical measurements and theoretical calculations details are provided in the Supporting Information[END_REF] The double helicene 2 crystallized in the orthorhombic centrosymmetric space group Pbca with one independent molecule in the asymmetric unit. The most peculiar structural feature of 2 consists in the almost planarity of the thiadiazole-4[helicene] fragment as indicated by the value of the corresponding dihedral angle of only 8.28°, while the carbo-[4]helicene part shows a twist of 29.13° (Figure 2a and Table S3). Interestingly, the intramolecular N1•••H4 distance is 2.28 Å, suggesting the establishment of a hydrogen bonding interaction, which may be at the origin of the quasi-planarity of this part of the molecule. Moreover, a similar interaction is likely to occur also in the stilbene type intermediate cis-9 (Scheme 1), stabilizing the conformer providing 2 by cyclization, with respect to the one giving the [7]helicene 1. This could possibly partially explain the regioselectivity of the photocyclization reaction affording both compounds 1 and 2, besides the steric hindrance which is also a factor to be taken into account. In the packing of 2 a "segregation" between the carbocyclic part of the molecule and the thiadiazole ring is observed, with establishment of - interactions and short intermolecular S•••N contacts (Figure S6). Suitable single crystals for X-ray analysis have been obtained as well for the thiadiazole-benzanthracene 4, having the same thiadiazole-4[helicene] pattern as 2. It crystallized in the monoclinic system, centrosymmetric space group P2 1 /n, with one independent molecule in the asymmetric unit. Here again the intramolecular hydrogen bond N1•••H4 (2.27 Å) certainly contribute to the planarity of the molecule, with a dihedral angle of only 3.77° between the thiadiazole ring and the C5-C10 benzene ring (Figure 2b and Table S4). The packing is governed by short intermolecular N•••S distances of 3.28 Å providing dyads of 4 through formation of N2S2 rings, as often observed in solid state structures of thiadiazole derivatives, [30a,40] together with aromatic - and CH- interactions (Figure S7).

We have further preliminary estimated the coordinating ability of the thiadiazole-[5]helicene 3 towards Cu(II), as a few examples of benzothiadiazole-Cu(II) complexes have been described in the literature. [41] In thiadiazole-helicenes only the nitrogen atom outside the helical curvature is in principle available for coordination, thus precluding the formation of coordination polymers. Reaction of 3 with the precursors [Cu(hfac) 2 ]•xH 2 O (hfac = hexafluoro-acetylacetonate) in classical conditions [42] provides the complex Cu(hfac) 2 (3) 2 5 isolated as pale yellow-green crystals (Scheme 2).

Scheme 2. Synthesis of Cu(hfac) 2 (3) 2 complex 5.

The Cu(II) ion is surrounded by four oxygen atoms of the two hfac ligands at Cu-O distances of 1.930 -1.979 Å and two nitrogen atoms N1 and N3 of thiadiazole ligands disposed in trans, with Cu-N bond lengths of 2.42 and 2.49 Å (Table S5), forming an axially distorted octahedral coordination sphere (Figure 3 and S8). for 1 and3, and-1.39 V vs SCE for 2 and4, respectively (Figure S9 andTable S6). [START_REF]Experimental details and characterization of compounds, X-ray data, CV, UV-vis, ECD, VCD, photophysical measurements and theoretical calculations details are provided in the Supporting Information[END_REF]

Chiroptical properties of thiadiazole-[7]helicene 1

As [7]carbohelicenes show strong VCD activity thanks to their rigid helical structure, [43] we have set out to determine VCD spectra for both enantiomers of 1 and compare them with theoretical data. Significant VCD signals are observed between 1000 and 1600 cm -1 (Figure 4, see also Figures S10 for IR spectra and S11 for the optimized geometry of (P)-1), comparable with those in the spectra of [7]helicene. Particularly the bands between 1500 and 1600 cm -1 , associated with ring deformation modes, have the same sign in the spectra of 1 and [7]helicene (positive for M and negative for P enantiomer). Furthermore, all the bands in the range between 1100 and 1300 cm -1 have the same sign (negative for P enantiomer) both for 1 and [7]helicene. These modes are largely associated with C-H bending modes in the aromatic ring plane. The most prominent band of opposite sign around 1350 cm -1 (positive for P enantiomer) is associated with C-C stretching vibrations radial to the helical axis coupled with C-H bending modes in the aromatic ring plane. The antisymmetric N-S stretching vibration of 1 has about one order of magnitude stronger VCD signal than the bands shown in Figure 4, according to the calculations. However, this band arises below 850 cm -1 and is therefore not accessible to our VCD measurements. The calculated VCD for the optimized (P)-1 matches very well the experimental curve for the dextrorotatory enantiomer, thus confirming once again the assignment done by single crystal X-ray analysis. Comparison between the experimental and theoretical VCD spectra for (P)-1 (bottom).

ECD measurements for the two enantiomers (P)-(+) and (M)-(-) of 1 show the expected object mirror-image relationship, with a series of positive bands at 444, 370, 323 nm and two negative bands at 297 and 241 nm for (P)-1, while the opposite is observed for (M)-1 (Figure 5 top, including the corresponding experimental absorption spectrum). The UV-vis absorption spectra (see also Figures S12-13) show, as in the case of the ECD, the series of high energy bands, with the most intense one centered at 260 nm, very likely arising from a helicene based -* transition, [22] and then at 300 nm and 360 nm. In the lower energy region the weak band appearing at 445 nm ( = 1300 M -1 cm -1 ) corresponds to an intramolecular charge transfer (ICT) transition from helicene to the thiadiazole acceptor unit (vide infra). The calculated ECD and absorption spectra (Figure 5 bottom) excellently reproduce the experimental spectra of (P)-1, although peak maxima appear blue shifted by about 40 nm. All bands are ππ* excitations that involve different orbitals. The 444 nm band is a HOMOLUMO charge transfer transition from the helicene structure to the thiadiazole unit. The 370 nm band is also a charge transfer transition from the helicene to the thiadiazole, which can be primarily described as HOMO-2LUMO and HOMO-1LUMO+1. The band at 323 nm is primarily a HOMOLUMO+3 transition, and finally, the negative band at 297 is a transition from the thiadiazole to the helicene backbone (predominantly HOMO-2LUMO+1 and HOMO-1LUMO+2). All calculations were performed with the GAUSSIAN suite of programs. [START_REF] Frisch | Gaussian Development Version, Revision H.09+[END_REF] Figure 5. Experimental UV-vis and ECD spectra of 1 in DCM (5×10 -5 M) at room temperature (top) and theoretical UV-vis and ECD spectra of (P)-1 (bottom); DFT/CAM-B3LYP [START_REF] Yanai | [END_REF] /aug-cc-pVDZ, solvent: CH 2 Cl 2 , solvation model: PCM [46] with SMD radii. [47] Excitations appear at 410.67 nm, 342.84 nm, 301.5 nm, 285.19 nm, 262.54 nm, and 243.2 nm.

Interestingly, in the present case the ICT could be operative either through bond, as the molecule is conjugated, or through space thanks to the intramolecular  stacking interaction between thiadiazole and the two terminal benzene units, as suggested by the solid state structure (see Figure 1). This hypothesis is supported by the composition of the frontier orbitals, with the HOMO especially developed on the three terminal benzene rings while the LUMO is concentrated on the BTD part (Figure 6). All of the orbitals and their respective energies are reported in Figure S14 and Table S7 of the supporting information.

Photophysical properties and theoretical investigations of 1-4

Since the benzothiadiazole (BTD) unit was introduced into the structures of 1-4 for its luminescence, beside the electron poor character, we have set out to determine the emission properties of the helical thiadiazoles. However, as already mentioned, the luminescence of helicenes can be strongly decreased by an intersystem crossing (ISC) mechanism. Upon excitation at  ex = 365 nm both enantiomers of 1 show a weak emission band centered at 490 nm (20400 cm -1 ) as shown in Figures S12 for (M)-1 and S13 for (P)-1. The corresponding excitation spectra do show the main features of the absorption spectra but they are not totally superimposable, probably indicating some underlying impurity luminescence. Thus the experimental quantum yield of < 0.1% is to be regarded as upper limit for the luminescence of compound 1, which is evidently much lower than the usual values for BTD derivatives. [48] In order to investigate the quenching of the fluorescence, we optimized the geometry of the first excited singlet state of (P)-1, and we report the energy levels of singlet and triplet states in Figure 7. Assuming that intersystem crossing (ISC) occurs from the relaxed S1 geometry, the plot in Figure 7 shows a favorable energy alignment with nearby triplet states. In order to study the effect of spatial superposition of the terminal rings, we performed similar calculations with smaller congeners both by removing the terminal benzene ring or the thiadiazole ring. The data for these smaller system, reported in the SI (Figures S15-16 and Tables S8-S10) show a worse energy alignment for ISC, in agreement with results for [5]helicene and monoaza[5]helicenes. [49] Figure 7. Calculated energy levels (eV) of ground and excited singlet and triplet states for (P)-1, evaluated at the ground state optimized geometry (S0 Geom) and at the first singlet excited state optimized geometry (S1 Geom).

The ground state energy at S0 Geom is used as reference. The energy levels of the triplet states are slightly shifted for clarity. All excited singlet and triplet energy levels are computed with linear response methods.

In striking contrast to 1, the S-shaped double helicene compound 2 exhibits luminescence in dichloromethane solution both at room temperature, with a broad band centred at 528 nm (Figure 8 top), and a slightly structured band at 77 K (Figure S17), with relatively high emission quantum yield of 5.4% at room temperature when excited at  ex = 420 nm. The corresponding excitation spectrum is perfectly superimposed with the absorption spectrum, indicating that the luminescence is indeed exclusively from compound 2. The ground state and also the lowest excited state of 2 have been optimized (Figure S18). The calculated absorption and emission spectra reproduce accurately the experimental ones (Figure 8 bottom). From the calculated spectra we are able to assign all of the transitions as primarily ππ* charge transfer transitions. The absorption peak at 330 nm is due to a transition from the HOMO to the LUMO+2 on the helicene unit (see Figure S19 and Table S11). The lower energy charge transfer transitions are all from the helicene to the thiadiazole unit, differing only in the orbitals involved. The large peak at 360 nm comes primarily from HOMO-1LUMO transition, and the broad set of peaks around 400 nm originates from a similar charge transfer between HOMO and LUMO. For the emission spectrum, the calculated peak at  em = 491.3 nm (corresponding to the experimental peak at  em = 528 nm) is characterized by a charge transfer from the LUMO on the thiadiazole unit back to the HOMO on the helicene.

As for 1, a very weak emission band with maximum at 474 nm was recorded for 3 by excitation in solution at 386 nm (Figure S20). The slight differences of the absorption spectrum with the excitation spectrum (measured at emission wavelength 474 nm) might suggest that this luminescence is probably not real. By cooling the solution at 77 K the luminescence becomes stronger and the spectrum more structured with a slight shift to lower energy (Figure S21). Likewise, the excitation spectrum does not fit perfectly the absorption spectrum, therefore the assignement of this luminescence to some impurity cannot be excluded. Nevertheless, the calculated absorption and emission spectra of 3 are in good agreement with the experimental ones (see Figure S20 and Figure S22 for the optimized geometries of 3 in the ground state and singlet excited state), leaving open the question on the origin of the experimentally observed luminescence.

The photophysical properties of compound 4 resemble those of its longer congener 2, exhibiting luminescence in dichloromethane solution both at room temperature (Figure 9 top) and at 77 K (Figure S24) with emission quantum yield of 6.5%, slightly higher when compared to 2. The emission band in solution is centred at 515 nm for an excitation at  ex = 396 nm. Since the excitation spectra are perfectly superimposed with the absorption spectra, this compound exhibits real luminescence which is further supported by theoretical calculations (Figure 9 bottom and Figure S25 for the optimized geometries of 4 in the ground state and singlet excited state). Once again, the agreement between the experimental and calculated spectra allows us to assign the peaks in the absorption and emission spectra. Similar to the longer helicene 2, the transitions in 4 are primarily ππ*. The peak at 406.7 nm is mainly charge transfer from the HOMO localized on the helicene unit to the LUMO localized on the thiadiazole (see Figure S26). The peak at 341.5 nm is a transition from the HOMO-1, which is more spread out across the entire molecule, to the LUMO. And finally, the peak at 296 nm is a combination of two transitions, both from the HOMO but one to the LUMO+1 and the other to the LUMO+2.

The emission peak is a LUMO to HOMO charge transfer from thiadiazole to the helicene unit.

Conclusions

In summary, thiadiazole fused [7]helicene 1 and [5]helicene 3, together with their S-shaped double [4]helicene 2 and thiadiazole-benzanthracene 4 isomers have been synthesized and structurally characterized. A copper(II) complex (5) containing two hfac and two thiadiazole-

[5]helicene ligands has been crystallized. The vibrational and electronic chiroptical properties of the enantiomerically pure [7]helicenes have been investigated and supported by theoretical calculations. The photophysical properties of compounds 1-4 are highly dependent on the helicity. The DFT calculations accurately reproduced the experimental spectra, allowing the assignment of the transitions between the thiadiazole and helicene units, being mostly of ππ* type. In addition, the calculations give a strong rationale for the fluorescence quenching of thiadiazole-[7]helicene 1, occurring through ISC from the minimum of the S1 state to one of the many triplet states. The shorter [5]helicene 3 is hardly more emissive. In striking contrast, the S-shaped isomer 2 and benzanthracene 4 show strong luminescence.

These results open the way towards several directions of investigations such as the use of thiadiazole-helicenes as precursors for other functional helicenes and as ligands for transition metal complexes, the possibility of enhancing the emission properties through oxidation of the sulfur atom in sulfoxide or sulfone and thus observation of CPL, [25] or the exploitation of the electron poor character by association with electron rich moieties into helical donor-acceptors.

Experimental Section

General comments

Dry THF and ether were obtained from a solvent purification system (LC Technology Solutions Incorporated). Nuclear magnetic resonance spectra were recorded on a Bruker Advance DRX 300 spectrometer operating at 300 MHz for 1 H and 75 MHz for 13 C. Chemical shifts are expressed in parts per million (ppm) downfield from external TMS. The following abbreviations are used: s, singlet; d, doublet; t, triplet; m, multiplet. MALDI-TOF MS spectra were recorded on a Bruker Biflex-IIITM apparatus, equipped with a 337nm N 2 laser.

Elemental analyses were recorded using Flash 2000 Fisher Scientific Thermo Electron analyzer. IR spectra were recorded on Bruker FT-IR Vertex 70 spectrometer equipped with a Platinum diamond ATR accessory. UV-Vis absorption spectra for 1 were recorded in solution using either a Lambda 19 or a Lambda 250 PERKIN ELMER spectrometer. Corresponding circular dichroism (CD) measurements were performed with a JASCO Corp. J-715 or J-810 apparatus. UV-Vis absorption spectra for 2 and 4 in CH 2 Cl 2 solution were recorded on a Cary 5000 spectrometer. Luminescence and excitation spectra in CH 2 Cl 2 solution were recorded on Fluorolog 3 fluorescence spectrophotometer. All luminescence spectra were corrected for the spectral response of the spectrometer and the detector as well as for the irradiation intensity.

5-(2-(benzo[c]phenanthren-2-yl)vinyl)benzo[c][1,2,5]thiadiazole 9:

To a suspension of 2-Benzo [c]phenanthrylmethyl-triphenylphosphonium bromide 6 (0.6 g, 1.03 mmol) in dry THF (20 mL) and cooled to -78°C under argon, was added dropwise n-butyllithium 1.6 M in hexanes (675 µL, 1.05 eq.) and the resulting solution (orange) was stirred at -78°C for 10 min, warmed to RT over 30 min (turn to red color), then re-cooled to -78°C. A solution of 2,1,3benzothiadiazole-5-carbaldehyde 8 (169 mg, 1.03 mmol) in THF (5 mL) was then added dropwise, and the resulting solution (brown/yellow) was stirred at -78°C for 10 min, warmed to RT and stirred for 4 hours. The reaction mixture was filtered over celite and concentrated in vacuo. After chromatography over SiO 2 (CS 2 /dichloromethane 2:1 as eluent, Rf = 0.7), 9 was obtained as a yellow solid (cis+trans mixture) (323 mg, 81%). 1 H NMR (CDCl 3 , 300 MHz) δ (ppm) 9.20 (s, 1H), 9.15 (d, J = 8.7 Hz, 1H), 8.07-7.67 (m, 12H), 7.57 (d, J = 16.2 Hz, 1H), 7.43 (d, J = 16.2 Hz, 1H); Anal. Calc. for C 26 H 16 N 2 S: C, 80. 38; H, 4.15; N, 7.21; S, 8.25. Found: C, 80.11; H, 4.02; N, 7.09; S, 8.45%.

5-(2-(naphthalen-2-yl)vinyl)benzo[c][1,2,5]thiadiazole 10:

To a suspension of methyl-2naphthyltriphenylphosphonium bromide (0.7 g, 1.45 mmol) in dry THF (30 mL) and cooled to -78°C under argon, was added dropwise n-butyllithium 1.6M in hexanes (0.95 mL, 1.05 eq.) and the resulting solution (orange) was stirred at -78°C for 10 min, warmed to RT over 30 min (turn to red color), then re-cooled to -78°C. A solution of 2,1,3-benzothiadiazole-5carbaldehyde 8 (238 mg, 1.45 mmol) in THF (5 mL) was then added dropwise, and the resulting solution (brown/yellow) was stirred at -78°C for 10 min, warmed to RT and stirred for 4 hours. The reaction mixture was filtered over celite and concentrated in vacuo. After chromatography over SiO 2 (hexane/dichloromethane 1:1 as eluent, Rf = 0.5), 10 was obtained as a yellow solid (359 mg, 86%). 1 H NMR (CDCl 3 , 300 MHz) δ (ppm) 8.03-7.69 (m, 6H), 7. 35-7.56 (m, 4H), 7.00 (d, J = 12.2 Hz, 1H cis ), 6.85 (d, J = 12.2 Hz, 1H cis ); { 1 H} 13 C NMR (CDCl 3 , 75 MHz) δ (ppm); Anal. Calc. for C 18 H 12 N 2 S: C, 74.97; H, 4.19; N, 9.71; S, 11.12. Found: C, 74.71; H, 4.24; N, 9.59; S, 11. [c]phenanthren-2yl)vinyl)benzo [c][1,2,5]thiadiazole 9 (cis+trans mixture), (300 mg, 0.77 mmol) and iodine (catalytic amount) were dissolved in toluene (700 mL) and placed in a photoreactor equipped with an immersion lamp (150 W). The mixture was irradiated for 40 hours. After evaporation of the solvent and a flash chromatography over SiO 2 (dichloromethane as eluent), a mixture of the two regioisomers 1 and 2 (1:1) was obtained as a yellow solid (176 mg, 59%). A chromatography over SiO 2 (CS 2 as eluent) allowed to separate the two isomers, both as a yellow solid.

1: 1 H NMR (CDCl 3 , 300 MHz) δ (ppm) 8.21 (d, J = 8.3 Hz, 1H), 8.16-7.93 (m, 7H), 7.77 (d, J = 8.5 Hz, 1H), 7.70 (d, J = 9.1 Hz, 1H), 7.54-7.46 (m, 2H), 7.02 (t, J = 7.4 Hz, 1H), 6.55 (t, J = 7.7 Hz, 1H); { 1 H} 13 C NMR (CDCl 3 , 75 MHz) δ (ppm) 153.8, 152.3, 133.2, 132.3, 132.1, 132.0, 131.9, 130.5, 129.91, 129.86, 129.3, 128.4, 128.3, 127.2, 127.1, 127.0, 126.8, 126.4, 126.2, 126.0, 125.6, 124.8, 124.5, 124.3, 123.6, 119.5 , 80.80; H, 3.65; N, 7.25; S, 8.30. Found: C, 80.64; H, 3.58; N, 7.15; S, 8.58%.

2:

1 H NMR (CDCl 3 , 300 MHz) δ (ppm) 11.10 (s, 1H), 9.82 (s, 1H), 9.39 (d, J = 8.6 Hz, 1H), 8.39 (d, J = 8.7 Hz, 1H), 8.35 (d, J = 8.8 Hz, 1H), 8.21-7.90 (m, 7H), 7.83 (t, J = 7.7 Hz, 1H), 7.77-7.69 (m, 1H); { 1 H} 13 C NMR (CDCl 3 , 75 MHz) δ (ppm) 155.4, 153.8, 133.63, 133.62, 132.93, 132.87, 131.3, 131.0, 129.0, 128.9, 128.8, 128.3, 128.2, 128.0, 128.8, 127.8, 127.6, 127.5, 128.2, 126.9, 126.7, 126.4, 125.9, 120.3 153.6, 133.8, 133.4, 133.0, 132.9, 129.74, 129.69, 129.67, 129.0, 128.4, 127.54, 127.49, 127.20, 126.0, 123.9, 123.1, 119.8 4, 153.8, 133.3, 132.9, 132.8, 131.8, 131.2, 131.0, 129.5, 128.2, 127.8, 127.0, 126.5, 126.3, 126.0, 119.8 Then, the solution was allowed to return to room temperature, and pale yellow/green crystals of 5 were obtained after slow evaporation of solvents (20 mg, 90%

  optical rotation   25 D  = +22581±1% °cm 2 /dmol), corresponding to the clockwise (P)-1 enantiomer, according to the single crystal X-ray analysis (vide infra). Note that these values for specific optical and molar optical rotations are of the same order of magnitude as those for the pure [7]carbohelicene, i.e.,

Figure 1 .

 1 Figure 1. Molecular structures of enantiomeric (M)-1 (left) and (P)-1 (right) together with atom numbering scheme.

Figure 2 .

 2 Figure 2. a) Molecular structure of the S-shaped double [4]helicene 2 with an emphasis on the N1•••H4 interaction (top) and on the helical twist (bottom); b) Molecular structure of the thiadiazole-benzanthracene 4 with an emphasis on the N1•••H4 interaction (top) and side view (bottom).

Figure 3 .

 3 Figure 3. Molecular structure of complex 5 with a partial numbering scheme (top) and a side view showing the two helicities of the ligand (bottom).

Figure 4 .

 4 Figure 4. VCD spectra of the two enantiomers of thiadiazole-[7]helicene 1 for solutions of 5 mg in 200 l CD 2 Cl 2 (path length: 0.2 mm, 14000 scans accumulation, a VCD spectrum of the solvent was subtracted) (top).

Figure 6 .

 6 Figure 6. HOMO (-6.85 eV) and LUMO (-1.47 eV) orbitals for (P)-1 (DFT/CAM-B3LYP/aug-cc-pVDZ).

Figure 8 .

 8 Figure 8. Absorption, emission and excitation spectra of 2 (c = 2.58×10 -5 M in CH 2 Cl 2 solution) at RT; for emission  ex = 420 nm, for excitation  em = 528 nm (top). Calculated absorption and emission spectra of 2; DFT/CAM-B3LYP/aug-cc-pVDZ, solvent: CH 2 Cl 2 , solvation model: PCM with SMD radii and nonequilibrium solvation; absorption peaks at 316.1 nm, 357.7 nm, and 404.7 nm with a  em of 491.3 nm (bottom).

Figure 9 .

 9 Figure 9. Absorption, emission and excitation spectra of 4 (c = XXX M in CH 2 Cl 2 solution) at RT; for emission  ex = 396 nm, for excitation  em = 515 nm (top). Calculated absorption and emission spectra of 4; DFT/CAM-B3LYP/aug-cc-pVDZ, solvent: CH 2 Cl 2 , solvation model: PCM with SMD radii and nonequilibrium solvation;absorption peaks at 406.7 nm, 341.5 nm and 296 nm with a  em of 495.2 nm (bottom).

R1

  F o ) = F o  -F c /F o ; R w (F o 2 ) = [[w(F o 2 -F c 2 ) 2 ]/[w(F o 2 ) 2 ]] 1/2

  ; MS (MALDI-TOF) m/z = 386 (M + ), calcd. 386.09; Anal. Calc. for C 26 H 14 N 2 S: C

Table 1 .

 1 ). Anal. Calc. for C 46 H 22 CuF 12 N 4 O 4 S Details about data collection and solution refinement are given in Table1. X-ray diffraction measurements were performed on a Bruker Kappa CCD diffractometer, operating with a Mo Kα (λ=0.71073 Å) X-ray tube with a graphite monochromator. The structures were solved (SHELXS-97) by direct methods and refined (SHELXL-97) by full matrix least-square procedures on F 2 .[50] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations but not refined. Crystallographic data for the five structures have been deposited with the Cambridge Crystallographic Data Centre, deposition numbers CCDC 1487949 ((M)-1), 1487950 ((P)-1), 1487951 (2), 1500339 (4) and 1500340 (5). These data can be obtained free of charge from CCDC, 12 Union road, Cambridge CB2 1EZ, UK (e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk). Crystal Data and Structure Refinement for (M)-1, (P)-1, 2, 4 and 5.

	Compound	(M)-1	(P)-1	2	4	5
	empirical formula	C26H14N2S	C26H14N2S	C26H14N2S	C18H10N2S	C46H22CuF12N4O4S2
	fw	386.45	386.45	386.45	286.34	1050.34
	T (K)	293(2)	293(2)	293(2)	293(2)	293(2)
	wavelength (Å)	0.71073	0.71073	0.71073	0.71073	0.71073
	cryst syst	Orthorhombic	Orthorhombic	Orthorhombic	Monoclinic	Monoclinic
	space group	P212121	P212121	Pbca	P21/n	Cc
	a (Å)	8.4500 (9)	8.4492 (7)	8.8179 (7)	13.7510 (9)	27.195 (2)
	b (Å)	13.7216 (19)	13.7240 (13)	17.3503 (17)	6.1357 (3)	9.2166 (7)
	c (Å)	16.265 (3)	16.2628 (13)	24.0917 (15)	15.8590 (10)	18.863 (9)
	α(deg)	90.00	90.00	90.00	90.00	90.00
	β(deg)	90.00	90.00	90.00	99.759 (6)	111.230 (4)
	γ(deg)	90.0	90.0	90.0	90	90.0
	V (Å 3 )	1885.9 (4)	1885.8 (3)	3685.9 (5)	1318.69 (14)	4407.1 (5)
	Z	4	4	8	4	4
	Dc (g cm -3 )	1.361	1.361	1.393	1.442	1.583
	abs coeff (mm -1 )	0.186	0.186	0.191	0.238	0.691
	Flack parameter	0.11(16)	0.12(19)
	GOF on F 2	1.049	1.089	1.026	1.085	1.048
	final R indices [I >
	2σ(I)]

2 :

C, 52.60; H, 2.11; N, 5.33; S, 6.11. Found: C, 52.45; H, 2.18; N, 5.25

; S, 6.32%.

X-Ray structure determinations:

Acknowledgements

This work was supported in France by the CNRS through the GDR 3712 Chirafun, the University of Angers and the French Ministry of Education and Research (grant to T.B.). The investigation was supported in part by the University of Kansas General Research Fund allocation #2302049, by the University of Kansas startup fund (T.A. and M.C.), the University of Geneva and by the Swiss National Science foundation (grant No 200020_152780). J.C. and N.A. warmly thank Chengshuo Shen (University of Rennes 1) and Flavia Pop (University of Angers) for technical help.


Benoit Grosjean

Clarisse Pean

Alessandro Siria

Lydéric Bocquet

Rodolphe Vuilleumier

Marie-Laure Bocquet

Lyderic Bocquet

Chemisorption of Hydroxide on 2D Materials from DFT Calculations: Graphene versus Hexagonal Boron Nitride

come   L'archive ouverte pluridisciplinaire

The successful isolation of single-layer graphene sheet 3 has led to tremendous progress in the discovery of new 2D materials including boron nitride, silicene and transition metal dichalcogenides to cite a few. In particular hexagonal boron nitride monolayers share the planar honeycomb structure of graphene, with boron and nitrogen atoms alternating in the vortices of the honeycomb structure. Despite of their similar crystallographic structure, the electronic structure of BN and graphene nano sheets are drastically different : semimetallic for graphene and insulating for BN. Until recently both pristine layered materials were assumed to present inherently low chemical reactivity. However recent experimental works showed that covalent chemical fonctionalization could be achieved under drastic reactive conditions, either via reduction [START_REF] Sun | Charge-Controlled Switchable CO2 Capture on Boron Nitride Nanomaterials[END_REF][START_REF] Shin | Covalent Functionalization of Boron Nitride Nanotubes via Reduction Chemistry[END_REF] or reaction with oxidative reagents (H 2 O 2 , 6 oxygen radical [START_REF] Sainsbury | An Introduction to Statistical Thermodynamics[END_REF] ) or via fluorination. 8 Furthermore recent nanofluidic experiments have altered the long-standing picture of the chemical inertness of BN nanotubes by showing a large negative surface charge when the BN materials is in contact with aqueous saline solutions. In contrast, similar experimental investigations based on nanofluidic investigations showed that carbon nanotubes 2 and planar graphitic surfaces 9 exhibit smaller and even minute surface charge.

The magnitude of the maximum charging -ranging from 0.1 C / m 2 for C to 1 C/m 2 for BN depending on the solution pH -rules out point defects as the sole origin of the measured surface charging. These experimental results therefore call for two challenging questions:

(i) what are the potential chemical species present in basic and saline solutions that could adhere on the surface and strongly charge the pristine nanotubes ? (ii) how can we explain the different charging behavior of graphene and BN ideal nanotubes ?

A recent DFT study has investigated the dissociation barrier of a water molecule in contact with pristine graphene and BN nano sheets and concluded that the process is highly endothermic with a larger barrier on graphene than on BN. 10 They also proposed facile ways to enhance the dissociation, either via including substitutional defects or either via pre-adsorption of H, which again amounts to the presence of defects.

Here in this DFT work we consider pristine nano sheets and take into account the presence of hydroxide ions in water. We demonstrate that adding one electron (via the charged adsorbate) to these 2D materials permits to tune the adsorption strength of one hydroxide : a strong ionic adsorption on BN and a weak ionic adsorption on graphene. Our atomistic results corroborate the acidity (pK a ) of the BN sheet, pointing to the hydroxide anion as the best candidate to explain the charging of BN material immersed in a saline electrolyte. The total energies of the systems have been calculated using the periodic DFT code Vienna Ab initio Simulation Package VASP (see Methods). The graphene and h-BN single layers were modeled using the same supercell corresponding to an (3×5) orthorhombic unit cell containing 60 atoms in Periodic Boundary Conditions (PBC) (see figure 1). The slab monolayer is separated by a vacuum space of 15 Å to avoid the interaction between the periodic images. The gamma point has been used in the k-point sampling of the first Brillouin zone of the supercell. The graphene and h-BN bond distances are evaluated to 1.43 Å and 1.45 Å with the PBE functional in agreement with previous calculations with the same method. 11 We study the adsorption of anionic hydroxide molecule onto one B (or C) atom of the sp 2 BN (graphene) layer respectively. The resulting adsorption state is therefore negatively charged and an homogeneous background charge is automatically added in the periodic supercell to ensure its neutrality. As a result the total energy of a charged solid system includes an additional electrostatic interaction with the charged background. To compute the first adsorption energy of OH -called E ads1 , we consider the adsorbed OH on a negatively charged surface (by adding an extra electron to the total valence electrons) as the final state and the non-interacting species taken in the same supercell as the initial state as shown in Figure 2A-B. By doing so, the additional electrostatic interaction with the charged vacuum cancels out in the resulting adsorption energy.

E ads1 = E(OH a ) -E(OH f ), (1)
where the indices a and f stand for adsorbed and desorbed OH -respectively (see figure caption 2). The second adsorption energy proceeds similarly as illustrated in Figure 2 C-D.

E ads2 = E(2 OH a ) -E(OH a + OH f ) (2)
Table 1 summarizes the energetics computed within periodic DFT for the first OH -adsorption on pristine BN and graphene monolayers subjected to different environments. In vacuum the adsorption is strongly favorable with -2.3 eV on BN layer, while it is less favored on the graphene layer (-0.5 eV). The obtained values are strikingly different for both materials. The difference can be assigned to the charged ligand. Comparatively the neutral adsorption computed with spin-polarized calculations leads to similar moderate values (around -0.6 eV) in full agreement with previous studies. 10,12 Interestingly the adsorption values do not depend on the inclusion, or not, of van der waals (vdw) corrections (see E ads1 (vdw) in Table 1 ). Indeed in the final adsorption state the species are short-range bonded precluding vdw interactions and in the initial desorbed state the species are positioned to avoid any spurious vdw interaction.

In contrast the adsorption values in implicit water are greatly reduced by around 1 eV maintaining a favorable adsorption value for BN while turning to an unfavorable adsorption value for graphene. Notably including the solvent in the DFT modelling permits to discriminate both materials regarding the adsorption of one anionic hydroxyl : BN will spontaneously attach, while graphene should remain inert at least at low temperatures. We note in passing an alternative way to model the adsorption of a charged ligand with zero net charge in the supercell : the insertion of a counterion such as one potassium cation in the vacuum space at long distance from the adsorbate and the surface. The resulting energetics including the potassium cation are very similar to the charged procedure used in this study (see Supporting Information). Hence the discriminative energetics between the two materials are robust against different modeling schemes of the charging effect. The detailed DFT geometries of the adsorbed structures are displayed in Figure 3. Side views permit to visualize the sp 3 rehybridization of the bonding atom inducing an out-ofplane buckling somewhat larger for BN than for graphene. The bonding distances between the O atom and the anchoring site on the substrates are 1.5 Å for BN and 1.54 Å for graphene, typical for single C-O bonds. A pyramidalization angle θ p 13 can be determined and amounts to 18 • for BN and 15 • for graphene corresponding to 93 % and 77 % of sp 3 hybridization respectively. The geometrical differences are not as striking as the energy differences but it confirms a weaker adsorption of OH -on graphene than on BN. We have investigated the spatial distribution of the additional electron using the Mulliken charge analysis (see Methods). It strongly differs between BN and graphene nano sheets (see supplementary Table ). The grafted C atom on graphene gains only 0.06 electron and the rest of C atoms remains practically non-charged. On graphene the additional charge is totally delocalized over the nano sheet and the possible charge-activating role to bind hydroxyl is therefore masked. In contrast, on BN the grafted B gains 0.22 electron and becomes less positively charged than the rest of electron-deficient B atoms. The additional charge on BN is therefore substantially localized on the ligand and the grafted atom and the local reduction of the binding B atom can explain its improved reactivity.

We then studied the adsorption of a second OH -ion onto BN and graphene monolayers. eV. The difference in energy between the first adsorption and the plateau is however reduced as compared to vacuum. One may attribute this effect to the screening of charge.

Table 2: Thermodynamic corrections and the Gibbs free energy of the first adsorption evaluated in eV at 297,15 K and 1 bar. b designates the dielectric permittivity of the solvent, G(OH a ) is the Gibbs free energy of the system with OH -adsorbed on the surface and G(Sub + OH f ) is the Gibbs free energy of the system with a free OH -. All energies are expressed in eV. In the following we compute the thermodynamics of adsorption of OH -on the BN or graphene layers following successful approaches adapted to solid interfaces [14][15][16] (see Supporting Information). The correction terms that link E sol ads1 to ∆G ads were evaluated at T = 293.15 K and considering a solute standard concentration c 0 = 1 mol L -1 . The thermody-namic corrections are summarized in Table 2 while more details are available in Supporting

OH

Information. As a result we find a favorable ∆G ads1 = -0.46 eV for BN and a non-favorable ∆G ads1 = + 0.79 eV for graphene. Thermodynamic corrections also worsen the scenario for the graphene material. This result rules out this charging mechanism on bare graphene but permits to envision it for BN.

We further quantified how the predicted adsorption energies ∆G ads1 compare with the surface charge data extracted from the BN measurements in. [START_REF] Siria | Giant Osmotic Energy Conversion Measured in a Single Transmembrane Boron Nitride Nanotube[END_REF] From the value of ∆G ads1 , one can define the surface pK a as

pK a = 14 + ∆G ads1 k B T × log(10) . (3)
As an overall result, Eq. ( 15) predicts pK a = 6 for BN and pK a = 27.7 for graphene. The value for BN agrees well with the post-treatment of experimental data, 1 in which the electrostatic potential at the surface was evaluated using the Poisson-Boltzmann model. The value proposed for graphene is non-realistic precluding the same mechanism on graphene.

Our results support the strong reactivity of BN monolayer in the presence of hydroxyl anions contained in saline water. The contrasted result on graphene confirms the differential chemical behavior between the two materials. This echoes the experimental results using nanofluidic tools where much smaller adsorption is found on carbon nanotubes and planar graphitic surface. [START_REF] Secchi | Scaling Behavior for Ionic Transport and its Fluctuations in Individual Carbon Nanotubes[END_REF]9 Still, the presence of a surface charge on carbon tubes, may suggest that other species than OH -could adsorb onto the graphene surface and charge it.

Another interesting parameter is the maximum coverage for the ions on the surface. Such a quantity enters the global thermodynamics when considering for example the charge-pH relationship in the experiments. The adsorption equilibrium

Sub + OH - aq <=> Sub -OH - ads (4)
is usually modeled by a Langmuir adsorption model, which introduces a maximal fraction θ max < 1 of B or C atoms (for h-BN or graphene respectively) that constitute possible adsorption sites. This value for θ max fixes the maximal charge expected on the surface, which is an interesting quantity per se. Denoting θ the fraction of bonded B or C atoms to an adsorbed OH -, the chemical potential of OH -at the surface is then modelled as

µ s = ∆G ads1 -eφ + k B T log θ θ max -θ , ( 5
)
where φ is the electrostatic potential at the surface. The chemical potential of OH-in the bulk at the concentration [OH -] expressed in mol.L -1 is fixed at equilibrium by its bulk value.

µ b = k B T log[OH -], (6)
so that

∆G ads1 k B T - eφ k B T + log θ θ max -θ = log[OH -]. (7)
The equilibrium condition then becomes

log θ θ max -θ = eφ k B T + log(10) (pH -pK a ) , (8)
which relates the surface coverage to pH of the solution and the electrostatic potential at the surface.

A rough estimate for θ max can be done for BN on the basis of our DFT calculations. To this aim, we performed preliminary investigations of further adsorption of hydroxyl anions keeping the same minimum favorable distance revealed in Fig. 4. First results are presented on Fig. 5. Considering a third adsorption on the same hexagon leads to an energetically unfavorable configuration (plot B). The adsorption of three adsorbates in a raw is energetically favorable, although the zig zag configuration (plot C) is less favorable than the straight segment (plot D). Finally completing the lines with a fourth adsorbate becomes non-favorable (plots E and F). Of course these are first configurations and the stability of many other configurations should be evaluated. This goes beyond the present work and will be the object of a forthcoming study. However, on the basis of these results, one may estimate that 3 hydroxides can be grafted at most in a rectangle of (L+l 3OH )× L, which amounts to a surface of 50 Å 2 . This corresponds to a site density of 0.06 per Å 2 , and the associated full coverage site density is ∼ 0.18 per Å 2 (30 B atoms per supercell). This leads to θ max 0.33 and Σ max 0.96 Cm -2 . These values are in good agreement with the values reported for the maximum charge mesured on BN surfaces. [START_REF] Siria | Giant Osmotic Energy Conversion Measured in a Single Transmembrane Boron Nitride Nanotube[END_REF] In conclusion, we have investigated comparatively within ab initio DFT framework a monolayer of BN and graphene embedded in implicit water and in contact with one hydroxyl anion. We predict for the first time a contrasted chemical reactivity for graphene and BN :

a chemisorption of OH -on BN while this mechanism is not highlighted on graphene. This striking difference between the two pristine materials is linked to their electronic structure and the lack of screening for graphene surface as compared to the insulating BN. Furthermore, on the basis of atomistic thermodynamics including all the vibrational contributions to energy and entropy, we have derived a free energy of adsorption of -0.46 eV on BN which corresponds to a pK a 6, in quantitative agreement with experimental measurements. [START_REF] Siria | Giant Osmotic Energy Conversion Measured in a Single Transmembrane Boron Nitride Nanotube[END_REF] Hence this charging scenario is viable on BN and our study leans strong support to the recently-revealed non-inertness of the BN material in contact with aqueous saline phase. On the other hand, the case of graphene requires further investigations. Indeed planar graphitic surfaces show minute surface charge (as deduced from surface conductance measurements) 9

while the experiments on carbon nanotubes still exhibit a surface charge, 2 although much weaker as compared to BN tubes. [START_REF] Siria | Giant Osmotic Energy Conversion Measured in a Single Transmembrane Boron Nitride Nanotube[END_REF] This suggests the possibility of adsorption of other charged species on the carbon surfaces, not considered in the present study. Future work will involve the exploration at finite T of the charging mechanism using explicit water solvent and the inclusion of counterions using Ab Initio Molecular Dynamics AIMD, in order to evaluate a more realistic pK a value. Also taking the full stacking of these 2D materials into account, their curvature effect (since the measurements are made on multi-wall nanotubes of different lengths and diameters) are other ingredients which will be of high interest in understanding the molecular mechanisms at stake in the chemical reactivity of these 2D materials.

Methods

Ab initio calculations

In the VASP code, [START_REF] Kresse | Ab Initio Molecular Dynamics for Liquid Metals[END_REF][START_REF] Kresse | Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF] the wave functions of the system contained in the supercell have been expanded in a plane wave basis set with an energy cutoff of 800 eV. The common cutoff for hybrid interfaces is 400 eV in vacuum and taking twice the value is requested for sake of precision when dealing with the implicit solvent scheme (see below).The electronic cores are described by the projector augmented wave method. [START_REF] Kresse | From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method[END_REF] The exchange-correlation energies have been evaluated within the generalized gradient approximation (GGA).We employ GGA functionals as suggested by Perdew, Burke, and Ernzerhof (PBE). [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF][START_REF] Ziesche | Density Functionals from LDA to GGA[END_REF] However we have also verified that the key results (adsorption energies) obtained here are not particularly sensitive to the choice of exchange-correlation functional, in particular using the vdW-inclusive optB86b-vdW functional [START_REF] Klimeš | Van der Waals Density Functionals Applied to Solids[END_REF][START_REF] Klimeš | Perspective: Advances and Challenges in Treating Van der Waals Dispersion Forces in Density Functional Theory[END_REF] which include vdw interactions. We use the smearing technique using a Gaussian broadening of 0.2 eV to achieve electronic convergence. To check the possible influence of the calculation settings, we have modified sequentially the smearing value to 0.1 eV, the k-mesh to 3x3x1 and the vacuum space to 20 Å. The adsorption values varie between 0.1 to 0.2 eV but the striking difference between graphene and BN adsorption energies holds true.

In a second step the solvation energies in implicit water (E sol ) are evaluated within the joint density functional theory framework as implemented into VASP by Mathew and Hennig [START_REF] Mathew | Implicit Solvation Model for Density-Functional Study of Nanocrystal Surfaces and Reaction Pathways[END_REF] and successfully used recently. [START_REF] Lespes | Using Implicit Solvent in Ab Initio Electrochemical Modeling: Investigating Li + /Li Electrochemistry at a Li/Solvent Interface[END_REF][START_REF] Sakong | Density Functional Theory Study of the Electrochemical Interface Between a Pt Electrode and an Aqueous Electrolyte using an Implicit Solvent Method[END_REF][START_REF] Steinmann | Modeling the HCOOH/CO 2 Electrocatalytic Reaction: When Details Are Key[END_REF][START_REF] Garcia-Ratés | Multigrid-Based Methodology for Implicit Solvation Models in Periodic DFT[END_REF] In this technique the dielectric permittivity of the medium is defined as a functional of the electronic density. Indeed dielectric permittivity approaches asymptotically the bulk water value at 1 bar, b = 78.4, in the region where the electronic density is lower than ρ cut = 0.0025 Å 3 (the default cutoff charge density), while b =1 in the region when the electronic density is high (inside the slab treated as a solute). The cavitation energies which account for the solvant contributions are calculated with a surface tension parameter of 0.525 meV/Å 2 . The systems are fully optimized in vacuum with atomic forces lower than 0.02 eV per Å. Then the optimized vacuo geometries are subjected to the implicit solvant interaction and the total energies named E sol are corrected after an energy and geometry optimization. For the second adsorption E ads2 however we use the frozen geometry from vacuum for sake of computational time. As a result the adsorption energy in implicit water transforms into :

E sol ads1 = E sol (OH a ) -E sol (Sub + OH f ) (9)
To evaluate the vibrational entropic corrections to this adsorption energy, we have run a vibrational analysis of state A and B (see figure 2) in order to determine the vibrational modes and frequencies. The classical procedure in VASP is to calculate and diagonalize the Hessian matrix using the finite-difference method.

In addition we derived Mulliken atomic charges from a static Dmol3 calculation [START_REF] Delley | An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules[END_REF][START_REF] Delley | From Molecules to Solids with the DMol 3 Approach[END_REF][START_REF] Delley | Fast Calculation of Electrostatics in Crystals and Large Molecules[END_REF] based of the VASP optimized structures in order to understand the spatial location of the excess charge.

Oxygen Radical Functionalization of Boron Nitride Nanosheets. J. Am. Chem. Soc.

Figure 1 :

 1 Figure 1: (A) BN hexagonal primitive unit cell. (B) BN Orthorhombic primitive unit cell. (C) BN orthorhombic supercell adopted in the DFT calculations. Distances, expressed in Å, are written in red (black) for the case of BN (graphene). Boron, Carbon, Nitrogen and Oxygen are respectively displayed in yellow, grey, blue and red.(D) is the matrix to transform cell (A) into cell (B) and (E) the matrix to obtain cell (C) from cell (A).

Figure 2 :

 2 Figure 2: (top) Simulation cells for the BN monolayer (same on graphene not shown) used to determine the energy of the adsorption of the first anionic hydroxide. (A) Sub + OH f : substrate and one desorbed anionic hydroxide in gaseous or aqueous phase. (B) OH a : substrate with an adsorbed hydroxide onto either a B atom (BN layer) or a C atom (graphene layer). (bottom) Simulation scheme used to determine the energy of the adsorption of a second hydroxide (C) OH a + OH f : a surface with an adsorbed hydroxide and a desorbed hydroxide in gaseous or aqueous phase. (D) 2 OH a : a substrate with two adsorbed hydroxides.

Figure 3 :

 3 Figure 3: DFT adsorption structures. Top views of BN (A) and graphene (B). Insets with side views of the bonding site for each material with key distances and angles expressed in Å and degrees, respectively.

Figure 4

 4 Figure4displays the second adsorption energy in vacuum E ads2 (dashed lines) and in implicit

Figure 4 :

 4 Figure 4: A) Circles display possible sites (labelled with letters) for the second adsorption of OH -at a given distance from the first adsorption site labelled A. B) Same for graphene. C) Adsorption energy of a second hydroxide versus the distance between the two adsorption sites. Results for BN are displayed by circles connected by blue lines while results for graphene are represented by squares connected by red lines. Horizontal lines represent the adsorption energies for single OH -ions (blue for BN, red for graphene). Plain and dashed lines respectively correspond to adsorptions in implicit water and in vacuum.

Figure 5 :

 5 Figure 5: Beyond the second adsorption on BN monolayer : from two adsorbates in the same honeycomb at the minimum favorable distance (A) exploration of three adsorbates (B-C-D) and four adsorbates (E-D) keeping the same favorable distance. The indicated energy is the adsorption energy of the last adsorbate.

Table 1 :

 1

	OH -(OH . )	BN layer	Graphene layer
	E ads1	-2.3 (-0.63, M = 1) -0.5 (-0.57, M=1)
	E sol ads1	-0.89	+.33
	E ads1 (vdw)	-2.1	-0.52
	E sol ads1 (vdw)	-0.9	+.33

Adsorption energies (in eV) of the first OH -species onto BN and Graphene layers in vacuum and in implicit water. Negative (positive) values mean favorable (unfavorable) cases. For the adsorption in vacuum, values in parentheses are spin-polarized calculation for the uncharged system, i.e the radical OH . adsorption that induces magnetism in graphene with M the electron magnetism.

  2012, 134, 18758-18771. (8) Tang, C.; Bando, Y.; Huang, Y.; Yue, S.; Gu, C.; Xu, F.; ; Golberg, D. Fluorination and Electrical Conductivity of BN Nanotubes. J. Am. Chem. Soc. 2005, 127, 6552-6553. (9) Radha, B.; Esfandiar, A.; Wang, F. C.; Rooney, A. P.; Gopinadhan, K.; Keerthi, A.; Mishchenko, A.; Janardanan, A.; Blake, P.; Fumagalli, L. et al. Molecular Transport Through Capillaries Made With Atomic-Scale Precision. Nature 2016, 538, 222-225. (10) Al-Hamdani, Y. S.; Alfè, D.; von Lilienfeld, O. A.; Michaelides, A. Tuning Dissociation Using Isoelectronically Doped Graphene and Hexagonal Boron Nitride: Water and Other Small Molecules. J. Chem. Phys. 2016, 144, 154706. Haddon, R. C. Chemistry of Single-Walled Carbon Nanotubes. Acc. Chem. Res. 2002, 35, 1105-1113. (14) Reuter, K.; Scheffler, M. Composition and Structure of the RuO 2 (110) Surface in an O 2 and CO Environment: Implications for the Catalytic Formation of CO 2 . Phys. Rev.

	B 2003, 68, 045407.

(11) Graziano, G.; Klimeš, J.; Fernandez-Alonso, F.; Michaelides, A. Improved Description of Soft Layered Materials with Van der Waals Density Functional Theory. J. Phys.: Condens. Matter 2012, 24, 424216. (12) Ghaderi, N.; Peressi, M. First-Principle Study of Hydroxyl Functional Groups on Pristine, Defected Graphene, and Graphene Epoxide. J. Phys. Chem. C 2010, 114, 21625-21630. (13) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; (15) Loffreda, D. Theoretical Insight of Adsorption Thermodynamics of Multifunctional Molecules on Metal Surfaces. Surf. Sci. 2006, 600, 2103 -2112.

Acknowledgement B.G., C.P. and M.-L.B. thank the national computational center IDRIS, CINES and TGCC (Grant 2015-[x2015087364]) for the computer grant. L.B. and A.S. thank ANR Blue Energy for support. A.S. and B.G. acknowledge funding from the European Union's H2020 Framework Programme / ERC Starting Grant agreement number 637748 -NanoSOFT.

Supporting Information Available

Additional figures and information. This material is available free of charge via the Internet at http://pubs.acs.org/.


Andre Nel
email: anel@mnhn.fr

Ziann Perreau
email: perreauziann@yahoo.fr

Gaelle Doitteau
email: gaelle.doitteau@mnhn.fr

The oldest representative of the modern snipe fly genus Symphoromyia (Diptera: Rhagionidae)

Keywords: Diptera, Rhagionidae, Symphoromyia, Eocene, Oise amber

We describe and illustrate from the earliest Eocene amber of France the oldest representative of the rhagionid modern genus Symphoromyia, under the name of S. eocenica sp. nov. Its mouthpart structure shows that it was hematophagous as is the modern representatives of the genus, demonstrating a remarkable morphological stability for 53 Myr.

Introduction

As a very ancient brachyceran group, the rhagionid flies may provide insights into the early evolution of the Tabanomorpha. [START_REF] Kerr | Phylogeny and classification of Rhagionidae, with implications for Tabanomorpha (Diptera: Brachycera)[END_REF] provided an extensive phylogeny and classification of the snipe flies. Fossils are rather frequent, especially in the Mesozoic (see fossil record in Zhang 2013; [START_REF] Angelini | A new genus and species of snipe fly (Diptera: Rhagionidae) in Lebanese Cretaceous amber[END_REF]. The majority of the Cenozoic species were described from the Eocene Baltic amber [START_REF] Evenhuis | Catalogue of the fossil flies of the world (Insecta: Diptera)[END_REF]. Most of fossil snipe flies are compressions with crucial structures hardly visible. With seven described species, the modern blood-sucking genus Symphoromyia Frauenfeld, 1867 seems to be among those with the greatest number of fossil representatives (after Rhagio Fabricius, 1775), all from the Middle Eocene to Early Oligocene [START_REF] Evenhuis | Catalogue of the fossil flies of the world (Insecta: Diptera)[END_REF][START_REF] Evenhuis | Catalog of the fossil flies of the world (Insecta: Diptera) (Version 2.0)[END_REF]. Here we describe a new earliest Eocene fossil that corresponds to the oldest representative of this genus.

Material and methods

The Oise amber deposit is located near Creil at the place known as "Le Quesnoy" (Oise, northern France). The lignite layers containing the amber belong to the Lower Eocene of the Paris basin (~53 Ma). This age corresponds to Paleocene-Eocene Thermal Maximum (PETM), one of the most important periods of global warming. Data from the evaluation of flora and fauna in Oise amber suggest a semideciduous forest under a hot climate with a wet and a dry season. Representatives of the Combretaceae or Caesalpiniaceae are considered as amber producing trees in this region. Up to date, 20,000 amber inclusions have been collected from this site, including hexapods, mites, spiders, and two pseudoscorpions. Plant remains (leaves, seeds, flowers, pollen), fungi and microorganisms are also very abundant (for review see [START_REF] Nel | Un gisement sparnacien exceptionnel à plantes, arthropodes et vertébrés (Éocène basal, MP7): Le Quesnoy (Oise, France)[END_REF][START_REF] Brasero | Insects from the Early Eocene amber of Oise (France): diversity and palaeontological significance[END_REF].

The piece of amber with the type specimen of Symphoromyia eocenica sp. nov. was polished and included into Canada balsam between two glasses. The fossil was examined and measured using an incident light (Olympus SZX9), and a Leitz Wetzlar binocular microscope and combined using Adobe Photoshop software.

Systematic

Discussion

If we follow the key to fly families of [START_REF] Mcalpine | Key to families, adult[END_REF], this fossil would fall in the Rhagionidae for the following characters: empodia pulvilliform; head more than half as wide as thorax; eyes not holoptic (female); subscutellum weakly developed; wing with anal lobe well-developed; clypeus exposed and convex; distal flagellomeres forming an arista. The reniform first flagellomere would suggest possible affinities with the Athericidae (but also present in some Rhagionidae), but Athericidae are excluded for the absence of one main synapomorphy, viz. wing veins R 1 and R 2+3 meeting together at the wing margin [START_REF] Kerr | Phylogeny and classification of Rhagionidae, with implications for Tabanomorpha (Diptera: Brachycera)[END_REF] (note that R 1 and R 2+3 are also strongly approximate in some Rhagionidae, e.g., Chrysopilus balbii [START_REF] Santos | Chrysopilus (Diptera: Rhagionidae) from Brazil: redescription of Chrysopilus fascipennis Bromley and description of eleven new species[END_REF]. If we follow the key to adult genera of Rhagionoidea of [START_REF] Kerr | Phylogeny and classification of Rhagionidae, with implications for Tabanomorpha (Diptera: Brachycera)[END_REF], this fossil falls in the genus Symphoromyia for the following characters: Clypeus bulbous; postspiracular sclerite smooth and flat; hind tibial spur one; M3 present, reaching wing margin; laterotergite setose; macrochaetae of hind tibia absent; palpus two-segmented; scale-like thoracic setae absent; first antennal flagellomere kidney-shaped, with dorsoapical arista; and mandibles present. The enlarged scape and the aristate and kidney-shaped in profile flagellomere are autapomorphic characters of this genus, after [START_REF] Kerr | Phylogeny and classification of Rhagionidae, with implications for Tabanomorpha (Diptera: Brachycera)[END_REF].

Many modern species within the genus Symphoromyia are separated on the basis of body and leg coloration [START_REF] Turner | A revision of the genus Symphoromyia Frauenfeld (Diptera: Rhagionidae). I. Introduction, subgenera and species-groups, review of biology[END_REF], which are not visible in this fossil. Also the lack of information on the genitalia structures forbid us to compare this fossil with the different modern species.

Nevertheless as it is relatively old, it certainly belongs to an extinct new species. [START_REF] Cockerell | Fossil insects from Florissant, Colorado[END_REF] described Symphoromyia subtrita from the Early Oligocene of Florissant (Colorado) and [START_REF] Théobald | Les insectes fossiles des terrains oligocènes de France[END_REF] described Symphoromyia marginata from the Early Oligocene of Kleinkembs (Germany, Rhine Valley), both on the basis of compression fossils. These fossils show clearly less characters than the present one for an attribution to the genus Symphoromyia. They should be revised to verify their attribution. Our fossil has a wing shorter than those of S. marginata and S. subtrita, allowing to consider that it belongs to a different species. [START_REF] Meunier | Monographie der Leptiden und der Phoriden des Bernsteins[END_REF] described Atherix evecta, A. examinata, and A. exigua from the Middle Eocene Baltic amber, later transferred into the genus Symphoromyia [START_REF] Meunier | Sur quelques Diptères (Bombylidae, Leptidae, Dolichopodidae, Conopidae, Chironomidae) de l'ambre de la Baltique[END_REF][START_REF] Hennig | Die sogenannten "niederen Brachycera" im Baltischen Bernstein (Diptera: Xylophagidae, Xylomyidae, Rhagionidae, Tabanidae)[END_REF][START_REF] Stuckenberg | A new genus and two new species of Athericidae (Diptera) in Baltic amber[END_REF]. All these species have wings smaller than S. eocenica (2.5 to 3.5 mm long instead of 4.2 mm long). [START_REF] Paramonov | On the entomofauna of ambers[END_REF] described S. succini and S. tertiarica from the Baltic amber. The first flagellomere of S. tertiarica has a shape different from that of S. eocenica, i.e. with part below arista straight. Both S. succini and S. tertiarica have wings smaller than S. eocenica (3.0-3.5 mm long).

All fossils currently attributed to Symphoromyia should be revised to verify if they show all the characters diagnostic of this genus, especially the antennal structures.

Modern Symphoromyia species are Holarctic. Female take blood meals from vertebrate hosts [START_REF] Kerr | Phylogeny and classification of Rhagionidae, with implications for Tabanomorpha (Diptera: Brachycera)[END_REF]. The presence of acute mandibles and laciniae in S. eocenica indicates that it was probably also a hematophagous insect.

  Named after the Eocene period. Material. Holotype MNHN.F.A57537 (PA 2496, sex unknown), stored in the laboratory of palaeontology, MNHN, Paris. Type locality. Farm Le Quesnoy, 49°19'31" N, 2°40'22" E, Chevrière, Region of Creil, Oise Department (northern France). Type horizon. Lowermost Eocene amber, Sparnacian (53 Ma), level MP7 of the mammal fauna of Dormaal (Brasero et al. 2009). Diagnosis. Clypeus bulbous; postspiracular sclerite smooth and flat; hind tibial spur one; R2+3 well separated from R1 at apices; M3 present, reaching wing margin; laterotergite setose; macrochaetae of hind tibia absent; palpus two-segmented; scale-like thoracic setae absent; first antennal flagellomere kidney-shaped, convex below arista, arista subterminal and anterodorsal; mandibles present; wing ca. 4.2 mm long; body ca. 5.1 mm long. Description. Body ca. 5.1 mm long. Head 0.71 mm long, ca. 1.0 mm high. Clypeus bulbous. Face bare between eyes. Scape 0.07 mm long, 0.08 mm high, clearly larger than pedicel. First flagellomere of antenna kidney-shaped, convex below arista, laterally compressed, enlarged, bearing a long subterminal anterodorsal arista. Eyes large, without visible setae; distinctly separated. Labella ca. 0.79 mm long, slightly shorter than height of head. Palpus twosegmented, distal segment setose, 0.24 mm long, longer than proximal segment, ca. 0.2 mm long. Lacinia longer than palpus, with serrated tip. Mandibles long and acute. Thorax 1.6 mm long. Dorsocentral bristles of mesonotum absent; all dorsal setae of equal length. Anepisternum setulose throughout posterior half. Laterotergite setose. Postspiracular sclerite smooth and flat. Prescutellum present. Subscutellum inconspicuous. Wing apparently hyaline, without markings or membrane lightly to moderately infuscate, brownish. Lower calypter reduced. Upper calypter well developed, with broad curvature, lobe-like. Wing ca. 4.2 mm long, 1.6 mm wide. Apex of costa not preserved. Humeral crossvein well developed. Crossvein sc-r weakly developed, positioned distal to h a little less than length of h. R 1 and R 2+3 separated at wing margin. Dorsal side of R 1 setulose, ventral side bare. All other wing veins without setulae. Apical third of R 2+3 ultimately bends anteriorly slightly, toward leading edge of wing margin. Length of R 2+3 probably shorter than R5. Base of R 4+5 fork proximal distal end of cell dm. R 4 nearly straight apically. R 5 probably posterior to wing tip, clearly longer than R 4+5 . Crossvein r-m at proximal one-third of discal cell. M 3 present. Crossvein mcu present. Origin of CuA 1 at cell bm near discal cell. M 3 cell at margin convergent. CuA 2 length ca. 2/3 the length of posterior vein of cell bm. Alula with broad, evenly rounded curvature. Anal lobe well developed. Cell cup open. Halter knob approximately 3/5 length of stem. Tibial spur formula 0:2:1. Hind tibial macrochaetae absent. Abdomen ca. 2.4 mm long, 1.2 mm high. Abdominal segments evenly tapered. Segmentation not clearly visible, last 3 abdominal segments telescoped; genitalia not visible.

FIGURE 1 .

 1 FIGURE 1. Symphoromyia eocenica sp. nov., holotype MNHN.F.A57537. Habitus. A, left

FIGURE 2 .

 2 FIGURE 2. Symphoromyia eocenica sp. nov., holotype MNHN.F.A57537. A, head, white

FIGURE 3 .

 3 FIGURE 3. Symphoromyia eocenica sp. nov., holotype MNHN.F.A57537. Wing.

FIGURE 4 .

 4 FIGURE 4. Symphoromyia eocenica sp. nov., holotype MNHN.F.A57537. A, lateral view of

Acknowledgements

We sincerely thank Charles Morphy D. Santos and Neals Evenhuis for their very useful comments on the first version of the paper. We also thank the Lafarge-Granulat company for help with the fossil sampling and the Langlois-Meurinne family for the authorization of working on their property.