Thierry Buffeteau

Delphine Pitrat

Nicolas Daugey

Nathalie Calin

Marion Jean

Nicolas Vanthuyne

Laurent Ducasse

Frank Wien

Thierry Brotin

Chiroptical properties of cryptophane-111 †

The two enantiomers of cryptophane-111 (1), which possesses the most simplified chemical structure of cryptophane derivatives and exhibits the highest binding constant for xenon encapsulation in organic solution, were separated by HPLC using chiral stationary phases. The chiroptical properties of [CD(+) 254 ]-1 and [CD(À) 254 ]-1 were determined in CH 2 Cl 2 and CHCl 3 solutions by polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) experiments and were compared to those of cryptophane-222 (2) derivative. Synchroton Radiation Circular Dichroism (SRCD) spectra were also recorded for the two enantiomers of 1 to investigate lowlying excited states in the 1 B b region. Time-dependent density functional theory (TDDFT) calculations of the ECD and SRCD as well as DFT calculations of the VCD and ROA allowed the [CD(À) 254 ]-PP-1 and [CD(+) 254 ]-MM-1 absolute configurations for 1 in CH 2 Cl 2 and CHCl 3 solutions. Similar configurations

were found in the solid state from X-ray crystals of the two enantiomers but the chemical structures are significantly different from the one calculated in solution. In addition, the chiroptical properties of the two enantiomers of 1 were independent of the nature of the solvent, which is significantly different to that observed for cryptophane-222 compound. The lack of solvent molecule (CH 2 Cl 2 or CHCl 3 ) within the cavity of 1 can explain this different behaviour between 1 and 2. Finally, we show in this article that the encapsulation of xenon by 1 can be evidenced by ROA following the symmetric breathing mode of the cryptophane-111 skeleton at 150 cm À1 . † Electronic supplementary information (ESI) available: Synthesis of (rac)-1.

Separation of the two enantiomers of 1 by HPLC using chiral stationary phase. 1 H and 13 C NMR spectra of the two enantiomers of 1 in CD 2 Cl 2 solution. Crystallographic data and pictures of the X-ray crystals of the two enantiomers of 1. UV-vis, ECD, SRCD, IR, VCD, Raman and ROA spectra of the two enantiomers of 1 in various solvents. ROA spectrum calculated at the B3PW91/6-31G** level (IEFPCM = CHCl 3 ) for conformer A of MM-1. Experimental SOR values measured at several wavelengths in various solvents and SOR values calculated at the B3PW91/6-31G** level (IEFPCM = CHCl 3 and CH 2 Cl 2 ) for conformer A of MM-1. CCDC 1537585 and 1537591.

Introduction

The cryptophane backbone displays a very simple and easily recognizable chemical structure, which is composed of six aromatic rings positioned into a rigid molecular frame. 1,2 The six aromatic rings are assembled into two independent cyclotribenzylene (CTB) sub-units connected together by three linkers, whose length and nature can be varied. This structure generates a lipophilic cavity that can accommodate a large variety of guest molecules, such as halogenomethanes and ammonium salts, or atoms in organic or aqueous solutions. 2 The cryptophane-111 skeleton (compound 1 in Scheme 1) appears as the most simplified structure of cryptophane derivatives and its synthesis has been reported for the first time in 2007. 3 This compound exhibits the highest binding constant (10 4 M À1 at 293 K) for xenon encapsulation in organic solvent but it does not bind halogenomethanes due to its small internal cavity. 3,4 In 2010, Rousseau and co-workers published a high-yielding scalable Scheme 1 Chemical structures of PP-1 and PP-2. [START_REF]IUPAC Tentative Rules for the Nomenclature of Organic Chemistry. Section E. Fundamental Stereochemistry[END_REF]19 a Bordeaux University, Institut des Sciences Mole ´culaires, CNRS UMR 5255, 33405 Talence, France. E-mail: t.buffeteau@ism.u-bordeaux1.fr b Lyon 1 University, Ecole Normale Supe ´rieure de Lyon, CNRS UMR 5182, Laboratoire de Chimie, 69364 Lyon, France. E-mail: thierry.brotin@ens-lyon.fr c Aix-Marseille University, CNRS, Centrale Marseille, iSm2, Marseille, France d Synchrotron SOLEIL, L'Orme des Merisiers, 91192 Gif sur Yvette, France synthesis of this derivative by optimizing the cyclotriphenolene unit dimerization, 5 whereas Holman and co-workers reported the X-ray structure of the racemate of 1 and the first water-soluble cryptophane-111 with Ru complexes. 6 Later, in 2011, Rousseau and co-workers published the synthesis of a metal-free watersoluble cryptophane-111. 7 Finally, Holman and co-workers reported the first rim-functionalized derivatives of cryptophane-111 ((MeO) 3 -111 and Br 3 -111), which limit the range of achievable conformations of the cryptophane-111 skeleton, 8 and they also showed the very high thermal stability (up to about 300 1C) of the Xe@1 complex in the solid state. 9 Besides their interesting binding properties, most of the cryptophane derivatives exhibit an inherently chiral structure due to the anti arrangement of the linkers or to the presence of two different CTB caps. Thus, the anti arrangement of the methylenedioxy linkers makes 1 a chiral molecule. During the past decade, we have thoroughly investigated enantiopure cryptophanes using several techniques such as polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) because the chiroptical properties of these derivatives are extremely sensitive to the encapsulation of guest molecules. [10][11][12][13][14][15][16][17] For instance, water-soluble cryptophanes display unique chiroptical properties depending on the nature of the guest (neutral or charged species) present within the cavity of the host. [10][11][12][13][14] In addition, cryptophane-222 (compound 2 in Scheme 1) possesses unusual chiroptical properties in organic solvents never observed before with cryptophane derivatives. 17 Indeed, a very different behaviour of the specific optical rotation (SOR) values was observed in the nonresonance region above 365 nm in CHCl 3 and CH 2 Cl 2 solutions. This feature was related to conformational changes of the three ethylenedioxy linkers upon encapsulation of the two solvent molecules by 2. This explanation could be confirmed by investigating the chiroptical properties of the new derivative 1. Indeed, 1 differs from 2 only by the length of the three linkers connecting the two CTB units, leading to a smaller size of the cavity. Moreover, the three portals of 1 are too small to allow any solvent molecules to enter the cavity of the host. Even CH 2 Cl 2 (V vdW = 52 Å 3 ) is too large to cross the portals of 1, leaving the cavity only accessible for smaller guests such as methane or xenon. 4 In addition, the replacement of ethylenedioxy by methylenedioxy linkers presents the advantage to decrease the number of conformations for the three bridges. Thus, we believe that the two enantiomers of 1 are important compounds for understanding the role of the solvent on the chiroptical properties of cryptophane derivatives in general. A change of the chiroptical properties of 1 regardless of the nature of the solvent will tend to demonstrate that the bulk solvent plays an important role in the chiroptical properties of cryptophane. In contrast, a lack of modification on the chiroptical properties of 1 will show that only the solvent molecule present within the cavity of the cryptophanes (that is the case for 2) has an effect on their chiroptical properties.

In this article we focus our attention on the chiroptical properties of 1 since they have never been reported in the literature, probably due to the difficulties encountered for the optical resolution of 1 into its two enantiomers (+)-1 and (À)-1.

In addition, the simplified chemical structure of 1 (87 atoms) allows more sophisticated theoretical calculations (better basis set) for the prediction of the VCD, ROA and ECD spectra by using density functional theory (DFT and time-dependent DFT) methods.

We report in this article the separation of the two enantiomers of 1 by high-performance liquid chromatography (HPLC) using chiral stationary phases and the detailed study of their chiroptical properties in CHCl 3 and CH 2 Cl 2 solutions by polarimetry, ECD, VCD, and ROA spectroscopy. Synchrotron Radiation Circular Dichroism (SRCD) spectra of the two enantiomers of 1 were also recorded in the two solvents to investigate the low-lying excited states in the 1 B b region (190-220 nm). The chiroptical properties of 1 were compared to those recently published for 2. 17 DFT and TD-DFT calculations were performed to predict SOR values as well as the ECD, VCD, and ROA spectra for several geometries of 1. The X-ray structures of these two enantiomers were also reported and compared to the optimized geometries of 1 calculated by DFT. Finally, the xenon encapsulation by 1 was followed by VCD and ROA spectroscopy.

Experimental

X-ray crystallography X-ray structures of the two enantiomers of 1 were obtained from crystals mounted on a Kappa geometry diffractometer (Cu radiation) and using the experimental procedure previously published. 16 CCDC 1537591 and 1537585 contain the crystallographic data of [CD(+) 254 ]-1 and [CD(À) 254 ]-1, respectively. †

Polarimetric, UV-vis and ECD measurements

Optical rotations of the two enantiomers of 1 were measured in two solvents (CHCl 3 , CH 2 Cl 2 ) at several wavelengths (589, 577, 546, 436, and 365 nm) using a polarimeter with a 10 cm cell thermostated at 25 1C. Concentrations used for the polarimetric measurements were typically in the range 0.22-0.27 g/ 100 mL. ECD spectra of the two enantiomers of 1 were recorded in four solvents (CHCl 3 , CH 2 Cl 2 , tetrahydrofuran (THF) and CH 3 CN) at 20 1C with a 0.2 cm path length quartz cell (concentrations were in the range 5 Â 10 À5 -1 Â 10 À4 M). Spectra were recorded in the wavelength ranges of 210-400 nm (THF and CH 3 CN) or 230-400 nm (CH 2 Cl 2 and CHCl 3 ) with a 0.5 nm increment and a 1 s integration time. Spectra were processed with standard spectrometer software, baseline corrected and slightly smoothed by using a third order least square polynomial fit. UV-vis spectra of the two enantiomers of 1 were recorded in CH 2 Cl 2 (230-400 nm) and THF (210-400 nm) at 20 1C with a 0.5 and 0.2 cm path lengths quartz cell, respectively.

SRCD measurements

Synchrotron Radiation Circular Dichroism (SRCD) measurements were carried out at the DISCO beam-line, SOLEIL synchrotron. 20,21 Samples of the two enantiomers of 1 were dissolved in CH 2 Cl 2 and CHCl 3 . Serial dilutions of the concentrations in view of data collection in three spectral regions were chosen between 100 g L À1 , 10 g L À1 to 2.5 g L À1 . Accurate concentrations were reassessed by absorption measurements allowing the scaling of spectral regions to each other. Samples were loaded in circular demountable CaF 2 cells of 3.5 mm path lengths, using 2-4 mL. 22 Two consecutive scans for each spectral region of the corresponding dilution, were carried out for consistency and repeatability. CD-spectral acquisitions of 1 nm steps and 1 nm bandwith, between 320-255 nm, 260-216 nm and 232-170 nm were performed at 1.2 s integration time per step for the samples. Averaged spectra were then subtracted from corresponding averaged baselines collected three times. The temperature was set to 20 1C with a Peltier controlled sample holder. Prior, (+)-camphor-10-sulfonic acid was used to calibrate amplitudes and wavelength positions of the SRCD experiment. Data-treatment including averaging, baseline subtraction, smoothing, scaling and standardisation were carried out with CDtool. 23

IR and VCD measurements

The IR and VCD spectra were recorded on an FTIR spectrometer equipped with a VCD optical bench, 24 following the experimental procedure previously published. 16 Samples were held in a 250 mm path length cell with BaF 2 windows. IR and VCD spectra of the two enantiomers of 1 were measured in CDCl 3 and CD 2 Cl 2 solvents at a concentration of 0.015 M. Additional spectra were measured in CDCl 3 in presence of xenon.

ROA measurements

Raman and ROA spectra were recorded on a ChiralRAMAN spectrometer, following the experimental procedure previously published. 15 The two enantiomers of 1 were dissolved in CDCl 3 and CD 2 Cl 2 solvents at a concentration of 0.1 M and filled into fused silica microcell (4 Â 3 Â 10 mm). The laser power was 200 mW (B80 mW at the sample). The presented spectra in CDCl 3 (CD 2 Cl 2 ) are an average over about 32 (52) h. Additional experiments were performed in the two solvents in presence of xenon.

Theoretical calculations

All DFT and TDDFT calculations were carried out with Gaussian 09. [START_REF] Frisch | Gaussian 09[END_REF] Preliminary conformer distribution search of 1 was performed at the molecular mechanics level of theory, employing MMFF94 force fields incorporated in ComputeVOA software package. Twenty one conformers were found within roughly 8 kcal mol À1 of the lowest energy conformer. Their geometries were optimized at the DFT level using B3PW91 functional [START_REF] Perdew | [END_REF] and 6-31G** basis set, 27 leading to ten different conformers within a energetic window of 7.5 kcal mol À1 . Finally, only the three lowest energetic geometries were kept, and reoptimized with the use of IEFPCM model of solvent (CH 2 Cl 2 and CHCl 3 ). 28,29 Vibrational frequencies, IR and VCD intensities, and ROA intensity tensors (excitation at 532 nm) were calculated at the same level of theory. For comparison to experiment, the calculated frequencies were scaled by 0.968 and the calculated intensities were converted to Lorentzian bands with a full-width at half-maximum (FWHM) of 9 cm À1 . Optical rotation calculations have been carried out at several standard wavelengths (365, 436, 532 and 589 nm) by means of DFT methods (B3PW91/6-31G**) for the three conformers reoptimized with the use of PCM solvent model.

ECD spectra were calculated at the time-dependent density functional theory (TDDFT) level using the MPW1K functional 30 and the 6-31+G* basis set. Calculations were performed for the three conformers reoptimized with the use of PCM solvent model (IEFPCM = CH 2 Cl 2 ), considering 120 excited states. For comparison to experiment, the rotational strengths were converted to Gaussian bands with a FWHM of 0.1 eV.

Results

Synthesis and HPLC separation of the two enantiomers of 1

The racemic mixture of 1, (rac)-1, was prepared according to a known procedure (Fig. S1 in the ESI †). 3 Compound 1 does not possess any substituent that could be exploited for separating the two enantiomers of 1 by the formation of diastereomeric derivatives. Consequently, the two enantiomers of 1 were separated using a chiral HPLC column (Chiralpak ID, eluent: heptane/EtOH/CHCl 3 50/30/20, 1 mL min À1 ), which allowed the efficient separation of the two enantiomers of 1 with an excellent resolution factor (R s = 3.24), as shown in Fig. 1. A circular dichroism detector provided the sign of each enantiomer at 254 nm. It was observed that enantiomer [CD(À) 254 ]-1 was first eluted at t = 6. ESI †). Thus, from 350 mg of racemic material, 160 mg of each enantiomer were obtained with an excellent enantiomeric excess (ee 4 99% for [CD(À) 254 ]-1 and ee 4 99.5% for [CD(+) 254 ]-1). In order to improve the chemical purity of the two compounds, an additional purification step was conducted on both enantiomers. Thus, compounds [CD(+) 254 ]-1 and [CD(À) 254 ]-1 were purified on silica gel (eluent: CH 2 Cl 2 /acetone 90/10) and then recrystallized in a mixture of CH 2 Cl 2 /EtOH. These additional purification steps provide both enantiomers with high chemical purity. The 1 H NMR and S1).

Compounds [CD(À) 254 ]-1 and [CD(+) 254 ]-1 crystallize in P2 1 2 1 2 1 and P2 1 space groups, respectively. No disorder was observed in the two X-ray structures and the cavities do not contain any substrate (solvent or gas molecules). The two enantiomers adopt a contracted conformation of the bridges that minimizes the internal cavity volume. Using a probe radius of 1.4 Å, the estimated cavity volume of [CD(+) 254 ]-1 and [CD(À) 254 ]-1 were 30 and 32 Å 3 , respectively. It is noteworthy that these two X-ray structures are significantly different from the one reported for the racemate. 6 Indeed, the X-ray structures of the two enantiopure derivatives adopt a more flattened shape with respect to the X-ray structure of the racemate, characterized by a large twist angle of 55.31 between the two CTB caps. [START_REF]average dihedral angles between the arene ring centroids of OCH 2 O-connected arenes with respect to the C 3 axis of the host[END_REF] For the racemate, a twist angle of 18.11 was found between the two CTB caps, associated with a cavity volume of 72 Å 3 . 6 Interestingly, these structures are also less symmetrical than the one observed for racemate and a topview of these two structures reveals that the six benzene rings are totally eclipsed (Fig. S8 in the ESI †). In contrast, the X-ray structure of the racemic derivative shows a strong overlapping of the phenyl rings.

Polarimetry and electronic circular dichoism

The two enantiomers of 1 are well soluble in CH 2 Cl 2 and CHCl 3 but unfortunately they show poor solubility in other organic solvents. Thus, polarimetric experiments were performed only in CH 2 Cl 2 and CHCl 3 solutions. The specific optical rotation (SOR) values of the two enantiomers of 1 are reported in the ESI, † (Table S2) and the wavelength dependence of [CD(+) 254 ]-1 is shown in Fig. 2. In CH 2 Cl 2 , the SOR values of [CD(+) 254 ]-1 are slightly positive at 589 and 577 nm, close to zero at 546 nm and significantly negative at 436 and 365 nm. Nevertheless, despite the low values measured for this compound at 589, 577 and 546 nm, SOR values with opposite sign are obtained for the two enantiomers of 1 (Table S2, ESI †). In CHCl 3 , the wavelength dependence of the SOR values evolves similarly with values slightly higher. This result contrasts with the measurements performed with compound 2 that exhibited significant differences in CH 2 Cl 2 and CHCl 3 solutions. Finally, as previously observed with compound 2, 17 a change of the SOR sign is observed in the nonresonant region (around 546 nm in CH 2 Cl 2 and 475 nm in CHCl 3 ).

UV-Vis and ECD experiments require lower concentration of solute and consequently this allows us to extend the range of solvents. Thus, UV-Vis and ECD spectra of [CD(+) 254 ]-1 and [CD(À) 254 ]-1 were successfully recorded in CH 2 Cl 2 , CHCl 3 , THF, and CH 3 CN solvents. The UV-Vis spectra measured in THF and CH 2 Cl 2 solvents are reported in Fig. S9 in the ESI. † These spectra are very similar to those published for compound 2. 17 The ECD spectra of the two enantiomers are reported in Fig. S10 in the ESI, † for the four solvents. A perfect mirror image is observed in all solvents for the two enantiomers as expected for enantiomers with high enantiomeric excess. For CH 2 Cl 2 and CHCl 3 solutions, the ECD spectra give only access to the bands corresponding to the two forbidden 1 L a and 1 L b transitions in the UV-visible region (230-300 nm). For THF and CH 3 CN solutions, the spectral range can be extended up to 210 nm. This allows us to have access to another spectral region corresponding to the allowed 1 B b transition. This spectral region usually gives rise to intense ECD signals in organic solution. It was observed that the ECD spectra of [CD(+) 254 ]-1 and [CD(À) 254 ]-1 are very similar in shape and intensity, regardless of the nature of the solvent used in these experiments. For instance, in CH 2 Cl 2 the ECD spectrum of the [CD(+) 254 ]-1 enantiomer shows four bands, as shown in Fig. 3. Three ECD bands (two negative and one slightly positive from high to low wavelengths) are observed in the spectral region related to the 1 L b transition (260-300 nm). At shorter wavelengths, only a single positive ECD band was observed between 230 and 255 nm ( 1 L a transition). Interestingly, it can be noticed that the ECD spectra of [CD(+) 254 ]-1 and [CD(À) 254 ]-2 show a lot of similarities even though some significant spectral differences are observed especially in the 1 L a region. Indeed, the bisignate ECD signal usually observed in the 1 L a region for cryptophane derivatives and observed for [CD(À) 254 ]-2 is no longer present in the ECD spectrum of [CD(+) 254 ]-1. In the past, the sign of this bisignate ECD signal was exploited to determine the absolute configuration (AC) of cryptophane-A molecule. [START_REF] Canceill | [END_REF] Then, we have confirmed that this approach could be used to assign the AC of other cryptophane derivatives in organic solution. This study shows that the approach can not be used for cryptophane-111.

Synchrotron radiation circular dichroism experiments were also performed to obtain additional information at lower wavelengths, in the 1 B b region (180-230 nm). The SRCD spectra of the two enantiomers of 1 recorded in CH 2 Cl 2 and CHCl 3 are reported in Fig. S11 in the ESI. † For wavelengths higher than 230 nm, the SRCD spectra of [CD(+) 254 ]-1 and [CD(À) 254 ]-1 are identical in shape and intensities to the ECD spectra described above. For wavelengths lower than 230 nm, the SRCD spectra reveal two additional bands with opposite sign. For instance, the [CD(+) 254 ]-1 enantiomer exhibits in CH 2 Cl 2 a positivenegative bisignate pattern from short to long wavelengths related to the 1 B b transition. It is noteworthy that similar (in shape and in intensities) SRCD spectra were recorded in CHCl 3 solution, in contrast to what was observed for compound 2. 17

VCD and ROA spectroscopy

The chiroptical properties of enantiopure cryptophane 1 have been also investigated by VCD in CDCl 3 and CD 2 Cl 2 solutions. The IR spectra of the [CD(+) 254 ]-1 enantiomer are similar in the 1700-1000 cm À1 region for the two solutions (Fig. S12 in ESI †). In addition, the presence of xenon in the CDCl 3 solution does not modify the IR spectrum in this spectral range. The VCD spectra of the two enantiomers of 1 measured in CDCl 3 and CD 2 Cl 2 solvents are reported in Fig. S13 in ESI, † whereas the comparison of experimental VCD spectra of [CD(+) 254 ]-1 in the two solvents is presented in Fig. 4. As shown in Fig. 4, the VCD spectra of 1 seem independent of the nature of the solvent, even though slight spectral differences are observed in the 1050-1010 cm À1 region. In addition, a slightly lower intensity of the VCD bands is observed in CD 2 Cl 2 solution, which can be related to the lower molar absorptivities measured in CD 2 Cl 2 with respect to CDCl 3 solution. Finally, the presence of xenon in the CDCl 3 solution does not modify the VCD spectrum of [CD(+) 254 ]-1 (Fig. S14 in ESI †).

The ROA spectra of the two enantiomers of 1 measured in CDCl 3 solution (0.1 M), in presence or not of xenon, are shown in Fig. S15 in ESI. † These ROA spectra are nearly perfect mirror images (Fig. S15a andb in ESI †), as expected for enantiopure materials. The ROA spectra measured in CD 2 Cl 2 solution were similar (Fig. S15c andd in ESI †), indicating that the ROA spectra of 1 is independent of the solvent, as already mentioned for ECD and VCD experiments. On the other hand, the ROA spectra of [CD(+) 254 ]-1 in CD 2 Cl 2 solution in presence or not of xenon reveal a clear spectral difference at wavenumbers lower than 200 cm À1 , as shown in Fig. 5. Indeed, in presence of xenon, we observe an important decrease of the intensity of the band at 150 cm À1 . The same effect is observed on ROA spectra measured in CDCl 3 solution (Fig. S16 in ESI †). The vibrational assignment of this mode was made by visual inspection of modes represented and animated by using the Agui program. All the displacement vectors of carbon atoms point towards the center of the cavity, indicating that this mode corresponds to the symmetric breathing mode of the cryptophane-111 skeleton. This result clearly indicates that the presence of a guest inside the cavity of a cryptophane derivative modifies the intensity of its symmetric breathing mode. The examination of this mode could be used in the future to reveal the complexation of guest molecules by cryptophane derivatives. However, it is noteworthy that it would not be possible to observe this effect for the compound 2 in CHCl 3 or CH 2 Cl 2 solutions, since these two solvent molecules can enter the cavity of 2 and would be therefore strong competitors for xenon.

Discussion

AC and conformational analysis of 1

As it is now recommended, different techniques have been used to assign unambiguously the absolute configuration (AC) of the two enantiomers of 1. [33][34][35] Thanks to the determination of the Flack and Hooft parameters, the X-ray crystallography analysis provides an easy way to determine the AC of the two [CD(+) 254 ]-1 and [CD(À) 254 ]-1 enantiomers. Thus, based on the analysis of the two X-ray structures, the following assignment [CD(+) 254 ]-MM-1 and [CD(À) 254 ]-PP-1 has been found for the two enantiomers of 1. Consequently, considering the specific optical rotation measured at 589 nm the AC become (+) 589 -MM-1 and (À) 589 -PP-1. It is noteworthy that these last descriptors are identical to those determined for compound 2, as suggested by the similarity observed in their experimental ECD spectra (Fig. 3).

To confirm the AC of the two enantiomers of 1, determined by X-ray crystallography, we have used VCD and ROA spectroscopy associated with DFT calculations, which are known to be a valuable approach to assign the AC of organic compounds. Conformer distribution search was performed at the molecular mechanics level of theory for the MM-1 configuration, starting from the more symmetrical structure obtained from X-ray analysis of the racemic compound. 6 Twenty one conformers within roughly 8 kcal mol À1 of the lowest energy conformer were found and their geometries optimized at the DFT level (B3PW91/6-31G**), leading to ten different conformers. The electronic and Gibbs energies as well as the twist angle between the two CTB caps for the three most stable conformers are reported in Table 1 and compared to those calculated from the optimized geometries of the enantiomer crystals. The conformer A leads to the lowest Gibbs free energy and represents more than 99% of the Boltzmann population of conformers at 298 K. This conformer exhibits the most symmetrical structure with an average value of the twist angle between the two CTB caps of 19.11 (dihedral angles [START_REF]average dihedral angles between the arene ring centroids of OCH 2 O-connected arenes with respect to the C 3 axis of the host[END_REF] As shown in Fig. 6, the VCD spectrum predicted for the MM configuration of conformer A reproduces very well the sign of most of the bands observed in the experimental spectrum of [CD(+) 254 ]-1, confirming the AC assignment [CD(+) 254 ]-MM-1, determined by X-ray crystallography. A very good agreement between predicted ROA spectrum of conformer A and experimental ROA spectrum is also obtained (Fig. S17 in ESI †). This conformational analysis shows that only one conformer is present for 1, contrary to the conformational equilibrium observed for 2 due to the ethylenedioxy linkers (i.e. possibility of trans and gauche conformations of the three linkers). This lack of conformational equilibrium for 1 may explain the overall higher intensities of the VCD bands for 1 and the lowest FWHM (9 cm À1 for 1 vs. 14 cm À1 for 2) used to reproduce the experimental VCD spectrum for 1.

As above mentioned, the bisignate pattern observed in the 1 L a region (230-260 nm) of the ECD spectra of cryptophane derivatives can be another way to determine the AC of these derivatives in organic solvents. 16,17 Using the Khun-Kirkwood excitonic model, Gottarelli and co-workers have shown that this bisignate resulted from different excited states (one A 2 and two degenerate E components) for cryptophane possessing a D 3 -symmetry. [START_REF] Canceill | [END_REF] For the 1 L a transition, the A 2 component is always located at lower energy and the two E components show opposite rotational strengths. This model leads to a positive/ negative bisignate pattern (from short to long wavelength) for the MM configuration of cryptophane-A derivatives. For cryptophane-111, this bisignate pattern is not observed and this rule does not apply. Indeed, in the case of compound 1, TD-DFT calculations show that the A 2 component located at high wavelength possesses a lower negative rotational strength (R = À0.38 cgs) than the one measured for compound 2 (R = À0.70 cgs). Thus, the contribution of the A 2 component in the experimental ECD spectrum is embedded in the two E components exhibiting a larger rotational strength (R = 1.05 cgs), leading to a broader positive band in the 1 L a region. The strong decrease of the negative A 2 component of the 1 L a transition suggests that the classical excitonic coupling model can not be used to determine the AC of cryptophane-111 molecule and that other contributions should be involved in the interpretation of the ECD spectrum. For instance, as it has been reported by Pescitelli and co-workers in some cases, 36,37 the coupling between the electric and magnetic transition moments (mm term) can contribute significantly to the overall rotational strength for a given excited state. This contribution, which is usually neglected in the case of the classical excitonic coupling model, can dominate the electric-electric coupling (mm term). Nevertheless, the bisignate pattern observed in the 1 B b region (190-230 nm) of the SRCD spectra can be used to determine the AC of 1. Indeed, the positivenegative sequence from short to long wavelength observed for [CD(+) 254 ]-1 was associated with the MM configuration by TD-DFT calculations (Fig. S18 in ESI †).

Comparison between the chiroptical properties of 1 and 2

In a recent article, different behaviours of the chiroptical properties (in particular, polarimetric properties) were observed for 2 in CHCl 3 and CH 2 Cl 2 solutions. 17 These modifications were interpreted by a subtle conformational equilibrium change of the ethylenedioxy linkers upon encapsulation of CHCl 3 and CH 2 Cl 2 molecules. A preferential G À conformation of the linkers was found in CH 2 Cl 2 solution, in order to decrease the cavity size and to favour hostguest interactions. In contrast, a higher proportion of G + conformation of the linkers was found in CHCl 3 solution, increasing the size of the cavity suitable for the complexation of chloroform molecule. The comparison of the chiroptical properties of 1 and 2 is very interesting because these two compounds possess identical CTB units and differ only by the nature of the alkyl linkers. The conformational equilibrium observed for compound 2 due to the possibility of trans and gauche (G + and G À ) conformations of the three ethylenedioxy linkers is not possible for compound 1 which possess methylenedioxy linkers. In addition, it has been shown that (rac)-1 does not bind halogenomethane molecules so that neither CH 2 Cl 2 nor CHCl 3 can enter its cavity. 3,4 Thus, no spectral modification in the ECD (or SCRD) and VCD (or ROA) spectra is expected for 1 in these two solvents. This assumption is confirmed by our experiments, as shown in the result section.

Our results reveal also that the SOR values of 1 behave similarly in the two solvents. We observe a change of the sign of the SOR values in the nonresonant region, as shown in Fig. 3. This surprising effect has been previously reported with compound 2 for experiments in chloroform, acetone and dimethylformamide. Calculations of the SOR at the B3PW91/6-31G** level (IEFPCM = CHCl 3 ) reproduce perfectly the experimental data measured in CHCl 3 solution (Fig. S19 in ESI †).

Conclusions

This article reports a thorough study of the chiroptical properties of the two enantiomers of cryptophane-111 (1) by X-ray crystallography, polarimetry, ECD (SRCD), VCD, and ROA spectroscopy. The absolute configuration of the two enantiomers has been determined based on X-ray crystallographic data. Thus, the (+) 589 -MM-1 ((À) 589 -PP-1) AC has been assigned. This result has been confirmed by the combined analysis of the VCD (ROA) spectra and DFT calculations. In a second part of this article, the chiroptical properties of 1 have been compared to those of cryptophane-222 (2). Despite the similarity in the two structures, derivatives 1 and 2 exhibit different behaviours of their chiroptical properties with respect to CH 2 Cl 2 and CHCl 3 solvents. In these two solvents, polarimetric measurements and SRCD spectra are clearly different for compound 2, whereas they remain almost unchanged for 1. This different behaviour can be explained by the incapacity of compound 1 to encapsulate a solvent molecule within its cavity, regardless of the nature of the solvent. Consequently, the nature of the solvent has almost no influence on the conformation of the methylenedioxy linkers. This result confirm our previous assumption that the different chiroptical properties observed for 2 in chloroform and dichloromethane solutions are certainly due to the conformation equilibrium change of the ethylenedioxy linkers upon encapsulation of CH 2 Cl 2 or CHCl 3 molecules.

Thus, the comparison of the chiroptical properties of cryptophane 1 and 2 sheds light on the importance of the solvent present within the cavity to understand the chiroptical properties of the cryptophane derivatives in general. In addition, our results shows that the bulk solvent has no significant effect on the chiroptical properties of 1.

  Fig.1Separation of the two enantiomers of 1 using an analytical chiral HPLC column.

13 C

 13 NMR spectra (Fig. S3-S6 in the ESI †) are identical to those reported for (rac)-1. X-ray crystallographic structures of the two enantiomers of 1 X-ray crystals of [CD(+) 254 ]-1 and [CD(À) 254 ]-1 were obtained in a CH 2 Cl 2 /EtOH mixture and in pyridine, respectively (Fig. S7a and b in the ESI †). The crystallographic data of the two X-ray crystal structures are reported in the ESI, † (Table

Fig. 2

 2 Fig. 2 Specific optical rotation values (10 À1 deg cm 2 g À1 ) of [CD(+) 254 ]-1 recorded at several wavelengths (365, 436, 546, 577 and 589 nm) in chloroform (c = 0.22), and dichloromethane (c = 0.27) solvents.

Fig. 3

 3 Fig. 3 Comparison of experimental ECD spectra of [CD(+) 254 ]-1 (black spectrum) and [CD(À) 254 ]-2 (red spectrum) in CH 2 Cl 2 solution.

Fig. 4

 4 Fig. 4 Comparison of experimental VCD spectra of [CD(+) 254 ]-1 in CDCl 3 (black spectrum) and in CD 2 Cl 2 (red spectrum) solutions.

Fig. 5

 5 Fig. 5 Comparison of experimental ROA spectra of [CD(+) 254 ]-1 in CD 2 Cl 2 solution in presence (red spectrum) or not (black spectrum) of xenon.

  of 19.0, 19.1 and 19.1). Conformers B and C present higher twist angles with average values of 23.61 and 28.71, respectively. It is noteworthy that the structure is less symmetrical than for conformer A with one dihedral angle, which differs from the two others (21.11, 21.51 and 28.31 for conformer B and 23.81, 31.11 and 31.21 for conformer C).

Fig. 6

 6 Fig. 6 Comparison of the experimental VCD spectrum of [CD(+) 254 ]-1 recorded in CDCl 3 solution with the calculated spectrum at the B3PW91/ 6-31G** level (IEFPCM = CHCl 3 ) for conformer A of MM-1.

Table 1

 1 Conformations, twist angles and energies of the three conformers of MM-1 calculated from the crystal of (rac)-1, and of the one calculated from the crystal of MM-1

	Conformers	Twist angle	Energy (hartrees) Electronic	Gibbs	DG (kcal mol À1 ) %
	A	19.1 À2187.01467627 À2186.367759 0	99.7
	B	23.6 À2187.00811518 À2186.362379 3.38	0.3
	C	28.7 À2187.00412196 À2186.359201 5.26	0.0
	Crystal MM-1 55.3 À2187.00057700 À2186.355790 7.51	0.0

Acknowledgements

Supports from the French Ministry of Research (project ANR-12-BSV5-0003 MAX4US) is acknowledged. The authors are indebted to the CNRS (Chemistry Department) and to Re ´gion Aquitaine for financial support in VCD and ROA equipments. They also acknowledge computational facilities provided by the MCIA (Me ´socentre de Calcul Intensif Aquitain) of the Universite de Bordeaux and of the Universite ´de Pau et des Pays de l'Adour, financed by the ''Conseil Re ´gional d'Aquitaine'' and the French Ministry of Research and Technology. The GDR 3712 Chirafun is acknowledged for allowing a collaborative network between the partners of this project.


Dominique Massiot
email: massiot@cnrs-orleans.fr

Franck Fayon

Valérie Montouillout

Nadia Pellerin

Julien Hiet

Claire Roiland

Pierre Florian

Jean-Pierre P Coutures

Laurent Cormier

Daniel R Neuville

Structure and dynamics of Oxyde Melts

whether they are published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Oxide glasses are known and used for thousands of years and tuning of properties like colour, durability, viscosity of the molten state were mostly known and dominated by glass makers. Despite this millenary knowledge, the range of glass forming system of interest is still expanding and many non-elucidated points remain in the understanding of the glass and melts structure and properties. The aim of this contribution is to underline, from the experimental point of view provided by Nuclear Magnetic Resonance, the relations existing between the structure and dynamics of the high temperature molten oxide systems and the short and medium range order of their related glasses.

The strength of Nuclear Magnetic Resonance for describing structure and dynamics of amorphous or disorganised system like oxide glasses or melts, comes firstly from its ability to selectively observe the environment of the different constitutive atoms (providing that they bear a nuclear spin) and secondly from its sensitivity to small variation in the first and second coordination sphere of the observed nucleus. This often provides spectral separation of the different types of environment [START_REF] Mackenzie | MultiNuclear Solid State NMR of Inorganic Materials[END_REF]. The information derived from NMR experiments are then complementary to those obtained by other means : optical spectroscopies, IR or Raman, X-Ray absorption, X-Ray or neutrons elastic or inelastic scattering etc… It is important to remark that NMR has a much slower characteristic time (ranging from Hz to MHz) than most of the above mentioned methods, leading to fundamental differences in the signatures of the viscous high temperature molten states.

One dimensional NMR experiments

In liquid state in general, and in the high temperature molten state in the case of oxide glass forming systems, the mobility is such that only the isotropic traces of the anisotropic interactions express in their NMR spectra. Fluctuation of these interactions leads to relaxation mechanisms that can allow discussion of the characteristic times of rearrangement and overall mobility of the system. In solid state materials and in glasses the anisotropy of the different interaction fully express in the static NMR spectra giving broad and often featureless line shapes accounting for all the different orientations of the individual structural motifs of the glass. Although these broad spectra contain many different information on the conformation of the structural motifs (Chemical Shift Anisotropy -CSA), spatial proximity between spins (homo-and hetero-nuclear Dipolar interactions), chemical bonds (indirect J coupling), electric field gradient at the nucleus position (Quadrupolar interaction for I>1/2 nuclei), these information are often hardly evidenced. Magic Angle Spinning is this unique tool that solid state NMR has at hand to average out all (or most) of the anisotropic part of the interactions only leaving their traces mimicking (or giving a coarse approach of) the Brownian reorientation of the liquid phase. Under rapid Magic Angle Spinning, Chemical Shift is averaged to its isotropic value and distribution directly given by the line position and width in the case of a dipolar (I=1/2) spin, while the traceless Dipolar interaction is averaged out, and the scalar (or isotropic) part of J-coupling is usually small enough to be completely masked in a 1D spectrum, even in crystalline phases.

Phosphates, silicates, alumino-silicates or aluminates oxide glasses structures are mostly based on tetrahedral species whose polymerization is characterized by their number of bridging oxygens (Q n : Q=P,Si,Al and n the number of bridging oxygens). Figure 1 presents the 31 P MAS NMR 1D spectra of a (60% PbO-40% P 2 O 5 ) glass. It shows two broad but resolved resonances in a 1/1 ratio that can unambiguously ascribed to end-chain groups (Q 1 750Hz 6.2 ppm width) and middle-chain groups (Q 2 1100Hz 9 ppm width) environments.

Both these lines are considerably broader than that of the corresponding crystalline sample (Pb 3 P 4 O 13 linewidth < 1 ppm) due to the disorder in the glass structure and the loss of long range order. In the case of simple binary glasses of phosphates or silicate the broad lines corresponding to the various Q n tetrahedral sites are often resolved enough to allow quantification of their respective abundance and evaluation of the disproportionation equilibrium constants (K n : 2Q n <->Q n-1 +Q n+1 ) [START_REF] Stebbins | [END_REF]. Figure 2 reports these quantitative results for PbO-SiO 2 [3]and PbO-P 2 O 5 [4] binary systems. In lead-phosphate glasses the K n values remain very small which correspond to a binary distribution and indicates that only two types of Q n environments can co-exist at a given composition, while in lead-silicate glasses the equilibrium constant are much higher, close to that of a randomly constructed network with a competition between lead based and silicon based covalent networks. 207 Pb NMR and L III -EXAFS experiments confirmed this interpretation by showing that the coordination numbers of Pb in silicate is of 3 to 4 oxygen with short covalent bonds and a very asymmetric environment (pyramid with lead at the top) while it is of more than 6 in phosphate glasses with a more symmetric environment, behaving more as a network modifier [3][4][5].

Polyatomic molecular motifs

Although these information already give important details on the structure of these phosphates or silicate binary glasses, it would be of great interest to obtain a larger scale image of the polyatomic molecular motifs constituting these glasses and especially to evaluate the length of phosphate chains possibly present in the glass, that makes the difference between the long range ordered crystalline phase and the amorphous phase. That type of information can be obtained by implementing multidimensional NMR experiments that allow to evidence Dipolar [4] or J-coupling [6,7,8] interaction and further use them to build correlation experiments separating the different contributions of well defined molecular motifs. Figure 1 gives a general picture of the possibilities offered by the J-coupling mediated experiments that allow to directly evidence the P-O-P bonds bridging phosphate units through J 2 P-O-P interaction. Let us consider the example of the 60% PbO-40% P 2 O 5 glass already introduced above. Its 1D spectrum (fig 1a) shows partly resolved Q 1 and Q 2 lines with strong broadening (750 and 1100Hz) signing the to be understood glass disorder. Because the Q 1 and Q 2 line width is essentially inhomogeneous, due to distribution of frequencies for each individual motif, this broadening can be refocused in an echo which is modulated by the small (and unresolved) isotropic J 2 P-O-P coupling [7]. Figure 1b shows the J-resolved spectrum of the glass that reveals the J coupling patterns consisting in a doublet for Q 1 , and to a triplet for Q 2 , thus justifying the spectral attribution previously made based on the 31 P isotropic chemical shift. It is also of importance to notice that this experiment clearly shows that the isotropic J-coupling does vary across the 1D lines, typically increasing with decreasing chemical shift. The new type of information provided by this experiment is likely to bear important information on the covalent bond hybridisation state and geometry. Because this isotropic J-coupling can be measured, it can also be used to reveal -or to spectrally edit -different polyatomic molecular units in the glass. Figure 1c and 1d respectively show the two-dimensional correlation spectra that enable the identification of through-bond connectivity between two linked PO 4 tetrahedra (fig. 1c) [6] and between three linked PO 4 tetrahedra (fig. 1d) [8]. These experiments, fully described in the referenced papers allow spectral separation of dimers, end-chain groups, and middle-chain groups when selecting Q-Q pairs (fig. 1c) and trimers, end-chain triplets or centre-chain triplets when selecting Q-Q-Q triplets (fig. 1d composition [11]. They showed that while the two Q 3 and Q 4 contributions can be resolved from their different chemical shift anisotropy or from their isotropic chemical shift, below glass temperature, they begin to exchange just above glass transition with characteristic times of the order of seconds [12] and finally end up into merging in a unique line in the high temperature molten state. This experiment underlines two important points. First, although silicate glasses can be regarded as SiO 2 based polymer, the melting of silicate glasses implies rapid reconfiguration of the structural motifs through a mechanism that was proposed to involve a higher SiO 5 coordination state of silicon with oxygen, second that the characteristic time scales of NMR spectroscopy allow to explore a large range of time scales involved in this mechanism. We can remark that this has been recently extended to below T g structural reorganisation of BO 3 and BO 4 configurations in borosilicate glasses [START_REF] Sen | XI International Conference on Physics on Non Crystalline Solids[END_REF]. The existence of higher (and previously unexpected) SiO 5 coordination state of silicon was proved experimentally by acquiring high quality 29 Si NMR spectra [START_REF] Stebbins | [END_REF] with clear effects of quenchrates and pressure stabilizing these high coordination silicon environments.

The high temperature NMR setup developed in our laboratory, combining CO 2 laser heating and aerodynamic levitation allows acquisition of 27 Al resolved NMR spectra in molten oxide at high temperature with a good sensitivity [15,16]. Figure 3a shows the experimental setting and an example of a 27 Al spectrum acquired in one scan for a liquid molten sample CaAl 2 O 4 at ~2000°C [17]. The sensitivity of this experiment is such that one can follow in a time-resolved manner the evolution of the 27 Al signal when cooling the sample from high temperature, until disappearance of the signal when the liquid becomes too viscous.

As in the case of the high temperature molten silicate discussed above, we only have a single sharp line giving the average chemical shift signature of the rapidly exchanging chemical species. This later point is confirmed by independent T 1 (spin-lattice) and T 2 (spin-spin) relaxation measurements giving similar values and reliably measured in the 1D spectrum from the linewidth. This relaxation time can be modelled using a simple model of quadrupolar relaxation which requires the knowledge of the instantaneous quadrupolar coupling that can be estimated from the 27 Al MAS NMR spectrum of the corresponding glass at room temperature. The obtained correlation times, corresponding to the characteristic time of the rearrangement of aluminium bearing structural units, can be directly compared to characteristic times of the macroscopic viscosity with a convincing correspondence in the case of aluminates melts [18] (Figure 3b&c).

Structure and dynamics of alumino-silicates

In alumino-silicate glasses of more complex composition, aluminium is able to substitute silicon in tetrahedral network forming positions, providing charge compensation by a neighbouring cation. In such case, the NMR signature of 29 Si spectra is much more complex and difficult to interpret [19]. Because 29 Si Q n species isotropic chemical shifts depend upon show proofs of the depolymerization of the AlO 4 based network [20]. In alumino-silicate glasses, aluminium species with higher coordination were evidenced [22] and quantified [21] using a detailed modelling of the 27 Al MAS and MQMAS NMR spectra obtained at high principal fields. One can also remark that no SiO 5 environments have ever been evidenced in aluminosilicate compositions. Going further and examining the whole SiO 2 -Al 2 O 3 -CaO phase diagram [23], we showed that these AlO 5 environments are not confined to the charge compensation line or to the hyper-aluminous region of the ternary diagram, where there exist a deficit of charge compensators, but that AlO 5 species are present, at a level of ~5%, for any alumino-silicate composition of this ternary diagram, including those presenting the smallest fraction of alumina but excluding the Calcium Aluminates of the CaO-Al 2 O 3 join which nearly exclusively show aluminium in AlO 4 coordination state. For C3A composition XANES unambiguously shows that Al occupy Q 2 environments both in crystal, and glass [20,23].

These finding that there exist no or very few AlO 5 in compositions close to CaO-Al 2 O 3 composition is somehow in contradiction with our previous interpretation of chemical shift temperature dependence with a negative slope [17]. At that time we proposed to consider that there could exist significant amounts of five fold coordinated aluminium in the high

Conclusion

From the above discussed experimental results we can draw several important points about the relations between structure and properties of oxide glasses and their related molten states which appear to be closely related. It first clearly appears that in many cases, even if most of the structure of the glasses, and consequently of their related high temperature molten states are built around a network of µ 2 connected tetrahedra (P, Si, Al…), there exist in many cases unexpected environments showing up as minor contributions in the glass structures (~5% or less) but significantly present and relevant to molecular motifs that can be identified. This is the case of SiO 5 species in binary alkali silicates [START_REF] Stebbins | [END_REF], AlO 5 (AlO 6 ) [21][22][23] in aluminosilicates, violations to Al avoidance principle [25] or tricluster µ 3 oxygens [9]. This implies that modelling of these complex materials in their solid or molten state will often be difficult using limited box sizes. Just consider that 5% of Aluminum species in a glass containing 5% Al 2 O 3 in a Calcium Silicate only represent 1 to 2 atoms over 1000 or that 5% µ 3 oxygens in a CaAl 2 O 4 composition represent less than 3 occurrences in a box of 100 atoms.

Furthermore Charpentier and coworkers [26] recently showed that a proper rendering of NMR parameters from all electrons ab-initio computations in glasses requires a combination of classical and ab-initio MD simulation. Going further we also emphasize that an important part of what we qualify with the general term of disorder can be described in terms of distribution of poly-atomic molecular motifs extending over a much larger length scale than the usual concept of coordination.

Figure Captions :

Summary of NMR experiments on a 60%PbO-40%P 2 O 5 glass evidencing polyatomic molecular motifs with : (a) 1D spectrum, (b) the J resolved spectrum showing doublet for Q 1 and triplet for Q 2 [7], (c) the INADEQUATE experiment evidencing pairs of phosphates (Q-Q) [6], and (d) the 3Quantum spectrum evidencing triplets of phosphates (Q-Q-Q) [8].

Figure 2

Quantitative interpretation of 29 Si and 31 P 1D spectra allowing the measurement of disproportionation constants for (a) lead silicate [3] and (b) lead phosphate glasses [4].

Q 1 -Q 1 Q 1 -Q 2 Q 2 -Q 2 Q-Q pairs Chemical bond Q 1 Q 2 (a) (b) Q-Q-Q triplets Q 1 -Q 2 -Q 1 Q 2 -Q 2 -Q 2 Q 1 -Q 2 -Q 2 (d) (c)

  ). From these experiments it becomes possible to identify the different structural motifs constituting these glasses in terms of molecular building blocks extending over 6 chemical bonds (O-P-O-P-O-P-O) over lengths up to nearly 10Å if we consider a linear chain. Other experiments of the same type now allow to describe hetero-nuclear structural motifs of different types involving Al-O[9], Al-O-Si, P-O-Si[10] or opening the possibilities of more detailed description of glasses or disordered solids at large length scale.High Temperature NMR experimentsEven if most of the resolution is lost when going to static NMR spectra in the general case, the very different chemical shift anisotropy of Q 3 and Q 4 silicon environment can be source of enough resolution for evidencing dynamic process occurring close to or above glass transition temperature as shown by Stebbins and Farnan in the case of a binary K 2 O-4SiO 2

  Al substitution in neighbouring tetrahedra,29 Si silicon spectra are usually broad Gaussian lines covering the full range of possible environment. Similarly 27 Al aluminium spectra are broadened by the combination of a distribution of chemical shifts and a distribution of second order quadrupolar interaction[21] and give only average pictures of the structure with possible resolution of different coordination states but no resolution of the different Al based Q n species except in the case of binary CaO-Al 2 O 3 glasses in which NMR and XANES both

  temperature molten state, based on the thermal dependence of chemical shift and on state of the art MD computations. A more detailed study shows that, across the CaO-Al 2 O 3 join, the slope of the temperature dependence of the average chemical shift in the high temperature molten state drastically changes from a positive value for Al 2 O 3 to very negative (~-4 to -5ppm) for composition around CaAl 2 O 4 . Indeed we can even remark that all compositions able to vitrify in aerodynamic levitation contactless conditions have a slope smaller than -2ppm/1000°C (Figure4a). Stebbins and coworkers recently studied the17 O NMR signature of similar composition[24]. They evidenced a significant amount of non bridging oxygen atoms and discussed the possibility of a seldom observed µ 3 tricluster oxygen linking three tetrahedral Al sites that exists in the closely related CA 2 (CaAl 4 O 7 -Grossite) crystalline phase. Thanks to the development of new methods of hetero-nuclear correlation between quadrupolar nuclei through J-coupling at high principal field (750MHz)[9], we could reexamine this question and show that a {17 O} 27 Al experiment carried out on a CaAl 2 O 4 glass was able to clearly evidence the signature of ~5% µ 3 tricluster oxygen linked to aluminium with chemical shift decreased by 5 ppm per linked tricluster (Figure 4b). It thus appears that molecular motif of type µ 3 [AlO 3 ] 3 can be quenched in the glass and do exist in the molten state while AlO 5 remains negligible, rising a new interpretation of the thermal dependence of the 27 Al isotropic chemical shift in the CaO-Al 2 O 3 melts.

Figure 3 (

 3 Figure 3 (a) High Temperature aerodynamic levitation NMR setup and a characteristic one shot spectrum, (b) temperature dependence of the chemical shift and (c) viscosity and NMR correlation times[adapted from ref.17]

Figure 4 (

 4 Figure 4 (a) Slope of the thermal dependence of average chemical shift in high temperature versus composition for the CaO-Al 2 O 3 join. (b) { 17 O} 27 Al HMQC experiment of CaO-Al 2 O 3 glass at 750 MHz showing clear signature of µ 3 tricluster oxygens[adapted from ref.9].

  Figure 1

D

  .Massiot, XI PNCS, Rhodos Nov 2006

  Figure 2

  Figure 3

Figure 4 dδ

 4 Figure 4

Acknowledgements

We acknowledge financial support from CNRS UPR4212, FR2950, Région Centre, MIIAT-BP and ANR contract RMN-HRHC.


Hatim Alnoor
email: hatim.alnoor@liu.se

Adrien Savoyant

Xianjie Liu

Galia Pozina

Magnus Willander

Omer Nur

An effective low-temperature solution synthesis of Co-doped [0001]-oriented ZnO nanorods

Keywords: Low-temperature aqueous chemical synthesis, ZnO NRs, Co-doping, EPR, intrinsic point defects

We demonstrate an efficient possibility to synthesize vertically aligned pure zinc oxide (ZnO) and Co-doped ZnO nanorods (NRs) using the low-temperature aqueous chemical synthesis (90 ºC). Two different mixing methods of the synthesis solutions were investigated for the Codoped samples. The synthesized samples were compared to pure ZnO NRs regarding the Co incorporation and crystal quality. Electron paramagnetic resonance (EPR) measurements confirmed the substitution of Co 2+ inside the ZnO NRs giving a highly anisotropic magnetic Co 2+ signal. The substitution of Zn 2+ by Co 2+ was observed to be combined with a drastic reduction in the core-defect (CD) signal (g ~ 1.956) which is seen in pure ZnO NRs. As revealed by the cathodoluminescence (CL) the incorporation of Co causes a slight red-shift of the UV peak position combined with an enhancement in the intensity of the defect-related yellow-orange emission compared to pure ZnO NRs. Furthermore, the EPR and the CL measurements allow a possible model of the defect configuration in the samples. It is proposed that the as-synthesized pure ZnO NRs likely contain Zn interstitial (Zni + ) as CDs and oxygen vacancy (VO) or oxygen interstitial (Oi) as surface defects. As a result, Co was found to likely occupy the Zni + leading to the observed CDs reduction, and hence enhancing the crystal quality. These results open the possibility of synthesis of highly crystalline quality ZnO NRs-based diluted magnetic semiconductors (DMSs) using the low-temperature aqueous chemical method.

1-Introduction

Zinc Oxide (ZnO) is a direct wide band gap (3.4 eV at room temperature) semiconductor with a relatively large exciton binding energy of 60 meV and possesses a significant luminescence covering the whole visible region. [1][2][3][4] Moreover, ZnO can be easily synthesized in a diversity of one-dimensional (1D) nanostructure morphologies on any substrate being crystalline or amorphous. [1][2][3][4][5][6][7] In particular, 1D ZnO nanostructures such as nanowires (NWs) and nanorods (NRs) have recently attracted considerable research due to their potential for the development of many optoelectronic devices, such as light-emitting diodes (LEDs), ultraviolet (UV) photodetectors and solar cells. [2][3][4][8][9][10] Also, ZnO NRs-based diluted magnetic semiconductors (DMSs), where a low concentration of magnetic elements (such as manganese (Mn) and cobalt (Co)) is diluted in the ZnO crystal lattice, show great promise for the development of spintronics and magneto-optical devices. [11][12][13][14] Among the different synthesis techniques utilized for ZnO NRs, the low-temperature solution-based methods are promising due to many advantages, i.e., low-cost, large-scale production possibility and the properties of the final product can be varied by tuning the synthesis parameters. [5][6][7] However, synthesizing ZnO NRs with optimized morphology, orientation, electronic and optical properties by low-temperature solution-based methods remains a challenge. The potential of ZnO NRs in all above-mentioned applications would require synthesis of high crystal quality ZnO NRs with controlled optical and electronic properties. [2][3][4]15 It is known that the optical and electronic properties of ZnO NRs are mostly affected by the presence of the native (intrinsic) and impurities (extrinsic) defects. [1][2][3][4] Therefore, understanding the nature of these intrinsic and extrinsic defects and their spatial distribution is critical for optimizing the optical and electronic properties of ZnO NRs. [1][2][3][4][16][17][18] However, identifying the origin of such defects is a complex matter, especially in nanostructures, where the information on anisotropy is usually lost due to the lack of coherent orientation. Recently, we have shown that by optimizing the synthesis parameters such as stirring times and the seed layer properties, the concentration of intrinsic point defects ( i.e. vacancies and interstitial defects ) along the NRs and at the interface between the NRs and substrate can significantly be tuned. 8,19,20 Thus, the ability to tune such point defects along the NRs could further enable the incorporation of Co ions where these ions could occupy such vacancies through substitutional or interstitial doping, e.g. a Co ion can replace a Zn atom or be incorporated into interstitial sites in the lattice. 21 Here, by developing theses synthesis methods, we obtained welloriented ZnO NRs, and by studying them at low temperature, we can access the magnetic anisotropy of such defects. Furthermore, by incorporating a relatively low amount of diluted Co into ZnO NRs the crystal structure of the as-synthesized well-oriented ZnO NRs can significantly be improved. The well-oriented pure ZnO and Co-doped ZnO NRs were synthesized by the lowtemperature aqueous chemical synthesis (90 ºC). The structural, optical, electronic, and magnetic properties of the as-synthesized well-oriented NRs have been systematically investigated by mean of field-emission scanning electron microscopy (SEM), X-ray powder diffraction (XRD), electron paramagnetic resonance (EPR), cathodoluminescence (CL) and X-ray photoelectron spectroscopy (XPS).

2-Experimental

The pure ZnO and Co-doped ZnO NRs were synthesized by the low-temperature aqueous chemical synthesis at 90 ºC on sapphire substrates. For pure ZnO NRs, a 0.075 M synthesis solution was prepared by dissolving hexamethylenetetramine (HMTA) and zinc nitrate hexahydrate in a deionized (DI) water and then stirred for three hours at room temperature (later denoted as M0 sample). After that, a sapphire substrate precoated with ZnO seed layer, 8,19,20 were submerged horizontally inside the above-mixed solutions and kept in a preheated oven at 90 °C for 5 hours.

Afterward, the samples were rinsed with DI-water to remove any residuals and finally, dried using blowing nitrogen. The synthesis process of the pure ZnO NRs is described in more details in Ref. 8,19,20 The Co-doped ZnO NRs were grown under similar conditions where two different approaches were used to prepare the synthesis solution. The first synthesis solution was prepared by mixing a 0 .075 M concentration of HMTA and zinc nitrate and stirred for 15 hours. Then a diluted solutions of Cobalt(II) nitrate hexahydrate with an atomic concentration of 7% was added dropwise to the above solution and stirred for extra 3 hours (later denoted as M1). The second synthesis solution was prepared by mixing a 7% diluted solution of Cobalt(II) nitrate hexahydrate with 0.075 M HMTA and stirred for 15 hours, and then a 0.075 M solution of zinc nitrate hexahydrate was added dropwise to the above-mentioned solution and stirred for extra 3 hours (later denoted as M2).

The morphology of the as-synthesized pure ZnO and Co-doped ZnO NRs was characterized using field-emission scanning electron microscopy (FE-SEM, Gemini LEO 1550). The crystalline and electronic structure were investigated by XRD using a Philips PW1729 diffractometer equipped with Cu-Kα radiation (λ = 1.5418 Å) and EPR, respectively. The EPR measurements were performed using a conventional Bruker ELEXSYS continuous wave spectrometer operating at X-band (ν = 9.38 GHz) equipped with a standard TE102 mode cavity. The angle between the static magnetic field and the NRs axis, denoted by θ, was monitored by a manual goniometer. The optical properties were examined by cathodoluminescence (CL) using Gatan MonoCL4 system combined with Gemini LEO 1550 FE-SEM. The CL measurements were performed on aggregated nanorods using an acceleration voltage of 5 kV. The chemical composition was analyzed by XPS measurements recorded by Scienta ESCA-200 spectrometer using monochromator Al Kα X-ray source (1486.6 eV). All the measurements were carried out at room temperature (RT) except the EPR measurements which were performed at 6 K.

3-Results and discussion

Fig. 1 shows the top-view FE-SEM images of the as-synthesized pure ZnO (M0) and Codoped ZnO NRs (M1) and (M2), respectively. The SEM images reveal that all the as-synthesized NRs were vertically-aligned with a hexagonal shape. The average diameter of the NRs was found to be ~160, ~400 and ~200 nm, for the M0, M1, and M2, respectively. The significant and slight increase in NRs average diameter in the case of M1 and M2 compared to M0 is likely due Co doping. 22,23 Fig. 1: SEM images of pure ZnO (M0) and Co-doped ZnO NRs as-synthesized using approaches M1 and M2, respectively.

The structural quality of the as-synthesized pure ZnO and Co-doped ZnO NRs have been confirmed by the XRD measurements as illustrated in Fig. 2. The XRD patterns showed that all the as-synthesized samples have a wurtzite structure and possess a good crystal quality with preferred growth orientation along the c-axis, as demonstrated by the intensity of the (002) peak. 15,[23][24][25] Also, it should be noted that no secondary phase related to Co was observed in the XRD patterns of all three NRs samples. As shown in the inset of Fig. 2 the position of the (002) peak is slightly shifted toward lower 2θ angle in M1, and toward higher 2θ angle in M2 as compared to M0. The peak position shift toward lower and higher 2θ angle are reported to be a confirmation of the successful incorporation of Co into the ZnO crystal lattice. 15,23,26 Also, the peak position shift is reported to be due to the variation of oxygen vacancies (Vo) and zinc interstitials (Zni) caused by Codoping. 27,28 In this study, the Co concentration in the synthesis solution is the same (7 %) for both M1 and M2. Thus, the observed shifts in the peak position could be attributed to either Co incorporation or to the variation of the defects concentration, e.g. such as vacancies and interstitials induced by Co doping. These results show that the way of preparing the synthesis solution have a significant influence on the Co incorporation in the synthesized ZnO NRs. The inset shows the normalized XRD data for the (002) peaks, indicating peak shifts.

Further, to confirm the crystal quality and the incorporation of Co into the ZnO crystal lattice as suggested by the XRD results, EPR spectra were recorded at 6 K, and the results are shown in Fig. 3 (a)-(b). The EPR spectra of pure ZnO NRs (M0) is characterized by the well-known defect signal from ZnO apparent at 350 mT (g ~1.956) [29][30][31][32][33][34] as shown in Fig. 3 (a), and commonly attributed to core-defects (CDs) arising from ZnO nanostructures rather than shell defects. 32,33 However, the identification of the exact nature of this CDs (1.956) signal is controversial, 35 and up to date, no experiment can give a concerete answer. Previously, many defect signals close to this value (1.956) have been reported and Zn interstitials (Zni + ) and the so-called D * center were proposed. 31,34 Indeed, the angle-dependent spectra of the CD signal shown in Fig. 3(a) display a slight easy-axis magnetic anisotropy and are composed of two overlapped lines. This observed anisotropy is compatible with a Zni + defect (easy-axis) but not with D* defect (easy plane), so that Zni + appears to be the most probable defect. 31,34 In our previous study, these CDs were characterized by three lines, which supports our hypothesis that these lines are likely a variation of the same defect, i.e. the same defects with slightly different parameters. 36 The successful substitution of Co 2+ was confirmed by the observed Co-related signal characterized by an eight-lines structure at g ~ 2.239 (θ = 0º ) and a broad asymmetric signal at g ~ 4.517 (θ = 90º ), 21,36,37 as shown in Fig. 3 (b). The observed magnetic anisotropy of the Co 2+ signal is a clear indication that the as-synthesized NRs are single crystalline,well-aligned and that Co is highly diluted along the NRs. 36 Interestingly, the substitution of Co 2+ caused a drastic reduction of the CD signal (g ~1.956) as indicated by dashed line (Fig. 3(b)) compared to that in the pure ZnO NRs (M0) (as shown Fig. 3 (a)), as previously observed in similar samples. 36 In fact, this could suggest that a certain amount of the incorporated Co is involved in the CDs neutralization. This neutralization could be due to substitutional doping, where a Zn atom is replaced by a Co atom (Cozn), or to interstitial doping, where a Co atom is incorporated into interstitial sites in the lattice (Coi). 21 As shown in Fig. 3(b) the intensity of the Co 2+ signal of M2 at θ = 90º and θ = 0º is significantly higher as compared to that of M1. Moreover, the line width of the Co 2+ signal at θ = 0º for M2 is found to be a slightly smaller (4 G) than that of M1(5 G). As the Co concentration in the synthesis solution is the same (7 %) for both M1 and M2, and by assuming a uniform doping and same coverage of the NRs, these results clearly show that the way of preparing the synthesis solution have a significant influence on the Co incorporation in the synthesized ZnO NRs in agreement with the XRD results shown in Fig. 2. It should be noted that the hyperfine constant (the spacing between two hyperfine line) is ~15.3 G in both samples, that is the same value for the bulk Co-doped ZnO. 36 Thus, we can deduce that the observed EPR signal comes from the substitutional Co 2+ inside the NRs, and not from ions on the surface. However, such an observation did not make exclusive evidence that the presences of Co is on the surface of the as-synthsized Co-doped ZnO NRs. In the solution-based synthesis method, it is possible that Co 2+ can be incorporated in the core of ZnO nanostructures or can be adsorbed at their surface. 21 Furthermore, in order to get more information on the defects in the as-synthesized pure ZnO and Co-doped ZnO NRs, room-temperature CL spectra were carried out, and the results are shown in Fig. 4. The emission spectra of all samples were dominated by UV emission peak centered at ~382 nm (3.24 eV) due to near-band-edge (NBE) emission and a strong broad yellow-orange emission centered at ~ 610 nm (2.03 eV) associated with deep-level defects related emission in ZnO. [1][2][3][4][38][39][40] Apparently, the CL spectra of Co-doped NRs exhibited a small red-shift of the UV peak position from 382 nm to 384 nm (as shown in the inset of Fig. 4) as compared to pure ZnO NRs, which is likely due to the change in the energy of the band structure as a result of doping. 22,41 It is important to note that the CL defect-related yellow-orange emission intensity decreases from M1 to M2 (Fig. 4) and the Co EPR signal increases from M1 to M2 (Fig. 3 (b)). This observation suggests that the way of preparing the synthesis solution have a significant influence on the Co incorporation and defect formation in the as-synthesized ZnO NRs in agreement with the XRD results shown in Fig. 2.

The physical origin of intrinsic defects-related yellow-orange emission is controversial, and it is proposed to be associated to the Vo, Oi and Zni. 23,[38][39][40] Recently, it was proposed that the defects-related orange emission is to be likely from the Zni in the core of ZnO NRs. 39 In this study, we believe that the defect-related yellow-orange emission is likely to originate from the Zni in the core of ZnO NRs or the Vo and Oi on the surface of ZnO NRs. As a consequence, the intensity of the defects-related yellow-orange emission is significantly enhanced by the Co doping (Fig. 4), which probably due to the increase in Vo and Oi in the NRs or to the Co-related defect. 15,23,41 Moreover, this suggests the above-observed red-shift of the UV peak could be attributed to the variation of the Zni + concentration in the Co-doped samples (M1 and M2) compared with the pure ZnO NRs (M0). In fact, these results indicate that the bulk quality of the ZnO NRs is improved by the substitution of Co, while the doping has the adverse effect on the surface defects related emission, in agreement with previous results. 15,41 Fig. 4: CL spectra of the as-synthesized pure ZnO and Co-doped ZnO NRs synthesized using different synthesis preparation approaches as indicated. The inset shows the red-shift in the UV peak. For clarity, the spectra are normalized to the near band edge intensity.

In view of the EPR and the CL results, a defect distribution model for ZnO NRs is shown in Fig. 5, 32 which propose that the incorporation of Co during the synthesis process could probably result in occupying Zni + through substitutional or interstitial doping and, subsequently, enhance the crystal quality. The other possibility is that a substitutional Co 2+ very close to a Zni + interstitial may form a non-magnetic complex, which is then not anymore EPR detectable. Also, the incorporation of Co was found to lead to the increase concentration of surface defects such as VO and Oi. Further experimental study combined with detailed theoretical calculations are necessary to fully understand the observed phenomena.

To elaborate more on surface related defect concentration, XPS spectra of all samples have been investigated. Figure 6 (a) shows the Zn 2p core level spectra of all samples, which is composed of two peaks centered at ~ 1022.2 and 1045.0 eV corresponding to binding energy lines of the Zn 2p3/2 and Zn 2p1/2, respectively, with a spin-orbital splitting of 23.1 eV suggesting that Zn is present as Zn 2+ . 22 Co signal in the ZnO NRs was not detected by the XPS; this could be attributed to the surface sensitive XPS technique with the Co 2+ at the inner core of ZnO NRs, as indicated in Fig. 5, and also due to the low Co concentration, as suggested by the EPR measurements in Fig. 3 (b). The O1s core level peak for all samples exhibits an asymmetric profile, which can be decomposed into three Gaussian peaks, donated to OI, OII, and OIII, respectively, as shown in Fig. 6 (b). The OI peak at low binding energy at ~530.9 eV is attributed to the Zn-O bond within the ZnO crystal lattice.

Whereas the OII peak centered at ~532.2eV is commonly assigned to oxygen-deficiency in the ZnO crystal lattice. 16,42 Finally, the OIII peak centered at ~ 533.1 eV is related to the absorbed oxygen on the ZnO surface, e.g. H2O, O2. 16,42 Fig. 5: Schematic illustration of the cross-sectional view of the as-synthesized pure ZnO and Codoped ZnO NRs containing Zni + as core-defect and oxygen vacancies/interstitials as surface defects, respectively.

The relative concentration of oxygen vacancies is estimated from the intensity ratios of OII/OI using the integrated XPS peak areas and the element sensitivity of the O and Zn. 42 The ratios of OII/OI were found to be 0.54, 0.52 and 0.49 for M0, M1, and M2, respectively, suggesting that M2 has a lower concentration of oxygen vacancies compared to M1 and M0, respectively. However, there is no obvious relationship between the samples defect composition estimated from the CL and XPS measurements. For instance, M0 shows lower CL defect emission intensity and higher OII/OI ratio.

4-Conclusions

The optical properties of ZnO NRs are commonly dominated by the presence of native intrinsic point defects, and identifying these defects is a difficult matter, especially in the nanostructure, where the information on anisotropy is usually lost due to the lack of coherent orientation. Here, by studying well-oriented ZnO NRs at low temperature, we were able to access the magnetic anisotropy of theses defects. Furthermore, by incorporating a relatively low amount of diluted Co inside ZnO NRs the crystal structure of the as-synthesized well-oriented ZnO NRs is significantly improved. Pure ZnO and Co-doped ZnO NRs were synthesized by the lowtemperature aqueous chemical method, where the crystal structure, orientation, and incorporation of the Co ion is tuned by the preparation procedure of the synthesis solution. The SEM and XRD measurements showed that the as-synthesized pure ZnO and Co-doped ZnO NRs are vertically aligned along c-axis and have a wurtzite crystal structure of high quality, as demonstrated by the intensity of the (002) diffraction peak. Moreover, the (002) peak position was observed to be shifted to lower or higher 2θ angle depending on the synthesis solution mixing procedure used. This is probably attributed to either Co incorporation or to the variation of the defect concentration in the samples, e.g. such vacancies and interstitials induced by Co doping. EPR measurements have confirmed the substitution of Co 2+ inside ZnO NRs giving a highly anisotropic magnetic Co 2+ signal characterized by eight lines indicating that the as-synthesized NRs are single crystalline, well-aligned and the Co is homogeneously distributed along the NRs. Also, the substitution of the Co 2+ was observed to be accompanied by a drastic reduction in the CD signal (g ~ 1.956) found in pure ZnO NRs. As revealed by CL, the incorporation of Co causes a red shift in the UV peak position with an observed enhancement in the intensity of defect-related emission as compared to pure ZnO NRs. In view of the different results from these complementary measurements, we proposed that the as-synthesized pure ZnO NRs likely contain Zn interstitial (Zni + ) as CDs and oxygen vacancy (VO) or interstitial (Oi) as surface defects. These results open for the possibility of synthesis of highly crystalline quality ZnO-based DMSs using the low-temperature aqueous chemical method.

Fig. 2 :

 2 Fig. 2: XRD patterns of the as-synthesized pure ZnO (M0) and Co-doped ZnO NRs (M1 and M2).

Fig. 3 :

 3 Fig. 3: (a) EPR spectra show anisotropy of the CD signal in the pure ZnO sample (M0). (b) EPR spectra of Co-doped ZnO NRs ( M1 and M2 ) for parallel (θ = 0º) and perpendicular (θ = 90º) orientation of magnetic field, recorded at T = 5 K. The upper axis gives corresponding g factor values.

Fig

  Fig. : XPS core level spectra of the (a) Zn 2p peak and (b) O 1s peak of the as-synthesized pure and Co-doped ZnO NRs as indicated.

Acknowledgement:

This work was supported by the NATO project Science for Peace (SfP) 984735, Novel magnetic nanostructures.