Structural characterisation of single crystals and thin films ofa,v-dihexylquaterthiophene

Massimo Moret,*a Marcello Campione,*a Alessandro Borghesi,a Luciano Miozzo,a Adele Sassella,a

Silvia Trabattoni,a Bernard Lotzb and Annette Thierryb

Received 24th March 2005, Accepted 19th May 2005

First published as an Advance Article on the web 31st May 2005

DOI: 10.1039/b504233j

Single crystals and thin films of the p-type organic semiconductor a,v-dihexylquaterthiophene

have been studied using X-ray diffraction and TEM, respectively. The single crystal analysis

reveals the presence of monomolecular layers of quaterthiophene cores with terminally bound

hexyl chains. A herringbone molecular packing akin to that exhibited by the parent

quaterthiophene molecule is observed. Electron diffraction of thin films on silica clearly shows the

crystalline polymorph of the solution-grown crystals and the crystallites deposited by organic

molecular beam deposition are identical. However, thin film crystallites are severely affected by

multiple twinning.

Introduction

In field effect transistors that include active organic layers,

alkyl-substituted oligothiophenes have superior properties

compared to the non-substituted ones: easier processability

of the material, enhanced performance of the devices.1–3

Previous studies have demonstrated promising performance of

electronic devices based on these materials. The structure and

morphology of thin films, kinetically grown from the vapour

phase, have been reported.1,4 In organics, the conduction

mechanism involves charge transport across p–p stacked

molecules. A high charge mobility thus requires high conjuga-

tion along the main molecular axis and favourable molecular

stacking.5 The typical herringbone organisation, present for

example in all crystalline structures of non-substituted

oligothiophenes, minimizes the repulsion between p orbitals

and favours p–p interactions between neighbour molecules. In

this respect, dialkyl-substituted oligothiophenes are among the

most promising p-type organic materials. They are able to self-

assemble in close packed crystalline structures and are soluble

in common organic solvents.

Since for solid organic semiconductors the carrier mobility

depends critically on the molecular order and on the p–p

interactions, a thorough understanding of the structural

properties in the condensed phase of a,v-dialkyl terminated

oligothiophenes is mandatory.2a,3,6 Particular attention has

been devoted to a,v-dihexylquaterthiophene (a,v-DH4T) for

which many experimental findings indicate considerable order

in the crystalline phase.1,3,7 Recently, Katz et al.1 established

the conditions for growing highly crystalline thin films of

a,v-DH4T. Electron diffraction of films deposited at 100 uCon carbon grids displays a single crystal-like pattern. Detailed

knowledge of the molecular organisation in the crystal is

necessary for the fundamental interpretation of the physical

properties in the solid state of semiconducting organic

materials.5 However, the crystalline structure, and hence the

molecular conformation and packing in the solid state, have

not yet been determined for a,v-DH4T. This motivated us to

study the growth of a,v-DH4T single crystals from solution

in order to determine the molecular and crystal structure,

opening the possibility to better understand the structural

features of thin films.

Results and discussion

The existence of a liquid crystal mesophase above 84 uChinders the growth of large single crystals of a,v-DH4T from

the vapour phase.3,8 Platelet crystals several millimetres

long and hundreds of micrometres thick were obtained from

anisole solutions. The present combined single crystal X-ray

diffraction study of a,v-DH4T and transmission electron

microscopy (TEM) characterisation of its thin films on silica

provide fundamental information on structural features and

self-assembling properties of these systems for bulk crystals

and for thin films. It demonstrates also that the crystalline

polymorph of the solution-grown crystals and the crystallites

deposited by organic molecular beam deposition (OMBD)

(under the experimental conditions of the present work) are

identical.

Crystal and molecular structures

The a,v-DH4T molecule crystallizes in the monoclinic space

group P21/a with one half molecule in the asymmetric unit;

pairing through an inversion centre (Wyckoff position a)

between atoms C(1) and C(1a) with another independent half

molecule completes the molecule (Fig. 1), giving rise to two

molecules per unit cell.

With the present unit cell choice, the molecular packing is

consistent with that of the low temperature polymorph of

quaterthiophene (4T/LT),9 i.e. a herringbone (HB) packing of

a,v-DH4T molecules in the ab plane. HB layers are stacked

along the [001] direction to generate the 3D motif of the crystal

structure (Fig. 2). As previously reported1,8 the c axis length is*massimo.moret@mater.unimib.it (Massimo Moret)marcello.campione@mater.unimib.it (Marcello Campione)

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compatible with a 4T central unit plus two terminal fully

extended hexyl chains, with the long molecular axis slightly

tilted with respect to the (001) plane. Characterisation of the

crystalline samples of a,v-DH4T has been previously per-

formed on thin films by using selected-area TEM electron

diffraction and X-ray diffraction.1,4,8 The unit cell lengths

found by Katz et al.1,8 are in good agreement with those

reported in the present paper. We are therefore dealing with

the same crystal polymorph. In contrast, partial crystal data

published by Garnier et al.4 based on X-ray powder diffraction

are not consistent with our cell parameters, crystal symmetry

and crystal density.

The molecular structure of a,v-DH4T is closely related to

that of the unsubstituted nT oligomers (n 5 2–8)2c,10 and

a,v-dimethylquaterthiophene (a,v-DM4T),11a previously

characterized by X-ray diffraction. It differs significantly from

that of c-dihexylquaterthiophene11b due to the position of the

alkyl groups. Intramolecular features for the a,v-DH4T

molecule remind those of the unsubstituted oligomers. The

thienyl rings display the usual flat all-anti conformation with a

dihedral angle between a central thienyl ring and an outer one

of 2.3(5)u, compatible with a high degree of p conjugation.

Structural features of interest are due to the co-presence in the

molecule of a rigid quaterthiophene core and two flexible

terminal hexyl chains in a and v positions. As discussed later,

the conformation adopted by the alkyl chains relative to the

thiophene moiety is highly relevant in determining the

molecular packing. In crystals of a,v-DH4T, monomolecular

layers parallel to the (001) plane (Fig. 2) correspond to the

largest macroscopic face exhibited by the crystals (i.e. the faces

with the slowest growth rate). Each (001) layer contains a HB

packing of molecules typical of most planar molecules12 and

in particular of oligothiophenes.2c A helpful geometrical

descriptor of the HB pattern is the angle between the least-

squares plane of the quaterthiophene moiety of two adjacent

molecules. The value found for a,v-DH4T is 62.8(1)u, very

close to the values of 63.1u and 62.1u determined for 4T/LT9

and a,v-DM4T,11a respectively. This result indicates only

minor perturbations of the 4T core HB packing caused by the

presence of the alkyl substituents in a/v positions. The analogy

between a,v-DH4T and 4T/LT crystals is further evidenced by

the tilt angle of the rigid core (i.e. the axis passing through the

a,v carbon atoms of the 4T core) with respect to the normal of

the (001) plane. The tilt angle is 24.2u and 24.8u for a,v-DH4T

and 4T/LT, respectively. The situation is very different

for a,v-DM4T, which is the archetypal structure for the

a,v-dialkyloligothiophenes, with methyl end groups.11a The

molecules are tilted in opposite directions in successive layers

(at + and 223.3u to the layer normal), thus generating a double

layer sequence in which the molecules are arranged in a

herringbone manner, too. The marked difference between

a,v-DM4T and a,v-DH4T highlights the impact of the alkyl

side chains on the crystal packing. Chain length, odd/even

number of carbon atoms (and also presence of impurities) that

are known to affect the growth of n-alkanes13 appear to play a

similar role in the growth of a,v-dialkyloligothiophenes.

The hexyl chains attached in the a position to the two outer

thienyl rings of a,v-DH4T exhibit an all-trans conformation,

that is, a flat zigzag of carbon atoms (rms deviation from least-

squares plane of fitted atoms C(9)–C(14) 5 0.0221 A). The

plane of the C6 chains makes an angle of y6u with the

pertinent thienyl ring, hence the alkyl groups are well aligned

with the 4T core. The chain conformation is well determined

and shows no trace of disorder even if the mobility of the alkyl

chains at room temperature is high. As expected, anisotropic

displacement parameters of the chain carbon atoms increase

when moving from C(9), closest to the molecule centre of

mass, to the terminal atom C(14) (Fig. 1), with a significant

component normal to the plane of the zigzag.

As already mentioned, the HB pattern is a ‘‘strong’’ packing

motif. It is present in several rod-like organic p-conjugate

molecules,12 in the low temperature polymorphs of 4T9 and

6T14 and is preserved with little change also in the high

temperature polymorphs of 4T15 and 6T.16 In the present case,

the oligothiophene core dictates the basic packing require-

ments that take precedence over the hexyl chain ones, but the

4T-like herringbone is compatible with an efficient packing of

the hexyl chains, too. This is achieved through a conformation

of the alkyl chains in which the ipso methylene group C(9) is

staggered relative to the sulfur S(2) and not to the b aromatic

carbon C(7). The latter conformation has been suggested in

previous attempts to infer the molecular conformation and

solid state or mesophase packing of alkyl-substituted oligo-

thiophenes.3,8,17 Moreover, the S(2)–C(8)–C(9) bond angle

(122.0(13)u) is not significantly different from the 120.5(6)uangle of a,v-DM4T. It is however significantly smaller than

that predicted by molecular mechanics modelling3 with the

‘‘correct’’ S-staggering. A brief survey of the a,v-DH4T

geometry with molecular mechanics18 did not reveal any

significant anomaly or strain for this intramolecular bond

parameter.

At this stage, it may be worth comparing structures

predicted by molecular modelling and experimentally observed

crystal structures. The apparently minor variations in the

chain conformation just mentioned cause indeed dramatic

variations of the molecular ‘‘shape’’ and length, and hence of

Fig. 1 ORTEP drawing (ellipsoids at 30% probability level) and labelling scheme for the centrosymmetric a,v-DH4T molecule. Hydrogen atoms

were given arbitrary radii. The all-anti and all-trans conformations for the quaterthiophene and hexyl moieties, respectively, are shown. Bond

distances: S(1)–C(1) 1.753(8), S(1)–C(4) 1.729(12), S(2)–C(5) 1.755(11), S(2)–C(8) 1.694(15), C(1)–C(1a) 1.47(2), C(1)–C(2) 1.335(13), C(2)–C(3)

1.428(15), C(3)–C(4) 1.383(15), C(4)–C(5) 1.422(14), C(5)–C(6) 1.383(14), C(6)–C(7) 1.401(15), C(7)–C(8) 1.378(18), C(8)–C(9) 1.503(19), C(9)–

C(10) 1.39(2), C(10)–C(11) 1.50(2), C(11)–C(12) 1.45(2), C(12)–C(13) 1.54(2), C(13)–C(14) 1.39(3).

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the magnitude of the molecular tilt within the layers. The

experimental molecular conformation is significantly more

straight than the predicted ones. The latter display a kinked

shape arising from the staggering of the C6 chain relatively to

the C(b)–H moiety17 and/or the wider S–C(a)–C(chain) angle.3

The linear shape taken by the a,v-DH4T molecule makes it

very similar to an n-alkane, as previously anticipated. Given

the proper overall molecular conformation an even nT central

unit (n 5 4 in this case) terminated by two a,v (even

numbered) alkyl chains is nearly equivalent to an even

n-alkane in terms of symmetry and molecular packing. As

for the lateral packing of the alkyl chains, a good arrangement

of the chains can usually be achieved only if a hydrogen atom

of one molecule fits into a hollow formed by H atoms of an

adjacent molecule. For chains a,v-connected to 4T cores and

HB packed, a shift along the chain axis is needed to fulfil this

condition. As shown in Fig. 2, neighbour chains are shifted

along their major axis in order to match nearby convex parts

with neighbour hollows. In the present case, the shortest

distances between H atoms belonging to different chains

(y2.6 A) is within accepted van der Waals distances19 and is

analogous to those found in e.g. n-C20H42.20

Normal-paraffin crystals adopt a stacking of molecular

layers with parallel chains. In the so-called oblique structure

(triclinic and monoclinic system) the chains are nearly parallel

to the c axis and tilted with respect to the molecular layers.

Therefore, in the presence of the HB pattern determined by the

oligothiophene core and the concomitant steric requirements

of the paraffinic moieties, packing efficiency for a,v-DH4T

results in a crystal having a density which is exactly that arising

from the average of n-hexane21 and 4T/LT9 (weighted on the

basis of type and number of involved atoms). This is an

indirect but simple proof that the packing of the hexyl ends in

a layer is similar to that of the crystalline free paraffins (note

however that the melting temperature of n-hexane is 294.3 uC,

whereas when attached to the 4T moieties the hexyl substituent

is ‘‘crystalline at room temperature’’).

The monomolecular layers just described cannot stack

exactly one above the other because protruding terminal

CH3 groups of one layer would impinge in protruding CH3

groups of the adjacent layers instead of fitting into hollows.

This potential steric conflict is relieved by a lateral displace-

ment of successive lamellae and a slight misalignment of c*

(perpendicular to the (001) plane) relative to the c axis. The

resulting distance between the plane defined by methyl C

atoms of molecules belonging to adjacent layers is 2.82 A,

larger than the value of 2.58 A found in the short chain

n-hexane21 at low temperature but well within the range of

2.70–2.85 A observed in the longer alkanes n-C18H38,22

n-C20H42,20 and n-C24H5023 (all crystalline at room tempera-

ture). The subtle role exerted by the terminal chains is

exemplified by the different bilayered crystal packing adopted

by a,v-DM4T. Similar bilayered packings are observed in

polymorphs and polytypes of n-alkanes, the ‘‘best’’ packing of

layers depending on many subtle parameters (structural,

chemical, thermodynamical, and kinetic factors).13

Thin film structural characterisation

Organic thin films are essential in the design of devices. The

aforementioned structural characterisation of bulk a,v-DH4T

single crystals is a crucial prerequisite in the analysis of

molecular arrangements observed in thin films of a,v-DH4T

Fig. 2 View down the b axis of the a,v-DH4T crystal structure

highlighting the monomolecular layered structure parallel to (001).

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deposited in ultra-high vacuum (UHV) on inert substrates to

be described now. These thin films grown on silica exhibit an

island-like morphology in the full range of nominal thicknesses

so far observed, with some fluctuations in thickness.

Fig. 3 shows an AFM image of the surface of a 10 nm thick

film of a,v-DH4T deposited under UHV at room temperature

on silica. From the image contrast and the cross-sectional

profile reported in Fig. 3, different layers can be distinguished.

The zero level in the cross-sectional profile represents the bare

substrate surface, and shows up with the darkest contrast in

the AFM image. The same profile reveals terraces about

11.6 nm high, a value that is close to the average film thickness

(10 nm), 14.6 nm and 17.5 nm. The height difference between

different terraces is within experimental error equal to one or

multiples of the c 6 sin b value of the single crystal structure.

A preliminary study carried out by electron microscopy for

a,v-DH4T thin films was reported in reference 24. A more

thorough investigation of these thin films reveals two

characteristic selected area electron diffraction patterns, as

shown in Fig. 4a and c. The sharpness and organisation of the

diffraction spots indicates that we are dealing with well-

ordered crystalline domains.

A simulated pattern with the zone axis [201] parallel to the

electron beam is shown in Fig. 4b. The calculated positions of

the diffraction spots are in excellent agreement with the

experiment (Fig. 4a). It is worth noting that some weak

diffraction spots appear in the experimental patterns whereas

they are absent in the simulated ones. This is frequently

observed in electron diffraction, and is usually associated with

multiple scattering, which is more likely for thicker specimens.

The contact plane associated with a [201] zone axis is (1 0 12).

The (1 0 12) plane is not a natural cleavage or growth plane for

a,v-DH4T single crystals, since it intersects the molecules. This

analysis therefore indicates that we are not dealing with a

single crystalline domain, but rather with some more complex

crystal. The analysis becomes straightforward when noting

that the (1 0 12) plane is normal to the long axis of the DH4T

molecule. This suggests that the crystals are actually multiple

twins with a (24 0 11) twin plane. This plane is indeed a

‘‘natural’’ twin plane. In addition, it contains the b axis. As a

result of the twinning, two c axis orientations relative to the

substrate are generated at + and 223.3u. Note that this

twinning does not therefore affect the diffraction pattern as

seen along the [201] zone axis. As a result of twinning, the rigid

core of the a,v-DH4T molecules stands up normal to the

substrate. In the composite twin, the (001) ‘‘natural’’ cleavage

plane is tilted by 21u from the substrate plane alternately

clockwise and anticlockwise. A side view along b of the

orientation of the molecules for this corrugated contact plane

is represented in Fig. 5. The micro-twinning with a twinning

plane parallel to the long axis of the rigid core of the molecules

gives rise to an apparent macroscopic contact plane different

from a natural cleavage plane. A similar example has been

reported previously.25 In that case another highly conjugate

rigid molecule, sexiphenyl, was deposited on GaAs at high

temperature (150 uC).

Fig. 4c reports another type of diffraction pattern occasion-

ally obtained on the same sample. In this case, we are dealing

with a ‘‘real’’ (untwinned) single crystal. The zone axis is now

[105]. The corresponding simulated diffraction with the zone

Fig. 3 10 6 10 mm2 AFM image of the surface of a 10 nm thick film

of a,v-DH4T deposited on silica at room temperature, and cross-

sectional profile along the white line.

Fig. 4 (a and c) Experimental electron diffraction patterns for two

different islands of a a,v-DH4T thin film deposited on silica. (b and d)

Calculated electron diffraction patterns with zone axes [201] and [105]

parallel to the electron beam, respectively. The calculation is based on

our crystalline structure for a,v-DH4T grown from solution.

Fig. 5 Structure of the twinned domains of a,v-DH4T grown on

silica as viewed along the b axis of the unit cell.

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axis [105] parallel to the electron beam is reported in Fig. 4d.

The contact plane corresponding to the zone [105] is the

‘‘natural’’ and expected (001) plane. The molecular axis is at an

angle of 23.3u to the substrate normal. A side view along b of

the orientation of the molecules relative to the substrate is

represented in Fig. 2. As a consequence also the steps in these

single crystalline domains should be 28.5 A high. Contrary to

the previously described case the surface of the contact plane is

molecularly flat and smooth.

In the light of the above results, the UHV deposition of

a,v-DH4T on inert substrates gives rise to crystalline domains

with the same crystalline structure as determined for solution

grown single crystals. The a,v-DH4T thin films are composed

of island-like domains. These islands correspond to two

orientations of the molecules relative to the substrate surface.

In one type of domain a multiple twinning parallel to (24 0 11)

generates a corrugated contact plane. The molecules are

standing up normal to the substrate surface. Another type of

domain is made of single crystals with a (001) contact plane

and the molecules tilted by 23.3u to the substrate surface. The

growth of the different domains is probably kinetically

controlled and depends on the substrate temperature and the

rate of evaporation. Further experiments are needed to

understand the origin of this behaviour.

Conclusion

We have determined the crystal structure of a,v-DH4T and

the morphology and structure of thin films vapour-deposited

on silica. The main feature of the crystal structure is the

herringbone packing of the molecules, which is similar to that

of other archetypal oligothiophenes. It gives rise to a mono-

molecular layered structure, i.e. to a 2D system in which

carrier mobility is characterized by negligible interchain

transfer integrals between molecules lying in adjacent layers.5

The present results are useful to correctly interpret experi-

mental data such as partial structural data obtained from

diffraction and optical spectroscopy which strongly depends

on the transition dipole orientation.26 Hence, some previous

attempts to figure out molecular conformation and orientation

for alkyloligothiophenes in crystals and mesophases have to be

re-examined in the light of the above results.2a,8,17 Considering

the structural similarities between alkyloligothiophenes and

n-alkanes,27 further work will be devoted to crystal growth

mechanisms in relation with polymorphism and twinning. As a

matter of fact, potential applications in opto-electronics of

efficient thin films require a strict control of the molecular

organisation throughout the film thickness. It is known from

the literature that devices exhibit better performances when

a,v-DH4T is used as active layer instead of 4T. Since these two

materials display the same packing of the conjugate cores, the

enhanced carrier mobility of thin film devices of a,v-DH4T is

principally attributable to a different mechanism of growth,

leading to the formation of uniform and well-connected

crystalline domains. Therefore, mastering the growth of films

based on promising active polymorphs with uniform orienta-

tion and entirely uniform thickness at the molecular level is

mandatory to reach valuable macroscopic characteristics for

organic films.

Experimental

a,v-DH4T crystals were grown from an anisole solution at

room temperature with a novel technique (described in a

forthcoming paper28) that did succeed in producing single

crystals suitable for a complete X-ray diffraction study. A

Bruker SMART CCD area-detector diffractometer was used

to collect data using Mo-Ka radiation (l 5 0.71073 A) with

the v-scan method, within the limits h ¡ 24u. Data were

corrected for Lorentz, polarisation and absorption effects

using the SADABS program. The structure was solved by

direct methods (SIR9729) and refined by full-matrix least-

squares on Fo2 (SHELX-9730). Anisotropic displacement para-

meters were assigned to all non-hydrogen atoms. Hydrogen

atoms were located by difference Fourier maps and sub-

sequently introduced in the refinement as isotropic scatterers

in idealized positions. No solvent accessible voids were

revealed by PLATON.31 ORTEP and packing figures were

generated with PLATON and SCHAKAL,32 respectively. A

summary of crystallographic data and refinement parameters

are listed in Table 1.

a,v-DH4T thin films were grown by means of OMBD,

starting from a,v-DH4T microcrystalline powder33 in a

Knudsen effusion cell at 190 uC, keeping the substrate

temperature at 25 uC. The pressure in the growth chamber

was kept below 8 6 1028 Pa. The film nominal thickness was

monitored by means of a quartz oscillator and was set to

10 nm. The substrate adopted was silica 1 mm thick and

1 6 1 cm2 in size, with a surface roughness of about 0.4 nm, as

measured by AFM over a 10 6 10 mm2 area. Before the

introduction in the growth chamber the substrates were

cleaned using various solvents and blown with dry nitrogen.

AFM images were obtained with a Nanoscope IIIa (Digital

Instruments) in Tapping2 mode using silicon cantilevers. For

electron microscopy observation the a,v-DH4T films were

backed with a thin carbon film. The a,v-DH4T/carbon system

Table 1 Summary of crystal data and refinement parameters fora,v-DH4T

Formula C28H34S4

Formula weight/g mol21 656.16Crystal system monoclinicSpace group P21/a (No. 14)a/A 6.049(5)b/A 7.814(7)c/A 28.532(25)b/u 92.39(2)V/A3 1347(2)Z 2T/K 298Dc/g cm23 1.229m/mm21 0.367Crystal size/mm3 0.25 6 0.22 6 0.18Minimum relative transmission 0.64h Range/u 2.7–24.1Crystal decay (%) 0Index ranges, h k l 26/6 28/8 232/32Reflections collected 7882Unique reflections, R(int) 2089, 0.156Data/parameters 2089/145Goodness-of-fit on Fo

2 0.905R1, wR2 [Fo . 4s(Fo)] 0.1230, 0.3082R1, wR2 [all data] 0.2013, 0.3740Max. diff. peak/hole/e A23 0.709/20.604

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was floated on a dilute HF solution and transferred on copper

grids. A Philips CM12 electron microscope working at a

voltage of 120 kV was used. Diffraction patterns were obtained

at a spot size of 50 nm, using a condenser aperture of 50 mm

and a selected-area aperture of 30 mm. Indexation of the

experimental diffraction patterns was carried out using the

program Cerius2 (Accelrys) on the basis of the crystal

structure here reported.

Acknowledgements

The authors kindly acknowledge Dr. F. Garnier and

Prof. A. Papagni for providing the a,v-DH4T molecules.

M. C. acknowledges the EEC contract Eurofet Nu RTN2-

2001-00382 for partial financial support, and Prof. P. Fantucci

for the availability of the program Cerius21. M. M. thanks

Dipartimento di Chimica Strutturale e Stereochimica

Inorganica (Universita degli Studi di Milano) for availability

of the X-ray diffractometer.

Massimo Moret,*a Marcello Campione,*a Alessandro Borghesi,a

Luciano Miozzo,a Adele Sassella,a Silvia Trabattoni,a Bernard Lotzb andAnnette Thierryb

aDepartment of Materials Science, Universita degli Studi di MilanoBicocca, Via R. Cozzi 53, I-20125 Milano, Italy.E-mail: massimo.moret@mater.unimib.it;marcello.campione@mater.unimib.itbInstitut Charles Sadron – CNRS, 6 rue Boussingault, F-67083,Strasbourg Cedex, France

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