Dr Francesca Terenziani

Céline Le Droumaguet

Dr Claudine Katan

Dr Olivier Mongin

Dr Mireille Blanchard-Desce
email: mireille.blanchard-desce@univ-rennes1.fr

Effect of Branching on Two-Photon Absorption: the Case of Triphenylbenzene Derivatives

Keywords:

The two-photon absorption (TPA) cross sections of octupolar branched compounds and corresponding dipolar branches are measured by varying the nature of the end-groups (donor versus acceptors) and the length of the branches. A correlation is found between the fluorescence solvatochromic behavior and the TPA cross section, ascribed to the intramolecular charge-transfer character of the transitions.

Introduction

Molecular two-photon absorption (TPA) has attracted a lot of interest over recent years both from theoretical and practical points of view. Various applications include spectroscopy, [1] three-dimensional optical data storage, [2] microfabrication, [3] laser upconversion, [4] high-resolution 3-dimensional imaging of biological systems, [5] and photodynamic therapy. [6] Using a two-photon excitation process (i.e. a nonlinear process involving the simultaneous absorption of two photons) instead of a conventional one-photon excitation offers a number of advantages. These include the ability for a highly confined excitation (and intrinsic three-dimensional resolution) and increased penetration depth by replacing typical one-photon excitation in the UV-visible blue region by two-photonexcitation in the visible red-near IR region, owing in particular to the reduction of scattering losses. Most of these applications require the design of compounds displaying high TPA cross-sections in order to significantly decrease the excitation intensity and provide improved excitation selectivity. This goal has stimulated the study of a variety of chromophores ranging from dipolar [7] to quadrupolar, [START_REF] Verbouwe | [END_REF][9][10] octupolar, [START_REF] Verbouwe | [END_REF][10][11][12][13][14][15][16][17][18][19] multipolar and branched structures including dendrimers. [15,18,20] A number of factors influence the TPA magnitude, among which electronic delocalization and intramolecular charge transfer (ICT) phenomena play an important role. [14] This is a critical issue in the case of branched systems where interbranch coupling plays a major role. Several structural parameters are expected to influence this coupling such as the nature of the branches, of the core, node and peripheral moieties. Given the huge synthetic effort needed to build such structures, detailed understanding of the effect of branching of molecular entities on the linear and nonlinear optical properties is of major interest. Among the many questions that are currently debated, coherent coupling between the arms (building blocks) is of particular interest. This has prompted several recent theoretical and experimental studies using different techniques. [18,19,21] Depending on the nature of this coupling, the ground or the excited state may be either localized or delocalized and specific optical properties -including TPA -may be either enhanced or suppressed. [18] In recent work, we have shown that branched systems based on the assembly of dipolar chromophores via a common triphenylamine core give rise to a sizeable electronic coupling between branches. [10] The interaction between branched chromophores is responsible for splitting between degenerate excited states, so that the amount of coupling can be extracted within the Frenkel exciton scheme. A concerted multidimensional ICT takes place from the central electrondonating moiety to the electron-withdrawing periphery upon excitation and the inter-branch electronic coupling gives rise to major TPA enhancement. A breakage of the coherent coupling between the arms is observed after excitation which then localizes on one of the branches, prior to emission, leading to highly dipolar emitting excited states. [10,[START_REF] Katan | Proc. SPIE-Int. Soc. Opt. Eng[END_REF], [17] The aim of this paper is to investigate how the photophysical and TPA properties can be modified by playing on the nature of the coupling core moiety. In that perspective we have studied a series of branched systems where dipolar units are connected via a common triphenylbenzene core (Figure 1). Such three-branched systems, bearing either strong electron-donating or electron-withdrawing groups were previously shown to allow either periphery-to-core or core-to-periphery multidimensional charge transfer leading to large first hyperpolarizabilities (i.e. second-order optical response). [16] Earlier structure-property studies carried out on different series of octupolar molecules showed that such ICT phenomena can lead to large TPA cross-sections. [12,13,[START_REF] Yang | [END_REF] These features prompt us to investigate the TPA properties of a series of octupolar derivatives built from a triphenylbenzene core in correlation with the ICT process. The ability of the triphenylbenzene core to promote electronic coupling between branches and possibly lead to enhanced TPA is investigated by comparing the versus TPB[(PV) 1 NHex 2 )] 3 ) on TPA responses is investigated. Moreover, the solvatochromic behavior of these systems is addressed in order to characterize the nature of the emitting excited state and assess the ICT.

Results and Discussion

Molecular geometry

Octupolar three-branched molecules derived from the TPB core adopt a propellershape conformation (Figure 2) where the three phenyl substituents on the central phenyl unit are twisted by about 45°. Interestingly, the ground-state optimized geometry of BP(PV) 1 NHex 2 can be superposed to that of one branch of TPB[(PV) 1 NHex 2 ] 3 showing identical geometrical parameters except for a small dissymmetry at the terminal phenyl ring of BP (Figure 3A). This suggests that the three branches behave as nearly independent subchromophores. The nitrogen atom is slightly out of plane and the twist angle between the two phenyl rings of BP is about 45° while that between the two phenyl rings on each side of the double bond is closer to 40°. BP(PV) 1 SO 2 CF 3 shows similar geometrical parameters with respectively slightly reduced and increased twist angles, and the terminal CF 3 almost perpendicular to the molecular backbone (Figure 3B). The bond-length alternation (BLA) parameter, defined as the difference between single and double bonds in the vinyl bridge, is 0.15 Å for all chromophores in the ground state. This parameter reflects the low degree of charge transfer in the ground state. This is confirmed by the large value of the 1 H-1 H coupling constant (≈ 16.5 Hz) in the vinylic bond of compounds BP(PV) 1 NHex 2 and BP(PV) 1 SO 2 CF 3 in solution, indicative of a full double bond character.

Excited-state geometry optimization of BP(PV) 1 NHex 2 leads to an almost planar molecule: carbons bonded to nitrogen and nitrogen itself are in the same plane, as well as adjacent phenyl and phenylene-vinylene moieties. BP shows a reduced twist angle of about 25° (Figure 3A). A similar planarization of the excited state is observed for BP(PV) 1 SO 2 CF 3 (Figure 3B). This change of geometry is associated to a vanishing BLA, which indicates that the electronic delocalization significantly increases in the emitting excited state.

Spectroscopic investigations

Photophysical properties. The absorption and fluorescence properties of dipolar and octupolar derivatives are collected in Table 1. All compounds show an intense absorption band in the near-UV region and emission in the visible region. A net bathochromic and hyperchromic shift of the absorption band is observed both for dipolar and octupolar chromophores bearing electron-donating end-groups as compared to their counterparts bearing electron-withdrawing end-groups (Figure 4). The donor derivatives also show significantly red-shifted emission and much larger fluorescence quantum yields (Table 1). At opposite, the acceptor derivatives display larger Stokes-shifts, indicative of a more pronounced nuclear reorganization after absorption prior to emission.

As shown in Figure 5, the absorption spectra of the octupolar derivatives are slightly red-shifted as compared to those of their dipolar analogues. The slight red-shift (about 0.04-0.05 eV in toluene) indicates that only weak coupling between the branches occurs in triphenylbenzene derivatives. This suggests that the branches behave as nearly independent subchromophores in the ground state, as suggested by the calculated ground state geometries.

Such conclusion was also derived from recent Hyper-Raman studies conducted on other octupolar derivatives built from a TPB core. [24] The quasi-proportionality of the molar extinction coefficient with respect to the number of branches further corroborates this analysis (Table 1). It should be stressed however that the coupling (as measured by the red-shift of the absorption band) increases with increasing solvent polarity (Figure 5). This indicates that the coupling between the dipolar branches slightly increases in polar environments, as due to increased polarity and polarization of the dipolar branches in response to the solvation interaction.

In low-polarity solvents (where the coupling between branches is vanishing), we observe that branching causes a noticeable enhancement of the fluorescent quantum yield, in correlation with a definite increase of the radiative decay rate (Table 1). Indeed the radiative lifetime of the three-branched octupoles are significantly shorter than that of the corresponding dipolar branches. Such behavior contrasts with what was observed for octupolar derivatives with similar branches based on the triphenylamine core. [10] This behavior suggests that a certain degree of delocalization is occurring in the relaxed excited state of the three-branched systems having a TPB core, leading to larger transition dipole moments and thus shorter radiative lifetimes than analogous dipolar derivatives with a BP moiety. [25] Indeed calculations conducted on the dipolar analogue bearing a TPB terminal moiety instead of a BP moiety (TPB[(PV) 1 NHex 2 ] 3 ) predict larger transition dipole moment (Table 2). This suggests that the TPB core is involved in the delocalization process in the case of three-branched derivatives, explaining the difference in radiative lifetimes.

In the case of three-branched derivatives bearing donor end-groups, the radiative decay rate can be further enhanced by increasing the length of the branches (Table 1). This increase cannot be accounted for only on the basis of the increase in molar absorption coefficient (i.e. oscillator strength), [25] and indicates that the transition dipole of the emitting state is significantly enhanced as compared to that of the ground state. This suggests that the planarization process and reduction of BLA in the conjugated branch is also operative for the elongated three-branched system and extends over the full length of the conjugated branch, leading to an increase of the transition dipole in the emitting state. Indeed, such an increase of transition dipole moments is also predicted by calculations for the dipolar derivatives (see Table 2).

Interestingly, in the case of donor derivatives, a decrease of the non-radiative decay rate is also observed for three-branched derivatives. The combination of these effects leads to a significant increase in the fluorescence quantum yield, the longest three-branched molecule having by far the largest fluorescence quantum yield in the series. The lengthening of the branches also produces a significant bathochromic shift of both the absorption and emission bands (Table 1).

Solvatochromism. Both three-branched octupolar and dipolar chromophores show a definite positive solvatochromic behavior. As illustrated in Figure 6, increasing solvent polarity induces a slight red-shift of the absorption band and a marked bathochromic shift of the emission band. In the case of dipolar systems, such behavior is typical of an ICT transition with an increase of dipole moment upon excitation. Indeed, transition orbitals calculations (Figure 3) confirm that a definite charge transfer from the biphenyl terminal group to the acceptor end-group occurs in BP(PV) 1 SO 2 CF 3 upon excitation, whereas a charge transfer from the donor end-group to the biphenyl terminal group occurs in BP(PV) 1 NHex 2 . This is correlated with the reduced BLA obtained in the excited state as compared to the ground state.

In the emitting excited state, the conjugated system becomes almost planar and the BLA vanishes. Such phenomenon has already been observed in other conjugated systems such as conjugated polymers. [26] As a consequence of this charge transfer phenomena, both molecules have a higher dipole moment in the excited state (Table 2). As shown in Figure 7, the solvatochromic behavior of both dipolar chromophores can be fitted with a Lippert-Mataga relationship: [27] const hca f eff em abs

+ ∆ ∆ = - ) /( 2 ~3 2 µ ν ν (1)
where abs ν ~ ( em ν ~) is the wavenumber of the absorption (fluorescence) maximum, h is the Planck constant, c is the light velocity, a is the radius of the solute spherical cavity, and ∆f = (ε -1)/(2ε + 1) -(n 2 -1)/(2n 2 + 1), where ε is the dielectric constant and n is the refractive index of the solvent. In the simple case where the Franck-Condon and relaxed excited state dipoles are identical (as well as relaxed and Franck-Condon ground state dipoles),

eff µ ∆ simply corresponds to ( ) G E µ µ -
, µ G and µ E being the ground-state and excited-state dipole moments. More generally,

( ) (

)

G E G f G f E f E eff µ µ µ µ µ µ µ - - - = ∆ ) ( ) ( ) ( 2 (3)
where the (f) superscript indicates the equilibrium excited-state geometry (relevant to fluorescence). In the present study, calculations show that µ E and µ G are pretty sensitive to geometrical changes occurring between absorption and emission processes (Table 2), so that the exact expression in ( 3) is for sure safer to be used.

We observe that the solvatochromic behavior of octupolar chromophores can also be fitted with a Lippert-Mataga relationship (Figure 7). This is a strong evidence that the emission process occurs from a dipolar excited state. The slope values are smaller for threebranched octupoles as compared to their dipolar counterparts (Figure 7 and Table 1). The dipolar nature of the emitting excited state clearly indicates that excitation localizes, prior to emission, on one of the dipolar branches, as already observed for triphenylamine-based multibranched structures. [10,[START_REF] Katan | Proc. SPIE-Int. Soc. Opt. Eng[END_REF] Nevertheless, the smaller value of the Lippert-Mataga slope with respect to dipolar analogues suggests that this phenomenon of localization is not complete, as also previously guessed from lifetime values. suggest that both dipolar and octupolar derivatives bearing donor end-groups may lead to larger TPA cross-section. [13] The longest three-branched derivative with donor end-groups show an even more pronounced solvatochromism (Figure 7) although the cavity radius is significantly increased.

Indeed this suggests that the ICT is still operative along the total length of the branch in TPB[(PV) 2 NHex 2 ] 3 , leading to a very large relaxed excited-state dipole. The enhancement of eff µ ∆ , in addition to the hyperchromic and bathochromic shift induced by the increase of the length of the branch, is expected to lead to significant enhancement of TPA.

Two-photon absorption. Wavelength-dependent TPA cross sections are shown in Figures 8 and9 (normalized per branch) and main characteristics are summarized in Table 3. Figure 8 clearly shows that the TPA response is nearly additive with respect to the number of branches:

indeed TPA cross sections normalized per branch are very similar for octupoles and corresponding dipoles. This is a further confirmation of the almost independency of each branch in octupolar compounds based on the triphenylbenzene central core, corresponding to weak (almost vanishing) electronic coupling. This result is markedly different from that obtained for structures based on a triphenylamine central core, where a strong non-additivity was found for quadrupolar and octupolar branched structures with respect to the dipolar branch. [10,[START_REF] Katan | Proc. SPIE-Int. Soc. Opt. Eng[END_REF] So, while triphenylamine is able to promote sizeable electronic coupling between branches, triphenylbenzene is not. While in triphenylamine-based branched structures the coupling leads to mixing of single-branch excited states, in triphenylbenzene-based structures each branch is almost unperturbed by other branches, so that properties are nearly additive.

The different behavior of the two series (triphenylamine-based and triphenylbenzene-based)

gives important hints on the nature of the coupling between branches. Indeed, if coupling was only due to electrostatic interactions between the transition dipole moments of the branches (Frenkel-exciton model), [START_REF] Davidov | Theory of molecular excitons[END_REF] the strength of the coupling should be comparable in the two series (since transition dipole moments are comparable). But this is not the case, indicating that other sources of coupling are needed to explain the behavior of the triphenylamine-based series. Indeed, the need of interactions beyond the Frenkel-exciton model was already suggested in Ref. [10] in order to explain the strong enhancement of TPA cross sections as a consequence of branching. As a matter of fact, the simple excitonic model was not able to reproduce this enhancement, while more refined quantum chemical calculations did. This observation suggested that not only dipolar interactions but also coherent interactions were important in defining the total coupling and hence the spectroscopic properties of branched structures. [10] While triphenylamine has been proved to be able to promote this kind of coupling, results presented here allow to conclude that the triphenylbenzene core is not.

The end-group effect on the TPA activity deserves some comments. Compounds with donor end-groups display higher TPA cross sections than corresponding structures with acceptor end-groups (Table 3). Even if (for solubility reasons) measurements have been carried out in different solvents for donors and acceptors (toluene for donors and DMSO for acceptors), the observed increase for donor-based compound is reliable. In fact, the effect of changing solvent polarity can be evaluated by means of the linear absorption spectra: only a very weak shift is observed when going from toluene to DMSO, accompanied by a slight broadening of the band and slight increase of the molar extinction coefficient on the maximum (of the order of 10%). This means that the oscillator strength slightly increases with increasing solvent polarity, due to the increase of the transition dipole moment (the molecule is polarizable so that its properties depend on environmental parameters). Despite the polar solvent DMSO favors an increased transition dipole with respect to toluene, compounds with acceptor end-groups (dissolved in DMSO) have lower TPA cross sections (σ 2 ) than corresponding compounds with donor end-groups (dissolved in toluene). This means that this behavior is not linked to the particular choice of solvents. This trend can be actually confirmed by the σ 2 values obtained through a two-state model, starting from calculated dipole moments (Table 2). In fact, for dipolar molecules the TPA cross section can easily be expressed by the aid of a two-state model:

( ) Γ - = 2 2 2 002076 . 0 G E eg µ µ µ σ (4)
where σ 2 is obtained in GM (10 -50 cm 4 .s.photon -1 ) when transition and state dipole moments are used in D and the half-bandwidth at half maximum Γ in eV. Thus, σ 2 is proportional to the product of the squared transition dipole moment and the squared difference between excitedstate and ground-state dipole moments (

)

G E µ µ -
. Data in Table 1 suggest that the increased σ 2 of BP(PV) 1 NHex 2 with respect to BP(PV) 1 SO 2 CF 3 is due only for one half to the increased transition dipole moment, while the further increase must be linked to the increased (

)

G E µ µ -
value. This is confirmed by the more pronounced solvatochromic behavior of BP(PV) 1 NHex 2 , as described by the slope values in Table 1. Indeed, while the solvatochromic slope is proportional to 2 eff µ ∆ and the TPA cross section to (

)

2 G E µ µ -
, it is reasonable to suppose that these two quantities behave very similarly.

For octupolar structures the behavior is similar, the compound having donor endgroups displaying higher TPA cross section than the analogous compound with acceptor endgroups. This is naturally explained on the basis of results for corresponding dipoles, since all data demonstrate that branches in octupolar structures behave as nearly independent subchromophores.

The longest compound, TPB[(PV) 2 NHex 2 ] 3 , has by far the highest TPA cross section among all the studied structures (doubled with respect to the shorter analogue). This is in agreement with predictions based on photophysical data. In fact, not only the transition dipole moment is increased with respect to the shorter analogue, but also the ICT from the donor end-groups to the central core upon excitation is more pronounced, as demonstrated by the much stronger solvatochromic behavior. Indeed, while the increase of oscillator strength can only account for ~ 20% (see Table 1) of the total enhancement of the TPA response, the increased ICT is responsible for most of the observed enhancement. Actually, for chromophores derived from the TPB core, a correlation between the solvatochromic magnitude and the peak TPA response of lowest energy is observed (Figure 10). This provides an interesting way to evaluate the TPA response of other derivatives built from the TPB core.

Conclusions

In this paper we presented a study of the photophysical and TPA properties of octupolar three-branched systems based on the triphenylbenzene core and corresponding dipolar systems with a biphenyl motif (branches). Compounds bearing donor and acceptor end-groups were synthesized and investigated. The lowest-energy electronic transition corresponds in all cases to a strong ICT from the triphenylbenzene or biphenyl motif towards the acceptor end-groups or vice versa in the case of donor end-groups. This charge transfer character is more pronounced in the case of donor end-groups, as revealed by the stronger solvatochromic behavior of fluorescence spectra with respect to corresponding compounds with acceptor end-groups.

All spectral properties indicate that the three branches in octupolar compounds behave nearly independently: only small deviations from the additive behavior are observed in fluorescence, absorption and TPA spectra when comparing the octupolar systems with the corresponding dipolar branches. A clear indication of localization of the excitation on one branch in octupolar compounds prior to emission is given by the strong fluorescence solvatochromism, only slightly reduced with respect to the corresponding dipoles. This slight effect is probably due to a certain degree of delocalization occurring in the relaxed excited state of the three-branched systems, as also suggested by excited-state lifetimes.

The comparison of results presented here with results on analogous compounds based on a triphenylamine central core [10,[START_REF] Katan | Proc. SPIE-Int. Soc. Opt. Eng[END_REF] allows to identify the nature of the coupling between branches in these and other octupolar branched systems. In fact, at difference with respect to the present compounds, analogous triphenylamine-based systems were characterized by strongly non-additive properties. This different behavior is observed despite the comparable values of transition dipole moments, and hence comparable values of the electronic coupling due to electrostatic interactions (Frenkel-exciton model). The comparison allows then to conclude that, while in branched systems based on the triphenylbenzene core only electrostatic interactions contribute to the electronic coupling between branches, in the case of the triphenylamine core other sources of coupling are present, such as coherent interactions.

We also show that a correlation exists between the solvatochromic behavior and the TPA cross section of these compounds, as due to the ICT character of the transitions. This is true not only inside the investigated series, but also with respect to analogous systems based on the triphenylamine central core. [17,[START_REF] Katan | Proc. SPIE-Int. Soc. Opt. Eng[END_REF] In fact, triphenylamine-based compounds having the same branches as presented here show a more pronounced solvatochromic behavior and corresponding higher TPA cross sections. [17,[START_REF] Katan | Proc. SPIE-Int. Soc. Opt. Eng[END_REF] Hence results presented here, together with results on analogous series allow to conclude that the study of the solvatochromic behavior gives an estimation of the relative TPA cross section for systems built from phenylenevinylene branches and dominated by the ICT phenomenon upon excitation.

Experimental Section

Chemicals

Octupolar compounds TPB[(PV) 1 NHex 2 ] 3 and TPB[(PV) 2 NHex 2 ] 3 were prepared according to litterature. [16] Their dipolar analogue BP(PV) 1 NHex 2 was obtained as a pure E compound from the Wittig condensation of biphenyl-4-carboxaldehyde (1) with phosphonium salt 2, [START_REF] Porrès | [END_REF] and subsequent isomerization of the obtained mixture of stereoisomers using a catalytic amount of iodine under illumination (Scheme 1). A similar methodology did not afford BP(PV) 1 SO 2 CF 3 as a pure E isomer.

Its synthesis was therefore achieved by means of the Heck coupling of 4-ethenyl-1,1'-biphenyl [30] (3) with bromo derivative 4. Octupolar compound TPB[(PV) 1 SO 2 CF 3 ] 3 was prepared analogously via the threefold Heck coupling of trivinyl 6 (obtained from the condensation of trialdehyde 5 [16] and methyltriphenylphosphonium iodide) with three equivalents of 4 (Scheme 1).

4-[(1E)-2-[1,1'-Biphenyl]-4-ylethenyl]-N,N-dihexylbenzenamine (BP(PV) 1 NHex 2 ).

To a solution of biphenyl-4-carboxaldehyde (1) (0.500 g, 2.74 mmol) and phosphonium salt 2 [START_REF] Porrès | [END_REF] (2.37 g, 3.57 mmol) in anhydrous CH 2 Cl 2 (25 mL) t-BuOK (0.553 g, 4.93 mmol) was added. The mixture was stirred at 20 °C for 24 h. After addition of water, extraction with CH 2 Cl 2 , and drying (Na 2 SO 4 ), the solvent was evaporated. The residue was purified by filtration through a short pad of silica gel (heptane/CH 2 Cl 2 50:50), to afford a mixture of isomers, which was dissolved in Et 2 O (18 mL). A catalytic amount of I 2 (0.4%) was then added and the solution was stirred at 20 °C for 8 h under light exposure (60 W lamp).

The organic layer was washed with aqueous Na 2 S 2 O 3 and dried (Na 2 SO 4 ). After evaporation of the solvent, the raw product was purified by column chromatography (heptane/CH 2 Cl 2 85:15) to yield 0.990 g (82%) of BP(PV) 1 NHex 2 ; 1 H NMR (200.13 MHz, CDCl 3 ) δ 7. 64-7.58 (m, 2H), 7.55 (m, 4H), 7.46-7.30 (m, 3H), 7.39 and 6.62 (AA'XX', J AX = 8.7 Hz, 4H), 7.08 (d, J = 16.4 Hz, 1H), 6.90 (d, J = 16.4 Hz, 1H), 3.28 (t, J = 7.5 Hz, 4H), 1.59 (m, 4H), 1.32 (m, 12H), 0.91 (m, 6H); 13 C NMR (75.47 MHz, CDCl 3 ) δ 147. 8, 140.8, 139.0, 137.4, 129.0, 128.7, 127.8, 127.1, 127.0, 126.7, 126.3, 124.4, 123.0, 111.6, 51.0, 31.7, 27.2, 26.8, 22.7, 14.

4-[(1E)-2-[4-[(Trifluoromethyl)sulfonyl]phenyl]ethenyl]-1,1'-biphenyl (BP(PV) 1 SO 2 CF 3 ).

Air was removed from a solution of 4-ethenyl-1,1'-biphenyl [30] (3) (200 mg, 1.109 mmol) and 1-bromo-4-[(trifluoromethyl)sulfonyl]benzene (4) (353 mg, 1.22 mmol) in anhydrous DMF (5 mL) by blowing argon for 20 min. Then triethylamine (146 mg, 1.44 mmol), tri-o-tolylphosphine (34 mg, 0.111 mmol) and palladium diacetate (14.5 mg, 0.055 mmol) were added. The mixture was stirred at 110°C for 15 h. The solvent was removed by distillation, and the raw product was purified by column chromatography (heptane/CH 2 Cl 2 30:70) to yield 311 mg (72%) of BP(PV) 1 SO 2 CF 3 as a pale yellow solid; 1 H NMR (200.13 MHz, CDCl 3 ) δ 8.01 and 7.76 (AA'XX', J AX = 8.5 Hz, 4H), 7.64-7.61 (m, 6H), 7.46 (m, 1H), 7.43 (d, J = 7.5 Hz, 2H), 7.37 (d, J = 16.3 Hz, 1H), 7.18 (d, J= 16.3 Hz, 1H); 13 C NMR (75.47 MHz, CDCl 3 ) δ 145. 8, 141.9, 140.3, 134.9, 134.1, 131.2, 128.9, 127.7, 127.7, 127.6, 127.5, 127.4, 127.0, 125.9, 119.9 (q, J = 324.1 Hz); 19

1,3,5-Tris(4-ethenylphenyl)benzene (6).

To a stirred solution of 1,3,5-tris(4formylphenyl)benzene [16] (220 mg, 0.563 mmol) in anhydrous THF (7.5 mL), methyltriphenylphosphonium iodide (1.37 g, 3.38 mmol) and sodium hydride (135 mg, 5.63 mmol)

were added. The solution was stirred for 48 h, filtered on Celite and the solvent was evaporated. The raw product was purified by column chromatography (CH 2 Cl 2 ) to yield 210 mg (97%) of 6 as a yellow solid; 1 H NMR (200.13 MHz, CDCl 3 ) δ 7.79 (s, 3H), 7.68 and 7.53 (AA'XX', J AX = 8.3, 12H), 6.81 (dd, J = 17.6, 10.9, 3H), 5.83 (d, J = 17.6, 3H), 5.31 (d, J = 10.9, 3H); 13 C NMR (75.47 MHz, CDCl 3 ) δ 141. 9, 140.4, 136.9, 136.4, 127.4, 126.7, 124.8, 114

Optical experiments

UV-vis. UV-vis spectra were recorded on a Jasco V-570 spectrophotometer using spectroscopic grade solvents.

Fluorescence. Steady-state and time resolved fluorescence measurements were performed at room temperature in dilute solutions (ca. 10 -6 M) using an Edinburgh Instruments (FLS 920) spectrometer in photon-counting mode. Emission spectra were obtained, for each compound, at λ ex = λ max (abs) with A λex ≤ 0.1 to minimize internal absorption. Fluorescence quantum yields were measured on degassed samples at room temperature; fluorescein in 0.1 N NaOH was used as a standard (quantum yield Φ = 0.90). [31] The lifetime values were obtained from the reconvolution fit analysis of the decay profiles with the F900 analysis software and the fitting results were judged by the reduced chi-square value. The chi-square analysis indicates that τ values longer than ~ 0.3 ns are known with a good confidence (the uncertainty decreasing when τ increases, and always less than ~ 10%), while very short lifetimes (shorter than the excitation pulse duration) are less reliable. For this reason we mainly concentrate our discussion based on lifetimes on the donor-series, which has well reliable τ values.

Two-photon excitation cross-sections of dipolar and octupolar chromophores were determined by investigating their two-photon-excited fluorescence (TPEF) in solution. These measurements provide the TPEF action cross-section σ 2 Φ. The corresponding σ 2 (TPA cross section) values were then obtained by using the fluorescence quantum yield Φ values derived from standard fluorescence measurements. We emphasize that experiments were conducted in the femtosecond regime thus preventing contribution from excited absorption that is known to lead to artificially enhanced "effective" TPA cross-sections when measurements (in particular nonlinear transmission) are conducted in the nanosecond regime.

TPEF measurements were conducted using a mode-locked Ti:sapphire laser operating between 700 and 990 nm and delivering 150 fs pulses at 76 MHz, following the experimental protocol described in detail by Xu and Webb. [32] The quadratic dependence of the fluorescence intensity on the excitation intensity was verified for each data point, indicating that the measurements were carried out in intensity regimes in which saturation or photodegradation do not occur. TPEF measurements were calibrated relative to the absolute TPEF action cross-section determined by Xu and Webb for fluorescein in water (pH = 11) in the 690-1050 nm range. [32,33] Equations for relative determination of σ 2 were adopted as reported in Ref. [34] For solubility reasons TPEF experiments were conducted in toluene for dipolar and octupolar derivatives with NHex 2 end-groups and in DMSO for dipolar and octupolar derivatives bearing SO 2 CF 3 end-groups. These two solvents have very similar refractive index, so that any possible problem linked to refractive index correction for the evaluation of TPA cross section is avoided.

Theoretical modeling

For the sake of simplicity, hexyl chains of the donor derivatives have been replaced by methyl groups and solvation effects have been neglected. Ground-state optimized geometries have been obtained using the Gaussian 98 package [35] at the Hartree-Fock (HF) level using the 6-31G* basis set.

Excited-state geometry optimization has been performed using the TURBOMOLE package [36] at the time dependent HF (TD-HF) level using the all SV basis set. TURBOMOLE was used to calculate all subsequent properties at the TD-B3LYP level of theory with the all SVP basis set. Different basis sets have been tested for both ground-state geometry optimization and properties. These calculations revealed the well known sensitivity of dipole moment calculations to basis sets: the problem is especially relevant for permanent dipole moments, while transition dipole moments are much less basis-set dependent. Trends are nevertheless robust and are used to assist interpretation of experimental data.

The nature of the excited states has been investigated through natural transition orbital analysis of the excited states [37] based on the calculated transition densities. Figures showing molecular geometries and transition orbitals have been obtained with XCrySDEN. [38] Table 1. Photophysical properties of three-branched octupolar chromophores derived from the triphenylbenzene (TPB) core and of their 1-D dipolar analogues bearing a biphenyl (BP) terminal moiety (in toluene).

D D D Π Π Π Π Π Π Π Π Π Π Π Π D D D Π Π Π Π Π Π Π Π Π Π Π Π D D D Π Π Π Π Π Π Π Π Π Π Π Π D D D Π Π Π Π Π Π Π Π Π Π Π Π A A A Π Π Π Π Π Π Π Π Π Π Π Π A A A Π Π Π Π Π Π Π Π Π Π Π Π A A Π Π Π Π A A Π Π Π Π D Π Π Π Π D Π Π Π Π
Π Π Π Π Π Π Π Π Π Π Π Π D D D Π Π Π Π Π Π Π Π Π Π Π Π A A A Π Π Π Π Π Π Π Π Π Π Π Π A A A Π Π Π Π Π Π Π Π Π Π Π Π A A Π Π Π Π A A Π Π Π Π D Π Π Π Π D Π Π Π Π

  photophysical and TPA properties of the three-branched systems (octupolar compounds TPB[(PV) 1 NHex 2 ] 3 and TPB[(PV) 1 SO 2 CF 3 ] 3 ) with those of their dipolar counterparts where the triphenylbenzene (TPB) core has been replaced by a biphenyl (BP) moiety (BP(PV) 1 NHex 2 and BP(PV) 1 SO 2 CF 3 ). In addition, the effect of the nature of the peripheral groups (donor versus acceptor) and of the length of the conjugated arms (TPB[(PV) 2 NHex 2 ] 3

  Dipolar and octupolar chromophores bearing donor end-groups show a more pronounced solvatochromic behavior (larger slope values) as compared to those bearing acceptor end-groups. This is indicative of either a more pronounced charge transfer (i.e. larger eff µ ∆ values) or smaller cavities for derivatives with donor end-group as compared to those with acceptor end-groups. By estimating the cavity volume to about 300 Å 3 , a eff µ ∆ of 7 D is derived for chromophore BP(PV) 1 NHex 2 and 6 D for chromophore BP(PV) 1 SO 2 CF 3 from the solvatochromic slopes. Calculated values in Table 2 lead to comparable results when using expression (3). Similar values can be derived for the corresponding octupolar threebranched systems TPB[(PV) 1 NHex 2 ] 3 and TPB[(PV) 1 SO 2 CF 3 ] 3 . The more pronounced charge transfer for molecules with donor end-groups as compared to that of acceptor endgroups correlates with the bathrochromic and hyperchromic shift of the absorption band and

  F NMR (282.38 MHz, CDCl 3 ) δ -78.41; HRMS (EI) calcd for C 21 H 15 O 2 F 3 S (M + ·) m/z 388.0745, found 388.0746.

Figure 1 .

 1 Figure 1. Structure of three-branched octupolar chromophores derived from the triphenylbenzene (TPB) core and their 1-D dipolar analogues bearing a biphenyl (BP) terminal moiety.

Figure 2 .Figure 3 .

 23 Figure 2. Ground-state optimized geometry of TPB[(PV) 1 NHex 2 ] 3 .

Figure 4 .

 4 Figure 4. Comparison of absorption and emission spectra of dipolar (left) or octupolar (right) chromophores bearing donor or acceptor end-groups (in toluene).

Figure 5 .

 5 Figure 5. Comparison of absorption and emission spectra of dipolar and octupolar chromophores bearing acceptor end-groups (left), or donor end-groups (right) in toluene (top) and in DMSO (bottom).

Figure 6 .

 6 Figure 6. Absorption and emission spectra of dipolar and octupolar studied chromophores in different solvents : in toluene (), in THF (….) and in DMSO (---).

Figure 7 .

 7 Figure 7. Lippert-Mataga correlations for dipolar (left), and octupolar (right) chromophores.

Figure 8 .

 8 Figure 8. TPA spectra of BP(PV) 1 SO 2 CF 3 and TPB[(PV) 1 SO 2 CF 3 ] 3 in DMSO (left); BP(PV) 1 NHex 2 and TPB[(PV) 1 Nex 2 ] 3 in toluene (right).

Figure 9 .

 9 Figure 9. TPA spectra of TPB[(PV) 1 NHex 2 ] 3 and TPB[(PV) 2 NHex 2 ] 3 : length effect.

Figure 10 .

 10 Figure 10. Correlation between maximum TPA cross-section amplitudes and solvatochromic behavior for dipolar (red line) and octupolar (blue line) chromophores.

  0; HRMS (ES + , MeOH/CH 2 Cl 2 ) calcd for C 32 H 42 N ([M+H] + ) m/z 440.3317, found 440.3323. Anal. Calcd for C 32 H 41 N (439.69): C, 87.42; H, 9.40; N, 3.19. Found: C, 87.79; H, 9.47; N, 2.92.

PV) 1 SO 2 CF 3 ] 3 as

  .1; HRMS (EI) calcd for C 30 H 24 (M + ·) m/z 384.1878, found 384.1877. a pale yellow solid; 1 H NMR (200.13 MHz, CDCl 3 ) δ 8.03 and 7.80 (AA'XX', J AX = 8.6 Hz, 12H), 7.86 (s, 3H), 7.78 and 7.70 (AA'XX', J AX = 8.6 Hz, 12H), 7.42 (d, J = 16.2 Hz, 3H), 7.23 (d, J =16.2 Hz, 3H); 19 F NMR (282.38, CDCl 3 ) δ -78.36 ; HRMS (ES + , MeOH/CH 2 Cl 2 ) calcd for C 51 H 33 O 6 F 9 S 3 K ([M+K]

	1,3,5-Tris[4-[(1E)-2-[4-[(trifluoromethyl)sulfonyl]phenyl]ethenyl]phenyl]benzene
	(TPB[(PV) 1 SO 2 CF 3 )] 3 ). Air was removed from a solution of 1,3,5-tris(4-ethenylphenyl)benzene (6)
	(170 mg, 0.442 mmol) and 4 (447 mg, 1.547 mmol) in anhydrous DMF (4 mL) by blowing argon for
	20 min. Then triethylamine (179 mg, 1.768 mmol), tri-o-tolylphosphine (40 mg, 0.133 mmol) and
	palladium diacetate (17 mg, 0.066 mmol) were added. The mixture was stirred at 100°C for 15 h. The
	solvent was removed by distillation, and the raw product was purified by column chromatography
	(heptane/CH 2 Cl 2 80:20 then 30:70) to yield 168 mg (38%) of TPB[(

+ ) m/z 1047.0933, found 1047.0937.

  3 cm -1 ) h

	BP(PV) 1 SO 2 CF 3	351	37 (38) a	8	424	4.91	0.04 0.19	4.52	0.22	5.04	7.7
	TPB[(PV) 1 SO 2 CF 3 ] 3	355	116 (132) a	14	420	4.33	0.10 0.17	1.72	0.58	5.30	6.6
	BP(PV) 1 NHex 2	378	44	9	435	3.47	0.11 0.37	3.36	0.30	2.41	10.4
	TPB[(PV) 1 NHex 2 ] 3	384	131	15	438	3.21	0.36 0.63	1.75	0.57	1.02	9.7
	TPB[(PV) 2 NHex 2 ] 3	412	159	16	477	3.34	0.82 1.08	1.32	0.76	0.17	16.7

a In DMSO. b Stokes-shift = (1/λ abs -1/λ em ). c Fluorescence quantum yield in toluene determined using fluorescein in 0.1 N NaOH as a standard.

d Experimental fluorescence lifetime measured by TCSPC. e Radiative excited-state lifetime (τ 0 = τ /Φ). f Radiative decay rate. g Non-radiative decay rate. h Slope derived from the linear dependence of the Stokes-shift on the polarity-polarizability function of the solvent (∆f = (ε -1)/(2ε + 1) -(n 2 -1)/(2n 2 + 1) where ε is the dielectric constant and n the refractive index).

Table 2 .

 2 Calculated properties for ground-state and excited-state (f) geometries of dipolar chromophores BP

(PV) 1 SO 2 CF 3 , BP(PV) 1 NHex 2 , TPB(PV) 1 NHex 2 and

  ground-state properties of the branched octupolar chromophore TPB

[(PV) 1 NHex 2 ] 3 .

		ω eg	µ ge	µ gg	µ ee	ω eg	(f)	µ eg (f)	µ gg (f)	µ ee (f)	σ 2 max	a
		(eV)	(D)	(D)	(D)	(eV)	(D)	(D)	(D)	(D)
	BP(PV) 1 SO 2 CF 3	3.64	9.3	6.6 19.6	2.87	12.3 9.6 16.1	100
	BP(PV) 1 NHex 2	3.52	9.5	3.1 16.4	2.85	12.5 5.0 10.4	120
	TPB(PV) 1 NHex 2	3.48	9.9	3.2 19.0	2.80	13.2 5.2 12.0	170
	TPB[(PV) 1 NHex 2 ] 3	3.42 11.9 b	-	-	-		-	-	-	-

a Derived from a two-state model when considering experimental half-bandwidth at half maximum (0.30 eV for BP(PV) 1 SO 2 CF 3 , 0.27 eV for BP(PV) 1 NHex 2 ). For TPB(PV) 1 NHex 2 0.30 eV has been fixed. b For each of the two degenerate excited states.

Table 3 .

 3 TPA properties of three-branched octupolar chromophores derived Synthesis of dipolar and octupolar chromophores.

	from the triphenylbenzene (TPB) core and their 1-D dipolar analogues bearing
	a biphenyl (BP) terminal moiety.
						2λ abs (nm)	Φ	(max) TPA λ	(nm)	σ 2 (GM) a @ (max) TPA λ
	BP(PV) 1 SO 2 CF 3	b			710	0.09	710	95
	TPB[(PV) 1 SO 2 CF 3 ] 3	b	728	0.15	710	290
	BP(PV) 1 NHex 2	c				756	0.11	740	130
	TPB[(PV) 1 NHex 2 ] 3	c		768	0.36	735	470
	TPB[(PV) 2 NHex 2 ] 3	c		824	0.82	820	975

a 1 GM = 10 -50 cm 4 .s.photon -1 ; b in DMSO; c in toluene. Scheme 1.

Acknowledgements

We acknowledge financial support from Région Bretagne (« Renouvellement des Compétences » Program) and Rennes Métropole. FT acknowledges Université de Rennes 1 for an invited assistant professor position. Calculations have been performed at the "Centre Informatique National de l'Enseignement Supérieur" (CINES-France).


Dominique Fréard
email: dominique.freard@telecom-paristech.fr

Alexandre A J Denis
email: denis@loria.fr

Françoise Détienne
email: francoise.detienne@telecom-paristech.fr

Michael Baker
email: michael.baker@telecom-paristech.fr

Matthieu Quignard
email: matthieu.quignard@univ-lyon2.fr

Flore Barcellini
email: flore.barcellini@cnam.fr

The role of argumentation in online epistemic communities: the anatomy of a conflict in Wikipedia

Keywords: Online communities, interactive profiles, argumentation, collaborative knowledge production, conflict

INTRODUCTION

Over the previous decades, new forms of distant computer-mediated collaborative work have emerged, termed "online epistemic communities", whose raison d'être is the creation of some kind of knowledge-object (e.g. a programming language, an encyclopaedia, an online course, a building design...). Our research aims to study the interactive processes by which such communities function. We focus on both discussions and the way that they are interwoven with the evolution of the knowledge-object. Our analysis approach aims to integrate both human interpretation and automatic language processing. Our ulterior practical aim is to propose software tools for teamwork management and for self-awareness in the design process, with a specific focus on conflict detection and resolution.

Given its known role in collaborative work, we focus here on one genre of interaction, argumentative interaction, in the case of an online epistemic community whose aim is to produce documents (Wikipedia: WP) relating to astronomy, presenting an illustrative discussion relating to the question "should the WP page previously titled 'Pluto (planet)' be (scientifically) renamed or not?" We will show that the argumentative dialogue proceeds by a subtle interplay between arguments drawing on different types of knowledge (scientific, everyday) and the images of their respective competencies that both specialists and enlightened 'amateurs' project in the interaction.

After presenting our theoretical background, we introduce the context of the interactive process studied. In what follows, the dual coding approach used for the analysis is explained. Then, the results obtained on this basis are presented. The discussion focuses on the limits and perspectives of our approach.

THEORETICAL BACKGROUND

In this study we focus on forms of participation and conflicts in an epistemic online community from the point of view of argumentation dialogue. [START_REF] Baker | Argumentation and Constructive Interaction[END_REF] and [START_REF] Quignard | A Collaborative Model of Argumentation in Dyadic Problem-Solving Interactions[END_REF] have pointed out that argumentative interactions can be associated with coconstruction of knowledge, within the interplay of socio-relational and task-related processes. To the extent that online epistemic communities function interactively and dialogically, argumentative processes should therefore play an important role in them.

Our general framework refers to a participant-centred approach that concerns the analysis of individual contributions, summarised by the concept of role (C.f. [START_REF] Gleave | A Conceptual and Operational Definition of 'Social Role' in Online Community[END_REF]. We use an analytical framework developed in a study of architectural design [START_REF] Baker | A socio-cognitive analysis of online design discussions in an Open Source Software community[END_REF]) that aims to bring out the nature of participants' contributions in a way that distinguishes content and functional dimensions (presented below in the part dedicated to our dual coding approach). The resulting global vision of contribution is termed an "interactive profile". A related approach has been applied to collaborative knowledge production in the Open Source Software (OSS) community dedicated to the Python programming language (Barcellini, Détienne, Burkhardt, & Sack, 2008).

Collaboration for knowledge production in online communities raises issues of group management, governance and conflicts between participants or subgroups. For example, in the case of WP, [START_REF] Kittur | He Says, She Says: Conflict and Coordination in Wikipedia[END_REF] analysed the statistics on editing of articles and the links between articles in order to detect conflicting articles. They showed that detection was possible based on these indicators. Similarly, [START_REF] Auray | La négociation des points de vue : une cartographie sociale des controverses dans Wikipedia francophone[END_REF] localised conflicting articles and study policies for conflict resolution. In these papers, the discussion pages are not investigated in themselves. They are only used to extract examples. We think that a systematic study of the content of discussions should provide a finergrained view of conflict types and their processes. We aim to analyse the processes with specific discussions, across discussions, and their relations with the production of evolving wiki-texts to which they relate.

A WIKIPEDIAN CONFLICT

The Wikipedia project has been elevated to the status of "a prolific, cooperatively-authored online encyclopedia" [START_REF] Bryant | Becoming Wikipedian: transformation of participation in a collaborative online encyclopedia[END_REF]. In this project, we study participation oriented towards astronomy given that this famous scientific discipline involves the collaboration of professional scientists with amateurs and novices with respect to a common project: developping a basis for general public knowledge.

Data

The data presented originates from the French WP project: "Projet:Astronomie"1 , in which several professionals are involved with other WP participants (administrators, users, IP addresses). In this project, the case of Pluto is particularly interesting, firstly, given its shared imaginary potential for all kinds of participants, and secondly, given the recent changes in scientific views on this celestial body, that require knowledge negotiation between the different types of participants in this community.

The Pluto debate context

The conflict concerning Pluto that we discuss here could be considered to be purely scientific, or else simply a matter of nomenclature: following a decision taken on the 24 th of August 2006 by the International Astronomical Union (IAU), "planet Pluto" was reclassified as an "asteroid" and integrated into the asteroid category: "dwarf planet". Nevertheless, this decision sparked off a heated debate in the wikiastronomy community. The debate was settled in the French WP between this date and its resolution in a WP internal vote the 15 th of April 2008. On the most general level, the discussion turned on the question as to whether to follow the new academic nomenclature ("(134340) Pluto") for the title page or else to use a more specific title (e.g. "Pluto (dwarf planet)").

The Pluto's article in the French WP2 was created on the 11 th of September 2002. The writing of the article itself (editing space) involved 445 contributors with respect to 1012 edits until the present (mean per year = 134). In the discussion page attached to the article (discussion space) the group was more restrictive, where only 40 participants were involved in the whole debate. Amongst the participants, three WP statuses were present: IP addresses, usual WP users and WP administrators. Six participants were IP addresses, with respect to whom no information is available. One was a WP user who never created a personal page and about whom no information is available. 24 were regular WP users. Nine were administrators. We call these latter 33 persons "wikipedians". Amongst them, two were scientific researchers in astronomy engaged in Projet:Astronomie. They both belonged to the category of regular WP users. The other 31 wikipedians had varying levels of knowledge in astronomy (seeing their contributions), but no statistical information is available. We found evidence of a high degree of knowledge for at least five of them. Within this 33 wikipedians group, the date of subscription to WP varied between August 2002 and June 2008. Five of them subscribed after the beginning of the debate.

Corpus

During the complete period of the debate, the Pluto discussion page was organised in different themes, with titles given by the discussants themselves. Eighteen were relevant to the question of page title, thus shaping a corpus of 18 threads. In this corpus, the means were 11.2 dialogue turns per thread (min = 1; max = 48) and 56.7 words per turn (min = 1; max = 258). The 40 participants produced 202 interventions in this debate, with 11463 words. The 14 most important participants produced 88% of contributions. Only one of the two professionals was among these 14 participants.

Most of the edits made on the Pluto page were not relevant for the debate. For example, lot vandalism took place (e.g. erasing the page content, adding bad comments, etc.), generally coming from IP addresses and compelling wikipedians to re-establish them (which was done, the most often, in less than two minutes). A lot of edits also concerned subjects out of the scope of our analyses, such as the editing of figures, the telling of the story of Pluto's exploration, etc. The edits we kept in the corpus in addition to the 18 discussion threads were only the title modifications and the text modifications about Pluto's nomenclature and status. In the editing space, 4 title edits occurred the day of IAU meeting (24 th of August 2006), made by regular WP users. The last of these 4 edits (for "Pluto (dwarf planet)") was made by one of the two professionals, who had previously rejected the modification because the IAU vote was not yet over (see Table 1). The next title edit (for "(134340) Pluto") was made the 15 th of September 2006 by a regular WP user, who is not a professional, with a series of page content edits to make the article conform with the new IAU definition. The same participant also made an update after the IAU creation of the plutoid sub-category the 12 th of June 2007. The next title edits were performed the 11 th and 12 th of March 2008 for, respectively, the same titles as just indicated.

Overview of the debate

In the discussion space, the grey line along the fourth thread indicates its particularly long duration. This thread was the spinal column of the discussion as it made the transition between the two important periods of the debate. Globally, the three first threads allowed participants to exchange knowledge in Astronomy. Only one title was evoked in here. In the fourth thread, a novice in Astronomy opened the deep debate with an argument against the asteroid nomenclature. The different possibilities were then examined and developed in this long thread (48 contributions). After the modifications of 11 th and 12 th of March 2008, the debate was restricted to the two main solutions. The discussion turned to opposition and a participant proposed a vote that took place in a series of threads from the 14 th to the 18 th of April 2008.

In the whole debate, we identified eight threads with clearly conflicting exchanges. This shows that conflicts are frequent in WP discussion pages. The first thread retained for the results (white dot in the first period) implies no conflict. Contrarily, the second thread retained (white dot in the second period) is the major conflict in the complete debate. We shall thus illustrate both cases.

ANALYSIS: A DUAL CODING APPROACH

Our approach to this data is multi-disciplinary, involving argumentative interaction analysis in Cognitive Ergonomics, based on the notion of interactive profile, and argumentation formalisation in Natural Language Processing. The final goal is the development of a tool dedicated to the automatic characterisation of participations.

Figure 1: Overview of the 18 discussion threads and corresponding edits

Research strategy

We used a dual coding approach: (1 st step) a complete coding of argumentation processes was performed by a human analyst in order to analyse argumentative process and obtain a basis for comparison with the automatic analysis. (2 nd step) Automatic coding of contents was performed in order to test specifications for domain recognition using content analyses. This coding was then compared with the first one. We hope to obtain minimal descriptive features that would allow automatic detection of conflicts and their categorisation.

Both codings are used in convergence in order to propose the analysis of participants interactive profiles in the discussion (see below). For this, they provide complementary elements that deepen the analysis.

st step: Manual analysis

Our manual analysis is based on the coding of the dialogue units across two main dimensions: argumentative and epistemic. The argumentative dimension characterises the type of speech act performed (see [START_REF] Searle | Speech Acts[END_REF] with the unit and the participant's commitment with respect to the solution discussed as a thesis (T). The epistemic dimension characterises the knowledge domain used by the participant and the degree of expertise associated with this knowledge. The discourse units were obtained on the basis of dialogue turns (see [START_REF] Sacks | A simplest systematics for the organization of turntaking for conversation[END_REF]. When a turn involved several functions, it was segmented into the different units necessary to describe the acts performed. Table 1 presents our coding for (a part of) the first thread of the corpus (first white dot in Figure 1).

The argumentative dimension is described with two categories. Firstly, a dialogic function was attributed to each proposition depending on what is performed in the argumentative process (see [START_REF] Quignard | A Collaborative Model of Argumentation in Dyadic Problem-Solving Interactions[END_REF]. Secondly, when needed, the thesis defended was coded. The dialogic functions found in the discussion were: (1) inform (content is proposed), ( 2) synthesise (several previous contents are summarised), (3) request (a question is asked or a demand to do something or take something into consideration is expressed), ( 4) manage (the act organizes the dialogue or the task), ( 5) argument +/-(a content is expressed in relation to a thesis) and ( 6) evaluate +/-(a personal attitude is expressed about a thesis, a person or a previous act). For argumentation and evaluation the polarity indicated which position was expressed by the participant.

The epistemic dimension reveals what the protagonists said in terms of knowledge domain and expertise. We assumed four domains of interest for the debate: (1) knowledge of astronomy (scientific institutions or celestial bodies), (2) knowledge about WP (the article discussed, other articles or the policies organizing wikipedians' life), (3) knowledge about the group (coordination, dialog or task management) and ( 4) knowledge about individual persons (expertise or intentions). When another knowledge domain was used, it was noted 'Else'. For astronomy and WP, two levels of expertise were distinguished (expert vs. general public).

After the coding was completed for a thread, counting of the different occurrences in each category of the different dimensions was possible. This operation relating to specific participants allowed building their interactive profiles in the thread. Thus, the interactive profile describes the participant's behaviour in regard to the different analytical dimensions.

nd step: Automatic Language Processing

Whilst it is possible to develop automatic semantic analysis in constrained domains [START_REF] Denis | A Deep-Parsing Approach to Natural Language Understanding in Dialogue System: Results of a Corpus-Based Evaluation[END_REF], large scale, domain independent automatic analysis is notoriously hard. Building argumentation structure automatically requires a very detailed ontological representation of the task at hand. Moreover, given that most of the argumentation remains tacit, inferential tasks are particularly hard to model. To tackle this complexity, we focus on what is said rather than what is meant. This is indeed easier to implement, and although not as fine-grained as a deep semantic analysis, it proves helpful in identifying interactive profiles on the basis of content.

Conflicting opinions -On one hand, we explore the argumentation by mining the opinions of the contributors [START_REF] Liu | Sentiment Analysis and Subjectivity[END_REF]. Most opinion mining approaches start from the surface features conveyed by utterances. Firstly, polarities of lexical choices made by participants -positive or negative -are clues about their commitment to their partner's contributions. For instance, uttering "that is wrong" is a domain independent way to attack a thesis. Nonetheless, considering lexical choices does not prevent deep syntactic analysis (for instance "I don't think this is wrong" is not an attack). Second, subjectivity markers such as personal pronouns are also clues about the contributors' commitments. Both markers, negative lexical polarity and subjectivity markers, are strong evidence for conflicting discussions and thus can provide discrimination of participants throughout their participation in conflicting discussions.

Conflicting domains -On the other hand, the domain of the contribution can be automatically retrieved. First the domain will help distinguishing the contributions that are related to the Pluto article itself, and in particular its name, from the contributions related to other problems such as scientific questions (typically: "Pluto is no longer a planet but a star"). This enables us to discriminate contributors interested in the article itself from the others. Secondly, during the naming conflict, participants make use of different arguments. Whilst some of them appeal to the official naming by the IAU to title the article "(134340) Pluto", others refer to the Wikipedia principle of "least astonishment" in order to favour the thesis "Pluto (dwarf planet)". Given that

Eval -

Else 0 these domains are contradictory, we can further discriminate the participants according to their position in the conflict by assigning them the most frequent domain they employ in their contributions.

RESULTS

We use manual and automatic coding to study participants' interactive profiles in the interaction. Since all results cannot be presented in detail, we focus here on two threads (Cf. Figure 1). They illustrate two different kinds of argumentative interaction: a constructive one and a conflicting one.

Thread: "Pluto is no longer a planet"

The first thread was partially presented in Table 1. This is the one that opened the debate. This is an example of a collaborative exchange where participants shared and proposed information in order to coordinate the production of the page, which is the rule of discussion pages in Wikipedia.

st step: Manual analysis

Eight participants participated in this thread: 4 IP addresses, 1 WP administrator and 3 WP users including a professional researcher in astronomy. As the automatic coding is presented in chronological order (see below) we focus here on roles that were adopted. The four IP addresses only informed (dialogic function) with respect to general public knowledge in astronomy (weak expertise) coming from external Media (domain "else"). Amongst the wikipedians, two of them produced negative evaluations: Mrick in the personal domain, VIGNERON about the information source (domain "else"). In addition, Mrick informed with an expert proposition in astronomy. Furthermore, a regular WP user, Markov, informed with expert knowledge, once about astronomy, once about WP, with the thesis that "Pluto (dwarf planet)" would be a good title for the page (T1). Alain_r, the professional astronomer, once gave a negative argument against T1, and once managed the updating task. He used expert knowledge in astronomy and knowledge about WP.

These results reveal what was performed during this thread. Some content was proposed, coming from external information sources and afforded specifically by non wikipedian users. This content was appropriated by the group of wikipedians and evaluated in terms of consequences for the artefact they are working on (the encyclopaedia itself). A proposition was made by a wikipedian not expert in the domain and controlled by an expert. It is necessary to notice that this particular exchange arrived between 2:00 p.m. and 2:30 p.m. and that the expert made at 8:10 p.m. the title modification he refused earlier with a temporary argument.

nd step: Automatic Language Processing

The automatic analysis of this thread is displayed in Figure 2. First, we observe a domain shift from the first contribution, which is related to the astronomy domain (noted domAstro) to the next contribution in the Wikipedia domain (noted domWP). The manual analysis confirms that the domain shift is related to the uptake of the scientific fact that argues in favour of modifying the article's title. This remark is enhanced by the subjective marker found in Markov's contribution. However, the next contribution being negative, the proposal by Markov is rejected by Alain_r. Secondly, whilst the first contribution of Mrick carries all the clues for an argumentative attack (subjective-2nd and negative) the manual analysis rejects this hypothesis, since the contribution has an informative function instead of an evaluative function. Thirdly, it is worth noting the exchange of 81.49.140.14 and MRick, in which there is no domain shift, but rather a negative contribution of MRick. Although the automatic analysis cannot determine this, this negative contribution is a personal attack ("Only a complete ignorant") instead of an argued attack. Finally, the contribution from VIGNERON seems to carry a positive polarity, however, it is wrongly attributed from "It's funny", which in the context has to be interpreted clearly as irony and thus should be treated as negative. The second thread we discuss in detail is the most conflicting one in the whole debate. It was the second thread after the debate was reactivated by the title modifications of March 2008. A professional in astronomy (Meodudlye) gave an exposition of the results obtained in research in scientific databases to indicate which solution is the most frequent. He thus invoked scientific authority for the asteroid nomenclature. Following this, a WP administrator (VIGNERON) remarked that the debate was probably closed, but then, the main defender of the opposed thesis (TheRedBurn) discussed the argument. The discussion continued in a defence-attack game between the two opponents. It closed when a second WP administrator (Phe) modulated a defensive argument from TheRedBurn that convoked Markov's title modification proposition (studied in the previous part).

The argument was refused as the scientific authority had then changed its nomenclature. This counter-argument closed the thread, but the vote was nevertheless launched.

st step: Manual analysis

The analysis of participants' interactive profiles gives more indications about TheRedBurn and Meodudlye because as they made more contributions in the thread, the categorisation of their acts indicated more manifest profiles. Figure 3 is an illustration of this with respect to dialogic functions. It shows that Phe produced two propositions and VIGNERON only one while Meodudlye produced 15 and The RedBurn produced 21.

For the two participants who contributed less, complete profiles in this thread can be described in a few words. VIGNERON produced a management act in a knowledge category relating to the group ("Thanks. I think this closes the discussion."). Phe evaluated once and argued once, negatively, with knowledge about the group. He evoked one thesis only ("Very instructive discussion. Markov proposed Pluto (dwarf planet) before the correct name was known."). Concerning the epistemic dimension, Meodudlye's messages were characterised by contents in the scientific domain (e.g. "Ceres and Eris the two other dwarf planets are named with the same convention: (number)_NameOfPlanet, which is one more reason, if needed, to keep Pluto's present name."), always with expert knowledge. Contrary, TheRedBurn made more specific statements about the WP domain (e.g. "An article title in Wikipedia doesn't have to respect the official name of the subject treated.") and group and dialogue management ("In fact, my question was rhetorical", "I think discussion cannot reach consensus"). The only time he used scientific knowledge was on the general public level. Furthermore, Meodudlye also used knowledge about the person, to criticise TheRedBurn's expertise (e.g. "Thanks to show so clearly that finally you don't know much in astronomy" or "(…) _you_ want impose your title coming from no where"). TheRedBurn had first to defend himself with respect to his expertise ("Thanks not to consider me as more ignorant than I am"). Thus, both participants showed very different profiles. These results show the potential of interactive profile calculation for revealing the interplay between taskrelated arguments and negotiation of participants' projected and perceived degrees of expertise (a form of ad hominen argument).

nd step: Automatic Language Processing

The automatic analysis of this thread is shown in Figure 3. It shows a striking pattern of a conflict. First, the polarity analysis shows that, amongst the 11 messages exchanged, 8 of them carry a negative value and that all messages of Meodudlye are negative. This observation itself provides an interesting indication about the attitude of Meodudlye and TheRedBurn: they both are positioning themselves against a thesis. However, this remark alone is not sufficient enough to deduce that they are antagonists: it could be the case that both of them are opposed to the same thesis. The subjective analysis enables to confirm the conflicting status by showing a remarkable alternation between the first (subjective-1st) and the second person (subjective-2nd).

While Meodudlye is exclusively using the second person (e.g. "you don't know much in astronomy", "we don't have to invent other names because you've decided you don't like"), TheRedBurn is exclusively using the first person (e.g. "Thanks not to consider me as more ignorant than I am", "I thought there were a lot of dwarf planets"). This corroborates that there is a conflict: while TheRedBurn defends a position that Meodudlye attacks. The attack is not present in the first message where Meodudlye is only being negative about the thesis without attributing it to someone (no subjective marker). But TheRedBurn then commits himself to the attacked thesis by using the first person. This is why, after the TheRedBurn's answer, the thesis is now attributed to him, and as such Meodudlye can use the second person to directly oppose to him. This use of persons persists throughout the thread.

It is also interesting to notice that the antagonists do not make use of the same kind of arguments, and that the automatic analysis confirms the manual annotation. The domains of their contributions are found to be different. TheRedBurn mentions terms related to the Wikipedia and encyclopaedias domain (e.g. "Maybe that's because they are encyclopaedias", "This is what do the other wikis"), and to the astronomy domain, noted domAstro (e.g. "Pluto is the only dwarf planet, right ?", "I thought there were a lot of dwarf planets").

On the other hand Meodudlye only refers to the astronomy domain without invoking the status of Wikipedia. These different uses of domains persist in all messages except the last exchange between them. In that exchange no real argument is exchanged, the conflict that was argumented at first evolved to a purely personal conflict. Meodudlye says "Notice that if there is no consensus, that's only because _you_ want to impose your title coming from no where" while TheRedBurn comments "Let's say that if there is no consensus, that's because we're only two to discuss".

Limits of the automatic analysis

The automatic analysis enables a shallow classification of contributions based on the occurrence of polarised lexical items (modulo syntactic variations) or subjective markers. However, this simple approach is not sufficient to provide a fine-grained analysis of argumentation. The participants may, in the same contribution, be positive about an argument while being negative about another.

Or they could provide mixed polarities about the same argument. For instance in the sentence "We can find many [names] reflecting the truth but not corresponding to its true name" (thread 17), the first part is positive about choosing some names for they "reflect the truth", but the second part moderates this positive argument by being negative. The difficulty of the shallow approach on this kind of examples is twofold. First, as we do not represent explicitely the arguments on a fine-grained level but only as domains of contribution, we lack the ability to express mixed polarities about one single argument, or even the possibility to be positive about a given argument while being negative about another. Nevertheless, this limit can be circumvented by refining the relation between domain and polarity. For instance, instead of assuming an overall polarity for a contribution, we can link the polarity to a given propositional content in a sentence. The extent to which this link can be computed automatically without having to deeply model the astronomy domain has yet to be investigated. The example above shows the second major difficulty of the approach. The contributor both says that some names reflect the truth but at the same time that they do not correspond to the true name. The blatant contradiction of the argumentation can only be understood by assuming that "truth" in the first part means "physical truth" (e.g. Pluton is indeed a dwarf planet), while "true" in the second part means "official".

Word-sense disambiguation is a difficult task. There is much existing research dedicated to this task (the classical work being [START_REF] Lesk | Automatic sense disambiguation using machine readable dictionaries: how to tell a pine cone from an ice cream cone[END_REF], but not only our general approach has to be amended with word-sense disambiguation, it also has to take into account phenomena such as irony, which is still an open issue in language processing.

DISCUSSION AND PERSPECTIVES

The results presented illustrated that the way online communities function goes beyond knowledge-based discussion and argumentation, to involve negotiation of competencies of so-called 'experts' and 'amateurs', and more specifically in a conflicting exchange.

The dual coding approach we used is promising since the elements provided are complementary in highlighting these negotiation processes. The coding used allows us to consider participants' interactive profiles relating to the participants in different threads.

We will apply the same method on the complete corpus to address globally the participants' roles in the group and their evolutions during the debate depending on discussion episodes (such as conflicts). We are now working on these elements to build a broader view of roles assumed in online epistemic communities.

Furthermore, our automatic coding converges with this cognitive ergonomic analysis and contributes to enlarging the overall vision of the way that online epistemic communities work. The semantic analyses of opinions and domains formalise the dialectic and epistemic categories in depth. These analyses also consider additional indicators, such as personal pronouns, which would be too long and error-prone if coded manually in a large corpus, or question-answer interplay which is difficult to build in manual analysis. These elements are integrated into the description and contribute to build richer interactive profiles.

This work should lead to the specification of participants' needs during collaborative process. On this basis, the development of the NLP tool should be proposed for automatic conflict detection. In this case, it is necessary to reduce the analysis to the more stable elements in the interactive profiles which should allow such conflict detection.

Figure 1

 1 Figure 1 gives a general overview of the debate distributed over time (August 2006 to June 2008). This Figure reveals that the debate settled into two main periods. The first occurred after an external event: the IAU decision to modify the planet definition. The second occurred after an internal event: the editing of the page title by a wikipedian. Figure 1 illustrates this movement in the two distinct spaces: the editing space of the article and the discussion space.

Figure 2 :

 2 Figure 2: Automatic annotation of thread "Pluto is no longer a planet"

Figure 3 :

 3 Figure 3: Dialogic functions in thread "Opinion from a professional in the domain" Concerning the argumentative dimension, Figure 3 shows different uses of dialogic functions. TheRedBurn was the only participant who informed, and he requested and managed more than Meodudlye. Conversely, he argued and evaluated less than Meodudlye. Concerning theses, TheRedBurn was less open since he only evoked the solution he put forward on two occasions (T1). Meodudlye evoke the solution he defended four times (T2) and three times the other thesis (T1).

Figure 4 :

 4 Figure 4: Automatic annotation of thread "Opinion from a professional in the domain"

Table 1 : Extract from the first thread in the corpus "Pluto is no longer a planet"

 1

	User Turn Unit	Content	Funct° Thesis Domain Exp.
	IP1	4.1	[a]	Pluton ne fait plus partie comme étantplanète du systeme solaire Pluto is no longer included in the planets of the solar system	Inform	K(Astro)	0
				Je propose le renommage en "Pluton (planète naine)" plutôt que "Pluton
	Markov 4.2	[a]	(corps céleste)". --Markov (discut.) 24 août 2006 à 16:01 I propose renaming as "Pluto (dwarf planet)" rather than "Pluto	Inform	T1 K(WikiP) Expert
				(celestial body)"
				Rien n'est officiel pour l'instant, puisque le vote n'est pas terminé, donc
	Alain_r 4.3 [a1]	on attend.	Arg -	(T1) K(Astro) Expert
				Nothing is official at present, since the vote didn't end, so we wait.
				Merci de ne pas tout déranger prématurément. Alain r 24 août 2006 à
	Alain_r 4.3 [a2]	16:17	Manage	K(WikiP)	0
				Thanks for not disturbing everything prematurely
				Selon les dépêches ([1]), la proposition 5A lors du congrès de l'IAU est
	Markov 4.4	[a]	passé, mais pas 5B. (cf http://...)--Markov 24 août 2006 à 16:26 According to dispatches, the proposition 5A in the IAU congress was	Inform	K(Astro) Expert
				accepted, but not the 5B.
				Il y a une petite erreur dans ton lien Markov, voici la news officielle de
	Mrick	4.5	[a]	l'UAI : http://... MRick 24 août 2006 à 17:20 There is a small mistake in your Markov link, here is the official news	Inform	K(Astro) Expert
				from the IAU:…
	IP2	4.6	[a]	I>TELE : Pluton n'est plus une planete mais une étoile. ITV. Pluto is no longer a planet but a star.	Inform	K(Astro) GP
	Mrick	4.7	[a]	Seul un ignare complet dirait que Pluton est une étoile. Only a completely ignorant person would say Pluto is a star.	Eval -	K(Person)	0
	Mrick	4.7	[b]

Si c'est de I>TELE que vient cette info, ça ne donne pas envie de regarder cette chaine. MRick 24 août 2006 à 17:20

If this news comes from ITV, it doesn't make one feel like watching that channel.

http://fr.wikipedia.org/wiki/Projet:Astronomie

http://fr.wikipedia.org/wiki/(134340)_Pluton

ACKNOWLEDGMENT

This study has been conducted in the framework of the CCCP Prosodie Project partly funded by the French National Research funding Agency (ANR).