Jean-Pierre Malval
email: jean-pierre.malval@uha.fr

Sylvain Achelle
email: sylvain.achelle@univ-rennes1.fr

Loic Bodiou

Arnaud Spangenberg

Laura Chia Gomez

Olivier Soppera

Robin-Le Guen

Two-Photon Absorption in a Conformationally Twisted D--A Oligomer : A Synergic Photosensitizing Approach for Multiphoton Lithography

interchromophore coupling within the oligomer which exhibits a highly distorted geometry resulting in a strong reduction of the effective conjugation length. The recursive implementation of nine monomers into a three-dimensional architecture leads however to a cooperative enhancement of the two-photon absorption (2PA) cross-section with a  MAX of 5093 GM at 800 nm. This very high 2PA ability has been oriented to improve the two-photon induced polymerization efficiency of a bicomponent photoinitiator system implying a hexaarylbiimidazole used as H-abstractor and an aliphatic amine used as H-donor. The photosensitizing mechanism is investigated and we clearly show that the intrinsic photoinitiation efficiency with the oligomer is increased by a factor 3 as compared to its corresponding monomer. We therefore demonstrate that such a two-photon sensitizing strategy leads to a synergy effect combining a higher photoinitiation reactivity and a very large two-photon absorption cross-section.

TABLE OF CONTENT ENTRY

The synthesis and the photophysical properties of a -conjugated oligomer alternating pyrimidine and triphenylamine fragments were described. This oligomer exhibits very high two photon absorption ability and was efficiently employed as photosensitizer for two photon lithography.

ABSTRACT.

A comparative study of the linear and nonlinear optical properties of a novel triphenylaminepyrimidine alternated oligomer and its corresponding V-shaped quadrupolar monomer is presented. Both chromophores strikingly exhibit the same spectral shape when considering their respective one-and two-photon absorption spectra. This effect was attributed to a weak

INTRODUCTION

Two-photon absorption (2PA) materials are of primary importance in a wide range of promising applications such as three-dimensional optical storage memory, 1 photodynamic therapy, 2 high-resolution imaging 3 or multiphoton stereolithography. 4 In the case of twophoton polymerization (2PP), a tight confinement of the excitation volume around the focal point is obtained allowing the fabrication of intricate 3D structures with feature sizes as small as 100 nm. 1c,4a The 2PP technology also implies the use of photoactivable systems with both large 2PA cross sections () and high photoinitiating reactivity. Despite their very low 2PA cross-sections, 5 commercially available photoinitiators can be successfully employed for twophoton polymerization when associated with two-photon activable sensitizers. 6 From a practical point of view, such a bi-component approach constitutes one of the simplest and reliable processing methods since it takes benefit of the very high reactivity of well known UV-curing initiators [START_REF] Fouassier | Photoinitiation, Photopolymerization, and Photocuring[END_REF] and of advances in engineering researches for the design of chromophores with large 2PA cross-section. 3a-b,8 In this domain, several strategies [8][9] have been proposed to establish some relevant structure/property relationships to enhance  In the so-called "multi-branching" approach, 9b,9e-f,10 various electron-accepting and -donating groups can be implemented within the core or at the periphery of multibranched -conjugated structures used as "electron relays". The nature of donor and acceptor substituents, 8 the delocalization length, 9c,11 the strength of the interbranch coupling, 9b,9e-f the spatial conformation and the electronic symmetry 12 of the global architecture 9a,13 are all distinctive parameters that can be methodically tuned to optimize the 2PA ability of the final chromophore. In this context, -conjugated oligomers and -conjugated polymers constitute the typical molecular platform which can rationalize such fundamental issues. 14 In this recursive synthesis approach, the generation of -conjugated macromolecules can offer a very high degree of electronic delocalization resulting in a strong enhancement of the two-photon absorption cross section. 14b-d,15 For instance, Williams-Harry et al. 16 have elaborated a series of macrocyclic thiophene n-mers integrating more than twelve thienylene units which exhibit giant 2PA cross sections up to 11 x 10 5 GM ( 1 GM = 10 -50 cm 4 s photon -1 ). In the same manner, Yoon et al. 17 have synthesized fused Ni 2+ porphyrin arrays containing up to 5 macrocycles leading to a  MAX of about 4.1 x 10 5 GM. In this latter example, the -conjugated chains with enforced coplanarity constitute the good guarantee for a great effective conjugation length 14a with a maximizing effect of the 2PA cross-section. The dendrimer architecture 18 also offers the possibility of increasing the density of effective chromophores.

Even though such a highly ordered confined geometry can potentially disrupt the conjugation, 12a,b the strong interchromophore coupling can promote a cooperative enhancement of the nonlinear optical response. For instance, Varnavski et al. 12c have reported the photophysical characterization of tolane-based dendritic chromophores which undergo a strong enhancement of  over the additive behaviour when going from the trimer derivative ( 9chromophores) to higher generations (up to 21 chromophores).

In this contribution, we report the synthesis and the photophysical characterization of a new -conjugated oligomer alternating triphenylamine (D) and pyrimidine (A) moieties connected with vinylene (--) bridges (Scheme 1). Based on a large set of experimental results such as absorption, emission, excitation anisotropy or 2PA spectra associated with theoretical calculations, a comparison of the linear and the nonlinear optical properties of the oligomer (Og) and its D--A--D model chromophore (M) is performed in order to highlight the role of dimensionality as well as the strength of interchromophore coupling. Finally we will take benefit of the very large 2PA ability of Og to demonstrate the potential interest of this new oligomer as efficient photosensitizer for one and two-photon polymerization applications.

Scheme 1.

Molecular structures of chromophores.

RESULTS AND DISCUSSION

2.1 Synthesis.

The synthesis and full characterization of the model chromophore M has been previously

reported by some of us. 19 In order to obtain an oligomer with enough solubility, a long alkoxy chain was added to the pyrimidine heterocycle. Starting from commercially available 2chloro-4,6-dimethylpyrimidine, a nucleophilic substitution with lithium octylate was carried out. The desired product was obtained with good yield (Scheme 2).The polymerization was carried out by condensation reaction between dimethylpyrimidine derivative 3 and 4,4"diformyltriphenylamine 4 in boiling aqueous 5M NaOH using aliquat ® 336 as a phase transfer catalyst according to the method initially described by Vanden Eynde. 20 Indeed it has been shown that such condensation of methylpyrimidine derivatives with aldehyde is complete.

The non quantitative yield sometimes obtained is mainly due to substantial loss during the purification process of the compounds. 19,21 The desired oligomer was obtained in good yield after purification by precipitation. Other conditions were tested using potassium tertbutoxyde in THF, but in that case a material which was completely insoluble in common organic solvent was obtained. The oligomer has been analyzed by NMR spectrometry, the 1 H spectrum is given in supporting information (Figure S1). The two doublets that are observed Hereafter, we will take an average value of 9 M-units for the oligomer. This oligomer has a good solubility in various solvents such as chloroform, dichloromethane or tetrahydrofuran. It does not show any self-aggregation as confirmed by checking Beer Lambert law within mM concentration range. Finally the X-ray diffraction spectrum of the powder of Og (Figure S2) exhibits a broad peak at 2θ= 19.5° which suggests an amorphous form for this oligomer.

Scheme 2.

Reaction scheme for the synthesis of the oligomer Og. units leads to a strongly distorted conformation that should drastically limit the extent of electronic delocalization all along the -conjugated backbone. Figure 2 displays the normalized absorption and fluorescence spectra of M and Og in dichloromethane along with their respective steady-state excitation anisotropies. Table 1 gathers the corresponding spectroscopic data. The calculated absorption spectrum of M using TD-DFT method is also presented on the top of the experimental spectrum and Table 2 reports all related calculated parameters. The low energy part of the absorption spectrum of M is dominated by an intensive and unstructured band which shows a very slight shoulder at its blue edge (i.e.

Geometry and Electronic

around 410 nm). Interestingly the anisotropy remains constant within this long wavelength absorption band but strongly decreases below 420 nm and reaches a plateau below 375 nm.

Such a change in anisotropy is clearly corroborated by TD-DFT calculations which indicate the presence of two well separated electronic transitions with distinctive polarization (see Table 2). The calculated energy of the S 0 → S 1 transition reasonably agrees with the experimental value of the 0-0 transition energy (E 00 ). This latter energy was measured from the intercept of the normalized absorption and fluorescence spectra recorded in apolar medium (hexane) in order to minimize the solvent induced relaxation process at S 1 state. In this condition, a mirror-image relationship is observed between absorption and fluorescence spectra which leads to a more reliable value for E 00 (Figure S3). According to our TD-DFT calculations, the lowest energy transition is a pure HOMO-LUMO transition which is strongly one-photon allowed (f >1) and polarized along the long axis of the molecule (x-axis in Figure 1A). The S 0 → S 2 transition is weakly allowed and corresponds to a pure HOMO -1 -LUMO transition. It is interesting to note that the polarization of this S 0 → S 2 transition is perpendicular to that of the S 0 → S 1 one which is in line with the strong change in anisotropy within this spectral region. Moreover, it should be noted that the average value for the anisotropy of the S 0 → S 1 is about 0.25 ± 0.01. This indicates that the transition moment at the emitting state is not collinear to that of the ground state. Such a change in the orientation of the absorption and emission moments should be presumably ascribed to a photoinduced planarization affecting the triphenylamines at the relaxed S 1 state. Such a relaxation effect has been previously observed for the excited-states geometry of multibranched D--A dipoles. 6d-e

In the 315-370 nm range (see Figure 2), the anisotropy remains constant due to the equivalent polarizations of the dominant transitions S 0 → S 3 and S 0 → S 5 respectively. The electronic distributions of the frontier orbitals are shown in Figure 3. The two lowest energy transitions clearly correspond to * transitions implying a charge transfer (CT) from the periphery (triphenylamino group) to the core (pyrimidine ring) of the dye. Interestingly, the occurrence of these two CT transitions can be predicted within the framework of the Frenkel exciton model. 9a-b,22 For a two-branched V-shaped chromophore, it is assumed that interbranch electrostatic interactions lead to the splitting of the one-exciton transition relative to the molecular synthon as illustrated in Scheme 3. Moreover for a homo-dimeric chromophore, this twofold degeneracy occurs symmetrically and leads to a S 1 -S 2 gap of 2V where V denotes the interbranch coupling. In our case and according to our TD-DFT calculations, the interbranch coupling which corresponds to half value of the S 1 -S 2 energy gap (see Scheme 3) can be theoretically estimated to ca. 0.13 eV. Such a value is comparable to those obtained from other stilbenyl quadrupolar chromophores symmetrically substituted with amino end groups. 9b,9e-f It is also noteworthy that this interbranching coupling is noticeably stronger than that calculated for a similar V-shaped structure with the same 4,6-bis(styryl)pyrimidine core but associating moderate pyran-based donors at the periphery. [START_REF] Achelle | [END_REF] Table 1.

Summary of optical data and parameters for M and Og.

 abs / nm

 MAX / 10 3 M -1 cm -1  fluo / nm  fluo  fluo / ns k r b / 10 8 s -1  MAX   MAX ) / GM N eff /N
H-L +1 S 0 → S 5 3.91 0.43 x H -2 -L S 0 → S 6 3.93 0.05 x,z H -1 -L +3 (0.34), H-L +2 (0.38) c
a The 0-0 transition energy was measured in hexane (apolar medium) b xyz axis are displayed in Figure 1A, c The two principal natural transitions are displayed.

Figure 1.

Optimized geometries of M and its oligomeric model with three M-units (PBE0/6-31G(d) level).

Scheme 3.

Schematic electronic level diagrams within the excitonic coupling model for a dipolar unit and its corresponding V-shaped quadrupolar structure.

Figure 3.

Representation of the molecular orbitals involved in the S 0 → S 1 and S 0 → S 2 electronic transitions of M.

L H H -1

Surprisingly, increasing the number of repeat units (from n = 1 to 9) leads to a very strong hyperchromic effect with a 10-fold increase of  MAX (Table 1) but hardly affects the shape of the absorption spectrum. Indeed, on going from M to Og, the longest wavelength absorption band undergoes a slight broadening (x 1.2) and is hardly redshifted by about 930 cm -1 . This very weak bathochromism contrasts, for instance, with that observed for oligo(pphenylvinylene)s 14e or oligo(-thiophene)s 14c -d,24 which exhibit a band redshift of about ~ 8000 cm -1 upon increasing the number of repeat units up to 12-mers. Contrary to these relatively planar oligomers, the severe steric interactions between each adjacent M-unit result in a serious disturbance of the -orbital overlap along the oligomer backbone. Such similar structural effects have been nicely rationalized by the working groups of Osuka and Kim 25 who have elaborated two series of porphyrin-based oligomers (n = 2 to 12) which show respectively a planar and an alternatively twisted geometry. Whereas the planar fused porphyrin series display strongly red-shifted absorption spectra upon increasing the size of the oligomer ( ~ 6100 cm -1 for n = 2 to 12), the twisted series undergo a 5-fold weaker redshift. This structure-property relationship is clearly in line with the effects observed in our case and presumably confirms the highly distorted structure of our oligomer. It turns out that the effective conjugation length 14a within our D--A alternating oligomer is reached very fast upon increasing the number of repeat units. As shown in Figure 2, both M and Og exhibit a second distinctive band near 300 nm. This band is usually observed for N-phenylated systems 10,26 and should be ascribed to the S 0 → 1 L a transitions 27 localized on the triphenylamine moieties. 28 Interestingly, the intensity of this band relative to the lowest energy one only decreases from 0.68 to 0.50 on going from M to Og. Such a slight effect illustrates a weak mixing of the localized 1 L a states with the low lying 1 CT ones 29 and corroborates the assumption that Og can be viewed as a composite macromolecule comprising molecular units which preserve their electronic individuality. According to the point-dipole exciton model proposed by Kasha et al.,30 the coupling between consecutive Munits should lead to a resonance splitting of the excited state composite molecule energy levels which were initially degenerate. For a strong exciton coupling, the splitting energy should give rise to a clear separation of the new S 0 →S n transitions. However, in our case, a slight band broadening (x 1.2) is only observed and the corresponding anisotropy spectrum of Og displays globally the same changes as those observed for M (Figure 2). Such a weak interchromophore exciton coupling is also corroborated by the equivalent fluorescence bands of both dyes. Moreover, for the oligomer, it should be indicated that the population of emitting conformers is clearly monodispersive since the fluorescence band shape of Og does not show any excitation wavelength dependencies and the corresponding excitation spectra collected at various emission wavelengths match perfectly the absorption spectrum. The fluorescence decay of Og can be confidently fitted with two exponential functions but exhibits a major long-lived component whose contribution is larger than 95 % of the total emission (see Figure S4). Even though the fluorescence quantum yield ( fluo ) decreases on going from M to Og (see Table 1), the oligomer remains highly emissive with  fluo = 0.63 in dichloromethane. The radiative rate constant (k r ) of Og is comparable to that calculated for M which indicates an electronic equivalency of the two emitting states and therefore confirms that the radiative deactivation of Og presumably stems from a localized excited state centered a single M-unit. This electronic localization we proposed for the emitting state of Og is also in line with the significant change in the orientation of the absorption and emission moments as indicated by the relatively low value of the S 0 → S 1 emission anisotropy which exhibits an average value of ca. 0.14 ± 0.02.

Hence the oligomer Og clearly exhibits a similar photophysical fingerprint as that observed for its model chromophore M. This electronic effect should be attributed to a highly distorted geometry which dramatically reduces the effective conjugation length (ECL) of this donor-acceptor -conjugated oligomer.

Two-photon Absorption (2PA) Properties.

The equivalency observed for the electronic properties of M and Og can be extended to the shape of their respective two-photon absorption (2PA) spectra. Such an effect has been previously reported by when comparing the 2PA spectra of the 4,4"bis(diphenylamino)stilbene and several generations of dendrimers elaborated through recursive association of this latter molecular building block. Here also, the successive alternation of planar and highly distorted domains should result in a loss of conjugation all long the macromolecular structure. Figure 4 shows the 2PA spectra of our compounds. The two-photon absorption cross-sections () have been measured in the 680-1000 nm range by means of two-photon excited fluorescence. 31 The 2PA bands are plotted against half the excitation wavelength to have a direct comparison with the one-photon absorption (1PA) spectra. According to our spectral resolution, both spectra present two distinctive bands localized in the same spectral range as the longest wavelength 1PA band. Their respective maximum absorption wavelengths (i.e.  2PA ≈ 900 nm and 800 nm) nicely correspond to the position of the S 0 → S 1 and S 0 → S 2 transitions. Moreover one clearly observes that the highenergy band is more intense than the low-energy one. This effect should be related to the dipole selection rule for nearly centrosymmetric molecular symmetry which brings the S 1 state to be almost 2PA forbidden whereas the S 2 becomes 2PA allowed. Even though this trend is globally observed it is clear that the S 0 → S 1 transition becomes partially allowed for both compounds probably due to a relaxation of the molecular structures to geometries with a lower symmetry. The most important effect observed upon increasing the dimensionality is the very strong enhancement of  MAX which increases from 360 GM to 5093 GM on going from M and Og. Therefore the value of  MAX per repeat unit is about 566 GM (n ~ 9) which clearly indicates a cooperative enhancement of the 2PA properties of the oligomer. However it should be emphasized that  MAX depends quadratically on the effective number of electrons (N eff ) which contribute to the non linear response of the molecule. 32 This parameter can be determined according to the counting method proposed by Kuzyk et al. 32 Therefore, when normalizing the 2PA cross-section by the square of the effective number of electrons (N 2 eff ), a direct comparison can be made between the intrinsic 2PA properties within a series of chromophores. This methodology allows a better quantification of the cooperative enhancement or weakening of the nonlinear optical response of the macromolecular structure.

According to this normalization approach, a moderate cooperative effect is observed with a 1.8-fold increase of the intrinsic cross-section of Og as compared to M. This enhancement is comparable to the 1.4-fold increase of N 2 eff on going from the 4,4"-bis (diphenylamino)stilbene ( MAX = 320 GM , N eff = 18) to its second-order generation dendrimer with 13 repeat units 12a ( MAX = 4500 GM, N eff = 56). It is worth stressing that the strategy consisting in increasing the dimensionality of a macromolecule does not lead necessary to a significant improvement of its intrinsic 2PA performance. For instance the working groups of Goodson and Iyoda 16,33 have studied the 2PA properties of several very large macrocyclic thiophene n-mers comprising up to 30 thienylene-acetylene repeat units. In this precise case, the optimal value for N 2 eff was observed for the derivative with 18-mers (N 2 eff ~ 3.2) whereas the largest one ( MAX = 107 800 GM, N eff = 240) clearly leads to a decrease of N 2 eff by a factor 1.7 with respect to the optimal structure. From a conceptual point of view, it is clear that the intrinsic performance of a macromolecule can not be scaled through several orders of magnitude but from a practical approach a very large enhancement of  MAX undoubtedly constitutes the guarantee to efficiently promote any two-photon activation processes. 1PA (full lines) and 2PA spectra (plotted against half of the excitation wavelenghth) of chromophores in dichloromethane.

Therefore, in order to take benefit of the very large 2PA cross-section of Og, we will demonstrate hereafter that this chromophore can be used as an efficient two-photon sensitizer for multiphoton stereolithography applications.

2.4

One-and Two-photon Sensitized Polymerization.

Figure 5 shows the FT-RTIR (i.e. Fourier transform real-time infrared) kinetic curves during visible irradiation of four distinctive diacrylate formulations which all contains a hexaarylbiimidazole derivative (HABI) (Scheme 4). This chromophore is a one of the most widely patented free-radical UV-Visible photoinitiator. [START_REF] Dessauer | Advances in Photochemistry[END_REF] Upon direct excitation, 35 HABI undergoes a homolytic cleavage resulting in the formation of two lyophyl radicals (L•) 36 as shown in Scheme 4. However this latter radical adds very slowly to acrylic monomer in such manner that HABI is intrinsically a very poor free radical photoinitiator as illustrated in Figure 5. Moreover, it should be indicated that the photopolymerization is performed at 440 nm irradiation which corresponds to the extreme red edge of the absorption spectrum of HABI (inset Figure 5). Therefore HABI is usually associated with a hydrogen donor coinitiator. 34b,37 For instance, in presence of N-methyl diethanolamine (MDEA), the photopolymerization is significantly improved with 28 % conversion of acrylate functions after 150 s of irradiation. In this case, the mechanism is somewhat different since a H-transfer reaction occurs from MDEA to L• which generates a -aminoalkyl radical. This latter species exhibits a very high reactivity upon addition reactions 38 onto double bonds of acrylate monomers. The sequential steps for the generation of primary radicals can be described as follows: However such a reducible radical 39 is also sensitive to the presence oxygen dissolved in monomer medium. This inhibition process is typically illustrated on the kinetic profile by the long induction time (~ 25 s) observed during the first step of photopolymerization. It corresponds to the time necessary for the total consumption of the dissolved oxygen.

HABI 1 HABI 2 L• (Homolytic dissociation) L• + MDEA LH + MDEA• (H abstraction) h
Interestingly, the presence of photosensitizers (PS) drastically reduces this inhibition time by a factor 6 and leads to a strong increase of the polymerization rates which are multiplied by a factor 2.9 for M and 4.4 for Og. This clear improvement of the photoinitiation should be connected to the high absorptivity of M and Og at the irradiation wavelength (see inset of process can also occur from the lowest triplet state of the sensitizers. However for both chromophores, the PeT from T 1 states is presumably a minor route since excited chromophores mainly deactivate through fluorescence pathways. Moreover, the photosensitization of HABI through a triplet-triplet energy transfer mechanism should be excluded due to the relatively high triplet energy of HABI whose value, 2.93 eV, 37b is already similar to S 1 state energies of both sensitizers. Therefore, both chromophores constitute interesting one-photon sensitizers for free radical polymerization in the visible range. As shown in Figure 5, the maximum polymerization rate of the formulation with Og is 1.5-fold h higher than that measured for M even though the Og-formulation clearly exhibits a weaker absorbance at the irradiation wavelength. This effect suggests that Og should lead to a more efficient photoinitiation process. Indeed, the initiation efficiency  i of a photoinitiating system can be connected to the maximum polymerization rate R p by the following relationship: 7a

abs p i I R K 2 1   (1)
Where K 1 is a global parameter which comprises the concentration of the acrylate monomer as well as the rate constants for the propagation and the termination steps of the polymerization reaction. I abs corresponds to the amount of light absorbed by the sample. As the maximum polymerization rate (R p ) is observed at the initial steps of photopolymerization, we assume that I abs is reasonably proportional to (110 A0 ) where A 0 denotes the initial absorbance of the formulation. Under this approach, we can show that the initiation efficiency of Og is 3.5 higher as compared to that of M. This higher reactivity can be also demonstrated locally upon two-photon polymerization. Figure 6 shows several polymerized lines generated upon two-photon excitation at 800 nm using the formulations with M and Og. Note that the photopolymerization is not observed without the presence of sensitizers which obviously underlines the two-photon activation role of PS. The incident laser power is gradually decreased to estimate the power threshold (P th ) necessary to induce the two-photon polymerization. This fundamental parameter should be related the minimal amount of aminoalkyl radicals (n th ) that effectively lead to the polymerization reaction. Due to the equivalent photoinitiation mechanism for M and Og, the value for n th should remain constant for both formulations and the initiation efficiency  i can be expressed as follows: .

th PS th i P N n K    (2)
Where K 2 is a global parameter which implies all the photonic conditions: exposure time, pulse shape parameter, laser temporal pulse width, and repetition rate of the laser. N PS and  are the number density of the photoinitiator and the 2PA cross section. As shown in Figure 6, the values for P th are about 5 mW and 3.6 mW for M and Og respectively which leads to a 2-fold higher initiation efficiency for Og with respect to M. Therefore, both one-and twophoton polymerization measurements nicely converge to the same conclusion that the photosensitization of the bicomponent system HABI/MDEA exhibits a much higher reactivity when using Og. It is clear that the efficiency for the generation -aminoalkyl radicals strongly depends on the amount of L• used as hydrogen abstractors. However, the back electron transfer (BeT) between the primary generated PS •+ / HABI • radical ion pair can clearly impact the efficiency of all subsequent reactions. As a consequence, the electronic stabilization of PS •+ is of prime importance to counterbalance such a radical ion pair recombination. Even though Og presents a distorted geometry, its amine centered radical cation Og •+ should present a more planar conformation around its electron-deficient nitrogen atom 44 allowing an extended charge delocalization along two adjacent subunits which presumably guarantees a better stabilization as compared to M •+ . Therefore, the better stability of Og •+ with respect to M •+ should indirectly promote a higher quantum yield for the generation of L•. Hence, with the use of Og as a two-photon sensitizer one clearly promotes a synergy effect with a high photoinitiation reactivity and very large two-photon absorption cross-section ( MAX = 5093 GM).

CONCLUSION

The synthesis of a new conjugated oligomer alternating 4,6-bis(styryl)pyrimidine and Nphenylamine subunits has been reported. The oligomer exhibits very high one-and twophoton absorption ability with corresponding spectra that strikingly resemble those measured for its D--A V-shaped monomer. These spectral effects are ascribed to the highly distorted geometry of the oligomer along its -conjugated backbone which reduces the effective conjugation length. Therefore O g can be compared to a composite macromolecule which preserves the electronic integrity of each M-unit due to a weak interchromophore coupling.

The recursive implementation of several D--A subunits into a three-dimensional architecture leads to a  MAX of about 5093 GM. Taking advantage of the very large linear and non-linear absorption properties of the oligomer, we show that this new system can be used as an efficient sensitizer for free-radical photopolymerization and multiphoton fabrication. The photoinitiation reactivity is two-times higher than its model chromophore which demonstrates that increasing dimensionality not only induces a cooperative enhancement of the nonlinear optical response but can specifically enhance the photopolymerization performance due to the further stabilization of intermediate species involved in the photoinitiation mechanism. Such a synergetic strategy can be presumably extended to other -conjugated oligomers with comparable symmetries. Occidentale". The chemical shifts  are reported in ppm relative to the residual solvent (CHCl 3 ). The coupling constants J are given in Hz. In the Where I is the fluorescence intensity, the subscripts denote the orientation (horizontal H or vertical V) of the excitation and emission polarizers, respectively and g is an instrumental correction factor. The proper calibration of the set-up was checked using a recent standard method with rhodamine 101 in glycerol. 45 The anisotropy spectra were collected in viscous medium (i.e. triacetin, η 20°C = 25 mPa.s) in order to prevent the depolarization of the fluorescence emission due to the rotational diffusion of the molecules. [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF] The fluorescence lifetimes were measured using a Nano LED emitting at 372 nm as an excitation source with a nano led controller module, Fluorohub from IBH, operating at 1MHz. The detection was based on an R928P type photomultiplier from Hamamatsu with high sensitivity photon-counting mode. The decays were fitted with the iterative reconvolution method on the basis of the Marquardt/Levenberg algorithm. [START_REF] Connor | Time correlated single photon counting[END_REF] Such a reconvolution technique allows an overall-time resolution down to 0.15 ns. The quality of the exponential fits was checked using the reduced  2 ( 1.2).

The two-photon absorption (2PA) measurements were performed with femtosecond modelocked laser pulse using a Ti: Sapphire laser (Coherent, Chameleon Ultra II: pulse duration: ~140 fs; repetition rate: 80 MHz; wavelength range: 680-1040 nm). A relative two-photon excited fluorescence (2PEF) method 31 was employed to measure the two-photon absorption cross-sections, . The measurements of 2PA cross-sections were performed relative to reference molecules (r) such as fluorescein 31,[START_REF] Albota | [END_REF] in water at pH = 11. The value of δ for a sample (S) is given by:

r S S S r r r r S S c S c S     .   
Where S is the detected two-photon excited fluorescence integral area, c the concentration of the chromophores, and  is the fluorescence quantum yield of the chromophores. η is the collection efficiency of the experimental set-up and accounts for the wavelength dependence of the detectors and optics as well as the difference in refractive indices between the solvents in which the reference and sample compounds are dissolved. The measurements were conducted in a regime where the fluorescence signal showed a quadratic dependence on the intensity of the excitation beam, as expected for two-photon induced emission. For the calibration of the two-photon absorption spectra, the two-photon excited fluorescence signal of each compound was recorded at the same excitation wavelength as that used for standards (i.e.  exc = 782 nm for fluorescein). The laser intensity was in the range of 0.2-2 x 10 9 W/cm 2 .

The experimental error on the reported cross section is 15 %.

The photopolymerization was monitored in situ by Fourier transform real-time infrared spectroscopy (FT-RTIR) with a Thermo-Nicolet 6700 instrument IR-spectrometer. The laminated formulations are sandwiched between two polypropylene films (10 m thick) separated by a round teflon spacer of 25 µm thickness. The sample is then positioned between two BaF 2 pellets and irradiated at 440 nm using 100 W Mercury-Xenon Lamp (Hamamatsu, L2422-02) equipped with a band pass filter (FB440-10 from Thorlabs). The conversion rates are obtained from the disappearance of the progressive vinyl C=C stretching vibration band at 1630 cm -1 .

The 3D lithographic microfabrication was carried out using a Zeiss Axio Observer D1 inverted microscope. The two-photon excitation was performed at 800 nm using respectively a mode-locked Ti: Sapphire oscillator (Coherent, Chameleon Ultra II: pulse duration: ~140 fs; repetition rate: 80 MHz). The incident beam was focused through a 0.65 NA objective (40 X) which leads to a radial spot size 600 nm at  exc = 800 nm (1/e 2 Gaussian). A drop of the resin is deposited on a cover slip which is mounted on a 3D piezoelectric stage allowing the translation relative to the laser focal point. The intensity of the incoming laser is controlled with the use of an acoustooptic modulator. The displacement of the sample and all photonic parameters (i.e. excitation power and irradiation times) are computer-controlled. This entire lithography set-up was purchased from Teemphotonics Company.

The theoretical absorption spectra have been computed based on Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT). The overall computation strategy was defined as follows: After initial AM1 optimization calculations (vacuum), subsequent optimization of geometrical structures of the derivatives were carried out using the PBE0/6-31G(d) level of calculation. (Note that PBE0 is a good functional for the description of photofunctional molecules 48 ). Finally, the TDDFT vertical transitions have been computed using the same level of calculations. All calculations have been performed using GAUSSIAN 09 package. [START_REF] Frisch | Gaussian 09, Revision B.01[END_REF] X-ray powder diffraction (XRD) measurements were performed on a D2 Phaser Bruker AXS equipped with a Linxeye detector using a Ni-filtered CuKα radiation (λ=0.15406 nm).

XRD was operated at an accelerating voltage of 30 kV and a current of 10 mA with the 2θ angle ranging from 5° to 40° at the scanning speed of 0.8°/min (0.02° intervals). Refractive indices were measured at room temperature using a Prism Coupler (Model: 2010/M, Metricon) at three different wavelengths (λ=632.8, 1320 and 1550 nm).

at 7 .

 7 85 and 6.91 ppm with a coupling constant of 15.5 Hz are consistent with an oligomer having double bonds with an E configuration. No signal corresponding to an aldehyde function is observed but a singlet at 2.44 ppm corresponds to methyl groups borne by a pyrimidine ring indicating the nature of the end groups of the oligomer. Comparison of the integration of this signal with the integration of the signal of one of the hydrogen of the double bond at 7.85 ppm indicates an average number of repeat motifs of 13.6, which corresponds to an average molecular weight of around 7000 g.mol -1 . Size exclusion chromatography analysis were also carried out giving a number-average molecular weight M n = 4650 g.mol -1 and a polydispersity indice PDI = 1.63 (M w = 7200 g.mol -1 ). Therefore the oligomer Og corresponds to a recursive association of around 8 to 10 M-units (see Scheme 1).

  Properties. The fully optimized structure of the model chromophore calculated at DFT level is presented in Figure 1A. The core of the dye which corresponds to a 4,6-bis(styryl)pyrimidine subunit adopts a quasi-planar conformation. The sterical hindrance around the external nitrogen atoms leads to a twist angle of about ~ 34° around the CN bonds connecting the diphenylamine and styryl moieties. Moreover all external phenyls are tilted of about ~ 65° with respect to the central plane. Therefore the geometry of M can be presented as a successive alternation of planar and highly twisted domains. Such a structural alternation can be generalized when extending the molecular dimensionality. For instance, Figure 1B displays the fully optimized structure of an oligomer associating three M-units. Note that methoxy substituents have been implemented in position 2 of each pyrimidine to demonstrate that this substituent hardly influences the planarity of the 4,6-bis(styryl)pyrimidine fragments. The oligomer clearly exhibits a helical shaped geometry which globally preserves the structural characteristics of each repeat motif: i) a quasi-planar 4,6-bis(styryl)pyrimidine subunit ii) the interplane angles between two adjacent fragments have an average value of ~ 63° iii) the twist angles of the connecting CN bonds slightly increases by about 3°. Hence increasing the number of repeat

Figure 4 .

 4 Figure 4.1PA (full lines) and 2PA spectra (plotted against half of the excitation wavelenghth) of chromophores in dichloromethane.

Figure 5 .

 5 Figure 5. Real-time FTIR kinetics conversion curves for visible-curing at 440 nm of diacrylate resins with 'HABI' : HABI : 1.5 wt % , 'HABI+MDEA' : HABI / MDEA : 1.5 wt % / 3 wt %, "M' HABI / MDEA / M : 1.5 wt % / 3 wt % / 0.12 wt %, "Og' : HABI / MDEA / Og : 1.5 wt % / 3 wt % / 0.07 wt %. Inset : Absorption spectra of the corresponding diacrylate resins (25-µm-thick films).

Figure 5 ) 1 PS 1  4 .

 5114 Figure5) which leads to a larger amount of primary excited species. The photosensitizing process in presence of PS typically corresponds to a sequential reaction34b,35b,37b-c,40 starting from a photoinduced electron transfer from the singlet excited state of PS ( 1 PS) to HABI

42

 42

Figure 6 .

 6 Figure 6. Polymer lines fabricated at various laser exposure powers ( exc : 800 nm,  exp = 10 ms) using diacrylate resins with HABI/MDEA : 1.5 wt % / 3 wt % and containing 2PA sensitizers Og ( c = 0.09 mM) and M ( c = 2 mM).

Figure 7 .

 7 Figure 7.SEM image of a µ-structure fabricated upon excitation at 800 nm of the diacrylate resin with Og as 2PA sensitizer. Inset: 2D model master.

  The starting materials were acquired from Sigma-Aldrich or Acros and used without purification. In air and moisture-sensitive reactions, all glassware was flame-dried and cooled under nitrogen. All the solvents employed for absorption and emission analysis were Aldrich or Fluka spectroscopic grade. The absorption and fluorescence of all solvents were checked for impurities and have been subtracted from the sample spectra. Ebecryl 605, a difunctional monomer with a high viscosity ( ~ 1000 cps at 25°C), was purchased from Cytec. This viscous monomer used for one-and two-photon polymerization is a formulation based on bisphenol A epoxydiacrylate and tripropyleneglycol diacrylate (75/25 in weight). The 2,2"di(orthochlorophenyl)-4,4",5,5"-tetraphenyl biimidazole (HABI) was purchased from Tokyo Chemical Industry Company.NMR spectra were recorded in CDCl 3 , with a Bruker DRX500 spectrometer. ( 1 H at 500 MHz and13 C at 125 MHz) at the "Service Commun de Recherche de Résonance Magnétique Nucléaire et de Résonance Paramagnétique Electronique de l"Université de Bretagne

  CH 2 ), 22.7 (CH 2 ), 14.1 (CH 3 ). Anal. Calcd for (C 35 H 37 N 3 O) n : C, 81.52; H, 7.23; N, 8.15.

	Found C, 81.60; H, 7.54; N, 7.76.
	2.3 Instrumentation and Methods.
	The absorption measurements were carried out with a Perkin Elmer Lambda 2 double
	beam UV-vis spectrophotometer. Steady-state fluorescence spectra were collected from a
	FluoroMax-4 spectrofluorometer. Emission spectra are spectrally corrected, and fluorescence
	quantum yields include the correction due to solvent refractive index and were determined
	relative to quinine bisulfate in 0.05 molar sulfuric acid ( = 0.52). VH VV VH VV I g I gI I r . . 2    with HH HV I I g 
	1 H NMR spectra, the following
	abbreviations are used to describe peak pattern: s (singlet), d (doublet), dd (doublet of
	doublet), t (triplet), m (multiplet), In the 13 C NMR spectra, the nature of the carbons (C, CH,
	CH 2 or CH 3 ) was determined by recording the JMOD experiment. Elemental analyses were
	performed at the "Centre Régional de Mesures Physiques de l"Ouest" (CRMPO, University or
	Rennes 1) using a Thermo-Finnigan Flasch EA 1112 CHNS/0 analyser. Analytical thin layer
	chromatographies (TLC) were performed on silica gel plates (Merk 60F254) and compounds
	were visualized by irradiation with UV light at 254 and 365 nm. Flash column
	chromatography was performed using silica gel silica Acros SI 60 (60-200 mesh ASTM).

Average molar mass (M n ) and molar mass distribution (M w ) values of the oligomer were (

44

For the steady-state anisotropy measurements, two Glan-Thompson polarizers are placed in the excitation and emission beams. The anisotropy r is determined as follows:

ACKNOWLEDGEMENTS

We thank the meso-centre "EQUIPEX@MESO" of Strasbourg University for access to HPC resources. We also gratefully thank Dr Sophie Guillaume and Cedric Jaffredo, from Institut des Sciences Chimiques de Rennes for SEC analysis. Dr Christine Baudequin from Université de Rouen as well as Adrien Morrelec-Le Men and Benoît Quinton, students at IUT Lannion, are acknowledged for preliminary synthesis and optical measurements. Bresmat network is also acknowledged for funding.


N Doassans-Carrere

G Mauviel

J-H Ferrasse

J Lédé

O Boutin

BIOMASS TO OIL: FAST PYROLYSIS AND SUBCRITICAL HYDROTHERMAL LIQUEFACTION

Keywords: Pyrolysis, liquefaction, bio-oils, energy

The present abstract deals with the comparison of two biomass-to-oil processes: fast pyrolysis and subcritical hydrothermal liquefaction. Using the same biomass (beech sawdust), fast pyrolysis was led thanks to the cyclone reactor (wall temperature between 870 and 1040 K) and subcritical hydrothermal liquefaction thanks to a 150-ml-batch-reactor (temperature between 420 and 600 K).

Mass balances and analysis (ultimate analysis, HHV, pH, Karl-Fischer, gas chromatographies, H 1 NMR) allow the comparison of both processes and the characterization of the main fractions of pyro-oils (heavy oils, light oils and aerosols) and liq-oils (heavy oils and water soluble organics).

Introduction

The global energy demand is continuously increasing, due to the growth of both population and industrialization. The supply of CO 2 -neutral energies has to become more important in the future, and bio-fuels from biomass may be a part of those energies.

Different processes have been developed to produce bio-oils from different biomass, among them fast pyrolysis and subcritical hydrothermal liquefaction are studied here. Fast pyrolysis can produce until 75%wt of bio-oil (pyro-oil), composed typically of 56%wt of C, 38%wt of O and 6%wt of H [START_REF] Huber | Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering[END_REF]. Subcritical hydrothermal liquefaction, which consists in thermal decomposition of biomass in water in high pressure and temperature condition but below its critical point (647 K and 22.1 MPa), can reach 35%wt of bio-oil (liq-oil). This liq-oil, less oxygenated than pyro-oil, is composed typically of 74%wt of C, 17%wt of O and 9%wt of H [START_REF] Huber | Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering[END_REF]. Those two types of bio-oils must be upgraded to obtain bio-diesel, thanks to catalytic hydrodeoxygenation (HDO) which reduces the proportion of oxygen.

Experimental apparatus

In order to compare those processes, experimentations were led from the same biomass, beech sawdust. Fast pyrolysis is achieved thanks to the cyclone reactor [START_REF] Lédé | The cyclone: A multifunctional reactor for the fast pyrolysis of biomass[END_REF] which is a continuous reactor with an oil recovery system separating three fractions of pyro-oil. Up-stream of the cyclone, the installation is composed of the biomass (tank and endless screw) and nitrogen (inert gas) feeds. Particles, driven by nitrogen, are pyrolysed along the cyclone wall, heated by induction. A by-pass system allows preheating and equilibrium set-up before feeding of biomass. Thanks to the separator properties of the cyclone, solids are isolated from gaseous products and recovered in the solids collector. Gaseous products, evacuating through the upper part of the cyclone, pass through three water cooled heat exchangers in series to recover the first fraction of oil in a collector, "heavy oils" (HO).

Then the gas flows through refrigerated coils inside which the "light oils" (LO) fraction is recovered. In order to recover the last fraction, "aerosols" (AE), both electrostatic and membrane filters are used in series. At the exit, clean gases are finally sampled and analyzed by micro-GC. Subcritical hydrothermal liquefaction is achieved in a 150 ml autoclave where biomass is introduced prior to the catch of the reactor with the locking straps. Thanks to a volumetric pump, nitrogen or distilled water can be introduced into the autoclave at a desired pressure. After inerting the reactor with nitrogen and setting the initial working pressure, 100 ml of distilled water are added into the reactor, heated by an electric resistance. Once achieved, the working temperature is maintained at the experiment duration time, then the reactor is cooled down by a refrigerating system using air and then water. Once the reactor at the ambient temperature, gases are sampled and analyzed by GC-TCD. The reactor is opened in order to collect the products (liquids and solids).

Based on [START_REF] Sun | Analysis of liquid and solid products from liquefaction of paulownia in hotcompressed water[END_REF], pure acetone is used to rinse the reactor and wash solids. After filtration, three phases are obtained: solids, aqueous phase and organic phase. Solids are placed into a drying oven. Evaporation at reduced pressure is used to remove acetone from the organic phase and water from the aqueous phase, in order to get respectively two fractions of oil, "heavy oils" (HO) and "water soluble organics" (WSO).

Results and discussion

In order to confirm and complete previous works on the cyclone [START_REF] Lédé | Properties of bio-oils produced by biomass fast pyrolysis in a cyclone reactor[END_REF], new experimentations were led at different temperatures (870-1040 K) and different ratios between nitrogen and biomass mass flow rates (7.3-18.8). These experiments show the presence of maximum mass yield of pyro-oil near 65%wt (including HO, LO and AE) with 20%wt of gas and 15%wt of char. This maximum is observed near 920 K, with a ratio of 18.8, and explained by a partial conversion of biomass below this temperature and a too high gas production above it. Above 1040 K the gas yields are higher than pyrooil ones; process is rather called pyro-gasification than pyro-liquefaction.

For subcritical hydrothermal liquefaction, temperature influence was studied within a range of 420 to 600 K, for 15 to 60 minutes at the working temperature. Using this installation, near to 40%wt of liq-oils (including HO and WSO) were obtained, at 570 K, with 45%wt of gas and 15%wt of char. The first similitude with fast pyrolysis is the existence of an optimal temperature to reach high conversion of biomass with a minimum of gas mass yield.

Ultimate analysis and HHV-measurement are made on solids (biomass, mild-reacted biomass and char) and fractions of pyro-oils and liq-oils. First, those analyses allow confirming the use of a correlation Dulong-like between HHV (Higher Heating Value) and ultimate analysis. Combined with mass balances, ultimate analysis led to atomic balances close to 92% for C, 97% for H and 108% for O. Then, ultimate analysis is very useful to compare products and especially fractions of pyrooil and liq-oil: liq-oil HO are closer to pyro-oil AE while liq-oil WSO are closer to pyro-oil HO and LO. Ultimate analysis of solids confirms that the existence of pyro-oils maximum yield is partly due to the partial conversion of biomass: the composition of solids recovered at 870 K (CH 1.26 O 0.57 ) is closer to initial biomass one (CH 1.48 O 0.72 ) than chars ones obtained at 920 K (CH 0.44 O 0.17 ) and 1040 K (CH 0.24 O 0.17 ).

Gas chromatographies and H 1 NMR allow identifying major compounds of bio-oils and/or chemical families like phenolic compounds, acids, sugars ... Other analyses like pH and Karl-Fischer allows a complete characterization of bio-oils.

Thanks to those results, energy balances of both reactors were made with a focus on the HHV fraction recovered in bio-oils. Besides, properties and major components of each kind of bio-oils allow understanding their behavior during the HDO step, with a focus on the required hydrogen, directly correlated to bio-oil oxygen content.

For future work, those results are used as a basis for energy and exergy analysis which must be achieved to ensure the viability of the two processes for the production of bio-diesel. Those analysis are led thanks to a commercial simulation software, in which different parameters can be studied like biomass humidity or bio-oil recovery system.


Simon Allier

Olivier Barais

Benoit Baudry

Johann Bourcier

Erwan Daubert

Franck Fleurey

Martin Monperrus

Hui Song

Maxime Tricoire

Multitier diversification in Web-based software applications

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

Multi-tier diversification in Web-based software applications

Simon Allier 1Olivier Barais 1,3 Benoit Baudry 1 Johann Bourcier 1,3 Erwan Daubert 1 Franck Fleurey 4 Martin Monperrus 1,2 Hui Song 4 Maxime Tricoire 1 (1) Inria, France (2) University of Lille, France (3) University of Rennes 1, France (4) Sintef, Norway

Reuse! Modularity! Among all computing domains, Internet Computing has long been an example of the application of those two software engineering mottos.

Web applications are formed from reusable components all over the software stack. Both on the client and the server side, web-specific libraries and frameworks enable creative developers to wrap up rich applications within very short time. Massive reuse happens both on the client side (on the browser) and on the server side (in the data-center). For instance, jQuery is a popular client-side library written in JavaScript, which drastically improves the development of responsive web applications. Spring is an example of a server-side technology which is used on millions of servers. The open-source philosophy and ecosystem is one of the backbones of this massive reuse in web applications.

In addition some very modular web applications have become extremely successful. For instance, according to our empirical data, more than 100k+ websites use Wordpress, a blogging and content management system (cf. Sidebar 2). The architecture of Wordpress is extremely modular. Every single point of the application can be tweaked or extended through "plugins" in an easy way. Combined with a high-quality documentation and a vibrant community, this results in thousands of plugins that can be instantly installed on any server running Wordpress.

Application monoculture

Reuse and modularity are key for liberating creativity and entrepreneurship in the Internet world. However, this bright world has a darker side. The problem is that they participate in creating a new form of massive-scale monoculture. This is what we discuss and address in this paper. 0 BB, FF, MM wrote the paper; OB, BB, JB, FF designed the MDMS application; SA, OB, JB, ED, FF, HS, MT all participated in the development, the deployment and the diversification of the MDMS web application What is monoculture in computing? The concept of software monoculture refers to a computing environment that is largely dominated by the same software application [START_REF] Stamp | Risks of monoculture[END_REF]. For instance, the Windows operating system has long been considered as a monoculture on desktop machines. The term "monoculture" comes from agriculture, where it has been shown that exploiting the very same species on large areas is a bad practice. Similarly, "software monoculture" has a negative connotation [START_REF] Stamp | Risks of monoculture[END_REF]. The main problem of software monoculture are so-called BOBE attacks: break once, break everywhere. With a large monoculture, attackers can exploit flaws and common failure modes on a massive scale.

Operating systems (OS) and database monoculture is known for a long time [START_REF] Lorge | Which is riskier: OS diversity or OS monopoly?[END_REF], both are key components of the general computing infrastructure. However, Internet Computing has introduced a new kind of monoculture, which could be called "application monoculture". The novelty is that monoculture appears in application-level code (libraries, frameworks, the application itself), where developers rely on leaky abstractions and are more concerned about time-tomarket and useful features for their clients than securing their code This monoculture might present even more risks than the monoculture of operating systems, which are developed over long periods of time, with security concerns in mind. Let us consider the example of the content management application Wordpress. In the top 500.000 web sites 1 , we found 106.412 sites running Wordpress. Among these sites, 65.558 (64%) use the Akismet plugin, which checks potential spams in Wordpress comments. 21.849 web sites (22,6%) use the Jetpack plugin, which has a known SQL injection vulnerability, even in the latest version. This demonstrates two levels of application monoculture: at the level of the application (Wordpress), and at the level of plugins. A single attack on a zero-day flaw on Wordpress is able to compromise thousands of websites.

On the one hand, reuse and modularity favor much writing the next killer application. On the other hand, reuse and modularity facilitates much the next massive BOBE attack. There is a tension between reuse and dependability. Should we give up one or the other? Not necessarily.

In this paper, we propose using software diversification in multiple components of web applications to reconcile those two aspects. We identify key enablers for the effective diversification of software, especially at the application-code level. Our vision is that it is possible to combine different software diversification strategies, from the deployment of different vendor solutions, to fine-grained code transformations, in order to provide different forms of protection. We report on an innovative diversification experiment consisting in injecting multi-tier software diversity in a prototypical web application.

Multi-tier software diversity

A web application is typically composed of server-side and client-side code working in concert. The client side code is mostly written in JavaScript and runs in a browser. The server side of a web application is a software stack composed of an operating system and a web server, a set of libraries, frameworks, and application-specific code.

Software diversity has long been promoted to enhance software dependability. Since the seminal work of Cohen [START_REF] Frederick B Cohen | Operating system protection through program evolution[END_REF] and Forrest [START_REF] Forrest | Building diverse computer systems[END_REF], there has been significant efforts towards automatic software diversification (see Sidebar 1 for more details). Current automated software diversification techniques operate at assembly-code level and are meant to mitigate memory safety vulnerabilities (e.g., stack overflow). Some of these techniques are successfully implemented in mainstream operating systems, making each operating system installation different from the others and thus mitigating the massive reuse of exploits. For example, address space layout randomization is implemented in all recent versions of Windows, Linux, Mac OS.

We believe that software diversification must address software layers beyond assembly code, to face the emergence of a new form of monoculture. The novelty of our proposal, with respect to the software diversity state of the art, is to diversify the application-level code (for example, diversify the business logics of the application), focusing on the technical layers found in web applications. In particular, web server deployment usually adopts a form of the Reactor architecture pattern, for scalability purposes: multiple copies of the server software stack, called request handlers, are deployed behind a load balancer, which dispatches all incoming requests. Currently all handlers are deployed as clones, but this Sidebar 1: Software Diversification Cohen [START_REF] Frederick B Cohen | Operating system protection through program evolution[END_REF] described 14 code diversification techniques to be combined for protecting operating systems. Forrest and colleagues [START_REF] Forrest | Building diverse computer systems[END_REF] emphasized the need for building diverse computing systems and suggested diversification based on code manipulation. Since these seminal works, several approaches have implemented automatic diversification transformations, at machine-code level, and have combined them to increase the dependability of software systems. Each kind of transformation targets a specific kind of vulnerability, and several work have started to combine them to get a more complete protection. These 'ìntegrated" software diversity techniques are particularly interesting with respect to multi-tier diversification. We now have a brief look at some of them and their defense objectives. Bhatkar et al. [START_REF] Bhatkar | Address obfuscation: an efficient approach to combat a broad range of memory error exploits[END_REF] thwart code injection attacks by integrating various forms of randomization: randomize base addresses of memory regions to make the address of objects unpredictable; permute the order of variables in the stack; and introduce random gaps in the memory layout. Jacob et al. [START_REF] Jacob | The superdiversifier: Peephole individualization for software protection[END_REF] target tamper-resistance, through superoptimization to identify semantically equivalent instruction sequences, and other transformations that change the set of instructions and operands. The GENESIS project mitigates return-to-libc attacks and code injection by implementing a virtual machine that integrates calling sequence diversity and instruction set randomization through software dynamic translation [START_REF] Williams | Security through diversity: Leveraging virtual machine technology[END_REF]. Wang et al. [START_REF] Wang | Protection of software-based survivability mechanisms[END_REF] obfuscate critical software modules in survivability infrastructures by combining control-flow flattening and introduction of aliases. For the readers interested in more references, we refer to the recent survey by Larsen and colleagues [START_REF] Larsen | Automated software diversity[END_REF], and to the different levels of moving target defenses summarized by Okhravi and colleagues [START_REF] Okhravi | Finding focus in the blur of moving-target techniques[END_REF].

kind of architecture provides a natural setting for diversification.

We call multi-tier diversification the fact of diversifying several application software components simultaneously. This approach can rely on both natural software diversity and automatic diversity. Natural software diversity designates diverse software modules that provide equivalent functionalities but that are developed by different communities or companies. For example, there is a natural diversity of Java Virtual Machines. This can be exploited through the simultaneous deployment of some request handlers that run the IBM JVM and others that run the Oracle JVM. Application-level automatic diversity is provided by code transformations that generate diverse versions of some application components. Examples include method body intermixing [START_REF] Frederick B Cohen | Operating system protection through program evolution[END_REF], randomization of database query language [START_REF] Boyd | Sqlrand: Preventing sql injection attacks[END_REF] or our recent work on the synthesis of sosie programs (variants that are functionally similar but computationally diverse) [START_REF] Baudry | Tailored source code transformations to synthesize computationally diverse program variants[END_REF].

"Multi-tier software diversification is a way to break the multi-level monoculture of web applications."

The diversification of application software code is expected to provide a diversity of failures and vulnerabilities in web server deployment. Thanks to the multiplicity of request handlers running in a web server, we can simultaneously deploy multiple combinations of diverse software components. Then, if one handler is hacked or crashes, the others should still be able to process client requests. The natural diversity of some components, exhibits diverse failure modes. For example there exist vulnerabilities that are present in the Oracle JVM only (CVE-2014-4244 and CVE-2014-2490). Meanwhile, automatic diversity can produce large quantities of local changes in the code, which affect specific kinds of vulnerabilities. For example method bodies merging changes the layout of the binary code, preventing the reuse of a single buffer overflow attack, or monkey patching; SQL queries randomization mitigates SQL injections; in the source code it is possible to diversify filesystem paths or urls to prevent certain forms of settings hijacking; sosies, which modify the flow of computation, can modify vulnerabilities such as lack of input validation or business logic vulnerabilities.

Proof-of-concept: multi-tier diversity on a blogging system

As a proof of concept, we have diversified a prototypical web-application that uses common, off-the-shelf components. The application is a multi-user blog-Sidebar 2: Wordpress, a fragile monoculture? Wordpress is a Web content management system that supports massive customization through "plugins". While the vibrant Wordpress community keeps growing the number and the diversity of plugins for all possible tasks, we observe a paradoxical trend towards a monoculture of some popular plugins. This specific plugin monoculture is a potential threat for the Wordpress ecosystem, in particular when the plugins that are widely used have a defect. As an example of massive threat, we found that 22,6% out of a 100000 sites sample, use the Jetpack plugin that is known to be vulnerable to attacks. Jetpack provides users who deploy their own installation of Wordpress with features that are available on wordpress.com, such as advanced handling of social media or multimedia documents. Yet, it is possible to exploit, the natural diversity of Wordpress plugins to mitigate this threat. For example, it is possible to replace Jetpack with wp-symposium or disqus, which offer compatible functionalities with a completely different implementation. This example shows that, despite the emergence of monocultures in the Wordpress ecosystem, the community actively continues to develop diverse solutions. This provides fertile ground to experiment with the automatic diversification of Wordpress sites, by exploiting the natural diversity of functionally similar plugins, to break this fragile application monoculture. All results about the monoculture in Wordpress are publicly available 2 1a. This architecture reflects the characteristics of a large number of web applications (server side scripting language, server-side framework, NoSQL database).

In terms of deployment, we deploy MDMS following a typical Reactor pattern, to allow the application to elastically scale over time. MDMS is deployed on request handlers, which are arranged behind a load balancer. Following current practices, our initial deployment is as follows: all handlers provide the same RingoJS environment, running on top an identical Java virtual machine. This usual way of deploying the Reactor pattern results in a "non-diversified" deployment of the MDMS system, illustrated in Figure 1b. This is an example of what we call an "application monoculture": all servers behind the load balancer are clones, from the operating system level, up to the libraries and the application software. Now, we inject diversity in the MDMS system at the following levels: deployment infrastructure, virtual machine, JVM and JavaScript library. Diversification at each level exploits either natural diversity or novel techniques for automatic diversity.

OS diversification: MDMS has no dependencies to any particular operating system and a mix of different distributions of Linux, BSD and Windows can be used. In our experiments, MDMS is randomly deployed on Windows and Linux.

JVM diversification: The next layer is the Java virtual machine. There are 3 major suppliers of Java virtual machines (IBM, Oracle and OpenJDK) with several versions for each of them. While the OpenJDK and the Oracle JVM share the same code base, the latter has some built-in commercial or open source tools that are not integrated in OpenJDK. These differences are significant enough to have vulnerabilities that are present only in the Oracle one. In our experiments, RingoJS alternatively runs on Oracle-jdk1.7.0_45 (Sun/Oracle), IBM-Java-x86-71 (IBM), and Java-7-openjdk (GNU).

Framework diversification: The next layer on the stack is the RingoJS framework. This is an opensource component for which there is no competing functionally equivalent alternatives. To diversify this level, we use our recent technique for the automatic synthesis of sosie programs [START_REF] Baudry | Tailored source code transformations to synthesize computationally diverse program variants[END_REF]. A sosie is a vari-ant a program, which exhibits the same functionality (passes the same test suite) and a diverse computation (different control or data flow). Sosie synthesis is based on the transformation of the original program through statement deletion, addition or replacement. We synthesized 70 RingoJS sosies for MDMS using this new diversification technique.

Deployment diversification: Using the Cloud and its deployment interface, MDMS can be randomly deployed in 2 different data centers that are geographically distant (in Europe and in the US). In our experiments, we have used the Amazon cloud and a private cloud running LXC container. We use the CloudML (Cloud Modelling Language) platform [START_REF] Ferry | Towards model-driven provisioning, deployment, monitoring, and adaptation of multi-cloud systems[END_REF] in order to model and automatically deploy the application among different Cloud provider and operating systems.

In total we have: 2 versions of the operating system; 3 diverse JVMs; 70 Rhino sosies; 2 datacenters. Since the layers are mostly independent from each others, those alternatives for each layer can be combined together to create an exponential number of possible server deployments. For example our setup has the potential to run 840 different diversified web servers for delivering the MDMS application.

Then, we deployed 17 instances of the server stack behind a Nginx web server. In particular, we deployed 17 diverse versions of the RingoJS library, a functional component that is never diversified in other diversification approaches. We designed 8 functional test scenario for validating the global functionality of MDMS. We checked that these scenario actually execute the diversified part of RingoJS, i.e., that the modified statements in sosie programs is executed. All scenario pass correctly on the original, non-diversified server, as well as on the diversified server. Yet, some of the modified statements in RingoJS (added, removed or replaced) are executed thousands of times (max is 148000 times for a sosie in the setLP method that is used by the JS interpreter to handle switches). The diversified system is available for use at http://cloud.diversify-project. eu/. This result demonstrates the ability to generate large quantities of software diversity by combining natural and automatic diversity in a realistic webbased architectural setting.

Insights from application diversity

This initial proof-of-concept demonstrates the feasibility of multi-tier diversification in web applications. It also illustrates the challenges to operationalize this vision. We discuss three of them here.

Automatic diversification. This is the key ingre-dient for a true diversity radiation. The main factor for the 840 versions of MDMS handlers is provided by the automatic synthesis of 70 sosies. However, there exist few solutions for the automatic diversification of application-level source code, because it requires transformations that are on the edge between functional correctness and quality of service [START_REF] Okhravi | Finding focus in the blur of moving-target techniques[END_REF]. throw new I l l e g a l A r g u m e n t E x c e p t i o n ( ) ;

To explain the potential effects of these transformations, we look at the RingoJS sosies. All of them exhibit a difference in the control or data flow, with respect to the original RingoJS. Looking in more details, we observe different reasons for this diversification. An attribute can be assigned a different value than in the original program, yet this side effect has no visible impact on the application; some method calls can be removed, removing a complete part of the program's computation, yet the program still provides the service. Both these cases are explained either because the changes have no side effect (e.g., the variable is never used), or, more interestingly, because they occur in parts of the computation that tolerate variations, which we call plastic zones. These plastic zones can appear in algorithms that compute some form of heuristic or in redundant code. Plastic zones are interesting from a security point of view because they indicate zones in which some potentially vulnerable code can be removed. Code replacement has a different impact. For example, Listing 1 illustrates a statement replacement, which ends up adding an input validation. In general, such reduction of the input space, if it still provides the service, is good for security. Eventually, we need to understand how to navigate the functional neighborhood of programs in order to provide different degrees of application level diversity while maintaining functional compatibility.

Integrating multi-tier diversification in development processes. The second challenge of multi-tier diversification is about its integration in web software engineering practices, to master its impact and leverage the full potential for dependable web applications. First, we can deploy the diverse handlers in different ways. In our experiment we pick 17 different handlers out of 840 and deploy them, to provide a form of spatial diversity. Then the incoming requests are sent to one of the handlers, round-robin. Several other strategies are possible, for example: pick a single handler and deploy it 17 times; constantly deploy new versions of the handler, providing a temporal diversity to form a moving target defense [START_REF] Goldrich | Moving target [guest editors' introduction[END_REF]; use the diverse handlers in a multi-version fashion, with a voting mechanism. We now need to understand what strategy best fits a given dependability goal. Second, diversification has an impact on distribution and maintenance. For example, when the binary code of an application must be signed by a third-party, the production of millions of diverse variants becomes a challenge. One solution consists in diversifying the bytecode at installation time (after the signed version has been shipped), and have the transformation itself recognized as legitimate (and not detected as a malware). Another example, is dump trace analysis or incremental updates. This will require accurate traceability of variants and reversible code transformations, as well as new forms of code analysis for automatic patching.

Runtime environments. Our proof-of-concept implementation taught us that the integration of multiple levels of diversification poses several technical challenges. We need a reconfigurable load balancer, which can reason about the number of handlers it sends a request to, detect potentially vulnerable handlers and stop using them or decide to replace them; architectures that allow the composition of multiple levels of diversity; manage the application state consistency between the diverse handlers; support dynamic reconfiguration and deployment of software in a distributed infrastructure (VM, containers, software modules). The MDMS architecture has been designed to natively support multi-tier diversification. The diverse request handlers store data in a Redis database, a distributed nosql solution, instead of a file system. We have adapted the NGinx load balancer with specific distribution and recovery policies when one of the handlers fails. We experimented with both Kevoree 4 and CloudML 5 to manage the deployment of software modules on diverse and distributed 4 http://kevoree.org/ 5 http://cloudml.org/ virtual machines. Both frameworks provide utilities to seamlessly handle the heterogeneity of technologies for virtual machines (e.g., Vmware, VirtualBox), system containers (e.g.,docker, lxc, jails), app containers (e.g., servlet, android, osgi). They provide flexible configuration models with built-in architecture model exploration capabilities and the ability to orchestrate coherent and transactional reconfiguration of the platform, infrastructure and service levels. In the future, we plan to experiment also with Mesos to manage virtual machines or container deployment.

The end of monoculture?

We have emphasized the growing monoculture in web applications, which emerges from the success of a few frameworks and libraries. These reusable components are essential to support the engineering of large applications. Yet, they also come with potential drawbacks due to the massive distribution of bugs or vulnerabilities that is associated to monoculture. We propose to counter this phenomenon by extending software diversification beyond the machine-code level. The way to go is to diversify the different layers, up to the functional code. We have experimented with a realistic blog application to demonstrate the feasibility of multi-tier diversification. This experiment highlights the challenges that are ahead of software engineers if they want to systematically break the application monoculture of web applications. In particular, we believe that unsound program transformations open the way for the true explosion of application code diversity. May multi-tier diversification be the end of multi-tier monoculture!

Figure 1 :

 1 Figure 1: The MDMS architecture: (a) the standard MDMS software stack; (b) homogeneous deployment of identical MDMS configurations; (c) multi-tier diversified deployment of different MDMS configurations

according to http://www.alexa.com/

all the code for this experiment is available at http:// diversify-project.github.io/

Acknowledgement

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreements No. 600654, DIVERSIFY project, FP7-ICT-2011-9 and No. 611337, the HEADS project, FP7-ICT-2013-10.