Julio Fernandez-Cestau

Benoit Bertrand

Anna Pintus

Manfred Bochmann
email: m.bochmann@uea.ac.uk

Synthesis, Structures and Properties of Luminescent (C^N^C)gold(III) Alkyl Complexes: Correlation between Photoemission Energies and C-H Acidity

Keywords: Gold complexes, Pincer complexes, Metal alkyls, Photoluminescence, Crystal structure

The reaction of (C^N pz ^C)AuCl with C-H acidic compounds H2CR 1 R 2 in the presence of base readily affords the corresponding alkyl complexes (

) and (C^N py ^C)AuMe (8b), respectively. The crystal structures of 2a, 2b, 3a, 6a, and 7a have been determined. The photophysical properties of the new complexes and those of the previously reported gold hydride (C^N pz ^C)AuH (AuH) are reported. The lower-energy absorption and the emission maxima follow the energy sequence 2a < 3a < 4 < 5 < 6a < AuH ≈ 8a for the pyrazine series, and 2b < 3b < 6b ≈ 8b for the pyridine series. These values provide a correlation with the pKa values of the corresponding ancillary ligand precursors. In agreement with DFT calculations, the emissions are assigned to 3 IL(C^N^C)/ 3 ILCT {(C6H4 t Bu-4')→pz/py*} transitions, dominated by the HOMO and the LUMO orbitals. The LUMO is located in the heterocycle (py/pz) in trans position to the ancillary ligand, which makes this orbital more sensitive to the electronic nature of the ancillary ligand than the HOMO. The calculations establish that the charge transfer from the tBuC6H3 ligand fragments to the central heterocycle ring in the dominant transition explains the modulation of the properties with the σ-donor characteristics of the alkyl or hydride ligands.

Introduction

Cyclometalated Gold(III) complexes, particularly those based on cyclometalated bidentate 2arylpyridine (C^N) and tridentate 2,6-diarylpyridine ligands (C^N^C) have proved to be highly successful ligand platforms in Au(III) chemistry, since they impart good thermal stability and protect the gold(III) centers against reduction. 1 Complexes of this type also exhibit rich photophysical properties that form the basis for application as sensors, bioimaging agents and as emitter materials in organic light-emitting diodes (OLEDs). 2 However, in (C^N^C)AuY complexes the presence of 3 LMCT (ligand-to-metal charge transfer) and 3 dd excited states can provide effective deactivating pathways for the photoemissions in these systems. 3,4 This has limited the range of photoluminescent complexes almost exclusively to those with strong carbon-based σ-donors in the ancillary positions, such as Y = alkyl, aryl, alkynyl, cyanide or Nheterocyclic carbenes. [5][6][START_REF] Blacque | For luminescent gold(III) C^N chelate complexes see: (a) Garg[END_REF] These strong σ-donating ligands push the gold 5d σ* orbital to higher energies, so that the emissive 3 IL(C^N^C) transitions centered on the pincer π-system become accessible. The colour of the emission is determined by the electronic properties of the cyclometallated ligand. The successful modulation of the photoluminescence (PL) wavelengths by ligand design and varying the combination of substituens has also been reported. 8,9 However, with this approach the synthesis protocol of each complex has to be optimized individually, including the time consuming optimization of the mercuration and transmetallation steps.

In some of these cases, and depending on the ancilliary ligand Y, a contribution of ligand-toligand charge transfer (LLCT) to the emission has been also observed. This is particulary true for the extensively studied family of cyclometalated (C^N py ^C) gold alkynyl complexes. 10,11 A wide range of alkynyl substituents has been used to modulate emission energies. 12 However, the success of this strategy has been limited for two main reasons: (a) if the emission of the system is purely centered on the cyclometallated ligand, the presence of different substituents on the alkynyl ligand has little effect on the relative energy of the C^N^C centered orbitals; and (b) if there is a relevant LLCT (C^N^C → C≡CR) contribution, it becomes difficult to predict the colour of the emission on the basis of the electronic properties of the substituents of the alkynyl ligands, as many other factors affect the transition energy.

We recently demonstrated that the pyrazine-based (C^N pz ^C)Au III Y pincer complexes show a wider reactivity than the pyridine analogues, since Y may readily be varied under mild conditions. Also, Y = aryl, alkynyl or NHC donors are not necessarily required in order to generate photoemissive materials; e.g. for the pyrazine series even the chloride is photoemissive at room temperature. 13,[START_REF] Au ; Kumar | III) halide complexes based on C^C^N pincers also show phosphorescence at room temperature[END_REF] Beyond photoluminescence, (C^N pz ^C)Au III compounds are of interest as anti-tumor agents. 15 A remarkable case for the difference in PL of pyridine-vs. pyrazine-based (C^N^C) gold complexes are the thiolates, (C^N^C)AuSR, which are strongly photoemissive whereas their pyridine analogues are non-emissive at room temperature. The photoluminescence of (C^N pz ^C)AuSR was due to 3 IL(C^N pz ^C) emissions which, unusually, proved to be modulated by pyrazine-pyrazine aggregation leading to bimolecular emissive states. 16 As an extention to these studies, we became interested in the luminescence behavior of gold(III) alkyl complexes and report here the synthesis and PL properties of a family of compounds

(C^N pz ^C)AuY [2a, Y = CH(CN)2; 3a, Y = CH{C(O)Me}2; 4, Y = CH(CO2Et)2; 5, Y = CH{C(O)Ph}2; 6a, Y = CH2C(O)Me; 8a, Y = Me; AuH, Y = H]
and the analogous pyridine-pincer complexes 2b, 3b, 6b and 8b (C^N pz ^C = [2,6-bis(4'-t-BuC6H3)2pyrazine] 2-; C^N py ^C = [2,6-bis(4'-t-BuC6H3) 2pyridine] 2-). The synthesis can be carried out under mild conditions and offers a good alternative to the alkynyl and NHC ancilliary ligands used to activate photoluminescence in cyclometallated Au(III) systems. The emission energy can be modulated and predicted by the double strategy of altering the heterocyclic core of the cyclometallated ligand, as well as by fine-tuning the electronic properties of the Y ligand. 17

Results and Discussion

Synthesis and Characterization. Gold(III) alkyl complexes bearing pyrazine-and pyridine-based C^N^C pincer ligands were prepared from C-H acidic hydrocarbons in the presence of base. In all cases C-coordinated complexes were obtained. The activation of (C^N^C)AuCl with potassium tert.butoxide in toluene and subsequent reaction with an excess of hydrocarbons H2CR 1 R 2 proved quite facile and general and follows a procedure reported previously for the synthesis of (C^N pz ^C)Aupyrazole (Scheme 1). 18 While potassium carbonate proved to be a strong enough base to generate 6a, formation of the pyridine complex 6b required a stronger base, KO t Bu, which demonstrates the higher reactivity of the pyrazine complex 1a compared to its pyridine analogue 1b, a feature previously observed in ligand substitutions with thiols. 16 On the other hand, less acidic hydrocarbons like ethyl acetate and acetonitrile proved unreactive even in the presence of KO t Bu for both systems.

Scheme 1: Synthesis of complexes 2 -6, showing the numbering scheme used for NMR assignments.

The formation of these compounds was confirmed by 1 H, 13 C{ 1 H} NMR and IR spectroscopy techniques. All 1 H NMR spectra showed signals for H 8 in the region of δ 7.9. Complexes 2-4 show singlets integrating for one proton for the alkyl-CH moiety (H 10 ) at δ 3.64 (2b), 3.68 (2a), 4.69 (3b) and 4.96 ppm (3a and 4). While both methyl groups appeared equivalent in the 1 H NMR spectra of both 3a and 3b, as indicated by a single resonance integrating for six protons at δ 2.29 and 2.31, respectively, in the 1 H NMR spectrum of 4 the ethyl-CH2 groups are inequivalent, giving rise to a multiplet at δ 4.09, instead of a quadruplet, as was previously found in case of Pt(II) complexes. 19 In the infrared spectra of 2 -4, the respective CN and C=O stretching frequencies are observed at 2237 cm -1 (2a), 2233 cm -1 (2b), 1677 cm -1 (for 3a and 3b), and 1737 and 1732 cm -1 (4). The presence of two C=O bands in the IR spectrum of 4 confirms the inequivalence of the ester moieties observed in the 1 H NMR spectrum.

Complexes 5, 6a and 6b, show singlets integrating for two protons corresponding to H 10 , at δ 3.65 (5) and 3.11 (6a and 6b). The compounds show intense C=O vibrations at 1654 cm -1 and 1674 cm -1 for 5 and 6a/b, respectively. Crystals suitable for X-ray diffraction were grown for complexes 2a, 2b, 3a, 6a and 7a. There are three different molecules of 3a in the asymmetric unit. One of them is depicted in Figure 1b, and selected distances and angles are given in the legend. The other two molecules show similar structural parameters. The metal center has a square-planar environment, as expected for Au(III), distorted to accommodate the strained cyclometallated ligand. The fourth coordination site is occupied by the acetylacetonate. Unlike R2Au(acac) complexes (R = Me, aryl), where the pentanedionato ligands are O-bonded, 20 the structure of 3a confirms the coordination through the σ-C bond of the central carbon.

The Au-C25 distance (2.078(5) Å) is in the range of the observed for other complexes with Au(III)-C(sp 3 ) bonds. 21,22 The main difference between the three independent molecules in the asymmetric unit cell is the N1-Au-C25-H25 torsion angle, which is 25 for molecule 1 (Figure 1b), and 24 and 58 for the other two molecules.

The disposition of the acetylacetonyl ligands in these molecules is determined by the requirement to accommodate intermolecular interactions in the solid state. These interactions are mainly O•••H hydrogen bonds that involve the oxygen atoms of the acetylacetonyl groups and H atoms of neighboring molecules. In addition, the crystal lattice shows the presence of large voids, which are occupied by hexane molecules, with positional disorders so that they could not be included in the model. The analysis with PLATON reveals a volume of 591 Å 3 for this void. Using SQUEEZE to address these disorder problems resulted in two hexane molecules in the unit cell.

The crystal structure of 6a is depicted in Figure S2, and selected distances and angles are given in the legend. The structural parameters are very similar to those of complex 3a, including the Au-C25 distance of 2.058(3) Å, typical for Au-C alkyls.

In the crystal structure of 2a the first feature to note is the existence of a positional disorder in the carbon atom of the CH(CN)2 ligand. This atom is equally positioned in two very similar locations that have been defined as C25A and C25B. The structural parameters are very similar between both; in Figure 1a we show the one corresponding to C25A. For both malonitrile complexes 2a (Figure 1a) and 2b (Figure S1) the reduced donor character of the [CH(CN)2] -ligand is reflected in the relatively long Au-C(alkyl) distances of 2.11( 2) Å (2a) and 2.084( 4) Å (2b), compared to an Au-Et distance of 2.042(8) Å in (C^N py ^C)AuEt. To extend the series of alkyl subsituents to more strongly electron-donating alkyls we synthesized the methyl derivatives 8a and 8b by reacting the corresponding trifluoroacetate complexes 7a and 7b with MeMgBr in THF, by adaptation of previously reported protocols (Scheme 2). 22,23 The X-ray structure of 7a (see SI, Figure S3) is very similar to the one previously reported for the pyridine analogue. Compared to the chloride 1a, complex 7a is characterized by a shift in the 1 H NMR signal of the H 8 proton by Δδ -0.48 ppm. The IR spectrum shows the typical C=O band of trifluoroacetate at 1727 cm -1 . The formation of complex 8a was confirmed by 1 H NMR spectroscopy, the data resemble those of the previously reported complex 8b. 22 Photophysical Properties. The photophysical properties of the complexes are conditioned by the cyclometalated ligand. The most relevant feature in the UV-Vis absorption spectra is the low energy band characteristic of this type of complexes and typically attributed to 1 IL(C^N^C) transitions (Table 1).

The pyrazine complexes are significantly more strongly emissive than their pyridine analogues.

In polymeric media the complexes show emission profiles that resemble those observed in the solid state, but with narrower bands, higher intensities and better resolution. The photophysical properties of the complexes in the solid state and in dichoromethane solution are collected in Table 2, those in poly(methylmethacrylate) (PMMA) containing 10 wt-% Au compound are given in Table 3. The emissions of 2b and 3b in a PMMA matrix were too weak to be measured. Both the low-energy absorptions and the emissions of the pyrazine complexes are red-shifted compared the pyridine analogues, as expected. This shift reflects the differences in the π-π* energy gap, which is 0.95 eV lower for pyrazine than for pyridine. 24 The emissions are assigned to 3 IL(C^N^C) transitions. This is in line with the C^N^C vibronic progression of the spectra in all cases, which is similar in energy to the vibronic progression of the low energy absorptions.

When comparing complexes of the same cyclometalated C^N^C ligand, the wavelengths of the low energy absorption band follow the trend: 2a < 3a < 4 < 5 < 6a < 8a in the pyrazine series. This parallels the pyridine series: 2b < 3b < 6b < 8b (see Figure 2a). The emission maxima are similarly affected by the electronic nature of the ancillary ligand Y. In the solid state the shift is not clearly noticeable, presumably due to the fact that the bands do not show a good spectral definition. However, in CH2Cl2 glass at 77 K the emissions show the same trend as the absorptions: 2a (548 nm) < 3a (540 nm) < 4 (532nm) < 5 (528 nm) < 6a (524 nm) < 8a (520 nm) (Figure 2b). The recently reported 25 gold(III) hydride complex (C^N pz ^C)AuH (AuH) also fits into this series.

AuH in PMMA (10 wt-%) shows an intense green emission due to a 3 IL(C^N pz ^C) transition, with a high quantum yield (10.3 %) (Figure 3). This is, to the best of our knowledge, the first case of a gold hydride complex (in any oxidation state) showing photoluminescent properties. These absorption and emission data show that there is a correlation of the low-energy absorption bands and of the emission energies (in CH2Cl2 at 77K and in PMMA at room temperature) with the pKa values of the alkyl ligand precursors in DMSO and of H2 [pKa = 11.1 (malodinitrile; 2a/b), 13.3 (acetylacetone; 3a/b), 16.4 (diethylmalonate; 4), 24.7 (acetophenone; 5), 26.5 (acetone; 6a/b), 36 (AuH)] (Figure 4). As will be discussed in the computational section, this is a reflection of the fact that the LUMO is more destabilized in energy compared to the HOMO, in response to the electrondonating properties of the ancillary ligands. On the other hand, increasing the pKa further, to 56 (CH4) as in compounds 8a/b, does not result in significant further shifts in the absorption and emission maxima. In CH2Cl2 solution at room temperature the situation is more complex. As can be seen in Figure 5 for some illustrative examples, the emission is the result of the sum of two components: the The energy of the 3 IL(C^N^C) band in fluid media also follows the trend of the electronic properties of the ancilliary ligands found in PMMA and in frozen CH2Cl2 at 77 K (for example 528 nm 2a < 520 nm 5 < 511 nm 8a). As mentioned before, the pyridine series shows higher energy emissions compared to the pyrazine analogues. This is true not only for the triplet emissions (for example 511 nm 8a vs. 471 nm 8b) but also for the higher energy 1 IL(C^N^C) transitions (484 nm 2a vs. 440 nm 2b) and further supports this assignment. (C^N pz ^C)Au{CH(C(O)Me)2} 3a 293 (19.5), 319 (15.5), 340sh (9.0), 399 (4.0), 419 (6.2), 442 (6.2).

(C^N pz ^C)Au{CH(CO2Et)2} 4 286 (19.5), 317 (13.1), 340sh (7.8), 397 (4.1), 418 (6.0), 442 (5.8).

(C^N pz ^C)Au{CH2(COPh)} 5 286 (18.2), 320 (12.8), 339sh (8.0), 395 (4.2), 415 (5.6), 435 (5.4).

( Theoretical Calculations. Computations have been performed at the density functional theory (DFT) level with the aim of rationalizing the correlation of the low energy absorption and the emission maxima with the electronic properties of the ancilliary ligand. To reduce the complexity of the systems, the t Bu substituents of the cyclometallated ligands have been omitted. We refer to this with an asterisk next to each complex number. Details of the calculations can be found in the SI.

These calculations establish that the lower energy absorption, related to the S0→S1 transition, can be assigned in all cases to an almost pure HOMO-LUMO monoelectronic excitation (see Table S15). As can be seen in Figures S6 andS7, in all cases the HOMO is localized in the pincer with only a small contribution from the Au center. The LUMO, in contrast, is mainly centered on the heterocyclic central ring (see Figure 6 for an illustrative example for 2a*).

Figure 6. Frontier molecular orbitals of complex 2a*.

Both frontier molecular orbitals are raised in energy as the electron-donating character of the σbonded ligand Y increases (Table 4). However, since the LUMO is centered on the central heterocycle (pyridine or pyrazine) the ligand trans to this ring has a strong influence on its energy. The LUMO is therefore destabilized to a larger extent than the HOMO, resulting in an increased HOMO-LUMO gap with increasing electron-donating character of Y (for example 4.14 eV 2a* vs. 4.20 eV 6a*). -6.92 -6.80 -6.78 -6.75 -6.74 -6.72 -6.68 -6.76 -6.64 -6.58 As can be seen in Figures 7a and7b, for both the pyrazine and the pyridine series there is a linear correlation between the experimental lowest energy absorption band of the complexes in CH2Cl2 with the calculated HOMO-LUMO gaps. Given that, as mentioned above, the S0→S1 transitions were found to be almost pure HOMO-LUMO excitations, a similar correlation is found between the experimental absorptions and these transitions (see Figures S8 andS9). While these calculations work provide a good model for such subtle chemical modifications, we did notice a general tendency to slightly overestimate (by about 8%) the energy of the S0→S1 transition compared with the experimental absorption bands; point checks showed that this overestimation is reduced to ~ 5% if the tBu substituents are included in the model. The change of the heterocycle in the pincer ligand defines the energy band of the emissions but, for a given pincer, the emission color can be fine-tuned by changing the electronic properties of the alkyl ligand. These subtle changes in emission energies can be rationalized on the basis of a simple chemical parameter; the pKa values of the hydrocarbons HY. The (C^N pz ^C) gold hydride was also shown to be photoluminescent and fits within the pKa series. The effect is related to the trans influence of the Y ligand and the fact that the LUMOs in these complexes are mainly located on the heterocycle trans to Y, which raises the LUMO level and increases the HOMO-LUMO gap.

HOMO

Experimental

When required, manipulations were performed using standard Schlenk techniques under dry nitrogen or using an M. Braun glove box. Nitrogen was purified by passing through columns of supported P2O5 with moisture indicator, and of activated 4 Å molecular sieves. Anhydrous solvents were freshly distilled from appropriate drying agents. Infrared spectra were recorded using a Perkin Elmer Spectrum 65 FT-IR spectrometer with a diamond ATR attachment. Elemental analyses were carried out at London Metropolitan University. 1 H, 13 C{ 1 H} and 19 F spectra were recorded using a Bruker Avance DPX-300 spectrometer. CDCl3, CD2Cl2 and DMSO-d6 were dried over CaH2, degassed by three freeze-pump-thaw cycles and stored on 4 Å molecular sieves prior to use. 1 H NMR spectra (300.13 MHz) were referenced to the residual protons of the deuterated solvent used. 13 C{ 1 H} NMR spectra (75.47 MHz) were referenced internally to the D-coupled 13 C resonances of the NMR solvent.

19 F{ 1 H} NMR spectra (282.38 MHz) were referenced to CFCl3. Complexes 7b and 8b were synthesized by modifiying reported procedures. 22 The assignment of the signals for the C^N^C ligand framework is as shown in Figure 8. Compounds 2 -6 were synthesized without exclusion of air and moisture, compounds 7 and 8 were prepared under dry nitrogen.

(C^N pz ^C)Au{CH(CN)2} (2a)

A mixture of (C^N pz ^C)AuCl (50 mg, 0.087 mmol), malodinitrile (57 mg, 0.87 mmol) and KO t Bu (15 mg, 0.13 mmol) in toluene (5 mL) was stirred overnight at room temperature. The solvent was evaporated and the yellow solid residue dissolved in dichloromethane and filtrated. The solvent was then removed and the resulting solid washed with cold hexane to give the pure product as a yellow powder (35 mg, 0.058 mmol, 67 % yield). (s, C 8 ), 125.5 (s, C 5 ), 123.7 (s, C 6 ), 35.5 (s, CMe3), 34.9 (s, C 10 ), 30.9 (s, CMe3), 30.5 (s, C 12 ).

Crystals suitable for X-Ray diffraction analysis were grown by slow evaporation of a concentrated solution in acetone.

Synthesis of (C^N py ^C) Au{CH2C(O)Me} (6b)

A mixture of (C^N py ^C)AuCl (0.050 g, 0.087 mmol) and KO t Bu (0.015 g, 0.13 mmol) in acetone (10 mL) was heated to reflux for 2 d. The volatiles were evaporated under vacuum.

The residue was dissolved in dichloromethane (10 mL) (C^N pz ^C)AuCH3 (8a)

A mixture of (C^N pz ^C)AuOAc F (50 mg, 0.077 mmol) was dissolved in THF (10 mL). A solution of MeMgBr in 2-MeTHF (0.11 mL, 0.38 mmol) was added dropwise at -78 C. After completion of the addition the reaction was kept at room temperature for 3 h. THF was

22

 22

Figure 1 .

 1 Figure 1. (a) Structure of 2a. Selected bond distances (Å) and angles (º): Au-C25A 2.11(2), Au-C6 2.073(9), Au-C16 2.09(1), Au-N1 2.009(8), N1-Au-C25A 166.8(6), N1-Au-C6 81.0(3), N1-Au-C16

Scheme 2 :

 2 Scheme 2: Synthesis of complex 8a -b.

Figure 2 .

 2 Figure 2. (a) Low energy UV-Vis absorption spectra of complexes 2a, 2b, 3a, 3b, 8a and 8b in CH2Cl2 (10 -5 M) at 298 K. (b) Emission spectra of 2a, 2b, 3a, 4, 5, 6a, 6b in CH2Cl2 (10 -4 M) at 77 K.

Figure 3 .

 3 Figure 3. Emission (black solid line) and excitation (black dotted line) bands of (C^N pz ^C)AuH (AuH) in PMMA (10 wt.-%), and UV-Vis absorption spectrum (grey line) of AuH in CH2Cl2 (10 -5 M) at 298 K.

Figure 4 .

 4 Figure 4. Correlation of the pKa values of hydrocarbons in DMSO with the lowest energy absorption for each complex (eV; calculated by averaging the three low energy absorption bands for each complex) and with the emission maxima in CH2Cl2 solution at 77 K. Grey lines represent the linear regression for each series of data with slopes of about 0.004 eV/pKa.

Figure 5 .

 5 Figure 5. Emission spectra of complexes 2a, 5, 8a, 2b, 8b in CH2Cl2 (10 -4 M) at 298 K.

Table 1 :

 1 Absorption data for complexes 2-8 in CH2Cl2 (10 -5 M) Complex Absorbance [nm] (10 3 /M -1 cm -1 ) (C^N pz ^C)Au{CH(CN)2} 2a 281 (19.5), 319 (10.7), 345 (5.9), 406 (3.8), 425 (5.3), 447 (5.6).

Figure 7 .

 7 Figure 7. Correlation between the calculated HOMO-LUMO energy difference ΔE (eV) (IEF-PCM SCRF model) and the experimental lowest energy absorption bands in CH2Cl2 (nm) for (a) 2a*, 3a*, 4*, 5*, 6a*, 8a* and AuH* (R 2 = 0.958). (b) 2b*, 3b*, 6b*, and 8b* (R 2 = 0.966).

Figure 8 .

 8 Figure 8. Numbering scheme of the C^N^C ligand.

Table 2 .

 2 Emission properties in the solid state and in CH2Cl2 solution (10 -3 M) at 298 K and 77 K.

			λem / nm (λex / nm) [τ / ns]
	Complex
			298 K	77 K
	2a	Solid	520max, 541, 586sh (300-450)	519max, 553, 585sh (380)
		CH2Cl2	484max, 528, 553sh, 600sh (377, 477)	546max, 576sh (380)
			[31±1 (50%), 6.05±0.25 (50%)]
	3a	Solid	476sh, 520max, 546sh (350-460)	518max, 550sh, 582sh (370)
		CH2Cl2	480, 522, 550sh (369, 467)	492sh, 540max, 562sh (380)
			[16±1 (75%), 3.76 (25%)]
	4	Solid	475sh, 520max, 545sh (300-450)	517max, 552, 588sh (370)
		CH2Cl2	484, 532max, 554sh (370, 464)	532max, 558sh, 602sh (370)
			[95±2 (90%), 6.06±0.41 (10%)]
	5	Solid	520max, 546sh (325, 345, 397, 416, 440) 518, 560 (370)
		CH2Cl2	475sh, 520max, 542sh (365, 459max)	484sh, 528max, 556sh (380)
			[92±20 (85%), 2.33 (15%)]
	6a	Solid	475sh, 520max, 546sh (378, 403, 430-	518max, 549sh, 580sh (370)
			455)
		CH2Cl2	503sh, 518max, 551, 590sh (380-420)	524max, 552sh (370)
			[5.73±0.01]
	8a	Solid	520max, 542sh (300-420)	518max, 550sh, 580sh (370)
		CH2Cl2	511max, 536sh, 582sh (300-420)	520max, 550sh (370)
	AuH	Solid	517max, 542, 577sh (320-430)	520max, 550sh, 577sh (370))
		CH2Cl2		522max, 55204750sh, 5780sh (370))
	2b	Solid	No emissive	536max, 562sh (320-430)
		CH2Cl2	440br (370) a	494max, 529, 555sh (370)
	3b	Solid	449max, 473sh (370) a	478max, 514, 542sh (370)
		CH2Cl2	451br (370) a	448sh, 477max, 506sh (380)
	6b	Solid	474max, 510, 542, 572sh (370)	474max, 510, 542sh, 562sh (370)
		CH2Cl2	442, 473, 498sh, 533sh (350-420)	477max, 505sh, 542sh (380)
			[5.2±0.2]
	8b	Solid	472max, 507 (310-400)	472max, 507, 540sh (370)
		CH2Cl2	423max, 471, 505, 535sh (300-400)	474max, 500, 530sh (370)

a Weak emission.

Table 3 .

 3 Emission properties of complexes 2 -8 in PMMA (10 %) at 298 K. a

	Complex	λem / nm (λex / nm)	τ / ns (contribution / %) [λem /
			nm] {ϕ / %}
	(C^N pz ^C)Au{CH(CN)2} 2a	528max, 557sh (320-430)	111(11), 668(89) [528] {6.2}

a Complexes 2b and 3b are too weakly emissive in PMMA to be measured reliably.

Table 4 .

 4 Calculated energy values of frontier Kohn-Sham molecular orbitals (KS-MOs) and energy difference ΔE between KS-HOMO and KS-LUMO (eV) in dichloromethane solvent model (IEF-PCM SCRF model).

	Complex	2a*	3a*	4*	5*	6a*	AuH*	8a*	2b*	3b*	6b*	8b*

C^N py ^C)Au{CH(CN)2} (2b)

  10 ),4.09 (m, 4 H, H 12 ), 1.40 (s, 18 H, t Bu), 1.13 (t, 6 H, H 13 ).13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 171.3 (s, C 11 ), 167.8 (s, C 9 ), 157.0 (s, C 3/4 ), 156.6 (s, C 3/4 ), 145.3 (s, C 7 ), 138.7 (s, C 2 ), 132.3 (s, C 8 ), 125.9 (s, C 5 ), 124.5 (s, C 6 ), 60.9 (s, C 12 ), 38.1 (s, C 10 ), 36.2 (s, CMe3), 31.4 (s, CMe3),14.6 (s, C 13 ).

	(C^N pz ^C) Au{CH(C(O)Me)2} (3a)
	The complex was made following the procedure for 2a, from (C^N pz ^C)AuCl (50 mg, 0.087
	mmol), acetylacetone (87 mg, 0.87 mmol) and KO t Bu (12 mg, 0.10 mmol) as a yellow
	powder (48 mg, 0.075 mmol, 86 % yield). Anal. Calcd for C29H33N2O2Au (638.6): C, 54.55;
	H, 5.21; N, 4.39. Found: C, 54.66; H, 5.30; N, 4.55. IR (cm -1 ): (C=O) 1677 (i). 1 H NMR
	(CD2Cl2, 300.13 MHz): δ 8.85 (s, 2 H, H 2 ), 7.95 (d, 4 JH-H = 1.9 Hz, 2 H, H 8 ), 7.68 (d, 3 JH-H = Synthesis of (C^N pz ^C)Au{CH2C(O)Ph} (5)
	8.2 Hz, 2 H, H 5 ), 7.36 (dd, 3 JH-H = 8.2 Hz, 4 JH-H = 1.9 Hz, 2 H, H 6 ), 4.96 (s, 1 H, H 10 ), 2.29 (s, A mixture of (C^N pz ^C)AuCl (0.027 g, 0.047 mmol), KO t Bu (0.007 g, 0.056 mmol) and
	6 H, H 12 ), 1.40 (s, 18 H, t Bu). 13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 205.9 (s, C 11 ), 166.7 (s, acetophenone (0.056 g, 0.470 mmol) was heated at 80 C in toluene (3 mL) overnight. The
	C 9 ), 156.8 (s, C 3/4 ), 156.6 (s, C 3/4 ), 145.8 (s, C 7 ), 138.9 (s, C 2 ), 131.3 (s, C 8 ), 126.1 (s, C 5 ), solution was evaporated. The yellow solid residue was dissolved in dichloromethane and
	124.7 (s, C 6 ), 59.9 (s, C 10 ), 36.4 (s, CMe3), 31.9 (s, C 12 ), 31.4 (s, CMe3). Crystals of 3a were filtrated through celite. After evaporation of the dichloromethane and addition of cold hexane
	grown by slow evaporation of a hexane solution. (2 mL) a yellow precipitate formed which was collected and dried to give the pure product as
	a yellow powder (0.029 g, 0.044 mmol, 94 % yield). Anal. Calcd for C32H33N2OAu (658.6):
	(C^N py ^C)Au{CH(C(O)Me)2} (3b) C, 58.36; H, 5.05; N, 4.25. Found: C, 58.47; H, 5.15; N, 4.30. IR (cm -1 ): (C=O) 1654. 1 H
	A mixture of (C^N py ^C)AuCl (50 mg, 0.087 mmol), acetylacetone (87 mg, 0.87 mmol) and NMR (CD2Cl2, 300.13 MHz): δ 8.79 (s, 2 H, H 2 ), 8.11 (dm, 3 JH-H = 9.0 Hz, 2 H, H 13 ), 7.93
	2 H, H 6 ), 3.68 (s, 1 H, H 10 ), 1.39 (s, 18 H, t Bu). 13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 167.3 KO t Bu (12 mg, 0.10 mmol) was stirred in toluene (5 mL) overnight. The solution was evaporated. The yellow residue was dissolved in dichloromethane and filtrated through celite. After evaporation to dryness, the solid residue was washed with light petroleum to give the pure product as a pale-yellow powder (40 mg, 0.075 mmol, 86 % yield). Anal. Calcd for (d, C 7 ), 139.3 (s, C 12 ), 138.7 (s, C 2 ), 132.6 (s, C 15 ), 131.3 (s, C 8 ), 129.0 (s, C 13 ), 128.6 (s, C 14 ),
	(s, C 9 ), 157.4 (s, C 3/4 ), 157.1 (s, C 3/4 ), 144.9 (s, C 7 ), 139.2 (s, C 2 ), 130.9 (s, C 8 ), 126.6 (s, C 5 ), C30H34NO2Au (637.6): C, 56.52; H, 5.37; N, 2.20. Found: C, 56.38; H, 5.46; N, 2.28. IR 126.0 (s, C 5 ), 124.3 (s, C 6 ), 36.2 (s, CMe3), 31.5 (s, CMe3), 30.2 (s, C 10 ).
	125.4 (s, C 6 ), 116.1 (s, C 11 ), 36.2 (s, CMe3), 31.3 (s, CMe3), -3.2 (s, C 10 ). (cm -1 ): (C=O) 1677. 1 H NMR (CD2Cl2, 300.13 MHz): δ 7.97 (d, 4 JH-H = 1.8 Hz, 2 H, H 8 ),
	7.83 (t, 3 JH-H = 8.0 Hz, 1 H, H 1 ), 7.56 (d, 3 JH-H = 8.1 Hz, 2H, H 5 ), 7.45 (d, 3 JH-H = 8.0 Hz, 2H, (
	H 2 ), 7.31 (dd, 3 JH-H = 8.1 Hz, 4 JH-H = 1.8 Hz. 2 H, H 6 ), 4.69 (s, 1 H, H 10 ), 2.31 (s, 6 H, H 12 ), (A mixture of (C^N py ^C)AuCl (50 mg, 0.087 mmol), malodinitrile (57 mg, 0.87 mmol) and 1.41 (s, 18 H, t Bu). 13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 205.8 (s, C 11 ), 165.2 (s, C 9 ),
	KO t Bu (15 mg, 0.13 mmol) in toluene (5 mL) was heated overnight at 80 o C. The product was 163.3 (s, C 3/4 ), 154.9 (s, C 3/4 ), 147.5 (s, C 7 ), 142.3 (s, C 1 ), 130.3 (s, C 8 ), 125.1 (s, C 5 ), 123.8
	isolated as described for 2a, as a yellow powder (41 mg, 0.068 mmol, 78 % yield).). Anal. (s, C 6 ), 116.3 (s, C 2 ), 59.2 (s, C 10 ), 35.7 (s, CMe3), 31.3 (s, C 12 ), 31.0 (s, CMe3).
	Calcd for C27H27N3Au (603.5): C, 55.72; H, 4.68; N, 6.96. Found: C, 55.32; H, 4.54; N, 7.09.
	IR (cm -1 ): (C≡N) 2233 (w). 1 H NMR (CD2Cl2, 300.13 MHz): δ 7.89 (t, 3 JH-H = 8.2 Hz, 1 H, (C^N pz ^C)Au{CH(CO2Et)2} (4)
	H 1 ), 7.85 (d, 4 JH-H = 1.9 Hz, 2 H, H 8 ), 7.55 (d, 3 JH-H = 8.2 Hz, 2 H, H 5 ), 7.45 (d, 3 JH-H = 8.2 Following the procedure given for 2a, the compound was made from (C^N pz ^C)AuCl (50
	Hz, 2 H, H 2 ), 7.35 (dd, 3 JH-H = 8.2 Hz, 4 JH-H = 1.9 Hz, 2 H, H 6 ), 3.64 (s, 1 H, H 10 ), 1.43 (s, 18 mg, 0.087 mmol), diethylmalonate (139 mg, 0.87 mmol) and KO t Bu (15 mg, 0.13 mmol) as a
	H, t Bu). 13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 166.4 (s, C 9 ), 164.6 (s, C 3/4 ), 156.1 (s, C 3/4 ), yellow powder (37 mg, 0.053 mmol, 61 % yield). Anal. Calcd for C31H37N2O4Au (698.6): C,
	147.2 (s, C 7 ), 143.6 (s, C 1 ), 130.3 (s, C 8 ), 126.1 (s, C 5 ), 125.1 (s, C 6 ), 117.2 (s, C 2 ), 116.6 (s, 53.30; H, 5.34; N, 4.01. Found: C, 53.17; H, 5.35; N, 4.03. IR (cm -1 ): (C=O) 1737; 1732 (i).
	C 11 ), 36.1 (s, CMe3), 31.5 (s, CMe3), -3.6 (s, C 10 ).

). Anal. Calcd for C27H27N4Au•H2O (622.5): C, 52.09; H,

4.50; N, 9.00. Found: C, 52.43; H, 4.10; N, 9.

74. IR (cm -1 ): (C≡N) 2237 (w). 1 H NMR (CD2Cl2, 300.13 MHz): δ 8.81 (s, 2 H, H 2 ), 7.85

(d, 4

JH-H = 1.9 Hz, 2 H, H 8 ), 7.65

(d, 3

JH-H = 8.2 Hz, 2 H, H 5 ), 7.39 (dd,

3

JH-H = 8.2 Hz,

4

JH-H = 1.9 Hz, 1 H NMR (CD2Cl2, 300.13 MHz): δ 8.81 (s, 2 H, H 2 ), 8.03

(d, 4

JH-H = 1.9 Hz, 2 H, H 8 ), 7.65

(d, 3

JH-H = 8.2 Hz, 2 H, H 5 ), 7.34 (dd,

3

JH-H = 8.2 Hz,

4

JH-H = 1.9 Hz, 2 H, H 6 ),

4.96 (s, 1 H, H 4

JH-H = 1.9 Hz, 2 H, H 8 ), 7.64

(d, 3

JH-H = 8.2 Hz, 2 H, H 5 ), 7.38 (tm,

3

JH-H = 7.5 Hz, 1 H, H 15 ),

7.27-7.34 (m, 4 H, H 6

+ H 14 ),

3.65 (s, 2 H, H 10

), 1.38 (s, 18 H, t Bu).

13

C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 201.4 (s, C 11 ), 166.6 (s, C 9 ), 156.2 (s, C 3/4 ), 156.1 (s, C 3/4 ), 146.0 (s,

C^N pz ^C)Au{CH2C(O)Me} (6a)

  JH-H = 8.2 Hz, 2 H, H 5 ),7.33 (dd, 3 JH-H = 8.2 Hz,4 JH-H = 1.9 Hz, 2 H, H 6 ),3.11 (s, 2 H, H 10 ), 2.23(s, 3 H, H 12 ), 1.40 (s, 18 H, t Bu).13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 208.8 (s, C 11 ), 167.3 (s, C 9 ), 155.8 (s, C 3/4 ), 155.7 (s, C 3/4 ), 145.2 (s, C 7 ), 138.2 (s, C 2 ), 131.4

	A mixture of (C^N pz ^C)AuCl (50 mg, 0.087 mmol) and K2CO3 (0.024 g, 0.174 mmol) was
	stirred in refluxing acetone (5 mL) overnight. The solution was evaporated to dryness, and
	the yellow residue was dissolved in dichloromethane and filtrated through celite. After
	evaporation to dryness, the yellow solid was washed with cold hexane to give the pure
	product as a yellow powder (46 mg, 0.077 mmol, 87 % yield). Anal. Calcd for C27H31N2OAu
	(596.2): C, 54.36; H, 5.24; N, 4.70. Found: C, 54.28; H, 5.30; N, 4.75. IR (cm -1 ): (C=O)
	1667 (i). 1 H NMR (CD2Cl2, 300.13 MHz): δ 8.82 (s, 2 H, H 2 ), 7.97 (d, 4 JH-H = 1.9 Hz, 2 H,
	H 8 ), 7.66 (d, 3

  and filtrated through celite. Slow diffusion of EtOH (10mL) into the dichloromethane solution gave the product as a paleyellow microcrystalline solid (0.028 g, 87 % yield). Anal. Calcd for C28H32NOAu.H2O (613.6): C, 54.81; H, 5.59; N, 2.28. Found: C, 54.81; H, 5.78; N, 2.44. IR (cm -1 ): (C=O) 1674 (i). 1 H NMR (CD2Cl2, 300.13 MHz): δ 7.96 (s, br, 2 H, H 8 ), 7.76 (t,3 JH-H = 7.9 Hz, 1 H, H 1 ), 7.53(d, 3 JH-H = 8.1 Hz, 2H, H 5 ), 7.40(d, 3 JH-H = 7.9 Hz, 2H, H 2 ), 7.28(d, br, 2 H, H 6 ), 3.02 (s, 2 H, H 10 ),2.23 (s, 3 H, H 12 ), 1.40 (s, 18 H, t Bu).13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 209.4 (s, C 11 ), 166.5 (s, C 9 ), 162.9 (s, C 3/4 ), 154.4 (s, C 3/4 ), 147.6 (s, C 7 ), 141.7 (s, C 1 ), 130.9 (s, C 8 ), 125.0 (s, C 5 ), 123.4 (s, C 6 ), 116.0 (s, C 2 ), 35.4 (s, CMe3), 34.6 (s, C 10 ), 31.0 (s, CMe3), 30.4 (s, C 12 ).A mixture of (C^N pz ^C)AuCl (200 mg, 0.348 mmol) and AgOAc F (92 mg, 0.417 mmol) was stirred overnight in CH2Cl2 (20 mL). The solution was filtrated through celite. Evaporation to dryness afforded a yellow solid that was washed with Et2O to give the pure product(190 mg, 0.291 mmol, 84 % yield). Anal. Calcd for C26H26N2O2F3Au (652.6):C, 47.86; H, 4.02; N, 4.29. Found: C, 47.75; H, 3.91; N, 4.17. IR (cm -1 ): (C=O) 1727 (m). 1 H NMR (CD2Cl2, 300.13 MHz): δ 8.70 (s, 2 H, H 2 ), 7.61(d, 3 JH-H = 8.2 Hz, 2 H, H 5 ), 7.43(d, 4 JH-H = 1.7 Hz, 2 H, H 8 ), 7.38 (dd,3 JH-H = 8.2 Hz,4 JH-H = 1.7 Hz, 2 H, H 6 ), 1.33 (s, 18 H, t Bu).13 C{ 1 H} NMR (CD2Cl2, 75.48 MHz): δ 170.6 (s, C 9 ), 158.2 (s, C 3/4 ), 157.3 (s, C 3/4 ), 141.9 (s, C 7 ), 138.5 (s, C 2 ), 130.0 (s, C 8 ), 125.8 (s, C 5 ), 125.3 (s, C 6 ), 35.5 (s, C(CH3)3), 30.7 (s, C(CH3)3).19 F{ 1 H}NMR (CD2Cl2, 282.38): δ -73.75 (s, CF3CO2Au).

	(C^N pz ^C)Au(trifluoroacetate) (7a)

IL(C^N^C) emission together with a high-energy feature. This high-energy feature, marked with an asterisk in Figure5, appears in a different proportion with respect to the main band depending on the complex. For the pyrazine complexes in some cases it is very weak (8a) or appears as a small shoulder (5), but is prominent in some complexes (2a). For the pyridine series the 3 IL(C^N^C) emission is only present in some cases (8b) and in others the weak emission that is observed has only the high energy component (2b). This high energy band can be tentatively ascribed to 1 IL(C^N^C) transitions. This is confirmed by the fact that bubbling air through the solution increases, in all cases, the proportion of this band with respect to the phosphorescence, and becomes the only emissive pathway for all the complexes under aerobic conditions.

■ ACKNOWLEDGMENTS

This work was supported by the European Research Council, M. B. is an ERC Advanced Investigator Award holder (grant no. 338944-GOCAT). We would like to thank the 'Centro de Investigación en Síntesis Química', Universidad de La Rioja, E-26006, Logroño, Spain, and very especially Prof. E.

Lalinde, for their kind assistance with the determination of some of the PL properties of the complexes.

evaporated and the residue dissolved in CH2Cl2. The dichloromethane phase was washed with water, dried over Na2SO4 and filtered. Evaporation to dryness afforded a yellow solid that was washed with hexane to give the pure product (31 mg, 0.056 mmol, 73 % yield).

Anal. Calcd for C25H29N2Au (554.5): C, 54. 15;H,5.27;N,5.05. Found: C,54.21;H,5.18;N,5.16. 1 H NMR (CD2Cl2,300.13 MHz): δ 8.84 (s, 2 H, H 2 ), 7.74 (d, 4 JH-H = 1.7 Hz, 2 H, H 8 ), 7.68 (d, 3 JH-H = 8. 2 Hz,2 H,H 5 ),7.32 (dd,2 H,H 6 ),1.39 (s,21 H,t Bu + H 10 ). 13 C{ 1 H} NMR (CD2Cl2,75.48 MHz): δ 168.1 (s,C 9 ),155.8 (s,C 3/4 ),155.5 (s, C 3/4 ), 146.2 (s, C 7 ), 138.6 (s, C 2 ), 131.2 (s, C 8 ), 126.0 (s, C 5 ), 124.0 (s, C 6 ), 35.9 (s, CMe3), 31.5 (s, CMe3), 3.7 (s, C 10 ).

(C^N py ^C)AuCH3 (8b)

The procedure described for 8a gave 8b as a pure yellow solid (37 mg, 0.067 mmol, 87 % yield). 1 H NMR (CD2Cl2, 300.13 MHz): 7.81 (t, 3 JH-H = 8.2 Hz, 1 H, H 1 ), 7.75 (d, 4 JH-H = 1.9

Hz, 2 H, H 8 ), 7.60 (d, 3 JH-H = 8.2 Hz, 2 H, H 5 ), 7.49 (d, 3 JH-H = 8.2 Hz, 2 H, H 2 ), 7.28 (dd, 3 JH-H = 8.2 Hz, 4 JH-H = 1.9 Hz, 2 H, H 6 ), 1.39 (s, 18 H, t Bu), 1.33 (s, 3 H, H 10 ).

■ ASSOCIATED CONTENT

Supporting Information

For details about X-ray crystallography, photophysical properties, theoretical calculations. See DOI: CCDC 1552153 (2a), 1552155 (3a), 1521267 (6a), 1552154 (2b), 1552156 (7a) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

■ AUTHOR INFORMATION

Corresponding

Notes

The authors declare no competing financial interest.


Antoine Lesieur
email: antoine.lesieur@inria.fr

Patrick Maillé
email: patrick.maille@imt.fr

Bruno Tuffin
email: bruno.tuffin@inria.fr

Non-Neutrality Detection of Video Streaming: Wehe Tool Demonstrator and Efficiency

Keywords: Network neutrality, Monitoring tool, Video streaming

With network neutrality regulation imposed worldwide at different extents, there is a need for monitoring tools to verify if Internet service providers (among other actors) comply with the existing rules. Among the very few maintained tools, Wehe has been developed by Northeastern University and has been publicized by the French regulator, ARCEP. The tool runs traces and produces a positive or negative response about service differentiation, but users have to trust that result without any visual perception of the degradation. Our goal in this paper is to describe a demonstrator of Wehe for video streaming, from which we can simultaneously see a baseline video and its manuallydifferentiated counterpart, as well as run Wehe to evaluate its response with respect to the perceived quality degradation. The demonstrator allows to study the efficiency of Wehe in order to propose potential improvements, if needed.

I. INTRODUCTION

Since the late 90s, network neutrality has been the subject of a sensitive and lasting debate. Basically, network neutrality means that Internet Protocol (IP) packets are treated equally within the network, regardless of the type of content, origin or destination, and type of platform, application or device they are related with. This notion was defined after actions from Internet Service Providers (ISPs) who were wishing to differentiate service and/or to charge (distant) content providers for the traffic going through their network to earn extra revenue and fund their infrastructure [START_REF] Lenard | Net Neutrality or Net Neutering: Should Broadband Internet Services be Regulated[END_REF]- [START_REF] Maillé | From Net Neutrality to ICT neutrality[END_REF]. That move provoked strong reactions from user associations and Content Providers (CPs) who were rather wanting to keep an open and neutral network. Different regulations have been passed throughout the world based on different interpretations about what should be considered a neutral behavior [START_REF] Maillé | From Net Neutrality to ICT neutrality[END_REF]. We even observed changes of mind such as with the 2017 neutrality repeal under the Trump administration in the US. Because of this, and due to evolving technologies and new issues popping up, the debate is often seen as never ending. As illustrative and important examples entering the debate, we can cite sponsored data, or slicing in 5G networks [START_REF] Maillé | From Net Neutrality to ICT neutrality[END_REF].

But as soon as rules are enforced, there is a need for measurement tools to ensure that the telecommunication actors comply with defined neutrality principles, or just to monitor activities to point out potentially harming behaviors. Actually few tools have been developed, mostly designed by the academic community, but often restricted in terms of infringement detection, and rarely maintained; see [START_REF] Castoreo | Weaknesses and Challenges of Network Neutrality Measurement Tools[END_REF], [START_REF] Garrett | Monitoring network neutrality: A survey on traffic differentiation detection[END_REF] for recent surveys. Though, a tool called Wehe may be seen as standing out, jointly designed by Northeastern University and the French regulator ARCEP [START_REF] Molavi Kakhki | Identifying Traffic Differentiation in Mobile Networks[END_REF]. This tool is maintained and distributed on Apple and Android application stores [START_REF]Wehe[END_REF] and its code is open source 1 . The tool is analyzing potential traffic throttling on the Internet. Due to its diffusion and official role, we focus on it in this paper.

We carried out a first analysis of Wehe in [START_REF] Castoreo | Analyzing the Wehe Network Neutrality Monitoring Tool[END_REF], investigating the playroom left for ISPs to differentiate traffic before being detected: By computing the detection probabilities and estimating the potential benefit of a revenue-interested operator, we compared the main differentiation types (throughput, packet loss and delay) that an operator could implement.

The present paper has a different goal. Wehe runs like a black box, by basically launching two similar traces: a baseline corresponding to a traffic an ISP can identify and differentiate, and an encrypted one which should be treated neutrally since impossible to identify. An analysis of the difference between the outputs of the two traces is performed by the tool, but unfortunately no output traffic is "visualized" by the end user, who has to trust the simple (positive, negative or inconclusive) answer provided by the tool. We propose here to present a demonstrator of Wehe on video streaming, showing simultaneously a baseline video and the same video degraded (the level of degradation being tuned by the user thanks to buttons) and running Wehe for the selected level of differentiation. The goal is to relate the observed degradation and the potential differentiation detection. It allows to evaluate the practical level of efficiency of Wehe.

The remaining of the paper is organized as follows. Section II briefly describes how Wehe works, then Section III presents the demonstrator and provides links to download it and/or to observe how it operates. The analysis of the detection algorithm for video streaming is presented in Section IV. Finally we conclude in Section V and give direction of future research.

II. WEHE

Wehe is an active measurement tool targeting mainly wireless networks and mobile terminals, on which most neu-trality infringements have been observed. Thanks to an app available from Android or Apple application stores, the tool tests traffic usually subject to differentiation (such as, among others, Hangouts, Netflix, Skype, Spotify, Viber, Amazon and Youtube). Basically, it replays twice a prerecorded traffic between the client and a Wehe server. The first replay is the original one while the second one is encrypted so that an ISP cannot identify it, and therefore not discriminate it. In both cases, the replayed traffic has the same shape, the same packet sizes, protocol headers and inter-packet times, the only difference being the encryption or not of the payload, so that the outputs should be the same if no differentiation is performed. Statistical tests are then performed to compare the throughputs of the output traffic by determining if a difference is significative: this would mean a discrimination between the identified and encrypted traffic. More details can be found in [START_REF] Molavi Kakhki | Identifying Traffic Differentiation in Mobile Networks[END_REF] and Wehe python source code can be found at https://github.com/NEU-SNS/wehe-server.

The app is practical and easy to launch: the user selects as an input which traffic should be tested and after a few seconds a positive, negative or inconclusive response on non-neutrality is provided. But it works like a black box: users do not have any visual perception on the level of difference between the two traces: they have to trust the tool without any view on how efficient it is.

III. A DEMONSTRATOR

In order to study/illustrate the efficiency of Wehe, we have designed a demonstrator coded also in python and which can be freely downloaded from https://files.inria.fr/wehe demonstrator/package wehe.zip It contains a "Readme" file giving installation instructions.

The demonstrator generates a topology made of three virtual machines. One is acting as the Wehe server and one is acting as the client device (or end user). Communications between client and server are done thanks to the third virtual machine called the middlebox on which is run a Linux Traffic Control utility with a netem queueing discipline [START_REF]netem[END_REF] for classifying and differentiating traffic. Netem allows to limit or deteriorate three performance characteristics, potentially combined: throughput, packet loss rate and delay applied to classified packets.

In order to visualize the correspondence between a chosen level of differentiation and Wehe detection, we consider the impact on video streaming, since it is the type of traffic of most applications targeted by the Wehe app. The demonstrator simultaneously displays two screens: one for the nominal video (not discriminated, corresponding to the encrypted one), and one for the potentially differentiated, with discrimination levels on throughput packet loss or delay selected by the user thanks to buttons. This is illustrated in Figure 1.

The videos are played over HTTP using adaptive bitrate (ABR) where the content is segmented and encoded at multiple bit rates, and the segment displayed corresponds to the available throughput. To facilitate the analysis, the demonstrator displays on the videos their resolution and the actual throughputs. When the differentiation parameters are selected, the user can directly see the impact on the video quality. A Wehe test can then be performed by selecting a trace on the bottom right part of Figure 1, for which a (similarly) differentiated and nominal traffic are sent and compared and the result (differentiation detected, not detected, or inconclusive test) of the test presented.

As illustrative examples, a positive test is presented in Figure 2 and a negative one in Figure 3 (despite a reduced resolution with the selected parameters for this last example).

For those not wanting to install the demonstrator but still interested in getting a more detailed idea about how it works, a short demonstration video is available at https://youtu.be/SgB4gIBvX8w

IV. EFFICIENCY RESULTS

This section relates the Wehe detection threshold with the ABR resolution degradation. To do this, Figure 4 displays the Wehe detection threshold and ABR resolution degradation in terms of the actual available data rate when the differentiation is done by reducing the sending rate. More precisely, the x-axis is for the available throughput, and the points give the median value of threshold, over ten independent sets of experiments, of the (reduced) throughput under which differentiation is detected. For each set of experiments the threshold is determined by dichotomy, as follows: we start with a search interval between half of-and the nominal throughput (assuming reducing the throughput to its half would mean detection), and reduce that search interval by performing a new experiment with its middle value as the degraded throughput, stopping when the length of the interval is below a given relative value of the nominal throughput; we then consider the last found point as the threshold of the set of experiments. The intervals around the points represents the interdecile ranges (that is, the difference between the first and the ninth deciles over the ten experiments). Detection is indeed a random variable since there is some randomness when traces are played, A linear regression curve (with coefficient of correlation close to 1) on the detection threshold with respect to the available throughput is computed, and shown in Figure 4. The curve y = x, that would correspond to perfect detection, is displayed to better grasp the percentage of degradation for which a detection is observed: the vertical difference with the diagonal gives the level of throughput degradation an ISP can operate without being detected. The regression and y = x curves are close to parallel, with a leading coefficient 1.02 for the linear regression meaning that an absolute difference of throughput is detected while we could have expected to detect a relative one. Basically with the YouTube trace presented here, a difference of 800 kbit/s is the upper limit before being detected. The blue curve shows the actual streaming rate of ABR, representing the rate of the segment observed by the streaming tool for the nominal video. It is of course a stairstep function of the available data rate, due the discrete number of resolutions. To see the effect of differentiation on the actual streaming rate, the streaming rate of the differentiated video (at the threshold value) is "just" the blue curve translated by a value of 800 kbit/s.The colored sections indicate the domain where degradation is detected by Wehe and actually does not result in a loss in video resolution. This shows that for most values of data rate, the streaming (ABR) rate of the differentiated (at the threshold level) and nominal videos are the same, so visually no differentiation is observed but Wehe still detects it, suggesting the tool is sensitive enough to prevent ISPs from implementing harmful differentiation. There are short intervals though, when the streaming segment rates reach the data rate, where we do not have a detection but the streaming rate is different. They are described in Table I. Note that our previous analysis is based on running Wehe on a YouTube trace, but several exist and we need to test if detection is trace-sensitive. Figure 5 shows how different the detection threshold is compared to the different traces (Here, YouTube, Amazon and Netflix). Hence, the shape of the trace, its length, its sequenciation, etc. have an impact on the efficiency of the differentiation detection. For instance, the detection threshold with the Amazon trace is much lower than with the Youtube trace (2.5 Mbit/s against 800 kbit/s at a rate of 15 Mbit/s). This means that Wehe is much more sensitive to a differentiation applied to the YouTube trace than to the Amazon one. But would it be the same if we take other YouTube and Amazon traces? Is it something worth being investigated in the future even if the impact is more quantitative than qualitative. The threshold is therefore trace-dependent; considering several traces for the same source/application would be worthwhile to investigate if the same type of gaps are observed for the same type of traffic.

V. DISCUSSION AND CONCLUSIONS

Our work allows to analyze the efficiency of the Wehe non-neutrality detection tool on streaming applications, which are the applications the most tested by the tool. We are able to relate the differentiation detection event to the observed streaming rate, something not possible with the application, which just had to be trusted.

In summary, Wehe seems to work fine for a large part of the data rate values (with a possible differentiation detection even if no difference in the streaming rate is observed), while for a small range of values close to the jumps in streaming rate, Wehe does not detect a differentiation even if the streaming rates of the two videos are different. In addition, since the traces are different, the detection rates are different for each trace and it is impossible to set a fixed threshold for all the websites.

Even if Wehe seems efficient, we want to point out that it actually does not play and compare two streamed flows as if they were going to be displayed, the nominal and the differentiated. It tests a pre-recorded trace that is sent without differentiation, and also sent potentially in a "deteriorated" way, but not interacting with the ABR algorithm. A challenge in a near future would be to imagine a procedure comparing two streaming videos played with ABR: an encrypted one (normally not differentiated), and a nominal one (potentially differentiated).

Another issue not discussed in this paper but worth further investigation is the detection test done in Wehe. What is implemented is a modification of the rigorous Kolmogorov-Smirnov test which looks at the difference between two empirical cumulative distribution functions of observed "instantaneous" throughputs and tests if it is statistically significant. Wehe modifies this test by using a variant with less theoretical foundations, based on heuristics, but probably conservative to avoid problematic false positives. Designing theoreticallyproven and robust tests is one of the goals of our future research.

Fig. 1 .

 1 Fig. 1. Components of Wehe demonstrator: screens of the differentiated and non-differentiated videos, box for discrimination parameters, and corresponding Wehe test.

Fig. 2 .

 2 Fig. 2. Wehe test, with positive detection.

Fig. 3 . 4 nominalFig. 4 .

 344 Fig. 3. Wehe test without detection.

Fig. 5 .

 5 Fig. 5. Variation of the detection threshold for different traces.

See https://dd.meddle.mobi/codeanddata.html