T Laurencin

Pierre J J Dumont
email: pierre.dumont@insa-lyon.fr

Laurent Orgéas

Steven Le Corre

Florian Martoïa

Sabine Rolland

Du Roscoat

Patrice Laure

Sabine Rolland Du Roscoat

3D real time and in situ observation of the fibre orientation during the plane strain flow of concentrated fibre suspensions

Keywords: Short-fibre composites, Concentrated fibre suspension, Rheology, X-ray microtomography, 3D in situ observations, Fibre kinematics

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

Introduction

Owing to their interesting mechanical and physical properties, short-fibre reinforced polymer composites are increasingly used as structural and/or functional components in many engineering fields. These composite parts are usually fabricated using cost-efficient forming processes such as injection and compression moulding [START_REF] Orgéas | Sheet Molding Compounds[END_REF][2] [START_REF] Advani | Manufacturing techniques for polymer matrix composites[END_REF]. A wide range of composite materials with tailored physical and mechanical properties can be designed by combining various types of fibrous reinforcements made of synthetic (e.g., carbon, glass, etc.) or natural (e.g., annual plant fibres such as flax and hemp fibres) fibres with varied morphology (shape, aspect ratio) with thermoplastic or thermoset polymer matrices . Short-fibre reinforced polymer composites with high fibre contents (>20-50 wt%) are used as structural parts adapted for highperformance applications (e.g., for the energy and transport industries) [START_REF] Ferré-Sentis | Rheological response of compressible SMCs under various deformation kinematics: Experimental aspects and simple modelling approach[END_REF]. Hence, during their forming, these composites behave as concentrated fibre suspensions with a non-Newtonian rheology [START_REF] Orgéas | Sheet Molding Compounds[END_REF][5] [START_REF] Guiraud | Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests[END_REF][7] [START_REF] Férec | 4 -Macroscopic modeling of the evolution of fiber orientation during flow[END_REF]. In addition, the fibrous microstructures undergo large flow-induced restructurations that affect the end-use properties of the produced parts. However, these phenomena are still not well characterised and modelled because of experimental challenges related to the fibre scale observation and analysis of evolving fibrous microstructures and flow mechanisms in non-Newtonian suspending polymer matrices.

Fibres are commonly characterised using several geometric descriptors, namely the fibre aspect ratio 𝛽 = 𝑙/𝑑 (where 𝑙 and 𝑑 are the length and diameter of the fibre, respectively) and its orientation through the unit tangent vector 𝐩 𝑖 = sin𝜃 𝑖 cos𝜑 𝑖 𝒆 1 + sin𝜃 i sin𝜑 𝑖 𝒆 2 + cos𝜃 𝑖 𝒆 3 associated to the fibre main direction, as depicted in Figure 1:

Coordinate system and definition of fibre descriptors. Note that 𝜃 𝑖 ∈ [0, 𝜋] and 𝜑 𝑖 ∈ [0, 2𝜋].

The study of fibre kinematics was initiated by Jeffery [START_REF] Jeffery | The motion of ellipsoidal particles immersed in a viscous fluid[END_REF] who determined the motion of an ellipsoidal particle immersed in a linear and infinite laminar flow field of an incompressible Newtonian fluid. The Jeffery's theory also assumes that the velocity 𝐯 𝑖 of the centre of mass of an ellipsoid 𝑖 is an affine function of the macroscale velocity gradient ∇v of the suspending fluid. Hence, for homogenous macroscale fluid flow situations without rigid body motions, the position 𝐱 𝑖 of the centre of mass of an ellipsoid 𝑖 can be predicted by integrating the ellipsoid velocity 𝐯 𝑖 = ∇𝐯 • 𝐱 𝑖 . In addition, using the first expansion of the fluid velocity field around the particle, the evolution of the ellipsoid orientation can be predicted from the well-known Jeffery's equation, which describes the rate of the unit tangent vector 𝐩 i : 𝐩̇i = 𝛀. 𝐩 i +  𝑏 (𝐃. 𝐩 i -(𝐩 i . 𝐃. 𝐩 i )𝐩 i )

Eq. 1

where 𝛀 = (∇𝐯 -∇𝐯)/2 𝒕 and 𝐃 = (∇𝐯 + ∇𝐯)/2 𝒕 ) are the macroscale vorticity and strain rate tensors, respectively, and  𝑏 is the effective shape factor that can be written as  𝑏 = 1 -16.35 ln 𝛽 4𝜋𝛽 2 for straight cylindrical fibres with aspect ratio 𝛽 = 𝑙/𝑑 [START_REF] Brenner | Rheology of a dilute suspension of axisymmetric Brownian particles[END_REF]. The validity of Jeffery's theory was reported by numerous authors [START_REF] Taylor | The motion of ellipsoidal particles in a viscous fluid[END_REF][12] [13][14] and extended to any asymmetrical particles by Bretherton [START_REF] Bretherton | The motion of rigid particles in a shear flow at low Reynolds number[END_REF]. Many rheological models that account for the kinematics of fibres in polymer composites during their forming are based on Jeffery's theory [START_REF] Jeffery | The motion of ellipsoidal particles immersed in a viscous fluid[END_REF]. For these situations, a compact description of the orientation of a collection of fibres is often achieved by computing the fibre orientation distribution function 𝜓 or its moments such as the second 𝐀 and fourth order 𝔸 orientation tensors, respectively [START_REF] Advani | The Use of Tensors to Describe and Predict Fiber Orientation in Short Fiber Composites[END_REF].

The predictions of Jeffery's model have been validated [START_REF] Petrich | An experimental determination of the stressmicrostructure relationship in semi-concentrated fiber suspensions[END_REF][17] [18][19] to deal with the fibre kinematics in dilute Newtonian fibre suspensions, where the fibre volume fraction is 𝜙 ≪ 1/𝛽 2 and the average distance between neighbouring fibres is larger than the fibre length 𝑙. When the fibre concentration is increased to the semi-dilute regime, defined as 1 𝛽 2 ⁄ ≪ 𝜙 ≪ 1 𝛽 ⁄ , the fibre kinematics is affected by the presence of neighbouring fibres. In this regime, the average distance between fibres ranges between one fibre length and one fibre diameter. Thus, fibre-fibre interactions can occur, especially long-range hydrodynamic interactions [START_REF] Rahnama | Observations of fiber orientation in suspensions subjected to planar extensional flows[END_REF][20] [START_REF] Stover | Observations of fibre orientation in simple shear flow of semi-dilute suspensions[END_REF]. In that case, the presence of a fibre within the suspension disturbs the fluid flow, which affects the kinematics of fibres that are in its vicinity. The theoretical estimation of the fibre orientation disturbance induced by long-range hydrodynamic interactions was studied using slender body theory for extensional [START_REF] Rahnama | Observations of fiber orientation in suspensions subjected to planar extensional flows[END_REF][22] and squeeze flows [START_REF] Stover | Observations of fibre orientation in simple shear flow of semi-dilute suspensions[END_REF] [START_REF] Rahnama | Hydrodynamic, translational diffusion in fiber suspensions subject to simple shear flow[END_REF]. These studies showed an increase in this disturbance with the increase in fibre concentration. However, this effect was limited by short-range particle screening effects due to multiple interactions that occur with the surrounding fibres (the neighbouring fibres playing a role like a cage) [START_REF] Fredrickson | Heat and mass transport in composites of aligned slender fibers[END_REF] [START_REF] Shaqfeh | The hydrodynamic stress in a suspension of rods[END_REF]. When the fibre concentration is above 𝜙 > 1/𝛽, the concentrated regime is reached. Shortrange interactions induced by fibre-fibre contacts have a predominant effect on the misalignment of fibre trajectories from Jeffery's predictions [START_REF] Orgéas | 5 -Rheology of Highly Concentrated Fiber Suspensions[END_REF][26] [START_REF] Le | X-ray phase contrast microtomography for the analysis of the fibrous microstructure of SMC composites[END_REF][28] [START_REF] Férec | 4 -Rheological Modeling of Non-dilute Rod Suspensions[END_REF]. Thus, for the semi-dilute regime, several modifications of Eq. 1 have been proposed in the literature based on the following phenomenological approach of Folgar and Tucker [START_REF] Folgar | Orientation Behavior of Fibers in Concentrated Suspensions[END_REF][31]:

𝐀 ̇= (𝛀. 𝐀 -𝐀. 𝛀) +  𝑏 (𝐃. 𝐀 + 𝐀. 𝐃 -2 𝔸: 𝐃) + 2𝐶 𝐼 |𝛾|(𝛅 -α𝐀) Eq. 2 where 𝛅 is the identity tensor, |𝛾| = √2𝐃: 𝐃 corresponds to the magnitude of the generalised strain rate, 𝐶 𝐼 is the (isotropic) orientation diffusivity and with α = 2 in two dimensions and 3 in three dimensions. Several expressions of the orientation diffusivity 𝐶 𝐼 have been proposed , leading to isotropic scalar functions of the fibre concentration and fibre geometry [START_REF] Bay | Fiber orientation in injection-molded composites: a comparison of theory and experiment[END_REF] [START_REF] Phan-Thien | Folgar-Tucker constant for a fibre suspension in a Newtonian fluid[END_REF] or more complex expressions including anisotropic tensorial expressions of the orientation diffusivity [34][35][36]. Solving Eq. 2 requires the expression of the fourth-order orientation tensor 𝔸 which is constructed from a closure approximation from the second-order orientation tensor 𝐀 [START_REF] Hand | A theory of anisotropic fluids[END_REF] Jeffery's theory was established to describe the fibre kinematics for flow situations with a good scale separation, meaning that the fibre length should be small compared to the typical dimensions of the flow. However, considering the typical dimensions of the mould gaps, this restrictive condition scarcely occurs during composite forming. Hence, in most situations, the scale separation is low and the flow of short-fibre composites has to be considered as confined [START_REF] Schiek | Oscillatory shear of a confined fiber suspension[END_REF][43] [START_REF] Diamant | Hydrodynamic interaction in confined geometries[END_REF]. Deviations from Jeffery's orbits for fibres in the vicinity of the mould walls or rheometer platens were reported by several authors for particles in suspension in Newtonian fluids [START_REF] Russel | Rods falling near a vertical wall[END_REF][46] [START_REF] Holm | Shear influence on fibre orientation[END_REF], or non-Newtonian fluids [START_REF] Laurencin | 3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow[END_REF][49] [START_REF] Stover | The motion of rodlike particles in the pressure-driven flow between two flat plates[END_REF]. Only few studies described the effects of fibre-mould mechanical interactions on the fibre kinematics by proposing a direct modification of Jeffery's model [START_REF] Ozolins | Simple Models for Wall Effect in Fiber Suspension Flows[END_REF] or using a modified dumbbell approach [START_REF] Perez | A multi-scale description of orientation in simple shear flows of confined rod suspensions[END_REF] [START_REF] Laurencin | Fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated squeeze flow: Direct numerical simulation and analytical modelling[END_REF].

Considering the complex rheology of polymer composites, several authors also studied the kinematics of single ellipsoidal particles immersed in viscoelastic fluids and subjected to shear flow [START_REF] Bartram | Particle motions in non-newtonian media[END_REF][54][55] [START_REF] Johnson | Dynamics of colloidal particles in sheared, non-Newtonian fluids[END_REF][57] [58][59][60] [START_REF] Borzacchiello | Orientation kinematics of short fibres in a second-order viscoelastic fluid[END_REF]. Deviations from the Jeffery's orbits were observed, showing bistable particle orientations related to the fluid elasticity. However, only a few experimental and theoretical studies accounted for the shear-thinning behaviour of the suspending fluids or elongational flow situations [START_REF] Dumont | Processing, characterisation and rheology of transparent concentrated fibre-bundle suspensions[END_REF] [START_REF] Sinha-Ray | The internal structure of suspensions in uniaxial elongation[END_REF], although these fluids and flow conditions are commonly encountered in short fibre-reinforced polymer composites. In a previous study [START_REF] Laurencin | 3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow[END_REF], the authors showed that during the compression of dilute suspensions with non-Newtonian shear-thinning power-law fluids (𝑛 = 0.2), the fibre kinematics were not affected by the fluid rheology and were well described by Jeffery's equation (Eq. 1) for nonconfined flows. These experimental results were in accordance with theoretical and numerical models [START_REF] Laurencin | Fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated squeeze flow: Direct numerical simulation and analytical modelling[END_REF].

Owing to the difficulty to observe and characterise in 3D the evolution of the fibrous microstructures during the flow of fibre suspensions, the complex flow mechanisms induced at the fibre scale still remain not well understood. Most of the experimental observations of fibre suspensions were performed using 2D optical techniques [11][12][17] [START_REF] Yasuda | A new visualization technique for short fibers in a slit flow of fiber suspensions[END_REF][63], 2D wide-angle X-ray diffraction [START_REF] Menendez | A Wide-angle X-ray diffraction method of determining chopped fiber orientation in composites with application to extrusion through dies[END_REF], high speed X-rays phase-contrast 2D imaging [START_REF] Sinha-Ray | The internal structure of suspensions in uniaxial elongation[END_REF] and video camera with isorefractive systems [START_REF] Petrich | An experimental determination of the stressmicrostructure relationship in semi-concentrated fiber suspensions[END_REF] [18] [START_REF] Stover | Observations of fibre orientation in simple shear flow of semi-dilute suspensions[END_REF]. These techniques work well for studying dilute suspensions. However, they are inappropriate for the characterisation of semi-dilute and concentrated fibre suspensions. In previous studies [START_REF] Laurencin | 3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow[END_REF][49], 3D in situ and real time observations of non-Newtonian shear-thinning dilute and semi-dilute fibre suspensions (𝜙 ≪ 1/𝛽 2 ) subjected to lubricated compression experiments were performed using 3D X-ray synchrotron images. Despite the non-Newtonian fluid behaviour and regardless of the investigated experiment conditions (fibre aspect ratio and orientation), Jeffery's theory was also found to be efficient for predicting fibre kinematics when fibres were far enough from compression platens (around 3 times a fibre diameter). Deviations from the affine motion of the fibre centre of mass were also observed experimentally but were not predicted by the simulation. The potential origins of these deviations were attributed to unexpected rigid body motions of the compressed sample or fluid viscoelastic effects. Confinement effects were also observed experimentally and numerically on the translation and rotation when fibres entered in contact with the compression platens.

In this context, the objective of this study was to investigate fibre scale flow-induced microstructure changes occurring in concentrated fibre suspensions with non-Newtonian suspending fluid subjected to confined elongational flows. Hence, using a similar procedure to that previously reported in [START_REF] Laurencin | 3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow[END_REF][49], model concentrated non-Newtonian fibre suspensions were prepared and compressed using a micro-rheometer that was installed in a synchrotron Xray microtomograph. 3D in situ and real time observations of the evolving fibrous microstructure were then performed during sample compression. The analysis of the 3D images acquired during these experiments revealed for the first time translation and rotation motions of the fibres during plane strain compression of a concentrated fibre suspension. The kinematics of fibres in concentrated fibre suspensions were compared with the predictions of Jeffery's model.

Experimental procedure

Materials

A hydrocarbon gel was used as suspending fluid [START_REF] Guiraud | Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests[END_REF]. At a temperature of 120°C the gel behaved as a Newtonian fluid and its shear viscosity was approximately 1 Pa s. At room temperature, the gel was solid which enabled the fibrous microstructure to be "frozen" and facilitated the handling of the suspension. At a temperature of 50°C, the gel exhibited a non-Newtonian behaviour close to that of industrial polymer matrices used in composites. The gel viscosity in shear could be modelled as a shear-thinning fluid of power law type, i.e., 𝜂 𝑚 = 𝐾|𝛾| 𝑛-1 with a consistency 𝐾 = 440 Pa. s 𝑛 and a power-law index 𝑛 = 0.2. The parameters 𝐾 and 𝑛 were determined using classical rheometry experiments (Anton Paar MCR301 rheometer equipped with a cone-plane geometry).

Short fibres used in the model suspensions were obtained from continuous elastic fishing wire (diameter 𝑑 = 200 µm, length 𝑙 = 1500 µm, i.e., 𝛽 = 7.5, Young's modulus 𝐸 = 2 GPa). Model concentrated fibre suspensions with 3D random fibre orientation were prepared manually using an experimental procedure that slightly differed from that already reported by Laurencin et al. [START_REF] Laurencin | 3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow[END_REF]. Briefly, this procedure consisted in projecting fibres inside the cavity of a mould. This allowed obtaining a fibre mat with a fibre orientation comprised between a 2D in-plane random fibre orientation and a 3D one. Then this fibre mat was impregnated by the hydrocarbon gel heated at 120°C and subjected to vibration to remove the air bubbles that could possibly be entrapped. Finally the as-obtained suspensions were cooled down to ambient temperature. To determine the fibre volume fraction 𝜙 (and thus the total number of fibres 𝑁) required to obtained fibre suspensions in the concentrated regime, i.e., suspensions with a mean coordination number 𝑧 (mean number of fibre-fibre contacts per fibre), greater than 1, we used the statistical tube model [START_REF] Toll | Note: On the tube model for fiber suspensions[END_REF] the estimates of which for these types of fibrous media were shown to be relevant [START_REF] Guiraud | Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests[END_REF][66]:

𝑧 = 4𝜙 ( 2 𝜋 𝛽Φ 1 + Φ 2 + 1) Eq. 3
where the orientation functions Φ 1 and Φ 2 can be estimated from the knowledge of the tangent unit vectors 𝐩 i and the total number 𝑁 of fibres as follows:

Φ 1 = 1 𝑁 2 ∑ ∑ ‖𝐩 𝑖 × 𝐩 𝑗 ‖ 𝑁 𝑗=1 𝑁 𝑖=1 and Φ 2 = 1 𝑁 2 ∑ ∑ |𝐩 𝑖 ⋅ 𝐩 𝑗 | 𝑁 𝑗=1 𝑁 𝑖=1
Eq. 4

Note that Φ 1 = π/4 and Φ 2 = 1/2 for fibrous structures with a 3D random fibre orientation [START_REF] Toll | Note: On the tube model for fiber suspensions[END_REF].

Following this procedure, several cuboids (in-plane dimension 𝑤 0 × 𝑤 0 = 8 × 8 mm² and height ℎ 0 = 2.2 mm) with 𝜙 = 40% (or conversely 𝑁 = 2770).

Plane strain compression experiments with real-time 3D in situ X-ray imaging

Samples were subjected to lubricated plane strain compression loading at a temperature of 50°C using a specific micro-rheometer [START_REF] Latil | Towards the 3D in situ characterisation of deformation micro-mechanisms within a compressed bundle of fibres[END_REF][48] [START_REF] Laurencin | Fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated squeeze flow: Direct numerical simulation and analytical modelling[END_REF]. The micro-rheometer was mounted on the rotation stage of a synchrotron X-ray microtomograph (TOMCAT beamline, Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland) which enabled 3D in situ images of the evolving fibrous microstructure to be acquired in real-time conditions. To maintain the fibre suspension at a constant temperature of 50°C, heaters and thermocouples were inserted inside the compression platens. Before compression, the platens were lubricated with silicone oil (47 V1000-80026, Chimie-Plus Laboratoires), and the suspensions were subjected to a small precompression to ensure a good mechanical contact. After a stress relaxation and temperature homogenisation (~10 min) the mechanical loading was applied continuously through a piezoelectric actuator attached to the lower compression platen. Plane strain compression experiments were performed at a constant velocity ℎ ̇= 11 µm s -1 of the lower platen, corresponding to an initial strain rate 𝐷 33 = ℎ ̇ℎ0 = 0.005 ⁄ s -1 along the 𝐞 3 -direction. A load cell of 5 N was also placed under the upper compression platen and used to measure precisely the axial force 𝐹 exerted on the sample in the 𝐞 3 -direction. Then, by assuming the suspension incompressibility, the axial compression stress 𝜎 33 = |𝐹|ℎ/(ℎ 0 𝑤 0 2 ) was plotted as a function of the axial compression Hencky strain |𝜀 33 | = |ln (ℎ/ℎ 0 )| (up to 0.5). Note that the suspension flow was considered to be confined because of the poor scale separation, i.e., 𝑙/ℎ = 𝒪(1). To acquire 3D X-ray microtomography images of the suspension during compression, the X-ray energy and the number of radiographs (dimensions of the images: 1776 × 1776 pixels) were set to 20 keV and 600, respectively. The scan duration was 0.42 s. 3D images of the scanned specimen with a size of 1776 × 1776 × 450 voxels for a voxel size of 11 × 11 × 11 µm 3 were obtained using the phase-contrast Paganin reconstruction mode [START_REF] Paganin | Simultaneous phase and amplitude extraction from a single defocused image of a homogeneous object[END_REF][69] and suitable reconstruction algorithms.

Image analysis and data post-treatment

To detect and quantitatively follow the evolution of the fibrous microstructure during lubricated plane strain compression, several image analysis operations were performed following the methodology reported by Latil et al [START_REF] Latil | Towards the 3D in situ characterisation of deformation micro-mechanisms within a compressed bundle of fibres[END_REF]. After a segmentation step of images on the fibrous phase (Figure 2a), the numerous contacts between fibres did not enable each fibre to be clearly distinguished. To individualise the fibres, a 3D distance analysis between the two phases (i.e., fibre and fluid) was applied (Figure 2b). This stage consisted in calculating the distance of each voxel of the fibrous phase with the fluid phase and attributing a new grey level value to the considered voxel. Then, the as-obtained 3D Euclidian distance map was segmented. This operation enabled the removal of all the fibre-fibre contacts by reducing the fibre diameter. The fibre centrelines were extracted using a skeletonisation algorithm [START_REF] Pudney | Distance-Ordered Homotopic Thinning: A Skeletonization Algorithm for 3D Digital Images[END_REF], namely the distance ordered homotopic thinning algorithm implemented in Avizo software (Figure 2c). The centreline of each fibre 𝑖 was smoothed and fitted using a linear parametric curve with coordinate 𝐱 𝑖 (𝑠) along the curvilinear abscissa 𝑠 of the considered fibre 𝑖. Then, a local Frenet basis was associated to each coordinate 𝐱 𝑖 (𝑠), using the procedure reported in [START_REF] Latil | Towards the 3D in situ characterisation of deformation micro-mechanisms within a compressed bundle of fibres[END_REF] [START_REF] Balbinot | In situ 3D observations of capillary-driven flows in parallel arrangements of rigid fibres using X-ray microtomography[END_REF]. In this local basis, 𝐭 𝑖 is the unit tangent vector, 𝐧 𝑖 and 𝐛 𝑖 are the unit normal and binormal vectors, respectively. Hence, the set (𝐱 𝑖 , 𝐭 𝑖 , 𝐧 𝑖 , 𝐛 𝑖 ) enabled the description of the local geometry of the fibres, their orientation (𝐩 𝑖 corresponds to the average of all unit vectors 𝐭 𝑖 along the curvilinear abscissa 𝑠 of the fibre 𝑖), and the orientation and position of each fibre-fibre contact. The set 𝐶 of fibre-fibre contacts was extracted from the 3D images using the procedure reported in [67][66]. This procedure was based on the calculations of the distances between the fibre centrelines. A fibre-fibre contact was detected when the local distance between two neighbouring fibre centrelines was less than or equal to a fibre diameter 𝑑. Hence, it was possible to estimate the mean coordination numbers 𝑧 in the studied suspensions for each compression stage. In addition, the unit normal vector 𝐜 𝛼 of each contact 𝛼 (1 ≤ 𝛼 ≤ 𝐶) between two contacting fibres 𝑖 and 𝑗 was calculated as follows: 𝐜 𝛼 = 𝐩 𝑖 × 𝐩 𝑗 . Doing so, it was also possible to analyse the overall orientation of fibres and fibre-fibre contacts by using deterministic estimates of the second 𝐀 and the fourth 𝔸 order fibre orientation tensors [START_REF] Guiraud | Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests[END_REF] Eq. 6

In addition, fibre positions, orientations and fibre-fibre contacts were tracked between each compression step. The rigid motion of fibres was followed using a fibre centreline tracking algorithm based on a correlation distance function [START_REF] Le Corre | A 3D image analysis method for fibrous microstructures: discretization and fibers tracking[END_REF]. Briefly, the principle of the correlation method consists in recognizing the fibre 𝑖 from a so-called initial configuration of the fibre suspension at time 𝑡 among the 𝑁 fibres 𝑗 of a so-called deformed configuration of the suspension at time 𝑡 + 𝑑𝑡. This method relies on the minimisation of the following discrete correlation function 𝑀 𝑖𝑗 defined as follows:

𝑀 𝑖𝑗 = ∑ ∑ ∑ ‖𝐱 𝑖 𝑡 (𝑠 𝑘 ) -𝐱 𝑗 𝑡+𝑑𝑡 (𝑠 𝑘 )‖ 𝑘 𝑁 𝑗 𝑁 𝑖 Eq. 7
For each couple (𝑖, 𝑗), the function 𝑀 𝑖𝑗 represents the inter-fibre distance calculated from the spatial coordinates 𝐱 𝑖 𝑡 (𝑠 𝑘 ) and 𝐱 𝑗 𝑡+𝑑𝑡 (𝑠 𝑘 ) along the curvilinear abscissa 𝑠 𝑘 of the fibre centrelines. Minimising this function 𝑀 𝑖𝑗 enables finding for each fibre 𝑖 its corresponding fibre 𝑗. The validity of each fibre association is checked by using again the correlation technique to recognize each fibre 𝑗 at time 𝑡 + 𝑑𝑡 among the 𝑁 fibres 𝑖 at time 𝑡. The correlation is validated if the couples (𝑖, 𝑗) of associated fibres obtained from both minimisation processes is identical.

The relevance of Jeffery's model was assessed by testing the validity of the affine assumption (i.e., 𝐯 𝑖 𝐽𝑒𝑓𝑓 = ∇𝐯 • 𝐱 𝑖 𝐽𝑒𝑓𝑓 ), and the orientation equation (Eq. 1) for the predictions of the position of the centre of mass 𝐱 𝑖 𝐽𝑒𝑓𝑓 and the orientation vector 𝐩 𝑖 𝐽𝑒𝑓𝑓 of each fibre 𝑖 of the model suspensions. These equations were numerically solved using a fourth order Runge-Kutta integration scheme implemented in Matlab. Previous measurements made on dilute fibre suspensions [START_REF] Laurencin | 3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow[END_REF] revealed that the macroscale suspension flow was homogeneous, incompressible (∇. 𝐯 = 𝟎) and irrotational (𝛀 = 𝟎). Therefore, the strain rate tensor 𝐃 was estimated from the height of the suspension ℎ measured using the 3D images with an error of ±1 voxel. The initial positions 𝐱 𝑖 0 and the initial orientation vectors 𝐩 𝑖 0 were taken from the initial undeformed configuration of the suspension measured on the 3D images, with maximum errors of ±1 voxel and ±0.15°, respectively. Then, two thousand integrations per experiment were computed using errors associated with ℎ, 𝐱 𝑖 0 and 𝐩 𝑖 0 that were randomly chosen within the aforementioned ranges. Hence, the errors on the corresponding integrated parameters could be estimated. The errors were considered to correspond to the difference between the 2 nd and 98 th percentile values of the computed distributions. Finally, the calculated orientation vectors 𝐩 𝑖 𝐽𝑒𝑓𝑓 from the integration of Eq. 1 enabled the second order fibre orientation tensor 𝐀 𝐽𝑒𝑓𝑓 to be directly calculated again from Eq. 5. The as-calculated tensor 𝐀 𝐽𝑒𝑓𝑓 could be compared to the tensor 𝐀 measured from the 3D images.

Results and discussion

Flow mechanisms at the sample scale

Figure 3 shows the typical evolution of the stress 𝜎 33 measured along the 𝒆 3 axis during the plane strain compression of a model concentrated fibre suspension. 3D views of the fibrous structure obtained at various strain |𝜀 33 | are also shown in this figure. The compression curves of concentrated fibre suspensions exhibit two regimes: a first regime where the stress increases non-linearly up to a flow stress 𝜎 33 ≈ 2.3 × 10 4 Pa followed by a plateau-like regime. Note that in these flow conditions, the contribution of the suspending fluid scales as 295 Pa which demonstrates the crucial of fibres on the rheology of the studied suspensions. In the same time, the 3D images reveal that the fibre microstructure is compressed along the 𝒆 3 axis but also flows along the channel's axis in the 𝒆 2 direction. At the macroscale, the flow of the suspension is homogeneous and corresponds to an incompressible plug-like flow without any phase separation between the fibres and the polymer matrix. This tends to show that despite the high concentration of the suspension, the entanglement of the fibrous network is sufficiently low to enable the fibres to move relatively to each other under the applied compression stress.

Plane strain compression of a model concentrated suspension containing 2770 fibres (𝜙 = 40%). The graph shows the evolution of the compression stress 𝜎 33 measured along the 𝒆 3 axis as a function of the compression strain |𝜀 33 |. 3D images numbered from 1 to 4 show the evolution of the fibre microstructure during lubricated plane strain compression. Numbers on the curves show the corresponding stress and strain states to these images.

Figure 4 shows the evolution with 𝜙 of the dimensionless compression stress 𝜎 33 * = 𝜎 33 (𝜙)/𝜎 33 (𝜙 = 0), where 𝜎 33 (𝜙) and 𝜎 33 (𝜙 = 0) are the axial compression stresses of the fibre suspension and the polymer matrix, respectively, that were measured at a compression strain |𝜀 33 | = 0.3. For comparison purpose, we have also plotted the dimensionless compression stresses 𝜎 33 * obtained for various model fibre suspensions in the dilute and semidilute regimes prepared in the same conditions and with the same fibre aspect ratio 𝛽 = 7.5 [48][49]. This shows that the compression stress required for the flow establishment of a concentrated fibre suspension (i.e, suspensions with 𝜙 = 0.4) is ten times higher than that measured for semi-dilute fibre suspensions. In addition, this figure emphasises the weak (resp. pronounced) effect of the compression strain rate 𝐷 33 (resp. 𝜙) on the dimensionless compression stress 𝜎 33 * .

Evolution with the fibre content 𝜙 of the dimensionless compression stress 𝜎 33 * for various model fibre suspensions (𝛽 = 7.5) that were prepared following the procedure described in subsection 2.1.

The graphs in Figures 5a,b show the evolution of the diagonal components of 𝐀 and 𝐂 tensors. Note that it was checked that the diagonal components could reasonably be associated to the principal components of these two tensors. These graphs reveal that the fibre orientation of the initial suspension exhibited a 3D transverse isotropy (𝑖. 𝑒. , 𝐴 11 ~𝐴22 ≠ 𝐴 33 ) with a high density of unit normal vectors 𝐜 𝛼 of fibre-fibre contacts oriented along the 𝒆 3 axis (i.e., 𝐶 33 > 𝐶 11 ≈ 𝐶 22 ). These features were presumably inherited from the sample preparation since each fibre of the suspension was introduced into the suspending fluid from the top, along the 𝒆 3 axis. The evolution of the diagonal components of 𝐀 and 𝐂 tensors with the compression strain |𝜀 33 | revealed that the fibres tended to reorient along the channel's axis 𝒆 2 . This is illustrated by the increase of the components 𝐴 22 and 𝐶 11 with the increase in the compression strain |𝜀 33 |. Then, it is interesting to notice that 𝐶 33 did not evolve, proving that the density of unit normal vectors of fibre-fibre contacts remains almost the same. Figure 5c shows the evolution of the mean coordination number 𝑧 as a function of the sample compression strain |𝜀 33 |. The error bars shown in the graph correspond to an error of ±1 voxel that was related to the extraction procedure of the centerlines of each fibre of the suspension. This figure shows that the mean coordination number 𝑧 was initially close to 2. This proves that the studied suspension was in the concentrated regime and that the fibres formed a connected network. In addition, this graph shows that 𝑧 slightly decreased with increasing the compression strain |𝜀 33 |. This evolution could be related to the flow-induced orientation of fibres along the 𝒆 1 axis. It is [START_REF] Toll | Note: On the tube model for fiber suspensions[END_REF][75] [START_REF] Orgéas | On the role of fibre bonds on the elasticity of low-density papers: a micro-mechanical approach[END_REF] that the mean number of fibre-fibre contacts decreases as the fibres orientate along a preferred/principal direction. It is important to notice that this trend is also well reproduced by the statistical tube model [START_REF] Toll | Note: On the tube model for fiber suspensions[END_REF] even if the theoretical predictions of 𝑧 are slighly higher than the measurements. This results is in agreement with several observations made from 3D images of fibrous materials by Orgéas et al. [START_REF] Orgéas | In-plane conduction of polymer composite plates reinforced with architectured networks of Copper fibres[END_REF] and Guiraud et al. [START_REF] Guiraud | Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests[END_REF]. 3 and that predicted by the statistical tube model (Eq. 3), respectively.

Flow mechanisms at the fibre and fibre-fibre scale

Using the correlation technique [START_REF] Le Corre | A 3D image analysis method for fibrous microstructures: discretization and fibers tracking[END_REF], it was also possible to analyse the flow mechanisms and the complex microstructure of fibre suspensions under flow at the fibre and contact scale. In particular, it was possible to describe the kinematics of single fibres that could be recognised, identified and followed during plane strain compression (Figure 6). The graph in Figure 7a shows the performance of the used semi-discrete correlation procedure. This graph shows the evolution during a plane compression experiment of the percentage of good fibre associations obtained by correlation between discretised 3D images at a compression deformation state |𝜀 33 | and that at |𝜀 33 | + 𝛥|𝜀 33 |. Thanks to the recent progress offered by synchrotron radiation facilities, 3D high resolution images were obtained at a sufficiently high frequency, i.e., with deformation increments between images Δ|𝜀 33 |< 0.03, thus leading to a high percentage (>90%) of fibre associations. Hence, as shown in Figure 6, it was possible to track the positions and orientations of a selection of neighbouring fibres taken at the centre of the sample as well as the positions and the number of contacts between them. The 3D images on the left-hand side show all fibre centrelines and their evolution for various deformation state |𝜀 33 | of the sample subjected to plane strain compression. The images on the right-hand side are zoomed views in the core of the sample where the fibre centrelines of tracked fibres appear in the form of coloured thick lines and their discrete contacts with the neighbouring fibres (represented by thin grey lines) are symbolised by red dots (Figure 6). For comparison purpose, the evolution of the orientation angles predicted by Jeffery's model (continuous lines) are also plotted in this figure. These three graphs clearly illustrate the erratic motion of these fibres in their network. The graph b reveals that the translation motion of tracked fibres was different from that they should follow under the affine field assumption. These deviations are much more pronounced than those observed for dilute and semi-dilute fibre suspensions [48][49]. As discussed in these previous studies, it is difficult to determine whether the general trend of the deviations had physical origins (e.g. small shear flow related to non-ideal lubrication conditions between the samples and the compression platens) or were induced by experimental artefacts (e.g. unexpected rigid body motions of samples). However, the evolution of Δx 𝑖 of the analysed fibres also revealed that they were subjected to several events (related for instance to the loss or creation of fibre-fibre contacts) that induced pronounced and erratic variations in the position of their centres of mass. Some of these events are clearly visible for the blue fibres in the graph b of Figure 7 and are now indicated by arrows.

Note also that all the analysed fibres from graph b were affected by this type of events. In addition, the graphs of Figures 7c-d reveal that the rotation of each fibre was extremely erratic and showed large fluctuations, i.e., a behaviour far from that predicted by Jeffery's equation (depicted by the continuous lines), as revealed by the fluctuations of the 𝜃 𝑖 and 𝜑 𝑖 angles measured during compression. The rotation of these fibres was presumably hindered by their contacts with their neighbouring fibres (zoomed views of Figure 6). For the highest compression strain, almost all the fibres tended to align along the main flow direction (increase of 𝜃 𝑖 and 𝜑 𝑖 angles). However, some fibres exhibited an inverse trend, see for instance the yellow, green and red fibres. These results obtained in the core of the sample, i.e., in a zone far from the two platens, are different from those observed for dilute and semi-dilute fibre suspensions [48][49]. In the present case, the erratic translation and rotation motions of fibres were induced by the multiple interactions between fibres. However, in spite of the chaotic kinematics of individual fibres it is interesting and surprising to notice that the macroscopic deformation of the sample remained homogeneous and the overall fibre orientation of the suspension could be well predicted by the second order fibre orientation tensor 𝐀 𝐽𝑒𝑓𝑓 (Figure 5a).

Conclusion

The objective of this study was to observe and characterise at the fibre scale, the flow mechanisms occurring in concentrated fibre suspensions. For that purpose, model concentrated fibre suspensions with a non-Newtonian suspending fluid and 3D random fibre orientations were prepared and deformed in confined flows by performing continuous tests with a compression rheometer that was installed on a synchrotron X-ray microtomograph. The image acquisition parameters enabled the acquisition of 3D images with high spatial resolution and short scanning time (i.e., less than 0.5 s). An image analysis procedure was specifically developed and enabled the extraction of several key microstructure descriptors such as the positions and orientations of fibres and their contacts. The evolution of these descriptors was followed using a semi-discrete correlation technique of the spatial positions of fibre centrelines [START_REF] Le Corre | A 3D image analysis method for fibrous microstructures: discretization and fibers tracking[END_REF].

These experiments enabled the 3D observation and characterisation of a confined plane strain compression test of a concentrated fibre suspension. The quantitative analysis of the 3D images led to original results that clearly emphasised the complex kinematics of individual fibres. Large fluctuations in the translational and rotational fields of the fibres were observed during plane strain compression. These fluctuations were shown to be on the same order of magnitude than the mean fields. The kinematics of fibres was found to be different from that observed for dilute and semi-dilute fibre suspensions under similar confined flow conditions [48][49]. We also observed that contacts between fibres induce locally very strong perturbations in the fibre motion, leading to fibres kinematics that are far from the affine field and local Jeffery's equation predictions. This emphasises the central role played by the interactions between fibres on the rheology of these concentrated suspensions.

Surprisingly, in spite of the chaotic kinematics of the fibres, the macroscopic deformation of the suspension was quasi homogeneous and the overall fibre orientation in the suspension was found to be well described by the second order orientation tensor estimated from the integration of Jeffery's equation for each fibre of the suspension during compression. In addition, our results showed that the statistical tube model [START_REF] Toll | Note: On the tube model for fiber suspensions[END_REF] was relevant to estimate the mean number of fibre-fibre contacts per fibre in concentrated fibre systems. The knowledge of the number of contacts is crucial to feed rheological models that enable estimating the predominant contribution of contacts to the overall stress field [START_REF] Orgéas | 5 -Rheology of Highly Concentrated Fiber Suspensions[END_REF][29] [START_REF] Djalili-Moghaddam | A model for short-range interactions in fibre suspensions[END_REF] within such systems.

The experimental results obtained in this study are promising and would enable improving micromechanical models and upscaling approaches related to the rheology modelling of concentrated fibre suspensions [START_REF] Orgéas | 5 -Rheology of Highly Concentrated Fiber Suspensions[END_REF]. This approach would also enable an in-depth characterisation of the fibre mesostructures (fibre aggregates) present in these suspensions (Figure 8). This would help understanding the complex micromechanics of these mesostructures and would help revealing the role they play on the rheology of concentrated fibre suspensions.

2D map showing the fibre density in the suspension (for a compression strain of |𝜀 33 | = 0.2) obtained thanks to the "Bivariate Kernel Density Estimator" function implemented in Matlab from the fibre centrelines projected in the (𝐞 2 , 𝐞 3 ), (𝐞 1 , 𝐞 2 ) and (𝐞 1 , 𝐞 3 ) planes, respectively. These maps show the presence of fibre mesostructures or aggregates within the fibre suspension, in particular in the (𝐞 1 , 𝐞 2 ) plane.

  [38][16][39][START_REF] Dupret | Modelling and simulation of injection molding[END_REF][29][START_REF] Chung | Invariant-based optimal fitting closure approximation for the numerical prediction of flow-induced fiber orientation[END_REF].

  Illustration of the procedure used to individualise each fibre. (a) Initial 3D image of a concentrated fibre suspension segmented on the fibre phase. (b) Same 3D image obtained after segmentation on the Euclidean distance map calculated on the fibre phase and (c) after skeletonisation and smoothing of fibre centrelines.

  (a) Evolution of the diagonal components 𝐴 ii of the measured second-order fibre orientation tensor 𝐀 with the sample compression strain |ε 33 |. The continuous lines represent the predictions of the calculated second order orientation tensor 𝐴 𝑖𝑖 𝐽𝑒𝑓𝑓 . (b) Evolution of the diagonal components 𝐶 𝑖𝑖 of the second order orientation contact tensor C with the sample compression strain |ε 33 |. (c) Evolution of the mean coordination number z with the sample compression strain |ε 33 |. The blue and red curves represent the mean coordination number 𝑧 that was directly measured from the 3D images shown in Fig.

  (a-d) Left-hand side 3D images: identified fibre centrelines during the plane strain compression of a concentrated fibre suspension at various compression strains |𝜀 33 |. (a-d) Right-hand side images: zoomed views in the core of the sample. These zoomed images show the evolution of the kinematics (positions and orientations) of nine fibres (represented by thick coloured lines) that were selected in the centre of the sample. The evolution of their discrete contacts with their neighbouring fibres (represented by thin grey lines) is also shown and represented by red dots. The graph in Figure 7b shows the evolution with |𝜀 33 | of the deviation Δx 𝑖 = ‖𝐱 𝑖 -𝐱 𝑖 𝐽𝑒𝑓𝑓 ‖ of the experimental position of the fibre centres of mass 𝐱 𝑖 with respect to 𝐱 𝑖 𝐽𝑒𝑓𝑓 of the nine tracked fibres. In addition, the two graphs c-d in Figure 7 show the evolution of the orientation angles 𝜃 𝑖 and 𝜑 𝑖 of the nine tracked fibres as a function of the macroscopic compression strain |𝜀 33 |.

  (a) Graph showing the percentage of fibre associations obtained using the semidiscrete correlation method of the fibre centrelines as a function of the sample deformation |ε 33 |. (b-d) Graphs showing the evolution of the deviations Δx i (b), angles 𝜃 i (c) and 𝜑 i (d) of nine tracked fibres taken in the centre of the sample as a function of the compression strain |ε 33 |.

Acknowledgments

This research was supported by IDEX Université Grenoble Alpes and Labex Tec21. The authors gratefully acknowledge LabEX Tec 21 and the research site of INSA Lyon at Oyonnax for administrative and technical support. The laboratories 3SR is part of the LabEx Tec 21 (Investissements d'Avenir-grant agreement n•ANR-11-LABX-0030) and the Carnot Institute Polynat (n• ANR16-CARN-0025).


Mariia Hruzd

Sebastien Gauthier
email: sebastien.gauthier@univ-rennes1.fr

Julien Boixel

Samia Kahlal

Nicolas Le Poul

Jean-Yves Saillard

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

Francoise Robin-Le Guen

Sébastien Gauthier

Nicolas Le Poul

Jean- Yves Saillard

Françoise Robin-Le Guen

N^N^C platinum (II) complexes based on phenylpyridin-2-ylpyrimidine ligands : synthesis, electrochemical and photophysical properties

Keywords: Platinum (II) complexes, Pyrimidine, N^N^C Ligand, Phosphorescence, Alkynyl complexes

published or not. The documents may come

Introduction

During the past two decades, with the development of second generation organic light emitting diodes (OLEDs) based on phosphorescent emitters, there has been a great interest in the design of luminescent transition metal complexes. 1 Indeed, 75% of electrically generated excitons are triplets and for classical fluorescent materials, the external quantum efficiency is therefore limited to 25%. 2 In this context, cyclometallated iridium (III) complexes have been extensively developed. 3 Platinum (II) complexes, even if less explored, also exhibit particularly interesting emission properties. [START_REF] Cebrián | Recent advances in phosphorescent platinum complexes for organic light-emitting diodes[END_REF] They display tunable emitting color over the entire visible spectrum and beyond depending of the structure of the ligands, 5 high photoluminescence quantum yield and long live excited state with triplet nature. [START_REF] Cebrián | Recent advances in phosphorescent platinum complexes for organic light-emitting diodes[END_REF] For luminescent cyclometallated Pt (II) complexes, tridentate ligands based on azaheterocycles can be used. [START_REF] Haque | Cyclometallated tridentate platinum(II) arylacetylide complexes : old wine in new bottles[END_REF] Complexes with N^C^N ligand are generally highly luminescent. [START_REF] Cebrián | Recent advances in phosphorescent platinum complexes for organic light-emitting diodes[END_REF][START_REF] Colombo | Platinum(II) complexes with cyclometallated 5-delocalized-donor-1,3-di(2-pyridyl)benzene ligands as efficient phosphors for NIR-OLEDs[END_REF] With N^N^C ligands, Pt(II) complexes can be less emissive due to a large structural distortion of the emitting triplet state, 8 but appropriate structural modifications of the tridentate ligand and choice of ancillary ligand permit to overcome this problem and high emission quantum yield can be obtained especially in solid state. [START_REF] Cebrián | Recent advances in phosphorescent platinum complexes for organic light-emitting diodes[END_REF][START_REF] Haque | Cyclometallated tridentate platinum(II) arylacetylide complexes : old wine in new bottles[END_REF] In particular the extension of -conjugation of cyclometalated ligand 9 and the use of phenylacetylide bearing electron-withdrawing group as ancillary ligand [START_REF] Chow | Luminescent pincer platinum(II) complexes with emission quantum yields up to almost unity: photophysics, photoreductive C-C bond formation, and materials applications[END_REF] tend to significantly enhance the emission quantum yield.

Pyrimidine is a six membered heterocycle bearing two nitrogen atoms in position 1 and 3. This aza-heterocycle exhibits a more pronounced -deficient character than pyridine. This fragment has been extensively used as electron-withdrawing group for organic push-pull chromophores for luminescent and nonlinear optical properties. [START_REF] Le Poul | Nonlinear optical properties of pyrimidine chromophores[END_REF] The pyrimidine core can be easily substituted in position 2 and 4 by (hetero)aromatic groups to form tridentate ligands. [START_REF] Abori | A new route for the synthesis of 2,4-bis(2-pyridyl)-6-(pyridyl)pyrimidines: synthesis ad characterization of Co(II), Ni(II) complexes of 2,4,6tris(2-pyridyl)pyrimidine[END_REF] Some phosphorescent Pt(II) complexes bearing bidentate [START_REF] Tang | An efficient heterodinuvlear Ir(III)/Pt(II) complex: synthesis, photophysics and application in light-emitting electrochemical cells[END_REF] or tridendate 14 pyrimidine ligands have been described in the literature.

In this work, we designed a series of Pt(II) complexes based on phenyl-pyridin-2ylpyrimidine ligands (Chart 1). The effects of the position of substituents on the pyrimidine core and the nature of the ancillary ligands on the photochemical and electrochemical properties of complexes were thoroughly investigated experimentally and theoretically.

Chart 1. Structural formulas of N^N^C cyclometalated Platinum(II) complexes 8-17.

Results and discussion

Synthesis and characterization

The synthesis of the complexes requires the cyclometalating tridentate ligands 2, 3, 6 and 7 whose preparation was accomplished in a two-step synthesis as shown in Scheme 1. In a first step, the 2-chloro-4-methyl-6-(pyridin-2-yl)pyrimidine 1 was produced by palladiumcatalyzed Stille cross-coupling reaction of 2,4-dichloro-6-methylpyrimidine with 2-(tributylstannyl)pyridine in the presence of Pd(PPh 3 ) 4 under reflux of toluene in moderate yield. [START_REF] Hadad | 4-Arylvinyl-2,6di(pyridine-2-yl)pyrimidines: synthesis and optical properties[END_REF] In the second step, the mono-halogen derivative 1 was converted into the corresponding ligands 2 and 3 under Suzuki-Miyaura cross-coupling reaction in presence of the appropriate boronic acids in good yields. [START_REF] Klikar | Dipolar NLO Chromophores Bearing Diazine Rings as π-Conjugated Linkers[END_REF] The ligands 6 and 7 were prepared by reactions similar to those described for 2 and 3, but using the Suzuki-Miyaura cross-coupling reaction as initial step, followed by a Stille cross-coupling reaction. As already observed, the coupling reactions using one equivalent of stannyl or boronic acid derivatives with 2,4-dichloro-6methylpyrimidine exhibit good selectivity with only the mono-coupling product in position 4 of the pyrimidine being obtained. 17 The N^N^C cyclometalated chloro-platinum(II) complexes precursors 8, 9, 14, and 15 were obtained in a good yield after a 3-day reaction between the corresponding ligands and potassium tetrachloroplatinate (K 2 PtCl 4 ) in a refluxed solution of glacial acetic acid (Scheme 2). [START_REF] Vezzu | Highly Luminescent Tridentate N ∧ C*N Platinum(II) ComplexesFeatured in Fused Five Six-Membered Metallacycle and Diminishing Concentration Quenching[END_REF] The chloro-platinum(II) 9 and 15 with tBu group were selected as precursors for the preparation of σbonded platinum(II) acetylide. This kind of functionalization with a tBu group usually provides complexes that are more soluble in organic solvents and reduces aggregation, thus facilitating the purification and chemical characterization of the compounds.

Thereby, N^N^C cyclometalated alkynyl-platinum(II) complexes 10-13, 16, and 17 were prepared under conventional cross-coupling condition by reaction of chloro-platinum(II) precursors 9 and 15 with the corresponding organic alkynes in the presence of a catalytic amount of copper(I) iodide and triethylamine at 50°C. [START_REF] Lu | Light-Emitting Tridentate Cyclometalated Platinum(II) Complexes Containing σ-Alkynyl Auxiliaries: Tuning of Photo-and Electrophosphorescence[END_REF] All complexes are air-stable and soluble in CH 2 Cl 2 , CHCl 3 , THF, and toluene, except the chloro-platinum(II) precursors 8, 9, 14, and 15 which display low solubility in common organic solvents.

Electrochemical properties

The redox properties of the N^N^C cyclometalated platinum complexes were investigated by cyclic voltammetry (CV) at a Pt working electrode in CH 2 Cl 2 /NBu 4 PF 6 0.1 M.

The electrochemical data are gathered in Table 1. All complexes display common CV features, including one reversible system in reduction at ca. E 1/2 (1) = 1.6 V vs. Fc +/0 , and one quasi-reversible or irreversible anodic peak at ca. E pa (2) = 0.5 V in oxidation (Figure 1). The values of the redox potentials are fully dependent on the nature of the ligands, for both reduction and oxidation processes. The chloro-platinum complexes 8, 9, 14, 15 were investigated first since they offer the possibility to rationalize the influence of the substituents on the pyrimidine moiety only on the electrochemical properties. As shown in Table 1, Figures 1A and S21, complexes 14 and 15 display E 1/2 (1) values which are 150-170 mV more negative than those found for complexes 8 and 9. This clearly suggests that the reduction process is more impacted by the position of the pyridyl group (2 or 4) on the pyrimidine than by the nature of the substituent (tBu or H) on the phenyl ring. This result is in agreement with previous studies on analogous chloro-platinum complexes bearing various nitrogen and substituted-phenyl. 13b,14b In oxidation, voltammetric studies of complexes 8, 9, 14 and 15

show that the process is dependent on the tBu or H substituents, since complexes 8 and 14 display lower E pa (2) values than 9 and 15 (120-160 mV difference, see Table 1). This result is consistent with DFT calculations which give computed HOMO energies at - 6.38, -6.32, -6.49 and -6.37 eV for 8, 9, 14, and 15¸ respectively. The computed ionization energies of the four complexes (Table 1) confirm this tendency. Additionally, cyclic voltammetry shows that the oxidation is partially reversible at v = 0.1 V s -1 for 8 and 14 (Figure 1A), whereas it is totally irreversible for 9 and 15 under strictly identical conditions. Such difference of reversibility in oxidation can be explained based on previous studies on cyclometalated platinum complexes. 14b The oxidation process is partially localized on the metal center with a major N^N^C cyclometalated-ligand contribution (see DFT calculations, vide supra) and generates a transient Pt(III) species, which may be stabilized by the solvent. The lifetime of the oxidized complex is thus close to the timescale of the CV experiment, yielding partially-reversible or irreversible anodic peaks.

Voltammetric studies of alkynyl-platinum complexes bearing various substituted phenylacetylide ligands were also performed. As shown in Table 1, E pa (2) increases slightly with the electron-withdrawing properties of the substituent (OCH 3 vs. CN or CHO) for the 10-12 and 16-17 series. This suggests that the oxidation occurring on alkynyl-ligand with of a small contribution of the metal. However, this trend should be taken with caution given the broadness of the irreversible peaks. Furthermore, the R 2 -dependent orientation of the phenylacetylide ring with respect to the Pt coordination plane (see DFT calculations, vide supra) may induce deviation from the expected linear variation Epa(2) with electronwithdrawing or donor properties in this series. Finally, comparison between complexes 11 and 16, or 12 and 17 confirms that the location of the pyridyl group in position 2 on the pyrimidine induces a negative shift (vs. 4-positionned) of the reduction potential E 1/2 (1) by 150-160 mV (Figure S21), as found for chloro-Pt complexes, without significantly affecting the oxidation potential. This effect is can be clearly seen in Figure 1B. a Irreversible anodic peak whose low potential value can be ascribed to the high contribution of the dimethylamino moiety to the HOMO (see Fig. S7).

Photophysical properties

The photophysical properties of the synthesized chloro-and alkynyl-platinum(II)

complexes have been investigated in solution using UV-visible absorption and luminescence spectroscopies. The steady-state absorption and luminescence spectra of complexes 10-13, 16

and 17, are presented in Figures 2,3, and 4, respectively, and the related data are summarized in Table 2. to 16 and17 (Figure 1B and Table 1).

With the exception of complex 13, all the new platinum complexes exhibit luminescence at room-temperature in diluted solutions, in frozen matrices and in the solid state. Additional non-radiative deactivation pathway through photo-induced electron transfer can explain the luminescent-silent character of 13. [START_REF] Chang | The synthesis, crystal structure and photophysical properties of molonuclear platinum(II) 6-phenyl-[2,2']bipyridynyl acetylide complexes[END_REF] As far as the room-temperature luminescence is concerned, chloro-platinum complexes 14 and 15 (2-pyridyl series) display structured emission profile ( max = 580 and 585 nm, respectively) with vibrational spacing of about 1100 cm -1 , substantiating a ligand-centered character of the emissive states. Complexes 8 and 9 (4-pyridyl series) show broad emission bands centered at 608 nm due to a charge transfer emissive state. Similarly to the observation made for their absorption features, the tBu groups do not play a significant role in the luminescence energy at room-temperature.

Complexes 8 and 9 exhibit bi-exponential decays as previously observed with other platinum complexes excited states. [START_REF] Golden | Tuning State Energies for Narrow Blue Emission in Tetradentate Pyridyl-Carbazole Platinum Complexes[END_REF] The photoluminescence quantum yields (PLQY) are about one percent for the 4-pyridyl chloro-platinum complexes 8, 9 and of about the double for the 2pyridyl analogues 14, 15. One can notice that these PLQY (1 to 2 %) are in accordance with those reported for related chloro(N^N^C)-platinum complexes. 22 When comparing the alkynyl-complexes of the 2-pyridyl series, 16 and 17, the OMe electron-donating group (in 16) red-shifts the  max of  = 762 cm -1 and drastically diminishes the emission intensity by a factor 5.5. This observation can be ascribed to additional nonradiative deactivation pathways through population of low-lying states ( 3 ILCT and 3 LLCT). Similarly, 11 bearing the p-anisole alkyne ligand in the 4-pyridyl series, exhibits red-shifted emission ( max = 940 cm -1 ) and a diminished PLQY (2.5 folds) with regards to the benzonitrile analogue 12. Replacing the benzonitrile group by the benzaldehyde (10) gives similar PLQY and luminescence decay times, revealing comparable photophysical behaviors.

Interestingly, the isomers 17 and 12 carrying the benzonitrile ligand in the 2-and 4-pyridyl series, respectively, display large differences in their emission features. The PLQY ratio is about 2.6 for 17/12 and the  max shift reaches 975 cm -1 , from 17 to 10. An even more pronounced red-shift of 46 nm is observed between 16 and 11, while they display similar low PLQY.

Figure S22 shows the luminescence spectra of complexes 8-12 and complexes 14-17

recorded in EPA mixture at 77 K. All the latter complexes display highly structured emission profile with vibrational spacing from around 1280 to 1380 cm -1 , 100 to 200 cm -1 higher than those reported for related 6-phenyl-2,2′-bipyridine platinum(II) complexes. [START_REF] Lu | Light-emitting tridentate cyclometalated platinum(II) complexes containing -alkynyl auxiliaries : tuning of photo-and electrophosphorescence[END_REF] The 0-0 emission bands of the chloro-platinum complexes are distributed according the electron richness of the ligand, starting with the complexes from the 2-pyridyl series (14 < 15) then the two of the 4-pyridyl series (8 < 9). One can notice that unlike room-temperature emission, the inductive effect of the tBu groups induces a slight bathochromic shift on the emission energy.

Computational studies

Density functional theory (DFT) calculations at the PBE0/LANL2DZ level of theory were performed to optimize the geometries of complexes 8-17. Solvent (CH 2 Cl 2 ) effect corrections were included (see Computational Details). The optimized geometries, characterized as true minima by vibrational frequency calculations, are shown in Figure 6.

Selected computed data are provided in Table 1 andS4. The ten complexes exhibit comparable optimized geometries, with rather similar metrical data. One can note that the chloro-platinum complexes 8, 9, 14 and 15 have slightly stronger Pt-N and Pt-C bonds than their phenylacetylide relatives. Within the phenylacetylide series, the major structural differences lie in the orientation of the phenylacetylide ring with respect to the Pt coordination plane. When this ring bears an electron-attracting group (CN or CHO), it is perpendicular to the rest of the molecule (complexes 10, 12 and 17), whereas in all the other compounds the molecule is close to ideal planarity. Obviously, the 16-electron metal center is maximizing its back-donation to the phenylacelylide ligand by using its π in-plane lone pair. This stronger conjugative effect is in line with the lowest C≡C Wiberg index values for 10, 12 and 17.

Consistently, the three complexes have the largest dipole moments of the series (Table 1 on the phenylacetylide chain.

The electron affinities (EI) of complexes 8-17, computed as the energy differences between the complexes and their anions (both in their equilibrium geometries), are given in Table 1. They correlate reasonably well with the CV-recorded E 1/2 (1) values and confirm the major influence of the position of the pyridyl group (Figure S33, left). A very similar correlation can be found between E 1/2 (1) and the LUMO energies (Figure S34), indicating that no significant structural or electronic rearrangement occurs upon reduction. The correlation between the recorded E pa (2) potentials and the ionization energies (IE) calculated as the energy differences between the complexes and their anions (both in their equilibrium geometry) is more approximate (Table 1 and Figure S33, right). This can be attributed to the non-reversible nature of E pa (2) and/or the perpendicular or parallel orientation of the phenylacetylide ring with respect to the Pt coordination plane according to the R 2 substituent.

Conclusion

In conclusion, a new class of luminescent N^N^C-cyclometalated chloro-and alkynylplatinum(II) complexes has been synthesized and characterized. The electrochemical and photophysical properties of these complexes have been carefully studied with the help of (TD-

)DFT calculations and except for complex 13, all exhibit yellow-to-red emission in solution and solid state. The following structure-property relationships can be highlighted:

-For chloro-platinum complexes, the absence of tBu substituent on the phenyl ring of N^N^C ligand induces a lower oxidation potential (E pa = 0.12-0.16 V), as well as a red-shifted excimer emission in solid state. In solution, the presence of tBu has no significant effect on the photophysical properties.

-The presence of alkynyl ligand shifts the oxidation potential of complexes. The Even if this new series of complexes display relatively low quantum yields, these results represent a useful study in the development of efficient phosphorescent emitters.

Experiments are currently underway to modify the auxiliary and the tridentate ligands of new cyclometalated platinum(II) complexes to improve the photoluminescence properties.

Experimental part

General information

In air-and moisture-sensitive reactions, all glassware was flame-dried. All reactions were conducted under a dry nitrogen atmosphere using Schlenk techniques, but workups were used as received. The starting materials were purchased from Sigma-Aldrich, TCI or Alfa-Aesar and were used as received. The solvents and reactants were used as received except tetrahydrofuran that was distilled under a dry nitrogen atmosphere over sodium and benzophenone. Organic solutions were concentrated under reduced pressure using a rotary (CRMPO, University of Rennes 1, France) using a Bruker MicroTOFQ II apparatus.

Cyclic voltammetry

The electrochemical studies were performed in a glovebox (Jacomex

) (O 2 < 1 ppm, H 2 O < 1 ppm)

Photophysical details

UV/vis absorption spectra were recorded with a Jasco V-770 UV-Vis-NIR spectrophotometer using quartz cuvettes of 1 cm pathlength. Steady-state luminescence spectra were measured using an Edinburgh FS920 Steady State Fluorimeter combined with a FL920 Fluorescence Lifetime Spectrometer. The spectra were corrected for the wavelength dependence of the detector, and the quoted emission maxima refer to the values after correction. Luminescence quantum yields were determined using [Ru(bpy) 3 ]Cl 2 (Φ = 0.042 in air-equilibrated aqueous solution) [START_REF] Demas | The measurement of photoluminescence quantum yields[END_REF] as standard and correcting for the refractive index. Life-times measurements were conducted with 375 nm diode laser excitation (EPL-series) plugged to a TCSPC pulsed source interface.

Computational details

DFT and TD-DFT calculations were carried out using the Gaussian16 package 26 employing the PBE0 functional, 27 together with the Def2TZVP basis set. [START_REF] Aschaefer | Fully optimized contracted Gaussian basis sets for atoms Li to Kr[END_REF] Solvent (dichloromethane) effects were included through the PCM approximation [START_REF] Acossi | Ab initio study of solvated molecules: A new implementation of the polarizable continuum model[END_REF] in all the calculations. The optimized geometries were fully characterized as true minima by analytical frequency calculations (no imaginary values). The composition of the Kohn-Sham orbitals was calculated using the AOMix program. [START_REF] Gorelsky | AOMix: program for molecular orbital analysis[END_REF] The geometries obtained from DFT calculations were used to perform natural atomic charge analysis with the NBO 6.0 program. [START_REF] Glendening | [END_REF] Only singlet-singlet transitions were taken into account in the TD-DFT calculations. The graphical GaussView interface 32 was used for simulating UV-vis spectra.

Synthesis

2-chloro-4-methyl-6-(pyridin-2--yl) pyrimidine (1), Method A:

In a Schlenk flask, aromatic 2,4-dichloro-6-methylpyrimidine (815 mg, 5 mmol, 1 equiv) was degassed by bubbling with nitrogen for 15 minutes. Toluene (60 mL) was added to the flask and stirred. Whereupon 2-(tributylstannyl)pyridine (2 g, 5.5 mmol, 1.1 equiv) was added and the mixture was stirred under nitrogen for 10 min. Pd(PPh 3 ) 4 (290 mg, 5 mol%, 0.05 equiv) was quickly added. The reaction was stirred under nitrogen at reflux for 24h. The mixture was allowed to cool to room temperature. The mixture was diluted with water, and the aqueous layer was extracted with EtOAc (2 × 25 mL). The separated organic phase extracts were dried over MgSO 4 , filtered and the solvents were evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (Petroleum ether to petroleum ether/AcOEt, 1:10, v/v) to

give compound 1 as a colorless crystalline solid in 52% yield (535 mg). NMR (δ (ppm),

CDCl 3 ): 1 H (300 MHz): 8.71 (d, 3 J HH = 3.9 Hz, 1H), 8.47 (d, 3 H, 2.85; N, 8.49, found: C, 38.93; H, 2.46; N, 8.38.

[Pt(C ∧ N ∧ N)Cl] 9: The product was synthesized by method C according to a similar procedure employed for complex 8, except the tridentate ligand 3 (91 mg, 0.3 mmol, 1 equiv) was used in place of ligand 2. The product 8 was obtained as as yellow powder in yield 98 % (157 mg [Pt(C ∧ N ∧ N)(CCC 6 H 4 -OCH 3 -p)] 16: The product was synthesized by method D according to a similar procedure employed for complex 10, except 4-Ethynylanisole (40 mg, 0.3 mmol) and complex 15 (54 mg, 0.1 mmol) were used in place of 4-Ethynylbenzaldehyde and complex 8. The product 16 was obtained as orange powder in 46 % yield (29 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 9.23 (d, 3 J HH = 5.9 Hz, 1H), 8.31 (d, 3 J HH = 7.9 Hz, 1H), 8. 18 -7.99 (m, 2H), 7.70 -7.63 (m, 1H), 7.50 (d, 4 J HH = 8.3 Hz, 2H), 7.31 (d, 3

Scheme 1 .

 1 Scheme 1. Synthesis of the N^N^C ligands 2, 3, 6 and 7.

Scheme 2 .

 2 Scheme 2. Synthesis of N^N^C Cyclometalated Platinum(II) Complexes 8-17.

Figure 1 .

 1 Figure 1. CVs at a Pt electrode of complexes 8 and 14 (A) (negative scanning), complexes 12 and 17 (B) (positive scanning) in CH 2 Cl 2 /NBu 4 PF 6 0.1 M (v = 0.1 V s -1 , C = 0.5 mM). The arrow indicates the initial sweeping direction.

  Chloro-platinum complexes 8, 9, 14 and 15, show characteristic UV-visible absorption features with a set of intense absorption bands in the UV region, from 250 to 370 nm, assigned to intra-ligand (IL) 1 -* transitions on the functionalized cyclometalated (N^N^C) ligands and moderately intense absorption bands from 385 to 420 nm, attributed to an admixture of intra-ligand 1 ILCT  Ph (N^N^C)→* bpy (N^N^C) and metal-to-ligand 1 MLCT d(Pt)→*(N^N^C) charge transfer. The two 4-pyridyl complexes 8 and 9 exhibit additional lower-lying absorption bands tailing up to 550 nm, in line with their smaller computed (see below) and voltammetric (Table1) HOMO-LUMO gaps. For both 2-pyridyl and 4-pyridyl chloro-platinum series, the tBu group on the phenyl part of the cyclometalated ligand has a minor influence on the absorption features.The alkynyl-platinum complexes(10-13, 16 and 17) show broad low-energy absorption bands of large ligand-to-ligand charge transfer nature ( 1 LL'CT (phenylacetylide)→*(N^N^C), see below), which strongly depend on the nature of the alkynyl ligand. Complexes 16 and 17, belonging to the 2-pyridyl series, show different absorption spectra ascribed to the opposed electronic characters of the alkynyl ligands; i.e.: electron-withdrawing (EWG: p-benzonitrile, for 16) and electron-donating (EDG: p-anisole, for 17) groups. Intense absorption bands, both in the UV (centered at 311 nm) and in the visible (from 375 to 500 nm) regions exhibiting an absorptivity twice that of the complex 16 bands, are observed for complex 17. However, complex 16 exhibits lower-lying transitions up to 575 nm attributed to its destabilized HOMO (see Figure S36), induced by the electron-donating character of its alkynyl ligand. Similar influences of the terminal function of the alkynyl ligand are observed for the 4-pyridyl series: Complex 11, bearing the p-anisole group, shows extended absorption tailing to 600 nm, while complexes 10 and 12, with p-benzaldehyde and p-benzonitrile respectively, display CT bands from 475 to 550 nm. When comparing the two 2-and 4pyridyl series with the same alkynyl ligand (12 vs 17 and 11 vs 16), the 4-pyridyl series exhibits bathochromic shifts of the charge-transfer transitions ( ≈ 20 nm) in accordance with the less negative reduction potential E 1/2 (1) recorded for complexes 11 and 12 compared

  17 552, 589 [61(43%), 338(57%)], 636 a  ex = 420 nm. b  ex = 375 nm. sh = shoulder, br = broad.

Figure 2 .

 2 Figure 2. (A): Absorption spectra in dichloromethane of chloro-platinum complexes (8, 9, 14 and 15) (C ≈ 10 -5 M); (B): TD-DFT-simulated UV-vis absorption spectra of chloro-platinum complexes (8, 9, 14 and 15).

Figure 3 .

 3 Figure 3. (A): Absorption spectra in dichloromethane of alkynyl-platinum complexes (10-13, 16 and 17) (C ≈ 10 -5 M); (B): TD-DFT-simulated UV-vis absorption spectra of alkynylplatinum complexes (10-13, 16 and 17).

Figure 4 .Figure 5 .

 45 Figure 4. Emission spectra in degassed dichloromethane at 298 K of chloro-platinum complexes 8, 9, 14 and 15 (A) and alkynyl-platinum complexes 10-12, 16 and 17 (B) (C ≈ 10 -

Figure 6 :

 6 Figure 6: DFT-optimized geometries of complexes 8-17.

  alkynyl-platinum complexes display broad low-energy absorption bands of long-range ligand-to-ligand charge transfer nature ( 1 LL'CT), which strongly depend on the R 2 substituent of the alkynyl ligand. CN and CHO electron withdrawing substituents induce hyspochromc shift of the less energetic absorption band with regards to methoxy analogues. Similarly, electron-withdrawing substituents lead to blue-shifted emission with increased PLQY.-The position of the pyridyl substituent also plays a significant role on the electrochemical and photophysical properties: complexes with electron-poorer 4pyridin-2-ylpyrimidine ligands exhibit a more positive reduction potential that the potential found for the 2-pyridyl complexes. Moreover, they show additional lowerlying absorption bands and red-shifted emission with reduced PLQY in solution with regards to 2-pyridin-2-ylpyrimidine analogues.

  evaporator. Thin layer chromatography (TLC) was conducted on pre-coated aluminum sheets with 0.20 mm Merck Alugram SIL G/UV254 with fluorescent indicator UV254 and 0.25 mm Merck silica gel (60-F254). Column chromatography was carried out using Acros silica gel 60 (particle size 63-200 m) and Macherey Nagel Aluminum neutral oxide 40 (particle size 40-160 m). Nuclear magnetic resonance (NMR) spectra were acquired at room temperature on a Bruker AC-300 spectrometer ( 1 H at 300 MHz,[START_REF] Tang | An efficient heterodinuvlear Ir(III)/Pt(II) complex: synthesis, photophysics and application in light-emitting electrochemical cells[END_REF] C at 75 MHz,) and referenced as follows: 1 H NMR, residual CHCl 3 (δ = 7.26 ppm); 13 C{ 1 H} NMR, internal CDCl 3 (δ = 77.16 ppm). The chemical shifts δ are reported in parts per million relative to TMS ( 1 H, 0.0 ppm) and CDCl 3( 13 C, 77.16 ppm). The coupling constant J is given in Hz. In the 1 H NMR spectra, the following abbreviations are used to describe the peak pattern: s (singlet), d (doublet), dd (doublet of doublet), t (triplet), and m (multiplet). Acidic impurities in CDCl 3 were removed by treatment with anhydrous K 2 CO 3 . IR spectra were recorded on a Perkin-Elmer spectrum 100 spectrometer with an ATR sampling accessory. High Resolution Mass Spectrometry (HRMS) analyses were performed at the "Centre Régional de Mesures Physiques de l'Ouest"

  with a home-made 3-electrode cell (WE: Pt, RE: Ag wire, CE: Pt). Ferrocene was added at the end of each experiment to determine the redox potential values. The standard potential of the Fc + /Fc couple in CH 2 Cl 2 /NBu 4 PF 6 was measured experimentally with reference to the standard calomel electrode (SCE): E 0 (Fc + /Fc) = 0.47 V vs. SCE. The potential of the cell was controlled by an AUTOLAB PGSTAT 100 (Metrohm) potentiostat monitored by the NOVA © software (Metrohm). Dichloromethane was freshly distilled from CaH 2 and kept under Ar in the glovebox. Tetrahydrofuran (99.9%, HPLC) was stored in the glovebox and dried over molecular for 48h before use. The supporting salt NBu 4 PF 6 was synthesized from NBu 4 OH (Fluka) and HPF 6 (Aldrich). It was then purified, dried under vacuum for 48 hours at 100°C, and then kept under N 2 in the glovebox.

Table 1 .

 1 Top: Electrochemical data (E 0 (1) and E pa (2)) for complexes 8-17 (0.5 mM) at a Pt working electrode in CH 2 Cl 2 /NBu 4 PF 6 0.1 M (E / V vs. Fc +/0 , v = 0.1 V s -1 ). Bottom: DFTcomputed electron affinities (EA), ionization energies (IE) in eV HOMO-LUMO gap (eV) and Dipole moment (D) in CH 2 Cl 2 .

		8	9	10	11	12	13	14	15	16	17
	E 1/2 (1)	-1.57 -1.61 -1.60 -1.61 -1.59 -1.61 -1.74 -1.76 -1.76 -1.75
	E pa (2)	0.49 0.61 0.56 0.46 0.51 0.02 a 0.50 0.66 0.47 0.55
	EA	3.07 3.05 3.07 3.01 3.06 2.98 2.88 2.85 2.83 2.89
	IE	6.15 6.02 5.76 5.22 5.80 4.78 6.17 6.12 5.22 5.80
	HOMO-LUMO gap 3.67 3.63 3.92 3.89 3.35 2.89 3.38 2.47 3.09 3.58
	Dipole moment	14.34 13.43 11.68 10.79 17.34 12.91 18.94 10.24 9.39 17.07

Table 2 .

 2 emission 24 as shown by the concentration dependent experiments demonstrating the respect of the Beer Lambert law (Figures S27-S30). The other complexes do not exhibit such excimer emission due to the hindering tBu group. Complexes 9-12 and 15-17 display structured solid-state emission with comparable 0-0 emission energy than that observed in frozen matrices (Figure 5). Complexes 11, 15 and 16 present both structured and broad excimer emission bands even though they bear a tBu group. Absorption and emission data in solution for platinum complexes 8-17. Measured in CH 2 Cl 2 solution at 298 K (C ≈ 10 -5 M). b In degassed CH 2 Cl 2 solution at 298 K (C ≈ 10 -5 M,  ex = 420 nm). c  ex = 375 nm. d With Ru(bpy) 3 Cl 2 as reference. e in EPA at 77K ( ex = 420 nm). sh = shoulder

	decays similar to that of 6-phenyl-2,2′-bipyridine analogues with about 7 -8 µs, while the 4-
	pyridyl complexes exhibit twice the decay time with around 15 -18 µs. Similar trends of
	longer emission life-time are observed in the alkynyl platinum complexes series with 40 %
	increased decays for the 4-pyridyl complexes.

Within the alkynyl-platinum series, the nature of the cyclometalated ligand impacts the emissive energy level with lower  max for the 2-pyridyl series, complexes 16 and 17, than for the 4-pyridyl analogues complexes 11 and 12. Also, the electron-richness of the aryl alkyne ligand induces drastic shifts of the emission energy, i.e.; within the 2-pyridyl series 16 and 17 that bear benzonitrile and anisole groups respectively show a  max of 41 nm (from  max = 526 to 567 nm), and for the 4-pyridyl series 10/12 (benzonitrile group:  max = 548/545 nm) versus 11 (anisole group  max = 589 nm) a  max of 43 nm is observed. Regarding the emission lifetimes of the chloro-platinum complexes, the 2-pyridyl series shows luminescence The solid-state luminescence of the chloro-platinum complexes 8 and 14 displays broad emission bands at 687 and 711 nm, respectively. These structureless low-energy emission originate probably from excimer formation more than from metal-metal-to-ligand charge transfer MMLCT a

Table 3 .

 3 Emission data in the solid state (powder) at 298 K for platinum complexes 8-12 and complexes 14-17.

		 em a / nm [ b /ns]
	8	711 [/]
	9	547, 584, 624 [169(74%), 848(26%)], 682sh
	10	612 [370(80%), 59(20%)], 644
	11	601 [61(18%), 208(82%)], 639, 730br
	12	543, 583, 605 [83(76%), 565(24%)], 661sh
	14	687 [105(25%), 254(75%)]
	15	517, 559, 605 [98(75%), 455(25%)], 700br
	16

627

[20(9%)

, 103(31%), 468(60%)]

  J HH= 7.3 Hz, 1H), 8.19 (s, 1H), 7.87 (t, 3 J HH= 7.8 Hz, 1H), 7.46 -7.41 (m, 1H), 2.63 (s, 3H);[START_REF] Tang | An efficient heterodinuvlear Ir(III)/Pt(II) complex: synthesis, photophysics and application in light-emitting electrochemical cells[END_REF] C{ 1 H} and JMOD (75 MHz) mg, 5.5 mmol, 1.1 equiv) were used in place of 1 and phenylboronic acid and the reaction time is 24h. The crude product was purified by silica gel column chromatography (Petroleum ether to petroleum ether/AcOEt, 10:1, v/v) to give compound 5 as a pale yellow The crude product was purified by neutral Al 2 O 3 column chromatography (Petroleum ether to petroleum ether/CH 2 Cl 2 , 3:2, v/v) to give compound 6 as a pale yellow oil in 40% yield (607 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 8.86(d, 3 J HH = 4.0 Hz, 1H), 8.66 (d, J HH = 7.9 Hz, 1H), 8.13(d, 3 J HH = 8.5 Hz, 2H), 7.86 (td, 3 J HH = 7.8, 1.7 Hz, 1H), 7.54 (75 mg, 0,3 mmol, 1 equiv) and K 2 PtCl 4 (125 mg, 0.3 mmol, 1 equiv) was degassed and backfilled with nitrogen three times. Then glacial acetic acid (60 ml) was introduced into the reaction flask by syringe. The reaction mixture was stirred under nitrogen protection under reflux for 72 h. The precipitate was filtered off and washed subsequently with water, methanol and diethyl ether. The product 8 was obtained and used without further purification as red powder in 95 % yield (136 mg). HRMS (ESI) m/z calculated for C 16 H 12 N 3 35 ClNa 195 Pt [M + Na] + : 499.0259, found: 499.0259 (0 ppm). Anal. Calcd. for C 16 H 12 N 3 ClPt·1H 2 O: C, 38.84;

	2-(4-(tert-butyl)phenyl)-4-methyl-6-(pyridin-2-yl)pyrimidine (3): The product was
	synthesized by method B according to a similar procedure employed for compound 2, except
	the (4-(tert-butyl)phenyl)boronic acid (980.0 mg, 5.5 mmol, 1.1 equiv) was used in place of
	171.6 (C), 165.4 (C), 160.9 (C), 152.6 (C), 149.6 (CH), 137.2 (CH), 125.9 (CH), 122.2 (CH), 115.2 (CH), 24.2 (CH 3 ). HRMS (ESI) m/z calculated for C 10 H 8 N 3 35 ClNa [M + Na] + : 228.0298, found: 228.0298 (0 ppm). 4-methyl-2-phenyl-6-(pyridin-2-yl)pyrimidine (2), Method B: In a Schlenk flask, 1 (1.0 g, 5 mmol, 1 equiv) and phenylboronic acid (671 mg, 5.5 mmol, 1.1 equiv) were introduced. The mixture was degassed by bubbling with nitrogen for 15 minutes. Whereupon toluene (48 mL), Na 2 CO 3 saturated solution (3 ml) and EtOH (2 mL) were added. The reaction mixture was stirred under bubbling with Nitrogen into the solution for 10 min. Then Pd(PPh 3 ) 4 (290 mg, 5 mol%, 0.05 equiv) was quickly added. The reaction was stirred under nitrogen at reflux for 48h. The mixture was diluted with water, and the aqueous layer was extracted with EtOAc (2 × 25 mL). The separated organic phase extracts were dried over MgSO 4 , filtered and the solvents were evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (Petroleum ether to petroleum ether/AcOEt, 3:7, v/v) to give compound 2 as a colorless crystalline solid in 97 % yield (1.2 g). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 8.65 (d, 3 J HH = 4.6 Hz, 1H), 8.63 -8,55 (m, 3H), 8.06 (s, 1H), 7.76 (t, 3 J HH = 7.0 Hz, 1H), 7.55 -7.41 (m, 3H), 7.32 -7.24 (m, 1H), 2.62 (s, 3H); 13 C{1H} NMR (JMOD, 75 MHz, CDCl 3 ) 168.3(C), 163.8(C), 162.3 (C), 154.4 (C), 149.2 (CH), 137.9 (C), 136.8 (CH), 130.4 (CH), 128.3 (CH), 128.3 (CH), 125.0 (CH), 121.7 (CH), 114.3 (CH), 24.5 (CH 3 ). HRMS (ESI) m/z calculated for C 16 H 14 N 3 [M + H] + : 248.1182, found: 248.1179 (1 ppm). phenylboronic acid. The crude product was purified by silica gel column chromatography (Petroleum ether to petroleum ether/AcOEt, 10:1, v/v) to give compound 3 as a colorless crystalline solid in 87 % yield (1.1 g). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 8.69 (d, 3 J HH = 4.8 Hz, 1H), 8.65 (d, 3 J HH = 7.0 Hz, 1H), 8.51 (d, 3 J HH = 8.6 Hz, 2H), 8.08 (s, 1H), 7.82 (td, 3 J HH = 7.7, 1.8 Hz, 1H), 7.54 (d, 3 J HH = 8.6 Hz, 2H), 7.34 ( ddd, 3 J HH = 7.5, 4.8, 4 J HH = 1.2 Hz, 1H), 2.65 (s, 3H), 1.39 (s, 9H); 13 C{1H} NMR (JMOD,75 MHz, CDCl 3 ) 168.4 (C), 164.2 (C), 162.4 (C), 154.7 (C), 153.7 (C), 149.3 (CH), 137.0 (CH), 135.4 (C), 128.2 (CH), 125.4 (CH), 125.1 (CH), 121.9 (CH), 114.1 (CH), 34.9 (C), 31.3 (CH 3 ), 24.6 (CH 3 ). HRMS (ASAP) m/z calculated for C 20 H 22 N 3 [M + H] + : 304.1808, found: 304.1809 (0 ppm). 2-chloro-4-methyl-6-phenylpyrimidine (4): The product was synthesized by method B according to a similar procedure employed for compound 2, except the 2,4-dichloro-6-methylpyrimidine (815 mg, 5 mmol, 1 equiv) was used in place of 1 and the reaction time is 24h. The crude product was purified by silica gel column chromatography (Petroleum ether to petroleum ether/AcOEt, 1:10, v/v) to give compound 4 as a white solid in 72 % yield (737 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 8.08 -7.96 (m, 2H), 7.50 -7.42 (m, 4H), 2.55 (s, 3H). The spectroscopic data are consistent with those reported in the literature. 33 4-(4-(tert-butyl)phenyl)-2-chloro-6-methylpyrimidine (5): The product was synthesized by oil in 50 % yield (652 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 7.77 (d, 3 J HH = 8.6 Hz, 2H), 7.32 -7.23 (m, 3H), 2.32 (s, 3H), 1.15 (s, 9H); 13 C{1H} NMR (JMOD, 75 MHz, CDCl 3 ) 170.1 (C), 165.8 (C), 160.65 (C), 154.6 (C), 131.7 (C), 126.6 (CH), 125.4 (CH), 113.7 (CH), 34.3 (C), 30.6 (CH 3 ), 23.5 (CH 3 ). HRMS (ESI) m/z calculated for C 15 H 17 N 2 35 ClNa [M + Na] + : 283.0972, found: 283.0970 (1 ppm). to petroleum ether/AcOEt, 3:7, v/v) to give compound 6 as a pale yellow oil in 72 % yield (891 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 8.86 (d, 3 J HH = 4.0 Hz, 1H), 8.66 (d, 3 J HH = 7.9 Hz, 1H), 8.25 -8.12 (m, 2H), 7.86 (td, 3 J HH = 7.8, 4 J HH =1.8 Hz, 1H), 7.58 (s, 1H), 7.55 -7.45 (m, 3H), 7.42 -7.35 (m, 1H), 2.72 (s, 3H); 13 C{1H} NMR (JMOD, 75 MHz, CDCl 3 ) 168.8 (C), 164.1 (C), 163.5 (C), 155.5 (C), 150.1 (CH), 137.0 (C), 136.9 (CH), 130.9 (CH), 129.0 (CH), 127.4 (CH), 124.7 (CH), 124.0 (CH), 115.5 (CH), 24.9 (CH 3 ). HRMS (ESI) m/z calculated for C 16 H 14 N 3 [M + H] + : 248.1182, found: 248.1185 (1 ppm). 4-(4-(tert-butyl)phenyl)-6-methyl-2-(pyridin-2-yl)pyrimidine (7): The product was synthesized by method A according to a similar procedure employed for compound 1, except the compound 5 (1.3 g, 5 mmol, 1 equiv) was used in place of 2,4-dichloro-6-75 MHz, CDCl 3 ) 168.6(C), 164.1(C), 163.5(C), 155.6(C), 154.4(C), 150.1(CH), 136.9(CH), 134.2(C), 127.2(CH), 126.0(CH), 124.7(CH), 124.0(CH), 115.2(CH), 35.0(C), 31.3(CH 3 ), 24.9 (CH 3 ). ). HRMS (ESI) m/z calculated for C 20 H 22 N 3 [M + H] + : 304.1808, found: 304.1811 methylpyrimidine. (d, J HH = 8.6 Hz, 3H), 7.42 -7.33 (m, 1H), 2.73 (s, 3H), 1.36 (s, 3H); 13 C{1H} NMR (JMOD, (1 ppm).

method B according to a similar procedure employed for compound 2, except the 2,4dichloro-6-methylpyrimidine (815 mg, 5 mmol, 1 equiv) and (4-(tert-butyl)phenyl)boronic acid (

980

4-methyl-6-phenyl-2-(pyridin-2-yl)pyrimidine (

6

): The product was synthesized by method A according to a similar procedure employed for compound 1, except the compound 4 (1.0 g, 5 mmol, 1 equiv) was used in place of 2,4-dichloro-6-methylpyrimidine and the reaction time is 48h. The crude product was purified by silica gel column chromatography (Petroleum ether [Pt(C ∧ N ∧ N)Cl] 8, Method C: A 250 ml Schlenk flask, charged with the tridentate ligand 2

  ). HRMS (ESI) m/z calculated for C 20 H 20 N 3 35 ClNa 195 Pt [M + Na] + : 555.0886, found: 555.0887 Synthesis of complex [Pt(C ∧ N ∧ N)Cl] 14: The product was synthesized by method C according to a similar procedure employed for complex 8, except the tridentate ligand 6 (75 mg, 0.3 mmol) was used in place of ligand 2. The product 14 was obtained as yellow powder in 75 % yield (108 mg). HRMS (ESI) m/z calculated for C 16 H 12 N 3 35 ClNa 195 Pt M + : 476.0362, found: 476.0358 (1 ppm). Anal. Calcd. for C 16 H 12 N 3 ClPt: C, 40.30; H, 2.54; N, 8.81, found: C, 40.04; H, 2.45; N, 8.56. The product was synthesized by method C according to a similar procedure employed for complex 8, except the tridentate ligand 7 (91 mg, 0.3 mmol) was used in place of ligand 2. The product 15 was obtained as green (brown) powder in 72 % yield (116 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 9.19 (d, 3 J HH = 4.8 Hz, 1H), 8.38 (d, 3 J HH = 8.5 Hz, 1H), 8.21 -8.10 (m, 1H), 7.90 (s, 1H), 7.77 (t, 3 J HH = 7.7 Hz, 1H), 7.38 (d, 3 J HH = 8.5 Hz, 1H), 7.24 (s, 1H), 7.17 (d, 3 J HH = 8.0 Hz, 1H), 2.62 (s, 3H), 1.37 (s, 9H). HRMS (ESI) m/z calculated for C 20 H 20 N 3 35 ClNa 195 Pt [M+Na] + : 555.0886, found: 555.0887 (0 ppm).

Synthesis of complex [Pt(C

∧ N ∧ N)Cl] 15:

CCC 6 H 4 -CN-p)] 17:

  J HH = 8.2 Hz, 1H),7.25 (s, 1H), 7.07 (dd, 3 J HH = 8.2, 1.9 Hz, 1H), 6.85(d, 3 J HH = 8.4 Hz, 2H), 3.83 (s, 3H), 2.62 (s, 3H), 1.37 (s, 9H). IR (ATR, cm -1 ): 2102 (ν C≡C ), 1590 (ν C=C ), 1526 (ν C=N ) 1032 (ν C-O ). HRMS (ESI) m/z calculated for C 29 H 28 N 3 O 195 Pt [M + H] + : 629.1875, found: 629.1876 (0ppm). The product was synthesized by method D according to a similar procedure employed for complex 10, except 4-Ethynylbenzonitrile (39 mg, 0.3 mmol) and complex 15 (54 mg, 0.1 mmol) were used in place of 4-Ethynylbenzaldehyde and complex 8. The product 17 was obtained as yellow powder in 77 % yield (48 mg). NMR (δ (ppm), CDCl 3 ): 1 H (300 MHz): 9.19(d, 3 J HH = 4.5 Hz, 1H), 8.42(d, 3 J HH = 7.8 Hz, 3H), 8.24 -7.96 (m, 2H), 7.77 -7.68 (m, 1H), 7.64 -7.51 (m, 4H), 7.45 (d, 3 J HH = 8.3 Hz, 1H), 7.32 (s, 1H), 7.18 (dd, 3 J HH = 8.3, 2.0 Hz, 1H), 2.67 (s, 3H), 1.35 (s, 9H). IR (ATR, cm -1 ): 2221(ν C≡N ), 2095 (ν C≡C ), 1591 (ν C=C ), 1562 (ν C=N ). HRMS (ESI) m/z calculated for C 29 H 24 N 4 Na 195 Pt [M + Na] + : 646.1541, found: 646.1542 (0 ppm).

[Pt(C ∧ N ∧ N)(

M,  ex = 420 nm).

Acknowledgments M. H. acknowledges the Région Bretagne, France and Conseil Départemental des Côtes d'Armor, France for her Ph. D. funding (MMLum project).