I Cazic

J Zollinger
email: julien.zollinger@univ-lorraine.fr

S Mathieu

M El Kandaoui

P Plapper

B Appolaire

New insights into the origin of fine equiaxed microstructures in additively manufactured Inconel 718

Keywords: Icosahedral Short-Range Order, Microstructure, Additive Manufacturing, Solidification, Nickel alloys Inconel 718 is

Fine equiaxed grain regions are frequently observed in additively manufactured Inconel 718 alloy. Based on a grainto-grain orientation analysis using EBSD, many assemblies of neighboring grains in these regions have been found to display multiple-twins orientation relationships sharing common 110 directions and exhibiting 5-fold symmetry. This is an experimental proof that the grain refinement observed in inconel 718 is due to Icosahedral Short-Range Order (ISRO) mediated nucleation mechanism. This is the first report of ISRO-mediated nucleation of fcc nickel which widens the perspectives on the microstructures control in additive manufacturing of Ni-based superalloys.

1

nuclear industries due to its excellent fatigue and corrosion resistance up to 650 • C. Hence, this alloy has been used 2 early in the development of additive manufacturing (AM) of metallic alloys [1]. Although intense research has been 3 devoted to achieve the best possible properties, controlling the grain structure established during solidification re-4 mains critical for that purpose [2]. Solidification microstructure impacts strongly both properties and processing; 5 primary dendrite arm spacing, grain size and grain texture affect the yield strength, fracture toughness and high cy-6 cle fatigue life, while having a grain refined equiaxed microstructure increases resistance to solidification cracking [START_REF] Kou | Welding metallurgy[END_REF]. Different strategies have been proposed to control the grain structure ensuing from solidification. Contrary to 9 conventional casting, it is possible with AM to play with the process parameters, such as the input energy, scanning 10 velocity and building strategy in order to establish thermal conditions promoting equiaxed grains, i.e. low temperature 11 gradients and fast solidification fronts. Along this line, the most successful solution relies on preheating the substrate, 12 mainly in electron beam melting (EBM) technology [START_REF] Körner | MATEC Web of Conferences[END_REF][START_REF] Dehoff | [END_REF]6,7,8,9,10], and predicted as potentially efficient in other 13 technologies such as direct energy deposition (DED) and selective laser melting (SLM) [10]. This solution is often 14 associated with high energy inputs contributing to establish low temperature gradients [11]. It seems more difficult to 15 identify general trends for the scanning speed because the relation with the velocity of the solidification front is far 16 from direct [10,12,13,14]. The same statement can be done about the building strategies, although some success has been achieved for direct laser sintering [15]. Enhancing the nucleation of new grains in the supercooled liquid pool is another promising route for controlling the grain structure. Hence, solutions similar to usual practices in conventional casting or welding have been identified, such as addition of grain refiners in Ti64 [16], dendrite fragmentation in high entropy alloys [17] and endogenous nucleants generated by the remelting of intermetallic in Al alloys with Sc [18]. In some studies using DED process, it has been suggested that partially melted powder can act as heterogenous nucleation sites and promotes equiaxed grains [9,19].

In both welding and additive manufacturing technologies, fine equiaxed grains (typically ≤ 10 µm) have been observed by several authors for low to medium linear energy densities ranging from 0.11 J/mm (selective laser melting) to 117 J/mm (direct energy deposition) in AM processes such as SLM [20,21], DED [13,22], EBM [START_REF] Benn | Proceedings of the 7th International Symposium on Superalloy 718 and Derivatives[END_REF] and coaxial laser-wire [START_REF] Bambach | [END_REF] processes. When this information is provided, the equiaxed microstructure is localised at the bottom of the layer in all the AM processes mentioned below and for a wide range of scanning velocities from 3 to 1000 mm/s. Different explanations to the equiaxed regions have been proposed so far. Bambach et al. have attributed the formation of equiaxed grains to local recrystallisation of the solidification microstructure [START_REF] Bambach | [END_REF]. Mostafa et al. and Choi et al. both have explained the formation of equiaxed grains by a higher concentration of impurities in remelted areas that could act as heterogeneous nucleation sites for equiaxed grains [20,21]. Other authors have pointed the effect of thermal gradient G and solidification velocity V, particularly the ratio G/V evolution during solidification that could change the growth morphology [START_REF] Benn | Proceedings of the 7th International Symposium on Superalloy 718 and Derivatives[END_REF]22]. In addition, Parimi et al. have underlined the possible local fluctuations in G due to Marangoni effect and in nucleation sites due to powder injection [13]. They have also shown that increasing the power prevents the equiaxed microstructure to appear. Finally, they have demonstrated that the building strategy does not play any role in the appearance of equiaxed microstructures, whereas increasing the power lead to a fully columnar microstructure.

In this work, the fine equiaxed microstructure observed in AM Inconel 718 was reproduced using a coaxial laserwire process. Based on detailed EBSD analysis, it is shown that the fine equiaxed grain regions consists of several assembly of nearest-neighbour grains having multiple-twins orientation relationship with a common 110 direction leading to a 5-fold symmetry and compatible with an icosahedron. This is the signature of the Icosahedral Short-Range Order (ISRO) mediated nucleation mechanism revealed by Kurtuldu et al. in Al-based and Au-based alloys [25,26]. This makes nickel the third fcc metal where such ISRO-mediated nucleation mechanism can operate, leading to a drastic reduction of the solidification grain size. It opens new prospects for obtaining fully equiaxed additively manufactured parts.

Samples of 150 x 27 x 7 mm 3 of Inconel 718 alloy were manufactured using a common commercially available Inconel 718 wire for welding applications (1.2 mm diameter). A Precitec CoaxPrinter laser head was used with a continuous laser beam to create an annular spot focused on the substrate, where the wire is coaxially fed into the melt pool, allowing to deposit the alloy layer by layer. Laser power of 2000 W, scanning speed of 1.2 m/min and wire feeding rate of 2 m/min were used for laser and wire metal deposition of the samples. Each deposited layer is in average 1 ± 0.2 mm in height. The laser source is a IPG 10 kW Yb 3+ -doped fibre solid-state laser with an operating wavelength of 1070 nm. The samples were deposited in an inert gas (Ar) atmosphere along a continuous bidirectional scanning strategy using an ABB IRB 6640-185 robot. The bidirectional scanning strategy consists in alternating building direction between each layer. Samples were cut in the x-z plane, z being the building direction and y the laser scanning direction, and prepared for both electron microscopy and electron back-scattered diffraction (EBSD) analysis by polishing respectively with 3 and 1 µm diamond solution then 0.5 µm Silica suspension (OPS).

These analysis were performed with 20 kV on a Zeiss Gemini SEM500 equipped with an Oxford Instrument Symmetry camera for high resolution maps. EBSD analysis were performed with a 300 nm step for macro mapping, and 200 nm step for micro mapping. Figure 1(a) shows an optical micrograph (OM) in the x-z plane, y being the laser scanning direction. The macroscopic "zigzag" grain structure is typical from bidirectional deposition [13]. Some areas, indicated by white arrows in the figure, appears in darker contrast and correspond to the fine equiaxed microstructure areas. In order to confirm that the grains are indeed equiaxed and not rotated columnar dendrites following the thermal gradient in the y-z plane of the molten pool, OM were taken in the y-z plane containing the scanning direction (not shown). The micrographs confirm that the fine grains are equiaxed, and surprisingly we have found that they form a continuous layer in the y direction. One of the equiaxed zones is shown in more details in Figure 1 1 shows the average grain size deduced from EBSD measurements and the relative twin frequency.

For columnar grains, the minimum Feret diameter was chosen to evaluate the grain size since their length can vary significantly depending on the observation plane. The columnar grains are ≈ 37 µm in width and their length can reach hundreds of micrometers, or even millimeters. The equiaxed zone shows a significant grain refinement with a size < 5 µm. Moreover, the twin frequency is about 1.3% in the columnar zone, close to the value in a MacKenzie distribution [27]. On the contrary, it is about 13% in the equiaxed zone, close to the value reported by Kutuldu et al.

when 200 ppm Ir is added to Au-Cu-Ag alloys in which an ISRO-mediated nucleation mechanism has been demonstrated [26]. In order to prove that this nucleation mechanism is similar to that identified in Al-Zn-Cr alloys [25] and Au-Cu-Ag-Ir alloys [26,28], multiple-twin orientation relationships between several fcc grains must be identified.

Several aggregates of nearest-neighbour grains having multiple-twin orientation relationships (OR) have been found in the equiaxed zone. In Al-Zn and Au-Cu-Ag alloys, the occurence of ISRO in the liquid is induced by minute additions of Cr and Ir, respectively. Approximant phases exhibiting icosahedral building blocks have been found in the Al-Cr system [30] as well as a few Au-rich Tsai-type icosahedral quasicrystals (i-QC) [28], but there is no known Ni-rich i-QC. Also, no specific element has been added and the microstructures have been obtained in this work with a commercial Inconel 718 wire. As shown by Zollinger et al. [28], rapid processing of ISRO-containing alloys can lead to a spinodal-like decomposition of the liquid phase, leading to local heterogeneities in the liquid phase. The effect of ISRO on heterogeneous local ordering has also been evidence by ab-initio molecular dynamics simulations in Al-Zn-Cr [31]. In the present work, the local heterogeneity leading to icosahedral clusters in the liquid may not be intrinsic, i.e. induced by a specific chemical element (such as Cr additions in Al-Zn alloys leading to Cr-centered icosahedra [31]), but induced by the rapid melting and solidification conditions inherent to AM processes. With heating rate in the general range of 1 to 8.10 4 K/s [32], the n -1 layer that partially remelt has little time for the ordered fcc phase to disordered liquid transition that could lead to partially disordered liquid or even metastable short-range order configurations including a certain fraction of ISRO. This hypothesis can be corroborated by an analysis based on the columnar-to-equiaxed transition (CET). According to Hunts steady state model [33], the CET mainly depends on three factors: the thermal gradient G, the solid/liquid interface velocity V and the number of potent nucleants ahead of the columnar front. Such analysis has been performed for AM Inconel 718 by Raghavan et al. for electron-beam melting process [6]. Based on a heat-transfer model and considering a constant nucleation density of 2.10 15 nucleus.m -3 , the authors showed that CET is more likely to occur at the end of the solidification of the layer, i.e. close to the free surface. Contrariwise, the high G and low V at the interlayer boundary is not favorable to a CET. Yet this is the position where the CET has been observed in this work, about 100 µm from the interlayer boundary. Assuming the G-V path determined by Raghavan et al. is valid, this means that the number of potent nucleus varies in the layer thickness, and is much more significant close to the interlayer boundary where the liquid phase comes from remelting of the n -1 layer. From the twin frequency measured in this work, and following the analysis of Kurtuldu and Rappaz [34], ≈ 13% of twin GBs relates to fraction of icosahedral nucleants over the total number of nucleants of 10%, each icosahedral cluster giving birth to potentially 20 fcc grains. The ISRO-mediated nucleation mechanism induced by the fast melting can explain why a CET occurs in high G and low V regions, by increasing strongly the local number of nucleation sites close to the interlayer boundary. This shows that the melting step can be employed beyond its usual role in the building process.

Our explanation for the occurence of small grains in Inconel 718 suggests a new route to refine the grain structure in alloys displaying ISRO mediated nucleation, in between the current strategies based on either the control of the thermal conditions [START_REF] Körner | MATEC Web of Conferences[END_REF][START_REF] Dehoff | [END_REF]6,7,9,10,11], or the enhancement of nucleation [9,16,17,19]. Indeed, the route based on ISRO mediated nucleation requires some melting that remains to be optimized. It is similar in spirit to what has been observed in Al alloys containing Sc [18], but with a different mechanism. Finally, it is worth noting that the reported grain sizes associated with the ISRO mediated nucleation seem in average lower than the sizes achieved by controlling the thermal conditions in Inconel 718. Even if this observation must be consolidated by quantitative comparisons, it suggests that ISRO mediated nucleation is worth to be further investigated for AM of Ni base superalloys.

To conclude, Inconel 718 alloy has been additively manufactured with a laser-wire co-axial process. Multiple equiaxe zones were observed in the sample. Based on detailed EBSD analysis, it has been shown that the equiaxed grains ensue from an ISRO-mediated nucleation mechanism, previously observed in Al-based and Au-based fcc alloys only. The icosahedral symmetry deduced from EBSD analysis of the fcc phase is expected to have originated from the fast remelting of the previous layer, providing metastable configurations in the liquid favorable to the nucleation of the fcc phase. These results open up new prospects for additive manufacturing of nickel based alloys. Indeed, controlling the ISRO nucleation mechanism in these alloys so as to obtain homogeneously fine equiaxed grains would (i) avoid solidification cracking, (ii) weaken texture to get isotropic mechanical properties, and (iii) benefit from the grain refinement to increase room temperature mechanical properties.

Figure 1 :

 1 Figure 1: (a) OM in the x-z plane of the deposited beads of Inconel 718, (b) SEM-BSE image corresponding to the dashed rectangle in (a), (c) EBSD false-color map of (b). The white arrows in (a) correspond to the equiaxed zone areas. (i-iv): see text for details.

  (b) on a SEM-BSE image. The area corresponds to the white dashed rectangle in Figure 1(a). Four distinct zones can be observed: (i) the n -1 layer in which growth direction is indicated by an arrow; (ii) columnar growth in the n th layer by epitaxy from layer n -1 where the change of growth direction is indicated with a white arrow; (iii) columnar-to-equiaxed transition leading to a fine equiaxed microstructure; (iv) equiaxed-to-columnar transition in which oriented growth leads to a columnar microstructure. Figure 1(c) is the false-colour EBSD map corresponding to Figure 1(b) where the different transitions are clearly visible. In this figure, identified twin grain boundaries are outlined with white lines whereas random grain boundaries (GB) with a misorientation > 10 deg. are shown with black lines: the twin boundaries are only detected in the equiaxed zone. The distributions of the grain size and orientation have been obtained from the EBSD and grain data. Table

Figure 2 (

 2 a) shows an assembly of grains in twinning orientation relationships, obtained from an high-resolution EBSD map, and labeled from 1 to 7. The 110 pole figures showing their mutual twin orientations are given in Figure 2(b-d) where the red arc of a circle is the trace of their common {111} plane. Between each pair of grains in Figure 2(b), the common {111} plane is rotated by 70.5 deg. around a common 110 direction, which is indicated by the red circle where all the pole figures in (b) are superimposed. The pairs of grains 1-2, 2-3, 3-4, 4-5 show perfect twin orientation relationships (OR). The orientation relationship between grains 5 and 1 correspond to what Kurtuldu et al. define as near-twin OR [25]: a twin OR and a rotation of about 7 to 8 deg around a common 110 direction. This assembly of 5 grains is compatible with the symmetry of a decahedron with grains 1 and 5 that accommodate the default aperture angle of 7.5 deg. This rotation corresponds to the cumulated angle difference between 5 tetrahedra of the icosahedron (360 deg) and 5 regular {111} tetrahedra of the fcc structure (352.5 deg) [29].

Figure 2 :

 2 Figure 2: (a) EBSD map of seven nearest-neighbour grains with multiple-twin orientation relationships taken from the equiaxed zone. The 110 pole figures in (b) shows the orientation relationships between pairs of grains, and the red arcs of a circle correspond to the trace of the common {111} plane (they are all drawn in the last pole figure). (c-d) 110 pole figures of grains 1-2-3-6 and 3-4-5-7. At the crossing of common {111} twin planes, there are 3 common 110 directions for grains 1-2-3, 1-2-6, 2-3-6 and 3-4-5, 3-4-7, 4-5-7, respectively. (e) Stereographic projections of the 5 common 110 directions of the fcc grains found (red squares), together with the calculated stereographic projection of the fivefold symmetry axes of the icosahedron (blue pentagons). (f) A perfect icosahedron with facets from which the fcc phase formed have the same colour and number as the grains in (a) and 5-fold axis have the same letter as in (e).

2

  figure). The 110 pole figure in Figure2(e) shows the 5 common 110 directions of the fcc grains, together with the calculated stereographic projection of the fivefold symmetry axes of the icosahedron. A perfect icosahedron is also shown in Figure2(f) with facets that have given rise to the fcc grains with the same color and number in Figure2(a). This confirms as in Al-based and Au-based alloys that ISRO-mediated nucleation mechanism applies for fcc Nickel in Inconel 718 alloy.

Table 1 :

 1 Grain sizes and twin frequencies for the columnar and equiaxed regions. For columnar grains, the grain size corresponds to the width of the grain corresponding to the minimum Feret diameter.

		Columnar region Equiaxed region
	Twin frequency (%)	1.3	12.8
	Grain size (µm)	37.76 ±7.14	4.54 ±3.62

Acknowledgements

This work was funded by the INTERREG VA "Grande Region" program "FAFIL" No. 050-4-08-126, and by the ANRT France under a CIFRE Ph.D. fellowship (Grant Number 2018-1630). The authors would like to thank C. Bernardi, D. Knispel and M. Schmitt for the assistance with the AM process.


Zakariae Asbai

Anna Bonfiglio

Pierluigi Mercandelli

Federico Polo

Matteo Mauro
email: mauro@unistra.fr

Cationic rhenium(I) complexes bearing a pi-accepting pyridoannulated N-heterocyclic carbene ligand: Synthesis, photophysical, electrochemical and theoretical investigation

Keywords: carbonyl complexes, density functional theory, N-heterocyclic carbenes, phosphorescence, rhenium complexes

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

Introduction

The photophysics and photochemistry of metal carbonyl complexes have been matter of intense investigation due to their interesting optical and redox features, [1] since early works of Wrighton, [2] Rillema, [3] and Demas [4] . A particularly appealing class of photoactive complexes is represented by the family of mononuclear Re(I) fac-triscarbonyl derivatives bearing N-donor heteroaromatic ligands with accessible * orbitals featuring -accepting properties. The interest in such compounds was mainly driven by their appealing application in photocatalysis (notably for CO2 reduction), [5] imaging probes in bio-medicine, [6] and emitters in organic light emitting diodes (OLEDs). [7] In this framework, derivatives of general formula [Re(N^N)(CO)3L] n+ , where N^N is a bidenatate N-heterocyclic ligand such as 2,2'-bipyridine, 1,10-phenanthroline and related scaffolds, and L is either an anionic monodentate ancillary ligand (L = halogen, cyanide, alkoxy, alkynyl, etc.; n = 0) or a neutral ligand such as pyridines, phosphines, isonitriles (with n = 1), are the most investigated ones by far. Several studies have shown that these compounds display broad and featureless photoluminescence in the green-to-red portion of the electromagnetic spectrum with excited state lifetime ranging from hundreds of nanoseconds to a few microseconds time scale. The nature of the emissive excited state may vary from purely triplet metal-to-ligand charge transfer ( 3 MLCT) to ligand-to-ligand charge transfer ( 3 LLCT) and up to (sizable) triplet ligand-centered ( 3 LC)

character depending on the nature and electronic properties of both the N^N and the ancillary ligand, with often a certain degree of mixing between the two states. [2,3] Hence, a metal-ligand-to-ligand character is more often associated to the emissive triplet manifold ( 3 MLLCT). Photoluminescence quantum yield (PLQY) values largely varies as well, being cationic rhenium(I) complexes typically more efficient than neutral counterparts with values almost one order of magnitude higher for the former. [4] Furthermore, neutral dinuclear species of general formula [Re2(-X)2(CO)6(-diaz)], where X = halogen and diaz = 1,2-diazine type of ligands, have shown superior properties and efficient electroluminescence, as reported by some of us. [8] Surprisingly, the vast majority of the phosphorescent derivatives containing the Re(CO)3 unit investigated so far features poly-pyridine type of ligands. On the other hand, limited attention has been devoted to investigate alternative ligands, in particular those containing stronger -donating moieties, such as N-heterocyclic carbenes (NHCs). This is in spite of the fact that NHC are outstanding ligands in organometallic chemistry, [START_REF] Mauro | gratefully acknowledge the Université de Strasbourg and CNRS for financial support[END_REF] and have demonstrated to play pivotal role in the preparation of efficient emitters with other transition metals, [10] including Ir(III), [11] Pt(II), [12] Au(I), [13] and Cu(I) [14] .

In this respect, Che and co-workers firstly described a series of [Re(N^N)(CO)3(NHC)]X complexes featuring an NHC as the ancillary ligand. [15] It is only in 2011 that Massi and co-workers [16] described the first examples of photoactive Re(I) tris-carbonyl complexes bearing an NHC as the chromophoric ligand, namely [ReX(CO)3(C^N)] with X = Cl -, Br -and C^N = 3-butyl-1-(2'pyridyl)benzimidazolin-2-ylidene NHC ligand. The complexes displayed 3 MLCT emission in the green-yellow region with PLQY 1%. Energetic stabilization of the * orbitals located onto the heteroaromatic ligand, achieved through extension of the -system and/or introduction of additional heteroatoms, [17] in related derivatives gave rise to a bathochromic shift of the 3 MLCT emission wavelength, as expected. This shift was accompanied in some cases by a prolongation of the excited state lifetime up to  = 1.07 s and an increase of the PLQY, with values falling in the range 0.1-13%. On the other hand, employment of the 3-(pyrid-2-yl)dimethylthiazol-2-ylidene as the NHC ligand gave a yellow phosphorescent complex displaying PLQY = 4% and  = 399 ns. [18] Concomitantly, Zheng and co-workers described related derivatives with either -conjugated or methylene-bridged C^N ligands containing pyridyl-benzimidazolin-2-ylidene, pyridyl-imidazolin-2ylidene and 2-pyridyl-1,2,4-triazoline-5-ylidene NHC scaffolds. [19] They found that the rupture of the -conjugation between the -accepting pyridyl group and the carbene moiety is detrimental for the emission properties of the final complex. Whereas, conjugated counterparts display photophysical properties that agrees with that earlier reported by Massi. [16][17][18] Prompted by our interest in the photophysics of novel rhenium(I) carbonyl compounds and encouraged by our previous results, [20] we aim at investigating the effect of neutral vs. cationic nature of the complexes onto both optical and redox properties in this family of tris-carbonyl rhenium(I) complexes bearing the pyridyl pyridoannelated NHC ligand, namely [pyipy]PF6. The variation of the overall charge of the complex is known to have profound effects on the photophysical properties of diimine-based counterparts. [2][3][4] However, to the best of our knowledge, it has not been investigated

for Re-NHC complexes. Herein, the synthesis, chemical, electrochemical and photophysical characterization of a series of [Re(CO)3(pyipy)(L)]PF6, where L = pyridine, PPh3, is presented along with the experimental data, which have been thoroughly rationalized also by means of computational approaches.

Synthesis and X-ray characterization

The schematic synthetic pathway employed for the synthesis of the target cationic complexes 3•PF6

and 4•PF6 is displayed in Scheme 1. The tris-carbonyl rhenium halo-complex 1-2 from [pyipy]PF6

and [ReX(CO)5] (X = Cl, Br), previously described by us elsewhere, [20] represent the starting material to synthesize the novel Re complexes. De-halogenation procedure is carried out by treatment with an Ag(I) source and metathesis with a PF6 -salt in a coordinating solvent, such as CH3CN, yielding the intermediate solvo-complex [Re(CO)3(pyipy)(CH3CN)]PF6. This latter is readily reacted with a monodentate ligand in THF to give the desired cationic complex 3•PF6 and 4•PF6, where L is pyridine and triphenylphosphine, respectively, as pale-yellow solids. The target complexes were fully characterized by means of 1 H and 13 C{ 1 H} NMR with spectra matching the predicted set of signals.

Elemental analysis and high-resolution electron spray ionization mass spectrometry (HR-ESI-MS)

techniques further confirmed the nature and the purity of the obtained compounds. The corresponding spectra are displayed in Figure S1-S6 of the Supporting Information. Suitable crystals of this latter were obtained by slow diffusion of n-hexane into a THF solution of the complex, which allowed to establish atom connectivity and geometrical parameters by solving the single crystal X-ray structure of derivative 3•PF6. The corresponding ORTEP diagram is displayed in Figure 1 and the crystallographic refinement parameters and metrics data are listed in Table S1-S2 of the Supporting Information. The X-ray structure shows a distorted octahedral arrangement of the coordinated ligands around the metal center, with the three carbonyls adopting a mutual facial arrangement, as expected. Among the three different Re-CO bond, the one lying in trans to the carbene moiety displays the longer Re-C bond distance, reflecting the larger trans influence exerted by the strong -donating moiety. A pyridine ligand completes the coordination sphere occupying remaining position.

Electrochemistry

The electrochemical behaviour of 3•PF6 and 4•PF6 was assessed by cyclic voltammetry (CV) in N,Ndimethylformamide (DMF)/0.1 M tetra-n-butylammonium perchlorate (TBAP) and showed the same pattern previously observed for neutral parental complexes 1 and 2. [20] In fact, as displayed in Figure 2, compound 3•PF6 and 4•PF6 showed an irreversible oxidation process (O1) in the positive bias, which is commonly observed for Re carbonyl complexes, at EO1 = +1.340 V and +1.391 V vs SCE, respectively. In the negative bias, one main irreversible reduction process (R1) occurs at ER1 = -1.285

V and -1.479 V vs SCE for 3•PF6 and 4 PF6, respectively. To better appreciate the redox processes, O1 and R1 were recorded separately and the voltammograms are shown in Figure 2. The electrochemical data are summarized in Table 1. The redox potential values for both O1 and R1 processes of 3•PF6 and 4•PF6 are shifted anodically, compared to 1 and 2, [20] due to the overall cationic character of rhenium(I) complex upon substitution of chloride with the neutral pyridine or triphenylphosphine ancillary ligand. In fact, O1 and R1 are related to the oxidation of the metal-NHC moiety and to the reduction of the chelating NHC ligand, respectively. With respect to 3•PF6, O1 occurs at more positive potentials (ca. 200 mV) than that of 1 and 2, whereas the electrochemical band gap is only slightly affected (ca. 0.1 eV), as reported in Table 1. Instead, with respect to 4•PF6, a Potential values are referred to SCE at the scan rate of 0.1 Vs -1 . b Ep/2 is defined as Ep-Ep/2, where Ep/2 is the potential value when the current i is at half its peak value, ip, for the processes under investigation (O1 or R1, respectively).

O1
The effect of the scan rate was investigated over the range 50-500 mV s -1 , and the peak current was found to depend linearly on the square root of scan rate (data not shown) thus witnessing that the heterogeneous electron transfer process is diffusion-controlled. On the other hand, Ep-Ep/2 varied in the range of 45-60 mV regardless of the scan rate, while peak potentials shifted towards more positive potentials (by ca. 20 mV) for O1 and more negative potentials (by ca. 40 mV) for R1. These data are in agreement with the occurrence of a diffusion-controlled heterogeneous electron transfer reaction, followed by a very fast chemical reaction (EC process). [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF]

Photophysics

The photophysical properties of complexes 3•PF6 and 4•PF6 were investigated at concentration of 2×10 -5 M in both air-equilibrated and degassed acetonitrile solution at room temperature, and at 77 K in 2-MeTHF glassy matrix. The electronic absorption and the normalized emission spectra are displayed in Figure 3 and the corresponding photophysical data are listed in Table 2. In the UV range, an intense absorption band in observed with λabs,max = 237 nm ( = 2.61×10 4 M -1 cm -1 ) and λabs,max = 231 nm ( = 4.22×10 4 M -1 cm -1 ) for compound 3•PF6 and 4•PF6, respectively. This band can be mainly ascribed to ligand-centered transitions localized onto the coordinated NHC-ligand in agreement with computational investigation (vide infra) as well as previously reported investigation on the neutral congeners. [20] The additional contribution of the -* transitions involving the phenyl rings of the PPh3 ligand in compounds 4•PF6 accounts for the increased molar extinction coefficient observed for this complex. At lower energy, the absorption band observed in the region at abs ca. reported previously. [20]

Computational investigation

To support the interpretation of the photophysical and electrochemical data, the electronic structures of the cationic complexes 3 + and 4 + and their absorption and emission spectra were computed employing density functional and time-dependent density functional theory. Molecular orbital plots and electron density difference maps (EDDM) computed for compound 3 + are displayed in Figure 4 and Figure 5, respectively; whereas, tables of calculated electronic transition properties are available as Supplorting Information in Table S3. For compound 4 + , corresponding data are available in Figure S9 and Table S4.

occupied molecular orbitals, close lying in energy. The HOMO and HOMO-3 are the two combinations (anti-phase and in-phase, respectively) of one of the d orbitals and the highest lying π orbital of the heterocyclic carbene moiety. On the contrary, the HOMO-1 and HOMO-2, on the contrary, do not show contributions from the organic ligand. In these four orbitals no contribution from the pyridine ancillary ligand is present. Lying at lower energy, HOMO-4 is a ligand π orbital extended over both the pyridyl and the heterocyclic carbene rings. The lowest unoccupied molecular orbital is a π* orbital delocalized all over the pyridylcarbene ligand. In addition, the LUMO+1 shows a substantial contribution from the lowest π* orbital of the pyridine ancillary ligand. The bands at lower energies in the absorption spectrum of complex 1 can be assigned to the superposition of two electronic transitions, S0→S1 and S0→S2 at 403 and 337 nm, corresponding mainly to the HOMO→LUMO and HOMO-1→LUMO monoelectronic excitations, respectively.

According to the nature of the orbitals involved, and looking at the corresponding electron density difference maps, both transitions imply a significant metal-to-ligand charge transfer (MLCT)

character. The lower-energy transition, however, shows also a noticeable intraligand character (IL).

In particular, it displays a net charge transfer from the heterocyclic carbene ring to the pyridyl moiety.

Two intense electronic transitions, S0→S19 and S0→S29 at 255 and 229 nm, dominate the region at higher energy and correspond to the two peaks observed experimentally at 265 and 237 nm.

They correspond to some complex admixtures of many monoelectronic excitations, mainly HOMO-3→LUMO+1, HOMO-4→LUMO+1 and HOMO→LUMO+10 (among many others). None of these transitions can be described as strictly ligand-centered (LC) only, since the redistribution of the electron density within the pyridylcarbene ligand is accompanied also by a charge transfer between the carbene and the ancillary ligand and by some redistribution of the electrons within the set of rhenium d orbitals. The complex nature of these transitions can also be appreciated from the corresponding electron density difference maps reported in the Supporting Information.

Geometry optimization of the lowest-lying triplet for cation 3 + leads to the individuation of a species analogous to that previously found for the neutral complexes 1 and 2. [20] In particular, with respect to the ground state, the triplet T1 shows an altered alternation of short and long bond lengths within the heterocyclic carbene moiety, leaving the rest of the molecule almost unaltered, in agreement with the LC nature of the T1→S0 transition. The same conclusion is reached when considering the corresponding electron density difference map, in which only a limited contribution of a rhenium d orbital can be detected, while the orbitals of the pyridine ancillary ligand are not involved at all. The electron density difference map computed for the cationic species 3 + shows an enhanced LC character of the emission with respect to the neutral species 1, in agreement with an increased contribution of the π orbital of the carbene ligand to the HOMO (69% and 52% in 3 + and 1, respectively).

Both vertical and adiabatic energies have been computed for the T1→S0 transition (see Figure 5). As expected, due to the substantial difference between the triplet and the ground-state geometry, Results for the triphenylphosphine derivative 4 + are analogous to the ones already described for the pyridine counterpart 3 + . However, in 4 the set of π orbitals of the phenyl rings of the phosphine ligand partially overlaps in energy with some of the rhenium d orbitals and with the π orbitals of the carbene ligand. The bonding and antibonding π orbital of the phenyl groups do not contribute to the lowest energy bands in the absorption spectra. Triphenylphosphine π-π* monoelectronic excitations, however, leads to an even more complex nature of the bands at higher energy. On the other hand, the computed emission energy remains almost unaltered on going from 3 + to 4 + , in agreement with the nature of the T1→S0 transition that does not show contributions from the orbitals of the ancillary ligand.

Experimental section

General considerations

[ReCl(CO)5] was purchased from Acros. The synthesis of complex 1 was carried out following a procedure reported elsewhere by us. [20] All procedures involving rhenium complexes were carried out under an argon atmosphere using standard Schlenk techniques. Silica gel for column chromatography was purchased from Sigma-Aldrich. Nuclear magnetic resonance spectra were recorded using a Bruker Avance III HD 500 spectrometer equipped with a N2 cryo-probe CPPBBO Prodigy at 298 K.

1 H and 13 C{ 1 H} NMR spectra were calibrated to residual solvent signals. Infrared spectra were recorded using a Fourier-transformed attenuated total reflectance infrared (FT-ATR-IR) spectrometer from PerkinElmer. Elemental analyses were obtained at the AMS Fédération de Chimie Le Bel, University of Strasbourg on a Flash 2000 ThermoFischer Scientific apparatus. HR-ESI-MS spectra were recorded on a MicroToF Bruker equipped with an electrospray ionization source.

Synthesis

3•PF6.

Complex 1 (100 mg, 0.20 mmol) and AgPF6 (87 mg, 0.26 mmol) were dissolved in 10 mL of CH3CN and refluxed in the dark for 18 h. The yellow solution was filtered on celite to remove AgCl and then the solvent was evaporated. The crude was dissolved in 5 mL of dry THF and pyridine ( 16mg, 0.20 mmol) was added under an argon atmosphere. The mixture refluxed for 2 hours. After cooling down the solution, THF was evaporated and the crude was purified on chromatographic column on SiO2 using CH2Cl2 as eluent mixture. A yellow solid was obtained. Yield 25% (38 mg, 0.05 mmol). 1 H NMR (500 MHz, 298 K, CD2Cl2) δ: 9.07 (ddd, J = 5.6, 1.5, 0.6 Hz, 1H), 8.49 (dd, J = 7.3, 1.0 Hz, 1H), 8.39-8.34 (m, 2H), 8.24 (m, 2H), 8.19 (d, J = 8.3 Hz, 1H), 7.83 (tt, J = 7.7, 1.5 Hz, 1H), 7.69 (ddd, J = 7.6, 5.6, 1.1 Hz, 1H), 7.64 (dt, J = 9.4, 1.0 Hz, 1H), 7. 33-7.29 (m, 2H), 7.14 (dd, J = 9.4, 6.5 Hz, 1H), 7.05-7.01 (m, 1H). 13 C{ 1 H} NMR (126 MHz, 298 K, CD2Cl2) δ: 206.5, 196.9, 189.4, 180.5, 153.5, 143.7, 139.2, 133.2, 126.9, 126.2, 124.9, 119.1, 117.5, 114.8, 108.6, 53.5, 30.6. Elemental analysis for C20H14F6N4O3PRe Calcd C 34.84 H 2.05 N 8.13, found C 34.12 H 2.08 N 7.87.

4•PF6.

For complex 4•PF6, a modified version of that previously reported for the synthesis of related [Re(CO)3(N^N)(PPh3)]PF6 derivatives was employed. [22] Complex 1 (100 mg, 0.2 mmol) and AgOTf (66 mg, 0.26 mmol) were dissolved in 10 mL of CH3CN and heated at 85 °C under the dark for 12 h.

The yellow solution was filtered over celite, then concentrated until 2 mL. A volume of 5 mL of saturated aqueous KPF6 was added dropwise to the solution. The suspension was kept stirring for 1 hour, then dried on the rotary evaporator to eliminate CH3CN. The precipitate was filtered, washed with ethanol and diethyl ether and air-dried. The obtained solid was dissolved in 10 mL of dry THF and PPh3 (1.0 g, 4 mmol) was added. The resulting mixture heated at 77 °C for 12 h under argon. The solvent was evaporated and the crude was purified by chromatographic column on Al2O3 using hexane/DCM (20:80) as eluent mixture to obtain a green oil. The oil was dissolved in 1 mL of DCM and diethyl ether was added dropwise under stirring until a solid precipitate. The solution was carefully removed with a syringe and the solid dried. A green solid was collected. Yield 25% (50 mg, 0.05 mmol). 1 H NMR (500 MHz, CD2Cl2) δ: 8.46-8.43 (m, 1H), 8.29 (s, 1H), 8. 18-8.13 (m, 1H), 8.00 (d, J = 8.3 Hz, 1H), 7.86 (dd, J = 7.4, 1.0 Hz, 1H), 7.52 (d, J = 9.4 Hz, 1H), 7.38 (td, J = 7.4, 1.8 Hz, 3H), 7.28 (td, J = 7.7, 2.5 Hz, 6H), 7.22-7.18 (m, 1H), 7.16-7.10 (m, 6H), 7.01 (dd, J = 9.4, 6.5 Hz, 1H), 6.76 -6.72 (m, 1H). 13 C{ 1 H} NMR (126 MHz, CD2Cl2) δ: 206.9, 206.4, 195.95, 195.9, 195.6, 195.5, 185.7, 185.2, 175.4, 175.3, 153.7, 152.5, 142.5, 133.4, 132.5, 132.4, 131.0, 130.3, 130.0, 129.0, 129.0, 126.6, 125.1, 124.5, 119.0, 116.5, 114.4, 108.6. Elemental analysis for C33H24F6N3O3P2Re Calcd C 45.42 H 2.77 N 4.81, found C 44.44 H 3.05 N 4.33.

X-ray diffractometric analysis

The crystals were placed in oil, and a single crystal was selected, mounted on a glass fibre and placed in a low-temperature N2 stream. X-ray diffraction data collection was carried out on a Bruker PHOTON III DUO CPAD diffractometer equipped with an Oxford Cryosystem liquid N2 device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 37 mm. The cell parameters were determined (APEX3 software) [START_REF]M86-EXX229V1 APEX3 User Manual[END_REF] from reflections taken from 1 set of 180 frames at one second exposure time. The structure was solved using the program SHELXT-2014. [START_REF] Sheldrick | [END_REF] The refinement and all further calculations were carried out using SHELXL-2014. [25] The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F 2 . A semiempirical absorption correction was applied using SADABS in APEX3. [START_REF]M86-EXX229V1 APEX3 User Manual[END_REF] Photophysical characterization Germany), while data analysis was performed using the built-in software FluoFit (PicoQuant GmbH, Germany). Data fitting was performed by employing the maximum likelihood estimation (MLE) methods and the quality of the fit was assessed by inspection of the reduced  2 function and of the weighted residuals. Luminescence quantum yields were measured in optically dilute solutions (optical density <0.1 at the excitation wavelength) and compared to reference emitter by following the method of Demas and Crosby. [26] The Ru(bpy)3Cl2 complex in air-equilibrated water solution at room temperature was used as reference (PLQY = 0.04). [27] All the solvents were spectrophotometric grade. Dearated samples were prepared by the freeze-pump-thaw technique by using a home-made quartz cuvette equipped with a Rotaflo stopcock.

Instrument

Electrochemistry setup

Anhydrous N,N-dimethylformamide (DMF, Sigma-Aldrich, 99.8%) and tetra-n-butylammonium perchlorate (TBAP, Fluka, 99%) were used as received. The working electrode was a glassy-carbon disk electrode (2 mm diameter, Princeton Applied Research GO224). The electrode was polished as already described elsewhere. [28] Before experiments, the electrode was further polished with a 0.05 μm polycrystalline diamond suspension (Buehler, MetaDI) and electrochemically activated in the background solution by means of several voltammetric cycles at 0.5 V s -1 between the anodic and the cathodic solvent/electrolyte discharges, until the expected quality features were attained. [29] A platinum wire served as the counter electrode and a silver wire, separated from the main electrolytic compartment by a Vycor® frit, was used as a quasi-reference electrode. At the end of each experiment, its potential was calibrated against the ferricenium/ferrocene couple, used as an internal redox standard. In DMF/0.1 M TBAP, ferricenium/ ferrocene has a formal potential of 0.464 V against the KCl saturated calomel electrode (SCE), [28] which allows to reference all the potentials against SCE. The cyclic voltammetry (CV) experiments were carried out in DMF/0.1 M TBAP under an Ar atmosphere, using a 1 mM concentration for the electroactive compound. A CHI 760D

Electrochemical Workstation (CH Instruments) was used. For the CV experiments, we employed the feedback correction to minimize the ohmic drop between the working and the reference electrodes.

Computational details

Ground state and low-lying triplet geometries were optimized by means of density functional calculations. The parameter-free hybrid functional PBE0 [START_REF] Adamo | Called PBE1PBE in Gaussian[END_REF] was employed along with the standard valence double-ζ polarized basis set 6-31G (d,p) for C, H, N, O and P. For Re the Stuttgart-Dresden effective core potentials were employed along with the corresponding valence triple-ζ basis set. All the calculations were done in the presence of solvent (acetonitrile, used in the photophysical characterizations) described by a polarizable continuum model (PCM). [31] The nature of all the stationary points was checked by computing vibrational frequencies and all the geometries were found to be true minima. In order to simulate the absorption electronic spectrum down to about 220 nm the lowest 40 (or 60) singlet excitation energies were computed for 3 + (or 4 + ) by means of timedependent density functional calculations. The vibrationally-resolved emission spectra were simulated in the framework of the Franck-Condon principle, [32] shifting the 0-0 energy to its observed value. All the calculations were done with Gaussian 16 software package. [33]

Conclusion

Two novel cationic tris-carbonyl rhenium complexes bearing the pyridoannulated NHC ligand pyipy is reported. For one of the derivatives, namely compound 3•PF6•THF, atom connectivity was unambiguously ascertained by solving the single-crystal X-ray structure. Both compounds display long-lived emission with structured profile in the red portion of the electromagnetic spectrum. The photoluminescence is attributed to an exited state with mainly 3 LC character as jointly supported by photophysical characterization and TD-DFT analysis. While the charged nature of the complex seems to do not affect the photoluminescence properties to a large extent, modulation of the redox properties was clearly achieved instead. Indeed, oxidation and reduction potential shifted anodically and cathodically, respectively, when compared to the parental neutral congeners. Overall, these features make the described class of photoactive Re-NHC complexes suitable candidates for potential application in photocatalysis under visible light irradiation.

Scheme 1 .

 1 Scheme 1. Schematic synthetic pathway used for the preparation of the cationic complexes 3•PF6

Figure 1 .

 1 Figure 1. ORTEP diagram of compound 3•PF6•THF with thermal ellipsoids shown at 50%

  occurs at even more positive potential compared to 3•PF6 (ca. 50 mV), whereas R1 is sizeably shifted cathodically (ca. 200 mV), owing to the triphenylphosphine ligand. The latter one led the bandgap to widen to 2.85 eV. Overall, the electrochemical data are in qualitative agreement with the widening of the optical bandgap observed in the UV-visible spectrum going from the neutral complex 2 to the cationic derivatives 3•PF6 and 4•PF6, as demonstrated by the hypsochromic shift of the onset of the 1 MLCT band recorded for these latter (see photophysics section).

Figure 2 .

 2 Figure 2. CVs recorded showing O1 and R1 redox processes for 1 mM of compounds 3•PF6 (black)

  400 nm is ascribed to an electronic transition with 1 MLCT character, namely d(Re)NHC →  * py, admixed with intraligand charge transfer ( 1 ILCT) involving the cyclometalating ligand with NHC →  * py nature, where NHC is the benzannulated NHC moiety and py is the pyridyl ring. Further details concerning the frontier orbitals involvement in the electronic transitions are provided in the theoretical section.Upon irradiation at the 1 MLCT band, complexes 3•PF6 and 4•PF6 show similar photophysical properties. The photoluminescence spectrum of both samples in degassed CH3CN show a structured emission with maxima at λem= 602 and 606, 660 and 725 nm at room temperature that are Stokes shifted by ca. 6500 and 6650 cm -1 for 3•PF6 and 4•PF6, respectively. The photoluminescence profile is independent of the presence of dioxygen molecules and excitation wavelength. Furthermore, similar vibronic progression is observed with fundamental spacing of ca.1350 cm -1 that is attributable to intraligand modes of the cyclometalatated pyipy scaffold. Upon removing dioxygen, increase of the photoluminescence quantum yield (PLQY) values (from 0.14% to 0.72%) as well as prolongation of the excited-state lifetime (from τ = 204 ns to 19.3 μs) is observed for compound 3•PF6. Likewise, PLQY and lifetime increase from 0.04% to 1.60% and τ = 209 ns to 30.0 μs, respectively, for derivative 4•PF6. Going from room temperature to 77 K glassy matrix, the photoluminescence spectra display a minor hypsochromic shift and band profile becomes even more structured. Overall, these findings along with the small kr values, being in the range 3.8-5.310 3 s -1 , are indicative for an excited state with largely triplet ligand centered ( 3 LC) character, in agreement with related complexes 1-2

Figure 3 .

 3 Figure 3. Electronic absorption and photoluminescence spectra of compound 3•PF6 (blue traces) and

Figure 4 .

 4 Figure 4. Isodensity surface plots and energies computed for some relevant molecular orbitals of fac-

  vertical and adiabatic values are quite different (0.376 eV). The adiabatic energy computed for the T1→S0 transition slightly underestimate the experimental value (624 vs 606 nm, corresponding to a difference of -0.058 eV or -464 cm -1 ). The vibrationally resolved emission spectra computed within the framework of the Franck-Condon principle reproduces reasonably well the observed additional maxima (673 and 752 nm vs 660 and 725 nm). As expected for a LC transition, the normal mode primarily contributing to the vibronic structure of the band is an IL one, and, in particular, an in-plane ring deformation (ν97, see FigureS10-S11 of the Supporting Information).

Figure 5 .

 5 Figure 5. Electronic density difference maps computed (at the optimized geometry of the

  details. Absorption spectra were measured on a Varian Cary 100 double-beam UV-VIS spectrophotometer and baseline corrected. Steady-state emission spectra were recorded on a Horiba Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp, doublegrating excitation, and emission monochromators (2.1 nm mm -1 of dispersion; 1200 grooves per mm) and a Hamamatsu R13456 red sensitive Peltier-cooled PMT detector. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Time-resolved measurements were performed using either the time-correlated single-photon counting (TCSPC) or the Multi Channel Scaling (MCS) electronics option of the TimeHarp 260 board installed on a PicoQuant FluoTime 300 fluorimeter (PicoQuant GmbH, Germany), equipped with a PDL 820 laser pulse driver. A pulsed laser diode LDH-P-C-375 (exc= 375 nm, pulse full width at half maximum FWHM <50 ps, repetition rate 200 kHz-40 MHz) was used to excite the sample and mounted directly on the sample chamber at 90°. The photons were collected by a PMA Hybrid-07 single photon counting detector. The data were acquired by using the commercially available software EasyTau II (PicoQuant GmbH,

Table 1 .

 1 Electrochemical

	Gap [eV]	Ref.

data for compound 3•PF6 and 4•PF6 obtained by cyclic voltammetry (CV) carried out in DMF and 0.1 M TBAP as the supporting electrolyte. Data for compound 1-2 and the chelating ligand [pyipy]PF6 are also reported from Ref. 20 for comparison. Cmpd Ep|O1 [V] a Ep/2|O1 [mV] b Ep|R1 [V] a Ep2|R1 [mV] b

Table 2 .

 2 Photophysical data for complexes 3•PF6 and 4•PF6 recorded in air-equilibrated and degassed CH3CN solution at room temperature and 77 K 2-MeTHF glassy matrix.

	Cmpd	max,abs ()	em	PLQY	τobs		kr	knr	em
		[nm, (10 3 M -1 cm -1 )]	[nm]	(%)
								[10 3 s -1 ]	[10 4 s -1 ]	[nm]
		405 (2.77),	air	air		air
				degassed	degassed		degassed		77 K
		339 (4.07),	equilibrated	equilibrated		equilibrated
	3•PF6	265 (14.88), 237	606, 660,	606, 660,					592,
				0.14	0.72	204 ns	19.3	3.79	5.23
		(26.09)	725	725					647, 710
		394 (3.00),
	4•PF6	329 (3.53),	602, 660,	602, 660,					592,647,
				0.04	1.60	209 ns	30.0 μs	5.33	3.28
		261 (19.29), 231	725	725
		(42.15)