Sriswaroop Dasari

Varun Chaudhary

Bharat Gwalani

Abhinav Jagetia

Vishal Soni

Stephane Gorsse

Raju V Ramanujan

Rajarshi Banerjee
email: raj.banerjee@unt.edu

Highly Tunable Magnetic and Mechanical Properties in an Al0.3CoFeNi Complex Concentrated Alloy

Keywords: Complex Concentrated Alloys, High Entropy Alloys, Nano-lamellar microstructure, Eutectoid decomposition, Magnetic properties

Electrical rotating machines, including motors, account for a significant portion of total energy consumption in the world. Improving the magnetic materials used in motors is a key challenge to increase their performance. Specifically, higher rotation frequency requires appropriate site specific magnetic properties as well as good mechanical properties. Hence, we studied both the magnetic and mechanical properties of an Al0.3CoFeNi complex concentrated alloy (CCA). Heat treatment, guided by phase diagram modeling, was employed to develop a novel eutectoid-like nano-lamellar (FCC+L12) / (BCC+B2) microstructure as well as a coarser FCC+B2 microstructure. The coarser microstructure exhibits soft magnetic properties with saturation magnetization (Ms) of ~127 emu/g, coercivity (Hc) of ~151 A/m and microhardness of ~ 195 VHN. On the other hand, the semi-hard nano-lamellar microstructure exhibits Ms ~138 emu/g, a high Hc ~12732 A/m and a very high microhardness ~ 513 VHN. This corresponds to more than eighty times increase in Hc and double the hardness in the same alloy. These results demonstrate the feasibility of producing a range of mechanical and magnetic properties by thermo-mechanical treatment of a single CCA composition, making them potential candidates for metamorphic manufacturing.

Introduction

Metamorphic manufacturing is a novel manufacturing approach to produce engineering components [START_REF] Spanos | Metamorphic Manufacturing: Shaping the Future of On-Demand Components[END_REF], which relies on incremental forming to achieve site-specific properties dictated by local microstructures. This new manufacturing approach requires materials whose properties can be varied over a wide range via thermomechanical treatment. Complex concentrated alloys (CCA) are potentially suitable materials for such manufacturing methods since the structural and functional properties of a single alloy composition can be varied over a wide range by suitable processing strategies [START_REF] Gwalani | Tensile yield strength of a single bulk Al0.3CoCrFeNi high entropy alloy can be tuned from 160 MPa to 1800 MPa[END_REF][START_REF] Su | Hierarchical microstructure design to tune the mechanical behavior of an interstitial TRIP-TWIP high-entropy alloy[END_REF]. In 3d transition (Co, Cr, Fe, Ni) based CCAs property manipulation is often achieved by systematically changing the phase stability and microstructure. Alloying elements such as Al and Ti can also be added both these elements have large atoms and can induce chemical ordering. With increasing Al content in such CCAs, the phases present typically change from single phase FCC (or FCC + L12), to a two phase FCC + BCC mixture (possibly also with L12 and/or B2 precipitates), and eventually to BCC + B2 [START_REF] Wang | Phases, microstructure and mechanical properties of AlxCoCrFeNi high-entropy alloys at elevated temperatures[END_REF][START_REF] Yeh | Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes[END_REF][START_REF] Manzoni | Phase separation in equiatomic AlCoCrFeNi high-entropy alloy[END_REF]. This effect of changing composition on the phase stability and consequently on the mechanical and magnetic properties has been previously demonstrated in AlxCoCrFeNi [START_REF] Wang | Effects of Al addition on the microstructure and mechanical property of AlxCoCrFeNi high-entropy alloys[END_REF], AlxCuCrFeNi2 [START_REF] Borkar | A combinatorial assessment of AlxCrCuFeNi2 (0< x< 1.5) complex concentrated alloys: Microstructure, microhardness, and magnetic properties[END_REF], and AlCoxCr1-xFeNi [START_REF] Borkar | A combinatorial approach for assessing the magnetic properties of high entropy alloys: role of Cr in AlCoxCr1-xFeNi[END_REF] CCAs. Chang and Yeh [START_REF] Chang | The evolution of microstructures and high temperature properties of AlxCo1.5CrFeNi1.5Tiy high entropy alloys[END_REF] reported microstructural changes with varying Al content and its influence on high temperature mechanical properties in AlxCo1.5CrFeNi1.5. Yang et al. [START_REF] Yang | Multicomponent intermetallic nanoparticles and superb mechanical behaviors of complex alloys[END_REF] recently developed an Al0.25Ti0.25CoFeNi CCA, based on the medium entropy CoFeNi system, with a high L12 volume fraction, this composition exhibited an excellent combination of tensile yield strength and ductility. These results suggests that we can systematically tune the mechanical as well as magnetic properties in Al containing CoFeNi CCAs. Hence, we selected an Al0.3CoFeNi alloy composition and employed systematic heat treatment to vary the microstructure. This microstructural variation was shown to dramatically influence magnetic coercivity and hardness, while retaining the magnetization values.

Soft magnetic materials are vital for many applications such as electrical power generation, rotating electrical machines, magnetic shielding, electromagnets etc. The global soft magnetic materials (SMM) market was ~ $47 billion in 2018, and is expected to reach ~$87 billion by 2026. SMM are vital in power electronics systems such as inductors, transformers, generators, electric motors. Electric Power Research Institute (EPRI) estimated that 60% to 65% of electrical energy is consumed by motor drive applications. Emerging electrical transportation systems also require better electrical conversion systems. Developing high performance, high frequency, high temperature electric motors require magnetic materials possessing high mechanical strength and good magnetic properties [START_REF] Silveyra | High speed electric motors based on high performance novel soft magnets[END_REF][START_REF] Collocott | Magnetic Materials: Domestic Applications[END_REF][START_REF] Bertotti | General-Properties of Power Losses in Soft Ferromagnetic Materials[END_REF][START_REF] Silveyra | Amorphous and Nanocomposite Materials for Energy-Efficient Electric Motors[END_REF][START_REF] Filippini | Magnetic Loss Analysis in Coaxial Magnetic Gears[END_REF][START_REF] Mu | New Core Loss Measurement Method for High-Frequency Magnetic Materials[END_REF][START_REF] Bozorth | Ferromagnetism, Ferromagnetism[END_REF][START_REF] Lefik | High temperature machines: topologies and preliminary design[END_REF][START_REF] Karthaus | Static and cyclic mechanical loads inside the rotor lamination of high-speed PMSM[END_REF][START_REF] Saidur | A review on electrical motors energy use and energy savings[END_REF].

The power of an electrical rotating machine is the product of the speed, thermal utilization, magnetic utilization and volume [START_REF] Ohodnicki | Soft Magnetic Alloys for Electrical Machine Applications: Basics[END_REF]. This implies that we need a material with high magnetic saturation, low core loss, high thermal conductivity and high strength. Conventional metallic SMM possess good magnetic properties but suffer from poor strength. Further, in rotors, the mechanical strength needs to be higher on the periphery compared to the central regions due to the larger centrifugal stresses. Site specific properties will facilitate spatial optimization of the properties.

Metamorphic manufacturing [START_REF] Spanos | Metamorphic Manufacturing: Shaping the Future of On-Demand Components[END_REF] can achieve site-specific properties by changing the local microstructures in a single material. Here, we demonstrate a large tunability in the magnetic and mechanical properties of Al0.3CoFeNi CCA via thermo-mechanical processing. This study could support in designing a next generation electric motor where site specific properties within a same material is highly desired.

Semi hard magnetic materials with high mechanical strength can also be useful in other applications, e.g.,, electronic article surveillance materials [START_REF] Liu | Semi-hard magnetic elements and method of making same[END_REF]. CCAs can open up a materials space exhibiting promising combinations of mechanical strength and magnetic properties. There have been a few investigations relating the Al content to the magnetic properties of CCAs.

Examples include AlxCoCrFeNi alloys exhibiting ferromagnetic behavior for x =0.5, 1.25 and 2, and paramagnetic behavior for x =0, 0. 25 and 0.75 [START_REF] Kao | Electrical, magnetic, and Hall properties of AlxCoCrFeNi high-entropy alloys[END_REF]. Zuo et al. [START_REF] Zuo | Effects of Al and Si addition on the structure and properties of CoFeNi equal atomic ratio alloy[END_REF], tuned the magnetic properties by changing the Al content in AlxCoFeNi alloys, the Hc was found to be ~215 A/m and 344 A/m for x = 0.25 and 0.50, respectively. Despite these previous reports it is important to note that metamorphic manufacturing will require alloys whose properties can be substantially changed via thermo-mechanical processing without changing the alloy composition. This study demonstrates that for a single Al0.3CoFeNi CCA, guided by thermodynamic modeling, the type and morphology of the precipitates in the complex multi-phase microstructure can be tuned via thermo-mechanical processing. This leads to large range of microhardness values (195 VHN to 513 VHN), and a very wide range of coercivity values (151 A/m to 12732 A/m). The extremely high microhardness and coercivity can be attributed to a novel eutectoid-like nano-lamellar microstructure.

Experimental Procedure

Al0.3CoFeNi alloy was produced using conventional arc melting. The composition of the cast ingot was found to be 9.4Al-30.1Co-31.9Fe-28.6Ni (at. %) by Energy dispersive spectroscopy in a Scanning electron microscope (SEM-EDS). The cast alloy was homogenized at 1250°C for 30 min, followed by 85% reduction in thickness by cold rolling. The samples were then encapsulated in quartz tubes backfilled with argon for subsequent heat treatments. The rolled samples were solution annealed at 1250°C for 5min (CRSA condition), followed by heat treatment at either 600°C for 50 h (CRSA600) or 700°C for 50 h (CRSA700). Some samples were only heat treated at 700°C for 50 h after cold rolling (CR700). Here, the acronym CR stands for cold rolled to 85% reduction in thickness and SA corresponds to solution annealing treatment at 1250°C for 5 min.

The temperature for solutionizing, to obtain a single phase microstructure was chosen based on the isopleth in Fig. 1(a). Microstructural characterization was performed using a FEI Nova-NanoSEM 230™ Scanning electron microscope coupled with an energy dispersive spectrometer (EDS) and Hikari Super Electron Backscattered Diffraction (EBSD) detectors, FEI Tecnai G2 TF20™ transmission electron microscope operated at 200 kV and a Cameca LEAP 5000XS 3D Atom Probe Microscope operated at 30K with a pulse fraction of 20% and detection rate of 0.5 in Laser mode. Microhardness measurements were carried out in a Vickers microhardness tester using a load of 500 gf. The magnetic properties were measured using a Lakeshore 7404 Vibrating Scanning Magnetometer (VSM).

Results and Discussion

1 Phase stability modeling

An isopleth for the AlxCoFeNi system was calculated using Thermocalc (TCHEA3 database) and is shown in Fig. 1(a). This isopleth can be treated as a pseudo-binary section of the complex phase diagram. The composition of Al0.3CoFeNi was chosen since this composition traverses several phase fields at different temperatures. Two different heat treatment temperatures were identified, 600°C and 700°C. The isopleth predicts a substantial change in phase stability from a three-phase FCC + L12 + B2 region at 600°C to a two-phase FCC + B2 region at 700°C. The calculated phase fractions as a function of temperature is shown in Fig. 1(b).

Scanning Electron Microscopy

In the single phase CRSA condition, large recrystallized grains (~150 m) of the FCC phase were observed with no intragranular or grain boundary precipitation, Fig. 2(a). A backscatter SEM image from the CRSA600 condition is shown in Fig. 2(b). The predicted phase stability at this temperature of 600°C, is a three-phase FCC+L12+B2 field (refer to Fig. 1). The image in Fig. 2(b) reveals a highly refined lamellar microstructure in all the grains, resembling the pearlite-like microstructure observed in steels. The inset in Fig. 2(b), recorded at higher magnification, indicates further decomposition within these lamellae.

The CRSA700 and CR700 samples correspond to heat treatment at 700°C in a different region of phase stability (FCC + B2). A backscattered SEM image from the CRSA700 condition is shown in Fig. 2(c), revealing recrystallized FCC grains, similar in size to those observed in the CRSA sample. The higher magnification inset in the same figure shows a small fraction of grain boundary precipitates, which are likely to be B2 precipitates [START_REF] Gwalani | Modifying transformation pathways in high entropy alloys or complex concentrated alloys via thermo-mechanical processing[END_REF]. Previous reports on Al0.3CoCrFeNi rationalized that the high nucleation barrier of the B2 phase within the FCC matrix makes it rather difficult to homogeneously nucleate intra-granular B2 precipitates, favoring heterogeneous nucleation at grain boundaries [START_REF] Gwalani | Tensile yield strength of a single bulk Al0.3CoCrFeNi high entropy alloy can be tuned from 160 MPa to 1800 MPa[END_REF][START_REF] Gwalani | Modifying transformation pathways in high entropy alloys or complex concentrated alloys via thermo-mechanical processing[END_REF]. Therefore, the number density of heterogeneous B2 nucleation sites has been increased by cold-rolling the sample to 85% reduction in thickness, followed by heat treatment only at 700°C for 50 h (CR700 condition). Fig. 2(d) shows a backscatter SEM image of a CR700 condition sample. The microstructure consists of substantially refined FCC grains compared to the CRSA700 condition with coarse grain boundary precipitates in darker contrast (inset shows a high magnification image). Coupled EBSD-EDS analysis of this microstructure (Fig. 3) identifies the darker contrast regions as BCC based precipitates, based on the phase map (they were found to be B2 precipitates based on subsequent TEM analysis) with a volume fraction of ~21%. The EDS mapping in Fig. 3(b) shows the enrichment of Al in grain boundary precipitates. The refined FCC matrix grains in the CR700 condition samples compared to the CRSA700 condition samples can be attributed to the concurrent recrystallization of the matrix grains and B2 precipitation occurring in CR700 samples, pinning the growing FCC grains.

3.3.Transmission Electron Microscopy

Further characterization of the CR700 and CRSA600 conditions has been carried out via transmission electron microscopy (TEM). Fig. 4(a) shows a bright-field TEM image of a sample in the CR700 condition revealing multiple FCC grains and a precipitate in the center. The diffraction pattern obtained from the [001] zone axis of the BCC based precipitate exhibits {100} superlattice reflections (inset in Fig. 4(a)), confirming that the crystal structure is B2. In the CRSA600 condition, this alloy exhibits a periodic lamellar structure (lamellae thickness of ~100-150 nm) as revealed by the bright field TEM micrograph shown in Fig. 4(b). Electron diffraction analysis revealed that {011} BCC // {111} FCC and <001> BCC // <011> FCC, indicating the Nishiyama-Wasserman orientation relationship between the lamellae [START_REF] Koumatos | A theoretical investigation of orientation relationships and transformation strains in steels[END_REF]. The dark-field image in Fig. 4(c), recorded from a {100} FCC superlattice reflection in the [011] FCC zone axis microdiffraction pattern (inset) highlights the L12 regions within the FCC lamella. Similarly, the darkfield image in Fig. 4(d), obtained from the {100} superlattice reflection in the [011] BCC zone selected area diffraction pattern (inset) highlights the B2 regions within the BCC lamella.

Therefore, the CRSA600 condition exhibits a complex nanolamellar pearlite-like microstructure, consisting of alternating FCC+L12 and BCC+B2 lamellae. This indicates that concurrent growth of FCC/L12 and BCC/B2 lamellae in the form of eutectoid-like colonies can overcome the nucleation barrier for the formation of B2 precipitates within the parent FCC matrix. Dark-field images (Figs. 4(c) and(d)) indicate that within each lamella, the FCC and L12 phases and the BCC and B2 phases can be co-continuous, suggesting a synergistic spinodal decomposition coupled with chemical ordering within both FCC and BCC lamellae [START_REF] Banerjee | Phase transformations: examples from titanium and zirconium alloys[END_REF][START_REF] Soffa | Decomposition and ordering processes involving thermodynamically first-order order → disorder transformations[END_REF].

Atom probe tomography

Atom probe tomography (APT) was performed to determine the constitution of phases in the fourphase microstructure of the CRSA600 condition. Figs. 5(a) and (b) show images of the two different APT reconstructions exhibiting several interphase interfaces. The composition of each phase was determined by averaging the volume of the phase within a sphere of 10 nm diameter, placed such that it is within that phase. The BCC phase, as expected, was lean in Al and Ni and rich in Fe and Co with a composition of 4Al-45Co-47Fe-4Ni. The B2 precipitates were rich in Ni and Al with a composition of 30Al-11Co-10Fe-49Ni. Within the FCC based lamella, the continuous matrix appears to be rich in Ni and Al and has thus been assigned as the L12 phase, with a composition of 23Al-5Co-5.5Fe-66.5Ni. The APT reconstructions and analysis clearly reveal Ni and Al lean regions within the L12 lamellae, and these regions are expected to be FCC with the composition 6Al-30Co-30Fe-34Ni. The compositions of each phase are tabulated in Supplementary Table I. Based on these compositions obtained from APT, the site occupancies in L12 regions could be represented as (Ni,Fe,Co)3(Al,Fe) [START_REF] Allaverdova | Atomic distribution of alloying additions between sublattices in the intermetallic compounds Ni3Al and NiAl II: Microscopic calculations and X-ray diffraction analysis[END_REF] and the B2 regions as (Ni,Co)(Al,Fe,Co) [START_REF] Allaverdova | Atomic distribution of alloying additions between sublattices in the intermetallic compounds Ni3Al and NiAl II: Microscopic calculations and X-ray diffraction analysis[END_REF][START_REF] Hao | Concentration of point defects and site occupancy behavior in ternary NiAl alloys[END_REF].

Mechanical properties

Fig. 6 shows the engineering stress-strain curves for the four conditions in tension and Table 1 shows the results from microhardness measurements. The single-phase FCC condition (CRSA) of Al0.3CoFeNi exhibited a yield stress (YS) ~457 MPa, an ultimate tensile stress (UTS) ~852 MPa and a net tensile strain ~76%. The sample did not fail during the test as it reached the maximum elongation. The microhardness of this condition was measured to be ~196 VHN. When compared to conventional solid solutions, the relatively high yield stress and hardens in CRSA can be rationalized based on the lattice distortion theory in HEA/CCAs and the solid solution strengthening caused by the size and modulus misfit of Al atoms in CoFeNi matrix. The nanolamellar microstructure, with substantial phase fractions of L12 and B2 intermetallic phases within the lamellae in CRSA600 condition, exhibited a high yield stress of ~1074 MPa, UTS of ~1302 MPa and a net plastic strain of ~8%. This condition also showed the highest hardness ( ~513 VHN) among the four conditions studied.

Interestingly, the CRSA700 condition exhibits an yield stress marginally lower than CRSA condition. This could potentially be due to reduced solid solution strengthening in the FCC matrix because of the depletion of Al and Ni caused by B2 precipitation. Additionally, the volume fraction of B2 phase is too low to have a substantial strengthening contribution. The hardness of CRSA600 was found to be similar to that of CRSA condition. This similarity in hardness can be attributed to two factors. First, the diameter of hardness indent was around 65 m while the grain size was around 171 m. The plastic deformation, mediated by dislocation glide, may not have spread to the grain boundaries to encounter the effect of the B2 precipitates. However, in the case of the CR700 (direct annealed at 700°C after cold-rolling) sample, a large number of heterogenous defects, such as deformation bands (containing a high density of dislocations) were produced during rolling, resulting in a higher, near equilibrium volume fraction of the B2 phase. The fine recrystallized FCC grain structure provided Hall-Petch strengthening and B2 phase provided precipitation strengthening. This resulted in a good balance of tensile properties with yield stress ~893 MPa, ultimate tensile strength ~1187 MPa and a tensile ductility ~26%. The hardness of CR700 was found to be ~232 VHN, which is also substantially higher than CRSA and CRSA700 conditions.

Magnetic properties

The magnetization curves were obtained by plotting magnetization (M) as a function of applied magnetic field (H), for each of the four conditions, CRSA, CRSA600, CRSA700, and CR700 (Fig. 6). The inset shows a magnified view of the region near zero applied field, revealing that the area under the magnetization curve is maximum for the CRSA600 condition. Table I summarizes the magnetization properties (saturation magnetization, Ms, and coercivity, Hc) and the microhardness values.

The value of Ms varies from 126.8 emu/g in the case of the CRSA700 condition to 138.7 emu/g in the case of the nano-lamellar CRSA600 condition. The value of Ms is not a strong function of the microstructure. The coercivity (Hc), on the other hand, varies dramatically and is dependent on the microstructure. The coercivity of the single-phase FCC solid solution condition in the case of CRSA is 199 A/m, increases to 604 A/m in the case of the CR700 sample, and finally becomes 12732 A/m in the case of the nano-lamellar CRSA600 condition. This corresponds to more than eighty times increase in the value of Hc in the same alloy. The nano-lamellar structure with sharp chemical/phase change in CRSA600 results in high resistance to magnetic domain wall movement and therefore higher value of coercivity. Lowe et al. also reported high value of coercivity in Alnico samples which exhibited a spinodal microstructure consisting of FeCo rich and AlNi rich phases with sharp chemical contrast [START_REF] Lowe | Microstructure and magnetic properties of melt-spun Alnico-5 alloys[END_REF].

For convenience, magnetic materials can be divided in three categories based on the value of coercivity; (i) soft (Hc< 25 Oe (2000 A/m)), (ii) semi hard (25 Oe<Hc<700 Oe (55720 A/m)) and (iii) hard (Hc > 700 Oe (55720 A/m)) [START_REF] Constantinides | Semi-Hard Magnets: The important role of materials with intermediate coercivity[END_REF]. CRSA, CRSA700 and CR700 belong to the soft magnetic category whileCRSA600 belongs to the semi hard magnetic category. Fig 6 (b) shows the Ms and Hc for our Al0.3FeCoNi along with other reported FeCoNi based soft magnetic FeCoNi(MnAl)x (x = 0.25, 0.50, 0.75 and 1) alloys [START_REF] Li | Composition dependence of structure, physical and mechanical properties of FeCoNi(MnAl)x high entropy alloys[END_REF], FeCoNi(AlSi)x (x = 0.1, 0.2 and 0.5) [START_REF] Zhang | High-entropy Alloys with High Saturation Magnetization, Electrical Resistivity and Malleability[END_REF] and FeCoNi(MnAl)0.25 (as cast, cold rolled and annealed) [START_REF] Li | A ductile high entropy alloy with attractive magnetic properties[END_REF] CCAs. CRSA and CRSA700 exhibit an excellent combination of low Hc and high Ms (highlighted in green color in Fig. 6(b)). The low coercivity for CRSA700 can be attributed by the fact that it has an average grain size higher than that of CRSA and CR700. Fig 6(c) shows the Ms and Hc for CRSA600 along with other FeCoNi based semi hard magnetic FeCoNi(MnSi)x (x = 0.25, 0.50 and 0.75) [START_REF] Sahu | Microstructure and magnetic behavior of FeCoNi(Mn-Si)x (x = 0.5, 0.75, 1.0) high-entropy alloys[END_REF] and FeCoNi(AlSi)x (x = 0.3, 0.4 and 0.8) CCAs. Recently, Zhang et el., explored Al0.5CoFeNi CCAs, however, the Hc values were changed only in the range of 60 Oe to 90 Oe [START_REF] Xu | Annealingdependent microstructure, magnetic and mechanical properties of high-entropy FeCoNiAl0.5 alloy[END_REF]. Some of the earlier reported CCAs exhibit larger values of Hc than CRSA600. However, the Ms for those CCAs is inferior (<100 emu/g). High Ms is a key requirement for both soft and semi (hard) magnetic applications. Our CRSA600 exhibits high saturation magnetization (138.7 emu/g) along with reasonable high Hc (12720 A/m). Fig. 6(d) shows the Vickers hardness of some of the commercialized soft and hard magnetic materials along with our Al0.3FeCoNi studied in four different conditions. The hardness of CRSA, CRSA700 and CR700 are higher or close to the hardness of famous soft magnetic materials, e.g., Fe50Ni50, permalloy and permendur. On the other hand, hardness of CRSA600 is very close to the values for Alnico, Sm-Co and Nd-Fe-B types of permanent magnets. The high hardness value coupled with high saturation magnetization and coercivity in CRSA600 condition makes this alloy a potential non-rare earth containing candidate for semi hard magnet applications which also require good mechanical strength. While the tensile ductility decreased with increase in yield stress, all four conditions exhibited a ductility reasonable for magnetic applications. Our results clearly demonstrate the feasibility of tailoring the site specific mechanical and magnetic properties within a component via site-specific thermo-mechanical treatment of a single alloy composition for room temperature applications. Such an approach has been recently referred to as metamorphic manufacturing [START_REF] Spanos | Metamorphic Manufacturing: Shaping the Future of On-Demand Components[END_REF][START_REF] Daehn | Metamorphic manufacturing: The third wave in digital manufacturing[END_REF]. In addition to room temperature applications, this alloy can also be potentially used for intermediate temperature (<500°C) applications. In particular, the two-phase microstructure in CR700 (FCC+B2) and the four-phase microstructure in CRSA600 (FCC+L12+BCC+B2) are in agreement with the equilibrium thermodynamic modeling shown in Fig. 1(a). The obtained microstructures in these two conditions are expected to have reached equilibrium because of the long-term annealing (50 hrs at 700°C and 600°C) carried out in this study. However, the high temperature properties of these microstructures is a subject of future study.

Summary and Conclusions

The mechanical and magnetic properties of an Al0.3CoFeNi alloy can be tailored over a wide range, paving the way for metamorphic manufacturing of mechanically strong magnetic components.

1. A novel eutectoid-like nano-lamellar microstructure, consisting of complex hierarchically decomposed alternating lamellae of FCC+L12/BCC+B2, has been found in an Al0.3CoFeNi CCA. This structure arises from Al alloying addition, which results in a competition between the tendency for L12 ordering within the FCC lattice and BCC/B2 phase formation. The experimentally observed phase stability as a function of temperature could be largely rationalized based on CALPHAD modeling (ThermoCalc with TCHEA3 database).

2. The nano-lamellar microstructure exhibits a hardness of ~ 513 VHN, saturation magnetization Ms ~138 emu/g and coercivity Hc ~12732 A/m, making this a mechanically and magnetically semi hard alloy.

3. On the other hand, the microstructure with fine recrystallized FCC grains and near equilibrium B2 phase fraction exhibited saturation magnetization of ~132 emu/gm, reasonably low coercivity 604 A/m and high hardness ~232 VHN.

Figure Captions

Fig. 1 :

 1 Fig. 1: (a) Psuedo-binary isopleth for varying Al in ternary CoFeNi system (b) Phase fraction vs

Fig. 2

 2 Fig. 2 Backscattered SEM micrographs of (a) CRSA (b) CRSA600 (c) CRSA700 and (d) CR700.

Fig. 3 :

 3 Fig. 3: Coupled EBSD-EDS mapping of CR700 (a) Phase map from EBSD (b) Al map showing

Fig. 4 Fig. 5 :

 45 Fig. 4 TEM of CR700 in (a) and CRSA600 in (b-d), (a) Bright field image showing ultra-fine FCC

Fig. 6 (

 6 Fig. 6 (a) Magnetization versus applied magnetic field curves of Al0.3FeCoNi studied at four

Acknowledgements

The work is supported by the US Air Force Office of Scientific Research under grant FA9550-17-1-0395. The authors acknowledge the Materials Research Facility (MRF) at the University of North Texas for use of microscopy facilities. This research is partially supported by AME Programmatic Fund by the Agency for Science, Technology and Research, Singapore under Grant No. A1898b0043.

Competing interests

None.


Kassem Amro

Anil K Thakur

Marc Rolland

Arie Van Der Lee

Vincent Lemaur

Roberto Lazzaroni

Joëlle Rault-Berthelot

Cyril Poriel

Lionel Hirsch

Sébastien Clément

Arie Van

Der Lee

Philippe Gerbier

Linking triptycene to silole: a fruitful association

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

Introduction

Limited to very few applications for a long time because of their complicated synthetic chemistry, [START_REF] Dubac | Group 14 metalloles. 1. Synthesis, organic chemistry, and physicochemical data[END_REF] siloles or silacyclopentadienes experienced a spectacular rebirth thanks to the works of K. Tamao and S. Yamaguchi in the late 90s. [START_REF] Yamaguchi | Synthesis of a Series of 1,1-Difunctionalized Siloles[END_REF][START_REF] Yamaguchi | Silole-containing σ-and π-conjugated compounds[END_REF] Indeed, the general and versatile synthesis via an intramolecular reductive cyclization of diethynylsilanes they proposed allowed the preparation of various 2,5-difunctional siloles that would be otherwise very difficult to obtain. Shortly afterwards, on account of their unique electronic properties, i.e., their very low LUMO energy levels combined with high fluorescence quantum yields in the solid state, siloles were used successfully as both emissive and electron-transporting materials in organic light emitting diodes (OLED) manufacture. 4- 7 The origin of their electronic properties is the consequence of an efficient interaction between the σ* orbital of the silole silicon atom with the π* orbital of the butadiene fragment [START_REF] Yamaguchi | Toward new materials for organic electroluminescent devices: synthesis, structures, and properties of a series of 2,5-diaryl-3,4-diphenylsiloles[END_REF][START_REF] Amro | 2,5-Thiophene substituted spirobisilolessynthesis, characterization, electrochemical properties and performance in bulk heterojunction solar cells[END_REF] whereas the origin of their outstanding high solid-state fluorescence is attributed to the aggregation induced emission (AIE) phenomenon that was described for the first time with 1methyl-1,2,3,4,5-pentaphenylsilole. [START_REF] Luo | Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole[END_REF] Since then, AIE materials are the subject of intense scientific research and have found many applications that encompass various domains going from organic electronics to theranostic bioprobes. [START_REF] Feng | Aggregation-induced emission (AIE) dots: Emerging theranostic nanolights[END_REF][START_REF] Wang | Aggregation-Induced Emission Luminogens for Activity-Based Sensing[END_REF][START_REF] Zhao | Aggregation-induced emission of siloles[END_REF][START_REF] Arribat | Silole amino acids with aggregation-induced emission features synthesized by hydrosilylation[END_REF][START_REF] Mei | Aggregation-Induced Emission: Together We Shine, United We Soar![END_REF] Moreover, with the growing number of AIE materials described in the literature, rather new and uncommon phenomena such as reversible mechanofluorochromism have started to be reported since their first observation with tetraphenylethene (TPE) and diphenyldibenzofulvene (DPDBF) derivatives. [START_REF] Xie | The development of mechanoluminescence from organic compounds: breakthrough and deep insight[END_REF][START_REF] Chi | Recent advances in organic mechanofluorochromic materials[END_REF][START_REF] Zhu | A polymorphic fluorescent material with strong solid state emission and multi-stimuli-responsive properties[END_REF][START_REF] Wilbraham | Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals[END_REF][START_REF] Luo | Reversible Switching of the Emission of Diphenyldibenzofulvenes by Thermal and Mechanical Stimuli[END_REF][START_REF] Zhao | Benzothiazolium-functionalized tetraphenylethene: an AIE luminogen with tunable solid-state emission[END_REF] In small molecules-based organic electronics, besides the device efficiency, the thermal and structural stabilities of the used organic materials are an important concern. [START_REF] Walzer | Highly Efficient Organic Devices Based on Electrically Doped Transport Layers[END_REF][START_REF] Kordt | Modeling of Organic Light Emitting Diodes: From Molecular to Device Properties[END_REF] Indeed, for instance, an operating OLED releases heat that may alter quickly its efficiency. [START_REF] Zhou | Real-time observation of temperature rise and thermal breakdown processes in organic LEDs using an IR imaging and analysis system[END_REF] This is the reason why materials with high melting points and high glass transition temperatures (Tg) are desirable to achieve the fabrication of devices with good durability. One of the strategies used to increase the thermal stability of the organic layers is to crosslink the adequately functionalized molecules after their deposition. [START_REF] Wantz | Stabilizing polymer-based bulk heterojunction solar cells via crosslinking[END_REF] Another strategy involves the control of the spatial organization, i.e., distance and relative orientation of the constitutive molecules through the chemical synthesis and/or the use of an appropriate construction scaffold. [START_REF] Swager | Iptycenes in the Design of High Performance Polymers[END_REF] Triptycene with its fascinating paddlewheel shape has been widely used for this purpose in supramolecular chemistry, polymer chemistry and materials science. [START_REF] Chong | Iptycenes in supramolecular and materials chemistry[END_REF][START_REF] Jiang | Recent Developments in Synthesis and Applications of Triptycene and Pentiptycene Derivatives[END_REF][START_REF] Locke | Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry[END_REF] However, in spite of the thermal

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Please do not adjust margins stabilization afforded by its presence in the molecular structures, only few examples of the use triptycene-based luminescent materials in OLEDs have been reported so far. [START_REF] Chou | Triptycene derivatives as high-Tg host materials for various electrophosphorescent devices[END_REF][START_REF] Jhulki | Benzophenone-imbedded benzoyltriptycene with high triplet energy for application as a universal host material in phosphorescent organic light-emitting diodes (PhOLEDs)[END_REF][START_REF] Wang | Novel phosphorescent triptycene-based Ir(III) complexes for organic lightemitting diodes[END_REF] With this in mind we report herein the synthesis and structural characterization of two novel silole derivatives incorporating triptycene moieties at the 2,5-positions (Tp-DMTPS and Tp-MPTPS, Fig. 1) with the first aim to use them as emissive layers in OLEDs. We will focus on the interplay between the triptycene side-groups and the silole core that induces an original behavior in terms of thermal and structural stability, luminescence and mechanofluorohypsochromism, phenomenon that was still rarely observed up to now. [START_REF] Nagura | Distinct Responses to Mechanical Grinding and Hydrostatic Pressure in Luminescent Chromism of Tetrathiazolylthiophene[END_REF][START_REF] Tanioka | Reversible Near-Infrared/Blue Mechanofluorochromism of Aminobenzopyranoxanthene[END_REF][START_REF] Wang | Luminescent Chromism of Boron Diketonate Crystals: Distinct Responses to Different Stresses[END_REF][START_REF] Wu | Molecular Engineering of Mechanochromic Materials by Programmed C-H Arylation: Making a Counterpoint in the Chromism Trend[END_REF][START_REF] Ying | Unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism of long-alkylated simple DPP dyes[END_REF][START_REF] Zhang | Enabling DPP derivatives to show multistate emission and developing the multifunctional materials by rational branching effect[END_REF] We show that the modulation of the structure in the solid state resulting from the incorporation of triptycene groups affords an enhancement of OLED performance by formation of morphologically stable non-crystalline emissive layers.

Results and discussion

Synthesis

The siloles Tp-DMTPS and Tp-MPTPS were conveniently prepared by the method previously described by Tamao et al. that involves the intramolecular reductive cyclization of 1,1bis(phenylethynyl)silane, followed by a palladium-catalyzed cross-coupling reaction with the appropriate bromoaryl compound (Scheme 1). [START_REF] Tamao | Silole Derivatives as Efficient Electron Transporting Materials[END_REF][START_REF] Uchida | Structural Optimization of 2,5-Diarylsiloles as Excellent Electron-Transporting Materials for Organic Electroluminescent Devices[END_REF] 9-(p-bromophenyltriptycene) was prepared via a Diels-Alder reaction between in-situ generated benzyne, and 4'-bromophenylanthracene. [START_REF] Murphy | Cyanine Borate Salts that Form Penetrated Ion Pairs in Benzene Solution: Synthesis, Properties, and Structure[END_REF] Tp-DMTPS and Tp-MPTPS were isolated in 54 and 49% yields, respectively, as pale yellow powders.

Crystal structure.

As shown in Fig. 2 (and Fig. S3-S6), the molecular packing of Tp-DMPTS is mainly governed by the paving of the triptycene moieties in alternating up (one of the phenyl ring parallel to the c-axis) and down (antiparallel to the c-axis) columns leading to a layered structure. Inside the triptycene double layer, the 3,4diphenylsilole moieties also adopt an up and down columnar packing corresponding to the orientation of the triptycene sidegroups. Interestingly, the structure shows that the silole rings could be inclined either on the right or on the left with respect to a plane going through the middle of a column (Fig. 3). This situation leads to the co-existence of two conformers in the crystal, which will be named R and L. In the R-conformer, the methyl group C69 (see Fig. 3) is nestled between two phenyl rings of the molecule immediately below. Short CH/π hydrogen bonds are present between two hydrogen atoms belonging to the methyl group and the interacting phenyl rings (dCH/π = 2.43 Å and 2.97 Å). [START_REF] Nishio | The CH/π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates[END_REF] In the L-conformer (Fig. S5 andS6), the other methyl group (C70 is nestled between the phenyl rings and shorter CH/π hydrogen bonds are observed (dCH/π= 2.55 Å and 2.85 Å). The distance and the orientation of the CH/π hydrogen bonds observed in each conformer are in the range of what is frequently observed in the literature (dCH/π = 2.73 ± 0.13 Å). [START_REF] Nishio | The CH/π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates[END_REF] Since it is only the 1,1-dimethyl-3,4-diphenylsilole moiety that is involved in this swinging, this induces torsion angle modifications between the silole core and the four phenyl rings at its periphery (Table 1). The phenyl rings located at the 2,5position of the silole core are locked within the structure because of a strong CH/π interaction between their aromatic system and a hydrogen atom belonging to the triptycene moieties (dCH/π = 2.44 Å and 2.59 Å). This CH/π interaction leads to a moderate bending of the phenyl moiety with respect to the threefold axis of the triptycene by ca. 17° (a bending angle of 6° is found in the crystal structure of 9-phenyltriptycene [START_REF] Nikitin | Severe Energy Costs of Double Steric Interactions: towards a Molecular Clamp[END_REF] ). If present in solution, this strong CH/π interaction should lead to an appreciable shift of the 1 H NMR signal of the corresponding hydrogen atom compared to the two other ones, because of the shielding anisotropy cones of the adjacent phenyl ring. [START_REF] Sicard | C1-Linked Spirobifluorene Dimers: Pure Hydrocarbon Hosts for High-Performance Blue Phosphorescent OLEDs[END_REF] However in none of the molecules incorporating triptycene moieties we have synthesized (see Fig. S30 -S32), this feature was observed. In every cases, the hydrogen atoms of the C(2) carbon atoms (see Fig. 1 for labelling) belonging to the triptycene paddlewheels are equivalents, indicating a free rotation of the later in solution at ambient temperature.

Si R 1 H 3 C Si Li Li R 1 H 3 C Si R 2 R 2 R 1 H 3 C

Tp-DMTPS

: R The crystal packing of Tp-MPTPS (Fig. S7) does not show any of the features found with Tp-DMTPS. Probably because of the presence of a phenyl ring attached to the silicon atom, the triptycene moieties are not able to organize themselves in regular layers. Consequently, the molecular packing is less dense in Tp-MPTPS and CH2Cl2 molecules are found in the structural voids. Notably short CCl/π halogen interactions are also observed between one chlorine atom belonging to these molecules and one of the phenyl rings adjacent to the silole ring (dCCl/π = 3.51 Å, see Fig. S2). [START_REF] Imai | Cl-π interactions in proteinligand complexes[END_REF] Because of the chirality of the silicon atom in Tp-MPTPS, both (R) and (S) enantiomers were found in the crystal lattice. The molecular structure of Tp-DMTPS shows a paddlewheel arrangement of the 2, 3, 4 and 5phenyl rings around the silole core, whereas a more conventional propeller-like disposition of the phenyl rings is found in Tp-MPTPS (Fig. 4). In this case, the structure also reveals that the phenyl moieties are bent off the threefold axis of the triptycene by angle of 16° on average.

Hirshfeld surface analysis.

The Hirshfeld surfaces [START_REF] Spackman | A novel definition of a molecule in a crystal[END_REF] of both conformers of Tp-DMTPS (Fig. 5) were calculated with the CrystalExplorer program 45 and are mapped with the dnorm property [START_REF] Mckinnon | Towards quantitative analysis of intermolecular interactions with Hirshfeld surfaces[END_REF] where the color scheme goes from red (dnorm = -0.2664 and -0.2795 for conformers Land R-, respectively) to blue (dnorm = 2.2715 and 2.2586 for conformers Land R-, respectively). The most notable interactions shown in red in Fig. 5 are indeed in both cases between the methyl hydrogens and the aromatic rings of adjacent molecules. Interestingly, in R-conformer, the secondary interactions between the CH systems of one ring to the π-system of an adjacent ring seem to be less important than in L-conformer. An analysis of the elemental fingerprint plots shows that the relative contributions of the different interactions in the two conformers are anyhow approximately equal: 31.7% / 31.3% for H…C/C…H interactions and 64.8 / 65.2% for hydrophobic H…H interactions. The Hirshfeld surface analysis of Tp-MPTPS (Fig. S8) shows dnorm ranging from -0.1149 to 2.3526 with the following relative elemental interactions: H…H (62.4%), H…C (31.5%), H…Cl (3.6%) and C…Cl (0.7%). However, it is clearly seen that the intermolecular interactions in the case of Tp-MPTPS are more dispersive than in the case of Tp-DMTPS.

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ARTICLE

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Thermal properties

The thermal properties of Tp-DMTPS and Tp-MPTPS were examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. As shown in Fig. 6, the TGA of Tp-DMTPS exhibits a weight loss in one step leading to a high decomposition temperature (Td) of 486°C. In contrast, the weight loss of Tp-MPTPS takes place in two steps. The first weight loss of ca. 10% could be ascribed to the loss of interstitial solvent molecules, as observed in the crystal structure of Tp-MPTPS (see above). This high temperature of desolvation may be due to the strong CCl/π halogen interactions that have been characterized in the crystal lattice. The second weight loss is due to the decomposition of Tp-MPTPS molecule. As for Tp-DMTPS, a high Td of 489°C was noticed highlighting the high thermal stability of these two siloles. It is noteworthy that these Td are higher than their parent compounds, namely, 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS) : 272 °C and 1-methyl-1,2,3,4,5-pentaphenylsilole (MPTPS) : 309 °C. [START_REF] Yu | Structures, Electronic States[END_REF] The same trend is observed with the melting point, measured at 431°C for Tp-DMTPS; 250 °C above that of the DMTPS parent (Fig. S13). 47

Photophysical properties.

The UV-visible absorption and fluorescence spectra were measured in dilute dichloromethane solution (Fig. 7). The data are gathered in Table 2. Tp-DMTPS exhibits an absorption maximum at 357 nm corresponding to the π-π* transition involving mainly the HOMO and the LUMO energy levels of the silole core (see below and Fig. 13). [START_REF] Yamaguchi | Silole-containing π-conjugated polymers. 5. Theoretical study of the electronic structure of 2,2'-bisilole in comparison with 1,1'-bi-1,3-cyclopentadiene: σ*-π* conjugation and a low-lying LUMO as the origin of the unusual optical properties of 3,3',4,4'-tetraphenyl-2,2'bisilole[END_REF] For Tp-MPTPS this transition is slightly red-shifted at 369 nm. This shift could be explained by the stabilization effect of the LUMO induced by the presence of the phenyl group on the silicon atom. [START_REF] Yamaguchi | Toward new materials for organic electroluminescent devices: synthesis, structures, and properties of a series of 2,5-diaryl-3,4-diphenylsiloles[END_REF] The absorption maximum of thin films is slightly red-shifted to 365 nm for Tp-DMTPS (Fig. S9), but remains unchanged for Tp-MPTPS.

The photoluminescence spectra in solution show faint PL signals with maxima at 486 nm (φf = 1.4%) and 491 nm (φf =1.2%) for Tp-DMTPS and Tp-MPTPS, respectively (Fig. 7). In comparison with the corresponding siloles without triptycene , DMTPS (λ f max = 470 nm, φf = 0.2%) and MPTPS ( λ f max = 471 nm, φf = 0.1%), [START_REF] Yu | Structures, Electronic States[END_REF] Tp-DMTPS and Tp-MPTPS both shows a small red shift in the emission maxima and a much higher quantum yield. Since it is known that it is the intramolecular rotation of the phenyl rings in 2,3,4,5-tetraphenylsiloles that makes them essentially non-luminescent when molecularly dissolved, [START_REF] Chen | Synthesis, Light Emission, Nanoaggregation, and Restricted Intramolecular Rotation of 1,1-Substituted 2[END_REF] the enhancement of the quantum yields is probably due to the restriction of such intramolecular rotation (RIR) afforded by the triptycene moieties.

In contrast to what is observed in solution, the microcrystalline powders of Tp-DMTPS and Tp-MPTPS display a rather good fluorescence with emission maxima at 561 and 498 nm, and quantum yields of 43 and 24%, respectively. The same trend is also observed with DMTPS and MPTPS and was related to the Aggregation Induced Emission (AIE) phenomenon that was reported for the first time with hexaphenylsilole. [START_REF] Hong | Aggregation-induced emission: phenomenon, mechanism and applications[END_REF] However, the significant red-shift found in the Tp-DMTPS PL spectrum in comparison with what was observed both in solution (λmax = 486 nm) or deposited as 100 nm thick films (λmax = 485 nm, Fig. S10) is surprising.

Aggregation induced emission (AIE).

The AIE properties of Tp-DMTPS and Tp-MPTPS were studied in THF/water mixtures with different water fractions (fw) in view of fine-tuning the aggregation extent (Fig. 8). As expected, the larger the water fraction, the stronger the fluorescence; molecular aggregation inducing a red shift of 12 nm in the emission spectrum. As shown in Fig. 8, when fw varies from 0 to 60 vol.%, the fluorescence of Tp-DMTPS remains very low. A remarkable 50-fold increase of fluorescence intensity is observed at a water fraction of 90 vol.%. The same trend is observed with Tp-MPTPS with a 60-fold increase of the fluorescence intensity when fw also reached 90% (Fig. S12).

Moreover, we found that unlike Tp-MPTPS, the position of the emission of Tp-DMTPS in the solid state depends strongly on its degree of crystallization. Whereas rapid precipitation of Tp-DMTPS by addition of pentane affords a solid exhibiting blue emission under excitation at 365 nm, slow crystallization from acetone/dichloromethane mixtures leads to a yellow-orange emission for the obtained microcrystals (Fig. S11). These changes in emission features should mainly originate from the transition from the crystal to the amorphous state as well as the changes in the molecular conformations. [START_REF] Hong | Aggregation-induced emission: phenomenon, mechanism and applications[END_REF][START_REF] Cao | Aggregation-tuned dual emission of silole derivatives: synthesis, crystal structure, and photophysical properties[END_REF][START_REF] Heng | Fabrication of hexaphenylsilole nanowires and their morphology-tunable photoluminescence[END_REF][START_REF] Yang | New AIEgens containing tetraphenylethene and silole moieties: tunable intramolecular conjugation, aggregation-induced emission characteristics and good device performance[END_REF][START_REF] Wang | A stable tetraphenylethene derivative: aggregation-induced emission, different crystalline polymorphs, and totally different mechanoluminescence properties[END_REF][START_REF] Dong | Vapochromism of Hexaphenylsilole[END_REF]

Mechanofluorochromism.

The dependence of the position of the Tp-DMTPS emission maximum on its procedure of isolation (precipitation / crystallization), suggests that this compound could display mechanofluorochromism. [START_REF] Wilbraham | Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals[END_REF][START_REF] Mutai | Switching of polymorph-dependent ESIPT luminescence of an imidazo[1,2-a]pyridine derivative[END_REF][START_REF] Sagara | Mechanically induced luminescence changes in molecular assemblies[END_REF] This hypothesis was confirmed by simply grinding a sample of microcrystalline Tp-DMTPS. As Please do not adjust margins Please do not adjust margins shown in Fig. 9, the emission turned from orange to blue-green upon UV illumination. It is important to mention that this phenomenon called mechanohypsochromism is rarely observed, a red-shift being observed in almost all cases. [START_REF] Nagura | Distinct Responses to Mechanical Grinding and Hydrostatic Pressure in Luminescent Chromism of Tetrathiazolylthiophene[END_REF][START_REF] Tanioka | Reversible Near-Infrared/Blue Mechanofluorochromism of Aminobenzopyranoxanthene[END_REF][START_REF] Wang | Luminescent Chromism of Boron Diketonate Crystals: Distinct Responses to Different Stresses[END_REF][START_REF] Wu | Molecular Engineering of Mechanochromic Materials by Programmed C-H Arylation: Making a Counterpoint in the Chromism Trend[END_REF][START_REF] Ying | Unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism of long-alkylated simple DPP dyes[END_REF][START_REF] Zhang | Enabling DPP derivatives to show multistate emission and developing the multifunctional materials by rational branching effect[END_REF] Finally, the initial orange emission could be restored by heating the ground solid at 140 °C for 10 min. The choice of this annealing temperature was based on the DSC analyses of Tp-DMTPS after grinding, which shows an exothermic signal associated to a crystallization process (Fig. S13). The corresponding PL spectra (Fig. 10) shows that the grinding process is not only affecting the position of the emission maximum (λem(crystals) = 561 nm, λem(after grinding) = 533 nm), but is also modifying the peak shape, the solid state quantum yield of emission remaining the same. Interestingly, the emission of the aggregates generated by adding water to a THF solution shows a sharper emission peak (λem = 498 nm), which is very similar to the one obtained in solution (see Fig. 7). These emission changes should mainly be due to their different molecular conformations that are affected by the the crystallinity in these solids. [START_REF] Wang | A stable tetraphenylethene derivative: aggregation-induced emission, different crystalline polymorphs, and totally different mechanoluminescence properties[END_REF][START_REF] Yang | Three polymorphs of one luminogen: how the molecular packing affects the RTP and AIE properties?[END_REF] To evaluate the degree of crystallinity, we performed powder XRD analyses, the patterns indicate that grinding microcrystalline Tp-DMTPS leads to a dramatic decrease of its crystallinity, but not reaching complete amorphization (Fig. S14). On the other hand, fast precipitation from a mixture of THF with ''high'' water content should lead to the formation of amorphous aggregates, as previously observed by Tang et al.. [START_REF] Hong | Aggregation-induced emission: phenomenon, mechanism and applications[END_REF][START_REF] Dong | Vapochromism of Hexaphenylsilole[END_REF] Therefore, there is a clear dependency between the long-range molecular order and the emission: the higher the order, the stronger the emission is red-shifted. Interestingly, similar experiments conducted with microcrystalline Tp-MPTPS powders showed that the position of the emission maximum remains unaffected upon grinding. Solid state NMR experiments. [START_REF] Arribat | Silole amino acids with aggregation-induced emission features synthesized by hydrosilylation[END_REF] C CP-MAS NMR spectra of either microcrystalline or ground powders of Tp-DMTPS are shown in Fig. 11. In comparison with the spectrum recorded in CDCl3 solution, the peaks were observed at very similar chemical shifts. The signal arising from the resonance of the methyl groups SI(C1,C1') at δ = -3.6 ppm is split into two, as well as the signals from the carbon atoms belonging to the silole ring SI(C3,C4) δ = 154.8 ppm and SI(C2,C5) δ = 141.8 ppm (see Fig. 1 for labelling). The same splitting is also observed for the carbon atoms PH2(C1) and PH5(C1) (δ = 139.0 ppm), which are directly connected to the silole ring. It is noteworthy that this peak splitting is also noticed, in the [START_REF] Arribat | Silole amino acids with aggregation-induced emission features synthesized by hydrosilylation[END_REF] C CP-MAS NMR spectrum of DMTPS but with a smaller extent (Fig. S15). As shown in Fig. 11, grinding the microcrystalline powder does not affect the spectrum pattern to a large extent; only a peak broadening is observed.

These observations are in line with what has been learned from the crystal structure analysis, ie. the splitting of the methyl group signals reflects their non-equivalent environment in the crystal lattice, one being nestled between two phenyl rings. As mentioned above, going from the R-to the L-conformer brings modifications of the torsion angles ϕ2, ϕ3, ϕ4 and ϕ5. Because of them, the conjugation between the surrounding phenyl rings and the silole core should be affected and the resonance peaks corresponding to the carbon atoms involved in the conjugation pathway split into two. This is also what was observed for DMTPS, its crystal structure exhibiting two molecules in the asymmetric unit that mainly differ in the rotation of the phenyl groups with respect to the central silole ring. 47 From the 13 C CP-MAS NMR experiments, we have shown that the spectrum of the ground powder is very similar to that of the crystalline one. The only difference consists in the broadening of some resonance peaks that should originate from the loss of crystallinity, as observed by XRD. Therefore, the two methyl groups remaining non-equivalent, this should indicate that a methyl group is always nestled between two phenyl rings both in the crystalline and the ground form. Interestingly, since the emission is blue-shifted, disordering the crystal packing upon grinding appears to favour the emission originating from

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Please do not adjust margins Please do not adjust margins the less conjugated L-conformer. This preference should be due to the stronger CH/π interactions found with this conformer. Finally, when vacuum-deposited as thin films, the PL and EL spectra showed similarities with that measured in solution. This indicates that this deposition process does not promote a good molecular organization even at a very small scale. Contrastingly, Tp-MPTPS displayed a more conventional behavior since the conformation of the molecules do not show any noticeable change depending on their physical state as it is observed with its parent MPTPS.

Electrochemical properties.

Cyclic voltammetry (CV) of Tp-DMTPS and Tp-MPTPS were recorded in CH2Cl2/ Bu4NPF6 (0.2 M) both in oxidation and reduction. In oxidation (Fig. 12, up), both compounds show similar successive oxidation steps with maxima closed to 1.26, 1.60, 1.90 and 2.10 V. Only the first oxidation step is reversible for the two compounds (see inset Fig. 12,top). From the onset potential of this first oxidation (1.14 V for both compounds), the HOMO levels were calculated at -5.54 eV. [START_REF] Kulkarni | Electron Transport Materials for Organic Light-Emitting Diodes[END_REF][START_REF] Cardona | Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications[END_REF] Thus, in the light of theoretical calculation (see further, Fig. 15), this first electron transfer is centered on the phenyl/silole moiety. Recurrent cycles recorded up to the fourth oxidation step, lead for the two compounds to electrodeposition processes with the appearance and growth of new redox systems along the cycles and to the coverage of the electrode surface by insoluble electroactive deposits (Fig. S16 -S17). Although this observation is not directly of interest in the topic of the present work, it is worth mentioning that the deposit obtained from Tp-MPTPS present both p-and a n-doping processes with a gap as low as 1.80 eV (see Fig. S17, right). As electropolymerization is a useful technique to design low gap materials, [START_REF] Paul-Roth | Electroactive films of poly(tetraphenylporphyrins) with reduced bandgap[END_REF] the present silole derivatives may be investigated in the future for that purpose. In reduction, both compounds exhibit a similar irreversible reduction wave (Fig. 12, bottom) from which the LUMO levels were calculated around -2.5 eV (-2.48 eV for Tp-DMTPS (Eonset red : -1.92 V) and -2.51 eV for Tp-MPTPS (Eonset red : -1.89 V)). The electrochemical gaps (Eg el ) for both compounds were calculated to be 3.06 eV for Tp-DMTPS and 3.03 eV for Tp-MPTPS. These Eg el values and their contraction of 0.03 eV from Tp-DMTPS to Tp-MPTPS are in accordance with the optical gaps calculated from absorption spectra recorded in CH2Cl2 (Eg el = 2.89 eV for Tp-DMTPS and 2.85 eV for Tp-DMTPS).

Electroluminescence.

To evaluate the benefit brought by tritycene groups, we compared Tp-DMTPS, Tp-MPTPS and DMTPS as both emissive and electron transporting layer in OLEDs. Since silole derivatives are good electron transporting materials, particular efforts must be done to have efficient hole injection at the anode for getting high luminescence. [START_REF] Huby | Injection and transport processes in organic light emitting diodes based on a silole derivative[END_REF] Therefore, we have used an ITO/PEDOT-PSS/NPB/silole/Ca simplified architecture to evaluate the performances of these compounds without trying to especially improve them. Because of its tendency to form highly crystalline thin films (Fig. S18), it was very difficult to collect reproducible data from devices made with DMTPS. However, we were able to make measurements showing very low performances with a quantum efficiency of 0.08 Cd/A at 6.0 V (Fig. S19) which is in line with what has been previously described. [START_REF] Tang | Efficient blue emission from siloles[END_REF] Contrastingly, Tp-DMTPS displays good filmforming ability, the thin films remaining amorphous even after being annealed at 80°C for 1h. As seen above, this excellent film stability is due to the drastic enhancement of both thermal and structural properties afforded by the triptycene moieties. Normalized electroluminescence (EL) spectra collected from two different devices are shown in Fig. 13. The EL spectrum an OLED with the simple architecture ITO/PEDOT-PSS/NPB/Tp-DMTPS/Ca displays a maximum at 516 nm, which is not affected by the applied voltage (Fig. S20). The insertion of a MoO3 layer between PEDOT-PSS and NPB layers (ITO/PEDOT-PSS/MoO3/NPB/Tp-DMTPS/Ca) induces a blue

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Please do not adjust margins Please do not adjust margins shift of ca. 20 nm in the EL spectrum. This could be the consequence of a micro cavity effect, the emission wavelength being affected by the thickness of the active layer. [START_REF] Dodabalapur | Microcavity effects in organic semiconductors[END_REF] The current density and luminuous intensity are shown in Fig. 14 for an ITO/PEDOT-PSS/MoO3/NPB/Tp-DMTPS/Ca device. The stability and repeatability for these devices are quite good, the quantum efficiency which is 2.8 Cd/A at 6.0 V is 35 fold higher than with DMTPS and the CIE color coordinates are x = 0.228 and y = 0.412 (Fig. S21). As anticipated from its lower solid state quantum yield, OLEDs with an ITO/PEDOT-PSS/NPB/Tp-MPTPS/Ca architecture show lower performances than those obtained with Tp-DMTPS, i.e., 0.8 Cd/A with CIE color coordinates of x = 0.301 and y = 0.312 (Fig. S22 and Fig. S23).

Electronic structure calculations.

The HOMO and LUMO energy levels of both optimized, R-and L-conformers of Tp-DMTPS and Tp-MPTPS were estimated at the Density Functional Theory (DFT) level using the B3LYP functional and the 6-31G* basis set. Calculations have been done on single molecules, either fully unconstrained, referred here as "optimized structures" or on structures for which the torsion angles have been frozen to their crystal values during the optimization, noted here as "constrained structures". The optimized structures and the shape of the HOMO and LUMO orbitals of Tp-DMTPS and Tp-MPTPS are depicted in Fig. 15. As previously observed, the four phenyl rings decorating the central silole ring adopt a propeller-like arrangement. Surprisingly, the calculations took rather well into account the strong CH/π interactions found in the crystal structures of both siloles with calculated bending angles of ca. 11° (Tp-DMTPS) and 15° (Tp-MPTPS) between the threefold axis of the triptycene group and the adjacent phenyl moiety. The HOMO orbitals of both derivatives are mainly located on the central silole core and phenyl rings at 2,5-positions while the substituents located at the 3 and 4-positions bear much weaker contributions. Calculated values are -5.56 and -5.58eV for Tp-DMTPS and Tp-MPTPS, respectively, values that are close to the ones estimated from CV measurements (see Table 2). This indicates that the optimized structures are certainly close to the ones present in solution. For the LUMO orbitals, a significant σ*-type orbital density is found on the two exocyclic Si-C bonds. This is typical for siloles and the resulting σ(Si-Cexocyclic)*π(butadiene)* hyperconjugation is responsible for the stabilization of the LUMO level; making those compounds good candidates for optoelectronic applications. [START_REF] Cai | Siloles in optoelectronic devices[END_REF] The triptycene moieties are not involved in the frontier orbitals; Tp-DMPTPS and Tp-MPTPS being therefore similar from an electronic point of view to their DMPTPS and MPTPS parents. Concerning R-and L-conformers of Tp-DMTPS (Fig. 16), as expected from the increased values of the torsion angles with respect to fully optimized structures (Table 1), both conformers display larger Eg values than for the optimized structure. Indeed, the L-conformer that is the less conjugated, displays the largest Eg value of 4.24 eV. Consequently, the emission of Tp-DMTPS is expected to be blue-shifted when passing from solution (calculated Eg = 3.37eV) to the solid state, in contrast to experimental observations. This clearly suggests that the redshift observed experimentally does not stem from the intrinsic properties of the molecules, but rather from the strong CH/π intermolecular interactions that occur in the solid. [START_REF] Huby | Evidence of environmental strains on charge injection in silole-based organic light-emitting diodes[END_REF][START_REF] Kokkin | π-π stacking vs. C-H/π interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9'-dimethylfluorene[END_REF] Charge transport calculations.

Finally, we decided to perform quantum-chemical calculation to evaluate both the role the CH/π intermolecular interactions and

Materials Chemistry Frontiers Accepted Manuscript

View Article Online DOI: 10.1039/D0QM00087F

Please do not adjust margins Please do not adjust margins the conformational modifications in a supramolecular property such as charge transport. Therefore, the estimation of the hole and electron mobilities for both R-and L-conformers of Tp-DMTPS as well as DMTPS as a reference were carried out. [START_REF] Niebel | Thienoacene dimers based on the thieno[3,2-b]thiophene moiety: synthesis, characterization and electronic properties[END_REF]68 Interestingly, our methodology reproduces nicely the measured electron-transporting character of DMTPS [START_REF] Yu | Structures, Electronic States[END_REF] with the highest calculated electron mobilities of 0.134 cm²/(V.s), compared to 0.038 cm²/(V.s) for holes (see Fig. S24). In contrast to DMTPS for which high transport properties are almost isotropic, Tp-DMTPS only exhibits a two-dimensional charge transport character within the bc plane, especially along the b axes. It is worthy to note that this is also in this direction that the main CH/π intermolecular interactions are running (see above). The two-dimensional behavior is a result of the 'insulating character' of the triptycene moiety: since the HOMO and LUMO orbitals are localized on the TPS subunit and not on the triptycene, hole and electron hopping events are highly unlikely along the triptycene direction. Surprisingly, our simulations reveal that, compared to DMTPS, hole mobilities are slightly enhanced while electron mobilities are significantly lowered, down to values never exceeding 0.008 cm²/(V.s), see Fig. 17. Moreover, an increase of the mobilities is observed when passing from the R-to the L-conformer of Tp-DMTPS. This result is in good agreement with the conclusion drawn by the Hirshfield surface analysis (see Fig. 5), indicating that the Lconformer display stronger CH/π intermolecular interactions than the R-. These calculations also show that these interactions are likely involved in the charge transport pathways. Grafting a triptycene fragment on a TPS central core therefore strongly modifies its charge transport properties by (i) reducing its charge transfer character from 3-D to 2-D and by (ii) inducing a stronger hole-transporting character while keeping a moderate electron mobility. These results imply that Tp-DMTPS is able to transport both types of charge carriers reasonably well, consistent with the device performances described above.

Conclusions

Experimental and theoretical studies of two silole-triptycene derivatives Tp-DMTPS and Tp-MPTPS are presented. Both compounds display a very good AIE activity as well as increased thermal and structural stabilities in comparison with other silole congeners. Consequently, a substantial improvement is found in the formation of stable, amorphous thin films as well as in the performance of simple OLED. Further improvement of the devices performance should be reached through the correction of the charge-transport balance by molecular engineering. [START_REF] Aubouy | Synthesis of new dipyridylphenylaminosiloles for highly emissive organic electroluminescent devices[END_REF][START_REF] Chen | Creation of Bifunctional Materials: Improve Electron-Transporting Ability of Light Emitters Based on AIE-Active 2,3,4,5-Tetraphenylsiloles[END_REF] Among the two molecules, Tp-DMTPS displays a very interesting photophysical behavior. In contrast to what is frequently observed, an appreciable red-shift is observed when passing from either the solution or the tiny aggregates to the crystals.

Moreover a pronounced reversible mechanohypsochromic phenomenon with a color emission changing from yellow-orange to blue-green is observed when microcrystalline powders are ground. Based on single crystal structural analysis, [START_REF] Arribat | Silole amino acids with aggregation-induced emission features synthesized by hydrosilylation[END_REF] C CP-MAS NMR and theoretical calculations this behavior was rationalized. We have demonstrated the significance of both molecular conformation and intermolecular CH/π interactions. [START_REF] Arribat | Silole amino acids with aggregation-induced emission features synthesized by hydrosilylation[END_REF] C CP-MAS NMR experiments conducted either on microcrystalline or ground powders confirmed that despite of the loss of crystallinity upon grinding, strong intermolecular CH/π interactions are preserved. This reflects the strong structure-directing properties brought by the triptycene groups. The origin of the red-shift observed when passing from isolated molecules to the crystal may be explained by the stabilization of the frontiers energy levels through CH/π intermolecular interactions. On the other hand, the origin of the mechanohypsochromism could be explained by the fact that the more energetic conformer is favored when the crystal environmental strain is diminished. Our results therefore indicate that the attachment of triptycene moieties to π-conjugated molecules is an interesting way to improve their thermal, structural and photophysical properties for sensing or optoelectronic applications. However, further experiments and sophisticated calculations are needed to fully understand this very original behavior.

Materials Chemistry Frontiers Accepted Manuscript

Scheme 1

 1 Scheme 1 Synthetic Route to Tp-DMTPS and Tp-MPTPS (see more details in SI).

Fig. 1

 1 Fig. 1 Labelling scheme for Tp-DMTPS (R1' = Me) and Tp-MPTPS (R1' = Ph).

Fig. 2

 2 Fig. 2 Views of the crystal packing of Tp-DMTPS showing the triptycene paving in the bc plane for a) R-conformers and b) L-conformers.

  LiNp, RT, (ii) [ZnCl2(TMEDA)], 1 h, RT, (iii) R2 -Br, PdCl 2(PPh3)2 , 18h reflux ARTICLE Please do not adjust margins Please do not adjust margins

Fig. 3

 3 Fig. 3 View of the R-conformers of Tp-DMTPS in the ab plane showing the methyl C69 nestling between the aromatic rings. Intra and intermolecular CH/π interactions are marked with green dotted lines.

Fig. 4

 4 Fig. 4 View of Tp-MPTPS molecular structure.

Fig. 5

 5 Fig. 5 Hirshfeld surfaces for the L-conformer (top) and the R-conformer (bottom) of Tp-DMTPS.

Fig. 6

 6 Fig. 6 TGA plots of Tp-DMTPS and Tp-MPTPS under nitrogen at a heating rate of 10 °C min -1 .

Fig. 7

 7 Fig. 7 Absorption (solid lines) and photoluminescence (PL) normalized spectra of Tp-DMTPS and Tp-MPTPS in CH2Cl2 solution (10 -5 M, dashed lines) and as microcrystalline powders (lines and dotted lines).

Fig. 8

 8 Fig. 8 Top: PL spectra of Tp-DMTPS in THF/water mixtures with different water fractions (fw). Concentration: 10 -5 M; excitation wavelength: 365 nm. Bottom: Plot of (I/I0) values versus water fractions in THF/water mixtures. I0 is the PL intensity in pure THF solution. Inset: photographs Tp-DMTPS in THF/water mixtures (fw = 0 and 90%) taken under UV lamp illumination.

6 |

 6 J. Name., 2012, 00, 1-3

Fig. 9

 9 Fig. 9 Photographs of Tp-DMTPS under UV illumination before grinding (left), after grinding (middle) and after annealing at 140°C for 10 min (right).

Fig. 10

 10 Fig. 10 PL spectra of Tp-DMTPS in various forms.

Fig. 11

 11 Fig. 11 13 C CP-MAS NMR spectra of Tp-DMTPS (microcrystalline and ground). Green bars represent the peaks observed in the 13 C{ 1 H} NMR spectrum recorded in CDCl3 solution. See Fig. 1 for labelling.

Fig. 12

 12 Fig. 12 Top :cyclic voltammetry in CH2Cl2/Bu4NPF6 (0.2 M) of Tp-DMTPS (in black) and Tp-MPTPS (in red). Exploration in the anodic direction (top) and in the cathodic direction (bottom). Working electrode: platinum disk, sweep-rate: 100 mV.s -1 .

Fig. 13

 13 Fig. 13 Normalized EL spectra of Tp-DMTPS.

Fig. 14 J

 14 Fig. 14 J-V characteristics and luminous intensity versus voltage of an ITO/PEDOT-PSS/MoO3/NPB/Tp-DMTPS/Ca device.

Fig. 15

 15 Fig. 15 Energy and shape of the HOMO and LUMO orbitals of optimized structures of Tp-DMTPS and Tp-MPTP, as calculated at the B3LYP/6-31+G* level.

Fig. 16

 16 Fig.16 Energy and shape of the HOMO and LUMO levels of constrained R-and Lconformers of Tp-DMTPS as calculated at the B3LYP/6-31+G* level.

Fig. 17

 17 Fig. 17Anisotropy of the hole (top) and electron (bottom) mobilities (in cm²/(V.s)) in the R-(left) and L-(right) conformers of Tp-DMTPS. The horizontal [vertical] direction corresponds to the c-[b-] unit cell axis. Note that the mobility scale is different for holes and electrons.

  Fig. 17Anisotropy of the hole (top) and electron (bottom) mobilities (in cm²/(V.s)) in the R-(left) and L-(right) conformers of Tp-DMTPS. The horizontal [vertical] direction corresponds to the c-[b-] unit cell axis. Note that the mobility scale is different for holes and electrons.

Table 1

 1 Selected torsion angles and calculated orbital energies of

Tp-DMTPS and Tp-MPTPS.

			φ2 (°)	φ 3 (°)	φ 4 (°)	φ 5 (°)	HOMO (eV) LUMO (eV) Eg (eV)
		Optimized a	42.6	59.5	58.2	42.6	-5.56	-1.99	3.57
	Tp-DMTPS	R-conformer b	54.0	70.2	50.4	58.6	-5.65	-1.81	3.84
		L-conformer b	74.2	76.0	71.4	67.5	-5.88	-1.64	4.24
	Tp-MPTPS	Optimized a X-ray	43.9 51.4	59.4 53.6	58.1 56.2	43.6 33.1	-5.58	-2.00	3.58

a B3LYP/6-31G optimization. b Determined from XRD data.

Table 2

 2 Optical and electronic properties of Tp-DMTPS and Tp-MPTPS. a

		Soln	λabs (nm) Thin film b	Soln (φF)	λem (nm) Solid (φF)	Eg opt (eV)	HOMO (eV) c	LUMO (eV) c	Eg el (eV)
	Tp-DMTPS	357	365	486 (1.4)	561 (43) -533 (43) d	2.89	-5.54	-2.48	3.06
	Tp-MPTPS	369	370	491 (1.2)	498 (24)	2.85	-5.54	-2.51	3.03

a λabs = absorption maximum; λem = emission maximum; φF = fluorescence quantum yield in CH2Cl2 solutions (Soln) and in the solid state (Solid); Eg opt = energy gap calculated from the absorption edge; Eg el = electrochemical gap obtained from HOMO and LUMO; b 100 nm thick film; c HOMO and LUMO levels determined from CVs in CH2Cl2 / Bu4NPF6. d First values : microcrystalline powders, second values : ground powders. Excitation wavelength: 365 nm.

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Acknowledgements

The authors thank the CNRS and the Université of Montpellier for financial support. The authors are also grateful to the Agence Nationale de la Recherche (ANR) for financial support of the research project PSICO (# ANR-07-BLAN-0281-01). The authors also thank technical staff of the Plateforme d'Analyses et de Caractérisations (PAC) de l'Institut Charles Gerhardt de Montpellier (ICGM) for their help and fruitful discussions. The work in Mons was supported by the European Commission and Region Wallonne FEDER program (BIORGEL project), and by the FNRS-FRFC (Consortium des Équipements de Calcul Intensif -CÉCI). P. Gerbier and S. Clément are grateful to Mr. Mehdi Mokhtari for his experimental work during his DUT internship.

Conflicts of interest

There are no conflicts to declare.