A Dorle

R Gillard

E Menargues

M Van Der Vorst

P. Martín-Iglesias E De Rijk

P Martin-Iglesias

M García-Vigueras

A Dorlé

Circularly Polarized Leaky-Wave Antenna Based on a Dual-Mode Hollow Waveguide

Keywords: Additive manufacturing, circular polarization, slotted waveguide, dual-mode, K-band, leaky waves, selective laser melting

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I. INTRODUCTION

Circular polarization (CP) is often required for space applications, being implemented for telecom links between satellites and base stations, end-users on Earth, or landers for deep space missions [START_REF] Hodges | A Deployable High-Gain Antenna Bound for Mars: Developing a new folded-panel reflectarray for the first CubeSat mission to Mars[END_REF]. CP is also used for potential future inter-satellite links (ISLs) within constellations of small satellites [START_REF] Radhakrishnan | Survey of Inter-Satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View[END_REF]. In addition to CP, the space environment also demands low loss components and to avoid the use of dielectric materials that are directly exposed to free space [START_REF] Chahat | All-Metal Dual-Frequency RHCP High-Gain Antenna for a Potential Europa Lander[END_REF]. As a result, structures based on metallic waveguides (WGs) are typically preferred in this context. Arrays of horns are the most commonly employed solutions, providing robust performance and wideband operation at the cost of relying on cumbersome WG beamforming networks. Promising alternatives presenting higher compactness are based on leaky-wave antennas (LWAs) and slotted arrays, whose profile is significantly lower since they avoid the need for complex feeding networks [START_REF] Oliner | Leaky-wave antennas[END_REF]. The trade-off in this case concerns the bandwidth of operation, which is narrower [START_REF] Lenormand | Compact switchable RHCP/LHCP mobile Ka-band antenna[END_REF], [START_REF] Hirokawa | Low-Profile Mobile DBS Receiving System using a Single-Layer Slotted Leaky Waveguide Array[END_REF]- [START_REF] Hougardy | Arbitrarily polarized slot array[END_REF].

Two main techniques can be identified in the literature to conceive compact CP antennas based on the radiation from leaky waves (LWs) on hollow WGs. On the one hand, single linearly-polarized slotted arrays can be combined with external polarizers [START_REF] Lenormand | Compact switchable RHCP/LHCP mobile Ka-band antenna[END_REF]. However, the co-design of the polarizer and the slotted array is not obvious and needs iterative steps to find the best solution. Besides, such compound architectures require further assembly steps and may result in costly and bulky prototypes. On the other hand, a second technique can be found using CP-polarized LW radiating elements, Manuscript submitted March 16, 2020. This work was supported in part by the European Space Agency under the grant NPI 486-2016, Rennes Métropole for the funds AIS 17C0481.

A. Dorlé, M. García-Vigueras and R. Gillard are with IETR, UMR CNRS 6164, INSA Rennes, 35708 Rennes, France (e-mail: aurelie.dorle@insarennes.fr, maria.garcia-vigueras@insa-rennes.fr).

P. Martín-Iglesias and M. Van Der Vorst are with the European Space Agency (ESA), ESTEC, Noordwijk, The Netherlands.

E. Menargues and E. De Rijk are with SWISSto12, CH-1020 Renens, Switzerland (e-mail: e.menargues@swissto12.ch).

Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2016.xxx thus avoiding the need for external polarizers. The LWA presented in [START_REF] Hirokawa | Low-Profile Mobile DBS Receiving System using a Single-Layer Slotted Leaky Waveguide Array[END_REF] has the advantage to be simple to design and manufacture, but its monomodal structure and asymmetric aperture do not allow dualpolarization. Furthermore, the cross-pol levels attained in K-and Kabands remain above -20 dB. Other authors have proposed to generate CP from symmetric LWAs basing on dual-mode operation [START_REF] Hougardy | Arbitrarily polarized slot array[END_REF]. In this case, the antenna has a compound feeding network consisting of a magic-T, phase shifters and attenuators. This assembly is needed to tune the LWA axial ratio (AR) by controlling the amplitude and phase of both modes at the antenna input. This article presents a LWA that induces CP from two degenerated modes together with a design methodology based on dual-mode dispersion analysis. Such an original methodology allows understanding the LW nature of the antenna and synthesizing CP while using a very simple transition (in contrast to the bulky network needed in [START_REF] Hougardy | Arbitrarily polarized slot array[END_REF]. In particular, the propagating modes TE 01 and TE 10 , are excited simultaneously at the input of the LWA with same power and phase. Single CP is achieved by designing properly the dimensions of the WG section and the aperture perforations. The structure can be interpreted as a combination of transversely slotted waveguide and longitudinally slotted waveguide LWAs. This proposal evolves from a previous contribution from these authors that concerned the generation of dual-linear polarization and sidelobe-level reduction [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF], [START_REF] García-Vigueras | Cost-effective dual-polarized leaky-wave antennas enabled by three-dimensional printing[END_REF]. It should be noted that we restrict here to the case of single-CP, but extension to dual-CP is straightforward by only adding an orthomode transducer (OMT). This easy extension is not possible in other similar structures [START_REF] Hirokawa | Low-Profile Mobile DBS Receiving System using a Single-Layer Slotted Leaky Waveguide Array[END_REF].

In order to validate the antenna operation principle and the design methodology, a prototype is conceived at K-band. The LWA has been monolithically manufactured using a metal Additive Manufacturing (AM) process based on Selective Laser Melting (SLM) combined with metal plating. Previous work by these authors considered a different technique based on polymer 3D-printing [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF], [START_REF] García-Vigueras | Cost-effective dual-polarized leaky-wave antennas enabled by three-dimensional printing[END_REF]. To their best knowledge, this is the first proof of successful SLM prototyping of a leaky-waveguide. Moreover, the enhancement of the antenna radiation efficiency is proven here for the first time through the post-processing of the piece by metal plating.

Section II introduces the antenna structure and the study of the modes dispersion. Section III presents the design process to generate CP. Section IV discusses the employed AM manufacturing technique. Section V shows the antenna measured performance. Good agreement is observed compared with full wave simulations, thus validating the proposed methodology based on the control of the LW propagation constants. Finally, conclusions are given in Section VI.

II. DUAL-MODE LWA: STRUCTURE AND DISPERSION ANALYSIS

The structure of the proposed LWA is shown in Fig. 1. The radiating aperture is presented in Fig. 1 (a) and is created on the top wall of a rectangular and hollow WG of length 𝐿 𝐴 . The aperture is composed of periodically perforated crossed slots centered with respect to the WG lateral walls. The two arms in a cross may be of . different length, and their dimensions may vary along the y-axis [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF]. The LWA is fed through a WR42 waveguide. As it is evidenced in Fig. 1 (a) and (b), a WG feed transition of length 𝐿 𝑇 couples the energy from the input port to the TE 10 and TE 01 modes of the hollow WG [START_REF] Rebollar | Dual-band compact square waveguide corrugated polarizer[END_REF]. The precise design of this transition is explained in Section III. Finally, the LWA is closed at its end by a metal plate, operating as a short circuit.

The basic underlying principle of the considered dual-mode LWA is explained in [START_REF] Hougardy | Arbitrarily polarized slot array[END_REF], [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF] and [START_REF] García-Vigueras | Cost-effective dual-polarized leaky-wave antennas enabled by three-dimensional printing[END_REF]. In brief, the degenerated TE 10 and TE 01 modes excite two LWs which propagate along the y-axis with respective complex wavenumbers: 𝑘 𝑦 10 01 ⁄ = 𝛽 10/01 -𝑗𝛼 10/01 [START_REF] Oliner | Leaky-wave antennas[END_REF]. The leakage rates 𝛼 10/01 express the amount of energy leaked through the slots, thus determining the magnitude of the antenna aperture illumination for each polarization. The phase constants 𝛽 10/01 govern the antenna pointing angle 𝜃 𝑅𝐴𝐷 10/01 for each polarization. Independent control of 𝛽 10/01 (𝑦) and 𝛼 10/01 (𝑦) is possible at a fixed frequency in order to taper the antenna illumination [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF].

A. Radiation from 𝑇𝐸 01 mode

When the leaky hollow WG is excited by the TE 01 mode, the near field at 𝑧 = 𝑏 may be derived from classical WG theory [START_REF] Pozar | Microwave Engineering[END_REF].

𝐻 𝑦 01 = -𝐴 01 𝑒 -𝑗𝑘 𝑦 01 𝑦 (1)
It is hence evidenced that the slotted aperture is excited by a single 𝐻 𝑦 01 component. Equivalence principle then allows for expressing the final radiating current (2) [START_REF] Balanis | Antenna Theory: Analysis and Design[END_REF]. A complex coefficient 𝑐 01 is added in the expression, representing the coupling between the guided and radiating currents. The current has a single x component that interacts with the cross arm along y (see Fig. 1(c)). The resulting far field is found by integrating the current (2) [START_REF] Balanis | Antenna Theory: Analysis and Design[END_REF]. By evaluating it in the plane of interest 𝜑 = 𝜋 2 ⁄ , it is theoretically confirmed that the TE 01 LW generates a φ-polarization: = 𝑐 01 𝐴 01 𝑤 𝑠 01 𝜂𝑘 0 𝑒 -𝑗𝑘 0 𝑟 4𝜋𝑟 𝑒 𝑗𝐿 𝐴 (𝑘 0 sin 𝜃-𝑘 𝑦 01 ) -1 𝑘 0 sin 𝜃 -𝑘 𝑦 01 𝑒 𝑗𝑘 0 𝑏cos𝜃 (3)

B. Radiation from 𝑇𝐸 10 mode

A similar procedure may be followed to express the far field generated by the TE 10 LW. The corresponding near field propagating at the top of the leaky hollow WG, at 𝑧 = 𝑏, is given in ( 4) and ( 5

C. Dual-Mode Dispersion Analysis

The frequency dispersion of the two LWs propagating in the antenna has been computed with a full-wave solver (HFSS). From the simulated results, the following well-known expressions are used in order to compute the phase and attenuation constants: . Main results are shown in Fig. 2. In each dispersion diagram, the leakage rate is represented only from the cutoff frequency, since this is the regime in which our LWA will operate. More specifically, these graphs show the impact of the perforations dimensions on the wavenumbers of both LWs when considering uniform LWAs (all the crosses are identical along the y-axis). Illustrative cases are considered where the lengths of the crossed-slot arms (𝐿 10 and 𝐿 01 , see Fig. 1) vary while their widths are kept constant. The main structure dimensions are given in the caption of Fig. 2.

According to Fig. 2 (a) and (b), the two LWs phase constants exhibit an identical behavior, and it is coherent with the basic theory on perforated hollow WGs [START_REF] Oliner | Leaky-wave antennas[END_REF]. Especially, they increase from zero at the cutoff frequency, and tend asymptotically to the value 𝑘 0 as the frequency increases. The specific dimensions of the hollow WG cross section (namely, a, b) have been chosen in order to assure that only the desired degenerated modes (the TE 10 and the TE 01 ) propagate within the frequency band of interest. Besides, the period P between two consecutive crosses is chosen to avoid the excitation of propagative higher-order Floquet harmonics below 25 GHz [START_REF] Oliner | Leaky-wave antennas[END_REF]. Fig. 2 (a) and (b) show that 𝐿 10 and 𝐿 01 have no effect on 𝛽 01 and 𝛽 10 , respectively. On the contrary, 𝐿 10 and 𝐿 01 may affect 𝛽 10 and 𝛽 01 , respectively, as shown by Fig. 2 (c) and (d). Indeed, each phase constant is sensitive to the cross arm which is defining the boundary condition of the associated guided mode. The evolution of 𝛼 01 and 𝛼 10 with frequency obeys quite different rules. On the one hand, Fig. 2 (a) and (d) show that 𝛼 01 is greater than zero after the cutoff and then decreases smoothly as 𝛽 01 increases [START_REF] Oliner | Leaky-wave antennas[END_REF]. As the frequency increases, the trend is reversed, and 𝛼 01 increases again to reach a maximum level (above 23 GHz). This evolution is related to the resonance of the crossed slot along the ydirection, thus to 𝐿 01 . This arm determines the level of transparency experienced by the TE 01 mode at the aperture, since the slot is perpendicular to 𝐽 𝑆 01 ⃗⃗⃗⃗⃗⃗ (2). This effect is further demonstrated in Fig. 2 (d), where it is clear that 𝐿 01 allows controlling 𝛼 01 [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF]. It should be noted that 𝛼 01 is not impacted by 𝐿 10 , as shown in Fig. 2 (a). On the other hand, Fig. 2 (b) and (c) present the frequency dispersion of 𝛼 10 [13]. The behavior close to the cutoff is clearly different. At the cutoff of the TE 10 , 𝛼 10 is null and then increases progressively together with 𝛽 10 . This relation is justified by the presence of the factor 𝑘 𝑦 10 multiplying the y-oriented component of the surface current 𝐽 𝑆 10 ⃗⃗⃗⃗⃗⃗ , as also seen in ( 6). It should be noted that such a multiplication factor does not appear in the case of 𝐽 𝑆 01 ⃗⃗⃗⃗⃗⃗ . The curves in Fig. 2 (b) and (c) also show that 𝛼 10 keeps on growing as the frequency increases until reaching a maximum level (which appears above 23 GHz in the case of Fig. 2 (b)), and then decreases asymptotically towards zero. This progression was also observed in the case of 𝛼 01 and is related to the resonance of the crossed slot along the x-direction, and therefore, to 𝐿 10 . By inspecting Fig. 2 (b), it is also clear that 𝐿 01 has almost no impact on 𝛼 10 .

Finally, it can be concluded that the frequency dispersion to both radiating LWs is very similar regarding phase constants, but has significantly different trends for leakage rates. Such differences come mainly from the fact that the aperture current associated to the TE 10 mode is somehow proportional to its phase constant, contrary to the one associated to the TE 01 mode. At a fixed frequency, there is relatively wide margin to control independently the leakage rates associated to both radiating LWs: 𝛼 10 and 𝛼 10 can be tuned through 𝐿 10 and 𝐿 01 respectively. Analogous independent control is achievable for the phase constants 𝛽 10 and 𝛽 10 , using the WG cross section dimensions a and b to modify the cutoff frequencies. Such dispersion graphs have not been presented here for the sake of conciseness [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF], [START_REF] García-Vigueras | Cost-effective dual-polarized leaky-wave antennas enabled by three-dimensional printing[END_REF].

III. DESIGN METHODOLOGY FOR CP ANTENNA

CP radiation can be pursued in the dual-mode LWA through the simultaneous excitation of both LWs analyzed in the previous section. To this aim, the objective is to generate and combine properly the two radiated orthogonal linear polarizations expressed in (3) and ( 6), which is carried out in two consecutive steps. Firstly, the perforated WG is designed, assuming equal excitation of both degenerated LWs. In coherence with the scenario considered in Section II.C., the perforated WG is assumed to be ended by a matched load in this first step. Secondly, a WG feed transition is designed to assure such dual-mode excitation from a single monomodal port. Additionally, several actions are taken to mitigate possible diffraction produced by the two LWs at the input and output of the perforated WG. These two steps are explained in the next subsections, which are followed by the performance of a design operating around 20 GHz.

A. Design of Dual-Mode Perforated WG

As explained above, the WG is here assumed to be ended by a matched load. Desired CP radiation is thus produced by the two progressive LWs on their first propagation path from 𝑦 = 0 up to 𝑦 = 𝐿 𝐴 (see Fig. 1 ). The analysis of ( 3) and [START_REF] Hirokawa | Low-Profile Mobile DBS Receiving System using a Single-Layer Slotted Leaky Waveguide Array[END_REF] shows that CP conditions are not met by just exciting the transverse and longitudinal slots with equal orthogonal modes. Indeed, the coupling coefficients 𝑐 10/01 have also to be taken into account. Additionally, those field expressions already suggest that, though such conditions for CP may be met at one frequency, they will not be satisfied on a broad bandwidth. This is a consequence of the distinct frequency behaviors observed for 𝑘 𝑦 10/01 in Section II, and the fact that 𝑐 10/01 may also be frequency dependent.

1. The starting point for the design is analytical. From the central operation frequency (𝑓 0 =20 GHz in our case), the WG cross section is preliminary dimensioned to be squared (a=b). Its size is chosen to achieve a desired pointing angle 𝜃 𝑅𝐴𝐷 𝐶𝑃 = 𝜃 𝑅𝐴𝐷 10 = 𝜃 𝑅𝐴𝐷 01 , by applying the well-known approximate relation with 𝛽 10 and 𝛽 01 found from a closed WG (𝜃 𝑅𝐴𝐷 𝐶𝑃 = 35° results in a=b=9.16 mm) [START_REF] Pozar | Microwave Engineering[END_REF]. Next, the period between perforations is selected in order to avoid higher-order harmonics at 𝑓 0 (P = 7 mm) [START_REF] Oliner | Leaky-wave antennas[END_REF]. The size of the aperture (𝐿 𝐴 = 136 mm) is selected to attain high beam efficiency. It considers useful CP radiation coming only from the progressive LWs, and is defined as:

𝜂 𝑏 𝐶𝑃 = 𝑃 𝑅𝐴𝐷 (𝐿 𝐴 ) 𝑃 𝑖𝑛 ⁄ (7)
where 𝑃 𝑖𝑛 is the incident power in the aperture, and 𝑃 𝑅𝐴𝐷 (𝐿 𝐴 ) is the power leaked between 𝑦 = 0 and 𝑦 = 𝐿 𝐴 . High values of 𝜂 𝑏 𝐶𝑃 are thus attainable with high values of 𝛼 10 01 ⁄ . As concluded in Section II, such scenario is possible when working at frequencies close and below the resonance of the crossed slots, when the aperture is highly transparent. The lengths of the crosses arms are thus initially chosen to be as long as possible, while still remaining below their first resonance. The value of 𝐿 01 is additionally limited by the period P.

2. This preliminary design is then simulated with a full-wave solver, being excited at its input and loaded at its end with ideal dualmode matched WG ports. A parametric analysis is carried out in order to tune the thickness of the metal walls (t) and the widths of the crossed slots (𝑤 10 and 𝑤 10 ). The objective is to trade off high attainable value of 𝜂 𝑏 𝐶𝑃 for manufacturability. 3. The resulting design is modified afterwards through a coarse full-wave tuning of parameters 𝑎, 𝑏, 𝐿 10 and 𝐿 01 , with the objective to reach 𝛼 10 ≈ 𝛼 01 and 𝜃 𝑅𝐴𝐷 10 ≈ 𝜃 𝑅𝐴𝐷 01 = 𝜃 𝑅𝐴𝐷 𝐶𝑃 . Interpolation between simulated values is used to reach an approximate solution at a reasonable computational cost.

4. Finally, the previous design provides a robust starting point for a final full-wave optimization. The considered parameters are 𝑎, 𝑏, 𝐿 10 and 𝐿 01 and the objective is to reach the minimal AR and the desired 𝜃 𝑅𝐴𝐷 𝐶𝑃 .

B. WG Feed Transition and Diffraction Mitigation

Once the cross section of the perforated WG is defined, the appropriate WG feed transition can be designed in order to excite simultaneously both modes with equal power and phase. It should be noted that this feed component is very simple and does not implement any orthomode function. The feed has been designed to discard any direct radiation from the source, and discontinuities have been minimized to avoid diffraction. In this way, the leaky modes are the main source of radiation from this antenna. As shown in Fig. 1 (b), we consider a WG transition made of 5 consecutive sections. The first one is a standard WR42 rectangular WG section, allowing propagation of the fundamental TE 10 mode. Then, a smooth taper of length 𝑠 2 transforms the rectangular cross section to a square one of size a (the width of the perforated WG). The third part of length 𝑠 3 is inspired from [START_REF] Rebollar | Dual-band compact square waveguide corrugated polarizer[END_REF] and enables dual-mode propagation. More precisely, the output port is rotated by 45° relatively to the input one so that the fundamental mode is decomposed into the two degenerate TE 10 and TE 01 modes, with same power and phase. Next, a ridged waveguide of length 𝑠 4 is implemented to transform the a×a cross section to a a×b rectangular one. Tapered ridges allow for maintaining the same phase for the two output modes, despite the rectangular section. The last section, of length 𝑠 5 , is a classical rectangular waveguide with a cross section a×b. This feed transition has been optimized using full-wave simulations, its final dimensions are given in the caption of Fig. 1. The obtained performance is shown in Fig. 3. It is evidenced that the desired dual-mode output is obtained, with modes of almost equal powers and phases. The transition is also well matched with S 11 < -15 dB over the frequency band of operation.

Additionally, it has been observed that diffraction occurs at the beginning of the perforated WG when connected to the feed transition, due to the exponential illumination of the aperture and its associated abrupt initial radiation [START_REF] Oliner | Leaky-wave antennas[END_REF]. Such phenomena may not only degrade the shape of the pattern but also the AR, since it does not affect equally the 𝜑 and 𝜃 polarizations. A modulation of the four first crosses is applied and thus mitigates such deterioration. The leakage rates are therefore tuned along the y-axis, with the crosses dimensions found to reach the specified 𝛼 10 01 ⁄ while keeping the same beam efficiency 𝜂 𝑏 𝐶𝑃 as the non-tapered design [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF]. Finally, the LWA end is loaded by a short circuit, as illustrated in Fig. 1. Such decision aims again at diffraction mitigation and keeping the LWA compact and short (to be compatible with small platforms). The energy remaining at the end of the antenna may radiate if it is left open. Non-negligible radiation from such discontinuity would affect differently 𝜃 and 𝜑 polarizations [START_REF] Dorlé | Additive Manufacturing of Modulated Triple-Ridge Leaky-Wave Antenna[END_REF], thus potentially spoiling the AR. Additionally, the short circuit allows reducing uncertainty in the antenna performance, since it isolates it from external interferences, and has a simple implementation. The power reflected backwards by the short circuit is radiated as crosspolarization in a direction opposite to the main beam. An alternative matched load can be employed in applications where such backward beam is not acceptable.

C. Example of CP Performance

The designed antenna is first simulated using an ideal input port exciting both degenerate modes (without the transition). Two different configurations are compared: one with a matched termination at the output and the other with a short circuit. The radiation pattern at 20 GHz is exhibited in Fig. 4. Due to the high efficiency (𝜂 𝑏 𝐶𝑃 > 85%), both terminations evidence very close performance in the main beam. The short circuit is then chosen, although a high backward cross-polarized beam is produced. The achieved pointing angle is 36.2° with a maximum directivity of 16 dBi. Low cross-polarization is observed in the main beam. The AR in the main beam is given in Fig. 5, and it is better than 3 dB on a 4 % bandwidth with a minimal value of 0.98 dB at 20.2 GHz. Fig. 5 plots the AR, considering three different configurations: with ideal feeding, with the transition designed in III. B, and finally, with application of the tapering procedure. The AR degradation associated to the use of the feed transition is evident in this figure, with a shift in the frequency of operation towards higher values. The taper allows for a slight improvement of the AR performance and bandwidth widening. The final design with crosses modulation provides LHCP polarization with AR of 0.3 dB at 20.6 GHz. The 3 dB AR bandwidth is 3.3 % with a pointing angle at 39.3° and a directivity of 15.9 dBi. The antenna is matched below -10 dB over the full usable band, from 19 GHz to 21.5 GHz. Further optimization of the final assembled antenna, or scaling of the metallic structure, are still possible actions to center the operation at 20 GHz.

IV. METAL ADDITIVE MANUFACTURING

The manufacturing technique selected for the prototyping of the CP LWA is the metal Additive Manufacturing (AM) process developed by the company SWISSto12. This process, which is presented here to the academic community for the first time, consists in two steps. The first step is the manufacturing of the component itself using Selective Laser Melting (SLM), while the second step is an electroless chemical treatment for roughness reduction followed by copper plating and silver passivation.

SLM can be used with different materials, but when using metals such as aluminum alloys, the raw manufactured part can already meet the typical mechanical and thermal requirements for space applications. This, as well as the fact that the raw part is nearly RFfunctional, is the main reason why SLM is typically the preferred AM process for WG hardware in the microwave industry [START_REF] Peverini | Selective Laser Melting Manufacturing of Microwave Waveguide Devices[END_REF], [START_REF] Booth | Enhancing the Performance of Waveguide Filters Using Additive Manufacturing[END_REF].

The main disadvantage of SLM is a poor surface roughness compared to common machining procedures. Additionally, the high porosity of the raw parts makes it difficult to plate the internal surfaces by standard methods. This often results in unacceptably high losses above Ku-band, even if a material with high bulk conductivity such as an aluminum alloy (AlSi10Mg) is used.

In order to overcome the poor surface roughness inherent to SLM, SWISSto12 has been active in developing a proprietary surface treatment process that leads towards significant RF loss reduction. This process consists of an electroless surface treatment that reduces the effective surface roughness and porosity, followed by copper plating for further effective conductivity improvement and silver passivation. The raw and plated equivalent conductivities of AM parts are below 0.5e7 S/m and 2e7 S/m, respectively. These two values are estimated from the comparison between measurements and simulations of a large sets of waveguides from WR90 to WR28. It must be highlighted that the metal AM route developed by SWISSto12, and presented here, has been extensively tested through several qualification processes for flight programs between Ku-band and Q-band.

The SLM tolerances range from 50 m to 150 m, depending on how the object is oriented inside of the 3D-printer. Generally, the best results are obtained when the structure does not present unsupported overhanging faces. If this is not the case, the structure is 3D-printed under certain angle, which leads to deterioration of the manufacturing tolerances as a consequence of the staircase effect [START_REF] Charles | Effect of Process Parameters on the Generated Surface Roughness of Down-Facing Surfaces in Selective Laser Melting[END_REF]. Fig. 6 shows the CP LWA presented in this work. Two measurement jigs (90° bend and twist) have also been produced using AM to facilitate the measurements in the anechoic chamber. To enable the best quality inside the dual-polarized waveguide, the antenna has been printed with the slots facing the printing platform. The cross-slots (see the shiny finish surrounding the slots in Fig. 6) and all the flanges have been re-worked after the SLM process, thus ensuring the best possible quality from this manufacturing run. The CP LWA has been measured in a compact anechoic chamber between 19 and 22 GHz. According to Section IV, the following simulation curves have been updated compared to Fig. 4 and5, taking into account additional metal thickness due to plating. Fig. 7 shows the normalized radiation pattern at 20.5 GHz. The figure compares full-wave simulation and measurement. Both curves agree very well, with a pointing angle shifting of 0.9°. The measured cross-polarization has the expected magnitude, around -22 dB. There is also a significant backward beam, created by the presence of the short circuit at the antenna end. For applications where this beam is not allowed, a waveguide load can be used. The reflection coefficient of the single LWA is shown in Fig. 8 (a). The measured value is higher than the simulated one. The differences are most probably caused by the fabrication tolerances and the precision of the measurement, done within a non-anechoic environment. The LWA is anyway well matched below -10 dB above 20 GHz. The pointing angle at the operation frequency is 35.9°. The complete frequency behavior is shown in Fig. 8 (b), and it is in very good agreement with simulations (differences are always below 1 degree). The antenna absolute gain is shown in Fig. 8 (b). In this case, the difference must be attributed to the precision of the test setup. The cross-polarization discrimination is presented in Fig. 8 (c). The axial ratio is better than 3 dB within a 500 MHz band centered at 20.55 GHz. This result, as well as the cross-polarization of Fig. 7, experimentally confirm that single-CP generation

.

mechanism works well, and that the realization of the antenna is accurate. Finally, the radiation efficiency of the prototype is presented in Fig. 8 (d). It is worth reminding that the radiation efficiency differs from the beam efficiency, and takes into account the radiation of the LWs both in their forward path along +y and backward one along -y. The conductivity enhancement achieved by the plating process represents a nearly 10%-increasing of the radiation efficiency. The CP LWA shows a radiation efficiency of around 90%. The device under test includes the bend and twist (which have not been plated) and the coaxial-to-WR42 setup excitation. Therefore, the CP LWA breadboard itself has an efficiency larger than 90%.

The main features of this CP LWA are compared in TABLE I with other LWAs in the literature providing CP and based on hollow WGs. From this table it can be summarized that the proposed antenna provides better CP quality and bandwidth than previous proposals. Besides, it is the only one benefiting from monolithic prototyping, thus allowing for very high radiation efficiency, higher precision and robustness while avoiding the need for any assembly process. Furthermore, the LWA topology here proposed is the only one that allows a straightforward extension to dual-CP performance. This upgrade can be attained through AM easily by integrating an OMT in the feeding transition, which may excite the two TE 01 and TE 10 modes with either 0 or π phase difference at the leaky guide input. It should be mentioned that such an antenna will have higher profile and less integration easiness compared to the existing reports on CP LWA employing dielectric substrates.

VI. CONCLUSION

A CP dual-mode LWA is presented here that is built only from metal. An original comparative dispersion study of the two leakymodes excited in the structure has been carried out for the first time, and evidences the different nature of both waves. Such differences justify that the combination of the radiation from both modes produces CP in a narrow band. A prototype operating at 20 GHz is designed and built based on SLM additive manufacturing. For the first time, a plating post-processing step is conducted, proving its interest regarding the radiation efficiency improvement. The plated prototype shows a satisfactory measured performance with an AR of 0.92 dB at 20.5 GHz.

Fig. 1 .

 1 Fig. 1. Topology of the proposed LWA. (a) Full structure, (b) Single-to-dual mode WG feed transition, (c) Unit cell. Dimensions for the K-band design are: 𝑃 = 7 mm, metal thickness = 1.4 mm, 𝑎 = 9 mm, 𝑏 = 8.8 mm, 𝐿 01 = 6.2 mm, 𝐿 10 = 7.3 mm, 𝐿 𝐴 = 136 mm, 𝑠 1 = 7.9 mm, 𝑠 2 = 11.8 mm, 𝑠 3 = 23.7 mm, 𝑠 4 = 4.6 mm, 𝑠 5 = 5.5 mm. The widths of the cross arms are 1 mm along x, and 1.76 mm along y.

  𝑧 ⃗⃗⃗ × 𝑐 01 𝐻 ⃗ ⃗ = -𝑐 01 𝐻 𝑦 01 𝑒 𝑥 ⃗⃗⃗ = 𝑐 01 𝐴 01 𝑒 -𝑗𝑘 𝑦 01 𝑦 𝑒 𝑥 ⃗⃗⃗ (2)

  to the previous TE 01 LW, two components excite now the slotted aperture: 𝐻 𝑥10 and 𝐻 𝑦 10 (as 𝐸 𝑧 10 is orthogonal to the surface). The radiating currents, being either xor y-oriented, are derived straightforwardly. The far field radiated by the TE 10 LW in the ZY-plane is a pure θpolarization, expressed in[START_REF] Hirokawa | Low-Profile Mobile DBS Receiving System using a Single-Layer Slotted Leaky Waveguide Array[END_REF], which is provided by the symmetry of the perforations with respect to the lateral walls.

  𝑦 10 𝜂𝑘 0 𝑒 -𝑗𝑘 0 𝑟 4𝜋𝑟 𝑒 𝑗𝐿 𝐴 (𝑘 0 sin 𝜃-𝑘 𝑦 10 ) -1 𝑘 0 sin 𝜃 -𝑘 𝑦 10 𝑒 𝑗𝑘 0 𝑏cos𝜃[START_REF] Hirokawa | Low-Profile Mobile DBS Receiving System using a Single-Layer Slotted Leaky Waveguide Array[END_REF]

Fig. 2 .

 2 Fig. 2. Normalized phase constants (dashed lines) and leakage rates (continuous lines). Subfigures (a) and (d) show the dispersion for the perturbed TE 01 mode while (b) and (c) concern the case of the TE 10 . (a) and (c) present the effect of varying 𝐿 10 (𝐿 01 = 6 mm) whereas (b) and (d) show results sweeping 𝐿 01 (𝐿 10 = 6 mm). Other dimensions of the simulated LWA are: 𝑃 = 7 mm, thickness of metal 1.4 mm, 𝑎 = 𝑏 = 8.5 mm, slots widths 0.7 mm and 𝐿 𝐴 = 84 mm.

  (a)). Pure CP is theoretically achieved when |𝐸 𝜃 10 | = |𝐸 𝜑 01 | and arg(𝐸 𝜃 10 ) = ± 𝜋 2 ⁄ + arg(𝐸 𝜑 01

Fig. 3 .

 3 Fig. 3. Performance of the WG feed transition for dual-mode excitation.

Fig. 4 .

 4 Fig. 4. Normalized radiation pattern (dB) simulated at 20 GHz in the 𝜑 = 𝜋 2⁄ plane. The perforated WG is excited at its input by an ideal dualmode port and ended by a short circuit or a matched load.

Fig. 5 .

 5 Fig. 5. AR (dB) in the main beam direction (𝜑 = 𝜋 2 ⁄ plane).

Fig. 6 .

 6 Fig. 6. Images of the prototypes (a) CP LWA before plating, (b) bend and twist for test (c) CP LWA after

Fig. 7 .

 7 Fig. 7. Normalized radiation patterns (dB) in 𝜑 = 𝜋 2 ⁄ plane at 20.5 GHz. Continuous lines represent LHCP whereas dashed lines are RHCP. Grey lines are measured patterns while black curves are full-wave ones.

Fig. 8 .

 8 Fig. 8. Frequency performance of the prototype versus simulation, (a) matching (dB), (b) absolute gain (dBi) on the left and pointing angle (deg) on the right, (c) axial ratio (dB) in the main beam direction for 𝜑 = 𝜋 2 ⁄ , (d) radiation efficiency (%), raw versus plated prototype.

ACKNOWLEDGMENTS

The authors would like to thank Dr. L. Le Coq, from IETR, and L. Simon for their help in the experimental activities. This publication has been supported by the European Space Agency through an NPI grant, Rennes Metropole under Grant AIS 17C0481, the European Union through the European Regional Development Fund, and by Ministry of Higher Education and Research, Brittany & Rennes Metropole, through the CPER Project SOPHIE / STIC & Ondes.


Dr Etienne S Gauthier

Dr Marie Cordier

Dr Vincent Dorcet

Dr Nicolas Vanthuyne

Dr Ludovic Favereau

J A Gareth Williams

Jeanne Crassous
email: jeanne.crassous@univ-rennes1.fr

Prof. J Williams A Gareth

Helically Chiral NHC-Gold(I

Keywords:

published or not. The documents may come

Introduction

For three decades since the isolation of the first free carbene by Arduengo et al., [1] N-heterocyclic carbenes (NHCs) have become popular strongly -donating ligands in organometallic chemistry, thanks to their ability to form stable carbene-metal bonds. [2] The complexes obtained have been shown to offer applications in homogenous catalysis, [3] medicinal chemistry, [4] and materials science. [5] The highly versatile structure of NHCs enables access to complexes with a plethora of transition metal centers in different coordination modes. For instance, many classes of luminescent M-NHC complexes have been developed that have potential in photonic and optoelectronic devices, in bioimaging and cancer therapy, or as photosensitizers in photoredox catalysis. [6] Among the d 10 coinage metals, gold(I) gives chemically stable complexes [7] that have been applied as luminescent materials, [6d,e] catalysts, [8] and therapeutic agents. [9] Recently, organometallic complexes bearing chiral NHC ligands have been increasingly studied, mainly owing to interest in enantioselective catalysis, [10,11] but also for their chiroptical features, i.e. Electronic Circular Dichroism (ECD) [11] and Circularly Polarized Luminescence (CPL), [6h,11b,d,e] that are appealing in the development of chiral molecular materials. [START_REF] Sanchez | For other examples of helicene-NHCs see: a) I[END_REF] While several classes of chiral NHC-gold(I) complexes have been developed for asymmetric catalysis, [13] results displaying the use of CPL-active NHC-gold(I) complexes are still limited. [14] Helicenes are ortho-condensed polyaromatic molecules; they possess inherent chirality due to the steric repulsion of their skeleton termini. [15] They are known to display strong chiroptical properties such as huge optical rotation values, strong ECD responses and substantial CPL activity. [15i,j] Our group has recently developed chiral versions of several different classes of organometallic compounds by incorporating helicenes within ligands. [16] Of particular interest are helicene-based NHC ligands which have yielded several types of complexes displaying strong ECD spectra [11a,c] and long-lived circularly polarized phosphorescence. [11b,d,e] Herein, we report the synthesis and the structural characterization of two-coordinate helically chiral [5]helicene-NHC gold(I) complexes with diverse ancillary ligands (chloride 1, acetylenide 2, carbazolide 3, Figure 1). They were prepared using N- (3,5-dimethylphenyl)-[5]helicenic-imidazolium salt 6▪H + as NHC precursor. The intrinsic electronic properties of the carbenic system (-donating and -accepting) and the influence of the helicenic unit were determined using classical quantitative methods based on IR and NMR spectroscopies applied to appropriate derivatives (i.e. Ir(NHC)(CO)2Cl complexes and selenoureas). [17,18] Finally, the gold(I) complexes were obtained as enantiopure complexes by chiral HPLC (with the exception of 3 which was unstable during the separation) and their chiroptics (optical rotation OR and ECD), emission properties and CPL activity were analyzed.

Results and Discussion

Synthesis

First, the imidazolium salt used as NHC proligand was prepared by functionalization of the already reported [5]heliceneimidazole 4 [11b] (Scheme 1). Indeed, the Chan-Lam-Evans coupling of 4 with commercially available 3,5-dimethylphenylboronic acid in the presence of copper(II) acetate, triethylamine and activated 4 Å molecular sieves in CH2Cl2 under aerobic conditions gave 5 in 83% yield. Then reaction with an excess of methyl iodide in acetonitrile at reflux afforded pentahelicenic imidazolium iodide [(rac)-6]▪HI in 89% yield. Finally, the chloride derivative [(rac)-6]▪HCl was obtained by anion-metathesis with a Dowex© Cl-exchange resin in 98% yield. The obtention of iodide and chloride imidazolium salts was confirmed by 1 H NMR with the presence of a downfield signal in CD2Cl2 (10.77 and 11.86 ppm, respectively), diagnostic of the imidazolium protons. [19] In addition, two singlets were observed between 2.5 and 4 ppm corresponding to the two equivalent CH3 groups of the 3,5dimethylphenyl group and of the N-Me group. Characteristic signals of the helicenic unit were also observed; for example, the H 13 proton appears as a multiplet at 8.34 ppm for (rac)-5 and as a doublet at 8.30 ppm for [(rac)-6]▪HI in CD2Cl2. Single crystals of [(rac)-6]▪HI were obtained by slow evaporation of pentane into a dichloromethane solution and were analysed by X-ray crystallography. It crystallizes in the C2/c centrosymmetric space group and displays a helicity (dihedral angle between the terminal rings) of 47.40°, a typical value for [5]helicenic derivatives. [15a,11b] Both (M) and (P) helices are present in the crystal and arranged in heterochiral columns along the b axis (see Supplementary Information (SI) for details).

Determination of the Tolmann electronic parameter [20,21] (TEP), based on the average IR-stretching frequency of the carbonyl ligands of [Ir(NHC)Cl(CO)2] complexes, enables the evaluation of the overall electron donation of an NHC ligand. Meanwhile, the measurement of the coupling constant 1 J(C,H) at the precarbenic position of the imidazolium precursor gives the donating ability of the imidazolylidene, as it is inversely proportional to the sigma character of the C-H bond, according to Ganter and co-workers. [22] In addition, the 77 Se NMR shift of the related selenium adduct provides clear information about the accepting properties of a carbene. [17,18] In order to ascertain the influence of the ortho-fused unit on the electronic properties of the NHC, a set of data was also obtained for a model carbene without the helicenic unit. Model salt [8]▪HI was thus prepared from already known benzimidazole 7 [23] and its X-ray structure was also obtained (Scheme 1).

Table 1. Electronic parameters of the studied NHCs NHC 1 J(C,H) (Hz) [a] ( 77 Se) (ppm) [b] TEP [average, CO] (cm -1 ) [c] In CDCl3 [21] 225 27 35 2051 i Pr-bimy [21] 218 -67 2054 BMes [21] ---2052 [(rac)-15] + [20] 224 -65, 49 [d] 2055 [a] Measured on the corresponding imidazolium precursor in CDCl3; [b] Measured on the corresponding selenoureas; [c] Measured on the corresponding [NHC-Ir(CO)2Cl] complex in CH2Cl2, TEP was calculated from the average value of the CO bands' frequency (in brackets for the first two complexes) following Nolan et al. : TEP (cm -1 ) = 0.8475average, CO(cm -1 ) + 336.2 (cm -1 ). [21,22,26] ; [d] The two resonances arise from the presence of two atropoisomers. [14b 10.1002/ejoc.202100722

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European Journal of Organic Chemistry Complexes (rac)-9 and 10 depicted in Scheme 2 were prepared by a one-pot silver-iridium transmetalation method, using Ag2O then [IrCl(COD)]2 (COD = cycloctadiene) and obtained in 43% and 72% yield, respectively, from [(rac)-6]▪HI and [8]▪HI. [24] Interestingly, this synthetic method through transmetalation using silver enables to eliminate anion scrambling. The deshielded signal around 192 ppm in 13 C NMR for (rac)-9 and 10 confirmed the presence of the carbenic carbon. These complexes were fully characterized by NMR, HRMS and X-ray analysis (see Figure 2 and Supporting Information, SI). It should be noticed that (rac)-9 was obtained as a mixture of conformers: (rac)-9 and (rac)-9' with a ratio of 79:21 according to NMR (see Scheme 2 and SI for details). This is in accordance with our earlier work on cyclooctadienyl-iridium(I) complexes bearing a helicenic-NHC. [11c] Indeed, due to the asymmetric nature of the helicenic ligand, the COD ligand can adopt two distinct positions, resulting in the existence of two conformers that are in equilibrium in solution and non-separable by column chromatography. This was fully supported by X-ray crystallography, 2D NMR and theoretical analyses. [11a,c] Single crystals suitable for the analysis of the iridium complexes were obtained by slow evaporation of pentane into a dichloromethane solution. (Rac)-9 and (rac)-10 crystallize in the P-1 and Pbca space groups, respectively (Figure 2). Both structures confirmed the presence of the COD and the chloride ligand coordinated to the pseudo-tetrahedral iridium(I) center with the NHC acting as a monodentate ligand. Regarding the bond lengths, the Ccarbene-Ir bond is slightly longer for the model complex 10 than for the helicenic complex (rac)-9 (2.028 and 2.005 Å, respectively), while no significant difference is observed for the length of the Ir-Cl bond (2.379(2) and 2.361(6) Å, respectively). These values are falling within the range of already published NHC-iridium(I) complexes. [24,25] The helicity of (rac)-9 is 43.52° in a similar range to its pentahelicenic analogues. [11b] It should be noted that the COD ligand is directed toward the helicenic backbone (P or M) and is the only conformer observed in the crystal, which is in accordance with the data obtained previously. [11c] The supramolecular packing consists of heterochiral columns formed by - interactions (3.881 Å) along the a axis (Figure 2). Finally, (rac)-11 and 12 were obtained in quantitative yields by using a balloon filled with carbon monoxide into a solution of (rac)-9 and 10 in CH2Cl2 at room temperature for 15 min (Scheme 2). [32] The formation of the [Ir(NHC)Cl(CO)2] complexes was confirmed by IR spectroscopy (ATR) showing the presence of two distinct bands around 2000 cm -1 , characteristic of the stretching modes of the two carbonyl ligands. Complex 12 was also characterized by X-ray diffraction, revealing the square planar geometry around the Ir(I) center and the two CO ligands, one of them being trans to the NHC monodentate ligand (Figure 2).

Next, deprotonation of [(rac)-6]▪HI and [8]▪HI with sodium bis(trimethylsilyl)amide (NaHMDS, as a 1.0 M solution in THF) at -78°C in distilled THF under argon and subsequent trapping of the free carbene with selenium powder afforded selenoureas (rac)-13 and 14 (Scheme 2). Compound 14 was obtained in 81% yield after filtration while (rac)-13 was isolated in 32% yield after column chromatography. Very recently, Nolan and co-workers presented a milder approach to access selenoureas and thioureas by reaction of the imidazolium salt with selenium powder and NEt3 under air in acetone at 40°C then 60°C overnight. [26] The use of these conditions enabled (rac)-13 to be prepared in 63% yield without any purification. The deshielded signal around 169 ppm in 13 C NMR and the observation of one signal in 77 Se NMR in d6acetone confirmed the presence of the C=Se moiety and the nature of (rac)-13 and 14.

Determination of the electronic properties of the [5]helicenic carbene and comparative study with known NHCs

The TEP, 1 J(C,H) and 77 Se NMR values (in CDCl3 and in d6acetone) for (rac)-6 and 8 are gathered in Erreur ! Source du renvoi introuvable.. Data of already described NHCs (including the well-known IMes) were also added for comparison (see structures in Scheme 2). [17] Concerning the donating properties,

Accepted Manuscript

European Journal of Organic Chemistry the coupling constants are similar for (rac)-6 and 8 (222 Hz) indicating that they have comparable -donating properties. On the other hand, the substitution pattern of the NHC appears to play a more important role, since the i Pr-bimy (without aryl groups) and the IMes have a lower and a higher coupling constant (218 and 225 Hz respectively). Interestingly, [(rac)-15] + possesses a higher coupling constant (224 Hz) than (rac)-6 owing to the cationic nature of the [4]helicenium arm and its withdrawing properties. Additionally, TEP values for (rac)-6 and 8 were found to fall in the same range as IMes, i Pr-bimy, BMes and [(rac)-15] + , thus reinforcing the point that the helicenic backbone has no pronounced influence on the donor character of the NHC. The absence of any clear backbone effect on the TEP was already highlighted by Huynh. [17] The influence of the aromatic backbone on the accepting properties was shown by the disparities in the 77 Se NMR shifts in d6-acetone. A clear trend can be established within the values: the IMes without backbone has the lower value (35 ppm) followed by helicenium [(rac)-15] + (65, 49 ppm) and finally the i Pr-bimy (67 ppm). Hence, it seems to indicate that a conjugated backbone has a more pronounced influence on the -accepting properties than the peripheral aryl groups. Indeed, this is supported by the model carbene 8, which combines both structural aspects, and has an even higher 77 Se shift (113 ppm). Finally, it is confirmed with the [5]helicenic-NHC (rac)-6, which possesses the highest value (133 ppm) of the series. Including [(rac)-15] + in the study highlighted the importance of the position of the helicene with respect to the carbene: higher -accepting character has been obtained with a fully ortho-fused helicenic NHC in comparison with an N-substituted helicenyl NHC.

In the end, due to an easier delocalization of the electron density over the helicenic unit, it can be easily understood that the orthofused helicenic carbene is a better -acceptor than the other carbenes.

Preparation and characterization of the enantiopure [5]helicene-imidazolylidene-gold(I) complexes

It should be noted that the helical chirality (P/M) is not stable in the [5]helicene-imidazole derivatives 4 and 5 as previously demonstrated. [11b] After the quaternarization, the N-Me group inside the helical groove hampers the inversion of the helix thus resulting in a configurationally stable helicene [(rac)-6]▪HX (X = I, Cl). However, due to their cationic nature, the enantiomeric resolution using chiral HPLC is not straightforward under conventional methods. The salts were therefore subjected to gold(I) metalation in their racemic forms. The metalation was conducted using [(rac)-6]▪HCl following a convenient procedure that requires neither inert atmosphere nor strong base. [27] [(Rac)-6]▪HCl was stirred with 1 equivalent of [AuCl(Me2S)] and a base (K2CO3) in acetone at 60°C for one hour in the dark and under air. After filtration over silica gel and precipitation, the desired NHC-Au-Cl complex (rac)-1 was isolated as a stable white solid in 94% yield. Then, (rac)-1 was used as a common precursor to prepare the phenyl-acetylide and the carbazolide derivatives, (rac)-2 and (rac)-3, respectively. (Rac)-2 was prepared by a ligand-exchange reaction by mixing the gold chloride (rac)-1, phenylacetylene and K2CO3 in methanol at 70°C for 2h under air and was obtained in 81% yield after filtration over basic alumina and precipitation. [28] (Rac)-3 was prepared by mixing (rac)-1, 9H-carbazole and NaO t Bu in distilled THF under argon at 70°C overnight and was obtained in 69% yield after filtration over basic alumina and precipitation (Scheme 3).

Scheme 3. Preparation of the helicenic-gold(I) complexes 1-3. i) [AuCl(Me2S)], K2CO3, Acetone, 60°C, 1h, in the dark under air, 94%; ii) phenylacetylene, K2CO3, MeOH, 70°C, 2h, under air, 81%; iii) 9H-carbazole, NaO t Bu, distilled THF, 70°C, overnight under argon, 69%. X-ray structures of (rac)-1, (rac)-2 and (rac)-3. CCDC numbers and crystallographic details are in the SI. Hydrogens have been omitted for clarity. Only one enantiomer is shown.

All the complexes were fully characterized using 1D and 2D NMR. 13 C NMR signals for the expected carbenic carbons were observed around 180-190 ppm in line with the literature. [8,29] The absence of any signal around 11-12 ppm in the 1 H NMR spectra confirmed the consumption of the imidazolium precursor, while the protons from the [5]helicenic unit were observed, notably H 13 (see numbering in Scheme 1) which appears as a doublet at 8.32 ppm for (rac)-1, as a multiplet at 8.36 ppm for (rac)-2 and as a doublet at 8.43 ppm for (rac)-3 (see SI for further details). Single crystals suitable for X-ray characterization of the complexes 1-3 were obtained which confirmed their monomeric nature (Scheme 3). They all crystallized in centrosymmetric space groups, i.e. C2/c, P-1 and P21/c for (rac)-1, (rac)-2 and (rac)-3, respectively, with helicities close to 50° in accordance with other [5]helicenic derivatives (see Table S1 for details). [11b,15a] Regardless of the ancillary ligand, the geometry around the gold(I) center is nearly linear (dihedral angles Ccarbene-Au-Y between 173. 24-178.30° where Y = Cl, C or N) in accordance with already reported twocoordinate NHC-Au(I) complexes. [6d,29] It should be mentioned that none of the complexes exhibit metallophilic interactions in the solid state due to the distance between two neighboring gold centers being too long (3.893 Å for (rac)-1 and 4.026 Å for (rac)-3, with respect to the van der Waals radius of the gold atom established at 1.66 Å), except for (rac)-2 which has a shortest gold-gold distance of 3.567 Å, sufficiently short to infer weak aurophilic interactions. [6d,30] The absence of strong metallophilic interactions is probably related to the bulkiness of the helical ortho-fused unit in addition to the presence of a moderately bulky aryl substituent attached to the NHC ring. [30b] The chiral HPLC separation of the enantiomers of 1, 2 and 3 was then examined. Complexes 1 and 2 were eluted on a Chiralpak IH column and a Chiralpak IB column respectively to afford isolated (P) and (M) enantiomers with excellent enantiomeric excesses (ee values > 97.5 %, see SI for details). The carbazolide complex 3 was eluted on a Chiralpak IA column but the compound decomposed during the separation, forming an impurity visible on the chromatograms (see SI). It suggests that the Au-Ncarbazolide bond in NHC-gold-carbazolide complexes is only moderately stable over stationary phases (silica gel or basic alumina). Thus, only the photophysical and chiroptical properties of complexes 1 and 2 were investigated.

10.1002/ejoc.202100722

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European Journal of Organic Chemistry

Photophysical properties

The UV/Vis absorption spectra of (rac)-1 and (rac)-2 were recorded in CH2Cl2 at concentrations around 10 -4 M (Figure 3a). Both complexes exhibit intense and highly structured bands < 330 nm attributable to the extended -helical unit. For example, 1 exhibits well-resolved bands at  = 265 ( = 36 100), 290 (41 700), 298 (4000) and 316 nm (33 700 M -1 cm -1 ). At lower energies, there is a shoulder at 352 nm (2400 M -1 cm -1 ) and a pair of weaker bands at 370 (2500 M -1 cm -1 ) and 388 nm (2200 M -1 cm - 1 ). The spectrum of the acetylenide derivative 2 is essentially identical to that of 1 in this region, whilst it has slightly augmented absorption at higher energies, i.e. around 280 nm, owing to the presence of the phenylacetylene -* transitions which usually occur at  < 340 nm.

Both complexes are luminescent in CH2Cl2 solution at room temperature (Figure 4 and Table S3). In deoxygenated solution, they display two well-separated sets of vibrationally structured bands, with the 0,0 vibrational components at 393 and about 515 nm. In air-equilibrated solution, only the higher-energy set is observed. This behaviour is suggestive of dual emission from singlet and triplet states, with the latter being quenched efficiently by dissolved molecular oxygen. Such an assignment is reinforced by time-resolved measurements that reveal a lifetime of the lowerenergy band of around 1 ms (0.8 ms for 1 and 1.5 ms for 2); lifetimes of this order are typical of triplet states. With such a long triplet lifetime, considerable vibrational quenching would be anticipated, and indeed, at 77 K, the intensity of the phosphorescence band drastically increases, dwarfing the fluorescence.

The phosphorescence lifetimes under these conditions are increased by more than an order of magnitude to 52 and 19 ms for 1 and 2 respectively. The vibronic progression of both the fluorescence and phosphorescence bands at room temperature is around 1350 cm -1 for both complexes, no doubt corresponding to C=C stretching modes of the helicene and indicative of an excited state of primarily ligand-centred character. The main difference between 1 and 2 is the higher overall quantum yield of the latter: the values are 0.50 and 1.6 % respectively in deoxygenated solution. Moreover, the relative intensity of the phosphorescence (P) compared to the fluorescence (F) is higher in the acetylide derivative 2: the ratio of the integrated intensities P/F is approximately 1.9 and 4.5 for 1 and 2 respectively (compare Figure 4 and Figure S32). Coupled with the phosphorescence lifetime of 2 being around double that of 1, it thus appears that the augmented quantum yield of 2 is associated with less severe non-radiative decay of the triplet state. Chloride to acetylide metathesis has previously been found to have such an effect in other gold complexes, often attributed to the higher ligand field strength of the acetylide helping to ensure that potentially deactivating d-d states are kept at bay, to high energy. [31] It is informative to compare the behaviour of 1 and 2 with other Au(I) NHC complexes. Non-helicenic NHC complexes that do not have extended conjugation in their ligands exhibit only phosphorescence, [28] whilst the recently reported helicenic complex [(rac)-15]Au I Cl.PF6 (Scheme 2)featuring a highly extended conjugated systemshows only fluorescence in the red region, reminiscent of the N- [4]helicenium-imidazolium ligand. [14b] The dual emissionphosphorescence and fluorescenceof 1 and 2 places them between these two extremes. Observation of phosphorescence requires spin-orbit coupling to be sufficiently efficient not only to promote the formally forbidden T1→S0 radiative process, but also to facilitate the S1→T1 intersystem

D / M -1 cm -1  / nm EGA329-1 EGA329-2 EGA308-1 EGA308-2 (P)-1 (M)-1 (P)-2 (M)-2 a) b)
10.1002/ejoc.202100722

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European Journal of Organic Chemistry crossing. It has been found in other systems {e.g. with Os(II), Pt(II) and Ir(III)} that, as the conjugated ligand system becomes more extended, the associated  orbitals increase in energy and mix less efficiently with metal orbitals, leading to more purely ligand-centred character to the excited state and thus to less efficient spin-orbit coupling. [32] Fluorescence then becomes more likely to be able to compete with intersystem crossing. Such an effect may be at work in the present case, though it must be acknowledged that spin-orbit coupling pathways are complex and may also be heavily influenced by, for instance, the relative energies of higher-lying singlet states relative to T1. [33]

Chiroptical properties

The chiroptical properties (OR, ECD, and CPL) of 1 and 2 were investigated in CH2Cl2. The molar rotation values at 25°C were similar for both complexes, around 5-6 × 10 3 which is quite typical for [5]helicenic complexes. [11b] The combination of the polarimetry (sign of the optical rotations) with ECD spectroscopy (alternation of bands with opposite signs, characteristic of a helicenic system, Figure 3b) helped to assign the (P)-(+) / (M)-(-) absolute stereochemistry of each enantiomer, by analogy with other complexes. The structured profile observed in UV-vis spectroscopy is evident also in ECD, confirming the strong involvement of the helically chiral ortho-fused unit in the observed polarized and unpolarized excitations of the complexes. For example, (P)-1 exhibits a first positive band at 247 nm (D = +21 M -1 cm -1 ) followed by a negative one at 268 nm (-54). Next, the complex displays a series of positive and structured bands at 298 (+75), 315 (+98) and 328 (+60). Finally, two bands of lowest energies can be found at 369 (+4) and 388 nm (+2). The spectrum of (P)-2 is identical in terms of the alternation of bands with the exception of the band at 266 nm which appears slightly red-shifted (about 10 nm) and more intense (D = -75 M -1 cm -1 ). For comparison, the gold(III) complex [(P)-15]Au III L.(PF6)2 (L = 2,6-diphenylpyridine) recently described [14b] displays ECD bands in acetonitrile in the UV region diagnostic of helicenic derivatives -237 nm (D = +23 M -1 cm -1 ), 287 nm (+52), 320 nm (+21)-in addition to observable bands in the red region of somewhat lower intensity -600 nm (D = + 8 M -1 cm -1 ). The lower intensities of [(P)-15]Au III L.(PF6)2 with respect to (P)-1 can be explained by the smaller length of the helical unit. It proved possible to measure weak CPL signals for the phosphorescence of enantiopure complexes of 2 in deoxygenated solution (glum ~ +1 × 10 -3 and -1 × 10 -3 for (M)-2 and (P)-2, respectively, at em = 550 nm, see Figure 4). The weakness of the detected signal arises in part from the low quantum yields and from inefficient interaction between the helicenic and the metallic part. Furthermore, it can be seen from the higher-energy region of the CPL spectra that the fluorescence displays no CPL activity, at least within the sensitivity of our apparatus.

Conclusion

In conclusion, we have reported helically chiral [5]helicenicimidazolylidene gold(I) complexes bearing various ancillary ligands. Using a straightforward synthetic approach, the targeted complexes were obtained in good yields and were fully characterized, notably using X-ray diffraction. Additionally, en route to the complexes, the electronic properties of the constituent carbene with an ortho-fused -conjugated system as backbone were highlighted. These quantitative analyses revealed that the presence of the helicenic backbone impacts only the accepting properties of the NHC, with no clear effect on the donating properties. Except for the carbazolide derivative, the NHC-gold(I) complexes exhibit high chemical stability on chiral stationary phases and were separated into their enantiomers by semipreparative chiral HPLC. Their chiroptical and photophysical 10.1002/ejoc.202100722

Accepted Manuscript

European Journal of Organic Chemistry properties revealed their strong ECD activity due to the presence of the [5]helicenic unit as well as dual luminescence from singlet and triplet states at room temperature together with long lifetimes for the triplet phosphorescence. Finally, CPL activity could be clearly detected for complexes (P) and (M)-2. Considering the good chemical and configurational stability of these chiral NHCgold(I), this work paves the way to further developments of gold complexes or assemblies. [34]

Figure 1 .

 1 Figure 1. Chemical structures of [5]helicene-NHC-gold(I) complexes 1, 2 and 3 and of [5]helicene-imidazolylidene 6, Ir(NHC)(CO)3Cl complex 9 and selenourea 13 examined herein.

Scheme 1 .

 1 Scheme 1. Preparation of [5]helicene-imidazolium precursors. i) 1,3-Dimethylphenylboronic acid, NEt3, Cu(OAc)2, CH2Cl2, 4 Å MS, under air, r.t., 65 h; ii) MeI, MeCN, reflux, overnight; iii) Dowex Cl-exchange resin, MeOH/Acetone. Yields are indicated under parenthesis.

Scheme 2 .

 2 Scheme 2. Preparation of the [5]helicene-imidazolylidene derivatives i) a) Ag2O, CH2Cl2, r.t., 2h under Ar in the dark then b) [IrCl(COD)]2, CH2Cl2, r.t., overnight under Ar in the dark; ii) CO (balloon), CH2Cl2, r.t., 15 min; iii) Method A: Se powder, NaHMDS (1.0 M in THF), THF, -78°C to r.t., overnight under Ar or Method B: a) Se powder, acetone, 40°C, 15 min under air, b) NEt3, acetone, 60°C, overnight, under air. Yields are indicated in parenthesis.

Figure 2 .

 2 Figure 2. X-ray structures of complexes (rac)-9 (two neighbouring molecules are shown to illustrate their relative disposition), 10 and 12.

  Figure 1

Figure 3 .

 3 Figure 3. a) UV-Vis absorption and b) ECD spectra of enantiopure complexes (P) and (M)-1 (blue lines) and 2 (red lines) recorded in CH2Cl2 at around 10 -4 M.

Figure 4 .

 4 Figure 4. Emission spectra of 1 and 2 (left and centre panels respectively) in deoxygenated and air-equilibrated CH2Cl2 at 295 K (red and green lines respectively, plotted on the same intensity scale, ex = 350 nm) and in EPA at 77 K (blue lines, maximum intensity normalised to that of the deoxygenated 295 K spectrum). EPA = diethyl ether / isopentane / ethanol (2:2:1 v/v). Right panel: CPL spectra of (M) and (P)-2 measured at 298 K in degassed CH2Cl2, exc = 345 nm (15 and 30 scans respectively).

Acknowledgements

We thank the Centre National de la Recherche Scientifique (CNRS) and the University of Rennes. This work was supported by the Agence Nationale de la Recherche (ANR-16-CE07-0019 "Hel-NHC" grant). Dr. Clément Orione (ScanMat-ISCR), Dr. Antoine Vacher (ISCR), Pr. François-Hugues Porée (ISCR) are acknowledged for their assistance respectively with NMR measurements, IR measurements and for reactions with carbon monoxide.

Keywords: carbene, circular dichroism, CPL, gold(I), helicene, organometallics

COMMUNICATION 10

The golden twist: Monodentate [5]helicene-imidazolylidene gold(I) complexes were prepared in good yields and structurally characterized. Benefiting from the presence of the configurationally stable [5]helicenic unit, they exhibit appealing chiroptical features, such as strong circular dichroism, moderate circularly polarized phosphorescence and dual emission with phosphorescence lifetimes up to the millisecond range.

Institute and/or researcher Twitter usernames: @EGA_Chem, @JeanneCrassous, @chimie_ISCR 10.1002/ejoc.202100722

Accepted Manuscript

European Journal of Organic Chemistry