Yu-Chao Yuan
email: yuchao.yuan@xzhmu.edu.cn

Qiu-Li Lu

Xiao-Tong Zhu

Sergio Posada-Pérez

Miquel Solà

Albert Poater
email: albert.poater@udg.edu

Thierry Roisnel

Rafael Gramage-Doria
email: rafael.gramage-doria@univ-rennes1.fr

Precise Control of the Site-Selectivity in Ruthenium-Catalyzed C-H Bond Amidations by Cyclic Amides as Powerful Directing Groups

Selective C-H functionalizations aiming at the formation of new C-N bonds is of paramount importance in the context of step-and atom-economy methodologies in organic synthesis. Although the implementation of noble metal catalysts is prevalent, more benign cobalt pre-catalysts have recently appeared promising. However, they sometimes feature selectivity issues that limits its applicability in late-stage functionalization. Herein, we report on a highly reactive ruthenium-based catalytic system displaying excellent levels of mono-, regio-and site-selectivity exploiting a series of biologically-relevant cyclic amides as weak directing groups. The use of dioxazolone derivatives as the amidating reagent overcomes the issues encountered in the use of unstable azide derivatives for such transformations and it enables to perform these reactions under very mild reaction conditions (air, 40 o C). Moreover, a combination of deuteration experiments and a comparative study with different types of directing groups highlight the relevance of weak amide directing groups for enabling formation of six-membered cycloruthenate intermediates in key elementary steps of the catalytic cycle. In addition, DFT computational calculations were carried out for the first time for studying ruthenium-catalyzed C-N bond-forming processes via C-H activation assisted by weak directing groups, thereby elucidating the origin of the regio-and site-selectivity.

Introduction

The selective formation of C-heteroatom bonds is a fundamental transformation with direct consequences in the streaming synthesis of highly elaborated molecules with important applications ranging from pharmacology to materials sciences. 1 In this context, C-N bond-forming processes are significantly relevant as they can be targeted in a sustainable manner using transition metal complexes as homogeneous catalysts. 2 Since the pioneering discoveries of the Ullmann coupling with copper catalysts 3 to the most recent Buchwald-Hartwig aminations with palladium ones, 4 these and analogous transformations have completely revolutionized the way of conceiving organic synthesis. 5 However, most of these protocols require the use of pre-activated starting materials which impose synthetic limitations together with undesired over-stoichiometric formation of hazardous by-products. [START_REF] Ullmann | Ueber eine neue Bildungsweise von Diphenylaminderivaten[END_REF][4][5] Consequently, new promising approaches based on transition metal-catalyzed C-H bond functionalizations have appeared in the last decades for amination reactions that can directly be performed on low functionalized starting materials. 6 In this scenario, the selectivity is typically controlled by the presence of a directing group (DG) nearby the C-H bond desired to be functionalized. 7 Therefore, this leads typically to orthoselectivity for aromatic C-H bond aminations 7 although few cases of remote, meta-and para-C-H bond aminations have been reported. 8 Scheme 1 General case of a transition metal-catalyzed C-H bond amidation on aromatic scaffolds. DG = directing groups.

From the many known C-N bond-forming reactions, transition metal-catalyzed ortho-C-H amidation ones are particularly interesting given the ubiquitous nature of amide bonds. 9,10 Catalysts based on Ir, Rh, Pd, Ru or Co are of choice due to their high reactivity 11 and amidating agents derived from dioxazolone are preferred instead of azides because of their benchmark stability. 12 Taken this into account, it appears that all reports from the literature dealing with transition metalcatalyzed C-H bond amidations focused exclusively on substrates containing only one aromatic C-H bond available for reaction (Scheme 1). 7,11,12 However, for implementation in latestage functionalization for instance, the methodologies may Please do not adjust margins Please do not adjust margins consider also the compatibility with other aromatic C-H bonds prone to react. Unfortunately, the site-selectivity of aromatic C-H bond amidation reactions have been rarely studied to date with low levels of selectivity so far reported. In a first study in 2017, Chang and co-workers demonstrated for a single example (N-phenylbenzamide) that a C-H bond amidation could be siteselective with a cobalt catalyst and unproductive with a ruthenium catalyst, albeit with a modest yield (Scheme 2A). 11d Later, Whiteoak and co-workers showed, as it could be expected, that a statistical mixture of amidated products is obtained using a cobalt catalyst with the assistance of an acyclic amide directing group in yields not exceeding 30% (Scheme 2B). These examples undoubtedly show the difficulty to reach site-selective C-H bond amidations in the presence of multiple aromatic fragments. Herein, we show that both regio-and siteselectivity is achieved when using cyclic amides as directing groups in the presence of two aromatic sites prone to react by means of a ruthenium catalyst that additionally displays a large functional group tolerance (Scheme 2C). We found that ortho-C-H bond amidations selectively took place in the N-aryl fragment rather than in the C(O)-aryl site. Control experiments, preliminary mechanistic studies and thorough DFT calculations unambiguously support that cyclic amides enable formation of catalytically productive six-membered ruthenacycles as unique intermediates [START_REF] Engle | Weak coordination as a powerful means for developing broadly useful C-H functionalization reactions[END_REF] as it was evoked but never demonstrated in C-O and C-C bond forming reactions. 15 It is worthy to note that cyclic amides, such as the isoindolinones employed in this study, are prevalent motifs encountered in several daily-life chemicals (Scheme 2C, framed), 16 thereby the presented methodology paves the way towards the use of ruthenium catalysts in C-H bond late-stage functionalization strategies. [START_REF] Yamaguchi | Emerging Synthetic Tools for Natural Products and Pharmaceuticals[END_REF] An in-depth comparative study with other type of common directing groups is presented indicating the suitability of weak amide directing groups over the more coordinating ones, such as pyridines, for ruthenium-catalyzed C-H bond amidations. In addition, cyclic amides were found to outperform acyclic ones as directing groups for this relevant transformation.

Results and discussion

Our initial efforts started by using N-arylisoindolinone (1a) as the model substrate for the optimization of the C-H bond amidation reaction with 3-phenyl-1,4,2-dioxazol-5-one (2a) as the amidating agent in the presence of [RuCl2(p-cymene)]2 as the pre-catalyst. After screening a number of parameters (Table 1 and Table S1 in the Supporting Information), we found suitable reaction conditions that afforded exclusively the amide derivative 3a, in which the C-H bond amidation occurred in the hydrogen atom Ha (ortho position with respect to the nitrogen atom). The C-H bond functionalization taking place at the other possible hydrogen atom Hb was not observed (Table 1). The reaction conditions consist of 5 mol% of [RuCl2(p-cymene)]2, AgSbF6 as a chloride scavenger, PivOH as additive in 2,2,2trifluoroethanol (TFE) as the solvent at 40 o C under air atmosphere during 24 hours (Table 1, entry 1). In this way, 3a was obtained in 80% isolated yield. Control experiments indicated the need of both the chloride scavenger AgSbF6 and the ruthenium pre-catalyst (Table 1, entries 2-3). Additives influenced in a different manner the reactivity and selectivity of the C-H bond amidation reaction. Among the different protic additives evaluated, PivOH was found to be the most suitable one (Table 1, entries 4-7). Screening other solvents such as 1,2dichloroethane (DCE), 1,4-dioxane and tetrahydrofuran (THF) was detrimental to the catalysis compared to TFE (Table 1, entries 8-10). Lowering the loading of the amidating agent 2a to 1.2 equivalents, afforded the corresponding amidated product 3a in a modest 60% yield (Table 1, entry 11). Gratifyingly, we found that the reaction was also operating when decreasing the temperature to room temperature, leading to 70% yield of 3a in a reaction conducted at an ambient temperature of 25 o C (Table 1, entry 12). Swapping the ruthenium pre-catalyst for a cobalt complex largely used in C-H bond amidations [START_REF] Chirila | Towards a Sequential One-Pot Preparation of 1,2,3-Benzotriazin-4(3H)-ones Employing a Key Cp*Co(III)catalyzed C-H Amidation Step[END_REF] did not afford any product so far (Table 1,entry 13). The use of tosyl azide (TsN3), which is a well-known nitrene source, 7 as amidating agent led to traces of the corresponding N-tosylated product analogue of 3a (NHTs instead of NHCOPh, Table 1, entry 14). Overall, these findings highlight the suitability of ruthenium Please do not adjust margins Please do not adjust margins catalysts over the more expensive iridium, palladium or rhodium ones, 18 as well as dioxazolone 2a as ideal and safe partner for this type of transformation. After reaction optimization (Table 1, entry 1), the scope of the catalysis was evaluated with different synthetically useful functional groups at different positions in both compound 1 as well as the dioxazolone partner 2 (Table 2). para-Substitution patterns in the N-phenyl ring with electron-donating groups, such as methyl and methoxy, were tolerated for the catalysis yielding the corresponding ortho-amidated products 3b and 3c in 76% and 83% yields, respectively. In addition, X-ray diffraction studies performed on single crystals of 3c (Table 2) established without ambiguity the site-and regio-selectivity observed in this C-H bond amidation reaction. 19 Similarly, introducing electron-withdrawing groups in the para-position such as fluoro and ester groups was also tolerated by the catalysis and afforded the corresponding ortho-amidated products 3d and 3e in 45% and 66% isolated yields, respectively. Notably, although ester groups have been identified as suitable directing groups in several ruthenium-catalyzed C-H bond activations, 20 in the case of 3e, it is the cyclic amide group (and not the ester group) which dictates the exclusive site-selectivity observed in the C-H bond amidation. Other halide groups such as chloro and bromo were compatible leading to products 3f and 3g in 62% and 73% yields, which is relevant for further derivatization sequences by cross-coupling chemistry. 21 Methyland methoxy-substituted isoindolinones in meta position of the N-phenyl ring afforded the ortho-amidated products 3h and 3i in 81% and 90% yield, respectively, in a selective manner, without any functionalization occurring in the other ortho-C-H bond positions. Analogously, the catalysis was found compatible with a significant number of functional groups attached to the dioxazolone core. For instance, aryl-substituted dioxazolone de-rivatives bearing either electron-donating or electron-withdrawing groups at para position such as methyl, tert-butyl, trifluoromethyl and chloro reacted smoothly with 1a affording the corresponding ortho-C-H bond amidated products 3j-3m in an excellent range of 83-92% isolated yields. Electron deficient substituents on the aryl-substituted dioxazolone such as 3-Cl was also tolerated under the developed catalysis leading to the amidated product 3n in a remarkable 91% yield, which makes the halide site available for post-functionalization. 21 Multisubstituted groups on the dioxazolones partner were also employed in this catalysis as shown in the successful synthesis of compound 3o in 66% yield that contains two chloride substitutents at both meta positons. Additionally, the reaction was also compatible with a very bulky polycyclic aromatic hydrocarbon fragment such as 1-naphthalene substituted dioxazolone leading to 3p in 38% yield. Interestingly, heteroaromatic-containing amidating agents can also be employed. In the case of furan-containing dioxazolone, the corresponding product (3q) was obtained in 32% yield. However, the more coordinating thiophene one inhibited the catalysis. Gratifyingly, further exploration of the scope of this C-H bond amidation reaction revealed that the aliphaticsubstituted dioxazolones were also reactive. For instance, methyl-, tert-butyl-and n-heptyl-substituted dioxazolones led to the ortho-C-H bond amidated products 3r, 3s and 3t in 80%, 76% and 80% yields, respectively.

Overall, a panel of more than twenty different functional groups in different positions around both reagents 1 and 2 were tolerated with no products resulting from other site-and/or regio-selectivity. The absence of bis-ortho-amidated products might be rationalized by a plausible intramolecular hydrogen bonding between the NH group from the amide and the carbonyl group from the directing group that forbids the second ortho-C-H bond amidation. [START_REF] Kim | Hydrogen-bond-assisted controlled C-H functionalization via adaptive recognition of a purine directing group[END_REF] We additionally noted that the catalysis was sensitive to the steric hindrance found in the coupling partners as evidenced in the lack of reactivity observed for the ortho-tolyl derivative from 1 and the ortho-chlorocontaining aryl-substituted dioxazolone derived from 2, respectively. [START_REF] Leitch | Ruthenium(II)-Catalyzed C-H Functionalization Using the Oxazolidinone Heterocycle as a Weakly Coordinating Directing Group: Experimental and Computational Insights[END_REF] Further limitations were encountered when using hydroxyl-or cyano-substituted coupling partners. Importantly, the reaction was scalable and the same excellent results of yield and selectivity were obtained when conducting the ruthenium catalyzed C-H bond amidation at gram scale starting with 5 mmol of 1a leading to ca. 1.30 g of product 3a, thus showing the robustness of the methodology (Scheme 3). For comparison purposes, the catalysis was applied to other substrates featuring cyclic amides as potential directing groups in view to address the versatility of this transformation (Scheme 4). Using pyrrolidone, the simplest cyclic amide, as the directing group, afforded the corresponding ortho-C-H bond amidated product 4 in 60% yield (Scheme 4). Note that the synthesis of 4 applying this methodology is more sustainable and efficient compared to previous examples in the literature, 25 that required long and tedious reaction sequences using hazardous rea-gents that have so far limited their exploitation. Interestingly, a remarkable difference of reactivity was encountered exploiting acyclic amide directing groups. Indeed, whilst acetanilide still afforded the corresponding orthoamidated product 5 in 51% yield, no reactivity was observed for the N-methylated version nor for benzanilide as the substrates (Scheme 4). Analogously, a complete absence of reactivity was encountered with N-phenylphthalimide, which features a cyclic imide as a potential directing group, as well as with the more strongly coordinating 2-pyridyl directing group (Scheme 4), which is rather counterintuitive considering previous contributions. 26 Please do not adjust margins Please do not adjust margins Scheme 4 Evaluation of the catalysis with different types of relevant directing groups.

Consequently, the reported ruthenium-catalyzed transformation appears to need a compromise between the geometry, the steric and electronic parameters as well as the coordinating ability of the directing groups. Furthermore, these findings establish that the directing ability of cyclic amides outperforms that of acyclic amides following a clear trend as shown in Scheme 5.

Scheme 5 Order of efficiency for amides as directing groups in rutheniumcatalyzed ortho-C-H bond amidations.

After having demonstrated the enhanced efficiency of cyclic amides as superior directing groups in ruthenium-catalyzed aromatic C-H bond amidations, we performed several experiments to better understand the mechanism operating in the catalysis (Scheme 6). Firstly, we verified the necessity of the carbonyl group by attempting a catalytic reaction using a carbonyl-free substrate such as isoindoline (Scheme 6, top). In this case, no C-H bond functionalization was detected, indirectly demonstrating the importance of the carbonyl group to assist the catalysis likely via coordination to ruthenium. [START_REF] Engle | Weak coordination as a powerful means for developing broadly useful C-H functionalization reactions[END_REF]15 In addition, deuteration experiments were performed under the catalytic conditions but in the absence of the amidating partner 2 with a mixture of solvents TFE:D2O (Scheme 6, bottom). In these conditions, 17% deuteration incorporation was observed in the ortho-C-H bonds of the phenyl ring attached to the nitrogen atom with no deuteration observed elsewhere in the molecule (Scheme 6, bottom).

Scheme 6 Control experiment in the absence of a carbonyl directing group (top) and a deuteration experiment (bottom).

The overall above-described findings strongly suggest that the cyclic amides prefer to accommodate six-membered ruthenacycle species within the catalytic cycle over the commonly found five-membered ones for C-N bond forming reactions via C-H bond activation (Scheme 7). This may account for the observed regio-and site-selectivity observed in the catalysis.

Scheme 7 Mechanistic consideration highlighting stabilization of sixmembered ruthenacycles over five-membered ones in the key C-H bond activation event.

Due to the difficulty on obtaining reproducible data for kinetic studies ascribed to solubility issues and the unsuccessful attempts to detect reaction intermediates, we turned our attention to further unveil the precise reaction mechanism by DFT computational calculations at the M06-D3/Def2TZVP∼sdd(smd)//BP86-D3/Def2SVP level of theory (Fig. 1). 24 In order to keep the whole reaction pathway in the same cationic level, the preactivation started from the cationic intermediate I0, derived from the [RuCl2(p-cymene)]2, after releasing the chlorides by means of the reagent AgSbF6 (a known halide scavenger). As such, SbF6 -is considered the weak coordinating anion throughout the whole catalytic cycle. In the next intermediate I0' the anionic SbF6 -ligand is substituted by the PivO -anion that comes from the other reagent, PivOH, with HSbF6 release. This step is thermodynamically unfavoured by 13.1 kcal/mol, although the formation of complex I0' is essential to stabilize the intermediate I0" by 10.7 kcal/mol after the combination of I0' with substrate 1a. From I0" there are two potential aryl C-H activations to explore: from the phenyl on the nitrogen, or the aryl ring annulated to the five-membered ring. [START_REF] Fernández-Salas | For examples of activation of aryl C-H bonds in Ru-based catalysts[END_REF] Our simulations show energy barriers of 18.3 and 20.3 kcal/mol, respectively. Besides to the 2 kcal/mol of difference, the C-H bond activation of the aryl ring leads to a worsening of 9.7 kcal/mol for the resulting intermediate I' as compared to I, as is illustrated in the catalytic cycle in Fig. 1 (red pathway). The resulting ruthenacycle intermediate with assistance of the cyclic amide as the directing group has literature precedents. Please do not adjust margins Please do not adjust margins

In the catalytically productive pathway (Fig. 1, blue pathway), species I coordinates to dioxazolone forming species II followed by extrusion of CO2, overcoming an energy barrier of 13.4 kcal/mol, giving rise to the formation of Ru IV -imido species III. The migratory insertion of imido species III, with the C-N bond formation associated, leads to the generation of ruthenacycle IV, with a low energy barrier of only 8.4 kcal/mol.

To close the catalytic cycle, the protodemetalation of species IV occurs with a new molecule of substrate 1, which is in excess in the reaction mixture compared to the catalyst. This step could also be performed with HSbF6 or PivOH furnishing the amidated product, as well. However, instead of the regeneration of the active ruthenium catalyst I, the process would end up in intermediates I0 or I0', respectively. The most difficult step in the pre-activation sequence is the C-H bond activation that leads to the ruthenacycle, either I or I' selectively (Fig. 1). On the other hand, the rate determining step (rds) of the catalytic pathway corresponds to the transition state II-to-III. Considering the catalytic cycle for an elusive C-H bond activation at the aryl ring annulated to the five-membered ring of the substrate 1 (red pathway, Fig. 1), the corresponding transition state II-to-III is 6.1 kcal/mol higher in energy compared to the C-H bond activation at the phenyl ring linked to the nitrogen atom in 1 (blue pathway, Fig. 1). Thus, it confirms the selectivity towards the reaction profile with I as the catalytic active species. The reason is not steric, but it is related with the fact that I forms a rather stable 6-membered ruthenacycle whereas I' has a 5-membered ruthenacycle (Fig. 2). In addition, in the transition state the pivalate ligand forms a less strained interaction to remove the proton, and actually the distances in the transition state leading to I are closer to an agostic interaction, with a more activated aryl C-H bond (by nearly 0.1 Å). For the sake of consistency and validate mainly the rds, we explored all the possibilities considering the intermediates in the preactivation and the two substrates. Interestingly, from intermediate I0' the coordination of the dioxazolone before 1 is omitted since the kinetic cost of the CO2 release increases by 5.3 kcal/mol (Fig. 3). Such findings strongly contrast with the initial elementary steps of the reaction mechanism associated to the ruthenium-catalyzed C-H bond amidation with azides, in which the ruthenium-nitrene species are postulated to form before the C-H bond activation step in the substrate. [START_REF] Choi | C7-Indole Amidations and Alkenylations by Ruthenium (II) Catalysis[END_REF] Please do not adjust margins Please do not adjust margins To further check the importance of the different anions we performed additional analyses with the initial [RuCl2(pcymene)]2. The stability of this dimer is not relevant as its cleavage only requires 1.1 kcal/mol. The substitution of one or both chlorides by SbF6 -is affordable, consuming only 3.1 and 2.8 kcal/mol, respectively. Moreover, for the reaction profile described in Fig. 1, the counter anion SbF6 -, that would neutralize the system, was omitted assuming that it does not affect significantly. Preliminary calculations show that the counter anion destabilizes intermediates I and II by 4.1 and 0.5 kcal/mol, respectively.

Conclusions

In summary, we have developed efficient site-selective C-H bond amidation reactions to form unprecedented C-N bonds exploiting the directing group ability of cyclic amides via sixmembered ruthenacycles formation. A simple ruthenium(II) pre-catalyst and safe amidating agent have been employed leading to a versatile catalytic system compatible with a large number of synthetically useful functional groups (more than 20 examples). This "close to room temperature" methodology offers a convenient route to access ortho-amidated cyclic amides, that might be potentially relevant in medicinal chemistry. 16 This study also shows the subtlety associated to C-H bond amidations because in the present case ruthenium outperforms cobalt as catalyst and dioxazolone outperforms tosylazide as amidating agents, respectively, which is somehow unexpected as regards of precedents in the literature. 6-13,28 Moreover, this contribution establishes that cyclic amides are more powerful directing groups than acylic ones or other coordinating groups such as imides and pyridines, at least for ruthenium-catalyzed C-H bond amidations. In addition, we provide the first mechanistic considerations of ruthenium-catalyzed C-H bond aminations supported by in-depth DFT calculations. Although the rate-determining step is the extrusion of CO2 in order to form ruthenium species coordinated to both the substrate and the nitrene group, the site-selective step is determined by the strong destabilization associated to the formation of a fivemembered ruthenacycle over a six-membered one. In conclusion, further research directed to new carbonheteroatom bond forming pro-cess via ruthenium-catalyzed C-H bond functionalization strategies should provide appealing methodologies for direct implementation into organic synthesis.

Experimental

General procedure for the ruthenium-catalyzed C-H bond amidation. A suspension of substrate 1 (0.3 mmol, 1.0 equiv.), dioxazolone 2 (0.45 mmol, 1.5 equiv.), [RuCl2(p-cymene)]2 (5 mol%), AgSbF6 (20 mol%), and PivOH (0.06 mmol, 0.2 equiv.) in anhydrous TFE (1.5 mL) was stirred at 40 o C for 20 hours under air. At ambient temperature, then the solvent was evaporated in vacuo, and the resulting crude reaction mixture was purified by flash column chromatography to afford the corresponding product 3-5 as analytically pure solids.

Computational Details. All the DFT static calculations were performed with the Gausian16 set of programs, 29 using the BP86 functional of Becke and Perdew, 30-32 together with the Grimme D3 correction term to the electronic energy. [START_REF] Grimme | A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[END_REF] The electronic configuration of the molecular systems was described with the double- basis set with polarization of Ahlrichs for main-group atoms (Def2SVP keyword in Gaussian), [START_REF] Schäfer | Fully optimized contracted Gaussian basis sets for atoms Li to Kr[END_REF] whereas for ruthenium the small-core quasi-relativistic Stuttgart/Dresden effective core potential, with an associated valence basis set (standard SDD keywords in Gaussian09) were employed. [START_REF] Haeusermann | Accuracy of energy-adjusted quasirelativistic ab initio pseudopotentials: All-electron and pseudopotential benchmark calculations for Hg, HgH and their cations[END_REF][START_REF] Küchle | Energy-adjusted pseudopotentials for the actinides. Parameter sets and test calculations for thorium and thorium monoxide[END_REF][START_REF] Leininger | The accuracy of the pseudopotential approximation. II. A comparison of various core sizes for indium pseudopotentials in calculations for spectroscopic constants of InH, InF, and InCl[END_REF] The geometry optimizations were performed without symmetry constraints, with analytical frequency calculations for the characterization of the located stationary points. These frequencies were used to calculate unscaled zero-point energies (ZPEs) as well as thermal corrections and entropy effects at 298.15 K. Energies were obtained by single-point calculations on the optimized geometries with the M06 functional, 38 with the Grimme D3 correction term, 33 and the Def2TZVP basis set. [START_REF] Kendall | Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions[END_REF] The reported Gibbs energies in this work contain electronic energies obtained at the M06-D3/Def2TZVP~sdd level of theory corrected with zero-point energies, thermal corrections and entropy effects evaluated at 298.15 K, achieved at the BP86-D3/Def2SVP~sdd level plus a solvation contribute evaluated by means of the SMD continuum solvation model based on the quantum mechanical charge density of the solute interacting with a continuum description of the solvent (2,2,2-trifluoroethanol, TFE). [START_REF] Marenich | Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions[END_REF]

13 Scheme 2

 132 Scheme 2 Transition metal-catalyzed C-H bond amidations on aromatic scaffolds using amides as directing groups: state-of-the-art (A and B) versus present work (C).

  TsN3 instead of 2a traces a Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), [RuCl2(p-cymene)]2 (5 mol%), AgSbF6 (20 mol%), PivOH (0.02 mmol, 0.2 equiv.), TFE (0.5 mL), 40 o C, 20 h, air. b Determined by 1 H NMR spectroscopy against dibromomethane as an internal standard. c Isolated yield shown in parentheses after purification by column chromatography. d Reaction performed without PivOH. e The product for this reaction is expected to bear a NHTs in place of a NHCOPh fragment in 3a.

a

  Evaluation of the scope for the ruthenium-catalyzed ortho-C-H bond amidation of both 1 and dioxazolone 2. a,b Reaction conditions: 1 (0.3 mmol), 2 (0.45 mmol), [RuCl2(p-cymene)]2 (5 mol%), AgSbF6 (20 mol%), PivOH (0.06 mmol, 0.2 equiv.), TFE (1.5 mL), 40 o C, 20 h, air. b Isolated yield by column chromatography.

Scheme 3

 3 Scheme 3 Scale-up reaction for the ruthenium-catalyzed ortho-C-H bond amidation.

Fig. 1

 1 Fig. 1 Computed Gibbs energy reaction profile in kcal/mol (in brackets) of the ruthenium-catalyzed ortho-C-H bond amidation with cyclic amides as weak directing groups. L = p-cymene. Black pathway = catalyst pre-activation, red color = disfavoured pathway, blue color = favoured pathway.

Fig. 2

 2 Fig. 2 Transition states I0'' → I (left) and I0'' → I' (right) with selected distances given in Å.

Fig. 3

 3 Fig.3Reaction profile of the extrusion of CO2 with the coordination of dioxazolone in I0'. Relative Gibbs energies relative to I (in kcal/mol). L = pcymene.

Table 1

 1 Optimization of the site-selective ruthenium-catalyzed ortho-C-H bond amidation of 1a with dioxazolone 2a. a

	Entry	Deviation from above conditions	3a (%) b
	1	none	82 (80) c
	2	no [RuCl2(p-cymene)]2	0
	3	no AgSbF6	0
	4	no PivOH	73
	5	AcOH instead of PivOH	76
	6	AdCO2H instead of PivOH	79
	7	H2O instead of PivOH	54
	8	DCE instead of TFE	65
	9	1,4-dioxane instead of TFE	0
	10 d	THF instead of TFE	19
	11	1.2 equiv. of 2a	60
	12	25 o C instead of 40 o C	70

7,11-13 ARTICLE Journal Name

6 | J. Name.,

2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Acknowledgements

Y.-C.Y. acknowledges Science and Technology of XuZhou (KC19042) for financial support. We also thank the Public Experimental Research Center, Xuzhou Medical University, for NMR determination. R.G-D. acknowledges the ANR-JCJC scheme (ANR-19-CE07-0039), CNRS and University of Rennes 1 for financial support. A.P. is a Serra Húnter Professor and thanks ICREA Academia 2019 and the Spanish MINECO for project ref.

PGC2018-097722-B-I00. S.P.P. thanks Marie Curie fellowship (H2020-MSCA-IF-2020-101020330). A.P. is a Serra Húnter Fellow and ICREA Academia Prize 2019. A.P. and M.S. thank the Spanish Ministerio de Ciencia e Innovación for projects PID2021-127423NB-I00 and PID2020-13711GB-I00, and the Generalitat de Catalunya for project 2017SGR39.

Conflicts of interest

There are no conflicts to declare.

Author contributions


Daniele Romanini

A A Kachanov

N Sadeghi

F Stoeckel

CW Cavity Ring Down Spectroscopy

CW cavity ring down spectroscopy

Introduction

Cavity Ring Down Spectroscopy (CRDS) is a versatile high sensitivity absorption technique, exploited in an increasing number of investigations these last few years. Some of the published CRDS spectra correspond to absorption as small as 10 -7 for one pass through the sample (a gas or a supersonic jet). With respect to the sensitivity, CRDS has a performance comparable to Photo-Acoustic Spectroscopy (PAS) [1] and Intra Cavity Laser Absorption Spectroscopy (ICLAS) [2]. One of the attractive features of CRDS is its conceptual and practical simplicity. This may well be considered as a multipass technique affording 10 to 1000 times longer pathlengths than traditional multiple reflection cells, therefore reliable absorption coefficients can be obtained directly [3][4][5]. In addition, even if the standard CRDS setup uses a pulsed laser source, the signal is not affected by the pulse to pulse intensity fluctuations, which are a serious source of noise when using a normal multipass cell. This allows CRDS to access the broad tunable range of pulsed lasers, which easily extends into the UV by the use of doubling crystals. Thus it is clear that this is a powerful and very versatile method. In fact, since the demonstrative detection of forbidden transitions of O 2 in the visible [6], CRDS was used to study metal clusters produced by laser ablation in supersonic jets [7], for the spectroscopy of weak transitions in a static gas cell [3,[8][9][10], to detect radical species (in the UV) in flames [11] or diamond deposition reactors [12], ions in a discharge [13], in association with Fourier transform spectral multiplexing [14] and, by exploiting its millisecond time resolution, to investigate reaction kinetics [15].

A detailed technical description of this method was given by Romanini and Lehmann [3], who discussed also the problem of the fundamental shot noise limitations of the sensitivity. Here, it is sufficient to outline what we will refer to as 'standard CRDS', where a pulsed laser is used to inject monochromatic radiation into a stable optical resonator of length L (see e.g. Yariv [START_REF] Yariv | Quantum Electronics[END_REF] or Siegman [START_REF] Siegman | Lasers[END_REF] for the theory of optical cavities and their mode structure). This "ringdown cavity" may be vacuum tight to confine the sample of interest (a gas, a supersonic jet, etc). Injection and excitation of the ringdown cavity is effected by direct transmission of the laser pulse through one of the mirrors, along the cavity axis. Transmittivity of good dielectric mirrors [START_REF] Rempe | [END_REF] in the visible may be as small as T = 10 -5 or even 10 -6 . However, the small injected fraction of the laser pulse still may consist of more than 10 10 photons, which remain trapped as the pulse propagates back and forth between the mirrors. At each pass, part of the trapped pulse is retransmitted outside the cavity and produces an easily detectable signal. The relevant feature of this ringdown signal is its first order decay envelope exp(-t/τ ), whose time constant τ may be as large as 100 µs. Considering the surface and diffraction losses L of the mirrors (usually, L < T ) and the frequency dependent absorption coefficient of the sample α s , the decay rate of the trapped photons is 1/τ = (T +L+α s L)/(L/c), where c is the speed of light. This decay rate is the sum of the slowly frequency dependent term cα c = c(T + L)/L (in the visible, mirrors have more than 50 nm working range), plus the sample absorption spectrum cα s (ν). The first term may be considered as a broad-band smooth baseline which can be easily distinguished from the rapidly varying spectral features of the sample. If needed (e.g. in case of relatively broad-band spectral features), the baseline may also be independently measured after removing the sample. Finally, the sample absorption α s (over the baseline α c ) is obtained directly in units of 1/cm simply from α tot = α c + α s = 1/cτ , where L eq = cτ is an equivalent absorption pathlength. For small signals, we can write δτ /τ = δα tot /α tot , so the noise equivalent absorption (detection limit) is immediately deduced from the noise on the measurement of τ , as δα noise = δτ noise /τ L eq .

Spectral resolution in CRDS

For the following, it is important to notice that impulsive ringdown cavity excitation is efficient only if the laser pulse spectrum overlaps one or more of the transmission modes of the cavity [19,20]. This is usually granted if one uses a commercial pulsed dye laser having a linewidth of about 0.1 cm -1 =3 GHz, which is larger than the free spectral range (FSR) of a ringdown cavity few tens centimeters long. Achieving spectral resolution higher than the cavity FSR would open for CRDS new applications such as sub-Doppler supersonic slit-jet absorption spectroscopy. As it has been discussed [11], one way to increase the resolution could be to exploit the high spectral density of high order transverse modes of the cavity. However, it has been pointed out that side effects such as transverse mode beating, multiexponentiality of the ringdown or fluctuations of the decay rate would pose substantial experimental complications [5,19,20]. In order to avoid such problems, a Fourier-transform limited pulsed laser could be used to selectively excite one cavity mode [19], after accurate mode matching and frequency locking. The laser should be narrower than the cavity FSR, and its frequency should be tracked as the cavity length is tuned to maintain efficient single-mode cavity excitation. This would allow recording spectra with resolution limited by the mode width [20], which is as narrow as 60 kHz for a ringdown having τ = 10µs.

Another possibility for very high resolution CRDS is to use a single frequency CW laser tuned to one of the cavity resonances. In this case a substantial buildup of the intracavity field occurs [21], and a strong ringdown signal may then be observed after the laser beam is quickly interrupted (faster than τ ). In fact, the intensity of the beam transmitted through the cavity after buildup may be close to the incoming intensity, which permits to detect the ringdown signal with high signal to noise ratio. With respect to the resolution, this is in principle not determined by the laser linewidth but by the width of the excited cavity resonance, as in the single-mode pulsed case mentioned above. Increased performance should also be granted by the high repetition rate of the ringdown events one could generate, since this is limited by the ringdown time itself, and not by the repetition rate of a pulsed laser. This type of CRDS (CW-CRDS) looks especially attractive if tunable diode lasers are considered as a source, in particular externalcavity single-frequency models which are continuously tunable over more than 50 nm in the near IR and 10 nm in the visible. Compact and relatively inexpensive trace gas detector systems could be built based on diode-laser CW-CRDS [START_REF] Lehmann | Ring-down cavity spectroscopy cell using continuous wave excitation for trace species detection[END_REF].

Preliminary results, where ringdown signals were obtained using optical feedback locking of a diode laser to a ringdown cavity, where reported [START_REF] Romanini | Ohio State University International Symposium on Molecular Spectroscopy[END_REF]. However, we are not aware of anybody being able to obtain an absorption spectrum by CW-CRDS. There are several reasons for that. The sub-MHz short term frequency jit-ter of most single frequency lasers is much larger than the linewidth of the cavity formed by two supermirrors. This makes frequency locking together the laser and the cavity extremely difficult. Even after succeeding in this task, for example by the especially performant technique of Drever et al. [START_REF] Drever | [END_REF], one needs to switch off the laser beam each time the ringdown signal should be sampled. This would strongly disturb the locking electronics, introducing possibly long recovery times and other problems. Other methods of sensitive absorption detection by CW locking to optical cavities have been shown to work, which altogether avoid the problem of interrupting the laser beam and the locking loop [25,[START_REF] Hall | Ohio State University International Symposium on Molecular Spectroscopy[END_REF]. However, we devised a simpler approach, where locking is replaced by a less demanding and robust tracking scheme, and one can use directly the ringdown signal to measure the intracavity sample absorption.

It is curious to note that back in 1984, one of the first ringdown applications was based on a CW source, without locking. Anderson, Frisch and Masser [START_REF] Anderson | [END_REF] exploited the occasional coincidences of a He-Ne laser with the cavity modes. When these coincidences would produce by buildup sufficient intensity transmitted by the cavity, a threshold circuit was interrupting the laser and allowing to observe the free ringdown signal. These authors could not tune their laser to obtain absorption spectra, nor they tried to control the occurrence of buildup-ringdown events. In fact, this pioneering work is usually not considered as a first demonstration of CRDS since the authors neither mentioned the potential of the technique for spectroscopy nor were interested in other than characterizing the reflectivity of mirror coatings. In contrast to that, our intent was to use CW-CRDS for spectroscopic applications. Therefore, in order to obtain a high repetition rate in sampling the ringdown signals while tuning the laser we had to introduce a regularity into the laser to cavity mode coincidences.

Experimental

Our experimental setup (figure 1) was very similar to that of Anderson et al. [START_REF] Anderson | [END_REF], except that we used an acoustooptic modulator (AOM) to interrupt the laser beam and our cavity was linear with two supermirrors of reflectivity 99.990% around 570 nm, where we tested the technique using weak acetylene overtone transitions. The mirrors had a radius of curvature of 100 cm, were placed at a distance of 50 cm, and one of them was mounted on a piezoelectric transducer. We used a Coherent CR-699 single frequency ring dye laser, stabilized within about 1 MHz to an external etalon and capable of continuous 30 GHz scans. We also used a threshold circuit to interrupt the beam when sufficient buildup would be detected. Interruption of the laser beam is essential to allow observing a clean ringdown decay [28].

The ringdown signals were registered by a silicon photodiode and digitized by a 1 MHz 12 bits PC interface card (model CIO-DAS 16/M1 by ComputerBoards Inc.). The AOM was used in deflection to have maximum attenuation contrast, and served also the purpose of isolating the laser from the strong optical feedback coming from the input mirror of the cavity. The beam reflected from this mirror back to the laser is shifted in frequency by twice the acoustic modulation of the AOM (80 MHz), therefore falling in between two of the laser cavity modes. Such an isolation was sufficient, as we did not observe any perturbation of the dye laser by the optical feedback.

We mode matched the laser to the TEM 00 modes of the cavity, and piezoelectrically modulated the cavity length to make the frequency of one of these modes to oscillate around the laser line. The modulation amplitude was chosen to be less than the mode spacing, so that ringdown events could be observed twice per modulation period, each time the mode passed through the laser line. This provided a good and steady repetition rate for signal averaging, as long as the ringdown events were centered with respect to the modulation range. When the laser frequency was fixed, these regular ringdowns could be observed during the time of a few seconds, until the thermally unstabilized cavity would drift out of the laser frequency. We could compensate this drift by a feedback circuit, which measures the distance of the ringdown events from the center of the modulation range and by integration provides a correction bias voltage to the piezoelectric transducer. This circuit makes the cavity to follow reliably the laser for scanning speeds up to 10 GHz per minute. Given the limited piezoelectric excursion, the circuit also resets the bias voltage by applying a jump which corresponds to about one full cavity FSR, so that the tracking continues without appreciable interruptions. This system allowed to observe stable and regular ringdown signals during the whole 30 GHz excursion of one laser scan, but it could work for arbitrar- The absorption lines belong to a weak overtone transition of HCCH [1], which was placed in the ringdown cavity at the pressure of 5 Torr. Each 30 GHz scan took 6 minutes. Two successive scans of the same spectral region show the good reproducibility. Their relative shift is for easier visual comparison.

ily large scans. We stress the fact that these ringdown decays are due exclusively to excitation of the TEM 00 longitudinal modes. With the data acquisition card installed into a 486DX33 PC computer, we could record and analyze separately each of the ringdown signals, at a rate of about 200 Hz. After baseline subtraction and conversion to logarithm, a linear fit would give τ for each decay. Shot to shot reproducibility of τ was about 0.5%, but some events showed larger deviations. It turned out that events having larger deviations had also a larger standard deviation of the fit. In this cases it was possible to observe some oscillations in the decay, which are likely produced by high order transverse modes lying within a laser bandwidth from the excited TEM 00 mode [19]. Even if mode matching was effective in reducing the excitation of higher order modes to 1% of the fundamental mode, this residual excitation is sufficient to explain the observed ringdown profile deviations from exponentiality. Fast fluctuations in the laser frequency may be responsible for variations in the degree of simultaneous excitation of these modes from event to event. To significantly reduce this effect, we used the standard deviation of the fit to discriminate against the bad events during the data acquisition.

Another (occasional) problem we should mention, was the presence of baseline oscillations, having period close to the FSR of the cavity. At this time, we are not sure of the origin of these oscillations. However, they seem to depend on the alignment of laser beam with respect to the center of the cavity mirrors. By aligning the beam so that it would pass closer to the edge of the mirror coatings, we observed that this effect disappeared.

To demonstrate the performance of this CW-CRDS system, we recorded part of the overtone transition of HCCH near 570 nm, which was previously studied by PAS [1]. The pressure in our sample was 5 Torr, for which the collisional broadening of acetylene is negligible compared to the Doppler width. In figure 2 we show the ringdown time recorded as described above for 30 GHz scans. Each data point was averaged over 50 ringdown events, after elimination of bad events, and the tuning rate was 20 GHz/minute. The excellent repeatability of CRDS measurements is demonstrated by superposing two scans taken in the same conditions (shifted in frequency for clarity). In order to record a larger portion of the HCCH spectrum we took five consecutive (partially overlapping) scans together with iodine fluorescence spectra for absolute calibration. The voltage ramp used to tune the laser was also recorded during the scan. Since this voltage controls linearly the length of the temperature stabilized external cavity to which the laser is locked, it is a reliable reference for linearization of the frequency scale. The joined spectrum, after conversion to the total absorption coefficient α tot = 1/cτ , is shown in figure 3. We would like to point out that no baseline adjustments for different pieces of this spectrum were done. The repeatability and baseline stability of this technique is a very valuable feature for quantitative absorption measurements, both in the determination of transition intensities and in the realization of accurate trace element detection systems.

To illustrate the linearity of the recorded signal amplitude, in figure 4 we show a Gaussian fit of one of the isolated lines in the recorded HCCH spectrum. One can well appreciate that the lineshape did not suffer any distortion. The FWHM is found to be 1.275 GHz, which is less than 1% away from the calculated HCCH Doppler width at 25 o C. We also did a fit of the water line (due to sample impurity) at 17551.46 cm -1 , and obtained a Doppler FWHM of 1.54 GHz. The ratio of this width to that of the HCCH line is only 1% larger than the expected value, which depends only on the molecular masses and not on the temperature.

The noise level in this HCCH spectrum, measured as the standard deviation of a baseline section from a straight line (rms noise level), corresponds to an absorption coefficient of 10 -9 /cm. This is almost the same as the detection limit previously obtained by the first author using a pulsed CRDS system with 2 or 3 times longer ringdown time [8]. While the ringdown signal level is here much larger than in that previous work, the number of averages per point are about the same. Since the pulsed CRDS data were recorded using a boxcar, we are tempted to attribute the failure to obtain much better performance to extra noise in our digitization system. In fact, we tested our data acquisition system by feeding to it synthetic ringdown signals generated by the discharge of a capacitor. The pulse to pulse fluctuations we observed were 0.3%, practically the same as in the real

Fig. 3. Section of the HCCH overtone transition [1] near 570 nm at 5 Torr and room temperature. As pressure broadening was negligible, lines are Doppler broadened.

The line marked by an asterisk is due to water impurity in the sample. No baseline correction was applied to the five 30 GHz laser scans which were joined to give this spectrum.

ringdown signal measurements. While measuring this spectrum, we averaged only 50 ringdown events per data point, at the repetition rate of 200 Hz. We tested that averaging up to 1000 events gives a signal to noise ratio improvement that goes like the square root of the number of samples. Since the maximum repetition rate can be of the order of a few kHz, limited by the ringdown time, we can expect a factor of 3 improvement in the signal to noise ratio by sampling 500 events per point at the same scanning rate. Further improvements are to be expected both by optimizing the electronics and by using better mirrors. It is difficult to estimate the performance of different digitizing or alternative analog data processing schemes. However, better mirror coatings are certainly available in the range from 1.5 µm to about 400 nm. Decay times of 100 µs would give at once another factor 5 improvement of the detection limit. Here, we are scaling the sensitivity linearly with the ringdown time since we are limited by the digitizing electronics and not by detector or shot noise.

Discussion

While no ICLAS spectra were available for direct comparison, we could compare our data to the same spectral portion recorded by PAS which was kindly provided to us by Halonen and co-workers [1]. Even if their signal to noise ratio appears to be 2 times better, one should take into account that their pressure was 50 mbar (37 Torr) and that in conditions of negligible collisional broadening line intensities are proportional to pressure. We should also take into account that Halonen et al. used slightly longer averaging per data point. While it is diffi- cult to generalize from this single comparison, we point out that Halonen's intracavity resonant acoustic cell is one of the most performant PAS schemes. Therefore, we believe that CW-CRDS is as performant or better than PAS in terms of sensitivity.

With respect to spectral resolution, we stated above that this is in principle limited by the width of the excited cavity mode. It is clear however that in our setup the optical frequency averaged over several ringdown events was determined by the laser, since all we know is that these events occur somewhere inside the laser bandwidth. An effect that should also be considered is that the frequency of the trapped light adiabatically follows the mode frequency, which in our measurements shifts in time. Given the slow cavity modulation we used, we estimated that during the time of the ringdown, the total mode frequency shift was still much smaller than the laser linewidth. In any case, one should recognize that MHz resolution is indeed more than sufficient for most applications, in particular slit jet spectroscopy where the residual Doppler width for transitions in the visible is typically 100 MHz. Indeed, we believe that our setup is well adapted for jet spectroscopy as it is relatively insensitive to vibrations induced by the vacuum system. Also, we would like to point out that it is quite simple to exploit the passive cavity stability to reach the limit of the cavity mode width. It is sufficient to passively stabilize the cavity from fast temperature drifts and vibrations and then dither the laser frequency instead of the cavity modes. The same electronics would then allow to track the laser as the cavity is smoothly tuned. There are some applications where sub-MHz resolution would be needed. Among those few, we think that one could use the high intracavity power to drive nonlinear phenomena, such as Doppler-free saturated absorption or two-photons electronic transitions. UV rovibronic states could be studied with visible tunable CW sources with a resolution sufficient to study even large molecules. Twophoton transition measurements were already obtained in the past using fluorescence detection [29]. However, important information would be deduced from the direct CW-CRDS absorption measurement of two-photon transition strengths.

The high intracavity power depends on the efficiency of the buildup. In the limit when the relative modulation of cavity and laser is slow, buildup is limited by fast frequency fluctuations of the laser. This is equivalent to consider the average laser linewidth compared with the mode width. In our case, with a laser power before the ringdown cavity of 100 mW, we could observe buildups such that the transmitted cavity intensity was on the order of 0.1 mW, which corresponded to about 1 W intracavity. Using the transverse section of the TEM 00 mode, this gives an intensity of about 300 W/cm 2 . Use of other single frequency laser sources and mirror coatings could provide substantially different buildup performance, so that much larger intensities could also be achieved.

Conclusions

We have demonstrated a new version of cavity ring down spectroscopy employing a tunable CW single frequency laser source. A tracking system has been developed which locks automatically and then enables the ringdown cavity to follow the laser frequency for arbitrarily large scans. The experimental setup is rather simple and any commercial single frequency laser source can be used without modification, with its linewidth determining the spectral resolution. In particular, the combination of CW-CRDS with diode lasers is very promising for application to trace detection. By measuring Dopplerlimited weak absorption spectra in acetylene, we demonstrated a detection limit (rms noise) of 10 -9 /cm, or 5×10 -8 /pass, which is as good as in the standard pulsed CRDS. As we are limited at the moment by noise in the digitizing electronics, the detection limit improves linearly with the mirror quality. We are confident that faster data acquisition rate, optimized electronics, and better mirror coatings will allow at least a factor 10 further performance improvement. Our CW-CRDS setup is quite insensitive to external perturbations and vibrations, which makes it promising for supersonic jet spectroscopy. Finally, together with the high power fluxes (> 100 Watts/cm 2 ) to which the sample may be subjected, CW-CRDS should be considered for the investigation of nonlinear phenomena such as Doppler-free saturated absorption or Doppler-free two-photon absorption. acknowledgments D. R. would like to acknowledge Prof. K. K. Lehmann for extensive discussions on the CRDS subject, and a grant of the European Community for the Formation and Mobility of Researchers, (contract no. ERBFM-BICT950331). Also we are grateful to Prof. L. Halonen for sending us a figure of a portion of the HCCH overtone spectrum.

Fig. 1 .

 1 Photo diode

Fig. 2 .

 2 Fig. 2. Ringdown time as a function of laser frequency.The absorption lines belong to a weak overtone transition of HCCH[1], which was placed in the ringdown cavity at the pressure of 5 Torr. Each 30 GHz scan took 6 minutes. Two successive scans of the same spectral region show the good reproducibility. Their relative shift is for easier visual comparison.

Fig. 4 .

 4 Fig. 4. Gaussian fit of one of the isolated lines in the recorded CW-CRDS HCCH spectrum. The FWHM is found to be 1.275 GHz, in perfect agreement with the HCCH Doppler width at 25 o C.


N D Lanco

M Lanzillotti-Kimura

P Esmann
email: m.esmann@uni-oldenburg.de

Senellart

Three-Dimensional Electrical Control of the Excitonic

Fine Structure for a Quantum Dot in a Cavity

Three-dimensional electrical control of the excitonic fine structure for a quantum dot in a cavity H. Ollivier, 1 P. Priya, 1 A. Harouri, 1 I. Sagnes, 1 A. Lemaître, 1 O. Krebs, 1 L. Lanco, 1, 2 N. D. Lanzillotti-Kimura, 1 M. Esmann, 1, 3, * and P. Senellart The excitonic fine structure plays a key role for the quantum light generated by semiconductor quantum dots, both for entangled photon pairs and single photons. Controlling the excitonic fine structure has been demonstrated using electric, magnetic, or strain fields, but not for quantum dots in optical cavities, a key requirement to obtain high source efficiency and near-unity photon indistinguishability. Here, we demonstrate the control of the fine structure splitting for quantum dots embedded in micropillar cavities. We propose a scheme based on remote electrical contacts connected to the pillar cavity through narrow ridges. Numerical simulations show that such a geometry allows for a three-dimensional control of the electrical field. We experimentally demonstrate tuning and reproducible canceling of the fine structure, a crucial step for the reproducibility of quantum light source technology.

Semiconductor quantum dots (QDs) have emerged as excellent emitters of single photons [1] and entangled photon pairs [2,3] with exciting prospects for both optical quantum networks and processors. The neutral exciton state in an epitaxially grown QD exhibits a fine structure splitting (FSS) arising from a reduced symmetry of the nanostructure as well as valence band mixing (see Fig. 1(b)) [4,5]. Cancelling this FSS has long been identified as a key requirement for the on-demand generation of polarization-entangled photon pairs via the radiative biexciton-exciton cascade [6,7]. Similarly, near-zero FSS was shown to enable high fidelity initialization of long-lived quantum dot hole spin qubits [8]. Conversely, it was very recently shown that the brightness of an exciton based single-photon source under resonant pumping could be optimized for an optimal finite FSS value [9]. All these perspectives have led to the development of a large variety of techniques to control the FSS. Material growth and processing are tailored to reduce the average FSS, for instance growing small QDs [10], performing post growth annealing [11] or using droplet epitaxy [12]. Fine tuning of the residual FSS was explored through the application of both magnetic and electric fields [10,13,14] as well as strain [15,16]. However, the control of a single degree of freedom only leads to zero FSS if the applied field points along the major axis of QD asymmetry [17][18][19]. To universally eliminate the FSS, it was theoretically shown that two independent and non-parallel external controls are needed [20], e.g. biaxial strain or two vector components of an applied electromagnetic field. Adding a third control further allows independent tuning of the FSS and the average exciton transition energy [21].

Over the years, the toolbox for controlling the excitonic fine structure has become quite diverse and efficient, leading to record entanglement fidelity for entangled photon pairs [22]. However, such control has not yet been achieved for QDs inserted in optical cavities, a crucial step to ensure high collection efficiency of the single and entangled photons [23][24][25], and for reaching near unity photon indistinguishability [23,24,26].

Here, we propose a structure that allows controlling the excitonic FSS for quantum dots inserted in monolithic micropillar cavities. We define three independent control knobs by remotely applying three electric potentials on large p-i-n diodes that are connected to the pillar cavity through narrow ridges. The vertical p-i-n doping profile extends over the full device. Finite element simulations are conducted to monitor both the stationary electric field and current in the structure for various bias situations, evidencing a control of the electric field at the position of the QD in all three directions of space. We experimentally explore this approach and demonstrate both a control of the FFS amplitude and of the orientation of the exciton transition dipoles. We find that various combinations of applied voltages reduce the FSS close or below the radiative linewidth. Furthermore, similar values of FSS can be obtained for different excitonic wavelengths.

The structure proposed to control the excitonic FSS for a QD in a cavity is illustrated in Fig. 1(a). We start from a planar GaAs/AlGaAs microcavity grown on a GaAs substrate. For the sample under investigation, the bottom (top) distributed Bragg reflector (DBR) consists of 34 ( 16) pairs of λ/4-thick GaAs/AlGaAs layers. The DBRs surround a λ-thick GaAs cavity spacer with a layer of annealed InGaAs QDs in its center. Vertically, the multilayer structure defines a p-i-n junction with p(n) doping for the top (bottom) mirror and an intrinsic region within the cavity spacer embedding the QDs. Laterally, the planar structure is etched to define circular pillar cavities with 10 µm diameters that are connected through three 3 µm wide, 50 µm long ridges to large mesas where titanium-gold pads are defined for electrical connection. Each pad can be connected to an independent voltage source with the gold-coated back of the substrate defining a common ground potential. By applying various values for the three biases labelled V A , V B and V C , one can control the amplitude and direction of the electric field at the center of the pillar cavity in the three directions of space.

To evidence such a possibility, we first performed Finite Elements Method (FEM) simulations (COMSOL Multiphysics). For computational efficiency, we replaced the vertical DBR layer structure by a single GaAs slab with a vertical p-i-n doping profile. Such a simplification allows for a qualitative understanding of the physics at play disregarding the complex modulation doping structure that one needs to implement in the DBR structure. Fig- ures 2(a-c) display the three electric field components at the center of the micropillar cavity -both in the vertical and horizontal direction -as a function of two applied electric potentials V A and V B , while V C is not fixed by an external potential. Along the vertical direction, the calculated electric field E z presents a large amplitude for negative values of V A and V B , and progressively decreases when going to the passing regime for both diodes A and B. The in-plane electric field components, on the other end, show a more subtle behavior and four regimes can be identified as indicated in the four areas labelled 1 through 4 in Fig 2(b-c). They essentially correspond to the regimes of non-passing and passing p-i-n junction for each of the two electric potentials as evidenced by Fig. 2(d) presenting the total current flowing through the device (in logarithmic scale).

Figures 2(e-h) present the in-plane electric field components in polar plots for each regime. For area 1 (Fig. 2(e)), the amplitude of the field depends on the applied voltage but its direction remains normal to the unconnected ridge C: the current flows along the p-doped upper DBR with an in-plane field parallel to it, dictated by the Ohmic resistivity of the p-doped material. The direction of the in-plane electric field can be modified when the junctions are in the passing regime. This is the case for area 2 (Fig. 2(f)) for which the accessible values of in-plane electric field cover an angular range of almost π. Effectively, we generate two independent nonparallel current components which are both drained to the common ground on the back side of the p-i-n junction. Finally, there are two intermediate cases found in areas 3 and 4 of the field maps for which one diode only is in the passing regime. For area 3 (4), the field mostly points in the direction of diode A (B), yet with a noticeable steering effect resulting from the second applied voltage presenting a current drain on the top surface.

Based on the above discussion, we find that there are at least two ways to generate in-plane electric fields with directional control at the position of a QD centered in the micropillar device. First, in the limit of a non-passing p-i-n junction, using the third control knob V C (in addition to V A and V B ) would allow to choose the field amplitude in the three directions that are normal to each one of the ridges as it was the case in the direction normal to ridge C with only one pair of voltages (V A and V B ) in Fig. 2(e). Second, in the limit of large forward bias, two electric potentials suffice to control both orientation and magnitude of the field. The maximum attainable magnitude of the in-plane electric field differs by roughly a factor of four between the regimes. The link between the observed in-plane electric field behavior and current flows is shown in supplementary figure 2. In all regimes, except for area 2, the current and electric field are roughly parallel. For area 2, two non-parallel current components are created which are both drained through the p-i-n junction in the passing regime. We underline however that the absolute value of the calculated current is largely over-estimated numerically, because of the strongly simplified doping structured considered in the z direction, as experimentally evidenced hereafter.

We tested this FFS control scheme experimentally on 10 µm diameter pillar cavities. Such large diameter cavities do not show discrete optical modes, so that any cavity bi-refringence effect that could perturb the polarization analysis of the excitonic fine structure is avoided [START_REF] Moczała-Dusanowska | ACS Photonics[END_REF]. The FSS is optically probed through polarization-resolved micro-photoluminescence (µPL) studies performed at 4K, under quasi-resonant (p-shell) continuous-wave excitation [28]. We present experimental data for two QDs (QD1 and QD2) inserted in two distinct cavities. Figure 3 shows the data set obtained for QD1. Figure 3(a) presents the measured sum of currents flowing through the connections to the two independent voltage sources. This sum corresponds the portion of the current that flows through the p-i-n junction, since the horizontal current flowing on top of the device cancels out. Figure 3(a) allows one to identify the various regimes for the FSS control and evidences maximal current in the µA range, orders of magnitudes lower than predicted by our simplified numerical simulations. The passing regime is obtained for V A ≥ 2 V and V B ≥ 4 V . This asymmetry, that is not accounted for in our numerical simulations, can be due to the imperfections of the 1D ridges exhibiting different Ohmic resistances.

The small FSS of our QDs is not spectrally resolved by our setup, however the exciton photoluminescence peak energy undergoes a clear sinusoidal shift of amplitude ∆ FSS in the 10 µeV range as a function of the angle θ of the detected linear polarization. This measurement is achieved here by rotating a half-wave-plate before a fixed polarizer (See supplementary). In Fig. 3(be), polar plots of this shift are reported for different voltages V B at a fixed voltage V A = 3 V. For each voltage, the angular dependence is fitted by the function ∆ FSS (cos[2(θ -θ 0 )] + 1)/2 where θ 0 refers to the polarization angle of the high energy exciton line. A reduction of ∆ FSS down to 2.4 ± 0.6 µeV is observed at V B =3.5 V, at the edge of the passing regime, together with a rotation of the exciton eigenaxes on the order of π/2. This is the characteristic signature of near cancellation of the FSS in such a way that the eigenaxes of the high and low energy exciton lines are exchanged when (ideally) crossing the zero FSS point [19,20]. A further reduction down to 1.55 ± 0.20 µeV is observed at a fixed voltage V A = 0 V (non-passing regime) when varying V B , as seen in Fig. 3(f-j). For both the passing and nonpassing regimes, a significant reduction of the FSS near or below the radiative linewidth is thus demonstrated, whereas over the whole voltage scans, we observed significant variations of FSS amplitude up to ∼ 20 µeV.

To complete the experimental picture, we report similar results for QD2 in Figure 4. Here, the exciton peak energy is measured for each value of (V A , V B ) for the ex-citon eigenaxes identified for V A = V B = 0 V. We deduce an algebraic value for FSS defined as the energy difference between these two energies. Figure 4(a) shows that the algebraic FSS varies over more than 20 µeV and exhibits a clear transition at the voltage V B ≈ 1 V where the algebraic FSS changes sign. To determine whether it is due to FSS cancellation (and not only to a rotation of the QD eigenaxes [19,20]), the exciton peak energy is measured as a function of the angle of detected polarization, for 6 different points as indicated in Fig. 4(a). The sinusoidal dependence reported in Fig. 4(d) does not show any significant rotation of the eigenaxes in the probed region of positive FSS and a vanishing FSS of 1.3 ± 0.7 µeV, below the radiative linewidth for point 1 in Fig. 4(a). Besides, when considering a fixed value of V A = 0 V and increasing V B , the FSS turns out to vary non monotonously, showing notably a local minimum at V B = 2.75 V, a behavior also observed for QD1. Such an observation is not directly explained by the electrical field which, according to our numerical simulations, should evolve monotonously. This rather reveals the complex dependence of FSS on the electric field, as already reported in Ref. 14 for an in-plane application of an electric field. Combined with the average exciton energy shift due to the quantum confined Stark effect, covering a range around 80 µeV as shown in Fig. 4(b), this behaviour is of particular interest to reduce or modify the FSS for different central exciton energies. Such a possibility is evidenced in Fig. 4(b) where similar FSS values are obtained at different exciton energy for voltage combination 3 and 4 as well as for 5 and 6. Finally, we underline that the method proposed to control the exciton FSS relies on a geometry very similar to the one already used to obtain highly indistinguishable photons under resonant excitation. This was reproducibly demonstrated from QDs deterministically positioned in micropillar cavities controlled by a single electrical pad [9,24], in a large applied voltage range with limited current. We thus anticipate that the efficient and versatile control of the FSS obtained here in the nonpassing regime would allow high indistinguishability. We note, however, that significant emission linewidth broadening is observed here in the passing regime (V B > 3 V) in the present time-integrated measurements obtained under non-resonant excitation (see supplementary figure S3).

In conclusion, we have proposed a new scheme to control the exciton fine structure of a QD inside a micropillar cavity based on three independent electric potentials. Numerical simulations provide important insight on how the proposed geometry allows for a threedimensional control of the electrical field at the center of the pillar cavity. Experimentally, we observe a tuning of the FSS in the 10-20 µeV range [21,29] which is shown to be sufficient to cancel the FSS of annealed InGaAs QDs. The mean-exciton energy is controlled over a significantly lower range than the one obtained for strain tuning techniques in bulk samples. However, several parameters can be explored to increase the amplitude of the electric field variation in the pillar center: length and width of the connecting ridges, vertical doping structures, etc. Here, we reported an 80 µeV exciton tuning range -a value that is already sufficient to tune the QD exciton into resonance with the pillar cavity mode when combined with the in-situ lithography technique [30]. Indeed, this tech-nique has been shown powerful to define a quasi-resonant pillar cavity around a chosen quantum dot for bright single photon sources or entangled photon pairs [24,25]. Our work thus opens the path toward the fabrication of bright sources of single photons and entangled photon pairs, with control of both the QD-cavity detuning and the excitonic fine structure. [9,24]. However, an additional broadening is observed in the passing regime (VB > 3 V, VA > 1 V), most probably induced by the current flowing through the structure in the passing regime.

1 , † 1

 11 Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies (C2N), 10 Boulevard Thomas Gobert, 91120 Palaiseau, France 2 Université de Paris, Centre de Nanosciences et de Nanotechnologies (C2N), 91120 Palaiseau, France 3 current affiliation: Institute of Physics, Carl von Ossietzky University, 26129 Oldenburg, Germany (Dated: 2nd December 2021)

Figure 1 .

 1 Figure 1. (a) SEM image of a 10 µm diameter micropillar cavity with QDs embedded at its center. Ridges connect the pillar to three electrical contacts defined via gold pads. The image has been artificially colored to evidence the different components of the structure (b) Schematic of the exciton energy levels. (c) Schematic zoom of the pillar structure highlighting the top and bottom cavity mirrors defining the cavity in the vertical direction. The QD layer is vertically centered in the cavity.

Figure 2 .

 2 Figure 2. Finite elements method (FEM) simulations of the electric field at the cavity center as a function of two applied electric potentials VA and VB. The third contact is simulated without fixed potential. (a-c) Calculated electric field Ez Ex and Ey. We define 4 regions labelled 1 to 4 corresponding to (1) non passing regime (2) two-diode passing regime (3,4) one diode passing regime as evidence by (d). The in-plave field values in region (1) has been multipled by 15 for better vizualization with the linear color scale in (b) and (c). All fields are in units of V /m. (d) Calculated net current flowing through the device in units of A, logarithmic scale. (e-h) Polar map of the in-plane electric field for the four (VA,VB) voltage regions labelled 1,2, 3 and 4 as indicated in (b),(c). The red solid lines correspond to the orientation of the three narrow ridges.

Figure 3 .

 3 Figure 3. (a) Measued net current flowing through the p-i-n junction as a function of VA and VB. (b)-(j) Polar plots of QD1 exciton energy shift as a function of the linear polarization angle of detection (circles) at VA = 3 V (b-e) and VA = 0 V (f-j) for different voltages VB as indicated. The solid lines are fits of the function ∆FSS(cos[2(θ -θ0)] + 1)/2. Unit of the radial scale as indicated in (b) is µeV.

Figure 4 .

 4 Figure 4. Measurements for QD2. (a-c) Color maps as a function of VA and VB of (a) algebraic ∆FSS measured in a fixed basis (see text), (b) energy shift of the average exciton transitions with respect to energy at VA = VB = 0. (c) Energy shift as a function of detection polarization angles for the 6 points indicated in (a).

Figure S2 .

 S2 Figure S2. Simulated current and electric field a Schematic of the structure modelled in the Finite Elements Method (FEM) simulations (COMSOL Multiphysics) b-e Polar plots of the in-plane electric field and corresponding in-plane current for the voltage regions labelled 1 to 4 in the main text Figure 3b and 3c.

Figure S3 .

 S3 Figure S3. QD2 linewidth Color map of the average linewidth measured for QD2 as a function of VA and VB. The observed values, larger than our apparatus spectral resolution of about 23.4 µeV (confirmed) reveal a significant broadening mostly due to the charge noise induced by the non-resonant excitation. Such a broadening would disappear under resonant excitation as demonstrated in[9,24]. However, an additional broadening is observed in the passing regime (VB > 3 V, VA > 1 V), most probably induced by the current flowing through the structure in the passing regime.

Acknowledgements

This work was partially supported by the H2020-FET OPEN project number 899544 -PHOQUSING, the French RENATECH network, and the European Research Council Starting Grant No. 715939, Nanophennec. H. O. acknowledges support from the Paris Ile-de-France Région in the framework of DIM SIRTEQ. M.E. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project 401390650, and by the University of Oldenburg through a Carl von Ossietzky Young Researchers fellowship.

Supplementary Information:

Three-dimensional electrical control of the excitonic fine structure for a quantum dot in a cavity


Amélie Forget

Matthieu Regnacq

Christophe Orain

Ewen Touzé

Evan Lelong

Christophe Brandily

Hélène Bernard

Raphaël Tripier

Nicolas Le Poul

Electrocatalytic reduction of CO 2 in water by a C -functionalized Ni-cyclam complex grafted onto carbon

teaching and research institutions in France or abroad, or from public or private research centers.

We present here a novel strategy based on the C-functionalization of the well-known Ni-cyclam complex to afford heterogeneous electrocatalytic CO 2 reduction in neutral water at low overpotential (-1.2 V vs. Ag/AgCl), with high turnover number (TON = 564), high selectivity (85 % CO produced) and good faradic efficiency (56% for CO 2 to CO conversion).

The continuous growing of carbon dioxide content in atmospheric and marine media has become one of the main human's concerns for the last years, due to the environmental and societal consequences associated with the greenhouse effect. Hence, cheap and environment-friendly catalytic devices, which can effectively and selectively reduce CO 2 into industrial feedstocks (CO, HCO 2 H, CH 3 OH) are highly seeked. [1][2][3][4] So far, many metal-based molecular catalysts have been developed for more than thirty years as both homogenous and heterogeneous catalysts. [5][6][7] Nevertheless, activation of CO 2 by molecular-based heterogeneous catalysts has remained less investigated, despite being very challenging. One main reason is the difficulty to design a catalyst, which displays a sufficiently low overpotential value, such that it does not deactivate upon CO 2 reduction in aqueous media. Moreover, the heterogeneous catalyst must be selective, not only against the hydrogen evolution reaction, but also against the formation of various CO 2 -related by-products. Several recent works have shown the possible immobilization of metal-based catalysts onto various surfaces. Ni II cyclam complexes onto glassy carbon (GC) surfaces, 13 whereas Fontecave and co-workers described the electrocatalytic CO 2 reduction by N-functionalized Ni cyclam complexes immobilized onto multi-walled carbon nanotubes (MWCNTs).

14

Considering this, we found interesting to develop devices based on the Ni-cyclam complex, since it is an effective and selective catalyst for CO 2 reduction, which would display all the following features: (i) covalent grafting of the electrode surface to stabilize the catalyst; (ii) C-functionalization (meaning via a carbon atom of the macrocyclic skeleton) 15 of the anchoring group on the cyclam for a better control of the metal-ion coordination sphere; (iii) easy procedure of immobilization involving soft conditions. For such purpose, we have based our device on a newly synthesized cyclam ligand which is functionalized by a 4-aminobenzyl (BnNH 2 ) group in a β-C position of the macrocycle, as shown in Scheme 1 (complex 1). [16][17][18] In suitable conditions, the aromatic group can be converted into a diazonium moiety, which itself can be electrochemically reduced to allow covalent binding.

19-21

This procedure was previously reported for the direct grafting of N-functionalized cyclam ligands. Table 1 Spectroscopic and electrochemical data for complexes 1, 2 and 3. ). Spectroscopic and electrochemical data are gathered in Table 1 and compared to those obtained from the parentcomplex [Ni(cyclam)]Cl 2 (2) and the N-functionalized analogue (3) in the same conditions (Scheme 1). For all complexes, UV-Vis data are consistent with a Ni(II) complex in a square planar geometry featuring a weak absorption d-d band in the 430-460 nm wavelength range. 27 CV of 1 at a GC electrode (v = 0.1 V.s -1 ) displayed a diffusion-controlled reversible system at E 1/2 = -1.41 V vs. Ag/AgCl in 12, 13, 28 When the solution was saturated with CO 2 , a significant increase of the current density was observed for 1, as shown by the ratio j pc (CO 2 )/j pc (Ar) = 3.3 (Table 1), consistent with an electrocatalytic reduction of CO 2 by the complex 1 (Fig. S16-A). For comparison (Table 1), the complex 2 displayed a similar enhancement of the current density (Fig. S16-B) under the same experimental conditions, whereas it was found to be smaller for 3 (Fig. S16-C).

The grafting of the complex 1 onto gold or glassy carbon electrodes was carried out by in-situ generation of the aryl-diazonium moiety from the reaction of complex 1 with sodium nitrite (1.1 equiv.) in diluted acid. 22 XPS analysis of Au wafers functionalized with 1 displayed signals at 400 eV, 855 eV and 872 eV, corresponding to Ni(1s), Ni(2p 3/2 ) and Ni(2p 1/2 ) respectively, whereas bare Au surfaces yielded no signal (Fig. S18-A). Moreover, the presence of the Ni-cyclam complex on the surface was confirmed by the relative atomic percentage of Ni(2p) (1.6%) vs. N(1s) (8%) (Fig. S18-B). Redox properties of complex 1 grafted onto a glassy carbon electrode (GC-1) were obtained from voltammetric studies in H 2 O/CH 3 CN (1:4) / NBu 4 PF 6 0.1 M. Under Ar, GC-1 displayed one reversible system upon oxidation at E 1/2 = 0.58 V vs. Ag/AgCl. ) under CO2 of GC-CNT-1 (red) and GC-CNT (black) electrodes in NaHCO3/H2O (pH = 6.8).

Increasing the scan rate (0.02 V.s

-1 < v < 5 V.s -1
) induced a linear variation of the peak current density j (Fig. S20-A), in agreement with the expected response for a surface-bound species.

29, 30 Plots of anodic current density j pa vs. v (inset, Fig. S20-A) allowed the direct determination of the surface concentration assuming a one-electron oxidation. [START_REF] Bard | Electrochemical methods. Fundamentals and applications[END_REF] The resulting value,  = 0.8 10 -10 mol.cm -2

, is consistent with the grafting of one monolayer of Ni complex on the surface, supposing a compact rearrangement of 11 Å x 7 Å ellipsoids. [START_REF] Orain | [END_REF] Plots of peak potential (E pa and E pc ) vs. log v for the GC-1 electrode allowed the determination of the standard electron transfer rate constant k 0 by using Laviron's formalism for non-interacting redox centers (Fig. S22). 32 We found that k 0 = 33 s -1 , which indicates a relatively fast electron transfer by comparison with existing data on analogous grafted complexes.

31, 33

Voltammetric studies under CO 2 showed a significant increase of the current density at -1.4 V by comparison with an unmodified GC electrode (Fig. S20-B). Such a behavior evidences that CO 2 reduction is mediated by the reduced form of the grafted complex 1, in agreement with solution studies (vide infra, Fig. S16). The same current enhancement was observed in water at neutral pH (Fig. S21).

In order to increase the number of Ni complexes on the electrode surface for further analysis of the products of the electrocatalytic reaction, we modified the GC surface with MWCNTs prior to Ni-cyclam grafting (see SI for details). The diazonium immobilization of 1 and 3 was then carried out according to the procedure used for GC-1. The resulting electrodes (GC-CNT-1 and GC-CNT-3) were analyzed by CV in water at neutral pH. As shown in Fig. 1 for GC-CNT-1 (red curve), the Ni III /Ni II redox process was clearly detected upon oxidation, hence evidencing the effective covalent grafting of the complex on MWCNTs. Integration of the peaks for this redox system allowed the determination of the surface concentration (1.6 10 -9 mol.cm -2

), hence showing a 200-fold increase of the current density compared to GC-1. In reduction, an intense Faradaic signal starting at -1.2 V was obtained compared to that obtained with a GC-CNT modified electrode. The cathodic wave found for GC-CNT-1 under CO 2 disappeared when CV was performed under argon at neutral pH (Fig. S23). This result clearly evidenced that the catalytic wave under CO 2 is fully correlated to carbon dioxide rather than proton reduction. Similarly, the N-functionalized GC-CNT-3 displayed a current enhancement under CO 2 as in the homogeneous case (Fig. S24). The stability of the grafted complexes was then examined by CV. Upon cycling at v = 0.1 V.s -1

, the voltammetric signal of GC-CNT-1 was shown to be relatively stable (<15 % loss after 5 cycles, Fig. S25-A), whereas GC-CNT-3 displayed no stability (complete loss after 2 cycles) as shown by the Ni III /Ni II signal (Fig. S25-B). Clearly, these results demonstrate the superiority of the C-functionalized complex for CO 2 electrocatalytic reduction over the N-functionalized one. In order to analyze the reaction products, controlled potential electrolysis was carried out over 1h30 with GC-NT-1 at -1.25 V vs. Ag/AgCl. The Faradaic current density was found to stabilize at approximatively 0.7 mA.cm -2 over this time range (Fig. S26). Noteworthy, carbon monoxide was generated as the main product according to GC analysis (Fig. S27 and Table S2) yielding 85%/15% CO-H 2 ratio with high turnover number (TON = 564) and high faradic efficiency (FE = 56%) for CO production. By comparison, the same experiment with GC-CNT electrodes produced uniquely H 2 (Fig. S27 and Table S2).

In conclusion, we have successfully performed the synthesis followed by the covalently immobilization of Cand N-functionalized Ni-cyclam complexes onto electrode surfaces. The grafted C-functionalized Ni-cyclam species displays remarkable electrocatalytic properties in aqueous media for the reduction of CO 2 . These promising results constitute an important first entry in the field of CO 2 reduction catalysis since it demonstrates that covalent electrode functionalization of molecular complexes can be easily carried out for further use in electocatalysis. Furthermore, it evidences the main benefits which can be obtained from the C-functionalization of Ni-cyclam species. This now encourage us to accelerate the development of a new family of C-functionalized cyclam complex catalysts for which various N-chelating arms will be introduced on the macrocyclic core in order to decrease the overpotential for CO 2 reduction, as well as to increase of the kinetics for the catalytic reaction. This work was supported by SATT-Ouest Valorisation (DV3755).

Scheme 1

 1 Scheme 1 Covalent grafting of complex 1 onto Au or GC electrode (top) and chemical representation of complexes 2 and 3 (bottom).

1 2 3 1 ,

 31 max /nm (max /M -1 cm -1 ) 439 (22) a 450 (36) a 461 (13) a E1/2(Ni II/I ) /V (Ep /mV) -1.41 (100) b -1.40 (100) b -1.30 (100) b E1/2(Ni III/II ) /V (Ep /mV) 0.67 b,c 0.78 (140) b 0.89 b,c jpc(CO2)/jpc(Ar) 3.3 d 3.4 d 2.1 d a In H2O. b In H2O:CH3CN (1:4)/NBu4PF6 0.1 M, E / V vs. Ag/AgCl/NaCl 3M. c Irreversible anodic peak. d Determined from the ratio of peak current densities under CO2 and Ar from CVs obtained in H2O:CH3CN (1:4)/NBu4PF6 0.1 M (v = 0.1 V.s - C = 1 mM) (see Fig. S10). The C-functionalized p-H 2 N-Bn-cyclam was synthesized through one-step reaction 23, 24 starting from the previously-reported di-N-benzylcyclam-BnNO 2 compound 25 (Ligand L1, Scheme S1 and details in ESI). The new Nfunctionalized analogue N-(p-H 2 N-Ph-(CH 2 ) 2 )-cyclam was prepared from cyclamPO 26 (Ligand L3, Scheme S2). The complex 1 was then synthesized from the stoichiometric reaction between NiCl 2 and the ligand (see SI for details). The resulting beige-brown solid was analyzed by UV-Vis spectroscopy, high-resolution mass spectrometry, and cyclic voltammetry (CV) (Fig.s S11, S14-S17

H 2 O

 2 /CH 3 CN (1:4) as well as an irreversible oxidation peak at E pc = 0.67 V (Fig.s S15 and S17). These processes are ascribed to the Ni III /Ni II and Ni II /Ni I redox couples, respectively, by analogy with complex 2 and from previous studies.

Fig. 1

 1 Fig. 1CVs (v = 0.1 V.s -1

selective electrocatalytic reduction of CO 2 in neutral water by a C- functionalized Ni-cyclam complex grafted onto carbon

  Laboratoire de Chimie, Electrochimie Mole´culaires et Chimie Analytique, Universite´ de Bretagne Occidentale, 6 avenue Le Gorgeu 29238 Brest, Cedex 3, France. E-mail: nicolas.lepoul@univ-brest.fr, raphael.tripier@univ-brest.fr b Laboratoire Environnement Profond, IFREMER Brest, Technopole Brest -Iroise,BP70, 29280 Plouzane´, France

	Version submitted to Chem. Commun. on 23	rd March 2022
	DOI: 10.1039/d2cc01667b
	Highly-Amélie Forget, Hélène Bernard, a Matthieu Regnacq, a Raphaël Tripier,* a Nicolas Le Poul* a Christophe Orain, a	a Ewen Touzé,	a Evan Lelong,	a Christophe Brandily,	b
	a

  7, 8 Inspired by the seminal works of Sauvage et al.

	on Ni-cyclam complexes (cyclam = 1,4,8,11-tetraazacyclotetradecane),	9-11 Neri et al. have attached these
	derivatives onto TiO 2 nanoparticles.	12 More recently, Kubiak et al. reported the grafting of N-alkynyl-terminated

Conflicts of interest

There are no conflicts to declare.