Keywords: porphyrinoids, carbene ligands, cage compounds, self-assembly, host-guest systems

Four porphyrins equipped with imidazolium rings on the para positions of their meso aryl groups were prepared and used as tetrakis(N-heterocyclic carbene) (NHC) precursors for the synthesis of porphyrin cages assembled from eight NHC-M bonds (M = Ag + or Au + ). The conformation of the obtained porphyrin cages in solution and their encapsulation properties strongly depend on the structure of the spacer -(CH2)n-(n = 0 or 1) between meso aryl groups and peripheral NHC ligands. In absence of methylene groups (n = 0), porphyrin cages are rather rigid and the short porphyrin-porphyrin distance prevents encapsulation of guest molecules like 1,4diazabicyclo[2.2.2]octane (DABCO). By contrast, the presence of methylene functions (n = 1) between meso aryl groups and peripheral NHCs offers additional flexibility to the system allowing the inner space between the two porphyrins to expand enough to encapsulate guest molecules like water molecules or DABCO. The peripheral NHC-wingtip groups also play a significant role on the encapsulation properties of the porphyrin cages.

INTRODUCTION

Coordination driven self-assembly of supramolecular complexes is an active field of research to devise original scaffolds with different sizes and shapes. [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF][START_REF] Chakrabarty | Supramolecular Coordination: Self-Assembly of Finite Two-and Three-Dimensional Ensembles[END_REF] Many fascinating supramolecular architectures built upon the formation of metal-ligand bonds were reported in the literature like helicates [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF][START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] and knots. [START_REF] Dietrich-Buchecker | Knotted Heterodinuclear Complexes[END_REF][START_REF] Meyer | The Dynamic Chemistry of Molecular Borromean Rings and Solomon Knots[END_REF][START_REF] Domoto | Self-Assembly of Coordination Polyhedra with Highly Entangled Faces Induced by Metal-Acetylene Interactions[END_REF] Metallocages are supramolecular complexes possessing confined nanospaces that may be used for host-guest chemistry [START_REF] Fujita | Coordination Assemblies from a Pd(II)-Cornered Square Complex[END_REF][START_REF] Fulong | A Self-Assembled Iron(II) Metallacage as a Trap for Per-and Polyfluoroalkyl Substances in Water[END_REF] and several applications ranging from the encapsulation of molecular species for sensing [START_REF] Shanmugaraju | Self-Assembled Discrete Molecules for Sensing Nitroaromatics[END_REF] or catalysis [START_REF] Pluth | Acid Catalysis in Basic Solution: A Supramolecular Host Promotes Orthoformate Hydrolysis[END_REF][START_REF] Kaphan | Enabling New Modes of Reactivity via Constrictive Binding in a Supramolecular-Assembly-Catalyzed Aza-Prins Cyclization[END_REF][START_REF] Gao | Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages[END_REF][START_REF] Yoshizawa | Diels-Alder in Aqueous Molecular Hosts: Unusual Regioselectivity and Efficient Catalysis[END_REF][START_REF] Yoshizawa | Functional Molecular Flasks: New Properties and Reactions within Discrete, Self-Assembled Hosts[END_REF] to the design of drug delivery systems for biomedical applications. [START_REF] Yu | A discrete organoplatinum(II) metallacage as a multimodality theranostic platform for cancer photochemotherapy[END_REF][START_REF] Sepehrpour | Biomedically Relevant Self-Assembled Metallacycles and Metallacages[END_REF][START_REF] Casini | The Promise of Self-Assembled 3D Supramolecular Coordination Complexes for Biomedical Applications[END_REF][START_REF] Cook | Biomedical and Biochemical Applications of Self-Assembled Metallacycles and Metallacages[END_REF] Incorporating -conjugated systems into the ligands structure can bring several advantages such as structural rigidity and optoelectronic properties. In this context, porphyrins are particularly appealing molecular scaffolds to design innovative metallocages. [START_REF] Durot | Multiporphyrinic Cages: Architectures and Functions[END_REF] Obviously, porphyrins with their large structure and their D4h symmetry are attractive building blocks to design metallocages with expanded three-dimensional cavities which are suitable for guest encapsulation [START_REF] Garcia-Simon | Size-selective encapsulation of C60 and C60-derivatives within an adaptable naphthalene-based tetragonal prismatic supramolecular nanocapsule[END_REF][22][START_REF] Black | Catenation and encapsulation induce distinct reconstitutions within a dynamic library of mixed-ligand Zn4L6 cages[END_REF][START_REF] Nakamura | A C60-Templated Tetrameric Porphyrin Barrel Complex via Zinc-Mediated Self-Assembly Utilizing Labile Capping Ligands[END_REF][START_REF] Meng | A Self-Assembled M8L6 Cubic Cage that Selectively Encapsulates Large Aromatic Guests[END_REF] and catalysis. [START_REF] Wang | Tuning the Porphyrin Building Block in Self-Assembled Cages for Branched-Selective Hydroformylation of Propene[END_REF][START_REF] Garcia-Simon | Enantioselective Hydroformylation by a Rh-Catalyst Entrapped in a Supramolecular Metallocage[END_REF][START_REF] Oliveri | Supramolecular Allosteric Cofacial Porphyrin Complexes[END_REF] Porphyrins are also stable photoand electroactive -conjugated systems and there are well-established synthetic procedures to functionalize their meso and -pyrrolic positions. [START_REF] Hiroto | Synthesis and Functionalization of Porphyrins through Organometallic Methodologies[END_REF] These macrocycles can also incorporate metal ions into their cavity, which can coordinate different ligands. This strategy has been exploited by Osuka's group to synthesize metallocages containing several porphyrins. [START_REF] Aratani | Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna Acc[END_REF] Cage-like structures containing two porphyrins assembled face-to-face, namely cofacial porphyrin dimers, are particularly investigated since they can be used as model compounds for photoinduced energy/electron transfer processes between a donor and an acceptor. [START_REF] Camus | Acceleration of the through space S1 energy transfer rates in cofacial bisporphyrin bio-inspired models by virtue of substituents effect on the Förster J integral and its implication in the antenna effect in the photosystems[END_REF][START_REF] Gros | Modulation of the singlet-singlet through-space energy transfer rates in cofacial bisporphyrin and porphyrin-corrole dyads[END_REF][START_REF] Tanaka | Persistent Electron-Transfer State of a π-Complex of Acridinium Ion Inserted between Porphyrin Rings of Cofacial Bisporphyrins[END_REF] Cofacial porphyrin dimers are also suitable structures to investigate encapsulation processes. [START_REF] Durot | Multiporphyrinic Cages: Architectures and Functions[END_REF][START_REF] Mondal | Cyclic metalloporphyrin dimers: Conformational flexibility, applications and future prospects[END_REF][START_REF] Kieran | Inclusion of C60 into an adjustable porphyrin dimer generated by dynamic disulfide chemistry[END_REF][START_REF] Nakamura | Silver-Mediated Formation of a Cofacial Porphyrin Dimer with the Ability to Intercalate Aromatic Molecules[END_REF] In this case, porphyrins play a key role in the encapsulation process since their large -delocalized systems and their inner metal centers can stabilize aromatic guest molecules inside the cavity through --stacking interactions or suitable ligands through the formation of coordination bonds. For these reasons, cofacial porphyrin dimers are particularly investigated in catalysis, notably for small molecule activation reactions like O2 reduction reactions (ORR) [START_REF] Durand | Mechanistic aspects of the catalytic reduction of dioxygen by cofacial metalloporphyrins[END_REF][START_REF] Fukuzumi | Mechanism of Four-Electron Reduction of Dioxygen to Water by Ferrocene Derivatives in the Presence of Perchloric Acid in Benzonitrile, Catalyzed by Cofacial Dicobalt Porphyrins[END_REF] and CO2 reduction reactions. [START_REF] Mohamed | Efficient electrocatalytic CO2 reduction with a molecular cofacial iron porphyrin dimer[END_REF] Compared to systems built upon the formation of covalent bonds, the synthesis of cofacial porphyrin dimers through self-assembly processes is appealing because the yields are usually higher and purification procedures are easier. Most of coordination driven self-assembled cofacial porphyrin dimers are Werner-type coordination compounds built upon formation of metal-ligand bonds with nitrogen, oxygen, sulfur or phosphorus donor atoms of polydentate ligands. Some of them were proven to be efficient for ORR. [START_REF] Oldacre | A Self-Assembled Cofacial Cobalt Porphyrin Prism for Oxygen Reduction Catalysis[END_REF][START_REF] Oldacre | Tuning the Activity of Heterogeneous Cofacial Cobalt Porphyrins for Oxygen Reduction Electrocatalysis through Self-Assembly[END_REF] Recently, we reported the synthesis of the first cofacial porphyrin dimers assembled from eight N-heterocyclic carbene-metal (NHC-M) bonds. [START_REF] Rose | Cofacial porphyrin dimers assembled from N-heterocyclic carbene-metal bonds[END_REF] The synthetic strategy consists in the self-assembly of the two porphyrins through labile NHC-Ag + bonds followed by the replacement of Ag + by Au + (transmetalation reaction). This two-step procedure allowed us to easily obtain very stable porphyrin cages, although an initial self-assembly process was used for their synthesis. Inspired by these preliminary findings, we describe here the synthesis and characterization of novel porphyrin cages assembled from NHC-M bonds (M = Ag + or Au + ). In this study, two types of porphyrins equipped with imidazolium rings on the para positions of their meso aryl groups were used as tetrakis(NHC) precursors. They differ by the nature of the spacer -(CH2)n-(n = 0 or 1) between the meso aryl groups and the peripheral NHCs.

In absence of methylene group (n = 0), porphyrin-based tetrakis(NHC) are rather rigid, while the presence of methylene functions (n = 1) between the meso aryl groups and the peripheral NHCs brings additional flexibility. The aim of this work is to investigate the impact of the introduction of these methylene groups (i) on the conformation of the obtained porphyrin cages in solution, and (ii) on their encapsulation properties. Indeed, the flexibility offered by the sp 3 carbon atoms of the methylene groups allows the two porphyrins of the cages to move away from each other enabling guest molecules to be encapsulated if the cavity is large enough. The role played by the peripheral NHC-wingtip groups on the conformation of the obtained porphyrin cages and their encapsulation properties is also investigated.

EXPERIMENTAL SECTION

Materials. Reactions needing inert atmosphere were performed under argon using ovendried glassware and Schlenk techniques. All solvents were obtained from commercial suppliers and used as received. Dry acetonitrile and DMSO were purchased from Alfa Aesar. DMF was purchased from Sigma-Aldrich. Dry CH2Cl2 and THF were obtained by a PureSolve MD5 solvent purification system from Innovative Technology. Silver(I) oxide 99% (Ag2O) was purchased from Fluorochem and gold(I) complex [AuCl(tht)] (tht = tetrahydrothiophene) was prepared according to the procedure described in the literature. [START_REF] Usón | Tetrahydrothiophene)gold(I) or Gold(III) Complexes[END_REF] Pyrrole (>99%) was purchased from TCI and distilled under reduced pressure before use. Imidazole (99.5%), 4-fluorobenzaldehyde (98%), iodomethane (99%), potassium hexafluorophosphate (>99%) and zinc(II) acetate dihydrate (>98%) were purchased from Sigma-Aldrich. Hydrogen tetrachloroaurate(III) trihydrate, ACS 99.99% (metals basis), Au 49.0% min was used as starting material and purchased from Alfa Aesar. Porphyrins 2a and 2c were prepared according to the procedures described in the literature. [START_REF] Subbaiyan | Near Unity Photon-to-Electron Conversion Efficiency of Photoelectrochemical Cells Built on Cationic Water-Soluble Porphyrins Electrostatically Decorated onto Thin-Film Nanocrystalline SnO2 Surface[END_REF][START_REF] Jin | A new route of water soluble porphyrins: phosphonium and ammonium type cationic porphyrins and self-assembly[END_REF] Porphyrins 4a-d were prepared according to procedures described in the Supporting Information. TLC were carried out on Merck DC Kieselgel 60 F-254 aluminium sheets and spots were visualized with UV-lamp ( = 254/365 nm) if necessary. Preparative purifications were performed by silica gel column chromatography (Merck 40-60 μM).

Instruments and methods. NMR spectroscopy and MS spectrometry were performed at the Laboratoire de Mesures Physiques (LMP) of the University of Montpellier (UM). 1 H, COSY, ROESY, HSQC, HMBC, 13 C{ 1 H}, 31 P{ 1 H}, [START_REF] Cook | Biomedical and Biochemical Applications of Self-Assembled Metallacycles and Metallacages[END_REF] Abbreviation used: sh, shoulder. All spectra can be found in the Supporting Information.

Experimental procedures used for DFT calculations are described in the Supporting Information.

General procedure for the preparation of porphyrin cages [Ag4(1a-d)2](PF6)4.

Porphyrin 4a-d (around 0.04-0.06 mmol) was dissolved in dry CH3CN (10 mL) and degassed with argon for 10 minutes. Then, Ag2O (4.0 or 6.0 eq) was added and the reaction mixture was vigorously stirred under argon at 70°C for 24 or 48 hours protected from light. The reaction was monitored by UV-visible absorption spectroscopy (in CH2Cl2) to verify the formation of the porphyrin cage (hypsochromic shift of the Soret absorption band). After cooling at room temperature, the reaction mixture was evaporated. Dichloromethane was added (10-15 mL) and the solution was filtered through a pad of celite. The obtained solution was concentrated under reduced pressure and precipitated from CH2Cl2/Et2O. The solid was filtered off, washed with Et2O and dried under vacuum to afford the porphyrin cage [Ag4(1a-d)2](PF6)4 as a dark solid.

Porphyrin cage [Ag4(1a)2](PF6)4. The title compound was prepared according to the above general procedure from porphyrin 4a (80 mg, 0.0429 mmol, 1.0 eq), Ag2O (40 mg, 0.1718 mmol, 4.0 eq) and CH3CN (10 mL). The reaction mixture was heated at 75 °C for 24 hours. The porphyrin cage [Ag4(1a)2](PF6)4 was obtained in 77% yield (59 mg, 0.0165 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (400 MHz, DMSO-d6): δ = 8.41 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 8.0 Hz, Ho out), 8.36 (br s, 16H, Hβ-pyrr), 8.24 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 8.0 Hz, Hm out), 8.15 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 5 ), 8.00 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 4 ), 7.80 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 8.0 Hz, Hm in), 7.66 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 8.0 Hz, Ho in), 4.54 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.2 Hz, Ha), 2.15-2.06 (m, 16H, Hb), 1.54 -1.36 (m, 24H, Hc, Hd, He) 0.97 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, -CH3). Porphyrin cage [Ag4(1b)2](PF6)4. The title compound was prepared according to the above general procedure from porphyrin 4b (80 mg, 0.0457 mmol, 1.0 eq), Ag2O (43 mg, 0.1828 mmol, 4.0 eq) and CH3CN (10 mL). ). The reaction mixture was heated at 75 °C for 24 hours. The porphyrin cage [Ag4(1b)2](PF6)4 was obtained in 68% yield (52 mg, 0.0155 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (600 MHz, DMSO-d6): δ = 8.42 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.7 Hz, Ho out), 8.36 (br s, 16H, Hβ-pyrr), 8.25 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.7 Hz and 4 JH,H = 2.5 Hz, Hm out), 8.14 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 5 ), 7.98 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7

Hz, H 4 ), 7.79 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.9 Hz and 4 JH,H = 2.6 Hz, Hm in), 7.66 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.9 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 2.6 Hz, Ho in), 4.55 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.3 Hz, Ha), 2.10 (quint, 16H, , [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Hb), 1.53 (sext, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Hc), 1.09 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, -CH3). [START_REF] Gao | Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages[END_REF] C{ 1 H} NMR (150.9

MHz, DMSO-d6): δ = 173.4, (CNHC), 148.9 (Cα-pyrr), 142.7 (Cp), 138.9 (Ci), 135.2 (Co out), 134.8 (Co in), 130.2 (Cβ-pyrr), 123.4 (C 4 ), 122.9 (C 5 ), 121.5 (Cm in), 121.0 (Cm out), 116.5 (Cmeso), 51.9 (Ca) 33.5 (Cb), 19.6 (Cc), 13.8 ppm (-CH3), CNHC not observed. [START_REF] Cook | Biomedical and Biochemical Applications of Self-Assembled Metallacycles and Metallacages[END_REF] Porphyrin cage [Ag4(1c)2](PF6)4. The title compound was prepared according to the above general procedure from porphyrin 4c (100 mg, 0.0521 mmol, 1.0 eq). Ag2O (75 mg, 0.3126 mmol, 6.0 eq) and CH3CN (10 mL). The reaction mixture was heated at 75 °C for 48 hours. Indeed, extended reaction time (48 hours) and more Ag2O (6 eq) were needed to successfully achieve the synthesis of porphyrin cage [Ag4(1c)2](PF6)4 in 71% yield (68 mg, 0.0185 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (400 MHz, CD2Cl2): δ = 7.88 (s, 16H, Hβ-pyrr), 7.64 (d br, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Hm in), 7.52 (d br, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Ho out), 7.41 (d br, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz Hm out), 7.34 (s br , H 4 ), 7.33 (s br, H 5 ), 7.22 (d br, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Ho in), 5.74 (s, 16H, CH2), 4.41 (t br, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, Ha), 2.09 (quint br, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, Hb), 1.60-1.40 (m, 48H, Hc, Hd, He), 0.97 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, -CH3). Porphyrin cage [Ag4(1d)2](PF6)4. The title compound was prepared according to the above general procedure from porphyrin 4d (80 mg, 0.0443 mmol, 1.0 eq), Ag2O (41 mg, 0.1771 mmol, 4 eq) and CH3CN (10 mL). The porphyrin cage [Ag4(1d)2](PF6)4 was obtained in 82% yield (63 mg, 0.0182 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (600 MHz, CD3CN): δ = 8.24 (br s, 16H, Hβ-pyrr), 8.07 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.3 Hz, Ho out), 7.82 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.3 Hz, Hm out), 7.77 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.3 Hz, Hm in), 7.53 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.9 Hz, H 5 ), 7.42 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.9 Hz, H 4 ), 7.41 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.3 Hz, Ho in), 5.69 (s, 16H, CH2), 4.40 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.1 Hz, Ha), 2.00 (quint, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.1 Hz, Hb), 1.49 (sext, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.1 Hz, Hc), 1.01 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.1 Hz, -CH3); 13 C{ 1 H} NMR (150.9 MHz, CD3CN): 149.9 (Cα-pyrr), 144.1 (Cp), 136.7 (Ci), 136.0 (Co in), 134.4 (Co out), 131.9 (Cβ-pyrr), 127.6 (Cm out), 126.3 (Cm in), 123.7 (C4), 122.6 (C5), 119.9 (Cmeso), 56.2 (CH2), 52.5 (Ca), 34.4 (Cb), 20.6 (Cc), 14.0 ppm (CH3), CNHC not observed; [START_REF] Cook | Biomedical and Biochemical Applications of Self-Assembled Metallacycles and Metallacages[END_REF]

General procedure for the preparation of porphyrin cages [Au4(1a-d)2](PF6)4.

Porphyrin cages [Ag4(1a-d)2](PF6)4 (around 0.01-0.02 mmol) was dissolved in dry CH3CN (5 mL) and degassed with argon for 10 minutes. Then, [AuCl(tht)] (6 eq) was added and the reaction mixture was vigorously stirred under argon and protected from light at room temperature for 24 hours. The reaction was monitored by ESI-TOF(+) mass spectrometry. Once finished, the reaction mixture was evaporated. Dichloromethane was added (10-15 mL) and the solution was filtered through a pad of celite. The obtained solution was concentrated under reduced pressure and precipitated from CH2Cl2/Et2O. The solid was filtered off, washed with Et2O and dried under vacuum to afford the porphyrin cage [Ag4(1a-d)2](PF6)4 as a dark solid.

Porphyrin cage [Au4(1a)2](PF6)4. The title compound was prepared according to the above general procedure from the porphyrin cage [Ag4(1a)2](PF6)4 (42 mg, 0.0117 mmol, 1.0 eq), [AuCl(tht)] (23 mg, 0.0702 mmol, 6.0 eq), and CH3CN (5 mL). The porphyrin cage [Au4(1a)2](PF6)4 was obtained in 68% yield (32 mg, 0.0081 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (400 MHz, DMSO-d6): δ = 8.39 (br s, 16H, Hβ-pyrr), 8.37 (dd ,8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.8 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 2.5 Hz, Ho out), 8.20 (dd ,8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.8 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 2.5 Hz, Hm out), 8.16 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 5 ), 8.06 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 4 ), 7.83 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.8 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 2.5 Hz, Hm in), 7.74 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.8 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 2.5 Hz, Ho in), 4.63 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Ha), 2.14 (m, 16H, Hb), 1.51-1.41 (m, 48H, Hc, Hd, He), 0.98 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, -CH3); [START_REF] Gao | Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages[END_REF] = 1.7 Hz, H 4 ), 7.82 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.9 Hz and 4 JH,H = 2.4 Hz, Hm in), 7.72 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.9 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 2.4 Hz, Ho in), 4.63 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.4 Hz, Ha), 2.13 (quint, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz, Hb), 1.54 (sext, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.4 Hz, Hc), 1.09 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.4 Hz, -CH3). [START_REF] Gao | Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages[END_REF] C{ 1 H} NMR (150.9

MHz, DMSO-d6): δ = 180.8 (CNHC), 147.2 (Cα-pyrr), 142.9 (Cp), 137.9 (Ci), 135.6 (Co out), 134.9 (Co [AuCl(tht)] (29 mg, 0.0894 mmol, 6.0 eq), and CH3CN (5 mL). The porphyrin cage [Au4(1c)2](PF6)4 was obtained in 72% yield (43 mg, 0.0107 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (600 MHz, CD2Cl2): δ = 7.97 (s, 16H, Hβ-pyrr), 7.65 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 1.5 Hz, Hm in), 7.56 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 1.8 Hz, Ho out), 7.41 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 1.5 Hz, Hm out), 7.36 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 4 ), 7.34 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 1.7 Hz, H 5 ), 7.22 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.5 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H = 1.8 Hz, Ho in), 5.81 (s, 16H, CH2), 4.49 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, Ha), 2.13 (quint, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, Hb), 1.60-1.40 (m, 48H, Hc, Hd, He), 0.98 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.0 Hz, -CH3); [START_REF] Gao | Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages[END_REF] [AuCl(tht)] (20 mg, 0.0624 mmol, 6.0 eq), and CH3CN (5 mL). The porphyrin cage [Au4(1d)2](PF6)4 was obtained in 86% yield (34 mg, 0.0089 mmol). [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR (400 MHz, CD3CN): δ = 8.24 (br s, 16H, Hβ-pyrr), 8.09 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.9 Hz and 4 JH,H = 1.7 Hz, Ho out), 7.95 (br d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.9 Hz, Hm in), 7.83 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.4 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H =1.9 Hz, Hm out), 7.54 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 2.0 Hz, H 5 ), 7.43 (d, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 2.0 Hz, H 4 ), 7.39 (dd, 8H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.4 Hz and [START_REF] Caulder | Superamolecular Self-Recognition and Self-Assembly in Gallium(III) Catecholamide Triple Helices[END_REF] JH,H =1.9 Hz, Ho in), 5.78 (s, 16H, CH2), 4.48 (t, 16H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 6.9 Hz, Ha), 2.05 (m, 16H, Hb), 1.55-1.45 (m, 16H, Hc), 1.01 ppm (t, 24H, [START_REF] Krämer | Self-recognition in helicate selfassembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions[END_REF] JH,H = 7.3 Hz, -CH3); [START_REF] Gao | Supramolecular catalysis based on discrete heterometallic coordination-driven metallacycles and metallacages[END_REF]

RESULTS AND DISCUSSION

Preparation of porphyrin-NHC precursors. Deprotonation of azolium salts with a base in a presence of metal ions is one of the most simple and efficient synthetic pathways to obtain NHC-metal complexes. [START_REF] De Frémont | Synthesis, properties, and organometallic chemistry[END_REF][47][START_REF] Díez-González | N-Heterocyclic Carbenes in Late Transition Metal Catalysis[END_REF] For this purpose, we synthesized zinc(II) porphyrins with imidazolium groups on the para positions of the four meso positions (Scheme1) as porphyrin tetrakis(NHC) precursors. [START_REF] Rose | Cofacial porphyrin dimers assembled from N-heterocyclic carbene-metal bonds[END_REF] Rigid (1a and 1b) and flexible (1c and 1d) porphyrin tetrakis(NHC) ligands were investigated. Rigid tetrakis(NHC) ligands 1a and 1b were obtained by directly attaching the NHC on the para positions of the four meso aryl groups. Flexible tetrakis-NHC ligands 1c and 1d were obtained by introducing methylene groups between the NHCs and the para positions of the four meso aryl groups. Long alkyl chains were used as peripheral NHC-wingtip groups in order to improve the solubility of the obtained complexes. Finally, zinc(II) was used as inner metal to protect the macrocycle from unwanted metalation reaction with silver(I) during the synthesis of cofacial porphyrin dimers. tetrakis(4'-chloromethylphenyl)porphyrin 2c was prepared as previously reported. [START_REF] Jin | A new route of water soluble porphyrins: phosphonium and ammonium type cationic porphyrins and self-assembly[END_REF] It was then reacted with 1-hexylimidazole and 1-butylimidazole to obtain the corresponding porphyrins 3c and 3d, respectively. These compounds were directly engaged in the anion metathesis reaction with KPF6 to obtain the corresponding imidazolium salts 3cPF6 and 3dPF6. Finally, metalation reaction with Zn(OAc)2•2H2O afforded the zinc(II) porphyrins 4c and 4d. Their spectroscopic data are also in agreement with the proposed structures.

Preparation of porphyrin cages. Porphyrins 4a-d were used as tetrakis-NHC precursors for the synthesis of porphyrin cages (Scheme 2). NHCs are able to bind strongly to transition metal ions in low and high oxidation states. [START_REF] De Frémont | Synthesis, properties, and organometallic chemistry[END_REF][47][START_REF] Díez-González | N-Heterocyclic Carbenes in Late Transition Metal Catalysis[END_REF] This is not advantageous to obtain coordination driven self-assembled supramolecular complexes, because reversible coordination bonds are needed to allow self-correction leading to the formation of thermodynamic compounds. However, Ag + -CNHC bonds are notable exceptions and are rather labile leading back to the formation of free NHCs and Ag + ions. Therefore, NHC ligands combined with Ag + ions behave like Werner-type compounds and are suitable building blocks to obtain coordination driven self-assembled supramolecular complexes. Moreover, the reversibility of the Ag + -CNHC bonds is routinely used to transmetalate NHCs from Ag + to other transition metal ions like Au + . [START_REF] Garrison | I) N-Heterocyclic Carbene Complexes: Synthesis, Structure, and Application[END_REF] This strategy was applied for the synthesis of coordination-driven metallacycles and metallacages from poly-NHC ligands. [START_REF] Ga | Preparation and Post-Assembly Modification of Metallosupramolecular Assemblies from Poly(N-Heterocyclic Carbene) Ligands[END_REF][START_REF] Sinha | Metallosupramolecular Architectures Obtained from Poly-Nheterocyclic Carbene Ligands[END_REF][START_REF] Rit | Self-Assembly of Molecular Cylinders from Polycarbene Ligands and Ag I or Au I[END_REF][START_REF] Segarra | Hexanuclear Cylinder-Shaped Assemblies of Silver and Gold from Benzene-Hexa-N-heterocyclic Carbenes[END_REF] The Conformations of the porphyrin cages in solution. [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR and UV-visible absorption spectroscopies were used to determine conformations adopted by porphyrin cages in solution. We conducted these studies on gold(I) complexes which are more stable and less sensitive to daylight compared to the corresponding silver(I) complexes. [START_REF] Rose | Cofacial porphyrin dimers assembled from N-heterocyclic carbene-metal bonds[END_REF] Moreover, well-resolved 1 H NMR spectra were obtained for all gold(I) complexes. Porphyrins have characteristic UV-visible absorption spectra and changes observed in their absorption bands indicate modifications in porphyrins structure or, as it is the case here, in their environment and spatial organization. Indeed, Haggregates exhibit a Soret absorption band blue-shift with respect to the one of the monomer, corresponding to a face-to-face stacking of the monomeric species. In contrast, J-aggregates are edge-by-edge or side-to-side assemblies that produce a Soret absorption band red-shift with respect to the one of the monomer. [START_REF] Maiti | J-and H-Aggregates of Porphyrin-Surfactant Complexes: Time-Resolved Fluorescence and Other Spectroscopic Studies[END_REF] Here, ZnTPP was taken as a reference compound and three solvents were used with increasing polarity in the order CH2Cl2 < CH3CN < DMSO. In Figure 2 These spectral features are attributed to the presence of several cofacial conformations in solution due to the incorporation of the more flexible meso benzyl groups. [START_REF] Zanetti-Polzi | Interpretation of Experimental Soret Bands of Porphyrins in Flexible Covalent Cages and in Their Related Ag(I) Fixed Complexes[END_REF] It is also remarkable that Soret and CD2Cl2, the presence of guest molecules like solvent or H2O between the two porphyrins is very unlikely (Figure 3a-d). By contrast, an upfield singlet at  = -6.27 ppm was observed in the 1 H NMR spectrum of the flexible porphyrin cage [Au4(1d)2](PF6)4 in CD3CN (see Figure 3g). 1 H 2D diffusion-ordered spectroscopy (DOSY) NMR showed that this upfield signal and signals due

to the porphyrin cage have the same diffusion coefficient indicating that they diffuse as a single entity (Figure 4a). These data strongly suggest the formation of a host-guest system between the porphyrin cage [Au4(1d)2](PF6)4 and encapsulated molecular species. Thus, the upfield singlet at  = -6.27 ppm may be attributed to encapsulated CH3CN (solvent used for the synthesis of porphyrin cages) or H2O molecules (present in CD3CN or generated in the course of the deprotonation of imidazolium salts by Ag2O). Since no correlation with any carbon could be observed by 1 H-13 C{ 1 H} HSQC NMR spectroscopy, the signal at  = -6.27 ppm is not due to the methyl group of CH3CN. Moreover, it was observed even if the synthesis of the cages was performed in CD3CN. Hence, it may be assumed that this upfield signal is due to protons of encapsulated H2O molecules, although it was not possible to observe them by mass spectrometry.

The most convincing evidence arises from the NMR experiment where H2O is added in excess to a solution of the porphyrin-cage [Au4(1c)2](PF6)4 with n-hexyl NHC-wingtip groups in CD3CN.

Initially, this porphyrin cage is in its "closed" conformation in CD3CN at a concentration in the millimolar range used for 1 H NMR spectroscopy and no high upfield signal could be observed (Figure 4b). Upon addition of H2O (10% v/v) to a solution of the porphyrin cage [Au4(1c)2](PF6)4 in CD3CN, a new set of signals is observed by 1 H NMR spectroscopy (Figure 4c). These new signals are very similar to those observed in the 1 H NMR spectrum of the porphyrin cage [Au4(1d)2](PF6)4 in CD3CN (Figure 4d). Moreover, a new high upfield signal is observed at  = -6.22 ppm and this signal is attributed to encapsulated H2O molecules (Figure 4c, red arrow).

Integration values are above 2H (~2.5-3H) and suggest that probably two H2O molecules are axially coordinated to zinc(II) porphyrins (one H2O per zinc(II) ion) and directed inside the cavity.

Indeed, many ligands and molecules show a preference for binding inside the cavity of porphyrin cages. [START_REF] Wylie | Unexpectedly selective ligand binding within the cavity of a cyclic metalloporphyrin dimer[END_REF][START_REF] Djemili | Positive Allosteric Control of Guests Encapsulation by Metal Binding to Covalent Porphyrin Cages[END_REF][START_REF] Nakash | Structure-Activity Relationships in the Acceleration of a Hetero Diels-Alder Reaction by Metalloporphyrin Hosts[END_REF][START_REF] Mondal | Controlling the Photophysics of Aromatic Guests Using a Cyclic Porphyrin Dimer: Synthesis, Structure, and Encapsulation-Mediated "ON-OFF[END_REF] The H2O proton signals appear as singlets because of the quick dynamic behavior on Investigated gold(I) complexes are displayed in Figure 5. The calculated Au + -CNHC distances of ~2.05-2.06 Å are in agreement with distances measured in X-ray crystal structures of linear bis(NHC) gold(I) complexes. [START_REF] Ga | Preparation and Post-Assembly Modification of Metallosupramolecular Assemblies from Poly(N-Heterocyclic Carbene) Ligands[END_REF][START_REF] Sinha | Metallosupramolecular Architectures Obtained from Poly-Nheterocyclic Carbene Ligands[END_REF][START_REF] Rit | Self-Assembly of Molecular Cylinders from Polycarbene Ligands and Ag I or Au I[END_REF] Porphyrin cages [Au4(1a/b)2] 4+ are considered without guest molecule between the two porphyrins as guest inclusion was not observed in the conditions used for this study (Figure 5a). The two porphyrins have a face-to-face orientation, but are not completely flat and adopt a slight out-of-plane saddle-shaped distortion. For this complex, the calculated Zn-Zn distance is ~5.53 Å. DFT calculations were further performed to explore the flexible porphyrin cages [Au4(1c/d)2] 4+ . In a first approach, no guest was included as it was experimentally observed in dichloromethane (Figure 5b). Interestingly, incorporation of the flexible benzyl group between the porphyrin cores and the peripheral NHCs allows the porphyrinporphyrin distance to be shorter. Indeed, the calculated Zn-Zn distance is ~3.82 Å. The two porphyrins also adopt a stronger out-of-plane saddle-shaped distortion. Then, we considered the inclusion of two H2O molecules within the flexible porphyrin cages [Au4(1c/d)2] 4+ (Figure 5c).

Both H2O molecules are coordinated to zinc(II) porphyrins and linked together by a hydrogen bond, as it was previously observed and/or calculated for other H2O dimers encapsulated within discrete 3D-molecular hosts such as superphane [START_REF] Li | Superphane: a new lantern-like receptor for encapsulation of a water dimer[END_REF] or fullerene C70. [START_REF] Zhang | Synthesis of a distinct water dimer inside fullerene C70[END_REF] The calculated Zn-Zn distance of 7.44 Å is significantly longer compared to the one calculated for [Au4(1a/b)2] 4+ (~+1.9 Å) and in agreement with the less remarkable blueshift of the Soret absorption band observed by UV-Visible absorption spectroscopy. Following these observations, we turned our attention to the bifunctional ligand 1,4diazabicyclo[2.2.2]-octane (DABCO), a typical guest used as probe to investigate encapsulation properties of cofacial porphyrin dimers [START_REF] Hunter | Dabco-metalloporphyrin binding: ternary complexes, host-guest chemistry and the measurement of pi-pi interactions[END_REF] or as template for their synthesis. [START_REF] Taesch | Templated synthesis of a large and flexible covalent porphyrinic cage bearing orthogonal recognition sites[END_REF][START_REF] Kocher | Control of the cavity size of flexible covalent cages by silver coordination to the peripheral binding sites[END_REF][START_REF] Ding | Highly-efficient synthesis of covalent porphyrinic cages via DABCO-templated imine condensation reactions[END_REF] First, we investigated the encapsulation properties of the rigid porphyrin cage [Au4(1a)2](PF6)4 (Figure 6ac). One equivalent of DABCO was added to a solution of the porphyrin cage [Au4(1a)2](PF6)4 in CD3CN at 25 °C and a weak upfield singlet was observed by [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] H NMR spectroscopy at  = -5.21 ppm (Figure 6b, red arrow). This signal is attributed to the protons of DABCO sandwiched between two porphyrins (free DABCO in CD3CN,  = 2.65 ppm). However, it may correspond to two possible structures (Figure 7), i.e. inclusion complexes (coordination of DABCO inside the porphyrin cage) or ternary complexes (coordination of DABCO outside bridging two porphyrin cages). To answer this question, the DABCO/[Au4(1a)2](PF6)4 ratio was increased up to 5 and we observed that the weak upfield signal due to DABCO disappeared (Figure 6c). This is due to the decomposition of ternary complexes upon coordination of additional DABCO as it was previously observed by Sanders and coworkers with analogous systems. [START_REF] Hunter | Dabco-metalloporphyrin binding: ternary complexes, host-guest chemistry and the measurement of pi-pi interactions[END_REF] No additional upfield singlet could be observed by [START_REF] Northrop | Self-Organization in Coordination-Driven Self-Assembly[END_REF] are able to encapsulate DABCO with a calculated Zn-Zn distance of 7.14 Å and structural features including porphyrin cores distortion similar to those calculated for the previous system with encapsulated H2O molecules (Figure 5d). The porphyrin cage [Au4(1c)2](PF6)4 was mixed with one equivalent of DABCO in CD3CN at 25 °C and a new set of signals was observed within minutes in the 1 H NMR spectrum of the reaction mixture (Figure 6e). The singlet due to the pyrrolic protons is significantly shifted downfield ( = 0.41 ppm) upon inclusion of DABCO suggesting a greater porphyrin-porphyrin distance in good agreement with DFT optimized geometries. An upfield signal integrating for 12H is observed at  = -5.51 ppm and corresponds to the signal of the protons of DABCO sandwiched between the two porphyrins of the cage (Figure 6e, red arrow). This complex is stable and did not decompose when the DABCO/[Au4(1c)2](PF6)4 ratio was increased up to 5 confirming the formation of the inclusion complex [DABCO@Au4(1c)2] 4+ (Figure 6f). The D4h symmetry of the porphyrin cage [Au4(1c)2] 4+ is retained after inclusion of DABCO with D3h symmetry. It is in agreement with a fast rotation of DABCO inside the porphyrin cage along the Zn---Zn axis.

The same upfield signal of DABCO at  = -5.51 ppm and identical signals in the aromatic region were observed by 1 H NMR spectroscopy when the porphyrin cage [Au4(1d)2](PF6]4 was mixed with one equivalent of DABCO in CD3CN at 25 °C (Figure 8a). Here, the appearance of this singlet at  = -5.51 ppm is concomitant with the disappearance of the signal at  = -6.27 ppm due to encapsulated H2O molecules suggesting the occurrence of a guest exchange reaction.

Compared to the previous system, the inclusion of DABCO in the porphyrin cage [Au4(1d)2](PF6)4 is significantly slower because signals of the remaining starting material could be observed after 4 hours (Figure 8a, middle):1 H NMR signals of both porphyrin cages [Au4(1d)2] 4+ including H2O molecules (green dots) and DABCO (red dots) were observed independently. After 16 hours, only one species corresponding to the inclusion complex [DABCO@Au4(1d)2] 4+ was observed in solution (Figure 8a, bottom). DOSY 1 H NMR experiment shows that the porphyrin cage [Au4(1d)2] 4+ and DABCO diffuse at the same rate, which provides strong evidence that DABCO is encapsulated (Figure S129). 2D 1 H ROESY NMR spectrum showed a cross-peak of DABCO protons with meso aryl protons pointing inside the cavity, namely Ho in, confirming the encapsulation of DABCO between the two porphyrins (Figure S130). High-resolution ESI-TOF(+) mass spectrometry also supports the encapsulation of DABCO by the presence of a peak at m/z = 836.2723 Da, which could be assigned to the tetracationic species [DABCO@Au4(1d)2] 4+ (Figure 8b). Finally, binding of DABCO was investigated by UV-visible absorption spectroscopy. containing H2O molecules and the inclusion complex [DABCO@Au4(1d)2] 4+ (Figure 8c). Satisfyingly, this is in agreement with NMR studies (Figure 8a). The absence of spectral evolution with DABCO > 1 equivalent confirms that the inclusion complex [DABCO@Au4(1d)2] 4+ persists in solution even in the presence of an excess of DABCO. A binding constant of Ka = 1.3  10 5 M -

CONCLUSION

In summary, four porphyrins equipped with imidazolium rings on the para positions of the meso aryl groups were prepared and used as NHC precursors for the synthesis of porphyrin cages assembled from eight metal-carbene bonds. The reversibility of the Ag + -CNHC bonds was used to our advantage to allow two porphyrins to self-assemble with a face-to-face orientation and to further transmetalate NHC ligands from Ag(I) to Au(I). In solution, we found that the conformation and the encapsulation properties of these cage-like architectures strongly depend on the structure of the -(CH2)n-NHC side arms on the para positions of the meso aryl groups. Indeed, in absence of methylene functions (n = 0), the two porphyrins are in close proximity and the obtained rigid systems are not able to act as host to encapsulate a typical guest like DABCO. In this case, external coordination of DABCO on zinc(II) porphyrins was observed. By contrast, the presence of methylene functions (n = 1) between meso aryl groups and peripheral NHCs bring additional flexibility to the systems allowing the inner space between the two porphyrins to expand enough to encapsulate H2O molecules or DABCO. More surprisingly, the length of the peripheral alkyl chains used as NHC-wingtip groups (n-hexyl vs. n-butyl) also dramatically modifies the conformation of the flexible porphyrin cages in solution and their encapsulating properties. To conclude, we demonstrate here that host-guest chemistry is feasible with porphyrin cages built upon formation of NHC-metal bonds and encapsulation of other guests may be considered in the future. The possibility to use porphyrin cages assembled from NHC-metal bonds for catalysis is also currently under investigation in our group.

  F{ 1 H} and DOSY NMR spectra were recorded on Bruker 400 MHz Avance III HD, 500 MHz Avance III or 600 MHz Avance III spectrometers at 298 K. Deuterated solvents DMSO-d6, CD3CN and CD2Cl2 were used as received (purchased from Sigma-Aldrich). 1 H and 13 C{ 1 H} NMR spectra were calibrated to TMS on the basis of the relative chemical shift of the residual non-deuterated solvent as an internal standard. Chemical shifts () are expressed in ppm and coupling constants values ( n J) are expressed in Hz. Abbreviations used for NMR spectra are as follows: s, singlet; d, doublet; t, triplet; quint, quintuplet; sext, sextuplet; sept, septuplet; m, multiplet; br, broad. No satisfactory elemental analysis data were obtained for porphyrin cages. High Resolution Mass spectra (HRMS) were recorded on a Bruker MicroTof QII instrument in positive/negative modes (ESI). UV-Visible absorption spectra were recorded in CH2Cl2, CH3CN or DMSO with a JASCO V-750 UV-Visible-NIR spectrophotometer in 10 mm quartz cells (Hellma); molar extinction coefficients  (L.mol -1 .cm -1 ) are expressed as log .

13 C{ 1 2 (

 1312 H} NMR (150.9 MHz, CD2Cl2): δ = 179.2 (dd, 1 J107Ag,C = 185 Hz, 1 J109Ag,C = 216 Hz, CNHC), 148.9 (Cα-pyrr), 143.0 (Cp), 140.0 (Ci), 136.8 (Co out), 136.6 (Cm out), 131.7 (Cβ-pyrr), 123.4 (Co in), 123.1 (Cm in), 122.5 (C 4 ), 122.1 (C 5 ), 118.2 (Cmeso), 54.0 (Ca) 32.5 (Cb), 32.2 (Cc), 27.2 (Cd), 23.Ce), 14.4 ppm (-CH3); 19 F{ 1 H} NMR (376.5 MHz, CD2Cl2): δ = -73.8 ppm (d, 1 JF,P = 711.5 Hz); 31 P{ 1 H} NMR (162 MHz, CD2Cl2): δ = -144.5 ppm (sept, 1 JP,F = 711.5 Hz). UV-Vis (DMSO): λmax (log ε) = 413 (5.75), ~420-430 (sh), 561 (4.46), 603 nm (4.17); HRMS (ESI+): m/z calcd for C160H168N24Zn2Ag4 4+ [M  4PF6] 4+ : 747.2170; found: 747.2181; MS (ESI-): m/z calcd for PF6 ¯: 144.96, found: 144.96.

  F{ 1 H} NMR (376.5 MHz, DMSO-d6): δ = -70.1 ppm (d, 1 JF,P = 711.7 Hz); 31 P{ 1 H} NMR (162 MHz, DMSO-d6): δ = -144.0 ppm (sept, 1 JP,F = 711.4 Hz); UV-Vis (DMSO): λmax (log ε) = 413 (5.51), ~420-430 (sh), 562 (4.51), 603 nm (4.35); HRMS (ESI+): m/z calcd for C144H136N24Zn2Ag4 4+ [M  4PF6] 4+ : 691.1534; found: 691.1505; MS (ESI-): m/z calcd for PF6 ¯: 144.96, found: 144.96.

  F{ 1 H} NMR (376.5 MHz, CD3CN): δ = -73.9 ppm (d, 1 JF,P = 706.5 Hz); 31 P{ 1 H} NMR (162 MHz, CD3CN): δ = -144.7 ppm (sept, 1 JP,F = 706.5 Hz); UV-Vis (DMSO): λmax (log ε) = 422 (5.83), 564 (4.69), 604 nm (4.52) ; HRMS (ESI+): m/z calcd for C152H152N24Zn2Ag4 4+ [M  4PF6] 4+ : 719.1857; found: 719.1875; MS (ESI-): m/z calcd for PF6 ¯: 144.96; found: 144.96.

  C{ 1 H} NMR (150.9 MHz, DMSO-d6): δ = 180.7 (CNHC), 147.2 (Cα-pyrr), 142.9 (Cp), 138.0 (Ci), 135.6 (Co out), 134.9 (Co in), 130.3 (C-pyrr), 123.8 (Cm in), 123.7 (Cm out), 122.3 (C 4 ), 122.1 (C 5 ), 116.6 (Cmeso), 55.0 (Ca) 31.4 (Cb), 31.2 (Cc), 26.0 (Cd), 22.2 (Ce), 14.1 ppm (-CH3); 19 F{ 1 H} NMR (376.5 MHz, DMSO-d6): δ = -71.1 ppm (d, 1 JF,P = 711.5 Hz); 31 P{ 1 H} NMR (162 MHz, DMSO-d6): δ = -144.3 ppm (sept, 1 JP,F = 711.5 Hz); UV-Vis (DMSO): λmax (log ε) = 414 (5.53), 562 (4.43), 604 nm (4.19); HRMS (ESI+): m/z calcd for C160H168N24Zn2Au4 4+ [M  4PF6] 4+ : 836.2786; found: 836.2804; MS (ESI-): m/z calcd for PF6 ¯: 144.96, found: 144.96. Porphyrin cage [Au4(1b)2](PF6)4. The title compound was prepared according to the above general procedure from the porphyrin cage [Ag4(1b)2](PF6)4 (60 mg, 0.0179 mmol, 1.0 eq), [AuCl(tht)] (35 mg, 0.1074 mmol, 6.0 eq), and CH3CN (5 mL). The porphyrin cage [Au4(1c)2](PF6)4 was obtained in 60% yield (40 mg, 0.0107 mmol). 1 H NMR (600 MHz, DMSO-d6): δ = 8.39 (br s, 16H, Hβ-pyrr), 8.38 (dd ,8H, 3 JH,H = 7.8 Hz and 4 JH,H = 2.4 Hz, Ho out), 8.20 (dd ,8H, 3 JH,H = 7.8 Hz and 4 JH,H = 2.4 Hz, Hm out), 8.15 (d, 8H, 3 JH,H = 1.7 Hz, H 5 ), 8.04 (d, 8H, 3 JH,H

3 (

 3 in), 130.3 (C-pyrr), 123.7 (Cm in), 123.6 (Cm out), 122.2 (C 4 ), 122.0 (C 5 ), 116.5 (Cmeso), 51.2 (Ca) 33.Cb), 19.5 (Cc), 13.8 ppm (-CH3); NMR (376.5 MHz, DMSO-d6): δ = -70.1 ppm (d, 1 JF,P = 711.8 Hz); 31 P{ 1 H} NMR (162 MHz, DMSO-d6): δ = -144.0 ppm (sept, 1 JP,F = 711.8 Hz); UV-Vis (DMSO): λmax (log ε) = 414 (5.65), 562 (4.29), 603 nm (3.95); HRMS (ESI+): m/z calcd for C144H136N24Zn2Ag4 4+ [M  4PF6] 4+ : 780.2152; found: 780.2101; MS (ESI-): m/z calcd for PF6 ¯: 144.96, found: 144.96. Porphyrin cage [Au4(1c)2](PF6)4. The title compound was prepared according to the above general procedure from the porphyrin cage [Ag4(1c)2](PF6)4 (55 mg, 0.0149 mmol, 1.0 eq),

5 (

 5 C{ 1 H} NMR (150.9 MHz, CD3CN): δ = 184.8 (CNHC), 148.7 (Cα-pyrr), 142.3 (Ci), 136.9 (Co in), 135.6 (Cp), 135.Co out), 131.1 (Cβ-pyrr), 126.1 (Cm in), 126.1 (Cm out), 123.0 (C4 or C5), 122.9 (C4 or C5), 119.7 (Cmeso), 55.6 (CH2), 52.5 (Ca), 32.2 (Cb), 32.1(Cc), 27.0 (Cd), 23.2 (Ce), 14.5 ppm (CH3); 19 F{ 1 H} NMR (376.5 MHz, CD3CN): δ = 72.6 ppm (d, 1 JF,P = 706.9 Hz); 31 P{ 1 H} NMR (162 MHz, CD3CN): δ = -144.1 ppm (sept, 1 JP,F = 706.9 Hz); UV-Vis (DMSO): λmax (log ε) = 425 (5.54), 562 (4.39), 602 nm (4.23); HRMS (ESI+): m/z calcd for C168H184N24Zn2Au4 4+ [M  4PF6] 4+ : 864.5595; found: 864.5514; MS (ESI-): m/z calcd for PF6 ¯: 144.96, found: 144.96. Porphyrin cage [Au4(1d)2](PF6)4. The title compound was prepared according to the above general procedure from the porphyrin cage [Ag4(1d)2](PF6)4 (36 mg, 0.0104 mmol, 1.0 eq),

  C{ 1 H} NMR (150.9 MHz, CD3CN): δ = 184.1 (CNHC), 149.9 (Cα-pyrr), 144.2 (Cp), 136.4 (Ci), 136.1 (Co in), 134.3 (Co out), 131.9 (Cβ-pyrr), 127.8 (Cm out), 126.4 (Cm in), 123.9 (C 4 ), 122.7 (C 5 ), 120.0 (Cmeso), 55.5 (CH2), 52.1 (Ca), 34.3 (Cb), 20.6 (Cc), 13 14.1 ppm (CH3); 19 F{ 1 H} NMR (376.5 MHz, CD3CN): δ = -73.8 ppm (d, 1 JF,P = 706.6 Hz); 31 P{ 1 H} NMR (162 MHz, CD3CN): δ = -144.7 ppm (sept, 1 JP,F = 706.6 Hz); UV-Vis (DMSO): λmax (log ε) = 424 (5.71), 562 (4.58), 604 nm (4.42); HRMS (ESI+): m/z calcd for C152H152N24Zn2Au4 4+ [M  4PF6] 4+ : 808.2473; found: 808.2485; MS (ESI-): m/z calcd for PF6 ¯: 144.96; found: 144.96.

Scheme 1 .

 1 Scheme 1. Right: structures of porphyrins 1a-4a, 1b-4b, 1c-4c and 1d-4d. Left: synthesis of

Scheme 2 .d) 2 ](PF6) 4 ,

 224 Scheme 2. Synthesis of cofacial porphyrin dimers [M4(1a-d)2](PF6)4 (M = Ag or Au, Hex = n-

Figure 1

 1 Figure 1 Left: stacked partial 1 H NMR (400 MHz) spectra in DMSO-d6 of porphyrin 4a (a), and

  are gathered UV-visible absorption spectra of porphyrin cages [Au4(1a)2](PF6)4 and [Au4(1c)2](PF6)4 with n-hexyl NHC-wingtip groups in the range of wavelengths where the Soret absorption bands of porphyrins can be observed. Almost identical UV-visible spectra were obtained for porphyrin cages [Au4(1b)2](PF6)4 and [Au4(1d)2](PF6)4 with n-butyl NHC-wingtip groups (see the Supporting Information, Figure S128). As it is illustrated in Figure 2, Soret absorption bands of porphyrins belonging to cofacial porphyrin dimers [Au4(1a)2](PF6)4 and [Au4(1c)2](PF6)4 are blue-shifted with respect to the one of ZnTPP in all solvents. On the contrary, Q-bands of porphyrin cages are slightly red-shifted with respect to those of ZnTPP. These spectral evolutions are consistent with the proposed face-to-face orientation between the two porphyrins.

Figure 2 . 4 .

 24 Figure 2. UV-Visible absorption spectra of ZnTPP (dotted black), and porphyrin cages

  absorption bands of the flexible porphyrin cage [Au4(1c)2](PF6)4 is blue-shifted by only max = 4 nm with respect to the one of ZnTPP in polar solvents like CH3CN and DMSO, while this hypsochromic shift is more important for [Au4(1a)2](PF6)4 (max = 18 and 13 nm in CH3CN and DMSO, respectively). This observation suggests that the porphyrin-porphyrin distance of the flexible porphyrin cage [Au4(1c)2](PF6)4 is greater in CH3CN and DMSO ("open" conformation) than in CH2Cl2 ("closed" conformation). It contrasts with the fact that the porphyrin-porphyrin distance of the rigid porphyrin cage [Au4(1a)2](PF6)4 remains relatively the same whatever the nature of the solvent. Conformation of porphyrin cages was also investigated by 1 H NMR spectroscopy. Aromatic regions of 1 H NMR spectra in CD3CN and CD2Cl2 of porphyrin cages [Au4(1ad)2](PF6)4 are represented in Figure 3. 1 H NMR spectra of both porphyrin cages [Au4(1a)2](PF6)4 and [Au4(1b)2](PF6)4 with n-hexyl and n-butyl NHC-wingtip groups, respectively, are very similar in CD3CN (Figures 3a vs.

  3c) and in CD2Cl2 (Figures 3b vs.

  Figures 3a vs.3b for porphyrin cage [Au4(1a)2](PF6)4, and Figures 3c vs.3d for porphyrin cage

  Figures 3a vs.3b for porphyrin cage [Au4(1a)2](PF6)4, and Figures 3c vs.3d for porphyrin cage

  Figures 3a vs.3b for porphyrin cage [Au4(1a)2](PF6)4, and Figures 3c vs.3d for porphyrin cage

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectra (400 MHz) of porphyrin cages [Au4(1a-d)2](PF6)4 in CD3CN and CD2Cl2 ( = signals of meso aryl protons ; = signals of H 4 and H 5 , R = n-hexyl or n-butyl groups).

  3f). The singlets due to -pyrrolic protons are in the same range in CD3CN ( = 7.91 ppm) and in CD2Cl2 ( = 7.97 ppm) showing that porphyrin-porphyrin distances

  Figures 3f vs.3h). It corresponds to the "closed" conformation observed by UV-visible

  Figures 3f vs.3h). It corresponds to the "closed" conformation observed by UV-visible

3 .

 3 Figures 3g vs.3e). All signals of the aromatic region are significantly shifted downfield ( = 8.24

the 1 H

 1 NMR timescale. Both signals of free H2O and encapsulated H2O are independently observed as sharp signals. This indicates that the binding and release of H2O at room temperature is slower than the NMR timescale. Interestingly, we also observed that no (or very weak) sign of D/H exchange was observed upon the addition of D2O to a solution of [Au4(1d)2](PF6)4 in CD3CN. We presume that inner H2O molecules surrounded by the porphyrin cage [Au4(1d)2] 4+ are protected from D/H exchange with outer D2O molecules. This observation is in line with the fact that, in some cases, chemical stability of encapsulated molecular species is remarkably improved within hosts. For example, Stoddart and coworkers recently reported the high chemical stability of a freebase porphyrin encapsulated within an X-shaped cyclophane receptor: protonation, D/H exchange and solvolysis of the encapsulated free-base porphyrin are blocked by the cyclophane host.[START_REF] Liu | Cyclophane-Sustained Ultrastable Porphyrins[END_REF]

Figure 4 . 1 H

 41 Figure 4. 1 H 2D DOSY NMR spectrum (600 MHz) of porphyrin cage [Au4(1d)2](PF6)4 in CD3CN (a). 1 H NMR spectra (400 MHz) of porphyrin cage [Au4(1c)2](PF6)4 in CD3CN (b) and in CD3CN +10% H2O (v/v) (c). 1 H NMR spectrum (400 MHz) of porphyrin cage [Au4(1d)2](PF6)4 in CD3CN

  When comparing the two flexible porphyrin cages [Au4(1c)2](PF6)4 and [Au4(1d)2](PF6)4, it is remarkable to note that the length of the peripheral akyl chains (n-hexyl vs. n-butyl) dramatically modifies the encapsulating properties. The porphyrin cage [Au4(1d)2](PF6)4 with peripheral nbutyl groups has a high affinity for H2O in CD3CN and it was not possible to observe the "closed" conformation (i.e. without H2O molecules): the amount of H2O present in commercially available CD3CN is sufficient and it is not necessary to add more H2O in the NMR sample. On the contrary, the "closed" conformation of the porphyrin cage [Au4(1c)2](PF6)4 with peripheral n-hexyl groups is the only conformation which is observed in CD3CN. The "open" conformation encapsulating H2O molecules is observed when a large amount of H2O is added in the NMR sample. Thus, it is obvious that the length of the peripheral alkyl chains also plays an important role in the encapsulation of H2O.Density Functional Theory (DFT) calculations were performed to optimize the geometries of the rigid and flexible porphyrin cages, namely [Au4(1a/b)2] 4+ and [Au4(1c/d)2] 4+ , in view of comparing their porphyrin-porphyrin distances (see the Supporting Information). Long alkyl chains are replaced by methyl groups to construct model systems and shorten computing time.

Figure 5 .

 5 Figure 5. DFT optimized geometries of empty porphyrin cages [Au4(1a/b)2] 4+ (a), empty

  H NMR spectroscopy by reacting the porphyrin cage [Au4(1a)2](PF6)4 with DABCO. The singlets due to -pyrrolic protons remain in the same range ( = 8.55-8.58 ppm) upon addition of DABCO. This shows that the porphyrin-porphyrin distance is not modified and that there is no encapsulation of DABCO even after several hours. The rigidity of the porphyrin cage [Au4(1a)2](PF6)4 and the short porphyrin-porphyrin distance (~5.5 Å according to DFT calculations) can explain this result.

Figure 6 .

 6 Figure 6. 1 H NMR spectra (400 MHz) in CD3CN of the porphyrin cages [Au4(1a)2](PF6)4 (a) and

Figure 7 .

 7 Figure 7. Schematic representation of the reactions of porphyrin cages with DABCO.

  Addition of DABCO leads to a slight redshift of the Soret absorption band from 421 to 423 nm. UV-Visible titration experiment with 0-3 equivalents of DABCO shows the presence of isobestic points indicating that only two species exist in solution: the starting material [Au4(1d)2] 4+

Figure 8 .

 8 Figure 8. (a) 1 H NMR spectra of porphyrin cage [Au4(1d)2](PF6)4 in CD3CN (top), 4 hours after

4 (1c) ](PF 6 ) 4 in CD 3 CN [Au 4 (1c) 2 ](PF 6 ) 4 in CD 2 Cl 2 [Au 4 (1d) 2 ](PF 6 ) 4 in CD 3 CN [Au 4 (1d) 2 ](PF 6 ) 4 in CD 2 Cl 2

					H -pyrr
		H -pyrr	[
	a.	H -pyrr	e.	H -pyrr
	b.		f.
		H -pyrr		H -pyrr
	c.	H -pyrr	g.	H -pyrr
	d.		h.
	8.8 8.6 8.4 8.0 7.8 7.6	-6.25 -6.3	8.2	8.0	7.8	7.6	7.4	7.2	-6.25 -6.3
		 / ppm				 / ppm

[Au

Au 4 (1a) 2 ](PF 6 ) 4 in CD 3 CN [Au 4 (1a) 2 ](PF 6 ) 4 in CD 2 Cl 2 [Au 4 (1b) 2 ](PF 6 ) 4 in CD 3 CN [Au 4 (1b) 2 ](PF 6 ) 4 in CD 2 Cl 2

	are similar in both solvents. Interestingly, a striking difference was observed when considering the
	porphyrin cage [Au4(1d)2](PF6)4 with n-butyl NHC-wingtip groups in CD3CN (Figure 3g

was calculated using a Benesi-Hildebrand plot. Coordination of a second DABCO was not observed.

ASSOCIATED CONTENT Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/. Additional experimental details and experimental procedures for the preparation of porphyrins 4a-d, NMR spectra, mass spectra and UV-visible absorption spectra for all compounds.

AUTHOR INFORMATION Corresponding Author

Sébastien Richeter -ICGM, Univ Montpellier, CNRS, ENSCM, Montpellier 34293, France; Email: sebastien.richeter@umontpellier.fr

Authors

ACKNOWLEDGMENT

The authors are grateful to the French Agence Nationale de Recherche (ANR-19-CE07-0009-01 and ANR-15-CE29-0018-01), the University of Montpellier and the Centre National de la Recherche Scientifique (CNRS) for financial support.

Entry for the Table of Contents

Porphyrin cages assembled from eight N-heterocyclic carbene-metal (NHC-M) bonds were synthesized and characterized (M = Ag + or Au + ). Their conformation in solution and their encapsulation properties strongly depend on the nature of the linkers between porphyrins and NHCs.