Much research has been devoted to molybdenum octahedral clusters Mo 6 since the discovery of the A x Mo 6 Y 8 solidstate series (Y = S, Se, Te) in the early 1970s.[1] Indeed, their interesting physical properties and potential applicationse.g., superconductivity at high critical field, thermoelectric, catalysis, or redox intercalation processes -have stimulated the research of many groups. [2] (Fig. 1). The physical properties of Mo 6 solid-state compounds are related to the number of electrons available for metal-metal bonding within the cluster (valence electron count, VEC) and to the strength of interaction between the units. Mo-centered electrons are located on twelve metal-metal bonding molecular orbitals of the molecular orbital diagram. Their full occupation leads to a closed-shell configuration with a VEC of 24.[ [8,9] that can be used for the formation and organization of supramolecular assemblies as well as hybrid materials. Hybrids can be synthesized either by the grafting of functional donor ligands in apical position or through the association of anionic cluster units with organic or organometallic cations by cation metathesis or electrochemical techniques.[10]The large emission region of the [Mo 6 X 14 ] 2-anion in the red and near infrared (580-900 nm) is particularly interesting for biotechnology applications as it is selectively transmitted through tissues owing to the relatively low absorption at these wavelengths.[11] Anionic Mo 6 cluster units are usually associated with alkali counter cations within inorganic solids. Indeed, the use of inorganic cluster compounds as luminescent dyes, for instance in bio-imaging strategies, presupposes that both clusters and counter cations are embedded in an inert matrix in order to avoid ionic diffusion, oxidization of the cluster, or apical ligand exchanges in aqueous media, which will precipitate the cluster as a hydroxo species.

Octahedral metal atom clusters in which metallic atoms are held together by metal-metal bonds are commonly found in solid-state compounds prepared by high-temperature synthesis.[1] The metallic octahedron is surrounded by eight facecapping and six terminal ligands to form a [M 6 units (X = halogen) that exhibit, either in the liquid or solid state, specific electronic, magnetic, and photophysical properties related to the number of metallic electrons available for metal-metal bonds. [3] In particular, they are highly emissive in the red-NIR region, have photoluminescence quantum yields of up to 0.23, [3d] display long excited-state lifetimes, [3d, 4] and undergo facile ground-and excited-state electron transfer by electrogenerated luminescence.[5] Owing to the stronger covalent nature of the M À Q i bond relative to the M À X a one, halogen apical atoms can be replaced by inorganic or organic ligands without any alteration of the (M 6 Q i 8 ) m+ core, leading to functional building blocks usable for the design of supramolecular architectures, polymeric frameworks, or nanomaterials with unique properties.[6] Although many examples of hexasubstituted [M 6 xÀ units (L = organic ligand) have been reported, [6d, 7] their integration in macroscopic devices by a bottom-up approach remains a challenge. This task requires systems with self-organization abilities on the one hand and fluidity on the other hand, to correct automatically the positioning errors that can occur during the assembly process. Metal-containing liquid crystals (metallomesogens) are the typical examples in which the unique properties of anisotropic fluids are combined with the specific properties of metals (e.g. geometry of coordination, optic, electronic, magnetic).[8] However, mesomorphic materials containing covalent metal-metal-bonded entities are rare, and all examples described up to now, since the pioneer work of Marchon and co-workers, [9] are based on dinuclear metalmetal-bonded species.[10] The association of mesomorphism with the peculiar properties of metallic clusters should lead to clustomesogens that offer great potential in the design of new electricity-to-light energy conversion systems, optically based sensors, and displays.In the scope of our work dedicated to transition-metalcluster based multifunctional materials, [11] we report herein the elaboration and characterization of liquid-crystalline materials based on a Mo 6 cluster. The synthesis is straightforward and consists of the one-step reaction of [Mo 6 Br 8 F 6 ] 2À units with carboxylic acid derivatives (Scheme 1), which results in the in situ exchange of apical F À by carboxylate anion along with the formation of HF.Owing to the bulkiness of the cluster unit and to its octahedral coordination, [12] we used a strategy based on the