Superatom Regiochemistry Dictates the Assembly and Surface Reactivity of a Two-Dimensional Material

The area of two-dimensional (2D) materials research would benefit greatly from the development of synthetically tunable van der Waals (vdW) materials. While the bottom-up synthesis of 2D frameworks from nanoscale building blocks holds great promise in this quest, there are many remaining hurdles, including the design of building blocks that reliably produce 2D lattices and the growth of macroscopic crystals that can be exfoliated to produce 2D materials. In a manuscript published in JACS, we reported the regioselective synthesis of the cluster [trans-Co₆Se₈(CN)₄(CO)₂]^3–/4–, a “superatomic” building block designed to polymerize and assemble into a 2D cyanometalate lattice whose surfaces are chemically addressable. The resulting vdW material, [Co(py)₄]₂[trans-Co₆Se₈(CN)₄(CO)₂], grows as bulk single crystals that can be mechanically exfoliated to produce flakes as thin as bilayers, with photolabile CO ligands on the exfoliated surface. As a proof of concept, we show that these surface CO ligands can be replaced by 4-isocyanoazobenzene under blue light irradiation. This work demonstrates that the bottom-up assembly of layered vdW materials from superatoms is a promising and versatile approach to create 2D materials with tunable physical and chemical properties.

Controlling Ligand Coordination Spheres and Cluster Fusion in Superatoms

In a manuscript published in JACS, we have shown that reaction pathways from a single superatom motif can be controlled through subtle electronic modification of the outer ligand spheres. Chevrel-type [Co₆Se₈L₆] (L = PR₃, CO) superatoms are used to form carbene terminated clusters, the reactivity of which can be influenced through the electronic effects of the surrounding ligands. This carbene provides new routes for ligand substitution chemistry, which was used to selectively install cyanide or pyridine ligands which were previously inaccessible in these cobalt-based clusters. The surrounding ligands also impact the ability of this carbene to create larger fused clusters of the type [Co₁₂Se₁₆L₁₀], providing underlying information for cluster fusion mechanisms. We used this information to develop methods of creating dimeric clusters with functionalized surface ligands with site specificity, putting new ligands in specific positions on this anisotropic core. Finally, adjusting the carbene intermediates can also be used to perturb the geometry of the [Co₆Se₈] core itself, as we demonstrated with a multicarbene adduct that displays a substantially anisotropic core.

Highly twisted azobenzene ligand causes crystals to continuously roll in sunlight

Direct conversion of solar energy to mechanical work promises higher efficiency than multistep processes, adding a key tool to the arsenal of energy solutions necessary for our global future. The ideal photomechanical material would convert sunlight into mechanical motion rapidly, without attrition, and proportionally to the stimulus. In a manuscript published in JACS, we describe crystals of a tetrahedral isocyanoazobenzene copper complex that roll continuously when irradiated with broad spectrum white light, including sunlight. The rolling results from sequential bending and straightening of the crystal due to blue light-driven trans-to-cis isomerization of a highly twisted azobenzene ligand. These findings introduce geometrically constrained crystal packing as a new strategy for manipulating the electronic properties of chromophores. Furthermore, the continuous, solar-driven motion of the crystals demonstrates that it is possible to achieve direct conversion of solar energy to continuous physical motion using easily-accessed molecular systems.

A few-layer covalent network of fullerenes

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp³- and sp²-hybridized carbon atoms, respectively. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic crystalline allotropes. In a manuscript published in Nature, we introduce graphullerene, a new allotrope that bridges the gulf between molecular carbon structures and extended carbon materials, combining three- and four-coordinate carbon atoms. Its constituent C₆₀ fullerene subunits arrange hexagonally in a covalently interconnected molecular plane. We present for the first time, charge-neutral, purely carbon-based macroscopic crystals, that are large enough for mechanical exfoliation and materials testing. The synthesis entails two steps: the growth of single crystals of two-dimensional, polymeric Mg₄C₆₀ by chemical vapor transport; and the subsequent removal of magnesium with dilute acid. Using this method, we can produce exfoliated flakes with clean van der Waals interfaces, a critical requirement for the creation of heterostructures and optoelectronic devices. As a proof of concept, we explore the optoelectronic and thermal conductivity properties of this new layered material. We find that the thermal conductivity of this remarkable new material is significantly higher than that of molecular C₆₀, a consequence of the in-plane covalent bonding. Furthermore, imaging using transmission electron microscopy and near-field nano-photoluminescence spectroscopy of few-layer flakes reveals the existence of moiré-like superlattices. The ability to chemically synthesize new extended carbon structures by polymerization of superatomic molecular precursors charts a clear path to the systematic design of new materials for the construction of 2D heterostructures with tunable optoelectronic properties.

Last Modified: 01/02/2024
Modified by: Xavier Roy