From Binary to Ternary Hydrogen-Bonded Solids with Anisotropic Thermal Expansion

Thermal expansion (TE) describes the behavior of a solid material as it responds to a change in temperature. The behavior is affected by the components of the material, and the strength of the bonds used to construct it. For materials that are held together by strong covalent bonds, TE is often reduced as the dimensionality increases. For example, diamond, which is covalently bonded in three dimensions, undergoes less expansion than fullerene, which is a discrete molecule. In molecular solid materials assembled through noncovalent bonds, TE is generally larger because the bonds are weaker. Here, we demonstrate synthesis of a series of hydrogen-bonded solids with differing dimensionality of the hydrogen-bonding network. Notably, the same molecular building blocks were used to construct all solids and dimensionality differences were achieved by modifying the stoichiometric ratio of the starting materials. All solids exhibit anisotropic TE behavior, and systematically increasing the dimensionality affords corresponding control over TE. Moreover, based on unexpected hydrogen-bonding behavior in one solid, a shape-size mimicry approach was successfully used to prepare a ternary molecular solid. Lastly, one of the two-dimensional (2D) hydrogen-bonded networks described here exhibits TE behavior that is similar to graphite and black phosphorus, classic 2D covalent-bond-based materials. The strategy of using identical molecular building blocks to construct multicomponent solids with differing dimensionalities is uncommon and offers a way to control TE in solid-state materials.