Engineering Colossal Anisotropic Thermal Expansion into Organic Materials through Dimensionality Control

Thermal expansion (TE) behavior in solid-state materials is influenced by both molecular and supramolecular structure. For solid-state materials assembled through covalent bonds, such as carbon allotropes, solids with higher dimensionality (i.e., diamond) exhibit less TE than solids with lower dimensionality (e.g., fullerene, graphite). Thus, as the dimensionality of the solid increases, the TE decreases. However, an analogous and systematic variation of the dimensionality in solid-state materials assembled through noncovalent bonds with a correlation to TE has not been studied. Here, we designed a series of solids based on dimensional hierarchy to afford materials with zero-dimensional (0D), 1D, and 2D hydrogen-bonded structures. The 2D materials are structural analogues of graphite and covalent-organic frameworks, and we demonstrate that these 2D solids exhibit unique biaxial zero TE with anisotropic and colossal TE along the π-stacked direction (α ∼ 200 MK^–1). The overall behavior in the 2D hydrogen-bonded solids is similar to 2D covalent-bonded solids; however, the coefficient of TE along the π-stacked direction for these hydrogen-bonded solids is an order of magnitude higher than in 2D graphite or phosphorus allotropes. The hierarchal materials design strategy and correlation to TE properties described herein can be broadly applied to the design and synthesis of new solid-state materials sustained by covalent or noncovalent bonds with control over solid-state behaviors.