HOFs

Hydrogen‑bonded organic frameworks (HOFs) are a class of crystalline porous materials formed through reversible, directional hydrogen bonding between organic molecules. Unlike metal–organic frameworks (MOFs) that rely on metal nodes, HOFs depend solely on non‑metallic linkers, enabling metal‑free synthesis and often lower toxicity. The hydrogen bonds provide structural stability while allowing for dynamic linkages that can be leveraged for stimuli‑responsive behavior.

Typical synthesis of HOFs involves solvothermal or slurry methods, where organic building blocks are dissolved in solvents such as dimethylformamide, ethanol, or water, then slowly cooled or precipitated. The choice of functional groups (e.g., carboxylates, amides, imidazolates) dictates the geometry of the hydrogen bonds, which in turn controls pore size and topology. Post‑synthetic modification is possible, allowing for tuning of surface chemistry.

HOFs exhibit surface areas ranging from a few hundred to over 2000 m² g⁻¹, with pore sizes adjustable from microporous (<2 nm) to mesoporous (>2 nm). Their lightweight, often fully organic composition confers high thermal stability (typically 150–300 °C) and chemical resilience to acids and bases, though they can be less mechanically robust than MOFs. Recent advances include high‑yield diffraction‑grade crystals, covalent HOFs that combine covalent bonds with hydrogen‑bond networks, and computational design of HOFs with targeted functionalities.

Applications under investigation encompass gas storage and separation (e.g., CO₂ capture), catalysis (as host matrix for metal nanoparticles), drug delivery (high payloads due to large pores), and environmental remediation (adsorption of pollutants). Although still less mature than MOFs, HOFs offer a promising, metal‑free alternative for sustainable porous materials.