Supercritical Solvothermal Synthesis of Single-Crystalline Covalent Organic Frameworks and Their Applications

Abstract

Covalent organic frameworks (COFs) are a rapidly evolving class of crystalline porous materials with customizable topologies, tunable functionalities, and a broad scope of applications ranging from catalysis to optoelectronics. Despite substantial progress in framework design, the controlled growth of single-crystalline COFs remains a formidable challenge due to the relatively poor reversibility of covalent bond formation and the difficulty in modulating nucleation and growth kinetics. Traditional solvothermal strategies often yield polycrystalline powders and require prolonged reaction times, limiting access to defect-free structures essential for in-depth structural characterization and advanced functional applications.

In this Account, we present the supercritical solvothermal method as a transformative strategy that simultaneously achieves ultrarapid synthesis and high crystallinity of COFs. By leveraging the unique physicochemical properties of supercritical carbon dioxide (sc-CO₂), notably its low viscosity, high diffusivity, and tunable solvent density, this method overcomes the trade-off between synthesis duration and crystal quality. This approach enables the synthesis of single-crystalline COFs in a few minutes, compared to hours or days in conventional systems. Mechanistically, sc-CO₂ facilitates dynamic mass transport and enhanced molecular mobility, which accelerate nucleation while promoting defect self-healing during framework propagation. Time-resolved characterization combined with template infiltration experiments reveals that the exceptional penetrability of sc-CO₂ enables framework formation even within confined micropores and allows for precise morphological tuning of COFs. Furthermore, we demonstrate that weak intermolecular forces such as interlayer electrostatic repulsions and hydrogen bonding can be amplified under supercritical fluid conditions to modulate crystal morphology, leading to the formation of rare helical COF crystals and enabling structure manipulation via rational side-group engineering.

Single-crystalline COFs exhibit specific properties and potential applications, particularly in nonlinear optics, optoelectronics, and chemical sensing. These crystals display high second harmonic generation efficiencies due to their noncentrosymmetric packing, as well as robust third-order nonlinear responses enabled by chromophore alignment and π-electron delocalization. In optoelectronic applications, dual-state COF phototransistors demonstrate room-temperature responsivity of ∼4.6 × 10¹⁰ A·W^–1 and detectivity of 1.62 × 10¹⁶ Jones, enabling high-contrast neuromorphic imaging under low-light and aqueous conditions. In chemical sensing applications, COF/graphene heterostructures synthesized via this method deliver unprecedented detection limits, down to 10^–19 M for methylglyoxal and 10^–10 M for mercury ions in biofluids, by integrating photochemical gating effects and exploiting ordered charge-transfer interfaces.

Overall, this Account establishes the supercritical solvothermal method as a general and scalable platform for single-crystal COF synthesis. It not only broadens the synthetic scope to previously inaccessible topologies and linkage chemistries but also paves the way for the integration of COFs into high-performance photonic, electronic, and biomedical devices. Future opportunities lie in tuning fluid dynamics for dimensionality control, coupling with nucleation or competitive reaction strategies to access new architectures, and extending the kinetic framework to guide crystallization in other polymeric and supramolecular materials.