Laser Ultrafast In Situ-Integrated COF Crystals.

Crystallization of covalent organic frameworks (COFs) is fundamentally limited by the high activation energies and slow bond-exchange dynamics of imine linkages, rendering solvothermal synthesis intrinsically sluggish (hours to days) and difficult to control. Here, we report a nonequilibrium crystallization strategy in which a 473 nm continuous-wave laser creates a confined photothermal microreactor at the solid-liquid interface. Gaussian focusing generates microsecond local hotspots and steep temperature gradients, producing instantaneous supersaturation in the focal region and driving dynamic covalent exchange into an ultrafast, far-from-equilibrium regime. As a result, COF-300 nucleates and grows in situ within seconds, without catalysts and with submicrometer spatial confinement. Laser power and exposure provide real-time, deterministic control over nucleation and crystal growth, enabling the on-demand, in situ integration of COF microcrystals. Moreover, the highly localized photothermal pathway allows direct laser writing of ordered COF microstructures without transfer or postprocessing. This work establishes an accessible nonequilibrium synthesis route for dynamic covalent materials and offers a generalizable strategy for rapid, programmable crystallization toward integrated photonic architectures.