Structural Dynamics of a COF-Based Soft Actuator: Molecular Motion and Macroscopic Response

Translating molecular-level dynamics of crystalline frameworks into macroscopic function remains a central challenge in the design of responsive materials. Here, we report that the intrinsic flexibility of COF-300 can be systematically characterized in the bulk powder and then harnessed in a processable composite film to achieve coupled optical and mechanical actuation. In powder form, COF-300 exhibits distinct responses to applied stimuli, including solvent vapors, acid-base environments, and temperature changes. Variable-temperature solid-state NMR reveals the thermally activated coalescence of the phenylene ring signals, and variable-temperature powder X-ray diffraction uncovers colossal anisotropic thermal expansion. Periodic TDA-DFT calculations trace the acid-induced chromism to both geometric distortion and protonation of framework. Embedding COF-300 into a poly(vinylidene fluoride) matrix produces a PVDF@COF film in which solvent vapors drive reversible bending-unbending motion synchronized with a visible color shift, and heating drives the same reversible motion. Exposure to HCl turns the film orange and locks both responses through imine protonation, while thermal deprotonation unlocks them, establishing a reversible chemical switch over framework dynamics. These findings show how molecular flexibility in a crystalline framework can translate into programmable macroscopic actuation, advancing the design of multifunctional solid-state actuators with integrated optical and mechanical output.