Controlling COF stacking architecture with light at soft interfaces.

Controlling stacking architecture in covalent organic framework (COF) films remains a fundamental challenge in interfacial polymerization because interlayer alignment directly governs π-π coupling and band structure. While dynamic covalent chemistry enables structural error correction, externally addressable stimuli capable of biasing stacking architecture during growth-without altering framework composition-remain scarcely explored. Here, we demonstrate that photonic activation functions as an independent process parameter that directs stacking architecture during surfactant-monolayer-assisted interfacial synthesis (SMAIS) at the air-water interface. Using a porphyrin-based COF as a model system, wavelength-selective illumination (420-640 nm) induces a reproducible hypsochromic shift of the Q-band relative to dark-grown films and increases the optical band gap by 110 meV. Spectroscopy of films collected at defined growth times reveals earlier emergence of COF-characteristic signatures under illumination. Quantum chemical calculations disentangle the electronic contributions of linker functionalization, lateral network extension, and interlayer coupling, identifying the Q-band as a sensitive probe of stacking geometry. The calculations predict a weaker bathochromic response for AB stacking compared to slipped AA' stacking. Consistent with this prediction, transmission electron microscopy reveals a light-dependent lattice contraction from 2.50 nm to 1.76 nm, supporting the assignment of preferential AB-type stacking under illumination. Atomic force microscopy further demonstrates wavelength- and time-dependent modulation of vertical film build-up under illumination, yielding thinner multilayer films and revealing a shift in the balance between interfacial growth and structural reorganization. Together, these results establish light as a controllable stimulus for directing stacking registry and tuning interlayer electronic coupling during dynamic covalent growth at soft interfaces.