Sum-Frequency Spectroscopy Provides Insight into the Formation of Covalent Organic Framework by Probing Interfacial Water Structure

Extensive efforts have focused on synthesizing new covalent organic frameworks (COFs) and expanding their applications; however, a molecular-level understanding of their formation processes and the associated COF-water interfacial structure remains limited. Here we report the light-driven synthesis of a two-dimensional COF thin film at the air-water interface, tracked in situ by vibrational sum-frequency generation spectroscopy and supported by MALDI and molecular dynamics simulations. We show that the catalyst accelerates macromolecular growth but does not yield crystallinity, while photoactivation directs reversible imine condensation toward crystalline COF films. Vibrational sum-frequency generation reveals pronounced ordering and strengthened hydrogen bonding of interfacial water adjacent to the growing framework, serving as a real-time probe of the crystallinity development. These findings advance molecular-level insight into COF formation at aqueous interfaces and establish the interfacial water structure as a sensitive probe of crystallinity and reaction pathways.