Abstract
While the chemical vapor deposition (CVD) process promises high-quality films of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), strategic approaches are limited. Here we report a new strategic approach to synthesizing imine-linked COF films through acid-amine salt films by CVD. Acids are known to catalyze imine condensation between amines and aldehydes in the solution phase and have been believed to do the same in the vapor phase reaction. Upon the attempts to synthesize COF-LZU1 thin films using p-phenylenediamine, 1,3,5-triformylbenzene, by CVD, highly crystalline and smooth COF-LZU1 films are obtained only in the presence of p-toluenesulfonic acid, proving the conventional role of acid during vapor phase imine condensation. The COF-LZU1 film has a mixed orientation according to the structure analysis by grazing-incidence wide-angle X-ray scattering. In contrast, amorphous films with a face-on orientation were obtained in the absence of an acid. The role of acid in modulating structure is confirmed from the reaction using a different acid, benzenesulfonic acid, which results in the COF-LZU1 film having an edge-on orientation. The control experiments using benzoic acid and succinic acid that failed to make either acid-amine salt or COF films explain that the acid-amine-salt film formation is crucial to the formation of imine-linked COF films by CVD. Our findings not only explain the limited kinds of active acids for imine condensation in the vapor phase but also widen the field for the facile synthesis of highly crystalline COF films by CVD and the structure control of COF films through the combination of an acid-amine salt.