Crystal Engineering of Covalent Organic Frameworks Based on Hydrazine and Hydroxy-1,3,5-Triformylbenzenes

Covalent organic frameworks (COFs) were prepared through imine condensation reaction of hydrazine hydrate with hydroxy-1,3,5-triformylbenzenes, containing a varying number of hydroxyl groups, affording the microporous materials called RIO-11, RIO-12, and RIO-13. The role of intramolecular hydrogen bonding formation (conformational locking effects) in the crystallinity of the resulting COFs was evaluated. The results indicate that the increase of the number of conformational locks increases the symmetry of moieties during nucleation and crystal growth, resulting in less defects in the product structure. The use of aniline as modulator, with in situ formation of an intermediate imine, was also evaluated and proved to be beneficial in the case where the number of conformational locks is insufficient to afford high crystallinity. The use of the modulator for RIO-11 resulted in greater crystallinity and a 5.3-fold increase of its pristine BET surface area. Narrower monomodal pore size distributions, with cylin...