Stable hydrazone-linked chiral covalent organic frameworks: Synthesis, modification, and chiral signal inversion from monomers

The designed synthesis of chiral covalent organic frameworks (COFs) featuring intriguing properties is fairly scant and remains a daunting synthetic challenge. Here we develop a de novo synthesis of an enantiomeric pair of 2D hydroxyl-functionalized hydrazone-linked chiral COFs, ( S )- and ( R )-HthBta-OH COFs, using enantiopure 2,5-bis(2-hydroxypropoxy)terephthalohydrazide (Hth) as monomers. The formation process of hydroxyl-functionalized chiral COFs was monitored using rigorous time-dependent PXRD, vibrational circular dichroism (VCD), and electronic circular dichroism (ECD) studies. Remarkably, VCD spectra indicated a unique chiral signal inversion from the positive Cotton effect of ( S )-Hth monomer to the negative Cotton effect of ( S )-HthBta-OH COF, which has never been reported in chiral COFs. Moreover, two unprecedented carboxyl-functionalized chiral COFs, ( S )- and ( R )-HthBta-COOH, were constructed by a post-synthetic modification of the corresponding hydroxyl chiral COFs with succinic anhydride. Notably, carboxyl-functionalized COFs retained homochirality and crystallinity without linker racemization and structural collapse after the chemical modification due to the chemically robust nature of pristine hydrazone-linked chiral COFs.