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 2 D hydroxyl-functionalized hydrazone-linked chiral COFs,(S)-and(R)-HthBta-OH COFs,using enantiopure 2,5-bis(2-hydroxypropoxy)terephthalohydrazide(Hth) as monomers.The fo rmation 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.

Third-order nonlinear optical adjusting behavior in azobenzene metal complexes

The third-order nonlinear optical(NLO)materials with stimuli-responsive properties have received extraordinary attention due to their controllable photophysical properties.In this work,two attractive metal complexes third-order NLO switches,which are far superior to congeneric optical switches in terms of their performance conversion,versatility,and fast response,were successfully designed and synthesized.The test of their third-order NLO properties proves that the metal complexes exhibit reverse saturable absorption and self-defocusing refraction.After light irradiation,the third-order NLO behavior turns quickly into self-focusing refraction.The relation between the molecular structures and the third-order NLO properties was investigated via1H nuclear magnetic resonance and ultraviolet-visible absorption.The results show that the metal ions have a significant influence on the NLO behavior and reveal the origin of third-order NLO properties via Z-scan determinations,pump-probe technology,and density functional theory calculations.These metal complexes can be used as third-order NLO switches with excellent fatigue resistance and broaden the application range of third-order NLO materials with adjustable performances.