posted on 2020-12-24, 18:35authored byChen Yuan, Shiguo Fu, Kuiwei Yang, Bang Hou, Yan Liu, Jianwen Jiang, Yong Cui
While crystalline covalent organic
frameworks (COFs) linked by
C–C bonds are highly desired in synthetic chemistry, it remains
a formidable challenge to synthesize. Efforts to generate C–C
single bonds in COFs via de novo synthesis usually afford amorphous
structures rather than crystalline phases. We demonstrate here that
C–C single bond-based COFs can be prepared by direct reduction
of CC bond-linked frameworks via crystal-to-crystal transformation.
By Knoevenagel polycondensation of chiral tetrabenzaldehyde of dibinaphthyl-22-crown-6
with 1,4-phenylenediacetonitrile or 4,4′-biphenyldiacetonitrile,
two olefin-linked chiral COFs with 2D layered tetragonal structure
are prepared. Reduction of olefin linkages of the as-prepared CCOFs
produces two C–C single bond linked frameworks, which retains
high crystallinity and porosity as well as high chemical stability
in both strong acids and bases. The quantitative reduction is confirmed
by Fourier transform infrared and cross-polarization magic angle spinning 13C NMR spectroscopy. Compared to the pristine structures,
the reduced CCOFs display blue-shifted emission with enhanced quantum
yields and fluorescence lifetimes, while the parent CCOFs exhibit
higher enantioselectivity than the reduced analogs when be used as
fluorescent sensors to detect chiral amino alcohols via supramolecular
interactions with the built-in crown ether moieties. This work provides
an attractive strategy for making chemically stable functionalized
COFs with new linkages that are otherwise hard to produce.