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submitted on 20.07.2019, 12:12 and posted on 22.07.2019, 17:11by Shengxian Cheng, Xiaoxia Ma,, Yonghe He, Jun He, Matthias Zeller, Zhengtao Xu
We report a curious porous molecular
crystal that is devoid of the common traits of related systems. Namely, the molecule
does not rely on directional hydrogen bonds to enforce open packing; and it
offers neither large concave faces (i.e., high internal free volume) to
frustrate close packing, nor any inherently built-in cavity like in the class
of organic cages. Instead, the permanent porosity (as unveiled by the X-ray
crystal structure and CO2 sorption studies) arises from the strong
push-pull units built into a Sierpinski-like molecule that features four
symmetrically backfolded (SBF) side
arms. Each side arm consists of the 1,1,4,4-tetracyanobuta-1,3-diene acceptor
(TCBD) coupled with the dimethylaminophenyl donor, which is conveniently installed
by a cycloaddition-retroelectrocyclization (CA-RE) reaction. Unlike the poor/fragile
crystalline order of many porous molecular solids, the molecule here readily
crystallizes and the crystalline phase can be easily deposited into thin films
from solutions. Moreover, both the bulk sample and thin film exhibit excellent
thermal stability with the porous crystalline order maintained even at 200 °C. The intermolecular forces underlying this robust porous molecular crystal likely
include the strong dipole interactions and the multiple C···N and C···O short
contacts afforded by the strongly donating and accepting groups integrated
within the rigid molecular scaffold.