Switching on and off Interlayer Correlations and Porosity in 2D Covalent Organic Frameworks

Two-dimensional
covalent organic frameworks (2D COFs) attract great
interest owing to their well-defined pore structure, thermal stability,
high surface area, and permanent porosity. In combination with a tunable
chemical pore environment, COFs are intriguing candidates for molecular
sieving based on selective host–guest interactions. Herein,
we report on 2D COF structures capable of reversibly switching between
a highly correlated crystalline, porous and a poorly correlated, nonporous
state by exposure to external stimuli. To identify COF structures
with such dynamic response, we systematically studied the structural
properties of a family of two-dimensional imine COFs comprising tris­(4-aminophenyl)­benzene
(TAPB) and a variety of dialdehyde linear building blocks including
terephthalaldehyde (TA) and dialdehydes of thienothiophene (TT), benzodithiophene
(BDT), dimethoxybenzodithiophene (BDT-OMe), diethoxybenzodithiophene
(BDT-OEt), dipropoxybenzodithiophene (BDT-OPr), and pyrene (Pyrene-2,7).
TAPB-COFs consisting of linear building blocks with enlarged π-systems
or alkoxy functionalities showed significant stability toward exposure
to external stimuli such as solvents or solvent vapors. In contrast,
TAPB-COFs containing unsubstituted linear building blocks instantly
responded to exposure to these external stimuli by a drastic reduction
in COF layer correlation, long-range order, and porosity. To reverse
the process we developed an activation procedure in supercritical
carbon dioxide (scCO2) as a highly efficient means to revert
fragile nonporous and amorphous COF polymers into highly crystalline
and open porous frameworks. Strikingly, the framework structure of
TAPB-COFs responds dynamically to such chemical stimuli, demonstrating
that their porosity and crystallinity can be reversibly controlled
by alternating steps of solvent stimuli and scCO2 activation.