Tunable Porosity through Cooperative Diffusion in a Multicomponent Porous Molecular Crystal

A combination
of different molecular simulation techniques was used to begin to
uncover the mechanism behind the compositional tuning of gas sorption
behavior in a multicomponent porous molecular crystal, CC1·CC3n·CC41–n, where 0 < n < 1. Gas access
to formally occluded voids was found to be allowed through a cooperative
diffusion mechanism that requires the presence of the guest for the
channel to briefly open. Molecular dynamics simulations and dynamic
void analysis suggest two putative diffusion mechanisms. We propose
that the gas diffusion is controlled by the cage vertices that surround
the void, with the slightly smaller and more mobile cyclopentane vertices
in CC4 allowing more facile nitrogen diffusion than the
cyclohexane vertices in CC3. A combination of sorption
simulations, void analysis, and statistical calculations suggests
the diffusion mechanism may rely upon the presence of two CC4 molecules adjacent to the occluded voids.