Impact of Shape Persistence on the Porosity of Molecular Cages

Porous materials provide a plethora
of technologically important
applications that encompass molecular separations, catalysis, and
adsorption. The majority of research in this field involves network
solids constructed from multitopic constituents that, when assembled
either covalently or ionically, afford macromolecular arrangements
with micro- or meso-porous apertures. Recently, porous solids fabricated
from discrete organic cages have garnered much interest due to their
ease of handling and solution processability. Although this class
of materials is a promising alternative to network solids, fundamental
studies are still required to elucidate critical structure–function
relationships that govern microporosity. Here, we report a systematic
investigation of the effects of building block shape-persistence on
the porosity of molecular cages. Alkyne metathesis and edge-specific
postsynthetic modifications afforded three organic cages with alkynyl,
alkenyl, and alkyl edges, respectively. Nitrogen adsorption experiments
conducted on rapidly crystallized and slowly crystallized solids illustrated
a general trend in porosity: alkynyl > alkenyl > alkyl. To understand
the molecular-scale origin of this trend, we investigated the short
and long time scale molecular motions of the molecular cages using
ab initio molecular dynamics (AIMD) and classical molecular dynamics
(MD) simulations. Our combined experimental and computational results
demonstrate that the microporosity of molecular cages directly correlates
with shape persistence. These findings discern fundamental molecular
requirements for rationally designing porous molecular solids.