Exploration of the Solid-State Sorption Properties of Shape-Persistent Macrocyclic Nanocarbons as Bulk Materials and Small Aggregates

Porous
molecular materials combine benefits such as convenient
processability and the possibility for atom-precise structural fine-tuning
which makes them remarkable candidates for specialty applications
in the areas of gas separation, catalysis, and sensing. In order to
realize the full potential of these materials and guide future molecular
design, knowledge of the transition from molecular properties into
materials behavior is essential. In this work, the class of compounds
termed cycloparaphenylenes (CPPs)shape-persistent macrocycles
with built-in cavities and radially oriented π-systemswas
selected as a conceptually simple class of intrinsically porous nanocarbons
to serve as a platform for studying the transition from analyte sorption
properties of small aggregates to those of bulk materials. In our
detailed investigation, two series of CPPs were probed: previously
reported hoop-shaped [n]­CPPs and a novel family of
all-phenylene figure-8 shaped (lemniscal) bismacrocycles, termed spiro­[n,n]­CPPs. A series of nanocarbons with
different macrocycle sizes and heteroatom content have been prepared
by atom-precise organic synthetic methods, and their structural, photophysical,
and electronic attributes were disclosed. Detailed experimental studies
(X-ray crystallography, gas sorption, and quartz-crystal microbalance
measurements) and quantum chemical calculations provided ample evidence
for the importance of the solid-state arrangement on the porosity
and analyte uptake ability of intrinsically porous molecular nanocarbons.
We demonstrate that this molecular design principle, i.e., incorporation
of sterically demanding spiro junctions into the backbone of nanohoops,
enables the manipulation of solid-state morphology without significantly
changing the nature and size of the macrocyclic cavities. As a result,
the novel spiro­[n,n]­CPPs showed
a remarkable performance as high affinity material for vapor analyte
sensing.