Investigating the Copolymerization of Ligands into Metal–Organic Nanotubes Using Small-Angle Neutron Scattering: Implications for Nanostraws

Although
metal–organic nanotubes (MONTs) are the one-dimensional
analogue of metal–organic frameworks (MOFs), in their crystalline
form they exist almost exclusively as aggregates, where their assembly
is driven by intertube π–π stacking. This aggregation
of tubes leads to bulk solids (on the μm scale), producing MONTs
with properties that are reminiscent of anisotropic MOFs. In contrast,
if MONTs could be dispersed as individual tubes or small bundles,
then their unique anisotropic attributes at the nanoscale can be realized,
for instance, in possible applications as “nanowires”
or “nanostraws”. To date, all studies of MONTs contain
a reaction of a single metal with a single ligand, but introducing
a sterically bulkier ligand during the reaction to form the MONTs
may allow for MONT bundles to be broken up or reduced in size by disrupting
the intertube π–π stacking. In this study, MONT
formation reactions were examined with varying ratios of two similar
ligands with slightly different steric bulk to elucidate the co-reaction
of the two ligands with the metal and potentially determine the effects
of steric disruption on the π–π stacking. The chemical
reactions were monitored by small-angle neutron scattering (SANS)
which has proven highly effective for studying the growth of nanoscale
materials from polymers to zeolites. The SANS measurements suggest
that the two ligands co-react into a single MONT structure that is
analogous to the copolymerization of two monomers into a single macromolecular
chain.