Supramolecular Proton Conductors Self-Assembled by Organic Cages

Proton conduction
is vital for living systems to execute various
physiological activities. The understanding of its mechanism is also
essential for the development of state-of-the-art applications, including
fuel-cell technology. We herein present a bottom-up strategy, that
is, the self-assembly of Cage-1 and -2 with
an identical chemical composition but distinct structural features
to provide two different supramolecular conductors that are ideal
for the mechanistic study. Cage-1 with a larger cavity
size and more H-bonding anchors self-assembled into a crystalline
phase with more proton hopping pathways formed by H-bonding networks,
where the proton conduction proceeded via the Grotthuss mechanism.
Small cavity-sized Cage-2 with less H-bonding anchors
formed the crystalline phase with loose channels filled with discrete
H-bonding clusters, therefore allowing for the translational diffusion
of protons, that is, vehicle mechanism. As a result, the former exhibited
a proton conductivity of 1.59 × 10–4 S/cm at
303 K under a relative humidity of 48%, approximately 200-fold higher
compared to that of the latter. Ab initio molecular dynamics simulations
revealed distinct H-bonding dynamics in Cage-1 and -2, which provided further insights into potential proton
diffusion mechanisms. This work therefore provides valuable guidelines
for the rational design and search of novel proton-conducting materials.