Redox-Active Macrocycles for Organic Rechargeable Batteries

Organic
rechargeable batteries, composed of redox-active molecules,
are emerging as candidates for the next generation of energy storage
materials because of their large specific capacities, cost effectiveness,
and the abundance of organic precursors, when compared with conventional
lithium-ion batteries. Although redox-active molecules often display
multiple redox states, precise control of a molecule’s redox
potential, leading to a single output voltage in a battery, remains
a fundamental challenge in this popular field of research. By combining
macrocyclic chemistry with density functional theory calculations
(DFT), we have identified a structural motif that more effectively
delocalizes electrons during lithiation events in battery operationsnamely,
through-space electron delocalization in triangular macrocyclic molecules
that exhibit a single well-defined voltage profilecompared
to the discrete multiple voltage plateaus observed for a homologous
macrocyclic dimer and an acyclic derivative of pyromellitic diimide
(PMDI). The triangular macrocycle, incorporating three PMDI units
in close proximity to one another, exhibits a single output voltage
at 2.33 V, compared with two peaks at (i) 2.2 and 1.95–1.60
V for reduction and (ii) 1.60–1.95 and 2.37 V for oxidation
of the acyclic PMDI derivative. By investigating the two cyclic derivatives
with different conformational dispositions of their PMDI units and
the acyclic PMDI derivative, we identified noticeable changes in interactions
between the PMDI units in the two cyclic derivatives under reducing
conditions, as determined by differential pulse voltammetry, solution-state
spectroelectrochemistry, and variable-temperature UV–Vis spectra.
The numbers and relative geometries of the PMDI units are found to
alter the voltage profile of the active materials significantly during
galvanostatic measurements, resulting in a desirable single plateau
for the triangular macrocycle. The present investigation reveals that
understanding and controlling the relative conformational dispositions
of redox-active units in macrocycles are key to achieving high energy
density and long cycle-life electrodes for organic rechargeable batteries.