Stabilizing the Naphthalenediimide Radical within a Tetracationic Cyclophane

Organic
radicals are of importance in developing smart materials
that have paramagnetic and/or near-infrared optical properties. Their
practical applications, however, are limited by the labile nature
of the radicals. Here, we demonstrate that by using a tetracationic
cyclophane, namely, cyclobis­(4,4′-(1,4-phenylene)­bispyridine-p-phenylene) (ExBox4+), to encapsulate
a naphthalene­diimide (NDI) guest, the redox properties of NDI
can be modulated. In organic solvents such as MeCN or DMF, ExBox4+ is able to provide the surrounding Coulombic attraction
to the NDI•– radical anion and therefore
enhance its stability toward oxidation. In water, NDI•– is prone to dimerization, forming its (NDI•–)2 dimer. Under UV-light irradiation, the (NDI•–)2 dimer is observed to disproportionate and yield the
dianionic NDI2–. ExBox4+ is able to encapsulate the NDI•– radical
anion and prevent its dimerization, and as a consequence, the radical
anion is protected from further reduction in a noncovalent manner.
We believe that our strategy of modulating the redox properties of
NDI by either host–guest recognition or mechanical interlocking
can aid and abet the development of radical-based materials, which
could be employed in pursuit of applications in many areas, such as
transporting spin and charges.