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 towards 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, as a consequence of which, the radical anion is protected from further reduction in a noncovalent manner. We believe th...