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 NDI^2-. 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.