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) (ExBox(4+)), to encapsulate a naphthalenediimide (NDI) guest, the redox properties of NDI can be modulated. In organic solvents such as MeCN or DMF, ExBox' is able to provide the surrounding Coulombic attraction to the NDI center dot- radical anion and therefore enhance its stability toward oxidation. In water, NDI center dot- is prone to dimerization, forming its (NDI center dot-)(2 )dimer. Under UV-light irradiation, the (NDI center dot-)(2) dimer is observed to disproportionate and yield the dianionic NDI2-. ExBox(4+) is able to encapsulate the NDI center dot- 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.