Photoinduced Emissive Naphthalenediimide Radical Anion in the Confinement of Cucurbituril Nanocavity; in situ Generation of Gold Nanoparticles

Abstract Supramolecularly assembled building blocks, especially those based on π stacked organic or inorganic redox active materials have found encouraging applications in recent years due to their electron-accepting nature and easy structural manipulations. In this regard, by employing various spectroscopic, thermodynamic and imaging measurements we establish the generation, stabilization and in situ reaction of a novel supramolecularly supported emissive radical anions through host–guest interaction of N, Ń-bis[2-(2-aminoethyl)benzimidazolyl] naphthalene diimide (BzNDI) with Cucubit[n]uril (CB7 and CB8) macrocycles in aqueous medium. While the CB7 and CB8 hosts revealed distinct interactions with the BzNDI, the CB8 complexation displayed a novel excimer emission band in the 523 nm region, which originated from dimeric inclusion of BzNDI dyes in to the larger CB8 cavity. These CB7/CB8 stabilized BzNDI complexes undergo photoinduced electron transfer process to generate respective radical anions. These transient radical anions held in the cucurbituril nanocavities displayed characteristic emission spectra in the visible region and longer lived (τav ~ 6.5 ns) radical excited states and the radical anions are found to be reversible to their parent complex under ambient condition in ~1–2 h. This dynamic stability window of the radical anions has been used for the in situ reduction of Au3+ to generate stable AuNPs. It is noteworthy that this chemistry was not observed with BzNDI in the absence of CB7/CB8 host interaction. We foresee that the demonstrated stimuli responsive supramolecular approach has the potential for the controlled assembly of smart materials, radical polymerization, molecular electronics and other gated radical induced reactions.