Self-Assembly Directed Organization of Fullerene-Bisporphyrins into Supramolecular Donut Structures for Excited State Charge Stabilization

Abstract

Supramolecular structures formed from intermolecular association of repeated units of redox- and photo-active molecular monomers, in solution or in bulk, are of greater significance due to their importance in biology performing various functions making life on earth possible. One such supramolecular organization is ‘photosynthetic antenna-reaction centre’ complex found in green plants and bacteria which is involved in converting sunlight into chemical energy. The core antenna complex in bacteria form concentric rings, LH1 and LH2. LH1 consists of 16 ab protomers with a 68 Å hole in the middle while the LH2 consists of 9 ab protomers. These work in synergy in terms of light capture, transport and generating charge separated states.Several covalent and supramolecular structures containing fullerenes have been described in literature, however, only a few examples where the components are either redox or photochemically amphoteric. This property becomes especially important in light energy harvesting where the presence of an electron donor-acceptor pair is a basic necessity with at least one of them being photo-active to promote photoinduced charge separation leading to the formation of radical ion-pairs. Here, we describe the synthesis, by tether-directed functionalization, of a [60]fullerene e -bisadduct carrying two Zn-porphyrins (see figure inset), its supramolecular organization, and photophysical events. Remarkably, the supramolecular assembly of the present bisporphyrin-C60 forms donuts-shape aggregates (see figure for AFM image), primarily via pi-pi type charge transfer interactions, as demonstrated by means of optical absorption and emission, variable temperature 1H NMR and AFM and supported by theoretical calculations. More importantly, upon photoexcitation, the supramolecular assembly generates long-lived charge separated states of ~1 ms lifetime due to electron/hole delocalization within the supramolecular structure. The unprecedented results of the present study demonstrate the success of supramolecular organization of donor-acceptor pairs in biomimetic light energy harvesting. Figure 1