Topological effects of a rigid chiral spacer on the electronic interactions in donor-acceptor ensembles

Two triads (donor-spacer-acceptor), exTTF-BN-C-60 (6) and ZnP-BN-C-60 (7), in which electron donors (i.e., exTTF or ZnP) are covalently linked to C-60 through a chiral binaphthyl bridge (BN), have been prepared in a multistep synthetic procedure starting from a highly soluble enantiomerically pure binaphthyl building block (1). Unlike other oligomeric bridges, with binaphthyl bridges, the conjugation between the donor and the acceptor units is broken and geometric conformational changes are facilitated. Consequently, distances and electronic interactions between the donor and C-60 are drastically changed. Both donor-spacer-acceptor (D-s-A) systems (i.e., 6 and 7) exhibit redox processes that correspond to all three constituent electroactive units, namely, donor, BN, and C-60. Appreciable differences were, however, observed when comparing triad 6, in which no significant exTTF-C-60, interactions were noted, with D-s-A 7, whose geometry favors donor pi-pi interactions that result in ZnP-C-60 electronic communication. This through-space interaction is, for example, reflected in the redox potentials. Excited-state studies, carried out by fluorescence and transient absorption spectroscopy, also support through-space rather than through-bond interactions. Although both triads form the corresponding radical-ion pair, that is, exTrF(.+)-BN-C-60(.-), and ZnP.+-BN-C-60(.-), dramatic differences were found in their lifetimes: 165 mu s and 730 ns, respectively.