Donor-Acceptor Conjugates of Lanthanum Endohedral Metallofullerene and π-Extended Tetrathiafulvalene

Stable donor acceptor conjugates (2, 3) involving an endohedral metallofullerene, La-2 @ l(h)-C-80, and pi-extended tetrathiafulvalene (exTTF) have been synthesized by highly regioselective 1,3-dipolar cycloadditions of exTTF-containing azomethine ylides to the endofullerene, yielding exclusively [5,6] metallofulleropyrrolidines with C-1 symmetry in high yields (68-77%). The cyclic voltammograms (CVs) of the conjugates reveal the redox active character of the system due to the presence of both donor and acceptor groups, that is, exTTF and La-2 @ l(h)-C-80, respectively. Furthermore, the electrochemically reversible character of the endofullerene confirms the presence of the [5,6] adduct. Despite the relatively close proximity between the exTTF and the endohedral metallofullerene (EMF), only a weak electronic interaction was observed in the ground state, as evidenced by absorption spectroscopy and CV measurements of 2 and 3. On the other hand, in the excited state the fast formation of a radical ion-pair state (i.e., 6.0 x 10(10) s(-1)), that is, the reduction of the electron accepting La-2@C-80 and the oxidation of exTTF, evolves with lifetimes as long as several ns (3.0 x 10(8) s(-1)) in toluene. Transient absorption spectroscopy experiments confirmed these observations.