Charge-transfer interactions in face-to-face porphyrin-fullerene systems: Solvent-dependent luminescence in the infrared spectral region

The cyclophane-type molecular dyads 1.2H and 1.Zn, in which a doubly bridge d porphyrin donor adopts a close, tangential orientation relative to the su rface of a fullerene acceptor, were prepared by Bingel macrocylization. The porphyrin derivatives 2.2H and 2.Zn with two appended, singly linked C-60 moieties were also formed as side products. NMR investigations revealed tha t the latter compounds strongly prefer conformations with one of the carbon spheres nesting on the porphyrin surface, thereby taking a similar orienta tion to that of the fullerene moiety in the doubly bridged systems. Cyclic voltammetric measurements showed that the mutual electronic effects exerted by the fullerene on the porphyrin and vice versa are only small in all fou r dyads, despite the close proximity of the donor and acceptor components. The steady-state and time-resolved absorption and luminescence properties o f 1.Zn and 2.Zn were investigated in toluene solution and it was shown that , upon light excitation, both the porphyrin- and the fullerene-centered exc ited states are deactivated to a lower-lying CT state, emitting in the IR s pectral region (lambda(max) = 890 and 800 nm at 298 and 77 K, respectively) . In the more polar solvent benzonitrile, this CT state is still detected b ut, owing to its very low energy (below 1.4 eV), is not luminescent and sho rter-lived than in toluene. The remarkable observation of similar photophys ical behavior of 1.Zn and 2.Zn suggests that a tight donor-acceptor distanc e cannot only be established in doubly bridged cyclophane-type structures b ut also in singly bridged dyads, by taking advantage of favourable fulleren e-porphyrin ground-state interactions.