Synthesis, Conformational Interconversion, and Photophysics of Tethered Porphyrin-Fullerene Dyads with Parachute Topology

The synthesis of a porphyrin-fullerene dyad with parachute topology is reported. To determine whether the dyad is flexing at room temperature, low-temperature NMR experiments were used. Computational modeling has shown the low-energy conformation of the dyad to be non-symmetric. Although, H-1 NMR spectroscopy at room temperature is consistent with a molecule with C-2v symmetry, the spectrum changes on lowering the temperature consistent with wind-shield wiper-like motion, in which the porphyrin moiety rotates from one side of the C-60 sphere to the other. Nanosecond and picosecond fluorescence lifetime experiments show two components contribute to the fluorescence decay, also consistent with the presence of more than one conformer.