Photoinduced Processes in Self-Assemblies of Bis-Porphyrinic Tweezers with an Axially Coordinated Bispyridinofullerene

Self-assembled bis(zinc porphyrin)-bispyridinopyrrolidinofullerene coordination complexes were obtained in solution. Two [5]polynorbornane-bridged bis-porphyrins were used that differed in the arms containing porphyrin units: whereas 1 has rigid [5]polynorbornane linkers, compound 2 has additional flexible propyl chains. The different geometries of the two hosts affect both the complexation process and the photoreactivity of the final product. Formation of the complexes, characterized by absorption, emission, and NMR spectroscopy, occurs with association constants in the order of 10(4) and 10(6)m(-1) for bis-porphyrin tweezers 1 and 2, respectively. The higher flexibility of tweezers 2 accounts for the greater association ability. Full photophysical characterization of the complexes, as well as of suitable models, has been performed by means of steady-state and time-resolved optical spectroscopy. Ultrafast luminescence detection and pump-probe transient absorption analysis were used to investigate photoinduced processes within the complexes. The results provide evidence that an electron-transfer process from the bis-porphyrin host to the fullerene guest occurs in both complexes, and a slightly longer lifetime of the charge-separated state is observed in the complex with more flexible host 2.