Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C_(60)

摘要

Two new conjugated porphyrin-based systems (dimers 3 and 4) endowed with suitable crown ethers have been synthesized as receptors for a fullerene-ammonium salt derivative (1). Association constants in solution have been determined by UV-vis titration experiments in CH_2Cl_2 at room temperature. The designed hosts are able to associate up to two fullerene-based guest molecules and present association constants as high as ～5 × 10~8 M~(-1). Calculation of the allosteric cooperative factor α for supramolecular complexes [3•1_2] and [4•1_2] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, first, on the complementary ammonium-crown ether interaction and, second, on the π-π interactions between the porphyrin rings and the C_(60) moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C_(60) molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C_(60)-C_(60) interactions that take place in the more stable syn- disposition of [4•1_2].