Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C60

Abstract

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 UVvis titration experiments in CH2Cl2 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 × 108 M‒1 . Calculation of the allosteric cooperative factor  for supramolecular complexes [3·12] and [4·12] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, firstly, on the complementary ammonium-crown ether interaction and, secondly, on the π−π interactions between the porphyrin rings and the C60 moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C60 molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C60-C60 interactions that take place in the more stable syn-disposition of [4·12].