Supramolecular Chirality in Self-Assembled Porphyrin Architectures

The expression of chirality in supramolecular assemblies is a highly investigated topic either because of the strict relationship with biological systems and for its fundamental importance in understanding the origin of life itself. In this framework a very debated issue, but nevertheless fascinating, is featured by the self-assembling of achiral porphyrins able to provide supramolecular structures whose chirality may be induced both by chemical chiral biases or by means of spontaneous chiral symmetry breaking. J-aggregates of the water soluble porphyrin tetrakis-(4-sulfonatophenyl)porphyrin (TPPS4) feature an iconic example of such behavior. Here, we will present a survey of our recent work aimed to address this issue, highlighting as a multitude of parameters may affect the expression and transmission of chirality in these self-assembled porphyrin architectures. Firstly, we have emphasized the primary role played by kinetic parameters, showing as mixture protocol of reactants strongly affects the size of these nanoassemblies and the chiral induction. Recently, being hydrogen bonding in aqueous solution a key factor in the stabilization of the final aggregates, we have focused our attention on the crucial role of isotopic substitution as well as on the hydrogen bonding network of the solvent in presence of chaotropic or kosmotropic ions. In both studies, the rates and extent of J-aggregate formation, as well as the final observed chirality are widely affected. Most recently, we have highlighted the key role of adventitious metal ions, like Zn(II), in the self-assembly of TPPS4, so providing some insights into controversial findings on J-aggregate chirality.