Kinetic investigations and chirality of porphyrin J-aggregates

The remarkable recent progress in nanotechnology is mainly due to the ability to synthesize, investigate, and exploit materials with structural modulation on the nanometer scale. Porphyrins are well suited building blocks since they i) are relatively easy to synthesize; ii) show interesting and peculiar photophysical properties that can be tuned through careful choice of peripheral substituents and inserted metal ions and iii) are able to self-assemble spontaneously into dimers or higher aggregates through non-covalent interactions. Due to the self-correcting nature of the supramolecular interactions, defect-free porphyrin structures whose shape, size and photophysical properties are high sensitivity to external stimuli, can be achieved. Water soluble porphyrins are very interesting systems since their aggregation can be conveniently controlled by screening the charge repulsion through changes in the ionic strength and pH. In this framework, the water soluble tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) provide an iconic example of such behavior, since its diacid form, in particular experimental conditions, is able to self-arrange in well defined J-aggregates supramolecular structures. In the last decades, our investigations have been manly focused on TPPS4 self-aggregation phenomena, supramolecular organization on biological matrices and in confined enviroments. The data reported on this system have allowed gaining important information in the field of supramolecular architectures, highlighting the importance of the role of experimental parameters such as concentration and/or mixing order of the reagents. Recently, we reported on a detailed kinetic investigations to demonstrate the fundamental role of kinetic parameters in assemblies sizing and in expression and transmission of chirality at nano and mesoscopic scale.1-3 Despite many reports on this subject, the onset of chirality in J-aggregates of TPPS4 is still an open and intriguing question. Being hydrogen bonding in aqueous solution a key factor in the stabilization of the final aggregates, we focused our attention on the important role of isotopic substitution4 as well on the hydrogen bonding network of the solvent in presence of chaotropic or kosmotropic ions5 on both the rates and extent of J-aggregate formation, and the final observed chirality.