Tuning supramolecular chirality in nano and mesoscopic porphyrin J-aggregates

In natural light-harvesting systems, chirality represents a crucial point being one of the recurrent motifs. The ability to tune supramolecular chirality both on nano and meso scale is a key topic on the design of mimetic antenna systems. Here we describe a non-covalent approach to achieve mesoscopic clusters, based on spermine and the diacid form of TPPS4 porphyrin, having a sea urchin-like morphology, whose chirality can be induced by the presence of L- or D-tartrate under mild acidic conditions. The mechanism proposed for the formation of these clusters deals with the formation of peculiar optically active J- nanoaggregates, which serve as nuclei for further growth into mesoscopic structures. Poly(- vinylsulfonate) has been used to scavenge the polyamine and thereby foster disaggregation. The kinetics of this process show a biphasic behavior characterized by a rapid removal of spermine externally bound to porphyrin monomeric units followed by a stripping-out of the polyamine which is tightly bound to the J-nanoaggregates. A careful choice of experimental conditions allows precise control of the disaggre- gation kinetics, leading to the isolation of J-nanoaggregates. These latter species exhibit chirality even in the absence of a chiral inducer. This feature, together with the anomalous behavior of the CD signal of the mesoscopic aggregates containing optically active tartrate, have been explained in terms of screening hypochromism, differential scattering and structural rearrangements.