Chiral self-organization of the TPPS₄ porphyrin assisted by molecular rotations

Self-assembly strategies are attracting considerable interest for the development and design of advanced chiral materials from the nano- up to the macroscale. In particular, the spontaneous chiral self-organization of achiral p-conjugated molecules has gained significant attention due to their versatile optical and electronic properties. The 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin (TPPS4) has shown an interesting ability to self-assemble into chiral supramolecular structures. Understanding the fundamental principles behind the generation of chirality can guide rational fabrication and control of the chiral assembly mechanism of the TPPS4 scaffold. Axial chirality due to the side chain rotations of the sulphonato-phenyl groups may propagate chiral information through specific interactions along the whole supramolecular structure during the non-covalent self-assembly interactions. Therefore, starting from the atropisomers of TPPS4 in its diacid form (H4TPPS42-), enhanced sampling simulations have been performed on this species considering its monomeric, dimeric, trimeric and tetrameric aggregates. The free-energy profiles have been reconstructed for all the porphyrin aggregates as a function of the pyrrole improper torsions of the porphyrin ring, allowing evaluation of how the symmetry or the asymmetry of the H4TPPS42- supramolecular system can be selectively affected by increasing the aggregate size. The formation of a specific twisted arrangement has been detected during the self-assembly process depending on the odd (destabilizer of a twisted arrangement) or even (stabilizer of a twisted arrangement) number of structural units forming the aggregate. The results in this study could help to create accurate predictive models for the generation of chiral supramolecular structures.