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

Abstract

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 π-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.