Origins of curvature in meso-tetra(4-sulfonatophenyl) porphine aggregation: molecular dynamics and electronic spectroscopy†

meso-Tetra(4-sulfonatophenyl) porphine (TPPS₄) is a significant theranostic agent for photodynamic therapy (PDT) and a model system of molecular nanowires. The zwitterionic forms of TPPS₄ tend to form large chiral nanotubes in acidic conditions at pH ≈1. However, it is still not clear how these aggregates are structured at the molecular level. We describe a computational strategy to model the TPPS₄ aggregation of small clusters using a molecular dynamics (MD) approach. Two possible forms of zwitterionic TPPS₄ tetramers were considered, and their absorption and circular dichroism (CD) spectra were calculated using the Frenkel exciton model. Possible molecular packing is suggested as a candidate for the formation of large aggregates.