TPPS4 nanotube molecular structure and linear spectra: Matching theory and experiment

Extended organic molecular aggregates are promising materials for nonlinear optics, photodynamic therapy, and other photonic applications. Meso-tetra(4-sulfonatophenyl) porphine (TPPS4) molecules form two zwitterionic species (Z1 and Z2) at solution pH = 1, which self-assemble into chiral molecular nanotubes exhibiting strong optical activity. While the macroscopic structure of these nanotubes is well characterized experimentally, their atomistic organization remains unresolved. A theoretical model of TPPS4 nanotubes based on the Frenkel exciton model is constructed to determine molecular packing characteristics that reproduce experimental spectra. Matching calculated and experimental absorption and circular dichroism spectra allows determining the orientation of the molecular plane within the nanotube.