Structure vs. properties - chirality, optics and shapes - in amphiphilic porphyrin J-aggregates

The structure of the meso-tetrakis(4-sulfonatophenyl) porphyrin (TPPS4) J-aggregates could be determined by X-ray and electron diffraction methods. A sheet-like architecture reveals the relationship between structure and chirality, optics and shapes of the J-aggregates of the meso 4-sulfonatophenyl- and phenyl-substituted porphyrins. The structure of the J-aggregates of H4TPPS4 belongs to the chiral space group P2(1) and includes four porphyrin molecules in its unit cell. The intermolecular stabilization of the zwitterionic units by hydrogen bonding and electrostatic interactions between the positively charged central NH groups and the periphery anionic sulfonato groups results in a structure of porphyrins sheets along the [(1) over bar 01] plane direction. The structure of the sheet on the [(1) over bar 01] plane is already chiral and its molecular architecture explains the simultaneous presence of H- and J-aggregate bands in their absorption spectra. This structure also accounts for the high similarity observed between the absorption spectra of different mesomorphs of the same substance and even between different members of the series of meso-4-sulfonatophenyl- and aryl-substituted diprotonated porphyrins. The possibility, or not, of the sheet-like structure on [(1) over bar 01] to interact with other layers, either through ionic or hydrophobic interactions, depends on the substitution pattern at the meso-positions of the porphyrin ring. Thus, the different morphologies of the particles [mono- bi- and multilayered] of this series of J-aggregates are explained taking into account the role that the fourth meso-substituent plays in the interlayer stabilization. The results suggest that supramolecular helicity, previously detected in several J-aggregates, is not the explanation of their chirality but would be the expression of the intrinsic chirality of the packing between building blocks.