Aggregation and supramolecular chirality of achiral amphiphilic metalloporphyrins.

Metalloporphyrin derivatives with three hydrophobic dodecyl chains and a hydrophilic ester or carboxylic acid substituent were designed in order to clarify the effect of the central metal ions on the aggregation as well as the supramolecular chirality in the Langmuir-Schaefer films. All the metalloporphyrins showed good spreading behavior on water surface and can be transferred onto solid substrates. The transferred films were characterized by a variety of methods including UV-visible spectroscopy, circular dichroism (CD) spectroscopy, FTIR spectroscopy, atomic force microscopy (AFM) and scanning electron microscope (SEM) measurements. It has been found that the copper derivative forms J-aggregates as well as H-aggregates in the film. Moreover, the film showed strong CD signals. Change from the ester substitution to carboxylic acid caused the decrease of the supramolecular chirality. On the contrary, the zinc derivative showed only a negligible CD signal although the corresponding free base could assemble into a chiral assembly. A possible mechanism for the subtle relationship between supramolecular chirality and molecular structures has been proposed.