Molecular Organisation and syn-anti Conformational Equilibria in Ethane-Bridged Bis(ZincPorphyrin) Floating Films at the Air-Water Interface

The behavior of the symmetrical ethane-bridged bis(Zn porphyrin) (1) has been investigated at the air–water interface. The molecular organization of floating films of pure 1 and its mixture with amphiphilic substances, such as arachidic acid and n-octadecylamine, was inspected by means of surface pressure–area isotherms, Brewster angle microscopy and reflection spectroscopy in the UV–Vis region. The overall results suggest the presence in all cases of mainly the anti conformer of 1 even when the dimer was spread on pure water, through the ligation of water molecules to the zinc atoms in the axial positions. It was also demonstrated for the first time that the syn-to-anti conformational transition of the porphyrin dimer can be accelerated by the ligation of suitable amphiphiles even at the liquid-air interface. In particular, it is noted that n-octadecylamine, and arachidic acid (to a lesser extent), added to the system as amphiphiles, drive the syn«anti equilibrium of 1 towards the anti form through the axial ligation of the amino or carboxylate group to the zinc atoms