Organisation in monolayers at the air–water interface of butadienyl dyes containing benzodithiacrown-ether or dimethoxybenzene

The novel amphiphilic butadienyl dye, containing a dimethoxybenzene unit instead of benzocrown-ether moiety (dye 1) and the amphiphilic benzodithia-15-crown-5 butadienyl dye (dye 2) form stable insoluble monolayers on distilled water. In order to evaluate the influence of the ionophore size on the molecular organisation, the known amphiphilic benzodithia-18-crown-6 butadienyl dye (dye 3) has also been used. The monolayers have been characterized by surface pressure–area and surface potential–area isotherms as well as Brewster angle microscopy and reflection spectroscopy. Besides monomers of the dyes two types of associates exist in the pure dye monolayers, i.e. dimers and aggregates. The spectroscopic properties of the monomers were obtained by studying mixed monolayers of the dyes with the lipid dimyristoyl-phosphatidyl-cholin (DMPC). The dimers may be organised as head-to-tail dimers with the intermolecular distances 0.38, 0.45, and 0.49 nm for dyes 1, 2, and 3, respectively. According to the extended dipole model, we propose formation of aggregates in which the chromophores are parallel to each other with the same intermolecular distances as in the dimers, and the centers of their transition moments shifted by 0.95 nm (dye 1) and 1.2 nm (dyes 2 and 3).