Langmuir Monolayers of Sheet-Shaped Multialkyne Amphiphiles

The molecular organization characteristics of amphiphilic sheet-shaped multialkynylbenzene derivatives containing polar terminal substituents have been investigated at the air/water interface. Surface pressure (π)-area, surface reflection (ΔR)-area, and surface potential (ΔV)-area isotherms along with reflection and fluorescence spectroscopy of monolayers of the amphiphilic disklike pentaalkynes 1 give rise to the conclusion that the multiynes form solid condensed islands withanedge-on orientationanda two-dimensional nematic-discotic (N_D)-like arrangement of the molecules immediately after spreading. These islands are pushed together during the compression. The molecular organization characteristics are dictated by the amphiphilic nature of the molecules and by strong π-π interactions between the rigid form anisotropic aromatic cores. Furthermore, the influence of chemical structure modifications on the amphiphilic properties of sheet-shaped multiynes has been investigated. The novel tetrakis(phenylethynyl)benzene compounds 2, which are characterized by two terminal hydroxy groups fixed at the ortho position of the central benzene nucleus via flexible spacers, form edge-on-oriented Langmuir monolayers as well. Compared to the case of the pentaynes 1 containing just one hydrophilic head group, the monolayer stability rises significantly through the incorporation of the second polar substituent.