CRYSTALLINE MONOLAYER AND MULTILAYER SELF-ASSEMBLIES OF OLIGOTHIOPHENES AT THE AIR-WATER-INTERFACE

The formation of Langmuir monolayers at the air-water interface has lo ng been believed to be limited to amphiphilic molecules containing a h ydrophobic chain and a hydrophilic headgroup. Here we report the forma tion of crystalline mono- and multilayer self-assemblies of oligothiop henes, a class of aromatic nonamphiphilic molecules, self-aggregated a t the air-water interface. As model systems we have examined the depos ition of quaterthiophene (S-4), quinquethiophene (S-5). and sexithioph ene (S-6) from chloroform solutions on the water surface. The structur es of the films were determined by surface pressure-area isotherms, by scanning force microscopy (SFM) after transfer of the films onto atom ically smooth mica, by cryo-transmission electron microscopy (Cryo-TEM ) on vitreous ice, and by grazing incidence synchrotron X-ray diffract ion (GID) directly on the water surface. S-4 forms two polymorphic cry stalline multilayers. in polymorph alpha, of structure very similar to that of the three-dimensional solid, the molecules are aligned with t heir long molecular axis tilted by about 23 degrees from the normal to the water surface. In polymorph beta the long molecular axis is perpe ndicular to the water surface. S-5 self-ageregates at the water surfac e to form mixtures of monolayers and bilayers of the beta polymorph; S -6 forms primarily crystalline monolayers of both alpha and beta forms . The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports, The relevance of the prese nt results for the understanding of the early stages of crystal nuclea tion is presented.