Steroid-bridged thiophenes: Synthesis and self-organization at the solid/liquid interface

Abstract

The synthesis of the oligothiophene 6a as well as the terminally mono- and diformylated oligothiophenes 6b and 6c, in which mono- and bithiophene units are bridged by androstene, is described. Starting from epiandrostane the thiophene units were linked in positions 3 and 17 by Grignard reaction. The synthesis was accomplished by introduction of formyl groups. The self- organization of compounds 6 on highly oriented pyrolytic graphite at the liquid/solid interface was studied by STM. Derivatives 6a,b,c spontaneously formed ordered monolayers on graphite, although neither are they planar nor do they have alkyl chains. Due to their different energy gaps in all cases the thiophene units were imaged as bright areas and the steroid unit as a dark area representing a novel type of surface structuring. While 6a and 6b are loosely packed, resulting in an area per molecule of 3 nm2, the dialdehyde 6c self-assembles to form a closely packed monolayer (area per molecule 2 nm2) obviously owing to interactions between the formyl groups.