The a-conjugated and structurally defined dodecithiophene 1 well crystallizes from soln. into single crystals which were studied by x-ray structure anal. Mol. conformations and packing of the longest oligothiophene ever structurally characterized were detd. Surprisingly, rather rare syn-conformations at the terminal thiophene rings and partial twisting of alkyl side chains for compact packing were found, whereby the mols. form a layer-like arrangement with partial overlap of their heteroarom. systems. In comparison, the mol. properties of the oligomer in a 2D self-assembled monolayer adsorbed at the liq.-solid interface were studied by scanning tunneling microscopy (STM). In comparison to the 3D bulk material, on graphite, a different mol. arrangement was found. A lamella-type ordering of molecularly resolved oligomers which exhibit an all-anti conformation could be imaged over wide areas. Larger lattice consts. were detd. which originate from mol.-substrate interactions.