Abstract.Monolayers of the organic molecules perylene-3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and -diimide (PTCDI) on graphite and MoS 2 have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations.

Thin films of the organic molecule perylene-3,4,9,10-tetracarboxylic-dianhydride ("PTCDA") on graphite(0001) have been investigated from the mono-to the multilayer regime with low energy electron diffraction (LEED), X-ray-diffraction in Bragg-Brentano geometry, and high resolution scanning tunneling microscopy (STM). These different methods proved epitaxial growth in a coincident superstructure and yielded congruent results concerning details of the crystallographic structure of the epilayer. In addition it was possible to resolve submolecular structures in high resolution STM images; a comparison of the 10 resolved maxima of the tunneling current with the molecular structure leads us to question the conventional model description of tunneling.

Monolayers of the nonplanar organic molecule lead-phthalocyanine ͑PbPc͒ on MoS 2 have been prepared by organic molecular beam epitaxy and investigated with video scanning tunneling microscopy. Three different crystallographic phases have been observed. Two of them, the close packed and the rowlike phase, have already been observed in an earlier study of the planar Cu-Pc. With PbPc we have now observed an additional phase, where three close packed rows alternate with one or two isolated single rows. In contrast to Cu-Pc, in submolecularly resolved images the PbPc appear in two different states, with either a dark or a bright center. This is attributed to the nonplanar molecular geometry, which allows the formation of two different adsorption geometries, the Pb above or below the molecular plane. In video sequences the transition from one state into the other could be observed.

Monolayers of copper-phthalocyanine (Cu-Pc) on highly oriented pyrolytic graphite (HOPG) and MoS 2 prepared by organic molecular beam epitaxy have been investigated by scanning tunneling microscopy. On both substrates there exist well defined preparation conditions leading to ordered two-dimensional arrays of flat lying molecules. On HOPG they form a close-packed structure with a nearly quadratic unit cell, whereas on MoS 2 we found two phases, one close-packed and one rowlike phase. This rowlike phase can be explained by a long range interaction due to an adsorbate induced superstructure of the substrate, which also can be seen in the scanning tunneling microscopy images. In images with submolecular resolution, the molecules appear different on the two substrates. On MoS 2 they look like a four-leaved clover, on graphite they show a more detailed inner structure.

Basic research aimed at developing molecular electronic devices tends to focus more and more on thin-if possible epitaxially ordered-organic films. Such films are produced preferentially under high-vacuum conditions by vapor deposition onto well-characterized substrate surfaces, with careful control of deposition rate, substrate temperature, and film thickness. This development requires organic compounds that vaporize without decomposition, but for applications they must also have a negligible vapor pressure at room temperature, that is, they must have strong intermolecular binding forces. Moreover, suitable molecules must have a shape and symmetry that allows two-dimensional close packing.". With the aim of producing adsorbate films composed of molecules that resemble larger sections of graphite layers, we here describe a structure-specific synthesis of the extended polybenzoid hydrocarbons 3 and 13 and report the study of the ordered adsorbate films of 3 by electron diffrdction analysis and scanning tunneling microscopy (STM) . Evaluation of the shape of larger molecules by ultrahigh-resolution STM and special diffraction methods becomes increasingly important as their characterization by classical spectroscopic methods is impaired by decreasing solubility.Polybenzoid arenes that can be represented by at least one KekulC form that consists only of electron sextets as a mesomeric limiting structure[31 can be obtained-as has been described previously-by a cycloaddition-cyclodehydrogenation sequence (Scheme 1) .I4] Starting from the ene-diene component 1, a double intramolecular Diels-Alder cycloaddition and dehydrogenation leads to the phenylene-bridged bistriphenylene derivative 2, which exists as two rotamers 2 a and 2 b. Therefore the synthesis sequence describedf41 must lead to the two isomers 3 and 4 as cyclodehydrogenation products.