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We directly follow the evolution of the absorption spectrum from a single molecule to a dimer and further to a one-dimensional molecular stack: We determine the optical absorption properties of ordered monolayer to multilayer films of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) on muscovite mica(0001) surfaces by in situ differential reflectance spectroscopy. The data clearly show the transition from the single molecule to a dimer spectrum, followed by the exciton delocalization to a molecular crystal exciton. The accompanying spectral shifts compare favorably with recent model concepts.

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The structure of [4-(phenylazo)phenoxy]hexane-1-thiol (AzoC 6 ) self-assembled monolayers (SAMs) on Au(111) has been investigated with scanning tunneling microscopy (STM) and Fourier transform infraredreflection absorption spectroscopy (FTIR-RAS). The FTIR-RAS results yield a tilt angle of the molecules close to 0°, which significantly differs from the 30°tilt angle of linear n-alkanethiols. In STM images two types of domains are observed that have equal unit cell dimensions and two molecules per unit cell but show different tunneling contrasts, which is attributed to a different arrangement of the molecules within the unit cell. The relationship of the molecular lattice to the substrate lattice is found to be commensurate.