Scanning tunneling microscopy (STM) and X-ray diffractometry (XRD) were applied to investigate the interfacial and bulk structures of 4-[[4-(alkoxy)phenyl]azo]benzoic acids by systematically varying the alkyl chain length. Complete wetting allowed us to perform STM on the crystalline ultrathin film, which was in the thermodynamic state that could be probed by XRD. XRD of the single crystalline, short chain compounds indicates a layered structure with a dimeric, head-to-head configuration with each azobenzene group tilted 70° with respect to the layer boundary. A bilayer model based on XRD spacings of longer homologs suggests that the tilt angle tends to decrease as the length of the alkyl chain increases. STM revealed various images of ordered molecular assemblies crystallized on the graphite surface. Although a head-to-head, bilayer configuration with the azobenzene units lying nearly parallel to the graphite surface was recognized for all homologs, no regularity in the chromophore orientation with respect to the alkyl chain length was observed. These studies show that while the length of alkyl chain controls the molecular orientation in the bulk, interaction of the alkyl chain with the graphite surface produces a variety of the aggregate structures at the interface.
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