Hourglass-Shaped Dendrimers on Surfaces: A Comparison of Different Scanning-Tunneling-Microscopy Approaches

)Laboratory of Organic Chemistry, ETH Hˆnggerberg, HCI, CH-8093 Z¸richDedicated to Professor DieterSeebachon the occasion of his 65th birthdayLarge molecules adsorbed on surfaces can be analyzed by scanning tunneling microscopy (STM) undervariousenvironmentalconditions:onadrysurfaceinairorvacuum,andatthesolid-liquidinterface.However,canmeasurementsunderdissimilarconditionsbecompared,e.g.,whensampleAwasstudiedatthesolid-liquidinterfaceandsample Binadryenvironment?Onlyrarely canthesame substance beexamined withmore thanone approach, since completely different set-up and preparations are necessary. Furthermore, few substancesare suitable for several methods of sample preparation and characterization. We have chosen a large, flexible,nonplanar molecule, namely an alkoxy-substituted second-generation dendritic compound with a chiral coreunit, which is peculiar for its −hourglass× conformation. The assembly properties have been explored by STMboth in solution-cast self-organized monolayers (SOMs) and multilayer films, as well as at the solid-liquidinterface. The complexity and limits of the three approaches applied to our hourglass-shaped dendriticcompound are discussed. Depending on the approach and environmental conditions, several quality levels ofimageresolution couldbeachieved;measurementscarriedoutatlowtemperaturesledtohighestresolutiononthe aromatic parts of the molecule. A comparison of equally sized images obtained under these varyingconditions reveals not only different packing arrangements, but also spots of unlike shape. Therefore, when theapproach, preparation, and/or environmental conditions are not the same, STM measurements of differentcompounds have to be compared with greatest care.