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

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Large molecules adsorbed on surfaces can be analyzed by scanning tunneling microscopy (STM) under various environmental conditions: on a dry surface in air or vacuum, and at the solid-liquid interface. However, can measurements under dissimilar conditions be compared, e. g., when sample A was studied at the solid-liquid interface and sample B in a dry environment? Only rarely can the same substance be examined with more than one approach, since completely different set-up and preparations are necessary. Furthermore, few substances are 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 core unit, which is peculiar for its −hourglass× conformation. The assembly properties have been explored by STM both in solution-cast self-organized monolayers (SOMs) and multilayer films, as well as at the solid-liquid interface. The complexity and limits of the three approaches applied to our hourglass-shaped dendritic compound are discussed. Depending on the approach and environmental conditions, several quality levels of image resolution could be achieved; measurements carried out at low temperatures led to highest resolution on the aromatic parts of the molecule. A comparison of equally sized images obtained under these varying conditions reveals not only different packing arrangements, but also spots of unlike shape. Therefore, when the approach, preparation, and/or environmental conditions are not the same, STM measurements of different compounds have to be compared with greatest care.

Self‐Organized Monolayers: A Route to Conformational Switching and Read‐Out of Functional Supramolecular Assemblies by Scanning Probe Methods

“…There is a contrast here, since the STM image is dominated by what we interpret as side-by-side extended octyl chains in this model, while in our previous work on 2,2′-bipyridine [36] and chiral [35] Fréchet dendrons, the aromatic residues dominated.…”

“…[22] It was instructive to conduct an STM study on the same compound both at a solid/liquid interface and in a dry environment (prepared by solution-casting with quickly evaporating solvent). [35] The (S,S)-enantiomer of the second-generation n-octyl-substituted dendritic compound (see Fig. 4a) has a chi- …”

“…d) STM image of a self-organized monolayer of (again) the dendrons on HOPG on a dry surface also with a unit cell in white (U bias = 700 mV, I t  = 0.01 nA [47]). Reprinted with permission from [35].…”

“…Even dynamic reorganization processes may be detected. The many advantages of good surface stabilization by this self-organization linker are apparent when one looks at our results [35][36][37][38][39][40] on a number of Fréchet dendrons, as discussed below.…”

Shape‐Switchable Azo‐Macrocycles

“…[99] Numerous further investigations have been reported in the field of self assembled monolayers. [100]- [102] Shape persistent two dimensional oligomers such as spoked wheels [103] as well as shape persistent macrocycles based on a phenyl ethynyl backbone have been immobilized on graphite. [104]- [106] In Figure 5 the deposition from solution to the surface of highly oriented pyrolytic graphite (HOPG) is shown.…”