Abstract
Molecules with tripodal anchoring to substrates represent a versatile platform for the fabrication of robust self-assembled monolayers (SAMs), complementing the conventional monopodal approach. In this context, we studied the adsorption of 1,8,13-tricarboxytriptycene (Trip-CA) on Ag(111), mimicked by a bilayer of silver atoms underpotentially deposited on Au. While tripodal SAMs frequently suffer from poor structural quality and inhomogeneous bonding configurations, the triptycene scaffold featuring three carboxylic acid anchoring groups yields highly crystalline SAM structures. A pronounced polymorphism is observed, with the formation of distinctly different structures depending on preparation conditions. Besides hexagonal molecular arrangements, the occurrence of a honeycomb structure is particularly intriguing as such an open structure is unusual for SAMs consisting of upright-standing molecules. Advanced spectroscopic tools reveal an equivalent bonding of all carboxylic acid anchoring groups. Notably, density functional theory calculations predict a chiral arrangement of the molecules in the honeycomb network, which, surprisingly, is not apparent in experimental scanning tunneling microscopy (STM) images. This seeming discrepancy between theory and experiment can be resolved by considering the details of the actual electronic structure of the adsorbate layer. The presented results represent an exemplary showcase for the intricacy of interpreting STM images of complex molecular films. They are also further evidence for the potential of triptycenes as basic building blocks for generating well-defined layers with unusual structural motifs.
Highlights
One has to keep in mind that a purely geometrical interpretation of scanning tunneling microscopy (STM) images can be misleading due to the crucial influence of the electronic structure.[52−55] Geometrically, a structure such as the one shown by model 2 on the left half is conceivable, and it might be even preferred over the structure of model 1 as this arrangement suggests increased π−π and van der Waals interactions between molecules
Along with frequently detected hexagonal molecular arrangements, a porous honeycomb-like structure is observed. Such a structure is not unusual for molecules adopting a flat adsorption geometry,[64,69−71] but it is rather unexpected for self-assembled monolayers (SAMs) comprised of upright-standing molecules
According to the spectroscopic data, all molecules in this structure are adsorbed in a well-defined tripodal geometry, with all three carboxylate docking groups bonded to the substrate as bidentate
Summary
Self-assembled monolayers (SAMs) are an important part of modern nanotechnology, with versatile applications ranging from corrosion protection and design of biointerfaces to lithography, nanofabrication, molecular electronics, organic electronics, and photovoltaics.[1−9] SAMs usually consist of rodlike molecules featuring an anchoring group, mediating the bonding to a specific substrate, a (functional) tail group, constituting the SAM−ambient interface, and a backbone, connecting both groups and building the SAM matrix.[1−3] Usually, each SAM-forming molecule comprises only a single docking group, but molecules with potentially dipodal,[10−12] tripodal,[10,13−32] and tetrapodal[32−34] building configurations (bearing a suitable number of anchoring groups) have been designed as well.
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