Self-Assembly Growth of an Upright Molecular Precursor with a Rigid Framework

Abstract

Upright orientation of a molecular adsorbate is one of the most practical keys for controlling surface functionalities by using self-assembled monolayers. However, lateral interactions between the upright molecules become more complex than planar molecules because they are not confined in a single plane. Here, a scanning tunneling microscope was used to study the intermolecular interactions among self-assembled 1,8,13-tris­(mercaptomethyl)-triptycenes (TMMT) on Ag(111) with atomic spatial resolution. TMMT chemisorbs with a three-point contact by the S atoms leading to upright orientation, which has been verified by X-ray photoelectron spectroscopy. The resulting chiral stereoisomers self-assemble into highly ordered structures, suggesting stereochiral recognition because of intermolecular interactions. Theoretical simulations show weak C–H···π intermolecular interactions and a strong S–Ag molecule–substrate interaction, which suggests the former being responsible for the self-assembly formation with the latter being the stabilizing force. This provides fundamental insights into molecular assemblies with intermolecular interactions not confined within a single plane.

Porous Honeycomb Self-Assembled Monolayers: Tripodal Adsorption and Hidden Chirality of Carboxylate Anchored Triptycenes on Ag

“…, phenylene rings), which is not surprising considering that the highest-occupied electronic states in the triptycenes possess π-character. This is also consistent with the three-leaved clover type shape of isolated, thiolate-bonded triptycenes in the STM images by Chaunchaiyakul et al Second, the brightness of the π-lobes on the two sides of each ring is not the same. A similar asymmetry of STM-related features has been observed, for example, for anthraceneselenolate SAMs, and this has been attributed to the molecular tilt .…”

“…Among these phases, only H 2 and P can be prepared reliably as majority structures under suitable experimental conditions. Even though the H 2 phase is characterized by a particularly high packing density, it exhibits a “conventional” hexagonal molecular arrangement, similar to those reported already for Trip-SH and Trip-CH 2 -SH SAMs on Au(111) and Trip-CH 2 -SH SAM on Ag(111). , In contrast, the honeycomb-like P-phase represents a structure distinctly different from all others that have so far been observed for triptycene based SAMs. Such open structures, frequently described as porous networks, are rather typical of molecules with flat adsorption geometry, such as trimesic acid, , 1,3,5-benzenetribenzoic acid, or hexaazatriphenylene-hexanitrile on noble metal substrates.…”

“…This requires the molecules to adopt the nested packing shown in Figure 3 c and reported earlier for other triptycene based systems. 37 , 41 , 60 However, the packing of the molecules in the Trip-CA SAMs on Ag is significantly denser than in the analogous thiol SAMs on Au(111) and Ag(111) (∼65 Å 2 per molecule) 37 , 41 and also denser than that reported for thin multilayer films of tripodal paraffinic triptycenes (56.8 Å 2 per molecule). 60 …”

“…Another aspect worth mentioning is that, in contrast to the studies of the thiolate-bonded triptycene SAMs, , for the P phase of the Trip-CA SAM the maximum (apparent) height is located in the center of the triangular shape, i.e. , at the bridging carbon atom, an aspect that will also be addressed below, when discussing the STM simulations.…”

“…Most importantly, these layers feature a tripodal bonding configuration, yielding SAMs of high crystallinity in the Trip-CH 2 -SH case and resulting in a less perfect but still acceptable film quality in the case of Trip-SH . Similar considerations apply to Trip-CH 2 -SH on Ag(111) substrates …”