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

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.


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from the normal (90°) to grazing (20°) incidence, which is additionally emphasized by the respective positive peaks in the difference spectrum. Considering that these orbitals are directed perpendicular to the planes of the phenyl and carboxylate groups in the Trip-CA SAM, an upright orientation of triptycene moieties can be assumed, which agrees well with the carboxylate-type bonding of all three anchoring groups of Trip-CA molecules, as follows from the XPS data (see above).  Figure S1. C K-edge NEXAFS spectra of the Trip-CA SAM UPD-Ag/Au/mica (P-phase) acquired at the different X-ray incidence angles, along with the difference between the spectra collected under the normal (90°) and grazing (20°) incidence (bottom curve). Individual absorption resonances are marked (see text for details). The horizontal dashed line corresponds to zero.
A quantitative evaluation of the dependence of the *ph intensity on the X-ray incidence angle, performed within the standard theoretical framework for a vector-type orbital, 1 adapted specifically to the triptycene case, 5,6 gives an average tilt angle of ~9° with respect to the surface normal for the axis of the Trip-CA, which is nearly perpendicular to the substrate. This value is quite close to the analogous parameter for the SAMs of 1,8,13-trimercaptomethyltriptycene (~7.5°) also exhibiting a tripodal bonding to the substrate, mediated by the thiolate anchoring groups connected to the triptycene framework over the methylene linker. 6 The deviation from in the 20° spectrum. This peak has been frequently observed in different CA SAMs on UPD-Ag/Au/mica 3,9 but not rationalized so far.

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In agreement with the C K-edge spectra and the proposed molecular orientation, the intensities of the *COO and 2* resonances in Figure S2 decrease significantly at going from the normal to grazing incidence, corresponding to the positive peaks in the difference spectrum. In contrast, the 1* resonance exhibits an opposite behavior, emphasized by the negative peak in the difference spectrum. This suggests a strong hybridization of a part of the * orbitals of the carboxylate groups with the electronic states of the substrate, resulting in their reorientation and lowering of energy.

Details on the Employed Basis Functions
The basis functions employed in the FHI-aims simulations have the format in spherical coordinates (r, Θ, Φ). They are defined relative to a given atomic center and are provided together with the FHI-aims code. Default settings for four different levels of accuracy are contained in the species_defaults subdirectory. For the production calculations, "tight" settings (i.e., the second-highest setting) have been used. They comprise the functions described in Table S1. Table S1. Basis functions that have been used for the calculations performed with FHI-aims.
The abbreviations read as follows (as described in FHI-aims: A User's Guide: February 10, 2018): X(nl, z), where X describes the type of basis function where H stands for hydrogen-like functions and ionic for a free-ion like radial function. The parameter n stands for the main/radial quantum number, l denotes the angular momentum quantum number (s, p, d, f, …), and z denotes an effective nuclear charge, which scales the radial function in the defining Coulomb potential for the hydrogen-like function. In the case of free-ion like radial functions, z specifies the onset radius of the confining potential. If auto is specified instead of a numerical value, the default onset is used.