Adsorption of Terarylenes on Ag(111) and NaCl(001)/Ag(111): A Scanning Tunneling Microscopy and Density Functional Theory Study

Abstract

Photoswitching materials are building blocks of next generation optoelectronic devices which may require molecule deposition on a solid substrate. However, molecule properties change upon adsorption due to surface-molecule interactions and symmetry considerations. Scanning tunneling microscopy (STM) and density functional theory (DFT) offer techniques to address interactions of functional molecules down to the single-molecular level on solid substrates. In this paper, we present a combined STM and DFT study of a tert-butyl functionalized terarylene molecule on Ag(111) and NaCl(001)/Ag(111) at ∼5 K. tert-Butyl groups aided in identifying three conformations of the compound upon adsorption on the surface. DFT calculations showed that two of these conformations refer to different adsorption geometries of the trans conformation in the gas phase. The other was assigned to the nonreactive cis conformation. For the first time, this conformation was isolated and imaged at the single-molecular level. Calculations further showed that aside from the electronic structure of the molecule, methyl groups sticking out of the surface are the origin of bright spots observed on the STM. On NaCl(001)/Ag(111), only the trans conformation was found and the mapping of occupied and unoccupied states of terarylenes was accomplished for the first time.

Photochromic Diarylethenes Designed for Surface Deposition: From Self‐Assembled Monolayers to Single Molecules

“…TD‐DFT calculations of the molecular orbitals (MOs) of the molecule in the gaseous state did show that such tert ‐butyl groups do not contribute to the highest occupied MOs and the lowest‐occupied MOs, leading the authors to conclude that such absence of the tert ‐butyl groups in the STM images was an indication of mapping occupied and unoccupied states. A rough approximation of 5.0 V was found for when the molecule was adsorbed on NaCl(001)/Ag(111), which was higher than that observed in the solution state . This was the first time that such occupied and unoccupied electronic states of terarylethenes were directly mapped at the single molecular level thanks to the UHV conditions.…”

“…Further, the low symmetry due to the central aromatic ring could give rise to varied surface structures due to surface chirality . This was suggested for terarylenes but was not confirmed so other derivatives having chiroptical switching capabilities may be investigated as well.…”

“…Deposition and imaging of 19 on Ag(111) at 5 K led to observation of three surface conformations using the tert ‐butyl groups to identify the differing molecules on the surface. This molecule has been shown to keep its reactive anti‐parallel conformation via intramolecular forces of interaction even at room temperature, so finding multiple conformations was surprising. Torsion angle scans (DFT), however, predicted that other stable conformations could be found if the molecule was exposed at higher temperatures, which is exactly the case as the molecule was deposited on the surface by sublimation at high temperature under UHV .…”

“…This molecule has been shown to keep its reactive anti‐parallel conformation via intramolecular forces of interaction even at room temperature, so finding multiple conformations was surprising. Torsion angle scans (DFT), however, predicted that other stable conformations could be found if the molecule was exposed at higher temperatures, which is exactly the case as the molecule was deposited on the surface by sublimation at high temperature under UHV . One such conformation is the cis conformation which is equivalent to the parallel conformation.…”

“…Torsion angle scans (DFT), however, predicted that other stable conformations could be found if the molecule was exposed at higher temperatures, which is exactly the case as the molecule was deposited on the surface by sublimation at high temperature under UHV. [93] One such conformation is the cis conformation which is equivalent to the parallel conformation. (Figure 10b) With the aid of DFT and ESQC calculations, [50] two of these conformations were assigned to the reactive antiparallel conformation (labelled as trans, referring to the direction of the methyl groups with respect to the central benzothiophene ring) while the other was assigned to the unreactive parallel (labelled as cis) conformation.…”