Stereoisomeric selection upon adsorption: A structural and optical study of curcuminoid derivatives on ultrathin films of KCl on Au(111) and on bulk KCl(001)

Abstract

We present a combined Scanning Tunneling Microscopy (STM), non-contact Atomic Force Microscopy (nc-AFM) and Differential Reflectance Spectroscopy (DRS) study aiming at the characterization of the relationship between structural and optical properties of a supramolecular assembly of a curcuminoid derivative adsorbed on two kinds of substrates: bilayers of KCl on Au (111) (2 ML KCl/Au(111)) and KCl(001) bulk. The molecule features a difluoroboron (-BF 2 ) complex as well as cyano (-CN) end groups, which induces an internal dipole moment of a few Debyes. The comparative study of the structural properties of the molecules adsorbed on 2 ML KCl/Au(111), which allows for STM characterization, and KCl bulk, which allows for nc-AFM and DRS characterization, shows that the molecular adsorption is similar on both substrates. STM and nc-AFM give evidence that the on-surface condensation of the molecules into supramolecular assemblies is steered by two main interactions: the molecule-substrate interaction between the -CN end groups of the molecule and the cationic K + species, as well as an in-plane intermolecular interaction involving -BF 2 and phenyl groups. Based on Density Functional Theory calculations, we propose an epitaxial molecular structure described by a two-molecules unit cell forming a 8 × 2 supercell on the KCl surface that grows in form of ribbons along the < 100 > or < 010 > directions of the substrate, which are polar. Among several possible stereo-isomers for the molecule, it is found that the balance between vertical and in-plane interactions selects the one whose size matches the distance between K + species to condense on the substrate and not the most stable one stemming from the gas phase. Finally, optical spectroscopy in solvent and DRS measurements performed on the supramolecular assembly exhibit similar features identified as three absorption peaks tracing a π → π * electronic transition followed by a vibronic progression, plus a broader band featuring a set of higher energy transitions. Due to both, the weak electrostatic interactions and a lack of π − π interactions in the condensed molecular phase, DR spectra exhibit no major excitonic effect, but a non-rigid redshift of the molecular absorption band compared to the situation in solvent. This observation is found to be consistent with the isomer selection due to the transition from the gas phase to the on-surface condensed phase.

Hydrogen-bonded one-dimensional molecular chains on ultrathin insulating films: Quinacridone on KCl/Cu(111)

“…For instance, the intrinsic luminescence of individual molecules has been measured using STM-induced luminescence [38][39][40][41] and STM tip-enhanced photoluminescence spectroscopy [42][43][44] on ultrathin NaCl films grown on silver and gold crystal surfaces. Ultrathin films of KCl have also been used to electronically decouple organic nanocrystals [45,46] and molecular layers [47] from silver and gold surfaces for STM-based experiments. However, the growth of hydrogen-bonded molecular chains on ultrathin alkali-halide films is still a challenge.…”

“…First, we describe the epitaxial growth of KCl on Cu(111), which has rarely been considered so far [68]. Ultrathin KCl film growth on Cu(111) exhibits some structural characteristics that it does not have on other metal and semiconductor surfaces studied so far, i.e., on Au(111) [47,69], Ag(100) [70], stepped Cu surfaces [71,72], and Si(111) [73]. Then, we determine the in-plane orientation of the QA chains relative to the main crystal axes of the Cu(111) substrate and the KCl(100) islands.…”