Subtle Fluorination of Conjugated Molecules Enables Stable Nanoscale Assemblies on Metal Surfaces

Abstract

In molecular self-assembly on surfaces, the structure is governed by the intricate balance of attractive and repulsive forces between molecules as well as between molecules and substrate. Frequently, repulsive interactions between molecules adsorbed on a metal surface dominate in the low coverage regime, and dense self-assembled structures can only be observed close to full monolayer coverage. Here we demonstrate that fluorination at selected positions of conjugated molecules provides for sufficiently strong yet non-rigid H⋅⋅⋅F bonding capability that enables (i) the formation of stable nanoscale molecular assemblies on a metal surface and (ii) steering the assemblies' structure. This approach should be generally applicable and will facilitate the construction and study of individual nanoscale molecular assemblies with structures that are not attainable in the high coverage regime.

Dipolar Substitution Impacts Growth and Electronic Properties of Para‐Sexiphenyl Thin Films

“…Therefore, our XRD‐based finding of upright standing molecules strongly supports the notion of randomly distributed dipole moments in the m ‐2F‐6P film, as this likewise minimizes the dipole–dipole interaction energy between neighboring molecules. We note that the marked differences in the structural properties of the ortho ‐ and meta ‐substituted 6P derivatives further agree with what has recently been reported for monolayers of these compounds on Ag(111) . There, the differences have been explained by substrate‐mediated interactions and ground‐state charge transfer, which, however, can only play a minor role on SiO x both due to the upright molecular orientation and as charge transfer is not expected with the dielectric substrate.…”

“…However, the impact of partial substitution on the growth behavior has so far only been investigated in the monolayer regime on metals for fluorination at the meta ‐ ( m ‐2F‐6P) and ortho ‐positions ( o ‐2F‐6P), as well as for cyano‐substitution at the para ‐position (CN‐6P); for the chemical structures, see Figure a. In monolayers on Ag(111) it has been found that the subtle difference in fluorine position at one terminal phenyl ring has an eminent impact on the supramolecular organization . This is related to the highly different net dipole moments and their opposite orientation in m ‐2F‐6P and o ‐2F‐6P, the absolute values of which have been derived for the gas phase to amount to 2.2 D and 1.1 D, respectively .…”

“…In general, partial nonsymmetric fluorination allows introducing a net molecular dipole moment, the direction and strength of which depends on the position of respective fluorine substitutions. For rod‐like molecules, the addition of dipolar terminal groups at only one end of the molecule therefore results in a dipole moment along the long molecular axis . In thin films, the orientation of these dipole moments with respect to both the substrate and the surface plane of the film has a strong impact on the thin film IE, which is altered by ΔIE as compared to a film of the parent compound.…”

Indexing grazing-incidence X-ray diffraction patterns of thin films: lattices of higher symmetry

“…The experimental GIXD data for the o-F 2 -6P film [the synthesis of o-F 2 -6P is reported by Niederhausen et al (2018)] as grown on HOPG are shown in Fig. 3.…”

“…The transition from inclined to parallel molecular planes in these structures has been ascribed to the impact of intramolecular polar bonds by the authors. For o-F 2 -6P, no bulk crystal structures have been published yet, and structural characterization was limited to sub-monolayers on Ag(111) by low-temperature scanning tunnelling microscopy (Niederhausen et al, 2018). There, in contrast to non-fluorinated 6P molecules which individually adsorb in the sub-monolayer regime on the metal surface without packing, the authors find a flat lying stack arrangement of the o-F 2 -6P molecules with small lateral shifts along the row direction.…”