Polymorphism at the Metal/Organic Interface: Hybrid Phase with Alternating Coplanar and Vertical Adsorption Geometry

Abstract

Polymorphism of organic molecular materials plays a prominent role in organic (opto)­electronic devices, where the structural properties of metal/organic interfaces determine performance-critical parameters such as charge carrier injection rates. Here, we show that (sub)­monolayers of a 2D-prochiral aromatic dinitrile with a bent quaterphenyl backbone engage in pronounced polymorphism on a Ag(111) surface and form a unique phase with alternating polar orientation of the molecules. In the saturated monolayer, the molecules are nearly coplanar with the substrate (flat-lying) and form an ordered racemic structure consisting of chiral dimers stabilized by H···NC hydrogen bonds. At submonolayer coverages, two ordered phases coexist with a dilute mobile phase. One phase forms homochiral domains with a quasi-Kagome structure. The other phase has a unique structure consisting of alternating rows of flat-lying and upright-standing molecules. The two different polar orientations can be distinguished by scanning tunneling microscopy. The upright-standing molecules have a straight and brighter shape and show increased packing density compared to their flat-lying counterparts. These results reveal the great local variability of molecular arrangements at metal/organic interfaces, even for fixed global parameters such as temperature and coverage.