Adsorption and Ordering of Merocyanines on the Ag(100) surface

Abstract

Merocyanines (MCs) constitute pro-chiral donor-acceptor (D-A) molecules that exhibit high optical transition dipole moments. Although MCs are used in many optoelectronic devices, their chemical bonding to metal interfaces has not yet been investigated. To overcome this knowledge gap, this work presents a first surface science study that characterizes the structure formation of MC monolayers in dependence on the interplay of interfacial and intermolecular interactions. <br /> The analysis was performed for a series of five structurally related, prototypical MCs on the Ag(100) surface. These exhibit a conjugated π-system that consists of a thiophene ring and a thiazole ring connected by a methine bridge. The D-A character of the molecules is induced by a tertiary amine (D) attached to the thiophene ring and a malononitrile group (A) attached to the thiazole ring. Alkyl side chains attached to the tertiary amine and the thiazole ring mainly impact the sterical demand of the MCs. <br /> It could be demonstrated that the interfacial interactions cause the face-on adsorption of the MCs and the commensurate structural growth, while the intermolecular interactions determine the growth of enantiopure structures. Investigations were performed by a series of surface-sensitive techniques including STM, SPA-LEED, PES, and the NIXSW technique. The intermolecular interactions were probed by a systematic variation of the alkyl side groups, leading to different MC phases due to the different sterical demands of the MCs. <br /> The results reveal that hydrogen bonds between the D and A groups of neighboring MCs lead to enantiopure molecular tetramers on the surface. These tetramers are the primary building units of all observed MC phases. On a larger scale, the tetramers arrange in enantiopure long-range ordered structures that grow commensurate with the Ag(100) surface. Tetramers of different handedness are found in the respective mirror domains. <br /> The commensurate growth is induced by strong interfacial interactions of the MCs, accompanied by charge exchange between the molecules and the Ag surface. The interfacial bonding is measurable in molecular adsorption distances that are, for HB238-A, 15% (C) to 24% (N) smaller than the sum of the respective van der Waals radii. In particular, a strong, covalent-like Ag-S binding was found. This manifests in a high vertical and lateral order of the S atoms in combination with short Ag-S bond lengths of 2.5 to 3.1 Å. <br /> The Ag-S bond determines the adsorption sites of the tetramers and thus the commensurate growth of the MC structures. In general, the Ag atop sites were found to be the preferred adsorption sites of the S atoms. However, the intermolecular tetramer-tetramer interactions in the MC structures lift the four-fold symmetry of the tetramers for two of the three observed MC phases, as two MC molecules of the tetramer interact more strongly with the neighboring tetramers. These MC molecules are laterally distorted, resulting in altered intramolecular S S distances of up to +31% and adsorption sites that are symmetrically shifted by up to 1 Å away from the atop adsorption sites towards the bridge positions. Smaller vertical adsorption distances of the distorted molecules to the Ag(100) surface indicate that the distortions weaken the molecular π-system, leading to a stronger interfacial binding of these MCs.