Abstract

Molecular self-assembly, governed by the subtle balance between intermolecular and molecule-surface interactions, is generally associated with the thermodynamic ground state, while the competition between kinetics and thermodynamics during its formation is often neglected. Here, we present a simple model system of a benzoic acid derivative on a bulk insulator surface. Combining high-resolution noncontact atomic force microscopy experiments and density functional theory, we characterize the structure and the thermodynamic stability of a set of temperature-dependent molecular phases formed by 2,5-dihydroxybenzoic acid molecules, self-assembled on the insulating calcite (10.4) surface. We demonstrate that a striped phase forms before the thermodynamically favored dense phase, indicating a kinetically trapped state. Our theoretical analysis elucidates that this striped-to-dense phase transition is associated with a distinct change in the chemical interactions involved in the two phases. The striped phase is characterized by a balance between molecule-molecule and molecule-substrate interactions, reminiscent of the molecular bulk. In contrast, the dense phase is formed by upright standing molecules that strongly anchor to the surface with a comparatively little influence of the intermolecular interactions, i.e., in the latter case the substrate acts as a template for the molecular structure. The kinetic trapping stems from a relatively strong intermolecular interaction between molecules in the striped phase that need to be broken before the substrate-templated dense phase can be formed. Thus, our results provide molecular level insights into two qualitatively different bonding motifs of a simple organic molecule on a bulk insulator surface. This understanding is mandatory for obtaining predictive power in the rational design of molecular structures on insulating surfaces. © 2016 American Chemical Society.