Breaking pi-pi Interactions in Carboxylic Acid Monolayers on Rutile TiO2(110) Leads to Unexpected Long-Range Ordering

摘要

Carboxylic acids are ubiquitous building blocks in supramolecular and surface chemistry because of their strong but reversible binding to metal cations. In these applications, the relative ordering and orientation of the acids can affect performance. We present a rational approach to tuning intermolecular interactions with the goal of maintaining a favorable molecular conformation while also enabling long-range ordering. In particular, we show that scanning tunneling microscopy (STM) images of m- or p-fluorobenzoate monolayers on rutile (110) produced in aqueous solutions display very large (2 x 1) grains without the intermolecular pairing observed in similarly prepared benzoate monolayers. The lack of pairing is attributed to the electronegative fluorine substituent, which reduces pi-pi or quadrupolar interactions between the phenyl groups on the adjacent molecules and stabilizes a favorable configuration between the aromatic head group and the carboxylate-binding moiety. In spite of the reduced interactions between the head groups, the large grain sizes are indicative of intermolecular interaction energies that significantly exceed thermal energy. These strong interactions are surprising given the 6.6 angstrom separation between the adjacent molecules. Quantitative measurements of the intermolecular interaction energies from molecularly resolved STM images are a factor of similar to 7 larger than those predicted by dispersion-corrected density functional theory. A number of possible origins for this discrepancy are discussed. This finding suggests a new path to the production of highly ordered monolayers and superstructures of large molecules.