Origin of Solvent-Induced Polymorphism in Self-Assembly of Trimesic Acid Monolayers at Solid-Liquid Interfaces

Encoding information in the chemical structure of tectons is the pivotal strategy in self-assembly for the realization of targeted supramolecular structures. However, frequently observed polymorphism in supramolecular monolayers provides experimental evidence for a decisive additional influence of environmental parameters, such as solute concentration or type of solvent, on structure selection. While concentration-induced polymorphism is comparatively well understood, the thermodynamic and molecular origins of solvent-induced polymorphism remain elusive. To shed light on this fundamental aspect of self-assembly, we explore the solvent-induced polymorphism of trimesic acid (TMA) monolayers on graphite as a prototypical example. Using the homologous series of fatty acids as solvents, TMA self-assembles into the anticipated chickenwire polymorph for longer chain fatty acids, whereas the more densely packed, but still porous flower polymorph emerges in shorter chain fatty acids. According to our initial working hypothesis, the origin of this solvent-induced polymorphism lies in the solvent dependence of the free energy gain. Utilizing an adapted Born-Haber cycle constructed from measured TMA sublimation and dissolution enthalpies as well as density functional theory-calculated monolayer binding energies, we quantitatively assessed the self-assembly thermodynamics of both polymorphs in hexanoic, heptanoic, and nonanoic acid. Yet, in contrast to the experimental findings, these results suggest superior thermodynamic stability of the chickenwire polymorph in all solvents. On the other hand, additional experiments comprising variable-temperature scanning tunneling microscopy corroborate that the flower polymorph is thermodynamically most stable in hexanoic acid. To resolve this apparent contradiction, we propose a thermodynamic stabilization of the flower polymorph in hexanoic acid through the stereochemically specific coadsorption of shape-matched solvent molecules in its unique smaller elongated pores. This alternative explanation gains further support from experiments with side-substituted hexanoic acid solvents. Combination of a quantitative thermodynamic analysis and studies with systematic variations of the solvent's molecular structure holds great promise to enhance the understanding of thus far underexplored solvent effects.