Solvent-dependent stabilization of metastable monolayer polymorphs at the liquid-solid interface

Self-assembly of 1,3,5-tris(4′-biphenyl-4″-carbonitrile)benzene monolayers was studied at the liquid-solid interface by scanning tunneling microscopy. Application of different fatty acid homologues as solvents revealed a solvent-induced polymorphism. Yet, tempering triggered irreversible phase transitions of the initially self-assembled monolayers, thereby indicating their metastability. Interestingly, in either case, the same thermodynamically more stable and more densely packed monolayer polymorph was obtained after thermal treatment, irrespective of the initial structure. Again, the same densely packed structure was obtained in complementary solvent-free experiments conducted under ultrahigh vacuum conditions. Thus, self-assembly of metastable polymorphs at room temperature is explained by adsorption of partially solvated species under kinetic control. The irreversible phase transitions are induced by thermal desolvation, that is, desorption of coadsorbed solvent molecules.