Hydroxyl versus Carboxyl Substituent: Effects of Competitive and Cooperative Multiple Hydrogen Bonds on Concentration-Controlled Self-Assembly

The self-assembled behaviors of two fluorenone derivatives, 2,7-bis((11-hydroxyundecyl)oxy)-9-fluorenone (BHUF) and 2,7-bis((10-carboxydecyl)oxy)-9-fluorenone (BCDF), were investigated at the liquid–solid interface by scanning tunneling microscopy. Two solvents, 1-octanoic acid and 1-phenyloctane, were employed in consideration of their distinct polarity and solubility. It is observed that the BHUF molecules self-assemble into seven different polymorphs upon adsorption, while only two different polymorphs are observed in the BCDF monolayer. The theoretical calculation is performed to reveal the underlying mechanism. As compared to that of C═O···HO hydrogen bonds, the enhanced binding energy of intermolecular C═O···HOOC hydrogen bonds in the BCDF monolayer would dominate the intermolecular van der Waals (vdWs) interactions and the molecule–solvent interactions, thereby resulting in a limitation of expression of structural polymorphism. In addition, the concentration-dependent polymorphism as well as the relative phase transition is discussed in terms of the stability and packing density of different polymorphs. Furthermore, the different self-assembled behaviors of BHUF molecules in these two solvents at lower concentrations are associated with the different energy gain upon solvent coadsorption. The investigation provides a simple and alternative strategy to construct the structural polymorphs by utilizing multiple hydrogen bonds at the liquid–solid interface.