Tale of Alkyl Chains: Chain-Length Effect-Directed Formation of Complex Self-Assembly Behaviors at the Liquid/Solid Interface for Unsymmetrically Substituted Fluorenone Derivatives

Structural diversity induced by the chain-length effect in the field of two-dimensional self-assembly has gained immense attention because of its potential application in nanoscience and crystal engineering. By modifying the two side chains in a certain molecule by one carbon atom and gradually increasing the alkyl chain length, seven fluorenone derivatives (F-CnCn+1, n = 11-17) were synthesized. At the 1-octanoic acid/graphite interface, diverse nanostructures of hexamer-I, tetramer, dimer, alternate-I, hexamer-II, and alternate-II were recorded. The arrangement for the two side chains which differ from each other only by one carbon atom was discussed from the viewpoint of thermodynamics and kinetics. The alkyl chain in the same length was speculated to show selective identification during the self-assembly process, which was favored in consideration of dense packing and maximizing the molecular interplay. Three forces such as dipole-dipole, hydrogen bonding, and van der Waals (vdWs) interactions cooperatively or competitively exert their roles on stabilizing the assembled monolayers. For the purpose of further understanding the self-assembly mechanisms, we performed force field calculations, which revealed that the strength of the hydrogen bonds was related to the arrangement of the fluorenone units, whereas the vdWs interaction showed a close relationship with the alkyl chain length. This work displays an efficient method on fabricating complex self-assembly networks, and we believe that it will promote the study of the chain-length effect in supramolecular chemistry and interfacial science.