Steric matching and the concentration induced self-assembled structural variety of 2,7-bis(n-alkoxy)-9-fluorenone at the aliphatic solvent/graphite interface

Abstract

Controlling and unraveling structural polymorphism has received special attention in 2D self-assembled monolayers. In this work, we investigated the steric matching and solution concentration controlled structural variety in the self-assembly of 2,7-bis(n-alkoxy)-9-fluorenone (F–OC_n) at the n-tetradecane and n-tridecane/graphite interface under different concentrations, respectively. Scanning tunneling microscopy (STM) revealed that the coadsorbed adlayers of F–OC_n and solvents (n = 12 to 16) were formed and exhibited concentration dependent 2D phases due to the steric matching. The self-assembled monolayer of F–OC_n (n = 12 to 16) evolved from a low-density coadsorbed linear lamellar packing, which was formed at low concentrations, to higher-density patterns at relatively high concentrations. F–OC₁₄ exhibited a complex structural variety, in which a systematic trend of decrease in the molecular density per unit cell with decreasing concentration was obtained. Except for F–OC_n (n = 13, 15, 17), the zigzag structure showing the linear lamella with dimers was observed. Systematic experiments revealed that the self-assembly of F–OC_n was chain-length dependent. The results provide insight into the structural variety exhibited by a series of organic molecules and furnish important guidelines to control the morphology by changing the solution concentration.