Exploration of Chirality and Achirality of Self‐Assembled Monolayer Formed by Unsymmetrically Substituted Fluorenone Derivative at the Liquid/Solid Interface

Abstract

noncovalent forces, such as dipole-dipole interaction, [3] electrostatic interaction, [4] van der Waals interaction, [5] hydrogen bonds, [6] and π-π stacking. [7] Through physical adsorption, achiral molecules can self-assemble into chiral configurations on the achiral substrate due to the reduced freedom and constraint of the substrate lattice. [2d,8] Because of the rotation characteristics, porous networks show great potential in the process of chiral recognition and have received wide attentions. [9] However, nonporous networks also possess possibility of forming chiral polymorphs as long as the molecules are properly designed. Thus obtaining chirality via tiny structure modification will be a new attempt on fabricating chiral structures and will provide a new method on designing advanced functional materials in the field of surface science.Fluorenone derivatives have been reported to be used as liquid crystals. [10] Moreover, they were also good candidates for studying the self-assemblies at the liquid/highly oriented pyrolytic graphite (HOPG) interface, which have been widely explored in our group. We have previously reported that structural diversity was common to be observed and could be induced via changing the length of the alkyl chain, the concentration, and the solvent. For the bis-substituted fluorenone derivatives, molecules were arranged into a variety of nanopatterns which were regularly ordered but not chiral. [3,11] For the monosubstituted fluorenone derivatives, molecules adopted both chiral and achiral configurations and structural transition could be regulated between them. [12] Therefore, fabrication of chiral nanostructures using unsymmetrically substituted fluorenone derivatives seems to be a challenging but meaningful target. Fortunately, we realized this goal by efficient modification of the side chains.When the π-conjugated fluorenone core was substituted at the equal positions but substituted by unequal alkyl chains, chiral nanostructures were observed in its self-assembled monolayer on HOPG. The molecule we use in this study is 2-decyloxy-7-pentadecyloxy-9-fluorenone (DPF), as shown in Figure 1a. The first typical feature induced by the two different alkyl chains (C 10 H 21 and C 15 H 31 ) is that the long chains and the short chains are orderly interdigitated. As a result of this interdigitation, DPF molecules are usually notThe self-assemblies of 2-decyloxy-7-pentadecyloxy-9-fluorenone (DPF) are characterized by scanning tunneling microscopy at the liquid/solid interface. Achiral Dimer and chiral S-like structures are observed in 1-octanoic acid. The solvent molecule takes part in the self-assembly via forming COOH⋯OC and COOH⋯COOH hydrogen bonds. When 1-phenyloctane and n-tetradecane are used as the solvents, DPF self-assembles into chiral Z-like structure, which are classified into Types I-IV according to the packing styles. For the purpose of exploring the effect of solvent and the competition between these three structures, the self-assemblies of DPF in mixed solvents and ...

Scanning Tunneling Microscopy and Computational Simulation Studies of a Fluorenone-Based Organic Optoelectronic Material

“…Every two molecules from the two trimers can be regarded as a molecule pair that can offset their dipole moments, favoring the dipolar interaction. ,,, Between the adjacent S-like hexamers, there are two coadsorbed solvent molecules. In this case, the two carboxyl groups are believed to be arranged in a head-to-head fashion, forming −COOH···COOH– hydrogen bonds. ,, Figure c is the proposed structural model for this S-like structure. The coadsorbed 1-OA molecules are in red color.…”

“…The self-assembly monolayer of TTF on the HOPG surface is stabilized by the noncovalent forces of hydrogen bonds, van der Waals interaction, and dipolar interaction. The TTF molecule displays polarity because of the electron affinity of the carbonyl group in the fluorenone cores . Molecule pairs that can offset their dipole moments are formed in the alternate-I, alternate-III, S-like, and tetramer structures, as shown in Figure S12.…”

“…In this case, the two carboxyl groups are believed to be arranged in a head-to-head fashion, forming −COOH•••COOH− hydrogen bonds. 32,33,40 Figure 5c is the proposed structural model for this S-like structure. The coadsorbed 1-OA molecules are in red color.…”

“…31,33 Moreover, the ability of selective identification for the alkyl chains in the same length has been observed for the fluorenone derivatives that are substituted by two chains with an obvious length difference. 32,34,37 Combining these two principles with the consideration of dipole offset and maximization of the molecule−molecule van der Waals interaction, we proposed the structural models for all self-assembly patterns.…”

“…Previously, we have studied a series of fluorenone derivatives and their self-assembled behaviors by STM at the liquid–solid interface. We unveiled at the nanoscale level the solvent, the chain length, and concentration effects as well as the structural transformation between chirality and achirality. − In this work, we systematically investigated a fluorenone derivative of 2-tridecyloxy-7-tetradecyloxy-9-fluorenone (TTF), which is a molecule functionalized by two different side chains of −C 13 H 27 and −C 14 H 29 (Scheme ). It has been investigated in the solvents of 1-octanoic acid (1-OA), 1-phenyloctane (1-PO), and n -tetradecane ( n -TD) under different concentrations (Scheme ).…”