Self-Assembled Supramolecular Architecture Based on an Achiral Bolaamphiphilic Diacid in Binary System

Abstract

A new family of polymers that includes topologically complex polymers and self-assembled nanostructures has recently attracted much attention. The non-covalent selfassembly technique, unlike chemical reaction or polymerization methods, has the advantage of directly producing well-defined twoand three-dimensional structures such as sheets, rod, fibers, tubes, disks, and spheres. Notably, these nanostructures can be constructed with high efficiency and accuracy, and yet with minimum energy. Organic amphiphiles typically possess a chiral moiety as well as functional groups with potential to form multiple hydrogen bonds, such as sugars, peptides, or nucleotide bases. However, only a few examples for self-assembled supramolecular structures of organic amphiphiles consisting of achiral moieties have been described. Therefore, our initial motivation to prepare bolaamphiphilic diacids was to develop self-organized pseudo-linear polymers stabilized by intermolecular hydrogen-bonds, and that incorporated suitable guest additives with achiral functional groups. With this idea in mind, we employed a simple bola-form amphiphile (“bolaamphiphile”) 1, consisting of one diphenyl ether group as a hydrophobic moiety at the center, flanked by two carboxylic acids as the terminal hydrophilic groups. In 1, the linkage we introduce between the hydrophobic and the hydrophilic moieties is an amide group, known to form a linear hydrogen bond chain. We herein report the assembling behavior of 1 with and without guest additives. The additives such as 4,4'-dipyridyl (4,4'-bpy) and 2,2'-dipyridyl (2,2'-bpy) are so common that their interaction behaviors have been well-defined especially in the coordination polymer networks. We have investigated the influence of the dipyridyl additives on the morphologies of self-assembled 1 and self-assembled 1 combined by bpy which are denoted with [1] and [1+bpy], respectively, by optical microscopy, SEM, FT-IR, powder-XRD and differential scanning calorimetry (DSC). Achiral bolaamphiphile 1 was synthesized in three steps (Scheme 1). The monomethyl sebacate 2 was chlorinated with oxalyl chloride to give methyl-10-chloro-10-oxadecanoate 2. Reaction of 3 with 4,4'-oxydianiline afforded diamide derivative 4. Treatment of 4 with NaOH gave the desired product 1 as a brown powder. The self-assembly reactions of 1 (3-5 mg) with and without dipyridyl additives (3 equiv) were accomplished by dissolving the reactants in acidic, neutral, and basic conditions in water/ethanol (4:1 v/v, 50 mL) at 80 C, respectively. The pH of each solution was adjusted with 0.1 M NaOH or 0.1 M HCl aqueous solution. After the incubation of the solutions at room-temperature for several hours, the formation of stable supramolecular assemblies was confirmed. In the absence of additives, SEM pictures for [1] (0.50 mM) obtained at pH = 4 show a film-like structure (Figure 1a) whereas at pH = 7, the structures are comprised of bundles Scheme 1. Synthetic route for achiral bolaamphiphilic diacid 1.