Template‐Guided Ionic Self‐Assembled Molecular Materials and Thin Films with Nanoscopic Order

Abstract

,I dunnP restholm, IlirianaQ oqaj, ChristinaB.R iel, To bias V. Rostgaard, Nora Saleh, HannibalM.S chultz, Mark Standland,Jens S. Svenningsen, RasmusTruels Sørensen, JesperV isby,E milie L. Wolff-Sneedorff, Malte Hee Zachariassen, Edmond A. Ziari, Henning O. Sørensen, and Thomas Just Sørensen* [a] To Professor Klaus Bechgaard and Professor ThomasB jørnholm for always teaching to think outside the box Abstract: Ionic self-assembly (ISA) is ap rovenm ethod that exploits non-covalenti nteractions to generate supramolecular materials. Here, we have expanded the scope of this approach fabricating thin films with nanoscopic order maintained over centimeters. Cationiclayers of benzalkonium surfactants form al amellar template. The template is able to host layers of negatively charged polyaromatic functional units, hered emonstrated with b-naphthol-derived azo-dyes. We show that av arietyo ft hese functional building blocks can be incorporated in the lamellar templatet hrough ISA. Sixteen different materials were produced,c haracterized, and processedi nto thin films, with lamellar order perpendicular to the substrate. Thus, ad esign concept is demonstrated in which diverse functional motifs can be isolated and ordered in a2 Dl attice between layers of alkyl chains in bulk and in thin films, in which the molecular orderi sm aintained and alignedt othe substrate.

Using Polarized Spectroscopy to Investigate Order in Thin-Films of Ionic Self-Assembled Materials Based on Azo-Dyes

“…As previously reported [ 37 ], the materials have a lamellar structure in bulk and thin films (see Figure 2 ). While investigations using X-ray diffraction (XRD) and atomic force microscopy (AFM) show that the materials in bulk and films have crystalline order, and that the material in thin films are ordered with respect to the substrate, these ordered thin films of the materials do not show activity in polarized optical microscopy [ 37 ]. A fact that implies: (a) the material is isotropic in the plane of the film [ 2 , 38 ]; or (b) the films are just a single domain.…”

“…Ionic self-assembled (ISA) materials are often made by mixing a solution of a functional building block with a solution of a suitable surfactant, and then collecting the precipitating nanomaterial (see Figure 2 ) [ 22 , 23 , 25 , 37 ]. In the process of preparing materials from benzalkonium surfactants BZK with tail lengths from ten to eighteen, i.e.…”

“…Building on the seminal work of Faul and Antonietti [ 24 , 36 ], we have developed a ISA system based on benzalkonium surfactants BZK that allow for facile formation of ordered materials and thin films upon ISA with polyanionic functional units [ 37 ]. Here, we have undertaken a systematic investigation of three rectangular functional units (red) with five benzalkonium surfactant templates (green), only differentiated by an increasing chain length of the hydrophobic alkyl group from ten carbons in BZK10 to eighteen in BZK18 (see Figure 1 ).…”

“…Using polarized spectroscopy, we have determined the orientation of the transition moment with respect to the long axis of the molecule as defined by the angle ϕ fz in the four dye molecules. Knowing the orientation of the transition dipole moment in the molecular framework, we can confirm the presence of order in the structure of the thin films using polarized absorption spectroscopy [ 37 ]. Thus, we were able to determine that the films made from all investigated materials have nanoscopic order, although we were not able to determine the exact molecular structure.…”

Key Developments in Ionic Liquid Crystals

“…Examples of applications of ILCs are anisotropic tribology [ 146 ] and ion conductivity to be used in molecular electronics (solar cells, optoelectronic devices, etc .) [ 35 , 37 , 38 , 147 , 148 ], in directed synthesis approaches [ 149 ], including controlled nanoparticle synthesis, [ 150 , 151 ], alignment [ 152 ] or deposition [ 153 ], in ionic self-assembly [ 154 , 155 ] that builds complex structures from simple charged organic species and possibly in bio-catalysis [ 156 ] or sensing applications [ 25 , 98 ]. The near future will demonstrate which roles ILCs can play.…”