Layer-by-Layer Assembly of Semiconducting and Photoreactive Bolaform Amphiphiles
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In the early 1990s, Decher and Hong extended the pioneering work of Iler, and introduced the layer-by-layer method of constructing multilayers on surfaces. They demonstrated that multilayers could be prepared simply by dipping a charged substrate sequentially into solutions containing a negatively and positively charged polyelectrolyte. Because of the ease of processability and wide array of molecules that can be incorporated into these multilayers, the layer-by-layer method has become one of the most useful and widespread techniques for fabricating ultrathin films. Successful assembly has been demonstrated for a variety of materials, some of which include DNA, latex particles, clay, proteins, nanoparticles, dyes, semiconducting oligomers, conjugated polymers, and bolaform amphiphiles.
Amphiphiles are interesting molecules that contain both hydrophilic and hydrophobic parts. A conventional water-soluble amphiphile usually consists of a long hydrocarbon chain tail and a hydrophilic head group, which is either highly polar or ionically charged, to impart some water solubility to the molecule. If the molecule bears a positive charge, such as a quaternary ammonium group, or a negative charge, such as a sulfate group, the amphiphile is categorized as a cationic or anionic amphiphile, respectively. Bolaform amphiphiles, or bolaamphiphiles, are described as molecules containing two hydrophilic moieties connected by a hydrophobic chain. It has been shown that bolaform amphiphiles have unusual aggregation behavior at the air–water interface and in lyotropic and thermotropic phases. Their self-assembly via physical adsorption on charged surfaces has been demonstrated and characterized using a variety of methods.
Bolaform amphiphiles with ionic end groups can form layers that produce charged surfaces upon their adsorption. These charged surfaces can be used to electrostatically assemble multilayer structures. The first account of the layer-by-layer assembly of bolaform amphiphiles was reported by Decher et al., where ordered multilayers of more than 70 layers were fabricated. Electrostatically attractive interactions of the hydrophilic head groups and appropriately charged substrates (such as mica, quartz, glass, Si/SiO2, gold, etc.) force molecules to pack closely on the substrate. Short-range van der Waals forces affect the packing density and orientation of the molecules within the layered structure.
The synthetic design of the bolaamphiphile must take into account these intermolecular forces to achieve a buildup of multiple layers. End-on adsorption of the molecules would maintain a maximum charge density on the surface and is crucial to make well-ordered films. The flexible alkyl spacer in the amphiphile must be of a certain length for end-on adsorption to occur. When the molecule is too short, the charges are separated by a small distance and adsorption occurs flat on the surface. At short chain length, electrostatic effects are dominant and the bolaform amphiphiles lie down on the substrate. If the length of the alkyl spacer is too long, a loop-type adsorption may occur. Either type of adsorption would lead to a reduction of charged groups on the surface making it difficult to build the consecutive adsorption of layers.
The aim of this article is to provide an overview of the layer-by-layer electrostatic adsorption and characterization of bolaform amphiphile multilayers. The rest of the article will be divided into three parts: (1) semiconducting bolaform amphiphiles, (2) photoreactive bolaform amphiphiles, and (3) photochemical isomerization of bolaform amphiphiles. Factors such as the aggregation behavior of the molecules in solution and as ultrathin films will be discussed.