Amphiphilic surfactants are the principal components of soaps and detergents which are responsible for their ability to lower the surface tension of water to and solubilize hydrophobic agents. Their action in aqueous solutions are due, in part, to their ability to undergo a concentration dependent self-assembly process which leads to the formation of larger molecular ensembles called micelles. The self-assembly of these micellar aggregate superstructures is driven by the hydrophobic effect. In the case of charged amphiphiles, the size of the micelles formed is limited by the electrostatic repulsion that builds among contiguously assembled molecules that comprise the micelle itself. It is possible however to design amphiphile systems which employ molecular recognition motifs to order the self-assembly of molecules in a programmed manner. Towards this end, we have constructed a series of alkyl-bridged (5, 7, and 9 hydrocarbon chain) and glycol-bridged (tetraethylene glycol chain) bis-bipyridinium-based gemini amphiphiles. The bipyridinium units of these amphiphiles are strongly π-electron deficient and can form donor-acceptor π-π stacked charge transfer (CT) complexes with π- electron rich molecules in aqueous solution, in this case, the neurotransmitter melatonin. In this system, the π-electron rich donor molecular template, melatonin, serves as “molecular glue”, capable of decreasing the electrostatic repulsion among the charged assembled amphiphiles. The addition of the melatonin molecular template to solutions of the bipyridinium-based amphiphiles resulted in: 1) the formation of larger micelles; 2) a more thermodynamically favorable self- assembly process; and 3) an overall increase in the efficiency and effectiveness of the amphiphile to lower the surface tension of water. This supramolecular system was characterized by 1 H-NMR, 13 C-NMR, UV-Vis spectroscopy and various 2D NMR techniques. The formation of CT complexes was confirmed by UV-Vis, 2D DOSY, and 2D ROESY NMR spectroscopies. This work augers well for further investigation into soft matter capable of undergoing template-directed self-assembly.

Development of bis-bipyridinium-based gemini surfactants for template-directed self-assembly

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