research

Photoresponsive Supramolecular Systems

Active research in materials chemistry is concentrated on the development of new devices with novel biological, electronic, and photonics properties. Supramolecular chemistry is an attractive tool for the manufacturing of such devices because it is different from traditional covalent synthesis. It uses non-covalent, weak interactions to build highly ordered architectures by self-assembly. This process is more economical than covalent synthesis since it does not require multiple sequential steps to obtain a macromolecule, instead only one step is sufficient to gather the building blocks. These building blocks contain the information relative to properties of interest (eg. pH, electronegativity, etc), binding mode and shape.

In our research, we use hydrogen-bonds as the cement of our structures because they are selective, directional and sturdy (eg. DNA). The building block used is azobenzene, which is more stable in its trans than in its cis-conformation. This molecule is interesting because its isomerization can be controlled by light. Irradiation with UV-light gives the cis isomer and irradiation with visible light the trans isomer. This photosensitivity is interesting in terms of applications because the conformational change induced by shining light onto trans-azobenzene units alters its physico-chemical properties (pH, mechanical, dipole). In our case, irradiation controls self-assembly because of the difference in shape between the isomers. In addition, self-assembly causes photoamplification as the hydrogen-bonds 'lock' the cis-isomer into a highly ordered macrocycle. The macrocycles then stack up to produce nanotubes (see figures).

This phenomenon can be used in photoimaging, which uses light-sensitive materials to record information. In the case of azobenzene, the image is formed directly as a pattern during UV irradiation. This process is reversible: the recorded information can be erased by irradiating with visible light. Subsequently, the trans-azobenzene can be re-irradiated for another set of images.

F. Rakotondradany, M. A. Whitehead, H. F. Sleiman*; "Photoresponsive Supramolecular Systems: Self-Assembly of Azobenzoic Acid Linear Tapes and Cyclic Tetramers", Chemistry, a European Journal, 2003, 9, 4771-4780. (Full paper), pdf