The photoswitching capability of azobenzenes has recently been used extensively in photoreactive supramolecular materials. One of the most astonishing uses of azonenzene photoswitching is the reversible association of these molecules with certain cyclodextrines. Azobenzenes change their structure reversibly under irradiation. There’s a cis-form and a trans-form, and photoisomerisation happens reliably wavelengths of 350 (trans –> cis) and 455 (cis –> trans) nanometer.
As it happens the trans-azobenzene fits snugly in the cyclodextrine cavity, while the bent cis-form doesn’t. This offers a number of interesting possibilities, ranging from the inevitable switchable hydrogels to rather more amazing structures, such as switchable ion channels and functionalized surfaces.
The latest invention, just published in Angewandte Chemie by the Münster-based chemists Nalluri and Ravoo, uses this principle to influence the collective behavior of vesicles in solution. It is possible to make micelles and vesicles from cyclodextrines. I didn’t know that, but it’s fairly obvious: The sugars of the cyclodextrine ring are water-soluble, so if you just attach some long-chained hydrocarbon to each sugar moiety you get amphiphilic building blocs for vesicles or bilayers.
What the researchers did then is rather straightforward: They connected two azobenzene moieties with a PEG linker and added the resulting molecule to a solution of monodisperse cyclodextrine vesicles, which coagulate nore or less immediately, since the trans-azobenzenes find a suitable resting place in the cyclodextrine cavity. This shows as an increase in optical density of about an order of magnitude. This macroscopic change can, of course be reversed by irradiation with light at a suitable wavelength. There are two interesting aspects here. For one, this reminds me of certain biological Systems, like certain sponges that can be separated into their component cells which then re-aggregate into a fully functional animal.
The other, probably far more significant issue is the fact that this degree of control over a macroscopic system fits snugly into the general pattern of progress made in this area of science. Self-organising macromolecular systems may well be the next Really Big Thing in materials technology – incremental steps slowly adding up to a level of knowledge that enables, at one point, a really huge leap in technology. We may be closing in on that point.
Nalluri, S., & Ravoo, B. (2010). Lichtgesteuerte molekulare Erkennung und Adhäsion von Vesikeln Angewandte Chemie DOI: 10.1002/ange.201001442