Chem. Commun. 50, 5615–5618 (2014)

Light-induced reactions on the molecular scale, such as the photoisomerization of azobenzene between its cis and trans configurations, are often used to induce macroscopic material changes such as self-assembly, flexing or other motions. Although light is a commonly used stimulus for effecting supramolecular self-assemblies, it is more unusual for audible sounds to elicit any changes in structure, but macromolecules have been observed to respond to sound waves through local effects on the surrounding solvent, which can combine and be amplified along the molecular chain.

Now, a collaborative team from Kobe University, Kansai University and the Kyoto Institute of Technology, led by Akihiko Tsuda, has shown that the assembly of supramolecular fibres can be induced by light, and mutual alignment of the fibres induced by audible sound. They synthesized a linear molecule with a central azobenzene moiety and three alkyl 'tails' at each end that each featured 12 carbon atoms. With the azobenzene in its trans form, this molecule was found to stack to form double-stranded supramolecular nanofibres. On UV irradiation to convert to the cis azobenzene, amorphous aggregates were instead formed. The aggregation behaviour was found to be strongly dependent on the length of the alkyl tails, with only a 12-carbon chain producing ordered aggregates from the trans molecule, and 6- and 16-carbon chains aggregating amorphously in both azobenzene conformations.

Linear dichroism was used to assess whether the supramolecular assemblies were preferentially oriented in solution. Neither the cis nor trans assemblies showed any anisotropy in their orientation. However, when audible sound waves were applied to the solution, the nanofibres were found to align with each other; no change was observed in the amorphous cis aggregates. The degree of ordering was found to vary with the frequency of sound waves, with lower frequencies eliciting a larger response. Both visible light and sound were required to induce the assembly and subsequent ordering of the fibres, akin to a stimulus-AND logic gate. The relative ease of application of both sound and light opens up interesting possibilities for multistimuli responsive materials in the future.