Writing With Light In A Polymer Mixture

With a pen-like laser, chemists in Finland can sketch letters into a dispersion of a new polymer. Light flips the polymer between its trans and cis isomers, changing its solubility and creating clear characters in a cloudy mixture (Macromolecules 2013, DOI: 10.1021/ma4011457).

Vladimir Aseyev at the University of Helsinki and his colleagues discovered this novel application while investigating the basic properties of a little-explored class of polymers based on poly(azocalix[4]arene). The azobenzene groups in these polymers’ backbones enable the macromolecules to switch between a cis and trans isomer when excited by light.

Aseyev and his team synthesized a variation of the polymer with tetraethylene glycol monomethyl ether side chains. They found that the two isomers of this new polymer have different solubility properties at the same temperature: At 20 °C, the trans form is barely soluble in ethanol, but the cis isomer dissolves easily.

When Aseyev used 365-nm laser light to trace the shape of letters through a cloudy dispersion of the trans isomer, the polymer isomerized to the cis version only where the light shone, producing clear letters.

The researchers can erase the writing by heating the mixture, shaking it to disperse the cis isomer, or waiting up to four hours for the polymer to relax back to the trans form. This ability to write with light is well known for solid polymers but not dispersions, Aseyev says. “It’s a new chemistry.” He hopes next to unravel why the cis form is more soluble than the trans and to investigate possible applications of the material, such as memory devices that store information using light.