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A Polymer that Changes its Optical Properties
November 3, 2014   
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Electrically controlled glasses with continuously adjustable transparency, new polarization filters, and even chemosensors capable of detecting single molecules of specific chemicals could be manufactured thanks to a new polymer combining optical and electrical properties.

An international team of chemists from Italy, Germany and Poland has developed a polymer with unique optical and electric properties. Components of this polymer change their spatial configuration depending on the electric potential applied. This, in turn, affects the polarization of transmitted light. The material can be used, for instance, in polarization filters and glass windows with continuously adjustable transparency. Due to its mechanical properties, the polymer is also suitable for the production of chemical sensors for selective detection and for the determination of optically active (chiral) forms of the same substances.

The research findings of the international team, headed by Prof. Francesco Sannicolo from the University of Milan, were recently published in Angewandte Chemie International Edition magazine.

"Until now, to give polymers chiral properties, chiral pendants were attached to the polymer backbone,” says Prof. Włodzimierz Kutner from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw, one of the initiators of the research project. “In such designs, the polymer was used as a scaffold only. Our polymer is exceptional, with chirality inherent to it, and with no pending groups. The polymer is both a scaffold and an optically active chiral structure. Moreover, the polymer conducts electricity.”

Chirality can be best explained by referring to mirror reflection. If two varieties of the same object look like their mutual mirror images, they differ in chirality. Human hands provide perhaps the most universal example of chirality, and the difference between the left and right hands becomes obvious if we try to place a left-handed glove on the right hand. The difference between two chiral molecules with identical chemical composition is the same as the difference between the left and right hands. Each of the molecules shows different optical properties and rotates plane-polarized light differently. In such a case, chemists refer to one chemical compound existing as two optical isomers called enantiomers.

The polymer presented by Prof. Sannicolo’s team was developed on the basis of thiophene, an organic compound composed of a five-member aromatic ring containing a sulfur atom. Thiophene polymerization gives rise to a chemically stable polymer of high conductivity. The basic component of the new polymer—its monomer—is made of a dimer with two halves each made of two thiophene rings and one thianaphthene unit. The halves are connected at a single point and can partially be rotated with respect to each other by applying electric potential. Depending on the orientation of the halves, the new polymer either assumes or loses chirality. This behavior is fully reversible.

The development of the new polymer was initiated thanks to research on molecular imprinting pursued at the Institute of Physical Chemistry in Warsaw. The research resulted, for instance, in the development of polymers used as recognizing units (receptors) in chemosensors, capable of selective capturing of molecules of various substances, for instance nicotine, and also melamine, a chemical detrimental to human health that is reportedly used as an additive to falsify protein content in milk and dairy products produced in China.

Generally, molecular imprinting is based on creating template-shaped cavities in polymer matrices with the molecules in question used first as cavity templates. Subsequently these templates are washed out from the polymer. As a result, the polymer contains traps with a shape and size matching those of the molecules of the removed template. To be used as a receptor in a chemosensor, the polymer imprinted with these cavities must show sufficient mechanical strength. Kutner said, "The three-dimensional networks we attempted to build using existing two-dimensional thiophene derivatives just collapsed after the template molecules were removed. That's why we asked for help from our Italian partners, who specialize in the synthesis of thiophene derivatives. The problem was to design and synthesize a three-dimensional thiophene derivative that would allow us to cross-link our polymers in three dimensions. The thiophene derivative synthesized in Milan has a stable three-dimensional structure, and the controllable chiral properties of the new polymer obtained after the derivative was polymerized turned out to be a nice surprise for all of us.”

Spectroelectrochemical studies on the new polymer were carried out by researchers from the Leibniz Institute of Solid State and Materials Research (IFW) in Dresden, Germany.
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