Molecular engineering makes optoelectronic product more efficient

Alkyl-pi engineering, created by researchers at NIMS, Japan, will allow molecules to have specific properties.
Molecules used to make optoelectronic devices can be engineered to have specific properties. Strategies proposed by researchers at the National Institute for Materials Science (NIMS) in Japan, could make the production of high-performance optoelectronic devices more efficient.

The molecules used to make optoelectronic devices can be engineered to balance the chemical interactions within them and to optimise their properties for specific applications, puts forward the paper, published in Science and Technology of Advanced Materials.

Optoelectronic devices are used in, for example, TVs and mobile device displays. Many materials used to make optoelectronics consist of 'pi-conjugated' molecules that feature a complex form of chemical bonding in which many electrons are shared between many atoms. This bonding confers electronic and optical properties that although ideal for optoelectronics, also lead to limitations. For example, at room temperature, most of these materials are solid and not suitable for flexible devices. The pi-conjugated molecules tend to be insoluble in solvents and difficult to work with in printing technology.

These properties can be changed by attaching alkyl chains to the pi-conjugated molecules. Quite how these alkyl chains, which have a backbone of carbon atoms, and varying length and branching structures, affect the properties of pi-conjugated molecules, is not known. Fengniu Lu and Takashi Nakanishi of NIMS have reviewed studies to determine the fundamental rules of the process. Dr. Nakanishi invented a way to control the self assembly of linear alkyl chains, such as alkylated-fullerenes, to pi-conjugated molecules in 2005. He recently developed a technique to create luminescent, room temperature 'liquid' pi-conjugated molecules by wrapping the pi-moiety up with several branched alkyl chains.

To assess the effects of attached alkyl chains, the team collated research that studied the properties of pi-conjugated molecules modified with specific alkyl chains. Some studies demonstrated that different types of alkyl chains, solvent polarity, temperature and chain-substrate interactions led to the assembly of pi-conjugated molecules into various two- and three-dimensional structures. Other studies showed that alkyl chains with certain structures allowed the formation of 'thermotropic' liquid crystalline materials which have properties between those of hard solids and soft liquids, as well as the formation of materials that were 'isotropic' liquids at room temperature and from which photoconducting liquid crystals or gels could be formed. The authors describe this strategy as alkyl-pi engineering.

They conclude that changes in the properties of alkylated-pi molecules depend upon the precise balance of the interactions among the pi-conjugated units as well as static interactions (known as van der Waals forces) among the alkyl chains. Different alkyl chains affect the balance of these interactions, leading to different molecular structures and properties. Researchers will be able to deliberately engineer pi-conjugated molecules to have specific properties.

Caroline Hayes

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