Amazingly 'green' synthesis method high-tech dyes

Amazingly 'green' synthesis method for high-tech dyes

5:28 AM, 6th September 2018
TU Wien-high-tech dyes.

Dyes that are also of great interest for organic electronics have recently been prepared and crystallised at TU Wien. All that is required is just water, albeit under highly unusual conditions.

They not only impress due to their radiant and intense colour, they also have an important technological significance: organic dyes are a class of materials with extremely special properties. From flat screens to electronic paper through to chip cards: in future, many technologies are likely to be based on organic molecules like these.

Previously, such materials could only be prepared using complex synthesis methods that are incredibly harmful to the environment. However, researchers at TU Wien have now successfully synthesized several typical representatives of this materials class in an entirely new and different way: toxic solvents have been replaced by plain water. But how is this done? When water is heated to extremely high temperatures, its properties change significantly. Details of the new preparation method recently published in the prestigious scientific journal ‘Angewandte Chemie’.

The researchers at TU Wien used water heated to at least 180° C in special pressure vessels.

“The properties of cold, liquid water are strongly influenced by what is known as hydrogen bonding,” explained Unterlass.

“These are weak bonds between water molecules that are constantly broken and reformed.” On average, each water molecule is linked to three or four other water molecules at any time at room temperature. In a pressure cooker, the number of these hydrogen bonds per molecule decreases.

“This also means that many more ions are present in water at high temperatures than under standard conditions – a certain amount of H2O molecules can become H3O+ or OH-,” explained Unterlass. And this dramatically changes the properties of the water.

Amongst other things, the higher number of ions in the water at elevated temperatures is a key cause for allowing the dissolution of organic substances that are entirely insoluble under normal conditions. Consequently, the dye molecules studied can not only be synthesised in water, but also crystallised: they dissolve at sufficiently high temperatures and then crystallise as they cool down.

“Normally, toxic solvents are needed to prepare or crystallise such dyes. In our case, though, pure water adopts the desired solvent properties – all you need is pressure and heat,” said Unterlass.

For the crystals obtained, however, there are also some quite different potential applications. “They can be used wherever the requirements for dyes are rather demanding,” said Unterlass. “One such application would be car paint, or other areas where extreme chemical or thermal conditions prevail, as the materials also become more stable the more crystalline they are.”

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