Amazingly 'Green' Synthesis Method for High-Tech Dyes

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

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At room temperature the dye indigo is completely water-repellent. A droplet of water easily pearls off. (TU Wien)At room temperature the dye indigo is completely water-repellent. A droplet of water easily pearls off. (TU Wien)

They not only impress due to their radiant and intense color, they also have an important technological significance: organic dyes are a class of materials with extremely special properties. From flat screens, to electronic paper, to chip cards; in the 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several typical representatives of this materials class in an entirely new and different way: toxic solvents have been replaced by plain water.

The Properties of the Water Change Without the Need for Additives

"If you were to listen to your initial gut feeling, you would actually suspect that water is the worst solvent imaginable for synthesizing and crystallizing these molecules," said Miriam Unterlass from the Institute of Materials Chemistry at TU Wien. "The reason for this expectation is that the dyes we produce are extremely water-repellent."

When a water droplet is placed on the dye powder, the droplet just rolls off. The dye cannot be mixed with water.

But this behavior only applies to water as we know it from everyday use. However, the researchers used water heated to at least 180°C in special pressure vessels. Under these conditions, pressure rises drastically, so that the majority of water remains liquid despite the elevated temperatures. The chemical and physical properties of water change drastically under these conditions.

Too Hot for Hydrogen Bonding

"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-," said Unterlass. And this dramatically changes the properties of the water: it can act both as an acidic and a basic catalyst and therefore accelerate certain reactions or even enable them in the first place.

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 synthesized in water, but also crystallized: they dissolve at sufficiently high temperatures and then crystallize as they cool down.

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

Crystals for the Electronics of Tomorrow

"In a highly crystalline state—i.e. at a high degree of order at the molecular level—the electronic properties of these materials improve. It is therefore particularly important for applications in organic electronics to have a high level of control over the crystallisation process," 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."

(Source: Vienna University of Technology)

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