Functional dyes and pigments are produced in small volumes compared to compounds used for dyeing textiles. However, they are the subject of much research and interest and are being developed for a variety of purposes. Some of these are illustrated below.
(a) Liquid crystal displays
Liquid crystals have played an important part in our lives for many years in various forms of information displays e.g. calculators. Initially they could only display differences between light and dark. It was found that by using dyes this contrast could be increased and coloured screens produced. They have now largely replaced the traditional display technologies of light emitting diodes and cathode ray tubes. The dyes used have been specifically designed to change orientation with the liquid crystal molecules and therefore offer a higher intensity of colour. These dyes are said to exhibit dichroism.
(b) Laser dyes
The term laser is an acronym referring to light amplification by stimulated emission of radiation.
Commonly inorganic lasers were used but only had the ability to produce radiation at a few selected wavelengths and in very narrow bands. The use of dyes has allowed for the production of light throughout the spectrum from wavelengths of 320 to 1200 nm. The application of dye lasers includes communication technology, and microsurgery.
(c) Ink jet printing
Ink jet printing is a non-impact technique to produce images by directing small droplets of ink, ideally under computer control, in rapid succession onto a substrate. It has found many applications. Because of the size requirements for the droplets to be able to achieve good definition the use of dyes has been favoured over pigments. Droplets are smaller (pigments tend to block the nozzles) and aqueous solubility reduces the environmental impact and keeps the price low. Early dyes were those already used in other industries but were characterized by poor water fastness. This has led to the development of specific dyes and unique fluid systems. These dyes are designed to be soluble in slightly alkaline systems (pH 7.5 to 10) which are made insoluble by the slightly acidic conditions (pH 4.5 to 6.5) on the paper or other substrate. This technology is having a great impact on high volume industrial printing for packaging, textiles, wallcoverings and advertising displays.
(d) Photodynamic therapy
This is a treatment for cancer that uses a combination of laser light, a photosensitizing compound (the dye) and molecular oxygen. The dye is administered to the patient intravenously and over time enters the cancerous cells. Irradiation of the cells with laser light can start their destruction.
The laser interacts with the dye and promotes it to its excited state. Through a complex process, excited (more reactive) oxygen molecules are produced which react with unsaturated centres in the proteins and lipids in the cell membrane. This method of treatment avoids the use of invasive surgery.