Over one hundred and sixty have passed since Perkin obtained the first commercial synthetic dye maueveine in 1856. Today numbers of synthetic colored molecules are known. The functional role of colored molecules which mediate the specific portion of electromagnetic radiation in many biological processes, sensing, and communications have been to some extent fully understood with the developments in physics, chemical biology as well as with the advent of technology. The colored molecules which usually contain a host of pi-electrons as well as non-bonding electrons mobilize the electron on interaction with electromagnetic radiation manifesting in several functional applications particularly in biology and electronics.
Traditionally colorants have been used as coloring matters for polymer substrates like textiles and plastics for enhancing the aesthetics of the materials. Today in electronics industry they are used as key materials which interact with light in an efficient manner. The sources of light used in such circumstances are monochromatic light, or light with narrow bandwidth, such as light produced by a halogen lamp. In such devices dyes with special properties are desired.
Usually in traditional applications colorants must generally possess satisfactory color, good fastness properties against external agencies like heat, light, and washing as well as chemical tolerance. However, different properties are need of the functional dyes. These properties are for example, near infrared absorption, pleochroism, non-linear optical properties, photochromism, photoconductivity and other similar properties. The relationship between chemical constitution and these properties has to be understood to some extent in order to delve in to the design of such kinds of dyes.
Advances in the knowledge of color-constitution relationships of chromophoric molecules are attributable to the developments in quantum chemistry both theoretical and computational. Quantitative Molecular Orbital theory and Density Functional Theory have been of great tool for chemists in this context.
In functional applications demands have arisen from the electronics industry to develop new types of colorants or precursors that change color as a result of small amount of external energy produced by light, heat, or an electric current or such stimuli.
Usually attempt is made at the first instance to use latent properties of traditional colorants, such as their photoconductivity for electrophotography and solar batteries, sublimation for thermal transfer printing, diffusion and thermal printing recording, photolysis for thermal recording and charging for ink-jet recording systems. In an another approach traditional dyes structures are modified to develop new functions, for example dichroic dyes for liquid crystal displays, heat sensitive dyes for thermal recording systems and near-infra-red active dyes for optical recording applications. In a totally different approach totally novel molecules have been developed with specific functions in mind. To develop such new special dyes it is essential to be able to have a clear understanding of color-constitution as well as color-physical constitution relations.
Chromophoric systems based on perylene, phthalocyanine, indigo and azo systems are used as organic photoconductors for electrophotography. Many kinds of fluorescent dyes are used for dye lasers covering radiation wavelength from ultraviolet to infra-red regions. Triphenylamines, phthalocyanine, squarylium, croconium cyanine dyes and quinine dyes are used as infrared absorbing dyes for optical recording systems in semiconductor lasers. Crystal violet lactone and fluoranes are used as heat sensitive and pressure sensitive dyes for use in carbonless copying paper and electric recording systems such as facsimiles. In fact almost all chromophoric systems known has in a way or other many electro-optical applications.
Easily sublimable dyes from quinine, quinophthalone, styryl and indonaphthol series find application in thermal-printer recording systems to give full colour copies from electronic systems. Erasable recording systems make use of spiropyrans and fulgides. Azo, quinine, indigo and cyanine dichroic dyes are used in guest-host liquid crystal display systems. Phthalocyanines and some merocyanine dyes are studied as energy transfer dyes for solar batteries. The geometrical photoisomerism of stilbene, azo, and indigo dyes can have potential applications in solar energy transfer and storage systems. Several dyes with non-linear optical properties are being extensively investigated for several functional applications.
Prof. N. Sekar
Dyestuff Technology Department, Institute of Chemical Technology, Matunga, Mumbai - 400 019.