1.0 INTRODUCTION
Dyes have many practical applications in a wide variety of contexts. Examples of their use range from providing simple, stable colours on paper and textiles, to providing switchable colours in more advanced technology applications, such as those found in flat-panel displays. The annual global dye market is in the multi-$billions, and there is a continuing demand from industry for new dyes that are designed to meet the stringent requirements of increasingly more advanced applications as high-technology devices evolve. From a research perspective, this continuing demand for new dyes requires the development of better molecular design strategies that are firmly based on an improved understanding of the fundamental chemistry that defines the properties of such dyes. It is important to develop a new strategy for designing dyes for advanced materials applications by using a combination of experimental studies and computational modelling, and which is based on understanding the way in which subtle variations in molecular structure can affect and enhance the properties of dyes. Dyes, and related ultraviolet and particularly infrared active molecules, which have been specifically designed for these hi-tech applications, are generally called functional dyes.
Japanese scholars in 1981 proposed ‘‘functional dyes’’ as materials that exhibit specific properties other than colouring. Functional dyes are materials which express new features by light, heat, electric fields, pressure, or other stimuli, in addition to the coloring functions provided by conventional dyes and pigments. This new term has come out of renewed interest and activity in the field of dye chemistry closely related with high-technology (hi-tech) applications different from the well-know traditional applications.
1.2 APPLICATIONS OF HIGH TECHNOLOGY DYES
The last 30 years, and particularly the last decade, has witnessed a phenomenal rise in the growth of dyes for high-technology (hi-tech) applications. Hi-tech dyes are applicable in wide areas like optoelectronics, such as dye sensitized solar cells, photo-chromic materials, liquid crystal displays, and the newer emissive displays such as organic light emitting devices; electronic materials, such as organic semiconductors; imaging technologies, such as electrophotography (photocopying and laser printing), thermal printing, and especially ink-jet printing; “invisible” imaging by using infrared absorbers in optical data storage, computer-to-plate and security printing; biotechnology as dye-affinity chromatography for the purification of proteins and enzymes; biomedical applications, such as fluorescent sensors and anticancer treatments such as photodynamic therapy, created the need for novel dyes to meet new and demanding criteria. Today, ‘‘Functional Dyes’’ have become popular to apply various industrial products such as materials of display, energy and light controlling with development of electronic field. Some applications of hi-tech dyes are stated and explained below in detail:
Optoelectronic application of hi-tech dyes
Solar Cells
The forecasts for the next 50 years predict that human energy needs are likely to double while fossil energy reserves are shrinking. At the same time, on earth, per year, plants and bacteria capture and convert light into a quantity of biological material greater than 100 times the food needed for mankind.
The natural light harvesting systems of plants and photosynthetic bacteria are one of the most fascinating functional molecular assemblies, which inspired many scientists for mimicking such process to harvest the solar energy. The first requirement for a light-harvesting system is, of course, its capacity to absorb light. If the system has to be used for artificial solar energy conversion, the absorption spectrum of its components should cover a substantial part of the visible spectral region. Another essential property of the light-absorbing units of an antenna system is their chemical and photochemical stability. The conversion of sunlight into electricity is a clean, abundant, and renewable energy source. The efficiency of conventional solar cells, or photovoltaic devices, made from inorganic materials reached up to 24%, using very expensive materials of high purity and energy intensive processing techniques.
New ways of manufacturing solar cells that can scale up to large volumes and low cost are required. In this interest a new generation of solar cells called dye-sensitized solar cells (DSCs) has been reported in 1991 by O’Regan and Grätzel. In contrast to conventional systems, in which the semiconductor assumes both the task of light absorption and charge carrier transport, the two functions are separated in DSCs.
A dye-sensitized solar cell of Grätzel type, typically consists of a dye-adsorbed nanoporous TiO2 film prepared on transparent conductive oxide, such as conductive glass. The film is immersed in an electrolyte containing a redox couple and placed on a platinum counter electrode. After absorption of a photon, the excited electron within the sensitizer molecule is transferred to the conduction band of TiO2 and diffuses through the porous TiO2 network to the contact. The oxidized sensitizer molecule is reduced to the original state by supply of electrons through a liquid electrolyte redox couple within the pores.
Photochromic Dyes
Recent progress in information technologies toward increasing capacity of optical information carriers and information processing rates makes urgent the development of new light sensitive media for use in super high-capacity optical memory. Compared to the thermal principle of data recording on modern optical disks, the photochemical principle of optical data recording on photochromic multilayer optical disks can be expected to ensure a higher density of data recorded on the pico- or femtosecond scale. Therefore, photonic devices that incorporate photochromic molecules represent the future of digital optical storage where the recording of information is an all photo-mode recording method that allows the read, write, and erase functions to be controlled by light energy. In photo-mode recording, light characteristics such as wavelength, polarization, and phase can be multiplexed to enable data storage and potentially increases the memory density.
One promising approach is the development of photochromic materials. These materials can interconvert between two distinct isomeric states when stimulated by two different wavelengths of light, where each isomer of the photo-chromic materials can represent “0” and “1” of a digital mode. Molecules with this behaviour are the best promising candidate optical storage materials as opposed to heat-mode recording employed with the optical media currently in use. It has been reported that photo-chromic compounds suitable for photonic applications must meet the following requirements: high cross section for two-photon absorption, high efficiency of photo-chromic transformations, thermal stability and fatigue resistance to irreversible photo- and thermo induced transformations of both forms (colorless and colored), and nondestructive readout by any suitable method (via fluorescence, refraction, reflection, polarization, etc.). Ongoing efforts in this area have focused on increasing the thermal stability of photo-chromic dyes by mainly two ways, first by designing new dye molecules with structural features to fit the necessary criteria mentioned above, and the second is by incorporating the dye molecules in a rigid matrix. It has been reported that organic photochromic compounds which exhibits valent isomerization such as diarylethenes have the above requirements to an utmost extent. Visible light-absorbing materials have been employed as materials in order to color tuning of color filters which liquid crystal displays have built-in. Spectroscopic properties peculiar to dye-based materials, such as good transmissivity and steep spectrum, absorb only unnecessary light emitted from backlight and contribute to high colorseparability. These properties of specific wavelength absorbing dyes improve color reproducibility of displays.
Smart Textiles
Photo-chromic dyes, such as spirooxazines, are disperse dyes. Structurally, there are similarities with traditional disperse dyes for textiles in that they are small- to medium-size neutral molecules with a balance of hydrophilic and hydrophobic character. The major difference, however, is that they are nonplanar in their ring-closed form because the two ring systems of the molecules are connected through a spiro linkage (sp3 carbon) so that they are orthogonal to one another.
Since the color change of these organic photochromic dyes is triggered by UV light, these dyes also have a UV protection function. Hence, applying photochromic dyes to a fabric can impart to the fabric, enhanced UV protection, smart fashion effects, functional effects such as camouflage, security printing, and for use in “smart” textile applications in general. In this prospective, recent research activity is growing for the application of photo-chromic compounds on textiles.
Biomedical and fluorescent sensor application of hi-tech dyes
Photodynamic Therapy
Photodynamic therapy (PDT) is a light-activated treatment which is used in the clinic to destroy diseased tissues and is in trials for the eradication of localised bacterial infections. The photodynamic action arises from the combined use of a photosensitiser and a light dose; independently, each of these components is harmless. The dual selectivity of the treatment is achieved through preferential uptake of the photosensitiser by diseased tissues and by controlled delivery of the light.
Fluorescent Sensors
The practical importance of functional dyes used in optical chemical sensor development has received increased attention in recent years. Fluorescent sensors represent a versatile tool for visualizing specific molecular targets and events in vitro and increasingly in vivo. As a consequence, an increasingly large number of fluorescent sensors, available in different colors, are required. A fluorescence-based technique for sensor applications offers significant advantages over other techniques due to its generally nondestructive character, high sensitivity, and specificity. Chemosensors are molecules or molecular systems changing their optical or electrical properties upon interaction with small anions, cations, or neutral molecules.
Visible light-absorbing materials have been employed as materials in order to color tuning of color filters which liquid crystal displays have built-in. Spectroscopic properties peculiar to dye-based materials, such as good transmissivity and steep spectrum, absorb only unnecessary light emitted from backlight and contribute to high colorseparability. These properties of specific wavelength absorbing dyes improve color reproducibility of displays. Visible Light-Absorbing Materials can also be used as the materials for optical filters, optical recording materials, organic solar cells and organic electrochromic among other things.
Near-infrared light absorbing materials are used for shielding heat rays (heat shielding) due to absorbing light of infrared region. These materials are used to window glass such as automotive and architectural, therefore attract a great deal of interest as energy-saving materials. These materials having satisfactory absorbability of near-infrared light make possible to apply to versatile purposes for example optical filters, laser welding, anti-counterfeiting inks and sensors.
Luminescent Materials (Fluorescent Dyes) and Leuco dyes can be applied for like organic light emitting diodes (OLED) and dye laser, and "Leuco Dyes" can be applied for such as stationery products as thermosensitive materials.
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