**2. Functional dyes**

Color plays an important role in the world in which we are living. Color can sway thinking, change actions, and cause reactions. If properly used, color can even save on energy consumption. The colors are characterized by their ability to absorb light in the visible spectrum (from 380 to 750 nm). The dyeing industry is in existence since 2000 years BCE wherein dyes were obtained from natural sources *viz*., plants, insects/animals, and mineral [5, 6]. A drastic development occurred after the discovery of the dye Mauveine by W.H. Perkin in 1856 while trying to synthesize quinine [7]. Dyes are the organic compounds with three essential groups in their molecules *viz*., the chromophore, the auxochrome, and the matrix. The chromophore is an active site of the dye which may be an atom or group whose presence is responsible for the color of a dye. The auxochrome is responsible for the intensity of the color of the dye with lone pairs of electrons.

It was Yoshida and Kato who used the term "functional dye" for the first time in 1981 due to the advancements and growth of dye chemistry related to high-technology (hi-tech) applications that are divergent from the well known traditional applications [8]. Hi-tech applications of dyes include the fields *viz*., optoelectronics (i.e. Dye-sensitized solar cells), photochemical materials, liquid crystal displays (LCD), and the newer emissive displays i.e. organic light-emitting diodes (O-LED), electronic materials (organic semiconductors), imaging technologies (electrophotography which includes photocopying and laser printing), thermal printing, and especially ink-jet printing, biotechnology (in dye-affinity chromatography for the purification of proteins and enzymes), biomedical applications (fluorescent sensors and anticancer treatments such as photodynamic therapy). All these fields were responsible for the design and synthesis of newer dyes to meet new and demanding criteria. Dyes, and related ultraviolet and particularly infrared active molecules,

**59**

discussed.

*Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

**2.1 Dyes (sensitizers) used in solar cells**

*2.1.1 Dye-sensitized solar cells (DSSCs)*

functional dyes.

which have been specifically designed for these hi-tech applications, are called

synthesized only under microwave irradiation are discussed.

containing metal-free organic dyes as sensitizers [9].

electrolyte redox couple within the pores [11].

Common dyes have been synthesized by applying mainly the conventional methods and also by microwave assistance. In the following sections the functional dyes used in solar cells, fluorescent sensors, fluorescent dyes to print on fibres, photochromic materials, O-LEDs, and dyes with advanced applications which were

To prevent harmful impact on the environment by conventional energy sources

In the conventional systems, the semiconductor does the task of light absorp-

The organic dye sensitizers consist of three important components *viz*., electron donor (D), π-conjugated spacer (π), and electron acceptor (A). Electron acceptors are generally acid ligands which also act as anchoring groups for loading the dye on TiO2 surface. The π-conjugated spacer (*viz*., conjugated double bonds, phenyl rings, thiophene, polythiophenes, etc) acts as a bridge to transfer electrons between the donor and the acceptor group and it is the key part which can induce a shift of both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels so that the photophysical properties may be tuned. The organic dyes/sensitizers belong to different classes depending on the donors such as triphenylamine, phenothiazine, fluorene, coumarin, carbazoles, etc. which have been profusely synthesized, and their power conversion efficiency as sensitizers have been reported and reviewed exclusively [12]. The structures of the dyes/ sensitizers synthesized under microwave irradiation along with the parameters such as short-circuit current (*Jsc*), open-circuit voltage (*Voc*), Fill Factor (FF), and power conversion efficiency (PCE) of the solar cells fabricated using these dyes are

tion as well as charge carrier transport. However, these two functions are separated in DSSCs by the metal-free organic dye and TiO2 in presence of an electrolyte. Hence, new ways of manufacturing the solar cells that can be scaled economically up to large volumes are essential. In this regard, a new generation of DSSCs also known as "Grätzel cells" has been fabricated by O'Regan and Grätzel [10]. A Grätzel cell consists of nanoporous titanium dioxide applied on transparent conducting oxide which is further made to absorb the dye from its solution. This film loaded with dye/sensitizer is immersed in an electrolyte containing a redox couple and placed on a platinum counter electrode. After irradiation, the excited electron from the dye (sensitizer) is transferred to the conduction band of TiO2 and diffuses through its porous network to the contact. Thus oxidized dye is further reduced to the original state by the supply of electrons through a liquid

it is necessary to use the alternative energy sources, specially, the solar cells. The conversion of sunlight into electricity is a clean, abundant, and renewable energy source. The amount of energy available from the sun to the earth is of the order of 3 × 1024 joules/year thus making it the best among sustainable energies. Photovoltaic devices have been fabricated using inorganic materials of high purity and energy-intensive processing techniques. The fabrication using these inorganic materials is not economical and often used scarce toxic materials. Therefore, such solid-state junction devices have been challenged by the 3rd generation dye-sensitized solar cells (DSSCs) which are based on interpenetrating network structures

#### *Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

when compared to a reaction on an oil bath and a microwave reactor [3].

reduce the processing time and enhance the overall quality [4].

ing useful materials such as dyes possessing hi-tech applications.

the color of the dye with lone pairs of electrons.

**2. Functional dyes**

nanotechnology, biochemical processes, thermal food processing, hydrothermal and solvothermal processing, etc. [2]. The energy efficiency is higher in the case of microwave heating in comparison with the conventional heating as evidenced by one such Suzuki reaction in which there is an 85 fold reduction in energy demand

During a chemical reaction under the conventional heating, the energy is introduced by convection, conduction, and radiation of heat from the surfaces of the reactants in the solution, and the energy transfer occurs due to thermal gradients. But in the case of the microwave irradiation, the energy is introduced through the electromagnetic field interaction into the molecules and the transfer of electromagnetic energy to thermal energy is energy conversion instead of heat transfer. This variation in the mode of introduction of energy leads to the advantages of using microwaves during chemical reactions. The microwaves penetrate easily into the bulk and, hence, heat evolves throughout the volume of the reaction mixture. As a result, fast and uniform heating of the reaction mixture can be advanced. In conventional heating, it is necessary to slow rates of heating to minimize the steep thermal gradients and obviate the process-induced stresses. As microwaves can transfer energy into all volumes of the reaction mixture, the potential exists to

Although the use of microwaves for organic synthesis is widespread, the documentation of this technology to the synthesis of the functional dyes is a relatively new development. The use of microwave energy for their synthesis has the potential to offer similar advantages in reduced reaction times and energy savings for obtain-

Color plays an important role in the world in which we are living. Color can sway thinking, change actions, and cause reactions. If properly used, color can even save on energy consumption. The colors are characterized by their ability to absorb light in the visible spectrum (from 380 to 750 nm). The dyeing industry is in existence since 2000 years BCE wherein dyes were obtained from natural sources *viz*., plants, insects/animals, and mineral [5, 6]. A drastic development occurred after the discovery of the dye Mauveine by W.H. Perkin in 1856 while trying to synthesize quinine [7]. Dyes are the organic compounds with three essential groups in their molecules *viz*., the chromophore, the auxochrome, and the matrix. The chromophore is an active site of the dye which may be an atom or group whose presence is responsible for the color of a dye. The auxochrome is responsible for the intensity of

It was Yoshida and Kato who used the term "functional dye" for the first time in 1981 due to the advancements and growth of dye chemistry related to high-technology (hi-tech) applications that are divergent from the well known traditional applications [8]. Hi-tech applications of dyes include the fields *viz*., optoelectronics (i.e. Dye-sensitized solar cells), photochemical materials, liquid crystal displays (LCD), and the newer emissive displays i.e. organic light-emitting diodes (O-LED), electronic materials (organic semiconductors), imaging technologies (electrophotography which includes photocopying and laser printing), thermal printing, and especially ink-jet printing, biotechnology (in dye-affinity chromatography for the purification of proteins and enzymes), biomedical applications (fluorescent sensors and anticancer treatments such as photodynamic therapy). All these fields were responsible for the design and synthesis of newer dyes to meet new and demanding criteria. Dyes, and related ultraviolet and particularly infrared active molecules,

**58**

which have been specifically designed for these hi-tech applications, are called functional dyes.

Common dyes have been synthesized by applying mainly the conventional methods and also by microwave assistance. In the following sections the functional dyes used in solar cells, fluorescent sensors, fluorescent dyes to print on fibres, photochromic materials, O-LEDs, and dyes with advanced applications which were synthesized only under microwave irradiation are discussed.
